FUEL NITROGEN CONVERSION DURING FUEL RICH COMBUSTION OF PULVERIZED COAL AND CHAR
KeywordsCoal, Pulverized -- Combustion.
Char -- Combustion.
Nitrogen compounds -- Combustion.
AdvisorWendt, J. O. L.
MetadataShow full item record
PublisherThe University of Arizona.
RightsCopyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.
AbstractThe conversion of coal and char nitrogen has been investigated during fuel rich combustion. The experiments were done with the objective of clarifying the roles of NO, HCN, and NH₃, and char nitrogen in the post-combustion gases in the first, fuel rich stage of a staged combustor. The experimental apparatus includes a downflow combustor of 15 cm internal diameter and 180 cm length constructed of fibrous alumina insulation surrounding a central tube composed of vacuum- formed alumina cylinders. The combustion gases and solids were sampled in situ with a water-cooled and -quenched probe. Neither the combustor nor the sample probe were found to be reactive towards NO. Temperatures of the gases and walls were measured with Type K thermocouples and the particle temperatures were determined with a seven wavelength infrared pyrometer. Gas compositions were measured chromatographically using a 5A molecular seive for permanent gases (H₂, O₂, N₂, CO, and CH₄) and Poropak T for polar gases (CO₂ and HCN). A chemiluminescent analyzer measured NO. NH₃ and HCN were measured in the quench water with ion electrodes. The C, H, N, ash compositions of the char were measured with an elemental analyzer. Experiments of the fuel rich conversion of char nitrogen show that at all stoichiometries (SR = 0.8, 0.4) the concentrations of HCN and NH₃ in the post-flame gases are small compared to the concentration of NO. Char nitrogen conversion was stoichiometric or greater. NO destruction was found to be controlled by a heterogeneous mechanism involving the char carbon surface. The mechanism is deactivated by oxygen, an effect demonstrated by others. The fuel rich conversion of coal nitrogen was investigated with a Utah bituminous coal. At moderate fuel rich conditions (SR = 0.8), the residual char nitrogen conversion is 90 percent or greater and NH₃ and HCN concentrations were less than 20 ppmv. NO peaked at 1200 ppmv (1850 K) and declined to 600 (1580 K) ppmv over 1.8 seconds. Coal nitrogen conversion is dominated by NO formation at this stoichiometry. At extreme fuel rich conditions (SR = 0.4), coal nitrogen conversion is 85 percent. The gas is dominated by HCN, NO, and NH₃. HCN decayed from 600 ppm to 300 ppmv, NO from 350 to 50, and NH₃ increased from 200 to 375 ppmv, indicating that interconversion reactions in the gas phase are dominating. The kinetics which govern the volatile nitrogen reactions can be described by global homogeneous kinetics as follows: UNFORMATTED TABLE/EQUATION FOLLOWS: r₁ = d/dt[HCN] = -5.5x10¹⁷ exp(-83.3 K/RT)[HCN][H₂O]/[H₂]¹/² mole/cm³s r₂ = d/dt[NO] = -2.2x10¹⁶ exp(-54.4 K/RT)[NO][NH₃]/[H₂]¹/² d/dt[NH₃] = d/dt[NO] - d/dt[HCN] UNFORMATTED TABLE/EQUATION ENDS These yield rates for free radical reactions very similar to those determined in gas flame experiments, lending credence to their validity. A one-dimensional combustor model has been formulated which accounts for the heterogeneous combustion and gasification of the coal and char. This model includes the devolatilization of the coal and homogeneous oxidation of carbon monoxide and devolatilized species. The water-gas shift reaction is assumed to be equilibrated. The model also includes the mass, momentum and energy balances of the particles but obviates the solution of the combustor heat balance by using the measured gas temperature in the solution. The model accurately predicts the gas and elemental conversions and particle temperatures observed in the experiments, and supports the homogeneous and heterogeneous kinetics of post-combustion fuel nitrogen conversion.
Degree ProgramGraduate College
Degree GrantorUniversity of Arizona
Showing items related by title, author, creator and subject.
