Industrial-scale investigations on effects of tertiary-air declination angle on combustion and steam temperature characteristics in a 350-MW supercritical down-fired boiler

Abstract

Industrial-scale experiments were conducted to study the effects of tertiary air declination angle (TDA) on the coal combustion and steam temperature characteristics in the first 350-MW supercritical down-fired boiler in China with the multiple-injection and multiple-staging combustion (MIMSC) technology at medium and high loads. The experimental results indicated that as the TDA increased from 0° to 15°, the overall gas temperature in the lower furnace rose and the symmetry of temperature field was enhanced. The ignition distance of the fuel-rich coal/air flow decreased. In near-burner region, the concentration of O₂ decreased while the concentrations of CO and NO increased. The concentration of NO decreased in near-tertiary-air region. The carbon in fly ash decreased significantly from 8.40% to 6.45% at a load of 260 MW. At a TDA of 15°, the ignition distances were the shortest (2.07 m and 1.73 m) at a load of 210 MW and 260 MW, respectively. The main and reheat steam temperatures were the highest (557.2°C and 559.4°C at a load of 210 MW, 558.4°C and 560.3°C at a load of 260 MW). The carbon in fly ash was the lowest (4.83%) at a load of 210 MW. On changing the TDA from 15° to 25°, the flame kernel was found to move downward and the main and reheat steam temperatures dropped obviously. The change of TDA has little effect on NO_x emissions(660–681 mg/m³ at 6% O₂). In comprehensive consideration of the pulverized coal combustion characteristics and the unit economic performance, an optimal TDA of 15° is recommended.

Keywords

supercritical down-fired boiler / industrial-scale experiment / tertiary air declination angle / coal combustion / steam temperature

Introduction

Nowadays, more than 80% of down-fired boilers in the world are in service in China [¹]. Characterized by a high temperature and a long pulverized-coal residence time, this kind of boiler has been widely used for burning lean coal and anthracite that are hard-to-burn but rich in reserves [²,³]. However, the down-fired combustion technologies introduced have been presenting various problems including late ignition [⁴], asymmetrical combustion [⁵], poor burnout [⁶,⁷], and high NO_x emissions(generally in the range of 1200–1600 mg/m³ at 6% O₂ [⁸–¹⁰], which is contrary to China’s policy of energy conservation and emission reduction [¹¹]. Researchers have conducted some further studies and proposed corresponding solutions, such as burning the anthracite blended with bituminite [¹²], retrofitting the combustion system to enhance burnout [¹³], and adjusting air-staging to restrain NO_x emissions [¹⁴]. Li et al. have developed a comprehensive method, named the multiple-injection and multiple-staging combustion (MIMSC) technology, to simultaneously deal with the above problem [¹⁵]. A series of studies on this technology have been conducted [¹⁵,¹⁶], whose results indicate that the technology has great strength in improving pulverized coal burnout and stabilizing the combustion, forming a symmetric flame distribution and decreasing NO_x generation.

Supercritical once-through boilers are extensively applied because of their high cycle thermal efficiency, low coal consumption for generation, sensitive adjustment, and quick start-up and shut-down [¹⁷]. For once-through boilers, because the drum is cancelled and the working fluid passes through each heating surface at one time, there are no fixed boundaries for evaporating and superheating surfaces of working fluid. As a result, the corresponding thermal storage capacity is greatly reduced, and the influence of the disturbance caused by any change in working condition on steam and water wall temperature is much greater than that of the boiler with a drum [¹⁸]. Therefore, besides adopting the conventional adjustment means like adjusting the ratio of fuel to water supply in operation [¹⁹], it is of great significance to change operating parameters such as air and fuel staged combustion ratio and air injection angle to adjust the distribution of combustion and thermal load in furnace, to control the water wall temperature, to raise the main and reheat steam temperature, and to ensure the safe and economic operation of the boiler.

