Controlled mineralisation of carbon dioxide: Laboratory assessment

Abstract

NCG reinjection has been implemented in several geothermal power plants to reduce greenhouse gas emissions and support New Zealand's net-zero emissions targets. A major challenge for this system is that there is a potential for NCG breakthrough to the surrounding wells and leakage to the ground surface, which could result in high CO2 gas emissions. Therefore, effective CO2 mineralisation is essential to mitigate those risks. This study presents a series of laboratory experiments designed to evaluate suitable and cost-effective materials for CO? mineralisation.
Hydroxide compounds—including calcium hydroxide (Ca(OH)?), potassium hydroxide (KOH), and sodium hydroxide (NaOH)—and industrial waste products were tested by injecting CO? into aqueous solutions of these materials. The efficiency of mineralisation was assessed via the composition and mass of solid CaCO? deposits formed. The tests were performed under varied conditions of temperature (25–60 °C), CO? flow rate (0.1–0.2 L/min), and reactant concentrations.
Based on the results, the most effective hydroxide mineral that has been tested for CO2 mineralisation is Calcium Hydroxide (Ca(OH)2) since it produces solid carbonate. However, due to the relatively high cost of pure Ca(OH)2, other alternatives could be used for the CO2 capturing. Waste products (e.g., burned lime and NSSC) containing 75-85% Ca(OH)2 are proven to be effective in CO2 mineralisation by precipitating CaCO3 minerals from the carbonation of Ca(OH)2 aqueous solutions. These waste-based sources produced significant CaCO? precipitation and present a promising low-cost alternative.
From these laboratory experiments, it is concluded that waste products containing significant Ca(OH)2 can be effective for CO? sequestration in geothermal reinjection systems, helping to reduce the risk of NCG breakthrough by promoting near-complete mineralisation before subsurface injection.