**1. Introduction**

Human beings find themselves at the beginning of the 21st century in a contradictory situation in which, on the one hand, significant growth in global demand for energy is expected while, on the other hand, human activities have posed a dangerous rise in the global average temperature by approximately 1.0 ± 0.2 ◦C above pre-industrial levels. Global warming is likely to reach 1.5 ◦C in the period between 2030 and 2050 if the consumption of fossil fuels continues to increase at the current rate [1]. It is generally accepted that a grea<sup>t</sup> share of greenhouse gas emissions is anthropogenic and originated from utilizing fossil fuels, with contributions coming from manufactured materials (e.g., concrete), deforestation, and agriculture (including livestock). Societies around the world actively support measures towards a flexible and low-carbon energy economy to attenuate climate change and its devastating environmental consequences. These measures include process improvement, new thermochemical conversion technologies, such as gasification or combustion of alternative energy sources, such as biomass [2,3], implementation of carbon capture and storage/utilization technologies [4,5], and promotion of renewable energy sources for power generation and district heating or cooling [6,7], as briefly described below:


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• methods, namely, oxy-fuel, pre-combustion, and post-combustion [9]. In the oxy-fuel process, fossil fuel is combusted using pure oxygen with circulated flue gas to obtain lower adiabatic combustion temperature. The generated flue gas consists of carbon dioxide, where the steam can be easily separated by a condensation process. The main drawback is separating oxygen from air using an air separation unit that is energy-intensive [10]. The chemical-looping process is considered an energy-efficient oxy-fuel method [11,12]. Solid particles of metal oxide are applied as oxygen carriers and these particles circulate between two coupled fluidized beds, namely, air, and fuel reactor. In the pre-combustion method, the solid fuel is gasified using steam and oxygen as a gasification agen<sup>t</sup> (usually at higher-pressure levels in a fluidized bed system or an entrained-flow gasifier). The produced gas consists essentially of hydrogen, carbon monoxide, carbon dioxide, and trace gases. Using a gas-cleaning unit, the carbon dioxide and the trace gases can be separated and the producer gas can be converted into value-added chemicals or combusted in a combined-cycle power plant (integrated gasification combined cycle (IGCC)) [13]. The post-combustion approach has the advantage that existing processes can be retrofitted with CO2 capture. Two technologies can be used, namely, the chemical scrubbing of flue gas or the carbonatelooping process. The latter uses limestone as a solid sorbent, circulating between interconnected fluidized bed reactors (carbonator and calciner) [14].

•The increased use of renewable energy sources (e.g., biomass, wind power, and photovoltaics) contributes to a decrease in CO2 emissions in the power generation sector. Through the substitution of fossil fuels by using alternative energy sources such as refuse-derived fuel (RDF), solid recovered fuel (SRF), tire-derived fuel (TDF), and sewage sludge, a considerable reduction in emissions can be further achieved [15]. The electrification of heating and transport sectors offers also a grea<sup>t</sup> opportunity for achieving zero emissions. However, variable renewable energy sources can lead to a seemingly paradox situation of negative electricity prices at times of high renewable electricity output and/or low demand, as well as peak electricity prices at times of low renewable electricity output and/or high demand. To maintain the security of supply, there are several potential solutions such as the expansion of high-voltage transmission infrastructure, the use of flexible power plants with CCS/U technologies, and the implementation of large-scale energy storage [16]. The solutions differ in their potential impact, technological maturity, and economic viability so that according to the opinion of authors, the future electricity system will contain all of these concepts to varying degrees with the possible integration of value-adding processes beyond electricity such as the power-to-fuel technology. The carbon-neutral fuels (e.g., hydrogen, methane, gasoline, diesel fuel, or ammonia) can be generated from renewable energy sources by the electrolysis of water to make hydrogen that hydrogenates carbon dioxide or nitrogen captured from thermal power plants or air.

According to the above background and in support of the development of thermochemical conversion processes for solid fuels and renewable energies, this Special Issue contains nominated contributions to:


The Editors are pleased to bring the best and recent advancements in this field of research to the scientific community in this compact, peer-reviewed Special Issue. Manuscripts that included the latest research progress in terms of development and optimization of conversion processes and concepts, especially for intermittent renewable energy sources, with thermodynamic analysis, CFD and process simulation of these systems were submitted and reviewed by recognized and expert reviewers. In the Special Issue, manuscripts of high quality and that made an explicit contribution to the technical and scientific knowledge were accepted, highlighting the main developments and the new findings. Accordingly, 10 papers were accepted and published in this Special Issue. All articles can be accessed freely online.

#### **2. Special Issue Findings**

In the following, a summary of the accepted papers with their most relevant contributions is illustrated.


ating conditions [21]. Olivine was used as bed material and the steam/fuel ratio was maintained at approximately 0.65. The influence of temperature and air injections in the freeboard was evaluated in terms of the conversion efficiencies, gas composition, and tar produced. Furthermore, the obtained ashes during the gasification tests were analyzed with X-ray Diffraction (XRD) and Scanning Electron Microscope/Energydispersive X-ray Spectroscopy (SEM/EDS) analysis, and an affinity between calcium and sulfur was reported. The authors stated that the increase in the operating temperature leads to an improvement of the gas quality and a lower amount of tar produced. The experiments with air injections in the freeboard did not result in the desired effect on tar reduction. Compared to other tests performed with biomass at similar operating conditions, the amount of tar produced was, however, lower.


decrease in the gas/liquid interfacial area and thus decreases the absorption rate of carbon dioxide.


**Figure 1.** Schematic of the chemical looping gasification process.

**Figure 2.** Simplified flow diagram of the 1 MWth pilot plant at the Technical University of Darmstadt.

**Figure 3.** Schematic diagram of sorption enhanced gasification (SEG) process (up to 750 ◦C) and extended steam gasification mode (up to 850 ◦C); option of oxy-fuel operation.
