Scholarly article on topic 'Assessing the Potential of Mineral Carbonation with Industrial Alkalinity Sources in the U.S'

Assessing the Potential of Mineral Carbonation with Industrial Alkalinity Sources in the U.S Academic research paper on "Earth and related environmental sciences"

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Energy Procedia
{"CO2 Mineralization" / Alkalinity / "Industrial byproducts" / "Carbon capture and storage"}

Abstract of research paper on Earth and related environmental sciences, author of scientific article — Abby Kirchofer, Adam Brandt, Sam Krevor, Valentina Prigiobbe, Austin Becker, et al.

Abstract The availability of industrial alkalinity sources is investigated to determine their potential for the mineral carbonation of CO2 from point-source emissions in the United States. The available aggregate markets are investigated as potential sinks for the mineralized CO2 products. Additionally, a life-cycle assessment of aqueous mineral carbonation suggests that a variety of alkalinity sources and process configurations are capable of net CO2 reductions. The CO2 storage potential of mineral carbonation was estimated using the life-cycle assessment results and alkalinity source availability.

Academic research paper on topic "Assessing the Potential of Mineral Carbonation with Industrial Alkalinity Sources in the U.S"

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Energy Procedia 37 (2013) 5858 - 5869


Assessing the Potential of Mineral Carbonation with Industrial Alkalinity Sources in the U.S.

Abby Kirchofera, Adam Brandtb, Sam Krevorc, Valentina Prigiobbed, Austin

Beckere, and Jennifer Wilcoxb*

a Earth, Energy, and Environmental Sciences, Stanford University b Energy Resources Engineering, Stanford University c Petroleum Engineering, Imperial College d Petroleum and Geosystems Engineering, University of Texas at Austin _e Emmett Interdisciplinary Program in Environment & Resources, Stanford University_


The availability of industrial alkalinity sources is investigated to determine their potential for the mineral carbonation of CO2 from point-source emissions in the United States. The available aggregate markets are investigated as potential sinks for the mineralized CO2 products. Additionally, a life-cycle assessment of aqueous mineral carbonation suggests that a variety of alkalinity sources and process configurations are capable of net CO2 reductions. The CO2 storage potential of mineral carbonation was estimated using the life-cycle assessment results and alkalinity source availability.

© 2013 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of GHGT

Keywords: CO2 mineralization; alkalinity; industrial byproducts; carbon capture and storage

1. Introduction

Approximately 30 gigatons (Gt) of CO2 are emitted in the atmosphere worldwide with approximately 6 Gt sourced from the United States alone. [1] It is widely accepted that a portfolio of solutions will be required for mitigation of CO2 at scale. [2] Mineral carbonation has been proposed as a technology to reduce greenhouse gas (GHG) emissions from fossil fuel combustion in a scalable manner. [3] Mineral carbonation is based on chemical reactions that are analogous to the silicate weathering cycle responsible for CO2 uptake on geologic time scales.[4] Mineral carbonation refers to the reaction of CO2 with alkali divalent cations (e.g., Ca2+ or Mg2+) to produce carbonate minerals that are stable at atmospheric

* Corresponding author. Tel.: 1-650-724-9449; fax: 1-650-725-2099. E-mail address:

1876-6102 © 2013 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of GHGT doi:10.1016/j.egypro.2013.06.510

conditions. Both natural and industrial alkalinity sources exist and have been investigated for mineral carbonation. While naturally available alkalinity sources are abundant, their use as alkalinity resources is associated with high-energy costs due to the mining and pre-processing (e.g., grinding) required. [5] Renforth et al. investigated industrial alkalinity source (e.g., aggregate and mine waste, construction and demolition waste, iron and steel slag, and fuel ash) availability for mineral carbonation, and estimate global production and sequestration potential are 7 - 17 Gt/yr and 67 - 1217 Mt-CO2/yr, respectively; however, the sequestration potential results are based on the assumption that the divalent cation content of the material is completely converted to carbonate. [6] Despite the promise of mineral carbonation technologies, to date accounting of alkalinity source production and sequestration potential is limited due to lack of accurate alkalinity source availability/production data, variation in chemical and mineralogical content of alkalinity sources, and inconsistent methods of estimating potential. The present work focuses on industrial alkalinity sources, due to their availability and reactivity, and assesses the feasibility of using industrial alkalinity sources based upon availability, geography, and a life-cycle assessment of their carbonation via aqueous processes.

