Reasons for Increasing Exhaust Gas Temperature of Boiler

The heat loss of exhaust gas is the biggest one of all the heat losses of the boiler. Generally, the lost heat is 6% of the heat input into the boiler furnace. The heat loss of exhaust gas increases by 0.5% when the temperature of exhaust gas increases by 12-15%. Therefore, exhaust gas temperature is one of the most important indicators of boiler operation. The following are the possible reasons for the increase in exhaust gas temperature in the furnace.

How to reduce the operating cost of the boiler? Understand the reasons for the rise of boiler flue gas temperature.

·         Slagging or ash deposition on the heat surfaces. Whether the water wall of the furnace slagging ash or the superheater, convection tube bundles, economizer, or preheater with ash deposit will increase the thermal resistance of flue gas measurement, heat transfer deterioration will make the cooling effect of flue gas deterioration, resulting in higher exhaust temperature.

·         Excessive air coefficient is too large. Under normal conditions, with the increase of excess air coefficient at the furnace outlet, the exhaust gas temperature will rise. With the increase of excess air coefficient, the increase of flue gas volume, flue gas velocity, and convective heat release is less than that of flue gas volume. That means when the flue gas velocity increases, the flue gas will no time to transfer its heat to medium before it leaves the heating surface.

·         The air leakage coefficient is too large. Air leakage in the furnace and shaft flue of negative pressure boiler is unavoidable, and the air leakage coefficient allowed by a certain heating surface is stipulated. When the air leakage coefficient increases, the influence on exhaust gas temperature is similar to that of the superheated air coefficient. Moreover, the closer the leaking air away from the furnace is, the greater the impact on the exhaust gas temperature.

·         Feed water temperature. If the turbine load is too low or the high-pressure heater is disassembled, the feedwater temperature of the boiler will be reduced. Generally speaking, when the feedwater temperature rises, if the fuel quantity remains unchanged, the heat transfer temperature difference of the economizer decreases, the heat absorption of the economizer decreases, result in the flue gas temperature rises.

·         Moisture in the fuel. Because of the increase in fuel moisture caused smoke volume increase, so that the flue gas temperature increased.

·         Boiler Loading. Although the boiler loading increase, flue gas, steam capacity, water supply, air increased proportionally, the furnace outlet flue gas temperature increased so that the exhaust temperature increased. When the load increases, the outlet temperature of the furnace increases, and then the temperature difference of the convective heating surface increases make the heat absorption increase. Therefore, the more convective heating surface, the less influence of the boiler load change on the exhaust gas temperature.

·         Fuel varieties. When burning low calorific value gas, because the furnace temperature is low, the radiation heat transfer in the furnace decreases, and the non-combustible components of low calorific value gas, mainly as N2, CO2, H2O, which increase the flue gas volume, so the flue gas temperature rises. After changes the pulverized coal boiler to burn oil, although the excess air coefficient at the furnace outlet is lower than when burning coal, there is less ash content in the fuel oil, nor big size ash particles, so there is no cleaning effect of the larger ash particles in the flue gas on the heating surface, and the convection heating surface pollution is more serious. So the burning is not good, the exhaust gas with black smoke and temperature is high. When the tail is equipped with a steel ball ash removal device, which tail is cleaner, so the exhaust gas temperature is slightly lower than that of coal burning.

How to reduce the operating cost of the boiler? Understand the reasons for the rise of boiler flue gas temperature.