PFBC demos operate successfully
Advances in high-pressure fluidized bed combustion are the leading development in this interesting new solid-fuel combustion technology
By Robert Smock
News of successful debugging of the first-generation pressurized fluidized bed combustion (PFBC) units was the highlight of the conference on fluidized bed combustion in Atlanta, Ga., USA, sponsored by the US Electric Power Research Institute (EPRI).
Interest in new PFBC plants appears to be largely in Europe which has most of the operating units and most of the plans for new ones. Plans in the United States for new projects are being slowed and there has been little interest in the technology in Asia, with the exception of Japan.
Progress on atmospheric pressure fluidized bed combus- tion (AFBC) was also discussed at the conference. The major AFBC development is success- ful operation of new, large circulating bed units and construction of even larger ones.
Developers of fluidized bed combus- tion see it as the best new technology for burning solid fuels such as coal. AFBC is already firmly rooted in the power industry in unit sizes up to approximately 150 MW.
PFBC is enter-ing the commercial phase and is seen as the primary alternative to integrated gasification combined-cycle technology as a high-efficiency, gas-turbine based, coal- fueled technology for the 21st century.
EPRI`s John Wheeldon, a PFBC veteran, explained that PFBC is attractive because it can convert coal to electricity at relatively high efficiency with low emissions. PFBC heat rates of 8,200 are attainable, he claimed, at 3,500 psi steam conditions (8,500 at 2,400 psi). Wheeldon said that NOx emissions as low as 0.05 lb per million Btu have been demonstrated at one of the operating PFBC demonstration plants.
There are four operating PFBC demos, two in Europe, one in Japan and one in the United States. All are approximately 80-MW units and are based on the ABB Carbon design. Two more projects based on this design are under active development–a 60-MW combined-heat-and-power unit in the Czech Republic and a 350-MW unit being designed for Kyushu Electric in Japan.
Progress at the Swedish unit
The first of the ABB 80-MW PFBC demos began operating in 1990 in Vartran, Sweden. Charlotte Thedeen, Stockholm Energi AB, which operates the Vartran unit, told the EPRI conference that there have been some initial operating problems, but they`re being solved and availability is rising. The unit is used for district heating and availability is defined as the achieved district heating load divided by the desired district heating load. Availability of the PFBC unit in the 1993/1994 heating season (October to April) was 82 percent, according to Thedeen, up from 50 percent the first season it operated. Goal for the current season is to exceed 90 percent.
The Vartran plant has two PFBC boilers that, combined, produce 135 MW of electricity and 220 MW of heat. They burn Polish coal (0.6 percent sulfur) in a bed of Swedish dolomite.
NOx emission is reduced 80 percent by injecting ammonia in the freeboard area of the bed and by a selective catalytic reduction system on the back end of the Vartran unit. Emissions of 20 milligram/megajoule (mg/MJ) (0.05 lb per mmBtu) have been achieved at full load, said Thedeen.
SO2 emissions at Vartran has been reduced 90 percent at a calcium to sulfur ratio of 2.0. Thedeen said annual average SO2 emission is 14 mg/MJ (0.035 lb/mmBtu).
Thedeen said that problems are still encountered when changing coal or limestone. She said that problems with erosion in the bed tube bundle remain to be solved.
Sven Jansson, ABB Carbon chief engineer in charge of PFBC, provided an update on the second of the demos to operate, the 80-MW unit at Escatron, Spain, operated by Endesa, the Spanish state utility. It was built as a small-scale precursor to larger, commercial-size, coal-fired generating units, Jansson explained. It has operated for approximately 10,000 hours during 1991, 1992 and 1993. The main operating problem, said Jansson, was plugging of the cyclone outlet piping in 1993. Average availability at Escatron in 1993 was 56.5 percent. The unit was scheduled to complete a 5,000-hour availability test in 1994.
SO2 emission reduction during 1993 operations was 95 percent and NOx emissions, with no ammonia injection, were 300 mg/normal cubic meter. Particulate emissions were less than 10 mg/normal cubic meter. The plant has an electrostatic precipitator.
Jansson said there has been no boiler tube erosion at Escatron. He said low-pressure-turbine blade cracking in the gas turbine found at all the ABB demos has been solved by a redesign.
There has been some work on a feasibility study for a large, 350-MW PFBC plant, and Endesa is considering whether to move ahead.
The remaining European PFBC project is a district heating and power project similar to Vartran that is being designed by ABB for the city of Ostrava in the Czech Republic. It is scheduled to be in commercial operation in 1997.
“ABB Carbon is working on a large number of new potential projects in many different parts of the world,” said Jansson, adding, “Utility interest in this power plant concept is increasing dramatically now that the first plants are starting to demonstrate reliable operation.”
PFBC projects in Japan
The most recent of the PFBC demos started operating at the Wakamatsu site of Japan`s Electric Power Development Co. (EPDC) in October, 1993.
