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Methanation of syngas from biomass gasification: An overview
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Review Article
Methanation of syngas from biomass gasification: An overview Jie Ren a,b, Yi-Ling Liu b, Xiao-Yan Zhao b, Jing-Pei Cao b,* a Lehrstuhl fu¨r Heterogene Katalyse und Technische Chemie, Institut fu¨r Technische und Makromolekulare Chemie (ITMC), RWTH Aachen University, Aachen, 52074, Germany b Key Laboratory of Coal Processing and Efficient Utilization (Ministry of Education), China University of Mining & Technology, Xuzhou, 221116, Jiangsu, China
highlights Detailed review of gasification and methanation development. Discussion of high active catalyst for gasification and methanation. It sheds light on effective reactors for biomass gasification. Large-scale commercial development of gasification and methanation are discussed.
article info
abstract
Article history:
Traditional fossil fuel overuse could lead to global warming and environmental pollution.
Received 30 July 2019
As a renewable energy, biomass energy is a sustainable and low pollution carbon energy,
Received in revised form
which has a wide range of sources. Syngas production from biomass thermochemical
27 October 2019
conversion is a promising technology to realize effective utilization of the renewable en-
Accepted 5 December 2019
ergy. Syngas produced from gasification could be further converted into value-added
Available online xxx
chemicals via the method of Fischer-Tropsch synthesis. Syngas and CO2 methanation could transform renewable energy into feasible transport and high-density energy. How-
Keywords:
ever, tar formation and catalyst deactivation are the main problem during the biomass
Gasification
gasification and methanation. This review sheds light on the development of biomass
Renewable energy
gasification and syngas methanation. Firstly, we presented the common reactors and some
Methanation
other factors during gasification. Secondly, we provide a comprehensive introduction of
Tar removal
the advanced active catalyst for gasification and syngas methanation. Finally, some
Biomass
representative large-scale and commercial plants and companies for biomass gasification were compared and discussed in details. Then the prospective developments in combination of gasification and methanation were concluded to give an outlook for biomass gasification and its downstream development. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. E-mail address: [email protected] (J.-P. Cao). https://doi.org/10.1016/j.ijhydene.2019.12.023 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Ren J et al., Methanation of syngas from biomass gasification: An overview, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.023
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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biomass gasification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Other parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ni-based catalyst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Alloy based catalyst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Other catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Syngas methanation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Integration and challenge on the syngas methanation and biomass gasification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Large-scale commercial development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Nomenclature EDS Energy dispersive X-ray spectrometry FE-SEM Field emission scanning electron microscopy GPSP Great plains synfuels plant H2-TPR H2-temperature-programmed reduction MSW Municipal solid waste PAHs Polycyclic aromatic hydrocarbons PtG Power-to-gas SEM Scanning electron microscope SNG Synthetic natural gas STEM-HDDF Scanning transmission electron microscopy-high-angle annular dark-field SV Space velocity TEM Transmission electron microscope TPSR Temperature programmed surface reaction TPD Temperature programmed desorption REs Renewable energy resources UVeviseNIR Ultra-violet-visible-and near infra-red WGSR Water-gas shift reaction XPS X-ray photoelectron spectroscopy XRD X-ray diffraction
Introduction Energy is a driving power for national economy development [1e3]. At present, the main energy sources of the world are remaining coal, oil and nature gas. As the increasing of the life quality, population and industrialization country, fossil fuels consumption is dramatically increase. However, excessive consumption of fossil fuel seriously destroy the environment, and leads to global warming, environmental pollution and
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ecological balance damage [4e6]. Recently, reducing consumption of fossil fuel and release the energy crisis by using renewable energy alternatives are very important to meet the growing energy needs [7]. The future renewable energy consumption from 2001 to 2040 all over the world was shown in Fig. 1. From Fig. 1, we concluded future energy structure and use has gradually transformed to clean renewable energy, especially the biomass energy. Renewable energy resources (REs) occupied about 13.8% of the world energy consumption at 2018 [1], which including hydropower, biomass energy, geothermal energy, solar energy, wind energy, ocean energy, material, etc. REs are the primary inexhaustible resources, which supplies about 20% electricity of all over the world. The development of REs could resolve the presently crucial energy problems, improve energy supply, and promote live standard of the local population. Moreover, it could ensure the improvement of sustainable energy in the remote regions (desert or mountain zones). It is possible that create job opportunities to rural city and country, and then the problem of biomass wastes could be well solved. Among the REs, biomass usually come from the living species of plants or animals, which absorb or convert sunlight to bioenergy. Biomass is also a representatively sustainable energy, which has a wide distribution in everywhere (Rural and urban) [8]. Table 1 listed the basic classifications based on biomass materials origin. Rural wastes included sugarcane, cocoa, maize, coconut, rice, oil palm, millet, etc. The urban category is subdivided into municipal solid waste (MSW), food industry wastes, and industrial wastes. As observed from Table 2, biomass from different origin contains abundant carbon resources, which would be potential to convert biomass into valuable chemicals [9,10]. Lapuerta et al. [11] employed grapevine, olive pruning, sawdust etc. to evaluate their gasified behavior, and the results shown that grapevine and olive pruning have the higher gas production and gasification efficiency than pinaster pruning and sawdust at relatively high biomass/air ratio. Therefore, if the industrial and forestal
Please cite this article as: Ren J et al., Methanation of syngas from biomass gasification: An overview, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.023
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Fig. 1 e Future renewable energy consumption from 2001 to 2040.
wastes were employed for gasification in agricultural wastes gasification system, the external energy (natural gas, etc.) or more biomass supply are required to obtain a high gasification efficiency. Physical conversion, thermochemical conversion and biochemical conversion are the three main ways of the biomass conversion [25e30]. Biomass combustion normally was used to produce heat or electric power [31]. However, serious pollution from biomass combustion, like the release of CO, soot and polycyclic aromatic hydrocarbons (PAHs), NOx and particles pollutants need to handle. Liquid, solid and gaseous products could be produced from biomass pyrolysis technology under different atmospheres at low temperatures [32,33]. However, these products were usually consisted of organic oxygen species, which limited industrial development of this technology [34].
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Currently, biomass gasification was used to produce H2rich gas through partial oxidation or other reaction when favorable gasification agent and catalyst were selected [35]. The main process in the biomass gasification are as follows: (1) Biomass drying: the moisture of biomass was converted into steam; (2) Tar forming: heavy tar, light tar and little amount of gas were released between 400 and 700 C. (3) Gas releasing: heavy tar and light tar cracking, more gaseous products produced at temperature above 700 C. An optimized reactor, highly active catalyst could decrease the gasification temperature and improve the quality of produced gas. Furthermore, biomass types, residence time, temperature, pressure, etc. are also important for biomass gasification. Additionally, biomass gasification is also a valuable technology to produce H2-rich combustible gases or value-added fuels via FischerTropsch synthesis [36]. To realize the industrialized development of biomass gasification, the operate parameters should be optimized for tar removal during the biomass gasification. Selection of the excellent gasifier, gasification agent, etc. have gradually became the key issues should be overcome. To some extent, a highly active and stable catalyst is always the research topic in recent years. A detailed discussion were summarized in the next section. Although REs are promising energy to use in the future, an appropriate method for energy generation and storage is deserved to discuss with the expansion of REs. Considering a renewable production of electrical energy via various energy (wind, solar, biomass, etc.), more efforts should be paid attention to find the solutions for electricity storage. Except for high storage batteries, chemical storage is also an alternative method for effective utilization of REs, which has high volumetric storage density and easily transport or
Fig. 2 e Recent development in methanation and gasification.
Table 1 e Biomass classifications based on their origin (Rural and urban). Main categories Rural wastes
Urban wastes
Sub-categories Agricultural resources Energy crops Forest resources Animal wastes MSW Food industry wastes Industrial wastes
Representatives Straw, shell, grass, wood, seeds, etc. Sorghum, maize, soybean, wheat, oil palm, sugarcane, etc. Fat, oil, grease, chicken litter, bones, meat, manures, etc. Wastewater, sewage, waste papers, wood pallets and boxes, bio-solids
Please cite this article as: Ren J et al., Methanation of syngas from biomass gasification: An overview, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.023
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Table 2 e Proximate and ultimate analyses of different biomass materials. Sample
Rice straw Wheat straw Walnut shells Hazelnut shell Miscanthus grass Olive wood Soft wood Pepper plant Forest residue Poplar Willow Christmas trees Rice husk Cotton husks Pig moisture Sewage sludge
Proximate analysis (wt.%)
Ultimate analysis (wt.%)
Ref.
VM
FC
A
C
O
H
N
64.3 74.8 59.3 69.3 81.2 79.6 70.0 64.7 79.9 85.6 82.5 74.0 68.9 78.4 66.1 34.6
15.6 18.1 37.9 28.3 15.8 17.2 28.1 20.9 16.9 12.3 15.9 20.8 11.1 18.2 14.7 2.2
20.1 7.1 2.8 1.4 3.0 3.2 1.7 14.4 3.2 2.1 1.6 5.2 20.0 3.4 19.2 69.7
43.0 49.4 49.9 52.9 49.2 49.0 52.1 42.2 52.7 51.6 49.8 51.6 47.4 50.4 49.3 47.4
5.7 43.6 42.4 42.7 44.2 44.9 41.0 49.0 41.1 41.7 43.4 36.7 45.1 39.8 >38.1 34.2
1.0 6.1 6.2 5.6 6.0 5.4 6.1 5.0 5.4 6.1 6.1 5.6 6.7 8.4 6.8 7.7
0.2 0.7 1.4 1.4 0.4 0.7 0.2 3.2 0.7 0.6 0.1 0.5 0.8 1.4 5.1 8.3
[12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [21] [12] [19] [23] [24]
Note: FC¼Fixed carbon; VM¼Volatile matter; A ¼ Ash.
distribution. Potential pathways including biomass catalytic gasification (Biomass gasification/CO þ CO2 þ H2 þ CH4 þ Tar) to deliver H2 and CO or CO2, followed by e.g. CO2 metanation via Fischer-Tropsch technology were proved effective for energy valued development. Recent develpment in biomass gasification and methanation of syngas or CO2 was depicted in Fig. 2. Following this line of argument, a discussion has started to combine gasification and methanation to realize energy value-added utilization. Like the biomass gasification, economical catalyst with high catalytic performance and stability is most important. The syngas or CO2 methanation (Sabatier reaction) is an exothermic reaction as following Eqs. 1 and 2: CO2 þ 4H2 /CH4 þ 2H2 O DH ¼ 165 kJ=mol
(1)
3H2 þ CO/CH4 þ H2 O DH ¼ 206 kJ=mol
(2)
Hence, heating removal is crucial to the syngas/CO2 methanation, and prefect reactor design could increase CH4 yield and decrease the electricity consumption. In this review, we first introduce the development of biomass gasification and methanation in section Biomass gasification and section Syngas methanation, respectively. Secondly, we review the main gasifier types, some effective catalysts and related gasification mechanism, and then we propose and prove the possibility of syngas methanation from biomass gasification. Some representative catalysts and pathways were concluded during the syngas methanation. Finally, we take some examples of large-scale gasification, methanation and combination of these two technologies.
Biomass gasification Syngas production from biomass catalytic gasification is an effective method for renewably value-added utilization of biomass at high temperature of 800e1000 C. However, high content of tar, volatile organic compounds, solid residues,
char, and soot would prevent gasification development. Biomass gasification process could be classified into material drying, pyrolysis, oxidation, and gasification. The moisture of the biomass material would be removed through first heating around 200 C, and dried biomass could be cracked into heavy tar, light tar, biochar, H2-rich gases during the whole pyrolysis process. The condensable “heavy tar” including complex and hard evaporate organic compounds. As Fig. 3 shows, the main processes of biomass gasification could be divided into primary cracking, secondary cracking and tertiary cracking. Syngas, CO2, CH4 and H2O would be produced at 400e700 C. The primary heavy tar below 700 C is rich in oxygenated vapors, such as levoglucosan, furfural, hydroxyacetaldehyde and methoxy phenols [37]. Some gaseous products, light olefins and aromatics are generated from the secondary cracking of methoxy phenols. Furthermore, PAHs including indene, anthracene, pyrene, etc. would be continuously formed, as well as H2-rich syngas at 850e1000 C. Biomass would be transformed to biochar, which still retain the property of lignocellulose during the primary pyrolysis. The soot could be produced from the homogeneous nucleation of the hydrocarbons, and coke formed from the thermal cracking of the heavy or light tar at relatively high temperature.