Tertiary air declination angle (TDA) is an important adjustment method to change the position of flame kernel in the lower furnace of down-fired boilers by using the airflow guiding blades [²⁰–²³]. As the position of the flame kernel in the lower furnace changes, the heat absorption of the heating surface of the lower furnace, the upper furnace, and the horizontal flue change. Accordingly, the main and reheat steam temperature varies. Simultaneously, changing the TDA also affects the pulverized coal ignition, combustion, and NO_x generation. Kuang et al. [²⁰] have taken industrial-size measurements to study the characterization of coal combustion and steam temperature with respect to staged-air (also called tertiary air) angle in a 600 MW supercritical down-fired boiler. It is found that increasing the staged-air angle from 0° to 45° has little effect on the main and reheat steam temperature. However, the gas temperature in the furnace is reduced, the carbon in fly ash sharply increased, and boiler efficiency decreased. Li et al. [²¹] have conducted experimental investigations on a 660 MW down-fired boiler. The results show that on increasing the F-layer secondary air (similar to tertiary air) declination angle from 0° to 20°, the ignition of the primary air and pulverized coal mixture is advanced, the gas temperatures rise in lower furnace, the residence time of pulverized coal in the fuel-burning zone is extended, and the carbon in fly ash decreases significantly from 9.55% to 3.82%, but the NO_x emissions increases from 1937 to 2594 mg/m³ at 6% O₂ . Fang et al. [²²] have performed a numerical investigation on a 300 MW down-fired boiler. The results indicate that increasing the F-layer secondary air angle can increase the flame penetration depth and lower the flame center. The boiler efficiency is improved and the NO_x emissions is reduced concurrently. Liang et al. [²³] have employed a cold model test to study the effect of F-layer secondary air on the aerodynamic characteristics of a 600-MW boiler. It is revealed that as the declination angle increases from 0° to 40°, a higher penetration depth and a fullness degree of airflow, as well as a larger recirculation region are obtained. Adjusting the F-layer secondary air declination angle is more beneficial to improving the distribution of the airflow within the furnace.

In this paper, industrial-scale experiments have been conducted to study the effects of TDA (0°, 15° and 25°) on the coal combustion and steam temperature characteristics in the first 350 MW supercritical down-fired boiler in China with the MIMSC technology at medium and high loads. The boiler units mainly supply power to the local industrial zone. Due to the actual production needs, the boiler units often operate at medium and high loads. Therefore, the experiments have been performed at a load of 210 MW and 260 MW. Several parameters have been measured and analyzed, including the gas and water wall temperature distributions in the lower furnace, the heating process of fuel-rich coal/air flow, the main and reheat steam temperatures of the boiler, the concentrations of gas species in the zone near the wing wall, the carbon in fly ash, and NO_x emissions at the furnace exit. The optimum TDA is obtained, which can provide theoretical basis for optimizing combustion system and guiding boiler operation.

Utility boiler

Figure 1(a) presents the structure and combustion system of the furnace. The whole furnace is divided by the arches into the rectangular upper furnace and the octagonal lower furnace (including four wing walls with small widths). Figure 1(b) demonstrates the overall arrangement of the burners on the arches. There are 16 burners evenly and symmetrically located on the front and rear arches. Each burner is connected to one cyclone pulverized-coal concentrator dividing the primary air/fuel mixture into fuel-rich and fuel-lean coal/airflow used to organize the fuel rich/lean combustion. Each burner group consists of 4 fuel-rich flow nozzles, 4 fuel-lean flow nozzles, 4 inner secondary air ports, 4 outer secondary air ports, and 5 other secondary air ports. In addition, 3 secondary air ports are added to the burner located at each arch corner near the wing wall. The fuel-rich flow, the inner secondary air, the fuel-lean flow, and the outer secondary air are located in order from the furnace center to the front/rear wall. 10 OFA groups are uniformly arranged on the front/wall in the upper furnace near the throats of the furnace. Figure 1(c) illustrates the layout of the tertiary air slot (only half of the slots are shown along the width of the furnace). 8 groups of tertiary air slots corresponding to burner groups are located at the lower parts of the front and rear walls. The airflow guiding blades are installed near tertiary air slots to adjust the declination angle, with an adjusting range of 0°–30° (with a scale of 5°) to the horizontal direction. As seen in Fig. 1(a), the water wall middle mixing system is arranged in the upper furnace to reduce the temperature deviation of the water wall. The temperature measuring points are arranged at the entrance of the mixing system, and the front/rear water wall temperature measuring point layout is exhibited in Fig. 1(d). The measuring points are evenly and symmetrically arranged, with points 9 and 26 being located along the symmetry axis of the furnace width. The boiler design parameters at boiler maximum continuous rating (BMCR) are listed in Table 1 while the design coal characteristics are tabulated in Table 2.

