Waste Heat Recovery Technologies

Waste Heat Recovery Technologies

Waste heat is the energy which is associated with waste streams of air, exhaust gases, and/or process products which leave a process and enter the atmosphere. It is the energy which is generated in various processes and which is not put into any practical use and is lost or wasted into the atmosphere. It is the energy which is rejected from a process at a temperature high enough to permit the recovery of some fraction of the energy for useful purposes in an economic manner

In the definition of waste heat, it is implicit that the waste streams carrying the heat eventually mix with atmospheric air or groundwater and that the energy contained within these streams becomes unavailable as useful energy. The absorption of waste energy by the environment is often termed as thermal pollution.

Recovery of waste heat can be conducted through different waste heat recovery (WHR) technologies to provide valuable energy sources and reduce the overall energy consumption. There are several WHR technologies which are available and which can be used for capturing and recovering the waste heat.

A considerable amount of energy used in the industrial processes is wasted as heat in the form of exhaust gases, air streams, and liquids/solids leaving the process. It is not technically and economically feasible to recover all the waste heat. An increased use of WHR technologies also serves to mitigate greenhouse gas (GHG) emissions.

WHR technologies consist of capturing and transferring the waste heat from a process with a gas, liquid, or solid back to the system as an extra energy source. The energy source can be used to create additional heat or to generate electrical and mechanical power. Waste heat can be rejected at any temperature. Usually the higher the temperature of the waste heat, the higher is the quality of the waste heat and the easier is the optimization of the WHR process. Hence, it is important to discover the maximum amount of recoverable heat of the highest potential from a process and to ensure the achievement of the maximum efficiency from a WHR system

Sources of waste heat normally include heat loss transferred through conduction, convection, and radiation from the products, equipment and processes and heat discharged from combustion processes. Heat loss can be classified into (i) high temperature heat, (ii) medium temperature heat, and (iii) low temperature heat. WHR technologies are available for each type of waste heat so as to have the most optimum efficiency of WHR which can be achieved.

High temperature WHR consists of recovering waste heat at temperatures higher than 400 deg C, the medium temperature waste heat range is 100 deg C to 400 deg C and the low temperature waste heat range is for temperatures which are lower than 100 deg C. Generally most of the waste heat in the high temperature range comes from direct combustion processes, in the medium temperature range from the exhaust of combustion units, and in the low temperature range from parts, products and the equipment of process units.

Depending on the type and source of waste heat and in order to justify which waste heat recovery system can be used, it is essential to examine the amount and grade of heat recoverable from the process. There are three important parameters used in the quantification of waste heat. These parameters are (i) quantity, (ii) quality, and (iii) temporal availability.

The quantity or the amount of available waste heat can be calculated using the equation Q = V x d x Cp x (T1-T2). Here, Q is the heat content, V is the flow rate of the substance carrying the heat, d is the density of the substance, Cp is the specific heat of the substance, and  (T1-T2) is the difference in substance temperature between the final highest temperature in the outlet (T2) and the initial temperature in the inlet (T1) of system. The quantity of available waste heat can also be expressed in terms of the enthalpy flow of the waste stream and is given by the equation H = m x h where H is the total enthalpy rate of waste stream, m is the mass flow rate of waste stream and h is the  specific enthalpy of waste stream.

The quality can be roughly expressed in terms of the temperature of the waste stream. The higher the temperature, the more waste heat is available for the recovery. WHR from lower temperature sources, such as cooling water from machines and condensers, is generally somewhat more difficult, and typically involves the use of heat pumps to increase the temperature to a suitable temperature for its recovery.

The temporal availability is a measure of the availability of waste heat at times when it is needed. Matching the availability of the waste heat to the ultimate load is an important consideration in the effectiveness of WHR. Hence, the usefulness of waste heat depends not only on the quantity available but also on whether its quality fits the requirements of the potential load and whether it is available at the times when it is required (temporal availability).

Cost-effective WHR and reuse involves the identification of waste heat sources of sufficient quality, quantity, and temporal availability, and heating loads which can reuse the waste heat recovered. There are several processes in the low to medium temperature range which can reuse waste heat. These processes are used in different industries. For example, certain distillation operations are ideal for open-loop heat pump systems which mechanically recompress the ‘overhead’ distillation vapour which is then allowed to condense in the reboiler where it vapourizes the ‘bottoms’ product in the distillation column. These applications typically involve small temperature differences and are often more cost-effective than using fuel combustion to heat the reboiler and a cooling tower to reject the heat in the distillate.

Evaluation of the feasibility of WHR needs characterization of the waste heat source, and the stream to which the heat is going to be transferred. Important waste heat stream parameters which are required to be determined include (i) heat quantity, (ii) heat temperature/quality, (iii) composition, (iv) minimum allowed temperature, and (v) operating schedules, availability, and other logistics. These parameters allow for analysis of the quality and quantity of the stream and also provide insight into possible materials/design limitations. As an example, corrosion of heat transfer media is of considerable concern in WHR, even when the quality and quantity of the stream is acceptable.

WHR options and technologies

Approaches for WHR include (i) transfer of heat between gases and/or liquids, (ii) transfer of heat to the load entering furnaces, (iii) generation of mechanical and/or electrical power, or (iv) using waste heat with a heat pump for heating or cooling facilities. The terminology for WHR technologies frequently varies among different industries. The major WHR technologies are described below.

