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Environmental impact of the subcritical production of silica aerogels
Journal Pre-proof Environmental impact of the subcritical production of silica aerogels Isabel Pinto, José D. Silvestre, Jorge de Brito, Maria F. Júlio PII:
S0959-6526(19)34566-4
DOI:
https://doi.org/10.1016/j.jclepro.2019.119696
Reference:
JCLP 119696
To appear in:
Journal of Cleaner Production
Received Date: 11 September 2018 Revised Date:
23 October 2019
Accepted Date: 11 December 2019
Please cite this article as: Pinto I, Silvestre JoséD, de Brito J, Júlio MF, Environmental impact of the subcritical production of silica aerogels, Journal of Cleaner Production (2020), doi: https:// doi.org/10.1016/j.jclepro.2019.119696. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Environmental impact of the subcritical production of silica aerogels Isabel Pinto1, José D. Silvestre2*, Jorge de Brito3, Maria F. Júlio4
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Master in Industrial Engineering and Management, IST, UL; [email protected] Assistant Professor, CERIS, Department of Civil Engineering, Architecture and Georresources (DECivil), Instituto Superior Técnico (IST), Universidade de Lisboa (UL), Av. Rovisco Pais 1, 1049-001, Lisbon, Portugal; [email protected], Telephone: +351 218419709 3 Full Professor, CERIS, DECivil, IST, UL; [email protected] 4 PhD Student, Centro de Química-Física Molecular and IN - Institute of Nanoscience and Nanotechnology, IST, UL; [email protected] * Corresponding author 2
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5042 words Environmental impact of the subcritical production of silica aerogels Isabel Pinto1, José D. Silvestre2*, Jorge de Brito3, Maria F. Júlio4 Abstract: This study evaluates for the first time the environmental impacts of three different syntheses of subcritical silica aerogels. These aerogels are produced by subcritical drying, a more economical and safer process than that of the supercritical aerogels. The environmental evaluation was performed using the standardised Life Cycle Assessment (LCA) methodology, using a cradle to gate approach. The life cycles of these materials were modelled using production data collected in laboratory and a LCA software. These aerogels are produced to be used as aggregates in wall coating mortars for buildings, conferring them thermal insulating properties. It was concluded that the most critical stages in the LCA of these aerogels are the production of raw materials and the production in the laboratory, the latter mostly due to the high energy consumption. The results presented in this paper correspond to a laboratory-scale production, which makes them expectedly higher than if the aerogels were produced at an industrial scale. It was also found that aerogels obtained using subcritical drying generally lead to lower environmental impacts than supercritical aerogels. Key-words: aerogel, environmental impact, life cycle assessment, thermal mortars Symbols and acronyms: λ Thermal conductivity (W/(m K)) ρ Density (kg/m3) ADP Abiotic Depletion Potential AP Acidification Potential CED Cumulative Energy Demand CFC Chlorofluorocarbon EP Eutrophication Potential ETICS External thermal insulation composite system EU European Union GWP Global Warming Potential HTSCD High Temperature Supercritical Drying
LCA Life Cycle Assessment LTSCD Low Temperature Supercritical Drying ODP Ozone Depletion Potential PE Polyethylene PE-NRe Consumption of primary energy, nonrenewable PE-Re Consumption of primary energy, renewable POCP Photochemical Ozone Creation Potential R-value Thermal resistance [(m2.K)/W] U-value Thermal conductivity coefficient [W/(m2 K)]
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Master in Industrial Engineering and Management, IST, UL; [email protected] Assistant Professor, CERIS, Department of Civil Engineering, Architecture and Georresources (DECivil), Instituto Superior Técnico (IST), Universidade de Lisboa (UL), Av. Rovisco Pais 1, 1049-001, Lisbon, Portugal; [email protected], Telephone: +351 218419709 3 Full Professor, CERIS, DECivil, IST, UL; [email protected] 4 PhD Student, Centro de Química-Física Molecular and IN - Institute of Nanoscience and Nanotechnology, IST, UL; [email protected] * Corresponding author 2
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1 INTRODUCTION The European Union (EU) has been establishing weather- and energy-related policies in order to reduce the carbon footprint and, in October 2014, it established goals to be achieved until 2030. These measures are part of the EU objective of reducing the emissions of greenhouse gases by 8095% (relative to the 1990 levels), so that the Earth temperature increase does not exceed 2 ºC (EC, 2014), and include: • Reducing the greenhouse gases emissions by at least 40%; • Guaranteeing that renewable energy represents at least 27% of the overall energy consumption; • Improving the energy efficiency by 27%. In order to achieve these objectives, the cooperation of all sectors is needed. The industrial, construction, transport and agriculture sectors are the greatest contributors to the high energy consumption (Pargana et al., 2014). In the EU, the construction sector is responsible for around 40% of the natural resources (Pinheiro, 2006), 25% of the water, and 40% of the energy consumption, and 1/3 of the greenhouse gases emissions (Bragança et al., 2013). It is clearly a sector with an unsustainable environmental profile and there is a need for a change and to search for new solutions in: the raw materials used in buildings; the production processes (concerning raw materials and the final product); and the end-of-life procedures of the various building components. In Europe, buildings’ energy certification already had positive repercussions (Pargana et al., 2014). When the buildings are adequately designed and operated, it is possible to obtain significant energy savings, which makes the design decisions important to the building’s sustainability. Among these decisions, the choice of construction materials and components, such as thermal insulation materials and systems, is highlighted. These materials establish a barrier to the passage of heat between two spaces, maintaining the inner temperature and contributing to improve the building’s energy efficiency. The study presented in this paper was part of a national research project in which inorganic and hybrid silica-based aerogels, subcritically dried, were synthesized (Júlio and Ilharco, 2014; Júlio et al., 2016a). A cradle-to-gate environmental Life Cycle Assessment (LCA) of three laboratorial syntheses of silica-based aerogels (one purely inorganic and two hybrid - one as powder (HYBA) and the other in monolithic form (HYB-C)), which avoids post-synthesis hydrophobization reactions and only uses subcritical drying, was completed. LCA is important, since it allows identifying opportunities of improvement of the production stage, and reduction of the environmental impacts, as well as comparing the performance of a product with that of other alternatives. The results presented here contribute therefore to compare the environmental impacts of this innovative production process with the traditional supercritical production of these materials, thus contributing to a cleaner production process. These aerogels are to be used as a thermal insulation material, namely by being incorporated as aggregates in wall thermal renders. The incorporation of these aggregates in coating mortars allows improving the building’s thermal performance, since these materials have a reduced thermal conductivity and lower the thermal conductivity of the mortars to values under than that of conventional ones (Júlio et al., 2016b). The environmental evaluation of the mortars in which these aerogels were incorporated is outside the scope of this paper, but the evaluation of the economic performance of these aerogels and mortars was already made (Garrido et al., 2017, 2019). Therefore, the contribution to sustainability is provided in two dimensions (environmental and economic), and the thermal comfort contributes to the third, social, one. This paper comprises five sections, including this introduction. The second section in2
cludes a literature review of similar studies. The LCA methodology is described in the third section. The following section presents the LCA results of the three silica aerogel syntheses. The last section presents the main conclusions of this study.
