Maximizing LFG capture and utilization requires a wide range of different but compatible equipment to perform the tasks of collecting, extracting, cleaning, storing, transporting, and flaring the gas. Their proper application and coordination can help a landfill largely avoid significant environmental issues and safety concerns while providing the landfill with an opportunity for additional income. The following is an introduction to these systems and their proper engineering applications.
LFG Characteristics
What we typically think of as landfill gas is the product of the fourth stage of a five-stage life cycle of alternating aerobic and anaerobic activity that takes place in the landfill. This first stage is driven by aerobic bacteria and begins almost immediately after waste disposal. While producing both carbon dioxide and water vapor, aerobic microbes consume the available oxygen in the newly deposited waste.
Once the oxygen has been almost completely removed, an anaerobic state is achieve and triggers the next stage. Stage two involves hydrolysis by anaerobic bacteria that produce organic acids, hydrogen, carbon dioxide, water vapor, and ammonia nitrogen.
During this stage, acidogenesis of the simpler organic monomers previously produced by aerobic hydrolysis during the first stage occurs. Also during this stage, sulfur reducing bacteria produce hydrogen sulfide. Stage three continues with anaerobic bacteria converting the volatile fatty acids produced by the previous stage’s acidogenesis activities into acetic acid, carbon dioxide and hydrogen.
The first three stages are of relatively short duration and set the stage for the long term production of landfill gas in the fourth stage. During the fourth stage, available acetate is converted to methane and carbon dioxide while using up hydrogen. This stage lasts the longest, often lasting as long as or longer than all the other phases combined and constituting the bulk of the landfill’s operational lifetime and post-closure care period. Following stage four, the landfill is ready to return to stage five when the initial aerobic stage is reestablished as methane production drops off. Most landfills have not been in existence long enough for this last stage to represent a significant portion of their lifetime.
The landfill gas produced during stage four consists primarily of carbon dioxide (50%-55%) and methane (45%-50%). In addition to these two primary components, approximately 1% of landfill gas consists of trace chemicals (hydrogen sulfide, benzene, ethyl benzene, toluene, vinyl chloride, dichloromethane, trichloroethylene, 1,2, -cis-dichloroethylene, tetrachloroethylene, etc.). Oddly enough, neither methane nor carbon dioxide has an odor (which is why the gas company adds an odor to its gas pipelines to warn people of leaks and why every house should be protected by carbon dioxide detectors). What people smell when they pass a landfill is the hydrogen sulfide and various esters that give off a “rotten egg” odor. Of these components only the methane has any real energy potential. The aim of any efficient landfill gas to energy system would be to separate the methane for m the rest of the gas stream prior to its utilization as a fuel.
LFG Energy Potential
So how much potential energy is there in the methane portion of the landfill gas stream that can be utilized as a fuel source? Predicting how much landfill gas will be generated by a given waste mass over a set period of time is more art than science. This is because municipal solid waste (and its organic contents generating the landfill gas) is heterogeneous with highly variable constituents.
A computer model developed by the EPA, “Landfill Gas Emissions Model,” or LandGEM, makes some simplifying assumptions based on empirical data in order to project gross estimates of landfill gas production over time. First it assumes for simplicity that exactly 50% of the landfill gas stream is methane and the other 50% is carbon dioxide, with trace elements assumed to be insignificant. The projected methane generation rate is a factor that determines the rate of methane production for each sub-mass of waste in the landfill and varies depending on several landfill characteristics (climate, waste moisture content, the abundance of nutrients for the anaerobic microbes, the pH value of the waste and the temperature of the waste, operational procedures such as leachate recirculation, etc.).
The model’s assumptions are put into an equation that estimates the amount of gas generated annually as new waste is deposited and some portion of the old waste decomposes. These estimates continue until the end of the landfill’s operational lifetime, when it stops receiving new waste. Peak production of landfill gas occurs at this point, and the rate of landfill gas formation then slowly declines. Given typical assumptions concerning the characteristics of the deposited waste, the average annual methane production rate would equal to approximately 272 cubic feet per ton of waste. Methane (not including the other landfill gas components) has a heat value of 1012 Btus per cubic foot. The 272 cubic feet of methane produced each year by a ton of disposed waste would therefore have a potential heat value of over 275,000 Btus. This figure represents potential maximum energy content. The actual energy recovered and put to use will depend on the overall efficiencies of the landfill gas to energy system equipment.
