Relationship between CO and Efficiency...Boilerpro

Glen

in my last example - theoretical as it may be - CO was 6.6% of final combustion products. If you truly want a little hydrogen on the right side of this exothermic event then it would look like this:

2CH4 + 2O2 producing CO2 + H20 + CO + 3H2 (and heat of course)

CO is now a staggering 16.6% of the flue gas products - which is well above the flammability of CO - further combustion would then happen (quickly) again consuming available hydrogen (3H2). Now I am just playing with the basic physics of combustion which our dear Maker decided to bless us with - and theorietical or not - the laws of physics can not be changed. I hold fast on my submission that it is unlikely that there is measurable hydrogen available - even after extremely poor combustion. What say thee?? Just recently I serviced a BEA 58 (oil) - 3 pulls on my smoke tester and it was plugged solid - black - no grey just black. CO was approaching 5000 ppm before I pulled the hose of the PCA. Efficiency was rock steady at 85%. After service, CO was a paltry 25 ppm while efficiency only climbed 1.5%. This was after a thorough brushing and cleaning. Before pics attached. So back to the earlier question - is efficiency affected - yes it is - but not to the extent we think. Ya gotta love the WALL - great place for this sort of discussion. Thanks Dan.

Boilerpro_3

Question?

As I understand it, Combustion analyzers do not take CO levels into account when calculating combustion efficiency. Obviously CO is an indication of unburnt gas, so this is reducing efficiency. So, what is the relationship between CO levels in PPM Air Free and drop in efficiency? This would be a useful tool to help convince people to replace equipment because a better idea of savings can be figured. Oh course if they won't fix or can't fix the equipment, they are taking a huge risk.

Boilerpro

jp_2

1000ppm CO

good question, I've been wondering that myself.

1,000ppm is 1,000 in 1,000,000 "things" or .1%

so right off hand the efficiency could dip .1%

also CO is partially combusted product. roughly 75% energy lost when CO does not goto CO2.

so then the efficiency drops to 75% of .1% ???

my estimate of a guess?

[Deleted User]

Interesting question...

but all I have is observations. I commished a modcon today, and the manufacturer wanted to see the 02 around 7.5 %. When started, o2 was at 10%, air free CO was around 50. I increased gas (no air adjustments possible) until the O2 was within factory spec, and the CO ended up out side of factory spec > 150 ppm a.f. CO.

So to be sure, I'm NOT sure what affect CO has on the combustion efficiency (spelled reducing the 02) of certain gas fired appliances. YMMV...

Jim Davis?

ME

Henry_5

CO leads to sooting

Anytime your oxygen level drops under four percent, your combustion becomes very unstable and instead of producing CO2 produces CO.

The more your percentage O2 goes down, the more CO / less CO2 / greater soot because there is not enough oxygen for complete combustion.


Incomplete combustion ie sooting is a prime factor in the need for service as soot is an insulator, so it robs the system of efficiency.


Henry

Jim Bergmann_2

CO

The combustion equation used in Testo analyzers does take into account CO and the loss of efficiency. THe effect of CO is not as much as you might think. Every 300 ppm lowers the efficiency about .1%, Soot however will compound the problem by not allowing heat to transfer to the heat exchanger and raising the stack temperature.

[Deleted User]

Henry...

Enlighten me please.

If CO2 is the inverse of 02, (Co2 is read from the O2 cell, no?) how can CO2 go down with O2?

Why would CO be produced more with less residual O2 in the products of combustion? Isn't residual O2 an indication of how efficiently you oxidized the fuel and converted it into thermal energy? (Of course, I'm speaking of my experience with natural gas here, not oil...)

Always yearnin' for a learnin':-)

Thanks for the enlightenment.

ME

Glen

Great question!

I have the good privilege to work on all fuels. And have had high CO with low oxygen - the O2 levels being within factory specs & have had high CO where oxygen levels were not to fact specs. This observation comes from mostly vitolas, rondomats, and vertomats. In most cases - I have failed to notice much impact on the efficiency. I use two different analysers (pca and quintox) and while each enjoys regular service - they never have agreed on combustion analysis values. And that has raised my eyebrows a few times. This may be dependent on the fuel analysis formulas programmed into each - even perhaps varying calorific values. But back to your question - it is my belief that you can enjoy (sic) unbelieveably high CO under a vast variety of conditions - so I must really answer - it depends! All fuels can soot a boiler - and that will have a greater impact than just the CO.

Mark Hunt

CO


CO, as you all know, is a by-product of incomplete combustion. Determining what is causing elevated levels of CO is one part of a complete combustion analysis. Since CO is the gas that will hurt or kill people, we pay very close attention to those numbers.

If, for instance, I have a gas fired boiler that produces high CO(>100ppm) and O2 <6% the unit is over-fired. We would also see high stack temperatures. Short-cycling of the burner is not efficient yes? Compound that with an already over-sized piece of equipment and the problem escalates.

