Lignocellulosic biomass pyrolysis mechanism: A state-of-the-art review
Introduction
Consumption of fossil fuels worldwide has increased tremendously in last few decades, which leads to several environmental issues, including greenhouse gas emissions and deteriorating air quality caused by pollutants such as SOx, NOx and fine particulate matter. Moreover, the fluctuation of fossil fuel prices and the depletion of fossil resources have shadowed the global economy. The production of carbon-neutral and low-emission fuels from renewable resources, such as biomass, is of growing importance in the gradual substitution of conventional fossils. Biomass is biological material from living, or recently living organisms produced directly or indirectly by photosynthesis, most often plants or plant-derived materials [1], [2], [3]. Biomass resources are widely available in nature. It is estimated that the global biomass production is approximately 100 billion tons per year [4]. As the only renewable carbonaceous resource, biomass has the potential to produce heat, electricity, fuel, chemicals, and other products [5], [6]. The International Energy Agency (IEA) suggests that bioenergy has the potential to provide 10% of the world's primary energy supply by 2035, and biofuels can replace up to 27% of world transportation fuel by 2050 [7].
Biomass can be converted into fuels and chemicals through biochemical or thermochemical processes. Digestion (anaerobic and aerobic) and fermentation are typical biochemical processes used to produce methane and alcohols [8], [9]. The main thermochemical processes include pyrolysis, gasification, combustion, hydrothermal liquefaction and hydrothermal carbonization [10], [11]. Among these thermochemical pathways, pyrolysis, the thermal decomposition of organics in the absence of oxygen, has been extensively developed as a promising platform to produce fuels and chemicals from various types of biomass. Pyrolysis produces char, liquid and gas products, the distribution of which strongly depends on the reaction conditions. Fast pyrolysis of biomass at rapid heating rates and short hot vapor residence times (< 1 s) produces liquid with yield up to 75 wt.% [12], [13]. The pyrolysis liquid, which is normally called bio-oil, can be further upgraded to transportation fuels and value-added chemicals. The char and gaseous products can be combusted to provide energy for the pyrolysis reaction or heat/power generation [12]. Many potential agricultural and environmental applications of char are also being explored to enhance the value chain of the pyrolysis process [12], [14]. Techno-economic analysis showed that transportation fuel production from biomass via pyrolysis-based pathways had economic advantages over other conversion pathways, such as gasification and biochemical pathways [15], [16], [17], [18]. Due to the huge demand for liquid transport fuels, biomass pyrolysis technology will attract increasing interest from both academia and industry [19].
Biomass slow pyrolysis, particularly wood carbonization and distillation, has been used by humans for more than a thousand years. Nevertheless, the pioneering studies on biomass pyrolysis were initiated in 19th century [20]. The effect of the reaction conditions on the yields of the solid, liquid and gas pyrolysis products was first reported in 1875 by Gruner [21]. However, little progress was made until the 1980s. During this stage, the kinetics of biomass/cellulose pyrolysis received significant attention. In 1956, Stamm [22] reported the kinetics of wood and cellulose thermal degradation and suggested that thermal degradation followed a first-order reaction model. In 1970, Roberts [23] reviewed the pyrolysis kinetics of biomass and related substances. The results showed that pyrolysis may proceed through different reaction routes. In 1979, the classic Broido–Shafizadeh (B‒S) model for cellulose pyrolysis based on the formation of char, volatiles and gas via different pathways was proposed [24]. These findings provide some fundamentals for further development of biomass pyrolysis technology.
Since the 1980s, a growing body of research has focused on biomass fast pyrolysis for liquid production due to the oil crisis. A number of research units in Europe and America conducted biomass fast pyrolysis in the period 1980–2000 [25], [26], [27], focusing on the development of various reactors for biomass fast pyrolysis. Those reactor technologies include the bubbling fluidized bed, circulating fluidized bed, transported bed, ablative reactor and rotating cone reactor [26], [28]. Meanwhile, the effect of the reaction conditions was investigated to optimize the fast pyrolysis process and to maximize the bio-oil yield. Modified kinetic reaction models were also proposed in the same period [29], [30], [31], [32], [33], [34], [35]. Additionally, possible formation mechanisms of typical pyrolysis products were proposed [36], [37], [38], [39], [40], [41].
Biomass pyrolysis has received considerably more attention from the academic field in the past decade. The number of scientific journal papers published per year on this topic increased from less than 200 in 2005 to more than 1800 in 2016, based on the statistical result from Web of Science. The research is currently focused on revealing the reaction mechanisms of pyrolysis and the development of novel pyrolysis-based pathways to produce transportation fuels. Advanced experimental technologies, such as spatiotemporally resolved diffuse reflectance in situ spectroscopy of particles (STR-DRiSP) [42], electron paramagnetic resonance (EPR) [43], and photoionization mass spectrometry (PIMS) [44], have emerged and have been used to detect reaction intermediates during pyrolysis, improving the understanding of biomass pyrolysis chemistry. Additionally, advanced kinetic methods (the detailed lumped kinetic model [45], chemical percolation devolatilization model [46] and kinetic Monte Carlo model [47]) and molecular simulation methods (density functional theory [48] and Car–Parrinello molecular dynamics [49]) were developed to elucidate the biomass pyrolysis mechanism. During the past decade, various technologies, such as catalytic pyrolysis, have also been developed to improve the quality of bio-oil, making it more compatible with the current petroleum industry [50], [51]. Pretreatment of biomass feedstock to remove or modify the undesired functional groups and/or structures in the biomass matrix has been explored as a way to improve the conversion efficiency. Considerable pretreatment achievements have been reported, such as torrefaction [52], [53], steam explosion [54], [55], acid/alkali pretreatment [56] and biological pretreatment [57].
