It is particularly important to realistically model landfill gas generation rates inclusive of the hydrogen sulfide generation potential in order to design a cost-effective H₂S treatment system.

By Jean Bogner and Doug Heguy

Many landfills now accept large quantities of construction-and-demolition (C&D) debris in addition to municipal solid waste. The strategic decision to obtain incremental revenue from C&D wastes can set in motion a number of factors that lower landfill gas (LFG) quality and increase operations-and-maintenance costs through the postclosure period. Gypsum wallboard in C&D debris can result in the generation of highly toxic hydrogen sulfide gas (H₂S). In sufficient amounts, this will result in the need for sulfur abatement systems, which can be expensive and complex. Fortunately the technology for such systems is well developed and has been in commercial use for the last 30-plus years. In this article we review the consequences of increased H₂S in LFG and discuss practical H₂S treatment options for LFG recovery operations ranging from 0.5 to more than 5.0 million scfd (standard cubic feet per day).

Hydrogen Sulfide Generation From C&D

C&D debris may include substantial percentages of gypsum (CaSO₄^.2H₂O) in discarded wallboard materials. Also, some sites in the United Stateshistorically have used ground-up C&D debris as daily cover. Under anaerobic landfill conditions (absence of air), sulfate-reducing bacteria produce H₂S from the sulfate (SO₄^-2) in gypsum and the organic carbon in waste material as follows:

SO₄^-2 + 2CH₂O = 2HCO₃^-1 + H₂S

From the above reaction, 100 tons of landfilled sulfate has the potential of producing 35 tons of H₂S. Most of this "potential" likely will be realized during the active LFG production phase. Furthermore, since the sulfate-reducing microorganisms tend to outcompete the methane-producing microorganisms (methanogens) until substantial sulfate depletion occurs, methane production for commercial gas recovery - especially during the first few years - may be reduced at sites with high H₂S production.

Increasing concentrations of H₂S in LFG can have several detrimental effects: (1) the onset of odor problems, (2) acid gas corrosion of gas recovery hardware, (3) increased SO_x emissions from flaring or other combustion processes, and (4) possible health consequences for workers. The odor threshold for H₂S is extremely low (0.05 to 0.10 parts per million by volume, or ppmv), and levels of H₂S above 10 ppmv are considered toxic, exceeding the threshold limit value. Moreover, levels of H₂S above 1,000 ppmv in a breathing zone can rapidly lead to unconsciousness and death. Thus, worker health and safety issues might require special attention at sites with high H₂S. It should perhaps be pointed out that there are other odorous reduced sulfur gases that might be present in LFG, including dimethyl sulfide, ethyl mercaptan, i-propyl mercaptan, t-butyl mercaptan, methyl n-propyl disulfide, dimethyl trisulfide, and thiophene; these typically are found in lower concentrations than H₂S but are also generated under anaerobic conditions.

Historically, concentrations of H₂S in LFG have tended to be less than 100 ppmv. Indeed, a recent compilation by the Waste Industry Air Coalition, an ad hoc waste industry group, indicated that the average H₂S from 40 sites across the US was 23.6 ppmv (Huitric et al., 2000). The current AP-42 default value for H₂S in LFG from the US Environmental Protection Agency is similar at 35.5 ppmv. At sites that have taken large volumes of C&D debris with municipal waste, however, H₂S concentrations above 100 ppmv are beginning to be measured. Some sites have noticed increased H₂S within a few months of accepting C&D debris on an emergency basis; for example, after a major hurricane. Recent experience at nine US sites showed H₂S ranging from 0.4 to 116 ppmv. A similar range (7-100 ppmv) was found, based on data from several southern California landfills.

Developing a Quantitative Basis for a Gas Processing Decision

How high can H₂S concentrations get in LFG? Several landfills in different parts of the US that have been collecting large amounts of C&D debris have installed, or are installing, gas processing equipment to treat H₂S concentrations in excess of 3-5% (30,000-50,000 ppmv). Percentage levels of H₂S will require treatment to prevent acid corrosion of gas recovery hardware, reduce odors, and minimize worker safety concerns. Landfill operators, however, might need to consider commercially available treatment processes when H₂S concentrations exceed about 75 ppmv, depending on equipment specifications and warranties from gas compressor, engine, or turbine vendors. When needed, treatment can achieve compliance with gas recovery hardware specifications, environmental regulations regarding combustion emissions, and local planning guidelines with respect to nuisance odor issues.

