Waste materials for carbon capture and storage by mineralisation (CCSM) – A UK perspective

Applied Energy 99 (2012) 545–554

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Waste materials for carbon capture and storage by mineralisation (CCSM) – A UK perspective Aimaro Sanna a,b,⇑, Marco Dri b,c, Matthew R. Hall c, Mercedes Maroto-Valer a a

School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS, UK Energy and Sustainability Research Division, Faculty of Engineering, University of Nottingham, Nottingham, NG7 2RD, UK c Nottingham Centre for Geomechanics, Faculty of Engineering, University of Nottingham, Nottingham, NG7 2RD, UK b

h i g h l i g h t s " This work illustrates the potential of using mineral wastes for CO2 abatement by mineral carbonation in the UK. " Mineral wastes have the potential to capture 0.1–1 Mt/year CO2 as CCSM resource in the UK. " Wastes’ volumes and chemical composition represent a major obstacle for their deployment in CCSM. " Use of waste resources for CCSM should be considered as a niche market in the introductory demonstration stage of CCSM.

a r t i c l e

i n f o

Article history: Received 18 March 2012 Received in revised form 14 June 2012 Accepted 15 June 2012 Available online 23 July 2012 Keywords: Clean energy Mineral carbonation CCS Solid waste Waste reuse

a b s t r a c t This work reviews the advantages and disadvantages of using mineral wastes for CCS and their potential in CO2 abatement, highlighting the potential applications and scenarios. This study indicates that a variety of inorganic waste materials such as pulverised fuel ash, municipal solid waste ash, cement kiln dust, biomass and paper sludge ash and sewage sludge ash are available feedstocks for Carbon Capture and Storage by Mineralisation (CCSM) in the UK. The high variability of both the waste amounts and chemical composition represent a major obstacle to the deployment of these materials in CCSM. Currently, mineral waste resources for mineral carbonation have the theoretical potential to capture about 1 Mt/year CO2 in the UK, considering only the materials not recycled that are currently sent to landfill. Moreover, inorganic waste as a CCSM resource is in many ways more complex than the use of natural minerals due to uncertainty on future availability and high chemical variability and might be viable only in niche applications. For example, the use of inorganic wastes (concrete waste and steel slag) and buffer solutions in spray trickle bed systems (able to sequester 50% of the CO2 entering the system) was estimated to have costs competitive with geological storage. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Fossil fuels account for 80–85% of the total of global energy production. However, their use is facing significant challenges due to the vast amounts of CO2 released during combustion and calcination in heavy industries and power generation [1,2]. Carbon Capture and Storage (CCS) is a technology that can significantly reduce the impacts of climate change by capturing the CO2 produced from the combustion of fossil fuels. The captured CO2 must then be transported to an underground storage site, where it will be stored away from the atmosphere for a very long time [2]. However, full lange-scale integrated demonstration ⇑ Corresponding author. Address: Energy and Sustainability Research Division, Faculty of Engineering, University of Nottingham, Nottingham NG7 2RD, UK. Tel.: +44 (0)115 9514198. E-mail address: [email protected] (A. Sanna). 0306-2619/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2012.06.049

projects and numerical simulations are still required to enhance the knowledge on long-term geological storage of CO2 before the wider deployment of CCS in geological formations takes place [3,4]. Due to slow progress in the deployment of technologies that store CO2 underground [5], and the fact that access to underground storage may not be entirely feasible, or even possible in some parts of the world, there has been an increasing interest in mineral carbonation. Mineral carbonation (referred also as mineralization or mineral sequestration) is one of the latest technologies to join the CCS portfolio. Mineral carbonation may be deemed advantageous because it is an ex situ and permanent process, however its costs are currently considered not competitive with those of geological storage [6]. The process consists of reacting CO2 with a divalent metal oxide (MO) to produce a metal carbonate (MCO3) and release heat.

MO þ CO2 ! MCO3 þ Heat

ð1Þ

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A. Sanna et al. / Applied Energy 99 (2012) 545–554

Unfortunately, the basic solid–gas chemical reaction takes place on geological time-scale and speeding it up has been the focus of researchers. In light of this, mineral carbonation technologies can be divided into two main groups: single-step processes and multi-step processes. Single-step processes involve the reaction of feedstock material with CO2, which is usually injected in a reactor maintained at a controlled temperature and pressure. Minerals require energy-intensive pre-treatments, such as fine grinding, heat treatment, and chemical activation to provide adequate conversions and reaction kinetics [2]. Multi-step processes have also been developed using chemicals to first extract the reactive fraction (e.g. pure metal oxide) from the feedstock, and secondly react it with CO2. For both routes efficiency depends on the nature of the feedstock and the parameters employed in carbonation (pressure, temperature, solid/liquid ratio, particle size) [6]. One of the advantages of CCS by Mineralisation (CCSM) is that the raw material is estimated to have carbon storage capacity one order of magnitude larger that global fossil reserves [7]; where minerals rich in magnesium and calcium can be easily mined. However, extraction of reactive Mg and Ca from the silicate-minerals matrix requires high energy demand [8]. Alkaline industrial waste residues represent an alternative source of readily available and reactive Ca and Mg oxides suitable for CCSM. Recently, new investigations have indicated that industrial wastes require a lower degree of pre-treatment and less energy intensive carbonation conditions (at same initial content of Ca/Mg oxides), in comparison to raw minerals [9,10]. Alkaline waste residues present other distinct advantages over natural mineral feedstocks for CCSM, as mining is not required and the related issues such as consumption of raw materials and environmental issues have less of an impact [11]. Also, mineral wastes are typically low in value (price per tonne) and generally located near the CO2 source [6,12,13]. So far, a detailed assessment of the potential waste streams in the UK, which could be employed for mineral carbonation, has not yet been conducted. This paper aims to give a detailed review of suitable candidate waste resources that are available in the UK, predicted future trends for their production. Moreover, the chemical composition of the different waste streams and how this may affect their potential for storing CO2 is also discussed.

