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This article – part 3 of a series that will run throughout 2023 – discusses the methods available to eliminate and/or reduce CO₂ emissions from process heating systems.

Methods of Elimination or Reduction of CO₂ Emissions

Several options are available to reduce or eliminate CO₂ emissions from heating processes. The most common methods include: use of carbon-free fuels; use of electricity as source of heat; collection of CO₂ and store it or transform it to materials that capture carbon; change the manufacturing processes to alternate technologies that do not use fuels and eliminate formation of CO₂; and use of alternate materials for the end product that do not require thermal processing. Technologies for implementation of some of these steps are available, and adaptation of these technologies becomes an economic issue involving capital investment, manpower retraining and limitations on availability of energy resources.

Total conversion to carbon-free processes requires availability of alternate fuels with zero carbon content at an economically justified cost and in sufficient quantity to meet the process heat requirement. When electricity is used to supply the process heat, it is necessary to select a proper electro-technology and to develop materials that can be used at high temperature for some of the commonly used electrical heating systems such as resistance or induction heating. It is also necessary to develop associated energy supply systems with controls that can be used for high energy capacity (usually in megawatts); establish energy supply and distribution infrastructure; develop alternate equipment designs; and, in some cases, develop alternate processes. Application of alternate energy sources for existing systems may also require equipment retrofit, changes in process parameters and recertification of some of the operating practices.

Reduction of CO₂ Emissions from Existing Systems

In all cases where conventional fuels are used as a source of primary energy, CO₂ emissions can be reduced by taking actions that would result in reduction of energy intensity (energy used per unit of production) and improvements in thermal efficiency. Many low-cost and no-cost action items have been suggested for improvement of thermal efficiency in process heating systems. They include improvements in combustion efficiency through better control of excess air, use of preheated combustion air, reduction of air leaks, use of oxygen enrichment of combustion air, reduction of heat losses from the system, use of alternate materials for construction of furnaces and ovens, and use of better controls. Many are no- or low-cost actions. They should be considered as a first step to reduce CO₂ emissions. However, not one of them would result in total elimination of CO₂. In all cases, it is necessary to eliminate use of carbonaceous fuels and use alternate fuels or energy sources.

Elimination of CO₂ Emissions from Process Heating Systems

Although total elimination of CO₂ emissions from process heating systems is a long-term goal, it is useful to review available and practically applicable methods to achieve this goal. Steps required to get there depend on many factors, such as the type of process, energy requirement for the process heating equipment and overall economics for the selected system. Each of the four categories discussed in part 2 of this series, which appeared in February 2023, requires consideration of a few unique options.

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Fig. 8. Decarbonization strategy for category 1 systems

Category 1: Heating System with Combustion-Generated GHG Emissions Only

In this case, the obvious option is to replace the carbonaceous fuels with non-carbonaceous fuels or use one of the electro-technologies to supply heat as shown in Fig. 8. Although several fuels – such as hydrogen, ammonia and gases produced from gasification of biomass – have been considered, hydrogen is the best available and practical option at this time. For use of hydrogen, it is necessary to convert or change the burners and the associated combustion system components. This is technically and practically possible for all fuel-fired systems.

Fuels with high hydrogen content have been used successfully with some modifications in many industries. Burner suppliers can supply the required combustion system with necessary components with some design modifications and offer training for engineers and operators. However, the infrastructure to produce, store, transport and distribute hydrogen in large quantities to meet industrial demand is nonexistent. Currently, this does not exist. Other issues – such as cost of hydrogen, safety considerations, approval by regulatory agencies and the effect of additional moisture in combustion products – become major concerns when large quantities of hydrogen fuel is used in a plant. This will require major efforts and cost for the foreseeable future, and no roadmap to get there exists.

Another option is to convert the system to use one of the many available electrical heating systems or electro-technologies. In the past, when natural gas was in short supply or its price was considered too high or it was simply not available, many plants in the United States modified their furnaces. They installed electrical heating systems to replace fuel-fired burners or used dual-fuel systems for an existing furnace. European industries used fuel oil or electrically heated equipment since natural gas was not easily available.

Many of these systems used resistance heating to replace direct- or indirect-fired burners. Use of other electro-technologies – such as induction heating for metal heating and melting, use of microwave or radio frequency for drying and electric-arc furnaces (EAFs) for metal melting – have been used in selected industrial heating applications. However, conversion from fuel-fired systems to electric systems requires major changes in equipment design, total replacement of system components and, in many cases, reconfiguration of the associated material-handling system. Other considerations include converting continuous processes to batch processes, cost of electricity, availability of required power and infrastructure for transmission of electricity, and limitations on space for installation of electric heating elements. Conversion to electric heating may not always be possible for large systems used in industries such as petroleum refining, iron and steel and chemical production.