Collection and Extraction
Often neglected in the evaluation of landfill gas collection technologies because it is not directly part of the landfill gas management system is the necessity for an impervious final cap and cover. This prevents atmospheric infiltration into the landfill (with potentially catastrophic consequences as the intake of oxygen makes fires within the waste mass much more likely). This barrier greatly improves the efficiency and the radius of influence for individual gas extraction wells. In turn, this means that fewer gas extraction wells will be necessary to provide complete coverage of a landfill’s limits of waste.
Active landfill gas extraction wells are typically located within the limits of waste and set at depths of approximately 75% of the waste thickness at the well locations. The deeper wells tend to be located in the center, with shallower wells around the waste perimeter. The changes in depth also result in changes in the zones of influence of the respective wells, so placing them to ensure proper overlap and coverage is critical. The wells themselves usually consist of 6- to 8-inch-diameter polyvinyl chloride (PVC) pipes, both solid a slotted segments with the lower portion of the piping consisting of the slotted segments. The pipe assembly is set in a borehole with a typical diameter of 3 feet.
The borehole outside of the lower, slotted pipe section is backfilled with gravel to allow both the passage of gas to the well and provide the well with a filter medium. Above the gravel backfill is an isolation ring topped by a plug of bentonite to provide a tight seal against air infiltration. The seal usually extends up into the final cap-and-cover layer and ties off to this layer in order to provide a complete seal at the well borehole. The well piping extends up and through both the seal and the final cap-and -cover to an exposed wellhead assembly. The wellhead consists of a valve to adjust flow rates, a port for sampling the landfill gas and a flexible connection to the adjacent lateral pipeline.
The lateral pipeline is set in a horizontal trench either above or below the final cap-and-cover system. Placing the pipeline above the final cap and cover allows for ease of repair, while setting it below allows for ease of construction. The pipe itself is solid-walled and is typically 8 to 12 inches in diameter and constructed of either Schedule 80 PVC or SDR-11 high-density polyethylene (HDPE). The lateral pieces act as branch lines, connecting the wellheads to a main header pipe that is typically encircling the landfill. The header pipe is of similar size or larger diameter than the lateral pipelines and is serviced by cleanout risers that extend to the surface and are located at regular intervals along the length of the header pipe.
The header pipe is connected to the landfill gas management system typically via one or more condensate knockout traps. Condensate is a liquid produced in the landfill gas extraction system. According to the EPA: “Production of condensate may be through natural or artificial cooling of the gas or through physical processes such as volume expansion. Condensate is composed principally of water and organic compounds. Often the organic compounds are not soluble in water and the condensate separates into a watery (aqueous) phase and a floating organic (hydrocarbon) phase. This organic fraction may comprise up to 5% of the liquid.“ Therefore, condensate typically contains toxic impurities at much higher concentrations than the gas itself. Condensate is collected in knock-out drip legs that either discharge the condensate back into the waste where it becomes leachate (if allowed) or is piped off separately to a storage tank for eventual treatment and disposal.
After discharging its condensate, the landfill gas continues along the header pipeline to the blower flare assembly used by most landfills. The blower is the mechanism that applies the negative atmospheric pressure to the entire landfill gas pipeline system and extracts the LFG from the various wells comprising the extraction system. These are high-volume industrial blowers adapted to landfill work from their original purpose of rapid removal of noxious or corrosive gases and fumes. Blower performance is typically rated in terms of flow rate (scfm of LFG), minimum negative pressure applied at the base of the flare stack (measured in inches of water column), maximum noise level while operating (decibels measured at a fixed distance, typically 3 feet from the blower) and electrical power requirements (typically three-phase where available). Blower fan production should be relatively smooth across the flare’s range of flow rates to ensure efficient combustion and operate at minimum rates without surging.
Blower inlets and outlets are typically oriented horizontally. The blower impellor is rated by maximum tip speed (feet per second) with the blower limited to a maximum vibration during operation (with equipment movement measured in mils). The housing and general construction of the blower unit should allow for long-term exterior operations in adverse environments. Often, extracted landfill gas is simply flared away as a safety precaution, eliminating the methane as a danger (sufficiently high levels of accumulated methane can lead to explosions or asphyxiation in confined spaces) and at least partially combusting some of the trace constituents. However, for energy production a more complex operation is required that produces a purified gas stream consisting mostly of energy-bearing methane, without the noncombustible carbon dioxide or other toxic impurities.
