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Five commercially available high volatile bituminous coals from different origins were studied with the objective of characterizing their petrographic nature with respect to emissions of NOx and N2O. The chars produced [at temperatures ranging from 700 to 1000 °C] from these coals were also petrographically analyzed to assess the contribution of char to NOx and N2O formation during combustion. Vitrinite-rich coals produced higher porous chars (cenospheres and tenuinetworks) than those that are rich in inertinite. The former coals were, however, found to release lower concentrations of NO. Consistent with previous works, N2O emissions were observed to decrease significantly with temperature, however, on the whole, the N2O emissions from vitrinite-rich high volatile coals were less than those from inertinite-rich coals. Additionally, hig...
This paper describes an improved model of char burning during black liquor combustion that is capable of predicting net rates of sulfate reduction to sulfide as well as carbon burnup rates. Enhancements include a proper treatment of CO2 and H2O gasification, reactions between oxygen and combustibles in the boundary layer, and integration of sulfate reduction and sulfide reoxidation into the char burning process. Simulations using the model show that for typical recovery boiler conditions, char burning behavior is independent of oxygen concentration up to the point of carbon depletion. Under these conditions, H2O and CO2 gasification reactions are primarily responsible for carbon removal. The H2 and CO coming from the gasifying particle consume oxygen in the boundary layer and help protect against sulfide reoxidation. After carbon deple...
Experimental work was carried out to study the kinetics of the carbon dioxide gasification of a coal char. The coal char was prepared from an Australian bituminous coal by pyrolyzing it at 900 °C in a tube furnace for 9 min in inert environment. The coal char was gasified in a thermo-gravimetric analyzer (TGA) with a mixture of carbon dioxide and nitrogen. The effects of gasification temperature (800-875 °C) and carbon dioxide concentration (10-80%) were studied. In the present study the reaction rate of the pyrolyzed coal was obtained and interpreted by Random pore model.
The emphasis on reduction of NOx as a precursor to street level ozone has increased the need for technologies capable of reducing NOx (>95%) to very low levels in major metropolitan areas from a wide variety of sources. Technology offerings available today may not always be appropriate for every desired application in the utility and industrial sectors. This paper will discuss a new technology under development that has promise to address many of the specialized needs of some of these applications. The technology is directed at NOx reduction but may also address other pollutants like SO2. The technology employees char, a heat treated and devolitilized form of coal, to adsorb NOx from the flue (or waste) gas. Adsorption of greater than 99% has been demonstrated on a lab scale and appears very feasible for scale-up. Microwave energy prop...
Two processes, namely shrinkage and primary fragmentation are known to be the major causes of size reduction of wood during its devolatilization in a fluidized bed combustor. A simple phenomenological model incorporating these effects to compute the average char size at the end of devolatilization is proposed. Experiments are conducted in a bubbling fluidized bed combustor using wood having three different shapes namely, cylinder, cuboid and sphere, to measure the average char size at the end of devolatilization. The model prediction of average char size agrees with the measured values within a deviation of 15%. An experimental correlation is derived to determine the number of fragments and is used to estimate the mean char size. © 2008 Elsevier Ltd. All rights reserved.
As one of the currently available thermo-chemical conversion technologies, biomass gasification has received considerable interest since it increases options for combining with various power generation systems. The product gas or syngas produced from biomass gasification is environmental friendly alternatives to conventional petrochemical fuels for the production of electricity, hydrogen, synthetic transportation biofuels and other chemicals. The product gas normally contains the major components such as CO, H2, CO2, CH4 and H2O, in addition to some organic (e.g., light hydrocarbon species, tar) and inorganic (e.g., H2S, HCl, NH3) impurities depending on operational conditions and gasification processes. Among these impurities, tar can hamper filtration operation and cause equipment fouling due to condensation at lower temperatures, wh...
A generalised model for the prediction of single char particle gasification dynamics, accounting for multi-component mass transfer with chemical reaction, heat transfer, as well as structure evolution and peripheral fragmentation is developed in this paper. Maxwell-Stefan analysis is uniquely applied to both micro and macropores within the framework of the dusty-gas model to account for the bidisperse nature of the char, which differs significantly from the conventional models that are based on a single pore type. The peripheral fragmentation and random-pore correlation incorporated into the model enable prediction of structure/reactivity relationships. The occurrence of chemical reaction within the boundary layer reported by Biggs and Agarwal (Chem. Eng. Sci. 52 (1997) 941) has been confirmed through an analysis of CO/CO2 product rati...
The transition from forward smoldering to flaming in polyurethane foam is observed using in-depth thermocouples and ultrasound probing. The experiments are conducted with small parallelepiped samples vertically placed in an upward wind tunnel. Three of the vertical sample sides are maintained at elevated temperature and the fourth is exposed to an upward oxidizer flow and a radiant heat flux. An ultrasound probing technique is used to measure the line-of-sight average permeability of the sample at the same heights as the thermocouples. The smolder front propagation is tracked by both the thermocouples and ultrasound data, which show an increase in temperature and permeability upon passage of the smolder front. The permeability data also show that the transition to flaming is preceded by rapid fluctuations in permeability in the char ...
Le « gisant », qui a motivé le rapprochement de Char et de Deguy - qui rend souvent hommage au premier -, est chez ces auteurs à la fois une personne et une forme littéraire : on écrit un gisant. D'abord, la scène d'agonie ou la vision du mort constituent une scène originaire fortement intériorisée par le poète et qui recouvre la perception du réel, qui fait office de comparant et impose une vision du monde mélancolique. La représentation endeuillée précède la perception du réel. Mais quand le sujet lyrique donne ainsi son énergie au mort, c'est « donnant donnant » (Michel Deguy) et le tombeau littéraire devient berceau du poème. La défiguration à l'œuvre dans le texte rend possible une autre figuration : le poème s'élabore dans le processus même de lente décomposition d'un corps, d'une figure (père, épouse). Les motifs composant la sc...
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