Heat exchangers

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

Recuperator – Recuperators recover waste heat of exhaust gas in medium to high temperature applications. Recuperators can be based on radiation, convection, or combinations of the two.

A simple radiation recuperator consists of two concentric lengths of ductwork. 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. The convective or tube type recuperator (heat exchanger) passes the hot gases 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. 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 below1100 deg C, while heat recovery at higher temperatures is better suited to ceramic tube recuperators. These can operate with hot side temperatures as high as 1550 deg C, and cold side temperatures of around 1000 deg C.

Regenerator – Regenerators are of two types namely (i) furnace regenerators, and (ii) rotary regenerators or heat wheel. In case of furnace regenerator, regenerative furnaces consist of two brick checker work’ chambers through which hot and cold airflow alternately. As combustion exhausts pass through one chamber, the bricks absorb heat from the combustion gas and there is increase in the temperature. The flow of air is then adjusted so that the incoming combustion air passes through the hot checker work, 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 after an interval of time. Regenerators are most frequently used with coke ovens, and were historically used with open hearth furnaces, used earlier for steelmaking. Regenerators are also used to preheat the hot blast provided to blast stoves used in ironmaking. 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 especially suited for high temperature applications with dirty exhausts. One major disadvantage of furnace regenerators is the large size and high capital costs.  

In case of rotary regenerators, they operate similar to the 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 are also sometimes referred to as air preheaters and heat wheels. They use a rotating porous disc placed across two parallel pipes, one containing the hot waste gas, the other containing cold gas. The disc, composed of a high heat capacity material, rotates between the two pipes and transfers heat from the hot gas pipe to the cold gas pipe. 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 pipes can lead to differential expansion and large deformations, compromising the integrity of pipe wheel air seals. In some cases, ceramic wheels can be used for higher temperature 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.

One advantage of the heat wheel 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 pipe to the other pipe. This makes the heat wheels useful particularly 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, heat wheels are also used to a limited extent in medium temperature applications.

Passive air preheaters – Passive air preheaters are gas to gas heat recovery equipments for low to medium temperature uses where cross contamination between two gas streams are to be prevented. Passive preheaters can be of two types namely (i) plate type, and (ii) heat pipe.

The plate type exchanger consists of multiple parallel plates which create separate channels for hot and cold gas streams. Hot and cold flows alternate between the plates and allow substantial areas for heat transfer. These systems are less susceptible to contamination compared to heat wheels, but they are often bulkier, more costly, and more susceptible to fouling problems.

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. In this heat exchanger, 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.

Regenerative/recuperative burners – These burners incorporate regenerative or recuperative systems. They are 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 self-recuperative burner incorporates heat exchange surfaces as part of the burner body design in order to capture energy from the leaving flue gas, which passes back through the body. Self-regenerative 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 medium temperature exhaust gases for heating liquids. The finned tube consists of a round pipe with attached fins which maximize surface area and heat transfer rates. Liquid flows through the tubes and receive heat from hot 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’.

Waste heat boiler – Waste heat boiler (WHB) is a water tube boiler which uses medium to high temperature exhaust gases to generate steam. WHBs are available in a variety of capacities, allowing for gas intakes from 1500 cum/hour to 1.5 million cum/hour. In cases where the waste heat is not sufficient for producing desired levels of steam, auxiliary burners or an afterburner is normally added to attain higher steam output. The steam can be used for process heating or for power generation. Generation of superheated steam generally needs addition of an external super-heater to the system.

Load pre-heating

Load pre-heating refers to any efforts to use waste heat leaving a system to preheat the load entering the system. The most common example is boiler feed water preheating, where an economizer transfers heat from hot combustion exhaust gases to the water entering the boiler. Other applications utilize direct heat transfer between combustion exhaust gases and solid materials entering the furnace.

While boiler feed water preheating is a standard practice, load preheating of material prior to melting in direct fired systems is not as widely used. This is due to a variety of reasons, including difficulties in controlling product quality, issues associated with environmental emissions, and the increased complexity and cost of building advanced furnace loading/heat recovery systems. Nevertheless, heat recovery via load preheating has received increased attention in recent years. The available technologies and barriers for different load preheating furnaces vary substantially depending on the type of furnace and load in question.

Low temperature energy recovery options and technologies

While economics often limit the feasibility of low temperature WHR, there are several applications where low grade waste heat has been cost effectively recovered for use. The large quantities of waste heat is available in the range of 40 deg C to 200 deg C and there is the inherent challenges to its recovery and use which require a separate and in depth investigation of low temperature WHR.

Most of the industrial waste heat is in the low temperature range. For example, combustion systems such as boilers frequently use recovery technologies which exhaust gases at around 150 deg C to 180 deg C. Also, large quantities of waste heat can be found in industrial cooling water and cooling air. For example, cooling of air compressors alone accounts for substantial quantity of waste heat per year. One integrated steel plant (ISP) in Japan has successfully installed a power generation plant with a 3.5 MW capacity using cooling water at only 98 deg C.

In the case of combustion exhaust gases, considerable heat can be recovered if water vapour contained in the gases is cooled to lower temperatures. Minimum temperature limits around 120 deg C to 150 deg C are often employed in order to prevent water in the exhaust gases from condensing and depositing corrosive substances on the heat exchanger surface. However, cooling the flue gas further can considerably increase heat recovery by allowing the latent heat of vapourization to be recovered. The latent heat comprises a significant portion of the energy contained in exhaust gases. Technologies, which can minimize chemical attack while cooling exhaust gases below the condensation point, can achieve substantial increases in energy efficiency via recovering the latent heat of evaporation. Fig 1 shows the energy recovery with different stack exit temperatures. If gases are cooled from 150 deg C to 60 deg C, then there is a 3 % efficiency increase. Cooling gases further to 38 deg C captures a portion of the latent heat and can provide an 11 % efficiency increase.

