Evaluation of Low-Emissions Wood Stoves (Shelton & Gay)

[precaud]

This 1986 study has been provoking some interesting discussions in another thread. In the interest of keeping that one on topic, I'm starting a new one here. The study in question is here: http://www.arb.ca.gov/research/apr/past/a3-122-32.pdf
Continuing with our discussion already in progress...

[quote author="Battenkiller" date="1294357162"]Yes, the green wood needed more air to achieve maximum burn rates, he even says so in the text. However, the overall efficiency numbers don't lie. There was not a significant loss of sensible heat up the flue due to the extra air being introduced. Yes, some dilution effect is evident, especially is the open stove, but that doesn't support anecdotal evidence of massive amounts of creosote deposition in residential chimneys venting stoves burning green wood. Just the opposite is reported here, and the dilution effect is clearly aiding this... to the benefit of green burners everywhere. [/quote]

True. And I think I found the reason why the "green" oak burned better than expected (in addition to what you wrote further down about the moisture content being less than what we might think of as green), with less air than one would expect. Look at table 5-2, Summary of Fuel Load Properties, see the Avg. Piece Mass. For the green oak it hovers around 1.1kg, or 2.4 lbs. That is a very small piece of wood. Most here would consider that large kindling. It would be in the same size range as a 2x4. In the airtight stove, the load was six of these pieces. Put pieces that small on a live 20% charcoal bed and it is not going to be difficult to burn a 38%MC load like that.

The largest pieces were used in the Blaze King - about 2.5kg, or 5.5 lbs. All the other stoves burned pieces half that weight, four to six pieces at a time. I'd bet my boots than almost nobody on this forum regularly loads their stove only with pieces that small - we're talking maybe a 3" diameter round or split. That's about the size of wood I use in the X33, because the firebox is very shallow.

Considering the stated +/-25% uncertainty in the Oregon weighted averages, the only really significant fuel factors across all the tests and burn cycles was for the production of elemental carbon (soot), and NOx production (assumed… based solely the performance of the Blaze King - a catalytic stove commonly thought to produce high NOx anyway). The major contributing factor in this study was the appliance effect. The better stoves burned both cleaner and more efficiently regardless of the fuel type - green oak, seasoned oak, or doug fir. Seeing as how that was what was being investigated to begin with, we have a clear winner:

The stove is much more important than the quality of the wood… at least in this study.

I think much of that too is explained by the piece size. Also, the thing I find most significant about the Oregon weighting is not so much the uncertainty factor, but the heavy weighting of lower burn rates. Throw out the lowest burn rate tests, and results improve dramatically for all of the non-cats.

Add in to that a basic discomfort about the sourcing of the elements used in catalysts.

I hear you, mate, but we are pretty much stuck using precious metals as long as the population continues to increase and the technology necessary to reduce our contribution to pollution lags behind. The fact is that catalysts are a crude but highly effective club that can be applied after the fact to cleanup the mess left behind from inferior design. As long as financial motives are the driving force behind scientific development, industry will continue to use whatever crude and inelegant solution they come up with providing that it satisfies the bottom line. The nice thing about the metals themselves is that they are recoverable once the appliance craps the bed. The combustors and converters themselves decline in efficacy with use, but the metals themselves can be largely recovered during recycling of the spent devices.

I didn't know the rarer metals could be recovered in recycling. Is it a high percentage recovery?

There's no denying the effectiveness of cats, especially for round-the-clock burners. They're not well suited to my heating needs so I've never used one - odd, perhaps, in 30+ years of burning, but true.

Anyway, keep working on your projects, they may lead to the next breakthrough in this technology. Me… I’m more of a thinker than an inventor. I like to know how things work, but leave it to the smarter guys to figure out how to make them work better.

There's a place for both in the scheme of things. It's a pleasure to be able to delve in to the depths of things with you.

 

[Renovation]

It took me a while to find the post Precaud is replying to. Here it is:

Battenkiller

Posted: 06 January 2011 02:39 PM [ Report ] [ Ignore ] [ # 23 ]
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precaud - 06 January 2011 12:21 AM

Two thoughts: First, see note in “wet wood†thread about the burn cycle. Long time spent in the charcoal phase, where all stoves perform about the same. Second, lower PM with green oak in the closed VC test says to me there was TONS of air pouring through that sucker in order to get it to burn. That is confirmed by the air-to-fuel ratio chart. Lots of dilution going on.