Flip that around. High CO and O2 >9% and I have an under-fired piece of equipment. Low stack temperatures and a boiler that can't keep up on design day. Again, efficiency suffers.

So the short answer is, CO itself is only an indicator of how well the appliance is combusting the fuel. Efficiencies will be at max potential when the combustion is correct.

Hope this helps.

Mark H

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Ron Schroeder

CO is more of an indication that something else is wrong rather than a problem in itself.

Something like the wrong nozzle pattern can have the same stack temperature, CO2 and O2 but a compleatly different CO than the proper nozzle.

Ron

Boilerpro_3

What I am seeing here.....

is that we all have a huge amount still to learn about combustion. Sort of like our recent heat loss calc discussion. I work only with gas,so that is my area of experience. I agree that high CO is a symtom, and yes high CO can result from both excessive air or not enough air. I had a Burnham steamer with a Gordon Piatt power burner recently that had a CO reading of 4500 PPM and about 300% excess air...very high O2. We cut down the air and the CO dropped like a rock....i suspect the excess air was cooling the flame so much it could not burn properly(very low NO). Efficiency jumped from about 68% to about 83%. NO sooting goin on. At about the same time I had a little conversion burner in a 50 year old 2 pass Kewanee with about 300% excess air and CO under 100PPM, we cut the air to about 100% and CO only dropped slightly, But efficiency on the meter jumped from 68% to 81%. Unit appears to still be underfired, but couldn't get anymore input out of the ancient burner.

I have found about a third of the older atmoshperic boilers I test(almost always if not always with cast iron burners) come up with High CO...From 700PPM to 2400PPM. None have shown signs of sooting. Usually a slight reduction in input brings the CO down nicely. Typically these older models seem to work at 30% excess air and about 200PPM CO. Newer models seem to run about 50% excess air and about 30 to 50PPM CO. This may explain why older atmoshperics had less trouble in gravity conversions with no low temp protection than newer models....they are less likely to condense at such low excess air numbers.


Let's keep this going!

Like Mark said Yearnin' for learnin'....


Boilerpro

[Deleted User]

I concur re: older cast iron burners...

Been there, done that. Now throw altitude into the picture and the waters get even muddier.

One interesting thing that I have seen, is on Buderi atmospherics, operating at altitude, I've found that I can crank the input up, ignoring the typical altitudinal deration models, and end up with an appliance with a full snoot of useable btu's and minimal CO production!

Put that in your stoichiometric pipe and smoke it!

The only thing I can think that has drastically changed is the secondary free air issue. The newer German slotted burners are much more in tune with the required primary air, and the design of the burner front basically blocks most of the secondary air from cooling down the process.

Maybe the right reverend Timmy Mac will drop in on the conversation and tell us his opinion on altitudinal deration.

Now, here's more conversation for the mix that I know Tim knows about. With all the imported natural gas coming into this country and coming "into the pipeline", what was good this week in combustion analysis may change next week...

Whassamutha to do??

Good conversations.

ME

Ron Schroeder

Fortunatly the electronic combustion analysers actually measure O2 and calculate CO2. O2 is more important than CO2 for tuning a burner, especially if the altitude and fuel type is a variable. Imagine trying to tune a burner running on pure H2 with a CO2 analyser....there is no C in the fuel to even create CO2.

Ron

Glen

velocity

while I can not comment on the blue boxes - their silver cousins seem to like the same treatment. I don't think you can completely ignore the hi alt issue - but I too have noticed that at altitude - above 3000 ' - higher gas velocities allow the "slotted" burners to mix gas/air very effectively - and as you have observed and measured - with little CO - while maintaining published Ef values and expected CO values. Especially true with LP. And the large Riello R series and Weishaupt GL series - regardless of firing rate - also imitate this finding. High gas velocities make for great combustion.

Jim Davis_3

CO & Efficiency

I just downloaded an engineering report a few months back that addressed the difference in BTU's lost by producing CO versus CO2 and it is much greater according to this report than the calculations I have seen. I will try to find it next week but jp was on the right track. What I found interesting is that the breaking point was around 100ppm. I have seen commercial burners run as low as 1% O2 and make less than 50ppm of CO. Lennox pulse furnaces ran below 3% O2 with less than 30ppm of CO. It is up to the equipment, the installation(including altitude) and the fuel to determine what fuel-air mixture it can operate. Our job is to figure out what it is using our analyzer. But never let the analyzer mislead you with it's the efficiency calculations because they are rarely even close.

Henry Nichols

I musta misspoke

When O2 goes up, CO2 goes down. But when O2 goes below 4%, in my experience, CO production goes way up and sooting occurs.

I probably do not know what I am talking about, but I have seen the sooting that occurs and watched the CO monitor go off because O2 was so low.

Henry

jp_2

next question,

if you are producing fairly large amounts of CO, next questions is: how much fuel is not even being combusted, but going up the vent? that would help reduce efficiency.

can you measure raw fuel in the vent with your equipment?

