Chemical methods to reduce emissions
Development work has been done on large stationary engines using cyanuric acid to reduce NOx emissions. Cyanuric acid is a low-cost solid material that sublimes in the exhaust flow. The gas dissociates, producing isocyanide that reacts with NOx to form N2, H20, and CO2• Operating temperature is about 500°C. Up to 95% NOx reduction has been achieved with no loss of engine performance. At present, this system is not practical for vehicle engines because of its size, weight, and complexity. Research is being done using zeolite molecular sieves to reduce NOx emissions. These are materials that absorb selected molecular compounds and catalyse chemical reactions. Using both SI and CI engines, the efficiency of NOx reduction is being determined over a range of operating variables, including AF, temperature, flow velocity, and zeolite structure. At present, durability is a serious limitation with this method.
Various chemical absorbers, molecular sieves, and traps are being tested to reduce HC emissions. HC is collected during engine startup time, when the catalytic converter is cold, and then later released back into the exhaust flow when the converter is hot. The converter then efficiently burns the HC to H20 and CO2• A 35% reduction of cold-start HC has been achieved.
H2S emissions occur under rich operating conditions. Chemical systems are being developed that trap and store H2S when an engine operates rich and then convert this to S02 when operation is lean and excess oxygen exists. The reaction equation is
H2S + 02 = S02 + H2
Ammonia Injection Systems
Some large ship engines and some stationary engines reduce NOx emissions with an injection system that sprays NH3 into the exhaust flow. In the presence of a catalyst, the following reactions occur:
4 NH3 + 4 NO + 02 = 4 N2 + 6 H20 6 N02 + 8 NH3 = 7 N2 + 12 H20
Careful control must be adhered to, as NH3 itself is an undesirable emission. Emissions from large ships were not restricted for many years, even after strict laws were enforced on other engines. It was reasoned that ships operated away from land masses most of the time and the exhaust gases could be absorbed by the atmosphere without affecting human habitat. However, most seaports are in large cities, where emission problems are most critical, and polluting from all engines is now restricted, incl.Mdingship engines.
Ammonia injection systems are not practical in automobiles or on other smaller engines. This is because of the needed NH3 storage and fairly complex injection and control system.