d

Energy 229 (2021) 120818
Contents lists available at ScienceDirect
Energy
journal homepage: www.elsevier.com/locate/energy
Study on products characteristics from catalytic fast pyrolysis of
biomass based on the effects of modified biochars
Chun Chang a, d, Zihan Liu a, d, Pan Li a, b, d, *, Xianhua Wang c, Jiande Song d,
Shuqi Fang a, b, d, Shusheng Pang a, e
a
School of Chemical Engineering, Zhengzhou University, Zhengzhou, China
School of Mechanical and Power Engineering, Zhengzhou University, Zhengzhou, China
State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan, China
d
Henan Key Laboratory of Green Manufacturing of Biobased Chemicals, Puyang, China
e
Department of Chemical and Process Engineering, University of Canterbury, Christchurch, New Zealand
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 10 March 2021
Received in revised form
21 April 2021
Accepted 28 April 2021
Available online 3 May 2021
Biochar was produced from peanut shells pyrolysis and then modified by HCl and MnCl2, and the effects
of modified biochars on the pyrolysis products characteristics were investigated. The catalytic effect of
the biochar prepared at 650 C is more suitable, and the products under 650 C have lower acid content
and higher phenol content than that under other temperatures. It was found that the modification effect
of low concentration of MnCl2 was not obvious, which manifested as a higher phenol content under the
catalysis of the HCl modified biochar. Then, different concentrations of MnCl2 were adopted and the
highest phenol yield was obtained at 1.5 mol/L, which was higher than that of direct pyrolysis, about
78.9%. Besides, its acid yield was also the lowest, and the reduction was about 37.5%. The phenol products
were analyzed under different concentrations of MnCl2 modified biochar and the results showed that the
selectivity of alkylated phenol substances was greatly improved. When MnCl2 is 0.8 mol/L, the CO2
content was lowest, while the H2 content was highest, the increment was up to about 92.8% compared
with that of direct pyrolysis. Therefore, the co-modified biochar with HCl and MnCl2 can better promote
lignin cracking and generate more phenols.
© 2021 Elsevier Ltd. All rights reserved.
Keywords:
Modified biochar
Pyrolysis
MnCl2
Characteristic analysis
1. Introduction
With the increasing demand of fossil fuels, two major issues of
negative environmental impacts and energy shortage have become
serious concerns world wide [1,2]. It is reported that at the present
consumption rate of crude oil, the proven reserves can last only
about 50 years [3]. In addition, the consumption of fossil energy
leads to greenhouse gas (GHG) emissions, and haze and acid rain.
Therefore, extensive efforts are being made to explore renewable
energy sources to mitigate the environmental impacts and secure
future energy supplies. Biomass, as a renewable and carbon-neutral
energy resource, has the potential to replace fossil energy [4,5]. The
global biomass availability is found to be about 100 billion tons per
annum, however, present utilization of the biomass is only 3%, and
* Corresponding author. School of Chemical Engineering, Zhengzhou University,
Zhengzhou, China.
E-mail addresses: [email protected], [email protected] (P. Li).
https://doi.org/10.1016/j.energy.2021.120818
0360-5442/© 2021 Elsevier Ltd. All rights reserved.
most of them are wasted which also generates GHG emissions [6].
Therefore, it is of great significance to increase the utilization of
biomass resources by developing more efficiency and more
environmentally-friendly technologies and processes. Catalytic
pyrolysis has been identified as a promising technology that can
effectively convert the biomass to solid (biochar), liquid (bio-oil)
and gas products. With varying the operation conditions, product
distribution from pyrolysis can be optimized.
It is known that the application of an appropriate catalyst in the
pyrolysis can improve the target product quality and yield, and
increase the reaction rate [7]. Catalysts used in previous research
for biomass pyrolysis include metal salt compounds [8], metal oxides [9] and molecular sieve catalysts [10,11]. Biochar was also used
as a catalyst in the biomass pyrolysis [12,13]. The metal salt compounds and metal oxides as catalysts have a low specific surface
area, and most of them are non-porous or have undeveloped voids.
Therefore, these catalyst materials are usually loaded on porous
supporters as active additives [14]. Molecular sieve catalysts have
high catalytic activity, and different molecular sieves can be used in
C. Chang, Z. Liu, P. Li et al.
Energy 229 (2021) 120818
periodic table. It is not only abundant in the earth's crust but also an
indispensable trace element for plant growth and hence it is cost
effective to purchase. Meanwhile, it is speculated that peanut shells
may contain a high content of manganese due to peanuts are one of
the food sources of manganese. Therefore, it is necessary to explore
the effect of manganese on the pyrolysis of peanut shells. Moreover,
in some researches it has been confirmed that manganese can
catalyze reactions such as dehydrogenation, hydrogenation, and
deoxygenation [29,30], which are beneficial to the upgrading of
bio-oil. In addition, the use of peanut shell as the raw material and
the catalyst precursor was executed to generate higher yields of
bio-oil products by pyrolysis. The influence of MnCl2 and HCl on
biochar and the influence of biochar and its modification on the
pyrolysis process of peanut shell were discussed in details through
analyzing the changes of product components, and meanwhile the
methods of the preparation of biochar catalyst were optimized
through the results of the pyrolysis process of peanut shell.
different reaction environments. However, in the process of
biomass pyrolysis, coke deposition on molecular sieves catalyst is
the main reason of catalyst deactivation [15e18]. Biochar is derived
from the pyrolysis of biomass and is generally used as a fuel to
supply energy [19]. Studies have shown that biochar has abundant
surface functional groups, a large specific surface area, and an
excellent void structure [20], which indicates that biochar can be
used as a catalyst for bio-oil upgrading following the pyrolysis
process.
