Di Lauro, Francesca (2023) Energetic valorisation of sludges from civil and industrial sources. [Tesi di dottorato]

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Abstract

Technological and economic development, population growth and the progressive urbanization of vast areas of the planet have caused an increase in global energy demand. The aim of this PhD Thesis is the energetic valorisation of a particular type of biomass, i.e., sludges from civil and industrial sources, aiming at the reduction of the material destined to disposal, and for their reuse as energetic vector. In particular, two types of sewage sludge were selected for these studies: i) a sludge from tannery industry, ii) a municipal sludge. Among the thermochemical valorisation paths, the energy recovery of this waste has been first investigated through fluidised bed (FB) gasification, where the target product is syngas. In fact, there is a limited literature concerning the FB gasification of sludges, in particular when they come from specific industrial sources. Then, the innovative process known as hydrothermal liquefaction (HTL), where the target product is a bio-oil, has been applied to sludges. Both represent “StE” technologies (“sludge-to-energy”). For the gasification tests, the tannery sludge was considered. With its 34% C-content (dry basis), the material is appropriate for energetic valorisation. Since the sludge came from the tannery industry, particularly relevant was its chromium content. In the parent material, we did not observe the harmful hexavalent Cr species. Under the best conditions, a syngas with 42% H2 (dry and N2-free basis) and a lower heating value of 12 MJ/kg was obtained (in line with values commonly reported for syngas for energetic purposes), with a limited tar concentration and the production of bottom ash whose gasification degree was, for all practical purposes, complete. The Cr(VI) concentration in bottom and fly ashes was not higher than 10 mg/kg, while the Cr(total) concentration was 3−4 orders of magnitude greater, showing the right choice of operating conditions that did not promote the oxidation of Cr(III) to Cr(VI). Degrees of chromium oxidation in ash ranged, as a function of the operating conditions, from 0.008% to 0.120%. For HTL, activities were conducted to limit thermal transients, to reduce the extent of collateral reactions that could lead to the increased formation of co-products. In addition, the effect of separation method of products, reaction time and temperature on products yield (Y) was assessed. Results showed that the upgrades to the heating system allowed to attain an average heating rate of about 8°C/min, determining improved performances vs. configurations characterized by lower heating rates. The centrifugation method was more effective, with respect to liquid-liquid extraction, yielding a bio-crude recovery up to 5.2% greater. The analysis of the effect of time and temperature on products yield has showed that higher temperatures, coupled with short reaction times, favour the distribution of products towards bio-oil formation. At 10 min and 350°C, an optimal value of both Ybio-crude=43% and energy recovery of 59% was obtained (higher heating value HHV=30 MJ/kg) for the municipal sludge (Ybio-crude=29.5% and energy recovery of 44% was observed for the tannery sludge, HHV=36 MJ/kg). The results obtained show that both processes are promising for the energy valorisation of sludge from both civil and industrial sources, and in the specific case of tannery sludge it is possible to limit the formation of hexavalent Cr in the reaction products. The product of interest obtained is in the case of gasification a syngas useful both as an energy carrier and as chemicals, while in the case of hydrothermal liquefaction a liquid fuel, easily integrated into the production chain of liquid fuels in the transport sector.

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