Many review papers in the field of biomass pyrolysis have been published, focusing on only one aspect of biomass pyrolysis, such as the reaction process [58], [59], [60], reaction mechanism [61], [62], [63], pyrolysis kinetics [64], [65], [66], [67], catalytic pyrolysis [68], [69], [70], [71] and effects of pretreatment [72], [73]. This work provides a broad review of the state-of-the-art advances in multiple aspects of biomass pyrolysis research and development. The complexity of the biomass structure is one of the challenges in elucidating biomass pyrolysis behavior. Therefore, the biomass structure and the correlated pyrolysis behavior are reviewed in Section 2. To gain deep insight into the biomass pyrolysis behavior, some advanced experimental technologies have been developed and are covered in Section 3. In Sections 4 and 5, macroscopic kinetic modeling and microscopic pyrolysis pathways based on molecular simulations of biomass pyrolysis are reviewed, respectively. Section 6 coverts biomass catalytic pyrolysis, including the catalytic reaction chemistry and the effects of various types of catalysts. Section 7 reviews biomass pretreatment methods, including torrefaction, acid/alkali pretreatment and steam explosion. Future research and opportunities are then outlined in Section 8.
Section snippets
Composition of lignocellulosic biomass
Lignocellulosic biomass is the most abundant non-edible biomass, mainly composed of forestry and agricultural wastes, such as woodchips and rice straw [74], [75]. Woody biomass can be classified into two broad categories, softwood and hardwood [76]. Softwood originates from conifers and gymnosperm trees, including evergreen species, such as fir, pine, cedar, hemlock, spruce and cypress [77]. Softwood grows faster and is less dense than hardwood. Hardwood comes from angiosperm plants, most of
Latest experimental methods to unravel the biomass pyrolysis mechanism
Research and development of biomass pyrolysis in the 20th century mainly focused on maximizing the bio-oil yield and improving the quality of the target products (bio-oil, gas or char) by optimizing the operating conditions and reactor configurations [67]. The most common experimental method is to perform applied pyrolysis in reactors and to characterize the collected products. Several types of reactors including fixed bed [197], fluidized bed [198], ablative [199], rotating cone [200],
Macroscopic kinetic modeling for biomass pyrolysis
Macroscopic kinetic modeling is often used to simulate the thermo-degradation rates of feedstocks or to predict the formation rates of products during biomass pyrolysis. It can provide valuable information for reactor design and process optimization for biomass pyrolysis [272]. With the development of kinetic theories and computer technologies, several macroscopic kinetic modeling methods with comprehensive mechanism schemes have been developed, such as DAEM, isoconversional method, lumped
Molecular simulation of the pyrolysis pathways of biomass and bio-based model compounds
The development of a fundamental description of biomass pyrolysis chemistry is beneficial to process optimization and feedstock flexibility [353]. However, molecular-level insight into the pyrolysis mechanism via experiments has been hindered due to the unavailability of analytical technologies to monitor the detailed process of biomass pyrolysis [49]. Currently, with the rapid advancement in computational technology and computational chemistry, molecular simulation, which can provide detailed
Catalytic pyrolysis of biomass
Due to the complexity of the biomass structure and various pyrolysis reaction pathways, several hundred types of oxygenated compounds with distinct properties are generated from biomass pyrolysis. Those compounds in bio-oil include acids, ketones, aldehydes, phenols, and anhydrosugars [13], [423], [424]. The complexity of bio-oil makes it challenging for further utilization as intermediate to produce transportation fuels or biochemicals. Moreover, bio-oil has high oxygen, water and acids
The effect of pretreatment on biomass pyrolysis
The low bulk density, high oxygen content, and high alkali metal content of raw lignocellulosic biomass are critical challenges in biomass pyrolysis. Different types of biomass, even the same type of biomass collected from different parts of a plant, present diverse morphological structures and physicochemical characteristics. The irregularity and differences in biomass feedstock can adversely affect the biomass conversion efficiency [529]. Biomass pretreatment can improve its quality by
Challenge and perspectives
Although the mechanism of biomass pyrolysis has been intensively investigated, there are still a lot of challenges. To obtain breakthroughs and unravel the intrinsic complexity of the biomass pyrolysis network, continued efforts are suggested to focus on the following issues.
- (1)
Convenient methods to extract, alter or synthesize specific structural fragments of biomass. Extensive work is needed to reveal the correlation between the biomass structure and the pyrolysis reaction. The introduction of
Conclusion
The state-of-the-art biomass pyrolysis research has been reviewed in this work. Thermochemical characteristics of three major components of biomass, namely cellulose, hemicellulose and lignin, the correlations between the pyrolysis behavior and the distribution of the component building blocks and functional groups are first discussed. The DP and crystal morphology of cellulose, the polysaccharide and side branch of hemicellulose, and the basic units and ether linkages of lignin are the main
Acknowledgments
The authors are grateful for the financial support from the National Natural Science Foundation of China (51621005, 51336008, 51622604, 51476142, 51276166, 50676085, 50476057, 50176046, and 51661145011) and the National Basic Research Program of China (2013CB228100).
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