I’ll go back to the green wood thing again only because of what your comments imply about the test results, but I don’t want to continue to hash this out here on your thread about the Kent stove itself. Maybe on another thread…?


Yes, the green wood needed more air to achieve maximum burn rates, he even says so in the text. However, the overall efficiency numbers don’t lie. There was not a significant loss of sensible heat up the flue due to the extra air being introduced. Yes, some dilution effect is evident, especially is the open stove, but that doesn’t support anecdotal evidence of massive amounts of creosote deposition in residential chimneys venting stoves burning green wood. Just the opposite is reported here, and the dilution effect is clearly aiding this… to the benefit of green burners everywhere.

If you examine the air:fuel ratios, there was not a significant amount more air actually needed to burn the green oak in either stove tested with it. Even at a burn rate as low as 0.7 kg/hr, the conventional air-tight was using less air burning green oak than the Lopi used burning the dry fir test load. Except for a few outliers, the data set shows that the Blaze King used almost an identical amount of air at all burn rates with either the green or the seasoned oak. Which, as I mentioned above, is reflected in the overall efficiency numbers.

Similarly, if massive amounts of PM were exiting the flue unburned, this would have been reflected in both the combustion efficiency and overall efficiency numbers, and this is clearly not the case. Besides, the sensors and filters catch all of the emissions, so dilution wouldn’t affect either the g/hr or g/kg numbers.

One last thing to say is that, although it was called “green†oak in the report, that doesn’t mean it was fresh-cut. Newly harvested oak generally has a MC of about 80%, not the 41% stuff they used in the tests. And 41% dry-basis is not 41% water by weight, it is 29% water by weight. That’s only 9% more water per pound of wood than the “seasoned†stuff. I find it frustrating that, even in a scientifically controlled study, the author included both ways of expressing moisture content in different places within the same report. No wonder everybody gets confused about this essential bit of information.

And that’s all I have to say here about burning green wood.


Considering the stated +/-25% uncertainty in the Oregon weighted averages, the only really significant fuel factors across all the tests and burn cycles was for the production of elemental carbon (soot), and NOx production (assumed… based solely the performance of the Blaze King - a catalytic stove commonly thought to produce high NOx anyway). The major contributing factor in this study was the appliance effect. The better stoves burned both cleaner and more efficiently regardless of the fuel type - green oak, seasoned oak, or doug fir. Seeing as how that was what was being investigated to begin with, we have a clear winner:


The stove is much more important than the quality of the wood… at least in this study.

Add in to that a basic discomfort about the sourcing of the elements used in catalysts.

I hear you, mate, but we are pretty much stuck using precious metals as long as the population continues to increase and the technology necessary to reduce our contribution to pollution lags behind. The fact is that catalysts are a crude but highly effective club that can be applied after the fact to cleanup the mess left behind from inferior design. As long as financial motives are the driving force behind scientific development, industry will continue to use whatever crude and inelegant solution they come up with providing that it satisfies the bottom line. The nice thing about the metals themselves is that they are recoverable once the appliance craps the bed. The combustors and converters themselves decline in efficacy with use, but the metals themselves can be largely recovered during recycling of the spent devices.

Anyway, keep working on your projects, they may lead to the next breakthrough in this technology. Me… I’m more of a thinker than an inventor. I like to know how things work, but leave it to the smarter guys to figure out how to make them work better.
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[Battenkiller]

I think I found the reason why the "green" oak burned better than expected (in addition to what you wrote further down about the moisture content being less than what we might think of as green), with less air than one would expect. Look at table 5-2, Summary of Fuel Load Properties, see the Avg. Piece Mass. For the green oak it hovers around 1.1kg, or 2.4 lbs. That is a very small piece of wood. Most here would consider that large kindling. It would be in the same size range as a 2x4. In the airtight stove, the load was six of these pieces. Put pieces that small on a live 20% charcoal bed and it is not going to be difficult to burn a 38%MC load like that.

Certainly, piece size is a major contributing factor. Still, it has been widely reported that the creosote effect with green wood is only actually seen in controlled studies when wood is burned in open air (AKA" fireplace"). For all other types of burning, green wood produces less creosote than wood that is too dry (below 15% wet-basis) except at the very lowest burn rates, and even then, the effect is not that significant. This little nugget will certainly never be accepted here in "the drier the better" land, but it has been a known fact for at least 30 years. I'm going to get my hands on the studies cited (TVA, etc.) in the footnotes of the Shelton report. He wasn't the first one to demonstrate this. He does go into this in some detail in "Solid Fuels", complete with graphs and such, but I'm too tired to dig the book out and scan those pages. Maybe tomorrow.

