Process heat: Industrial applications and decarbonization

Overview

Process heat is defined as the application of thermal energy during various industrial processes, serving as a fundamental component in the manufacturing of many common products. This concept encompasses the thermal requirements necessary to transform raw materials into finished goods across multiple sectors, including the production of concrete, glass, steel, and paper. The integration of process heat is critical for achieving the desired physical and chemical properties of these materials, ensuring quality and efficiency in industrial output.

Industrial Applications

The manufacture of concrete, glass, steel, and paper each relies on specific thermal profiles to achieve optimal results. In the production of concrete, heat is often utilized in the curing process, where temperature control affects the hydration of cement and the subsequent strength development of the final product. For glass manufacturing, high-temperature process heat is essential for melting silica sand and other raw materials, allowing for the formation of a homogeneous molten mass that can be shaped and cooled into various glass products. Steel production involves significant thermal energy inputs, particularly in the reduction of iron ore and the refining of molten steel, where precise temperature management influences the metallurgical structure and mechanical properties of the final steel. Similarly, paper manufacturing utilizes process heat in stages such as drying, where moisture is removed from the paper web to achieve the desired thickness and texture.

Utilization of Byproducts

Where byproducts or wastes of the overall industrial process are available, those are often used to provide process heat, enhancing energy efficiency and reducing reliance on primary fuel sources. Examples include black liquor in papermaking or bagasse in sugarcane processing. Black liquor, a byproduct of the pulping process in paper manufacturing, is rich in organic compounds and inorganic chemicals, making it a valuable fuel source for generating steam and electricity within the mill. Bagasse, the fibrous residue left after extracting juice from sugarcane, is commonly burned in boilers to produce steam for process heat and power generation in sugar refineries. These practices not only optimize energy usage but also contribute to the sustainability of industrial operations by minimizing waste and lowering carbon emissions.

What are the temperature requirements for industrial process heat?

Industrial process heat requirements vary significantly across sectors, defined by three primary temperature grades: low, medium, and high. These classifications determine the suitability of different thermal technologies, including combustion, electric resistance, and thermodynamic cycles. Understanding these thermal thresholds is critical for optimizing energy efficiency and selecting appropriate heat sources for specific manufacturing stages.

Temperature Grades and Applications

Low-temperature process heat typically operates below 150 °C. This range is common in the food and beverage industry for pasteurization, drying, and evaporation, as well as in textiles for ironing and drying. Medium-temperature heat spans from 150 °C to 400 °C. This grade is essential for chemical processing, such as distillation and crystallization, and in the metal industry for annealing and tempering. High-temperature process heat exceeds 400 °C and is dominant in energy-intensive sectors like steelmaking, cement production, and glass manufacturing. In these applications, heat is used for melting, sintering, and calcination, often requiring temperatures above 600 °C.

The 400 °C Threshold

The 400 °C mark represents a critical technological dividing line in industrial decarbonization. Below this threshold, thermal energy can often be supplied by electric heat pumps or medium-temperature steam systems. Above 400 °C, the efficiency of conventional heat pumps drops significantly, making direct electric resistance heating or high-temperature steam more viable. This threshold influences the choice between fossil fuel combustion and electrification strategies. Industries operating above 400 °C, such as steel and cement, face greater challenges in replacing natural gas and coal with electricity due to the higher exergy requirements.

Limitations of Heat Pumps

Heat pumps are highly efficient for low-temperature applications, leveraging the Carnot efficiency principle. The coefficient of performance (COP) is defined as COP=Thot​/(Thot​−Tcold​), where temperatures are in Kelvin. As the temperature lift (Thot​−Tcold​) increases, the COP decreases. Below 100 °C, heat pumps can achieve COPs of 3 to 5, meaning three to five units of heat are delivered per unit of electricity consumed. However, above 100 °C, the efficiency gains diminish, and the capital cost of high-temperature heat pumps increases. This limits their widespread adoption in medium-temperature processes, where electric boilers or thermal storage may be more economical.

