From the EPA
About Landfill Gas and How It Affects Public Health, Safety, and the Environment.

Turning Sour Landfill Gas into Sweet Electricity

John G. Carlton, P.E.
Camp Dresser & McKee, Inc.
Edison, New Jersey

James J. Williams
Pollution Control Financing Authority of Warren County
Oxford, New Jersey

Dave Graubard
Merichem
Schaumburg, Illinois

Abstract

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 landfill cover. Landfill gas (LFG) sampling conducted early in 2004 indicated H2S levels as high as 11,400 parts per million by volume (ppmv). In contrast, the U.S. Environmental Protection Agency estimates the average H2S in LFG is 35.5 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 3.8 megawatt LFG to electricity facility (LFGTE) at the landfill. One contractual requirement of the project was the supply of LFG with less than 500 ppmv H2S.

To achieve the required H2S concentration, the PCFAWC needed to install an LFG sulfur scrubbing system, capable of removing approximately 1 ton per day of sulfur from LFG. After a thorough review of commercially available technologies, the PCFAWC selected LO-CAT by Gas Technology Products.

Construction on both the LO-CAT system and LFGTE facility took place during 2005 and 2006. Commercial operation of the LO-CAT system and LFGTE facility commenced in November 2006.

This paper presents the background to the selection of the LO-CAT LFG sulfur scrubbing system. The paper also discusses the specific challenges encountered during construction and system operation. Data is presented on construction and operation costs as well as system performance.

Background

Warren County District Landfill

The Warren County District Landfill (WCDL) is owned and operated by the Pollution Control Financing Authority of Warren County. The WCDL began operation in September 1990 and accepts municipal solid waste, construction and demolition debris, non-hazardous dry industrial waste, vegetative waste, animal/food processing waste and ash from the nearby waste-to-energy facility. The WCDL is permitted for a 45-acre disposal area and a capacity of 4.5 million cubic yards.

Flow Control

The WCDL was financed with $42 million in revenue bonds issued by the PCFAWC. From 1990 until 1997, the PCFAWC relied on New Jersey’s system of regulatory flow control to ensure the finances of the landfill and nearby waste-to-energy facility. In November 1997, New Jersey’s system of regulatory flow control was ruled an unconstitutional of interstate commerce.

Beginning in 1998, the PCFAWC had to compete with local Pennsylvania landfills and reduced the disposal fee from $99.43 per ton to $45.00 per ton. In order to raise sufficient landfill revenues, the PCFAWC significantly increased the quantity of solid wastes accepted in the WCDL.

Construction and Demolition Debris Screenings

An additional revenue source sought by the PCFAWC was alternate daily cover (ADC). In addition to ADCs such as contaminated soil, paper mill by-products, crushed glass and tarps, the PCFAWC began to accept construction & demolition debris screenings (C&D screenings) as ADC. C&D screenings had the following benefits to the PCFAWC:

  • Revenue (approximately $10/ton);
  • Performance (especially during periods of wet weather due to good drainage and traction); and,
  • Convenience (the material was delivered to the WCDL working face.

The C&D screenings were approved by the New Jersey Department of Environmental Protection (NJDEP) for use as ADC. The NJDEP required testing of the C&D screenings as a condition of uses. Among other required tests, the NJDEP required a sulfate analysis to be performed and set a “target goal” of 3% of sulfate by weight for the C&D screenings. Monthly testing of C&D screenings routinely indicated sulfate levels within the NJDEP goal.

Sour Gas

In the anaerobic environment of a landfill, sulfate reducing bacteria produce H2S for wastes containing sulfate. In the case of C&D screenings, the small particle size and corresponding increased surface area expose the sulfate reducing bacteria to a greater quantity of sulfate (from the gypsum) in a shorter period of time.

The resulting combination of C&D screenings and sulfate reducing bacteria created high quantities of H2S in the WCDL LFG. The reaction takes places as follows:

SO42– + 2CH2O 2HCO31– + H2S

High levels of H2S can be problematic in several ways. First, H2S is an exceptionally malodorous gas. Second, high H2S can complicate LFG to energy (LFGTE) projects due to the H2S’s corrosive nature. Third, high H2S levels in LFG when combusted will create high SOx, which may create regulatory challenges. Lastly, H2S can create health and safety issues at a landfill.

Sweet Gas

The removal of H2S from LFG was therefore critical to the success of the WCDL, including an LFGTE project. The PCFAWC committed to installing a scrubber to turn sour gas (containing H>sub>2S) into sweet gas (without H2S).

