Industrial process heating systems supply necessary heat to produce goods used by consumers and industries. The heat is supplied mainly from fuels, steam and electricity used in various types of heating equipment, such as furnaces, ovens, heaters and kilns.
This article – the final installment of a four-part series that started in December 2022 – discusses approaches to CO2 emission reduction and elimination from industrial process heating systems.
Approaches to CO2 Emission Reduction
As mentioned in previous parts of this series, CO2 generation and emission can be reduced or eliminated by using several different approaches and appropriate actions. Earlier discussions touched on only a few possibilities, such as:
- Energy-efficiency improvements that result in reduction in energy demand and corresponding reduction in CO2 for existing fuel-fired systems.
- Use of alternate fuel such as hydrogen and, in a few cases, ammonia if available at justifiable cost. As of 2020, the majority of hydrogen (∼95%) is produced from fossil fuels by steam reforming of natural gas and other light hydrocarbons, partial oxidation of heavier hydrocarbons and coal gasification. Future use of hydrogen in process heating systems will require major advancements in production and delivery of “green” hydrogen.
- Use of biofuels that produce CO2 but can be considered as having little or no effect on overall CO2 cycle. In 2020, about 5% of total U.S. energy production was in the form of biofuels. However, almost all of this was used as automobile fuel or feedstock.
- Use of electrical heating systems to supply the process heat. In 2021, about 39% of total U.S. electricity generation was by using renewable sources or nuclear plants.
- Development and use of alternate processes for production of materials that do not require fuel-generated heat and, thus, elimination of the associated CO2 emissions.
These approaches are discussed below.
Energy-Efficiency Improvements Resulting in CO2 Reduction in Existing Plants
Most of the energy used for process heating is supplied by natural gas and other fossil fuels in furnaces, ovens and other types of heating equipment. This equipment-discharge exhaust gas contains heat that depends on the gas temperature and its oxygen content. This heat is often wasted. In addition to the exhaust gas loss, heat is lost from the equipment through a variety of means. Depending on the temperature and oxygen content of the exhaust gases, as well as the equipment design, operation and maintenance, the combined losses can represent 15% to as high as 60% of the heat input, resulting in correspondingly high CO2 emissions. Reducing these heat losses is the most cost-effective way to reduce energy use and CO2 emissions.
Over the last three decades, the industry has taken many steps to reduce energy use that resulted in significant reduction of energy intensity, defined as energy use per unit of production, in manufacturing facilities. Specific steps to reduce energy intensity in process heating depend on many parameters, but the recommended steps are common.
The first step is to evaluate performance of the existing equipment by preparing a heat balance for the heating system and identify the heat losses. This may sound like a difficult task, but it can be done by using a few measurements and analysis of the collected data. Over the last two decades, many plants have used analysis tools such as PHAST, which was developed by collaborative efforts of the industry and the Department of Energy’s Advanced Manufacturing Office. The Process Heating Assessment and Survey Tool (PHAST) introduces methods to improve thermal efficiency of heating equipment. The tool helps plant personnel make simple measurements on heating equipment and identifies areas of heat usage. The results are displayed in the form of a pie chart or a Sankey diagram. It also suggests possible areas of improvements and the effects of each of these improvements on final energy use. The energy savings can be easily translated into reduction in CO2 emissions using Table 3 when the fuel type, composition or type of energy supply is known.
For many plants, good operating and maintenance practices that require no or low upfront expenses can result into substantial energy savings and a corresponding reduction in CO2 emissions. In some cases, it may be beneficial to make modifications to the system, such as installation of heat-recovery equipment (recuperators or regenerators) or advanced combustion systems such as regenerative burners. Depending on the fuel cost and availability of incentives to reduce energy use and CO2 emissions, the payback period can vary.
The application of selected energy-saving methods could result in 20-40% reduction in CO2 emissions while the heating systems are using fossil fuels. Reaching total elimination of CO2 emissions is impossible without making major changes in the heating systems. These are discussed below.
Fig. 12. Colors of hydrogen
Use of Hydrogen to Replace Fossil Fuels
If there is no carbon in the fuel, there cannot be CO2 present in the combustion products and, in most cases, no CO2 in exhaust gases. Hydrogen is one of the most commonly discussed fuels that does not contain carbon, and it can be used to eliminate emission of CO2 generation in combustion process. Hydrogen is used in steel plants, petroleum refineries and chemical plants as a feedstock or a reactant in production of variety of products. Use of hydrogen as a substitute to fossil fuel to eliminate CO2 emissions from heating systems has been discussed over last two decades. Alternatively, it has also been proposed to use a mixture of natural gas and hydrogen to reduce CO2 emissions. However, use of hydrogen in industrial heating applications has been almost nonexistent because it offers several challenges.
