| Nitrogen oxides (NOx) is one of the major pollutant emission from coal-fired power plant boilers and gives a serious threat to the atmosphere and man-kind's health. The research on NOx control has been one of the hot topics around the world in the past 2 decades. Many technologies have been developed to reduce NO emission, among which reburning is one of the most promising technologies. Usually natural gas and low-rank coal are used as the typical reburning fuel. But natural gas is very expensive and coal is a fossil fuel that is not renewable. Therefore, it's important to find a new and efficient alternate reburning fuel for boilers taking place of natural gas and low-rank coal.Waste tires are increasing rapidly as solid waste disposal and it's difficult to carry out the management problems associated with the waste tire disposal. It's necessary to investigate new techniques for its utilization as resources. After reviewing briefly on the present techniques for the recycle of waste tires, the author proposed the use of waste tires as a fuel to reduce both air emission and solid waste by applying them as a reburning fuel for NOx reduction. Ultimate analysis of typical waste tire showed that it contains more than 80% carbon content, less than 2% nitrogen and sulfur, very little moisture and ash. The heating value of waste tires per kilogram is 69% more than that of wood, 10% more than that of bituminous and 4% more than that of coke. Waste tires meet the volatility and reactivity requirements and are low in both nitrogen and sulfur. Beyond its good combustion characteristics, it's also very attractive using waste tires as an alternative fuels for reburning because there are large mass of waste tires requiring disposal. These properties make waste tires a potentially good fuel for reburning NOx control. This technology can dispose waste tires at a maximum extent by clean utilization of waste tire as a useful energy resource.First, a thermo-gravimetric analyzer (TGA) was used to study the pyrolysis performance experimentally at the beginning step. In a thermo-gravimetric analyzer, a series of experiments were conducted. The pyrolysis tests of pulverized waste tire and pulverized coal in an inert atmosphere of nitrogen gas were conducted respectively to study their pyrolysis characteristics. The results show that the lost of weight of pulverized waste tire is as high as 64.3%. Compared to pulverized coal, the pyrolysis of pulverized waste tire is much easier, it can be carried out at relatively lower temperature. A DSC combined with a mass-spectrum was used to study the volatile release during the tire's pyrolysis. It's found that the yield of volatile gases is about 40% during pyrolysis of waste tires and the content of the volatile gases are mainly CO,H2,CH4,C2H4 etc, which are all good reburning fuels. A mathematical model was established to calculate the parameters of its pyrolysis.Second, experiments were carried out on an ceramic flow reactor to study the reburning performance of waste tire powder at 1150℃. The results show that under the typical reburning conditions, the NO reduction efficiency of pulverized waste tire can achieve about 95%, which is as high as that by nature gas at the same conditions. The best stoichiometric ratio in the reburning zone of waste tire is 0.8～0.9, the residence time of waste tire is much shorter than pulverized coal's, the best reduction efficiency of waste tire appears at residence time is 0.3s, and the best temperature of waste tire reburning is 1150℃. In order to study the burnout of waste tire, a 2-stage test rig was established and several mixed fuel based on waste tire were experimentally investigated. The results showed that the NO emission increased when the OFA add to the furnace for tire only, which is also true for natural gas and coal reburning due to the burnout of HCN and/or coal char-N. When special matters were added to the tire, the mixes fuel gave very good results. At the typical reburning and burnout conditions, the reduction efficiency for one mixed fuel was as high as 85%, which is far beyond that by coal and similar to that by natural gas. The results showed that waste tire can be used as a new and efficient reburning fuel for NO reduction..Finally, the author used FLUENT software to simulate the combustion process of pulverized waste tire and coal. Compared to pulverized coal, the combustion of waste tire can be much easier, the NO separate out characteristics of both fuel is resemble, the NO concentration will increases firstly, and decreases later along the axis of the furnace. 2). When the temperature increases, the NO concentration increases a little bit when pulverized coal is burned, while the NO concentration decreases when waste tire is burned. The NO concentration of both fuel increase when the stoichiometric ratio increase; When the particle diameter decreases, the NO concentration of waste tire decreases.The conclusion could be drawn from this thesis as the following: the waste tire powder is as good as natural gas for reburning NO control. When mixed with a special matter, the final NO reduction ratio can be as high as 85% during the reburning and burnout 2-stage combustion experiment, which is similar to that by natural gas and much high than that by typical coal reburning. The results suggested that waste tire powder can be used as a new and efficient reburning fuel for NO reduction. |
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