c and at a pressure of for example 2 9 atmospheres absolute the residual no x concentration of the exhaust gas amounts for example to 2 500 ppm the exhaust gas is heated in heat exchanger 20 to about 270 u00b0 c additional burner 21 raises the temperature to 320 u00b0 c approximately 42 kg h gaseous ammonia is supplied to reactor 13 the distribution ratio of the gaseous ammonia to pipes 17 18 and 19 is 70 20 10 and the ratio of the amounts of catalyst in catalyst beds 14 15 and 16 is 15 20 65 the reaction between the residual nitric oxides in the exhaust gas and the ammonia is exothermal and the thus generated heat of the purified exhaust gas passing from the outlet of reactor 13 is yielded in heat exchanger 20 to the exhaust gas coming from heat exchanger 2 at the outlet of heat exchanger 20 the purified exhaust gas has a gas flow of 20 500 norm m 3 h at a temperature of 180 u00b0 c and a pressure of 2 8 ata the remaining residual no x concentration is lower than 200 ppm the purified exhaust gas is expanded i e the pressure thereof is relieved in connection with recovery of energy in the expansion turbine 10 and is then discharged through the.