F. Okasha

Formation rates of characteristic carbon phases during fuel-water slurry injection in a hot fluidized bed

Abstract The interaction of fuel-water slurries (FWS) with a hot fluidized bed has been studied and the allocation of fixed carbon resulting after FWS dehydration and devolatilization has been investigated. An experimental technique has been purposely set up. A pneumatic injector has been used to disperse FWS into an atmospheric fluidized bed reactor, 140 mm ID. Silica sand has been used as bed material. The experiments have been carried out at typical temperatures of atmospheric fluidized bed combustion (AFBC), but under inert conditions to prevent combustion. Nitrogen as inert gas has been used for fluidization and for dispersion of FWS. Experiments provide results in terms of relative formation rates of carbon aggregates (A-phase), tiny carbon deposits on individual sand particles (S-phase) and carbon fines (F-phase). Relative formation rates are controlled by injection conditions, i.e. by the velocity of dispersing gas at the nozzle and the gas-to-slurry mass feed ration. A further operating parameter is the hydrodynamics of the bed. Finally, the nature of the parent fuel in FWS is also of great importance. The above findings are critically reviewed in the paper in order to determine the rates controlling the formation of carbon phases in an actual FWS-fired FBC unit.

Clean Combustion of Low Quality Fuel in Fluidized Bed Combustor

Combustion characteristics for rice straw and mazut in a fluidized bed combustor have been investigated. Rice straw has been prepared as pellets in order to increase its bulk density and control feeding flow rate. Rice straw pellets have been burnt in bubbling fluidized combustor operating at atmospheric pressure. Over-bed fuel feeding of fuel is applied to provide steady condition of performance. Mazut combustion in the fluidized bed has been also investigated. In-situ desulfurization is considered for the case of mazut combustion. It is concluded that post-combustion of volatiles in the fluidized bed combustor results in a peak temperature values in the freeboard zone. The peak temperature value and position shifts based on the operating condition of the fluidized bed. Carbon monoxide and nitrogen oxides emissions are measured for the presented cases of fuel combustion. Nitrogen oxides emission measurements are reported as 175–270 ppm which is considered relatively low. The effect of fluidization velocity, static bed height and excess air on emissions of carbon monoxide and nitrogen oxides is also investigated. Improvement in combustion of mazut is achieved with the increase in bed temperature, static bed height, and with excess air. Adding limestone particles to the fuel caused sulfur retention up to 90 %.

Prediction of coal-water slurry dispersion in a fluidized bed combustor

Coal-water slurry (CWS) injection in a fluidized bed leads to formation of three carbon phases: aggregates (A phase), tiny carbon deposits on individual bed particles (S phase), and carbon fines (F phase). The mechanism by which CWS is dispersed in a bubbling fluidized bed is analyzed by considering fundamental aspects and key stages underlying interaction of a CWS spray with hot bed solids. A mathematical model has been developed for the formation rates of carbon phases inside the jet zone from the fixed carbon injected with CWS. The model, which takes into consideration fluid dynamics, heat, and mass transfer of the jetting region, has a predictive character and uses no adjustable parameter. A key role is played by the submodel that predicts the rate of CWS deposition on sand particles entrained by the jet. Results show that the formation rates of the three carbon phases can be controlled by adjusting injection conditions. The mathematical model may complement a code aimed at predicting and controlling combustion efficiency of CWS in actual fluidized bed boilers. Model predictions are compared with experimental results obtained by Miccio et al. [8]. In spite of the complexity of the phenomena, the general trends of the predicted and experimental results as a function of injection conditions are the same. The agreement between the model and experimental results is not always satisfactory, but it tends to be better under injection conditions leading to generation of finer CWS droplets. Reasons for the discrepancies are discussed thoroughly.