Industrial-scale measurements

Industrial-scale measurements were performed at a load of 210 MW and 260 MW. The damper openings of secondary air, tertiary air, and OFA were kept unchanged at each load. The TDA was adjusted to 0°, 15°, and 25°. Before industrial-scale measurements, all data-acquiring apparatuses, including the thermometer, the thermocouple device, and the fuel gas analyzer were calibrated to ensure the accuracy of measurements. During the measurements, great efforts were made to maintain the coal characteristics (as listed in Table 2) stable and avoid any interferential operation, such as soot blowing and pollution discharge. The parameters of the instruments in the control center corresponding to each operating condition were recorded, including main operating parameters such as the main and reheat steam temperature, the superheat degree, and the front and rear water wall temperature. The main boiler operating parameters during the measurements are also given in Table 1. Under each operating condition, the parameters were recorded every ten minutes, and the mean values of the corresponding data were calculated. The methods in industrial-scale data acquiring are as follows:

(1) Measuring ignition distances of fuel-rich and fuel-lean coal/air flows

The thermocouple device (K-type, 8 mm diameter, 10 m length, with a measurement range from 0°C to 1200°C and an accuracy of±0.75%) was vertically inserted into the furnace along the measuring pipes on the fuel-rich coal/air flow nozzles to obtain the gas temperature under the arch burner. The locations of the measuring points are displayed in Fig. 2. As the thermocouple was long, it was placed in a 20 mm-diameter stainless steel pipe for protection and for keeping its rigidity and stability when inserted into the furnace. The thermocouple swung violently because of the flowing fuel gas inside the furnace when deeply inserted, which made the readings fluctuate greatly, thereby the inserted depth was kept within a 4 m distance to burner nozzles to get stable readings. For more accurate measurements, the average temperature of each measuring point was recorded in 1 min. To avoid the error caused by soot or ash deposition on the thermocouple, the thermocouple was frequently examined and cleaned.

(2) Measuring gas temperature distribution in lower furnace.

The furnace temperature was measured with a hand-held pyrometer (Raytek 3i pyrometer, USA, with a measurement range from 300°C to 3000°C and an accuracy of±1°C or±0.5%) through inspection ports. As shown in Fig. 2, there are 5 floors of inspection ports in the lower furnace. The measurement was taken 10 times within 5 min at each inspection port.

(3) Measuring gas species concentrations in the zone near the wing wall.

The compositions of gas species were sampled using a water-cooled stainless-steel probe, 4.5 m in length, for analysis of the local mean concentrations of O₂, CO, and NO. The probe featured a centrally located 10 mm (inner diameter) tube, which was surrounded by a tube for cooling. The water-cooled probe was vertically inserted into the furnace through inspection ports 1 and 4 (shown in Fig. 2). High-pressure firefighting water with a large flow rate was used as cooling water. Once the flue gas is sucked into the sampling pipe, the temperature of flue gas can be cooled sharply and the reactions of various components stop immediately, which makes sure that the results of the measurements accurately reflect the compositions of real local gas. The gas samples captured were analyzed online by a Testo 350 M instrument (with an accuracy of ±1% for O₂, ±5% for CO, and 50 ppm for NO) to obtain the concentration of gas species in the near-wall region.

(4) Measuring fuel gas species concentrations at furnace exit and carbon in fly ash.

The flue gas composition at the horizontal outlet of economizer was measured by using a Testo 350 flue gas analyzer every 10 min to calculate mean values during each experimental run. An isokinetic sampling device was used to extract samples of fly ash at the horizontal outlet of the air preheater.

