Large Volume Inorganic Chemicals - Ammonia ... - ammk-rks.net

Chapter 33.4.9 Selective catalytic reduction of NO x (SCR)DescriptionFor a more extensive description of SCR, see [11, European Commission, 2003]. The selectivecatalytic reduction of NO x utilises the reaction of ammonia with nitrogen oxides, producingnitrogen and water vapour according to the following equations:6 NO + 4 NH 3 d 5 N 2 + 6 H 2 O6 NO 2 + 8 NH 3 d 7 N 2 + 12 H 2 ONO + NO 2 + 2 NH 3 d 2 N 2 + 3 H 2 O4 NO + O 2 + 4 NH 3 d 4 N 2 + 6 H 2 OAmmonia or (evaporated) ammonia solution is injected, in the required stoichiometricamounts, into the waste gas stream. The ammonia reacts preferentially with the nitrogenoxides in the tail gas using a catalyst to initiate the reaction. The tail gas needs heating to theoperational temperature depending on the catalyst being used, i.e. between 120 and 400 °C. Thisis normally carried out by passing the tail gas through a heat exchanger using heat recoveredfrom the ammonia oxidation unit. An SCR facility can be used both before and after theexpander.NO x removal from nitric acid plants using SCR presents significant differences in comparison toremoval of this contaminant from, e.g. power plants, mainly due to the differing compositionsof the untreated gases. The high NO 2 concentration in the tail gases of a nitric acid plant, i.e. thepercentage of NO 2 in the NO x , varies between 50 – 75 %, and this significantly affects thecatalyst behaviour. As a result of this, some catalysts used in power plants cannot be used innitric acid plants.Achieved environmental benefits• a NO x conversion of 80 – 97 % can be achieved [11, European Commission, 2003]• achievable emission levels are 74 – 100 ppm [11, European Commission, 2003]• ammonia reacts preferentially with nitrogen oxides so less reducing agent is neededcompared to NSCR, however, energy might be recovered in the case of NSCR.For achieved emissions levels, see also Table 3.8. See also Section 3.4.7 "Combined NO x andN 2 O abatement in tail gases".132 Large Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers