how to evaluate environmental benefits in circular
HOW TO EVALUATE ENVIRONMENTAL BENEFITS IN CIRCULAR ECONOMY: ATTRIBUTIONAL VS A CONSEQUENTIAL LIFE CYCLE APPROACHES FOR A CASE STUDY IN THE STEEL INDUSTRY A. DI MARIA*, K. VAN ACKER* * KU Leuven, Department of Material Engineering, KasteelPark Arenberg 44, 3000 Leuven, Belgium
SUMMARY: Industrial symbiosis between steel and concrete sectors represents one of the most promising strategies for circular economy implementation in waste management. Residues from steel industry can be processed and used to produce new construction blocks, substituting traditional Portland cement. Through an life cycle assessment, the study analyses the environmental benefits obtained when producing construction blocks from steel production residues. Two LCA approaches are compared: attributional and consequential. For both approaches, results show a clear reduction in CO2-equivalent emissions for residues-derived construction blocks. While attributional approach provides a product-level analysis, the consequential approach enlarge its boundaries to a wider system-level analysis. The system analysis allows to assess for possible consequences that the introduction of a new wastederived product into wall established markets may cause.
1. INTRODUCTION Current national and European waste management policies aim at the implementation of Circular Economy (CE) strategies in the waste sector. Circular Economy (CE) is a concept popularized in China in the 1990s, aiming to balance the economic growth with environmental pollution and resources use. (Fujii et al., 2016; Naustdalslid, 2014; Winans et al., 2017). A key concept in the development of CE is Industrial Ecology (IE), The general idea behind IE is that industrial systems must reproduce the way natural systems function. Therefore, outputs of energy and materials produced by an organism, represented by an industry in the industrial system, constitute inputs for other organisms, namely other industries (Kronenberg and Winkler, 2009). The energy and materials exchanges among different industries can mitigate resource consumption and lead to a closed material cycle. In the waste sector, implementation of CE and IS strategies is achived by stimulating materials recycling across different economic sectors. In this context, the symbyosis between steel and cement industries represents already a virtous example of industrial ecology. Steel and cement (the last as a component of concrete) are two of the most used materials in the world. They are made from primary raw materials at high temperature in large industrial reactors. Traditional cement is used mainly as a binder in concrete production. The most Proceedings Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium/ 2 - 6 October 2017 S. Margherita di Pula, Cagliari, Italy / © 2017 by CISA Publisher, Italy
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
common form of cement is the Ordinary Portland Cement (OPC), which consists for a 95-97% of a material called clinker. The clinker is composed primarily of calcium silicate minerals. The clinker production consists in various steps, from a raw material mining to a manufacturing process, in which the raw materials are crushed before entering a rotary kiln, where the cement clinker is produced. In the kiln, the materials reach temperatures higher than 1400 °C (Huntzinger and Eatmon, 2009). In order to reach those temperatures, high-energy consumption is required, either as electricity or as fuel, typically accounting for 30-40% of total production costs (Szabó et al., 2006). Although energy consumption represents a high source of CO2 emissions an important additional source of CO2 is originated from the limestone dissociation occurring in the kiln: at temperature above 900°C, the calcium carbonate CaCO3 contained in the limestone decomposes forming CaO and CO2, which is release in the atmosphere (Worrell et al., 2001). According to (van Oss and Padovani, 2002), cement is thought to be responsible for the 5% of the global anthropogenic CO2 emissions and substantial emissions of SO2, NOx and other pollutants (van Oss and Padovani, 2003, 2002). Steel is an alloy made of almost pure iron, produced by reducing iron ores at high temperature using coal as reducing agent. This reduction reaction produces CO2 (Birat, 2012). The steel production industry has traditionally been a pioneer in the implementation of IS strategies, owing to the material and energy intensive processes and to the large amount of residues and energy released (Geiseler, 1996). A steel plant is well suited to be the core of an eco-industrial network, exchanging flows at a large scale (Dong et al., 2013). The potential for synergy between the steel and cement industry is exploited when steel slag is used in cement production as additive (Tsakiridis et al., 2008). Steel slag is a co-product in steel production. It can be categorized as “(carbon) steel slag and stainless steel slag (SSS) according to the type of steel. The use of some steel slags, as BFS, from steel production in the cement industry, for instance, is one of the most well established example of IS between steel and cement sectors (Johansson and Söderström, 2011; Van den Heede and De Belie, 2012). Some of the additions made during blast furnace go beyond the requirements of steel making and they are meant to adapt the composition of the slag to the cement industry requirements (Birat, 2012). However, BFS represents only a small portion of all slags produced by the steel industry and the potential for the valorization of other residues from steel and stainless steel production is not fully explored at present. Stainless Steel Slag (SSS) is produced during the stainless-steel making process. Since chromium is an essential constituent of stainless-steel, a fraction of it appears also in the slag, together with other heavy metals. The presence of those hazardous compounds, together with the huge volumes generated each year, poses environmental and health threats when managing the SSS (Huaiwei and Xin, 2011a). Moreover, SSS occurs in a very fine texture (a few µm diameter), giving to the slag the shape of a fine powder, which arises problems of leaching and makes the