Waste Heat Recovery Devices
Waste Heat Recovery Devices
Industrial furnaces are used for carrying out certain processes which requires heat. Heat in the furnace is provided by (i) fuel energy, (ii) chemical energy, (iii) electrical energy or (iv) a combination of these energies. Gases which are generated during the process leaves the furnace at a temperature which is the inside temperature of the furnace and hence have a high sensible heat content. Sometimes the exhaust gases carries some chemical energy, which raises the temperature of exhaust gases further due to post combustion because of this chemical energy. The heat energy contained in the exhaust gases is the waste energy since it gets dumped in the environment. However, it is possible to recover some part of this energy if investments are made in waste heat recovery devices (WHRDs).
Methods for waste heat recovery include (i) transferring heat between exhaust gases and combustion air for its preheating, (ii) transferring heat to the load entering furnaces, (iii) generation of steam and electrical power, or (iv) using waste heat with a heat pump for heating or cooling facilities.
WHRDs work on the principle of heat exchange. During heat exchange the heat energy of the exhaust gases gets transferred to some other fluid medium. This exchange of heat reduces the temperature of the exhaust gases and simultaneously increases the temperature of the fluid medium. The heated fluid medium is either recycled back to the process or utilized in the production of some utilities such as steam or power etc.
The benefits of WHRDs devices are multiple namely (i) economic, (ii) resource (fuel) saving, and (iii) environmental. The benefits of these devices include (i) saving of fuel, (ii) generation of electricity and mechanical work, (iii) reducing cooling needs, (iv) reducing capital investment costs in case of new facility, (v) increasing production, (vi) reducing greenhouse gas emissions, and (vii) transforming the heat to useful forms of energy.
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 WHRDS used for air preheating include recuperators, furnace regenerators, recuperative and regenerative burners, passive air preheaters, shell and tube heat exchangers, finned tube heat exchangers or economizers, rotary regenerator or heat wheel, preheating of load, waste heat boilers, and heat pumps.
In a recuperator there is a direct transfer of heat. The two fluids (exhaust gases and the combustion air) are separated by a heat transfer surface and they do not mix. The heat exchange takes place between the exhaust gases and the air through metallic or ceramic walls. Duct or tubes carry the air for combustion to be pre-heated, while the other side contains the waste heat stream.
Recuperators generally recover heat from the exhaust gases of a furnace of medium temperature or high temperature and transfer it to incoming combustion air. Recuperators can be categorized by the relative directions of gas flow such as (i) ‘in parallel-flow heat exchangers’ where both the gases flow in the same general direction, (ii) ‘in counter flow exchangers’ where both the gases flow in opposite directions, or (iii) ‘in cross-flow’ where the gases flow at right angles to each other. Counter flow heat exchangers have the greatest effectiveness while the parallel flow arrangement has the lowest effectiveness.
Recuperators can be based on the principle of heat transfer by radiation, convection, or combinations. Recuperators are constructed out of either metallic or ceramic materials. Metallic recuperators are used in applications with temperatures below 1050 deg C, while heat recovery at higher temperatures is better suited to ceramic tube recuperators which can operate with hot side temperatures as high as 1500 deg C and cold side temperatures of around 950 deg C. The principle of recuperation and different types of recuperators are shown in Fig 1.
Fig 1 Principle of recuperation and different types of recuperators
The simplest configuration for a recuperator is the metallic radiation recuperator. It consists of two concentric lengths of metal tubing. Hot waste exhaust gases pass through the inner tube 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. The radiation recuperator gets its name from the fact that a substantial portion of the heat transfer from the hot gases to the surface of the inner tube takes place by radiative heat transfer. The cold air in the outer shell, however, is almost transparent to infrared radiation so that only convection heat transfer takes place to the incoming air. The two gas flows are usually parallel, although the configuration would be simpler and the heat transfer more efficient if the flows were opposed in direction (or counter flow). The reason for the use of parallel flow is that recuperators frequently serve the additional function of cooling the duct carrying away the exhaust gases and consequently extending its service life.
