[Stoves] combustion of char -- spontaneous and dangerous

[Crispin Pemberton-Pigott]

Dear Kevin

 

Wow. Well I had not considered either of those two. What is perhaps the
logical approach is to look at the processes that could account for 100 g of
gain.

 

You can take it that the initial fire was with 5 kg of coal and then a
reload with 4.6 = 9.6 kg of coal, 25% moisture, roughly 10% ash. From that
we could calculate the missing fraction of fuel mass which would be most of
the non-ash portion. I can’t see there being enough carbon left to pick up
that much O2.

 

There is no doubt that the reversal idea is not correct because there is a
temperature record and the interior air temperature remained well above the
outdoor temperature (the building is heated).

 

Here is the set of temperatures with the Mass, test date 12 Jan 2010:

 



You can save and stretch that. The 1 deg rise in temperature to -12.5° as
the sun comes up is visible at the end. The temperature in the stove is
slightly higher than the air in the room. The mass curve is generally the
same shape as the charts by Battacharyya for coal (with a different time
scale – but remember this is coke).

 

It was hardly an ideal set of measurements.  We were just looking to see the
cooling. The thermal efficiency of the stoves drops to -250% at times
because it pulls in more cold air than it generates as heat – the fire just
acts as a fan to throw heat from the room up the chimney.

 

I am still prepared to write off the mass change as an instrument artifact –
wind or something but it is a curious phenomenon to consider.

 

Regards

Crispin

 

 

Dear Crispin

 

I can think of two possibilities for the possible weight gain:

 

1: If there was backdraft, as a result of the windy conditions, it might be
possible that particles of char or creosote fell from the chimney, or were
blown back, to the stove system, creating an increase in weight of the stove
system. This material then burned off. Note that the weight at about 2815 is
the same as at the very end of the chart, where the red line continues past
6567

 

2: The oxygen that was picked up would be not only as oxygen, but as CO2.
What might have happened is that is that the ash, containing FeO could
easily pick up oxygen and become oxidized to Fe3O4, and limestone components
in the ash could have been calcined to CaO, which could easily react with
CO2 from the atmosphere, to "airslake" back to CaCO3. The "jiggle in the
weight" may be a result of "local fire" re-calcining" the lime fraction, or
"re-reducing the iron components.

 

Any likelihood that either of these guesses might be accurate?

 

Best wishes,

 

Kevin

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