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Flash pyrolysis of biomass in a bench scale fluidized bed plant


In this project, the flash pyrolysis of biomass will be investigated in a fluidized bed bench scale plant. Wood from fast growing species as well as from cotton residues will be used as biomass feedstock. The objective is to investigate the operating conditions of the pyrolytic unit at which the yield of pyrolysis oil is maximised.
Amongst various thermochemical processes of biomass, flash pyrolysis is known to maximize liquid products yield. Flash pyrolysis is usually exerted in inert gaseous atmospheres in entrained beds or fluidized bed reactors.

In order to combust the pyrolysis derived char in a fluidized bed reactor, it is clear that char should be lead to the bottom of the bed just above the gas distributor and the fluidizing air quantity should be restricted to that amount necessary to carry out stoichiometric char combustion thus avoiding undesired oxidation of the volatile products. Since char is lighter than the sand (heat carrier), this can only be achieved if the entire char and sand mixture is recirculated resulting in guiding the entrained char in the lower oxidizing zone of the reactor.

The pyrolysis reactor to achieve such a solids recirculation consists of a circulating fluidized bed (CFB), where solids are entrained in a vertical tube (riser), while air (fluidizing gas) quantity is restricted to that necessary to carry out the derived charcombustion. After the riser, the solids are separated from the gas stream and are recycled to the combustion chamber, where fluidizing air is fed and the combustion reaction provides the heat necessary for the biomass pyrolysis.

The pyrolysis product vapours leave the reactor through the cyclone and are supplied to the downstream processing equipment (scrubber) for quenching and collection. The char is separated in the cyclone(s) and is recycled via the downcomer and the solids reinjection system (L-valve) to a bubbling bed, operating in the slugging mode, where it is combusted. The hot sand from the lower fluidized bed enters the riser section of the CFB due to the slugs, the sand being essentially the heat carrier for biomass pyrolysis process. In order to determine the hydrodynamic behaviour of the CFB reactors, a cold model was designed, constructed and operated. The design parameters were derived from mass and energy balance c alculations based on an approximate 10 kg/h biomass input, for both the cold model and the hot unit.
It is proposed to design, construct and operate a fluidized bed flash pyrolysis bench scale plant in order to determine the process parameters by which the yield of oil products is maximised. The plant will be constructed and operated in the Agricultural University of Athens, while the design and engineering will be carried out by CRES in cooperation with Free University of Brussels (B), Egemin (B) and Aston University (UK). Moreover, the technical economic assessment will be carried out in cooperation with Aston University.

The feedstock to be examined are fast growing wood species found in Greece and other southern EC countries as well as cotton stalks which are found (as an agricultural residue) in abundance in Greece, Italy and Spain.

The plant consists of a continuous feeding system, a fluidized bed and cyclones, two heat exchangers (condensers) in series, a mist eliminator, a filter, a compressor, a preheating oven, a gas flare as well as instrumentation and controls.

The subjects to be investigated in this research project are:

a) residence time of gases in the reactor;
b) particle size of the feedstock;
c) operating temperature;
d) inert gas;
e) bed material;
f) specific throughput.

The work program includes the following main phases:

1) design of the plant;
2) ordering of equipment;
3) construction of plant parts;
4) assembly of the plant;
5) commissioning;
6) operation;
7) evaluation and technical economic assessment.


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Centre for Renewable Energy Sources
EU contribution
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KM 19TH,Marathonos Avenue
19009 Pikermi Attiki

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Participants (4)