The second common type of recuperator is convective or tube type recuperator. In this type of recuperator, the hot exhaust gases are carried through a number of parallel small diameter tubes contained in a larger shell, while the incoming air to be heated enters the shell and passes over the hot tubes one or more times in a direction perpendicular to the axes of the tubes. The incoming combustion air is generally baffled around the tubes, picking up heat from the waste gas. If the tubes are baffled to allow the gas to pass over them twice, the recuperator is called a two-pass recuperator and if two baffles are used, it is called a three-pass recuperator. Although baffling increases both the cost of the exchanger and the pressure drop in the combustion air path, it increases the effectiveness of heat exchange. Shell and tube type recuperators are generally more compact and have a higher effectiveness than radiation recuperators, because of the larger heat transfer area made possible through the use of multiple tubes and multiple passes of the gases.
For maximum effectiveness of heat transfer, combinations of radiation and convective designs are used. The recuperator includes a radiation section followed by a convection section in order to maximize heat transfer effectiveness. These are more expensive than simple metallic radiation recuperators, but are less bulky.
Ceramic recuperator is normally used to overcome the principal limitation of the metal recuperators which is the reduced life at the inlet temperatures of more than 1100 deg C. Ceramic tube recuperators allow operation on the gas side at 1550 deg C and on the preheated air side at 810 deg C on a more or less practical basis. The ceramic recuperators consist of short silicon carbide tubes which are joined by flexible seals located in the air headers.
Regenerators are generally used for large capacity furnaces. Regenerators consist of two brick ‘checker work’ chambers through which hot and cold air flow alternately. As the combustion exhausts pass through one chamber, the bricks absorb heat from the combustion gas and there is an increase in its temperature. After the bricks pick up heat, the flow is then reversed so that the incoming combustion air passes through the hot checker work, which transfers heat to the combustion air entering the furnace.
A minimum of 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 a fixed interval of time.
Important relations exist between the size of the regenerator, time between reversals, thickness of brick, conductivity of brick and heat storage ratio of the brick. The time between the reversals is an important aspect in a regenerator. Long periods mean higher thermal storage and hence higher cost. Also long periods of reversal result in lower average temperature of preheat and consequently reduce fuel economy.
Regenerators are especially suited for high temperature applications with dirty exhaust gases. The major disadvantages are its large size and the capital costs, which are significantly greater than costs of recuperators. Other disadvantages of regenerator are the accumulation of dust and slagging on the surfaces which reduce efficiency of the heat transfer. Heat losses from the walls of the regenerator and air in leaks during the gas period and out-leaks during air period also reduces the heat transfer efficiency. Regenerators along with furnace are shown in Fig 2.
Fig 2 Regenerators with furnace
Regenerative and recuperative burners
A regenerative burner is with a heat recovery system that recovers the waste heat of the furnace exhaust gas to heat up the combustion air needed for the burning of the fuel at the burner. Use of regenerative burners for reheating furnaces can provide significant energy savings.
The regenerative burners are designed to recover the heat to the inlet air by transferring the heat from the exhaust gas to the inlet air which is to be used in the combustion. The regenerative burner has two set of burners each with a regenerator and the reversing valve. The regenerator uses the ceramic (usually alumina) balls to collect the heat. While the first regenerative burner is firing, the other is exhausting the furnace gases. The exhaust gas is passed through the regenerative burner body and transfers the heat to the ceramic balls. Hence, the heat from exhaust gas is transferred to the inlet air since it is passed through the heated ceramic balls. The reversing valve sets the direction of the air flow that enters into the burner head, which makes the inlet air temperature similar to the operating temperature. Due to a high preheat combustion air temperature, the regenerative burner can save the fuel and make the combustion highly efficient.
In case of a recuperative burner, the structure of the burner is the similar to the radiation heat exchanger tube which heats the inlet air up to the higher temperature (about 750 deg C) by recovering the heat from the exhaust gas to the inlet air. Hence, the exchanged heat in the burner can improve the combustion efficiency and save the fuel cost approximately 25 % to 30 %.
In case of the regenerative burner, the first burner is in the firing mode while the second burner is in the exhausting mode. The first burner is firing with the warm combustion air blowing across its burner. The second burner is receiving the hot exhaust gas out from the furnace to its ceramic balls in order to keep the heat in the burner. Only after passing its heat, the exhaust gas gets released. After a period of half a minute to one minute, the second burner is switched to fire mode while the first burner starts receiving the hot exhaust gas. The firing and receiving mode of burner operates alternatively and continuously until the reheating furnace is stopped. The high preheated air temperature makes the combustion process very efficient.
