Fuel flexibility and a smaller carbon footprint are behind renewed interest in circulating fluidized bed technology
By Steve Blankinship, Associate Editor
Back in the 1980s, circulating fluidized bed (CFB) technology was a little like integrated gasification combined cycle (IGCC) today: coal generation’s technology of the future. Back then, CFB was touted as a means of using abundant supplies of coaland other fuelsin a more environmentally attractive way than was possible by burning pulverized coal in a conventional boiler.
CFB never took the industrial boiler or power generation sectors by storm. Since its reemergence, however, CFB has steadily provided a viable and valuable alternative to pulverized coal (PC) in the United States and around the world. Today, as conventional coal plants face intensified opposition, there’s reason to believe that CFB can help the United States, China and other countries better use coal and coal-related materials to fuel industrial processes and generate electricity.
A key advantage of CFB technology is that pollution control is built right into the combustion process. By adding low-cost limestone into the CFB, SOx is captured and removed right at the point where it is formed as the fuel burns. The CFB’s low combustion temperature (about 1,000 F less than a conventional PC peak temperature) minimizes NOx formulation. And by injecting ammonia into the CFB, NOx can be further reduced by half. CFB was promoted in the 1980s as a solution to deal with sulfur dioxide (SO2), nitrous oxides (NOx) and other pollutants produced by burning coal. But emission technology improvements in the PC plant sector, along with CFB’s relatively small plant size, limited units to sites where conditions were just right. That mainly meant the availability of difficult-to-use fuels (often called “opportunity fuels”) such as waste from bituminous coal mines (gob or boney), anthracite coal mine waste (culm) or petroleum coke (petcoke).
Thermal spray processing of CFB heat transfer panels. Photo courtesy of Foster Wheeler Global Power Group.
Low-value coal waste discarded by mining operations provided one major incentive to build CFBs. For decades, mountains of gob/boney and culm piled up in Pennsylvania, West Virginia and other states. That waste coal contains a significant amount of heating valuegenerally ranging from 5,000 to 6,000 Btu/lb. That’s not far below the heating value of low-grade lignite, which supplies power in North Dakota, Texas, Mississippi and other places around the world.
Waste coal, high sulfur coal and even discarded tires play to the strengths of CFB. That’s because CFB boilers can use these fuels while producing low emissions. Furthermore, CFB can use a wide variety of opportunity fuels almost interchangeably without major, if any, plant modifications.
How It Works
Conventional coal plants use pulverized coal, ground to the consistency of facial powder, and burn it at temperatures between 2,200 F to 2,400 F. In contrast, CFBs use coal in chunks about 3/8-inch in size mixed with limestone and burn it at lower temperatures1,500 F to 1,650 F. Air is blown into the boiler to suspend, or fluidize, the mixture. Heat in the boiler converts the limestone to lime that absorbs SO2, removing most of it in the furnace.
A CFB’s lower burn temperature produces less NOx. Overall, the CFB process removes some 98 percent of SO2 and produces roughly half the NOx compared with conventional coal plants. A cyclone turbulent air system returns ash and unburned fuel to the boiler to be burned again, making the combustion more thorough and reducing the volume of pollutants in the flue gas. CFB reduces or eliminates entirely equipment that conventional coal plants need to capture SO2 and NOx.
The 460 MW Lagisza plant, one of the largest single boiler CFBs in the world, will enter commercial operation next year. Photo courtesy of Foster Wheeler.
Early-generation CFB technology emerged in the late 1970s and early 1980s as bubbling fluidized bed technology using the same boiler floor grid as circulating designs. Ash or river sand was introduced to create a fluidized media that produced an active thermal zone within the first 15 feet of the combustor floor. But the media did not circulate out of the boiler and back into it for re-burning.
With CFB, solids are circulated all the way to the top of the combustor, into the cyclone return system, then back to the fluidized bed combustion chamber. The cloud of circulating solids creates a “flywheel” effect that generates massive thermal inertia. Whether the unit is at full or partial load, around 2 percent of the circulating mass is consumed as fuel.
