Basic precautions before re-starting Boiler 2

[maypot]

Hi,

We had side wall damage on our Water Tubes boiler 90 T/Hr 35 Bar 475 Deg C at re-start after a stoppage of 30 mins. We presumed the damage was caused by sudden explosion due to accumulation of gas inside the furnace when fire was resumed on the boiler.
We are proposing to implement procedures after a hot re-start so that the accumulation of gas inside the boiler is exhausted by the ID Fan before firing back again.
Do these make sense ?

Bob

 

[yogibear1]

Any time there is a flame out all fuel must be shut off and the unit purged to at least 30% of max air flow for a minimum of three to five minutes. Both FD and ID fans should be used. In the case where you had a "puff" the unit should be throughly examined to ascertain any damage which may may preclude normal, proper, and safe operation.

 

[rmw]

Bob,

Do you use any auxiliary fuel in your boiler?

Describe what fuel you are using to restart it.

Define stoppage. To me, stoppage in your industry means that the mill stopped, but that you did not necessarily take the boilers off line. It is unclear if your mill stopped, or if this particular boiler stopped, and to what degree you took the fuel out of the boiler or took this boiler off line.

If you kept a bagasse fire in it during a mill stoppage in order to maintain steam pressure for the mills upon restart, you may have overfed the bagasse and had a fuel gassification problem that filled the furnace with gas (fuel rich) from the bagasse which torched off when when it you introduced air upon the restart and it went stociometric again.

We need to know a little more about your restart process, and what was happening while the boiler was down.

rmw

 

[maypot]

Hi rmw,

Thanks for your kind reply.
We stopped the firing because the bagasse conveyor failed, we only used bagasse as fuel.Normaly, during a mill stoppage, we continue to operate the boiler as our power plant is still functioning.
Effectively, we have noticed that there was a very high positive pressure in the combustion after restarting and we presume that might be the cause for the damage.
We wish to define a procedure for a hot re-start.

Bob

 

[maypot]

rmw,

What happened in a boiler, bagasse fired , when fire is suddenly killed for example due to the admission of wet bagasse.I noticed that in this very situation, there is a very sharp increase in pressure inside the combustion chamber.
Thanks for your input.

Bob

 

[yogibear1]

Like I said above the unit MUST be purged to free it of any latent combustible gases. Your burner controls should have had this logic in it from day one; such is required by the NFPA, and state/local codes. Without such logic your insurance company will take a very dim view of paying any claims and may even void your policy.
I don't know sugar mill operation, but if you have any fuel laying in the furnace - maybe as on a traveling grate stoker - that fuel also has to be removed before the purge cycle begins as it is prone to offgassing and the intent is to totally evacuate all combustibles before reintroducing fuel for lite-off.

If you have kept the steam side active there should be no problem with overheating any elements on startup, though there will have to be some water level makeup.

Any puff merits close examination of the pressure parts. Once a pressure part is deformed (the wall is thinned) the pressure required to go to complete failure is less than before. A reduction in working pressure (depending upon the amount of thinning) is merited; the weakest link scenario. When it doubt, cut it out.

 

[maypot]

Thanks yogibear1 for your reply. You mean the boiler must be purged to free, does it stand true even for a few miniutes say 10 mins stop. You mentioned the burner controls should heve this logic, could you please be more explicit.

Bob

 

[yogibear1]

YES, it applies for any/every flame out condition, all fuel sources must be shut off and a purge initiated before (re)lite off.
The burner logic controls are those controls which manage and safeguard the firing, and safety shut down of the flame envelope. This includes contacts (input) from the combustion controls, specifically air flow greater than 30%, and multiple contacts in the fuel system such as all fuel valves closed (limit switches). Depending upon your unit there can be many other safeguards integrated, i.e. fuel dampers closed, furnace to windbox differential proper, furnace pressure not high, etc. etc. All of this type of flame management logic should be there, if not, your installation may well be in violation of any number of regulations.

 

[maypot]

Thanks again yogibear1 . I would like to brief the management about these recommendations. Where can I find these regulations ?
Besides , I noticed that after a fire kill in the boiler as a consequent of moist bagasse drawing into the furnace, there is a sort of explosion particularly when the FD and ID are fully ON.Is there a sort of chemical rection which takes place ?