Purification Methods
Purification of the landfill gas is a necessity before it can be effectively utilized as a fuel or an energy source. A comparison between typical landfill gas properties and standard utility pipeline quality gas specifications illustrates why this is necessary. Landfill gas typically has about half the Btu value of an equivalent amount of pipeline gas. Its hydrogen sulfide content can be as much as 250 times the maximum allowed under pipeline specifications, with 500 times the allowed amount of water vapor. The landfill gas can have carbon dioxide, oxygen, and nitrogen quantities 10 to 12 times greater than those allowed by the gas utilities. Lastly, the purified methane has to be delivered under pressure that meets pipeline requirements (100 psi to 600 psi).
The goal of a landfill gas purification process is to produce a stream of methane with levels of purity as high as is physically and chemically possible. The purification process usually involves several steps. First, intrusion of atmospheric nitrogen and oxygen into the well field must be limited as much as possible. Second, the moisture content of the landfill gas stream must be removed. Next to be eliminated is the hydrogen sulfide and the other trace contaminants. The last-and most significant-component to be removed before the conversion to useable methane is complete is the carbon dioxide, which can constitute the bulk of the landfill gas stream. Various methods are available for removal of each type of unwanted compound from the gas stream.
Solvent absorption is a preferred method of extracting carbon dioxide from industrial gases in general and for purifying natural gas in particular. In this process, liquid chemicals are used to absorb carbon dioxide, which is then released when the absorbing liquids are subject to higher temperatures. The first step occurs in an absorption column filled with the extracting liquid. As the landfill gas is bubbled through this column from below, the carbon dioxide is stripped from the gas stream, the rest of which continues to the top of the column in purified form. The carbon dioxide laden liquid is pumped to a desorption column where it is heated to 120°C, releasing the carbon dioxide. The stripped liquid is then recycled back into the absorption column.
Pressure swing adsorption (PSA) is a non-cryogenic separation process that functions under near ambient temperatures. It is based on the fact that different gases have different propensities to be attracted to different materials and surfaces. A nitrogen removal process utilizing PSA would involve the use of an adsorption bed consisting of porous materials with a natural inclination to attract nitrogen. Most of the nitrogen will remain in the bed, allowing the passage of a purified gas stream. Later, the nitrogen can be removed for the adsorption bed by means of reducing its pressure. A variation of this process is temperature swing adsorption where gas release is triggered by changes in temperature, not pressure.
Membrane separation is often used in conjunction with other gas separation techniques. This technique is an absorption process that utilizes hollow fiber membranes as contact media for gas flows. Porous polymer membranes are the preferred contact media. The membranes can operate at 2,000 psi pressure differential and at temperatures as high as 200°F to 400°F. Operationally, membrane separation is based on the physical principle that certain gases permeate across a barrier faster than others. The hollow fibers utilized by these membranes are no bigger than twice the diameter of a human hair. By applying gas flows at given operational pressures and temperatures the desired impurities can be forced through the membrane (or conversely retained by the geomembrane) for later sequestration.
“CO2 washing” is an innovative new approach that takes advantage of the different reaction of the landfill gas components to lowered temperatures. At the same pressures, carbon dioxide gas tends to liquefy at a higher temperature than methane. So when the landfill gas stream is subjected to refrigeration, carbon dioxide will transition to liquid while methane remains gaseous. In a vent stack, the now liquid carbon dioxide falls backwards, counter to the upward direction of the gas stream, as a mist of small droplets. As they fall, the droplets act as a carbon filter, picking up the trace impurities as they drop. Meanwhile, the now almost pure stream of methane continues upward through the vent stack to its final destination. The chilled liquid carbon dioxide can be utilized as a commercial refrigerant (dry ice).
Storage and Transportation
The preferred form of transportation for methane recovered from landfill gas is the standard natural gas pipeline utilized by the local gas utility. This is practical only in those situations where a landfill generates enough gas in sufficient quantities to make it worthwhile for the utility to include it in the pipeline. Even for large landfills, only those with a conveniently close pipeline can hope to make an economical connection. However, the piping of even purified methane into natural gas pipelines is often forbidden by many states due to the potentially high nitrogen content of the methane. The utility will have very stringent quality standards for the purity of the methane that it accepts. Even purified natural gas can have significant nitrogen content.
