Power From Trash, It's A Gas, Gas, Gas!

Neal Miller

Landfill gas (LFG) isn't a new problem. Most landfills already have passive systems in place to vent the gas, reducing the risk of fire and gas migration. But EPA's new standards, which are required by the Clean Air Act, call for active systems to collect and manage LFG.

One Minnesota site, the Burnsville Sanitary Landfill, is taking advantage of the new regulations by recovering LFG and converting it into electricity. A network of landfill wells and pipeline collects LFG and delivers it to an on-site generating facility that produces about 3.2 megawatts of electricity - enough methane to create power for about 2,000 homes.

While recovering LFG for conversion to energy can be an attractive alternative, it is not feasible for every landfill. Factors such as size, content and location can affect a landfill's ability to produce methane.

Weighing The Options One of two mixed municipal solid waste (MSW) landfills operating in the seven-county Minneapolis-St. Paul metropolitan area, the Burnsville Sanitary Landfill opened in the early 1960s. With a permitted capacity of 98.5 acres, the landfill accepts about 110,000 tons of MSW, construction and demolition debris and non-hazardous commercial and industrial waste per year. Edward Kraemer & Sons Inc. (EKS), a construction company in Plain, Wis., owns and operates the facility.

Composite liner and leachate collection systems have been installed under 11 acres of the landfill. More than 50 acres have been capped with compacted clay or synthetic barriers, with additional areas capped each year. After an extensive remedial investigation, the landfill was removed from the state Superfund process. Under current infill rates, the landfill has less than four years of capacity remaining, but EKS is seeking permits for a 63-acre, 6.1 million-cubic-yard horizontal expansion.

Two years before its methane recovery system began in May 1994, EKS evaluated the economic feasibility of using LFG as an energy source. In addition to anticipating the new source performance standards, the company believed that drawing contaminants out of the landfill with active LFG collection might reduce the potential for groundwater pollution.

"We hoped that selling methane would offset the cost of installing and operating the system," said Bob Miller, vice president and general manager of operations for EKS in Burnsville. "It also made sense to install and expand the LFG collection system during cap construction each year, instead of installing it retroactively after the entire site was capped."

With help from Mike Michels, a senior environmental engineer with Camp Dresser & McKee Inc., Milwaukee, EKS identified three basic options: sell LFG directly to nearby commercial businesses for use in boilers, space heating and other applications; upgrade and sell LFG to a local gas utility; or convert LFG to electricity for sale to the local power utility.

Selling the electricity to the local power utility, Northern States Power Co. (NSP), proved to be the most viable option. The $3.5 million capital costs and estimated $268,000 yearly operating costs could be offset not only by revenue from the sale of electricity, but possibly by IRS Section 29 tax credits. The tax credits, a direct dollar-for-dollar offset to federal taxes otherwise paid by EKS, are available for LFG-to-energy systems that become operational prior to 1997 and will continue through 2007. With the federal tax credits, payback of the estimated capital costs was calculated at nine years, well within the facility's life.

EKS conducted a month-long pump test to verify that the landfill would produce enough LFG to make the project economically worthwhile. Using three gas extraction wells and 12 monitoring probes, the test estimated the gas recovery rate for the entire capped landfill at 1,950 cubic feet per minute, with an average heating value of 520 Btu per cubic foot. This was much higher than the 1,200 cfm and 450 Btu/cf estimated for the beneficial use study, and increased the chances of success.

EKS negotiated an agreement with Minnesota Methane LLC, a joint venture between NEO Corp., a non-regulated subsidiary of NSP, and Ziegler Power Systems Inc. of Minneapolis. EKS agreed to own and operate the LFG collection system and Minnesota Methane would own and operate the generating facility on property leased from EKS, converting LFG to electricity for sale to NSP. The arrangement allowed EKS to qualify for tax credits without limiting the return on investment.

Since the project was the first of its kind in the state, Minnesota Methane was a newly-formed company and the 15-year break-even period was relatively long, negotiations and financing took 19 months. This resulted in two 20-year contracts: a gas sales agreement between EKS and Minnesota Methane and a power purchase agreement between Minnesota Methane and NSP. The agreements define LFG quality standards, establish mechanisms for compensation if LFG isn't supplied or used and provide for system expansion.