In recent years, researchers have been conducted on the catalytic effect of biochar on biomass pyrolysis. Yang et al. [21] studied
the catalytic effect of bamboo biochar on biomass pyrolysis, and
found that the biochar, with larger specific surface area and
oxygen-containing functional groups, showed catalytic effect to
promote conversion of pyrolysis intermediates to phenols, however, the catalytic effect of the biochar largely depends on the
oxygen-containing functional groups. During the pyrolysis, the
oxygen-containing functional groups will be consumed in the catalytic process, which is not conducive to recycling. To improve the
catalytic effect of biochar, biochar treatments have been reported,
which include alkali modification, acid modification, metal oxide
modification and metal salt modification. The introduction of nitrogen atoms into biochar can increase the electronic effect and
alkalinity of biochar, and thus enhance the adsorption capacity of
CO2 [22,23]. The studies of Chen et al. [24] have found that nitrogen
doping in biochar is beneficial to the adsorption of acid gases or
volatiles, however, the form of N in nitrogen-doped biochar is uncertain, therefore, controlling the content of various nitrogencontaining functional groups is impossible. Chellappan and Nair
[25] found that the hydrophobicity of biochar increases with
increasing the pyrolysis temperature, and the introduction of sulfonic acid groups can promote esterification and transesterification,
but its preparation may cause more pollution. Huo et al. [1] used
phosphoric acid and MgO to activate biochar, which improved the
yield of bio-oil and the selectivity of alkylated phenol, however,
biochar production through microwave pyrolysis is difficult to scale
up, and the stability of microwave-prepared biochar in a tube
furnace needs to be considered.
From previous studies, it is found that the catalytic effect of
biochar and its modified form can be applied in the biomass pyrolysis to promote the yield of phenols and to inhibit the generation
of acids [1,21,24,26]. However, the composition of liquid product
components varies significantly due to different raw materials to
produce the biochar and the biochar modification method. The
objective of this study is to investigate the catalytic effects of biochar from catalytic fast pyrolysis of peanut shells.
Peanut shell (PS) is one of the residue of the peanut industry.
China is the first leading producer of peanuts in Asia. The resource
of peanut shells in China is about 5 million tons per annum, but its
utilization is limited to animal food and combustion fuel and even
most of them are abandoned. In addition, Peanut shell may be
transformed into bioenergy products in an environmentally
friendly way due to its lower content of nitrogen and sulfur which
just causes a small amount of acid gas to be released during pyrolysis and combustion [27]. According to Varma reports [28], the
activation energy of peanut shells is lower than that of most
biomass raw materials, so it has great potential for pyrolysis to
produce bio-energy products. Therefore, peanut shell was selected
as feedstock of pyrolysis process as a lignocellulosic biomass.
In this study, MnCl2 supported on peanut shell biochar or peanut
shell biochar treated with HCl, were prepared in two step through
first carbonizing peanut shell and secondly impregnating biochar
with HCl/MnCl2. Then the prepared biochar and its modification
were used for catalyst during the pyrolysis process of peanut shell.
Manganese is a transition metal which is adjacent to iron in the
2. Materials and methods
2.1. Experimental materials
Dry peanut shells were collected from local farms near
Zhengzhou City, Henan Province, China, and used as feedstock
materials for pyrolysis. After received, the peanuts shells were oven
dried and ground to particles with particle sizes in the range of
0.25e0.425 mm screened with 40 and 60 meshes. Proximate
analysis (according to the GB/T 28,731-2012 standards) and ultimate analysis (using the Vario EL III element analyzer) were then
performed and the results are given in Table 1. From Table 1, it is
observed that the fixed carbon content of the peanut shells is
21.29% which is higher than wood biomass, indicating that there is
a high biochar yield during the preparation of biochar catalyst.
Meanwhile, the low ash content (2.40%) means that pickling pretreatment of the raw materials is not necessary, because pickling
cannot remove more ash based on 2.4% ash content. In addition, the
volatile, hydrogen and oxygen contents of peanut shells are
68.07%,5.83% and 49.59%, respectively, which are positively related
to the formation of liquid and gaseous of organic compounds.
Moreover, 0.23% sulfur and 1.04% nitrogen contents indicate it may
cause little acid gas to be released during pyrolysis.
2.2. Preparation and modification of peanut shell biochar
In preparation of the biochar, a certain mass, 20 g, of peanut
shell particles was first weighed, and then put into the tube furnace
reactor as shown in Fig. 1. After purging with nitrogen gas (purity of
99.9%), the reactor was heated up through tube furnace at 5 C/min
to 550 C, 650 C and 750 C, respectively, and was maintained at
the target temperature for 1 h. During the heating up and the
operation, the nitrogen gas was injected to the reactor top as carrier
gas at a flow rate of 200 mL/min. After the experiment, the reactor
was cooled down to room temperature and the solid product,
biochar, was collected for analysis and modification. The biochar
was modified in three ways: (1). HCl modification (HC), (2). MnCl2
modification at moderate concentration of 0.2 mol/L (MC); (3).
Table 1
Proximate analysis and ultimate analysis of peanut shells.
Feedstock
Peanut shells
a
2
Proximate analysis/%
ultimate analysis/%
M
A
V
FCa
N
C
H
S
Oa
8.24
2.40
68.07
21.29
1.04
43.31
5.83
0.23
49.59
Estimated by difference.
