Research On Doosan Babcock Down-fired Boilers And Multi-injection Multi-stage Combustion Technology

Anthracite and lean coal account for more than40%of power coal in China.Down-fired boilers, designed specifically for the industrial firing of anthracite and leancoal, have become popular in China in recent years. Currently, approximately130down-fired boilers are either in service or are under construction in China. Operatingresults reveal that down-fired boilers suffer from various problems such as late coalignition, heavy slagging, asymmetric combustion, poor burnout, and high NO_xemissions. According to newly promulgated emission standards for air pollutants fromthermal power plants （GB13223–2011）, the allowed NO_xemission concentration fordown-fired boilers in power plants in China is200mg/m³at6%O₂. These newguidelines require that most down-fired boilers must be retrofitted with low-NO_xcombustion and flue gas denitration processes. Again, problems of fin cracking andtube bursting in water-cooled walls, caused by large differences in water-cooled walltemperatures and these temperatures exceeding allowable values excessively, affect thesafe operation of down-fired supercritical utility boilers. In this dissertation, a newdown-fired combustion technology, i.e., multi-injection multi-stage combustiontechnology, has been developed to resolve these problems. This new technologycombustion system is characterized by three features:（i）: in the burner configuration,the fuel-rich coal/air flow nozzles, inner secondary-air ports, fuel-lean coal/air flownozzles, and outer secondary-air ports are located in order on the furnace arches fromthe side near the furnace center to the side near the front and rear walls;（ii） tertiary airis fed into the lower furnace through the lower part of the front and rear walls; and （iii）over-fire air （OFA） is positioned on the furnace arches in the zone near the furnacethroat. Three technical principles govern the new down-fired combustion technology.（i）Air-driving mechanism—Because of static-pressure differences between the low-speedfuel-rich coal/air flow and three high-speed jets （i.e., inner secondary, outer secondary,and tertiary air）, the fuel-rich coal/air flow is guided downstream by these high-speedjets into the hopper region to prolong the residence time of the pulverized coal in thefurnace and enable efficient fuel combustion.（ii） Symmetrical combustion—Thedeclivitous tertiary air and unique burner configuration establish a symmetricalcombustion pattern in the furnace.（iii） Deep-air-staging combustion—Multiple airinjections consisting of inner and outer secondary air, tertiary air, and OFA alongthe flame travel form deep-air-staging conditions in the furnace, which inhibitsignificantly the NO_xformation. Moreover, a centralized layout of the high-speedinner and outer secondary air in the zone near the front and rear walls, the high-speedtertiary air jet in the near-wall zone in the hopper, and the symmetrical combustionformation, prevent large differences in water-cooled wall temperatures and these temperatures exceeding allowable values significantly. Besides the investigations intovarious parameter optimizations and industrial applications for the new down-firedcombustion technology, this dissertation also provides a study on the flowcharacteristics and operation status of Doosan Babcock （DB） down-fired boilers,determined by cold single-phase and gas/solid two-phase flow experiments, industrialmeasurements, and numerical simulation. In comparison with DB down-firedcombustion technology, down-fired boilers with this new technology exhibit goodburnout, low-NO_xproduction, low slagging tendency and safe operation.Industrial measurements in three DB down-fired boilers （i.e., two subcriticalboilers with capacities of350and300MWeand a600MWesupercritical boiler） revealthat severely asymmetric combustion characterized by gas temperature differences ashigh as300°C arising between the regions near the front and rear walls, developed inall three furnaces. Primary measures such as parametric tuning of operating conditionsfailed to eliminate this asymmetric combustion phenomenon. At normal full load, thehighest carbon content in fly ash, the lowest boiler efficiency, and NO_xemissions were15%,83%and1100–1700mg/m³（at6%O₂）, respectively. Heavy slagging occurredin the lower furnaces of the350and300MWeboilers, especially the wing-andside-wall regions with refractory coverage. Additionally, large differences inwater-cooled wall temperatures occurred in the600MWesupercritical boiler and thesetemperatures exceeded the allowable values significantly.Using cold single-phase and gas/solid two-phase flow experiments in small-scalemodel test facilities, it was found that a deflected flow field appeared in all three boilers.This explains the severely asymmetric combustion in real furnaces. Unreasonabledesigns in air distribution, burner arrangement, and staged-air supply direction favor theflow-field deflection formation. Cold small-scale airflow experiment results for the300MWeboiler showed that various methods such as reducing the tertiary-air ratio,inclining downward tertiary air, shortening the secondary-air port area in the side nearthe furnace center, and constructing an asymmetric