DB Riley Inc., Worcester, MA, is developing a low emission boiler system (LEBS) supercritical boiler that integrates advanced low nitrogen oxide coal burners with furnace air staging and coal reburning, and with a proven U-fired slagging furnace configuration. In pilot-scale tests, the new boiler reduced NOx emission to less than 0.2 pound per million Btu on some American coals compared to 0.8 to 1.2 pounds per million Btu from conventional burners. For the US Department of Energy (DOE) LEBS project, DB Riley drew on the related experiences of the advisory board it formed of major utilities, state energy and economic agencies, and research organizations. The DB Riley engineers set a NOx emission limit of 0.2 pound per million Btu for the firing system alone to minimize the amount of NOx to be treated by the post combustion emissions control system. The U-fired design provides sufficient resident time for reburning to effectively reduce NOx.
Coal is a Domestically plentiful and affordable fuel, accounting for more than 56 percent of this country's electricity, and even higher percentages in developing economies like China and India. However, even coal's staunchest supporters know that it generates nitrogen modes, sulfur dioxide, and particulates, all of which are under increasingly restrictive environmental regulation.
The U.S. Department of Energy is seeking to improve the environmental performance of this venerable form of carbon by sponsoring the development of technologies that will burn coal but generate less pollution. One such effort is under way as part of the DOE's low emission boiler system, or LEBS, program.
DB Riley Inc. of Worcester, Mass., is leading a team developing a LEBS supercritical boiler that integrates advanced low nitrogen oxide coal burners with furnace air staging and coal reburning, and with a proven U-fired slagging furnace configuration. In pilot-scale tests, the new boiler reduced NOx emission to less than 0.2 pound per million Btu on some American coals compared to 0.8 to 1.2 pounds per million Btu from conventional burners.
DB Riley expects the new boiler to operate at net efficiency levels of 42 percent or more, compared to the 33 to 35 percent efficiencies typical of most power plants. Lastly, the new boiler converts virtually all the ash in coal into a vitrified byproduct that can be used in the construction industry as road building material.
DB Riley presented its results at the International Joint Power Generation Conference and Exposition of ASME held at San Francisco Airport in Burlingame, Calif., in July.
"The DOE was primarily driven by environmental considerations when it initiated the LEBS program in the early 1990s," said Larry Ruth, a chemical engineer and senior management and technical advisor at the DOE's Federal Energy Technology Center in Pittsburgh.
"The agency wanted to rethink coal-fired boilers to produce a boiler system that greatly reduced emissions, raised efficiency, and would be affordable to power generators." Ruth was involved in the LEBS program from its inception until recently.
At the heart of the low emission boiler system is the Controlled Combustion Venturi dual air zone burner, which is designed to create a fuel-rich flame that minimizes the formation of nitrogen oxides.
The DOE put out a request for proposals for a low emitting coal-fired boiler, and initially accepted proposals from DB Riley, ABB, and Babcock & Wilcox in 1992. "The agency selected DB Riley's U-fired boiler in 1997 from among the three projects to conduct the final phase of the project: the design, construction, and operation of a LEBS proof-of-concept plant," Ruth recalled.
The emission limits that the DOE set for its LEBS project are 0.1 pound per million Btu of NOx' the same limit for sulfur dioxide, and 0.01 pound per million Btu of particulates.
The DB Riley LEBS design team included, as it does currently, Sargent & Lundy LLC of Chicago, Thermo Electron Corp. of Waltham, Mass., Reaction Engineering International of Salt Lake City, and the University of Utah in Salt Lake City.
The engineers at DB Riley have specialized in controlling NOx emissions from utility boilers for years. Since 1990, the company has sold more than 1,500 of its Controlled Combustion Venturi burners in different configurations to reduce NOx generated by a wide variety of coal-burning utility boilers.
For the DOE LEES project, DB Riley drew on the related experiences of the advisory board it formed of major utilities, state energy and economic agencies, and research organizations. They included American Electric Power in Columbus, Ohio, the Illinois Department of Commerce and Community Affairs, and the Electric Power Research Institute of Palo Alto, Calif.
The DB Riley tea m designed a 400-MW commercial generating unit of the LEES that includes a supercritical Benson boiler fired with a low NOx' slag tap, U-firing system, and a regenerating, copper oxide flue gas desulfurization system with de-NOx capability and high-efficiency particle removal. The commercial generating unit, or CGU, is based on an efficient, supercritical steam cycle originally developed by the Electric Power Research Institute and Sargent&Lundy. This cycle operates at 4,500 pounds per square inch and 1,100°F, with double reheat to 1,1 00F.
"We calculate that the net plant efficiency for a LEES CGU firing high sulfur coal is 42.2 percent based on the higher heating value of the steam cycle," said Terry Ake, staff engineer and chemical engineer at DB Riley.
The generating unit borrows the same U-fired slagging boiler design used more than 50 times by utilities, primarily in Germany. Pulverized coal is fired down into a refractory chamber so that slag forms on the chamber walls and bottom, as well as on the slag screen covering the refractory chamber exit. The slag captures about half the ash in the coal and is continuously removed, or tapped, from the combustion chamber, quenched in a bath, and dewatered by gravity as it is conveyed out of the bath. Hot combustion gases flow up through the slag screen and final heated air is added to complete combustion.
A benefit of the U-firing system is that it can fire a wide variety of coals under a range of different operating conditions. The quenched slag forms an inert, vitreous granular material with only about one-third the volume of fly ash. The firing system recycles the remaining fly ash back to the boiler so that it is captured in slag.
