Scholarly article on topic 'Carbonation of Industrial Residues for CO2 Storage and Utilization as a Treatment to Achieve Multiple Environmental Benefits'

Carbonation of Industrial Residues for CO2 Storage and Utilization as a Treatment to Achieve Multiple Environmental Benefits Academic research paper on "Materials engineering"

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{"accelerated carbonation" / aggregates / "biogas upgradating" / "CO2 storage and utilization" / "industrial residues" / "leaching behaviour"}

Abstract of research paper on Materials engineering, author of scientific article — Renato Baciocchi, Oriana Capobianco, Giulia Costa, Milena Morone, Daniela Zingaretti

Abstract This paper presents and discusses the application of accelerated carbonation of industrial residues in three different contexts (a landfill site, a steel manufacturing plant and a Brownfield site) aimed at storing CO2 and obtaining additional specific environmental objectives, i.e.: the improvement of the leaching behavior of the treated residues, biogas upgrading, the production of aggregates to use in construction applications and in situ remediation of Brownfield sites.

Academic research paper on topic "Carbonation of Industrial Residues for CO2 Storage and Utilization as a Treatment to Achieve Multiple Environmental Benefits"

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Energy Procedia 63 (2014) 5879 - 5886

GHGT-12

Carbonation of industrial residues for CO2 storage and utilization as a treatment to achieve multiple environmental benefits

Renato Baciocchi*, Oriana Capobianco, Giulia Costa, Milena Morone, Daniela Zingaretti

Laboratory of Environmental Engineering, Department of Civil Engineering and Computer Science Engineering, University of Rome "Tor

Vergata",via delPolitecnico 1, 00133 Rome, Italy

Abstract

This paper presents and discusses the application of accelerated carbonation of industrial residues in three different contexts (a

landfill site, a steel manufacturing plant and a Brownfield site) aimed at storing CO2 and obtaining additional specific

environmental objectives, i.e.: the improvement of the leaching behavior of the treated residues, biogas upgrading, the production

of aggregates to use in construction applications and in situ remediation of Brownfield sites.

© 2014The Authors.Publishedby ElsevierLtd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.Org/licenses/by-nc-nd/3.0/).

Peer-review under responsibility of the Organizing Committee of GHGT-12

Keywords: accelerated carbonation; aggregates; biogas upgradating; CO2 storage and utilization; industrial residues, leaching behaviour

1. Introduction

Injection in deep geological formations is considered as the most promising option for CO2 storage. However, limitation of available storage capacity with acceptable leaking rates may limit its application, at least in some geographical locations, thus prompting the need for developing alternative storage options. Among these, accelerated carbonation has been proposed as an effective method for ex situ carbon dioxide sequestration [1]. This process mimics natural weathering, where CO2 reacts exothermically with alkaline elements present in natural metal-oxide bearing materials, forming thermodynamically stable and benign carbonates. This process has been typically applied to alkaline earth minerals, such as serpentine, olivine and wollastonite; despite these materials may exhibit a

* Corresponding author. Tel.: +39-06-72597022; fax: +39-06-72597021. E-mail address: baciocchi@ing.uniroma2.it

1876-6102 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/3.0/).

Peer-review under responsibility of the Organizing Committee of GHGT-12

doi:10.1016/j.egypro.2014.11.621

high carbon sequestration potential, energy intensive operating conditions are typically required to accelerate the reaction kinetics in order to make carbonation feasible for an industrial application [1]. In the last decade, accelerated carbonation has been also applied to other feed alkaline materials, such as different industrial solid stream, e.g. cement kiln dust, municipal waste incineration ash, steel slag, that are characterized by high calcium or magnesium (hydr)oxide or silicate contents and hence prove more reactive than minerals, especially at mild operating conditions [2,3]. Furthermore, carbonation has shown to affect some of the properties of these waste materials (i.e.: main mineralogy, porosity and mobility of specific elements) improving their long-term technical performance and/or environmental behavior. Thus, the application of this treatment to alkaline industrial residues may be indeed considered as a CO2 storage and utilization option, since accelerated carbonation may be also employed as a valorization technique for widening the reuse options of these materials and also to achieve other environmental benefits.

