Anthony J. Marchese

A Semi-Empirical Reaction Mechanism for n-Heptane Oxidation and Pyrolysis

A new semi-empirical mechanism for n-heptane oxidation and pyrolysis has been developed and validated against several independent data sets, including new flow reactor experiments. Previous semi-empirical chemical kinetic mechanisms assumed that a generic n-alkyl radical, formed by abstraction of an H-atom from the parent fuel, thermally decomposes into a fixed ratio of methyl and propene. While such an approach has been reasonably successful in predicting premixed, laminar flame speeds, the mechanism lacks sufficient detail to quantitatively capture transient phenomena and intermediate species distributions. The new chemical kinetic mechanism retains significantly more detail, yet is sufficiently compact to be used in combined fluid-mechanical/chemical kinetic computational studies. The mechanistic approach is sufficiently general to be extended to a wide variety of large linear and branched alkane fuels.

Numerical modeling of isolated n-alkane droplet flames: initial comparisons with ground and space-based microgravity experiments

Transient, spherically symmetric, combustion of single and multi-component liquid n-alkane droplets is numerically simulated with a model that includes gas phase detailed, multi-component molecular transport and complex chemical kinetics. A compact semi-detailed kinetic mechanism for n-heptane and n-hexadecane oxidation consisting of 51 species (including He, Ar, and N2) and 282 reactions is used to describe the gas phase. Non-luminous, gas phase radiative heat transfer and conservation of energy and species within the liquid droplet interior are also considered. Computed quasi-steady flame structure for pure n-heptane droplets is compared with that produced using the kinetic mechanism of Warnatz (frequently used in the past for modeling both premixed and diffusion flame properties). Transient calculations are also compared with the numerical results of King, which consider infinite rate chemical kinetics, but temperature dependent molecular diffusion. Modeling results are in reasonable agreement with small-diameter, drop tower experiments, though slow convective effects and droplet sooting effects exist in the experimental data. Comparisons with isolated large-diameter free droplet data (1 atm, He/O2 mixtures and air) from recent space experiments are reasonable for droplet gasification rate, flame position, and flame extinction. Very small extinction diameters are predicted for small initial diameter droplets (<1 mm). As droplet size is increased, or oxygen index is decreased, the model predicts decreasing gasification rates and for an appropriate range of parameters, radiative flame extinction. Bi-component droplet combustion of n-heptane and n-hexadecane is also considered. Modeling results qualitatively reproduce experimentally observed, multi-stage burning, resulting from the volatility differential and diffusional resistance of the liquid components. Internal liquid convection effects are examined by parametrically varying an effective liquid mass diffusivity. Flame extinction can occur either in the initial or the secondary droplet heating period, with subsequent, continuing vaporization of the more volatile component from the residual heat within the liquid phase.

The effect of liquid mass transport on the combustion and extinction of bicomponent droplets of methanol and water

The time-dependent combustion of isolated, bicomponent liquid droplets of methanol and water is simulated using a spherosymmetric, finite-element, chemically reacting flow model. The computations consider multi-component molecular transport and detailed chemical kinetics (19 species, 89 forward reactions) in the gas phase; semiempirically formulated vapor-liquid equilibrium with water vaporization/condensation at the liquid surface; and liquid species mass transfer and energy conservation in the liquid phase. The results are compared with previously reported data from microgravity drop tower, freely falling isolated droplet, and suspended droplet combustion experiments, all of which show significant departure from d2-law burning behavior and/or substantial water accumulation in the liquid phase. The liquid mass Peclet number (defined as the ratio of the droplet surface regression velocity to the effective diffusion velocity of water within the liquid droplet) is greater than 20 for methanol burning in air. The numerical model predicts little or no water accumulation for such circumstances. However, numerical results are consistent with experiments when it is speculated that sufficient internal liquid phase motion is present to reduce the effective liquid mass Peclet number to the order of one. Such internal motion has been noted in droplet combustion experiments and most likely arises from droplet generation/deployment techniques and/or surface tension gradients. Calculations show that the droplet combustion characteristics of methanol/water mixtures of 0%–50% initial water content are extremely sensitive to the liquid mass transport effects and might be used to better determine the magnitude of liquid mass transport present in microgravity experiments.