2. Potential waste materials for CCSM Waste materials from a wide range of industrial processes are rich in calcium and magnesium oxides and hydroxides, which are desirable source materials for CCSM. However, the consideration of waste is in many ways more complex than natural rocks. Rocks are a finite resource in a specific location that stay essentially unchanged for long periods of time and could act as a resource for hundreds of years. In contrast, a material previously considered as waste may find a use due to developments in technology. Second, technology changes can lead to the cessation of waste production. A third factor that can radically change things is legislation with increasing charging on the disposal of waste that could create markets for ‘waste’ as low value by-products. The use of waste products is therefore very unpredictable. Waste streams characterized by a high content of calcium oxides that can be used for mineral carbonation include: Recycled concrete aggregate (RCA), steel slag (SS), blastfurnace slag (BFS), pulverised fuel ash (PFA)including oil shale pulverised ash, secondary steel slags such as argon-oxygen decarburisation slag (AOD), desulphurisation slag and ladle slag, incinerator bottom ash (IBA), air pollution control (APC) residue, cement kiln dust (CKD), incinerator sewage sludge ash (ISSA), paper sludge ash (PSA), biomass ash (BA), clinical waste ash (CWA) and quarry waste and

fines. CCSM using all the above materials has been demonstrated in several recent studies [13–19]. The assessment of inorganic waste as a resource material for CCSM technology has been carried out here considering the volumes available in the UK, their chemical composition and their location. It should be mentioned the difficulties in retrieving long-term data for past production and the unpredictable nature of future sources and availability of waste. This is far more difficult to evaluate compared to the availability and access of silicate rocks suitable for CCSM. A discussion on the future availability of waste resources is the focus of this work, along with the possibility to employ the CCSM technology as an intermediate process towards the re-use of the mineral wastes. The parameters used to assess the current and the future potential of the mineral wastes as a resource for CCSM are: 1. Amounts available (including predicted future trends); 2. Content of calcium, magnesium and other elements. The ‘theoretical’ maximum CO2 uptake (TCO2 uptake) expressed in wt.% was calculated for each potential UK CCSM resource from its total Ca and Mg concentrations using a modified Steinour formula [20,21].

TCO2

uptake

¼ 0:785  ð%CaO  0:56  %CaCO3  0:7  %SO3 Þ þ 1:091  %MgO þ 0:71  %Na2 O þ 0:468  ð%K2 O  0:632  %KClÞ

ð2Þ

This method is based on the assumption that the total amount of Ca and Mg can be extracted from the waste and subsequently carbonated [22]. TCO2 uptake and the annual production of the waste in the UK were then used to calculate its ‘theoretical’ annual maximum CO2 storage potential (TCO2 capture; expressed in Mt CO2/year) as follows;

TCO2

capture ðAÞ

¼ Waste availableðAÞ  TCO2

uptake ðAÞ

ð3Þ

Where (A) represents potential waste resources in the UK for CCSM and the waste available stands for the amount of waste that is currently not reused in other applications (i.e. sent to landfill). The experimental CO2 capture (ECO2 capture) considering the experimental CO2 uptake (ECO2 uptake) was calculated. This parameter is strictly dependent on the composition of the residues that may vary quite considerably, whereas for the same type of material applying a set process route, similar conversions may be expected. However, for this study, the ECO2 uptake and not the conversion yield is considered of primary importance, because its final aim is to evaluate the potential of UK wastes as feedstock for CCSM and not to compare the different mineral carbonation technologies available, where the consideration of the conversion yield would be more appropriate. 2.1. Chemical composition and co2 uptake of the identified waste materials A summary of the chemical composition and theoretical and experimental CO2 uptake of the CCSM resources (and potential resources) is presented in Table 1. The table shows the high variability of the same material that directly depends on the starting material (e.g. coal, iron ore). Particularly, the variation of the reactive calcium and magnesium oxides can be associated with the efficiency of the carbonation process. All steel slag, CKD, biomass ash, oil shale ash, AOD process slag, APC residue and paper incineration ash present high CaO content (40–70%) that is reflected in a high theoretical CO2 uptake. MW incinerator bottom ash presents a large amount of Cl and

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A. Sanna et al. / Applied Energy 99 (2012) 545–554 Table 1 Chemical composition of the carbonation resources and their theoretical CO2 uptake (TCO2uptake) and the experimental uptakes (ECO2uptake). Chemical composition (wt.%)

Blastfurnace slag [19,46,69] Steel slag (BOF)[22,69] Steel slag (EAF)[22,69] AOD process slag [69] Cement kiln dust[15,60] Biomass and wood ash [15] Recycled concrete aggregate [25,30] Inceneritor sewage sludge ash [23] MW incinerator bottom ash [15,16,70] Pulverized-fuel ash [15,36,39] Oil shale ash [37,38] Air pollution control residue [55,57] Papersludge incineration ash [15] a

TCO2

CaO

MgO

Fe2O3

SiO2

15–41 34–55 25–47 60.7 34.5–46.2 24–46 15–24 9–37 32–53 1.3–10 42–50 50–60 45–69

8–11 1.5–10 4–15 5.83 1.5–2.1 8–9 2–3 3 2.8 1–3 5–6.5 8 1.3–5.3

0.5–0.9 1.6 0.21 2.9 1–1.3

34–36 0.8 27 27.6 16.4 5.17

5.6 1–7.9 13.8 4 0.5–1.5 1–4.7

40 4–30 56 22 10 10–25

Cl

SO3

Na2O

K2O

1.4 8

0.3

0.6 0.1

0.5 0.5 0.1 0.7 2.2–5.7 0.5

4–5.8 14–21 0.2 2.3 0.8–2 0.1

0–1

2–6 0–2

22 0.63–3 0.3 27.9 4 2.1

19

1.8–38 1–7

uptake

(wt.%)

