How Low Can You Go (flue gas temps)

[pybyr]

Jebatty's thread about controlling the draft fan to keep flue temps from going inefficiently high makes me want to ask a question that is on my mind:

How low could you drive the flue temps (assuming that you are doing it by catching more BTUs in the boiler's heat exchanger) without negative consequences (condensing flue gas moisture in the boiler is obviously to be avoided, as is significant moisture build up in flue pipe and chimney)?

I was reading some literature on some commercial boiler retrofit turbulator manufacturers' web sites, along with some academic papers on turbulators (I embrace my inner nerd...), and the only specific figure that I found- for an oil boiler- was 265 at the boiler flue collar, plus an additional half degree farenheit for every foot of distance between the boiler and the final chimney outlet.

In my case, between flue pipe and chimney height, that would be 265 + 40 / 2 = 285 degrees (using the oil boiler formula) at the boiler flue collar- which is way lower than my boiler actually runs on flue gas temps, and way lower than I have heard people report from other makes and installations.

Interested in peoples' thoughts on this

 

[Chris Hoskin]

I think it is reasonable to say that a near boiler stack temp of 300F is going to result in condensation in most chimneys. That's why we normally recommend a chimney liner with the Froling boilers - especially the P4 pellet boiler.

 

[Chris Hoskin]

also why we don't recommend turbulators in a Solo Plus unless you are seeing stack temp of 500 or so as with long burn time and thermal storage.

 

[ewdudley]

...
I was reading some literature on some commercial boiler retrofit turbulator manufacturers' web sites, along with some academic papers on turbulators (I embrace my inner nerd...), and the only specific figure that I found- for an oil boiler- was 265 at the boiler flue collar, plus an additional half degree farenheit for every foot of distance between the boiler and the final chimney outlet.


Interested in peoples' thoughts on this

Thanks, it's nice to have a firm guideline to refer to.

In my case I've got just eleven feet from the neck of the boiler to the top of the flue, with eight feet being class A insulated flue, or 132 deg C / 270 degF by the formula.

I've settled on a flue neck setpoint of 145 degC/ 295 degF just because it seems to be as low as I can go and still have a nice little turbulent gasification jet and enough heat output to keep up with load and make progress towards recharging storage, which is about 17.5 kW / 60000 btu per hour with my particular boiler.

--ewd

 

[sdrobertson]

I've wonder this many times myself as I always see other operators with much higher stack temps. I run mine in the 300 range. I've never had any condensation issues that I am aware of, but I guess I don't know what to look for. I get fly ash in the bottom of my "t" and it doesn't show any moisture. What would a problem look like, or act like so I will know for sure?

 

[RobC]

I noticed I started to loose gasification at around 425F at flue.

 

[jebatty]

I would be wary of trying to control flue temp on the low end by draft fan control. My effort was to control high end flue temp. On the other hand, lowering flue temp while maintaining a draft that sustains good gasification is something else, provided the resulting flue temp is not so low as to result in condensation. I burn very dry aspen/pine and experienced flue temps at one time as high as 800-900F. Through adjusting the damper on the draft fan and adding my homemade chain turbulators, I now burn the same wood and have flue temps in the 380-480 range, and with the experiment on the K-type temperature controller for the draft fan have lowered the high end to 430F. As mentioned before, the Tarm purrs very nicely at the 380-430F range.

I am able through more draft adjustment to bring this down even further, but I have found that boiler output also decreases. This may be OK for a system that cannot use the higher output and experiences idling, but with storage I can take the full 140,000 btuh output of my Tarm without idling, and can push storage to 185-190F without idling.

At this point I am a pretty strong believer in the statement that gasifiers operate most efficiently when allowed to operate close to their rated output. With that belief I think that's about where operation should be and effort should be directed at capturing that output rather than letting it go up the stack.

