The state of the art in biosolids gasification.

Biosolids is a by-product of wastewater treatment that needs to be further processed. Traditional biosolids treatment and disposal technologies are inefficient under the current demanding standards. Thermochemical conversion technologies have been employed for biosolids management, with gasification being the most promising due to the production of syngas, a gaseous product that may be used for the production of energy or high-added-value substances through reforming reactions. Gasification is a complex thermochemical process; its performance and yield are strongly affected by the type of feedstock, but also by the system configuration and process conditions. Gasification usually takes place at temperatures between 700 and 1,200 °C, but it may also occur at lower temperatures (above 375 °C: supercritical water gasification) or at higher temperatures (above 3,000 °C: plasma gasification). The present review briefly presents the biosolids management practices, focusing on the gasification process and syngas treatment, while the state of the art in biosolids gasification is critically presented and discussed. A number of types of gasifiers (more frequently fluidized bed, but also fixed bed, rotary kiln, downdraft, etc.), gasifying agents, and operational conditions have been used for biosolids gasification. The key results of the study regarding biosolids gasification are: (i) the increase of temperature and equivalence ratio enhances the gasification performance, resulting in high syngas yield and quality, high cold gas efficiency, and low tar and char production; (ii) the calorific value of the obtained syngas tends to decrease with the increase of equivalence ratio; and (iii) the use of catalysts has been proven to substantially improve the gasification performance, compared to non-catalytic gasification. The proper selection of technical parameters determines the effectiveness of biosolids gasification, which is considered as a promising technology for the energy recovery from biosolids, so to upgrade wastewater treatment and improve environmental quality.