Process heat

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Process heat refers to the application of heat during industrial processes. [1] Some form of process heat is used during the manufacture of many common products, from concrete to glass to steel to paper. Where byproducts 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 in sugarcane processing.

Contents

Requirements

Process heating: Precious-metal tubing is shown proceeding through a heating process in an industrial oven. HiResBetterCrop.png
Process heating: Precious-metal tubing is shown proceeding through a heating process in an industrial oven.

The required temperature of the process varies widely, with about half the industrial process heat having operating temperatures above 400 °C (752 °F). These higher-temperature processes can generally only be supplied by dedicated supplies like natural gas or coal, although pre-heating from other sources is also common in order to reduce fuel use. Those processes operating below the median can draw on a much wider variety of sources, including waste heat from other processes in the same industrial process. Resistive heating 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) is cheap and plentiful. Heat pumps which are commonly employed for home heating, warm water and other heat applications below 100 °C (212 °F) 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 provide the required process heat by the same electricity that is also needed to keep the endothermic 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 catalysts 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 (c.f. 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, in 2018 process heat accounted for approximately 50% of energy use in the manufacturing sector, as well as 30% of Greenhouse gas emissions. [1] Accordingly, it is the target of significant efforts to introduce new forms of carbon neutral 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. [2] Other potential lower-carbon sources include Biomass, which is already in widespread use in industry, while geothermal, concentrated solar power and nuclear power remain experimental as of 2024.

One problem with using nuclear power for process heat is that pressurized water reactors, commonly used for electric power generation, have an operating temperature well below 400 °C (752 °F) [3] and boiling water reactors work at even lower temperatures, around 285 °C (545 °F). [4] Other reactor designs, particularly high-temperature gas-cooled reactors (HTGR), may be suitable for process heat generation. The Advanced Gas-cooled Reactors, constructed in the United Kingdom, had a high coolant outlet temperature (610 °C) as an explicit design goal for increased thermal efficiency. [5] Of particular interest are small modular reactor designs, which could be built onsite for process heat generation. The Chinese HTR-PM, a 250 MWt Generation IV HTGR, features an outlet temperature of 750 °C (1,380 °F). [6] 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. [7] [8]

A stopgap solution for decarbonization at the price of increased costs (ignoring carbon pricing) and lower round trip efficiency is the replacement of currently used fossil fuels by Power to X derived fuels.[ citation needed ] 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 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 is often proposed as an alternative to current sources of process heat.[ citation needed ] Hydrogen is already in widespread use in industry today but is mostly derived from fossil fuels via processes such as steam reforming 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.[ citation needed ]

Related Research Articles

Syngas, or synthesis gas, is a mixture of hydrogen and carbon monoxide, in various ratios. The gas often contains some carbon dioxide and methane. It is principally used for producing ammonia or methanol. Syngas is combustible and can be used as a fuel. Historically, it has been used as a replacement for gasoline, when gasoline supply has been limited; for example, wood gas was used to power cars in Europe during WWII.

<span class="mw-page-title-main">Power station</span> Facility generating electric power

A power station, also referred to as a power plant and sometimes generating station or generating plant, is an industrial facility for the generation of electric power. Power stations are generally connected to an electrical grid.

<span class="mw-page-title-main">Gasification</span> Form of energy conversion

Gasification is a process that converts biomass- or fossil fuel-based carbonaceous materials into gases, including as the largest fractions: nitrogen (N2), carbon monoxide (CO), hydrogen (H2), and carbon dioxide (CO2). This is achieved by reacting the feedstock material at high temperatures (typically >700 °C), without combustion, via controlling the amount of oxygen and/or steam present in the reaction. The resulting gas mixture is called syngas (from synthesis gas) or producer gas and is itself a fuel due to the flammability of the H2 and CO of which the gas is largely composed. Power can be derived from the subsequent combustion of the resultant gas, and is considered to be a source of renewable energy if the gasified compounds were obtained from biomass feedstock.