When determining H₂S concentrations in LFG, it is important to obtain representative samples of the composite LFG, retain those samples in appropriate inert containers (lined stainless steel cylinders or black-layered Tedlar bags), and analyze them according to standard methods. For example, EPA Methods 15 and 15A can be accessed at www.epa.gov/ttn/emc/promgate.html. There are other standard methods recommended by the American Society of Testing and Materials, the American Gas Association, and other industry or governmental agencies, including the South Coast Air Quality Management District in California. Because LFG is a complex mixture of 200 or more gases, it is not appropriate to use field analyzers or colorimetric tubes when attempting to quantify H₂S and other reduced sulfur gases.

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For large sites with elevated and increasing levels of H₂S in LFG, it is critically important and challenging to predict H₂S generation in the context of routine LFG-generation modeling. These systems can be capital-intensive, and a good model and forecast are necessary to design a system that meets the facility¹s requirements and makes efficient use of the invested capital. Modeling H₂S generation is more complex than traditional LFG modeling and still is evolving technically. It is common practice in the LFG industry to apply a first-order kinetic model for methane generation using annual waste quantity and composition data for a specific landfill cell or site. For predicting H₂S generation, the same kinetic model is not likely to be appropriate because the timing and rates for H₂S generation differ from methane. Thus it is recommended that a good consultant be retained to assist in this effort. It also is important to inventory all potential sulfur sources, including sewage sludge, local soils used as cover materials, landfills developed in high-sulfate geologic materials, high-sulfate groundwater contributions, and - at bioreactor sites - recirculation of high-sulfate groundwaters, surface waters, or leachates. This inventory will require site-specific data that then can be used to develop a sulfur mass balance for a particular site; for example, summing the input mass and form of various materials in order to more realistically predict H₂S production rates. Historical data on H₂S generation rates and yields over time can be used to fine-tune preliminary predictive models. In some cases, supporting laboratory studies might be needed to determine kinetics of specific waste fractions. In the future, we might well have standard models for H₂S generation that can be matched to potential gas treatment options at sites with high-sulfate inputs, but these models do not currently exist.

Commercial H₂S Removal Processes

Above critical levels, H₂S might need to be removed via commercially available treatment processes. This critical level can be reached by:

• exceeding sulfur emissions above permitted levels,
• receipt of odor and corrosion complaints from neighbors,
• the need to meet inlet gas-quality specifications for compressors, engines, turbines, or microturbines.

For sites with relatively low sulfur concentrations and gas flow (1 million-2 million scfd @ 50 ppmv H₂S), the recommended sulfur abatement would consist of a low capital investment scavenger system (see table). Modeling expected H₂S generation for sizing scavenger systems is important but not as critical as for larger sulfur recovery systems. If and when the landfill crosses into the range where more sophisticated sulfur recovery techniques become economic, landfill modeling becomes critically important, and more extensive modeling is required for proper and efficient design.

The level at which gas-quality specifications are exceeded and sulfur abatement is required will vary by application, equipment, and vendor. Internal combustion engines for LFG-to-electricity projects can tolerate levels as high as 1,000-1,500 ppmv (total sulfur in gas). Properly specified turbine generators can tolerate in excess of 10,000 ppmv. Gas specifications for microturbines have a very wide range, depending on the manufacturer. The sulfur limit for gas turbine systems, however, often is determined by the gas compressor upstream of the turbine, which might tolerate only 75-100 ppmv. This is because a highly corrosive liquid condensate can form during the higher compression required for turbines. Thus, many landfills generating electricity require sulfur limits to be restricted to 75-100 ppmv.

Removal of H₂S from gas streams has been an issue in the energy industry for years, so currently there are a number of commercial processes to remove H₂S from LFG, including solid and liquid scavengers (e.g., triazine, Sulfur-Rite, SulfaTreat) and regenerable catalyst processes, such as iron-redox systems (e.g., LO-CAT). These products/processes are in use treating LFG and removing H₂S in concentrations of less than 100 ppm to 50,000-plus ppm, in gas flows of less than 1 million scfd to more than 5 million scfd. Sulfur removal rates range from a few pounds per day to greater than 5 tpd.

The smaller systems, appropriate for the great majority of LFG treatment applications, typically will be scavenger (nonregenerable) systems and will be simple to operate. The costs of removing the sulfur, while small in total terms, can be quite large in terms of dollars per unit of sulfur removed. But these systems have low capital cost, and more units can be added easily, so extensive gas design and landfill modeling are not as critical as with much larger levels of sulfur removal.