Hydrogen being considered as a substitute fuel can be produced by various methods. Depending on how it is produced, it has been described as green hydrogen, blue hydrogen and grey hydrogen. The so-called color of hydrogen is attributed to the method by which it is produced and its environmental impact. A graphical description of these different types of hydrogen is given in Fig. 12.
Green hydrogen is produced by electrolysis of water using electricity produced from renewable energy sources. It does not involve production of CO2 at any stage of hydrogen production. Blue hydrogen is produced by using the grid electricity, which is generated from fossil fuels. CO2 produced from fossil fuels used for electricity generation is captured or utilized by using an appropriate carbon-capture system (CCS). Blue hydrogen can also be produced by using steam methane reforming of natural gas and use of CCS to prevent its emission to the atmosphere. Hydrogen produced using steam methane reforming without the use of CCTS technology is called grey hydrogen.
At this time, almost all hydrogen used in the U.S. is produced by reforming natural gas reaction with steam or from coal gasification. However, these methods result in CO2 emissions from the reaction itself as well as from the burners supplying the heat required for the reaction. Although use of grey hydrogen does not result in CO2 emissions at the point of end use, the overall effect is often more CO2 emissions than what could be from the direct use of natural gas to supply heat to the process. Therefore, the use of grey hydrogen or even blue hydrogen is not recommended as an acceptable method of decarbonization of heating processes since it does not reduce CO2 emissions on a global basis.
Another method being considered is to use biomass materials to produce hydrogen. This method can produce hydrogen with net zero or even negative emissions of CO2. However, this route to produce hydrogen is not considered practical in the foreseeable future.
Lately, it has been suggested to blend hydrogen with natural gas to produce fuel that contains approximately 20% hydrogen (by volume) with the expectation of reducing CO2 emissions. This mixture has 86% of the energy of an average natural gas, meaning that we have to burn 14% more volume of gas to deliver the same BTU of heat. However, the reduction in CO2 is not nearly 20% but only closer to 7% when we look at the combustion process, assuming we use green hydrogen, and even less than that when you consider energy use and associated CO2 emissions associated with the compression of the gas required to compensate pressure loss in transmission and distribution of hydrogen.
Use of Biofuels
Biofuels, or biomass, is the term used for all organic material originating from plants, trees and crops. These materials contain carbon and hydrogen together with other elements and can be used as a source of heat. The release of CO2 through the combustion of biofuels contributes little or no net carbon (CO2) to the atmosphere. Biomass in the as-derived form contains large amounts of moisture and requires drying to reduce moisture before it can be used as acceptable fuel. For example, wood waste products may contain 15-20% moisture, while other products may contain even higher moisture values. The heating value of biomass varies from 4,000-7,000 BTU/pound as received to between 8,000-9,500 BTU/pound for totally dry (% moisture) wood. This is very low compared to typical bituminous coal, which contains about 14,000 BTU/pound.
At this time, biomass or waste products are used as fuel in direct or indirect form for producing steam and electricity in many industries. The combustion of biomass for process heating furnaces is very rare due to high moisture content, low heating value and inorganic residues resulting from combustion of such fuels. However, biomass has been proposed as a fuel for cement kilns and even electric-arc melting furnaces. Conversion of biomass into gaseous fuels through gasification or production of liquid fuels such as alcohols is other routes of using biomass. Biomass offers the only possibility of producing carbon-negative CO2. However, the use of biofuels in industrial heating processes is not considered practical, except in only a few cases, at this time.
One of the exceptional cases is use of biomass fuels or sewage sludge in cement and lime kilns, where combustion residues are not of major concerns. Some of the cement plants in China and India are using biofuels as a supplement to coal-firing. The lesson learned is that such sources of heat are acceptable. However, the biggest hurdles are availability of constant and reliable supply, consistency of the available materials, logistics of collection, transportation and preparation of these materials, and controls to maintain acceptable moisture in the feed material. The combined effect of this prevents wide-scale use of biomass as fuel in cement kilns and other applications in the U.S. and other countries.
