Ethanol production from clean chips and whole-tree chips poplar feedstocks: effects of preprocessing on ethanol yield and process economics

Abstract

Biomass feedstock represents up to 40% of the operating costs of ethanol production. The overarching goal of this work is to develop an economically viable process to convert poplar biomass to ethanol. Two different types of poplar feedstocks were compared: poplar clean pulp chips (CPC) with low non-structural components (NSCs) content and thus more expensive; and poplar whole-tree chips (WTC) with high NSCs content and consequently cheaper. NSCs can be very detrimental to the overall ethanol production yields, so a biomass preprocessing step can be used to partially remove these components from the biomass prior to the pretreatment. The objective of the present work was to evaluate and compare the technical and economic feasibility of three biorefinery scenarios: 1) CPC feedstock via pretreatment, enzymatic hydrolysis, overliming, and sugars to ethanol fermentation, 2) WTC feedstock via preprocessing, pretreatment, enzymatic hydrolysis, and sugars to ethanol fermentation, and 3) CPC feedstock via preprocessing, pretreatment, enzymatic hydrolysis, and sugars to ethanol fermentation. Three different preprocessing conditions were tested (acidic, alkaline, and neutral washes at 80ºC for 3 hours). All untreated and preprocessed samples were subjected to steam pretreatment, enzymatic hydrolysis, and fermentation. An additional detoxification step (i.e., overliming) was investigated for the CPC samples. All preprocessing conditions tested were effective in removing NSCs from both types of biomass at different extents. Acidic preprocessing was the most effective, removing 81% and 66% of ash from CPC and WTC, respectively; and removing 42% and 57% of extractives from CPC and WTC, respectively. At the same time, acidic preprocessing achieved the highest improvement in monomeric sugar yield after steam pretreatment and enzymatic hydrolysis (85 kg and 38 kg increase per tonne of WTC and CPC, respectively, when compared to the untreated). This is due to the removal of ash during preprocessing, which lowered the buffering capacity of the biomass and resulted in better solubilization of the monomeric sugars during pretreatment. Fermentation of the liquid fraction after steam pretreatment showed that similar improvements on ethanol yield can be achieved via preprocessing (49-56% ethanol yield) and overliming (45% ethanol yield). However, sugar losses of 20-30% were observed during the overliming step, which reduced the final ethanol yield. Finally, a preliminary economic assessment showed that using WTC via preprocessing (scenario 2) resulted in a return on investment of 920% when compared to scenario 1. This is due to the low price of WTC, and to the high monomeric sugar yield from acidic preprocessed WTC, which in turn increased the ethanol production. Thus, this work will have important implications for increasing the ethanol yield and lowering the lignocellulosic ethanol production costs.