Biomass to energy conversion is particularly attractive on the microscale were internal combustion engines such as microturbines may be utilized coupled to an indirect gasification system. The authors have developed the IPRP technology based on rotary kiln pyrolisys and a pilot plant was built in Italy powered by an 80 kWEl microturbine fired by pyrolysis biomass syngas. This paper describes CFD numerical investigations carried out to study the combustion process occurring inside the annular rich-quick-lean combustion chamber of the given microturbine. A RANS analysis has been performed in order to simulate both natural gas and syngas combustion. A mechanisms based on tworeduced and detailed chemical kinetic were taken into accountand applied to carry out the CFD simulations. The numerical results obtained for NG are presented and compared with the experimental data on emission to validate the numerical assumptions. The combustion mechanism are used also in pyrolysis gas combustion case to investigate the operation of the microturbine fuelled with this biomass derived fuel.