This paper aims to briefly overview gasification technologies of biomass and heterogeneous wastes as a means for syngas production. For this purpose, an overview of the existing technologies, their main advantages, limitations, and costs, as well as commercial plants and projects (lower TRL) operating with these technologies and syngas applications is presented. The type of technology and operating parameters should be selected considering the quality of the syngas as it will dictate its end use. Syngas quality is determined by the combination of feedstock properties, type of technology and process operating conditions, and the scale of operation. For smaller projects with a capacity of up to 10 MWth, fixed-bed technologies have been a recurring choice, while fluidized bed reactors can have an installed capacity above 100 MWth and are, therefore, more suitable for medium- and large-scale projects. Fluidized bed gasification technology supports feedstock flexibility, has scale-up potential, and presents relatively low cost, making it a suitable solution and a frequent choice for heterogeneous waste gasification in medium- or large-scale projects. Commercializing waste gasification technology is already a reality. However, more efforts need to be made so that pilot and demonstration projects can overcome the technological and economic problems and move towards commercialization.

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A large part of municipal solid waste (MSW) still goes to landfills, representing an environmental concern. A circular economy approach can enable safe management of MSW while mitigating the increasing energy needs when waste is used as a feedstock in energy production processes (waste to energy). Currently, MSW can be converted into refuse-derived fuel (RDF) through mechanical and biological treatment processes. This study analyzes the status of MSW and RDF production, as well as its main destinations in Portugal and Europe. The legislation applied, possible energy-recovery routes, and challenges associated with energy recovery are discussed throughout this paper. This research finds that the production of RDF in Portugal has been neglected, mostly because of RDF composition being quite heterogeneous and its poor fuel properties. Therefore, the need to improve and upgrade the characteristics and properties of RDF for waste-to-energy processes was detected. RDF can be pretreated to be further applied to waste-to-energy and waste-to-gas processes, such as incineration and gasification. The technology readiness level data, costs, and SWOT analysis allowedto assess that although incineration is the most mature and widely used technology, gasification becomes more attractive, having lower costs and gaseous emissions, proving to be more efficient and sustainable for MSW and RDF conversion.

co-gasification experiments with mixtures in various proportions of waste wood (WW) and sewage sludge (SS) were performed in a downdraft gasifier, in order to evaluate process performance and properties of gas and chars. Addition of SS improved the energy conversion efficiency and product gas yield, and minimised the formation of tars. Char production was greater, with formation of agglomerations inside the reactor. The optimal waste mixture contained 87.5 wt.% WW and 12.5 wt.% SS, generating a product gas yield of 2.9 m3/kg waste with a calorific value of 4.8 MJ/m3, and a tar production of 4.7 g/kg waste.