20–44 29–52 [22] 24–48 [22] 54 U.5[60]-30[15] 50 [15] 6a [30]-22 15 [15] 25 [15] 6 [11,36]-9 15 [37,38]-45 50–58 50 [15]

ECO2

uptake

(wt.%)

7[30]-22.7[46] 21 [11,54] 12–18 [24,47] 16 [48] 10 [15] 8 [15] 7.5 [28]-16.5 [29] 2 [15] 3–14 [54,55] 1.7–6.7 [39,40] 9 [38] 7–25 [28,55,57] 17 [15]

Assuming 75% of Ca in the RCA binds to CO2.

SO3 that lower the MW ash theoretical CO2 uptake. The state of the art of the CCSM using wastes indicates that the maximum ECO2 uptake reached 20–25 wt.% when BFS, steel slags, and APC residue are used in the CCSM processes, while largely available RCA and PFA present low ECO2 uptake mainly because of low Ca content.

RCA is not considered waste and it is currently re-used in construction applications as an aggregate, mainly for low-end applications such as ‘hard core’ for building products and land reclamation and thereby, reducing the amount of material that is sent to landfill (that attracts a tax) and reducing the need for virgin rock aggregate in new constructions [31].

2.2. Recycled concrete aggregate 2.2.1. Assessment of UK resources Recycled concrete aggregate is increasingly being seen as a valuable source of engineering materials for the construction industry in the UK. Using RCA materials potentially reduces reliance on primary aggregates and lowers the environmental impact of construction. An annual average of about 53 Mt of RCA is generated in the UK [32,33]. The UK construction sector consumes about 380 CO2 capture (Mt)Mt of resources every year and is facing a number of fundamental changes over the next few years to develop a sustainable construction industry capable of delivering a low carbon future and to meet the current carbon, water and waste reduction targets [31]. Economic and legislative developments driven by increasing emphasis on reducing energy, water consumption and waste generation, and recycling and disposal issues, will influence future aggregate production trends, driving a radical change in the extraction and processing aggregates industry [31]. The wide distribution of RCA production facilities might be an advantage for the integration of a mineralisation plant, reducing costs of moving materials over long distances [32]. However, RCA is currently fully reused and cannot be considered a CCSM resource. Fig. 1 shows the distribution of construction and demolition (C&D) waste in the UK from 1999 to 2008. The data suggests that

Spread on exempt sites

100 90 80 70 60 50 40 30 20 10 0

Used or disposed in landfill

Recycled by crushers/screeners

Million tonnes

Recycled concrete aggregate (RCA) results from the processing of inorganic material previously used in construction and principally comprising crushed concrete. The cement component of RCA consists of a series of calcium silicate hydrate and calcium aluminate hydrate compounds, as well as calcium hydroxide, which is highly alkaline. The alkalinity of RCA can decrease due to age-related natural carbonation. Chloride ions from the application of de-icing salts to roadway surfaces or sulphates from contact with sulphate-rich soils can sometimes be present [23]. The maximum TCO2 uptake (6–22 wt.%) of the ‘active’ fraction (Ca oxides) of construction and demolition waste (50% for Portland cement) depends on the variable oxide content [24,25]. Portland cement ECO2 uptake is between 5% and 37% by weight and RCA ECO2 uptake of 7.5–16.5 wt.% was found [14,18,26,27,29]. The higher ECO2 rating of 16.5% was achieved by direct carbonation at ambient temperature, 4 bar pressure, 48 min duration, and a mean particle diameter of 80 lm [29]. In this process, the cement powder was mixed with the desired amount of water (50 wt.%) and then moulded into bricks and cured with CO2 then dried overnight. Gunning et al. treated the waste with pure CO2 in a pressurized vessel (2 bar) and in the presence of saturated NaCl solution to maintain a relative humidity of 75% [14]. Kashef-Haghighi and Ghoshal investigated the carbonation of a fresh concrete block using a flowthrough reactor under a constant flow of CO2 (20% in nitrogen balanced) at 20 °C and 60 min duration [26]. Other investigations have attempted direct carbonation in a pressurized vessel purged with 2 bar of pure CO2 for 60 min [27], a two-step process for the extraction of Ca+ from cement waste (30 bar, 50 °C) and sequent carbonation (1 bar) in a stirring tank vessel [18,25], and direct carbonation in a 500 ml glass reaction chamber filled with water and concrete powder to guarantee a Ca content of 0.01 mol/l [30]. Waste cement represents a potentially large source of mineral carbon sequestration feedstock with recent annual waste concrete production of 1100 CO2 capture (Mt)Mt in the EU, China and the USA altogether [6]. Considering the highest ECO2 uptake of 16.5 CO2 capture (Mt)wt.% [29] and taking the waste concrete produced in the above mentioned countries, RCA could collectively sequester a maximum of 60 Mt of CO2/year, accounting for 0.2% of global CO2 emissions from fuel combustion [6]. However, the vast majority of

1999

2001

2003

2005

2008

Year Fig. 1. Construction and demolition waste management from 1999 to 2008 [29].

