Section 2. ANIMAL/AGRICULTURAL WASTES
USE OF METHANE GAS FROM ANAEROBIC TREATMENT OF
STILLAGE FOR FUEL ALCOHOL PRODUCTION
Enos L. Stover, Professor
Ganapathi Gomathinayagam, Graduate Student
Reinaldo Gonzalez, Graduate Student
Bioenvironmental and Water Resources Engineering
School of Civil Engineering
Oklahoma State University
Stillwater, Oklahoma 74078
INTRODUCTION
In recent years fermentation of ethanol from agricultural feed stocks, such as corn and milo, has
been demonstrated to be a promising energy alternative for the United States. Gasohol, a blend of
10% anhydrous ethanol and 90% unleaded gasoline, has been shown to be compatible with 100%
unleaded gasoline and has received widespread public acceptance. However, the gasohol industry
has been plagued with many problems, and in this day and time of economic turmoil and energy
shortages, a fledgling industry such as the fuel alcohol industry must capitalize on every economic
advantage available.
Two very serious problems of this industry center around the energy consumption requirements to
produce the alcohol and the production of high-temperature, high-strength, acidic wastewaters called
thin stillage. Heat energy is required at every stage of the production process including grain drying,
cooking, saccharification, fermentation, and distillation. Unless careful design and operation
constraints are imposed, the energy consumption per gallon of alcohol produced can be greater than
the energy available from one gallon of alcohol. Thus, the total energy balance around the production
plant becomes a critical factor, and innovative improvements to reduce the total process energy
demands are needed. Ethanol production by fermentation also requires considerable quantities of
water with substantial amounts ending up in the thin stillage as wastewater. Although a portion of this
water can be recycled, complete reuse is generally not possible due to the build-up of salts and toxic
by-products of the fermentation reactions. These high strength wastewaters can be treated biologically
to high treatment efficiencies in both aerobic and anaerobic systems [1,2,3,4]. The anaerobic systems
offer the advantage of methane gas production which can be used as a fuel source back in the alcohol
production process.
Independently, the two problems described in the previous paragraph can have a significant
negative impact on the cost of fuel alcohol production. However, when considered jointly, a synergistic effect can be realized by application of anaerobic treatment of the stillage and use of the
methane gas produced to supplement the fuel requirements of the alcohol plant. Research at Oklahoma State University with both fixed-film and suspended growth anaerobic treatment systems has
demonstrated very high treatment efficiencies of thin stillage. The biological treatment kinetics and
methane production rates have been defined such that full-scale facilities can be designed and
operated for reliable methane production. The impacts of employing anaerobic treatment at a one
million gallon per year alcohol production plant and utilizing the methane gas from treatment of the
thin stillage have been evaluated. The results of this investigation are presented in the following
sections.
ALCOHOL PRODUCTION
Material and energy balances were conducted around a small commercial or industrial ethanol
(fuel alcohol) plant capable of producing one million gallons of anhydrous ethanol (200 proof) per
year. When operating 24 hours per day for 300 days out of the year at the production rate of 150
gallons per hour (3,600 gallons per day), this plant can produce one million gallons of pure ethanol
per year. The feedstock consists of corn and milo, and under good operating and management
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