 

[steam man]

Good info here. I especially like the idea of compensating for the height of the stack to ensure condensing temps are not reached before the flue gases reach the stack outlet. This almost mkes me think that having a stack probe higher up the chimey would give an operator better info. I have seen large marine boilers whose stacks literally rotted right off and almost fell over from operating at too low a stack temp for extended periods of time. We typically run at just under 300 deg F measured from high up the stack after extracting excess heat by the use of an air preheater. We actually bypass the air preheater at load loads so not to cause condensation in the stack. My take on trying to get best efficiency is that you have to realize that using the stack temp setting alone is only good as long as your boiler is clean and really under a load. I have seen many time where operators compensate for dirty heat exchangers that let heat go up the stack and cut back on the air. The irony is that cutting back on the air makes the boiler dirtier even faster. There are other variables such as if your stack is insulated, inside or outside, etc.. that determins how warm it stays. The only real way to ensure optimal efficiency is with an O2 measurement as some boilers have but still maintaining a noncondensing stack temp hopefully near the top of the stack.

 

[jebatty]

a stack probe higher up the chimey would give an operator better info.... We typically run at just under 300 deg F measured from high up the stack

Interesting point. I have Class A Duravent chimney -- any problem with drilling a small hole high on the stack to insert a probe to measure temp?

using the stack temp setting alone is only good as long as your boiler is clean and really under a load

Another good point. I brush the HX tubes twice/month, leave a good ash layer in the firebox but do not let it build up excessively, and keep the gasification tunnel quite clean of any ash in the bottom of the tunnel. Also, every burn is high load, with full output to storage and no idling.

 

[steam man]

I don't know why you couldn't put a stack probe any place as long as it is accessible and do a proper job of sealing. I would think you could just find some high temperature insulated wire and drop it down your stack from the top. Can't say I've done this but the readings would be interesting compared to the flue near the boiler.

 

[jebatty]

high temperature insulated wire and drop it down your stack from the top

One of my probes may have this -- good idea. Unfortunately, with all the snow on the roof, the top of stack is not accessible now. Will have to wait for the top down approach.

 

[slowzuki]

Stack temp isn't the key to efficiency. Dropping your burn chamber temps to chase low stack temp will reduce your efficiency. Keeping the same burn chamber temp while decreasing stack temp increases efficiency.

 

[steam man]

Stack temp isn't the key to efficiency. Dropping your burn chamber temps to chase low stack temp will reduce your efficiency. Keeping the same burn chamber temp while decreasing stack temp increases efficiency.

I agree and disagree. LOL Stack temp is the number one heat loss of your boiler and is one indicator of boiler efficiency. However it has to be kept within optimum parameters that being low condensing temps and high heat loss. Think about why all the gassifier manufacturers recommend boiler water return temp of app 140deg F. This seems to be the magic number which infers you have flue gas temps above condensing. Keeping the burn chamber temp high enough to ensure all combustibles are consumed is the goal to efficiency as you state. The only way I know to do this is by measuring the stack gases for lack of CO by 02/CO2 measurements. I think I read elsewhere on this forum where 7% seems to be the goal for 02 in the stack gas for biomass boilers. Burning wood seems to be a best guess as far as complete combustion is concerned. If I remember correctly in heavy fuel boiler, 2% or so equates to about 5% excess air. Excess air ensures that CO release is minimal and basically all you have is CO2. Once you start producing CO you have non-combustibles going up the stack. I do believe each stack adds a unique variable to each boiler and its efficiency.

 

[slowzuki]

I sort of agree and disagree too, but you should only reduce stack temp by increasing heat exchanger efficiency. The highest efficiency of your burning will not produce the lowest stack temps, they will be higher!

Anything that produces lower stack temps via incomplete burn or too much excess air will lower the boilers overall efficiency. Turbulators, cleaning ash etc, all affect the hx efficiency more than the combustion process.

