Down-fired Boiler Research Articles

ICOPE-15-C078 Effects of vent air angles on combustion and NO emission characteristics for a down-fired 600MW pulverized-coal utility boiler retrofitted with a novel combustion system

ICOPE-15-C078 Effects of vent air angles on combustion and NO emission characteristics for a down-fired 600MW pulverized-coal utility boiler retrofitted with a novel combustion system

Numerical simulations of flow, combustion characteristics, and NOx emission for down-fired boiler with different arch-supplied over-fire air ratios

This paper presents research on a 660 MWe Foster Wheeler (FW) type down-fired boiler. The arch-supplied over-fire air (OFA) plan and high efficiency measures are proposed for the boiler. The influence of different OFA ratios on flow field, temperature distribution, and O2 and NOx concentration distributions inside the furnace are investigated by numerical simulation using Fluent 6.3.26 software. Also, the average temperature, the carbon content in the fly ash, and the O2 and NOx concentrations at the exit of the furnace are calculated. Three high-temperature zones are identified in the furnace: at the front and rear wall under the arch, and at the bottom of the upper furnace. As the OFA ratio increases, the gas temperature, the carbon content in the fly ash, and the O2 concentration increase, but the NOx concentration decreases significantly at the exit of the furnace. In particular, when the OFA ratio increases from 20% to 25%, the amplitude of the decrease of the NOx emissions is smaller, but the combustible content in the fly ash increases more obviously. Considering the NOx emissions and the combustible content in the fly ash, an OFA ratio of 20% is advisable for the arch-supplied OFA program.

Characterization of Coal Combustion and Steam Temperature with Respect to Staged-Air Angle in a 600 MWe Down-Fired Boiler

The staged-air angle was adjusted from 45° to 20° on a down-fired 600 MWe supercritical utility boiler suffering from superheat and reheat steam temperatures operating at below their designed values. This modification was expected to elevate the flame kernel position and increase the combustion share in the upper furnace, thereby raising superheat and reheat steam temperatures. To evaluate the effect of this modification on coal combustion, NOx emissions, and the aforementioned steam temperatures, normal full-load industrial-size measurements were taken within the furnace before and after the angle reduction. A comparison of the data from the two angle settings revealed that there were slight changes in superheat and reheat steam temperature levels as well as NOx emissions. Unexpectedly, the angle reduction caused several adverse effects, including a reduction of gas temperatures in the furnace, a sharp increase of carbon in fly ash, and a decrease in boiler efficiency. In conclusion, the staged-air angle...

Effect of the arch-supplied over-fire air ratio on gas/solid flow characteristics of a down-fired boiler

An OFA (over-fire air) program and matched measures supporting efficient-combustion are proposed for the FW (Foster Wheeler)-type down-fired boiler, where the OFA is supplied from the arches of the boiler. A gas/solid cold test was conducted at a scale of 1:15, using a two-dimensional laser phase Doppler analyzer to measure parameters in the model furnace, such as the two-phase gas/solid flow velocity, fluctuation velocity, particle size, particle volume flux and particle number concentration. The effects of the OFA ratio on the gas/solid flow characteristics in the furnace were investigated. With an increase in the OFA ratio, the jet depth of the fuel-rich flow decreased. For an OFA ratio of 25%, the jet depth of the fuel-rich flow was much less than that for other OFA ratios, and the residence time of the gas/solid flow in the lower furnace was shorter. An increase in the OFA ratio can increase the penetration depth and allow fuller mixing with the upward airflow, while an OFA ratio that is too great will hinder the upward flow back to the burner nozzle region. An OFA ratio of 20% is considered appropriate for the proposed OFA technical scheme.

