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See the fire - windows and other designs

Rufus Laggren


Joined: Feb 23, 2012
Posts: 322
Location: Chicago/San Francisco
    
    4
Following up Matt Walker's post about a window at the end of the burn tube.

Re-purpose an oven door window? Steel angle frame (welded/bolted)? Any old hinges? Can't right off see why it would have to be air tight - just well fitted; you have a relative gale blowing down through the feed hole anyway so a little draft from the window-door probably wouldn't mess anything up.

But a question: Why wouldn't a "hearth" work at the end of the burn tube instead of an "overhead" hole? Ie. just end the burn tube in a little open "fire place". It wouldn't self feed which is a really nifty function of the burn hole, but... You could see the fire scurry away from you. Is keeping heat an issue?

Another approach might be to mount a piece of polished stainless steel at the good angle above the fire hole - a mirror.


Rufus
Ernie Wisner
volunteer

Joined: Oct 16, 2009
Posts: 788
Location: Tonasket washington
    
  23
you actually need the mixing of the vertical feed and you dont want to feed the stove constantly. the ss mirror has been done it works pretty well. As i said i am not endorsing anything until it has been in use a year.

Perhaps i need to be clear about something to everyone. the plans are proven designs. i will not endorse anything till its been in use for a year at minimum, if i suspect someone bent the numbers even a little bit the device or design will be retested.
Sorry but thats the way I have to play it. My goal is to make good stoves that are dependable , with in a range i will add stuff that pretties it up for some esthetic but the base is the same a consistent good stove folks can build for them selves that wont cost you an arm and leg..



Sorry Rufus this is not pointed at you. I just have to say it now and again.


Need more info?
Ernie and Erica
Wood burning stoves, Rocket Mass Heaters, DIY,
Stove plans, Boat plans, General permiculture information, Arts and crafts, Fire science, Find it at www.ernieanderica.info


Rufus Laggren


Joined: Feb 23, 2012
Posts: 322
Location: Chicago/San Francisco
    
    4
Thanks for your response. Your testing approach seems about the only responsible way and I'm glad somebody's taking the time/effort. Also, the 1)passive 2)reliable criteria seem pretty good also.

FWIW I'll pass on a link you might find worthwhile some time - www.heatinghelp.com. A small business vehicle, it attracts mostly smart professionals who love their craft. Doesn't relate directly (lot of digital whiz-bang there), but there are some good discussions of heating in general and quality people are good to know; they might have some interest in your version of HVAC, too. Dan Hollohan, the owner, is quite ecletic and a most decent person.

Which makes me think of a Q: Do RMH installs have any condensation issues where the surfaces' temps get below the condensation point for the wood fire gasses?
IIRC 137F. is where natural gas combustion products (don't know about wood fuel) condense onto a surface and lots of thought goes into dealing with that phenomenon. Or do the RMH flue gasses always remain hot enough all the way through to the atmosphere to prevent any condensation?

Rufus


Ernie Wisner
volunteer

Joined: Oct 16, 2009
Posts: 788
Location: Tonasket washington
    
  23
You do get a little condensation if it gets to low; but that is usually not an issue after the first burn of the season. The time you get condensation is just after you build and the stove is curing in. this can look like the flood of Noah. Its temporary but it can be scary.

the thing to remember with wood heat is its dry; the exhaust is steam and CO2 but it still has the ability to pick up other water so a bit of damp in the system will just be absorbed by the exhaust column and...... Exhausted. this is what i have seen but conditions i have not met yet might be different.

Gas heat is kinda wet and icky IMO. I've used it on boats and in campers and its always wet. so i dunno maybe there is less water in our burns. but i dont think we burn as wet as Gas and dont have much of that concern.

Ive not had coffee yet so thats the best i can do at the moment.
Erica Wisner
volunteer

Joined: Feb 10, 2009
Posts: 689
Location: Okanogan Highlands, Washington
    
  79
Rufus Laggren wrote:Thanks for your response. Your testing approach seems about the only responsible way and I'm glad somebody's taking the time/effort. Also, the 1)passive 2)reliable criteria seem pretty good also.

