Via Errera 11, Trieste
The city of Trieste, currently served by the plant known as "Errera 3", has a very long tradition of waste management using energy recovery plants.
The local story in the daily newspaper "Il Piccolo" on 23 February 1915 was already talking about the inauguration of a "waste incinerator", which for the modest sum of 1 million crowns was already designed in those days for energy recovery.
After a long break, in 1972 the new Giarizzole incinerator was launched, which served the city of Trieste until the end of 1999. In the meantime, legislative changes induced the Trieste Municipality to speed up the building of new waste incineration plant with energy recovery, which incorporated the best technologies available.
The "ERRERA2" waste-to-energy plant was constructed on two incineration lines with a capacity of 204 t/day each and a thermal cycle dedicate to energy recovery serving a 4.85 MW steam turbine capable of producing 32 GWh/year of electricity. This plant made it possible to respond to the needs of an area which, because it is densely populated, limited in size and includes a limestone plateau region, made it and still makes it exceedingly difficult to locate a landfill there.
"Errera 3" was the natural development from "Errera 2". The original plant was modernised and altered by constructing a third line (also with a capacity of 204 t/day) for waste treatment and an entirely new thermal cycle to cover all three energy recovery lines servicing a 14.9 MW steam turbine capable of producing 90 GWh/year.
Page updated 25 August 2015
The waste-to-energy plant makes it possible to dispose of a variety of waste through combustion. The heat this generates is exploited to produce heat and electricity.
1 - Waste receipt and storage.
The waste enters the plant after passing through the radiometric gate and the entrance weighing station to calculate the gross weight; the vehicles unload the waste into the pit and leave the plant after being weighed a second time to determine the unladen weight.
The plant is equipped with a pit with a capacity of around 10,500 m3 for the unloading and storage of waste, of which 7,000 m3 can be exploited. This area is sealed and kept at a constant vacuum thanks to the intake of the primary combustion air produced. There are 2 bridge cranes inside the pit with a capacity of 10 t each, both equipped with 5 m3 capacity polyp-grabs, which are operated by trained waste management crane operators.
2 - Combustion.
The section relating to the waste-to-energy plant is composed of three separate waste disposal lines each with a theoretical capacity of 204 t/day making a total capacity of 612 t/day (with a reference LCV of 2,280 kcal/kg - 9,544 kJ/kg).
Each line is composed of an incinerator, a boiler and a combustion fumes treatment system.
The grate installed on Line 2 is Martin technology, air-cooled horizontal type and the incinerator is the semi-adiabatic type with a reduced area for the membrane-lined walls in order to optimise energy recovery, which essentially takes place in the recovery boiler installed downstream of the incinerator.
On the first and third lines the grate is also the Martin technology, horizontal moving plane type, but it is a development of Line 2: there is a mixed cooling system, both air and water and, the boiler, which acts as an incinerator, is installed above the grate. This is known as an integrated incinerator-boiler system, where energy recovery is higher.
On line 2 at the combustion chamber outlet the gases are directed towards the post-combustion chamber where the oxidation reactions begun earlier are completed. The fumes are kept at a temperature of 850 °C for more than two seconds, as required by Legislative Decree 152/06.
On lines 1 and 3 the temperature control takes place in the actual combustion chamber (there is no proper physically separate post-combustion chamber, only a post-combustion area) where there are two burners which, if there is a fall in temperature, are activated to keep the process temperature above the regulatory limits.
A burner (one for each line), running on methane, ensures the starting and stopping stages so that the minimum temperature is 850 °C before the waste is introduced and it guarantees it for the entire time the waste is present.
3 - Steam generation.
Downstream of the post-combustion chamber of the incinerator for Lines 1 and 2 there is a vertical recovery boiler; above the grate on Lines 1 and 3 there is a vertical and horizontal recovery boiler (integrated incinerator-boiler system).
4 - Fume purification.
The fume capacity is approximately 50,000 Nm3/h for each of the three lines. The fume treatment is broken down, for each line, into different phases:
5 - Cogeneration of electricity and heat.
The plant is equipped with a single steam turbine serving the three lines coupled with an alternator designed to produce electricity at 10 kV.
The theoretical gross electricity power generated is 14.9 MW. The three incineration lines operate independently of one another in order to guarantee the incineration process if there is a stoppage at one of them. The thermal cycle, in addition to the main condenser, is equipped with an auxiliary condenser which enables the steam produced at the three lines to be absorbed even if there is no turbine in order to be able to fulfil the waste disposal function if the turbine is out of service.
The condensation of the steam takes place through an evaporation tower supplied by mains water.
Until September 2013, the electrical configuration of the plant involved the total transfer to the 27.5 kV grid of the energy produced, while all the energy needed to cover the consumption of the plant was taken from the AcegasApsAmga external 10 kV grid. From October 2013, this configuration was changed and the current arrangement involves the electricity produced supplying all the plant consumers with only the excess transferred to the 27.5 kV grid. As a result energy is no longer drawn from the 10 kV grid.