Hydrolytic and Pyrolytic Technologies of Pretreatment Lignocellulose for Production of Ethanol Fuels - A Comparative Review

The development of lignocellulose-based liquid fuels has enormous potential to replace the over-consumption of traditional fossil fuels while reducing carbon emissions. Lignocellulosic ethanol, as one of the most important green liquid fuels, is predominantly produced by the biorefining process of biomass, mainly including biomass pretreatment, saccharification, detoxification, fermentation, etc. Among them, the saccharification of lignocellulosic biomass is one of the most vital steps to produce fermentation sugars. Here, the conversion technologies of both biomass hydrolytic sugars (glucose) and pyrolytic sugars (levoglucosan) are first compared and evaluated together. The effect of biomass types (poplar, softwood, grass), chemical compositions (cellulose, hemicellulose, lignin), two conversion technologies of sugars, pretreatment technologies (physical, chemical, biochemical and combinations), detoxification (pretreatment, oxidization, extraction, and adsorption), and fermentation (all kinds of enzymes) process on the production of sugar and ethanol are systematically investigated and summarized. Chronologically, the development and achievement of conversion technologies for pyrolytic and hydrolytic sugars are first studied and analyzed in detail. The novel routes of pretreatment coupled with hydrolysis or pyrolysis and the new methods of detoxification and fermentation are summarized and analyzed. Life cycle assessment and techno-economic analyses show that the fermentation of pyrolytic sugars would attract and supply capital markets in the future. The advantages and disadvantages of the two conversion technologies are systematically compared and analyzed. Based on previous research work, the subsequent development of two saccharification technologies is respectively outlooked for biomass-based liquid fuels.