A series of techno-economic analyses were performed for pretreatment processing technologies using low acid, low severity process options to increase the overall pretreatment, enzymatic saccharification, and fermentation yields. Results from bench-scale experiments for corn stover compositions and sugar and ethanol yields were applied as well as compared to the current 2011 NREL design base case model, and simulations were run using the various combinations of process options. The MESP for bioethanol produced from corn stover was estimated for incorporation of the following process options:
Different corn stover varieties with different levels of recalcitrance
Control cases using low severity pretreatment
Deacetylation of the corn stover feedstocks prior to pretreatment, with or without washing
Mechanical refining prior to enzymatic hydrolysis, with or without washing following pretreatment and prior to enzymatic hydrolysis.
Bench-scale experiments showed that deacetylation, mechanical refining, and hydrolysate solids washing all improved the final sugar and ethanol yields for the low severity pretreatment conditions used in this study. In general, the calculated MESP decreased from high numbers ($2.92/gallon for the low severity pretreatment control case) when the various process options were incorporated, with the exception of the washing option. Because of the expense of the solid–liquid separation step, the washing process option showed MESP increases of up to $0.15 per gallon of bioethanol produced, even when both the enzymatic saccharification yields of glucose and xylose and the fermentation yields increased substantially by decreasing the inhibitor levels. Deacetylation not only improved xylose monomer yield during pretreatment but also increased the digestibility of residual xylan in the low severity pretreated corn stover feedstocks studied. By removing acetyl groups prior to pretreatment, less acetic acid is released into the downstream process, leading to lower amounts of neutralizing chemicals added during enzymatic saccharification and higher ethanol yields in fermentation. Mechanical refining benefits the enzymatic saccharification significantly. By applying high shearing force, the pretreated corn stover was made more digestible due to multiple property changes in the cellulose structure. Washing solids is known to be expensive because of the additional cost of solid–liquid separation equipment. However, washing also overcomes the inhibition of hemicellulase brought on by high xylose concentration in hydrolysates, resulting in further conversion of unreacted xylan in the solids. By combining two of the above options or all three of them, higher sugar/ethanol yields were indeed achieved.
The MESP of corn stover ethanol produced by current process technology is significantly driven by the final ethanol yield. For Pioneer 34M95 corn stover, deacetylation increased the annual ethanol production by about 8 million gallons, and correspondingly the MESP of ethanol decreased by about $0.23 per gallon. For Pioneer 33B51 corn stover, deacetylation increased the annual ethanol production by about 7 million gallons and lowered the MESP by about $0.29 per gallon of ethanol. Similar to deacetylation, mechanical refining decreased the MESP mainly by improving ethanol yields. Also, combining these two options increased the ethanol yield significantly and lowered the MESP, offsetting the combined cost increases for capital and operating costs. If a less expensive reactor can be utilized in these lower severity pretreatments, it may be possible to lower the MESP by an additional $0.11 to $0.14 per gallon of ethanol if a 50% reactor cost reduction is assumed. Washing of the solids was shown to improve ethanol yields for all cases studied; however, large increases in the calculated MESP were found due to the high capital costs of solid–liquid separation equipment. An increase in MESP of $0.07 per gallon of ethanol was calculated if the energy requirements for disk refining were increased 10-fold.
There are still many uncertainties in the current research that need further investigation. More accurate quotations on industrial scale equipment, more accurate energy consumption measurements on mechanical refining, accurate sugar and ethanol yield data from pilot-scale experiments, and more accurate solid–liquid washing equipment costs will improve the MESP estimates. In the future, optimization of deacetylation and mechanical refining process options could improve the economic feasibility of using low acid, low severity pretreatment process options.