Is Flue Gas Dryer A Better Choice for Heat Recovery in Bagasse-Fired Boiler Than Air Heater?

Bagasse-fired boiler is an essential component of cogeneration system in the cane sugar factory, which also consists steam turbine, condenser, evaporation process, and other minor components. Main components of bagasse boiler are furnace, evaporator, boiler bank, superheater, and economizer. Combustion of fuel in furnace results in thermal energy that is used for water evaporation in evaporator. Heat transfer from flue gas results in additional water evaporation in boiler bank, increases steam temperature in superheater, and increases feed water temperature in economizer. Boiler efficiency is defined as the ratio of the difference between enthalpies of superheated steam and feed water to the heating value of fuel. Boiler efficiency equals 100% minus various losses in percentages of fuel heating value. The major efficiency loss is due to the fact that the temperature of flue gas exhausted from boiler is much higher than the ambient temperature. If this temperature is reduced, boiler efficiency will increase, and so will the energy efficiency of the cogeneration system. There are two choices of doing so. The first choice is adding flue gas dryer after economizer. The second choice is adding air heater after economizer. Both choices should be critically examined in order to reach the best results.

Prof. Somchart Chantasiriwan, Ph.D.

1. Flue Gas Dryer

Bagasse is the remainder of sugar cane after sugar juice is extracted. Because juice extraction process requires a lot of water addition, bagasse contains a high moisture content. Most sugar factories feed moist bagasse to furnace directly. Bagasse moisture becomes water vapor during combustion process in boiler. Since water vapor has a higher specific heat capacity than other gases in furnace, it contributes to more flue gas loss than other gases. If bagasse moisture content is reduced before combustion, flue gas will contain less water vapor, and boiler efficiency will increase.

The removal of moisture in bagasse requires an energy source, which may be obtained from flue gas if flue gas temperature is sufficiently high. Contact between flue gas and bagasse in flue gas dryer results in heat transfer from flue gas to bagasse, which increases bagasse temperature, decreases bagasse moisture content, and decrease flue gas temperature. Dried bagasse and flue gas are then separated at the outlet of flue gas dryer before bagasse is fed to boiler. Two types of flue gas dryer that have been installed in sugar factories are rotary drum dryer or pneumatic flash dryer. Rotary drum dryer consists of a slowly rotating horizontal cylindrical drum, provided with internal deflectors to promote mixing of flue gas and bagasse. Flue gas and bagasse flow in the same direction. Airlock systems must be provided at inlet and outlet to avoid air ingress, which may cause a fire. Pneumatic flash dryer consists of long vertical tubes. Flue gas and bagasse flow in at the bottom, and flow out at the top. The velocity of flue gas must be sufficient to carry along bagasse. Heat transfer occurs between flue gas and bagasse as they flow along the tubes. This type of dryer is characterized by high heat transfer rates, short residence times, and uniform drying.

2. Air Heater

Complete combustion of bagasse requires a sufficient amount of air. In theory, the minimum amount of air can be determined if the composition of bagasse is known. However, excess air, which is the difference between the actual amount and the minimum amount, is needed in practice. Too little excess air leads to incomplete combustion, which results in carbon monoxide in flue gas and unburned bagasse in bottom ash. Combustion is more likely to be complete if there is more excess air. However, too much excess air leads to an unnecessary decrease in boiler efficiency. The appropriate amount of excess air depends on air temperature among other factors. Required excess air is lower if air temperature is higher. Therefore, air temperature should be increased before combustion. The device used for this purpose is known as air heater.

rotary drum dryer

Two types of air heater are regenerative air heater and recuperative air heater. Regenerative air heater consists of a slowly rotating wheel filled with porous material known as matrix. Flue gas and air flow through the opposite halves of the matrix the opposite directions. The cooler half of the matrix absorbs heat from flue gas, becomes hotter, and decreases flue gas temperature. The hotter half of the matrix transfers heat to air, becomes cooler, and increases air temperature. Recuperative air heater consists of many tubes contained in a large rectangular metal box. Flue gas flows inside these tubes, whereas air flows outside across these tubes. Heat transfer from flue gas to air through metal tube walls results in an increase in air temperature and a decrease in flue gas temperature. Regenerative air heater is smaller and cheaper than recuperative air heater of the same heat transfer capacity. However, the major disadvantage of regenerative air heater is the possibility of flow blockage if there is a lot of particulate matter in flue gas. Since bagasse boiler produces flue gas that contains a lot of particulate matter, the type of air heater commonly found in sugar factories is recuperative air heater.

3. Comparison

Using flue gas dryer is equivalent to adding more heat exchanger surface in boiler from the viewpoint of boiler efficiency. In other words, flue gas dryer and air heater can equally increase boiler efficiency if exhaust flue gas temperatures from both devices are the same. Therefore, comparison of both devices must be based on their characteristics and costs.

3.1 Operation and Maintenance
Sugar engineers are more familiar with air heater because its operation is similar to other heat exchangers in sugar factories. The operation of flue gas dryer is mechanically more complicated. Safety concerns are greater with flue gas dryer because there are risks of fires due to contact between flue gas and bagasse. Maintenance of flue gas dryer also requires more effort because the operation of flue gas dryer causes wear on equipment due to impact of fast-moving solid particles on equipment surface.

Bagasse boiler retrofitted by installing (a) flue-gas dryer, (b) air heater

3.2 Power Requirement
Fans are required for the operation of either flue gas dryer or air heater. Flue gas dryer requires more powerful fans because of higher pressure losses. Furthermore, cyclone separators may have to be installed at the outlet of flue gas dryer to separate bagasse from flue gas. Power consumption of cyclone separators may be quite significant.

3.3 Exhaust Flue Gas Temperature
The temperature of flue gas at boiler exit should not be lower than the dew-point temperature to avoid the formation of acids that cause corrosion. Even though flue gas temperature is higher than the dew-point temperature, corrosion may still occur in air heater because the temperature of air heater tube walls is approximately equal to the average of air temperature outside tubes and flue gas temperature inside tubes. If air temperature is too low, it is possible that the temperature of air heater tube walls may be lower than the dew-point temperature, and the condensation of water vapor in flue gas may occur on tube walls. Therefore, the flue gas temperature at air heater outlet must be sufficiently high to prevent this occurrence. This means that the exhaust flue gas temperature of flue gas dryer can be lower than that of air heater, and flue gas dryer has more potential to increase boiler efficiency than air heater.

3.4 Cost
Air heater cost depends primarily on its surface area. Materials for its construction are relatively inexpensive due to the low operation pressure and the low risk of corrosion. However, the surface area of air heater is quite large as a result of low heat transfer coefficients of air and flue gas. Therefore, air heater size may be large. Flue gas dryer cost increases with moisture removal rate, and decreases with increasing inlet flue gas temperature. It is possible to perform a detailed economic analysis in comparing the costs of air heater and flue gas dryer of which installations result in the identical increase in boiler efficiency. Such an analysis will provide the cost of flue gas dyer relative to that of air heater that justifies the installation of flue gas dryer.