Daily Question (December 17)

1. When the boiler load increases, the temperature in the furnace and in the flue gases rises, as does the volume of flue gases. Although both radiant heat transfer and convective heat transfer increase (leading to evaporation and superheating), convective heat transfer increases more than radiant heat transfer does. Given that the characteristic of convective superheaters is an increase, the steam temperature rises as the boiler load increases. 2. When the boiler load decreases, the furnace temperature and the temperature of the flue gases at the furnace outlet drop, and the volume of flue gases decreases. Although both radiation and convection heat transfer (evaporation and superheating) decrease, the reduction in convection and radiation heat transfer is greater. The characteristics of the convective superheater deteriorate as well; therefore, a decrease in the boiler load leads to a drop in the steam temperature.

Answer: When the boiler load increases, more fuel is used, resulting in an increase in smoke volume and smoke velocity. This leads to an increase in the convective heat transfer coefficient on the smoke side, as well as a larger temperature difference for heat transfer, thereby increasing the amount of heat released by the smoke. Additionally, an increased load leads to an increase in steam flow, which in turn increases the heat absorbed by the steam. However, the increase in heat transfer in the superheater is greater than the heat required to accommodate this increased steam flow; hence, the superheater temperature rises, or it falls if the opposite occurs.

As the boiler load increases, the temperature in the furnace and the flue gases rises, and the volume of flue gases increases. Although both radiant heat and convective heat increase (leading to evaporation and superheating), convective heat increases more than radiant heat does. Given that the characteristic of convective superheaters is an upward trend, the steam temperature rises as the boiler load increases.

When the boiler load increases, more fuel is used, resulting in an increase in smoke volume and smoke velocity. The convective heat transfer coefficient on the smoke side increases, as does the temperature difference for heat transfer, which leads to an increased amount of heat released by the smoke. Additionally, an increase in load results in an increase in steam flow, thereby increasing the heat absorbed by the steam. However, the increase in heat transfer in the superheater is greater than the heat required to accommodate this increase in steam flow; hence, the superheater temperature rises, or it falls if the opposite occurs.

The temperature of the superheated steam and reheat steam generated by power plant boilers is not constant; it is influenced by various factors such as design, operation, and installation. To control the steam temperature during boiler operation, it is necessary to analyze the factors that affect it. 1. Influence of boiler load: During operation, the load of a boiler changes frequently. When the load changes, the steam temperature also changes accordingly; for different types of superheaters and reheaters, the way in which their temperature changes with the boiler load varies as well. For radial heating surfaces, as the boiler load increases, it is necessary to increase the amount of fuel and air supply in order to enhance combustion; this leads to an increase in the furnace temperature, and as a result the radiant heat transfer amount Q also increases. However, since the increase in furnace temperature is not significant, and the steam production is proportional to the fuel supply, the increase in radiant heat transfer cannot keep up with the increase in evaporation volume. Furthermore, as the load increases and combustion intensifies, the flue gas temperature at the furnace exit rises, indicating that more heat is carried out of the furnace by the flue gas produced per kilogram of fuel, which also shows a relative decrease in the heat absorbed by radiation within the furnace. Therefore, the temperature of the radial heating surface decreases as the boiler load increases. For convective heating surfaces (low-temperature and high-temperature superheaters, as well as medium- and high-temperature reheaters), as the boiler load increases, fuel consumption also rises, which increases the flow velocity of the flue gas passing through these convective heating surfaces and thus raises their convective heat transfer coefficients ； Additionally, this is due to the increase in the flue gas temperature at the furnace exit; that is, the increase in heat absorbed by the convective heating surfaces exceeds the increase resulting from the steam flow passing through those surfaces. Therefore, the steam temperature at the convective heating surface increases as the boiler load increases. The boilers in our plant are equipped with a combined superheater consisting of radiant, semi-radiant (rear screen), and convective superheaters. The reheat steam is generated through a combination of radiant and convective elements. When the boiler load changes, the steam temperature characteristics are relatively good according to the design; at loads above 75%, the main steam temperature can reach its rated value. At lower loads, the superheating characteristics of these boilers still tend to be convective in nature, increasing as the load rises, although there are some deviations in actual operation. 2. Effect of the temperature at the flame center in the furnace: When the flame center in the furnace rises, the radiant heat transfer within the furnace decreases, which leads to an increase in the temperature of the flue gas at the furnace outlet, and thus an increase in the ambient air temperature. On the contrary, when the flame center drops, it will cause the temperature to fall. An effective way to change the height of the flame center in the furnace is to adjust the inclination angle of the burner. Of course, the operation to change the flame center should be carried out slowly to ensure the safe and economical operation of the boiler. 3. Influence of the cleanliness of the heat-exchanging surfaces: Slag formation on the water wall leads to an increase in the temperature of the superheated steam. This is because slag formation on the water wall reduces its heat absorption capacity, resulting in less steam generation; at the same time, the temperature of the flue gases at the furnace outlet rises. All these factors contribute to an increase in steam temperature. Moreover, slag formation and severe ash accumulation in the superheater itself will cause the steam temperature to drop. During operation, necessary soot blowing and coking removal tasks should be carried out to ensure that the temperature remains within the specified range. 4. Effect of cooling water volume: Spray cooling is used, and the cooling water comes mainly from the feedwater system. When the pressure in the feedwater system increases, although the opening degree of the desuperheating water control valve remains unchanged, the amount of desuperheating water used increases, thereby reducing the temperature. If a spray-type water cooler leaks, it will also increase the amount of water used for temperature reduction and lower the temperature, even without operating the water temperature control valve. 5. Effect of feedwater temperature: Increasing the feedwater temperature will lower the superheated steam temperature. This is because the amount of fuel required to produce each kilogram of steam decreases, and the amount of flue gas flowing through the reheater also decreases. Whether the high-pressure feedwater heater is activated during turbine operation results in a significant difference in feedwater temperature, which has a noticeable impact on the temperature of the superheated and reheat steam. Sometimes, when starting the boiler, in order to maintain the superheated steam temperature, it is possible to use the high-pressure heating circuit to reduce the feedwater temperature, thereby increasing the superheated steam temperature. 6. Impact of changes in main steam pressure: Changes in main steam pressure will directly affect the saturation steam temperature in the drum. The saturated steam exiting the vapor pocket always contains a small amount of moisture; under normal operating conditions, the temperature of the saturated steam changes very little. However, when operating conditions change or the boiler load suddenly increases, the amount of water carried by the saturated steam will **increase**. Since the additional moisture requires heat to be absorbed in order to vaporize in the superheater, and with the combustion conditions remaining unchanged, the amount of heat available to superheat the dry saturated steam decreases, which leads to a drop in the temperature of the superheated steam.