Also, a charcoal bed that is 20% of the mass of the reload is not out of line to me. I believe the EPA test protocol specifies 20-25% of the mass of the test load be left in the stove when the main load is added. Personally, I don't wait until the stove has cycled down to a handful of coals and 300-400º temps to reload. When you drop semi-seasoned wood onto a substantial coal bed in a hot stove, it takes right off and burns great. No low flue temps or excessive air diluting the gases. It burns in a very even and controlled fashion. I can't prove this will be the same in an EPA stove, but long-standing hearth professionals I am acquainted with tell me that the new stoves handle marginal wood just fine if you don't try to choke them down too much. I'm not about to drop three grand on a fancy new stove just to prove this to myself, but anyone who wants to drop off a spare EPA stove they aren't using at the moment, I'll be only to glad to slide my stove out of the way for a few weeks to put it to the test.

... the thing I find most significant about the Oregon weighting is not so much the uncertainty factor, but the heavy weighting of lower burn rates. Throw out the lowest burn rate tests, and results improve dramatically for all of the non-cats.

Without a doubt. If folks would ever get their heads out of their asses about burning at low rates to save wood and get super long burns, we wouldn't even need cats, because the non-cats are superior when burned at high output. I never did understand the smoldering burn. Let it rip or let it go out. I wouldn't burn an oil burner with just the pilot light because I wanted to save fuel. I want heat out of the appliance. But getting back to the study, my take is that wood "quality" is not important as long as it will actually burn. The best stoves seem to have the ability to burn well over a wide range of conditions, and they deliver the best numbers across all of them.

I didn't know the rarer metals could be recovered in recycling. Is it a high percentage recovery?

Near 100%. My wife used to be a bench jeweler for a local goldsmith. He told me that the big commercial jewelry manufacturers sent their carpets out every ten years or so to be burned out. There was always enough gold and platinum that escaped the benches that it paid for new carpeting. Good thing, too. All of the gold ever mined would supposedly fit into a cube only 80' wide, and all the platinum mined since the dawn of man would fit into a 20' cube.

 

[madrone]

When you drop semi-seasoned wood onto a substantial coal bed in a hot stove, it takes right off and burns great. No low flue temps or excessive air diluting the gases. It burns in a very even and controlled fashion. I can't prove this will be the same in an EPA stove, but long-standing hearth professionals I am acquainted with tell me that the new stoves handle marginal wood just fine if you don't try to choke them down too much.

Fascinating study. My personal experience is that green(er) wood doesn't burn as well in an EPA stove because the primary air must be opened wide until the moisture is burned off, which is pretty close to the coaling stage. There's not enough air. Can you do it? Sure. I've never been able to maintain secondary combustion with green wood, but I have been able to burn down a load. The difference in heat output, however, is big, as is the duration of the burn. Dry wood goes right to secondary combustion.

There's a lot of data here and I admit I don't follow all of it. I'm not really surprised that green wood burns fine in an open fireplace. I'm not sure how the Kent compares to stoves on the market today. I do know from personal experience, though, that my stove puts out more heat, longer, with dry wood.

 

[begreen]

Still, it has been widely reported that the creosote effect with green wood is only actually seen in controlled studies when wood is burned in open air (AKA†fireplaceâ€).

The creosote accumulation debate is an interesting one, but with respect, I have a very hard time swallowing this one, even with sugar.

I have seen too many stoves, EPA and not, choke up caps and flues, burning green wood, often with the air controls and sometimes the door wide open, just trying to produce meaningful heat. The steam produces by the water in the wood cools down the flue gases to the point where they condense. Theory is one thing, but evidently what is happening in real burning doesn't always support the lab tests.

 

[richg]

I'm way too hung over to read all of this....can somebody give me the Cliff Notes version? :red:

 

[Pagey]

I'm way too hung over to read all of this....can somebody give me the Cliff Notes version? :red:

Burning green and or seasoned wood may/may not/could possibly/does not/will not/definitely does/there's no way in hell! produce more/less/significantly more/omg no wayz! creosote. Epic lulz.