<span class="mw-page-title-main">Energy development</span> Methods bringing energy into production

Energy development is the field of activities focused on obtaining sources of energy from natural resources. These activities include the production of renewable, nuclear, and fossil fuel derived sources of energy, and for the recovery and reuse of energy that would otherwise be wasted. Energy conservation and efficiency measures reduce the demand for energy development, and can have benefits to society with improvements to environmental issues.

<span class="mw-page-title-main">Combined cycle power plant</span> Assembly of heat engines that work in tandem from the same source of heat

A combined cycle power plant is an assembly of heat engines that work in tandem from the same source of heat, converting it into mechanical energy. On land, when used to make electricity the most common type is called a combined cycle gas turbine (CCGT) plant, which is a kind of gas-fired power plant. The same principle is also used for marine propulsion, where it is called a combined gas and steam (COGAS) plant. Combining two or more thermodynamic cycles improves overall efficiency, which reduces fuel costs.

<span class="mw-page-title-main">Environmental impact of electricity generation</span>

Electric power systems consist of generation plants of different energy sources, transmission networks, and distribution lines. Each of these components can have environmental impacts at multiple stages of their development and use including in their construction, during the generation of electricity, and in their decommissioning and disposal. These impacts can be split into operational impacts and construction impacts. All forms of electricity generation have some form of environmental impact, but coal-fired power is the dirtiest. This page is organized by energy source and includes impacts such as water usage, emissions, local pollution, and wildlife displacement.

<span class="mw-page-title-main">Alternative fuel</span> Fuels from sources other than fossil fuels

Alternative fuels, also known as non-conventional and advanced fuels, are fuels derived from sources other than petroleum. Alternative fuels include gaseous fossil fuels like propane, natural gas, methane, and ammonia; biofuels like biodiesel, bioalcohol, and refuse-derived fuel; and other renewable fuels like hydrogen and electricity.

<span class="mw-page-title-main">Cogeneration</span> Simultaneous generation of electricity and useful heat

Cogeneration or combined heat and power (CHP) is the use of a heat engine or power station to generate electricity and useful heat at the same time.

<span class="mw-page-title-main">Steam reforming</span> Method for producing hydrogen and carbon monoxide from hydrocarbon fuels

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 hydrogen production. The reaction is represented by this equilibrium:

<span class="mw-page-title-main">Sabatier reaction</span> Methanation process of carbon dioxide with hydrogen

The Sabatier reaction or Sabatier process produces methane and water from a reaction of hydrogen with carbon dioxide at elevated temperatures and pressures in the presence of a nickel catalyst. It was discovered by the French chemists Paul Sabatier and Jean-Baptiste Senderens in 1897. Optionally, ruthenium on alumina makes a more efficient catalyst. It is described by the following exothermic reaction:

<span class="mw-page-title-main">Fossil fuel power station</span> Facility that burns fossil fuels to produce electricity

A fossil fuel power station is a thermal power station which burns a fossil fuel, such as coal, oil, or natural gas, to produce electricity. Fossil fuel power stations have machinery to convert the heat energy of combustion into mechanical energy, which then operates an electrical generator. The prime mover may be a steam turbine, a gas turbine or, in small plants, a reciprocating gas engine. All plants use the energy extracted from the expansion of a hot gas, either steam or combustion gases. Although different energy conversion methods exist, all thermal power station conversion methods have their efficiency limited by the Carnot efficiency and therefore produce waste heat.

<span class="mw-page-title-main">District heating</span> Centralized heat distribution system

District heating is a system for distributing heat generated in a centralized location through a system of insulated pipes for residential and commercial heating requirements such as space heating and water heating. The heat is often obtained from a cogeneration plant burning fossil fuels or biomass, but heat-only boiler stations, geothermal heating, heat pumps and central solar heating are also used, as well as heat waste from factories and nuclear power electricity generation. District heating plants can provide higher efficiencies and better pollution control than localized boilers. According to some research, district heating with combined heat and power (CHPDH) is the cheapest method of cutting carbon emissions, and has one of the lowest carbon footprints of all fossil generation plants.

