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Biomass thermochemical conversion: A review on tar elimination from biomass catalytic gasification
Journal Pre-proof Biomass thermochemical conversion: A review on tar elimination from biomass catalytic gasification Jie Ren, Yi-Ling Liu, Xiao-Yan Zhao, Jing-Pei Cao PII:
S1743-9671(19)30833-5
DOI:
https://doi.org/10.1016/j.joei.2019.10.003
Reference:
JOEI 650
To appear in:
Journal of the Energy Institute
Received Date: 16 July 2019 Revised Date:
3 October 2019
Accepted Date: 7 October 2019
Please cite this article as: J. Ren, Y.-L. Liu, X.-Y. Zhao, J.-P. Cao, Biomass thermochemical conversion: A review on tar elimination from biomass catalytic gasification, Journal of the Energy Institute, https:// doi.org/10.1016/j.joei.2019.10.003. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Energy Institute. Published by Elsevier Ltd. All rights reserved.
1
Biomass thermochemical conversion: A review on tar elimination from biomass catalytic
2
gasification Jie Ren a,b*, Yi-Ling Liu b, Xiao-Yan Zhao b, Jing-Pei Cao b**
3 4
a
5
Makromolekulare Chemie (ITMC), RWTH Aachen University, Aachen 52074, Germany
6
b
7
University of Mining & Technology, Xuzhou 221116, Jiangsu, China
8
Abstract:
Lehrstuhl für Heterogene Katalyse und Technische Chemie, Institut für Technische und
Key Laboratory of Coal Processing and Efficient Utilization (Ministry of Education), China
9
Biomass is promising renewable energy because of the possibility of value-added fuels
10
production from biomass thermochemical conversion. Among the thermochemical conversion
11
technology, gasification could produce the H2-rich syngas then into value-added chemicals via F-T
12
(Fischer-Tropsch) synthesis. However, a variety of difficulties, such as tar formation, reactors
13
impediment, complex tar cracked mechanism, etc. make it difficult to develop for further
14
application. This paper sheds light on the developments of biomass thermochemical conversion, tar
15
classifications, tar formation, and elimination methods. Secondly, we provide a comprehensive the
16
state-of-the-art technologies for tar elimination, and we introduce some advanced high activity
17
catalysts. Furthermore, many represent tar models were employed for explanation of the tar-cracked
18
pathway, and real tar-cracked mechanism was proposed. Following this, some operational
19
conditions and effective gasified models were concluded to give an instruction for biomass catalytic
20
gasification.
21
Keywords: Biomass; Thermochemical conversion; Gasification; Tar elimination; Tar-cracked
22
mechanism.
*
Corresponding author. Tel.: +49 80 26595 E-mail address: [email protected]; [email protected] Corresponding author. Tel./fax: +86 516 83591059. E-mail address: [email protected]
**
1
23 24
Contents 1. Introduction .................................................................................................................................... 3
25
1.1. Biomass energy ...................................................................................................................... 3
26
1.2. Biomass thermochemical conversion..................................................................................... 6
27
1.2.1. Biomass combustion .................................................................................................... 6
28
1.2.2. Biomass pyrolysis ........................................................................................................ 7
29
1.2.3. Biomass gasification .................................................................................................... 8
30
2. Tar elimination ............................................................................................................................. 12
31
2.1. Physical purification ............................................................................................................ 12
32
2.2. Chemical purification. .......................................................................................................... 13
33
2.2.1. Thermochemical reduction......................................................................................... 13
34
2.2.2. Catalytic reduction ..................................................................................................... 15
35
3. Gasified mechanisms and models ............................................................................................... 19
36
3.1. Biomass tar ........................................................................................................................... 19
37
3.2. Tar model compounds .......................................................................................................... 20
38
3.2.1. Toluene ....................................................................................................................... 20
39
3.2.2. Naphthalene ............................................................................................................... 23
40
3.2.3. Benzene ...................................................................................................................... 25
41
3.2.4. Lignin, cellulose and hemicellulose ........................................................................... 26
42
3.3. Mathematical models ........................................................................................................... 27
43
4. Conclusions and outlooks ............................................................................................................ 31
44
Acknowledgments ............................................................................................................................ 32
45
Reference ........................................................................................................................................... 32
46
2
47
1. Introduction
48
1.1. Biomass energy
49
Energy is an indispensable resource for the national economic construction, a driving force for
50
development [1-3]. The main energy sources of the world are remaining coal, oil and nature gas.
51
The primary energy consumption all over the world was presented in Fig. 1. Although traditional
52
energy occupied the most part of energy consumption, traditional energy overuse could lead to
53
global warming, environmental pollution, ecological balance damage and other issues [4-6]. The
54
present research focus has gradually shifted to efficient use of clean renewables and improved
55
energy structure development.
56
Renewable energy mainly includes water, solar geothermal wind, ocean, and material, etc.
57
Among these major renewable natural resources, biomass energy is a representative carbon energy,
58
which has a wide range of sources, sustainable and low pollution [7]. Biomass conversion contains
59
physical conversion, biochemical conversion and thermochemical conversion [8-10]. Table 1 listed
60
the basic classifications based on biomass materials origin, source, and biological diversity.
61
Combined with Table 2, we could observe biomass contains abundant carbon resources, which
62
would be potential to convert biomass into H2-rich syngas and valuable chemicals. To update,
63
biomass thermochemical conversion is a valuable technology to transform biomass into
64
value-added fuels or H2-rich syngas then to chemicals via F-T (Fischer-Tropsch) synthesis [11].
3
65
Fig. 1. The current global primary energy consumption.
66 67
Table 1. Biomass classifications based on their origin, source and biological diversity [12]. Biomass groups
Biomass sub-groups, varieties and species
Wood and woody biomass
Stems, branches, foliage, bark, chips, etc. of trees or plants; Various wood species including lumps, briquettes, sawdust, sawmill, pellets, etc.
Aquatic biomass
Marine or other algaes including blue, seaweed, water hyacinth, brown, green, kelp, weed, etc.
Herbaceous and
Grasses and flowers including alfalfa, bamboo, arundo, brassica, bana,
agricultural biomass
switchgrass, cane, etc.; Straws including bean, sunflower, rice, wheat, mint, etc.; Other residues including husks, pits, cobs, pips, bagasse, grains, food, fruits, shells, hulls, seeds, etc.
Animal wastes
Chicken litter, Bones, meat, manures, etc.
Contaminated biomass
Municipal solid waste, sewage sludge, waste papers, paper-pulp
and industrial biomass
sludge, hospital waste, fiberboard, wood pallets and boxes, paperboard
4
wastes
waste, etc.
Biomass mixtures
Blends from the above varieties
68
Table 2. Proximate (dried basis, wt.%) and ultimate analyses (dried and ash-free basis, wt.%) of
69
different varieties of biomass. Proximate analysis
Ultimate analysis
Sample
Reference VM
FC
A
C
O
H
N
Forest residue
79.9
16.9
3.2
52.7
41.1
5.4
0.7
[13, 14]
Oak sawdust
86.3
13.4
0.3
50.1
43.9
5.9
0.1
[15]
Poplar
85.6
12.3
2.1
51.6
41.7
6.1
0.6
[15]
Willow
82.5
15.9
1.6
49.8
43.4
6.1
0.1
[16]
Olive wood
79.6
17.2
3.2
49.0
44.9
5.4
0.7
[17]
Christmas trees
74.2
20.7
5.1
54.5
38.7
5.9
0.5
[15]
Soft wood
70.0
28.1
1.7
52.1
41.0
6.1
0.2
[18]
Bana grass
73.6
16.6
9.8
50.1
42.9
6.0
0.9
[15]
Miscanthus grass
81.2
15.8
3.0
49.2
44.2
6.0
0.4
[19]
Switch grass
80.4
14.5
5.1
49.7
43.4
6.1
0.7
[15]
Rice straw
64.3
15.6
20.1
43.0
5.7
1.0
0.2
[20]
Oat straw
80.5
13.6
5.9
48.8
44.6
6.0
0.5
[21], [22]
Wheat straw
74.8
18.1
7.1
49.4
43.6
6.1
0.7
[23]
Rice straw
64.3
15.6
20.1
50.1
43.0
5.7
1.0
[24]
Coconut shells
73.8
23.0
3.2
51.1
43.1
5.6
0.1
[25]
Pistachio shell
81.6
17.0
1.4
50.9
41.8
6.4
0.7
[15]
Hazelnut shell
69.3
28.3
1.4
52.9
42.7
5.6
1.4
[26]
Cotton husks
78.4
18.2
3.4
50.4
39.8
8.4
1.4
[25]
Corncob
88.9
20.0
1.0
44.3
48.5
6.4
0.7
[27]
5
Pepper plant
64.7
20.9
14.4
42.2
49.0
5.0
3.2
[25]
Rice husk
68.9
11.1
20.0
47.4
6.7
0.8
45.1
[20]
Soya husks
74.3
20.3
5.4
45.4
46.9
6.7
0.9
[25]
Walnut shells
59.3
37.9
2.8
49.9
42.4
6.2
1.4
[28]
Sewage sludge
34.6
2.2
69.7
47.4
34.2
7.7
8.3
[29]
Pig moisture
66.1
14.7
19.2
49.3
>38.1
6.8
5.1
[30]
70
Note: FC=Fixed carbon; VM=Volatile matter; A=Ash.
71
1.2. Biomass thermochemical conversion
72
1.2.1. Biomass combustion
73 74
Three main thermochemical conversion of biomass, the intermediate energy carriers and the final energy products were presented in Fig. 2.
75 76
Fig. 2. Three main thermochemical conversions of biomass, the intermediate products, and the final
77
products.
78
Among the existing thermochemical conversion technologies (combustion, pyrolysis, and 6
79
gasification), combustion technology is the only technology to produce heat and electric power.
80
Biomass combustion has a high efficiency for heat production, which is economically feasible.
81
However, the complex combustion process included consecutive solid-solid and solid-gas reactions.
82
The main steps for biomass combustion are followed by drying, gasification, char combustion, and
83
gas oxidation.
84
As a renewable energy source, biomass comes from plant, organic matter, animal waste, etc. For
85
the reason of high efficiency and reasonable transport distances, biomass and coal co-combustion is
86
promising, which could continuously produce energy for residents and factories utilize. However,
87
serious pollution from biomass combustion needs to cope with. There have two reasons for
88
pollutant formation: (1) Although optimized furnace was designed to reduce the incomplete
89
combustion of biomass, and it still could bring the release of CO, soot and polycyclic aromatic
90
hydrocarbons (PAHs); (2) As an existence of the N, P, S, K, Na, Cl, Mg, and P, NOx and particles
91
pollutants are formed after biomass combustion. The current main solution is to classify air and fuel,
92
which has been proven an effective measure to reduce NOx, and its potential activity can be reduced
93
by 50% to 80%. Specific measures to reduce biomass particles have not yet appeared so far, but
94
some researchers have proposed new ways to reduce air significantly, which may lead to new
95
furnace design and used for decreasing particulate emissions. In addition, more efforts should be
96
given to help plant operations optimizing, which could ensure low emissions and high efficiency of
97
biomass combustion under realistic conditions.
98
1.2.2. Biomass pyrolysis
99
It is a consensus that biomass pyrolysis could be transformed into liquid, solid and gaseous
100
fractions, by heating the biomass in the absence of oxygen under low temperatures [31,32].
101
However, the bio-oils mainly composed of complicated organic oxygen species preventing its
102
further industrial application [33]. If the biomass was applied for rapid pyrolysis, the pyrolysis
103
product can be used to primarily produce bio-oil with an efficiency of up to 80%. The produced
104
bio-oil can be used in the operation of engines and turbines and can be used as a feedstock for 7
105
refineries. However, some problems should be overcome during the conversion process and
106
subsequent utilization, such as the poor thermal stability and corrosively of pyrolysis products [34].
107
In some cases, hydrogenation and catalytic cracking of the oil removes alkali to reduce the oxygen
108
content to upgrade the quality of the bio-oil. Recently, major technical opportunities were
109
transformed to design the catalyst for biomass catalytic pyrolysis and subsequently upgrade and
110
produce the bio-oils, light aromatics, olefins and gases [35,36]. Biomass pyrolysis is certainly
111
understudied and in its infancy, more applications and development for pyrolysis should be focused
112
on the novel catalyst design, solid mixtures selection, and other exploring of related approaches.
113
1.2.3. Biomass gasification
114
o
At high temperature range of 800-1000
C, gasification is an effective method for
115
thermochemical conversion of biomass into combustible gases (CO, CO2, CH4, and H2), through
116
partial oxidation when air/oxygen was employed as gasification agent [37,38]. Furthermore,
117
biomass also could be gasified in the presence of steam and produce gas with high H2/CO ratio as
118
well as significant heating value for the reason of endothermic reactions occurrence (steam
119
gasification). The limitation is the external source should be provided for temperature improvement.