Reactors Biomass catalytic gasification could obtain gaseous products with different types and concentrations in the specific structure reactor. Here we presented some usual gasifiers, which were used according to the biomass size, ash, moisture, and the feed way. It could be concluded to fixed-bed, fluidized-bed, entrained flow reactor, rotary kiln reactor and plasma reactor, etc. Fixed-bed could be classified as “updraft”, “downdraft” and “crossdraft” gasifier, which is the simplest reactor for gasification. Biomass is fed from the top of the reactor and decomposed through drying, pyrolysis, reduction and
Please cite this article as: Ren J et al., Methanation of syngas from biomass gasification: An overview, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.023
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Fig. 3 e Biomass gasification pathways.
combustion area. The air/gasification agent enter into reactor from the different side. For “updraft” gasifier, the gasifying agent or air rose from the bottom and react with biomass material. This reactor shows the high thermal efficiency, low pressure drop and low slag formation. However, long startup time, low syngas yield, and high tar sensitivity are the main problems. Biomass and gasification agent from the top to bottom, and wastes and gaseous products also move in the same direction inside the “downdraft” gasifiers. This reactor could produce high quality and low tar content gaseous products. However, “downdraft” reactor usually used for power generation at a small scale. “Crossdraft” gasifier is also useful for syngas production from biomass gasification for the reason of great response for flexibility. The shorter start-up time, lower reactor height are the advantages for biomass gasification. The key problem for this kind of reactor is hard to treat the small size and high amount of biomass material. Fluidized-bed reactor could be classified as “bubbling” and “circulating”, the inert materials were stirred continuously to produce gas bubbles, which shows an excellent heat exchange ability between biomass and gas. Currently, “bubbling” and “circulating” were commonly used for biomass gasification because the high flexibility of biomass, high heat transfer and cold gas efficacy, etc. However, the ash and alkali metals in biomass could break gasification equipment and lead to low quality of gaseous products. Therefore, highly active catalyst should be designed for biomass gasification due to the high temperature requirement during the gasification process. There are some other well-designed reactors used for biomass tar removal. Entrained flow reactor usually used at the high temperature (1300e1500 C) for coal residues and plastic wastes gasification. Moisture in biomass should be avoided during the biomass pretreatment. Recently, plasma reactor was widely used in the upstream gasification process, which could improve the tar cracking into light tar and increase the amount and quality of syngas products. Most importantly, this
reactor could be used for biomass gasification with various flow rate, biomass moisture, size and composition because of the excellent temperature control in different types and qualities of biomass. In industry, rotary kiln gasifiers were the main equipment for industrial waste combustion, which have the temperature resistance metal shell. Biomass would have a great contact with gasification agent due to the existence of the continually rotated reactor. Therefore, lower heat exchange between the biomass and gas would bring the lower gasification efficiency. As we already reported, the detailed comparison of different gasifiers was presented in Table 3.
Other parameters To obtain the excellent yield and composition of gaseous products, biomass type, space velocity (SV), temperature, pressure, etc. are also important for gasification evaluation. These factors have a great influence on energy conversion efficiency. Therefore, the gasification operating conditions should be optimized, and some effects on the biomass gasification were discussed as follows: Less feeding would bring the less syngas content, and biomass overfeeding could result in reactor plunge and gasification efficiency decrease. A proper biomass feeding rate could make the maximize energy conversion. Biomass composition (lignin, cellulose and hemicellulose ratio) is another main determination of gasification. Hanaoka et al. [39] claimed the tar conversion of lignin, cellulose and xylan reached 52.8%, 97.9% and 92.2%, and cellulose gasification could produce the highest yield of gas. Equivalence ratio and SV are also the important factors for biomass gasification. Different air flow rate with various O2 content could result in gasification temperature change and then leads to high biomass conversion and fuel quality. However, excessing combustion of biomass would decrease produced gas energy [40,41]. High SV would lead to the decrease in the yields of
Please cite this article as: Ren J et al., Methanation of syngas from biomass gasification: An overview, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.023
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Table 3 e Advantages and disadvantages of different reactor types [38]. Reactor types
Advantages
Disadvantages
Fixed bed 1. High thermal efficiency, solid residence time and carbon conversion; 2. High capacity of handling different biomass; 3. Excellent contact between the feedstock and the react atmosphere; 4. Feasibility of large-scale production; 5. Low entrainment of dust and ash; 6. Simple construction. Fluidized 1. High of contact between feedstock and atmosphere; bed 2. High carbon conversion and thermal loads; 3. Excellent for temperature control, feed and process, handling biomass with different properties; 4. Possible to realize the large-scale utilization; 5. Reduced residence times, etc.
1. 2. 3. 4. 5. 6.
High content of tar in the syngas; Low content of produced CO and H2; Limited flexibility to feed and react process; Hard to start and control temperature; Mobile grates should be installed; Catalysts are easily be poisoned and deactivated.
1. Carbon loss in the ashes; 2. Dragging of dust and ashes; 3. Relatively low process temperature is required to avoid phenomena of de fluidization of the bed; Restrictions on the size; 4. High costs of investment and maintenance; 5. Technology complex and difficult to control, etc. 1. Abundant oxidants are required; 2. Small size and preparation of biomass are required; 3. Heat recovery is required to improve efficiency; 4. Low cold gas efficiency; 5. Short life of system components and high costs of investment and maintenance, etc.
Entrained 1. Materials flexibility and uniform react temperature during the flow gasification; 2. High carbon conversion and low tar content; 3. Short reactor residence time; easy to control the parameters process; 4.High-temperature slagging operation; 5.Possibility for large-scale utilization, etc. Rotary 1. Excellent capacity of handling biomass with different properties, 1. Significant hard to control the starting temperature and the kiln flexible loading; Suitable for waste easily be melted; Possibility for leakage and wear of movement part; large-scale use; 2. High of refractory consumption, dust and tar content; Low 2. Simplicity of construction and high reliability of operation; capacity of heat exchange; 3. Low investment costs, etc. 3. Low efficiency of heat; Limited flexibility process; 4. High costs of maintenance, etc. Plasma 1. Production of vitrified non-leachable slag including heavy metals; 1. Presence of nanoparticles in the syngas, the maintenance of movement part and heat shocking for the start-up and reactor 2. Recovery of leachable waste; shout-down; 3. Clean H2-rich syngas production; 2. Consumption of refractory and electrodes; 4. Short reaction times; 3. Removal of molten material in the ducts is required; safety 5. Possibility for large-scale use, etc. problems, etc.
gaseous products because of the short contact with catalyst [42,43]. Steam could increase the H2 production via water-gas shift reaction (WGSR) and methane reforming reaction at high temperature (above 750 C). Temperature is also the most important influence for gaseous yield and composition. The high gas yield and tar conversion could be obtained from gasification at high temperature. Steam reforming of tar and WGSR are the main reason for H2 production and CH4 conversion at the relatively low temperature of 750e800 C. With the temperature increased from 850 to 900 C, CO content will be improved from steam reforming and the Boudouard reactions. Moreover, high temperature also in favor of tar reforming and destruction [44e47].
obtain more useful products. The main criteria and roles for syngas production by using tar cracking catalyst were presented as follows: Main criteria for gasification catalyst design: (1). (2). (3). (4).
Highly effective and active for tar removal; Highly capable for CH4 reforming; Providing a suitable syngas ratio for next utilization; Easily regenerated and anti-coke activity for spent catalyst; (5). Easily obtained and inexpensive for catalyst development. Catalyst roles in biomass gasification:
Catalysts Although various parameters were adjusted to reduce tar production during biomass gasification, the tar amount in the gaseous products is still difficult to realize the industrial application. From middle of 1980s, the interest of many researchers started focus on the biomass catalytic reduction (see Fig. 2). Previous literatures screened some tar-cracked catalysts for biomass gasification. To conclude, the catalyst mainly plays three roles on biomass gasification, i.e., reduce the activation energy, usage of gasification medium, and
(1). (2). (3). (4).
Decreasing the activation energy of reforming reaction; Reducing the heat consumption; Reducing the input of gasification medium; Achieving directional production of H2-rich syngas for value-added chemicals via Fischer-Tropsch synthesis.
As shown in Fig. 3, heavy tars and other hydrocarbons can be directionally reformed into value-added gases in the presence of catalyst. Normally, these complex reactions were taken place on the surface of catalyst or support with the
Please cite this article as: Ren J et al., Methanation of syngas from biomass gasification: An overview, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.023
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Fig. 4 e Possible cracked mechanism and reaction steps of complex tar. excellent metal site and support structure. The simplified mechanism for tar catalytic cracking was described in Fig. 4. Firstly, heavy tars and light tars were cracked into light hydrocarbons at high temperature over high active catalyst. Secondly, some hydrocarbons and CH4 dissociated and dehydrogenated on the active metal/support site. Finally, these hydrocarbon fragments were oxidized by OH radicals (from hydroxylation of inner/formed water) and produce the H2-rich syngas, CO2, C1eC4, etc. Heterogeneous catalysts for biomass catalytic gasification or methanation could divided into mineral, alkali metal, single/bimetallic metal, acid/basic catalysts, etc. [48]. Active metal and support design and modification are the key for heterogeneous catalyst preparation. To update, Ni-based catalyst with the cheap price and high activity is widely investigated for gasification [49,50]. Assuredly, some precious catalyst, like Pt, Ru, Ir, and Pd based catalysts were also proved to be effective for tar reforming [51,52]. There have many researchers concentrated on the support design, and they considered special structure could provide an excellent metal dispersion and metal-support interaction [53e55]. Recently, the investigation on the gasification catalyst is gradually focused on the catalyst with high activity, anti-coke, anti-S and lifetime extension.
Ni-based catalyst Nickel-based catalysts, usually loaded on some supports (Al2O3, limonite, olivine, ZrO2, zeolite, TiO2, etc.) were extensively used for syngas production and biomass gasification. Ni-based catalyst also proved has excellent activity for CH4 reforming and WGSR. The coke formation over the catalyst surface is the main limitation for Ni-based catalyst during the biomass catalytic gasification process. Therefore, more researches should be focused on the stability and anti-coke activity of Ni-based catalyst. Previously researcheres mentioned low-rank coal have abundant porous structures and oxygen-containing species, which could provide an excellent carrier for Ni-based catalyst preparation [56,57]. Previous work [58] employed sewage sludge to perform gasification experiment over Ni/lignite char prepared by ion-exchange method in a two-stage fixed-bed reactor and investigated the nitrogen distribution and H2 yields under different temperatures and atmospheres in details. Furthermore, our research team compared the activity of commercial Ni/Al2O3 and Ni/lignite char on corncob catalytic gasification [56]. We proved Ni/lignite char showed a higher activity than commercial ones, and syngas yield reached to 43.9 mmoL/g under Ar atmosphere. To investigate the influence of minerals on this kind of catalyst preparation, the
Fig. 5 e Preparation and recycle of biomass/coal char supported catalyst. Please cite this article as: Ren J et al., Methanation of syngas from biomass gasification: An overview, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.023
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Fig. 6 e Heavy tar yield (a), syngas yields and composition (b) over char and char-supported catalyst, and photo of char and char-supported catalyst (c) Redrawn from Ref. [68].
Fig. 7 e SEM images of NieFe/CNF before (a) and after (b) 13 recycle biomass gasification, N2 adsorption isotherm (c) and syngas yield during biomass gasification (d). Redrawn from Ref. [69].
internal and external minerals of lignite was removed by using HCl/HF and HCl and supported Ni catalyst was used for corncob gasification [59]. Interestingly, H2O2 could improve the oxygen containing species then increase the specific space area and reduce the Ni crystallite size of the catalyst, which are most effective for corncob tar reforming. Finally, the Ni/ lignite char was characterized by SEM, XPS, HR-TEM, CO-pulse chemisorption to reveal the layered delocalized structure [60]. To extend the lifetime of Ni/lignite char catalyst, the NieCe/ lignite char catalyst was designed with different Ni/Ce ratio (1e100) to evaluate their activity and stability during corncob tar reforming [61]. It was found that NieCe/lignite char with the Ni/Ce ratio of 50 shows stable gas yield (69.1 vol%), which is higher than commercial Ni/Al2O3 catalyst. To investigate the anti-coke ability of the Ni-based catalyst, Zhang et al. [62]
added Ce promoter on Ni/olivine catalyst and used for toluene reforming. The promoted NieCeeMg/olivine catalyst is more active and stable than NieCe/olivine alone. Furthermore, they claimed the addition of Ce and Mg promoter have a great H2S resistance for 400 min measurement. Except for Ce, La is also a great promoter in high activity catalyst preparation. Garbarino et al. [63] compared the performance of Ni/Al2O3 and NieLa/ Al2O3 by using temperature programmed surface reaction (TPSR) for the phenol and ethanol reforming. They mentioned metal La could reduce the Al2O3 acidity and promote the NieLa interactions by the characterization of X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS), ultra-violet-visibleand near infra-red (UVeviseNIR), etc. Biomass/coal char is an economical material to support active metal, which is
Please cite this article as: Ren J et al., Methanation of syngas from biomass gasification: An overview, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.023
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Fig. 8 e Simulated equilibrium structures and distribution densities (along the Z direction) of different NiIr ratio based catalyst (a); Activity and stability of the different NiIr ratio catalysts for tar reforming under steam atmosphere (b); Binding energies (c) and reaction energy (d) for the intermediates during methane dissociation and CO formation on Ir clusters and bulk (111) surface. Redrawn from Ref. [82].
possible to achieve large-scale development of biomass gasification [64e66]. Importantly, inherent energy of charsupported catalysts could easily be recovered by combustion or other ways after deactivation. As Fig. 5 shows, biomass/coal char material can not only used for high activity and dispersion catalyst preparation, but also used for value-added “green recycle”. Wang et al. [67] mechanically mixed NiO and char particles and used for sawdust tar reforming in a lab-scale fixed-bed gasifier. They obtained a high conversion (>97% tar) and great stability (8 h tests) over the catalyst with a NiO loading of 15%
Fig. 9 e Schematic diagram of syngas from biomass catalytic gasification to SNG.
under the residence time of 0.3 s at 800 C. As shown in Fig. 6, Shen et al. [68] used rice husk char supported NieFe for rice husk gasification and obtained a high tar conversion about 92.3% and 93% before and after calcination under their optimized conditions (pyrolysis temperature: 800 C, feedstock size: 0.125e0.5, biomass weight: 5 g, catalyst weight: 5 g, 1 L/ min N2 as the carrier gas).
