Alkaline pretreatment of spruce and birch to improve bioethanol and biogas production

TLDR

Alkaline pretreatment of spruce and birch with 7 % NaOH at temperatures ranging from –15 °C to 100 °C for 2 h was followed by enzymatic hydrolysis and either fermentation to ethanol or anaerobic digestion to biogas. The pretreatment yielded higher ethanol and biogas production, especially for birch (ethanol 0.17 g g⁻¹, methane 83 % increase) compared to spruce (ethanol 0.08 g g⁻¹, methane 74 % increase), due to reduced hemicellulose and cellulose crystallinity.

Abstract

Alkaline pretreatment with NaOH under mild operating conditions was used to improve ethanol and biogas production from softwood spruce and hardwood birch. The pretreatments were carried out at different temperatures between minus 15 and 100ºC with 7.0% w/w NaOH solution for 2 h. The pretreated materials were then enzymatically hydrolyzed and subsequently fermented to ethanol or anaerobically digested to biogas. In general, the pretreatment was more successful for both ethanol and biogas production from the hardwood birch than the softwood spruce. The pretreatment resulted in significant reduction of hemicellulose and the crystallinity of cellulose, which might be responsible for improved enzymatic hydrolyses of birch from 6.9% to 82.3% and spruce from 14.1% to 35.7%. These results were obtained with pretreatment at 100°C for birch and 5°C for spruce. Subsequently, the best ethanol yield obtained was 0.08 g/g of the spruce while pretreated at 100°C, and 0.17 g/g of the birch treated at 100°C. On the other hand, digestion of untreated birch and spruce resulted in methane yields of 250 and 30 l/kg VS of the wood species, respectively. The pretreatment of the wood species at the best conditions for enzymatic hydrolysis resulted in 83% and 74% improvement in methane production from birch and spruce.