- THE MAGAZINE
Renewable fuels have long had a "green" characteristic associated with the environmental benefits of their use. With the rising cost of fossil fuel, renewable fuels are beginning to show a green economic benefit by lowering operating costs in industrial facilities with thermal processes. Landfill gas (LFG) is a renewable energy resource containing methane collected from solid biodegradable municipal waste. In major industrialized urban populations, solid biodegradable waste is collected and concentrated in nearby prepared areas known as landfills. During the compaction and decomposing process, various gases, including methane, are released into the atmosphere.
Methane (CH4) is a greenhouse gas that remains in the atmosphere for approximately 9-15 years. Over 20 times more effective in trapping heat in the atmosphere than carbon dioxide (CO2) over a 100-year period, methane is emitted from a variety of natural and human-influenced sources. One form of methane generation is the decomposition of waste under anaerobic (without oxygen) conditions. The amount of methane created depends on the quantity and moisture content of the waste and the design and management practices at the site. Municipal solid waste landfills are the largest human-generated source of methane emissions in the U.S., accounting for 34% of all methane emissions.
Harnessing LFGLFG is created as solid waste decomposes in a landfill. This gas consists of about 50% methane (the primary component of natural gas), about 50% carbon dioxide, and a small amount of non-methane organic compounds. Instead of being allowed to escape into the air, LFG can be captured, converted and used as an energy source. The alternative use of LFG helps reduce odors and other hazards associated with LFG emissions, and it helps prevent methane from migrating into the atmosphere and contributing to local smog and global climate change.
Landfill gas is extracted from landfills using a series of wells and a blower/flare (or vacuum) system, which directs the collected gas to a central point where it can be processed and treated depending on the ultimate use for the gas. From this point, the gas can be simply flared or used to generate electricity, replace fossil fuels in industrial and manufacturing operations, fuel greenhouse operations, or be upgraded to pipeline-quality gas.
Pressurized methane gas can be piped underground and burned as a renewable fuel for manufacturing facilities at a lower cost than other fuels, such as natural gas. In North America and the U.S. specifically, landfill property that is in close proximity to manufacturing facilities can be a ready source of energy.
LFG CharacteristicsFrom a given landfill, LFG’s heating value (BTU per cubic foot), specific gravity (the mass compared to air) and corrosive material content are nearly constant over time (see Table 1). However, these LFG characteristics vary widely from one landfill to another, and the type of gas conditioning affects the characteristics. Gas conditioning includes cleaning or filtering, drying, and transporting.
Given its nature of formation and the variability between landfills, it is prudent to have an independent laboratory analyze the constituents of the gas as part of the initial project development. The lab analysis will provide insight into the fuel characteristics and very often determine the scope and details of a project's requirements. With a specific gas analysis and knowledge of the process, a burner manufacturer can make the proper burner selection and determine what modifications are necessary for a particular fuel gas.
LFG commonly has low parts per million (ppm) concentration levels of corrosive material, particularly molecules containing sulfur. Depending on the ppm of sulfur-containing molecules, it might be necessary to select or construct burners, piping and control components of higher-grade materials like high-grade stainless steel.
The presence of water and oxygen are factors that can increase the corrosive behavior of sulfur molecules, particularly hydrogen sulfide (H2S). Water can increase the rate of sulfur corrosion by more than 10 times. A good LFG supply should be thoroughly dried, but there is always the potential for some amount of water to be present.
The compatibility of safety devices, such as gas shut-off valves, is the greatest concern. While LFG installations often use burners with standard material construction or with minor upgrades, safety shut-off valves are commonly upgraded from those typically used with natural gas. In many instances, LFG contains one or more species of siloxanes, which are non-toxic organosilicates that are used in many consumer and industrial products to enhance certain product characteristics. Organosilicates volatilize and are carried with the landfill gas as part of the organic decomposition process. As the LFG is combusted, the organosilicate reduces to silica dioxide (SiO2), often creating deposits on the combustion and heat transfer equipment. Siloxane filtration technology can remove various siloxane species to below detection limits.
LFG OpportunitiesSince all landfills generate methane, the beneficial use of LFG makes economic and social sense. But not all landfills generate the same amount of LFG. The quantity and quality of the LFG from a candidate landfill should be investigated to ascertain its suitability for specific purposes.
Because methane is both potent and short-lived, reducing methane emissions from landfills is one of the best ways to achieve a near-term beneficial impact in mitigating global climate change. The greenhouse gas reduction benefits of using 1,000,000 BTU/hour of LFG in a typical production facility are the equivalent of planting almost 1200 acres of forest per year or removing the annual carbon dioxide emissions from more than 800 cars.* But compared to the cost of natural gas, the true benefit may be the economic savings achieved from using a green fuel.
* The EPA calculates the total equivalent emissions reduction based on the reduction of methane emitted directly from the landfill, plus the offset of carbon dioxide from avoiding the use of fossil fuels ( www.epa.gov/lmop/res/lfge_benefitscalc_022806.xls).
For more information regarding LFG for use as an alternative fuel, contact DŸrr Systems, Inc., 40600 Plymouth Rd., Plymouth, MI 48170; (734) 254-2228; fax (734) 459-4837; e-mail firstname.lastname@example.org; or visit www.durrusa.com. Additional information regarding the EPA's LMOP is available at www.epa.gov/lmop.