Since industrial drying is used by almost all industries, the use of biofuels and other renewable energy sources is becoming an area of great interest. Use of solar energy, either directly or in concentrated form, has been proposed for drying applications in agriculture and forest products industries. Use of direct solar energy, without conversion to electricity, requires the use of concentrators, energy storage at high temperatures and other backup energy sources. Many research institutes are working on developing these components and integrated systems for drying and other applications.
Use of Electrical Heating Systems
Use of electricity to replace fossil fuels for industrial heating applications has been one of the most attractive options for decarbonization of process heating systems. Systems that use electricity directly to supply heat or use other electricity-based technologies for thermal processing of materials are commonly known as electrotechnologies (ETs).
Electricity can be used to supply heat in two primary methods. In one method, electricity is used to produce heat within the material being heated by electrical resistance of the material itself or by affecting molecular movement within the material. In the second method, heat is generated by electrical resistance and is then transferred to the materials being heated by thermal conduction, convection or radiation. In each case, there are several options available and currently used.
The most commonly used ETs for industrial heating applications are resistance heating, induction heating, electric-arc heating, electric infrared processing, microwave heating, radio-frequency heating, electron-beam processing, ultraviolet processing, plasma heating and laser heating. Some heating systems may use a combination of one or more these heating methods. The selection of an electric heating system for a specific application depends on many considerations, including:
- Process temperature
- Type of heating – batch or continuous, direct heating versus indirect heating, type of process atmosphere used, product contamination considerations, material-handling system
- Production rate, energy supply system capacity and physical size of the heating system.
- Process characteristics – drying, melting, heat treating, etc.
- Comparative cost and availability of energy sources – electricity, hydrogen or biomass as fuel, other fossil fuels, etc.
These systems, their components and selection criteria are not described in detail for brevity. There are several references, such as a chapter on electric heating in Marks’ Standard Handbook for Mechanical Engineers that gives a short description of each of these systems and their operating characteristics.
The most widely used ETs for process heating in large-scale production facilities (several tons/hour) include resistance, induction and electric-arc heating. The newer generation of ETs – such as microwave and RF heating, laser heating and ultraviolet heating – are used mostly for niche markets and are relatively small in capacity (usually in hundreds of kilowatt hours). Many companies and research institutions are working on developing radically different and disruptive thermal ETs, developing the required components (e.g., power supply, frequency modulators) for large heating applications and to meet demand of higher-capacity heating systems.
Table 5 shows the application of ETs for various categories of processes used in manufacturing. As shown in this table, a number of options are available for each of the process categories. At first glance, it may not be obvious that some of these technologies can be applicable to the processes indicated in this table. For example, use of resistance heating or electric infrared heating are easy to understand, but there may be a question about use of induction and microwave heating since they are not commonly considered or used in processes such as fluid heating and drying. Use of susceptors and alternate material-handling systems has been proposed for heating of crude in refineries. Many research-and-development organizations are working on developing innovative systems to meet industry demand.
Table 5 gives an overview of the possible methods of application for generic process categories. Tables 6a and 6b describe commonly used or proposed electrotechnologies for specific applications in various industries. This is not meant to be a complete list of all technologies and their applications in various industries.
For those looking to use electricity for high-temperature industrial heating, either through induction or the use of resistive heating elements, it is important to consider the source of electrical power. While using electricity in place of fossil fuels will always reduce or eliminate CO2 directly discharged from the local industrial heating equipment, it is necessary to trace back the primary energy source used to produce the electricity and understand the overall carbon footprint associated with this energy source.
As per Department of Energy data (2020), 20% of electricity is generated by using renewable sources, 20% by nuclear plants, 40% by natural gas and 19% by coal, with 1% by petroleum products. The average CO2 emission rate with the use of electricity in the U.S. is 0.85 pounds/kWh or 250 pounds per MM BTU. This compares to 115 pounds of CO2 emissions from combustion of natural gas and 237 pounds of CO2 emissions from combustion of bituminous coal. Things get more complicated when we consider the effective efficiency of use of the supplied energy to the process or material being heated.
From these numbers it may seem that use of natural gas is a better choice for reduction of CO2 emissions for heating processes. However, the reality is somewhat different. With use of natural gas, we have to consider combustion efficiency or available heat and also other heat losses from a furnace. Combustion efficiency is simply the portion of usable energy that goes to the process compared to the total energy supplied.
