Process heating systems deliver heat at the temperatures needed to transform materials into manufactured goods. They fall into three categories:
Fuel-fired systems can be direct or indirect. In direct fuel-fired systems, burners with open flames place hot combustion gases directly in contact with the material. In indirect fuel-fired systems, flames are enclosed in a sealed chamber with the heated gas running through tubes or panels which radiate the heat to the material, keeping the exhaust gases and material separate.
These systems are used in high-temperature processes such as calcining, a critical step in cement making. Direct-fired systems are typically more efficient, have greater temperature control, suitable for smaller equipment, and can be used when fumes containing exhaust gases, water vapor, and particulates (soot) are not detrimental to the material being processed.
Steam-based systems can efficiently transfer large quantities of latent heat at a constant temperature, which is helpful for many low-temperature processes (less than 212°F or 100°C). The heat from steam is transferred indirectly through a container wall (such as rollers used in papermaking) or injected directly into water or a product to rapidly transfer heat (such as cooking food).
Steam is used in paper products, chemicals, petroleum refining, food processing, and many other manufacturing industries, accounting for about 30% of all process heat, according to the 2018 Manufacturing Energy and Consumption Survey (2018 MECS). Most steam is generated through the combustion of fuels, although electric boilers are also used to take advantage of low electricity prices.
Electricity-based systems use electricity to power technologies that apply heat directly to a material or run an electric current to heat a material. For example, resistive electric heating elements take advantage of a material’s intrinsic electrical resistivity to convert electricity into heat, which is applied to a target material (similar to an electric coil on a stovetop).
Alternatives to resistive heating include: infrared ovens that use high intensity lighting to transfer energy to the product without heating the air; induction heats that use an electric current to create a magnetic field that surrounds and heats conductive materials (metals and semi-conductors) in a container; and other electromagnetic heating mechanisms. Electric systems account for less than 5% of industrial process heating today. Eliminating industrial GHG emissions will involve identifying opportunities to closely couple the deployment of low-carbon sources of electricity with increased build out of electricity-based process heating systems.