Technology Selection

There are several technologies available to remove H2S from LFG. The technologies fall into the following major categories:

  • Adsorption
  • Biofiltration
  • Caustic Scrubbers
  • Chemical Oxidants
  • Clauss Systems
  • Liquid-Redox Systems (LO-CAT)
  • Liquid Scavengers
  • Solid Scavengers

LO-CAT, a patented process developed by Gas Technology Products (GTP), is a liquid redox system that uses a chelated iron solution to convert H2S to innocuous elemental sulfur. Gas Technology Products, based in Schaumberg, Illinois, is a company dedicated to removing H2S from various gas streams, including landfill gas, natural gas and digester gas. LO-CAT systems treat sulfur loads between 1,000 kg/day and 25+ tons/day.

An initial analysis was performed on the technical feasibility of the technologies to adequately remove H2S from LFG. Based on the analysis, caustic scrubbers, liquid-redox systems, liquid scavengers and solid scavengers were selected for further investigation for control effectiveness, cost effectiveness, energy impacts and environmental impacts.

Control Effectiveness

The technologies were evaluated according to the ability to remove H2S from LFG. Table 1 summarizes the control effectiveness of the various technologies.

Table 1. Control Effectiveness

TECHNOLOGYH2S REMOVAL

LO-CAT

99.7%

Caustic Scrubbers

96.7%

Liquid Scavengers

93.3%

Solid Scavengers

99.0% to 99.8%

Economic Impacts

The technologies were evaluated according to the cost of H2S per ton as outlined in Table 2.

Table 2. Economic Impacts

TECHNOLOGYCONTROL COST
($/ton H2S)

LO-CAT

$1,089

Caustic Scrubbers

$1,437

Liquid Scavengers

$11,737

Solid Scavengers

$2,155 to $4,236

Energy Impacts

The technologies were evaluated according to electrical useage as outlined in Table 3. Electricity was assumed to cost $0.08/kWh.

Table 3. Energy Impacts

TECHNOLOGYELECTRICITY USE
(kWh/yr)
LO-CAT

911,040

Caustic Scrubbers

192,188

Liquid Scavengers

0

Solid Scavengers

0

Environmental Impacts

The technologies were evaluated for environmental impacts. Table 4 outlines the expected H2S emissions rate. Table 5 outlines the impacts from the system by-products.

Table 4. Environmental Impacts—Emissions Rate

TECHNOLOGYH2S EMISSIONS RATE
(tons/yr)

LO-CAT

1.0

Caustic Scrubbers

10.2

Liquid Scavengers

20.4

Solid Scavengers

1.0 to 3.1

Table 5. Environmental Impacts—System By-Products

TECHNOLOGYBY-PRODUCT

LO-CAT

Sulfur cake, non-hazardous and non-toxic

Caustic Scrubbers

Hydrophilic sulfur, non-hazardous and non-toxic

Liquid Scavengers

Granular sulfur, non-hazardous and non-toxic

Solid Scavengers

Non-hazardous to hazardous by-products.

LO-CAT

The LO-CAT system was selected by the PCFAWC based on the control effectiveness, the long-term cost control, and environmental impacts. LO-CAT is a liquid-redox system that absorbs incoming H2S into a water-based solution containing a chelated iron catalyst. The redox reaction converts H2S into elemental sulfur while the catalyst is regenerated using ambient oxygen. The reaction proceeds through the following steps:

Absorption H2S(gas) + H2O(liq) H2S(liq) + H2O(liq)
Ionization H2S(liq)) 2H+ + S2–
Oxidation S2– + 2Fe3+ S0 + 2Fe2+
Regeneration ½ O2 + H2O + 2Fe2+ 2OH + 2Fe3+

The elemental sulfur settled out of the solution and is removed. Currently, the sulfur is being marketed as a soil amendment. GTP can guarantee a 99% removal efficiency.

LO-CAT Chemicals

There are five (5) primary chemicals and two (2) secondary chemicals utilized by the LO-CAT system:

Primary Chemicals
  • KOH: Potassium Hydroxide
  • ARI-340: Catalyst Concentrate
  • ARI-350: Make-Up Solution
  • ARI-400: Biostat
  • ARI-600: Surfactant
Secondary Chemicals
  • Antifoam
  • ARI-360K: Thiosulfate Solution

KOH: To maintain good adsorption of the H2S into the LO-CAT circulating solution, the pH of the solution must be maintained in the mildly alkaline range. Increasing the pH will improve removal efficiency, however the rate of formation of thiosulfate will also increase and reoxidation of the catalyst becomes more difficult. The pH of the system is maintained by adding NaOH or KOH to the system.