Fig 1 Energy recovery with different stack exit temperatures 

There are three challenges which are faced by the low temperature heat recovery. These challenges are as given below.

  • There is corrosion of the heat exchanger surface. As the water vapour contained in the exhaust gas cools, some of it condenses and deposits corrosive solids and liquids on the heat exchanger surface. The heat exchanger is to be designed to withstand exposure to these corrosive deposits. This generally needs use of advanced materials, or replacement of the components of the heat exchanger frequently, which is often uneconomical.
  • Large heat exchange surfaces are needed for heat transfer. Heat transfer rates are a function of the thermal conductivity of the heat exchange material, the temperature difference between the two fluid streams, and the surface area of the heat exchanger. Since low temperature waste heat involves a smaller temperature gradient between two fluid streams, larger surface areas are needed for heat transfer. This limits the economics of heat exchangers.
  • There is necessity of a requirement for the low temperature heat. Recovering heat in the low temperature range makes sense only if there is a need in the plant for the low temperature heat. Potential end use is low temperature process heating. Other options include using a heat pump to ‘upgrade’ heat to a higher temperature to serve a load requiring higher temperatures. Also, low temperature power generation technologies are slowly emerging.

Low temperature heat exchange technologies

Low temperature heat exchange technologies are available which can cool gases below dew point temperatures to recover low temperature waste heat. Technology options include deep economizers, indirect contact condensation recovery, direct contact condensation recovery, and recently developed transport membrane condensers. Commercialization of these technologies are limited due to high costs and because facilities lack an end use for the recovered heat. When facilities lack an end use for waste heat, other means for recovery are used which include heat pumps and low temperature power generation. These technologies are also frequently limited by economic constraints.

Deep economizers – Deep economizers are designed to cool exhaust gas to around 70 deg C and to withstand the acidic condensate depositing on its surface. Design of the economizers can have different alternatives. It can have installation of a ‘throwaway’ section on the cold end of the economizer. The pipes at the cold end degrade over time and needs to be replaced very often. The frequency of replacements depends on the flue gas composition and the material of construction. One of the alternatives consists of designing the economizer with stainless steel pipes. Stainless steel can withstand acidic gases better than the mild steel typically used in construction. In another design, using C steel for the majority of the heat exchanger, but using stainless steel pipes in the cold end where acidic deposits occur. Using of glass pipe heat exchangers (mainly for gas-gas applications such as air preheaters) or advanced materials such as Teflon can be other alternatives.

Indirect contact condensation recovery – Indirect contact condensation recovery units cool gases to around 40 deg C. In this range, the water vapour in gases condenses almost completely. Indirect contact exchangers consist of a shell and tube heat exchangers. They can be designed with stainless steel, glass, Teflon, or other advanced materials.

Direct contact condensation recovery – Direct contact condensation recovery involves direct mixing of the process stream and cooling fluid. Since this type of recovery does not involve a separating wall across which heat is to be transferred, it avoids some of the challenges of large heat transfer surfaces required for indirect contact recovery units. In this type of recovery, as flue gases enter the heat exchanger, they are cooled by cold water introduced at the top of the unit. The heated water stream leaves through the bottom of the exchanger and provides heat to an external system. A challenge with direct contact condensation is that the water can be contaminated by substances in the flue gas.

Transport membrane condenser – Transport membrane condenser (TMC) is a developing technology for capturing water (along with water’s latent heat) from the water vapour in gas exhaust streams. Water is extracted from the flue gas at temperatures above dew point by employing capillary condensation and recycled into the boiler feed water. As in the direct contact heat recovery, TMC extracts hot water directly from the flue gas. However, since TMC recovers the water via transport through a membrane, the recovered water does not become contaminated as in the direct contact recovery. The technology has been demonstrated for clean exhaust streams in a natural gas fired boiler. However, TMC requires further development in advanced materials before widespread implementation for dirtier waste streams is possible.

Heat pump or upgrading of low temperature waste heat – Heat exchange technologies mentioned above involve flow of energy ‘downhill’ from a high temperature to a low temperature end use. This can place limitations on opportunities for heat recovery when the waste heat temperature is below the temperature needed for a given heating load. As an example, waste heat can be available in the form of hot water at around 35 deg C, while hot water at around 85 deg C is needed. In such case, a heat pump can provide opportunities for ‘upgrading’ heat to the desired end use temperature. Heat pumps use external energy inputs to drive a cycle which absorbs energy from a low temperature source and rejects it at a higher temperature. Depending on the design, heat pumps can serve two functions. They can either upgrade waste heat to a higher temperature, or use waste heat as an energy input for driving an absorption cooling system. Heat pumps are most applicable to low temperature product streams found in process industries.

Upgrading heat can be economical in some cases depending on the temperature differential required and the relative costs of fuel and electricity. If a facility has a heat load at a slightly higher temperature than the waste heat source, the heat can sometimes be provided more efficiently by a heat pump than if it is to be obtained from burning additional fuel. The coefficient of performance (COP) is a measure of heat pump performance, determined from the heat output and work input and given by equation COP = Q/W where Q is the useful heat output from heat pump, and W is the work input.