Electric heating at the point of use can be considered 100% efficient. However, there are losses between the point where electricity is generated and delivered from the grid and the point of use. Even at the point of use there are losses in power conversion or supply system and losses in other components such as cooling coils used for induction heating systems. The typical efficiencies for commonly used electric heating systems are given in Table 7. These losses, in general, would increase the threshold temperature. The technology of power-supply system components is improving rapidly in all types of heating systems, so the efficiency values given in this table should be updated when used in any specific case.
Use of Alternate Processes
Industrial heating processes used in manufacturing of basic materials such as iron, copper, glass, ceramics and basic chemicals have been developed over several centuries and have traditionally used locally available fuels such as coal, wood and other biomass products to extract the required base materials. Over the centuries, the scale of production, equipment size and design, nature of fossil fuels and operating practices have changed, but the basic process – such as reduction of iron oxide, melting and heat treatment – has remained mostly unchanged. During the last century, however, many new materials – such as advanced ceramics and sources of energy like natural gas and electricity – have emerged and are used in manufacturing processes. We have also seen advancement in research activities and the use of advanced analytical tools that have resulted in better understanding of fundamental processes and drive new developments in materials.
These activities have allowed researchers to develop new process routes – often non-thermal processes in production and processing of materials. The use of electricity to produce aluminum from alumina was a radical change from conventional practices. Many other developments, such as membranes and nanotubes for separation of gases and liquids, offer potential to replace highly energy-intensive thermal processes in multiple industries.
Currently used fossil-based processes result in a large amount of CO2 emissions. Researchers are working to develop alternate processes that replace the carbonaceous energy sources or use alternate reducing agents such as hydrogen. One research approach taken to extract basic material such as iron from iron oxides and copper from copper sulfide is to use an electrolysis process without using carbon-based reducing agents to avoid emission of CO2 either in the process itself or in the downstream processes.
The other approach being pursued is to replace the use of carbon-containing reducing agents as CO or hydrocarbons with a reducing agent such as hydrogen. Private industries are developing non-thermal separation processes using membrane technology. Other researchers and private organizations are using electromagnetic technologies such as microwave and radio-frequency heating to replace traditional fuel-based drying technologies, which reduces overall energy intensity, increases productivity and, in many cases, improves product quality. Many of these processes are at the development or pilot stage and may take several years before they are used at the mass production scale.
Two specific examples of alternate processes are mentioned here to illustrate the approaches taken by researchers.
- In the steel industry, the biggest consumer of energy is the blast furnace operation where iron ore is smelted using coke as a reducing agent. Two alternative pathways are being considered to the traditional blast furnace and BOF steelmaking: iron production using an electrolysis process to decompose iron ore to iron using electricity as energy source and production of direct-reduced iron (DRI) for iron and steel production using hydrogen.
- There is a trend to use alternate processes to replace thermal processes used in the distillation of liquids and gases. A specific example is dewatering of ethanol that uses steam or hot gases. The development of a separation process that uses membranes and nanotubes is under way to reduce energy use and associated CO2 emissions.
Reducing global carbon emissions will likely involve most, if not all, the approaches discussed in this four-part article.
There are hundreds of process heating furnaces and boilers used by the manufacturing sector. Their energy use rate varies from a few million BTU/hour to hundreds of MM BTU/hour. Their age varies from a few years to several decades. Conversion of this equipment to non-carbon-containing energy sources is a huge challenge in terms of cost and logistics. It is likely that manufacturing plants would use several approaches to achieve the final goal of total decarbonization of the process heating systems.
The first step in reducing CO2 emissions is to consider and implement all possible measures to reduce energy consumption for the existing systems. These steps will not eliminate CO2 emissions, but it offers a cost-effective method of reduction. These steps will also help when the system is converted to use alternate fuel such as hydrogen while maintaining fuel-firing or when the system is converted to use electric heating.
The next consideration should be to use non-carbonaceous fuel, primarily hydrogen, for existing thermal processes and heating equipment. Conversion of existing combustion systems or thermal processes to use hydrogen is an option to replace natural gas or other forms of fossil fuels for almost all types of heating equipment. It can be used on existing heating systems without making changes to other auxiliary systems such as the furnace structure, material handling, exhaust gas handling and treatment.
It would be necessary to change the combustion system, primarily the burners and related controls. Combustion system suppliers and burner suppliers can develop or modify existing burners and other components to use hydrogen in a relatively short time, but supply of cost-justifiable hydrogen in sufficient quantity is a big issue. We are not aware of any manufacturing plant receiving hydrogen by a pipeline or on-site hydrogen production for use in heating equipment. Almost all hydrogen produced in the United States is used by chemical and petroleum industry, and it is not available in most regions. This being the case, it is difficult to imagine that hydrogen will replace natural gas for industrial heating in the foreseeable future. It is likely that hydrogen or other non-carbon-based reducing agents may be introduced for a few metallurgical processes as a replacement or supplement to coal, coke and natural gas.
