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Μass and energy balances of novel system for the exploitation of the energy content of biosolids through gasification

Tsamoutsoglou Konstantinos

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Summary

Wastewater Treatment Plants (WWTPs) cause according to their planning requirements direct environmental impacts due to energy use, chemical consumption and excess sludge production. Consequently, there is a growing interest in optimizing the relationship between energy and wastewater quality, as well as in finding innovative technologies aiming to convert waste into energy resources.The scope of this thesis is to assess the sustainability and energy self-sufficiency of a novel system for the exploitation of the energy content of biosolids through gasification, as well as to calculate the mass and energy balances with the view of protecting the environment. The system will be installed at the WWTP of Rethymnon, comprising of 3 stages (microscreening, drying and gasification) and it will treat about 1/3 (5,000m^3) of the inlet flowrate of the above WWTP. The synthesis gas (syngas) which will be produced during gasification, will be used for the production of thermal and electric energy through an internal combustion engine. The produced energy is expected to cover the energy needs of the pilot plant, thus further reducing the net energy consumption of the WWTP, and consequently eliminating the greenhouse gases emissions. The determination of the balances of the system has been carried out in two stages. The first stage is referred to the analysis of mass balances of the microscreen, while the second one to the analysis of mass and energy balances of the drying unit and gasification plant with simultaneous production of electric and thermal energy.The solution of the microscreen balances aimed to find the estimated removal rates of Biochemical Oxygen Demand over five days (BOD5), Chemical Oxygen Demand (COD), and Total Suspended Solids (TSS), as well as to determine the expected amount of produced biosolids which will be used as gasification feedstock. The results showed that the estimated effluent concentrations of TSS, BOD5 and COD from the microsieve range from 55-164 mg/L 290-309 mg/L , and 520-558 mg/L , respectively, while the production of biosolids (on a dry basis) is expected from 7 to 18 (kg-PSS)/h. On the contrary, the solution of the balances of the second stage aimed to check the energetic self-sufficiency of the demonstration plant. In this case, two scenarios were considered (worst and best case scenario). For the first scenario, it was found that the energy produced would marginally cover the drying needs and for this reason two alternatives were proposed to ensure the energy self-sufficiency of the system (recycling of hot air and consumption of generated electricity). However, for the second scenario the excess thermal energy was calculated 61 MJ/h and the electricity 21.5kW, which will be both used to cover the energy needs of the other parts of the plant, maintaining its energetic self-sufficiency.

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