Progress of combustion in the furnace of a circulating fluidised bed boiler

Paper in proceeding, 1996

Gas concentrations were measured in the combustion chamber of a circulating fluidized bed boiler(FBB ). Sieved fractions of solid material sampled at different heights were analyzed for combustible matter. Together with the bulk density profile obtained from pressure drop measurements, these data were used to obtain the distribution of combustible matter in the combustion chamber. High concentrations of combustible gases and low concentrations of oxygen Were found in the bottom part, indicating that the bed material to a large extent is subject to reducing conditions. From the gas concentration profile, the degree of combustion/fuel conversion versus boiler height was deduced. This procedure, however, involves a major error since measured concentrations gin, a misrepresentation of the concentrations of the gas flow. This is explained by a through flow of gas through bubbles or voids in the bottom bed with high velocity and high concentration of oxygen. A correction for the hn1ass flow was attempted, which increases the 02 concentration in the lowest position from 2.5 to 11%. This correction compared favorably with the fuel conversion versus height, derived from the distribution of solid combustibles (char) in the combustion chamber. The results show that the concentrations measured in the bottom part of the combustion chamber do not represent the flow and cannot, accordingly, be directly used in mass balanees. This misrepresentation has consequences for the interpretation of gas measurements in the bottom part of FBBs.