英文文献翻译
1；文献原文
(1)Coal-Fired, Circulating Fluidized-Bed Boilers in Action
Electric utilities burning coal continueto search for cost-effective ways toincrease electricity generation whilestill meeting increasingly stringent emissionstandards. Over the last several years,fluidized-bed combustion has emerged as aviable option. One company with significant experience in the area of industrial andutility boiler design has developed a compact atmospheric internal recirculation circulating fluidized-bed (IR-CFB) boiler forcommercial application.
Performance data for Babcock & WilcoxIR-CFB installations at Southern IllinoisUniversity (SIU) and an industrial facility inIndia are reported in a recent paper preparedby S. Kavidass and Mikhail Maryamchik ofBabcock & Wilcox (Barberton, Ohio),C. Price of SIU (Carbondale, Illinois), andA. Mandal of Kanoria Chemicals & Industries Ltd. (Renukoot, India). The paper, entitled ―B&W’s IR-CFB Coal-Fired BoilerOperating Experiences,‖ was presented atthe Fifteenth Annual International Pittsburgh Coal Conference, held September 14–18, 1998 inPittsburgh, Pennsylvania.
IR-CFB Boiler Design
In a fluidized-bed boiler, crushed coal isintroduced into a furnace containing a bedof either an inert material (like sand orcrushed limestone) or dolomite. Pressurizedair, fed into the bottom of the furnace, blowsupward through the bed and causes the coaland bed materials to ―fluidize‖ in a highlyturbulent, suspended state. Figure 1 profilesa typical IR-CFB furnace, demonstrating thechange in bed density with increasingheight. The turbulence of the fluidized-bedsystem allows prolonged contact betweenthe air and the particles of coal, resulting inmore complete combustion at a lower temperature than older systems (which reducesnitrogen oxides). If sorbent material such aslimestone is used as bed material, emissionsof sulfur dioxide are likewise reduced due toconversion to calcium sulfate. Further, because combustion occurs at a lower temperature, the process is relatively insensitiveto the type of fuel burned. This allows theuse of alternative fuels such as coal waste,biomass fuels, petroleum coke, and otherlower British thermal unit (Btu) material.
A circulating fluidized bed captures thesolids carried out of the furnace and returnsthem to the primary combustion chamber.This recycling feature increases the fuel residence time in the furnace, which increasescombustion efficiency. The Babcock &Wilcox IR-CFB boiler provides two stagesof solids recirculation, maximizing fuelburnout and sulfur capture. Also, design
velocities at the furnace exit are relativelylow, which significantly reduces erosion ofthe upper furnace and primary solids separator.
Unique Design Features
One of the features of Babcock & Wilcox’s IR-CFB design is the use of a U-beamsolids separation system. As shown in Figure 2, the U-beam system consists of rowsof U-shaped vertical rods attached to theroof of the furnace that interrupt the flow ofthe gases exiting the furnace. Two rows ofU-beams are placed inside the furnace itself,and four rows of U-beams are installed behind the furnace rear wall
plane. The in-furnace U-beams capture about 75% of thesolids, which slide down the length of thebeams back into the combustion chamber.The remaining solid particles captured in theexternal U-beams are collected in a particlestorage hopper, which is periodically emptied back into the furnace forreburning.Theflue gas velocity across the U-beams isaround 8 meters/second (26.5 ft/sec) or less,producing a relatively low gas-side pressuredrop (less than 1 inch of water column) ascompared to conventional cyclone-typeseparators (6 to 10 inches of water column).
The IR-CFB furnace is made of gas-tightmembrane enclosure water-cooled wallswith studded tubing spaced every fourinches. The lower furnace walls (up to aheight of 7.3 meters [24 ft]) are protectedwith an ultra high-strength, abrasion-resistant, low-cement refractory material lessthan 1 inch in thickness, which is placedover the studs protruding from the coolingtubes. A band of metal spray is typicallyapplied to further protect against erosion atthe point where the refractory material ends.The very thin application of refractory material means faster startup and less maintenance cost.
Other beneficial characteristics of the IR-CFB boiler design include:
* Use of in-furnace surfaces (division andwing walls) for furnace temperature control;
* Gravity fuel feed and simplified secondary ash recycle system;
* Absence of hot expansion joints, allowingsignificantly reduced maintenance; * Smaller footprint, which allows retrofitinside existing structural steel. Operating Experience at Two Installations
The IR-CFB design has been installed attwo locations—one at SIU in Carbondale, Illinois, and the second at the KanoriaChemicals & Industries Ltd. (Kanoria) sitein Renukoot, India. The SIU installation is a35-megawatt (MW) boiler that burns high-sulfur, low-ash Illinois coal, while the 81-MW Kanoria unit uses low-sulfur, high-ashcoal. The SIU boiler has a crushed limestone
bed to combat the higher sulfur content ofthe fuel, while the Kanoria boiler uses a sandbed.
