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Padua waste-to-energy plant

Viale della Navigazione Interna 34, Padua

  • 1 / 1   Padua waste-to-energy plant

"Hestambiente"

The first waste-to-energy plant in Padua was built in the San Lazzaro district in the 50s and became operational in 1962. It was a real innovation at the time: the first Italian plant where energy was recovered. The nominal capacity of the incinerator was 140 t/day and the boiler and thermal unit generated 1.4 MWh/day. A second combustion line with a capacity of 150t/day was built at the end of the 60s and later updated to adapt it to the increasingly more stringent regulations and inspections until it took on its final format in 2000.

The plant currently comprises three modernised incineration lines integrated with the Third Line, which began producing electricity on 18 June 2010.

Page updated 25 August 2015

 
    Number of waste-to-energy lines
    3
    Total thermal capacity
    79.8 MWt
    Combustion technology
    Water-cooled moving grate incinerators
    Waste disposal capacity
    approx. 600 t/d with LCV of 2,750 kJ/kg
    Annual operation
    8,000 hours
    Nominal electric power
    17.9 MWe
    Disposal and recovery codes
    D10; R1
    Type of waste accepted
    Urban waste, special non-hazardous waste and hospital waste with infection risk

    Environmental compatibility in compliance with regulations in force (Legislative Decree 152/06)

    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 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 20,000 m3 for the unloading and storage of waste.
    The waste is moved to the waste-to-energy lines by means of two bucket crane systems operating on a single level of runways.
    The hospital waste is delivered to the incinerator separately from the remaining waste on dedicated conveyor belts, without going via the waste pit.

    2 - Combustion.

    Each line at the plant is composed of an incinerator, a boiler and a combustion fumes treatment system.
    The grate installed on Lines 1 and 2 is an air-cooled inclined moving rungs 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 third line the grate is the horizontal moving plane type, but there is a mixed cooling system, which uses both air and water, and the boiler, which acts as an incinerator, is installed above the grate. In this case, it is possible to speak of an integrated incinerator-boiler system, where energy recovery is higher.
    On lines 1 and 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 maintained for over two seconds at a temperature above 850 °C. Methane-fuelled support burners, which turn on automatically based on the fume temperature, ensure that this temperature is maintained, even if there is waste with a low calorific value.
    On line 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 three burners which, if there is a fall in temperature, are activated to keep the process temperature above the regulatory limits.

    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 Line 3 there is a vertical and horizontal recovery boiler (integrated incinerator-boiler system).

    • the steam generator on Line 1 has a capacity of approximately 18 t/h of steam at a temperature of 380 °C and a pressure of 44 bar;
    • the steam generator on Line 2 has a capacity of 18 t/h of steam at a temperature of 380 °C and a pressure of 44 bar;
    • the steam generator on Line 3 has a capacity of 51 t/h of steam at a temperature of 380 °C and a pressure of 46 bar.

    4 - Fume purification.
    SNCR DeNOx - Selective non-catalytic reduction of NOx

    The SNCR of nitrogen oxide system is similar on all three lines and consists of the injection of urea or ammonia into the post-combustion chamber. This system comes into operation in emergencies, during the start-up phase of the system, following a stoppage, or when the SCR system described below is out of service.

    INITIAL DRY STAGE - Line reactor with calcium hydroxide and activated carbon
    The fumes coming out of the boiler enter a line reactor where they come into contact with the calcium hydroxide and activated carbon, which are injected dry, separately on Line 3 and mixed with sorbalite on Lines 1 and 2 in the current of fumes.
    The calcium hydroxide initially neutralises the pollutant acids. The activated carbon reduces the pollutant substances like dioxins (PCDD), furans (PCDF) and heavy metals.