 

[DanCorcoran]

No surprise that the catalyst is recoverable: a catalyst, by definition, enables or facilitates a chemical reaction without taking place in the reaction. Every auto junk yard in the country removes the catalytic converter to recycle, because all that platinum (or whatever the current catalyst is) is still in there.

 

[precaud]

Since I have never cut live trees for firewood, the whole green wood thing doesn't register much. I'll let you guys hammer that aspect out.

Also, a charcoal bed that is 20% of the mass of the reload is not out of line to me... Personally, I don’t wait until the stove has cycled down to a handful of coals and 300-400º temps to reload.

And that was my point exactly. Few burners let their stoves dwindle back down to a 20% coals bed before reloading. And in the test, they basically added kindling to replenish/re-enliven the coals and to restore internal temps. There's a lot of fudging going on in those tests. But it's not fudging with the intent to purposely obscure anything - it's an attempt to limit the number of variables.

Theory is one thing, but evidently what is happening in real burning doesn’t always support the lab tests.

Absolutely. For non-cats, a low burn rate (which is typical) + green wood = rapid creosote buildup. Even the tests show that.

It appears there's a movement afoot to address this and other discrepancies in the next standards update. Details of the proposed changes are supposed to be released this September.

I think it would be valuable for everyone to burn a load that resembles the test conditions for their stove. On an established coals bed, a 15-lb load of 3" splits with an inch spacing between them is a very easy burn. (Larger stoves get 25 lbs and a few 4 inchers mixed in.) Even some pre-EPA stoves do pretty well at it, as the test shows.

 

[oldspark]

The cresote comes from too low a flue temp correct, so if you keep the flue temp up you will not form so much cresote even with green wood, which is one of the points BK makes.
BK I think you should change your name to WoodRebel, keep it coming.

 

[Battenkiller]

Just got back inside from taking care of the snow deposition problem I was having in my driveway, so...

The cresote comes from too low a flue temp correct, so if you keep the flue temp up you will not form so much cresote even with green wood, which is one of the points BK makes.

Not exactly. OK... not at all. Creosote production occurs in the firebox, not in the flue... at any temperature. Creosote deposition is caused by low flue temps, tall stacks as well. Just because someone has a 14' tall chimney and doesn't see visible smoke coming out the top of the stack, doesn't mean they aren't releasing large total amounts of PM and creosote into the great outdoors. My 25' interior masonry chimney gets creosote in it way up past where many folks' chimneys end. If I had a 14' chimney, I might never see a speck of creosote in it, but it doesn't mean I never made any.

The creosote itself is a complex and variable compound that increases dramatically in production at low burn rates, and most folks burning green have a real hard time (or don't bother to even try) increasing the burn rate in their stoves. The whole idea for the green-burning set is to slow down the burn, not increase it. As I said, I've never understood that notion. It comes from folks that are too lazy or too cheap to use a little extra wood. First thing some folks noticed when they came out with the air-tight stoves was that you could starve the fire and it would burn forever. Not a good idea for a heater IMHO.

The creosote accumulation debate is an interesting one, but with respect, I have a very hard time swallowing this one, even with sugar.

I have seen too many stoves, EPA and not, choke up caps and flues, burning green wood, often with the air controls and sometimes the door wide open, just trying to produce meaningful heat. The steam produces by the water in the wood cools down the flue gases to the point where they condense. Theory is one thing, but evidently what is happening in real burning doesn't always support the lab tests.

As far as theory vs. reality, this ain't theory at all, it's observation. Actually, combustion theory might have predicted a lot more creosote than was observed in the Shelton and other studies. And if real life burners can't reproduce these results in their own stoves, well, maybe it's because they haven't been burning them correctly?

I've burned plenty of lesser wood (some quite wet) in my life and I have never personally noticed any correlation between the seasoned years and the unseasoned years regarding creosote in the flue. It is much more dependent on how much wood I actually tossed through the stove. If I burned 3 cord instead of 4, I got less creosote when the sweep was done, regardless of the MC of the wood I used. And chances are that if I only burned 3 cord, it was because I couldn't get the green wood to burn quite as fast as the dry stuff would have, so I ended up using less total wood. And because I used less wood, I was not able to get as much heat out of the stove throughout the season.

