Solid Waste Landfills are Producing Pipeline Quality Natural Gas – WIH Resource Group

With improved processing technologies and attractive natural gas prices, more landfills are producing pipeline-quality gas.

After several years of inactivity, the number of high-Btu landfill gas energy projects is increasing. Since 2006, 11 construction startups and three expansions of high-Btu projects have taken place, according to the U.S. Environmental Protection Agency’s (EPA) Landfill Methane Outreach Program (LMOP).

Today’s high-Btu projects employ the latest advancements in technology to remove carbon dioxide and other impurities from landfill gas, resulting in a gas that is more than 95 percent methane and has a heating value equivalent to natural gas. Recent improvements in technology and higher returns on the finished gas have made high-Btu projects a viable option for more landfills — including facilities with lower gas flows.

Ordinary landfill gas consists of roughly 50 percent methane — which is the primary component of natural gas — 50 percent carbon dioxide, a small amount of nonmethane organic compounds (NMOCs) and other trace impurities. Removing the carbon dioxide and other impurities from landfill gas doubles its heating value, making it comparable to natural gas, which has a heating value of 1,025 Btu per cubic feet (ft3) to 1,095 Btu/ft3.

High-Btu landfill gas is most often injected directly into natural gas pipelines. Once in the pipeline, the gas blends with the natural gas and is distributed to a gas utility’s customers.

The Trends

Up until 2006, the growth of high-Btu projects was flat, averaging around one startup or expansion per year, as illustrated in Figure 1 on p. 46. In 2007, however, the number of startups and expansions began to grow steadily. In fact, 18 of the 25 high-Btu projects operating as of August 2009 were started or expanded during the last three years. LMOP expects a total of 10 projects to begin operation in 2009, including five that have come on line already. (Those five are Oak Grove Landfill in Winder, Ga.; Live Oak Landfill in Conley, Ga.; Carter Valley Landfill in Church Hill, Tenn.; Greenwood Farms Landfill in Tyler, Texas; and Turnkey Recycling & Environmental Enterprises in Rochester, N.H.)

In 2007, the Rumpke SLF Landfill in Cincinnati completed a $15 million expansion of its high-Btu processing plant, which began operating at the site in 1986. The expansion increased the processing of landfill gas from 9 million cubic feet per day (mmft3/day) to 15 mmft3/day. According to Rick O’Mahony, vice president of operations of Pittsburgh-based Montauk Energy, which is the developer and owner/operator of the plant, the plant was expanded to take advantage of increasing volumes of landfill gas. “We could either flare the gas at some marginal cost or expand the plant to provide increased gas processing with the associated sales of high-Btu gas to the gas utility,” says O’Mahony, who has helped develop six high-Btu landfill gas projects.

The three key factors that have contributed to the recent growth of high-Btu landfill gas projects are low wholesale electricity prices, high natural gas wholesale prices and improvements in gas separation technologies.

Wholesale prices for electricity and natural gas significantly affect which type of project will be profitable. Over the last several years, national wholesale electricity prices have been relatively steady, as illustrated in Figure 2 on p. 46. Meanwhile, natural gas prices have increased significantly since 2001 (see Figure 3 on p. 46). When natural gas prices rose from 2005 through 2008 — approaching $9 per million Btus — many landfills were able to pursue high-Btu projects.

Improvements in technology also have helped spur the growth of high-Btu landfill gas projects by making these projects feasible at landfills that provide less than 3,000 standard cubic feet per minute (scfm) of landfill gas. For example, manufacturers have reduced the cost to build gas-processing equipment and have reduced pressure requirements, which have decreased operating costs.

David Mauney, an experienced landfill gas project developer and consultant with The Hunter Group, explains how high-Btu projects are not just for larger landfills any more. “Initially, high-Btu projects required at least 3,000 scfm. But with improvements in the technology today, you can go as low as 1,500 to 2,000 scfm.” LMOP has recorded at least 10 high-Btu projects at landfills that provided less than 3,000 scfm of landfill gas to the project.

Meeting the Specifications

Landfill gas must be thoroughly cleaned and upgraded before it can be sold to a natural gas utility. To meet the typical specifications for pipeline-quality landfill gas (see chart on p. 48), several steps are required.

The final step, carbon dioxide removal, is the main component of a high-Btu project. This is because carbon dioxide constitutes approximately half of the raw landfill gas, and its removal requires specialized separation equipment. Removal of NMOCs, hydrocarbons and siloxanes often is achieved concurrently with carbon dioxide removal. To create pipeline quality gas, a combination of technologies may be necessary, depending on the composition of the landfill gas. Here are the four steps to creating pipeline quality gas:

  1. Minimize oxygen and nitrogen.To minimize nitrogen and oxygen levels, a landfill may need to upgrade its gas collection system to prevent air intrusion. Landfill gas collection systems operate under a vacuum, and oxygen and nitrogen from the atmosphere can be drawn through the surface of the landfill and into the gas collection system. Air intrusion can be minimized by adjusting well vacuums and repairing leaks in the landfill cover.
  2. Remove moisture.To remove moisture, many landfills employ a chiller or desiccant. Compressing and cooling the landfill gas further removes moisture. For high-Btu projects, removing the moisture (measured as water vapor) prevents interference with the subsequent compression and separation processes.
  3. Remove hydrogen sulfide.Hydrogen sulfide can be removed using liquid absorption, either with chemical or physical solvents and usually in a continuous process, or through adsorption on a solid reactant product in a batch process.
  4. Remove carbon dioxide.Four carbon dioxide removal technologies, described below, have been applied at landfills in the United States: scrubbing, membrane separation, molecular sieve (pressure swing adsorption) and CO2Wash.

