Guangkui Liu

Improved NOx emissions and combustion characteristics for a retrofitted down-fired 300-MWe utility boiler.

A new technique combining high boiler efficiency and low-NO(x) emissions was employed in a 300MWe down-fired boiler as an economical means to reduce NO(x) emissions in down-fired boilers burning low-volatile coals. Experiments were conducted on this boiler after the retrofit with measurements taken of gas temperature distributions along the primary air and coal mixture flows and in the furnace, furnace temperatures along the main axis and gas concentrations such as O(2), CO and NO(x) in the near-wall region. Data were compared with those obtained before the retrofit and verified that by applying the combined technique, gas temperature distributions in the furnace become more reasonable. Peak temperatures were lowered from the upper furnace to the lower furnace and flame stability was improved. Despite burning low-volatile coals, NO(x) emissions can be lowered by as much as 50% without increasing the levels of unburnt carbon in fly ash and reducing boiler thermal efficiency.

Influence of the overfire air ratio on the NO(x) emission and combustion characteristics of a down-fired 300-MW(e) utility boiler.

Down-fired boilers used to burn low-volatile coals have high NO(x) emissions. To find a way of solving this problem, an overfire air (OFA) system was introduced on a 300 MW(e) down-fired boiler. Full-scale experiments were performed on this retrofitted boiler to explore the influence of the OFA ratio (the mass flux ratio of OFA to the total combustion air) on the combustion and NO(x) emission characteristics in the furnace. Measurements were taken of gas temperature distributions along the primary air and coal mixture flows, average gas temperatures along the furnace height, concentrations of gases such as O(2), CO, and NO(x) in the near-wall region and carbon content in the fly ash. Data were compared for five different OFA ratios. The results show that as the OFA ratio increases from 12% to 35%, the NO(x) emission decreases from 1308 to 966 mg/Nm(3) (at 6% O(2) dry) and the carbon content in the fly ash increases from 6.53% to 15.86%. Considering both the environmental and economic effect, 25% was chosen as the optimized OFA ratio.

Numerical Simulation of Low NO x Combustion Technology in a 100 MWe Bituminous Coal-Fired Wall Boiler

Computational fluid dynamics (CFD) has been applied to evaluate two NO x reducing schemes in a 100 MWe per hour (p/h) boiler that uses double volute burners without over-fire-air (OFA). The new schemes involve: a) changing the double volute burners for centrally fuel rich (CFR) burners, and b) using the OFA system in conjunction with a). In analyzing the results of these two schemes, various conclusions were drawn: 1) gas temperatures and related rise rates in the central zone of burners were higher, O2 and NO x concentrations were lower; and 2) cross-sectional gas temperature distributions through the burner centers in scheme employing b) is higher than that of original furnace set-up, and lower than that of scheme employing a). Comparing the b) scheme with those of the a) scheme and the original set-up, which is 277 mg/m3 (at 6% O2) at the furnace outlet, the peak value of NO x concentration has decreased 571 mg/m3 (67.4%) and 436 mg/m3 (61.2%), respectively.

Influence of Angled Secondary Air on Combustion Characteristics of a 660-MWe Down-Fired Utility Boiler

To overcome the problem of high carbon content in the fly ash of down-fired utility boilers using low-volatility coals, the combustion system of a 660 MWe full-scale down-fired boiler was retrofitted, with the direction of the secondary air under the arch being changed from horizontal to an angle of declination of 20°. Industrial experiments were performed using the boiler before and after the retrofit to determine the reconstruction effect. Data are reported for the gas temperature distribution along the primary air and coal mixture flow, furnace temperature, gas compositions, such as O2, CO, CO2, and NO x , and gas temperatures in the near-wall region. Comparisons between the two cases were made, and the results show that with the angled secondary air under the arches, ignition of the primary air and pulverized coal mixture was brought forward in the boiler. Gas temperatures rose in the fuel-burning zone, and the residence time of pulverized coal in the fuel-burning zone was extended. Thus, the quantity of unburned carbon in fly ash and the gas temperature at the furnace outlet decreased, and the boiler efficiency increased.

Airflow and Combustion Characteristics and NOx Formation of the Low-Volatile Coal-Fired Utility Boiler at Different Loads

In order to solve the problems of lower heating rate of the primary air/fuel mixture and higher NO x emission of the boiler, both the small-scale cold air experiment of a single burner and industrial-scale experiments of the centrally fuel-rich (CFR) swirl burner on a retrofitted 300 MW wall-fired boiler fed with low-volatile coal under deep air staging were performed. The aerodynamic characteristics, flue gas temperature, and temperature distribution of the furnace and carbon content in the fly ash were measured at loads of 300, 230, and 150 MW. Results illustrate that a central recirculation zone appeared close to the outlet the CFR burner under deep air staging. Compared with the enhanced ignition-dual register (EI-DR) burner, the gas temperature and the heating rate of the CFR burner are much higher, so it can guarantee the timely ignition and stable combustion of the pulverized coal. As for the CFR burner, with a decreasing load, the heating rate of the gas temperature decreases and the ignition position of the primary coal/air mixture becomes backward. The overall temperature of the furnace also decreases with decreasing load, as does the difference between the temperatures in the burning region and the lower position of the burnout region. After the retrofitting of the combustion system, the exhaust gas temperature decreases from 149 to 146 °C, the NO x emissions at the air preheater exits decrease from 1354 to 778 mg/m3 (6 % O2), and the boiler thermal efficiency increases from 89.75 to 90.57 % at the rated load.