Remember, correlation does not prove causation. It is incorrect to just assume that the reason you can't get a lot of heat out of green wood is because all of the heat goes up the flue, or the moisture in the wood cools the stove off. The real reason why you can't get maximum heat output from your stove using lesser wood has been demonstrated in many tests and studies to be merely because you can't burn the wood as fast, so less total heat/per hour is generated. The physical losses aren't that great, but the heat output is lower because the burn rate is lower. So, sure, burning green won't let you maximize the heat output of your burner, but for different reasons than many suppose. That fact is enough to get me to get my wood dry. If I didn't think it made any difference at at, I wouldn't be crowding my wood shop up with the stuff every winter.

BTW steam doesn't cool flue gases. Does steam cool off a radiator? No, radiant and convective heat transfer cool off a radiator. The steam is a highly efficient way to achieve mass transfer of heat from the boiler to the radiator. When it does condense, it always releases heat, never takes it away. It's the evaporation of water that uses heat, not the other way around. At long as that water vapor in the flue stays as water vapor, it ain't changing the temperature of anything.

Besides, even if the condensed water in the flue deposits were to evaporate and cause temps to lower, the more moisture in the gases, the less cooling can occur. That's why sweat cools you off better on dry summer days and excess water vapor (stream) in the air on muggy days won't let you cool off.

Low flue temps are not caused by water condensation inside the flue, they are low because they never got up there to begin with, because the burner didn't administer the proper amount of air into the burning mass at the proper time to achieve an efficient burn. Opening the doors on a stove is the surest way to lower flue temps. With all the fuss folks make about tiny air leaks getting past gaskets and seams and such cooling off the flue gases, what do you think happens when you open the doors all the way up?


Anyway, I'm not advocating burning green wood, and I don't want to make this the focal point of an otherwise extremely interesting study. The whole creosote thing reminds me of the lead up to the Iraq conflict. There may or may not have been numerous good reasons why going in there may have been a good idea, but the one we were sold was the WMD. Turns out there weren't any WMD after all, but the idea scared the Bejezzus out of us enough to go along with the plan. The end justifying the means? I prefer the truth any day.

 

[Battenkiller]

Theory is one thing, but evidently what is happening in real burning doesn't always support the lab tests.

Touching on this concept again, BG, I thought I'd include this statement from the author of this study (taken from his book "Solid Fuels"):

A chart from the same book is attached below. I find this one that graphically compares the dependence on moisture content of various types of efficiencies to be very revealing.

Combustion efficiency actually increases in a linear fashion as moisture content increases, up to about 20% MC, then continues to increase at a slower rate up to a peak combustion efficiency of close to 90% between 25% and 30% MC, after which it begins to decline sharply. However, at no point does combustion efficiency drop below 85%, even with wood that is 50% MC (33% water by weight, or "wet-basis").

Heat transfer efficiency OTOH drops rapidly in an almost liner fashion as moisture content increases beyond 10% MC, and drops to less than 60% efficiency at 50% MC. This is because an increasingly larger proportion of sensible heat is sent up and out the flue as MC increases.

Overall efficiency (the heat you actually receive in the living space) increases as moisture content increases up to about 18% MC, then drops precipitously with increasing MC after that point. Still, the overall efficiency remains at 60% or higher throughout the range of MC from 10% all the way to 30%. Enlightening to see that the highest overall efficiency of a conventional air-tight stove is about 63%... the same figure the EPA assigns to all non-cat stoves that pass the emissions testing. Dirtier burn than the new stoves, but same basic overall efficiency.

Notice how he includes both dry-basis (top) and wet-basis (bottom) figures in the chart. You can use this chart to visually convert between these two commonly used ways of expressing MC.;-)

 

[precaud]

Good points as usual, BK. But let's look at the real-world conditions under which this data set is true. Specifically, an airtight stove (no secondary air), air inlet setting varied to maintain a 17k btu/hr output, and small wood piece size relative to what is normally used. With combustion efficiency peaking at 30% MC, that says to me that the moisture content was essentially the main mechanism being used to slow down the pyrolysis rate to match the capabilities of the stove to burn it. Change the target output power, or change the appliance to a modern one, and a completely different set of curves would result - for the latter, more likely peaking at a lower moisture rate and a higher overall efficiency.

So, as you said earlier, it really shows that the appliance factors predominate, and matching the fuel MC to the stove capabilities will give the best efficiency.

 

[begreen]

Are the flue gas temps and emissions data included for those same test charts?