2 Literature review Nanomaterials are characterized by their reduced structural size and specific properties. When added to mortars, they modify the latter’s properties, affecting their strength, durability and shrinkage, but also improving their thermal resistance and that of the buildings (Soares et al., 2014). One of the most important characteristics of a thermal insulation material is its low thermal conductivity [λ - W/(m.K)]. Silica aerogels are among the best existing thermal insulation materials (with thermal conductivity between 0.01 and 0.02 W/(m.K )), due to the combination of extremely low density (3-500 kg/m3), high porosity (> 90%) and small mesopores (sizes between 4 and 20 nm) (Koebel et al., 2012). They can be purely inorganic, organic or hybrid. Their application in construction is mostly due to their high thermal performance. Recently, they proved their importance as aggregates in cementitious materials, such as improved thermal performance mortars and concrete (Buratti et al., 2014; Gao et al., 2014; Júlio et al., 2016a; Stahl et al., 2014). A review of the literature only turned up one study of the environmental performance of an aerogel. In this study, Dowson et al. (2012) produced a silica aerogel in laboratory, using supercritical drying conditions at high and low temperatures (HTSCD and LTSCD, respectively). The LCA was developed from cradle to gate, excluding the impact of the transportation of raw materials and finished products, as well as its end-of-life processing. This study was performed according to the ISO 14044 and ISO 14040 standards (ISO, 2006a; 2006b), and the functional unit was the energy spent (kWh) and the CO2 released (kg CO2) to produce 40 ml of solid aerogel (non-granular). The authors concluded that: • The production method of LTSCD has a longer environmental return, with a total energy consumption of 62.6 MJ/40 ml and 6.64 kg of CO2 released/40 ml; • The total energy consumption and CO2 released by HTSCD were 29.3 MJ/40 ml and 0.73 kg CO2/40 ml, respectively; • The ageing of the aerogel was the production stage with the highest energy spent. When this process becomes optimized for industrial production, the energy spent and CO2 released values would significantly decrease. However, the LTSCD method is the one where greater improvement can be achieved. 3 METHODOLOGY As the awareness in terms of the environmental impacts of our decisions increases, there is a need to create tools to evaluate the potential damage associated to products daily used. LCA is a technique whose aim is the evaluation of the potential environmental impact of a process, product or service throughout its life cycle. When applied from cradle to grave, this approach analyses all the stages of the life cycle, including: from the extraction of raw materials to the end-of-life treatment and final destination of the product, also comprising the intermediate stages of auxiliary materials manufacture and energy production, transportation and use. LCA is regulated by ISO 14040 (ISO, 2006a), which defines the principles and the framework, and by ISO 14044 (ISO, 2006b), which defines the requirements and guidelines that were followed in this study. The life cycle stages of the construction materials and products are standardized in European standards (CEN, 2011a, 2013). The life cycle boundaries of a construction material can be 3
defined as: from cradle to gate (including the extraction and processing of raw materials and the production); from cradle to grave (including also the transportation, distribution and assemblage, use, maintenance and final disposal); or from cradle to cradle (including the reuse, recovery and/or the recycling potential) (Ferrão, 2009; Ortiz et al., 2009). In this work, the LCA approach used was from cradle to gate (of the laboratory). The environmental evaluation was performed using the SimaPro software and applying international standards (ISO, 2006a; 2006b; CEN, 2011a; 2011b; 2013). The environmental impact analysis starts with the extraction and production of raw materials (A1), includes their transportation to the laboratory (A2) and ends at the production stage (A3). 3.1 LCA of the aerogels 3.1.1 Declared unit The declared unit’s main objective is to provide a reference (common base) that relates the inputs and outputs of the system. In this study, the declared unit considered for the aerogels is 1 kg of finished product. 3.1.2 System boundaries The main objective of the system boundaries is to define the unit processes to be included in this LCA study. Figure 1 represents the generic life cycle from cradle to grave of a thermal insulation material.
Figure.1. Generic life cycle of a thermal insulation material (adapted from Pargana et al., 2014).
This study considered the raw materials extraction and processing (A1), transportation to the production site (A2), and production (including waste from production and raw materials packaging, and final product packaging - A3). The production, application and end-of-life stages of the mortar are not considered relevant to the study because they are identical for all the aerogels compared and, therefore, were not considered. However, the LCA study of the three aerogel syntheses ends at the laboratory gate and includes all stages from the production of raw materials until their packaging (stage A1-A3). European standard EN 15643-2 (CEN, 2011b) and Silvestre (2016) classify these production stages as follows: • A1 - extraction and processing of the raw materials, and entry processing of secondary materials; • A2 - transportation to the production site; • A3 - production and production waste: o A3.1 - product packaging; o A3.2 - production; o A3.3 - waste generated in production, including packaging waste.
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3.1.3 Choice of the environmental impact categories According to Ferrão (2009), the choice of the method of evaluation of the environmental impacts must be related with the selection of the environmental impact categories. In this study, the approach used was based on intermediate indicators focused on the environmental loads and final consequences. The CML 2001 method (developed in the Netherlands by the Institute of Environmental Sciences of the Leiden University) was used, with indicators such as: Global Warming Potential (GWP); Acidification Potential (AP), Eutrophication Potential (EP), Ozone Depletion Potential (ODP); Photochemical Ozone Creation Potential (POCP); and Abiotic Depletion Potential (ADP). Two additional environmental impact categories were added, related with energy consumption, based on the Cumulative Energy Demand (CED) method. The CED provides the values of the consumption of six primary energy sources (non-renewable consumption: fossil, nuclear, biomass; and renewable consumption: biomass, hydric and wind, solar, geothermic) grouped and presented in two categories with the same unit (MJ - mega joule): Consumption of primary energy, renewable (PE-Re); Consumption of primary energy, non-renewable (PE-NRe). 3.2 Production process of the aerogels This section describes the production process of the three silica aerogels for which the LCA study was completed: inorganic (AI), hybrid in monolithic form (HYB-C) and hybrid in powder form (HYB-A). Table 1 identifies the raw materials and processes used in SimaPro to model the production of these three aerogels. Table 1. Raw materials needed for the production of the three alternatives of aerogel. Raw-material Aerogel Process used in SimaPro (Ecoinvent database) Inorganic HYB-C HYB-A Designation TEOS Tetraethyl orthosilicate Tetrachlorosilane X X i-PrOH 2-Propanol Isopropanol X X HCl Hydrogen chloride Hydrochloric acid X X NH4OH Ammonium hydroxide Ammonia, liquid X X HMDZ Hexamethyldisilazane Hexamethyldisilazane X X Sodium silicate Sodium silicate Sodium silicate X HNO3 Nitric acid Nitric acid X n-hexane Hexane Hexane X H2 O Distilled water Water, ultrapure X X
3.2.1 Inorganic aerogel The inorganic aerogel synthesis starts with the mixing of a tetraethyl orthosilicate (TEOS) as silica precursor with 2-propanol (i-PrOH) as co-solvent, to which water is added drop by drop as it is stirred. The reactional mix is acidified with a hydrochloric acid (HCl) solution. After it is closed, the reactor is heated at 60 ºC and stirred (120 rpm) for 60 minutes, in an incubator with orbital stirring with temperature and speed control. The stirring is then interrupted, the sol is neutralized by adding ammonia (NH4OH) and the resulting homogeneous sol was left to gel, with no further stirring (closed, at 60 °C). The alcogel remains closed in the reactor at 60 ºC for 48 hours: the first 24 h in the residual solution and the other 24 h after addition of an ageing solution (TEOS/H2O/i-PrOH in the same proportions as the initial sol), to age the silica network. Finally, it is washed and dried at atmospheric pressure, at 60 °C in a solvent-saturated atmosphere, until their weight loss became negligible. After drying, a translucent material of variable size is obtained. The aerogel is reduced to grains, by manual grinding in a ceramic vase, and sieved to different sizes (Fidalgo et al., 2007;
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Júlio et al., 2016a). 3.2.2 Hybrid aerogel in powder form (HYB-A) In this production process, the HYB-A aerogel is prepared through a co-precursor method, using a solution of sodium silicate with no previous treatment as silica precursor, and hexamethyldisilazane (HMDZ) as organic co-precursor (Júlio and Ilharco, 2014). The combination of the sodium silicate precursor (low cost because it is a sub product of the sodium carbonate industry) with drying at atmospheric pressure is possibly the most promising way for low cost aerogels. This aerogel is prepared by addition of nitric acid (HNO3) and HMDZ to the sodium silicate solution, under constant stirring. The gelation proceeds at ambient temperature and is followed by a stage of solvent exchange, by immersion of the gel in n-hexane to replace any water included in the pores, whose surfaces are expected to be essentially hydrophobic. This exchange proceeds for 3 h, after which the two liquid phases (excess n-hexane and displaced pore water) are removed. The silylated hydrogels are dried at ambient pressure in two steps: at 60 οC for 24 h followed by 24 h at 100 οC. The samples are repeatedly washed with water (to remove the NO and Na+ ions) and finally dried under the above conditions, until aerogel powders are obtained. As the purpose of this aerogel is its incorporation as aggregates in renders, and since for that it has to be grinded, it is beneficial to obtain it already powdered. 3.3 Hybrid aerogel in monolithic form (HYB-C) The synthesis of HYB-C is similar to that of inorganic aerogel, except for the aging step Júlio et al., 2016a. After 24 h in the residual liquid, the aged alcogels were further hydrophobized by chemical surface modification with an ageing solution of HMDZ in i-PrOH, and left to dry until their weight loss became negligible. 4 RESULTS OF THE LCA STUDY OF THE AEROGELS In this section, the cradle to gate (of the laboratory) LCA results are presented, concerning the production of the three silica aerogels. These LCA results of the aerogels are compared per environmental impact category. 4.1 Inorganic aerogel The cradle to gate (of the laboratory) LCA results for the production of the inorganic aerogel are presented in this section. Table 2 shows the results per environmental impact category of stages A1-A3. Figure 2 shows the relative contribution, in percentage, of these stages for each of the environmental impact categories selected. Stage A1’s contribution is higher than 70% in all environmental impact categories (except for acidification -AP, and eutrophication - EP, with 56% and 67%, respectively). The production stage (A3.2) contributes with around 40% to category AP and 26% to category EP. The amount of packages used in the final product (A3.1) is very small and, therefore, it practically does not contribute to the impacts of this process.
