Improved chemical looping process for greener fossil fuels

Jo McCulty, Ohio State University

ENGINEERS at Ohio State University, US, are developing a chemical looping process which can create clean energy, liquid fuels and chemical feedstocks from fossil fuels without releasing CO₂ into the atmosphere.

The team, led by chemical and biomolecular engineering professor Liang-Shih Fan, has announced two breakthroughs in chemical looping, which essentially releases the energy from coal and other fossil fuels without burning it. Instead of coming from the air, the oxygen required for the reaction comes from metal oxide particles, which are heated with the fuel until the reaction occurs. The clean stream of CO₂, which is fully contained within the reactor, can be easily separated for storage or use and is not released to the environment. The metal particles, which are generally larger than the ash particles produced, are also easily separated and reoxidise in air.

This is known as chemical looping gasification (CLG). Using, rather than just storing captured CO2 is being increasingly promoted as a greener option.

The system the team uses is a co-current moving bed reactor and an iron-titanium metal oxide. Syngas is produced by reforming methane with oxygen into hydrogen and carbon monoxide. In Fan and the team’s new process, CO₂ can be used to replace some of the methane. In conventional syngas production the ratio of hydrogen and carbon monoxide can be quite low and requires an air separation process.

In the chemical looping process, the methane is partially oxidised into syngas with CO₂, water, and the oxygen from the metal oxide particles, eliminating the need for air separation and increasing the hydrogen and carbon monoxide purity. The process is also more robust than the dry reforming process. The syngas can then be used to create liquid fuels, plastics or ammonia using conventional processes.

The researchers found that in simulations in a commercial scale gas-to-liquids (GTL) plant, using the chemical looping process with CO₂ to create syngas could reduce methane consumption by 23%, producing more liquid fuels from conventional syngas processes, whilst using less methane, and using CO₂. The researchers claim that the process could produce syngas for around half the cost of conventional dry reforming processes.

One of the major problems preventing commercialisation of chemical looping technology is that the metal oxide particles do not generally have a very long life. However, Fan and his laboratory team have found a way to hugely extend the life of the particles. The particles now last for more than 3,000 chemical looping cycles, or more than eight months’ continuous use in laboratory tests at pilot and sub-pilot scales. Initially, the particles would only last for 100 cycles, or eight days.

The redox reactions at the high temperatures (over 1,000^oC) cause structural deterioration of the particles. Fan and the team found a way to confine ilmenite (iron-titanium oxide) particles, into an aluminium-based skeleton. The particles are 1–1.5 mm in diameter and are around 65 mol% ilmenite. The researchers say the skeleton is resistant to structural changes, shielding the metal oxide from the impacts of the reactor walls during solid circulation, without impeding gas flow. The particles can be used in CLC and CLG.

Fan stresses that his chemical looping technology, which he describes as his “life’s work”, is meant to be a stop-gap.

“Renewables are the future,” he said. “We need a bridge that allows us to create clean energy until we get there – something affordable we can use for the next 30 years or more, while wind and solar power become the prevailing technologies.”

The researchers are already collaborating with Linde & Babcock and Wilcox to develop the technology. They hope to set up more industry partnerships.

“The [coal direct chemical looping] CDCL process is the most advanced and cost-effective approach to carbon capture we have reviewed to date and are committed to supporting its commercial viability through large-scale pilot plant design and feasibility studies. With the continued success of the collaborative development programme with Ohio State, B&W believes CDCL has potential to transform the power and petrochemical industries,” said David Kraft, a technical fellow at Babcock & Wilcox.

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