<span class="mw-page-title-main">Sulfur–iodine cycle</span> Thermochemical process used to produce hydrogen

The sulfur–iodine cycle is a three-step thermochemical cycle used to produce hydrogen.

<span class="mw-page-title-main">Thermal power station</span> Power plant that generates electricity from heat energy

A thermal power station, also known as a thermal power plant, is a type of power station in which the heat energy generated from various fuel sources is converted to electrical energy. The heat from the source is converted into mechanical energy using a thermodynamic power cycle. The most common cycle involves a working fluid heated and boiled under high pressure in a pressure vessel to produce high-pressure steam. This high pressure-steam is then directed to a turbine, where it rotates the turbine's blades. The rotating turbine is mechanically connected to an electric generator which converts rotary motion into electricity. Fuels such as natural gas or oil can also be burnt directly in gas turbines, skipping the steam generation step. These plants can be of the open cycle or the more efficient combined cycle type.

<span class="mw-page-title-main">Supercritical carbon dioxide</span> Carbon dioxide above its critical point

Supercritical carbon dioxide is a fluid state of carbon dioxide where it is held at or above its critical temperature and critical pressure.

<span class="mw-page-title-main">Electric heating</span> Process in which electrical energy is converted to heat

Electric heating is a process in which electrical energy is converted directly to heat energy. Common applications include space heating, cooking, water heating and industrial processes. An electric heater is an electrical device that converts an electric current into heat. The heating element inside every electric heater is an electrical resistor, and works on the principle of Joule heating: an electric current passing through a resistor will convert that electrical energy into heat energy. Most modern electric heating devices use nichrome wire as the active element; the heating element, depicted on the right, uses nichrome wire supported by ceramic insulators.

<span class="mw-page-title-main">Steam-electric power station</span> Power station whose electric generator is steam-driven

A steam-electric power station is a power station in which the electric generator is steam-driven: water is heated, evaporates, and spins a steam turbine which drives an electric generator. After it passes through the turbine, the steam is condensed in a condenser. The greatest variation in the design of steam-electric power plants is due to the different fuel sources.

Hydrogen gas is produced by several industrial methods. Nearly all of the world's current supply of hydrogen is created from fossil fuels. Most hydrogen is gray hydrogen made through steam methane reforming. In this process, hydrogen is produced from a chemical reaction between steam and methane, the main component of natural gas. Producing one tonne of hydrogen through this process emits 6.6–9.3 tonnes of carbon dioxide. When carbon capture and storage is used to remove a large fraction of these emissions, the product is known as blue hydrogen.

<span class="mw-page-title-main">Energy recovery</span>

Energy recovery includes any technique or method of minimizing the input of energy to an overall system by the exchange of energy from one sub-system of the overall system with another. The energy can be in any form in either subsystem, but most energy recovery systems exchange thermal energy in either sensible or latent form.

The Glossary of fuel cell terms lists the definitions of many terms used within the fuel cell industry. The terms in this fuel cell glossary may be used by fuel cell industry associations, in education material and fuel cell codes and standards to name but a few.

References

  1. 1 2 "Process Heat Basics". United States Department of Energy Industrial Efficiency & Decarbonization Office.
  2. "Von Altreifen zum Ersatzbrennstoff". 2 June 2022.
  3. "Pressurized Water Reactor (PWR) Systems" (PDF), Reactor Concepts Manual, USNRC Technical Training Center, archived from the original (PDF) on 2022-08-12
  4. "Boiling water reactor - Energy Education".
  5. "Advanced Gas Reactor - an overview | ScienceDirect Topics".
  6. Jaradat, Mustafa; Schunert, Sebastian; Ortensi, Javier (April 2023), Gas-Cooled High-Temperature Pebble-Bed Reactor Reference Plant Model (PDF), Idaho National Laboratory
  7. "Geothermal Source Temperature - an overview | ScienceDirect Topics".
  8. Finger, John; Blankenship, Doug (December 2010). Handbook of Best Practices for Geothermal Drilling (PDF) (Report). Sandia National Laboratories. Archived (PDF) from the original on 2022-04-18.

Further reading