3.1 Closed-circuit schemes of waste heat utilization

Furnace metal heating uses only 10-15% of the entering heat. Up to 60 -70% of the energy consumed is ultimately lost via waste heat contained in streams of hot exhaust gases, external loss (masonry, emissions, etc.) is about 10-20%. Increasing of the temperature of incoming combustion air enhances the efficiency and reduces fuel consumption.

Closed-circuit schemes of waste heat utilization include preheating combustion components and load preheating.

Heat exchangers are most commonly used to transfer heat from combustion exhaust gases to combustion air entering the furnace. Since preheated combustion air enters the furnace at a higher temperature, less energy must be supplied by the fuel. Typical technologies used for air preheating include recuperators, furnace regenerators, burner regenerators, rotary regenerators, and passive air preheaters.

Recuperators

Recuperator recovers exhaust gas waste heat in medium­ to high­temperature applications. It can be based on radiation, convection, or combinations:

• A simple radiation recuperator consists of two concentric lengths of ductwork, as shown in Fig. 2. Hot waste gases pass through the inner duct and heat transfer is primarily radiated to the wall and to the cold incoming air in the outer shell. The preheated shell air then travels to the furnace burners.

• In the convective or tube­type recuperator (Fig. 3) the hot gases passes through relatively small diameter tubes contained in a larger shell. The incoming combustion air enters the shell and is baffled around the tubes, picking up heat from the waste gas.

• Another alternative is the combined radiation/convection recuperator, shown in Fig. 4. The system includes a radiation section followed by a convection section in order to maximize heat transfer effectiveness.

Recuperators are constructed out of either metallic or ceramic materials. Metallic recuperators are used in applications with temperatures below 1093 ⁰C, while heat recovery at higher temperatures is better suited to ceramic­tube recuperators. These can operate with hot­side temperatures as high as 1538 ⁰C and cold­side temperatures of about 982 ⁰C.

F ig.2 Metallic Radiation Recuperator

Fig.3 Convection Recuperator Fig. 4 Combined Radiation/Convection

Recuperator

Regenerators

The mostly used are furnace and rotary regenerators.

Regenerative furnaces consist of two brick “checkerwork” chambers through passes which hot and cold alternately (Fig. 5). As combustion exhausts pass through one chamber, the bricks absorb heat from the combustion gas and increase in temperature. The flow of air is then adjusted so that the incoming combustion air passes through the hot checkerwork, which transfers heat to the combustion air entering the furnace. Two chambers are used so that while one is absorbing heat from the exhaust gases, the other is transferring heat to the combustion air. The direction of airflow is altered about every 20 minutes.

Regenerators are most frequently used with glass furnaces and coke ovens, and were historically used with steel open­hearth furnaces, before these furnaces were replaced by more efficient designs. They also used to preheat the hot blast provided to blast stoves used in iron-making; however, regenerators in blast stoves are not a heat recovery application, but simply the means by which heat released from gas combustion is transferred to the hot blast air. Regenerator systems are specially suited for high­temperature applications with dirty exhausts. One major disadvantage is the large size and capital costs, which are significantly greater than costs of r ecuperators.

Fig. 5 Regenerative Furnace Diagram

Rotary regenerators operate similar to fixed regenerators in that heat transfer is facilitated by storing heat in a porous media, and by alternating the flow of hot and cold gases through the regenerator. Rotary regenerators, sometimes referred to as air preheaters and heat wheels, use a rotating porous disc placed across two parallel ducts, one containing the hot waste gas, the other containing cold gas (Fig. 6). The disc, composed of a high heat capacity material, rotates between the two ducts and transfers heat from the hot gas duct to the cold gas duct. Heat wheels are generally restricted to low­ and medium­temperature applications due to the thermal stress created by high temperatures. Large temperature differences between the two ducts can lead to differential expansion and large deformations, compromising the integrity of duct­wheel air seals. In some cases, ceramic wheels can be used for higher­temperature applications. Another challenge with heat wheels is preventing cross contamination between the two gas streams, as contaminants can be transported in the wheel’s porous material.

O ne advantage of the rotary regenerator is that it can be designed to recover moisture as well as heat from clean gas streams. When designed with hygroscopic materials, moisture can be transferred from one duct to the other. This makes it particularly useful in air conditioning applications, where incoming hot humid air transfers heat and moisture to cold outgoing air. Besides its main application in space heating and air conditioning systems, rotary regenerators are also used to a limited extent in medium­temperature applications. They have also been developed for high­temperature furnace applications such as aluminum furnaces, though they are not widely implemented due to cost. They are also occasionally used for recovery from boiler exhausts, but more economical recuperators and economizers are usually preferred.

