shen2016 cleaning syngas

Renewable and Sustainable Energy Reviews 59 (2016) 1246–1268
Contents lists available at ScienceDirect
Renewable and Sustainable Energy Reviews
journal homepage: www.elsevier.com/locate/rser
By-products recycling for syngas cleanup in biomass pyrolysis
– An overview
Yafei Shen a,n, Junfeng Wang a,b, Xinlei Ge a,n, Mindong Chen a
a
Jiangsu Engineering and Technology Research Center of Environmental Cleaning Materials (ECM), Jiangsu Key Laboratory of Atmospheric Environment
Monitoring and Pollution Control (AEMPC), Collaborative Innovation Center of Atmospheric Environment and Equipment Technology (AEET), School of
Environmental Science and Engineering, Nanjing University of Information Science & Technology (NUIST), Nanjing 210044, China
b
Department of Environmental Toxicology, University of California at Davis, 1 Shields Avenue, Davis, CA 95616, USA
art ic l e i nf o
a b s t r a c t
Article history:
Received 5 September 2015
Received in revised form
27 December 2015
Accepted 13 January 2016
Available online 30 January 2016
Bio-char and bio-oil have a potential to be used for gas cleaning in biomass pyrolysis/gasification. On one
hand, tar in producer gas could be removed by physical treatment, such as oil absorption and char
adsorption; on the other hand, tar could be eliminated by chemical treatment, such as catalytic conversion over char-supported catalysts. This paper reviewed the recent progress in gas cleaning especially
for tar removal during biomass pyrolysis/gasification by using the by-products (i.e. bio-char, bio-oil, lowviscosity tar). In general, bio-char could effectively adsorb the light tar compounds such as volatile
organic compounds (VOCs), while bio-oil is normally benefit for the absorption of heavy tars. Additionally, catalytic reforming is considered as one of the promising alternatives for the removal of tars,
because it converts the tars into the additional gas products. Bio-char could be used as a carbon catalyst
or support with fair performance in tar removal. It is noteworthy that the char-supported catalysts could
be gasified to recover energy of char without the need of frequent regeneration after deactivation.
Furthermore, the carbon-based catalysts derived from bio-chars could be urgently developed for the
removal of contaminants including NH3, H2S and tar simultaneously in the producer gas from the real
biomass gasification processes.
& 2016 Elsevier Ltd. All rights reserved.
Keywords:
Biomass gasification
Tar removal
Bio-char
Bio-oil
Catalytic reforming
Contents
1.
2.
3.
4.
5.
6.
7.
n
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.1.
Bio-energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2.
Pyrolysis/gasification of biomass. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.3.
The purpose of this review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Biomass tar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Gas-cleaning technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.
Primary Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.
Secondary methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.
Catalytic conversion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Tar removal with bio-char. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1.
Adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.
Catalytic reforming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Tar removal with bio-oil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
N-containing tar removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Concluding remarks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Corresponding authors.
E-mail addresses: [email protected], [email protected] (Y. Shen), [email protected] (X. Ge).
http://dx.doi.org/10.1016/j.rser.2016.01.077
1364-0321/& 2016 Elsevier Ltd. All rights reserved.
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Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1266
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1266
1. Introduction
1.1. Bio-energy
Biomass is carbon based and is composed of a mixture of
organic molecules containing hydrogen, usually including atoms of
oxygen, often nitrogen and small quantities of other atoms,
including silica, alkali, alkaline earth and heavy metals. Biomass is
a renewable energy resource derived from biological sources, such
as energy crops, agricultural residues, forestry residues, algae and
municipal solid wastes. Biomass utilization is recognized as one of
the most promising solutions for current energy and environmental problems. Alternative renewable energy technologies such
as hydropower energy, solar energy and wind power, which often
suffer from the intermittent power generation issue, are less reliable in term of security of supply. Meanwhile, biomass is the only
renewable energy source that could be converted into liquid fuels
and used as feedstock in chemicals synthesis [1]. For instance, the
biomass power generating industry in the United States, consisting
of approximately 11,000 MW of summer operating capacity
actively supplying power to the grid, can produce about 1.4% of the
U.S. electricity supply. Fig. 1 illustrates the trends of the top five
countries in generating electricity from biomass. It suggests that
the electricity generated from biomass significantly increase,
especially for China, therefore requiring a further development of
biomass utilization for electricity and biofuels.
1.2. Pyrolysis/gasification of biomass
Thermochemical processes including combustion, pyrolysis
and gasification can convert biomass into the useful bio-energy
(i.e., fuel gas, bio-oil) and bio-char. Biomass pyrolysis or gasification is recognized as one of the most promising technologies for
producing sustainable fuels that could be used for power generation systems or syngas applications. For instance, biomass pyrolysis at relative higher temperatures could produce the bio-char,
bio-oil and syngas for boiler and power generation (Fig. 2).
Moreover, the proportion of the derived products, to some extent,
depends on pyrolysis temperature and other conditions. Compared
with the partial oxidative gasification process, the inert pyrolysis
of biomass has low process efficiency, but can produce fuel gas
with a high heating value [2]. Pyrolysis can be divided into three
subclasses; slow pyrolysis, fast pyrolysis and flash pyrolysis [3].
The main operational parameters are summarized in Table 1.
Significantly, gasification of biomass has several environmental
merits over fossil fuels, namely lower emission of CO2 and other
flue gases, such as H2S, SO2, NOx [3–7]. Biomass gasification is a
thermochemical process in which biomass undergoes the incomplete combustion to yield a gas product referred to syngas that
mainly consists of H2, CO, CH4, CO2, and N2 (if air or N2 is used as
the carrier gas) in various proportions. Biomass pyrolysis or gasification has considerable advantages compared to the direct
combustion. The main reasons are that it can convert the lowvalue feedstocks to high quality combustible synthesis gas, which
can be not directly burned for electricity generation but turned
into liquid transportation fuels [8].
Processes occurring in biomass gasification are often distinguished: drying and devolatilization, volatile and char combustion, and gasification and tar reforming with steam and CO2.
These processes can be identified in certain spatial regions in fixed
bed gasifiers [9]. As for biomass pyrolysis illustrated in Fig. 3,
during transient heating of the solid particle, temperature
increases locally, leading first to the evaporation of moisture
(drying stage) and then to the progressive release of pyrolytic
volatiles (primary pyrolysis stage). The primary volatiles are produced from the thermal scission of chemical bonds in the biomass
individual constituents, which are cellulose, hemicellulose, lignin
and extractives, and comprise permanent gas species (e.g., CO2, CO,
CH4) with the condensable species at ambient conditions (i.e.,
some organic compounds and water). Although each of the biomass constituents decompose at faster rates in different temperature ranges, the primary pyrolysis stage is complete at lower
Fig. 1. Trends in the top five countries generating electricity from biomass.
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temperatures ( o500 °C), thereby yielding a carbon-rich nonvolatile solid that is called char or charcoal. The produced char also
contain a significant part of the mineral matter originally present
in the parent fuel. Nevertheless, if the fuel is converted at relative
higher temperatures, some of the primary volatiles released inside
the particle can further participate in a variety of secondary
reactions to form product “2”. Serial and parallel reactions can
occur, either heterogeneously or homogeneously, such as the
cracking, the reforming, the dehydration, and so on [10–12].
During the devolatilization, biomass is thermally decomposed
into a carbonaceous solid named char, while it releases volatiles,
expressed in a simplified route by R1 in Table 1 and Fig. 4. The
volatiles consist of non-condensable gases, such as CO2, H2, CO,
CH4, H2, and so on, condensable gases (i.e. tar) and water vapor
(both from chemically bound and from the free water in the fuel).
The devolatilization stage, chemical decomposition by heating in
the absence of oxygen, is also termed as pyrolysis. Herein, volatile
material is released from the fuel. This is a thermochemical process driven by the heating of the fuel and oxygen is supposed not
to penetrate into the fuel particle, so the process proceeds in an
atmosphere free of oxygen. This is due to the high release of
volatiles from biofuels, which obstructs the transport of oxygen
from the bulk fluidization agent to the interior of the fuel particle.
However, oxygen possibly existing in the gas around the particle
can react with the residual volatiles or the char particles after
devolatilization. After the primary decomposition, various gas–gas
and gas–solid reactions take place: secondary pyrolysis, during
which the tar may be reformed (R12 and R13), oxidized (R11), and
Fig. 2. Pyrolysis of biomass for bio-char, oil and syngas production.
cracked (R15). The light hydrocarbons (e.g., CH4, C2þ ) and other
combustible gases (e.g., CO, H2) may react with O2 by reactions
(R7-R9). The carbon in char can be burnt (R2 and R3) or gasified
[reforming with CO2, H2O, and less with H2, by reactions (R4-R6)],
mainly depending on the oxygen concentration in the specific
location of gasifier where the char is present.
1.3. The purpose of this review
In this paper, the syngas contaminates mainly in the form of
biomass tar would be introduced. Then, the gas cleaning technologies such as primary and secondary methods would be presented. Based on these technologies, bio-char and bio-oil have a
potential to be used for gas cleaning in biomass pyrolysis/gasification. On one hand, tar in producer gas could be removed by
physical treatment, such as oil absorption and char adsorption; on
the other hand, tar could be eliminated by chemical treatment,
such as catalytic conversion over the char-supported catalysts. This
paper reviewed the recent progress in gas cleaning especially for
tar removal during biomass pyrolysis by using the by-products (i.e.
bio-char, bio-oil, low-viscosity tar).
2. Biomass tar
In common, condensable aromatic organics (e.g., polycyclic
aromatic hydrocarbons) referred to as “tar” are generated with the
producer gas during biomass gasification and their contents vary
from 0.5–100 g/m3 mainly depending on the design of a gasifier,
feedstock types, and operating conditions, etc. [13]. Tar is a generic
term comprising all organic compounds in syngas except for gaseous hydrocarbons. Tars can condense or polymerize to more
Fig. 3. Thermal decomposition of a solid biomass particle in inert atmosphere:
drying, primary pyrolysis and secondary pyrolysis [12].
Table 1
Main thermochemical reactions in biomass gasification.
Stoichiometry
Biomass-charþ light gas (COþ H2 þCO2 þCH4 þC2 þ N2 þ…) þ H2Oþtar
C þ 1/2 O2-CO
C þ O2-CO2
C þ CO2-2CO
C þ H2O-COþ H2
C þ 2H2-CH4
COþ1/2O2-CO2
H2 þ 1/2O2-H2O
CH4 þO2-CO2 þ2H2O
COþH2O-CO2 þ 2H2
CnHm þ(n/2)O2-nCOþ(m/2)H2
CnHm þnCO2-(2n)COþ(m/2)H2
CnHm þnH2O-nCOþ(m/2þ n)H2
CnHm þ(2n-m/2)H2-nCH4
CnHm-(m/4)CH4 þ (n-m/4)C
(R1)
(R2)
(R3)
(R4)
(R5)
(R6)
(R7)
(R8)
(R9)
(R10)
(R11)
(R12)
(R13)
(R14)
(R15)
Heat of reaction (kJ/mol)
Name
Endothermic
111
394
þ 173
þ 131
75
283
242
283
41
Highly endothermic (200–300)
Biomass devolatilization
Complete combustion
Partial combustion
Boudouard combustion
Steam gasification
Hydrogen gasification
Carbon monoxide oxidation
Hydrogen oxidation
Methane oxidation
Water-gas-shift reaction
Partial oxidation
Dry reforming
Steam reforming
Hydrogen reforming
Thermal cracking
Y. Shen et al. / Renewable and Sustainable Energy Reviews 59 (2016) 1246–1268
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Fig. 4. Thermochemical reactions occurring in the coal or biomass gasification.
Fig. 5. Two typical classification methods of biomass tar.
complex structures (e.g., coke) in pipes, filters, or heat exchangers
of downstream equipment and processes, which may result in the
mechanical breakdown of the entire system. Additionally, tars may
deactivate catalysts in the refining process. Tar removal by the
adsorption and reforming to syngas should be important and
indispensable to commercialize this technology for applications in
power generation and synthetic fuel production [14].
Tars can be classified by solubility and condensability [15],
categorized into five classes (Fig. 5). Class 1 refers to the GC
undetectable heaviest tars, which can condense at high
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Fig. 6. Proposed mechanisms for primary, secondary, and tertiary tar formation.
Fig. 7. Comparison of primary and secondary measures for gas cleaning.
temperatures and very low concentrations; Class 2 refers to the
heterocyclic aromatic compounds with high water solubility (e.g.,
phenol and cresol); Class 3 refers to the light hydrocarbons singlering aromatic compounds (e.g., toluene and xylene); Class 4 refers
to the light poly-aromatic hydrocarbons (2–3 rings), which can
condense at relatively high concentrations and intermediate
temperatures (e.g., indene and naphthalene); Class 5 refers to the
heavy poly-aromatic hydrocarbons (4–7 rings), condensing at high
temperatures and low concentrations (e.g., pyrene and coronene).
