This Science Short, written by Maxwell Pisciotta, summarizes a section of: Pisciotta, M., Pilorgé, H., Feldmann, J., Jacobson, R., Davids, J., Swett, S., Sasso, Z., & Wilcox, J. (2022). Current state of industrial heating and opportunities for decarbonization. Progress in Energy and Combustion Science, 91, 100982. https://doi.org/10.1016/j.pecs.2021.100982.
Carbon dioxide (CO2) emissions in the atmosphere have been contributing to the increased global average temperature rise and climate change. To avoid the worst effect of climate change, the temperature rise needs to be limited to 1.5ºC, which will require reducing CO2 emissions, or decarbonization. In some sectors, decarbonization will require replacing existing industries or factories with others. An example would be replacing coal or natural gas power plants with solar or wind energy. These renewable sources would generate and deliver electricity without the CO2 emissions that would have been generated from the use of fossil fuels. In other sectors, such as heavy industry, the replacement is not as straightforward.
Take cement for example, cement is the building block for concrete and asphalt, and is used in the foundations of buildings, sidewalks, and streets. Cement is a mixture of calcium (Ca) and silica (SiO2), but not one that exists naturally; it must be manufactured. Cement is manufactured by taking limestone (CaCO3), a very abundant resource, and clay, heating them up in a large kiln, and fusing them together to get the calcium-silicate mixture for cement. This process results in CO2 emissions that come from the fossil fuels (coal or natural gas) used to heat this large kiln, but also from the limestone feedstock. As the limestone is heated, it breaks apart into calcium oxide (CaO) and CO2. This means that replacing the fossil fuel energy with renewable energy will not eliminate the CO2 emissions from this process. The emissions from cement production account for 5-8% of all CO2 emissions globally. The chemistry of the limestone feedstock makes this a tricky industry to decarbonize, not to mention, how important cement is in the built environment.
There is still a reason for optimism. There are naturally occurring rocks and minerals that may be used in concrete to replace cement, which would mean that less of it would need to be produced. Also, although carbon capture and storage (CCS) has come under great scrutiny for its proposed use for fossil-derived electricity generation, the cement industry would benefit greatly from this technology. The flue gas from the cement kiln would enter a column that houses specific chemistry that would selectively extract the CO2 from this stream, allowing the other gases; oxygen, nitrogen, and water; to pass through the stack into the atmosphere. When the chemistry is “full” of CO2 and cannot capture any more, it will be heated to release the CO2 into a storage cylinder. The chemistry can then be recycled and used again to continue capturing additional CO2.
Furthermore, designing cement kilns that can be run with an enriched-oxygen atmosphere can simplify the process for separating the CO2 from the environmentally-benign gases. When the oxygen, nitrogen, and water are present in the cement flue gas, one of the difficulties is separating the CO2 from the nitrogen because the molecules are very close in size and have very low condensing temperatures. However, with the use of an enriched-oxygen or pure oxygen atmosphere in the cement kiln, the flue gas will no longer have nitrogen, but rather, just CO2 and water. This means that the flue gas can be taken down to room temperature, where the water can be easily separated from the CO2 because it will be a liquid.
Both of these methods have the potential to address the CO2 generation from cement, but they do not necessarily reduce the use of fossil fuels in this process. At the volume that cement is currently produced, fossil fuel use is one of the only methods to heat the vast quantities of limestone that are needed for this purpose. However, replacing some of the coal that is currently used in cement production with waste biomass has proven to result in the same quality of cement production and has the potential to reduce fossil fuel use. Nearly 20% of the coal can be replaced with drop-in waste biomass, but if that percentage is to go up, the biomass may need to be pretreated to make its properties more similar to that of coal. Some of the waste biomass that has high carbon-content and is positioned as a beneficial replacement to coal includes nut shells and fruit pits. When these are burned, they still release CO2, but the difference is that this CO2 comes from the carbon within the biomass, or the carbon that was absorbed by the plants during their lifetimes. So, this ultimately does not add more CO2 to the atmosphere. To fully account for the CO2 reduction that comes from using waste biomass, a lifecycle assessment would need to be conducted to address any emissions that came from biomass pretreatment and transport.
Decarbonizing the cement industry is not as straightforward as some of the other proposed climate solutions and may benefit from technologies that are controversial, such as carbon capture and storage. However, it is important that cement facilities can evaluate the resources that are near their operations and make decisions based on those available to them in their efforts to decarbonize. This is particularly important if CO2 storage sites are not nearby or if there is very little waste biomass available. These natural constraints may indicate that a specific decarbonization strategy is more attractive than another in different regions. The difference in resources, and therefore, available strategies will also contribute to differences in the state’s roadmap for decarbonizing their industrial activities.
Max Pisciotta is a 3rd year PhD candidate in the chemical engineering department and their research is focused on decarbonization, carbon capture, and carbon removal technologies. Prior to Penn, they earned their B.S. and M.S. in Mechanical Engineering from Colorado School of Mines.
The Clean Energy Conversions Lab at Penn focuses on carbon management, specifically looking to limit atmospheric accumulation of carbon dioxide, opportunities for storing and using carbon dioxide once it's captured, and the industrial scale up of these solutions.