Measurements of Methane Emissions from Natural Gas Gathering Facilities and Processing Plants: Measurement Results

Facility-level methane emissions were measured at 114 gathering facilities and 16 processing plants in the United States natural gas system. At gathering facilities, the measured methane emission rates ranged from 0.7 to 700 kg per hour (kg/h) (0.6 to 600 standard cubic feet per minute (scfm)). Normalized emissions (as a % of total methane throughput) were less than 1% for 85 gathering facilities and 19 had normalized emissions less than 0.1%. The range of methane emissions rates for processing plants was 3 to 600 kg/h (3 to 524 scfm), corresponding to normalized methane emissions rates <1% in all cases. The distributions of methane emissions, particularly for gathering facilities, are skewed. For example, 30% of gathering facilities contribute 80% of the total emissions. Normalized emissions rates are negatively correlated with facility throughput. The variation in methane emissions also appears driven by differences between inlet and outlet pressure, as well as venting and leaking equipment. Substantial venting from liquids storage tanks was observed at 20% of gathering facilities. Emissions rates at these facilities were, on average, around four times the rates observed at similar facilities without substantial venting.

Constructing a Spatially Resolved Methane Emission Inventory for the Barnett Shale Region

Methane emissions from the oil and gas industry (O&G) and other sources in the Barnett Shale region were estimated by constructing a spatially resolved emission inventory. Eighteen source categories were estimated using multiple data sets, including new empirical measurements at regional O&G sites and a national study of gathering and processing facilities. Spatially referenced activity data were compiled from federal and state databases and combined with O&G facility emission factors calculated using Monte Carlo simulations that account for high emission sites representing the very upper portion, or fat-tail, in the observed emissions distributions. Total methane emissions in the 25-county Barnett Shale region in October 2013 were estimated to be 72,300 (63,400-82,400) kg CH4 h(-1). O&G emissions were estimated to be 46,200 (40,000-54,100) kg CH4 h(-1) with 19% of emissions from fat-tail sites representing less than 2% of sites. Our estimate of O&G emissions in the Barnett Shale region was higher than alternative inventories based on the United States Environmental Protection Agency (EPA) Greenhouse Gas Inventory, EPA Greenhouse Gas Reporting Program, and Emissions Database for Global Atmospheric Research by factors of 1.5, 2.7, and 4.3, respectively. Gathering compressor stations, which accounted for 40% of O&G emissions in our inventory, had the largest difference from emission estimates based on EPA data sources. Our inventorys higher O&G emission estimate was due primarily to its more comprehensive activity factors and inclusion of emissions from fat-tail sites.

The Effect of Non-Luminous Thermal Radiation in Microgravity Droplet Combustion

The effect or radiative heat loss from isolated droplet flames is usually assumed to be negligible. For the small droplet sizes studied in most isolated droplet combustion experiments conducted to dale (< 1.0 mm), this assumption has been shown to be reasonable. For example, by neglecting radiation, a detailed numerical model accurately predicts the burning rate, flame position and extinction diameter for 1 millimeter-sized methanol/waler droplets. However, recent space-based 3 to 5 millimeter methanol/water droplet combustion data show an increase in extinction diameter and a decrease in burning rate with increasing initial diameter. These results suggest that, at larger initial droplet diameters, the effect of radiative heat loss cannot be neglected. By including a radiation sub-model, the modified numerical model predicts that at droplet diameters greater than about 1 mm the effect of radiation results in a decrease in burning rate and a non-linear increase in extinction diameter with increasing initia...

Reconciling divergent estimates of oil and gas methane emissions

Significance Past studies reporting divergent estimates of methane emissions from the natural gas supply chain have generated conflicting claims about the full greenhouse gas footprint of natural gas. Top-down estimates based on large-scale atmospheric sampling often exceed bottom-up estimates based on source-based emission inventories. In this work, we reconcile top-down and bottom-up methane emissions estimates in one of the country’s major natural gas production basins using easily replicable measurement and data integration techniques. These convergent emissions estimates provide greater confidence that we can accurately characterize the sources of emissions, including the large impact that a small proportion of high-emitters have on total emissions and determine the implications for mitigation. Published estimates of methane emissions from atmospheric data (top-down approaches) exceed those from source-based inventories (bottom-up approaches), leading to conflicting claims about the climate implications of fuel switching from coal or petroleum to natural gas. Based on data from a coordinated campaign in the Barnett Shale oil and gas-producing region of Texas, we find that top-down and bottom-up estimates of both total and fossil methane emissions agree within statistical confidence intervals (relative differences are 10% for fossil methane and 0.1% for total methane). We reduced uncertainty in top-down estimates by using repeated mass balance measurements, as well as ethane as a fingerprint for source attribution. Similarly, our bottom-up estimate incorporates a more complete count of facilities than past inventories, which omitted a significant number of major sources, and more effectively accounts for the influence of large emission sources using a statistical estimator that integrates observations from multiple ground-based measurement datasets. Two percent of oil and gas facilities in the Barnett accounts for half of methane emissions at any given time, and high-emitting facilities appear to be spatiotemporally variable. Measured oil and gas methane emissions are 90% larger than estimates based on the US Environmental Protection Agency’s Greenhouse Gas Inventory and correspond to 1.5% of natural gas production. This rate of methane loss increases the 20-y climate impacts of natural gas consumed in the region by roughly 50%.