Combustion and NOx Emission Characteristics of a Down-Fired Furnace with the Hot Air Packing Combustion Technology

With the focus on improving coal burnout and controlling NOx emissions in down-fired boilers, a new deep air-staging combustion technology, the hot air packing technology (HAPT), was investigated by experiments and numerical simulations. The effects of the special secondary air ports added in the furnace ash hopper (SA-H) and the furnace bottom (SA-B) were analyzed by comparing the three cases: the HAPT, no-SA-H, and no-SA-B cases. The experiments with Guizhou anthracite coal in a down-fired 3.5 MW pilot facility showed that the HAPT case presented a good performance of both NO emissions of 683 mg/Nm3 at O2 = 6% and coal burnout, 3.07% of unburned coal in fly ash. Simulation results using Fluent software satisfactorily coincided with the experiment results of the HAPT case. It was found by simulation that the HAPT case formed a rational aerodynamic field in the furnace, refrained dead recirculation zones from emerging in the ash hopper, and implemented an air-packed and deep air-staging coal combustion inside the furnace. SA-H flows took the responsibility of destroying dead recirculating zones in the ash hopper, and SB-H flows affected the penetration depth of primary air flow and the utilization rate of the ash hopper.

Combustion Flexibility of a Large-Scale Down-Fired Furnace with Respect to Boiler Load and Staging Conditions at Partial Loads

Down-fired boilers, designed especially for industry-firing low-volatile coals such as anthracite and lean coal, suffer similarly from various problems, such as poor burnout, high NOx emissions, and asymmetric combustion. A new down-fired combustion technology was developed specially for these problems and finally trialed in a 600-MWe supercritical boiler. To evaluate the flexibility of this technology application with respect to boiler load and air-staging conditions at partial loads, industrial-size measurements (taken of gas temperatures and species concentrations in the furnace, CO and NOx emissions in the flue gas, and carbon in fly ash) were performed within the furnace at different boiler loads (i.e., 370, 450, and 600 MWe) and staged-air damper openings (i.e., 10, 30, and 45% at 450 MWe), respectively. It was found that a relatively symmetric gas temperature distribution pattern developed for all settings, despite asymmetric burner operation models at 450 and 370 MWe. Decreasing boiler load reduce...

A novel coal combustion technology for a down-fired boiler: Aerodynamic characteristics

A new combustion technology, the hot air packing (HAP) technology, was proposed and researched to improve the coal burnout and control NOx emission in down-fired boilers. Compared with the prior technology, HAP technology necessitates the addition of special secondary air ports added in the furnace ash hopper (SA-H) and the furnace bottom (SA-B). Cold-air-flow experiments in a 1:4 scale model and fluid dynamics numerical simulations were carried out to reveal the aerodynamic characteristics in a furnace with HAP technology. The experimental and simulation results coincided well with each other; and both showed that HAP technology had rational aerodynamic characteristics in the furnace. Compared with a base case, common down-fired combustion technology, HAP technology forms a larger W shape and bigger recirculation zones, as well as a larger penetration depth of primary air flow. Moreover, HAP technology destroys the dead recirculation zones and avoids air flow washing against the wall. The angle and location of SA-H have great impacts on the aerodynamic characteristics in the furnace, especially in the ash hopper. Within the six cases, the SA-H configuration with an angle of 0° and the upper-located position yielded optimal results.

Effect of angle of arch-supplied overfire air on flow, combustion characteristics and NOx emissions of a down-fired utility boiler

A new overfire air (OFA) technology was proposed by the authors for a Foster Wheeler down-fired boiler, and a small-scale cold experimental system for a 660-MWe unit was established. The velocity field distribution was measured in the furnace to study the characteristics of single-phase flow in the furnace at different OFA nozzle angles. The furnace temperature and distributions of the O2 concentration and NO concentration at different OFA nozzle angles were simulated using Fluent 6.3.26, and the average temperature of the furnace outlet, the O2 and NOx concentrations and carbon content in the fly ash were calculated. As the OFA nozzle angle increased, the vertical penetration depth of OFA increased and the horizontal penetration depth of OFA gradually decreased. The carbon content in the fly ash and NOx concentration initially rose and then fell at the furnace outlet, and they were lowest when the OFA angle was set at 20°. Thus, according to the findings of this article, an optimized OFA angle of 20° was chosen.

