Abstract

Norway spruce (Picea abies) is an industrially important softwood species available in northern Europe and can be used to produce bio-methane after proper pretreatment to overcome its recalcitrant complex structure. Hot water extraction (HWE) pretreatment at two different conditions (170 °C for 90 min (severity 4.02) and 140 °C for 300 min (severity 3.65)) was applied to extract hemicellulosic sugars from Norway spruce for thermophilic anaerobic digestion (AD) of the hydrolysate. The methane yield of hydrolysate prepared at the lower pretreatment severity was found to be 189 NmL/gCOD compared to 162 NmL/gCOD after the higher pretreatment severity suggesting higher pretreatment severity hampers the methane yield due to the presence of inhibitors formed due to sugars and lignin degradation and soluble lignin, extracted partially along with hemicellulosic sugars. Synthetic hydrolysates simulating real hydrolysates (H170syn and H140syn) had improved methane yield of 285 NmL/gCOD and 295 NmL/gCOD, respectively in the absence of both the inhibitors and soluble lignin. An effect of organic loadings (OLs) on the methane yield was observed with a negative correlation between OL and methane yield. The maximum methane yield was 290 NmL/gCOD for hydrolysate pretreated at 140 °C compared to 195 NmL/gCOD for hydrolyate pretreated at 170 °C, both at the lowest OL of 6 gCOD/L. Therefore, both pretreatment conditions and OL need to be considered for efficient methane production from extracted hydrolysate. Such substrates can be utilized in continuous flow industrial AD with well-adapted cultures with stable organic loading rates.

Highlights

  • Biofuels are of great interest to reduce excessive dependence on fossil fuels that trigger issues related to global warming and energy security

  • This paper aims to evaluate the methane yield in thermophilic anaerobic digestion (AD) condition from hot water extract of Norway spruce pretreated at two different pretreatment severities, by testing corresponding synthetic hydrolysates and the effect of organic loadings in AD

  • Hot water extraction of Norway spruce led to higher organic concentrations in the hydrolysate when treated at 170 ◦ C than at 140 ◦ C, with total COD (CODt) and soluble COD (CODs) concentrations of 30.7 and 26.9 g/L respectively when treated at 170 ◦ C compared to 22.3 and 20.0 g/L at 140 ◦ C

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Summary

Introduction

Biofuels are of great interest to reduce excessive dependence on fossil fuels that trigger issues related to global warming and energy security. Lignocellulosic biomass such as wood, food wastes, energy crops, and agricultural and forest residues are the most abundant renewable sources for biofuels and do not compete with food production. Its inherent recalcitrant structure and complex composition must be overcome to enhance hydrolysis and further conversion by anaerobic microorganism [2] Various pretreatments, such as mechanical, thermal, chemical, and biological methods, have been attempted on lignocellulosic biomass prior to anaerobic digestion (AD) to enhance methane production.

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