handling of the SSS problematic. The fine texture is due to a phenomenon called “dusting”, which is caused by a volume expansion of the slag during the cooling phase (Kim et al., 1992). In order to avoid the problem of dusting, boron oxide (B2O3) is commonly added during the cooling process of SSS in a quantity equal to 2% of the total mass of the slag. Boron addition prevents the formation of the fine particles (Durinck et al., 2008b). Stabilized SSS grains present a bigger texture (few mm) and a more stable chemical status, which allows their disposal in hazardous waste landfills or their reuse as low quality aggregates. However the valorisation as aggregates represents a low-value application in view of the high quality oxides (CaO, MgO, AlO2) contained in the SSS, whose chemical potential can be activated and exploited. Together with landfilling or reuse as low quality aggregates after boron stabilization, a third possible end-of-life route for SSS is represented by its valorization as alternative binder in
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construction materials. Recent research has in fact shown that SSS can react with alkali activators and be used as binder to create high-quality construction materials(Huaiwei and Xin, 2011b; Motz and Geiseler, 2001; Panda et al., 2013; Salman et al., 2014a, 2015a, 2015b; Sheen et al., 2016) . Alkali activation can be simply described as a process where latent binding properties of slag are activated by mixing the slag with alkali activators (carbonates or silicates). Therefore, the alkali activation of SSS to produce new construction materials can represent the new frontier to increase the level of synergies between steel and cement industries. However, the debate about the environmental impacts of these policies has brought into focus the effectiveness of environmental analysis techniques to support public decision making (Rajagopal, 2014). Among these techniques, Life Cycle Assessment (LCA) is a tool to assess the potential environmental impacts and resources used throughout a product’s life cycle, which has been applied in a wide range of economic sectors (Finnveden et al., 2009; Vázquez-Rowe et al., 2013). Several authors made a clear distinction between attributional approach and consequential approach in LCA. Attributional-LCA (A-LCA) aims at describing the environmentally relevant flows to and from the investigated life cycle, while Consequential-LCA (C-LCA) aims at describing how environmentally relevant physical flows to and from the technological system will change in response to possible changes in the investigated life cycle (Ekvall, 2000a; Ekvall and Andrae, 2006; Ekvall and Weidema, 2004; Zamagni et al., 2012). A-LCA presents many advantages in reporting and understanding the environmental impacts directly related to the system under study. However it shows limitations when it comes to tackle support policy decisions consequences, since it does not consider any indirect effect arising in the markets from changes in the output of a product (Brander et al., 2009; Vázquez-Rowe et al., 2013). A-LCA is indeed based on the underlying assumption that the process involved in the life cycle are operated under steady-state conditions and the investigated life cycle is not connected with other markets. Moreover, the amount of product investigated does not affect the results, since it can be scaled up or down by any scaling factor. This assumption also implies fully elastic markets, meaning that for any chosen quantity of investigated product, there will always be a consumer willing to buy the additional product put in the market, and there will always be a supplier able to meet this demand (Marvuglia et al., 2013). This intrinsic limitation make attributional approach debatable for tackle industrial ecology issues, since the fundamental role of policy related question and interconnections between different markets. The scope of this paper is therefore to assess the environmental benefits of substituting OPC concrete with SSS construction blocks, analyising the differences between attributional and consequential approaches in analyzing the environmental performances of a new waste-derived product.
2. METHODS 2.1 Consequential LCA vs attributional LCA The effectiveness of the attributional approach in evaluating industrial symbiosis strategies has been discussed (Marvuglia et al., 2013). Since most of the activities on the global technological system are interrelated, an action may have indirect effects that propagate through the whole technological system, involving several product life cycle (Ekvall, 2000b). By taking into account how markets react to a change in the demand or supply of a product, the consequential approach allows to anticipate the environmental consequences of an action considering also the interrelated systems (Ekvall and Finnveden, 2001). The result of a C-LCA study depends on the magnitude of the change and they are not linearly dependent on the functional unit. Moreover, while in the early development IE strategies were thought able to develop autonomously as a result of the quest to minimize operation costs, experiences showed
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how governance, can offer a fundamental contribute for promotion of IS mode of operation (Lehtoranta et al., 2011). 2.2 Goal and scope The goal of the LCA study is to assess the environmental changes due to a shift on the recovery of SSS, from a low quality application –low quality aggregates for road construction – to a high quality application – binder to make new construction blocks, called SSS Blocks -. In particular, two different approach will be used (i) Attributional approach and (ii) consequential approach. 2.3 Functional unit and life cycle inventory The functional unit chosen is the treatment of the total amount of SSS produced in 2011 in Belgium. The inventory data for producing 1m² of SSS Blocks production and of traditional OPC concrete are described in (Kellemberg et al., 2007; Salman et al., 2015c). As reported in (Salman et al., 2015c), the compressive strength of 1 m² of SSS Blocks is comparable to the compressive strength of 1m² of traditional OPC concrete. The inventory data are reported in table 1:
