The power station of Dushanzi Petrochemical Thermal Power Plant has successfully achieved mixed combustion of high-sodium coal and low-sodium coal
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The power station of Dushanzi Petrochemical Thermal Power Plant has successfully achieved mixed combustion of high-sodium coal and low-sodium coal. Author/Source: Dushanzi Online. Date: February 18, 2022. Clicks: 16. In order to achieve greater improvements in quality and efficiency, Dushanzi Petrochemical has adopted mixed combustion of high-sodium coal and low-sodium coal in its power station; currently, 40% of the coal used in the power station is high-sodium coal, and the system is operating steadily. On the morning of February 16, reporters learned from the power generation operation department of the thermal power plant that, in order to ensure stable operation when high-sodium coal is used in combustion, the plant has developed a \"Plan for Using High-Sodium Coal in Power Station Boilers and Safety Protocols\" to provide further guidance on the process of using such coal as well as on how to handle emergency situations that may arise during this process. During the combustion of high-sodium coal, it is essential to maintain a stable boiler load. Samples of the high-sodium coal from various mines are taken for testing and analysis, key parameters are monitored, and daily summaries are prepared to ensure that all parameters and operating conditions remain within normal ranges. To address the potential risks associated with the blending process, thermal power plants establish corresponding control measures. Various departments and workshops cooperate in implementing these controls, with dedicated personnel responsible for ensuring even mixing. This is done to prevent various risks that could arise from a large amount of high-sodium coal entering the furnace, thereby affecting the normal operation of the boiler. Chen Liang, an operation engineer in the power generation operation department of the thermal power plant, explained that the initial proportion of high-sodium coal used in combustion was 3%, and it has gradually increased to 40% at present. In the next step, depending on the results of subsequent shutdown inspections, the proportion of mixed firing will be gradually increased to 50%, or even higher. It is understood that the power generation operation department of the thermal power plant plans to increase the proportion of high-sodium coal used in combustion 30 days before the boiler is shut down for maintenance. After the shutdown, the boiler will be inspected for coking; once it is confirmed that safety conditions are under control, this practice will be adopted on a wider scale.
The Zhundong coal in Xinjiang, China, features large reserves, low mining costs, high volatility, and low sulfur content, making it an excellent coal for power generation. However, the coal from the Jungdong area has a high sodium content, and when burned, it tends to form sintering ash deposits on the heated surfaces, leading to severe slagging and greatly restricting the development and utilization of high-sodium coal. Therefore, to achieve clean and efficient utilization of high-sodium coal, it is necessary to fully understand the sintering properties of high-sodium coal ash.
Coal in Xinjiang has a high volatile matter content and low ash content, making it easy to catch fire and burn completely. However, Xinjiang coal has a high content of alkali metals, as well as a relatively high sulfur content. The chlorides and sulfates of alkali and alkaline earth metals have low melting points, and may be in a gaseous or molten state at high temperatures. Furthermore, during combustion, alkali metals tend to form eutectics at low temperatures with substances such as SiO2-Al2O3; for example, the melting point of SiO2-CaO-Na2O is 720°C, while that of Na3Al(SO4)3 is 920°C. Within a certain temperature range (in the area of high-temperature convective heating surfaces where the flue gas temperature is between 750–900°C), gaseous alkali metal compounds or low-temperature eutectics adhere to the heating surfaces when cooled, and ash particles in the flue gas also gradually stick to the surface of these convective heating surfaces. This is a type of accumulating dust that, if not removed in time, will affect the proper installation, stability, and efficient operation of the boiler. The high moisture content, severe slagging and ash accumulation, as well as contamination associated with coal in Xinjiang pose significant challenges to the safe operation of boilers in existing power plants. Based on the operation of boilers currently using Xinjiang coal, severe problems such as coking, ash deposition, slag formation, and fouling have been observed in the furnace, high-temperature convective heating surfaces, medium-temperature convective heating surfaces, and low-temperature convective heating surfaces.
The main characteristics of coal coking and fouling problems in Xinjiang are: (1) A wide distribution area, with severe coking or fouling occurring on the heating surfaces at all stages along the combustion (flue gas) path； (2) During the use of Xinjiang coal, the locations of the heated surfaces where coking and fouling occur vary among different power plants ； (3) Boilers burning Xinjiang coal mostly exhibit the characteristic of coking growing and developing in the windward direction. In Xinjiang coal-fired boilers, coking, slagging, fouling, and ash accumulation on the high-temperature convective heating surfaces are inevitable phenomena during boiler operation. These issues affect the heat exchange in the boiler’s heating surfaces, reduce the boiler’s output, cause metal corrosion of the tube walls in these surfaces, and in severe cases, can even lead to tube explosions due to overheating. Currently, the main method for addressing the slagging and fouling characteristics of coked Xinjiang coal is blending it with other fuels. It reduces the ash accumulation on the heated surfaces, but it does not fundamentally solve the problems of slag formation and fouling associated with burning Xinjiang coal. At the current stage, there is no effective method in boiler structural design to address the problem of fouling and ash accumulation on the heat-exchanging surfaces. Using the LYWS high-sodium coal coking agent can help solve the problem of fouling on the high-temperature heating surfaces of boilers. Due to the low ash melting point of high-sodium coal, the ash melting point of anti-coking and anti-fusing agents can reach 1900–2000°C, while their boiling point can reach 2900–3000°C. To improve the low ash melting point and anti-coking combustion properties of high-sodium and high-potassium coal, 0.5–1% of LYWS, a high-sodium high-potassium coal low ash melting point anti-coking and anti-fusing agent, is added to the combustion process in boilers using such coal. It can increase the ash fusion temperature of high-sodium and high-potassium coal by 150–200 degrees, **reducing the coking tendency of such coal during combustion and improving economic efficiency.