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Comparative Life-cycle Assessment of Slurry and Wet Accelerated Carbonation of BOF Slag
Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 114 (2017) 5393 – 5403
13th International Conference on Greenhouse Gas Control Technologies, GHGT-13, 14-18 November 2016, Lausanne, Switzerland
Comparative life-cycle assessment of slurry and wet accelerated carbonation of BOF slag Sara Ghasemia,b, Giulia Costaa, Daniela Zingarettia, Matthäus U. Bäblerb, Renato Baciocchia* a
Laboratory of Environmental Engineering, Dept. Civil Engineering and Information Technology Engineering, University of Rome “Tor Vergata,” Via del Politecnico 1, I-00133 Rome, Italy b Dept. Chemical Engineering and Technology, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden
Abstract This work reports the results of the life cycle assessment (LCA) of two carbonation processes aimed at permanent CO2 storage, employing Basic Oxygen Furnace (BOF) slag from steel manufacturing as alkalinity source. Specifically, the performance of the slurry phase and wet carbonation routes were compared assuming to store the CO2 emitted from a 10 MW conventional natural gas power plant. The LCA was based on the material and energy requirements for each of the involved process steps, i.e. pretreatment and transport of raw materials, CO2 compression, carbonation, post treatments and management of the obtained products. The slurry and wet route resulted in a net avoided greenhouse warming potential (GWP) of 473 and 384 kg CO2/MWhel, respectively. Nevertheless, both routes affected the other environmental impact categories. In general, the wet route had approximately two times higher impact than the slurry route, due in particular to the higher material and energy requirements. An exception was the abiotic resource depletion which resulted higher for the slurry route due to greater water requirement with respect to the wet route. The contributions to all mid-point impact categories were mainly due to energy requirements. A sensitivity analysis showed that the environmental impacts are affected by the energy mix and by the transport distance of slags and carbonation products.
© ©2017 2017The TheAuthors. Authors.Published Publishedby byElsevier ElsevierLtd. Ltd.This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of GHGT-13. Peer-review under responsibility of the organizing committee of GHGT-13. Keywords: Mineral carbonation, CO2 storage, steelmaking slag, life cycle assessment.
* Corresponding author. Tel.: +39-06-72597022; fax: +39-06-72597021. E-mail address: [email protected]
1876-6102 © 2017 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/4.0/). Peer-review under responsibility of the organizing committee of GHGT-13. doi:10.1016/j.egypro.2017.03.1675
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1. Introduction Carbonation, a naturally occurring process by which gaseous carbon dioxide reacts with minerals containing alkali earth metal oxides to form Ca or Mg carbonates, has been proposed and investigated in the last couple of decades as a CO2 ex situ storage method [1-4]. Storing CO2 in the form of solid carbonates is in fact a stable and safe solution that does not present the disadvantages of geological storage such as long-term monitoring and the risk of CO2 leakage. Instead of employing mineral phases (e.g. Mg and Ca silicates) as alkalinity source, which are available in large amounts but present a slow reactivity, several studies have tested the applicability of carbonation to alkaline industrial residues such as steelmaking slag, or other types of residues from thermal processes [5,6]. Carbonation in fact has shown not only to be an effective CO2 storage technique but also to affect the leaching properties of the treated material and other of its properties such as e.g. porosity, making the product potentially suitable for utilisation [7-10]. Moreover, the exothermicity of the carbonation reaction generates heat that could be recovered and used to lower the energy penalties of the process [11-12]. In fact, even though the carbonation reaction is thermodynamically favored, the kinetics is slow; thus in order to apply this process in industrial applications to either minerals or alkaline residues, the carbonation reaction kinetics must be enhanced (i.e. accelerated) through feedstock pretreatment, optimized reaction conditions, increased CO2 concentration and/or use of additives [6]. These intensifications result in additional material and energy requirements which under certain circumstances can lead to higher net CO2 emission, making accelerated carbonation unviable and leading also to other non negligible potential environmental impacts [13,14]. Specifically, accelerated carbonation for CO2 storage has been investigated using different process routes that can be broadly classified in gas-solid and aqueous. The latter route‚ by far the most investigated one, is attained by mixing the alkaline feedstock with water‚ applying Liquid to Solid ratios (L/S) above 2 l/kg (slurry-phase) or between 0.1 and 1 l/kg (wet route) [15]. While the first type of process has been investigated mainly as a CO2 sequestration technique for minerals or residues applying enhanced operating conditions (e.g. T=50-225 °C and pCO2=5-10o bar) [3,4,9]‚ the wet route has been tested on a wide range of residues at mild operating conditions (30-50 °C and 1-10 bar CO2) for CO2 storage‚ but also as a valorization treatment to generate a potentially recyclable product [16]. This study presents a comparative LCA of the slurry-phase and wet route processes applied to steelmaking slag, one of the types of residues that has been indicated to have the highest potential for CO2 storage, both in terms of availability and chemical and mineralogical composition [16]. The operating conditions selected for each process route, as well as the resulting CO2 uptake and reactivity of the residues were taken from a previous study in which slurry phase and wet route experiments were tested on Basic Oxygen Furnace steel slag samples [15]. The layout of the two carbonation processes was assumed based on [17], while material and energy requirements specific to each route were derived from Zingaretti et al., (2014), that assessed slurry-phase and wet route carbonation applied to different types of alkaline residues at several operating conditions [12]. 2. Problem formulation and methodology We assessed the performance of two carbonation processes aimed at storing the CO2 resulting from the capture unit of a 10 MW natural gas fired power plant using basic oxygen furnace (BOF) steelmaking slag as alkalinity source. A 90% capture rate of the plant’s CO2 emissions was assumed, while the size of the power plant was chosen based on the annual amount of slag generated by the plant from which the BOF residues were sampled (§ 700000 ton) . The flow exiting the capture unit was assumed to consist of pure CO2.. The power plant and the capture unit were placed outside the system boundaries. Fig. 1 and 2 show the schematic flow diagrams of the two carbonation routes assessed in this study. In the slurry route (Fig. 1), the slag particles are ground and fed to a unit where they are mixed with water to form a slurry. The slurry is pumped into a heat exchanger where it flows countercurrently to the hot carbonated slurry leaving the reactor. The heated slurry and compressed CO2 are fed to the carbonation reactor. The slurry with the carbonated slag returns to the heat exchanger before entering the solid-liquid separation unit. The separated water is sent to treatment while the solid product is properly managed. The wet route (Fig. 2) also starts with size reduction of the slag. The ground slag enters the reactor, which is assumed to be a rotary kiln, where it is humidified with a water
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sprayer and heated to the reaction temperature. The slag reacts with compressed CO2 fed to the reactor, thus forming carbonated products that are properly managed. Four scenarios were examined, i.e. two base scenarios and two improved scenarios. In the base scenarios, referred to as Slurry1 and Wet1, all the carbonation products were assumed to be landfilled. In the improved scenarios, referred to as Slurry2 and Wet2, half of the products were assumed to be landfilled while the other half to substitute limestone (for the slurry route) and natural aggregates (for the wet route). In all cases the transport distance of the products was assumed to be 20 km independently from the use of the products. Operating conditions for both the slurry-phase and the wet route were taken from [15]. Namely, for both routes the starting material was assumed to be collected downstream the iron recovery unit of the steel mill and to be hence characterized by an average particle size equal to 1.19 mm. It was then assumed to further reduce the particle size to below 0.15 mm by milling. For both routes, the CO2 partial pressure and residence time in the carbonation reactor were assumed to be equal to 10 bar, and 1 hour, respectively. For the slurry-phase route, the L/S ratio was set at a 5 dm3/kg slag and the reaction temperature at 373 K; whereas in the wet route ,the L/S ratio was 0.3 dm3/kg slag and the reaction temperature 323 K [15]. The results of the carbonation experiments indicated that in the slurry phase route a CO2 uptake of 0.25 kg CO2/kg slag was achieved, while in the wet route 0.1 kg CO2/kg slag were stored. In order to compare the two process routes an LCA based on ISO 14040 standards was performed. The functional unit was set to 1 MWh electricity produced from the natural gas power plant with 10 MW full capacity. Material and energy balances for the operation of the two carbonation plants were based on Zingaretti et al. [12], making reference specifically to experiment codes S19 for the slurry-phase process and W20 for the wet route one. In addition, the capital goods (i.e. materials and energy) necessary for the construction of the two types of plants were also assessed [18]. The results from these calculations were implemented in SimaPro v8.0.5. Inventory data was also taken from the Ecoinvent 3.1 database while the CML2001 method was selected for performing the impact assessment.
