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Aircraft de-icer: Recycling can cut carbon emissions in half
Environmental Impact Assessment Review 32 (2012) 156–164
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Environmental Impact Assessment Review journal homepage: www.elsevier.com/locate/eiar
Aircraft de-icer: Recycling can cut carbon emissions in half Eric P. Johnson ⁎ Atlantic Consulting, Obstgartenstrasse 14, 8136 Gattikon, Switzerland
a r t i c l e
i n f o
Article history: Received 30 June 2011 Received in revised form 6 August 2011 Accepted 6 August 2011 Available online 17 September 2011 Keywords: Recycling Aircraft de-icer Carbon footprint Propylene glycol
a b s t r a c t Flight-safety regulations in most countries require aircraft to be ice-free upon takeoff. In icy weather, this means that the aircraft usually must be de-iced (existing ice is removed) and sometimes anti-iced (to protect against ice-reformation). For both processes, aircraft typically are sprayed with an ‘antifreeze’ solution, consisting mainly of glycol diluted with water. This de/anti-icing1 creates an impact on the environment, of which environmental regulators have grown increasingly conscious. The US Environmental Protection Agency (EPA), for example, recently introduced stricter rules that require airports above minimum size to collect deicing effluents and send them to wastewater treatment. De-icer collection and treatment is already done at most major airports, but a few have gone one step further: rather than putting the effluent to wastewater, they recycle it. This study examines the carbon savings that can be achieved by recycling de-icer. There are two key findings. One, recycling, as opposed to not recycling, cuts the footprint of aircraft de-icing by 40–50% — and even more, in regions where electricity-generation is cleaner. Two, recycling petrochemical-based de-icer generates a 15–30% lower footprint than using ‘bio’ de-icer without recycling. © 2011 Elsevier Inc. All rights reserved.
1. Introduction De-icing and anti-icing are common procedures in colder-weather airports. Both aircraft and runways are sprayed to keep them ice-free for takeoffs and landings. With current economics and technologies, it would not make sense to recover runway de-icers, but economics and technologies to recycle aircraft de-icers already exist. Airports in Munich and Zurich have been recycling aircraft de-icer for some years now. Oslo plans to join the recycling club, and other airports are known to be considering a similar move. The question addressed by this study is: by recycling de-icer rather than discharging it to wastewater treatment, what is the impact on the carbon footprint of de-icing? The system examined is conventional de-icing in Europe, the main ingredient of which is propylene glycol. The method is that of a life-cycle assessment or carbon footprint. This question – whether recycling is better than not recycling – is not a trivial one. Although for decades the mantra of ‘reduce, reuse, recycle’ has been chanted by numerous policy-makers, it has been refuted in specific cases, e.g. Berkhout et al. (1999) or Johnson and Arne (2010). The report is presented as such: after a brief text about the method, a definition of the de-icing system is presented, followed by the
⁎ Tel.: + 41 44 772 1079. E-mail address: [email protected]. 1 For simplicity's sake, hereafter, de-icing and anti-icing are referred to as de-icing, unless explicitly stated otherwise. 0195-9255/$ – see front matter © 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.eiar.2011.08.001
base case footprint, a sensitivity analysis, a comparison of recycling and use of ‘bio’ de-icer, and a discussion of the applicability of the results to ethylene glycol, which is also used as a de-icer in some regions. 2. Method of this analysis The method of this study was to compile the carbon footprint of aircraft de-icing, with and without recycling, and then to compare the two. Scope of the analysis is ‘cradle-to-grave’, from production of the raw materials and energy used in de-icing on through to disposal of its components. All greenhouse gases, as defined by the Intergovernmental Panel on Climate Change (IPCC), are included. The method used in this study is believed to be consistent with current, global best-practice, and it is compliant with the current standard for carbon footprinting, Publicly Available Standard 2050 (BSI et al., 2008), commonly referred to as PAS 2050. Carbon footprints are a summation of the greenhouse-gas emissions of a product or service across its lifetime (or life cycle). A carbon footprint is a subset of a life-cycle assessment (LCA), which is a summation of all emissions of a product or service. The approach to footprinting is identical to that for LCA; the only difference is that in a carbon footprint, a smaller scope of emissions – i.e. greenhouse gases only – is covered. This study's method is also consistent with current guidance for LCA, such as ISO (2006). In line with current best-practice, future emissions in this analysis have not been discounted. All greenhouse gases emitted over the
E.P. Johnson / Environmental Impact Assessment Review 32 (2012) 156–164
lifetime of the heat pump are summed and multiplied by the same global warming potentials (GWPs); in effect, the emissions over time are treated mathematically as if they were a single pulse to the atmosphere.
Table 1 Composition of aircraft de-icer. Component Weight fraction of de-icer
Comment
Glycol
Concentration depends on type of de-icing application and on weather conditions
Additives
50–80% in Europe 50–88% in N America b1%
Water
11+% to 49+%
3. Definition of the aircraft de-icing system Flight-safety regulations in most countries require aircraft to be ice-free upon takeoff. In icy weather, this means that aircraft usually must be de-iced (existing ice is removed) and sometimes anti-iced (to protect against ice-reformation). For both processes, aircraft typically are sprayed with an ‘antifreeze’ solution, consisting mainly of glycol diluted with water. The life-cycle of aircraft de-icing (Fig. 1) starts with production and blending of the raw materials, follows with usage in de-icing and concludes with disposal. Disposal follows a mixture of three paths: wastewater treatment, recycling and dispersal to the environment. This mix depends mainly on the installed disposal facilities, which vary by airport. For airports that recycle, spent de-icer collected in drainage is typically discharged to wastewater when its glycol is at a low concentration that is uneconomic to recycle. Recycling can be conducted either onsite or offsite. The onsite/offsite choice rests mainly on three factors: • Economics, especially economies of scale — i.e. the volume of de-icer used versus the cost of recycling • Available space for a recycling plant • Anti-competition rules — to prevent contamination and potential product failure, an onsite plant requires a single source of de-icer supply. In some cases, this is not allowed under anti-competition rules. Aircraft de-icing is defined in seven parts: composition of the deicer; raw materials production; blending; use; disposal; electricity production; and transport. These are described in the following sections. 3.1. Composition of the de-icer De-icers consist of three components (Table 1): glycol, additives and water. In Western Europe and the US, the typical glycol used is propylene glycol, sometimes referred to as MPG (for mono-propylene glycol). In Canada and Russia, the more common choice is ethylene glycol. 3.2. Raw material production Production methods and sources of emission-data are reviewed for glycol, additives and water. 3.2.1. Glycol (propylene glycol) Propylene glycol (MPG) is the only existing de-icer currently recycled, so we have used this as the base case glycol in this analysis.
De-icer onsite blending
157
Primary functions of the additives are to: prevent foaming, keep fluid on the aircraft, inhibit corrosion, prevent combustion, buffer the solution (to prevent acid formation) and give colour to the de-icer. This can be added at the production site, at a regional blending plant and/or at the airport, with the aim being to minimise water transport.
