Publications

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Oxygen/carbon dioxide recycle coal combustion is actively being investigated because of its potential to facilitate CO2 sequestration and to achieve emission reductions. In the work reported here, the effect of enhanced oxygen levels and CO2 bath gas is independently analyzed for their influence on single-particle pulverized coal ignition of a U.S. eastern bituminous coal. The experiments show that the presence of CO2 and a lower O2 concentration increase the ignition delay time but have no measurable effect on the time required to complete volatile combustion, once initiated. For the ignition process observed in the experiments, the CO 2 results are explained by its higher molar specific heat and the O2 results are explained by the effect of O2 concentration on the local mixture reactivity. Particle ignition and devolatilization properties in a mixture of 30% O2 in CO2 are very similar to those in air.

Over the years, researchers have been investigating large-scale pool fores, both experimentally and numerically, because of the risk they pose during transport accidents. In the course of developing and validating computational models, researchers have come to realize that knowledge of the soot concentration, temperature and optical properties within fires is required to quantify the amount of heat transferred. In turn, such knowledge may help in understanding the dynamics of fires, particularly large accidental ones.

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Oxygen-fuel fired glass melting furnaces have successfully reduced NO x and particulate emissions and improved the furnace energy efficiency relative to the more conventional air-fuel fired technology. However, full optimisation of the oxygen/fuel approach (particularly with respect to crown refractory corrosion) is unlikely to be achieved until there is improved understanding of the effects of furnace operating conditions on alkali vaporization, batch carryover, and the formation of gaseous air pollutants in operating furnaces. In this investigation, continuous online measurements of alkali concentration (by laser induced breakdown spectroscopy) were coupled with measurements of the flue gas composition in the exhaust of an oxygen/natural gas fired container glass furnace. The burner stoichiometry was purposefully varied while maintaining normal glass production. The data demonstrate that alkali vaporization and SO2 release increase as the oxygen concentration in the exhaust decreases. NOx emissions showed a direct correlation with the flow rate of infiltrated air into the combustion space. The extent of batch carryover was primarily affected by variations in the furnace differential pressure. The furnace temperature did not vary significantly during the measurement campaign, so no clear correlation could be obtained between the available measurements of furnace temperature and alkali vaporization.

Future energy systems based on gasification of coal or biomass for co-production of electrical power and gaseous or liquid fuels may require gas turbine operation on unusual fuel mixtures. In addition, global climate change concerns may dictate the production of a CO2 product stream for end-use or sequestration, with potential impacts on the oxidizer used in the gas turbine. In this study the operation at atmospheric pressure of a small, optically accessible swirl-stabilized premixed combustor, burning fuels ranging from pure methane to conventional and H2-rich and H2-lean syngas mixtures is investigated. Both air and CO2-diluted oxygen are used as the oxidizers. CO and NOx emissions for these flames have been determined over the full range of stoichiometrics from the lean blow-off limit to slightly rich conditions (φ ∼ 1.03). The presence of hydrogen in the syngas fuel mixtures results in more compact, higher temperature flames, resulting in increased flame stability and higher NOx emissions. The lean blowoff limit and the lean stoichiometry at which CO emissions become significant both decrease with increasing H2 content in the syngas. For the investigated mixtures, CO emissions near the stoichiometric point do not become significant until (φ > 0.95. At this stoichiometric limit, where dilute-oxygen power systems would preferably operate, CO emissions rise more rapidly for combustion in O2-CO2 mixtures than for combustion in air.

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Oxygen-enhanced and oxygen-fired pulverized coal combustion is actively being investigated, to achieve emission reductions and reduction in flue gas cleanup costs, as well as for coal-bed methane and enhanced oil recovery applications. To fully understand the results of pilot-scale tests and to accurately predict scale-up performance through CFD modeling, fundamental data are needed concerning coal char combustion under these conditions. In the work reported here, the effect of enhanced oxygen levels and CO2 bath gas are independently analyzed for their influence on a single-particle pulverized coal ignition of a U.S. bituminous coal and its char. The experiments show that the presence of CO2 and a lower O2 concentration increase the ignition delay time of both coal and char particles. The char particle results are explained by the difference in the mass diffusivity of CO 2 and N2, whereas the coal particle results require further analysis. © (2005) by the International Pittsburgh Coal Conference.

