Overall Evaluation of Combustion and NO_<i>x</i> Emissions for a Down-Fired 600 MW_e Supercritical Boiler with Multiple Injection and Multiple Staging

To achieve significant reductions in NO_<i>x</i> emissions and to eliminate strongly asymmetric combustion found in down-fired boilers, a deep-air-staging combustion technology was trialed in a down-fired 600 MW_e supercritical utility boiler. By performing industrial-sized measurements taken of gas temperatures and species concentrations in the near wing-wall region, carbon in fly ash and NO_<i>x</i> emissions at various settings, effects of overfire air (OFA) and staged-air damper openings on combustion characteristics, and NO_<i>x</i> emissions within the furnace were experimentally determined. With increasing the OFA damper opening, both fluctuations in NO_<i>x</i> emissions and carbon in fly ash were initially slightly over OFA damper openings of 0–40% but then lengthened dramatically in openings of 40–70% (i.e., NO_<i>x</i> emissions reduced sharply accompanied by an apparent increase in carbon in fly ash). Decreasing the staged-air declination angle clearly increased the combustible loss but slightly influenced NO_<i>x</i> emissions. In comparison with OFA, the staged-air influence on combustion and NO_<i>x</i> emissions was clearly weaker. Only at a high OFA damper opening of 50%, the staged-air effect was relatively clear, i.e., enlarging the staged-air damper opening decreased carbon in fly ash and slightly raised NO_<i>x</i> emissions. By sharply opening the OFA damper to deepen the air-staging conditions, although NO_<i>x</i> emissions could finally reduce to 503 mg/m³ at 6% O₂ (i.e., an ultralow NO_<i>x</i> level for down-fired furnaces), carbon in fly ash jumped sharply to 15.10%. For economical and environment-friendly boiler operations, an optimal damper opening combination (i.e., 60%, 50%, and 50% for secondary air, staged-air, and OFA damper openings, respectively) was recommended for the furnace, at which carbon in fly ash and NO_<i>x</i> emissions attained levels of about 10% and 850 mg/m³ at 6% O₂, respectively

Industrial Application of an Improved Multiple Injection and Multiple Staging Combustion Technology in a 600 MW_e Supercritical Down-Fired Boiler

To solve the water wall overheating in lower furnace, and further reduce NO_<i>x</i> emissions and carbon in fly ash, continuous improvement of the previously proposed multiple injection and multiple staging combustion (MIMSC) technology lies on three aspects: (1) along the furnace arch breadth, changing the previously centralized 12 burner groups into a more uniform pattern with 24 burners; (2) increasing the mass ratio of pulverized coal in fuel-rich flow to that in fuel-lean flow from 6:4 to 9:1; (3) reducing the arch-air momentum by 23% and increasing the tertiary-air momentum by 24%. Industrial-size measurements (i.e., adjusting overfire air (OFA) damper opening of 20–70%) uncovered that, compared with the prior MIMSC technology, the ignition distance of fuel-rich coal/air flow shortened by around 1 m. The gas temperature in the lower furnace was symmetric and higher, the flame kernel moved upward and therefore made the temperature in near-wall region of furnace hopper decrease by about 400 °C, the water wall overheating disappeared completely. Under the optimal OFA damper opening (i.e, 55%), NO_<i>x</i> emissions and carbon in fly ash attained levels of 589 mg/m³ at 6% O₂ and 6.18%, respectively, achieving NO_<i>x</i> and carbon in fly ash significant reduction by 33% and 37%, respectively

Industrial Experiments on Anthracite Combustion and NO_<i>x</i> Emissions with Respect to Swirling Secondary Air for a 300 MW_e Deep-Air-Staged Down-Fired Utility Boiler

A new deep-air-staging and low-NO_<i>x</i> technology has been introduced to a 300 MW_e anthracite- and down-fired boiler with swirl burners. Industrial experiments were performed at different outer secondary air vane angles (defined as β) (i.e., 20°, 30°, 40°, and 50°) to evaluate the environmental and economic performance for the retrofitted boiler. Furthermore, combining with the previous investigations on the inner secondary air vane angle (defined as α), the influence degrees of β and α on anthracite combustion and NO_<i>x</i> emissions for the retrofitted boiler were further analyzed and compared. The experimental results revealed that the main factors affecting the ignition and the flame fullness for β and α are different. Compared with α, β had a relatively greater influence on NO_<i>x</i> emissions for the retrofitted boiler. Compared with the orignianl boiler, a strong reducing atmosphere was formed in the primary combustion zone for the retrofitted boiler, and for the β of approximately 30°, the arithmetic mean of NO_<i>x</i> emissions in the whole measurement range was reduced by 1073 mg/m³ at 6% O₂. Taking consideration of the environmental and economic effects, the optimal β for the retrofitted boiler was 20°

Combustion and NO_<i>x</i> Emission Characteristics with Respect to Staged-Air Damper Opening in a 600 MW_e Down-Fired Pulverized-Coal Furnace under Deep-Air-Staging Conditions

Deep-air-staging combustion conditions, widely used in tangential-fired and wall-arranged furnaces to significantly reduce NO_<i>x</i> emissions, are premature up to now in down-fired furnaces that are designed especially for industry firing low-volatile coals such as anthracite and lean coal. To uncover combustion and NO_<i>x</i> emission characteristics under deep-air-staging conditions within a newly operated 600 MW_e down-fired furnace and simultaneously understand the staged-air effect on the furnace performance, full-load industrial-size measurements taken of gas temperatures and species concentrations in the furnace, CO and NO_<i>x</i> emissions in flue gas, and carbon in fly ash were performed at various staged-air damper openings of 10%, 20%, 30%, and 50%. Increasing the staged-air damper opening, gas temperatures along the flame travel (before the flame penetrating the staged-air zone) increased initially but then decreased, while those in the staged-air zone and the upper part of the hopper continuously decreased and increased, respectively. On opening the staged-air damper to further deepen the air-staging conditions, O₂ content initially decreased but then increased in both two near-wall regions affected by secondary air and staged air, respectively, whereas CO content in both two regions initially increased but then decreased. In contrast to the conventional understanding about the effects of deep-air-staging conditions, here increasing the staged-air damper opening to deepen the air-staging conditions essentially decreased the exhaust gas temperature and carbon in fly ash and simultaneously increased both NO_<i>x</i> emissions and boiler efficiency. In light of apparently low NO_<i>x</i> emissions and high carbon in fly ash (i.e., 696–878 mg/m³ at 6% O₂ and 9.81–13.05%, respectively) developing in the down-fired furnace under the present deep-air-staging conditions, further adjustments such as enlarging the staged-air declination angle to prolong pulverized-coal residence times in the furnace should be considered to improve the deep-air-staging combustion configuration