Alloy based catalyst Xie et al. [69] prepared NieFe/carbon nanofibers (NieFe/CNF) catalyst and used for wood chips catalytic gasification. As Fig. 7 shows, they found alloy formation in NieFe/CNF catalyst showed a high activity (85.76% tar removal efficiency and 0.947 L/g syngas yield) and anti-coke stability, which can effectively reformed tar compared with non-catalytic test. More mesopores were created when carbon nanofibers were calcined, which could adsorb and reduce macromolecular tar components. Wang et al. [70] proved NieFe alloys could prevent carbon deposition during the cracking of cedar wood tar. They prepared NieFe/Al2O3, Ni/Al2O3 and Fe/Al2O3 catalyst, among which NieFe/Al2O3 was proved has a relatively high activity. Li et al. [71] prepared NieFe/Mg/Al catalysts by the calcination and reduction of NieFeeMgeAl hydrotalcite-like compounds and found that NieFe/Mg/Al with the Ni/Fe ¼ 0.5 was homogeneously distributed on the carrier, and NieFe/Mg/Al with Fe/Ni ¼ 0.25 exhibited the best performance for the cedar wood tar steam reforming. They claimed that the synergy between Ni and Fe nanoparticles were the main reason for the high tar cracked activity. Moreover, the NieFe/Mg/Al catalyst
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Fig. 10 e Calculated equilibrium constants of 8 reactions (a), gas compositions for CO (b) and CO2 (c) during the methanation process (0.1 MPa). Redrawn from Ref. [92].
showed better anti-coke and oxidation-reduction performance than conventional NieFe/Al2O3 for the reason of Mg(Ni, Fe, Al)O formation and reduction. Optional noble metal doping could prevent Ni sintering and increase catalyst activity. Nishikawa et al. [72] employed co-precipitation, precipitation and impregnation method to introduce noble metal Pt, Rh and Pd to Ni-based CeOeAl2O3 and used for cedar wood gasification. They found that Pt and Fe doped Ni/CeOeAl2O3 catalyst exhibited high tar conversion. Moreover, they mentioned that the addition of Pt on Ni/ CeO2/Al2O3 exhibited the best performance because of activation by the tar during the gasification. Noble metal Pt, Ru,
Pd, etc. could enhance the activity of tar reforming, WGSR and anti-coking performance over the catalyst surface. Oh et al. [73] added Ru and Mn to commercial and self-design Nibased catalysts and investigated their performance on toluene steam reforming at 400e800 C. They observed the high stability and toluene conversion occurred above 600 C over Ni/RueMn/Al2O3 catalyst. Ni/RueMn/Al2O3 catalyst with the Mn loading of 2.6 wt% shown the maximum toluene conversion (100%) at 600 C. Especially, they provided the pathways of gaseous products formation according to the experiment data and a series of characterization under steam atmosphere.
Fig. 11 e Possible pathways for CO2 methanation over zeolites (a); Evolution of the relative concentrations of different gas component during TPSR over 14% NiUSY zeolite (b); In situ FTIR spectra over 14% NiUSY under CO2 methanation conditions (c), Redrawn from Ref. [104]. Please cite this article as: Ren J et al., Methanation of syngas from biomass gasification: An overview, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.023
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Other catalysts As mentioned above, Ni-based catalysts, precious metal catalysts, and alloy catalysts have certain activity for biomass catalytic gasification. Recently, there are some researchers developed various novelty catalysts, which could overcome the disadvantages of conventional catalysts, and show relatively high tar conversion. Mineral catalyst, including some metal oxides (Al2O3, CaO, Fe2O3, etc.), are commonly used for biomass catalytic gasification. Berrueco et al. [74] employed forest residues and dolomite to investigate spruce and forest residues gasification behavior. They observed this kind of catalyst enhanced syngas production above 850 C. Delgado et al. [75] prepared calcite and magnetite with different particle size (1e4 nm) for pine sawdust tar removal at 780e910 C. They employed kinetic model and calculated the apparent energies according to the reaction results. The as-prepared calcined dolomite with apparent energies of 42e47 kJ/mol showed the best performance for sawdust tar reforming. Zhao et al. [76] investigated corncob volatiles reforming over calcined limonite, reduced and supported Ni limonite at 400e650 C, and found that the limonite structure changed from FeOOH to Fe2O3 when the temperature was increased. Alkali metals, classified as group IA, are the primary component in biomass, which could catalyze itself to improve the production of gaseous products. Alkali metals are usually added into reactor directly with the biomass fuels. However, alkali metals are difficult to recovery after the tar reforming. Alternatively, alkali metals could also be introduced into supported catalyst to realize coal or biomass catalytic gasification [77]. Lu et al. [78] developed CeO2 and n (CeZr)xO2 and compared the activity for syngas production from wood sawdust, rice straw, rice shell, wheat stalk, etc. gasification. They also employed Pd/C and Ru/C catalyst to study the lifetime and stability, and discussed the influence on biomass size and types, reactor types, and heating rate systematically. Tomishige et al. [79] designed the catalyst of metal Rh, Pd, Pt, Ru and Ni loaded on CeO2/SiO2 and investigated the catalytic activity for cellulose gasification. They claimed that Rh and Pd based catalyst were more active than Ni and Pt catalyst at 550 C during the process of gasification. The tar conversion increased from 88% to 97% when Rh/CeO2/SiO2 was used for gasification at 600 C, and it showed a high anti-coke ability during tar reforming. Asadullah et al. [80,81] compared the influence of the support (CeO2/ZrO2, CeO2/SiO2 and CeO2/Al2O3) on Ru based catalyst and investigated the wood catalytic gasification. They claimed the Ru-based catalyst is easily deactivated when SiO2 was chosen as the carrier, and they reported that CeO2/SiO2 is an excellent support for H2-rich gas production. Dagle et al. [82] employed MgAl2O4 as carrier to load Ni, Rh, Ir, Ru, Pt, Pd, Ni, and investigated the hydrocarbons steam reforming. They designed and controlled Ni/Ir ratio of bimetallic IrNi based catalyst to obtain a stable benzene and naphthalene reforming catalysts. Dagle et al. [82] employed scanning transmission electron microscopy with high-angle annular dark-field mode (STEM-HAADF) and energy dispersive X-ray spectrometry (EDS) technology to reveal the presence of individual Ir atoms. They proved that Ir clusters loaded on Ni particles have a high anti-coke and anti-sintering ability through theoretical calculations as Fig. 8 shows.
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Keller et al. [83] compared the activity of La/ZrO2, Sr/ZrO2, Fe/ZrO2 and LaSrFe catalyst for tar model compounds (benzene and ethylene) reforming. The result showed that the addition of Sr in the La and Fe with the LaSrFe ratio of 0.8/0.2/1 exhibited a significant high conversion during benzene reforming. However, it is not stable for the reason of La2Zr2O7 and SrZrO3 perovskite formation, which would interact with the ZrO2 support and reduce the activity of La0$8Sr0$2Fe based catalyst. Recently, more researches focus on the modification and active metal loading of zeolites to realize biomass catalytic gasification, which have complex and variable 3dimensional network structure[84,85]. After reviewed above, active metal lost and oxidation, coke formation and catalyst sintering, make the gasification catalyst easy to inactivate during the long time testing of real tar or model compounds [86,87]. Furthermore, the changes of support structure are another reason for catalyst deactivation. Finally, fly ash, S poisoning, and unstable feed rate are also the main consideration for catalyst design in future large-scale utilization [38,88]. Here, we concluded some focus for future researchers and industrial users.
Fig. 12 e Normally used (Marked in Green) methanation catalyst (Excerpt from the periodic system of elements). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) (1). Ru is known to be the most active metal for CO, CO2, or mixtures methanation, and the activity is reported 120 times higher than Ni-based catalyst [81]. However, it is not possible for large-scale methane production because of the expensive investment [115]. (2). Ni is the most selective for methanation catalyst preparation due to the high activity and low price [116,117]. There it is the most promising active metal for commercial applications. (3). Co exhibits methanation activity similar to Ni. However, they are not that widely used in large-scale utilization for the reason of expensive price. (4). Fe is proved to be the high reactivity catalyst, but the methane selectivity is low in methanation reaction. Normally, Fe-based catalysts are usually used in ammonia synthesis or Fischer-Tropsch process. (5). Mo shows the low methanation activity compared with Fe, Co, Ni-based catalyst. Mo-based catalyst could exhibit high selectivity of C2þ hydrocarbons. In addition, Mo-based catalyst could exhibit highest S-resistance than other active metal-based catalyst.
Please cite this article as: Ren J et al., Methanation of syngas from biomass gasification: An overview, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.023
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Fig. 13 e (a) The specific side view for depicting the orbitals with electronic densities in S vacancy, (b) The density of states of S-vacancy, and (c) The energy change on the activated (001) surface when S-vacancy formation. Redrawn from Ref. [123].
(1). Choosing and designing a suitable active metal site over the specific structure carrier, and decrease the metal (Ni, Ru, Pt, Pd, etc.) loading to obtain the single-atom catalyst, which could reduce the resource consumption and realize the biomass gasification in the lab scale. (2). Developing the recyclable catalyst, like carbon-based catalyst, which is the cheapest catalyst and could be used for power generation after deactivation and poisoning. (3). Understanding of crack mechanism of biomass tar. At beginning, selecting some tar model compounds as the tar model to study the reaction pathways, and then explore the actual tar cracking mechanism, which could provide a useful guidance for gasification catalyst design. (4). Adding and optimizing the accessories of the reactor. To realize large-scale gasification, the impurity and water of the received feedstock should be well cleaned.
Syngas methanation Until now, synthetic natural gas (SNG) production from biomass catalytic gasification is considered as a possible
method because of its low costs for CH4 production. However, the produced gas from gasification should be treated as follows: tar removal, water removal, gas conditioning and cleaning before syngas methanation. Except for H2-rich syngas, the synthetic gaseous products also contains a few H2S or HCl components. Cyclone and ceramic cartridge filters are the main system for ash and other particles removal [89,90]. Organic scrubbers or water could be used for tar removal at low/high temperature. Steam could be collected by water scrubber then poured into methanation reactor, which has the ability for preventing coke during gasification and methanation process. Interestingly, it is not required that tar removal before SNG process if the methanation temperature is higher than tar condensation temperature. However, HCl and H2S must be removed to prevent catalyst deactivation during the syngas methanation. Normally, HCl could be removed by Na2CO3$NaHCO3$2H2O at around 400 C and H2S removal could occur at a ZnO-fixed bed for further methanation process [91]. Fig. 9 shows the whole combination concept for methanation of syngas from biomass gasification. Before methanation, the H2S produced from gasification should be removed to prevent catalyst S-poisoning. Above all, a high activity catalyst with excellent anti-coke and anti-S is the urgent issues for future discussion.
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Fig. 14 e CO conversion and CH4 selectivity for ZSM-5 catalyst (a, b) and FSZSM-5 catalyst (c, d) Ref.[129].
Fig. 15 e Pilot-scale biomass gasification (Volkswagen Audi) and syngas methanation (PHG Energy).
Table 4 e Global syngas methanation projects at large-scale [130]. Project name GAYA (Engie) € teborg Energi) GOBIGAS (Go BioSNG (EU project) Great Plains Synfuels Plant (Dakota Gasification Company) DemoSNG (EU project) CPI project (CPI Xingjiang Energy Co.) Keqi project (Datang) Fuxin project (Datang) Huineng project (Huineng Coal Electricity Group) Xinwen project (Xinwen Mining Group) Qinghua project (Qinghua Group) POSCO Project (POSCO)
Location
Capacity
Methanation
Material
Saint Fons (France) Gothenburg (Sweden) Gu¨ssing (Austria) Beulah (North Dakota, USA)
400 kW SNG output 20 MW SNG output 1 MW SNG output 1500 MW fuel input
n.s. TREMP PSI Lurgi methanation
Biomass Biomass Biomass Coal
€ ping (Sweden) Ko Yili City (China) Chifeng (China) Fuxin (China) Ordos (China) Yili City (China) Yili City (China) Gwangyang (South Korea)
50 kW SNG output 6 billion m3 a1 SNG output 4 billion m3 a1 SNG output 4 billion m3 a1 SNG output 1.6 billion m3 a1 SNG output 4 billion m3 a1 SNG output 5.5 billion m3 a1 SNG output 0.7 billion m3 a1 SNG output
KIT (honeycomb) TREMP HICOM HICOM TREMP HICOM TREMP TREMP
Biomass Coal Coal Coal Coal Coal Coal Coal
Note: n.s.: not specified.
Please cite this article as: Ren J et al., Methanation of syngas from biomass gasification: An overview, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.023
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Fig. 16 e Diagram of direct melting system for large-scale biomass gasification. Adapted from Ref. [134].
