CO lean ue gas
Hydrogen peroxide H O
Alkali
Mixing tank
Fresh alkali NaOH or KOH make-up
Centrifuge
CO rich raw ue gas
Filter
Cooling water
Mineral powder Na CO , NaHCO , K CO or KHCO
Figure 7 Airovation - alkali mineralisation of CO 2 using the superoxide radical
The carbonate or bicarbonate mineral solution from the process is enriched using a centrifuge and hydrocyclone: conventional mineral solution processing equipment. If required, a dryer may be incorporated to yield a solid mineral product. Heat exchangers, filters, pumps, flue gas fans, feedstock tanks, and product storage make up the balance of plant. The mineralisation reaction stoichiometry governs the consumption of the NaOH or KOH alkali feedstock. For sodium carbonate formation, a 2:1 ratio of NaOH to CO 2 is required, equivalent to 1.8 kg of NaOH per kg of CO 2 captured. For sodium bicarbonate, a 1:1 NaOH to CO 2 molar ratio is required, equivalent to 0.9 kg of NaOH per kg of CO 2 captured. Per kg of CO 2 sequestered by the mineralisation process, the formation of bicarbonate has a lower alkaline feedstock input requirement and, therefore, lower operating cost. NaOH is added as a highly concentrated aqueous solution. This can either be delivered to site or made on-site from solid NaOH mineral salt crystals, which are dissolved in water prior to being introduced into the process. The use of highly concentrated or
solid NaOH reduces the volume of feedstock required to be transported by road or rail tanker to the CO 2 capture location (see Figure 8 ). Process design, operating conditions, and the catalytic dosage can be adjusted to determine whether NaHCO 3 or Na 2 CO 3 is produced. A crucial factor used to control the product formation pathway is the alkalinity, requiring a pH greater than 8. Airovation Technologies will be able to finely tune the process to control the selectivity of the reactions and avoid the need for any subsequent separation processes between the mixed formation of Na 2 CO 3 and NaHCO3 . Economic considerations At present, a major driver for operators to reduce their CO 2 emissions is taxation, with Emissions Trading Schemes in place in the European Union and many other jurisdictions. Pull-through from customers is also a strong motivation to decarbonise, and many companies selling their products in European, Japanese, South Korean or US markets will face pressure to reduce the CO 2 footprint of their products. Corporate strategies and national decarbonisation targets being implemented by government-owned entities are also powerful decarbonisation drivers. If the Airovation Technologies process is integrated into a chloralkali electrolysis site focused on chlorine gas or hypochlorite production, the co-produced sodium hydroxide may be regarded as a low-value by-product. Under these circumstances, the economics for the Airovation CCM process can become highly favourable. The main balance of plant operating costs is
CO captured and utilised from another process
Cement, sand agglomerate and water
CO lean ue gas
NaOH (aq)
CO rich ue gas
CO cured concrete
Na CO (aq)
Figure 8 Mineralisation processes for CO 2 capture and utilisation from steam methane reforming
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