Cost-Effective Technologies for Removing H2S from Landfill Gas

D. Graubard*, William Rouleau*, and Jean Bogner**

* Gas Technology Products Division,
Merichem Chemicals & Refinery Services LLC, Schaumburg, IL, USA
** Landfills +, Inc., Wheaton, IL, USA

Summary: Many landfills are experiencing higher hydrogen sulfide (H2S) concentrations in landfill gas as a result of increased inputs of C&D (construction and demolition) debris and the use of C&D fines as an alternate daily cover. Concentrations may exceed upper limits for engine or turbine gas quality specifications resulting in acid gas corrosion of gas recovery hardware. Exceedances of emission limits and local environmental issues (mostly odors) are also resulting from high H2S concentrations. Commercialized technologies are available to remove the H2S safely and economically, using either liquid or solid media.

1. INTRODUCTION

Many landfills in the U.S. and other countries are accepting larger quantities of construction and demolition (C&D) waste. Although this can be a valuable revenue stream, the C&D waste contains gypsum wallboard (CaSO4•2H2O) which results in elevated sulfate in the leachate and elevated H2S in the landfill gas (LFG). Sulfate-reducing microorganisms produce H2S in parallel with the oxidation of organic carbon contributed by other waste fractions:

SO4-2 + 2CH2O

→

2HCO3-1 + H2S

Sulfate-reducing bacteria require a sulfate source (gypsum), a carbon source (organic material), anaerobic conditions, and moisture. Large quantities of gypsum can be added as either bulk C&D materials or, in some cases, as C&D “fines” from recovery/recycling facilities which are used as an alternative daily cover. Because of their small particle size and large surface area, the use of these fines as daily cover has led to rapid increases in H2S concentrations in the LFG, in some cases to less than or equal to 1% (v/v) over several months. This results in odor nuisance complaints, health and safety issues for landfill workers, and exceedance of maximum H2S specifications for LFG recovery hardware resulting in acid gas corrosion under combustion conditions for flares, engines and turbines. Moreover, post-combustion SO2 emissions may exceed air quality specifications and trigger additional odor complaints.

There are several strategies to prevent or mitigate high concentrations of H2S in LFG. One possibility is to ban or limit the disposal of high sulfate waste materials (including C&D fines, bulk C&D waste, bio-solids from sewage treatment, and some local soils and geologic materials) (Reinhard, et al., 2004). A second strategy is to dispose of high sulfate wastes in monofills without co-disposal with organic waste fractions, since the reaction above is driven by available organic carbon. However, in many cases these options are neither practical nor cost-effective. Wallboard contains paper backing as well as gypsum which together can generate H2S. One can also mitigate complaints of fugitive odors through active gas collection. However, in most cases, the recovered LFG will require treatment for H2S removal prior to combustion and utilization. For example, Table 1 summarizes gas quality specifications for H2S and total S for four major engine vendors in the LFG/biogas market.

Table 1. H2S and Total S Specifications for LFG/Biogas Engines

Constituent	Jenbacher	Deutz	Caterpillar	Waukesha
Total S Content	2000 mg/10 kWh (with catalyst) 1150 mg/10 kWh (without catalyst) calculated as H2S	<2.2 g/Nm3	<57 mg H2S /MJ <60 µg H2S /Btu (total S as H2S)	<0.1 vol % total S bearing compounds
H2S Content		<0.15 vol.%

The purpose of this paper is to review cost-effective and appropriate strategies at various scales for H2S removal from LFG, including both regenerable and non-regenerable systems. Typically, H2S treatment systems are sized relative to the total mass of sulfur (S) produced per day, which is a function of both the gas flow rate and the H2S concentration.

2. STRATEGIES FOR H2S REMOVAL FROM LANDFILL GAS

2.1 Liquid H2S scavenger systems

Liquid scavengers are used for smaller quantities of H2S removal, with equivalent sulfur loads below 25 kg/day. The scavengers are typically amine-based resins manufactured from monoethanolamines and formaldehyde. The resulting “scavenger” product is a hexahydrotriazine, and is commonly called “triazine” in the industry. The “triazine” is typically offered in a water-based solution. In most applications, the reaction products are also water-soluble, have very low toxicity, and are biodegradable, making this a relatively simple system to handle.

Liquid scavengers can also be employed in two different processing schemes. After separation of liquid hydrocarbons and water from the gas, the liquid scavenger can be either injected directly into the gas stream (Figure 1A) or the system can be operated in a batch mode by passing the gas through a vessel (bubble tower) filled with the scavenger liquid (Figure 1B). The direct injection method is much more efficient and has a lower capital cost compared to the bubble tower option.

