[Gasification] heat required for drying

[Paul Francesco]

I thought I posted this to the list, but apparently I just hit "reply" 
and it went out as an off list post. Can anyone tell me why drying 
cannot be accomplished as follows:

This is probably a little off the wall but:

     If you had 2 downdraft gasifier main hoppers, - #1 filled with 
conditioned chips with the hearth lit- directing the raw gas up through 
#2 so as to act as a chip conditioner and partial gas conditioner "tar 
removal" then when #1 is spent. #1 gets filled with "wet" chips and the 
hearth in #2 is lit and the flow positions reversed, wouldn't that  
effectively and efficiently act to cool  the gas, remove tar and 
condition the upcoming fuel load?

    Some setback would have to be observed between the 2 hoppers to 
avoid ignition is the second unit but the heaviest tars would be 
deposited nearest the hearth to be lit on the next load. and the char 
bed may already be prepared for the "new run".

    Granted extra attention may have to be paid to later removal of 
moisture. Anyway just a thought from the woodwork.
Paul


Mark Ludlow wrote:

>Hi Toby,
>The most fruitful approach may be a condensing air-to-air heat exchanger on
>the exhaust stream that recovers at least apportion of the evaporative
>enthalpy and transfers it to the incoming air. I've seen some industrial
>installations where a significant amount of warm air flow was exhausted and
>replenished with unheated fresh make-up air that used large surface area
>aircoils as well as honeycomb exchangers to advantage.
>Mark
>
>-----Original Message-----
>From: gasification-bounces at listserv.repp.org
>[mailto:gasification-bounces at listserv.repp.org] On Behalf Of Toby Seiler
>Sent: Wednesday, December 13, 2006 5:33 AM
>To: gasification at listserv.repp.org
>Subject: [Gasification] heat required for drying
>
>Dear Erik, 
>   
>  I'm probably repeating something that you know.  It takes just over 1000
>btu to turn one pound of water (liquid) into vapor (after reaching 212f) and
>in the condensation gives back about 970 btu. Of course one pound of water
>raised one degree is a btu., but ambient to 212f is relatively small when
>considering phase change, but not inconsequential.  Then come issues like
>the molecular bound water versus free water, when dealing with woody
>material.  Generally, I believe drying must target a 10 to 16% moisture
>content which is about the end of free water and beginning of bound water in
>wood.  Calculating the incoming weight of the water content in the fuel less
>the outgoing desired will give the weight desired to be removed (as vapor),
>times the 1000 btu/lb (plus ambient to 212f), gives the energy required to
>come out of the process, divided by efficiency, for input energy.
>   
>  One would think that some practical use would exist for the energy given
>up and not just discharge to the atmosphere as steam.  It takes a lot of
>energy to dry, but the wise was of the energy from condensation seems
>prudent.
>   
>   
>  Regards,  Toby Seiler
>   
>   
>
> 
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