However, a drawback of U-firing systems for pollution control is that high temperatures needed to keep slag flowing also raise NOx formation as high as 1.6 pounds per million Btu. The designers of other U-fired boilers applied air staging techniques and advanced burner designs to reduce the emission level to 0.8 pound per million Btu, eight times the limit set by DOE for the LEBS project. The challenge was designing a low NOx system to satisfy the LEBS goals while operating at the high temperatures required by slagging.
The DB Riley engineers set a NOx emission limit of 0.2 pound per million Btu for the firing system alone to minimize the amount of NOx to be treated by the post combustion emissions control system. They combined advanced air staging and coal reburning techniques with the company's own Controlled Combustion Venturi dual air zone coal burner.
Air staging refers to the introduction of air downstream of the burner. "The U-fired shape of the burner offers a unique opportunity to introduce air well downstream of the burner. This allows for longer, fuel-rich residence time in the combustion chamber, inherently minimizing NOx formation ," Ake explained. Similarly, coal reburning involves injecting a small portion of fuel downstream of the burner. The U-fired design provides sufficient resident time for reburning to effectively reduce NOx.
DB Riley's burners are designed to create a fuel-rich flame core that minimizes the formation of both fuel and thermal NOx ' The main combustion air side of the dual air zone burner is divided into secondary and tertiary air passages. These passages contain swirl vanes that provide spin control, and burner shrouds and dampers that independently control the airflow to each passage.
In addition, a movable shroud diverts secondary and tertiary air from the primary combustion zone, further minimizing NOx ' The secondary airflow is measured by individual burner airflow probes designed by Air Monitor Corp. of Irvine, Calif. These are Pitot-tube type instruments that measure the air flowing through them.
DB Riley divided the burner throat of its dual zone burner to strengthen control of the stoichiometry at the burner discharge, making it more flexible to regulate NOx emissions.
In 1997, DB Riley built a 100 million Btu/hour U-fired boiler system at its Worcester research facility to validate the boiler design. It conducted tests through th e summer of this year. During the same period, the University of Utah conducted parametric tests of the effects of air staging and coal reburning in a 15 million Btu/hour unit to test DB Riley's design. Reaction Engineering International performed computational fluid dynamics simulations to study the parameters of the coal reburning jet designs.
The test facility consists of a single, refractory-lined combustion chamber fired by one roof- mounted burner, a slag tap system, a slag screen, and an up flow section that corresponds to the lower part of the radiant furnace in a commercial scale boiler.
Most of the LEBS tests at the Worcester facility used Illinois No. 5 coal from the Turris mine in Elkhart, Ill. Illinois No. 5 is a high (3 percent) sulfur, high volatile bituminous C coal. Some tests were completed using a medium (1 percent) sulfur, high volatile bitunlinous A coal. In recent tests, a sub-bituminous, Powder River Basin coal was fired in the facility.
The Worcester researchers tested several variations of their coal nozzle tips to achieve the requisite low NOx emission and both slag production and carbon combustion goals. They also modified the test burner to simulate the burners that are installed in commercially opera ting U-fired slagging boilers. This modified, or baseline, burner provided a comparison between the performance of the LEBS design and conve ntional Ufired boilers. In addition, videotapes were made of the burner flame in each case to compare the flame shape of baseline burner and Controlled C ombustion Venturi burner operation. To tal excess air in both cases was maintained at 15 percent.
The results bore out the efficacy of the LEBS system. For example, when the more turbulent baseline burner produced in excess of 0.65 pound of NOx per million Btu, the CCV burner achieved 0.2 pound of NOx per million Btu. The videos showed that the baseline burner produced a wide, detached flame with rapid nlixing of the burner air and coal, while the CCV produced a narrower, well-attached flame.
DB Riley lowered the NOx emissions of the CCV burner by using extended air staging and by introducing coal reburning. "We preferred coal reburning because it can be controlled separately from the burner stoichiometry, which optimizes slag production," Ake said. Coal reburning provides the same amount of NOx reduction as air staging, but with a smaller portion of the furnace at substoichio-metric conditions.
Tests showed that over half the coal ash was converted into slag, minimizing the overall furnace heat loss due to unburned carbon, averaging less than 1 percent. Unburned carbon content can be further reduced in a commercial scale LEBS by recycling the fly ash into the firing chamber.
Researchers found that levels of heavy metals in the furnace slag that might leach into groundwater, including lead, arsenic, and mercury, were well below the 1990 Resource Conservation Recovery Act toxicity limits. This means the slag can be safely used for roadfill without further treatment.
The next step for DB Riley is to build a proof-of-concept facility to demonstrate its design on a commercial scale.
"We have already secured DOE and State of Illinois funding ($31 million and $25 million, respectively) for the proof-of-concept plant, and are seeking private financing," said Robert Lisauskas, an ASME member and director of research and development at DB Riley.
Once private funding is secured, DB Riley will construct a U-fired boiler, and a contractor will be selected to build the remainder of a commercial scale LEBS plant adjacent to the Turris Coal Co. mine in Elkhart, Ill. The unit will consume Illinois No. 5 coal to generate 80 MW of electricity in single-cycle operation. Four DB Riley 200 million Btu/hr CCV dual air zone burners will fire the coal. Coal and overfire air will be injected in the up-flow section of the furnace along the slag screen. Slag will be continuously tapped.
The major challenges for the new plant's designers include understanding the interaction of the multiple burner system. "We must also avoid corrosion of the furnace walls caused by burning high sulfur coals at high temperatures," Lisauskas noted. "Then, too, we must maintain precise control of the combustion process within different physical dimensions than our research scale LEBS."
He added that his colleagues at the University of Utah are providing computer modeling of the commercial plant to assist in controlling the combustion process, as they did during the laboratory phase of the LEBS project.