This work presents the main findings of the activities carried out in our research group in the last few years with the aim of developing and investigating new ways for scaling-up accelerated carbonation of alkaline residues in different contexts for CO2 storage while targeting also other specific environmental objectives, i.e.: biogas upgrading, the production of aggregates to use in construction applications and in situ remediation of Brownfield sites. Here below, for each different context, i.e.: landfill site, steel manufacturing plant and Brownfield site, the proposed layout of the carbonation process, its main applications and the results obtained by laboratory or pilot-scale tests are reported and discussed.

2. Application of carbonation in a landfill

2.1. Process concept

Typically, depending on the type of waste management strategies adopted in the area, several different types of waste streams may be found in a municipal solid waste landfill. These may include waste incineration residues and residual municipal solid waste after at-source separation. This latter type of waste undergoes anaerobic digestion in the landfill leading to the generation of landfill biogas containing around 50-60% CH4 and 40-50% CO2, which may be directly exploited for heat and/or electricity production or pre-treated by upgrading processes in order to increase its methane content for use as vehicle fuel or injection into the natural gas distribution grid. Various commercial biogas upgrading processes are available for separating CO2 from biogas; however these are basically capture processes after which the separated CO2 is typically re-emitted to the atmosphere. Hence, in this context, the application of carbonation of industrial residues may be exploited as a biogas upgrading process to produce bio-methane, but also to achieve the permanent storage of the separated CO2 in a solid and stable form, as well as the improvement of the environmental properties of alkaline residues, such as waste incineration residues that may be also disposed of in the same landfill site.

We have investigated this option in the framework of the EU-funded UPGAS-LOWCO2 Life+ project (LIFE08 ENV/IT/000429), through the development of the Absorption with Regeneration (AwR) treatment. In this process, carbonation of specific alkaline industrial residues was applied to regenerate the spent solution produced from an absorption column adopted for capturing carbon dioxide contained in landfill biogas (see Fig. 1). In particular, the AwR process consists in a first stage in which CO2 is removed from the biogas by means of chemical absorption with KOH or NaOH solutions, followed by a second stage in which the spent absorption solution, rich in alkali carbonates, is contacted with the residues, rich in calcium hydroxides. The reaction occurring in the second step leads on the one hand to the regeneration of KOH or NaOH in the solution, that can be reused in the absorption process, and on the other hand to the precipitation of calcium carbonate in the solid product, which is also characterized by an improved environmental behavior compared to the untreated residues.

Fig. 1.Application of carbonation in a landfill context.

2.2. Experimental procedure adopted and main results

In the first phase of the UPGAS-LOWCO2 project, preliminary lab-scale experiments allowed to select the type of residues to employ for the regeneration process as well as to define the unit processes and operating conditions required by the AwR process in order to achieve the targeted objectives [4]. Specifically, Air Pollution Control (APC) residues resulting from the treatment of waste incineration flue gas with calcium hydroxide were selected since, due to their chemical, physical and mineralogical composition, they exhibited the highest regeneration capacity among the tested waste materials [5]. These preliminary tests also allowed to define the layout of the regeneration process, that mainly includes three steps: a preliminary washing step to reduce the content of soluble salts of the residues which showed to hinder the yield of the regeneration reaction; a regeneration step, in which the washed residues are contacted with the spent solution produced in the absorption process and a final washing step aimed at improving the environmental properties of the solid product [5]. Furthermore, on the basis of these preliminary experiments, the sizing, design and construction of a pilot-scale regeneration/carbonation unit was performed [6]. This unit was installed in the same operating landfill site where an absorption pilot unit designed to treat 20 m3/h of landfill gas was located, and several AwR tests were carried out employing different KOH or NaOH concentrations in order to verify the technical feasibility of the proposed process. Results showed that a high CH4 content (88% in vol.) in the outlet gas can be obtained using a 3.8 mol/L of NaOH or KOH aqueous solution with a solution/landfill gas ratio of about 9 L/m3 (gas at standard conditions of 273 K and 1001 kPa). The regeneration process proved to be feasible, but its efficiency was limited by several factors (e.g. dilution effect due to the initial and final washing steps) to maximum values in the range of 50-60%, showing to decrease with higher KOH or NaOH concentrations in the absorption solution [6]. The pilot-scale tests also showed that a further asset of the AwR process is that the final solid product presents a significant improvement of its leaching behavior compared to the untreated APC residues, which are classified as hazardous waste, and hence may be applied also as a treatment strategy for this type of waste material [7].

