# Special Issue \"Thermochemical Conversion Processes for Solid Fuels and Renewable Energies\"
## 2 Special Issue Findings
Furthermore, the obtained ashes during the gasification tests were analyzed with X-ray Diffraction (XRD) and Scanning Electron Microscope/Energy-dispersive 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. * The main contribution of the fifth paper is related to a solar-driven air-conditioning system utilizing absorption technology. In this study, the authors [PERSON]; [PERSON]; [PERSON], [PERSON] from the Technical University of Darmstadt, Germany, proposed a solar driven-absorption cooling system as an alternative technology to the conventional air conditioning of a house under hot and dry climate in Baghdad, Iraq [22]. The effect of different parameters on the solar cooling performance was evaluated. The results show that the weather conditions have a crucial influence on the performance of the solar absorption air-conditioning system, with the peak loads during the summer months. The highest performance was achieved in August with an average coefficient of performance (COP) of 0.52 and a solar fraction of 59.4%. The authors claimed that this study provides a roadmap for engineers, showing that all of the operating and design variables should be considered when developing a solar-driven air-conditioning system under the Iraq climate. * The sixth paper included in this Special Issue dealt with an important topic that is now under research investigation as an effective gasification technology. By avoiding the use of the costly air separation unit, chemical looping gasification (CLG, see Figure 1) is a novel gasification method, allowing for the production of a nitrogen-free high calorific synthesis gas from solid hydrocarbon feedstocks (e.g., biomass and refugee-derived fuel). An equilibrium process model for an autothermal chemical looping gasification process of biomass was developed by [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON] at the Technical University of Darmstadt, Germany [23]. The results show that pursuing continuous CLG operation leads to challenges in terms of the oxygen carrier (OC) circulation, which is responsible for both, oxygen and heat transport between the air and fuel reactor. According to the authors, the CLG faces an essential dilemma. Here, higher OC circulation rates are necessary to fulfill the process heat balance (i.e., retain constant temperatures in the fuel reactor), whereas significantly lower circulation rates are required in terms of the necessary oxygen transport. Therefore, two strategies to achieve the autothermal CLG behavior through a de-coupling of oxygen and heat transport were suggested and evaluated. The findings of this study encourage deeper numerical modeling of the chemical looping gasification of biomass, as only through the deployment of elaborate models considering hydrodynamics and reaction kinetics can in-depth inferences regarding the process efficiency be offered. * The authors of the seventh paper, published by [PERSON]; [PERSON], [PERSON]; [PERSON]; [PERSON], [PERSON], presented a combined experimental/numerical study on CO\({}_{2}\) absorption [24]. Here, the effect of pressure on the gas/liquid interfacial area was investigated experimentally in the pressure range of 2 to 3 bar using an absorber ray column test rig, erected at the author's institute. Furthermore, a rate-based model was generated based on the design data of the real test rig. A simulated waste gas, consisting of 30% carbon dioxide and 70% air, and distilled water as an absorbent were used in this work. Two gas flow rates were applied. The results predicted by the rate-based model agrees very well with the experimental data.