Energy Requirements & CO2 Generation
The basis of modern electrochemical aluminium smelters, the Hall-Heroult process, was originally invented in 1886. In each production plant, the process is based on numerous electrolysis cells, arranged in series, all of which are operating under DC voltage at 960°C.
In the Hall-Heroult process, carbon dioxide is formed through the oxidation of carbon anodes and is a product of the chemical reaction between the carbon anode and alumina. Furthermore, CO2 can also be produced through an effective “burning” of the anodes at elevated ambient temperatures. As a result, the process-related carbon consumption for each kilogram of metal is between 0.45 kg and 0.6 kg in most modern Hall-Heroult cells. However, recently, some smelters operating at “World’s Best Practice” have reported lower figures in the range 0.42kg - 0.43 kg carbon per kg aluminium production.
Compared to the theoretical value of 0.33 kgC/kgAl, it is clear that electrochemical smelting technology is approaching a technological limit for the reduction of carbon consumption and the consequent emission of CO2. Based on these data, it is expected that the level of CO2 generated from a typical smelter should range between 1.5 to 2.2 kgCO2/kgAl.
The production of electricity used in the smelting process is another major, though indirect, source of CO2 generation. According to the Environmental Protection Agency (USA EPA) and the International Aluminium Institute (IAI), “the current average electricity requirement for smelting purposes is about 15.25 kWh per tonne of aluminium (year 2008 data)”.
In a power plant, the level of CO2 emission is dependent on the nature of the fossil fuel that is used to generate electricity. Coal burning power plants emit 1.0kg - 1.1kg of CO2 per kilowatt-hour (kWh) produced, while gas fired plants emit 0.35kg - 0.4kg CO2 per kWh. By comparison, hydroelectric or nuclear plants do not emit significant CO2. Therefore, CO2 emissions per tonne of aluminium produced can range from approximately 16 tonnes CO2 (if coal is used), down to 5.7 tonnes CO2 (if natural gas is used). If an aluminium smelter is purchasing its electricity requirements from the grid, the electricity is likely to be generated from mix of resources. According to IAI, there has been about 10% reduction in average energy consumption since 1990, although the rate of progressive reduction has dropped significantly in recent years.
In the Thermical™ process, the chemical reaction for aluminium production is the direct source of CO2, while the electricity required to carry out the reaction is an indirect source.
Based on the carbon source used in the Thermical™ process and the application of the produced gas CO / (CO + 2H2) in power production, three process options can be considered for the Thermical™ process.
The Thermical™ process is able to achieve a 35% to 64% emission reduction, if coal based electricity is used. If the source of electricity is natural gas, then the Thermical™ process shows a 24% to 58% reduction in emissions, whereas, if the purchased energy is from a hydro-electrical or nuclear source, the reduction is of the order of 15% to 54%.
The CO2 emission reduction numbers shown in Figure 1 are based on PFCs equivalent to 0.24 - 1.00 kgCO2/kg Al produced in the electrolytic process.
For comparison, data for the generated CO2 gas for 1 kg plain carbon steel is also shown in Figure 1. Based on these data, using an aluminium process without producing PFC, and using hydro- or nuclear-sourced electricity, will produce approximately the same level of CO2 gas per 1 kg aluminium as the steel process does for 1 kg steel.
Greenhouse Emissions
Another source of significant direct emissions produced in the Hall-Heroult process are perfluorocarbons (PFCs). These gases are created during a process condition called the “Anode Effect”. This condition results in the electrolytic decomposition of cryolite, a material used to dissolve alumina for electrolysis in the cell. The decomposition leads to the formation of CF4 and C2F6. These PFCs both have very high global warming potential.
Primary aluminium production has been identified as the largest source of PFC emissions today. The Aluminium industry continues to seek means to mitigate the problem through its use of more advanced process control techniques and feeding systems. Since 1990, a number of systematic programs have been undertaken by major producers to meet this objective. As a result, the average global value of PFC emissions has been reduced from about 5.00 kg CO2-eq per 1 kg of aluminium down to about 2 kgCO2-eq per 1 kg Al by the year 2000 and to about 1 kgCO2-eq/kgAl in year 2006. The target value for year 2010 is 0.99 kgCO2-eq/kgAl.
Emissions of CF4 and C2F6 do vary significantly from one aluminium smelter to the next, depending on the specifics of cell type and the consequent anode-effect parameters. The IAI has collected data for different cell designs, all of which are currently in operation. According to these data, the average equivalent CO2, related to PFC generation, varies for the different cell designs and, as examples, the value may range from 10.9 kgCO2-eq/kgAl for the “Side Work Prebake” type to 0.24 kgCO2-eq/kgAl for the “Point Fed Prebake” type.
However, Calsmelt considers that it is quite unlikely that existing processes will be able to eliminate PFCs completely. Nor is it considered likely that all smelters will be powered by clean, low-cost, electricity sources within the foreseeable future. Therefore, Calsmelt believes that its technology, which does not use cryolite and so has no resultant PFC emissions, offers very significant potential to both reduce the cost of aluminium production and to limit energy usage and greenhouse gas emissions.