Prakash
Prashanth
,
Raymond L.
Speth
,
Sebastian D.
Eastham
,
Jayant S.
Sabnis
and
Steven R. H.
Barrett
*
Laboratory for Aviation and the Environment, Department of Aeronautics and Astronautics, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA. E-mail: sbarrett@mit.edu
First published on 7th December 2020
Emissions of nitrogen oxides (NOx) from aircraft cause air quality degradation and climate change. Efforts to improve the efficiency of aircraft propulsion systems are leading to small, power-dense engine cores with higher overall pressure ratios and combustion temperatures, which can result in higher NOx emissions. The trend towards smaller engine cores with smaller mass flow rates in the core stream, presents new opportunities for emissions control. Specifically, we propose and assess using a selective catalytic reduction (SCR) system that was previously infeasible when mass flow rates in the core were an order of magnitude larger than heavy-duty diesel engines for road based applications. SCR systems would reduce NOx emissions at the cost of increased aircraft weight and specific fuel consumption due to the pressure drop in the core stream induced by the catalyst. We quantify the effects of these trade-offs in terms of emissions reduction and fuel burn increase using representative engine cycle models provided by a major aero-gas turbine manufacturer. Due to its size, any SCR system will likely need to be housed in the aircraft body, potentially making it most suitable for future hybrid- or turbo-electric aircraft designs. Furthermore, SCR systems require ultra-low sulfur (ULS) fuel to prevent catalytic fouling. We find that employing an ammonia-based SCR results in an approximately 95% reduction in NOx emissions in exchange for a ∼0.5% increase in block fuel burn. The performance of the post-combustion emissions control (PCEC) system is shown to improve for smaller-core engines, such as those proposed in the NASA N + 3 time-line (2030–2035). Using a global chemistry-transport model we estimate that PCEC used with ULS fuel, could avert ∼92% of aviation air pollution related early deaths each year. Using a simplified climate model and accounting for changes in emissions (including life cycle emissions) and radiative forcing we estimate that PCEC with ULS fuel increases climate damages by ∼7.5%. We estimate that the net benefit of using PCEC accounting for air quality and climate impacts is 304 USD (2015) per metric tonne of jet fuel burned, or a reduction of ∼52% in monetized air quality and climate damages.
Broader contextEmissions of nitrogen-oxides (NOx) from the aviation industry have an impact on global climate change and air quality. It is well documented that NOx is a precursor to fine particulate matter and ozone, which have an adverse impact on human health. The continued growth of the aviation industry will further increase the absolute and relative contribution of aviation emissions to global pollution. Moreover, the current techniques used to reduce NOx emissions from aero-gas turbine engines are approaching their limit. Leveraging the trends in aircraft engine design and novel aircraft configurations such as turbo-electric designs, our work is the first proposal and assessment of post-combustion emissions control methods for aircraft gas turbine engines for a future commercial aircraft. Our findings indicate that using post-combustion emissions control can virtually eliminate aviation related air-quality damages at the cost of small increase in aviation climate impacts. While detailed investigations of various aspects and implications of post-combustion emissions control need to be undertaken, this work opens up a new area of study in the design of the next generation of aircraft and maybe a step towards the sustainable development of the aviation industry. |
In the commercial aviation sector, gas turbines have been the primary choice of power plant since the early 1950s6 due to their high power density (relative to reciprocating engines) and suitability for high subsonic speeds. The thermodynamic efficiency of the gas turbine increases with higher overall pressure ratio (OPR). A higher OPR leads to increased thermal NOx production as the compressor exit temperature increases with OPR.7 Various combustor design strategies such as RQL (rich-quench-lean) combustion chambers have provided ∼50% reduction in NOx emissions compared to annular combustors8 but their effectiveness decreases as OPR of the engines increase.9 We propose that post-combustion treatment of the NOx emissions could offer a solution by eliminating >90% NOx emissions. It may also expand the design space for new engine architectures by partly decoupling combustor design from NOx control.