Common industrial-sourced alkaline byproducts include coal fly ash (FA) [5], electric arc furnace (EAF) dust and steel-making slag[7-11], waste concrete[7, 12] and cement kiln dust (CKD)[12-14], municipal waste incinerator (MSWI) ash[15-17], asbestos mine tailings[5, 18] and bauxite residue[19]. In addition to the potential CO2 mitigation associated with the mineral carbonation of industrial alkalinity sources, this process adds significant environmental benefit in the handling of industrial byproducts that may otherwise be considered as waste (or hazardous waste) materials. For instance, mineral carbonation has been shown to immobilize trace metals in alkaline waste byproducts[20, 21]. Due to the abundance of the alkaline byproduct sources from coal-fired power, cement manufacturing, and steel production industries, the focus of the current study is the potential mitigation of CO2 via mineralization using CKD, FA, and SS.

Cement kiln dust is an alkali-rich dust produced during cement manufacturing at a ratio of approximately 0.15 - 0.20 tons CKD per ton cement.[22] Approximately 5.2 million metric tons (Mt) of CKD are produced annually in the U.S. The typical weight percent ranges of calcium oxide (CaO) and magnesium oxide (MgO) in CKD are 38 - 50 and 0 - 2, respectively.[23] Cement kiln dust is a finegrained solid, with particle size on the order of micrometers (^m)[14], and is an ideal source of alkalinity for mineral carbonation due to its composition and small particle size. [24] Huntzinger et al. investigated carbonation of CKD at conditions of approximately 98% relative humidity, 25°C, and 1 atm with a partial pressure of CO2 of 0.8 atm, and found that the degree of carbonation correlates directly with the mass fraction of calcium oxide and hydroxide content of the CKD.[14] The degree of carbonation at a given time, t, is defined as the mass of CO2 taken up by the sample, MCo2(t), divided by the maximum theoretical carbonation of the sample. The average degree of carbonation was found to be approximately 77% over 8 days, with 90% of the carbonation occurring in less than 2 days.[14]

Fly ash is a residue generated from the combustion of coal, and is typically captured after coal combustion by air pollution control devices such as fabric filters or electrostatic precipitators. Fly ash comprises approximately 60% of all coal combustion waste, and in the U.S. alone coal-fired power plants produce approximately 42.4 Mt of FA annually. [25] Fly ash is a complex, amorphous, and chemically heterogeneous material, and its physicochemical properties depend on the composition of the feed coal and the operating conditions of the coal-fired power plant. In the U.S., coal is ranked as one of four broad categories, listed in order of increasing rank (purity): lignite, subbituminous, bituminous, and anthracite. While FA is often classified based on these ranks, it is important to note that FA composition even within these categories varies greatly due to coal heterogeneity. In general, inorganic minerals comprise approximately 90-99% of fly ash, while organic compounds makeup up approximately 1-9%. [26] The inorganic minerals consist primarily of silicon dioxide (SiO2) and CaO, along with other metal oxides

such as Fe2Ü3 and MgO. The typical weight percent ranges of CaO and MgO in FA are 1 - 37 and 1 - 15, respectively. [27-29] In general, Ca-rich minerals are much less abundant than alumino-silicates and Fe-oxides in high-rank coals; however, in lower rank coals, Ca-rich minerals dominate the inorganic crystalline fraction of FA. [24] Montes-Hernandez et al. investigated the aqueous carbonation of FA and found that 82% of the FA CaO content is converted to CaCO3 after reacting for 2 hours at 30°C or 60°C.[30] The authors report that carbonation conversion is independent of initial CO2 pressure, but did investigate high-pressure conditions ranging from approximately 10 to 39 atm of CO2. Additional investigations have considered the carbonation of fly ash or fly ash-brine mixtures, and have also found that CaO present in FA is readily converted to CaCO3. [27, 31]