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A. Sanna et al. / Applied Energy 99 (2012) 545–554

in the future there will be an increase of the amount of cement waste that will be recycled. The above consideration is reflected in the increased fraction of C&D waste that has been recycled in recent years and the trend shows that it will increase in the future. However, all RCA is currently used for low-end applications [32] and is therefore not available for CCSM. 2.3. Pulverised fuel ash and furnace bottom ash Pulverized-fuel ash (PFA), also known as coal fly ash (FA), is extracted by electrostatic precipitation from the flue gases of coal-burning power stations. PFA is a fine powder made up of individual fused ash particles with a diameter of about 10–15 lm. A certain proportion of PFA is formed as cenospheres, which are hollow glass spheres. Si, Al and O combine together to form an amorphous aluminosilicate glass matrix (SixAlyOz); with Al2O3 ratio highly depending on the mineral content of the feed coal. PFA also contains recrystallized minerals, such as quartz, cristobalite and mullite [34,35]. Furnace bottom ash (FBA) is the coarser fraction of ash produced in coal burning power stations resulting from the fusion of pulverized-fuel ash particles which fall to the bottom of the furnace. It varies in size from fine sand to coarse gravel and has a porous nature. The principal components of bituminous coal fly ash are silica, alumina, iron oxides, magnesium oxides (1–3%) and calcium oxides (5–10%), with varying amounts of carbon [36]. The chemical properties of fly ash are influenced to a great extent by those of the coal burned and the techniques used for handling and storage and also by the type of flue gas system adopted [35]. Also, PFA from combustion of oil-shale has been investigated as feedstock material for mineral carbonation. In countries such as Estonia, where the primary energy source is oil shale (low-grade carbonaceous fossil fuel) pulverised fuel ash contains up 12–30% free Ca, leading to 9 wt.% ECO uptake using ambient pressure and temperature in a continuous flow column reactor and stirring at 1000 rpm [37,38]. The global production of fly ash (600 Mt in 2009) exceeds the potential uses with only about 30% being used as construction material and therefore, new applications are needed. The maximum amount of CO2 (TCO2 uptake) that can be sequestrated by coal PFA is about 9 wt% but the ECO2 uptake has been reported to be only about 7% by weight [39,40]. Both direct aqueous carbonation of 40 lm PFA particles at 30 °C, 10 bar, 18 h [39] and aqueous carbonation under 2.7 bar of pressure, 20% moisture and 120 h [40] have been investigated. Moreover, PFA carbonation at 30–90 °C, 10 and 40 bar, using a mean particle diameter of between 20 and 150 lm and a solid to liquid rations of between 0.1 and 1 L/kg, was carried out in a stainless steel autoclave reactor [36]. Other direct aqueous carbonation experiments were performed at 25 °C, 2.8 bar, 120 h, particles <250 lm and 20% moisture [11]. Whilst the quantities of PFA generated are large, the low Ca and Mg contents limits the amount of CO2 that can be converted to carbonates (0.25% CO2 emissions from coal fired power plants could be sequestered). Therefore, PFA cannot significantly reduce CO2 emissions from coal fired power plants [6]. 2.3.1. Assessment of UK resources About 5.6 Mt of PFA and 1 Mt of FBA are produced every year in the UK, of which about 50% is sold for varied applications including cement manufacturing (main use), asphalt, hydraulically bound mixtures, uses without any binding agent (e.g. fill material in embankments and as capping layers) or as grouts, where the material is hydraulically pumped or injected into the ground to fill void space. The remaining 50% of PFA is normally sent to landfill as conditioned ash in either a monofill or a lagoon and might be available for CCSM [31,41].

2.4. Steel and iron slags Steel slag is the by-product of the manufacture of steel from pig iron (blast furnace) and metal scrap (electric arc furnace). Steel slag production can be subdivided in Basic Oxygen Furnace slag (BOF) (62%), Electric Arc Furnace slag (EAF) (29%) and secondary metallurgical slag such as ladle slag (9%) [42]. EAF slag generally has lower free lime content than BOF, as can be seen in Table 1. Steel furnace slags typically present high Ca content (34–55%) and elevated alkalinity (pH between 11.3–12.4) making them suitable candidates for CCSM. The global production of iron and steel slag was about 350 Mt/year in 2000 and the maximum TCO2 uptake (25 to 52 wt.%) would be enough to sequester about 40% and 10% of the CO2 emissions from EAF and BOF, respectively [22,43,44]. Blast furnace slag (BFS) is a by-product from the production of iron, resulting from the fusion of fluxing stone (fluorspar) with coke, ash and the siliceous and aluminous residues remaining after the reduction and separation of iron from the ore [31]. The chemical composition of slag is usually expressed in terms of simple oxides calculated from elemental analysis determined by X-ray Fluorescence (XRF). The predominant compounds are dicalcium silicate, tricalcium silicate, dicalcium ferrite, merwinite (calcium magnesium disilicate), calcium aluminate, calcium-magnesium, iron oxide, and some free lime and free magnesia. Secondary processes for further refinement of stainless steel through reduction of carbon content and pollutants such as sulphur also produce slags. To further refine the steel after coming out of the BOF or EAF, fluxes are added to the molten steel while in a ladle. The slag from this process is usually called ladle slag. The chemical composition of ladle slag is significantly different from that of steel furnace slag in that the former has a very low FeO content and a higher Al2O3 content. Also, a mixture of argon and oxygen, with the addition of cleaning agents to eliminate impurities, is added to the molten metal to decrease its carbon content. The oxygen combines with carbon in the unrefined steel to reduce the carbon level. The presence of argon enhances the affinity of carbon for oxygen and thus facilitates the removal of carbon forming argon oxygen decarburization slag (AOD) [45]. BFS typically contains calcium oxide in the range of 15–41% and magnesium oxide content ranging from 8% to 11% (Table 1). It consists primarily of silicates, alumino-silicates, and calcium-aluminosilicates. The molten slag, which absorbs much of the sulphur from the charge, is equivalent to about 20 wt.% of iron production [23]. An ECO2 uptake of 7–23% by weight has been reported for BFS [24,46]. Electric arc furnace slag, blast furnace slag and ladle slag showed an ECO2 uptake on the order of 12 wt.%, 7 wt.% and 4.6 wt.%, respectively [24,38]. The BFS and EAF powders were subjected to 100% CO2 at a constant pressure of 5 bar for 2 h [24]. BFS indirect carbonation experiments were run in a batch reactor at 70 °C, 40 bar, 20% acetic acid solution (Ca extraction step) using particles between 125 and 250 lm. This was followed by carbonate precipitation at 30 °C and ambient pressure whilst being stirred at 600–700 rpm for 2 h in the presence of pure CO2 and sodium hydroxide solution [46]. EAF accelerated carbonation tests were performed in stainless steel reactor at 50 °C, 10 bar pressure, a liquid to solid ratio of 0.4 L/kg and 100% CO2 and at 75% relative humidity that was maintained using a saturated NaCl solution [47]. EAF carbonation under ambient pressure and temperature using 15% CO2 balance in air for 65 min and a solid to liquid ration of 0.1 L/kg [38] was also attempted. The ECO2 rating of EAF and BOF slags was also calculated after Ca+ extraction from <150 lm powders, using a solid to liquid ratio of 10 kg/kg under continuous stirring and at 22 °C in the presence of 0.5 M NaOH, 0.5 M H2SO4 or 0.5 M HNO3 [22]. AOD carbonation was performed at 30 °C, 20% CO2, at a solid to liquid ratio of 0.5 L/kg for 7 days [48].

