Therefore, as shown in Table 2, gas quality and gas yield
(1.9 Nm3/kg) increase. Increasing the steam to biomass ratio
reduces the methane concentration and also the temperature
at the candle outlet, because endothermic reforming reactions
occur inside the candles. However, the carbon monoxide
concentration increases less than proportionally to the
methane reacted: this behaviour is expected, because inside
the candles the water gas shift reaction also occurs, which is
enhanced at low temperature, whereas methane is preferentially
reformed when the operating temperature is increased.
Tar concentrations are predicted to be negligible in the candles
output.
At the inlet of the HT-WGS reactor, water has been added
in different concentrations, as functions of temperature and
steam to biomass gasification ratio. In this way, the hydrogen
conversion efficiency was further improved by increasing the
HER from 87%, at the catalytic filter candle outlet, to 99%, at
the WGS reactors outlet, as obtained by simulations at 850 C
and S/B ¼ 2. The dry product gas from the WGS reactors (at
800 C and S/B ¼ 0.5) is characterized by a calculated composition
of 62% H2, 6%CH4, 0.4% CO and 31% CO2 (by volume), in
line with results reported in the literature [51] with a corresponding
gas yield of 1.8 Nm3/kg daf biomass.