In summary, biomass tar is a complex organic mixture of the
condensable or non-condensable hydrocarbons comprising 1- to
5-rings aromatic compounds along with other oxygen-containing
hydrocarbons and polycyclic aromatic hydrocarbons (PAHs) produced during the thermochemical conversion. The most frequent
individual tar species studied experimentally and as modeling tar
compounds include acetol, acetic acid and guaiacols (primary
tars), phenols, cresols and toluene (secondary tars), and naphthalene (tertiary tar). Biomass is composed of lignin, cellulose, and
hemicellulose. Lignin fraction, which is aromatic form in nature,
normally consists of 20–40 wt% dry-based biomass [16]. Fig. 6
illustrates the mechanism of tar evolution assuming lignin units as
precursors. Three lignin-units of vanillin (C8H8O3), guaiacol
(C7H8O2), and catechol (C6H6O2) are pyrolyzed or react with
hydrogen. Moreover, the reaction pathway could be determined by
the most thermodynamically favorable reactions. Reaction R1
represents the vanillin pyrolysis, and reaction R1a is the vanillin
reacting with hydrogen. Phenol is transformed into cyclopentadiene, and CO is abstracted from the phenol according to the
reaction R4. Afterward, cyclopentadiene combines to form naphthalene in accord with reaction R7 [16,17].
Y. Shen et al. / Renewable and Sustainable Energy Reviews 59 (2016) 1246–1268
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Fig. 8. Effect of the temperature adjustment on parameters and processes in gasification [13].
3. Gas-cleaning technologies
It is essential to reduce the level of tar for widespread utilization of syngas. Considerable efforts have been directed to tar
removal from fuel gas. In general, tar removal technologies could
be divided into two approaches: gas downstream cleaning after
the gasifier (secondary methods) and treatment inside the gasifier
(primary methods). Fig. 7 presents the difference between the
primary and the secondary methods. The primary methods
include measures taken during the gasification process to prevent
or convert tar formed in the gasifier. Secondary methods are
measures to improve the hot product gas issuing from the gasifier.
An ideal primary method concept could eliminate the requirement
for the secondary treatment. The secondary methods have been
studied widely and are well understood. In contrast, the primary
methods have not yet been fully understood and are currently
investigated a great deal.
economic feasibility of the gasification process. Therefore, the
composition of the gasification agent must be assessed by appreciating the technical benefits and the economical drawbacks. Inbed additives used in the fluidized-bed gasifiers have been also
attracted much concern. Addition of various catalysts, such as
carbonate rocks (e.g., dolomite and limestone), olivine, and metalbased catalysts can improve significantly the gas quality by reducing tar content. Nevertheless, loss of catalyst materials by
entrainment, the inhibition caused by carbon and sulfur, or the
contamination of ashes greatly limits these options. Again, an
economic evaluation of the process determines which technique is
feasible in the practical use. The measure that yields the maximum
reduction of tar in the bed is not necessarily the best: primary
measures can be considered as a primary action in conjunction
with secondary measures. The overall process should be optimized
in terms of the technical reliability and the economy feasibility.
3.2. Secondary methods
3.1. Primary Methods
The primary methods include the selection of operating conditions, bed additives or catalysts in the gasifier, and the gasifier
design. The operating conditions can play a crucial role in biomass
gasification process in many ways, such as the carbon conversion,
the producer gas composition, the tar formation, and the tar
reduction. The most significant influencing parameters are the
temperature, the gas concentration, and the residence time. For a
given gasifier design and biomass type, these variables are the
result of setting the following variables: the air ratio (i.e., oxygen
feed in relation to the stoichiometric), the composition of gasifying
medium (flow rates of oxygen/steam in relation to the flow rate of
biomass), and the addition of catalysts and additives. The key
variable in gasification is the temperature. Fig. 8 summarizes the
effect of temperature in the gasifier on key variables (i.e., the
heating value of the gas and tar and char conversion) and processes, such as the sintering, when gasifying different biomasses.
The temperature range of 800–900 °C is normally employed in
biomass gasification, considering the balance of the benefits and
the drawbacks associated with the thermal level [13].
Addition of steam could enhance the reforming reaction of tar
and char gasification, improving the gas quality and reducing the
tar content. However, these reactions consume heat, which must
be added to the reactor, for example, by addition of oxygen. If the
oxygen is added by air, the gas is diluted with nitrogen and the
heating value of the gas produced is reduced. If pure oxygen is
used, a better gas quality is achieved, but oxygen affects the
Secondary methods are conventionally used to treat with the
hot product gas of the gasifier. The methods can be chemical or
physical treatments. Secondary chemical methods are composed
of tar cracking in downstream of the gasifier, either thermally or
catalytically. Secondary physical methods include the use of
cyclones, filters of various types (baffle, ceramic, and fabric),
rotating particle separators, electrostatic filters, and scrubbers. Tar
removal by means of condensation is, in principle, the least complicated way to remove tars. Various cooling methods have been
used in biomass gasification systems, including scrubbers, venturies, humidified packed beds, and so on. In the direct water-cooled
tar-removal systems, some of the dust, HCl, sulfur oxides, and
alkaline metals are also removed by water (e.g., venturi scrubber).
A significant drawback of the direct water-cooled systems is the
downstream of wastewater contaminated by tar, which needs
further treatment [13].
In methods based on scrubbing with water, an important factor
to consider is the amount of soluble tars in the water stream,
mainly phenols. Phenols are relatively easily destroyed at a relative
high temperature in the gasifier. Properly linked primary measures
with this secondary wet cleaning concept could be a sound technical solution for small- to medium-scale plants for power generation. Conversion of tar in the hot gas by thermal cracking/
reforming is generally preferred, since the energetic value of tar
remains in the gas phase, while it is decomposed into light fuel
gases. Thermal cracking is only effective at relative high temperatures, whilst a catalytic technique is more effective at the
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Fig. 9. Tar removals by oil absorption (A) and char adsorption (B).
thermal level of hot gas. However, the latter is more expensive and
has some technical shortcomings, such as inactivation caused by
carbon deposition. Some novel catalysts can overcome these
obstacles, but they still require much demonstration prior to the
industrial implementation. Thus, tar cracking systems have not
been widely commercially available, but are still under development. In summary, although some secondary gas cleaning methods are reported to be effective, they are usually not economically
viable.
In our previous works, the researchers have preliminary studied the tar removal by oil absorption and char materials
adsorption as illustrated in Fig. 9 [18–20]. Paethanom et al. [21]
combined an oil scrubber with the chestnut wood char adsorption
bed from the lab-scale to pilot-scale facility of integrated pyrolysis
regenerated plant. It showed that the optimum system required
for 0.045 m3/h pyro-gas about 1 L of oil. The oil scrubber charged
with 1 L of vegetable oil was combined with a 41 g of chestnut
wood pyrolysis char adsorption bed and reached 97.6% of the
gravimetric tar removal. It is noteworthy that the service life,
detailed mechanism and kinetics of various tar compounds
adsorption on chars are unclear and needed to be further studied.
Biochars from the pyrolysis and the carbonization of biomass
normally contain many nitrogen or oxygen functional groups such
as –NH2/–OH, C–O, C˭O, etc (Fig. 7A). Firstly, –NH2 on the char
surface is one kind of basic functional groups, while phenol is a
Lewis acid; accordingly, phenol is prone to combine with –NH2
through the acid-base interaction. Secondly, –OH, C–O and C˭O
groups could interact with phenol via hydrogen bond. Additionally, the strong electron-donating ability of the hydroxyl group can
cause the aromatic ring of phenol to be a π-electron rich system
(Fig. 7B). Thereby, the aromatic rings of different phenol molecules
can easily form π–π* stacking interactions to offer a multilayer
adsorption system. Compared to the Van der Waals force, which is
the primary driving force in adsorption of activated carbon, the
chemical interactions between the functional groups and phenol
molecule are more effective to enhance the adsorption [22].
Fig. 10. Dry reforming of tar compounds over the palygorskite (PG) supported Ni
catalyst [51].
3.3. Catalytic conversion
Several approaches for tar elimination in the primary or secondary treatment, such as physical treatment [18–21], thermal
cracking [23], plasma-assisted cracking [24], and catalytic
reforming [25–29], have been extensively employed. Physical
treatment such as adsorption and absorption is a sustainable and
feasible method for tar removal. Moreover, physical processes used
for tar removal are uncomplicated adaptable to any gasification
system. However, it always depends on gas quality specifications
for specific downstream applications. In general, char adsorption
could adsorb the light tar compounds, while the heavy tar is
normally eliminated by chemical methods, such as the catalytic
and non-catalytic thermochemical conversion. Among these, the
catalytic conversion has been considered to be one of the most
promising in the large-scale applications due to its fast reaction
rate and reliability [29] and its ability to transform tar into highvalue added gases such as CO and H2 in the presence or absence of
steam. Various types of catalysts such as calcined rocks [30],
zeolites [31], iron ores [32], alkali metals [33], nickel-based catalysts [34,35], and noble metal-based catalysts [36–40] have been
developed for their usefulness on tar reforming in biomass gasification. With regards to the catalytic reactivity and economic
Y. Shen et al. / Renewable and Sustainable Energy Reviews 59 (2016) 1246–1268
reasons, nickel-based catalysts are considered to be one of the
most promising catalysts for tar removal and gas upgrading [41–
47]. Nickel-based catalysts are commonly supported by natural
materials (e.g., dolomite, olivine, charcoal and palygorskite) (Fig. 10)
or metal oxides (e.g., Al2O3 and MgO) (Fig. 11) [48–54]. Table 2
summarizes the advantages and disdvantages of different catalysts
used for tar removal [55]. These catalyst supports are relative
expensive and unsustainable. Besides, the catalyst preparation
steps are time and energy consuming. Therefore, these factors
would inhibit the extensive applications of nickel-based catalysts.
As an alternative, the by-product of char has been used as a sustainable catalyst with fair performance in tar removal [56–59] and
an excellent carbon-based adsorbent [60]. The char-supported
catalysts with low cost can be simply gasified to recover energy
of char without the need of frequent regeneration after deactivation. However, char could be normally consumed due to the participation of gasification reactions and its properties are not fixed
mainly depending on biomass types and process conditions.
Fig. 12 shows the schematic diagram of the interaction among
char, vapor and tar in biomass pyrolysis. On one hand, carbon
derived from char can interact in a wide range of pyrolysis reactions. On the other hand, carbon derived from char can interact at
the later stage of pyrolysis reactions. The attractiveness of biochars as catalysts originates from low cost and natural production
1253
inside the gasifier. However, it could be consumed by steam or CO2
in the producer gas. The requirement for a continuous external
char supply or withdrawal mainly depends on the balance of biochars production and consumption in the gasification system.
Interestingly, bio-chars showed a good catalytic performance for
tar conversion as well.
Z. Abu El-Rub [56] comparatively studied the biomass char and
other catalysts for tar compounds reduction. As shown in Fig. 13A,
dolomite and nickel catalyst had the highest phenol conversion
efficiencies at 700 °C (90 and 91 wt%, respectively). However, the
commercial biomass char (C.B. char) showed a moderate phenol
conversion (82 wt%). Since the heterocyclic tars (e.g., phenol) at a
gasification temperature of 800 °C or above are thermally cracked,
only a small amount of the heterocyclic tars (e.g., phenol) remains
in the producer gas to be reformed catalytically. Naphthalene is a
major tar compound at 900 °C. In summary, the catalytic reactivity
obtained at 40 g/Nm3 is followed by the order of commercial
nickel 4 dolomite4olivine4silica sand. The relative low reactivity of the dolomite can be attributed to the low iron content. The
activity of olivine can be improved by a pre-treatment to make the
iron active present on the surface of olivine. The ranking of catalytic activity is obtained: C.B. char4 biomass char4 ash 4fluid
cracking catalyst (FCC) (Fig. 13B).
4. Tar removal with bio-char
Fig. 11. Steam reforming of tar compounds over the metal oxides supported Ni
catalyst [32].
Tar removal by carbonaceous adsorbents/catalysts includes
physical adsorption, thermochemical cracking/reforming, and
integration of adsorption and catalytic conversion. The tar conversion by char-supported catalysts can be performed by in situ
catalysis in the gasifer or ex situ catalysis in the reformer. Herein,
catalysts placed in contact with the feedstock inside the pyrolysis
reactor (in situ) improve conversion efficiency of the solid biomass, and adjust the distribution of volatilized products. Meanwhile, tar catalytic reforming has been proved to be effective for
thermochemical conversion of biomass to improve gas yield,
reduce tar contents, and enhance conversion rate. Biomass is
decomposed in the first stage, and then the derived pyrolysis
Table 2
Advantages and disdvantages of various catalysts for tar removal [52].