An important consideration in determining the feasibility of heat pumps is the waste heat temperature and the desired lift in the temperature. The type of cycle used and the type of working fluid chosen influence the temperatures at which the heat pump can receive or reject heat, as well as determine the maximum lift in temperature achievable. The efficiency of a heat pump decreases as the desired lift in temperature increase.

Closed compression cycle – In closed compression cycle, a heat pump is used to lower the temperature of the cooling water, while using the heat extracted to increase the temperature of process water used elsewhere in the plant. The heat pump consists of evaporator, compressor, condenser, and expansion valve. In the evaporator, energy is transferred from the waste heat source to the refrigerant. Then the refrigerant enters the compressor, where its temperature increases. Superheated refrigerant then enters the condenser and transfers heat to the heat sink. Finally, refrigerant is throttled in an expansion valve before returning to the evaporator.

Open cycle vapour recompression – The open cycle vapour recompression uses compression to increase the pressure (and consequently the temperature) of the waste vapour. Mechanical vapour recompression uses a mechanical compressor, while thermal vapour recompression uses a steam ejector, and hence is heat driven rather than mechanically driven

Absorption heat pumps – Absorption heat pumps are very similar to the closed compression cycle, except the compressor is replaced by a more complex, heat driven absorption mechanism. Depending on the plant needs, the system can be configured in multiple ways. In one type, heat pump can use a lower and a higher temperature heat input to reject heat at an intermediate level (e.g. upgrade the low temperature heat). In another type, heat pump can use a medium temperature input to reject heat in one lower temperature stream and one higher temperature stream. This second application can be used for air conditioning and/or refrigeration.

Power generation

Generating power from waste heat typically involves using the waste heat from boilers to create mechanical energy which then drives the electric generator. These power cycles are well developed. However, new technologies are being developed which can generate electricity directly from heat, such as thermoelectric and piezoelectric generation. When considering power generation technologies for WHR, an important factor to be kept in mind is the thermodynamic limitations on power generation at different temperatures. The efficiency of power generation is heavily dependent on the temperature of the waste heat source. In general, power generation from waste heat has been limited to only medium to high temperature waste heat sources. However, advances in alternate power cycles can increase the feasibility of generation at low temperatures. While maximum efficiency at these temperatures is lower, these schemes can still be economical in recovering large quantities of energy from waste heat.

The three methods for the generation of power by the use of mechanical energy are described below.

Steam Rankine cycle – The most frequently used system for power generation from waste heat involves using the heat to generate steam, which then drives a steam turbine. The traditional steam Rankine cycle is the most efficient option for waste heat recovery from exhaust streams with temperatures above 340 deg C. At lower waste heat temperatures, steam cycles become less cost effective, since low pressure steam needs larger equipment. Moreover, low temperature waste heat cannot provide sufficient energy to superheat the steam, which is a requirement for preventing steam condensation and erosion of the turbine blades. Hence, low temperature heat recovery applications are better suited for the organic Rankine Cycle or Kalina cycle, which use fluids with lower boiling point temperatures compared to steam.

Organic Rankine cycle – The organic Rankine cycle (ORC) operates similar to the steam Rankine cycle, but uses an organic working fluid instead of steam. Alternatives include silicon oil, propane, halo-alkanes (e.g. freons), iso-pentane, iso-butane, p-xylene, and toluene, which have a lower boiling point and higher vapour pressure than water. This allows ORC to operate with significantly lower waste heat temperatures. The most appropriate temperature range depends on the fluid used, as fluids’ thermo-dynamic properties influence the efficiency of the cycle at various temperatures. In comparison with water vapour, the fluids have a higher molecular mass, enabling compact designs, higher mass flow, and higher turbine efficiencies. However, since ORC functions at lower temperatures, the overall efficiency is low and depends on the temperature of the condenser and evaporator. While the efficiency is lower than a high temperature steam power plant, it is important to remember that low temperature cycles are inherently less efficient than high temperature cycles. Limits on efficiency can be expressed according to Carnot efficiency which is the maximum possible efficiency for a heat engine operating between two temperatures. A Carnot engine operating with a heat source at 150 deg C and rejecting it at 25 deg C is only about 30 % efficient. In this light, a low efficiency in the range of 10 % to 20 % in case of ORC is a substantial percentage of theoretical efficiency, especially in comparison to other low temperature alternatives, such as piezoelectric generation, which are only 1 % efficient.

Although the economics of ORC, heat recovery need to be carefully analyzed for any given application, it is a useful alternative in those industries which do not have in-house use for additional process heat or no neighbouring plants which can make economic use of the heat.

Kalina cycle – The Kalina cycle is a variation of the Rankine cycle, using a mixture of ammonia and water as the working fluid. A key difference between single fluid cycles and cycles which use binary fluids is the temperature profile during boiling and condensation. For single fluid cycles, the temperature remains constant during boiling. As heat is transferred to the working medium (water), the water temperature slowly increases to boiling temperature, at which point the temperature remains constant until all the water has evaporated. In contrast, a binary mixture of water and ammonia (each of which has a different boiling point) increases its temperature during evaporation. This allows better thermal matching with the waste heat source and with the cooling medium in the condenser. Consequently, these systems achieve considerable greater energy efficiency. The cycle was invented in the 1980s.

