Post-combustion Gas Research Articles

Formation of polychlorinated naphthalenes from chlorophenols

To test the hypothesis that polychlorinated naphthalenes (PCNs) can be formed from chlorinated phenols via cyclopentadienyl radicals in post-combustion gas containing 8% oxygen, the slow combustion of three chlorophenols (CPs) at 600 °C was studied in a laminar flow reactor. Contrary to the a priori hypothesis, different distributions of PCN isomers were produced from each CP. To explain these distributions and the observed correlation between polychlorinated dibenzofuran (PCDF) and PCN yields, a mechanism is proposed that builds on published mechanisms of PCDF formation from chlorinated phenols and naphthalene formation from dihydrofulvalene. This mechanism involves phenoxy radical coupling at unsubstituted ortho-carbon sites followed by CO elimination to produce dichloro-9,10-dihydrofulvalene intermediates. Naphthalene products are formed by loss of H and/or Cl atoms and rearrangement. Of the three chlorophenols, 3-CP produced the highest PCN and PCDF yields, as well as the greatest number of PCN and PCDF product; 2-CP produced the lowest PCN and PCDF yields. Phenol, formed by hydrodechlorination of CP, led to the formation of naphthalene and monochloronaphthalene products. This work provides a better understanding of isomer-specific PCN formation in post-combustion gas and may be developed and used as a tool for mechanism attribution in combustion systems.

Effects of mixing on ammonia oxidation in combustion environments at intermediate temperatures

Ammonia often occurs in combustion gases as a product of fuel-nitrogen. Since either NO or N 2 may predominate as the product of ammonia oxidation, there is considerable interest in understanding the factors responsible for selecting the final products. In flames, the selectivity is known to depend on whether the reaction zones are premixed or nonpremixed. This paper reports on a combined experimental and modeling investigation of ammonia chemistry in a hot combustion environment that is below flame temperatures, such as in post-combustion gases. Experiments that mix highly diluted ammonia–methane and oxygen–water streams are interpreted in terms of a plug-flow model, a simplified mixing reactor model, and a two-dimensional direct numerical simulation. The study finds that the final products of ammonia oxidation remain sensitive to mixing even at temperatures below those of self-sustaining flames. At low temperatures, ammonia oxidation occurs in a premixed reaction zone, but at sufficiently high temperatures a nonpremixed reaction zone may develop that produces significantly less NO than the equivalent premixed system. A direct numerical simulation is required to predict the behavior over the full range of conditions investigated experimentally, while a simplified mixing reactor model captures the essential features as long as the radial gradients are not too steep.

Performance analysis of combined microgas turbines and gas fired water/LiBr absorption chillers with post-combustion

The integration of microgas turbines (MGT) and absorption chillers is an emerging technology that uses a wide range of fuels to produce electricity, cooling and heating simultaneously for small scale distributed generation in grid connected or isolated locations. This paper studies the performance of MGTs of different power capacities directly coupled to double-effect water–LiBr absorption chillers. In these systems the MGT exhaust gas is the heating medium to drive the chiller. Also post-combustion natural gas is used to increase the cooling capacity of the system. The paper analyses the effect of the post-combustion degree on the integrated system performance of four MGT power sizes. Two cases are considered. In the first, fresh air is added together with the post-combustion natural gas and in the second it is not added. In the latter case the oxygen necessary for the combustion reaction is extracted from the MGT exhaust gas stream. For the sake of comparison a study is also made of the performance of a more conventional system consisting of an MGT and a hot water heat exchanger to drive an absorption chiller. The main advantages of the new technology over this conventional system are that the COP of the chillers is higher because they are driven by higher temperatures, the production of electricity and chilled water is decoupled and there is a wider range of chilled water production capacity.