The scavenger can be a liquid or solid system. The solid system has several advantages for landfill applications:

• No operators are needed to treat the gas (though the H₂S concentration at the outlet of the system will need to be monitored).
• Media change-out often can be done by contractors.
• Disposal of spent solid media is often easier than liquid waste.
• The system can expand easily by adding another "box" of media.

On the downside, the part of the system that is undergoing the media change-out is out of service during that time, and the media change-out process can be messy and allow noxious odors into the surrounding environment. Some systems are more susceptible to this than others are.

The liquid scavenger requires more operator attention to make sure the gas is being treated appropriately but has the advantage that the liquid is generally easier to handle than the solid media, and the system can be designed so that showdowns for media change-out are not required. Triazine-based liquid scavengers will efficiently treat several of the sulfur compounds in LFG, including H₂S and some mercaptans. Ease of disposal of the liquid is a site-specific issue, however, and might be more difficult than the solid media. Caustic treatment systems, commonly used to remove sulfur in the energy industry, usually are not a good choice for LFG because of the relatively high amount of carbon dioxide in the gas. The carbon dioxide consumes caustic and creates sodium carbonate. So this is an inefficient treatment system for LFG. In general, cost, ease of operation, and disposal options favor the solid scavenger over the liquid scavenger for landfill applications.

Solid Scavenger

Sizing a solid scavenger system is straightforward. The design parameters of a solid system are typically a maximum gas velocity over the media bed, minimum residence time, and an acceptable pressure drop. Once those parameters are met, the system volume can be adjusted to manage media change-out frequency. The solid media-bed system scales linearly with the gas. Should gas flow double over time, one can double the number of vessels treating the gas. Should the H₂S concentration increase, the media volume can be increased or the media can be changed out more often. The most common forms of solid scavengers used for treating LFG are iron sponge, iron-based solid scavenger systems like Sulfur-Rite and SulfaTreat, and activated carbon.

The oldest commercial process for removing H₂S is iron sponge, which has been available for more than 100 years. Iron sponge has a relatively low initial cost. The iron sponge concept is quite simple: Hydrated iron oxide is impregnated onto redwood chips. The wood chips are placed in a vessel where the gas flows over the wood chips. The H₂S reacts with the iron oxide to form iron sulfide, and the treated gas exists the vessel. The iron sponge system has one significant drawback: During media change-out, a highly exothermic oxidation reaction can take place, causing the media to spontaneously catch fire. To control the oxidation reaction and make the spent iron sponge suitable for transportation and disposal, the spent iron sponge is spread out on a concrete pad and kept moist for eight to 10 days. This material allows the spent material to oxidize slowly. As iron sponge itself can absorb odors from the LFG being treated, odorous compounds can be released to the atmosphere during this process, resulting in complaints if this is being done in a high-population-density area. So the change-out process can be messy, last for a number of days, and generate the very complaints the system was installed to prevent.

A newer, though well-established, form of solid media - SulfaTreat and Sulfur-Rite - uses iron-based chemistry but a different media base to address the change-out issues associated with iron sponge. In these systems, the solid medium is an inorganic, ceramic material coated with an iron oxide. The iron oxide reacts with H₂S to form iron pyrite. This product is more expensive than iron sponge but has the following advantages:

• More uniform particle size for better controlled gas flow
• Nonflammable
• Easier change-out
• Easier transportation and disposal

For these reasons, the acceptance of these products is better than iron sponge and the savings in change-out and disposal should, at least partially, offset the higher media cost.

The scavenger system shown in the preceding table is a prepackaged, preengineered system appropriate for LFG applications. This unit is well suited for 1 million scfd LFG. One can easily see that a doubling of the H₂S level in the feed-gas concentration doubles the consumption rate of the media and therefore doubles the cost per unit time. This unit can handle double the gas flow at lower H₂S concentrations, but at higher concentrations another unit can be added and the gas flow split between the two units. At larger gas flows, equipment savings could be achieved by optimizing the vessel design, but these savings would at least be partially offset by increased engineering and vessel fabrication costs.