Thus, this innovative upgrading process integrated with the carbonation/regeneration step allows to achieve multiple environmental benefits compared to the traditional treatments, i.e. separation of CO2 from the biogas, storage of the separated carbon dioxide in a solid form and improvement of the leaching behavior of the treated residues.

3. Application of carbonation in a steel manufacturing industry

3.1. Process concept

Steel manufacturing plants are typical examples of industrial sites in which relevant flows of both CO2 and alkaline industrial residues are generated and therefore represent one of the potentially most interesting contexts for accelerated carbonation application. In addition, several studies have recently shown that a number of different types of steel slag present a significant reactivity with CO2, allowing to achieve, for specific process routes and operating conditions, CO2 uptake values above 200 g/kg residue (see e.g.: [8],[9]). However, especially for steel manufacturing plants employing the integrated production cycle, which includes blast furnace production of pig iron and its subsequent treatment in basic oxygen furnace (BOF) units, but also for those based on electric arc furnace (EAF) treatment of scrap iron, typical production yields of CO2 emissions are significantly higher than those of the solid residues exploitable for the treatment. In addition, since in order to achieve significant CO2 uptakes in reasonable timeframes, milling of the residues and operation of the carbonation reactor at enhanced temperature and CO2 pressure are necessary, the process presents an associated energy penalty which should be assessed and accounted for. Therefore, the potential benefits achievable by applying accelerated carbonation in terms of only CO2 emissions reduction may be quite limited. On the other hand, some of the types of residues generated in steel manufacturing plants such as BOF, EAF and argon oxygen decarburization (AOD) slag, are typically not valorized and generally landfilled or employed only for low-end applications, owing for the significant content of free calcium and magnesium (hydr)oxides that may result in poor volumetric stability and hence in a low technical performance in construction applications. In addition, although these slags present a similar main composition to natural aggregates, they are generally characterized by a higher content of potential contaminants such as Cr, V or Ba, as well as an inadequate particle size, since they are often ground at the plant for iron and steel recovery. Hence, in this context, the idea is to apply accelerated carbonation to different types of un-valorized steel slags with the aim of producing a product that may present suitable characteristics to be employed in civil engineering applications, while at the same time storing part of the CO2 emitted during the steel manufacturing process, see Fig. 2. Since, carbonation affects some of the properties of the slag (e.g. mineralogy and leaching of major and trace compounds), but does not exert a relevant influence on its particle size distribution, coupling of carbonation with granulation processes, that consist basically in contacting the material with a liquid binder in a dynamic device such as a disc granulator or a rotating drum, is being currently investigated with the aim of producing artificial aggregates presenting suitable characteristics for use in civil engineering applications. The additional assets of the proposed process thus rely in the potential reduction of the consumption of abiotic natural resources and of the impacts related to the disposal of the residues.

I S (Carbonation |i S

Iron minerals

Blast Furnace I=> Pig Iron l=>

BOF/EAF/AOD units

BOF/EAF/AOD

Quarry

Blast Furnace Slags

Solid Products/ Aggregates

Fig. 2.Application of carbonation in the context of a steel manufacturing plant.

3.2. Experimental procedure adopted and main results

The proposed process was tested on the slag produced by a steel making plant employing the basic oxygen furnace (BOF) process. The sample, collected from the slag storage site directly after crushing and magnetic separation for iron and steel recovery, presented an initial particle size lower than 2 mm. The combined carbonation and granulation process was tested in a bench-scale rotary drum reactor (diameter of 0.3 m and height of 0.23 m) operated at 24 rpm applying a tilt of 50° and equipped with a blade and lid with a CO2 feeding system [10]. In each test, around 500 g of wet slag (liquid/solid ratio of 0.12 l/kg) were treated at ambient temperature and pressure for reaction times varying between 30 and 120 minutes under a 100% CO2 flow. The products of each test were subjected to curing under atmospheric conditions for 7 days and then analyzed to determine their main physical properties, mineralogy, calcite content, leaching behavior and mechanical performance.

