The application of strong electric fields in water and organic liquids has been studied for several years, because of its importance in electrical transmission processes and its practical applications in biology, chemistry, and electrochemistry. More recently, liquid-phase electrical discharge reactors have been investigated, and are being developed, for many environmental applications, including drinking water and wastewater treatment, as well as, potentially, for environmentally benign chemical processes. This paper reviews the current status of research on the application of high-voltage electrical discharges for promoting chemical reactions in the aqueous phase, with particular emphasis on applications to water cleaning.

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Electrohydraulic Discharge and Nonthermal Plasma for Water Treatment

Biomass is an important primary energy source as well as renewable energy source. As the most promising biomass utilization method, gasification/pyrolysis produces not only useful fuel gases, char and chemicals, but also some byproducts like fly ash, NOx, SO2 and tar. Tar in the product gases will condense at low temperature, and lead to clogged or blockage in fuel lines, filters and engines. Moreover, too much tar in product gases will reduce the utilization efficiency of biomass. Therefore, the reduction or decomposition of tar in biomass derived fuel gases is one of the biggest obstacles in its utilization for power generation. In this paper, we review the literatures pertaining to tar reduction or destruction methods during biomass gasification/pyrolysis. On the basis of their characteristics, the current tar reduction or destruction methods can be broadly divided into five main groups: mechanism methods, self-modification, thermal cracking, catalyst cracking and plasma methods.

The reduction and control technology of tar during biomass gasification/pyrolysis: An overview

Near infrared spectroscopy for on/in-line monitoring of quality in foods and beverages: A review

Experimental data are presented of the development of PD activity in artificial air-filled spherical voids in epoxy resin over a period of /spl sim/6 months. The data are presented in the form of phase averaged plots of the charge distribution over the time (aging plots) and phase resolved histograms (PRPD patterns) at various aging states. They are interpreted by comparison to numerical simulations which are based on a physical discharge model. This provides insight into the physical changes in the void during aging. The simulation is the basis for a physically meaningful interpretation of the PD data. It provides rules for the identification of the defect type, the defect quantification, and an assessment of the aging stage. >

Measurement and simulation of PD in epoxy voids

The potential to commercialize the non-thermal pulsed electric field (PEF) technology as a new method to preserve food products has caught the attention of the food industry that wishes to fulfil consumers demands for fresh products. In recent years, numerous research groups have demonstrated the possibility to inactivate a range of microorganisms both in buffer systems and in food products using different PEF systems. In this review, we survey the critical process factors and main characteristics of food products that determine the microbial inactivation kinetics.

Critical factors determining inactivation kinetics by pulsed electric field food processing

Two pilot pulsed power sources were developed for fundamental investigations and industrial demonstrations of treatment of conducting liquids. The developed heavy-duty power sources have an output voltage of 100 kV (rise time 10 ns, pulse duration 150 ns, pulse repetition rate maximum 1000 pps). A pulse energy of 0.5-3 J/pulse and an average pulse power of 1.5 kW have been achieved with an efficiency of about 80%. In addition, adequate electromagnetic compatibility is achieved between the high-voltage pulse sources and the surrounding equipment. Various applications, such as the use of pulsed electric fields (PEFs) or pulsed corona discharges for inactivation of microorganisms in liquids or air, have been tested in the laboratory. For PEF treatment, homogeneous electric fields in the liquid of up to 70 kV/cm at a pulse repetition rate of 10-400 pps could be achieved. The inactivation is found to be 85 kJ/L per log reduction for Pseudomonas fluorescens and 500 kJ/L per log reduction for spores of Bacillus cereus. Corona directly applied to the liquid is found to be more efficient than PEF. With direct corona we achieve 25 kJ/L per log reduction for both Gram positive and Gram negative bacteria. For air disinfection using our corona pulse source, the measured efficiencies are excellent: 2 J/L per log reduction.

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A fast pulsed power source applied to treatment of conducting liquids and air

The relation between the motion of charges in an electrode gap and the current induced in the external circuit is described on the basis of energy considerations and on the basis of the Ramo-Shockley theory. It is shown that both approaches are applicable, and mutually consistent, not only for gas-filled gaps but, if applied properly, also when the gap is partially filled with a solid dielectric. Energy considerations are especially useful for two-electrode gaps. If more than two electrodes are involved, the Ramo-Shockley theory has to be invoked. Based on the theoretical description, fast measuring systems with a subdivided measuring electrode can be constructed (typically 400 MHz bandwidth, 1 ns risetime). Fundamental and electrotechnical bandwidth limitations are discussed. The derived techniques and insights are applied to perform and interpret time-resolved measurements of currents induced by avalanches in homogeneous fields corona discharges in inhomogeneous fields and partial discharges in voids. >

Prebreakdown currents: Basic interpretation and time-resolved measurements

We studied the propagation of partial discharge pulses in a stator winding by means of measurements on a dismantled 35-MW generator and found that a stator winding acts as a transmission line. Therefore, a partial-discharge (PD) signal manifests itself at the generator terminals after a transit time that depends on the location of the discharge. Due to capacitive and inductive couplings in the end-winding region, a second propagation mode is present for higher frequencies. This "fast mode" manifests itself at the terminals without appreciable time delay. The capacitive and inductive couplings also cause crosstalk between the phases. As a consequence, a signal measured in one phase does not necessarily originate from a discharge in that phase. The amplitudes of the fast mode and, to a lesser extent, the slow mode (or transmission-line mode) are heavily damped when the discharge occurs further away from the measuring terminal. The consequence is that only discharges close to the measuring terminal can be observed. The propagation of a PD signal is not only influenced by the construction of the generator but also by its external connections.

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Propagation of partial discharge signals in stator windings of turbine generators

In practice the problem of comparing several treatments can often be formulated as which treatment can be considered best. Selection procedures have been designed specifically to answer questions like this one. In many situations selection procedures are more realistic than the usual testing procedures. In this paper an introduction and an overview of selection of populations will be given with special reference to Normal, Binomial, Poisson and Multinomial populations. In the field of agricultural applications these distributions play an important role.



Bechhofer's and Gupta's approach are presented side by side for a number of problems. After that the relatively new procedures of Somerville are discussed. Finally, some recent results are presented.

Selection of populations: An overview and some recent results.

Metallic conduits provide mechanical support for cables, and may also protect the cables against interference caused by currents induced by external electromagnetic fields. The conduit transfer impedance Z/sub t/ then is a good measure for the provided protection. We studied Z/sub t/ by means of conformal transformations and of the method of moments in a quasistatic, two-dimensional (2-D) approach. Practical U-, H-, L-, and T-shaped conduits are discussed. Conformal transformations provide simple engineering expressions for the mutual inductance part of Z/sub t/ for cable positions inside the conduit. The method of moments formulation results in useful approximations for the crossover frequencies in Z/sub t/ between DC and high frequency. Measurements on U-shaped conduits confirm the calculations.

P.C.T. van der Laan

Papers

A fast pulsed power source applied to treatment of conducting liquids and air

Prebreakdown currents: Basic interpretation and time-resolved measurements

Propagation of partial discharge signals in stator windings of turbine generators

Selection of populations: An overview and some recent results.

Transfer impedance of nonmagnetic conduits of various shapes