SO₂-Catalyzed Steam Fractionation of Aspen Chips for Bioethanol Production:  Optimization of the Catalyst Impregnation

Abstract

The pretreatment step has a key role in the enzymatic conversion of lignocellulosic biomass to bioethanol. Steam pulping of biomass has long been recognized as being effective in producing high biomass fractionation. In this step, the preliminary impregnation of biomass with acid catalysts, including SO 2 , has been shown to further improve the hydrolytic effect and increase the digestibility of the fibers by enzymes. This work focused on developing an experimental setup for the SO 2 impregnation, enabling accurate control of the process variables. The final purpose was the assessment of a method to minimize the quantity of SO 2 used, and that could be applicable to continuous operations. Aspen chips were impregnated with the acid catalyst at room temperature in a stainless steel batch reactor, and the influence of the biomass humidity and contact time on SO 2 uptake was explored. After the catalyst adsorption, biomass was fed into the steam explosion batch reactor and steamed at 205°C for 3 and 10 min to obtain slurries. The addition of catalyst (∼0.9% w/w raw material dry matter) reduced the degree of polymerization of the cellulose by 50%, on average. The highest yield of xylose (10.3 g/100 dry chips) was obtained via water extraction following a steam pretreatment of 3 min. The amount of monomer xylose was 80% of the total extracted dry matter. Simultaneous saccharification and fermentation (SSF) of the washed fibers yielded 37 g of glucose/100 g dry chips, 96% of which fermented to ethanol.

Sulfite pretreatment to overcome recalcitrance of lignocellulose (SPORL) for robust enzymatic saccharification of hardwoods

“…Table 3 also indicates that the terminal process enzymatic cellulose conversion (PECC) of SPORL was equivalent to that achieved by the ethanol organosolv process, 16 between 85 and 90%. The PECC of the steam explosion study 17 was below 70%. The PECC of sulfuric acid catalyzed steam 17 in 72 h but lower than that achieved by the SPORL process in just 4-12 h. Figure 2 indicates that increasing the pH of pretreatment liquor by reducing acid application in SPORL pretreatment decreased enzymatic hydrolysis rate and cellulose conversion of SPORL substrate because of less hemicellulose removal and insignificant depolymerization (prehydrolysis) of cellulose, which agrees with finding in our previous study with spruce.…”

“…Table 3 compares the enzymatic hydrolysis efficiencies of SPORL with those of organosolv and steam explosion processes. The steam explosion 17 used a shorter pretreatment time but a higher pretreatment temperature than the present SPORL. The steam-exploded aspen 17 achieved a lower enzymatic cellulose conversion of substrate (ECCS), about 80% after about 72 h, when compared with the more than 90% conversion of the present SPORL substrates in 4-12 h, even though the steam explosion used slightly higher enzyme dosages.…”

“…The steam explosion 17 used a shorter pretreatment time but a higher pretreatment temperature than the present SPORL. The steam-exploded aspen 17 achieved a lower enzymatic cellulose conversion of substrate (ECCS), about 80% after about 72 h, when compared with the more than 90% conversion of the present SPORL substrates in 4-12 h, even though the steam explosion used slightly higher enzyme dosages. The ethanol organosolv process 16 achieved an equivalent enzymatic hydrolysis conversion of about 95% after 24 h enzymatic hydrolysis that the present SPORL process achieved at the same pretreatment temperature of 180 C and similar enzyme application dosages in just 4-12 h hydrolysis, however, the ethanol organosolv process 16 used a relatively long pretreatment time of 1 h at acid charge of 1.25%.…”

“…Different terms have been used in the literature for describing substrate enzymatic digestibility and pretreatment process performance (for example, cellulose-to-glucose conversion yield 16 and enzymatic hydrolysis yield 17 ). To more easily compare our results with published data (as listed in Table 1), we define the following terminologies.…”

“…[17][18][19] Dilute acid pretreatment can achieve satisfactory conversion after significant size reduction prior to pretreatment. 6,20 We believe that SPORL, with its excellent performance in overcoming softwood recalcitrance, is more effective than existing processes for pretreating hardwoods.…”