Direct electrical conversion technologies

Whereas traditional power cycles involve using heat to create mechanical energy and ultimately electrical energy, new technologies are being developed which can generate electricity directly from heat. These include thermoelectric, thermionic, and piezoelectric technologies. However, these technologies are in development stage. A few have undergone some prototype testing in applications such as heat recovery in automotive vehicles.

Thermoelectric generation – Thermoelectric (TE) materials are semiconductor solids which allow direct generation of electricity when subject to a temperature differential. This technology is based on a phenomenon known as the Seebeck effect which states that when two different semiconductor materials are subject to a heat source and heat sink, a voltage is created between the two semi-conductors. Conversely, TE materials can also be used for cooling or heating by applying electricity to dissimilar semiconductors. Thermoelectric technology has existed for a long time (the thermoelectric effect was first discovered in 1821), but has seen limited use due to low efficiencies and high cost. Most TE generation systems in use have efficiencies in the range of 2 % to 5 %. These have mainly been used to power instruments on spacecraft or in very remote locations. However, recent advances in the nano-technology have enabled advanced TE materials which can achieve conversion efficiencies 15 % or higher.

In a recent study, it has been concluded that advanced TE packages are appropriate in medium to high temperature, high flow rate exhaust streams where facilities have little use for recovered waste heat. However, more development work is needed in this area. Low cost, high volume production methods for TE materials need to be developed in order to achieve this goal. Also, maintaining a high temperature differential across thin TE devices present a significant engineering challenge. Obtaining high heat transfer rates require advances in heat transfer materials and heat exchange systems with high heat transfer coefficients.

Piezo-electric power generation – Piezo-electric power generation (PEPG) is an option for converting low temperature waste heat in the range of 100 deg C to 150 deg C to electrical energy. Piezo-electric technology converts mechanical energy in the form of ambient vibrations to electrical energy. A piezo-electric thin film membrane can take advantage of oscillatory gas expansion to create a voltage output. However, there are several technical challenges associated with PEPG technologies. These include (i) low efficiency (only around 1 % efficient), (ii) difficulties remain in obtaining high enough oscillatory frequencies (current devices operate at around 100 Hz, and frequencies needed are close to 1,000 Hz), (iii) high internal impedance, (iv) complex oscillatory fluid dynamics within the liquid/vapour chamber, (v) need for long term reliability and durability, and (vi) high costs.

While the conversion efficiency of PEPG technology is currently very low (1 %), there can be prospects to use PEPG cascading, in which case efficiencies can reach about 10 %. Other key issues are the costs of manufacturing piezoelectric devices, as well as the design of heat exchangers to facilitate sufficient heat transfer rates across a relatively low temperature difference.

Thermionic generation – Thermionic devices operate similar to thermo-electric devices. However, whereas thermoelectric devices operate according to the Seebeck effect, thermionic devices operate via thermionic emission. In these systems, a temperature difference drives the flow of electrons through a vacuum from a metal to a metal oxide surface. One key disadvantage of this technology is that it is limited to applications with high plying electricity to dissimilar semiconductors. Thermo-electric technology has existed for temperatures above 1,000 deg C. However, some development has enabled their use at around 100 deg C to 300 deg C range.

Thermo photo voltaic generator – Thermo photo voltaic generators can be used to convert radiant energy into electricity. This technology involves a heat source, an emitter, a radiation filter, and a photo voltaic (PV) cell (like those used in solar panels). As the emitter is heated, it emits electro-magnetic radiation. The PV cell converts this radiation to electrical energy. The filter is used to pass radiation at wave-lengths which match the PV cell, while reflecting remaining energy back to the emitter. This technology can potentially enable new methods for WHR. A small number of prototype systems have been built for small burner applications and in a helicopter gas turbine.

WHR and iron and steel industry

The iron and steel industry employs several high temperature furnaces for coke, sinter, hot metal, and steel production and accounts for high energy consumption. While recovery from clean gaseous streams in the industry is common, heavily contaminated exhaust gases from coke oven, blast furnace (BF), basic oxygen furnace (BOF), and electric arc furnace (EAF) continues to present a challenge for economic WHR. Heat recovery techniques from these dirty gaseous streams are available, yet implementation has been limited due to high capital investment costs.

The steel industry has made the biggest progress in reducing its energy intensity. Such progress has been achieved by continuous casting and optimization of BF operation, and also through steel recycling and replacement of fossil fuels with recycled by-product gases (coke oven gas, blast furnace gas, and converter gas). In-situ waste heat recovery has been implemented wherever possible, for example, by recirculating hot flue gases inside the furnace where they were created to lower external energy demand, or by using hot flue gases to preheat combustion air or fuel gas. Such energy efficiency improvements still leave residual waste heat recovery opportunities, e.g. to produce steam for other parts of the process or to produce electricity.

WHR in case of steel plants is described below.

Coke production

Production of coke is an essential burden material for BF operation. Coke is produced in coke ovens, where coal is heated in an oxygen limited atmosphere. There are two methods for producing coke namely (i) the byproduct process, and (ii) the non-recovery process. In the byproduct process, chemical byproducts (crude tar, ammonia, and light oils) in the coke oven gas are recovered, while the remaining coke oven gas (COG) is cleaned and recycled within the steel plant. In the non-recovery process, the entire COG is burned in the process. The most common type of process is still the byproduct process and this is discussed below.