This leads to consideration of electricity to replace fuel-fired systems. Conversion of some existing combustion-based heating systems to electrotechnology is possible for certain applications without replacing the entire system. For example, retrofitting a radiant-tube-heated furnace with electrical resistance elements is relatively simple if the required electric power is available or can be added. In other cases, it is necessary to make major modifications to the existing system. An example is metal heating by induction. However, use of induction heating requires a totally different material-handling system, heating method, equipment and controls.
For each fuel-fired system there may be more than one electrotechnology option available. Selection of a specific technology depends on many factors. In many cases, the energy requirement for the selected system would be different from the current energy input rating, so it is not advisable to convert a rating expressed in terms of BTU/hour to kWh. In all cases, however, it is necessary to make detailed evaluations of available power, transformers and power distribution systems within the plant and requirements of additional utilities such as cooling water for induction systems.
At this time, there are many furnace or heating system suppliers capable of designing and delivering appropriate electricity-based heating equipment for small- to medium-sized systems. For large systems used in modern plants, however, it is necessary to develop innovative methods for the application of many electrotechnologies. For example, the use of induction heating for steel reheating will require innovative approaches, and research may take several years or decades. A good example is the development and use of electric-arc furnaces for steel production. The concept of using an electric arc to melt steel was proposed in the 1880s with a production-scale furnace installed in 1909 in Europe, but it took more than 50 years for the steel industry to accept it as a viable option to conventional coal-based steel production.
Conversion to electric heating would result in increased demand for electricity. To meet CO2 emission goals most, if not all, electricity has to be generated using renewable sources. According to the Energy Information Agency (EIA), electricity production by renewable sources was 20% in 2021. It is expected to reach 44% in 2050. This indicates that a large percentage of electricity generation will continue to be dependent on the use of fossil fuels for the foreseeable future, and it will be necessary to use carbon capture and storage (CCS) or other technologies to eliminate emission of CO2 at the point of generation before transmission of to the grid.
Thermal processes such as reduction of iron ore where metallurgical coke is used as a reducing agent present special challenges. In these cases, three options have been considered: use of alternative reaction agent such as hydrogen to replace carbon-based reactant, use of biomass or other non-carbon-based feedstock and development of non-thermal options. A number of research projects are under way to use hydrogen as an alternate reducing agent or reactant. One example is the use of hydrogen as a reducing agent in the production of direct-reduced iron (DRI), which can be used in EAFs for steel production.
Fig. 13. The contributing organizations and their responsibilities to achieve zero carbon emissions for industrial heating systems
The use of additive manufacturing and innovative heat-treating processes are a few examples of alternate manufacturing processes that can eliminate or make substantial reductions in CO2 emissions.
The achievement of near-zero carbon emissions in a manufacturing plant through conversion of fuel-fired thermal processes or use of carbon-free technologies will take many years with a large capital expenditure, even after the appropriate technologies and associated auxiliary support systems are available. It will also require changes in the infrastructure of manufacturing facilities for transmission and distribution of electricity and green fuels such as hydrogen. As various options are being pursued, it is necessary to develop advanced materials and design new components together with training the workforce with different engineering skills.
Achieving the goal of decarbonization of process heating systems within a manufacturing plant requires contributions from many organizations together with the government. Figure 13 shows the contributions each of these institutions can make to achieve this goal within the time frame established by the U.S. government and international organizations – by 2050.
The information used in this article is developed by the author and also collected from a report “Thermal Process Intensification: Transforming the Way Industry Uses Thermal Process Energy,” co-authored by the author and the following individuals: Sachin Nimbalkar and Kiran Thirumaran of Oak Ridge National Laboratory (ORNL) and Joe Cresco of the Advanced Manufacturing Office of the U.S. Department of Energy (DOE). As of May 2022, these reports are available on the DOE’s website: https://www1.eere.energy.gov/library/.
For more information: Arvind Thekdi is president of E3M Inc., which he founded 22 years ago, in Gaithersburg, Md. He has over 55 years of experience in combustion, energy-efficiency improvements, emission reduction and waste-heat recovery in industrial heating systems. He can be reached at firstname.lastname@example.org.
All graphics courtesy of the author except where noted.