The effects of various factors on NOx reduction by urea solution injection in the fuel-rich zone (UIFR) under a reducing atmosphere at high temperature were experimentally investigated in a 330 MW tangentially pulverized coal-fired boiler. The experimental results indicated that the NOx emission of the boiler could be effectively reduced by using the UIFR method, and the NOx reduction efficiency was mostly affected by the operating load of the boiler, the air distribution condition, and the boiler operating oxygen content (BOOC). The higher the load was, the larger the optimal normalized stoichiometric ratio (NSR) and the lower the NOx reduction efficiency became. As compared with the condition of conventional air distribution mode, the reducing atmosphere in the combustion zone could be enhanced under the condition of limiting air distribution (LAD) mode, which thus increased the NOx reduction efficiency of UIFR and reduced the optimal NSR value. A low BOOC could further increase the NOx reduction efficiency of UIFR. When the BOOC was, respectively, reduced to 1.71 and 1.85 vol % under 210 and 240 MW loads, the corresponding NOx reduction efficiencies of UIFR reached 45.3 and 41.3% in the LAD mode, respectively. However, the low BOOC increased the CO emission concentration, which could be avoided by the combined use of UIFR and high-velocity over-fire air. These experimental results can provide guidance for the ultra-low NOx emission of coal-fired boilers.

The effects of various factors on NOx reduction by urea solution injection in the fuel-rich zone (UIFR) under a reducing atmosphere at high temperature were experimentally investigated in a 330 MW tangentially pulverized coal-fired boiler. The experimental results indicated that the NOx emission of the boiler could be effectively reduced by using the UIFR method, and the NOx reduction efficiency was mostly affected by the operating load of the boiler, the air distribution condition, and the boiler operating oxygen content (BOOC). The higher the load was, the larger the optimal normalized stoichiometric ratio (NSR) and the lower the NOx reduction efficiency became. As compared with the condition of conventional air distribution mode, the reducing atmosphere in the combustion zone could be enhanced under the condition of limiting air distribution (LAD) mode, which thus increased the NOx reduction efficiency of UIFR and reduced the optimal NSR value. A low BOOC could further increase the NOx reduction efficiency of UIFR. When the BOOC was, respectively, reduced to 1.71 and 1.85 vol % under 210 and 240 MW loads, the corresponding NOx reduction efficiencies of UIFR reached 45.3 and 41.3% in the LAD mode, respectively. However, the low BOOC increased the CO emission concentration, which could be avoided by the combined use of UIFR and high-velocity over-fire air. These experimental results can provide guidance for the ultra-low NOx emission of coal-fired boilers.

The distribution, composition and yield of coal-water slurry pyrolysis products have an important impact on the efficient combustion/gasification of coal-water slurry. In this paper, the coal-water slurry made of Shenmu coal (bituminous coal) is rapidly pyrolyzed by a high-frequency heating furnace, and the yield, composition and composition of pyrolysis gas were measured and analyzed. The effects of pyrolysis temperature, heating rate and residence time on the pyrolysis characteristics of coal-water slurry were studied. The results have shown that as the temperature increases, the yields of volatile matters and pyrolysis gas continue to increase. The compositions of pyrolysis gas are mainly H2, CO, CH4 and CO2. With the increase of temperature, the yields of H2, CO and CH4 increase first and then decrease, and peaks appear at around 1100℃. The CO yield continues to increase with increasing temperature. The rate of temperature increase affects the yield of volatiles. The research results provide a reference for understanding and mastering the formation characteristics of primary pyrolysis products of coal-water slurry. © 2022, Tsinghua University Press.

The influence of industrial processing parameters, especially high temperature, on NOx abatement in the absence of oxygen has been experimentally. NOx reduction efficiency is significantly promoted with increasing residence time and NSR and optimal residence time and NSR are 0.7s and 1.5, respectively, when temperature exceeds 1400 ℃. NOx reduction is strongly dependent on temperature. When temperature is lower than 1000 ℃, NO consumption is hindered due to lack of O radicals. The denitrification efficiency is significantly promoted with the increase of temperature because thermal decomposition of CO2 and NO is quite sensitive to temperature. However, NO formation from pyrolysis of HNO begins plays an important role since temperature exceeds 1400 ℃, which results in decline in NOx reduction efficiency. And the peak value of NO reduction efficiency can reach almost 100% at temperature range of 1300–1400 ℃ with NSR of 1.5. Four chemistry mechanisms have been adopted to simulate NOx reduction by ammonia. Validation shows that results calculated by POLIMI chemistry mechanism agrees better with experimental data than other 3 mechanisms. © 2022, Tsinghua University Press.