120
Biomass gasification is a complex thermochemical process, which contains many closely
121
interconnected reactions [39,40]. As Fig. 2 shows, this scheme begins with biomass drying through
122
heated around 150 oC, where moisture was converted into steam. Secondly, biomass volatiles in the
123
dried biomass samples would be vaporized and produced H2-rich gaseous products and water under
124
Air/O2 atmospheres. Thirdly, other hydrocarbons would be transformed into liquid tars then
125
condensed inside of the gasifiers. CO and CO2 could be produced from pyrolysis gases, tars and
126
char react with gasification agent, and H2 could be oxidized then produce water [41].
127
An optimized reactor design for biomass gasification is important. As a consequence of
128
oxidation reactions taken place of the biomass gasification, which is exothermic reactions would
129
generate more heating and increase the temperature [42,43]. For fixed-bed gasifier, it could be
130
classified as updraft gasifier and downdraft gasifier. Biomass materials are fed from the top or 8
131
bottom of the reactor and air/O2/steam (gasification agents) was supplied from the reactor side.
132
High gasification temperatures (>1200 oC) is required for both updraft and downdraft, and the hot
133
efficiencies were really significant (85-95%). However, for the reason of poor transfer of mass and
134
heat, and the temperature distribution of the reactor is not uniform [44,45]. Another fluidized bed
135
reactor (bubbling or circulating) have a great mixing between biomass and bed materials during the
136
gasification process, and the mass and heat transfer was improved then obtaining a significant
137
carbon conversion and gas yield. However, high temperatures could lead to biomass and bed
138
material particles agglomeration. Therefore, the additives were usually employed to control the
139
gasification temperature during the actual biomass gasification process. Rotary kiln reactor is a
140
important gasifier for biomass gasification recently [46]. The biomass could be rotated with a
141
proper speed improve the gasification between the reacting gas and biomass, and the exchange of
142
matter and heat could be fully exchanged. Biomass degradation could take place in a plasma reactor
143
under different atmospheres. Oxidizing agents are not required in this kind of reactor, and the
144
energy for biomass gasification would be provided plasma process. The plasma reactor usually
145
applied in clean production of gaseous products, then improve syngas yield from the light tar
146
decomposition during the gasification process.
147 148
In addition, biomass types, residence time, gasifying agents, temperature, pressure, etc. also significant for gasification evaluation.
149
As Fig. 3 presented, biomass gasification is a promising method to convert solid fuel into
150
syngas, then into value-added chemicals via F-T synthesis. However, the gasification process
151
produces not only useful gases but also some fly ash and tar. Tar as one of the contaminants in the
152
producer gas is the main concern of many researchers. Tar is the major problem that has not been
153
completely solved when the tar vapor condensed and blocked pipelines, turbines and engines. The
154
mechanism of tar formation has not fully understood until now.
9
155 156
Fig. 3. Concept of value-added chemicals production from gasification over high activity catalyst.
157
As the previous research already mentioned, biomass would be decomposed into primary tar
158
after dried and heated to 200 to 500 oC, which was the mixture of oxygenates and condensable
159
organic molecules [47].
160 161 162
Fig. 4. “Tar” component distribution as the temperature increasing at 0.3 s of residence time.
163
Redrawn from [48].
164
As shown in Fig. 4, tar components during gasification were changed with the temperature 10
165
rising. Primary tars and heavy tars would be cracked when temperature reached to 500 oC, and then
166
small molecules, non-condensable gas released. Finally, primary tar and heavy tar from biomass
167
could be completely destroyed to tertiary tar.
168
Tar formation should be considered in the development of biomass gasification, which could
169
condense into the reactors and then destroyed the gasifiers and decrease the gasification efficiency.
170
Many researchers give some definition of tar according to the different standard, which are as
171
follows:
172
1. The organics produced under thermal or partial-oxidation regimes (gasification) of any organic
173
materials are called “tar” and are generally assumed to be largely aromatics.
174
2. The complex mixture of condensable hydrocarbons, which includes monocyclic to bicyclic
175
aromatics along with other oxygen-containing species and complex polycyclic aromatics [49].
176
3. Tar as hydrocarbons with molecular weight higher than benzene.
177
As Fig. 5 concluded, previous researchers classified the tar to two categories according to the
178
release sequence of tar cracking, and the solubility, condensability. Therefore, we could eliminate
179
tar based on their components and properties. To elaborate the importance and development of tar
180
removal, the present paper first discusses the strategies for tar elimination including physical,
181
thermal and catalytic removal. A direct comparison of the literatures for tar removal and reactor
182
types was concluded in detail.
11
183 184
Fig. 5. Categories of biomass tar according to the light of the solubility, condensability and release
185
sequence.
186
2. Tar elimination
187
The hazard of tar is enormous in the biomass gasification systems. Previously researchers
188
employed a variety of methods for removing or reducing tar generated during gasification process.
189
The tar elimination methods can be categorized depending on the tar removed location, one is in the
190
gasifier itself, anther is in outside of the gasifier. Tar removed from outside of the reactor is suitable
191
for produced gas treatment, and it could be divided into three categories: 1. Physical purification
192
method; 2. High-temperature thermal cracking; 3. Catalytic cracking.
193
2.1. Physical purification
194 195
The physical purification methods of tar are divided into wet and dry purification [47]. Wet method (washing) could also divide into spray method and bubble blowing.
196
The main disadvantages of the wet purification method are as follows: 1. Liquid mist in the gas
197
is easily entrained; 2. Low temperature is needed during the gasification operation; 3. Difficult
198
cleaning of the equipment; 4. Hard to liquid recovery; 5. Large circulation equipment. Furthermore,
199
the sewage after the wet purification will cause secondary pollution. In addition, the tar amount 12
200
about 5%-15% of the total energy in gasification will be lost with water, and thus the energy is
201
wasted, so the wet purification system will eventually be eliminated.
202
Dry purification (filtration), utilizes the strong adsorption of the substance to filter out the tar
203
through the adsorption layer and remove tar, which was installed inside the container [50]. The
204
filtration method has the advantages for wide adaptability of tar, high removal efficiency of tar,
205
wide source of filter material and lower price than wet method. Although dry purification could
206
solve the problem of water pollution, the complex filtration equipment, high cost and inconvenient
207
operation-running lifetime and short operating life hindered its development. This method also
208
requires a low gas flow rate and generally used in the end-stage separator and other
209
high-demanding occasions. Most important, it is still not solve the problem of tar energy utilization.
210
Finally, the filtration will be gradually replaced by other purification methods.
211
2.2. Chemical purification.
212
2.2.1. Thermochemical reduction
213
Thermochemical reduction technology is a promising chemical method for tar removal.
214
Biomass tar could be cracked into different chemical compounds under different temperatures. As
215
Fig. 6 presented, benzenes, phenols, catechols, etc. could be formed during the fast pyrolysis
216
process. Besides, the other gases derived from pyrolysis would be released at a high temperature
217
above 700 oC, where light tar can be cracked into light gases [51-53]. Previous study reviewed that
218
tar could be cracked by high-temperature cracking in a fixed, fluidized bed and other gasifiers.
219
In addition, the researchers also mentioned that biomass tar was hardly cracked by thermal
220
treatment. They suggested the following methods could effectively decompose the biomass tar:
221
optimal residence time, optimal reaction temperature, optimal gasification agents and gasifier, etc.
222
To achieve a high biomass tar cracking efficiency, Han and Kim reported that 1250 oC was the
223
lowest temperature for sufficient tar cracking. Fagbemi et al. [53] employed wood, straw, and
224
coconut shell to evaluate the effects of reaction temperature and residence time on tar cracking.
13
225 226
Fig. 6. Composition of biomass tar at different temperatures.
227 228 229 230
Fig. 7. Effect of temperature on the gasified products of straw (a), coconut shell (b), wood (c) and tar cracked kinetics (d). Redrawn from Ref. [53]. From Fig. 7, it could be found that the tar yield decrease and gas increase with the temperature 14
231
increasing and pyrolysis temperature around 1000 oC is suitable for tar cracking. Longer residence
232
in favor of tar yield decrease.
233
2.2.2. Catalytic reduction
234
Although various measures were taken to control the production of tar during the gasification
235
process of biomass, the tar contents in the gas is still far beyond the allowable level in practical
236
application. In order to deal with the tar in the gas and improve the hydrogen yield, the addition of
237
high activity catalyst is an essential process to realize the effective use of combustible gas at low
238
temperature. During the process of biomass gasification, the catalyst mainly plays three roles:
239
(1) Reducing the activation energy which required for the pyrolysis reaction, and then reducing the
240
source consumption;
241
(2) Reducing the input of gasification medium;
242
(3) Achieve directional catalytic cracking of tar by directed catalytic cracking, and obtain more
243
useful products, which then synthesized for F-T synthesis to prepare abundant highly value-added
244
chemicals. As Table 3 presented, Ni-based catalysts are the commonly used catalyst for biomass
245
catalytic gasification [54].
246
Table 3. Performance comparison of different catalysts and synthetic methods. Biomass Catalyst H2 yields Gasification conditions
Ref.
Ni loading: 12%, Ce loading: 7.5%; Rice hull
Corncob
Ni/CeO2-ZrO2
69.7%
Ni/lignite char
60.0
Ni loading: 17.32%; 1 h; Steam: 30 kPa;
mmol/g
650 oC; 1 g biomass
Pig
W/B=4.9, 800 oC
69.1
Corncob
[27]
[56] Ni loading: 19±1 wt.%; 650 oC; Ar
Ni/lignite char manure
[55]
mmol/g Limonite
70.4 vol.%
700 oC, 3600 h-1, 30 kPa steam
Wood
[57] [58]
NiO/MgO
51.0 vol.%
o
NiO loading: 7.2 wt.%; 850 C
sawdust 15
Ni:Mg:Al=1:1:1; T1=400 oC, T2=800 oC; Corn stalk
Ni-Mg-Al
[59]
56.5% S/C=3.54; 30 min
Wood NiZnAlOx
48.1 vol.%
T1=535 oC, T2=800 oC;
[60]
sawdust 750 oC, Equivalence ratio of 0.30, Steam
Pine Calcined dolomite
52.8 vol%
sawdust
[61] for 0.4 kg/h S/CH4=2; Particle diameter: 2-3 mm; 800
Sawdust
Ni/MgO, dolomite
81.1%
[62] o
C; Catalyst: 15.0 g; GHSV= 3600 h−1
Maize
Ni loading: 14.9%, Ce loading: 2.0%; 900 Ni-Ce/Al2O3
71.4%
stalk Almond
[63] o
-1
C, S/C= 6; WHSV=12 h
Tri-metallic 63.7 vol%
900 oC,
[64]
shells
perovskites
Corncob
Ni/Al2O3
25.4 vol.%
650 oC, Ni loading: 20 wt%, 3600 h-1
[65]
Ni/La2O3-αAl2O3
96%
Ni loading: 9.92%; 700 oC; S/C=12;
[66]
61.2
Ni loading: 18.0%; 1 h; 650 oC; 1 g
mmol/g
biomass; Steam: 30 kPa
Pine sawdust
Corncob
Ni/Resin
Pine
[67]
130.3 g/kg 0.4 kg/h steam, 820 oC, 2.14 h-1,
Calcined dolomite sawdust
[68]
biomass
247
Note: W/B=Water/biomass, WHSV=Weight hourly space velocity, GHSV=Gas hourly space
248
velocity, S/C=Steam/carbon, T1=Temperature of biomass pyrolysis, T2= Temperature of catalytic
249
bed
16
250 251
Fig. 8. Classification of common high tar cracking activity catalysts.