The granules produced by the tested combined treatment exhibited a tenfold larger particle size compared to the untreated slag, i.e. the d50 value increased from 0.4 mm to 4 mm after 30 min and to 10 mm after 120 min. Significant CO2 uptake values (between 120 and 144 g CO2/kg) were measured even after short reaction times for granules with diameters below 10 mm and for the coarser particle size fractions after reaction times of 90 minutes [10]. The density, mineralogical composition and leaching behavior of the obtained granules were also investigated and results indicated that the combined granulation-carbonation process may be a promising option for BOF slag valorization, particularly in terms of decreasing the Ca hydroxide content of the slag. Another interesting finding was that the leaching behaviour of the product of the combined treatment appeared to be significantly modified respect to that of the untreated slag only for coarse uncrushed granules, an indication that the carbonation reaction occurs mainly on the outer layer of the formed granules [10]. The mechanical resistance of the product, as obtained applying the aggregate crushing value test, proved however quite poor, as compared to natural aggregates. Hence, current experimental activities are aimed at testing if by adding specific additives, such as alkaline activators, or modifying the conditions applied during the granulation/carbonation step and/or the following curing process, the mechanical performance of the aggregates may be enhanced to comply with typical engineering requirements.

Another aspect under investigation concerning accelerated carbonation of steel slag in general, is the effect, in terms of CO2 storage and product properties, of directly employing diluted sources of CO2 in the carbonation step instead of 100% flows, which would imply the possibility of skipping the energy-intensive capture step and hence of possibly greatly reducing the energy requirements associated to the accelerated carbonation process [11].

4. Application of carbonation in the context of Brownfield regeneration

4.1. Process concept

Brownfield sites are defined as sites that have been affected by former uses of the site or surrounding land, are derelict or underused, are mainly in fully or partly developed urban areas, require intervention to bring them back to beneficial use and may have real or perceived contamination problems [12]. In the case of former industrial sites, brownfields are often characterized by the presence of industrial soil, i.e. a mixture of natural soil with different types of industrial by-products such as demolition waste or steel slags, whose presence may have a detrimental effect on the groundwater quality due to the leaching of heavy metals [13]. The management of industrial soil is usually based on excavation and disposal of the contaminated material. Nevertheless, this approach is poorly sustainable as it leads to soil consumption for the construction of the landfill and for the replacement of the excavated soil and generates emissions to the atmosphere of gas pollutants (including CO2!) and particulate matter as a result of excavation and transport of the material to the landfill. An alternative approach consists in leaving the soil in-situ, while reducing the mobility of the heavy metals with a proper treatment, thus obtaining in principle a relevant reduction of the remediation environmental footprint. In this regard, carbonation could be exploited in the case of industrial soils characterized by the presence of alkaline industrial residues, allowing to improve the leaching behavior of the materials, as already discussed in the previous sections. Differently from the other contexts discussed in this paper, the CO2 needed for the carbonation process cannot be obtained from an industrial process, considering that Brownfields are abandoned sites. Nevertheless, CO2 fluxes may derive as a consequence of treatments aimed at

the remediation of groundwater contaminated by organic compounds, such as oxidation or stripping using CO2; besides, CO2 may also be generated by treatments aimed at improving the structural properties of the subsoil in view of Brownfield sites redevelopment [13].

The concept of the proposed in-situ carbonation process in the context of brownfield regeneration is summarized in Figure 3, which refers to a site, whose conceptual model includes a layer of industrial soil and groundwater contaminated by organic contaminants. As shown in Figure 3, a flux of pure CO2 is used for stripping the organic contaminants, thus allowing to clean-up the groundwater body. Alternatively, an oxidant could be injected, fostering the mineralization of the organic contaminants. CO2 may also evolve from innovative treatments for the improvement of the subsoil in view of Brownfield sites regeneration: this is the case of the Ecogrout process, where a solution of calcium chloride and sodium bicarbonate is injected underground, leading to the precipitation of calcium carbonate, that enhances the structural properties of the soil [13]. In all these cases, an upward CO2 flux is produced, that can be employed for the carbonation of already existing or specifically prepared layers of alkaline residues and/or industrial soil to improve the environmental and technical properties of the materials as well as to permanently store CO2.

