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).
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 5.4 shows the conditions for ideal pretreatment of lignocellulosic biomass for dilute acid, steam explosion, AFEX and liquid hot water.
|Pretreatment Process||Dilute Acid||Steam Explosion||AFEX||Liquid Hot Water|
|Particle Size Reduction Required||Yes||No||Nob||No|
|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||
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