The methanation reaction is a reversible exothermic reaction, and it is important to design catalysts to reach the maximum conversions at around 350 C. As Fig. 10 shows, the calculated equilibrium of different H2/CO ratio and temperature have the great influence on syngas methanation. Nibased and other noble metal-based catalysts (Ru [93], Rh [94], Pt [95], etc.) were proved to be active for CO2 methanation, and the common supports included SiO2 [96,97], hydrotalcites [98,99], ZrO2 [100,101], TiO2 [102,103], zeolites [104], etc. Main reactions (Eqs. 3, 4, 5 and 6): COþ3H2 #CH4 þ H2 O DH298K ¼ 206 kJ=molCO methanation (3) 2CO þ 2H2 #CH4 þ CO2
DH298K
¼ 247 kJ=mol Reverse dry reforming
(4)
CO2 þ H2 #CO þ H2 OD H298K ¼ 41kJ=mol Reversewatergas shift (5) CO2 þ 4H2 #CH4 þ 2H2 O DH298K ¼ 165 kJ=mol CO2 methanation Side reactions (Eqs. 7, 8, 9 and 10):
(6)
2CO#C þ CO2
DH298K ¼ 172 kJ=mol Boudouard reaction (7)
CO þ H2 #C þ H2 O
DH298K ¼ 131 kJ=mol CO reduction
(8)
CO2 þ 2H2 #C þ 2H2 O DH298K ¼ 90 kJ=mol CO2 reduction (9) CH4 #C þ 2H2
DH298K ¼ 75 kJ=mol Methane pyrolysis (10)
Ni/Al2O3 is an excellent catalyst for syngas and CH4 production, which shows a high activity during biomass gasification and syngas methanation. However, easy carbon deposition or poor stability on syngas catalytic methanation are the disadvantage of Ni/Al2O3 [105,106]. Therefore, developing a catalyst with high activity and anti-coke ability is a crucial point. Zhao et al. [107] designed Ni/Al2O3 catalysts with Ni loading of 10e50 wt% and used for syngas methanation. The activity of Ni/Al2O3 were sensitive to Ni particle size and the productivity reached the maximum Ni crystals around 41.8 nm by the characterization of XRD, H2-temperature-programmed reduction (H2-TPR), Scanning electron microscope
Please cite this article as: Ren J et al., Methanation of syngas from biomass gasification: An overview, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.023
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(SEM) and Transmission electron microscope (TEM), etc. Furthermore, because metal Ni was separated by support, Ni/ Al2O3 catalysts could exhibit great stability when the Ni loading was above 20 wt%. Hu et al. [108] systematically investigated some operate parameters (H2/CO ratio, reaction pressure, NiO and MgO loading, SV, etc.) during syngas methanation over Ni/Al2O3. The addition of MgO (2 wt%) could reduce the carbon deposition and improve the catalyst stability. 400 C could promote the formation of NiO and reducible b-NiO of the catalyst with Ni loading of 20 wt%. They obtained the high CO conversion (100%) and CH4 selectivity at 3.0 MPa, 20e40 wt% NiO and 2e4 wt% MgO supported on Al2O3 at 300e550 C. These results are helpful for developing and optimizing Ni/Al2O3 catalysts for syngas methanation. To improve the activity of Ni/Al2O3, some promoters were added to modify the structural or electronic properties. For example, as an electronic and structural promoter, Ce could improve the activity of Ni-based catalysts for many catalytic reactions [109,110], due to the transformation of Ce3þ to Ce4þ during the gasification and methanation [111]. In addition, CeO2 could improve the thermal stability of Al2O3, and promote the dispersion of active metal on the support and modify the Ni/Al2O3 properties because of strong metal-support interaction [94,112,113]. Westermann et al. [104] employed USY zeolites to load Ni (Ni loading of 5e14 wt%), and they investigated the pathways involved in CO2 methanation by operando IR spectroscopy, insitu FTIR and TPD experiments. As Fig. 11 shows, the CH4 formation was started from dissociated hydrogen reacts with carbonates and/or physisorbed CO2, and then monodentate and carbonyls formed on Ni0 particles. n 3þ )x/ Hydrotalcite-like compounds ([M2þ 1-xMx (OH)2](A $yH O), also named layered double hydroxides, which are 2 n noteworthy for designing CO2 methanation catalyst. M2þ and M3þ are any active or not active metal cations, and An is an exchangeable interlayer anion. Wierzbicki et al. [99] designed various metals, such as Ni, Al, Mg and La, supported on hydrotalcite catalyst by the method of thermal decomposition. They determined periclase-like materials formation, and the La addition promoted the catalytic activity (46.5e75%) and CH4 selectivity (99-98%) for CO2 methanation at 250e300 C. For the reason of basicity improvement, the activity was increased when La dopped into the hydrotalcite-derived catalysts. Marocco et al. [114] also prepared Ni/Al hydrotalcite derived catalyst and characterized by atomic adsorption spectroscopy, XRD, H2-TPR, CO chemisorption etc. They performed methanation experiments at different H2/CO2 ratios, temperatures, and the flow rates. In particular, they developed different kinetic rates from the reaction results, performed nonlinear regression analysis to find kinetic parameter values between measured and calculated values and explained the formation of CO by the reverse WGSR. Except for commercial Ni-based catalyst, there are also other highly active catalysts proved to be active for methanation. As shown in Fig. 12, many metals mainly focus on the groups of VI B to I B, which were found to be highly active for methanation. Metal Mo was also investigated for CO2 methanation. Saito and Anderson [118] compared the activity of some Mo compounds (Mo, Mo2C, Mo3N, MoS2, MoO2, and MoO3) for CO2
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methanation at 350 C, and found Mo could improve hydrocarbons production and catalyze the WGSR. They indicated the activity order of these catalysts as follows: Mo2C > Mo > Mo3N > MoO2 > MoS2, > MoO3. Yao et al. [119] claimed that Mo could prevent the carbon deposition due to the highly efficient hydrogenation of the coke. Wang et al. [120] reported MoP/Al2O3 for S-resistant methanation at 550e650 C. It indicated that MoP catalyst could show the high activity under the atmosphere with high H2/CO ratio and low H2S content. Liu et al. [121] reported the preparation of sulfur-resistant catalyst and used for CO methanation. They employed ammonium heptamolybdate as the precursor for Mo-based catalyst preparation, and they considered that the weight ratio of S/ammonium heptamolybdate and S atmosphere play an important role in the structure and morphology of the catalyst. The catalyst treated under N2 atmosphere existed more weakly-bonded S species on the surface of catalyst and more S vacancies on the edge planes of MoS2 particles, which exhibited a high activity for methanation (88.2% of CO methanation). Zhang et al. [122] employed DFT simulations to elaborate methanation mechanism over the cleaved (001) surface of the MoS2 catalyst. They demonstrated the possibility of CO and H2 adsorbtion and reaction. The electronic orbitals and densities for depicting S vacancies were presented in Fig. 13a. Three of the exposed Mo atoms were activated from the blue field (conduction states in orbitals). As reported [123], the formation of S vacancy could introduce more bands to the gap around 0 eV (fermi level) when S-vacancy formed. For the MoS2 catalyst, the active sites inosculate with the new bands from the high density of d-states in Fig. 13b. The formation mechanism of the S-vacancy is presented in Fig. 13c, and the formed Svacancy stability is also confirmed. MoeH and SeH (P1) will be formed from the heterolytic fission of the activated H2. For the reason of the MoeH and SeH are the electron acceptor and donor, respectively, the exposed Mo atoms would produce unoccupied adsorption orbitals when S atom was removed. The process is exothermic, and the reaction heat and the activation energy are 0.48 eV and 0.85 eV, respectively.
Integration and challenge on the syngas methanation and biomass gasification Syngas and methane production from biomass gasification and methanation are promising alternatives of value-added chemicals generation under catalytic conditions [124]. It also provides an opportunity to reduce the green house by CO2 capture and hydrogenation. Compared to other synthetic transportation fuels (dimethyl ether, ethanol, etc.), methane uses the existing natural gas infrastructure, could really realize a smooth transition from fossil to synthetic energy [125]. The advantages of methane are the high conversion efficiency, and some basic infrastructures are already distributed over the world. For this reason, car, heating, power stations were found and used in our daily life, which utilize the SNG [126]. Solid biomass was converted into methane via gasification and subsequent methanation shown the comparable efficiency (65% energy from wood to methane) [127].
Please cite this article as: Ren J et al., Methanation of syngas from biomass gasification: An overview, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.023
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Duret et al. [127] employed a fast internally circulated fluidized bed to investigate the difference between the reaction temperature and the thermodynamic equilibrium for gasification and methanation. They mentioned only about 7% of the extra energy needs should be imported and the other energies could be obtained from the excess of heat produced in their system. As a result of the methanation is the exothermic reaction, catalyst chosen for increasing CH4 content and decreasing the reaction heat will improve the overall chemical efficiency in the methanation step. Increasing the H2/CO2 or H2/CO ratio from gasification could promote the CH4 production and CO2/ CO selectivity. H2/CO2 or H2/CO ratio varied with the temperature, pressure and catalyst type. He et al. [128] employed MSW to produce the H2-rich gas in a bench-scale downstream fixed bed reactor. They found the best H2/CO2 ratio was 3:1 at the temperature between 700 and 950 C when dolomite was used as catalyst. Hussain et al. [129] found that CH4 mole fraction increased with the increasing of the H2/CO ratio from 1:1e5:1. High H2/ CO ratio could improve the methanation activity and resist carbon deposition (see Fig. 14). Furthermore, as Table 2 presented, N and little amount S (Neglected in Table 2) are existed in the biomass. SOx and NOx are also required to remove because of the activity of catalyst would be poisoned by them. As a consequence, the optimal gas compositions from biomass gasification are important to realize the integration of gasification and methanation. Considering process and reactor design to quantify the overall conversion efficiency are important to improve the integration possibility of biomass gasification and methanation. Consequently, great care has to be taken in ways of valorizing and optimizing the energy conversion efficiency to avoid investing more energy than what is produced during the production process.
Large-scale commercial development To obtain more electricity and effectively treat sewer sludge, PHG Energy of Nashville Tennessee and Covington signed the waste-to-energy gasification contract in 2012. The gasification plant for gasification was presented in Fig. 15. Table 4 listed an overview of methanation projects all over the world at pilot or commercial scale. This system utilized 12 t/d to supply 6 million Btu/h of producer gasas the central technology. This gasifier contained wood chipping and material handling, and it solved around 10 t wood wastes and 2 t sewer sludges every day. PHG Energy company designed the wood pretreating, material handling, material mixing and drying process during biomass gasification. At the 1980s, the first large-scale plant for coal to SNG was built in North Dakota, USA, which named as Great Plains Synfuels Plant (GPSP) during the oil crisis phase. This gasification plant equipped with Lurgi fixed-bed gasifiers and methanation reactors. GPSP produced around 153 mm scf/d of SNG [56 billion scf/y] from 6 million t/y of coal. Another biomass-based SNG project was found by Austria in 2008 with 1 MW SNG production in pilot-scale, which employed wood
chips as the biomass materials [131]. In the last years, some other methanation projects were also build at pilot-scale in France, which named as GAYA project. Climate change and energy shortage are the main reasons for biomass-gas projects development in Europe. There are some other countries started to realize this combination. The first industrial power to gas plant was built at Volkswagen Audi Company in Germany, which transform CO2 to methanation. The project intends to improve the efficiency of a combination of power-to-gas (PtG) and methanation process. They claimed PtG plant equipped electrolyzer and methanation unit, which is promising to produce a variety of heat sources for CO2 separation, fermenter heating and hygienization of wastes. Japan and South Korea already have more than 30 commercial gasification plants by using fluidized-bed type gasifier, which equipped with melting technology and direct melting system [132,133]. As the flow diagram of this technology shows (Fig. 16), the plant of this company was consisted of a melting furnace, gasification furnace, MSW charging system, combustion chamber and gas cleaning system. Furthermore, some subsystems for raw material and fly ash handling system were also designed for excellent operation.
Conclusions Biomass, as a representative REs, is a significant promising and alternative energy for syngas and other chemicals production via biomass gasification and methanation. An indepth discussion throughout this review provided guidance about high efficiency gasification of biomass. Some highperformance gasification catalyst and their catalytic pathways were concluded, and the influence of various parameters (gasifiers, S/C ratio, reaction temperature, etc.) were discussed as a whole. A deep understanding and the knowledge of alternative approaches about gasification and methanation process are required for its optimization and advancements. Especially, the gasification products were employed to realize the syngas methanation, and some operate conditions and catalyst design were also reviewed briefly. Furthermore, the large-scale multi-stage gasification, methanation and their combination plants or small centralized gasification equipment were reviewed as a whole to explain value-added utilization of biomass. The advancements on tar-cracked elimination and new catalyst were mentioned to promote the clean and high-efficiency operation of gasification process. This paper presents a positive future for integration of biomass catalytic gasification and methanation as a promising and economically technology.
Acknowledgments This work was funded by the National Key R&D Program of China (Grant 2017YFE0124200), the National Natural Science Foundation of China (Grants U1710103, 21676292 and 21978317), and the project “Power to Fuel” of JARA Energy from
Please cite this article as: Ren J et al., Methanation of syngas from biomass gasification: An overview, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.023
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German federal and state governments. We are also thankful to China Scholarship Council who provides scholarship for Jie Ren (No. 201806420028) to develop his PhD research at the RWTH Aachen University.
