Cost-Effective Hydrogen Sulfide Treatment Strategies for Commercial Landfill Gas Recovery: Role of Increasing C&D (Construction and Demolition) Waste
There are more than 350 commercial landfill gas recovery operations in the U.S. which generate electricity on-site, supply industrial gas-fired boilers, or produce substitute natural gas fuels such as CNG. Historically, very few have required H2S treatment. However, many landfills are now accepting large quantities of construction and demolition (C&D) debris in addition to municipal solid waste (MSW), which can set in motion a number of factors that lower landfill gas quality and increase O&M (operations and maintenance) costs through the post-closure period. Gypsum wallboard in C&D debris generates 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+ years. In this paper we review the consequences of increased H2S in landfill gas and discuss practical H2S treatment options for commercial landfill gas recovery.
Hydrogen Sulfide Generation from C&D
C&D debris may include substantial percentages of gypsum (CaSO4.2H2O) in discarded wallboard, and some sites in the U.S. have historically used ground-up C&D debris as daily cover. Under anaerobic landfill conditions (absence of air), sulfate-reducing bacteria produce hydrogen sulfide (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” will likely be realized during the active landfill gas production phase.
Increasing concentrations of H2S in landfill gas can have several detrimental effects: 1) onset of odor problems; 2) corrosion of gas recovery hardware; 3) increasing SOx 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 0.1 ppmv), and levels of H2S above 10 ppmv are considered toxic, exceeding the Threshold Limit Value (TLV). Moreover, levels of H2S above 1000 ppmv (0.1 V%) in a breathing zone can rapidly lead to unconsciousness and death. Thus, worker health and safety issues may require special attention at sites with high H2S.
Predicting and Measuring Hydrogen Sulfide: Developing a Quantitative Basis for a Gas Processing Decision
How high can H2S concentrations get in landfill gas? Several landfills in different parts of the United States that have been collecting large amounts of C&D debris are installing gas processing equipment to treat H2S concentrations in excess of 3% to 5% (30,000-50,000 ppmv) H2S. Percentage levels of H2S require treatment to prevent acid corrosion of gas recovery hardware, reduce odors, and minimize worker safety concerns. However, landfill operators may 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 determining H2S concentrations in landfill gas, it is important to obtain representative samples of the composite landfill gas, 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, U.S. EPA standard methods 15 and 15A can be accessed at www.epa.gov/ttn/emc/methods.html and www.epa.gov/ttn/emc/methods.html, respectively. Because landfill gas 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.
For large sites with elevated and increasing levels of H2S in landfill gas, the need for landfill gas modeling becomes more important. Larger treatment systems can be capital intensive, and a good model and forecast is necessary to design a system that meets treatment goals and makes efficient use of the invested capital. Modeling hydrogen sulfide generation is more complex than traditional landfill gas modeling, and is still evolving technically. It is recommended that a good consultant with a background in sulfur generation mechanisms be retained to assist in this effort. It is also important to inventory all potential sulfur sources, including sewage sludge, local soils used as cover materials, landfills developed in high sulfate geologic materials, and high sulfate groundwater contributions or recirculated leachates.
Commercial H2S Removal Processes
Above critical levels, H2S may 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
- Need to meet inlet gas quality specifications for compressors, engines, turbines or microturbines.
For sites with relatively low sulfur concentrations and gas flow (1-2 million SCFD @ 50 ppmv H2S), the recommended sulfur abatement would consist of a low capital investment scavenger system. (See Table 1.) 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 landfill-gas-to-electricity projects can tolerate levels as high as 1000-1500 ppmv (total sulfur in gas). Properly specified turbine generators can tolerate in excess of 10,000 ppmv. Gas specifications for microturnbines have a very wide range depending on the manufacturer. However, the sulfur limit for gas turbine systems is often determined by the gas compressor upstream of the turbine, which may 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.
The smaller sulfur removal systems, appropriate for the great majority of landfill gas treatment applications, will typically be scavenger (non regenerable) systems, and 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 additional units can be added easily, so extensive gas design and landfill modeling is 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 can often be done by contractors
- Disposal of spent solid media is often easier than liquid waste
- The system can easily be expanded 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.
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 landfill gas are iron sponge and iron-based solid scavenger systems like Sulfur Rite® and SulfaTreat®.
The oldest commercial process for removing H2S is iron sponge, which has been available for over 100 years. Iron sponge consists of hydrated iron oxide impregnated onto redwood ships. The main drawback of this system is that during media changeout, the unreacted iron oxide can react exothermically with the air and catch fire. The SULFUR RITE® and SulfaTreat® products address this problem by using an inert ceramic base. The initial cost of the SULFUR RITE® and SulfaTreat® products is higher than the iron sponge product, but that cost is at least partially offset by easier changeout procedures and transportation and disposal costs.
The scavenger system shown in Table 1 is a pre-packaged, pre-engineered SULFUR RITE® system suitable for landfill gas applications. This unit is well suited for 1 million SCFD landfill gas. 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.
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 site-specific design suitable for 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 are shown in Table 1. The operating cost of removing 1 pound of sulfur drops from over $3.00 per pound for the scavenger to under 10¢ per pound for the regenerable system. However, the capital cost for the system is typically between $1 million and $2 million.
The economic tradeoff point between the simple scavenger systems and the more capital-intensive regenerable systems are determined by long-term comparison of projected capital vs. operating costs. Representative trade-off points are shown in Table 1. Table 1 assumes a payback requirement of 2-3 years, which occurs at about 400 pounds 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 the level of capital expenditure required for large regenerable systems such as LO-CAT®, developing the proper design basis for the gas processing system is critical to efficient capital utilization and cost effective operation. This process takes time and careful planning: initial gas analysis, modeling, design, and capital appropriation can easily take 12-18 months, with the detailed design and construction requiring another 9-11 months. Thus the complete process of data collection and modeling to start-up can take 2-3 years and must be planned for well in advance of reaching allowable sulfur limits.
How can H2S production in your landfill be anticipated and prevented? Here are some
steps to follow:
Limit the total amount of gypsum wallboard accepted with C&D waste.
Do not use ground-up C&D for daily or interim cover.
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 landfill gas consultant to quantify and model H2S production.
If H2S is present in the landfill gas, 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 landfill gas.
Model the H2S generation mechanisms to:
Be sure the incremental revenue from the sulfur generating collection stream covers the increase in operating cost.
When it becomes clear that some form of sulfur abatement system will need to be installed:
Project sulfur gas and sulfur levels, consistent with current trends and future business strategy.
Establish project team to evaluate system and investment options.
Plan ahead if the facility will require a regenerable system for cost effective sulfur control.
|Table 1. Comparison of Solid Scavenger System to Iron-Redox Regenerable System For Landfill Applications|
Solid Scavenger System
Iron-Redox Regenerable System
Raw gas is saturated with water.
Saturated gas passes over media bed, iron pyrite formed.
Treated gas exits system
Raw gas is “scrubbed” with catalyst solution, sulfur formed, Treated gas exits column.
Catalyst is regenerated using air, returned to scrubber
Sulfur is separated from catalyst.
System Cost: $ 41,000
50 ppm: $ 3,800/year
System Cost: $1 million – 2 million
1 MMSCFD: 4,500 ppm