[Gasification] Fluidised bed reactor

Sat Nov 30 02:06:27 CST 2013

On Fri, Nov 29, 2013 at 10:00 PM, Tom Miles <tmiles at trmiles.com> wrote:

> . The best use of the exhaust gas heat may be
> in preconditioning the fuel. Danish Technological University (DTU Viking
> gasifier, 1990-2010,  that is now being commercialized by Weiss) and later
> All Power Labs (Power Pallet, 2009) have used heat from the exhaust to
> indirectly dry and pyrolyze incoming fuel. The dried and partially
> devolatilized wood, along with preheated combustion air, appears to
> contribute to a stable oxidation zone. Both units make a very good quality
> gas. These are both fixed bed downdraft gasifiers with pre-pyrolysis zones.
> Peak temperatures reach 1000 C-1200 C in the oxidation zone and 800-900C in
> the reduction zones.
>
> http://www.btgworld.com/en/references/publications/handbook-biomass-gasifica
> tion-second-edition
>
>

this sort of heat recycling can certainly improve the total thermal budget,
but i believe the more important impact is improvement of pyrolysis control
and resulting tar species you have to deal with downstream.

in a non-heat regenerating downdraft, the pyrolysis is driven parasitically
from heat radiating and convecting up from the combustion zone.  in
practice, pyrolysis ends up happening in a very thin layer right above the
nozzles.  the result is pyrolysis time is very short and the temps are high.

the result is that most pyrolysis is happening above the 600C threshold
where there is significant evolution to refractory/teritary tars.  these
are the tars with lots of double carbon bonds, making them more difficult
to crack downstream.  these tars are also high in PAH species, which are
the nuclei precursors of soot.  thus high PAH gas streams tend to increase
your soot load.

exhaust gas driven pyrolysis let's you run pyrolysis over a longer period
of time, and at a much lower temp.  IC exhaust is around 600-700c, so
through a double jacket you end up with pyrolysis staying under 600c.  this
keeps the tars in primary fragment forms, or minimally evolved.  cracking
conditions are a bit more flexible downstream as a result, and the
reduction in PAHs reduces soot in the final gas.

i can't provide quantitative numbers on this sadly, but by observation we
regularly see a large difference in gas quality between using and not using
the exhaust gas driven pyrolysis.  the soot load difference is particularly
surprising.

also, if you are trying to drive a given reactor to high throughput, the
rate limitation is usually the pyrolysis rate.  "externally" driving the
pyrolysis gives you much more residence time to complete pyrolysis before
things drop into the hearth.  this is particularly the case with chunkier
and wet fuels.

this externally driven pyrolysis is likely the most impactful thing we've
done so farwith the GEK to improve the traditional imbert.  more than the
air preheating or the fuel predrying.  each helps, but the driven pyrolysis
section changes the chemical species you are dealing with, not just the
total thermal budget.

many gasifiers have done this.  the viking did it most impressively in
recent years, and with the best documentation of the benefit.  the main
thing we added was to well separate the drying from the pyrolysis stage.
 this keeps the mess down in the hopper, and better segments needed heat to
specific input heat.  we also tried to do this in a compact space to keep
the total unit size down.

the specific relationships of waste heat to process input points is
explained in more detail here:
http://wiki.gekgasifier.com/Tower-of-Total-Thermal-Integration.

thanks to tom reed for doing the work to document how all tars are not
created equally.

j

> We have worked with many fluidized bed gasifiers in research and in
> industrial applications, for producer gas, and for synthesis gas. The
> fluidized bed is an intriguing reactor that has its use in industrial
> applications but they are expensive to build and operate.
>
> Fluidized bed (FB) gasifiers are unique because distributor plates or
> nozzles in the bed uniformly distribute the reactant (air, steam) in the
> sand (or media) bed. It is therefore imperative that the fuel be
> distributed
> uniformly for good gasification. This good distribution affords good
> temperature control as the fuel goes through the exo- and endo- thermic
> reactions that Kevin described. The ideal arrangement is to feed the fuel
> into the bed and allow sufficient time (depth) for it to completely react
> before breaking the surface into the vapor space above the bed. Of course
> there are variations on fluidized bed reactors such as spouted beds, fast
> beds, or entrained flow reactors. The latter are used for fast pyrolysis to
> liquid fuels by companies like Ensyn. I think that Dynamotive is the only
> company that uses a bubbling bed for pyrolysis. Many of the biomass to
> liquid reactors consumes the char to drive the process so there is no
> excess
> char
>
> When used for gasification fluidized beds can be pretty stable in the
> 650-750 C range. The higher temperatures are needed to provide thermal
> inertia when wetter fuel (>20% MC) is used. Higher temperatures are
> generated by adding oxidants as Kevin has described. Higher vapor space
> temperatures (750 C) can be achieved by adding air above the bed. The
> partial oxidation can reduce NOx precursors in the fuel gas. You can think
> of the heat balance as consuming between 25% and 33% of the fuel to convert
> the remainder to chemical (producer gas or syngas) and sensible energy.
> Producer gas is burned directly in a boiler or reformed for use in engines.
> Synthesis gas is usually made using enriched air or oxygen as the reactant
> or by indirect heating, as in a dual fluid bed. The variety of catalytic
> reformers used to make synthesis gases can be seen online in the
> presentations at the TC Biomass 2011 and 2013 conferences.
> http://www.gastechnology.org/tcbiomass2013/Pages/2013-Presentations.aspx
>
> There are a few small commercial (50-100 tpd) fluidized bed/entrained flow
> pyrolyzers that are making specialized products (liquid smoke) for the food
> industry. In general there are no commercial small scale fluidized bed
> gasifiers or combustors.
> Attempts by US boiler makers and several small entrepreneurs have failed to
> sustain commercial operation of fluidized bed gasifiers or combustors at
> the
> small scale.
>
> Tom  Miles
>
>
>
>
>
>
>
>
>
>
> -----Original Message-----
> From: Gasification [mailto:gasification-bounces at lists.bioenergylists.org]
> On
> Behalf Of Kevin
> Sent: Friday, November 29, 2013 9:23 AM
> To: Discussion of biomass pyrolysis and gasification
> Subject: Re: [Gasification] Fluidised bed reactor
>
> Dear Rex
>
> ----- Original Message -----
> From: "Rex Zietsman" <rex at whitfieldfarm.co.za>
> To: <gasification at lists.bioenergylists.org>
> Sent: Friday, November 29, 2013 4:18 AM
> Subject: [Gasification] Fluidised bed reactor
>
>
> > Kevin,
> >
> > What you say is correct. However, there is benefit in returning some
> > exhaust
> > back to gasification as you force a higher production of CO relative to
> > CO2
> > production. I suspect it is an equilibrium thing...
>
> # For a gasifier having a relatively high exit temperature, then engine
> exhaust additions to the gasifier intake air could indeed be a way to both
> lower the gasifier exit temperature, and to raise the CO level.
>
> There is a CO/CO2 equilibrium consideration, and also a kinetics or "speed
> of reaction" consideration.
>
> Best wishes,
>
> Kevin
> >
> > Rex
> >
> >
> > ---
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