Effect of steam content in the air–steam flow on biomass entrained flow gasification

In this work, several experimental schedules have been carried out in order to study the effect of the addition of steam to air as gasifying agent in biomass entrained flow gasification, a promising technology due to its commercial availability, high efficiency and high potential for the production of biofuels and chemicals from biomass. Dealcoholised marc of grape (an abundant residue in the southern regions of Europe with interesting physical and thermochemical properties) has been used as fuel in all the tests. Firstly, the steam content of the gasifying mixture has been varied from 0% (air gasification) to 100% (steam gasification) by keeping the molar fuel/gasifying agent ratio (F) constant, in order to determine the role of air and steam in the gasification process. On the other hand, the effect of the steam/biomass (S/B) ratio has been determined both for steam and air–steam gasification. Lastly, the effect of the operating temperature has been compared both for air and air–steam gasification. The performance of the gasification process has been evaluated through some characterisation parameters, such as the producer gas composition and heating value, the gas yield, the cold gas efficiency, and the product gas ratios and production. The results obtained show that there is an optimal range in the steam content of the gasifying agent (found for air–steam mixtures containing 40–70% mol steam) for which a trade-off between gas quality, gas production, and cold gas efficiency is reached. In general, the addition of steam proved positive for the process performance, not only because of the lower dilution of gas in N₂ from air, but also because of the promotion of the steam reforming and the WGS reactions. An increase in the operating temperature has different effects depending on the gasifying agent used. Thus, a higher temperature increases the CO and H₂ content in the product gas for air gasification, whereas air–steam gasification leads to a boost in the H₂ production at higher temperatures, as well as an increase in the CH₄ content.