Technology

Clean-burning fossil fuels

 By RP Siegel

Is it possible to produce electricity from fossil fuels such as natural gas or coal, without emitting carbon dioxide?

We’ve all heard about carbon capture and sequestration (CCS), which has been demonstrated on a large scale, but it has been costly and unwieldy for full commercial implementation.
Now there is another way, thanks to Professor LS Fan and his team at Ohio State University. Fan has been working in the area of air pollution control and mitigation his entire career and is now focused intensely on the carbon dioxide problem.

Innovations to chemical looping

Fan and his team have come up with innovations to a process called chemical looping, which, while somewhat complicated, open up an entirely new path for not only producing electricity but also valuable chemicals such as hydrogen and various hydrocarbons that form the basis of many common materials. The versatility of the chemical looping technology platform, combined with the recent innovations of Fan and his team, can potentially provide cost-efficient solutions to several of the grand challenges associated with global energy consumption.
In a power generation application, chemical looping, like more conventional power generation sources, produces heat, which is then converted to electricity using turbines. However, the path used to get there is entirely different.
Traditional combustion occurs when fuel combines rapidly with oxygen from the air. With chemical looping, the oxygen is provided in the absence of air, by a metal oxide which is regenerated at the end of the process. Using chemical looping for combustion applications provides electricity along with the production of pure, sequestration-ready CO2 at a significantly lower cost than conventional CCS technologies. One surprising aspect of this looping process is the fact that there is no flame involved. Rather, metal and fuel are burned in a separate reactor without a flame, while still giving off an amount of heat comparable to burning carbon. The process is much cleaner than conventional combustion systems because in addition to CO2 capture, it can also reduce NOx emissions due to its unique reaction scheme.

Sankey diagram of energy fluxes in a reversible Chemical looping combustion system (NiallMcG, Wikimedia)

Synthesis gas

The utilization of CO2 as a co-feedstock along with the fossil fuels enables the chemical looping technology to be operated in a carbon-negative/neutral mode. In this configuration, the CO2 is converted into useful products (by way of syngas, or synthesis gas), with considerable high-quality heat being given off along the way. That heat can be utilized to produce electricity as a form of cogeneration. The syngas, which consists primarily of carbon monoxide and hydrogen, can be used as a fuel or as a feedstock for a variety of useful chemicals.
According to Chris Doherty, CFO at ThermoChem Recovery International, in Baltimore, “Syngas has the building blocks to create all the products and chemicals currently generated in the petrochemical industry.”
The chemical looping system, when configured for CO2 utilization, is guided by similar principles as the power generation version. Differences arise in the target products and process operation, most notably, the former can be a carbon negative process. This difference stems from the fact that the fuel is partially oxidized in this process rather than being fully oxidized as it is in conventional power generation systems. Partial oxidation leads to the production of carbon monoxide and hydrogen whereas full combustion produces carbon dioxide and water. When the carbon monoxide is used to produce a durable item, such as a polymer material, the net result is a carbon negative process. There may be a small amount of CO2 generated, which can be recycled back such that the consumption rate of CO2 is higher than the emission rate, maintaining the CO2 negative nature of the system.
Fan says he prefers the term gasification or reforming to describe this chemical looping syngas generation process rather than combustion, which can be misleading. As novel as this is, it is far from a laboratory curiosity. Fan has several 25 kW (thermal) power plants using this process in his lab, along with a 250 kW to 3 MW system at the National Carbon Capture Center (NCCC), which is configured to produce hydrogen as a main product, and 250 kW system at the Babcock and Wilcox Research Center, which is configured to produce heat as a main product.
Fan says that “people have been trying to develop this type of system, which could transform the industry, for 110 years without commercial success.”

Syngas Products (Kopiersperre, Wikimedia)

A new application to the process

The breakthrough that has made this possible today has to do with the metal oxide portion of the process. Until now, the metal oxide particles would expand and contract as they were cycled, causing them to break down rapidly. However, the team at Ohio State was able to synthesize a new form of iron-based composite oxygen carrier that would stand up to many, many months of thermal cycling while maintaining desired reactivity. This major breakthrough by Fan and his team renders the chemical looping technology now commercially realizable.
Fan shared what he saw as the most promising application of this technology. He said that the replacement of steam methane reforming (while producing power) as a means of producing hydrogen looks very good. His team has already have engaged with Linde for a plan on commercializing this process.
The other is the production of syngas, which according to Fan, will see a 50 percent reduction in capital cost when a looping process is utilized. At the same time the process can be carbon negative, (not counting emissions associated with getting the fuel out of the ground and to the plant) while at the same time, it can be used for cogeneration of electricity due to the amount of heat being generated.
Fan considers chemical looping a type of “carbon emission control,” noting that it can be applied to shale gas, coal, or biomass as well. Using biomass says Fan, “is already carbon neutral, but with this process, it would be strongly negative because of the CO2 consumed by the process.”
Fan, who has spent his entire career working on coal, primarily developing processes to reduce and control pollutants, believes that coal can be burned cleanly, though he acknowledges that the economics, the concerns about pollution and the ability to treat the exhaust may tend to favor other fuels.
Fan says he sees continued growth of renewables, though he says he cannot foresee a time when fossil fuels will not continue to be used, “it is just a matter of to what degree?”

READ MORE: Removing carbon from the air by Amanda Saint

about the author
RP Siegel
Skilled writer. Technology, sustainability, engineering, energy, renewables, solar, wind, poverty, water, food. Studied both English Lit.and Engineering at university level. Inventor.