Boiling and distillation processes often require precise temperature control within the low to medium range. Distillation columns, for instance, rely on latent heat of vaporization, making them sensitive to temperature fluctuations. Annealing metals requires slow heating and cooling cycles to relieve internal stresses, typically occurring between 600 °C and 800 °C, depending on the alloy. These diverse requirements underscore the need for flexible heat supply systems that can adapt to varying thermal demands across different industrial sectors.

How is process heat currently generated?

Process heat is generated through the application of thermal energy during industrial manufacturing processes, which are essential for producing common materials such as concrete, glass, steel, and paper. When byproducts or wastes from the overall industrial process are available, they are frequently utilized to provide the necessary process heat. While the provided grounding highlights these organic byproducts, current industrial practice also relies heavily on fossil fuels, including natural gas and coal, to generate the required thermal energy. These fuel sources are burned to produce heat, which is then transferred to the industrial process through various mechanisms.

Efficiency of Resistive Heating

The efficiency of heat generation methods varies significantly depending on the source and the mechanism of transfer. Resistive heating, which involves passing an electric current through a resistive element to generate heat, is generally less efficient than the direct use of fuel for process heat. This is because electricity is a secondary energy source, meaning it has already undergone conversion losses during generation and transmission. When electricity is used for resistive heating, the efficiency is limited by the thermodynamic limits of the power plant that generated the electricity, as well as transmission losses. In contrast, direct fuel use, such as burning natural gas or coal on-site, can achieve higher overall thermal efficiency because the heat is generated closer to the point of use, reducing intermediate conversion losses.

However, there is an exception to this general rule: hydropower. Hydropower is a renewable energy source that converts the potential energy of water into electricity with relatively high efficiency. When hydropower is used for resistive heating, the overall efficiency can be competitive with direct fuel use, particularly in regions with abundant hydroelectric resources. This is because the conversion process in hydropower plants is more efficient than in many fossil fuel power plants, and the transmission losses are often lower due to the proximity of hydroelectric dams to industrial centers. Therefore, while resistive heating is generally less efficient than direct fuel use, it can be a viable option when powered by hydropower.

The efficiency of resistive heating can be expressed using the following formula: ηresistive​=Ein​Qout​​, where Qout​ is the heat output and Ein​ is the electrical energy input. In contrast, the efficiency of direct fuel use is given by ηdirect​=Fin​Qout​​, where Fin​ is the fuel energy input. Comparing these two efficiencies helps to determine the most cost-effective and energy-efficient method for generating process heat in a given industrial setting.

What are the challenges in decarbonizing process heat?

Decarbonizing process heat presents distinct challenges compared to electricity generation, primarily due to the vast scale of thermal energy demand across industrial sectors. Process heat is integral to the manufacture of foundational materials, including concrete, glass, steel, and paper. In many cases, industries rely on byproducts or wastes from the overall process to supply this thermal energy. For instance, black liquor is commonly used in papermaking, while bagasse serves as a primary heat source in sugarcane processing. These integrated waste-heat systems complicate decarbonization efforts because they are tightly coupled with specific feedstocks and production cycles.

Energy Use and Emissions Statistics

The energy intensity of industrial processes results in significant global emissions. While specific aggregate statistics on total thermal energy use vary by region and year, the reliance on fossil fuels remains high in sectors where waste byproducts are insufficient to meet total thermal demand. The transition requires replacing these entrenched thermal sources with low-carbon alternatives without disrupting the continuous nature of industrial operations.

Alternative Fuels and Technologies

Several alternative fuels and technologies are being evaluated to reduce the carbon footprint of process heat. Waste tires and biomass are considered potential substitutes for traditional fossil fuels. Biomass, such as bagasse in the sugar industry, offers a relatively direct pathway for decarbonization where the feedstock is abundant. Waste tires can provide high calorific value, though their use requires specific handling and combustion infrastructure to manage emissions effectively.

Geothermal energy and concentrated solar power (CSP) are also explored as sources for high-temperature process heat. However, as of 2024, these technologies face notable limitations. Geothermal resources are often geographically constrained, requiring specific tectonic conditions to achieve the necessary temperatures and flow rates for industrial use. CSP systems, while capable of providing high-temperature heat, depend heavily on direct normal irradiance, making them less versatile in regions with variable solar resources or where space constraints exist. These limitations mean that geothermal and CSP cannot serve as universal solutions for all industrial process heat needs, necessitating a mixed approach that may include biomass, waste fuels, and emerging thermal storage technologies.

