Mats Galbe

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.

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.

Ethanol production from enzymatic hydrolysates of sugarcane bagasse using recombinant xylose-utilising Saccharomyces cerevisiae

Sugarcane bagasse was pre-treated by steam explosion at 205 and 215degreesC and hydrolysed with cellulolytic enzymes. The hydrolysates were subjected to enzymatic detoxification by treatment with the phenoloxidase laccase and to chemical detoxification by overliming. Approximately 80% of the phenolic compounds were specifically removed by the laccase treatment. Overliming partially removed the phenolic compounds, but also other fermentation inhibitors such as acetic acid, furfural and 5-hydroxy-methyl-furfural. The hydrolysates were fermented with the recombinant xylose-utilising Saccharomyces cerevisiae laboratory strain TMB 3001, a CEN.PK derivative with over-expressed xylulokinase activity and expressing the xylose reductase and xylitol dehydrogenase of Pichia stipitis, and the S. cerevisiae strain ATCC 9658 1, isolated from a spent sulphite liquor fermentation plant. The fermentative performance of the lab strain in undetoxified hydrolysate was better than the performance of the industrial strain. An almost two-fold increase of the specific productivity of the strain TMB 3001 in the detoxified hydrolysates compared to the undetoxified hydrolysates was observed. The ethanol yield in the fermentation of the hydrolysate detoxified by overliming was 0.18 g/g dry bagasse, whereas it reached only 0.13 g/g dry bagasse in the undetoxified hydrolysate. Partial xylose utilisation with low xylitol formation was observed. (Less)

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.

Comparison of SO2 and H2SO4 impregnation of softwood prior to steam pretreatment on ethanol production

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)

The effect of Tween-20 on simultaneous saccharification and fermentation of softwood to ethanol

Fuel ethanol can be produced from softwood through hydrolysis in an enzymatic process. Prior to enzymatic hydrolysis of the softwood, pretreatment is necessary. In this study two-step steam pretreatment by dilute H2SO4 impregnation to improve the overall sugar and ethanol yield has been investigated. The first pretreatment step was performed under conditions of low severity (180°C, 10 min, 0.5% H2SO4) to optimise the amount of hydrolysed hemicellulose. In the second step the washed solid material from the first pretreatment step was impregnated again with H2SO4 and pretreated under conditions of higher severity to hydrolyse a portion of the cellulose, and to make the cellulose more accessible to enzymatic attack. A wide range of conditions was used to determine the most favourable combination. The temperatures investigated were between 180°C and 220°C, the residence times were 2, 5 and 10 min and the concentrations of H2SO4 were 1% and 2%. The effects of pretreatment were assessed by both enzymatic hydrolysis of the solids and with simultaneous saccharification and fermentation (SSF) of the whole slurry, after the second pretreatment step. For each set of pretreatment conditions the liquid fraction was fermented to determine any inhibiting effects. The ethanol yield using the SSF configuration reached 65% of the theoretical value while the sugar yield using the SHF configuration reached 77%. Maximum yields were obtained when the second pretreatment step was performed at 200°C for 2 min with 2% H2SO4. This form of two-step steam pretreatment is a promising method of increasing the overall yield in the wood-to-ethanol process. (Less)