| Stalks based fuel ethanol is a type of renewable fuel and can be used for vehicles. The utilization of stalks based fuel ethanol decreases the air pollution from vehicle exhaust emissions and directly burning of stalks in the countryside, and improves our energy consumption structure. The effect of different pretreatment methods on conversion efficiency of cellulose to ethanol was investigated with special focus on the mechanism of the ethanol production from organic acid treated samples. The feasible process for the production of cellulosic ethanol was obtained. Developing the potential application of residual solids and liquid can improve the utilization levels for raw materials and energy, which can provide theoretic and technical guidance in the industrial application of the cellulosic ethanol.In this study, different dilute acid-steam pretreatments were investigated to enhance the efficiency for converting lignocellulosic biomass to ethanol. The raw materials were first impregnated by different solvents, including pure water,1% acetic acid,0.2% sulfuric acid, and 0.4% phosphoric acid. The solid yield, sugar recovery rate and ethanol yield from these pretreatment methods were compared. High-solid simultaneous saccharification and fermentation (SSF) of treated solids were discussed, especially for the utilization of both pentose and hexose. We analyzed the yields of the end-products and energy recovery rates based on the fermentation processes. Finally, we made techno-economic analyses for the co-generation of ethanol and electricity. The main conclusions are as follows:The hemicellulose content after steam pretreatment decreased dramatically. The degradation of hemicellulose was significantly affected by the change of temperature. The concentration of inhibitors from different pretreatment methods and investigated the kinetics for the production of inhibitors had been compared. High concentrations of fermentation inhibitors in the pretreatment hydrolysate were observed with increasing treatment intensity. The concentration of acetic acid was the highest among all the inhibitors. The highest concentration of acetic acid reached 8-9 g/L. In addition, there are some furfural and Hydroxymethylfurfural (HMF) in the pretreatment liquid. Phosphoric acid impregnation, following by steam pretreatment, resulted in high concentration of furfural and low concentration of acetic acid.We compared the total recoveries of glucose and xylose. It was found that the glucose mainly came from enzymatic hydrolysis of cellulose and xylose came from the degradation of hemicellulose during the pretreatment process. The highest glucose yield of 92% was obtained when 0.4% phosphoric acid was first used for corn stalks, following by steam pretreatment. However, the xylose yield is low in this condition. A glucose yield of 80% can be obtained when corn stalks were pretreatment with 0.2% sulfuric acid or 1% acetic acid, while the xylose yield reached 70%. The sulfuric acid impregnated samples after steam pretreatment and washing resulted in a xylose yield of over 90%.The SSF of unwashed or washed residues after steam pretreatment was compared. It was suggested that washed samples had higher ethanol concentration and ethanol yield. The fermentation rate of washed samples was higher than that of unwashed samples. Our results showed that sulfuric acid impregnated samples had a higher ethanol yield of 80.8% compared with other solvent impregnated samples at the same condition for steam pretreatment (200℃,10 min). The highest ethanol concentration was 28.4 g/L from unwashed samples with phosphoric acid impregnation and stream pretreatment.Some by-products from the production of cellulosic ethanol were analyzed in this study. It was found that the concentrations of acetic acid and lactic acid from unwashed samples were higher than those from washed samples. The concentration of acetic acid was associated with the value of pH of broth. PH 5.0 resulted in 7.7 g/L acetic acid from sulfuric acid impregnated samples after 24 h of fermentation. The concentration of lactic acid was lower than that of acetic acid. The highest concentration of lactic acid (2.6 g/L) was obtained from 24 h of fermentation of sulfuric acid impregnated samples.High-solid fermentation improved the final ethanol concentration in the fermentation broth, but decreased the ethanol yield as well. Our results found that a solids-loading of 20% led to a high ethanol concentration of 49.9 g/L, but the ethanol yield was low (68.6%). A high ethanol yield of 82.7% was obtained at a solids-loading of 12.5%. One type of strains (KE6-12), which can specifically degrade pentose, was used for fermentation of unwashed samples after acid impregnation and steam pretreatment. The efficiency of ethanol production was significantly improved by changing the feeding process and using KE6-12. A high ethanol concentration (27.6 g/L) and high ethanol yield (78%) were obtained when samples were fermented at pH 5.5 and 30℃ for 144 h.Furthermore, the end-products yields and energy recovery rates were discussed between two processes for the production of cellulosic ethanol. Process I:raw material-acid impregnation-steam pretreatment-washing; the obtained residues were used for SSF, and the obtained liquids, as well as the residual liquids after fermentation, were used for anaerobic fermentation; Process II:raw material-acid impregnation-steam pretreatment; the obtained slurries were directly used for SSF; the residual liquids after fermentation were used for anaerobic fermentation. Our results indicated that the methane yield and total energy recovery from Process I were higher than those from Process II. The highest methane yield of 11.6 g/100 g-corn stalks was obtained from acetic acid impregnation and steam pretreatment, following by Process I. The highest energy recovery rate of 88% was also obtained from this process.The co-generation of ethanol and electricity was simulated by Aspen Plus for techno-economic analysis. It was found that the lowest cost of 4.9 RMB/L for an annual output of ethanol of 30,000 tons could be achieved under Process I, while a 30MW biomass power plant needed to be supplied as well. This process will bring great social and environmental benefits.In this paper, it was the first time to combine the production of ethanol, bio-gas and electricity by integrated utilization of components of cellulosic materials. In addition, the techno-economic analysis was conducted for the conversion process; One new type of strain, which can utilize both hexose and pentose, was developed based on the SSF. Furthermore, high-solids SSF and different feeding process were also discussed. |