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Fig. 1. Scheme of the slurry-phase route carbonation process using BOF steel slag considered in the LCA; the dashed line indicates the boundaries of the system.
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Fig. 2. Scheme of the wet route carbonation process using BOF steel slag considered in the LCA; the dashed line indicates the boundaries of the system.
3. Results and discussion 3.1. Energy and material requirements Tab.1 reports the material requirements for construction and operation of both slurry and wet routes, normalized to the functional unit chosen for the LCA, i.e. 1 MWh electricity produced from a 10 MW natural gas plant, with 608 kg CO2/MWhel emissions, 90% of which (i.e. 548 kg CO2/MWhel) are conveyed to the carbonation plant and stored in mineral form. The amount of slags needed to store the CO2 (Slag in Tab. 1) was calculated starting from the selected operating conditions for each route. Clearly this figure is lower for the slurry phase than for the wet route, as the calcium to carbonate conversion in the BOF slags achieved for the former one was higher than for the latter one. On the other hand, the amount of water needed is lower for the wet route, as the L/S ratio adopted is in this case much lower than for the slurry route. The three other terms (steel, glass wool, concrete), which refer to the construction step, were normalized to the functional unit assuming a 25 years lifetime of the 10 MW natural gas power plant. Interestingly, the wet route presented a higher requirement for construction materials despite the fewer units employed in this process route. Table 1. Material requirements for carbonation and construction for the slurry route and the wet route. Material
Slurry route
Wet route
Slag [kg/MWhel]
2.19×103
5.48×103
Water [l/MWhel]
1.14×104
2.67×103
Steel [kg/MWhel] Glass wool [kg/MWhel] 3
Concrete [m /MWhel]
1.87×10
-3
9.90×10-6
7.07×10
-6
1.69×10-5
5.83×10
-6
5.40×10-3
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Tab. 2 reports the energy requirements of the different process steps for both the investigated routes, as electric energy per functional unit. As for the slurry route, it is worth pointing out that no heat terms are provided in Tab.2. This happens because the energy needed to heat the slurry to the required temperature in the carbonation reactor is completely provided by the heat recovered from the hot slurry leaving the reactor and the heat evolved from the carbonation reaction, with no need for any other external heat source. Therefore, all the terms reported in Tab. 2 are associated to the electric energy needed to run the different unit operations. The most relevant terms are related to CO2 compression and BOF slag grinding, which contribute to around 80% of the overall energy requirements, whereas the contribution of slurry mixing, wastewater treatment and of the clarifier are negligible. As for the wet route, also in this case, all energy requirements are given in terms of electric energy with respect to the functional unit. The term associated to heating in the carbonation reactor was calculated assuming to use an electric heater, characterized by a 95% conversion efficiency from electricity to heat. Differently from the slurry route, in this case the term associated to heating in the carbonation reactor is quite relevant, despite the carbonation temperature is lower than for the slurry route (323K against 373K). For the wet route, in fact, heat recovery from the wet carbonated products leaving the rotary kiln was neglected. This is a rather conservative assumption, also done in [12], which stems from the consideration that, differently from the slurry phase process, it may result difficult to recover the heat from a solid product. The overall energy requirements associated to grinding, compression and rotation of the carbonation reactor are close to those estimated for the slurry route. As a result, clearly the overall energy penalty for the slurry route is just below 8% of the electric energy produced by the power plant, whereas it increases to 22% for the wet route. Considering that the heat term associated to the carbonation reactor corresponds roughly to 60% of the overall energy requirement, a solution allowing to recover the heat from the solid products from the rotary kiln could allow to achieve a substantial reduction of the energy penalties associated to the wet route. Table 2. Electrical energy requirements of the different units considered in the slurry route and wet routes (a) Slurry route Process step
(b) Wet route
Energy requirement
Process step
(kWhel/MWhel)
Energy requirement (kWhel/MWhel)
CO2 compression
44.14
Carbonation reactor (heating)
131.01
BOF slag grinding
17.54
CO2 compression
44.14
Carbonation reactor (mixing)
6.18
BOF slag grinding
35.08
Slurry pumping
5.10
Carbonation reactor (rotation)
8.88
Solid liquid separation (centrifuge)
2.86
.
.
Slurry mixing
0.62
.
.
Wastewater treatment
0.40
.
.
Solid liquid separation (clarifier)
0.027
.
.
