The WTA technology An advanced method of processing and drying lignite

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1 Position: January 2008 RWE Power Aktiengesellschaft Essen Köln The An advanced method of processing and drying lignite

2 RWE Power With all our power RWE Power Energy is life. It is the nervous system of modern industrial society. We, the electricity producers in the RWE Group, do our considerable bit to ensure that the wheels don t stand still. We produce electricity and heat, and we extract coal on a secure, economically efficient, and environmentally sound basis. We are a global player today. Our roots lie by the Rhine and Ruhr rivers. We have traditionally had close links with the locations where we operate. That is because we have grown with the regions and the regions with us. Cologne Aachen Mainz Bremen Saarbrücken Stuttgart Dortmund Frankfurt Hard coal Lignite with integrated opencast mines Natural gas Nuclear power stations Other conventional power plants Hydropower stations Munich Our commitment rests on this identity. Here, we are talking more than just electricity and heat. As important employers and investors, we underpin economic growth and jobs. In numerous projects and in close partnership with the regions, we support the residents and the economy at our locations. We assume responsibility for things big and small. So we are just as committed to the environment on our doorstep as we are to global climate protection. Being Germany s biggest electricity producer, we are proactively involved in designing concepts for the energy supply of the future. Our aim: to square the triangle of economic efficiency, security of supplies, and environmental compatibility. We provide impetus with our know-how, innovative technologies, and substantial investment in ultramodern power plants. We are continuously working on making power generation even more efficient, while seeking solutions to the worldwide problem of a rise in energy needs and the growing scarcity of raw materials. We combine all energy sources under one roof: from renewables like hydropower, wind and solar energy, via coal and gas, all the way to nuclear energy. With this balanced mix, we can create the best basis for long-term energy security. A workforce of 17,000 employees inside and outside Germany is committed to ensuring energy supply in Germany and Europe. With all their power. The importance of drying in lignite processing All industrial uses of lignite require upstream fuel drying. Owing to the high amount of water to be removed, lignite drying is a process that needs a lot of energy. This is why energy efficiency is of great significance here. In conventional lignite-fired power plants, lignite is dried by hot flue gases which are recirculated from the steam generator furnace at temperatures between 900 and 1,000 C. Decoupling of the combined milldrying process applied so far allows drying at low temperature levels in an exergetically more efficient way and, hence, helps to achieve a substantial increase in overall power plant process efficiency. When power is generated in an integrated coal gasification combined-cycle power plant or in an oxyfuel process, lignite predrying is needed as a matter of principle. Here too, an energetically efficient drying process can contribute to further efficiency enhancement. For so-called low-rank coals, which are not only characterized by high moisture contents but also have high ash contents, the calorific value can be raised by predrying to such an extent that these coals can be used for combustion in conventional steam generators without needing back-up fuels. 2 3

3 The importance of drying in lignite processing To allow lignite to be processed into gaseous and liquid products as well as high-grade solid fuels, its moisture content must be reduced to 10 to 20 % wt depending on the production goal. Being a basic operation, drying plays a key role in the overall coal upgrading process as well. An energetically efficient drying process improves the energy balance of the overall process. As an advanced method for processing and drying lignite, the (WTA = German abbreviation standing for fluidized-bed drying with internal waste heat utilization) can be employed in all of the above processes and adjusted to the specific requirements in each case. It contributes significantly to optimizing the energy efficiency and reducing the emissions of the overall coal use process. As the energy amount required for drying increases with rising moisture contents, the efficiency enhancement that can be achieved by the in the power plant process is not a constant value, but a function of the moisture content: The higher the amount of water to be removed by drying, the greater the net caloric value-based efficiency enhancement potential that can be obtained by the. The flue gas emissions associated with power generation (incl. CO 2, SO 2, NO x ) are directly proportional to the fuel input or proportional to the power plant s efficiency. The efficiency enhancement achieved through the thus makes a direct contribution to reducing emissions and further improving the environmental compatibility of power generation. Fig. 1 illustrates by way of example for Rhenish lignite the emission reduction potential as a function of efficiency enhancement. for lignite drying Process fundamentals The is based on the principle of a stationary fluidized bed with low expansion. The energy required for drying is supplied via heat exchangers that are integrated in the fluidized-bed drier and heated with steam. Drying takes place in quasi 100% pure steam which is slightly superheated. At constant pressure, equilibrium is reached depending on the steam temperature between the steam temperature and the residual moisture of the dried lignite. Fig. 2 shows this dependence for Rhenish and Australian lignite at a system pressure of approx. 1.1 bar. For a temperature of approx. 110 C (Rhenish) and of approx. 107 C (Australian lignite), a residual moisture content of some 12 % is reached. By controlling the fluidized-bed temperature, the moisture content can be adjusted and maintained constant at the desired value. spec. CO ² emissions in t CO ² MWh 1,5 1 0,5 Past (Power plants without predrying ) 150 MW 300 MW 600 MW 40% reduction in CO ² emissions Status quo: BoA Future Power plants with WTA drying TBK TBK C Moisture content of dry lignite (%) Australian lignite Rhenish lignite Efficiency in % Fluidized bed temperature ( C) Pressure ca. 1,1 bar Fig. 1: Specific CO 2 emissions as a function of efficiency Fig. 2: Steam-coal equilibrium 4 5

