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

Chapter 22.4.10 SNCR at the primary reformerDescriptionThe selective non-catalytic reduction (SNCR) process is a secondary measure to reduce nitrogenoxides already formed in the flue-gas of a combustion unit. Within the SNCR unit, the injectionof the additive and the reaction of nitrogen oxides to nitrogen and water take place. It isoperated without a catalyst at a temperature of between 850 and 1100 °C. At ammonia plants,normally ammonia is used as the reducing agent, as it is available on-site.The temperature window is of considerable importance as, above this, ammonia is oxidised andso even more NO x is produced, and below this, the conversion rate is too low and unconvertedammonia is emitted to air. Moreover, with load changes the temperature pattern in theconvection section will change. To adjust the required temperature window with the ammoniainjection, several levels of injection are necessary.The reaction of nitrogen oxides and ammonia/urea into water and nitrogen is strongly dependenton temperature and retention time within the required temperature range, as well as on theammonia to nitrogen oxides ratio. As already mentioned, the temperature window for ammoniaand caustic ammonia is 850 to 1000 °C, the optimum temperature being 870 °C. In comparison,the temperature window when using urea is wider (800 to 1100 °C) with an optimal temperatureat 1000 °C.The retention time within the required temperature window is in the range 0.2 – 0.5 seconds.Once more, optimisation is required as regards the molar ratio of NH 3 to NO x . The NO x removalrate is favoured by an increased ratio, but in the meantime the ammonia slip increases too,leading to increased pollution of subsequent units (e.g. heat exchangers, flue-gas ducts). In orderto neutralise these two contrary effects, a NH 3 to NO x ratio between 1.5 and 2.5 has been foundto be the optimum.Achieved environmental benefitsGenerally, with SNCR a reduction rate of 40 – 70 % is achievable [11, European Commission,2003].Austrian ammonia plants, which are equipped with SNCR at the primary reformer, achieve NO xreduction rates of 30 – 50 % and emission levels of 140 – 160 mg/Nm 3 . The limit for theammonia slip is 10 mg/Nm 3 [9, Austrian UBA, 2002].Ammonia slip is 1 – 5 mg/Nm 3 [17, 2nd TWG meeting, 2004].Cross-media effectsMost problems with SNCR applications are related to non-uniform distribution of the additivewithin the combustion chamber. Therefore, optimisation of the distribution system is required.Special distribution systems are used to gain an optimal distribution of ammonia and flue-gas.In order to achieve a high abatement rate and a low NH 3 slip, additive and NO x in the flue-gasmust undergo sufficient mixing.• potential for N 2 O formation• NH 3 consumption.Large Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers 73