In case of the recuperative burner the temperature of the inlet air is preheated before the combustion in the furnace by the heat exchange technique. The exhaust gas flows through the burner equipped with a heat exchanger installed inside the burner. The heat from the exhaust gas is exchanged to the inlet air before it flows out from the burner. The exhaust gas runs through the area around the outside of the burner and the heat is exchanged inside the burner.
The cross sections of the recuperative and the regenerative burners are shown in Fig 3
Fig 3 Cross sections of recuperative and regenerative burners
Passive air preheaters
Passive air preheaters are gas to gas heat recovery devices for low to medium temperature applications where cross contamination between gas streams are required to be prevented. Passive preheaters are usually of two types namely (i) the plate type and (ii) the heat pipe type. The plate type preheater 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. The heat pipe heat exchanger consists of several pipes with sealed ends. Each pipe contains a capillary wick structure which facilitates movement of the working fluid between the hot and cold ends of the pipe. 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 passive plate type preheater is shown Fig 4.
Fig 4 Passive plate type preheater
Shell and tube heat exchangers
These are similar in construction to convective recuperators, but are liquid to liquid heat exchangers. Shell and tube heat exchanger is normally used, when the medium containing waste heat is a liquid or a vapour which heats another liquid. This is because both paths must be sealed to contain the pressures of their respective fluids. The shell contains the tube bundle, and usually internal baffles, to direct the fluid in the shell over the tubes in multiple passes. Baffles are normally installed parallel to the axis of the shell, causing shell-side flow along the length of the shell. The shell is inherently weaker than the tube, so that the higher pressure fluid is circulated in the tubes while the lower pressure fluid flows through the shell. When a vapour contains the waste heat, it usually condenses, giving up its latent heat to the liquid being heated. In this application, the vapour is almost invariably contained within the shell. If the reverse is attempted, the condensation of vapours within small diameter parallel tubes causes flow instabilities. Tube and shell heat exchangers are available in a wide range of standard sizes with many combinations of materials for the tubes and shells. Fig 5 shows typical shell and tube heat exchangers without and with baffles.
Fig 5 Typical shell and tube heat exchangers without and with baffles
Finned tube heat exchanger or economizer
Finned tube heat exchanger is used to recover heat from low to medium temperature exhaust gases for heating liquids. Applications include boiler feed water preheating and air preheating etc. The finned tube consists of a round tube with attached fins which maximize surface area and heat transfer rates. Liquid or air flows through the tubes and receives heat from hot exhaust gases flowing across the tubes. A finned tube exchanger where boiler exhaust gases are used for feed water preheating is generally referred to as a boiler economizer.
In the case of boiler systems, an economizer is generally provided to utilize the exhaust gas heat for pre-heating the boiler feed water. On the other hand, in an air pre-heater, the waste heat is used to heat the combustion air. In both the cases, there is a corresponding reduction in the fuel requirements. A finned tube heat exchanger of a steam boiler is shown in Fig 6.
Fig 6 Finned tube heat exchanger of a steam boiler
Rotary regenerator or a heat wheel
Rotary regenerator is similar to fixed regenerator since the 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. 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 medium temperature applications due to the thermal stress created by high temperatures. Large temperature differences between the two ducts sometimes lead to differential expansion and large deformations, compromising the integrity of duct wheel air seals. In some cases, ceramic wheels are used for higher temperature applications. Another issue with heat wheels is preventing cross contamination between the two gas streams, as contaminants can be transported in the wheel’s porous material. A rotary generator is shown in Fig 7.
Fig 7 Rotary heat generator
Load preheating is the direct heat recovery to the product and refers to the use of waste heat leaving a system for preheating the load entering the system. It is done for direct transfer of heat between combustion exhaust gases and solid materials entering the different furnace. Direct heat recovery to the product has the highest potential efficiency because it does not require any ‘carrier’ to return the energy to the product. The most common example is reheating furnace where the exhaust gases leaving the furnace preheat the furnace charge material. This is shown in Fig 8.