Also we have placed this base case in Europe, where existing de-icing is located. Various process routes to MPG are possible; emissions data are available for only some of them. 3.2.1.1. Process routes. MPG production unfolds in four major steps (Fig. 2). Natural gas and crude oil are processed, creating gas liquids and naphtha, which are steam cracked to create, among other things, propylene. Refineries also generate propylene directly. Purified propylene is then converted to propylene oxide (PO) via one of three processes: peroxidation, chlorohydrins processing; or hydroperoxidation. PO is then hydrated to create propylene glycol. In principle, the molecules can proceed along any of the routes shown in this chain. And in fact, the route to propylene alone can be even more complicated than this (Fig. 3). In practice, however, nearly all the propylene that ends up as MPG comes through steam cracking, and in Europe, the majority feedstock for this is naphtha. For the propylene-to-PO step, capacity is split roughly 47/47 between peroxidation and chlorohydrin processing, with the remainder coming from hydroperoxidation. 3.2.1.2. Emissions: sources and data. For the base case we have used the only public, cradle-to-gate emissions data-source for MPG. This is (ecoinvent, 2010), which models a European petrochemical-production chain. It presumes a European-average steam cracker feeding propylene only to chlorohydrin processing (Fig. 2); peroxidation and hydroperoxidation are not covered. The ecoinvent footprint is 4.06 kg CO2e/kg MPG. 3.2.2. Additives A supplier of de-icers has provided us with a detailed description of typical de-icer additives on a confidential basis, and their functions on a non-confidential basis. For each of these, we have found a proxy carbon footprint in ecoinvent (2010), which has been used in the system model. 3.2.3. Water In the base case we have used industrial process water, as defined by ecoinvent. Its footprint is 0.8 kg CO2e/tonne of water. As noted in
Dispersal to the environment Glycol production Additives production Diluent water
Concentrate blending
Natural gas
Wastewater treatment De-icer blending
De-icing use
Drain
Recycling offsite
Recycling onsite
Gas processing
NGLs Propylene Steam cracking
Crude oil
Oil refining
Peroxidation or Chlorohydrin or
PO
Hydration
Hydroperoxidation
LPG Naphtha Propylene
Fig. 1. The life-cycle of aircraft de-icing.
Fig. 2. Propylene glycol (MPG) production chain.
MPG
158
E.P. Johnson / Environmental Impact Assessment Review 32 (2012) 156–164 Thermal-cracked propylene Fluid-catalytic-cracked propylene
Upgrading Ethylene Naphtha Crude oil
Refinery
Butylenes
Metathesis
Gas oil Steam Cracker Propylene Natural gas
Ethane
Gas processing
Propane
Propane dehydro Hydro treating
Vegetable oil
Fig. 3. Propylene production chain (with extra detail).
Table 1, this can be blended at the production site, at a blending site and/or at the airport itself.
3.3. Blending to de-icer concentrate (DIC) and to de-icer MPG is shipped 900 km by railcar from Dow's Stade plant to Clariant Gendorf for blending. De-icer is blended in two stages: first the MPG and additives are blended to create de-icer concentrate, which is shipped long-distance to a regional de-icing plant; at the regional de-icing plant, water is blended to bring the de-icer to its desired concentration. (For Type I de-icers – low viscosity liquids, typically used at 55–80 °C – hot water is also blended at the airport itself.) Regional de-icing plants are located at Gothenburg and Amsterdam, which respectively are 1126 km and 900 km distant from Gendorf. For the base case we have used a distance of 1000 km. For concentrate blending and filling to an IBC container, the power required per tonne of concentrate, as reported by a supplier, is 9.818 kWh.
De-icer onsite blending 35% of spent de-icer
Recycled de-icer
De-icing use
Dispersal to the environment
3.5. Disposal: discharge to wastewater and/or recycling Spent de-icer can be disposed of in four ways: dispersal to the environment, in wastewater treatment, by onsite and offsite recycling. These are described in the following sub-sections.
Table 2 Recycling base case, mass-balance and MPG concentrationsa.
Drain
Recycling onsite 60% of
Recycling offsite
To MPG
In use, aircraft de-icers are in some cases mixed with hot water and in all cases sprayed on to the aircraft. This involves transport, heating and pumping, which – due to a lack of data – have not been included in this analysis. However, because these inputs would be equal, whether or not the de-icer was recycled, this gap is believed not to dent the robustness of this study.
55-60% Glycol
Wastewater treatment
65% of spent de-icer
3.4. Use
3.5.1. Process routes After use spent de-icer either is collected in drainage ponds or tanks, or it is dispersed to the local environment (Fig. 4, Table 2). The amount collected varies, depending on drainage design and
5% of spent de-icer Make-up de-icer
The filled concentrate is shipped in a 22-tonne truck to the regional deicing plant. For de-icer blending, we estimate half as much power is required, i.e. 5 kWh/tonne. The final product is shipped by tank-truck to the airport; for that distance we have presumed a journey from Gothenburg to Oslo, i.e. 300 km.
spent de-icer to recycling
65% Glycol
Disposal route of MPG
Fraction (weight) of spent de-icer disposed this way
Concentration of MPG in this fraction
To the environment To wastewater treatment To recycling
35% 5%
NA b5%
60%
15% average
a
Fig. 4. De-icer recycling flow sheet (Data as reported by an airport that runs a recycling operation.).
Comment
Corresponds to TOC of about 2.5%b Corresponds to TOC of about 7.5%
Data as reported by an airport that runs a recycling operation. Pure MPG has a TOC (total organic content) of 47%. So we have applied a simple 1:2 ratio of TOC to MPG concentration. b
E.P. Johnson / Environmental Impact Assessment Review 32 (2012) 156–164 Table 3 Inputs to onsite recycling (per unit of diluted de-icera). Process
Power
Units
Losses of MPG
Ultrafiltration Ion exchange Distillation to 60% MPG Distillation to 65% MPG
15 1 0.1000 0.1015
kWh/m3 kWh/m3 kWh/l kWh/l
1 l/m3 0.072 l/m3 8.4 l/m3 8.4 l/m3
a
Assumed to consist of 10% MPG. This is lower than the 15% average reported above, to give a safety margin to the calculation.
spray operations. For airports without recycling, the collected effluent is discharged to wastewater treatment. For economical recycling, collected de-icer needs to have an MPG concentration above 5%, which corresponds to a TOC of about 2.5%. If below this, it is instead discharged to wastewater treatment. Concentration depends on the mix of de-icers being sprayed as well as the amount of local precipitation. In onsite recycling, spent de-icer is: filtered to remove particles and sediment; run through an ion exchanger to remove salts; and pH is adjusted. Then it is distilled to 55–60% (weight) glycol and sent for re-blending to finished de-icer. In offsite recycling, the spent de-icer is first concentrated in an evaporator to 65% glycol concentration. The 65% solution is then sent to an offsite recycling plant, where it is purified to 99.9% glycol via steam distillation and thin-film evaporation. From these figures we have estimated mass-balances and concentrations for the recycling base case (Table 2). Both recycling operations are considered closed-loop according to the (ISO, 2006) definition. For economic and commercial reasons, Munich and Zurich/Oslo approach de-icer recycling somewhat differently. Munich operates a closed loop, i.e. de-icer is recycled onsite back into its original use. By contrast, Zurich (and presently in Oslo) operate open loops, i.e. effluent rich in de-icer is sent to an offsite treatment plant that recovers pure glycol, which can be used in de-icer or other glycol applications. 3.5.2. Emissions: sources and data The four disposal routes are presented in the following subsections. 3.5.3. To the environment In this study, we assume that propylene glycol dispersed to the environment is fully biodegradable. This is based on a Screening Information Dataset published by the OECD, 2 which states: “Propylene glycol is not volatile, but is miscible with water. Air monitoring data are not available, but concentrations of propylene glycol in the atmosphere are expected to be extremely low because of its low vapor pressure and high water solubility. It is readily biodegraded in water or soil. Four studies reported ≥60% biodegradation in water in 10 days. PG is not expected to bioaccumulate, with a calculated BCF b 1. Measured freshwater aquatic toxicity data for fish, daphnia and algae report LC/EC50 values of N18,000 mg/l. Therefore, PG is not acutely toxic to aquatic organisms except at very high concentrations. Using an assessment factor of 100 and the Ceriodaphnia data (48-hour EC 50 = 18,340 mg/ l), the PNEC is 183 mg/l.” So we presume that each molecule of propylene glycol, C3O2H8, is converted to three molecules of CO2: i.e. per tonne of MPG, biodegradation creates 1.74 tonnes of carbon dioxide. 3.5.4. To wastewater treatment In the study we assume that propylene glycol to wastewater treatment also biodegrades fully. Again, we presume that each molecule of 2 SIDS Initial Assessment Report for 1,2-Dihydroxypropane. Available at http:// www.inchem.org/documents/sids/sids/57-55-6.pdf.
