Controlling mechanisms and performance of coal burning rijke type pulsating combustors

This paper describes an investigation which has been concerned with the determination of the performance and mechanisms which control the operation of coal burning, Rijke type pulsating combustors. The combustor consisted of a vertical tube open at both ends. Air entered the combustor through its lower end and unpulverized coal was burned on a metal grid located at the center of its lower half. Heat released by the combustion process excited the fundamental acoustic mode of the combustor. The interaction between the acoustic oscillations and the combustion process resulted in high combustion efficiencies and high combustion intensities. Furthermore, the presence of pulsations in the flow increased convective heat transfer rates to the combustor walls. Results obtained in this study showed that the amplitude of the pulsations increased as the steady temperature ratio between the combustion products and the cold air, TH/Tc, at the combustion bed increased. Also, the amplitude of pulsations depended upon the acoustic losses in the combustion bed. For a given air/fuel ratio, most of the combustion occurred in the bed for low coal feed rates. However, as the coal feed rate increased, a fraction of the coal particles burned in spouting type combustion above the bed. Combustion efficiencies higher than 95% were attained when the combustor was operated with 13% excess air; performance which compares very favorably with other combustors (e.g., stokers) which burn unpulverized coal. Measurements indicated that NOx production in the developed combustor could be controlled by combustion stating. Finally, the simplicity of the combustor, its high combustion intensity, high convective heat transfer rate and low excess air operation suggest that the utilization of this combustor would involve low capital investment and low operating costs.