**2.2. Premixed and non-premixed mixture**

PM may work with a premixed flow or with a non-premixed fuel flow. Premixed porous burners consist of two zones: the premixed fuel–air mixture first enters a hot solid matrix, where it is heated until it enters to the second hot solid matrix. Depending on its application, a third section, a compact heat exchanger may be added to the burner. The schematic of a two-layer premixed combustor is shown in Fig. 1.

A premixed mixture of methane-air and hydrogen-air were used in different burners with two section PM. Measurements show considerable reduction in the concentrations of NOx, CO in the fluid gases. Also the effects of hydrogen addition to methane, were investigated. For the porous burners hydrogen was found to lower the NOx emissions slightly, while for the other burners an increase, or no obvious effect, was found. The enhancement of the radiation flux from PM burners operating with non-premixed flames by using a vane-rotary burner, in which the swirling fuel flow was confined by an air duct, is necessary. They also studied emission characteristics of ceramic foam burners operating with non-premixed flame.

**Figure 1.** Schematic of a premixed combustor (M. A. Mujeebu et al. , 2009)
