| This thesis focused on the process simulation,optimization and environmental evaluation of biorefinery systems for producing lignocellulosic ethanol.It proposed a new biorefinery system for production of bio-ethanol,xylose,electricity and steam from corncob,which was investigated and assessed by a systematic methodology of combining the process simulation,pinch analysis,exergy analysis and life cycle assessment(LCA).In addition,the co-gasification of acid hydrolysis residues(AHR)and sewage sludge(SS)were carried out to produce high-quality fuel gas,aiming to find a new way to solid waste disposal for light industry with agricultural feedstock.The work are summarized as follows:(1)First,a new biorefinery system for production of ethanol,xylose,heat and power from corncob was introduced.Two other biorefinery systems with promising conversion technologies were constructed as basecases.The three biorefinery systems,involved with the same corncob processing capacity 875 kt/a with the same operating time 8400 h/a,were all simulated by Aspen Plus and optimized according to pinch analysis.The yield of bio-ethanol in the new biorefinery system was 193.1 L/t dry corncob(153.2 kg/t dry corncob),which was lower than that of basecase 2.Basecase 2performed the highest ethanol yield of 334.2 L/t dry corncob(265.2 kg/t dry corncob)because hemicellulose and cellulose in the raw material were both extracted to produce bio-ethanol.However,considering the economic value of the products,the new biorefinery system presented the highest 611.8 USD/t dry feedstock,much higher than 145.2 USD/t dry feedstock in basecase 1 and 237.8 USD/t dry feedstock in basecase 2.The output value of per kg corncob in new biorefinery system was quite competitive mainly because the hemicellulose in corncob was employed to produce high valued xylose.The new biorefinery system of ethanol,xylose,electricity and heat was much more competitive than those considered before.In addition,the heat exchanger network(HEN)was designed according to the pinch principles to reduce the external utilities consumption of the systems.The Aspen Energy Analyzer software was applied to identify the pinch point and energy targeting for three systems.Data of the hot and cold streams was extracted from the models in Aspen Plus.Through process integration,the biorefinery systems reduced 24.8~36.8%of cooling utilities and 46.3~60.6%of heating utilities,achieved by 10~11 heat exchangers installation.Although the heat exchangers increased the capital cost,the operating cost can be decreased significantly.The energy effiency of the biorefinery systems could be improved by the HEN for heat integration.The units of consuming steam located in the pretreatment,xylose concentration(only in the new scenario)and ethanol distillation.The pretreatment accounted for 91%of steam consumption in basecase 1 and 2.While in the new scenario,the xylose concentration contributed to 85%of steam consumption.Xylose concentration consumed a large amount of steam due to the multiple-effect evaporation,which equaled to 11.9 kg steam consumption for per kg xylose production.While,the ethanol distillation units contributed to 2~9%of the total steam consumption,equivalently8302~8915 kJ heat/kg ethanol production.(2)Then,three simulated biorefinery systems were investigated by the exergy analysis.The NER ranged from 1.4 to 1.8,indicating that the biorefinery systems were feasible from energy aspect because they were energy self-sufficient and furthermore can provide excess electricity and steam.The general exergy efficiencies of the three systems were 64.0~71.7%and the production processes as a whole presented the highest exergy efficiencies of 85.0~89.9%.CHP unit presented the lowest exergy efficiency of 65.5~67.0%because of heat transferring over a temperature difference and heat losses.The exergy efficiency in WWT was 69.4~71.8%.The work was valuable to find key unit of improving the process exergy efficiency.These results suggested some technology improvement in the CHP and effluent decrement in WWT be necessary to increase the exergy efficiency.(3)Next,three biorefinery systems were investigated using LCA method in SimaPro software.Bioethanol-blended E10 and E85 fuels were compared with a gasoline reference system against a suite of life cycle impact categories.The study also compared the three biorefinery systems with a high quality reference lignocellulosic ethanol production process model from the U.S.National Renewable Energy Laboratory(NREL).The results were presented using per distance driven(1 km)as the functional unit to facilitate analysis.Chemical exergy allocation was applied to the raw inputs(corn,corn stalk and corncob)and also the co-products(ethanol,xylose,mother liquor,electricity and steam)in the simulated multifunctional biorefinery processes.Results showed that regardless of the configuration of the ethanol-biorefinery,ethanol-blended fuels performed better in terms of fossil fuels depletion(E10 was 6%lower and E85 was 67 to 72%lower than gasoline),global warming potential(E10 was 3 to12%lower and E85 was 21 to 129%lower than gasoline)and human toxicity potential(E10 was 6%lower and E85 was 74%lower than gasoline)and E85 was a better option than E10 in terms of these impacts.But bio-ethanol fuels contributed more to ODP,AP and EP than gasoline.(4)There were two kinds of solid waste obtained from the lignocellulosic ethanol plant,AHR and SS.The last section was trying to find a way not only for solid waste disposal but also for potential energy recovery.Co-gasification of AHR and SS was carried out in a downdraft fixed-bed reactor in the presence of CaO at atmosphere pressure.The gasifier bed temperature(T),SS content in blends and equivalence ratio(ER)were investigated.The composition and low heating value(LHV)of the syngas product,as well as tar content in crude gas and cold gas efficiency(CGE)have been evaluated.The results showed that co-gasification of AHR with SS was applicable to produce high quality syngas in the downdraft fixed-bed gasifier.The optimal LHV of syngas of 6.83 MJ/Nm~3,CGE of 70.68%and tar content in the crude gas of 5.84 g/Nm~3were obtained at 800°C,SS content of 50 wt.%,CaO/C(molar ratio)of 1.0 and ER of0.22.Synergetic effects occurred in the co-gasification,and syngas quality could be improved by blending 50 wt.%of SS with AHR.When SS content was 50 wt.%,the LHV of the gas and CGE respectively reached the maximum values of 6.42 MJ/Nm~3and 67.33%,respectively.It was likely that the large amount of metal elements in SS acted as catalysts,promoting the thermochemical conversions of AHR and SS.The detailed mechanism of synergistic effects should be investigated in the future work.The results indicated the potential technology for air co-gasification of AHR and SS as an option for energy recovery and waste disposal as well as for light industry with agricultural feedstock. |
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