In general, the NOX generated by waste incineration can be divided into two types: thermal NOX formed by oxidation of N in air under high temperature fuel condition and waste NOX formed by incineration of N element in waste. When the temperature of fuel in the furnace is low, the NO_x produced by garbage accounts for 70%-80%, and the NO_x produced by N in air is very little. The content of N in garbage is about 0.5%~1%. When garbage and sludge are heated at low temperature, they usually begin to decompose at about 300 C. At first, ammonia is generated. Nearly 450 C, HCN is also generated at the same time. When the temperature is above 500 C, hydrocarbons, CO, CO2 and other gases are formed. When nitrogen is decomposed in less air, it becomes a stable nitrogen molecule. As long as the high temperature is avoided as far as possible, the nitrogen molecule can be successfully inhibited to produce NOx and the concentration of ammonia can be controlled to react with the generated NOx, and it can also be turned into a stable nitrogen molecule and removed.
There are four ways to control NOx:
Combustion control method. In general, low oxygen concentration combustion is used to avoid high temperature, so as to control the production of NOx. However, when oxygen concentration is low, incomplete combustion is easy to occur, resulting in CO and dioxins.
Catalyst-free denitrification. Urea or ammonia water is sprayed into the incinerator to react with NOx to form nitrogen gas to remove NO 2. The removal rate of this method is about 30%-50%, but when too much agent is sprayed, ammonium chloride will be produced and the smoke from the chimney will turn purple. This method is simple and low cost. Because the reduction reaction can be carried out at high temperature, it can be completed in the incinerator chamber. High temperature is conducive to reducing dioxin emissions. At present, it is widely used in developed countries.
Catalytic denitrification. The method is to reduce NOx to nitrogen in the presence of ammonia on the catalyst surface. Because of the need of flue gas treatment in the front section, the flue gas temperature is low, so the low temperature catalyst of about 200 C is generally used. In theory, the reaction efficiency of this method can reach 100%, but in practice, the treatment efficiency is generally 59% – 95%. Low-temperature catalysts are expensive, and ammonia supply equipment should be built, so careful comparison and research should be carried out before selection. Catalytic denitrification process, reduction reaction is completed in a special catalytic denitrifier behind the ESP.
Natural gas reburning method. The principle of this technology is to use the combustion of nitrogen and hydrogen to react directly with NOx to produce harmless nitrogen. The specific method is to inject natural gas containing a large amount of hydrocarbon gas into the incinerator. Natural gas and NOx are directly oxidized and reduced to form harmless nitrogen or nitrogen compounds, which can achieve the purpose of removing NOx. This method not only removes NOX, but also removes CO. This method is still in the research and development stage.