SIU Unit data
The SIU boiler is located close to the OldBen II coal mine in southern Illinois. Theplant was completed in 1996 and startedoperation in mid-1997. Performance testingwas completed in September 1997. Table 1shows the design and performance data forthe SIU boiler.
Raw coal, delivered by truck, is movedby drag chain conveyor to a crusher. A24-hour capacity silo stores the pulverizedcoal. The coal is introduced into the furnaceby one gravimetric feeder through the sidewall. Two 60-MMBtu/hr gas-fired, over-bed burners and two 25-MMBtu/hr gas-fired, in-bed lances provide heat for startup.A multi-cyclone dust collector is used as asecondary solids separator (downstreamfrom the U-beams). The overall solids collection efficiency exceeds 90% and solidscollected in the cyclone are returned to thefurnace via an air fluidized conveyor. Abaghouse provides final particulate control.
The bed material is periodically drainedfrom the furnace to control bed solids build-up and to remove any oversized material.The SIU unit has a single 8-inch
diameterdrain pipe to remove the bed, which iscooled with a screw ash cooler using recirculated plant water supply.
Cold startup to 100% maximum continuous rating (MCR) can be achieved withinfive hours and the observed boiler dynamicload response is 5%–6% per minute. Aboiler turndown of 5:1 has been achievedwithout auxiliary fuel (a turndown ratio of3.5:1 to 4:1 is guaranteed). Further, all majorequipment has performed reliably whilemeeting or surpassing permitted emissions.A soot blower installed at the horizontalconvection pass floor has experienced plugging with ash and residual moisture. Whilethe boiler can operate successfully withoutthe soot blower, more investigation isneeded to overcome this operational glitch.
Kanoria Unit Data
The Kanoria facility is located within thestate of Utter Pradesh, India, in close proximity to the Singaroli coal mine. The boilerwas constructed in 1996 and began commercial operation in February 1997. Performance testing continued until September1997. Design and performance data for theKanoria boiler are also shown in Table 1.
In contrast to the Illinois coal, the Kanoriafuel is erosive, low in sulfur, and high in ash.
Crushed coal is introduced via two volumetric drag chain feeders through the front wallof the furnace. Two 60-MMBtu/hr oil-firedover-bed burners provide heat for startup.Solids collected by the U-beams are reinjected by gravity into the furnace at fourlocations. The Kanoria unit uses an electro-static precipitator for final particulate control. Bed draining is accomplished throughtwo bed drain pipes and ash coolers; finematerial is returned to the furnace, whileoversize particles are diverted to the ashdisposal system.
The observed boiler efficiency of 88.8%is higher than originally anticipated andcombustion efficiency has exceeded 99%,due to very low unburned carbon and lowflue gas outlet temperatures. However, theerosive nature of the fuel initially causedtubing leaks in the water-cooled furnacewall, which have been remedied by applyingadditional metal spray at the refractory interface and adjusting the interface angle.Also, furnace temperature exceeded design
value on several occasions due to insufficient upper furnace inventory caused by failures of the first fields of the electrostaticprecipitator and the ash conveying system.Adjustments to the precipitator rectifier andthe ash silo backpressure have solved theseproblems.
In summary, two examples of IR-CFBboilers are successfully operating at 100%MCR with varying fuel types. IR-CFB appears reliable and incorporates several verylow-maintenance features that reduce operating costs.
（2）Why Build a Circulating Fluidized Bed Boiler
to Generate Steam and Electric Power
Abstract
In Asia, demand for electric power continues to rise steeplydue to population growth, economic development, and progres-sive substitution of alternate technology
with clean forms ofenergy generation. Atmospheric circulating fluidized bed (CFB)echnology has emerged as an environmentally acceptable technology for burning a wide range of solid fuels to generate steamand electricity power. CFB, although less than 20 years old, is amature technology with more than 400 CFB boilers in operation worldwide, ranging from 5 MWe to 250 MWe.
Electric utilities and Independent Power Producers must nowselect a technology that will utilize a wide range of low-costsolid fuels, reduce emissions, reduce life cycle costs, and provide reliable steam generation for electric power generation.Therefore, CFB is often the preferred technology. Even thoughpulverized coal (PC) fired boilers continue to play a major roleworldwide, they have inherent issues such as fuel inflexibility,environmental concerns and higher maintenance costs.