Catalyst
Advantage
calcined rocks
fragile materials and quickly eroded from fluidized beds
1. inexpensive and abundant;
2. attain high tar conversion 95% conversion with dolomite;
3. often used as guard beds for expensive catalysts most
popular for tar elimination
1. inexpensive;
lower catalytic activity than dolomite
2. high attrition resistance
1. lower catalytic activity than dolomite;
1. inexpensive and abundant;
2. fewer disposal problems
2. most natural clays do not support the high temperatures (800–850 °C)
needed for tar elimination (lose pore structure)
inexpensive and abundant
1. rapidly deactivated in the absence of hydrogen;
2. lower catalytic activity than dolomite
1. consumption because of gasification reactions;
1. inexpensive and abundant;
2. its properties are not fixed depending on biomass type and process
2. sustainable (natural production inside the gasifier);
conditions
3. high tar conversion compared to dolomite;
4. neutral or weak base properties
1. rapid deactivation by coke;
1. relatively inexpensive but not cheaper
2. lower catalytic activity than dolomite
2. than the above;
3. more known about it from experience with FCC units
1. natural production in the gasifier;
1. particle agglomeration at high temperatures;
2. reduce ash-handling problems
2. lower catalytic activity than dolomite
high tar conversion comparable to that of dolomite
rapid deactivation by coke
1. able to attain complete tar elimination at 900 °C;
1. rapid deactivation because of sulfur and high tar content in the feed;
2. increase the yield of CO2 and H2;
2. relatively expensive;
3. higher tar reforming activity (Ni-based catalysts are 8–10 3. relatively easier regenerated
times more active than dolomite)
olivine
clay minerals
iron ores
char
fluid cracking catalysts
(FCC)
alkali-metal-based
activated alumina
transition-metal-based
Disadvantage
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Y. Shen et al. / Renewable and Sustainable Energy Reviews 59 (2016) 1246–1268
Fig. 12. Schematic diagram of the interaction among char, vapor and tar in biomass pyrolysis.
Fig. 13. (A) Effect of various catalysts on the phenol conversion. T ¼ 700 °C, τ ¼ 0.3 s, feed gas composition: 6 vol% CO2, 10 vol% H2O and balance N2, inlet phenol concentration: 8–12 g/Nm3; (B) Naphthalene conversion T¼ 900 °C, initial naphthalene concentration: 40 g/Nm3 and 90 g/Nm3 (other conditions are the same as phenol
conversion) [56].
gases, volatiles, and tar are reformed in the second stage. The
addition of different catalysts with/without steam in the second
stage has presented a positive effect on biomass to gas conversion
and tar reduction [61].
Surface morphology, such as pore distribution, surface area and
surface functionality, are key properties to effectively reuse biochar as catalysts and absorbents. The intrinsic ash content of biochar, such as alkaline and alkali metal, may affect its catalyst
performance. Organic structure on bio-char is often acquired by
Fourier transform infrared (FT-IR) and Raman spectroscopy (RS)
[62]. FT-IR analyzes the chemical properties of char by assigning
peaks of interest to functional groups based on characteristic
absorption regions (as shown in Table 3) [63]. And RS can identify
structural features of highly disordered carbonaceous materials,
such as chars [64–66]. Raman spectral characteristics of the G
(graphite band) and D (disordered structure) bands are usually
used to investigate coal structure.
Due to highly porous textural structures [67–70], bio-chars
have already been used as adsorbents or catalysts for tar removal.
Their macroporous and mesoporous structures significantly
improve the dispersion of metal ions, and facilitate transport of
reactant molecules (e.g., toluene, 0.68 nm molecular size) into the
Table 3
FT-IR characteristic absorption of bio-char [63].
Functional group
Absorption (cm 1)
Alkyl C–H stretch
Aromatic C–H bending
Aromatic C˭H bending
Aromatic C, indicative of lignin C˭C
Alcohol/phenol O–H stretch
Aldehyde, ketone, ester, carboxylic acid
Phenol O–H bending
C–O stretching C–O–C groups and aryl ethers; phenolic C–
O associated with lignin
Phosphines and phosphine oxides, silican oxid, C–O–C
stretching
2950–2850
860–680
1600–1500
1440, 1510
3550–3200
1780–1700
1375
1270–1250
1100–950
internal surfaces of the catalyst. The activated char has a high
affinity and adsorption selectivity to hydrocarbon compounds
[68]. In particular, the mesopores of activated char play a vital role
for effectively converting heavy hydrocarbons into lighter fractions, while restricting the coke formation. Besides, the charsupported catalysts, because of their neutral or weak base
Y. Shen et al. / Renewable and Sustainable Energy Reviews 59 (2016) 1246–1268
properties, were superior to the solid acid catalysts with respect to
the resistance to deactivation by deposition of coke and heavy
metals.
4.1. Adsorption
As mentioned above, bio-char could be used for tar adsorption
[18,19,21]. However, tar is a very complex organic mixture in
biomass gasification resulting in the complex adsorption
mechanism, which can be studied by using tar model compounds
such as toluene, phenol with carbonaceous materials, including
bio-chars. Some researchers thought that bio-chars normally
contain various organic functional groups. Firstly, -NH2 on the biochar surface is a basic functional groups, while phenol is a Lewis
acid; accordingly, it is prone to combine with –NH2 via the acidbase interaction. Secondly, some organic functional groups (e.g., –
OH, C–O and C˭O) can interact with phenol via hydrogen bond.
Moreover, the strong electron-donating ability of the hydroxyl
group causes the aromatic ring of phenol to be a π-electron rich
system. Thereby, the aromatic rings of different phenol molecules
easily form π–π* stacking interactions to offer a multilayer
adsorption system. Compared to the Van der Waals force (as the
primary driving force in adsorption of activated carbon), the chemical interactions between the functional groups and phenol
molecule are more effective to enhance the adsorption [22]. Furthermore, Hadi et al. [71] investigated the role of the pore geometry in adsorption of phenol. The significance of micropore
surface area, rather than total surface area, has been proved in
adsorption of phenol. In addition, activated carbons with similar
surface areas, the one with higher fraction of microporosity results
in higher phenol uptake. It is attributed to the π–π London dispersion forces between the graphitic carbon basal planes and the
phenol aromatic ring, indicating that hydrogen bonding does not
occur between the phenol molecules and the functional groups on
the carbon surface.
To enhance the adsorption capacity of tar, bio-chars could be
fabricated into the desired carbon materials. Kong et al. [72] prepared a hierarchical porous char (SCCA/Zn) from sewage sludge by a
new two-step pore-fabricating process coupling citric acid (CA)
with ZnCl2 in a pyrolysis process. Coupling CA and ZnCl2 can
synergistically fabricate pores in the pyrolysis process, resulting in
a hierarchical porous char (i.e., SCCA/Zn), with the largest SBET of
867.6 m2/g due to the fact that the former contributes to the fabrication of macro-pores, which provides more space for fabricating
meso- and micro-pores by ZnCl2 activation. It is known that water
vapor has a negative effect on the adsorption of carbon materials
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[73,74]. The toluene adsorption capacity of activated carbon fiber
(ACF) was decreased significantly with the increase of humidity.
SCCA/Zn possesses similar adsorption breakthrough curves, and its
adsorption capacity is slightly deceased (6%) in air of 90% relative
humidity (RH) (Fig. 14A). The adsorption capacity of SCCA/Zn is
4.4 times higher than that of ACF in wet air with 90%. The
inconsistence between SBET and adsorption capacity is ascribed to
the strong hydrophobic property of SC CA/Zn [72].
It can be concluded that SCCA/Zn possesses both pore-driven
adsorption and hydrophobic partition adsorption for toluene as
illustrated in Fig. 14B. The solid tar oil ocates in these hierarchical
pores and covers the non-microporous surfaces. Therefore, the
micropore, mesopore, macropore and non-microporous hydrophobic surfaces of SCCA/Zn could all adsorb the hydrophobic
toluene gas by pore-driven and partition-driven factors, whereas
the toluene was adsorbed in the micropores and mesopores for
ACF. It could well explain the higher adsorption capacity of SCCA/Zn
to toluene, although its microporous surface is smaller than that of
ACF. Herein, SCCA/Zn had a more superior adsorption capacity to
toluene, due to pore and partitioning adsorption mechanisms [72].
To better understand the interaction difference between
organics and carbonaceous materials on site energy respect, Shen
et al. [75] recently conducted the site energy distribution analysis
to probe sorption behaviors of naphthalene, lindane, and atrazine
on ten sorbents with different physical structure and chemical
composition. Herein, sorbents with different surface chemistry
were used to evaluate roles of hydrophilic groups introduced to
their surfaces in organic sorption. The carbonaceous materials
with similar elemental composition but different physical structures, such as graphene, graphite, mesoporous carbon (MC) and
carbon nanotubes (CNTs) were chosen to better understand roles
of the physical structure (e.g., surface area and pore volume) of
these sorbents in their sorption for organics.
The sorption site energy distributions could be calculated by
the Dubinin–Ashtakhov (DA) model (as shown in Fig. 15). The
average sorption site energy and standard deviation of the site
energy distribution were deduced and applied to analyze the
interaction between sorbents and sorbates, and the sorption site
heterogeneity. The introduction of oxygen-containing functional
groups to the sorbents caused a decrease in the average sorption
energy. However, relative to the decrease in average site energy,
the reduction in number of sorption sites as indicated by surface
area more strongly reduced their sorption capacity to the carbonaceous materials based on the result of the linear regression
analysis. Sorption site heterogeneity of the sorbents decreased as
the oxygen contents increased, which is attributed to the better
Fig. 14. (A) Adsorption characteristics of toluene (initial concentration of 70 mg/L; flow rate¼ 200 ml/min; T ¼298 K) on SCCA/Zn at varied RH%; (B) schematic adsorption
model for toluene on ACF and SCCA/Zn (T: toluene molecule) [72].
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Y. Shen et al. / Renewable and Sustainable Energy Reviews 59 (2016) 1246–1268
dispersion of the oxygen-containing materials. The quantification
of the average sorption site energy and heterogeneity is helpful for
a better understanding of the sorption mechanisms of organics
(e.g., aromatic compounds) with carbonaceous materials [75,76].
4.2. Catalytic reforming
Physical treatment such as adsorption and absorption is a
sustainable and feasible method for tar removal. Physical process
Fig. 15. Sorption mechanisms of organic compounds by carbonaceous materials:
quantification the average sorption site energy and heterogeneity [75].
Fig. 16. Key parameters in catalytic reforming of biomass tar with bio-char based
materials.
for tar removal is uncomplicated adaptable to any gasification
system. However, it depends on gas quality specifications required
for specific downstream applications. In general, bio-char could
adsorb the light tar compounds, while heavy tar is widely eliminated by chemical methods, such as catalytic and non-catalytic
thermal conversion. Compared with other technologies, the catalytic reforming is considered as one of the most feasible technologies since it completely transfers tar compounds into syngas
including H2 and CO. Bio-char plays multiple roles in functionalization of char-supported metal catalysts. Firstly, bio-char acts as a
carbon reductant for converting metal oxides into metallic states
at relative high temperatures, thereby enhancing the catalytic
performance [77]. Secondly, bio-char has both co-catalyst and
adsorbent effects [78]. Bio-char as a catalyst is attractive because
of low cost and natural production inside the gasifier. However, it
could be consumed by steam or CO2 in the producer gas. The need
of a continuous external char supply or withdrawal depends on
the balance of char consumption and production in the gasification system. Bio-chars show a certain catalytic activity on tar
conversion due to their surface characteristics including alkali or
alkaline earth metallic species (AAEMS). Catalytic reforming of
biomass tar with bio-char based materials can be mainly influenced by char characteristics (e.g., biomass types, char surface
characteristics), reforming ways (e.g., in situ, ex situ), reformer
characteristics (e.g., dimensions), metal loading (e.g., Ni, Fe),
gasification agents (e.g., steam, CO2), and reforming conditions (as
illustrated in Fig. 16). Therefore, it is difficult to compare the
results from different char supported catalysts under different
conditions.
In general, the main concern with regard to the release of tar
during biomass gasification is the heavy molecular weight compounds in the condensable phase. The condensable yield separates
itself into a light fraction, containing water and other water
soluble products, and a heavier, more viscous fraction, which is the
main bulk of the tar. Table 4 shows the effects of temperature and
catalytic cracking in the tar cracking zone on the product yields.
The heavy condensable phase had a larger degree of decomposition at 700 °C both in the heterogeneous and homogeneous cases.