FWIW I'll pass on a link you might find worthwhile some time - www.heatinghelp.com. A small business vehicle, it attracts mostly smart professionals who love their craft. Doesn't relate directly (lot of digital whiz-bang there), but there are some good discussions of heating in general and quality people are good to know; they might have some interest in your version of HVAC, too. Dan Hollohan, the owner, is quite ecletic and a most decent person.

Which makes me think of a Q: Do RMH installs have any condensation issues where the surfaces' temps get below the condensation point for the wood fire gasses?
IIRC 137F. is where natural gas combustion products (don't know about wood fuel) condense onto a surface and lots of thought goes into dealing with that phenomenon. Or do the RMH flue gasses always remain hot enough all the way through to the atmosphere to prevent any condensation?

Rufus


I like that link a lot - we've been getting requests for water-heater information and I would love to send folks to take a Dead Men Steam School workshop. http://www.heatinghelp.com/seminar/73/Dead-Mens-Steam-Night-School-Parsippany-NJ Wonder if they're open to the public, or if he requires builder qualifications?

Condensation:
The original Rocket Mass Heater designs relied entirely on the super-clean combustion design to prevent any problems with creosote.
Ianto likes his exhaust gases as cool as possible, down around 60 degrees F for cob buildings with a lot of thermal mass. This exhaust rolls out and tends to drift downward as a dense fog, and the exit pipe does drip. They exhaust slightly downward to allow drainage of ordinary flue-gas condensation. (CO2 and steam, condenses to carbonated water, plus a few stray ketones.) With adequate drainage, the warm air flowing past at the end of a burn cycle can dry things out pretty nicely. He's working with cob buildings, though, so the walls are completely non-flammable, and going out that way is much easier than creating a custom thru-roof for his EPDM/living roof designs. He's also in a forested valley with pretty convenient (and minimal) prevailing winds.

We tend to keep the exhaust at somewhat higher temperatures for conventional buildings, and use a conventional manufactured chimney through-roof to bring the exhaust out above the roof. It's easier than trying to establish a reliable downwind horizontal draft (the owner-builder may not be able to remember prevailing winds or predict storm winds / eddies). To get reliable vertical draft, you have to maintain warmer exhaust temperatures, as dropping below the dew point also makes the exhaust quite dense. So you do a shorter heat-exchanger, like the masonry heaters' rough working limit of about 20-25 feet, and lose a little more heat to atmosphere, or you bring the last pass back between the barrel and wall to re-heat the chimney gas on its way out. These systems tend to exhaust a clear gas, which turns to white fog a few inches beyond the chimney cap, then dissipates and disappears.

Another situation where the cooler, downward exhaust might be worth doing is in a greenhouse, where conditions (such as damp garden beds) make a cold exhaust temperature highly likely.

There's not a lot of time or space on a modern building site to tweak slopes for drainage, and in the US there's this expectation that all the working bits will be hidden in the basement or attic (like the servants of yesteryear). So high-tech fans or high-temp exhaust is the preferred solution, over-building for reliable installation with minimal disruption of the owner's happy home. These are like taking a pill instead of healthy exercise or diet.
I recently lived in an apartment that was cobbled together over time, and central heat was added as an afterthought: insulated tubes of flexible duct crawling through the attic like a monstrous squid. When we started heating with our rocket mass heater, we had warm air circulating in the house, and the usual levels of damp for a Pacific NW winter. Moisture would condense in the furnace air ducts even from ordinary room air, in the colder attic, and we had a drip from one of the vents once that we thought was a leak in the roof. We had all the vents closed except the intake. Ended up blocking the air intake haphazardly with brown paper - allowing enough air to circulate to hopefully prevent mold, but limiting that condensation cycle.

Building cheap and thoughtless gives marginal results at best. The larger house on the same property was modeled on an older New England home, and had a number of highly functional features - but still suffered from leaks where a gable gutter had no drainspout and back-flowed up the roof in heavy weather.

One thing to understand about natural building (building with natural materials) is that it traditionally handles damp very differently from industrial building.
Most natural materials are breathable. Softer mortars are used to protect stone, instead of needing expansion joints as with cement-type materials. Aesthetics often include practical elements that make sense with traditional materials, like the blunt trim of thatch creating a softer drip-line. (Replicating this curve in asphalt shingle creates some unique challenges for a roofer.)

