Search

A two dimensional, steady-state model for describing a variety of reactive and nonreactive flows, including pulverized coal combustion and gasification, is presented. The model, referred to as 93-PCGC-2 is applicable to cylindrical, axi-symmetric systems. Turbulence is accounted for in both the fluid mechanics equations and the combustion scheme. Radiation from gases, walls, and particles is taken into account using a discrete ordinates method. The particle phase is modeled in a lagrangian framework, such that mean paths of particle groups are followed. A new coal-general devolatilization submodel (FG-DVC) with coal swelling and char reactivity submodels has been added.

An important requirement in many industries is the ability to perform on-line monitoring and control of harsh, multi-phase process streams. During the last ten years, significant progress has occurred in the hardware and applications for Fourier transform infrared (FT-IR) spectroscopy. Instrumentation is now available which can perform, in harsh environments, continuous unattended and simultaneous measurements of absorbed (or reflected) and emitted radiation. The applications of FT-IR include: (1) concentrations of multiple species and phases (gases, liquid, particles, surfaces) as low as ppb; (2) temperatures of multiple species and phases (gases, liquid, particles, surfaces) with accuracies as good as +/- 1 degree(s)C at any elevated temperature; (3) measurement of particle sizes; (4) measurement of film thickness; (5) in-situ line-of-sight data; (6) in-situ spatially resolved data using tomography; (7) data on extracted samples; and (8) data on time scales as short as a few milliseconds.

Publisher Summary This chapter discusses a study to examine the importance of key factors, such as oxygen functional group content, water content, and cation content, that are important in retrograde reactions that occur under pyrolysis conditions. Three coals from the Argonne premium sample bank were used. The results from various techniques suggested that nearly all the hydroxyls and carboxyls were methylated for each coal. The results of the X-ray analysis of the exchanged, demineralized, and methylated samples confirmed that the concentrations of exchangeable metal cations (Ca, Mg, Na, and K) were significantly reduced. The results of programmed pyrolysis in the TG-FTIR apparatus of the raw, ion-exchanged, demineralized, and methylated coals indicated significant increases in the yield of tar for the latter two modifications in the case of the two low rank coals. After the samples were characterized, experiments were done under coal liquefaction conditions in a microautoclave reactor, which allows for gas analysis. The extent of retrogressive reactions in pyrolysis and liquefaction for low rank coals (lignite, subbituminous) is significantly reduced by methylation and demineralization.

A new ignition phenomenon has been observed during studies of coal combustion. The experiment, which was described in a recent publication [1], injects coal particles suspended in a roomtemperature carrier gas as a jet into a flowing preheated air stream. The new phenomenon is shown in Fig. 1, which is a photograph of several ignited particles traveling upward. The particles of interest are at the edge of the cold particle stream and have first contact with the surrounding hot preheated gas stream. They ignite and appear to burn on the particle's surface, as indicated by the fact that the width of the luminosity is roughly that of the particle diameter. The particles become hotter with increasing distance. The new ignition phenomenon occurs after the particles have burned for a short time. Under these conditions there is often a secondary ignition event. To the unaided eye, it looks like a bright flash of light at the end of a long luminous trajectory. When viewed through a microscope, this event has the following characteristics.

Utilisation des donnees provenant de trois types de reacteur ayant differentes caracteristiques de chauffe afin de developper une serie de parametres cinetiques

The kinetics of vapor-phase secondary reactions of newly formed coal pyrolysis tars were studied at temperatures and residence times of 500-900/sup 0/C and 0.6-3.9 s, respectively. Tar vapors, generated by heating a helium-swept, shallow packed bed of Pittsburgh No. 8 bituminous coal from room temperature to 550/sup 0/C, at 3/sup 0/C/min, were either rapidly conveyed to collection traps for subsequent characterization or else passed through an adjacent reactor for controlled thermal treatment prior to trapping. Results are described. A multiple, independent, parallel, first-order reaction model also performed well but could not predict step-like behavior in conversion at 0.6-s residence time. A single-reaction first-order decomposition model correlated the conversion data poorly at 0.6- and 1.1-s residence times, and use of the resulting best fit parameters gave totally inadequate predictions of conversions measured at 2.5 and 3.9 s.

To develop accurate predictive models for coal combustion or gasification, it is necessary to know the rate and amount of volatiles release as a function of the particle temperature. The volatiles, which can account for up to 70% of the coal’s weight loss, control the ignition, the temperature and the stability of the flame, which can, in turn, affect the subsequent reactivity and burnout of the char. Unfortunately, there is still controversy concerning the rate of coal pyrolysis. For example, at particle temperatures estimated to be 800°C, rates reported in the literature (derived using a single first-order process to define weight loss or tar evolution) vary from less than 1 s-1 1–4 to more than 100 s-1 5–11 with values in between.12–14

A computer model was developed to relate pyrolysis and soot formation to fuel properties. A Fourier Transform-Infrared Spectrometer (FT-IR) was used to characterize the effect of temperature, pressure, reaction time, and hydrogen and oxygen concentration of fuels undergoing pyrolysis. The problem of how to identify and measure the hydrogen available for release during pyrolysis was investigated. A detailed data base of a variety of fuels undergoing pyrolysis was developed. The data base conditions included: temperature 800-1500 C; pressure 1-15 atm; and reaction time 100ms-1 sec. Experimental data results show that fuels decompose to form small hydrocarbon species prior to the onset of soot formation. A hydrocarbon model was extended to include Rice-Kossiakoff-Herzfeld mechanism to predict the decomposition of long chain aliphatics to small molecules. A free radical soot model was developed to predict the growth of soot precursors from the small molecules. Keywords: Combustion pyrolysis model; Soot formation; Fourier transform-infrared spectroscopy; Rice-Kossiakoff-Herzfeld mechanism; Free radical.

Extensive measurements of flaming and smoldering fires and nuisancelenvironmental sources were performed with Fourier Transform Infrared (lT-IR) spectroscopy of gas phase products. A neural network model was formulated using the so-called Learning Vector Quantization (LVQ) network approach. The LVQ approach contains input and output layers with a hidden layer being a Kohenen layer. The hidden layer learns and performs classification. The inputs to the network are concentrations (from FT-IR measurements) of eighteen (18) gas species. The outputs of the network are classification of the input data as a flaming fire, smoldering fire, nuisance or environmental source. The network was trained and tested using the test data collected during this project. The results were very successful as, among the 248 cases tested, only 12 cases were misclassified, mostly due to the difficulties in classifying the modes of combustion during a transition from a smoldering to a flaming fire. Each case represents the gas phase concentration data at a time step from one of the validation fires, which were different types of fires from the training set. A first generation fire detection system using FT-IR gas measurements and neural networks has been built and implemented.

Considerable controversy exists concerning the rate of coal pyrolysis. At 800 °C, the reported rates derived using a single first-order process to define weight loss vary from