[Stoves] Inverted top lit updraught

[ajheggie at gmail.com]

On Sunday 15 January 2012 03:50:48 rongretlarson at comcast.net wrote:

> ----- Original Message -----
> From: "Agua Das" <aguadas at onebox.com>
> To: rongretlarson at comcast.net
> Sent: Saturday, January 14, 2012 12:43:29 AM
> Subject: Re: [Stoves] Inverted top lit updraught
>
> The ejector conveys the gas and secondary air to the forge. This
> allows access to the molten metal without the need for a flue.
> I can heat bronze to 2150 F in 20 to 40 minutes ( as fast as propane).
> Using 1 to 3 lb wood per pound of metal melted. 

We haven't had quite the discussion I had hoped for about comparing 
conventional down draught with inverted down draught and I'll try and add 
a few thoughts below but first the dasifier:

http://www.cd3wd.com/cd3wd_40/EMGroups/WasteWatts/DASIFIER.htm

I have been fascinated by this device since it was first described because 
of the apparently high temperatures that can be achieved[1]. I hope 
everyone understands the principle of the device, it uses a small 
compressor which injects high pressure air into the tube between the 
gasifier and the burner, this compressor is a luxury that most people 
wouldn't consider for a stove and the ejection it forces is lossy 
compared with a mechanical fan moving the same amount of air. Its 
advantage is not having moving parts in a hot dirty environment. This air 
is directed  such that it acts as an ejector forming a depression on the 
gasifier side and entrains 3 other air streams. The largest amount is the 
secondary air that oxidises the offgases from the gasifier sections to 
form the flame that can melt bronze in the furnace section.

Its the dual gasifier part that is relevant to my earlier questions.

The fuel falls through the upper section under gravity and air is sucked 
by the depression at the ejector down with, and through, the fuel where 
it is pyrolysed and gasified at the throat at the bottom of the chamber. 
This throat is crucial to DD gasifiers as it allows for the gradual 
shrinkage of the chunks of wood as they descent into the first oxidation 
zone and gradually shrink. This zone has to be hot enough to cause any 
CO2 to be reduced to CO when it meets hot char, the gases leaving this 
zone should be mostly nitrogen from the primary air and CO and Hydrogen.

Most DD gasifiers would add sufficient air at this stage ( often by 
blowing through pipes ( tuyeres) into the throat region) to gasify all 
the char so only ash falls below the throat. The design and configuration 
of this section is what bugs most wood gasifiers.

If we invert this top its action is what forms a tlud and if the primary 
air is delivered such that it is just sufficient to heat the pyrolysis 
zone then pyrolysis offgas is driven off, a small amount of char is burnt 
in the process, if the fuel is dry, and a char residue remains. The 
essential difference in the first half of the dasifier is the fuel moves 
down to the pyrolisis area and in the char making tlud the pyrolysis 
front moves down through the fuel I think the off gas composition should 
be about the same, given the same limited primary air.

If so then the particulates from the secondary flame should be about the 
same with primary air controlled TLUD or Down Draught. The DD offers 
continuous refueling and as long as the air path is not constricted by 
extra fuel then emissions should be constant and the cooking surface 
convenient, the TLUD suffers from limited means to refuel ( or does 
it ? ;-)) and  it has a height problem. 

With a conventional DD the char is all consumed and only charry ash falls 
through the grate but Aqua Das doesn't attempt to gasifiy the char at the 
throat using the stratified approach of all the primary air descending 
through the fuel or adding air via tuyeres, he allows the char to fall 
into a second gasifier that is a simple updraught char gasifier, this 
needs again to operate at high temperature, the third part of the 
entrained air is sucked up into the hot char, turns to CO2 and reaches a 
high temperature before it is reduced to CO in the hot char and low 
oxygen environment. The output from this is mostly CO and N2. the two 
offgas streams ( N2 H2 and CO from the downdraught section and N2 and CO 
from the updraught section) combine in the area below the throat and exit 
the gasifier via the tube with the ejector. In practice not all the wood 
is perfectly gasified and there will be steam, methane CO2 and tars going 
to the burner.


Mass flow considerations mean that all the heat released in the gasifier 
and the heat from the secondary flame less losses to the gasifier walls 
will be distributed in the flame  and thence the flue gases and it looks 
like Aqua Das gets above the melting point of bronze, say 1000C. The 
flame must be much higher than this to allow for heat transfer and 
losses. In fact I think with dry wood about 1600C is achievable but in 
practice we know wood cannot melt iron ( 1500 C  and a bit lower for iron 
with high carbon content).

One of the benefits of gasification is in reducing the need for excess air 
necessary in a conventional fire. The gasification uses controlled 
primary air to produce hot simple gases that will easily burn and if the 
output is first cooled ( recycling the heat) then the combustion gases 
can be premixed before the flame. I'm not sure what is happening in the 
dasifier regarding pre mixing but I suspect the gases meet the secondary 
air above their spontaneous ignition temperature and secondary combustion 
starts before much premixing can occur, the ejector causes enough 
turbulence to  keep everything mixed in the flame.

Add too much secondary air and the heat released in the flame has to be 
spread over a larger mass flow and the temperature drops, the same 
happens if the wood is wet, the water robs the flame temperature with 
both its mass flow and its latent heat of vaporisation.

Getting back to my question and Alex's recent posting on that ingenious 
method of inserting a burner into a conventianal stove without cutting 
the body about; the problem with DD is that unprocessed wood does not 
flow, the grate or throat areas need agitation. Which is why Alex uses 
pellets ( an expensive, pre processed fuel) and I don't think the air 
controlled DD to produce a clean flame can compete with TLUD without a 
means to constantly remove char from the bottom.

That was too much finger bashing and probably too long to read but 
thoughts anyone?

Further musings:

[1] it should be just about possible to melt steel with a wood flame if 
the wood is perfectly dry, in practice it doesn't happen and iron was not 
produced until charcoal was used, the reason is simple; whilst the 
charcoal only has <50% of the heat of the raw wood it was produced from, 
it is dry nearly pure carbon with about 30MJ per kg available to be 
released on full oxidation. C+O2+4N2=>4N2+ CO2+30MJ heat per kg of C is a 
low massflow reaction and heats of over 2000C are achievable. 

Consider a propane torch flame, when premixed with the correct amount of 
air the flame is two blue flames, a bright blue inner flame surrounded by 
a paler blue enveloping flame, the hottest part is at the tip of the 
inner cone where nearly all of the premixed oxygen is consumed burning 
hydrogen and some carbon to H2O and CO/CO2, remaining CO is then burned 
in the outer enveloping flame by oxygen that has survive the first flame 
and oxygen that diffuses in from the surrounding air.

Can we consider doing something similar with the dasifier, using the high 
mass flow ~1600C pyrolysis offgas flame to form a hot envelope around the 
lower heat capacity but higher temperature CO+N2 flame? Without 
destroying the burner pipe from the heat?

AJH