Prior to 1991, diesel engines in automobiles in the United States (US) did not require after-treatment and the average engine out NOx emissions were 4.6 g kW−1 h−1. By 2013 emissions regulations required all on-road engines in the US to use after-treatment measures to control emissions. The average NOx emissions from diesel engines was reduced to 0.27 g kW−1 h−111 over 20 years using SCR. This corresponds to approximately a 94% reduction in NOx emissions. SCR systems in modern engines remove 95% to 98%12 of NOx across the catalyst.
4NO + 4NH3 + O2 → 4N2 + 6H2O |
NO + NO2 + 2NH3 → 2N2 + 3H2O |
Greater than 90% of NOx emissions from typical diesel engines (and gas turbines) consists of NO.13 Since gas turbine emissions are also predominantly NO (approximately 95%),15 except at low thrust conditions16 as used in approach and taxi operations, the first of the two reactions is the primary reaction for deNOx (conversion of NOx to N2 and H2O) with ammonia.13
Today the core size of the engine is becoming smaller in new engine architectures such as the Pratt and Whitney geared turbofan and proposed small core engines.26 The smaller, power dense core implies that a smaller mass of exhaust gas needs to be treated for a fixed engine thrust. This reduces the impact of a pressure drop in the core stream on the engine SFC. Furthermore, these cores contribute little to the overall engine thrust. For example, approximately 8.0% of the gross thrust in the modeled geared turbofan comes from the core exhaust and we estimate that for a small core engine as described by Lord et al.26 the core flow will contribute 3.6% of the gross thrust.
Approaches designed to improve engine efficiency such as increased pressure ratios also increase NOx emissions. Present low-NOx combustor designs, which attempt to change the flame structure within the combustor to reduce residence time in high temperature regions9 will be less effective as the OPR increases.9 Post-combustion emissions control could provide an alternative approach.
Efforts to improve the overall efficiency of the aircraft have led to novel architectures and configurations. For example propulsion, airframe integration, distributed propulsion, turbo-/hybrid-electric propulsion, and boundary layer ingestion. The work done in these areas have been primarily aimed at improving the system level efficiency of the aircraft. These changes in configuration also present a new opportunity to implement an SCR based system to reduce the NOx emissions from the engine. For example, an SCR based system could be used in a turbo-/hybrid-electric aircraft with fuselage embedded gas turbines, or mechanical transmission in other configurations.
This work quantifies the additional fuel burn (which is proportional to CO2 emissions) incurred as a function of NOx reduction relative to a baseline design. We evaluate the environmental costs and benefits of lower NOx and increased CO2 emissions by quantifying air quality and climate impacts. We include the life cycle emissions of CO2 for the fuel (accounting for the desulfurization process) and ammonia (for SCR based post-combustion emissions control) in the analysis. The uncertainties in the analysis are propagated using a Monte Carlo approach, where feasible.
In this section we describe a lumped parameter model of the monolithic reactor. Tronconi and Forzatti28 showed the adequacy of lumped parameter models for simulating SCR reactors, finding an average error between experiments and the lumped one-dimensional model of 1.3%. In this model, average values of velocity and non-dimensional species concentration over the channel cross-section are used. The non-dimensional NO concentration is represented by Γ = [NO]/[NO]0, where [NO] is the local concentration of NO and [NO]0 is the concentration of NO at the inlet to the catalyst channel.
Based on the work done by Tronconi and Forzatti28 we can express the efficiency of the catalyst in removing NOx in the exhaust (deNOx) as
(1) |
Kin = −0.415 × OFA + 1.08 |
Kout = (1 − OFA)2, |
The implications for three engines were assessed, a representative turbofan (110 kN (25000 lbf) thrust class), a geared turbofan for the same thrust class and a small core engine (58 kN (13000 lbf) thrust class). The lower thrust of the small core engine reflects the lift to drag ratio (L/D ≈ 20)30 benefits from future airframes.
