The Potential of LFG

Sept. 1, 2009

Last year, when the cost of gasoline at the pump almost reached $5 per gallon in many regions of the country, there was a silver lining on the energy storm clouds. The increased price at the pump made renewable energy and alternative fuels economically attractive even without the further research necessary to make them more competitive. Everything from wind to solar thermal to photovoltaic cells to ethanol to algae biofuels stood ready to take the place of ever more expensive fossil fuels. Then came the fuel price collapse later in the year, coinciding with the economic meltdown and recession and pushing the price of gasoline back down below $2 per gallon. And like a receding tide, the falling gasoline prices left many an alternate energy plan high and dry. The questions that need asking now are: What landfill-gas-to-energy (LFGTE) technologies remain economically viable, and which ones remain technologically promising?

Landfill Gas Production
Landfill gas (LFG) is a byproduct of waste decomposition. Like any other soil, municipal waste can be described by its physical fractions of solids, liquids, and gases. The solids consist of the waste matter itself along with soils utilized for daily and intermediate cover. Waste gets deposited with relatively small moisture content but has the ability to retain significant quantities of percolating precipitation (up to about 33% by volume). The air-void fraction of the waste mass volume depends on the type of waste and its level of compaction achieved after deposition in the current disposal cell. Initially, the air voids have an oxygen and nitrogen composition identical to that of the surrounding atmosphere.

However, aerobic bacteria present in the waste and soil start to work, breaking down the organic fraction of the waste while consuming the available oxygen and producing carbon dioxide. This sets the stage for the true phase of LFG production by anaerobic bacteria. This gas consists primarily of methane (45%–50%), carbon dioxide (45%–50%) as well as volatile organic compounds (trace gases). It is the methane component that interests us as a source of energy.

The rate of production can vary widely from landfill to landfill or between different areas of the same landfill, due to a variety of conditions. Moisture content can greatly accelerate the production of LFG as increased availability of water aids bacterial activity, up to a point. Too much water, however, and the landfill could be completely saturated with liquids filling all available air voids. The amounts of organics in the waste mass can vary widely as well. Some municipal solid waste landfills receive organic sludges, with their very high organic content, from wastewater treatment plants. Other landfills may receive construction-and-demolition (C&D) debris consisting primarily of inorganic concrete and structural steel. In addition to precipitation, other climate variables, such as temperature, can affect LFG production. So can the daily waste receipts, compaction rates achieved, and other operational factors. So it pays to remember that almost any value sited for landfill waste and LFG characteristics represents an overall average across many sites. A specific value at a particular landfill may vary greatly from a quoted amount.

On average, waste is deposited at the landfill‘s current working face in its current operational disposal cell with a standard density of about 33 pounds per cubic foot (0.45 tons per cubic yard). After it is deposited and spread out over the work face, waste compaction equipment works to reduce the volume and increase the in-place density of the waste. The goal is to reduce the volume by half (doubling the density to 0.90 tons per cubic yard). This still leaves the waste with an equivalent porosity (volume of voids per total volume) of about 67%. With these physical characteristic established, the waste mass can begin producing LFG at a more or less predictable rate.

Projecting LFG production rates is a tricky business due to the wide variety of facility, material, and situational characteristics that impact the amount of LFG produced per amount of waste in a given time frame. The best anyone can hope to do is come up with a range of values for production estimates with a standard value based on typical characteristics for the purpose of planning.

This is what the EPA’s software program “Landfill Gas Emissions Model” (LandGEM) does. LandGEM relies on several model parameters with assumed values to estimate landfill emissions. These parameters include the projected methane generation rate (K), the potential methane generation capacity (L), assumed NMOC (non methane organic compounds) concentrations, and the assumed methane content of the overall LFG emissions. For the purposes of modeling, methane is typically assumed to be 50% of the total LFG emissions by volume (with the other 50% being carbon dioxide). NMOC concentrations are usually assumed to be insignificant (4,000 parts per million as hexane being a typical value). The first two values, K and L, have the greatest impact on projected annual LFG production rates.

