1. Introduction

Heat recovery systems are a vital part of modern gas manufacturing. They enable the sensible recovery of heat that would otherwise be discharged to the atmosphere, allowing it to be used in another area of the process. Applications of heat recovery in gas manufacturing include preheat the feedstocks gases, create process heat, or provide heat for space heating.

Heat exchangers, heat pumps, and combined heat and power (CHP) systems are the three major types of heat recovery systems in use today. Heat exchangers allow for sensible heat recovery and transfer from sources such as furnace or boiler stack gases, ventilation or flue gases from fires, and hot process gases, liquids, or vapors. Heat pumps can transfer heat from low-temperature gases to higher-temperature firing, combustion, steam raising, or water systems at elevated temperature. CHP systems recover sensible heat from prime mover jacket or coolant water, lubricating oil, and exhaust gases, creating useable heat for industrial processes or space heating.

2. Overview of Gas Manufacturing Processes

Gas manufacturing processes produce industrial or commercial gases. These gases can be produced through chemical reactions (e.g., chlor-alkali process) or by separation from another substance (e.g., air separation to obtain oxygen, nitrogen, or argon).

Many gas manufacturing operations require a supply of energy for producing and distributing their products. Heat Recovery Systems can be used to recover and reuse much of the heat energy generated by these processes. Heat Recovery Systems can benefit from the high-temperature exhaust gases generated by furnaces, ovens, and turbines.

3. Importance of Energy Efficiency in Manufacturing

Energy efficiency—the practice of using less energy to provide the same service—plays a major role in reducing operational costs, environmental impact (particularly carbon emissions), and demands on the local power grid. In most manufacturing industries, such as gas manufacturing, a significant portion of the energy consumed is used to add or remove heat, either to or from process streams and ancillary spaces. The most common form of energy in these industries is fuel, and directly combusting that fuel produces a by-product of waste heat. To avoid wasting thermal energy, heat recovery systems can be installed.

A heat-recovery system captures waste or excess heat and uses it elsewhere within the facility, or stores this energy for later use. Heat recovery can be applied to support a variety of needs, including preheating feedstocks, providing process heat, and space heating. Systems that capture and reuse heat often also utilize power generation or heat pumps to increase the applicability of the recovered heat. Although heat-recovery systems require an initial investment—and may increase operational complexity and maintenance requirements—they nonetheless result in significant savings or revenue generation when compared with the alternatives.

4. Heat Recovery Systems: Definition and Functionality

Modern gas manufacturing facilities incorporate several processes in which heat transfer is a fundamental part of operations. When transfer of heat occurs in either direction, the energy that is leaving the system can be recovered and reincorporated into the plant cycle by using heat recovery systems. Carrying out early identification of heat recovery opportunities within an industrial plant is highly advantageous in terms of environmental and economic impact. Heat recovery systems may be defined as energy-conserving systems built to utilize conveniently available heat in an industrial process for heating purposes elsewhere within the site. In a typical gas manufacturing facility, heat recovery systems might be applied to achieve several tasks, including the preheating of feedstocks in catalytic processes, generation of process heat such as reforming gas, and space heating within the manufacturing plant. Realization and development of heat recovery from flue gases can markedly reduce the problems of air pollution and global warming potential, contributing to a better economy through reduced fuel consumption.

The main idea of heat recovery is to utilize heat that would otherwise be wasted, and to reuse it for some beneficial purpose. Common types of heat recovery systems include heat exchangers, heat pumps, and combined heat and power systems. Heat exchangers are typically applied for preheating of feedstocks in catalytic processes. Heat pumps may be employed for process heating, and combined heat and power systems for space heating. There is great interest nowadays in various heat recovery systems, although the high capital investment cost acts as a deterrent and many industries avoid undertaking such projects. Nevertheless, the long-term benefits in terms of energy savings and reduced primary fuel consumption justify the investment in heat recovery systems.