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Table 2. LCA results for stages A1-A3 for 1 kg of inorganic aerogel.
Life cycle assessment stages (for 1 kg of finished product) A1 A2 A3.1 A3.2 A3.3 6.78E+03 2.08E+02 1.02E-02 6.64E+02 2.06E+02 9.93E+02 1.08E+00 1.63E-04 1.67E+02 1.51E+00 3.26E+00 9.00E-02 4.80E-06 3.88E-01 9.24E-02
Indicator
Unit
PE-NRe PE-Re ADP
MJ MJ kg Sb eq
A1-A3 7.86E+03 1.16E+03 3.83E+00
AP
kg SO2 eq
1.36E+00
7.57E-01
2.60E-02
1.12E-06
5.51E-01
2.59E-02
EP
PO43-
eq
3.25E-01
2.13E-01
1.40E-02
7.02E-08
8.40E-02
1.40E-02
GWP
kg CO2 eq
4.54E+02
3.84E+02
6.00E+00
2.89E-04
5.78E+01
6.17E+00
ODP
kg CFC-11 eq
5.01E-05
4.60E-05
3.00E-07
0.00E+00
3.56E-06
2.40E-07
POCP
kg C2H4
1.16E-01
8.75E-02
6.80E-03
5.81E-08
1.50E-02
6.74E-03
kg
100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% PE-NRe
PE-Re
A1 - raw material
ADP
A2 - transport
AP
EP
A3.1 - packaging
GWP
A3.2 - production
ODP
POCP
A3.3 - production waste
Figure 2. Relative contribution of stages A1-A3 for each environmental impact category (inorganic aerogel).
4.2 Hybrid aerogel in powder form (HYB-A) The cradle to gate (of the laboratory) LCA results for the production of HYB-A aerogel are presented in this section. Table 3 shows the contribution of stages A1-A3 to the selected environmental impact categories. The relative contribution of these stages is represented in percentage in Figure 3, for the same environmental impact categories. The extraction and production of raw materials (A1) gives the greatest contribution to categories PE-NRe, POCP and ODP (51%, 67.92% and 85.31%, respectively). For AP and EP, the contribution of this stage is small (17.88% and 21.37%, respectively). Transportation has the greatest impact on global warming (GWP, 41%), abiotic depletion potential (ADP, 29%) and consumption of primary non-renewable energy (PE-NRe, 28%). These impacts are due to the extraction and use of fossil combustibles and also to the greenhouse gases emissions to the atmosphere resulting from the transportation of raw materials to the laboratory.
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Table 3. LCA results for stages A1-A3 for 1 kg of HYB-A aerogel.
Indicator
Unit
PE-NRe PE-Re ADP
MJ MJ kg Sb eq
Life cycle assessment stages (for 1 kg of finished product) A1-A3 A1 A2 A3.1 A3.2 A3.3 5.83E+02 3.22E+02 1.63E-03 2.39E+02 8.01E+00 1.15E+03 1.20E+01 4.34E-01 1.63E-04 4.48E+01 1.19E-01 5.74E+01 2.60E-01 1.58E-01 4.80E-06 1.25E-01 3.70E-03 5.47E-01
AP
kg SO2 eq
5.51E-01
9.85E-02
1.06E-01
1.12E-06
3.42E-01
5.25E-03
3-
EP
kg PO4 eq
1.17E-01
2.50E-02
2.40E-02
7.02E-08
6.67E-02
1.30E-03
GWP
kg CO2 eq
5.60E+01
1.73E+01
2.29E+01
2.89E-04
1.39E+01
1.92E+00
ODP
kg CFC-11 eq
7.08E-06
6.04E-06
5.00E-08
0.00E+00
9.33E-07
5.67E-08
POCP
kg C2H4 eq
2.65E-02
1.80E-02
2.60E-03
5.81E-08
5.83E-03
7.38E-05
100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% PE-NRe
PE-Re
A1 - raw material
ADP
A2 - transport
AP
EP
A3.1 - packaging
GWP
A3.2 - production
ODP
POCP
A3.3 - production waste
Figure 3. Relative contribution of stages A1-A3 for each environmental impact category (HYB-A aerogel).
The stage corresponding to production (A3.2) has an impact of 78% on the consumption of renewable resources, 62% on acidification potential (AP) and 57% on eutrophication potential (EP). The consumption of non-renewable energy resources is approximately five times lower relative to the consumption of renewable energy resources, for stage A3.2. As for the inorganic aerogel, the impact of the packages used in the final product (A3.1) is small and, therefore, their contribution to the environmental impacts is not significant. 4.3 Hybrid aerogel in monolithic form (HYB-C) In this section, the cradle to gate LCA results of HYB-C aerogel are presented. Table 4 identifies the contributions, for each environmental impact category, of stages A1-A3. The relative contribution of these stages is presented in percentage in Figure 4. For each impact category, stage A1 shows values above 80% (except for AP and EP, with 52% and 62%, respectively). The production of aerogel (A3.2) has a more significant environmental impact in categories AP and EP, with 48% and 38%, respectively. Due to the high 8
environmental impact of the raw materials (stage A1), stages A2, A3.1 and A3.3 show insignificant values. It is found that the production of the TEOS raw material is the greatest contributor to indicators AP and GWP. For categories EP and POCP, the production of TEOS and i-PrOH presents the most significant environmental impact. Table 4. LCA results for stages A1-A3 for 1 kg de HYB-C aerogel.
Indicator
Unit
PE-NRe PE-Re ADP
MJ MJ kg Sb eq
Life cycle assessment phases (for 1 kg of finished product) A1-A3 A1 A2 A3.1 A3.2 A3.3 6.62E+03 0.00E+00 1.02E-02 8.00E+02 -6.84E-04 7.42E+03 9.96E+02 0.00E+00 1.63E-04 1.58E+02 -3.93E-06 1.15E+03 3.19E+00 0.00E+00 4.80E-06 4.70E-01 -2.83E-07 3.66E+00
AP
kg SO2 eq
1.43E+00
7.41E-01
0.00E+00
1.12E-06
6.89E-01
-2.95E-08
EP
PO43-
eq
3.29E-01
2.03E-01
0.00E+00
7.02E-08
1.26E-01
2.00E-08
kg CO2 eq
4.42E+02
3.80E+02
0.00E+00
2.89E-04
6.20E+01
2.27E-05
0.00E+00
0.00E+00
3.60E-06
-1.16E-12
1.00E-04
5.81E-08
2.05E-02
-1.76E-09
GWP
kg
ODP
kg CFC-11 eq
5.08E-05
4.72E-05
POCP
kg C2H4 eq
1.02E-01
8.14E-02
4.4
Comparative environmental evaluation of the aerogels life cycle
Figures 5, 6 and 7 compare the impact per kg of the production of each of the three aerogels to the stages that contribute the most to the overall impact of each environmental category. 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% PE-NRe
PE-Re
A1 - raw material
ADP
A2 - transport
AP
EP
A3.1 - packaging
GWP
A3.2 - production
ODP
POCP
A3.3 - production waste
Figure 4. Relative contribution of stages A1-A3 for each environmental impact category (HYB-C aerogel).
It is found that the HYB-A aerogel has the lowest environmental impact in all categories. For stage A1-A3, the inorganic aerogel shows the greatest environmental impact in categories ADP, GWP, POCP, PE-NRe and PE-Re, while HYB-C has the greatest impact in the remaining categories. This difference is due to the raw materials used in the synthesis of each aerogel. The inorganic aerogel and HYB-C have a dissimilar composition but, since they use the same silica precursor
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(TEOS), they have similar impacts in stage A1. HYB-A does not use TEOS as silica precursor, the raw material responsible for most of the environmental impacts, which justifies its better environmental performance. Therefore, in stage A1, the inorganic aerogel causes the greatest environmental impact, followed by HYB-C and finally HYB-A. At the production stage, A3.2, the HYB-C aerogel has the greatest environmental impact, followed by the inorganic aerogel and HYBA. The HYB-A aerogel needs less drying time, thus spending less energy and reducing the impacts. 60% 50% 40% 30% 20% 10% 0% PE-Nre
PE-RE
ADP
AP
Inorganic
EP HYB-A
GWP
ODP
POCP
HYB-C
Figure 5. Comparison of the environmental impacts of stages A1-A3 for inorganic aerogel, HYB-A and HYB-C (per kg of finished product) 60% 50% 40% 30% 20% 10% 0% PE-Nre
PE-RE
ADP Inorganic
AP
EP HYB-A
GWP
ODP
POCP
HYB-C
Figure 6. Comparison of the environmental impacts of stage A1 for inorganic aerogel, HYB-A and HYB-C (per kg of finished product)
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60% 50% 40% 30% 20% 10% 0% PE-Nre
PE-RE
ADP Inorganic
AP
EP HYB-A
GWP
ODP
POCP
HYB-C
Figure 7. Comparison of the environmental impacts of stage A3.2 for inorganic aerogel, HYB-A and HYB-C (per kg of finished product)
Dowson et al. (2012) produced two versions of a silica-based aerogel with supercritical drying, at high and low temperature (HTSCD and LTSCD, respectively). In their study, they performed a cradle to the laboratory gate LCA and determined the energy consumption and CO2 amount during the aerogels’ production. Figure 8 shows the comparison of the environmental impacts for the GWP category of the three subcritical temperature’ aerogels analysed here with the two supercritical temperature’ alternatives of Dowson et al.. It was necessary to assume values for the aerogel density (based on the minimum and maximum current densities of this material), since this parameter is not quantified in the above-mentioned paper.