Bob

 

[yogibear1]

NFPA 8502 is the Standard for the Prevention of Furnace Explosions and is availabe directly from NFPA.org. I think it is like $35.00. This may refer you to other codes too so don't stop too short. Your state/local codes often invoke NFPA or mimic their language- so you will have to do that search in your locale.
The moist bagasse must off-gas enough, and finding some hot spot/ignition source - explodes. Can't be sure but that is my best guess at this time.
Your interlocks will have to be made to block the bagasse from being drawn into the boiler, a knife valve or something akin to it, will have to be used.
I know coal plants best so with your situation I am extrapolating to some extent.

 

[douglasaudfroid]

Once the boiler flamed out, the BMS logic is manually reset and goes into a full furnace purge unless it is a multi gun application in which case it would steam or air purge the particular gun. The purge times are usually based on volume of air changes and is usually at least 5 times or volumes. Which in most cases based on standard fan selection and sizing would be at least 3 minutes, but more than likely 5 minutes. Better safe than sorry.

 

[rmw]

Maypot, Sorry, I am returning to this thread late.

YogiBear1, Bagasse boilers are solid fuel stoker fed over sloped or dump grates normally, and in his case, without auxiliary fuel, more than likely without flame safety systems and probably only the most rudimentary of combustion controls (proportional only). What you are recommending is way beyond the scope of this boiler.

However your approach is right on, and I see you agree with my earlier post about the potential for the gasification of the fuel bed.

Maypot, the controls logic that Yogibear1 is recommending is a sequence in boilers where before the igniters can be engaged a sequence of operating the fans and dampers in order to pass a certain amount of fresh air through the boiler to purge out any remaining fuel is gone through.

Since your boiler probably lacks the control systems that control that it is not applicable to your situation.

Regarding your fuel situation on an emergency stoppage,you have two choices in this occasion. You can either burn out the bed by continuing to run the ID fan and burn the fuel and make what steam you can until the fuel on the grate all burns up, or you can manually remove the fuel.

This can be done by dumping it into the ash pit if you have dump grates, or by raking it out through the furnace doors onto the floor. I have done it both ways. I prefer raking it out, because in the ashpit it still can burn and cause problems in the furnace. I would note that I hate handling those hot rakes and dragging that stuff all the way to the door, and then having to work over the pile of burning fuel as I dragged the next rake load out the door.

The increase in pressure you asked about is most likely due to the moisture and/or volatile gas coming off the fuel pile which being wet and thick enough to put the fire out, most likely blocked the airflow through the grates as well. To make this statement, I am assuming that you would have stopped or closed the dampers on your ID fan upon encountering the stoppage.

The situation that YogiBear1 and I am referring to is the introduction of fuel into a hot furnace without adequate airflow to promote complete combustion and removal of the gasses, but with enough residual heat in the furnace due to radiant heat from the refractory as well as heat in the fuel bed below that it sat down on top of to provide sufficient heat to dry it and to drive the moisture and volatile gasses off of it. Without adequate airflow and purging to remove it, if the volatile gas builds up and finds a glowing ember, hot piece of ash, refractory or flame source of any kind, maybe something is still burning over in a corner of the furnace, it will ignite and if there is enough of it, explode.

YogiBear1 and I suspect that this is what puffed your boiler.

If you ever look into the furnace when you have a good fire and see a real blue flame near an air jet or air port, you are seeing the volatile gasses burning at that point.

Bagasse, as well as bark, sawdust, etc, solid fuels that are around 50% moisture go through the following steps. The fuel enters the furnace basically at room temperature or slightly above, (depending to some degree on the temperature of your imbibition water). In order for the bagasse to ignite, first it has to heat to 212F in order to boil off all the water in the fuel and dry out. From that point, then it has to heat up to the igniton point I(I don't remember what that is for bagasse) where it then catches fire, and begins to burn.

As it gets lighter as it looses moisture and volitales, it will try to lift off the grate if there is sufficient air flow. If it lifts, and is entrained in the upward furnace gas flow and enters the boiler tube banks, there the temperature is not sufficient to maintain combustion, and the flame will be quenched resulting in LOI or CO.