5. Types of Heat Recovery Systems

Gas manufacturing consumes energy for all of these operations and more. A significant amount of energy, in the form of heat, flows out of the process, mainly at higher temperatures and is lost. Heat recovery systems enable the process to recover some of these heat losses and use them for useful work—thus increasing the process efficiency. Heat recovery systems include, but are not limited to, heat exchangers, heat pumps, combined heat and power (CHP) systems. The use of heat recovery systems depends on the process conditions and the integration of the different heat requisites within the manufacturing process.

Heat exchangers transfer heat from one medium to another to preheat feedstocks or other process requirements. Heat pumps draw heat from one source (for example, the atmosphere) and transfer it to another medium (for example, heat energy delivered at a higher temperature). Combined heat and power (CHP) systems generate heat and power while recovering heat losses from the fuel combustion/ power generation processes.

5.1. Heat Exchangers

Heat exchangers are devices that transfer thermal energy from a hot fluid to a cold one without mixing the two. They consist of a shell that encloses tubes or passages through which the fluids move. Heat from the hot fluid is conducted through the walls of the tubes or passages, thereby heating the cold fluid. The fluids may be liquids, gases, or a mixture of the two. Heat exchanger types include double-pipe, shell-and-tube, plat-and-frame, spiral, and waste-heat boilers.

Heat exchangers recover usable energy from the hot streams within a manufacturing facility. The recovered energy is commonly used to preheat feedstocks and process heating. It can also be utilized for space heating in the manufacturing facility, which reduces costs in colder climates. Heat exchangers incur low capital and maintenance costs, and therefore, simple payback periods are achievable. Refer to "Applications of Heat Recovery in Gas Manufacturing" for further details.

5.2. Heat Pumps

Heat pumps are devices designed to move thermal energy from one space to another using mechanical energy, in accordance with the first law of thermodynamics. The operation of a heat pump cycle is analogous to that of a refrigerator. Heat pumps are capable of producing temperatures higher than could be naturally obtained from the source for heating purposes, or, conversely, temperatures lower than ambient for refrigeration. Heat pump technology is widely employed across various manufacturing industries due to its versatility.

Heat pumps have been implemented as heat recovery systems in gas manufacturing. For instance, waste heat from an advanced gasification process can be captured and used to drive a heat pump cycle, producing saturated steam at 10 bar for subsequent process heating. Alternatively, low-grade waste heat can be converted into higher temperature heat using a heat pump, and the resulting hot water directed to a district heating system. These applications demonstrate the utility of heat pumps in recovering waste heat and reducing the external energy demand of the manufacturing process.

5.3. Combined Heat and Power Systems

Finally a combined heat and power (CHP) system uses only one fuel source to provide efficiently both heat and power to a manufacturing plant or district heating system, and can act as the source of mechanical power to operate a plant or site. CHP systems are most efficient when the heat source is located close to the users of the heat being generated—hence the alternative name of “cogeneration.” The recoverable heat that emerges from the power-producing process can be used for process or space heating, with the power either exported to the plant or site, or sold to a local electrical utility. Process heat can also be used to preheat feedstocks, thereby reducing the plant’s overall fuel consumption required to manufacture products.

The applications of heat recovery in gas manufacturing appear most often when a heat exchanger is used to preheat feedstocks entering the process, to provide process steam for stripping or regeneration processes involved in refining natural gas liquids or nitrogen removal, or to heat a space using the thermal energy exiting an electrically powered chiller. Other sectors of the gas manufacturing industries use heat recovery in these ways, as well as using the heat contained in a combustion exhaust to supply heat to their manufacturing processes or space heating systems.

6. Applications of Heat Recovery in Gas Manufacturing

The manufacturing of ammonia, methanol, and unsaturated hydrocarbons is an energy-intensive process that requires a significant amount of thermal energy. These processes operate at high temperatures with a vast supply of combustion-based heated air. Heat recovery systems utilize waste or surplus heat from manufacturing processes to generate steam. The steam produced can be used to preheat fresh feedstocks into the reactors, generate steam necessary for internal operations, provide additional heating to the surrounding gases, or create space heating or process heating for plant use.

