Fluidized-bed drying with internal waste heat utilization (WTA)

Raw lignite has a high natural moisture content of up to 60%. This moisture content worsens coal's combustion properties, so that any industrial utilization must have upstream drying systems 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 costs 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 own 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 WTA is at a temperature of 110°C, ie much more favourable, energetically speaking. What is more, most of the heat spent on drying the coal is now recovered and used again. By way of comparison: to achieve a similar effect for the climate, nearly 500,000 mid-size cars would have to remain permanently in their garages.

Fluidized-bed drying has been successfully trialled in a small plant at Frechen since 1993 already. In 2009, a commercial-scale prototype was commissioned at the Coal Innovation Centre, the object being to prove the economic and technical benefit of fluidized-bed drying in continuous operations. The demonstration plant can generate 110 tons of dry lignite an hour – that's 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 is to 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.