[Bioconversion] Re "drying"

[Peter Singfield]

You might want some perspective -- costs of endothermic investment -- costs
of drying biomass for proper partial combustion.

There is a huge difference in regards to temepratures required -- but the
energy involved is still surprising.

The last posting was Tom Reed drawn from my old Gas Archives -- this one is
from Kevin --

Those were the "nuts and bolts" days ---

Peter/Belize


************appended**********

From: "Kevin Chisholm" <kchishol at fox.nstn.ca>
To: <gasification at crest.org>
Date: Sat, 10 Feb 2001 16:00:17 -0400
Subject: RE: GAS-L: Drying

Dear Samuel

100 pounds of "wet" wood would typically contain 50% moisture. This is the
first problem: 50% of what???

If it was "50% moisture wet basis", then you would have 50 pounds of bone
dry wood, and 50 pounds of water. On the other hand if it was "50% moisture
dry basis, then you would have 66.66 pounds dry wood, and 33.3 pounds of
water. Lets assume "wet basis" all the way.

To get to 20% moisture, you would have to remove some water, but how much?
If you removed 30 pounds of water, you would be left with 20 pounds of water
and 50 pounds of bone dry wood; the wood would then have a moisture content
of:
		20/(20 + 50) = 20/70 = 28.57%
and not the 20% one would guess at intuitively.

To get to 20% moisture, you would have to remove 37.5 pounds of water,
leaving 12.5 behind with the 50 pounds of bone dry wood. (I won't bore you
with the elemental math...)

Very roughly, it takes 1000 BTU to evaporate 1 pound of water.... in this
case, it would take 37,500 BTU. However, you also have to heat the 50 pounds
of wood and 12.5 pounds of water, but, at a specific heat of say .6, the
"wood sensible heat" is about 60 BTU/Degree.... say 6000 BTU for 100
degrees. This is small, in comparison to the theoretical water heat load,
but brings the theoretical total to about 43,500 BTU.

The major heat loss is in connection with the air required to
convey the heat to the wood, and heating furnace losses. It generally works
out that the heat input into the drying furnace is about 1.7 times the heat
required for water evaporation. In this case, the "General Rule of Thumb"
would indicate 1.7 x 37,500 = 63,750 BTU

This can be more or less depending on the specifics of the situation.

Hope this is helpful.

Kindest regards,

Kevin Chisholm


> -----Original Message-----
> From: samuel.martin at epfl.ch [mailto:samuel.martin at epfl.ch]
> Sent: Saturday, February 10, 2001 12:06 PM
> To: gasification at crest.org
> Subject: GAS-L: Drying
>
>
> Dear all,
>
> does anybody have some values about heat requiring to dry
> biomass. Typically,
> what heat quantity do I need to dry fresh cut wood until gasifier
> limits (20%
> dry basis) ?
>
> Thanking you
>
> Samuel Martin
>
>