Techno-economic comparison of hot water and dilute acid pretreatment for biochemical production of ethanol from corn stover and evaluation of alternative scenarios to purchasing cellulase enzymes

摘要

Techno-economic analysis was conducted to compare hot water pretreatment and dilute acid pretreatment for biochemical production of ethanol from corn stover, and to compare several enzyme production schemes as alternatives to on-site enzyme production. Each of these scenarios was modeled in detail and economic analysis was performed to estimate the total capital investment (TCI) and Minimum Ethanol Selling Price (MESP), and Equivalent Purchased Enzyme Price for the enzyme production scenarios.In Chapter 2 hot water and dilute acid pretreatment technologies are compared for both an nth plant design as well as a pioneer plant. Plants are modeled assuming they receive 2000 MT/day (metric tonne per day) of biomass. The dilute acid pretreatment process has the lowest MESP, which is estimated to be $3.40/gal EtOH, compared to $4.29/gal EtOH for the hot water pretreatment scenario. Sensitivity analysis shows that the MESP for the dilute acid pretreatment scenario is most sensitive to feedstock costs, enzyme loading, enzyme cost, and installed equipment costs. The MESP ranges from $3.37 to $3.93 under the assumed ranges for sensitivity parameters.Cellulosic ethanol production has yet to be commercialized and a pioneer plant is expected to be significantly more expensive than an nth plant. To assess the impact of technological maturity on pioneer plant capital cost and plant performance a cost growth analysis was performed using a method developed by the RAND Corporation. Pioneer plant costs are estimated for three scenarios: optimistic, most probable and pessimistic. The estimated range of MESPs for the pioneer plant were substantially larger than for the nth plant. The MESPs for the model with dilute acid pretreatment were $4.19, $5.22 and $6.68/Gal EtOH for the optimistic, most probable and pessimistic scenarios, respectively. The Total Capital Investmetn (TCI) for the three respective scenarios increased by 54%, 24%, and 97% above the nth plant TCI.Enzymes are one of the most significant costs of cellulosic ethanol production. Chapter 3 analyzes two enzyme production schemes as alternatives to purchasing enzymes. The first is the production of enzymes on-site for a stand-alone plant. The competitiveness of on-site enzyme production with purchasing enzymes is compared among plant scales varying from 500 MT/day to 3000 MT/day. The second scheme is the production of both ethanol and excess enzymes at a central plant for export to satellite plants producing only ethanol. Two cases were examined for this scheme--one in which the central plant supplies enzymes to two satellite plants and another with four satellite plants. Both the central plant and satellite plants in this scheme receive 2000 MT/day of corn stover.For the on-site enzyme production scheme it was found that the competitiveness of on-site enzyme production with purchasing enzymes varies significantly with plant scale. At the 500 MT/day scale on-site enzyme production is economically advantageous at enzyme prices above $1.07/gal EtOH ($7.82/kg protein). The competitive price drops to $0.66/gal EtOH ($4.81/kg protein) for the 3000 MT/day plant scale. The MESPs for the 500 MT/day and 3000 MT/day are $4.70 and $3.24/gal EtOH, respectively.The results from the central enzyme production scheme are similar to those of the 2500 MT/day on-site enzyme plant. This scheme is more economically advantageous than purchasing enzymes at enzyme prices higher than $0.71 and $0.69/gal EtOH ($5.15 and $5.02/kg protein) for the cases with two and four satellite plants, respectively. The central enzyme production scheme with both two and four satellite plants has lower MESPs than a stand-alone 2000 MT/day plant with on-site enzyme production. This is due primarily to the economy of scale that exists with the enzyme production equipment, resulting in a lower capital cost per gallon of ethanol production capacity.