Maximizing limestone dissolution rate and alkalinity enhancement in an open-system benchtop reactor

Reducing CO₂ emissions is crucial for mitigating climate change and its environmental impacts. A promising strategy for CO₂ reduction involves enhancing limestone dissolution in seawater by reacting it with industrial CO₂ waste gas to form dissolved bicarbonate, which prevents the CO₂ from being released into the atmosphere. This process of limestone dissolution and atmospheric CO₂ reduction occurs naturally over time-scales of 10⁵ – 10⁶ years and serves as “Earth's thermostat”. Enhancing this natural process could serve as an efficient way to remove man-made atmospheric CO₂. To adapt this process to perform at industrial scales and rates suitable for mitigating climate change it is essential to scrutinize limestone dissolution rates and assess the parameters governing this process under controlled laboratory conditions. To assess the potential of limestone dissolution rates and characterize the conditions required to maximize dissolution rates and CO₂ removal, we constructed a versatile benchtop reactor that mimics the natural limestone dissolution process and allows for experimenting with different materials and dissolution conditions. This experimental setup affords control over gas and recycled gas flow rates, as well as the mineralogy and grain size of the utilized limestones—parameters known to influence dissolution rates. The reactor is a 22 x 110 (cm) circular tube filled with a limestone medium. A continuous stream of seawater and CO₂ gas is introduced into the reactor where it reacts with the limestone. An air pump recycles CO₂ gas from the reactor head-space, in order to enhance the efficiency of CO₂ dissolution in seawater. Excess gas and seawater are removed continuously from the reactor, creating an open, through-flowing system. The system is monitored online using temperature, pCO₂ and pH meters. Total alkalinity (TA), dissolved inorganic carbon (DIC) and Calcium concentrations of the sea water in the reactor are sampled throughout the experiments and measured offline. To identify the parameters that achieve maximum limestone dissolution rates we performed experiments under different grain sizes, gas to seawater flows ratio, and recycled gas flow rate. Comparing our findings with previous studies reveals that a significant amount of limestone dissolution occurred in our system, leading to alkalinity enhancement in the sea water and removal of CO₂. In the presentation, we will discuss the effects of the different parameters on the final total dissolution rate and suggest the set of parameters that maximize the limestone dissolution rate.