Byproduct cokemaking process has two areas of sensible heat loss namely (i) COG which is cooled in the gas cleaning process, and (ii) waste gas leaving the coke oven. The coke making process employs several coke oven chambers separated by heating flues. Recycled COG, and sometimes other gases such as BF gas, are used as the fuel source in the heating flue and supply heat to the oven chamber where coal carbonization takes place. As coal is carbonized in the oven chamber, gas and moisture (accounting for around 8 % to 11 % of charged coal) are driven off and leave through the pipes. The COG has a high heat content ranging from around 4000 kcal/cum to 4400 kcal/cum and hence it can be recycled for use as a fuel after undergoing a cleaning process.

The temperature of the crude COG at the oven outlet ranges from 650 deg C to 1000 deg C.  At this point, the COG gas is a source of sensible heat. However, the heat is universally wasted due to the high amount of tar and other materials which can cause build up on heat exchanger surfaces. Upon leaving the oven, the COG is cooled by ammonia liquor spray followed by primary coolers. Different technologies are then used for removing tar, sulphur compounds, ammonia, and light oils. After cleaning, the COG is used as a fuel throughout the steel plant. In this arrangement, only the chemical energy of the COG is recovered when recycled, while the sensible heat is wasted.

While most of the steel plants do not employ heat recovery from COG, a limited level of heat recovery from COG is possible, as shown by the success of this practice in Japan. Coke oven facilities in Japan have successfully applied heat recovery through use of a low pressure heat transfer medium. In general, the minimum allowable temperature for the COG in the heat exchanger is around 450 deg C. At lower temperatures, tar condenses and leads to soot formation on the heat exchanger surface.  Cooling to 450 deg C enables only about one third of the sensible heat to be recovered. However, it is unlikely that ISPs in other countries are going to pursue new technologies for heat recovery from crude coke oven gas. This is since ISPs are facing cost barriers with heat recovery from dirty exhaust streams. Also, the byproduct coke making process can become irrelevant in future years. It is likely that the ISPs are going to move away from the byproduct process to the non-recovery process due to environmental considerations. In the non-recovery process, the COG gas is burned within the process, and a WHB used to recover the sensible heat in the off gases.

Another source of sensible heat loss in coke ovens is the waste gases from the combustion of recycled fuel gases. The recycled fuel gases are used in the heating flue, which is adjacent to the oven chamber. Combustion of the fuel gases generates hot exhaust gases which leave the oven flue and pass through a regenerator to transfer heat to incoming combustion air and/or fuel. Waste gases leave the regenerator at temperatures averaging around 200 deg C. In some plants, the heat content of the waste gases are further recovered by use of a heat pipe or for preheating coal charge and reducing its moisture content. In this case, the temperature of the exhaust gases drops to around 60 deg C.

Production of sinter

Sintering plant consists of two major sections, sintering section and sinter cooling section. Heat recovery from both parts has been developed namely (i) from sintering section exhaust gas, and (ii) from cooling section cooling gas. There is large temperature difference depending on the position of the section. Average gas temperature in both sections is in the level of 100 deg C to 150 deg C, too low for effective heat recovery. Heat recovery is to be limited to high gas temperature zone, the final part of sintering section and primary part of cooling section, where gas temperatures of 300 deg C or higher are available. Although heat recovery zone is limited, the gas volume of sintering process is large enough for practical heat recovery.

The waste gas energy recovery system consists of hood, dust catcher, heat recovery boiler, circulation fan and de-aerator. Sintering machine exhaust gas is corrosive containing some dusts. Heat recovery is generally limited to high gas temperature zone as aggregated average temperature is low for heat recovery. At the same time, due to its corrosiveness, the gas temperature after heat recovery is to be kept above acid due point of the gas. Cooling gas is basically atmosphere air containing some dust. In case of sinter cooler, it is same as sintering machine heat recovery. Due to gas temperature distribution along with the cooler, heat recovery is limited to high gas temperature zone.

Sintering machine exhaust gas heat recovery can be categorized to circulation type and non-circulation type. In circulation type, gas after heat recovery are circulated to sintering machine as cooling gas replacement, whereas in non-circulation type, the gas after heat recovery is lead to gas treatment facility directly. Circulation type is adopted to improve heat recover efficiency.

In case of cooler heat recovery, the cooler gas is air. The cooler heat recovery system can be categorized as circulation type and non-circulation type. In case of non-circulation type, after heat recovery from hot gas zone, cooling gas is released to the atmosphere. In case of circulation type, after heat recovery from hot gas zone, cooling gas is led to cooler and reused for sinter cooling. Cooler gas temperature rises through recirculation and consequently results to higher heat recovery. On the other hand, cooling gas temperature rises up to the level of 180 deg C, cooling capability can decrease. Sinter temperature at outlet of the cooler is higher around 30 deg C in circulation type. Temperature difference is small enough and does not affect sinter plant operation. Recovered energy increases by 50 % in circulation type compared to non-circulation type. Fan power consumption is larger in case of circulation type. However, recovered power is far larger.

