In the U.S. manufacturing sector, the steam and a large percentage of electricity is produced by using fuels such as natural gas, coal or other types of byproduct fuels. Ultimately, the primary source of heat for process heating is fuels with a varying amount of carbon content, with the exception of a small percentage of electricity generated by using renewables and nuclear plants. The carbon in these fuel sources ends up in the atmosphere as carbon dioxide (CO2), which is recognized as one of the prime greenhouse gases (GHG).
As reported in literature and media, a concerted effort is being made to reduce carbon emissions (CO2, CH4, hydrocarbons, etc.) to limit the global temperature rise within 2°C above pre-industrial levels. 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. Many corporations are increasingly making commitments to reduce carbon emissions as progressive steps toward corporate sustainability and compliance with any regulations that may be forthcoming. Considering the scale of manufacturing across the United States, achieving net-zero industrial GHG emissions while maintaining competitiveness is a grand challenge.
This article – part 1 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.
Process Heating Energy Use in U.S. Manufacturing
In 2018, thermal processes including fuels, on-site steam generation and cogeneration systems that supplied steam and electricity accounted for 67% of total manufacturing energy use in the U.S. Heat used for all thermal processes is supplied by fuel and steam or electricity. In most individual heating processes, however, it is supplied by a single source. In only a few cases the heating system may use two or more sources of heat at the same time.
It is useful to understand the energy-use patterns and the detail of the process heating system before discussing options for decarbonization. The total energy use for process heating varies considerably from industry to industry. Five industries account for more than 80% of all U.S. manufacturing energy consumption: petroleum refining, chemicals, forest products including pulp and paper, iron and steel, and food and beverages. About 36% of process heating energy supplied to furnaces, ovens and other thermal processing equipment is lost as waste heat. A portion of these losses can be recovered and can result in energy savings and reduced GHG emissions.
A review of the types of energy used in manufacturing (Fig. 1) shows that fuels supply 64%, steam supplies 29% and electricity supplies 7% of the total energy used for process heating and electrochemical processes. Steam is used mainly in forest products, chemicals and petroleum refining, while electricity is used mainly for metals (iron and steel). Note that every BTU from steam requires 20-40% higher fuel use in steam-generating equipment (boilers) and 30-50% higher fuel use for electricity generation, whether it is on-site or offsite, except for a small fraction of purchased electricity generated from renewable sources. This indicates that fuel combustion is the primary method of heat supply, and it is the main source of CO2 generation from the process heating equipment.
Fig. 1. Types of energy used for process heating in manufacturing industries
The CO2 emissions are usually reported as CO2 equivalent (CO2e) GHG gases. For process heating systems, they are mainly from combustion of carbonaceous fuels used directly or indirectly by manufacturing industries. In addition to process heating, process cooling and other processes may use steam and electricity generated by using fuels and become a contributor of CO2 emissions.
Thermal energy, either generated by fuel combustion or steam or electricity, is used for a variety of processes in each industry. For example, the iron and steel industry uses fuel and other types of energy for heating of iron ore, iron-making in blast furnaces, steelmaking in BOFs and EAFs, reheating of slabs or ingots, annealing or hardening of rolled sheets or wires, forming and forging of steel parts, heat treatment of finished parts, etc.
Many of these processes have common characteristics, and they can be divided into a few broad categories. These process categories, listed alphabetically with temperature range and the industries where they are used, are shown in Table 1. This categorization helps to understand important process requirements and approaches that can be taken to reduce GHG emissions.
Here is a very brief and generalized description of each of the thermal processes listed in Table 1.
Table 1. Types of thermal processes used for eight energy-consuming industries. Colors indicate temperature ranges. Blue = low temperature (<800°F); yellow = medium temperature (800-1400°F); red = high temperature (>1400°F).
This is a heating process for materials such as calcium carbonate (limestone) or certain reforming processes where the feed material breaks down when heated, releasing CO2 or other gases due to chemical decomposition of the base material. These heating processes use mostly direct-fired burners to supply the heat, and the combustion products are mixed with CO2 or other gases before they are discharged into the atmosphere. In a specific case in the cement industry, a series of heat-recovery steps (including drying of the raw “meal” charged into the kiln used for clinker production) are used before the gases are discharged, often with very high moisture content. A similar system is used for the production of lime.
Bonding, Curing and Forming
Heat is supplied to bond, set or cure materials (such as paints or coatings, plastics and rubber) when they are heated within a certain temperature range. The process is carried out at relatively low temperature in the range of 300-600°F. The process may release vapors of organic or inorganic materials, which may need further treatment to meet existing emission regulation requirements. These systems often use forced convection heating, where direct-fired burners supply the process heat and the combustion products are mixed with a large volume of recirculating gases. A large amount of dilution air or inert gases are used to avoid formation of flammable gas mixture within the heating system. The exhaust gases contain high oxygen content, ranging from 14-18%, with a possibility of organic vapors that need to be treated. They are incinerated in thermal oxidizers or other systems before they are discharged into the atmosphere.
This is a process where water or other liquid from the wet material is evaporated by increasing the material temperature. It also includes separation processes where liquids of different boiling points are separated by selective distillation. Examples include drying of grains, paper, metallic ores and textiles, and distillation of organic materials, crude oil and chemicals. A variety of equipment is used for drying the materials. Dryers use recirculated gases such as air or a mixture of air and combustion products to supply heat to the material. The recirculated air is heated directly mixing with combustion products from fuel-fired (usually natural gas) burners or in a heat exchanger where steam or hot combustion products are used as a heat source or by supplying heat from electrical heating elements. The dryer exhaust gases contain combustion products and part of the recirculated air to maintain the required humidity or vapor pressure level in the dryer. In addition to fuel fired systems, some specialized drying systems use electro-technologies such as ultrasonic drying, radio frequency (RF) or microwave drying. In thermal separation systems, vapors from a mixture of liquids are selectively separated (fractional distillation) using steam or heat from other sources. A drying process is used in all industries in one form or another.