System Operation The collection system consists of 26 LFG extraction wells installed on 52 acres of capped landfill and connected by a looped pipeline. Each vertical well, perforated PVC pipe surrounded by crushed stone, extends to within 10 to 15 feet of the landfill's bottom.

A centrifugal blower creates a vacuum in the line that draws LFG from the landfill into the extraction wells. So far, the flow of LFG has exceeded 1,100 cfm with a 54 percent methane content. "We figured that to run the engines we needed somewhere between 1,200 cubic feet per minute at 50 percent methane and 1,000 cfm at 54 percent methane, so we're happy with the quality of the gas," said EKS Environmental Affairs Specialist Sabina Kitts. Wells located in areas capped with synthetic liners tend to perform better than wells in areas capped with clay, said Kitts, who explained that the synthetic cap may allow for a tighter seal with the well head.

The system is fine-tuned at least once a week to maintain consistent methane content by adjusting individual wells. Measurements of methane, nitrogen and oxygen from each well are downloaded into a computer, which helps EKS technician Paul Pladsen calculate new settings based on current and past performance. Wells with high methane and low nitrogen content are opened more than wells with low methane and high nitrogen. The wells must be balanced against each other to achieve the methane content and LFG flow required by the generating facility.

The pipeline includes three 2,000-gallon, double-walled condensation tanks equipped with leak detection systems to help remove moisture from the gas. About 300 gallons of condensate are pumped from the tanks each week, reducing the moisture content of the LFG entering the generating facility.

At the generating facility, the gas fuels two pairs of Caterpillar 3516 reciprocating engines designed for low-Btu fuel. Each pair of engines is linked by a single drive train to power a generator. The start-up and ongoing operation of the generating facility have met expectations so far, according to Jerry Peterson of Ziegler Power Systems. "Our on-line time has steadily improved," he said. "We've always met the minimum required in our agreement with the utility. After running at 84 percent total capacity for the first month, we've climbed steadily to between 92 and 93 percent, including start-up performance."

Over time, the engines are expected to operate at 95 percent capacity. The engines efficiently convert methane to electricity and destroy more than 98 percent of the other non-methane organic compounds in the LFG. Compared to pipeline-quality natural gas, said Peterson, burning LFG poses some special challenges. In addition to its higher moisture content and lower Btu value, LFG often contains contaminants that can lead to engine problems.

"We've had some spark plug problems that have increased the need for maintenance during start-up, but we don't expect those problems to affect long-term operation," he said. The engines must deal with organic halides such as fluorine, chlorine and sulfur, which disassociate to form acids that contribute to engine wear. Alkaline additives in the engine oil help neutralize the acids.

Each month, Peterson expects to make changes and other routine maintenance adjustments. The company is experimenting with a new type of oil filter that may extend service intervals to two months. Head replacement will occur annually, and the engines will be fully overhauled every three years.

Otherwise, there have been few changes so far. Minnesota Methane switched from an infrared scanning system to a gas chromatograph to more accurately measure methane content in the LFG. Variable-speed drives have been added to the cooling system and gas compressor to conserve electricity.

When the generating facility is shut down for maintenance or repair, an alternate blower draws the LFG to an enclosed flare where the gas is burned off. The company also may purchase a Ziegler generator to restart one engine set, Kitts said. "That way, even when the power line shuts down, the facility could still generate some electricity to run the blower for the flare."

Potential Expansion EKS plans to add new extraction wells to the LFG collection system as additional areas of the landfill are capped. If the proposed landfill expansion occurs, a network of at least 42 wells are planned. An additional generator set could be added to the facility, increasing its generating capacity to 4.8 megawatts.

In June 1995, EKS and Minnesota Methane will consider adding a single gas engine to the existing pair to increase the generating capacity to 4.0 megawatts. According to Kitts, the decision may hinge on the landfill expansion. "If we actually have the amount of gas we're currently estimating, we may add a fifth engine. If the expansion is approved, the likelihood of adding additional engines would increase," she said.

Another factor affecting future methane production, said Kitts, is reduction in the amount of organic material disposed in the landfill. Yard waste is now diverted to an on-site composting operation, and recycling programs have reduced the amount of paper in the waste stream.

"We still receive a lot of food waste and other organic materials, but that might change in the future," she said. "If organic waste composting or other processing operations divert organic waste away from the landfill, the amount of methane produced will decline." In the meantime, EKS plans to continue to demonstrate how to make methane recovery a win-win proposition for sanitary landfills.