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Wang, W.;  Gu, Y.;  Zhou, C.;  Hu, C. Current Challenges and Perspectives for Biomass Catalytic Pyrolysis. Encyclopedia. Available online: https://encyclopedia.pub/entry/38392 (accessed on 20 June 2024).
Wang W,  Gu Y,  Zhou C,  Hu C. Current Challenges and Perspectives for Biomass Catalytic Pyrolysis. Encyclopedia. Available at: https://encyclopedia.pub/entry/38392. Accessed June 20, 2024.
Wang, Wenli, Yaxin Gu, Chengfen Zhou, Changwei Hu. "Current Challenges and Perspectives for Biomass Catalytic Pyrolysis" Encyclopedia, https://encyclopedia.pub/entry/38392 (accessed June 20, 2024).
Wang, W.,  Gu, Y.,  Zhou, C., & Hu, C. (2022, December 09). Current Challenges and Perspectives for Biomass Catalytic Pyrolysis. In Encyclopedia. https://encyclopedia.pub/entry/38392
Wang, Wenli, et al. "Current Challenges and Perspectives for Biomass Catalytic Pyrolysis." Encyclopedia. Web. 09 December, 2022.
Current Challenges and Perspectives for Biomass Catalytic Pyrolysis
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Lignocellulosic biomass is an excellent alternative of fossil source owing to the fact that it is low-cost, plentiful and environmentally friendly. Through pyrolysis, lignocellulosic biomass can be converted into the potential precusor of liquid fuel or platform chemicals. Therefore, the detailed knowledge and mechanism about biomass conversion should be overviewed and concluded. We have reviewed several factors which impact the conversion of biomass, including the properties of biomass, operational parameters of catalytic pyrolysis and different types of pyrolysis equipment. Meantime, some perspectives for future development have been proposed in the end.

biomass catalysts pyrolysis biofuel

1. Introduction

During the past several decades, research studies have mainly focused on three strategies for the optimal harnessing of biomass from pyrolysis, as shown in Figure 1. The first one is the pretreatment of raw biomass. It is reported that the characteristics of biomass can be influenced by different kinds of pretreatment methods [1][2][3], such as grinding, torrefaction, chemical or biological pretreatment. Therefore, the pretreated biomass may yield high quality bio-oil. Secondly, refinement of bio-oil is another choice for further utilization [4][5]. After the liquid products are obtained from direct pyrolysis of biomass, some oriented catalysts can be used to improve the quality. Or some separation and purification approaches can be applied to obtain the single chemicals. Thirdly, catalysts are introduced to the pyrolysis process to change the distribution of chemicals in bio-oil products directly [6][7][8]. Compared to the former two methods, catalytic pyrolysis outstands as the form of one-pot reaction, which simplifies significantly the technological process, especially avoiding the cooling and reheating of the obtained liquid products.

Figure 1. The three strategies for biomass pyrolytic conversion

2. Factors Affecting the Catalytic Pyrolysis of Biomass

There are some key factors for catalytic pyrolysis of biomass,  including biomass species, categories of reactor and catalyst as shown in Figure.6 .

Figure 6. The factors that affecting the pyrolysis of lignocellulosic biomass.

Lignocellulosic biomass: different kinds of biomass feedstocks contain different amounts of cellulose, hemicellulose and lignin, and the interactions between the three major components also varies. In addition, the polymer structure, cross-linkages, density, thermal conductivity, airflow permeability or specific heat capacity of different biomass also have large differences, which may lead to different pyrolysis performance [9].

Diversity of reactor: miscellaneous reactors have been used and explored for catalytic pyrolysis in a small scale, such as fluidized bed, circulating fluidized bed, fixed bed, ablative, rotative, auger, vacuum and microwave reactors. 

Process parameters (temperature, heating rate, and volatiles residence time): Taking these pyrolysis parameters into consideration, low temperature, low heating rate and long VRT are conducive to the formation of biochar; high temperature, high heating rate and long VRT are beneficial for the generation of gas products; and medium temperature, high heating rate and short VRT favor the production of bio-oil with a higher yield.

Position of catalysts: Compared with the ex-situ, the cost of in-situ mode is comparatively low, and the catalysts loaded not only participate in the catalytic process but also act as heat carriers. In addition, it is much more direct and quick for pyrolytic volatiles to undergo the further reaction. However, the coke formation and difficulties in catalyst recovery make the in-situ mode less competitive. Ex-situ is beneficial to control the secondary upgrading process flexibly, including catalyst species, reaction temperature, reaction gas atmosphere and so on. Before the catalytic pyrolysis technique is determined or improved, all aspects, for instance, the cost of device and catalysts, are needed to be taken into consideration.

The type of catalysts: Zeolites,  metal oxide catalysts, soluble inorganic salts, and other low-cost materials.

