In biomass gasification facilities different biomasses are used, with different proportions of hemicellulose, cellulose, and lignin. Several pretreatments are performed in these installations in an effort to change physical characteristics of the feedstock such as drying, size reduction, size fractionation, and leaching with water. Taking into account that lignin gasification produces more hydrogen than other components of the biomass, it could be of interest as a pretreatment that improves lignin content, Composting is a biological process that modifies biomass composition, increasing lignin content. Without going into any economic details, the present work studies the effect of biomass composting (biological pretreatment) on hydrogen production in air gasification. This laboratory-scale experience was completed, studying the effect of the heating rate on the gasification process. In this study, two different biomasses were used: Leucaena (Leucaena Leucocephala) and Tagasaste (Chamaecytisus palmensis). The experimental results show that combining slow heating rate and biomass composting, hydrogen production increases 20% (into studied range). This result can be related with feedstock composition (inorganic matter and lignin contents) and char properties.