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Two forms of ammonia monia, and (2) aqueous ammonia. Anhydrous ammonia is toxic, hazardous, and requires thick-shell, pressurized storage tanks and piping due to its high vapor pressure. Aqueous ammonia, NHstep three·H2O, is less hazardous and easier to handle. A typical industrial grade ammonia, containing about 27% ammonia and 73% water by weight, has nearly atmospheric vapor pressure at normal temperatures and can be safely transported on highways in the USA and other countries.
A number of chemical reactions occur in the ammonia SCR system, as expressed by Equations (1) to (5). All of these processes represent desirable reactions which reduce NOx to elemental nitrogen. Equation (2) represents the dominant reaction mechanism . Reactions given by Equation (3) through (5) involve nitrogen dioxide reactant. The reaction path described by Equation (5) is very fast. This reaction is responsible for the promotion of low temperature SCR by NO2 . Normally, NO2 concentrations in most flue gases, including diesel exhaust, are low. In diesel SCR systems, NO2 levels are often purposely increased to enhance NOx conversion at low temperatures.
It has been learned that the aforementioned responses is restricted because of the drinking water . Wetness is always found in diesel deplete or other flue gases. To obtain appropriate performance, water vapour are going to be constantly found in laboratory gas evaluation out-of SCR process plus procedure modeling.
In case the NO2 content has been increased to exceed the NO level in the feed gas, N2O formation pathways are also possible, as shown in Equation (6) and (7) .
Undesirable processes occurring in SCR systems include several competitive, nonselective reactions with oxygen, which is abundant in the system. These reactions can either produce secondary emissions or, at best, unproductively consume ammonia. Partial oxidation of ammonia, given by Equations (8) and (9), may produce nitrous oxide (N2O) or elemental nitrogen, respectivelyplete oxidation of ammonia, expressed by Equation (10), generates nitric oxide (NO).
Ammonia can also react with NO2 producing explosive ammonium nitrate (NHcuatroNO3), escort services Rochester Equation (11). This reaction, due to its negative temperature coefficient, occurs at low temperatures, below about 100-200°C. Ammonium nitrate may deposit in solid or liquid form in the pores of the catalyst, leading to its temporary deactivation .
Ammonium nitrate formation can be avoided by making sure that the temperature never falls below 200°C. The tendency of NH4NO3 formation can also be minimized by supplying into the gas stream less than the precise amount of NH3 necessary for the stoichiometric reaction with NOx (1 to 1 mole ratio).
A treatment rate which is excessive leads to discharge of undesired ammonia with the conditions
When the flue gas contains sulfur, as is the case with diesel exhaust, SO2 can be oxidized to SO3 with the following formation of H2SO4 upon reaction with H2O. These reactions are the same as those occurring in the diesel oxidation catalyst. In another reaction, NH3 combines with SO3 to form (NH4)2SO4 and NH4HSO4, Equation (12) and (13), which deposit on and foul the catalyst, as well as piping and equipment. At low exhaust temperatures, generally below 250°C, the fouling by ammonium sulfate may lead to a deactivation of the SCR catalyst .
The SCR process requires precise control of the ammonia injection rate. An insufficient injection may result in unacceptably low NOx conversions. These ammonia emissions from SCR systems are known as ammonia slip. The ammonia slip increases at higher NH3/NOx ratios. According to the dominant SCR reaction, Equation (2), the stoichiometric NH3/NOx ratio in the SCR system is about 1. Ratios higher than 1 significantly increase the ammonia slip. In practice, ratios between 0.9 and 1 are used, which minimize the ammonia slip while still providing satisfactory NOx conversions. 3/NOx ratio, NOx conversion, temperature, and ammonia slip . The ammonia slip decreases with increasing temperature, while the NOx conversion in an SCR catalyst may either increase or decrease with temperature, depending on the particular temperature range and catalyst system, as will be discussed later.
Addition Choosy catalytic protection (SCR) off NOx of the nitrogen substances, for example ammonia otherwise urea-commonly referred to as simply “SCR”-was developed for and you will well demonstrated inside the commercial stationary programs
In stationary applications, the maximum permitted NH3 slip is usually specified, with a typical specification at 5-10 vppm NH3. These concentrations of ammonia are generally undetectable by the human nose. Optionally, ammonia slip can be also controlled by a guard catalyst (oxidation catalyst) installed downstream of the SCR catalyst.