 

[precaud]

Not in the book. The word "emissions" is not even an entry in the index... how times have changed.

 

[precaud]

By the way, what did I find tucked into the back of my "Solid Fuels" book this morning? A Consumers Research article from January 1988 with a list of all of the stoves certified by the Oregon DEQ 1988 spec, which essentially became the Phase I EPA spec I believe. Interesting to note, cats dominate non-cats on the list by more than 2-to-1. And the focus of the article is on overall efficiency, and hardly at all on emissions output.

It is also likely that the Kent Sherwood I bought back then was the XLE model.

 

[Renovation]

A thought, which may reconcile the lab with real life (for example, BeGreen's points), for your ignore-al.

As I understand it, this experiment starts its burns with hot stoves with established coal beds, and burns relatively small loads of wood.

From what I've read here, my question is, in real life, do many people burn green wood that way?

From the posts, it seems that most people who burn green wood report hard-starting, cold, smokey fires, and would have a hard time getting to a hot clean stove with a 20% coal bed. Even if they can, doesn't it take a good while to get there, during which there is a lot of cool, smokey, creosote-producing burning unlike anything in the experiment?

The alternative would be to first burn a load of dry wood to get to the conditions at the start of the experiment, but maybe that is rare in practice, since most folks burn green wood because they don't have any dry?

Perhaps this is one of those "forest for the trees" situations, where practice itself holds the key to the difference from theory?

Just a thought--as you were.

 

[oldspark]

I was wrong before so I will do it again, yes george you will have to mix the seasoned with the green, burning green wood is not by choice.

 

[Battenkiller]

Are the flue gas temps and emissions data included for those same test charts?

Flue temps are only part of the equation. Exhaust residence times in the flue are much more important. Although temperature is providing the pressure that generates draft, if the opening into the stove is severely restricted, that gas ain't going anywhere too fast. In Shelton's testing procedure, instantaneous sensible heat losses were taken using a combination of temperature sensors, flue gas volume, plus all of the known energy losses from the contents of the gases themselves (the same "stack loss" determination universally used in the industry), then it was all integrated to get the total accumulated heat loss (the area under the curve). Some samples were also taken using a dilution tunnel, but I won't go into that, it's there in the report to read.


This is infinitely more accurate than trying to guess at heat loss just by looking at flue temps.


For example, I had a 475º flue temp yesterday with the air flapper opened just a bare crack. Stove top was 775º! Guess what I was burning? A smallish load of 100% black birch... straight from the BK kiln - at a split and measured 34% MC on the meter (~25% water by weight). There wasn't a hint of smoke seen coming from the stack. I even used the same wood (smaller splits) as kindling to start the fire. It was up to 775º on the stove top within 1/2 hour.

At this low rate of air intake (but high rate of combustion), the fire lasted for a few hours, even though the initial charge was small. And the whole time, I never closed the bypass on the Vigilant. Wood lasting hours, screaming hot stove, no visible smoke, scorching heat (86º) in the room.... where's all that heat coming from if all the wood energy is going up the flue?

Wasn't sure about the times, so I decided to do the same thing again today, but this time with curiosity as a motivator rather than irritation. By deliberately using my very best wet wood burning techniques, I got such quick and dramatic results that I was actually stunned and a little frightened at one point. Not only did the stuff light immediately, but at one point the stove actually started to run away on me for the first time since I've owned it. I had to shut the air all the way and open the griddle top to cool the flue gases, which within a minute's time raised the flue pipe over 250º to 785º as measured 20" up the pipe with the IR gun. I took pics of the whole process (when I had time), and recorded times and the mass of the main load (three progressively larger splits, the largest being 14 lbs, 4 oz) of the same black birch. At minute 35 I closed the bypass and dropped the air down to about a quarter open. After 5 minutes flue temps had finally come down and stabilized at around 370º on the IR. I grabbed the camera and went out to catch whatever was coming out the top of the stack. Crystal clear, not a hint of haze, just 100% heat waves. I will post the whole thing in another thread sometime in the next day or so. No one who was here would ever again doubt how powerfully and cleanly wood in the (estimated) 35% MC range can burn.


Back to the study...


As far as the PM emissions in the 1983 study, here is the chart that shows what went on. In the conventional air-tight, PM emissions/hr while burning green wood were almost half what the seasoned wood figures were. The open fireplace OTOH had 3 times as much PM burning green as it did burning seasoned. The green wood burned much slower in an uncontained (and therefore much cooler) environment, so again, burn rate dictated PM emissions.

