Dual-fuel engines use clean-burning natural gas much of the time and eliminate a high-cost maintenance item by using a pinch of diesel fuel instead of sparkplugs to ignite the gas. If natural gas becomes unavailable, however, the engines shift automatically and seamlessly into full-diesel operation, eliminating rigid dependence on natural gas. Even better is that they need not be brought off-line and shut down to switch fuels.

Dual-fuel is not a new engine technology. Fairbanks Morse Engine, an EnPro Industries company located in Beloit, WI, pioneered the concept in the late 1940s.

Considerations of familiarity, operational flexibility, economics, and environmental protection prompted Poplar Bluff Municipal Utilities to choose these engines. Poplar Bluff already had two dual-fuel engines that went into service in 1973, so the utility's decision-makers and staff already knew the benefits of this technology.

Poplar Bluff's new installation—three 18-cylinder, four-stroke Fairbanks Morse FM-MAN 32/40 dual-fuel engine generator sets rated at 6.72 MW apiece—add enough capacity so the city won't have to pay top dollar for extra electricity when extreme weather conditions boost demand to peak levels.

Indeed Poplar Bluff now has excess capacity. To help pay for the new $14,620,000 plant, it can sell power to nearby utilities when their demand peaks exceed what their regular sources can supply.

"In 1999, we had $1.8 million in wholesale power sales with 14 MW of generating capacity. Now we have 33 MW," says Doug Bagby, who was Poplar Bluff's utilities director when the project began and now is the city's manager. "The market has stabilized in the last couple of years, but, if there are some hot summers people don't anticipate, we now have more excess supply available to the market."

Meanwhile the additional peaking capacity lets Poplar Bluff Municipal Utilities optimize its own purchases of power from external sources. "We can reduce our firm capacity down to what we think a normal winter and summer will be," Bagby says. "We have to make sure we cover worst-case scenarios, but that's expensive if they don't happen.

"Now, if we're incorrect, we've got that peaking plant sitting there. It's going to reduce our costs just on the fact that we don't have to prepare for the worst day of the summer or winter anymore. Without it, having to buy enough power to cover ourselves even for three or four days a year would cost us a tremendous amount."

Carroll Foster, Poplar Bluff Municipal Utilities' plant superintendent, says the city's demand for electricity typically peaks at about 78 MW in July and August, with a secondary peak of 58-60 MW from mid-December through early February. Those peaks assume normal weather conditions—not the peak of the peak. In a hot, dry summer, demand might reach 80-82 MW; a cold winter could prompt demand of 68-70 MW.

Bagby points out that the new peaking plant also will help Poplar Bluff Municipal Utilities keep its retail electricity rates "more stable than they have been. We try to sell power pretty much at cost." Until a 10% increase went into effect in January 2004, rates had remained unchanged since 1987. "We might be pricing it slightly above cost in the near range," he explains, "and then before we do a rate increase, it will be a little below cost in the long term."

To obtain the vast majority of the megawatts Poplar Bluff distributes to its retail customers, the city has contracted with the Grand River Dam Authority and the Southwestern Power Administration.

The Grand River Dam Authority, based in Vinita, OK, has a primary service area that encompasses 24 counties in northeastern Oklahoma. It operates four power-generating facilities: the Pensacola and Robert S. Kerr Dams on the Grand River, which together can produce up to 239 MW of electricity; the Salina Pumped Storage Project; and a coal-burning power plant in Chouteau, OK. In 1992, Poplar Bluff signed a 15-year contract with Grand River.

The Southwestern Power Administration, based in Tulsa, OK, is an agency of the United States Department of Energy. It markets hydroelectric power produced at 24 US Army Corps of Engineers dams to more than 100 municipal utilities and rural electric cooperatives with more than 7 million customers. Southwestern operates and maintains 1,380 mi. of high-voltage transmission lines and 24 substations.

"Eighty percent of the power I buy comes from Grand River, and 20% [comes] from Southwestern," reports Foster. "I buy 40 megawatt-hours around the clock from Grand River. With Southwestern, I can buy up to 39.5 megawatt-hours, not to exceed 1,200 hours a year or 600 hours within four months, and I have to take a minimum of 60 megawatt-hours a month."

If those sources and Poplar Bluff's own peaking equipment fall short of demand, Foster says, he can buy extra power from the municipal utility in Sikeston, MO, 48 mi. east of Poplar Bluff.

Poplar Bluff is 153 mi. south of St. Louis and 22 mi. north of the Arkansas state line. A thriving industrial city with a population of about 17,000, it stands beside the Black River at the margin of the Mississippi Delta. East of town lie some of the world's most fertile farmland; to the west rise the rolling woodlands of the Ozark Mountains.

Health care is the city's largest single industry. A local hospital, Three Rivers Healthcare, employs 1,350 people. A Veterans Benefits Administration medical center has another 350 employees.

Other major employers include Briggs & Stratton Corporation, with 1,200 employees producing internal-combustion engines; Rowe Furniture Corporation, where 850 employees make upholstered living-room furniture; and a Gates Rubber Company radiator-hose plant with 460 employees. Three industrial parks in the city encompass a total of 415.79 ac.