Steel slag is a byproduct of iron and steel manufacturing and includes the impurities separated from iron during ore smelting. Slag is comprised of a heterogeneous mixture of crystalline components, including iron oxides, calcium and magnesium hydroxides, oxides, and silicates, and quartz. [32] Slag content varies depending on the ore and the smelting process; specifically, the type of furnace used, e.g., blast furnace (BF), basic oxygen furnace (BOF), EAF, and ladle furnace (LF). It is worth noting that data are rare on the actual average production of slag, as the amount of slag produced is not routinely measured. The amount of slag produced changes depending on the overall chemistry of the raw furnace feed, in particular the iron ore feed grade, and the type of furnace used. Approximately 0.2 tons of steel slag is produced for each ton of iron produced;[32] however, a significant portion of the slag is entrained metal and recovered during slag processing and the amount of marketable slag remaining after entrained steel removal is usually equivalent to between 10% to 15% of the crude steel output. [33] The SS production (after metal removal) in the U.S. is estimated to be 10.3 Mt/yr.

Steel slag is an ideal feedstock for mineral carbonation due to its high alkalinity and, more specifically, high Ca content. The typical weight percent ranges of CaO and MgO in SS are 32 - 58 and 3.9 - 10.0, respectively.[11, 32] Previous investigations suggest that SS is a viable feedstock for cost-effective CO2 sequestration via mineral carbonation.[7, 9, 34] Bonenfant et al. investigated aqueous carbonation of slag suspensions and found sequestration capacities of 0.02 - 0.25 t-CO2/t-slag.[11] Huijgen et al. investigated mineral carbonation of SS in aqueous suspensions and found reaction rate depends primarily on particle size and reaction temperature. [9] The authors report 74% extent reacted after 30 minutes for SS with particle size less than 38 цт at 19 bar and 100°C. Stolaroff et al. estimated a SS carbonation potential of 0.27 ton CO2 sequestered per ton SS, assuming that 75% of Ca content reacts with CO2, with an estimated cost of $8/ton CO2 sequestered. 6

The extent to which synthetic aggregate production from mineral carbonation using industrial alkalinity sources could replace mined aggregate has also been investigated. In addition to serving as a sink for the mineralized CO2, this synthetic aggregate has the potential co-benefit of preventing CO2 emissions associated with mining aggregate. For this reason, the size of the aggregate market in the U.S. has been investigated since this market will inevitably serve as an upper limit of the CO2 mitigation potential from reuse of CO2 mineralization products. Natural aggregates are traditionally sourced from either crushed stone or sand and gravel, and the various aggregates can frequently be interchanged with one another. The estimated annual outputs of crushed stone and sand and gravel produced for consumption in the U.S. in 2010 were 1.19 Gt and 820 Mt, respectively.[35] The 2010 market value of all natural aggregates was 17.5 billion dollars.[35] Natural aggregates are primarily used in the construction industry, and account for approximately half of U.S. mining industry output.[36] However, natural aggregates are not universally available and some areas lack quality and/or practically accessible natural aggregate.

To assess the CO2 mitigation potential of mineral carbonation using industrial alkalinity sources, the current work determines the abundance and geographic location of industrial alkaline sources. In addition, the potential U.S. production rates of synthetic aggregate produced from reaction of CO2 with industrial-

based alkalinity sources are compared on a state-by-state basis to the mined aggregate industry, to assess the extent to which synthetic aggregate could replace mined aggregate. Finally, the results of a life-cycle assessment of aqueous mineral carbonation are used to determine the mineral carbonation potential of CKD, FA, and SS.

2. Methodology

2.1. Industrial alkalinity source capacity and mapping

The carbonation capacity for a given alkalinity source depends on the total alkalinity available, the reactivity of the alkaline components, the kinetics of the reaction, and the reaction conditions. In the present work, available alkalinity is used as a direct measure of the carbonation capacity of a given resource, and is a function of the maximum theoretical carbonation capacity of the resource and the expected extent reacted. The maximum theoretical carbonation of a material is a measure of the alkalinity of the material; in the present work, alkalinity is defined to include Ca2+ and Mg2+.[37] The expected extent reacted is based on previous reported values, explained in more detail for each resource below. Extent reacted is typically reported as a percentage of the maximum theoretical reacted. The available alkalinity is defined as the total Ca2+ and Mg2+ alkalinity of a resource multiplied by the percentage of expected extent reacted. Given a generic alkaline resource (MO), the carbonation reaction can be expressed as:

MO + CO2 ** MCO3 + heat ^

with a 1:1 molar ratio of CO2 to mineral oxide and to the carbonate product formed.

