Originally intended as a way to minimize a nuisance or manage a potential hazard at a landfill, landfill gas (LFG) management systems have evolved into highly sophisticated engineering systems and a potential source of renewable energy. And while the engineering design and construction of a modern active landfill gas system is a technological achievement of the first order, it is the day-to-day operations of an LFG system that makes it so successful. Daily operations of an LFG system include both monitoring and maintenance. Monitoring is not just done at pressure gauges on wellheads throughout the landfill to ensure proper functioning; monitoring also takes place offsite in strategic locations around the landfill to ensure that no LFG is escaping. In addition to monitoring, continued maintenance is performed to ensure that the electrical power, instrumentation, and mechanical systems are functioning properly. The primary goal is to avoid red flags such as an underperforming wellhead, pipeline leaks, detection of LFG in adjacent probes, or the inflation of any geomembrane cover by the buildup of an LFG bubble.
Municipal Solid Waste and Landfill Gas Generation
The organic portion of MSW does not immediately begin producing LFG right after disposal. This is a long-term process that can last well beyond the operational lifetime of the landfill, with LFG being generated long after the landfill has stopped receiving waste and has achieved final closure. The LFG production process occurs in four distinct phases over a landfill’s lifetime. In larger landfills operating for decades, the most recently deposited waste can be at the first stage of LFG production while the oldest parts of the landfill are at phases 3 or 4. These stages are as follows:
Stage I – Aerobic Decomposition. After initial disposal and in-waste compaction, the air voids within the landfill’s waste mass are nearly identical to the atmosphere, with large amounts of oxygen. This oxygen supply allows for the first stage, which is driven by aerobic (oxygen-using) bacteria. This is a relatively short duration stage and it begins almost immediately after waste disposal.
Stage II – Acidogenesis. Anaerobic bacteria are poisoned by oxygen, and with the oxygen consumed, they quickly displace the previous aerobic microorganisms. The continuing hydrolysis by anaerobic bacteria is actually a form of fermentation that produces organic acids, hydrogen, carbon dioxide, water vapor, ammonia, and nitrogen. The hydrogen and carbon dioxide are produced as byproducts of the fermentation of the simpler organic material previously produced by the aerobic bacteria, creating volatile fatty acids.
Stage III – Acetogenesis. This last, preparatory stage involves the conversion of the volatile fatty acids produced by the previous stage’s activities into acetic acid, carbon dioxide, and hydrogen.
This continues under anaerobic conditions, requiring additional heat. So, by this stage, the waste’s temperature has typically fallen to less than 100°F from its peak in the first stage. With this stage, stable LFG production can now commence.
Stage IV – Methanogenesis. This fourth and longest stage converts available acetate to methane and carbon dioxide while consuming the last of the hydrogen in a process that also involves CO2 reduction by free hydrogen molecules. This phase is the longest duration, lasting for the bulk of the landfill’s operational lifetime and post-closure care period, and beyond (longer than all the other phases combined). While earlier phases last for several years, this fourth stage lasts for decades, often extending even beyond the site’s post-closure care period.
These four stages are followed by a Maturation Stage where methane production falls off and the waste goes back to producing oxygen and nitrogen (though this stage typically occurs long after landfill closure). As a practical concern, this last stage may not be achievable due to the design of the landfill’s final cover. Though it is possible that once all of the available acetate is converted into methane at the end of stage 4, the landfill can theoretically revert back to its initial aerobic stage requiring the influx of oxygen for further bacterial activity.
Landfill Gas System Operations
An active LFG management system consists of vertical wells (typically 6-inch diameter) set in a regular pattern within the top of the waste and installed at depths equal to a certain percentage of the waste depth at the location of the well. The pipe below grade consists of a solid pipe wall segment and a lower slotted or perforated pipe wall segment. The pipe length is encased in permeable stone and sealed off at the top by an impermeable plug. The gas wells are equipped with a header system on the exposed end of the pipe extending above the top of the waste. The header system consists of a smaller (typically 2-inch diameter) standpipe attached to the top of the well pipe by a reducer pipe connection, sampling ports, pressure gauge, and a wellhead flow control valve. The wellhead is connected via a flexible hose to a gas collection header or branch pipe installed in the top of the waste horizontally and adjacent to the wellhead.
