[Gasification] Practical gasifier ideas

[Daniel Chisholm]

Thanks for the comments, Jim!

On Mon, 2007-19-11 at 10:15 -0800, jim mason wrote:
> using the exhaust is a very good idea for the source of air blast for
> a fluidized bed.  i like it.  many details to work out though.  you
> are going to get large air % and gas flow volume into the fluidized
> bed as power varies.  both equivalency ratio and fluidization kinetics
> are going to swing in a challenging manner.

For a non-turbocharged engine, the air mass flow rate will be *very*
closely connected to RPM, which is an easily measured and easily
governed operating variable.

Air mass flow will have a small variation with changes in exhaust back
pressure, to the extent that this makes (small) changes in the engine's
volumetric efficiency.  This will probably be a small enough effect
though that it can be ignored.

Note that I am suggesting a diesel engine to be used solely as a
field-expedient blower.  It is not powered by gas that is produced, nor
is its shaft power used for anything.  The power (fuel flow) into the
diesel engine will only vary to the extent of its exhaust pumping load,
which will be quite a small fraction of the engine's rated power.  So it
should be very easy to control in a stable and responsive manner.

> in the work i've done with fluidized beds, i found the main challenge
> to be understanding the fluidization kinetics.  you need to be able to
> figure out your minimum bubbling velocity, the transition point to
> turbulent flow, then fast bed, then full pneumatic transfer.  if in a
> bubbling fluidized bed without recirculation, over pulling will of
> course entrain and pull the material out the top.  a typical fluidized
> bed has a rather narrow band in which the flowrate is tolerable.
> about a 4 or 5:1 turndown ratio typically.

Or even less.  Bubbling fluidized bed boilers have about a 2.5 or 3.0
turndown range.  To get wider control, they partition the bed into
zones, and shut down ("slump") the bed in some of the zones.

W.r.t. turndown ratios and using a diesel engine as a blower, the
available mass flow range is roughly the ratio of the engine's max RPM
to its idle RPM.  This ought to be at least 4:1 (e.g. max 2000RPM, idle
500RPM).  A turbocharged engine will give a somewhat wider exhaust flow
range than its RPM range alone would indicate.


-- 
- Daniel
Fredericton, NB  Canada