Influence of Inner Secondary Air Vane Angle on Combustion Characteristics and NOx Emissions of a Down-Fired Pulverized-Coal 300 MWe Utility Boiler

Using an IFA300 constant temperature anemometer system, cold air experiments on a quarter-scaled burner model sited in a 300 MWe down-fired boiler were conducted to investigate the influence of various inner secondary air vane angles on the flow characteristics in the burner nozzle region. By increasing vane angles, no central recirculation zone appeared, the axial velocity decay rate increased, radial velocities increased at the jet boundary, and tangential velocities clearly increased in the inner secondary air zone. Industrial experiments were also performed on a down-fired pulverized-coal 300 MWe utility boiler with swirl burners. Gas temperature, concentrations of gas components (O2, CO, and NOx) in the burning region, and carbon content in the fly ash were measured with inner secondary air vane angles of 42°, 47°, 53°, and 60°. By increasing vane angles up to 53°, NOx emission and boiler efficiency increased; when vane angles were increased to 60°, NOx emission increased, but boiler efficiency decreased.

Characterization of gas/particle flows with respect to staged-air ratio for a down-fired 600 MWe supercritical utility boiler with multiple injection and multiple staging: A lab-scale study

To achieve significant reductions in particularly high NOx emissions and to eliminate severely asymmetric combustion found in down-fired boilers, a multiple-injection and multiple-staging combustion technology was developed in our previous study. Representing great progress in this area, one of two newly-designed down-fired 600 MWe supercritical utility boilers using this technology had a recent trial run before its 168-h test. By phase-Doppler anemometry measurements within a cold 1:40-scaled model of the furnace, gas/particle flow characteristics were compared among various staged-air ratio settings (i.e., 20%, 25%, 30%, and 35%) to establish an appropriate staged-air ratio range for boiler operation. Meanwhile, to validate this first industrial application of the newly developed technology, industrial-size measurements were made at a 550 MWe load during the trial run. Experimental gas/particle flow results uncovered that at the highest setting, a severely asymmetric gas/particle flow field developed in the furnace. For the two lower settings, an essentially symmetric gas/particle flow field appeared. Moreover, gas/particle flow-field symmetries at the 30% setting were generally acceptable. Consequently, a staged-air ratio range of 20–30% is recommended for the boiler. As expected, industrial-size data revealed that excellent furnace performance, characterized by symmetric combustion and low levels of carbon in fly ash (5.1%) and NOx emissions (822 mg/m3 at 6% O2), appeared within the boiler. This was despite combustion optimization and a present lack of industrial-size data at full load.

Overall Evaluation of Combustion and NOx Emissions for a Down-Fired 600 MWe Supercritical Boiler with Multiple Injection and Multiple Staging

To achieve significant reductions in NOx 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 MWe 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 NOx emissions at various settings, effects of overfire air (OFA) and staged-air damper openings on combustion characteristics, and NOx emissions within the furnace were experimentally determined. With increasing the OFA damper opening, both fluctuations in NOx 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., NOx 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 NOx emissions. In comparison with OFA, the staged-air influence on combustion and NOx 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 NOx emissions. By sharply opening the OFA damper to deepen the air-staging conditions, although NOx emissions could finally reduce to 503 mg/m(3) at 6% O2 (i.e., an ultralow NOx 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 NOx emissions attained levels of about 10% and 850 mg/m(3) at 6% O2, respectively.

