Techno-economic study for a gasification plant processing residues of sewage sludge and solid recovered fuels.

The energetic valorisation of wastes through gasification is a promising solution with a better environmental impact in terms of pollutant emissions compared with incineration, landfilling, and heat and power generation from fossil fuels. However, techno-economic studies are imperative to define the viability of these technologies and to optimise heat and power consumptions and costs. This work intended to develop a techno-economic analysis for a small-scale gasification plant processing mixtures of solid recovered fuels and sewage sludge, assuming a capacity of 883 kg/h and two different sale scenarios: (A) production of electric energy, and (B) production of hydrogen. Gasification tests and mass and energy flow analyses were carried out for the economic assessment. The results showed that both scenarios presented viability for implementation. Although scenario A was more attractive in the short-term period due to the lower payback period (9 year) and higher internal rate of return (IRR, 7.5%), the other option was more favourable at the end of plant's life once the net present value was greater (1,801,700 €). Based on the results of a sensitivity analysis, a conclusion could be drawn that the economic indicators payback period and IRR were most influenced by capital expenditures applied in the plant.

Characterization of hydrochar and process water from the hydrothermal carbonization of Refuse Derived Fuel.

In this study, hydrothermal carbonization (HTC) was used as a thermochemical conversion process to upgrade Refuse Derived Fuel (RDF). The effect of process temperature (250 °C, 275 °C and 300 °C), residence time (30 min and 120 min), and RDF-to-water ratio (1:15 and 1:5) on the main characteristics of the produced hydrochars and process waters was assessed. The HTC process yielded hydrochars with enhanced fuel properties when compared to the original feedstock, namely higher carbon content and heating value. The hydrochars also presented reduced oxygen and ash contents. The hydrochar produced at 300 °C for 120 min presented the lowest ash content (3.3 wt%, db) whereas the highest heating value was found for the hydrochar obtained at 275 °C for 120 min (28.1 MJ/kg, db). The HTC process was also responsible for a significant reduction in chlorine concentration, showing dechlorination efficiencies between 69.2 and 77.9%. However, the HTC process generated acidic process waters with high COD values (maximum 27.2 gO2/L), which need to be further managed or valorized. Energy calculations were also performed, revealing that lower water amounts, lower temperatures, and longer residence times, represent optimal conditions for higher hydrochar yields and consequently good process efficiencies.

Torrefaction and carbonization of refuse derived fuel: Char characterization and evaluation of gaseous and liquid emissions.

Refuse derived fuel containing non-hazardous industrial wastes was subjected to torrefaction and carbonization in an industrial furnace. The RDF samples were heated at 300 °C and 400 °C, for 30 min, yielding solid products (chars) as well as gases and liquids. Proximate and ultimate composition, mineral composition, chlorine content and high heating value were determined for the original sample and the produced chars. Thermal treatment produced RDF chars with carbon contents of 61.6 and 80.2 wt%, and high heating values of 19.9 and 23.5 MJ/kg, that could be further upgraded by washing with water to reduce ash and chlorine concentrations and improve calorific value. Gas products were composed of carbon dioxide and carbon monoxide with minor amounts of hydrogen. Methane was only detected in the gas produced at 400 °C. The process generated liquid products rich in organic compounds that represent potential in further energy or material recovery.