Developing a robust, commercially viable biocatalyst (or microorganism) capable of fer­menting a large percentage of both the hemicellulose sugars as well as glucose to commer­cially viable ethanol titers at commercially viable fermentation times is a significant goal. Foust and coworkers (4) define near-term commercially viable targets as capable of fer­menting 85% of hemicellulose sugars and 95% of glucose to a concentration of at least 6% ethanol in 3 days in combined hybrid saccharification and fermentation, while maintain­ing the solids concentration necessary for the ethanol concentration target above. Specific research needed to accomplish these objectives is as follows:

• Identify strain candidates that exhibit superior “wild-type” performance

• Use metabolomics, proteomics, and other tools to understand metabolicbottlenecks in the carbon assimilation pathways that limit pentose sugar uptake and the ability to withstand fermentation inhibitors such as organic acids, low pH, and increased temperature

• Extend “omics” studies to identify and understand secondary pathway limitations related to reaction cofactors and regulation of metabolism

• Increase pentose uptake rates by applying protein and metabolic engineering to increase sugar transporter efficiency, pentose specificity, and expression

• Improve strain robustness by manipulating cell membrane composition to reduce its permeability to organic acids and improve its temperature stability

• Use a combination of metabolic engineering, mutagenesis, and long-term culture adap­tation strains on actual pretreatment hydrolyzate to achieve targeted fermentation per­formance

• Perform parametric analysis of such factors as lignin redeposition and the detrimental effects this can have on enzyme kinetics to minimize these effects

• Use information about the enzyme capabilities and fermenting strain’s performance to develop and test strategies for efficiently integrating enzymatic hydrolysis with biomass sugar fermentation to maximize cellulose hydrolysis and sugar fermentation rates and yields

• Quantify the effects of enzyme loading, strain inoculation time, and inoculum charge on batch process performance

• Use reactor designs and operational schemes to maximize the solids loading and conver­sion of cellulose and other carbohydrates to ethanol.

2.2.2.2.3 INTEGRATION/PROCESS ENGINEERING R&D NEEDS Finally, it is important to consider all the individual unit operations in the context of an integrated process. Although there is general agreement in the literature about necessary per­formance targets for individual unit operations as outlined above, overall integrated process targets are more difficult to define. Essentially, the overall process must be economically vi­able in the fuel marketplace, and the process must be demonstrated at some reasonable pilot scale of continuous reliable operation. Economic viability in the fuel marketplace is largely a function of gasoline prices, which are driven by crude oil price projections. Figure 2.6 shows the latest DOE Energy Information Agency 2008 Annual Energy Outlook projections for world crude oil prices out to 2050.

Foust and coworkers (4) picked $1.07 in 2002 dollars as a production cost target, which includes an IRR (internal rate of return) of 10% as an economically viable target for cellulosic ethanol. The rational provided for this target was based on historical fuel ethanol prices as shown in Figure 2.7. The $1.07 in 2002 dollars ($1.31 in 2007 dollars) per gallon value

Historic fuel ethanol prices

0

1985 1990 1995 2000 2005 2010

Figure 2.7 US list prices for ethanol.

represents the low side of the historical fuel ethanol prices, and, given historical price data, cellulosic ethanol would be commercially viable at this production cost.

In addition, the target price of $1.31 per gallon of ethanol is also in line with current gasoline rack, or pre-tax, prices. To compare the target ethanol price with the price of gasoline on an “apples to apples” basis at the pump, the ethanol price must be adjusted as follows:

1 Adjust the ethanol price from dollars per gallon of ethanol to dollars per gallon of gasoline equivalent by correcting for the two-thirds lower energy content of ethanol compared to gasoline. This increases the $1.31 per gallon ethanol to $1.96 per gallon gasoline equivalent.

2 Adjust ethanol price from plant gate to retail price. The price of gasoline includes, on average, $0.40 per gallon for taxes and $0.23 for distribution. Assuming the same costs for ethanol gives it a retail price slightly higher than that of gasoline when oil is at $65 per barrel, as shown in Figure 2.8. This price at the pump analysis does not assume any subsidy for ethanol.