Study On Ammonium Salt Decomposition And Low Temperature NO_x Reduction Characteristic Of Solid SCR Technology

Selective Catalytic Reduction(SCR) technology is the main stream technology of reducing the nitrogen oxides(NOX) emission of diesel engine, solid ammonium SCR(SSCR)technology heats ammonium salt outside the exhaust pipe, which can produce ammonia.Exhaust gas temperature will not be the limit of SSCR technology any longer. SSCR technology avoid the issues which Urea SCR technology may occur at low exhaust temperature, such as lack of reductant, deposits formation and so on. It is an important supplement and one of the development directions of the urea SCR technology. This thesis studied the characteristics of ammonium salt decomposed, decomposition kinetics,ammonium salt recrystallizaed, recrystallization decomposed characteristics, NOX low temperature reduction reaction, which were the key issues of solid ammonium SCR technology.The decomposition characteristics of ammonium bicarbonate, ammonium carbonate and ammonium carbamate were investigated based on thermal gravimetric analysis(TGA),decomposition kinetics of ammonium carbonate and ammonium carbamate were studied by combined with the integral method and the isothermal method. The test temperature range was 50?~100?, the results showed that: the decomposition of ammonium bicarbonate,ammonium carbonate and ammonium carbamate is a single process, without intermediate product; at the same heating rate, the decomposition rate in descending order was ammonium carbamate, ammonium carbonate and ammonium bicarbonate. Ammonium carbamate and ammonium carbonate can slowly decompose at room temperature, the initial decomposition temperature of ammonium bicarbonate was about 80?; during the isothermal decomposition process, the decomposition rate in descending order was ammonium carbamate, ammonium carbonate and ammonium bicarbonate; ammonium carbamate decomposition order of reaction was 1/2, the apparent activation energy was 56382J/mol,pre-exponential factor was 6.07×105s-1; ammonium carbonate decomposition order of reaction was 2/3, the apparent activation energy was 62614J/mol and the pre-exponential factor is 4.33×106s-1.In order to study the recrystallization of ammonium carbonate and ammonium carbamate, a solid ammonium SCR injection system was established, crystallization test bench was designed, The experimental results showed that under constant injection pressure,the temperature of somewhere of injection system falled below a certain value, the gas released by ammonium carbonate and ammonium carbamate would re-condensed, the injection pressure and crystallization temperature one correspondence; injection pressure,respectively to 40 kPa, 70 kPa, 100 kPa, 130 kPa, 160 kPa, 190 kPa, 220 kPa, ammonium carbonate recrystallization temperature of approximately were 56?, 60?, 63?, 66?, 68?,71?, 73?, ammonium carbamate recrystallization temperature of approximately were 57?,61?, 64?, 67?, 69?, 72?, 74?; the injection parameter didn't change crystallization temperature; crystals produced by the test system can be completely eliminated by heating.By TGA, the decomposition characteristics of crystallization and two ammonium salts which were repeated heating and cooling had been analyzed, and the results showed that: the decomposition characteristics of crystallization was consistent with ammonium salt;ammonium carbonate and ammonium carbamate still maintained the original decomposition characteristics after repeated heating and cooling, repeated heating and cooling didn't change its characteristics.The low-temperature characteristics of chemical reaction and ammonia storage were investigated quantitatively in a full-size Cu-zeolit SSCR system on a diesel engine test bench,compared to the urea SCR system conversion efficiency at low temperature, the effect of the mixer on the SSCR system conversion efficiency was studied, established the SCR control model based on Cu-zeolite, and proposed ammonia injection control strategy based on ammonia leak. The results showed that, NOX conversion efficiency was consistent with the change of the catalytic reduction reaction rate, increasing the reaction rate can improved the NOX conversion efficiency. The impact of catalyst temperature on the reaction rate was greater than space velocity; the lower the catalyst temperature was, the influence of space velocity on the catalytic reduction reaction rate was more obvious. With airspeed constant, as temperature increased, amount of saturated ammonia storage of catalyst reduced; with temperature constant, as the airspeed increased, the amount of saturated ammonia storagereduced. Below 210?, there was a large amount ammonia storaged in the catalyst, when the NOX concentration proportion of the catalyst outlet was back to 95% of the initial, ammonia stored in the catalytic converter can not be completely consumed, the lower the temperature and the greater the airspeed, the amount accounting for part of the saturated ammonia storage was higher. The NOX conversion efficiency of SSCR system was higher than the urea SCR system at low temperature, while more ammonia leakage. Mixer improved the conversion efficiency of SSCR system at low temperature, but the effect was not obvious.