The projected methane generation rate (K) determines the rate of methane production for each sub-mass of waste in the landfill. K is a constant that determines the rate of LFG generation. The value of K is a function of waste moisture content, the abundance of nutrients for the anaerobic microbes, the pH value of the waste and the temperature of the waste. The higher the value of K the faster the methane rate increases and then decreases over time. The model assumes that the value of K is the same before and after peak production of methane occurs (a point which coincides with the last receipt of waste and close out of the landfill). A standard value of K used by the LandGEM model is K = 0.05 per year. However, field observations indicate a wide range of potential values of K, from 0.003 per year in arid climates to 0.70 per year for wet bioreactor landfills (source: USEPA).

The potential methane generation capacity (L) depends on the type and composition of the waste in the landfill, specifically how much of the waste mass is cellulosic and prone to organic decomposition. The LandGEM model assumes a default value for L = 170 cubic meters per milligram (1 milligram = 1,000 kilograms, or 1 metric ton is equivalent to 2,205 pounds). The value of L can vary from 6.2 to 270 (source: USEPA 1991). With a value of 35.3 cubic feet per 1 cubic meter, the default value of L in English units is 2.72 cubic feet per pound.

By combining these parameters into a first-order decomposition rate equation, LandGEM can estimate the annual emission over a specified time period. Put simply, a certain portion of each year’s waste receipts decomposes and generates LFG. The undecomposed portion from the previous year is added to the new waste receipts from this year to provide the total amount of decomposable waste for this year. Each year this process is repeated until the last waste load is received and the landfill closes. Afterwards, with no new waste receipts, the amount of decomposable waste is reduced each year and landfill gas production along with it. For any given year in isolation, the amount of landfill gas generated is determined by the product of K and L. Utilizing the default values above, this is equivalent to an annual methane production rate of 0.136 cubic feet for every pound of waste (equal to 272 cubic feet per ton of waste). Assuming that methane is 50% of the total emissions, the total landfill-gas-generation rate for the same period would be twice this amount. But what concerns us is the energy-producing methane.

Energy Potential
Converting the annual methane production rate to cubic feet per minute results in 0.0005 cubic feet per minute per ton of waste, or approximately 1 cubic foot per minute per pound. Methane has an intrinsic heat value of 1012 Btus per cubic foot. The 272 cubic feet of methane produced each year by a ton of disposed waste would have a heat value of over 275,000 Btus.

Each of the 300 million citizens of the United States on average throws out approximately 4.5 pounds of municipal solid waste each day. This is equivalent to 675,000 tons per day or over 246 million tons of waste each year. Approximately 72 million tons of waste are recycled annually and never get sent to a landfill. The remaining 174 million tons that do get landfilled can be utilized for methane generation. Given an annual methane heat energy value of 275,000 Btus per ton of waste, the total amount of landfilled waste has a value of 47,850,000,000,000 Btus nationwide.

With a weighted average heat rate for LFG-fired engines, turbines, and boiler/steam turbines of 11,700 Btus per kilowatt-hour (source: USEPA Landfill Methane Outreach Program, LMOP), the heat value from the methane generated from a year’s worth of waste disposal operations has the potential to generate almost 4,100,000,000 kWh per year. Since the average American household utilizes 11,232 kWh per year, this is equivalent to the electricity used by over 365,000 homes. As a whole, the United States consumes approximately 3,656,000,000,000 kWh per year (source: Energy Information Administration). The potential electrical energy generated from landfill methane would be equal to slightly over 0.1% of this total. Note that this value does not factor in the left over nondecomposed waste that has been previously disposed of and is still capable of generating methane as well. This may seem an insignificant amount, but for local uses LFG-derived energy can play a significant role in niches markets.