 

[jebatty]

I will attempt data collection as outlined in the Gassifer Draft Control thread and see if anything is measurable by way of either amount of wood burned or temperature change in storage by reason of reducing maximum stack temp to 430F. I have no ability to measure O2, CO2 or any other combustion-related gases. Perhaps if nothing is measurable, there would be no reason to attempt stack temp reduction by the method suggested; or on the other hand, no reason not to. I already keep my boiler clean, have turbulators of my own design, and burn dry to very dry wood.

 

[pybyr]

Stack temp isn't the key to efficiency. Dropping your burn chamber temps to chase low stack temp will reduce your efficiency. Keeping the same burn chamber temp while decreasing stack temp increases efficiency.

I agree and disagree. LOL Stack temp is the number one heat loss of your boiler and is one indicator of boiler efficiency. However it has to be kept within optimum parameters that being low condensing temps and high heat loss. Think about why all the gassifier manufacturers recommend boiler water return temp of app 140deg F. This seems to be the magic number which infers you have flue gas temps above condensing. Keeping the burn chamber temp high enough to ensure all combustibles are consumed is the goal to efficiency as you state. The only way I know to do this is by measuring the stack gases for lack of CO by 02/CO2 measurements. I think I read elsewhere on this forum where 7% seems to be the goal for 02 in the stack gas for biomass boilers. Burning wood seems to be a best guess as far as complete combustion is concerned. If I remember correctly in heavy fuel boiler, 2% or so equates to about 5% excess air. Excess air ensures that CO release is minimal and basically all you have is CO2. Once you start producing CO you have non-combustibles going up the stack. I do believe each stack adds a unique variable to each boiler and its efficiency.

I've got an old-school (but spotless condition) Bacharach test kit, including the liquid Fyrite CO2 measurement device, and have always been curious what I could find out if I used that to measure the stack CO2 with my Econoburn- but I am unclear what percentages of CO2 would indicate with wood, rather than oil (I know there are parameters to aim for with oil, although I don't recall from memory at the moment). Can one of you folks with wide-ranging boiler experience shed light on what would be optimal CO2 percent with wood, and what higher and lower would indicate?

 

[pybyr]

Stack temp isn't the key to efficiency. Dropping your burn chamber temps to chase low stack temp will reduce your efficiency. Keeping the same burn chamber temp while decreasing stack temp increases efficiency.

My question is very much based on the assumption of trying to keep burn chamber temps high but to lower stack temperature by finding some means of achieving better heat transfer on the way from the firebox to the flue.

I'm interested in knowing what the lower limit would be where you'd start to have other problems such as flue condensation. That'd give a sense of how much might be gained/ is worth bothering with in trying to improve heat capture. It'd also allow being aware of "overshoot" in extracting too much heat before the combustion gases exit the boiler.

I'd be really intrigued to find out exit temperatures at the top of my stack (a masonry chimney), but don't know of any affordable way of finding that out. If anyone knows of a really modestly priced wireless sensor/ transponder that could survive out in the weather and with the temperature sensor hung in the top of the flue tile, and that can provide data without expensive stuff on the receiving end, I'd be interested to know about it.

 

[jebatty]

BBQ wireless thermometers read into the high 400'sF. Might be able to fix the probe to the top of the flue, or on a metal support right into the flue. That's the best I know off hand. Here is one to look at, but check ebay "wireless bbq thermometer" for more.
BBQ

 

[steam man]

Thinking about it, I could move an extra solar sensor from my evacuated tubes and mount it on the top tile and read it right off the controller.

 

[DaveBP]

So now we want to put a temperature probe at the top of the chimney (and it does seem like a very pertinent bit of data up there)... where would we put it?

In the center of the pipe? Or would it be more relevant nearer the wall of the of the flue where the condensation would be happening.

Probably cooler out there near the outside.

I know from my own flue thermometer that near the fire it is hotter at the center of the stove pipe than out near the wall. Seems that would be true anywhere along the chimney, regardless of construction.