Three generator sets driven by advanced natural-gas-fueled Caterpillar (Peoria, Illinois, U.S.) engines are helping Greenwood Utilities, a municipal utility based in Greenwood, Mississippi, U.S., to maintain its status as one of the lowest-cost retail electricity providers in the United States.

Since installing the new generation, Greenwood's operating strategy has been to dispatch engine-driven units as needed when demand for power increases with warm weather. During peak months, the utility base-loads one or both of its coal-fired units, then uses the engine-driven generators as necessary for peaking or price hedging.

On a daily basis, Greenwood personnel maintain a spreadsheet that enables quick calculation of the gas and diesel units' generating costs as fuel prices fluctuate with the markets. In summer 2002, the gas-fueled generator sets were dispatched first based on their generating costs of US$50 per megawatt-hour, versus $62 for the low-BSFC diesels and $67 for the low-emissions diesels. The gas units logged approximately 800 operating hours during the summer, producing from 10% to 40% of the utility's self-generated power at any given time.

The gas-fueled units are part of a generating facility commissioned in early 2001 that delivers 13.175 MW of capacity used for summer peaking and for hedging against high spot market rates. The utility contracted with Thompson Power Corp. (LaVergne, Tennessee, U.S.) in 2000 to install the three generator sets. The units are rated 1.25 MW (with standard radiators) at 1800 rpm. Thompson Power increased the output to 1.35 MW by adding a separate aftercooler circuit-cooling loop using available well water.

The electronically controlled Cat® G3516B gas units meet Mississippi Department of Environmental Quality (MDEQ) emissions regulations without exhaust aftertreatment. The facility also includes five diesel-fueled generator sets: three Cat low-emissions 3516B units and two Cat low brake specific fuel consumption (BSFC) 3516B units. All five diesel units (9.125 MW total capacity) have selective catalytic reduction (SCR) exhaust aftertreatment using urea for NO_x removal. Installing the SCR units on each diesel-fueled generator set cost about $150,000 per unit, which enabled Greenwood to increase their annual operating hours from 500 to 2000.

The new generating plant augments Greenwood Utilities' existing capacity, which includes two coal- or gas-fired boilers totaling 30 MW; two gas-fired boilers at Wright Generating Station totaling 17 MW; and a 13.5 MW combustion turbine with a heat-recovery steam generator (HRSG), capable of burning natural gas or diesel fuel.
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