159
Table 4 Footprint of airport de-icing, recycling versus not recycling (kg CO2e/tonne 65% deicer). Case
No recycle Offsite recycle Onsite recycle
Total
3917 2288 1857
Of which De-icer at airport
MPG to environment
MPG to wastewater
2792 1838 1408
393 393 393
732 56 56
% reduction to no recycling
0% − 42% − 53%
Table 5 Contributions to airport de-icing footprint (kg CO2e/tonne 65% de-icer). Contribution to footprint
No recycling
Offsite recycling
Onsite recycling
Total MPG to wastewater Propylene, at plant/RER U MPG to environment Electricity UCTE Other contributions
3917 731 565 393 15 2213
2288 56 242 393 247 1349
1857 56 234 393 241 934
propylene glycol, C3O2H8, is converted to three molecules of CO2: i.e. per tonne of MPG, biodegradation creates 1.74 tonnes of carbon dioxide. For wastewater treatment, we have used inputs defined in ecoinvent for transport, electricity, fuel oil and natural gas to operate the plant itself. 3.5.5. To recycling As noted previously, MPG can be recycled onsite or offsite. Either way, the spent de-icer (average 15% MPG concentration) is first treated by ultrafiltration and ion exchange to remove impurities, and then it is concentrated by distillation to 60% or 65% MPG purity (Table 3). In offsite recycling, the 65%-MPG concentrate from onsite-treatment is transported by a 22-tonne truck to a recovery plant. There it requires 16 kWh and 1.72 tonnes of medium-pressure steam per tonne of 99.9% pure MPG product. For the distillation from 65% to 99.9%, losses of 4% MPG are assumed in the model. In both the onsite and offsite recycling, it is presumed that the distilled water (generated by purifying the MPG) is not used further. Because of its minor impurities, it is discharged to wastewater treatment as well. 3.6. Electricity production In the base case, power throughout the production-recycling chain is assumed to come from a continental Western European mix, socalled UCTE 3 electricity. The UCTE mix is an average for Western Europe (west of the former Soviet Union, excluding the British Isles and the Nordics except for the western half of Denmark). The footprint for 1 kWh of UCTE power is 0.523 CO2e. 3.7. Transport For road and rail transport, the model uses standard modules from ecoinvent. 4. Base case footprint Using the definitions from the previous chapter, footprints of the system were calculated using SimaPro 7 software. The normalisation factor, or functional unit, is 1 tonne of de-icer at the airport with an average 65% concentration of propylene glycol.
3
Union for the Co-ordination of Transmission of Electricity.
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E.P. Johnson / Environmental Impact Assessment Review 32 (2012) 156–164
1p De-icer life cycle, NO recycling 3.92E3
1E3 kg De-icer at airport 2.79E3
650 kg De-icer concentrate, at plant 2.64E3
646 kg Propylene glycol, liquid, at plant/RER U 2.62E3
519 kg Propylene oxide, liquid, at plant/RER U 2.31E3
1.09E4 MJ Electricity, medium voltage, production UCTE, at 1.59E3
1.12E4 MJ Electricity, high voltage, production UCTE, at 1.6E3
1.13E4 MJ Electricity, production mix UCTE/UCTE U 1.6E3 Fig. 5. No recycling, de-icing carbon footprint (kg CO2e/tonne 65% de-icer).
E.P. Johnson / Environmental Impact Assessment Review 32 (2012) 156–164
The ‘no recycle’ case shows a much higher footprint that recycling either on or offsite (Table 4). Compared to a ‘no recycle’ footprint of 3,917 kg CO2e/tonne of de-icer, recycling offsite drops that by 42% to 2,288 kg CO2e/tonne, while recycling onsite lowers it 53% to 1857 kg CO2e/tonne.
The recycling reduction comes down to two factors: • Displacement: recycling at these concentrations generates a considerably lower footprint than does manufacturing fresh MPG — this accounts for about two-thirds of the difference to the ‘no recycling’ case.
1p De-icer life cycle, Recycling 1.86E3
413 kg De-icer at airport
226 kg MPG to environment 393
1.15E3
388 kg MPG to recycling -1.38E3
269 kg De-icer concentrate, at plant 1.09E3
267 kg Propylene glycol, liquid, at plant/RER U 1.08E3
214 kg Propylene oxide, liquid, at plant/RER U 956
161
4.49E3 MJ Electricity, medium voltage, production UCTE, at grid/UCTE U 659
4.63E3 MJ Electricity, high voltage, production UCTE, at grid/UCTE U 663
4.68E3 MJ Electricity, production mix UCTE/UCTE U 661 Fig. 6. Onsite recycling, de-icing carbon footprint (kg CO2e/tonne 65% de-icer).
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• Avoidance: recycling also avoids significant emissions of MPG to wastewater, which contribute considerable to the ‘no recycling’ footprint — this accounts for about one-third of the difference to the ‘no recycling’ case. The difference between the two recycling approaches is simply a trade-off between energy and yield. Although distilling to 99.9% MPG generates a more valuable product, it requires proportionately more fuel and power, mainly for distillation but also more for transport.
The differences also can be seen by comparing the major contributors to each footprint (Table 5). In the ‘no recycling’ case, MPG to wastewater is the single biggest contributor, whereas in the recycling cases it is reduced dramatically. Much less propylene is needed in the recycling cases, thanks to displacement, but this is roughly offset by increased demand for electricity in recycling. Meanwhile, MPG to the environment stays constant in all cases. Graphic representations of the base case footprints are presented (Fig. 5, Fig. 6, Fig. 7) as supplementary material for interested readers.
1p De-icer life cycle, Recycling 2.29E3
226 kg MPG to environment
1E3 kg De-icer at airport 2.79E3
388 kg MPG to recycling -954
393
650 kg De-icer concentrate, at plant 2.64E3
277 kg Propylene glycol, liquid, at plant/RER U 1.12E3
222 kg Propylene oxide, liquid, at plant/RER U 992
4.71E3 MJ Electricity, medium voltage, production UCTE, at grid/UCTE 689
4.85E3 MJ Electricity, high voltage, production UCTE, at grid/UCTE 694
692
4.9E3 MJ Electricity, production mix UCTE/UCTE U
Fig. 7. Offsite recycling, de-icing carbon footprint (kg CO2e/tonne 65% de-icer).
E.P. Johnson / Environmental Impact Assessment Review 32 (2012) 156–164
163
Table 6 Sensitivity results to the base case, in descending order of significance. Scenario
Description
Total footprint sensitivity
Variation in propylene glycol footprint
Propylene glycol can be produced by 3 processes, which also have some natural variation.
Variation in electricity footprint
National and regional power footprints vary, from nearly 0.8 kg CO2e/kWh in the US to 0.01 in Norway.
Variation in the recycling rate
More de-icer goes to wastewater
The maximum estimated downward variation in the propylene glycol footprint is about one-third lower than the base case. Compared to no recycling (for the base case, see Table 4), offsite recycling still cuts the footprint by 34% and onsite by 50%. In areas with higher power footprints the gap between ‘no recycling’ and either recycling case narrows, while in areas with lower power footprints it widens. Not enough, however, to change the fundamental relationship of the cases. If the base case recycling rate, 60%, is halved to 30%, offsite recycling still has a significantly lower footprint.
