Process heat refers to the application of

heat In thermodynamics, heat is energy in transfer between a thermodynamic system and its surroundings by such mechanisms as thermal conduction, electromagnetic radiation, and friction, which are microscopic in nature, involving sub-atomic, ato ...

during

industrial processes Industrial processes are procedures involving chemical, physical, electrical, or mechanical steps to aid in the manufacturing of an item or items, usually carried out on a very large scale. Industrial processes are the key components of heavy ...

. Some form of process heat is used during the manufacture of many common products, from

concrete Concrete is a composite material composed of aggregate bound together with a fluid cement that cures to a solid over time. It is the second-most-used substance (after water), the most–widely used building material, and the most-manufactur ...

glass Glass is an amorphous (non-crystalline solid, non-crystalline) solid. Because it is often transparency and translucency, transparent and chemically inert, glass has found widespread practical, technological, and decorative use in window pane ...

steel Steel is an alloy of iron and carbon that demonstrates improved mechanical properties compared to the pure form of iron. Due to steel's high Young's modulus, elastic modulus, Yield (engineering), yield strength, Fracture, fracture strength a ...

paper Paper is a thin sheet material produced by mechanically or chemically processing cellulose fibres derived from wood, Textile, rags, poaceae, grasses, Feces#Other uses, herbivore dung, or other vegetable sources in water. Once the water is dra ...

. Where

byproduct A by-product or byproduct is a secondary product derived from a production process, manufacturing process or chemical reaction; it is not the primary product or service being produced. A by-product can be useful and marketable or it can be cons ...

s or wastes of the overall industrial process are available, those are often used to provide process heat. Examples include black liquor in papermaking or

bagasse Bagasse ( ) is the dry pulpy fibrous material that remains after crushing sugarcane or sorghum stalks to extract their juice. It is used as a biofuel for the production of heat, energy, and electricity, and in the manufacture of pulp and building ...

in sugarcane processing.

Requirements

The required

temperature Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...

of the process varies widely, with about half the industrial process heat having operating temperatures above . These higher-temperature processes can generally only be supplied by dedicated supplies like

natural gas Natural gas (also fossil gas, methane gas, and gas) is a naturally occurring compound of gaseous hydrocarbons, primarily methane (95%), small amounts of higher alkanes, and traces of carbon dioxide and nitrogen, hydrogen sulfide and helium ...

coal Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other Chemical element, elements, chiefly hydrogen, sulfur, oxygen, and nitrogen. Coal i ...

, although pre-heating from other sources is also common in order to reduce fuel use. Those processes operating below the

median The median of a set of numbers is the value separating the higher half from the lower half of a Sample (statistics), data sample, a statistical population, population, or a probability distribution. For a data set, it may be thought of as the “ ...

can draw on a much wider variety of sources, including

waste heat Waste heat is heat that is produced by a machine, or other process that uses energy, as a byproduct of doing work. All such processes give off some waste heat as a fundamental result of the laws of thermodynamics. Waste heat has lower utility ...

from other processes in the same industrial process.

Resistive heating Joule heating (also known as resistive heating, resistance heating, or Ohmic heating) is the process by which the passage of an electric current through a conductor produces heat. Joule's first law (also just Joule's law), also known in countr ...

would in theory be a possible source of process heat but even as it converts nearly 100% of the supplied electricity to heat, it is obviously less efficient to burn a fuel in a thermal power plant to produce electricity only to use that electricity for process heat than to use the fuel directly. Thus this source of heat is only used where electricity from non-thermal sources (such as

hydropower Hydropower (from Ancient Greek -, "water"), also known as water power or water energy, is the use of falling or fast-running water to Electricity generation, produce electricity or to power machines. This is achieved by energy transformation, ...

) is cheap and plentiful.

Heat pump A heat pump is a device that uses electricity to transfer heat from a colder place to a warmer place. Specifically, the heat pump transfers thermal energy using a heat pump and refrigeration cycle, cooling the cool space and warming the warm s ...

s which are commonly employed for home heating, warm water and other heat applications below have too low a Carnot efficiency at high temperature differences between "hot" and "cold" end to be worthwhile. Some processes such as molten salt

electrolysis In chemistry and manufacturing, electrolysis is a technique that uses Direct current, direct electric current (DC) to drive an otherwise non-spontaneous chemical reaction. Electrolysis is commercially important as a stage in the separation of c ...

provide the required process heat by the same electricity that is also needed to keep the

endothermic An endothermic process is a chemical or physical process that absorbs heat from its surroundings. In terms of thermodynamics, it is a thermodynamic process with an increase in the enthalpy (or internal energy ) of the system.Oxtoby, D. W; Gillis, ...

reaction going. Heat is usually described by "grade" with higher temperatures having a higher "grade". This is because heat naturally flows from hot to cold and it is thus always possible to use a high temperature source of heat for lower temperature applications but not vice versa. As higher grade heat is more cumbersome and expensive to produce and as materials have limited heat resistance, there are efforts to reduce working temperatures wherever possible through the use of

catalyst Catalysis () is the increase in rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quick ...

s and fluxes. In equilibrium reactions where temperature is one of the factors influencing the equilibrium, temperature requirements can be reduced by removing the desired products in a continuous process. For example, if an equilibrium reaction between AB and CD produces AC and BD and the equilibrium can be shifted rightward by increasing temperature, continuously removing AC or BD from the reaction can serve to reduce the temperature requirements (cf. principle of Le Chatelier). However, there are limits to this as the speed of reaction is also temperature-dependent. Catalysts can serve to increase the speed of reaction at any given temperature but they, by definition, do not shift the equilibrium.