Moisture is generally handled with a combination of primary protection (roof overhangs, tall dry foundations), ventilation, and breathability. Higher airflows can be tolerated, or even encouraged, to protect the building materials and inhabitants. Air movement is often handled within the rooms, with cross-breezes (doors or windows that open and shut); hidden cavities are minimized or kept above the ceiling as places for bugs and mice to collect.
Modern building seems enamored of 'waterproof' materials like cement, that can actually still wick moisture or ground-damp, and damp-proof materials like tarpaper and plastic, that can cause condensation problems as moisture migrates toward a cold surface and is stopped there. Trying to limit airflow sometimes results in condensation exactly where you don't want it (wall sockets and chimney liners). Insulating a roof rather than a ceiling can create mold problems due to inadequate ventilation. There are limits to how well-sealed a home can be, and still be durable and healthy.

In most traditional buildings, only secondary heat is circulated as air. Primary heat is radiant, and conductive, stored within thermal mass solutions like passive solar orientation and masonry heaters/chimneys.

We have a decent solar aspect on our current home by accident (it was a converted garage belonging to my in-laws), but are giving a lot of thought to passive-solar design for a permanent home. We would like to have a natural building showcase to live in (who wouldn't). But we currently travel for weeks at a time for work, and Ernie dreams of doing coastal disaster relief from a big live-aboard boat, so realistically we will need to design a small home that can take care of itself while we are gone for extended absences. Or content ourselves to live where we are, not where we own.

The passive-solar option can be harder to implement, but it's safer and more sustainable than something that lights itself on fire while you're gone.
If the building isn't passively comfortable, I'd be tempted to install a small electric heater for the pipes, or hire a responsible neighbor's kid, rather than an electrically-controlled combustion device.

Why do you need to heat the house while you're not in it?
To keep pipes thawed from a distance?
Commuting and literally only there when you're asleep?
Or could you find time to supervise a fire 3 evenings a week, while doing other things like cooking dinner or surfing online?
(Or is it just the lure of tinkering - spending hundreds of hours to design a workable self-feeding device is its own reward, as opposed to the same hours spent enjoying a good book by a simpler fire?)

I'm going on about this, kinda ponderously. You're right about us preferring simple and solid-state to whizbangery, although I can appreciate a good whizbang as much as the next person when I'm in the mood. But there's something about the hopper-fed idea that looks like a limited solution to me, and I'm trying to put my finger on it.

Just seems to rely on too many complex interacting factors - including waste materials, components, and cheap electricity that may become scarce tomorrow.
Coal-fired furnaces didn't last that long, historically speaking, even when coal was still cheap. District electricity or steam was convenient and tidy in cities, and wood fuel more easily obtained / cheaper, for rural folks. I can see some of the same supply-line issues with pellet fuels. They depend on the grid / home electrical generation, and pellets won't store for very long without being subject to deformation or rot ...

Another factor might be the number of conversions of power. You lose something like 30-70% of your potential energy with every conversion, so a system that depends on both electrical and a processed fuel seems like it is going to have hidden energy costs that are currently masked by cheap electricity, transport, and pellet mass-production. I wouldn't be surprised if pellets are being shipped from China or Mexico at a cost in fuel oil that outshines their fuel value.

A lot of people like pellet stoves, or like the idea of biofuels other than wood.
(Non-wood biofuels is another issue that may be part of my hesitation. It sounds 'green' in theory to save trees by burning agricultural wastes instead, but ... our soils are being depleted while forests are choking on ladder-fuels. Most alternative biofuels are grown on cleared lands, that were once productive forest or prairie. There's also a good argument for the idea that a healthy forest, even a timber or fuel-wood plantation, does more for biomass, oxygen, and tree conservation than a field of ethanol-corn or grassy chaff. Modern paper waste is unsuitable for fuel for the same reasons it's not much good in compost: too much clay-sizing and heavy metals. It's easier to collect large dung patties than small pellets.