4 Drier design Variants of energetic vapour utilization Both variants can be integrated into the WTA pro- used as water in industrial processes. The selection Coal feeding is fed via a star feeder into the slightly Compared with other processes, lignite drying in a cess and allow the energy efficiency of the drying of the vapour utilization variant depends, among pressurised drier. A system specifically developed steam atmosphere has the advantage, among others, process to be further improved and emissions to be other things, on the drying task and the integration for the which is installed in the that the evaporated coal water can be condensed reduced. The produced vapour condensate can be into the overall process. drier s upper section distributes the raw coal onto isothermally and, hence, utilized in an energetically the fluidized-bed surface. The actual fluidized bed with the integrated heat exchangers is located in efficient way. RWE has developed two vapour utilization concepts to industrial-scale: 0 80 mm Electrostatic precipitator milling Vapour Vapour Freeboard Vapour compressor Revolving chute distributer preheater Drier Heating steam (compressed vapour) Heat exchanger Fluidized bed Circulation blower Fluidizing steam Fluidizing bottom Fixed bed Dry lignite cooler Dry lignite milling Dry lignite 0 1 mm Dried lignite Fig. 4: Process principle of WTA fine-grain drying with vapour recompression Fig. 3: Schematic diagram of the drier 0 80 mm Electrostatic precipitator Vapour to Atm the central section of the drier. Heating is by low- Mechanical vapour recompression as an open milling pressure steam or alternatively (depending on the heat pump process for heating the drier s heat process variant) by recompressed vapour, the pres- exchangers with and without integrated lignite sure ranging between around 3 and 4 bar. Fluidization is achieved by a system that is adjusted to the specific conditions of lignite drying. Below the fluidizing bottom, the dried coal is discharged from the pre-heating Vapour condensation for preheating of, e. g., boiler feed water in the power plant process Drier Circulation blower Boiler feed water Vapour condenser fixed bed via star feeders. Fig. 3 shows the schematic design of the drier. It is characterized by a high Vapour condensate specific capacity and a particularly compact structure. Dry lignite cooler LP-steam for heating Dry lignite milling Dry lignite 0 1 mm Fig. 5: Process principle of WTA with vapour condensation 6 7