252
Fig. 8 concluded the main tar cracking catalyst which proved to be active for biomass tar
253
reforming, such as natural catalysts, Ni-based catalyst, carbon-based catalyst, noble metal-based
254
catalyst, etc. Natural catalysts, like minerals, consisted of different metal oxides, which already
255
proved to be effective for biomass gasification. Natural minerals are the economic support for
256
supporting active metal, while they are unstable and easily deactivated when employed for
257
long-term utilization. Metal oxides (MgO, CeO2, etc.) are the main components in the natural
258
catalyst. Miyazawa et al. [69] reviewed the literatures and reported that the activities of these
259
natural catalysts are as follows: Ni/Al2O3>Ni/ZrO2>Ni/TiO2>Ni/CeO2>Ni/MgO. Alkali metals, like
260
Li, Na, K, etc. could be the primary catalysts and enhance the biomass gasification reactions, which
261
are also the components of the biomass. Alkali metals are usually used as a gasification or catalyst
262
additives and directly feed into the reactor with the biomass fuels. Carbon-based catalyst with the
263
advantages of self-reduction and easily recovery for power generation are detailed studied recently
264
[70]. Noble metal catalysts exhibit an excellent performance for gas production, but they are
265
expensive to obtain and apply for industrial utilization. In addition to the Co and Fe based catalysts,
266
Ni-based catalysts are widely used for biomass tar reforming and tar model compounds cracking
267
[71,72]. Ni-based catalysts belong to the transition metals (group VIII), which are widely 17
268
investigated for H2-rich gas production during biomass tar elimination. Commercial Ni-based
269
catalysts are easily deactivated through carbon deposition, Ni(CO)4 formation, S-poisoning
270
sintering, pore blockage, support broken and Ni oxidation by produced water [73,74]. Hence,
271
develop a suitable support with special structure and modifying active metal is the focus in recent
272
research, which could improve the activity and stability of Ni-based catalyst. The detailed activity
273
of these catalysts for biomass tar elimination would be discussed below.
274
Natural minerals, such as olivine, clay minerals, calcines rocks, etc., which contains Mg, Fe, Si,
275
Al, and other active metal could provide a great tar-cracked ability. Rapagna et al. [75] compared
276
the catalytic activity of olivine and calcined dolomite, they concluded that the performance of
277
olivine was better than dolomite in terms of tar elimination and the gasification activity. In addition,
278
the doping of other active metals to natural minerals could improve the tar reforming activity.
279
Tursun et al. [76] employed the olivine and NiO/olivine as the catalyst for tar in-situ removal and
280
observed that the gas yield of 1.59 Nm3/kg with 56.1 vol% of H2 concentration. Especially, tar
281
yields were reduced by 55% and 94% when olivine and NiO/olivine catalysts were used. Zhang et
282
al. [77] prepared different loading of NiO/olivine, and NiO-CeO2/olivine via the method of wet
283
impregnation to investigate the activity of benzene and toluene steam reforming at 700 and 830 °C
284
and S/C of 5.
285
Virginie [78] studied the activity of Fe/olivine for tar removal, and they found that the changes
286
of Fe2+ to Fe3+ in Fe/olivine could promote volatiles burning. Similarly, Zhao et al. [57] also
287
indicated the balance between the Fe2O3 and Fe3O4 shows a better activity for corncob tar cracking,
288
which is the reason for high H2 yields (Fig. 9).
18
289 290
Fig. 9. TEM images of limonite before (a) and after (b) calcination, XRD patterns (c) of limonite
291
calcined at different temperatures, and gas yields (d) from corncob tar cracking. Redrawn from Ref.
292
[57]
293
Raheem et al. [79] employed ZnO-Ni-CaO (16.4 wt.% loading) to obtain a high H2 fraction of
294
48.95 mol% at 851 oC. Furthermore, the development of biomass tar cracking catalysts should be
295
focused on the catalyst with high activity, cost and easy recovery at low temperatures.
296
3. Gasified mechanisms and models
297
3.1. Biomass tar
298 299
Real biomass tar is a complex oxygenated hydrocarbon, the main deposition pathways under inert and steam atmospheres were as follows: Cracking: CxHyOz (tar)→mCO+nCO2+pH2+qCH4
(1)
Steam reforming: CxHyOz (tar)+H2O→CO+H2
(2)
CO+H2O→CO2+H2 ∆Η=+41 MJ/kmol
(3) ∆Η=+260 MJ/kmol
CH4+CO2→2CO+2H2 19
(4)
Carbon formation: CxHy→xC+y/2H2
(5)
C+2H2O→CH4+CO2
∆Η=+103 MJ/kmol
(6)
C+H2O→H2+CO
∆Η=+130 kJ/mol
(7)
300
In our previous studies, we designed various catalysts for biomass tar cracking under Ar and
301
steam atmospheres. As we already presented, the volatiles from biomass gasification would be
302
oligomerized on the catalyst surface and formed carbon deposition. Simultaneous reactions, such as
303
thermal cracking, water-gas shift, tar steam reforming, CO2 dry reforming, coke formation, etc.
304
could take place, and light and heavy tar were cracked and reformed to light tar molecules and
305
syngas on the active sites of as-prepared catalysts via several simultaneous reactions Especially,
306
dissociate hydroxyl radicals (OH·) produced on the surface of catalyst when steam was introduced
307
into reaction, and then OH· react with the tar volatiles to produce H2, CO2 and CO. As we described
308
in section 2, tar model compounds such as toluene, benzene, etc. are usually employed for
309
mechanism investigation for the reason of complexity of real tar.
310
3.2. Tar model compounds
311
Biomass tar is a mixture of condensable hydrocarbons, which includes complex ring aromatics,
312
PAHs and O-containing hydrocarbons [80-82]. These resulted in the difficulty to understand the
313
decomposition mechanism of real tar, because of the wide range of different compounds presented
314
in tar. For easy understanding, toluene, benzene, naphthalene, etc. were usually employed as the
315
representative tar model compounds to discuss the possible crack pathway of the tar [83-90].
316
3.2.1. Toluene
317 318
Zou et al. [91] reported the pathways of steam forming of toluene over Fe-Ni/Palygorskite in a fixed-bed reactor.
20
319 320
Fig. 10. HADDF, EDS images (a), particle size distribution (b) of Fe3Ni8/Pal catalyst; Stability of
321
the Fe3Ni8/Pal catalyst for toluene steam reforming (c) and Arrhenius plot (d) for apparent
322
activation energy calculation. Redrawn from [91].
323
As Fig. 10 shows, they evaluated the different parameter influences of catalytic temperatures
324
and S/C ratios on the toluene steam reforming. The catalytic activity of Fe3Ni8/Palygorskite reached
325
maximum when S/C molar ratio=1.0, the pre-exponential factor and apparent activation energy of
326
this catalyst were 41.55 kJ/mol and 1350 m3 kg−1 h−1, respectively. The H2 yield, CO yield, toluene
327
conversion, and H2/CO molar ratio were >63%, >60%, >97% and 1.55, respectively, when 0.5 g
328
catalyst was tested at S/C=1 and 700 oC. From the characterization of the catalysts, they mentioned
329
coke formed on the active metal sites. Especially, they reported the formation of graphitic carbon
330
was the main reason for Fe3Ni8/Palygorskite deactivation. Water-gas shift reaction improved the
331
activity recovery of Fe3Ni8/Palygorskite. The molar ratio of H2/CO (1.41-1.66) and CO/CO2
332
(7.4-12.6) fluctuated with the increase of reaction time. The Fe3Ni8/Pal catalyst could exhibit a 21
333
relatively stable activity when H2/CO molar ratio was fixed. From the characterization results of the
334
catalyst, the HADDF (High-angle annular dark-field) image, Fe and Ni EDS (Energy dispersive
335
X-ray spectrometry) mapping (Fig. 10) show the strong interaction and they uniformly distributed
336
on the surface of palygorskite. Evidently, the carbon deposition of the catalyst could be clearly
337
observed from the C mapping.
338
Ren et al. [92] developed a Ni/C layered carbon catalyst using modified lignite char, and
339
employed for corncob tar and toluene reforming, and then investigated the effects of pH value of
340
solution and calcination temperature on the catalyst activity. Finally, they calculated the reaction
341
activation energy Ni/C catalyst. They claimed the Ni (111) plane and layered structure of the
342
layered carbon load Ni catalyst have a superior activity for corncob tar and toluene reforming. Liu
343
et al. [93] employed a gliding arc discharge reactor for the toluene conversion under N2 atmosphere.
344
They investigated the effects of S/C ratio, toluene feed rate and specific energy input on their
345
reactor. The toluene conversion reached 51.8% when S/C=2, toluene flow rate= 4.8 mL/h and
346
specific energy input=0.3 kWh/m3. The syngas yield was 73.9% contains 34.9% of H2 and 39% of
347
CO. Liu et al. [93] also mentioned a new stepwise oxidation route for the toluene conversion when
348
steam was introduced into the plasma reaction, and steam could produce OH radicals resulting in a
349
significant improvement of syngas and light tar during the conversion of toluene.
350 351
Fig. 11. Possible cracked mechanism of toluene reforming in gliding arc discharge reactor. Redrawn 22
352 353
from [93] The primary pathway of toluene decomposition in gliding arc discharge reactor might be as
354
follows:
355
(1) H was abstracted from the methyl group, and the generated benzyl radicals would react with OH
356
to obtain benzaldehyde (ii), and then benzaldehyde was oxidized to form benzoic acid.
357
(2) Phenyl radicals (iii) could be obtained through the energetic electrons collided with reactive
358
species from the aromatic intermediates. Furthermore, phenyl radicals (iii) also could be produced
359
from C-C bonds between methyl and benzene through N2 excited species and energetic electrons.
360
(3) Benzene (iv), phenol (v), aniline (vi) and benzonitrile (vii) could be formed from the reaction
361
between phenol radicals, H, OH, NH2 and CN radicals, respectively.
362
(4) The aromatic ring of toluene was cracked to produce acetylene and methyl-cyclobutadiene, and
363
then a peroxide bridge radical formed.
364
(5) The ring of toluene was opened and formed a relatively stable epoxide (xii).
365
(5) The epoxide radicals were decomposed to syngas and H2O.
366
3.2.2. Naphthalene
367
Furusawa et al. [94] investigated the naphthalene steam reforming by using the Co/MgO and
368
found that 12 wt.% Co/MgO exhibited the best activity, and then activity decrease resulted from the
369
CnHm radicals deposition and the oxidation of catalysts by steam introduction. Sato et al. [95]
370
developed a novelty Ni-WO3/MgO-CaO, and WO3 was used as a sulfur-resistant promoter. The
371
catalysts showed a high activity and stability for naphthalene reforming even in gas containing
372
hydrogen sulfide. Furthermore, they investigated the concentration of H2S from 0-500 ppm and
373
compared with the commercial Ni/Al2O3 and Ru/Al2O3 catalyst. The result shows the
374
Ni-WO3/MgO-CaO catalyst has a great sulfide resistance. The possible reaction mechanism for
375
naphthalene reforming under H2S atmosphere was drawn in Fig. 12.
23
376 377
Fig. 12. Possible pathways of S elimination over Ni-WO3/MgO-CaO catalyst.
378
Firstly, Ni catalysts combined with S and produce Ni-S, and then S in Ni-S was replaced by W
379
at high temperature, Finally, WSx on Ni catalyst was converted to H2S. Their catalyst activity for
380
naphthalene conversion reached 90% during 100 h test in the presence of H2S at 800-850 oC, and
381
this kind of catalyst was already applied in industry for sewage sludge containing-biomass
382
gasification. Josuinkas et al. [96] chosen the hydrotalcite-like with 10 and 20 wt.% NiO for benzene,
383
toluene and naphthalene steam reforming. The catalysts presented the same activity for benzene and
384
toluene steam reforming over 10% NiO-hydrotalcites. However, naphthalene is hard to convert and
385
inhibits the toluene reforming. Their catalyst showed great stability, coke and Ni sintering resistance.
386
The effects of temperatures on tar model compounds reforming were also investigated in detail as
387
Fig. 13 presented. As shown in Fig. 13, naphthalene reforming is more difficult than benzene or
388
toluene reforming under steam atmosphere. They found that the toluene conversion decreased due
389
to naphthalene strongly adsorbed on the catalyst surface. The conversion of toluene reforming alone
390
(100%) is higher than together with naphthalene (85%) at 700 oC. Formed H2 could be detected at
391
high temperatures (above 800 oC), and the main products are CO and CO2 even the temperature
392
reached 900 oC.
24
393 394
Fig. 13. Conversion (a) of toluene and naphthalene over 10NiHT and 20NiHT and product
395
composition for benzene reforming over 10NiHT. Redrawn from [96]
396
3.2.3. Benzene
397
Kaisalo et al. [97] employed Ni/Al2O3 to investigate the behaviors of the benzene steam
398
reforming kinetics at 750-900 oC, and they discussed the qualitative effect of the gaseous products
399
compounds on the reforming kinetics. The first-order kinetic model was built to explain the H2 or
400
CO2 accelerated/decelerated the steam reforming of benzene. Furthermore, they used the
401
Langmuir-Hinshelwood type model to describe the effect of H2 on the benzene decomposition. As
402
Fig. 14 presented, they employed a linearization method to calculate the Arrhenius plots from
403
benzene inlet concentration between 600 to 3500 ppm and H2O concentration varied from 4.3 to
404
12.8%. Moreover, the linearization method was used to explain the behavior of different gas
405
compositions (Fig. 14b), these points are drawn from the experiments with different catalyst
406
packings, stream time and gas compositions. Although the effects of gaseous species were
407
qualitatively investigated in this study, more experimental data are required for explaining the
408
influence of CO2 on the benzene reforming and that reason of H2S poisoning.