C. Chang, Z. Liu, P. Li et al.
Energy 229 (2021) 120818
then out of the reactor from the bottom. Before the experiment, 4 g
of biochar or modified biochar obtained from section 2.2 was first
placed on the bed in bottom of the furnace reactor, and the reactor
was purged using N2 gas. Then the reactor was heated up at heating
rate of 22 C/min to the target temperature of 550 C. Once this
temperature was reached, the peanut shell particles were continuously added to the reactor from the top of the tube furnace, and N2
gas was also injected to the reactor top as carrier gas at flowrate of
200 mL/min. The gas and vapors produced in the pyrolysis flew
with N2 downwards, first passed through the biochar catalysis bed
at the reactor bottom, and then flew through the condenser at
temperature from 10e13 C. The condensed liquid was collected
in the conical flask, and the non-condensed gas was subsequently
dried and filtered, and finally sampled in an airbag. After the
experiment, the quartz tube reactor was cooled down to room
temperature. Then the bio-oil and the biochar products from the
pyrolysis as well as the biochar catalyst were collected, respectively,
and weighed using an analytical balance to determine the product
yields.
Fig. 1. Biomass catalytic pyrolysis reaction system. 1-Nitrogen cylinder, 2-Mass flow
meter, 3-Feeder, 4-Quartz tube, 5-Raw material, 6-Tube furnace, 7-Catalyst, 8Condenser, 9-Conical flask, 10-Scrubber (containing silica gel and absorbent cotton),
11-Air bag, 12-Cold trap.
2.4. Instrumental analysis
BK100-01 automatic surface area and pore analyzer based on N2
adsorption and desorption isotherm, manufactured by Beijing
Jingwei Gaobo Science and Technology Co., Ltd., was used to
determine the pore structure parameters of biochar and the
modified biochar. Before the analysis, the biochar sample was
degassed in vacuum at 300 C for 5 h, using N2 as the adsorption
gas, and its specific surface area was calculated according to the
Brunauer-Emmett-Teller (BET) formula [31]. The result is the
average of three measurements.
Fourier Transform Infrared Spectrometer (FT-IR), using KBr
compressed tablets to prepare samples, was used to analyze the
biochar and modified biochar in spectral analysis range of
400 cm1e4000 cm1 with resolution of 1 cm1.
X-ray diffraction (XRD) of model X0 Pert PRO X-ray diffractometer of PANalytical, Netherlands, was used to analyze the crystalline
phase characteristics of the biochar and modified biochar. Parameter settings for the analysis were 40 kV voltage, 40 mA current, Cu
as anode target with scan step of 0.0167 and scan speed of 5 /min
in the scan range of 10 ~80 .
Gas chromatograph (GC-14C, Shimadzu Corporation) was used
to analyze the gas products. Standard gas control procedure was
firstly used to identify the gas species and the external standards
were used to quantitatively analyze the main components of gas
products (CO, CO2, H2, CH4). The instrument conditions for the
measurement: Column is of Shimadzu TDX-01 (2 m 3 mm),
carrier gas is Ar, carrier(M) pressure is 0.1 MPa, injection volume is
100 mL, injection port temperature is 100 C, column box temperature is 80 C, detector temperature is 150 C. The result is the
average of three measurements.
Gas Chromatography/Mass Spectrometer (GC-8860/MS-5977A,
Agilent Technologies Co., Ltd.) was used to analyze liquid product
components. The instrument conditions for the measurement: 1)
GC conditions: Column is of HP-5-MS (30 m 0.25 mm 0.25 mm),
carrier gas is He, gas flow rate is 1.0 mL/min, split ratio is 50:1,
injection volume is 0.2 mL, injection port temperature is 275 C,
column heating program is that 40 C for 0.5 min, then raise the
temperature to 270 C at a heating rate of 5 C/min and keep it for
5 min; 2) MS conditions: MS interface temperature is 280 C,
scanning range is from 30 to 500 (m/Z), solvent delay time is 2 min.
Shake the liquid product before dilution and shake the solution
Further modification of the biochar obtained in (1) with MnCl2 at
concentration of 0.2, 0.8 and 1.5 mol/L, respectively (HMC). The
modification (3) was to investigate the effect of combination of HCl
and MnCl2, and the loading of biochar in MnCl2 solution.
In preparation of HC, the biochar obtained above was mixed
with 1 mol/L HCl according to a solid-liquid mass ratio of 1:20 in a
mixer which was stirred with a magnetic stirrer at 600 r/min for
6 h. Then the solution was filtered and washed with deionized
water to near neutral. The solid was then oven-dried under vacuum
at 105 C for 24 h.
In preparation of MC, the biochar obtained above was mixed
with MnCl2 at concentration of 0.2 mol/L according to the solidliquid mass ratio of 1:20 in a mixer which was stirred at 600 r/
min for 6 h. Then the solution was filtered and washed with the
same amount of deionized water 3 times. The solid was then ovendried under vacuum at 105 C for 24 h. After this the modified
biochar was heated to 550 C at 22 C/min under N2 (99.9%, 200 mL/
min) atmosphere, and calcined for 1 h.