Fig 8 Preheating of billets by exhaust gases
Waste heat boiler
Waste heat boiler is a normal water tube boiler in which the hot exhaust gases from a furnace pass over a number of parallel tubes containing water. Waste heat boiler generally uses medium to high temperature exhaust gases to generate steam. Waste heat boilers are available in a variety of capacities allowing for gas intakes ranging from 30 cum/min to 25000 cum /min. In cases where the waste heat is not sufficient for producing desired levels of steam, auxiliary burners or an afterburner are usually added for obtaining higher output of steam. The steam can be produced for process purpose or for generation of power. Generation of superheated steam normally needs addition of an external super heater to the boiler. A typical two pass water tube waste heat boiler is shown in Fig 9.
Fig 9 Typical two pass water tube waste heat boiler
In the various commercial heat transfer devices previously discussed, the waste heat is transferred from a hot fluid to a fluid at a lower temperature. In these devices heat must flow freely ‘downhill’, that is from a system at high temperature to one at a lower temperature. When energy is repetitively transferred, it becomes less and less available for use. Eventually that energy has such low intensity (resides in a medium at such low temperature) that it is no longer available at all to perform a useful heat transfer function. It is generally considered that fluids with temperatures less than 150 deg C, as limit for waste heat recovery because of the risk of condensation of corrosive liquids. However, as fuel costs continue to rise, even such waste heat can be used economically for space heating and other low temperature applications. It is possible to reverse the direction of spontaneous energy flow by the use of a thermodynamic system known as a heat pump.
The majority of heat pumps work on the principle of the vapour compression cycle. In this cycle, the circulating substance is physically separated from the source (waste heat, with a temperature of T-in) and user (heat to be used in the process, T-out) streams, and is re-used in a cyclical fashion, therefore called ‘closed cycle’. In the heat pump, the following processes take place.
- In the evaporator the heat is extracted from the heat source to boil the circulating substance.
- The circulating substance is compressed by the compressor, raising its pressure and temperature. The low temperature vapour is compressed by a compressor, which requires external work. The work done on the vapour raises its pressure and temperature to a level where its energy becomes available for use.
- The heat is delivered to the condenser.
- The pressure of the circulating substance (working fluid) is reduced back to the evaporator condition in the throttling valve, where the cycle repeats.
The heat pump was developed as a space heating system where low temperature energy from the ambient air, water, or earth is raised to heating system temperatures by doing compression work with an electric motor-driven compressor. The arrangement of a heat pump is shown in Fig 10.
Fig 10 Arrangement of a heat pump
Benefits and other aspects of waste heat recovery devices
The benefits of the WHRDs devices can be broadly classified in two categories, namely (i) direct benefits, (ii) indirect benefits.
- Direct benefits are reflected by the reduction in the consumption of the resources and utilities and also the operating costs, since recovery of waste heat improves the energy productivity of the process and has a direct effect on the efficiency of the process. In the present scenario of global climate change, the biggest benefit of the waste heat recovery is that it is a green-house gas free source of energy.
- The indirect benefits are reduction in environmental pollution, reduction in the consumption of energy for auxiliary uses and reduction in the equipment sizes. WHRDs reduce the fuel consumption, which leads to reduction in the production of exhaust gas. This results in reduction in equipment sizes of all fuel gas handling equipments such as fans, stacks, ducts, burners, etc. Reduction in equipment sizes gives additional benefits in the form of reduction in auxiliary energy consumption like electricity for fans, pumps etc.
The other aspects of the WHRDs are that there is need of additional space, capital and operating cost which need to be justified from the benefits gained in terms of heat recovered.
WHRDs, although currently employed to varying degrees at many places in the industry, face technical and economic barriers that impede their wider applications. Though many of the devices are already well developed for waste heat recovery (e.g. recuperators and regenerator etc.) yet there is the challenge that these devices are not always economical for a given application (e.g. application with dirty exhaust streams).
There are many barriers which impact the economy and effectiveness of heat recovery devices and impede their wider installation. Many of these barriers are interrelated, but can generally be categorized as related to cost, temperature restrictions, chemical composition, application specifics, and inaccessibility/transportability of heat sources.