6.1. Preheating Feedstocks

Preheating of feedstock to various unit operations using heat recovery systems is one of the common energy saving activities in gas manufacturing facilities. Various streams in different manufacturing operations contain heat and can be used for preheating feedstock of other operations. Heat recovery from process vents and rejected gas can provide low-grade heat to gas manufacturing facilities. Operations use different sorts of heat recovery systems:

The heat exchanger system is the most common heat recovery system; it uses the hot gases produced during a manufacturing operation to help in preheating the feedstock. A heat pump is an excellent alternative method of waste heat recovery. The combined heat and power (CHP) technique is used to generate useful heat for heating requirements in manufacturing facilities as well as electrical power. The few important applications of heat recovery in gas manufacturing are preheating of feedstock, process heating, and demand for space heating.

6.2. Process Heating

Heat recovery systems are designed to capture the remaining thermal energy within the process stream and deliver it to a second process through work, heating or cooling. Combined heat and power systems capture heat generated during the production of electricity (including from waste heat streams), otherwise known as cogeneration. Heat recovered using this method is mostly used in pre-heating for combustion or space heating. Waste heat recovery encompasses heat generated by other processes, including the compression of gas or air, or steam generation, and delivered to a process or space heating application.

Two additional methods for recovery through heating are heat pumps and heat exchangers. Heat pump systems are used to “upgrade” heat through raising heat to higher temperature levels, with the lowest possible temperature of waste heat generally being above the ambient temperature of the surrounding land or water. Heat exchangers transfer heat from a high-temperature source to a lower temperature process requiring heat. Heat exchangers are most commonly applied in pre-heating their raw materials using waste heat from another process, such as compression of gases. The major types of heat exchangers include shell and tube, spiral, brazed plate, double pipe and plate heat exchangers. The selection is generally based on the pressure rating required by the application, the temperature rating and the space available in the process area.

6.3. Space Heating

Heat Recovery System

A. Definition and Operational Principles

Heat recovery refers to the process of reconverting heat used in one stage of an operation for use in another stage. A heat-recovery system captures exhaust heat and uses it for preheating feedstocks or plant-space heating, thereby reducing the amount of prime energy required in a gas-manufacturing operation.

B. Principal Types

The principal types of heat-recovery systems are heat exchangers, heat pumps, and combined heat and power (CHP) systems. Heat exchangers are custom-designed to transfer heat from one process stream to another. Heat pumps raise the temperature of waste heat to a more useful level, while combined heat and power systems recover heat from waste gases of the prime mover or generator.

C. Applications of Heat Recovery in Gas Manufacturing

Heat created by various components (e.g., heaters, compressors) can be captured and reutilized for space heating in the gaseous-fuels production facility. A heat-recovery system captures exhausted heat and uses it for plant-space heating, reducing the amount of prime energy required in the manufacturing operation.

7. Benefits of Implementing Heat Recovery Systems

Modern technology has significantly improved the energy efficiency of gas manufacturing processes. Improved operation and waste recovery reduce overall energy demand, but additional improvement can be achieved through the use of heat recovery systems. The primary benefit of heat recovery systems is often the reduction in energy cost. Recovering low-grade heat can significantly displace purchased fuel needed for preheating feedstocks, process heating, and space heating. Waste heat can also be used to reduce electrical consumption for process heating by means of electrically driven heat pumps or to produce electrical power by fitting a turbine and generator to an exhaust gas stream in a combined heat and power (CHP) arrangement. A simplified representation of the variation in efficiency with temperature for several types of heat recovery systems is shown in Figure 1.

