Zero Emission Heat Technologies for Industry
Heat for the iron and steel industry contributes the most GHG emissions
Industrial heat, often operating behind the scenes of our modern society, plays a pivotal role in our everyday life. From the food we eat to the buildings we live in, our everyday life would look very different without industrial heat. But with this utility comes a sizeable emissions footprint.
The Problem in Context
Industrial heat, which refers to the energy used in production processes like melting metals or producing steam for factories, makes up about 29% of global energy consumption. Now, if we factor in non-process heat — that's the energy used for things like space heating in factories or warming water in commercial laundries — this number jumps to 35%. Combined, they account for about 15-20% of global GHG emissions. To give you some perspective, that's on par with the emissions churned out by the entire transportation sector. Breaking down the contributors, the iron and steel industry is the largest emitter, followed by the chemicals and plastics industry, and then cement.
Industrial heat accounts for 29% of global energy consumption (source)
What is Industrial Heat?
Many industries need heat for thermal manufacturing processes. These include sectors like iron, steel, cement, chemicals, food, beverages, paper, to name just a few. The type of heat these industries require can be broken down into three primary classifications: low, medium, and high temperature.
Low-Temperature Heat (<150 C)
At the coolest end of the scale, low temperature heat is everything up to around 150 degrees Celsius. These kinds of temperatures are needed by processes primarily within the chemical, food and beverage and paper industries, for processes like drying, evaporation, pasteurization, baking, and sterilization.
Medium-Temperature Heat (150-400 C)
A notch higher, the medium-temperature range of 150 to 400 degrees Celsius is vital to other chemical processes, such as more advanced separations and reactions. This range is also used by many refining processes, like in biofuel production where distillation is used to separate impurities. And, in the textile sector, heat treatments for dyeing and finishing, as well as in specific drying processes, often fall within this range.
High-Temperature Heat (>400 C)
Above 400 degrees Celsius is where things intensify. In the high-temperature heat domain, we find the backbone of heavy industries such as iron, steel, aluminium, and cement production, along with the conversion of hydrocarbons into plastics and other petrochemicals, and high-temperature hydrogen electrolysis. For example, in the steel industry, blast furnaces that transform iron ore into molten iron operate at temperatures exceeding 1650°C. Similarly, the cement industry requires extreme temperatures for the process that converts limestone and other materials into clinker, a primary component of cement. These processes require not just the attainment of high temperatures but also intense, sustained heat to drive the necessary chemical reactions and transformations. When analysing the heat used in industry based on temperature range, the energy demand is almost evenly divided between low-plus-medium and high temperatures.
So, how is this heat currently produced? The answer, as of 2018, is that over 90% comes from fossil fuels. Coal was the largest fuel source for industrial heat, supplying 41%. This was followed by gas at 23%, and oil at 11%. Renewables, despite their increasing importance in other sectors, contributed only 9% to the industrial heat source in 2018, with the majority coming from bioenergy sources such as biomass and waste. The present state of industrial heat is predominantly powered by fossil fuels, but this does not have to remain the case.
Zero-Emissions Solutions for Industrial Heat
There are a range of zero emissions technologies available for heating, and their difficulty and cost vary roughly in line with the temperature needed.
One of the most fundamental approaches to heating doesn't involve generating heat at all, but rather extracting it directly from the earth. In geographically suitable areas, direct utilization of geothermal resources is a viable option. This method is typically employed for lower temperature processes, such as drying fish, aquaculture, and heating greenhouses.
Heat pumps are known for their ability to heat homes to a comfortable temperature, but you may not be aware that the technology has advanced a lot in recent years, and they are now supplying a lot of industrial heat. The latest heat pump technologies have surpassed the traditional limits, with recent prototypes achieving temperatures as high as 180 degrees Celsius, a significant increase from the previous maximum of around 130 degrees Celsius.
The technology is a little more advanced than heat pump you’re probably familiar with in your home, but the principle is the same. A heat pump operates based on the principle of refrigeration: it exploits the phase changes of a refrigerant, compressing it to release heat at one end (the condenser) and expanding it to absorb heat at the other end (the evaporator). The house I live in has both gas heating and an air conditioner. We use the air conditioner as a heater in winter, because it’s cheaper than turning on the ducted gas heating. Not to mention lower greenhouse gas emissions. Similarly, many industries are now transitioning from traditional fossil fuel-based heating solutions to heat pumps, attracted by the potential for significant cost savings and reductions in greenhouse gas emissions.
