Laboratory study of co-firing renewable (wood granules) with fossil fuel (propane) is carried out using a compact design (Fig.1), which includes a laboratory-scale premixed propane/air burner (3), a wood biomass gasifier (1), charged with wood granules and a water-cooled channel (6), downstream of which the dominant combustion of the volatiles is developing. Combustion conditions in the system are varied by varying the mass flow rate of the primary air (4) into the bottom part of the combustor and the mass flow rate of secondary tangential air flow (5), introduced from the two tangential air nozzles of inner diameter D=5mm. The primary airflow ignites the volatiles and initiates combustion, while the secondary airflow completes the fuel combustion. Between the co-combustor and water-cooled channel sections and so between the channel sections the diagnostic sections (7) with peepholes are located to provide the local injection of the diagnostic tools (Pt/Pt-Rh thermocouples, Pito tube, gas sampling probe) into the flame of volatiles and so providing the local measurements of the flame temperature, velocity and composition. The residual ash is removed from the bottom part of the combustor (8).

The electric control of co-firing the wood granules with propane is carried out using the central electrode, axially inserted downstream the flame of volatiles. The bias voltage of the central electrode in this study could be varied within a range from –3kV to +3kV, while the ion current is limited to 1 mA, producing the evident variations of the flame shape (Fig.2), provoked by the field-enhanced variations in a rate of unsteady heating and volatilisation of wood granules and burnout of volatiles and wood char.

Figure 2: The electric field effect on the shape of the free flame of volatiles: a- U=+3kV, b-U=0, c-U=-3kV