Summary
The site in Psyttaleia
A significant chance to utilize biomass in Greece is the exploitation of biogas produced in the biological treatment plants in which a part of the produced biogas is used to cover heating requirements during the biological treatment processes and another is used in the CHP plant.
One such example is the CHP plant on biogas in the wastewater treatment plant Psyttaleia serving the capital city of Athens both situated on a small island located at 1500 m distance from the mainland coastline. The biogas is being produced from the sludge digesters at a daily rate of 72 000 Nm³/day and can be used for producing 64 GWh of useful energy per year. The project includes biogas burning in specially designed turbines for electricity production, and the associated heat coming from the flue gas and the cooling water circuit of the turbines which will be used for sludge heating (inside the digesters) and drying (of the final product).
The produced electrical energy will be consumed for satisfying the site consumption, and any surplus energy will be sold directly to the grid.
Results
The environmental benefits arising from the realisation of this project are significant in terms of air emission reduction. The daily methane (CH4) emissions will be reduced from 20 000 Nm³ to 0,2 Nm³, hydrocarbons emissions from 120 Nm³ to 0,2 Nm³ when carbon monoxide (CO) will be held below 650 mg/m³ and NOx below 500 mg/ m³. In addition to the reduced or avoided air emissions a significant reduction of solid wastes volume will be realised as dewatering and sludge drying will help in reducing its volume by a factor of 0,8. Currently the sludge is being deposited to Athens main landfill at Liosia, which is facing significant capacity problems. The project will also create 20 new jobs on site contributing hence to the acute unemployment problem of the wider area.
Lessons learned and repeatability
During the design and construction phase a number of issues have been raised which are briefly described below. At the early stages of the project significant issues arose as this was the first power plant of that kind ever installed in Greece and in addition the nominal power output was quite significant. The solution given was to send EYDAP personnel abroad to UK and Denmark where they had accumulated experience from existing similar type power plants. The location of the power plant and the wastewater treatment facility is quite unique as they are both situated on a small island located at 1500 m distance from the mainland coastline. Although part of the electrical energy production is consumed locally for the site needs, the energy surplus, which is a significant part of the overall production, is sold to the grid. This itself created a significant technical barrier, which had to be overcome namely the construction of an underwater cable connecting the island with the grid at the mainland and the associated control system. A under water cable connection was constructed with an XLP insulated 355 cable and a second one 3x 120 with paper insulation. Initially a small building was constructed on the island containing the hardware necessary for system control, and in addition for coupling and synchronising the generation facility with the grid. At a later stage PPC (the Public Power Company which was at that time the grid sole owner and supervisor) requested a second similar facility to be constructed at the mainland; a request, which was also fulfilled from EYDAP. PPC also constructed a cable connection between a nearby combined-cycle natural gas fed power plant and the control centre located at the mainland which made possible to control the whole island based generation system from distance (i.e. the system is now fully automated and unmanned).
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Case Study (95 Kb PDF)