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Aviation emits pollutants that affect the climate, including CO2 and NOx, NOx indirectly so, through the formation of tropospheric ozone and reduction of ambient methane. To improve the fuel performance of engines, combustor temperatures and pressures often increase, increasing NOx emissions. Conversely, combustor modifications to reduce NOx may increase CO2. Hence, a technology trade-off exists, which also translates to a trade-off between short-lived climate forcers and a long-lived greenhouse gas, CO2. Moreover, the NOx-O3-CH4 system responds in a nonlinear manner, according to both aviation emissions and background NOx. A simple climate model was modified to incorporate nonlinearities parametrized from a complex chemistry model. Case studies showed that for a scenario of a 20% reduction in NOx emissions the consequential CO2 penalty of 2% actually increased the total radiative forcing (RF). For a 2% fuel penalty, NOx emissions needed to be reduced by >43% to realize an overall benefit. Conversely, to ensure that the fuel penalty for a 20% NOx emission reduction did not increase overall forcing, a 0.5% increase in CO2 was found to be the "break even" point. The time scales of the climate effects of NOx and CO2 are quite different, necessitating careful analysis of proposed emissions trade-offs. [ABSTRACT FROM AUTHOR]

The control of toxic chromate (Cr6+) formation is still a significant challenge in solid fuel combustion. In particular, the mechanism of chromium transformation from Cr3+ to chromate or other unoxidized forms remains unclear. The present study confirms the formation of a significant unoxidized Cr-containing compound CaCr2O4(Cr3+) during solid fuel combustion. Experiments were conducted, for the first time, to clarify the mechanism of CaCr2O4 oxidation, which is quite different from Cr2O2 oxidation. The findings demonstrate that CaCr2O4 was formed at temperatures above 1200 K, through rapid decomposition of CaCrO4 or slow and direct interaction between CaO and Cr2O2. Compared to Cr2O3, CaCr2O4 could be oxidized at lower temperatures under the influence of free CaO. In the absence of free CaO, the oxidation of CaCr2O4 was minimal; however, in the presence of CaSO[sub 4], calcium in the form of CaCr2O4 participated in the oxidation of CaCr2O4. Thus, chromium in the form of CaCr2O4 was more likely to be oxidized when CaCr2O4-containing fly ash was reheated. Fortunately, CaCr2O4 showed slight basicity on the surface, allowing it to react with acidic gases. Accordingly, measures were proposed to suppress the oxidation of CaCr2O4 by stimulating the reactions between CaCr2O4 and acidic substances, like SO2 and Si/Al-compounds. These compounds competed with chromium at high temperatures to react with calcium in the fly ash and in CaCr2O4. As a result, the unoxidized chromium was transformed into highly stable Cr2O2 or Ca3Cr2 (SiO4)3. [ABSTRACT FROM AUTHOR]

The University of Denver repeated its 2013 fuel specific gaseous and particle emission measurements on two California heavy-duty vehicle fleets. In 2015 1456 measurements at the Port of Los Angeles and 694 measurements at the Cottonwood weigh station in northern California were collected. The Port fleet changed little since 2013, increasing the average age (+1.8 years), accompanied by an increase in particle mass (PM) by +266% (0.03 ± 0.01 to 0.11 ± 0.01 gPM/kg of fuel) and black carbon (BC) by +300% (0.02 ± 0.003 to 0.08 ± 0.01 gBC/kg of fuel). Particle number (PN) also increased (1.5 × 1014 ± 2.5 × 1013 to 2.8 × 1014 ± 2.8 × 1013 PN/kg of fuel) by a smaller percentage (+87%). Chassis model year 2008 and 2009 vehicles currently dominate the fleet, accounting for the majority of these increases. The long-haul Cottonwood fleet decreased in fleet age (-0.6 model years), where half the decreases in fuel specific PM (-66%), BC (-65%), and PN (-19%) emissions are due to the newer fleet; an increased fraction of pre-2008 chassis model year vehicles with retrofit diesel particulate filters (DPFs) account for the remaining reductions. These opposing emissions trends emphasize the importance of fully functional DPFs. [ABSTRACT FROM AUTHOR]