Total
76.87
Total
219.11
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3.2. Impact assessment The LCA results are summarized in Fig. 3, where the different panels compare the impacts of the slurry route and the wet route for the different categories accounted for by the CML2001 method and report graphically the contribution of each unit operation to each impact. For sake of simplicity, only the results corresponding to the base scenarios (Slurry1 and Wet1) are shown, whereas the improved scenarios (Slurry2 and Wet2) are discussed in the next section. The impact on global warming potential (GWP) is shown in Fig. 3a. The most important contribution to GWP for the slurry route results from CO2 compression, followed by mixing, grinding and transportation of the residues and of the products to/from the carbonation plant; the other unit operations provide a low or negligible contribution. The GWP is mainly made by CO2 emissions directly related to energy production. The only exception is the mixing unit, for which the GWP is only indirectly related to the generation of energy, which is needed for producing the water required to prepare the slurry. As for transportation, fuel combustion is the main process contributing to GWP. In the case of the wet route, the main GWP contribution is related to the CO2 emissions associated with the production of the electric energy needed to satisfy the energy requirements for the different unit operations, i.e. carbonation, compression and grinding. For transportation, also in this case fuel combustion is the main process contributing to the GWP. In agreement with the results provided in Tab.2 for the energy requirements, the wet route has an overall impact on GWP which is roughly twice the one of the slurry route. The impact on abiotic depletion (AD) is shown in Fig. 3b. AD is related to the consumption of raw materials, such as water, metal and minerals. The main contribution to this category is related to the water needed for preparing the slurry or for humidifying the residues. The slurry route has clearly a higher impact with respect to the wet route due to the higher water consumption in the mixing step. The second contribution in both cases comes from transportation; in this case, the wet route impacts twice than the slurry route due to the higher amount of slags treated in the plant needing to be transported from/to the plant. The abiotic depletion of fossil fuel (ADF) shown in Fig. 3c is connected to the extraction of fossil fuels. The contributions to this impact category are clearly related to the energy requirements shown in Tab. 2, as the energy mix considered for this study mostly relies on this kind of energy source. This also explains the higher impact of the wet route, which is characterized by an overall higher energy requirement. As to the contribution of the different unit operations, the main term for the wet route is related to the electricity input for the heating of the slag which is even higher than the total contribution of the slurry route. The difference between the slurry and the wet route regarding grinding and transportation is due to the higher amount of slag processed in the wet route. Acidification (AC) and photochemical oxidation (PO) are shown in Fig. 3d and 3e, respectively. The main contribution to AC and PO is related to NOx and SOx in the flue gas emitted by the power plants running on fossil fuels such as natural gas, oil and hard coal. Accordingly, the impact on acidification and photochemical oxidation shows a similar pattern as the ADF in Fig. 3c. A difference is observed for the slurry route, for which a more relevant contribution of the mixing step is observed. This higher impact is due to the emission of chloride in the production of water needed for this unit operation. The impact on ozone layer depletion (ODP) is shown in Fig. 3f. For the slurry route the mixing step gives the main contribution to ODP, which is connected to the upstream processes for hydrochloric acid production needed for the production of demineralized water. The wet route also shows a contribution related to the production of demineralized water; however, it is not significant in comparison to the contribution from the energy input. An additional contribution comes from pipeline transportation of natural gas for electricity production. Therefore, with the exception of mixing (for the slurry route), the main contribution is related to the unit operations characterized by the higher energy requirements for both routes. Impacts on human toxicity (HT) and ecotoxicity are shown in Fig. 3g and 3h, respectively. The emission of chromium containing compounds in upstream processes, such as metal alloy production and steel production for construction materials, is the primary contribution to this impact category. This is followed by the spoil from hard coal mining which is related to electricity production. For the slurry route, the deionized water consumption gives the main contribution to the overall impact on human toxicity. The impacts on fresh water aqueous ecotoxicity (FWAE), marine aqueous ecotoxicity (MAE), and terrestrial ecotoxicity (TE) are shown in Fig. 3i to 3k. It can be noticed that the impacts result significantly higher for MAE than for the other two categories. The main contribution
Sara Ghasemi et al. / Energy Procedia 114 (2017) 5393 – 5403
to these impact categories is related to mining of hard coal for electricity production, which leads to acid mine drainage and, in addition to acidification, contaminates ground water with heavy metals. Landfill of carbonated products also provide a small contribution due to leaching of the heavy metals from the raw material (BOF slag). The impact on Eutrophication (EU) is shown Fig. 3l and is mainly associated to emissions of phosphorus and nitrogen compounds. In this case it is mostly related to energy production, and this is why it is mostly associated to the unit operations characterized by the higher energy requirements (see Tab. 2).
Acidification (AC)
Fig. 3. Impact assessment of the slurry route and the wet route carbonation process in the base scenario. Each panel shows a specific impact category with the colors indicating the different process steps.
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3.3. Improved scenarios The improved scenarios (Slurry 2 and Wet2) differ from the base scenarios (Slurry1 and Wet1) only for the landfill step, as in the improved scenarios it was assumed that half of the products are not landfilled (as in the base scenario) but used to replace limestone and natural aggregates as building materials. Tab. 3 presents the impact of the landfill step for each simulated scenario. Looking first at the base scenarios (Slurry1 and Wet1) it can be observed that landfill of carbonated slag has an effect only on four impact categories, i.e. HT, FWAE, MAE and TE. The impact on this four categories is mainly due to leaching of metals into ground water and acid drainage caused by mining of hard coal for energy production. It must be considered that the emissions due to leaching are based on data for un-reacted BOF slag. According to literature [10] carbonation is expected to affect the leaching behavior of steelmaking slag and this could have an impact on the LCA results. Accounting for the modified leaching behavior of carbonated slag disposed in landfills remains open for future studies. Turning to the improved scenarios (Slurry2 and Wet2), it can be observed that the replacement of limestone and natural aggregates with the carbonated products leads to a negative impact (positive from an environmental point of view) for the AD, ADF, GWP, ODP, PO, AC and EU categories, for whom there was no impact in the base scenarios. This result is primarily due to the reduced energy requirement involved in the lifecycle of limestone and aggregate production, while avoiding the landfilling of 50% of BOF slag provides only a small contribution to reducing the impacts. Namely, replacing limestone and natural aggregates leads to a GWP reduction of 3.35 and 14.2 kg CO2/MWhel for the slurry and the wet route, respectively. The difference in GWP reduction between the slurry route and the wet route is partially due to the higher amount of slag processed in the wet route, resulting in a higher amount of aggregates that are substituted. Besides, it must be also considered that the impacts associated to aggregate production (replaced by the products of the wet route) are higher in all categories with respect to limestone (replaced by the products of the slurry route). Table 3. Impact assessment of the landfill step in the base scenario and in the improved scenario. Impact category