A comparative study of high-temperature erosion wear of plasma-sprayed NiCrBSiFe and WC–NiCrBSiFe coatings under simulated coal-fired boiler conditions

A comparative study was carried out of the behaviour of plasma sprayed NiCrBSiFe and WC–NiCrBSiFe alloys subjected to conditions which simulate a post-combustion gas atmosphere from a coal-fired boiler combustor. The study first evaluates the effects of thermal exposure at high temperatures on the microstructure of the coatings and on the adherence between substrate (austenitic stainless steel) and coatings. The oxidation rates of these coatings in atmospheres with 3–3.5% of free oxygen at 773 and 1073 K were then evaluated. The effect of WC on the low-velocity corrosion–erosion behaviour produced by the impact of fly ashes in the gas stream at high temperatures (773 and 1073 K) was assessed under impact angles of 30° and 90°. Finally, the eroded surfaces were analysed using scanning electron microscopy in order to characterize the ash embedment phenomena and the operating erosive micromechanisms.

PAH and soot emissions from burning components of medical waste: examination/surgical gloves and cotton pads

This is a laboratory investigation on the emissions from batch combustion of representative infectious (“red bag”) medical waste components, such as medical examination latex gloves and sterile cotton pads. Plastics and cloth account for the majority of the red bag wastes by mass and, certainly, by volume. An electrically heated, horizontal muffle furnace was used for batch combustion of small quantities of shredded fuels (0.5–1.5 g) at a gas temperature of ≈1000°C. The residence time of the post-combustion gases in the furnace was ≈1 s. At the exit of the furnace, the following emissions were measured: CO, CO 2, NO x , particulates and polynuclear aromatic compounds (PACs). The first three gaseous emissions were measured with continuous gas analyzers. Soot and PAC emissions were simultaneously measured by passing the furnace effluent through a filter (to collect condensed-phase PACs) and a bed of XAD-4 adsorbent (to capture gaseous-phase PACs). Analysis involved soxhlet extraction, followed by gas chromatography–mass spectrometry (GC–MS). Results were contrasted with previously measured emissions from batch combustion of pulverized coal and tire-derived fuel (TDF) under similar conditions. Results showed that the particulate (soot) and cumulative PAC emissions from batch combustion of latex gloves were more than an order of magnitude higher than those from cotton pads. The following values are indicative of the relative trends (but not necessarily absolute values) in emission yields: 26% of the mass of the latex was converted to soot, 11% of which was condensed PAC. Only 2% of the mass of cotton pads was converted to soot, and only 3% of the weight of that soot was condensed PAC. The PAC yields from latex were comparable to those from TDF. The PAC yields from cotton were higher than those from coal. A notable exception to this trend was that the three-ring gas-phase PAC yields from cotton were more significant than those from latex. Emission yields of CO and CO 2 from batch combustion of cotton were, respectively, comparable and higher than those from latex, despite the fact that the carbon content of cotton was half that of latex. This is indicative of the more effective combustion of cotton. Nearly all of the mass of carbon of cotton gasified to CO and CO 2, while only small fractions of the carbon in latex were converted to CO 2 and CO (20% and 10%, respectively). Yields of NO x from batch combustions of latex and cotton accounted for 15% and 12%, respectively, of the mass of fuel nitrogen indicating that more fuel nitrogen was converted to NO x in the former case, possibly due to higher flame temperatures. No SO 2 emissions were detected, indicating that during the fuel-rich combustion of latex, its sulfur content was converted to other compounds (such as H 2S) or remained in the soot.

High temperature erosion wear of flame and plasma-sprayed nickel–chromium coatings under simulated coal-fired boiler atmospheres

This study concerns the behaviour of plasma and flame sprayed modified nickel–chromium alloy (with small aluminium and titanium additions) subjected to the action of simulated post-combustion gases from a coal-fired boiler combustor. The study first evaluates the effects of thermal exposure at high temperature on the adherence between the substrate (austenitic stainless steel) and coatings. Then, the oxidation rates of these coatings in atmospheres with 3–3.5% of free oxygen at 500 and 800°C (773 and 1073 K) were evaluated. The low velocity corrosion–erosion behaviour produced by the impact of fly ashes in the gas stream at high temperatures (773 and 1073 K) was also evaluated under impact angles of 30 and 90°. Finally, the eroded surfaces were analysed using scanning electron microscopy in order to characterise the ash embedment phenomena and the operating erosive micromechanisms.

