The Study On Ethanol Production From Corn Stover

The depletion of the world's petroleum supply and the increasing problem of greenhouse gas effects have resulted in an increasing worldwide interest in alternative non petroleum-based sources of energy. It is reported that the stockpiles of China's traditional energy resource are not optimistic as we think. The exploitable year of oil is about 50 years. Another main energy sources——natural gas is 70 years. In 2004, the newest data showed that the coal stockpiles had been plunged into its lowest level after 20 years'digging. Ethanol produced from biomass is one attractive alternative for partial replacements of fossil fuels. Large scale application of fuel ethanol will contribute to reduction of CO2 and other emissions from the transport sector. Unlike fossil fuels, ethanol as a renewable energy source can be produced through fermentation of sugars.Lignocellulosic residues such as corn stover are considered as attractive raw materials for the production of fuel ethanol because of their availability in large quantities at low costCorn stover is an abundant agricultural residue and consists of cellulose,hemicellulose and lignin. Corn stover is ubiquitous in China comprising more than 100 million tonnes every year. Only small part of corn stover is collected and used and the most part is buried in the air in our country. This causes great pollution and waste. This method of direct combustion results in low thermal efficiency, only about 10%. If direct combustion is converted into gas or liquid fuel (ethanol,hydrogen,diesel,etc.), thermal efficiency can reach up to 30%.This will not only alleviate the resource crisis,food shortages, environmental pollution and other issues, but also provide a guarantee for the sustainable development of society.The cost of raw material dominates the cost of total ethanol production.To attain commercial interest, the costs of bioethanol production mustbe reduced, and a sufficient amount of cheap and readily available rawmaterial is a necessity. Therefore, it has potential as an industrial fermentation substrate.In order to make full use of corn stover resources, find out the appropriate process for ethanol production from corn stover and ultimately to realize industrialization, the study on ethanol production from corn stover was carried out. Main contents are divided into five parts. The first part is that optimization of different hydrothermal pretreatment conditions.The second part is that ethanol production from wet oxidized corn stover by simultaneous saccharification and fermentation. The third part is that ethanol production from corn stover by Pichia stipitis. The four part is that effect of different detoxification methods on ethanol production from corn stover hydrolysate. The fifth part is that the study on ethanol production from corn stover adopting hydrothermal pretreatment and detoxification1. Optimization of different hydrothermal pretreatment conditionsDue to the close association to lignin in the plant cell wall, pretreatment is necessary to make the carbohydrates available for enzymatic hydrolysis. Hydrothermal pretreatment was carried out in high temperature. The pretreatment temperature and residence time were chosen to be 195℃and 15min, which was reported to be optimum. Compared to other pretreatment processes, Hydrothermal pretreatment has been proven to be more efficient for treating some lignocellulosic materials because of the advantage of producing fewer by products.Hydrothermal pretreatment conditions was optimized by this work. Recovery of cellulose and hemicellulose,enzymatic hydrolysis of cellulose were studied under the three conditions.The results showed:(l)Under three different hydrothermal pretreatment conditions, A (195℃,15min),B (195℃,15min,O2 12bar) and C (195℃,15min,Na2CO3 2g/L,O2 12bar),about 90% of cellulose was remained in the solid cake and most of hemicellulose and lignin were solubilized and degraded. Pretreatment significantly increased the digesty of cellulose.(2)Under the best condition(195℃,15min,Na2CO32g/L,O2 12bar), the recovery of cellulose was 95.87% which was higher than those under other pretreatment conditions. After 24h hydrolysis using cellulase (Cellubrix L), the achieved conversion of cellulose to glucose was 67.6% compared with 16.2% for raw corn stover2. Ethanol production from wet oxidized corn stover by simultaneous saccharification and fermentation At present, many experiments about ethanol production from lignocellulosic materials were not made on real fibrous substrate but only in filtrate using added glucose and xylose as sugar components. In order to test the real process, the glucose and xylose should be derived from enzymatic treatment of the wet oxidized fiber material as performed in the