Heavy dark black smoke from Frame 9E GT Exhaust


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We have four GE Frame9 gas turbines (MarkIV) that always run on base load. I have noticed that while GT is running on furnace oil, sometimes (at least once in 2 hours) heavy dark black smoke comes out of boiler stack (GT running in combined cycle mode) which lasts from 5 to 10 minutes & then becomes normal. And sometimes this smoke comes out of stack in such a way as it was being restricted by something & then suddenly released (I mean like a quick burst). During these periods of heavy dark smoke, GT spread & exhaust temperature always remain normal. Moreover, this phenomenon is observed on all 4 GTs.

Any such observation by someone else?
Do you know where heavy furnace oil comes from? (I never heard that term until I read it in a post on control.com. It sounds like a PC (politically correct) term for what is a really nasty fuel. Some "heavy furnace oils" I've worked around will eat a hole through the leather upper of shoes in a couple of hours when a small spill gets on your shoes!)

It's literally the bottom of the barrel (tank), what's left over from the refining process. Some sites actually use centrifugal separators to try to remove water and other non-combustible substances (including some heavy metals and rocks and sand) from the heavy fuel oil (yet another term for residual fuel oil). Usually, an additive has to be added to the fuel stream to prevent the plating of heavy metals on hot gas path parts (nozzles and buckets). I've heard they can't even use this stuff to make asphalt for roads and parking lots.

Black smoke from a heavy duty gas turbine usually means excessive load or combustion system problems, but you say the exhaust temperature and spread are always "normal". What about the power output? Is it increasing or decreasing or remaining relatively stable during these events?

It's most likely some really nasty stuff that doesn't burn very well that's in the "heavy furnace oil" that you're now buying and burning. If you're getting your "heavy furnace oil" from a new or different supplier, it may be that it's just gotten worse, because this stuff is, again, literally the bottom of the barrel (tank) from the refinery. You probably have to heat the "heavy furnace oil" just to get it to flow (it usually has a very high viscosity at ambient temperature) and to atomize properly to burn. Is the temperature of the fuel oil remaining relatively constant during these smoking periods or is it lower than normal?

Have you compared the fuel analysis of the fuel you are burning now compared to some other fuels that you have previously burned? How does it differ?

They don't also call it "residual fuel oil" for nothing.
It seems that your heat recovery boilers are equipped with soot blowing equipment. When the GTs burn furnace oil, their exhaust gases tend to contain ash, and a small amount (very small) of unburnt fuel which is generated by the GT burners. This dust content tends to deposit itself on the boiler tubes. If left there the heat absorption of the boiler will fall as more and more of the boiler tubes are covered with this dust. To prevent this, soot blowing equipment is installed in the boiler gas side. The soot blowing equipment uses either steam from the boiler itself or compressed air (depends on which system you have installed) to blow off the accumulated dust. it is during this operation that you see the heavy dark smoke that is coming out of your boiler stack.

If this black smoke is creating problems (complaints from people nearby, dust deposits on building, etc., problems with the environmental regulator) you can try to increase the frequency at which the soot blowing operations are made, to reduce the amount of dust that needs to be blown off at each operation.

Having said that, are you sure that:

a. the fuel quality and viscosity are as per GT specification, or are you using higher viscosity fuels that as per design?
b. the fuel nozzle tips are within tolerance? When was the last time that the burners were inspected for wear?
c. the fuel pre-heating system is functioning properly and that the fuel coming out of it is of the right viscosity?

When one burns furnace oils, one should expect a certain amount of ash and unburnt fuel to be generated. However I must state that your statement that 'heavy dark black smoke' comes out of your stacks, leads one to think that something is not perfectly right.

By the way, has this been the norm ever since the GTs were started, or has this problem been observed recently?

In addition how often is the soot blowing procedure done during the day (i.e. how many times during the day do you see the black smoke bursts coming out)?

You bring up a very good possibility; this poster has never told us if the GE-design heavy duty gas turbine-generators at his site have HRSGs in the exhaust. And, if they burn residual fuel oil it's a good bet that the HRSGs have soot blowers. It would definitely explain the "bursts" of soot/smoke seen coming from the stack and also address the period and frequency (he said it occurs every couple of hours). It would also explain why the exhaust temperature and spreads don't change when this happens.