The environmental benefits of heat recovery are directly related to fuel usage and cost savings. Reduced fuel usage means reduced emissions of greenhouse gases, acid rain precursors, and other harmful air pollutants. Given that heat recovery systems continue to consume fuel, albeit at reduced quantities, the environmental benefits will never be as great as those of a nonfuel-consuming process, such as recycling. The investment cost, additional complexity, and maintenance requirements of heat recovery systems need to be weighed against the financial and environmental savings. However, there are situations where energy costs either are not high enough or the capital investment required for the heat recovery system is not low enough to justify the implementation of such a project.

7.1. Cost Savings

The use of heat recovery systems is particularly suitable when the cost of the fuel required for process heating exceeds the cost of the fuel used for generating power. Gas manufacturing processes in which a fuel-fired furnace is needed to provide heat to the process can recover the heat rejected in the gas turbine exhaust to generate power, which can be exported to an external source or used to reduce the internal load of the plant. The heat recovered in the gas turbine exhaust gases can also be utilized effectively to satisfy certain space- and process-heating needs within a gas manufacturing plant. The use of heat recovery systems, especially combined heat and power systems, significantly minimizes the fuel consumption (and the associated operating costs) of a gas manufacturing process.

The use of heat recovery systems in gas manufacturing minimises fuel consumption and thereby reduces the cost of energy and raw materials. Facility A uses two heat recovery boilers to preheat the constituent feed before it enters the main process heating unit. The feed is preheated to temperatures of about 2 50 °C using external steam, which is produced by firing the flare gas that would otherwise be burned at the gas flare centre. Heat recovery boiler 1 provides steam to the heat recovery boiler 2 by recovering the heat lost in the exothermic portion of the process, resulting in an Net Heat of Reaction of 207 kJ/mol. Facility B utilizes heat recovery boilers to preheat the constituent feed before it enters the main process heating coil. The heat recovered from the exothermic reaction is used to produce the steam needed for this application.

7.2. Reduced Environmental Impact

Heat recovery systems are extensively used in destructive distillation plants of coal, coke oven plants and in liquefaction, vaporization and “cold” plants and in other additional industries in which the process generates and consumes heat at the same time. Combined heat and power systems (CHP) recover a major part of the internal heat requirements of a plant, which may then be met at a very low cost.

Heat recovery enables the use of waste heat of one process to meet the heat requirements of another process. Waste heat can be used for preheating combustion air, combustion gases, or boiler feedwater; for certain gas manufacturing process requirements (e.g., preheating of feedstocks); for space or process heating requirements; and for other purposes (e.g., preheating oven charge). Application of waste heat also helps in maintaining flue-gas temperature and avoiding the formation of acidic gases in the exhaust.

7.3. Enhanced Operational Efficiency

Gas manufacturing processes require large amounts of heat. In fact, it often accounts for a significant portion of operating costs. Larger amounts of heat are required for pre-heating feedstocks or process fluids, process heating or distilling, feedstock cracking, and space heating.

A large amount of heat is usually discharged to the environment. Heat recovery systems can capture some of this heat and reuse it. They can generally be categorized into the three major types: heat exchanger systems, heat pump systems, and combined heat and power systems (CHP). More detailed information on the three types of heat recovery systems can be found in Section 6.2. Heat recovery applications in gas manufacturing (Table 3-3) are discussed in Section 7.2. Helping industries address the following heat recovery issue can bring drum-positive energy and cost-saving results.

8. Challenges in Implementing Heat Recovery Systems

Despite the benefits realized from heat recovery systems, there are considerations and difficulties that can be encountered when implementing such systems in the gas manufacturing industry. Investment costs for heat recovery systems can be high, and the decision to install one needs to be carefully analysed to determine if it is cost-and energy-effective. Waste heat may be generated at a much lower temperature than what is useful for the target process. The larger the difference in the temperature values, the larger the heat exchanger required for recovery. The external temperature of the heat source and/or the internal temperature of the target process may also vary during operations.

The complexity and flexibility of the heat recovery system can also be an issue. The heat exchange network of the facility must be able to respond to its heat demand without being constrained by the heat recovery system. As the heat generated for the intended purpose is limited, the source may produce excess heat when demand is very small. This may affect the performance of the machine and lead to breakdowns. Therefore, having a makeshift method of dissipating excess heat or stopping the flow of heat may also be necessary during the design and planning process.