Activated carbon can be used to treat LFG by itself or in combination with other systems. Activated carbon adsorbs most types of sulfur compounds onto the carbon, not just H₂S, so the carbon does a more thorough job of treating LFG odor. Since the spent activated carbon is classified as a hazardous waste, however, disposal cost, landfill location, and logistics will play a key role in the total cost of using carbon to treat LFG. For this reason, activated carbon often is used in combination with the previously mentioned products/technologies. The iron-based product is used to remove the H₂S (the largest portion of the sulfur compounds - and the most dangerous). In many cases this is sufficient treatment of the gas odor. If an additional remedy is required, a carbon canister can be added to further treat the LFG. The combined system is characteristically more cost-effective than the carbon system alone.

Optimized, Large-Scale Sulfur Recovery

At large levels of sulfur removal the cost of the solid media becomes prohibitive, and it makes economic sense to invest in a system with a regenerable catalyst. These systems are capital-intensive, and care must be taken to develop a design basis suited to the landfill for both present and future operations.

An example of a large-scale H₂S removal system with a regenerable catalyst is the iron-redox process, such as LO-CAT. A description of the LO-CAT process, as well as cost comparisons to the solid scavenger system, is shown in the table. The operating cost of removing 1 lb. of sulfur falls from more than $3/lb. for the scavenger to less than $0.10/lb. for the regenerable system. The capital cost for the system, however, can run between $1 million and $2 million.

The economic tradeoff point between the simple scavenger systems and the more capital-intensive regenerable systems is determined by capital/operating cost tradeoffs of the owner firm. Representative tradeoff points are shown in the table, which assumes a payback requirement of two to three years, which occurs at about 400 lb. of sulfur removed per day. This payback requirement is quite typical of many industrial firms. Municipalities often have a longer payback investment criterion, which would make the regenerable system more attractive at lower sulfur removal levels.

Clearly at this level of capital expenditure, developing the proper design basis for the gas processing system is critical to efficient capital utilization and cost-effective operation. The process takes a lot of time and careful planning. The gas analysis, modeling, design and option analysis, and capital appropriation can easily take 12-18 months. The detailed design and construction can take another nine to 11 months. Therefore the complete process of data collection and modeling to start-up of the unit can take two to three years. Obviously this is not an overnight project and must be planned for before reaching the allowable sulfur limits.

Conclusions/Summary

How can H₂S production in your landfill be anticipated and prevented? Here are some guidelines:

• Limit the total amount of gypsum wallboard accepted with C&D waste.
• Do not used ground-up C&D for daily or interim cover; this fine-grained material promotes rapid sulfate reduction with H₂S production in landfill environments.
• Do a "sulfur balance" on your landfill, considering all sources of sulfur, including native soils used for cover and geologic materials into which landfilling occurs.
• Retain a qualified LFG consultant to quantify and model H₂S production.

If H₂S is present in the LFG, and if the landfill has made the business decision to accept sulfur-laden C&D debris, what steps need to be taken to the manage sulfur levels?

• Be aware of the sulfur limitations of the downstream equipment.Monitor H₂S levels in the LFG.
• Model the H₂S generation mechanisms to be sure the incremental revenue from the sulfur-generating collection stream covers the increase in operating cost.
• Plan ahead.

When it becomes clear that some form of sulfur abatement system will need to be installed, follow these steps:

• Project sulfur gas and sulfur levels, consistent with current trends and future business strategy.
• Establish a project team to evaluate system and investment options.
• Plan ahead if the facility will require a regenerable system for cost-effective sulfur control.

In summary, landfill operators should pay attention to the quantity of gypsum board in the C&D waste coming into their sites. H₂S is odorous, toxic, and corrosive. Once H₂S production is initiated, it must be managed, and we anticipate that treatment of H₂S in LFG will become necessary at more sites in the future. H₂S can be removed from LFG to avoid corrosion of gas recovery hardware and minimize emissions and odors. The application of treatment processes requires periodic gas testing and cost-efficient designs based on the total daily sulfur quantity. Landfill operators should consider developing a sulfur balance for their sites, considering both waste sources and natural materials.

Reference

Huitric, R., P. Sullivan, and A. Tinker. Waste Industry Air Coalition Comparison of Recent Landfill Gas Analyses with Historic AP-42 Values, Draft Report. October 2000.

Jean Bogner is president of Landfills + Inc. in Wheaton, IL, and an adjunct associate professor with the Department of Earth and Environmental Sciences at the University of Illinois in Chicago. Doug Heguy is sales and marketing manager with Gas Technology Products LLC in Schaumburg, IL. Table 1. Comparison of Solid Scavenger System to Iron-Redox Regenerable System for Landfill Applications

MSW - March/April 2004