van Ejm Bert Heesch

From Chemical Kinetics to Streamer Corona Reactor and Voltage Pulse Generator

This paper discusses the global chemical kinetics of corona plasma-induced chemical reactions for pollution control. If there are no significant radical termination reactions, the pollution removal linearly depends on the corona energy density and/or the energy yield is a constant. If linear radical termination reactions play a dominant role, the removal rate shows experimental functions in terms of the corona energy density. If the radical concentration is significantly affected by nonlinear termination reactions, the removal rate depends on the square root of the corona energy density. These characteristics are also discussed with examples of VOCs and NOx removal and multiple processing. Moreover, this paper also discusses how to match a corona plasma reactor with a voltage pulse generator in order to increase the total energy efficiency. For a given corona reactor, a minimum peak voltage is found for matching a voltage pulse generator. Optimized relationship between the voltage rise time, the output impedance of a voltage pulse generator, and the stray capacitance of a corona reactor is presented. As an example, the paper discusses a 5.0-kW hybrid corona nonthermal plasma system for NOx removal from exhaust gases.

Analysis of streamer properties in air as function of pulse and reactor parameters by ICCD photography

Streamer properties such as their velocity, diameter, intensity and density, can be obtained by analysis of temporal and spatial resolved ICCD imaging. In this paper, experimental results on streamer generation and propagation as a function of several high-voltage pulse and reactor parameters are described. Experiments were performed on a large scale wire–plate reactor in ambient air. The set-up allows for independent variation of the parameters over wide ranges. The minimum gate time of the ICCD camera is 5 ns, allowing for a high temporal resolution. The camera can be triggered with a precision of 1 ns. Both negative and positive polarity pulses are investigated. The most important conclusions are as follows. (1) The streamer velocity ((0.5–2.5) × 106 m s−1) increases if the applied electric field and/or the voltage rise rate is increased. (2) The same is true regarding the velocity ((0.2–1.2) × 105 m s−1) with which the streamer diameter (0.7–3.0 mm) increases during propagation. (3) Typical properties (velocity, diameter, etc) of negative and positive polarity streamers vary less than 25%, especially when the applied electric field is high. (4) As long as the dc bias voltage is below the dc corona onset value it does not have a separate effect on the visual streamer properties. Only the total voltage (peak voltage + dc) is of importance. (5) A simple model was used to determine the electric field in the secondary streamer channel. It was found that in the light emitting part of the secondary streamer the electric field is approximately 21.5 kV cm−1. In the remainder (dark part) of the channel the electric field is around 6.5 kV cm−1. This paper shows mainly experimental findings. Not all observed relations and phenomena could be explained. This is partly caused by the fact that current theoretical and numerical models are not yet able to describe the experimental situation as used during this study.

Evaluation of pulsed streamer corona experiments to determine the O* radical yield

The production of O* radicals in air by a pulsed streamer plasma is studied by integration of a large set of precise experimental data and the chemical kinetics of ozone production. The measured data comprise ozone production, plasma energy, streamer volume, streamer length, streamer velocity, humidity and gas-flow rate. Instead of entering input parameters into a kinetic model to calculate the end products the opposite strategy is followed. Since the amount of end-products (ozone) is known from the measurements the model had to be applied in the reverse direction to determine the input parameters, i.e. the O* radical concentration.

Long, lifetime, triggered, spark-gap switch for repetitive pulsed power applications

In this article a critical component for pulsed power applications is described: the heavy-duty switch. The design of a coaxial, high repetition rate, large average power, and long lifetime spark-gap switch is discussed. The switch is used with a fail-free LCR trigger circuit. Critical issues for switch design are presented together with experimental results. It is observed that the switch has a good stability, and its lifetime is estimated to be in the order of 1010 shots (∼106C) at 10J∕pulse, 60kV and 100ns pulses. Measurements were performed with 20 and 34kV average switching voltage (100ns pulses, energy per pulse 0.4 and 0.75J, respectively). For up to 450pulses∕s (pps), pre-firing can be prevented by increasing the gap pressure (up to 2.5 and 7bars, respectively), no gas flush is required. Above 450pps, up to 820pps, a forced gas flow of maximal 35Nm3∕h, is required for stable operation. Measurements on the time delay and jitter of the switch demonstrate that these values are influenced by pressure,...

Tar removal from biomass-derived fuel gas by pulsed corona discharges

Tar removal from fuel gas obtained from biomass gasification offers a significant challenge in its deployment for power generation as well as for other applications such as production of chemicals by processes such as Fischer–Tropsch. The present investigation focuses on pulsed corona discharges for the mentioned objective. The paper is meant to give an overview of our developments in the area of pulsed power development for large-scale plasma processing. In addition, lab-scale results as well as pilot-scale results for tar removal on an actual gasifier are presented.

