[Gasification] Low and high temp gasifiers

[Bruce Southerland]

I agree, my experience is more power with a hotter reaction, with or without introducing water/steam. Someone recently counseled us to move away from the term "crack" for what goes on with water in a gasifier because, he argued, water/steam doesn't 'crack' until it reaches 2200 C. He suggested that reactive C rips the O away from the H. I'm trying to wrap my tiny mind around this. If this is correct, then this may be a more accurate way of expressing the process:
As steam that is superheated in the reaction zone passes through the hot glowing char in the surrounding reduction zone, O will be pulled off by the C to form CO and release H. 
Does that sound right? 
We don't need the right explanation to experience the increase in gas power from dripping water or spraying steam into the reaction zone, but I would like to understand what is going on if I can. 

-----Original Message-----
From: Gasification [mailto:gasification-bounces at lists.bioenergylists.org] On Behalf Of Big Daddy
Sent: Sunday, September 14, 2014 9:11 PM
To: 'Discussion of biomass pyrolysis and gasification'
Subject: Re: [Gasification] Low and high temp gasifiers

It has more to do with complete oxidation/reduction, and in my opinion, surface area of the feedstock. 

You'll find that at 1,200 C+, CO2 is almost non-existent, because of its near-complete conversion to CO. That means ~10% of the typical CO2 (a non-combustible dilution) is no longer in the entrained gas flow.
Additionally, if you spray H2O in the form of steam into the reaction zone, you'll "crack" that steam at high temps, which also has the added side-effect of lowering nitrogen levels in the gas flow -- also a non-combustible.

Charcoal gasification is best for this "cracking" effect, as pure carbon burns hotter with few impurities, and more consistency. There are, however, less BTU's (energy) in pyrolyzed biomass, than in typical dried biomass.

There is no free lunch for high-temperature gasification. It all depends on which aspect of the "holistic lifecycle" is most important. 

Troy

-----Original Message-----
From: Gasification [mailto:gasification-bounces at lists.bioenergylists.org] On Behalf Of Gao Pronove
Sent: Sunday, September 14, 2014 7:27 PM
To: gasification at lists.bioenergylists.org
Subject: [Gasification] Low and high temp gasifiers

There seems to be a split between low temp gasifiers (700 C and below) and high temp (1,000 C and above). The high temp group claims to have cleaner gas because they "burn" off most of the hydro carbons while the low temp leave a lot of tar in the gas. Is this the common view?

Thanks
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