Combustion adjustment in boiler technology

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The tasks of automatic regulation in the boiler combustion process are as follows: ① Maintain a balance between the heat load and the electrical load, adjust the steam volume by controlling the fuel amount, and keep the steam pressure stable. ② Maintain sufficient combustion; when the fuel changes, adjust the air supply accordingly to maintain an appropriate air-to-coal ratio. ③ Keep the negative pressure in the furnace constant by adjusting the ratio of exhaust air to supply air, in order to maintain that negative pressure. 02 Coordination between boiler air volume and fuel supply: An excessive or insufficient air volume can have adverse effects on the safe and efficient operation of the boiler. The air supply volume to the boiler is adjusted via the inlet damper of the air blower. The air volume delivered by the adjusted supply fan can better meet the requirements of combustion only through the coordinated adjustment of primary and secondary air; the distribution of air volumes between these two types of air should be adjusted according to their respective functions. The primary air should meet the requirements for the combustion of the volatiles in the air-powder mixture entering the furnace, as well as for the oxidation of solid carbon particles. The amount of secondary air not only needs to meet the requirements of combustion but also to compensate for the lack of air at the rear part of the furnace. More importantly, the energy from the secondary air must mix with the combustible materials that have just entered the furnace; this requires a high velocity of the secondary air in order to facilitate mixing within the high-temperature flame. The better the mixing, the faster and more complete the combustion will be. I. The primary and secondary air can also be adjusted to address variations in the air flow to each burner caused by differences in the resistance of the coal powder pipelines or burners, as well as the air flow required due to variations in the fuel concentration within those pipelines or burners. In addition, factors such as the deviation of the flame within the furnace, variations in flue gas temperature, and the position of the flame center all require adjustment of the air volume. 03 Adjustment of secondary air in corner-cut circular boilers: These boilers use a large air box for supplying air; the 18 nozzles at each corner are connected to a single large air box. This air box contains 18 separate chambers, each of which is connected to one of the 18 nozzles. Each compartment entrance is equipped with louver-type adjustment baffles. The adjustment of secondary air is based on maintaining the optimal oxygen level. Auxiliary air is the most important component of secondary air. Its function is to adjust the pressure difference between the secondary air blower and the furnace chamber (which should in principle be no less than 380 Pa). This ensures that the secondary air entering the furnace has an appropriate flow rate, thereby enabling effective disturbance and mixing of the coal powder airflow once it enters the furnace, resulting in optimal combustion conditions. The total secondary air volume is adjusted according to the fuel amount and oxygen level, while the opening degrees of the auxiliary air valves for each burner are adjusted based on the furnace/chamber pressure difference specified in the relevant regulations. Each oil layer has its own oil-air distribution system, and there are two ways to control the opening degree of this system: before the oil gun is activated, the air distribution flap for that oil gun is set to over 20% opening ； When the oil gun is not in use, it is adjusted based on the furnace/bellows pressure difference, just like the auxiliary air. Arranging a perimeter wind around the primary air inlet can increase the rigidity of the primary air ； It can keep the coal powder afloat, prevent its separation, and avoid the primary air sticking to the walls ； It can also supply the oxygen needed in the early stages of a fire caused by primary air in a timely manner. Generally speaking, for coals with a high volatile content, the baffles of the peripheral air can be set slightly wider; this helps to prevent the coal powder with high volatility from mixing with the flue gases inside the furnace, thereby delaying ignition and avoiding overheating of the nozzles as well as slag formation. At the same time, due to its high volatile content, it catches fire quickly, and perimeter winds can supply oxygen in a timely manner. However, for coals with lower volatile matter content, it is best to reduce the proportion of peripheral air, as excessive peripheral air can affect the stability of ignition of the primary air. The perimeter air can also be used to cool the burner; the opening degree of the perimeter air corresponding to the operating coal mill should be no less than 20%, while it can be adjusted to 10% when the coal mill is not in operation. The adjustment of the upper burnout air is primarily used to adjust the