Decarbonization

According to the

United States Department of Energy The United States Department of Energy (DOE) is an executive department of the U.S. federal government that oversees U.S. national energy policy and energy production, the research and development of nuclear power, the military's nuclear w ...

, in 2018 process heat accounted for approximately 50% of energy use in the manufacturing sector, as well as 30% of

Greenhouse gas emissions Greenhouse gas (GHG) emissions from human activities intensify the greenhouse effect. This contributes to climate change. Carbon dioxide (), from burning fossil fuels such as coal, petroleum, oil, and natural gas, is the main cause of climate chan ...

. Accordingly, it is the target of significant efforts to introduce new forms of

carbon neutral Global net-zero emissions is reached when greenhouse gas emissions and Greenhouse gas removal, removals due to human activities are in balance. It is often called simply net zero. ''Emissions'' can refer to all greenhouse gases or only carbon diox ...

or at least lower carbon process heat supplies. Some wastes - including waste tires - are commonly used as replacement fuels or mixed into conventional fuel at appropriate ratios. Other potential lower-carbon sources include

Biomass Biomass is a term used in several contexts: in the context of ecology it means living organisms, and in the context of bioenergy it means matter from recently living (but now dead) organisms. In the latter context, there are variations in how ...

, which is already in widespread use in industry, while geothermal,

concentrated solar power Concentrated solar power (CSP, also known as concentrating solar power, concentrated solar thermal) systems generate solar power by using mirrors or lenses to concentrate a large area of sunlight into a receiver. Electricity is generated whe ...

and

nuclear power Nuclear power is the use of nuclear reactions to produce electricity. Nuclear power can be obtained from nuclear fission, nuclear decay and nuclear fusion reactions. Presently, the vast majority of electricity from nuclear power is produced by ...

remain experimental as of 2024. One problem with using nuclear power for process heat is that

pressurized water reactor A pressurized water reactor (PWR) is a type of light-water nuclear reactor. PWRs constitute the large majority of the world's nuclear power plants (with notable exceptions being the UK, Japan, India and Canada). In a PWR, water is used both as ...

s, commonly used for electric power generation, have an operating temperature well below and

boiling water reactor A boiling water reactor (BWR) is a type of nuclear reactor used for the generation of electrical power. It is the second most common type of electricity-generating nuclear reactor after the pressurized water reactor (PWR). BWR are thermal neutro ...

s work at even lower temperatures, around . Other reactor designs, particularly high-temperature gas-cooled reactors (HTGR), may be suitable for process heat generation. The

Advanced Gas-cooled Reactor The advanced gas-cooled reactor (AGR) is a type of nuclear reactor designed and operated in the United Kingdom. These are the generation II reactor, second generation of British gas-cooled reactors, using Nuclear graphite, graphite as the neutron ...

s, constructed in the

United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Northwestern Europe, off the coast of European mainland, the continental mainland. It comprises England, Scotlan ...

, had a high coolant outlet temperature (610 °C) as an explicit design goal for increased thermal efficiency. Of particular interest are

small modular reactor The small modular reactor (SMR) is a class of small nuclear fission reactor, designed to be built in a factory, shipped to operational sites for installation, and then used to power buildings or other commercial operations. The term SMR refers t ...

designs, which could be built onsite for process heat generation. The Chinese HTR-PM, a 250 MW_t Generation IV HTGR, features an outlet temperature of . As of 2024, it is the only high-temperature small modular reactor currently in operation. Likewise, geothermal heat sources often have relatively low temperatures, sometimes even requiring binary cycles for electricity generation. A stopgap solution for decarbonization at the price of increased costs (ignoring

carbon pricing Carbon pricing (or pricing) is a method for governments to Climate change mitigation, mitigate climate change, in which a monetary cost is applied to greenhouse gas emissions. This is done to encourage polluters to reduce fossil fuel combustion, ...

) and lower round trip efficiency is the replacement of currently used

fossil fuels A fossil fuel is a flammable carbon compound- or hydrocarbon-containing material formed naturally in the Earth's crust from the buried remains of prehistoric organisms (animals, plants or microplanktons), a process that occurs within geologica ...

by Power to X derived fuels. While this approach has the advantage of being usable with existing technology with minimal or no modification, it is less efficient than even resistive heating as the chemical processes required to turn electric energy into artificial fuels are less efficient than resistive heating. In processes where the fuel provides both heat and a chemical function (e.g. coke as a

reducing agent In chemistry, a reducing agent (also known as a reductant, reducer, or electron donor) is a chemical species that "donates" an electron to an (called the , , , or ). Examples of substances that are common reducing agents include hydrogen, carbon ...

in steelmaking) a power-to-x fuel may however be the only feasible low carbon alternative for some time to come. Hydrogen derived via processes such as

electrolysis of water Electrolysis of water is using electricity to Water splitting, split water into oxygen () and hydrogen () gas by electrolysis. Hydrogen gas released in this way can be used as hydrogen fuel, but must be kept apart from the oxygen as the mixture ...

is often proposed as an alternative to current sources of process heat. Hydrogen is already in widespread use in industry today but is mostly derived from fossil fuels via processes such as

steam reforming Steam reforming or steam methane reforming (SMR) is a method for producing syngas (hydrogen and carbon monoxide) by reaction of hydrocarbons with water. Commonly, natural gas is the feedstock. The main purpose of this technology is often hydrogen ...

as of 2022. As some proposed processes for hydrogen production like the sulfur-iodine cycle themselves require high temperatures, their feasibility for generating hydrogen as a fuel for process heat as opposed to the direct use of the heat needed for the process seems questionable.

Requirements

Decarbonization

References

Further reading