3. Challenges and Future Perspectives

Catalytic pyrolysis of lignocellulosic biomass is a promising technology for the generation of biofuels, and it is still in the initial stage, which is far from commercialization. Although huge progress in catalyst modification and catalytic pyrolysis has been made, the yield and quality of fuel-grade bio-oil still faces significant challenges. The following should be investigated, in order of priority.
(1) Since lignocellulosic biomass varies in composition and structure in terms of species, it is necessary to develop analytical methods and technology to characterize raw biomass or its single component at the molecular level. Then, these biomass species can be classified into particular categories, which is beneficial to design oriented catalysts, thereby obtaining the high conversion rate of biomass and high-quality biofuels. For example, for the efficient conversion of woody biomass species, one catalytic system might be designed and developed, while for algae, it is needed to develop another one. Furthermore, it should be estimated whether the raw biomass or the isolated component from biomass is more conductive to catalytic pyrolysis for desirable products, achieving the effective conversion and utilization of biomass to the greatest extent.
(2) How to fully exert the role of solid catalysts to solid biomass is another tough task. When the solid catalyst is mixed with biomass evenly, the pyrolytic volatiles can arrive at the active sites on catalyst quickly for the following secondary reactions towards aromatic hydrocarbons or deoxygenated compounds. However, the solid catalysts only have an impact on the secondary reactions, while the initial thermal decomposition stage of biomass is not affected. The introduction of soluble salts might be a promising approach, which can be mixed evenly with biomass and attached to the biomass surface, thereby changing the intrinsic structure and boosting the thermal conversion rate of biomass. Nevertheless, soluble salts as catalysts are accompanied with the introduction of anions, such as Cl, SO42−, NO3, CO32− and other organic acid ions. These anions pose a threat on the safety and stability of bio-products. For example, it can be difficult to remove the SxOy, NxOy or other undesirable gas products from pyrolytic gas, and some toxic chemicals, such as chlorinated hydrocarbons and dioxins, might be generated during the pyrolysis of biomass.
(3) The accurate analytical technology for the reaction intermediates in pyrolysis needs to be urgently developed. These intermediates could not be identified clearly and the information about the real catalytic pyrolysis process is still scarce. Therefore, the development of analytical technology that holds the ability to qualify and quantify the large-molecular weight fragments in-situ is quite promising for the overall comprehension of the reaction mechanism. Only when the structure and properties of pyrolytic volatiles or reaction intermediates are understood clearly, could the oriented catalysts be conceived and developed. Furthermore, the comprehensive analysis and characterization of bio-oil derived from catalytic pyrolysis is of great importance. Precisely figuring out the chemical composition of the liquid phase is the prerequisite of further upgrading and utilization.
(4) The stability and reutilization of catalyst is another significant issue for catalytic pyrolysis technique feasibility and economy, and it is vital to develop the ability of coke resistance. To achieve this goal, the first step is to clarify the detailed catalytic conversion mechanism of pyrolysis vapor that occurred on the active site of the surface or channels of catalysts. On the other hand, the design of pyrolysis reactor that allows for the in-situ regeneration of catalysts may help reduce the capital invested in the technical routes of biomass pyrolysis.
(5) The cost or investment of catalyst determines the overall economics of the biomass pyrolysis process, so it is necessary to exert techno-economic analysis and life cycle analysis of the selected catalyst. Furthermore, whether the corresponding technical route is suitable for the environment, or allowed with national policies, should be taken into account first before scaling up.
(6) Industrialization of biomass catalytic pyrolysis technology still faces many challenges. Firstly, the mixing of catalysts and biomass particles is tough to be guaranteed in large scale, which makes catalysts invalid; at the same time, the recovery of the used catalysts could not be easily realized, which increases the cost. Secondly, the heat and mass transfer become significantly difficult owing to the properties of lignocellulosic biomass, so large amounts of energy need to be supplied to the large-scale pyrolysis device. Last but not least, the diversity of biomass species and wide distribution of biomass both make the biomass pyrolysis industry tough to develop towards a large scale.

References

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  2. Chen, Z.; Wang, M.; Jiang, E.; Wang, D.; Zhang, K.; Ren, Y.; Jiang, Y. Pyrolysis of Torrefied Biomass. Trends Biotechnol. 2018, 36, 1287–1298.
  3. Chen, D.; Mei, J.; Li, H.; Li, Y.; Lu, M.; Ma, T.; Ma, Z. Combined pretreatment with torrefaction and washing using torrefaction liquid products to yield upgraded biomass and pyrolysis products. Bioresour. Technol. 2017, 228, 62–68.
  4. Han, Y.; Gholizadeh, M.; Tran, C.-C.; Kaliaguine, S.; Li, C.-Z.; Olarte, M.; Garcia-Perez, M. Hydrotreatment of pyrolysis bio-oil: A review. Fuel Process. Technol. 2019, 195, 106140.
  5. Tabassum, N.; Pothu, R.; Pattnaik, A.; Boddula, R.; Balla, P.; Gundeboyina, R.; Challa, P.; Rajesh, R.; Perugopu, V.; Mameda, N.; et al. Heterogeneous Catalysts for Conversion of Biodiesel-Waste Glycerol into High-Added-Value Chemicals. Catalysts 2022, 12, 767.
  6. Wang, Y.; Akbarzadeh, A.; Chong, L.; Du, J.; Tahir, N.; Awasthi, M.K. Catalytic pyrolysis of lignocellulosic biomass for bio-oil production: A review. Chemosphere 2022, 297, 134181.
  7. Qiu, B.; Tao, X.; Wang, J.; Liu, Y.; Li, S.; Chu, H. Research progress in the preparation of high-quality liquid fuels and chemicals by catalytic pyrolysis of biomass: A review. Energy Convers. Manag. 2022, 261, 115647.
  8. Liu, R.; Sarker, M.; Rahman, M.M.; Li, C.; Chai, M.; Nishu; Cotillon, R.; Scott, N.R. Multi-scale complexities of solid acid catalysts in the catalytic fast pyrolysis of biomass for bio-oil production—A review. Prog. Energy Combust. Sci. 2020, 80, 100852.
  9. Pasangulapati, V.; Ramachandriya, K.D.; Kumar, A.; Wilkins, M.R.; Jones, C.L.; Huhnke, R.L. Effects of cellulose, hemicellulose and lignin on thermochemical conversion characteristics of the selected biomass. Bioresour. Technol. 2012, 114, 663–669.
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