BIOMASS PYROLYSIS
IN DENSE FLUIDIZED BED
Preliminary information

Preliminary information


March 14, 2008

Contents

1 The context

At a time when finding fossil fuels is becoming more and more of an issue and when CO2 emissions have to be limited, the use of biomass is considered as an interesting alternative solution for future energy supply. Biomass pyrolysis was discovered in the 1950s and essentially consists in reducing raw materials at a very high temperature. Although it has not been intensively studied nor used in the past decades, the evolution of the energy market has brought back this technique to the industrial sector. The yields obtained in biomass pyrolysis fluidized bed reactors are much higher than conventional biomass burning reactors [1] and the ability to transport the produced fuels is a major argument in favor of pyrolysis.As a consequence, research on biomass has to be developped in order to optimize the operating conditions of reactors and maximize the profitability of this type of reactors.

2 Fluidization

The fluidization of particles is a complex phenomenon that relies on a simple principle : stir up particles thanks to a gazeous flow. This principle is illustrated on figure.


PIC

Figure 1: Scheme of the fluidization principle


Each particle is stressed by its weight, the buoyancy and its drag. Thanks to the forces balance, the limit stirring velocity can be established but it depends on many parameters such as the pressure losses, the porosity or the particles’ size and their density. Many complex correlations were developped, but as it is not the aim of our study, just the final model used to calculate the limit stirring velocity is given here. The minimum fluidization velocity Umf is calculated from the particular Reynolds number :

U    =  μfRemf--
  mf     dp ρf

With:

dp

particles diameter [m]

ρf

fluid density [kg.m-3]

μf

fluid dynamic viscosity [Pa.s]

The following correlation is used to determine Remf:

             2               0.5
Remf =  (31.6 +  0.0 421 *Ga )   - 31.6

Where Ga is the Galilée number. This number depends on the fluid and particles properties:

         (ρp - ρf )ρfg
Ga =  dp3-------2-----
              μ f

With:

ρp

particles density [kg.m-3]

g

gravitationnal acceleration [m.s-2]

The fluidization enables a better mixing of particles that improves heat and mass transfers between the phases. In this study, a bed of inert heavy particles (olivine or sand) is fluidized thanks to a gazeous flow (water steam or nitrogen N2) to improve the pyrolysis phenomenon of biomass particles.

3 The pyrolysis

Pyrolysis is a thermal decomposition process that occurs at moderate temperatures with limited oxygen.This process theorically include no oxygen to avoid burning. The pyrolysis of biomass produces a liquid product, pyrolysis oil or bio-oil that can be stored and transported.

The whole pyrolysis scheme is showed on figure 2. First the virgin unactive biomass is introduced in a cell with an inert solid phase (sand, olivine) which has been heated through the combustion and provides heat to biomass particles. Then, when heated at about 550C, virgin biomass particles become active and react to produce solid particles called char but also two gaseous phases of gas and tar. The heat necessary to the pyrolysis comes from the combustion of the char or of a part of the gases produced.


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Figure 2: Scheme of a pyrolysis reaction


4 The advantages of biomass pyrolysis

Many types of biomass can be used with this technology. However, most of the biomass that is taken into consideration for this technology is "wasted biomass". Indeed, in lots of industrial process, biomass is the residual part of the primary product. For example, in Spain, the olive oil industry leave tons of olive kernels unused in their industrial process. The idea is to use this wasted biomass in order to produce electricity, fuel and heat while keeping low C02 global emissions.

Even if emissions are similar for gasifiers and combustors, on the contrary to direct biomass combustion, biomass pyrolysis provides transportable and storable products. Moreover, gasifiers are cheaper than combustors [1]. The costs involved in such an installation is listed in table 1. Finally, if considering an already running powerplant in Austria, one can notice that biomass pyrolysis is both ecologically and economically profitable.




Cost category Amount




Investment cost 10 Millions Euros


Funding (EU, national) 6 Millions Euros


Operation cost / year 1,5 Million Euros


Price for heat 3,9 Euro-cents/kWhth


Price for electricity 16,0 Euro-cents/kWhel


Electrical output 2.000 kW


Thermal output 4.500 kW


Table 1: Economic balance of the biomass power plant of Gussing (Austria)

5 Conclusion

As regards to the better efficiency, the ease of storage and handling of the pyrolysis products compared to the direct combustion ones, it seems necessary to improve the modelization of the pyrolysis process to get the best configuration for the fluidized bed reactors.

References

[1]   Luo : Renewable energy : A model of wood flash pyrolysis in fluidized bed reactor.