A. Sanna et al. / Applied Energy 99 (2012) 545–554

2.5. Assessment of UK resources A total of 1.25 Mt of steel slag was produced in the UK in 2002 and a similar amount was generated in 2009 as shown in Fig. 2. BOF and EAF accounted for 0.99 Mt (2001) and 0.26 Mt (2005), respectively. Currently, the total production is mostly used as aggregates. About 3 Mt of BFS is generated in England and Wales. At present, about 75% of the slag is processed to produce ground granulated blast furnace slag (GGBS), which is used by the concrete industry as a cement replacement material. The remaining fraction (25%) is air-cooled and is used as an aggregate. The split between the two uses is dictated by production choices, economics and demand [31,32]. Overall, 100% of the steel slag and the blast-furnace slag is reused into aggregates [49,50] and therefore cannot be practically considered as CCSM resource in the near future. Only assuming that the mineralisation technology will produce an intermediate product with enhanced properties (and therefore market value) compared to the raw slags, these materials might represent a resource for CCSM in future. The future availability of steel slag might decline considering that the production of electric arc furnace (EAF) slag is expected to rise in the South East and can balance the decline of the basic oxygen furnace (BOF) slag. The latter is used for aggregate use and considering that the demand is expected to continue, there is little necessity to find alternative uses [49]. It is believed that the global steel market will rise from 1.4 bn tonnes in 2010 to 1.6 bn tonnes in 2014 and the iron market will raise by 0.2 bn tonnes in the same period [51]. The distribution of the steel slag is limited to only a few areas in the country (Teesside, South Wales, Kent), and the majority of the works are close to port facilities that would allow the movement of materials in and out.

2.6. Municipal waste bottom ash and air pollution control residue

Million tonnes

Incinerator bottom ash aggregate (IBA) is processed from the material discharged into the burning grate of municipal solid waste (MSW) incinerators and comprises 80% to 90% of the total MSW ash production [31]. The most abundant elements in municipal waste combustor ash are silicon, calcium and iron. BA is a heterogeneous material whose composition depends on the feed waste, combustion and quenching conditions used. BA presents variable mineralogical structure, with amorphous and crystalline phases and (hydr)oxides, (alumino)silicates such as Gehlenite (Ca2Al2SiO7), hydrocalumite (Ca2Al(OH)6[Cl1xOx]3H2O), Ca(OH)2, calcite (CaCO3) formed during quenching and storage, forsterite (Mg2SiO4), dicalcium silicate (Ca2SiO4) and hematite (Fe2O3) have been identified in bottom ashes [52]. Also, these ashes present heavy metals in both the non-hazardous bottom ash and in the hazardous fly ashes (in high concentration) [15,53]. Although ash

20 18 16 14 12 10 8 6 4 2 0

UK steel production

UK steel slag

1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2009

Year Fig. 2. UK crude steel production from 1990 to 2009 and steel slag production in 1990, 2002 and 2009 [69].