Temporal development and chemical efficiency of positive streamers in a large scale wire-plate reactor as a function of voltage waveform parameters

In this paper a large-scale pulsed corona system is described in which pulse parameters such as pulse rise-time, peak voltage, pulse width and energy per pulse can be varied. The chemical efficiency of the system is determined by measuring ozone production. The temporal and spatial development of the discharge streamers is recorded using an ICCD camera with a shortest exposure time of 5 ns. The camera can be triggered at any moment starting from the time the voltage pulse arrives on the reactor, with an accuracy of less than 1 ns. Measurements were performed on an industrial size wire-plate reactor. The influence of pulse parameters like pulse voltage, DC bias voltage, rise-time and pulse repetition rate on plasma generation was monitored. It was observed that for higher peak voltages, an increase could be seen in the primary streamer velocity, the growth of the primary streamer diameter, the light intensity and the number of streamers per unit length of corona wire. No significant separate influence of DC bias voltage level was observed as long as the total reactor voltage (pulse + DC bias) remained constant and the DC bias voltage remained below the DC corona onset. For those situations in which the plasma appearance changed (e.g. different streamer velocity, diameter, intensity), a change in ozone production was also observed. The best chemical yields were obtained for low voltage (55 kV), low energetic pulses (0.4 J/pulse): 60 g (kWh)−1. For high voltage (86 kV), high energetic pulses (2.3 J/pulse) the yield decreased to approximately 45 g (kWh)−1, still a high value for ozone production in ambient air (RH 42%). The pulse repetition rate has no influence on plasma generation and on chemical efficiency up to 400 pulses per second.

An Industrial Streamer Corona Plasma System for Gas Cleaning

For pulsed corona plasma applications, it becomes important to develop pilot systems with large average power and high-energy conversion efficiency. Since the beginning of 2000, we have been working on an industrial corona plasma system with tasks of 10-30 kW in average power and higher than 90% of total energy conversion efficiency. The pulsed-power source should have the following specifications: rise time of 10-25 ns, pulsewidth of 50-150 ns, pulse repetition rate of up to 1000 pulses per second, peak voltage pulse of 70 kV, peak current of 3.5 kA, dc bias voltage of 10-35 kV, and energy per pulse of up to 30 J. Sixteen parallel wire cylinder reactors are used to match the source. Gas and reactor temperatures can be controlled individually with water flow around the outside of those cylinders. The system is designed for gaseous oxidation and electrostatic dust precipitation. The system has been used for up to 17 kW in average power. This paper reports the system in detail, discusses issues related to the matching between the source and the reactor, and presents an example of industrial demonstrations on odor abatement at 1000 m3/h. Finally, this paper also gives a general guideline for design of corona plasma systems

Chemical Processes in Tar Removal from Biomass Derived Fuel Gas by Pulsed Corona Discharges

Cleaning or conditioning of fuel gas from biomass gasification is perhaps one of the main obstacles for utilization of biomass as a source of power generation. Various methods exist, but, so far, none of them have been reported to be reliable for long-term operation. In our present research, we try to couple our advancements in pulsed power technology for industrial applications to the application mentioned. Here we focus on the chemical processes that occur during pulsed corona fuel gas cleaning. Experimental results at 200°C show that the main process for tar (heavy aromatic hydrocarbons) removal is mainly via oxidation.

A high-voltage pulse generator for corona plasma generation

This paper discusses a high-voltage pulse generator for producing corona plasma. The generator consists of three resonant charging circuits, a transmission line transformer, and a triggered spark-gap switch. Voltage pulses in the order of 30-100 kV with a rise time of 10-20 ns, a pulse duration of 100-200 ns, a pulse repetition rate of 1-900 pps, an energy per pulse of 0.5-12 J, and the average power of up to 10 kW have been achieved with total energy conversion efficiency of 80%-90%. Moreover, the system has been used in four industrial demonstrations on volatile organic compounds removal, odor emission control, and biogas conditioning.

Oxidative degradation of toluene and limonene in air by pulsed corona technology

The oxidative degradation of two volatile organic compounds, i.e. toluene (fossil fuel based VOC) and limonene (biogenic VOC), has been studied. A hybrid pulsed power corona reactor with adjustable energy density has been utilized for degradation of ppm level target compounds in large air flows. The observed oxidation product range features an energy density-dependent spectrum of oxygen-functional hydrocarbons, which has been qualitatively discussed on the basis of literature studies. Typically, observed stable oxidation products for both target compounds are the biocompatible carboxylic acids acetic and formic acid. Measured degradation G-values are 23 nmol J−1 at 74% conversion of 70 ppm toluene and 181 nmol J−1 at 81% conversion of 10 ppm limonene.