The role of particles in the inhibition of premixed flames by iron pentacarbonyl

Laser light scattering has been used to investigate particle formation in Fe(CO) 5-inhibited premixed flames in order to understand the influence of metal and metal oxide condensation on flame inhibition. In premixed CH 4–air flames, particles form early in the flame zone, nucleate and grow to a peak scattering cross section, then disappear as the temperature increases to its peak value. Downstream in the postcombustion gases, the peak scattering signal is several orders of magnitude larger than the peak value near the main reaction zone of the flame. Thermophoretic particle sampling and numerical estimates indicate nanoparticles with diameters between 10 and 30 nm. As the mole fraction of iron pentacarbonyl in the flame is increased, a concentration is reached at which both the burning velocity becomes constant and particle nucleation begins. A model of an ideal heterogeneous inhibitor shows that radical recombination on particle surfaces alone cannot account for the magnitude of the observed inhibition. Measurements in three CO-H 2 flames with similar adiabatic flame temperatures but different burning velocities demonstrate the importance of residence time for particle formation in premixed flames.

Gas-phase transformations of mercury in coal-fired power plants

Because mercury enters the food chain primarily through atmospheric deposition, exposure models require accurate information about mercury emission rates and mercury speciation from point sources. Since coal-fired power plants represent a significant fraction of the anthropogenic emissions of mercury into the atmosphere, the speciation of mercury in coal-fired power plant flue gas is currently an active topic of research. We have demonstrated that the assumption of gas-phase equilibrium for mercury-containing species in coal-fired power plant exhaust is not valid at temperatures below approximately 800 K (500°C). Chlorine-containing species have been shown to be the most important for oxidation of elemental mercury in the post-combustion gases. The conversion of HCl to Cl 2 in the flue gas of a coal-fired power plant is kinetically limited. Kinetic calculations of the homogeneous oxidation of elemental mercury by chlorine-containing species were carried out using global reactions from the literature. The levels of mercury oxidation, while of comparable magnitude to field observations, are still below the 40% to 80% oxidation typically observed in field measurements.

Radiation and velocity fields induced by localized temperature fluctuations

A theory describing the coupling between radiative transport, thermal conduction, and velocity fluctuations in postcombustion gases is described. The emission and absorption of radiant energy are taken to be dominated by soot, distributed uniformly in space. The theory is local in the sense that the gas is assumed to be unbounded. However, the temperature, velocity, and radiation fields can be both three-dimensional and time-dependent. Moreover, the model can be thought of as describing any postcombustion scenario in which the absorption coefficient is spatially uniform. Within the framework of the low Mach number combustion equations, an exact representation of the velocity and radiation fields instantaneously induced by fluctuations of any magnitude in the temperature is presented. This result is used to derive a single scalar integro-differential equation for the temperature that incorporates the conservation of mass and energy, together with an exact solution of the radiative transport equation. Some consequences of the theory are illustrated by studying the response generated by a spherically burning fuel mass.

Remote sensing of gas emissions on natural gas flares

Emissions from operational natural gas flares are examined by a remote sensing technique using a commercial moderate-resolution Fourier-transform infrared spectrometer. The thermal radiation emitted by the post-combustion gas is analysed to determine plume temperatures and concentrations of and . The multicomponent air pollution software (MAPS) is applied which is based on radiative transfer line-by-line calculations and least-squares fit procedures. Emission rates and combustion efficiencies are calculated which indicate that the local environmental impact of methane emissions from natural gas flares is small, while significant amounts of carbon dioxide are released.

