Guido Zacchi

A review of the production of ethanol from softwood

Abstract. 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.

The generation of fermentation inhibitors during dilute acid hydrolysis of softwood

The influence of the severity of dilute sulfuric acid hydrolysis of spruce (softwood) on sugar yield and on the fermentability of the hydrolysate by Saccharomyces cerevisiae (Bakers yeast) was investigated. Fermentability was assessed as the ethanol yield on fermentable sugars (mannose and glucose) and the mean volumetric productivity (4 h). The hydrolysis conditions, residence time, temperature, and sulfuric acid concentration were treated as a single parameter, combined severity (CS). When the CS of the hydrolysis conditions increased, the yield of fermentable sugars increased to a maximum between CS 2.0-2.7 for mannose, and 3.0-3.4 for glucose above which it decreased. The decrease in the yield of monosaccharides coincided with the maximum concentrations of furfural and 5-hydroxymethylfurfural (5HMF). With the further increase in CS, the concentrations of furfural and 5-HMF decreased while the formation of formic acid and levulinic acid increased The yield of ethanol decreased at approximately CS 3; however, the volumetric productivity decreased at lower CS. The effect of acetic acid, formic acid, levulinic acid furfural, and 5-HMF on fermentability was assayed in model fermentations Ethanol yield and volumetric productivity decreased with increasing concentrations of acetic acid, formic acid, and levulinic acid. Furfural and 5-HMF decreased the volumetric productivity but did not influence the final yield of ethanol. The decrease in volumetric productivity was more pronounced when 5-HMF was added to the fermentation, and this compound was depleted at a lower rate than furfural. The inhibition observed in hydrolysates produced in higher CS could not be fully explained by the effect of the by-products furfural, 5-HMF, acetic acid, formic acid: and levulinic acid

Pretreatment of Lignocellulosic Materials for Efficient Bioethanol Production

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.

Techno-economic evaluation of producing ethanol from softwood: comparison of SSF and SHF and identification of bottlenecks.

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.

A techno-economical comparison of three processes for the production of ethanol from pine

Abstract This study comprises a technical and economic comparison of three different processes for the production of fuel ethanol from pine. The processes were based on the same technical and economic assumptions. The main difference between the processes lies in the front end of each process, i.e. in the pretreatment of the raw material and in the hydrolysis of the hemicellulose and cellulose to sugars. The three processes compared are a concentrated hydrochloric acid process, a two-step dilute acid process with sulphur dioxide in the first and hydrochloric acid in the second hydrolysis step, and an enzymatic hydrolysis process, including steam pretreatment. The ethanol production costs found in this study are, excluding the capital costs, 2·43, 1·89 and 1·80 SEK/l, respectively, and including the capital costs 4·22, 4·29 and 4·03 SEK/l, respectively. Based on a sensitivity analysis, it was concluded that none of the processes can be eliminated as less economical than the others.

Process Engineering Economics of Bioethanol Production

This work presents a review of studies on the process economics of ethanol production from lignocellulosic materials published since 1996. Our objective was to identify the most costly process steps and the impact of various parameters on the final production cost, e.g. plant capacity, raw material cost, and overall product yield, as well as process configuration. The variation in estimated ethanol production cost is considerable, ranging from about 0.13 to 0.81 US

Reduced inhibition of enzymatic hydrolysis of steam-pretreated softwood.

per liter ethanol. This can be explained to a large extent by actual process differences and variations in the assumptions underlying the techno-economic evaluations. The most important parameters for the economic outcome are the feedstock cost, which varied between 30 and 90 US

The effect of water-soluble inhibitors from steam-pretreated willow on enzymatic hydrolysis and ethanol fermentation

per metric ton in the papers studied, and the plant capacity, which influences the capital cost. To reduce the ethanol production cost it is necessary to reach high ethanol yields, as well as a high ethanol concentration during fermentation, to be able to decrease the energy required for distillation and other downstream process steps. Improved pretreatment methods, enhanced enzymatic hydrolysis with cheaper and more effective enzymes, as well as improved fermentation systems present major research challenges if we are to make lignocellulose-based ethanol production competitive with sugar- and starch-based ethanol. Process integration, either internally or externally with other types of plants, e.g. heat and power plants, also offers a way of reducing the final ethanol production cost.

Two-step steam pretreatment of softwood by dilute H2SO4 impregnation for ethanol production

Softwood constitutes the main source of lignocellulosic material in Sweden which can be used for ethanol production from renewable resources. To make the biomass-to-ethanol process more economically feasible, it is preferable to include the sugar-rich prehydrolysate, i.e. the liquid obtained after the pretreatment step, in the enzymatic hydrolysis of the solid fraction. This study shows that the prehydrolysate inhibits cellulose conversion in the enzymatic hydrolysis step. When the prehydrolysate was included in the enzymatic hydrolysis, the cellulose conversion was reduced by up to 36%. However, this inhibition can be overcome by fermentation of the prehydrolysate prior to enzymatic hydrolysis.

Techno-economic evaluation of 2nd generation bioethanol production from sugar cane bagasse and leaves integrated with the sugar-based ethanol process

In the process of producing ethanol from lignocellulosic materials, compounds inhibitory to enzymatic hydrolysis and fermentation are generated during the pretreatment of the wood. In an industrial process, these compounds will accumulate due to the recirculation of process streams. The inhibitory effects of the accumulation of volatile and nonvolatile compounds released during stream pretreatment on enzymatic hydrolysis and fermentation were studied. The volatile compounds did not affect either the enzymatic hydrolysis or the fermentation significantly even at high concentrations. In contrast, the nonvolatile compounds severely affected both the hydrolysis and the fermentation: the effect was more pronounced in the latter case. For the effective use of a lignocellulosic material as a substrate for ethanol production, the nonvolatile compounds must thus be removed. (Less)