Slurry1
Slurry2
Wet1
Wet2
GWP [kg CO2 eq]
0
-3.35
0
-14.17
AD [kg Sb eq]
0
-4.89E-06
0
-2.52E-06
ADF [MJ]
0
-42.56
0
-177.2
ODP [kg CFC-11 eq]
0
-1.72E-07
0
-3.10E-07
HT [kg 1,4-DB eq]
1.3
-3.84
1.37
-10.76
FWAE [1,4-DB eq]
10.55
-25.4
11.09
-71.33
MAE [1,4-DB eq]
1.31E3
-5.81E+03
1376.69
-1.90E+04
TE [1,4-DB eq]
8.64E-20
-1.43E-03
9.08E-20
-6.95E-03
PO [kg C2H4 eq]
0
-1.51E-03
0
-6.18E-03
AC [kg SO2 eq]
0
-0.05
0
-0.12
EU [kg PO42-eq]
0
-0.01
0
-0.02
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3.4. CO2 reduction potential and total GWP Tab. 4 reports the CO2 amount conveyed to the carbonation process and stored as carbonate (548 kg CO2/MWhel), the overall greenhouse gas emissions associated to the carbonation process (GWP carbonation) and the overall CO2 emissions associated to the carbonation process (CO2 emissions carbonation). These data are provided for both the slurry and the wet route with reference to the base scenarios (Slurry1 and Wet1) and to the improved scenarios (Slurry2 and Wet2). These data were used to estimate the net CO2 reduction potential of the carbonation process. This is given as: “Total GWP”, i.e. the difference between the CO2 stored by carbonation (Stored CO2) and the overall greenhouse gas emitted by the carbonation process (GWP carbonation); or as “Avoided CO2”, i.e. the difference between the CO2 stored by carbonation (Stored CO2) and the CO2 emitted by the carbonation process (CO2 emissions carbonation). As can be seen from Tab. 4 carbonation of BOF slag can lead to an overall negative contribution to GWP, meaning that CO2 emissions are actually avoided. However, there is a difference in the amount of CO2 avoided with the different routes due to the difference in energy consumption and material requirements. For the base scenario, the slurry route is characterized by a total CO2 avoided (GWP) around 480 (470) kg CO2/MWhel , whereas the same figures for the wet route are roughly 80-90 kg CO2/MWhel lower in absolute terms. The difference is slightly smaller if the improved scenarios are considered, due to the reduction in GWP achieved through the partial replacement of natural aggregates with the products of the process. Table 4. CO2 reduction potential (negative number means that the CO2 is not emitted) Stored CO2
GWP Carbonation
[kg/MWhel]
[kg CO2 eq/MWhel]
Slurry1
-547.82
Slurry2
-547.82
Wet1 Wet2
Scenario
CO2 emissions Carbonation [kg CO2/MWhel]
Total GWP
Avoided CO2
[kg CO2 eq/MWhel]
[kg/MWhel]
74.91
69.05
-472.91
-478.78
71.57
66.09
-476.26
-481.73
-547.82
164.30
141.75
-383.52
-396.08
-547.82
150.13
139.32
-397.69
-408.51
3.5. Sensitivity analysis The sensitivity of total GWP with respect to two process parameters (transportation distance and energy source) was evaluated, in order to get some more insights into the main factors affecting the impact on this category. The results of this sensitivity analysis are shown in Fig. 4. The influence of the transportation distance is analyzed in Fig. 4a. So far, the transportation distance of the raw slag and products was set at 20 km. To assess the influence of this parameter, the GWP was estimated assuming transport distances of 10, 20, 40, 60 and 100 km. These distances were applied for each transportation step, i.e., transport of the BOF slags to the carbonation plant and transport of the carbonated products from the plant to the landfill site. As expected, transport distance has a relevant effect on the GWP for both routes. However, the wet route is more sensitive to changes in distance due to the larger amount of residues treated, needing to be transported. The impact of the energy source on the calculated GWP is reported in Figure 4b. As expected, higher GWP values (lower in absolute term), around -350 kg CO2 eq/MWhel are obtained when the electricity is assumed to be produced by fossil fuels, hydropower (pumped storage) and NGC (Natural Gas Combined Cycle). On the other hand, assuming to have an energy mix completely relying on renewable energy (such as Hydro or Wind) would actually lead to a much lower GWP (higher in absolute term) equal to -530 kg CO2 eq/MWhel. As a reference, it can be recalled that the GWP reported for the base scenarios, calculated assuming the energy mix of Italy, were around 380 and -470 kg CO2 eq/MWhel for the wet and slurry route, respectively.
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(a) (b)
Fig. 4. Sensitivity of the total global warming potential (GWP) of the wet route and slurry route carbonation for the base scenario (Slurry1 and Wet1) and the improved scenario (Slurry2 and Wet2) as a function of: a) transport distance; b) electricity source.
4. Conclusions A comparative LCA study was performed on direct carbonation of BOF slag through a slurry route process and wet route process. The functional unit for the LCA was defined as 1 MWh electricity produced by a conventional natural gas power plant. In addition, a sensitivity analysis was carried out to investigate the effect of transportation distance and energy source on the outcome. The results show that both routes are capable of storing CO2 with a negative net contribution to global warming potential. The slurry route showed a higher reduction potential when compared to the wet route. The difference is mainly due to the higher energy requirements for heating of the slag. Even though a negative GWP was observed, both routes affected the other environmental impact categories. In general, the wet route had approximately two times higher impact than the slurry route. One exception was the impact category of abiotic depletion, where the slurry route appears to have the highest impact. The contributions to these mid-point impact categories were mainly due to energy requirements. In the case of the slurry route mixing, the main contributors to the energy requirements are (in descending order) compression, grinding and transportation, while for the wet route the order is: carbonation, compression, grinding and transportation. By performing a sensitivity analysis, a dependency on transportation distance and electricity source was observed. As the distance increased the GWP also increased. If electricity was fossil fuel based, the GWP would increase but still result in a total negative contribution. Since the contribution to the other impact categories were connected to electricity requirements, similar trends were observed. References [1] Seifritz W. CO2 disposal by means of silicates. Nature 1990; 345:486. [2] Lackner KS, Wendt CH, Butt D et al. Carbon dioxide disposal in carbonate minerals. Energy 1995; 20:1153–1170. [3] Gerdemann SJ, O’Connor WK, Dahlin DC, et al. Ex Situ Aqueous Mineral Carbonation. Env Sci Technol 2007;41:2587–2593. [4] Huijgen WWJ, Witkamp GJ, Comans RNJ. Mechanisms of aqueous wollastonite carbonation as a possible CO2 sequestration process. Chem Eng Sci 2006;61:4242–4251. [5] Bobicki ER, Liu Q, Xu Z et al. Carbon capture and storage using alkaline industrial wastes. Prog Energ Combust 2012;38:302–320. [6] Pan S, Chang EE, Chiang P. CO2 capture by accelerated carbonation of alkaline wastes: A review on its principles and applications. Aerosol Air Qual Res 2012;12:770-791.. [7] Huijgen W, Comans R. Carbonation of steel slag for CO2 sequestration: Leaching of products and reaction mechanisms. Environ Sci Technol 2006;40:2790-2796. [8] Van Gerven T, Van Keer E, Arickx S et al. Carbonation of MSWI-bottom ash to de-crease heavy metal leaching, in view of recycling. Waste Manage 2005;25:291–300.