Scrubbing uses a solvent that preferentially absorbs carbon dioxide and other gases into the liquid phase (also known as liquid absorption or physical solvent treating). Early high-Btu plants relied primarily on scrubbing technology, including the plants at the Fresh Kills Landfill in New York City and the McCarty Road Landfill in Houston, both of which began operations in the 1980s.

For these projects, large-scale cleanup technology was built on site, modeled after natural gas processing plants. Significant capital was required to build the processing plants and compress the landfill gas to appropriate pressure levels — 600 pounds-force per square inch gauge (psig). Scrubbing technology has therefore been applied historically at high-volume landfills that could generate enough gas (i.e., greater than 6,000 scfm) to ensure long-term returns. Recent technology advancements, however, have lowered the pressure requirements to around 400 psig and made smaller projects (those less than 3,000 scfm) possible.

One of the first applications of a gas treatment system on a smaller scale occurred at the Johnson County Landfill in Shawnee, Kan., in 2001. The landfill installed a modular scrubbing plant on a skid, specifically developed for smaller applications. According to the project developer, South Tex Renewables, the skid-mounted design is less expensive and easier to install and operate than previous technology. Pressure requirements are 400 psig. LMOP’s national database shows that five physical solvent high-Btu landfill gas projects have come on line since 2000 at landfills with gas flow as low as 1,000 scfm.

With membrane separation, different gases pass through porous membranes at different rates based on molecule size, allowing for the separation of compounds. Carbon dioxide passes through the membrane approximately 20 times faster than methane. In the past, a drawback to this technology’s application in the landfill gas industry was that pressure of around 600 psig was required to push the carbon dioxide through the membranes. Recent technology advancements, however, have lowered the pressure requirements to around 200 psig, thereby making smaller project sizes (less than 3,000 scfm) possible.

Membrane separation is relatively easy to operate and maintain. LMOP’s national database shows that nine of the high-Btu projects that started up since 2000 use membrane separation technology to remove carbon dioxide. More than half of those projects use technology that consists of a pre-treatment skid to remove NMOCs and a series of membrane modules to remove carbon dioxide and some oxygen.

Molecular sieve (pressure swing adsorption) uses vapor phase activated carbon for NMOC removal and a molecular sieve for carbon dioxide removal. The molecular sieve media preferentially adsorbs carbon dioxide on the media surface. When the media is loaded with carbon dioxide, the molecular sieve is taken off line, and the media is regenerated through a depressurization and purge cycle. (The process is therefore often referred to as pressure swing adsorption or PSA.) Molecular sieve media that selectively remove nitrogen also are available. PSAs can achieve some incidental oxygen removal. Four projects that started up or expanded in 2007 and 2008 use PSA, including the Rumpke Landfill in Cincinnati.

CO2Wash is a patented process that uses liquid carbon dioxide obtained from landfill gas as a solvent. After moisture removal and compression, landfill gas moves upward through a three-story column. Refrigeration at the top of the column condenses the carbon dioxide into liquid form. A portion of the liquid carbon dioxide washes down the column, cleansing volatile organic contaminants from the gas. The process produces two products: food-grade carbon dioxide (meaning the gas is 99.99 percent carbon dioxide) and medium-Btu gas (70 percent methane) that is virtually free of siloxanes and volatile organic compounds. The medium-Btu gas can be used as fuel, or it can undergo membrane processing to produce high-Btu gas.

Looking Ahead

The price of natural gas will continue to influence the number of new high-Btu projects. For 2009, the U.S. Department of Energy’s Energy Information Administration (EIA) predicts an average annual price of $4.67 per million Btus, and for 2010, $5.87 per million Btus. Beyond 2010, EIA projects a steady increase, nearing $8 per million Btus by 2020.

Two new projects have already come on line in 2009, with six more expected. The upcoming projects employ three different carbon removal technologies: scrubbing, membrane, and molecular sieve. Developers and technology providers will continue to demonstrate and refine the technology, paving the way for even more projects at landfills of all sizes.

What is LMOP?

The U.S. Environmental Protection Agency’s Landfill Methane Outreach Program (LMOP) promotes landfill gas as a renewable local energy resource. As of June 2009, LMOP has encouraged and facilitated the development of approximately 410 landfill gas (LFG) energy projects since the voluntary program’s inception in 1994. These projects have prevented the release of more than 33.8 million metric tons of carbon equivalent into the atmosphere over the past 14 years. This reduction has the same environmental benefit as preventing the carbon dioxide emissions that would result from the consumption of nearly 197 million barrels of oil.

As of December 2008, more than 790 LMOP Partners have signed voluntary agreements to work with EPA to help promote and advance LFG energy. Today, approximately 480 LFG energy projects are operating nationwide, and about 130 projects are under construction or development. LMOP estimates that roughly 520 additional landfills present attractive opportunities for project development.

Sources: Waste Age Magazine and WIH Resource Group

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