Figure 8. Environmental impacts of the aerogels produced in this study and of those produced by Dowson et al. for the GWP impact category (densities assumed for the latter: 175 kg/m3 and 305 kg/m3, in blue and red, respectively).
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It was found that the production of aerogel in supercritical conditions at low temperature (LTSCD) has higher environmental impacts than that of the remaining aerogels, for the two density values assumed for LTSCD and HTSCD. However, the aerogel produced in supercritical conditions but at high temperature (HTSCD) has a better environmental performance than the inorganic and HYB-C aerogels, for both density values assumed, but worse than HYB-A. These results are explained by the long drying time needed by the inorganic aerogel and by the HYB-C (20 days for both), and by the LTSCD (twice that of HTSCD), thus increasing the energy consumption, the main cause of the environmental impact at the production stage of the aerogels. 5 CONCLUSIONS This work was developed within a research project with the objective of promoting the use of silica-based aerogels as aggregates in wall coating mortars, synthesized by the sol-gel method and dried at atmospheric pressure. The final goal of this paper was to develop an environmental Life Cycle Assessment (LCA) of three different syntheses of this nanomaterial (aerogel), focused on their production stage. The cradle to (laboratory) gate LCA results were analysed and the materials and processes that contribute the most to each environmental impact category were identified. Some conclusions are highlighted: • In stages A1-A3, the HYB-A aerogel shows the lowest environmental impact in all categories, followed by HYB-C and by the inorganic aerogel; • The inorganic aerogel and HYB-C have a dissimilar composition but, since they use the same silica precursor (TEOS), they have similar environmental impacts in stage A1; • The raw material with the greatest environmental impacts is TEOS; the silica precursor for the HYB-A aerogel is not TEOS, which explains its better environmental performance in stage A1; • In the production stage (A3.2), the HYB-C aerogel has the greatest environmental impact, followed by the inorganic aerogel and by HYB-A; the HYB-A aerogel needs less drying time (around 1/5 of the others), spending less energy, thus lowering its impacts; • The environmental impacts-related data of the aerogel concern its production in laboratory; when the production is upscaled to the industry, it is estimated that these values will decrease, even though it is difficult to estimate the exact scale of that reduction. Comparing the production of aerogels using subcritical drying with the results available in literature for supercritical aerogels, and besides the economic and safety advantages, it was found that the former may lead to the consumption of less energy resources and cause lower environmental impacts. The LCA performed is in accordance with the family of standards ISO 14040 and used a consistent methodology. The results are innovative since there are not many LCA studies in the literature concerning thermal insulation materials, namely silica aerogels. 6 ACKNOWLEDGEMENTS The authors acknowledge the support of the CERIS-ICIST research centre, from Instituto Superior Técnico, Universidade de Lisboa, and of FCT (Fundação para a Ciência e Tecnologia), since this work was developed within project FCT PTDC/ECM/11826/2010 NANORENDER Development of coating mortars with silica nanoaerogels.
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7 REFERENCES Bragança, M., Pinheiro, M. and Mateus, R., “Portugal SB 2013: Contribution of sustainable building to meet EU 20-20-20 targets”, MULTICOMP - Artes Gráficas, Lda, Guimarães, pp. V-VI (2013). Buratti, C., Moretti, E., Belloni, E. and Agosti, F., Development of innovative aerogel based plasters: preliminary thermal and acoustic performance evaluation, Sustainability, 6, 5839-5852 (2014). CEN, Sustainability of construction works - Assessment of environmental performance of buildings Calculation method, EN 15978, Comité Européen de Normalisation, Brussels, Belgium (2011a). CEN, Sustainability of construction works - Assessment of buildings - Part 2: Framework for the assessment of environmental performance, EN 15643-2. Brussels, Belgium, Comité Européen de Normalisation (2011b). CEN, Sustainability of construction works - Environmental product declarations - Core rules for the product category of construction products. EN 15804:2012+A1. Brussels, Belgium, Comité Européen de Normalisation (2013). Dowson, M., Grogan, M., Birks, T., Harrison, D. and Craig S., Streamlined life cycle assessment of transparent silica aerogel made by supercritical drying, Applied Energy, 97, 396-404 (2012). EC, GREEN PAPER: A 2030 framework for climate and energy policies, COM (2013) 169, European Commission, Brussels, Belgium (2014). Ferrão, P. C., Industrial ecology - principles and tools (in Portuguese), IST Press. Lisbon, Portugal (2009). Fidalgo, A., Farinha, J.P.S., Martinho, J.M.G., Rosa, M. E. and Ilharco, L.M., Hybrid silica/polymer aerogels dried at ambient pressure, Chemistry of Materials, 19, 2603-2609 (2007). Gao, T., Jelle, B.P., Gustavsen, A. and Jacobsen, S., Aerogel-incorporated concrete: An experimental study, Construction and Building Materials, 52, 130-136 (2014). Garrido, R., Silvestre, J. and Flores-Colen, I., Economic and energy life cycle assessment of aerogel-based thermal renders, Journal of Cleaner Production, 151. 537-545 (2017). Garrido, R., Silvestre, J., Flores-Colen, I., Júlio, M. and Pedroso, M., Economic assessment of the subcritical production of silica-based aerogels. Journal of Non-Crystalline Solids, 516, pp. 26-34 (2019). Gurav, J. L., Jung, I.-K., Park, H.-H., Kang, E. S. and Nadargi, D. Y., Silica Aerogel: Synthesis and Applications, Journal of Nanomaterials, 2010, pp.1-11 (2010). ISO, Environmental management - Life cycle assessment - Principles and framework, ISO 14040:2006(E), International Organization for Standardization (2006a). ISO, Environmental management - Life cycle assessment - Requirements and guidelines, ISO 14040:2006(E), International Organization for Standardization (2006b). Júlio, M.F. and Ilharco, L.M., Superhydrophobic hybrid aerogel powders from waterglass with distinctive applications, Microporous and Mesoporous Materials, 199, 29-39 (2014). Júlio, M.F., Soares, A., Ilharco, L.M., Flores-Colen, I. and de Brito, J., Silica-based aerogels as aggregates for cement-based thermal renders, Cement and Concrete Composites, 72, 309-318 (2016a). Júlio, M.F., Soares, A., Ilharco, L.M., Flores-Colen, I. and de Brito, J., Aerogel-based renders with lightweight aggregates: Correlation between molecular/pore structure and performance, Construction and Building Materials, 124, 485–495 (2016b). Koebel, M., Rigacci, A. and Achard, P., Aerogel-based thermal superinsulation: an overview, Journal of Sol-Gel Science and Technology, 63, 315-339 (2012). Ortiz, O., Castellsa, F. and Sonnemann, G. Sustainability in the construction industry: A review of recent developments based on LCA, Construction and Building Materials, 23 (1), 28-39 (2009). Pargana, N., Pinheiro, M.D., Silvestre, J.D. and de Brito, J., Comparative environmental life cycle assessment of thermal insulation materials of buildings, Energy and Buildings, 82, 466-481 (2014). Pinheiro, M. D., Environmental and sustainable construction (in Portuguese), Environment Institute, Amadora, Portugal (2006). Silvestre, J. D., Pargana, N., de Brito, J., Pinheiro, M. D. and Durão, V., Insulation Cork Boards Environmental Life Cycle Assessment of an Organic Construction Material, Materials Journal, 9 (5), 394 (2016). Soares, A., Flores-Colen, I. and Brito, J., Nanorenders on buildings facades: technical, economic and environmental performance”, XIII International Conference on Durability of Building Materials”, São Paulo, Brazil (2014). Stahl, T., Brunner, S. and Zimmermann, M., Thermally insulating aerogel based rendering materials, WO2014090790 A1 (2014).