The pattern and spacing of the wells depend on the zone of influence (ZOI) each well can generate when vacuum pressure is applied to the wellhead. The ZOI is a function of the well depth, refuse permeability (which decreases with increased levels of in-place compaction), the permeability of the waste cover, and the thickness of the cover. The ZOI is defined by a particular radius which forms a cylindrically shaped volume of influence along the length of the buried well pipe length. Since the ZOIs of adjacent wells tend to overlap to ensure complete coverage, the actual volume of influence is reduced somewhat from the original radius of influence. By combining the effective volume of influence by the projected LFG generation rate (typically cubic feet per minute) per unit of waste, the minimum required flow rate from each wellhead can be determined.
To achieve the necessary flow rate, consistent minimum pressure must be applied to each wellhead location. This is transmitted to the wellheads through a system of buried pipes. The pipe system connecting the wells typically consists of one or more pipelines (6-inch to 12-inch diameter) encircling the landfill and running more or less parallel with the landfill’s perimeter. These pipes form a complete circle with branch lines extending out to the individual well locations and connecting them to the main header pipeline. The header is also connected to the source of applied negative pressure. This negative pressure is applied by a central blower facility. However, pressure losses occur along the header pipeline length. These are a function of the specific gravity of LFG (0.98), the pipeline length, and the flow rate. Usually, a minimum pressure of 10 inches of water needs to be applied at each wellhead to properly extract LFG. A further 12 inches or more is added to account for a flare stack or other mechanism for deposing or storing the extracted LFG. Therefore, the minimum pressure generated by the blower must be equal to these 22 inches of pressure plus the worst-case pipeline pressure losses incurred along the header pipe, along with a factor of safety. Once the flow rate is determined through each pipeline, the required flow velocity of the LFG can be determined.
Lastly, the condensate that accompanies the LFG must be dealt with. The liquid flows with the LFG and can be thought of as an extremely concentrated form of landfill leachate. To remove it, the LFG flow passes through a larger diameter pipe which reduces the LFG temperature, causing the condensate to liquefy and drop out of the gas flow. It leaves the pipeline via a drip leg to be carried away for treatment by its own separate pipeline system, or it is allowed to percolate back down through the waste via a mound of permeable stone.
Landfill Gas Monitoring
In addition to the day-to-day operation of the landfill gas extraction system (maintaining and operating the blower and flare units; measuring the applied pressure at the wellheads and adjusting the control valves accordingly to ensure the system is properly balanced; measuring flow rates of LFG; collecting and treating condensate; etc.), a crucial function is monitoring for LFG that could be migrating beyond the limits of the landfill. There are five broad categories of LFG monitoring: soil gas monitoring for LFG migrating through the soil; near surface gas monitoring for airborne LFG; emissions monitoring directly at the LFG vent or flare stack; ambient air monitoring for general outdoors LFG content; and indoor air monitoring for LFG that may have accumulated in confined spaces or buildings. Depending on what is being monitored, the sampling equipment can be handheld portable units or permanently installed sampling stations, and either consists of continuous monitoring or one-time measurements by grab samplings.
Monitoring is performed by LFG monitors located in three areas: subsurface systems that measure soil gas concentrations; surface monitoring performed within a few inches of the soil surface; and enclosed space systems to monitor indoor air quality. Monitoring is performed either in an instantaneous mode or by integrated sampling. The first involves taking samples and testing them individually while walking over the landfill surface or just outside its limits, with testing performed in the field. The second pumps multiple samples into a storage back for later testing of the integrated samples in a laboratory.
All LFG monitoring techniques rely on flame ionization detectors (FID) for testing. An FID measures the content of analytes in a gas stream via gas chromatography. This technique detects ions formed during combustion of organic compounds (like methane) in a hydrogen flame. The resultant proportion of ions indicates the amount of organics present. Measurements are reported as equivalent amounts of methane (the method does not directly detect carbon dioxide) though typically labeled as “total hydrocarbons.”
