oxygen control for garn

[TCaldwell]

I have been working on putting the puzzle together for about 18 months, but i think it has finally proved out. I am able to control the combustion process in the garn so as to maintain a desired o2 percent in the fluegas. Ideal combustion of wood takes place at an excess air between 1.4 and 1.6. The oxygen percentage in the fluegas will equate to 7.5% with a corresponding co2 of 13%. this comes from chapter 6 ,the theory of wood firing pages 30,31,32, out of the wood for energy production. In order to achieve this i sealed off the existing combustion air inlet, fabricated fixed overlapping coverplate over the existing door hole. i installed 2 air inlets side by side in the bottom of the coverplate, one for primary and one for secondary air. now the inducer pulls air from the new inlets, the balance of the door hole opening has a hinged gasketed positive latching loading door. Unfortunately this is smaller than i would have wanted,it works without cutting into the garn. I fixed the coverplate to the garn by drilling and tapping into the aircollar. after measuring the cfm flow of the original setup i calculated that 4 inch round ducting connected to the new primary and secondary combustion air inlets would suffice.On the inside of the firebox i ducted the primary to the firebox floor and the secondary at the top of the firebox, essentially where they are stock.The 2 four inch ducts on the outside of boiler have modulating dampers, the primary controlled by a scaled co2 signal and the secondary controlled by a scaled o2 signal. the o2 signal is referenced from a 4 wire wideband bosch o2 sensor in the fluestack that monitors the burn cycle and gives feed back to my {controller} that controls the dampers. this system would work on any gasifier that has p/s air control, dampers could be replaced with small fans. One of the unexpected benefits of this system with the garn is that you actually get better results with longer wood in the firebox. With actual o2 feedback you get control of almost all wood conditions. This controller will allow you to adjust the 02/co2 setpoints in scale to burn at any desired o2 percentage to control your flue temp. Heaterman stated about a month ago garn was going to announce their o2 control anyday now?

 

[brad068]

Tom I was wondering where you went. So you are pulling combustion air right out of the room? Can you control the puffing? I have inspection cover plates on my air collar. i could make new ducting and insert them right into and through my air collar? Wonder if a guy could slide a piece of 3.5 or 4" pipe in the combustion air intake tube so as to have two separate air intakes all the way from the outside. Top nozzle would use the smaller dia. pipe and the lower nozzle will use the remaining volume of the original pipe.

I installed some new temp gauges in the air collar. One at 2:00 and one at 8. The 2, highest temp was 300*F and the 8 was 400*F. Does preheating the air affect the process at all? Do you have a thermal couple tied in to the control?

 

[TCaldwell]

hi brad, i initally tried 3inch ducting , the velocity was too intense, espically on the secondary. you really only get a benefit from heating the secondary air, i think it is 1% effiency gain for 40degf increase in secondary combustion air temp. this would allow you to use the existing inlet for the secondary and install a primary. my thought was to install 2 four inch internally , from the combustion chamber out through the side of the boiler, no puffing unless the settings are wrong.

 

[heaterman]

You two crack me up. WAAAAAAYYYYYY to much time on your hands. But I do get quite a bang out of it.

Last I knew Garn is experimenting with combustion control using an O2 sensor but instead of dampers and a constant rpm motor they were talking about using a variable freq drive on the draft inducer to control the amount of excess air over the course of a burn. We'll see....Martin was questioning the cost/benefit ratio of the added expense vs efficiency gained.

 

[brad068]

I know heaterman, just can't leave well enough alone. Its like High E and I were talkin' the other day. The company he is currently working for built this one of a kind monstrosity cranberry pump and one of the loads were shipped to a different plant. The plant was amazed at how clean the load was 99.98% clean. And we both agreed that there is still room for improvement!

If I ain't tinkering I'm dead!

