Current Challenges and Perspectives for Biomass Catalytic Pyrolysis: History
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Subjects: Chemistry, Applied
Contributor:
Wenli Wang
,
Yaxin Gu
,
Chengfen Zhou
,
Changwei Hu

Lignocellulosic biomass is an excellent alternative of fossil source because it is low-cost, plentiful and environmentally friendly, and it can be transformed into biogas, bio-oil and biochar through pyrolysis; thereby, the three types of pyrolytic products can be upgraded or improved to satisfy the standard of biofuel, chemicals and energy materials for industries. The bio-oil derived from direct pyrolysis shows some disadvantages: high contents of oxygenates, water and acids, easy-aging and so forth, which restrict the large-scale application and commercialization of bio-oil. Catalytic pyrolysis favors the refinement of bio-oil through deoxygenation, cracking, decarboxylation, decarbonylation reactions and so on, which could occur on the specified reaction sites. Therefore, the catalytic pyrolysis of lignocellulosic biomass is a promising approach for the production of high quality and renewable biofuels. This review gives information about the factors which might determine the catalytic pyrolysis output, including the properties of biomass, operational parameters of catalytic pyrolysis and different types of pyrolysis equipment. Catalysts used in recent research studies aiming to explore the catalytic pyrolysis conversion of biomass to high quality biooil or chemicals are discussed, and the current challenges and future perspectives for biomass catalytic pyrolysis are highlighted for further comprehension.

  • biomass
  • catalysts
  • pyrolysis
  • biofuel

1. Introduction

The modern industrial development and considerable increase in global population resulted in the substantial growth in worldwide energy consumption. However, the large-scale consumption of fossil resources has brought a series of eco-environmental crises, such as global warming, destruction of the ozone layer, acid rain, land desertification and so forth, which challenged the sustainable development of all walks of the world. These related questions necessitate the development and use of renewable resources as alternatives, such as solar, wind, tidal, geothermal and biomass. Among these choices, biomass, the only renewable carbon resource, has attained a large amount of attention due to its abundant reservation and low cost [1]. Approximately 1700 billion tons of biomass can be harvested each year, which is equivalent to 850 billion tons of standard coal or 600 billion tons of oil [2]. According to the data published by the International Energy Agency, 10% of global basic energy consumption will be supplied by biomass by 2023, and approximately 27% of worldwide transportation fuel will be supplied by biomass by 2050 [3]. Since the development and utilization of biomass have the potential and feasibility in handling sustainable development and environmental problems, effective policies promoting the use of biomass have been issued in many countries. For example, the Dutch Ministry of Economic Affairs requires that approximately 30% of transportation fuel should be derived from biomass and 20–45% of non-renewable resource are replaced by biomass until 2040 [4]. The U.S. Department of Agriculture and U.S. Department of Energy set goals that biofuels must take the place of one-fifth of transportation fuel and one quarter of oil-based platform chemicals by 2030 [2]. Thus, there is an urgent demand to integrate renewable biomass energy into contemporary energy systems for modern society towards a sustainable process.
The conversion methods of biomass consist of direct combustion, liquefaction, fermentation, pyrolysis, gasification and supercritical fluid conversion [5][6]. Among these ways, pyrolysis is one of the most promising methods due to its feasibility in industrialization, low cost in installation and convenience in operation [2][5][7]. Pyrolysis of biomass refers to the process where biomass is converted to bio-oil, biogas and charcoal in an inert atmosphere at high temperatures of approximately 400–600 °C. Among these products, bio-oil has been widely used to produce transportation fuel or platform chemicals through refinery technology, for example, the hydrodeoxygenation process [8]. However, pyrolysis of lignocellulosic biomass is a quite complex process, because a range of parameters contribute to the quality of bio-oil in the meantime, such as biomass species, pyrolysis conditions and so on [9]. The raw bio-oil derived from direct pyrolysis of biomass has several drawbacks: high corrosivity and viscosity, low heating value, high oxygen and water contents [10][11][12], which hinder the direct utilization as transportation fuels. 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 [13][14][15], 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 [16][17]. 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 [18][19][20]. 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.
Catalysts 12 01524 g001 550
Figure 1. The three strategies for biomass pyrolytic conversion.
Numerous studies have focused on the catalysts used for the catalytic conversion of lignocellulosic biomass, for example, metal oxides [21][22][23], inorganic salts [24][25][26], and zeolites [27][28][29]. The addition of catalyst is expected to decrease the reaction activation energy and change the reaction pathway, yielding high-quality bio-oil and high-value platform chemicals.

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

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 might be generated during the pyrolysis of biomass, such as chlorinated hydrocarbons and dioxins.

(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 technique 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 technique 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.

This entry is adapted from the peer-reviewed paper 10.3390/catal12121524

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