| For many years, the cement production and consumption of China are always the top one in the world, however, with more and more cement pre-calciner kiln having been built, NOx produced by the cement kiln has become the third sources of exhaust gas. In addition, taking wastes as a part of fuel and raw materials for cement is very significant. But as the waste is widely used in cement production, many minor components are introduced, and it has an important impact on gas emissions, including the formation of NOx. Therefore, researching factors for fuel-NOx formation in cement calciner process with using waste, and realizing effectively controlling NOx emissions from the source, has not only great scientific significance, but also great value for environmental protection.The present paper, combining laboratory experiment with factory production, and taking quantum chemistry calculation, Fluent for fluid simulation, x-ray photoelectron (XPS), GC/MC, TG-DSC, TG-FTIR, and Hydrochloride Naphthodiamide, studies nitrogen oxide formation mechanism as waste is used in cement calciner and the technology for decreasing NOx from source. The context includes the pyrolysis mechanism of nitrogen forms in the fuel and its affect on the formation of NOx, the harmless treatment of sludge in cement kiln and the effect on nitrogen oxides emissions, the automotive waste such as tires, scrap leather, real leather effect on nitrogen oxides emissions, as well as the study of raw material of cement effect on fuel nitrogen oxide formation. Through research, the following results have been achieved:(1) Based on the analysis of nitrogen forms in the raw material and fuel, the differences between fuel-NOx formations which are caused by the nitrogen form have been studied. Four main nitrogen forms of raw material and fuel are combusted at 900 ℃ in cement kiln. The result shows that nitrogen form of amino acid in waste leather has the worst thermal stability, while the fuel-NOx concentration it gets is the highest, followed by pyrrole and pyridine of coal. The toluene diisocyanate which comes from waste-PU has the best thermal stability and its fuel-NOx is the smallest. In addition, under the same experimental conditions, the number of nitrogen oxide will be increased with the pyrrole content rising in coal. Therefore, the differences between nitrogen formations in the raw material and fuel have an obvious effect on fuel-NOx formation.(2) Quantum chemistry method has been used to calculate the reaction process when pyrrole, pyridine, amino acid and toluene diisocyanate are heated. The Mulliken bond orders show that the strength of C-N is weak, and it will be broken first if being heated and this will cause thermal decomposition; therefore the activation energy orders:glycine acid<pyrrole<pyridine<toluene diisocyanate,while thermal stability has the opposite orders under the same conditions. Glycine acid has the most active chemical properties and the lowest stability of the studied compounds. Additionally, the precursor to fuel-NOx is NH3, which is more likely to be oxidized. Pyrrole has lower activation energy and poorer thermal stability than pyridine, which will be broken down more easily than pyridine; and it has HCN and NH3, while pyridine has HCN. Toluene diisocyanate is the most stable. HCN is derived from the isocyanic acid groups (-N=C= O) by depolymerization of Toluene diisocyanate. Samples containing nitrogen release NH3 and HCN as intermediate products in thermal decomposition reactions. The reaction rates of oxidizing radicals (O, OH) and NH3 are greater than the reaction rate of HCN under the same conditions. When it is heated, the rapid oxidation of NH3 is very good for pyrrole amino acid decomposition, the reaction carries to the right, so there will be more nitrogen oxide form.(3) Fuel-NOx of coal can be decreased when waste tire is used in cement kiln. Fuel-NOx emissions of waste tire are much less than waste leather, while they have the same nitrogen form of amino acid, under the same combustion condition. The analysis structure internal elements shows that, the restore elements content of C and H of waste tire is bigger than waste leather, but the oxidation elements content of O is smaller. On the other hand, it shows that the overlap of material weightlessness is processed by TG-DSC curve, when volatile of coal is just released and a little part of fixed carbon burn, but volatile of waste tire has been released, and a large amount of restore material such as CH4, C2H4, C2H2, CH and CO, is released. it is unfavorable conditions for fuel-NOx emissions. Therefore, NOx emissions of waste tire is far less than the waste leather, and waste tire can reduce NOx emissions of coal when waste tire is mixed with coal in combustion.(4)At 900℃,cement raw materials would obviously promote fuel-NOx formation. Coal has been pickled and mixed with mineral for combustion, compared to all of the samples, contribution for fuel-NOx emissions varies in the following order:CaO> MgO> Al2O3. However NOx emission is reduced by SiO2. In addition, fuel-NOx is decreased when the compound of sulfur has been mixed with coal for combustion, and the sulfur forms are one of the main factors. Ferrous sulfide (FeS) and pyrite (FeS2) can be decreased for fuel-NOx emissions, However the calcium sulfate and calcium sulfite have a small effect on fuel-NOx emissions. All in all raw materials of cement can promote NOx emissions to grow.(5) There is a large amount of hazardous of waste gas when sludge is dried. Organic compound has 31 types in waste gas with drying sludge, such as halogenated hydrocarbons, hexane, and also there has odorous substance, including NH3, H2S and mercaptan, and so on. The number of organic compounds will increase to 34 when drying deodorized sludge, including aromatic, halogenated hydrocarbons, hexane, and new compounds, for example, amines.(6) The mixture of waste gas and air is converted to furnace for coal combustion at 900 ℃ which has been studied, and the result shows that the exhaust odor substances and aromatic compounds, halogenated hydrocarbons of waste gas are removed, and NOx emissions can be reduced. Therefore, moisture content of sludge is tried to decrease, then sewage sludge will be dried by hot air of cooling kiln cylinder or cooling machine for clinker, and then the mixture of waste gas and air is conversed to the pre-calciner for combustion. That not only eliminates the harmful substances, but reduces fuel-NOx emissions.(7) Fuel-NOx concentration of sewage sludge is lower than that of coal under the same experimental conditions. Nitrogen form of sewage sludge include amino acid, nitrous compounds and aniline, among which amino acids are the main source of fuel-NOx because of poor thermal stability and full rich oxygen elements. Nitrous would compete with oxidizing free radicals and meanuhile conversion to nitro, and amine would be quickly released by aniline. All of these are not conducive to the formation of fuel-NOx. In addition, fuel-NOx can be reduced by the sludge which is a specially light porous structure. With volatile releasing at combustion, sludge structure would be formed into activity carbon, and gas (including NO, and HCN, and NH3, and CH4, and C2H2, and CiHi, hydrocarbons, and C) is adsorption on surface, so NO can be decreased through reaction with gas and solid or gas and gas.(8) The effect of sewage sludge on fuel-NOx in calciner is simulated with software of Fluent. According to the work of 6000t/d cement production line, results of simulation can be used for prediction and control of fuel-NOx emissions. The test of factory shows that fuel-NOx emissions will be reduced when sludge is injected into calciner, and that does not affect the production. The extent of reduction of fuel-NOx is from 18% to 39%, which depends on the position of injection. The extent of reduction of the central part in calciner is higher than the bottom.Reasonably co-processing waste cement kiln can make good use of waste and also reduce the emission of nitrogen oxides from the source. |
PDF Full Text Request