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Review Article
Methanation of syngas from biomass gasification: An overview Jie Ren a,b, Yi-Ling Liu b, Xiao-Yan Zhao b, Jing-Pei Cao b,* a Lehrstuhl fu¨r Heterogene Katalyse und Technische Chemie, Institut fu¨r Technische und Makromolekulare Chemie (ITMC), RWTH Aachen University, Aachen, 52074, Germany b Key Laboratory of Coal Processing and Efficient Utilization (Ministry of Education), China University of Mining & Technology, Xuzhou, 221116, Jiangsu, China
highlights Detailed review of gasification and methanation development. Discussion of high active catalyst for gasification and methanation. It sheds light on effective reactors for biomass gasification. Large-scale commercial development of gasification and methanation are discussed.
article info
abstract
Article history:
Traditional fossil fuel overuse could lead to global warming and environmental pollution.
Received 30 July 2019
As a renewable energy, biomass energy is a sustainable and low pollution carbon energy,
Received in revised form
which has a wide range of sources. Syngas production from biomass thermochemical
27 October 2019
conversion is a promising technology to realize effective utilization of the renewable en-
Accepted 5 December 2019
ergy. Syngas produced from gasification could be further converted into value-added
Available online xxx
chemicals via the method of Fischer-Tropsch synthesis. Syngas and CO2 methanation could transform renewable energy into feasible transport and high-density energy. How-
Keywords:
ever, tar formation and catalyst deactivation are the main problem during the biomass
Gasification
gasification and methanation. This review sheds light on the development of biomass
Renewable energy
gasification and syngas methanation. Firstly, we presented the common reactors and some
Methanation
other factors during gasification. Secondly, we provide a comprehensive introduction of
Tar removal
the advanced active catalyst for gasification and syngas methanation. Finally, some
Biomass
representative large-scale and commercial plants and companies for biomass gasification were compared and discussed in details. Then the prospective developments in combination of gasification and methanation were concluded to give an outlook for biomass gasification and its downstream development. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. E-mail address: [email protected] (J.-P. Cao). https://doi.org/10.1016/j.ijhydene.2019.12.023 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Ren J et al., Methanation of syngas from biomass gasification: An overview, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.023
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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biomass gasification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Other parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ni-based catalyst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Alloy based catalyst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Other catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Syngas methanation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Integration and challenge on the syngas methanation and biomass gasification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Large-scale commercial development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Nomenclature EDS Energy dispersive X-ray spectrometry FE-SEM Field emission scanning electron microscopy GPSP Great plains synfuels plant H2-TPR H2-temperature-programmed reduction MSW Municipal solid waste PAHs Polycyclic aromatic hydrocarbons PtG Power-to-gas SEM Scanning electron microscope SNG Synthetic natural gas STEM-HDDF Scanning transmission electron microscopy-high-angle annular dark-field SV Space velocity TEM Transmission electron microscope TPSR Temperature programmed surface reaction TPD Temperature programmed desorption REs Renewable energy resources UVeviseNIR Ultra-violet-visible-and near infra-red WGSR Water-gas shift reaction XPS X-ray photoelectron spectroscopy XRD X-ray diffraction
Introduction Energy is a driving power for national economy development [1e3]. At present, the main energy sources of the world are remaining coal, oil and nature gas. As the increasing of the life quality, population and industrialization country, fossil fuels consumption is dramatically increase. However, excessive consumption of fossil fuel seriously destroy the environment, and leads to global warming, environmental pollution and
00 00 00 00 00 00 00 00 00 00 00 00 00 00
ecological balance damage [4e6]. Recently, reducing consumption of fossil fuel and release the energy crisis by using renewable energy alternatives are very important to meet the growing energy needs [7]. The future renewable energy consumption from 2001 to 2040 all over the world was shown in Fig. 1. From Fig. 1, we concluded future energy structure and use has gradually transformed to clean renewable energy, especially the biomass energy. Renewable energy resources (REs) occupied about 13.8% of the world energy consumption at 2018 [1], which including hydropower, biomass energy, geothermal energy, solar energy, wind energy, ocean energy, material, etc. REs are the primary inexhaustible resources, which supplies about 20% electricity of all over the world. The development of REs could resolve the presently crucial energy problems, improve energy supply, and promote live standard of the local population. Moreover, it could ensure the improvement of sustainable energy in the remote regions (desert or mountain zones). It is possible that create job opportunities to rural city and country, and then the problem of biomass wastes could be well solved. Among the REs, biomass usually come from the living species of plants or animals, which absorb or convert sunlight to bioenergy. Biomass is also a representatively sustainable energy, which has a wide distribution in everywhere (Rural and urban) [8]. Table 1 listed the basic classifications based on biomass materials origin. Rural wastes included sugarcane, cocoa, maize, coconut, rice, oil palm, millet, etc. The urban category is subdivided into municipal solid waste (MSW), food industry wastes, and industrial wastes. As observed from Table 2, biomass from different origin contains abundant carbon resources, which would be potential to convert biomass into valuable chemicals [9,10]. Lapuerta et al. [11] employed grapevine, olive pruning, sawdust etc. to evaluate their gasified behavior, and the results shown that grapevine and olive pruning have the higher gas production and gasification efficiency than pinaster pruning and sawdust at relatively high biomass/air ratio. Therefore, if the industrial and forestal
Please cite this article as: Ren J et al., Methanation of syngas from biomass gasification: An overview, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.023
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Fig. 1 e Future renewable energy consumption from 2001 to 2040.
wastes were employed for gasification in agricultural wastes gasification system, the external energy (natural gas, etc.) or more biomass supply are required to obtain a high gasification efficiency. Physical conversion, thermochemical conversion and biochemical conversion are the three main ways of the biomass conversion [25e30]. Biomass combustion normally was used to produce heat or electric power [31]. However, serious pollution from biomass combustion, like the release of CO, soot and polycyclic aromatic hydrocarbons (PAHs), NOx and particles pollutants need to handle. Liquid, solid and gaseous products could be produced from biomass pyrolysis technology under different atmospheres at low temperatures [32,33]. However, these products were usually consisted of organic oxygen species, which limited industrial development of this technology [34].
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Currently, biomass gasification was used to produce H2rich gas through partial oxidation or other reaction when favorable gasification agent and catalyst were selected [35]. The main process in the biomass gasification are as follows: (1) Biomass drying: the moisture of biomass was converted into steam; (2) Tar forming: heavy tar, light tar and little amount of gas were released between 400 and 700 C. (3) Gas releasing: heavy tar and light tar cracking, more gaseous products produced at temperature above 700 C. An optimized reactor, highly active catalyst could decrease the gasification temperature and improve the quality of produced gas. Furthermore, biomass types, residence time, temperature, pressure, etc. are also important for biomass gasification. Additionally, biomass gasification is also a valuable technology to produce H2-rich combustible gases or value-added fuels via FischerTropsch synthesis [36]. To realize the industrialized development of biomass gasification, the operate parameters should be optimized for tar removal during the biomass gasification. Selection of the excellent gasifier, gasification agent, etc. have gradually became the key issues should be overcome. To some extent, a highly active and stable catalyst is always the research topic in recent years. A detailed discussion were summarized in the next section. Although REs are promising energy to use in the future, an appropriate method for energy generation and storage is deserved to discuss with the expansion of REs. Considering a renewable production of electrical energy via various energy (wind, solar, biomass, etc.), more efforts should be paid attention to find the solutions for electricity storage. Except for high storage batteries, chemical storage is also an alternative method for effective utilization of REs, which has high volumetric storage density and easily transport or
Fig. 2 e Recent development in methanation and gasification.
Table 1 e Biomass classifications based on their origin (Rural and urban). Main categories Rural wastes
Urban wastes
Sub-categories Agricultural resources Energy crops Forest resources Animal wastes MSW Food industry wastes Industrial wastes
Representatives Straw, shell, grass, wood, seeds, etc. Sorghum, maize, soybean, wheat, oil palm, sugarcane, etc. Fat, oil, grease, chicken litter, bones, meat, manures, etc. Wastewater, sewage, waste papers, wood pallets and boxes, bio-solids
Please cite this article as: Ren J et al., Methanation of syngas from biomass gasification: An overview, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.023
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Table 2 e Proximate and ultimate analyses of different biomass materials. Sample
Rice straw Wheat straw Walnut shells Hazelnut shell Miscanthus grass Olive wood Soft wood Pepper plant Forest residue Poplar Willow Christmas trees Rice husk Cotton husks Pig moisture Sewage sludge
Proximate analysis (wt.%)
Ultimate analysis (wt.%)
Ref.
VM
FC
A
C
O
H
N
64.3 74.8 59.3 69.3 81.2 79.6 70.0 64.7 79.9 85.6 82.5 74.0 68.9 78.4 66.1 34.6
15.6 18.1 37.9 28.3 15.8 17.2 28.1 20.9 16.9 12.3 15.9 20.8 11.1 18.2 14.7 2.2
20.1 7.1 2.8 1.4 3.0 3.2 1.7 14.4 3.2 2.1 1.6 5.2 20.0 3.4 19.2 69.7
43.0 49.4 49.9 52.9 49.2 49.0 52.1 42.2 52.7 51.6 49.8 51.6 47.4 50.4 49.3 47.4
5.7 43.6 42.4 42.7 44.2 44.9 41.0 49.0 41.1 41.7 43.4 36.7 45.1 39.8 >38.1 34.2
1.0 6.1 6.2 5.6 6.0 5.4 6.1 5.0 5.4 6.1 6.1 5.6 6.7 8.4 6.8 7.7
0.2 0.7 1.4 1.4 0.4 0.7 0.2 3.2 0.7 0.6 0.1 0.5 0.8 1.4 5.1 8.3
[12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [21] [12] [19] [23] [24]
Note: FC¼Fixed carbon; VM¼Volatile matter; A ¼ Ash.
distribution. Potential pathways including biomass catalytic gasification (Biomass gasification/CO þ CO2 þ H2 þ CH4 þ Tar) to deliver H2 and CO or CO2, followed by e.g. CO2 metanation via Fischer-Tropsch technology were proved effective for energy valued development. Recent develpment in biomass gasification and methanation of syngas or CO2 was depicted in Fig. 2. Following this line of argument, a discussion has started to combine gasification and methanation to realize energy value-added utilization. Like the biomass gasification, economical catalyst with high catalytic performance and stability is most important. The syngas or CO2 methanation (Sabatier reaction) is an exothermic reaction as following Eqs. 1 and 2: CO2 þ 4H2 /CH4 þ 2H2 O DH ¼ 165 kJ=mol
(1)
3H2 þ CO/CH4 þ H2 O DH ¼ 206 kJ=mol
(2)
Hence, heating removal is crucial to the syngas/CO2 methanation, and prefect reactor design could increase CH4 yield and decrease the electricity consumption. In this review, we first introduce the development of biomass gasification and methanation in section Biomass gasification and section Syngas methanation, respectively. Secondly, we review the main gasifier types, some effective catalysts and related gasification mechanism, and then we propose and prove the possibility of syngas methanation from biomass gasification. Some representative catalysts and pathways were concluded during the syngas methanation. Finally, we take some examples of large-scale gasification, methanation and combination of these two technologies.
Biomass gasification Syngas production from biomass catalytic gasification is an effective method for renewably value-added utilization of biomass at high temperature of 800e1000 C. However, high content of tar, volatile organic compounds, solid residues,
char, and soot would prevent gasification development. Biomass gasification process could be classified into material drying, pyrolysis, oxidation, and gasification. The moisture of the biomass material would be removed through first heating around 200 C, and dried biomass could be cracked into heavy tar, light tar, biochar, H2-rich gases during the whole pyrolysis process. The condensable “heavy tar” including complex and hard evaporate organic compounds. As Fig. 3 shows, the main processes of biomass gasification could be divided into primary cracking, secondary cracking and tertiary cracking. Syngas, CO2, CH4 and H2O would be produced at 400e700 C. The primary heavy tar below 700 C is rich in oxygenated vapors, such as levoglucosan, furfural, hydroxyacetaldehyde and methoxy phenols [37]. Some gaseous products, light olefins and aromatics are generated from the secondary cracking of methoxy phenols. Furthermore, PAHs including indene, anthracene, pyrene, etc. would be continuously formed, as well as H2-rich syngas at 850e1000 C. Biomass would be transformed to biochar, which still retain the property of lignocellulose during the primary pyrolysis. The soot could be produced from the homogeneous nucleation of the hydrocarbons, and coke formed from the thermal cracking of the heavy or light tar at relatively high temperature.