I think in general both biochar and pellets fail Descarte's ethical test: would it still be a good thing if everyone did it? Then again, none of the appropriate technologies really meet this test; the test itself assumes a universal 'good' when nature's 'waste into resource' involves a lot more local balancing of variables. Rocket stoves and rocket mass heaters do very well on small-wood that is currently abundant to excess in North American forests. Similar small fuels can be produced on very small permaculture plots worldwide through pruning and coppicing. This avoids killing any large trees, either directly for fuels, or while clearing land for fuel plantations. Dried invasive weed bundles make good tinder. But there may be places where noxious wastes can be safely burned as pellets, and there are no other locally abundant small fuels.)

We are in a weird in-between era where a lot of back-to-the-landers and starving farmers are commuting to pay the mortgage, but wanting more of a homestead self-sufficiency level just in case.
Pellet stoves are cool in the way that model trains are cool - a working, 'living' household example of industrial achievement.
As long as there is a local market big enough to support a local pellet-maker who is committed to sourcing and processing suitable local waste fuels, and they are cheaper to run than pure electric heat, they might be a viable option. Historically, commercial producers often have a much easier time sourcing virgin materials than bothering about collecting waste materials. Trees are pulped for paper, while any self-respecting permie can get truckloads of wood chip for free (arborist, mill wastes, etc). Most of us don't save our own sawdust for wood putty or fuel pellets. If a fuel is not something I am willing or able to procure for myself, or a trusted local source can make a living supplying to me, the chances of it being locally sustainable drop off quickly.

I am also biased toward things that have been locally sustainable over long periods of history - indigenous technologies. I can't recall any indigenous cultures where an auto-fed stove is a priority (aside from the slight tilt of a fox-stove, or downdraft stove). It comes down to the question of why would you need the fire to tend itself? Leaving fire alone is a form of disrespect that is anathema in many cultures - especially, in many of the cultures that have taught me cool things about efficient and effective fire.
Some cultures have truly ingenious and weird stoves, and the empires tend to develop bizarre furnaces for every conceivable purpose. But most cultures seem to gravitate toward intuitive up-draft stoves that are easy to understand and maintain. I've seen fuels processed into useful shapes to minimize fire tending for a cooking fire, like the Sterno-candle-stove, or the Japanese tea-stove with its special coal-powder briquettes (they look like a thick watering-can nozzle, you light the fire from below with a small amount of tinder), or our barbecue briquettes (minimize time from flame to coals).

But the self-feeding pellet thing is new. Is it a response to the dilemma of needing heat all night, but not staying awake to tend the fire?
Thermal mass does that better, in my opinion. Better return on time spent, if you count the pellet-processing or purchasing.

In the absence of the industrial subsidies for fossil-fueled electricity and processing waste into uniform fuel pellets, I think it will be easier to hand-feed minimally processed fuels than to produce uniform pellets and a hopper that can intelligently disburse them. So it would be a post-modern solution that is viable (fit) for a current niche, but might not remain practical for long. It would be a labor of love.

I have this concern about the rocket mass heaters too - in many places steel barrels and ducting simply aren't in the waste stream, and the expense goes up if you count the time to source or fabricate replacements out of masonry or local resources. With all-masonry materials, the benefits over other masonry heaters are reduced (you lose the quick radiant heat, owner-building becomes more difficult, and the costs go up). I think benefits from other, traditional, masonry heaters (like intuitive fire tending, or being able to see the flames) might outweigh the special benefits of the rocket mass heaters (very clean burn, horizontal heated mass for cheap foundations and comfy resting platforms).

@EAW - I like the idea of using goat or deer pellets as a natural fuel - until I think of the difficulty of potty-training the deer to concentrate their pellets in a useful area. Goats, however, might get really into the idea of lighting their poop on fire. Can it be burned wet? Because potty-training the goats onto the hopper would be a pretty simple solution. (Not sure the poop is a better fuel than a soil-builder, but if they are penned up and concentrating it, and you gotta burn something...)

Seems like the sort of job that Douglas Adams would have assigned to Marvin, the suicidal robot with a brain the size of a planet: would he be more depressed about being used as a ruminant's pooper-scooper, or about hand-feeding the little pellets to the fire?