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Fig. 9. Decarbonization strategy for category 2 systems

Category 2: Reactive Heating Systems Using Carbonaceous Material as a Heat Source

A few options are possible to eliminate CO₂ emissions in this type of system. One option (Fig. 9) is to use an alternative reducing agent such as hydrogen. Many European steel producers are experimenting with green hydrogen based DRI production that can be used in EAFs to produce steel and discontinue use of blast furnaces. In this case, iron-oxide pellets are fed into a reactor, where hydrogen reacts with iron oxide at about 1800°F, and produce pure iron, which can be fed into an EAF that uses electricity to produce molten steel. Companies such as Midrex are currently building pilot plants to demonstrate this technology with the intention of it use in the coming years.

Another option is to continue using carbonaceous material as a reducing agent. Collect a mixture of CO and CO₂ discharged from the process, use it as feedstock if possible or as fuel in an appropriate heating system followed by use carbon-capture technology at the end-use location. This is practical and economically justifiable only for large plants.

A third option is to develop an alternate non-thermal-based process that uses electricity to extract iron from iron ore. Research and pilot-scale work has demonstrated that an electrolysis process can be used to replace the existing blast-furnace route to produce iron and then steel. The replacement of blast furnaces and eventual use carbon-free electric melting of scrap will require considerable research-and-development efforts and capital investment before there is total elimination of CO₂ emissions from the existing industrial base.

Similar approaches can be used for other processes where carbon-based material is used to supply heat and act as a reducing agent. It is customary to collect exhaust gases from these processes and use them in secondary processes as fuel or as feedstock. Carbon is converted into CO₂ before the gases are discharged into the atmosphere. The solution to eliminate CO₂ emissions can be different for each process.

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Fig. 10. Decarbonization strategy for category 3 systems

Category 3: Heating Systems with Process-Generated CO₂ Emissions

In this category of industrial heating processes it is necessary to supply heat to bring the material to a temperature where carbon dioxide or other forms of carbon-bearing gases are released by breakdown of the material (Fig. 10). This produces free CO₂, which is usually mixed with the combustion products from the burners supplying heat to the process. CO₂ emissions from the combustion system that uses carbonaceous fuels can be reduced or eliminated by using alternate heating methods, but there is no way to eliminate process-generated CO₂ that comes from the feed materials.

Use of electric heating with an option of induction or resistance heating has been proposed as a possibility if a system can be developed to achieve a process temperature as high as 2650°F. There are other examples in iron, steel and aluminum industries where use of limestone or other carbonates that produce CO₂ have to be dealt with. Use of a carbon-capture system is a possibility to eliminate CO₂ emissions. Several approaches are being considered for CO₂ capture, such as storage or transformation.

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Category 4: Heating systems Where Carbon-Based Fuels are Used as Feedstock

Processes that fall under this category are very similar to those discussed in category 2 and 3, where the process-generated CO₂ and combustible gases have to be dealt with. Processes in category 4 (Fig. 11) include coke-making, petroleum coke processing, gasification of biomass and pyrolysis of organic materials. These processes are somewhat different because heat is supplied by partial combustion of the material itself, and the process discharges gases that contain hydrocarbons, CO, CO₂, H₂, etc. The exhaust gases need to be cleaned and used as fuel or feedstock for producing useful organic compounds. The resultant CO₂ has to be collected, cleaned, separated and stored in a carbon storage system.

In some cases, the offgases from these processes can be used as feedstock for certain chemical processes to produce materials that contain carbon, and there is very little or no CO₂ emission. It is possible to use electric heating to supply the heat and avoid dilution of exhaust gases, which will have a higher percentage of hydrocarbons and no presence of nitrogen. These gases are more suitable for use as feedstock than for some chemical processes. It is likely that such processes may be further developed to produce carbon and graphite that can be used in advanced structural materials, such as carbon-fiber-based shapes.

Summary

This four-part series describes the different types of process heating systems used by industries, CO₂ emissions from these systems and possible pathways to achieve net-zero carbon emissions. However, elimination or major reduction in CO₂ emissions is a very challenging task for established manufacturing operations due to its effects on economic and financial parameters. Considering the scale of manufacturing across the United States, achieving net-zero industrial GHG emissions while maintaining competitiveness is a grand challenge.

Stay tuned for the final installment of this series (part 4), which will appear in June’s issue.

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 arvindthekdi@gmail.com.