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 – part 2 of a series that will run throughout 2023 – describes the different types of process heating systems used by the industries, CO₂ emissions from these systems and possible pathways to achieve net-zero carbon emissions with citation of a few examples.

Greenhouse Gases (GHGs)

A large percentage of CO₂ emissions in manufacturing operations are directly from combustion of fuels fired in process heating equipment or indirectly from generation of steam and electricity. Table 3 gives a list of CO₂ emissions from fuel combustion in furnaces and boilers.

Combustion-generated CO₂ emission depends on carbon content of the fuel and the amount of oxidant, usually ambient air or oxygen, used for combustion. Any amount of combustion air equal to or higher than stoichiometric air (excess air) would result in a fixed amount of CO₂ expressed as pounds of CO₂ per MM BTU heat release during the combustion process. Commonly recommended energy-saving methods – such as reduction or control of excess air, use of combustion air preheating, oxygen enrichment of combustion air or use of oxy-fuel combustion – do not have a direct effect on combustion-generated CO₂. However, use of any one of these steps would result in energy savings and reduction of fuel use per unit of production, resulting in a reduction in CO₂ emissions per unit of production from the process heating equipment. Therefore, any one of these actions or other commonly recommended energy-saving measures could result in reduction of total GHG emissions from a plant.

CO₂ emissions from common fuels

Processes that use steam and electricity as a source of heat do not have direct emission of CO₂ at the site where the heating equipment is used. However, production of steam and electricity generated in fossil-fuel-fired plants do emit CO₂ at the generation site. Accounting for CO₂ discharge from steam-heated systems requires consideration for total heat content of steam used, type of fuel used in boilers and efficiency of steam-generation system.

For example CO₂ emission from a steam-heated system that supplies 1 MM BTU heat to a process and is derived from a natural-gas-fired boiler with 85% efficiency would have 135 pounds of CO₂ emission per MM BTU heat supplied by steam. It is also necessary to consider heat losses from the steam distribution, condensate return and process heating equipment when considering CO2 emission associated with heating equipment.

Calculations for CO₂ emission from an electrically heated system can be a little more complicated because it is necessary to consider the source of electricity. According to the U.S. Energy Information Agency (EIA), the average CO₂ emission in 2020 was about 0.85 pounds per kWh for the entire country. This accounts for electricity production from all sources mentioned in this article. Actual CO₂ emission from a heating process depends on the efficiency of electricity use.

For example, an induction system that uses 10,000 kWh electrical energy for melting iron or aluminum with 70% overall thermal efficiency would have 12,140 pounds of CO₂ emissions per hour. In many cases, CO₂ emissions per kWh for on-site electricity generation that uses combined heat and power (CHP) or cogeneration could be substantially lower than the national average value.

Most of the heating systems in U.S. use natural gas, which has the lowest CO₂ emission rates, as fuel. The CO₂ emissions from a fuel-fired system are not affected by the amount of excess air used or the exhaust gas temperature. However, use of energy-conservation measures – such as reduction of excess air, use of waste-heat recovery system and efficient operation through better controls – reduces energy use per unit of production and correspondingly reduces CO₂ emissions. If the combustion is at sub-stoichiometric (rich firing), CO₂ emissions will be lower. However, the combustion products contain CO, H₂, etc.

Carbon dioxide emissions are generated from various systems used in manufacturing plants. Figure 2 shows major areas of manufacturing energy use and CO₂ emissions from these areas, which include on-site generation of steam and electricity, process energy use and non-process energy use. Further breakdown of these three main areas and CO₂ emissions from U.S. manufacturing plants for 2018 are also shown Fig. 2.

Fig. 2. Total GHG emissions from U.S. manufacturing (MMT CO2e)

On-site generated steam and electricity may be used for process heating in electrically heated systems, steam-heated systems or other plant systems. Electricity and steam purchased from outside (shown outside the plant boundary line) is not included in this discussion, but they indirectly contribute to CO₂ emissions associated with the manufacturing operations. These additional CO₂ emissions are 385 MM tons and almost 50% of the emissions generated on site.

In addition to energy supply or combustion-related GHG emissions, some thermal processes produce GHG emissions that result from chemical reactions or from breakdown of products such as calcium carbonate (limestone) in production of lime. This is reported as part of CO₂ emissions from use of process energy. The decarbonization process requires different approaches for each category.

CO₂-Generating Systems

According to the DOE’s Energy Information Agency (EIA), the total CO₂ emissions from manufacturing operations (1,165 tons/year) was about 20% of total U.S. emissions (5,742 tons/year) in 2018. Total CO₂ emissions from manufacturing processes is shown in Fig. 3. Note that CO₂ emissions resulting from combustion systems used for process heating represents 38% of the total CO₂ emissions while the process-generated CO₂ that originates from the materials being processed within the heating equipment represents 23% of the total. Therefore, 61% of total emissions are from process heating equipment used in the plants. Examples of process-generated CO₂ include calcination of limestone in the cement industry and reformers used in petroleum refining operations. They cannot be eliminated by just replacement of fuels. They need technologies such as carbon capture, storage or transformation or replacement of the process raw materials and changes in process routes.

