In this work, biomass (eucalyptus spp.) gasification has been investigated in a two-stage downdraft reactor by using different gasifying agents such as: air, air + saturated steam, and O2 + saturated steam. The utilization of low quality saturated steam for gasification in different mixtures with air and O2 is the main novelty of this paper. Thus, the influence of different gasifying agents was assessed on the concentrations of CO, H2 and CH4 and hence, on the lower heating value (LHV), the syngas power and the cold gasification efficiency. Since the type and flow of gasifying agent control the biomass consumption in downdraft gasifiers, the equivalence ratio (ER) and the steam-to-biomass ratio (SBR) were calculated by means of a mass balance. The results showed that low quality steam can increase remarkably the H2 concentration and the LHV of syngas despite its low temperature. Particularly, when O2 + saturated steam is fed to the gasifier, the H2 concentration varied between 27.4 and 35.9 vol.%, while the LHV was between 7.13 and 8.35 MJ/N m3, i.e. almost twice the produced when only air or air + saturated steam is used. The optimum ER seems to be below 0.412, while the SBR is found between 0.495 and 1.032. Under this conditions, the cold gasification efficiency was between 70 and 75%. Moreover, the O2 seems to mitigate in a better way the negative effect of the low temperature of the saturated steam given the lower tar concentration in syngas (58.74–64.18 mg/N m3) respect to those found with air + saturated steam (91.41–91.95 mg/N m3). This paper concludes that saturated steam can be used blended either with air or O2 (being better with O2) for biomass gasification in two-stage downdraft reactors and therefore, the requirements of superheated steam are not so important as in other gasification processes.
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