Description of the material group.
The current material group includes plants to remove sulphur dioxide from flue gases produced in thermal power plants using coal combustion and/or fuel oil, based on a process with wet limestone and gypsum in an open spray absorber with a capacity of at least 1000000 Nm3/h in-situ oxidation of sulphites/bisulphites to calcium sulphate (sadur).
Flue-gas desulphurisation facilities shall be provided in full together with all mechanical and electrical automation equipment, the scope of the works shall include engineering, production, testing and supervision of both the workshop and the site activities.
Construction and commissioning activities may or may not be included in the scope of the works; in so far as it is not included, the contractor must provide all the information and instructions necessary for the third party to carry out these activities and must check and supervise by its own personnel that these activities are carried out in accordance with defined regulations.
Main technical characteristics.
Each of the exhaust gas desulphurisation systems must contain the following:
— absorber, a shower tower technology based on the damp limestone-sather process, with in-situ oxidation of sulphites to sulphates and recirculation of the absorption fluid prepared from limestone-kium suspension (the completion of the oxidation of sulphites/bisulphites into sulphates is also required for the absorption liquid). The efficiency of the removal of sulphur oxides and the maximum permitted concentrations of other impurities in purified (particulate) gases will be defined by Enel in the bidding dossier for each specific project. In general, the requirements will follow the best performance line that can be achieved by the process;
— absorber auxiliary equipment: flue-gas tubes and flaps, pipes and valves, oxidation air blowers, stirrers, pumps, plugs, dehumidifiers, etc.
Main recirculation absorber pumps and gypsum drainage systems (hydrocyclones and vacuum belt filters) may and may not be included in the supply range, depending on the requirements of the specific design; if they are not part of it, the contractor is required to develop the relevant detailed engineering, which will be made available in the ENEL tender documentation (Technical specifications of the order) and to provide full technical technical assistance to ENEL during the ordering, construction and commissioning phase;
— support structures,
— electrical and automation systems.
In general, supply/removal fans from/to the absorber, wet limestone shredding system, waste water management, buildings (but functional design to be provided) are excluded from the scope of the works.
Functional and production characteristics of the material group:
— flue gas desulphurisation must be designed with a lifetime of 25 years, 8 000 hours per year for operation and a large number of cold start-ups per year.
The operation of the flue gas desulphurisation plant shall preferably be automated from the power plant surveillance plant and shall require minimum operator intervention.
In the event of failure of the individual active components, the flue gas desulphurisation device shall be capable of operating correctly in accordance with the guaranteed performance. Therefore, all active components operating in normal operation must be represented by backup.
To the extent possible, component failures shall be recognised by the automated system and the deployment of the backup component shall be automated.
In general, the guaranteed performance must also be achieved with one closed recirculating absorber pump and without the use of chemical additives other than limestone. However, depending on the specific project, the addition of basic organic acids may be required to increase the efficiency of SO2 removal and/or to reduce energy consumption.
It shall be possible to change the operating number of the main recirculation pumps and the corresponding shower planes in order to manage the different concentration of the incoming SO2.
the pH of the absorption suspension must be controlled by injecting limestone into an absorber at less than 5.6; in particular, the absorber injection of limestone must be controlled, in addition to the pH-based feedback mode, by way of forward control, taking into account the gas flow and the SO2 concentration and their changes during transition conditions, in order to reduce pH fluctuations in the absorber and thus the passage of SO2 to the outlet of the absorber.
The concentration of chlorides should be controlled by the run-off of water directed to the waste water management to the maximum rate, which, depending on the specific project, can be up to 50000-60 000 ppm.
Absorber and components in contact with process media shall be fully corrosion/erosion resistant and selected by the supplier in accordance with the guidelines given in the ENEL Technical Specification. The main materials used are: corrosion resistant metal alloys with a high content of Cr-Mo, polypropylene, rubber, vinyl ester resin lining, GRP, silicon carbide.
Absorber.
