| China is one of the few countries that has an abundance of hard-to-burn coals taking up 40 % of the total coal power reserves. The main furnace types burning hard-to-burn coals are down-fired boilers, which have seen rapid development in China. Among these the most widely used is the Foster-Wheeler (FW) type. However, in the practical operation of these boilers, the problems of unstable combustion, high carbon content in fly ash and high NOx emission exist. Sponsored by the Hi-Tech Research and Development Program of China (863 program), a series of technology improvements for the retrofit of FW type down-fired boilers were developed. The technologies of declining secondary air (DSA) and high efficiency combustion (HEC) were progressively improved and finally a high-efficiency and low-NOx combustion (HELNC) technology emerged. By means of cold single-phase aerodynamic experiments, trajectory experiments, gas/solid two-phase flow experiments and numerical simulations, the mechanisms of these technologies were investigated and the operating parameters of the boilers were optimized. Finally, these technologies were applied to 660 MW and 300 MW down-fired boilers. By performing industrial experiments, their effects were evaluated and validated.Initially the characteristics of the original FW type down-fired boiler were investigated on a single phase and a gas/solid two phase 1:15 small scale model test facility. By regulating the E tier secondary air velocity and the fuel lean flow ratio, it was found that whatever the setting of these parameters, the fuel-rich flow could not penetrate the F-tier airflow zone and this lead to a relatively shorter coal residence time in the furnace. The industrial experimental results on a full scale 300 MW boiler unit show that the down-fired boilers of the original FW type had several problems such as the large resistance in the combustion system, the distance of the ignition point far from the nozzle, the flame kernel appearing in the upper furnace, the high level of unburnt carbon in the fly ash and the high NOx emissions. The current regulating method, including the regulation of the fuel lean air valve, secondary air damper, adjustable vane position and oil secondary air damper, was able to improve the combustion conditions over a limited range but could not solve the above problems entirely. As a consequence of the above problems, the application of the DSA technology was proposed. The results of cold single-phase and two-phase experiments on the small scale test facilities show that the rise of the secondary air angle increases the depth of reach of the fuel rich flow in the lower furnace. The turbulence intensity also increases. Considering both the coal residence time and the wall slagging, the angle of the F-tier secondary air was selected at 25°, and the technology was then applied to a 300MW boiler unit. It caused the NOx content to fall from 2101mg/m3 to 1926mg/m3 (at 6 % O2), the unburnt carbon in the fly ash to fall from 7.84% to 4.91% and the boiler efficiency to rise from 91.08 % to 93.25%.The HEC technology for FW type down-fired boilers was improved with the application of DSA. The results of the single phase experiments show that eliminating the original fuel-lean ports and guiding the flow through oil ports into the furnace does not significantly influence the flow field in the furnace. This technology was then applied to a 660 MW boiler unit. It caused the resistance of the combustion system to fall from 3724 pa to 1692 pa, the gas temperature at the furnace exit to fall from 136°C to 124°C, the unburnt carbon in the fly ash to fall from 9.55 % to 3.82 % and the boiler efficiency to rise from 84.54 % to 92.17 %. However, this technology had minimum influence on restricting the NOx emission.On the basis of the HEC technology, the low NOx retrofit was implemented on the down-fired boiler, thus creating the HELNC technology for FW type down-fired boilers. The results of the single phase experiments show that with the increase of the OFA ratio, the decay in fuel-rich flow slows down, the depth of reach for the fuel-rich flow in the lower furnace increases and the residence time for the coal in the furnace rises. By means of trajectory experiments, the flow characteristics for the OFA in the down-fired furnace under the condition of different OFA ratios, different nozzle angles and different nozzle structures were investigated. We determined that the OFA ratio should be at least 20 % and the nozzle angle 30°, the structures of the nozzles chosen were the inner direct and outer swirl types, and the vanes should be the chosen bending type located in the outer swirl nozzle. The results from the experiments on a two phase test facility indicate that 30°should be chosen as the optimized OFA nozzle angle in the furnace. Using Fluent 6.3, a numerical simulation was carried out for the combustion and NOx emission in the FW type down fired furnace. The results indicate that the boiler performance can be improved by applying the HELNC technology to the FW type down-fired boiler. Finally, this technology was applied to a 300 MW boiler unit. The results show that although the introduction of the OFA may increase the quantity of unburnt carbon in the fly ash, the decline of the secondary air and the guide of fuel lean flow through the oil port into the furnace promote the ignition of a fuel rich flow and improve the gas temperature distribution in the furnace. They eliminate the disadvantage of the OFA. After the retrofit, the amount of unburnt carbon in the fly ash reduced slightly and the boiler thermal efficiency increased. The NOx emission lowered by 50 %, which means that the HELNC technology for down-fired boilers can lower the NOx emission without reducing the efficiency of the boiler.
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