That means a plant is getting a lot of benefit from the latent heat and the solids that are circulating, said Jim Utt, vice president of utility steam generators for Foster Wheeler, a CFB technology pioneer.
The earliest CFB units tended to be less than 100 MW. Several CFB plants in the 50 MW size range sprang up in the waste coal regions of Pennsylvania, Ohio and West Virginia and began “gobbling gob” and accumulated culm. The units’ small size was due to the fact that the technology was evolving. Plus, no infrastructure existed to assure a steady stream of fuel over many decades. Such uncertainties limited investment to small units.
Foster Wheeler worked to increase unit size while adding advanced features to its CFB offerings. One of the earliest units to reach utility-scale application came in 1987 with the 110 MW Tri-State Nucla power project in Colorado. By 2001, CFBs had grown to utility scale when two 300 MW units went into service for Jacksonville Electric Authority in Florida. To date, Foster Wheeler has supplied more than 300 fluidized bed units to industrial and utility customers worldwide.
“Now that the technology has broken into the 300 MW size range, there’s been a real increase in the number of projects,” said Scott Darling, CFB product director for Alstom Power. And not just in the United States. China is seeing an explosion in the number of utility-size CFBs. Today, Alstom, Foster Wheeler, Babcock & Wilcox and Aker Kvaener are major CFB boiler manufacturers.
CFB’s advantage can be especially dramatic in repowering projects. SOx and particulate emissions often can be cut by more than 90 percent and NOx emissions by more than 50 percent. CO2 emissions are often cut by 25 percent or more due to boiler and plant efficiency improvements when older equipment is replaced with CFB units.
Between 1998 and 2004, Foster Wheeler delivered six CFB units totaling 1,500 MW for one of the largest CFB repowering projects to date: the Turów plant in Poland. Over six years, six Russian-designed, high-emissions pulverized coal units were replaced with CFB units. The last three units were of a more advanced “compact” design, a modular concept that became the basis for scaling Foster Wheeler’s CFB design. Whereas Turów 1, 2 and 3 used a conventional cyclone to send ash and other solids back into the boiler for re-burning, units 4, 5 and 6 used a compact cyclone that allows placement of cyclones on the front and rear walls of the boiler.
That set the stage for building one of the largest single-boiler CFB power plants in the world, the 460 MW Lagisza plant, which is nearing completion in southwest Poland. Owned by Poland’s largest electric utility, Południowy Koncern Energetyczny, Lagisza is billed as the world’s first supercritical CFB. Designed to fire bituminous coal as its primary fuel, Lagisza will enter its commissioning phase this year with commercial operation scheduled for early 2009. Foster Wheeler has a 600 MW version of the design and is looking at scaling it up to 800 MW.
Lagisza’s supercritical steam parameters will include an operating pressure of almost 4,000 psia and temperatures of 1,050 F to 1,075 F. The backend gas cooling system will cool the flue gas to 185 F, providing high cycle efficiency. Foster Wheeler’s Jim Utt said the supercritical process allows the combustion temperature to be the same for both supercritical and subcritical steam conditions, thus preserving the advantage that low heat confers in producing low NOx. That feat is achieved with a heat exchanger submerged in the return ash, which transfers heat from the fluidized bed ash to the superheat and reheat surfaces. It allows reheat and superheat temperatures to reach nearly 1,100 F, a temperature that can be achieved in the Foster Wheeler supercritical design but is a bit higher than what will be used at Lagisza. The 460 MW unit will replace power blocks built in the 1960s.
As with pulverized coal, potential carbon capture for CFBs could be accomplished post-combustion. But the nature of CFB seems to favor capturing CO2 during the combustion process.
“With our oxy-firing technology, primary air supplied to the fluidizing grid would contain a higher percentage of oxygen than the 21 percent contained in regular air,” said Utt. “Nitrogen is removed with an air separation unit, which alone produces a 75 percent pure stream of CO2 as exhaust. Our CFB technology is oxyfuel-ready today and allows 100 percent CO2 capture.” Even without carbon capture, CFB technology provides a net CO2 reduction through efficiency gains.
