| The fossil fuels will be increasingly exhausted and the environment pollution isbecoming much severe. Eighty percent of coal production is consumed by burningdirectly in our country, and thus the NOx emission is huge.The biomass resources arerich in China, and biomass energy is one kind of the renewable energy. Biomassgasification is a promising way of biomass utilization, but the tar, which is thebyproduct of the gasification process, is still the bottleneck of the technology. The tarspecies have the effect of NO reduction, thus the biomass gasification syngas with tarcan be used as the reburning fuel to reduce NOx from coal-fired boilers. It is a properway for the utilization ofthe biomass resources. Experimental and kinetic mechanismstudy of NO reduction by tar and syngas with tar are studied in this paper to providethe support theory and develop the reburning technology using biomass gasificationsyngas with tar as the reburning fuel.Using theoretical and experimental methods in this paper, a series of experimentson NO reduction by tar model compound and syngas with tar were first carried out tostudy the influence factors and the variation rules on NO reduction. Based on these,we further studied the chemical kinetic theory, and set up the detailed kineticmechanism of NO reduction by tar. At last, the simplified kinetic mechanismconcerning NO reduction by syngas with tar was set up, and was implemented in areal system to simulating and analyzing the NO reduction effect by syngas with tar.The main work is as follows:Because there is large difference in reactivity between benzene and phenol, theywere both selected as model compound to be studied. Experimental study on the NOreduction effect by benzene and phenol were carried out on the tubular flow reactorand the micro-reactor respectively. The effect of temperature and oxygen content(equivalence ratio) on NO reduction were analyzed, and from experimental results thechange rule of NO reduction by tar were concluded, that is: Under the hightemperature environment, cracking and polymerization reactions of tar are concurrentand competed. The crack products of tar are light gases, and the polymer products oftar are soot or soot precursors. In contrast with the light gases, the NO reduction by soot is less effective, so the conditions favored for tar cracking is appreciated for NOreduction by tar.Based on the tubular flow reactor in our lab, a small batch size updraft biomassgasifier was build, and thesmall batch size biomass gasification for reburning systemwas set up. A set of experiments were carried out aiming mainly at the volume flowrate of syngas with tar, oxygen content, temperature and equivalence ratio on the NOreduction, and the effect of tar were analyzed emphatically. It can be concluded that:Oxygen is needed in the reburning process using syngas with tar as the reburning fuel,but the oxygen content should be matched with the syngas flow rate. Excess oxygenwill lead to more combustion reactions, while too little oxygen will lead to too fuelrich environment, both hinder the NO reduction effect. Under proper oxygen content,it is favorable for NO reduction by increasing the temperature. In addition, the effectof tar on NO reduction should not be neglected. In contrast with the light gases, moreoxygen will be needed for tar to crack, especially for the tar species with lessreactivity.Based on the20kWe coal-fired boilers in our lab, a10kWthupdraft biomassgasifier which has a continued feeding system was build, and the pilot scale biomassgasification for reburning system was set up to simulate the full scale biomassgasification for reburning process. Aiming mainly at temperature, excess air ratio andthe reburn heat input ratio on NO reduction effect, a series of experiments werecarried out.The best operating condition for syngas reburning with tar was concluded,that is,when the reburning temperature was approximately1200℃，with the excessair ratio in the reburning zone of0.7and the reburn heat input ratio of15%，the NOreduction effect can be achievedapproximately84%.The tar analyzing results from the updraft gasifier showed that phenoliccompounds covered approximate40%in tar, so phenol（C6H5OH）was selected as tarmodel compound to study the detailed kinetic mechanism of NO reduction by tar. Themechanism was composed of two parts: one part is the tar cracking mechanism, andthe other part is the NO reduction mechanism by light gases. GRI3.0mechanism wasselected as the NO reduction mechanism by light gases because it’s popular andmature. The crack route of phenol was categorized depending on a lot of documents,and the pyrolysis mechanism of phenol was stressed in this paper concerning on theinitiation step of the phenol pyrolysis, the important intermediates reactions and thepreferences of the element reactions.The detailed NO reduction mechanism by phenol was verified by the experimental results. The calculating results matched well with the experimentalresults, so the mechanism can be used to analyze the NO reduction process. Theanalysis relying on the mechanism mainly from three aspects, which are the effect oftemperature and oxygen content on phenol cracking; the NO reduction route byphenol under different conditions and the most important radicals for NO reduction;thephenol cracking, the radicals generating and NO reduction with time.Based on sensitivity analysis, simplified mechanism was developed for NOreduction by syngas with tar, with phenol as tar model compound. The simplifiedmechanism was implemented in the20kWe coal-fired boiler to simulate the NOreduction effect. Aiming mainly on the effect of temperature and excess air ratio onNO reduction, the modeling results were compared with the experimental data. It isshown that the combustion and reburning process can be reasonably modeled with thekinetic mechanism under the experimental conditions. At last, the effect of NOreduction by changing the relative content of the main combustible gases and theresident time in the reburning zone was predicted using the simplified model. |
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