January 15, 2007

Focus on O&M (January 2007)

FUEL SWITCHING

Practical aspects of burning landfill gas

Converting an existing steam power plant to burn landfill gas may—in one fell swoop—capture and put to good use the methane generated by an adjacent or nearby landfill and lower the plant's fuel bill. But there are tradeoffs that need to be fully explored before jumping into a such a project. (See RENEWABLES to learn how AEP extended the life of an aging power plant by repowering it with landfill gas.)

Landfill gas is a renewable fuel with a heating value of 350 to 600 Btu per cubic foot—about one-half that of natural gas. Early repowering projects included installing either reciprocating engines or small gas turbines to burn the landfill gas. That was done more to solve an environmental problem than to capture the power. As it turns out, methane leakage from urban landfills that lack methane collection remediation often is problematic for neighbors of the plant and/or the landfill. At a minimum, the leakage is an irritant; at worst, it is a safety hazard. Collecting the methane through a series of wells, interconnecting piping, and a fan and flare stack was the next step in the evolution of landfill gas projects. The challenge then became operating the wells with a suction pressure equal to the rate of methane generation, which at a landfill decreases slowly over time.

Burning the captured methane productively became the next priority, for obvious economic reasons. Because naturally aspirated reciprocating engines operate at an intake pressure that matches the landfill gas pressure at the discharge of the collection fan, they were a natural choice. Others have tried using combustion turbines to burn landfill gas, but doing so accelerated corrosion of the standard materials used in turbines' compressors. Using compressors made of exotic alloys solved that problem but ruined project economics.

Larger landfill gas power plants—such as the 48-MW Puente Hills project operated by the Los Angeles County Sanitation District (LACSD)—have succeeded by taking a low-risk approach to reducing methane leakage. The gas collection system at Puente Hills uses more than 1,000 wells, 55 miles of underground trenches, and more than 30 miles of pipes to collect an average of 26,000 cubic ft/min of landfill gas. The fuel is burned by conventional boilers whose steam is collected and sent to a 50-MW steam turbine that has been in service since 1987. The plant's overall heat rate is about 11,000 Btu/kWh. LACSD also compresses landfill gas for use by the facility's water trucks and passenger vehicles, saving the equivalent of 1,000 gallons of diesel fuel per day.

The EPA's Landfill Methane Outreach Program (www.epa.gov/methane) reports that more than 70 steam plants have switched their boiler fuel to landfill gas and enjoy from 10% to 40% fuel savings as their reward. The plants' fuel demand ranges from 2 to 150 mmBtu/hr.

When taking boiler conversion costs into account, care must be taken to adapt the boiler for the higher fuel flow rate (about twice that of natural gas), variations in the heating value of the landfill gas, and its inherently higher corrosiveness, which can wreak particular havoc at the cold end of the evaporator and air preheater, as well as on the boiler stack. On the positive side of the ledger, the corrosive deposits have proven easy to remove by sootblowing and periodic manual cleaning. Finally, the lower flame temperatures produced when landfill gas burns generate lower levels of NOx emissions than natural gas, but operating at these temperatures may require increasing the surface area of the superheater.