LANDFILL: Landfill Lights the Arizona Sky

Across the country, commercial facilities are taking advantage of landfills' gaseous resources. One such facility is Arizona's Tucson Electric Power (TEP), Irvington Generating Station, which accepts methane and carbon dioxide (CO2) transported across a 3.5-mile pipeline from the Tucson-owned Los Reales Landfill. The landfill gas is used to fire a boiler, which supplies electricity to TEP's 3,000 customers.

According to Zahren Alternative Power Corporation (ZAPCO), Avon, Conn., which built the pipeline and operates the gas processing facility, using LFG has replaced the equivalent of 20,000 tons of coal per year, as well as increased incremental generation. ZAPCO initially was awarded the gas rights by the city of Tucson in 1995. SCS Field Services, Long Beach, Calif., was the general contractor for the pipeline construction and designed and constructed the gas collection system.

Currently the city-owned-and-operated Los Reales Landfill accepts approximately 500,000 tons of municipal solid waste (MSW) per year. To date, nearly 10 million tons of waste have been disposed of at the landfill. Each pound of waste produces approximately three cubic feet of LFG, and the landfill currently generates approximately 1,500 standard cubic feet of LFG per minute (scfm). The hourly gas production rate is about 48 million Btus, enough energy to provide power to 3,000 homes and as many as 1,000 more in six months. Additionally, the first of five new Subtitle D cells is under construction, which should help with future gas production. The 847-acre landfill is estimated to be open until 2016.

The process begins at the landfill where gas is collected by vacuuming more than 70 wells, which were drilled into the waste. The wells are connected to a main collection pipe that runs below the landfill's surface to the gas compression facility. The gas compression facility pressurizes and dehydrates the LFG before it enters the pipeline. If the compression facility is not available, a flare immediately burns the LFG. Both the flare and the compression facility have a capacity of approximately 2,700 scfm.

Once the LFG is collected, it is pressurized to 22 pounds per square inch gauge (psig) by two nine-stage centrifugal blowers installed in series, which also creates a vacuum equal to 80 inches of water column. The gas is cooled during and after pressurization by air-to-gas exchangers. To condense and separate any moisture in the gas, the LFG is chilled to 40 degrees Fahrenheit. Then, the chilled, condensed gas stream moves through a separator that removes and collects the water droplets for disposal.

Once the water is removed, LFG is reheated to 70 degrees. The dry gas then is piped to TEP where it passes through a 0.3-micron Filer before entering the metering station and boiler. TEP's boiler was converted to co-fire LFG with oil, coal and natural gas.

The LFG travels through 750 feet in the power plant before entering the boiler, which has a gas capacity of 120 megawatts (MW) of coal, 160 MW of natural gas and 6 MW of LFG. A meter measures the amount of gas consumed by the boiler, and a gas analyzer continuously tracks the Btu content. The meter and analyzer are linked to the centralized fuel monitoring system, which continuously controls the fuel amount.

Today, the four-year, $4 million project is a finely-tuned process. However, the project was not always problem-free. Obstacles included contract mitigation, negotiating, permitting and engineering. Other obstacles included:

* Obtaining rights of way and permitting for the 3.5-mile pipeline stretch;

* Working around protected land;

* Dealing with equipment failure after drilling into rock; and

* Having to dig deeper than expected because of highways and utility lines.

Despite the project's hurdles, the city and TEP are pleased with the outcome. The environmental benefits include less methane and CO2 greenhouse gas emissions and ozone depletions.

According to ZAPCO, extracting LFG prevents approximately 9,400 tons of methane per year from entering the atmosphere. And, because TEP no longer needs to use coal, sulfur dioxide emissions also are reduced significantly.

Currently, ZAPCO is financing the project through the sale of the LFG to TEP, federal tax credits and an environmental service fee paid by the city.

If higher gas flow rates are generated and new landfill cells become operational, a revenue share will offset the current fee.