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Suspension Burning
Published in Kenneth M. Bryden, Kenneth W. Ragland, Song-Charng Kong, Combustion Engineering, 2022
Kenneth M. Bryden, Kenneth W. Ragland, Song-Charng Kong
An arch in the upper part of the radiant section is used to provide more uniform flow around the upper corner of the combustion chamber, thereby increasing heat transfer in the convective section. The convection section contains tubes to superheat the steam and provide reheat for multistage turbines. The convective section is designed to extract as much heat in as small a space as possible. Gas velocities in the convective section are restricted to about 20 m/s for coal-fired furnaces to minimize tube erosion due to fly ash. If the ash content of the fuel is high or contains especially hard compounds such as quartz, the gas velocity must be reduced by enlarging the convective section. The flue gas temperature at the inlet to the convective section generally is limited to 1100°C to minimize tube corrosion. Soot blowers, consisting of either steam or compressed air jets, are used regularly on the convective tubes as well as the economizer and air preheater tubes to maintain effective heat transfer and reduce flue gas pressure drop due to deposition of ash on the tube surfaces. Elements such as sodium, potassium, and iron in the ash can cause ash deposits to sinter and fuse onto the boiler tubes and thus reduce the effectiveness of the soot blowers. Sulfate and chloride ions in the deposits can cause corrosion of the tube surfaces. When changing fuels, the effects of slagging, erosion, and corrosion must be considered in relation to the specific boiler design.
Over 100 Ways to Improve Efficiency
Published in Harry Taplin, Boiler Plant and Distribution System Optimization Manual, 2021
The rule of thumb applies: every 40 degree decrease in net stack temperature saves about one percent in fuel consumption. Soot is an excellent insulator which can retard heat transfer to a great degree, so soot blowers are used to keep heat exchange surfaces clean to maintain lower stack temperatures and higher efficiencies. Also, there is more of a tendency to form soot with the heavier residual type fuels. On the other hand, soot blowing is an expensive proposition when you consider the loss of energy involved to just keep the tubes clean. It can cost from 50 to 200 thousand dollars a year for a single boiler, so it is important to manage soot blowing properly. Both too little and too much soot-blowing can waste significant amounts of energy. When low excess air levels are maintained, keeping heat exchange surfaces clean becomes a greater challenge because the burners are operating closer to their smoke point. Soot blowers are usually installed on water tube boilers, but they are also available for fire tube type units on special order for applications such as black liquor and other dirty fuel type service. Some plants have even had successful results with a large ship’s fog horn to remove deposits with low frequency vibrations. They are less expensive to operate than the conventional units, but their performance has had mixed reviews.
Boilers
Published in Neil Petchers, Combined Heating, Cooling & Power Handbook: Technologies & Applications, 2020
Because refuse is a high fouling fuel, more rigorous and costly routine maintenance procedures are necessary versus conventional fossil fuel units. Designs should include good access to convection sections for frequent inspection and cleaning. To prevent plugging of gas passages, it is necessary to remove ash and slag deposits from external tube surfaces. Steam or air soot blowers are most commonly used. Corrosion prevention is an important consideration with refuse-fuel combustion. In addition to the corrosive materials present in conventional fossil fuels (such as sodium and sulfur chlorides, among other additional chemical elements), are other persistent corrosives that deposit on the various steam generator sections. Special corrosion resistant materials are used for protection in the furnace walls and tubes. Strict water treatment programs are also particularly important to minimize corrosion.
A framework of clustering based on W-EFC with updating strategy for power plant air preheater monitoring
Published in Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 2021
Hui Gu, Pan Chen, Hongxia Zhu, Kening Zhang
Low-temperature corrosion mitigation measures can help to reduce the ash fouling phenomenon in air pre-heater. Fine-grained sedimentary and grime on cold end of the heated surface can be cleared by changing the average temperature of the cold junction or soot blowing device. The main devices dealing with ash fouling problems in air pre-heater are steam soot blower and water flushing device. Steam soot blower, generally arranged on the cold side of the flue gas side, can be used to remove the fine ash deposits on the cold and hot sides. The device arrangement can clear fine ash deposits into the electrostatic precipitator. If it is set in the cold end of the air side, the removed fouling ash will be mixed with air entering the bellows in burner, causing fouling. When the ash content of the fuel increases and the fouling is more serious, both cold side and hot side of air pre-heater need steam soot blowers to be set. Steam blower is an effective device to control the fouling deterioration. However, when dealing with the large ash particles blocked on the air pre-heater heated surface, water flushing method should be taken to keep the pressure drop of both air and flue gas. Under normal circumstances, the water flushing should be arranged during the boiler scheduled shutdown or maintenance. Boiler load decreases during the process with a baffle isolated one heater, and water flushes the other air pre-heater. When the cleaning process is finished, the same water flushing method will be applied on the formerly isolated one.
Optimal soot blowing and repair plan for boiler based on HJB equation
Published in Optimization, 2022
Jie Wen, Yuanhao Shi, Xiaoqiong Pang, Jianfang Jia
Fortunately, the soot-blowers can clean the deposited soot by using water, air or steam to improve the boiler efficiency. In fact, when more soot blowing operations are performed, the soot thickness deposited in the boiler can keep thinner to obtain higher efficiency, but the cost of soot blowing will increase. On the contrary, if less soot blowing operations are performed, the cost of soot blowing can decrease, but the thickness of deposited soot will be too thick, which leads to low efficiency. Thereby, the cost of soot blowing and boiler efficiency are in conflict.
Investigation of aerosol and gas emissions from a coal-fired power plant under various operating conditions
Published in Journal of the Air & Waste Management Association, 2019
Zhichao Li, Yang Wang, Yongqi Lu, Pratim Biswas
Soot blowing is widely employed by coal-fired power plants to avoid deterioration of thermal efficiency due to soot accumulation in the coal boiler (Shi, Wang, and Liu 2015). Soot blowers using compressed air or steam are triggered periodically to blow away the ash attached on boiler tubes, which inevitably increases the amount of fly ash leaving the boiler. Figure 7 shows the size distributions measured by the SMPS and APS during soot blowing in boilers 5 and 7 compared with those obtained under the normal operating condition. During soot blowing in either boiler 5 or boiler 7, the measured size distributions showed no significant difference. Like the result of FGD bypass mode, most submicrometer particles displayed higher number concentrations at soot blowing mode. The total particle number concentrations reached 1.95 × 1010 and 1.88 × 1010 #/cm3 when conducting soot blowing in boiler 5 and boiler 7, respectively. These observations verified that soot blowing could increase the particle number concentration in the flue gas even though the gas passed through the ESP and FGD. Valmari et al. demonstrated in a field study that around 70% by weight of the total generated fly ash could be deposited on the surface of a heat exchanger between two consecutive soot blowing operations, and significant emissions would be expected during soot blowing (Valmari et al. 1999). This statement is consistent with the results shown in Figure 7. Their study also showed that nearly all particles with diameters larger than 10 µm were deposited, but only a small portion of particles smaller than 3 µm was deposited. Nonetheless, our study indicates that the smaller particles re-entrained in the flue gas by soot blowing could double the particle load emitted to the atmosphere. On the contrary, the large particles generated by soot blowing can probably be recaptured by the ESP and FGD, which both have high removal efficiency for larger particles (Li et al. 2009; Wang et al. 2008; Yang et al. 2010; Zhuang et al. 2000).