[Stoves] preheating air (and learning about how these devices work)

Mon Aug 1 01:19:13 CDT 2011

Stovers,

It is stimulating to read the discussions between Crispin, Tom R, and  
Andrew H.  It is great minds like these that carry us all forward.

I am not in that league, but I did attend a gasifier stove workshop in  
China about 5 or 6 years ago.  There, the leader/instructor was Prof.  
Dr. Mukunda of Bangalore, India, a noted world expert now retired.  I  
do remember him emphasizing the importance of heating the secondary air.

Another advantage of heating the secondary air in the 2-walled TLUDs  
is the cooling effect on the metal of the fuel chamber, thereby  
prolonging its life.

Sorry, that is all that I can contribute more to the quantitative discussion.

I hope that Crispin can come to the BEF Stove and CHAB camp in  
Massachusetts on 7 - 12 August.  Tom Reed will be there, and a growing  
list of others with significant experience and/or projects of  
interest.  Contact Hugh McLaughlin for further info.   Hugh McLaughlin  
     <wastemin1 at verizon.net>

For anyone who has not been to a stove camp and is in striking range  
of Massachusettes, I strongly recommend that you consider this 5-day  
event.  Stovers are working literally at the frontiers of knowledge  
and they have opportunities to interact.  This will be my 11th Stove  
Camp, and I am excited to be attenting!!!

Paul S. Anderson, PhD
Known to some as:  Dr. TLUD    Doc    Professor
Phone (USA): 309-452-7072   SKYPE: paultlud   Email: psanders at ilstu.edu
www.gtz.de/de/dokumente/giz2011-en-micro-gasification.pdf   (Best ref.)

Quoting ajheggie at gmail.com:

> On Sunday 31 July 2011 12:37:36 Thomas Reed wrote:
>> 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.
>
> OK so this may appear to be a small plus but the heat has to come from
> somewhere and generally it doesn't come from above the pot so mostly it
> is from heat that would have been wasted from the sides of the combustion
> area.
>
>>
>> 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.
>
> I don't see this. If we assume that we have forced air then the forcing
> takes place at ambient temperature, the subsequent heating by 300C
> doubles its volume so whilst the penetration may be less the increased
> surface area contact will compensate.
>>
>> So, in my (moderately) humble opinion, preheating secondary air is
>> counterproductive.
>
> One would have to qualify what one was expecting to achieve with
> preheating. Firstly in my locale the only wood found at 10%mc wwb is
> freshly packed pellets, in our humidity wood equilibrium moisture content
> is about 17% mc wwb at room temperature. So if we start with the idea
> that bone dry hard wood burned completely yields 18600J/g then  a quick
> calculation on stoichiometric massflow ( 1g wood : 6.1g air) suggests
> this is sufficient heat to raise the flue massflow temperature to 1950C
> if the specific heat of the flue is 1.4j/g/degC. This is close to the
> calculated adiabatic flame temperature so we're in the ball park.
>
> Alter the parameters to 17%mc and 50% excess air and the achievable
> combustion temperature drops to 1165C which is about the ideal I see.
> Couple that with the fact that as combustion temperature drops so does
> combustion completeness you'll see there is a temperature below which you
> start losing chemical energy with the flue gases. I suspect with wood
> this is in the 600C-800C region and in my scenario the combustion
> temperature drops to 800C when burning moisture content of 45% (freshly
> felled oak in UK), 50% excess air and no heat losses from the combustion
> chamber ( unrealistic as no burner is lossless).
>
> My conclusion is that preheating secondary combustion air is a means of
> capturing heat lost through the walls of the combustion chamber without
> recourse to expensive refractory insulation and keeping the combustion
> chamber at a sufficiently high temperature. As I recall I was typically
> preheating the secondary air to 250C from losses through a 3mm steel
> combustion chamber wall and it did enable combustion chamber exit
> temperatures of over 800C with fresh felled woodchip. I calculate that at
> this moisture content and excess air that preheating the secondary air re
> injects 17% of total heat flux into the combustion chamber, why do our
> figures differ by an order of magnitude?
>
>
>> You can preheat the primary air, but that is only 20% of the incoming
>> air, so, again not worth it.
>
> Trouble with that is you introduce an unwanted feedback, leading to a
> thermal runaway in a conventional updraught fire and worse in a TLUD in
> that the primary air is hot enough to evolve offgas and this fuel gas and
> air mix will cause the flame to run back to under the fuel.
>
>> > When replying, please edit your Subject line so it is more specific
>> > than "Re: Contents of Stoves digest..."
>
> and please try not to quote whole digests back to the list.
>
> AJH
>
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