As the boiler load increases, the temperature in the combustion chamber and the flue gas temperature rise, as well as the volume of flue gas, which leads to an increase in both radiant heat transfer and convective heat transfer. However, the increase in convective heat transfer is greater than that in radiative heat transfer, and the characteristic of the convective superheater is an upward trend; therefore, as the boiler load increases, the steam temperature rises.

Answer: When the boiler load increases, the temperature in the combustion chamber and the flue gas temperature rise, as well as the volume of flue gas. This leads to an increase in both radiation heat transfer and convection heat transfer. However, the increase in convective heat transfer is greater than that in radiative heat transfer, and the characteristic of the convective superheater is an upward trend; therefore, as the boiler load increases, the steam temperature rises.

When the boiler load decreases, the furnace temperature and the temperature of the flue gases at the furnace outlet drop, and the volume of flue gases decreases. Although both radiation and convection heat transfer (evaporation and superheating) decrease, convection heat transfer decreases more significantly than radiation heat transfer. The characteristics of the convective superheater also deteriorate; as a result, a decrease in the boiler load leads to a drop in the steam temperature. When the boiler load increases, more fuel is used, resulting in an increase in smoke volume and smoke velocity. The convective heat transfer coefficient on the smoke side increases, as does the temperature difference for heat transfer, which leads to an increased amount of heat released by the smoke. Additionally, an increase in load results in an increase in steam flow, thereby increasing the heat absorbed by the steam. However, the increase in heat transfer in the superheater is greater than the heat required to be absorbed due to the increase in steam flow, so the superheated steam temperature rises.

High load, high steam temperature! Low load, low steam temperature! Because under high load conditions, the furnace temperature is higher and the steam pressure is also greater; heat transfer is better in such situations! Conversely, when the load is low, the steam temperature is also lower!

1. Influence of boiler load: During operation, the load of a boiler changes frequently. When the load changes, the steam temperature also changes accordingly; for different types of superheaters and reheaters, the way in which their temperature changes with the boiler load varies as well. For radial heating surfaces, as the boiler load increases, it is necessary to increase the amount of fuel and air supply in order to enhance combustion; this leads to an increase in the furnace temperature, and as a result the radiant heat transfer amount Q also increases. However, since the increase in furnace temperature is not significant, and the steam production is proportional to the fuel supply, the increase in radiant heat transfer cannot keep up with the increase in evaporation volume. Furthermore, as the load increases and combustion intensifies, the flue gas temperature at the furnace exit rises, indicating that more heat is carried out of the furnace by the flue gas produced per kilogram of fuel, which also shows a relative decrease in the heat absorbed by radiation within the furnace. Therefore, the temperature of the radial heating surface decreases as the boiler load increases. For convective heating surfaces (low-temperature and high-temperature superheaters, as well as medium- and high-temperature reheaters), as the boiler load increases, fuel consumption also rises, which increases the flow velocity of the flue gas passing through these convective heating surfaces and thus raises their convective heat transfer coefficients ； Additionally, this is due to the increase in the flue gas temperature at the furnace exit; that is, the increase in heat absorbed by the convective heating surfaces exceeds the increase resulting from the steam flow passing through those surfaces. Therefore, the steam temperature at the convective heating surface increases as the boiler load increases. The boilers in our plant are equipped with a combined superheater consisting of radiant, semi-radiant (rear screen), and convective superheaters. The reheat steam is generated through a combination of radiant and convective elements. When the boiler load changes, the steam temperature characteristics are relatively good according to the design; at loads above 75%, the main steam temperature can reach its rated value. At lower loads, the superheating characteristics of these boilers still tend to be convective in nature, increasing as the load rises, although there are some deviations in actual operation.