The role played by the pre-treatment is to increase the ability for the enzymes to get inside the wood cells. As mentioned the lignin hemicellulose connections are largely responsible for this resistance that is why many of the pre-treatment techniques work to either destroy /dissolve the lignin or hemicellulose. The removal of either the lignin or hemicellulose greatly increases the ability for the wood to be converted into sugars and subsequently methane (Mendu et al., 2011; Miao et al., 2011;
Narayanaswamy et al., 2013; Potters et al., 2010; Zheng et al., 2014). Therefore, the pre-treatment is necessary to break down the lignin structure, recover cellulose, decrease cellulose crystallinity, partially remove or break down bond between hemicellulose, increase surface area and porosity of biomass, increase accessibility of enzymes and microbes. The effect of pre-treatment on lignocellulosic biomass is shown in a simplified form in Figure 3-5.
There are many treatments available that are divided into mechanical, e.g. by milling or grinding, physical by steam explosion or radiation, chemical by acids, alkali or solvents, biological by enzymes and fungi, thermal, combined mechanical, thermal
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Figure 3- 5:Impact of pre-treatment on lignocellulosic biomass (Liu and Fei, 2013)
and chemical(Chandra et al., 2012; Chen et al., 2014a; Salehian and Karimi, 2013;
Zhang et al., 2014a), Co-digestion with other organic wastes(Chen et al., 2014a)have been explored to improve the efficiency of degradation. Among the mechanical and physical pre-treatment, milling has a good potential to improve the hydrolysis by increasing the surface area of lignocellulosic materials (Miao et al., 2011; Potters et al., 2010) which makes this as a prerequisite before any other pre-treatment to apply.
Among the chemical treatments, the alkaline pre-treatment is one of the most important methods to break down the ester bonds between amorphous and cellulose contents by saponification and cleavage of lignin-carbohydrate linkage, decrease in polymerization and crystallinity, which increases porosity, internal surface area and structural swelling (Salehian and Karimi, 2013; Zheng et al., 2014). Sodium hydroxide (NaOH) has been extensively used in the studies to improve biogas from lignocellulosic wheat straw (Chandra et al., 2012), rice straw (He et al., 2009), corn stover (Wang et al., 2013a), hardwoods and softwoods (Salminen and Rintala, 2002;
Zheng et al., 2014), paper and pulp sludge (Lin et al., 2009), oil palm empty fruit branches (Nieves et al., 2011). Studies showed that NaOH pre-treatment has increased methane yield from feedstocks like softwood, hardwood and pulp and paper. However, various treatment conditions, i.e. NaOH concentration, time, and temperature can show different results along with wood types. For example, NaOH
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pre-treatment was more effective on hardwood (birch) than softwood (spruce) for methane yield in anaerobic digestion (Mohsenzadeh et al., 2012).
Cellulase and hemicellulose are the most commonly used enzymes for lignocellulosic biomass. However, the cost of enzymes and effectiveness in biogas production limited its applications in anaerobic digestions (Zheng et al., 2014).
Although several studies have been done on the effectiveness of pre-treatment, the promising results were found with the combination of two or more pre-treatment methods. Moreover, the effectiveness and selection of pre-treatment mostly depend on the types of lignocellulosic materials. In addition, the use of enzymes still needs to be investigated as it accelerates the cellulose and hemicellulose decomposition.
3.4.1 Effects of chemical pre-treatment
Chemical pre-treatment has been done by alkaline, acid, catalyst, wet oxidation and ionic liquid. A summary of literature review on chemical pre-treatment are given in Table 3-12. Therefore, depending upon the nature of the lignocellulosic biomass, chemical pre-treatment should be chosen carefully.
Table 3- 12: Summary of most effective chemical pre-treatment Chemical pre-treatments summary (Serna et al., 2015)
Pre-treatment Reagents Treatment effects Reference Alkaline Sodium hydroxide, potassium
hydroxide, calcium hydroxide, ammonium hydroxide, lime among others
Lignin removal (Park and Kim, 2012)
Dilute acid Sulphuric acid, hydrochloric acid, nitric acid, phosphoric acid among others
Hemicellulose
fractionation (Chandel et al., 2012) Organosolv Ethanol, acetic acid, formic acid,
per acetic acid with organic and inorganic catalysts Ionic liquids Anions from chloride, formate,
acetate or alkyl phosphorate Cellulose crystallinity reduction and partial hemicellulose and lignin removal
(Li et al., 2010)
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3.4.2 Comparison of different pre-treatments
Different pre-treatment techniques have been discussed individually in the previous section. Now summary of all the pre-treatment has been provided in Table 3-13 showing the effect of pre-treatment on the compositional and structural alteration of lignocellulosic biomass.
The effect of pre-treatment has been divided in six categories which are accessible surface area, decrystallization of cellulose, solubilisation of hemicellulose, solubilisation of Lignin, alteration of lignin structure and formation of any inhibitory ingredient such as furfural. The effect was also categorised on the scale of 1-4 where 1 being the major effect and 4 would be no effect.
From Table 3-13, alkaline pre-treatment has major effect on lignin solubilisation and lignin structural alteration whereas acid pre-treatment has major effect on solubilisation of hemicellulose. Therefore, depending upon the lignocellulosic structure, the type of pre-treatment should be chosen. Sometimes, combination of two pre-treatment would be effective if the structural lignin and hemicellulose is required to be sequentially removed.
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Table 3- 13: Effect of pre-treatment on the compositional and structural alteration of lignocellulosic biomass (Zheng et al., 2014)
Pre-treatment Primary effect Accessible
Surface area Decrystallization
of cellulose Solubilisation of
hemicellulose Solubilisation
of lignin Alteration of
lignin structure Formation of furfural/(HMF) Particle size/
mechanical -increase microbial access
and activity ❶ ❶ ❹ ❹ ❹ ❹
Ultrasonic -disintegrates the particles -creates favourable
conditions for biodegradation
❶ ❸ ❶/❷ ❶/❷ ❶/❷ ❸
Irradiation ❶ ❷ ❷ ❹ ❹ ❷
Steam explosion -organic and inorganic compounds are partially solubilized
❶ ❹ ❶ ❷ ❶ ❶
Liquid hot water -partially solubilized ❶ ❸ ❶ ❷ ❷ ❷
Catalysed
steam-explosion - solubilisation of organic
and inorganic compounds ❶ ❹ ❶ ❶/❷ ❶/❷ ❶
Acid -higher solubilisation and biodegradation of
pre-treatment - degrade lignin and
hemicelluloses ❶ ❸ ❶ ❶ ❶ ❹
❶= Major effect ❷= Minor effect ❸= Not determined ❹= no effect
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