In an exclusive interview with CemWeek, Doctor Alexander Schopf shares his insights on chemical looping combustion, a carbon sequestration process for carbon capture and storage, particularly on its role in rendering the cement production process greener.

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Having 17 years of experience in Mineralogy and eight years in Chemical Looping Combustion, Alexander Schopf is a PhD professor at the University of Stuttgart (Germany), where he studied Geology and Mineralogy. Doctor Schopf has a diploma in Mineralogy and received his Doctoral degree from the Faculty of Chemistry. The focus of his diploma thesis was the mineralogical analysis of fly ash obtained from the combustion of brown coal via the oxy-fuel process.


What is chemical looping combustion?

Chemical Looping Combustion (CLC) is a large-scale combustion technology for power generation with a steam turbine, not very innovative up to this point. The special thing about CLC is, that it is a carbon sequestration process for "Carbon Dioxide Capture and Storage" (CCS).

By switching to innovative CLC power plants, CO2 could be disposed of in an energy-saving manner and stored permanently or used in chemical processes such as methanation. The biggest difference with conventional large-scale combustion processes is the use of solids instead of air as oxygen carriers. These oxygen carriers have formed the core of my work.

How does CLC work? The fuel reactor is designed as a stationary fluidized bed, in which the solid oxygen carriers are fluidized with water vapor together with fuel. Here the oxygen carriers are reduced setting free the oxygen used for the oxidation of the fuel. The next station of the oxygen carriers is the air reactor, a circulating fluidized bed, generated by air where the oxygen carriers are oxidized again.

A cyclone follows, separating gases and solids, with solids falling back into the fuel reactor. The flue gas consists mainly of CO2 and water vapor.


schematic representation of chemical looping combustion process

Schematic representation of the chemical looping combustion process. Courtesy of Doctor Alexander Schopf


From the choice of fuel to the oxygen carrier, what factors ensure chemical looping combustion is as highly efficient and cost-effective as possible?

There´s a large variety of fuels. You can use solid, liquid or gaseous fuels. Everything from natural gas, coal, oil to woodchips is under research.

In my work I had a closer look at natural metal oxides as oxygen carriers, especially iron and manganese ores. They are considered as „low cost materials”. Synthetically produced oxygen carriers cost about EURO 20000 per ton, a ton of iron ore, however, only EURO 65. Natural oxygen carriers offer a high availability and a good price/performance ratio. Also environmental neutrality is important to keep the cost low and people healthy who live next to a CLC-power station. Due to the hypothesis of Bowman (2014) that manganese could play a role in neurodegenerative diseases, manganese ores might not be the first choice. Natural iron ores, more precise iron oxides, seem to be the best choice also because of their direct recyclability.


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How does it compare to other carbon capture technologies?

For CLC, the calculated efficiency is over 96 %. Modern gas and steam power plants without CO2 capture show an efficiency of just 65 %. CCS processes like Pre-Combustion, oxyfuel and Post-Combustion show even a much worse efficiency. Compared e.g. to the oxyfuel process the overall electrical efficiency drops by approximately 10 % compared to a power plant without CO2 separation. This means that there is a much larger fuel requirement to produce the same amount of electricity.

Other advantages of CLC are that the production of nitrogen oxides (NOx) is avoided due to the process, instead there is a possibility to produce elemental hydrogen by adding another step. By using biomass as fuel, it will be possible to reduce the amount of CO2 in the atmosphere by storing it.


What role can chemical looping combustion play in rendering the cement production process greener?

In the age of climate change wind turbines, photovoltaic and solar heat are not going to produce the fly ash needed for the production of cement. CLC might be the only source left where fly ash can be produced in a high-efficient green way by the use of biomass (“green carbon”) as a fuel or even coal (“black carbon”) under the circumstance that the CO2 is stored. Also fly ash contains the remains of the used oxygen carriers that offer the iron needed for the production of cement.

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At first, carbon capture and storage (CCS) had a reputation problem, with some opponents deeming it unavailable, immature and/or expensive. Now, research from Mark Z. Jacobson at Stanford University, published in Energy and Environmental Science, notes CCS reduces only a small fraction of carbon emissions, usually increases air pollution, and has a capture equipment cost. Where does CCS stand as compared to renewables?

I think CLC is the only possibility at the moment to reduce the CO2 content in the atmosphere while at the same time energy is produced. A wind mill e.g. produces electricity but doesn´t reverse man made climate change. CLC does by the use of renewables like biomass or biofuels. Even though not every carbon sequestration process for CCS seems to show a good price/performance ratio for the energy produced and cares for resources and is furthermore sustainable. In my opinion renewables and CCS complement each other quite well. For any application there´s a solution that either CCS or renewables can offer.


Considering its required investment and divisive reputation among the scientific community, how can CCS represent a financially attractive option for cement companies and the manufacturing industry at large?

It is out of the question that the cost for CO2 certificates will increase. The industry needs to take action now to keep upcoming costs down. By the urge to comply with the climate target, economies will need to switch to CCS and renewables for energy production. For this reason, there will be a lack of fly ash which will worsen the overall carbon footprint of the cement companies significantly. They have to innovate in order not to disappear from the market because their products become too expensive.

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In November 2019, ExxonMobil and FuelCell Energy announced a USD 60 million, two-year expanded joint-development agreement to further enhance carbonate fuel cell technology for the purpose of capturing carbon dioxide from industrial facilities. Does this development somehow contribute to validate CCS as a viable option?

I think so! If USD 60 million are invested in a CCS technology by a giant like ExxonMobil it might “infect” others in the industry to do the same and help to improve the reputation of CCS. Whether it´s greenwashing or not, it is a good start.

CLC e.g. was first introduced in 1983. And it´s a very long way for innovations like the CCS technologies from invention to incubation, introduction and diffusion. At the moment we are stuck somewhere in between incubation and introduction. Now it´s the turn of the energy producers worldwide to start building that new generation of power plants flanked by the will of governments to give CCS the chance to succeed.


Does cement have a place in the low-carbon economy of the future?

In my opinion that´s for sure! About 10 years ago I spoke at HeidelbergCement about fly ash produced in the oxyfuel process, I have worked on during that time. In 2017 they made the first cement clinker with an oxyfuel cooler. I have read that the test phase will cost around Euro 80 million. But the industry has to keep breaking new ground to succeed.


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