EDI to unveil biomass digester system

By Anna Austin
Posted December 3, 2009, at 12:28 p.m. CST

California-based Environmental Developers Inc. plans to showcase a new biomass digestion system for the first time at Biomass Magazine's Pacific West Biomass Conference & Expo Jan. 11-13 in Sacramento, Calif. The company believes the process is three to 10 times faster than other digestion designs on the market.

The Vacuum Retort Anaerobic Digestion system has been in development since 1977, according to EDI President Herman Miller III, after he initially observed an anaerobic digester. "Typically, its instrumentation was out of service and deep rust has penetrated everything ferrous in the vicinity, eaten up by the hydro-sulfurous acid products of raw digest gas," he said. "Realizing the potential energy possibilities for anaerobics and the disastrous impediment of the hydrogen sulfide byproduct as well as its usefulness, I was intrigued and challenged into a new avocation of work and study, while building other people's plant designs. Every solution highlighted another problem until 30 years and four or five patents later, we had VRAD."

At a typical plant, municipal solid waste (MSW) is weighed, sorted, crushed and ground as finely as possible. Miller said EDI has a design for a grinder which the company can build, but added that it would be more economical to buy a grinding system that is already in the market though none have yet met the system's exact specifications.

The MSW is hydrolyzed, enzymes are added and the material then goes through three digestion stages. Hydrogen sulfide and inert materials are removed. "The sodium sulfate and sodium bicarbonate byproducts are taken off as part of the gas separation process," Miller explained. "We end up with clean water, a certain amount of which is used for washing, hydrolization, fire protection and general cleanup around the plant." The water is continually recycled, he added, leaving about 80,000 gallons per day for sale.

"The pure biomethane and carbon dioxide products are put through two stages of compression," Miller said. "The low-pressure stage (300 pounds per square inch) gas is used for mixing, pH control and to feed the high-pressure stage (3,000 per square inch) gas that is bottled and sold, based on the highest and best use principle-to power electric cars, trucks and buses. Brazil and Argentina are already well into these vehicles, and they make the most sense on every count."

Miller said the most significant benefit of the VRAD process is its gas separation capability. "It not only allows us to sell a cleaner biomethane product with a higher Btu per cubic foot than pipeline natural gas, but also allows us to reduce greenhouse gasses and carbon emissions to zero," he said. This offers a serious carbon credit advantage over other processes. We didn't design the process with the carbon credit game in mind, it just made good sense."

As far as the cost is concerned, Miller said EDI expects it will take $35 million to get the first 1,000-ton-per-day plant on line. The plant would have a 10-acre footprint and generate a return on investment in five to seven years.