Technical Review - The process of finding new alternative fuels

The AFS Process - turning air into a sustainable fuel

i) Air is blown up into a tower and meets a mist of a sodium hydroxide solution. The carbon dioxide in the air is absorbed by reaction with some of the sodium hydroxide to form sodium carbonate. Whilst there are advances in CO2 capture technology, sodium hydroxide has been chosen as it is proven and market ready.

ii) The sodium hydroxide/carbonate solution that results from Step 1 is pumped into an electrolysis cell through which an electric current is passed. The electricity results in the release of the carbon dioxide which is collected and stored for subsequent reaction.

iii) Optionally, a dehumidifier condenses the water out of the air that is being passed into the sodium hydroxide spray tower. The condensed water is passed into an electrolyser where an electric current splits the water into hydrogen and oxygen. Water might be obtained from any source so long as it is or can be made pure enough to be placed in the electrolyser.

iv) The carbon dioxide and hydrogen are reacted together to make a hydrocarbon mixture, the reaction conditions being varied depending on the type of fuel that is required.

v) There are a number of reaction paths already in existence and well known in industrial chemistry that may be used to make the fuels.

(1) Thus a reverse-water-gas shift reaction may be used to convert a carbon dioxide/water mixture to a carbon monoxide/hydrogen mixture called Syn Gas. The Syn Gas mixture can then be further reacted to form the desired fuels using the Fisher-Tropsch (FT) reaction.

(2) Alternatively, the Syn Gas may be reacted to form methanol and the methanol used to make fuels via the Mobil methanol-to gasoline reaction (MTG).

(3) For the future, it is highly likely that reactions can be developed whereby carbon dioxide and hydrogen can be directly reacted to fuels. 

vi) The AFD product will require the addition of the same additives used in current fuels to ease starting, burn cleanly and avoid corrosion problems, to turn the raw fuel into a full marketable product. However as a product it can be blended directly with gasoline, diesel and aviation fuel.

Energy requirements

It will be the energy requirements that largely determine the cost of the fuel. From the descriptions below it takes 21.4KWh to make 1L of fuel (petrol or similar). This equates to an energy input of 2.2kWh to synthesise fuel with an energy content of 1 kWh, which might be interpreted as a conversion efficiency of 45%.

i) Carbon dioxide capture

For the carbon dioxide capture step, the tower arrangement described by Keith et al(10) required approximately 30kJ/mol CO2 captured .

This gives an energy requirement of 0.44 kWh of electricity to capture the CO2 required for 1L of fuel.

ii) Carbon dioxide release

From experimental data by others we believe it is possible to recover the CO2 from the sodium hydroxide/carbonate solution. The reported energy requirement represents a real-world figure.

Energy to release CO2 for 1L of fuel is 4.6 kWh.

iii) Hydrogen generation

The theoretical electrical energy required to make hydrogen from water is 237.1kJ/mol H2. Current electrolysers operate at about 70% efficiency. Again a real-world statement of the energy required can be made.

For the amount hydrogen needed to make 1L of fuel, the energy requirement is 14.6kWh.

iv) Fuel synthesis reaction – assuming the use of a Fischer Tropsch (FT) process

The FT reaction is exothermic overall so that no energy needs to be put in to drive the reaction. We have estimated the energy needed to compress the CO2 and H gases for the reaction using industry figures for the compressor efficiencies and made allowance for control equipment. The compression requires 0.74 kWh/L. An additional 1 kWh/L has been added to allow for start-up energy and electronic controls.

Energy to be put in to the FT reactor for 1L fuel is 1.74 kWh.