Carbon Deposits in Boiler Chimney

my experience is limited for boilers, but

unburnt carbon deposition

what do you mean by this, is it carbon and ash build up, or unburnt oil.

efficiency may be poor, is there some control to increase the heat on the boiler

this will have to maintain immediate......chimney fire is eminent.

What are the typical Orsat Analysis (or other combustion gas analyses) of the combustion gases of your boiler when firing each of these fuels? That should tell you a lot about your situation.

What is the colour of the gases coming out of the chimney when the boiler is in operation ? This will give an idea whether the combustion and the air-fuel ratio are optimum. This is a layman's way of finding out.

Rajan

Simple ... the unburnt carbon should zero, ideally. Unburnt carbon means that fuel with some liquid or solid content isn't completely burnt (purely gaseous fuel, if unburnt or partially burnt (to CO instead of CO2), will go up the stack). So it's your oil that's not burning fully all the time.

This can arise from various sources: dirty burners, oil-pressure too low, insufficient excess air, etc. There are either times when not enough air is mixed with the oil in the burner (so insufficient excess air), the turbulence of oil-air mixing is poor (low oil pressure, poor oil-atomization (possibly from insufficient steam flow or pressure if steam is used to atomize the oil), etc.). All of this could also arise from bad programming of process controls, or from faulty control equipment (air dampers, oil-flow control valves).

Have the oil-combustion section checked by a qualified engineer or technician.

And understand that it's NEVER a good idea to have unburnt carbon on stack and heat-transfer surfaces. Sometimes it's inevitable (when burning solid fuel, like waste wood or garbage), but having some when burning oil should never be toilerated.

DZ

Some of these have already suggested,

you need to do the following

1) Measure CO and oxygen in flue gas. This will give the type of combustion you are having.

2) Analyse the soot removed for % carbon and % ash ( inert) This will tell you how much of fuel is unburnt.

3) You have to analyse based on fuel burnt what is the amout of soot/ ash generated and what quantity is retained in the chimney.

4) You may have to look up the chimney design that you are maintaining sufficient velocity so that soot is not collected in chimney.

5) Part load operation will result in lower velocity and more deposits.

6) If possible you can schedule your change over from fuel oil to NG.

Firstly, there is no such thing as complete combustion. Even with the correct stoichiometric mixture and correct velocity and the best burner systems that can be bought with vaporized fuel there will still be an amount of unburnt combustibles. After one year these will accumalate to an extent in the chimney. Probably most of your accumalation is ash from the oil with a little bit of carbon black or soot mixed in. Put it in an oven and see what the potential is for combustion.

You should be having 2 types of burners. If not then the mixtures will not be adequate for either gas or oil, as oil requires more air (oxygen) for combustion then gas. Since we don't know what type of oil or gas it is that you are using we can't comment on what the mixtures should be and it also depends on the type of burner(s). I would suggest you have a burner company come in and take a look. He should tell you the mixtures, how much air at what velocities, fuel and at what velocities, spud and spud designs, sizes for both fuels and for the burners, design of the piping, flow controls, by passes and safeties. If you have access to North American Burners, they publish a book on all points of design, safeties (according to US requirements), everything else you might need. There is a represetative in India for them.

is soot

It would also be of interest to know the sulphur content of the heavy oil..........just to know what other crap you are polluting the atmosphere with.

As pointed out carbon is a product of incomplete combustion.........how often do you clean your sprayers (burners), are you getting correct atomisation of the fuel, are you getting sufficient air, is it mixing correctly with the fuel etc, etc.

........we get about 1200kg of carbon.........

On a ship that I served in we used to burn somewhere between 24-36 tons of heavy fuel oil/day. Although we used to blow soot regularly and maybe carry out two external boiler cleans a year...........being water tube boilers with very high forcing rates you dare not run the risk of soot build up between the tubes.........and I doubt whether we would have created that much soot in twelve months............the fuel was burnt relatively efficiently.

In my humble opinion your boiler is either terribly inefficient, or is in need of a major refit.

Thanks friends for all the inputs.

Though I agree theoretically there should not be unburnt carbon, some soot and ash would result as produt of combustion and part of it will deposit at chimney bottom. In this case it works out to about 0.5% of fuel used. I feel now this is more and I need to concentrate on burner and atomization of fuel. This might have also resulted due to our efforts to limit excess air to increase boiler efficiency.

This will inprove in a week or so becuase the natural gas supply will be sufficient and we will not have to change over to oil every day.

Rajesh