Improving carbohydrate recovery from uncatalyzed steam pretreated hybrid poplar

Abstract

The presence of SO₂ in steam pretreatment creates a series of problems in environmental protection, equipment corrosion, chemical catalysis and waste water treatment. Moreover, SO₂ increases the pretreatment severity, resulting in more sugar degradation and increased inhibitor formation. In this study, hybrid poplar chips were steam exploded using 6 different conditions with or without the addition of SO₂. The steaming temperature ranged from 190 °C to 212 °C, and the residence time ranged from 5 min to 10 min, resulting in a range of fermentation inhibitors, including furfural, 5-hydroxymethyl furfural (HMF), acetic acid and phenolic. It was found that certain concentration of inhibitors, particularly acetic acid, could promote the ethanol yield in hydrolysate fermentation, but always impairs the xylitol yield due to the acetyled xylan in hybrid poplar. At the lowest pretreatment severity condition at 205 °C, 10 minutes, without SO₂, the best inhibitor concentration for ethanol yield and the highest overall sugar recovery following pretreatment were achieved. However, since no SO₂ was applied, poor digestibility during enzymatic hydrolysis of cellulose reduced the post-hydrolysis sugar recovery. For that reason, mechanical refining was applied to the solid fractions and improved the enzymatic hydrolysis for solids pretreated at 205 °C for 10 minutes, without SO₂ as much as 23 %. Similar improvements were observed for different enzyme loadings and solid consistencies. However, refining did not improve the hydrolyzability of solids pretreated at five other conditions. Reduced particle sizes were found to be correlated to increased sugar yields in enzymatic hydrolysis. Solids pretreated at 205 °C 10 minutes, without SO₂ exhibited the largest size reduction after refining and correspondingly achieved the highest overall sugar recovery improvement after steam pretreatment and enzymatic hydrolysis. In general, refining can enable a catalyst free, low inhibitor concentration, high overall sugar recovery bioconversion system based on the steam pretreatment and enzymatic hydrolysis method.