Located just south of Provo, Utah, the town of Springville has about 16,000 residents and just one major industry in the form of a Stouffer/Nestle plant.

Springville Municipal Power's expansion disproved two myths of emissions control: that two-stoke engines run too "dirty" for catalytic converters, and that large, two-stroke engines are too big for catalytic converters. Shown here are exhaust silcencers at the Springville station, where the expansion was completed in November 2003.

Still, the Springville Power Company has initiated a power station to meet the 5-mW needs of the Stouffer plant and provide standby power for the town’s 16,000 residents.

In 1985, an opportunity arose when a TVA nuclear plant project was abandoned and the engines it had acquired for back-up power generation became available as government surplus.

As a result, Springville Power was able to acquire two 16-cylinder, four-stroke, 7-mW Enterprise diesel engines for about $1 million each. Because of the relatively low cost of natural gas in Utah at that time, the company decided to convert these diesel engines to dual-fuel engines, with natural gas being the primary fuel and diesel fuel being relegated principally to ignition.

The company had an Oakland, CA-based firm do the conversion at a cost of another $1 million each. However, the company was well satisfied with its $4 million investment because the operating costs were also quite low and, with the use of natural gas, the emissions were cleaner than they would have been with the use of diesel fuel.

The company’s permit imposed a 220-tpy limit on CO and NOx emissions, a level that the system could meet.

The Expansion Begins
In 1990, Springville began an expansion of this engine complex when surplus bargain prices became irresistible. The company acquired two more 7-mW Enterprise engines for just $50,000 each and then seized the opportunity to acquire three General Motors EMD 20-cylinder, two-stroke, 2.8-mW units that had become available as government surplus from a nuclear research facility near Albany, NY. The purchase price for these units was just $10,000 each.

One of the 7-mW engines had already been converted to dual-fuel operation by its previous owner, who had used it at a privately owned plant in Missouri. Still, the other four were diesel-only engines, so Springville Power faced costs to convert them all to dual-fuel operation. Fortunately, conversion costs were coming down by that time, and the company’s technical expertise had grown. Springville Power employees did the conversion of the fourth Enterprise engine, so the out-of-pocket cost for this conversion was limited to just $300,000 for parts. These employees also did the conversion of the three EMD engines, so the company’s out-of-pocket cost for the conversion of all three engines totaled just $800,000.

At a remarkably modest cost, then, Springville Power had built up a seven-engine complex with an output capacity of more than 36 mW. However, the company faced an unexpected difficulty in emissions limit compliance. Despite having almost tripled its output capacity since its original permit was issued in 1985, Springville Power was mandated to limit the CO and NOx emissions of its entire seven-engine complex to the same 220-tpy limit set for the original two-engine complex.

Addressing the Need for Emissions Compliance
“That meant that our expansion from two to seven engines would require a reduction in per-unit emissions,” said Springville Power company project engineer Jeff Davis. “We mitigated emissions somewhat by adjustments and improvements to the engines. For example, we can now adjust the air-to-fuel ratio in our Enterprise engines to add air and thereby reduce NOx emissions. And we are using less diesel fuel now. Today, the diesel fuel injector just initiates combustion as a sort of ‘liquid spark plug’.

MIRATECH designed and installed this 72-inch diamter IQ catalytic converter for one of Springville Power's 20-cylinder EMD engines. Despite the size of this unit, the pressure drop across the catalyst face at full load was only 1.8 inches w.c., 60% less than the engine manufacturer's maximum allowable pressure drop of 4.5 inches w.c.

“Also, we installed Cooper Manufacturing’s ‘Clean Burn’ units. Instead of the diesel fuel being injected directly into the cylinder, it is now injected into a small pre-combustion chamber where the air/natural gas mixture is ignited. By controlling ignition in a confined space, we achieve combustion with a lesser amount of diesel fuel. We’re using only 1% for pilot fuel now. Not only does this significantly reduce our diesel fuel costs, but it also reduces NOx emissions.”

Still, it was clear that since the Utah Division of Air Quality was not going to relax the 220-tpy limit, some emissions control equipment would be needed. Scott Jensen of Seattle-based Energy Conversions, which handles natural gas conversions, recommended that Springville Power contact the MIRATECH Corporation of Tulsa, OK.

MIRATECH analyzed the Springville Power complex and concluded that it offered technical challenges that would require customized emission control equipment. MIRATECH Engineering Manager John Sartain explains why.

“Most air quality regulations limit output of three main classes of air pollutant: NOx, CO and hydrocarbons (HC), including volatile organic compounds (VOCs) and EPA- listed Hazardous Air Pollutants (HAPs). With a rich-burn engine, a “three-way” non-selective catalytic reduction (NSCR) system can usually handle the job, simultaneously reducing NOx, CO, and HC to compliance levels. But with lean-burning engines, like those used at Springville Power and most distributed generation facilities, emissions control is more complicated.

“With lean-burn engines, there’s more air in the air-fuel mix than combustion requires. That means there’s more air – and oxygen – in the exhaust stream than you find in rich-burn engines. This excess oxygen keeps a three-way catalyst process from breaking NOx down into harmless nitrogen and water. While lean-burn engines normally produce far less NOx per hour of operation than rich-burn engines, the total volume of NOx output without after-treatment often exceeds regulatory limits. That’s why you need a ‘Selective Catalytic Reduction’ (SCR) system or an oxidation catalyst with many larger-load, lean-burning engines.

