Abstract: In view of that the aqueous urea solution used in traditional selective catalytic reduction (SCR) system is difficult to produce ammonia by thermal decomposition at low exhaust temperature in lowspeed and light load conditions of diesel engine and is easy to form sediments, leading to the blockingup of piping and hence lowering the conversion rate of NOx, two ammonium salts (ammonium carbonate and ammonium carbamate) are studied in this paper as candidate reductants of solidstate SCR system. Firstly the theoretical analysis of ammonia content and thermogravimetric experiments are conducted with the results showing that the obtainable masses of ammonia by thermal decomposition from ammonium carbonate and ammonium carbamate are 192 and 237 times as more as that from aqueous urea solution respectively, and both ammonium salts can completely decompose in 8 mins at 90°C, a temperature much lower than that of exhaust gas (150°C) in diesel engine at low speed/light load conditions. Then decomposition equilibrium pressure and recrystallization experiments are performed and the results indicate that in the normal range of working temperatures of coolant and lubricant oil in diesel engine, both ammonium salts can provide rather high injection pressure for injection system, and they do not tend to recrystallize So it is demonstrated that both ammonium salts meet all the requirements for reductants in solidstate SCR system, in which ammonium carbamate is slightly superior to ammonium carbonate in overall property. Finally isothermal and integral methods are used to calculate the decomposition kinetics parameters of ammonium salts, including their decomposition reaction orders, activation energies and apparent preexponential factors, to provide theoretical basis for the design of ammonia generation and supply devices in solid SCR system.

Key words: solidstate SCR, ammonium salt, thermal decomposition characteristics, decomposition kinetic parameters, crystallization