As Figure 1B shows, the scavenger is injected into the pipeline through a nozzle and either a static mixer or a long length of pipe mixed the gas/liquid mixture. The mixture is then separated in a coalescing filter or a separator vessel. Direct injection of liquid scavengers does have problems. First, the degree of gas/liquid contact is dependent on the type of contacting device, the gas velocity and the residence time. Consequently, the degree of mixing and hence efficiency is sensitive to changing gas flow.

As the most common triazine products are water-based, they are water soluble as are their biodegradable reaction products, the system has the potential for a relatively simple disposal option. In many applications, the amount of water requiring treatment is very large relative to the amount of liquid scavenger consumed. Thus, the incremental cost of treating this small, low toxicity, biodegradable waste stream can be very small.

Capital Expense (CAPEX) for these systems is typically low, with only a small list of equipment required. Operating Expense (OPEX) however is significantly high, as replenishment of fresh scavenger chemical is frequently required. Assuming efficient mixing and reaction, the cost of the liquid scavenger can range from a low of US $8.80/kg of H2S to a high of US $22/kg. This relatively high chemical cost can often be offset by the low capital cost and often small incremental disposal costs

2.2 Solid H2S scavenger systems

Typical media consists of a ceramic base that is impregnated with iron oxide (Fe2O3). When the water-saturated LFG containing H2S comes in contact with the media, the H2S is converted to Iron Pyrite (FeS2). Figure 2A below shows the cut-away profile of a solid media system in which the LFG passes in a down-flow direction. Sweet gas exits the system at the bottom of the vessel. This type of system is cost-effective for equivalent sulfur loads between 25 kg – 135 kg. Systems are designed for either batch processing (single vessel) or Lead-Lag operation (2 vessels) that allows for continuous treating of the H2S (Figure 2B).

Solid scavenger systems are typically higher in CAPEX than a liquid scavenger system, due to the larger vessels required for containing the media. However, OPEX are significantly lower, as replacement of the spent media is less frequent and less costly than that of a liquid scavenger product.

In a properly operating solid scavenger system, approximately 5 kg of spent media is required per kg of H2S removed. This equates to a media cost of approximately US $7.73/kg of H2S removed. As the spent media is stable iron pyrite, the waste generated is non-hazardous, non-flammable, and disposed of in a landfill. This is generally not a problem in the Western Hemisphere, but it is a big problem in Europe and parts of the Far East, where disposal in landfills is severely restricted.

Media change-out can typically be completed in one day. This can be a messy operation with high pressure water hoses required to cut the spent material out of the vessel, but the infrastructure for media change-out is well developed in the Western Hemisphere with a number of service companies capable of performing this operation.

2.3 Iron-redox processes

Iron has long been known to be an effective catalyst for the oxidation of H2S to elemental sulfur. Iron sponge systems have been in service for over 100 years. The “trick” to getting the cost down to treat larger volumes of gas was to make the iron catalyst regenerable, and to put it in solution.

In the iron-redox process, iron, in its ferric state (+3), is held in solution by chelating agents. The intent of the process is to oxidize hydrosulfide (HS-) ions to elemental sulfur by the reduction of the ferric (Fe+3) iron to ferrous (Fe+2) iron, and the subsequent reoxidation of the ferrous ions to ferric ions by contact with air. The chemistry of all chelated iron processes is summarized as follows with (l) and (v) representing the liquid and vapor states, respectively;

H2S (v) + H2O (l)	→	H2S (l)	(1)
H2S (l)	→	H+ + HS-	(2)
HS- + 2Fe+3	→	S° + 2Fe+2 + H+	(3)
1/2 O2 (air) + H2O (l)	→	1/2 O2 (l)	(4)
2Fe+2 + 1/2 O2 (l) + H2O	→	2Fe+3 + 2OH-	(5)
Overall Reaction
H2S (v) + 1/2 O2 (v)	→	S° + H2O	(6)

Equations 1 and 2 represent the absorption of H2S into the aqueous, chelated iron solution and its subsequent ionization, while equation 3 represents the oxidation of hydrosulfide ions to elemental sulfur and the accompanying reduction of the ferric iron to the ferrous state. Equations 4 and 5 represent the absorption of oxygen into the aqueous solution followed by oxidation of the ferrous iron back to the ferric state.

Equations 3 and 5 are very rapid. Consequently, iron-based systems produce small amounts of by-product thiosulfate ions. However, equations 1 and 4 are relatively slow and are the rate-controlling steps in all chelated iron processes.

It is interesting to note that the chelating agents do not appear in the process chemistry, and in the overall chemical reaction, the iron cancels out. So why is chelated iron required at all, if it doesn’t take part in the overall reaction? The iron serves two purposes in the process chemistry. First, it serves as an electron donor and acceptor, or in other words, a reagent. Secondly, it serves as a catalyst in accelerating the overall reaction. The sole purpose of the chelating agents is to solubilize iron.