steam temperature difference between sides A and B; moreover, increasing the amount of upper burnout air can also help reduce NO levels. The amount generated. When the unit is operating at full load, the valves on the upper two levels should be set to over 60% of their capacity; at low load, they can also be set to over 20% of their capacity, provided that the differential pressure in the air box permits it. 04 Adjustment of secondary air for front-and-back wall opposed boilers: The method for adjusting the secondary air in front-and-back wall opposed boilers is as follows: ① When adjusting the secondary air dampers, it is preferable to first reduce the opening degree of the secondary air dampers of the idling coal mills; the opening degree of these dampers can be reduced to at least 20%. After that, the lower layer combustion air dampers should be adjusted as well, with their opening degree being set to over 50% under full load conditions. ② During operation, the opening degree of the secondary air damper of the coal grinder should be controlled between 70% and 85%. The opening degree of this damper is adjusted according to the amount of coal fed to the grinder; when the coal feed rate exceeds 60 t/h, the opening degree of the secondary air damper should be above 80%. ③ Adjust the boiler’s operating air volume in a timely manner based on changes in the carbon content of the fly ash; prioritize adjusting the opening degree of the dampers, and after the dampers are fully open, increase the air volume by raising the pressure in the secondary air main. ④ The upper burnout air should be kept fully open to control nitrogen oxides at the denitration inlet and reduce the amount of ammonia used for denitration. ⑤ The operation of the boiler follows the principle of supplying air first and then increasing the load; when the load rises, the amount of secondary air should be increased promptly. Before shutting down a coal mill, the amount of secondary air for the coal mill that is still in operation should be increased first, after which the amount of secondary air for the coal mill to be shut down should be reduced. Before starting a coal mill, the secondary air valve should be opened first, and then the coal mill can be started. ⑥ During unit operation, pay attention to the matching relationship between the opening of the secondary air dampers and the air volume. If a significant discrepancy is observed, it may be due to the disconnection of the damper control actuator; conduct an immediate on-site inspection, and contact the maintenance team for handling in case of any abnormalities. 05 At low loads, more upper burners should be used. The superheaters in most medium and high-pressure boilers rely primarily on convective heat transfer; the steam temperature characteristic of convective superheaters is that it decreases as the load falls. When the boiler load is low, it is possible for the desuperheating water control valve to be fully closed yet the steam temperature to remain below the lower limit. Although increasing the excess air coefficient at the furnace outlet or raising the negative pressure in the furnace can be used to raise the steam temperature, these methods lead to an increase in the exhaust gas temperature and thus an increase in the excess air coefficient of the exhaust gases, which results in higher heat losses from the exhaust gases and a decrease in the boiler’s thermal efficiency. If the lower burners are turned off as much as possible and more of the upper burners are used, the flame center in the furnace moves upward, resulting in a decrease in the heat absorbed by the furnace, and the temperature of the flue gas at the furnace outlet rises. Due to the increased radiation heat absorption and the greater temperature difference for heat transfer, the total heat absorption by the superheater increases, which raises the steam temperature. This method of adjusting steam temperature is cost-effective; it should be the first method to be employed when the steam temperature is low due to low load. 06 Adjustment of boiler oxygen supply: When the boiler is operating at high load, the high furnace temperature ensures stable combustion but results in higher flue gas losses. To improve the boiler’s efficiency, it is possible to reduce the excess air coefficient appropriately, taking into account factors such as the actual quality of the coal used. By reducing the oxygen level, smoke loss is decreased, which improves the boiler’s efficiency. However, experimental data are necessary to guide these adjustments; different oxygen levels can be tried at full load to measure the carbon content in the fly ash as well as the CO content. More rigorous tests can also be conducted, such as measuring the boiler’s efficiency at various oxygen levels under high load conditions, in order to determine the optimal oxygen level for boiler operation. Furthermore, since the oxygen meter is located after the economizer and not at the furnace outlet, air leakage in the flue gas duct from the furnace outlet to the economizer outlet causes the oxygen level reading to be higher than the actual oxygen level at the furnace outlet. Therefore, this air leakage must be taken into account when adjusting the oxygen level in the boiler. 