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. 1247 1247 1247 1248 1248 1251 1251 1251 1252 1253 1255 1256 1260 1264 1266 Y. Shen et al. / Renewable and Sustainable Energy Reviews 59 (2016) 1246–1268 1247 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 . 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 . Pyrolysis can be divided into three subclasses; slow pyrolysis, fast pyrolysis and ﬂash pyrolysis . 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 . 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 . 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. 1248 Y. Shen et al. / Renewable and Sustainable Energy Reviews 59 (2016) 1246–1268 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. . 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 . 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 1249 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 . Tars can be classiﬁed by solubility and condensability , categorized into ﬁve classes (Fig. 5). Class 1 refers to the GC undetectable heaviest tars, which can condense at high 1250 Y. Shen et al. / Renewable and Sustainable Energy Reviews 59 (2016) 1246–1268 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 . 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 1251 Fig. 8. Effect of the temperature adjustment on parameters and processes in gasiﬁcation . 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 . 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 . 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 1252 Y. Shen et al. / Renewable and Sustainable Energy Reviews 59 (2016) 1246–1268 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.  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 . Fig. 10. Dry reforming of tar compounds over the palygorskite (PG) supported Ni catalyst . 3.3. Catalytic conversion Several approaches for tar elimination in the primary or secondary treatment, such as physical treatment [18–21], thermal cracking , plasma-assisted cracking , 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  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 , zeolites , iron ores , alkali metals , 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 . 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 . 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  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 . 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 . 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 1254 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) . 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 . 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) . 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) . 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 . 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 . 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 . Furthermore, Hadi et al.  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.  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 1255 [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 . 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 . To better understand the interaction difference between organics and carbonaceous materials on site energy respect, Shen et al.  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) . 1256 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 . 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 . Secondly, bio-char has both co-catalyst and adsorbent effects . 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 . 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%)  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 1257 Fig. 17. Mechanism illustration of tar compounds (i.e., toluene and naphthalene) conversion over a carbonaceous surface . 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 . Fuentes-Cano et al.  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  is smaller than that of heterogeneous conversion over carbonaceous surfaces . Reforming of aromatic molecules such as benzene and naphthalene over char cannot generate other aromatics . In contrast, alkyl- and heteroatomic compounds produce lighter compounds by dealkylation and decarboxylation . 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 . 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.  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 1258 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 . 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 . 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 . 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 . The char-supported iron or nickel catalyst exhibited much higher activity for reforming tar than the char itself did. Han et al.  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  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 . Wang et al.  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 , 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.  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 . 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.  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 . 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.  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 1260 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 . 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 . With regards to biomass gasiﬁcation, the quality of syngas can be improved with the biomass ashes as well . 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 . 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 1261 Fig. 23. Possible impact of ash on the catalyst and on the products in catalytic fast pyrolysis of biomass . Fig. 24. Outline of the OLGA process . 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 . 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 . 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 . 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 . 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 . 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.  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 1262 Y. Shen et al. / Renewable and Sustainable Energy Reviews 59 (2016) 1246–1268 Fig. 25. Schematic diagram of bench-scale wet packed bed scrubbing set-up . 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 . 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 . 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 1263 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 . 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 . 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 . 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 1264 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.  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.  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.  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) . 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.  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 . Y. Shen et al. / Renewable and Sustainable Energy Reviews 59 (2016) 1246–1268 1265 Fig. 30. Absorption for typical N-tar compounds . 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) . quinoline with different absorbents is concluded as follows: diesel oil ¼cooking oil 4BDF 4water . 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 . 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 . 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 . 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.  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.  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.  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 . 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). 1266 Y. Shen et al. / Renewable and Sustainable Energy Reviews 59 (2016) 1246–1268 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 . 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. 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