The difference between coking and slagging

Scaling is a chemical reaction between impurities in the water and water pipes, while sludging is a physical reaction.

Coking – refers to the properties of coal； Slagging – refers to a characteristic of ash. In practice, slagging is commonly referred to as coking. Measures to prevent slagging in furnace design: 1. Properly design the furnace structure and arrange the radiant heating surfaces appropriately ； For the 2 pairs of burners using coal with a low ash melting point, to prevent slag formation during operation, the vertical distance between them can be increased, thereby reducing the temperature level in the burner area ； 3. The soot blower is an essential device to prevent severe slagging in the furnace; it also improves boiler efficiency and is an important means of saving energy. Therefore, soot blowers must be installed in the furnace, and the quality of their manufacturing and installation must be ensured. 4. In areas of the furnace where slag formation is likely to occur, such as at the primary air nozzles, on the left and right side walls of the burner area, near the flame deflection angle, and close to the slope of the cold ash hopper, observation holes and slag removal holes should be provided to enable operators to access these areas easily for inspection and maintenance purposes. The design angle of the cold ash hopper in the furnace should be no less than 50°; the water wall tubes and support structures at the cold ash hopper must be capable of withstanding the impact of large pieces of cinder falling from above, as well as the load resulting from the accumulation of large amounts of cinder during abnormal operation. The reliable operation of the slag removal equipment at the bottom of the boiler cannot be ignored. During the initial operation of the 600MW boilers at Yuanbaoshan Power Plant and Shidongkou No. 2 Plant, severe slag accumulation at the boiler bottom occurred, and this was related to design flaws and poor performance of the slag removal equipment, as well as issues with slag discharge. Therefore, full attention should be paid to the design, manufacturing, installation, and construction quality of such slag removal equipment. Measures to prevent slagging during operation: 1. Strengthen fuel management ； 2 Establish reasonable combustion conditions through combustion adjustment tests ； 3 Strengthen the inspection and analysis of boiler operation conditions ； 4 Strengthen the operation and maintenance management of soot blowers and slag removal equipment.

What are the hazards of boiler fouling, and how can it be prevented? Answer: (1) The main hazards of boiler fouling include: ① It causes the steam temperature to rise excessively； ②Disrupting the water cycle ； ③Increased smoke exhaust loss ； ④It reduces the boiler’s output. (2) To prevent coking, it is necessary to adjust the combustion properly during operation, so as to ensure a uniform flame distribution within the furnace and to keep the flame center in an appropriate position ； Ensure an appropriate excess air amount to prevent oxygen-deficient combustion ； When dust accumulation and coking are detected, they should be removed promptly ； Avoid operating beyond capacity ； Improve the quality of maintenance work ； Ensure the burner is installed correctly ； The boiler must have good tightness, and any issues with the boiler equipment should be addressed promptly. What are the hazards of boiler slagging? Answer: Slagging has adverse effects on both the economic efficiency and safety of boiler operation, which are manifested in the following aspects: (1) Decreased boiler thermal efficiency: ① Slagging on the heat transfer surfaces deteriorates heat transfer, raises the flue gas temperature, and leads to a decrease in the boiler’s thermal efficiency ； ②Slag formation at the burner outlet causes the airflow to deviate, leading to poor combustion; this may result in increased heat losses due to mechanical inefficiencies in combustion as well as incomplete chemical combustion ； ③It increases the ventilation resistance of the boiler, leading to higher plant electricity consumption. (2) Impact on boiler output: ① Slag formation on the water wall leads to a decrease in evaporation rate ； ②An increase in the flue gas temperature at the furnace outlet, an increase in the steam outlet temperature, an increase in the tube wall temperature, and an increase in ventilation resistance can all become factors that limit output. (3) Impact on the safety of boiler operation: ① After slag formation, both the flue gas temperature and steam temperature in the superheater increase; in severe cases, this can lead to overheating of the tube walls ； ②Slagging is often uneven, which increases the thermal deviation in the superheater and has an adverse effect on the water circulation safety in natural-circulation boilers as well as on the thermal deviation of the water wall in forced-circulation boilers ； ③When slag masses at the upper part of the furnace fall, they may damage the water-cooled wall tubes in the cold ash hopper, leading to the furnace going out of operation or the slag discharge outlet becoming blocked, thereby forcing the boiler to stop running ； ④When the slag removal operation takes a long time, too much cold air leaks into the furnace, causing unstable combustion or even extinguishing the fire.

The chemical composition of cinder is 40%–50% SiO2, 30%–35% Al2O3, 4%–20% Fe2O3, and 1%–5% CaO; the ash contains high levels of silicon and aluminum, which makes it difficult for slag to form. It is high in sodium, potassium, and calcium, and tends to form slag. Coal with good coking properties must have good cohesion, but coal with good cohesion does not necessarily enable the production of high-quality coke on its own. This is why coal blending for coking is necessary. Cohesiveness is the main indicator for the industrial classification of coal; it is generally expressed by the thickness of the colloid formed as a result of the thermal decomposition and softening of the organic matter in coal, and is often referred to as the thickness of the gum layer. The thicker the gel layer, the better the adhesiveness. There are many methods for measuring cohesion and coking behavior; in addition to the gum layer measurement method, there are also the Roga index method, the Oya swelling test, and others. Adhesiveness is influenced by various factors such as the degree of coalification, coal-rock composition, degree of oxidation, and mineral content. Coals with the highest and lowest degrees of coalification generally lack cohesion, and the thickness of their vitrinite layer is also very small. Measures to prevent coking in gas stoves: 1. Increase the amount of saturated steam. 2. Use a soldering iron to make multiple punctures at the coking area in order to reduce the ventilation there. 3. Strengthen testing; the melting point of coal ash must be above 1250 degrees.