By retrofitting older boilers with flue gas condensing heat exchangers, it is possible to turn old boilers green.

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A Super Boiler with a flue gas heat recovery system operates at 95 percent efficiency at Del Monte Foods in Modesto, Calif.

Replacing an aging, energy-guzzling boiler with a Super Boiler, the ultra high efficiency boiler developed in part by the U.S. Department of Energy (DOE), is not the only way to make progress toward meeting the agency’s aggressive goals for increasing boiler efficiency and reducing emissions. Other options seek to improve efficiency by utilizing existing technology - condensing heat exchangers - in previously unexpected ways. In fact, says one control system developer, any boiler or burner combination can be turned into a low NOX boiler by installing a different control strategy to operate the fan and the combustion control system. In this type of setup, the operation is based on an air-to-fuel ratio instead of using a damper on a fixed speed fan. By circulating a small amount of flue gas in older boilers, it is possible to attain NOX emissions similar to those of low-NOX burners.

“The easiest way to reduce greenhouse gases is to improve efficiency. So, by reducing the amount of fuel that is burned, it is possible to reduce carbon dioxide emissions and lower energy costs at the same time,” says Robert Benz, president of Benz Air Engineering in Las Vegas, which has developed such a system.

The technology, a flue-gas condensing heat exchanger, uses a two-stage heat exchange design, recovering sensible heat without the waste associated with direct-contact heat exchangers, says Benz. This allows the system to preheat makeup water to more than 220°F (104°C) while keeping stack temperatures at less than 110°F (43°C).

To create the system, the company maintains the existing boiler and adds a different control system. Included in the retrofit is a condensing heat exchanger that is engineered for the specific application, taking into consideration the approach temperature, pressure drop and other variables.

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SunOpta Aseptic in Modesto, Calif., retrofitted a boiler with a flue control system to increase efficiency.

Many of the boilers used in industry today were designed 50 to 75 years ago to burn No. 6 diesel fuel, which produces high quantities of ash. As a result, heat exchangers were made with wide fin spacing, which reduced clogging of the fins and coils with ash and dirt, but the wide spacing also made the unit less efficient. Even as many of these older boilers were converted to burning lower cost natural gas, they kept the same, less efficient, design in heat exchangers.

By using a condensing heat exchanger made with a stainless steel coil and fins set close together, the flue gas temperature can be brought down below the dewpoint by extracting heat from the exhaust. The flue gas condensate collected with the dropout volume under the downstream coil is pumped back to the makeup water line coming out of the upstream coil, contributing up to 7 percent of the total makeup water flow to the boiler.

Pacific Coast Producers in Woodlands, Calif., a tomato processing plant with five high-pressure steam boilers, contacted Benz Air to retrofit one of the plant’s boilers with the boiler combustion control system. At Pacific Coast, the capacity of the watertube boiler - 100,000 lb/hr before the retrofit - increased to 150,000 lb/hr after the system was in place. Annual fuel savings were more than $400,000 for the tomato processing plant, while the NOX and CO2 emissions were reduced 13 ppm and 2,380 tons per year, respectively.

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A stainless steel condensing heat exchanger typically is used in the boiler control system.

Super Boiler Designs

Following the success of the Pacific Coast Producers project, Benz Air knew it had a solution to many processors’ problems, but the company needed assistance with the development of a flue gas heat recovery system. Benz Air turned to Super Radiator Coils, a manufacturer of heat exchangers headquartered in Chaska, Minn., which has an experienced design engineering team focused on custom designs for heat exchanger applications. One of the first projects the two firms worked on together was a new Super Boiler that went online in 2007 at the Del Monte Foods plant in Modesto, Calif.

The project involved an additional, condensing heat exchanger between the makeup water and the feedwater that would provide a reduction in the needed heat transfer of the condensing heat exchanger. Again in this installation, the flue gas condensate is collected within the dropout volume under the downstream stainless coil and pumped back to the makeup water line coming out of the upstream coil. Capturing this condensate contributes up to 7 percent of the total makeup water flow to the boiler. In addition, capacity for preheating up to 50 gal/min of process or makeup water was added. The saturated flue gas at 110°F (43°C) still contains nearly 7 percent of the total energy input to the boiler. The additional flow capacity of the last coil increases efficiency to 95 percent. As a result, CO2 emissions were reduced by 1,171 metric tons per year.

Subsequently, Benz Air designed a Super Boiler for Seneca Foods, also in Modesto, where a 96 percent efficiency rate was achieved. NOX emissions were reduced to 5 ppm and CO2 emissions were reduced by 562 metric tons and the stack temperature was 97°F (36°C).

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An efficiency rate of 96 percent was achieved with a Super Boiler at Seneca Foods in Modesto, Calif.

Presently, California is the only state with a CO2 emissions mandate. However, metro areas in other states, including many along the upper East Cost, have mandates, most of which start by requiring all new boiler installations to maintain emissions below a certain level. Other state and local governments are currently considering such actions.