EPDC`s Masao Fujita told the EPRI conference that the unit ran into several problems with plugging in the coal preparation and coal feed systems, and the unit was shut down for debugging in February, 1994.
Fujita noted sev-eral differences between the Japanese unit and the earlier demos, including use of a higher pressure steam turbine, a reheat cycle in the boiler and use of a full-scale, ceramic-filter, hot gas cleanup system between the combustor and the gas turbine. These innovations will do much to advance the promise of PFBC technology.
Conceptual design work is under way on a 350-MW PFBC unit at EPDC. Fujita said negotiations are proceeding with the government of Yokohama to finance and build the unit, but nothing is definite yet.
US PFBC progress
Focal point of PFBC development in the United States is the federal government`s Department of Energy and its Clean Coal Technology Program. Program head Larry Carpenter told the EPRI conference that six of the 46 active CCT projects involve new or retrofit large-scale fluidized bed combustion systems, two atmospheric pressure and four high pressure.
Carpenter`s concept of a development cycle for the technology is shown in Figure 1. He expects to see a large, first-generation project under way in the United States by the year 2000. The ultimate goal is a clean, coal-fired generation system with a thermal efficiency well above 50 percent.
The showpiece of the US PFBC program is the 70-MW demonstration project at the Tidd Station of American Electric Power Co. This is one of the four operational demonstrations of the ABB Carbon technology. The boiler was built by a joint venture of Babcock & Wilcox and ABB.
“The Tidd project,” said DOE`s Carpenter, “is the first large-scale demonstration of a PFBC system in the United States. The project`s original goal was to demonstrate that PFBC was suitable for use in the commercial marketplace. The specific objective was to show, in a large-scale setting, that PFBC combined-cycle technology is a cost-effective, reliable and environmentally superior alternative to conventional coal-fired electric power generation options.”
Tidd has completed 7,000 hours of testing during the past three years and encountered the same sort of initial problems as the other demonstration units. “It`s running fine now,” said Carpenter, adding, “They`ve worked the bugs out.” Availability, he said, is at the 40 percent to 45 percent level. A fourth, and possibly final, year of testing is under way that will conclude in February, 1995.
David Bauer, AEP senior engineer, described the operational problems at Tidd in some detail and concluded that all the problems have been resolved except for a bed sintering problem, adding, “We`re starting to get a handle on that.” NOx emission, said Bauer, was better than the design level at 0.17 lb per mmBtu. Bauer noted that Tidd has no normal plant redundancy and was not designed for commercial operation.
AEP is still evaluating whether or not to proceed with the next step in their PFBC program with DOE: the 340-MW Mountaineer project.
A third US PFBC project is also still on the drawing boards. It is intended to be a 70-MW plant at Midwest Power Co.`s Des Moines, Iowa, station. The PFBC portion will be a circulating bed design from Pyropower. The original plan, explained Midwest Power`s Paul Licht, was to repower an old, 70-MW generating unit with the new PFBC boiler. Unfortunately, the economics currently look less attractive to Midwest Power than when the project was launched several years ago and it is currently under consideration with nothing definite.
The fourth and newest US PFBC project was recently awarded funding under the DOE clean coal technology program. It will be located at a factory operated by Air Products and Chemicals Co. in Calvert City, Ky. It will be based on what Carpenter calls a “second generation” PFBC design by Foster Wheeler Corp. The Four Rivers project will produce approximately 70 MW of electricity and 300,000 lb/hr of process steam.
Another interesting feature of the US PFBC program is the demonstration of hot gas cleanup technology at Tidd. The ability to remove particulate matter from the gas in one step, prior to combustion, and meet ultra-strict emission limits is essential to commercial adoption of PFBC and remains the major, undemonstrated technology associated with PFBC.
A commercial sized test module of a hot gas filter using ceramic tubes was installed at Tidd by Westinghouse to clean one-seventh of the combustion gas flow. The filter operates at a maximum temperature of 1,670 F and 164 pounds per square inch pressure. Westinghouse`s Tom Lippert told the conference, “We`re making a lot of progress, but there are still some problems to be solved.” A successful 1,300-hour run was completed in 1994.
As mentioned, the technology is also being developed and demonstrated at commercial scale in Japan at the Wakamatsu site.
Atmospheric pressure update
EPRI`s Ellen Petrill provided the conference with an overview of AFBC. She said there are more than 600 operating fluidized bed combustion units operating throughout the world (counting units larger than 20,000 lb/hr of steam). Of those, 98 percent are AFBC, 23 are percent bubbling bed and 75 are percent circulating bed.
The largest concentration of AFBC units, which is in North America, has 170 units totaling 6 GW in capacity. The installation pattern in North America is shown in Figure 2. Independent power producers have built approximately 130 of the 170 North American units. Less than 10 percent of the fluidized bed combustion capacity in the United States is owned by utilities.
AFBC, said Petrill, is a mature technology being offered in unit sizes up to 400 MW.