The Blaze King results were the reverse of the conventional stove, with half the PM emissions burning seasoned as it did burning green. But look how low both numbers are compared to the conventional air-tight. Th BK burned the green oak at 1/12th the emission rate as the conventional burned the seasoned wood. Clearly, this shows how much more important the appliance is compared to the wood. Which is why the author said so in the report.



Precaud, as you can see in the attached chart below that was taken from the report being discussed here (which BTW was written three years earlier than the book), the term "emissions" was alive and well in 1983. Times really haven't changed that much after all. Wood burns the same as it always has, but the appliances and appliance operators have changed over the years.

 

[oldspark]

Less emissions mean cleaner chimney? Burnt dry wood for 30 years in an old airtight with no creosote problems what so ever.

 

[begreen]

It has less to do with the amount of crap coming out of the stack and more to do with the temperature of the flue gases exiting the flue.

If I might borrow an example from BrotherBart. Here's an old airtight with a very clean stack. But not necessarily burning cleanly.

 

[Battenkiller]

I was wrong before so I will do it again, yes george you will have to mix the seasoned with the green, burning green wood is not by choice.

Well, you weren't really wrong, it's a matter of semantics.

Creosote as a physical substance is, in fact, formed as a result of low flue temps allowing the smoke to condense on the walls of the flue. On that you were 100% correct. What I was pointing out is that the actual components of that creosote are formed by a slow, smoldering burn inside the stove. When the burn rate is stepped up (not necessarily by adding way too much excess air BTW, reducing the amount of excess air can actually increase the burn rate in many cases), PM (smoke) emissions drop dramatically, so creosote formation will drop proportionately, even at identical flue temps. And when the rate of pyrolosis exceeds the amount of air that the stove can take in, excess smoke is created in the box. That leads to an increase in PM emissions and an increase in creosote components. This happens when too much dry wood is added to the coal bed, and not the reverse. The green wood burns cleaner at high burn rates in almost every situation, but in the slow, smoldering burns that people end up using in real life, the introduction of secondary air can usually tip the balance in favor of the drier wood... as long as the wood isn't too dry.

That doesn't mean that slow, smoldering burns are the most efficient (except in cat stoves). In fact, the opposite has been demonstrated to be true. What it does mean is that folks are going to do it anyway, so the EPA stepped in and forced the industry to design stoves that burned better when burned in a less efficient manner in order to try to stretch (effectively) their burn times and (mistakenly) their available heat from the wood.

So I guess we're both correct.

 

[Battenkiller]

If I might borrow an example from BrotherBart. Here's an old airtight with a very clean stack. But not necessarily burning cleanly.

Perfect example, BG. I wonder what that stack would look like on the inside if it was 30' tall.

 

[Battenkiller]

Less emissions mean cleaner chimney? Burnt dry wood for 30 years in an old airtight with no creosote problems what so ever.

We're into semantics again, OS. How wet is green, and how dry is dry?

In this study, "green" was wood at 41% MC dry-basis (29% wet-basis, or % water by weight). That ain't like any green oak I've ever seen. Most oak around here will sink in water if you drop it in when first cut. It's up around 80% MC dry-basis (44% wet-basis). You can see in the graph I posted above how much more severely that extra water impacts overall efficiency. About 12% loss in overall efficiency. That is, if you can get the damn stuff to burn at all, never mind maintain a steady 17K BTU heat output.

 

[precaud]

As far as the PM emissions in the 1983 study, here is the chart that shows what went on. In the conventional air-tight, PM emissions/hr while burning green wood were almost half what the seasoned wood figures were.

Yes, but again, the context is important. Look at Fig. 7-16, which shows creosote vs burn rate. Green oak burned in the conventional stove (CG) at low rate turned in the worst readings of the entire group. Only two loads were burned in the CG, that's partly why it averaged low. This falls under the "missing data" that Sheldon describes.

Precaud, as you can see in the attached chart below that was taken from the report being discussed here (which BTW was written three years earlier than the book), the term "emissions" was alive and well in 1983. Times really haven't changed that much after all. Wood burns the same as it always has, but the appliances and appliance operators have changed over the years.

Yes, but context again, my friend... my comment was about the book, not the paper. Of course the term was around, it just wasn't a major topic in the book.