The amount of heavy condensable phase at 800 °C in the homogenous case was 8.87 wt% of biomass input, which corresponds to
52% of that from pyrolysis at 500 °C (18.4 wt%). In the presence of
char, the heavy phase further decreased to 8.0 wt%. This suggests
that the thermal cracking was the main mode of tar decomposition. Char contributed to promote the tar cracking [59]. Furthermore, the main mode of tar cracking was homogeneous and the
presence of char was helpful in further removing the tar. The total
carbon yield was over 100% for the cases in the presence of char. It
can be understood as the result of additional carbon input from
gasification of char by water vapor releasing from biomass drying
Table 4
Effect of temperature and heterogeneous cracking on the product yields (wt%) [59]
Presence of char
Heterogeneous (with char)
Homogeneous (without char)
Temperature (°C)
Char amount (wt%)
Heavy condensable (wt%)
Light condensable (wt%)
Producer gases (wt%)
H2
CO2
CO
CH4
C2H4
C3H6
C3H8
Subtotal
Product total
600
20.69
19.30
28.33
700
20.56
8.33
17.46
800
21.28
8.00
12.16
600
20.76
29.80
29.80
700
21.06
23.59
23.59
800
20.94
14.98
14.98
0.31
12.26
10.72
2.23
0.84
0.64
0.51
27.51
95.81
1.05
19.79
22.64
5.02
2.55
1.39
1.11
53.55
99.89
1.52
25.30
30.62
5.60
3.13
0.55
0.33
67.05
108.51
0.16
9.92
8.98
1.99
0.75
0.53
0.63
22.96
89.82
0.43
11.58
17.12
3.95
2.20
0.78
0.94
37.00
91.06
0.55
14.48
22.85
5.01
3.00
0.40
0.29
46.58
91.35
Y. Shen et al. / Renewable and Sustainable Energy Reviews 59 (2016) 1246–1268
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Fig. 17. Mechanism illustration of tar compounds (i.e., toluene and naphthalene) conversion over a carbonaceous surface [79].
Fig. 18. (A) Roles of pore structures of a carbon-supported catalyst in tar cracking and (B) TEM micrograph of the Ni–Mo–BAC catalyst, EDX shows the atomic contents at
points a and b are 80% C, 3–4% Ni and 4–5% Mo [28,91].
and tar cracking (C(s) þH2O2COþH2). The char gasification was
confirmed by the change in the weight of char in the tar cracking
zone. The char used for tar cracking/reforming can improve the gas
production from tar conversion and char further decomposition. In
addition, the oxygen-containing functional groups in the surface of
activated char can play a significant role in syngas reforming [79].
Fuentes-Cano et al. [78] investigated the catalytic decomposition of two model tars (toluene and naphthalene) over the char
materials (coconut char, coal char and dried sewage sludge char).
The main mechanisms for tar conversion over carbon material are
deposition, dehydrogenation (soot formed over the char surface)
and soot gasification [80–83], which are similar to tar conversion
over porous particles [84–88]. However, the AAEMS in the char
can impact significantly the soot gasification rate [81–83]. The rate
of homogeneous aromatic tar reforming below 900–1000 °C [89]
is smaller than that of heterogeneous conversion over carbonaceous surfaces [56]. Reforming of aromatic molecules such as
benzene and naphthalene over char cannot generate other aromatics [80]. In contrast, alkyl- and heteroatomic compounds produce lighter compounds by dealkylation and decarboxylation [60].
As illustrated in Fig. 17, the tar compounds initially contact a fresh
char with a certain number of active sites distributed over the
surface. The tar is adsorbed on the char matrix and undergoes
polymerization reactions, producing hydrogen and soot, the latter
staying over the char surface as solid deposits. This soot blocks the
active sites, hindering the interaction of the active sites with the
gaseous tar. If the carbon deposition rate is higher than the carbon
consumption rate, soot accumulation over the surface will occur,
decreasing the number of active sites available for reaction with
tar molecules and then the char reactivity. The active sites could be
attributed to the presence of AAEMS in the fresh char, since these
species are known to be active during steam gasification of soot
generated after tar deposition on char surfaces [90].
Due to their porous textural structures, activated carbons from
biomass or coal have been widely used as catalyst supports for
hydrocarbons and tar cracking, not only because their macropores
and mesopores can significantly improve the dispersion of metal
ions, but facilitate transport of reactant molecules (e.g., toluene,
0.68 nm molecular size) into the internal surfaces of the carbon
catalyst, as illustrated in Fig. 18(A). Xu et al. [91] proved that the
highly dispersed metallic nickel nanoparticles in the porous carbon matrix of the Ni–Mo–BAC (biomass derived activated carbon
in Fig. 18B) can effectively act as the active adsorption sites for
both the solvent and the polycyclic aromatic compounds. Although
bio-char itself shows a fair catalytic performance, the catalytic
conversion of tar can be improved in addition of metal actives. In
particular, metal actives such as iron present in the char can play
an important role for improving the catalytic activity. The further
work can comparatively study the catalytic conversion of tar with
different char materials.
Li et al. [92–94] comparatively studied the char-supported
catalysts for tar reforming. In general, reforming in the presence
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Y. Shen et al. / Renewable and Sustainable Energy Reviews 59 (2016) 1246–1268
Fig. 19. The yields of (A) tar and (B) coke after the reforming of mallee wood volatiles with steam by char and char-supported catalysts as a function of reforming
temperature [92].
of steam improve the H2 yield via tar reforming and water-gasshift reaction. The formed CO in syngas reacts with H2O to generate H2 and CO2. Fig. 19A shows the yields of tar after the
reforming of mallee wood volatiles with steam by char-supported
catalysts. The tar yields always decreased with the increase of
temperature due to the enhanced thermal cracking and reforming.
However, the tar yields after the pyrolysis were similar to those
after the steam reforming without catalyst at lower temperatures.
The external steam has weak influences on the thermal cracking
and reforming of tar in the gas phase because of its low reactivity
at low temperature [92]. The iron/nickel nanoparticles highly
dispersed in the char lead to high activities. Although the effects of
the char-supported iron catalyst on the tar yields were inconspicuous at low temperatures, its catalytic activity became very
prominent at high temperatures ( Z800 °C). It is noteworthy that
nickel has higher activity for tar reforming than iron even at
temperature as low as 500 °C [92]. In the case of char-supported
catalysts, there was a net gasification of char in the char-supported
catalysts (Fig. 19B). The iron (or nickel) nanoclusters were never
covered by coke, allowing their catalytic activities to be maintained during reforming.
In general, the tar reforming activity of char is determined by
three aspects. Firstly, as the active points, AAEM dispersed in the
carbon matrix of char promote the condensations of hydrocarbons
in volatile phase to form coke. Secondly, the porous structures in
char enable the tar adsorption efficiently, resulting in the good
residence time of tar reacting with catalyst. Finally, abundant
oxygen-containing functional groups are present on the char surface, forming some acidic centers, possibly combining with the
poly-aromatic rings (i.e. tar precursor) possessing negatively
charged π electron system and activate their thermal cracking
reactions [95]. The char-supported iron or nickel catalyst exhibited
much higher activity for reforming tar than the char itself did. Han
et al. [96] proved that the catalytic activity of such metal-char
catalysts for tar cracking is subject to an order of Co-char 4 Nichar 4Cu-char 4Zn-char, if based on their realized tar cracking
capability. In addition, the nickel loading of coal chars such as
nickel loaded brown coal char [97–99] and nickel loaded on lignite
char [100] show a good catalytic performance even at lower
temperatures along with the resistance ability of coke formation.
Significantly, as the nickel-loaded brown coal char was added as a
catalyst, most of the N-containing species in the volatiles converted into N2 gas. It suggests that nickel-loaded coal char has high
catalytic activity for the conversion of N-containing compounds
[99].
Wang et al. [101] investigated the Ni-based catalysts prepared
by mechanically mixing NiO and char particles at various ratios.
The catalyst preparation is inexpensive and convenient without
impregnation or calcination steps. In this work, under the same
reaction conditions, the Ni/coal char catalyst showed the highest
tar removal efficiency (ranging from 91% to 99%). Ni/wood char
achieved a lower tar removal efficiency (86–96%). In the absence of
Ni, coal char and wood char could also remove 75–90% tars
depending on the temperature (Fig. 20A). This indicates that char
alone is a reasonably effective catalyst for tar removal. Therefore,
the coal char supported nickel would be a good catalyst design to
take advantage of the high reactivity of nickel for heavy tar and
effectiveness of char for light tar cracking in syngas cleaning.
Furthermore, the Ni/coal char catalyst showed better performance
than the Ni/wood char catalyst at all NiO loadings because of the
better dispersion of Ni on coal char particles (as shown in
Fig. 20B). A rapid increase of tar removal from 10% to 20% NiO
loading on wood char can indicate that wood char requires higher
loadings of Ni because of poor adhesion of Ni particles on the char
surface. This suggests that better mixing of Ni and wood char
particles is required, especially to improve adhesion capability and
porosity of wood char.
Compared to the iron based catalysts, the nickel based catalysts
show a higher catalytic activity for the tar reforming. However, the
char supported iron catalysts have been widely studied due to
their relative low costs [92–94]. Bio-char catalytically removes tar
compounds partly due to the presence surface alkali metals such
as Na, Ca, K and potentially Fe. However, tar removal by using biochar is lower than metal supported catalysts, such as Ni/olivine
and Ni/dolomite. Attaching an active metals to the surface of biochar can improve its catalytic performance. Nickel supported catalysts have shown to remove tar efficiently, yet rapidly deactivate
due to coke deposition and H2S poisoning, and pose an environmental risk upon disposal. Alternatively, as an earth abundant,
non-toxic metal, iron has a highly potential to be used for tar
removal. It has been proved that iron and iron oxides (hematite,
Fe2O3, reduced to magnetite, Fe3O4) can act to catalytically oxidize
tar and ammonia as well [102], potentially by promoting steam
reforming and the water gas shift reaction. Most research using
iron oxide or iron as a tar removal catalyst has been studied using
aluminum, olivine, or coal char as a support. In addition, effectiveness of the bio-char to remove tar can be increased by
improving its surface area and chemical properties [92,103].
Kastner et al. [104] studied the reaction kinetics for catalytic
cracking of toluene using iron supported bio-char and evaluate the
longevity of the catalyst for tar cracking. Toluene conversion and
Y. Shen et al. / Renewable and Sustainable Energy Reviews 59 (2016) 1246–1268
1259
Fig. 20. Effects of reaction temperature (A) and NiO loading (B) on tar removal with wood or coal char NiO catalysts [101].
decomposition rates increased linearly with the increase of temperature and catalyst loading from 600 to 700 °C. Relative to biochar alone, the iron supported catalysts lowered the activation
energy by 47% and decreased the formation of benzene.
Except for the catalytic cracking effect, the adsorption of tar
molecule on the surface is of great importance, which is as a result
of the similar chemical structure that lead to form π–π bond or
Lewis acid structure [22,105]. In small-scale fixed bed biomass
gasifier, partial oxidation and char bed reduction are two main
solution for tar conversion and syngas cleaning inside gasifier.
Char is continuously produced in gasifier. But in char bed the
reduction reaction is endothermic reaction and needs external
heat provided to maintain the high temperature. A new idea is to
couple the two methods. Although there are several researches on
oxidation and char bed reduction for tar elimination, there are still
too few work on the mechanism of char as a catalyst for tar
removal and no report on the synergy effect of oxygen and biomass char on the conversion of tar. Zhao et al. [106] investigated
the biomass tar catalytic reduction by the synergy of char and
oxygen. The coupling of char and oxygen could result in the significant improvement of tar conversion rate (89.32%) than both
two separated method (85.1% and 86.14%) at 700 °C. At 900 °C, the
highest conversion rate of 95.84% can be achieved due to the
synergy effect. High oxygen concentration coupling with char
could result in carbon deposition and bring down tar conversion
rate at 800 °C. However, a light amount of oxygen significantly
promoted the formation of porosity. The reaction between tar and
bio-char at high temperatures ( 4800 °C) was benefit for the
toluene conversion. The coupling of char and partial oxidation
benefited the removal of larger PAHs tar compounds as well as
toluene. Slight amount of oxygen would contribute the char pore
development, but high oxygen concentration leads to the carbon
deposition on char pore surface at 800 °C. The coupling of partial
oxidation and char catalysis is a feasible method for tar reduction.
Bio-char can be used as the catalyst support for the tar removal
due to its porosity and thermal stability. Consequently, exploring
the role of bio-chars in the heterogeneous conversion of tars can
provide optimization strategies for the real gasifier [107].
