Stadtwerke Dusseldorf AG is the utility company responsible for the energy supply of the City of Dusseldorf. To ensure the supply of power and district heat, it operates power plants with a total capacity of around 1100 MW, 600 MW of which are generated by coal-fired power plants with 13 boiler installations located at three different sites. In addition, it operates the refuse incineration plant of the city of Dusseldorf with a capacity of 450,000 tons per year of municipal refuse and industrial waste. Within the last few years, all of the above plants had to be retrofitted with flue gas cleaning systems for the removal of SO2, HCl, HF, NO(x), chlorinated hydrocarbons, vaporous heavy metals, etc. The three-stage semi-dry flue cleaning process ＇＇System Dusseldorf＇＇ meets these requirements, in addition to the following: simple and space-saving technology; flexible capacity; no waste water, no sewage problems; attractive investment and operating cost; utilization of residual products; adaptability to other flue gas cleaning technologies. As anticipated the dust, SO2, HCl, HF, etc., emissions after the lignite-activated coke filter as the second stage, reach zero or near-zero. However, using the same material as a low temperature catalyst in the downstream third stage, the denitrogenisation resulted in emissions of not lower than 200 mg NO(x) per m3. Consequently, to achieve lower NO(x) emissions, without reheating the flue gases, other materials were found, such as endowed synthetic carbon, zeolite, or a type of titanium dioxide, all of which are currently in long-term tests. To date, plants for more than 3 million m3 flue gas per hour have been successfully built, and been proven in operation now for more than three years. The only residue from this type of cleaning system, a kind of gypsum, is reused for fabricating lime sand bricks and filling coal and ironpits.