RE: law of mass action

["Michael J. Antal, Jr." <mantal@hawaii.edu>]

Dear Paul: I agree with Tom's comments.  Most of my work has emphasized somewhat lower temperatures, where thermochemical equilibrium is not often achieved.  It is usually instructive to compare observed results with thermochemical equilibrium predictions.  As one approaches equilibrium, dramatic changes in the product slate become less likely.  But in my experience (at lower temperatures without catalysts) the equilibrium calculation is only a rough guide, that can offer insights in the absence of experiments (i.e. its cheaper to run STANJAN than to do an experiment).  For example, detailed thermochemical equilibrium calculations predict a negligible influence of pressure (above 1 atm) on charcoal yields from cellulose at 400 C.  As my work has shown, this prediction could be quite misleading.  Best regards, Michael.

-----Original Message-----
From: Paul S. Anderson [mailto:psanders@ilstu.edu]
Sent: Friday, January 25, 2002 5:37 AM
To: Reedtb2@cs.com; mantal@hawaii.edu; stoves@crest.org; gasification@crest.org
Subject: Re: law of mass action

Tom and Mike,

You guys are GREAT resources.  But I need some additional help with this topic.  My question is mainly to Tom.

So what does this reversability stuff mean to me when I am involved with a gasification process such as with a modified IDD gasifier (ala Reed - Larson)?

I want to heed your warnings about the iron rules of thermodynamics.  But what does this reversability info mean about making charcoal in an IDD style gasifier?  What should I be trying to do, or trying NOT to do, especially in terms of heat and fuels and etc.?  More air, less air, when, where, what type of charcoal is best and how to get it.

Or what else should I know.....?? (I am not sure even how to ask the question.) 

Please, guide me along a little more on the very practical side of this topic.

Paul

At 10:06 AM 1/25/02 -0500, Reedtb2@cs.com wrote:

Dear All:

Mike is quite correct about the irreversability of pyrolysis reactions which occur in the 300-500C range.  

However, most of the gasification reactions occur in the 700-1000C range and ARE reversible (some quenchable) so that equilibrium predictions are often surprisingly close to observed.  

AND I have been pleasantly surprised that while not predictive of specific components at lower temperatures, they do predict the magnitude of char formation (33%) even at 600 C, the "equilibrium pyrolysis temperature" predicted by the thermodynamic calculations of Prof. Ray Desrosiers in our 1980 "Survey of Biomass Gasification", soon to be RE PUBLISHED by the BEF PRESS.

(To paraphrase, "Those who don't understand the predictions of thermodynamics are doomed to continual frustration trying to break its iron rules").  

Look for the book soon, new title,

    ENCYCLOPEDIA OF BIOMASS THERMAL CONVERSION  

Yours truly,                                TOM REED                       BEF PRESS




In a message dated 1/22/02 12:11:53 PM Mountain Standard Time, mantal@hawaii.edu writes:

As noted in the discussion, the law of mass action pertains to reversible
chemical reactions.  Unfortunately, biomass pyrolysis involves irreversible
reactions.  Try to adjust temperature and pressure a little to recover wood
from charcoal, pyrolysis oils, and gas!  For this reason, the law of mass
action is largely irrelevant to any analysis of charcoal formation.
Regards, Michael Antal.

Paul S. Anderson, Ph.D.,  Fulbright Prof. to Mozambique 8/99 - 7/00

Dept of Geography - Geology (Box 4400), Illinois State University

Normal, IL  61790-4400   Voice:  309-438-7360;  FAX:  309-438-5310

E-mail: psanders@ilstu.edu - Internet items: www.ilstu.edu/~psanders