Recent progress on catalytic pyrolysis of lignocellulosic biomass to high-grade bio-oil and bio-chemicals
Introduction
Fossil fuels have satisfied the fast growing energy demand for transportation fuel and industrial revolution. However, increasing concern on the fossil fuel depletion and global climate change are the impetuses for the search for an alternative and carbon neutral energy resources. Attention has shifted to non-food lignocellulosic biomass, as one of the most significant renewable energy resource for liquid fuel and chemicals synthesis. The lignocellulosic biomass has been carbon neutral to fulfill the global reduction in carbon dioxide emission. Also, they are abundant and readily available to meet the global fuel demands [1]. As the precursor of fossil feedstock, the use of biomass in energy application for heating, power and transportation requires multiple steps conversion [2]. Therefore, lignocellulosic biomass from forest, agricultural and agroindustries are dedicated to liquid fuel production via thermochemical decomposition such as pyrolysis and liquefaction processes [3], [4]. Depending on the desired pyrolysate (biochar or liquid oil), lignocellulosic matters are decomposed by slow or fast pyrolysis. Slow pyrolysis produces primarily biochar, the residual tar and syngas are rarely attended for any uses; the pyrolysis carbonisation is facilitated with conventional heating element or microwave irradiation [5], [6].
Fast pyrolysis converts lignocellulose biomass to mainly bio-oil and other chemicals in contrast to slow pyrolysis [7]. Low cost makes focus shift to fast pyrolysis of lignocellulose biomass for bio-oil production because high-pressure liquefaction gives low yield of bio-oil at high cost [8]. Conventional fast pyrolysis produces pyrolysate liquid with fractions widely distributed. However, catalytic biomass fast pyrolysis narrows the bio-oil fractions to high-grade bio-oil. The major constituents of the pyrolysis oil are phenolic and alkylated (poly-) phenols, along with relatively small amounts of phenol, eugenol, cresols and xylenols [9]. The pyrolysis reactions involve cracking of different side-chain structures and methoxy groups of aromatic ring of the lignin component of the biomass. Heating values for the bio-oils range from 16.79 to 19 MJ/kg, close to that of oxygenated fuel such as ethanol; but lower than 40 - 45 MJ/kg for conventional fossil fuels. The bio-oils low heating values are affected by the high degree of oxygenated compounds [10]. Also, the water in the bio-oils is problematic for the direct use of the oil as a transportation fuel. The water and oxygenated compounds promoted non-volatility, high acidity, corrosiveness and aging during storage. The bio-oil experienced extreme instability and tendency to repolymerise during storage as a response to the aging, responsible for drawbacks in the bio-oil fuel properties [11]. Water and oxygenated compounds in the pyrolysis oil (bio-oil) are relatively in large quantities; therefore it is difficult to suppress their repolymerisation that can overburden the bio-oil fuel characteristics. Water removal can increase stability and viscousity, and reduces acidity of the bio-oil, but requires expensive, sophisticated technique. To produce high-grade bio-oil and chemicals, Gong et al. [12] and Hussain et al. [13] used catalytic pyrolysis of lignocellulose biomass. Catalysts such as Zeolite (ZSM-5, HZSM-5 and FCC) [12], [13], and alkaline (Na2CO3/γ-Al2O3, K2CO3, Ca(OH)2 and MgO) [16,17] in biomass pyrolysis influences the bio-oil quality. The catalysts upgrades the quality of the pyrolysis oil through cracking, deoxygenation, oligomerisation, cyclization, aromatisation, alkylation, isomerization and polymerisation [18]. The catalytic upgrade mainly increases the heating value (energy density) of the bio-oil. Catalysts tailor thermally driven fast pyrolysis reactions to yield high quality and more stable bio-oil [19]; also, the consequences has significant influence on product selectivity by the catalysts [20]. In association with catalyst loading, pyrolysis variant conditions also influence the yield and composition of the pyrolysate. The conditions (operating parameters) are tempered to direct the pyrolysis reaction to the desired quality and quantity of bio-oil. The typical operating parameters for fixed, fluidised bed, batch and auger pyrolysers can include; Biomass loading, Biomass particle size, Nitrogen flow rate, Pyrolysis temperature and Time [21]. Also, Taarning et al. [10] reported that bio‐oil compositions rest on the type of pyrolyser system, biomass feedstock, and catalyst type and the reaction conditions.