Table 7 Carbon footprints of propylene glycol (MPG) production. Process
MPG, cradle-to-gate Propylene oxide, conversion only, chlorohydrins process MPG, conversion only Comment
Carbon footprint
Comment
Ecoinvent
SRI Consulting
Du Pont/Tate & Lyle
4.06 3.36
NA, but inferred to be 2.7 2.22
3.75
0.48 Chlorohydrin process, European production
NA Chlorohydrin process, US production
5. Sensitivity analysis Three main sensitivities were identified and tested (Table 6). None changes the fundamental findings of the base case. 5.1. Variation in the propylene glycol footprint There are two types of variation in the MPG footprint. One is variation by process route, i.e. chlorohydrins versus peroxidation or hydroperoxidation (see Section 3.2.1.1). This study's model is based on chlorohydrins, because that is the only process for which a full, public dataset is available. One other footprint for MPG has been published by Kilfrost and DuPont Tate and Lyle BioProducts (2009). This is more a data point than dataset: the value is 3.75 kg CO2e/kg MPG. It appears to be an average value for US production, which consists of approximately 40% chlorohydrins-based and 60% peroxidation-based MPG (SRI Consulting, 2009b). If so, then the imputed footprint for peroxidation-based MPG is about 3.5 kg CO2e/kg MPG. The other variation is within estimates for the chlorohydrins process itself. One other public estimate is known. This is from (SRI Consulting, 2009a, p A-209) is, however, not from cradle-to-gate, but for only part of the process: for the conversion from propylene to propylene oxide. SRI presents neither the build-up to propylene nor the conversion from propylene oxide to propylene glycol (Table 7). A rough estimate of SRI's cradle-to-gate footprint is 2.7 kg CO2e/kg MPG 4. So it would appear that the biggest downward variation in MPG's footprint would be as low as 2.7 kg CO2e/kg MPG, which is one-third lower than the base case figure of 4.06 kg CO2e/kg MPG (Table 7). What would this do to the overall results? Plugging this lower MPG footprint into the base case model, the recycling benefit declines, but is still significant. Compared to no recycling (for the base case, see Table 4), offsite recycling still cuts the footprint by 34% and onsite by 50%. 5.2. Variation in electricity footprints, for example: Norway The base case assumes power from a continental Western European mix (see Section 3.6), where 1 kWh of this so-called UCTE power generates a footprint of 0.523 CO2e. 4
That is, 2.22 + 0.48 = 2.7.
Ecoinvent assumes higher power input and much higher fuel input than SRI Unclear what process, probably US production
Of course national and regional footprints vary. However, if the entire operation were moved to various locations in Europe or even the US (Table 8), the basic findings of the comparison remain unchanged. As expected, in areas with higher power footprints the gap between ‘no recycling’ and either recycling case narrows, while in areas with lower power footprints it widens.
5.3. Variation in the recycling rate What if the process routes (see Section 3.5.1) were different, i.e. if considerably more de-icer than the base case scenario of 5% (Table 2) went to wastewater treatment, rather than recycling? Obviously, the footprint of the recycling case would increase (Table 9). The inverse relationship between decrease in recycling and increase in footprint (Table 9) is more linear in the life-cycle model than it would be in reality. This is because the life-cycle model was defined on a unit basis at a 60% recycling rate. Nonetheless, it is clear that even a considerable drop in recycling rate would not change the basic finding: recycling generates a lower footprint than not recycling. The author hypothesises that a more-relevant limit here is economics. Too-small volumes of captured de-icer – even if recycled offsite – would not justify the cost of recycling in the first place. This limit is difficult to define precisely: it will depend on a specific airport's cost base, Table 8 De-icing footprint sensitivity to national/regional electricity footprint. Country or region
United States The Netherlands Germany Italy UK UCTE, Western Europea Spain France Sweden Norway a
Power footprint, kg CO2e/kWh
0.76 0.693 0.671 0.642 0.606 0.523 0.517 0.09 0.0405 0.011
This is the base case, also shown in Table 4.
Recycling case No recycling
Offsite recycling
Onsite recycling
3924 3922 3921 3921 3920 3917 3917 3905 3903 3902
2400 2368 2358 2344 2327 2288 2285 2083 2060 2046
1966 1936 1925 1912 1896 1857 1855 1658 1635 1622
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Table 9 De-icing footprint sensitivity to recycling rate. Case
Recycling rate
Total
% reduction versus no recycling
Kg CO2e/tonne 65% de-icer No recycle Offsite recycle
3917 2288 3103 3374 3646
60% 30% 20% 10%
0% − 42% − 21% − 14% − 7%
Table 10 Footprint of airport de-icing, bio de-icer versus recycling (kg CO2e/tonne 65% de-icer). Case
Total Of which % reduction to bio footprint de-icer, no recycling De-icer MPG to MPG to at airport environment wastewater
Bio de-icer, 2704 no recycling Offsite recycle 2288 Onsite recycle 1857
1579
393
732
0%
1838 1408
393 393
56 56
− 15% − 31%
neutral’ assumption about biofuels. Nonetheless, to compare bio deicer to recycling of conventional de-icer, the 2.18 kg CO2e/kg Susterra® has been substituted into this study's model, assuming that the bioproduct can replace the conventional product one for one. The result (Table 10) is that, even with the questionably low footprint of Susterra®, recycling either offsite or onsite generates a significantly lower footprint than using a bio de-icer without recycling. 7. Are the findings valid for ethylene glycol? Ethylene glycol is also used as a de-icer. Although that system was not modelled in this study, a rough estimate of recycling versus no recycling is possible, simply by substituting ethylene glycol for propylene glycol in the carbon-footprint model. Doing so – using ecoinvent values for ethylene glycol – gives a very similar result to that of propylene glycol: 47% footprint reduction with offsite recycling, and a 55% reduction with onsite recycling. There is another incentive for recycling ethylene glycol de-icers that does not apply to propylene glycol. Unlike propylene glycol, ethylene glycol is more problematic in the environment and in wastewater treatment plants (Nitschke et al., 1996). References
and particularly on its availability of land for capture and recycling operations. 6. Comparison to bio de-icer How does recycling conventional de-icer compare to using ‘bio’ de-icer, i.e. fluid made not from petrochemicals but from plants? A bio de-icer is commercially available. The glycol – sold under the trade name of Susterra® – has the same chemical composition as propylene glycol, C3O2H8, but rather than its isomer being 1,2 propanediol, instead it is 1,3 propanediol. Its manufacturers have estimated the ‘cradle-to-gate’ or manufacturing footprint to be 2.18 kg CO2e/kg Susterra® (Kilfrost and DuPont Tate and Lyle BioProducts, 2009). This is nearly half the 4.06 kg CO2e/kg of conventional propylene glycol, used in the base case of this study. It appears that the difference is almost entirely based on the use of renewable energy in the Susterra® production process, and that this renewable energy is assumed to be carbon-neutral. Increasing numbers of researchers, e.g. (Johnson, 2009) or (McKechnie et al., 2011), are rejecting this ‘inherently carbon
Berkhout F, Howes R, Johnson E, Smith D. Adoption by Industry of Life Cycle Approaches: Its Implications for Industry Competitiveness and Trade. Kogan Page; 1999. BSI, Carbon Trust & UK DEFRA (2008) PAS 2050. Specification for the assessment of the life cycle greenhouse gas emissions of goods and services. Publicly available specification 2050:2008. ecoinvent. LCI database. Switzerland: St Gallen; 2010. ISO. 14044: environmental management — life cycle assessment — requirements and guidelines; 2006. Johnson E. Goodbye to carbon neutral: getting biomass footprints right. Environ Impact Assessment Rev 2009;29:165–8. Johnson E, Arne M. Reduce, reuse, don't recycle? Chem Ind 2010:16–7. Kilfrost & DuPont Tate & Lyle BioProducts. Eco-friendly de-icing fluid DFsustain™ & ABC4sustain™. Airline and airport deicing management conference; 2009. Cincinnati, Ohio, USA. McKechnie J, Colombo S, Chen J, Mabee W, MacLean HL. Forest bioenergy or forest carbon? Assessing trade-offs in greenhouse gas mitigation with wood-based fuels. Environ Sci Technol 2011;45:789–95. Nitschke L, Wagner H, Metzner G, Wilk A, Huber L. Biological treatment of waste water containing glycols from de-icing agents. Water Res 1996;30:644–8. SRI Consulting. In: Arne M, editor. Greenhouse gases handbook; 2009a. SRI Consulting. Propylene oxide. CEH Marketing Research Report; 2009b.