Tar removal can be enhanced by the coupling of partial oxidation and char catalysis. However, the char characteristics could
be changed with the char reaction with oxidation. For instance,
slight amount of oxygen can promote the char pore development,
but high oxygen concentration leads to the carbon deposition on
char pore surface, thereby reducing the surface area. The catalytic
synergy effect can also take place between different metal actives
in the bio-char. Shen et al. [108] embedded the nickel
nanoparticles into the carbon matrix of rice husk char by
impregnation and pyrolysis method. The bio-char supported
bimetallic Ni-Fe catalyst shows a higher tar conversion compared
with the monometallic catalysts (i.e. Ni or Fe). Additionally, catalysts placed in contact with the feedstock inside the pyrolysis/
gasification reactor (i.e. in situ) can inhibit the nascent tars polymerization, thereby reducing the macromolecular tar formation
and condensation at sources. Fig. 21 illustrated the tar in-situ
conversion mechanism by the char supported Ni-Fe catalysts. In
the pyrolysis stage, biomass is initially decomposed to the small
molecular gases, tar, char and water via the thermochemical
reactions. The formed tar is cracked and reformed simultaneously
by co-pyrolysis with the char supported Ni-Fe catalysts at high
temperatures. After that, partial the char supported catalysts are
converted to the silica-based catalysts by the char gasification. Biochar plays two significant roles during biomass pyrolysis. On one
hand, it provides the carbon atom to transform metal oxides into
metallic state via the carbothermal reduction; on the other hand, it
is considered as an adsorbent to adsorb metal ions and biomass
tar. Consequently, metal species present in the solid residues to be
further regenerated or the direct catalytic gasification to the
additional syngas.
The integrated strategy of catalytic biomass pyrolysis/gasification includes different key reaction steps as illustrated in Fig. 22,
consisting of (step 1) metal precursor (e.g., Ni2 þ ) insertion into
solid biomass, (step 2) catalytic pyrolysis of biomass, (step 3) the
catalytic active nanoparticles (e.g., Ni°) in situ generated and highly
dispersed in the carbon matrix, (step 4) catalytic conversion of
nascent tars by the formed nanocomposites, (step 5) catalytic
gasification of the char residues, and (step 6) recycling and reuse
of the metal nanoparticle species in the ash (e.g., Ni/SiO2). Each of
these reaction steps requires a thorough fundamental understanding to develop an innovative high-efficiency biomass pyrolysis/gasification process at nanoscales. Furthermore, it is critical
to develop breakthrough conversion technologies followed by the
design of the future intensified gasification processes [109,110].
Among the steps of integrated strategy, it is a sustainable route
to recycle and reuse of the metal nanoparticle species in the ash.
The indigenous and catalytically active minerals included in the
biomass structure, such as AAEMs (e.g., Ca, K, Na) and other loaded
metal species (e.g., Ni, Fe) can catalyze cracking and so thermolysis
reactions in the vapor phase and remold the chemical composition
of a resulting pyrolysis liquid, and change the distributions of
pyrolysis products. Parameters such as the type of the catalyst, and
the presence of inorganic constituents in the biomass, could be
used to alter the relative rates of the biomass decomposition and
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Y. Shen et al. / Renewable and Sustainable Energy Reviews 59 (2016) 1246–1268
Fig. 21. Schematic of in-situ tar conversion by char catalysts for biomass gasification [108].
Fig. 22. Integrated strategy of biomass catalytic pyrolysis/gasification.
subsequent vapor phase reactions (e.g., tar reforming), and eventually produce pyrolysis liquids or gases with an improved composition [111]. With regards to biomass gasification, the quality of
syngas can be improved with the biomass ashes as well [112].
Fig. 23 illustrates the possible effects of the ash on the catalytic
activity in fast pyrolysis of biomass. Hypothetically, four different
pathways can be distinguished on how the ash influences the
vapor phase chemistry and the activity of the catalyst. It should be
noted that the combined effects are possible too as follows: (1) the
catalytic effect of ash itself on the primary pyrolysis vapors
increases the production of non-condensable gases (NCGs) and
char; (2) Ash cracks some larger vapor molecules inaccessible to
the interior of the catalyst, to smaller ones which are capable of
entering the catalyst pores; (3) Cracked vapors then are further
reformed by the catalyst (a) or not (b); (4) Ash particles poison the
catalyst and negatively affect the vapor conversion and the reaction chemistry [111]. Application of biomass ash as a catalyst to
improve the gasification rate is a promising way for the effective
utilization of waste ash as well as for the reduction of cost.
5. Tar removal with bio-oil
Biomass tar is classified as heavy and light tars. Condensation
behavior of tar compounds mainly depends on type and associated
concentration and resultant dew-point temperature of the tar
Y. Shen et al. / Renewable and Sustainable Energy Reviews 59 (2016) 1246–1268
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Fig. 23. Possible impact of ash on the catalyst and on the products in catalytic fast pyrolysis of biomass [111].
Fig. 24. Outline of the OLGA process [113].
compounds. Tar compounds start condensing as the producer gas
temperature drops below the tar dew-point temperature. Catalytic
and thermal cracking and mechanism methods are the main
downstream methods for tar removal [9]. Mechanism methods,
such as scrubbers, cyclones, and filters, are preferred due to low
energy requirements. Among these mechanism methods, wet
scrubbing technologies are widely reported as an effective method
for the tar removal [9]. Upon cooling of the product gas, the
temperature decreases below the tar dew-point. Furthermore, tar
condensation gradually occurs until the product gas is not cooled.
At the resulting temperature, between the dew-point of tars and
water, a liquid/gas (L/G) phase system is obtained (L: liquid tar).
The scrubbing liquid acts as the medium to collect these liquid
tars. The remaining gaseous tars are removed from the gas by
absorption into the scrubbing liquid (i.e. the scrubbing liquid acts
as absorption medium). The degree of absorption could be well
controlled by changing the operation conditions and be determined by the desired tar dew-point of the outlet product gas. In
the regeneration of the scrubbing liquid the tar is removed, upon
which some scrubbing liquid may evaporate. The outline of the
OLGA process is illustrated in Fig. 24 [113]. The liquid tars are
separated from the scrubbing liquid and returned to the gasifier;
also a small amount of the scrubbing liquid is bleed and recycled
to the gasifier. For the absorption step, scrubbing columns were
selected that are interacting with each other in a classical
absorption-regeneration mode. The scrubbing liquid from the
Absorber with the dissolved tars is regenerated in the Stripper. In
case of air-blown gasification, air is used to strip the tar. Subsequently, the air with the stripped tars is used as a gasifying
medium. The loss of scrubbing liquid in the Stripper by volatilization is minimized [113].
Wet scrubbing of tars involves absorption of gaseous tar compounds in the scrubbing solvents. Water is the most common
scrubbing solvent [114–117]. The major drawbacks in using water
as a scrubbing solvent is the low solubility of tar compounds and
costly wastewater treatment [118]. Vegetable, engine, and waste
cooking oils and biodiesel have been studied as solvents for the
removal of tars and volatile organic compounds (VOCs) such as
benzene, toluene, and, methanol in a bubble column [114,119–
121]. Tar compounds including benzene, toluene, and ethylbenzene, being lipophilic in nature, can mix comparatively well with
vegetable oils because these oils have saturated (palmitic and
steric acids) and unsaturated (oleic, linolenic and linoleic acids)
fatty acids which are also lipophilic in nature. Phuphuakrat et al.
[114] reported that fuel oils, such as diesel and plant-based biodiesel fuels, in a bubble column reactor showed high removal
efficiency for tar compounds; however, the loss of solvent due to
high volatility was the major issue, resulting in fuel oils not
recommended as solvents.
The major influencing parameters consist of liquid flux, gas
flux, concentration of pollutants, and concentration of absorbent
solution. Furthermore, type and size of packing material, method
of packing, liquid distribution, viscosity of solvent, solvent temperature and bed height also affect the performance of the
absorption system [122–126]. As an alternative, bio-oils such as
vegetable oils has a high potential as solvents used for the removal
of tar owing to their characteristics of high absorption capacity for
tar compounds. Significantly, vegetable oils are renewable in nature and, being plant-based, are CO2 neutral, less volatile, low cost
and hazard free. Recently, Rhoi et al. [127–129] conducted a
comprehensive study to evaluate the performance of vegetable oil
as a solvent in a wet packed bed scrubbing system for the removal
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Fig. 25. Schematic diagram of bench-scale wet packed bed scrubbing set-up [128].
Fig. 26. Effect of bed height on the removal efficiencies of (A) toluene, (B) benzene, and (C) ethylbenzene at a solvent temperature of 50 °C, solvent flow rate of 53 ml/min,
and soybean oil as a solvent [127].
of tar compounds. The packed bed wet scrubbing set-up includes
two major sections: gas mixing and packed bed absorption column
(Fig. 25). In this study, bed height and solvent temperature had
highly significant effects on tar removal efficiencies. Bed height,
solvent temperature and solvent flow rate had highly significant
effects on liquid holdup and pressure drop across the column
[127]. As shown in Fig. 26B, as the bed height increased from 0.5 to
1.1 m, the removal efficiency of benzene increased from 90% to
over 97%. This trend increased with the increase of bed height
until 18 min of operation at which time the trend reversed due to
saturation of solvent that occurs earlier at the highest bed height.
The mass transfer area of tar compounds is directly proportional to
Y. Shen et al. / Renewable and Sustainable Energy Reviews 59 (2016) 1246–1268
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Fig. 27. Effect of solvent flow rate on the removal efficiencies of (A) toluene, (B) benzene, and (C) ethylbenzene at a bed height of 1.1 m, solvent temperature of 30 °C, and
soybean oil as a solvent [127].
Fig. 28. Effect of solvent temperature, bed height and solvent flow rate on (A) the pressure drop across the column and (B) liquid holdup for soybean oil as a solvent [127].
the bed height for the given packing materials. In addition, an
increased bed height increases liquid holdup which reduces a
cross sectional area of the column, in turn increasing gas and
liquid velocities. Bed height beyond 1.1 m would increase the cost
of packing materials and the pressure drop across the column with
marginal improvement in tar removal efficiency. For toluene and
ethylbenzene, the effect of bed height on removal efficiencies was
far less compared to benzene due to low equilibrium ratio
(Fig. 26A and C).
Due to the effect of interactions, the incremental change in tar
removal efficiencies as a function of bed height increases with the
increase of solvent temperature and decreases with the increase of
solvent flow rate. It is attributed to the significant reduction in the
solubility of tar compounds, as the solvent temperature increases
from 30 to 50 °C and liquid holdup increases as the solvent flow
rates increase [127]. Solvent flow rate did not have a significant
effect on the tar removal efficiencies except for benzene at 40 and
50 °C solvent temperatures and at the bed height of 1.1 m (Fig. 27).
Solvent flow rate will have a significant effect on the removal
efficiency because the driving force for the mass transfer of tar
compounds significantly improves with the increase of solvent
flow rate from 53 to 73 ml/min. However, as the solvent flow rate
increases, the liquid film thickness increases, leading to higher
mass transfer resistance on the liquid side. In addition, an increase
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Y. Shen et al. / Renewable and Sustainable Energy Reviews 59 (2016) 1246–1268
in solvent flow rate increases liquid holdup which reduces the
cross sectional area of the column; accordingly, gas velocity
increases which reduces the residence time of the gas–liquid
contact. In general, the effect of increased solvent flow rate could
be balanced by increased mass transfer resistance and reduced
residence time.
High mass transfer efficiency and low pressure drop are
essential for wet packed bed scrubbing systems. The solvent
temperature had a highly significant effect on the pressure drop
across the column for solvent flow rates and bed heights. The
effect of solvent temperature on the pressure drop across the
column is shown in Fig. 28A. At bed height of 1.1 m, as the solvent
temperature increased from 30 to 50 °C, the pressure drop across
the column decreased from 16.07 to 14.67 and 18.79 to 16.61 mm
of water column (WC) for the solvent flow rate of 53 and 73 ml/
min, respectively. Similar trends were observed at the bed heights
of 0.8 and 0.5 m. The reduction in the pressure drop was ascribed
to reduction in the density and viscosity of the solvent causing a
reduction in the liquid holdup as the solvent temperature
increased from 30 to 50 °C (Fig. 28B). The solvent temperature had
a highly significant effect on the liquid holdup for the given solvent flow rate and bed height. The void fraction of the bed
increases as the liquid holdup reduces that leads to reduction in
gas and liquid velocities, consequently a lower frictional drop as
the solvent temperature increases. In summary, bed height, solvent temperature and solvent flow rates had a highly significant
effects on the pressure drop across the packed bed column.
The type of absorbent is a considerable influence parameter on
tar removal. Different absorbents show different physical and
chemical properties, such as viscosity, solute, which can significantly influence the absorption performances of tar compounds. Phuphuakrat et al. [114] found that only 31.8% of gravimetric tar was removed by the water scrubber, whereas the
highest removal efficiency (60.4%) of gravimetric tar was achieved
by a vegetable oil scrubber. As for light PAH tar removal, the
adsorption efficiency can be ranked as follows: diesel
fuel 4vegetable oil 4biodiesel fuel 4engine oil 4water. Therefore, the vegetable oil was highly recommended as a tar absorbent
in biomass gasification systems. In the view of adsorption process,
some tar compounds can be easily absorbed by water (i.e. hydrophilic), while other tar compounds are tended to be absorbed by
oily materials (i.e. oleophylic). The further investigation on the
interfacial interactions between water, oil and tar compounds
should be developed by using the two-phase absorbents, such as
emulsified absorbents with the aim to enhance the tar uptake.