Iron-based liquid oxidation has developed into a very versatile processing scheme for treating gas streams containing moderate amounts of H2S. Advantages of these systems include the ability to treat both aerobic and non-aerobic gas streams, removal efficiencies in excess of 99.9%, essentially 100% turndown on H2S concentration and quantity, and the production of innocuous products and by-products.

The most common processing schemes encountered in iron-based, liquid oxidation systems are illustrated in Figure 3. It shows a “conventional” unit, which is employed for processing gas streams, which are either combustible or cannot be contaminated with air such as carbon dioxide, which is being treated for beverage purposes. In this scheme, equations 1 through 3 are performed in the Absorber while equations 4 and 5 are performed in the oxidizer (Nagl, 2006).

2.4 Safe Operation

Iron redox processes are run at maximum temperatures of 60°C, with minimum temperatures as low as 7°C. The catalyst solution is non-toxic and non-hazardous, as is the sulfur produced from the process for most applications. Iron redox is a very friendly system to the environment. The chemistry is water-based with dissolved iron and chelate, so there are no chances for chemical injuries to local plant operators.

2.5 Sulfur Product

The sulfur product is typically a sulfur “cake”, with entrained water and catalyst solution. The entrained catalyst solution acts as a “blow-down” stream for the aqueous process. Depending on the type of sulfur filter used, the sulfur cake can have a sulfur concentration between 30% (bag filter) to 90% (filter press). The vacuum belt filter is quite common, and produces a sulfur cake that is 60–65% sulfur, with the remainder being water and entrained chemicals from the liquid-redox process.

Both the organic chelates and the iron make the liquid-redox sulfur product a perfect fit for agricultural use for soil pH adjustment, as a soil nutrient, and as a fungicide. Because of the small particle size of between 8–45 microns, the sulfur product has a high surface area promoting rapid reactions in soils for crops (Heguy et al., 2002).

3. COST COMPARISONS OF H2S REMOVAL OPTIONS

First, it must be determined how much equivalent sulfur will be produced by the amount of H2S that is present. A quick calculation can be performed using the following equation:

(Nm3/hr gas flow x ppm H2S) / 29,671 = kg/day sulfur

Once the sulfur load is determined, then one can develop comparative economics for different processes for removing H2S from the landfill gas. Compared to both liquid and solid H2S scavenger processes, the CAPEX of iron-redox systems is significantly more costly. Multiple vessels, pumps, air blowers, a chemical addition skid, and sulfur filter are required. The OPEX of a iron-redox system is drastically lower than that of either the liquid or solid scavenger systems. Table 1 below gives a cost comparison between liquid and solid scavenger systems and an iron-redox regenerable system.

Table 1. Quick Reference Guide for Technology Selection

Technology	When Normally Used	Relative Capital Cost (US $)	Operating Cost
Liquid Scavenger	< 25 kg/day	$25,000–$100,000	$8.80 to $22.00 per Kg of S
Solid Media	25–135 kg/day	$15,000–$500,000	$6.60 to $13.25 per kg of S
Iron Redox	135 kg to 15.24 metric tons/day	$500,000–$2MM	$0.45 to $0.88 per kg of S

When evaluating a system, it is important to not only look at short-term CAPEX and OPEX, but also at long-term OPEX. A typical landfill will have a 20-year period of sulfur-recovery required once they have taken C&D waste or used C&D fines. Figure 4 shows a typical landfill sulfur recovery and when the peak of H2S removal will be required during a 20-year period.

For a shorter-term project (where the landfill will only need 1-2 years of H2S treatment), it will be more cost-effective to install a solid scavenger system. However, if the landfill has a longer time-period where H2S needs to be removed, the iron-redox process can save the operator almost US $10 Million in total project costs (see Table 2).  

Table 2. Cost Comparison for Solid Scavenger vs. Iron Redox Systems for H2S Removal from LFG for 20 Years

Technology Type	Capital Cost (US $)	Treatment Cost (US $) over 20 years	Total Costs (US $)
Aerobic Iron Sponge	$350,000*	$13,025,000	$13,375,000
Iron Redox (LO-CAT®)	$2,100,000*	$1,450,000	$3,550,000
20-Year Total Savings: US $9,825,000

* Installed Cost

4. Regenerable Iron Redox Systems: Landfill Gas Case Studies

Waste Management Landfill, Pompano Beach, Florida, USA

When a massive hurricane struck southern Florida in 1993, a local landfill saw a huge increase in the amount of C&D waste. Shortly after this, the levels of H2S started dramatically rising. This company evaluated several technologies for the removal of H2S, which was estimated to be as high as 5,000 ppmv, with up to 2-3 tons/day of sulfur being removed with the landfill gas. This presented a problem, since the landfill planned to generate electricity on-site using gas turbines. All combustion equipment has a certain tolerance for H2S and its corrosive combustion products, but turbines have the lowest tolerance. The equipment at the facility could only tolerate 100 ppmv inlet H2S, or well below the current levels in the landfill gas.

