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Ethanol produced from various lignocellulosic materials such as wood, agricultural and forest residues has the potential to be a valuable substitute for, or complement to, gasoline. One of the major resources in the Northern hemisphere is softwood. This paper reviews the current status of the technology for ethanol production from softwood, with focus on hemicellulose and cellulose hydrolysis, which is the major problem in the overall process. Other issues of importance, e.g. overall process configurations and process economics are also considered.

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Second-generation bioethanol produced from various lignocellulosic materials, such as wood, agricultural or forest residues, has the potential to be a valuable substitute for, or a complement to, gasoline. One of the crucial steps in the ethanol production is the hydrolysis of the hemicellulose and cellulose to monomer sugars. The most promising method for hydrolysis of cellulose to glucose is by use of enzymes, i.e. cellulases. However, in order to make the raw material accessible to the enzymes some kind of pretreatment is necessary. During the last few years a large number of pretreatment methods have been developed, comprising methods working at low pH, i.e. acid based, medium pH (without addition of catalysts), and high pH, i.e. with a base as catalyst. Many methods have been shown to result in high sugar yields, above 90% of theoretical for agricultural residues, especially for corn stover. For more recalcitrant materials, e.g. softwood, acid hydrolysis and steam pretreatment with acid catalyst seem to be the methods that can be used to obtain high sugar and ethanol yields. However, for more accurate comparison of different pretreatment methods it is necessary to improve the assessment methods under real process conditions. The whole process must be considered when a performance evaluation is to be made, as the various pretreatment methods give different types of materials. (Hemicellulose sugars can be obtained either in the liquid as monomer or oligomer sugars, or in the solid material to various extents; lignin can be either in the liquid or remain in the solid part; the composition and amount/concentration of possible inhibitory compounds also vary.) This will affect how the enzymatic hydrolysis should be performed (e.g. with or without hemicellulases), how the lignin is recovered and also the use of the lignin co-product.

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The aim of the study was to evaluate, from a technical and economic standpoint, the enzymatic processes involved in the production of fuel ethanol from softwood. Two base case configurations, one based on simultaneous saccharification and fermentation (SSF) and one based on separate hydrolysis and fermentation (SHF), were evaluated and compared. The process conditions selected were based mainly on laboratory data, and the processes were simulated by use of Aspen plus. The capital costs were estimated using the Icarus Process Evaluator. The ethanol production costs for the SSF and SHF base cases were 4.81 and 5.32 SEK/L or 0.57 and 0.63 USD/L (1 USD = 8.5SEK), respectively. The main reason for SSF being lower was that the capital cost was lower and the overall ethanol yield was higher. A major drawback of the SSF process is the problem with recirculation of yeast following the SSF step. Major economic improvements in both SSF and SHF could be achieved by increasing the income from the solid fuel coproduct. This is done by lowering the energy consumption in the process through running the enzymatic hydrolysis or the SSF step at a higher substrate concentration and by recycling the process streams. Running SSF with use of 8% rather than 5% nonsoluble solid material would result in a 19% decrease in production cost. If after distillation 60% of the stillage stream was recycled back to the SSF step, the production cost would be reduced by 14%. The cumulative effect of these various improvements was found to result in a production cost of 3.58 SEK/L (0.42 USD/L) for the SSF process.

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