The AGRAL (Advanced Green Aluminum Anodes) project aims at developing the manufacturing technologies of a specific cermet (called AGRAL cermet ) that has shown (during lab scale tests) outstanding properties in high temperature and corrosive media. This new technology could reduce carbon emission during the aluminium production.More precisely, if the objectives are met within the project, this AGRAL cermet will enable the aluminium industry to envisage the replacement of their current carbon anode by this inert anode thus decreasing to zero the direct CO2 emission during electrolysis process.
Consequently this AGRAL cermet will be developed and tested for two applications:
- the inert anode for the aluminium electrolysis (up to industrial scale in Aluminium Pechiney (AP) premises)
- hydrogen and fuel cell interconnect protection (up to pre-prototype scale).
Primary aluminium production
Worldwide Aluminium production is 53 million tons per year. European Aluminium industry (including Iceland and Norway) represents 4 millions of tons of primary Aluminium produced in 2013.
Currently, the production of primary Aluminium uses the Hall-Héroult electrolysis process. It is working with consumable carbon anodes which are responsible of direct CO2 emissions: 7.04 millions of tons of CO2 per year in EU and other Greenhouses Gas GHG. The reaction consumes the carbon of the anodes and these need to be changed every 25 days.
European Environment policy targets to decrease global CO2 emission by 20% until 2020.
To reach this objective, the quotas of carbon for industry, including Aluminum one, are managed by the ETS (European Trading System). Today 1 ton of CO2 ETS costs 7€, meaning 49M€ of compliance costs. In 2020, it is estimated that the cost of 1 ton of CO2 will increase to reach 20€, representing more than 140M€ of compliance costs for European aluminium industry.
Hall-Heroult and Inert anode reactions
The electrolysis process
Industrial production of primary aluminum is carried out in alumina reduction cells by the Hall-Héroult process. This process developed and patented in 1886 is an electrolytic process by which alumina (Al2O3) is dissolved in an electrolyte consisting mainly of molten cryolite (Na3AlF6) at 960°C. A feature of Hall-Héroult process is the fact that the anodes are consumable, made of a mixture of petroleum coke aggregate and coal tar pitch binder, molded into blocks and baked in separate anode baking furnaces at about 1100°C. Standard Hall Héroult process leads to CO2 emissions at the carbon anode and liquid aluminum at the cathode.
As a result of the process design, 1.76 tons of CO2 are emitted by ton of aluminum (assuming no indirect emissions: electricity from hydraulic or nuclear sources). Carbon oxidation is beneficial to the specific energy consumption but goes with several drawbacks in addition to direct CO2 emissions:
- The consumption of the carbon anode induces its replacement every 25 days, the mass production of the anodes has issues of raw material supply of good quality leading to Polycyclic Aromatic Hydrocarbon (PAH) emission management.
- A lack of Al2O3, close to carbon anode, results in anode effect, with production of perfluorocarbon gases (CF4 and C2F6). The later impacts much more the environment that CO2, the equivalent effect could reach more than 2 t eq. CO2/t Al.
The AGRAL project is aiming at developing at the industrial level material inert anode. Introduction of inert anodes (or non–consumable) anodes will eliminate the consumption of carbon and therefore atmospheric pollution issues (CO2, SO2, PAH, CF4, C2F6 …).