I don't think I got an answer to a question in another thread as to whether this boiler had FD fans, and/or air preheaters. Air preheaters help the combustion process of moist fuels immensily, since they heat the air to the range of 4-500F which helps drive off the moisture in the fuel much more rapidly so combustion can begin.

Therefore, I recommend for any stoppage that is going to amount to more just a very few minutes, (fixing a broken conveyor is not going to be rapid) that you keep your ID fan in service until you burn the bed out, and then start over when you get fuel flow back. With a hot furnace it won't be as difficult starting a cold boiler. It will restart quite rapidly.

Ask if you don't understand any of this.

rmw

PS, what is your mill's TPD grinding rate?

 

[maypot]

Thanks again rmw for your long and valuble post.
We had a severe explosion causing boiler roof damage in the combustion chamber on Monday night.All brick works and expansion joint went away. We presumed the explosion was a consequence of wet bagasse into the furnace.Repairs were undertaken and the we started firing back the boiler yesterday at noon and full steam pressure was achieved at 5.00 pm and the Mills back in operation at 6.00 pm. The roof broke down again at 10.30 pm and the damage seemed to be more pronounced.
Since we started the crop in June, I noted that were always operating with a very high CO and puffs in the combustion chamber were noted every now and then. I am just trying to link the high CO, that is incomplete combution , with the damage.
We used a FD Fan and the air is preheated before combustion.

Bob

 

[yogibear1]

Maypot: RMW had a real good post. It gave me a better picture of the type of unit. I am concerned about any pressure part damage since repairs to same were not metnioned in your post. Was the damage limited to casing, refractory, expansion joints and the like? What happened when the roof broke down?

 

[maypot]

yogibear1,fortunately the damage was limited only to the casing viz refractory and joints. Obviously, when the roof collapsed , we stopped the boiler for verification and repairs are on going now.We expect to resume operation early saturday morning.
I noted that two to three minutes before the knock out, the mills stopped for some reasons and the boiler was fed with mostly moist bagasse from the stock.It is also worth mentioning that it has been raining heavily here for the last two weeks.My question is why did the casing collapsed ?
Could moist bagasse be the sole culprit ? We used to run many times under these circumstances previously without much problem except that we experienced sudden drop in the steam pressure.
Your concern about my problem is very much appreciated.

Bob

 

[yogibear1]

Generally, roofs, that I am familiar with, have finned tubes which are overlain with a pourable lite weight refractory/insulation. Casing, which is generally flat steel plates can be seal welded over this. Is this what your roof looks like?

 

[rmw]

Maypot,

I looked into this thread a little earlier today. You have a very interesting situation.

If you are feeding bagasse from your stock, how old is it?

Bagasse in storage tends to ferment because of the sugar still in it. Do you see any obvious changes to the stored fuel, like orange mold growing on it? I suspect that when you fed this wet partially (or fully) fermented bagasse into the hot furnace now with a low load (meaning low furnace velocities) because of a mill stoppage, the alcohols in it gassed off rapidly and gave you an excess of a detonatable fuel.

I have never heard of that happening just as I described it, but you can bank on one thing, the moisture in your fuel didn't ignite. Water can disassociate into H and O, and the H will explode, but this requires temperatures higher than I think you have in your furnace. Since you don't have natural gas or oil, or any other fuel that can quickly get out of control, and bagasse on a normal day is hard to burn (note your question re: high CO) it has to be something very volatile. Alcohol from fermentation is all I can think of.

Regarding puffing, I consider that to be normal. Your fuel quality changes minute by minute, and sometime it is dryer than others. As that is 'chunked' into the furnace, (I has asked a question about your distributors in another thread that you didn't answer) some of it burns immediately in suspension, and some of it falls on the grate and burns there. Some of the interactions in that process cause puffing. One learns soon not to open a furnace door and look in without being very very light on their feet.

I got burned one time (singed hair, eye brows, clothes, etc.) while looking into a bark fired furnace with a cobalt glass which filters out all the yellow and makes the flame real observable. The thing puffed real good. I didn't see it coming because the cobalt filtered it, and I was literally standing in a blow torch.

Now, I suspect your high CO is because you are pushing this boiler very hard, using information gleaned from other threads that you have posted.

