Particle charging generated by particle-wall, particle-particle and particle-gas contacts in pneumatic transport pipelines contain rich information on gas-solid flow hydrodynamics. In this paper, experiments were performed in a dense phase pneumatic conveying system, and electrostatic fluctuation signals were detected by a ring-shape electrostatic sensor that is based on electrostatic induction theory. Power spectrum analysis and Hilbert-Huang transform (HHT) were applied to the electrostatic fluctuations, namely outputs of the electrostatic sensor, to extract and characterize the intrinsic features of dense-phase gas-solid flow. Results show that the dominant peak of the power spectrum of the electrostatic fluctuations moves toward higher frequency with the increasing gas superficial velocity. The Hilbert-Huang transform reveals the non-linear and non-stationary intrinsic nature of the electrostatic fluctuations. By using the non-linear and non-stationary signal processing method (HHT), non-linear inter-modulation characteristics in the dense phase gas-solid flow were analyzed, and the relations between the energy distribution transmissions in intrinsic mode functions (IMFs) with different orders and the flow characterization of the dense phase gas-solid flow, was investigated as well, which can represent the behavior, stability and regime transitions of the gas-solid flow in the dense-phase pneumatic conveying system at high pressure. (C) 2009 Elsevier Ltd. All rights reserved.