Approaches for CO2 Capture and Sequestration Inspired by Biological Systems
Authors: Carpenter Chris 
Published: July 2017

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 180718, “Approaches for CO2 Capture and Sequestration Inspired by Biological Systems,” by Z. Ouled Ameur and S. Gupta, Cenovus Energy, and Hector De La Hoz Siegler, University of Calgary, prepared for the 2016 Canada Heavy Oil Technical Conference, Calgary, 7–9 June. The paper has not been peer reviewed.

In this study, several process alternatives for the permanent sequestration of carbon dioxide (CO2) as solid carbonates are evaluated. Although the formation of mineral carbonates is thermodynamically favorable, it does not occur significantly because of kinetic limitations and the formation of products that hinder the evolution of the process. In the complete paper, the authors propose biomimicking approaches to precipitate solid carbonates while limiting the amount of energy required or using the byproducts to generate valuable materials.

Introduction

Permanent sequestration of CO2 as solid carbonates is a feasible solution to the increased levels of CO2 in the atmosphere. Mineral carbonation—the process of capturing CO2 in the atmosphere in the form of solid carbonates through the reaction of CO2 with silicates—is a spontaneous, thermodynamically favorable process. Unfortunately, the kinetics of natural mineral carbonation is very slow and the process is only significant over geological time periods (millions of years).

Accelerated formation of solid carbonates is, nonetheless, widely observed in biological systems, particularly in corals, bivalve molluscs, echinoderms, and foraminifera. These organisms have developed mechanisms to induce and accelerate the precipitation of carbonates, required for their skeletons, in natural saline waters.

Biomimicking is the imitation of biological processes in other contexts for achieving a result not originally present in the mimicked biological systems. While corals need to precipitate carbonate to build their exoskeletons, they do not significantly modify the concentration of CO2 in the atmosphere. The authors propose use of the mechanisms for carbonate precipitation relied upon by corals and other organisms to develop a large-scale process for accelerating the sequestration of CO2 in the form of stable mineral rock.

Process Alternatives

The conversion of CO2 into a mineral form involves the transformation of gaseous CO2 into ionic form and the further reaction of the carbonate ions (CO32−) with divalent cations [e.g., calcium (Ca2+) and magnesium (Mg2+)] to form an insoluble precipitate (e.g., CaCO3). Because the reaction system must maintain electroneutrality, an exchange of ions is usually required. In the case of corals, the carbonate-precipitation reaction occurs in the presence of sea water, which naturally contains Ca2+ and chlorine (Cl−) ions. The Ca2+ ions react with CO32− ions, which are present because of the reaction of CO2 with water. The process results in the reduction of one mole of Ca2+ ions per mole of CO2 removed, and the production of 2 moles of hydrogen ions (H+), thus maintaining electroneutrality. To preserve a favorable pH, however, corals couple the production of H+ with photosynthesis, which provides a net sink of H+ ions.