Fig. 3.Application of carbonation in the context of a brownfield (reprinted with permission from [13])

4.2. Experimental procedure adopted and main results

The feasibility of in-situ carbonation of industrial soils was first investigated through lab-scale column carbonation experiments, in which 100% CO2 was fed through humidified stainless steel slag, applying operating conditions expected at Brownfield sites (T=22-25°C and atmospheric pressure) and evaluating the effects of the treatment in terms of CO2 uptake and modifications in mineralogy and leaching behavior of the residues. The results,

obtained and discussed in more detail by Baciocchi et al. [13] showed that, even at the mild operating conditions tested, a significant degree of carbonation could be achieved. Namely, an average CO2 uptake of 6% was obtained for the intermediate size fraction of the material (0.177-0.84 mm) after 8 hours of reaction. This value compared reasonably well with the conversion obtained on the same fraction in batch carbonation tests performed at enhanced conditions (T=50°C, P=10 bar, L/S ratio=0.4 l/kg). The tested column carbonation treatment significantly affected the mineralogy and environmental properties of the slag. The XRD patterns indicated a clear decrease of portlandite and a slighter one of Ca-Al silicate and melilite peaks, as a result of the carbonation reaction, that was confirmed by the relevant increase of calcite peaks as well as by the results of IC analysis of the treated samples. As far as the leaching behavior is concerned, the results of the compliance test showed a decrease of the eluate pH well below the limit for reuse set by the Italian national legislation.

The possibility of feeding the in-situ carbonation process with the CO2 evolved from one of the process discussed above, namely the Ecogrout process, was also tested at lab-scale. Despite the promising results obtained in one of the tested experimental set-ups, the obtained CO2 uptakes were very dependent on the effective mixing of the reagents in the Ecogrout process and on the formation of preferential CO2 pathways, especially in the larger reactor set-up, that probably induced a strongly heterogeneous carbonation of the slag layer [14]. More tests are hence required to better elucidate these effects and to definitively assess the feasibility of the proposed in-situ carbonation process.

5. Conclusions and perspectives

The case studies discussed in this paper show that carbonation of alkaline industrial residues can be usefully applied in different contexts, allowing to provide multiple environmental benefits, besides permanent storage of CO2. Nevertheless, the results obtained allowed to identify the main bottlenecks that may hinder the implementation of accelerated carbonation in the investigated contexts. Specifically, in the landfill context, the main limitation of the AwR process is related to the requirement of a specific type of residue, characterized by a high calcium hydroxides content, whose availability is rather limited. Besides, although a regeneration of alkali hydroxide for the absorption step above 50% is achieved, the required make-up of the alkali reagent and consequently the CO2 emissions associated to its production are significant, thus making the overall process barely carbon-neutral. As for the steelmaking context, although the proposed carbonation process operates at rather mild conditions, the associated energy requirements are still too high to make the process attractive, unless the preliminary capture step could be avoided. Thus, direct carbonation of steel slags using a rather concentrated stream (i.e. syngas or biogas) could represent a viable option for scaling-up the technology. The other issue to be still addressed is related to the mechanical properties of the product obtained from the combined carbonation-granulation process, whose solution could be achieved by selecting a proper binder to be added during the process. Finally, the in-situ process is a rather visionary application of carbonation in a very specific context, whose feasibility still needs to be assessed through properly designed field-scale tests.

Acknowledgements

The authors wish to acknowledge the support received by the Life+ Programme and European Commission within the activities of the UPGAS-LOWCO2 project (LIFE08/ENV/IT/000429) and the funding received by the Seventh Framework programme of the European Commission within the project Holistic Management of Brownfield Regeneration (HOMBRE).

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