Table 1: Inventory data for 1 m² Density (g/cm³) Weight of 1 m² (kg) Quantity of slag (kg)
SSSblocks (1 )
Traditional OPC Concrete
2,22
2,6
111
134
39
/
1,4
/
3
/
60
108
0,04
/
Water (kg)
/
7
Portland Cement (kg)
/
20
NaOH (kg) Na silicate (kg) River Sand (kg) Energy (kWh)
Since SSS is used as binder in the new SSS Blocks, a more consistent analysis should focus on the market for binders (e.g. OPC) instead of the market for construction blocks. However, while it is possible to define a standard equivalent binding capacity between BFS and OPC (AFNOR, French Normalisation Organization, 2004), such equivalence is not available for AOD and for stainless steel slag in general. Therefore, due to the novelty of the technology and the differences in the chemical properties of the two binders (activated SSS and OPC), it is not possible to define the quantity of activated SSS which is necessary to have the same binding properties of 1 kg of OPC. On the other side, the properties of SSS blocks (compressive strength, leaching, etc.) have been well studied (Salman et al., 2015a, 2015c, 2014a, 2014b) and compared with traditional concrete made from OPC. Therefore, for the present study, the
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SSS blocks will be considered as the competitor of the traditional OPC concrete, and cement making process will be included as a step of the concrete production process. This choice will not affect the result since the cement production is the main source of CO2 emission during the whole concrete making process (Blankendaal et al., 2014; Chen et al., 2010; Van den Heede and De Belie, 2012). Due to the differences in the LCA approaches, system boundaries for A-LCA and C-LCA must be defined separately. 2.3.1 Attributional scenarios and system boundaries Two different scenarios are analysed in the A-LCA: 1. Scenario A-1: the SSS are valorized through high quality recycling, providing AA constructin blocks. The low quality natural aggregates are provided from natural resources. 2. Scenario A-2: the SSS are stabilized through boron stabilization and recycled as low quality aggregates. The construction blocks are provided by traditional OPC concrete. The system boundaries for the two scenarios in A-LCA are defined in Figure 1. As it can be seen, both A-1 and A-2 scenarios provide equivalent functions: construction blocks for the construction market and low quality aggregates for the road construction market.
Figure 1. Attributional LCA system boundaries
2.3.2 Consequantial scenarios and system boundaries Consequential LCA allows to accounts for the consequences that the introduction of a certain product into a well established market may carry. In other words, it requires a system expansion of the analysis, taking into account the markets involved in the life cycle of the
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analysed product. In order to model the conseqiential scenarios and system boundaries, three possible approaches have been found in literature: (1) simplified approach; (2) Partial Equilibrium Modelling (PEM) approach; (3) Compute General Equilibrium (CGE) approach (Earles and Halog, 2011; Ekvall, 2000a; Marvuglia et al., 2013). The PEM and CGE approaches can reduce the uncertainty of the hypothesis on the consequences of certain actions, but at the same time require a deep knowledge of the market affected by the change and price elasticity of demand and supply should be clearly identified (Ekvall and Weidema, 2004). Unfortunately, in most of the case of open loop recycling, this is not likely the case. Therefore a simplified approach has been developed in ((Earles and Halog, 2011; Ekvall, 2000a; Ekvall and Andrae, 2006; Marvuglia et al., 2013; Zamagni et al., 2012)), considering a set of default assumptions and decisions based on given criteria (e.g. long term vs short term or constrained vs unconstrained market). The simplified approach can be seen as a collection of special case of dynamic economic modelling (Marvuglia et al., 2013). Since the uncertainty related to the identification of price elasticity for cement and concrete market in Belgium, for the current study the simplified approach is adopted. . As first line of simplification, it is assumed that the recycled material, in this case the SSS Blocks, competes only with virgin material- concrete made with OPC- in construction blocks market, while it will not compete with other green concretes that use recycled materials as OPC substitute- BFS or fly ashes. Since the need for the cement and concrete industries to reduce their total CO2 emissions, it is reasonable to assume that the market share for “low-emissions green concrete” will stay the same. As suggested by (Ekvall and Weidema, 2004), it is reasonable to develop different scenarios based on various assumptions. The scenarios developed for the study are the following: 1. Scenario C-1: SSS slag is stabilized with boron and replaces natural aggregates in the market for low quality aggregates. 2. Scenario C-2: The SSS blocks replace virgin OPC concrete in the market for construction blocks. This scenario assumes a short term analysis, were demand for
construction blocks is considered unelastic, and the quantity demanded by the market will not change. The assumption here is that the concrete or cement industries buy the SSS and produced SSS Blocks, reducing their own production of clinker or OPC concrete. Therefore, cement and concrete industries (interested by the possibility of reducing the CO2 emissions) convert part of its production to green concrete. The market for low quality aggregates replaces the SSS with natural aggregates. 3. Scenario C-3: The SSS blocks replaces completely OPC concrete in the market for construction blocks. This scenario considers a long-term analysis, where elasticity of concrete supply is considered higher. Therefore, concrete producers have the potential to keep the same level of production and to replace other competing products in other markets. In this scenario, OPC is assumed to replace gypsum plasterboard in the market for low strength structural components. Figure 2 and table 1 depicts the main assumptions in building the consequential scenarios.