Guidance plates and/or routing rods at the inlet to the absorber and/or finger on the absorber walls must be installed in such a way as to ensure an even distribution of the gas flow to the liquid/gas contact zone and to prevent the phenomenon of bypass/overflow of the flue gas inside the absorber.
The tank shall be designed to minimise the deposition of particulates inside the absorber: The shape of the absorber, the location of the stirrers, etc. must be designed in such a way as to avoid dead areas.
For both of the reasons mentioned above, fluid dynamics must be optimised within the engineering stage using fluidised dynamic models.
As far as possible, the combustible gas velocity inside the absorber shall be selected in order to minimise the amount of moisture flowing downstream through the drop separators.
The absorber shall be equipped with a discharge rated to discharge the full flow of air blowers when flaps in the flue gas supply line are closed. The discharge shall be automatically opened, whenever the flaps are closed and high pressure is produced. Air blower shall be stopped whenever high pressure occurs. In addition, the absorber must be protected against overpressure by other appropriate devices.
Emergency cooling system.
The system shall provide for water injection into the absorber estuary to ensure a sufficient reduction of the gas temperature, assuming an inlet gas temperature passage and a failure of the main circulation of the suspension. The water shall be stored in an increased reservoir at atmospheric pressure or in an air-pressure reservoir.
The amount of water to be stored and sprayed must be determined assuming a conventional full nominal gas flow rate for the entire duration of the temperature passage.
Oxidation and blowers.
The blower head must be designed with a suitable margin (around 2 m H20), taking into account the maximum level of suspension and density.
The amount of air required for sulphite oxidation must be determined assuming a maximum SO2 content and a minimum O2 content in the flue gas.
The operating time elapsed with the blower stopped shall not exceed 15 seconds.
Oxidising air distribution system
Complete oxidation of sulphites to sulphates is required for the whole circulating suspension.
Before distribution to the suspension, oxidation air must be cooled by saturation with high-quality industrial water. The temperature of the outflow of shower water shall be monitored.
Air should be injected into a suspension through oxidation pipes or by another system that ensures perfect air distribution to the suspension (e.g. wide-open pipes prior to the stirrers), creating a dispersal of air bubbles in the suspension.
Each distribution line shall be equipped with continuous air flow measurement and alarm for low values detection.
Injector tubes shall be equipped with shut-off valves and connections for compressed air/pressure water in the event of a pipe wash if locking occurs (a manual system is also acceptable).
An alternative oxidation system configuration may be proposed, provided that these alternative solutions are very well verified, with a reference record of long operating time, and that the oxidation air distribution network is located in the liquid zone above the liquid interface/fixed phase resulting from solid deposition.
Stirrers.
Absorbers, tanks and waste pits containing a suspension with more than 1 % solids shall be equipped with mechanical stirrers, powered by motor.
For absorber and suspension containers without reserve, the number and location of the agitators shall be defined in such a way as to ensure, in the event of failure of a single stirrer, the other minimum mixing intensity necessary for the continued operation of the plant. Repair of the damaged stirrer (engine and seal) shall be allowed during the operation of the absorber/container.
Waste pit stirrers will be individual without reserve and emergency mixing will be provided with air.
The stirrer blades should be stored in the liquid zone above the liquid interface/fixed phase resulting from solid deposition in case of shutdown of stirrers. In any case, the design of the stirrer shall consider the maximum torque when starting with shovels covered with fixed material.
Drop separators.
Each absorber must have a moisture eliminator consisting of two or three degrees (depending on the maximum residue of the liquid required by the specific project).
The profile of the dehumidifiers shall ensure that solid deposition is minimised and allow full penetration of water from the shower nozzles used for cleaning.
A homogenous gas velocity throughout the entire section of the dehumidifiers at each stage shall be ensured to the extent possible.
The drop separator shall be designed with an appropriate margin between the average ‘front velocity’ at maximum gas load and the ‘portable speed’ (the rate above which particles separated by the moisture eliminater are transmitted).