 

[TCaldwell]

Heaterman, martin and i discussed the two different approaches, variable speed inducer vs dampers and stock inducer. The cost associated with the cleanup of the 3 phase power for control signals in unknown residential enviornments is a marketing headache, i went through it and decided on single phase , much easier to control results for installation of such a add-on. I am pretty confident that taking a { clean burn} boiler , that you have posted results during high burn, and being able to say those results are attainable for 80% of the burn rather than 30% might be of intrest to some, not all. Admittedly when i bought a garn , it was for a integrated storage gasification boiler that was bulletproof, i was hoping that would be enough. I guess the pyro part of me took intrest in combustion control, you are right it took alot of time, trial and error, hey, the day goes by waaay to slow without a challenge! Also these results are not restricted to a garn , but any gasification boiler that has primary and secondary air inlets.

 

[Como]

What is the end result?

More btu's from a given burn?

What percentage increase?

 

[TCaldwell]

Durango, If you can regulate your residual fluegas o2 to approx 7.5%, you will achieve the highest btu transfer possible for any wood, maybe not the fastest but the most btus for the load of wood. When you lower the o2 % you will burn your load faster , richer mixture if you will, however sending unburned fuel up the flue. And the opposite holds true with a higher o2 , leaner mix but too much excess air {heat} up the flue. Some manufacturers controls are regulated by flue temp, to control primary and secondary air delivery, basically low flue temp alot of primary and a little secondary air, and a high flue temp,little primary and more secondary, there can be a approximate relationship to o2%, often enough to pass muster. To date for wood, o2 feedback is the most accurate

 

[Nofossil]

Very cool. I'm jealous, but this might just give me the impetus to finish mine in my nonexistent spare time.

 

[sdrobertson]

I understand the theory of increasing or decreasing air through the boiler, but my confusion comes in the relationship between primary and secondary air. Are you guys changing both or only the secondary? I thought that once you get both air supplies dialed in, just changing the speed would increase or decrease burn rates or is this what your shooting for?

 

[TCaldwell]

this is not quite a inverse relationship, the o2 range is 0-20.9% and the co2 is 20.6 to 0% close but not quite, with watching the dampers based on o2 feedback there are times when both dampers are nearly closed , and vise versa, with actual feedback and not just a inverse relationship dictates damper positioning. this comes into play when you are in a high temp gasification mode, you actually need very little of primary or secondary to attain the correct ratio

 

[heaterman]

Durango, If you can regulate your residual fluegas o2 to approx 7.5%, you will achieve the highest btu transfer possible for any wood, maybe not the fastest but the most btus for the load of wood. When you lower the o2 % you will burn your load faster , richer mixture if you will, however sending unburned fuel up the flue. And the opposite holds true with a higher o2 , leaner mix but too much excess air {heat} up the flue. Some manufacturers controls are regulated by flue temp, to control primary and secondary air delivery, basically low flue temp alot of primary and a little secondary air, and a high flue temp,little primary and more secondary, there can be a approximate relationship to o2%, often enough to pass muster. To date for wood, o2 feedback is the most accurate

Off the beaten path here a bit Tom but I think you'll find it interesting none the less. Viessmann has just introduced their second generation condensing gas boiler and it has a piece of really cool technology on board. This boiler is not sensitive to the type of gas and to a certain extent gas pressure. It will burn LP gas from 12" w.c. down to 5-6" and still provide full rated output. Natural gas goes from just over 2" to almost 6" and neither fuel requires any modification to the burner whatsoever or even an orifice conversion. I figured they were doing this with some sort of Lambda or O2 sensor but such is not the case. What they have developed is a burner that uses flame rectification signal as the input to the onboard processor. The processor is able to vary not only fan speed but also fuel flow rate by some magic only the German guys in the white coats understand. To me that is absolutely incredible technology. It eliminates the service issues of an O2 sensor and makes the boiler impervious to variations in fuel btu content as well as fuel pressure fluctuations which are beginning to be a real problem in areas of the country using LNG. Leave it to Viessmann to think outside the box.

I wonder how a person would use flame rec on a wood fire?..........Probably impossible as the position of the flame rod would have to move with the fuel load.

 

[Como]

Vorsprung dirch technique?

Probably do able for chips and pellets.

 

[kabbott]

To me that is absolutely incredible technology. It eliminates the service issues of an O2 sensor and makes the boiler impervious to variations in fuel btu content as well as fuel pressure fluctuations which are beginning to be a real problem in areas.