Landfill Gas Collection and Extraction
But how much of this potential energy is recoverable? That depends on the efficiency of the landfill collection and extraction systems at the individual landfills. The overall efficiency of any landfill gas extraction system depends on several design factors. These include well spacing and depth, applied negative pressures, head losses due to friction, the permeability of the final cap and cover, and the amount of exposed waste during the current operational stage of the landfill. The various factors affect each other. For example, the amount of gas that can be extracted per well, and the spacing between the wells, can be greatly increased with an impermeable cap that completely covers the waste mass. This impermeable cap prevents in infusing of outside air from entering the landfill waste mass. This allows greater efficiencies since the applied pressure heads can be much greater than one atmosphere. Furthermore, by preventing the intrusion of oxygen, an impermeable cap prevents the short-circuiting of the methanogenic process by keeping the waste mass anaerobic. Also, keeping oxygen out of the waste mass reduces the potential for a fire to occur within the landfill.

However, a complete impermeable cover over all of the waste mass represents an ideal condition. For most of the operating history of any landfill, the waste surface can include those surfaces that have achieved final grades and slopes which have received a final cap and cover layer, those that have achieve final grades but are still awaiting a final cap, those intermediate slopes of previous cells that will be exposed for significant periods of time before additional waste is piled up against them, and the daily working face slopes of the current disposal cell. Intermediate slopes typically have on 12 inches of soil cover, while daily operating slopes have 6 inches of soil cover (or some sort of synthetic alternate cover layer). In either case, both intermediate and daily slopes, the predominant topographic features in any landfill until the later stages of its operational lifetime, remain without an impermeable cover for years or even decades.

Furthermore, each previous operational area and associated waste mass will be at a different stage in gas production. More recent waste volumes associated with the current disposal cell will be in the initial aerobic production stages (though these may be relatively short lived).

Older disposal cells that have entered the anaerobic regime will be a different methane production stages, some will just be starting to produce measurable amounts of methane, while adjacent cells will be a full or even declining production. As such, the variable operational nature of a landfill combined with the variable organic content of the waste itself make predicting specific methane production rates, let alone achieving maximum collection efficiencies, nearly impossible.

For this reason, most landfill-gas-to-energy (LFGTE) plans are not initiated until the later stages of landfill operations, when methane production is peaking and impermeable cover is at its greatest extent.

Methods of Utilizing LFG-Derived Energy
According to the EPA’s Landfill Methane Outreach Program (LMOP), there are approximately 480 LFGTE projects are currently in operation in the United States. These vary in size from local community self help projects to multimillion-dollar capital investments—depending on the size and location of the landfill.

LFG (specifically the methane content of the landfill gas) can be used in any number of ways to produce energy. The simplest and easiest method is to simply flare off the LFG to destroy the methane and associated nonmethane organic compounds (many of which are toxic). While this activity is not normally thought of as a means of energy production, the utilized energy does perform useful work. In this case, the combustion of methane (which produces water vapor and carbon dioxide) removes a significant greenhouse gas from the atmosphere. Methane has 21 times the greenhouse potential of carbon dioxide. The NMOC trace compounds found in landfill gas emissions include vinyl chloride, dichloromethane, benzene, toluene, and other organic compounds. Combusting the landfill gas at a flare stand effectively volatilizes these trace organic compounds and performing the work of pollution control.

If a simple flare is the traditional method of utilizing LFG, a more recent development is the microturbine. Capable of generating between 25 and 500 kW, with cycle efficiencies of 25% to 30%, these units are about the size of a refrigerator and have been used at landfills since 2001. It is estimated that over 100 microturbine systems are in use at landfills in the United States. Like any other turbine electric generator, a microturbine system includes a compressor, combustor, turbine, alternator, recuperator (to recover waste heat in the exhaust vent), and generator. The LFG is prepared for use prior to arriving at the microturbine. The LFG is compressed, dehydrated, and filtered to remove particulates and other nonusable organic compounds. This need for pretreatment will increase the cost of the system’s operation and/or reduce its overall power generation potential.

A simpler concept is the utilization of LFG to generate direct heat. These direct thermal applications have been put to use in many creative and innovative applications, such as providing infrared heat in nearby facilities via heated tubes filled with either air or water, firing up brick kilns and the flash drying of clays and other construction materials, the evaporation of paint and other liquid wastes to dry powders for safe disposal, the incineration of sludges and other dangerous wastes, lime and steel production, evaporation of landfill leachate, and the processing of animal feed and pet foods.

