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The Story of Electrical Energy, Pt.16 .Recent advances in thermal power station design include Pressurised Fluidised Bed Combustion, or PFBC systems. This new type of boiler, together with improved turbo alternator sets, increases the overall efficiency while decreasing costs. By BRYAN MAHER Thermal power stations have grown in capacity and improved in efficiency during their evolution over the past century. Yet boilers designed from about 1970 to the present are now regarded by some engineers as physically bigger, less efficient and more cumbersome than necessary, and thus too expensive. The designs we have seen previously in this series first pulverise the coal to a fine powder. This is then blown into a fire chamber lined with water/steam pipes. The flue gases then must be scrubbed and filtered to remove fly ash, sulphur dioxide, oxides of nitrogen and other pollutants. These scrubbers and filters have to be designed for the expected type of coal. That presents big problems. Australian power stations are sited close to coal mines, so the fuel properties for each station are well defined and the boilers are designed to suit. Things are different in many other countries. Some or all of the coal burned in Norway, Sweden, England, Japan, Peru and other countries must be imported. Changing world economics leads to a mixed variety of coals arriving at some power stations. For example, since 1984, up to 4% of all the coal burned in English power stations comes from foreign countries. In Japan, the figure is close to 100%. It would be a great help if boilers could accommodate a wide range of coals with a varying content of ash, sulphur, tar, gas and impurities. This is where the PFBC boiler comes in. PFBC boiler A radically new power station concept emerged from experiments during the 1970s, culminating in the building of a 15 megawatt test facility by Asea/ ABB at Malmo, Sweden in 1984. This consisted of a Pressurised Fluidised Bed Combustion (PFBC) boiler, as shown in the diagram of Fig.1. Coal in the form of 5mm particles is first mixed with dolomite (to add calcium) and then injected into the bottom of the fire chamber. When sufficient air is blown in from below, the mass of particles (coal, dolomite and ash) will be loosened up. At sufficient air velocities, all particles lose contact with their neighbours and float like a fluid in the airstream. This suspension is called a fluidised bed (the idea comes from coal gasification plants in the 1920s ). The vessel baseplate has a large This photo shows the huge size of a modern double-flow low pressure steam turbine. The largest blades are just over one metre long and the whole assembly is just on four metres in diameter and weighs close to 50 tonnes. 76 SILICON CHIP You can now afford a sate IIite TV system STACK For many years you have probably looked at satellite TV systems and thought "one day'· You can now purchase the following K band system for only $995. 00 This is about 1/3 the price of corn parable systems Here's what you get: ~ A 1.8 metre pressed steel prime focus dish antenna, complete with all the mounting hardware - as well as a self supporting ground stand. 1J+ One super low noise LNB (low noise block converter) 1.4dB or better. w+ One KU band feedhorn and all the mounting hardware as well as a magnetic signal polariser. ltt 30 metres oflow loss coaxial cable with a single pair control line. ASH STORAGE 11+ A 99 channel infrared control satellite receiver with adjustable IF and audio bandwidth, polarity, and dual digital readout . The IR control unit has a range of approx. 10 metres. Before you receive your system the unit is pre-programmed to the popular AUSSAT transponders via the internal EEPROM memory. This unit is also suitable for C band applications. Fig.1: this diagram shows the principal components in a PFBC (pressurised fluidised bed combustion) demonstration power station built in Malmo, Sweden in 1984. The entire boiler furnace is housed in a pressurised container and the flue gases are used to drive a gas turbine. CALL, FAX or WRITE to AV-COMM PTY LTD. PO BOX 386, NORTHBRIDGE NSW 2063 PHONE (02) 949 7417 number of air nozzles. In operation, the fluidised bed can be four metres deep, consisting of about 1 % coal and 99% inert ash and dolomite. The ignition of the coal particles floating in the air stream raises the bed temperature to 850°C. Steam, generated in water pipes buried in the bed, is returned to a vertical pressure drum. From this, steam flows in more pipes laid in the furnace, returning to the drum in a superheated condition. Output steam from the boiler is at lOMPa (1500psi) and a temperature of 530°C. In the 15MW test facility, coal was fed in at 0.58kg/s to form a fluidised bed weighing 3.6 tonnes. Flue gas was produced at a rate of 8.1kg per second. First commercial unit The first commercial unit was rated at 206MW and was built for the Vartan CHP plant, and is similar to a 200MW unit installed for the