Source: Zhe Jiangang Circulating fluidized bed power generation
In recent years, the continent's new energy technology and its installed power generation capacity have developed rapidly. In 2020, the cumulative total installed capacity of wind and solar power reached 534 million kW, and it is expected that by 2030, the total installed capacity will reach more than 1.2 billion kW. Due to the randomness and intermittency of new energy, the power supply of the power grid is unstable. In order to comprehensively improve the capacity of new energy consumption, China has put forward the requirements of improving the flexible operation ability of thermal power units and tapping the peak regulation potential of coal-fired units.
Circulating fluidized bed (CFB) power generation technology has been widely used in mainland China due to its superior performance of efficient desulfurization and nitrogen suppression in the furnace and wide fuel adaptability. Due to its unique ash circulation system, the CFB unit can maintain a high bed temperature in the furnace and have a large heat storage, which ensures the stable combustion of ultra-low load, and generally its minimum stable combustion load can reach 30%, which has a strong advantage of deep peak regulation. Most of the CFB boiler NO x ultra-low emission technologies choose the technical route of "furnace combustion optimization adjustment + selective non-catalytic reduction ammonia injection (SNCR)" flue gas denitrification. However, when the CFB boiler is running in depth peak regulation, the CFB boiler is affected by its fluidization safety characteristics, and the primary air volume is not lower than the critical fluidized air volume, and the primary air volume remains unchanged, and the excess air coefficient at the furnace outlet is controlled by adjusting the secondary air volume, resulting in excess oxygen in the dense phase area of the boiler furnace, and the reduction of the secondary air share leads to the poor combustion effect of the boiler classification, resulting in the generation of NOX original emissions. For the NOx denitrification technology outside the furnace, the SNCR system is mainly arranged at the inlet of the separator, and the denitrification reducing agent is mainly urea solution, and the optimal reaction temperature range is 800~1 100 °C. With the decrease of boiler load, the operating bed temperature of the boiler furnace and the inlet temperature of the separator also decrease, and when the flue gas temperature in the separator is lower than 800 °C, the denitrification effect outside the furnace is reduced. Therefore, it is difficult to control NOx ultra-low emissions during the deep peak regulation of CFB boilers.
Flue gas recirculation technology is a technology that realizes the replacement of part of the flue gas with the primary air volume in the low-load operation of CFB boilers. According to the application effect of early industrial furnaces, the flue gas recirculation technology can not only ensure the safe fluidization air volume of CFB boilers, but also reduce the oxygen content in the dense phase zone of the boiler, increase the reducing atmosphere in the dense phase zone, and reduce the generation of NOx. At the same time, the flue gas recirculation technology can also delay the combustion in the dense phase zone of the boiler furnace, increase the upper flue gas temperature, and increase the inlet temperature of the separator, which can simultaneously improve the SNCR denitrification efficiency at low load, but the operation experience of this technology on large CFB boiler units is less. Therefore, for a 300 MW subcritical CFB unit that has been renovated flue gas recirculation system, the thermal test of the flue gas recirculation system before and after the commissioning and shutdown of the flue gas recirculation system was carried out to study the influence of flue gas recirculation technology on the main operating parameters such as boiler combustion and pollutant emissions, and the operation effect and economy of flue gas recirculation after transformation were compared and analyzed through performance tests, so as to provide a technical reference for the flue gas recirculation operation of large CFB units during deep peak regulation.
1 Research Objects and Methods
1.1 Boilers and environmental protection facilities
A 300MW subcritical CFB boiler model is HG-1065/17.5-L.MG44, and the overall arrangement is trouser leg type, double air plate single furnace structure, and H-shaped arrangement. The section of the boiler combustion chamber is rectangular, with a width× depth × height of 16.617 m× 16.269 m × 44.700 m, respectively. The furnace is equipped with a screen-type II-stage superheater tube screen, a final reheater tube screen, and a water-cooled evaporation screen; The four separators are high-temperature insulated separators with an inner diameter of about 8 m, and one non-mechanical recirculation valve is arranged under the return leg of each separator, and the recirculation is self-balancing. The tail flue adopts a double flue structure, the front flue is arranged with a low-temperature reheater, the rear flue is arranged with a high-temperature superheater and a low-temperature superheater, the upper flue is covered by a membrane-type wall-clad superheater, the lower part is a single flue, and the economizer and the air preheater are arranged sequentially from top to bottom, the economizer adopts the H-type economizer arranged in sequence, and the air preheater adopts the tubular air preheater. The boiler adopts 4 coal feeding lines to send the raw coal to the return leg of the return valve, and the left and right sides of the boiler are equipped with 3 drum type slag coolers, and the slag discharge temperature does not exceed 150 °C, and the main design parameters of the boiler are shown in Table 1.