PAH and Soot Emissions from Combustion of Coal and Waste Tire-derived-fuel in Fixed Beds

This is a laboratory investigation on the emissions from batch combustion of waste tire-derived-fuel (TDF) and coal in fixed beds. The goal was to compare the magnitude of the emissions of this mode of combustion to that of continuous combustion of streams of particles of the same fuels in pulverized form, conducted previously (Levendis et al., 1996; Levendis et al., 1998). In the experiments herein waste tire chunks (in the range of 3-9 mm), tire crumb (180 212 μm) and bituminous coal (63-75 μm) were burned in a horizontal muffle furnace, at a gas temperature of 1000°C. The fuel mass loading in the furnace was varied; the residence time of the post-combustion gases was 1 s. At the exit of the furnace CO, CO 2 , NO,, polynuclear aromatic hydrocarbon (PAH) and particulate emissions were measured. Results showed that the PAH emissions from batch combustion of tire chunks were higher than those from tire crumb, which in turn were an order of magnitude higher than those from coal, under similar combustion conditions. Increasing the mass loading for both tire crumb and pulverized coal, increased the specific emissions of PAHs. Increasing the segmentation of the tire, at a fixed mass loading also increased the specific PAH emissions. Emissions of particulates closely followed the trend of PAH emissions. Batch combustion of tire chunks resulted in higher particulate yields than tire crumb, with pulverized coal a distant third. Specific CO emissions from batch combustion of TDF were an order of magnitude higher than those from coal combustion. For both fuels, CO emissions were detected during the release and combustion of the volatiles only. On the other hand, specific CO 2 , and, especially, NO x emissions from batch combustion oftire crumb were lower than those from coal. The comparison of the emissions of batch combustion of pulverized fuels in fixed beds with those of continuous combustion of streams of particles was conducted under laminar flow conditions, similar gas temperatures and similar post combustion residence times. Results showed that (i) the PAH and CO emissions of tire crumb were much lower in the latter mode of combustion, even at bulk equivalence ratios as high as 1.4; (ii) to the contrary, for coal the mode of combustion had little influence on the PAH and CO emissions.

Pyrolysis product absorption by burning benzene droplets

By burning droplets of benzene in a single-droplet combustor and performing phase-discriminating sampling of the liquid and gas phases of the droplet system, we have found that gas-phase pyrolysis products arise in the liquid phase of the droplet. The experiments are conducted at 1000 K and 21 mol % O 2 in the postcombustion gas from an oxygen-rich premixed methane flame. Disruptive burning, which has not previously been reported for a pure hydrocarbon in normal gravity conditions, is observed at the end of the droplet residence time (∼92 ms). Samples of the liquid phase have been taken at various times throughout the combustion lifetime and analyzed by high-pressure liquid chromatography. Compositional analysis using ultraviolet-visible absorbance spectra of the separated components of the samples reveals a wide variety of pure polycyclic aromatic hydrocarbons (PAH), substituted PAH, and cyclopenta-fused PAH. In addition, recent synthesis of new reference standards has enabled identification of cyclopenta-fused PAH—cyclopent[ hi ]acephenanthrylene, cyclopenta[ cd ]fluoranthene, and dicyclopenta[ cd, jk ]pyrene—which have never before been identified as benzene products. Because the droplet remains relatively cold (∼350 K) with respect to the gas phase in the oxygen-deficient zone between the droplet and the flame (∼2000 K), we conclude that these compounds are gas-phase pyrolysis products that are obsorbed into the droplet, rather than products of reactions within the droplet. These heavier species may play a role in observed terminal disruptive burning events by acting as additional droplet components that promote multicomponent effects. Analysis of species concentrations over time reveals the dominance of both ring rupture pyrolysis products such as phenylacetylene, triacetylene, and acenaphthylene, and biaryl pyrolysis products such as biphenyl. These four products in particular represent 70% of the identified mass of absorbed pyrolysis products, which accounts for up to 5% of the droplet mass at the end of its lifetime.