American Electric Power Company (AEP) in 1990. In all its power stations, AEP burns 40 million tonnes of high sulphur content coal annually, mined in Ohio and Indiana. This new Pressurised Fluidised Bed Combustion technology was chosen for an upgrade of AEP's Tidd power plant at Brilliant, Ohio. FAX (02) 949 7095 All items are available seperately. Ask about our low noise "•c· band LNB, and other interesting products. All systems are provided with dish pointing details. ----------- 1 Yes Garry, I Please send me more information I on your l{ band satellite systems. I II Name ..................................... .. II I Address ................................... I I ................................................... I I ........................... P/Code., ........ I I I :Ce~~~~~::a . . . . . ... . . . . . . .~;;~~ 11. _ _ _ _ _ as ____ _ DECEMBER 1991 _.1 77 10 20 30 40 in 1990 by Asea Babcock, a consortium of the US companies ABB Carbon and Babcock Wilcox. Phase 2 of the Ohio clean coal program helps the funding of a 330MW PFBC unit being installed at AEP's Philip Sporn power-plant at New Haven, West Virginia. This unit, replacing two older 150MW boilers, will be in operation by 1995. 50m Pressure sphere COAL FIRED BOILER WITH DRY SCRUBBER PULVERISED COAL FIRED BOILER WITH REGENERATIVE AIR PREHEATING LIME ELECTRO S02 PREPARATION STATIC - REACTOR PRECIPITATOR FABRIC FILTER FLUE STACK GAS FAN PRESSURISED FLUIDISED BED BOILER PRESSURISED FLUIDISED GAS TURBINE BED Fig.2: here's how the size of the boiler and ancillary equipment for a conventional pulverised coal power station (top) compares with a PFBC station (above). Note that not only is the PFBC system smaller but it does not need separate precipitators or S0 2 reactors, leading to considerable cost savings. Fig.3 (below): a modern steam turboset made by ABB, Sweden. This has a single flow high pressure turbine (1), a double flow intermediate pressure turbine (2), and two double flow low pressure turbines (3 & 4). 78 SILICON CHIP The Tidd power plant is a working demonstration power station funded partly by the US Department of Energy and the Ohio State Coal Department's clean technology programs. The PFBC system at Tidd was installed But there's much more to the story. As Fig.1 shows, the complete combustor is enclosed within a cylindrical or spherical pressure vessel. The feed air is forced into the fluid ised bed until the pressure above the bed is 1680kPa (255psi). The products of combustion are forced out into multiple sets of 3-stage passive cyclones. Here the finest bed particles, caught up in the draught, are extracted from the flue gases. By controlling the fuel flow, the bed temperature can be maintained between 800°C and 900°C. The depth of the fluidised bed is regulated by the removal of ash from the bottom or by re-injecting stored ash. The deep bed and low fluidisation rate in the PFBC system assure a long contact time between bed and gas, yielding a very high combustion efficiency of 99%. This is about 3% better than conventional plants using pulverised coal. Fig.2 shows the greatly reduced size of a PFBC boiler installation compared to a conventional boiler of the same AIR PRESSURScD fLUIDScD 8!:0 BOILER CLEANED FLUE GAS TUHHtNf CONDENSE!l COAL ANO DOLOMITE !NTEfi-. COOLER 51 M\N power capacity. In both drawings, the auxiliaries required to clean the flue gases to government requirements are shown. Low sulphur emission Notice that the PFBC boiler does not need any electrostatic precipitator for fly ash removal, as these particles are caught by the internal cyclones. Nor is a sulphur dioxide (SO 2 ) reactor required. This is a big bonus, as many high quality coals (ie, those having energy ratings above 24 gigajoules per tonne) have a high sulphur content. In older types ofboilers, sulphur dioxide (SO 2 ) is produced in copious quantities by the burning of the fuel. This toxic gas, ifreleased into the atmosphere, reacts with airborne water vapour to produce sulphurous acid. This leads to acid rain and the resultant ruination ofrivers, fish, lakes, crops, trees and so on. Hence, huge SO 2 reactors are normally installed in power station flue lines to remove this gas. So bad was the problem in Europe that, in June 1988, the environmental ministers of the European Community agreed on a directive: chemical reactors were to be installed despite the huge cost; and sulphur dioxide was to be progressively reduced in existing plants by 20% in 1993, by 40% by 1998, and by 60% by the year 2003. Coal also contains ammonium compounds as impurities. These break down in the high furnace temperatures of conventional boilers to form nitrogen oxides. These may constitute as much as 500ppm (parts per million) of the flue gases. As well as being toxic, these gases also attack many common metals. Furthermore, if released ihto the atmosphere, they react with water vapour to form nitric and nitrous acids. Severe acid rain can result unless catalytic denoxing equipment is included in the power station