EGEE 439
Alternative Fuels from Biomass Sources

5.3h Summary

Of the methods we’ve discussed, there are pretreatment options that lead the others (some under commercialization). The current leading pretreatment options include dilute acid, AFEX, liquid hot water, lime, and aqueous ammonia, with dilute acid and water, AFEX, and lime under commercialization. Figure 5.28 shows switchgrass before pretreatment and after several pretreatment options, i.e., AFEX, dilute acid, liquid hot water, lime, and soaking in aqueous ammonia (SAA).

Resulting Switchgrass Solids after Different Pretreatment Technologies
Figure 5.28: Resulting Switchgrass Solids after Different Pretreatment Technologies.
Credit: Donohoe et al., 2011. Bioresour. Technol., in press

To summarize the methods of pretreatment, Table 5.3 shows some of these pretreatment methods and the major and minor effects on lignocellulosic biomass. All methods (AFEX, dilute acid, lime, liquid hot water, soaking aqueous ammonia, and treatment with SO2) have an effect on increasing surface area, removing hemicellulose, and altering lignin structure. Only AFEX, lime, and SAA pretreatment remove lignin, and AFEX and SAA decrystallize cellulose.

table showing Effects of Pretreatment of Biomass Recalcitrance
Table 5.3: Effects of Pretreatment of Biomass Recalcitrance.
Pretreatments: AFEX, Dilute Acid, Lime, Liquid H2O, SAA, and SO2
Increases Accessible Surface Area - all methods major effect
Decrystallizes Cellulose - Major effect AFEX and SAA, Not Determined for Lime and Liquid H2O
Removes Hemicellulose - All effect this category, minor effects for AFEX, Lime, and SAA, rest major effect
Removes Lignin - AFEX, Lime, and SAA
Alters Lignin Structure - All effect this category, minor effects for liquid H2O and SO2, rest major effect
Credit: Mosier et al., 2005. Bioresour. Technol., 96: 673-686

Table 5.4 shows the conditions for ideal pretreatment of lignocellulosic biomass for dilute acid, steam explosion, AFEX and liquid hot water.

Table 5.4: Comparison of Pretreatment Processesa
Pretreatment Process Dilute Acid Steam Explosion AFEX Liquid Hot Water
Reactive Fiber Yes Yes Yes Yes
Particle Size Reduction Required Yes No Nob No
Hydrolyzate Inhibitory Yes Yes No Slightly
Pentose Recovery Moderate Low High High
Low Cost Materials of Construction No Yes Yes Yes
Production of Process Residues Yes No No No
Potential for Process Simplicity Moderate High Moderate High
Effectiveness at Low Moisture Contents Moderate High Very High

Not Known

a Modified from (86); AFEX ratings from Bruce Dale (personal communication).

b For grasses, data for wood not available.

Credit: Lynd, 1996. Annual Rev. Energy Environ., 21: 403-465