Tech question: Flue gas analysis

[dh1989]

Over the summer I acquired an electrochemical carbon monoxide flue gas analyzer. I recently used it with a reagent type CO2 tester to tune up my oil boiler. I'm curious if anyone has used such an analyzer on a wood stove flue with a catalytic combustor, and if so what the readings are for a working catalyst. I've read some papers that showed wood smoke from an open fire (no catalyst) in the 3000-5000 ppm range for CO. I know CO is one of the main combustion products that the catalyst is intended to reduce.

I would fire my stove up and check it out, but it's about 85F today. What kind of CO reduction (percentage/ppm) should a working catalyst provide at operating temperature?

 

[peakbagger]

I cant answer your questions for small home units but I do have some experience with gas analyzers on large biomass boilers. Most oil type analyzers dont work well with wood boilers as the required range of CO is much higher for wood than oil and the oil type analyzer sensor will get damaged by CO spikes. Flue gas temp also can be an issue. When I do biomass boiler tests I use a testo that has high CO range protection and sensors. Usually it has an internal air dilution system to dilute strong CO spikes. I also run the gases through ice bath bottles to knock out any condensables with a downstream vacuum pump pulling the sample before letting it enter the analyzer. In order to do a tune up, I intentionally swing the boiler out of tune and I can see spikes of 20,000 ppm while normal operation is down in the 20 PPM range if I can runt he boiler hot which usually means high NOx. . Some of the boilers do have NOx catalysts and occasionally a few have CO catalysts so I can tune them hot and let the NOx catalyst do its thing. Testo has (or had) a dedicated biomass tuning unit designed specifically for this use but they arent cheap.

 

[dh1989]

It's cold enough for me to start burning, so I wanted to follow up on my thread:

I ran a test with the CO meter and also the CO2 wet tester. I should note that my catalyst has 2 seasons of full time burning (probably around 8000 hrs). I just adjusted the bypass gasket tension and it is barely passing the dollar bill test and may need a new gasket next year. The cat is a Durafoil steel cat and it slides loose in the housing when cold (probably also needs a new cat gasket).

I loaded 18 lbs of oak with 13.5% avg. moisture content. I lit a top down fire with a super cedar right under the cat. 25 minutes after lighting it I closed the bypass. The cat meter still read inactive but the flames were going strong. I noticed the exhaust stream outside went from 10-15' gray smoke to clean almost instantly. I cut the thermostat down to 3 o'clock right away.

15 minutes after bypass closed I had light flames and lots of smoke in the firebox. Cat meter read on first active notch. I drew a sample from the flue thermometer hole. It looked clear with a smoky smell. CO meter read >2000PPM.

30 minutes after bypass closed I still had light flames and lots of smoke in the box. Cat meter 1/2 between 1st and 2nd notch and cat had a light glow. Exhaust sample looked clear with a faint smoke odor. CO meter read the sample at 534 PPM.

45 minutes after bypass closed the flames were out and lots of smoke and glowing charred wood. Cat still glowing with meter at 2/3 to 2nd notch. Exhaust sample clear with little odor. CO meter read the sample at 224 PPM.

60 minutes after bypass closed the flames were out and still lots of smoke and faint glowing wood. Cat meter hasn't moved since 45 min and cat still glowing. Exhaust sample clear with little odor. CO meter read the sample at 132 PPM.

After the 60 minute sample I opened the bypass and drew a sample. The sample was thick gray smoke with strong campfire odor and peaked the CO meter >2000 PPM. That confirmed a firebox full of smoke was being converted down to the clear exhaust stream by the cat. Between the 45-60 minute mark I ran a CO2 test which showed 5.0% and a net flue gas temperature of 360F. Using an oil combustion efficiency table I had 74.5% efficiency. I couldn't find a wood combustion efficiency table, but I imagine it's close.

If we assume the thick gray smoke of a smoldering wood fire is 5000 PPM CO, the cat is burning 89% of emissions at 30 minutes after light off and 97% after 60 minutes.

As a side note I tested the exhaust after the fire burned out and there were only tiny glowing coals left. I had ~500 PPM CO with an inactive cat and no visible smoke. This just shows how a draft reversal during the coaling stage can be dangerous, not to mention leaving ash buckets/pans full indoors. I have a CO/smoke detector above the stove as well as throughout the house.

 

[WoodyIsGoody]

I imagine with a newer combustor the CO levels would drop more quickly after closing the bypass. Looking forward to any future reports!