Results and discussion

Gas temperature distributions in the lower furnace

Figure 3 plots the distributions of gas temperature in the lower furnace at a load of 210 MW. All conditions show that from the 1st floor ports located in the burner region (shown in Fig. 2), with the decrease of furnace height, the lower furnace temperatures gradually increased, reaching the maximum at the 3rd and 4th floor ports located in the main combustion area, and then gradually decreased in the hopper region. Table 3 lists the mean temperature of each floor of measuring ports. Comparing the temperature distributions at different TDAs, it can be seen that: (1) with the increase of the TDA from 0° to 25°, the mean temperature of the first floor ports increased from 1016°C to 1104°C at first, and then decreased to 1010°C. (2) At a TDA of 0 °, the maximum value of mean temperature appeared on the 3rd floor of 1216°C. With the decrease of furnace height, the temperatures of the lower furnace gradually decreased and the mean temperature of the 5th floor ports dropped to 1079°C. At a TDA of 15°, the maximum value of mean temperature appeared on the 3rd floor of 1213°C, and the mean temperature of the 5th floor ports dropped to 1127°C. Compared with the TDA of 0°, the peak temperature of the furnace at a TDA of 15° changed little, but the overall temperature of the lower furnace rose. The reason for this is that when the pulverized coal flow reached the tertiary air region, it was carried to the deeper part of the furnace by the tertiary air after increasing the TDA. The residence time of pulverized coal was prolonged, and the flame fullness and temperature in the lower furnace increased. At a TDA of 25°, the maximum value of mean temperature appeared on the 4th floor of 1161°C while the mean temperature of the 3rd and 5th floor ports were 1159°C and 1160°C. The mean temperatures of the 3rd, 4th, and 5th floor ports were close to each other, which indicated that the flame kernel of the lower furnace moved downward at the TDA of 25° compared with 0° and 15°. (3) The influence of the TDA on the temperature of the lower furnace can also be explained by the temperature changes of ports 2 ('), 3 ('), and 4 (') on the 4th floor and port 5 (') on the 5th floor. The temperatures at port 2 (') were low at different TDAs, because port 2 (') was located near the tertiary air slot, and the tertiary air injected into the furnace decreased the temperatures near the front and rear wall. The temperatures at port 3 (') were higher than that at port 2 ('), due to the fact that the pulverized coal flow further burned and released heat at this position because of the oxygen-supplement and combustion-supporting effect of the tertiary air. On increasing the TDA from 0° to 25°, the temperature at port 4 (') decreased and the temperature at port 5 (') increased gradually, which demonstrated that the trajectory of the pulverized coal flow carried by the tertiary air was declined gradually. (4) Comparing the mean temperatures of the ports in the front and rear sides, the difference between them on the 1st floor decreased from 117°C to 9°C as the TDA changed from 0° to 25°, and the difference on the 3rd floor decreased from 112°C to 18°C. With the increase of TDA, the gas temperature deviation between the front and the rear sides gradually decreased, and the temperature field in the lower furnace tended to be symmetric. The reason for this is that at a TDA of 0°, the tertiary air entered the furnace horizontally, and the momentum exchange occurred when the tertiary airs of the front and rear sides met in the furnace center, which resulted in the fact that the airflow on one side extruded the airflow on the other side. As a result, the asymmetric flow and temperature fields in the lower furnace formed. Increasing the TDA decreased the horizontal momentum of the tertiary air and reduced the deflection degree.

