Air heaters are used in a few waste heat boilers for preheating combustion air. Incineration plants and reformer furnaces also use preheated air. Decades ago they were used in boilers that fired solid, liquid, and gaseous fuels. However, with NOx limitations for all kinds of fuels, they are now used only if the combustion of the fuel warrants it. If the gaseous fuel has a low heating value or if the solid fuel has a significant amount of moisture, then hot air is required for drying the fuel and also to ensure combustion with a stable flame. A gas to gas heater, which is similar to an air heater, is also used in incineration heat recovery plants where waste fuel is heated by the flue gases from the incinerator before entering the thermal or catalytic incinerator. In gas-fired or liquid fuel-fired packaged boilers, air heaters are not generally used. An economizer is the main heat recovery equipment. There are several types of air heaters, including tubular, regenerative, and heat pipes, the latter being a recent development. In all these heat exchangers, air is preheated by using hot flue gases from the boiler or heater. The flue gases could flow outside or inside the tubes. If the flue gases contain dust or ash particles, it is preferable to make them flow inside the tubes so that the shell or casing is not fouled, because it is more difficult to clean the exterior surfaces. The air takes a multipass route outside the tubes as shown iN Fig. 3.27. Q8.28 shows the sizing procedures.
One of the concerns with air heaters is low temperature corrosion at the cold end. The tube wall temperature or the plate temperature at the cold end falls below the acid dew point of the flue gases if the incoming air temperature is low. Also, tube wall and plate temperatures are lower at lower loads because of their low heat transfer coefficients. Steam is often used to increase the incoming air temperature and thus mitigate this concern.
Figure 3.27 (a) Rotary regenerative air heater. (b) Tubular air heater.
There are two types of regenerative air heaters, one in which the heater matrix rotates, and one in which the connecting air and flue gas duct work rotate. The first type is called the Ljungstrom air heater. The energy from the hot flue gases is transferred to a slowly rotating matrix made of enamel or alloy/carbon steel material, which absorbs the heat and then transfers it to the cold air as it rotates. The elements are contained in baskets, which makes cleaning or replacement easier. Regenerative air heaters are more compact than tubular air heaters, which are heavy and occupy a lot of space. The gas — and air-side pressure drops are high in both these types of air heaters, adding to the fan power consumption. Due to the low heat transfer coefficients of air and flue gases and a low log-mean temperature difference (LMTD), surface area requirements are large for air heaters. However, a lot of surface area can be packed into each basket of a regenerative air heater, so they are more compact than the tubular heater.
One of the problems with regenerative air heaters is the leakage of air from the flue gas side that affects the power consumption and efficiency of the fan. Though the leakage may be low, on the order of 5-10% depending upon the seal design, it is significant in large plants. In tubular air heaters, failure of the tubes or expansion joints could result in leakage from the air side to the gas side, but this is minimal.
In regenerative air heaters, corrosion concerns are addressed by using enamel or corten materials at the cold end. In the case of tubular air heaters, the entire section of tubes may have to be replaced. In some designs of regenerative air heaters, a selective coating of catalytic materials is given to the heating elements to promote the reaction of NOx with ammonia or urea, which is injected upstream of the air heater. NOx is thus reduced. The ammonium bisulfate formed is removed periodically by online soot blowing.
Both tubular and regenerative air heaters are widely used in pulverized coal-fired or fluid bed coal-fired boiler plants.