Carbon dioxide sequestration by mineral carbonation

Abstract

The increasing atmospheric carbon dioxide (CO2) concentration, mainly caused by fossil fuel combustion, has lead to concerns about global warming. A possible technology that can contribute to the reduction of carbon dioxide emissions is CO2 sequestration by mineral carbonation. The basic concept behind mineral CO2 sequestration is the mimicking of natural weathering processes in which calcium or magnesium containing minerals react with gaseous CO2 and form solid calcium or magnesium carbonates: (Ca,Mg)SiO3 (s} + CO2 (g) ->(Ca,Mg)CO3 (s) + SiO2 (s)Potential advantages of mineral CO2 sequestration compared to, e.g., geological CO2 storage include (1) the permanent and inherently safe sequestration of CO2, due to the thermodynamic stability of the carbonate product formed and (2) the vast potential sequestration capacity, because of the widespread and abundant occurrence of suitable feedstock. In addition, carbonation is an exothermic process, which potentially limits the overall energy consumption and costs of CO2 emission reduction. However, weathering processes are slow, with timescales at natural conditions of thousands to millions of years. For industrial implementation, a reduction of the reaction time to the order of minutes has to be achieved by developing alternative process routes.The aim of this thesis is an investigation of the technical, energetic, and economic feasibility of CO2 sequestration by mineral carbonation.In Chapter 1 the literature published on CO2 sequestration by mineral carbonation is reviewed. Among the potentially suitable mineral feedstock for mineral CO2 sequestration, Ca-silicates, more particularly wollastonite (CaSiO3), a mineral ore, and steel slag, an industrial alkaline solid residue, are selected for further research. Alkaline Ca-rich residues seem particularly promising, since these materials are inexpensive and available near large industrial point sources of CO2. In addition, residues tend to react relatively rapidly with CO2 due to their (geo)chemical instability.Various process routes have been proposed for mineral carbonation, which often include a pre-treatment of the solid feedstock (e.g., size reduction and/or thermal activation). The only available pre-treatment option that has proven to be energetically and potentially economically feasible is conventional grinding.Two main types of process routes can be distinguished; (1) direct routes in which carbonation takes place in a single step process, either in a gas-solid or a gas-liquid-solid process, and (2) indirect routes in which the Ca is first extracted from the silicate matrix and subsequently carbonated in a separate process step. The aqueous route in which Ca-silicates are directly carbonated in an aqueous suspension at elevated temperature and CO2 pressure is selected as the most promising process route for further investigation. The following key issues for further research are identified: the reaction rates and mechanisms of mineral carbonation as well as its energy consumption and sequestration costs. Another important aspect of mineral carbonation is the destination of the carbonated products.In Chapter 2 the mechanisms of aqueous steel s!ag carbonation are studied experimentally. Process variables, such as particle size, temperature, and carbon dioxide pressure are systematically varied and their influence on the carbonation rate is investigated. The maximum carbonation degree reached is 74% of the Ca content in 30 minutes at 19 bar CO2 pressure, 100 0C, and a particle size of<38 prn. The two most important factors determining the reaction rate are particle size (<2 mm to<38 pm) and reaction temperature (25-225 0C). The carbonation reaction is found to occur in two steps: (1) leaching of calcium from the steel slag particles into the solution and (2) precipitation of calcite on the surface of these particles. The first step and, more in particular, the diffusion of calcium through the solid matrix towards the surface, appears to be the rate-determining reaction step. The Ca-diffusion is found to be hindered by the formation of a CaCO3-coating and a Ca-depleted silicate zone during the carbonation process.In Chapter 3 the mechanisms of aqueous steel slag carbonation are further investigated, together with the environmental properties of the (carbonated) steel slag. Steel slag samples are carbonated to a varying extent and leaching experiments and geochemical modelling are used to identify solubility-controlling processes of both major and minor elements that are present in the slag. Carbonation is shown to reduce the leaching of alkaline earth metals (except Mg) by conversion of Ca-phases, such as portlandite, ettringite, and Ca-(Fe)-silicates into calcite, possibly containing traces of Ba and Sr. The leaching of vanadium increases substantially upon carbonation, probably due to the dissolution of a Ca-vanadate. The increased reactive surface area of AI- and Fe-(hydr)oxides after carbonation tends to reduce the leaching of sorption-controlled trace elements. Sorption on Mn-(hydr)oxides is found to be also required to adequately model the leaching of divalent cations, but is not influenced by carbonation, Consideration of these three distinct reactive surfaces and possible (surface) precipitation reactions resulted in adequate modelling