The effect of the pressure drop through the catalyst is modeled by varying the turbine exit duct pressure loss in a series of calculations. GasTurb was run iteratively such that the engine produces the same design point thrust for each turbine exit duct pressure drop by adjusting the combustor exit temperature. This corresponds to increasing the fuel flow rate and hence the SFC. The increase in the maximum landing mass and SFC, is used to calculate the percentage increase in fuel burn from eqn (2) as described in Section 2.4.
We size the catalyst by first considering effective bulk dimensions as shown in Fig. 2. The catalyst for this purpose is characterized by three parameters – the catalyst substrate, total frontal area (A) of the catalyst and the reacting length (l) of each channel in the catalyst. The catalyst substrate sets the hydraulic diameter of each channel, the bulk density and the open frontal area (OFA) of the catalyst. The total frontal area, A sets the local velocity of the flow in each channel by continuity and the reacting length of the channel sets the residence time of the exhaust gases within the catalyst.
Fig. 2 Bulk effective dimensions of the lumped catalyst model. Frontal area is defined as the area perpendicular to the flow through the catalyst. |
The above three parameters also indirectly affect the SFC of the engine. Once values are chosen for the substrate, flow through area and the reacting length we compute the pressure drop and the NOx conversion fraction. The pressure drop and additional weight is then used to calculate the increase in fuel burn from the baseline case (where no after-treatment is used and no additional weight is carried).
(2) |
The baseline impact of aviation on radiative forcing and surface air quality is determined by performing two GEOS-Chem simulations, one with and one without aviation emissions for 2015, such that the differences in atmospheric composition between the two cases (after a spin up period of one year) are attributable to baseline aviation emissions. Similarly, the impact of post-combustion emissions control (PCEC) and ultra-low sulfur fuel (ULS) is the difference in atmospheric composition between simulations where aviation emissions are at their baseline values and simulations where the aviation emissions have been adjusted for a comparison scenario of fleet-wide use of PCEC with ULS fuel. The emissions are obtained by scaling down aviation NOx emissions and introducing ammonia emissions (NH3) to capture the effect of ammonia slip (any ammonia that remains un-reacted downstream of the catalyst) when PCEC is used. The fleet-wide application is not intended to be representative of an introduction scenario, but as with comparable analyses22 to enable calculation of a representative average of the environmental impacts of PCEC. The effect of ULS fuel is modeled by reducing the fuel sulfur content from 600 ppm (typical jet fuel) to 15 ppm. The CO2 emissions from the life-cycle of the fuel and ammonia (in the PCEC scenario) are also considered in the analysis.
The anthropogenic, biogenic, and natural emissions inventories in GEOS-Chem used for all scenarios are shown in Table 2. However, we note that the marginal benefits of NOx reduction from aviation may be higher if a future cleaner atmosphere were used as the background.35,36 Details of the aviation emissions inventory for each scenario considered, including life-cycle emissions are provided in Table 3. In the simulation year (2015) aviation emissions accounted for 2.1% of the global NOx emissions from all sources compared to ∼12% from lightning. If we consider the Northern Hemisphere above 1 km in altitude, aviation accounts for ∼20% of all NOx emissions, with the remainder being produced by lightning. NOx emissions in this region are associated with increased ozone production and climate impacts relative to surface NOx emissions.37 The NOx burden is provided in Section 3.7.