Scholars extensively discuss the main aspects of catalytic pyrolysis such as pyrolysis technology and processes, catalyst type activity and deactivation, parameters influences, biomass feedstock, and reaction mechanism. Previously, Akhtar & Saidina Amin [22] evaluated the influence of severity on zeolite catalysed biomass pyrolysis on the maximum yield of the desired bio-oil. Lappas et al. [23] appraised the application of the zeolite acid catalyst in biomass pyrolysis for the production of transportation fuel with few attentions to base catalyst in pyrolysis of biomass. Similarly, Rezaei et al. [24] reviewed the catalytic activities of several acidic zeolite catalysts for the selective production of aromatics and olefins in relation to pyrolysis conditions. Furthermore, Dickerson and Soria [25] assessed the effect of various catalyst over pyrolysis reaction with particular attention to pyrolysis reaction pathway and mechanism. However, Isahak et al. [26] reviewed catalytic pyrolysis of biomass with an exclusive focus on the type of pyrolysis and pyrolyser, pyrolysis severity in the pyrolysis of biomass to bio-oil. Galadima and Muraza [27] discussed on zeolite coke deactivation from the perspective of the catalyst topology and coke deposition on the active catalyst sites. The effect primarily reduces the yield of aromatics in the resultant bio-oil; as a consequence, the bio-oil quality as fuel precursor is seriously compromised. Furthermore, Authours in [28] appraised the influences of parameters in fashioning the distribution of the final products of biomass pyrolysis with a special focus on the pyrolysis processes and yield of biochar. Exclusively presented are the reaction pathways that establish the pyrolytic devolatilisation of structural components of biomass and the attending kinetics models that explained the devolatilisation pathways [29], [30].
This paper elucidated on the prevalent dynamic innovations that expedites the production of renewable fuel and fine chemicals from the bio-oil precursor, a product of catalytic pyrolysis of lignocellulosic biomass. The report focused on research expeditions on different biomass precursors; catalysts and pyrolysis process that are susceptible to the improvement of catalytic biomass pyrolysis to high-grade bio-oil and chemicals.
Section snippets
Availability and energy prospects
The appetite for fossil fuels for energy is on the decline from the high cost, dwindling reserves and the problematic greenhouse effects. However, following the growing energy demand and drawbacks associated with fossil fuels gives particularly lignocellulosic biomass eminent role in the international energy policy for renewable energy. Lignocellulose biomass becomes increasingly a significant source of clean alternative energy. The feedstocks are unusually diverse, cheap and abundant non-food
Pyrolysis of lignocellulose biomass for bio-oil production
Pyrolysis embroils thermal degradation of lignocellulose biomass through a sequence of complex reactions in an oxygen-free environment, mostly created by sweeping nitrogen gas. The biomass structural components decompose to lower molecular weight products such as bio-oil, gases, and bio-char. The products of biomass pyrolysis are categorised as sources of renewable energy and chemicals; the biomass decomposed during slow pyrolysis to maximise char production [52], [53]. Previously, residual
Conventional and microwave heating of biomass pyrolysis
External heating systems provide the high and rapid heating that vigorously vibrate biomass molecules in fast pyrolysis of lignocellulose biomass to liquid oil [107], [108]. The heat sustains the molecular vibration for a precise time to overstretch and later breaks the binding forces between the biomass molecules. The outcome of conventional heating in a biomass pyrolysis is conspicuous from the product distribution. Often, the conventional heating promotes larger quantity of biochar and gases
Micropores acid catalysts
The biomass pyrolysis to bio-oil, gas and biochar has been a testing ground for testing the catalytic activities of several acid based catalyst. The purpose is to improve the fuel property of the bio-oil. Most of the catalysts fell in the class of microporous and mesoporous, depending on their pore sizes and nature. Heterogeneous acids catalysts are the primary catalyst in catalytic cracking in petroleum refining processes and attract attention for biomass pyrolysis. Acidic catalyst is
Base catalyst for lignocellulose biomass pyrolysis
Acids catalysts face fast deactivation by coke deposition because of the large molecules in the pyrolysis vapour. The molecules cannot penetrate the micropores of the catalysts but end up deposited as coke on the surfaces of the catalyst. Afterwards, the catalyst inhibition by the coke leads to low liquid pyrolysis yield and formation of polycyclic aromatic hydrocarbons (PAHs). The inhibition limit the suitability of the catalyst for biomass pyrolysis [141], [145]. However, mesoporous and
Conclusions
The pyrolysis of lignocellulosic biomass to high-grade renewable fuel and petrochemicals can minimise the dependence on fossil fuels and toxic chemicals. Of recent, most studies focused on the upgrading of the pyrolysis vapour by reducing the oxygenated compounds that affect the quality of the bio-oil as a precursor for petrochemical products with numerous catalysts. The catalysts presence in lignocellulosic biomass pyrolysis have upgraded the quality and in some instance quantity of the
Acknowledgements
The authors acknowledge the research grants provided by the Universiti Sains Malaysia, under Research University (RU) grant (Project No: 1001/PJKIMIA/814227) that resulted in this article.
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