Air Pollution Control Equipment Services, Emissions

Field Testing New “Super” Boiler

Issue 11 and Volume 110.

Gas Technology Institute (GTI) is developing a new industrial steam-generation system that provides increased efficiency, reduced emissions and lower fuel costs than steam systems presently available. GTI has teamed with Cleaver-Brooks – provider of packaged boilers for industrial, institutional and commercial applications – to focus efforts on bringing technological advances to the marketplace.

Now in field demonstration at Specification Rubber Products in Alabama, the advanced gas-fired boiler – dubbed the “super boiler” by its sponsors – has shown significant performance improvements over alternative technologies. In addition, the boiler has a 40 to 50 percent smaller footprint and half the weight of conventional boilers.

A critical part of the system design is its ability to meet future nitrogen oxide (NOx) emission regulations without the need for post-combustion flue-gas treatments. “The development of this technology is being driven primarily by rising energy prices combined with increasingly stringent emissions regulations,” said GTI’s Rick Knight, research and development manager in the institute’s power generation division. Industry, manufacturers and the government are teaming with GTI to develop cleaner, more efficient ways to use fossil fuels.

Sponsors for the super boiler project include the U.S. Department of Energy, Cleaver-Brooks, the Gas Research Institute, Utilization Technology Development NFP, GTI’s Sustaining Membership Program, the Southern California Gas Company, the California Energy Commission, the California Air Resource Board and the South Coast Air Quality Management District.

The two-stage, 90 horsepower laboratory-scale super boiler. Photo courtesy Gas Technology Institute
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While steam boilers account for about 35 percent of industrial energy use, some 80 percent of those currently operating are more than 25 years old. Energy efficiency for gas-fired boilers is typically in the 75 to 83 percent range and uncontrolled NOx emissions are about 70 parts per million (ppm). While NOx reduction measures are readily available, they generally increase capital and operating costs and consume additional energy. The goal for the super boiler is to achieve fuel-to-steam efficiency greater than 94 percent (HHV – higher heating value) while maintaining NOx and CO levels below 5 ppm.

Knight said that boilers are the single largest industrial sector energy consumer, particularly in paper, steel, chemical and food industries. Unfortunately, he said, 100-year-old technology is still in use.

In 2005, GTI and Cleaver-Brooks designed and fabricated a 12-million Btu/h (300-horsepower) prototype boiler system that includes the boiler, burner, fan, controls, fuel train and heat-recovery system. To achieve low emissions, researchers are using a patented combustion concept developed by GTI and derived from the forced internal recirculation burner. To achieve high efficiency, a heat-recovery system was developed and patented that employs a transport membrane condenser (TMC), a humidifying air heater (HAH) and ultra-compact dual economizers.

“The prototype super boiler has been laboratory tested with good results,” said Knight. “We have already confirmed that the first 300 horsepower field demonstration unit can achieve greater than 94 percent fuel-to-steam efficiency, which will save the facility more than $50,000 annually in fuel based on $12 per MMBtu.”

The super boiler consists of two main parts: the boiler and the heat recovery system. The boiler uses a split-combustion section (designed to address the low-emission goal), along with enhanced heat transfer innovations from Cleaver-Brooks, to achieve a compact design. The heat recovery system is designed to recover most of the remaining heat in the flue gas coming out from the boiler, including latent heat associated with water vapor, which accounts for two-thirds of the waste heat from natural gas combustion.

Two ultra-compact, high-efficiency economizers are incorporated into the system to recover sensible heat of the total flue-gas stream before it enters the TMC. In that device, water vapor passes selectively through a nanoporous ceramic membrane, giving up additional sensible heat and all of its latent heat to the boiler feed water.

The HAH plays a critical role in managing water temperatures in applications that return and re-use a major portion of the condensate from their steam system. The HAH also conditions the incoming combustion air to further reduce NOx emissions in the boiler.

“A unique feature of this system is the ability to recover flue-gas moisture for use as additional boiler feed water,” said Knight. “This reduces makeup-water requirements and recovers as much waste energy as possible.”

A field demonstration of a second prototype unit is being installed at a juice bottling plant in Ontario, Calif., with an additional demonstration of the heat-recovery system retrofitted to a conventional boiler scheduled for installation at a manufacturing site in mid-2007.