## 1 Introduction  
According to the sustainable development scenario, the use of biofuel produced by biomass for power production is expected to increase and provide 2% of the total world supply by 2030, equating to 4% within the Organization for Economic Cooperation and Development area (OECD) [1]. Due to the emerging policies for the reduction in greenhouse gas emissions, lignocellulosic biomass gasification coupled with solar photovoltaics [2] is classified as one of the renewable energy sources with the potential to replace conventional fossil fuels [3, 4]. Gasification is a complex technology that converts biomass into a synthesis gas called syngas. The use of syngas for power generation is a growing industry with high potential for reducing greenhouse gas emissions [5]. Biomass gasification for power generation has been implemented in developed countries across Europe, North America, and Asia in the form of multiple small-scale biomass gasification systems [6]. However, the potential for uptake of biomass gasification is more promising in developing countries where access to the electrical grid is possibly limited and the cost for the necessary extensions of the existing electrical network might be economically unviable [7]. Gasified biomass is a green and sustainable resource that can play a significant role in the context of a Circular Economy (CE). The CE has emerged as an opportunity to achieve meaningful decarbonization across the value chain where the three R's related to this concept (reduce, re-use, and recycle) aim to make the best possible use of society's resources. This can be accomplished through the utilization of biomass from raw materials to produce syngas, which can support the achievement of CE goals and prove beneficial to a range of biomass producers including forest owners. Hence, biomass gasification has the potential to turn waste into valuable green fuels [8]. Despite the benefits of gasified biomass, there are some challenges that still must be overcome. Mainly, the unpredictable availability of biomass resources and a lack of uniformity in its composition, in addition to the existence of a wide range of apparatus designs and set-ups, many of which are still at the research stage, are among the notable challenges [9; 10]. Moreover, previous studies showed biomass gasification as a technology not yet mature enough for large-scale applications and is still in need of costly and time-consuming research to achieve stable behavior and high performance [11]. Due to these factors, governmental subsidies and incentives are still crucial for the upscaling of syngas gasifiers' development [12]. In particular, the fixed-bed downdraft gasifier configuration is a well-established technology, characterized by the production of syngas with low tar content [13] but is difficult to scale up and better suited to small-scale power generation applications [14]. The influence of operational parameters are still the principal factors that need to be analyzed in order to achieve a stable and efficient process [15]. This research study explores the technology of biomass syngas production by using an experimental fixed-bed downdraft gasifier coupled to a two-cylinder engine. Further-more, it studies the experimental and analytical relationships between the driving thermodynamic parameters that control the gasification process, in order to contribute to the development of a theoretical model for the design of a small-scale gasification facility while helping to overcome the remaining technological barriers to its development. In-deed, the gasifier used in this research was built at the IIE-UPV lab as reactors of this kind are difficult to be found commercially.