[Gasification] Empirical data on dryer systems

[Rodenhuis, E.J. (Erik Jan, Student TBK)]

Dear Mark, list,

Thanks Mark for clarifying. 

When using the waste energy streams we have air and/or exhaust gases at
moderate temperatures. This was the main reason why I mentioned drum
drying and flash drying in my first posting on this topic. 

On first sight the temperatures of the waste heat streams seem to
combine better with a flash drying system. You need only some extra
centrifugal fans to overcome the pressure drop through the system. Is
the assumption that flash drying systems consume a lot of energy then
still true?
The CLEW project uses also the exhaust gases, but with long residence
times of the hogged wood.

When considering drum drying higher temperatures are mentioned, and then
we need to look for other sources that will generate this higher
temperatures. Indirect heating by use of (saturated steam) is an option.
Or we should combust part of the product gas to heat the incoming
(co-current/counter-current discussion is relevant here) air stream.
Also you could consider direct drying with superheated steam.

To justify the choice for any system we need some experience-based rules
of thumb for the different systems. And that's the reason I started this
discussion at all. 

Regards,

Erik 


-----Oorspronkelijk bericht-----
Van: Mark Ludlow [mailto:mark at ludlow.com] 
Verzonden: woensdag 13 december 2006 7:38
Aan: Rodenhuis, E.J. (Erik Jan, Student TBK);
gasification at listserv.repp.org
Onderwerp: RE: [Gasification] Empirical data on dryer systems


> To Mark L.: the big drawback of counter-current drying with hot air
> stream is a bigger fire hazard. Please explain your vision regarding
> possible fire hazards. 

Hi Erik,

Perhaps we are envisioning different scenarios. While I don't
necessarily
agree with your assumption about the 'big drawback', I think the very
fact
that one would have an air stream hot enough to ignite wood would call
into
question the source of this energy. I hope it's wood and not petroleum
based!

My assumption is that having a high-efficiency fuel drying system should
be
a primary design constraint for gasifier systems. With higher dryer
temperatures comes the increasing likelihood of higher dissipative
losses
across the dryer system boundary and assumed rates of drying that exceed
the
ability of the wood substrate to transport water from its interior to
its
surface where it can be evaporated. This can contribute to high exhaust
temperatures with lower water partial pressures than desirable and
therefore
lower efficiencies.

In many places in the US wood reaches an equilibrium moisture content of
12-15% after exposure to ambient air. Passing warmed air or, better yet,
waste heat across wood with lots of surface area per unit of mass seems
more
elegant than simply adding more driving potential without consideration
of
concomitant losses in system efficiency.

Cheers,
Mark