Experimental study on combustion and NOx emissions for a down-fired supercritical boiler with multiple-injection multiple-staging technology without overfire air

A deep-air-staging combustion technology was previously developed to achieve reduction in NOx emissions and to eliminate strongly asymmetric combustion found in down-fired boilers. Recently, one of two down-fired 600MWe supercritical utility boilers using this technology (without applying overfire air) began commercial operations. To understand coal combustion and NOx emissions characteristics within the furnace, full-load industrial-size experiments were performed at different air-staging conditions with measurements taken of gas temperatures in the burner region, gas temperatures and species concentrations in the near wing-wall region, carbon content in fly ash, and NOx emissions. As expected, the furnace performance characterized by relatively timely coal ignition, symmetric combustion, and low levels of carbon in fly ash, developed in the furnace at all three settings. Deepening the air-staging conditions could reduce NOx emissions by one-fifth, but varied slightly carbon in fly ash. In view of the still high NOx production (i.e., 1036mg/m3 at 6% O2), adding an overfire air system which was essentially a part of the technology, was recommended for the boiler to significantly reduce the present NOx emissions.

Study on Gas/particle Flows in a Down-Fired Boiler with Swirl Burners: Influence of Different Outer Secondary Air Vane Angle

The gas-solid multi-phase flow is measured in a 300MW down-fired bolier cold model with swirl burners by two-dimensional phase Doppler velocimetry(PDA), concluded the influence of different outer secondary air vane angles on the multi-phase flow characteristics in the furnace. For vane angles of 25°, the vertical direction average velocity is high and fluctuating velocity is low, the reach of the downward airflow is deeper, primary air and secondary air mix slowly, the horizontal direction average velocity is high and fluctuating velocity is low, solid phased particles spread slowly and mix with the rewind air weakly, this is against to the ignition of pulverized coal. For vane angles of 35°, the vertical direction average velocity decrease and decay faster, fluctuating velocity increase slightly, the horizontal direction average velocity and fluctuating velocity increased slightly, solid phased particles spread quickly and mixed with the rewind air quickly, the ignition of pulverized coal increase. For vane angles of 55°, the recirculation zone appear in the burner nozzle region, the vertical direction fluctuating velocity increase significantly, average velocity decrease and decay quickly, the downward airflow turn upwards before mixing with gas, the horizontal direction average velocity high and fluctuating velocity is higher, solid phased particles spread more quickly, mixed with the rewind air more quickly, this will erode the water wall and throat, cause the water wall slagging. Considering various factors, the best outer secondary air vane angle is 35° in the operation of boiler.

Aerodynamic characteristics within a cold small-scale model for a down-fired 350 MWe supercritical utility boiler at various primary air to vent air ratios

Down-fired boilers currently suffer from a number of different issues including high levels of fly ash in the combustible matter, serious slagging and a high level of NOx emissions. In this study, the combustion system of a down-fired 350 MWe supercritical utility boiler was designed with multiple injections and multiple staging to experimentally investigate the impact of the primary air to vent air ratio on the aerodynamic field in the furnace. We constructed a cold 1:10 scale model of the down-fired boiler, and used this to compare the distribution characteristics of the furnace vertical velocity component along the horizontal direction, the decay curves of the downward airflow, and the effect of the secondary air supply ejector on the primary air. It was found that changing the ratio of primary air:vent air had very little effect on the downward flow. Taking the pulverized-coal concentration, the primary air velocity, and other factors into account, we found that the optimal ratio of primary air:vent air was 5:5.

New over-fire air arrangement and its air ratio optimization determined by aerodynamic characteristics in a cold small-scale model for a down-fired 660-MWe utility boiler

A new over-fire air (OFA) program, which called arch-supplied OFA, is proposed for Foster Wheeler (FW) type down-fired boiler in this paper. The OFA nozzles are setting on arches near the furnace center and emitting OFA flow into the furnace at an inclined angle. Cold airflow experiments were conducted to determine the effect of the OFA ratio on the aerodynamic field in a small-scale furnace modeled for a 660-MWe down-fired boiler. As the OFA ratio increases, the penetration depth of the OFA flow increases in the level direction and the turning point from downward to upward of the OFA flow moves to the furnace center. At the same time, the velocity of the F-tier airflow decreases and its ability to bring the arch-injected airflow to deeper positions weakens and the turning point from downward to upward of the F-tier airflow moves to the wall. When the OFA ratio is 20%, the OFA flow could penetrate to the furnace center in the level direction and mix with the upflowing gas completely, while the arch-injected airflow have sufficient travel distance and enough residence time in the lower furnace. Thus, considering the penetration depth of arch-injected flow in the vertical direction and the OFA flow in the level direction, an OFA ratio of 20% is recommended for the new OFA arrangement according to the present work.