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[9] Santos, R.M., Van Bouwel, J., Vandevelde, E., Mertens, G., Elsen, J., Van Gerven, T.: Accelerated mineral carbonation of stainless steel slags for CO2 storage and waste valorization: Effect of process parameters on geochemical properties. Int. J. Greenh. Gas Con. 2013;17: 32–45. [10] Morone M, Costa G, Polettini A, Pomi R, Baciocchi R. Valorization of steel slag by a combined carbonation and granulation treatment. Miner Eng 2014;59:82-90. [11] Huijgen WWJ, Ruijg GJ, Comans RNJ et al. Energy Consumption and Net CO2 Sequestration of Aqueous Mineral Carbonation. Ind Eng Chem Res 2006;45:9184–9194 [12] Zingaretti D, Costa G, Baciocchi R. Assessment of accelerated carbonation processes for CO2 storage using alkaline industrial residues. Ind Eng Chem Res 2014;53:9311-9324. [13] Kirchofer A, Brandt A, Krevor S, Progiobbe V, Wilcox J. Impact of alkalinity sources on the life cycle energy efficiency of mineral carbonation technologies. Energy Environ. Sci. 2012; 8631-8641. [14] Giannoulakis S, Volkart K, Bauer C. Life cycle and cost assessment of mineral carbonation for carbon capture and storage in European power generation”, Int J Greenh Gas Con 2014;21:140-157. [15] Baciocchi, R, Costa, G, Di Gianfilippo, M, Polettini, A, Pomi, R, Stramazzo, A: Thin-film versus slurry-phase carbonation of steel slag: CO2 uptake and effects on mineralogy. J. Hazard. Mater. 2015; 283: 302–313 [16] Baciocchi R‚ Costa G‚ Zingaretti D Baciocchi R. Accelerated carbonation processes for carbon dioxide capture, storage and utilization. In: Bhanage, B.M., Arai, M. editors. Transformation and Utilization of Carbon Dioxide, Green Chemistry and Sustainable Technology. Berlin:Springer-Verlag, Berlin 2014; p. 263–299. [17] Zingaretti D, Costa G, Baciocchi R. Assessment of the energy requirements for CO2 storage by carbonation of industrial residues. Part 1: Definition of the process layout. Energy Procedia 2013;37:5850-5857.
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13th International Conference on Greenhouse Gas Control Technologies, GHGT-13, 14-18 November 2016, Lausanne, Switzerland
Comparative life-cycle assessment of slurry and wet accelerated carbonation of BOF slag Sara Ghasemia,b, Giulia Costaa, Daniela Zingarettia, Matthäus U. Bäblerb, Renato Baciocchia* a
Laboratory of Environmental Engineering, Dept. Civil Engineering and Information Technology Engineering, University of Rome “Tor Vergata,” Via del Politecnico 1, I-00133 Rome, Italy b Dept. Chemical Engineering and Technology, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden
Abstract This work reports the results of the life cycle assessment (LCA) of two carbonation processes aimed at permanent CO2 storage, employing Basic Oxygen Furnace (BOF) slag from steel manufacturing as alkalinity source. Specifically, the performance of the slurry phase and wet carbonation routes were compared assuming to store the CO2 emitted from a 10 MW conventional natural gas power plant. The LCA was based on the material and energy requirements for each of the involved process steps, i.e. pretreatment and transport of raw materials, CO2 compression, carbonation, post treatments and management of the obtained products. The slurry and wet route resulted in a net avoided greenhouse warming potential (GWP) of 473 and 384 kg CO2/MWhel, respectively. Nevertheless, both routes affected the other environmental impact categories. In general, the wet route had approximately two times higher impact than the slurry route, due in particular to the higher material and energy requirements. An exception was the abiotic resource depletion which resulted higher for the slurry route due to greater water requirement with respect to the wet route. The contributions to all mid-point impact categories were mainly due to energy requirements. A sensitivity analysis showed that the environmental impacts are affected by the energy mix and by the transport distance of slags and carbonation products.
© ©2017 2017The TheAuthors. Authors.Published Publishedby byElsevier ElsevierLtd. Ltd.This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of GHGT-13. Peer-review under responsibility of the organizing committee of GHGT-13. Keywords: Mineral carbonation, CO2 storage, steelmaking slag, life cycle assessment.
* Corresponding author. Tel.: +39-06-72597022; fax: +39-06-72597021. E-mail address: [email protected]
1876-6102 © 2017 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/4.0/). Peer-review under responsibility of the organizing committee of GHGT-13. doi:10.1016/j.egypro.2017.03.1675
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1. Introduction Carbonation, a naturally occurring process by which gaseous carbon dioxide reacts with minerals containing alkali earth metal oxides to form Ca or Mg carbonates, has been proposed and investigated in the last couple of decades as a CO2 ex situ storage method [1-4]. Storing CO2 in the form of solid carbonates is in fact a stable and safe solution that does not present the disadvantages of geological storage such as long-term monitoring and the risk of CO2 leakage. Instead of employing mineral phases (e.g. Mg and Ca silicates) as alkalinity source, which are available in large amounts but present a slow reactivity, several studies have tested the applicability of carbonation to alkaline industrial residues such as steelmaking slag, or other types of residues from thermal processes [5,6]. Carbonation in fact has shown not only to be an effective CO2 storage technique but also to affect the leaching properties of the treated material and other of its properties such as e.g. porosity, making the product potentially suitable for utilisation [7-10]. Moreover, the exothermicity of the carbonation reaction generates heat that could be recovered and used to lower the energy penalties of the process [11-12]. In fact, even though the carbonation reaction is thermodynamically favored, the kinetics is slow; thus in order to apply this process in industrial applications to either minerals or alkaline residues, the carbonation reaction kinetics must be enhanced (i.e. accelerated) through feedstock pretreatment, optimized reaction conditions, increased CO2 concentration and/or use of additives [6]. These intensifications result in additional material and energy requirements which under certain circumstances can lead to higher net CO2 emission, making accelerated carbonation unviable and leading also to other non negligible potential environmental impacts [13,14]. Specifically, accelerated carbonation for CO2 storage has been investigated using different process routes that can be broadly classified in gas-solid and aqueous. The latter route‚ by far the most investigated one, is attained by mixing the alkaline feedstock with water‚ applying Liquid to Solid ratios (L/S) above 2 l/kg (slurry-phase) or between 0.1 and 1 l/kg (wet route) [15]. While the first type of process has been investigated mainly as a CO2 sequestration technique for minerals or residues applying enhanced operating conditions (e.g. T=50-225 °C and pCO2=5-10o bar) [3,4,9]‚ the wet route has been tested on a wide range of residues at mild operating conditions (30-50 °C and 1-10 bar CO2) for CO2 storage‚ but also as a valorization treatment to generate a potentially recyclable product [16]. This study presents a comparative LCA of the slurry-phase and wet route processes applied to steelmaking slag, one of the types of residues that has been indicated to have the highest potential for CO2 storage, both in terms of availability and chemical and mineralogical composition [16]. The operating conditions selected for each process route, as well as the resulting CO2 uptake and reactivity of the residues were taken from a previous study in which slurry phase and wet route experiments were tested on Basic Oxygen Furnace steel slag samples [15]. The layout of the two carbonation processes was assumed based on [17], while material and energy requirements specific to each route were derived from Zingaretti et al., (2014), that assessed slurry-phase and wet route carbonation applied to different types of alkaline residues at several operating conditions [12]. 2. Problem formulation and methodology We assessed the performance of two carbonation processes aimed at storing the CO2 resulting from the capture unit of a 10 MW natural gas fired power plant using basic oxygen furnace (BOF) steelmaking slag as alkalinity source. A 90% capture rate of the plant’s CO2 emissions was assumed, while the size of the power plant was chosen based on the annual amount of slag generated by the plant from which the BOF residues were sampled (§ 700000 ton) . The flow exiting the capture unit was assumed to consist of pure CO2.. The power plant and the capture unit were placed outside the system boundaries. Fig. 1 and 2 show the schematic flow diagrams of the two carbonation routes assessed in this study. In the slurry route (Fig. 1), the slag particles are ground and fed to a unit where they are mixed with water to form a slurry. The slurry is pumped into a heat exchanger where it flows countercurrently to the hot carbonated slurry leaving the reactor. The heated slurry and compressed CO2 are fed to the carbonation reactor. The slurry with the carbonated slag returns to the heat exchanger before entering the solid-liquid separation unit. The separated water is sent to treatment while the solid product is properly managed. The wet route (Fig. 2) also starts with size reduction of the slag. The ground slag enters the reactor, which is assumed to be a rotary kiln, where it is humidified with a water
