As mentioned before, the co-firing of coal and biomass creates problems for the use of mixed ash that contains fly ash as well as inorganic materials. Coal ash is used in the construction industries and biomass ash is used in the fer­tilizer industry. The ash content of a feedstock (biomass) has a major impact on gasifier operation. This type of impact depends on the gasifier type: slag­ging or nonslagging. For a moving bed (or fixed bed) nonslagging operation, ash below its fusion temperature forms clinker which stops the flow of feed­stock within the reactor. Although ash fusion temperature depends on the amount of sodium present in the feedstock, clinkers can become a signifi­cant problem for reactor operation. The ash can also affect the fuel’s reaction response. For high sodium content woody biomass such as birchwood, the formation of sticky sodium silicates by the interaction between sodium and silica bed materials used in a circulating fluidized bed can cause agglomera­tion and potential interruption in the gasification operation.

For an entrained flow gasifier, ash from biomass does not melt even at temperatures of 1,300-1,500°C because ash is rich in CaO, and alkali met­als are removed by the gas phase. Despite the high melting temperature of ash, the slagging entrained flow reactor is preferred because melt can never be avoided and the slagging entrained flow gasifier is more fuel flexible. Slagging co-gasification may require fluxing materials such as silica or clay in order to obtain proper slag properties at reasonable tem­peratures. By adding flux material to biomass, coal-based slag (generally coal ash with added limestone) is mimicked and slag properties become comparable. Solids handling is one of the important reasons why new gasifiers tend to be high-pressure, high-temperature entrained bed gasifi­ers. Such gasifiers will also handle solids produced with the mixed feed­stock well.