Experimental Study On Soot Formation Mechanism And Emission Control Methods Derived From Lump Coal Combustion In Stoker Fired Boiler

The soot particles derived from coal combustion affect the environmental quality and people's health, which are paid attention to by more and more people. Soot particles in air relate to people's acute and chronic diseases. Up to now, many works focus on the soot formation derived form simply gas hydrocarbons, liquid fuels and pulverized coals. Few works has been mentioned for soot formation derived from lump-coal combustion in stoker-fired boiler. There are more than 0.5 millions industrial boilers and the total installed capacity of the industrial boilers is more than 1.2 millions tons per hour; more than 400 million tons of coals are consumed in China; and the stoker-fired boiler is the dominating combustion style. The characters of this boiler style are low in efficiency and high in pollutants emission. It is significant to study the mechanism of soot formation in order to reduce the soot emission and increase the efficiency of stoker-fired boiler.In stoker-fired boiler, the volatiles of coal and air are flowed in the same direction, so the mix of them is poor. The mechanism of soot formation has been studied in pyrolysis process in stoker-fired boiler by movable-fixed bed reactor. The factors'effects on soot formation such as temperature, residence time of gas, and coal type, have been studied. The soot aerosol have been collected by glass fiber filter, tar and soot can be separated by methylene dichloride. Ashes content of soot aerosol can be test by Inductively Coupled Plasma Aomic Emission Spectrometer (ICP-AES); the compositions of sample gases from flue have been examined by Gas chromatograph mass spectrometer (GC-MS); the functional groups of soot aerosol can be tested by Fourier Transform Infrared Spectroscopy (FTIR); and the microscopic structure of soot aerosol have been examined by Scanning Electron Microscope (SEM). The results of study are recited as below: Yields of soot are connected with contents of aromatic hydrocarbon; some of aromatic hydrocarbons come from concentration and cyclic reaction of aliphatic hydrocarbon. Soot yields are increased, tar yields are decreased; contents of saturated aliphatic hydrocarbons are decreased and contents of aromatic hydrocarbons are increased in flue gas; contents of benzene loops are increased in soot aerosol; and the diameters of soot aerosol particles are augmented with temperature increase. Soot yields are increased, tar yields are decreased; contents of unsaturated aliphatic hydrocarbons are decreased and contents of saturated aliphatic hydrocarbons are increased in flue gas; contents of C=C are increased early and decreased late in soot aerosol; and the diameters of soot aerosol particles are augmented with residence time extension. More soot yields can be collected, more aromatic hydrocarbons can be tested in flue gases, more soot particles are agglomerated in high-volatile coal pyrolysis; and contents of benzene loops are connected with microcosmic structure of coals.The mechanism of soot formation has also been studied in incomplete combustion process in stoker-fired boiler by movable-fixed bed reactor. The factors'effects on soot formation such as temperature, residence time of gas, amount of oxygen and coal type, have been studied by orthogonal experiment. The experimental methods in combustion experiment are the same as those in pyrolysis experiment. The results of study are recited as below: The yields of soot are the product of formation and oxidation of soot in presence of oxygen and oxygen-containing radicles. Soot and tar yields are decreased respectively; contents of saturated aliphatic hydrocarbons aromatic hydrocarbons are decreased in flue gas; contents of benzene loops are decreased in soot aerosol; less soot particles are agglomerated with oxygen amount increase. Soot and tar yields are increased early and decreased late respectively; contents of saturated aliphatic hydrocarbons are decreased and contents of aromatic hydrocarbons are increased in flue gas; contents of benzene loops are decreased in soot aerosol; the diameters of soot aerosol particles are less with temperature increase. Soot yields are increased early and decreased late;tar yields are decreased; contents of unsaturated aliphatic hydrocarbons are decreased and contents of saturated aliphatic hydrocarbons are increased in flue gas; contents of benzene loops are decreased in soot aerosol; the diameters of soot aerosol particles are larger and less soot particles are agglomerated with residence time extension. More soot yields can be collected, more aromatic hydrocarbons can be tested in flue gases, more soot particles are agglomerated in high-volatile coal pyrolysis; and contents of benzene loops are connected with microcosmic structure of coals.Based on the mechanism study on soot formation, the effect on reduction of soot emission by secondary air has been simulated by the measurement in practical furnace and numerical methods. The content of volatile, composition and velocity as boundary condition have been tested in real furnace. PDF and soot model method of simple gas hydrocarbon have been used to simulate the effect on soot emission by secondary air. The secondary air may reduce the soot emission and that have been tested in furnace.The soot emission has been studied for different coal diameters in ignition or restart of combustion in the furnace. That the soot emission can be reduced by secondary air has been examined. The soot emission at the time of ignition of the coal at ambient temperature is more serious than that of restart of the combustion after the temporary flameout because higher temperature is helpful for the complete combustion of the volatiles. The secondary air can help to reduce the soot emission because the secondary air can enhance the turbulence intensity of the gas in furnace, in which the reaction between volatiles and air is easier. Selection of coal particle size is necessary because volatiles release of small particle is faster, and it needs more air for reaction in time; if the turbulence is not enough, the soot trends to be emitted. The effect on reduction of the soot emission by the secondary air with the small particles coal is more evident than that with the big particles coal. Based on results of study on soot formation mechanism derived from lump coal combustion and academic analysis, some measures can be taken to reduce the soot emission. Moreover, it has been tested in 2t/h stoker-fired boiler that reduction of soot emission by arranging the oxygen amount.