In a circulating fluidized bed (CFB) with an external heat exchanger (EHE) and a pneumatically controlled loop seal, it is important to measure the gas-solids flow in the loop seal and EHE, for high combustion efficiency. For this purpose, electrical capacitance tomography (ECT) has been applied. Two circular ECT sensors with eight electrodes each are mounted in the two exports of the pneumatically controlled loop seal to monitor the solids flow in the "cold" CFB system. Dynamics tests with different operating conditions were carried out. 2D and 3D images of the cross sections of the exports were reconstructed. The ECT results are analyzed with the pressure drop. The power spectral density (PSD) of signals is obtained by Welch's method and wavelet decomposition in the time domain is implemented for capacitance signals with different conditions. The experimental result indicates that the gas-solids flow can be controlled by pneumatically controlled loop seal based on ECT and pressure drop measurements. (C) 2014 Elsevier B.V. All rights reserved.