The pollutant emissions of the boiler are all subject to ultra-low emission standards. The desulfurization adopts the process of "desulfurization in the furnace + semi-dry method of circulating fluidized bed outside the furnace", the desulfurizer is limestone powder and slaked lime respectively, and the emission concentration of SO2 at the outlet of the chimney is less than 35 mg/Nm 3 (the benchmark oxygen content is 6%, the same below); Among them, the desulfurization in the furnace is sent to the boiler furnace by limestone powder pneumatic conveying, with 2 return legs on the left and right sides, and 2 left and right furnaces for the inner and lower secondary air, a total of 8 limestone addition points. The ultra-low emission of NOx adopts the method of "staged combustion + SNCR system denitrification", the SNCR uses urea as the reducing agent, and the emission concentration of NOx at the outlet of the chimney is less than 50 mg/Nm 3. The dust removal adopts the method of "pre-electrostatic precipitator + desulfurization bag dust collector", and the smoke and dust emission concentration is less than 10 mg/Nm 3. The process flow is shown in Figure 1. The flue gas treated by the ultra-low emission process is controlled at 70 °C at the outlet of the baghouse.
1.2 Flue gas recirculation system
The flow of the flue gas recirculation system is shown in Figure 2. The transformation mainly includes: using the outlet pressure head of the induced draft fan, extracting the flue gas from the confluence flue at the outlet of the induced draft fan, dividing it into 2 branch pipelines through the main flue gas main pipeline to access 2 flue gas recirculation fans, and then connecting to the air inlet of a and B primary fans through the supporting branch pipelines of the fan outlet respectively; Before entering the flue gas recirculation fan from the main flue gas main pipeline to the bifurcated branch pipeline, one electric adjusting door is installed for each of them, and one electric shut-off door is installed on the branch pipeline after the flue gas recirculation fan, and one electric adjustment door is installed before each branch pipeline enters the primary fan, so as to realize the cut-off and isolation function with the tail flue and the primary air system. The main design parameters of each fan and flue gas recirculation fan of the boiler are shown in Table 2.
1.3 R&D Methodology
In order to analyze the operation effect of flue gas recirculation after transformation, the thermal test and related performance test before and after the flue gas recirculation of CFB boiler were carried out, and the relevant data under stable working conditions were collected, and the influence of flue gas recirculation and shutdown on the combustion conditions and economic indicators of the boiler was analyzed and studied. The boiler has a long-term operating load of 150 MW and 300 MW, and the proportion of flue gas recirculation air volume to primary air volume is up to 40%. Therefore, the following conditions are selected to study the proportion of flue gas recirculation air volume to the primary air volume at 150 MW and 300 MW loads, as shown in Table 3, and the parameters of coal quality and limestone during the test period are shown in Table 4~5.
The operating parameters such as the operating bed temperature of the boiler furnace, the upper differential pressure, the inlet temperature of the separator, the exhaust gas temperature, the oxygen volume at the inlet of the air preheater, and the fan current are all from the DCS system value in the same time period under stable working conditions, the pollutant emission concentration is from the chimney CEMS meter value (the benchmark oxygen content is 6%), the bottom slag and fly ash combustible content and economic indicators are from the results of the test in accordance with the performance test requirements, and the instrument model and accuracy are shown in Table 6.