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Figure 2: Consequential LCA system boundaries Table 2: Consequential Scenarios
Scenario C-1
Scenario C-2
Scenario C-3
Slag_HighVal
0
142 kt
142 kt
Slag_LowVal
142 kt
0
0
∆X
0
404 kt
404 kt
∆Y
142 kt
0
0
D1
404 kt
404 kt
404 kt
S1_a
404 kton
0
0
S1_b
0
0
0
D2
142 kt
142 kt
142 kt
S2_a
0
142 kt
142 kt
S2_b
142 kton
0
0
S3
0
S4
Not considered
0
Not considered
404 kt -404 kt
2.4 Life cycle impact assessment The impact calculation methodology selected in this study is the global warming potential (GWP) assessed by the IPCC Fifth Assessment Report, which provides a scientific assessment of the main recent findings from physical climate metric research (Levasseur et al., 2016). The results for the GWP for the attributional and consequential approaches are shown in Figure 3 and 4
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Figure 3: Attributional LCA results
Figure 4: Consequential LCA results From the analysis of the results in Figure 2 and 4 it is possible to evaluate the effects of a possible change in the end of life of SSS slag, and how the environmental performances is differently evaluated by the attributional and consequential approaches. The attributional approach in figure 2 presents a product level analysis, showing the environmental advantages when substituting OPC construction blocks with SSS construction blocks, leading to a 75% reduction in CO2 –equivalent emissions. The consequential approach in figure 3, even if in scenarios C-1 and C-2 it confirms the same findings of the attributional approach, it gives a more system level analysis, considering the markets involved in the system. This system-level approach is more clear for C-3, where a third market, the market for low strength structural components, is involved. The results of C-3 highlight that, even if the long run, elasticity if concrete production is high and the level of production can stay the same, if OPC concrete substitute gypsum plasterboard, there is still an environmental advantage in producing SSS construction blocks. Therefore, even in the long run, the symbiosis between steel and cement/concrete industries can bring beneficial effects in term of GWPreduction.
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3. CONCLUSIONS AND FUTURE DEVELOPMENT The improvement of industrial symbiosis practices between steel and cement/concrete industries is likely to result in reduced overall GWP. The alternative use of SSS blocks made from SSS slag can avoid the production of concrete made from OPC, which carries a much higher GWP in its production phase. Since today SSS slag are stabilized and used as low quality aggregates, the use of SSS slag in SSS blocks production will lead to a replacement of SSS by natural aggregates. However, the increase use in natural aggregates cannot offset the gain obtained thanks to the avoided production of traditional concrete made from OPC, as the changes in the construction blocks market are the ones driving the results of the whole system. These results are confirmed from both attributional and consequential approach. While attributional approach provides a “product-level” analysis, the consequential approach enlarges its analysis to a wideer “system-level”, assessing the possible consequences that the introduction of new products into a market may cause. The consequential scenarios presented in this paper represent only some of the possible consequences the introduction of the SSS blocks can have. In the further development of the study, more scenarios are foreseen, where SSS blocks affect also the market of other construction materials. In fact, a future hypothesis will be based on the assumption that the overall production of OPC concrete will not be affected by the SSS Blocks. The extra unit of concrete replaced by the SSS blocks will be still produced and put into the market, and it will replace other materials used in lower quality applications.
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Rajagopal, D., 2014. Consequential Life Cycle Assessment of Policy Vulnerability to Price Effects. J. Ind. Ecol. 18, 164–175. doi:10.1111/jiec.12058 Salman, M., Cizer, Ö., Pontikes, Y., Snellings, R., Vandewalle, L., Blanpain, B., Balen, K.V., 2015a. Cementitious binders from activated stainless steel refining slag and the effect of alkali solutions. J. Hazard. Mater. 286, 211–219. doi:10.1016/j.jhazmat.2014.12.046 Salman, M., Cizer, Ö., Pontikes, Y., Vandewalle, L., Blanpain, B., Van Balen, K., 2014a. Effect of curing temperatures on the alkali activation of crystalline continuous casting stainless steel slag. Constr. Build. Mater. 71, 308–316. doi:10.1016/j.conbuildmat.2014.08.067 Salman, M., Dubois, M., Maria, A.D., Van Acker, K., Van Balen, K., 2015b. Construction Materials from Stainless Steel Slags: Technical Aspects, Environmental Benefits, and Economic Opportunities. J. Ind. Ecol. n/a–n/a. doi:10.1111/jiec.12314 Salman, M., Dubois, M., Maria, A.D., Van Acker, K., Van Balen, K., 2015c. Construction Materials from Stainless Steel Slags: Technical Aspects, Environmental Benefits, and Economic Opportunities. J. Ind. Ecol. n/a–n/a. doi:10.1111/jiec.12314 Salman, M., Van Balen, K., Blanpain, B., Vandewalle, L., 2014b. Sustainable Materialisation of Residues from Thermal Processes into Construction Materials (Duurzame valorisatie van residu’s van thermische processen tot bouwmaterialen). KU Leuven, Leuven, Belgium. Sheen, Y., Huang, L.-J., Sun, T.-H., Le, D.-H., 2016. Engineering Properties of Self-compacting Concrete Containing Stainless Steel Slags. Proceeding Sustain. Dev. Civ. Urban Transp. Eng. 142, 79–86. doi:10.1016/j.proeng.2016.02.016 Szabó, L., Hidalgo, I., Ciscar, J.C., Soria, A., 2006. CO2 emission trading within the European Union and Annex B countries: the cement industry case. Energy Policy 34, 72–87. doi:10.1016/j.enpol.2004.06.003 Tsakiridis, P.E., Papadimitriou, G.D., Tsivilis, S., Koroneos, C., 2008. Utilization of steel slag for Portland cement clinker production. J. Hazard. Mater. 