| Summary: | This review considers the selective studies on environmentally friendly, combustion-free, allothermal, atmospheric-pressure, noncatalytic, direct H<sub>2</sub>O/CO<sub>2</sub> gasification of organic feedstocks like biomass, sewage sludge wastes (SSW) and municipal solid wastes (MSW) to demonstrate the pros and cons of the approaches and provide future perspectives. The environmental friendliness of H<sub>2</sub>O/CO<sub>2</sub> gasification is well known as it is accompanied by considerably less harmful emissions into the environment as compared to O<sub>2</sub>/air gasification. Comparative analysis of the various gasification technologies includes low-temperature H<sub>2</sub>O/CO<sub>2</sub> gasification at temperatures up to 1000 °C, high-temperature plasma- and solar-assisted H<sub>2</sub>O/CO<sub>2</sub> gasification at temperatures above 1200 °C, and an innovative gasification technology applying ultra-superheated steam (USS) with temperatures above 2000 °C obtained by pulsed or continuous gaseous detonations. Analysis shows that in terms of such characteristics as the carbon conversion efficiency (CCE), tar and char content, and the content of harmful by-products the plasma and detonation USS gasification technologies are most promising. However, as compared with plasma gasification, detonation USS gasification does not need enormous electric power with unnecessary and energy-consuming gas–plasma transition.
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