Carbon dioxide sequestration by mineral carbonation

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

Mineral CO2 sequestration by steel slag carbonation

Mineral CO2 sequestration, i.e., carbonation of alkaline silicate Ca/Mg minerals, analogous to natural weathering processes, is a possible technology for the reduction of carbon dioxide emissions to the atmosphere. In this paper, alkaline Ca-rich industrial residues are presented as a possible feedstock for mineral CO2 sequestration. These materials are cheap, available near large point sources of CO2, and tend to react relatively rapidly with CO2 due to their chemical instability. Ground steel slag was carbonated in aqueous suspensions to study its reaction mechanisms. Process variables, such as particle size, temperature, carbon dioxide pressure, and reaction time, were systematically varied, and their influence on the carbonation rate was investigated. The maximum carbonation degree reached was 74% of the Ca content in 30 min at 19 bar CO2 pressure, 100 °C, and a particle size o

Development of Seaweed Biorefineries for Fuels and Chemicals

<p>Presentation of the development of seaweed biorefineries for fuels and chemicals, presented at EUBCE, the 24th European Biomass Conference and Exhibition, Amsterdam, The Netherlands, 6 - 9 June 2016.</p

Carbonation of steel slag for CO2 sequestration: Leaching of products and reaction mechanisms

Carbonation of industrial alkaline residues can be used as a CO2 sequestration technology to reduce carbon dioxide emissions. In this study, steel slag samples were carbonated to a varying extent. Leaching experiments and geochemical modeling were used to identify solubility-controlling processes of major and trace elements, both with regard to carbonation mechanisms and the environmental properties of the (carbonated) steel slag. Carbonation was 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 increased substantially upon carbonation, probably due to the dissolution of a Ca-vanadate. The reactive surface area of Al- and Fe-(hydr)oxides increased with the carbonation degree, which tends to reduce the leaching of sorption-controlled trace elements. Sorption on Mn-(hydr)oxides was found to be required to adequately model the leaching of divalent cations, but was not influenced by carbonation. Consideration of these three distinct reactive surfaces and possible (surface) precipitation reactions resulted in adequate modeling 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 sla

Mechanisms of aqueous wollastonite carbonation as a possible CO2 sequestration process

The mechanisms of aqueous wollastonite carbonation as a possible carbon dioxide sequestration process were 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 was 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 were 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 are probably limiting the conversion, due to a reduced (bi)carbonate activity. The mechanisms for the aqueous carbonation of wollastonite were shown to be similar to those reported previously for an industrial residue and a Mg-silicate. The carbonation of wollastonite proceeds rapidly relative to Mg¿silicates, with a maximum conversion in 15 min of 70% at , 20 bar CO2 partial pressure and particle size of . The obtained insight in the reaction mechanisms enables the energetic and economic assessment of CO2 sequestration by wollastonite carbonation, which forms an essential next step in its further development

Cost Evaluation of CO2 Sequestration by Aqueous Mineral Carbonation

A cost evaluation of CO2 sequestration by aqueous mineral carbonation has been made using either wollastonite (CaSiO3) or steel slag as feedstock. First, the process was simulated to determine the properties of the streams as well as the power and heat consumption of the process equipment. Second, a basic design was made for the major process equipment, and total investment costs were estimated with the help of the publicly available literature and a factorial cost estimation method. Finally, the sequestration costs were determined on the basis of the depreciation of investments and variable and fixed operating costs. Estimated costs are 102 and 77 Euro/ton CO2 net avoided for wollastonite and steel slag, respectively. For wollastonite, the major costs are associated with the feedstock and the electricity consumption for grinding and compression (54 and 26 Euro/ton CO2 avoided, respectively). A sensitivity analysis showed 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. The sequestration costs for steel slag are significantly lower due to the absence of costs for the feedstock. Although various options for potential cost reduction have been identified, CO2 sequestration by current aqueous carbonation processes seems expensive relative to other CO2 storage technologies. The permanent and inherently safe sequestration of CO2 by mineral carbonation may justify higher costs, but further cost reductions are required, particularly in view of (current) prices of CO2 emission rights. Niche applications of mineral carbonation with a solid residue such as steel slag as feedstock and/or a useful carbonated product hold the best prospects for an economically feasible CO2 sequestration process. (c) 2007 Elsevier Ltd. All rights reserved