Region | Inventory | Species |
---|---|---|
Global | EDGARv4340 | NOx, SOx, SO4, CO, NH3 |
Global | BOND41 | BC, OC |
Global | RETRO42 | NMVOCs except C2H6 and C3H8 |
Global | SHIP43 | NOx, SO2, CO |
Global | ParaNOx44 | O3, HNO3 |
Global | C2H6_201045 | C2H6 |
Global | POET46 | C2H5OH |
Asia | MIX47 | NOx, SO2, CO, BC, OC, NMVOCs, NH3 |
US | NEI201148 | NOx, SO2, CO, BC, OC, NMVOCs, NH3 |
Canada | APEI49 | NOx, SOx, CO, BC, OC, NMVOCs, NH3 |
Mexico | BRAVO50 | NOx, SO2, CO |
Europe | EMEP51 | NOx, SO2, CO |
Global | VOLCANO | SO2 |
Global | Lightning52 | NO |
Global | SoilNOx53 | NO |
Global | BROMOCARB54 | CHBr3, CH2Br2 |
Global | IODOCARB55 | CH3I, CH2I2, CH2ICl, CH2IBr |
Global | MEGAN56 | Biogenic hydrocarbons |
The well-to-tank emissions for conventional jet fuel and ULS fuel are taken from Stratton et al.38 While the combustion CO2 emissions of ULS fuel are lower (by ∼0.4%) than conventional jet fuel, due to a change in the hydrogen to carbon ratio during the desulfurization process, the life-cycle CO2 emissions (well-to-wake) from ULS fuel are ∼2% higher than conventional jet fuel, which we account for.22,38 The global average estimate of life cycle emissions for ammonia are taken from Bicer et al.39
In this work a single year of aviation emissions (for the year 2015) and its integrated impact into the future is considered. This is carried out for each scenario outlined in Table 3. While post-combustion emissions control may not be applicable in all aircraft, this analysis will allow future research to scale the benefits and costs based on the percentage of aviation fuel burn where post-combustion emissions control is practical. A discount rate of 3% is used to discount the damages occurring in the future and the NPV is used to compare the climate damages from the two scenarios.
The premature mortalities due to aviation attributable PM2.5 and ozone are estimated using log-linear concentration response functions (CRF). The ozone impacts are estimated using the relative risk from Jerrett et al.58 This study found a 4% [95% CI: 1.3% to 6.7%] increase in risk of respiratory disease related mortality per 10 ppbv increase in the daily 1 hour maximum ozone concentration (MDA1) during local ozone season. The health impacts due to PM2.5 exposure are estimated using the relative risk from Hoek et al.59 This meta-analysis of epidemiological studies reports a 11% [95% CI: 5.0% to 16%] increase in cardiovascular mortality rates per 10 mg increase in annual average PM2.5 exposure.
An EPA-recommended60 cessation lag of 20 years is used. It assumes that 30% of the premature mortalities occur in the first year, 50% of the mortalities in the next 4 years and the final 20% over the remaining 15 years. Consistent with the method used in AMPT-IC, a discount rate of 3% is used when monetizing impacts. The damages due to premature mortalities are calculated based on the US EPA estimates of the value of statistical life (VSL).61 The resulting mean US VSL (scaled from 1990 income levels using an income elasticity of 0.7) is $10.2 million (in 2015 US dollars). The VSL for other countries is calculated from the US value using the gross domestic product per capita (PPP basis) and adjusted using an income elasticity of 0.7.62
At the temperatures and pressures found downstream of the LPT, we find Da ≈ 1.6 × 1010, which indicates that the chemical reactions are several orders of magnitude faster than the mass transfer from the free stream to the wall.
DeNOx is thus only dependent on the non-dimensional parameter z* = (zDNO)/(ud2). Thus the required residence time (τ = z/u) for a certain level of deNOx is dependent only on the square of the hydraulic diameter of the channel, d2 (for a given diffusivity DNO). A smaller channel diameter implies a shorter residence time is required as compared to a larger channel diameter (see Fig. 3).
The capacity of the reductant storage tank and hence the weight of the storage system is estimated using eqn (2). The total mass of fuel spent for a 1500 km range mission is approximately 4.1 tonnes, which would require 21 kg of anhydrous NH3 to treat the NOx emissions (based on the FSRC calculated in Table 1). Using the density of anhydrous liquid NH3 the volume of the storage tank required is 35 L (9.25 gal). Storage tanks for anhydrous NH3 are typically filled to ∼85% of the total volume (∼15% vapour space must be maintained to account for expansion).68 Therefore for the design range the storage tank has a volume of ∼42 L (cylindrical tank of inner radius of 15 cm and a length of 0.6 m) and is designed for a gauge pressure of 250 psi (∼1725 kPa) (based on safety recommendations for ammonia storage69). This results in an empty tank weight of approximately 8 kg per aircraft. Anhydrous ammonia pumps for the required flow rates weigh approximately 60 kg70 per engine. Assuming that any additional mass requirements for piping and injectors are small, we use 128 kg (a pump for each engine in a two-engine aircraft and a single NH3 storage tank) as the total additional mass due to the reductant storage and delivery systems.