152, 805–811. doi:10.1016/j.jhazmat.2007.07.093 Van den Heede, P., De Belie, N., 2012. Environmental impact and life cycle assessment (LCA) of traditional and “green” concretes: Literature review and theoretical calculations. Cem. Concr. Compos. 34, 431–442. doi:http://dx.doi.org/10.1016/j.cemconcomp.2012.01.004 van Oss, H.G., Padovani, A.C., 2003. Cement Manufacture and the Environment Part II: Environmental Challenges and Opportunities. J. Ind. Ecol. 7, 93–126. doi:10.1162/108819803766729212 van Oss, H.G., Padovani, A.C., 2002. Cement Manufacture and the Environment: Part I: Chemistry and Technology. J. Ind. Ecol. 6, 89–105. doi:10.1162/108819802320971650 Vázquez-Rowe, I., Rege, S., Marvuglia, A., Thénie, J., Haurie, A., Benetto, E., 2013. Application of three independent consequential LCA approaches to the agricultural sector in Luxembourg. Int. J. Life Cycle Assess. 18, 1593–1604. doi:10.1007/s11367-013-0604-2 Winans, K., Kendall, A., Deng, H., 2017. The history and current applications of the circular economy concept. Renew. Sustain. Energy Rev. 68, Part 1, 825–833. doi:10.1016/j.rser.2016.09.123 Worrell, E., Price, L., Martin, N., Hendriks, C., Meida, L.O., 2001. CARBON DIOXIDE EMISSIONS FROM THE GLOBAL CEMENT INDUSTRY. Annu. Rev. Energy Environ. 26, 303– 329. doi:10.1146/annurev.energy.26.1.303 Zamagni, A., Guinée, J., Heijungs, R., Masoni, P., Raggi, A., 2012. Lights and shadows in consequential LCA. Int. J. Life Cycle Assess. 17, 904–918. doi:10.1007/s11367-012-0423-x
SUMMARY: Industrial symbiosis between steel and concrete sectors represents one of the most promising strategies for circular economy implementation in waste management. Residues from steel industry can be processed and used to produce new construction blocks, substituting traditional Portland cement. Through an life cycle assessment, the study analyses the environmental benefits obtained when producing construction blocks from steel production residues. Two LCA approaches are compared: attributional and consequential. For both approaches, results show a clear reduction in CO2-equivalent emissions for residues-derived construction blocks. While attributional approach provides a product-level analysis, the consequential approach enlarge its boundaries to a wider system-level analysis. The system analysis allows to assess for possible consequences that the introduction of a new wastederived product into wall established markets may cause.
1. INTRODUCTION Current national and European waste management policies aim at the implementation of Circular Economy (CE) strategies in the waste sector. Circular Economy (CE) is a concept popularized in China in the 1990s, aiming to balance the economic growth with environmental pollution and resources use. (Fujii et al., 2016; Naustdalslid, 2014; Winans et al., 2017). A key concept in the development of CE is Industrial Ecology (IE), The general idea behind IE is that industrial systems must reproduce the way natural systems function. Therefore, outputs of energy and materials produced by an organism, represented by an industry in the industrial system, constitute inputs for other organisms, namely other industries (Kronenberg and Winkler, 2009). The energy and materials exchanges among different industries can mitigate resource consumption and lead to a closed material cycle. In the waste sector, implementation of CE and IS strategies is achived by stimulating materials recycling across different economic sectors. In this context, the symbyosis between steel and cement industries represents already a virtous example of industrial ecology. Steel and cement (the last as a component of concrete) are two of the most used materials in the world. They are made from primary raw materials at high temperature in large industrial reactors. Traditional cement is used mainly as a binder in concrete production. The most Proceedings Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium/ 2 - 6 October 2017 S. Margherita di Pula, Cagliari, Italy / © 2017 by CISA Publisher, Italy
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
common form of cement is the Ordinary Portland Cement (OPC), which consists for a 95-97% of a material called clinker. The clinker is composed primarily of calcium silicate minerals. The clinker production consists in various steps, from a raw material mining to a manufacturing process, in which the raw materials are crushed before entering a rotary kiln, where the cement clinker is produced. In the kiln, the materials reach temperatures higher than 1400 °C (Huntzinger and Eatmon, 2009). In order to reach those temperatures, high-energy consumption is required, either as electricity or as fuel, typically accounting for 30-40% of total production costs (Szabó et al., 2006). Although energy consumption represents a high source of CO2 emissions an important additional source of CO2 is originated from the limestone dissociation occurring in the kiln: at temperature above 900°C, the calcium carbonate CaCO3 contained in the limestone decomposes forming CaO and CO2, which is release in the atmosphere (Worrell et al., 2001). According to (van Oss and Padovani, 2002), cement is thought to be responsible for the 5% of the global anthropogenic CO2 emissions and substantial emissions of SO2, NOx and other pollutants (van Oss and Padovani, 2003, 2002). Steel is an alloy made of almost pure iron, produced by reducing iron ores at high temperature using coal as reducing agent. This reduction reaction produces CO2 (Birat, 2012). The steel production industry has traditionally been a pioneer in the implementation of IS strategies, owing to the material and energy intensive processes and to the large amount of residues and energy released (Geiseler, 