Or how about a Rube Goldberg conveyor system that the goats poop onto, which first cycles the pellet up over the heat source for drying, and then down toward the hopper and individually toward its fiery doom...
This would be amazing just to see. People watch OMSI's Gravi-Tram do similar circulations for hours, driven by worm-gears; there's another one at Timberline Lodge still keeping kids and dates entertained in the chow line. But the staff know that the little steel ball bearings routinely fall off the tracks, and have to be rescued to keep things in motion. There wasn't even any fire or goats, and it still broke down.

Some things are fun to imagine, and amazing if brought into reality, no matter how briefly.


Play with nature, make nifty stuff:
www.ErnieAndErica.info
Matt Walker


Joined: Nov 27, 2011
Posts: 128
Location: North Olympic Peninsula
    
    3
Wow, what a great post Erica. Lots of food for thought there.

I just made a little video that talks a bit about my experience with my door/window combo so far, and touches on one of Rufus' questions.



http://www.permsteading.com
Erica Wisner
volunteer

Joined: Feb 10, 2009
Posts: 689
Location: Okanogan Highlands, Washington
    
  79
Matt Walker wrote:Wow, what a great post Erica. Lots of food for thought there.

I just made a little video that talks a bit about my experience with my door/window combo so far, and touches on one of Rufus' questions.



That's a gorgeous system you have there.
I have not seen an oven in the front like that; you've really dialed in the restriction you needed, to allow some extra air without things smoking back.

A lot of people who want outside air want to burn big wood too; you seem to have done enough tinkering to understand exactly what the rocket mass heater system demands, and you've gotten two great perks out of it that have stumped a lot of people. I think you even have a little bit of 'airwash' over the glass to keep it clean longer. Awesome work; mind if I link to it on our website?

I'm so accustomed to people asking for help, that I'm powerfully compelled to offer unsolicited advice. Creative collaboration. Homage.

Your door is working great, and I can see you have a gasket to give it good seal when closed. I imagine the hinges will loosen up over time.
I saw a cool latch in NZ that might be just the ticket for fine-tuning the door to stay closed, or ajar at the precise angle(s) you like for baking. It's more often used for opening hinged windows, so you can probably Google window latch + NZ and get some images.
The latch is a bar with holes drilled in, hinged onto on the window panel. There is a pin or peg on the fixed part of the frame. You just pick which hole in the latch to drop onto the stationary pin, to hold the panel closed or ajar. Slightly different angles for your outside-operated stove than an inside-operated window frame, but I can think of a couple adaptations that would work well on the same general idea.

Your glass fiber cob does seem to be working well. Is it as clay-heavy as it appears, or did you paint or burnish it to bring the clay up to the surface?
The glass fibers do worry me a bit as far as health and safety (any dusting off will have those fine fibers in it), but I don't suppose it's any more dangerous than other household applications of fiberglass, like attic insulation. You have one crack showing, not surprising in a high-heat bridge area. Could be due to too much clay in the cob, or just heat shock. We have found that about 4" thick is a good working minimum for areas around the metal parts, and we usually insulate between the hot areas and the outer cob. Perlite insulation doubles as an expansion joint; you might be able to make a very fiberglassy cob mix (or fiberglass-and-furnace-cement felt) that would offer the same expansion function around your oven.

I like how you set the door into a round capped pipe, so there is not the strain on the cob that would be there if a square metal fitting was trying to expand and poke sharp corners into the surrounding masonry.

Very, very nice project. I hope you are enjoying it as much as your cat.

-Erica
Matt Walker


Joined: Nov 27, 2011
Posts: 128
Location: North Olympic Peninsula
    
    3
Thank you so much for the thoughtful response and kind words. You are more than welcome to link to it on your site.

The air wash certainly does help keep the window clear. If I have to cover the wood, which isn't necessary now that I've restricted the feed, it would soot badly. By restricting the feed so the door has a constant gap, I'm having very few instances of soot.

That is a great idea on the latch, I've seen what you are describing. They are sometimes used on boats as well, to hold the window open a certain amount and keep it there. I will play with that idea, thank you.

