What are the main differences between drum boilers and once-through boilers?

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Main advantages of drum boilers: (1) Due to the large amount of water stored in the drum, they have a high heat storage capacity, which enables them to buffer the changes in steam pressure caused by variations in load； (2) In drum boilers, since there is a fixed boundary between water, steam, and superheated steam, changes in load result in only minor variations in the temperature of the superheated steam ； (3) Since there is a steam cleaning device in the drum, lower requirements are placed on the quality of the feedwater. Main disadvantages of drum boilers: (1) High metal consumption ； (2) Delay in the regulatory response ； (3) It can only be used at working pressures below the critical pressure. Main advantages of DC boilers: (1) Low metal consumption ； (2) Short start-up and shutdown time; sensitive regulation ； (3) Not restricted by pressure; it can be used in both subcritical-pressure boilers and supercritical-pressure boilers. Main disadvantages of DC boilers: (1) High requirements for feedwater quality ； (2) High power consumption of the feed water pump ； (3) High requirements for automatic control systems ； (4) A dedicated start-up bypass must be provided. Features of a once-through boiler: The critical point of water is 22.15 MPa and 374.15°C; above this pressure, it becomes a supercritical unit. Steam pressure exceeds 27 MPa; ultra-supercritical thermal power units. Since natural circulation cannot be maintained at supercritical pressures, meaning that drum boilers cannot be used, once-through boilers become the only option. Supercritical units not only have a **reduced coal consumption**, but also exhibit a lower level of pollutant emissions, resulting in significant economic benefits. Supercritical units differ significantly in structure and operational process from subcritical drum boilers. Their characteristics are as follows: 1. Supercritical once-through boilers do not have a drum; the feed water is heated, evaporated, and converted into superheated steam in one continuous process. Depending on the operating conditions, the boiler operates at subcritical or supercritical pressure, and the boiling point shifts spontaneously within one or more heating sections, with no clear boundary between steam and water. This requires stricter maintenance of the relationships between various ratios (such as water supply/steam output, fuel supply/water supply, and spray water volume/water supply, etc.). In a once-through boiler, there are no fixed boundaries between the heating surfaces in the hot water section, the evaporation section, and the superheating section. This is the fundamental reason why the operating characteristics of DC boilers differ significantly from those of drum boilers. 2. Due to the absence of a steam drum with energy storage capacity, the boiler’s energy storage capacity is significantly reduced; as a result, its load regulation is highly sensitive, enabling rapid start-up and shutdown as well as precise load adjustment, which makes it suitable for operation at variable pressure. However, steam pressure responds sensitively to load changes, resulting in poor variable-load performance and difficulty in maintaining the steam pressure. 3. In a DC furnace, the steam and water are generated in one step, so it is different from a drum furnace. During operation, in addition to serving as a steam-water separator, the drum also acts as a buffer for imbalances in the coal-to-water ratio. When the coal-to-water ratio becomes unbalanced, the water stored in the steam drum and its volume are utilized to temporarily maintain the balance of the working medium in the boiler, so as to keep each heating surface area unchanged. Operation characteristics of DC boilers: Dynamic characteristics refer to the changes in steam flow, steam temperature, and steam pressure when the water supply volume, fuel quantity, and power (valve opening degree) change, while all other conditions remain constant. 1. Water supply volume: When there is a disturbance in the water supply volume, the supply volume increases, assuming all other conditions remain unchanged. Since the wall heat load remains unchanged, the hot water section must be extended, and the steam flow gradually increases to the feedwater flow after the disturbance. During the transition process, since the steam flow rate is less than the feedwater flow rate, the mass stored of the working fluid continues to increase. As the steam flow rate increases gradually and the superheating section decreases, the outlet superheated steam temperature drops slowly. However, as the temperature decreases, the metal releases its stored heat, which helps to mitigate the change in temperature to some extent. The steam pressure gradually increases as the steam flow rate increases. It is worth noting that although the steam flow rate increases, the total heat absorbed per unit of working fluid remains unchanged after stabilization, as the fuel quantity does not increase; only the heat absorbed per unit of working fluid decreases (the outlet steam temperature drops). When the water supply is perturbed, there are delays in the changes in evaporation rate, steam temperature, and steam pressure. This is because, since the disturbance begins, it takes some time for the feed water to flow from the inlet to the end of the original hot water section; as a result, there is a delay in the amount of evaporation. This delay in evaporation in turn causes delays in steam pressure and steam temperature. 