U.S. maps of CO2 emissions and alkalinity source, mined aggregate, and synthetic aggregate production, were compiled using Geographical Information Systems (GIS) software ArcGIS.[38] The National Energy Technology Laboratory National Carbon Sequestration Database (NATCARB) was used to determine CO2 emissions by source for coal-fired power plants, cement kilns, and steel plants.[39] The production rate of FA, CKD, and SS alkalinity was estimated based on available data, as described below. Natural aggregate production by state was determined from the 'Mineral Operations - Sand and gravel' and 'Mineral Operations - Crushed Stone' of the National Atlas 2005 (map layers compiled by the Minerals Information Team of the USGS).[40, 41]

The alkalinity sourced from fly ash was estimated based upon the type of coal burned and the typical concentrations of Ca and Mg in each coal type. The coal types considered in this study include Appalachian Low-Sulfur bituminous, Appalachian Medium-Sulfur bituminous, Wyoming Powder River Basin subbituminous, Wyodak bituminous, North Dakota lignite, and Illinois #6 bituminous. Information regarding the U.S. power plants location in addition to capacity and type of coal burned was determined from electricity data files available from the U.S. Energy Information Administration. [42] The coal composition, including ash content and distribution of calcium, magnesium, and iron oxides, differs among the various coal types and was determined from the internal fuel library of the Integrated Environmental Control Module (IECM) developed by Rubin and colleagues at Carnegie Mellon University.[43] For each of the coal types considered, the ratio of fly ash produced to CO2 emitted was calculated from this software package assuming a 500-MW power plant. On average, power plants generate approximately 10 to 13 tons per hour of ash, which is small in comparison to the approximate 435 tons per hour of CO2 generated. The rate of fly ash production by each power plant was calculated by multiplying the CO2 emissions from the plant by the appropriate fly ash production to CO2 emissions ratio. Based on previous work by Montes-Hernandez, the expected extent reacted for FA is assumed to be 82%.[30]

For CKD and SS, the rate of alkalinity production was estimated based on the typical concentrations of Ca and Mg in the alkalinity source. The amount of CKD generated per source was calculated based on the CO2 emissions of the source, a clinker to CO2 production ratio of 1, and a CKD to clinker production ratio of 0.06, based on the assumption that a non-hazardous fuel kiln and dry process are used.[12, 44, 45] Based on data from Huntzinger et al. the expected extent reacted for CKD is assumed to be 77%. [14] The amount of SS generated per source was calculated based on the CO2 emissions of the source, a ratio of CO2 emitted to steel produced of 0.64, and a ratio of steel produced to slag generated of 8.33.[11, 46] Based on previous work by Huijgen et al. the expected extent reacted for SS is assumed to be 75%. [9]

2.2. Life cycle assessment

The LCA model allows for the evaluation of the tradeoffs between different reaction enhancement processes while considering the larger lifecycle impacts on energy use and material consumption. The LCA model methodology is described in detail in a previous publication by Kirchofer et al. [47] All main process stages are included in the tool, and comprehensive system boundaries are applied throughout the model. The process-model core of the mineral carbonation LCA tool includes 7 process stages defined generically to be applicable to a variety of mineral carbonation technologies (see Figure 1). Because our tool aims to compare process schemes that vary significantly in input resource and process design, the model was built at a general, first-order level. The methodology accounts for the following three types of energy consumption: on-site energy consumption, energy of material and energy inputs consumed in the sector of interest (embodied direct energy), and energy of material and energy inputs consumed in all other sectors (embodied indirect energy). Including both on-site and embodied energy allows a full accounting of the total greenhouse gas (GHG) reduction benefits of each process scheme.