8.1. Initial Investment Costs

Heat recovery is an essential consideration even in the earliest stages of the design of a new gas manufacturing facility, allowing immediate cost savings through a smaller boiler and increased efficiency. The investment cost of adding heat recovery to an existing plant can be high, only being justified by the cost savings obtained over the life of the equipment. Likewise, a system of heat recovery may be technically possible but not financially justifiable because of the high initial expense involved.

Owing to the complex construction of heat pumps for example, servicing is more difficult than for normal boilers or hot water systems. Servicing costs may, therefore, be higher. Heat exchangers are simple pieces of equipment, but potentially are exposed to very aggressive atmospheres, leading to high maintenance costs that may outweigh the savings achieved.

8.2. Technical Complexity

The sophisticated nature of the equipment and the intricacy of the integration process with the existing plant contribute to the complexity of heat recovery systems. Several factors warrant meticulous investigation prior to the design phase to guarantee the optimal performance of such recuperation systems. The selection of an appropriate working fluid represents a crucial decision, particularly in the case of heat pumps. Due to their scalable design, heat exchangers accommodate operating fluids of higher and lower temperatures for heat energy transfer, although they frequently necessitate preliminary treatment of these fluids, for instance through filtration. Additional considerations encompass the anticipated mode of operation, stability over time, thermal inertia, desired temperature levels, and area constraints.

The demanding nature and onerous maintenance requirements of heat recovery systems are also determining aspects in their design. This is especially true for combined heat and power (CHP) plants, which require a certain minimum amount of heat and an adequate usage of this heat during its implementation. Despite these impediments, heat recovery considerably contributes to economic benefits and the reduction of CO2 emissions. Consequently, modern manufacturing plants typically implement such systems, as exemplified by Facility A's use of a heat pump and Facility B's incorporation of flue gas heat exchangers, described elsewhere in this report.

8.3. Maintenance Requirements

Increasing investments in heat recovery systems have resulted in several technological improvements, which not only increase efficiency but also balance the costs associated with maintenance and operation. Establishing proper maintenance regimes is important because, despite the economic and ecological advantages of these systems, it remains essential to minimize downtime.

In particular, heat exchangers require systematic maintenance of their exchangers, pumps, cooling towers, and expansion joints. Within the heat pump, both the charging circuit and the discharge circuitry must be properly maintained. Combined cycle plants need careful maintenance of the gas turbine.

9. Case Studies of Successful Implementations

Facility A illustrates a gas manufacturing site that has integrated modern heat recovery technologies in its operation. The facility uses a two-stage combined cycle system to produce hydrogen sheeting gas from coke oven gas, reformed crude gas, and natural gas. Following the combined cycle process, methane-neutralised residue high-pressure steam at 10.4 bar absolute pressure is recovered via a heat recovery boiler. Additionally, steam at 3 bar absolute pressure is generated from the combined cycle exhaust, which helps satisfy the facility's basic steam needs. The waste heat recovery boiler can burn natural gas to maintain the required capacity during low loads in the combined cycle. The recovered heat is utilised to preheat feedstocks and products destined for other manufacturing processes. Employing heat recovery resources reduces natural gas consumption and ensures the provision of higher temperature feedwater.

Facility B employs the waste heat recovery and utilisation system to harness sensible heat from coke oven flue gas. Approximately 400 GJ/h of sensible heat is recovered to generate steam, which contributes to the facility's basic requirements. The use of waste heat for preheating significantly decreases fuel expenditure associated with coke production. Moreover, waste heat within the facility is recycled for space and process heating. The 1,000 kPa steam facilitates heating for the coke oven gas scrubber, ammonium sulphate plant, and sulphur plant, besides serving the boilers in the ammonia-plant. The recovered sensible heat further preheats the feed water to the heating boiler, effectively curbing the consumption of natural gas and coal gas. The annual natural gas savings amount to around 425,000 GJ, equivalent to 11.9 million RMB in monetary terms.