Fig. 6 Rotary Regenerator

Passive Air Preheaters

Passive air preheaters are gas­to­gas heat recovery devices for low­ to medium­temperature applications where cross­contamination between gas streams must be prevented. Applications include ovens, steam boilers, gas turbine exhaust, secondary recovery from furnaces, and recovery from conditioned air.

Passive preheaters can be of two types – the plate­ type and heat pipe. The plate­type exchanger consists of multiple parallel plates that create separate channels for hot and cold gas streams. Hot and cold flows alternate between the plates and allow significant areas for heat transfer. These systems are less susceptible to contamination compared to heat wheels, but they are often bulkier and more costly.

The heat pipe heat exchanger consists of several pipes with sealed ends. Each pipe contains a capillary wick structure that facilitates movement of the working fluid between the hot and cold ends of the pipe. As shown in Figure 7 hot gases pass over one end of the heat pipe, causing the working fluid inside the pipe to evaporate. Pressure gradients along the pipe cause the hot vapour to move to the other end of the pipe, where the vapour condenses and transfers heat to the cold gas. The condensate then cycles back to the hot side of the pipe via capillary action.

a b

Fig. 7 Heat pipe exchanger (a) and Heat pipe (b)

The heat pipes are used in the following industrial applications:

1. Process to Space Heating: The heat pipe heat exchanger transfers the thermal energy from process exhaust for building heating. The preheated air can be blended if required. The requirement of additional heating equipment to deliver heated make up air is drastically reduced or eliminated.

2. Process to Process: The heat pipe heat exchangers recover waste thermal energy from the process exhaust and transfer this energy to the incoming process air. The incoming air thus become warm and can be used for the same process/other processes and reduces process energy consumption.

3.Cooling: Heat pipe heat exchangers precools the building make up air in summer and thus reduces the total tons of refrigeration, apart from the operational saving of the cooling system. Thermal energy is supply recovered from the cool exhaust and transferred to the hot supply make up air.

Heating: The above process is reversed during winter to preheat the make up air.

The other applications in industries are:

• Preheating of boiler combustion air.

• Recovery of Waste heat from furnaces.

• Reheating of fresh air for hot air driers.

• Recovery of waste heat from catalytic deodorizing equipment.

• Reuse of Furnace waste heat as heat source for other oven.

• Cooling of closed rooms with outside air.

• Preheating of boiler feed water with waste heat recovery from flue gases in the heat pipe economizers.

• Drying, curing and baking ovens.

• Waste steam reclamation.

• Brick kilns (secondary recovery).

• Reverberatory furnaces (secondary recovery).

Regenerative/Recuperative Burners

Burners that incorporate regenerative or recuperative systems are commercially available. Simpler and more compact in design and construction than a stand­alone regenerative furnaces or recuperators these systems provide increased energy efficiency compared to burners operating with ambient air. A self­recuperative burner incorporates heat exchange surfaces as part of the burner body design in order to capture energy from the exiting flue gas, which passes back through the body. Self­regenerative burners pass exhaust gases through the burner body into a refractory media case and operate in pairs similar in manner to a regenerative furnace. Typically, recuperative burner systems have less heat exchange area and regenerative burner systems lower mass than stand­alone units. Hence, their energy recovery is lower but their lower costs and ease of retrofitting make them an attractive option for energy recovery.

Finned Tube Heat Exchangers/Economizers

Finned tube heat exchangers are used to recover heat from low­ to medium­temperature exhaust gases for heating liquids. Applications include boiler feedwater preheating, hot process liquids, hot water for space heating, or domestic hot water. The finned tube consists of a round tube with attached fins that maximize surface area and heat transfer rates. Liquid flows through the tubes and receive heat from hot gases flowing across the tubes.

Waste Heat Boilers

Waste heat boilers, such as the two­pass boiler, are water tube boilers that use medium­ to high­temperature exhaust gases to generate steam. In cases where the waste heat is not sufficient for producing desired levels of steam, auxiliary burners or an afterburner can be added to attain higher steam output. The steam can be used for process heating or for power generation. Generation of superheated steam will require addition of an external superheater to the system.