In preparation of HMC, the HC modified biochar was mixed with
MnCl2 at concentration of 0.2 mol/L, 0.8 mol/L and 1.5 mol/L,
respectively, according to the solid-liquid ratio of 1:20. The solution
was stirred with a magnetic stirrer at 600 r/min for 6 h, filtered and
washed with the same amount of deionized water 3 times. The
solid was then oven-dried under vacuum at 105 C for 24 h. After
this the over-dry solid was heated to 550 C at 22 C/min under N2
(99.9%, 200 mL/min) atmosphere, and calcined for 1 h. The biochar
modified using this method was named as 0.2HMC (with concentration of 0.2 mol/L MnCl2), 0.8HMC (concentration of 0.8 mol/L
MnCl2) and 1.5HMC (concentration of 1.5 mol/L MnCl2),
respectively.
2.3. Catalytic pyrolysis experiment
Catalytic pyrolysis experiments were conducted using the same
apparatus as shown in Fig. 1 in which the carrier gas, N2, and the
biomass were introduced from the reactor top and the gas and
vapor product flew downwards through a catalytic biochar bed and
3
C. Chang, Z. Liu, P. Li et al.
Energy 229 (2021) 120818
before testing to ensure the homogeneity of the solution.
concentrations, which will make biochar form more microporous
structures, instead biochar will form more mesoporous structures
at high concentrations. It may make different effects on the characteristics of peanut shell pyrolysis products.
3. Results and discussion
3.1. Characterization of biochar and modified biochar as catalysts
3.1.2. FT-IR
Fig. 2 shows the Fourier transform infrared (FT-IR) spectra of
treated and untreated biochars from which similar patterns are
observed for all of the biochars. It is found from the figure that the
peak intensity of the untreated biochar prepared at 650 C at
3438 cm1 is higher than that of the biochars prepared at 550 C
and 750 C, which may promote the interaction between biochar
and phenolic intermediates [21,37]. The peak intensity of aliphatic
CH2 (2924 and 2852 cm1) decreases continuously with the increase of temperature, indicating that organic aliphatic hydrocarbons continue to decompose at high temperatures. As can be
appreciated, 650C is different from 550C to 750C in that it has more
oxygen-containing functional groups, where the three peaks at
1631, 1123, and 585 cm1 are attributed to the tensile vibrations of
C]O, CeO and eOH out-of-plane bending vibrations, respectively,
and its peak of C]C (1400 cm1) is the weakest among the three
temperatures. The results show that 650C will promote the production of phenols more than 550C and 750C [24].
After the hydrochloric acid modification (HC), the peaks of eOH
moved from 3438 cm1 to 3161 cm1, from 585 cm1 to 534 cm1,
respectively, and they were strengthened. Meanwhile, the peaks of
C]C and CeO were strengthened, which is connected to hydrochloric acid washed away the ash content of biochar. In addition, a
new peak appeared at 871 cm1, which means aromatic eCH.
After MnCl2 modification (MC), the changes of various functional groups on biochar are almost the same as those of HCl
modification, especially the C]C peak, which is greatly enhanced,
and further promotes the conversion of C]O to CeO.
After combined modification of the biochar with 0.2 mol/L
MnCl2 on the basis of hydrochloric acid modification (0.2 HMC), the
strength of functional groups of biochar is almost unchanged,
except that the strength of C]O and CeO decreases, on the other
hand, these functional groups increase with the increase of MnCl2
concentration. What's more, it can be seen from the figure that
calcining weakens the peaks of eOH and CeO. Comparing the
Fourier transform infrared spectra before and after the catalysis of
various biochar, it is found that the peaks of eOH, CeO and C]C on
the biochar after catalysis are enhanced, which indicates that some
small molecules are transferred to biochar through polymerization
reaction during the catalytic pyrolysis of biomass.
3.1.1. BET
Table 2 shows the pore structure parameters of biochar and the
modified biochar. It can be seen from the table that the original
biochar has a large specific surface area of 154.710 m2/g and an
average pore diameter of 2.949 nm. Based on these results, these
biochar and modified biochar fall in the category of mesoporous
catalysts but close to the category of microporous catalysts. It is
obvious that the specific surface area of the biochar decreases
significantly with increasing the preparation temperature which
can be explained by the void structure collapse of the biochar at
high temperatures [32]. No consistent trends are found on the effect of preparation temperature on the pore diameter and pore
volume, which is due to the etching effect of high temperature has
two sides. On the one hand, the high temperature causes the
porous structure of biochar, that is, the pore-forming effect, and on
the other hand, it causes the micropores are enlarged into mesopores or macropores, that is, the voids collapse [33]. As the temperature rises, the pore-forming effect is still dominant from 550 C
to 650 C, but void collapse happening to part of micropores is
inevitable, and when it reaches 750 C, the void collapse is
enhanced beyond the pore-forming effect, which indicates excessive etching effects lead to the micropores are enlarged into mesopores or macropores at higher temperature.
After washing of the biochar with hydrochloric acid (HC biochar), Hþ exchanges with metal ions in the biochar to form metal
chloride, part of which is lost in the modified solution, and part of
which stays in the voids, forming a rough surface, thereby forming
a new mesoporous structure, which increases the specific surface
area of the biochar [34,35]. The average pore diameter of the HC
biochar is reduced slightly but the pore volume is significantly
increased. When the biochar is treated with MnCl2 of 0.2 mol/L and
then calcined (MC), its specific surface area is significantly
increased, but the average pore diameter is closer to the biochar
treated by HCl and less than that of the untreated biochar. Based on
the pore diameters, the HC and MC biochars are close to the category of micropores catalysts [36].
