Pressurized Steam Classification (PSC). The PSC process separates MSW into organic and inorganic components using a combination of steam, pressure, and agitation in a large rotating pressure vessel (autoclave). Steam in the autoclave sterilizes the material, while the pressure and agitation cause a pulping action, resulting in three outputs, with the mix depending on the composition of the MSW:

• 55-60 percent cellulosic biomass, which has been decontaminated and homogenized and is available for conversion to ethanol;

• About 25 percent separated recyclables (steel cans and other ferrous metals, aluminum cans, plastics and glass), which can be sold to recyclers; and

• 15-20 percent residual waste (rocks, fines, soils, textiles, and non-recyclable fractions), which must be sent to landfills.

The individual autoclave units can process up to 250 tons per day of MSW and can be combined in parallel to handle larger amounts of municipal solid waste. The technology has been used by an operator to generate cellulosic material from garbage on a commercial scale for the production of paper.

Brelsford Process. Brelsford Dilute-Acid Cellulose Hydrolysis (DACH) is a lower-cost acid hydrolysis process that uses low-pressure, high temperature oil to provide energy for the process rather than using more energy-intensive high temperature steam. The process a two-stage plug-flow-reactor system. It further reduces energy requirements by recovering heat and acid used in the first stage of the reaction and reusing them in the second stage.

CleanTech Biofuels cites a review of the technology by the National Institute of Science and Technology (NIST – Final Technical Evaluation Report No. 457) “the Brelsford process has a potential for achieving considerable economic savings in: (1) acid composition, (2) heat-energy supplied for cellulose hydrolysis, and (3) process-energy for fuel ethanol production”.

CleanTech estimates a reduction in total capital and operating costs of roughly 30% compared to other acid hydrolysis processes.

Green Tech America, Inc. is developing and commercializing a yeast-based cellulosic ethanol technology that was pioneered by Dr. Ho, Research Molecular Biologist/Group Leader of the Laboratory of Renewable Resources Engineering (LORRE) at Purdue University. (Earlier post.) During the 1980s and 1990s, researchers at LORRE altered the genetic structure of Saccharomyces yeast to enable the conversion of the two major sugars found in cellulosic materials—glucose and xylose—into ethanol.

The ability to co-ferment glucose and xylose to ethanol was enabled by cloning three highly modified xylose-metabolizing-genes—XR, XD and XK—on a high-copy-number plasmid, followed by transforming the yeast with the plasmid (incorporating the plasmid into the yeast cells). A high-copy-number plasmid is a plasmid capable of self-replicating in the host cells many times—the resulting host cells will contain many copies of the cloned genes via the plasmids.

The initial target was the yeast strain 1400(LNH-ST), which was owned by a company. LORRE began screening for better yeasts with no legal constraints for converting cellulosic sugars to ethanol. Among the yeasts tested and integrated with the XR-XD-XK genes (more than ten yeast strains), 424A (LNH-ST) and 259A (LNH-ST) are effective for industrial production of cellulosic ethanol.

Further genetic engineering of the best yeast, 424A (LNH-ST) improved its xylose fermentation and enabled it to ferment two other minor sugars effectively. The further improved yeast should be able to ferment xylose and other minor sugars 30 to 75% faster.