Flame heights of co-flowing cylindrical ethylene-air and methane-air laminar inverse diffusion flames were measured. The luminous flame height was found to be greater than the height of the reaction zone determined by planar laser-induced fluorescence (PLIF) of hydroxyl radicals (OH) because of luminous soot above the reaction zone. However, the location of the peak luminous signals along the centerline agreed very well with the OH flame height. Flame height predictions using Roper's analysis for circular port burners agreed with measured reaction zone heights when using values for the characteristic diffusion coefficient and/or diffusion temperature somewhat different from those recommended by Roper. The fact that Roper's analysis applies to inverse diffusion flames is evidence that inverse diffusion flames are similar in structure to normal diffusion flames.

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As part of the U.S. Department of Energy (DOE) Office of Industrial Technologies (OIT) Industries of the Future (IOF) Forest Products research program, a collaborative investigation was conducted on the sources, characteristics, and deposition of particles intermediate in size between submicron fume and carryover in recovery boilers. Laboratory experiments on suspended-drop combustion of black liquor and on black liquor char bed combustion demonstrated that both processes generate intermediate size particles (ISP), amounting to 0.5-2% of the black liquor dry solids mass (BLS). Measurements in two U.S. recovery boilers show variable loadings of ISP in the upper furnace, typically between 0.6-3 g/Nm{sup 3}, or 0.3-1.5% of BLS. The measurements show that the ISP mass size distribution increases with size from 5-100 {micro}m, implying that a substantial amount of ISP inertially deposits on steam tubes. ISP particles are depleted in potassium, chlorine, and sulfur relative to the fuel composition. Comprehensive boiler modeling demonstrates that ISP concentrations are substantially overpredicted when using a previously developed algorithm for ISP generation. Equilibrium calculations suggest that alkali carbonate decomposition occurs at intermediate heights in the furnace and may lead to partial destruction of ISP particles formed lower in the furnace. ISP deposition is predicted to occur in the superheater sections, at temperatures greater than 750 C, when the particles are at least partially molten.

A number of industrial combustion systems are adopting oxygen-enhanced firing to improve heat transfer characteristics and reduce emissions. The exhaust gas from these systems is dominated by H2O and CO2 and therefore has substantially different gas properties from traditional combustion exhaust. In the past, laser-induced breakdown spectroscopy (LIBS) has been successfully used for the evaluation of alkali aerosol concentrations in air-based combustion systems. This paper presents results of LIBS measurements of alkali concentrations in a laboratory calibration setup and in an oxygen/natural gas container glass furnace. It shows how both gas conditions (composition and temperature) and the molecular form of the alkali species affect the LIBS signals. The paper proposes strategies for mitigating these effects in future applications of LIBS in oxygen-enhanced combustion systems.

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The dimensionless extinction coefficient (K{sub e}) of soot must be known to quantify laser extinction measurements of soot concentration and to predict optical attenuation through smoke clouds. Previous investigations have measured K{sub e} for post-flame soot emitted from laminar and turbulent diffusion flames and smoking laminar premixed flames. This paper presents the first measurements of soot K{sub e} from within laminar diffusion flames, using a small extractive probe to withdraw the soot from the flame. To measure K{sub e}, two laser sources (635 nm and 1310 nm) were coupled to a transmission cell, followed by gravimetric sampling. Coannular diffusion flames of methane, ethylene and nitrogen-diluted kerosene burning in air were studied, together with slot flames of methane and ethylene. K{sub e} was measured at the radial location of maximum soot volume fraction at several heights for each flame. Results for K{sub e} at both 635 nm and 1310 nm for ethylene and kerosene coannular flames were in the range of 9-10, consistent with the results from previous studies of post-flame soot. The ethylene slot flame and the methane flames have lower K{sub e} values, in some cases as low as 2.0. These lower values of K{sub e} are found to result from the contributions of (a) the condensation of PAH species during the sampling of soot, (b) the wavelength-dependent absorptivity of soot precursor particles, and, in the case of methane, (c) the negligible contribution of soot scattering to the extinction coefficient. RDG calculations of soot scattering, in combination with the measured K{sub e} values, imply that the soot refractive index is in the vicinity of 1.75-1.03i at 635 nm.