The gas hourly space velocity (GHSV) is defined as the ratio of the volume flow rate per hour of the exhaust gas to the bulk volume of the catalyst and is inversely proportional to the residence time in the catalyst. A large catalyst corresponds to a smaller GHSV (longer residence time) and hence shows a greater conversion of NOx. Fig. 4 shows that post-combustion emissions control as evaluated here has the potential to reduce the NOx emissions by 95% in exchange for approximately a 0.5% increase in fuel burn. The catalyst total frontal area required for this conversion is approximately 19 m2.
The deNOx at take-off conditions is approximately 75%. The lower NOx conversion efficiency at take-off is due to the higher pressures at sea-level (relative to cruise altitude) which decreases the effective diffusivity (Deff) of the reacting species by ∼60% relative to cruise conditions. The increased NO2 emission fraction at low thrust conditions (such as at idle conditions) does not affect the results because the conversion of NOx is limited by the bulk mass transfer and not the chemical kinetics (Da ≫ 1).
Reduction in the conversion efficiency while the catalyst warms up has not been accounted for. In addition, the impact of the NOx reduction across each flight segment, especially idle and taxi warrant further analysis with respect to local air quality.
The deNOx during cruise is higher (∼97%) which results in an effective deNOx of ∼95% over the full flight (a 1500 km range mission is assumed here). Furthermore, according to Yim et al.,71 cruise emissions account for three-quarters of the premature mortalities attributable to aviation PM2.5 and ozone. The design point of our catalyst is therefore chosen to be the cruise condition, however to ensure catalyst performance at off-design conditions, we calculate the temperature of the gas entering the catalyst at take-off and idle to be ∼480 and ∼250 °C respectively which fall well within the operating range (150–600 °C)17 of the zeolite class of substrates chosen in our analysis.
As the size of the catalyst is increased the pressure drop incurred can be reduced (decreasing fuel burn). However, this comes at the cost of additional weight (increasing fuel burn). This tradeoff is shown in the graph on the right in Fig. 4, as the frontal area of the catalyst is increased from approximately 5 m2 to 10 m2 the fuel burn penalty decreases. This is a consequence of the lower flow velocity and hence smaller pressure drop downstream of the LPT. Further increase in the flow through area results in an increase in fuel burn penalty. This is due to the catalyst mass, which affects the maximum landing mass of the aircraft and hence the fuel required to fly the same mission.
The dashed blue lines in Fig. 5 show that as the reacting length (l) is decreased for a fixed catalyst frontal area (A) the pressure drop and the catalyst volume (and hence catalyst mass) decrease. This causes the deNOx and fuel burn penalty to monotonically decrease. However, if l is held constant and A is increased, the pressure drop decreases but the catalyst mass increases. This causes the fuel burn penalty to first decrease and then increase as explained above. Higher lift to drag ratio airframes will mitigate the impact that this additional weight has on the fuel burn penalty, shifting the optimum. This is seen from the modified range equation (eqn (2)). Details of the SCR system at the chosen design point are outlined in Table 4.
