"the correct Stoichiometric ratio"

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Monica in France

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Jan 22, 2014
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France
I haven't found this document in English but in the French version of a Ravelli RDS manual I found this phrase :
<< Le système RDS permet à l’installation dans laquelle est assemblé, une fois programmées les valeurs calculées, de fournir toujours la correcte quantité d’air pour obtenir le parfait rapport stœchiométrique. >>
Crudely translated this is claiming that once the system is set-up correctly it will always supply air to the combustion in the correct Stoichiometric ratio.

This obviously is a very good thing and on paper sounds impressive.

However I would like the views of this forum on the likelihood of ever attaining such an ideal.
I offer two evident problems :
A : different pellets will have different characteristics
B : varying pellet mass being dropped down the chute.

IS it possible ?

Does anyone know what the correct stoichiometric ratio is for pellets ?
 
Only thing I can tell you is that 14.7:1 is the correct stoichiometric for the air fuel mixture for an internal combustion motor.
It may be a start.
 
You need excess air beyond stochiometric combustion in real life.

It is usually measured in mass, and can vary on HOW the fuel is combusted.

My Kedel asks for 18% excess O2 ( different way to express excess air, and different reference) at idle, and 9% excess air at 100% fire. My work boiler only requires 4-5% excess O2 since we grind up the pellets and spray it in for combustion. Less air is required to combust all the fuel since its nearly atomized.

Out traditional way of burning it in our boilers/stoves require more air to do that though.
 
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You need excess air beyond stochiometric combustion in real life.

The ratio varies with the type of fuel, Gaseous fuels need lower amounts of excess air while solid fuel require more. About the only way to compensate for varying fuel characteristics is to install an O2 probe and vary the air flow to preset value. This is done on high end wood boilers but I am not aware of pellet stoves with this option. Generally a bit too much air does not hurt unless you have fan horsepower to deal with.
 
At the risk of "iceing this thread, ....

Having my own equipment and knowledge to set up my oil boiler, I have wondered how pellet burners get their efficiency...while blasting VAST amounts of "air" into the combustion chamber. Air consists of 21% oxygen, 79% nitrogen (generally accepted values) Of that mix, ONLY the oxygen is used in the combustion process. . The Nitrogen(inert= not used)is heated and sent on its way. In the "old days" , oil burner technicians would open up the air and get a good flame. Setting up the burner "by eye"( Old timers will argue but it is IMPOSSABLE to do this) This at the expense of the efficiency. (a very GOOD reason why NOT to let your oil delivery people service your boiler)
At this point we measure exhaust gasses for carbon dioxide, ....get that level as high as you can short of soot in exhaust (generally 13% + - any higher and insulating soot will form) MY point, too much air is good for flame...bad for efficiency....
 
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Your stove is set up for your installation set-up? Pelleting in NJ probably sent you both manuals... From there, you can make minor adjustments to air and pellet drop - press buttons 6 & 7 together to get pellet and draught. This would accommodate pellet change.

These stoves have multiple heat sensors but that really doesn't say much about the efficiency of the burn... I have noticed the combustion blower will change slightly but not quite sure what triggers that as the temp remains fairly steady (state stove).
 
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When calculating efficiency by flue gas analysis, I believe a ROUGH rule of thumb is 1% efficiency loss for every 1.5% increase in excess O2 above stochiometric combustion and that a 40*f increase in stack gas temp is roughly a percent as well.... this is for generating steam however. It changes with lower rejection temperatures and changes quick when you start condensing stack gasses.... but that's another massive conversation.
 
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Checked both manuals I have for the word stoichiometric .. not to be found. These manuals appear to be poorly set-up and hardly very informative on many of the technical details - no details on the board set-up, etc. Wonder if there is a technicians manual....

Phil Do's Palazzetti appears to have a more advanced burn system but then it has the rotary or star feed and secondary combustion. These are interesting.... http://en.rossatogroup.com/prodotti/generatori-biomassa/caldaie-a-pellet.html sound similar to the Palazzetti.
 
The only way to properly regulate the air-fuel ratio in a pellet stove would be to use an exhaust gas sensor (chemical content), like an oxygen sensor used in cars. As far as I know, there is no pellet stove that uses an oxygen sensor.

The Ecoteck/Ravelli "RDS" stove models use a mass air-flow sensor in the intake air path, used to regulate the combustion fan speed to acheive a constant amount of combustion air, automatically compensating for the restriction of the particular flue pipe configuration installed with each stove, and the normal increase of combustion air restriction that occurs as the burn-pot holes and the exhaust passages get restricted as the stove is used, between cleanings. This type of system does help maintain a more consistent air-fuel ratio (compared to a stove which does not have a mass-airflow sensor), but it does not have the precise control that an oxygen sensor type of system can acheive. It is a stretch for the stove manufacturers to claim that the stove maintains the ideal stoichiometric air-fuel ratio with this mass-airflow system, a bit of marketing hype.

