[Gasification] Why dry gassification fuel?

[Daniel Chisholm]

Hi Steve, welcome to the list.

I enjoyed looking at your VTHR heater, be sure to let us know how it
works over the winter.  (In fact I am a bit jealous of you building
this, since I had the exact same idea of burning wet wood chips for
residential heat - but I seem to have gotten sidetracked with the notion
of producing engine-grade gas...)

If you haven't already discovered it, I would highly recommend that you
spend some time playing with the "gasifier.exe" program from DTU (Danish
Technical University).  For me it was a real eye-opener to play with,
and learn the implications of all sorts of different design decisions or
operating parameters - e.g., what's the effect of 15%MC versus 45%MC
fuel?  What about preheated combustion air?  What about steam addition?
Heat losses?

Google tells me it is at

http://bgg.mek.dtu.dk/research/modelling/systems/

...however it seems to be down at this moment (let me know if you want
me to email the file to you - it's a 3.3MB .exe file for Windows)


On Fri, 2006-15-12 at 17:57 -0500, Steve Redmond wrote:

> I doubt that the 700 to 900 (F, I believe, not C) needed to thermally
> sustain pyrolisis is unattainable with 50% mc chips in a self (charcoal)
> consuming reactor.
> 
> In fact I'm certain it is attainable.
> 
> Excess heat and steam can be condensed from the gas flow and made to give up
> heat to the reactor. The air intake is one area to look at.

When playing with gasifier designs, cost-effectiveness has to be kept in
mind too - the smaller the system, the less justifiable it is to spend
money to make the system more efficient, since the fuel flows (costs) of
a smaller system make efficiency gains less attractive.  To use one
example that is near and dear to my heart (i.e. I'm struggling with it
right now), preheating combustion air to 600-700C does wonders for
thermal efficiency and gas quality, but the cost complexity and effort
of building a high temperature air to gas heat exchanger to accomplish
this might not be justifiable on a small gasifier.



> In fact by drying chips with process heat, the energy balance is the same.
> The question is, where do you want to spend your energy to
> evaporate water from the chip, before pyrolisis or during it? No free rides.

The thing is though that because downdraft gasifiers have a thermally
inefficient layout (in the sense that they are co-current devices), you
really don't want to use any heat at all from the hearth area for fuel
drying if you can possibly help it.  High-quality heat (i.e. high temp)
robbed from the hearth area will hurt your process, whether in the form
of higher tar production (giving up temp and residence time to crack
tars) or gas quality (reduction stops once the gas through the char bed
back-reacts down to ~800C).

No free rides in the sense of something-for-nothing, however it is much
better to use lower grade heat for either drying your wood (if
applicable to your gasifier design) or for preheating your combustion
air.  If a gas:air heat exchanger can be justified, you can preheat your
combustion air using the the ~800C gas that the gasifier is generating
(and remember that there is a fair bit more (about 1.5X) as much gas
(masswise) coming out of the gasifier as there is air going in, so this
helps getting your preheated air to cost-effectively approach your gas
temp).


> I'm suggesting not separating these processes.
> 
> Unless naturally air dried chips are used, heat input to dry the chips comes
> from the chips, at one stage or another. Even the engine exhaust manifold
> heat derives from the chips. It also could raise reactor temperature, rather
> than drying chips pre-pyrolisis.
> 
> The advantage is to eliminate a separate stage of chip handling: drying.

Agreed - but the question becomes, can a given gasifier design handle
excess moisture?  And even if it can, does the elimination of the chip
handling/drying outweigh the other tradeoffs (e.g. larger gas cooler
required, some loss of gasifier thermal efficiency due to the parasitic
heat loss from the "unnecessary" moisture in the gas, increased volumes
of condensate to deal with).

Engine exhaust isn't hot enough to provide heat to the reactor at its
hottest portion, but it could be used to preheat the wood chips
(directly or indirectly), or the gasifier's intake air.



-- 
- Daniel
Fredericton, NB  Canada