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sprayer and heated to the reaction temperature. The slag reacts with compressed CO2 fed to the reactor, thus forming carbonated products that are properly managed. Four scenarios were examined, i.e. two base scenarios and two improved scenarios. In the base scenarios, referred to as Slurry1 and Wet1, all the carbonation products were assumed to be landfilled. In the improved scenarios, referred to as Slurry2 and Wet2, half of the products were assumed to be landfilled while the other half to substitute limestone (for the slurry route) and natural aggregates (for the wet route). In all cases the transport distance of the products was assumed to be 20 km independently from the use of the products. Operating conditions for both the slurry-phase and the wet route were taken from [15]. Namely, for both routes the starting material was assumed to be collected downstream the iron recovery unit of the steel mill and to be hence characterized by an average particle size equal to 1.19 mm. It was then assumed to further reduce the particle size to below 0.15 mm by milling. For both routes, the CO2 partial pressure and residence time in the carbonation reactor were assumed to be equal to 10 bar, and 1 hour, respectively. For the slurry-phase route, the L/S ratio was set at a 5 dm3/kg slag and the reaction temperature at 373 K; whereas in the wet route ,the L/S ratio was 0.3 dm3/kg slag and the reaction temperature 323 K [15]. The results of the carbonation experiments indicated that in the slurry phase route a CO2 uptake of 0.25 kg CO2/kg slag was achieved, while in the wet route 0.1 kg CO2/kg slag were stored. In order to compare the two process routes an LCA based on ISO 14040 standards was performed. The functional unit was set to 1 MWh electricity produced from the natural gas power plant with 10 MW full capacity. Material and energy balances for the operation of the two carbonation plants were based on Zingaretti et al. [12], making reference specifically to experiment codes S19 for the slurry-phase process and W20 for the wet route one. In addition, the capital goods (i.e. materials and energy) necessary for the construction of the two types of plants were also assessed [18]. The results from these calculations were implemented in SimaPro v8.0.5. Inventory data was also taken from the Ecoinvent 3.1 database while the CML2001 method was selected for performing the impact assessment.
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Fig. 1. Scheme of the slurry-phase route carbonation process using BOF steel slag considered in the LCA; the dashed line indicates the boundaries of the system.
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Fig. 2. Scheme of the wet route carbonation process using BOF steel slag considered in the LCA; the dashed line indicates the boundaries of the system.
3. Results and discussion 3.1. Energy and material requirements Tab.1 reports the material requirements for construction and operation of both slurry and wet routes, normalized to the functional unit chosen for the LCA, i.e. 1 MWh electricity produced from a 10 MW natural gas plant, with 608 kg CO2/MWhel emissions, 90% of which (i.e. 548 kg CO2/MWhel) are conveyed to the carbonation plant and stored in mineral form. The amount of slags needed to store the CO2 (Slag in Tab. 1) was calculated starting from the selected operating conditions for each route. Clearly this figure is lower for the slurry phase than for the wet route, as the calcium to carbonate conversion in the BOF slags achieved for the former one was higher than for the latter one. On the other hand, the amount of water needed is lower for the wet route, as the L/S ratio adopted is in this case much lower than for the slurry route. The three other terms (steel, glass wool, concrete), which refer to the construction step, were normalized to the functional unit assuming a 25 years lifetime of the 10 MW natural gas power plant. Interestingly, the wet route presented a higher requirement for construction materials despite the fewer units employed in this process route. Table 1. Material requirements for carbonation and construction for the slurry route and the wet route. Material
Slurry route
Wet route
Slag [kg/MWhel]
2.19×103
5.48×103
Water [l/MWhel]
1.14×104
2.67×103
Steel [kg/MWhel] Glass wool [kg/MWhel] 3
Concrete [m /MWhel]
1.87×10
-3
9.90×10-6
7.07×10
-6
1.69×10-5
5.83×10
-6
5.40×10-3
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Tab. 2 reports the energy requirements of the different process steps for both the investigated routes, as electric energy per functional unit. As for the slurry route, it is worth pointing out that no heat terms are provided in Tab.2. This happens because the energy needed to heat the slurry to the required temperature in the carbonation reactor is completely provided by the heat recovered from the hot slurry leaving the reactor and the heat evolved from the carbonation reaction, with no need for any other external heat source. Therefore, all the terms reported in Tab. 2 are associated to the electric energy needed to run the different unit operations. The most relevant terms are related to CO2 compression and BOF slag grinding, which contribute to around 80% of the overall energy requirements, whereas the contribution of slurry mixing, wastewater treatment and of the clarifier are negligible. As for the wet route, also in this case, all energy requirements are given in terms of electric energy with respect to the functional unit. The term associated to heating in the carbonation reactor was calculated assuming to use an electric heater, characterized by a 95% conversion efficiency from electricity to heat. Differently from the slurry route, in this case the term associated to heating in the carbonation reactor is quite relevant, despite the carbonation temperature is lower than for the slurry route (323K against 373K). For the wet route, in fact, heat recovery from the wet carbonated products leaving the rotary kiln was neglected. This is a rather conservative assumption, also done in [12], which stems from the consideration that, differently from the slurry phase process, it may result difficult to recover the heat from a solid product. The overall energy requirements associated to grinding, compression and rotation of the carbonation reactor are close to those estimated for the slurry route. As a result, clearly the overall energy penalty for the slurry route is just below 8% of the electric energy produced by the power plant, whereas it increases to 22% for the wet route. Considering that the heat term associated to the carbonation reactor corresponds roughly to 60% of the overall energy requirement, a solution allowing to recover the heat from the solid products from the rotary kiln could allow to achieve a substantial reduction of the energy penalties associated to the wet route. Table 2. Electrical energy requirements of the different units considered in the slurry route and wet routes (a) Slurry route Process step
(b) Wet route
Energy requirement
Process step
(kWhel/MWhel)
Energy requirement (kWhel/MWhel)
CO2 compression
44.14
Carbonation reactor (heating)
131.01
BOF slag grinding
17.54
CO2 compression
44.14
Carbonation reactor (mixing)
6.18
BOF slag grinding
35.08
Slurry pumping
5.10
Carbonation reactor (rotation)
8.88
Solid liquid separation (centrifuge)
2.86
.
.
Slurry mixing
0.62
.
.
Wastewater treatment
0.40
.
.
Solid liquid separation (clarifier)
0.027
.
.