How can nuclear power provide high-grade process heat?

Nuclear power can provide high-grade process heat by utilizing reactor types that achieve higher outlet temperatures than standard Light Water Reactors (LWRs). The choice of reactor technology determines the thermodynamic efficiency and suitability for industrial applications.

Light Water Reactors: PWR and BWR

Pressurized Water Reactors (PWRs) and Boiling Water Reactors (BWRs) are the most common nuclear technologies. In a PWR, the primary coolant is kept under high pressure to prevent boiling, typically reaching temperatures around 300–320 °C at the steam generator outlet. BWRs produce steam directly in the core, with saturation temperatures generally ranging from 250–280 °C depending on pressure. These temperatures are suitable for low-to-medium grade process heat, such as district heating or certain chemical processes, but may require heat exchangers for higher-temperature industrial needs.

Advanced Reactor Technologies

For higher-grade process heat, advanced reactor designs offer superior thermal performance. High-Temperature Gas-cooled Reactors (HTGRs) use helium as a primary coolant, allowing outlet temperatures exceeding 500 °C, and potentially up to 750 °C. This high temperature is ideal for hydrogen production via thermochemical cycles and refining processes.

Canada Deuterium Uranium (CANDU) reactors, which use heavy water as both moderator and coolant, can achieve steam temperatures around 300 °C, offering flexibility for integration with industrial heat demands. The UK Advanced Gas-cooled Reactors (AGR) utilize carbon dioxide as a coolant, achieving higher temperatures than LWRs, typically around 300–350 °C, making them suitable for specific industrial applications.

The Chinese HTR-PM (High-Temperature Reactor - Pebble-bed Module) is a modular HTGR design. It features pebble-bed fuel elements and helium cooling, achieving outlet temperatures of approximately 500 °C. This design is specifically targeted at providing high-grade process heat for industries such as steel and chemical manufacturing, demonstrating the potential of next-generation nuclear technology for industrial decarbonization.

What is the role of hydrogen and Power to X in industrial heating?

Hydrogen and Power to X (PtX) fuels represent a critical pathway for decarbonizing industrial process heat, particularly in sectors where electrification via resistive heating is technically or economically challenging. Hydrogen can be produced through steam methane reforming (SMR) or via electrolysis, where water is split into hydrogen and oxygen using electricity. The efficiency of hydrogen production and subsequent utilization varies significantly depending on the primary energy source and conversion technology.

Hydrogen Production and Efficiency

Steam methane reforming (SMR) is currently the dominant method for hydrogen production. In this process, natural gas reacts with high-temperature steam in the presence of a catalyst to produce hydrogen, carbon monoxide, and a relatively small amount of carbon dioxide. The overall reaction can be summarized as:

Power to X Fuels vs. Resistive Heating

Power to X (PtX) involves converting electricity into various energy carriers, such as hydrogen, ammonia, or synthetic hydrocarbons, to store and utilize energy in different sectors. When comparing PtX fuels to direct resistive heating, efficiency losses are a key consideration. Resistive heating converts electricity directly into heat with nearly 100% efficiency at the point of use. However, when electricity is used to produce hydrogen through electrolysis, and that hydrogen is then burned for process heat, the overall round-trip efficiency is lower. This is due to losses in electrolysis, compression, storage, and the final combustion or fuel cell conversion. Despite these losses, PtX fuels offer advantages in terms of energy density and the ability to utilize existing infrastructure, making them suitable for high-temperature process heat applications.

Application in Steelmaking

In steelmaking, process heat is essential for reducing iron ore and refining the molten metal. Traditionally, coal has been the primary fuel source, contributing significantly to CO2 emissions. Hydrogen is emerging as a viable alternative to coal in direct reduced iron (DRI) processes. In this method, hydrogen reacts with iron oxide to produce iron and water vapor, thereby reducing the carbon footprint of steel production.