Cellulosic ethanol, in comparison with first generation biofuels, is substantially more advantageous with regard to feedstock abundance and greenhouse gas reduction [1, 2]. However, unlike the corn ethanol industry, lignocellulosic biomass processing requires higher severity pretreatments due to the inherent recalcitrance of plant material . The selection of pretreatment method has a far-reaching impact on the overall process, including feedstock handling, biological conversions, and downstream processing . The ability to generate steam and electricity from residual lignin is also crucial to maximize the economic profitability and environmental benefits of this industry .
Among potential pretreatment processes, dilute acid pretreatment and ammonia fiber expansion (AFEX) are regarded as promising candidates for large-scale cellulosic biofuel production. Dilute acid pretreatment has been extensively investigated and developed both in the laboratory and at pilot scale [5, 6] to pretreat lignocellulosic biomass for fuel production. This pretreatment is a dry-to-slurry process which effectively hydrolyzes hemicellulose to soluble sugars in the liquor stream . In contrast, AFEX is a dry-to-dry process at alkaline pH using anhydrous ammonia as the reaction catalyst. Although the macrostructure of the pretreated materials is preserved, AFEX reduces the degree of polymerization of cellulose and hemicellulose to increase enzyme accessibility for hydrolysis . High sugar recoveries for corn stover (CS) can be achieved by both pretreatments, as shown by a previous comparative study .
However, a comprehensive comparison of pretreatments with regard to their impacts on important processing units is required. Lignocellulosic biomass is a complex material consisting primarily of cellulose, hemicellulose, lignin, and protein . An ideal pretreatment should produce reactive biomass while minimizing the generation of inhibitory compounds that complicate bioconversions and downstream processes [4, 11]. Furthermore, lignin and biomass nutrients must be preserved for coproduct generation.
In this report, we examine the impacts of these two pretreatments from an overall process perspective. Specifically, we evaluate the interactions of dilute acid pretreatment and AFEX with enzyme requirements, hydrolysate fermentability and lignin preservation. The microbial platform used for the pretreatment comparison involves Saccharomyces cerevisiae 424A(LNH-ST) and Escherichia coli KO11. Comprehensive mass balances were also constructed around each pretreatment.