Characterizing fuels for atmospheric fluidized bed combustion

Abstract

A complete methodology for characterizing coal combustion in atmospheric fluidized bed reactors is presented. The methodology comprises studies of fragmentation and particle size variations during combustion, necessary to allow an accurate determination of kinetic parameters and attrition rates. Samples of three different carbonaceous materials (a medium-ash lignite, a medium-ash anthracite and a graphite) were pyrolyzed in N 2 and partially burned in air in a bench-scale fluidized bed reactor (5 cm i.d.) at different operating conditions (0.68 mm < d 0 < 3.50 mm, 750°C < T b < 950°C and U = 0.13 m/s). The samples were retrieved from the sand-bed by means of a basket. The particle size distribution, apparent density and number of particles were evaluated by Image Analysis. Thus, the evolution of these parameters with burnoff was determined for different materials. Additionally, the sphericity factors were calculated. Combustion studies were carried out in batch experiments in the laboratory-scale, fluidized bed reactor at the same operating conditions. The reactor outlet concentrations of O 2, CO 2, and CO were monitored continuously. The results indicate that only anthracite particles experienced both primary (due to devolatilization) and secondary (during char combustion) fragmentation. Graphite particles underwent secondary fragmentation, whereas lignite particles did not significantly vary in number during combustion. Size and density variations during combustion suggest that graphite particles burn under regime II, intraparticle diffusion being the rate controlling step. On the other hand, anthracite and lignite particles developed an ash layer, which may control combustion. The attrition constants of the medium-ash materials (lignite and anthracite) were found to be very low (0.3 × 10 −7 and 0.8 × 10 −7, respectively), whereas that of graphite was much higher (17.6 × 10 −7), due mainly to peripheral percolation during combustion.