The Effects Of Volatile Reaction And Stereo-Staged Combustion Technology On NO_x Emission

It is a primary goal of China, on the basis of its situation, to develop low-NO_x combustion technologies of low cost, high efficiency and good safety. To reach this goal, this dissertation investigated factors that affect NO_x control in utility boiler units and developed stereo-staged low-NO_x combustion technology for pulverised coal. which was validated via retrofit in a number of boilers.Firstly, experimental study was made on pyrolysis of bituminous coal with TG-FTIR. Coasts-Redfern quadrature is used to get the kinetic parameters of coal pyrolysis and it is found that the results are best correlated when reaction order equals to 1. Kinetic model for coal pyrolysis at the heating rates of 20, 40 and 80K/min respectively were proposed, and the modelling results agree well with experimental results. Effects of pyrolysis conditions on volatile components and their yields were studied, and mass fraction of the primary pyrolysis products (CH4, and CO et al) and N-component (NH₃) at different heating rates were determined.Secondly, it was confirmed that the FG-DVC model can be used to describe pyrolysis course of bituminous coal from China, since TG-FTIR data agree with FG-DVC modeling results well. The model was used to simulate the volatile components and yields. These data were used to GRI 3.0 chemical reaction mechanism combined with PFR reaction model to calculate transformation proportion (CR), which refers to Fuel-N converted into NO, at different equivalence ratios (ER). CR decreases with the increase of ER. The reaction time for stable Fuel-N conversion is about 0.2s by comparison with the experimental results. It is concluded that the suitable residence time for the fuel from primary combustion zone to the over-fire air zone should be 0.32 s at least. The parameters for the stereo-staged combustion technology can be established from this conclusion.Thirdly, the industry experimental investigations were carried out on 50 MW and 200 MW units, NO_x emission was decreased remarkably. In the 50MW units, it is no more than 450 mg/m³ by adopting stereo-staged combustion technology, and the result is 350 mg/m³ in the 200 MW units, with the minimum NO_x emission being 250 mg/m³, and enormous economic and social benifit are attained. The reliability of former mechanism is therefore validated in different utility boilers. At last, combustion and NO_x emission were simulated by using the models and computation method validated as above. It is shown that when NO emission was simulated on the FLUENT platform with pyrolysis data from FG-DVC model, NO_x emission agrees better with experimental results than using the pyrolysis data from FLUENT.This work provides theoretical and technical foundation for reducing NO_x emission from utility boiler, while improving combustion efficiency and saving energy consumption.