1996). A steel plant is well suited to be the core of an eco-industrial network, exchanging flows at a large scale (Dong et al., 2013). The potential for synergy between the steel and cement industry is exploited when steel slag is used in cement production as additive (Tsakiridis et al., 2008). Steel slag is a co-product in steel production. It can be categorized as “(carbon) steel slag and stainless steel slag (SSS) according to the type of steel. The use of some steel slags, as BFS, from steel production in the cement industry, for instance, is one of the most well established example of IS between steel and cement sectors (Johansson and Söderström, 2011; Van den Heede and De Belie, 2012). Some of the additions made during blast furnace go beyond the requirements of steel making and they are meant to adapt the composition of the slag to the cement industry requirements (Birat, 2012). However, BFS represents only a small portion of all slags produced by the steel industry and the potential for the valorization of other residues from steel and stainless steel production is not fully explored at present. Stainless Steel Slag (SSS) is produced during the stainless-steel making process. Since chromium is an essential constituent of stainless-steel, a fraction of it appears also in the slag, together with other heavy metals. The presence of those hazardous compounds, together with the huge volumes generated each year, poses environmental and health threats when managing the SSS (Huaiwei and Xin, 2011a). Moreover, SSS occurs in a very fine texture (a few µm diameter), giving to the slag the shape of a fine powder, which arises problems of leaching and makes the handling of the SSS problematic. The fine texture is due to a phenomenon called “dusting”, which is caused by a volume expansion of the slag during the cooling phase (Kim et al., 1992). In order to avoid the problem of dusting, boron oxide (B2O3) is commonly added during the cooling process of SSS in a quantity equal to 2% of the total mass of the slag. Boron addition prevents the formation of the fine particles (Durinck et al., 2008b). Stabilized SSS grains present a bigger texture (few mm) and a more stable chemical status, which allows their disposal in hazardous waste landfills or their reuse as low quality aggregates. However the valorisation as aggregates represents a low-value application in view of the high quality oxides (CaO, MgO, AlO2) contained in the SSS, whose chemical potential can be activated and exploited. Together with landfilling or reuse as low quality aggregates after boron stabilization, a third possible end-of-life route for SSS is represented by its valorization as alternative binder in
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construction materials. Recent research has in fact shown that SSS can react with alkali activators and be used as binder to create high-quality construction materials(Huaiwei and Xin, 2011b; Motz and Geiseler, 2001; Panda et al., 2013; Salman et al., 2014a, 2015a, 2015b; Sheen et al., 2016) . Alkali activation can be simply described as a process where latent binding properties of slag are activated by mixing the slag with alkali activators (carbonates or silicates). Therefore, the alkali activation of SSS to produce new construction materials can represent the new frontier to increase the level of synergies between steel and cement industries. However, the debate about the environmental impacts of these policies has brought into focus the effectiveness of environmental analysis techniques to support public decision making (Rajagopal, 2014). Among these techniques, Life Cycle Assessment (LCA) is a tool to assess the potential environmental impacts and resources used throughout a product’s life cycle, which has been applied in a wide range of economic sectors (Finnveden et al., 2009; Vázquez-Rowe et al., 2013). Several authors made a clear distinction between attributional approach and consequential approach in LCA. Attributional-LCA (A-LCA) aims at describing the environmentally relevant flows to and from the investigated life cycle, while Consequential-LCA (C-LCA) aims at describing how environmentally relevant physical flows to and from the technological system will change in response to possible changes in the investigated life cycle (Ekvall, 2000a; Ekvall and Andrae, 2006; Ekvall and Weidema, 2004; Zamagni et al., 2012). A-LCA presents many advantages in reporting and understanding the environmental impacts directly related to the system under study. However it shows limitations when it comes to tackle support policy decisions consequences, since it does not consider any indirect effect arising in the markets from changes in the output of a product (Brander et al., 2009; Vázquez-Rowe et al., 2013). A-LCA is indeed based on the underlying assumption that the process involved in the life cycle are operated under steady-state conditions and the investigated life cycle is not connected with other markets. Moreover, the amount of product investigated does not affect the results, since it can be scaled up or down by any scaling factor. This assumption also implies fully elastic markets, meaning that for any chosen quantity of investigated product, there will always be a consumer willing to buy the additional product put in the market, and there will always be a supplier able to meet this demand (Marvuglia et al., 2013). This intrinsic limitation make attributional approach debatable for tackle industrial ecology issues, since the fundamental role of policy related question and interconnections between different markets. The scope of this paper is therefore to assess the environmental benefits of substituting OPC concrete with SSS construction blocks, analyising the differences between attributional and consequential approaches in analyzing the environmental performances of a new waste-derived product.
