[Gasification] Empirical data on dryer systems

[Mark Ludlow]

Erik,

Purely from my perspective, a counter-current configuration makes the most
sense for a small scale wood chip dryer. The incoming wet feed is exposed to
moist air which has increased enthalpy and is much more effective at
transferring its latent and sensible heat to the cooler in-feed material.
While not much moisture is removed in the first contact zone, rapid heating
occurs because of the superior thermal characteristics of high RH warm air
versus its low RH equivalent. A second advantage is the lessening of a
tendency to case-harden the surface of the chips which subsequently retards
water vapor transmission.

Efficiencies improve when a portion of the exhaust is recycled to the input,
to the limit of having a saturated or condensing exhaust stream. Dissipative
heat losses from the dryer shell itself are a major factor in determining
overall system efficiency.

Mark

> -----Original Message-----
> From: gasification-bounces at listserv.repp.org [mailto:gasification-
> bounces at listserv.repp.org] On Behalf Of Rodenhuis, E.J. (Erik Jan,Student
> TBK)
> Sent: Saturday, December 09, 2006 2:25 AM
> To: gasification at listserv.repp.org
> Subject: Re: [Gasification] Empirical data on dryer systems
> 
> Hey all!
> 
> Is this topic not worth considering a thought? I hoped to get some get
> some ideas and figures about dryers. Couldn't find much discussion on
> this topic in the archives Or do you just not have a clue ;-)
> 
> Erik
> 
> -----Oorspronkelijk bericht-----
> Van: gasification-bounces at listserv.repp.org
> [mailto:gasification-bounces at listserv.repp.org] Namens Rodenhuis, E.J.
> (Erik Jan,Student TBK)
> Verzonden: donderdag 7 december 2006 13:46
> Aan: gasification at listserv.repp.org
> Onderwerp: [Gasification] Empirical data on dryer systems
> 
> Dear list,
> 
> 
> 
> For downdraught gasification of biomass generally is assumed that the
> moisture content of the biomass is below 15% (wb). As the moisture
> content of green wood is mostly higher and I have seen sawmills use
> water-cooled band saws, you need to dry the wood waste before it is
> suitable to gasify. Main options seem to be drum drying of flash/ring
> drying.
> 
> 
> 
> Drum Drying:
> 
> -        Co-current to reduce fire hazard
> 
> -        Tair,in = 230 - 1100 degr. Celsius
> 
> -        Tair, out = 70 - 110 degr. Celsius
> 
> -        Empirical data on energy consumption 5 - 12 GJ/ton of water
> evaporated, see poster presentation Mani:
> http://www.biocap.ca/images/pdfs/conferencePosters/Mani_S_P1.pdf
> 
> -        General statements made in diverse publications: "high"
> investment cost, "low energy use"
> 
> 
> 
>  Flash Drying
> 
> -        short residence time < 30 sec
> 
> -        drying at lower temperatures Tair,in = 100 - 650 degr Celsius
> 
> -        "low investment, high energy use"
> 
> 
> 
> A few other systems available:
> http://barr-rosin.com/english/pdf/airstream.pdf
> 
> 
> 
> As we have will have with a downdraught gasifier with an ic-engine some
> low quality waste energy streams are available, namely from gas cooling
> (gas-gas shell-and-tube heat exchanger) and from the exhaust gases, I am
> interested to know what the general ideas about which drying system
> suits best. There might be some other criteria like the controllability
> of the process and fire hazards.
> 
> 
> 
> Maybe there is somebody who can give some rule of thumb for investment
> cost and energy consumption of a 1 ton/h drying system. When assuming
> that we want to dry the wood waste (let's assume chips) from MCwet 30%
> to a moisture content of 15% we need to evaporate 115 kg H2O/h.
> 
> 
> 
> Any hints or leads to good literature on the web on this topic??
> 
> 
> 
> Regards,
> 
> 
> 
> Erik Jan Rodenhuis
> 
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