Industrial furnace is one of the most energy intensive processes in Europe and it has been the focus of multiple researches to address improvements in the competitiveness energy, environmental and cost performance. One of the alternatives is the reduction of fossil fuel consumption, such as natural gas, promoting its substitution by an alternative fuel (Co-firing). This paper gathers a wide evaluation of the possibilities of substitution of natural gas. A revision of the types of syngas with potential for use has been carried out, as well as the evaluation of the physical properties of the mixtures from the combustion operation point of view. Three types of syngas from biomass gasification (BSG) and three from process off-gases (POG) have been considered. The feasibility study has taken into consideration parameters such as the lower heating value, the Woobe index, adiabatic flame temperature, air preheating temperature, total flue flow and laminar flame velocity. Regardless of the type of syngas, significant increase in volumetric fuel flow occurs in all cases. This behaviour is directly related to the LHV of the syngas, so that using syngases with lower LHV than the NG will always produce an increase of the fuel flow. A weigh matrix has been obtained in order to evaluate the viability of burners use with the syngas/natural gas mixture. It has been considered a burner commonly used in ceramic sector and it has been concluded that relevant modifications are necessary in order to consider the combustion of syngas/natural gas. The modifications have been evaluated using CFD simulations to stablish the proper design of the mix zone to ensure the mixture between the air, syngas and natural gas. This evaluation has been carried out following the fuel to air ratio. (C) 2019 The Authors. Published by Elsevier Ltd.