Above ground piping collects the landfill gas (“LFG”) from the gas extraction points and gas wells and routs the gas to a thermal oxidizer, which processes the H2S at a high temperature and residence time. When the oxidizer heats the H2S, the H2S converts to sulfur dioxide (SO2), which is a primary pollutant subject to national ambient air quality standards (NAAQS) in the United States. To control high SO2 emissions, the thermal oxidizer emits to a scrubber which captures and treats the SO2.
These treatment systems can handle very high concentrations of H2S that may be present. However, the operation and maintenance of this system is demanding. The system requires continuous supervision, almost daily delivery of caustic material to operate the scrubber, and almost daily disposal (e.g., trucking off-site) of the wastewater generated from the scrubber.
H2S removal can be achieved using a liquid scavenger treatment system. In a liquid scavenger, the liquid media is pumped directly into the LFG and treats the H2S. Examples of liquid-media solutions are as follows:
Liquid scavengers require drums, vessels, or towers to store the liquid media. Typically, the liquid media is spent after treating the H2S and will have to be disposed. Liquid scavenger systems have low capital costs and additional units can be easily added to the system. However, these systems have a higher operational cost due to the frequency of changing the liquid media and the costs of transport to offsite disposal sites. The liquid media has a tendency to form solids that can deposit in pipes, valves, or tanks leading to additional costs.
Another available liquid treatment technology is one with a regenerable catalyst. These treatment systems operate similar to the simple liquid scavenger system with the exception of a regenerable media. These systems have a high capital cost and a substantial, sustained operation and maintenance cost due to the need to treat and replenish the regenerable media used in the process.
These systems are complex, require significant operator attention and are known to sustain high maintenance costs. As the concentration in most LFG streams declines with time, these high capital cost system, which of course must be sized for the initial H2S requirements, become highly cost inefficient over only a few years of use. This has been the recent experience in some landfill sites.
In a biological treatment process, H2S from the gas stream is removed by exposure to bacteria of the genus thiobacillus through systems of towers (packed columns) designed to serve as a home for the active bacteria. Sulfur and sulfuric acid is produced from this biological H2S oxidation reaction.
The key to obtaining an efficient reaction is to provide an ideal habitat for the growth of sulfide-oxidizing bacteria and the absence of competing microbes that are normally predominate in an aerobic treatment processes. The bacteria are capable of oxidizing H2S at low pH levels. To grow, the microbes need oxygen, a narrow range of temperature, nutrients, and suitable humidity. Efficient removal of H2S requires an inert media with enough surface area to maintain a large population of sulfide-oxidizing microbes. That surface area becomes coated with sulfur as part of the process, so the reactor chambers must be periodically cleaned.
These removal systems typically have:
However, these systems require either high volumes of liquid effluent disposal (solids content of a few percent) or the addition of a sulfur recover system which adds significantly to total capital cost. This makeup water and effluent treatment as well as the large footprint required by the towers is the primary reason why biological systems are not typically applied in landfill environments.
Control and supervision are required to operate the system because of the inherent intolerance to upset conditions in the incoming gas stream. Biological processes do not respond well to fluctuations in concentrations and flow rate or other conditions such as temperature or nutrient levels. Even moderate changes in these process variables can push a biological system out of compliance with the target outlet conditions.
These systems tend to have a large footprint because the size of system is designed based on the “worst-case” scenario. As a result, reductions in H2S concentration that typically occur over the life of a landfill yield little to no corresponding reductions in operating cost as much of that cost is based on “fixed” contributors of power consumption and operator attention.
H2S removal can be achieved using a solid scavenger treatment system (often referred to as a “dry scrubber” system). Solid scavengers utilize a simple design of tower(s) that contain a reactive media over which the “sour” gas passes. The H2S reacts with the media and is converted to stable compounds such as pyrite that can be placed in landfills. In carbon systems, the carbon pores remove the H2S from the gas stream primarily by capturing the molecules.
Solid Scavenger or dry scrubber systems typically:
Because solid scavenger systems are immune to fluctuations in flow rate and H2S concentrations without a change in performance they offer significant total performance advantages. If gas flow increases over time, the number of vessels treating the gas can be increased to meet the demand. If the H2S concentration increases, the media volume can be increased, or the media can be changed out more frequently. Correspondingly, if H2S concentrations fall, as is typical in a landfill gas life cycle, the operating costs fall in direct proportion as less media is consumed.
No operators are required to treat the gas and disposal of spent media is simple and 100% system uptime is virtually guaranteed. When part of the system undergoes media change-out, that portion is out of service during that time, but this can be easily addressed given that media change out cycles are short in duration and can be staggered across vessels while the system continues to operate.
As a result, in the past decade, dry scrubber systems have become the dominant technology applied in landfill gas treatment applications.
The performance effectiveness of dry scrubbers relies in major part on the reactive media employed. There are two basic categories to consider.
Activated Carbon Media
Activated carbon has long been used as a solid scavenger material in H2S management and is a good choice at low H2S concentrations (manufacturers typically recommend a concentration limit of less than 200ppm). “Enhanced” activated carbon includes the addition of metal oxides to improve overall performance but cost and reaction temperature considerations limit its use in landfill gas treatment.Metal Oxide Coated Media
This media type consists of a substrate material coated with metal oxides that react with the H2S to form Sulfur and stable sulfur compounds such as pyrite. The products of the reaction are disposed of in landfill.
The most common forms of solid scavengers used for treating landfill gas are “iron sponge” and clay based iron-based solid scavenger systems.
Iron sponge media is the oldest commercial process for removing H2S and consists of hydrated iron oxide impregnated into wood chips. The once perceived drawback of an iron sponge system was pyrophicity of the media when used in anaerobic applications. However, this issue has been eliminated for over a decade with system design and as a result, iron sponge systems have been readily accepted in several large LFG gas cleaning applications in the past three years, that now treat more than 35,000scfm across in U.S. sites, including three in New Jersey.
Clay based media offers the positive performance characteristics discussed above and is also widely used in landfill gas treatment applications. The on-going media cost per pound of H2S removed is greater than that of iron sponge media and media changes are typically more difficult due to bridging effects in the media towers.
Under some conditions, technologies can be cost effectively “paired”. For example, the H2S could undergo a chemical process, followed by a biological process, and further reduced with a solid-media process. First, the gas stream is reacted in a tower with a caustic solution (NaOH) to remove the H2S from the gas stream. In order to reach specified low concentrations of H2S, the landfill gas exiting the caustic tower then passes through a media vessel to achieve a desired outlet concentration. To minimize the caustic consumption, the caustic liquid solution passes through a separate reaction where biological matter reacts and regenerates the caustic and creates elemental sulfur as a byproduct, which is removed from the process. Aside from the parasitic loads associated with the system, only water and caustic are required to operate the equipment. As is implied, these kinds of systems are complex, carry a high capital cost and require significant operator attention.
Technologies can also be changed over time if the landfill’s sulfur content dramatically changes. For example, a thermal system can be replaced with a solid adsorption system if the LFG sulfur content decreases as less sulfur is placed in the landfill.