Steam pretreatment of spruce forest residues: Optimal conditions for biogas production and enzymatic hydrolysis

•Steam pretreatment enables both enzymatic hydrolysis and anaerobic digestion.•Steaming conditions for maximal biogas and enzymatic hydrolysis yield are identical.•Enzymatic hydrolysis can be used as a screening test for anaerobic digestion.•By-products formed during steaming are not inhibitive in the biogas process.

Steam refining of non-debarked spruce forest residues was investigated as pretreatment for enzymatic hydrolysis as well as for biogas production. Pretreatment conditions were varied in the range of 190–220 °C, 5–10 min and 0–3.7% SO2 according to a statistical design. For both applications highest product yields were predicted at 220 °C and 2.4% SO2, whereas the reaction time had only a minor influence. The conformity of the model results allows the conclusion that enzymatic hydrolysis is a suitable test method to evaluate the degradability of lignocellulosic biomass in the biogas process. In control experiments under optimal conditions the results of the model were verified. The yield of total monomeric carbohydrates after enzymatic hydrolysis was equivalent to 55% of all theoretically available polysaccharides. The corresponding biogas yield from the pretreated wood amounted to 304 mL/gODM. Furthermore, furans produced under optimal process conditions showed no inhibitory effect on biogas production.It can be concluded that steam refining opens the structure of wood, thus improving the enzymatic hydrolysis of the polysaccharides to fermentable monomeric sugars and subsequently enabling a higher and faster production of biogas. Anaerobic fermentation of pretreated wood is a serious alternative to alcoholic fermentation especially when low quality wood grades and residues are used. Anaerobic digestion should be further investigated in order to diversify the biorefinery options for lignocellulosic materials.