2. METHODS 2.1 Consequential LCA vs attributional LCA The effectiveness of the attributional approach in evaluating industrial symbiosis strategies has been discussed (Marvuglia et al., 2013). Since most of the activities on the global technological system are interrelated, an action may have indirect effects that propagate through the whole technological system, involving several product life cycle (Ekvall, 2000b). By taking into account how markets react to a change in the demand or supply of a product, the consequential approach allows to anticipate the environmental consequences of an action considering also the interrelated systems (Ekvall and Finnveden, 2001). The result of a C-LCA study depends on the magnitude of the change and they are not linearly dependent on the functional unit. Moreover, while in the early development IE strategies were thought able to develop autonomously as a result of the quest to minimize operation costs, experiences showed
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how governance, can offer a fundamental contribute for promotion of IS mode of operation (Lehtoranta et al., 2011). 2.2 Goal and scope The goal of the LCA study is to assess the environmental changes due to a shift on the recovery of SSS, from a low quality application –low quality aggregates for road construction – to a high quality application – binder to make new construction blocks, called SSS Blocks -. In particular, two different approach will be used (i) Attributional approach and (ii) consequential approach. 2.3 Functional unit and life cycle inventory The functional unit chosen is the treatment of the total amount of SSS produced in 2011 in Belgium. The inventory data for producing 1m² of SSS Blocks production and of traditional OPC concrete are described in (Kellemberg et al., 2007; Salman et al., 2015c). As reported in (Salman et al., 2015c), the compressive strength of 1 m² of SSS Blocks is comparable to the compressive strength of 1m² of traditional OPC concrete. The inventory data are reported in table 1:
Table 1: Inventory data for 1 m² Density (g/cm³) Weight of 1 m² (kg) Quantity of slag (kg)
SSSblocks (1 )
Traditional OPC Concrete
2,22
2,6
111
134
39
/
1,4
/
3
/
60
108
0,04
/
Water (kg)
/
7
Portland Cement (kg)
/
20
NaOH (kg) Na silicate (kg) River Sand (kg) Energy (kWh)
Since SSS is used as binder in the new SSS Blocks, a more consistent analysis should focus on the market for binders (e.g. OPC) instead of the market for construction blocks. However, while it is possible to define a standard equivalent binding capacity between BFS and OPC (AFNOR, French Normalisation Organization, 2004), such equivalence is not available for AOD and for stainless steel slag in general. Therefore, due to the novelty of the technology and the differences in the chemical properties of the two binders (activated SSS and OPC), it is not possible to define the quantity of activated SSS which is necessary to have the same binding properties of 1 kg of OPC. On the other side, the properties of SSS blocks (compressive strength, leaching, etc.) have been well studied (Salman et al., 2015a, 2015c, 2014a, 2014b) and compared with traditional concrete made from OPC. Therefore, for the present study, the
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SSS blocks will be considered as the competitor of the traditional OPC concrete, and cement making process will be included as a step of the concrete production process. This choice will not affect the result since the cement production is the main source of CO2 emission during the whole concrete making process (Blankendaal et al., 2014; Chen et al., 2010; Van den Heede and De Belie, 2012). Due to the differences in the LCA approaches, system boundaries for A-LCA and C-LCA must be defined separately. 2.3.1 Attributional scenarios and system boundaries Two different scenarios are analysed in the A-LCA: 1. Scenario A-1: the SSS are valorized through high quality recycling, providing AA constructin blocks. The low quality natural aggregates are provided from natural resources. 2. Scenario A-2: the SSS are stabilized through boron stabilization and recycled as low quality aggregates. The construction blocks are provided by traditional OPC concrete. The system boundaries for the two scenarios in A-LCA are defined in Figure 1. As it can be seen, both A-1 and A-2 scenarios provide equivalent functions: construction blocks for the construction market and low quality aggregates for the road construction market.
