Catalytic Tar Removal from Biomass Producer Gas with Secondary Air

Abstract

The effect of air addition on biomass tar conversion in catalytic packed bed crackers was studied using both an isothermal micro reactor and a fluidised bed bench scale biomass gasification set up with down stream tar crackers.

The micro reactor was applied for experiments with artificial biomass producer gas containing naphthalene as a model tar compound. Experiments were carried out with inert silica and catalytically active calcined dolomite bed material both with and without air addition. Kinetic results were modelled assuming three parallel first order naphthalene decomposition reactions:

$${R_{Naphth}} = k_1^{eff}{c_{Naphth,0}}(1 - {\zeta _{Naphth}})\;with:{k_1}^{eff} = [{k_1}\varepsilon + {k^*}_1(1 - \varepsilon ) + {k_1}^{ox}]$$

• k ₁: rate constant of the homogeneous thermal decomposition reaction [s^-1.]

• k ₁*: rate constant of the catalytic heterogeneous decomposition reaction [s^-1.]

• k ₁ ^ox: rate constant of the oxygen induced decomposition reaction [s^-1.]

With secondary air the results show a significant increase in tar decomposition rate both with dolomite and silica. With dolomite, the naphthalene decomposition reactions on the catalyst surface dominate the overall conversion rate making the relative effect of air addition less significant. However, with dolomite, secondary air feeding has the positive effect of significantly decreasing the catalyst deactivation rate.

Experimental results with real tar from the fluidised bed bench scale gasification set up were in qualitative agreement with results from the micro reactor experiments.