The presence of variability in life cycle analysis (LCA) is inherent due to both inexact LCA procedures and variation of numerical inputs. Variability in LCA needs to be clearly distinguished from uncertainty. This paper uses specific examples from the production of diesel and jet fuels from 14 different feedstocks to demonstrate general trends in the types and magnitudes of variability present in life cycle greenhouse gas (LC-GHG) inventories of middle distillate fuels. Sources of variability have been categorized as pathway specific, coproduct usage and allocation, and land use change. The results of this research demonstrate that subjective choices such as coproduct usage and allocation methodology can be more important sources of variability in the LC-GHG inventory of a fuel option than the process and energy use of fuel production. Through the application of a consistent analysis methodology across all fuel options, the influence of these subjective biases is minimized, and the LC-GHG inventories for each feedstock-to-fuel option can be effectively compared and discussed. By considering the types and magnitudes of variability across multiple fuel pathways, it is evident that LCA results should be presented as a range instead of a point value. The policy implications of this are discussed. [ABSTRACT FROM AUTHOR]

Recent U.S. climate change policy developments include aggressive proposals to reduce greenhouse gas emissions, including cap-and-trade legislation with a goal of an 83% reduction below 2005 levels by 2050. This study examines behavioral and technological changes required to achieve this reduction within the light-duty vehicle (LDV) sector. Under this "fair share" sectoral assumption, aggressive near-term actions are necessary in three areas: vehicle miles traveled (VMT), vehicle fuel economy (FE), and fuel carbon intensity (FCI). Two generic scenarios demonstrate the important role of FCI in meeting the 2050 goal. The first scenario allows deep reductions in FCI to compensate for relatively modest FE improvements and VMT reductions. The second scenario assumes optimistic improvements in FE, relatively large reductions in VMT and less aggressive FCI reductions. Each generic scenario is expanded into three illustrative scenarios to explore the theoretical implications of meeting the 2050 goal by relying exclusively on biofuels and hybrid vehicles, biofuels and plug-in hybrid vehicles, or hydrogen fuel cell electric vehicles. These scenarios inform a discussion of resource limitations, technology development and deployment challenges, and policy goals required to meet the 2050 GHG goal for LDVs. [ABSTRACT FROM AUTHOR]

Anaerobic mineralization of tert-butyl alcohol (TBA) and methyl tert-butyl ether (MTBE) were studied in sediment incubations prepared with fuel-contaminated aquifer material. Microbial community compositions in all incubations were characterized by amplified ribosomal DNA restriction analysis (ARDRA). The aquifer material mineralized 42.3 ± 9.9% of [U-14C]-TBA to 14CO2 without electron acceptor amendment, Fe(III), sulfate, and Fe(III) plus anthraquinone-2,6-disulfonate addition also promoted U-[14C]-TBA mineralization at levels similar to those of the unamended controls. Nitrate actually inhibited TBA mineralization relative to unamended controls. In contrast to TBA [U-14C]-MTBE was not significantly mineralized in 400 days regardless of electron acceptor amendment. Microbial community analysis indicated that the abundance of one dominant clone group correlated closely with anaerobic TBA mineralization. The clone was phylogenetically distinct from known aerobic TBA-degrading microorganisms, Fe(III)- or sulfate-reducing bacteria. It was most closely associated with organisms belonging to the alphaproteobacteria. Microbial communities were different in MTBE and TBA amended incubations. Shannon indices and Simpson indices (statistical community comparison tools) both demonstrated that microbial community diversity decreased in incubations actively mineralizing TBA, with distinct "dominant" clones developing. These data contribute to our understanding of anaerobic microbial transformation of fuel oxygenates in contaminated aquifer material and the organisms that may catalyze the reactions. [ABSTRACT FROM AUTHOR]

