Assessment of shock pretreatment and alkali pretreatment on corn stover using enzymatic hydrolysis

Abstract

This study investigates digestibility enhancements of lignocellulose from shock pretreatment, alkaline pretreatment, and combination. Shock pretreatment subjects aqueous slurries of lignocellulose to shock waves, which disrupts its structure rendering it more susceptible to hydrolysis. Alkaline pretreatment submerges the biomass in aqueous alkali (NaOH, Ca(OH) 2 ), which removes lignin and acetyl groups. As indicators of digestibility, cellulase (CTec3) and hemicellulase (HTec3) were used to saccharify the pretreated corn stover and the resulting filtrate which contains about 10% of the sugars. Shock is most effective when it precedes alkaline pretreatment, presumably because it opens the biomass structure and enhances diffusion of pretreatment chemicals. Lignocellulose digestibility from calcium hydroxide treatment improves significantly with oxygen addition. In contrast, sodium hydroxide is a more potent alkali, and thereby eliminates the need for oxygen to enhance pretreatment.At low hydroxide loadings (<4 g OH À /100 g dry biomass), both NaOH and Ca(OH) 2 provide similar increases in digestibility; however, at high hydroxide loadings, NaOH is superior. For animal feed, Ca(OH) 2 treatment is recommended, because residual calcium ions are valuable nutrients. In contrast, for methane-arrested anaerobic digestion, NaOH treatment is preferred because NaHCO 3 is a stronger buffer. At 50 C, shock pretreatment improves sugar yields at all NaOH loadings. The effect of shock is most pronounced when the no-shock control employed the same soakingand-drying procedure as the shock treatment. The recommended conditions are shock treatment (5.52 bar [abs] initial H 2 /O 2 pressure) followed by 50 C alkaline treatment with NaOH loading of 4 g OH À /100 g dry biomass for 1 h.

Assessment of corn stover pretreated with shock and alkali using methane‐arrested anaerobic digestion

“…19 Details for shock treatment are described in detail in previous studies. [30][31][32] In this study, corn stover was pretreated using the recommended two-step process from a previous study: (1) shock treatment of an aqueous slurry of raw corn stover with headspace containing stoichiometric mixture of hydrogen and oxygen, initial pressure 5.52 bar (abs), followed by detonation of explosive gas (H 2 + O 2 ) with a glow plug; and (2) NaOH treatment using 4 g OH À /100 g dry biomass maintained at 50 C for 1 h. 19 Shock treatment is estimated to cost $5/ton, 32 which is much smaller than conventional chemical pretreatments estimated to cost about $45/ton. 33 To maintain consistency in methodology across multiple pretreatment studies, the loading for NaOH was quantified based on the hydroxide group (g OH À /100 g dry biomass) according to Equation (1).…”

“…This platform uses indigenous enzymes present in a microbial community to convert biomass to carboxylic acids, which subsequently are chemically converted to fuels and industrial chemicals. 19 In traditional anaerobic digestion, carboxylic acids would be lost to methane and carbon dioxide. In the carboxylate platform, to prevent this loss, an inhibitor is added, hence the term methane-arrested anaerobic digestion (MAAD).…”

“…18 In a previous study, shock and alkaline pretreatment conditions were assessed using extracellular enzymes. 19 Enzymatic hydrolysis is rapid and precise, and therefore useful for screening multiple pretreatment conditions. However, extracellular enzymes are employed in the sugar platform, which has numerous industrial challenges elucidated earlier.…”

“…26 Using extracellular enzymes, previous experiments on corn stover surveyed numerous pretreatment approaches where parameters were systematically varied to determine operating conditions that maximize digestibility. 19 From these enzymatic studies, the following two-step pretreatment process is recommended: (1) shock treatment of an aqueous slurry of raw corn stover with headspace containing stoichiometric mixture of hydrogen and oxygen, initial pressure 5.52 bar (abs), followed by detonation; and (2) NaOH treatment using 4 g OH À /100 g dry biomass maintained at 50 C for 1 h. At this moderate sodium hydroxide concentration, shock treatment is presumed to enhance enzymatic digestion by creating microscopic fissures that improve diffusion of alkali into the biomass structure. At high sodium hydroxide concentrations, the alkali is powerful enough that shock treatment is not beneficial.…”

“…At high sodium hydroxide concentrations, the alkali is powerful enough that shock treatment is not beneficial. 19 MAAD requires sources of energy and nutrients. In a four-stage countercurrent fermentation, Rughoonundun et al studied the codigestion of sugarcane bagasse (energy source) and sewage sludge (nutrient source).…”