Reactors Biomass catalytic gasification could obtain gaseous products with different types and concentrations in the specific structure reactor. Here we presented some usual gasifiers, which were used according to the biomass size, ash, moisture, and the feed way. It could be concluded to fixed-bed, fluidized-bed, entrained flow reactor, rotary kiln reactor and plasma reactor, etc. Fixed-bed could be classified as “updraft”, “downdraft” and “crossdraft” gasifier, which is the simplest reactor for gasification. Biomass is fed from the top of the reactor and decomposed through drying, pyrolysis, reduction and
Please cite this article as: Ren J et al., Methanation of syngas from biomass gasification: An overview, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.023
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Fig. 3 e Biomass gasification pathways.
combustion area. The air/gasification agent enter into reactor from the different side. For “updraft” gasifier, the gasifying agent or air rose from the bottom and react with biomass material. This reactor shows the high thermal efficiency, low pressure drop and low slag formation. However, long startup time, low syngas yield, and high tar sensitivity are the main problems. Biomass and gasification agent from the top to bottom, and wastes and gaseous products also move in the same direction inside the “downdraft” gasifiers. This reactor could produce high quality and low tar content gaseous products. However, “downdraft” reactor usually used for power generation at a small scale. “Crossdraft” gasifier is also useful for syngas production from biomass gasification for the reason of great response for flexibility. The shorter start-up time, lower reactor height are the advantages for biomass gasification. The key problem for this kind of reactor is hard to treat the small size and high amount of biomass material. Fluidized-bed reactor could be classified as “bubbling” and “circulating”, the inert materials were stirred continuously to produce gas bubbles, which shows an excellent heat exchange ability between biomass and gas. Currently, “bubbling” and “circulating” were commonly used for biomass gasification because the high flexibility of biomass, high heat transfer and cold gas efficacy, etc. However, the ash and alkali metals in biomass could break gasification equipment and lead to low quality of gaseous products. Therefore, highly active catalyst should be designed for biomass gasification due to the high temperature requirement during the gasification process. There are some other well-designed reactors used for biomass tar removal. Entrained flow reactor usually used at the high temperature (1300e1500 C) for coal residues and plastic wastes gasification. Moisture in biomass should be avoided during the biomass pretreatment. Recently, plasma reactor was widely used in the upstream gasification process, which could improve the tar cracking into light tar and increase the amount and quality of syngas products. Most importantly, this
reactor could be used for biomass gasification with various flow rate, biomass moisture, size and composition because of the excellent temperature control in different types and qualities of biomass. In industry, rotary kiln gasifiers were the main equipment for industrial waste combustion, which have the temperature resistance metal shell. Biomass would have a great contact with gasification agent due to the existence of the continually rotated reactor. Therefore, lower heat exchange between the biomass and gas would bring the lower gasification efficiency. As we already reported, the detailed comparison of different gasifiers was presented in Table 3.
Other parameters To obtain the excellent yield and composition of gaseous products, biomass type, space velocity (SV), temperature, pressure, etc. are also important for gasification evaluation. These factors have a great influence on energy conversion efficiency. Therefore, the gasification operating conditions should be optimized, and some effects on the biomass gasification were discussed as follows: Less feeding would bring the less syngas content, and biomass overfeeding could result in reactor plunge and gasification efficiency decrease. A proper biomass feeding rate could make the maximize energy conversion. Biomass composition (lignin, cellulose and hemicellulose ratio) is another main determination of gasification. Hanaoka et al. [39] claimed the tar conversion of lignin, cellulose and xylan reached 52.8%, 97.9% and 92.2%, and cellulose gasification could produce the highest yield of gas. Equivalence ratio and SV are also the important factors for biomass gasification. Different air flow rate with various O2 content could result in gasification temperature change and then leads to high biomass conversion and fuel quality. However, excessing combustion of biomass would decrease produced gas energy [40,41]. High SV would lead to the decrease in the yields of
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Table 3 e Advantages and disadvantages of different reactor types [38]. Reactor types
Advantages
Disadvantages
Fixed bed 1. High thermal efficiency, solid residence time and carbon conversion; 2. High capacity of handling different biomass; 3. Excellent contact between the feedstock and the react atmosphere; 4. Feasibility of large-scale production; 5. Low entrainment of dust and ash; 6. Simple construction. Fluidized 1. High of contact between feedstock and atmosphere; bed 2. High carbon conversion and thermal loads; 3. Excellent for temperature control, feed and process, handling biomass with different properties; 4. Possible to realize the large-scale utilization; 5. Reduced residence times, etc.
1. 2. 3. 4. 5. 6.
High content of tar in the syngas; Low content of produced CO and H2; Limited flexibility to feed and react process; Hard to start and control temperature; Mobile grates should be installed; Catalysts are easily be poisoned and deactivated.
1. Carbon loss in the ashes; 2. Dragging of dust and ashes; 3. Relatively low process temperature is required to avoid phenomena of de fluidization of the bed; Restrictions on the size; 4. High costs of investment and maintenance; 5. Technology complex and difficult to control, etc. 1. Abundant oxidants are required; 2. Small size and preparation of biomass are required; 3. Heat recovery is required to improve efficiency; 4. Low cold gas efficiency; 5. Short life of system components and high costs of investment and maintenance, etc.
Entrained 1. Materials flexibility and uniform react temperature during the flow gasification; 2. High carbon conversion and low tar content; 3. Short reactor residence time; easy to control the parameters process; 4.High-temperature slagging operation; 5.Possibility for large-scale utilization, etc. Rotary 1. Excellent capacity of handling biomass with different properties, 1. Significant hard to control the starting temperature and the kiln flexible loading; Suitable for waste easily be melted; Possibility for leakage and wear of movement part; large-scale use; 2. High of refractory consumption, dust and tar content; Low 2. Simplicity of construction and high reliability of operation; capacity of heat exchange; 3. Low investment costs, etc. 3. Low efficiency of heat; Limited flexibility process; 4. High costs of maintenance, etc. Plasma 1. Production of vitrified non-leachable slag including heavy metals; 1. Presence of nanoparticles in the syngas, the maintenance of movement part and heat shocking for the start-up and reactor 2. Recovery of leachable waste; shout-down; 3. Clean H2-rich syngas production; 2. Consumption of refractory and electrodes; 4. Short reaction times; 3. Removal of molten material in the ducts is required; safety 5. Possibility for large-scale use, etc. problems, etc.
gaseous products because of the short contact with catalyst [42,43]. Steam could increase the H2 production via water-gas shift reaction (WGSR) and methane reforming reaction at high temperature (above 750 C). Temperature is also the most important influence for gaseous yield and composition. The high gas yield and tar conversion could be obtained from gasification at high temperature. Steam reforming of tar and WGSR are the main reason for H2 production and CH4 conversion at the relatively low temperature of 750e800 C. With the temperature increased from 850 to 900 C, CO content will be improved from steam reforming and the Boudouard reactions. Moreover, high temperature also in favor of tar reforming and destruction [44e47].
obtain more useful products. The main criteria and roles for syngas production by using tar cracking catalyst were presented as follows: Main criteria for gasification catalyst design: (1). (2). (3). (4).
Highly effective and active for tar removal; Highly capable for CH4 reforming; Providing a suitable syngas ratio for next utilization; Easily regenerated and anti-coke activity for spent catalyst; (5). Easily obtained and inexpensive for catalyst development. Catalyst roles in biomass gasification:
Catalysts Although various parameters were adjusted to reduce tar production during biomass gasification, the tar amount in the gaseous products is still difficult to realize the industrial application. From middle of 1980s, the interest of many researchers started focus on the biomass catalytic reduction (see Fig. 2). Previous literatures screened some tar-cracked catalysts for biomass gasification. To conclude, the catalyst mainly plays three roles on biomass gasification, i.e., reduce the activation energy, usage of gasification medium, and
(1). (2). (3). (4).
Decreasing the activation energy of reforming reaction; Reducing the heat consumption; Reducing the input of gasification medium; Achieving directional production of H2-rich syngas for value-added chemicals via Fischer-Tropsch synthesis.
As shown in Fig. 3, heavy tars and other hydrocarbons can be directionally reformed into value-added gases in the presence of catalyst. Normally, these complex reactions were taken place on the surface of catalyst or support with the
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Fig. 4 e Possible cracked mechanism and reaction steps of complex tar. excellent metal site and support structure. The simplified mechanism for tar catalytic cracking was described in Fig. 4. Firstly, heavy tars and light tars were cracked into light hydrocarbons at high temperature over high active catalyst. Secondly, some hydrocarbons and CH4 dissociated and dehydrogenated on the active metal/support site. Finally, these hydrocarbon fragments were oxidized by OH radicals (from hydroxylation of inner/formed water) and produce the H2-rich syngas, CO2, C1eC4, etc. Heterogeneous catalysts for biomass catalytic gasification or methanation could divided into mineral, alkali metal, single/bimetallic metal, acid/basic catalysts, etc. [48]. Active metal and support design and modification are the key for heterogeneous catalyst preparation. To update, Ni-based catalyst with the cheap price and high activity is widely investigated for gasification [49,50]. Assuredly, some precious catalyst, like Pt, Ru, Ir, and Pd based catalysts were also proved to be effective for tar reforming [51,52]. There have many researchers concentrated on the support design, and they considered special structure could provide an excellent metal dispersion and metal-support interaction [53e55]. Recently, the investigation on the gasification catalyst is gradually focused on the catalyst with high activity, anti-coke, anti-S and lifetime extension.
Ni-based catalyst Nickel-based catalysts, usually loaded on some supports (Al2O3, limonite, olivine, ZrO2, zeolite, TiO2, etc.) were extensively used for syngas production and biomass gasification. Ni-based catalyst also proved has excellent activity for CH4 reforming and WGSR. The coke formation over the catalyst surface is the main limitation for Ni-based catalyst during the biomass catalytic gasification process. Therefore, more researches should be focused on the stability and anti-coke activity of Ni-based catalyst. Previously researcheres mentioned low-rank coal have abundant porous structures and oxygen-containing species, which could provide an excellent carrier for Ni-based catalyst preparation [56,57]. Previous work [58] employed sewage sludge to perform gasification experiment over Ni/lignite char prepared by ion-exchange method in a two-stage fixed-bed reactor and investigated the nitrogen distribution and H2 yields under different temperatures and atmospheres in details. Furthermore, our research team compared the activity of commercial Ni/Al2O3 and Ni/lignite char on corncob catalytic gasification [56]. We proved Ni/lignite char showed a higher activity than commercial ones, and syngas yield reached to 43.9 mmoL/g under Ar atmosphere. To investigate the influence of minerals on this kind of catalyst preparation, the
Fig. 5 e Preparation and recycle of biomass/coal char supported catalyst. Please cite this article as: Ren J et al., Methanation of syngas from biomass gasification: An overview, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.023
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Fig. 6 e Heavy tar yield (a), syngas yields and composition (b) over char and char-supported catalyst, and photo of char and char-supported catalyst (c) Redrawn from Ref. [68].
Fig. 7 e SEM images of NieFe/CNF before (a) and after (b) 13 recycle biomass gasification, N2 adsorption isotherm (c) and syngas yield during biomass gasification (d). Redrawn from Ref. [69].
internal and external minerals of lignite was removed by using HCl/HF and HCl and supported Ni catalyst was used for corncob gasification [59]. Interestingly, H2O2 could improve the oxygen containing species then increase the specific space area and reduce the Ni crystallite size of the catalyst, which are most effective for corncob tar reforming. Finally, the Ni/ lignite char was characterized by SEM, XPS, HR-TEM, CO-pulse chemisorption to reveal the layered delocalized structure [60]. To extend the lifetime of Ni/lignite char catalyst, the NieCe/ lignite char catalyst was designed with different Ni/Ce ratio (1e100) to evaluate their activity and stability during corncob tar reforming [61]. It was found that NieCe/lignite char with the Ni/Ce ratio of 50 shows stable gas yield (69.1 vol%), which is higher than commercial Ni/Al2O3 catalyst. To investigate the anti-coke ability of the Ni-based catalyst, Zhang et al. [62]
added Ce promoter on Ni/olivine catalyst and used for toluene reforming. The promoted NieCeeMg/olivine catalyst is more active and stable than NieCe/olivine alone. Furthermore, they claimed the addition of Ce and Mg promoter have a great H2S resistance for 400 min measurement. Except for Ce, La is also a great promoter in high activity catalyst preparation. Garbarino et al. [63] compared the performance of Ni/Al2O3 and NieLa/ Al2O3 by using temperature programmed surface reaction (TPSR) for the phenol and ethanol reforming. They mentioned metal La could reduce the Al2O3 acidity and promote the NieLa interactions by the characterization of X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS), ultra-violet-visibleand near infra-red (UVeviseNIR), etc. Biomass/coal char is an economical material to support active metal, which is
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Fig. 8 e Simulated equilibrium structures and distribution densities (along the Z direction) of different NiIr ratio based catalyst (a); Activity and stability of the different NiIr ratio catalysts for tar reforming under steam atmosphere (b); Binding energies (c) and reaction energy (d) for the intermediates during methane dissociation and CO formation on Ir clusters and bulk (111) surface. Redrawn from Ref. [82].
possible to achieve large-scale development of biomass gasification [64e66]. Importantly, inherent energy of charsupported catalysts could easily be recovered by combustion or other ways after deactivation. As Fig. 5 shows, biomass/coal char material can not only used for high activity and dispersion catalyst preparation, but also used for value-added “green recycle”. Wang et al. [67] mechanically mixed NiO and char particles and used for sawdust tar reforming in a lab-scale fixed-bed gasifier. They obtained a high conversion (>97% tar) and great stability (8 h tests) over the catalyst with a NiO loading of 15%
Fig. 9 e Schematic diagram of syngas from biomass catalytic gasification to SNG.
under the residence time of 0.3 s at 800 C. As shown in Fig. 6, Shen et al. [68] used rice husk char supported NieFe for rice husk gasification and obtained a high tar conversion about 92.3% and 93% before and after calcination under their optimized conditions (pyrolysis temperature: 800 C, feedstock size: 0.125e0.5, biomass weight: 5 g, catalyst weight: 5 g, 1 L/ min N2 as the carrier gas).
