[Stoves] Fwd: Stoves Digest, Vol 95, Issue 26 top lit updraft burning

Norbert Senf norbert.senf at gmail.com
Sat Jul 28 06:36:30 MDT 2018

---------- Forwarded message ----------
From: Norbert Senf <norbert.senf at gmail.com>
Date: Sat, Jul 28, 2018 at 8:34 AM
Subject: Re: [Stoves] Stoves Digest, Vol 95, Issue 26 top lit updraft
To: "Ronal W. Larson" <rongretlarson at comcast.net>

On Fri, Jul 27, 2018 at 9:20 PM, Ronal W. Larson <rongretlarson at comcast.net>

> *(snip)*
> * 1a.  Because they can burn whole logs,  the cost per unit of supplied
> energy (absent initial cost) must be lower than any other fuel source. *

We can burn large logs (150mm - 200mm diameter) with low emissions. PM is
same range as pellet stoves. The cold start and getting everything up to
temperature is the main technical issue. Efficiency is roughly the same
(75% HHV, 80% LHV) as a good EPA stove, so fuel consumption per BTU would
be similar. The low temperature radiant heating has some additional
benefits compared to hot air.

* 1b.   Do some MH's ever recommend use of coal?*

In Poland, for example, they use coal and therefore have grates. This
causes air quality problems in urban areas.
I have a friend from Ukraine, where wood was scarce and they used it only
for starting the coal.

* Being able to use stored energy must be a big selling point, in terms of
> time spent in keeping a house warm.  *

That's the main benefit. We've heated exclusively with wood for 45 years,
and my wood heating chores amount to roughly 10 minutes per day. As a
bonus, my wife and I get to watch a nice big fire for 2 hrs every day.
Having started out as a mason building fireplaces, this is much more
successful as a fireplace than the traditional open one.

* 1c.  **Is there ever a cooking application (besides ovens) where mass is
> so important  (question for everyone). *

We have an oven option, and use ours a lot. It is a free byproduct of the
heating function, and is different from having a dedicated oven that you
would fire separately, although some people incorporate that style as well.
We also have a cookstove, which is a separate appliance and requires a
separate flue. In Hungary, it is traditional to add a brick "heating wall"
between the cookstove and the chimney. Crispin is looking at a version of
this in Mongolia(?) I believe.

> * 1d.  What heating efficiency is claimed by MHA members?  (can you get to
> 90%?)*

HHV we realistically get 75% (82% LHV). You can get roughly 5% more with a
system from Russia called "double bell" that takes advantage of buoyancy
rather than channeling of the gases.

Batch fuel loads are 15 - 30 kg., once or twice per day. Biochar has never
> been a consideration, and I only got interested in it recently.
> * [RWL2a:  Which is why I have to keep this dialog going a bit longer*
> They typically have a flue damper which needs to be closed to retain the
> heat. This means all the charcoal has to be burned, or you risk carbon
> monoxide poisoning. Some countries like Germany and Austria don't permit
> flue dampers due to the potential safety issue.
> *[RWL2b.  I have read that burning charcoal (not making it) is a favored
> form of suicide;  via CO.*

I had a Finnish neighbour who told me that as children, they were taught
"blue flame means death".

> With an underwire air system (grate), burning up charcoal is easy, but
> underfire air was found to be dirty once we started measuring PM and CO.
> *[RWL3:  Apologies for "underwire" - my computer doesn't have "underfire"
> in its dictionary.   You are giving here one of the main advantages of
> making and saving char in a cookstove - this is a topic broader than
> efficiencies.*
> For the last couple of years I have experimented with using an airtight
> system, like the Austrians, that allows the combustion to be stopped by
> shutting off the combustion air and also preventing flow through the
> chimney. This can be done at any time once the batch has burned the
> volatiles and is into charcoal mode. This is a huge convenience, because
> you can walk away and don't have to worry about closing a flue damper too
> early. Coincidentally, the tighter you can manage to make it, the more
> charcoal you can make. The next morning, there may still be some live coals
> and I scoop all the remains into a metal ash bucket with a tight lid, that
> extinguishes everything.  Somewhat similar to the bottom part of a Kon Tiki
> kiln. Charcoal yield is roughly 5% by weight.
> *[RWL4:   I suppose you are giving a reason (difficulty in achieving a
> tight fit) some TLUD cookstove manufacturers choose to burn up their char.
> I like the solution achieved by Julien Winter of a burnable string (sort of
> a fuse) that dumps the char automatically and immediately when the
> pyrolysis front hits the bottom.*
> * We promoting char-making stoves would not be satisfied with 5% (always
> hear more than 15% and sometimes up to 25% or more - by weight).  If I
> wanted a MH that maximized char production (to make more money while
> heating), what char-production do you think might be achievable?   *

Maybe 10% - 15% using existing techniques? To max it out maybe you'd be
looking at something like an airtight boiler made from refractory lined
steel, set up somehow to facilitate the handling of the charcoal from a
batch process. In my case, the fireplace function with a large glass door
is important, and probably the hardest thing to get airtight.

> * Gordon West and Bill Knauss are doing heating (not cooking) with a
> continuous TLUD design - but need (as do most all) pellets or maybe chips -
> vs your use of much cheaper logs.  So you might have a leg up in costs per
> kg of char production - but they might (?) have a leg up in being able to
> produce more char per log.  Any thoughts here on economics?*

I made about 300 lb of charcoal last winter from 7000 lb of cordwood (20%
moisture). There is a big push in the US Northeast from New York State and
others to push pellet biomass for domestic and small scale commercial.
Cordwood is considered too problematical with PM emissions. For EPA
certified woodstoves, the PM emissions are very operator dependent, and
there is still a big debate about this. The New Zealanders have done some
really cool field and laboratory studies of certified and uncertified
pellet and cordwood stoves, using the Condar portable dilution tunnel that
we use.

> Contraflow refers to a particular way of running the internal heat
> exchange channels, ie downdrafting. It is native to Finland. Combustion is
> complete by this time. In the traditional Austrian system, the firebox has
> less height and immediately downdrafts before entering additional heat
> exchange flue channels. With this setup you get very good mixing in the
> flames in the downdraft channel this way.
> *[RWL5:  I still haven't visualized the geometry.  Can you send us to a
> technical paper on this?    I think your use of "down drafting" here does
> not mean airflow through the fuel bed, so I am not sure how this might fit
> into char-making.*

Downdrafting in our case refers to heat exchange flue channels after the
firebox, and not to combustion in the firebox:

Austrian stove:

Finnish contraflow heater:

Norbert Senf
Masonry Stove Builders
25 Brouse Road, RR 5
Shawville Québec J0X 2Y0

Norbert Senf
Masonry Stove Builders
25 Brouse Road, RR 5
Shawville Québec J0X 2Y0
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