This paper discusses the benefits of CFB boilers for utilityand industrial applications. Specific emphasis is given to B&W’snternal Recirculation CFB (IR-CFB) technology, CFB technology comparisons, PC vs. CFB technology, emissions benefits,and economics including maintenance cost and boiler reliabilty. Introduction
Babcock & Wilcox (B&W) is a leading global supplier ofindustrial/utility boilers and has supplied more than 700 unitstotaling more than 270,000 MWe. Many of B&W’s CFB boilerdesign features have been adapted from vast experience designing and building boilers of all types and sizes for industrial andelectric 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–tightmembrane enclosure as commonly found in Pulverized Coal(PC) fired boilers. This CFB technology has been successfullyintroduced in the global market.
To date, B&W, including B&W joint ventures and licenseecompanies, has sold 16 CFB boilers worldwide, shown in Table 1.B&W offers IR-CFB boilers up to 175 MWe, both reheat and
non-reheat, with full commercial guarantees and warranties. TheIR-CFB boiler is simple in configuration and compact, requiresa smaller boiler foot print, has minimal refractory, requires lowmaintenance, features quick startup, and provides high avail-ability.
The modern way of burning solid fuels requires fuel flex-ibility and reliable technology, plus good combustion efficiencywith low emissions. CFB technology is well suited for a widerange of sold fuels. CFB technology is proven, mature and competitive.
What is CFB technology?
CFB technology utilizes the fluidized bed principle in whichcrushed (6 –12 mm x 0 size) fuel and limestone are injectedinto the furnace or combustor. The particles are suspended in astream of upwardly flowing air (60-70% of the total air) whichenters the bottom of the furnace through air distribution nozzles.The balance of combustion air is admitted above the bottom ofthe furnace as secondary air. While combustion takes place at840-900 C, the fine particles (<450 microns) are elutriated outof the furnace with flue gas velocity of 4-6 m/s. The particlesare then collected by the solids
separators and circulated back into the furnace. This combustion process is called circulatingfluidized bed (CFB). The particles’ circulation provides efficient heat transfer to the furnace walls and longer residence timefor carbon and limestone utilization. Similar to PC firing, thecontrolling parameters in the CFB combustion process are temperature, residence time and turbulence.
Designers and power plant operators have vast experience in PC-fired boiler design and operations. Adapting and under-standing CFB technology by those familiar with the PC environment requires time. CFB technology brings the capability ofdesigns for a wide range of fuels from low quality to high quality fuels, lower emissions, elimination of high maintenance pulverizers, low auxiliary fuel support and reduced life cycle costs.A PC vs. IR-CFB comparison is given in Table 2.
The combustion temperature of a CFB (840-900 C) is muchlower than PC (1350-1500 C) which results in lower Nox for-mation and the ability to capture SO2with limestone injectionin the furnace. Even though the combustion temperature of CFBis low, the fuel residence time is higher than PC, which resultsin good combustion efficiencies comparable to PC. The PC pulverizers, which grind the coal to 70% less than 75 microns, require significant maintenance expenses. These costs are virtually eliminated in CFB because the coal is crushed to 12 - 6 mmx 0 size. Even though CFB boiler equipment is designed forrelatively lower flue gas velocities, the heat transfer coefficientof the CFB furnace is nearly double that of PC which makes thefurnace compact. In an IR-CFB, auxiliary fuel support is neededfor cold startup and operation below 25% versus 40-60% MCRwith PC. One of the most important aspects is that CFB boilers release very low levels of SO2 and NOx pollutants compared to PC, as shown in Table 2. PC units need a scrubber system, whichrequires additional maintenance.
CFB is a fuel-driven and flexible technology
CFB can be the technology of choice for several reasons.The CFB can handle a wide range of fuels such as coal, wastecoal, anthracite, lignite, petroleum coke and agricultural waste,with low heating value (>1500 kcal/kg), high moisture content(< 55%), and high ash content (< 60%). The fuel flexibility provides use of opportunity fuels where uncertainty of fuel supplyexists and economics are an issue. If a CFB boiler is designedfor coal, the same boiler can be used to burn lignite or petroleum coke or anthracite. The material handling and feeding system should be properly designed to meet these fuel variations.Such fuel flexibility is not available in the competing conventional PC-fired boiler technologies. This is one of the importantfeatures of CFB that the customer needs to analyze carefullybefore selecting a technology.
Environmental benefits of CFB technology
The CFB combustion process facilitates steam generationfiring a wide range of fuels while meeting the required emissions such as sulfur dioxide (SO2 ) and nitrogen oxides (NO x)even more effectively than World Bank guidelines, as shown inTable 3.
The major environmental benefit of selecting CFB technology is the removal of SO2(90-95%) and NOx (emission is lessthan 100 ppm) in the combustion process without adding postcombustion cleaning equipment such as wet or dry flue gas