Source: Thermal Power Plant Technology Alliance
The CO detection device was introduced by boiler manufacturers very early and piloted on a small scale, but due to the high temperature in the furnace, sensor performance and other constraints, it has not been included in the reference index of boiler combustion adjustment. With the rapid development of manufacturing processes in China in recent years, it is possible to install CO devices and effectively balance the relationship between high-efficiency combustion, low nitrogen emission, and high-temperature corrosion/coking. The following is a discussion of the influencing factors affecting CO concentration based on field test data.
1. The influence of the opening of the burnout damper
Under the load condition of 600MW, the SOFA damper maintains the original opening, that is, 55%/100%/55%/75%, and the CO concentration on the furnace side is 70000-90000ppm, and the reducing atmosphere near the water wall is strong, and the risk of high temperature corrosion is greater. After the SOFA damper opening of each layer is reduced by 10%, the CO concentration of the furnace can be reduced to about 50,000ppm, while the change of denitrification inlet content is very small.
During the SOFA damper opening adjustment test, #1机组运行在600MW负荷工况时, when the SOFA damper was adjusted to 30%, 20% and 10% openings respectively, the unit was stable for half an hour, and the parameters such as the CO concentration level of the furnace were recorded, as shown in Table 1.
Table 1 CO concentration in the furnace chamber at different SOFA damper openings
| 负荷 | SOFA Damper opening | Furnace side CO concentration | Denitrification inlet NOx concentration | Tail flue CO concentration |
| % | ppm | mg/Nm3 | ppm | |
| 600MW | 55/100/55/75 | 80000 | 209 | 393 |
| 45/100/45/65 | 50000 | 237 | 383 | |
| 30/30/30/30 | 32000 | 222 | 325/ | |
| 20/20/20/20 | 25000 | 255 | 245 | |
| 10/10/10/10 | 20000 | 271 | 351 |
It can be seen that when the SOFA damper opening is more than 20%, it has a great influence on the CO concentration of the furnace, but the impact is very limited, and when the SOFA damper opening is 10%, the CO concentration of the furnace can be reduced to less than 20000ppm, indicating that reducing the burnout air rate is conducive to alleviating the high-temperature corrosion. In addition, when the opening of the SOFA damper is greater than 20%, reducing the opening has little effect on the concentration of denitrification inlet. Therefore, in the high load section, the SOFA damper opening is kept between 10% and 30%, which can not only reduce the CO concentration of the furnace, but also ensure that the denitrification inlet content is within a controllable range.
Second, the impact of air volume adjustment
When the stable load period is more than 600MW and the tail CO content is too high, the variable air volume test is carried out. The test parameters are shown in Table 2, and the main parameter curves before and after air volume adjustment are shown in Figure 1.
Table 2 Adjusting the test parameters under high CO conditions
| 负荷/ MW | Air/ T/h | tail CO/ ppm | entrance Nox/ mg/Nm3 | Induced draft fan Current/A | q2/% | q3/% |
| 630 | 1850 | 1000 | 230 | 321 | 4.75 | 0.35 |
| 1930 | 450 | 268 | 351 | 4.86 | 0.14 | |
| 620 | 1842 | 4000 | 178 | 301 | 4.71 | 1.38 |
| 1888 | 350 | 247 | 314 | 4.87 | 0.13 |
Under the stable working condition of 620MW, the total air volume is increased by 50t/h, and the tail CO is reduced from more than 4000ppm (the specific value is unknown, the range has been exceeded) to 350ppm. If the initial value is 4000ppm, the exhaust heat loss increases by 0.16%, and the chemical incomplete combustion heat loss decreases by 1.25%.
Under the stable working condition of 630MW, the total air volume is increased by 80t/h, and the tail CO is reduced from 1000ppm to 450ppm. At this time, the exhaust heat loss increased by 0.11%, and the heat loss of chemical incomplete combustion decreased by 0.21%.
In addition, for every 1000ppm reduction of CO, a reduction of 0.3459% will result in an approximate reduction of 1.11g/kWh. Separately comparing the impact of boiler efficiency improvement and induced draft fan consumption on economy, the calculated values are shown in Table 3.
Table 3 Comparative economic data
| 负荷/ (MW) | Induced draft fan Current (A) | Induced draft fan Electricity costs (RMB/hour) | Furnace efficiency improved energy conservation (RMB/hour) | synthesis earnings (RMB/hour) | |
| 630 | Before adjustment | 321 | 75.3 | 568.3 | 493.0 |
| After adjustment | 351 | ||||
| 620 | Before adjustment | 301 | 38.5 | 2273.3 | 2234.8 |
| After adjustment | 314 |
三、O2-CO协同控制逻辑及投运策略
Before putting into the CO control loop, the oxygen must be put into the automatic, the output value of the CO controller is superimposed on the oxygen set value, the control limit of the CO to the oxygen set value is ±0.5%, the CO set value changes with the actual load, and the correspondence between the load and the CO set value is shown in Table 4.
Table 4 Correspondence between load and CO setpoint
| 负荷 | CO setpoint |
| <300 | 300 |
| 300 | 300 |
| 320 | 300 |
| 400 | 400 |
| 510 | 400 |
| 590 | 500 |
| 640 | 500 |
In addition, the CO sensor has a sensitivity of 1%, a range of 4000 ppm, and a dead band of ± 100 ppm tentatively set for the control loop. In order to prevent random interference in the CO measurement loop, a 30s signal filter is set, and the CO measurement value below 50ppm is defined as a bad pixel, and when the CO measurement value on both sides is defined as a dead pixel, the CO automatic control loop exits.
IV. Conclusions
1) The opening of the four-layer SOFA damper is controlled at 10%-30%, and the CO concentration on the furnace side is controlled below 30,000ppm.
2) The inlet value of denitrification should not be controlled below 200ppm for a long time, and the linearity of the ammonia pipeline adjustment door is not good at low opening, which is not conducive to the control of ammonia escape, and the opening of the ammonia injection adjustment door is between 20%-30%.
3) Try to control the CO of the tail flue within 500ppm, especially under high load, adjust the air volume in time to avoid excessive incomplete combustion heat loss.
4) Under the premise of automatic oxygen production, CO automatic control loop is put into operation, and two automatic control long-term operations are put into operation.
5) The load is too fast, the total air volume is overshooted, in order to prevent the fan from stalling, if necessary, withdraw CO and automatic control.
By increasing the CO measurement point and increasing the parameter monitoring of boiler operation, the operator can control the incomplete local combustion, reduce the speed of high-temperature corrosion, and improve the boiler efficiency on the basis of taking into account the boiler efficiency, denitrification control and parameter deviation.