ARI-340: The iron concentration in the catalyst solution must be maintained at the design level to ensure that sufficient iron is available to oxidize the sulfide ions to sulfur. To compensate for iron lost from the system with the sulfur cake or the system blowdown stream, iron is continuously added to the system as a concentrated chelated iron complex.

ARI-350: Iron is held in solution by a mixture of Type Q and Type B chelating agents. The Type A chelating agents are effective at a solution pH below 8.5, while Type B chelates are effective above a solution pH of 8.5. Type A chelates are oxidized in the process and must be replaced on a continuous basis. ARI-350 is a special mixture of stabilized chelates used for this purpose.

ARI-400: Bacteria may enter the LO-CAT unit with the sour gas stream, oxidizer air or the water make-up. Once in the LO-CAT unit, colonies of bacteria may develop in the catalyst solution. In this event, bacteria will consume the organic chelates. To prevent this occurrence, a small amount of biostat is added on a continuous basis to control the growth of bacteria.

ARI-600: Sulfur particles may either become attached to minute air bubbles or become coated with hydrocarbons and float to the surface of the solution rather than settling out of the solution. Ordinarily, these floating particles will eventually be wetted by the solution provided there is no place in the system where floating material may accumulate. A surfactant is added continuously in small amounts to facilitate wetting.

Antifoam: Foaming tendencies are generally related to the introducing of foreign materials or excessive addition of surfactants. If foaming occurs, batch additions of an anti-foam agent can be made.

ARI-360K (At Initial Start-up Only):A small amount of thiosulfate is continuously generated in the process. Although thiosulfate is a by-product, it does have the beneficial side effect of reducing the degradation rate of the Type A chelate. Since there is very little thiosulfate in the solution during the initial start-up of the unit, a concentrated thiosulfate solution is added during the initial start-up to depress chelate degradation.

LO-CAT Process Flow

The absorption, ionization, and oxidation reactions listed above occur in the absorber vessel(s). When the H2S in the incoming gas contacts the LO-CAT iron catalyst solution, it is an instantaneous reaction to convert the H2S to elemental sulfur. The active iron catalyst (Fe+++) is reduced down to Fe++ (and thereby in-activated). As the gas passes through the absorber system, H2S is removed down to
The sulfur formed in the absorber is actually a slurry with the spent LO-CAT chemicals. This slurry flows from the absorber to the oxidizer vessel. The regeneration reaction occurs in the oxidizer vessel. Air is blown into the oxidizer, and this “reactivates” the iron from the Fe++ state to the Fe+++ state. The sulfur slurry settles in the cone bottom of the oxidizer, and is pumped to the vacuum belt filter.

The filter removes entrained water and chemicals, resulting in a 60 wt.% sulfur cake product. This sulfur has been used for over 10 years as an agricultural fertilizer. The iron and chelates remaining in the sulfur act as soil nutrients, pH adjusters, and as fungicides.

Fresh chemicals are added daily to make up for chemical degradation and the chemicals that are left in the sulfur product. Figure 1 depicts a basic flow diagram for a conventional LO-CAT system.

Figure 1. LO-CAT Basic Flow Diagram



Design and Construction

Schedule

Design started in the June 2005. Construction began in March 2006 and completed in October 2006. The design and construction process was therefore approximately 18 months.

Construction Observations

The construction of the LO-CAT was successful, but challenging for several reasons. First, the system is fairly complex. There are numerous parts to the system that must be assembled in the field. Cranes are required for part of the installation.

Second, the installation at the WCDL involved multiple contractors including civil/site, mechanical, electrical, motor control center (MCC), and building. The numerous contractors presented coordination and scheduling challenges.

Third, infrastructure needed to be developed at the site, including water supply, wastewater disposal, electrical service, and communications service.

Finally, the system installation took place at the same time as the construction of a LFGTE system. Additional coordination was required to keep both projects moving simultaneously.

Figure 2 shows the constructed LO-CAT vessels and Figure 3 shows the LO-CAT and LFGTE systems.

Figure 2. LO-CAT Vessels



Figure 3. LO-CAT and LFGTE Systems



Construction Cost

The construction cost for the LO-CAT system was comprised of two (2) components, equipment/license and installation. The PCFAWC contracted with GTP to supply the equipment/license and SCS Engineers to install the system. The final system costs are detailed in Table 6 below.