Alloy based catalyst Xie et al. [69] prepared NieFe/carbon nanofibers (NieFe/CNF) catalyst and used for wood chips catalytic gasification. As Fig. 7 shows, they found alloy formation in NieFe/CNF catalyst showed a high activity (85.76% tar removal efficiency and 0.947 L/g syngas yield) and anti-coke stability, which can effectively reformed tar compared with non-catalytic test. More mesopores were created when carbon nanofibers were calcined, which could adsorb and reduce macromolecular tar components. Wang et al. [70] proved NieFe alloys could prevent carbon deposition during the cracking of cedar wood tar. They prepared NieFe/Al2O3, Ni/Al2O3 and Fe/Al2O3 catalyst, among which NieFe/Al2O3 was proved has a relatively high activity. Li et al. [71] prepared NieFe/Mg/Al catalysts by the calcination and reduction of NieFeeMgeAl hydrotalcite-like compounds and found that NieFe/Mg/Al with the Ni/Fe ¼ 0.5 was homogeneously distributed on the carrier, and NieFe/Mg/Al with Fe/Ni ¼ 0.25 exhibited the best performance for the cedar wood tar steam reforming. They claimed that the synergy between Ni and Fe nanoparticles were the main reason for the high tar cracked activity. Moreover, the NieFe/Mg/Al catalyst
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Fig. 10 e Calculated equilibrium constants of 8 reactions (a), gas compositions for CO (b) and CO2 (c) during the methanation process (0.1 MPa). Redrawn from Ref. [92].
showed better anti-coke and oxidation-reduction performance than conventional NieFe/Al2O3 for the reason of Mg(Ni, Fe, Al)O formation and reduction. Optional noble metal doping could prevent Ni sintering and increase catalyst activity. Nishikawa et al. [72] employed co-precipitation, precipitation and impregnation method to introduce noble metal Pt, Rh and Pd to Ni-based CeOeAl2O3 and used for cedar wood gasification. They found that Pt and Fe doped Ni/CeOeAl2O3 catalyst exhibited high tar conversion. Moreover, they mentioned that the addition of Pt on Ni/ CeO2/Al2O3 exhibited the best performance because of activation by the tar during the gasification. Noble metal Pt, Ru,
Pd, etc. could enhance the activity of tar reforming, WGSR and anti-coking performance over the catalyst surface. Oh et al. [73] added Ru and Mn to commercial and self-design Nibased catalysts and investigated their performance on toluene steam reforming at 400e800 C. They observed the high stability and toluene conversion occurred above 600 C over Ni/RueMn/Al2O3 catalyst. Ni/RueMn/Al2O3 catalyst with the Mn loading of 2.6 wt% shown the maximum toluene conversion (100%) at 600 C. Especially, they provided the pathways of gaseous products formation according to the experiment data and a series of characterization under steam atmosphere.
Fig. 11 e Possible pathways for CO2 methanation over zeolites (a); Evolution of the relative concentrations of different gas component during TPSR over 14% NiUSY zeolite (b); In situ FTIR spectra over 14% NiUSY under CO2 methanation conditions (c), Redrawn from Ref. [104]. Please cite this article as: Ren J et al., Methanation of syngas from biomass gasification: An overview, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.023
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Other catalysts As mentioned above, Ni-based catalysts, precious metal catalysts, and alloy catalysts have certain activity for biomass catalytic gasification. Recently, there are some researchers developed various novelty catalysts, which could overcome the disadvantages of conventional catalysts, and show relatively high tar conversion. Mineral catalyst, including some metal oxides (Al2O3, CaO, Fe2O3, etc.), are commonly used for biomass catalytic gasification. Berrueco et al. [74] employed forest residues and dolomite to investigate spruce and forest residues gasification behavior. They observed this kind of catalyst enhanced syngas production above 850 C. Delgado et al. [75] prepared calcite and magnetite with different particle size (1e4 nm) for pine sawdust tar removal at 780e910 C. They employed kinetic model and calculated the apparent energies according to the reaction results. The as-prepared calcined dolomite with apparent energies of 42e47 kJ/mol showed the best performance for sawdust tar reforming. Zhao et al. [76] investigated corncob volatiles reforming over calcined limonite, reduced and supported Ni limonite at 400e650 C, and found that the limonite structure changed from FeOOH to Fe2O3 when the temperature was increased. Alkali metals, classified as group IA, are the primary component in biomass, which could catalyze itself to improve the production of gaseous products. Alkali metals are usually added into reactor directly with the biomass fuels. However, alkali metals are difficult to recovery after the tar reforming. Alternatively, alkali metals could also be introduced into supported catalyst to realize coal or biomass catalytic gasification [77]. Lu et al. [78] developed CeO2 and n (CeZr)xO2 and compared the activity for syngas production from wood sawdust, rice straw, rice shell, wheat stalk, etc. gasification. They also employed Pd/C and Ru/C catalyst to study the lifetime and stability, and discussed the influence on biomass size and types, reactor types, and heating rate systematically. Tomishige et al. [79] designed the catalyst of metal Rh, Pd, Pt, Ru and Ni loaded on CeO2/SiO2 and investigated the catalytic activity for cellulose gasification. They claimed that Rh and Pd based catalyst were more active than Ni and Pt catalyst at 550 C during the process of gasification. The tar conversion increased from 88% to 97% when Rh/CeO2/SiO2 was used for gasification at 600 C, and it showed a high anti-coke ability during tar reforming. Asadullah et al. [80,81] compared the influence of the support (CeO2/ZrO2, CeO2/SiO2 and CeO2/Al2O3) on Ru based catalyst and investigated the wood catalytic gasification. They claimed the Ru-based catalyst is easily deactivated when SiO2 was chosen as the carrier, and they reported that CeO2/SiO2 is an excellent support for H2-rich gas production. Dagle et al. [82] employed MgAl2O4 as carrier to load Ni, Rh, Ir, Ru, Pt, Pd, Ni, and investigated the hydrocarbons steam reforming. They designed and controlled Ni/Ir ratio of bimetallic IrNi based catalyst to obtain a stable benzene and naphthalene reforming catalysts. Dagle et al. [82] employed scanning transmission electron microscopy with high-angle annular dark-field mode (STEM-HAADF) and energy dispersive X-ray spectrometry (EDS) technology to reveal the presence of individual Ir atoms. They proved that Ir clusters loaded on Ni particles have a high anti-coke and anti-sintering ability through theoretical calculations as Fig. 8 shows.
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Keller et al. [83] compared the activity of La/ZrO2, Sr/ZrO2, Fe/ZrO2 and LaSrFe catalyst for tar model compounds (benzene and ethylene) reforming. The result showed that the addition of Sr in the La and Fe with the LaSrFe ratio of 0.8/0.2/1 exhibited a significant high conversion during benzene reforming. However, it is not stable for the reason of La2Zr2O7 and SrZrO3 perovskite formation, which would interact with the ZrO2 support and reduce the activity of La0$8Sr0$2Fe based catalyst. Recently, more researches focus on the modification and active metal loading of zeolites to realize biomass catalytic gasification, which have complex and variable 3dimensional network structure[84,85]. After reviewed above, active metal lost and oxidation, coke formation and catalyst sintering, make the gasification catalyst easy to inactivate during the long time testing of real tar or model compounds [86,87]. Furthermore, the changes of support structure are another reason for catalyst deactivation. Finally, fly ash, S poisoning, and unstable feed rate are also the main consideration for catalyst design in future large-scale utilization [38,88]. Here, we concluded some focus for future researchers and industrial users.
Fig. 12 e Normally used (Marked in Green) methanation catalyst (Excerpt from the periodic system of elements). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) (1). Ru is known to be the most active metal for CO, CO2, or mixtures methanation, and the activity is reported 120 times higher than Ni-based catalyst [81]. However, it is not possible for large-scale methane production because of the expensive investment [115]. (2). Ni is the most selective for methanation catalyst preparation due to the high activity and low price [116,117]. There it is the most promising active metal for commercial applications. (3). Co exhibits methanation activity similar to Ni. However, they are not that widely used in large-scale utilization for the reason of expensive price. (4). Fe is proved to be the high reactivity catalyst, but the methane selectivity is low in methanation reaction. Normally, Fe-based catalysts are usually used in ammonia synthesis or Fischer-Tropsch process. (5). Mo shows the low methanation activity compared with Fe, Co, Ni-based catalyst. Mo-based catalyst could exhibit high selectivity of C2þ hydrocarbons. In addition, Mo-based catalyst could exhibit highest S-resistance than other active metal-based catalyst.
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Fig. 13 e (a) The specific side view for depicting the orbitals with electronic densities in S vacancy, (b) The density of states of S-vacancy, and (c) The energy change on the activated (001) surface when S-vacancy formation. Redrawn from Ref. [123].
(1). Choosing and designing a suitable active metal site over the specific structure carrier, and decrease the metal (Ni, Ru, Pt, Pd, etc.) loading to obtain the single-atom catalyst, which could reduce the resource consumption and realize the biomass gasification in the lab scale. (2). Developing the recyclable catalyst, like carbon-based catalyst, which is the cheapest catalyst and could be used for power generation after deactivation and poisoning. (3). Understanding of crack mechanism of biomass tar. At beginning, selecting some tar model compounds as the tar model to study the reaction pathways, and then explore the actual tar cracking mechanism, which could provide a useful guidance for gasification catalyst design. (4). Adding and optimizing the accessories of the reactor. To realize large-scale gasification, the impurity and water of the received feedstock should be well cleaned.
Syngas methanation Until now, synthetic natural gas (SNG) production from biomass catalytic gasification is considered as a possible
method because of its low costs for CH4 production. However, the produced gas from gasification should be treated as follows: tar removal, water removal, gas conditioning and cleaning before syngas methanation. Except for H2-rich syngas, the synthetic gaseous products also contains a few H2S or HCl components. Cyclone and ceramic cartridge filters are the main system for ash and other particles removal [89,90]. Organic scrubbers or water could be used for tar removal at low/high temperature. Steam could be collected by water scrubber then poured into methanation reactor, which has the ability for preventing coke during gasification and methanation process. Interestingly, it is not required that tar removal before SNG process if the methanation temperature is higher than tar condensation temperature. However, HCl and H2S must be removed to prevent catalyst deactivation during the syngas methanation. Normally, HCl could be removed by Na2CO3$NaHCO3$2H2O at around 400 C and H2S removal could occur at a ZnO-fixed bed for further methanation process [91]. Fig. 9 shows the whole combination concept for methanation of syngas from biomass gasification. Before methanation, the H2S produced from gasification should be removed to prevent catalyst S-poisoning. Above all, a high activity catalyst with excellent anti-coke and anti-S is the urgent issues for future discussion.
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Fig. 14 e CO conversion and CH4 selectivity for ZSM-5 catalyst (a, b) and FSZSM-5 catalyst (c, d) Ref.[129].
Fig. 15 e Pilot-scale biomass gasification (Volkswagen Audi) and syngas methanation (PHG Energy).
Table 4 e Global syngas methanation projects at large-scale [130]. Project name GAYA (Engie) € teborg Energi) GOBIGAS (Go BioSNG (EU project) Great Plains Synfuels Plant (Dakota Gasification Company) DemoSNG (EU project) CPI project (CPI Xingjiang Energy Co.) Keqi project (Datang) Fuxin project (Datang) Huineng project (Huineng Coal Electricity Group) Xinwen project (Xinwen Mining Group) Qinghua project (Qinghua Group) POSCO Project (POSCO)
Location
Capacity
Methanation
Material
Saint Fons (France) Gothenburg (Sweden) Gu¨ssing (Austria) Beulah (North Dakota, USA)
400 kW SNG output 20 MW SNG output 1 MW SNG output 1500 MW fuel input
n.s. TREMP PSI Lurgi methanation
Biomass Biomass Biomass Coal
€ ping (Sweden) Ko Yili City (China) Chifeng (China) Fuxin (China) Ordos (China) Yili City (China) Yili City (China) Gwangyang (South Korea)
50 kW SNG output 6 billion m3 a1 SNG output 4 billion m3 a1 SNG output 4 billion m3 a1 SNG output 1.6 billion m3 a1 SNG output 4 billion m3 a1 SNG output 5.5 billion m3 a1 SNG output 0.7 billion m3 a1 SNG output
KIT (honeycomb) TREMP HICOM HICOM TREMP HICOM TREMP TREMP
Biomass Coal Coal Coal Coal Coal Coal Coal
Note: n.s.: not specified.
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Fig. 16 e Diagram of direct melting system for large-scale biomass gasification. Adapted from Ref. [134].
