[Stoves] Hydrogen from charcoal

Crispin Pemberton-Pigott crispinpigott at gmail.com
Mon Sep 9 10:50:11 CDT 2013


Dear Jaakko

 

This is very reasonable. Because most people have not been measuring H2
(which requires an electrochemical cell) there is very little information on
it. It just happens that the SeTAR Lab and sometimes the SEET lab in
Ulaanbaatar use combustion analysers from time to time we get this
information.

 

I was surprised to see two things in fires: H2S late in a fire, and H2 late
in a fire. One expects that H and S are long gone from a pile of red
charcoal. Not so.

 

We often see H2(EF) in the 15,000 ppm range with a dilute measurement of
about 2000 ppm. 

 

The correlation coefficient is so high that is must be as valid an indicator
as CO for incomplete combustion.

 

The only thing that might be messing this up is the possibility that it is
an autocorrelation because the H2 is measured on the same cell as the CO in
most machines. The CO is strongly affected by the presence of H2 and there
is (is good ones) a separate detector built in that determines the H2
concentration and factors the CO value using it. This is reported in detail
in the Testo 350/454 manual which has lots of useful info in it.

 

Outside this possibility (a machine error) I think the H2 really is present
and there is a lot of it.  It looks like ordinary gasification. Maybe in
some stoves it runs ahead of the other gases and escapes with greater
relative ease. 

 

Thanks

Crispin 

 

 

Dear Crispin, Karve and all,

 

H2 can be both from combustion air and fuel as charcoal contains also some
hydrogen. As discussed in the list, H2 can be formed both from gasification
and from water-gas shift reaction if temperature is high enough.
Temperature can rise high locally inside char bed in the location where O2
is completely consumed allowing reactions to take place there and H2 not to
be burned (because oxygen has been consumed). H2 is probably not from wet
charcoal, since moisture is driven out (by drying) as H2O at lower
temperature, except if combustion air comes from a dyer.

 

Hydrogen is very reactive. H2 burns faster than CO (if oxygen is present).
CO emissions correlate well with hydrocarbon emissions in fireplaces for
heating purposes. I believe that H2 is so reactive that emissions of it are
minor, if there is oxygen in the gas and temperature is high enough. Was
oxygen concentration in gas zero?  Both H2 and O2 could be averagely present
in the gas in high temperature (and found when gas is cooled), if H2
produced is channelled somehow so that it does not mix well with O2. 

 

Moisture in combustion air reduces CO emission from charcoal bed by two ways
(if there is enough oxygen): 

1)      The mixture of CO and H2 is ignited and burned better than just CO
due to high reactivity of H2 producing heat to sustain combustion (also of
CO) .

2)      The combustion chemistry of CO involves radicals formed from H2O
even the simple reaction CO+½O2=CO2 does not show it. So increase in H2O
accelerates burning rate of CO. 

 

Regards

 

Jaakko

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