25
409 410
Fig. 14. Arrhenius plot for the benzene and concentration of steam (a) and for the different gas
411
compositions (b). Redrawn from [97]
412
Furusawa et al. [98] compared the catalytic performances of Pt and Ni-based catalysts for the
413
naphthalene/benzene steam reforming. They concluded Al2O3 is an excellent carrier for H2
414
production when used for naphthalene/benzene steam reforming to produce H2. Krause [86]
415
employed natural catalyst of dolomite as the reformed catalyst for benzene reforming, they used
416
kinetic models to investigate the catalytic behavior of simulated gas mixture gasification at 750-925
417
o
418
of mechanistic models investigation show the rate-determining step of single-site adsorption of
419
hydrogen and benzene was the main reason for benzene decomposition. Colby et al. [99] develop
420
the Rh based catalysts to reduce benzene at 650-850 oC in a fixed-bed reactor. They increased the
421
dispersion and stability of Ru-based catalyst through Ce doping.
422
3.2.4. Lignin, cellulose and hemicellulose
C, and they develop Langmuir Hinshelwood models to discuss the benzene reforming. The results
423
Lignin, cellulose and hemicellulose are the main components of biomass. The contents of lignin,
424
cellulose, and hemicellulose in the biomass depend on the biomass types. Wu et al. [100] employed
425
a Ni-based catalyst to evaluate the H2 production in a two-stage fixed-bed reactor. They concluded
426
their catalyst is more active for H2 production during cellulose gasification. Yu et al. [101]
427
employed lignin, cellulose, and hemicellulose and discussed the tar formation mechanisms and 26
428
characteristics during the gasification. They reported tar yields were increased as temperature or
429
excess air ratio increasing during lignin, cellulose and hemicellulose gasification. Lignin has a high
430
tar yield of stable components because of its special molecular structure, and it is more impossible
431
to obtain the heavy tar, However, more PAHs produced from benzene, toluene, etc. Hosoya et al.
432
[102] mixed lignin, cellulose and hemicellulose and investigated their interactions during the
433
gasification process at 800 °C. They observed the improvement of lignin during the light tar
434
production, and inhibition during thermal polymerization of laevoglucose. Cellulose improves the
435
formation of lignin-derived products and the secondary char decomposition. Similar interactions
436
also could be observed in cellulose-hemicellulose pyrolysis. Finally, they mentioned lignin
437
gasification must be considered during tar removal.
438
Lv et al. [103] investigated the effects of alkali and alkaline earth metals in biomass materials
439
on gasification by using a thermogravimetric analyzer. They removed the alkaline earth metals
440
through acid treatment and found it consists of gaseous products depend on the ratio of biomass
441
components and alkaline earth metals. Carbon deposition was decreased with the improvement of
442
cellulose content, and the tar and gas yields were increased with the improvement of cellulose
443
content. They observed the first step is the decomposition of cellulose, and then lignin gasification
444
at high temperature. Furthermore, the influences of alkaline earth metals on biomass fuel
445
gasification were concluded, and alkaline earth metals could decrease gasification temperature and
446
increased the product yield.
447
3.3. Mathematical models
448
Models are helpful to evaluate the gasified behavior of various biomass feedstock without
449
actually different kinds of reactors at wanted temperatures and pressures. Tar models design for
450
biomass gasification could be divided into three models: single-compound models, kinetic models,
451
and lumped models. In general, the simulation of biomass gasification could be classed as 5
452
categories: (1) Kinetic Models (2) Tar Models (3) Computational Fluid Dynamics (CFD) Models (4)
453
Artificial Neural Network (ANN) Models (5) Thermodynamic Equilibrium Models. Toluene and 27
454
phenol were used as a single compound model and study a thermodynamic model for tar formation
455
[104,105]. The single-compound model is the simplest model of biomass gasification and they
456
reflect how biomass reacts with gasification agents. However, lumped and kinetic models in the
457
biomass gasification could utilize heat and mass to transfer information.
458
Kinetic model is more accurate than thermodynamic models in the field of gas yield and
459
composition at low temperature. The Arrhenius (k =Aexp(-Ea/RT)) plots are significant to this model,
460
which reflects the kinetic parameters. This equation contains the temperature, conservation of
461
energy and mass and momentum. Inayat et al. [106] developed a kinetic model to study the effects
462
of temperature and concentration for oil palm empty fruit bunch gasification at steady and dynamic
463
states. Sreejith et al. [107] provided a kinetic model and proved the steam atmosphere is better than
464
the air-steam atmosphere on gas production from wood gasification in a fluidized bed gasifier.
465
Khonde et al. [108] designed anther activation energy model for rice husk gasification, and they
466
claimed that the distributed activation energy model is more suitable than the single reaction model. dVi/dT=ki(V*i-Vi) Vi =V*i (
(8)
∞ t exp (- 0 ki dt)f(Ei)dEi) 0
(9)
467
Vi: Initial yield of tar or gas at t (time); V*i: Final yield of Vi; ki: Reaction rate equation; f(Ei):
468
Gaussian distribution of activation energy (mean); f(Ei)dEi: fraction of tar or gas formed with the
469
activation energies from E to E+dE.
470
The model of Khonde et al. [108] shows the minimum standard error of simulation, and their
471
models could reach maximum tar conversion through the variables of time and temperature.
472
However, it is complex to know the conversion reason from the time-temperature data by using the
473
distributed activation energy model.
474
Huang et al. [109] investigated thermal cracking behavior of soybean straw and compared the
475
kinetic parameters with experimental results in thermogravimetric analyzer. They utilized
476
iso-conversion
477
Coats-Redfern (CR) method to calculate the activation energy. The related results were shown in
Kissinger-Akahira-Sunose
(KAS),
28
Ozawa-Flynn-Wallmodels
(OFW),
and
478
Fig. 15.
479 480
Fig. 15. Thermogravimetry and derivative thermogravimetry curves (a), activation energy
481
changes with different conversion (b), and kinetic plot of KAS (c) and OFW (d) models of
482
soybean straw cracking [109].
483
Giltrap et al. [110] assumes CO2 is completely cracked products and solid carbon was presented
484
as char. To calculation convenience, they ignored the possible pyrolysis products form pyrolysis
485
and cracking reactions. They introduced the char reactivity factor and provided a downdraft
486
gasified model for the reduction zone of the biomass gasifier under steady-state operation.
487
Previous researchers built some computational fluid dynamics models to investigate cold gas
488
efficiency, conversion efficiency, products composition and temperature profile of biomass
489
gasification [111,112]. Furthermore, ANN modeling is a new tool for biomass gasification, which
490
proposed for the analysis of complex processes. ANN modeling is an environmental method for
491
biomass tar reforming, which is a very promising method for biomass gasification. However, this
492
model needs abundant data and large database to build a model for mechanism development.
493
CFD model is a useful tool to study the tar cracking of dispersed and gas phase during the 29
494
modeling simulation in thermochemical gasifiers. Normally, this CFD model is employed for
495
temperature, concentration, and other parameters predicting based on the solutions of series of
496
simultaneous equations, and further for calculating of the mass, energy and momentum
497
conservation. CFD model was proved to be very accurate for temperature and gas yield prediction,
498
but it depends on the biomass types, age, and location. Gao et al. [113] designed a CFD model for
499
sawdust pyrolysis and combustion of volatiles. Their model accurately predicted the gasification
500
temperature and gas composition. From the experiment results, they found that carbon conversion
501
and cold gas efficiency varied between 77.1-94.2% and 53.6-63.0% when the equivalence ratio was
502
varied from 0.23 to 0.35. In addition, this model was also employed for CO2 and CO concentration
503
prediction, and the predicted results are the almost same as the experiment results. Jakobs et al. [111]
504
used CFD model and equations to solve mass momentum, energy, and several species balance. They
505
employed the CFD model to predict the influence of drop size distribution on gasification quality in
506
a high pressure entrained flow gasifier. Janajreh et al. [112] used wood chips to investigate the
507
conversion efficiency in a small-scale downdraft gasification system. They performed the
508
high-fidelity numerical simulation to study the temperature field inside the gasifier, and Lagrangian
509
particle evolution was also simulated. High-resolution mesh was employed for numerical simulation,
510
and the different phases, turbulence, CFD model, the temperature distribution and species evolution
511
are compared with the actual results in zero-dimensional case and ideal equilibrium. It was
512
suggested that the developed numerical model could provide a good reference for the development
513
of biomass gasifier [114].
514
ANN modeling approach is proved to be a possible tool in signal processing and function
515
approximation, which could correlate data and form a prediction model [115]. ANN could be used
516
as a function approximator, and then approximate any continuous function to an arbitrary precision
517
even without a detailed knowledge of this function [114]. Guo et al. [116] predicted the product
518
yield and gas composition of biomass gasification in a fluidized bed gasifier using a hybrid neural
519
network. In their paper, they set bed temperature and residence time as input variables. The results 30
520
even showed this model could reflect the actual gasification process. Due to the ANN model should
521
be designed for every biomass material, it would be more attractive to develop one model for
522
gasification of different biomass feedstocks [115].
523
Sreejith et al. [117] employed this model to perform the prediction of gas concentrations and
524
temperature profile for biomass gasification. From the results, the predicted H2 yields were close to
525
the experimental data (29.1%) at the steam/carbon ratio of 2.53. To realize this ANN model for
526
biomass gasification successfully, collecting of the abundant data and large database are required
527
for model development.
528
4. Conclusions and outlooks
529
Biomass gasification is a promising technology to obtain highly valued H2-rich gas and for heat,
530
power generation. Especially, the fuel gas obtained from gasification can also be utilized for
531
producing value-added chemicals via F-T synthesis. However, gasification is a complex process,
532
and the tar produced from the gasification is harmful to reactors and gasification development.
533
Therefore, it is definitely important to understand the tar cracking mechanism. In this review, we
534
concluded the recent biomass utilizes and development and shed light on some excellent catalysts
535
and their catalytic mechanism for tar cracking. The main conclusions could be made as follows:
536
1. Biomass could be transformed into other fuels through combustion, pyrolysis, and gasification.
537
Gasification is an important conversion technique, and fasciation efficiency depends on numerous
538
parameters, such as gasified temperature, moisture content, etc.).
539
2. Heterogeneous catalysts are proved to be active for syngas production. Ni-based catalysts, noble
540
metal-based catalysts, natural catalysts, and char catalysts were proved to have high activity for tar
541
removal. More mechanism proved that the free radical reaction could take place with or without
542
catalyst. The volatiles are deposited on the active surface of the catalyst to form syngas and coke,
543
and the generated coke can be further degraded under the steam atmosphere.
544
3. Kinetic model is easily built to explain the gasification process. Many researchers considered tar
545
crack mechanism is a single reaction and first-order reaction. It is not enough to surmise the real tar 31
546
components gasification.
547
Based on this review, some productive measures should be taken for the development of
548
biomass gasification. More technique and tools of computational chemistry and quantum chemistry
549
technique could be employed for the mechanism optimization of biomass tar cracking. We could
550
concentrate on reforming the mixtures of different model compounds and increase the tar model
551
components to obtain the actual tar cracking mechanism, and then optimized the catalyst and
552
gasifier. In general, more investigation in the future should be focused on tar efficient removal and
553
promote the commercialization of biomass gasification.
554
Acknowledgments
555
This work was funded by the National Key R&D Program of China (Grant 2017YFE0124200),
556
the National Natural Science Foundation of China (Grants U1710103, 21676292 and 21978317),
557
and the project “Power to Fuel” of JARA Energy from German federal and state governments. We
558
are also thankful to China Scholarship Council who provides scholarship for Jie Ren (No.
559
201806420028) to continue his scientific research at the RWTH Aachen University.
560
Nomenclature ANN
Artificial neural networks
CFD
Computational fluid dynamics
CR
Coats-Redfern
EDS
Energy dispersive X-ray spectrometry
F-T
Fischer-Tropsch
GHSV
Gas hourly space velocity
HADDF
High-angle annular dark-field
KAS
Kissinger-Akahira-Sunose
OFW
Ozawa-Flynn-Wallmodels
PAHs
Polycyclic aromatic hydrocarbons
32
S/C
Steam/carbon
WHSV
Weight hourly space velocity
W/B
Water/biomass
561
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sawdust gasification in an air cyclone gasifier, Chem. Eng. J., 213 (2012) 97-103.
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biomass gasification in fluidized bed gasifiers, Biomass and Bioenerg., 49 (2013) 279-289.