well-known simultaneous saccharification and fermentation (SSF).In experiments, we chose S. cerevisiae in the form of dried baker's yeast because it has often been proposed as the best organism for the fermentation.Dried baker's yeast has the advantage of being quite robust and less sensitive to inhibitors than other cultivated yeast strains.This will be conductive to realize industrialization.Recovery of cellulose and hemicellulose and the achieved conversion of cellulose to glucose was studied after wet-oxidation pretreatment (195℃,15min,Na2CO32g/L, O2 12bar) with substrate concentration of 8%, simultaneous saccharification and fermentation (SSF) were adopted to produce ethanol by Dried baker's yeast.The results showed(1)After wet-oxidation pretreatment solid fraction and hydrolysate were filtered and collected respectively.90% of cellulose remained in the solid fraction and the recovery of cellulose was 95.87%, whereas recovery of hemicellulose was only 68.2% after pretreatment. The recovery of cellulose was obviously higher than that of hemicellulose. In wet oxidation, the hemicellulose was converted or degraded at high temperature. This relatively low recovery was owing to hemicellulose oxidation to other products, such as carboxylic acids, CO2 and H2O.(2)After 24h hydrolysis at 50℃using Cellulase, enzymatic conversion of pretreated cellulose in the remaining solid was 67.6% compared with 16.2% for the native corn stover. This was expected because this pretreatment most significantly modified the composition of the solid portion of pretreated corn stover compared with the untreated corn stover. The pretreatment more effectively increased the conversion of cellulose to glucose.(3)After 142 h of SSF with substrate concentration of 8%, ethanol yield of 79.0% of the theoretical was obtained. The estimated total ethanol production was 262.7 kg/ton raw material by assuming the consumption of both C-6 and C-5. No obvious inhibition effect occurred during SSF. No obvious inhibition effect occurred during SSF.High ethanol concentration was obtained with simultaneous saccharification and fermentation (SSF) by robust dry yeast.Detoxification procedures was not used in fermentation process. This will reduce process steps and low production costs. It is conductive to realize industrialization.These offered experiment evidences for ethanol production from corn stover.3. Ethanol production from corn stover by Pichia stipitisCellulose and hemicellulose can be hydrolyzed to mainly liberate glucose and xylose respectively.The ability of microorganisms to ferment both glucose and xylose in biomass to ethanol is of importance for an economically feasible process. Native strains of Saccharomyces cerevisiae are unable to utilize xylose. Naturally occurring yeasts such as Pichia stipitis, Candidashehatae, and Pachysolen tannopHilus are able to ferment both glucose and xylose to ethanol. Among the xylose-fermenting yeasts, P. stipitis has shown the most promise for industrial application. But Pichiia stipitis is sensitive to fermentation inhibitors and need to detoxify in advance for fermentation process. So the current study focuses on utilizing detoxified hydrolysate for fermentation. Simultaneous saccharification and fermentation (SSF) with the mixture of pretreated solid cake and diluted hydrolysate will not only lift the inhibition effect, but also low production costs because both glucose and xylose are derived from enzymatic treatment of the pretreated materials.The ratio of solid cake and hydrolysate of Hydrotherm-pretreated (195℃,15min) corn stover was optimized. With substrate concentration of 5%(W/V), Additional experiments were performed to investigate the effects of temperature (30℃,35℃,40℃), initial pH (5.5,5.5,6.0) and shaking rate (100 rpm,130 rpm) on the fermentation. The results showed:(1)After Hydrothermal pretreatment (195℃,15min) solid fraction and hydrolysate were filtered and collected respectively.86.5% of cellulose were remained in solid cake and most of hemicellulose were solubilized after pretreatment.(2)The highest ethanol concentration of 12.12 g/L was achieved at initial pH of 5.5, temperature 30℃and 130 rpm shaking rate with substrate concentration of 5%(w/v) by Pichia stipitis after 192 h fermentation. The corresponding ethanol yield and volumetric productivity were 0.34g/g (glucose+xylose) and 0.065g/L.h respectively.