The cogeneration facility is one of the most complex of all the District’s operations. It is also one of the most interesting. The term “cogeneration” is derived from the fact that two sources of energy are produced during its operation – electricity and heat.

The District’s cogeneration facility includes engine-generator units, a natural gas blending system, liquid-liquid heat exchangers, digester gas meters and piping, and a programmable logic control (PLC) – based monitoring system. 

One of the by-products of the treatment plant’s anaerobic solids digestion process is methane gas. This gas is combustible and has an average BTU content of 600-650 per standard cubic foot. At current solids production rates, the District’s digestion process produces approximately 300,000 cubic feet of gas per day. The gas is combustible and, therefore, provides an energy source which can be harnessed for a benefit. The District’s cogeneration facility is designed to harness the energy available in the digester gas and turn it into useable electricity and heat.

The digester gas is used to fuel engine-generators, which produce the electricity and heat. The cogeneration facility has three engine-generator units. Two of the units have a 650 kilowatt generating capacity. The third unit has a capacity of 923 kilowatts. Each unit is powered by a 12-cylinder internal combustion engine. The engines drive electrical generators that supply electrical energy for use at the treatment plant. On average the cogeneration facility supplies approximately 60 percent of the electrical needs of the treatment operations. This means that during normal operation only 40 to 50 percent of the treatment plant’s electrical needs are supplied by utility power.  State design requirements require that emergency power facilities be provided to allow continuity of operation during utility power outages. The cogeneration facility also satisfies this requirement.

Heat, the second energy source from cogeneration, is produced during the combustion process of the engines. This heat is also harnessed for beneficial uses. The heat of the exhaust is harnessed through the use of gas-liquid heat exchange units, which are part of the engine-cooling loop. The cooling loop also circulates through cavities in the engine blocks and receives heat directly. The cooling loop then circulates through liquid-liquid heat exchangers, which transfer heat to keep the total contents of the primary digesters at a constant temperature of 95˚F to 100˚F. The digesters must be kept in this temperature range to sustain the biological digestion process.

The cooling loop water is also used to heat building spaces through the use of hydronic heaters. The hot water is circulated through coils and a fan blows air through the coils to heat the air and keep the building room temperature as desired.

The District’s facilities are designed to maximize the use of digester gas and minimize dependency on natural gas to fuel the engine-generator units. However, digester gas production rates only supply about 90 percent of the total fuel required by the generators. The remaining 10 percent is supplied by natural gas. When natural gas is used, it is blended with the digester gas as a supplement. A natural gas blending system also dilutes the concentration of natural gas, which contains about 1,000 BTU’s per cubic foot, to a concentration that will match the digester gas BTU content of about 650 BTU’s per cubic foot. This blending and consistent BTU content of the fuel supply supports a more stable operation of the engine-generators without the need of dual fuel carburetion trains.

An emergency waste gas burner is part of the digester gas supply system. Its purpose is to burn excess digester gas that is not used by the engines. Although the goal is to minimize use of the waste gas burner because it is a non-beneficial use of the digester gas, wasting gas from time to time is unavoidable and the release of large quantities of unburned gas is not allowed.

© North Davis Sewer District 2006