2 Comparative analysis of the influence of flue gas recirculation on boiler combustion
2.1 Effect of NOX emissions at boiler start-up
As shown in Figure 3, during the hot start of the boiler, the SNCR system was not put into operation, the primary air volume was 244 kNm 3 /h, the secondary air volume was 202 kNm 3 /h, and the NO x instantaneous emission value was the original emission value. When the boiler load is 45 MW, the instantaneous value of NOx emission is 178 mg/Nm 3, the flue gas recirculation air volume is 83 kNm 3/h, and the operating oxygen volume is 9.5%. During this period, the boiler load was kept unchanged, and as the flue gas recirculation air volume further increased to 195 kNm 3/h, the oxygen inlet of the air preheater gradually decreased to 2.7%, and the NOx emission gradually decreased and stabilized to 26 mg/Nm 3.
During the start-up process of the boiler, the input of flue gas recirculation can significantly control the oxygen inlet of the air preheater to 3.0%, thus forming a strong reducing atmosphere in the dense phase zone of the furnace, and significantly inhibiting the original generation of NOx emission concentration. Therefore, in the boiler start-up or low-load operation, flue gas recirculation can be put into the flue gas in time according to the oxygen capacity of the boiler operation to ensure that the pollutants are discharged up to environmental protection standards.
2.2 Influence of boiler operating bed temperature
Compare the changes in bed temperature under the same load before and after flue gas recirculation is put into operation, as shown in Figure 4. After being put into operation, the bed temperature of the lower, middle and upper three layers of the boiler furnace decreased significantly. Under the stable working condition of 300 MW, the bed temperature of the lower part of the boiler furnace decreased from 897 °C before operation to 880 °C after operation, and the bed temperature decreased by 17 °C. At 150 MW load, the bed temperature in the lower part of the boiler furnace was 845 °C, which was 24 °C lower than 869 °C before commissioning. After the flue gas is recirculated, due to the decrease of oxygen content in the dense phase area of the boiler furnace, the anoxic combustion in the dense phase area of the furnace is caused, resulting in the decrease of the operating bed temperature of the boiler.
2.3 Effect of differential pressure in the upper part of the boiler
The increase of the amount of circulating material in the CFB boiler has a good effect on maintaining the uniformity of combustion in the furnace and inhibiting the emission of pollutants, as shown in Figure 5. After the flue gas recirculation is put in, there is basically no significant change in the upper differential pressure under the condition of high and low loads, indicating that although the total air volume of the boiler increases after the flue gas recirculation is put in, it has little impact on the upper differential pressure.
2.4 Effect of separator inlet temperature
The inlet and outlet temperatures of the separator play a crucial role in the operational efficiency of the SNCR denitrification system. Compare the flue gas temperature changes of the inlet and outlet of the separator under the same load conditions before and after the flue gas recirculation is put into operation, as shown in Figure 6. After the flue gas recirculation was put into operation, the flue gas temperature at the inlet of the separator was reduced by 5 °C at a load of 300 MW. At 150 MW load, the inlet temperature of the separator remains unchanged after commissioning compared to before the flue gas recirculation was put into operation. However, when the inlet temperature of the separator is above 800 °C under the loads of 300 MW and 150 MW, the flue gas temperature change of the separator flue gas caused by the flue gas recirculation and shutdown is very small, and has no effect on the SNCR denitrification efficiency.
2.5 Effect of exhaust gas temperature
From the analysis of Figure 7, it can be seen that when the load is 300 MW, the exhaust gas temperature after flue gas recirculation increases by 3 °C compared with that before commissioning. At a load of 150 MW, it is increased by 12 °C after flue gas recirculation. The increase of exhaust gas temperature after flue gas recirculation is mainly caused by the air leakage of the air preheater and its heat exchange efficiency, and the change of boiler combustion conditions.
2.6 Impacts of pollutant emissions
As shown in Figure 8, under the condition of stable operation of the boiler, under the condition of keeping the flow rate of SNCR denitrification urea solution unchanged, the SO 2 and NO x emissions can meet the ultra-low emission requirements before and after the flue gas recirculation is put into operation.