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Highlights
•
Environmental impact assessment of silica-based aerogels
•
Three synthesis methods for subcritically dried silica-based aerogels are compared
•
The phase of production of raw materials has the greatest environmental importance
•
The environmental optimization of the process should be done at the drying stage
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0959-6526(19)34566-4
DOI:
https://doi.org/10.1016/j.jclepro.2019.119696
Reference:
JCLP 119696
To appear in:
Journal of Cleaner Production
Received Date: 11 September 2018 Revised Date:
23 October 2019
Accepted Date: 11 December 2019
Please cite this article as: Pinto I, Silvestre JoséD, de Brito J, Júlio MF, Environmental impact of the subcritical production of silica aerogels, Journal of Cleaner Production (2020), doi: https:// doi.org/10.1016/j.jclepro.2019.119696. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Environmental impact of the subcritical production of silica aerogels Isabel Pinto1, José D. Silvestre2*, Jorge de Brito3, Maria F. Júlio4
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Master in Industrial Engineering and Management, IST, UL; [email protected] Assistant Professor, CERIS, Department of Civil Engineering, Architecture and Georresources (DECivil), Instituto Superior Técnico (IST), Universidade de Lisboa (UL), Av. Rovisco Pais 1, 1049-001, Lisbon, Portugal; [email protected], Telephone: +351 218419709 3 Full Professor, CERIS, DECivil, IST, UL; [email protected] 4 PhD Student, Centro de Química-Física Molecular and IN - Institute of Nanoscience and Nanotechnology, IST, UL; [email protected] * Corresponding author 2
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5042 words Environmental impact of the subcritical production of silica aerogels Isabel Pinto1, José D. Silvestre2*, Jorge de Brito3, Maria F. Júlio4 Abstract: This study evaluates for the first time the environmental impacts of three different syntheses of subcritical silica aerogels. These aerogels are produced by subcritical drying, a more economical and safer process than that of the supercritical aerogels. The environmental evaluation was performed using the standardised Life Cycle Assessment (LCA) methodology, using a cradle to gate approach. The life cycles of these materials were modelled using production data collected in laboratory and a LCA software. These aerogels are produced to be used as aggregates in wall coating mortars for buildings, conferring them thermal insulating properties. It was concluded that the most critical stages in the LCA of these aerogels are the production of raw materials and the production in the laboratory, the latter mostly due to the high energy consumption. The results presented in this paper correspond to a laboratory-scale production, which makes them expectedly higher than if the aerogels were produced at an industrial scale. It was also found that aerogels obtained using subcritical drying generally lead to lower environmental impacts than supercritical aerogels. Key-words: aerogel, environmental impact, life cycle assessment, thermal mortars Symbols and acronyms: λ Thermal conductivity (W/(m K)) ρ Density (kg/m3) ADP Abiotic Depletion Potential AP Acidification Potential CED Cumulative Energy Demand CFC Chlorofluorocarbon EP Eutrophication Potential ETICS External thermal insulation composite system EU European Union GWP Global Warming Potential HTSCD High Temperature Supercritical Drying
LCA Life Cycle Assessment LTSCD Low Temperature Supercritical Drying ODP Ozone Depletion Potential PE Polyethylene PE-NRe Consumption of primary energy, nonrenewable PE-Re Consumption of primary energy, renewable POCP Photochemical Ozone Creation Potential R-value Thermal resistance [(m2.K)/W] U-value Thermal conductivity coefficient [W/(m2 K)]
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Master in Industrial Engineering and Management, IST, UL; [email protected] Assistant Professor, CERIS, Department of Civil Engineering, Architecture and Georresources (DECivil), Instituto Superior Técnico (IST), Universidade de Lisboa (UL), Av. Rovisco Pais 1, 1049-001, Lisbon, Portugal; [email protected], Telephone: +351 218419709 3 Full Professor, CERIS, DECivil, IST, UL; [email protected] 4 PhD Student, Centro de Química-Física Molecular and IN - Institute of Nanoscience and Nanotechnology, IST, UL; [email protected] * Corresponding author 2
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1 INTRODUCTION The European Union (EU) has been establishing weather- and energy-related policies in order to reduce the carbon footprint and, in October 2014, it established goals to be achieved until 2030. These measures are part of the EU objective of reducing the emissions of greenhouse gases by 8095% (relative to the 1990 levels), so that the Earth temperature increase does not exceed 2 ºC (EC, 2014), and include: • Reducing the greenhouse gases emissions by at least 40%; • Guaranteeing that renewable energy represents at least 27% of the overall energy consumption; • Improving the energy efficiency by 27%. In order to achieve these objectives, the cooperation of all sectors is needed. The industrial, construction, transport and agriculture sectors are the greatest contributors to the high energy consumption (Pargana et al., 2014). In the EU, the construction sector is responsible for around 40% of the natural resources (Pinheiro, 2006), 25% of the water, and 40% of the energy consumption, and 1/3 of the greenhouse gases emissions (Bragança et al., 2013). It is clearly a sector with an unsustainable environmental profile and there is a need for a change and to search for new solutions in: the raw materials used in buildings; the production processes (concerning raw materials and the final product); and the end-of-life procedures of the various building components. In Europe, buildings’ energy certification already had positive repercussions (Pargana et al., 2014). When the buildings are adequately designed and operated, it is possible to obtain significant energy savings, which makes the design decisions important to the building’s sustainability. Among these decisions, the choice of construction materials and components, such as thermal insulation materials and systems, is highlighted. These materials establish a barrier to the passage of heat between two spaces, maintaining the inner temperature and contributing to improve the building’s energy efficiency. The study presented in this paper was part of a national research project in which inorganic and hybrid silica-based aerogels, subcritically dried, were synthesized (Júlio and Ilharco, 2014; Júlio et al., 2016a). A cradle-to-gate environmental Life Cycle Assessment (LCA) of three laboratorial syntheses of silica-based aerogels (one purely inorganic and two hybrid - one as powder (HYBA) and the other in monolithic form (HYB-C)), which avoids post-synthesis hydrophobization reactions and only uses subcritical drying, was completed. LCA is important, since it allows identifying opportunities of improvement of the production stage, and reduction of the environmental impacts, as well as comparing the performance of a product with that of other alternatives. The results presented here contribute therefore to compare the environmental impacts of this innovative production process with the traditional supercritical production of these materials, thus contributing to a cleaner production process. These aerogels are to be used as a thermal insulation material, namely by being incorporated as aggregates in wall thermal renders. The incorporation of these aggregates in coating mortars allows improving the building’s thermal performance, since these materials have a reduced thermal conductivity and lower the thermal conductivity of the mortars to values under than that of conventional ones (Júlio et al., 2016b). The environmental evaluation of the mortars in which these aerogels were incorporated is outside the scope of this paper, but the evaluation of the economic performance of these aerogels and mortars was already made (Garrido et al., 2017, 2019). Therefore, the contribution to sustainability is provided in two dimensions (environmental and economic), and the thermal comfort contributes to the third, social, one. This paper comprises five sections, including this introduction. The second section in2
cludes a literature review of similar studies. The LCA methodology is described in the third section. The following section presents the LCA results of the three silica aerogel syntheses. The last section presents the main conclusions of this study.