predictions of oxyanion and trace metal leaching from (carbonated) steel slag. Hence, these surfaces exert a major influence on the environmental properties of both fresh and carbonated steel slag.In Chapter 4, the mechanisms of aqueous wollastonite carbonation as a possible carbon dioxide sequestration process are investigated experimentally by systematic variation of the reaction temperature, CO2 pressure, particle size, reaction time, liquid-to-solid ratio, and agitation power. The carbonation reaction is observed to occur via the aqueous phase in two steps: (1) Ca leaching from the CaSiO3 matrix and (2) CaCO3 nucleation and growth. Leaching is hindered by a Ca-depleted silicate rim resulting from incongruent Ca-dissolution. Two temperature regimes are identified in the overall carbonation process. At temperatures below an optimum reaction temperature, the overall reaction rate is probably limited by the leaching rate of Ca. At higher temperatures, nucleation and growth of calcium carbonate is probably limiting the carbonation rate, due to a reduced (bi)carbonate activity. The mechanisms for the aqueous carbonation of wollastonite are shown to be similar to those of steel slag (Chapter 2) and of the Mg-silicate olivine. The carbonation of wollastonite proceeds rapidly relative to Mg-silicates, with a maximum conversion of 70% in 15 min at 200 0C, 20 bar CO2 partial pressure, and a particle size of<38 um.The obtained insight in the reaction mechanisms in Chapter 2 - 4 is used as the (experimental) basis for the energetic and economic assessment of CO2 sequestration by mineral carbonation in Chapters 5 & 6.The energy consumption of a mineral carbonation plant causes extra CO2 emssions and, thereby, reduces the net amount of CO2 sequestered by the process. Chapter 5 studies the energetic CO2 sequestration efficiency (i.e., the fraction of CO2 that is sequestered effectively) of the aqueous mineral carbonation in dependence of various process variables using either wollastonite or steel slag as feedstock. A flowsheet of a mineral carbonation plant is designed and the process is simulated to determine the properties of streams as well as the power and heat consumption of the process equipment. For woliastonite, the maximum energetic efficiency within the ranges of process conditions studied is 75% at 200 0C, 20 bar CO2, and a particle size of<38 ?m. The main energy-consuming process steps are the grinding of the feedstock and the compression of the CO2 feed. At these conditions, a significantly lower efficiency is determined for steel slag (69%), mainly due to the lower Ca content of the feedstock. The CO2 sequestration efficiency might be improved substantially for both types of feedstock by e.g. reducing the amount of process water applied and further grinding of the feedstock, In Chapter 6 a cost evaluation of CO2 sequestration by aqueous mineral carbonation is presented, using either wollastonite or steel slag as feedstock. On the basis of a basic design of the major process equipment, the total investment costs are estimated with the help of pubiic!y available literature and a factorial cost estimation method. Subsequently, the sequestration costs are determined on the basis of the depreciation of investments and variable and fixed operating costs. Estimated costs are 102 and 77 /ton CO2 net avoided for wollastonite and steel slag, respectively. For wollastonite, major costs are associated with the feedstock and the electricity consumption for grinding and compression (54 and 26 /ton CO2 avoided, respectively). The sequestration costs for steel slag are significantly lower due to the absence of costs for the feedstock. A sensitivity analysis shows that additional influential parameters in the sequestration costs include the liquid-to-solid ratio in the carbonation reactor and the possible value of the carbonated product. In the Epilogue the main conclusions of this thesis are summarised and recommendations for further research are given.This thesis shows that CO2 sequestration by carbonation of Ca-silicates is possible at technically feasible process conditions. Altough the energy consumption of current mineral carbonation processes is substantial, the identified possibilities to reduce the energy demands of the process suggest that mineral carbonation may become energetically feasible after further technology development. Finally, the costs of CO2 sequestration by mineral ore carbonation processes are relatively high compared to other CO2 storage technologies and (current) CO2 market prices. (Niche) applications of mineral carbonation based on the use of a solid residue as feedstock and/or the production of a carbonation product with positive value, hold significantly better prospects for an economically feasible process.Overall, mineral CO2 sequestration is (still) a longer-term option compared to other 'carbon capture & storage'-technologies and probably has limited potential in the short term. However, the possibilities identified for further process improvement, the permanent and inherently safe character of the CO2 sequestration, and the large sequestration potential warrant further research on mineral CO2 sequestration. This research should primarily focus on cost reduction, which is a prerequisite for mineral CO2 sequestration to become part of a broad portfolio of employable CO2 mitigation options