Contents lists available at SciVerse ScienceDirect
Environmental Impact Assessment Review journal homepage: www.elsevier.com/locate/eiar
Aircraft de-icer: Recycling can cut carbon emissions in half Eric P. Johnson ⁎ Atlantic Consulting, Obstgartenstrasse 14, 8136 Gattikon, Switzerland
a r t i c l e
i n f o
Article history: Received 30 June 2011 Received in revised form 6 August 2011 Accepted 6 August 2011 Available online 17 September 2011 Keywords: Recycling Aircraft de-icer Carbon footprint Propylene glycol
a b s t r a c t Flight-safety regulations in most countries require aircraft to be ice-free upon takeoff. In icy weather, this means that the aircraft usually must be de-iced (existing ice is removed) and sometimes anti-iced (to protect against ice-reformation). For both processes, aircraft typically are sprayed with an ‘antifreeze’ solution, consisting mainly of glycol diluted with water. This de/anti-icing1 creates an impact on the environment, of which environmental regulators have grown increasingly conscious. The US Environmental Protection Agency (EPA), for example, recently introduced stricter rules that require airports above minimum size to collect deicing effluents and send them to wastewater treatment. De-icer collection and treatment is already done at most major airports, but a few have gone one step further: rather than putting the effluent to wastewater, they recycle it. This study examines the carbon savings that can be achieved by recycling de-icer. There are two key findings. One, recycling, as opposed to not recycling, cuts the footprint of aircraft de-icing by 40–50% — and even more, in regions where electricity-generation is cleaner. Two, recycling petrochemical-based de-icer generates a 15–30% lower footprint than using ‘bio’ de-icer without recycling. © 2011 Elsevier Inc. All rights reserved.
1. Introduction De-icing and anti-icing are common procedures in colder-weather airports. Both aircraft and runways are sprayed to keep them ice-free for takeoffs and landings. With current economics and technologies, it would not make sense to recover runway de-icers, but economics and technologies to recycle aircraft de-icers already exist. Airports in Munich and Zurich have been recycling aircraft de-icer for some years now. Oslo plans to join the recycling club, and other airports are known to be considering a similar move. The question addressed by this study is: by recycling de-icer rather than discharging it to wastewater treatment, what is the impact on the carbon footprint of de-icing? The system examined is conventional de-icing in Europe, the main ingredient of which is propylene glycol. The method is that of a life-cycle assessment or carbon footprint. This question – whether recycling is better than not recycling – is not a trivial one. Although for decades the mantra of ‘reduce, reuse, recycle’ has been chanted by numerous policy-makers, it has been refuted in specific cases, e.g. Berkhout et al. (1999) or Johnson and Arne (2010). The report is presented as such: after a brief text about the method, a definition of the de-icing system is presented, followed by the
⁎ Tel.: + 41 44 772 1079. E-mail address: [email protected]. 1 For simplicity's sake, hereafter, de-icing and anti-icing are referred to as de-icing, unless explicitly stated otherwise. 0195-9255/$ – see front matter © 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.eiar.2011.08.001
base case footprint, a sensitivity analysis, a comparison of recycling and use of ‘bio’ de-icer, and a discussion of the applicability of the results to ethylene glycol, which is also used as a de-icer in some regions. 2. Method of this analysis The method of this study was to compile the carbon footprint of aircraft de-icing, with and without recycling, and then to compare the two. Scope of the analysis is ‘cradle-to-grave’, from production of the raw materials and energy used in de-icing on through to disposal of its components. All greenhouse gases, as defined by the Intergovernmental Panel on Climate Change (IPCC), are included. The method used in this study is believed to be consistent with current, global best-practice, and it is compliant with the current standard for carbon footprinting, Publicly Available Standard 2050 (BSI et al., 2008), commonly referred to as PAS 2050. Carbon footprints are a summation of the greenhouse-gas emissions of a product or service across its lifetime (or life cycle). A carbon footprint is a subset of a life-cycle assessment (LCA), which is a summation of all emissions of a product or service. The approach to footprinting is identical to that for LCA; the only difference is that in a carbon footprint, a smaller scope of emissions – i.e. greenhouse gases only – is covered. This study's method is also consistent with current guidance for LCA, such as ISO (2006). In line with current best-practice, future emissions in this analysis have not been discounted. All greenhouse gases emitted over the
E.P. Johnson / Environmental Impact Assessment Review 32 (2012) 156–164
lifetime of the heat pump are summed and multiplied by the same global warming potentials (GWPs); in effect, the emissions over time are treated mathematically as if they were a single pulse to the atmosphere.
Table 1 Composition of aircraft de-icer. Component Weight fraction of de-icer
Comment
Glycol
Concentration depends on type of de-icing application and on weather conditions
Additives
50–80% in Europe 50–88% in N America b1%
Water
11+% to 49+%
3. Definition of the aircraft de-icing system Flight-safety regulations in most countries require aircraft to be ice-free upon takeoff. In icy weather, this means that aircraft usually must be de-iced (existing ice is removed) and sometimes anti-iced (to protect against ice-reformation). For both processes, aircraft typically are sprayed with an ‘antifreeze’ solution, consisting mainly of glycol diluted with water. The life-cycle of aircraft de-icing (Fig. 1) starts with production and blending of the raw materials, follows with usage in de-icing and concludes with disposal. Disposal follows a mixture of three paths: wastewater treatment, recycling and dispersal to the environment. This mix depends mainly on the installed disposal facilities, which vary by airport. For airports that recycle, spent de-icer collected in drainage is typically discharged to wastewater when its glycol is at a low concentration that is uneconomic to recycle. Recycling can be conducted either onsite or offsite. The onsite/offsite choice rests mainly on three factors: • Economics, especially economies of scale — i.e. the volume of de-icer used versus the cost of recycling • Available space for a recycling plant • Anti-competition rules — to prevent contamination and potential product failure, an onsite plant requires a single source of de-icer supply. In some cases, this is not allowed under anti-competition rules. Aircraft de-icing is defined in seven parts: composition of the deicer; raw materials production; blending; use; disposal; electricity production; and transport. These are described in the following sections. 3.1. Composition of the de-icer De-icers consist of three components (Table 1): glycol, additives and water. In Western Europe and the US, the typical glycol used is propylene glycol, sometimes referred to as MPG (for mono-propylene glycol). In Canada and Russia, the more common choice is ethylene glycol. 3.2. Raw material production Production methods and sources of emission-data are reviewed for glycol, additives and water. 3.2.1. Glycol (propylene glycol) Propylene glycol (MPG) is the only existing de-icer currently recycled, so we have used this as the base case glycol in this analysis.
De-icer onsite blending
157
Primary functions of the additives are to: prevent foaming, keep fluid on the aircraft, inhibit corrosion, prevent combustion, buffer the solution (to prevent acid formation) and give colour to the de-icer. This can be added at the production site, at a regional blending plant and/or at the airport, with the aim being to minimise water transport.