In addition, Paethanom et al. [21] combined an oil scrubber and
a char adsorption bed from the lab-scale to pilot-scale facility of
integrated pyrolysis regenerated plant. The results showed that
the optimum system needed for 0.045 m3/h pyro-gas about 1 L oil.
The oil scrubber charged with 1 L oil was combined with a 41 g
chestnut wood char bed and reached 97.6% heavy tar removal. In
the large-scale application, Nakamura et al. [130] used the bio-oil
derived from the gasification process for the removal of tar from
the syngas. The results showed that the bio-oil absorbent was
effective for tar removal (in terms of 73.3% of the heavy tar
removal at 50°). It is notice that the service life and kinetics of tar
absorption with bio-oils including low-viscosity tar are unclear to
be further studied.
6. N-containing tar removal
In general, various contaminants are present in producer gas
generated from biomass gasification such as tars, ammonia (NH3),
and hydrogen sulfide (H2S) [131]. The nitrogen and sulfur contents
in biomass can contribute a lot to produce the N-containing and Scontaining contaminants. For example, compared with lignocellulosic biomasses, sewage sludge as one of typical high
nitrogen content bio-wastes could normally generate various Ncontaining components such as N-tar, NH3 after gasification. The
molecular structures of main N-tar components indicated by GC–
MS were shown in Fig. 29. Chen et al. [132] investigated the
characteristics of N-tar and NOx precursors during the pyrolysis of
sewage sludge. And absorption performance for N-tar and NOx
precursors were studied by using four kinds of scrubbing mediums: cooking oil, diesel oil, BDF and water.
Although some tar compounds containing nitrogen heteroatom
with high polarity and soluble in water, N-containing compounds
with two rings or three rings are difficult to dissolve in water. As
shown in Fig. 30, as the increase of the molecular size of N-tar
compounds, the decrease of their solubility and diffusivity in water
leads to the poor absorption performance, especially in terms of
carbazole. Similar to water, behaviors of these major N-tar compounds in oily solvents showed a similar trend. In view of N-tar
compounds removal, diesel oil showed good absorption capacity
as cooking oil except for quinoline. In addition, its absorption
capacity was even a little higher than cooking oil. Compared to
diesel oil and cooking oil, BDF only removes two rings N-tar
compounds such as quinolone and 1-cyanonaphtahlene efficiently.
The absorption capacity for major N-tar compounds except for
Fig. 29. Major components in N-tar identified by GC–MS [132].
Y. Shen et al. / Renewable and Sustainable Energy Reviews 59 (2016) 1246–1268
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Fig. 30. Absorption for typical N-tar compounds [132].
Fig. 31. (A) Toluene removal efficiency; (B) Breakthrough curves for ammonia and
(C) H2S with time for tested catalysts at temperature of 800 °C and gas residence
time of 0.03–0.1 kg m3/h (catalyst quantities were 1 g of bio-char, 0.35 g each of
activated carbon, acidic surface activated carbon, and mixed metal oxide) [131].
quinoline with different absorbents is concluded as follows: diesel
oil ¼cooking oil 4BDF 4water [132].
The absorption behavior mainly depends on physical and chemical properties of absorbents and tar compounds. The physical
properties such as density and viscosity of absorbents, solubility
and diffusivity of tar compounds in absorbents, and the chemical
properties, such as polarity of tar compounds and absorbents and
aliphatic (linear or nonlinear) or aromatic and hydrophobic or
hydrophilic nature of tar compounds can affect the absorption
performance [121]. According to ‘‘like dissolves like’’, oily absorbents showed better removal performances for N-tar compounds
than water. Additionally, by using water scrubber, a lot of contaminated water will be generated leading to another wastewater
disposal problem. Therefore, water is still not suitable absorbent
for N-tar compounds. In view of physical properties, viscosity of an
absorbent strongly influences mass transfer kinetics [133]. A low
viscosity minimizes the thickness of the interface layer on the
liquid side and increases diffusion kinetics in this layer. Thus, a
lower viscosity can enhance the absorption efficiency. It is reported that the viscosity and density of hydrophobic substances can
be ranked in the order of diesel fuel obiodiesel fuel ocooking oil
[114]. In general, cooking oil can be regarded as a good absorbent
for both gravimetric tar and N-tar compounds.
In the real applications, producer gas from biomass gasification
often contains tars, NH3, and H2S. The reducing gases including
NH3 and H2S are normally removed by catalytic oxidation. Shen
et al. [134] used the activated carbon desulfurizer for H2S removal.
The H2S was catalytically oxidized to sulfur (S°), which was
adsorbed in the carbon matrix. Xu et al. [135] comparatively studied the sewage sludge- and pig manure-derived bio-chars for H2S
removal. All of these demonstrate the biochars can be used as
good adsorbents and catalyst supports for gas cleaning including
tar and H2S removal. Bhandari et al. [131] studied the effectiveness
of four catalysts (biochar, activated carbon, acidic surface activated
carbon, and mixed metal oxide) for the simultaneous removal of
toluene, NH3, and H2S from the producer gas. In the absence of
NH3 and H2S, toluene removal efficiencies were 78.8%, 88.5%, and
88.1% for biochar, activated carbon, and acidic surface activated
carbon, respectively, at 800 °C [136]. In the presence of NH3 and
H2S, toluene removal efficiencies increased to 91.60%, 86.69%, and
83.66%, respectively (Fig. 31A). The increases of toluene removal
efficiency for the biochar-based catalysts (biochar, activated carbon, and acidic surface activated carbon) indicate that NH3 and
H2S played a role in increasing the toluene removal efficiencies.
The breakthrough times of NH3 with mixed metal oxide and
activated carbon catalysts (120 min) were lower than that with
acidic surface activated carbon (170 min). As shown in Fig. 31B, the
low C/C0 for acidic surface activated carbon sustained for an
extended time compared to that for the other catalysts indicated
that acidic surface contributes to sustained and high NH3 removal
efficiency. Furthermore, the breakthrough time for the mixed
metal oxide was around 100 min; whereas the breakthrough time
for the other three catalysts was around 70 min. All the other three
catalysts (biochar, activated carbon, and acidic surface activated
carbon) performed equally well for H2S removal with an adsorption capacity of 0.008 g-H2S/g-catalyst (Fig. 31C). In summary, the
carbon-based catalysts (biochar, activated carbons) can remove
contaminants simultaneously (86–97% of toluene removal efficiency; NH3 breakthrough time of 90–150 min; H2S breakthrough
time of 80–110 min).
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7. Concluding remarks
Biomass is one of the predominant renewable energy sources
including energy crops, agricultural residues, forestry residues,
algae and municipal solid wastes. Biomass utilization is recognized
as one of the most promising solutions for alleviating current
energy and environmental problems. In contrast to bio-energy,
alternative renewable energy technologies such as hydropower
energy, solar energy and wind power are less reliable especially in
term of supply security. Meanwhile, biomass is the only renewable
energy source that could be transformed into liquid or gas fuels.
Gasification is a key technology for the use of biomass offering
high flexibility and efficiency. New knowledge and more efficient
and cost-competitive industrial applications are urgently required
for the long run, showing evidence of promising developments
and cross-fertilization with sectors other than energy, which
provides ideas, experiences, technology contributions, new
approaches, innovative materials and skills [137].
Bio-char and bio-oil are predominant by-products from biomass pyrolysis. They have a potential to be used for gas cleaning in
biomass pyrolysis. On the one hand, tar in producer gas could be
removed by physical treatment, such as oil absorption and char
adsorption; on the other hand, tar could be removed by chemical
treatment, such as catalytic conversion over char-supported catalysts. This paper reviewed the progresses in tar removal for biomass pyrolysis/gasification by using the recyclable by-products
(i.e. bio-char, bio-oil). Generally, bio-char can effectively adsorb
the light tar compounds such as VOCs, while bio-oil is more
benefit for the adsorption of heavy tars. Additionally, catalytic
reforming is considered as a promising method for the removal of
tar compounds, since it converts the tar into the additional gas
products. Bio-char could be used as a carbonaceous catalyst or
support with fair performance in tar removal. It is noteworthy that
the char-supported catalysts could be directly gasified to recover
energy of char without the need of frequent regeneration after
deactivation. Therefore, the integrated strategy of catalytic biomass pyrolysis/gasification was proposed with the objective to
promote the environmentally friendly application of biomass
gasification in the future. In this frame, the development of
innovative catalysts, sorbents and high temperature filtration
media from the by-products was shown to represent a fundamental requirement to increase the yield and purity of the biomass gasification product, to allow efficient conversion into power
and further catalytic processing addressed to second generation
bio-fuels and chemicals. As the developments in new materials
preparation, characterization and testing at real industrial conditions, poly-generation strategies focused on biomass, adaptable to
different demands and markets, can be implemented and enforced
with success. Furthermore, the carbon-based catalysts derived
from bio-chars could be developed for the removal of contaminants including NH3, H2S and tar simultaneously in producer
gas from the real biomass gasification processes.
Acknowledgments
This work is supported by Startup Fund for Talents at NUIST
under Grant no. 2243141501046. This work is also financially
supported by the National Natural Science Foundation of China
(21407079, 21577065 and 91543115) and Jiangsu Natural Science
Foundation (BK20150042). Additionally, the authors are grateful to
the editors and anonymous referees for their helpful comments on
this paper.
References
[1] Chan FL, Tanksale A. Review of recent developments in Ni-based catalysts for
biomass gasification. Renew Sustain Energy Rev 2014;38:428–38.
[2] Ruiz JA, Juárez MC, Morales MP, Muñoz P, Mendívil MA. Biomass gasification
for electricity generation: review of current technology barriers. Renew
Sustain Energy Rev 2013;18:174–83.
[3] P. Basu, Gasification theory and modeling of gasifiers. In: Biomass Gasification Design Handbook. Boston: Academic Press; 117–165.
[4] Torres W, Pansare Jr SS, Goowin JG. Hot gas removal of tars, ammonia, and
hydrogen sulfide from biomass gasification gas. Catal Rev 2007;49:407–56.
[5] Stiegel GJ, Maxwell RC. Gasification technologies: the path to clean, affordable energy in the 21st century. Fuel Process Technol 2001;71:79–97.
[6] Ahmed II, Nipattummakul N, Gupta AK. Characteristics of syngas from
cogasification of polyethylene and woodchips. Appl. Energy 2011;88:165–74.
[7] Ahmed II, Gupta AK. Evolution of syngas from cardboard gasification. Appl
Energy 2009;86:1732–40.
[8] Bridgwater AV. The technical and economic feasibility of biomass gasification
for power generation. Fuel 1995;74:631–53.
[9] Han J, Kim H. The reduction and control technology of tar during biomass
gasification/pyrolysis: an overview. Renew Sustain Energy Rev 2008;12:397–
416.
[10] Mohan D, Pittman CU, Steele PH. Pyrolysis of wood/biomass for bio-oil: a
critical review. Energy Fuels 2006;20:848–89.
[11] Di Blasi C. Modeling chemical and physical processes of wood and biomass
pyrolysis. Prog Energy Combust Sci 2008;34:47–90.
[12] Neves D, Thunman H, Matos A, Tarelho L, Barea AG. Characterization and
prediction of biomass pyrolysis products. Prog Energy Combust Sci
2011;37:611–30.
[13] Gómez-Barea A, Leckner B. Gasification of biomass and waste. Handbook of
combustion. . Wiley; 2010. p. 365–99.
[14] Guan GQ, Chen G, Kasai Y, Lim EWC, Hao XG, Kaewpanha M, Abuliti A,
Fushimi C, Tsutsumi A. Catalytic steam reforming of biomass tar over iron- or
nickel-based catalyst supported on calcined scallop shell. Appl Catal B:
Environ 2012;115–116:159–68.
[15] Kiel JHA. Primary measures to reduce tar formation in fluidised-bed biomass
gasifiers. Final Report SDE Project P1999-012. Netherlands Energy Research
Foundation; 2004.
[16] Font C Palma. A model for biomass gasification including tar formation and
evolution. Energy Fuels 2013;27:2693–702.
[17] Palma Font C. Modeling of tar formation and evolution for biomass gasification: a review. Appl Energy 2013;111:129–41.
[18] Phuphuakrat T, Namioka T, Yoshikawa K. Tar removal from biomass pyrolysis
gas in two-step function of decomposition and adsorption. Appl Energy
2010;87:2203–11.
[19] Paethanom A, Nakahara S, Kobayashi M, Prawisudha P, Yoshikawa K. Performance of tar removal by absorption and adsorption for biomass gasification. Fuel Process Technol 2012;104:144–54.
[20] Hasler P, Nussbaumer T. Gas cleaning for IC engine applications from fixed
bed biomass gasification. Biomass Bioenergy 1999;16:385–95.