2 Literature review Nanomaterials are characterized by their reduced structural size and specific properties. When added to mortars, they modify the latter’s properties, affecting their strength, durability and shrinkage, but also improving their thermal resistance and that of the buildings (Soares et al., 2014). One of the most important characteristics of a thermal insulation material is its low thermal conductivity [λ - W/(m.K)]. Silica aerogels are among the best existing thermal insulation materials (with thermal conductivity between 0.01 and 0.02 W/(m.K )), due to the combination of extremely low density (3-500 kg/m3), high porosity (> 90%) and small mesopores (sizes between 4 and 20 nm) (Koebel et al., 2012). They can be purely inorganic, organic or hybrid. Their application in construction is mostly due to their high thermal performance. Recently, they proved their importance as aggregates in cementitious materials, such as improved thermal performance mortars and concrete (Buratti et al., 2014; Gao et al., 2014; Júlio et al., 2016a; Stahl et al., 2014). A review of the literature only turned up one study of the environmental performance of an aerogel. In this study, Dowson et al. (2012) produced a silica aerogel in laboratory, using supercritical drying conditions at high and low temperatures (HTSCD and LTSCD, respectively). The LCA was developed from cradle to gate, excluding the impact of the transportation of raw materials and finished products, as well as its end-of-life processing. This study was performed according to the ISO 14044 and ISO 14040 standards (ISO, 2006a; 2006b), and the functional unit was the energy spent (kWh) and the CO2 released (kg CO2) to produce 40 ml of solid aerogel (non-granular). The authors concluded that: • The production method of LTSCD has a longer environmental return, with a total energy consumption of 62.6 MJ/40 ml and 6.64 kg of CO2 released/40 ml; • The total energy consumption and CO2 released by HTSCD were 29.3 MJ/40 ml and 0.73 kg CO2/40 ml, respectively; • The ageing of the aerogel was the production stage with the highest energy spent. When this process becomes optimized for industrial production, the energy spent and CO2 released values would significantly decrease. However, the LTSCD method is the one where greater improvement can be achieved. 3 METHODOLOGY As the awareness in terms of the environmental impacts of our decisions increases, there is a need to create tools to evaluate the potential damage associated to products daily used. LCA is a technique whose aim is the evaluation of the potential environmental impact of a process, product or service throughout its life cycle. When applied from cradle to grave, this approach analyses all the stages of the life cycle, including: from the extraction of raw materials to the end-of-life treatment and final destination of the product, also comprising the intermediate stages of auxiliary materials manufacture and energy production, transportation and use. LCA is regulated by ISO 14040 (ISO, 2006a), which defines the principles and the framework, and by ISO 14044 (ISO, 2006b), which defines the requirements and guidelines that were followed in this study. The life cycle stages of the construction materials and products are standardized in European standards (CEN, 2011a, 2013). The life cycle boundaries of a construction material can be 3
defined as: from cradle to gate (including the extraction and processing of raw materials and the production); from cradle to grave (including also the transportation, distribution and assemblage, use, maintenance and final disposal); or from cradle to cradle (including the reuse, recovery and/or the recycling potential) (Ferrão, 2009; Ortiz et al., 2009). In this work, the LCA approach used was from cradle to gate (of the laboratory). The environmental evaluation was performed using the SimaPro software and applying international standards (ISO, 2006a; 2006b; CEN, 2011a; 2011b; 2013). The environmental impact analysis starts with the extraction and production of raw materials (A1), includes their transportation to the laboratory (A2) and ends at the production stage (A3). 3.1 LCA of the aerogels 3.1.1 Declared unit The declared unit’s main objective is to provide a reference (common base) that relates the inputs and outputs of the system. In this study, the declared unit considered for the aerogels is 1 kg of finished product. 3.1.2 System boundaries The main objective of the system boundaries is to define the unit processes to be included in this LCA study. Figure 1 represents the generic life cycle from cradle to grave of a thermal insulation material.
Figure.1. Generic life cycle of a thermal insulation material (adapted from Pargana et al., 2014).
This study considered the raw materials extraction and processing (A1), transportation to the production site (A2), and production (including waste from production and raw materials packaging, and final product packaging - A3). The production, application and end-of-life stages of the mortar are not considered relevant to the study because they are identical for all the aerogels compared and, therefore, were not considered. However, the LCA study of the three aerogel syntheses ends at the laboratory gate and includes all stages from the production of raw materials until their packaging (stage A1-A3). European standard EN 15643-2 (CEN, 2011b) and Silvestre (2016) classify these production stages as follows: • A1 - extraction and processing of the raw materials, and entry processing of secondary materials; • A2 - transportation to the production site; • A3 - production and production waste: o A3.1 - product packaging; o A3.2 - production; o A3.3 - waste generated in production, including packaging waste.
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3.1.3 Choice of the environmental impact categories According to Ferrão (2009), the choice of the method of evaluation of the environmental impacts must be related with the selection of the environmental impact categories. In this study, the approach used was based on intermediate indicators focused on the environmental loads and final consequences. The CML 2001 method (developed in the Netherlands by the Institute of Environmental Sciences of the Leiden University) was used, with indicators such as: Global Warming Potential (GWP); Acidification Potential (AP), Eutrophication Potential (EP), Ozone Depletion Potential (ODP); Photochemical Ozone Creation Potential (POCP); and Abiotic Depletion Potential (ADP). Two additional environmental impact categories were added, related with energy consumption, based on the Cumulative Energy Demand (CED) method. The CED provides the values of the consumption of six primary energy sources (non-renewable consumption: fossil, nuclear, biomass; and renewable consumption: biomass, hydric and wind, solar, geothermic) grouped and presented in two categories with the same unit (MJ - mega joule): Consumption of primary energy, renewable (PE-Re); Consumption of primary energy, non-renewable (PE-NRe). 3.2 Production process of the aerogels This section describes the production process of the three silica aerogels for which the LCA study was completed: inorganic (AI), hybrid in monolithic form (HYB-C) and hybrid in powder form (HYB-A). Table 1 identifies the raw materials and processes used in SimaPro to model the production of these three aerogels. Table 1. Raw materials needed for the production of the three alternatives of aerogel. Raw-material Aerogel Process used in SimaPro (Ecoinvent database) Inorganic HYB-C HYB-A Designation TEOS Tetraethyl orthosilicate Tetrachlorosilane X X i-PrOH 2-Propanol Isopropanol X X HCl Hydrogen chloride Hydrochloric acid X X NH4OH Ammonium hydroxide Ammonia, liquid X X HMDZ Hexamethyldisilazane Hexamethyldisilazane X X Sodium silicate Sodium silicate Sodium silicate X HNO3 Nitric acid Nitric acid X n-hexane Hexane Hexane X H2 O Distilled water Water, ultrapure X X
3.2.1 Inorganic aerogel The inorganic aerogel synthesis starts with the mixing of a tetraethyl orthosilicate (TEOS) as silica precursor with 2-propanol (i-PrOH) as co-solvent, to which water is added drop by drop as it is stirred. The reactional mix is acidified with a hydrochloric acid (HCl) solution. After it is closed, the reactor is heated at 60 ºC and stirred (120 rpm) for 60 minutes, in an incubator with orbital stirring with temperature and speed control. The stirring is then interrupted, the sol is neutralized by adding ammonia (NH4OH) and the resulting homogeneous sol was left to gel, with no further stirring (closed, at 60 °C). The alcogel remains closed in the reactor at 60 ºC for 48 hours: the first 24 h in the residual solution and the other 24 h after addition of an ageing solution (TEOS/H2O/i-PrOH in the same proportions as the initial sol), to age the silica network. Finally, it is washed and dried at atmospheric pressure, at 60 °C in a solvent-saturated atmosphere, until their weight loss became negligible. After drying, a translucent material of variable size is obtained. The aerogel is reduced to grains, by manual grinding in a ceramic vase, and sieved to different sizes (Fidalgo et al., 2007;
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Júlio et al., 2016a). 3.2.2 Hybrid aerogel in powder form (HYB-A) In this production process, the HYB-A aerogel is prepared through a co-precursor method, using a solution of sodium silicate with no previous treatment as silica precursor, and hexamethyldisilazane (HMDZ) as organic co-precursor (Júlio and Ilharco, 2014). The combination of the sodium silicate precursor (low cost because it is a sub product of the sodium carbonate industry) with drying at atmospheric pressure is possibly the most promising way for low cost aerogels. This aerogel is prepared by addition of nitric acid (HNO3) and HMDZ to the sodium silicate solution, under constant stirring. The gelation proceeds at ambient temperature and is followed by a stage of solvent exchange, by immersion of the gel in n-hexane to replace any water included in the pores, whose surfaces are expected to be essentially hydrophobic. This exchange proceeds for 3 h, after which the two liquid phases (excess n-hexane and displaced pore water) are removed. The silylated hydrogels are dried at ambient pressure in two steps: at 60 οC for 24 h followed by 24 h at 100 οC. The samples are repeatedly washed with water (to remove the NO and Na+ ions) and finally dried under the above conditions, until aerogel powders are obtained. As the purpose of this aerogel is its incorporation as aggregates in renders, and since for that it has to be grinded, it is beneficial to obtain it already powdered. 3.3 Hybrid aerogel in monolithic form (HYB-C) The synthesis of HYB-C is similar to that of inorganic aerogel, except for the aging step Júlio et al., 2016a. After 24 h in the residual liquid, the aged alcogels were further hydrophobized by chemical surface modification with an ageing solution of HMDZ in i-PrOH, and left to dry until their weight loss became negligible. 4 RESULTS OF THE LCA STUDY OF THE AEROGELS In this section, the cradle to gate (of the laboratory) LCA results are presented, concerning the production of the three silica aerogels. These LCA results of the aerogels are compared per environmental impact category. 4.1 Inorganic aerogel The cradle to gate (of the laboratory) LCA results for the production of the inorganic aerogel are presented in this section. Table 2 shows the results per environmental impact category of stages A1-A3. Figure 2 shows the relative contribution, in percentage, of these stages for each of the environmental impact categories selected. Stage A1’s contribution is higher than 70% in all environmental impact categories (except for acidification -AP, and eutrophication - EP, with 56% and 67%, respectively). The production stage (A3.2) contributes with around 40% to category AP and 26% to category EP. The amount of packages used in the final product (A3.1) is very small and, therefore, it practically does not contribute to the impacts of this process.
