[Stoves] preheating air

[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