Abstract
This review considers the selective studies on environmentally friendly, combustion-free, allothermal, atmospheric-pressure, noncatalytic, direct H2O/CO2 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 H2O/CO2 gasification is well known as it is accompanied by considerably less harmful emissions into the environment as compared to O2/air gasification. Comparative analysis of the various gasification technologies includes low-temperature H2O/CO2 gasification at temperatures up to 1000 °C, high-temperature plasma- and solar-assisted H2O/CO2 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.
Highlights
Modern society is faced with the problem of clean processing/utilization of organic wastes
The literature review indicates (Tables 2 and 3) that the main bottlenecks of existing allothermal, atmospheric pressure, noncatalytic, direct low-temperature H2O/CO2 gasification technologies of carbon containing materials (CCMs) consist in low-quality syngas due to high content of tar and CO2, low gasification efficiencies due to high char residues, difficult in-situ gas quality control due to the need in long Residence time (RT) of feedstock in the reaction zone, and low yields of syngas due to low gas yields, high tar and char contents and partial use of syngas for the production of heat required for gasification in the existing dual fluidized-bed (DFB) gasifiers [140]
The literature review (Table 4) indicates that the main advantages of existing allothermal, atmospheric pressure, noncatalytic, direct plasma and solar high-temperature H2O/CO2 gasification technologies of CCMs consist in high-quality syngas due to negligible or low content of tar and CO2, high gasification efficiencies with conversion efficiency (CCE) attaining 100% due to negligible or small tar and char residues, easy in-situ gas quality control due to relatively short RTs of feedstock in the reaction zone, and high yields of syngas due to the use of electric or solar energy for the production of heat required for gasification
Summary
Modern society is faced with the problem of clean processing/utilization of organic wastes. Thermal processing of these materials is considered the most suitable solution due to relatively low environmental impact and partial recovery of energy and material resources. Available technologies of thermal processing are based on combustion/incineration, pyrolysis, and gasification, as well as on their combinations [1,2,3,4]. Combustion is the transformation of the matter due to overall exothermic self-accelerating chemical reactions induced by molecular/turbulent mass and energy transport. Pyrolysis and gasification usually involve endothermic thermal degradation of the matter in the absence/presence of gasifying agent, respectively. A mild form of pyrolysis, torrefaction, is another emerging technology aimed at improving the energy density, calorific value, and grindability of feedstocks by their heating in the temperature range of 200–300 ◦C [5]
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