Heat pump operating principle
Many industries need hot water at temperatures that can easily be provided by heat pumps: food and beverage companies are using heat pumps to heat water for food processing, such as canning and cooking. Hot water from heat pumps is also being used in the textile industry for dyeing and finishing. These are just a couple of examples of the many industries that are moving from fossil fuel heat to heat pumps, and in every case, there is money to be saved. As the technology continues to improve and the cost of heat pumps continues to fall, we can expect to see even more industries making this switch in the future.
Heat pumps are widely recognized for their ability to supply low-temperature heat. While they are improving continuously and are starting to nudge the lower end of medium temperature heat, it's unlikely that they will extend beyond these ranges anytime soon. A key advantage of heat pumps is their incredible efficiency. For example, a heat pump used for home heating typically has a coefficient of performance (COP) of about 4, meaning that for every 1 kWh of electricity input, it can produce 4 kWh of heat. However, as the required temperature increases, the COP tends to decrease. At really high temperatures, the COP approaches closer to 1, where the input of 1 kWh of electricity results in just over 1 kWh of heat output.
At that point, there are other simpler and cheaper ways to get the higher temperatures, for example biomass. There are numerous examples of industry using biomass for heat, especially in paper processing or other industries that generate a lot of waste biomass that needs to be got rid of somehow.
Solar thermal is one option for high temperature heat. In the recent Engineering with Rosie video on cement I mentioned a couple of concentrating solar thermal companies, Heliogen in the US and SOLPART in France, who are attempting to get the energy for kilns directly from concentrated solar sunlight. Both have reported achieving temperatures exceeding 1000 degrees Celsius so far.
Perhaps simplest heating method of all is also one of the most versatile. Electric resistance heating is probably one you have in your home in the form of a kettle or electric hot water system, where an element is inserted directly into water and heats up when a current is applied to it. It can be used to heat other materials too. Electric resistance heating can easily reach temperatures of around 800 degrees Celsius. Above that it’s a little trickier simply because there aren’t many materials that don’t melt in that kind of heat. So, you need specialist materials and heating elements made of iron-chromium-aluminium alloy, molybdenum, carbon, silicon carbide or others.
For higher temperatures, technologies include electric arc furnaces, which are commonly used in the steelmaking industry for recycling purposes. They work by channelling electrical energy directly into heat through an electrical arc. This high-intensity heat can then be used to melt scrap steel, converting it into high-quality steel.
You might be familiar with induction, perhaps even having it in your kitchen. But its application goes beyond just cooking. In industry, induction can be harnessed to heat and melt metals. This can be either for direct metal processing or to use molten metals as a reagent, catalyst, or even as a heat transfer medium. Microwaves, while a common household item, have some high-temperature industrial applications too and one possibility currently being explored is to provide heat needed in mineral processing. Lastly, there's plasma – an incredibly versatile high-temperature technology. Beyond its prevalent use in high-tech applications like coatings, plasma finds its place in converting municipal solid waste into syngas, albeit with some reservations due to the presence of fossil and chlorinated compounds.
Challenges
In certain industrial applications, the challenge lies not only in achieving the required temperature but also in the quality of heat delivered. Heat can be transferred in three main ways: conduction, convection, and radiation. Each of these modes plays a unique role in industrial processes, with radiation being the most efficient, followed by convection, and conduction being the least efficient.
In the making of cement, one of the most critical steps is called 'clinkering.' Here, materials like calcium oxide mix with silica, alumina, and iron to form what's known as clinker minerals, the key ingredients that hold cement together. This reaction needs a very high temperature of around 1450°C.
In traditional cement kilns, this heat is provided by a flame that produces intense visible and infrared light. The kiln is a large, rotating tube lined with heat-resistant material. It's vital to use energy efficiently here, ensuring as much heat as possible goes into making the clinker and as little as possible is lost. The flame's proximity to the materials allows it to employ all three heat transfer modes - radiation, conduction, and convection, with radiation being the predominant and most effective method.
However, if we want to use electric resistance heating with the heat source outside the kiln, the scenario changes significantly. The absence of direct flame means losing the advantage of radiative heating. The heat must travel through the kiln's layers to reach the materials, mainly by conduction - the least efficient mode of heat transfer. This method would require the heating elements reach even higher temperatures than the process itself needs, leading to increased energy use and the challenge of finding materials that can withstand these extreme temperatures. These difficulties explain why, despite electric resistance elements being capable of reaching the necessary temperatures for clinkering, electric cement kilns are still mostly in the prototype and pilot stages.
