Endothermic and Exothermic Gasification

[Reedtb2@cs.com]

Dear Harry, Mike and All:

The question of degree of endothermic or exothermic reaction is very important in gasification.  The equilibrium calculations of Prof. Ray Desrosiers show that it requires only 1.5 kg of air/kg biomass to convert the biomass completely to gas and requires then 4.5 more air/kg to burn that gas. 
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As time goes by, whan I say "Gasification", I tend to include "pyrolytic gasification" with charcoal gasification, even though SLOW pyrolysis only produces ~ 1/3 permanent gas.

The HEAT OF PYROLYSIS can be exothermic, as shown best by some of Antal's measurements.  When included with the energy required to reach pyrolysis temperature, what I call the HEAT FOR PYROLYSIS is generally mildly endothermic to VERY endothermic, depending on moisture content. 

In downdraft gasifiers operating at peak intensity I have recently realized that there is a new category of gasification, 

SIMULTANEOUS PYROLYSIS AND GASIFICATION, (SPG) in which the inside of a large particle is still  pyrolysing, while the outside of the particle is above 800 C and reacting endothermically with the emerging CO2, H2O  etc in classic char gasification reactions.  Technically, this occurs when the BIOT number (external heat transfer/internal conduction) exceeds 1 and the particle is very non isothermal.  For lower BIOT number the particle is approximately isothermal.  

I actually measured this effect in 1996 in an experiment where rapid heating in a reducing atmosphere only yielded 5% charcoal rather than the expected 15-30% charcoal and the outside of the particle was incandescent, ie > 800 C.  I measured the heat of the SPG reaction as 2.8-3.3 MJ/kg, considerablly above the typical 0.5-1.5 MJ/kg with slower pyrolysis.  It wasn't until 1998 that I realized that the low charcoal yield signified Simultaneous Pyrolysis and Gasification.  

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It is widely (and naively?) believe that fluidized beds have very high heat transfer rates.  Heat transfer in an updraft gasifier is limited because increasing flows lead to fluidization.  Heat transfer by convection during pyrolysis in a fluidized bed tends to be self limiting, since the 1000/1 production of gases forms a boundary layer around each particle that reduces heat transfer effectiveness.   

In a downdraft gasifier the charge is held in a tight packed bed with about 25% porosity through which the reacting air passes and burns the exiting pyrolysis gases as they form.  Increased gas velocity doesn't increase the spaces, and so heat transfer rises well above that from a fluidized bed.  

It has always amazed me that our 1944 75 HP Hesselman gasifier had active dimensions of about 30 cm diameter by 20 cm deep - yet provided all the fuel for an engine much larger in size.  (The rest of the gasifier is primarily magazine for 50 miles of fuel plus an external cooling surface to reduce the gas temperature to a few hundred degrees).

So, with few exceptions gasification is mildly endothermic.  

ONWART,         TOM REED       THE BEF GASWORKS

In a message dated 1/25/02 8:42:02 AM Mountain Standard Time, Harry.Parker@ttu.edu writes:


> Hello Tom and all,
>  
> You might add that gasification reactions are highly endothermic --- they consume a lot of energy.
> This energy consumption during gasification is a major constraint on thermal efficiency and on the design of gasifiers.
>  
> In contrast pyrolysis reactions are near neutral or perhaps a bit exothermic.
>  
> Harry
>  
>