As mentioned above, biomass feedstock and its moisture
content are fixed, so steam flow rate has been changed according
to the value considered for the steam to biomass ratio
(S/B). Under these conditions, the simulations show how the
results depend on this ratio and on the gasification temperature.
Fig. 3 shows that the hydrogen energy ratio (HER) of the
gasifier at each gasification temperature level first increases
and then decreases when S/B is increased. This trend is clearly
shown on the Figure at 750 and 800 C and it is probably present
also at 850 C although the maximum would appear in
this case for higher steam to biomass ratio. At 750 C and
800 C the maximum HER of the gasifier corresponds to S/B ¼ 1.5 (37.5% and 41.0%, respectively). As it is known from the
literature [48e50], the hydrogen yield increases with the
gasification temperature and with the steam to biomass ratio.
The gas yield calculated at T ¼ 800 C and S/B ¼ 1.5 is 1.3 Nm3/
kg.
The catalytic filter candles (Fig. 4) improve the HER. This is
because methane steam reforming, tar steam reforming and
water gas shift reactions occur inside the candles [36]. The
steam reforming reactions are enhanced at high temperature,
whereas the water gas shift thermodynamic equilibrium is
more favourable at low temperature.