Author AcknowledgementsThe author would like to acknowledge the contributions of George Fritts and Jim Westin of Eclipse, and to thank Dara Leadford and the entire BMW Spartanburg team for their support of this project and paper.
SIDEBAR: BMW Case StudyThe Spartanburg, S.C., facility of BMW Manufacturing Co. LLC has been using "green energy" since January 2003, when it implemented one of the most ambitious landfill gas to energy (LFGTE) projects in North America (the project was recognized as the EPA LMOP Project of the Year in 2003). BMW entered into a long-term (20-year) contract with Ameresco to supply landfill methane gas at a fixed cost from the Palmetto Landfill almost 10 miles away from the plant. The 9.5-mile LFG supply pipeline crosses a river, two creeks, an interstate highway and BMW's test track.
LFG was originally used by BMW to run a co-generation (cogen) system that produced 8.8 Mw of power on-site, as well as hot and cold water, but the Palmetto Landfill had more LFG capacity than was being used by the cogen system. BMW wanted to find a way to displace more of its natural gas use with available LFG as a continuation of its environmental stewardship and to combat the rising cost of natural gas.
Expanding LFG UseDürr Systems, Inc, an industry partner in LMOP and the original supplier of the Spartanburg paint shop, worked together with BMW engineers to identify the potential for using the additional LFG as direct thermal energy in the Dürr paint shop process equipment and eliminate the requirement for natural gas as a fuel. Several pieces of existing process equipment were assessed as candidates for running on LFG as a primary fuel source. The primary evaluation criteria were consistent thermal load/fuel usage, and the products of combustion from LFG usage could have no detrimental effect on the equipment or the process.
The paint shop processes chosen were the regenerative thermal oxidizer (RTO), which is an air pollution control system that treats the paint shop process exhaust before discharge to the atmosphere, and the hot air supply units (also known as heater boxes) for the paint cure ovens and spray booth emission abatement system. Although the spray booths are the largest user of natural gas in the paint shop, the booths were eliminated from consideration for two reasons.
First, the thermal demand of heating the booth air, while very significant, is highly influenced by the seasonal outdoor air temperature, making for an inconsistent thermal demand. And secondly, the Air Supply House burners are directly fired into the air stream, so any byproducts of LFG combustion would be present in the air stream that the painted car bodies pass through and the workers spray in. BMW also requested the inclusion of one of the three Nebraska Boilers from the Energy Center, a separate building on the assembly complex grounds, in the project. The Energy Center boiler is used selectively throughout the year to supplement seasonal thermal demands and is used as a backup in the event of a turbine not running.
All of the process equipment chosen for this project had existing natural gas-fired burners and gas trains that required replacement or modification in order to accommodate LFG as a primary fuel. Because of the lower BTU content of the LFG and the increased volumetric flow requirements, 23 Eclipse process burners and Maxon gas trains were replaced in this project. Also, due to the moisture and corrosive potential of LFG, it was decided to supply the new gas trains (and supply pipe) in stainless steel.
In addition to process requirements, the production requirements, schedules and downtime were significant concerns for BMW when considering alternative renewable fuels. Synthetic gas blending systems based on propane standby technology were evaluated and required to automatically switch fuel feeds without manual intervention inside the paint shop. Propane standby systems routinely provide an alternate form of energy in the event that natural gas is curtailed or shut off.
Two piston-operated mixing systems (one in operation and one on standby), manufactured by Alternate Energy Systems and capable of blending and producing a low-BTU "synthetic gas" from high-pressure natural gas and compressed air, were installed as redundant fuel backup systems. The synthetic gas properties can be manually adjusted to closely match the calorific and Wobbe Index properties of the landfill methane gas (LFG). The usefulness of the Wobbe Index number is that for any given orifice, all mixtures having the same number will deliver the same amount of heat.
The blending systems provide a supplemental fuel supply to the paint shop LFG delivery system in the event that LFG service is interrupted or unable to meet the paint shop process equipment's thermal energy demands. Supplemental fuel is provided in an automatic, seamless manner so that no interruptions in fuel delivery are noticed by the process equipment.
Green BenefitsThe equipment installed during this project did not change the thermal energy requirements of the process equipment. Dürr was able to displace current process consumption of 27,000,000 BTU/hr of natural gas with LFG. The additional use of LFG in the BMW paint shop does not inhibit the production of electricity on-site, and has greatly reduced the paint shop's reliance on natural gas. Based on BMW's long-term contract with Ameresco for the fixed price supply of LFG and the rising cost of natural gas, BMW will save in excess of $1.0 million dollars per year in their paint shop operating costs, which is the equivalent of over $5 in cost savings per painted body.*
With over 63% of its total paint shop energy being supplied by landfill gas, BMW has successfully achieved the conversion of waste into a perfect paint job. In recognition of this superior innovation and achievement, the EPA's LMOP presented the 2006 Energy Partner of the Year jointly to BMW Manufacturing Co. and Dürr Systems. The project at Spartanburg was also selected to receive the 2007 Renewable Energy Project of the Year Award from the Association of Energy Engineers (AEE).