Heating process of fuel-rich coal/air flow

Figure 4 illustrates the heating processes of fuel-rich coal/air flows at different TDAs. As the coal used belongs to lean coal, this paper takes 900°C as its ignition temperature. It can be seen from Fig. 4 that: (1) on increasing the TDA from 0° to 15°, the heating of fuel-rich coal/air flows gradually accelerated at the two loads. The corresponding ignition distance decreased from 2.40 m to 2.07 m at a load of 210 MW and the ignition distance decreased from 2.65 m to 1.73 m at a load of 260 MW. The reason for this is that increasing the TDA increased the lower furnace temperature (shown in Fig. 3), which is favorable for absorbing heat from the high-temperature gas entrained into the recirculating zones below furnace arches and therefore enhances ignition. (2) As the TDA increased from 15° to 25°, the heating of the fuel-rich coal/air flows gradually retarded at the two loads. The ignition distances at the loads of 210 MW and 260 MW increased to 2.48 m and 1.94 m, respectively. According to the above analysis results of the temperature field in the lower furnace, the position of the high temperature area in the lower furnace moved down as the TDA increased from 15° to 25°, and the temperature of the recirculating gas in the burner area decreased. As a result, the heat of fuel-rich flow absorbed from the recirculating gas decreased, which was not conducive to ignition. In addition, the TDA is adjusted by the airflow guiding blades in the tertiary air duct, and when the TDA increased to 25°, the blade declination angle was large. Consequently, the local resistance of the tertiary air at the blade position was large. The secondary air, tertiary air, and OFA come from the main secondary air duct. With the increase of local resistance of the tertiary air, the flux of the secondary air and OFA increased to varying degrees. To reach the ignition temperature of the fuel-rich coal/air flow, the fuel-rich coal/air flow and the adjacent inner secondary air need to be heated together to the ignition temperature of the pulverized coal. The heat required for the two flows to be heated to the ignition temperature is defined as the ignition heat. Increasing the secondary air flux increased the ignition heat. Therefore, the heating of the fuel-rich coal/air flows retarded and the ignition was delayed. (3) The ignition distances at a load of 260 MW were shorter than those at a load of 210 MW when the TDAs were 15° and 25°. However, the ignition distance at a load of 260 MW was the longest at 2.65 m when the TDA was 0°. This is resulted from the deflection of the temperature field in the lower furnace at the TDA of 0°, which affected the heating of the fuel-rich coal/air flow.

Water wall and steam temperatures

The characteristics of the front/rear water wall temperature at a load of 210 MW are shown in Fig. 5. During the measurements, it was found that the distribution of the water wall temperature under various working conditions suggested that the temperature in the center of the furnace was high and that near the side walls was low. The water walls of supercritical down-fired boilers had a much weaker anti-interference ability of thermal load deviation than that of subcritical down-fired boilers because of the use of vertical internally ribbed water wall tubes at low mass flux [²⁴]. When the combustion distribution evenness was poor in the furnace, the phenomenon that the water wall temperature deviation was too large (the general allowable value is 89°C [²⁵]) and the wall temperature exceeded the allowable value would occur, which caused the water wall tube fin crack and water wall tube burst, affecting the safe operation of boilers. It can be seen from Fig. 5 that the maximum temperature differences of the front/rear water walls at different TDAs are less than 60°C. This indicates that the temperature deviation of the water wall is within the deviation permitted when the MIMSC technology is applied, satisfying the demands of the safe operation of the boiler. However, the variation of the TDA has a certain effect on the deviation of the temperature of the water wall. On increasing the TDA from 0° to 25°, the maximum temperature difference of the front water wall increased from 56.4°C to 59.5°C, and the difference of the rear water wall increased from 32.9°C to 56.9°C. This means that with the increase of the TDA, the thermal load increasingly tends to concentrate to the central part of furnace, and the uniformity of the distribution of water wall temperature worsens and the water wall temperature deviation increases. Therefore, it is not appropriate to adopt a large TDA. As the TDA increased from 0° to 25°, the difference of the maximum wall temperatures between the front and rear water wall decreased from 26.6°C to 11.6°C, which indicated that the deflection of the flow field in the lower furnace decreased gradually, corresponding to the analysis results of the temperature fields in Fig. 3. The maximum water wall temperature not only affects the safe operation of the boiler, but also influences the main and reheat steam temperatures. The regulation is analyzed below combining with the influence of the TDA on the main and reheat steam temperature.