9.1. Case Study 1: Facility A

This case study of a modern gas manufacturing facility illustrates the efficiency benefits achievable through the integration of a heat recovery system in a gas manufacturing process. A heat recovery system is an energy conservation measure that captures and uses heat from process exhaust streams. The system increases the overall energy efficiency of the production process by using heat that would otherwise be expelled into the environment.

Many gas manufacturing processes require high-temperature heat for process operations or space heating and cooling. Heat recovery systems can extract heat from hot exhaust gases, steam, or liquids and reuse the heat for the same or similar applications. The energy generated offsets the demand for purchased fuel and provides associated cost savings. Although the investment cost for heat recovery systems is usually higher than for traditional heating equipment, the energy savings during system operation can help to recover the added cost. Heat recovery systems can also reduce depleting energy resources, limit environmental effects, and reduce heat discharged into the environment.

9.2. Case Study 2: Facility B

Heat recovery systems in modern gas manufacturing provide both space heating and site heat demand (preheat, process). Two such systems—both of which incorporate modern air-source heat pumps, operating between a flue gas heat-recovery heat exchanger and a process—are presented here. A number of other sites have also been identified as suitable for implementation. This case study forms part of a broader examination of how gas manufacturing processes can meet increasing demands for environmental performance and affordability.

Facility B produces a gas stream by transforming a hydrocarbon resource through an endothermic reaction. Heat supplied by a burner raises the temperature of the hydrocarbon and an additional input, often air or oxygen, supporting the main reaction. The coke-type product is sent to a number of secondary processes, whilst the main, high-temperature hot gas is utilised in coke cooling. The flue gases from the burner are still approximately 600°C when discharged to the atmosphere. Energy is employed in different areas on the site: feedstock preheating requires low-grade heat (<200°C), the burners in the secondary coke-cooling operation require medium-grade heat (450°–500°C), and the coke dry quenching operation demands high-grade heat (900°C). The profiles of these loads during a typical campaign are not the same (i.e., they peak at different times), thus implementing a heat recovery system is challenging. The heat recovery system must therefore offer flexibility, and, ideally, be capable of storing heat. Before these technologies advance, the simplest and least capital-demanding approach involves site preheating using heat recovered from the flue gases.

10. Regulatory Framework and Standards

Implementing heat recovery systems in existing or planned manufacturing facilities may require engineering review, modification of existing equipment, or design of new equipment. Additionally, there may be compliance matters associated with the use, qualification, or certification of heat recovery systems associated with a production plant.

Heat recovery projects can significantly reduce production cost by offsetting the requirement for purchased fuel or electricity and reducing the installation charge. However, the initial capital investment is often the greatest challenge to adopting this technology. Due to space limitations, the introduction of an additional piece of equipment such as a heat pump can also be difficult to accommodate within an existing production facility. The operating complexity and maintenance considerations associated with a heat recovery system are typically covered by operators engaged in hazardous duty site manufacturing.

11. Future Trends in Heat Recovery Technologies

Ongoing developments in heat recovery equipment and technological advances across a wide spectrum of manufacturing processes continue to stimulate better options for reutilizing waste heat. The recently established architectural concept of a "net-zero building" reflects this trend by employing technologies for heat extraction and reuse that enable a structure to generate all of the heat energy it needs.

While heat recovery is a well-established technique in manufacturing processes that require large quantities of low-pressure saturation steam, there are limits to its general application. These limits arise principally from the investment cost involved in the waste-heat recovery system and from the problem of scheduling space heat requirements to coincide with the production of high-temperature waste products. Developments are expected to focus on decreasing investment costs and, when possible, on integrating the heat recovery system with cogeneration plants, a combination that produces high-temperature heat and power simultaneously.