Fig. 3. GHG emissions from U.S. manufacturing by end uses (MMT CO₂e)

The CO₂ emissions reported in Fig. 3 originate from hundreds of heating systems – small and large – each with unique process requirements, equipment design and operating procedures. Even though most of the emissions are from natural-gas-fired systems, they do include fuels such as manufactured gaseous fuels (coke oven gas, blast furnace gas, refinery gases), liquid fuels, coal and other solid fuels. It is a huge challenge to analyze and develop CO₂ emissions control systems for each type of heating process or equipment. When we consider the origin of CO₂ emissions and options to reduce or eliminate them, however, it is possible to group them into four categories – each with a similar pattern of CO₂ generation.

Depending on the source of CO₂ generation, type of heating system used and specific process requirements, the CO₂-generating systems can be classified into four categories.

Fig. 4. Heating systems with combustion-generated GHG emissions

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

In this type of system (Fig. 4), heat is supplied from a conventional fuel-fired burner located in a furnace or indirectly from a radiant tube or muffle, which is heated by a fuel-fired burner. These fuels contain carbon and hydrogen, and their combustion or oxidation reaction produce heat and combustion products that contain mainly CO₂, H₂O, N₂ and O₂ with trace amount of combustibles and SO2 if the fuel contains sulfur. Hot flue gases produced by stoichiometric or excess air combustion are discharged directly from a furnace or after flue-gas treatment, resulting in emission of CO₂ into the atmosphere.

Heat transfer from combustion products is primarily by convection with some radiation from the flame or combustion products that contains CO₂ and H₂O. For high temperature (>1400°F) furnaces, thermal radiation from the combustion products may account for a significant part of the total heat transferred to the material being heated. The heat-transfer considerations play a very important role in case of alternate methods of heat supply, such as electric heating, when it may be used to eliminate combustion-generated CO₂. A vast majority of industrial heating processes fall under this category.

Fig. 5. Reactive heating systems using carbonaceous material for heat and reactions

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

In this type of system (Fig. 5), fuel such as coal or metallurgical or petroleum coke is mixed or reacted with a source of oxygen (air or pure oxygen) to produce heat by complete or, more often, partial combustion. The reaction products, such as CO and H₂, are used as reducing agents or reactants for the required chemical reactions. The heat generated by combustion is used to raise and maintain the required reaction temperature of the material being processed.

 A good example of this is melting of iron in cupolas or reduction of iron ore in blast furnaces. In this case, use of non-carbonaceous fuel or an alternate heating system (such as one of the electric heating methods) still requires use of an alternate reducing or carburizing agent for the process. The exhaust gas stream contains a mixture of gases containing CO₂, CO, H₂, N₂ and combustible gases that are used as low-heating-value fuels in downstream processes. These processes are used in the chemical industry, petroleum refineries and the pulp and paper industry. The gases with combustible components (such as H₂, CO or other hydrocarbons) are used as fuel in other heating systems, such as furnaces, steam generators, gas turbines, or simply incinerated resulting in CO₂ discharged into the atmosphere.

Fig. 6. Heating systems using carbonaceous fuels and raw materials

Category 3: Heating systems Using Carbonaceous Fuels and Raw Materials

In this type of system (Fig. 6), combustion of carbonaceous fuels is used to raise the temperature of materials being processed. At a certain temperature the material “breaks down” and releases CO₂. A good example of this is production of lime in a kiln. In this case, limestone (CaCO₃) is heated to 2640°F by using heat from combustion of fuel when CO₂ is released from the calcination of limestone. This CO₂ is mixed with combustion products and other gases and leaves the heating system (kiln, in this case) as exhaust gas. The combustion-generated CO₂ can be reduced or eliminated by using low- or non-carbonaceous fuels, but the CO₂ produced from the calcination process cannot be eliminated. In this type of process, CO₂ has to be captured and converted or stored even when heat is supplied by an alternate source that does not produce CO₂.

Fig. 7. Heating systems where carbon-based fuels are used as feedstock

Category 4: Heating Systems Where Carbon-Based Fuels are used as Feedstock to Produce an Alternate Reactant or Other Type of Commodity

A few examples of this type of system (Fig. 7) include production of metallurgical and petroleum coke, carbon anodes and cathodes used in the aluminum industry, and graphite electrodes used in EAFs. These systems produce CO₂ from two sources: from the combustion system used to supply the process heat to raise the product temperature and from the volatile gases released from the material being heated or processed.

The elimination of combustion-generated CO₂ requires use of non-carbonaceous fuels. In this case, use of electric heating, particularly resistance or induction heating, is not practical due to the working environment, which contains condensable and flammable vapors and may shorten the life of heating system components. Emission of the volatile material and gases released from the material being heated cannot be avoided. These exhaust gases from the process side can be used as feedstock material in production of chemicals, or they can be used as fuel in other processes. In the latter case, the combustion process will produce CO₂, which has to be dealt with to avoid CO₂ emissions.

In some cases, the volatiles released from the product react with preheated air within the furnace or reactor, resulting in partial combustion of volatiles and heat that can meet the process heat requirement. In this case, some method of capturing CO₂ has to be used to avoid CO₂ emissions. It is necessary to have an externally fired heating system to raise the temperature of the material at the start-up condition, and CO₂ emissions during the start-up need to be captured.

Summary

Almost all thermal processes used in industry can be classified in one of these four categories. Table 4 shows examples of where some of the commonly used thermal processes fall within these four categories. The processes mentioned here are only a few examples of many processes used in manufacturing, and each of them should be evaluated by considering the process requirements, currently used heating systems and other process characteristics as described in one of the four categories. Decarbonization methods for each process have to be selected based on considerations discussed in part three of this series.

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.