In this paper thermal DeNox process for Selective Non-Catalytic removal (SNCR) of nitric oxide from engine exhaust gases has been discussed and optimum parameters i.e. optimum temperature, optimum residence time, and optimum molar ratio has been calculated by using CHEMKIN-II software. To run the code exhaust emission data has been obtained from reciprocating engine power plant operating on duel fuel system (40% Natural gas & 60% HFO). Finally it is concluded that if the conditions prescribed as predicted results are met in the exhaust stream, then with ammonia injection a substantial decrease in NOx (about 96 %) can be achieved. Sources of NOx formation and different NOx control techniques are also discussed in the paper.