However, with the combined treatment of biochar with MnCl2
of 0.2 mol/L (0.2 HMC), the specific surface area and average pore
volume are increased and the pore diameter is decreased in comparison with those of HC and MC biochars. It is increasing to note
that with increasing the concentration of MnCl2 solution, the specific surface area and the average pore volume of the biochars are
decreased and the pore diameter is increased. The above phenomenon shows that the influence of hydrochloric acid modification is greater than that of MnCl2 modification at low MnCl2
3.1.3. XRD
In order to compare the contents of carbon and MnCl2 in biochar
and the modified biochar, XRD analysis was performed. The results
of the biochar and three modified biochars, namely, biochar prepared at 650 C (650C), and modified biochars of HC, MC and
0.2HMC, are shown in Fig. 3(a) while those of the biochars of
0.8HMC and 1.5HMC are shown in Fig. 3(b). The characteristic
peaks of 2q were located near 22.5 and 43.5 correspond to (001)
crystal planes of disordered and amorphous carbon and (200)
crystal planes of graphitized carbon [38,39], respectively. As can be
appreciated, modification enhances disordered and amorphous
carbon peak. In 650C and HC, a 2q peak of 28.2 was detected,
which corresponds to the presence of silicon oxide [40,41]. The
peak here is enhanced after washing with hydrochloric acid
because the acid washing removes the ash in the biochar, exposing
the graphitized carbon and silicon oxide covered by the ash, which
was confirmed by the increase in the graphitized carbon peak and
the increase in specific surface area in the XRD pattern after pickling, while it almost disappeared in MC and 0.2HMC because of the
Table 2
Pore structure parameters of biochar.
Catalyst preparation
condition
SBET/(m [2]/g)
Average pore
diameter/nm
Average pore
volume/(cm [3]/g)
550C
650C
750C
HC
MC
0.2HMC
0.8HMC
1.5HMC
245.577
154.710
99.959
271.900
400.388
414.233
362.878
352.774
3.141
2.949
3.415
2.318
2.761
1.888
3.292
3.247
0.031
0.047
0.021
0.080
0.093
0.147
0.051
0.048
4
C. Chang, Z. Liu, P. Li et al.
Energy 229 (2021) 120818
Fig. 3. X-ray diffraction of biochar catalyst.
corresponding to the (311) crystal planes of MnO and the (224)
crystal plane of Mn3O4 [42,43], respectively. It is confirmed in the
different selectivity of MC and 0.2HMC to phenols in the catalytic
pyrolysis experiment. What's more, it can be seen from Fig. 3(b)
that as the concentration of MnCl2 increases, there is still no peak
other than the C peak in the XRD pattern of HMC, indicating that
MnCl2 is highly dispersed in biochar and the particles are smaller,
which is due to the fact that the hydrochloric acid washing is
beneficial to the dispersion of metal active sites [26].
Fig. 2. FTIR spectra of biochar catalyst (A means after catalysis pyrolysis, B means
before biochar calcination.).
3.2. Effect of temperature in the biochar preparation on its
characteristics
dual effects of MnCl2 and high-temperature calcination to bury
elemental silicon. After biochar modification with 0.2 mol/L MnCl2,
no characteristic diffraction peak of MnCl2 was observed, which
indicates that MnCl2 is highly dispersed in biochar. However, new
peaks (2q ¼ 35.5 , 59.2 ) appeared in the XRD pattern of MC,
One of the important operating parameters for biomass pyrolysis is the reactor temperature which has significant impact on the
product yields and the characteristics of each product, particularly
the biochar. From previous studies, it was found that in pyrolysis of
the peanut shell, high bio-oil yield can be achieved at pyrolysis
temperature of 500 C [44], and the maximum conversion at 580 C
5
C. Chang, Z. Liu, P. Li et al.
Energy 229 (2021) 120818
[45]. Therefore, in order to achieve the high yield of bio-oil and the
high conversion in pyrolysis of the peanut shells, 550 C was chosen
as the operation temperature in the experiments of the present
study for the biomass pyrolysis. However, the main objective of the
present study was to investigate the effect of temperature in biochar preparation on its characteristics and catalytic performance in
biomass pyrolysis and, therefore, the effects of pyrolysis temperature on the other products are not presented in this paper.
The distribution of peanut shells pyrolysis products by direct
pyrolysis or catalytic pyrolysis with biochar prepared at different
temperatures are shown in Fig. 4(a) whereas the composition of
gaseous product is shown in Fig. 4(b) and the key components in
the bio-oil area shown in Fig. 4(c). From Fig. 4(a), it can be seen that
the solid yield is reduced with application of the biochar which was
prepared at 550 C, while the effects of biochars prepared at 650 C
and 750 C did not show significant impacts in comparison with
direct pyrolysis without application of the biochar. These results
indicate that the biochars prepared at 650C and 750C have higher
stability and lower reactivity than that prepared at 550C. Therefore,
the biochar prepared at 550 C may be decomposed during the
catalytic pyrolysis process of peanut shells due to its instability,
which causes the calculated value of the solid product yield is low.
And to reduce the impact of biochar decomposition on the pyrolysis
products of peanut shells, the preparation temperature of biochar
should be higher than the temperature of the biomass pyrolysis
process.
From Fig. 4(b), it is observed that the four main components of
the gaseous product are H2, CH4, CO and CO2. It is also seen from the
figure that the content of H2 increases significantly with application
of the biochar as a catalyst, in particular for the biochars prepared at
650 C, reaching the maximum value of 9.24% which increased the
yield by 80.12% compared to direct pyrolysis. It is likely that biochar
may promote the cracking of CeH and C]C aromatic bonds [44].