[21] Paethanom A, Bartocci P, Amico BD, Testarmata F, Moriconi N, Slopiecka K,
Yoshikawa K. A low-cost pyrogas cleaning system for power generation:
Scalling up from lab to pilot. Appl Energy 2013;111:1080–8.
[22] Liu WJ, Zeng FX, Jiang H, Zhang XS. Preparation of high adsorption capacity
bio-chars from waste biomass. Bioresour Technol 2011;102:8247–52.
[23] Fagbemi L, Khezami L, Capart R. Pyrolysis products from different biomasses:
application to the thermal cracking of tars. Appl Energy 2001;69:293–306.
[24] Nair SA, Pemen AJM, Yan K, Van Heesch EJM, Ptasinski KJ, Drinkenburg AAH.
Chemical processes in tar removal from biomass derived fuel gas by pulsed
corona discharges. Plasma Chem Plasma Process 2003;23:665–80.
[25] Baker EG, Mudge LK. Mechanisms of catalytic biomass gasification. J Anal
Appl Pyrolysis 1984;6:285–97.
[26] Wang D, Yuan W, Ji W. Use of biomass hydrothermal conversion char as the
Ni catalyst support in benzene and gasification tar removal. Trans ASABE
2010;53:795–800.
[27] Abu El-Rub Z, Bramer EA, Brem G. Review of catalysts for tar elimination in
biomass gasification processes. Ind Eng Chem Res 2004;43:6911–9.
[28] Shen Y, Yoshikawa K. Recent progresses in catalytic tar elimination during
biomass gasification or pyrolysis – a review. Renew Sustain Energy Rev
2013;21:371–92.
[29] Huber GW, Iborra S, Corma A. Synthesis of transportation fuels from biomass: chemistry, catalysts, and engineering. Chem Rev 2006;106:4044–98.
[30] Yu QZ, Brage C, Nordgreen T, Sjöström K. Effects of Chinese dolomites on tar
cracking in gasification of birch. Fuel 2009;88:1922–6.
[31] Dou B, Gao J, Sha X, Baek SW. Catalytic cracking of tar component from high
temperature fuel gas. Appl Therm Eng 2003;23:2229–39.
[32] Tamhankar SS, Tsuchiya K, Riggs JB. Catalytic cracking of benzene on iron
oxide-silica: catalyst activity and reaction mechanism. Appl Catal
1985;16:103–21.
[33] Suzuki T, Ohme H, Watanabe Y. Alkali metal catalyzed carbon dioxide
gasification of carbon. Energy Fuels 1992;6:343–51.
[34] He M, Xiao B, Hu Z, Liu S, Guo X, Luo S. Syngas production from catalytic
gasification of waste polyethylene: influence of temperature on gas yield and
composition. Int J Hydro Energy 2009;34:1342–8.
Y. Shen et al. / Renewable and Sustainable Energy Reviews 59 (2016) 1246–1268
[35] Li D, Nakagawa Y, Tomishige K. Development of Ni-based catalysts for steam
reforming of tar derived from biomass pyrolysis. Chin J Catal 2012;33:583–
94.
[36] Furusawa T, Tsutsumi A. Comparison of Co/MgO and Ni/MgO catalysts for the
steam reforming of naphthalene as a model compound of tar derived from
biomass gasification. Appl Catal A: Gen 2005;278:207–12.
[37] Juutilainen SJ, Simell PA, Krause AO. Zirconia: Selective oxidation catalyst for
removal tar and ammonia from biomass gasification gas. Appl Catal B:
Environ 2006;1–2:86–92.
[38] Constantinou DA, Álvarez-Galván MC, Fierro JLG, Efstathiou AM. Lowtemperature conversion of phenol into CO, CO2 and H2 by steam reforming
over La-containing supported Rh catalysts. Appl Catal B: Environ 2012;117–
118:81–95.
[39] Cheah S, Gaston KR, Parent YO, Jarvis MW, Vinzant TB, Smith KM, Thornburg
NE, Nimlos MR, Magrini-Bair KA. Nickel cerium olivine catalyst for catalytic
gasification of biomass. Appl Catal B: Environ 2013;134–135:34–45.
[40] Azadi P, Afif E, Foroughi H, Dai T, Azadi F, Farnood R. Catalytic reforming of
activated sludge model compounds in supercritical water using nickel and
ruthenium catalysts. Appl Catal B: Environ 2013;134–135:265–73.
[41] Zhao H, Draelants KJ, Baron GV. Performance of a nickel-activated candle
filter for naphthalene cracking in synthetic biomass gasification gas. Ind Eng
Chem Res 2000;39:3195–201.
[42] Bangala DN, Abatzoglou N, Chornet E. Steam reforming of naphthalene on
Ni-Cr/Al2O3 catalysts doped with MgO, TiO2, and La2O3. AIChE J
1998;44:927–36.
[43] Kinoshita CM, Wang Y, Zhou J. Effect of reformer conditions on catalytic
reforming of biomass-gasification tars. Ind Eng Chem Res 1995;34:2949–54.
[44] Corella J, Orío A, Toledo J. Biomass gasification with air in a fluidized bed:
exhaustive tar elimination with commercial steam reforming catalysts.
Energy Fuels 1999;13(3):702–9.
[45] Blanco PH, Wu C, Onwudili JA, Williams PT. Characterization and evaluation
of Ni/SiO2 catalysts for hydrogen production and tar reduction from catalytic
steam pyrolysis-reforming of RDF. Appl Catal B: Environ 2013;134–135:238–
50.
[46] Li C, Hirabayashi D, Suzuki K. A crucial role of O2 and O22 on mayenite
structure for biomass tar steam reforming over Ni/Ca12Al14O33. Appl Catal B:
Environ 2009;3–4:351–60.
[47] Kimura T, Miyazawa T, Nishikawa J, Kado S, Okumura K, Miyao T, Naito S,
Kunimori K, Tomishige K. Development of Ni catalysts for tar removal by
steam gasification of biomass. Appl Catal B: Environ 2006;3–4:160–70.
[48] Pompeo F, Nichio NN, Ferretti OA, Resasco D. Study of Ni catalysts on different supports to obtain synthesis gas. Int J Hydro Energy 2005;30:1399–
405.
[49] Choudhary VR, Mamman AS. Energy efficient conversion of methane to
syngas over NiO-MgO solid solution. Appl Energy 2000;66:161–75.
[50] Wang TJ, Chang J, Wu CZ, Fu Y, Chen Y. The steam reforming of naphthalene
over a nickel-dolomite cracking catalyst. Biomass Bioenergy 2005;28:508–
14.
[51] Courson C, Udron L, Ski DW, Petit C, Kiennemann A. Hydrogen production
from biomass gasification on nickel catalysts: tests for dry reforming of
methane. Catal Today 2002;76:75–86.
[52] Lee IG, Ihm SK. Catalytic gasification of glucose over Ni/activated charcoal in
supercritical water. Ind Eng Chem Res 2009;48:1435–42.
[53] Le DD, Xiao X, Morishita K, Takarada T. Biomass gasification using nickel
loaded brown coal char in fluidized bed gasifier at relatively low temperature. J Chem Eng Jpn 2009;42:51–7.
[54] Chen T, Liu H, Shi P, Chen D, Song L, He H, Frost RL. CO2 reforming of toluene
as model compound of biomass tar on Ni/Palygorskite. Fuel 2013;107:699–
705.
[55] Abu El-Rub Z, Bramer EA, Brem G. Review of catalysts for tar elimination in
biomass gasification. Ind Eng Chem Res 2004;43:6911–9.
[56] Abu El-Rub Z, Bramer EA, Brem G. Experimental comparison of biomass
chars with other catalysts for tar reduction. Fuel 2008;87:2243–52.
[57] Abu El-Rub Z, Bramer EA, Brem G, Tar reduction in biomass gasification using
biomass char as a catalyst. In: Proceeding of conference and technology
exhibition on biomass for energy, industry and climate protection, Rome,
Italy; 2004. p. 1046–1049.
[58] Abu El-Rub Z. Biomass char as an in-situ catalyst for tar removal in gasification systems. PhD dissertation. Enschede: University of Twente; 2008.
[59] Gilbert P, Ryu C, Sharific V, Swithenbank J. Tar reduction in pyrolysis vapours
from biomass over a hot char bed. Bioresour Technol 2009;100:6045–51.
[60] Danny C, Ko K, Porter JF, McKay G. Optimised correlations for the fixed-bed
adsorption of metal ions on bone char. Chem Eng Sci 1999;55:5819–29.
[61] Shen Y. Chars as carbonaceous adsorbents/catalysts for tar elimination during biomass pyrolysis or gasification. Renew Sustain Energy Rev
2015;43:281–95.
[62] Qian K, Kumar A, Zhang H, Bellmer D, Huhnke R. Recent advances in utilization of biochar. Renew Sustain Energy Rev 2015;42:1055–64.
[63] Qian K, Kumar A, Patil K, Bellmer D, Wang D, Yuan W, et al. Effects of biomass feedstocks and gasification conditions on the physiochemical properties of char. Energies 2013;6:3972–86.
[64] Li X, Hayashi J-I, Li C-Z. FT-Raman spectroscopic study of the evolution of
char structure during the pyrolysis of a Victorian brown coal. Fuel
2006;85:1700–7.
[65] Li C-Z. Some recent advances in the understanding of the pyrolysis and
gasification behaviour of Victorian brown coal. Fuel 2007;86:1664–83.
1267
[66] Potgieter-Vermaak S, Maledi N, Wagner N, van Heerden JHP, van Grieken R,
Potgieter JH. Raman spectroscopy for the analysis of coal: a review. J Raman
Spectrosc 2011;42:123–9.
[67] Foo K, Hameed B. Recent developments in the preparation and regeneration
of activated carbons by microwaves. Adv Colloid Interface 2009;152:39–47.
[68] Xu CC, Hamilton S, Ghosh M. Hydro-treatment of Athabasca vacuum tower
bottoms in supercritical toluene with microporous activated carbons and
metal-carbon composite. Fuel 2009;88:2097–105.
[69] Fukuyama H, Terai S, Uchida M, Cano JL, Ancheyta J. Active carbon catalyst
for heavy oil upgrading. Catal Today 2004;98:207–15.
[70] Keiluweit M, Nico PS, Johnson MG, Kleber M. Dynamic molecular structure of
plant biomass-derived black carbon (biochar). Environ Sci Technol
2010;44:1247–53.
[71] Hadi P, Yeung KY, Barford J, An KJ, McKay G. Significance of microporosity on
the interaction of phenol with porous graphitic carbon. Chem. Eng. J
2015;269:20–6.
[72] Kong L, Xiong Y, Tian S, Luo R, He C, Huang H. Preparation and characterization of a hierarchical porous char from sewage sludge with superior
adsorption capacity for toluene by a new two-step pore-fabricating process.
Bioresour Technol 2013;146:457–62.
[73] Li SD, Tian SH, Du CM, He C, Cen CP, Xiong Y. Vaseline-loaded expanded
graphite as a new adsorbent for toluene. Chem Eng J 2010;162:546–51.
[74] Li SD, Tian SH, Feng YF, Lei J, Wang P, Xiong Y. A comparative investigation
on absorption performances of three expanded graphite-based complex
materials for toluene. J Hazard Mater 2010;183:506–11.
[75] Shen X, Guo X, Zhang M, Tao S, Wang X. Sorption mechanisms of organic
compounds by carbonaceous materials: Site energy distribution consideration. Environ Sci Technol 2015;49:4894–902.
[76] Zhu DQ, Pignatello JJ. Characterization of aromatic compound sorptive
interactions with black carbon (charcoal) assisted by graphite as a model.
Environ Sci Technol 2005;39:2033–41.
[77] Zhang X, Li Y, Li G, Hu C. Preparation of Fe/activated carbon directly from rice
husk pyrolytic carbon and its application in catalytic hydroxylation of phenol. RSC Adv 2015;5:4984–92.
[78] Fuentes-Cano D, Gómez-Barea A, Nilsson S, Ollero P. Decomposition kinetics
of model tar compounds over chars with different internal structure to
model hot tar removal in biomass gasification. Chem Eng J 2013;228:1223–
33.
[79] Wang Y-G, Sun J-L, Zhang H-Y, Chen Z-D, Lin X-C, Zhang S, Gong W-B, Fan MH. In situ catalyzing gas conversion using char as a catalyst/support during
brown coal gasification. Energy Fuels 2015;29:1590–6.
[80] Hosokai S, Kumabe K, Ohshita M, Norinaga K, Li C-Z, Hayashi J-I. Mechanism
of decomposition of aromatics over charcoal and necessary condition for
maintaining its activity. Fuel 2008;87:2914–22.
