Cogeneration - combined heat and power

Cogeneration is the concurrent production of power and heat from the same source.

Several different systems can be used in cogeneration. King County now has three systems for cogeneration:

• internal combustion engines
• gas turbines
• fuel cell (demonstration project, 2004-2006)

Cogeneration at the plants

West Point Treatment Plant

The West Point Treatment Plant at Discovery Park in northwest Seattle treats about 98 million gallons of wastewater per day from 750,000 people in the metropolitan area.

At West Point, four internal combustion engines provide power to run the main pumps for incoming wastewater, or influent. Digester gas is used as the fuel for four diesel-style engines that use pistons to drive a shaft creating the power to run pumps and produce electricity.

The exhaust heat from these engines also boils water to create hot water that is used to heat working spaces and the digesters. The entire system was installed in 1985. The engines generate about 1 megawatt of electricity.

South Treatment Plant

The South Treatment Plant in Renton treats about 90 million gallons of wastewater per day from about 700,000 people in the metropolitan area.

The plant has two cogeneration systems. One uses a fuel cell that began operating in 2004 for a two-year demonstration period. The other uses two gas turbines and a steam turbine, installed in 2005.

Fuel cell: Generating 1-megawatt of electricity, the fuel cell is different from the other systems because it does not combust the digester gas. Instead, the fuel cell uses the gas to create an electrochemical reaction, as a battery uses its components to produce electricity.

Generating electricity also creates heat. The fuel cell has a heat recovery unit attached to a system that makes steam, and this steam is added to the plants heating system for use by the digesters.

This two-year demonstration project is aimed at testing the technology and the claims of high energy-conversion efficiencies and low air emissions for fuel cells. At the end of the demonstration, the county may elect to continue to use the fuel cell as a permanent facility. King County can upgrade the power plant to 1.5 MW if the demonstration proves successful and the county wants to increase power-generation capacity.

Turbines: When operating, the gas turbines combust digester gas and turn fan blades. This combustion process creates lots of heat. The recovered exhaust heat is then used to boil water and create steam. The steam is used to heat the digester tanks and other places in the plant. The steam is also used to generate electricity in a steam turbine. The turbine uses the steam instead of gas to turn blades creating electricity.

This system, designed to provide standby power at the treatment plant, also can operate during months of high energy use to reduce peak load charges. If the total system were called into action, it would generate 8 megawatts of electricity under normal conditions, enough power to operate the plant most days of the year.

A sound investment

The turbines and engine generators create some air pollution. Although they produce a minimal amount of carbon monoxide and sulfur dioxide, they do produce some air pollution from nitrogen. King County has operating permits from the Puget Sound Clean Air Agency (external link) and monitors the amount of air emissions form the cogeneration plants.

The fuel cell is unique since the chemical reaction of methane to produce electricity changes the chemicals involved. Fuel cells emit only water and heat and negligible amounts of pollutants. One drawback of a fuel cell plant is the higher initial cost of the facility, but as the newer technology matures, the costs will decrease.

Using waste products to create energy is not a new idea. Human beings have always looked for ways to reuse "waste" to create energy. Such reuse ranges from the Plains Indians burning buffalo dung for heating and cooking to the late 1800s when British cities burned garbage to produce electricity to King County operating a fuel cell to produce electricity from municipal sewage.

King County's Wastewater Treatment Division is considered a model agency in the United States for efficiently reusing the products of wastewater treatment. By recycling and reusing much of the byproducts from wastewater treatment, King County is helping to create an environmentally sustainable region and save money.