A three-dimensional trial bed is established for a staged combustion boiler, and a modeling method based on similarity theory is proposed. The aerodynamic field of the 35 t/h layer combustion-composite combustion chamber-in the stoker boiler with staged combustion was evaluated. Further, a three-dimensional calculation model based on computational fluid dynamics (CFD) was used to simulate the aerodynamic field of the reformed boiler under normal operation, which facilitated convenience in the boiler design. Hot-wire anemometer and other instruments were used for the characteristic test of damper, a velocity field test in the furnace, wall wind test, temperature balance test at the outlet of the furnace, etc., and the law of motion for the flow field in the furnace was obtained. By analyzing the structure of staged combustion, the emission of nitrogen oxides and the combustion stability of a novel layer-fired boiler were studied. The calculated results are in excellent agreement with the experimental data. The results revealed that the combustion efficiency of the boiler and the reduction of nitrogen oxides were significantly improved by the staged combustion technology. There was no erosion on the water wall, and the flow velocity at the outlet of the furnace was uniform. This modeling method exhibits good adaptability to the combustion of stratified combustion boilers and is potentially useful for optimizing furnaces in a variety of applications.

This work indicates the functionality of injection reagent combined with air staging, which achieves a better NOx reduction than bare air staging in coal combustion. The key influencing factors of NOx reduction were investigated by experiments on a bench-scale combustion test system. The results showed that the NOx reduction efficiency could be increased by 22.2% compared to bare air staging. The primary stoichiometric ratio SR1 was the key parameter, and the optimum SR1 for NOx reduction by injection was 0.85. There was no benefit for NOx reduction by injection urea in the very fuel-rich zone, but when SR >= 1, the injected urea solution at the higher temperature leads to more NOx formation. A higher temperature greatly improved NOx reduction by injection when SR1 was less than 1. It is because that high temperatures were the necessary condition for a greater generation of OH free-radicals, which promoted NOx reduction with NH3 in the absence of oxygen. Therefore, this technology broken through the limitation of narrow temperature window in the traditional technologies with NO reduction by NH3 reagent. Moreover, deeper NOx reduction can be achieved by this technology combined with the existing low NOx combustion technologies.

A 3D computational fluid dynamic (CFD) simulation is performed on a 35t/h grate boiler to investigate the compound combustion, consisting of grate firing of lump coal on the fire grate and suspension combustion of pulverized coal sprayed in the middle of the stoker. A grate boiler-based low-nitrogen combustion method is proposed in the present study. The influences of this technology on the boiler temperature field and flue gas components as well as NOxare studied by comparing two different combustion modes before and after the transformation. Results show that the compound combustion technology can significantly improve the boiler efficiency and reduce the NOx concentration at the outlet. Therefore, the proposed method may have favorable applications in different industries. It is found that the boiler NOx emission can reach an optimal status, when the pulverized coal feed rate of the burner and the proportion of pulverized coal are set to 20m/s and 20%, respectively. It is observed that when the forward spinning arrangement form is adopted, the emission can be reduced up to 58.08%. The NOx at the outlet of a 35t/h boiler was reduced by 35.71% after retrofitting, which is in consistency with the obtained results.

In this paper, the characteristics of NOx reduction by reagent injection into the fuel-rich zone (RIFR) in coal combustion were investigated, under high temperatures and reducing atmosphere conditions. The stoichiometry and temperature have a major influence on the chemistry of NO and NH3 reagent (ammonia or urea). Therefore, experiments were conducted on a bench-scale test system with urea solution as the reagent to investigate the key factors influencing NOx reduction, including primary stoichiometric ratio (SR1), temperature in the reaction zone, and normalized stoichiometric ratio (NSR) of the injected reagent. The results indicated that the primary stoichiometric ratio SR1 was the key parameter affecting the reduction of NOx emissions. Better NOx reduction was achieved with a decrease in the SR1 for bare air staging. However, there was no benefit for NOx reduction by reagent injection in the very fuel-rich zone (SR1 <= 0.75), which depended on the distribution of N-intermediates and initial NO concentration. On the other hand, a negative NOx reduction was obtained by reagent injection when SR1 >= 0.95 because the added reagent was oxidized to form NO. The optimum SR1 for RIFR was found to be 0.85 in this study. A higher SR1 greatly improved the NOx reduction by RIFR only when SR1 was less than 1, and high temperatures (1473-1673 K) were required for the generation of more OH free-radicals in the fuel-rich zone, which promoted NOx reduction by NH3 in the absence of oxygen. Therefore, RIFR is different from the traditional selective non-catalytic reduction technology, which has a strong temperature dependency from 1100-1300 K. The NOx reduction efficiency was increased by 21.4% with RIFR compared to the bare air-staged method, under the optimum conditions of SR1 = 0.85, T = 1673 K, and NSR = 2. More urea solution led to greater NOx formation in the burnout zone but with no ammonia slip. This method can be applied as a new alternative technology to further reduce NOx in combination with the existing low NOx combustion technologies.