City powers plant with landfill methane

In 1995, the city of Greensboro, N.C., faced a major challenge: disposing of the methane gas that had escaped into the atmosphere from refuse buried in the city’s landfill.

Clean Air Act regulations mandated the disposal because of concerns about carbon dioxide and methane emissions that occur when landfill waste is digested by bacteria. As the gas builds in the landfill, pressure forces it up into the atmosphere. Once airborne, the gas is diluted and does not pose an explosive threat but has been identified as a potential contributor to the greenhouse effect.

A partnership between the city, Duke Engineering & Services, a Charlotte, N.C.-based company and subsidiary of Duke Energy Company; and Cone Mills Corporation, a local textile company, has resulted in an innovative and self-sustaining landfill gas recovery project that protects the environment, generates revenue for the municipality and supplies an economical source of supplemental energy to a large textile manufacturer.

By partnering with an engineering company and an industrial user, the city eliminated the need for capital outlay and taxpayer funding. This public-private partnership has saved local public dollars that would have been needed to meet regulatory landfill gas management requirements.

During the life of the landfill, the city (like nearly all landfill owners) had passively vented gas, a slow process that enabled the gas to migrate away from the landfill site. By the time the Clean Air Act was passed, the city’s landfill had buried more than 7 million tons of solid waste, far surpassing the 2 million-ton limit set forth by legislation.

Greensboro did not want to simply collect the gas and burn it off into the atmosphere where it could harm the ozone. Instead, the city wanted a more innovative and less wasteful solution.

With passage of the Clean Air Act, private companies interested in receiving tax credits for generating renewable energy contacted Greensboro.

The companies’ proposals, however, usually involved large capital outlays and provided only the environmental benefit of compliance with the act.

Unsatisfied with that option, Greensboro considered building a generating site to supply electricity to the city-owned Osborne Water Treatment Plant.

Greensboro contacted an engineering company to discuss the possibility of working together to construct and operate such a generating facility. Engineers analyzed the idea, gathered data and determined the amount of gas the landfill would generate each year.

Knowing the quantity of gas expected in the future, the engineers examined several technologies, including purifying the gas and producing electricity through a gas turbine. Neither option was considered economically feasible, yet preliminary studies showed that the gas was, in fact, a viable fuel.

While visiting other gas collection systems, city officials observed a system that transported landfill gas through a pipeline to a pharmaceutical corporation which used the gas for supplemental steam energy.

Knowing that one of the city’s major manufacturers, Cone Mills, had a nearby plant with a gas-fired broiler, city officials discussed the possibilities with their engineering firm, which, coincidentally, was conducting a boiler realignment at the plant.

If the landfill could pipe the gas without further treatment to the boiler, the capital investment could be kept low enough to make the project economical.

In order to collect the gas from the landfill, engineers drilled wells into the buried waste. A perforated length of pipe was centered in the hole, and gravel was added to hold it in place and to provide a porous space between the rocks where gas would collect.

A gas main was constructed from the landfill to a gas blower, which pulled a vacuum on the wells and compressed the gas to about 10 pounds-per-square inch.

As the gas was compressed in the blower, its temperature rose above 200 degrees, but, to keep it from damaging the plastic pipeline, it had to be cooled to below 150 degrees.

However, any cooling of the gas brought it below its dew point and caused water droplets to form. Therefore a knockout drum was required just before the blower to keep droplets of water from damaging the fast-moving impeller in the blower.

Under the current arrangement, gas is sold to the textile plant on the basis of its BTU content. A metering station was placed at the plant where the boiler is located to measure gas flow.

Another flow rate meter was placed at the landfill, where an employee of Natural Power, a Raleigh, N.C.-based subcontractor of Duke Engineering, works full-time monitoring the gas collection system. Although the gas is not being flared into the atmosphere, flares were installed to remove the gas if the boiler were to shut down or in the event of an emergency.

By mid-1997, 54 wells had been drilled, each designed for its specific location within the landfill. Within the next year, the company plans to drill and install 39 more wells as active sections of the landfill are closed.

The system currently generates approximately 2 million cubic feet of gas per day, which easily meets the supplemental power requirements of the plants’ boilers.

The project cost approximately $5 million, but the city paid only for the initial feasibility study. DE&S agreed to pay royalties on the gas it pumped from the landfill, and, when the city’s contract with the engineering company expires in 2007, the city will become the owner of the system. At that time, the city will have several options, including contracting out the operation of the system or operating it as part of the city government.

Who helped: Duke Engineering and Services, Cone Mills, Natural Power, Chevron Plexco, Hoffman, LFG Specialties