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[116] B. Guo, D. Li, C. Cheng, Z. Lü, Y. Shen, Simulation of biomass gasification with a hybrid
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neural network model, Bioresource Technol., 76 (2001) 77-83.
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283-304.
44
1. Overview of tar formation and elimination during biomass conversion. 2. In-depth discussion of tar-cracked mechanisms via tar model compounds. 3. A comprehensive review of tar cracked catalysts is presented. 4. Prospects and disadvantages of gasification reactions are discussed.
S1743-9671(19)30833-5
DOI:
https://doi.org/10.1016/j.joei.2019.10.003
Reference:
JOEI 650
To appear in:
Journal of the Energy Institute
Received Date: 16 July 2019 Revised Date:
3 October 2019
Accepted Date: 7 October 2019
Please cite this article as: J. Ren, Y.-L. Liu, X.-Y. Zhao, J.-P. Cao, Biomass thermochemical conversion: A review on tar elimination from biomass catalytic gasification, Journal of the Energy Institute, https:// doi.org/10.1016/j.joei.2019.10.003. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Energy Institute. Published by Elsevier Ltd. All rights reserved.
1
Biomass thermochemical conversion: A review on tar elimination from biomass catalytic
2
gasification Jie Ren a,b*, Yi-Ling Liu b, Xiao-Yan Zhao b, Jing-Pei Cao b**
3 4
a
5
Makromolekulare Chemie (ITMC), RWTH Aachen University, Aachen 52074, Germany
6
b
7
University of Mining & Technology, Xuzhou 221116, Jiangsu, China
8
Abstract:
Lehrstuhl für Heterogene Katalyse und Technische Chemie, Institut für Technische und
Key Laboratory of Coal Processing and Efficient Utilization (Ministry of Education), China
9
Biomass is promising renewable energy because of the possibility of value-added fuels
10
production from biomass thermochemical conversion. Among the thermochemical conversion
11
technology, gasification could produce the H2-rich syngas then into value-added chemicals via F-T
12
(Fischer-Tropsch) synthesis. However, a variety of difficulties, such as tar formation, reactors
13
impediment, complex tar cracked mechanism, etc. make it difficult to develop for further
14
application. This paper sheds light on the developments of biomass thermochemical conversion, tar
15
classifications, tar formation, and elimination methods. Secondly, we provide a comprehensive the
16
state-of-the-art technologies for tar elimination, and we introduce some advanced high activity
17
catalysts. Furthermore, many represent tar models were employed for explanation of the tar-cracked
18
pathway, and real tar-cracked mechanism was proposed. Following this, some operational
19
conditions and effective gasified models were concluded to give an instruction for biomass catalytic
20
gasification.
21
Keywords: Biomass; Thermochemical conversion; Gasification; Tar elimination; Tar-cracked
22
mechanism.
*
Corresponding author. Tel.: +49 80 26595 E-mail address: [email protected]; [email protected] Corresponding author. Tel./fax: +86 516 83591059. E-mail address: [email protected]
**
1
23 24
Contents 1. Introduction .................................................................................................................................... 3
25
1.1. Biomass energy ...................................................................................................................... 3
26
1.2. Biomass thermochemical conversion..................................................................................... 6
27
1.2.1. Biomass combustion .................................................................................................... 6
28
1.2.2. Biomass pyrolysis ........................................................................................................ 7
29
1.2.3. Biomass gasification .................................................................................................... 8
30
2. Tar elimination ............................................................................................................................. 12
31
2.1. Physical purification ............................................................................................................ 12
32
2.2. Chemical purification. .......................................................................................................... 13
33
2.2.1. Thermochemical reduction......................................................................................... 13
34
2.2.2. Catalytic reduction ..................................................................................................... 15
35
3. Gasified mechanisms and models ............................................................................................... 19
36
3.1. Biomass tar ........................................................................................................................... 19
37
3.2. Tar model compounds .......................................................................................................... 20
38
3.2.1. Toluene ....................................................................................................................... 20
39
3.2.2. Naphthalene ............................................................................................................... 23
40
3.2.3. Benzene ...................................................................................................................... 25
41
3.2.4. Lignin, cellulose and hemicellulose ........................................................................... 26
42
3.3. Mathematical models ........................................................................................................... 27
43
4. Conclusions and outlooks ............................................................................................................ 31
44
Acknowledgments ............................................................................................................................ 32
45
Reference ........................................................................................................................................... 32
46
2
47
1. Introduction
48
1.1. Biomass energy
49
Energy is an indispensable resource for the national economic construction, a driving force for
50
development [1-3]. The main energy sources of the world are remaining coal, oil and nature gas.
51
The primary energy consumption all over the world was presented in Fig. 1. Although traditional
52
energy occupied the most part of energy consumption, traditional energy overuse could lead to
53
global warming, environmental pollution, ecological balance damage and other issues [4-6]. The
54
present research focus has gradually shifted to efficient use of clean renewables and improved
55
energy structure development.
56
Renewable energy mainly includes water, solar geothermal wind, ocean, and material, etc.
57
Among these major renewable natural resources, biomass energy is a representative carbon energy,
58
which has a wide range of sources, sustainable and low pollution [7]. Biomass conversion contains
59
physical conversion, biochemical conversion and thermochemical conversion [8-10]. Table 1 listed
60
the basic classifications based on biomass materials origin, source, and biological diversity.
61
Combined with Table 2, we could observe biomass contains abundant carbon resources, which
62
would be potential to convert biomass into H2-rich syngas and valuable chemicals. To update,
63
biomass thermochemical conversion is a valuable technology to transform biomass into
64
value-added fuels or H2-rich syngas then to chemicals via F-T (Fischer-Tropsch) synthesis [11].
3
65
Fig. 1. The current global primary energy consumption.
66 67
Table 1. Biomass classifications based on their origin, source and biological diversity [12]. Biomass groups
Biomass sub-groups, varieties and species
Wood and woody biomass
Stems, branches, foliage, bark, chips, etc. of trees or plants; Various wood species including lumps, briquettes, sawdust, sawmill, pellets, etc.
Aquatic biomass
Marine or other algaes including blue, seaweed, water hyacinth, brown, green, kelp, weed, etc.
Herbaceous and
Grasses and flowers including alfalfa, bamboo, arundo, brassica, bana,
agricultural biomass
switchgrass, cane, etc.; Straws including bean, sunflower, rice, wheat, mint, etc.; Other residues including husks, pits, cobs, pips, bagasse, grains, food, fruits, shells, hulls, seeds, etc.
Animal wastes
Chicken litter, Bones, meat, manures, etc.
Contaminated biomass
Municipal solid waste, sewage sludge, waste papers, paper-pulp
and industrial biomass
sludge, hospital waste, fiberboard, wood pallets and boxes, paperboard
4
wastes
waste, etc.
Biomass mixtures
Blends from the above varieties
68
Table 2. Proximate (dried basis, wt.%) and ultimate analyses (dried and ash-free basis, wt.%) of
69
different varieties of biomass. Proximate analysis
Ultimate analysis
Sample
Reference VM
FC
A
C
O
H
N
Forest residue
79.9
16.9
3.2
52.7
41.1
5.4
0.7
[13, 14]
Oak sawdust
86.3
13.4
0.3
50.1
43.9
5.9
0.1
[15]
Poplar
85.6
12.3
2.1
51.6
41.7
6.1
0.6
[15]
Willow
82.5
15.9
1.6
49.8
43.4
6.1
0.1
[16]
Olive wood
79.6
17.2
3.2
49.0
44.9
5.4
0.7
[17]
Christmas trees
74.2
20.7
5.1
54.5
38.7
5.9
0.5
[15]
Soft wood
70.0
28.1
1.7
52.1
41.0
6.1
0.2
[18]
Bana grass
73.6
16.6
9.8
50.1
42.9
6.0
0.9
[15]
Miscanthus grass
81.2
15.8
3.0
49.2
44.2
6.0
0.4
[19]
Switch grass
80.4
14.5
5.1
49.7
43.4
6.1
0.7
[15]
Rice straw
64.3
15.6
20.1
43.0
5.7
1.0
0.2
[20]
Oat straw
80.5
13.6
5.9
48.8
44.6
6.0
0.5
[21], [22]
Wheat straw
74.8
18.1
7.1
49.4
43.6
6.1
0.7
[23]
Rice straw
64.3
15.6
20.1
50.1
43.0
5.7
1.0
[24]
Coconut shells
73.8
23.0
3.2
51.1
43.1
5.6
0.1
[25]
Pistachio shell
81.6
17.0
1.4
50.9
41.8
6.4
0.7
[15]
Hazelnut shell
69.3
28.3
1.4
52.9
42.7
5.6
1.4
[26]
Cotton husks
78.4
18.2
3.4
50.4
39.8
8.4
1.4
[25]
Corncob
88.9
20.0
1.0
44.3
48.5
6.4
0.7
[27]
5
Pepper plant
64.7
20.9
14.4
42.2
49.0
5.0
3.2
[25]
Rice husk
68.9
11.1
20.0
47.4
6.7
0.8
45.1
[20]
Soya husks
74.3
20.3
5.4
45.4
46.9
6.7
0.9
[25]
Walnut shells
59.3
37.9
2.8
49.9
42.4
6.2
1.4
[28]
Sewage sludge
34.6
2.2
69.7
47.4
34.2
7.7
8.3
[29]
Pig moisture
66.1
14.7
19.2
49.3
>38.1
6.8
5.1
[30]
70
Note: FC=Fixed carbon; VM=Volatile matter; A=Ash.
71
1.2. Biomass thermochemical conversion
72
1.2.1. Biomass combustion
73 74
Three main thermochemical conversion of biomass, the intermediate energy carriers and the final energy products were presented in Fig. 2.
75 76
Fig. 2. Three main thermochemical conversions of biomass, the intermediate products, and the final
77
products.
78
Among the existing thermochemical conversion technologies (combustion, pyrolysis, and 6
79
gasification), combustion technology is the only technology to produce heat and electric power.
80
Biomass combustion has a high efficiency for heat production, which is economically feasible.
81
However, the complex combustion process included consecutive solid-solid and solid-gas reactions.
82
The main steps for biomass combustion are followed by drying, gasification, char combustion, and
83
gas oxidation.
84
As a renewable energy source, biomass comes from plant, organic matter, animal waste, etc. For
85
the reason of high efficiency and reasonable transport distances, biomass and coal co-combustion is
86
promising, which could continuously produce energy for residents and factories utilize. However,
87
serious pollution from biomass combustion needs to cope with. There have two reasons for
88
pollutant formation: (1) Although optimized furnace was designed to reduce the incomplete
89
combustion of biomass, and it still could bring the release of CO, soot and polycyclic aromatic
90
hydrocarbons (PAHs); (2) As an existence of the N, P, S, K, Na, Cl, Mg, and P, NOx and particles
91
pollutants are formed after biomass combustion. The current main solution is to classify air and fuel,
92
which has been proven an effective measure to reduce NOx, and its potential activity can be reduced
93
by 50% to 80%. Specific measures to reduce biomass particles have not yet appeared so far, but
94
some researchers have proposed new ways to reduce air significantly, which may lead to new
95
furnace design and used for decreasing particulate emissions. In addition, more efforts should be
96
given to help plant operations optimizing, which could ensure low emissions and high efficiency of
97
biomass combustion under realistic conditions.
98
1.2.2. Biomass pyrolysis
99
It is a consensus that biomass pyrolysis could be transformed into liquid, solid and gaseous
100
fractions, by heating the biomass in the absence of oxygen under low temperatures [31,32].
101
However, the bio-oils mainly composed of complicated organic oxygen species preventing its
102
further industrial application [33]. If the biomass was applied for rapid pyrolysis, the pyrolysis
103
product can be used to primarily produce bio-oil with an efficiency of up to 80%. The produced
104
bio-oil can be used in the operation of engines and turbines and can be used as a feedstock for 7
105
refineries. However, some problems should be overcome during the conversion process and
106
subsequent utilization, such as the poor thermal stability and corrosively of pyrolysis products [34].
107
In some cases, hydrogenation and catalytic cracking of the oil removes alkali to reduce the oxygen
108
content to upgrade the quality of the bio-oil. Recently, major technical opportunities were
109
transformed to design the catalyst for biomass catalytic pyrolysis and subsequently upgrade and
110
produce the bio-oils, light aromatics, olefins and gases [35,36]. Biomass pyrolysis is certainly
111
understudied and in its infancy, more applications and development for pyrolysis should be focused
112
on the novel catalyst design, solid mixtures selection, and other exploring of related approaches.