Shower nozzle planes and main circulating pumps.
Shower nozzles shall be arranged on planes, each at different heights. An alternative solution with two planes at the same height may be accepted, provided that guaranteed performance can be achieved at the MCR with any pump and/or bench out of service.
The distance between each level shall allow the access of maintenance personnel. The layout of the shower nozzles at each level shall ensure that the shower nozzles overlap and complete coverage of the absorber section.
The shower nozzle system and the recirculation pump system shall be designed to ensure a constant flow to the nozzles under all operating conditions (i.e. failures of any pump, partial load operation with reduced number of pumps, lower SO2 inlet concentration than the design value, MPR with all pumps in operation).The system shall be designed to ensure correct operation under conditions where at least 3 % of the shower nozzles, randomly distributed on different planes, are not working correctly (completely clad nozzles or totally broken).
The contractor may optimise the head of each pump. In this case, all pumps shall be of the same type and shall differ only with the blade wheel.
The size of the pumps shall be defined by the supplier in conformity with the maximum value indicated by Enel in the tender documents.
Filters shall be installed on the suction side of the main recirculation absorber pumps.
Lime suspension dosing system
Two backup circuits must be provided (only one will be operational, the shutdown must be left empty).
Two backup lime suspension dosing stations, each connected to both circuits, shall be installed for each absorber. Each station shall consist of a control valve and shut-off valves. Back-up flow measurements shall also be provided.
The limestone suspension should be dosed automatically, taking into account its density and the calculation of the required amount based on the gas flow, inlet and outlet concentration of SO2. If the pH value of the suspension inside the absorber deviates outside the fixed range, the control of the addition of the limestone suspension shall be corrected accordingly.
Gypsum suspension extraction system.
The gypsum suspension shall be extracted with dedicated pumps (1 in operation + 1 reserve for each absorber) and delivered to the gypsum drainage station via loop circuit pipelines (1 for each absorber), maintaining the extraction pumps continuously in operation and diverting the gypsum suspension to the drain station or back for recycling to the absorber, to check the density of the suspension inside the absorber.
The loop circuit flow rates and diameters shall be so selected that the maximum/minimum suspension rate ratio at each loop point is not higher than 2/1 under all operating conditions (including full recirculation of the gypsum suspension into the ABS).
If the extraction system is switched off, the whole loop pipe shall be flushed automatically with water.
Drainage of gypsum.
The system shall consist of hydrocyclone (primary and secondary) arrays for solid phase concentration intensification and separation of fine solid phase particles and vacuum belt filters.
A vacuum pump shall be reserved for the belt filter, supplemented by the necessary accessories. The vacuum pumps shall be of the closed-circuit type.
The belt filters shall operate at variable speed to check the thickness of the filter cake on the belt measured with the appropriate instrumentation. The belt shall move on a tension drum and drive drum controlled by an electric motor controlled by an electronic frequency transducer to achieve different speeds. A monitoring system shall be provided for the complete automatic control of the belt speed, as a function of gypsum densification and operation time of the filter cycle.
The vacuum shall operate throughout the width of the belt through the drainage system made of grooves and holes. A system with effective vacuum tightness and low friction is required.
A hopper shall be installed at the inlet of the filter to ensure the homogeneous distribution of the gypsum based on the hydrocyclone system to the belt filter.
The gypsum cake formed on the belt filter must be washed two stages with industrial water to reduce as far as possible the content of soluble impurities in the gypsum.
After unloading of the cake, the filter fabric shall be washed with industrial water from each side using a shower-type body washing machine system, sliding over the entire length of the rubber belt.
Codes and standards.
The reference codes and standards for the installations covered by the scope of the work are given as follows:
— materials: EN, UNI, A8ME,
— pressure equipment: I8PE8L, PED;
— electrical equipment: IEE, EEI, NEMA,
— examinations and tests: A8ME (PTE 40), IEE,
— firefighting: UNI, NFPA.
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