The "cobposite" glass/clay/perlite mix is interesting isn't it? I thought of it as I was lamenting not being able to use straw in the hot areas. I really wanted insulated material for the feed area. Whenever I played with restricting the flow with bricks, I noticed that once they became hot, the bricks would start to work against the draw by creating hot spots at the top of the feed, right where it can do the most harm. Using insulated material there seems to improve draw quite a bit. Anyway, I share your concerns about the glass, and love the cob because it's natural. However, restricting cobposite use to the feed area is acceptable to me as I know how strong the air is flowing out of the house at that point. I expect others would choose to not use fiberglass at all if it's avoidable.

What you see on top there is a little bit of plaster I just smeared over it, the mix underneath has a lot of perlite in it, and my clay deposit is pretty sandy as well. The crack in the front is not in the cob that retains the window but rather the top layer that encases the cast iron pan and comes down to cover the trim there. Good tip about the expansion joint nature of perlite, I'll take your advice when I go for the final finish. I think the cast iron is going to be tough to avoid cracks around. That said, this thing is a work in progress for sure. I haven't decided what "final finish" will look like, and the little cracks here and there are okay with me at this stage. I do think the cobposite would be great for eliminating cracking in finish coats, but I don't want the chance of little itchy fibers poking out everywhere.

Thank you again for sharing your thoughts and experience, I truly appreciate it.
Rufus Laggren


Joined: Feb 23, 2012
Posts: 322
Location: Chicago/San Francisco
    
    4
Matt

That's the "envy of the block" stove. <g>

> CSA...

That sounds like the single most important parameter for any stove; the next being the shape. I'm still not clear on the details of possible various possible air flow, in particular how changing the CSA of the fill hole (resulting from larger or smaller loads or burning down) affects the burn. It'll become apparent with some more reading, I'm sure.

But it looks to me like air flow can be fairly easily controlled using traditional CI stove methods - ie. adjustable inlets, various holes, etc. And this would allow initial tuning more easily and also (if beneficial) changes in setup to account for different burn types (large, small, ending, etc). In particular, it appears to me that holes or slots around the top and/or bottom perimeter of the window case, regulated by a sliding cover, would allow repeatable settings more easily w/out leaving the door open. This might allow a less complex door latch and provide for keeping the door fully closed/latched in all circumstances which might be a safety feature.

Speaking of safety, i have a strong impression that the barrel is a danger point due to high temperatures. Putting "bars" around the barrel about 4-6" off might be a first cut at a safety improvement. Minimal cross section support can be bolted to the barrel itself; the small the cross section the less heat will be conducted to anything mounted on them. Or 1/8" all thread inside of glass or other non-conductive insulating spacers can be used. Maybe instead of bars, 1/2" square wire screen would be better... It's not just children or adults falling on the barrel which might cause problems; things touching it especially over long periods could cause problem. A big part of the safety is is that the barrels don't show their heat in any meaningful way to humans - we don't see in the IR region. The look the same stone cold as they do right up to just before red hot. What happens if somebody tosses their straw hat on top of the barrel while it's warming up and the owner doesn't notice it? A guest puts their plastic travel alarm clock there? These stoves are not something everybody grew up with and (duh!) know not to put their hands on.

That bread looks great. <g>

Rufus
Rufus Laggren


Joined: Feb 23, 2012
Posts: 322
Location: Chicago/San Francisco
    
    4
Erica

Ditto Matt - Thank you for all your thoughts. Take a bit to digest.

> pellets

I have the same feel about pellets. They depend on a certain infrastructure and the mechanicals are a bit complex.

> local building material

That is a very useful concept rule-of-thumb: Major difference between local materials building and conventional is the big difference in water/moisture control.

> tar paper

"Building wrap" has actually been tested extensively vv. moisture permeability and air sealing and water sealing. Tar paper came out at the top of the pile - it is quite permeable to water vapor so it allows drying and properly installed it's water proof and very wind proof (ie. it's a good primary weather barrier).


Rufus
Erica Wisner
volunteer

Joined: Feb 10, 2009
Posts: 689
Location: Okanogan Highlands, Washington
    
  79
Ernie Wisner wrote:
Gas heat is kinda wet and icky IMO. I've used it on boats and in campers and its always wet. so i dunno maybe there is less water in our burns. but i dont think we burn as wet as Gas and dont have much of that concern.