5 Input grain-size variants Overall process Capacity and plant equipment Advantages of the The WTA drying process has been developed for Fig. 4 shows the overall process for the fine-grain The WTA drying process is characterized by a high The advantages of the can be sum- two different input grain sizes: The so-called coarse- WTA variant with upstream fine milling and inte- specific capacity per square meter of the drier cross marized as follows: grain WTA plant operates with input coal grain sizes grated mechanical vapour compression to allow sectional area and a low heating steam pressure. between 0 and 6 mm, while the fine-grain WTA pro- the vapour energy to be used in the drying process. It is therefore possible to achieve a very high drying High energy efficiency thanks to drying at low cess uses grain sizes of 0 to 2 mm. The coarse-grain Following cleaning in an electrostatic precipitator, output per drier unit. The compact drier structure temperature levels and energetic utilization of the variant is employed where for reasons of the down- the evaporated coal water (fuel-laden vapour) is and the integrated fine milling system for the raw evaporated coal water (through vapour conden- stream process the coal to be dried must have a recompressed to about 4 bar in a compressor, so and the dry coal add up to a compact structure of sation or mechanical vapour compression) specific minimum grain size, e.g. for gasification in that the vapour can be used to heat the heat ex- the HTW process or for coke production from lignite. changer installed in the drier. The sensible heat of For all other processes, the fine-grain variant is the produced vapour condensate is used to preheat usually the more attractive option in technical and the raw lignite to about 65 to 70 C and, hence, economic terms. In particular, as a predrying stage contributes significantly to covering the drier s in conventional power plants, the fine-grain WTA energy demand. Part of the cleaned vapour is re- process offers advantages as the dried coal already circulated and employed for fluidizing the bed. has a fineness of some 0 to 1 mm, making it suitable The dried coal is cooled and where required for immediate use in the steam generator. milled a second time to a grain size of 0 to 1 mm in a mill integrated in the WTA plant to make it For fine milling of run-of mine lignite, RWE has directly suitable for combustion in the power plant. developed a specific process which is adjusted to the particular requirements of lignite and the sub- Fig. 5 shows the overall process for the fine-grain sequent fluidized-bed drying. It consists of two WTA variant with upstream fine milling and down- milling stages connected in series where the coal s stream vapour condensation for boiler-feed water grain sizes are reduced from approx. 0 to 80 mm preheating from the water steam-cycle in the to the desired 0 to 2 mm. power station. Fig. 6 shows a low-cost variant without vapour utilization that can be employed, e.g., for improving the 0 80mm calorific value of low-rank coals. milling Vapour cyclone Vapour to power station Fig. 7: Photo of the prototype WTA plant at Niederaussem the overall plant. Fig. 7 shows the prototype WTA, Safe plant operation as drying takes place in an the world s biggest drying plant for lignite, which inert atmosphere both during normal operation Circulation cyclone is currently being built; it has a raw coal input of and during start-up and shutdown (avoidance of LP-steam for heating Drier Circulation blower 210 t/h and an evaporation capacity of 100 t/h. As an alternative, all components of the raw coal flow can be arranged above each other in a steel explosive coal dust mixtures) High drying capacity per drier unit Compact design thanks to the integration of raw structure. coal fine milling and where required secondary dried lignite milling Dry lignite cooler Dry lignite 0 1 mm Thanks to the energetic vapour utilization large amounts of steam and dust emissions are avoided and the discharged vapour condensate is a water source that can be used in industrial processes Flexible adjustment of the plant equipment to the Fig. 6: Process principle of a low-cost WTA variant requirements of the individual drying task 8 9

6 Know-how and range of services WTA plants Know-how and range of services WTA plants completed and being planned The is a proprietary RWE development that is specifically adjusted to industrial-scale application in the lignite industry. To meet the particular requirements of lignite processing and drying, innovative process- and plant-specific solutions have been developed for which 18 patents have been filed or granted yet. The evaluation of plant operation with domestic and foreign coals allows RWE to continuously develop and optimize the process. Based on the comprehensive know-how gained in developing, planning, building, commissioning, and operating its own plants, RWE is able to offer a wide range of services: Feasibility studies incl. investment cost determination Bench-scale tests (processing, drying, fluidizing behaviour) Laboratory analyses Licensing of the Industrial-scale processing and drying tests in the WTA 2 plant Frechen as a basis for designing licensed plants Basic and detail engineering Support during commissioning WTA 1 Frechen Coarse-grain drying with integrated vapour compression and coal preheating Raw coal input: approx. 53 t/h; dry coal production: approx. 28 t/h WTA 1 Niederaussem Coarse-grain drying with integrated vapour compression Raw coal input: approx. 170 t/h; dry coal production: approx. 90 t/h WTA 2 Frechen Fine-grain drying with partial vapour condensation Raw coal input: approx. 27 t/h; dry coal production: approx. 14 t/h WTA 2 Niederaussem (under construction) Fine-grain drying with vapour condensation Raw coal input: approx. 210 t/h; dry coal production: approx. 110 t/h WTA Hazelwood (being planned) Fine-grain drying without vapour utilization Raw coal input: approx. 140 t/h; dry coal production: approx. 70 t/h The WTA 2 Frechen and WTA 2 Niederaussem plants were planned, built, and commissioned by RWE acting as its own general contractor. For the other plants, RWE did the basic engineering and parts of the detail engineering. Fig. 8 shows the WTA 2 plant Frechen which is used for large-scale drying and processing tests with foreign lignites. The test lignites are discharged in the semi-open unloading point and filled into the raw lignite bin by a wheel loader (centre of photo). Fig. 8: Photo of the WTA 2 plant Frechen 10 11