In 2003, four turbines located at BMW Manufacturing Co.’s Energy Center came alive with the combustion of methane gas piped in from a nearby landfill. BMW realized an immediate benefit from using this renewable fuel to generate power and hot water on site, but more fuel was available than the turbines could burn. BMW engaged its process partner, Dürr Systems Inc., to develop a project that converted its existing paint cure ovens to secure the direct beneficial use of landfill gas (LFG) in its paint shop without any impact to the signature BMW world-class surface finish or the productivity of the plant.
The success of the project and subsequent publicity has generated many questions about how other companies might accomplish a similar feat. If you’re considering a similar project, consider some of the most frequently asked questions, along with their answers.
1. What Is Landfill Gas?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 to 15 years. Methane is at least 20 times more effective in trapping heat in the atmosphere than carbon dioxide (CO 2) over a 100-year period and is emitted from a range of natural and human-influenced sources. Municipal solid waste landfills are the largest human-generated source of methane emissions in the United States, accounting for 34 percent of all methane emissions.
Landfill gas is a renewable energy resource that is created as solid waste decomposes in a landfill. Instead of allowing landfill gas to escape into the air, it can be captured, converted and used as an energy source. The greenhouse gas reduction benefits of using 1 million BTU/hr of landfill gas in a typical production facility are the equivalent of planting more than 1,100 acres of forest per year or removing the annual carbon dioxide emissions from nearly 800 cars, according to EPA statistics.
2. Where Can I Use Landfill Gas in my Facility?Many landfill developers often look at power generation first. However, the most likely candidates for direct, renewable fuel usage are those thermal processes that have fairly consistent and continuous loading/fuel usage profiles -- like industrial process ovens. Processes where fuel consumption fluctuates due to changes in production, weather or other process characteristics will place a difficult demand on the supply of gas from the landfill.
Landfill gas is typically constantly produced on a 24/7 basis. Any fuel not used is generally flared at the landfill. Indirect fired ovens are excellent candidates because the products of combustion do not impact the process. But, properly evaluated, a direct-fired process oven also may be an excellent candidate for landfill gas.
3. How Does Landfill Gas Differ from Natural Gas?Landfill gas usually consists of nearly 50 percent methane -- the primary component of natural gas -- and nearly 50 percent carbon dioxide, with a small amount of non-methane organic compounds. This typical composition gives landfill gas a lower heating value and requires the average user to use more gas on a volumetric basis to get the same amount of energy. From any given landfill, the heating value (BTU per cubic foot), specific gravity (the mass compared to air) and corrosive-material content of the landfill gas are nearly constant over time. However, these landfill gas characteristics will vary widely from one landfill to another.
Landfill gas commonly has low parts-per-million (ppm) concentration levels of corrosive material, particularly molecules containing sulfur. Depending on the concentration level of sulfur-containing molecules, it might be necessary to select or construct burners, piping and control components of higher-grade materials such as 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 landfill gas supply should be well dried, but there is always the potential for some amount of water to be present. In many instances, landfill gas contains one or more species of siloxanes. Siloxanes are nontoxic organosilicates that are used in many industrial and consumer products to enhance certain product characteristics. Organosilicates volatilize and are carried with the landfill gas as part of the organic decomposition process.
4.What Are the Typical Conversions Required to Use Landfill Gas?Because of the lower BTU content of the landfill gas and the increased volumetric flow requirements, most existing burners and gas trains typically require replacement or some sort of modification for the burners to run on landfill gas. For example, at BMW, Dürr replaced 23 paint shop process burners and safety valve trains. Consideration should be given to the corrosive potential of landfill gas, and the potential for the presence of moisture. It would be prudent to consider the strategic use of stainless steel in the landfill gas supply header and the construction of the gas trains.
Another consideration when selecting a burner for landfill gas is gas pressure requirements. Landfill gas requires six to 20 times more pressure than natural gas across a fixed device (e.g., burner, valve or piping); the exact value depends on the quality of the fuel. Some burners can provide the same heat with landfill gas as with natural gas by simply increasing the gas pressure. Others enlarge the gas ports to take less pressure drop. Some styles de-rate the heat capacity of a given burner size, thus requiring larger burners or more burners to retrofit an installation designed to run on natural gas.
Similar to burner sizing, gas pressure requirements must be considered when sizing piping and control components. All components will take a higher pressure drop than natural gas for a given heat requirement. The pressure requirement of each component must be evaluated, but most components will typically be one pipe size larger than found on a natural gas installation.