Microbial oxidation of BTEX compounds under iron-reducing conditions is an important attenuation process for fuel-contaminated sites. We evaluated the use of compound-specific isotope analysis for the identification and quantification of anaerobic toluene oxidation by Geobacter metallireducens. 13C and ²H enrichment of toluene was measured in laboratory batch systems and varied significantly for a solid vs a dissolved Fe(III) phase provided as terminal electron acceptor. 13C enrichment factors (∈C) in suspensions of a solid Fe(III) phase were between -1.0 and -1.3‰, whereas ∈C-values were significantly higher in solutions of Fe(III) citrate (-2.9 to -3.6‰). The same trend was observed for ²H fractionation. Solid phase reduction resulted in an ∈H-value of -34.6 ± 0.9‰, compared to -98.4 ± 3.0‰ for the reduction of dissolved Fe(III). The linear correlation of δ²H vs δ13C during toluene oxidation resulted in nearly identical slopes for both systems, confirming that the reaction mechanism, that is enzymatic methyl-group oxidation, was the same. We hypothesize that smaller ²H and 13C fractionation in suspensions is due to toluene transport limitations to cells of G. metallireducens at surfaces of solid Fe(III) phases. Enrichment factors determined in Fe(III) mineral suspensions should be more representative for anaerobic toluene degradation owing to the abundance of solid Fe(III) in soils and aquifers. [ABSTRACT FROM AUTHOR]

During the past century, increased biomass burning and fossil fuel consumption have drastically increased the input of black carbon (BC) into the environment, and that has been shown to influence the behavior of organic contaminants in soil. A study was conducted to investigate the effects of BC on the relationship between aqueous hydroxypropyl-β-cyclodextrin (HPCD) extraction and microbial mineralization (bioaccessibility) of 14C-phenanthrene (10 mg kg-1) in four soils amended with 0, 0.1, 0.5, 1, 2.5, and 5% (% dry wt soil) activated charcoal, a type of BC. Mineralisation was monitored over 20 d incubation, within respirometric assays, using an inoculum containing a phenanthrene-degrading pseudomonad and compared to HPCD extraction (24 h) using 50 mM aqueous solution; analyses were conducted after 1, 25, 50, and 100 d soil-phenanthrene contact time. Statistical analyses revealed that for each soil the addition of BC led to significant (P < 0.001) reductions in both HPCD extractability and microbial mineralization. Linear correlations for BC concentrations of 0% (r² = 0.95; slope = 0.89) and 0.1% (r² = 0.67; slope = 0.95) revealed a highly significant (P < 0.01) relationship between HPCD extractability and total mineralization (20 d), indicating a direct prediction of phenanthrene bioaccessibility by HPCD. However, in soils amended with 0.5, 1, 2.5, and 5% BC exhibited r< values ranging 0.51-0.13 and slopes of 2.19-12.73. This study has shown that BC strongly sorbs phenanthrene causing reductions in extractability and, to a lesser extent, bioaccessibility to degrading microorganisms. [ABSTRACT FROM AUTHOR]

Biodiesel is one of the most promising alternative diesel fuels. As diesel emission regulations have become more stringent, the diesel particulate filter (DPF) has become an essential part of the aftertreatment system. Knowledge of kinetics of exhaust particle oxidation for alternative diesel fuels is useful in estimating the change in regeneration behavior of a DPF with such fuels. This study examines the characteristics of diesel particulate emissions as well as kinetics of particle oxidation using a 1996 John Deere T04045TF250 off-highway engine and 100% soy methyl ester (SME) biodiesel (B100) as fuel, Compared to standard D2 fuel, this B100 reduced particle size, number, and volume in the accumulation mode where most of the particle mass is found. At 75% load, number decreased by 38%, D6N decreased from 80 to 62 nm, and volume decreased by 82%. Part of this decrease is likely associated with the fact that the particles were more easily oxidized. Arrhenius parameters for the biodiesel fuel showed a 2–3 times greater frequency factor and ∼6 times higher oxidation rate compared to regular diesel fuel in the range of 700–825 °C, The faster oxidation kinetics should facilitate regeneration when used with a DPF. [ABSTRACT FROM AUTHOR]

Highlights the efforts of engineers from around the world to develop technologies to comply with the International Maritime Organization's regulations limiting fuel pollution. Lubricant formulations; Engine operating modes; Modification of the combustion system; Water-emulsified fuels; Treatment of exhaust gas by selective catalytic reduction.