Total
76.87
Total
219.11
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3.2. Impact assessment The LCA results are summarized in Fig. 3, where the different panels compare the impacts of the slurry route and the wet route for the different categories accounted for by the CML2001 method and report graphically the contribution of each unit operation to each impact. For sake of simplicity, only the results corresponding to the base scenarios (Slurry1 and Wet1) are shown, whereas the improved scenarios (Slurry2 and Wet2) are discussed in the next section. The impact on global warming potential (GWP) is shown in Fig. 3a. The most important contribution to GWP for the slurry route results from CO2 compression, followed by mixing, grinding and transportation of the residues and of the products to/from the carbonation plant; the other unit operations provide a low or negligible contribution. The GWP is mainly made by CO2 emissions directly related to energy production. The only exception is the mixing unit, for which the GWP is only indirectly related to the generation of energy, which is needed for producing the water required to prepare the slurry. As for transportation, fuel combustion is the main process contributing to GWP. In the case of the wet route, the main GWP contribution is related to the CO2 emissions associated with the production of the electric energy needed to satisfy the energy requirements for the different unit operations, i.e. carbonation, compression and grinding. For transportation, also in this case fuel combustion is the main process contributing to the GWP. In agreement with the results provided in Tab.2 for the energy requirements, the wet route has an overall impact on GWP which is roughly twice the one of the slurry route. The impact on abiotic depletion (AD) is shown in Fig. 3b. AD is related to the consumption of raw materials, such as water, metal and minerals. The main contribution to this category is related to the water needed for preparing the slurry or for humidifying the residues. The slurry route has clearly a higher impact with respect to the wet route due to the higher water consumption in the mixing step. The second contribution in both cases comes from transportation; in this case, the wet route impacts twice than the slurry route due to the higher amount of slags treated in the plant needing to be transported from/to the plant. The abiotic depletion of fossil fuel (ADF) shown in Fig. 3c is connected to the extraction of fossil fuels. The contributions to this impact category are clearly related to the energy requirements shown in Tab. 2, as the energy mix considered for this study mostly relies on this kind of energy source. This also explains the higher impact of the wet route, which is characterized by an overall higher energy requirement. As to the contribution of the different unit operations, the main term for the wet route is related to the electricity input for the heating of the slag which is even higher than the total contribution of the slurry route. The difference between the slurry and the wet route regarding grinding and transportation is due to the higher amount of slag processed in the wet route. Acidification (AC) and photochemical oxidation (PO) are shown in Fig. 3d and 3e, respectively. The main contribution to AC and PO is related to NOx and SOx in the flue gas emitted by the power plants running on fossil fuels such as natural gas, oil and hard coal. Accordingly, the impact on acidification and photochemical oxidation shows a similar pattern as the ADF in Fig. 3c. A difference is observed for the slurry route, for which a more relevant contribution of the mixing step is observed. This higher impact is due to the emission of chloride in the production of water needed for this unit operation. The impact on ozone layer depletion (ODP) is shown in Fig. 3f. For the slurry route the mixing step gives the main contribution to ODP, which is connected to the upstream processes for hydrochloric acid production needed for the production of demineralized water. The wet route also shows a contribution related to the production of demineralized water; however, it is not significant in comparison to the contribution from the energy input. An additional contribution comes from pipeline transportation of natural gas for electricity production. Therefore, with the exception of mixing (for the slurry route), the main contribution is related to the unit operations characterized by the higher energy requirements for both routes. Impacts on human toxicity (HT) and ecotoxicity are shown in Fig. 3g and 3h, respectively. The emission of chromium containing compounds in upstream processes, such as metal alloy production and steel production for construction materials, is the primary contribution to this impact category. This is followed by the spoil from hard coal mining which is related to electricity production. For the slurry route, the deionized water consumption gives the main contribution to the overall impact on human toxicity. The impacts on fresh water aqueous ecotoxicity (FWAE), marine aqueous ecotoxicity (MAE), and terrestrial ecotoxicity (TE) are shown in Fig. 3i to 3k. It can be noticed that the impacts result significantly higher for MAE than for the other two categories. The main contribution
Sara Ghasemi et al. / Energy Procedia 114 (2017) 5393 – 5403
to these impact categories is related to mining of hard coal for electricity production, which leads to acid mine drainage and, in addition to acidification, contaminates ground water with heavy metals. Landfill of carbonated products also provide a small contribution due to leaching of the heavy metals from the raw material (BOF slag). The impact on Eutrophication (EU) is shown Fig. 3l and is mainly associated to emissions of phosphorus and nitrogen compounds. In this case it is mostly related to energy production, and this is why it is mostly associated to the unit operations characterized by the higher energy requirements (see Tab. 2).
Acidification (AC)
Fig. 3. Impact assessment of the slurry route and the wet route carbonation process in the base scenario. Each panel shows a specific impact category with the colors indicating the different process steps.
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3.3. Improved scenarios The improved scenarios (Slurry 2 and Wet2) differ from the base scenarios (Slurry1 and Wet1) only for the landfill step, as in the improved scenarios it was assumed that half of the products are not landfilled (as in the base scenario) but used to replace limestone and natural aggregates as building materials. Tab. 3 presents the impact of the landfill step for each simulated scenario. Looking first at the base scenarios (Slurry1 and Wet1) it can be observed that landfill of carbonated slag has an effect only on four impact categories, i.e. HT, FWAE, MAE and TE. The impact on this four categories is mainly due to leaching of metals into ground water and acid drainage caused by mining of hard coal for energy production. It must be considered that the emissions due to leaching are based on data for un-reacted BOF slag. According to literature [10] carbonation is expected to affect the leaching behavior of steelmaking slag and this could have an impact on the LCA results. Accounting for the modified leaching behavior of carbonated slag disposed in landfills remains open for future studies. Turning to the improved scenarios (Slurry2 and Wet2), it can be observed that the replacement of limestone and natural aggregates with the carbonated products leads to a negative impact (positive from an environmental point of view) for the AD, ADF, GWP, ODP, PO, AC and EU categories, for whom there was no impact in the base scenarios. This result is primarily due to the reduced energy requirement involved in the lifecycle of limestone and aggregate production, while avoiding the landfilling of 50% of BOF slag provides only a small contribution to reducing the impacts. Namely, replacing limestone and natural aggregates leads to a GWP reduction of 3.35 and 14.2 kg CO2/MWhel for the slurry and the wet route, respectively. The difference in GWP reduction between the slurry route and the wet route is partially due to the higher amount of slag processed in the wet route, resulting in a higher amount of aggregates that are substituted. Besides, it must be also considered that the impacts associated to aggregate production (replaced by the products of the wet route) are higher in all categories with respect to limestone (replaced by the products of the slurry route). Table 3. Impact assessment of the landfill step in the base scenario and in the improved scenario. Impact category