A word of caution is in order: When evaluating your processes, be sure to match the process burner requirements to the characteristics of any alternative fuel being considered. The basic requirements of process burner design and operation include reliable ignition, flame stability and flame supervision. The fuel type can affect any or all of these requirements, so burners must be evaluated and selected to match both the process and the fuel.
Because landfill gas is about half methane (the primary component of natural gas), not all natural gas burners can fire landfill gas without modifications, special selection or, at a minimum, special control and adjustment. Confirm with the burner manufacturer that the basic requirements of burner operation can be satisfied for a given landfill gas. Be sure to verify their historical landfill gas testing and experience.
5. What Happens If the Landfill Gas Supply Is Interrupted?In addition to process requirements, the production requirements, schedules and downtime concerns specific to your application are key concerns when considering alternative renewable fuels. Backup fuel systems should be considered and might be required. When manually switching a fuel supply is adequate, some burners are capable of burning landfill gas and natural gas with minor burner or control adjustments. However, for the most demanding production schedules, gas-blending systems (based on propane technology) exist to automatically switch fuels without manual intervention.
In the case of the BMW Spartanburg application, two piston-operated mixing systems (one operating and one standby), manufactured by Alternate Energy Systems Inc., Peachtree City, Ga., 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 adjusted to closely match the calorific and Wobbe Index properties of the landfill gas so that no burner adjustments are necessary. (The usefulness of the Wobbe Index number is that for any given orifice, all mixtures that have the same number will deliver the same amount of heat.) Supplemental fuel can be provided in an automatic, seamless manner so that no interruptions in fuel delivery are reflected in the performance of the process equipment.
6. How Close Does a Landfill Need to Be?The beneficial use of landfill gas is often termed a marriage of opportunity. An accessible landfill, in the vicinity of the plant, that consistently produces enough fuel to make economic sense for the facility that is currently purchasing natural gas, is necessary to make it a worthwhile endeavor. Market drivers such as natural gas costs, right-of-way availability and construction costs combine to give each project a unique maximum distance. But, unlike a decade ago when the typical maximum distance from a landfill was less than 5 miles, today projects are now being developed for much greater distances. At BMW, a 9.5-mile landfill gas supply underground pipeline was installed that had to cross a river, two creeks, an interstate highway and BMW’s test track. Given the right circumstances, a distance of even 15 miles may be economically feasible.
7. What Happens in the Future When the Landfill Gas Production Stops?This is my favorite question. My usual retort is, do we ever question what happens when the natural gas wells run out? No one ever questions the availability of fossil fuel. It is true that landfill gas production does have a finite life -- typically 10 to 20 years of significant landfill gas output. However, I foresee no change in our society’s ability to generate waste.
Given that all landfills generate methane, the beneficial use of landfill gas makes economic and social sense. Because methane is both potent and short-lived, reducing landfill methane emissions is an excellent way to achieve a near-term beneficial impact in mitigating global climate change. But compared to the cost of natural gas, the true green benefit may be the economic savings achieved from using a green fuel.
The equipment installed during the BMW Spartanburg paint shop project did not change the thermal energy requirements of the process equipment. Nevertheless, Dürr was able to displace a consumption of 27 million BTU/hr of natural gas with landfill gas. BMW’s additional use of landfill gas in the paint shop does not inhibit in any way the production of electricity on site, and it has greatly reduced the paint shop’s reliance on natural gas. Based upon BMW’s long-term contract with Framingham, Mass.-based Ameresco, its energy company, for the fixed price supply of landfill gas, and the rising cost of natural gas, BMW currently saves in excess of $1 million dollars per year in their paint shop operating costs. This savings is the equivalent of more than $5 in cost savings per painted body.
In conclusion, remember that not all landfills generate the same amount of landfill gas -- the amount of methane created depends on the quantity and moisture content of the waste, as well as the design and management practices at the site. However, it is a viable candidate for a renewable energy source that you may be able to use in your process application. Be sure to investigate the quantity and quality of the landfill gas from a candidate landfill in order to ascertain its suitability for your purposes. PH
The author would like to acknowledge the contributions of George Fritts and Jim Westin of Eclipse Inc., Rockford, Ill., and thank Dara Leadford and the entire BMW Spartanburg team for their support of this project and associated articles.
Sidebar:In recognition of their achievements, BMW Manufacturing Co. LLC, and
Dürr Systems Inc. were joint recipients of the 2006 “Energy Partner of
the Year” award. It was presented by the U.S. Environmental Protection Agency
at the 10th Annual Landfill Methane Outreach Program (LMOP) Conference.
Success Brings More than Energy Savings
The project also was recognized as the 2007 Renewable Energy Project of the Year by the Association of Energy Engineers (AEE).