flame intensity vs. temperature

[macmet]

I am wondering how the flame intensity affects combustion temperature.

We build solid fuel combustion systems (generally wood), how does the combustion temperature change as our firing rate changes? I am wondering about the combustion at the burning fuel and not simply the chamber temperature.

Does it change? Or is combustion temperature consistent and independent of flame intensity?

 

[mbeychok]

macmet:

I don't understand your question. Please define precisely what you mean by "flame intensity". How do you characterize or measure "flame intensity"?

The combustion temperature of any fuel depends primarily on the fuel's elemental composition, the amount of excess combustion air, and (for gas or oil fuel) on the amount of fuel preheat if any.

Milton Beychok
(Visit me at www.air-dispersion.com)
.

 

[macmet]

I am not sure what the exact definition of "flame intensity" is, or even if it can be measured. I would assume that the higher flame intensity has the fuel being oxidized at a higher rate.

We build wood burners and have firing rates from 10-100%. We have the same excess air regardless of the firing rate (ideally). At 20% is the temperature at the surface of the burning fuel the same as at 100%? (again we should have the same percentage of excess air).

Does the rate the fuel is burning have any bearing?

 

[mbeychok]

macmet:

I still don't understand what you mean by flame "intensity" and I suspect that you also don't know what that term means.

As I said before, the flame temperature depends primarily on the fuel's elemental composition, the amount of excess combustion air, and the preheat of the fuel and/or the combustion air. Burner type may have some effect on the flame temperature ... of that I am not sure.

As far as I know, and I am by no means an expert on this subject, the firing rate does not affect the flame temperature as long as the amount of excess combustion air remains constant.

Milton Beychok
(Visit me at www.air-dispersion.com)
.

 

[25362]

Trying to think about the query, being no expert myself, I might consider other factors such as flame cooling by radiation, cooling by mixing with gases present in the firing chamber, the effect that water and noncombustibles in wood may have on the combustion chemistry, the degree of turbulence and mixing attained to different firing rates, particle sizes that may affect the degree of water evaporation, air distribution, draft effects, etc., etc.

So many variables that it's my opinion that you, as builders of burners, would be better able to assess the expected flame temperature changes if any. We could only speculate.

 

[AbbyNormal]

maybe he means low excess air or burning with O2 enriched

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[macmet]

Milton, you're right, I do not completely understand it, I was taught nothing about fire other than it was hot during my time at uni, so I am still learning.

I believe that the intensity - what I call flame intensity - has no impact on the actual combustion temperatures. I believe this b/c none of the determining factors, which control comb. temp. change with firing rate (again ideally). However, I have had a few people tell me that when in low firing rate - what I consider a lower flame intensity - the temperature at the surface of the fuel is lower and vice versa for higher intensities. I do not understand why this would be and was hoping someone may know one way or another.

I consider a high flame intensity to be a situation where the flames are high and chaotic and the fuel burns much quicker. With a lowered rate we have flames but they are much smaller and seem almost tame. I do not know if you can actually measure this parameter, but if I could show the fire at the two different rates it is easy to see what I mean.

25362 - We have just started devoting serious time to emissions. In the past the most crucial thing was output and it's easy to tell how the temp. in the chamber changes, etc. I do not think anyone ever put thought into what happens while the fixed carbons oxidize.

 

[macmet]

AbbyNormal,

You submitted your post while I was typing mine I guess.

We try to maintain constant excess air. If we are at 50% firing rate we ahve 50% fuel and 50% air.

And we only use 'regular' air with no oxygen enrichment.

 

[rmw]

You are confusing 'flame intensity' what ever that is with heat release. The units of heat release are BTU/cu ft of furnace volume or BTU/sq ft of grate surface area or both if applicable. Your furnace temperature is a function of the heat released and the heat absorbed as mentioned earlier.

Obviously a match wouldn't heat a furnace very well-not very intense in your terms, but a temperature measuring device pointed directly at the match flame would see similar flame temperatures as a piece of wood burning in suspension in the furnace or on the grate if it could isolate on it.

It's just that a well designed wood burning furnace has lots and lots of little matches in it and if really well designed some good circulaton and o'fire air added just at the right point to get complete combustion and burn all the volatiles completely.

In a good furnace, the o'fire air entry ports can look like a natural gas flame-burning brightly blue.

rmw

 

[macmet]

rmw,

I think you're right, I am thinking of heat release. So the temp. would be the same and is independent of heat release?

One other question, would blue flames not indicate high hydrogen levels?

 

[25362]

Apart from the effect some ions (Cu, Bi, As, Sb, Pb) may have on the bluishness of a flame, the blue color is attributed to light emissions mainly by the heat-excited groups CH and C₂.

See:

http://en.wikipedia.org/Wiki/Flame

It is a matter of investigation whether the presence of CO influences color.

 

[Combustor1]

Macmet,

If you have constant air at any firing rate then you must consider that you have to heat that excess air up - so part of you btu power from what ever fuel you use will go to heat the excess air (if in a closed furnace) - in other words you only need so much Theroetical air to burn a fuel - the rest or excess air is a waste or inefficient if you are trying to get heat or intensity.

Combustor

 

[rajant2001]

Flame Intensity indicates the qulity of flame inside the chamber. This is normally measured by the Flame Scanners. There are three types Scanners available in the market viz. Ultravoilet(UV), Infrared(IR) & Visible Light. The selection of the type of Flame Scanner for the application depends on the Fuel being fired (Coal, oil, Gas etc.).

The temperature of flame depends upon the 1. Quantity of fuel burned, 2. Colorific Value(CV) of the Fuel, 3. The volume of excess air fed into the Furnace. (The more excess air will lead to decrease in Fluegas Temperature), 4. The effectiveness (Efficiency) of Combustion.

R.Thiyagarajan

 

[macmet]

Combustor,

Ideally our excess air percentage should remain constant. Our combustion air is not constant as it varies with fuel input. However, this is ideal and it will likely fluctuate somewhat.

Rajant,

Our calorific value is can be assumed to be constant for this question. But why does the quantity of fuel matter? The furnace temperature would fluctuate but I cannot see why the flame temperature would be dependent on this.

Regarding combustion efficiency, our CO levels are always below 100ppm.

I think rmw's analogy with a match answered my question. That is what I suspected and was looking to get some verification.

 

[athomas236]

If flame intensity means the radiation emitted by the flame then by the the old Stefan Boltzman equation it is temperature that affects intensity.

Just a thought.

athomas236

 

[pekjaer]

Hi Macmet,
The match analogy is nice, but all the truth.
If you imagine a match in an 500dgr. stove, and a match in snow pile, you would see a large difference in the flame tamperature. The socalled adiabatic flame temperature is the theoretical temperature calculated if it was not cooled - but it is. It is constantly mixed up with the surronding atmosphere, and the radiation is a power 4 function to the dT. Your flame temperature will (with everything even) not change with the throughput, but depends on how the flame is cooled.
Regards
Poul Erik