The same air-flow sensing/regulating system has been used by Rika-Austroflamm stoves for years, the Ravelli RDS feature is really nothing new, but it is certainly a desireable feature to have. I think the Pallazzetti stoves also use a mass-air-flow system.

I think Harman and Quad should update their products to use a mass-airflow system.....and at least catch-up to this early 1980s technology (electronic mass-airflow sensor).

http://www.wellsve.com/sft503/Counterpoint3_2.pdf
 
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Pelleting In NJ said:
The only way to properly regulate the air-fuel ratio in a pellet stove would be to use an exhaust gas sensor (chemical content), like an oxygen sensor used in cars. As far as I know, there is no pellet stove that uses an oxygen sensor.

The Ecoteck/Ravelli "RDS" stove models use a mass air-flow sensor in the intake air path, used to regulate the combustion fan speed to acheive a constant amount of combustion air, automatically compensating for the restriction of the particular flue pipe configuration installed with each stove, and the normal increase of combustion air restriction that occurs as the burn-pot holes and the exhaust passages get restricted as the stove is used, between cleanings. This type of system does help maintain a more consistent air-fuel ratio (compared to a stove which does not have a mass-airflow sensor), but it does not have the precise control that an oxygen sensor type of system can acheive. It is a stretch for the stove manufacturers to claim that the stove maintains the ideal stoichiometric air-fuel ratio with this mass-airflow system, a bit of marketing hype.

The same air-flow sensing/regulating system has been used by Rika-Austroflamm stoves for years, the Ravelli RDS feature is really nothing new, but it is certainly a desireable feature to have. I think the Pallazzetti stoves also use a mass-air-flow system.

I think Harman and Quad should update their products to use a mass-airflow system.....and at least catch-up to this early 1980s technology (electronic mass-airflow sensor).

http://www.wellsve.com/sft503/Counterpoint3_2.pdf
Click to expand...

Some of the European pellet boilers are utilizing lambda probe oxygen sensors like you mentioned for oxygen monitoring in the exhaust stream. The Kedel has a curve set for low, mid, and high fire o2 targets and some variable fan speeds built in for each of those points that are all fully adjustable by the user. The fan will either speed up or slow down to match that target for best efficiency over the whole firing range. If the fan can not take care of a large difference (change in pellet brands for example) the auger will speed up or slow down to help out as well.

I figured some of the higher end stoves were doing this too already...?
 
T
DZL_Damon said:
Some of the European pellet boilers are utilizing lambda probe oxygen sensors like you mentioned for oxygen monitoring in the exhaust stream. The Kedel has a curve set for low, mid, and high fire o2 targets and some variable fan speeds built in for each of those points that are all fully adjustable by the user. The fan will either speed up or slow down to match that target for best efficiency over the whole firing range. If the fan can not take care of a large difference (change in pellet brands for example) the auger will speed up or slow down to help out as well.

I figured some of the higher end stoves were doing this too already...?
Click to expand...
The harmons and others use a temp sensor(range,not on/off)to keep burn rate what the programming wants to see.Some european stoves use a maf sensor and a exhaust temp probe together,along with programming(key here programming).Europeans still ahead of us in pellet technology.Original idea for austroflamm maf sensor was so stove could be used at any altitude,just happens it works well.Original design came from BMW.I think mfg. are using heat sensors because would be almost impossible to keep an 02 sensor clean.As far as advertising,look at china stove web sites,they advertise over 90% efficency!Some truth streaching there!I do not know how the boilers are keeping a o2 sensor clean.
 
China with clean tech? Did you see the news tonight with the pollution in Bejing? I've seen cleaner air when cleaning the ash out of the shop vac. Worse visibility than here in the blizzard.
 
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I've been searching the web for information on the best way to burn pellets and I came across this thesis by a Polish guy who was experimenting with different control strategies for the combustion. Lots of formula and verbiage but the one thing stood out was that dynamic feedback control was not a great success. (It wasn't dynamic enough : the time lag screwed him )

So is the Lambda sensor the ultimate solution ? The cheap ones only tell you lambda <> l : and now you tell me they can get blocked up with the soot as well.
The Ecoteck/Ravelli MAF sells for just €24 : two temperature sensors in series – the second heated. Simple and perhaps even reliable.
But “stoichiometric” – you are right there <Pelleting in NJ> they are pushing it a bit : and worse it seems because for complete combustion of pellets convention wisdom states that you need a lambda of 1,2 – 1,3 ( I've even read of higher figures , and thank you <DZL_Damon> for your case study ( real ) figures ).
If Ecoteck are Hyping ( is this a word ? ) , I loved the page given to us by <Lake Girl> . I read it in the French translation as well – just as good. Are there any Italian readers on the forum to explain to us what the translators couldn't understand ? ( or swallow ?)


ChickenMan : I am disbelieving. 97% ???
Not because of the Chinese angle but because of those basic laws of physics.
You need to tell us more :
What is your definition of efficiency.
What sort of power levels are you talking about

For every Kg of fuel you burn you have to push out over (6,2 + 1) Kgs of hot air in the chimney.
How is the miracle performed ?
 