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Table 2. LCA results for stages A1-A3 for 1 kg of inorganic aerogel.
Life cycle assessment stages (for 1 kg of finished product) A1 A2 A3.1 A3.2 A3.3 6.78E+03 2.08E+02 1.02E-02 6.64E+02 2.06E+02 9.93E+02 1.08E+00 1.63E-04 1.67E+02 1.51E+00 3.26E+00 9.00E-02 4.80E-06 3.88E-01 9.24E-02
Indicator
Unit
PE-NRe PE-Re ADP
MJ MJ kg Sb eq
A1-A3 7.86E+03 1.16E+03 3.83E+00
AP
kg SO2 eq
1.36E+00
7.57E-01
2.60E-02
1.12E-06
5.51E-01
2.59E-02
EP
PO43-
eq
3.25E-01
2.13E-01
1.40E-02
7.02E-08
8.40E-02
1.40E-02
GWP
kg CO2 eq
4.54E+02
3.84E+02
6.00E+00
2.89E-04
5.78E+01
6.17E+00
ODP
kg CFC-11 eq
5.01E-05
4.60E-05
3.00E-07
0.00E+00
3.56E-06
2.40E-07
POCP
kg C2H4
1.16E-01
8.75E-02
6.80E-03
5.81E-08
1.50E-02
6.74E-03
kg
100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% PE-NRe
PE-Re
A1 - raw material
ADP
A2 - transport
AP
EP
A3.1 - packaging
GWP
A3.2 - production
ODP
POCP
A3.3 - production waste
Figure 2. Relative contribution of stages A1-A3 for each environmental impact category (inorganic aerogel).
4.2 Hybrid aerogel in powder form (HYB-A) The cradle to gate (of the laboratory) LCA results for the production of HYB-A aerogel are presented in this section. Table 3 shows the contribution of stages A1-A3 to the selected environmental impact categories. The relative contribution of these stages is represented in percentage in Figure 3, for the same environmental impact categories. The extraction and production of raw materials (A1) gives the greatest contribution to categories PE-NRe, POCP and ODP (51%, 67.92% and 85.31%, respectively). For AP and EP, the contribution of this stage is small (17.88% and 21.37%, respectively). Transportation has the greatest impact on global warming (GWP, 41%), abiotic depletion potential (ADP, 29%) and consumption of primary non-renewable energy (PE-NRe, 28%). These impacts are due to the extraction and use of fossil combustibles and also to the greenhouse gases emissions to the atmosphere resulting from the transportation of raw materials to the laboratory.
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Table 3. LCA results for stages A1-A3 for 1 kg of HYB-A aerogel.
Indicator
Unit
PE-NRe PE-Re ADP
MJ MJ kg Sb eq
Life cycle assessment stages (for 1 kg of finished product) A1-A3 A1 A2 A3.1 A3.2 A3.3 5.83E+02 3.22E+02 1.63E-03 2.39E+02 8.01E+00 1.15E+03 1.20E+01 4.34E-01 1.63E-04 4.48E+01 1.19E-01 5.74E+01 2.60E-01 1.58E-01 4.80E-06 1.25E-01 3.70E-03 5.47E-01
AP
kg SO2 eq
5.51E-01
9.85E-02
1.06E-01
1.12E-06
3.42E-01
5.25E-03
3-
EP
kg PO4 eq
1.17E-01
2.50E-02
2.40E-02
7.02E-08
6.67E-02
1.30E-03
GWP
kg CO2 eq
5.60E+01
1.73E+01
2.29E+01
2.89E-04
1.39E+01
1.92E+00
ODP
kg CFC-11 eq
7.08E-06
6.04E-06
5.00E-08
0.00E+00
9.33E-07
5.67E-08
POCP
kg C2H4 eq
2.65E-02
1.80E-02
2.60E-03
5.81E-08
5.83E-03
7.38E-05
100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% PE-NRe
PE-Re
A1 - raw material
ADP
A2 - transport
AP
EP
A3.1 - packaging
GWP
A3.2 - production
ODP
POCP
A3.3 - production waste
Figure 3. Relative contribution of stages A1-A3 for each environmental impact category (HYB-A aerogel).
The stage corresponding to production (A3.2) has an impact of 78% on the consumption of renewable resources, 62% on acidification potential (AP) and 57% on eutrophication potential (EP). The consumption of non-renewable energy resources is approximately five times lower relative to the consumption of renewable energy resources, for stage A3.2. As for the inorganic aerogel, the impact of the packages used in the final product (A3.1) is small and, therefore, their contribution to the environmental impacts is not significant. 4.3 Hybrid aerogel in monolithic form (HYB-C) In this section, the cradle to gate LCA results of HYB-C aerogel are presented. Table 4 identifies the contributions, for each environmental impact category, of stages A1-A3. The relative contribution of these stages is presented in percentage in Figure 4. For each impact category, stage A1 shows values above 80% (except for AP and EP, with 52% and 62%, respectively). The production of aerogel (A3.2) has a more significant environmental impact in categories AP and EP, with 48% and 38%, respectively. Due to the high 8
environmental impact of the raw materials (stage A1), stages A2, A3.1 and A3.3 show insignificant values. It is found that the production of the TEOS raw material is the greatest contributor to indicators AP and GWP. For categories EP and POCP, the production of TEOS and i-PrOH presents the most significant environmental impact. Table 4. LCA results for stages A1-A3 for 1 kg de HYB-C aerogel.
Indicator
Unit
PE-NRe PE-Re ADP
MJ MJ kg Sb eq
Life cycle assessment phases (for 1 kg of finished product) A1-A3 A1 A2 A3.1 A3.2 A3.3 6.62E+03 0.00E+00 1.02E-02 8.00E+02 -6.84E-04 7.42E+03 9.96E+02 0.00E+00 1.63E-04 1.58E+02 -3.93E-06 1.15E+03 3.19E+00 0.00E+00 4.80E-06 4.70E-01 -2.83E-07 3.66E+00
AP
kg SO2 eq
1.43E+00
7.41E-01
0.00E+00
1.12E-06
6.89E-01
-2.95E-08
EP
PO43-
eq
3.29E-01
2.03E-01
0.00E+00
7.02E-08
1.26E-01
2.00E-08
kg CO2 eq
4.42E+02
3.80E+02
0.00E+00
2.89E-04
6.20E+01
2.27E-05
0.00E+00
0.00E+00
3.60E-06
-1.16E-12
1.00E-04
5.81E-08
2.05E-02
-1.76E-09
GWP
kg
ODP
kg CFC-11 eq
5.08E-05
4.72E-05
POCP
kg C2H4 eq
1.02E-01
8.14E-02
4.4
Comparative environmental evaluation of the aerogels life cycle
Figures 5, 6 and 7 compare the impact per kg of the production of each of the three aerogels to the stages that contribute the most to the overall impact of each environmental category. 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% PE-NRe
PE-Re
A1 - raw material
ADP
A2 - transport
AP
EP
A3.1 - packaging
GWP
A3.2 - production
ODP
POCP
A3.3 - production waste
Figure 4. Relative contribution of stages A1-A3 for each environmental impact category (HYB-C aerogel).
It is found that the HYB-A aerogel has the lowest environmental impact in all categories. For stage A1-A3, the inorganic aerogel shows the greatest environmental impact in categories ADP, GWP, POCP, PE-NRe and PE-Re, while HYB-C has the greatest impact in the remaining categories. This difference is due to the raw materials used in the synthesis of each aerogel. The inorganic aerogel and HYB-C have a dissimilar composition but, since they use the same silica precursor
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(TEOS), they have similar impacts in stage A1. HYB-A does not use TEOS as silica precursor, the raw material responsible for most of the environmental impacts, which justifies its better environmental performance. Therefore, in stage A1, the inorganic aerogel causes the greatest environmental impact, followed by HYB-C and finally HYB-A. At the production stage, A3.2, the HYB-C aerogel has the greatest environmental impact, followed by the inorganic aerogel and HYBA. The HYB-A aerogel needs less drying time, thus spending less energy and reducing the impacts. 60% 50% 40% 30% 20% 10% 0% PE-Nre
PE-RE
ADP
AP
Inorganic
EP HYB-A
GWP
ODP
POCP
HYB-C
Figure 5. Comparison of the environmental impacts of stages A1-A3 for inorganic aerogel, HYB-A and HYB-C (per kg of finished product) 60% 50% 40% 30% 20% 10% 0% PE-Nre
PE-RE
ADP Inorganic
AP
EP HYB-A
GWP
ODP
POCP
HYB-C
Figure 6. Comparison of the environmental impacts of stage A1 for inorganic aerogel, HYB-A and HYB-C (per kg of finished product)
10
60% 50% 40% 30% 20% 10% 0% PE-Nre
PE-RE
ADP Inorganic
AP
EP HYB-A
GWP
ODP
POCP
HYB-C
Figure 7. Comparison of the environmental impacts of stage A3.2 for inorganic aerogel, HYB-A and HYB-C (per kg of finished product)
Dowson et al. (2012) produced two versions of a silica-based aerogel with supercritical drying, at high and low temperature (HTSCD and LTSCD, respectively). In their study, they performed a cradle to the laboratory gate LCA and determined the energy consumption and CO2 amount during the aerogels’ production. Figure 8 shows the comparison of the environmental impacts for the GWP category of the three subcritical temperature’ aerogels analysed here with the two supercritical temperature’ alternatives of Dowson et al.. It was necessary to assume values for the aerogel density (based on the minimum and maximum current densities of this material), since this parameter is not quantified in the above-mentioned paper.
