Carbon capture and sequestration (CCS) has long been an ultimate goal for many energy companies, for both money saving and environmental reasons. It is the process by which the carbon dioxide, which would otherwise be released as waste from power plants into the atmosphere, is captured, compressed and injected underground for permanent storage.
Now ExxonMobil has partnered up with FuelCell Energy, one of the largest suppliers of fuel cells worldwide, to integrate its efficient carbonate fuel cells into CCS technology to cleanse CO2 from the exhaust fumes from natural gas and coal-fired power plants.
“Carbon capture with carbonate fuel cells is a potential game-changer for affordably and efficiently concentrating CO2 for large-scale gas and coal-fired power plants,” says president and chief executive officer of FuelCell Energy, Chris Bottone.
Fuel cells have been a rapidly expanding option for mini grids and distributed generation because they produce electricity directly from a chemical reaction, and devices are generally fairly small and easy to install and transport.
In this case the fuel cells would run directly off the power plant emissions, removing them from the air and, in turn, producing additional electricity to feed back into the system or indeed sell off.
CCS plus fuel cells: how does it work?
The cells are conventionally powered with methane and ambient air, from which they produce hydrogen and CO2. Electricity is generated by combining with oxygen to create a current producing exhaust water vapour and CO2 as waste.
Instead of air, the fuel cells integrated into CCS technology will run on the flue gas expelled by gas and coal burning processes, as well as still using methane.
This process is more efficient because the chemical reaction can concentrate up to 90% of incoming CO2, which can be redirected into the CO2 generated in the reforming process and separated from water vapour.
It also destroys around two thirds of the mono-nitrogen oxides (NOx) in the flue gas, which are formed wherever combustion occurs, such as in car engines. As air polluters, NOx gases are significant, as they react to form smog and rain, as well as having a negative impact on the ozone layer.
“Ultra-clean and efficient power generation is a key attribute of fuel cells and the carbon capture configuration has the added benefit of eliminating approximately 70% of the smog-producing nitrogen oxide generated by the combustion process of these large-scale power plants,” says Bottone.
Carbonate ions are formed at the anode, which complete the electrical circuit across the electrolyte later of the fuel cell. CO2 is required for this reaction, and power plant exhaust is a worthwhile source because it contains 5% CO2 in natural gas plants.
FCE vice-president of applications and technology development Tony Leo says: “There’s an internal CO2 cycle within the carbonate fuel cell, which can be co-opted to separate CO2 for carbon capture without the need for a regeneration step, as is the case in amine-based carbon capture.”
A long history of mixed success
Many facilities have been able to capture CO2 emissions since the 1970s, but in very energetically exhaustive ways. Existing CCS technology actually consumes up to 25% of electricity at a power plant, equating to a large amount of money.
With power plants already facing financial challenges from the growing interest in renewable energy, falling oil prices, and an increasing unpopularity of coal, a 25% jump in power generation isn’t usually feasible.
For example, Southern Company built a coal-fired power plant in Kemper Mississippi which was supposed to incorporate CCS. The project ended up costing $7bn in total, three times the original estimate.
But thanks to technological advances and the combination with fuel cells, CCS has become a lot more realistic for big oil and gas.
In 2015, ExxonMobil claims that it captured 6.9 million metric tonnes of CO2 using the CCS process, which is the equivalent amount of fumes from over a million cars.
“Advancing economic and sustainable technologies to capture CO2 from large emitters such as power plants is an important part of ExxonMobil’s suite of research into lower emissions solutions to mitigate the risk of climate change,” says ExxonMobil Research & Engineering Company vice-president for research and development Vijay Swarup.
“Our scientists saw the potential for this exciting technology for use at natural gas power plants to enhance the viability of carbon capture and sequestration while at the same time generating additional electricity.
“We sought the industry leaders in carbonate fuel-cell technology to test its application in pilot stages to help confirm what our researchers saw in the lab over the last two years.”
Other benefits and future steps
Researchers think that pursuing new CCS technology could actually help to reduce costs. While current CCS processes are associated with added expenditures, by combining with FuelCell, the new method could increase the amount of electricity a power plant produces.
In addition to the cost savings and environmental benefits, carbonate fuel cells also produce a chemical feedstock called syngas, which is primarily made of hydrogen and can be used as a fuel for internal combustion engines.
"This is very clever, this is very unique," says Swarup. "You put this on the back end of a power plant; you concentrate the CO2 while creating some by-products that could be valuable in other processes."
Exxon and FCE are optimistic about the carbonate cell-CCS collaboration, but Swarup has said that the researchers have to be “ready for it not to work” and that it will probably take several years for these cells to be commercially available.
"We will get there when we get there, or we will not get there," he says. "Either way we want to understand why or why not."
One potential setback for the technology is that the amount of CO2 that can be captured depends heavily on how many fuel cells are in operation. For example, a 500MW combined-cycle plant would require at least 120MW of fuel cells to achieve a level of 90% carbon capture. An equivalent coal plant might need as much as 400MW of fuel cells because coal plants are generally less efficient and emit higher levels of CO2.
Either way, FCE and Exxon scientists will focus on increasing the efficiency in separating the CO2 from gas turbine exhausts, and are likely to learn a lot in the process. They are working to better understand the chemical processes and working out how they respond to different compositions and concentrations of the flue gas.
If successful, the next steps will be to launch a pilot project for more testing and then integrate to a larger scale pilot facility after. Eventually, the goal is to build a 2MW-3MW demonstration plant that would run alongside a coal-powered plant.