[81] Matsuhara T, Hosokai S, Norinaga K, Matsuoka K, Li C-Z, Hayashi J-I. Rapid
gasification of nascent char in steam atmosphere during the pyrolysis of Naand Ca-ion-exchanged brown coals in a drop-tube reactor. Energy Fuels
2009;24:76–83.
[82] Hosokai S, Norinaga K, Kimura T, Nakano M, Li C-Z, Hayashi J-I. Reforming of
volatiles from the biomass pyrolysis over charcoal in a sequence of coke
deposition and steam gasification of coke. Energy Fuels 2011;25:5387–93.
[83] Sueyasu T, Oike T, Mori A, Kudo S, Norinaga K, Hayashi J-I. Simultaneous
steam reforming of tar and steam gasification of char from the pyrolysis of
potassium-loaded woody biomass. Energy Fuels 2011;26:199–208.
[84] Ito K, Moritomi H, Yoshiie R, Uemiya S, Nishimura M. Tar capture effect of
porous particles for biomass fuel under pyrolysis conditions. J Chem. Eng.
Jpn. 2003;36:840–5.
[85] Namioka T, Yoshikawa K, Hatano H, Suzuki Y. High tar reduction with porous
particles for low temperature biomass gasification: effects of porous particles on tar and gas yields during sawdust pyrolysis. J Chem Eng Jpn
2003;36:1440–8.
[86] Hosokai S, Hayashi J-I, Shimada T, Kobayashi Y, Kuramoto K, Li C-Z, et al.
Spontaneous generation of tar decomposition promoter in a biomass steam
reformer. Chem Eng Res Des 2005;83:1093–102.
[87] Matsuoka K, Shinbori T, Kuramoto K, Nanba T, Morita A, Hatano H, et al.
Mechanism of woody biomass pyrolysis and gasification in a fluidized bed of
porous alumina particles. Energy Fuels 2006;20:1315–20.
[88] Kuramoto K, Matsuoka K, Murakami T, Takagi H, Nanba T, Suzuki Y, et al.
Cracking and coking behaviors of nascent volatiles derived from flash pyrolysis of woody biomass over mesoporous fluidized-bed material. Ind Eng
Chem Res 2009;48:2851–60.
[89] Mahishi MR, Goswami DY. Thermodynamic optimization of biomass gasifier
for hydrogen production. Int J Hydro Energy 2007;32:3831–40.
[90] Sueyasu T, Oike T, Mori A, Kudo S, Norinaga K, Hayashi J-I. Simultaneous
steam reforming of tar and steam gasification of char from the pyrolysis of
potassium-loaded woody biomass. Energy Fuels 2011;26:199–208.
[91] Xu C, Hamilton S, Ghosh M. Hydro-conversion of Athabasca vacuum tower
bottoms in supercritical toluene with highly porous biomass-derived activated carbon and metal-carbon composite. Fuel 2009;88:2097–105.
[92] Zhang S, Asadullah M, Dong L, Tay H-L, Li C-Z. An advanced biomass gasification technology with integrated catalytic hot gas cleaning. Part II: tar
reforming using char as a catalyst or as a catalyst support. Fuel
2013;112:646–53.
[93] Min Z, Zhang S, Yimsiri P, Wang Y, Asadullah M, Li C-Z. Catalytic reforming of
tar during gasification. Part IV. Changes in the structure of char in the charsupported iron catalyst during reforming. Fuel 2013;106:858–63.
1268
Y. Shen et al. / Renewable and Sustainable Energy Reviews 59 (2016) 1246–1268
[94] Min Z, Lin J-Y, Yimsiri P, Asadullah M, Li C-Z. Catalytic reforming of tar during
gasification. Part V. Decomposition of NOx precursors on the char-supported
iron catalyst. Fuel 2014;116:19–24.
[95] Wang F-J, Zhang S, Chen Z-D, Liu C, Wang Y-G. Tar reforming using char as
catalyst during pyrolysis and gasification of Shengli brown coal. J Anal Appl
Pyrolysis 2014;105:269–75.
[96] Han J, Wang X, Yue J, Gao S, Xu G. Catalytic upgrading of coal pyrolysis tar
over char-based catalysts. Fuel Process Technol 2014;122:98–106.
[97] Li L, Morishita K, Mogi H, Yamasaki K, Takarada T. Low-temperature gasification of a woody biomass under a nickel-loaded brown coal char. Fuel
Process Technol 2010;91:889–94.
[98] Xiao X, Cao J, Meng X, Le DD, Li L, Ogawa Y, Sato K, Takarada T. Synthesis gas
production from catalytic gasification of waste biomass using nickel-loaded
brown coal char. Fuel 2013;103:135–40.
[99] Li L, Takarada T. Conversion of nitrogen compounds and tars obtained from
pre-composted pig manure pyrolysis, over nickel loaded brown coal char.
Biomass Bioenergy 2013;56:456–63.
[100] Cao J, Huang X, Zhao X, Wang B, Meesuk S, Sato K, Wei X, Takarada T. Lowtemperature catalytic gasification of sewage sludge-derived volatiles to
produce clean H2-rich syngas over a nickel loaded on lignite char. Int J Hydro
Energy 2014;39:9193–9.
[101] Wang D, Yuan W, Ji W. Char and char-supported nickel catalysts for secondary syngas cleanup and conditioning. Appl Energy 2011;88:1656–63.
[102] Ohtsuka Y, Xu C, Kong D, Tsubouchi N. Decomposition of ammonia with iron
and calcium catalysts supported on coal chars. Fuel 2004;83:685–92.
[103] Bhandari PN, Kumar A, Bellmer DD, Huhnke RL. Synthesis and evaluation of
biochar-derived catalysts for removal of toluene (model tar) from biomassgenerated producer gas. Renew Energy 2014;66:346–53.
[104] Kastner JR, Mani S, Juneja A. Catalytic decomposition of tar using iron supported biochar. Fuel Process Technol 2015;130:31–7.
[105] Lillo-Ródenas M, Cazorla-Amorós D, Linares-Solano A. Behaviour of activated
carbons with different pore size distributions and surface oxygen groups for
benzene and toluene adsorption at low concentrations. Carbon
2005;43:1758.
[106] Zhao S, Luo Y, Zhang Y, Long Y. Experimental investigation of the synergy
effect of partial oxidation and bio-char on biomass tar reduction. J Anal Appl
Pyrolysis 2015;112:262–9.
[107] Zhang Y, Wu W, Zhao S, Long Y, Luo Y. Experimental study on pyrolysis tar
removal over rice straw char and inner pore structure evolution of char. Fuel
Process Technol 2015;134:333–44.
[108] Shen Y, Zhao P, Shao Q, Ma D, Takahashi F, Yoshikawa K. In-situ catalytic
conversion of tar using rice husk char-supported nickel-iron catalysts for
biomass pyrolysis/gasification. Appl Catal B: Environ 2014;152–153:140–51.
[109] Shen Y, Chen M, Sun T, Jia J. Catalytic reforming of pyrolysis tar over metallic
nickel nanoparticles embedded in pyrochar. Fuel 2015;159:570–9.
[110] Zhang S, Dong Q, Zhang L, Xiong Y. High quality syngas production from
microwave pyrolysis of rice husk with char-supported metallic catalysts.
Bioresour Technol 2015;191:17–23.
[111] Yildiz G, Ronsse F, Venderbosch R, van Duren R, Kersten SRA, Prins W. Effect
of biomass ash in catalytic fast pyrolysis of pine biomass. Appl Catal B:
Environ 2015;168:203–11.
[112] Rizkiana J, Guan G, Widayatno WB, Hao X, Li X, Huang W, Abudula A. Promoting effect of various biomass ashes on the steam gasification of low-rank
coal. Appl Energy 2014;133:282–8.
[113] Boerrigter H, van Paasen SVB, Bergman PCA, Könemann JW, Emmen R, Wijnands A, “OLGA” TAR REMOVAL TECHNOLOGY. ECN-C-05-009; 2005.
[114] Phuphuakrat T, Namioka T, Yoshikawa K. Absorptive removal of biomass tar
using water and oily materials. Bioresour Technol 2011;102:543–9.
[115] Bhave A, Vyas D, Patel J. A wet packed bed scrubber-based producer gas
cooling-cleaning system. Renew Energy 2008;33:1716–20.
[116] Dogru M, Midilli A, Howarth CR. Gasification of sewage sludge using a
throated downdraft gasifier and uncertainty analysis. Fuel Process Technol
2002;75:55–82.
[117] Baker E, Brown M, Moore R, Mudge L, Elliott D. Engineering Analysis of
Biomass Gasifier Product Gas Cleaning Technology. Richland, WA, USA:
Pacific Northwest Laboratory; 1986.
[118] Bhoi PR, Huhnke RL, Kumar S, Kumar A, Payton ME. Solubility enhancement
of producer gas tar compounds in water using sodium dodecyl sulfate as a
surfactant. Fuel Process Technol 2015;133:75–9.
[119] Boerrigter H, Van Paasen S, Bergman P, Könemann J, Emmen R, Wijnands A.
OLGA Tar Removal Technology. ECN-C-05-009. Energy Research Centre of the
Netherlands; 2005.
[120] Pierucci S, Del Rosso R, Bombardi D, Concu A, Lugli G. An innovative sustainable process for VOCs recovery from spray paint booths. Energy
2005;30:1377–86.
[121] Ozturk B, Yilmaz D. Absorptive removal of volatile organic compounds from
flue gas streams. Process. Saf. Environ. Prot. 2006;84:391–8.
[122] Yin F, Wang Z, Afacan A, Nandakumar K, Chuang KT. Experimental studies of
liquid flow maldistribution in a random packed column. Can. J. Chem. Eng.
2000;78:449–57.
[123] Wu H, Feng TC, Chung TW. Studies of VOCs removed from packed-bed
absorber by experimental design methodology and analysis of variance.
Chem. Eng. J. 2010;157:1–17.
[124] Heymes F, Manno-Demoustier P, Charbit F, Fanlo JL, Moulin P. A new efficient
absorption liquid to treat exhaust air loaded with toluene. Chem Eng J
2006;115:225–31.
[125] Doan H, Fayed M. Dispersion-concentric model for mass transfer in a packed
bed with a countercurrent flow of gas and liquid. Ind Eng Chem Res
2001;40:4673–80.
[126] Noureddini H, Teoh B, Davis Clements L. Viscosities of vegetable oils and fatty
acids. J Am Oil Chem Soc 1992;69:1189–91.
[127] Bhoi PR, Huhnke RL, Kumar A, Payton ME, Patil KN, Whiteley JR. Vegetable oil
as a solvent for removing producer gas tar compounds. Fuel Process Technol
2015;133:97–104.
[128] Bhoi PR, Huhnke RL, Kumar A, Patil KN, Whiteley JR. Design and development of a bench scale vegetable oil based wet packed bed scrubbing system
for removing producer gas tar compounds. Fuel Process Technol
2015;134:243–50.
[129] Bhoi PR, Huhnke RL, Whiteley JR, Gebreyohannes S, Kumar A. Equilibrium
stage based model of a vegetable oil based wet packed bed scrubbing system
for removing producer gas tar compounds. Sep Purif Technol 2015;142:196–
202.
[130] Nakamura S, Unyaphan S, Yoshikawa K, Kitano S, Kimura S, Shimizu H, Taira
K. Tar removal performance of bio-oil scrubber for biomass gasification.
Biofuels 2015;5:597–606.
[131] Bhandari PN, Kumar A, Huhnke RL. Simultaneous removal of toluene (model
tar), NH3, and H2S, from biomass-generated producer gas using biocharbased and mixed-metal oxide catalysts. Energy Fuels 2014;28:1918–25.
[132] Chen Hongfang, Namioka Tomoaki, Yoshikawa Kunio. Characteristics of tar,
NOx precursors and their absorption performance with different scrubbing
solvents during the pyrolysis of sewage sludge. Appl Energy 2011;88:5032–
41.
[133] Heymes F, Manno-Demoustier P, Charbit F, Fanlo JL, Moulin P. A new efficient
absorption liquid to treat exhaust air loaded with toluene. Chem Eng J
2006;115:225–31.
[134] Sun T, Shen Y, Jia J. Gas cleaning and hydrogen sulfide removal for COREX
coal gas by sorption enhanced catalytic oxidation over recyclable activated
carbon desulfurizer. Environ Sci Technol 2014;48:2263–72.
[135] Xu X, Cao X, Zhao L, Sun T. Comparison of sewage sludge- and pig manurederived biochars for hydrogen sulfide removal. Chemosphere 2014;111:296–
303.
[136] Bhandari PN. Synthesis and evaluation of novel biochar-based and metal
oxide-based catalysts for removal of model tar (toluene), NH3, and H2S from
simulated producer gas. Masters thesis. Oklahoma State University; 2012.
[137] Heidenreich S, Foscolo PU. New concepts in biomass gasification. Prog Energy
Combust Sci 2015;46:72–95.