Quantitative Measurement of Direct Nitrous Oxide Emissions from Microalgae Cultivation

Although numerous lifecycle assessments (LCA) of microalgae-based biofuels have suggested net reductions of greenhouse gas emissions, limited experimental data exist on direct emissions from microalgae cultivation systems. For example, nitrous oxide (N(2)O) is a potent greenhouse gas that has been detected from microalgae cultivation. However, little quantitative experimental data exist on direct N(2)O emissions from microalgae cultivation, which has inhibited LCA performed to date. In this study, microalgae species Nannochloropsis salina was cultivated with diurnal light-dark cycling using a nitrate nitrogen source. Gaseous N(2)O emissions were quantitatively measured using Fourier transform infrared spectrometry. Under a nitrogen headspace (photobioreactor simulation), the reactors exhibited elevated N(2)O emissions during dark periods, and reduced N(2)O emissions during light periods. Under air headspace conditions (open pond simulation), N(2)O emissions were negligible during both light and dark periods. Results show that N(2)O production was induced by anoxic conditions when nitrate was present, suggesting that N(2)O was produced by denitrifying bacteria within the culture. The presence of denitrifying bacteria was verified through PCR-based detection of norB genes and antibiotic treatments, the latter of which substantially reduced N(2)O emissions. Application of these results to LCA and strategies for growth management to reduce N(2)O emissions are discussed.

Hydroxyl radical chemiluminescence imaging and the structure of microgravity droplet flames

A procedure is outlined that uses hydroxyl (OH) radical chemiluminescence measurements along with detailed numerical modeling to determine flame position and gain further insight into the structure of microgravity droplet flames. To validate this procedure, microgravity n -heptane and methanol droplet combustion experiments were conducted using the 2.2-second drop tower and the ero Gravity Facility at NASA Lewis Research Center. The spontaneous emission from electronically excited hydroxyl radicals (OH o ) within the envelope diffusion flame was measured with a UV-sensitive video camera. The OH o emission data was deconvoluted using an inverse Abel transform to determine the time evolution of the location of peak intensity within the flame. Chemical reactions describing the production, emission, and quenching of OH o were incorporated into a transient, spherically symmetric droplet combustion model. The modeling and experimental results indicate differences in the route of OH o production between n -heptane and methanol flames. For n -heptane, the production of OH o emission-intensity profiles which agree well with experiment (both in terms of shape and location of maximum intensity) and are shifted from the position of maximum ground-state OH concentration. For methanol flames, which produce very little CH, the OH o appears to be the result of thermal excitation within the flame rather than from a specific chemiluminescent reaction. In both cases, the location of maximum OH o emission intensity is very near the location of maximum flame temperature, suggesting that OH o imaging is a good approach for measurement of the flame position.

Methane Emissions from United States Natural Gas Gathering and Processing

New facility-level methane (CH4) emissions measurements obtained from 114 natural gas gathering facilities and 16 processing plants in 13 U.S. states were combined with facility counts obtained from state and national databases in a Monte Carlo simulation to estimate CH4 emissions from U.S. natural gas gathering and processing operations. Total annual CH4 emissions of 2421 (+245/-237) Gg were estimated for all U.S. gathering and processing operations, which represents a CH4 loss rate of 0.47% (±0.05%) when normalized by 2012 CH4 production. Over 90% of those emissions were attributed to normal operation of gathering facilities (1697 +189/-185 Gg) and processing plants (506 +55/-52 Gg), with the balance attributed to gathering pipelines and processing plant routine maintenance and upsets. The median CH4 emissions estimate for processing plants is a factor of 1.7 lower than the 2012 EPA Greenhouse Gas Inventory (GHGI) estimate, with the difference due largely to fewer reciprocating compressors, and a factor of 3.0 higher than that reported under the EPA Greenhouse Gas Reporting Program. Since gathering operations are currently embedded within the production segment of the EPA GHGI, direct comparison to our results is complicated. However, the study results suggest that CH4 emissions from gathering are substantially higher than the current EPA GHGI estimate and are equivalent to 30% of the total net CH4 emissions in the natural gas systems GHGI. Because CH4 emissions from most gathering facilities are not reported under the current rule and not all source categories are reported for processing plants, the total CH4 emissions from gathering and processing reported under the EPA GHGRP (180 Gg) represents only 14% of that tabulated in the EPA GHGI and 7% of that predicted from this study.