You can;t beat the European pellet burning technology (Windhagger, froler, etc), no way the chinese will come up with a high efficiency number that is overstated!
 
chickenman said:
The stove tested was from the US with our flue system.
Click to expand...
If you don't mind...what is your "flue system"? PM me if your not comfortable posting it
 
Chinese are burning US coal as well. US shut down several power plants as they cant meet new standards and the coal is going over seas and being burnt without any cleaning and then still floats over here contributing to 10% of west coast pollution. Was a major environmentalist on the tube last weekend being very disappointed with Obama administration as it hits us three ways. Loss of jobs, loss of cheap power and still makes for pollution here:( What are the health issues the Chinese going to have with all that smog with unrestricted emissions? Hope we can get the stoves to burn better with some simple modifications. Studying simple rocket stove designs as now there is at least three designs for pellet burning without power.
 
Bioburner said:
What are the health issues the Chinese going to have with all that smog with unrestricted emissions
Click to expand...
Let me tell you...I for one am glad they are only polluting their air! and Japan is only altering their country with radioactivity ... Are you aware China has build a dam that when it completely fills its weight will shift the earth on its axis
 
iceguy4 said:
Let me tell you...I for one am glad they are only polluting their air! and Japan is only altering their country with radioactivity ... Are you aware China has build a dam that when it completely fills its weight will shift the earth on its axis
Click to expand...
They will have to have someplace to hide all those lung ailment victims:) Japan making glow in the dark tuna so its easier to catch em. Self cooking cows grazing the hot zones.
 
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Time to steer things back to pellet stove operation, or this is going to make a move to the ash can.
 
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Bioburner said:
Loss of jobs, loss of cheap power and still makes for pollution here
Click to expand...

Loss of job where: unemployment has decrease since he took office.

Cheap power does no longer exist. You have to realize that coal, oil and gas are only in a limited supply.

Pollution is here because some individual do not care about the environment.

JMHO
 
chickenman said:
Nothing special about our flue system and as I don't know how to PM so I will explain it here; THe inner flue connects to the stove exhaust as normal. The outer flue is sealed to the inner flue with a high temp flex hose going to the combustion air intake. The outer flue draws the combustion air from outside and exchanges heat from the hot inner exhaust tube.
You can see the flue adaptor on the back of our Nero stove on our website. If I can make them so can you.
THis makes a huge difference to the efficiency of the stove but, Like I have said you just need to be careful to not reduce the exit flue gas too much.
Click to expand...

This does work but its an uncontrolled route to condensation of flue gasses in your exhaust. I wouldn't advise this unless the homeowner understands the risk of creosote and checks for it often.
 
chickenman said:
The outer flue draws the combustion air from outside and exchanges heat from the hot inner exhaust tube.
Click to expand...
AAaahh. I have been doing just that for over a year ("in spite of neigh Sayers") I chose to make my own because my vent pipe was just onths old. when I replace my vent , I plan to use the "Selkirk" system Lake girl mentions. Common sense tells me I have improved its efficiency I have attached pictures....What your are looking at is a 6" regular pipe OVER the exhaust vent... 2014 march 001.JPG 2014 march 002.JPG 2014 march 003.JPG 2014 march 004.JPG

DZL_Damon said:
This does work but its an uncontrolled route to condensation of flue gasses in your exhaust. I wouldn't advise this unless the homeowner understands the risk of creosote and checks for it often.
Click to expand...

As for creosote....Not even close (on my application) Nothing but ash in my vent. I agree ...you should keep an eye out for it....on ANY device that has the potential to produce it....Just saying
 
iceguy4 said:
AAaahh. I have been doing just that for over a year ("in spite of neigh Sayers") I chose to make my own because my vent pipe was just onths old. when I replace my vent , I plan to use the "Selkirk" system Lake girl mentions. Common sense tells me I have improved its efficiency I have attached pictures....What your are looking at is a 6" regular pipe OVER the exhaust vent...View attachment 128585 View attachment 128586 View attachment 128587 View attachment 128588



As for creosote....Not even close (on my application) Nothing but ash in my vent. I agree ...you should keep an eye out for it....on ANY device that has the potential to produce it....Just saying
Click to expand...

Is the PB105 direct vent? That's one of the things I really wish the Kedel was....

Have you measured your Exhaust temps before and after? My flue temps are quite low already so I don't want to reduce the further... usually below 200*F at lower firing rates while making 170*F output water. If I had radiant floors or radiators I would run lower temps and get closer to condensation range which I suppose would not be the end of the world if you check your flue often and don't allow it to build up.

I lived with a wood stove my whole life and always took care of my chimney (dad owned a chimney sweep business and tought me well, young). However, I had the opportunity with a woodstove to heat it up well every morning and clean it up well. Not an option with a boiler.
 
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