Experimental Study On Fuel Nitrogen Migration Characteristics For The Macro Coal Particles And Numerical Modeling Of The No_x Prediction For The Grate Combustion

The primary energy structure of China is based on coal,most of which are utilized by direct combustion.Industrial boiler ranks in the second place of coal consumption,only less than the coal-fired power plant.Thus the NO_x emission problems should not be ignored for the coal-fired grate boiler.The process of macro coal particle grate combustion in the grate boiler is very different from the pulverized coal combustion.The fuel nitrogen migrations and conversions are very complicated in the macro coal particle utilization processes including pyrolysis,char oxidation and heterogeneous reduction over char.However these processes have not gained enough attentions and are lack of in-depth study.The nitrogen removal technologies in the grate boiler nowadays primarily learned from the power plant boiler,of which the nitrogen removal cost is high.The clean and effective ways of combustion in the grate boiler are needed.The trends of nitrogen function groups migration and release of HCN and NH₃ for macro coal particle pyrolysis were experimentally investigated.A horizontal fixed bed was used,combining the XPS and FTIR detection methods,to study effects of coal type,pyrolysis temperature and particle sizes.The nitrogen function groups mutual convert and decompose to light gas-N like HCN and NH₃ during coal pyrolysis.With the rising pyrolysis temperature,the total amount of nitrogen function groups decrease and the released HCN and NH₃ amount increase.When the pulverized coal was piled up during pyrolysis process,the more amount of NH₃ was released due to secondary reaction between char and HCN.The pyrolysis for the macro coal particle produced more NH₃.Because the macro char particle is denser,the residence time for the pyrolysis gases staying in the char pores becomes longer,the secondary reaction is more severe and more HCN converts to NH₃.In the study of grate combustion in coal-fired grate boiler,pulverized coal pyrolysis data cannot be directly applied to NO_x model.The NO_x model requires macro coal particle pyrolysis experiment to obtain related parameters of the distribution of light gas-N.The trends of char nitrogen migration for macro char particle oxidation process were experimentally investigated.A horizontal fixed bed was used,combining the XPS and testo flue gas analyzer,to study effects of combustion temperature,coal type,oxygen concentration and particle sizes.With the increasing temperature,The NO from char oxidation first increases then decreases.The low-temperature oxidation of char produces large amount of oxygenated function groups,promoting the NO reduction over char.At high temperatures,the gasification reaction on the char surface becomes more severe.Meanwhile on the char surface the CO reacts with C?N?yielding C?NCO?,which promotes the NO reduction.With coal rank increasing,the char reactivity decreases,the active sites for NO reduction in in char pores decreases and the NO produced by char nitrogen oxidation increases.In macro char particle oxidation process the NO reduced over char is due to the in-site reduction by carbon on the inner surface of the char pores.The macro char particle is denser and there are large amount of oxygenated function groups in the char pores with low oxygen environment,promoting the NO reduction over char.Meanwhile the residence time for the NO staying in the char pores becomes longer,and more NO converts to N₂.Based on experimental results,given the effects of char oxidation rate,temperature and particle sizes,a numerical model for the NO formation by the char nitrogen oxidation in the macro coal particle was established.The rates of NO heterogeneous reduction over macro char particle were experimentally investigated.A fixed bed reactor was used,combining the XPS,XRD and testo flue gas analyzer,to study effects of temperature,coal type,particle sizes and the height of char bed.With an increase in pyrolysis temperature,the crystal structure of char becomes ordered,while the active sites for heterogeneous reduces and the rate of NO reduction over char is lowered.Meanwhile the total amount of oxygenated function groups decreases,which lowers the promotion of the NO heterogeneous reduction by the oxygenated function groups.The reactive surface area for the larger particle with identical weight is less than the smaller particle,thus the NO reduction rate is slower for the larger one.With an increase in the char bed height,the involved carbon active sites increase,meanwhile the residence time for the NO in the char bed also increases,these factors are favorable for the NO reduction over char.Based on experimental results,a calculation model for the NO reduction over macro particle was built.In order to reveal the NO formation characteristics in the process of macro coal particle grate combustion in the grate boiler.A unit boiler reactor was used to study effects of coal bed height,air distribution ways,particle sizes and layered bed with multiple particle sizes.The NO formation on the char bed surface shows typical two-peak structure,the first peak is due to the oxidation of HCN and NH₃ in the volatile,the second peak is due to the char nitrogen oxidation during the burn out stage of the grate combustion.The lowest value between the two peaks is due to the NO reduction over char.The decrease in the height of coal bed lowers the char reduction layer,decreasing the NO reduction amount,and the NO from coal bed grows.The delay air distribution decreases the front oxygen,leading to less NO by oxidation.Meanwhile the reaction between char reduction layer and NO becomes more severe which is favorable for the NO reduction.With increasing char particle,the NO reduction rate decreases,the effect of char reduction layer is weakened and more NO formed.On the condition of layered bed with multiple particle sizes,the upper smaller char particles are favorable of NO reduction.By using delay air distribution and thicker coal bed height operation mode,the reaction between char reduction layer and NO becomes more severe and two sets of chain grate boiler's NO_x emission are reduced.The expected results of the low NO_x control test have been achieved.Finally a NO_x model for grate combustion in the grate boiler is built,based on previous works of numerical modeling.The numerical results were validated by the unit boiler experimental results.An in-use industrial grate boiler was numerically investigated.Some conclusions are obtained through the numerical analysis of the excess coefficient for coal bed,the arch shape and the air staged combustion.First reducing the excess coefficient and increasing the height of char reduction layer can reduce the NO formation from the char bed.Second,utilizing the back arch to get some of the NO formed by char oxidation to the front position beneath the front arch,forcing them to react with NH₃,lowers the NO in the gaseous space.Finally a mechanism of low NO_x combustion operation based on the combination of arch shape and air distribution in grate boiler is obtained,which offers a solid technical support for the low NO_x operation improvement of the realistic boilers.