2. Fuel quantity: When there is a disturbance in the fuel quantity, with all other conditions remaining constant, an increase in fuel quantity causes the evaporation rate to rise first after a short delay, then to fall, and finally to stabilize at a level that balances with the water supply volume. The reason is that at the beginning of the change, as the heat load changes immediately, the hot water section gradually shortens ； The evaporation section will evaporate more saturated steam, increasing the flow rate of superheated steam; its length also gradually decreases. When the lengths of the evaporation section and the hot water section are reduced to such an extent that the flow rate of superheated steam becomes equal to the water supply rate again, no further changes occur. During this period, since the amount of evaporation is always greater than the amount of water supplied, the amount of working fluid inside the boiler continues to decrease (part of the water volume is gradually replaced by steam volume). As mentioned earlier, the fuel volume increases, the superheating section lengthens, and the superheated steam temperature rises. However, in the initial stage of the transition process, since the evaporation rate and the heat released by combustion change almost proportionally, coupled with the delaying effect of heat storage in the tube wall metal, the superheated steam temperature takes some time to change gradually. If the rate and magnitude of fuel increase are both very rapid, it is possible for the boiler to release a large amount of steam instantly. In this case, the steam temperature will first decrease and then gradually rise. The steam pressure gradually increased after a brief delay, and finally stabilized at a higher level. The initial increase was due to increased evaporation, while the subsequent high levels were caused by rising steam temperature (with the turbine throttle valve opening remaining unchanged). 3. Power (throttle opening). Here, a power disturbance refers to the situation where the main steam throttle is activated to draw in some steam, thereby increasing the turbine’s power, while the amount of fuel and water supplied remains unchanged. If the throttle is suddenly opened wider, the steam flow increases immediately and the steam pressure drops. The steam pressure did not change as sharply as the steam flow rate. This is because when the steam pressure drops, the saturation temperature also drops; the boiler fluid \"flashves\", and the metal releases its stored heat, resulting in additional evaporation that prevents the steam pressure from falling further. Subsequently, the steam flow rate gradually decreases due to the drop in steam pressure, eventually becoming equal to the water supply rate and maintaining equilibrium; at the same time, the rate of decline in steam pressure also slows down, until a stable value is reached. Adjustment of operating parameters for once-through boilers: 1. Steam pressure control in once-through boilers. When the unit load increases, the turbine control valves open wider, causing an immediate increase in steam flow rate; consequently, the power output of the turbine also increases immediately. Since the boiler feedwater flow rate and combustion rate have not changed yet, the temporary increase in steam flow rate and turbine power is caused by the boiler releasing its stored heat due to the drop in steam pressure. Due to the low heat storage capacity of DC boilers, the pressure drops at a faster rate. After stabilization, the steam pressure remains at a relatively low level. This necessitates an increase in the amount of feedwater and fuel supplied to prevent any significant drop in main steam pressure, while ensuring that the opening degree of the turbine control valves does not fluctuate excessively. 2. Steam temperature control in direct-fired boilers: The main method for adjusting steam temperature is by controlling the ratio of coal feed to water feed, while auxiliary methods include water injection for temperature reduction or adjustments on the flue gas side. Due to the absence of a fixed steam-water interface, as the feedwater flow rate and fuel amount change, the lengths of the economizer section, evaporation section, and superheating section of the heating surfaces change, which in turn affects the steam temperature; as a result, it is difficult to regulate the steam temperature. Superheated steam temperature control. The superheated steam temperature is controlled by the coal/water ratio and two-stage spray desuperheating. The desuperheating water taken from the superheater comes from the outlet manifolds of the high-pressure heater and the economizer; the water injection rate for each stage of the desuperheater is 3% of the main steam flow rate at that load. The system maintains the outlet steam temperature at 569±5°C within the 35%–100% BMCR load range. Spraying is not allowed at loads below 20% BMCR. If the spray control valve remains closed for more than 10 seconds and the superheated steam temperature is below the target value, each isolation valve should be closed. Reheat steam temperature control. During slip operation, between 50% and 100% BMCR load, the steam temperature at the reheater exit is controlled at 569±5°C. During normal operation, the reheat steam temperature is controlled by flue gas dampers located in the tail flue. The dampers of the two flues operate in opposite directions. The linkage of the flue gas baffle is equipped with an actuator that allows the full-stroke limit to be adjusted, ensuring that at least 20% of the flue gas flow passes through it when in the closed position. When the reheat steam temperature is low, the flue dampers in the reheater are adjusted to the fully open position in order to reduce the resistance in the reheater flue, increase the amount of smoke flowing through it, and raise the reheat steam temperature. The reheater dampers are fully open when the load is below about 85%. Adjusting the superheater flue dampers to the closed position increases the resistance in the superheater flue, which in turn increases the amount of flue gas passing through the convective heating surfaces of the reheater, thereby raising the steam temperature at the reheater outlet. (The sum of the two opening degrees of the flue gas dampers in the superheater during normal operation must be greater than 120.) The response of the flue gas damper system has a certain lag; in transient conditions, an emergency temperature reducer installed on the cold reheat pipeline can be utilized. 