97/04902 Secondary effects in sampling ammonia during measurements in a circulating fluidised-bed combustor: Kassman, H. et al. J. Institute of Energy, September 1997, 70, 95–101

Ammonia often occurs in combustion gases as a product of fuel-nitrogen. Since either NO or N2 may predominate as the product of ammonia oxidation, there is considerable interest in understanding the factors responsible for selecting the final products. In flames, the selectivity is known to depend on whether the reaction zones are premixed or nonpremixed. This paper reports on a combined experimental and modeling investigation of ammonia chemistry in a hot combustion environment that is below flame temperatures, such as in post-combustion gases. Experiments that mix highly diluted ammonia–methane and oxygen–water streams are interpreted in terms of a plug-flow model, a simplified mixing reactor model, and a two-dimensional direct numerical simulation. The study finds that the final products of ammonia oxidation remain sensitive to mixing even at temperatures below those of self-sustaining flames. At low temperatures, ammonia oxidation occurs in a premixed reaction zone, but at sufficiently high temperatures a nonpremixed reaction zone may develop that produces significantly less NO than the equivalent premixed system. A direct numerical simulation is required to predict the behavior over the full range of conditions investigated experimentally, while a simplified mixing reactor model captures the essential features as long as the radial gradients are not too steep.

5547651 Process for production and use of deactivated gaseous atomic nitrogen for post combustion gas nitric oxide emissions control : Rolfe Richard Dana Point, CA, United States assigned to Bleiweis So

5547651 Process for production and use of deactivated gaseous atomic nitrogen for post combustion gas nitric oxide emissions control : Rolfe Richard Dana Point, CA, United States assigned to Bleiweis So

96/05315 The temperature dependence of H + OH recombination in phosphorus oxide containing post-combustion gases

96/05315 The temperature dependence of H + OH recombination in phosphorus oxide containing post-combustion gases

The temperature dependence of H + OH recombination in phosphorus oxide containing post-combustion gases

The temperature dependence of the rate of H + OH recombination is experimentally investigated using laser photolysis of high-temperature (1500–2500 K) water vapor. Parametric adjustment of reaction rate constants to fit a simulated OH signal with the experimental OH decay data gives a three-body reaction rate constant for H + OH + M → H 2O + M that is larger than expected at low temperature and lower at high temperatures. This result could have negative consequences for hypersonic propulsion systems since much of the thrust producing radical recombination occurs at high temperature. When PH 3 combustion products are present in the photolyzed gas mixture at low concentration, the OH decay is much faster and dominated by catalytic reactions that include phosphorus oxides and acids. Parametric reaction rate analysis indicates that two reactions, H + PO 2 → HOPO and H + HOPO → H 2 + PO 2 are responsible for the increase in the overall rate of H + OH recombination. The temperature dependence of the phosphine catalyzed recombination reaction is adequately described by the rate constant estimates previously reported.

The formation of SiO2 from hexamethyldisiloxane combustion in counterflow methane-air flames

Silica formation from hexamethyldisiloxane (HMDS) oxidation was studied by means of a CH 4 −N 2 /air opposed diffusion flame technique to which vaporized HMDS was added to the fuel flow. The CH 4 −N 2 /air flame changed color from a pale-blue flame to whitish pink color when small amounts of HMDS were introduced in the flame. Increasing concentrations (1.3 mol %) made the flame more luminous, and a second thin-flame zone, orange in color, appeared on the fuel side. Emission spectroscopy revealed the existence of Si−H and Si−O species. SiO 2 particles were observed only in the postcombustion gases, and the analysis of solid materials suggested the formation of fused silica particles, which were initially about 10 nm in size, forming outside the flame zone in these experiments. The overall mechanism in addition to that for methane oxidation is suggested as follows: C 6 H 18 Si 2 O+OH=2C 3 H 9 SiO+H C 6 H 18 Si 2 O+O 2 =2C 3 H 9 SiO+O C 6 H 18 Si 2 O+HO 2 =2C 3 H 9 SiO+OH C 3 H 9 SiO+M=3CH 3 +SiO+M SiO+HO 2 =HSiO+O 2 SiO+OH=SiO 2 +H SiO+O+M=SiO 2 +M SiO+O 2 =SiO 2 +O together with the formation of SiOH and SiO(OH) species. The agreement between the model predictions using Sandia code OPPDIF and the experimental data was found to be satisfactory. The model appears to be a useful tool in elaboration of chemistry of formation of SiO 2 in flames used to synthesize pure silica in this way.

