Cfbc vs Pulverrised Coal Boilers- Good Article
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Why Build a Circulating Fluidized Bed Boiler to Generate Steam and Electric Power S. Kavidass G.L. Anderson G.S. Norton, Jr. The Babcock & Wilcox Company Barberton, Ohio, U.S.A. BR-1708
Presented to: POWER-GEN Asia 2000 September 20-22, 2000 Bangkok, Thailand
Abstract In Asia, demand for electric power continues to rise steeply due to population growth, economic development, and progressive substitution of alternate technology with clean forms of energy generation. Atmospheric circulating fluidized bed (CFB) technology has emerged as an environmentally acceptable technology for burning a wide range of solid fuels to generate steam and electricity power. CFB, although less than 20 years old, is a mature technology with more than 400 CFB boilers in operation worldwide, ranging from 5 MWe to 250 MWe. Electric utilities and Independent Power Producers must now select a technology that will utilize a wide range of low-cost solid fuels, reduce emissions, reduce life cycle costs, and provide reliable steam generation for electric power generation. Therefore, CFB is often the preferred technology. Even though pulverized coal (PC) fired boilers continue to play a major role worldwide, they have inherent issues such as fuel inflexibility, environmental concerns and higher maintenance costs. This paper discusses the benefits of CFB boilers for utility and industrial applications. Specific emphasis is given to B&W’s Internal Recirculation CFB (IR-CFB) technology, CFB technology comparisons, PC vs. CFB technology, emissions benefits, and economics including maintenance cost and boiler reliability.
Introduction Babcock & Wilcox (B&W) is a leading global supplier of industrial/utility boilers and has supplied more than 700 units totaling more than 270,000 MWe. Many of B&W’s CFB boiler design features have been adapted from vast experience designing and building boilers of all types and sizes for industrial and
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electric utility applications. B&W’s design is an inherently compact, distinctive internal recirculation fluidized bed (IR-CFB) boiler featuring U-Beam solids separators. The furnace and convection pass of the IR-CFB boiler are within a single, gas–tight membrane enclosure as commonly found in Pulverized Coal (PC) fired boilers. This CFB technology has been successfully introduced in the global market. To date, B&W, including B&W joint ventures and licensee companies, has sold 16 CFB boilers worldwide, shown in Table 1. B&W offers IR-CFB boilers up to 175 MW e, both reheat and non-reheat, with full commercial guarantees and warranties. The IR-CFB boiler is simple in configuration and compact, requires a smaller boiler foot print, has minimal refractory, requires low maintenance, features quick startup, and provides high availability. The modern way of burning solid fuels requires fuel flexibility and reliable technology, plus good combustion efficiency with low emissions. CFB technology is well suited for a wide range of sold fuels. CFB technology is proven, mature and competitive.
What is CFB Technology? CFB technology utilizes the fluidized bed principle in which crushed (6 –12 mm x 0 size) fuel and limestone are injected into the furnace or combustor. The particles are suspended in a stream of upwardly flowing air (60-70% of the total air) which enters the bottom of the furnace through air distribution nozzles. The balance of combustion air is admitted above the bottom of the furnace as secondary air. While combustion takes place at 840-900 C, the fine particles (100 micron)
Single-stage (100% efficiency for particles of d>100 micron)
*Recycling finer particles increases furnace heat transfer rate, improves combustion efficiency and limestone utilization. Upper Furnace Density, (kg/m 3)
Furnace Temperature Control
Desired temperature can be maintained within +/-5C interval for wide range of fuels and operating conditions by adjusting secondary solids recycle rate.
Temperature is predetermined by furnace and heat exchanger design along with fuel and limestone properties/sizing.
Temperture is predetermined by furnace and heat exchanger design along with fuel and limestone properties/sizing.
Lower bed temperature is controlled by adjusting cold cyclone ash recycle rate. Temperature span across furnace height is up to 100C.
