[Stoves] Stoves Digest, Vol 11, Issue 38

Sun Jul 31 06:37:36 CDT 2011

Crispin and All

Let's explore the plus and minus of preheating secondary air.  

Certainly, preheating increases efficiency by adding Joules/gram-degree (heat capacity), about 0.7 J/g-C, back into the combustion process.  If you could preheat the air to 300C, that would be 210 J/g.

Consider that the heat of combustion of 10% moisture wood is about 20,000 J/g.  So, this is about 1% of the energy being released.  That's the plus.

<><><><>

One of the beauties of the TLUD forced draft stove is that all the volatiles are burned in a disk 4" in diameter and about 1" thick, so that all of the heat is available to the pot, no unburned gases are quenched into emissions.  

If you preheat the air 300C, 573K, degrees to capture 1% of the heat of combustion, you reduce its density by a factor of about 2.  This means that the jets of secondary air intended for combustion can only penetrate the gases half as far, and there will be a lot more unburned gases exiting the pot area.  

So, in my (moderately) humble opinion, preheating secondary air is counterproductive.  

You can preheat the primary air, but that is only 20% of the incoming air, so, again not worth it.

<><><>

I believe you are one of the most diligent and practical practitioners of TLUD stoves, so could balance the theoretical vs practical advantages in each situation, and look forward to your comments on this issue...

Your co-stover,

Tom Reed

Dr Thomas B Reed
President, The Biomass Energy Foundation
www.Woodgas.com

On Jul 29, 2011, at 7:58 AM, stoves-request at lists.bioenergylists.org wrote:

