Estimation of the solid circulation rate in circulating fluidized bed systems

The results of experiments performed in four different fluidized bed units (two pilot plants and two cold flow models, with a wide range of operating conditions) together with correlations from literature are used to establish methods to estimate the solid circulation rate in a circulating fluidized bed system. The estimation is based on the measured pressure drop in the upper part of the riser as well as particle properties and geometry of the riser exit. Investigations included smooth as well as abrupt riser exit configurations. In case of a smooth riser exit geometry (direct gas solids separation exit), the assumption of a uniformly upflow of solids in the riser is sufficient for the estimation of the solid circulation rate. Further, for an abrupt exit (L-shape exit), two methods were developed. Both methods are based on the assumption that the rate of reflected particles at the top of the riser increases with the so-called exit Froude number. One method is based on a correlation between interstitial gas velocity in the riser, slip velocity, mean particle velocity in the riser, and exit Froude number where it was possible to keep the difference between measured and estimated solid circulation rate in the range of approximately +75% and −40%. Further, this method is not able to consider the local mass flow distribution in the riser. For the other method, the output of a detailed mathematical model, based on information from literature, was used to establish a correlation between, exit Froude number, exit reflecting coefficient and local mass flow to estimate the solid circulation rate. Using this method it was possible to keep the difference between measured and estimated solid circulation rate in the range of ±40%. For both methods, the accuracy of the measurement of the pressure drop in the upper part of the riser is a significant parameter for the prediction of the solid circulation rate.