549

composition can be expected to vary from facility to facility, these elements are present within relatively predictable ranges. Generally, IBA presents about 35–50% of CaO and 3–4% of MgO [23]. The use of processed bottom ash in engineering applications is in its infancy in the UK and CCSM may be more attractive as a use for this material than for the production of aggregates, due to the described high content of Ca and Mg. Municipal waste IBA has showed ECO2 uptake on the order of 3– 14 wt.% [15,52,54,55]. IBA carbonation was carried out under different conditions: (1) using a pressurized reactor vessel (2 bar) with pure CO2 for 72 h maintaining 75% relative humidity inside the reaction vessel, (2) using a a stainless steel pressurized reactor (10 bar) under 100% CO2 atmosphere at 30 °C for 24 h at a solid/liquid ratio of 0.3 L/kg [52], and (3) direct gas–solid carbonation of incinerator bottom ash at ambient temperature and 3 bar pressure for 2.5 h in a stainless steel chamber [55]. Air pollution control (APC) residue from MSW incinerators consists of particulates that originate in the primary combustion zone area and are subsequently entrained in the combustion gas stream. Then, the particulates are carried into the boiler and air pollution control system together with reaction products and excess reagents resulting from flue gas treatment. Usually APC residues and fly ash are interchangeable [16]. The entrained particulates stick to the boiler tubes and walls or are collected in the air pollution control equipment, which consists of scrubber, electrostatic precipitator and baghouse. Ash extracted from the combustion gas consists of very fine particles, with a significant fraction measuring less than 0.1 mm in diameter [23]. The major elements in APC residues are Ca (4.1–36.1 wt.%), Cl (4.5–38.0 wt.%), Si (3.6–19.0 wt.%), Al, Fe, Ca, Mg, K and Na. As regards heavy metals, Zn (0.4–10.4 wt.%), Pb (0.02–2.7 wt.%), Cd, Cr, Cu, Hg, Ni were the most frequent. Also, trace quantities of very toxic organic compounds such as polycyclic aromatic hydrocarbons (PAH) and chlorobenzenes (CB) may be present in these materials [56]. Abundance of Ca and Cl is due to their use in excess for acid gas abatement [28]. APC residue is a ‘hazardous’ waste that can be reclassified, following CCSM applications, to ‘non-hazardous’ by decreasing their metal ion leaching. Obviously, if materials containing chlorine would be employed for CCSM, special precautions, with associated costs, would be required. However, the benefit of reclassification from ‘hazardous’ to ‘non-hazardous’ material would reduce the risk and cost associated with the disposal. The ECO2 uptake of the APC residues was found ranging from 7 to 25 wt.% [55,57]. The carbonation experiments were performed using a modified muffle furnace, under a constant 100% CO2 flow, with temperatures ranging from 200 to 500 °C and with a residence time of 6 h [57]. Also, direct gas–solid carbonation of APC residues was evaluated at ambient temperature and 3 bar pressure for 2.5 h in a stainless steel chamber [55]. 2.6.1. Assessment of UK resources The annual tonnage of incinerator bottom ash (IBA) produced in the UK is 0.75 Mt of which 0.4 Mt (55%) is currently used to produce aggregates [58]. Previously, from 1996 to 2000, about 90% of the incinerator ash produced in the UK was sent to landfill, but due to the strict EU landfill directive, the space used for these hazardous wastes such as APC residue (fly ash), has significantly decreased and alternatives such as CCSM are necessary [16]. The Waste Strategy 2000 predicted a rise of waste-to-energy of about 25% of municipal solid waste by 2020 [31]. This is likely to result in significant increases in the amount of IBA available for mineralisation. Overall, considering that the majority of MSW is sent to landfill in the UK [16], IBA represents a small resource for CCSM. This resource would be much larger if incineration were used more extensively to reduce the amount of waste going to the diminishing space available for new landfill.

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Air pollution control (APC) residues produced from cleaning gaseous emissions generated during combustion of wastes at incineration plants had an annual tonnage of 128,000 tonnes/year in the UK in 2008, and 85% was sent to landfill as hazardous waste. These residues are often transported over long distances for treatment and disposal, and alternative sustainable treatments would be beneficial [59]. APC residues are generated in the same locations as IBA and might be used together as feedstock for CCSM, increasing the uptake of CO2 in the incineration plant.

0.35 0.3

Million tonnes

550

0.25 0.2 0.15

0.1 0.05

2.7. Cement kiln dust

0 1999

Cement Kiln dust (CKD) is a fine by-product of Portland cement and lime high-temperature rotary kiln production that is captured in the air pollution control dust collection system (e.g., cyclones, electrostatic precipitators). CKD, composed of fine particulates of unburned and partially burned raw materials, is collected from the combustion gases within pre-heater and kiln systems. Due to its caustic nature and potential as a skin, eye and respiratory irritant, CKD is a potentially hazardous waste. Typically, for every 100 tonnes of cement produced, 15–20 tonnes of CKD is generated [60–62]. The chemical and physical characteristics of CKD mainly depend on the method of dust collection employed at the facility. The concentration of free lime in CKD is typically highest in the coarser particles captured closest to the kiln, while fine particles tend to exhibit higher concentrations of sulphates and alkalis. About 75% of the kiln dust particles are finer than 0.03 mm. CKD from wet-process kilns tends to be lower in calcium content and richer in salts than the dust from dry-process kilns. Chemically, CKD has a composition similar to conventional Portland cement. The principal constituents are compounds of lime, iron, silica and alumina. CaO usually varies between 34% and 46% and MgO between 1.5% and 2.1% [23]. The ECO2 uptake was evaluated as being about 9–11 wt.% (i.e. every kg of CKD has potential to capture 0.09–0.11 kg of CO2) [15,60]. A direct carbonation route at ambient temperature and pressure over 3 days (38% relative humidity) in a column reactor [60] and ambient temperature and 2 bar over 72 h in a pressurized reaction vessel [15] were employed in the experiments. 2.7.1. Assessment of UK resources The UK cement industry disposes of about 46,000 tonnes/year of cement kiln dust (CKD), and an additional 5000 tonnes of CKD was returned to manufacture in 2008 compared to 2007. The total annual tonnage of CKD being disposed of has fallen significantly since 1998. In that year some 289,207 tonnes went to landfill. The CKD produced in the UK per tonne of cement produced is very low (<1%) because the CKD is mainly recycled in the kiln during the production process. The production of CKD decreased considerably from 1999 to 2008, but it is expected to rise again in the near future as shown in Fig. 3. Before 2007, CKD together with sewage sludge could be used to create fertile soil in land reclamation projects, but changes to legislation now preclude this use. Therefore, the CKD sent to landfill is expected to rise to 75,000 tonnes and remain at this level from 2010 to 2015 [61]. It is difficult to estimate the amount of CKD that will be available in the future as global cement consumption continues to rise and the amount of CKD waste is likely to show a similar increase. However, the cement industry might decline in the UK due to the expected rise in the cost of producing cement as a consequence of EU regulations (EU ETS, climate change agreements, carbon reduction commitments and a carbon levy). Therefore, the availability of CKD in the UK years is unpredictable [63] but possibly will decrease.