For demanding heating processes where a flame is indispensable, low-emission options like biomass, biogas, and e-fuels present themselves as alternatives, albeit at varying additional costs.
How About Hydrogen?
Hydrogen is a versatile option for generating heat at any temperature, suitable for both home heating and more demanding industrial processes. However, there are reasons why it's not the first choice for these applications.
First, its efficiency is only around 50% when converting electricity to hydrogen and then to heat, so you need twice as much energy compared to electric resistance heating and the comparison to heat pumps is even worse. Additionally, switching from methane to hydrogen isn't totally straightforward. Hydrogen can cause materials to become brittle, and its combustion characteristics are also different, so it needs specific engineering adjustments and careful handling. It isn’t a matter of simply substituting hydrogen for gas in existing equipment.
Therefore, while hydrogen holds the technical capability for use in industrial heating, it's not frequently chosen. Other technologies, offering more practical and efficient alternatives, are more likely to be preferred for various industrial processes.
Transitioning from Fossil Fuels
Despite the availability of zero-emission options for industrial heat, over 90% of it still comes from fossil fuels. The primary reasons for this are cost and availability. Historically, heating with fossil fuels has been more cost-effective than renewable alternatives. However, this dynamic is shifting as renewable energy becomes more affordable and natural gas prices fluctuate. Renewable energy costs are decreasing, making options like heat pumps increasingly economical for businesses, especially in the context of recent spikes in gas prices. This shift is leading companies to reconsider their heating sources. But it's not just about cost. Industrial operations need reliable heat, delivered precisely where and when it's needed.
Geographic Limitations
Addressing the "where" aspect of heat delivery, renewable heat faces challenges in transportation. Unlike natural gas, which can be easily piped, renewable heat options like district heating are more geographically restricted. For instance, district heating is effective in Nordic countries for both residential and industrial use but is limited in the temperatures it can provide, often not meeting the higher requirements of certain industrial processes.
However, district heating usually operates at temperatures around 100°C, which is lower than many industrial processes require. District heating networks want to keep temperatures low to minimise losses, so it doesn’t really work that well to supply district heating hot enough for your hottest industrial need on the same network as building heating.
District heating pipelines Västerås, Sweden
Concentrated Solar Thermal (CSP) is another renewable heat technology limited by location. Unlike solar PV, which can be installed in many places, CSP needs strong, consistent sunlight. This restricts its placement. Some facilities might relocate to areas with abundant, cheap heat, but most won't want to move away from population centres.
Timing Limitations
While the 'where' of renewable heating is about geographic feasibility, the 'when' is equally crucial, especially as we rely more on wind and solar power. These sources are becoming some of the cheapest available, but their variability presents challenges. Industrial processes often require constant, uninterrupted operation, and aligning this with the intermittent nature of wind and solar isn't always possible.
In some areas, hydro or geothermal can provide this consistent power. Elsewhere, nuclear energy might be a cost-effective option. However, combining wind and solar with storage solutions is emerging as a common approach.
Combining Wind, Solar and Storage
The topic of electricity storage, particularly in the context of thermal storage methods, is a complex and crucial issue in the energy sector. These methods typically involve converting electricity into heat and then back into electricity. However, this process is not without its challenges and costs. Technologies like lithium-ion batteries, while effective, come with a high price tag. Additionally, thermal energy storage systems, which also convert electricity to heat and then back to electricity, operate with an efficiency of around 40%.
For industrial heat applications, the scenario is different. In these cases, there is no need to convert the stored heat back into electricity. Heat can be stored directly as heat, bypassing the energy losses associated with conversion. This method is significantly more efficient and aligns with thermodynamic principles, thereby minimizing energy loss.
Electricity storage holds significant implications for the decarbonization of electricity grids. As these grids expand to support the electrification of transport, buildings, and industry, the challenge of storing electricity intensifies. Electrifying industrial heat, when combined with thermal energy storage, could effectively complement wind and solar power in the electricity grid. With thermal storage in place, industrial heat users will have a buffer and will be able to primarily purchase electricity during surpluses of solar and wind energy when it is less expensive. They can then rely on stored heat rather than the grid when electricity prices are higher. This strategy will help to lower the peaks and elevate the troughs of the variable electricity supply and will be more cost-effective than using electricity storage technologies like lithium-ion batteries.
Watch “Zero Emissions Heat Technologies for Industry” on Engineering with Rosie on YouTube.