> Send Stoves mailing list submissions to
>    stoves at lists.bioenergylists.org
> 
> To subscribe or unsubscribe via the World Wide Web, visit
>    http://lists.bioenergylists.org/mailman/listinfo/stoves_lists.bioenergylists.org
> 
> or, via email, send a message with subject or body 'help' to
>    stoves-request at lists.bioenergylists.org
> 
> You can reach the person managing the list at
>    stoves-owner at lists.bioenergylists.org
> 
> When replying, please edit your Subject line so it is more specific
> than "Re: Contents of Stoves digest..."
> 
> 
> Today's Topics:
> 
>   1. Re: Fire Stump /  stump-incuts (Crispin Pemberton-Pigott)
>   2. Re: WorldStove replies to BioFuelWatyche's
>      latestimprecisereporting of facts. (Crispin Pemberton-Pigott)
> 
> 
> ----------------------------------------------------------------------
> 
> Message: 1
> Date: Thu, 28 Jul 2011 22:28:16 -0400
> From: "Crispin Pemberton-Pigott" <crispinpigott at gmail.com>
> To: "'Discussion of biomass cooking stoves'"
>    <stoves at lists.bioenergylists.org>
> Subject: Re: [Stoves] Fire Stump /  stump-incuts
> Message-ID: <0bdb01cc4d97$2d4ca560$87e5f020$@gmail.com>
> Content-Type: text/plain; charset="us-ascii"
> 
> Dear Roger
> 
> 
> 
>> Why isn't the secondary combustion process be applied in cooking? or
> another angle why could these appliances do both jobs?
> 
> 
> 
> Secondary and even tertiary combustion is commonly applied in stoves. In
> fact even an open fire has secondary combustion of the gaseous products that
> evolve from the solid fuel. In some cases, the secondary air enters the
> combustion area together with the primary, and sometimes it is brought in
> via a separate channel (esp through a number of holes). On occasion the
> secondary air is blown in with a fan or a chimney (which is a natural draft
> 'fan' in that it provides something like forced air).
> 
> 
> 
> There are combustors that provide most of the secondary air with the primary
> air, and add a small amount of secondary (sort of 'supplementary' air) above
> the primary combustion area. An example of this the GTZ 7 series stove (now
> renamed GIZ 7 because they changed the name of the organisation). It has a
> grate setup that allows most of the needed secondary air to pass through a
> thin layer of burning coke where it is preheated and partly used. There
> follows a small set of holes (about 8) that let in just enough secondary air
> to complete the burning. This has the result that the amount of air pissing
> into the fire is minimised, but enough. The result is a very high flame
> temperature.
> 
> 
> 
> Stoves that are known for having very restricted primary and lots of
> secondary are the gasifiers like the CampStove from Tom Reed. His stove has
> a small fan and most of the air, nearly all of it, is secondary. There are
> natural draft gasifiers and pyrolysers (that deliberately make char) which
> accomplish the same thing using slightly taller stoves that basically have a
> short chimney section inside. 
> 
> 
> 
> They are all built to create gases then burn them at a secondary stage. The
> GIZ 7 stoves can arguably be analysed as having three sets of air supplied:
> primary air to devolatilise the coal or wood, then secondary air preheated
> by passing through a thin layer of nearly burned fuel, then preheated
> secondary air that passes through a set of holes about in the middle of the
> flaming area (combustion chamber). This last 'air' is tertiary in that it is
> one of three supplies, however there are only really two burning areas: the
> primary one and the secondary one. Not every stove fits easily into simple
> categorisation.
> 
> 
> 
> I would be interested in hear from you what has been built. Is the primary
> or secondary air deliberately pre-heated before entering the fire? Do you
> control the amount of secondary air during the burn (for example closing it
> off when the fire gets low(er)?
> 
> 
> 
> Regards
> 
> Crispin
> 
> 
> 
> -------------- next part --------------
> An HTML attachment was scrubbed...
> URL: <http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/attachments/20110728/bf3ca157/attachment-0001.html>
> 
> ------------------------------
> 
> Message: 2
> Date: Fri, 29 Jul 2011 08:00:54 -0400
> From: "Crispin Pemberton-Pigott" <crispinpigott at gmail.com>
> To: "'Discussion of biomass cooking stoves'"
>    <stoves at lists.bioenergylists.org>
> Subject: Re: [Stoves] WorldStove replies to BioFuelWatyche's
>    latestimprecisereporting of facts.
> Message-ID: <0c1601cc4de7$2c50f8f0$84f2ead0$@gmail.com>
> Content-Type: text/plain; charset="us-ascii"
> 
> Dear Roger
> 
> 
> 
> As you are planning to get EPA certification for your stove I suggest that
> you look carefully at the way they are going to run the test. They are very
> particular about loading, lighting and operating the stove in 'a certain
> way' that they wrote down. It may well be that the way you fuel and or
> operate the stove is different from how they are going to operate it when it
> is tested. It is blindingly obvious that their method may not suit your
> stove, or anyone's stove. It is just how they are doing things and that is
> that.
> 
> 
> 
> The problem is of course that if you have a novel technology and a good way
> of operating that technology, and it is quite different from the EPA test
> procedure, there are going to be long faces when the results come back,
> maybe. So be prepared by looking at the test procedure and follow it to see
> how your stove performs when loaded with that wood type and size and
> operated in their manner. You might ask in advance if they will operate it
> according to your instructions (or not). The answer might make you happy,
> might not.
> 
> 
> 
>> The average stove at the time was reportedly dumping 60 to 70 grams pm10
> particulate emissions per hour, per stove, into our airshed. 
> 
> There is an assumption by the EPA that the stoves for domestic use are
> pretty much the same power, meaning the total heat delivered in MJ into the
> home is about the same for all of them. If you build a stove with a higher
> combustion efficiency, then there will be PM10 savings at the same power
> level which will translate into reduced fuel use, as you noted. 
> 
> If you build a stove with a lower power level and the same combustion
> efficiency, there will be a reduction in emissions not because of better