2008

2010

2015

Year Fig. 3. CKD production and forecast for 2010 and 2015 [55].

2.8. Other resources for carbonation A number of other inorganic materials such as incineration sewage sludge ash (ISSA), paper and biomass sludge ash can be currently considered as a secondary resource for carbonation in the UK because of the low quantities available, and might be of primary importance in other countries based on their availability. Sewage sludge ash is the by-product produced during the combustion of dewatered sewage sludge in an incinerator. It contains between 9% and 37% of CaO [23] and hence is highly variable in terms of chemical composition. About 0.2 Mt/year sewage sludge is produced in the UK [64] with an approximated 0.08 Mt/year of ash remaining after incineration (considering 40% of ash left after incineration) [23] and currently are sent to landfill in full. Paper sludge incineration ash (PS) is the residue from incineration of the wastewater sludge from paper-making. It contains residual fibres, fillers and chemicals and contains about 45–69% of CaO [15]. About 0.13 Mt/year of paper sludge ash is produced in the UK. Currently 70% (or 0.088 Mt) goes to end uses, such as brick and cement manufacturers and the remaining 30% is landfilled [64]. Biomass ash (BA) is a by-product of the combustion of biomasses such as spent grains after beer and bio-ethanol production, rape-cake after oil extraction. Wood ash (WA) is generated from coal power station and combined electricity and heat generation plants using wood sources. BA and WA contain between 24% and 45% of CaO [15]. These wastes have high contents of desirable elements but are only produced in low volumes in the UK. The production of biomass ash is of 0.062 Mt/year. All sixteen major UK power plants are now co-firing a proportion of biomass, at an average level of 3% (energy basis) making use of a range of fuels including wood (virgin and recycled), olive cake, palm kernal expeller, sewage sludge and energy crops [65]. The ECO2 uptake has been reported to be about 8 wt.% [15]. ISSA, PS and BA CO2 uptake were investigated using a reactor vessel in a 100% CO2 atmosphere held a 2 bar pressure and 75% relative humidity for 72 h [15].

2.8.1. Quarry waste as a potential resource Processing of crushed stone for use as construction aggregate consists of blasting, primary and secondary crushing, washing, screening, and stockpiling. These operations produce significant amounts of waste, often referred to as quarry waste and fines. Usually sand and gravel workings do not produce permanent waste, while hard rock quarries produce variable amounts of quarry waste and fines. Quarry fines are the inherent fraction of an aggregate below 63 lm, as defined by BS EN 1377:2. However more commonly quarry fines are defined as the sub-mm ‘sand’ fraction that is the undersize from screening coarse aggregate and the 63 lm fraction,

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as the airborne dust, collected by extraction filters [66]. Only waste and fines from operations processing ultramafic rocks should be considered as potential resources but there are no such operations in the UK. However, the tailing remaining from the mining industry can represent an important resource for CCSM in countries with large mining activities. For example, in South Africa, the calculated CCS capacity of the platinum industry is 14 Mt/year [67].

such as steel and blast furnace slags. The location of the mineral waste is widely distributed across the UK, and in many of the cases, the waste resource is located very close to the CO2 emitters. In particular, the South East, South Wales, the East Midlands and the North East are regions with higher potential for CCSM, considering the wastes available. Indeed, steel and cement works, incinerators, crude-oil refineries of small and medium–small scale represent the ‘‘ideal match’’ for the application of this technology considering that CCS by geological storage mainly targets large power emitters.

3. UK waste streams potential as resource for CCSM 3.1. Mineral carbonation challanges A total potential estimation of the mineral waste resource available in the UK for the CCSM was carried out with a total potential CO2 capture (TCO2 capture) value of 14.3 Mt of CO2/year, and representing less than 3% of the current annual UK emissions [68]. However, this hypothetical maximum potential would be reached only if all the inorganic wastes would be used as CCSM resource and the theoretical CO2 uptake would be reached. Table 2 reports the quantity (Mt/year) of the waste materials that is produced in the UK, the quantity (Mt/year) of the wastes that is available for CCSM because not currently reused/recycled, and the potential and effective CO2 that could be captured considering the TCO2 uptake and ECO2 uptake, respectively. Currently, recycled concrete aggregate (RCA), steel slag (SS)and blast furnace slag (BFS) do not present any tonnage available for CCSM, while pulverised fuel ash (PFA), air pollution control (APC) residue, incinerator bottom ash (IBA), Biomass Ash (BA), ISSA, paper sludge ash and cement kiln dust (CKD) could be potential resource for CCSM feedstock in the UK with a potential CO2 capture capacity of about 1 Mt/year or 0.2% of the total UK CO2 emissions. Currently, PFA represents the most abundant waste material to be used as CCSM resource. Table 2 also indicates that the effective CO2 capture capacity (it consider the ECO2 uptake from Table 1) of the UK waste materials, calculated considering the experimental CO2 uptake, would be about 0.1 Mt/year. It has to be stressed that the majority of these waste materials are currently re-used for low-end applications to avoid landfill disposal costs and the aggregate levy in the case of primary aggregate production for the construction industry. For example, the use of the RCA into CCSM could be technically viable integrating the CCSM plant after the crushing and sieving step of the construction and demolition waste, assuming that the properties of the carbonates produced could be compatible to commercial ones and also competitive in terms of cost. Also, SS and BFS that have high ECO2 uptake (20 wt.%), and are generally located close to the CO2 emitter could be attractive candidates for CCSM if the carbonation technologies will be in future proven to be economically competitive with other CCS technologies such as geological storage. Fig. 4 shows the distribution of all the waste resources and the main CO2 emitters in the UK, including potential future resources