Figure 8. Environmental impacts of the aerogels produced in this study and of those produced by Dowson et al. for the GWP impact category (densities assumed for the latter: 175 kg/m3 and 305 kg/m3, in blue and red, respectively).
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It was found that the production of aerogel in supercritical conditions at low temperature (LTSCD) has higher environmental impacts than that of the remaining aerogels, for the two density values assumed for LTSCD and HTSCD. However, the aerogel produced in supercritical conditions but at high temperature (HTSCD) has a better environmental performance than the inorganic and HYB-C aerogels, for both density values assumed, but worse than HYB-A. These results are explained by the long drying time needed by the inorganic aerogel and by the HYB-C (20 days for both), and by the LTSCD (twice that of HTSCD), thus increasing the energy consumption, the main cause of the environmental impact at the production stage of the aerogels. 5 CONCLUSIONS This work was developed within a research project with the objective of promoting the use of silica-based aerogels as aggregates in wall coating mortars, synthesized by the sol-gel method and dried at atmospheric pressure. The final goal of this paper was to develop an environmental Life Cycle Assessment (LCA) of three different syntheses of this nanomaterial (aerogel), focused on their production stage. The cradle to (laboratory) gate LCA results were analysed and the materials and processes that contribute the most to each environmental impact category were identified. Some conclusions are highlighted: • In stages A1-A3, the HYB-A aerogel shows the lowest environmental impact in all categories, followed by HYB-C and by the inorganic aerogel; • The inorganic aerogel and HYB-C have a dissimilar composition but, since they use the same silica precursor (TEOS), they have similar environmental impacts in stage A1; • The raw material with the greatest environmental impacts is TEOS; the silica precursor for the HYB-A aerogel is not TEOS, which explains its better environmental performance in stage A1; • In the production stage (A3.2), the HYB-C aerogel has the greatest environmental impact, followed by the inorganic aerogel and by HYB-A; the HYB-A aerogel needs less drying time (around 1/5 of the others), spending less energy, thus lowering its impacts; • The environmental impacts-related data of the aerogel concern its production in laboratory; when the production is upscaled to the industry, it is estimated that these values will decrease, even though it is difficult to estimate the exact scale of that reduction. Comparing the production of aerogels using subcritical drying with the results available in literature for supercritical aerogels, and besides the economic and safety advantages, it was found that the former may lead to the consumption of less energy resources and cause lower environmental impacts. The LCA performed is in accordance with the family of standards ISO 14040 and used a consistent methodology. The results are innovative since there are not many LCA studies in the literature concerning thermal insulation materials, namely silica aerogels. 6 ACKNOWLEDGEMENTS The authors acknowledge the support of the CERIS-ICIST research centre, from Instituto Superior Técnico, Universidade de Lisboa, and of FCT (Fundação para a Ciência e Tecnologia), since this work was developed within project FCT PTDC/ECM/11826/2010 NANORENDER Development of coating mortars with silica nanoaerogels.
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7 REFERENCES Bragança, M., Pinheiro, M. and Mateus, R., “Portugal SB 2013: Contribution of sustainable building to meet EU 20-20-20 targets”, MULTICOMP - Artes Gráficas, Lda, Guimarães, pp. V-VI (2013). Buratti, C., Moretti, E., Belloni, E. and Agosti, F., Development of innovative aerogel based plasters: preliminary thermal and acoustic performance evaluation, Sustainability, 6, 5839-5852 (2014). CEN, Sustainability of construction works - Assessment of environmental performance of buildings Calculation method, EN 15978, Comité Européen de Normalisation, Brussels, Belgium (2011a). CEN, Sustainability of construction works - Assessment of buildings - Part 2: Framework for the assessment of environmental performance, EN 15643-2. Brussels, Belgium, Comité Européen de Normalisation (2011b). CEN, Sustainability of construction works - Environmental product declarations - Core rules for the product category of construction products. EN 15804:2012+A1. Brussels, Belgium, Comité Européen de Normalisation (2013). Dowson, M., Grogan, M., Birks, T., Harrison, D. and Craig S., Streamlined life cycle assessment of transparent silica aerogel made by supercritical drying, Applied Energy, 97, 396-404 (2012). EC, GREEN PAPER: A 2030 framework for climate and energy policies, COM (2013) 169, European Commission, Brussels, Belgium (2014). Ferrão, P. C., Industrial ecology - principles and tools (in Portuguese), IST Press. Lisbon, Portugal (2009). Fidalgo, A., Farinha, J.P.S., Martinho, J.M.G., Rosa, M. E. and Ilharco, L.M., Hybrid silica/polymer aerogels dried at ambient pressure, Chemistry of Materials, 19, 2603-2609 (2007). Gao, T., Jelle, B.P., Gustavsen, A. and Jacobsen, S., Aerogel-incorporated concrete: An experimental study, Construction and Building Materials, 52, 130-136 (2014). Garrido, R., Silvestre, J. and Flores-Colen, I., Economic and energy life cycle assessment of aerogel-based thermal renders, Journal of Cleaner Production, 151. 537-545 (2017). Garrido, R., Silvestre, J., Flores-Colen, I., Júlio, M. and Pedroso, M., Economic assessment of the subcritical production of silica-based aerogels. Journal of Non-Crystalline Solids, 516, pp. 26-34 (2019). Gurav, J. L., Jung, I.-K., Park, H.-H., Kang, E. S. and Nadargi, D. Y., Silica Aerogel: Synthesis and Applications, Journal of Nanomaterials, 2010, pp.1-11 (2010). ISO, Environmental management - Life cycle assessment - Principles and framework, ISO 14040:2006(E), International Organization for Standardization (2006a). ISO, Environmental management - Life cycle assessment - Requirements and guidelines, ISO 14040:2006(E), International Organization for Standardization (2006b). Júlio, M.F. and Ilharco, L.M., Superhydrophobic hybrid aerogel powders from waterglass with distinctive applications, Microporous and Mesoporous Materials, 199, 29-39 (2014). Júlio, M.F., Soares, A., Ilharco, L.M., Flores-Colen, I. and de Brito, J., Silica-based aerogels as aggregates for cement-based thermal renders, Cement and Concrete Composites, 72, 309-318 (2016a). Júlio, M.F., Soares, A., Ilharco, L.M., Flores-Colen, I. and de Brito, J., Aerogel-based renders with lightweight aggregates: Correlation between molecular/pore structure and performance, Construction and Building Materials, 124, 485–495 (2016b). Koebel, M., Rigacci, A. and Achard, P., Aerogel-based thermal superinsulation: an overview, Journal of Sol-Gel Science and Technology, 63, 315-339 (2012). Ortiz, O., Castellsa, F. and Sonnemann, G. Sustainability in the construction industry: A review of recent developments based on LCA, Construction and Building Materials, 23 (1), 28-39 (2009). Pargana, N., Pinheiro, M.D., Silvestre, J.D. and de Brito, J., Comparative environmental life cycle assessment of thermal insulation materials of buildings, Energy and Buildings, 82, 466-481 (2014). Pinheiro, M. D., Environmental and sustainable construction (in Portuguese), Environment Institute, Amadora, Portugal (2006). Silvestre, J. D., Pargana, N., de Brito, J., Pinheiro, M. D. and Durão, V., Insulation Cork Boards Environmental Life Cycle Assessment of an Organic Construction Material, Materials Journal, 9 (5), 394 (2016). Soares, A., Flores-Colen, I. and Brito, J., Nanorenders on buildings facades: technical, economic and environmental performance”, XIII International Conference on Durability of Building Materials”, São Paulo, Brazil (2014). Stahl, T., Brunner, S. and Zimmermann, M., Thermally insulating aerogel based rendering materials, WO2014090790 A1 (2014).
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Highlights
•
Environmental impact assessment of silica-based aerogels
•
Three synthesis methods for subcritically dried silica-based aerogels are compared
•
The phase of production of raw materials has the greatest environmental importance
•
The environmental optimization of the process should be done at the drying stage
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: