This paper is presented as a case study on one of the potential, efficient uses of landfill gas, or LFG, as a renewable energy source. It is based on the detailed design of a nominal 26 MWe gas turbine combined-cycle power plant, which will run solely on 8,000 scfm of LFG. KPUD, the owner-operator of the plant, operates 5 reciprocating engines to generate about 10 MWe from LFG at the Roosevelt Regional Landfill, near Roosevelt, Wash. The landfill receives municipal waste from a number of communities throughout the Northwest U.S.
Based on the current LFG production rate at the landfill, it is projected that in 2013 there will be sufficient LFG to power two 10 MW gas turbines at near 100 percent capacity. A 2x2x1 combined-cycle power plant configuration would utilize the full power generation potential of the LFG and substantially shorten the pay-back period for the plant.
Typical LFG consists of approximately 50 percent methane and 50 percent carbon dioxide with a small amount of entrained air.
Despite the large inert gas percentage, low BTU methane-based gases
Acknowledgements Hands on contributions by John Clairmont, senior V.P. of WorleyParsons also are greatly appreciated. References
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Without prior treatment, LFG does not lend itself to direct combustion either in reciprocating engines or gas turbines because:
- It is laden with moisture
- It can include several hundred parts per million of H2S and mercaptans
- It can carry detrimental amounts of various siloxane species
- It is collected under negative gauge pressures and needs to be sequentially compressed in several stages prior to combustion
Moisture in gaseous fuels poses a corrosion risk, hinders combustion stability and increases the plant heat rate due to its latent heat absorption – requiring its removal.
Typically with a multi-stage process, first the water droplets are removed with cyclonic separators and coalescing filters, then the water vapor fraction is further reduced by chilling, then reheating the gas. A desiccant bed also can be used to reduce the water vapor fraction to even lower levels. A target relative humidity of the LFG of 40 percent or less is desired prior to combustion.
Sulfur carrying compounds, such as H2S and mercaptans, are corrosive in the presence of moisture.
With time they take their toll on the fuel delivery system and carbon steel engine components they come in contact with. Once burned, they result in sulfur oxides (SOX) emissions; which also requires pre-combustion removal. Typically, an iron sponge or iron-solution scrubber is used to capture and remove the sulfur in sour LFG. These systems require constant replacement or regeneration of the spent media, adding to the O&M cost of the plant.
Siloxanes are long chain, organic based silicon compounds and are commonly found in many cosmetic and household products. They are deposited in landfills through disposal with municipal wastes and volatilize into the LFG during anaerobic digestion of the landfill wastes. At a given landfill site, out of the 12 major siloxane species, at least three to four species would be present, most commonly octamethylcyclotetrasiloxane (D4), decamethlycyclopentasiloxane (D5), hexamethyldisiloxane (L2) and octamethyltrisiloxanes (L3), [Ref. 2, 4].
The total siloxanes concentrations can vary significantly from landfill to landfill, ranging from as low as 5 mg/Nm3 to as high as 135 mg/Nm3 [Ref. 3]. Newer landfills tend to contain higher concentrations of siloxanes. In untreated LFG, siloxanes are present in vaporous form and cannot be filtered out by simple mechanical means. Once the LFG is compressed and combusted, siloxanes convert into abrasive silicon dioxide (SiO2) powder, which deposits onto hot surfaces, like the piston heads and valves in reciprocating engines and turbine blades in gas turbines.