> burning, but because of reduced fuel, as you noted. It would of course be
> better to have both efficiencies: better combustion efficiency and better
> thermal efficiency. One will produce less PM10 per kg burned and the other
> will reduce the number of kg that need to be burned. Double plus!
> 
> As you mentioned it, I have attached a graph of the thermal efficiency of a
> Mongolian 'ger stove' (they call it a ger, pronounced 'gare' like 'tare').
> As you can see it varies a great deal from 100% when the stove body is cold
> to a negative number at minute 140. The stove was fuelled at minute 12 and
> refuelled at minute 60. The steep drops in efficiency are the top of the
> stove being opened to add coal. When the top is opened huge amounts of air
> pass into the stove, cooling the stack gases with sheer volume and carrying
> away all the heat (up the chimney) at a lot temperature.
> 
> There are three lines on the graph. The Green one is the instantaneous
> thermal efficiency which displays the efficiency as it changes in real time.
> It is titled the 'interval efficiency' and is calculated every 10 seconds.
> The Orange line is the power output in kilowatts and that number is the
> 'cumulative' or 'average so far' power during the test. The Blue one is the
> thermal efficiency, also calculated cumulatively. The reason for doing this
> is that we want to see the average heat output. We know in advance what the
> heat demand is for a building, say a very small two room house. So we test
> the stoves to see what the average power level is assuming that on average,
> the house will be warm. The stoves are produced in three power ranges (for
> the most part). This is a small one at 5 kW avg.
> 
> As you will see on the chart, the efficiency after ignition of this
> traditional Mongolian stove is about 65% (the cumulative average) until
> minute 100 and is still above 60% after 2 hours, which included a
> refuelling. By minute 210 (3 hours) the average efficiency had dropped to
> 50% because of the very low efficiency during the third hour. The stove can
> be made to operate at a higher efficiency with a small fire by watching the
> stack temperature and oxygen level in the chimney then closing the damper,
> but an operator would not have the equipment to do that, so they usually
> guess at a damper setting and see what they get. In the USA and Europe
> commercially produced domestic burners have a device that measures the
> oxygen (a Lambda controller) that operates the damper automatically to
> maintain the thermal efficiency. Nothing like that in Mongolia.
> 
> If the stack temperature is low, it could be because the heat exchanger (all
> the parts of the stove that warm the room) is really efficient, or it could
> also be because there is a lot of room air entering and passing through the
> stove and cooling the chimney (or some combination). Obviously there is no
> gain from such a waste which is why people put on a damper (to prevent a lot
> of excess air going through). If you know how much air is going through
> without helping the fire, you can determine the amount of dilution taking
> place (with room air). The Mongolian traditional stove at high power is
> quite efficient but it makes a lot of smoke when it is ignited and
> refuelled. At other times it is pretty good.  
> 
> Note that the thermal efficiency is above 70% when the power level is above
> 7.5 kW at minute 40. But later it lets way too much air in at low power,
> partly because the chimney is oversized and because they are generally
> leaky, letting in air all over the place which dilutes the stack gases,
> cools them, and carries the heat up the chimney. The note under the chart
> says that the thermal efficiency (at low power) drops to -250%. The power
> level at which this happens is on a different chart so you can't trace it on
> this one. The point is in that condition, the stove is cooling the room
> because it is pulling in air from the room which has to be replaced by cold
> air from outside coming under the door (etc). The heat lost up the chimney
> is not enough to warm the cold air entering so the efficiency goes negative.
> Almost all stoves reach this condition but usually very late in the burn
> when the CO is high (chemical loss) and the excess air level is high (heat
> transfer efficiency loss).
> 
> If the EPA is going to test your stove in a way that does not include
> operating the damper in the way you recommend, it is likely to have negative
> consequences. 
> 
> The producers for the American market do not necessarily make stoves that
> are optimized for clean burning and thermal efficiency. They build stoves
> that will pass the EPA test as the EPA chooses to test them. If your stove
> is highly innovative and uses shall we say, non-standard control methods, it
> may not give good results when operated the way they are going to. You may
> have to make some changes to get in as much fuel as they are planning to
> load, for example. When you explain that your stove doesn't need as much
> fuel they will tell you the regulation says 'load X-amount of split oak'.
> And then they will do it.
> 
> Forewarned is forearmed. Do you know which EPA test (the number) will be
> used in your area?
> 
> Regards
> 
> Crispin
> 
> -------------- next part --------------
> An HTML attachment was scrubbed...
> URL: <http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/attachments/20110729/a7b0eef2/attachment.html>
> -------------- next part --------------
> A non-text attachment was scrubbed...
> Name: 081.10.3.4 Thermal Efficiency and Mass Burned Traditional Baseline 2010-08-22 v2.58.JPG
> Type: image/jpeg
> Size: 128712 bytes
> Desc: not available
> URL: <http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/attachments/20110729/a7b0eef2/attachment.jpe>
> 
> ------------------------------
> 
> _______________________________________________
> Stoves mailing list
> 
> to Send a Message to the list, use the email address
> stoves at lists.bioenergylists.org
> 
> to UNSUBSCRIBE or Change your List Settings use the web page
> http://lists.bioenergylists.org/mailman/listinfo/stoves_lists.bioenergylists.org
> 
> 
> for more Biomass Cooking Stoves,  News and Information see our web site:
> http://www.bioenergylists.org/
> 
> 
> End of Stoves Digest, Vol 11, Issue 38
> **************************************