MSW Management | The Missing Step

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By David Graubard and Jean Bogner

Today, many municipal landfills accept construction and demolition debris. In fact, ground construction and demolition fines are frequently used as a daily cover, thus maximizing landfill use. These fines, however, typically include gypsum, composed of calcium sulfate, which reacts with sulfate-reducing bacteria in the landfill to produce hydrogen sulfide.

For more than 27 years, the core business of Gas Technology Products LLC, based in Schaumburg, IL, has included sulfur recovery from gases, as a logical companion technology to gas treatment and gas processing. Today, the company applies the same technology to low-volume sulfur recovery in landfill operations, making processes previously suited only for industrial recovery operations available to solid waste landfill operators. Removing hydrogen sulfide has typically been missing from the landfill gas (LFG) recovery process.

H₂S Removal From Landfill Gas
What is available today in the marketplace? Depending on the quantity of sulfur produced daily, Gas Technology Products can match several processes to a landfill’s specific requirements. These technologies include:

Eliminator: A liquid scavenger system for eliminating very low levels of H₂S, typically less than about 50 pounds sulfur per day
Sulfur-Rite: A solid media technology for removing low levels of H₂S, about 50 to 300 pounds per day
LO-CAT or MINI-CAT: A liquid oxidation catalyst process to remove higher levels of H₂S, from about 300 pounds to several tons per day. MINI-CAT is a smaller, modular version of LO-CAT with the same chemistry but reduced capital cost, suitable for many LFG applications.

Computing the daily sulfur load is the first step in choosing the most cost-effective process. The daily sulfur production rate is determined by the following equation:

PPDs = (SCFM x ppm H₂S)/8220

PPDs = pounds per day of sulfur generated

SCFM = Gas rate as SCF/Minute

ppm = H₂S concentration (in ppm)

Where: The constant is simply conversion factors to get the units correct, including the molecular weight of sulfur.

The liquid scavenger technology Eliminator is best suited to treating less than 50 pounds of sulfur daily by spraying liquid directly into the gas or employing a bubble tower. This technology results in a liquid wastestream that requires disposal. Sulfur-Rite—a solid media technology—effectively treats 50 to 300 pounds per day of sulfur. The spent solid media require disposal. Many landfills with H₂S problems are producing sulfur loads from about 300 pounds to a few long tons per day. The iron-redox technology offered by LO-CAT and MINI-CAT is best suited for these applications. These processes result in a small product stream of elemental sulfur and recover solid sulfur for use in agricultural applications.

In addition, operating costs for these two processes on a unit sulfur basis are a fraction of the cost of either Eliminator or Sulfur-Rite.

Putting LO-CAT Technology To Work
LO-CAT technology—and the smaller scale MINI-CAT systems based on this technology—converts hydrogen sulfide to elemental sulfur very effectively and efficiently by employing an iron chelate solution that is regenerated for continuing use in the process. The LO-CAT technology can also be used to process any gas stream containing hydrogen sulfide, including fuel gas, acid gas, digester biogas, syngas, and non-condensable gas—in addition to landfill gas. In fact, the system has been developed and refined through varied applications over the past 27 years in more than 160 operational units. Through this practical field experience, LO-CAT can be confidently configured for landfill applications.

The resulting technology is environmentally friendly and provides very high H₂S removal efficiencies of 99% and greater, making the system economically viable for recovering sulfur at typical landfill production levels. The system has a turndown rate of virtually 100% and has extremely low operating costs.

As an extension to LO-CAT, the company has developed the smaller scale MINI-CAT for removal requirements of 300 pounds to 2 tons of sulfur per day. This smaller system has a smaller footprint and is manufactured in a modular design for installation and operational simplicity. Also, the smaller system reduces initial capital investment without sacrificing results.

How MINI-CAT And LO-CAT Work
A brief overview of LO-CAT and MINI-CAT chemistry shows how these hydrogen sulfide treatment and sulfur recovery processes produce results. The landfill gas is bubbled through an iron chelate solution. In this process, the sulfide in H₂S is oxidized to elemental sulfur while the Fe is reduced from +3 to +2. Next, air from atmosphere is blown through the iron chelate solution to regenerate it by oxidizing Fe to +3 once again. There are no environmentally hazardous byproducts to handle. The only products are solid elemental sulfur and water.

As the flow diagram indicates, the LO-CAT process is accomplished in a few simple steps. In the gas contacting step, when sour gas containing H2S comes in contact with the ferric iron chelate solution, the gas is sweetened and a spent solution is created. Next, the spent solution is mixed with air, regenerating the solution by converting the ferrous iron chelate to the original ferric iron state. This regenerated solution is then clarified by decanting to clean the solution and to thicken the solid sulfur product. Finally, sulfur is filtered from the thickened sulfur product stream and is thus recovered for agricultural or other productive uses.

Operational Considerations
Since the LO-CAT and MINI-CAT processes are quite simple and continually regenerate the treatment solution, operational costs are very low and manpower requirements are minimal. In typical landfill operations, only a few hours of operator time are required daily. Chemical addition is done automatically through continuous chemical dosing, with only daily simple inspection and solution sampling necessary. Iron chelate must be added to make up for losses with the sulfur cake, and other chemical additives are used to adjust solution pH, aid in sulfur settling, and control bacteria growth in the solution, as well as prevent solution foaming. In total, manpower requirements and chemical costs are very low with these processes.

The mechanical equipment required for LO-CAT and MINI-CAT can include:

Venturi scrubber
Mobile bed absorber (scrubber)
Oxidizer vessel with cone bottom (for sulfur product settling)
Air blower (for solution regeneration)
Circulation pump(s)
Chemical addition pumps
Chemical storage

This equipment is configured as indicated in the schematic diagram below.

Two equipment items are particularly significant in the MINI-CAT and LO-CAT processes. First, in landfill gas service, these processes typically will use mobile bed absorbers, where the packing moves freely through the 30%–50% of free space in the absorber when the unit is in service.

This configuration eliminates fouling and deposits and leads to easier cleaning. In fact, the units are self-cleaning while in service. Because the packing is fluid, rather than the more rigid packed sections in traditional packed bed absorbers, the fouling that reduces efficiency is much reduced. This is especially important in processes such as liquid redox systems, where solids are produced.

Secondly, the oxidizer uses compressed air to regenerate the iron catalyst. In this system, air spargers provide uniform air addition, and air lift promotes circulation inside the vessel.

The sulfur slurry settles into the bottom of the cone—which is equipped with air blast rings to keep the sulfur in motion—where a slurry pump sends the sulfur to the filter.

Advantages of MINI-CAT And LO-CAT Technology
Cost effective LO-CAT and MINI-CAT systems—based on selecting the most appropriate level of technology – can offer the right size system to meet the specific H₂S removal requirements at many landfills. Operational and maintenance costs are low, and, since the recovered sulfur can be used for agriculture, this process does not create another solid wastestream requiring recovery.

The recovered LO-CAT sulfur finds agricultural uses as soil fertilizers and conditioners, as well as in plant fungicides. It is readily absorbed in soils and also contains micronutrients such as chelated iron and thiosulfate. For some crops, the sulfur cake provides a beneficial soil pH adjustment.

David Graubard is business development engineer for Gas Technology Products in Schaumburg, IL. Jean Bogner is president of Landfills+ Inc., located in Wheaton, IL.

MSW - January/February 2007