Large-scale energy production is possible at larger landfills with high peak LFG generation rates. These large methane quantities are usually used to fire industrial-size boilers. These in turn provide the energy necessary to drive large turbines that can provide power to industrial facilities or residential areas. LFG can also be combusted in an industrial process heater to provide heat for chemical reactions. More directly, LFG can be utilized to run reciprocating internal combustion engines that generate electricity without the need for a boiler and turbine setup.

One advanced concept being utilized for LFG-derived energy involves a carbon-dioxide wash unit, a system developed by Acrion Technologies of Ohio. This approach strips out the carbon dioxide and other impurities, leaving behind nearly pure methane indistinguishable from other natural-gas pipeline fuels. In this process, LFG is pretreated by being compressed, having hydrogen sulfide removed by means of a filter and then dehydrated. The LFG then enters the wash stack, which is effectively a refrigerated exhaust stack. The reduced temperatures take advantage of the fact that carbon dioxide has a different triple point than methane and will transition from a gas to a liquid at a higher temperature than methane. As the landfill gas (an approximate 50:50 mix of methane and carbon dioxide) rises through the refrigerant stack, the carbon dioxide becomes a liquid mist and falls back down counter current through the stack. As it falls, the liquid carbon dioxide acts as a filter to remove additional trace impurities. The liquid carbon dioxide is collected at the bottom of the stack and is either sent along with the accumulated trace impurities to a flare for destruction or reutilized as commercial-grade refrigerant (dry ice). What remains of the landfill gas is an almost pure stream of methane that can be compressed and reutilize as a natural-gas-like fuel. Such a system is in operation at the Solid Waste Authority of central Ohio’s (SWACO) landfill located outside of Columbus, OH. Designed, built, and operated by Firmgreen Inc., the extracted methane is stored and sent to pump stations, where it is used as fuel for vehicles that have been converted to natural gas operation.

Potential Markets for LFG-Derived Energy
The first potential market for energy derived from landfill gas is the landfill itself. Direct thermal systems can provide heat for onsite facilities. Microturbines can be used to generate electricity for use at the landfill itself, often resulting in significant reductions in daily operating cost. Purified methane can be compressed and used locally by site equipment and vehicles (usually, specialized vehicles owned and operated by the landfill itself would be the only users of such fuel). Indirectly, the use of LFG to volatilize leachate extracted from the landfill can avoid significant wastewater treatment costs.

Often, for reasons of politics, aesthetics, and land-use economics, landfills are located in isolated areas away from the population centers that could utilize the energy generated by LFG. As such, it would often be difficult to find a major offsite customer for LFG-derived energy. Proposals for selling electricity back to the grid, for example, would have to take into account the voltage drops along power lines. Piping of purified methane in existing or proposed natural-gas pipelines is often forbidden by many states due to the high nitrogen content of the methane. However, if the landfill is fortunate enough to be located within a reasonable distance of an industrial customer in need of a relatively inexpensive source of energy, it may make sense for the landfill to enter into a long-term agreement with the local industry at terms and prices that justify the capital expense of installing the LFGTE system.

In all cases, there are several economic rules of thumb that must be adhered to for an LFGTE project to make financial sense. First, all costs (including the capital and operating costs of LFG pretreatment subsystems) must be factored into the analysis. These can account for the majority of the overall costs, especially in small recovery systems. Second, continuous capital investments may be required throughout the system’s service life, including overhauls or replacements every few years as well as future expansions as the landfill gets bigger with age. Third, comparisons with local energy costs from standard utilities must be made and grants obtained from the government (if necessary) to offset the price differential. Finally, sales of renewable energy credits, carbon credit offsets, or green power tax deductions may make the LFGTE proposal financially viable.

It’s a perfect example of “doing well by doing good.”
About the Author

Daniel P. Duffy

Daniel P. Duffy, P.E., writes frequently on the topics of landfills and the environment.

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