[Gasification] Fluidised bed reactor

Tom Miles tmiles at trmiles.com
Sat Nov 30 00:00:46 CST 2013


If you want to use the engine exhaust in a gasifier you must consider the
heat and material balance for a gasifier and IC engine. 

Fuel input 100%
Heat loss in FB gasifier 5%
(Sensible) Heat loss to cooling the gas 25%
Cold clean gas efficiency to engine 70% (20% C0, 20% H2, 2% CH4, 12% CO2,
44% N).
Heat conversion in engine ~25%
33% to power (~14-17% fuel to power)
33% to engine jacket heat (17% net)
33% to exhaust. (17% net, mostly CO2 and N)

By injecting exhaust into the gasifier you are recycling a lot of inert gas
(CO2, N2) and very little heat. 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

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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