Ams-3277 Alternative

[Boyan Atanaschev]

Awide variety of chromated organic sealants is used to seal equipment against water ingress in applications such as electronics and interior and exterior panels in vehicles and aircraft. Chromates are incorporated into some sealants to inhibit corrosion of underlying metals.

ESOH IssuesSealants may contain Cr6+, solvents such as toluene.Cr6+ (CrVI, hexavalent chromium) is a known carcinogen that is strongly regulated under: EPA Clean Air Act regulations OSHA Occupational Exposure to Hexavalent Chromium (Cr6+ Permissible Exposure Limit is currently 5 μgm-3) European regulations (RoHS and REACH) DoD Policy Memorandum on Minimizing the Use of Hexavalent ChromiumToluene is a VOC (volatile organic compound) and a HAP (hazardous air pollutant).ExposurePersonnel may be exposed at the OEM level, during depot overhaul and repaint, and during operational level removal of access panels for inspection and repair. The primary exposure issue involves applying the sealant since the Cr6+ and HAP concentrations are highest when the material is new. Removing chromated sealant is an additional potential Cr6+ exposure source. Because sealants are usually well hidden, they do not present a significant source of exposure to users during service, but operational and depot maintainers will be exposed when removing panels, windshields, fasteners, etc. that may be sealed with chromated sealants.Alternatives, Applications, StatusPrimary alternatives are chrome-free, low-HAP versions, of which there are many on the market. Alternatives are commercially available and some have been authorized and implemented, especially in vehicles and commercial aircraft. Chromated sealants are not used in any Boeing commercial aircraft.

For detailed technical and performance information, validations, authorizations and implementations see Surface Engineering Database. Search for Chromate Sealant Alternatives or High VOC Material Alternatives.

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Energy transition scenarios are characterized by increasing electrification and improving efficiency of energy end uses, rapid decarbonization of the electric power sector, and deployment of carbon dioxide removal (CDR) technologies to offset remaining emissions. Although hydrocarbon fuels typically decline in such scenarios, significant volumes remain in many scenarios even at the time of net-zero emissions. While scenarios rely on different approaches for decarbonizing remaining fuels, the underlying drivers for these differences are unclear. Here we develop several illustrative net-zero systems in a simple structural energy model and show that, for a given set of final energy demands, assumptions about the use of biomass and CO2 sequestration drive key differences in how emissions from remaining fuels are mitigated. Limiting one resource may increase reliance on another, implying that decisions about using or restricting resources in pursuit of net-zero objectives could have significant tradeoffs that will need to be evaluated and managed.

A variety of modeling frameworks have been used to produce energy transition scenarios consistent with net-zero emissions objectives, including Integrated Assessment Models (IAMs)12,13 and Macro Energy System (MES) models14. IAMs exhibit broader geographic and sectoral scope, linking climate, land, energy and economic systems globally, and have been used extensively to produce energy transition scenarios used in global assessments1,15 as well as scenarios of regional and national GHG mitigation16,17. MES models tend to have greater spatial and temporal resolution and have been used widely to produce scenarios of net-zero energy systems for several different countries and regions6,18,19,20,21,22,23,24.

While prior studies have examined differences in electrification25 and remaining fuel use10 at net-zero, few have explicitly examined the range of pathways available to mitigate emissions from remaining fuels even though significant volumes remain in most scenarios, including in those with higher levels of electrification. This study aims to fill this gap by showing that, for a given set of final energy demands, assumptions about the use of biomass and CO2 sequestration drive key differences in how emissions from remaining fuels are mitigated. Because limiting one resource may increase reliance on another, decisions about using or restricting resources in pursuit of net-zero objectives could have significant tradeoffs that will need to be evaluated and managed. In addition to highlighting these tradeoffs for decision-makers, we also provide several actionable recommendations for the research community.

However, in most of the scenarios shown in Fig. 1, except when strongly constrained, synthetic fuels do not deploy or deploy only modestly. For example, in the EPRI LCRI Limited Options scenario, synthetic fuels deploy only when geologic CO2 storage is assumed not to be available and biomass supply is assumed to be limited, and in this case, the overall amount of remaining fuels is lower than in the other cases. In contrast, biofuels contribute to mitigation in all the cases shown, albeit to varying extents. Although the cases shown here represent only a subset of the scenarios in the literature that attain net-zero emissions by 2050, these scenarios are sufficient to illustrate key differences in potential mitigation pathways for remaining fuels. Scenarios differ not only in their relative reliance on CDR and alternative fuels, but also in whether those options utilize biomass-based or other synthetic routes. What is currently lacking is a unified explanation for these differences in terms of underlying drivers.