Compared with the flue gas recirculation, the instantaneous and hourly mean values of NOx emission concentration were reduced under the condition of flue gas recirculation. At 300 MW load, the instantaneous value of NOx emission decreased by 13 mg/Nm 3 and the hourly average value decreased by 6 mg/Nm 3 after flue gas recirculation. At 150 MW load, the instantaneous value of NOx emission decreased by 6 mg/Nm 3 and the hourly average value decreased by 7 mg/Nm 3 after flue gas recirculation. SO2 emissions vary depending on the factors of desulfurization in the furnace and the desulfurization system outside the furnace. After the flue gas recirculation is put into operation, due to the decrease of the operating bed temperature of the boiler and the anoxic combustion in the dense phase zone of the boiler chamber, the reducing atmosphere is enhanced, which can effectively inhibit the original generation of NOx.
3 Comparative analysis of the impact of flue gas recirculation on operation economy
3.1 Effect of fan power consumption
As shown in Fig. 9, compared with the non-flue gas recirculation, the secondary fan and induced draft fan currents increase significantly at 300 MW load, and the total boiler fan current increases by 260 A and 107 A under 150 MW load. Due to the flue gas recirculation input, a part of the flue gas replaces the original primary air volume, and the secondary air volume of the boiler is increased in order to ensure the oxygen required for boiler combustion under the condition that the total primary air volume of the boiler is basically unchanged; Due to the flue gas recirculation input, the total flue gas volume of the boiler increases, which leads to a significant increase in the current of the boiler induced draft fan.
3.2 Effect of combustible content of bottom slag and fly ash
As shown in Figure 10, compared with before commissioning, after the flue gas recirculation, the combustible content of bottom slag and fly ash increased significantly under 300 MW load, and the fuel content of bottom slag increased by 1.2%, mainly due to anoxic combustion in the dense phase area of the boiler. The combustible content of fly ash increased by 1.9%, mainly due to the obvious increase in the operating air volume of the boiler furnace, which led to the escape of fly ash that was not captured by the separator.
3.3 Impact of Economic Indicators
According to the relevant standards of performance test, the influence of flue gas recirculation under 300 MW load on the operating economy of the boiler was tested, as shown in Table 7. The test results show that, compared with the non-flue gas recirculation, after the 300 MW load is put into the flue gas recirculation, the power consumption rate of the unit plant increases from 9.41% to 10.26%, increases by 0.85%, the boiler efficiency decreases from 92.47% to 90.62%, and the boiler efficiency decreases by 1.85%, which leads to the decrease of boiler operation economy after the flue gas recirculation is put in.
4 Conclusions
Through the comparative analysis of flue gas recirculation in boiler start-up and hot state tests under high and low loads, it is concluded that:
(1) During the start-up process of the boiler, the flue gas recirculation can effectively reduce the oxygen inlet of the air preheater to 3.0%, enhance the reduction atmosphere in the dense phase zone, and thus inhibit the original generation of NOx.
(2) In the normal operation of the boiler, after the flue gas recirculation is put into operation, the operating bed temperature of the boiler decreases by 17 °C and 24 °C, the exhaust gas temperature increases by 3 °C and 12 °C, and the instantaneous value of NOx emission decreases by 13 mg/Nm 3 and 6 mg/Nm 3 under the load of 300 MW and 150 MW. However, the change of differential pressure and SO2 emission in the upper part was not obvious, and the flue gas temperature at the inlet of the separator was different, but above 800 °C, it had no effect on the SNCR denitrification efficiency.
(3) When the proportion of flue gas recirculation air volume to the primary air volume is 40%, the total current of the boiler fan increases by 260 A, the content of bottom slag and fly ash combustibles increases by 1.2% and 1.9% respectively, the power consumption rate of the plant increases by 0.85%, and the boiler efficiency decreases by 1.85% under the condition of 300 MW. Therefore, in the future, when the power plant unit is running at high load, the combustion can be adjusted through the flue gas recirculation system to reduce the operating bed temperature of the boiler. During deep peak shaving, it is necessary to further optimize the operation mode and circulating air volume of the flue gas recirculation system, and enhance the reducing atmosphere in the dense phase area of the boiler furnace, so as to facilitate the environmental protection emission standard during deep peak shaving.
Bibliographic information
ZED Jiangang,XIE Guowei,WU Wanzhu. Energy Science and Technology,2021,19(05):84-89.)