11.1. Innovations in Materials

Recent new developments in material science offer considerable improvements in heat-recovery systems for gas manufacturing processes. For example, new materials enable Heat-exchanger-type plasma waste heat-recovery systems to improve the unit capacity and the recovery ratio, since the heat exchange area per unit volume can be significantly enlarged. Application of Heat-pump heat-recovery systems is growing due to their capacity for efficacious increase in the variation of heat sources. The new technique of a Heat-and-power cogeneration system can not only save more energy but also reduce environmental pollution.

In any of the gas-manufacturing processes, considerable waste heat is exhausted, in some cases, at quite a high temperature. Heat recovery is used mainly for preheating the raw material, process heating, and room heating. Thermal energy must not be produced by consuming valuable primary energy more than required. Application of Heat-recovery systems for all of these processes saves primary energy, reduces the waste-heat discharge to the surroundings, and lowers the production cost considerably.

11.2. Integration with Renewable Energy

Renewable energies play a vital role in improving the performance of Heat Recovery Systems (HRS). High-grade heat input to the cycle can be supplied from solar energy and geothermal energy instead of conventional fuels. The availability of solar energy is seasonal and varies from year to year, but considering the large-scale utilization of solar energy, methods of solar thermal energy storage have been developed. Solar heat can be stored thermochemically, thermally, or chemically for use during hours when sunlight is unavailable. Solar collectors such as parabolic dishes, parabolic trough collectors, flat plate collectors, and others can be used to harness solar energy for different types of heat input and heat rejection duties of the cycle.

The cast of the brine is another ecological concern. Direct discharge of brine into the ocean can severely impact the environment, even with dilution. The brine from the system can be converted into useful energy by employing an OTEC plant, which produces power and simultaneously utilizes the oceanic temperature difference. The integration of OTEC with an HRS can provide high-grade heat in terms of additional power output and low-grade heat for low-temperature vapor generation in the cycle. Thus, the major input of coal required to operate an absorption system can be reduced by a considerable amount.

12. Comparative Analysis of Heat Recovery Systems

Thermal energy can be recovered and reused in virtually all gas manufacturing processes. Sources of waste heat include flue gases and hot process streams. With the exception of the gas production process in which the gas is manufactured in the gaseous state, heat recovery can have beneficial effects in most of the other processes. Waste heat may be used for space heating, combustion air preheating, feedstock preheating, and in the manufacture of other products such as steam and hot water. Heat recovery systems can be effectively applied to nearly all manufacturing processes involving combustion of fuels. Heat exchangers, heat pumps, and combined heat and power (CHP) systems are the three thermal energy-recovery technologies with the greatest potential for implementation in the gas manufacturing industry.

Although heat exchangers have been used for years in several plant processes, technological advances are making them more effective. New heat-exchanger designs enable more heat to be recovered from flue gases, with exit temperatures approaching the dewpoint, thereby generating relatively large quantities of hot water or steam. Heat pumps can be used as temperature transformers to convert thermal energy at low temperatures into thermal energy at usable higher-temperatures. Typically, heat pumps employ a current of low-temperature (usually below 30 °C) liquid or gaseous media, such as air, water, steam, or gas, to extract heat from an external source (from a collection of streams or from ambient air or water) and to supply the equivalent heat at a higher temperature. CHP systems operate on a thermodynamic cycle implemented to produce electric power or mechanical power from a combustion process, as well as to recover heat (by use of a gas boiler, heat exchanger, or heat pump). The recovered thermal energy can then be used for space heating, combustion air preheating, feedstock preheating, or in the manufacture of other products such as steam and hot water. For further discussion, see Applications of Heat Recovery in Gas Manufacturing.

