From the environmental point of view, an issue is any solid, liquid or gaseous substance introduced into the atmosphere that can cause air pollution.
Given the wide variety of substances present in the atmosphere, have been proposed several methods of classification:
Primary pollutants can be of gaseous or particulate.
Among the gases are:
• Sulfur compounds
The main compounds that contain sulfur in the atmosphere are: sulfur dioxide (SO2), carbonyl sulfide (COS), carbon disulfide (CS2), hydrogen sulfide (H2S), dimethyl sulfate ( CH3) 2SO4. The sulfur dioxide, which constitutes 95% of the total anthropogenic emissions of sulfur, derived from combustion processes, in dependence on the sulfur content of the fuel used.
• Nitrogen compounds
The main nitrogen-containing compounds are: N2O, NO, NO2, NH3, HNO3, HONO, N2O5 and salts of NO3, NO2, NH4. They are considered pollutants as carbon monoxide and nitrogen dioxide (NO and NO2). The first is produced by both natural and anthropogenic sources, and in particular in all combustion processes, NOx (NO + NO2) are formed by the fact largely through the fusion of molecules of oxygen and nitrogen. The mixture of emitted NOx is formed mostly of NO, while a good part of NO2 has secondary origin and formed in the atmosphere by oxidation of carbon monoxide.
• Carbon compounds
In this category the main inorganic compounds are carbon monoxide (CO) and dioxide or carbon dioxide (CO2). The carbon dioxide produced by human activities derived from combustion processes. The interest that has developed around this compound is due to climatic changes on a global scale for which it is responsible. The carbon monoxide instead is considered highly toxic. Its primary source is the exhaust fumes of cars and to a lesser extent the thermal power plants and heating plants;
Polycyclic aromatic hydrocarbons, a wide range of substances produced by burning wood, naphtha and gas oil, the most important and is the only standardized Benzo (a) Pyrene, considered a strong carcinogen.
• Halogenated compounds
(HCl, HF, HBr, CFC);
The particles are an aerosol of small solid particles classified according to their size. Atmospheric particles are usually measured in PTS (Total Suspended): PM10 when the mean aerodynamic diameter is less than 10 microns, PM2, 5 when their mean aerodynamic diameter less than 2.5 microns.
As said, the formation of nitrogen oxides increases almost exponentially with increasing combustion temperature. We can mention the equipment specifically provided for the removal of nitrogen oxides, for which the processes that are normally used are the type catalytic or non-catalytic.
The first of these technologies, called selective catalytic reduction (SCR), is the installation of a reactor downstream of the sewage line in which ammonia is injected spray, which, mixing with the smoke and through the layers of the catalysts, the temperature becomes of 300 ° C the nitrogen oxides in water and gaseous nitrogen, harmless gas that makes up about 79% of the atmosphere. Since it is possible that a certain amount of unreacted ammonia escapes from the chimney (“ammonia slip”), other methods have been developed that do not use ammonia as a reagent or involving the use of an additional catalyst to prevent its escape.
The second technology, called Selective Non Catalytic Reduction (SNCR), often preferred because it more economical, has the advantage of not having to dispose of the exhausted catalysts but has characteristics of efficacy lower than the SCR systems, and consists of the injection of a reactant (urea that high temperature dissociates into ammonia) in an aqueous solution in an area of the system in which the temperature is between 850 ° C and 1050 ° C with the consequent reduction of nitrogen oxides to nitrogen gas and water. Other processes not exploit the catalytic reduction with ammonia implemented through irradiation with electron beam or through the use of electrostatic filters.