Emigh is the president of Valley Fig Growers, a grower-owned marketing cooperative in Fresno, Calif. It processes, markets, and sells dried figs, and features a cold storage building that holds about 8,000 tons of figs for processing and packaging.

Steam from Valley Fig's boilers is used to run what looks like a large pressure cooker for steam processing the figs to rehydrate them. Steam also is used to extract the juice from the rehydrated figs to make fig concentrate.

All Steamed Up

The company's firetube boiler problem was brought to a head by California's strict limitations on the amount of oxides of nitrogen (NOx) that a gas-fired boiler can emit. Even though Valley Fig Growers replaced the boiler's burner several years ago with a state-of-the-art burner, at full capacity, it still emitted too much NOx.

Ziegenfuss, Valley Fig Growers' maintenance superintendent, explained the problems presented by the company's old 300 BHP firetube boiler. "We would have had plenty of capacity if we could have used the boiler's full potential, but then we would have been in violation of the local smog laws," he said.

"Here in California, we have very strict smog emission standards. To be able to meet the smog emission restrictions, we had to derate the boiler down so much that we weren't able to get 300 BHP.

"Derating is like adjusting the carburetor on your car. You lower the air and you lower the gas. You sit there and let it run for a while and measure the gas emissions. It is kind of trial and error. You just keep adjusting it and adjusting it until you get the most energy you can out of it while maintaining acceptable emissions. After derating the firetube boiler, the tests we ran showed that we were getting around 175 BHP out of it. So that kind of hurt us. There were times when we had to run everything in the plant and we had a hard time maintaining proper steam pressure."

The firetube boiler also took a long time to heat up, said Ziegenfuss. The boiler was in use 24 hours a day, five days a week. The shutdown on the weekends allowed it to cool off, but somebody would have to come in at 2 a.m. on Monday to start it, so that it would be ready when the workforce arrived at 6 a.m. That's how long it would take to produce steam from the firetube's 900 gallons of water.

"At that point, we ran across Nate Moehlman of Moehlman Boilers, and he suggested that we look at the Miura," said Emigh. "We found that we could replace our large firetube boiler with two 100 BHP Miuras."

Russ Binder of Miura's California office suggested they apply for a rebate offered by PG&E on qualifying fuel-efficient boilers.

According to Binder, "PG&E looked at the efficiency ratings in our brochures and approved the rebate for the project. Valley Fig went ahead with it, put it all in, and had the gas company come back. The gas company certified that everything proposed had been done, and hand delivered what amounted to an upfront reward ($16,000) for being fuel efficient."

As it stands now, if a boiler is under a 119 BHP rating, California's San Joaquin Air Pollution Control District doesn't require that it be tested every year. But, the state legislature is trying to change that, since many companies are replacing large boilers with three or four smaller ones. They are talking about requiring that the total on-site horsepower be tested. "But we're not worried about that because the Miuras are still so much more efficient than what we had that we would still pass," Ziegenfuss said. "It would just be a matter of having the state come in and do the testing - which we would have to pay for, of course."

Ziegenfuss noted that the new boilers have been tested at both low fire and high fire. "At high fire, the emissions from both of them were less than the firetube 300 BHP boiler at its derated 175 BHP," he said. "They were way less than the firetube boiler was at the best that it could be."

Fired Up On Mondays

No one seems to miss the middle-of-the-night treks to fire up the boiler on Monday mornings. "From dead cold, we can have 125 psi of steam in eight to 10 minutes," said Ziegenfuss. "If we were to valve off the boiler, it could heat itself up and be to temperature in about five minutes. But we're heating up the boiler and pressurizing our whole plant all at the same time and it still does it in under 10 minutes."

Mark Utzinger of Miura's North American headquarters, which is outside Chicago, explained how Miura engineers lowered the NOx emissions, "Miura uses the pre-mix method of NOx control," he said. "Pre-mix burners combine gas and air in a manifold. They are burned through a flat burner that has lots of little holes in it. This reduces flame temperature, which reduces NOx.

"This pre-mix method can achieve NOx levels under 20 ppm, and on some models, less than 12 ppm. The most stringent requirements in California, for boilers in this size range, limit emissions to 12 ppm. Miura developed the low NOx pre-mix burner and boiler design to lower the peak flame temperature, lowering NOx."

Miura's LX Model has published results that show a value of 85.7 percent efficiency, according to the company. A reduction in water volume was accomplished by reducing the size of the upper and lower headers, eliminating water from the upper header and partially eliminating water from the tubes by having a two-phase mixture of steam and water in the tubes. This design incorporates a floating header, which all but eliminates thermal shock, according to the company. The result is a small, compact boiler, which can be turned on and off quickly and efficiently. This reduces original water usage by 95 percent.

Boilers can be tied together in a multiple installation so they act like one large boiler. When load swings occur, boilers can be brought on and off line quickly, eliminating nonproductive steam generation.

According to Ziegenfuss, "The way our Miura boilers are set up, one boiler is adjusted slightly lower than the other, so if they did get to a point where they didn't need one of the boilers, one boiler shuts down and the plant runs on just one. Then, if the demand comes up, it automatically fires up again."

Emigh noted, "We found that we are running more efficiently, our exhaust gases meet the state requirements, and, to date, it looks like we are saving eight to 10 percent on our gas bills."

For more information, visit www.miuraboiler.com.

Publication date: 03/15/2004