113
1.2.3. Biomass gasification
114
o
At high temperature range of 800-1000
C, gasification is an effective method for
115
thermochemical conversion of biomass into combustible gases (CO, CO2, CH4, and H2), through
116
partial oxidation when air/oxygen was employed as gasification agent [37,38]. Furthermore,
117
biomass also could be gasified in the presence of steam and produce gas with high H2/CO ratio as
118
well as significant heating value for the reason of endothermic reactions occurrence (steam
119
gasification). The limitation is the external source should be provided for temperature improvement.
120
Biomass gasification is a complex thermochemical process, which contains many closely
121
interconnected reactions [39,40]. As Fig. 2 shows, this scheme begins with biomass drying through
122
heated around 150 oC, where moisture was converted into steam. Secondly, biomass volatiles in the
123
dried biomass samples would be vaporized and produced H2-rich gaseous products and water under
124
Air/O2 atmospheres. Thirdly, other hydrocarbons would be transformed into liquid tars then
125
condensed inside of the gasifiers. CO and CO2 could be produced from pyrolysis gases, tars and
126
char react with gasification agent, and H2 could be oxidized then produce water [41].
127
An optimized reactor design for biomass gasification is important. As a consequence of
128
oxidation reactions taken place of the biomass gasification, which is exothermic reactions would
129
generate more heating and increase the temperature [42,43]. For fixed-bed gasifier, it could be
130
classified as updraft gasifier and downdraft gasifier. Biomass materials are fed from the top or 8
131
bottom of the reactor and air/O2/steam (gasification agents) was supplied from the reactor side.
132
High gasification temperatures (>1200 oC) is required for both updraft and downdraft, and the hot
133
efficiencies were really significant (85-95%). However, for the reason of poor transfer of mass and
134
heat, and the temperature distribution of the reactor is not uniform [44,45]. Another fluidized bed
135
reactor (bubbling or circulating) have a great mixing between biomass and bed materials during the
136
gasification process, and the mass and heat transfer was improved then obtaining a significant
137
carbon conversion and gas yield. However, high temperatures could lead to biomass and bed
138
material particles agglomeration. Therefore, the additives were usually employed to control the
139
gasification temperature during the actual biomass gasification process. Rotary kiln reactor is a
140
important gasifier for biomass gasification recently [46]. The biomass could be rotated with a
141
proper speed improve the gasification between the reacting gas and biomass, and the exchange of
142
matter and heat could be fully exchanged. Biomass degradation could take place in a plasma reactor
143
under different atmospheres. Oxidizing agents are not required in this kind of reactor, and the
144
energy for biomass gasification would be provided plasma process. The plasma reactor usually
145
applied in clean production of gaseous products, then improve syngas yield from the light tar
146
decomposition during the gasification process.
147 148
In addition, biomass types, residence time, gasifying agents, temperature, pressure, etc. also significant for gasification evaluation.
149
As Fig. 3 presented, biomass gasification is a promising method to convert solid fuel into
150
syngas, then into value-added chemicals via F-T synthesis. However, the gasification process
151
produces not only useful gases but also some fly ash and tar. Tar as one of the contaminants in the
152
producer gas is the main concern of many researchers. Tar is the major problem that has not been
153
completely solved when the tar vapor condensed and blocked pipelines, turbines and engines. The
154
mechanism of tar formation has not fully understood until now.
9
155 156
Fig. 3. Concept of value-added chemicals production from gasification over high activity catalyst.
157
As the previous research already mentioned, biomass would be decomposed into primary tar
158
after dried and heated to 200 to 500 oC, which was the mixture of oxygenates and condensable
159
organic molecules [47].
160 161 162
Fig. 4. “Tar” component distribution as the temperature increasing at 0.3 s of residence time.
163
Redrawn from [48].
164
As shown in Fig. 4, tar components during gasification were changed with the temperature 10
165
rising. Primary tars and heavy tars would be cracked when temperature reached to 500 oC, and then
166
small molecules, non-condensable gas released. Finally, primary tar and heavy tar from biomass
167
could be completely destroyed to tertiary tar.
168
Tar formation should be considered in the development of biomass gasification, which could
169
condense into the reactors and then destroyed the gasifiers and decrease the gasification efficiency.
170
Many researchers give some definition of tar according to the different standard, which are as
171
follows:
172
1. The organics produced under thermal or partial-oxidation regimes (gasification) of any organic
173
materials are called “tar” and are generally assumed to be largely aromatics.
174
2. The complex mixture of condensable hydrocarbons, which includes monocyclic to bicyclic
175
aromatics along with other oxygen-containing species and complex polycyclic aromatics [49].
176
3. Tar as hydrocarbons with molecular weight higher than benzene.
177
As Fig. 5 concluded, previous researchers classified the tar to two categories according to the
178
release sequence of tar cracking, and the solubility, condensability. Therefore, we could eliminate
179
tar based on their components and properties. To elaborate the importance and development of tar
180
removal, the present paper first discusses the strategies for tar elimination including physical,
181
thermal and catalytic removal. A direct comparison of the literatures for tar removal and reactor
182
types was concluded in detail.
11
183 184
Fig. 5. Categories of biomass tar according to the light of the solubility, condensability and release
185
sequence.
186
2. Tar elimination
187
The hazard of tar is enormous in the biomass gasification systems. Previously researchers
188
employed a variety of methods for removing or reducing tar generated during gasification process.
189
The tar elimination methods can be categorized depending on the tar removed location, one is in the
190
gasifier itself, anther is in outside of the gasifier. Tar removed from outside of the reactor is suitable
191
for produced gas treatment, and it could be divided into three categories: 1. Physical purification
192
method; 2. High-temperature thermal cracking; 3. Catalytic cracking.
193
2.1. Physical purification
194 195
The physical purification methods of tar are divided into wet and dry purification [47]. Wet method (washing) could also divide into spray method and bubble blowing.
196
The main disadvantages of the wet purification method are as follows: 1. Liquid mist in the gas
197
is easily entrained; 2. Low temperature is needed during the gasification operation; 3. Difficult
198
cleaning of the equipment; 4. Hard to liquid recovery; 5. Large circulation equipment. Furthermore,
199
the sewage after the wet purification will cause secondary pollution. In addition, the tar amount 12
200
about 5%-15% of the total energy in gasification will be lost with water, and thus the energy is
201
wasted, so the wet purification system will eventually be eliminated.
202
Dry purification (filtration), utilizes the strong adsorption of the substance to filter out the tar
203
through the adsorption layer and remove tar, which was installed inside the container [50]. The
204
filtration method has the advantages for wide adaptability of tar, high removal efficiency of tar,
205
wide source of filter material and lower price than wet method. Although dry purification could
206
solve the problem of water pollution, the complex filtration equipment, high cost and inconvenient
207
operation-running lifetime and short operating life hindered its development. This method also
208
requires a low gas flow rate and generally used in the end-stage separator and other
209
high-demanding occasions. Most important, it is still not solve the problem of tar energy utilization.
210
Finally, the filtration will be gradually replaced by other purification methods.
211
2.2. Chemical purification.
212
2.2.1. Thermochemical reduction
213
Thermochemical reduction technology is a promising chemical method for tar removal.
214
Biomass tar could be cracked into different chemical compounds under different temperatures. As
215
Fig. 6 presented, benzenes, phenols, catechols, etc. could be formed during the fast pyrolysis
216
process. Besides, the other gases derived from pyrolysis would be released at a high temperature
217
above 700 oC, where light tar can be cracked into light gases [51-53]. Previous study reviewed that
218
tar could be cracked by high-temperature cracking in a fixed, fluidized bed and other gasifiers.
219
In addition, the researchers also mentioned that biomass tar was hardly cracked by thermal
220
treatment. They suggested the following methods could effectively decompose the biomass tar:
221
optimal residence time, optimal reaction temperature, optimal gasification agents and gasifier, etc.
222
To achieve a high biomass tar cracking efficiency, Han and Kim reported that 1250 oC was the
223
lowest temperature for sufficient tar cracking. Fagbemi et al. [53] employed wood, straw, and
224
coconut shell to evaluate the effects of reaction temperature and residence time on tar cracking.
13
225 226
Fig. 6. Composition of biomass tar at different temperatures.
227 228 229 230
Fig. 7. Effect of temperature on the gasified products of straw (a), coconut shell (b), wood (c) and tar cracked kinetics (d). Redrawn from Ref. [53]. From Fig. 7, it could be found that the tar yield decrease and gas increase with the temperature 14
231
increasing and pyrolysis temperature around 1000 oC is suitable for tar cracking. Longer residence
232
in favor of tar yield decrease.
233
2.2.2. Catalytic reduction
234
Although various measures were taken to control the production of tar during the gasification
235
process of biomass, the tar contents in the gas is still far beyond the allowable level in practical
236
application. In order to deal with the tar in the gas and improve the hydrogen yield, the addition of
237
high activity catalyst is an essential process to realize the effective use of combustible gas at low
238
temperature. During the process of biomass gasification, the catalyst mainly plays three roles:
239
(1) Reducing the activation energy which required for the pyrolysis reaction, and then reducing the
240
source consumption;
241
(2) Reducing the input of gasification medium;
242
(3) Achieve directional catalytic cracking of tar by directed catalytic cracking, and obtain more
243
useful products, which then synthesized for F-T synthesis to prepare abundant highly value-added
244
chemicals. As Table 3 presented, Ni-based catalysts are the commonly used catalyst for biomass
245
catalytic gasification [54].
246
Table 3. Performance comparison of different catalysts and synthetic methods. Biomass Catalyst H2 yields Gasification conditions
Ref.
Ni loading: 12%, Ce loading: 7.5%; Rice hull
Corncob
Ni/CeO2-ZrO2
69.7%
Ni/lignite char
60.0
Ni loading: 17.32%; 1 h; Steam: 30 kPa;
mmol/g
650 oC; 1 g biomass
Pig
W/B=4.9, 800 oC
69.1
Corncob
[27]
[56] Ni loading: 19±1 wt.%; 650 oC; Ar
Ni/lignite char manure
[55]
mmol/g Limonite
70.4 vol.%
700 oC, 3600 h-1, 30 kPa steam
Wood
[57] [58]
NiO/MgO
51.0 vol.%
o
NiO loading: 7.2 wt.%; 850 C
sawdust 15
Ni:Mg:Al=1:1:1; T1=400 oC, T2=800 oC; Corn stalk
Ni-Mg-Al
[59]
56.5% S/C=3.54; 30 min
Wood NiZnAlOx
48.1 vol.%
T1=535 oC, T2=800 oC;
[60]
sawdust 750 oC, Equivalence ratio of 0.30, Steam
Pine Calcined dolomite
52.8 vol%
sawdust
[61] for 0.4 kg/h S/CH4=2; Particle diameter: 2-3 mm; 800
Sawdust
Ni/MgO, dolomite
81.1%
[62] o
C; Catalyst: 15.0 g; GHSV= 3600 h−1
Maize
Ni loading: 14.9%, Ce loading: 2.0%; 900 Ni-Ce/Al2O3
71.4%
stalk Almond
[63] o
-1
C, S/C= 6; WHSV=12 h
Tri-metallic 63.7 vol%
900 oC,
[64]
shells
perovskites
Corncob
Ni/Al2O3
25.4 vol.%
650 oC, Ni loading: 20 wt%, 3600 h-1
[65]
Ni/La2O3-αAl2O3
96%
Ni loading: 9.92%; 700 oC; S/C=12;
[66]
61.2
Ni loading: 18.0%; 1 h; 650 oC; 1 g
mmol/g
biomass; Steam: 30 kPa
Pine sawdust
Corncob
Ni/Resin
Pine
[67]
130.3 g/kg 0.4 kg/h steam, 820 oC, 2.14 h-1,
Calcined dolomite sawdust
[68]
biomass
247
Note: W/B=Water/biomass, WHSV=Weight hourly space velocity, GHSV=Gas hourly space
248
velocity, S/C=Steam/carbon, T1=Temperature of biomass pyrolysis, T2= Temperature of catalytic
249
bed
16
250 251
Fig. 8. Classification of common high tar cracking activity catalysts.