And the increase in CH4 is duo to biochar promotes the cleavage of
methyl and methoxy groups [13]. Meanwhile, the content of CO2 in
the gas product reaches the maximum value of 31.91% with application of the biochar prepared at 650 C while the content of CO2 is
28.37% in direct pyrolysis, which indicates that biochar enhances
the decarbonylation and decarboxylation reactions [44]. On the
opposite trend, the content of CO decreases with application of the
biochar, which is connected with the secondary reactions [14].
Moreover, biochar may promote wateregas shift (CO þ H2O #
CO2 þ H2) reactions according to the change of composition of
gaseous product [33].
From Fig. 4(c), the main compounds in the liquid product of
peanut shell pyrolysis include acids, phenols, aldehydes, ketones,
esters, furans, etc. It can be seen from the figure that biochar can
promote the production of phenols and ketones, while inhibiting
the production of acids, which is due to the active sites in the
biochar promote the cleavage of the b-O-4 bond, and then the
oxygen-containing functional groups in the biochar react with
phenolic intermediates, which promotes the decomposition of
lignin [21,24]. As the temperature of biochar preparation increases,
the acid content decreased from 37.7% by direct pyrolysis to 28.5%
with biochar prepared at 550 C and then decreased to 25.3% with
biochar prepared at 650 C, but it was increased to 30.9% with
biochar prepared at 750 C. This trend corresponds to the change of
CO2 in Fig. 4(b), which indicates that biochar does promote the
decomposition of carboxyl and carbonyl groups. Only 650C
appeared to promote the production of phenols which is 21.8%
among biochar prepared at 550 C, 650 C and 750 C due to 650C
have larger average pore volume and more oxygen-containing
functional groups. The content of ketones increases as the temperature of biochar preparation increases due to dehydrogenation
reaction. In addition, the changing trend of aldehydes is similar to
Fig. 4. Effects of application of biochar prepared at different temperatures as a catalyst
in pyrolysis of peanut shells.
6
C. Chang, Z. Liu, P. Li et al.
Energy 229 (2021) 120818
that of acids, while the changing trend of esters and furans is
opposite to that of acids.
3.3. Catalytic effect of biochar modification on the pyrolysis of
peanut shell
The distribution of peanut shells pyrolysis products by direct
pyrolysis or catalytic pyrolysis with biochar and its modified biochar are shown in Fig. 5(a) whereas the composition of gaseous
product is shown in Fig. 5(b) and the key components in the bio-oil
area shown in Fig. 5(c). From Fig. 5(a), it can be seen that the solid
yield of peanut shell pyrolysis increased slightly with application of
the modified biochar. On the one hand, the properties of biochar are
relatively stable and the biochar will undergo a calcination process
after loading MnCl2, which excludes the effect of biochar decomposition on the experiment, so it can be assumed that the quality of
the biochar catalyst does not change. On the other hand, biochar
loaded with MnCl2 enhances the aromatization reaction and
polymerization reaction. The aromatic hydrocarbons are transferred to the biochar through aromatic polycondensation, so it
appears as an increase in solids yield and in fact the increase is
attributed to biochar catalyst. It is consistent with the increase in
the intensity of the C]C peak of the biochar catalyst. Modified
biochar significantly reduces the liquid yield and increases the gas
yield compared with direct pyrolysis, among which it obtains the
lowest liquid yield of 35.7% and the highest gas yield of 33.4% with
the catalysis of MC due to it has the highest inorganic content.
Hydrochloric acid treatment reduces its inorganic content [33e35],
while MnCl2 treatment increases its inorganic content.
From Fig. 5(b), it is observed that the four main components of
the gaseous product are H2, CH4, CO and CO2. It is also seen from the
figure that the modified biochar has a certain promotion effect on
the formation of CH4. The addition of Mn2þ enhances the ability of
biochar to crack methyl and methoxy groups. In section 3.2, it is
known that it can obtain high content of H2 with application of
650C as a catalyst. Among the three modified carbons, only MC can
reach a similar content to it, whose content of H2 accounts for 9.15%.
It may be due to the balance between the dehydrogenation reaction
and hydrogenation reaction [29] based on MC catalyst. The content
of H2 with application of HC as a catalyst has no significant change
compared with direct pyrolysis. It is due to some of the alkaline
earth metal elements are lost with the modifier, and the catalytic
cracking performance of macromolecules decreases after pickling
[46]. The content of CO in the pyrolysis gas with application of MC is
reduced in comparison with direct pyrolysis but increased in
comparison with 650C, while the change trend of content of CO2 is
opposite to that of CO. It indicates that MnCl2 modification enhances wateregas shift (CO þ H2O # CO2 þ H2) reaction and the
reverse reaction of Boudouard (2CO # CO2 þC) reaction, which can
also be verified by the increase in gas yield.
From Fig. 5(c), the main compounds in the liquid products of
peanut shell pyrolysis include acids, phenols, aldehydes, ketones,
esters, furans, etc. It is clearly seen that the content of phenols increases significantly with application of the modified biochar,
which indicates that modification enhanced the interaction between phenolic intermediates and biochar. First, the catalytic effect
of biochar on acids is analyzed. In section 3.2, it is known that the
acid content in the liquid product reaches a minimum of 25.31%
with application of 650C, which is 32.93% lower than direct pyrolysis. After washing with hydrochloric acid, although the specific
surface area of the biochar is increased, the basic site of the biochar
is neutralized, which causes the inhibitory effect on acids is
weakened in the liquid product of the catalytic pyrolysis of peanut
shells [46]. The distribution of liquid components obtained with
application of MC or 0.2HMC is similar to that of HC. The acid
Fig. 5. Effects of application of biochar prepared at different modification as a catalyst
in pyrolysis of peanut shells.