Numerical Simulation of Flow, Combustion, and NO x Emission Characteristics in a 300 MW Down-Fired Boiler with Different OFA Ratios

A computational fluid dynamics model of a 300 MW down-fired furnace equipped with over-fire air (OFA) has been developed using Fluent 6.3. The validated CFD model is applied to investigate the effects of several operating conditions at full load with different OFA ratios. Simulation results indicate that as the OFA ratio rises from 0% to 40%, the NO x emission falls and the combustible material content in the fly ash increases. On the whole, n, the optimal OFA ratio for the studied down-fired boiler is 20%. This study provides a basis to assess in depth future operations of down-fired boilers.

Inner and Outer Secondary-Air Distance-Effect Study within a Cold Small-Scale Model of a New Down-Fired 600 MWe Supercritical Utility Boiler

To achieve significant reductions in particularly high NOx emissions and to eliminate severe asymmetric combustion in down-fired boilers, a multiple-injection and multiple-staging combustion technology was developed in our previous study. That technology was trialed in a newly designed down-fired 600 MWe supercritical utility boiler, without applying overfire air. In consequence, a reconfiguring of the secondary air for the redesigned burners on the arches, composed of direct-flow inner and outer secondary air, had to be performed for the newly reconfigured furnace. The present work reports our experimental investigation on the impact of the distance between inner and outer secondary air on aerodynamic characteristics within the furnace, determined by cold airflow experiments within a cold 1:20-scaled model of the furnace. Aerodynamic field measurements were conducted at four different settings, each distinguished by the ratio (denoted by Cs) of the distance between the inner and outer secondary air to th...

Numerical Simulation of Flow and Combustion Characteristics in a 300 MWeDown-Fired Boiler with Different Overfire Air Angles

A computational fluid dynamics (CFD) model of a 300 MWe down-fired furnace equipped with overfire air (OFA) has been developed using Fluent 6.3. A level of confidence in the current CFD model has been established by performing mesh-independence tests and verified by comparing furnace temperature simulations with actual furnace data. The validated CFD model is then applied in the investigation of the effects of several operating conditions at full load with different OFA nozzle angles. In the down-fired furnace, the primary air/fuel was found unable to penetrate the horizontal secondary air flow zone and reach the furnace hopper; thus, no combustion occurs in the furnace hopper zone. The peak-temperature zone appears in the upper furnace, contrary to the original design concept. Simulation results also indicate that an OFA nozzle angle set below 30° moves the mixing point of the OFA flow and the up-flowing gas downward, further aiding combustion in the upper furnace. However, when the angle increases above...

Combustion and NO x emission characteristics of a retrofitted down-fired 660 MW e utility boiler at different loads

Industrial experiments were performed for a retrofitted 660 MW e full-scale down-fired boiler. Measurements of ignition of the primary air/fuel mixture flow, the gas temperature distribution of the furnace and the gas components in the furnace were conducted at loads of 660, 550 and 330 MW e. With decreasing load, the gas temperature decreases and the ignition position of the primary coal/air flow becomes farther along the axis of the fuel-rich pipe in the burner region under the arches. The furnace temperature also decreases with decreasing load, as does the difference between the temperatures in the burning region and the lower position of the burnout region. With decreasing load, the exhaust gas temperature decreases from 129.8 °C to 114.3 °C, while NO x emissions decrease from 2448 to 1610 mg/m 3. All three loads result in low carbon content in fly ash and great boiler thermal efficiency higher than 92%. Compared with the case of 660 MW e before retrofit, the exhaust gas temperature decreased from 136 to 129.8 °C, the carbon content in the fly ash decreased from 9.55% to 2.43% and the boiler efficiency increased from 84.54% to 93.66%.

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.