Table 6. Construction Cost

CONSTRUCTION TASKCAPITAL COST

Equipment/License

$1,328,832*

Installation

$1,883,500

TOTAL

$3,212,332*

Operation

H2S Removal

The LO-CAT immediately demonstrated the ability to achieve significant removal of H2S from LFG. The PCFAWC was obligated to provide a 98.0% removal of H2S to satisfy permit requirements and achieve ≤ 500 ppmv H2S in the LFG for the LFGTE system.

At the start of system operation, the incoming sour LFG contained approximately 5,000 ppmv H2S. The system operators initially obtained sweet LFG containing less than 10 ppmv H2S, a 99.8% removal efficiency. Because the system did not need to perform at that level, chemical use, particularly KOH, was able to be reduced. The LO-CAT is currently producing sweet LFG containing approximately 40 to 50 ppmv H2S, which is approximately a 99.0% removal efficiency.

Operational Costs

The LO-CAT system operational costs are comprised of labor, chemicals and electricity. Provided below are a description of the current operational costs of the LO-CAT system.

Labor Cost: The PCFAWC currently estimates that the system operation requires 8 hours per day, Monday through Friday, and 4 hours each on Saturday and Sunday. The estimated annual labor cost for the PCFAWC to operate the LO-CAT system is approximately $73,000.

Chemical Cost: Table 7 provides data on the monthly LO-CAT chemical consumption at the Warren County District Landfill.

Table 7. Chemical Cost

CHEMICALgpd$/gal$/yr

KOH

35.0

$3.48

$44,500

ARI-340

4.3

$19.75

$31,000

ARI-350

8.0

$9.38

$27,400

ARI-400

0.06

$10.66

$200

ARI-600

3.3

$12.90

$15,500

TOTAL

$118,600

Electrical Cost: The LO-CAT system is presently utilizing 6,960 kWh/day which projects out to 2,540,400 kWh/year. This electrical consumption is approximately 3 times the estimated electrical consumption for the system. Based on an electricity cost of $0.0925 per kWh, the annual electrical cost is approximately $235,000. Table 8 summarized the expected annual operational cost of the LO-CAT system.

Table 8. Expected Annual Operational Cost

ANNUAL EXPENSEAMOUNT

Labor

$73,000

Chemicals

$118,600

Electrical

$235,000

TOTAL

$426,600*

Sweet Electricity

The PCFAWC utilizes the sweet LFG to produce electricity though an agreement with WC Landfill Energy, LLC. WC Landfill Energy LLC is a partnership between DCO Energy LLC and Marina Energy LLC.

The LFGTE facility utilizes landfill gas to fuel two reciprocating engine generator sets (Jenbacher J616). The 3.8MW plant can produce approximately 28,000,000 kWh of electric energy. Approximately 1,000,000 kWh will be sold to the PCFA. The remaining 27,000,000 kWh will be sold to the electric wholesale market through the PJM Power Pool. DCO personnel operate and maintain the project on behalf of the partnership.

The LFGTE system is currently operating and producing approximately 3 MW of electricity. The LFGTE system performance is being evaluated and greater electrical production is expected in the near future.

*The capital and operating costs presented here are specific to the current operating conditions of this LO-CAT unit. Certain conditions (creation of infrastructure—sewers, electrical, etc.) dictated that the capital costs were higher than for a typical LO-CAT unit. The operating costs are higher than expected due to a last-minute design change. GTP is working with Warren County to change some equipment to reduce electrical consumption and chemical usage, along with reducing labor hours required to monitor the unit.

Summary and Recommendations

The PCFAWC selected the LO-CAT system to provide an environmentally sound, technically feasible and cost effective solution to sour LFG. The LO-CAT system was designed and constructed in 18 months at a construction cost of approximately $3.2 million.

The system annual operational costs are expected to be approximately $426,600 with the largest cost associated with electrical consumption. As the system operation is refined though operational and design modifications, labor costs should reduce.

The LO-CAT system is fairly easy to operate. The LO-CAT system is stable and requires primarily monitoring of solution chemistry and management of the sulfur production.

The LO-CAT system is highly effective in removing H2S from LFG. The system achieved 99.8% removal efficiency at the WCDL, but is currently operated at the 99.0% efficiency. The higher removal efficiency is possible with greater chemical use, but there is a diminished rate of return (i.e., operational costs increase substantially).

Based on the installation challenges experiences at the WCDL, it is recommended to use an experienced general contractor to manage the installation. Cost effective installation may be achieved through a well coordinated installation team.