252
Fig. 8 concluded the main tar cracking catalyst which proved to be active for biomass tar
253
reforming, such as natural catalysts, Ni-based catalyst, carbon-based catalyst, noble metal-based
254
catalyst, etc. Natural catalysts, like minerals, consisted of different metal oxides, which already
255
proved to be effective for biomass gasification. Natural minerals are the economic support for
256
supporting active metal, while they are unstable and easily deactivated when employed for
257
long-term utilization. Metal oxides (MgO, CeO2, etc.) are the main components in the natural
258
catalyst. Miyazawa et al. [69] reviewed the literatures and reported that the activities of these
259
natural catalysts are as follows: Ni/Al2O3>Ni/ZrO2>Ni/TiO2>Ni/CeO2>Ni/MgO. Alkali metals, like
260
Li, Na, K, etc. could be the primary catalysts and enhance the biomass gasification reactions, which
261
are also the components of the biomass. Alkali metals are usually used as a gasification or catalyst
262
additives and directly feed into the reactor with the biomass fuels. Carbon-based catalyst with the
263
advantages of self-reduction and easily recovery for power generation are detailed studied recently
264
[70]. Noble metal catalysts exhibit an excellent performance for gas production, but they are
265
expensive to obtain and apply for industrial utilization. In addition to the Co and Fe based catalysts,
266
Ni-based catalysts are widely used for biomass tar reforming and tar model compounds cracking
267
[71,72]. Ni-based catalysts belong to the transition metals (group VIII), which are widely 17
268
investigated for H2-rich gas production during biomass tar elimination. Commercial Ni-based
269
catalysts are easily deactivated through carbon deposition, Ni(CO)4 formation, S-poisoning
270
sintering, pore blockage, support broken and Ni oxidation by produced water [73,74]. Hence,
271
develop a suitable support with special structure and modifying active metal is the focus in recent
272
research, which could improve the activity and stability of Ni-based catalyst. The detailed activity
273
of these catalysts for biomass tar elimination would be discussed below.
274
Natural minerals, such as olivine, clay minerals, calcines rocks, etc., which contains Mg, Fe, Si,
275
Al, and other active metal could provide a great tar-cracked ability. Rapagna et al. [75] compared
276
the catalytic activity of olivine and calcined dolomite, they concluded that the performance of
277
olivine was better than dolomite in terms of tar elimination and the gasification activity. In addition,
278
the doping of other active metals to natural minerals could improve the tar reforming activity.
279
Tursun et al. [76] employed the olivine and NiO/olivine as the catalyst for tar in-situ removal and
280
observed that the gas yield of 1.59 Nm3/kg with 56.1 vol% of H2 concentration. Especially, tar
281
yields were reduced by 55% and 94% when olivine and NiO/olivine catalysts were used. Zhang et
282
al. [77] prepared different loading of NiO/olivine, and NiO-CeO2/olivine via the method of wet
283
impregnation to investigate the activity of benzene and toluene steam reforming at 700 and 830 °C
284
and S/C of 5.
285
Virginie [78] studied the activity of Fe/olivine for tar removal, and they found that the changes
286
of Fe2+ to Fe3+ in Fe/olivine could promote volatiles burning. Similarly, Zhao et al. [57] also
287
indicated the balance between the Fe2O3 and Fe3O4 shows a better activity for corncob tar cracking,
288
which is the reason for high H2 yields (Fig. 9).
18
289 290
Fig. 9. TEM images of limonite before (a) and after (b) calcination, XRD patterns (c) of limonite
291
calcined at different temperatures, and gas yields (d) from corncob tar cracking. Redrawn from Ref.
292
[57]
293
Raheem et al. [79] employed ZnO-Ni-CaO (16.4 wt.% loading) to obtain a high H2 fraction of
294
48.95 mol% at 851 oC. Furthermore, the development of biomass tar cracking catalysts should be
295
focused on the catalyst with high activity, cost and easy recovery at low temperatures.
296
3. Gasified mechanisms and models
297
3.1. Biomass tar
298 299
Real biomass tar is a complex oxygenated hydrocarbon, the main deposition pathways under inert and steam atmospheres were as follows: Cracking: CxHyOz (tar)→mCO+nCO2+pH2+qCH4
(1)
Steam reforming: CxHyOz (tar)+H2O→CO+H2
(2)
CO+H2O→CO2+H2 ∆Η=+41 MJ/kmol
(3) ∆Η=+260 MJ/kmol
CH4+CO2→2CO+2H2 19
(4)
Carbon formation: CxHy→xC+y/2H2
(5)
C+2H2O→CH4+CO2
∆Η=+103 MJ/kmol
(6)
C+H2O→H2+CO
∆Η=+130 kJ/mol
(7)
300
In our previous studies, we designed various catalysts for biomass tar cracking under Ar and
301
steam atmospheres. As we already presented, the volatiles from biomass gasification would be
302
oligomerized on the catalyst surface and formed carbon deposition. Simultaneous reactions, such as
303
thermal cracking, water-gas shift, tar steam reforming, CO2 dry reforming, coke formation, etc.
304
could take place, and light and heavy tar were cracked and reformed to light tar molecules and
305
syngas on the active sites of as-prepared catalysts via several simultaneous reactions Especially,
306
dissociate hydroxyl radicals (OH·) produced on the surface of catalyst when steam was introduced
307
into reaction, and then OH· react with the tar volatiles to produce H2, CO2 and CO. As we described
308
in section 2, tar model compounds such as toluene, benzene, etc. are usually employed for
309
mechanism investigation for the reason of complexity of real tar.
310
3.2. Tar model compounds
311
Biomass tar is a mixture of condensable hydrocarbons, which includes complex ring aromatics,
312
PAHs and O-containing hydrocarbons [80-82]. These resulted in the difficulty to understand the
313
decomposition mechanism of real tar, because of the wide range of different compounds presented
314
in tar. For easy understanding, toluene, benzene, naphthalene, etc. were usually employed as the
315
representative tar model compounds to discuss the possible crack pathway of the tar [83-90].
316
3.2.1. Toluene
317 318
Zou et al. [91] reported the pathways of steam forming of toluene over Fe-Ni/Palygorskite in a fixed-bed reactor.
20
319 320
Fig. 10. HADDF, EDS images (a), particle size distribution (b) of Fe3Ni8/Pal catalyst; Stability of
321
the Fe3Ni8/Pal catalyst for toluene steam reforming (c) and Arrhenius plot (d) for apparent
322
activation energy calculation. Redrawn from [91].
323
As Fig. 10 shows, they evaluated the different parameter influences of catalytic temperatures
324
and S/C ratios on the toluene steam reforming. The catalytic activity of Fe3Ni8/Palygorskite reached
325
maximum when S/C molar ratio=1.0, the pre-exponential factor and apparent activation energy of
326
this catalyst were 41.55 kJ/mol and 1350 m3 kg−1 h−1, respectively. The H2 yield, CO yield, toluene
327
conversion, and H2/CO molar ratio were >63%, >60%, >97% and 1.55, respectively, when 0.5 g
328
catalyst was tested at S/C=1 and 700 oC. From the characterization of the catalysts, they mentioned
329
coke formed on the active metal sites. Especially, they reported the formation of graphitic carbon
330
was the main reason for Fe3Ni8/Palygorskite deactivation. Water-gas shift reaction improved the
331
activity recovery of Fe3Ni8/Palygorskite. The molar ratio of H2/CO (1.41-1.66) and CO/CO2
332
(7.4-12.6) fluctuated with the increase of reaction time. The Fe3Ni8/Pal catalyst could exhibit a 21
333
relatively stable activity when H2/CO molar ratio was fixed. From the characterization results of the
334
catalyst, the HADDF (High-angle annular dark-field) image, Fe and Ni EDS (Energy dispersive
335
X-ray spectrometry) mapping (Fig. 10) show the strong interaction and they uniformly distributed
336
on the surface of palygorskite. Evidently, the carbon deposition of the catalyst could be clearly
337
observed from the C mapping.
338
Ren et al. [92] developed a Ni/C layered carbon catalyst using modified lignite char, and
339
employed for corncob tar and toluene reforming, and then investigated the effects of pH value of
340
solution and calcination temperature on the catalyst activity. Finally, they calculated the reaction
341
activation energy Ni/C catalyst. They claimed the Ni (111) plane and layered structure of the
342
layered carbon load Ni catalyst have a superior activity for corncob tar and toluene reforming. Liu
343
et al. [93] employed a gliding arc discharge reactor for the toluene conversion under N2 atmosphere.
344
They investigated the effects of S/C ratio, toluene feed rate and specific energy input on their
345
reactor. The toluene conversion reached 51.8% when S/C=2, toluene flow rate= 4.8 mL/h and
346
specific energy input=0.3 kWh/m3. The syngas yield was 73.9% contains 34.9% of H2 and 39% of
347
CO. Liu et al. [93] also mentioned a new stepwise oxidation route for the toluene conversion when
348
steam was introduced into the plasma reaction, and steam could produce OH radicals resulting in a
349
significant improvement of syngas and light tar during the conversion of toluene.
350 351
Fig. 11. Possible cracked mechanism of toluene reforming in gliding arc discharge reactor. Redrawn 22
352 353
from [93] The primary pathway of toluene decomposition in gliding arc discharge reactor might be as
354
follows:
355
(1) H was abstracted from the methyl group, and the generated benzyl radicals would react with OH
356
to obtain benzaldehyde (ii), and then benzaldehyde was oxidized to form benzoic acid.
357
(2) Phenyl radicals (iii) could be obtained through the energetic electrons collided with reactive
358
species from the aromatic intermediates. Furthermore, phenyl radicals (iii) also could be produced
359
from C-C bonds between methyl and benzene through N2 excited species and energetic electrons.
360
(3) Benzene (iv), phenol (v), aniline (vi) and benzonitrile (vii) could be formed from the reaction
361
between phenol radicals, H, OH, NH2 and CN radicals, respectively.
362
(4) The aromatic ring of toluene was cracked to produce acetylene and methyl-cyclobutadiene, and
363
then a peroxide bridge radical formed.
364
(5) The ring of toluene was opened and formed a relatively stable epoxide (xii).
365
(5) The epoxide radicals were decomposed to syngas and H2O.
366
3.2.2. Naphthalene
367
Furusawa et al. [94] investigated the naphthalene steam reforming by using the Co/MgO and
368
found that 12 wt.% Co/MgO exhibited the best activity, and then activity decrease resulted from the
369
CnHm radicals deposition and the oxidation of catalysts by steam introduction. Sato et al. [95]
370
developed a novelty Ni-WO3/MgO-CaO, and WO3 was used as a sulfur-resistant promoter. The
371
catalysts showed a high activity and stability for naphthalene reforming even in gas containing
372
hydrogen sulfide. Furthermore, they investigated the concentration of H2S from 0-500 ppm and
373
compared with the commercial Ni/Al2O3 and Ru/Al2O3 catalyst. The result shows the
374
Ni-WO3/MgO-CaO catalyst has a great sulfide resistance. The possible reaction mechanism for
375
naphthalene reforming under H2S atmosphere was drawn in Fig. 12.
23
376 377
Fig. 12. Possible pathways of S elimination over Ni-WO3/MgO-CaO catalyst.
378
Firstly, Ni catalysts combined with S and produce Ni-S, and then S in Ni-S was replaced by W
379
at high temperature, Finally, WSx on Ni catalyst was converted to H2S. Their catalyst activity for
380
naphthalene conversion reached 90% during 100 h test in the presence of H2S at 800-850 oC, and
381
this kind of catalyst was already applied in industry for sewage sludge containing-biomass
382
gasification. Josuinkas et al. [96] chosen the hydrotalcite-like with 10 and 20 wt.% NiO for benzene,
383
toluene and naphthalene steam reforming. The catalysts presented the same activity for benzene and
384
toluene steam reforming over 10% NiO-hydrotalcites. However, naphthalene is hard to convert and
385
inhibits the toluene reforming. Their catalyst showed great stability, coke and Ni sintering resistance.
386
The effects of temperatures on tar model compounds reforming were also investigated in detail as
387
Fig. 13 presented. As shown in Fig. 13, naphthalene reforming is more difficult than benzene or
388
toluene reforming under steam atmosphere. They found that the toluene conversion decreased due
389
to naphthalene strongly adsorbed on the catalyst surface. The conversion of toluene reforming alone
390
(100%) is higher than together with naphthalene (85%) at 700 oC. Formed H2 could be detected at
391
high temperatures (above 800 oC), and the main products are CO and CO2 even the temperature
392
reached 900 oC.
24
393 394
Fig. 13. Conversion (a) of toluene and naphthalene over 10NiHT and 20NiHT and product
395
composition for benzene reforming over 10NiHT. Redrawn from [96]
396
3.2.3. Benzene
397
Kaisalo et al. [97] employed Ni/Al2O3 to investigate the behaviors of the benzene steam
398
reforming kinetics at 750-900 oC, and they discussed the qualitative effect of the gaseous products
399
compounds on the reforming kinetics. The first-order kinetic model was built to explain the H2 or
400
CO2 accelerated/decelerated the steam reforming of benzene. Furthermore, they used the
401
Langmuir-Hinshelwood type model to describe the effect of H2 on the benzene decomposition. As
402
Fig. 14 presented, they employed a linearization method to calculate the Arrhenius plots from
403
benzene inlet concentration between 600 to 3500 ppm and H2O concentration varied from 4.3 to
404
12.8%. Moreover, the linearization method was used to explain the behavior of different gas
405
compositions (Fig. 14b), these points are drawn from the experiments with different catalyst
406
packings, stream time and gas compositions. Although the effects of gaseous species were
407
qualitatively investigated in this study, more experimental data are required for explaining the
408
influence of CO2 on the benzene reforming and that reason of H2S poisoning.
