1994

Leppälahti, Jukka; Koljonen, Tiina; Kilpinen, Pia; Hupa, Mikko

Decomposition of ammonia, NH3, and reaction products formed in decomposition were studied. In particular, reactions of NH3 in gas phase with addition of different oxidizers to the gas were studied. Test equipment consisted of a quartz reactor positioned in an electric oven. The synthetic mixture of the gasification gas and the oxidizers studied was fed into the reactor at atmospheric pressure. Thermal decomposition of NH3 was insignificant with no added oxidizers and the reaction time of 4 s and up to the temperature 1000 oC. However, possible impurities contained in gases cause easily catalytic deposits on reactor surfaces. These deposits may contribute to the decomposition of NH3. The NH3 content of the gas can be reduced by O2 addition. When 2.5 vol% of oxygen was added to the gas, 20 - 25 mol% of NH3 decomposed at high temperature. The decomposition of NH3 increased with an increased O2 addition. If CH4 was present in the gas, the variations in the concentrations of other gas components such as H2 had only an insignificant effect on the decomposition degree of NH3. If no CH4 was present in the gas, the H2 concentration had a significant effect on NH3 decomposition. When the H2 content was 7 vol%, about 65 % of NH3 was decomposed by adding 2.5 vol% oxygen. In the gas combustion tests NH3 decomposed nearly completely at air/fuel stoichiometric ratio of 0.5 - 0.6, if no CH4 was present in gas. If the gas contained CH4, the decomposition of NH3 was retarded, and at stoichiometric ratio of 0.5 more HCN and NO were formed than in the gas without CH4. If NO and O2 were added simultaneously to the gas at a reaction time of 0.65 s, the same result as with pure O2 addition was usually obtained. When the residence time was 4 s and temperature 1000 oC, the addition of both NO and O2 gave better result than that of pure O2. The efficiency of NO to decompose NH3 is reduced when the temperature falls. At temperatures below 700 oC solids are formed in the gasification gas, if the gas contains NH3, NO and O2. The possible solids formation in sampling lines should be considered in the detection of NH3.