Optimization of a biomass to bioethanol process using exergy

摘要

The use of residual biomass to produce bioenergy is a field of ever growing importance.One of the interesting residual biomass streams is corn stover.In some countries it is used as feed for animals,but in the United States of America an abundance of corn stover exists and the conversion of corn stover to ethanol is a subject of practice and research.In this paper,a design of a corn stover to ethanol process made by NREL [1] is evaluated based on exergy.The design consists of a hydrololysis section,a saccharification and co-fermentation section,a separation section,which includes distillation,a steam and power generation section,and other utilities.The chemical exergy of several biomass components was determined.This was used to evaluate the process using an exergy evaluation programme called OptiJoule.The results were used to calculate the exergetic costs and allocate CO2 costs using exergy.The exergetic inefficient processes were found to be the steam and power generation section,the saccharification and co-fermentation and the separation section,closely followed by the hydrolysis.It was found that the exergetic efficiency of the process as a whole is 50.6％.The exergetic efficiency of the ethanol production is 58.8％(expressed in exergetic costs),whereas the exergetic efficiency of the production of electricity(expressed in exergetic costs)is only 20％.To produce the ethanol from corn stover 0.93 kg CO2/kg is emitted,whereas 0.33 kg CO2/kWh is emitted for the production of electricity.The latter is still 43％lower than the average(Dutch)CO2 emission of 0.581 kg CO2/kWh for produced electricity.Optimization of the process is possible.The process has a large water demand,mainly for cooling.A location close to a waterway may reduce the water consumption considerably,and additionally gives easy access to the required water to be evaporated.An improved design is proposed using anaerobic digestion of all beer still bottoms of the distillation of ethanol,followed by a solid-liquid separation and subsequent combustion or gasification of the solid to steam.Given the scale of the plant it is proposed to combust the biogas in a small combined cycle.It can be concluded that biofuel production from corn stover has a positive effect on the CO2-emission.Additionally it can be concluded that this process can be optimized further and that exergy proved a useful tool to do so.