When you are firing hard, there are very high upward velocities in the furnace. There is information out there regarding furnace volumes and grate areas required for proper heat release for various fuels and you should do some research on that to see if your boiler was spec'd with adequate furnace volume.

I suspect not. A very high percentage of your type boiler either don't have enough furnace volume because of skimping on the front end (more volume means higher cost to build the boiler) or end up pushing the boiler past its design heat release optimal point.

Anyway, as you distribute fuel through this upward moving velocity in the furnace, some of the lighter particles will immediately ignite, and will lighten and will be swept into the boiler banks where they are quenched, as you noted.

Some of the fuel just can't find enough oxygen in that environment to burn adequately, and you get CO.

You have finally answered about the FD fan and APH, and those are good things to have in your situation. Now, I have another question (you still didn't tell me your grinding rate). Do you have overfire air?

If not, you should consider adding it, and tap off of the preheated air for it's source.

If so, then you should examine how the nozzles are aimed and what they are accomplishing.

There are several schemes that are popular with respect to o'fire air for this type of boiler, but I won't go into them until I know more about your situation.

The condition of your grates is also crucial. If there are holes or blocked areas, it can cause high FD air velocities in the open areas of the grates, tending to lift fuel and send it into the boiler banks before it completes combustion. All the while, robbing the fuel that is at the point of highest resistance to air flow through the grates, producing CO that you are concerned about.

I hope some of this information helps.

rmw

 

[maypot]

Many thks again rmw for the very useful information you provided.
We have been operating boiler for years and we never experienced anything of the sort, I mean two successive explosions in three days.You might be right concerning the fermented bagasse.
However, people on the plant are assuming that the real cause is wet bagasse causing successive back pressure inside the furnace.However, we only recorded a very positive pressure inside the furnace during or just after the explosion. I fully agree with you that moist bagasse would not ignite and the worst thing that could happen would cause the fire to kill.
Your concern about the type of fuel also is interesting. It is also worth mentioning that when we were raising the steam pressure following the repairs after the first epxlosion, we were mixing oil with the bagasse to speed up the process of pressure build up.This is a practice that has always been adopted particularly when the boiler is brought to pressure after a long shutdown.However, once the mills has started up, only bagasse is used.
I don`t know if fermentation bagasse is a detonable fuel but the two explosions occured when we were feeding from the stock and it happened that at each time it was practically the same stock.

Bob

 

[rmw]

I'll ask some of my clients in the sugar industry in this country if they have ever had a similar experience. It may be a while before I can post what I find out, as they are just completing steam trials and starting their grind here within the next few days, and are fighting the effects of one hurricane, and anticipating yet another one (Rita).

I'd be more worried about wet bagasse killing the fire, and causing a negative pressure excursion rather than causing a puff. Something with a lot of energy in it had to have caused the puff. Water vapor takes energy to make it evaporate. And, when you want it to evaporate rapidly in normal combustion conditions, it is agonizingly slow (the reason you invested in an air preheater).

And, mixing some oil (I would assume diesel) into the already wet bagasse doesn't give me a problem. If, for the point of discussion, you had had oil guns, as some earlier responders assumed you might, and had had a flame out, and filled the furnace with finely atomized fuel oil particles, then, yes, a puff would be in order. But just mixing some oil into the wet fuel doesn't present in my mind something that would cause this type of problem.

Now, the oil would accelerate the fire, which is what you wanted it to do, and that would accelerate the heating of the supposedly fermented bagasse, which might have driven the alcohols off faster than they could burn in a controlled conflagration, causing an excursion.

With one exception. Since you did not say exactly when the puff occurred in the restart, I have assumed you were well underway with a flame established, and building pressure.

If, on the other hand, you put the oil/bagasse mixture into the hot furnace, and it sat there and cooked for a while before igniting, the residual heat of the furnace would have driven off volatiles which could have filled the furnace in the same fashion as an oil gun would upon flameout. In such a case, I would be more worried about the boiler fireman who opened a furnace door in that condition and stuck the burning rag in to re-ignite the fire. But that is not the situation I see you describing.

More information about the precise sequence of events would be helpful.

Now, all the above is purely supposition, since neither you, nor my experience has presented a more plausable theory.

rmw

 

[maypot]

Thanks rmw. The boiler expert claimed the cause to be wet bagasse!!!!

Bob