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Title: Thermogravimetric analysis studies of low rank coals and modelling of combustion and gasification processes in entrained systems.
Author: Vamvuka, Despina.
ISNI:       0000 0001 3542 1276
Awarding Body: University of Manchester : UMIST
Current Institution: University of Manchester
Date of Award: 1988
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Thermogravimetric analysis studies were performed on three low rank coals and a lignite (average size 41um), as well as the dense media separated fractions of these in nitrogen and air. All experiments were carried out at 20°C/min and over the temperature range of 25 to 850°C. Such studies have been used to examine the effect of the different minerology of the coals upon the devolatilization and combustion processes, to investigate the effect of mineral matter on coal reactivity, and to provide data for mathematical models of pulverized coal combustion and gasification in entrained systems. The first-order kinetic model, used to characterise the devolatilization and combustion processes, seemed to correlate the experimental data reasonably well. The activation energy values corresponding to devolatilization, and ranging from 22 to 38 KJ/mole, were similar for all coals, while those corresponding to combustion of the devolatilized coal varied between 41 and 96 KJ/mole and were significantly higher. The presence of the mineral matter slightly increased the reactivity of the coals in nitrogen, whereas it affected greatly the temperature sensitivity of the reaction in air, A mathematical model, incorporating thermogravimetric analysis data of Whitwick coal, was developed in order to predict the burning history of a single entrained coal particle, and to study the effect of ambient gas temperature and boundarylayer thickness on the final conversion, This model included a set of ordinary differential equations, describing the reaction rates and the mass and heat transport processes, as well as a partial differential equation, for computing the temperature profile within the particle. The system of equations was solved numerically, The location of the reaction zone on the solid surface, where gas-phase and heterogeneous combustion could occur simultaneously, appeared to describe successfully the combustion mechanism of the particle. The combustion process was chemical reaction rate controlled. The particle behaved essentially isothermally and its lifetime was estimated to be approximately 1.23s. A higher ambient gas temperature or boundary-layer thickness resulted in shorter burn-out times. Finally, a one-dimensional, steady-state model, for an entrained flow coal gasifier, was developed, by using combustion data from thermogravimetric analysis of Whitwick coal. The model was based on mass and energy balances, heterogeneous reaction rates and homogeneous gas-phase equilibrium. The resulting set of nonlinear mixed ordinary differential-implicit algebraic equations was solved numerically, by using modified Euler's method in conjunction with a nonlinear algebraic equation solver. Parametric studies were made, in order to provide a better understanding of the reactor performance, in terms of coal conversion, product gas composition and temperature profiles along the reactor, under various operating conditions, such as feed flow rates and gasifier pressure. High conversion of carbon could be predicted only if the devolatilization reaction proceeded in parallel with the heterogeneous reaction at the coal surface, with oxygen and steam. The model suggested that a two-stage gasification with precombustion, followed by reaction with steam would be possible. The critical parameters in gasification were the steam-to-coal and oxygen-to-coal feed ratios. Data is presented showing their effect on total conversion, synthesis gas composition and calorific value, as a function of reactor pressure. No experimental data for the verification of the simulation was performed, but comparison of the results with those of previous investigations showed consistency.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available