Standards for allowable levels of methane vary internationally and from state to state with a typical regulatory limit of methane as 500 parts per million (ppm) by volume. The monitoring, sampling, and testing are performed to ensure that the landfill meets this and any other applicable regulatory requirement.
Landfill Gas System Capital and Operating Costs
A fully complete landfill gas management system consists of collection wells and wellheads, header pipes and fittings, branch pipelines and fittings, condensate drip legs, and at least one blower assembly and flare system for safe destruction of the extracted gas (usually one flare per 100 acres of landfill). Instead of a flare, more advanced systems at larger landfills can utilize waste to energy systems that use the heat from combusted LFG to run a turbine or use as a fuel for a reciprocating engine to generate electricity for the local grid. The pricing of each component is unique, but can be roughly prorated on a per acre basis:
- Gas probes typically cost from $5,000 to $10,000 each and are typically installed at a rate of one per 10 acres, giving a cost per acre between $500 and $1,000.
- Gas extraction wells and associated fittings cost between $8,000 and $12,000 each, depending on their depth. At a rate of one per acre, the costs per acre would be between $8,000 and $12,000.
- Assuming about a 200-foot average spacing interval, header pipelines are installed at a rate of 200 feet per acre. Costing $100 to $150 to install, their cost per acre varies from $20,000 to $30,000.
- A gas collection blower and flare assembly connected to the extraction well field by the header pipes will cost from $40,000 to $60,000 each. At an installation rate of one per 100 acres, per acre cost would be between $400 and $600.
The total cost per acre of the landfill gas management system would be between $30,000 and $54,000.
The annual costs of landfill gas system maintenance can also be prorated on a per acre basis. Annual maintenance averages $50 to $100 per well, with an average of one gas well per acre. Maintenance of the header pipelines and other appurtenances averages $2.00 to $2.50 per linear foot. At an average of 200 linear feet of header pipeline per acre, the cost of maintaining the pipelines varies from $400 to $500 per acre. Total annual gas system costs per acre will be $450 to $600. Over a 30-year post-closure care period, leachate system management costs will vary from $13,500 to $18,000 per acre.
Blackhawk Technology Company provides a range of reciprocating, positive displacement piston pump products for leachate pumping, gas-well dewatering, and gas-condensate pumping. Power sources for these pumps include pneumatic, electric (explosion proof), and solar-powered drive motors. Ruggedly built, they can run dry without damage and perform in all temperature and weather extremes. They are safely field-serviceable, requiring little maintenance. Their most powerful and reliable pump, the Atlas Pneumatic Piston Pump, excels in demanding pumping situations and harsh environments. It handles virtually all liquids, including coal tar, gases, and other difficult products. It is designed to operate without emitting greenhouse gases or exhaust air and can be customized for specific site requirements.
Parker Hannifin provides filter systems to solve the problems associated with LFG collection: system compressor damage, heat exchanger fouling, unpleasant odors, safety hazards, and other problems at energy usage sites. Their systems filter the collected landfill gas entering into the compressor to eliminate particles, liquid slugs, and aerosols that could otherwise damage downstream equipment. The Process Filtration Division of Parker Hannifin has launched a range of natural gas filtration products, developed to improve the efficiency of gas production processes and extend the service life of the equipment. Their ZA-, ZJ-, and FFC-Series filters are able to remove contaminants that would quickly impair the performance of conventional filters, considerably reducing maintenance requirements and costs for users. The range of filters are designed for natural gas flows and feature PED compliant housings constructed from carbon steel or stainless steel.
Perennial Energy manufactures Utility Flare and Enclosed Flare systems for LFG applications. Their standalone Utility Flares are designed with 100:1 turndown ratio (absolute minimum of 10 standard cubic feet per minute) for optimum utilization with landfill or digester gas destruction requirements. PEI Utility flares provide high-efficiency combustion and odor control applications while meeting all current air quality regulatory management agency requirements for non-verifiable, high-efficiency combustion devices. PEI Enclosed Flare Systems and standalone Enclosed Flares (EGF) meet or exceed most air quality regulatory management agency requirements for maximum byproduct of combustion emissions and minimum destruction and reduction efficiencies (DRE). Their flares operate with the lowest manifold and burner back pressures in the industry and offer very low noise levels and zero rumble.