Back Corona

Doesn't exist and the question as it is phrased makes no sense.

Background corona does exist but what do you really want to know?

Im sorry but it does exist.

If you don't know the answer better not to give a non sense reply.

That makes you non sense either.

I just want to elaborate further the phenomenon.

Guest, if you want to be unfriendly, logon with a proper name etc., otherwise please just read what goes on here.

Thanking you for your attention.

PS The reason of course being that you might be mistaken for anyone of the "Guests" who logon and have stuff attributed to you that was nothing to do with you at all!!!

My apologies. I've spent a lifetime studying corona in external and internal HV, EHV and UHV insulation products and systems and should have restricted my attempt to answer to those systems.

Google is your buddy

http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=885235

Back corona does indeed exist. It's a phenomenon that occurs within electrostatic precipitators filtering dust that has high resistivity. As a layer of dust with high resistivity (>10^8 ohm-cm) builds up, a high E-field may develop within the dust layer. If the E-field reaches 10-20 kV/cm, microdischarges may develop within the dust layer which, in turn, generate large numbers of positive ions which drift toward the negative corona-generating electrodes. This reduces the negative space charge in the air region, reducing precipitator collection efficiency and increased odds of sparkover. Back corona is a severe problem in ESP's with certain types of high resistivity dust. Does that sufficiently answer your question?

very well.

some people find it hard to understand. can you explain it in layman's term...

my next question is how to avoid back corona if you have high resistivity dust?

This is an engineering forum, so discussions are normally fairly technical. Without getting into the physics of back corona, the root cause is the high resistivity of the dust that adversely impacts dust collection efficiency and increases power consumption.

Common methods to address the problem include modifying the resistivity of the dust (by chemical conditioning, injecting sulfur trioxide gas or water droplets into the gas stream), actively reducing the high voltage supply voltage whenever back corona is detected, using a pulsed HV supply instead of HVDC, or making fundamental design changes to the shapes of the corona and collection electrodes. Since you may not have access to the technical literature (IEEE, etc.), here are a cople of readable online articles which discuss ESP operation, back corona, and various approaches that can be used to reduce/prevent it:

http://www.indigotechnologies.com.au/documents/icespviia.pdf

http://www.indigotechnologies.com.au/documents/csiro88a.pdf

Good luck,

Bert

With regard to the operation of Elestro-Static Precipitators:

Back Corona - A phenomenom that occurs when the gas within a high resistivity dust layer becomes ionized, which causes heavy positive ion backflows, which neutralizes negative ion current and reduces voltage levels.

(definition from "ESP Control & Electrical Systems Troubleshooting Guide" 2001 NWL, www.NWL.com)

One of the best ESP book you can read is "The Art of Electrostatic Precipitation" by Jacob Katz.

10^8 ohm-cm is a really low resistivity when you deal with Electrostatic Precipitators (ESPs). This resistivity is actually so low, that the voltage drop across the collected dust layer may not suffice to maintain the collected dust safely adhered on the ESP plates.

Back Corona will not be created in a mechanically well-designed ESP until a much higher resistivity is reached, with the worst cases from ash after coal fired boilers to be be found in India, Australia, Mongolia and in some parts of Russia. The highest resistivity example ash is probably the Tarong Power station in Australia, where the resistivity is often well above 10^15 ohm-cm, thus 10 ^7 (or 10 million) times higher than the value you mentioned. Back Corona usually starts to be a problem once the resistivity goes above 10^11 ohm-cm, meaning that from then and up the Intermittent Energization (Semipulse) will reduce emission. Note that although Indigo have one of their installations att Tarong, it is installed there to agglomerate fine particles to become more coarse, not to abate Back Corona.

Back corona is not abated by Indigo. The target of Indigo is to agglomerate small particles to larger particles which ESPs find easier to collect - that's how its supposed to work..(Does it ?). Google "Rod Truce" to find more papers on this technology.

There is an alternative method to abate back corona: Intermittent Energization or as some call it Semipulse. Back Corona is created by the voltage drop across the dust layer on the collection electrodes. With less current fed from the TRs the back corona can be kept low. But then the charging of the particles in the ESP gas gets bad. In the 1980ies some smart guys found out that by letting the TRs supply current in short intense bursts, the average current could be kept low to minimize back corona, while at the same time the particle charging remained excellent.

Simultaneously emission is reduced and TR power consumption becomes a fraction only. Amazingly the stack plume may disappear, the TR power consumption can go to less than 5 % of TR rated abnd the kV and mA meters hardly move from their zero position.

The experts on this technology are all within Alstom. First papers they wrote stem back from 1996. Google "alstom" and "Semipulse" to find papers. In the US abating back corona in this way has not yet come much into use, mainly because back corona occurrs mainly when the ash resistivity is very high. And most US coals give an ash that is not excessivley high-resistive - mainly because of a high sulphur content in the coal (mostly above 1 %).

Voltage-current characteristics are a basic tool to analyse the operational conditions of an electrostatic precipitator, and to detect the onset of Back-corona, that strongly affects the collection performances. In this investigation, a laboratory set-up has been arranged, with a classical ESP wireplate configuration; both static and dynamic voltage-current characteristics have been measured and analysed with different geometries, dust characteristics, and layer thickness. Similar results have been also obtained in an industrial ESP, and compared with the laboratory results.

In the laboratory ESP, the static voltage-current characteristics and the collection efficiency have been measured as functions of time; with high resistivity dust, back-corona takes place as layer thickness increase, with a decrease of the efficiency and a sharp raise of the current. The effect of electrode type and powder resistivity has been analysed.

See more on my paper at:
http://www.irsweb.it/en/FileAllegati/Static_dynamic_back-corona_characteristics.pdf
http://www.irsweb.it/en/FileAllegati/Back-Corona%20Model%20for%20Prediction%20of%20Esp%20Efficiency%20and%20Voltage-Current%20Characteristics.pdf