Laser-induced breakdown spectroscopy (LIBS) was used in the evaluation of aerosol concentration in the exhaust of an oxygen/natural-gas glass furnace. Experiments showed that for a delay time of 10 {micro}s and a gate width of 50 {micro}s, the presence of CO{sub 2} and changes in gas temperature affect the intensity of both continuum emission and the Na D lines. The intensity increased for the neutral Ca and Mg lines in the presence of 21% CO{sub 2} when compared to 100% N{sub 2}, whereas the intensity of the Mg and Ca ionic lines decreased. An increase in temperature from 300 to 730 K produced an increase in both continuum emission and Na signal. These laboratory measurements were consistent with measurements in the glass furnace exhaust. Time-resolved analysis of the spark radiation suggested that differences in continuum radiation resulting from changes in bath composition are only apparent at long delay times. The changes in the intensity of ionic and neutral lines in the presence of CO{sub 2} are believed to result from higher free electron number density caused by lower ionization energies of species formed during the spark decay process in the presence of CO{sub 2}. For the high Na concentration observed in the glass furnace exhaust, self-absorption of the spark radiation occurred. Power law regression was used to fit laboratory Na LIBS calibration data for sodium loadings, gas temperatures, and a CO{sub 2} content representative of the furnace exhaust. Improvement of the LIBS measurement in this environment may be possible by evaluation of Na lines with weaker emission and through the use of shorter gate delay times.

Fires pose the dominant risk to the safety and security of nuclear weapons, nuclear transport containers, and DOE and DoD facilities. The thermal hazard from these fires primarily results from radiant emission from high-temperature flame soot. Therefore, it is necessary to understand the local transport and chemical phenomena that determine the distributions of soot concentration, optical properties, and temperature in order to develop and validate constitutive models for large-scale, high-fidelity fire simulations. This report summarizes the findings of a Laboratory Directed Research and Development (LDRD) project devoted to obtaining the critical experimental information needed to develop such constitutive models. A combination of laser diagnostics and extractive measurement techniques have been employed in both steady and pulsed laminar diffusion flames of methane, ethylene, and JP-8 surrogate burning in air. For methane and ethylene, both slot and coannular flame geometries were investigated, as well as normal and inverse diffusion flame geometries. For the JP-8 surrogate, coannular normal diffusion flames were investigated. Soot concentrations, polycyclic aromatic hydrocarbon (PAH) laser-induced fluorescence (LIF) signals, hydroxyl radical (OH) LIF, acetylene and water vapor concentrations, soot zone temperatures, and the velocity field were all successfully measured in both steady and unsteady versions of these various flames. In addition, measurements were made of the soot microstructure, soot dimensionless extinction coefficient (&), and the local radiant heat flux. Taken together, these measurements comprise a unique, extensive database for future development and validation of models of soot formation, transport, and radiation.

The structure of laminar inverse diffusion flames (IDF) of methane and ethylene in air was studied using a cylindrical co-flowing burner. IDF were similar to normal diffusion flames, except that the relative positions of the fuel and oxidizer were reversed. Radiation from soot surrounding the IDF masked the reaction zone in visible images. As a result, flame heights determined from visible images were overestimated. The height of the reaction zone as indicated by OH LIF was a more relevant measure of height. The concentration and position of PAH and soot were observed using LIF and laser-induced incandescence (LII). PAH LIF and soot LII indicated that PAH and soot are present on the fuel side of the flame, and that soot is located closer to the reaction zone than PAH. Ethylene flames produced significantly higher PAH LIF and soot LII signals than methane flames, which was consistent with the sooting propensity of ethylene. The soot and PAH were present on the fuel side of the reaction zone, but the soot was closer to the reaction zone than the PAH. This is an abstract of a paper presented at the 30th International Symposium on combustion (Chicago, IL 7/25-30/2004).

Smoke is known to cause electrical equipment failure, but the likelihood of immediate failure during a fire is unknown. Traditional failure assessment techniques measure the density of ionic contaminants deposited on surfaces to determine the need for cleaning or replacement of electronic equipment exposed to smoke. Such techniques focus on long-term effects, such as corrosion, but do not address the immediate effects of the fire. This document reports the results of tests on the immediate effects of smoke on electronic equipment. Various circuits and components were exposed to smoke from different fields in a static smoke exposure chamber and were monitored throughout the exposure. Electrically, the loss of insulation resistance was the most important change caused by smoke. For direct current circuits, soot collected on high-voltage surfaces sometimes formed semi-conductive soot bridges that shorted the circuit. For high voltage alternating current circuits, the smoke also tended to increase the likelihood of arcing, but did not accumulate on the surfaces. Static random access memory chips failed for high levels of smoke, but hard disk drives did not. High humidity increased the conductive properties of the smoke. The conductivity does not increase linearly with smoke density as first proposed; however, it does increase with quantity. The data can be used to give a rough estimate of the amount of smoke that will cause failures in CMOS memory chips, dc and ac circuits. Comparisons of this data to other fire tests can be made through the optical and mass density measurements of the smoke.