1) Water-coal ratio: The higher the water-coal ratio, the lower the main steam temperature ； Conversely, the higher the main steam temperature: 2) Feedwater temperature – When the feedwater temperature is low (for example, when the high-temperature superheater is shut down), if the fuel supply remains unchanged, the main steam temperature will drop, and so will the load. 3) Excess air coefficient – When the excess air coefficient is high, since heat absorption in the furnace occurs through radiation, temperature has a significant impact on this process; therefore, an excessive amount of cold air entering the furnace reduces the heat absorbed there, resulting in a decrease in the temperature at the inlet of the superheater ； Conversely, an increase in the superheated steam temperature 4) Flame center height: When the flame center height rises, the heat absorption in the furnace decreases, resulting in a lower temperature at the inlet of the superheater ； Conversely, an increase in superheated steam temperature; 5) Slag formation on the heating surfaces – slag formation on the water wall heating surfaces leads to a decrease in the main steam temperature ； Slag formation on the heating surfaces of the superheater caused a significant drop in the main steam temperature. 3. Combustion control in once-through boilers: The excess air coefficient in the burner area varies with the boiler load, and it is also affected by the number of coal mills in operation. The air volume in the burner area refers to the amount of air that passes through the burners into the boiler, including the primary air and secondary air for operating burners, the leakage/cooling air for non-operating burners, and the central air for all burners. The air leakage rate of the shut-down burner is determined by the minimum position of the secondary air damper, and it varies depending on the pressure difference between the hot secondary air duct and the negative pressure in the furnace at that load. By controlling the ratio of the air volume in the burner air chamber to that of the combustion air based on the oxygen level signal, the tendency for rotation in the combustion air system is minimized. 4. Feedwater control for DC boilers: The boiler feedwater system is equipped with one electric pump with a capacity of 30% and two steam-driven pumps, each with a capacity of 50%. At low load and during startup, an electric feedwater pump or an air pump as a pre-pump is used to supply water to the boiler for startup, with the feedwater bypass control valve used to adjust the feedwater flow rate. Once the valve is opened to more than 75%, the bypass should be switched to operate on the main feedwater line. When two steam-driven pumps are in operation and one of them loses its load, with the load dropping below 50%, the combustion rate and the amount of water used for temperature reduction also decrease. At this point, the amount of water supplied should be equal to the flow rate of steam being delivered. During low-load operation, the feedwater flow rate and pressure decrease, resulting in an increased subenthalpy of the working fluid at the inlet to the heating surfaces, which facilitates hydrodynamic instability. Due to the reduced feedwater flow, the unevenness in the water wall flow distribution increases ； As pressure decreases, the change in the specific volume of vapor increases ； An increase in the sub-enthalpy of the working fluid causes a change in the resistance ratio between the evaporation section and the economizer section. Image: What are the differences between DC boilers and drum boilers during load adjustment and temperature control? When the load of a DC furnace or a drum boiler changes, both the fuel and feedwater amounts must increase or decrease accordingly. However, due to the effect of the drum water volume, drum boilers do not require a strict maintenance of a fixed ratio between feedwater and fuel supply during regulation. When only one of the feedwater or fuel conditions changes, it will only cause changes in the boiler output or the water level in the drum, with little effect on the superheated steam temperature. This is because the heating surfaces of the superheater in a drum boiler are fixed, and the steam parameters (saturated steam) at the superheater inlet do not change much. In a DC furnace, when the load changes, both the amount of feedwater and fuel must be adjusted simultaneously, while maintaining a strict constant ratio; otherwise, changes in either feedwater or fuel alone, or changes in both without maintaining a proportional relationship, will lead to significant fluctuations in the superheater temperature. This is because the boundary points between the heating, evaporation, and superheating stages in a DC furnace shift, meaning that the area of the heating surfaces in these three stages changes, which inevitably leads to variations in the superheated temperature. Therefore, strictly maintaining a fixed ratio between the fuel volume and the feedwater volume is the most fundamental difference in regulation between once-through boilers and drum boilers. Additionally, the regulation of steam pressure (i.e., the steam generation rate of the boiler): In drum boilers, since the drum has a certain water storage capacity, there is no direct relationship between the feedwater volume and steam pressure; instead, the feedwater volume is adjusted according to changes in water level. However, in a DC furnace, the steam production is determined directly by the water supply volume; therefore, changes in the fuel amount do not affect the boiler’s output, and only changes in the water supply volume can cause changes in the boiler’s evaporation rate. Obviously, when the feedwater is adjusted to maintain stable pressure, it inevitably causes changes in the superheated steam temperature; therefore, during the pressure regulation process, the superheated steam temperature must be corrected. In other words: regulate the feed water pressure, adjust the fuel to control the feed water temperature, focus on the intermediate point, and make fine adjustments by spraying water – these are the basic principles for operating and regulating a once-through boiler.