Trucks haul coal waste to Reliant Energy’s Seward plant as other trucks haul limestone-rich ash out of the Pennsylvania plant to neutralize alkaline soils produced by past coal mining. Photo courtesy of Reliant Energy
“Any CFB, PC or IGCC burning coal or petcoke is going to face CO2 issues,” said Nancy Mohn, director of marketing strategy for Alstom Power. “That’s why in the medium- to long-term time frame, I think you’ll see CFBs in either oxy-firing mode or with CO2 post-combustion capture.” Alstom is testing and developing oxy-firing and post-combustion carbon capture strategies on both PC and CFB boilers.
Consider the Opportunities
The ability to burn so-called “opportunity fuels” makes CFB something of an opportunity technology. Those opportunities include fuel diversity that extends to renewable fuel sources. And using renewable energy resources often can create the opportunity to generate a secondary revenue stream from renewable energy credits.
There are also 300 MW CFB plants being built to produce 600 MW by using two CFB boilers on one turbine generator (2 X 1). Utt said such plants are being designed to use three different fuels, any of which can supply all or some of the plant’s fuel. And typically, the portfolio includes a renewable fuel component
Mohn said that CFBs are amenable to burning higher percentages of biomass than pulverized coal units, which are limited to co-firing about 10 percent biomass. CFBs can run on 100 percent biomass.
Wisconsin-based Alliant is one utility that hopes to take advantage of CFB fuel flexibility. In December, Wisconsin regulators approved an application by Alliant subsidiary Wisconsin Power and Light to build a 300 MW CFB at the Nelson Dewey plant in Cassville, Wis. It would use several kinds of coal as well as renewables including switch grass, corn stalks (stover) and wood. Washington Group International will provide engineering, procurement and construction services for the project.
Among the most ambitious and successful CFB projects is the Seward Station 80 miles east of Pittsburgh. Owned by Reliant Energy, Seward consists of two Alstom CFB boilers feeding a single turbine generator to produce 521 MW of power solely with waste coal. Created in partnership with the Pennsylvania Department of Environmental Protection, Seward is among the largest waste coal-fired generating plants in the world and operates in the PJM merchant market.
Reliant acquired the 82-year-old, 200 MW Seward plantoriginally built and operated by GPUfrom Sithe Energy in 2000. The old unit was retired in 2003. The CFB unit produces two and a half times as much electricity as the plant it replaced, while reducing emissions.
Environmental remediation started with construction as 2 million tons of waste coal from the plant’s old mine-mouth site was used to build the new foundation. Once it entered service, Seward began consuming 3.5 million tons of coal a year; consumption eventually could grow to 100 million tons of waste coal near the plant. Heating value of the bituminous gob ranges from 5,000 to 6,000 Btu/lb. The waste coal is delivered to the plant by haulers who leave the plant loaded with limestone-laden fly ash from Seward’s CFB combustion process. The material neutralizes acidic soils created by past mining operations. The new Seward unit represents a decrease of about 7 percent of CO2 emissions per net megawatt-hour compared with the unit it replaced, as well as lower levels of SO2 and NOx. Mercury removal is 99.94 percent.
Large deposits of coal waste tailings will fuel Seward. Photo courtesy of Reliant Energy
Seward is one of the “very highest dispatched plants in the PJM,” said Dave Freysinger, senior vice president of generation operations for Reliant. Last summer, it operated at a 96 percent capacity factor and was in the money nearly all the time. That meant grid power prices were nearly always higher than Seward’s cost of generation. “Seward is one of the first fossil plants on PJM’s dispatch stack,” said Freysinger. Only hydro and nuclear dispatch higher.