7
C. Chang, Z. Liu, P. Li et al.
Energy 229 (2021) 120818
content is reduced in comparison with direct pyrolysis and
increased in comparison with 650C. In section 3.1.3, it can be known
that MnCl2 not only exists in the form of MnCl2 in MC and 0.2HMC
but also exists in the form of Mn oxide, in which MnO can promote
the formation of acids and phenols [47]. Secondly, the effect of
biochar on phenols is analyzed. There is a maximum phenol content of 23.79% with application of HC, which increases by 8.83%
compared with 650C. This is due to the hydrochloric acid washing
increases the oxygen-containing functional groups on the surface
of the biochar and promotes the reaction between the biochar and
phenolic intermediates. After loading MnCl2, both MC and 0.2HMC
showed stronger phenol selectivity than C, but it did not reach the
expected higher phenol content with application of 0.2HMC than
HC. This is related to the low concentration of MnCl2 and the
decrease of oxygen-containing functional groups on the surface of
0.2HMC during the calcination process.
3.4. Catalytic effect of biochar modified with MnCl2 at different
concentration
The distribution of peanut shells pyrolysis products by catalytic
pyrolysis with HMC prepared at different concentrations are shown
in Fig. 6(a) whereas the composition of gaseous product is shown in
Fig. 6(b) and the key components in the bio-oil area shown in
Fig. 6(c). From Fig. 6(a), it can be seen that as the concentration of
MnCl2 increases, the liquid yield gradually decreases, and the gas
yield generally increases. This shows that the amount of Mn loaded
on the biochar is positively correlated with the concentration of the
MnCl2 solution. And a higher Mn content means that more secondary reactions will happen on the biochar at the same time. As
can be appreciated, the solids yield first decreased and then
increased with the increase of MnCl2 concentration, but both were
smaller than when the MnCl2 concentration was 0.2, which indicated that the aromatization reaction and polymerization reaction
did not increase with the increase of MnCl2 concentration. It is
related to the properties of biochar, 0.2HMC is essentially different
from 0.8HMC to 1.5HMC because its average pore diameter is
1.888 nm, which is a microporous catalyst.
From Fig. 6(b), it is observed that the four main components of
the gaseous product are H2, CH4, CO and CO2. It is also seen from the
figure that as the concentration of MnCl2 increases, the content of
H2 increased from 5.26% with HC to 7.29% with application of
0.2HMC and then increased to 9.89% with application of 0.8HMC,
but it was decreased to 9.10% with application of 1.5HMC. A proper
amount of Mn can promote the dehydrogenation reaction of volatiles, while an excessive amount of Mn will increase the hydrogenation reaction. The change of the content of CO and CH4 shows a
similar trend to that of H2. Especially for CH4, it can be clearly seen
that when the MnCl2 concentration is 0.8 mol/L, it has a great increase compared to when the MnCl2 concentration is 0. This once
again confirms the cleavage effect of Mn on methyl and methoxy
groups. Correspondingly, when the MnCl2 concentration is 0.8 mol/
L, it obtained the lowest content of CO2. Moreover, as the Mn
content increases in biochar catalyst, more CO should be generated
due to the reverse reaction of Boudouard (2CO # CO2 þC) reaction,
but it is not obvious in Fig. 6. On the one hand, Mn promotes the
production of H2, CH4, and CO at the same time, so the difference
between them under the percentage content gets smaller. On the
other hand, the hydrogenation reaction of CO may have occurred
because of its high content or it always exists in the pyrolysis
process of peanut shell, because the hydrogenation reaction condition of CO is more severe than that of CO30
2 .
From Fig. 6(c), the main compounds in the liquid products of
peanut shell pyrolysis include acids, phenols, aldehydes, ketones,
esters, furans, etc. There is an interesting phenomenon in the figure
Fig. 6. Effects of application of biochar prepared at different concentration as a catalyst
in pyrolysis of peanut shells.
8
C. Chang, Z. Liu, P. Li et al.
Energy 229 (2021) 120818
that as the concentration of MnCl2 increases, the effect of HMC on
acids changes from a low-concentration promoting effect to a highconcentration inhibitory effect in comparison with 650C. It is due to
calcination reduces the oxygen-containing functional groups in the
biochar according to the result of FT-IR, and the loading of MnCl2 at
low concentrations is very low. The lowest acid content is obtained
with application of 1.5HMC as a catalyst, which is 37.5% lower than
that of direct pyrolysis. The content of phenols increases with the
increase of MnCl2 concentration, and reaches the maximum value
of 33.1% when the MnCl2 concentration is 1.5 mol/L, which is an
increase of 78.9% in comparison with direct pyrolysis. And its
phenol yield is at an intermediate level among biomass raw materials (Table S1). The relationship between the ketone content and
the MnCl2 concentration is similar to the change trend of the
phenol content, and it increases slightly with the increase of the
concentration. On the contrary, the content of esters and furans
decreases with the increase of MnCl2 concentration. From previous
studies, it is known that biomass is mainly composed of cellulose,
hemicellulose and lignin, among which cellulose and hemicellulose
are the origins of furan, acid, aldehyde, and ketone et al., while
lignin is mainly decomposed into phenols [18,48]. The above results
indicate that the biochar modified by MnCl2 can promote the
decomposition of lignin.