[Deleted User]

Your experience Henry...

is just as important as the next guys. However, your experience may also be coming from a completely different technology than that which I am familiar with. I'm positive that the matrix burner used in the Vitodens reacts differently than any other burner on the face of this planet, hence, your input is just as valid as the next guys.

My experience is primarily involving stainless steel ribbon burners, and the can type of post mix burner used in the Munchkin style of converter.

Thanks for the input.

jp, I'm not sure that there is a measurement for that which you are looking for, other than the obvious effect on the residual oxygen in the flue gasses. Obviously, if you are not doing a good job of oxidizing the fuel (natural gas in my case) then you will end up with a high 02 content in your flue products, along with a high CO factor. But I am not aware of any conversion factor that can tie it to a dollar or energy deficiency amount. We just have to do the best we can with what we've got to deal with...

ME

Glen

not raw fuel -

but varying amounts of incompletely burned fuel. Thus the high CO.

jp_2

the question,

certainly there 'can' be raw fuel within incompete combustion, question is can it be measured?

probably talking about CO way over 1,000.

Glen

I disagree - here's why

3.5O2 + CH4 produces 2CO2 + 3H2O for "perfect combustion"

if you plan to produce CO then I would adjust the equation like this:

9.5O2 + 5CH4 produces 4CO2 + 10H2O + CO

There is no "raw" (eg. unattached hydrogen or hydrogen attached to carbon) fuel left on the right side of the equation. But if the CO is high enough it too burns - quite violently. All that said - there may be one or two radical molecules of hydrogen kicking around after ignition and the resulting production of heat (unlikely) - and that could be measured: but I think we would need to take over the jet proplusion lab from NASA and use their finely tuned instruments.

jp_2

fine & good,

well glen, I agree you can balance the equation in any fashion. but this is on paper.

I'm questioning the different efficience numbers coming up?
you only answered "that depends", come on, put some muscle into it!

one posters tester gave readings 3 times higher than I predicted, I was giving a reason as to maybe why?

but to answer you directly, I still think you can have situtations where not 100% gas being combusted.

still waiting for jim davis to come back with that info?

lets get back to the orginal question anyway.

Jim Davis_3

Just for the record you can not attain 85% efficiency without condensing. The efficiency calculations on combustion analyzers absolutely will mislead you with their fictitious readings. 85% could actually be 45%, 55%, 65% or 75%.

jp_2

reading efficiency?

jim how does that work? does it assume a burner temperature, then when you measure flue temperature it decides what the efficiency is? hi - low ?? does seem to make some sense.

jp_2

physics

i do stand behind the sciences, but they are somewhat all empirical. they do play with statistics and often throw out data that does not comform to the theory, I have always questioned this? then theres curve fitting, so the data will fit within an equation???

all the laws have fudge factors.

but I was posing a question, not making a statement, I need to reread your posts and reply.

as others have said, an abundance of O2 can cause CO to form, this will not happen on paper!

Jim Davis_3

Efficiency

The calculations assume the BTU's of the fuel and deduct flue temperture from the ultimate flame temperature the fuel can attain versus actual flame temperature which can be radically reduced by O2. Theoretical flame temp = 3600 degrees. Flame temperature at 6% O2 = 2800 degrees, at 12% O2 2000 degrees. Analyzer will calculate higher efficiency when O2 is 12% because our flue temperature will also be lower. Excess air cools or quenches the flame. The temperature of the flame can be cool enough not to be able to competely combine the carbon in the fuel with the oxygen. In larger burners or oil burners, the excess air can actually move the fuel past the flame so fast that it is never heated to the temperature it needs to totally combust.

Mark Hunt

Try this


Set your testing equipment fuel selector to "oil" when you test a gas fired appliance.

See what your "efficiency" reading will be then. Your customers will be AMAZED!

Mark H

To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"

jp_2

I agree

i agree with what you are saying jim, but if you set the burner up to factory specs, 02,excess air,etc... wouldn't the efficience test give you a reasonable number?

wouldn't it at least tell you the burner still has some problems even though the other readings are ok?

Jim Davis_3

The efficiency is unpredictable. Many years ago when first helping in house boiler engineers set up their burners it became quite clear the efficiency calculation was mis-leading because more often than not the actual calculated efficiency went down as burners were tuned up. In one case on some 400HP boilers the efficiency went from 83% to 81% but after 6 months the customers realized a $12,000 savings in fuel!! On a residential oil burner the efficiency went from 86% to 78% and the homeowner saved 600 gallons of fuel. Have seen this happen thousands and thousands of times.

Glen

indeed

and that is partially my point - eff seemed to be static - while all manner of nasty things was happening in the combustion chamber. Ef is now one of the last things I look for ---

Glen

on paper -

all things can happen - what was the quote from Mark Twain - about statistics???? An over supply of air can cause CO - by cooling the flame front. On the other hand an absence of oxygen does the same thing - or too much fuel (excess, unburned or incompletely burned)And as Mr D points out - each analyser calculates a bit differently - based on known science - and can artificailly render really great numbers. All the while making CO, soot and burning a lot of fuel.