25
409 410
Fig. 14. Arrhenius plot for the benzene and concentration of steam (a) and for the different gas
411
compositions (b). Redrawn from [97]
412
Furusawa et al. [98] compared the catalytic performances of Pt and Ni-based catalysts for the
413
naphthalene/benzene steam reforming. They concluded Al2O3 is an excellent carrier for H2
414
production when used for naphthalene/benzene steam reforming to produce H2. Krause [86]
415
employed natural catalyst of dolomite as the reformed catalyst for benzene reforming, they used
416
kinetic models to investigate the catalytic behavior of simulated gas mixture gasification at 750-925
417
o
418
of mechanistic models investigation show the rate-determining step of single-site adsorption of
419
hydrogen and benzene was the main reason for benzene decomposition. Colby et al. [99] develop
420
the Rh based catalysts to reduce benzene at 650-850 oC in a fixed-bed reactor. They increased the
421
dispersion and stability of Ru-based catalyst through Ce doping.
422
3.2.4. Lignin, cellulose and hemicellulose
C, and they develop Langmuir Hinshelwood models to discuss the benzene reforming. The results
423
Lignin, cellulose and hemicellulose are the main components of biomass. The contents of lignin,
424
cellulose, and hemicellulose in the biomass depend on the biomass types. Wu et al. [100] employed
425
a Ni-based catalyst to evaluate the H2 production in a two-stage fixed-bed reactor. They concluded
426
their catalyst is more active for H2 production during cellulose gasification. Yu et al. [101]
427
employed lignin, cellulose, and hemicellulose and discussed the tar formation mechanisms and 26
428
characteristics during the gasification. They reported tar yields were increased as temperature or
429
excess air ratio increasing during lignin, cellulose and hemicellulose gasification. Lignin has a high
430
tar yield of stable components because of its special molecular structure, and it is more impossible
431
to obtain the heavy tar, However, more PAHs produced from benzene, toluene, etc. Hosoya et al.
432
[102] mixed lignin, cellulose and hemicellulose and investigated their interactions during the
433
gasification process at 800 °C. They observed the improvement of lignin during the light tar
434
production, and inhibition during thermal polymerization of laevoglucose. Cellulose improves the
435
formation of lignin-derived products and the secondary char decomposition. Similar interactions
436
also could be observed in cellulose-hemicellulose pyrolysis. Finally, they mentioned lignin
437
gasification must be considered during tar removal.
438
Lv et al. [103] investigated the effects of alkali and alkaline earth metals in biomass materials
439
on gasification by using a thermogravimetric analyzer. They removed the alkaline earth metals
440
through acid treatment and found it consists of gaseous products depend on the ratio of biomass
441
components and alkaline earth metals. Carbon deposition was decreased with the improvement of
442
cellulose content, and the tar and gas yields were increased with the improvement of cellulose
443
content. They observed the first step is the decomposition of cellulose, and then lignin gasification
444
at high temperature. Furthermore, the influences of alkaline earth metals on biomass fuel
445
gasification were concluded, and alkaline earth metals could decrease gasification temperature and
446
increased the product yield.
447
3.3. Mathematical models
448
Models are helpful to evaluate the gasified behavior of various biomass feedstock without
449
actually different kinds of reactors at wanted temperatures and pressures. Tar models design for
450
biomass gasification could be divided into three models: single-compound models, kinetic models,
451
and lumped models. In general, the simulation of biomass gasification could be classed as 5
452
categories: (1) Kinetic Models (2) Tar Models (3) Computational Fluid Dynamics (CFD) Models (4)
453
Artificial Neural Network (ANN) Models (5) Thermodynamic Equilibrium Models. Toluene and 27
454
phenol were used as a single compound model and study a thermodynamic model for tar formation
455
[104,105]. The single-compound model is the simplest model of biomass gasification and they
456
reflect how biomass reacts with gasification agents. However, lumped and kinetic models in the
457
biomass gasification could utilize heat and mass to transfer information.
458
Kinetic model is more accurate than thermodynamic models in the field of gas yield and
459
composition at low temperature. The Arrhenius (k =Aexp(-Ea/RT)) plots are significant to this model,
460
which reflects the kinetic parameters. This equation contains the temperature, conservation of
461
energy and mass and momentum. Inayat et al. [106] developed a kinetic model to study the effects
462
of temperature and concentration for oil palm empty fruit bunch gasification at steady and dynamic
463
states. Sreejith et al. [107] provided a kinetic model and proved the steam atmosphere is better than
464
the air-steam atmosphere on gas production from wood gasification in a fluidized bed gasifier.
465
Khonde et al. [108] designed anther activation energy model for rice husk gasification, and they
466
claimed that the distributed activation energy model is more suitable than the single reaction model. dVi/dT=ki(V*i-Vi) Vi =V*i (
(8)
∞ t exp (- 0 ki dt)f(Ei)dEi) 0
(9)
467
Vi: Initial yield of tar or gas at t (time); V*i: Final yield of Vi; ki: Reaction rate equation; f(Ei):
468
Gaussian distribution of activation energy (mean); f(Ei)dEi: fraction of tar or gas formed with the
469
activation energies from E to E+dE.
470
The model of Khonde et al. [108] shows the minimum standard error of simulation, and their
471
models could reach maximum tar conversion through the variables of time and temperature.
472
However, it is complex to know the conversion reason from the time-temperature data by using the
473
distributed activation energy model.
474
Huang et al. [109] investigated thermal cracking behavior of soybean straw and compared the
475
kinetic parameters with experimental results in thermogravimetric analyzer. They utilized
476
iso-conversion
477
Coats-Redfern (CR) method to calculate the activation energy. The related results were shown in
Kissinger-Akahira-Sunose
(KAS),
28
Ozawa-Flynn-Wallmodels
(OFW),
and
478
Fig. 15.
479 480
Fig. 15. Thermogravimetry and derivative thermogravimetry curves (a), activation energy
481
changes with different conversion (b), and kinetic plot of KAS (c) and OFW (d) models of
482
soybean straw cracking [109].
483
Giltrap et al. [110] assumes CO2 is completely cracked products and solid carbon was presented
484
as char. To calculation convenience, they ignored the possible pyrolysis products form pyrolysis
485
and cracking reactions. They introduced the char reactivity factor and provided a downdraft
486
gasified model for the reduction zone of the biomass gasifier under steady-state operation.
487
Previous researchers built some computational fluid dynamics models to investigate cold gas
488
efficiency, conversion efficiency, products composition and temperature profile of biomass
489
gasification [111,112]. Furthermore, ANN modeling is a new tool for biomass gasification, which
490
proposed for the analysis of complex processes. ANN modeling is an environmental method for
491
biomass tar reforming, which is a very promising method for biomass gasification. However, this
492
model needs abundant data and large database to build a model for mechanism development.
493
CFD model is a useful tool to study the tar cracking of dispersed and gas phase during the 29
494
modeling simulation in thermochemical gasifiers. Normally, this CFD model is employed for
495
temperature, concentration, and other parameters predicting based on the solutions of series of
496
simultaneous equations, and further for calculating of the mass, energy and momentum
497
conservation. CFD model was proved to be very accurate for temperature and gas yield prediction,
498
but it depends on the biomass types, age, and location. Gao et al. [113] designed a CFD model for
499
sawdust pyrolysis and combustion of volatiles. Their model accurately predicted the gasification
500
temperature and gas composition. From the experiment results, they found that carbon conversion
501
and cold gas efficiency varied between 77.1-94.2% and 53.6-63.0% when the equivalence ratio was
502
varied from 0.23 to 0.35. In addition, this model was also employed for CO2 and CO concentration
503
prediction, and the predicted results are the almost same as the experiment results. Jakobs et al. [111]
504
used CFD model and equations to solve mass momentum, energy, and several species balance. They
505
employed the CFD model to predict the influence of drop size distribution on gasification quality in
506
a high pressure entrained flow gasifier. Janajreh et al. [112] used wood chips to investigate the
507
conversion efficiency in a small-scale downdraft gasification system. They performed the
508
high-fidelity numerical simulation to study the temperature field inside the gasifier, and Lagrangian
509
particle evolution was also simulated. High-resolution mesh was employed for numerical simulation,
510
and the different phases, turbulence, CFD model, the temperature distribution and species evolution
511
are compared with the actual results in zero-dimensional case and ideal equilibrium. It was
512
suggested that the developed numerical model could provide a good reference for the development
513
of biomass gasifier [114].
514
ANN modeling approach is proved to be a possible tool in signal processing and function
515
approximation, which could correlate data and form a prediction model [115]. ANN could be used
516
as a function approximator, and then approximate any continuous function to an arbitrary precision
517
even without a detailed knowledge of this function [114]. Guo et al. [116] predicted the product
518
yield and gas composition of biomass gasification in a fluidized bed gasifier using a hybrid neural
519
network. In their paper, they set bed temperature and residence time as input variables. The results 30
520
even showed this model could reflect the actual gasification process. Due to the ANN model should
521
be designed for every biomass material, it would be more attractive to develop one model for
522
gasification of different biomass feedstocks [115].
523
Sreejith et al. [117] employed this model to perform the prediction of gas concentrations and
524
temperature profile for biomass gasification. From the results, the predicted H2 yields were close to
525
the experimental data (29.1%) at the steam/carbon ratio of 2.53. To realize this ANN model for
526
biomass gasification successfully, collecting of the abundant data and large database are required
527
for model development.
528
4. Conclusions and outlooks
529
Biomass gasification is a promising technology to obtain highly valued H2-rich gas and for heat,
530
power generation. Especially, the fuel gas obtained from gasification can also be utilized for
531
producing value-added chemicals via F-T synthesis. However, gasification is a complex process,
532
and the tar produced from the gasification is harmful to reactors and gasification development.
533
Therefore, it is definitely important to understand the tar cracking mechanism. In this review, we
534
concluded the recent biomass utilizes and development and shed light on some excellent catalysts
535
and their catalytic mechanism for tar cracking. The main conclusions could be made as follows:
536
1. Biomass could be transformed into other fuels through combustion, pyrolysis, and gasification.
537
Gasification is an important conversion technique, and fasciation efficiency depends on numerous
538
parameters, such as gasified temperature, moisture content, etc.).
539
2. Heterogeneous catalysts are proved to be active for syngas production. Ni-based catalysts, noble
540
metal-based catalysts, natural catalysts, and char catalysts were proved to have high activity for tar
541
removal. More mechanism proved that the free radical reaction could take place with or without
542
catalyst. The volatiles are deposited on the active surface of the catalyst to form syngas and coke,
543
and the generated coke can be further degraded under the steam atmosphere.
544
3. Kinetic model is easily built to explain the gasification process. Many researchers considered tar
545
crack mechanism is a single reaction and first-order reaction. It is not enough to surmise the real tar 31
546
components gasification.
547
Based on this review, some productive measures should be taken for the development of
548
biomass gasification. More technique and tools of computational chemistry and quantum chemistry
549
technique could be employed for the mechanism optimization of biomass tar cracking. We could
550
concentrate on reforming the mixtures of different model compounds and increase the tar model
551
components to obtain the actual tar cracking mechanism, and then optimized the catalyst and
552
gasifier. In general, more investigation in the future should be focused on tar efficient removal and
553
promote the commercialization of biomass gasification.
554
Acknowledgments
555
This work was funded by the National Key R&D Program of China (Grant 2017YFE0124200),
556
the National Natural Science Foundation of China (Grants U1710103, 21676292 and 21978317),
557
and the project “Power to Fuel” of JARA Energy from German federal and state governments. We
558
are also thankful to China Scholarship Council who provides scholarship for Jie Ren (No.
559
201806420028) to continue his scientific research at the RWTH Aachen University.
560
Nomenclature ANN
Artificial neural networks
CFD
Computational fluid dynamics
CR
Coats-Redfern
EDS
Energy dispersive X-ray spectrometry
F-T
Fischer-Tropsch
GHSV
Gas hourly space velocity
HADDF
High-angle annular dark-field
KAS
Kissinger-Akahira-Sunose
OFW
Ozawa-Flynn-Wallmodels
PAHs
Polycyclic aromatic hydrocarbons
32
S/C
Steam/carbon
WHSV
Weight hourly space velocity
W/B
Water/biomass
561
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