Current CCSM schemes present high energy requirements and high costs compared to geological storage. Direct gas–solid carbonation of mineral wastes presents reaction kinetics too slow to be practically deployed, which results in too large an amount of materials to be moved in/out from the mineral carbonation plant [6]. The direct and indirect aqueous mineral carbonation systems that have been investigated so far (using thermal pre-treatments and high temperature/pressure or strong and weak acids, salts, caustic agents and bioleaching agents) have in common a net energy demand for the grinding of the feedstock, the extraction of the reactive species, and the processing and separation/disposal of the reaction products [2,6]. For example, the energy penalty of direct high pressure and temperature mineral carbonation ranged from 430 kW h/6 tCO2 ($64/tCO2) to 2300 kW h/tCO2 ($210/tCO2) when activated Ca-silicates and lizardite-serpentine were used, respectively [8]. Moreover, the chemical recycling step of indirect processes that use strong acids (HCl, HNO3) would emit 2.6–3.5 times the amount of CO2 bound in the carbonation process [75]. However, the use of weak acid extraction processes (laboratory scale) was estimated to cost $27/tCO2 [76] and the use of inorganic wastes (concrete waste and steel slag) and buffer solutions in spray trickle bed systems (able to sequester 50% of the CO2 entering the system) was estimated to cost $ 8/tCO2; making it competitive with geological storage [30]. Also, CO2 sequestration using cement waste at 30 bar and 50 °C required a power consumption of 26 MW/100 MW of power generation with associated costs (excluding capital costs) of $22.6/tCO2 [18]. Local energy policy priorities will drive the development of competing technologies such as retrofitting CCS in existing fossil fuels power plants, polygeneration via coal gasification with CCS, coal oxyfuel technology with CCS, and renewable energy. [77– 79]. For example, in Germany, it has been assessed that renewable electricity generation costs are likely to approach that of CCS power plants in 10–20 years and CCS might not be economically competitive in their case [80]. Moreover, mineral carbonation of wastes might be viable where geological storage is not available or where natural silicates are readily available and closely located to the CO2 emission source, although the amount of materials to be moved might not be practical. However, the CCSM products are

Table 2 Primary potential waste resources for carbonation in the UK, considering a lifespan from 1990 to 2010 and diversification of the current materials reuse. These values represent the theoretical sum achievable considering the variability of the volumes produced from 1990 to 2010. Resource

Tot production (Mt/yr)

Waste available (Mt/yr)

Potential CO2 capture (Mt)

Effective CO2 capture (Mt)

RCA Steel slag (BOS + EAF) Blast furnace slag Coal power ash (PFA, FBA) MW (APC) MW Ash (IBA) CKD ISSA Paper sludge ash Biomass ash

53 [32] 1.18 [71] 3 [32,71] 6.61 [72] 0.13 [28,59] 0.75 [15,58] 0.046 [62] 0.08 [23,73] 0.13 [64] 0.02 [65]

0[32] 0 [49,50] 0 [49,50] 3.3 [15,72] 0.11 [59] 0.34 [58] 0.046 [62] 0.08 [23,73] 0.039 [64] 0.02 [65]

0 0 0 0.752 0.064 0.085 0.014 0.012 0.02 0.01

0 0 0 0.05 0.016 0.012 0.001 0.0002 0.01 0.001

Total

65

3.93

0.96

0.09

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Fig. 4. Waste resources and CO2 emitter locations considering a lifespan of 20 years and a future use for CCSM [74].

environmentally benign and could be potentially sold for profit reducing the overall costs [81], and also the synergic integration of several energy-intensive industries (e.g. metals refining plant, mineral beneficiation and power plants) can result in an industrial eco-complex with low net CO2 emissions [6,82]. Overall, the challenges for a successful deployment of CCS by mineral carbonation still remain as the need for acceleration of the reactions in direct processes, to economically recycle the chemicals used in indirect carbonation processes, and to minimise the energy and materials requirements.

4. Conclusions This work gives an up-to-date review of the potential applications of inorganic waste materials in mineral carbonation for the sequestration of CO2. Mineral waste resources, suitable for mineralization in the UK have the theoretical potential to capture 14 Mt/year CO2, if all waste products are used to CCSM.

However, this still only represents less than 3% of the current UK CO2 emissions per year. A wide range, but comparatively small quantity (1 Mt/year), of waste and industrial by-product resources is actually available to be used as CCSM resource and its potential in CO2 capture is considerably low in terms of annual UK emissions (about 490 Mt). This work indicates that RCA and PFA would be available for a life span of 100 years. The accessibility of steel slag, BFS, IBA and APC is highly uncertain over a 30–100 years’ time span due to market and environmental issues. However, similar amount of these wastes are expected to be produced in the next 5–30 years. Overall, the use of waste resources for CCSM should be considered as a niche market that could utilise relatively small amounts of feed material for CCSM. They may have the advantage to be often located close to the CO2 emission site. Moreover, for some of the waste materials (e.g. RCA) there are existing infrastructures for the recycling and reuse of these materials, where CCSM may be integrated. Although the total wastes resources are currently too small to substantially reduce CO2 emissions, waste resources

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