# Special Issue \"Thermochemical Conversion Processes for Solid Fuels and Renewable Energies\"
###### Abstract  
The world society ratifies international measures to reach a flexible and low-carbon energy economy, attenuating climate change and its devastating environmental consequences. The main contribution of this Special Issue is related to thermochemical conversion technologies of solid fuels (e.g., biomass, refuse-derived fuel, and sewage sludge), in particular via combustion and gasification. Here, the recent activities on operational flexibility of co-combustion of biomass and lignite, carbon capture methods, solar-driven air-conditioning systems, integrated solar combined cycle power plants, and advanced gasification systems, such as the sorption-enhanced gasification and the chemical looping gasification, are shown. thermochemical conversion technologies; combustion; carbon capture and storage/utilization; gasification; solar-driven air-conditioning; integrated solar combined cycle; energy and exergy analyses; thermodynamic modeling; dynamic process simulation +
Footnote †: journal: Applied Sciences  
10 February 2021 17 February 2021 22 February 2021  
methods, namely, oxy-fuel, pre-combustion, and post-combustion [9]. In the oxy-fuel process, fossil fuel is combusted using pure oxygen with circulated fuse 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 agent (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 CO\({}_{2}\) capture. Two technologies can be used, namely, the chemical scrubbing of flue gas or the carbonate-looping 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 CO\({}_{2}\) 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 great 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].