Poplar Bluff is the seat of Butler County, which has a total population of 40,867. According to the local chamber of commerce, Poplar Bluff's trade-area population is 175,000.

Poplar Bluff and Butler County have gone separate ways with respect to electric power, says Foster. The county's Ozark Border Electric Cooperative lacks generating capacity and buys all of its electricity elsewhere for resale to its retail customers outside the city limits, but Ozark does not buy from the city utility that its territory surrounds.

Instead Popular Bluff sells its surplus power to Sikeston; to Jonesboro, AR, 73 mi. distant; and to other small municipal systems elsewhere in southeastern Missouri and northeastern Arkansas.

This Balkanization of electric-power retailing in the region and of the modest scale of the individual public utilities there might seem quaint and arcane to readers accustomed to large investor-owned utilities with service areas that encompass thousands of square miles. "Our electric utility formed in 1918," Bagby explains. "St. Louis and Kansas City were getting lit up, but there wasn't a whole lot else here until the municipalities started forming their own utilities. Today they're still primarily distribution companies."

The process that led to Poplar Bluff's new plant began with the city's wholesaling success in 1999, which included supplying Jonesboro with 14 MWh of electricity for several days. Peak rates at that time were running as high as $1,250/MW. The potential for generating revenues and electricity impressed the seven-member Poplar Bluff City Council and the five-member Municipal Utilities Advisory Board, which is appointed by the city council. "The city council and the electric board said if we had more capacity available and this opportunity came up again, it would help pay for the plant," Foster recalls.

Another consideration was the age of the original 1973-vintage peaking units. Each consists of a 450-rpm Model RV Delaval Enterprise engine, rated at 9,704 bhp (brake horsepower, or actual horsepower at the flywheel), from Delaval Engine and Compressor Division of Oakland, CA, coupled with a 7.2-MW Westinghouse generator.

"We felt it was time we looked at that plant and our overall power supply," Bagby says. "You want some kind of mix that has peaking and capacity you're getting from other places. Unless you have your own baseload plant, it's not good to have all of your eggs in one basket."

The city council and advisory board commissioned a study by the St. Louis office of Burns & McDonnell, an engineering firm headquartered in Kansas City, MO. Burns & McDonnell determined that a new plant would be warranted, prepared the specifications for it, and remained with the project to perform inspections during construction.

The project attracted three bidders:

• Caterpillar Inc. of Peoria, IL
• The Annapolis, MD, office of Wärtsilä North America Inc., a subsidiary of Wärtsilä Corporation in Helsinki, Finland
• Poplar Bluff's own Huffman Inc., a general contractor offering design-build services for the complete power plant and substation improvements, using Fairbanks Morse equipment

Caterpillar proposed the installation of small turbines, but the Poplar Bluff officials didn't want those. Wärtsilä and Huffman/Fairbanks Morse both offered dual-fuel technology.

"The Wärtsilä bid was slightly lower," Bagby says, "but the proposals were very similar. Huffman being local and Fairbanks Morse's reputation as the industry standard are why the city council made the decision. Also we asked for a range of 15 to 20 megawatts. Fairbanks Morse was at the top of that range, and Wärtsilä was around 16 megawatts. At the time, the city council felt that looking for the most plant we could get was the right approach."

Fairbanks Morse received the order in May 2001 and delivered the equipment in the first quarter of 2002. Construction took about six months. The new peaking plant was operational for the summer generating season of 2003—four years after it first was proposed.

The FM-MAN 32/40 engines installed at Poplar Bluff are 720-rpm, medium-speed engines with a 320-mm bore and a 400-mm cylinder stroke. The product of a joint venture between Fairbanks Morse and MAN B&W Diesel in Augsburg, Germany, they use Fairbanks Morse's Enviro-Design low-emission dual-fuel technology, originally developed for the US Navy in a larger marine diesel engine (the 48/60). "As a licensee of MAN, Fairbanks Morse has exclusive rights to sell the 32/40 engine in the US," explains Jon Frey, a Fairbanks Morse marketing specialist.

Rated at 9,290 bhp, each 32/40 engine is coupled with a two-bearing, three-phase, 60-cycle generator made in Germany by AVK using Fairbanks Morse technology.

Each of the three 13,800-V generators can produce up to 6.72 MW of electricity, for a theoretical total of 20.16 MW. Operating at a full load in the dual-fuel mode, their efficiency is rated at 96.5%, equivalent to 6.48 MW apiece or 19.45 MW for the entire plant. Assuming that a single residential customer consumes 2-3 kW, the plant with all three of the new engines at full load could generate enough electricity for 6,483-9,725 homes.

Fairbanks Morse sold the engines and generators to Huffman complete with auxiliaries and controls. "We build our equipment with the engine and generator on a common sub-base 4 feet deep, a steel framework with an integral oil pan. The engine and generator are resiliently mounted to the top of the sub-base by means of conical vibration isolation supports," Frey says.

