A bioethanol process development unit: initial operating experiences and results with a corn fiber feedstock.

Interest in bioethanol production from lignocellulosic feedstocks for use as an alternative fuel is increasing, but near-term commercialization will require a low cost feedstock. One such feedstock, corn fiber, was tested in the US Department of Energy (DOE)/National Renewable Energy Laboratory (NREL) bioethanol pilot plant for the purpose of testing integrated equipment operation and generating performance data. During initial runs in 1995, the plant was operated for two runs lasting 10 and 15 days each and utilized unit operations for feedstock handling, pretreatment by dilute sulfuric-acid hydrolysis, yeast inoculum production, and simultaneous saccharification and fermentation using a commercially available cellulase enzyme. Although significant operational problems were encountered, as would be expected with the startup of any new plant, operating experience was gained and preliminary data were generated on corn fiber pretreatment and subsequent fermentation of the pretreated material. Bacterial contamination was a significant problem during these fermentations.

Dilute-sulfuric acid pretreatment of corn stover in pilot-scale reactor: investigation of yields, kinetics, and enzymatic digestibilities of solids.

Corn stover is a domestic feedstock that has potential to produce significant quantities of fuel ethanol and other bioenergy and biobased products. However, comprehensive yield and carbon mass balance information and validated kinetic models for dilute-sulfuric acid (H2SO4) pretreatment of corn stover have not been available. This has hindered the estimation of process economics and also limited the ability to perform technoeconomic modeling to guide research. To better characterize pretreatment and assess its kinetics, we pretreated corn stover in a continuous 1 t/d reactor. Corn stover was pretreated at 20% (w/w) solids concentration over a range of conditions encompassing residence times of 3-12 min, temperatures of 165- 195 degrees C, and H2SO4 concentrations of 0.5-1.4% (w/w). Xylan conversion yield and carbon mass balance data were collected at each run condition. Performance results were used to estimate kinetic model parameters assuming biphasic hemicellulose hydrolysis and a hydrolysis mechanism incorporating formation of intermediate xylo-oligomers. In addition, some of the pretreated solids were tested in a simultaneous saccharification and fermentation (SSF) process to measure the reactivity of their cellulose component to enzymatic digestion by cellulase enzymes. Monomeric xylose yields of 69-71% and total xylose yields (monomers and oligomers) of 70-77% were achieved with performance level depending on pretreatment severity. Cellulose conversion yields in SSF of 80-87% were obtained for some of the most digestible pretreated solids.

Carbon mass balance evaluation of cellulase production on soluble and insoluble substrates.

A methodology is described and applied for performing carbon mass balances across cellulase enzyme production processes using both soluble sugar and insoluble cellulose substrates. The fungus Trichoderma reesei was grown on either glucose, lactose, or cellulose in aerobic batch mode, and the evolution of the main carbonaceous components (cell mass, cellulose, soluble protein, adsorbed protein, sugars, and carbon dioxide) was followed. A variety of analytical techniques were utilized to measure these components, including (i) gravimetric analysis, (ii) near-infrared spectroscopy, (iii) bicinchoninic acid based soluble protein measurement, (iv) gas mass spectrometry and flow rate, (v) CHNS/O elemental analyses, and (vi) high-performance liquid chromatography. The combined set of measurements allowed carbon mass balances across the cellulase production process to be assessed to determine the consistency of the underlying kinetic data. Results demonstrate the capability to determine the levels and distribution of all major carbonaceous components during the cellulase production process on both soluble and insoluble substrates. Average carbon mass balance closures were near 100% during early stages (<72 h) of the cultivations using glucose, lactose, or cellulose as the substrates, but carbon mass closures trended high later in the cultivation. Analysis of carbon allocation results suggests that an error in the gas mass flow rate measurement was the primary cause for carbon mass balance closures to exceed 110% late in the process.