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

Chapter 33.4.2 Optimisation of the oxidation stepDescriptionThe aim of optimising the oxidation step is to get an optimum NO yield. This means that theformation of unwanted side products such as N 2 O is lower. The NO yield in the oxidation step ishighest at an optimum NH 3 /air ratio of 9.5 – 10.5 % ammonia. Additionally, a high NO yield isfavoured by low pressure (as low as possible) and optimum temperature (750 – 900 °C).Figure 3.5: Indicative relation of the ammonia burner efficiency and temperature at 1 and 4 bar[88, infoMil, 1999]Ammonia/air ratio. From an engineering point of view, the combustion of ammonia is one ofthe most efficient catalytic industrial reactions (the theoretical maximum conversion would be98 % in a one bar plant). The ammonia-air reaction mixture should contain 14.38 % ammoniaaccording to the equation stoichiometry. However, a lower NH 3 /air ratio is used for a variety ofreasons, the most important being that the conversion efficiency decreases at a high NH 3 /airratio. In addition, ammonia and air can form an explosive mixture. The “lower explosion level”(LEL) drops with pressure, so high pressure burners can only safely operate with up to 11 %ammonia, whereas 13.5 % ammonia is possible in low pressure systems. A safety margin isnecessary to deal with locally higher ratios as a result of imperfect mixing, so many plants add±10 % ammonia.Operating at low pressure increases the conversion of NH 3 to NO according to the laws ofthermodynamics.A high reaction temperature enhances ammonia combustion, but decreases the conversionefficiency due to the increased generation of N 2 and N 2 O. The oxidation generally takes place attemperatures of between 850 and 950 °C, resulting in NO yields above 96 %. Temperaturesabove 950 °C are possible, but catalyst losses, mainly due to vaporisation, then increase. Thereaction mechanism tends to be more selective towards nitrogen and N 2 O formation at lowercatalyst temperature. Nitrous oxide (N 2 O) is unstable at these temperatures (i.e. 850 – 950 °C)and partly reduces to N 2 and O 2 . A long residence time and a high temperature in the catalyticreactor promote this reduction. The reaction temperature is directly related to the ammonia/airratio: a 1 % increase in the proportion of ammonia increases the temperature by approximately68 °C.Large Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers 113