Electric Arc Furnace Electrode Size and AC to DC Conversion

[spikes2020]

Everyone, I posted here last time i had a project and got a lot of good experience from everyone and this time i'd like some more. If everyone could be so kind.


The Problem:

We are a 35MW electric arc furnace. Our arc furnace transformer is 45MVA 13.8kv to 800V secondary. We use 18" electrodes running about 30-35,000 amps with a online tap changer from 500-800V. We are having issues buying electrodes in the 18" range and are thinking of running 24" electrodes, they are significantly cheaper and more available.

We control the current via how much we dip the electrodes into the scrap/metal. Changing from 18" to 24" will significantly change how we arc, and throttle our current. It will also change how hot the electrodes get. What i am afraid of is that we will reduce the resistance and allow for higher current spikes to hit our transformer thus damaging it. I am also afraid of the electrode not responding correctly as it will be significantly under powered. I know that this entire system was engineered to be this way but in order to keep doors open we are looking at all avenues.

Another road I would also like to consider is switching our furnace from AC to DC. This would go from 3 electrodes to 1. The reduction of electrode surface area would reduce our electrode consumption as oxidation is our biggest waste. I know several furnaces have switched in the past, but i don't know what benefits it has for them or even if its worth it for us.

Anyway just peoples thoughts on electrode size change and/or AC to DC Arc Furnace conversion.


PS:

I understand that these projects cost a lot of money but at the moment this is brain storming and there are no stupid ideas!:thumbsup:

 

[Ingenieur]

how long are the electrodes?
what is the gap electrode to electrode (anode?)
since you can regulate v 500-800 won't that keep i ~ the same?

what is the resistivity of the media?

ac-dc
you need to do a cost analysis
cost of rectifiers + elec losses due to conversion - electrode savings (material and labor)
will dc use more, less or the same power as ac?

 

[spikes2020]

how long are the electrodes?
what is the gap electrode to electrode (anode?)
since you can regulate v 500-800 won't that keep i ~ the same?

what is the resistivity of the media?

ac-dc
you need to do a cost analysis
cost of rectifiers + elec losses due to conversion - electrode savings (material and labor)
will dc use more, less or the same power as ac?

Thanks for the great questions!

how long are the electrodes?

I am looking at how long 24" vs 18" electrodes are, we have limited head room above our furnace but atm someone else is working on that problem. If your asking how long our entire stick length is, it's roughly 15' from the gripper to the puddle.

what is the gap electrode to electrode (anode?)

The gap between phases would have to change and the entire electrode holder assembly would have to change. In fact we would also need a new roof to our furnace as the new electrodes wouldn't fit. Currently i believe there is 14" between phases and we would design it to be the same in the new system.

since you can regulate v 500-800 won't that keep i ~ the same?

Changing the voltage is like changing a tap on a transformer, we can operate it online but we have limited operations until it needs to be serviced. So its not infinitely variable, normally we pick 3 or so taps and change between them. We vary the resistance to control the current by lowering or raising the electrodes into our out of the scrap/metal puddle.


what is the resistivity of the media?

it changes as the scrap melts and we vary it to control the current, by dipping or raising the electrodes.



will dc use more, less or the same power as ac?

Everything i have read says that DC is 5% more efficient than AC. It also uses far less electrodes. But yes it seems to be much more expensive conversion. I believe its more efficient as it is sending the arc down into the metal instead arcing 3 phases on top of the puddle.

 

[Phil Corso]

Spikes,

It is unwise to alter parameters without input from the manufacturer. Altering electrode-separation. configuration, or depth can negatively impact its performance!

Regards, Phil Corso

 

[spikes2020]

Spikes,

It is unwise to alter parameters without input from the manufacturer. Altering electrode-separation. configuration, or depth can negatively impact its performance!

Regards, Phil Corso

Also shutting down the facility and putting over 300 people out of work would also negatively impact performance.

 

[GeorgeB]

Spikes, I've worked on hydraulics for electrode control systems in 7 mills of 4 companies. None of these companies re-engineered designs themselves. All had a furnace manufacturer involved for any significant modification.

