Qingyan Fang

Reducing NOx Emissions for a 600 MWe Down-Fired Pulverized-Coal Utility Boiler by Applying a Novel Combustion System

A novel combustion system was applied to a 600 MWe Foster Wheeler (FW) down-fired pulverized-coal utility boiler to solve high NOx emissions, without causing an obvious increase in the carbon content of fly ash. The unit included moving fuel-lean nozzles from the arches to the front/rear walls and rearranging staged air as well as introducing separated overfire air (SOFA). Numerical simulations were carried out under the original and novel combustion systems to evaluate the performance of combustion and NOx emissions in the furnace. The simulated results were found to be in good agreement with the in situ measurements. The novel combustion system enlarged the recirculation zones below the arches, thereby strengthening the combustion stability considerably. The coal/air downward penetration depth was markedly extended, and the pulverized-coal travel path in the lower furnace significantly increased, which contributed to the burnout degree. The introduction of SOFA resulted in a low-oxygen and strong-reducing atmosphere in the lower furnace region to reduce NOx emissions evidently. The industrial measurements showed that NOx emissions at full load decreased significantly by 50%, from 1501 mg/m3 (O2 at 6%) to 751 mg/m3 (O2 at 6%). The carbon content in the fly ash increased only slightly, from 4.13 to 4.30%.

Flexibility of a 300 MW Arch Firing Boiler Burning Low Quality Coals

Abstract Experimental investigations on the flexibility of a 300 MW Arch Firing (AF) coal-fired boiler when burning low quality coals is reported. Measurements of gas temperature and species concentration and char sampling using a water-cooled suction pyrometer were carried out along the furnace elevation. The carbon content and the size distributions of the char samples were obtained. The char morphology was examined using a field emission scanning electron microscope (FESEM). The char sampling was performed on this type of boiler for the first time. The results indicate that the flexibility of this boiler burning low quality coals under a moderate boiler load is better than its flexibility under a high boiler load. Because of the insufficient capacity of the coal pulverizers used, in case of low coal quality the pulverized coal fineness will drastically decrease under high boiler loads. This causes an increase in the loss due to incomplete mechanical and chemical combustion. This is the main cause of a low burnout degree of the pulverized coal and the decrease of the flexibility of this AF boiler under a high boiler load.

Modeling of Soot Formation in Gas Burner Using Reduced Chemical Kinetics Coupled with CFD Code

Abstract A computational study of soot formation in ethylene/air coflow jet diffusion flame at atmospheric pressure was conducted using a reduced mechanism and soot formation model. A 20-step mechanism was derived from the full mechanism using sensitivity analysis, reaction path analysis and quasi steady state (QSS) approximation. The model in premixed flame was validated and with computing savings in diffusion flame was applied by incorporating into a CFD code. Simulations were performed to explore the effect of coflow air on flame structure and soot formation. Thermal radiation was calculated by a discrete-ordinates method, and soot formation was predicted by a simple two-equation soot model. Model results are in good agreement with those from experiment data and detailed mechanism at atmospheric conditions. The soot nucleation, growth, and oxidation by OH are all enhanced by decrease in coflow air velocity. The peak soot volume fraction region appears in the lower annular region between the peak flame temperature and peak acetylene concentration locations, and the high soot oxidation rate due to the OH attack occurs in the middle annular region because of high temperature.

Experimental and Simulation Investigations of Combustion Optimization on a 125 MW Tangentially Anthracite-fired Boiler

Experimental and simulation investigations of combustion optimization were performed on a 125 MW tangentially, anthracite- fired boiler. The experiments including forty cases by burning three pure and five blended coals were firstly carried out under the different O2 concentrations in flue gas and distribution modes for secondary air. The results show that a reverse V-shaped operating for the secondary air mode and O2 concentration of 3%–4% is favorable to decrease the unburned carbon content in the fly ash and obtain the high boiler efficiency. After that, the BP artificial neural network technology was adopted to develop a simple network model between coal quality, boiler operating conditions and the boiler efficiency. The most important and representative parameters such as V ar and Q net, ar from approximate analysis, distribution mode for the secondary air and O2 concentration in flue gas at the furnace exit, were chosen as input parameters. The distribution mode for secondary air was denoted by the ratio of the damper opening values between upper and lower secondary air. Then a preprocessor method of approximate optimizing, based on the above model, was proposed to optimize the combustion performance of the boiler and establish an information library of combustion optimization for the boiler. Finally, validation tests were conducted to check the applications of the optimum information library.