Hot metal production in BF

BF is one of the main units in ISPs. It converts iron ore into hot metal. Raw materials are charged from the top, including iron containing materials (lump iron ore, sinter, or pellets), additives (flux), and coke, while hot air and supplemental fuels are injected through tuyeres at the bottom of the furnace. The burden moves down through the BF and meets a rising current of hot gases. The hot air entering the BF is provided by several auxiliary hot blast stoves. In the hot blast stove, mixed gas consisting of BF gas (BFG) and COG are combusted. The heat from the combustion exhausts is transferred to a checker work regenerator. When the regenerator reaches an appropriate temperature, the flow of air is reversed and cold air is forced through the regenerator, which transfers heat to the cold air. The heated air is then injected into the furnace. The system operates according to the same principles as a regenerator used for heat recovery. However in this case, the regenerator is not a ‘waste heat’ recovery unit, but rather the mechanism for transferring heat from the stove to the hot blast. Sources of off gas waste heat in BF include both the exhaust gases from the hot blast stove and the BFG leaving the BF.

There is sensible heat loss from BFG. New BFs are designed for efficient heat transfer, resulting into hot gases at the BF top in the low temperature range. The BFG is recovered for use as a fuel in blast air heating, rolling mill reheating furnaces, coke oven heating, power production, and steam generation. Since BFG has low calorific value, it is often mixed with COG or converter gas. BFG is required to be cleaned before it can be used as a fuel, and the sensible heat contained in the gas is rarely recovered. In some cases, BF operates at a sufficiently high pressure (2.5 atm or higher) to economically use a top pressure recovery turbine (TRT) for recovering of the ‘pressure energy’ of the BFG. The gas is to be cleaned before entering the TRT, which is generally accomplished via wet cleaning, with the result that sensible heat of the off gas is lost. An alternative to wet cleaning technology is dry cleaning, in which the temperature of the gas entering the TRT can be raised to around 120 deg C. Dry type TRT technology is already working in several places. However, it is more expensive.

Another opportunity for WHR is from the combustion exhaust gases leaving hot blast stoves. The gases are at temperatures of around 250 deg C. The blast stove exhaust gas is relatively clean and is more compatible with heat recovery devices, making heat recovery from blast stoves a more common practice. The heat can be used to preheat combustion air and/or fuel gas. Heat exchangers used include rotary regenerators, fixed plate heat exchangers, and circulating thermal medium systems.

Production of liquid steel in BOF

BOF uses oxygen to oxidize impurities in the hot metal. Operation is semi-continuous: hot metal and scrap are charged to the furnace, oxygen is injected, fluxes are added to control erosion, and then the metal is sampled and tapped. The temperature required to melt the metal is supplied by the exothermic oxidation reaction and hence, no external heat source is needed.

The off gases from the BOF are at a high temperature. It has a high concentration of CO (carbon monoxide). Like COG and BFG, BOF gases offer opportunities for recovery of chemical energy and sensible heat. Challenges to WHR include high capital costs and the substantial maintenance problems resulting from hot dirty gases. Contaminants include iron oxides, heavy metals, SOx, NOx, and fluorides.

Various commercial methods for WHR are available. The two main methods for heat recovery are ‘open combustion’ and ‘suppressed combustion’. In open combustion systems, air is introduced to the BOF gas duct to combust the CO. The heat generated is recovered with a waste heat boiler. In the ‘suppressed combustion’ method, a skirt is added to the converter mouth to reduce air infiltration and combustion of the CO. The gas is then cleaned, collected, and used as a fuel. It is also possible to recover both the gas and the sensible heat via a combined boiler/suppressed combustion gas recovery system. 

Liquid steel production by EAF

The steel industry has experienced significant growth in manufacture from recycled scrap via electric smelting. EAF and induction furnace are the two types of furnaces used to melt ferrous scrap for electric smelting. Out of these two, EAF is the prominent furnace. The furnace is refractory lined and typically covered by a retractable roof, through which C electrodes are lowered. Charge materials are lowered through the roof. Fluxes and alloying agents are also added to help control the quality of the material. The electrodes are then lowered to about an inch above the metal, and the current provides heat for melting the scrap. During furnace operation, several gases and particulate emissions are released, including CO, SOx, NOx, metal oxides, volatile organic compounds (VOCs), and other pollutants. Off gas temperatures at peak loads can equal anywhere from 1,350 deg C to 1,950 deg C. Exhaust gases are responsible for losses of around 20 % of the power input. Half of these losses are due to the chemical energy in the gases, while the other half is sensible heat. Additionally, around 8 % to 10 % of energy input is also lost to EAF cooling water ‘jacket’.

The most common method for heat recovery is scrap preheating, which has been widely used. The use of off gases to preheat scrap can save from 5 % to 10 % of total EAF energy consumption. Initial designs for scrap preheat required piping off gases to the charging bucket. Some of the challenges with these systems include the need to transport preheated scrap containing semi-burned non scrap materials (e.g., plastics), as well the evaporation of volatiles which create odour and environmental control problems. Alternatives to the bucket preheating system include the Consteel process, the Fuchs shaft furnace, and the Twin shell furnace. These processes have been installed at various places.

The Consteel process involves continuous charging of scrap and uses a scrap conveyer, a feeding system, and a preheater. The preheater is a refractory lined tunnel where off gases flow opposite the flow of scrap charge. Air is introduced into the preheater to burn the CO and CO2 and thus both the chemical and sensible heat in the off gas is used. An afterburner is sometimes installed to burn remaining CO and other compounds.The Fuchs shaft furnace involves a shaft immediately above the arc furnace roof. The charge is loaded via baskets in three stages. The baskets are refractory lined and designed with a seal which prevents the escape of fumes. Scrap heating is further assisted by auxiliary oxy-fuel burners. Additionally, afterburners are installed to completely combust all the CO. One additional benefit of the system is that charge acts as a dust filter, capturing around 40 % of the dust and returning it to the furnace, thus enabling slight increases in yield.