Figure 1. Attributional LCA system boundaries
2.3.2 Consequantial scenarios and system boundaries Consequential LCA allows to accounts for the consequences that the introduction of a certain product into a well established market may carry. In other words, it requires a system expansion of the analysis, taking into account the markets involved in the life cycle of the
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analysed product. In order to model the conseqiential scenarios and system boundaries, three possible approaches have been found in literature: (1) simplified approach; (2) Partial Equilibrium Modelling (PEM) approach; (3) Compute General Equilibrium (CGE) approach (Earles and Halog, 2011; Ekvall, 2000a; Marvuglia et al., 2013). The PEM and CGE approaches can reduce the uncertainty of the hypothesis on the consequences of certain actions, but at the same time require a deep knowledge of the market affected by the change and price elasticity of demand and supply should be clearly identified (Ekvall and Weidema, 2004). Unfortunately, in most of the case of open loop recycling, this is not likely the case. Therefore a simplified approach has been developed in ((Earles and Halog, 2011; Ekvall, 2000a; Ekvall and Andrae, 2006; Marvuglia et al., 2013; Zamagni et al., 2012)), considering a set of default assumptions and decisions based on given criteria (e.g. long term vs short term or constrained vs unconstrained market). The simplified approach can be seen as a collection of special case of dynamic economic modelling (Marvuglia et al., 2013). Since the uncertainty related to the identification of price elasticity for cement and concrete market in Belgium, for the current study the simplified approach is adopted. . As first line of simplification, it is assumed that the recycled material, in this case the SSS Blocks, competes only with virgin material- concrete made with OPC- in construction blocks market, while it will not compete with other green concretes that use recycled materials as OPC substitute- BFS or fly ashes. Since the need for the cement and concrete industries to reduce their total CO2 emissions, it is reasonable to assume that the market share for “low-emissions green concrete” will stay the same. As suggested by (Ekvall and Weidema, 2004), it is reasonable to develop different scenarios based on various assumptions. The scenarios developed for the study are the following: 1. Scenario C-1: SSS slag is stabilized with boron and replaces natural aggregates in the market for low quality aggregates. 2. Scenario C-2: The SSS blocks replace virgin OPC concrete in the market for construction blocks. This scenario assumes a short term analysis, were demand for
construction blocks is considered unelastic, and the quantity demanded by the market will not change. The assumption here is that the concrete or cement industries buy the SSS and produced SSS Blocks, reducing their own production of clinker or OPC concrete. Therefore, cement and concrete industries (interested by the possibility of reducing the CO2 emissions) convert part of its production to green concrete. The market for low quality aggregates replaces the SSS with natural aggregates. 3. Scenario C-3: The SSS blocks replaces completely OPC concrete in the market for construction blocks. This scenario considers a long-term analysis, where elasticity of concrete supply is considered higher. Therefore, concrete producers have the potential to keep the same level of production and to replace other competing products in other markets. In this scenario, OPC is assumed to replace gypsum plasterboard in the market for low strength structural components. Figure 2 and table 1 depicts the main assumptions in building the consequential scenarios.
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Figure 2: Consequential LCA system boundaries Table 2: Consequential Scenarios
Scenario C-1
Scenario C-2
Scenario C-3
Slag_HighVal
0
142 kt
142 kt
Slag_LowVal
142 kt
0
0
∆X
0
404 kt
404 kt
∆Y
142 kt
0
0
D1
404 kt
404 kt
404 kt
S1_a
404 kton
0
0
S1_b
0
0
0
D2
142 kt
142 kt
142 kt
S2_a
0
142 kt
142 kt
S2_b
142 kton
0
0
S3
0
S4
Not considered
0
Not considered
404 kt -404 kt
2.4 Life cycle impact assessment The impact calculation methodology selected in this study is the global warming potential (GWP) assessed by the IPCC Fifth Assessment Report, which provides a scientific assessment of the main recent findings from physical climate metric research (Levasseur et al., 2016). The results for the GWP for the attributional and consequential approaches are shown in Figure 3 and 4
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Figure 3: Attributional LCA results
Figure 4: Consequential LCA results From the analysis of the results in Figure 2 and 4 it is possible to evaluate the effects of a possible change in the end of life of SSS slag, and how the environmental performances is differently evaluated by the attributional and consequential approaches. The attributional approach in figure 2 presents a product level analysis, showing the environmental advantages when substituting OPC construction blocks with SSS construction blocks, leading to a 75% reduction in CO2 –equivalent emissions. The consequential approach in figure 3, even if in scenarios C-1 and C-2 it confirms the same findings of the attributional approach, it gives a more system level analysis, considering the markets involved in the system. This system-level approach is more clear for C-3, where a third market, the market for low strength structural components, is involved. The results of C-3 highlight that, even if the long run, elasticity if concrete production is high and the level of production can stay the same, if OPC concrete substitute gypsum plasterboard, there is still an environmental advantage in producing SSS construction blocks. Therefore, even in the long run, the symbiosis between steel and cement/concrete industries can bring beneficial effects in term of GWPreduction.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
3. CONCLUSIONS AND FUTURE DEVELOPMENT The improvement of industrial symbiosis practices between steel and cement/concrete industries is likely to result in reduced overall GWP. The alternative use of SSS blocks made from SSS slag can avoid the production of concrete made from OPC, which carries a much higher GWP in its production phase. Since today SSS slag are stabilized and used as low quality aggregates, the use of SSS slag in SSS blocks production will lead to a replacement of SSS by natural aggregates. However, the increase use in natural aggregates cannot offset the gain obtained thanks to the avoided production of traditional concrete made from OPC, as the changes in the construction blocks market are the ones driving the results of the whole system. These results are confirmed from both attributional and consequential approach. While attributional approach provides a “product-level” analysis, the consequential approach enlarges its analysis to a wideer “system-level”, assessing the possible consequences that the introduction of new products into a market may cause. The consequential scenarios presented in this paper represent only some of the possible consequences the introduction of the SSS blocks can have. In the further development of the study, more scenarios are foreseen, where SSS blocks affect also the market of other construction materials. In fact, a future hypothesis will be based on the assumption that the overall production of OPC concrete will not be affected by the SSS Blocks. The extra unit of concrete replaced by the SSS blocks will be still produced and put into the market, and it will replace other materials used in lower quality applications.
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