Turning captured carbon into nanotubes

POWER stations could one day capture CO₂ and convert it directly into sought-after carbon nanotubes and nanofibres, after engineers in the US proved that their method is thermodynamically feasible and financially favourable.

Capturing CO₂ from fossil fuel-fired power plant emissions is increasingly seen as a vital technology to reduce the world’s greenhouse gas emissions, but finding suitable storage sites for the captured gas can be a problem. Converting the gas into useful products, such as sodium bicarbonate, or carbon nanotubes (CNTs), which are used in batteries, consumer electronics, aeroplanes, and athletic equipment, would be a better alternative, but such processes can be costly, both economically and thermodynamically. Stuart Licht and his team at George Washington University, have found that their process to produce carbon nanotubes and nanofibres (CNFs) from the captured emissions of a combined cycle power station is feasible on both counts.

Last year, Licht and his team developed the process to convert CO₂ into carbon nanofibres. They estimated at the time that 1 t of the nanofibres would cost around US$1,000 to produce, far less than the value of the finished product. They have since discovered that adding a small amount of nickel catalyst to the process instead produces hollow carbon nanotubes.

The process uses a molten lithium carbonate (Li₂CO₃) electrolyser, at temperatures of around 750?C. The hot captured CO₂ dissolves into this. Using electrolysis, with a current generated by electrodes, the CO₂ splits to form oxygen gas and pure carbon. The carbon forms into nanotubes in the presence of the nickel and zinc catalyst. Oxygen generated in the process is fed back into the combined cycle power station, so although some electricity is needed for the electrolysis process, the oxygen has the benefit of boosting its electricity-production efficiency. Energy is saved in various other ways throughout the process, including by using hot CO₂ directly from the turbines, which more easily reacts and has significant enthalpy savings compared to capturing cool CO₂ in a conventional CCS plant.

Licht and the team say that per ton of methane burned, a conventional combined-cycle power station with no CNF facility produces 9,090 kWh of electricity, which at current rates in the US equates to US$909, and emits 2.74 t CO₂, while a so-called CC CNF (combined-cycle carbon nanofibre) plant would produce 8,350 kWh of electricity, worth US$835, and 0.75 t of CNT, worth around US$225,000 at current prices, and emit no CO₂.

In a conventional CCS system, around 7% of the electricity produced by a power station must be expended to regenerate the amine capture liquids, and energy is also expended in storing it, which does not happen in a CC CNF plant. In addition, companies making carbon nanotubes would avoid carbon tax charges.

'The CC CNF [plant] adds a high-temperature molten Li₂CO₃ electrolyser to a CC power plant design to remove exhaust CO₂ and transform it into a valuable carbon nanotube product. The present-day value of the carbon nanotubes product is around 10,000 times that of proposed, or in place, carbon tax costs of US$30/t, strongly incentivising CO₂ removal and spurring new public/private investment in this process to convert CO₂ into carbon nanotubes globally,' the researchers conclude.

Energy Conversion and Management DOI: 10.1016/j.enconman.2016.06.007