Techno-Economic Impacts of Pretreatment Options in Cellulose-Ethanol Production from Model Grass Analyzed

The production of ethanol from lignocellulosic biomass usually involves three sequential processes: (1) pretreatment, for lignin removal and liberation of complex carbohydrates in the biomass, (2) saccharification, for the conversion of the liberated complex carbohydrates into simple sugars, and (3) fermentation of simple sugars to ethanol. The pretreatment step offers the most number of technological options. Common pretreatment methods include the use of dilute acid, dilute alkali, hot water (hydrothermal treatment) or steam explosion treatment.

Cost-effectiveness is the primary consideration for the selection of a good pretreatment method, but techno-economic comparisons among different pretreatment options have been difficult. This difficulty has been attributed to the fact that process models have been developed only for individual pretreatment process, without looking at the "overall picture" of the cellulose-ethanol production process. A "lack of a consistent process modeling framework for the underlying ethanol production process" reportedly makes comparison difficult.

Researchers from Oregon State University (United States) attempted to develop process models for four common pretreatment processes under a "consistent underlying framework", in order to investigate "economic feasibility, compare energy consumption and sensitivity of the ethanol price to process parameters". Using a grass-based, tall fescue (Festuca arundinacea Schreb) straw as the model feedstock, they compared the techno-economic factors for the following pretreatment methods: dilute acid, dilute alkali, hydrothermal and steam explosion.

Among the highlights of the study are: (1) hot water (hydrothermal) pretreatment had the lowest unit ethanol production cost, due to non-use of chemicals while achieving comparably reasonable pretreatment efficiencies, (2) the capital cost of an ethanol production plant was lowest for steam explosion, due to a "high solids loading assumption" during pretreatment and hydrolysis, (3) ethanol production cost was shown to be sensitive to pentose fermentation efficiency, (4) energy from lignin residue was found to be sufficient to supply total steam requirement for the production plant (for all pretreatment options), (5) water use in the production process using steam explosion, (6) potentials for reducing ethanol production cost are in increasing pentose fermentation efficiency and reducing the costs of biomass (feedstock) and enzyme. The full article is published in the open-access journal, Biotechnology for Biofuels.