Extraction transportation processing conversion processing transportation or reuse

Fig. 1. Life cycle process model schematic for aqueous mineral carbonation; thickness of lines is scaled to the energy and mass fluxes (inputs enter from top, outputs leave through bottom)—Reproduced by permission of The Royal Society of Chemistry. [47]

Using the LCA model, the energy efficiency and the net CO2 storage potential of various mineral carbonation processes based on different feedstock material and process schemes was compared on a consistent basis by determining the energy and material balance of each implementation.[47] In particular, we evaluated the net CO2 storage potential (i.e. sequestration efficiency) of aqueous mineral carbonation CKD, FA, and SS across a range of reaction conditions and process parameters.

3. Results

In order to assess the potential for mineral carbonation using industrial alkalinity sources, the total U.S. production of CKD, FA, and SS and associated alkalinity were estimated, as shown in Table 1. The calculated production rates are in agreement with estimates reported in previous literature. [25, 33, 48]

The range of alkalinity produced is based on the low and high values of Ca and Mg in the alkalinity source (see Introduction) and the estimated uncertainty in the alkalinity source production rate.

Table 1. U.S. production of CKD, FA, and SS

Industrial alkalinity source U.S. Production (Mt/yr) Uncertainty8 (%) Ca alkalinity (Mt/yr) Mg alkalinity (Mt/yr)

Cement Kiln Dust 5.2 ±15 2.3 0.1

Fly Ash 52.8 ±10 2.5 0.7

Steel Slag 10.3 ±15 4.6 0.7

a Includes uncertainty of the NATCARB CO2 emissions data, assumed to be 5%, and for CKD and SS uncertainty in production rate of alkalinity source per CO2 emitted, assumed to be 10%, and for FA uncertainty in the IECM data, assumed to be 5%

The total alkalinity available from CKD, FA, and SS is approximately 10.9 Mt/yr, with a CO2 mineralization capacity of approximately 7.0 Mt-CO2/yr.

To determine the geographic relationship between CO2 emissions sources and industrial alkalinity sources, the locations of industrial alkalinity sources have been mapped in relation to U.S. CO2 emissions. Figure 2 provides the locations of stationary CO2 emissions in the conterminous United States overlain with circles representing CKD, FA, and SS production locations, scaled to the relative annual production.[39] Almost all locations where CKD, FA, or SS is produced also produce CO2 because FA is a byproduct of burning fossil fuels and because cement kilning and iron and steel manufacturing are energy intensive processes. This relationship is illustrated in Figure 2. States with no production of CKD, FA, and SS are: Alaska, Connecticut, Hawaii, Maine, Massachusetts, New Hampshire, Rhode Island, and Vermont.

CKD Fly Ash Slag CO, Sources (kton/year)

• 0-100 • 0-100 • 0-100 • 0-250

• 100-500 • 100-500 • 100-500 250-1000

• A • >1000

>500 W >500 >500

Fig. 2. Production of industrial alkalinity and CO2 (CO2 sources based on NatCarb, 2010).

Absolute mined aggregate volumes are illustrated in Figure 3 by state, with the darker shades representing the higher production areas. Examination of Figure 3 reveals that California, Pennsylvania and Texas have the highest volumes of mined aggregate. Only two states (Montana and New Hampshire)

have mined aggregate volumes below 10,000 kiloton in 2010. In addition, the synthetic aggregate produced from the reaction of C02 (from point-source power plant emissions) with industrial-based



' -v • • ; jRgggp

Potential aggregate from industrial alkalinity (kton/year) CKD based Fly Ash based Slag based

• 0-100 • 0-100 100-500 # 100-500

0-100 100-500

Mined aggregate production (mton/year)

No Data

0-25 25-50 50-75 75-100 ■ >100

Fig. 3. Potential aggregate from mineral carbonation of industrial alkalinity compared to mined aggregate production.

alkalinity sources is presented in Figure 3 and divided between slag, CKD, and fly ash sources. Given that the alkalinity source is the limiting resource for CO2 mineralization, the location of the synthetic aggregate production was assumed to be that of the alkalinity source rather than the CO2 source. From Figure 3 it becomes clear that the extent of synthetic aggregate production in total is an order of magnitude smaller than the mined aggregate volumes, and it can be assumed that most synthetic aggregate will find a market locally.

Ratio of potential synthetic to mined aggregate

No data

0.1 -1%

1 - 5%


H >10%

Fig. 4. Ratio of potential synthetic aggregate from mineral carbonation of industrial alkalinity to mined aggregate.

