Spontaneous Conversion of Power Plant CO2 to Dissolved Calcium Bicarbonate by The Planet Doctors

Intriguing proposal with lots of promise, it’s a technology the judges did not know. And the proposal was written up well. But it needs to be developed with respect to details. Some questions to answer: What happens when the residue gets pumped into the sea? How much calcium carbonate will be needed and how much CO2 would it absorb? Would also like to see more numbers re: technology, cost, economics. Presumably this technology is limited in its application to plants near the coast, so that raises the question of what countries have coal plants near sea water globally and how many are there? Given this, what is the overall mitigation potential? The proposed action is presumably a pilot plant, that could be specified more explicitly.

Thank you for your changes in response to the Judges comments. This is an interesting alternative approach to CCS, and the technology deserves a hard look.

“…the team still needs to show the relevant thermodynamic data (Delta H, Delta G) with equilibrium/mass action calculations.” THE PLANET DOCTORS REPLY: CACO3 + CO2 + H2O → CA^2+ + 2HCO3^-, DELTA G = -164.6 KJ/MOL THE MASS ACTION OF CARBONATE EQUILIBRIUM IN SEAWATER HAS RECEIVED CONSIDERABLE INVESTIGATION OVER DECADES, AND THE REACTION HAS BEEN SHOWN TO BE EXOTHERMIC. THUS, THIS SPONTANEOUS, EXOTHERMIC REACTION ALLOWED CONTINOUS, STEADY STATE REMOVAL OF CO2 (<=97% OF INPUT) IN LABORATORY EXPERIMENTS AT AMBIENT, ROOM T AND P (REF 7). GIVEN THE MASS AND HIGH HEAT CAPACITY OF SEAWATER, THE EXOTHERMIC REACTION WILL HAVE LITTLE IMPACT ON THE TEMPERATURE OF THE REACTOR FLUIDS. “They also need to present some rate estimates for the reaction with solid particles, which can be intrinsically slow.” REACTION RATES FOR CARBONIC ACID (CO2 + H2O) AND CARBONATE MINERALS ARE ORDERS OF MAGNITUDE FASTER THAN COMPARABLE REACTIONS FOR SILICATE MINERALS. OBSERVED RATES ARE ON THE ORDER OF 10^-6 MOLES PER M^2 OF LIMESTONE SURFACE AREA PER SEC (REF 5). WE ESTIMATE REACTION DENSITIES OF 10^-2 TONNES CO2 PER M^3 OF INTERNAL REACTOR VOLUME PER DAY. BUT MUCH HIGHER DENSITIES ARE POSSIBLE, DIRECTLY DEPENDENT ON HOW SMALL LIMESTONE PARTICLE SIZE CAN BE AND STILL ALLOW EFFECTIVE FLOW AND LIQUID CONTACTING. WE PROPOSE TO DETERMINE OPTIMAL PARTICLE SIZES AND REACTION DENSITIES. “The only time scale mentioned in Ref. 7 is "1-2 weeks" which is clearly unsuitable for an on-line real-time CO2 removal process.” 1-2 WEEKS WAS THE MAXIMUM REACTION TIME STUDIED UNDER STATIC, NON-STEADY STATE CONDITIONS, NOT CONDITIONS ENVISAGED FOR A REAL APPLICATION (REF 7). THE MAJORITY OF THE REACTOR EXPERIMENTS WERE PERFORMED UNDER CONTINUOUS FLOW STEADY STATE, SHOWING NEAR-INSTANTANEOUS (SECONDS) REMOVAL OF UP TO 97% OF INPUT CO2, HIGHLY DEPENDENT ON GAS/WATER INPUT FLOW RATIOS TO THE REACTOR (REF 7). THE EXPERIMENTAL SETUP WAS IN EFFECT A PLUG FLOW REACTOR. DESIGNING AN OPTIMUM REACTOR AT SCALE WILL REQUIRE MODEL PARAMETRIZATION THAT CAN ONLY BE ACCURATELY OBTAINED THROUGH MORE EXPERIMENTATION AT APPROPRIATE SCALES, AS PROPOSED. “In any case, a full Life Cycle Assessment is essential before any of the steps enumerated in the time line can be considered.” A LIFE CYCLE ANALYSIS IS REQUIRED FOR THIS TECHNOLOGY AND IS PART OF THE PROPOSED WORK. HOWEVER, AN ACCURATE LCA BEYOND THAT ALREADY PRESENTED [REF 7] IS NOT POSSIBLE WITHOUT APPROPRIATE PARAMETERS DETERMINED THROUGH EXPERIMENTATION. CONTROL VARIABLES (PARTICLE SIZE, SOLUTION FLUX, GAS FLUX, CO2 GAS CONCENTRATION, GAS PURITY) WILL BE MODIFIED IN THE TEST REACTOR AND THE OUTCOMES MEASURED (CO2 REMOVAL, WATER QUALITY). THESE DATA WILL BE USED TO REFINE MODELS AND TO CONSTRAIN AND OPTIMIZE SYSTEMS. AN LCA THEN WILL BE UNDERTAKEN (INCLUDING INTEGRATIVE ASSESSMENT MODELING AND REGIONAL CASE STUDIES) TO DETERMINE THE TRUE COST AND CARBON/ENVIRONMENTAL BENEFIT OF AWL AND IT'S FEASIBILITY COMPARED WITH OTHER MITIGATION TECHNOLOGIES.

The Planet Doctors would prefer to conduct an initial demonstration at a natural gas fired power plant because the flue gas is cleaner and hence the chemistry and the potential downstream environment issues will be simpler than in the case of coal-firing. Also, the CO2 in NG flue gas will be more dilute, so if we can successfully mitigate CO2 here, we can most certainly mitigate CO2 from more concentrated sources. The fact that wet limestone scrubbing or seawater scrubbing are already widely employed for coal SO2 mitigation suggests that this can adapted/scaled up for separate or additional CO2 mitigation, but let's find out with further R&D.