THE EFFECT OF COAL TYPE, RESIDENCE TIME AND COMBUSTION CONFIGURATION ON THE SUBMICRON AEROSOL COMPOSITION AND SIZE DISTRIBUTION FROM PULVERIZED COAL COMBUSTION (STAGED, FLYASH, SPECIES ENRICHMENT).Peterson, Tom; LINAK, WILLIAM PATRICK. (The University of Arizona., 1985)Pulverized samples of Utah bituminous, Beulah (North Dakota) low Na lignite, Beulah high Na lignite and Texas (San Miguel) lignite coals were burned at a rate of 2.5 kg/hr in a laboratory furnace under various (overall fuel lean) combustion conditions. Particle size distributions (PSD) and size segregated particle filter samples were taken at various positions within the convection section. Temperature and gas concentrations were measured throughout. The evolution of the submicron PSD within the convection section for the four coals was similar, although the location of the initial particle mode at the convection section inlet varied with coal type. While staged (.8/1.2) combustion of the Utah bituminous coal had a variable effect on the volume of submicron aerosol produced, staged combustion of two of the three lignites (Beulah low Na and Texas) caused a definite increase in the submicron aerosol volume. Vapor enhancement due to a localized reducing atmosphere, which would effect coals of higher ash volatility or higher inherent ash content, is thought to explain this behavior. Depressed combustion temperatures associated with the high moisture content of the Beulah high Na lignite are thought to offset the effects of staging. Increased combustion temperatures (through oxygen enrichment) caused staged volume increases for the Beulah high Na lignite. Combustion temperatures are a controlling factor even at more extreme staging conditions. Chemical analysis of the size segregated particle samples show the trace elements, As, Pb, Zn and the major elements, Na and K to be enriched in the submicron aerosol. Auger depth profiles show these small particles to be comprised of a core enriched in Fe, Si, Ca and Mg and surface layers enriched in Na and K. These results point to a mechanism of homogeneous nucleation of low vapor pressure species followed by successive layering of progressively more volatile species. Volatile species are enriched in the submicron aerosol due to the large surface areas provided. Modeling efforts show that while coagulation may be the dominant mechanism to describe the aerosol evolving within the convection section, it cannot be used solely to predict the PSD. Another mechanism, presumably surface area dependent growth (condensation) must be included.
On the path to elucidating the speciation of mercury in the flue gases of coal combustionBlowers, Paul; Wendt, Jost O. L.; Wilcox, Jennifer (The University of Arizona., 2004)The goal of this research is to understand the speciation of mercury in the flue gases of coal combustion. As the flue gas cools, thermochemical equilibrium calculations indicate that elemental mercury, Hg⁰, is converted to oxidized mercury, Hg²⁺, in the form of HgO or HgCl₂. Hg⁰ is insoluble in water, HgO has low solubility in water and HgCl₂ is highly soluble in water. Since HgCl₂ is water-soluble, it can be captured in wet chemical scrubbers to prevent its release to the atmosphere. Therefore, the understanding of the mechanisms of mercury's oxidation in flue gases is paramount when considering mercury capture. This research attempts to elucidate the mechanisms of oxidation through a detailed kinetic and thermodynamic analysis. The current research focuses specifically on the oxidation of mercury via chlorine-containing compounds. Future research will involve the oxidation via oxygen-containing compounds and the effect of SO₂ and NOₓ compounds on mercury's oxidation. Quantum chemistry is used to determine accurate transition structures, which are required for the calculation of activation energies and rate constants from theory. Simultaneous to the theoretical work, an experimental apparatus has been designed and fabricated with the inclusion of a quadrupole mass spectrometer. The mass spectrometer is used in conjunction with a laminar flow reactor to simulate the oxidation of mercury via chlorine-containing compounds in flue gases. The ultimate goal of this research is to obtain a potential mercury oxidation mechanism based upon theoretically predicted kinetic parameters, which are then validated through concentration profiles obtained from experimental measurements. In addition, results from the experimental work indicate that at ambient conditions, the oxidation of mercury via chlorine may result as a consequence of heterogeneous reactions involving the Pyrex reactor surface. This work not only allows for a more thorough understanding of mercury's speciation in the flue gas environment, but also questions current sampling devices and their potential interference with reactivity measurements involving mercury-chlorine species.