**2.3 Saturated and superheated steam as gasification agent**

Water gas (3) and water gas shift (6) reactions are the reasons steam can be introduced to oxygen/air gasification or wet biomass is accepted, of which moisture is more tolerated than that in direct combustion. Higher generation yields of H2 and CO are obtained so the final syngas mixture gets higher heating value. However, these two reactions are endothermic while the vaporization enthalpy of water has a large value (at atmospheric pressure that is 40.65 kJ/mol) so saturated steam or water can make the pyrolysis zone lose heat, drop temperature, leading to lower conversion yield. Lower quantity becomes a contrast to higher quality of syngas formation in this case. Subsequently, the process even gets faded if sufficient heat is not guaranteed. To achieve both quantity and quality of syngas, heat should be redeemed by using superheated steam instead of saturated steam or water in wet biomass so that the gasification temperature is maintained above 750–800°C [19].

The ratio of steam to carbon content of the biomass fuel (SCR) is used as a crucial operating parameter in biomass gasification with steam feeding [20]:

$$\text{SCR} = \frac{\text{Steam mass flow rate} \left(\frac{\text{kg}}{\text{s}}\right)}{\text{Carbon feed rate } \left(\frac{\text{kg}}{\text{s}}\right)} \tag{10}$$

Steam flow rate (kg/s) to biomass (kg/s) ratio (S/B) is also used like SCR [21]. Steam feeding makes the ratio of hydrogen to carbon in the whole reaction mixture increase, which was found to yield more H2, and increase the heating value of the syngas, while tar content decreases significantly [22]. This technique is positively meaningful in biomass gasification because it does not only increase the quality of the syngas but also reduce tar-clogging problems to sustain the process.