Small-scale smoldering combustion experiments in microgravity

Results from small-scale experiments of the smolder characteristics of a porous combustible material (flexible polyurethane foam) in microgravity and normal gravity are presented. The microgravity experiments were conducted in the Spacelab Glovebox on the USML-1 mission of the Space Shuttle Columbia, June/July 1992, and represent the first smolder experiments ever conducted under extended periods of microgravity. The use of the Glovebox limited the size of the fuel sample that could be tested and the power available for ignition but provided the opportunity to conduct such experiments in space. Four tests were conducted, varying the igniter geometry (axial and plate) and the convective environment (quiescent and forced). A series of comparative tests was also conducted in normal gravity. Measurements conducted included temperature histories at several locations along the fuel sample, video recording of the progress of the smolder, and postcombustion char and gas composition analyses. The results of the tests showed that smolder did not propagate without the assistance of the igniter, primarily because of heat losses from the reaction to the surrounding environment. In microgravity, the reduced heat losses caused by the absence of natural convection resulted in only slightly higher temperatures in the quiescent microgravity test than in normal gravity but a dramatically larger production of combustion products in all microgravity tests. Particularly significant is the proportionately larger amount of carbon monoxide and light organic compounds produced in microgravity, despite comparable temperatures and similar char patterns. This excessive production of fuel-rich combustion products may be a generic characteristic of smoldering polyurethane in microgravity, with an associated increase in the toxic hazard of smolder in spacecraft.

A real-time monitor for toxic metal emissions from combustion systems

Excimer laser fragmentation-fluorescence spectroscopy (ELFFS) simultaneously detects many airborne toxic metal species in real time and in situ. With ELFFS, sub-ppm single-shot detection limits are achieved for Ba, Cr, Mn, Ni, Pb, and Tl in postcombustion gases of methane-air flames. Spectra of metals both singly and in mixtures illustrate the response of ELFFS to metals injected into a flame. Background interference from combustion products is negligible. We investigate aspects of the technique relevant to application in real-world devices. The detection signal changes linearly with concentration for each metal injected in the postflame gases. The signal also changes with the aerosol size, which is a strong function of temperature. We find that the ELFFS signal depends on the aerosol surface area, suggesting that the signal evolves from surface ablation. In the cooled gases, where the temperature and aerosol size are constant, the signal is constant, and similar signal levels are observed for four different Pb species injected into the flame. These characteristics are discussed along with the detection limits, and we concluded that ELFFS is a technique capable of functioning as a real-time emissions monitor for the toxic metals listed above.

Kinetic modeling and sensitivity analysis for B/H/O/C/F combination systems

A kinetic model is presented to describe the high-temperature, gas-phase B/H/O/C/F chemistry associated with fluoroamino/nitroamino/B(s) mixtures. The results of thermodynamic constant temperature and pressure calculations for a prototypical fluoroamino/nitroamino-based oxidizer and solid boron are presented to characterize the equilibrium speciation. Key gas-phase reactants are selected and a reaction mechanism describing potential reaction pathways is formulated. Rate parameters for elementary reactions were obtained from published experimental/theoretical data or estimated using transition state argments and scaling relations. The results of kinetic calculations for several adiabatic systems are presented which illustrate the general high temperature mechanistic behavior of B/H/O/C/F systems. Standard reaction flux/pathway and gradient sensitivity analysis techniques are used to identify important reaction pathways. The model results consistently show that OBF(g) is the major high-temperature combustion product. Thermodynamic and kinetic calculations further indicate that for oxygen/fluorine mole ratios near unity, the postcombustion gases will not contain liquid B 2O 3. The model results indicate that relative to B/H/O/C systems, the gas phase kinetics are as fast or faster with fluorine enrichment and that the heat release rate is increased. However, the modeling results also indicate that this feature is sensitive to the oxygen/fluorine mole ratio.