Also we have placed this base case in Europe, where existing de-icing is located. Various process routes to MPG are possible; emissions data are available for only some of them. 3.2.1.1. Process routes. MPG production unfolds in four major steps (Fig. 2). Natural gas and crude oil are processed, creating gas liquids and naphtha, which are steam cracked to create, among other things, propylene. Refineries also generate propylene directly. Purified propylene is then converted to propylene oxide (PO) via one of three processes: peroxidation, chlorohydrins processing; or hydroperoxidation. PO is then hydrated to create propylene glycol. In principle, the molecules can proceed along any of the routes shown in this chain. And in fact, the route to propylene alone can be even more complicated than this (Fig. 3). In practice, however, nearly all the propylene that ends up as MPG comes through steam cracking, and in Europe, the majority feedstock for this is naphtha. For the propylene-to-PO step, capacity is split roughly 47/47 between peroxidation and chlorohydrin processing, with the remainder coming from hydroperoxidation. 3.2.1.2. Emissions: sources and data. For the base case we have used the only public, cradle-to-gate emissions data-source for MPG. This is (ecoinvent, 2010), which models a European petrochemical-production chain. It presumes a European-average steam cracker feeding propylene only to chlorohydrin processing (Fig. 2); peroxidation and hydroperoxidation are not covered. The ecoinvent footprint is 4.06 kg CO2e/kg MPG. 3.2.2. Additives A supplier of de-icers has provided us with a detailed description of typical de-icer additives on a confidential basis, and their functions on a non-confidential basis. For each of these, we have found a proxy carbon footprint in ecoinvent (2010), which has been used in the system model. 3.2.3. Water In the base case we have used industrial process water, as defined by ecoinvent. Its footprint is 0.8 kg CO2e/tonne of water. As noted in
Dispersal to the environment Glycol production Additives production Diluent water
Concentrate blending
Natural gas
Wastewater treatment De-icer blending
De-icing use
Drain
Recycling offsite
Recycling onsite
Gas processing
NGLs Propylene Steam cracking
Crude oil
Oil refining
Peroxidation or Chlorohydrin or
PO
Hydration
Hydroperoxidation
LPG Naphtha Propylene
Fig. 1. The life-cycle of aircraft de-icing.
Fig. 2. Propylene glycol (MPG) production chain.
MPG
158
E.P. Johnson / Environmental Impact Assessment Review 32 (2012) 156–164 Thermal-cracked propylene Fluid-catalytic-cracked propylene
Upgrading Ethylene Naphtha Crude oil
Refinery
Butylenes
Metathesis
Gas oil Steam Cracker Propylene Natural gas
Ethane
Gas processing
Propane
Propane dehydro Hydro treating
Vegetable oil
Fig. 3. Propylene production chain (with extra detail).
Table 1, this can be blended at the production site, at a blending site and/or at the airport itself.
3.3. Blending to de-icer concentrate (DIC) and to de-icer MPG is shipped 900 km by railcar from Dow's Stade plant to Clariant Gendorf for blending. De-icer is blended in two stages: first the MPG and additives are blended to create de-icer concentrate, which is shipped long-distance to a regional de-icing plant; at the regional de-icing plant, water is blended to bring the de-icer to its desired concentration. (For Type I de-icers – low viscosity liquids, typically used at 55–80 °C – hot water is also blended at the airport itself.) Regional de-icing plants are located at Gothenburg and Amsterdam, which respectively are 1126 km and 900 km distant from Gendorf. For the base case we have used a distance of 1000 km. For concentrate blending and filling to an IBC container, the power required per tonne of concentrate, as reported by a supplier, is 9.818 kWh.
De-icer onsite blending 35% of spent de-icer
Recycled de-icer
De-icing use
Dispersal to the environment
3.5. Disposal: discharge to wastewater and/or recycling Spent de-icer can be disposed of in four ways: dispersal to the environment, in wastewater treatment, by onsite and offsite recycling. These are described in the following sub-sections.
Table 2 Recycling base case, mass-balance and MPG concentrationsa.
Drain
Recycling onsite 60% of
Recycling offsite
To MPG
In use, aircraft de-icers are in some cases mixed with hot water and in all cases sprayed on to the aircraft. This involves transport, heating and pumping, which – due to a lack of data – have not been included in this analysis. However, because these inputs would be equal, whether or not the de-icer was recycled, this gap is believed not to dent the robustness of this study.
55-60% Glycol
Wastewater treatment
65% of spent de-icer
3.4. Use
3.5.1. Process routes After use spent de-icer either is collected in drainage ponds or tanks, or it is dispersed to the local environment (Fig. 4, Table 2). The amount collected varies, depending on drainage design and
5% of spent de-icer Make-up de-icer
The filled concentrate is shipped in a 22-tonne truck to the regional deicing plant. For de-icer blending, we estimate half as much power is required, i.e. 5 kWh/tonne. The final product is shipped by tank-truck to the airport; for that distance we have presumed a journey from Gothenburg to Oslo, i.e. 300 km.
spent de-icer to recycling
65% Glycol
Disposal route of MPG
Fraction (weight) of spent de-icer disposed this way
Concentration of MPG in this fraction
To the environment To wastewater treatment To recycling
35% 5%
NA b5%
60%
15% average
a
Fig. 4. De-icer recycling flow sheet (Data as reported by an airport that runs a recycling operation.).
Comment
Corresponds to TOC of about 2.5%b Corresponds to TOC of about 7.5%
Data as reported by an airport that runs a recycling operation. Pure MPG has a TOC (total organic content) of 47%. So we have applied a simple 1:2 ratio of TOC to MPG concentration. b
E.P. Johnson / Environmental Impact Assessment Review 32 (2012) 156–164 Table 3 Inputs to onsite recycling (per unit of diluted de-icera). Process
Power
Units
Losses of MPG
Ultrafiltration Ion exchange Distillation to 60% MPG Distillation to 65% MPG
15 1 0.1000 0.1015
kWh/m3 kWh/m3 kWh/l kWh/l
1 l/m3 0.072 l/m3 8.4 l/m3 8.4 l/m3
a
Assumed to consist of 10% MPG. This is lower than the 15% average reported above, to give a safety margin to the calculation.
spray operations. For airports without recycling, the collected effluent is discharged to wastewater treatment. For economical recycling, collected de-icer needs to have an MPG concentration above 5%, which corresponds to a TOC of about 2.5%. If below this, it is instead discharged to wastewater treatment. Concentration depends on the mix of de-icers being sprayed as well as the amount of local precipitation. In onsite recycling, spent de-icer is: filtered to remove particles and sediment; run through an ion exchanger to remove salts; and pH is adjusted. Then it is distilled to 55–60% (weight) glycol and sent for re-blending to finished de-icer. In offsite recycling, the spent de-icer is first concentrated in an evaporator to 65% glycol concentration. The 65% solution is then sent to an offsite recycling plant, where it is purified to 99.9% glycol via steam distillation and thin-film evaporation. From these figures we have estimated mass-balances and concentrations for the recycling base case (Table 2). Both recycling operations are considered closed-loop according to the (ISO, 2006) definition. For economic and commercial reasons, Munich and Zurich/Oslo approach de-icer recycling somewhat differently. Munich operates a closed loop, i.e. de-icer is recycled onsite back into its original use. By contrast, Zurich (and presently in Oslo) operate open loops, i.e. effluent rich in de-icer is sent to an offsite treatment plant that recovers pure glycol, which can be used in de-icer or other glycol applications. 3.5.2. Emissions: sources and data The four disposal routes are presented in the following subsections. 3.5.3. To the environment In this study, we assume that propylene glycol dispersed to the environment is fully biodegradable. This is based on a Screening Information Dataset published by the OECD, 2 which states: “Propylene glycol is not volatile, but is miscible with water. Air monitoring data are not available, but concentrations of propylene glycol in the atmosphere are expected to be extremely low because of its low vapor pressure and high water solubility. It is readily biodegraded in water or soil. Four studies reported ≥60% biodegradation in water in 10 days. PG is not expected to bioaccumulate, with a calculated BCF b 1. Measured freshwater aquatic toxicity data for fish, daphnia and algae report LC/EC50 values of N18,000 mg/l. Therefore, PG is not acutely toxic to aquatic organisms except at very high concentrations. Using an assessment factor of 100 and the Ceriodaphnia data (48-hour EC 50 = 18,340 mg/ l), the PNEC is 183 mg/l.” So we presume that each molecule of propylene glycol, C3O2H8, is converted to three molecules of CO2: i.e. per tonne of MPG, biodegradation creates 1.74 tonnes of carbon dioxide. 3.5.4. To wastewater treatment In the study we assume that propylene glycol to wastewater treatment also biodegrades fully. Again, we presume that each molecule of 2 SIDS Initial Assessment Report for 1,2-Dihydroxypropane. Available at http:// www.inchem.org/documents/sids/sids/57-55-6.pdf.