Pyrolysis of wheat straw-derived organosolv lignin

The cost-effectiveness of a lignocellulose biorefinery may be improved by developing applications for lignin with a higher value than application as fuel. We have developed a pyrolysis based lignin biorefinery approach, called LIBRA, to transform lignin into phenolic bio-oil and biochar using bubbling fluidized bed reactor technology. The bio-oil is a potential source for value-added products that can replace petrochemical phenol in wood-adhesives, resins and polymer applications. The biochar can e.g. be used as a fuel, as soil-improver as solid bitumen additive and as a precursor for activated carbon. In this paper we applied the pyrolysis-based LIBRA concept for the valorisation of wheat straw-derived organosolv lignin. First, we produced lignin with a high purity from two wheat straw varieties, using an organosolv fractionation approach. Subsequently, we converted these lignins into bio-oil and biochar by pyrolysis. For comparison purposes, we also tested two reference lignins, one from soda-pulping of a mixture of wheat straw and Sarkanda grass (Granit) and one from Alcell organosolv fractionation of hardwoods. Results indicate that ~80 wt% of the dry lignin can be converted into bio-oil (with a yield of 40–60%) and biochar (30–40%). The bio-oil contains 25–40 wt% (based on the dry lignin weight) of a phenolic fraction constituting of monomeric (7–11%) and oligomeric (14–24%) components. The monomeric phenols consist of guaiacols, syringols, alkyl phenols, and catechols. 4-vinylguaiacol is the major phenolic monomer that is formed during the pyrolysis of the straw lignins in yields from 0.5–1 wt%. For the hardwood-lignin Alcell, the predominant phenol is 4-methylsyringol (1.2 wt%). The ratio guaiacols/syringols seems to be an indicative marker for the source of the lignin

Lignin pyrolysis for profitable lignocellulosic biorefineries

Bio-based industries (pulp and paper and biorefineries) produce > 50 Mt/yr of lignin that results from fractionation of lignocellulosic biomass. Lignin is world's second biopolymer and a major potential source for production of performance materials and aromatic chemicals. Lignin valorization is a key-issue for enhanced profitability of sustainable bio-based industries. Despite a myriad of potential applications for lignin and decades of research, its heterogeneity and recalcitrance still preclude commercial value-added applications. Most lignin is utilized for heat and power. Unconventional solutions are needed to better exploit lignin's potential. Organosolv lignins are especially suitable as feedstock for high-value chemicals. At ECN, a lignin biorefinery approach (LIBRA) has been developed, involving a dedicated lignin pyrolysis protocol that is robust, continuous, and capable of processing different lignins. Typical product yields are 20% gas, 35% char, and 45% oil. The oil contains approximately 45% oligomeric phenolic substances, 23% monomeric phenols, and 33% water. The future perspective is scale-up of the process to produce larger lignin pyrolysis oil samples for separation, purification, and industrial application tests. Presently, small lignin pyrolysis oil samples are investigated as feedstock for extracting high-value chemicals, as a substitute for phenol in several applications, and as a feedstock for hydrotreating. The biochar is tested as growth enhancer and as substitute for carbon-black in rubber. Regarding the large lignin side streams from (future) bio-based industries, the LIBRA pyrolysis technology has ample potential to increase the profitability of lignocellulosic biorefineries provided that for both the liquid product and the solid char value-added applications are developed

Opportunities and challenges for seaweed in the biobased economy

The unique chemical composition of seaweeds and their fast growth rates offer many opportunities for biorefining. In this article we argue that cascading biorefinery valorization concepts are viable alternatives to only using seaweeds as carbohydrate sources for the fermentative production of biofuels. However, many challenges remain with respect to use of seaweeds for chemical production, such as the large seasonal variation in the chemical composition of seaweeds

Characteristics of Wheat Straw Lignins from Ethanol-based Organosolv Treatment

Non-purified lignins resulting from ethanol-based organosolv fractionation of wheat straw were characterized for the presence of impurities (carbohydrates and ash), functional groups (hydroxyl, carboxyl and methoxyl), phenyl-propanoid structural moieties, molar mass distribution and thermal behavior. In accordance with its herbaceous nature, the syringyl/guaiacyl-ratio of the wheat straw lignins was substantially lower than of Alcell lignin. In addition, the content of p-hydroxyphenyl and carboxyl groups is substantially higher for the wheat straw lignins. The non-purified organosolv lignins had a high purity with 0.4–5.2% carbohydrate impurities, both originating from lignin to carbohydrate complexes and residual organosolv liquor. The use of H2SO4 in the organosolv process improved the lignin yield, but at low acid doses increased the carbohydrate impurities. For applications where a low amount of carbohydrates is important, lignin from a high-temperature autocatalytic organosolv process was found to be preferred. The highest content of total hydroxyl groups was determined when lignins were produced using 30 mM H2SO4 as catalyst or 50% w/w aqueous ethanol as solvent for the organosolv process. Aliphatic hydroxyl groups, the most predominant type of hydroxyl groups present originating for a substantial part from residual carbohydrates, were found to decrease with reaction time and ethanol proportion of the organosolv solvent. The correlations between organosolv process conditions and lignin characteristics determined can facilitate the use of organosolv lignins in value-added applications such as in polymers and resins and as a feedstock for bio-based aromatics