[Stoves] re-kindling stoves

[ajheggie at gmail.com]

On Wednesday 22 December 2010 00:06:35 Crispin Pemberton-Pigott wrote:
> >>That means training people not to do what comes
> >> naturally (let the stove die out and refuel and relight from
> >> scratch).
>
> Yeah. That is what I meant.

...and that's where I saw a point for confusion.

It could read as letting the fire die out and then refuel being what comes 
naturally.

In fact what comes naturally is to dump another batch of fuel on top of 
the glowing embers, this quenches any existing flame as the new load 
heats up and subsequently the new load starts pyrolysing with insufficient 
temperature or flame to burn the offgas.

Pellet stoves get around the problem by feeding in a small amount of fresh 
fuel such that a continuous flame is maintained over the whole firebed. 
Everything has to pass through the flame.

As a tlud maintains a fairly constant power if the primary air is well 
controlled then extended burn time can be got by increasing the height of 
the fuel.

As this is often not practical there are a number of ways people have 
tried to get around it. Paul Anderson went for fuel cassettes that were 
pre loaded and alternated but it has not been mentioned recently.

Top lighting for a clean start has been advocated by many, I think the 
masonry stove association mentioned it when [stoves] was young. TLUD is a 
special case where the progressive descent of the pyrolysis front 
provides an air free offgas that shields the char formed above from 
combustion. Change the conditions slightly by slowing the front or 
increasing the air supply and this char is consumed. I've seen no mention 
of particulates under these "no residual char" conditions. Generally 
particulates are formed in the secondary combustion area. From a long 
term health point of view particulates remain the bigger problem over 
straight thermal conversion efficiency.

Richard asked:
> Please tell me why adding a side feed feature to this stove would not
> greatly improve the cool down start up problem /smoke/ you are
> experiecning with your batch feed  process ? 

Richard I don't know this stove, as Crispin has pointed out a modification 
made to steam locos was to auger a "molehill" of coal from under the 
firebed so the curtain of fire wasn't disturbed by a fresh cold batch of 
coal being dumped on it.

Given we seem to be looking at 5kW stoves, that's about 1kg of drymatter 
per hour. What does one of your holey briquettes weigh when dry? Part of 
the way a tlud works is to do with the change in buoyancy of the gases as 
they evolve and then heat up. As we have discussed before I'd like to try 
making a semi continuous burner using holey briquettes in much the same 
way a rocket fuel in fed by pushing longer sticks into the fire chamber. 
As I see it we need to keep it as close coupled as possible because with 
these small devices if heat is lost in the primary combustion zone then 
the gases in the secondary  zone have lost temperature and are diluted by 
CO2 and N2. The tlud avoids this problem by minimising the combustion 
products of char.

AJH