The correlation between the MnCl2 concentration and the
content of the major compositions of bio-oil (phenols, acids and
ketones) is shown in Fig. 7. Phenols and ketone content described a
positive linear correlation with the MnCl2 concentration, while
acids content described a negative linear correlation with the
MnCl2 concentration. It may be due to the developed porosity of the
MnCl2 modified biochar catalyst promoted pyrolysis intermediates
to diffuse more easily in biochar, which enhanced interaction between the active sites (MnCl2 and oxygen-containing functional
group) and intermediate. By the way, the large specific surface area
provides more space for the interaction between intermediate and
the active sites, but it is obvious that its influence on the distribution of pyrolysis products is far less than that of MnCl2 concentration. The negative linear correlation between acid content and
MnCl2 concentration is due to MnCl2 modified biochar promotes
decarboxylation, decarbonylation, dehydration and oligomerization reactions to product the ketones and phenols [1,5]. More
importantly, the phenols in pyrolysis products mainly include
phenol, benzenediol and alkylated phenol, and the results are
shown in Table 3. It is clear that the increase in phenols content is
Table 3
Main components of phenols.
compositions
phenol
catechol
resorcinol
Hydroquinone
2-Isopropoxyphenol
phenol,2-methylp-Cresol
phenol,3,5-dimethyl1,2-benzenediol,3-methylOrcinol
4-ethylcatechol
Formula
C6H6O
C6H6O2
C6H6O2
C6H6O2
C9H12O2
C7H8O
C7H8O
C8H10O
C7H8O2
C7H8O2
C8H10O2
Content%
0.2HMC
0.8HMC
1.5HMC
2.76
9.09
0.87
1.13
0.71
1.58
2.10
0.72
2.35
0.72
0.00
4.16
9.49
0.68
0.91
0.64
1.90
2.37
0.88
2.59
0.66
0.49
5.79
9.40
0.54
0.91
0.90
3.56
4.57
2.56
3.26
0.89
0.67
mainly contributed by the increase in alkylated phenols and phenol
content. Although the content of benzenediol is the highest among
phenols, it is almost unchanged.
According to the analysis of experimental data, the possible
catalytic pyrolysis mechanism of peanut shell on biochar catalyst is
shown in Fig. 8. Cellulose and hemicellulose product furans, sugars,
aldehydes, acids, ketones and other small organic molecules and
release H2, CO2, CO during the pyrolysis process of peanut shell
[1,19]. Then they are converted into alkylated phenol by decarboxylation, decarbonylation, dehydration and oligomerization reactions on biochar [1,5]. On the other hand, lignin contains a large
number of aromatic rings, which create conditions for the formation of phenols. Lignin is cracked into small molecular substances
(such as phenol, methyl phenol, etc.) and some larger molecular
weight fragments during the pyrolysis process of peanut shell,
which are cracked again on the biochar catalyst. The side chains and
b-O-4 bonds on the benzene ring will break down to generate
phenol and alkylated phenols and release light hydrocarbons [7,13].
Furthermore, studies have shown that Mn has a catalytic effect on
both dehydrogenation and hydrogenation [29]. It promotes intramolecular dehydrogenation of volatiles to form ketones, cyclic
compounds, etc., and intermolecular dehydrogenation to form
oligomers or transfer to biochar, resulting in the increase in ketone
content and solid yield. However, these H did not form H2 due to
the fact that the H2 yield is not improved, but played a role in
breaking the CeC and ether bonds [7], which promotes the cleavage
of the lignin fragment molecules and to generate more alkylated
phenols.
4. Conclusions
From the results of this study, the main conclusions are drawn as
follows:
Biochar prepared at 650 C has the highest content of oxygencontaining functional groups and pore volume but the smallest
pore diameter and medium specific surface area. The contents of H2
and phenols are increased in comparison with the direct pyrolysis
and are the highest with the biochar prepared at 650 C. Meanwhile, the content of acids in the liquid product is reduced with
application of the biochar.
The specific surface area and pore volume of the MnCl2 modified
biochar are significantly increased, however, the catalytic effect of
the biochar modification on the bio-oil composition is not consistent. It is worth mentioning that the hydrochloric acid modification
increases the oxygen-containing energy groups in the biochar and
can promote the uniform dispersion of metal active sites, which
improves the selectivity of phenols, but it is not conducive to the
generation of H2.
With the increase of MnCl2 concentration, biochar gradually
Fig. 7. Correlation between product yield peak area and MnCl2 concentration.
9
C. Chang, Z. Liu, P. Li et al.
Energy 229 (2021) 120818
Credit author statement
Chun Chang: Writing - Original Draft, Resources, Zihan Liu:
Investigation, Writing- Reviewing and Editing, Pan Li: Conceptualization, Funding acquisition. Xianhua Wang: Data Curation.
Jiande Song: Investigation. Shuqi Fang: Resources. Shusheng
Pang: Writing- Reviewing and Editing.
Declaration of competing interest
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
Acknowledgement
This work was supported by the National Natural Science
Foundation of China (No. 52006200), the Foundation of State Key
Laboratory of Coal Combustion (No. FSKLCCA2108), Henan Science
and Technology Think Tank Research Project (No. HNKJZK-202122C), and the Program of Henan Center for Outstanding Overseas
Scientists (No. GZS2018004).
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.energy.2021.120818.
Notes
The authors declare no competing financial interest.
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