[Stoves] re-kindling stoves

[Crispin Pemberton-Pigott]

Dear Andrew

 

>> The next issue is the excess air quantity.

>Yes I know you preheat secondary air but I was wondering what the effect
>would be on preheating as a test. Generally excess air acts to quench and
>dilute the reaction. I was just wondering if an experimental external
>heating element for the secondary air supply would make any changes to
>the pm. 

 

We are able with coal to demonstrate things that are really difficult with
biomass so the understanding of how the fire reacts is probably better.
There are two ways to increase the CO (which if there are volatiles left in
the coal, usually means an increase in PM). This observation extends the
piece I wrote a few months ago about getting secondary air through the
grate. One is to cut the primary air supply which reduces the burn rate,
which means there is more air left over at the end, which means there is a
greater quantity of excess air. This happens when a portion of the primary
air is in fact shot through to become secondary air above the grate.

 

The second is to increase the amount of secondary air that is provided
separately. It turns out there is always a minimum excess air rate
obtainable for each construction of grate, hopper and combustion chamber.
Remember the discussion about the thickness of the fuel on the grate being
used to determine the excess air level? When things are balanced for minimum
excess air (which means in consequence, minimum PM and CO) the is just the
right balance of fuel on the grate burning at a time, the right amount of
primary air to burn it, and the right quantity secondary air to complete the
burn.

 

The low PM we are seeing is because of the unusually low EA level (for a
domestic stove) and or the elimination of the hydrocarbons late in the burn
which happens eventually.

 

>It's really only spark ignition reciprocating engines that need
>air:fuel close to stoichiometric most other combustion processes trade
>off the extra air massflow with the better chance of a fuel, or Product
>of Incomplete Combustion, particle meeting an oxygen molecule.

>In fact I imagine the pm rate would drop dramatically if the secondary air
>were enhanced with O2, though of course the increased temperatures may
>have a non intuitive effect.



Testing of a cast iron coal stove last week showed a pretty low PM level
even when the EA was far too low by any normal standard. The secret was the
high temperature of the combustion area.


There will be a compromise to be made because the graphs of lambda, CO,
PICs and pm will not all map to a single point.



It might be a bit early to say that for all fuels. In other words it might
be true for certain fuels and not others.

 

>Is there evidence that these smaller than pm2.5 particles are as or more
dangerous? 

 

According to WHO and Kirk Smith, yes. The technology is available to find PM
0.020 (20 nanometres). I expect as soon as we look for them, all sorts of
things will show up and it will all be worse that we thought.

 

>>We have a long way to go...

>Yes but your recent pictures of the modification to an existing, in use,
stoves show there are big improvements to be made which should be readily
accepted. Eliminating PMs will reach a limit below which we will struggle
and we're not near yet



I sure hope so. The reduction is significant so I will post charts later
when they are confirmed. Prof Lodoysamba has been running a stove that was
built from scratch to optimise current understandings.  $70 made with new
3mm mild steel. He reports that it works really well and made three
refuelings yesterday to see how it affected the operation when adding duff
coal (powder and small bits). We will have Ulzii test it in his ger as soon
as possible. He has been surviving the bitter weather while learning how to
light and refuel his modified version made 8 days ago (attached). That means
we changed his existing stove by adding a 28 cm long pipe cut are 45 degrees
both ends. You can see the entrance of the pipe. It goes to the left end of
the heat exchanger on the back of the stove. The pictures some have
downloaded are of the 'professional' version.

 

[snip]

>An interesting boiler related problem, that has
>shown up laxness in installation, is that most urban dwellers have
>heating by gas condensing boilers which require a condensate drain. Where
>this drain has been poorly installed in an exposed position, and the
>acidic condensate frozen, it has caused the boiler to cut out.



Wow. There is a similar problem with toilet supply pipes and even water
heating systems in UB. The simple cure is to run a small stream of water
continuously into the drain to keep it above 0 C. This is widely used in the
far north of Canada and is a requirement for connection to the sewage
system.

 

List readers in the NE US may find this trick prevents problems during the
coming couple of weeks of Nor'Easters.

 

Regards

Crispin

 

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