By Rodolfo Marin Rivera
“Global warming is controversial, of course, but the controversy is mainly over whether human activity is driving it”. (Prof. Michio Kaku)
Are we the boiling frog?
It is well known that emissions of carbon dioxide (CO2), originating mainly from burning oil and coal, contribute enormously to the so-called greenhouse effect. Due to its increase in the atmosphere during the last two and a half centuries, it has been estimated that by 2100, atmospheric CO2 concentrations could reach approximately 1150 ppm, resulting in a global mean temperature increase of 5.5ºC. It doesn’t look like that much, eh? However, such a minor increase of temperature will have a harmful effect on water and food availability, human health, ecosystem, coastlines and biodiversity. Therefore, if we are not able to perceive the danger of this situation, we will not able to react to this threat on time… as the frog that it is placed in cold water and slowly begins to be cooked to death once the water is heated.
Today there is a worldwide concern about such drastic implications and governments have made commitment to reduce their greenhouse gas emissions. The EU has committed to reduce its greenhouse gas emissions by 80-95% by 2050. Moreover, countries like Sweden, Norway, the Netherlands, Denmark, Finland, Italy, the UK and Ireland have implemented regulations in the form of CO2 taxes. It must be taken into account that efficiency measures, reduction of carbon energy sources and increase in non-fossil energies can only reduce the specific emissions by around 30%. Whatever the way to bring the greenhouse effect under control, it is essential to concentrate our efforts on both the reduction of emissions and the levels of atmospheric CO2.
If we consider the increase in demand for energy and the associated rising demand for fossil fuels, we have to start developing technologies that fall under the concept of Carbon Capture and Storage (CCS). With the development of such technologies, it will be possible to separate CO2 from gaseous waste streams, transport CO2 to storage locations and secure its long-term isolation from the atmosphere. It has been estimated that such technologies may contribute up to 55% of the cumulative global climate change mitigation effort.
A number of technologies exist for each phase of CCS, which consider the use of sorbents, membranes and/or chemical-looping for storing, while transport can be achieved by using pipelines, rail and road tankers.
Putting CO2 into jail: Mineral Carbon Sequestration (MCS)
MCS is a technology based on the process of natural rock weathering where carbonic acid, formed through the dissolution of CO2 in water, is neutralized with high pH minerals to form stable mineral carbonates. The products remain in the solid state and there is no possibility of CO2 release after sequestration. It can be carried out by using one of two ways: direct or indirect carbonation. The main difference between them is the number of steps needed in each case. Thus, while direct carbonation requires only one step, indirect carbonation requires two or more steps. Both ways have demonstrated quite good results; however, their application at industrial scale is still being evaluated in terms of cost and benefit.
The most remarkable use of such technology at industrial level is the case of Alcoa at the Kwinana Refinery, in Australia, were they have developed a process capable of sequestering approximately 70,000 tonnes of CO2 per year from the air, which it is equivalent of taking 17,500 cars off the road! Basically, the process mixes waste CO2 with bauxite residue (red mud).
Bauxite residue is a by-product formed during production of alumina in the Bayer process and is considered a hazardous waste, especially due to its high alkalinity, and large quantities are produced annually. It is composed mainly of Fe2O3, Al2O3, SiO2, Na2O, CaO, TiO2 and minor elements (like scandium and rare earths elements). NaOH is its principal source of alkalinity in the liquid phase..
The use of bauxite residue for sequestering CO2 seems to be an interesting technology, due to the fact that the products formed after carbonation are environmentally benign and might be treated for recovering minor elements, for building materials and/or be sold for profit.
Currently, several metal producing industries must deal with such by-products and in most cases it represents an environmental liability that has to be deposited in special landfills, which must be isolated from the ground a with specialized leachate treatment systems. After the capacity of the landfill is reached, the deposit must be neutralized and stored, which requires great investment and strict security and environmental policy. So, why we don’t use MCS in other mineral by-products?
During the last decades, scientists have demonstrated with great success the use of MCS when applied to metallurgical slags, incineration ashes, mining tailings, asbestos containing materials, bauxite residue and oil shale processing residues. Nevertheless, despite of the great amount of research already developed, the use of such technology at industrial scale still remains uncertain. It is not possible to compare the results obtained with different waste materials, because the carbonation process depends significantly on their chemical, mineralogical and morphological properties. The chemical reaction rate of carbonation is also an issue. Hence, it is necessary to improve the reaction rate and the mineral conversion by considering innovative process intensification, such as ultrasound or high temperature processing. Currently, in the division of ProcESS at KU Leuven, we are using ultrasound in order to improve mixing and mass transfer of the carbonation process with bauxite residue. It is expected to considerably improve the carbonation process thank to the cavitation phenomena produced during the application of ultrasound, which will remove passivating layers allowing the exposure of unreacted particles to the reaction solution.
However, despite the efforts to make this technology feasible at industrial scale, it is also important to consider the focus that industry gives to the application of this technology. It is remarkable that the main target of the industry is the increase of production efficiency of the principal products, rather than the eventual treatment and management of waste material. For instance, it is necessary to start changing this attitude through a better understanding of new innovative and sustainable green technologies.