A system and process for modulating the carbon content of ash produced by a biomass gasification process, for example, to selected levels chosen by an operator, through the controlled injection of steam and controlled introduction of warm air during processing of a biomass feedstock. The system and process include delivering a carbon-containing biomass feedstock to a gasification reactor and producing a syngas and an ash from the biomass feedstock, and regulating the carbon content of the ash between a level at which carbon not present in the ash and a second level at which carbon is present in the ash. The regulating step entails selectively decreasing the moisture level of the biomass feedstock prior to the biomass feedstock being delivered to the gasification reactor and thereby increasing the carbon content of the ash, or increasing a moisture level of a mixture of the biomass feedstock, ash and gases within the gasification reactor and thereby decrease the carbon content of the ash.