Lime pretreatment

Pretreatment using lime has been studied as a low-cost process that primarily achieves acetyl and lignin solubilization (10,14,18,62,63). Lime pretreatment has been practiced at a wide range of temperatures, from 25°C to about 130°C, with lime loadings of about 10 wt% (on a dry feedstock basis) and solids loadings of 20% or less. At the higher temperatures, the pretreatment times are reasonably short (minutes to hours), but can extend to several weeks at lower temperatures. Because of the lengthy residence time at low temperatures, lime pretreatment can be conducted in a pile arrangement without expensive pressure reactors and can be performed as part of the feedstock storage system (18). Near-complete deacetylation generally occurs upon lime pretreatment of low-lignin herbaceous feedstocks and agricultural residues, with about 30% lignin removal. Much higher lignin removal (up to 80%) can be achieved by adding oxygen or air to the lime pretreatment system. In the lower temperature lime pretreatment pile arrangement, this can be accomplished by percolating air through the pretreatment pile. The additional lignin removal under oxidative conditions allows lime pretreatment to achieve reasonable enzymatic digestibility using more recalcitrant feedstocks, such as sugar cane bagasse and hardwoods (14). Lime pretreatment generally requires longer pretreatment times than other alkaline pretreatments, such as those using ammonia, but catalyst costs are lower. However, regeneration and reuse of the lime will probably still be necessary, and such recovery systems will add significant capital and operating costs to the lime pretreatment approach (21).

14.5.3 Solvent pretreatments

14.5.6.1 Organic solvents

Numerous organic or organic-aqueous solvent mixtures utilizing methanol, ethanol, ace­tone, ethylene glycol, triethylene glycol, and tetrahydrofurfuryl alcohol have been used as biomass pretreatment processes to solubilize lignin (10-13, 64, 65). Such processes are commonly referred to as organosolv processes. In some studies, inorganic acid catalysts, such as sulfuric or hydrochloric acid, are added to achieve significant levels ofhemicellulose hydrolysis and even cellulose hydrolysis (66) along with lignin solubilization. In some cases, the main components of biomass (cellulose, hemicellulose, and lignin) can be effectively fractionated, with each component potentially used for separate value-added products (67). Solvents must be effectively recovered and recycled using appropriate extraction and separa­tion techniques without leaving behind any inhibitory levels of residual solvents in process streams that undergo subsequent biological processing. While residual cellulose-rich pre­treated solids from such processes maybe highly digestible using cellulase enzymes, the cost of such processes and the potential value of the relatively pure fractions may make them better suited to higher-value applications.