12.1. Efficiency Metrics

Gas manufacturing processes are energy intensive. They use large quantities of fuel and operate at high temperatures. Generating that fuel and then heating reactor vessels emit wastes gases at very high temperature. Heat recovery systems use that waste heat to reduce manufacturing cost and production of green house gases by different heat sources in the gas manufacturing processes. In modern times, the systems are integrated with highly sophisticated technologies to improve their energy efficiency. Efficiency analysis of heat recovery systems is a valuable tool for comparison and design. This section establishes two separate energy efficiency criteria, analyze two complicated heat recovery system, and propose two simplified methodology to evaluate the energy efficiency of heat recovery system. Finally, gas manufacturing processes are operated in heat recovery systems. Heat recovery systems use heat from different sources for preheating feedstock, process heating and space heating in the manufacturing processes.

Heat recovery systems have been successfully integrated in several manufacturing processes to increase combined heat and power capacities, enhance direct-fired heating services, reduce CO2 emissions and decrease fuel consumption. Some recent implementations in gas manufacturing are discussed in this subsection. The advantages of adding heat recovery systems include using less fuel, simultaneously generating electricity and heat, and recovering waste heat. The benefits are reflected in reduced GHG emissions released into the environment, lower production costs for the gas manufacturer, better productivity due to less reliance on expensive grid electricity, and in some cases, the production of renewable fuel. A comprehensive review of the benefits and challenges associated with the technologies should be done before investment is finally made.

12.2. Cost-Benefit Analysis

Heat recovery refers to extracting waste heat generated from one process and repurposing it for an even heat load in the same process or for a different function in the facility, such as generating steam or preheating feedstock. Implementing a heat recovery system allows the use of the full conversion of fuel from fuel gas or steam to heat without expending extra fuel gas.

In the world of gas manufacturing, energy efficiency is among the most important components in gas production. One major use of the energy produced in a gas plant is heat. With new technological advancements, engineering teams have now designed heat recovery methods and sources to make gas manufacturing more efficient by reusing waste heat and using less fuel during manufacturing. Utilizing this waste heat essentially decreases fuel cost while conserving waste heat as preheated feedstock, for space heating, or PACU temperature maintenance.

13. Impact of Heat Recovery Systems on Sustainability

The Impact of Heat Recovery Systems on Sustainability

Heat recovery systems are a notable development in improving energy efficiency, cost savings, and environmental friendliness of gas manufacturing. These systems capture waste heat from manufacturing facilities, so the heat does not go unused. Various types of heat recovery systems exist. Nonetheless, regardless of specific configuration, they contribute to more sustainable manufacturing by absorbing and reusing heat in various manufacturing processes.

Heat exchangers are one common heat recovery system. They transfer heat from exhaust gases to natural gas or other feedstock, either before or after compression. Heat pumps can also be used to upgrade waste heat to a higher temperature, suitable for process requirements. Finally, Combined Heat and Power (CHP) systems generate electricity and capture the residual heat for other purposes. The recovered heat is generally applied to preheat gas feed or for general process or space heating needs. As a result, heat recovery systems have the potential to significantly reduce carbon emissions associated with natural gas manufacture.

14. Conclusion

Manufacturing processes for producing basic industrial gases are energy intensive and depend largely on fossil fuels. Heat recovery in gas manufacturing is or can be a key element in a strategy to achieve greater energy efficiency. Heat recovery helps to reduce operating costs, and the associated emission reductions may enable the facility to meet more stringent future emissions targets. Operating plants are well suited for retrofit applications in which high-quality steam or direct process heating is recovered. Design plants also usually have waste heat available for recovery. Typical requirements for process heating in manufacturing plants include preheating combustion air or feedstocks for the main manufacturing process, space or feedwater heating, or direct process heating.

Heat recovery can involve simple heat exchangers, heat pumps, or combined heat/cooling/power systems. Heat exchangers use recovered waste process heat to replace other heating fuels or natural process sources. Heat-pump systems collect heat at low temperatures and, through external work, deliver it at higher temperatures. Combined heat, cooling, and power (CHCP) systems employ waste-heat-recovery technologies such as absorption chiller/electric-generation-turbines to simultaneously deliver electrical power, process heat, and process cooling. Coproduction-CCHP plant can provide a net reduction in fuel consumption of 35–50% compared to conventional systems when heat and power are required simultaneously.