One of the major problems in air gasification is the dilution of the product gas by the nitrogen in the air. This could be overcome by using oxygen as the gasifying agent. However, the use of oxygen requires an oxygen generator and is expensive and highly energy extensive. Another route to achieve a product gas without diluting nitrogen is to separate the combustion process from the gasification process in two reactors in a so-called dual fluidized bed configuration.

A typical dual bed fluidized bed gasifier is made up of two reactor chambers, the gasifier and the so-called riser combustor. An example of this configuration is shown in Figure 6.8, where the fast internal circulating fluidized bed (FICCFB) gasifier, developed by Vienna University of Technology in Austria, is illustrated. The system combines a bubbling fluidized bed gasifier with a circulating fluidized bed combustor, where the gasifier in effect is a pyrolysis, indirectly heated with hot sand from the riser, which in turn is heated by burning the product char with air before

re-circulation back to the gasifier. Steam is also usually added for enhancing hydrogen generation via the shift reaction and to promote the carbon-steam reactions. Product quality is good from a heating value perspective, but poor in terms of the tar content. These gasification configurations normally give middle-caloric gas with the heating value (LHV) of 10-14 MJ/m3N as the resulting gas will be almost nitrogen-free.

Another concept with a combination of a gasifier and a combustor is the SilvaGas (Batelle) process shown in Figure 6.9 has been tested in several countries. The idea is that fuel is heated with solids from the combustor, and char is fed to the combustor after the gasification to get good conversion efficiency. The gasifier can in this case be operated with steam and by this achieve a richer gas.