11.08.2015   Sustainable Biotechnology

Pretreatment Methods

Many pretreatment processes have been tested for the capability to facilitate lig­nocellulosic biomass component separation and to aid in subsequent access for the hydrolytic enzymes [25, 26]. The more extensively studied methods are listed in Table 4, which includes AFEX (ammonia fiber explosion) and ARP (ammonia recycle percolation) [27, 28], lime [29], organosolv [30], liquid hot water, ionic liquid [31], dilute acid and steam explosion [32, 33], and enzyme treatment [34]. Additional information on pretreatments is available from Taherzadeh and Karimi [35] and Jorgensen, Kristensen, and Felby [27].

Table 4 Features of some pretreatment processes

Pretreatment

Conditions and solid load

Effects on biomass

Xylose yield (%)

Digestibility of cellulose

(%)

Dilute acid

0.5-2% H2SO4

140-210°C, 1-30 min; <40% solid

Remove and hydrolyze hemicelluloses, redistribute lignin, form furfural and HMF

75-90

<85

Steam explosion

170-250° C, 1-20 min; >50% solid

45-65

90

Liquid hot water

190-260° C min-h, 1-10% solid

Remove of some

hemicelluloses, lignin

80-98

80-90

AFEX and ARP

Anhydrous or 15% ammonia, 90 or 170°C; >50% solid

Remove lignin, partially hydrolyze hemicelluloses and cellulose, decrystalize cellulose

>90

>80-90

Lime

~0.1 g CaO/g biomass, 55°C a few weeks, 20-40% solid

Remove lignin

>90

>90

Alkaline

peroxide

1-7.5% H2O2, pH 11.5, 30-85°C 45 min-24 h; 15% solid

Solublize and oxidize lignin

>90

>95

Organosolv

Methanol, ethanol, acetone Remove of lignin and etc. +<0.1 M acid as some hemicelluloses catalyst, 160-200°C, h;

25% solid

>70

90-100

An effective practical pretreatment process should meet the following standards for use in future commercial facilities: (a) allow excellent cellulose digestibility by commercial cellulases, (b) good recoveries of cellulose and pentoses from hemicel — luloses, (c) minimal or no microbial inhibitory by-products, (d) good separation of lignin, (e) be easily managed at large volumes, (f) be relatively inexpensive (capex and opex), (g) not require large energy inputs, and (h) have environmentally acceptable features.

Published economic analysis has suggested that the MESP (minimal ethanol selling price) for cellulosic ethanol from corn stover, using different pretreatment technologies, ranges from $1.41/gallon for the AFEX process to $1.7/gallon for hot water treated corn stover [36]. More recently, Sendich et al. [37] indicated that the MESP for AFEX treated corn stover could be as low as $0.81/gallon due to reduced ammonia concentration and a simplified ammonia recycle process. However, we believe the assumptions used are perhaps overly-optimistic. For example, a feed­stock cost of $30/ton is very low, especially given the alternative nutrient and soil texture improvement values for corn stover. More recently, the DOE reported a 2007 cellulosic MESP of $2.43/gallon [38]. In any case, and despite many years of R&D, it is difficult to validate the assumptions since none of the conversion processes have been evaluated at practical scale.