In actual operation, the main and reheat steam temperature is affected by the superheat degree and the flame kernel position in the lower furnace. Superheat degree refers to the difference between the outlet temperature of water-steam separator and saturation temperature under corresponding pressure. The heat absorption of the working fluid is controlled by adjusting the ratio of fuel to water supply in operation, and then the superheat degree is controlled. With other conditions unchanged, as the superheat degree increases, the main and reheat steam temperature increases, and the water wall temperature increases, too. Operators often control the maximum water wall temperature by keeping the superheat degree beneath the allowable water wall temperature (design allowable value is 430°C), while leaving a certain safety margin to adapt to the impact of load change on water wall temperature. Therefore, the main and reheat steam temperatures are also constrained by water wall temperature. As far as the flame kernel position in the lower furnace is concerned, when the flame kernel position is lower, the heating surfaces of lower furnace absorb more heat, the heating surfaces of upper furnace and horizontal flue absorb less heat, and the main and reheat steam temperature is lower. At the TDA of 0°, influenced by the asymmetric temperature field (shown in Fig. 3), the front water wall absorbed more heat. The maximum front water wall temperature is 421.5°C (shown in Fig. 5), the superheat degree is 21.8°C (shown in Table 1), and the main and reheated steam temperatures are 555.1°C and 557.1°C respectively (shown in Table 1). On increasing the TDA from 0° to 15°, the temperature field tended to be gradually symmetric (shown in Fig. 3). The maximum temperature of front/rear water wall decreased to 417.3°C. At the same time, the superheat degree rose to 23.5°C, and the main and reheat steam temperatures rose to 557.2°C and 559.4°C. On changing the TDA from 15° to 25°, the flame kernel obviously moved downward (shown in Fig. 3). When the superheat degree was basically unchanged (the superheat degree is 23.9°C at the TDA of 25 °), the maximum water wall temperature rose to 420.2°C, and the main and reheat steam temperatures reduced to 551.2°C and 553.1°C. Compared with the situation at a load of 260 MW, the superheat degree decreased from 24.9°C to 21.0°C as the TDA changed from 15 ° to 25 °, and the main and reheat steam temperatures dropped from 558.4°C and 560.3°C to 546.1°C and 545.1°C, respectively, with a conspicuous reduction. Consequently, the TDA should not be too large, and an optimal TDA of 15° is recommended.

Gas species concentrations in the region near the wing wall

Figure 6 plots the concentrations of gas species in the region near the wing wall of inspections port 1 and 4. Inspection port 1 is located at the near-burner region, and the distance of 2.0–2.4 m to the wing wall is the below-burner region. Inspection port 4 is located at the near-tertiary air region (shown in Fig. 2), and the distance of 2.0–2.4 m to the wing wall is the tertiary air region. At inspection port 1, the concentration of O₂ decreased promptly after 0.2 m to the wing wall, and then increased due to the mixing of secondary air. The concentration of CO increased with the increase of the distance to the wing wall. The concentration of NO increased rapidly after 0.2 m to the wing wall, then decreased gradually, and increased again when approaching the below-burner region. In the below-burner region, on increasing of the TDA from 0° to 25°, the concentration of O₂ decreased first and then increased, while the concentrations of CO and NO increased first and then decreased. At the TDA of 15°, as the distance to the wing wall increased from 2.0 m to 2.4 m, the concentration of O₂ reduced rapidly to 8.5%, and the concentrations of CO and NO increased promptly to 2619 ppm and 452 ppm, respectively. Compared with the other two working conditions, the concentration of each gas specie in the below-burner region varied significantly at the TDA of 15°. The reason for this is that the ignition distance of the fuel-rich flow is short at the TDA of 15° (shown in Fig. 4). The combustion of pulverized coal consumed a large amount of O₂ and produced CO and NO products. At inspection port 4, with the increase of the distance to the wing wall, the overall O₂ concentration decreased first and then increased. In the tertiary air region, the O₂ concentration was higher at the TDA of 0°. Because the tertiary air was injected horizontally into the furnace at this work condition and inspection port 4 was located at the same height as the tertiary air slots, the concentration of O₂ was high under the influence of the tertiary air. Increasing the TDA decreased the concentration of O₂ gradually. The reason for this is that with the increase of the TDA, the tertiary air gradually flowed downward, which is consistent with the analysis of inspection ports 2 ('), 3 ('), 4 ('), and 5 (') in Fig. 3. The concentration of CO was low and the change is not apparent with the distance to the wing wall increasing at different TDAs. As the TDA increased from 0° to 25°, the overall concentration of NO decreased first and then increased. At the TDA of 0°, the concentration of NO is high in the range of 0.6–1.0 m to the wing wall, reaching a maximum of 371 ppm. Because under this condition, the ignition distance of the fuel-rich flow was long (shown in Fig. 4), the unburnt pulverized coal particles were rapidly burnt in the oxygen-enriched condition after mixing with the tertiary air, thus producing a large amount of NO.