For those situations where purified natural gas is to be used locally (usually by the landfill itself), storage in pressurized tanks is a necessity. Further refrigeration is often required for compacted storage of the now purified methane in standard liquid natural gas storage tanks. For the most part, the storage of large quantities of landfill gas onsite is often uneconomical and landfill gas not intended for direct and continuous use as fuel or as an energy source will typically be flared off. However, since landfill gas production rates can be highly variable over the operational lifetime of the landfill, onsite tank storage facilities can provide a useful surge capacity, evening out the process flow rates.
Electricity and Fuel
As mentioned above, for most moderate to small landfills, collected and extracted landfill gas is simply flared off as a nuisance. However, on larger landfills producing significant quantities of landfill gas, the heat from the flare can be utilized to heat a boiler, which in turn runs a turbine, or used directly as fuel to run an engine.
There are also situations where methane extracted from the landfill gas is of sufficient purity to meet local pipeline gas specifications and can be sold directly to a natural gas utility. In this scenario, purified methane that meets the specifications for commercial natural gas sale and transport is pumped under high pressure into a nearby natural gas pipeline system. Flow gauges measure the quantities of recovered methane that are distributed to the pipeline network. In addition to the technical hurdles described above, the economic viability of this approach depends greatly on natural gas prices, with higher prices making the use of methane from landfills more financially attractive.
Methane used as fuel in the form of compressed natural gas (CNG) in another option. CNG vehicles can be used as a clean alternative to gasoline and petrol. CNG is made by compressing natural gas to 1% of its volume under normal atmospheric pressures. CNG has a much higher octane rating than gasoline and utilizes a standard internal combustion engine, but CNG-powered vehicles require pressurized tanks for fuel storage. Fears of explosions, however, are unfounded. In fact, CNG vehicles are safer than gasoline vehicles, since CNG is lighter than air, and a leak would quickly disperse.
The simplest application of landfill gas energy is the production of direct heat. Even landfills that don’t produce large quantities of methane can utilize the heat from a modified flare to operate a radiant hot-water heating system for nearby offices and buildings. In such a system, the landfill gas flare is used to heat tubes filled with water or air that carry the heat to radiators located inside neighboring buildings. Such a system becomes economical when the capital cost of the piping and radiators is compensated by the reduction in the costs of other utilities providing heat. Other innovative direct-heat applications utilizing landfill gas as a fuel source include evaporation of industrial liquids, creation of dehydrated products and dry powders, incineration of sludges with a high moisture content, and the curing and processing of commercial and industrial products ranging from animal feed to bricks.
America’s Energy Mix
LMOP is the US EPA’s Landfill Methane Outreach Program. Its mission statement is to provide “voluntary assistance program that helps to reduce methane emissions from landfills by encouraging the recovery and use of landfill gas as an energy resource.” It does so by forming partnerships with landfill gas and energy industry players (communities, landfill owners, utilities, power marketers, states, project developers, tribes, and non-profit organizations) to encourage project development. They assess technical feasibility of landfill gas to energy proposals, investigate market demand for the resultant energy, and provide financing to underwrite the proposal. Its ultimate goals are to effectively develop landfill gas as a viable alternative fuel while reducing greenhouse gas emissions.
As an overall solution to America’s energy needs, the methane from landfill gas is far from a silver bullet. Overall quantities remain small compared to consumer energy needs and the sources of landfill gas (the landfills themselves) are often located in areas that make access to the power grid inconvenient. On a per capita basis, the amount of energy generated by methane from the waste thrown out by individuals is only a fraction of an individual’s energy needs. Bioreactor landfills (where water is deliberately introduced into the waste mass to accelerate rate and amount of waste decomposition and landfill gas formation) change the equation somewhat. But bioreactor landfills are relatively new and have significantly higher operational needs than a typical “dry tomb” landfill.
For typical landfill-gas-fired steam boiler turbines operating at acceptable efficiencies, approximately 11,700 Btus of heat energy are required for each kWh of electricity produced (source: USEPA Landfill Methane Outreach Program, LMOP). The average American household utilizes 11,232 kWh (the United States consumes approximately 3,656,000,000,000 kWh per year. Source: Energy Information Administration, 2007). If all the waste that Americans dispose in landfills could be efficiently tapped to run a steam boiler turbine, it could provide only 0.1% of America’s total electrical needs. Methane extracted from landfill gas by itself will not meet that growing energy need.
However, these nationwide averages are not very meaningful when evaluating individual landfill-gas-to-energy projects that could provide energy for local populations. In such local niche markets, landfill gas to energy can play a significant role in cutting overall costs and providing a more diversified energy mix. Local economics and energy needs will be deciding factors. For example, it may n