    FIRST BAG FILTER
    The gases coming out of the line reactor enter a bag filter where the reduction process continues. The bags are made up of a support felt pad which has a micro-porous membrane; both parts are made from PTFE (GORETEX®) which has excellent chemical and mechanical resistance properties, optimising the filtration efficiency and reducing the passage of particulates to a minimum. The bags are cleaned by blowing through the jets located on each bag with compressed air on a cyclical basis. The compressed air jet directs other air inside the bag causing rapid expansion with the consequent detachment of the dust which falls into the hopper. The dust that is collected and has dropped into the hoppers is unloaded onto the conveyor belt located under the bag filter hoppers; this dust, which still contains calcium which has not yet reacted (PCR - Residual Calcium Products), is partly recirculated to the line reactor and partly discharged onto a transport system, which carries it, together with the fly ash, to the dedicated storage system. The recirculation of the dust is designed to improve the efficiency of the reduction system and possibly reduce consumption of the reagent.

    SECOND DRY STAGE - Venturi reactor with sodium bicarbonate and activated carbon
    The fumes coming out from the first stage enter a dry absorption Venturi reactor, where they come into contact with the reagents, made up of sodium bicarbonate and activated carbon.

    SECOND BAG FILTER
    The gases coming out of the dry reactor enter the bag filter where the reactions described above continue. Bag filters, except for where the bags are made from heavier fabric in order to provide greater mechanical resistance and efficiency, are all similar to the one described previously for the first dry stage.

    NOx CATALYTIC REDUCTION (Line 1 only)
    The second bag filter for Line 1 is equipped with special filter bags (still in the development stage) which have a catalytic reduction capacity for the nitrogen oxides. These bags, developed by GORE and used for the first time at the Padua plant, carry out a totally similar function to traditional SCR systems. An aqueous solution of ammonia is injected upstream of the second bag filter in order to reduce the nitrogen oxides.
    Before entering the catalytic filter, the fumes are heated by a steam heat exchanger so they can reach the optimum temperature for catalytic reduction.

    FUME PRE-HEATING SYSTEM (For Lines 2 and 3 only)
    There is a fume/stem heat exchanger downstream of the second bag filter and upstream of the catalytic reactor, which has the task of heating the fumes before they enter the catalytic reduction system. The heat exchanger comprises a battery which is installed in line on the pipe connecting the second stage bag filter with the catalytic reactor. The heat exchanger is supplied with superheated steam.

    SCR DeNOx - The selective catalytic reduction of NOx (Lines 2 and 3 only)
    On leaving the pre-heater, the fumes are sent to a catalytic type denitrification system.
    The reduction of NOx, using an SCR system, involves a dry gas treatment process through the injection of ammonia (NH3) as the reduction agent. The ammonia (NH3) is added to the combustion gases upstream of a catalytic converter and reacts with the NOx on the catalytic bed producing nitrogen (N2) and water (H2O).
    Another effect that can be achieved, when the fumes pass over the catalytic bed, is the final reduction of any traces of PCDD and PCDF still present in the fumes.

    FUME COOLING
    The fumes are brought to a temperature of around 130-140 °C thanks to the outgoing energy recovery, obtained through the pre-heating of the condensates supplied to the degasser. On Lines 2 and 3 the fumes are then sent to the chimney via the induction fan and released into the atmosphere at a height of 80 metres.

    ACTIVATED CARBON TOWER (Line 1 only)
    Line 1 is equipped with a fume treatment system consisting of a filtration system on fixed activated carbon beds. This stage further reduces the organic micro-pollutants. From here the fumes are sent to the chimney via the induction fan and released into the atmosphere at a height of 80 metres.

    5 - Electricity generation
    The steam produced supplies the three turbines, one for each line, which operate the three-phase synchronous alternators for the generation of electricity. The energy produced, purified for self-consumption, is introduced into the national grid at 20kV (MV) and at 135 kV (HV).
    The installed generation power of the system is 17.9 MW.
    The steam discharged by the turbines condenses in shell and tube heat exchangers which use the water from the nearby Piovego canal as a coolant. The condensed water is reused when it re-enters the degasser.

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