Theory speaking to experience - but natural gas is mostly methane, right? Methane, ethane, propane, butane - one thing they have in common is a lot more of the fuel is hydrogen, molecularly speaking, than in a longer-chain carbon-based fuel like gasoline or wood. Though hydrogen theoretically burns fast and hot, in practice it's hard to concentrate the fuel. All hydrogen + oxygen combustion produces water vapor.

Gasoline has a lot of hydrogen by molecule, but more carbon by weight. ('Octane' is an 8-carbon chain; at most 18 hydrogens. Looking it up: octane ratings are based on comparing the fuel's performance with a 100 octane standard, (CH3)3CCH2CH(CH3)2 = C8H18, and a zero octane standard, C7H16. Benzene is 'high-octane' fuel at C6H6; ethanol is CH3CH2OH.) Wikipedia: gasoline. The general formula for hydrocarbons is CnH(2n+2), so the longer the chain, the more equal the carbon dioxide and water byproducts will be.

The cellulose (40-50% of the wood) is a carbohydrate, so it has theoretically got enough oxygen 'built in' to burn its hydrogen already. (C6H10O5)n. Hemicellulose is also a carbohydrate.
Lignin is maybe 30% or more of the wood (depends on how it grew); lignin is more complex (C9H10O2, C10H12O3, C11H14O4), but it burns better than sugars, and more like oils. (One stage lignins can go through as they burn is methylated phenols, the flavorful smoke compounds that are chemically very similar to the 'aromatic' benzene rings in gasoline).
Resin burns a lot like tar, and becomes tar if heated with limited oxygen. I would guess the bark has most of the minerals and unusual stuff.

The upshot is that wood burns slightly hotter than natural gas, and produces much less water from combustion - in addition, the hydrogen in cellulose is 'pre-burned' leaving more oxygen for the carbon. Combustion gases (by molecular volume), assuming adequate oxygen for complete combustion:
- from lignin, roughly 2/3 carbon dioxide and 1/3 water;
- cellulose, butane, gasoline, or parrafin/oils will produce roughly half water and half carbon dioxide;
- Hydrcarbon gases give off more water: hydrogen 100%, methane, 1 part carbon dioxide and 2 parts water; ethane, 2:3; propane, 3:4.

Solid fuels also give much more fuel density, so you can run more air past them while burning and maintain clean combustion. Gas flames can't have too much air or they go out, and because the exhaust is typically cleaner, they tend to be tuned so that less room air is exhausted along with the combustion products. So the exhaust of a natural gas device is going to be more water vapor, and likely more concentrated.
Compared to devices using the same solid fuels, rocket mass heaters and masonry heaters release a cleaner and more concentrated (steamier) exhaust than a woodstove, but they naturally go through drying period at the end of the burn where a lot of warm air is pulled past the small mass of embers, effectively drying out the heater and its surroundings. Metal woodstoves pull a lot of warm air through to dilute their smoke, throughout the burn cycle, because woodstoves with inadequate air burn so smoky that they pose serious health risks both indoors and out. (Many modern woodstoves are built with hidden air intakes to purposely prevent owners from damping down the air supply; even so, they produce more smoke than a properly designed furnace, incinerator, or masonry heater, and only slightly less than a modern outdoor barbecue.)

Another practical application: I learned today that the higher fuel value of lignin may be one reason why punky wood (decayed lignin) burns much more poorly than seasoned dry wood. The other reason is it's full of water.
Water is one of the impurities that must be removed in refining natural gas. Air compressors end up with a lot of water condensation, and I wouldn't be surprised if this happens when compressing or refining fuel gas too. And there might be other reasons you could get water vapor in the pipelines and tanks with the fuel. Wet wood can be up to 70% water by weight, and burning wet wood also slows down the fire and creates more problems with incomplete pyrolysis and smoke. Burning wet natural gas wouldn't be a huge smoke problem in most devices, but it could contribute a lot to the steam coming out the other end.

-Erica
 
 
subject: See the fire - windows and other designs
 
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