(3)Native strains of Saccharomyces cerevisiae are unable to utilize xylose. Pichia stipitis is able to ferment both glucose and xylose to ethanol and improve the utilization of raw materials. Detoxification procedures was not used in fermentation process. This will reduce production costs.4. Effect of different detoxification methods on ethanol production from corn stover hydrolysateDuring pretreatment process of corn stover, several compounds such as furfural,5-hydroxymethyl furfural, acetic acid are released as a result of sugar or lignin degradation processes.The degradation products inhibit the metabolism of fermentative microorganisms and negatively affect the efficiency of fermentation. At present, there are many detoxification methods. Among them, overliming and Na2SO3 addition detoxification are widely used and neutralization method is relatively simple.However, the effect of the different methods cannot be compared with each other because different hydrolysate and different microorganisms are used in the fermentations. In this study, the influence of three different detoxification methods (neutralization,overliming and Na2SO3 addition) on inhibitors were evaluated using corn stover hydrolysate prepared with hydrothermal pretreatment. (195℃,15min).Ethanol fermentability of detoxified corn stover hydrolysate was investigated by Pichia stipitis 58376. The results showed:(1)Total furan concentration in pretreated corn stover hydrolysate was affected by all three detoxification methods applied. All the employed detoxification methods resulted in a 41% reduction in average total furans. The highest removal of furans (44.3% removal) was achieved by Na2SO3 addition method.28.4% removal of total pHenols was achieved by overliming and 10.6% by neutralization, whereas no changes occurred by Na2SO3 addition.(2)Ethanol fermentability of detoxified corn stover hydrolysate was investigated by Pichia stipitis 58376. Fermentation performance was greatly enhanced by employed detoxification methods. Ethanol yield of 69.31% of the theoretical based on reducing sugar was obtained by overliming. The corresponding ethanol concentration and volumetric productivity were 12.2g/L and 0.056g/L.h. These values were obviously higher than those of other detoxification methods.(3)Overliming was the most efficient detoxification methods for Hydrotherm-pretreated corn stover hydrolysate5. The study on ethanol production from corn stover adopting hydrothermal pretreatment and detoxificationAfter hydrothermal pretreatment of corn stover, solid fraction and hydrolysate were collected separately. Ethanol production was evaluated from dried solid fraction and the hydrolysate was employed as liquid fraction by baker'yeast. The effect on different Ph and detoxification on ethanol production were investigated. The results showed:(1)When 100% hydrolysate was added, the ethanol concentration of 0.31g/L (9.48% of theoretical ethanol yield) was obtained due to existence of acetic acid and furans which are important inhibitor of the fermentation to microorganisms. After prehydrolysis, the initial pH was adjusted to 5.5,6.0 and 6.5 respectively. The best value obtained was 10.67g/L of ethanol concentration (56.4% of theoretical ethanol yield) with addition of 100% hydrolysate at pH of 5.5.The hydrolysate was overlimed, then prehydrolysized for 24h at 50C.After prehydrolysis the initial pH was adjusted again to 5.5. The ethanol concentration of 10.96g/L (57.9% of theoretical ethanol yield) was obtained. The values are almost 35 times high compared to those at initial pH 4.8.(2) Acetic acid is an important fermentation inhibitor basically due to its undissociated form. The utilization of increasing pH higher than pKa of acetic acid can partially overcome this problem. Overliming is an effective way to reduce the toxicity of hydrolysate.(3)The cost of fermentable sugar losses during detoxification process should be considered.Our study showed that considering its abundance and high cellulose content, corn stover could be an excellent substrate for ethanol production. The studies above provided the basis for fuel ethanol production from corn stover. However, we still face many challenges. Reducing the cost of ethanol production is the most fundamental that requires to further investigate all the aspects of ethanol fermentation.(1) Cheap and special pretreatment methods should be developed. The study on formation of fermentation inhibitors, mechanism of inhibitors and mechanism of tolerany-inhibitors of yeast should be strengthened.(2) To reduce cellulose cost, more efficient microorganisms to produce cellulose or genetically engineering microorganism for cellulose production should be screened.(3) Not only are the properties of the microorganism of importance in the process, but also the choice of fermentation strategies such as batch culture, continuous culture with cell recycling. For the realization of the ethanol production from lignocellulosic materials, the fermentation step has to be integrated with the rest of the process.In a word, exploring the best ways and production process from all aspects of ethanol production from lignocellulosic materials would obtain the best results.