Whether end use decisions are explicitly represented is another factor that would primarily affect the overall demand for fuels. Demand for fuels could also depend on additional assumptions, such as fossil fuel resources and prices, the availability, efficiency, and cost of substitutes (e.g., electricity and hydrogen consuming technologies in end uses), how end use decisions are represented, particularly at the consumer level, and the full range of policies implemented. Supplementary Table 1 reports remaining liquids and natural gas fuel volumes for the scenarios examined in Fig. 1, which vary considerably across studies.

As demonstrated by the range of results shown in Fig. 1, there is no easily identifiable consensus across the IAM and MES modeling communities about how emissions from remaining liquid fuels and natural gas would be mitigated in net-zero systems. BECCS, DACCS, biofuels, and other synthetic fuels would likely require different enabling conditions and could have different implications for the broader energy system or other societal outcomes. In the following sections, we explore key drivers and implications of these alternative mitigation options using a simple global energy system model designed to shed light on the range of outcomes observed in scenarios produced by more complex models.

Primary resources (primary energy) are shown on the left, energy conversion (secondary energy) is shown in the center, and final energy is shown on the right. Supply and demand for captured CO2 is shown in the bottom row. Lines with arrows represent inputs and outputs, with lines of the same color generally representing the same vector. Electricity is shown as blue, hydrogen as yellow, natural gas as red, and liquid fuels as gray. Primary biomass is shown as green, captured CO2 is shown as purple, fossil primary energy (oil and natural gas) is shown as black, and primary carbon-free energy (wind, solar, nuclear, and hydro) is shown as light blue. Electricity and natural gas inputs to the captured CO2 box are energy inputs for direct air capture (DAC).

In the unconstrained (UC) and biomass constrained (BC) cases, remaining liquids (91 EJ) are conventional oil-derived fuels. Emissions are offset by removals (CDR) in the form of either BECCS in hydrogen and/or electricity (in both the UC and BC cases) or DACCS (in the BC case only). In these scenarios, CDR is selected because it is less costly than replacing conventional fuels with biofuels or other synthetic fuels. BECCS accounts for all CDR when biomass is unconstrained (UC), whereas DACCS accounts for a significant share of CDR when biomass is constrained (BC). BECCS deploys preferentially in the unconstrained case because it is more cost-effective than DACCS, given the capital and energy requirements associated with DACCS.

While BECCS could be deployed in any sector, BECCS deploys preferentially in hydrogen and electricity generation under our default assumptions when there are no CO2 sequestration constraints. This preference arises, in part, because nearly all carbon in the biomass can be captured when the final product (electricity or hydrogen) does not contain carbon, thereby increasing the effective (negative) carbon intensity of these production technologies. Since BECCS electricity and BECCS hydrogen provide similar functions in a net-zero energy system (CDR), their relative deployment is determined primarily by cost differences. BECCS hydrogen is slightly more cost-effective under our default assumptions, leading it to deploy up until the demand for hydrogen is satisfied, followed by deployment of BECCS electricity. Other assumptions may result in a different cost ordering.

When only CO2 sequestration is constrained (SC), the amount of allowable CO2 storage is not sufficient to offset all emissions from remaining liquid fuels and natural gas if they are produced using conventional fossil sources, so biofuels displace a share of conventional liquids47. Applying CCS to biofuels production (i.e., deploying BECCS in liquids rather than biofuels without CCS) is preferable because this option provides CDR simultaneously, offsetting remaining emissions from fossil natural gas. Once the imposed CO2 sequestration limit is reached, the model can either deploy biofuels without CCS or synthesize fuels from hydrogen and captured CO2 to satisfy remaining liquids demand. The latter option is more cost-effective if the additional CO2 is derived from biofuels with CCS (rather than from DAC) because the incremental cost of CO2 capture on biofuels production is relatively small. Conceptually, this route is similar to recycling more CO2 and adding supplemental hydrogen within the biofuels production process48,49. In addition, there is a greater preference for synthetic fuels when more emissions are assumed to be associated with bioenergy production (compare Supplementary Figs. 1 and 3). When both CO2 sequestration and biomass are constrained (SBC), the share of liquids from the synthetic route increases, since biomass supply is constrained, with a greater share of the captured CO2 coming from DAC.

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