[Stoves] [biochar] Pine char gasification

[Crispin Pemberton-Pigott]

Dear Tom

 

I am not subscribed to the Char lists but I appreciate the copy.

 

I have a question:

 

"They point out that the mass loss with surface gasification occurs within
the smaller pores leading to pore widening."

 

Is this the same process that is used for clearing out the carbon from
ceramics in the 300-650 C range using CO? U understand the shuttling back
and forth was done primarily by CO at that temperature. The importance is
the time to tell people to hold the temperature steady while removing
char/carbon that is trapped inside the thicker ceramic parts. You may
remember some discussion about this with reference to making the POCA stoves
in Maputo.

 

Getting the carbon out is an important step before ramping to the final
temperature.

 

We were using crushed charcoal as a source of carbon for the creating of
pores, while not creating quasi-pores filled with sand.

 

Thanks
Crispin

 

From: Tom Miles [mailto:tmiles at trmiles.com] 
Sent: Monday, December 23, 2013 2:58 PM
To: biochar at yahoogroups.com; 'Crispin Pemberton-Pigott'
Cc: 'Gasification-Request'
Subject: RE: [biochar] Pine char gasification

 

Ron,

 

This work is very important for both the biochar and gasification lists.
Biochar will be produced at the large, or even small, scale as a co-product
of energy (liquid fuels and/or power). The most efficient way to generate
power from the gases and vapors from slow pyrolysis (50% of the energy) is
probably through charcoal gasification (e.g. run the pyrolysis gases through
a charcoal gasifier). There are commercial systems under development to make
char and power in this way. There are also commercial systems under
development to make liquid fuels through combinations of pyrolysis and
gasification. The char products from these and fast pyrolysis processes run
from 0% to about 15% of fuel input. I don't know the fuel or char yield for
Cool Planet. 

 

This particular study prepared the char with high temperature (826 C)
nitrogen.  Wood particles (chips, sawdust) and resultant char particles in
this study are larger than for other char studies. Observations about BET
surface area, particle size and the char morphology are very interesting.
The char morphology looks different than the SEM images that we typically
see. From gasification and pyrolysis we know that pine carbonizes
differently than hardwood so it is interesting to see the shredded fibrous
appearance of the pine char in this study compared to the neat geometric
structures that we often see, which is probably from hardwood chars. The
authors observe that the macropore volume is significantly greater than the
mesopore or micropore volume of the char. They observe "numerous wide tunnel
protruding into the char particles. . . [that] may provide pathways for bulk
transport of CO2 into the particle." 

 

Char conversion numbers are interesting. Only 10-12% of the char was
gasified at 726 C (BET 391 m3/g) while 98-100% was converted at 896 C.
Surface area increased with conversion but not much greater than the 35-47%
conversion at 776 C so CO2 gasification could be used to increase surface
area at the expense of half of char (660 m3/g). Meso and micro pore volume
doubles at the higher rate but stays pretty constant above 776 C.
Researchers conclude that a significant proportion of the pore volume is
within macro pores although the majority of the internal surface area is
within micro pores.  They point out that the mass loss with surface
gasification occurs within the smaller pores leading to pore widening. 

 

Researchers explain that the char gasification process involves three steps:
(1) adsorption of the gas-phase species to the char surface, (2) surface
reactions, and (3) desorption of the gasification products from the surface.
The latter is the rate limiting process. 

 

Recycling CO2 from gasification to gasify the char is an interesting concept
that may apply to modifying char properties (e.g. increase surface area)
from pyrolysis or recovering energy (heat, power, syngas) in an industrial
setting.   

 

There is very little information about gasification or combustion chars.
Sometimes it helps to step back from our char-philia (and gaso-phobia) to
see what products combined pyrolysis and gasification can produce.  

 

Tom

 

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