159
Table 4 Footprint of airport de-icing, recycling versus not recycling (kg CO2e/tonne 65% deicer). Case
No recycle Offsite recycle Onsite recycle
Total
3917 2288 1857
Of which De-icer at airport
MPG to environment
MPG to wastewater
2792 1838 1408
393 393 393
732 56 56
% reduction to no recycling
0% − 42% − 53%
Table 5 Contributions to airport de-icing footprint (kg CO2e/tonne 65% de-icer). Contribution to footprint
No recycling
Offsite recycling
Onsite recycling
Total MPG to wastewater Propylene, at plant/RER U MPG to environment Electricity UCTE Other contributions
3917 731 565 393 15 2213
2288 56 242 393 247 1349
1857 56 234 393 241 934
propylene glycol, C3O2H8, is converted to three molecules of CO2: i.e. per tonne of MPG, biodegradation creates 1.74 tonnes of carbon dioxide. For wastewater treatment, we have used inputs defined in ecoinvent for transport, electricity, fuel oil and natural gas to operate the plant itself. 3.5.5. To recycling As noted previously, MPG can be recycled onsite or offsite. Either way, the spent de-icer (average 15% MPG concentration) is first treated by ultrafiltration and ion exchange to remove impurities, and then it is concentrated by distillation to 60% or 65% MPG purity (Table 3). In offsite recycling, the 65%-MPG concentrate from onsite-treatment is transported by a 22-tonne truck to a recovery plant. There it requires 16 kWh and 1.72 tonnes of medium-pressure steam per tonne of 99.9% pure MPG product. For the distillation from 65% to 99.9%, losses of 4% MPG are assumed in the model. In both the onsite and offsite recycling, it is presumed that the distilled water (generated by purifying the MPG) is not used further. Because of its minor impurities, it is discharged to wastewater treatment as well. 3.6. Electricity production In the base case, power throughout the production-recycling chain is assumed to come from a continental Western European mix, socalled UCTE 3 electricity. The UCTE mix is an average for Western Europe (west of the former Soviet Union, excluding the British Isles and the Nordics except for the western half of Denmark). The footprint for 1 kWh of UCTE power is 0.523 CO2e. 3.7. Transport For road and rail transport, the model uses standard modules from ecoinvent. 4. Base case footprint Using the definitions from the previous chapter, footprints of the system were calculated using SimaPro 7 software. The normalisation factor, or functional unit, is 1 tonne of de-icer at the airport with an average 65% concentration of propylene glycol.
3
Union for the Co-ordination of Transmission of Electricity.
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E.P. Johnson / Environmental Impact Assessment Review 32 (2012) 156–164
1p De-icer life cycle, NO recycling 3.92E3
1E3 kg De-icer at airport 2.79E3
650 kg De-icer concentrate, at plant 2.64E3
646 kg Propylene glycol, liquid, at plant/RER U 2.62E3
519 kg Propylene oxide, liquid, at plant/RER U 2.31E3
1.09E4 MJ Electricity, medium voltage, production UCTE, at 1.59E3
1.12E4 MJ Electricity, high voltage, production UCTE, at 1.6E3
1.13E4 MJ Electricity, production mix UCTE/UCTE U 1.6E3 Fig. 5. No recycling, de-icing carbon footprint (kg CO2e/tonne 65% de-icer).
E.P. Johnson / Environmental Impact Assessment Review 32 (2012) 156–164
The ‘no recycle’ case shows a much higher footprint that recycling either on or offsite (Table 4). Compared to a ‘no recycle’ footprint of 3,917 kg CO2e/tonne of de-icer, recycling offsite drops that by 42% to 2,288 kg CO2e/tonne, while recycling onsite lowers it 53% to 1857 kg CO2e/tonne.
The recycling reduction comes down to two factors: • Displacement: recycling at these concentrations generates a considerably lower footprint than does manufacturing fresh MPG — this accounts for about two-thirds of the difference to the ‘no recycling’ case.
1p De-icer life cycle, Recycling 1.86E3
413 kg De-icer at airport
226 kg MPG to environment 393
1.15E3
388 kg MPG to recycling -1.38E3
269 kg De-icer concentrate, at plant 1.09E3
267 kg Propylene glycol, liquid, at plant/RER U 1.08E3
214 kg Propylene oxide, liquid, at plant/RER U 956
161
4.49E3 MJ Electricity, medium voltage, production UCTE, at grid/UCTE U 659
4.63E3 MJ Electricity, high voltage, production UCTE, at grid/UCTE U 663
4.68E3 MJ Electricity, production mix UCTE/UCTE U 661 Fig. 6. Onsite recycling, de-icing carbon footprint (kg CO2e/tonne 65% de-icer).
162
E.P. Johnson / Environmental Impact Assessment Review 32 (2012) 156–164
• Avoidance: recycling also avoids significant emissions of MPG to wastewater, which contribute considerable to the ‘no recycling’ footprint — this accounts for about one-third of the difference to the ‘no recycling’ case. The difference between the two recycling approaches is simply a trade-off between energy and yield. Although distilling to 99.9% MPG generates a more valuable product, it requires proportionately more fuel and power, mainly for distillation but also more for transport.
The differences also can be seen by comparing the major contributors to each footprint (Table 5). In the ‘no recycling’ case, MPG to wastewater is the single biggest contributor, whereas in the recycling cases it is reduced dramatically. Much less propylene is needed in the recycling cases, thanks to displacement, but this is roughly offset by increased demand for electricity in recycling. Meanwhile, MPG to the environment stays constant in all cases. Graphic representations of the base case footprints are presented (Fig. 5, Fig. 6, Fig. 7) as supplementary material for interested readers.
1p De-icer life cycle, Recycling 2.29E3
226 kg MPG to environment
1E3 kg De-icer at airport 2.79E3
388 kg MPG to recycling -954
393
650 kg De-icer concentrate, at plant 2.64E3
277 kg Propylene glycol, liquid, at plant/RER U 1.12E3
222 kg Propylene oxide, liquid, at plant/RER U 992
4.71E3 MJ Electricity, medium voltage, production UCTE, at grid/UCTE 689
4.85E3 MJ Electricity, high voltage, production UCTE, at grid/UCTE 694
692
4.9E3 MJ Electricity, production mix UCTE/UCTE U
Fig. 7. Offsite recycling, de-icing carbon footprint (kg CO2e/tonne 65% de-icer).
E.P. Johnson / Environmental Impact Assessment Review 32 (2012) 156–164
163
Table 6 Sensitivity results to the base case, in descending order of significance. Scenario
Description
Total footprint sensitivity
Variation in propylene glycol footprint
Propylene glycol can be produced by 3 processes, which also have some natural variation.
Variation in electricity footprint
National and regional power footprints vary, from nearly 0.8 kg CO2e/kWh in the US to 0.01 in Norway.
Variation in the recycling rate
More de-icer goes to wastewater
The maximum estimated downward variation in the propylene glycol footprint is about one-third lower than the base case. Compared to no recycling (for the base case, see Table 4), offsite recycling still cuts the footprint by 34% and onsite by 50%. In areas with higher power footprints the gap between ‘no recycling’ and either recycling case narrows, while in areas with lower power footprints it widens. Not enough, however, to change the fundamental relationship of the cases. If the base case recycling rate, 60%, is halved to 30%, offsite recycling still has a significantly lower footprint.
