Editor’s note: This article first appeared in the March/April 2016 issue of MSW Management.
Prior to conversion, limited treatment is required for direct use of landfill gas (LFG) in boilers or engines. Primary treatment steps may include dehydration and filtration to reduce moisture and particulates, as well as hydrogen sulfide and/or volatile organic compound (VOC) removal.
Advanced treatment is required to produce high-BTU gas for injection into natural gas pipelines or production of alternative fuels. Advanced treatment steps provide additional LFG processing and may employ multiple cleanup processes. The type of advanced treatment depends on the constituents that need to be removed for potential end use. This article describes the different approaches and technologies applicable to primary and advanced treatment options.
Electric power generation is the most widely applied landfill gas-to-energy (LFGE) technology in the United States. Microturbines, reciprocating engines, combustion turbines, steam cycle, and combined cycle power plants have been successfully used to convert LFG to electricity. Medium-BTU gas sale is the second most widely applied LFGE technology in the US. Medium-BTU gas projects usually provide limited treatment prior to conveying the LFG through a dedicated pipeline to an end user.
Electric power and medium-BTU gas projects employ primary treatment steps, as necessary, to condition the LFG prior to use. Primary treatment steps, including dehydration, filtration, hydrogen sulfide removal, and VOC removal, are discussed below.
Typically, LFG is saturated with water vapor within the relatively warm landfill. When extracted and subjected to relatively cool temperatures outside of the landfill, some of the water vapor in the LFG will condense and form liquid water or condensate. Condensate must be managed, or prevented from forming, throughout the entire LFGE process. Of most importance, liquid water cannot be allowed at the burner tip or at an engine’s fuel valve.
The simplest and most cost-effective method to prevent condensate from forming, after pressurization or compression of LFG, is temperature control. This approach is used by many electric power projects and a few medium-BTU projects. LFG is almost always cooled after pressurization or compression. The combination of pressure increase, and the subsequent cooling to close to ambient temperature, results in an LFG that is saturated with moisture.
Condensation downstream of the cooler can be avoided by setting the temperature of the LFG exiting the cooler above its dew point temperature (so that the temperature of the LFG in the downstream piping does not cool below its dew point), or by installing a LFG re-heater or heat tracing on the downstream piping (to prevent the LFG temperature from falling below the cooler’s LFG outlet temperature). Pipe insulation is employed in any of these cases. Setting the LFG temperature exiting the cooler higher than its dew point is often not possible due to temperature constraints imposed by the LFG use.
A typical dehydration system for LFGE projects removes most of the moisture in the LFG prior to delivery to the pipeline or onsite use. Typical dehydration systems lower the dew point of the LFG to 40°F and then re-heat the LFG to at least 20°F above the dew point. Typical equipment includes a reheat gas-to-gas heat exchanger, a chilled glycol-to-LFG heat exchanger, and a condensate knock-out. Pipe insulation and/or other heat exchangers maybe needed downstream to ensure that the LFG temperature does not fall below the dew point prior to combustion.
Condensate knockouts are typically located prior to the inlet of the compression equipment. Besides managing condensate, most knockouts will contain a stainless steel demister. The demister element typically is designed to remove 10 micron and above particulates, in addition to water droplets. For engine projects, a coalescing filter is also used to provide particulate removal down to 0.3 microns.
Removal of Hydrogen Sulfide
There are numerous hydrogen sulfide (H2S) removal systems that are commercially available. Typically, the goal for removal of H2S is to reduce corrosion for power generation equipment, to pre-treat LFG for high-BTU gas processing, and/or to reduce sulfur oxides (SOx) emissions, associated with LFG combustion. An evaluation of H2S treatment technologies typically hinges on analysis of life-cycle costs. Some treatment systems have high capital costs, but the operating and maintenance (O&M) costs are relatively low. Other treatment systems have relatively low capital costs, but the O&M costs are significantly higher. H2S treatment systems must be evaluated on a life-cycle basis to ascertain whether a high-capital cost or low-capital cost system is the best fit for an individual landfill.
H2S removal technologies are typically grouped per the following process categories:
- Physical Adsorption
- Solid Chemical Scavenging
- Liquid Chemical Scavenging
- Solvent Absorption
- Liquid Redox
- Biological Processes
The advantage for some of the processes is that regeneration of the scrubbing media can often be accomplished. In other cases, media can be regenerated by changing the media temperature or pressure, or by purging the media with air or a purge gas, to release H2S or sulfur from the media.
Due to the number of technologies available, only a few examples are provided in this article.
The SulfaTreat process is among the simplest of the H2S treatment technologies available for LFG applications. The SulfaTreat process and similar processes using proprietary media use a manufactured, granular, iron-based media that can achieve H2S removal efficiencies of 99% with low or high inlet H2S concentrations. There are no moving parts and no utility services are required, except for a condensate drain. The system essentially consists of large vessels filled with SulfaTreat media, through which LFG flows. The media chemically reacts with H2S to form a stable iron pyrite. Systems can be designed with either single vessels (once-through), or in a lead-lag configuration with two vessels.
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