Parameter | Value |
---|---|
NOx reduction (deNOx) | 95% |
Increase in fuel burn | 0.5% |
Catalyst frontal area | 19 m2 |
Reacting channel length | 1.25 cm |
Catalyst porosity | 0.56 |
Catalyst tortuosity factor | 2 |
Catalyst mass (per engine) | 91 kg |
Mass of reductant (for 1500 km mission) | 21 kg |
Additional system mass (pumps, storage tanks, etc. per aircraft) | 128 kg |
Pressure loss at cruise | 115 Pa |
Packed volume of catalyst | 1.57 m3 |
While ammonia slip at ground level results in the formation of PM2.5 which adversely affects human health,1 cruise altitude emissions of ammonia do not share the same risk, since neither the ammonia nor its products would reach population at ground level due to wet deposition and atmospheric transport phenomenon at cruise altitude. However, we do include these emissions. The impact of ammonia slip is captured by the GEOS-Chem simulations as presented in Section 3.7. As identified by Eastham et al.3 the transport of aviation attributable ozone from cruise altitude is the mechanism responsible for human exposure to both ozone and PM2.5. This is supported by the analysis presented in Section 3.7.
Fig. 6 shows the results of evaluating the after-treatment methods on three different engine architectures. The conventional turbofan is representative of a modern mixed flow turbofan, the geared turbofan represents the state of the art low fan pressure ratio geared turbofans, and the small core engine is representative of an advanced engine architecture that was proposed to be used on the MIT D8 aircraft.26
We see from Fig. 6 that the performance of the post-combustion control system improves as the core size decreases. Considering the core size (expressed as the corrected mass flow at compressor exit), current generation engines have a core size of 3.18 kg s−1 (7 lb s−1), geared turbofans have a core size of 2.27 kg s−1 (5 lb s−1) and the next generation engines may have smaller core sizes of ∼0.68 kg s−1 (1.5 lb s−1).26 The thrust size for the conventional and geared turbofan engines is 110 kN (25000 lbf) and the small core engine has the above core size is 58 kN (13000 lbf). The small core engine has a lower thrust rating since the airframe envisioned by Lord et al.26 (the MIT D8 design) has a higher L/D of approximately 20.30
The authors envision the proposed post-combustion emissions control methods could be implemented with a small core architecture that could be housed within the body of the aircraft in a turbo-electric configuration or possibly with a decoupled propulsor such as in the D8 aircraft.26 This could allow installation of the catalyst in the fuselage of the aircraft. The core flow in such a design would thus contribute little or no thrust, although the design may be configured such that the core ingests the airframe boundary layer, providing scope for further improvement of the post-combustion emissions control performance.
(3) |
Applying eqn (3) shows that we can fit this area of catalyst into a cylinder of length 2.2 m and outer diameter of 1 m (using 24 pleats and a pleat depth of 18 cm). Detailed analysis concerning the packing and manufacturing of the catalyst design will be subject of future research.
The efficiency of the catalyst in removing NOx from the exhaust decreases over time. The typical life time of a catalyst used in ground-based power plants is ∼40000–60000 hours.69 Assuming a similar life time for the catalyst used on board an aircraft and maintenance (C-check) intervals of ∼7500 hours,73 the catalyst will need to be replaced every 5–8 maintenance cycles.
We find that the contribution of global aviation to NOx emissions while using post-combustion emissions control along with ULS fuel is approximately 0.11%, while the baseline contribution of aviation as outlined in Section 2.5 is ∼2.1%. Furthermore, the combination of PCEC with ULS fuel reduces aviation's contribution to NOx emissions in the free-troposphere of the Northern Hemisphere to 0.81% from a baseline of ∼20%. Additionally, baseline aviation emissions are responsible for ∼34% of the Northern Hemisphere NOx mixing ratios at typical cruise altitudes (10–12 km) (i.e. zonally mass averaged across cruise altitudes and the Northern Hemisphere). The use of PCEC along with ULS fuel reduces the aviation attributable NOx mixing ratio at Northern Hemisphere cruise altitudes to approximately 0.25% (see ESI† for further information).