Slurry1
Slurry2
Wet1
Wet2
GWP [kg CO2 eq]
0
-3.35
0
-14.17
AD [kg Sb eq]
0
-4.89E-06
0
-2.52E-06
ADF [MJ]
0
-42.56
0
-177.2
ODP [kg CFC-11 eq]
0
-1.72E-07
0
-3.10E-07
HT [kg 1,4-DB eq]
1.3
-3.84
1.37
-10.76
FWAE [1,4-DB eq]
10.55
-25.4
11.09
-71.33
MAE [1,4-DB eq]
1.31E3
-5.81E+03
1376.69
-1.90E+04
TE [1,4-DB eq]
8.64E-20
-1.43E-03
9.08E-20
-6.95E-03
PO [kg C2H4 eq]
0
-1.51E-03
0
-6.18E-03
AC [kg SO2 eq]
0
-0.05
0
-0.12
EU [kg PO42-eq]
0
-0.01
0
-0.02
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3.4. CO2 reduction potential and total GWP Tab. 4 reports the CO2 amount conveyed to the carbonation process and stored as carbonate (548 kg CO2/MWhel), the overall greenhouse gas emissions associated to the carbonation process (GWP carbonation) and the overall CO2 emissions associated to the carbonation process (CO2 emissions carbonation). These data are provided for both the slurry and the wet route with reference to the base scenarios (Slurry1 and Wet1) and to the improved scenarios (Slurry2 and Wet2). These data were used to estimate the net CO2 reduction potential of the carbonation process. This is given as: “Total GWP”, i.e. the difference between the CO2 stored by carbonation (Stored CO2) and the overall greenhouse gas emitted by the carbonation process (GWP carbonation); or as “Avoided CO2”, i.e. the difference between the CO2 stored by carbonation (Stored CO2) and the CO2 emitted by the carbonation process (CO2 emissions carbonation). As can be seen from Tab. 4 carbonation of BOF slag can lead to an overall negative contribution to GWP, meaning that CO2 emissions are actually avoided. However, there is a difference in the amount of CO2 avoided with the different routes due to the difference in energy consumption and material requirements. For the base scenario, the slurry route is characterized by a total CO2 avoided (GWP) around 480 (470) kg CO2/MWhel , whereas the same figures for the wet route are roughly 80-90 kg CO2/MWhel lower in absolute terms. The difference is slightly smaller if the improved scenarios are considered, due to the reduction in GWP achieved through the partial replacement of natural aggregates with the products of the process. Table 4. CO2 reduction potential (negative number means that the CO2 is not emitted) Stored CO2
GWP Carbonation
[kg/MWhel]
[kg CO2 eq/MWhel]
Slurry1
-547.82
Slurry2
-547.82
Wet1 Wet2
Scenario
CO2 emissions Carbonation [kg CO2/MWhel]
Total GWP
Avoided CO2
[kg CO2 eq/MWhel]
[kg/MWhel]
74.91
69.05
-472.91
-478.78
71.57
66.09
-476.26
-481.73
-547.82
164.30
141.75
-383.52
-396.08
-547.82
150.13
139.32
-397.69
-408.51
3.5. Sensitivity analysis The sensitivity of total GWP with respect to two process parameters (transportation distance and energy source) was evaluated, in order to get some more insights into the main factors affecting the impact on this category. The results of this sensitivity analysis are shown in Fig. 4. The influence of the transportation distance is analyzed in Fig. 4a. So far, the transportation distance of the raw slag and products was set at 20 km. To assess the influence of this parameter, the GWP was estimated assuming transport distances of 10, 20, 40, 60 and 100 km. These distances were applied for each transportation step, i.e., transport of the BOF slags to the carbonation plant and transport of the carbonated products from the plant to the landfill site. As expected, transport distance has a relevant effect on the GWP for both routes. However, the wet route is more sensitive to changes in distance due to the larger amount of residues treated, needing to be transported. The impact of the energy source on the calculated GWP is reported in Figure 4b. As expected, higher GWP values (lower in absolute term), around -350 kg CO2 eq/MWhel are obtained when the electricity is assumed to be produced by fossil fuels, hydropower (pumped storage) and NGC (Natural Gas Combined Cycle). On the other hand, assuming to have an energy mix completely relying on renewable energy (such as Hydro or Wind) would actually lead to a much lower GWP (higher in absolute term) equal to -530 kg CO2 eq/MWhel. As a reference, it can be recalled that the GWP reported for the base scenarios, calculated assuming the energy mix of Italy, were around 380 and -470 kg CO2 eq/MWhel for the wet and slurry route, respectively.
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(a) (b)
Fig. 4. Sensitivity of the total global warming potential (GWP) of the wet route and slurry route carbonation for the base scenario (Slurry1 and Wet1) and the improved scenario (Slurry2 and Wet2) as a function of: a) transport distance; b) electricity source.
4. Conclusions A comparative LCA study was performed on direct carbonation of BOF slag through a slurry route process and wet route process. The functional unit for the LCA was defined as 1 MWh electricity produced by a conventional natural gas power plant. In addition, a sensitivity analysis was carried out to investigate the effect of transportation distance and energy source on the outcome. The results show that both routes are capable of storing CO2 with a negative net contribution to global warming potential. The slurry route showed a higher reduction potential when compared to the wet route. The difference is mainly due to the higher energy requirements for heating of the slag. Even though a negative GWP was observed, both routes affected the other environmental impact categories. In general, the wet route had approximately two times higher impact than the slurry route. One exception was the impact category of abiotic depletion, where the slurry route appears to have the highest impact. The contributions to these mid-point impact categories were mainly due to energy requirements. In the case of the slurry route mixing, the main contributors to the energy requirements are (in descending order) compression, grinding and transportation, while for the wet route the order is: carbonation, compression, grinding and transportation. By performing a sensitivity analysis, a dependency on transportation distance and electricity source was observed. As the distance increased the GWP also increased. If electricity was fossil fuel based, the GWP would increase but still result in a total negative contribution. Since the contribution to the other impact categories were connected to electricity requirements, similar trends were observed. References [1] Seifritz W. CO2 disposal by means of silicates. Nature 1990; 345:486. [2] Lackner KS, Wendt CH, Butt D et al. Carbon dioxide disposal in carbonate minerals. Energy 1995; 20:1153–1170. [3] Gerdemann SJ, O’Connor WK, Dahlin DC, et al. Ex Situ Aqueous Mineral Carbonation. Env Sci Technol 2007;41:2587–2593. [4] Huijgen WWJ, Witkamp GJ, Comans RNJ. Mechanisms of aqueous wollastonite carbonation as a possible CO2 sequestration process. Chem Eng Sci 2006;61:4242–4251. [5] Bobicki ER, Liu Q, Xu Z et al. Carbon capture and storage using alkaline industrial wastes. Prog Energ Combust 2012;38:302–320. [6] Pan S, Chang EE, Chiang P. CO2 capture by accelerated carbonation of alkaline wastes: A review on its principles and applications. Aerosol Air Qual Res 2012;12:770-791.. [7] Huijgen W, Comans R. Carbonation of steel slag for CO2 sequestration: Leaching of products and reaction mechanisms. Environ Sci Technol 2006;40:2790-2796. [8] Van Gerven T, Van Keer E, Arickx S et al. Carbonation of MSWI-bottom ash to de-crease heavy metal leaching, in view of recycling. Waste Manage 2005;25:291–300.
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