The methanation reaction is a reversible exothermic reaction, and it is important to design catalysts to reach the maximum conversions at around 350 C. As Fig. 10 shows, the calculated equilibrium of different H2/CO ratio and temperature have the great influence on syngas methanation. Nibased and other noble metal-based catalysts (Ru [93], Rh [94], Pt [95], etc.) were proved to be active for CO2 methanation, and the common supports included SiO2 [96,97], hydrotalcites [98,99], ZrO2 [100,101], TiO2 [102,103], zeolites [104], etc. Main reactions (Eqs. 3, 4, 5 and 6): COþ3H2 #CH4 þ H2 O DH298K ¼ 206 kJ=molCO methanation (3) 2CO þ 2H2 #CH4 þ CO2
DH298K
¼ 247 kJ=mol Reverse dry reforming
(4)
CO2 þ H2 #CO þ H2 OD H298K ¼ 41kJ=mol Reversewatergas shift (5) CO2 þ 4H2 #CH4 þ 2H2 O DH298K ¼ 165 kJ=mol CO2 methanation Side reactions (Eqs. 7, 8, 9 and 10):
(6)
2CO#C þ CO2
DH298K ¼ 172 kJ=mol Boudouard reaction (7)
CO þ H2 #C þ H2 O
DH298K ¼ 131 kJ=mol CO reduction
(8)
CO2 þ 2H2 #C þ 2H2 O DH298K ¼ 90 kJ=mol CO2 reduction (9) CH4 #C þ 2H2
DH298K ¼ 75 kJ=mol Methane pyrolysis (10)
Ni/Al2O3 is an excellent catalyst for syngas and CH4 production, which shows a high activity during biomass gasification and syngas methanation. However, easy carbon deposition or poor stability on syngas catalytic methanation are the disadvantage of Ni/Al2O3 [105,106]. Therefore, developing a catalyst with high activity and anti-coke ability is a crucial point. Zhao et al. [107] designed Ni/Al2O3 catalysts with Ni loading of 10e50 wt% and used for syngas methanation. The activity of Ni/Al2O3 were sensitive to Ni particle size and the productivity reached the maximum Ni crystals around 41.8 nm by the characterization of XRD, H2-temperature-programmed reduction (H2-TPR), Scanning electron microscope
Please cite this article as: Ren J et al., Methanation of syngas from biomass gasification: An overview, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.023
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(SEM) and Transmission electron microscope (TEM), etc. Furthermore, because metal Ni was separated by support, Ni/ Al2O3 catalysts could exhibit great stability when the Ni loading was above 20 wt%. Hu et al. [108] systematically investigated some operate parameters (H2/CO ratio, reaction pressure, NiO and MgO loading, SV, etc.) during syngas methanation over Ni/Al2O3. The addition of MgO (2 wt%) could reduce the carbon deposition and improve the catalyst stability. 400 C could promote the formation of NiO and reducible b-NiO of the catalyst with Ni loading of 20 wt%. They obtained the high CO conversion (100%) and CH4 selectivity at 3.0 MPa, 20e40 wt% NiO and 2e4 wt% MgO supported on Al2O3 at 300e550 C. These results are helpful for developing and optimizing Ni/Al2O3 catalysts for syngas methanation. To improve the activity of Ni/Al2O3, some promoters were added to modify the structural or electronic properties. For example, as an electronic and structural promoter, Ce could improve the activity of Ni-based catalysts for many catalytic reactions [109,110], due to the transformation of Ce3þ to Ce4þ during the gasification and methanation [111]. In addition, CeO2 could improve the thermal stability of Al2O3, and promote the dispersion of active metal on the support and modify the Ni/Al2O3 properties because of strong metal-support interaction [94,112,113]. Westermann et al. [104] employed USY zeolites to load Ni (Ni loading of 5e14 wt%), and they investigated the pathways involved in CO2 methanation by operando IR spectroscopy, insitu FTIR and TPD experiments. As Fig. 11 shows, the CH4 formation was started from dissociated hydrogen reacts with carbonates and/or physisorbed CO2, and then monodentate and carbonyls formed on Ni0 particles. n 3þ )x/ Hydrotalcite-like compounds ([M2þ 1-xMx (OH)2](A $yH O), also named layered double hydroxides, which are 2 n noteworthy for designing CO2 methanation catalyst. M2þ and M3þ are any active or not active metal cations, and An is an exchangeable interlayer anion. Wierzbicki et al. [99] designed various metals, such as Ni, Al, Mg and La, supported on hydrotalcite catalyst by the method of thermal decomposition. They determined periclase-like materials formation, and the La addition promoted the catalytic activity (46.5e75%) and CH4 selectivity (99-98%) for CO2 methanation at 250e300 C. For the reason of basicity improvement, the activity was increased when La dopped into the hydrotalcite-derived catalysts. Marocco et al. [114] also prepared Ni/Al hydrotalcite derived catalyst and characterized by atomic adsorption spectroscopy, XRD, H2-TPR, CO chemisorption etc. They performed methanation experiments at different H2/CO2 ratios, temperatures, and the flow rates. In particular, they developed different kinetic rates from the reaction results, performed nonlinear regression analysis to find kinetic parameter values between measured and calculated values and explained the formation of CO by the reverse WGSR. Except for commercial Ni-based catalyst, there are also other highly active catalysts proved to be active for methanation. As shown in Fig. 12, many metals mainly focus on the groups of VI B to I B, which were found to be highly active for methanation. Metal Mo was also investigated for CO2 methanation. Saito and Anderson [118] compared the activity of some Mo compounds (Mo, Mo2C, Mo3N, MoS2, MoO2, and MoO3) for CO2
15
methanation at 350 C, and found Mo could improve hydrocarbons production and catalyze the WGSR. They indicated the activity order of these catalysts as follows: Mo2C > Mo > Mo3N > MoO2 > MoS2, > MoO3. Yao et al. [119] claimed that Mo could prevent the carbon deposition due to the highly efficient hydrogenation of the coke. Wang et al. [120] reported MoP/Al2O3 for S-resistant methanation at 550e650 C. It indicated that MoP catalyst could show the high activity under the atmosphere with high H2/CO ratio and low H2S content. Liu et al. [121] reported the preparation of sulfur-resistant catalyst and used for CO methanation. They employed ammonium heptamolybdate as the precursor for Mo-based catalyst preparation, and they considered that the weight ratio of S/ammonium heptamolybdate and S atmosphere play an important role in the structure and morphology of the catalyst. The catalyst treated under N2 atmosphere existed more weakly-bonded S species on the surface of catalyst and more S vacancies on the edge planes of MoS2 particles, which exhibited a high activity for methanation (88.2% of CO methanation). Zhang et al. [122] employed DFT simulations to elaborate methanation mechanism over the cleaved (001) surface of the MoS2 catalyst. They demonstrated the possibility of CO and H2 adsorbtion and reaction. The electronic orbitals and densities for depicting S vacancies were presented in Fig. 13a. Three of the exposed Mo atoms were activated from the blue field (conduction states in orbitals). As reported [123], the formation of S vacancy could introduce more bands to the gap around 0 eV (fermi level) when S-vacancy formed. For the MoS2 catalyst, the active sites inosculate with the new bands from the high density of d-states in Fig. 13b. The formation mechanism of the S-vacancy is presented in Fig. 13c, and the formed Svacancy stability is also confirmed. MoeH and SeH (P1) will be formed from the heterolytic fission of the activated H2. For the reason of the MoeH and SeH are the electron acceptor and donor, respectively, the exposed Mo atoms would produce unoccupied adsorption orbitals when S atom was removed. The process is exothermic, and the reaction heat and the activation energy are 0.48 eV and 0.85 eV, respectively.
Integration and challenge on the syngas methanation and biomass gasification Syngas and methane production from biomass gasification and methanation are promising alternatives of value-added chemicals generation under catalytic conditions [124]. It also provides an opportunity to reduce the green house by CO2 capture and hydrogenation. Compared to other synthetic transportation fuels (dimethyl ether, ethanol, etc.), methane uses the existing natural gas infrastructure, could really realize a smooth transition from fossil to synthetic energy [125]. The advantages of methane are the high conversion efficiency, and some basic infrastructures are already distributed over the world. For this reason, car, heating, power stations were found and used in our daily life, which utilize the SNG [126]. Solid biomass was converted into methane via gasification and subsequent methanation shown the comparable efficiency (65% energy from wood to methane) [127].
Please cite this article as: Ren J et al., Methanation of syngas from biomass gasification: An overview, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.023
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Duret et al. [127] employed a fast internally circulated fluidized bed to investigate the difference between the reaction temperature and the thermodynamic equilibrium for gasification and methanation. They mentioned only about 7% of the extra energy needs should be imported and the other energies could be obtained from the excess of heat produced in their system. As a result of the methanation is the exothermic reaction, catalyst chosen for increasing CH4 content and decreasing the reaction heat will improve the overall chemical efficiency in the methanation step. Increasing the H2/CO2 or H2/CO ratio from gasification could promote the CH4 production and CO2/ CO selectivity. H2/CO2 or H2/CO ratio varied with the temperature, pressure and catalyst type. He et al. [128] employed MSW to produce the H2-rich gas in a bench-scale downstream fixed bed reactor. They found the best H2/CO2 ratio was 3:1 at the temperature between 700 and 950 C when dolomite was used as catalyst. Hussain et al. [129] found that CH4 mole fraction increased with the increasing of the H2/CO ratio from 1:1e5:1. High H2/ CO ratio could improve the methanation activity and resist carbon deposition (see Fig. 14). Furthermore, as Table 2 presented, N and little amount S (Neglected in Table 2) are existed in the biomass. SOx and NOx are also required to remove because of the activity of catalyst would be poisoned by them. As a consequence, the optimal gas compositions from biomass gasification are important to realize the integration of gasification and methanation. Considering process and reactor design to quantify the overall conversion efficiency are important to improve the integration possibility of biomass gasification and methanation. Consequently, great care has to be taken in ways of valorizing and optimizing the energy conversion efficiency to avoid investing more energy than what is produced during the production process.
Large-scale commercial development To obtain more electricity and effectively treat sewer sludge, PHG Energy of Nashville Tennessee and Covington signed the waste-to-energy gasification contract in 2012. The gasification plant for gasification was presented in Fig. 15. Table 4 listed an overview of methanation projects all over the world at pilot or commercial scale. This system utilized 12 t/d to supply 6 million Btu/h of producer gasas the central technology. This gasifier contained wood chipping and material handling, and it solved around 10 t wood wastes and 2 t sewer sludges every day. PHG Energy company designed the wood pretreating, material handling, material mixing and drying process during biomass gasification. At the 1980s, the first large-scale plant for coal to SNG was built in North Dakota, USA, which named as Great Plains Synfuels Plant (GPSP) during the oil crisis phase. This gasification plant equipped with Lurgi fixed-bed gasifiers and methanation reactors. GPSP produced around 153 mm scf/d of SNG [56 billion scf/y] from 6 million t/y of coal. Another biomass-based SNG project was found by Austria in 2008 with 1 MW SNG production in pilot-scale, which employed wood
chips as the biomass materials [131]. In the last years, some other methanation projects were also build at pilot-scale in France, which named as GAYA project. Climate change and energy shortage are the main reasons for biomass-gas projects development in Europe. There are some other countries started to realize this combination. The first industrial power to gas plant was built at Volkswagen Audi Company in Germany, which transform CO2 to methanation. The project intends to improve the efficiency of a combination of power-to-gas (PtG) and methanation process. They claimed PtG plant equipped electrolyzer and methanation unit, which is promising to produce a variety of heat sources for CO2 separation, fermenter heating and hygienization of wastes. Japan and South Korea already have more than 30 commercial gasification plants by using fluidized-bed type gasifier, which equipped with melting technology and direct melting system [132,133]. As the flow diagram of this technology shows (Fig. 16), the plant of this company was consisted of a melting furnace, gasification furnace, MSW charging system, combustion chamber and gas cleaning system. Furthermore, some subsystems for raw material and fly ash handling system were also designed for excellent operation.
Conclusions Biomass, as a representative REs, is a significant promising and alternative energy for syngas and other chemicals production via biomass gasification and methanation. An indepth discussion throughout this review provided guidance about high efficiency gasification of biomass. Some highperformance gasification catalyst and their catalytic pathways were concluded, and the influence of various parameters (gasifiers, S/C ratio, reaction temperature, etc.) were discussed as a whole. A deep understanding and the knowledge of alternative approaches about gasification and methanation process are required for its optimization and advancements. Especially, the gasification products were employed to realize the syngas methanation, and some operate conditions and catalyst design were also reviewed briefly. Furthermore, the large-scale multi-stage gasification, methanation and their combination plants or small centralized gasification equipment were reviewed as a whole to explain value-added utilization of biomass. The advancements on tar-cracked elimination and new catalyst were mentioned to promote the clean and high-efficiency operation of gasification process. This paper presents a positive future for integration of biomass catalytic gasification and methanation as a promising and economically technology.
Acknowledgments This work was funded by the National Key R&D Program of China (Grant 2017YFE0124200), the National Natural Science Foundation of China (Grants U1710103, 21676292 and 21978317), and the project “Power to Fuel” of JARA Energy from
Please cite this article as: Ren J et al., Methanation of syngas from biomass gasification: An overview, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.023
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German federal and state governments. We are also thankful to China Scholarship Council who provides scholarship for Jie Ren (No. 201806420028) to develop his PhD research at the RWTH Aachen University.
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Please cite this article as: Ren J et al., Methanation of syngas from biomass gasification: An overview, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.023