Species | Δozone [ppbv] | ΔPM2.5 [μg m−3] |
---|---|---|
Baseline aviation | 0.640 | 0.0702 |
ULS | 0.641 | 0.0625 |
PCEC with ULS | 0.0223 | 0.00911 |
Post-combustion emissions control along with desulfurized jet fuel leads to a reduction (87% from the baseline as defined in Table 3) in population exposure to PM2.5 of which approximately 11% of the reduction in population exposure to PM2.5 is due to the use of ULS fuel and the rest is attributable to the removal of NOx. ULS fuel is required to prevent fouling of the catalyst as detailed in Section 1.2.4. The reduction in surface concentration of PM2.5 is therefore primarily attributable to the post-combustion reduction of NOx emissions. The global distribution of PM2.5 and the reduction due to PCEC is shown in Fig. 8.
We find that using ULS fuel results in a reduction of sulfate aerosol in the lower stratosphere. This leads to a reduction in heterogeneous hydrolysis of N2O5 on sulfate aerosols and a subsequent reduction in ozone depletion by halogen catalysed cycles.74 This increases the ozone concentration in the lower stratosphere and in stratospheric air masses that enter the troposphere, thereby resulting in an increase in the surface concentration of ozone as seen in Table 5. This finding is consistent with the findings by Eastham et al.75
Furthermore, the identified pathway implies that this effect will reduce in future years as the concentration of halogens in the atmosphere decreases (since the adoption of the Montreal Protocol).
The average reduction in population exposure to ozone due to the use of post-combustion emissions control with ULS fuel is 97%. The reduction in surface ozone concentration is a consequence of the reduced NOx emissions due to post-combustion control through the mechanism described by Eastham et al.3
While reducing ground level ozone concentration has a health benefit, a reduction in column ozone can increase the risk of melanoma. However as estimated by Eastham et al.3 the avoided mortalities due to melanoma resulting from column ozone created by aviation is small (2.5%) compared to the PM2.5 and ozone related air quality impacts attributable to aviation.
These baseline values are consistent with previous estimates of aviation attributable premature mortalities [3] when accounting for the addition of new emission inventories in GEOS-Chem and the increase in aviation fuel burn by ∼30% (188 Tg in the AEDT-2005 inventory vs. 240 Tg in the AEDT-2015 inventory). The PM2.5 and ozone attributable premature mortalities in each of the scenarios outlined in Table 3 are shown in Fig. 10.
Fig. 10 Premature mortalities attributable to aviation under the scenarios considered. The error bars shown are the 95% confidence intervals from the Monte Carlo runs. |
Post-combustion emissions control used with ULS jet fuel (PCEC-ULS), decreases the population exposure to PM2.5 and ozone by reducing NOx and SOx emissions. Converting exposure to mortality using the concentration response functions described earlier, we estimate that ∼13000 premature mortalities (95% CI: 6300 to 19000) due to exposure to PM2.5 and ∼8500 premature mortalities (95% CI: 2800 to 14000) due to exposure to ozone are avoided by using PCEC and ULS fuel. Furthermore, ∼12000 (95% CI: 5900 to 18000) of the ∼13000 avoided premature mortalities due to decreased exposure to PM2.5 are attributable to the post-combustion removal of NOx emissions while the remaining avoided premature mortalities are attributable to reduced PM2.5 from the use of ULS fuel (see Section 2.5).
Therefore ∼22000 [95% CI: 13000 to 31000] total premature mortalities are avoided due to the use of PCEC with ULS fuel annually. This is approximately 92% of all premature mortalities attributable to aviation as calculated in this study. The air quality benefits of using PCEC-ULS are monetized as described in Section 2.5.2. The benefit associated with the averted premature mortalities by using PCEC (with ULS fuel), amounts to approximately 77 billion USD (2015) annually [95% CI: 45 to 110 billion USD], or $320 per tonne of fuel burned.
Baseline aviation | ULS | PCEC with ULS | |
---|---|---|---|
Sulfates | −6.21 | −1.41 | −0.114 |
Nitrates | −0.667 | −1.83 | −0.387 |
Black carbon | 0.537 | 0.534 | 0.568 |
The lower sulfate concentration when ULS fuel is used, reduces competition for available ammonium resulting in an increase in nitrate formation and therefore an increased cooling effect from nitrates in the ULS scenario as seen in Table 6. The changes in black carbon RF are negligible. The values of radiative forcing from Table 6 are used in APMT-IC to estimate the climate damages due to aviation.