I am curious if anyone has a good idea of the expected life of an O2 sensor in this environment. I have a little experience with wideband O2 sensors
in an automotive application and I know they can be quite fragile. Leaded fuel will kill them as will condensation on the element when the heater
is turned on. I have a wideband sensor and would love to mess with it when I get a gassifier , which may be before the seasons over.The bosch sensor
I have is not cheap and may not be cost effective if it would need to be replaced often. Can they be run hot enough to keep the nasties burnt off
without killing the sensor?
I would assume running for any length of time in non-gassification mode would kill it rather quickly.

Amazing the improvement in power AND fuel mileage obtained when fine tuning an internal combustion engine with one, be fun to see the improvement
with biofuels.

It would interface nicely with the NFCS analog input and a variable speed blower I would think.

 

[Jim K in PA]

Good work Tom. Glad you got this working. Where in the exhaust stream did you mount the O2 sensor?

I haven't begun to work on the low flue temp fan kill circuit yet, but I found a cheap controller for it.

 

[TCaldwell]

the probe and sensor are in the fluestack, centered in the flue about 3ft off the back of the garn, the bosch lsm11 is a industrial sensor with a 35 watt built in heater, the sensor is rated for 1480degf anything below that the heater cmpensates to keep the temp up , it stays clean that way, i am told to clean it with compressed air once a month and that it should last 2 years 175$

 

[pybyr]

Question 1: what is the device for the O2 sensor in the flue

Question 2: why motorized dampers instead of an air feed motor supplied by a variable speed drive that could output 3 phase with single phase input, using some form of signal to control the VFD

Question 3: my inescapable desire to tune and fiddle with such things has driven off one wife and at least two quality girlfriends (OK, the wife was a scold....): what kind of sensor can I deploy to find/ verify a woman who can smile or at least shrug at the fact that large ferrous items follow me home and that I spend lotso time in the basement, barn, Hearth.com, & etc. I need a babe [yes, I know that term is declasse: I truly like smart wom'n] who can appreciate that any of the foregoing, however nerdy, guarantee that I am not chasing anyone else, anywhere else.

 

[kabbott]

Question 1: what is the device for the O2 sensor in the flue

Question 2: why motorized dampers instead of an air feed motor supplied by a variable speed drive that could output 3 phase with single phase input, using some form of signal to control the VFD

Question 3: my inescapable desire to tune and fiddle with such things has driven off one wife and at least two quality girlfriends (OK, the wife was a scold....): what kind of sensor can I deploy to find/ verify a woman who can smile or at least shrug at the fact that large ferrous items follow me home and that I spend lotso time in the basement, barn, Hearth.com, & etc. I need a babe who can appreciate that any of the foregoing, however nerdy, guarantee that I am not chasing anyone else, anywhere else.

I guess I just got lucky but all I gotta tell her is "It Will keep you warmer" and she keeps me warm so to speak. ;-P
Plus she knows I cant catch them anymore and if I'm eyeballing something it's more than likely the iron not the feline.

edit: Did you get your tank in service yet?

 

[Jim K in PA]

the probe and sensor are in the fluestack, centered in the flue about 3ft off the back of the garn, the bosch lsm11 is a industrial sensor with a 35 watt built in heater, the sensor is rated for 1480degf anything below that the heater cmpensates to keep the temp up , it stays clean that way, i am told to clean it with compressed air once a month and that it should last 2 years 175$

Thanks Tom. Certainly easier to get to there. Other than maintaining a higher ambient temp, is there an advantage to mounting it further upstream in one of the inspection plates? I think doing so would increase the possibility of contamination of the sensor, but just wondering about performance. Those WB O2 sensors sure have come down in price . . .