secondary-air distributionbetween the front and rear arches （or asymmetric tertiary-air distribution between thefront and rear walls）, could mitigate or eliminate the flow-field deflection. Afterleaving the port outlet, secondary air with higher velocity than that of the fuel-richcoal/air flow mixes rapidly with the slower flow. This process dilutes the pulverizedcoal concentration, increases the fuel-rich flow velocity in the burner zone, andfacilitates the early ignition of pulverized coal in an oxygen-rich atmosphere, therebydelaying coal ignition and producing a large amount of fuel-NO_x. In conjunction withlow tertiary-air ratios （below15%） forming shallow staging conditions in these furnace,high levels of NO_xemissions are unavoidable. The high-velocity secondary air carryingthe fuel-rich coal/air flow to wash over the front and rear walls and thehigh-temperature flue gas entrained particles expanding towards the wing and side walls, are the main causes of the heavy slagging in the furnaces.Using cold small-scale airflow experiments and numerical simulations, theimpact of various parameters settings in the new technology such as tertiary-airratio, tertiary-air declination angle, OFA ratio, and OFA declination angle on theflow field was determined for the350MWeboiler. This was in addition to the effect ofOFA location and air stoichiometric ratio in the primary combustion zone （SRp） on thecombustion characteristics and NO_xformation in the furnace. Using a series ofoptimization investigations, the optimal setup for the new technology was finallydetermined, i.e.,25%and20%for the tertiary-air and OFA ratios,45°and40°forthe tertiary-air and OFA declination angles, the arch zone close to the furnace throatfor the OFA positioned location, and0.96for the SRpvalue when the airstoichiometric ratio in the burner zone is set at0.66, respectively. Application of thenew technology as a replacement for the prior DB art in the350MWeboiler led tothree furnace performance improvements as identified by numerical simulations.（i）The original asymmetric flow field, gas temperature, and gas componentdistributions in the furnace all develop a symmetrical pattern.（ii） Relatively low gastemperatures and high O₂concentrations exist in near-wall zones, therebymitigating slagging in the lower furnace.（iii） NO_xemissions decrease by as much as50%, without increasing levels of unburnt carbon in fly ash.Finally, the new technology was applied to two newly designed down-fired600MWesupercritical utility boilers. Before applying the OFA system, results from thecold single-phase and gas/solid two-phase flow experiments for one of the twoboilers revealed that a well-formed symmetrical flow field developed in the furnace.A high-particle-concentration and low-velocity zone existed in the burner zone inthe cold gas/particle flow field. This could facilitate timely coal ignition andfuel-NO_xreduction in the real furnace. Industrial measurements showed that thefurnace achieved well-formed symmetrical combustion and the symmetrical patternwas not affected by the latter OFA application and two retrofits in tertiary air.Before the OFA application, the furnace had below4%carbon in fly ash, relativelyhigh NO_xemissions （above1000mg/m³at6%O₂） and thermal fatigue ofwater-cooled walls in the hopper slope because the too deep flame penetration depthproduced gas temperatures of up to1350°C in the hopper region. After applying OFAand reducing the staged-air declination angle from45to20°, the hopper region stillmaintained relatively high gas temperature levels, despite gas temperaturesdecreasing in the lower furnace. With increasing the OFA damper opening in therange of40–70%, NO_xemissions decreased significantly and carbon in fly ashincreased sharply. Fixing the OFA damper at an opening of50%and increasing thestaged-air flux reduced carbon in fly ash and raised NO_xemissions slightly. Byadjusting the air distribution, NO_xemissions could be reduced to approximately878 mg/m³at6%O₂, with relatively high carbon in fly ash of approximately10%.Fortunately, the latter retrofit where tertiary air is supplied horizontally and thetertiary-air slot area is increased resulted in decreased gas temperatures in the hopperregion to safe levels of approximately1110°C and relatively low carbon in fly ash （i.e.,approximately5%） with high secondary-and tertiary-air damper openings of75%.This improved the previously deteriorated burnout and resolved the thermal fatigueproblem in the hopper region. Under these circumstances, opening OFA affected theburnout rate slightly but reduced s NO_xemissions significantly. The furnace couldfinally attain a well furnace performance with low NO_xemissions （867mg/m³at6%O₂） and relatively high burnout rate （carbon in fly ash of5.4%）. In addition, largedifferences in water-cooled wall temperatures and phenomenon of the water-cooledwall temperatures exceeding allowable values were absent at boiler loads of300,450,and600MWe. In comparison with the imported down-fired combustion technologiessuffering from the problems mentioned above, the proposed multi-injectionmulti-stage combustion technology has been confirmed to be excellent in attainingsafe boiler operation, symmetrical combustion, weak slagging tendency, goodburnout, and low NO_xemissions.