I suggest you approach the furnace manufacturer's sales/engineering staff. The preliminary evaluation from one who knows your equipment won't likely cost much.

 

[junkhound]

there are no stupid ideas!

You said the 24" electrodes are cheaper than the 18" Is that just due to standardization?

How about cutting the 24 ft to length, chucking in a big lathe and turning to 18" dia ?

I was at an auction that had a big old 20 foot by 3 foot ship prop shaft lathe that sold for $150.00, way less than scrap price! Way too big for most folks, cost a few K$ more than scrap price to move.

Carbon easy to cut, visualize even building a big lathe to just turn carbon ? Even a woodmizer bandsaw or something to cut the 24" down to 18"

 

[junkhound]

PS; was curious about sources, prices, etc, so took a quick look at alibaba.

e.g. https://www.alibaba.com/product-det...ml?spm=a2700.7724857.main07.10.2cd85bc9G32XUs


So, maybe the non-standard 18" is 100K for 20 tons, and the 24" is only 60K for 20 tons. Turning down 24" to 18" boosts the cost from $60K to $106/useful ton ? I have zero familiarity with electric furnace operating costs, so may be a non-workable approach. I'm guessing that the graphite electrode use per ton of steel may be lower for 24" vs. 18" also, so staying with (or going to) 24" electrodes has other economic benefits ?

Out of curiosity, what were the originalplant design trades that led to 18" vs. 24" originally, or even 32"?

 

[junkhound]

pps - interesting subject
;
http://www.amm.com/pdf/AMM Daily/Graphite Electrode_Oct2017.pdf

---

 

[Tony S]

Also shutting down the facility and putting over 300 people out of work would also negatively impact performance.

Not a nice situation, I know as I’ve been there. Not an arc furnace but an induction furnace. For three months I’d been looking for an intermittent earth fault on an earth free system. When I found it I wished I hadn’t, metal was seeping through the refractory on to the coil. All the production guys laid off for a week while us maintenance lads were working overtime to get the damned thing going.

Two guesses who got the blame for it? I’d redesigned the furnace control system about three years before therefore it was my fault. Not a stigma you want to carry with you.

 

[Tony S]

Back to the original topic, are you wanting a single DC electrode or to use the original three? I would guess a single electrode.

One of the subjects I had the misfortune to study at college was control of the AC arc (about as interesting as watching paint dry). From reading this topic you’ve jogged one of the remaining brain cells left.

A DC arc will create a more intense cathode (melting) pool but it will be localised. Also a single electrode will erode faster.

We’re down to pure theory and I’m not confident with my answers. As in my last post, you could end up as the pariah, are your shoulders broad enough to take the responsibility?

Inductotherm (US) refused to have anything to do with our furnace after I altered the control system. All I did was install a PLC.

 

[spikes2020]

Spikes, I've worked on hydraulics for electrode control systems in 7 mills of 4 companies. None of these companies re-engineered designs themselves. All had a furnace manufacturer involved for any significant modification.

I suggest you approach the furnace manufacturer's sales/engineering staff. The preliminary evaluation from one who knows your equipment won't likely cost much.

I am talking with them now about it. I think in 5-10 years a DC furnace would make more sense with batteries to level out our energy use. Right now we are looking at changing from 18" to 20" or 24" electrodes as they are much more common. We normally use UDHP electrodes and may could get away with a cheaper 20-24" one and just run less current is the thought.

 

[spikes2020]

there are no stupid ideas!

You said the 24" electrodes are cheaper than the 18" Is that just due to standardization?

How about cutting the 24 ft to length, chucking in a big lathe and turning to 18" dia ?

I was at an auction that had a big old 20 foot by 3 foot ship prop shaft lathe that sold for $150.00, way less than scrap price! Way too big for most folks, cost a few K$ more than scrap price to move.