"The lube-oil filtration, filters, strainers, heat exchangers, prelube pumps, cooling system, and cooling towers—all of the auxiliaries to make an operating plant—were shipped loose. Huffman did the building addition, foundations, mechanical piping, electrical wiring, and switchgear modifications and provided a gas compressor."

Huffman paid Fairbanks Morse $8.6 million for the three units, including delivery to Poplar Bluff and field service and customer support during the installation, says Kevin Lidbury, the commercial-engine sales manager in charge of the transaction.

Lidbury describes the FM-MAN 32/40 engines as "like a car engine, only on a much bigger scale," with a cast-iron block and V configuration. Building one takes seven or eight months.

The engines were built and tested in Augsburg, sent by ship to the Port of Houston, TX, and then placed on a special railcar for transport to Poplar Bluff. The closest rail siding was 2 mi. from the plant site. Heavy-haul riggers used cranes to mount the engines on special trucks with multiple wheels, which hauled them through town at a speed of about 20 mph. At the plant, big rollers were placed under the engines to transfer them from the trucks to the concrete foundation beneath their sub-bases.

The three engines sit parallel to each other in the plant. Each is 42.25 ft. long, 12.25 ft. wide, and 16.75 ft. tall. Each engine weighs 306,945 lb.—more than 153 tons.

Instead of a basement, the plant has trenches in the floor for piping that goes to the generators. At one end of the plant are the control room, the switchgear room, and a lean-to that houses the auxiliaries. Outside a wall at the opposite end are the air intake, filters, and silencers.

The plant operates within acceptable noise limits, even though it stands near a residential area. "One corner of the utility plant is 50 yards—half a football field—from the nearest residence," Lidbury says, "but there haven't been any complaints. The plant produces 108 decibel-amps at 3 feet vertical and 3 feet horizontal from the engine. That's engine-casing noise within the building. The exhaust noise outside is less—65 decibel-amps at the property line. We have a residential-grade silencer on the exhaust.

"We met all of the environmental requirements of the Missouri Department of Natural Resources and the US Environmental Protection Agency. We went through their permitting processes with no problems."

The plant's exhaust emissions vary with its load and fuel mix (see table), with dramatic reductions of most pollutants in the dual-fuel mode. All dual-fuel ratings improve as the plant's load increases, and the same is true in full-diesel mode for all emissions except oxides of nitrogen.

"The engine starts as a diesel," Lidbury explains. "At a 40% load it can switch over to dual-fuel. That takes about 30 minutes. In emergency situations, if you want to be on-line quicker from a dead start, you can get it on-line and fully loaded on diesel and then switch over."

In dual-fuel operation, which should occur most of the time the plant is running, the engines burn a mixture of 99% natural gas and 1% diesel pilot fuel. Ignition of the diesel fuel in a high-energy precombustion chamber in turn ignites the natural gas in a cylinder.

Lidbury describes the No. 2 diesel fuel as "a liquid sparkplug." An engine running solely on natural gas would need an electronic system with actual sparkplugs to initiate the combustion cycle. "With equipment this size," he explains, "the sparkplugs have a very low life expectancy, and they're expensive. You would have to change out a relatively expensive item quite often."

Moreover, a plant with a straight-natural gas engine can't realize the savings of an interruptible gas supply. It needs a more costly firm contract because it would shut down if gas became unavailable.

"With a dual-fuel engine, if you buy interruptible and the gas company curtails you, you still can run on No. 2 diesel fuel. If there's a gas failure and you're operating in dual-fuel, the engine automatically switches back to diesel and keeps running."

Operating at a full load as a straight-diesel engine, each unit burns 407 gal./hr. of No. 2 diesel fuel. In dual-fuel operation at a full load, a unit burns 4.16 gal./hr. of diesel fuel and 55,550 ft.³/hr. of natural gas.

Lidbury says these engines are designed to run continuously around the clock, but they typically aren't used that way in utility power plants. To generate peaking utility power, they might be turned on one by one as demand rises between 11 a.m. and 1 p.m. on a hot summer day and then turned off one by one in the evening as the temperature drops and demand falls.

By around 10 p.m., demand and rates would be down, enabling Poplar Bluff to obtain all of the electricity it requires from the Grand River Dam Authority and the Southwestern Power Administration at less cost than by generating its own.

When Foster has no need for peaking power and no opportunity to sell power elsewhere, each engine still gets exercise during a maintenance run once every five weeks. "I run one engine a week in rotation on Thursday for six hours at a 90% load," he explains.

With the installation of the new engines, Poplar Bluff now has more than enough peaking capacity for its current power needs. "We're really set for the future for peaking demand," Foster declares.

In addition, with five engine-generator sets, the city now has enough variety of peaking equipment to fine-tune its operations and adapt if mechanical problems arise. Indeed, as the new engines were being installed, just such a problem occurred with one of the older Enterprise engines.

"The number-one engine started knocking, so we shut her down," Foster says. "Her bolts broke, and her rods cracked. We had to x-ray the bolts to identify the problem. It cost $350,000 to replace the bolts, rods, and gaskets and for the labor to put her back together. The overhaul took us a little over a year."

GEORGE LEPOSKY is a science and technology writer based in Miami, FL.

DE - March/April 2004