Heating of a fluid (liquid and gas) to raise its temperature without phase change or breakdown of the base material is one of the most common requirements in many industries. Gases such as air are heated by directly mixing with combustion products or in a heat exchanger, where hot gas or steam is used as the source of heat. Other types of liquid are heated in a fired heater or a liquid boiler, where heat is supplied by fired burn-ers. In some cases, steam or electricity is used to heat specialized liquids. Fluid heating is carried out over a broad temperature range from less than 200°F to as high as 800°F in equipment such as air heaters, crude oil heaters and hot-water boilers. Exhaust gases from these heating systems include clean combustion products and rarely need any type of flue-gas treatment.
Heat Treatment of Metals, Nonmetals
This process heats a ferrous or nonferrous material to a temperature where its grain structure or composition changes, resulting in changes in its physical properties. Examples include carburizing (hardening) or annealing (softening) of steel parts and annealing and tempering of glass. Metal heat-treatment furnaces can be direct-fired (steel temper-ing) or indirectly heated using radiant tubes or muffles. The most widely used processes require a protective atmosphere, which is generated in an atmosphere generator using natural gas or ammonia as base material. Vacuum heat treatment of metals uses elec-tricity to supply the heat. Nonmetal heat treatment includes annealing and tempering of glass, where glass is heated to a certain temperature and cooled at a controlled cooling rate to achieve the desired physical properties. These furnaces are direct fuel-fired or use electricity to supply the heat.
Reheating of Metals, Nonmetals
This process heats a material (metal or nonmetal) to raise its temperature without its melting, breakdown or changes in its metallurgical properties. Examples include reheating of steel or aluminum ingots, billets or slabs. Equipment used for metal reheating vary from small slot forging furnaces to very large steel reheating furnaces. Metal heating furnaces use direct-fired burners, and the exhaust gases are directly discharged into the atmosphere. Nonmetal heating is used in many industries for processes such as glass heating for decorative glassware, ceramicware and bricks. These furnaces also use direct-fired burners or electric heat. In most cases, they do not require flue-gas treatment prior to discharging into the atmosphere.
Melting of Metals, Nonmetals
This process heats a material or a mixture of materials to high enough temperature to melt them and produce molten material that is cast in final end product. Examples include melting to produce glass, production of iron in blast furnaces, melting of scrap in electricarc furnaces (EAFs) or simply melting of metal/nonmetal ingots or other shapes to produce castings or other products. Each of these processes use a different method for supplying heat. Glass melting furnaces use direct-fired burners and electricity in some cases. Iron production requires the use of coke for reduction of iron ore in a blast furnace and to supply the necessary heat. Iron or steel melting can be carried out in an induction furnace or in an EAF, which uses electricity and some fossil fuels. Nonmetal melting us-es a variety of heating methods using fossil fuels and electricity. The temperature and nature of exhaust gases discharged from each of these processes vary considerably and need some type of flue-gas treatment before the gases are discharged into the atmos-phere.
Other Heating Processes
This includes heating processes not covered in the list in Table 1. A few examples that may fall into this category include refractory heating, ladle and tundish heating and welding of materials. Most of them use fuel-fired (natural gas) burners and discharge exhaust gases directly into the atmosphere.
Reactive Thermal Processing
This process heats materials, usually a mixture of more than one material, to promote a chemical reaction with or without the presence of a catalyst or other method of affecting (accelerating) a chemical reaction. These processes are used in industries such as chemical, petroleum refining, steel and aluminum and forest products. The heating method for this type of process depends on the process characteristics, production capacity and type of materials produced. For example, natural gas reformers use indirect heating from fuel-fired burners, while metal-ore reduction processes use air or oxygen supply and carbonaceous materials to produce heat within the reactor. The exhaust gases from these systems could be clean combustion products for indirect-heated systems or a mixture of combustible and noncombustible gases together with particulates that can be used as fuel in other processes and ultimately need to be treated before discharge into the atmosphere.
This process heats water at high pressure to produce steam in a boiler. The term boiler is used as a generic heating system used for heating or vaporizing a variety of liquids. Steam is required in almost all industries and used as a source of heat for production of mechanical or electrical power or as a feedstock in many chemical processes. In some cases, other liquids are heated to produce liquid vapors used in processes. Boiler capacity is reported as pounds of steam produced per hour or boiler horsepower (BHP), which represents 20,000 pounds/hour steam production capacity per BHP. Almost all boilers are fuel-fired and may use biofuels, coal and byproduct materials. The composition of exhaust gases from a boiler depends on the type of fuel used and may need further treatment if they contain pollutants such as SO2, soot, ash or other types of particles.
These process categories cover almost all thermal processes used in manufacturing plants. It should be noted that these categories are not compartmentalized. In some cases, they overlap. For example, reduction of iron ore to produce liquid iron includes heating, thermal reactions and melting. Certain petroleum refining and chemical processes may include fluid heating, thermal chemical reactions and even melting. This classification of thermal processes is very useful in identifying and deploying one or more decarburizing strategies. Not all processes are used in each plant. Some industries may use only drying, melting and annealing processes, while an integrated steel plant may use almost all of the processes listed in Table 1. Table 2 gives a list of thermal process-es under each applicable category for the iron and steel industry.
*This article will continue with Part 2 in February 2023.
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 email@example.com.
All graphics provided by E3M Inc. except where noted.