Fluidized-bed drying with internal waste heat utilization (WTA)
Raw lignite has a high natural moisture content of up to 60%. This moisture content impairs the coal's combustion properties. Prior to any industrial utilisation, drying systems must be used to remove as much moisture as possible from the coal.
Conventional lignite power plants dry the coal by removing some of the flue gases during combustion from the power plant boilers' flue-gas stream and combining it with the moist raw lignite. The evaporation heat spent on drying the coal is lost in the process. This process consumes a relatively high amount of energy that is not available for power generation. The result is that, altogether, more coal must be combusted and this, in turn, entails higher CO2 emissions.
Using WTA – RWE's proprietary development – the moisture content of the raw lignite is lowered from over 55% to 12%. The crucial edge: unlike the previous drying process involving 1,000°C hot flue gas, drying in the WTA at a temperature of 110°C, uses the energy far more efficiently. In addition, most of the heat spent on drying the coal is now recovered and used again. This new process is similar to taking , nearly 500,000 mid-size cars of the road.
Fluidised-bed drying has already been successfully trialled in a small plant at the Frechen site since 1993. In 2009, a commercial-scale prototype was commissioned at the Coal Innovation Centre, with the objective of proving the economic and technical benefit of fluidised-bed drying in continuous operation. The development project was successfully completed in 2014 and the pilot plant has continued to be operated by the power station since. The demonstration plant can generate up to 110 tons of dry lignite per hour – that is up to 30% of the overall coal requirements of the BoA unit at Niederaussem. Thanks to the WTA, the efficiency of future lignite power stations could be boosted by 10% to more than 47%. This will enable a power plant with a capacity of 1,000 MW to produce up to one million tons of CO2 less annually with the same amount of power being generated.