Table 7 Carbon footprints of propylene glycol (MPG) production. Process
MPG, cradle-to-gate Propylene oxide, conversion only, chlorohydrins process MPG, conversion only Comment
Carbon footprint
Comment
Ecoinvent
SRI Consulting
Du Pont/Tate & Lyle
4.06 3.36
NA, but inferred to be 2.7 2.22
3.75
0.48 Chlorohydrin process, European production
NA Chlorohydrin process, US production
5. Sensitivity analysis Three main sensitivities were identified and tested (Table 6). None changes the fundamental findings of the base case. 5.1. Variation in the propylene glycol footprint There are two types of variation in the MPG footprint. One is variation by process route, i.e. chlorohydrins versus peroxidation or hydroperoxidation (see Section 3.2.1.1). This study's model is based on chlorohydrins, because that is the only process for which a full, public dataset is available. One other footprint for MPG has been published by Kilfrost and DuPont Tate and Lyle BioProducts (2009). This is more a data point than dataset: the value is 3.75 kg CO2e/kg MPG. It appears to be an average value for US production, which consists of approximately 40% chlorohydrins-based and 60% peroxidation-based MPG (SRI Consulting, 2009b). If so, then the imputed footprint for peroxidation-based MPG is about 3.5 kg CO2e/kg MPG. The other variation is within estimates for the chlorohydrins process itself. One other public estimate is known. This is from (SRI Consulting, 2009a, p A-209) is, however, not from cradle-to-gate, but for only part of the process: for the conversion from propylene to propylene oxide. SRI presents neither the build-up to propylene nor the conversion from propylene oxide to propylene glycol (Table 7). A rough estimate of SRI's cradle-to-gate footprint is 2.7 kg CO2e/kg MPG 4. So it would appear that the biggest downward variation in MPG's footprint would be as low as 2.7 kg CO2e/kg MPG, which is one-third lower than the base case figure of 4.06 kg CO2e/kg MPG (Table 7). What would this do to the overall results? Plugging this lower MPG footprint into the base case model, the recycling benefit declines, but is still significant. Compared to no recycling (for the base case, see Table 4), offsite recycling still cuts the footprint by 34% and onsite by 50%. 5.2. Variation in electricity footprints, for example: Norway The base case assumes power from a continental Western European mix (see Section 3.6), where 1 kWh of this so-called UCTE power generates a footprint of 0.523 CO2e. 4
That is, 2.22 + 0.48 = 2.7.
Ecoinvent assumes higher power input and much higher fuel input than SRI Unclear what process, probably US production
Of course national and regional footprints vary. However, if the entire operation were moved to various locations in Europe or even the US (Table 8), the basic findings of the comparison remain unchanged. As expected, in areas with higher power footprints the gap between ‘no recycling’ and either recycling case narrows, while in areas with lower power footprints it widens.
5.3. Variation in the recycling rate What if the process routes (see Section 3.5.1) were different, i.e. if considerably more de-icer than the base case scenario of 5% (Table 2) went to wastewater treatment, rather than recycling? Obviously, the footprint of the recycling case would increase (Table 9). The inverse relationship between decrease in recycling and increase in footprint (Table 9) is more linear in the life-cycle model than it would be in reality. This is because the life-cycle model was defined on a unit basis at a 60% recycling rate. Nonetheless, it is clear that even a considerable drop in recycling rate would not change the basic finding: recycling generates a lower footprint than not recycling. The author hypothesises that a more-relevant limit here is economics. Too-small volumes of captured de-icer – even if recycled offsite – would not justify the cost of recycling in the first place. This limit is difficult to define precisely: it will depend on a specific airport's cost base, Table 8 De-icing footprint sensitivity to national/regional electricity footprint. Country or region
United States The Netherlands Germany Italy UK UCTE, Western Europea Spain France Sweden Norway a
Power footprint, kg CO2e/kWh
0.76 0.693 0.671 0.642 0.606 0.523 0.517 0.09 0.0405 0.011
This is the base case, also shown in Table 4.
Recycling case No recycling
Offsite recycling
Onsite recycling
3924 3922 3921 3921 3920 3917 3917 3905 3903 3902
2400 2368 2358 2344 2327 2288 2285 2083 2060 2046
1966 1936 1925 1912 1896 1857 1855 1658 1635 1622
164
E.P. Johnson / Environmental Impact Assessment Review 32 (2012) 156–164
Table 9 De-icing footprint sensitivity to recycling rate. Case
Recycling rate
Total
% reduction versus no recycling
Kg CO2e/tonne 65% de-icer No recycle Offsite recycle
3917 2288 3103 3374 3646
60% 30% 20% 10%
0% − 42% − 21% − 14% − 7%
Table 10 Footprint of airport de-icing, bio de-icer versus recycling (kg CO2e/tonne 65% de-icer). Case
Total Of which % reduction to bio footprint de-icer, no recycling De-icer MPG to MPG to at airport environment wastewater
Bio de-icer, 2704 no recycling Offsite recycle 2288 Onsite recycle 1857
1579
393
732
0%
1838 1408
393 393
56 56
− 15% − 31%
neutral’ assumption about biofuels. Nonetheless, to compare bio deicer to recycling of conventional de-icer, the 2.18 kg CO2e/kg Susterra® has been substituted into this study's model, assuming that the bioproduct can replace the conventional product one for one. The result (Table 10) is that, even with the questionably low footprint of Susterra®, recycling either offsite or onsite generates a significantly lower footprint than using a bio de-icer without recycling. 7. Are the findings valid for ethylene glycol? Ethylene glycol is also used as a de-icer. Although that system was not modelled in this study, a rough estimate of recycling versus no recycling is possible, simply by substituting ethylene glycol for propylene glycol in the carbon-footprint model. Doing so – using ecoinvent values for ethylene glycol – gives a very similar result to that of propylene glycol: 47% footprint reduction with offsite recycling, and a 55% reduction with onsite recycling. There is another incentive for recycling ethylene glycol de-icers that does not apply to propylene glycol. Unlike propylene glycol, ethylene glycol is more problematic in the environment and in wastewater treatment plants (Nitschke et al., 1996). References
and particularly on its availability of land for capture and recycling operations. 6. Comparison to bio de-icer How does recycling conventional de-icer compare to using ‘bio’ de-icer, i.e. fluid made not from petrochemicals but from plants? A bio de-icer is commercially available. The glycol – sold under the trade name of Susterra® – has the same chemical composition as propylene glycol, C3O2H8, but rather than its isomer being 1,2 propanediol, instead it is 1,3 propanediol. Its manufacturers have estimated the ‘cradle-to-gate’ or manufacturing footprint to be 2.18 kg CO2e/kg Susterra® (Kilfrost and DuPont Tate and Lyle BioProducts, 2009). This is nearly half the 4.06 kg CO2e/kg of conventional propylene glycol, used in the base case of this study. It appears that the difference is almost entirely based on the use of renewable energy in the Susterra® production process, and that this renewable energy is assumed to be carbon-neutral. Increasing numbers of researchers, e.g. (Johnson, 2009) or (McKechnie et al., 2011), are rejecting this ‘inherently carbon
Berkhout F, Howes R, Johnson E, Smith D. Adoption by Industry of Life Cycle Approaches: Its Implications for Industry Competitiveness and Trade. Kogan Page; 1999. BSI, Carbon Trust & UK DEFRA (2008) PAS 2050. Specification for the assessment of the life cycle greenhouse gas emissions of goods and services. Publicly available specification 2050:2008. ecoinvent. LCI database. Switzerland: St Gallen; 2010. ISO. 14044: environmental management — life cycle assessment — requirements and guidelines; 2006. Johnson E. Goodbye to carbon neutral: getting biomass footprints right. Environ Impact Assessment Rev 2009;29:165–8. Johnson E, Arne M. Reduce, reuse, don't recycle? Chem Ind 2010:16–7. Kilfrost & DuPont Tate & Lyle BioProducts. Eco-friendly de-icing fluid DFsustain™ & ABC4sustain™. Airline and airport deicing management conference; 2009. Cincinnati, Ohio, USA. McKechnie J, Colombo S, Chen J, Mabee W, MacLean HL. Forest bioenergy or forest carbon? Assessing trade-offs in greenhouse gas mitigation with wood-based fuels. Environ Sci Technol 2011;45:789–95. Nitschke L, Wagner H, Metzner G, Wilk A, Huber L. Biological treatment of waste water containing glycols from de-icing agents. Water Res 1996;30:644–8. SRI Consulting. In: Arne M, editor. Greenhouse gases handbook; 2009a. SRI Consulting. Propylene oxide. CEH Marketing Research Report; 2009b.