Hydrogen sourced from coke oven gas, a byproduct of steel production, is currently the most economically viable option as an input for "calcium looping," a type of carbon capture, at least until the price of hydrogen sourced from cleanly powered electricity significantly drops, researchers have concluded.
Their findings were published in the journal Carbon Future on December 13.
Even in a world with little or no combustion of fossil fuels, there would still be a handful of industrial processes such as cement, steel or glass production that are hard to decarbonize, yet the modern world still needs those products for vital social needs from the building of bridges to the construction of apartment buildings. There are a handful of fully clean alternatives for some limited use cases, such as wood in place of steel and concrete for some types of buildings, but for the most part, we can't go without such industrial products; instead, we need a way to capture their emissions.
One recent study explores the feasibility of an innovative solution, "calcium looping," that could help clean up these hard-to-decarbonize sectors while still meeting our need for their industrial products.
In essence, calcium looping just reverses the chemical reaction at the heart of cement production, but in a way that is of benefit to multiple industrial processes, not just cement.
The first step in production of cement involves turning limestone—or calcium carbonate, to describe it chemically—into quicklime, or calcium oxide. In a very simple chemical reaction called "calcination," the carbon atoms in limestone are ripped away and stuck onto oxygen atoms. But the process releases an enormous volume of CO2 into the atmosphere. In fact, cement production accounts for a full eight percent of global CO2 emissions.
Calcium looping turns this process on its head. It uses calcium oxide, the quicklime, to absorb CO2 from the flue gases produced during cement manufacturing—the opposite chemical reaction to calcination, this time called "carbonation."
But the desired product, quicklime, is the very substance used to capture the unwanted CO2, seeming to defeat the original purpose of quicklime production. The key then, at least for cement production, is to optimize the process so that there is a net positive manufacture of quicklime while still reducing or eliminating CO2 production. Other industrial processes such as steel production don't confront this careful balancing act between quicklime production and quicklime use, as with cement, but still would benefit from any optimization of calcium looping.
Key to this optimization may be a hydrogen-driven calcium looping process. When the carbonation reaction occurs in the presence of hydrogen, far higher efficiencies in capture of CO2 from flue gas are achieved.
In addition, the hydrogen-driven calcium looping can produce significant quantities of methane when hydrogen reacts with the CO2. This methane, also known as natural gas, can be used as a clean energy source. This may sound strange, as natural gas is a fossil fuel, but when synthetic methane is produced from CO2 captured from industrial processes, it can be considered part of a closed carbon loop. The CO2 released during the combustion of synthetic methane can be captured once again, creating a cycle that can potentially be carbon neutral. This is distinct from fossil methane, the combustion of which constantly adds new carbon to the atmosphere. And sale of the synthetic methane reduces the cost of carbon capture.
On top of all this, calcium looping is less energy intensive compared to other proposed carbon capture methods.
This all sounds great in principle, but for success, crucially, hydrogen-driven carbon looping depends upon an abundant, affordable and relatively clean source of hydrogen. So a group of Guangzhou-based researchers wanted to model two different sources of hydrogen to compare efficiencies and costs: the first from renewably produced using wind or solar powered electricity; and the second from coke oven gas—which can be as much as 60 percent hydrogen and is a byproduct of steel production.
Due to the characteristics of its geology, China enjoys an abundance of coal but little natural gas or oil. This has elevated coal to a pivotal role in the domestic chemical industry while other countries' chemical manufacturers tend to make greater use of natural gas or petroleum. This dominance of a coal-supported chemical industry in turn produces an abundance of cheap coke oven gas as a byproduct, only half of which is ever used in practical applications. The other half is simply released into the atmosphere, where it contributes to the warming of the planet. So if much of this coke oven gas that would otherwise be released can instead be used for carbon capture purposes—at least as a transitional method until inherently cleaner renewable-driven hydrogen production scales up and lowers its significant costs—then this is still an overall, if partial, climate win.
"But both options remain at the conceptual level," said Hao Yu, the lead researcher on the project and a chemical engineer with Guangzhou University. "We needed to perform a more thorough analysis, using models that involve a deep understanding of the technical and chemical processes as well as the economics surrounding them, to be able to better compare the two hydrogen sources."
The research group's modelling found that coke oven gas-sourced hydrogen for calcium looping would produce about four times more synthetic methane than renewably-sourced hydrogen, and with a significantly higher energy efficiency. In addition, a plant using coke oven-gas-sourced hydrogen would enjoy annual profits of about $26 million, while one that used the latter would incur annual losses of around $62 million—almost entirely due to the current high cost of renewably produced hydrogen.
"In the short to medium term, the coke oven gas route is plainly the most feasible option, but to achieve net-zero greenhouse gas emissions in the medium to long term, cleanly sourced hydrogen will be necessary, and that means that its costs will have to drop dramatically," said Dongliang Zhang, co-author of the analysis at the South China University of Technology.
In the meantime, the researchers advised that for optimal techno-economic performance, any calcium-looping carbon capture operation would best be sited in those regions that enjoy a healthy supply of both limestone and coke oven gas. In the Chinese case, northern China has a high concentration of both coal coking and limestone industries, along with highly developed transport infrastructure, and multiple hard-to-decarbonize industries needing carbon capture technology. They added that locations with similar characteristics may be worth considering for application of calcium-looping carbon capture in the United States, Australia, Russia and India.
About Carbon Future
Carbon Future is an open access, peer-reviewed and international interdisciplinary journal, published by Tsinghua University Press and exclusively available via SciOpen . Carbon Future reports carbon-related materials and processes, including catalysis, energy conversion and storage, as well as low carbon emission process and engineering. Carbon Future will publish Research Articles, Reviews, Minireviews, Highlights, Perspectives, and News and Views from all aspects concerned with carbon. Carbon Future will publish articles that focus on, but not limited to, the following areas: carbon-related or -derived materials, carbon-related catalysis and fundamentals, low carbon-related energy conversion and storage, low carbon emission chemical processes.
About SciOpen
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