COMBUSTION AND HEAT TRANSFER CHARACTERISTICS IN A SEMI INDUSTRIAL FLUIDIZED BED COMBUSTOR

Abstract

The present work is a detailed study for one of the most qualified furnaces either in heat transfer processes or reduction of pollution, named fluidized bed combustor. It is the first time in Egypt to carry out the experimental work of scientific research using industrial scale test rig. The study aims to investigate the influence of some operating parameters on the combustion characteristics of liquefied petroleum gas and light oil, and the heat transfer modes in relatively large scale fluidized bed combustor. The mechanism of formation of relatively low concentration ofNOx at low temperature in the fluidized beds, which is hampered by lack of fundamental work, is one of the main interests of the present work.. The concentration of CO and NOx emissions, through the narrow zone close to and above the point of stoichiometric air to fuel ratio in the different levels of the fluidized bed, is also studied carefully.

The study gives a full description for the behavior of CO and NOx concentration through the different levels of combustor body including explanation of the mechanisms of their formation and reduction. The study tried to optimize the operating conditions to get pollutant emissions under the standard limits of pollution, in addition to introduce some important knowledge which hould be taken into consideration during the design of fluidized bed combustors. The study facilitates, for the designer, to choose the suitable dimensions of combustor body depending on the required operating temperature and excess air factor. It also facilitates, for the operator, to choose the suitable operating conditions, since it introduces many alternative groups of bed temperature and excess air factor with lower pollutant emissions than the standard limits. Developing empirical formulae for the total outside heat transfer coefficient and the conductive heat transfer coefficient from bed materials to immersed cooling coil, is another main interest of the present study. The study tries to minimize the deviation between the experimental and correlation results as possible.

Experimental work was performed on the semi industrial fluidized bed combustor, which is located in the combustion laboratory of the Mechanical Power Engineering Department of Cairo University. The combustor of the test rig has a cylindrical shape with internal diameter of 500mm and height of I OOOmm. Water­ cooling coil, manufactured from steel tube of diameter 3/8" and consists of five rings of diameter 470mm, is located inside the combustor. Two groups of sand particles with mean diameter of about 0.5mm and lmm are used as a bed material.

The results of the present study show that: The quantity of heat absorbed by immersed cooling coil, in the fluidized bed, is highly influenced by the bed temperature and particle size. It increases by about 60% when the bed temperature increases from 650"C to 91 O"C in case of particle size of I mm, while it increases by about 52% when the bed temperature increases from 570"C to 670"C only in case of particle size of0.5mm. However, for the same particle size, the bed temperature does not influence the percent of heat absorbed by the immersed cooling coil, to the total heat liberated from the injected fuel. This percent of heat absorbed by the immersed coil, to the total heat liberated from fuel is greatly influenced by the bed particle size. The cooling coil absorbs about 46% of the total heat liberated from fuel when using sand with particle size of I mm as a bed material.