Mechanisms of liquefaction and pyrolysis reactions of biomass

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Abstract

In the liquefaction process, the micellar-like broken down fragments produced by hydrolysis are degraded to smaller compounds by dehydration, dehydrogenation, deoxygenation and decarboxylation. These compounds once produced, rearrange through condensation, cyclization and polymerization, leading to new compounds. Thermal depolymerization and decomposition of biomass, cellulose, hemicelluloses and products were formed as well as a solid residue of charcoal. The mechanism of pyrolytic degradation of structural components of the biomass samples was separately studied. Cleavage of the aromatic C–O bond in lignin led to the formation of one oxygen atom products, and the cleavage of the methyl C–O bond to form two oxygen atom products is the first reaction to occur in the thermolysis of 4-alkylguaiiacol at 600–650 K. Cleavage of the side chain C–C bond occurs between the aromatic ring and the α-carbon atom.

Introduction

Biomass contributes about 12% of today’s world energy supply, while in many developing countries, its contribution ranges from 40 to 50%. ‘Biomass’ is a generic term for all animate organic matter, excluding fossil fuels, in which not only crops, forestry and marine products but also organic wastes, such as municipal solid waste, sewage and pulp derived black liquor, are widely included. Biomass, as an energy source, has two striking characteristics. Firstly, biomass is the only renewable organic resource and is also one of the most abundant resources. Secondly, biomass fixes carbon dioxide in the atmosphere by photosynthesis. Among the various kinds of biomass, woody biomass has been used traditionally as an energy source, for a long time and, even to this day, in the form of firewood or charcoal. It is, however, next to impossible to use firewood or charcoal as an alternative fuel for commercial equipment and industrial processes where fossil fuels, in particular oil, are used at present. It is necessary to develop technologies which make possible conversion of biomass to a more suitable form, such as liquid or gas [1].

In an earlier study [2], the yields of oil products from thermochemical biomass conversion processes were investigated. This work deals with liquefaction and pyrolysis mechanisms of biomass in order to obtain synthetic liquid fuels. The liquefaction of biomass has been investigated in the presence of solutions of alkalies [3], [4], [5], [6], [7], [8], formate of alkaline metals [9], [10], propanol and butanol [1] and glycerine [11], [12], [13], [14] or direct liquefaction [15], [16].

A large number of research projects in the field of biomass pyrolysis have been performed [17], [18], [19], [20], [21], [22], [23], [24], [25]. Amongst the thermochemical processes, pyrolysis has received increasing attention, since the process conditions may be optimised to produce high energy density pyrolytic oils in addition to derived charcoal and gas. The product, in this case, is an intermediate energy gas that can be used for power generation or as a source of heat for a variety of processes. The energy ratio, heating value of the product gas to the heating value of the waste material, is somewhat below 0.7 [26].

Thermochemical conversion can be subdivided into gasification, pyrolysis and direct liquefaction. The last two processes are sometimes confused with each other, and a simplified comparison of the two follows. Both are thermochemical processes in which feedstock organic compounds are converted into liquid products. In the case of liquefaction, feedstock macro-molecule compounds are decomposed into fragments of light molecules in the presence of a suitable catalyst. At the same time, these fragments, which are unstable and reactive, repolymerize into oily compounds having appropriate molecular weights [27]. With pyrolysis, on the other hand, a catalyst is usually unnecessary, and the light decomposed fragments are converted to oily compounds through homogeneous reactions in the gas phase. The differences in operating conditions for liquefaction and pyrolysis are shown in Table 1. The liquefaction [1], [3], [9], [10], [15], [28], [29] and pyrolysis [17], [21], [22], [23], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47] mechanisms of biomass have been studied by many investigators.

According to the proposed mechanism, the heat variations associated with the thermal degradation reactions may affect the pyrolysis route. Experimental runs indicate several endothermic and/or exothermic peaks for biomass pyrolysis [24], [48], [49]. According to several researchers, cellulose pyrolysis is endothermic [30], [50], [51], but lignin pyrolysis is exothermic [52]. Wood pyrolysis is exothermic, and the main cause of heat generation is the secondary decomposition of the volatiles, possibly catalyzed by the remaining solid.

This present work has undertaken a study on the liquefaction and pyrolysis mechanisms of biomass structural constituents. The methods were based on analyses of the liquefaction and pyrolysis products from the biomass samples.

Section snippets

Mechanisms of liquefaction reactions

Concerning the catalytic effect of alkali metal salts, there has been little description about the roles that a catalyst plays in the liquefaction with some exceptions. Appell et al. proposed the following mechanism for sodium carbonate-catalyzed liquefaction of carbohydrate in the presence of carbon monoxide [54].

  • Reaction of sodium carbonate and water with carbon monoxide, to yield sodium formateNa2CO3+2CO+H2O→2HCOONa+CO2

  • Dehydration of vicinal hydroxyl groups in a carbohydrate to an enol,

Pyrolysis mechanism of cellulose

The destructive reaction of cellulose is started at temperatures lower than 325 K and is characterized by a decreasing polymerization degree. Thermal degradation of cellulose proceeds through two types of reaction: a gradual degradation, decomposition and charring on heating at lower temperatures, and a rapid volatilization accompanied by the formation of levoglucosan on pyrolysis at higher temperatures. The glucose chains in cellulose are first cleaved to glucose and from this, in a second

Pyrolysis mechanism of hemicelluloses

The hemicelluloses, which are present in deciduous woods chiefly as pentosans and in coniferous woods almost entirely as hexosanes, undergo thermal decomposition very readily. It was therefore to be expected that furan derivatives would readily be found among the decomposition products.

The hemicelluloses reacted more readily than cellulose during heating [67]. The thermal degradation of hemicelluloses begins above 373 K during heating for 48 h; hemicelluloses and lignin are depolymerized by

Pyrolysis mechanism of lignin

Lignin is one of the major constituents of biomass (typically 15–30% by weight of wood). It is produced in large amounts during wood pulping but only a very small share of it is isolated for chemical purposes [33]. The term lignin is used to mean the complex, naturally occurring phenolic phenylpropanoid polymer or ‘protolignin’ characterized by the general empirical formula [69], [35]:C9H8−xO2H2<1.0OCH3xAlmost all of the lignin produced is used as a low-grade fuel for the pulping process and as

Conclusion

The relative abundance of identified low molecular weight phenolic compounds decreased from lignin to wood to cellulose in the liquefaction products [29]. This is in agreement with the known phenolic nature of lignin. It confirmed the synthesis during reaction of such compounds from a carbohydrate substrate, as well.

Acetic acid is formed in the thermal decomposition of all three main components of wood. When the yield of acetic acid originating from the cellulose, hemicelluloses, and lignin is

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