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.
- Click Here for a Text Alternative of Figure 5.28
Effects of Pretreatments of Biomass Recalcitrance
Pretreatment Increases Accessible Surface Area Decrystallizes Cellulose Removes Hemicellulose Removes Lignin Alters Lignin Structure AFEX Major Effect Major Effect Minor Effect Major Effect Major Effect Dilute Acid Major Effect Major Effect Major Effect Lime Major Effect Not Determined Minor Effect Major Effect Major Effect Liquid H2O Major Effect Not Determined Major Effect Minor Effect SAA Major Effect Major Effect Minor Effect Major Effect Major Effect SO2 Major Effect Major Effect Minor 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|
|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