The total climate damages associated with ULS fuel and post-combustion emissions control is estimated using APMT-IC to be approximately 57 billion USD (or $238 per tonne of fuel burned) compared to a baseline climate damage of 53 billion USD (or $222 per tonne of fuel burned) due to global aviation without post-combustion emissions control. These damages include the life cycle emissions of CO2 as detailed in Table 7. Therefore the use of PCEC with ULS fuel results in a ∼7.5% increase in climate damages from all aviation. As seen in Table 7 the dominant contribution is from the decreased cooling effect due to a lower sulfate aerosol concentration when ULS fuel is used. The increase in climate damages due to an increase in fuel burn (∼0.5%) as a result of the additional weight and pressure losses introduced by the PCEC system is partially offset by the lower combustion CO2 emissions from the ULS fuel used22,38 as seen in Table 7.
Baseline | ULS | PCEC with ULS | |
---|---|---|---|
a Note that the total cost includes life cycle CO2 and consists of other short-lived forcers that are not shown in the table. | |||
Life cycle CO2 (of which combustion CO2) | 40.4 (33.7) | 41.0 (33.5) | 41.3 (33.7) |
NOx | −1.02 | −1.43 | −0.175 |
Sulfates | −2.19 | −0.0120 | −0.00100 |
Black carbon | 0.190 | 0.189 | 0.201 |
Total costa | 53.2 | 55.6 | 57.2 |
The net benefit (i.e. the monetized benefit due to avoided premature mortalities less the increase in climate damages) is therefore approximately 73 billion USD annually [95% CI: 40 to 100 billion USD] or a mean value of $304 per ton of jet fuel burnt. The environmental costs normalized by fuel burn (from degraded air quality and climate related damages) are shown in Fig. 11. The baseline costs are consistent with recent work by Grobler et al.76
The current work quantifies the impact that a fleet-wide adoption of post-combustion emissions control will have on air quality and climate. However, the size requirements of the SCR system, particularly of the catalyst, imply that they will have to be housed within the aircraft fuselage, making this unsuitable for certain classes of aircraft. Post-combustion emissions control systems might be better suited for a hybrid- or turbo-electric design with small core engines. A NASA N + 3 aircraft design such as the D8 with small core engines, and turbo-electric designs, may offer further potential for optimization. Additionally using post-combustion emissions control to reduce NOx could result in combustor design space benefits that improve combustor efficiency. Further analysis is required to quantify the performance of such an integrated aircraft system. The spatial distribution of aviation hubs and missions flown by aircraft where PCEC is feasible might result in spatial variations of the impacts, which also need to be quantified. Since the implementation cost of post-combustion emissions control technology is dependent on the aircraft configuration and specific design concepts, we do not include the cost of implementation in this analysis. However we estimate that the increase in annual fleet-wide operating cost due to the increased fuel burn (of ∼1.30 Tg per year) is approximately 875 million USD based on average price of $86 per bbl for Jet-A.77
Using GEOS-Chem it is estimated that approximately 87% of surface PM2.5 concentration and 97% of ozone concentration due aviation emissions is averted with the use of post-combustion emissions control with desulfurized jet fuel (as is required for PCEC) for a fleet-wide implementation (as a hypothetical analysis scenario). An analysis based on epidemiological studies shows that ∼22000 premature mortalities are avoided (∼92% of all premature mortalities attributable to aviation) due to exposure to PM2.5 and ozone, if post-combustion emissions control is used along with ULS jet fuel. The mean monetized air quality benefits due to this is estimated to be $77 billion annually. The increase climate damages associated with the use of post-combustion emissions control is estimated using APMT-IC to be $4 billion annually. An environmental cost-benefit analysis, therefore, indicates that the net benefit of post-combustion emissions control is approximately $73 billion annually or $304 per ton of jet fuel burned.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ee02362k |
This journal is © The Royal Society of Chemistry 2021 |