Study On Pretreatment Technology And Integrated Energy Utilization Of Wheat Straw

As the biggest country of population and agriculture in the world, China generated a total of more than 700 milloin tons of crop straw including 150 million tons of wheat straw for each year. Due to the disperse management pattern of agritulture, a lot of wheat straw are discarded irresponsibly and open bured, which lead to resource waste and environment pollution. As a major agriculture residue with high content of organic matter, wheat straw is an attractive potential low cost feedstock for biofuel production. However, its high lignin content limits enzymatic and microbial degradation. Considerable research efforts have been made to improve the degradability of lignocellulosic materials by different pretreatments. Alkaline pretreatment has been reported to improve the biodegradability of lignocellulosic biomass by causing cellulose swelling, increasing internal surface area, and disrupting lignin structure. Most previous investigations focused on pretreatments with sodium hydroxide (NaOH) solution at high temperatures, which lead to environment pollution of sodium ion discharge and generation of inhibitory products for subsequent fermentation process. In this study, potassium hysroxide (KOH) pretreatment with ambient temperature will be conducted to solve the problem of black liquor pollution and improve the enzymatic hydrolysis and anaerobic digestion performance of wheat straw. Alkaline pretreatment with black liquor recycle and the combination with enzyme addition are conducted and the utilization potential of black liquor will be also evaluated by membrane separation to improve the energy conversion efficiency and provide data base for industry application.The pretreatment of wheat straw with a wide range of KOH loading (2%-50% w/w dry basis) at ambient temperature (20℃) was investigated. Higher KOH loading resulted in increased cellulose content, higher hemicellulose and lignin removal from the straw and COD in the resulting black liquor. Maximum lignin removal of 54.7% was observed at 50% KOH loading. The pretreatment effects on chemical composition and physical structures, and subsequent enzymatic hydrolysis and anaerobic digestion were evaluated. In comparison to untreated straw, specific hydrolysis yields increased with increasing KOH loading and the maximum hydrolysis yiedd achieved 92.3%. Specific methane yields increased 16.7%-77.5% for KOH loadings of 10%-50%. Accounting for losses during pretreatment,20% KOH loading resulted in maximum overall reducing sugar yield of 489.5mg/g or higher methane yield and therefore the recommended loading for pretreatment under these conditions.An integrated system including pretreatment, biogasification and CHP cogeneration was proposed. The mass balance and energy balance were evaluated with three different feedstock throughputs of 50t/d、150t/d、450t/d. The results showed that the methane output increased and digested sludge decreased with increasing KOH loading. The net methane yield, electricity, and heat did not change much when KOH loading was more than 20%. The portion of net energy in total energy production was increased with increasing feedstock throughputs.Black liquor recycling and reuse was applied in this study to improve the cost-effectiveness of KOH pretreatment of wheat straw. The results indicate that a stready state was achieved after fove rounds of recycles that with reducing sugar, soluble COD and K+ in black liquor accumulating up to a steady state. Comparing with fresh chemical pretreatment, chemical consumption was 25% lower and water consumption 75% lower. The lignin removal of 32%-35%, total reducing sugar yields of 336-366 mg/g TS, specific methane yields of 290-303 mL/g VS were achieved for rounds 2 to 5 of black liquor recycling, which were significantly improved compared to raw straw, although not effective as fresh chemical pretreatment. Accounting for losses during pretreatment, the overall methane yields showed no differences for fresh chemical and black liquor recycling pretreatment and therefore black liquor recycling and reuse can improve the KOH pretreatment process effectively.The combination of alkaline pretreatment and enzyme addition was evaluated and the results showed that enzyme addition could not only improve the biogas production rate, but also enhance the biomethane yield. Three days of enzyme hydrolysis before anaerobic digestion showed higher biomethane yield than one day hydrolysis or directly adding enzyme to digester. F/M=0.5 showd best biogas production performance and too high or too low microorganism concentration could not improve biogasification and maybe inhibit the biodegradability. The mixture of cellulase and hemicellulase showed better hydrolysis yield and biogas yield comparing with single cellulase.Anaerobic digestion of black liquor was conducted and black liquor showed good biogas and methane production performance. With increasing KOH loading, the initial biodegradability of black liquor was inhibited and then gradually improved with acclimatization of microorganisms. The overall biogas and methane yields including pretreated wheat straw and black liquor were calculated based on mass balance after pretreatment, and the results showed that the overall biogas yield of 474.1mL/g VS and methane yiled of 263.4 mL/g were achieved at 20% KOH loading.The black liquor was treated through microfiltration to remove solid firstly and then reverse osmosis membrane to recycle potassium. The solid removal achieved 74.1% with microfiltration and reject factor for potassium achieved 90.3% with reverse osmosis membrane. Therefore, reverse osmosis membrane was an option for potassium recycle, which provide an efficient approach for black liquor utilization.This study focused the novel pretreatment technology of wheat straw from the view of industrialization. The pretreatment performance and the potential value of black liquor were evaluated to provide a feasible and sustainable way for efficient energy convertion of wheat straw in the future.