Coordinated development of leading biomass pretreatment technologies for the generation of bioethanol from Irish crops

Abstract

Increased environmental awareness, coupled with increasing global energy demands, is facilitating the emergence of a green economy; a low carbon, resource efficient and socially inclusive economy aimed at reducing polluting emissions, preventing loss of biodiversity and valuing ecosystem services. Irish energy crops have the potential to contribute to the national green economy for the production of second generation biofuels provided improved lignocellulosic deconstruction processes can be identified and developed with the necessary economic and environmental-impact characteristics. Four dedicated energy crops which can be grown in Ireland were selected for this study; switchgrass (Panicum virgatum L), miscanthus (x-giganteus), hemp (Cannabis sativa L) and willow (Salix). The primary aim was to explore, develop and compare biomass pretreatment approaches for these energy crops and to gain an insight into their potential for the production of second generation biofuels. The objectives of this study were to: - Conduct a comprehensive review of leading biomass pretreatment technologies and select prospective approaches for the bioconversion of the Irish energy crops. - Perform a comparative analysis of various chemical and enzymatic pretreatment approaches for lignocellulosic hydrolysis and bioethanol production using the four crops. - Evaluate the economic performance of the targeted pretreatment chemicals. - Conduct a Life Cycle Assessment (LCA) profile of the pretreatment technologies. Chemical and enzymatic pretreatment was demonstrated to be crop specific. Pretreatments employing ammonia proved most effective for willow and hemp saccharification with yields of 59% and 35.7%, respectively. Sulphuric acid pretreatment generated highest saccharification yields for miscanthus at 41.5%, while methanol pretreatment generated the highest yields for switchgrass at 69%. Through a series of process refinements and improvements, including the introduction of simultaneous saccharification and fermentation, these bioconversion yields significantly increased to 97% for switchgrass (methanol pretreatment), 80% for miscanthus (ammonia pretreatment), 98% for hemp (sulphuric acid pretreatment) and 99% for willow (ammonia pretreatment). Assessment of the cost of switchgrass pretreatment demonstrated that methanol was the most efficient pretreatment chemical at €0.55 kg-1 glucose and €0.50 L-1 ethanol. This compares to sodium hydroxide at €2.52 kg-1 glucose and €1.96 L-1 ethanol; sulphuric acid at €2.41 kg-1 glucose and €1.83 L-1 ethanol; ammonia at €0.92 kg-1 glucose and €0.80 L-1 ethanol. An LCA conducted for switchgrass pretreatment processes demonstrated that the environmental receptors are pretreatment-specific and that there is no one leading pretreatment technique. However, it is concluded that methanol generated the lowest emissions output contributing to the lowest Global Warming Potential. This is significant as methanol has the potential to be a leading pretreatment technology with commercial viability. The research has shown that the pretreatment step can be optimised to increase the yield of ethanol from energy crops grown in Ireland while minimising environmental impact.