[Bioconversion] Re: [Stoves] Grass Pellets

[AJH]

On Thu, 16 Feb 2006 18:12:16 -0500, Roger Samson wrote:

>
>I am plant scientist but will comment as an amateur combustion specialist.

Fine, I am a tree feller and tractor driver with amateur interest in
combustion technology.

>Clinker is formed through the combination of silica combining with the
>binders (chlorine and potassium) in the presence of heat, much like the
>concept of concrete being formed by sand, cement and water. Silica is not
>the problem it is the binder and a high temperature that causes the stuff to
>fuse.   

I'd like to change this interpretation around a bit, corrections and
comments welcome.

 Silica forms a large proportion of the ash and different biomass
feedstocks have differing levels of ash. Pure wood fibre is low in ash
and has little few other trace elements. This is significant both in
the fact that it depletes the soil little when harvested as mature
stem wood and the trace elements do little to interfere with
combustion.

Pure silica has a fusion temperature of over 1700C, which seldom is
reached in small furnaces burning wood. The problem arises when other
ash or non combustable materials ( such as contamination by soil) are
present near the grate. Tom Mile's has pointed out that the alkali
salts, mainly potassium, which are in the grate ash from biomass are
absorbed onto the surface of hot silica and fuse with it. The
resulting mixture is eutectic, that is it has a lower fusion
temperature than either of the two pure components, thus the two ashes
combine and fuse binding other components of the ash into a gooey mass
which solidifies as it falls to the cooler grate. This clinker then
hinders both primary air flow and removal of ash.

That ash that is entrained by combustion gases, fly ash, can have even
worse effects as it can settle and fuse onto heat exchange surfaces.
>
>Some suggestions to manage the combustion that are being used by companies
>
>1. keep a low temperature and low primary air where biomass is initially
>burning to avoid too high a temperature (you can combust the gases at high
>temperature later)

Yes this is achieved by a number of means, exhaust gas recirculation
being one. My pellet burner ash is brownish, which I interpret as
meaning there is char still in the ash and I think some of this is
down to mitigation of complete combustion in the burn pot.

In principle I can see that separately pyrolysing these high ash, low
fusion temperature, fuels and then burning the char with a low
superficial velocity air supply could be a work around but adding a
lot of extra complication. Though I haven't tried recently burning
wheat in a top lit updraught tube could not be sustained and the
resulting char was a matrix of part burnt grains glued by tars into
the tube.

>2. keep a live floor, and auger or push out fuel as it burns so that by the
>time it is withdrawn it is completed burned out and not mixed with the fresh
>fuel being added

I fail to see the reason for this, other than to keep the ash mobile.

>3. avoid excess air so that the alkali species migration (heavily induced by
>the presence of chlorine) is limited by not having too much air flow out of
>the initial combustion zone

Again I don't understand this, Is chlorine acting in synergism with
the alkali metals? I look on chlorine as being a problem in both being
a precursor to dioxin formation and hydrogen chloride, which can
potentially condense on and corrode boiler surfaces.

>Its easier to manage these processes in bigger units. 

Yes but also if the problem occurs in bigger units removing the
clinker becomes a serious headache. In smaller units, especially
pellet stoves with top fed burn pots, ash filling up the pot seriously
interferes with combustion and shortens the maintenance period, in the
case of my stove to about 5 hours rather than over 24 hours with wood
pellets.

>There are many
>companies in Canada now selling pretty good technology and exporting. 

Well pretty good can be faint praise. I was involved with importing
some Canadian pellet stoves and the problems were "interesting". The
particular one you indicated does not seem to be available outside of
Canada, as I would like to try one.

Andrew Heggie