07 Precautions for combustion adjustment during low-load operation: The precautions for adjusting combustion when the boiler is operating at low load are as follows: ① At low loads, it is necessary to use coal with a higher volatile content as much as possible. When the volatiles in coal are low and combustion is unstable, an ignition oil gun should be used to assist combustion in order to prevent potential extinguishment. ② The burners to be used at low load should be distributed evenly, and the number of burners should not be too small. ③ The speed of increasing or decreasing the load should be slow, and the air volume should be adjusted promptly. Pay attention to maintaining stable primary air pressure, and the volume of primary air should not be too large either. When starting up or shutting down the burner, an oil gun should be used to assist with combustion in order to prevent the flame from going out when adjusting the air volume. ④ Starting, stopping the coal grinding system, and carrying out ash flushing have a significant impact on the stability of combustion; all personnel involved must work closely together and operate carefully and slowly to prevent large amounts of air from entering the furnace. ⑤ When operating at low load, fuel furnaces have difficulty ensuring proper combustion of the fuel; therefore, care must be taken to prevent unburned oil droplets from causing reignition at the rear of the flue. ⑥ During low-load operation, use as little desuperheating water as possible (for mixed-type desuperheaters), but the desuperheating valve should not be closed completely either. ⑦ During low-load operation, the flue gas temperature is low, increasing the likelihood of corrosion at low temperatures. Therefore, a heater or hot air recirculation should be installed. 08 Causes of boiler fouling The causes of boiler fouling are as follows: ① Properties of the ash. The higher the melting point of ash, the less likely it is to coking ； Conversely, the lower the melting point, the easier it is to coking. ② Composition of the surrounding medium. During the combustion process, insufficient air supply or poor mixing of fuel and air prevents complete combustion of the fuel; incomplete combustion generates reducing gases, which **lower** the melting point of the ash. ③ Improper operation during running. The furnace flame is skewed due to improper combustion adjustment ； 1. The coordination between primary and secondary air is inadequate; the high velocity of the primary air prevents the coal particles from burning completely, causing them to remain in a softened state at high temperatures and adhere to the heated surfaces where they continue to burn, thus creating a vicious cycle. ④ The heat load per unit volume of the furnace chamber is too high. An unreasonable furnace design or excessive stress on the boiler leads to an excessive volumetric heat load in the furnace and high furnace temperatures, resulting in coking. ⑤ Soot blowing and coking removal are not carried out in a timely manner. Excessive ash accumulation on the heating surface of the furnace, inadequate cleaning, or failure to remove coke promptly once it forms, can all lead to an increase in the wall temperature of the heating surface, resulting in severe coking of that surface. 09 The impact of coking on boiler operation: The effects of coking on the economic efficiency and safety of boiler operation are as follows: (1) The thermal efficiency of the boiler decreases. ① Coking on the heat-exchanging surfaces deteriorates heat transfer, increases the flue gas temperature, and reduces the thermal efficiency of the boiler. ② Coking at the burner outlet causes airflow deviation and worsens combustion, which may increase mechanical heat losses due to incomplete combustion as well as chemical heat losses due to incomplete combustion. ③ It increases the ventilation resistance of the boiler, leading to higher plant electricity consumption. (2) Affects the boiler output. ① Coking of the water wall reduces the evaporation rate. ② An 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) Affects the safety of boiler operation. ① After coking, both the flue gas temperature and the steam temperature at the superheater increase; in severe cases, this can lead to overheating of the tube walls. ② Coking is often uneven, which increases the thermal deviation in the superheater and has an adverse effect on the water circulation safety of natural circulation boilers as well as on the thermal deviation of the water wall in forced circulation boilers. ③ When coking masses fall from the upper part of the furnace, they may damage the water-cooled wall tubes in the cold ash hopper, leading to furnace shutdown or blockage of the slag discharge outlet, thereby forcing the boiler to stop operating. ④ When the slag removal operation takes a long time, too much cold air leaks into the furnace, causing unstable combustion or even extinguishing the fire.