As people pay more and more attention to the living environment, the pollution problem caused by combustion has attracted more and more attention. Although gas is a relatively clean fuel, some of the combustion products produced through combustion at high temperatures will inevitably cause harm to the human body and the atmospheric environment. The combustion process also inevitably produces noise. Only by knowing the causes of many pollutions can we take effective measures to control them.
The harmful substances in smoke mainly include: CO₂, CO, NOₓ, SOₓ, etc. The combustion of hydrocarbons inevitably produces CO₂, which is not toxic in itself, but is an important substance that causes the atmospheric greenhouse effect and causes global warming. Therefore, it is important to save energy and reduce consumption, improve fuel utilization efficiency, reduce fuel consumption and thereby reduce the total amount of smoke emissions. Moreover, CO, NOₓ, and SOₓ are all highly toxic and very harmful to the human body, and their emission levels should be strictly controlled. Under normal conditions, gaseous fuel must undergo desulfurization and purification treatment before use, so that the sulfur content in the fuel can be effectively controlled. Therefore, for gaseous fuels, the amount of SOₓ produced after combustion is very small. Our focus is on studying how to reduce the CO and NOₓ content in exhaust smoke through effective methods.
CO is the earliest combustion pollutant recognized by humans. It is a colorless and odorless gas, the product of incomplete combustion of fuel. When CO is inhaled into the human body, it combines with hemoglobin to form carbon monoxide hemoglobin (COHb). It hinders the transportation of oxygen with the blood, causing hypoxia in human tissues, and then causing various diseases and even death.
Already known nitrogen oxides include NO, NO₂, N₂O, N₂O₃, N₂O₄, N₂O₅, etc. The nitrogen oxides generated during the combustion process are almost all NO and NO₂, and what we usually call NOₓ mainly refers to the NO and NO₂ generated during the combustion process.
90~95% of the NOₓ emitted by stationary combustion devices is NO. Therefore, the study of NO generation mechanism and inhibition pathways mainly refers to NO. There are three pathways for NO production:
1) Temperature NO (Thermal-NO, T-NO for short)
T-NO is produced by nitrogen molecules and oxygen molecules in the air at high temperatures. Since the activation energy required for the NO generation reaction is higher than the activation energy of the reaction between the combustible components of the gas and oxygen, the NO generation rate is slower than the combustion reaction, so a large amount of NO will not be generated in the flame surface. A large amount of NO is generated downstream of the flame surface. Especially in places downstream of the flame surface with local high temperatures, high local oxygen concentrations and long flue gas residence times, NO is more likely to be generated.
>The main factors affecting the generation of T-NO are combustion temperature, oxygen concentration and the time the flue gas stays in the high temperature zone.
2) Fast NO (Prompt-NO, P-NO for short)
P-NO is produced when the fuel concentration is high and the oxygen concentration is low. Therefore, all it takes to reduce P-NO is to supply enough oxygen. P-NO is produced in the flame plane and is a unique phenomenon when burning hydrocarbon-rich fuels. Typically the amount of P-NO produced is an order of magnitude smaller than the amount of T-NO produced. P-NO has little relationship with temperature.
3) Fuel-NO (Fuel-NO, referred to as F-NO)
F-NO is produced by the oxidation of nitrogen atoms present in the fuel as compounds. Its generation temperature is 600~900C and has medium temperature generation characteristics. Since the general combustion temperature is much higher than this value, the combustion temperature has little effect on the generation of F-NO. Since the N compound content in gaseous fuel is very small, F-NO can be ignored.
>Most of the NO generated by gaseous fuel combustion is T-NO. Therefore, suppressing the generation of T-NO is one of the main measures to reduce NO emissions.
>The nitrogen oxides generated during the combustion process are almost all NO and NO₂
>NO has strong binding ability with hemoglobin in human blood, causing hypoxia and causing central nervous system paralysis, spasm and other symptoms.
>NO also has carcinogenic effects and has adverse effects on cell division and the transmission of genetic information.
>NO₂ is more toxic than NO and is very harmful to human internal organs and hematopoietic tissues.
>NO₂ participates in the formation of photochemical smog and will produce stronger toxicity.
>The damage to forests and crops is also considerable.
>NOₓ is an important source of acid rain
1) Influence of excess air coefficient
As the excess air coefficient changes, the combustion temperature and oxygen concentration also change. Changes in these two are the main factors affecting the amount of NO generated. Therefore, the excess air coefficient is a comprehensive reflection of the impact of these two factors on NO production.
2) Influence of combustion heat load
It is generally believed that changes in combustion heat load will cause changes in flame temperature, which will also affect the amount of NO generated. Except for methanol, the NOₓ concentration of other fuels increases with increasing heat load.
All the carbon initially contained in the fuel will generate CO, so CO is an intermediate product inevitably produced by the oxidation of carbon-containing fuel. Therefore, attention to controlling CO emissions should focus on how to completely re-oxidize CO rather than on limiting its formation.
Provided there is sufficient oxygen and residence time at flame temperature, the CO concentration will drop to very low levels after the reaction. The CO concentration value actually measured in the flue gas is lower than the maximum value in the flame, but it is much higher than the equilibrium value under flue gas conditions, which exactly illustrates this problem.