Experimental results of the parameters at the furnace exit

Figure 7 depicts the experimental results of the parameters at the furnace exit at a load of 210 MW and 260 MW. As shown in Fig. 7, at a load 210 MW, the concentration of O₂ at furnace exit ranged from 5.07% to 5.14%, while at a load of 260 MW, the concentration of O₂ ranged from 4.80% to 5.05%. The NO_x emissions were in the range of 660–681 mg/m³ at 6% O₂ at both loads. At a load of 210 MW, increasing the TDA from 0° to 15° decreased the carbon in fly ash from 4.96% to 4.83%, while increasing the TDA from 15° to 25° increased the carbon in fly ash to 5.72%. At a load of 260 MW, increasing the TDA from 0° to 15° decreased the carbon in fly ash from 8.40% to 6.45%, showing a great variation. The carbon in fly ash was the lowest at the TDA of 15°, because, for one thing, the ignition distance of the fuel-rich flow at the TDA of 15° is shorter than that in other working conditions (shown in Fig. 4), and the ignition of pulverized coal was improved. For another, compared with the TDA of 0 °, the pulverized coal flow was carried to the deeper part of the furnace by the tertiary air after reaching the tertiary air region at the TDA of 15° (shown in Fig. 3). The residence time of the pulverized coal was prolonged, and the pulverized coal burnt more fully in the lower furnace. However, at the TDA of 25°, although the penetration depth of pulverized coal flow was prolonged, the ignition distance of the fuel-rich flow in this condition is long (shown in Fig. 4), and the temperature in the lower furnace is low (shown in Fig. 3), which is not conducive to the burnout of pulverized coal particles.

Conclusions

In this paper, industrial-scale experiments were conducted to study the effects of TDA (0°, 15° and 25°) on the coal combustion and steam temperature characteristics in the first 350 MW supercritical down-fired boiler in China with the MIMSC technology at medium and high loads. The results are as follows:

1) As the TDA increased from 0° to 15°, the overall gas temperature in the lower furnace rose and the symmetry of temperature field was enhanced. The ignition distance of the fuel-rich coal/air flow decreased. In the near-burner region, the concentration of O₂ decreased while the concentrations of CO and NO increased. The concentration of NO decreased in the near tertiary air region. The carbon in fly ash decreased significantly at a load of 260 MW.

2) At the TDA of 15°, the ignition distance of the fuel-rich flow is the shortest. The ignition distances at a load of 210 MW and 260 MW were 2.07 m and 1.73 m, respectively. In the near-burner region, as the distance to the wing wall increased from 2.0 m to 2.4 m, the concentration of O₂ reduced rapidly to 8.5%, while the concentrations of CO and NO increased promptly to 2619 ppm and 452 ppm. The main and reheat steam temperatures were the highest (557.2°C and 559.4°C at a load of 210 MW, 558.4°C and 560.3°C at a load of 260 MW). The carbon in fly ash was the lowest(4.83% and 6.45%) at a load of 210 MW and 260 MW, respectively.

3) On changing the TDA from 15° to 25°, the flame kernel was found to move downward, the temperature in the hopper region increased, and the main and reheat steam temperatures dropped obviously (551.2°C and 553.1°C at a load of 210 MW, 546.1°C and 545.1°C at a load of 260 MW). The carbon in fly ash increased.

4) On increasing the TDA from 0° to 25°, the maximum water wall temperature decreased first and then increased, the water wall temperature deviation increased (less than 60°C), and the difference of the maximum wall temperatures between the front and rear water wall decreased.

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