The potential total U.S. synthetic aggregate production using FA, CKD, and SS is estimated to be 20 Mt/yr, approximately 1.7% of total U.S. mined aggregate (i.e., 1.19 Gt/yr).[35] Figure 4 shows that the potential market share of synthetic aggregate from CO2 mineralization using industrial-based waste products varies greatly in the U.S. In addition to the CO2 mitigation from the use of emitted CO2 toward the production of the synthetic aggregate, it is interesting to also consider the CO2 emissions associated with mining aggregate and the CO2 mitigation potential of displacing the mined aggregate with synthetic. The CO2 emissions associated with mining of crushed granite, crushed limestone, and industrial sand have been determined based on the direct consumption of energy associated with mining. [49-52]. Table 2 shows the CO2 emissions per ton of mined material, and the potential CO2 emissions mitigation by replacing 1.7% of U.S. mined aggregate production with synthetic aggregate. The CO2 mitigation potential of replacing mined aggregate with synthetic aggregate depends on the source of the mined aggregate, and the greatest impact would be attained by replacing sand and gravel-based aggregate given the high CO2 emissions associated with sand and gravel mining.

Table 2. CO2 emissions and mitigation potential of mined aggregate

Mined aggregate CO2 emissions (kg CO2/t mined) Mitigated CO2a (Mt COJyr)

Crushed granite 3.6 0.07

Crushed limestone 4.5 0.09

Sand and gravel 23.9 0.48

a assuming 1.7% of U.S. mined crushed stone, crushed limestone, or sand and gravel aggregated replaced with synthetic aggregate

The life-cycle assessment of aqueous mineral carbonation suggests that a variety of alkalinity sources and process configurations are capable of net CO2 reductions. The maximum carbonation efficiency, defined as mass percent of CO2 mitigated per CO2 input, was 83% for CKD at ambient temperature and pressure conditions. The maximum carbonation efficiencies for SS and FA were 64% and 36%, respectively.

The CO2 storage potential of mineral carbonation is estimated using the life-cycle assessment results and alkalinity source availability. The annual storage potential for a given alkalinity source was calculated by multiplying its availability (Mt/yr) by the CO2 sequestration efficiency of mineral carbonation of that alkalinity source (t-CO2/t-alkalinity source). For industrial alkalinity sources, availability is based on U.S. production rates.37 The low estimate assumes the maximum sequestration efficiency of the alkalinity source obtained in the life-cycle assessment and the high estimate assumes a sequestration efficiency of 95%. The total CO2 storage potential for FA, CKD, and SS in the U.S. ranges from 4.2 - 7.0 Mt/yr (~ 0.1% of U.S. CO2 emissions), depending on the assumed efficiency of the mineral carbonation processes. Figure 5 shows the CO2 storage potential of mineral carbonation for the different alkalinity sources.

Fig. 5. CO2 storage potential of mineral carbonation of CKD, FA, and SS compared to U.S. CO2 emissions

4. Conclusions

This work shows that in the U.S., FA, CKD, and SS-based alkalinity has the potential to mitigate approximately 7.5 Mt CO2/yr, of which 7.0 Mt CO2/yr are CO2 captured through mineralization and 0.5 Mt CO2/yr are CO2 emissions avoided through reuse of synthetic aggregate. This represents a small share (0.1%) of U.S. total CO2 emissions; however, industrial byproducts may represent comparatively low-cost methods for the development of CO2 mineralization technologies, which could be extended to abundant but more expensive natural alkalinity sources.


The authors acknowledge the support of the Joint Institute for Strategic Energy Analysis, which is operated by the Alliance for Sustainable Energy, LLC, on behalf of the U.S. Department of Energy's National Renewable Energy Laboratory, the University of Colorado-Boulder, the Colorado School of Mines, the Colorado State University, the Massachusetts Institute of Technology, and Stanford University. The authors would like to express their gratitude to Patricia Carbajales, Geospatial Manager at the Branner Earth Sciences Library and Map Collections, and Jasper Van der Bruggen, Noemi Alvarez, Wenny W. Ng, and Emilia Dicharry, for assistance with the GIS resources and data compilation. Professor Charles F. Harvey and Dr. Kurt Z. House are also acknowledged for helpful discussions.


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