So the "smoke" may not be smoke at all; it might just be the soot (dust) being blown off the tubes on a regular basis by the soot blowers.
When burning furnace oils.... the viscosity becomes important because there will be an optimum viscosity for efficient combustion.

The fuel is atomised and if the viscosity is too high then the droplets formed will be larger than optimum, will travel through the optimum burning zone and burn less efficiency, will burn longer, and less completely.

Conversely, if the fuel viscosity is too low then a fine spray of droplets is produced which do not disperse well, do not mix well with the air flow and which again results in incomplete combustion.

Excess soot may result.

Residual fuels are notorious for their variable quality.

Many burners operate using fuel temperature to control fuel heating and it is usual to take frequent samples to ascertain the optimum temperature to which to heat the fuel prior to atomising. It is also common to run with excess oxygen and to conduct frequent flame inspections.

Some of these options may not be available with turbines.

If your fuel oil is heated using fuel temperature as a reference then you might consider switching to an inline viscometer. This will provide much more precise fuel heater control and changes in fuel quality will be far less significant.

In studies (e.g. at a petro chemical plant where fuel oil to burners was coverted from temperature control to viscosity control, the cost savings were considerable and the fuel combustion more efficient resulting in far less soot production, the ability to reduce the excess oxygen (and thus reduce the cooling effect) and reduced fuel consumption.

Visit www.viscoanalyser.com for details.
small comment, in the first sentence speedtronic mentions 'heavy dark black smoke comes out of boiler stack (GT running in combined cycle mode)'. I believe this means a combined cycle unit with HRSGs.

Having said that, it would be nice to have some feedback from speedtronic on what we shared with him.
As far as soot blowing frequency is concerned, it is performed after every 12 hours of GT operation on furnace oil & this schedule is followed very strictly. However, sometimes (though not very frequently) it happens that during soot blowing some soot blowers stuck due to mechanical trouble or limit switch problem & they have to be bypassed which means no soot is blown from a certain portion of boiler (tubes).


AS far as furnace oil viscosity is concerned, our Frame 9E GTs with MarkIV control have certain operating limits regarding furnace oil temperature (& may be like all other GTs which run on furnace oil). These operating limits are between 96degC & 118degC (& normally furnace oil temperature remains between 103degC to 110degC). Above or below these limits, GT coverts to HSD on auto. So I don't think that viscosity should be a problem.

In the end, thanks very much to all of you for replying to my post. You all have mentioned many important points to consider. I have started observing all these things & hopefully soon I will be able to reach a conclusion.
You're right; I misread the post and didn't pick up on the multiple references to combined cycle and boiler stack.

My bad.

And you're also right that it would be nice to hear back from speedtronic.
Some comments to speedtronic:

1. Sootblowing once every 12 hours may be too little. I understand that sootblowing wastes energy (in terms of steam and water being lost), but the accumulation of ash will also degrade the boiler performance. Try reducing the period to 8 hours, i.e. 3 sootblowing operations per day, rather than the 2 you make.

2. What is used to maintain the furnace oil temperature between 103 and 110 deg C? Do you correct the operating temperature relative to the viscosity of the fuel you have? Simply maintaining the temperature within a band is not the right approach to attain proper fuel viscosity.
Yes, definitely. You set up a plant using past experience, and theory obviously. However, operation of the new plant will practically always reveal new problems which weren't envisaged in the design stage, or which although thought of at design stage, were not completely solved. Other problems could crop up, like operational problems which were not envisaged/catered for at design stage.

A typical case that comes to mind is the famous case where the earlier combined cycle plant was designed for base load operation but then later was requested by the grid operators to operate in two shift mode. You had the operators of such plant asking what are the effects of 2-shift operation on boilers, steam turbines, etc.

One can mention a thousand other problems that are encountered after a plant is commissioned. It is the famous bath tube curve for plant maintenance (non-planned) requirements. In the first years of operation you have a number of such problems, and these progressively get lower as you learn your plant, and start solving problems. Then you get a quiet period, until age and wear and tear start having their toll, where maintenance increases again. Finally you reach a point where it is not worth it trying to maintain that plant, and you build a new power plant, and the whole cycle starts again.