East Kentucky Power Cooperative (EKPC) represents another CFB success story. The utility’s 268 MW Gilbert 3 CFB went into operation in 2005. The essentially-identical Spurlock 4 CFB goes into operation this year. A third unit, Smith 1, is planned for service in 2010. In addition to coal, the new Gilbert unit can burn more than 1 million tires a year and 150,000 tons of sawdust and other wood products. All three EKPC units can use “raw” coal, meaning coal delivered to the plant without having been washed and prepped at the mine. That lowers the fuel price since washing can be an expensive process. Pennsylvania Crusher, one of the largest supplier of coal handling equipment to the CFB sector, said it sees an increasing trend of new CFB projects specifying raw coal as their primary fuel. (See Sidebar, page 38.)
Entergy’s Little Gypsy plant near New Orleans is currently fired by natural gas, but plans call for a repowering to use petcoke. Little Gypsy will also be able to co-fire using various sources of biomass. The plant also will be able to fire high sulfur Illinois Basin bituminous, delivered almost straight from the source to the plant on the Mississippi River.
Lamar, Colo.-based Arkansas Power Authority is converting a plant from natural gas to coal. Babcock & Wilcox will design and deliver a 360,000 lb/hr, 38.5 MW CFB boiler that will burn Powder River Basin coal as part of the Authority’s plan to repower its Lamar Light and Power plant. The unit will produce steam for an existing 25 MW turbine generator and a new 18 MW turbine generator. Included in the $20.5 million contract are coal feeders, limestone feed ID fans, a multicyclone dust collector, an economizer, tubular air heater, baghouse for particulate control, stack, structural steel, boiler controls and service start-up support.
The Authority is replacing the gas-fired boiler due to unfavorable economics. B&W design engineer Mike Maryamchik said controllability and low maintenance are the key features of B&W’s CFB. “Its low and uniform exit furnace velocity and minimal use of refractory result in high availability and low maintenance,” he said. “The two-stage solids collection system, comprised of U-beam and multicyclone particle separators for efficient particle collection and controllable solids recycle, enables precise furnace temperature control as well as high combustion efficiency and limestone utilization. Combined with staged combustion, these features limit NOx emissions.”
CFB’s ability to take up to 20 percent biomass on a 600 MW supercritical CFB represents a big net CO2 reduction, said Foster Wheeler’s Jim Utt. “From a long-term viewpoint,” he said, “that’s where we see the future of CFB.”
Handling CFB Fuel
One CFB asset is its ability to burn a variety of fuels. But such boiler flexibility requires far more attention to fuel handling and crushing equipment than for a plant that will burn the same fuel over many decades.
Pennsylvania Crusher has supplied about half of all the fuel handling systems for the world’s current fleet of CFBs and has picked up a share of CFBs currently being built or planned. Lee Doyer, vice president of sales and marketing, explained how PennCrusher plans fuel handling systems for plants that may use a variety of fuels over their lifetime. That includes using multiple fuels at the same time. “We get many specifications that say ‘coal or gob.’ That means we have to handle both,” he said. And his colleagues are starting to see more coal-only jobs, including more orders for raw coalin distinct contrast to the gob and petcoke jobs the company has traditionally seen.
To handle the variables, PennCrusher offers several kinds of crushers for fluidized bed boilers. Because the coal enters the boiler in chunks, no pulverizers are needed. Forestry products go through chippers rather than crushers and PennCrusher sister company Jeffery Specialty Equipment makes them. Wood chips can be blended with coal or petcoke before entering the boiler. A New Hampshire plant uses wood chips as its primary fuel “sweetened” with a small amount of coal.
“Gob is more abrasive than petcoke so the handling system has to be tougher too,” said Doyer. “Gob has hard rock in it and anthracite culm has even harder rock. It’s the worst stuff.”
With the trend of modern CFBs burning more kinds of fuels comes the need for better planning of fuel handling systems. “For gob, we use reversal hammer mills for smaller plants,” he said. Larger coals need multiple mills to handle the large volumes. PennCrusher has recently used reversible impact crushers and supplied them to CLECO’s 660 MW Rodemacher petcoke-fired CFB in Louisiana.SB