MIRATECH’s Catalytic Converter Solution
“The MIRATECH Diesel Oxidation Catalyst simultaneously reduces CO, HC, VOCs, HAPs and particulates,” Sartain added. “For Springville Power, we developed a custom-version of our standard ‘IQ’ catalyst element design. The element is constructed of corrugated metal foil wash-coated with a slurry containing precious-metal–group catalysts. This wash-coated foil is tightly wound around a steel spool-post, forming a ‘honeycomb’ pattern that maximizes catalyst contact with exhaust gasses while resisting shock and vibration as well as fouling and catalyst “poisoning” by materials such as lead in the exhaust stream. Our patented banding and pinning process increases durability and strength.

“The distinctive ‘flat-top bonnet’ Y-design of the element casing eliminates exhaust gas blow-by around the catalyst, as well as sagging and telescoping. The design also features an ‘Easy-Grab’ lift shelf, which allows easier catalyst insertion and removal for maintenance and cleaning and helps prevent worker injury. Like the catalyst elements, the IQ catalyst housing was also designed for durability, easy maintenance and low service-life cost.”

A key challenge in the project was the large size of the engines, which required catalytic converters sized to match. For the 16-cylinder Enterprise engines, MIRATECH custom-built the largest IQ catalyst housings and elements in company history—96 inches in diameter. The units are so tall that there wasn’t enough overhead clearance to remove the catalyst elements for cleaning or maintenance by lifting them out of the top of the housing, as is done in most configurations.

“As a result, we installed the IQs on their sides, so the elements could be removed from either side of the housing,” Sartain said. “This also allowed the catalyst elements to be designed at a weight of less than 60 lbs., which makes it easier to remove the elements, without the need for heavy equipment.”

“One catalytic converter was installed on one of the 2.8-mW EMD engines, and two others were installed on two of our 7-mW Enterprise engines,” Davis said. “The catalysts were sized according to exhaust flow rates given by the engine manufacturers and from actual flow measured in previous emissions tests. The maximum allowable backpressure was also determined by the engine manufacturers’ recommendations. The maximum allowable pressure-drop for the first unit installed was 4.5 inches w.c. [water column]. It was a pleasant surprise to see that the actual pressure-drop across the unit, at full load, was only 1.8 inches w.c.”

For 16-cylinder Enterprise engines installed by Springville Power, MIRATECH custom-built the largest "IQ" catalyst housings and elements in company history—96"—in diameter shown here.

For Sartain, it wasn’t the size but the typical exhaust composition of the two-stroke EMD engines that presented the biggest engineering challenge. “The main problem in using a catalyst on a two-stroke engine is the poisoning of the catalyst by the lubrication oil. In two-stroke engines, the oil is mixed with either the fuel or the air to lubricate the piston. This oil can blow by and go through the exhaust ports and down the exhaust pipe. At Springville, we showed that our catalyst is designed to be durable and that it overcomes this problem.

“This project has disproved two common myths of emission control: that catalytic converters don’t work on two-stroke engines because such engines run ‘dirty,’ and that large two-stroke engines are too big for catalytic converters,” Sartain points out. “To reduce emissions from the Springville engines, we custom-designed and installed the largest catalytic converters in MIRATECH history. With them, Springville Power has been able to produce electricity within prescribed limits almost continuously.”

“With the resolution of the emissions control issues, the increased capacity at the generating station has given our operators more options to deliver reliable power at minimum costs,” Davis said. “Springville purchases power from a number of sources and has an allocation of hydroelectric power from the Colorado River Storage Project. In addition, the city has invested in coal-fired power plants in the area. The plant operators monitor hourly prices and the power demand for the city. A power exchange is maintained by a broker. Hourly price and capacity is available for the operator to decide the most economical means to deliver power to our customers.”

The Future for Dual-Fuel Engines
Is Springville Power’s solution applicable for many other distributed energy producers? “Probably not here in the United States at this time,” says Jensen of Energy Conversions. “The price of natural gas is simply too volatile. Seemingly without reason, natural gas prices here have continued to soar. It seems that whenever diesel fuel prices go up, natural gas prices follow suit. Overseas, in countries that also have indigenous natural gas supplies, the price of natural gas is half that of diesel fuel. In those countries, conversion from diesel to natural gas is a very viable alternative. In this country, with our inflated natural gas prices, the decision is much less obvious.

“That doesn’t mean that aren’t some organizations that are making the conversion. For example, Portland General Electric is taking its diesel engines and converting them so that the company can extend its operating time made possible by lower emissions and somewhat cheaper power. However, not too many utilities are looking seriously at the possibility of converting to natural gas. They ask themselves four key questions:

1. What are the capital costs of conversion and emission compliance?
2. How long will we generate power each month?
3. How much diesel fuel will we replace?
4. What is the fuel price differential?

“If they calculate from the answers to these questions that it will take five years or more to pay back the equipment investment, they probably won’t be interested. But if the cost differential of diesel fuel vs. natural gas goes up to 2:1, as it is in Europe, the conversion to natural gas will be a very attractive proposition for a wide range of current or prospective distributed energy generators in the US, too.”

L.A. author CHARLES D. BADER writes on diverse technological topics.

DE - May/June 2006