Short project completion was essential in the decision, and the project was awarded to Gas Technology Products LO-CAT Process. The LO-CAT Process is a proprietary, chelated iron liquid redox process that removes the H2S from landfill gas, converting it to solid elemental sulfur that can be used as a fertilizer. It is a stainless steel construction due to the iron used as catalyst. The unit was commissioned in 1994 and has been operating since, producing gas with less than 100 ppmv H2S. In 2002 an expansion of this LO-CAT unit was undertaken as the landfill had been receiving additional C&D waste along with increased gas flow. Currently the sulfur handling capacity of the LO-CAT unit is 10.8 tons/day, allowing the unit to treat gas containing up to 33,350 ppmv H2S, reducing it to less than 50 ppmv H2S.

Warren County Landfill, Warren County, New Jersey, USA

The Pollution Control Financing Authority of Warren County (PCFAWC) owns and operates a regional landfill for the disposal of non-hazardous solid wastes in northwestern New Jersey.  From 1998 to 2004, the PCFAWC utilized construction and demolition (C&D) debris screenings in significant quantities as alternate daily cover. Landfill gas (LFG) sampling conducted early in 2004 indicated H2S levels as high as 11,400 ppmv.

The PCFAWC attributes the high concentration of H2S to the use of C&D screenings. The PCFAWC discontinued the use of C&D screenings in the spring of 2004. In 2005, the PCFAWC contracted with an energy developer to construct, own and operate a LFG-to-electricity facility. One contractual requirement of the project was the supply of LFG with less than 500 ppmv H2S (Carlton, et al 2007).

To achieve the required H2S concentration, the PCFAWC needed to install an LFG sulfur scrubbing system capable of removing approximately 1 long ton per day of sulfur from LFG. After a thorough review of commercially available technologies, the PCFAWC selected MINI-CAT by Gas Technology Products. MINI-CAT is a system identical to the LO-CAT process, except it is designed for sulfur loads more common in landfill gas applications. Fiberglass Reinforced Plastic (FRP) is used to save on cost compared to stainless steel, and equipment is built in a modular fashion to save space.

Construction on both the MINI-CAT system and LFG-to-electricity facility took place during 2005 and 2006. Commercial operation of the MINI-CAT system and LFG-to-electricity facility commenced in November 2006 (Carlton, et al 2007).

Delaware Solid Waste Authority—Cherry Island Landfill, Delaware, USA

The Cherry Island Landfill of the Delaware Solid Waste Authority (DSWA) has been using C&D fines for alternate daily cover and accepting sludges and special wastes to this landfill. In 2004, elevated levels of H2S (2,000 ppmv) were noted; flaring the gas caused fugitive odor and SOx permit problems.
DSWA began an evaluation process of technologies that could remove the H2S and alleviate the odor and permit problems (along with the issues of public relations from people around the landfill). Solid scavenger systems were not considered at all due to the high operating costs for treating up to a metric ton/day of equivalent sulfur. A variety of regenerable systems were considered, both domestic and international. Upon the final review, DSWA chose the MINI-CAT system by Gas Technology Products. Construction on the MINI-CAT system began in late-2005, and operation commenced in December 2006 (Heck, 2007).

5. CONCLUSION

With many landfills using C&D fines for alternate daily cover, or accepting larger quantities of C&D waste, H2S has become a major problem. The processes detailed in this paper are commercially proven and have been used in landfills, along with many other industries, for 28+ years. Iron-redox systems provide the lowest operating costs and are now operating in three landfills in the United States, with more in the planning stages.

REFERENCES

Carlton, J., Graubard, D., Williams, J., (2007), Turning Sour Landfill Gas into Sweet Electricity, SWANA 30th Annual LFG Symposium, March 2007.

Heck, T., Delaware Solid Waste Authority, 2007

Heguy, D., Karr, J. & Neeley, F., (2002) At the crossroads of energy, environment and agriculture, Fertilizer International No. 388, May/June 2002.

Nagl, Gary (2006) Sweet selections, Gas Technology Products, MCRS LLC, Hydrocarbon Engineering, August 2006

Reinhard, D., Townsend, T. & al (2004), Control of Odors from Construction and Demolition (C&D) Debris Landfills, Florida Center for Solid and Hazardous Waste Management, University of Florida, November 2004