Heat and Power Production from Stover for Corn Ethanol Plants

Abstract

Supply of corn stover to produce heat and power for a typical 170 million L (45 million gallon) dry mill ethanol plant is proposed. The corn ethanol plant requires 5.6 MW electricity and 52.3 MW process heat, which creates an annual stover demand of as much as 150 million Mg (Mg = 1,000,000 g = 1 metric ton). The stover supply system consists of collection, pre-processing, transportation and on-site fuel storage and preparation to produce heat and power. Economics of the entire supply system was conducted using the Integrated Biomass Supply Analysis and Logistics (IBSAL) simulation model. Corn stover was delivered in three formats (square bales, dry chops and pellets) to the ethanol plant. Among the three formats of stover supply systems, cost of chopped biomass was highest due to the high transportation cost and low bulk density. The economics of the stover fired heat generation system was assessed using a discounted cash flow method and compared with coal- and natural gas-fired systems. Although the capital investment cost of natural gas-fired heat generation system was relatively low, annual operating costs were the highest compared to the stover and coal-fired heating system. The coal-fired heating system had the lowest annual operating cost due to the low fuel cost, but had the highest environmental and human toxicity impacts. Combined heat and power (CHP) generation systems have high thermal efficiency, onsite power utilization and lower environmental impacts than process heat generation systems. A corn-stover-fired CHP plant was proposed to supply both power and process heat for the corn ethanol plant. The proposed CHP system required 137,450 Mg stover to generate 9.5 MWe of power and 52.3 MWth of process heat with an overall CHP efficiency of 83.3%. Economic analysis of the stover-fired CHP system was compared with that of both coal and a combination of coal and stover fuel options along with an environmental impact analysis. Annual savings from the CHP plant were calculated by comparing the existing energy input conditions for the corn ethanol plant. The stover-fired CHP system would generate annual savings of US $3.61 million with a payback period of 6 years. The economics of the coal-fired CHP system were relatively attractive compared to the stover-fired CHP system. but greenhouse gas emissions for the coal-fired CHP system were 20 times greater than that of stover-fired CHP system. Co-firing of stover with coal may balance out the environmental impacts and economics of the CHP system. We envision that creating demand for biomass will build infrastructures to deliver biomass fuel, which will further reduce the cost of biomass