The benefits and drawbacks of scrap preheating systems depend on the specific operation. In some cases, it enables reduced electricity consumption and increased productivity. In other cases, scrap preheating systems are difficult to maintain. As EAFs become increasingly efficient and tap to tap times are reduced, scrap handling can reduce productivity and possibly create burdensome maintenance demands. In one case, the energy savings enabled by scrap preheating are reduced by about one half when tap to tap times are reduced by a third.

Power plant boilers

Boilers in ISPs normally use BFG and COG as fuel. The exhaust gas temperature for the boilers varies with the boiler’s age and the controls used. Temperatures can be fairly high (340 deg C to 450 deg C), with O2 content varying from 3 %–7.5 %. The waste heat is in the form of clean, contamination-free gases and does not require further conditioning.  The areas of waste heat and recovery from boilers and steam systems include (i) use of exhaust gases to preheat BFG and COG, (ii) use of low-temperature power generation if economically justifiable, (iii) preheating service water or river water for use in the plant, if possible and required, and low-pressure steam can be condensed and reused for the boiler water system instead of venting.

Reheating furnace

Reheating furnace is a key equipment of the hot rolling mills. Its function is to continuously heat billets, slabs or blooms of different sizes and grades upto 1,250 deg C. Most of the new reheat furnaces are ‘walking beams furnaces’ (WBF). On the WBF, the heating is done over and under the products which are handled from charging side to discharging side by means of insulated and cooled beams (skids). A key performance criterion for reheating furnaces is heating homogeneity. 20 % to 30 % of the energy input is typically wasted divided between several thermal losses namely (i) the temperature of the exhaust gas between the combustion air recuperator and the stack is at 250 deg C to 300 deg C with natural gas fuel and higher with lower calorific value fuel, (ii) the product handling systems inside the furnace with skids and post cooling system, and (iii) wall and doors losses, hardly recoverable.

Water is used to constantly cool the skid system which is in contact with a very hot atmosphere in the furnace. This water loop typically enters at 40 deg C and is heated by 15 deg C before being directed to a dedicated cooling system.

At several places, WHR is carried out on the skid cooling system by producing steam when it is needed in the plant for other purposes. On its own, this installation reduces losses through the skid system because of the use of water cool pipes used at higher temperature. If steam is not needed by the plant then an ORC (adapted for such temperatures around 200 deg C) can be installed on the steam circuit to produce electricity. This installation has the benefit of being easily and safely operable especially with high variability of the losses because of the constant temperature brought by the water phase change. Most of the time, this technology is not installed because of long payback, and the energy contained in exhaust gases is wasted.

An electricity production system is possible to recover energy from exhaust gases. Depending on the heat source temperature, either a ‘water-steam cycle’ (with low efficiency furnace) or an ORC (with better efficiency) are available. However, most of the time those technologies are not installed because of their long payback. This situation can have another solution. This solution combines heat from the skid cooling loop operated at higher pressure and temperature so as to produce a mixture of steam and water at around 215 deg C in a closed loop and heat from exhaust gases. The two heat sources are recovered separately thanks to organic heat fluid loops and then combined to form a common heat source.

The heat fluctuation from the exhaust gases (temperature and volume are modified) in case of furnace power variations (production or product variations) are balanced because of the constant temperature of the heat coming from the skid cooling system. Thus operation of the system is easy and makes the global heat source more stable especially with high fluctuations.

It is possible that the reheat furnace production can fluctuate in few minutes, which affects the heat content of exhaust gases entering the WHR system. The ORC is a rather flexible system which can accommodate such variations upto a certain point. An ORC can typically operate down to 30 % of its nominal capacity, and automatically shuts down when the heat input goes below that threshold. However, the economic aspect is affected as electricity production also decreases as well.

Heat storage solutions can be adapted to daily variations are becoming available for industrial applications and can be used in combination with an ORC to flatten its production. Oil is, for instance, is appropriate heat storage medium at that temperature level. Economic benefits need to be assessed on a case by case basis.

Waste heat from solid streams

In addition to waste heat losses from off gases, solid streams and cooling water are sources of additional sensible heat losses. Solid products and byproducts with significant waste heat losses include hot coke, hot sinter, BF slag, BOF slag, cast steel, and hot rolled steel. Though the heat from solid streams are often more difficult to recover, the heat losses are high. The sensible heat loss from coke is recovered in some plants coke dry quenching (CDQ) as an alternative to wet quenching. CDQ involves catching incandescent coke in a specially designed bucket, which is discharged into the CDQ vessel. An inert gas such as nitrogen passes over the coke and recovers its sensible heat. The hot gas is then passed through a waste heat boiler. Energy saving is in the range of 0.2 million to 0.25 million kcal per ton of coke. There have also been attempts to recover heat from other solid flows via radiant heat boilers. This was unsuccessful for BF and BOF slag, but has been commercialized for recovering heat from cast steel in a few locations in Japan and Germany.

Another option for reducing heat losses from cast steel is hot charging, in which cast products are charged to the reheating furnace while still hot. Hot charging can save about 0.12 million kcal per ton. Sensible heat loss from hot rolled steel can also be partially recovered by using water cooling. Since the final temperature of the cooling water is generally low (around 80 deg C), it can be upgraded for other heating applications with a heat pump.

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