flues. Photochemical smog can also result because of the absorption by nitrogen dioxide of ultraviolet light, releasing an oxygen radical which in turn reacts with the air and forms ozone. This oxidises hydrocarbons in the atmosphere to form photochemi- Fig.4: diagram of a proposed 332 megawatt power station in which the power will be extracted from both a steam turbine and a gas turbine. The net efficiency of this arrangement is expected to be close to 42%, a very worthwhile improvement over present day power stations. cal smog which is dangerous to human eyes and lungs. Benefits of PFBC stations A most interesting benefit of the fluidised bed technology is that over 93% of the sulphur dioxide is removed within the furnace by reaction with the dolomite added in with the coal. The end product is a dry gypsum solid, which has a ready market. Thus,'desulphurisation takes place in the fluidised bed and no SO 2 reactors are needed in the outside flue line. This easily meets Swedish standards which limit sulphur dioxide emissions to 100mg of SO 2 per megajoule of energy rating of the power station. Furthermore, the comparatively low DECEMBER1991 79 condenser. The exhaust from the gas turbine heats the feedwater in the economiser before these flue gases vent up the stack. Steam generated by the PFBC boiler will drive a modern 4-stage turbine set as shown in Fig.3. In this turboset, superheated steam first drives the high pressure (HP) turbine and is reheated before passing to the intermediate pressure (IP) turbine. Steam enters the middle of the IP stage and exhausts from both ends. To generate yet more shaft power, the IP exhaust steam drives two (not one) double-flow low-pressure (LP) turbines. These LP units are huge, having individual blades as long as 1050mm in the final stage. Each LP turbine bladeset rotor is approximately four metres in diameter and 10 metres long. So large are these final blades that the centrifugal force on them when they are rotating results in tensile stresses of 200-300 tonnes. They must not only be very securely anchored to the hub but blade resonances must be minimised if vibration stresses are not to become excessive. Efficiency enhancements This photo shows a PFBC boiler before it is enclosed in its spherical pressure vessel. This boiler is intended for a 200MW turboalternator set. temperature and high pressure in a fluidised bed furnace result in extremely small quantities of oxides of nitrogen being formed in the first place. So no denoxing plant is needed. The cost saving of the above is enormous. Currently, two billion pounds is being spent in England on equipment to reduce sulphur and nitrogen emissions from a handful of large existing power stations with a total output of 12 gigawatts. Because a PFBC power station automatically deals with pollutants , these stations can successfully burn almost any type of coal. This fact can lead to cost savings of a high order. If the coal contains more sulphur, the operators simply add slightly more dolomite. No problem! The almost complete combustion of the coal (99%) also results in considerable savings. Over the life of a large power station, this can amount to tens of millions of dollars. Furthermore, as the diagram ofFig.2 indicates, the PFBC system, being 80 SILICON CHIP smaller for the same power, represents large capital savings. Compared to a conventional pulverised coal burning plant, the PFBC steam generator is 70% smaller and uses 65% less steel for its construction. Finally, because of the lower combustion temperature, the ash is not softened. This fact eliminates the problem of high temperature corrosion within the plant and allows easy collection of fly ash in the passive cyclones. Commercial PFBC station The proposed commercial PFBC power station shown in Fig.4 contains even more innovations aimed at increased efficiency. The flue gases from the cyclones are so clean that they can be used to directly power a 3-stage gas turbine. This turbine drives both the boiler air compressor and a 74MW alternator. For economy, the air compressor interstage cooler is cooled by the feedwater returning from the turbine Any loss of steam pressure due to path friction represents a power loss. Therefore, in modern LP turbines, baffle plates are intentionally dispensed with to allow unimpeded passage of steam to the exhaust and prevent premature flow separation in the diffuser. The exhaust steam casing is also designed with a minimum of stiffeners and braces, as these would increase the pressure loss by friction. As a result of all these improvements, the overall efficiency of a complete large PFBC power station can be as high as 41. 5 % corn pared to a figure of 38.5% for a conventional station of similar size. If a 3% improvement does not sound much, consider that it represents 30MW in a lGW power station. That's a lot of extra power, provided at no extra expense. SC Acknowledgement Grateful thanks to ABB/ASEA Review and Action and National Power, England for data and photographs.