Carbon easy to cut, visualize even building a big lathe to just turn carbon ? Even a woodmizer bandsaw or something to cut the 24" down to 18"

Yes about 40% of the market or more use 24" electrodes. 18" are very rare for an arc furnace. Thus us having the issue finding them.

Yes we have turned down 20-24" electrodes to 18" to keep running, but it wastes about 50% of the material. So that we can buy 20-24" at half the cost, its all wasted by the time we turn them down.

 

[spikes2020]

PS; was curious about sources, prices, etc, so took a quick look at alibaba.

e.g. https://www.alibaba.com/product-det...ml?spm=a2700.7724857.main07.10.2cd85bc9G32XUs


So, maybe the non-standard 18" is 100K for 20 tons, and the 24" is only 60K for 20 tons. Turning down 24" to 18" boosts the cost from $60K to $106/useful ton ? I have zero familiarity with electric furnace operating costs, so may be a non-workable approach. I'm guessing that the graphite electrode use per ton of steel may be lower for 24" vs. 18" also, so staying with (or going to) 24" electrodes has other economic benefits ?

Out of curiosity, what were the originalplant design trades that led to 18" vs. 24" originally, or even 32"?

Our furnace is a small diameter and also small Ton furnace about 55-60 tons. Thus a bigger diameter electrodes will take more space in the center and will be closer to the out side walls. We can vary the arc length by changing the online tap changer and regulate the voltage. But still the electrodes can melt the walls if we are not carefully. Luckily we have expanded our furnace by 2' with the last upgrade so this might not be as much of a problem.

Last year electrodes were about $1 per pound, this year 18" electrodes are anywhere from $6 to $12... considering we use over a million pounds a year this is a big deal.

 

[spikes2020]

Back to the original topic, are you wanting a single DC electrode or to use the original three? I would guess a single electrode.

One of the subjects I had the misfortune to study at college was control of the AC arc (about as interesting as watching paint dry). From reading this topic you’ve jogged one of the remaining brain cells left.

A DC arc will create a more intense cathode (melting) pool but it will be localised. Also a single electrode will erode faster.

We’re down to pure theory and I’m not confident with my answers. As in my last post, you could end up as the pariah, are your shoulders broad enough to take the responsibility?

Inductotherm (US) refused to have anything to do with our furnace after I altered the control system. All I did was install a PLC.

Thanks for your insight, and experience.That would does suck when you get the blame.

It's my job to keep this facility going and take responsibility for the project. I have worked here for over 3 years and managed over 12 million dollars in projects, so far none have failed me yet. I may ask stupid questions here or sound lost, but i find it helps gain perspective. If i look like the fool, others don't mind asking more questions.


Right now i think we ruled out swapping over to DC furnace is out unless a lot of money plops down on my desk, but we are still considering changing electrode sizes. But this may take almost as much money and time to do.

Swapping over to a DC furnace is still on my bucket list for the future as batteries will soon be able to power the furnace and level out our power usage. This could easily save us 1 to 2 million dollars per year on energy alone, not including other efficiency such has half electrode use.

 

[Besoeker]

Swapping over to a DC furnace is still on my bucket list for the future as batteries will soon be able to power the furnace and level out our power usage. This could easily save us 1 to 2 million dollars per year on energy alone, not including other efficiency such has half electrode use.

I think your battery expectations are too optimistic.

 

[spikes2020]

I think your battery expectations are too optimistic.

I already got one quoted and it was about 2 million dollars... It wasn't the lithium ion, it was a liquid flow battery. But i am sure in 5 to 10 years there will be something that will give me a good enough payback to do this project.

 

[junkhound]

At the scale you are discussing, I'd watch the DARPA and Dept of Energy solicitations for load leveling technology and similar.

You may see a program that you could fit into, get the gov. to pay 1/2 of your cost on some cost sharing development program.

Our company actively watches for those opportunities.

 

[Besoeker]

I already got one quoted and it was about 2 million dollars...

How big was it physically?

 

[Sahib]

Instead of battery bank, is it not possible to use more economical capacitor banks to meet peak demand if it is practical?