Boiler Turndown Without Auxiliary Fuel
3.5 : 1
3.5 : 1
3.5 : 1
Lower furnace, U-beam zone enclosure walls
Lower furnace, cyclone, recycle loop (5-10 times more than B&W CFB)
Lower furnace, cyclone, recycle loop (3-5 times more than B&W CFB)
Entire furnace, cyclone (3-4 times more than B&W CFB)
Hot-Temperature Expansion Joints
3-5 per cyclone
Number varies with arrangement
Furnace Velocity, m/s
Furnace Exit Velocity, m/s
Required for J-valves
Required for J-valves
Required for siphons
Refractory: Thickness, mm Covered Areas
Total Pressure Drop Across 1.0 Solids Separator(s), kPa (U-beams + MDC) Aux. Power Consumption
Economics of CFB Technology
Reliability of CFB Technology
CFB technology can burn a wide range of low cost solid fuels and competes well with oil/gas fired plants. The decisionmakers often ask, “What are capital and operating costs and benefits of a CFB boiler?” The experience in Europe and North America suggests that for a sulfur fuel (>0.5%S) and less than 150 MW, a CFB boiler has 8-15 percent lower capital costs as well as 5-10% lower operating costs than a PC-fired boiler because of the FGD system. In general, CFB-based power plants provide low emissions control costs and low O&M costs, which lead to lower life cycle costs. In the end, owner profit margin increases and payback period improves as shown in Table 5. Costs not included in Table 5 are items such as land, project development, permitting, escalation, taxes and owner’s costs, since these costs are common for both PC and CFB-based power plants.
CFB boiler design is simple and proven, compared to other technologies. Experience indicates that operating and maintenance costs are relatively lower than PC-fired boilers because of the ability to burn lower rank fuels, thus reducing fuel costescalation uncertainty. Since maintenance areas are very minimal in the CFB boiler, the availability of the boiler is relatively higher. The CFB design allows emissions reduction without significant capital cost, since SO2 and NO x removal are inherent within the combustion process.
Conclusion Even though a number of competing technologies are available in the market for steam and electric power generation, CFB is an excellent choice due to its fuel flexibility, wider
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turndown without support oil/gas, superior environmental performance, lower operating and maintenance costs, and safe, reliable and simple boiler operation. The B&W IR-CFB boiler
design offers compact, superior performance due to two-stage solids separation, and is cost effective for multiple fuel firing in both PC retrofit and greenfield applications.
Table 5 Typical Economic Evaluation for 125 MW CFB vs. PC with FGD-Based Power Plant Description Unit size (gross) Unit size (net) EPC price Capacity factor Coal heating value Coal cost Limestone cost Ash disposal cost Annual O&M cost Percent financed Debt payment term Interest rate Discount rate Tariff to yield 20 years, 10% ROI Payback period, at $45/MWh Notes:
Units MW e MW e $ 000 % kcal/kg $/MT $/MT $/MT $ 000 % years % % $/MWh years
CFB-Based 125 112.5 120,000 85 5,550 35 8 10 3,000 100 10 9 10 39.50 6.8
PC w/ FGD-Based 125 112 134,375 85 5,550 35 8 10 3,300 100 10 9 10 41.60 7.6
1. The above analysis does not include escalation or taxes. 2. Heat rate and construction period are assumed same for both. 3. Land, project development, permitting fee, owner’s costs are not included.
References 1. Kavidass S., Alexander K.C., “Design Consideration of B&W IR-CFB Boilers,” Power-Gen Americas ’95, Anaheim, California, U.S.A., December 5-7, 1995. 2. Kavidass S., “Why CFB is perfect for India,” Powerline Magazine, India, February 1999.
3. Kavidass S., Walker D.J., Norton G.S., “IR-CFB bringing new life to old coal-fired boilers,” Modern Power Systems, February 2000. 4. Maryamchik M., Wietzke D.L.,“B&W IR-CFB Boiler Operating Experience Update and Design,” Power-Gen International ’99, New Orleans, Louisiana, U.S.A., December 2, 1999.
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