 

[TCaldwell]

Prybr, the o2 sensor works on a millivolt signal, the oxygen analyser hardware converts it to a milliamp signal that gets scaled to a 0-20.9% display and a output mA control signal, witch is the actual feedback of the % of o2 in the fluestack. I used dampers to maintain a negative pressure in the firebox, so when you open the loading door no smoke rolls out. The variable speed fan alone will not control combustion, all it will do is add or subtract air volume. Because cordwood in a boiler does not all burn at the same rate, and that the phases of wood combustion require different ratios and volumes of air to be efficient. With dampers or fans controlled by feedback signals the correct percentages and volumes of air are maintained. the basic principle is that the primary air control burns the load of wood producing wood gas, the secondary air burns the wood gas , and the end result is monitored by residual o2% in the flue. The near stoichimeteric or balanced burn produces 13% co2 with the primary air and a secondary air volume that when burns the woodgas has a 7.5% volume of o2 in the fluegas. In order to maintain the %s throught the everchanging burn the dampers or fans are always modulating to simultaneously manipulate the total air volume as well as p/s%s. I believe the wife girlfriend thing and being in the boiler room is a balancing act, its all about who wants what when. My daughters call my boiler the other woman, i think at times we are all on thin ice.

 

[RowCropRenegade]

I wish I was to the point of tinkering with the garn. My question is how much more efficiency are you trying to achieve by controling primary/secondary air? At what cost?

Very curious to see mine running. Oil boiler working hard tonite.

 

[TCaldwell]

The garn stock, if you follow o2%s goes as follows, within 5 minutes of ignition of wood o% o2, this lasts for 15 to 30 minutes, too much primary air producing wood gas for secondary to burn off, middle of burn 4 to 12% o2 lasts about 30 minutes , pretty good considering no control, last 30 minutes 12 to 19% 02 too much of both p/s air. It does really well for no feedback control. basically rich at beginning, good in middle and lean at end. It becomes complicated to determine exactly how much more efficient o2 control is, the formula is not difficult CFM x density of fluegas x delta t of flue temp- secondary burn temp, stock garn might go like this 375cfm x 1.08 x 1500=607,500btu/hr. then x length of burn. the numbers represent how the formula works, the delta t would need to be a average over the length of burn. for a modulating damper or fan setup you would need a average of the cfm also, without a gas flow meter it is just a guess. I would have to assume being able to control o2 to a efficient% throughtout the burn would be more efficient than stock, the cheapest way would be with a pitot tube flow meter but i am not sure how the fly ash would affect it.

 

[heaterman]

I'd love to monitor a full burn with my combustion analyzer both with and without the O2 sensor engaged and observe what CO and efficiency are doing over the course of a load. That would really tell the story. I'm betting that the O2 sensor controlled burn would show really low CO levels.

 

[pybyr]

edit: Did you get your tank in service yet?

The tank is fully built and full of water; I just need to run the silicone hoses from the tank over to the plate HX next to the boiler and then see how it does in service at max temp, and then, assuming all works well, put the foamboard around it. Projects multiply faster than time, so I need to get back to it.

 

[Jim K in PA]

I'd love to monitor a full burn with my combustion analyzer both with and without the O2 sensor engaged and observe what CO and efficiency are doing over the course of a load. That would really tell the story. I'm betting that the O2 sensor controlled burn would show really low CO levels.

Steve - This would be an interesting exercise. I just wonder how accurately you could duplicate a burn cycle. Even splits from the same tree aged in the same manner for the same amount of time are going to burn differently. I think you would need to do a longer study of multiple burns over the course of a season to get averaged results for both combustion efficiency, Delta T comparisons (thermal yield), and solid fuel consumption.

Tom - do you have any chrono data for the phases of a non-modified GARN burn? Let's say for a 2 hour burn, how much of the burn is occuring during rich, stoich, and lean phases? I'm a believer in the 80/20 rule for many things. I would offer a hypothesis that 80% of the burn is occurring within stoich, or within 20% of stoich on either side. If that assumption is correct, you are optimizing the burn phase during less than 20% of the burn. I am sure your system is modulating the input air continuously, as you say, but I just wonder how much "return" their may be on this type of investment. Can you pull data from the controller and plot the flow curves for P&S air flow over the course of a full burn?

I am not at all criticizing your effort. I commend it. I am just curious as to how much more thermal yield you are getting from optimizing the burn in this manner. If you can get more close to a 20% increase in Btu yield from the fuel using this method, I think it is well worth the effort. If it is less than 5-10% increase in thermal yield, it is more of a fun exercise (and maybe a cleaner flue). You sure have us thinking though! :coolsmile: