From waste incinerator to power plant

Siemens KWU has won a contract to convert the Bratislava municipal waste incineration facility into a cogeneration plant. The new plant will be designed for 7500 annual operating hours and it is to be capable of burning up to approximately 134 000 tonnes of waste per year, with a design heating value of 8250 kJ/kg.

The existing plant has three “waste lines”, one of which will continue in operation until completion of the reconstruction project. The upgraded plant will have two waste lines, each rated at 10.9t/h.

Process steam from the plant will be extracted for use in a refinery and as district heat for greenhouses. By installing a GK 26/40 industrial turbine in the steam cycle, the incinerator will generate 5.4 MWe, in addition to carrying out its function as a waste incinerator.

The contract for the conversion of the waste incineration plant, which is operated by the municipal waste disposal company Odvoz a Likvidacia odpadu (OLO), is worth around 60 million euro (DM 117 million). The Siemens scope will include supply of a package-unit industrial turbine from the Gorlitz manufacturing plant, a Teleperm XP I&C system and all the electrical equipment, while local companies will be extensively involved in civil engineering and erection activities.

Main components

Turbine

The Siemens KWU GK 26/40 industrial steam turbine is designed to handle a main steam flow of up to 27.7 t/h.

The turbine is constructed as a package unit, in which all of the associated components and systems are mounted on a single baseframe.

The baseframe of the unit also incorporates an oil tank located near the turbine. The turbine has a horizontally-split outer casing. Its HP valve chests and the control valves are connected to the upper section of the outer casing.

The turbine rotor is a monobloc forging that is supported by two plain bearings that are supplied with oil under pressure. Blading comprises:

One row of control stage blading.

Multi-stage low-reaction drum-type blading.

Standard low pressure stages.

The blades of the control stage and final stages fit into slots and are fixed with pins, while moving blades in the high pressure stages have interlocking inverted T-roots. Outer shrouds are provided on the blades of these stages.

The blades of the standard low pressure stages are free-standing, twisted conical blades fitted with damping wires.

Turbine gearbox

A single-stage gearbox is situated between the turbine and generator in order to step down the high turbine shaft speed to the low speed of the four-pole generator. This gerabox is of a single-stage design. Pinion and spur gears have double-helical involute gearing in order to cancel out axial thrust.

Generator

The generator is of a four-pole design, with both the stator and inductor windings being indirectly air-cooled. The thermal losses of other generator parts are also removed by air cooling.

Air-cooled condenser

The air-cooled condenser consists of the following:

A-frame condenser elements.

Cooling air fans.

Condensation cambers, each comprising several condenser elements.

Steam supply lines.

Condensate lines.

Air extraction line to the steam-jet ejector.

Steel support structure.

Grate firing/slag treatment

Charging

Waste charging equipment comprises a charging hopper, which is connected to a water-cooled charging chute and one charging ram per grate.

Reciprocating grate

The most important step of the waste treatment process occurs on the reciprocating grate.

The fuel travel is changed from the vertical to the horizontal. With a designed grate inclination of 26°, the reciprocating gate ensures that there is a good mixing of the materials to be combusted with matter on the grate which is already burning.

Owing to the characteristic motion of the reciprocating grate, burning matter is throughly tumbled, compounding the four classic process steps: drying, degassing, ignition and combustion. Combustion on the grate is concluded by the time the waste has traversed approximately two thirds of the grate’s length. There is a clear transition from combustion to burnt-out zone. Matter to be combusted is transported independently of the grate’s motion by the force of gravity which augments the effect of tumbling.

Unlike other firing systems, it is only possible to maintain constant dwell times with a reciprocating grate which intensively tumbles the fuel. As a consequence of this, the reciprocating grate ensures that there is a consistently high combustion quality and optimal burnout, despite the pronounced fluctuations in fuel quality which are inherent to the burning of municipal waste, which is of variable composition.

Boiler

The first boiler pass (radiant section) is located directly above the combustion grate, to which it is connected with a gas-tight expansion joint. The design of the three-pass radiant section ensures that there is a long flue gas dwell time in order to ensure that there is both complete combustion and cooling of flue gases to below 650°C before they reach the convection section.

The convection section of the boiler comprises the superheater, evaporator and economiser tube bundles.

Connection of the primary superheater and secondary superheater heat exchange surfaces in concurrent flow and reheater in counterflow minimises the susceptibility of these items to corrosion.

Using ample tubing in the convection section, with these tubes arranged in an in-line pattern, and reduced bundle depth, it is possible to guarantee optimal utilisation of heat-exchange surfaces.

Convection surfaces are cleaned by rapping gear in the superheater/evaporator region, and by brushes in the economiser section.

The boiler is suspended in a steel support structure, which ensures that thermal expansion can be accommodated.

Flue gas cleaning

The flue gas concept includes one effluent-free spray absorber for cleaning the waste incineration plant flue gas.

This flue gas cleaning concept reliably ensures that there is compliance with the imposed emissions limits that are given in the table of design data for the flue gas cleaning plant.

Flue gas cleaning equipment includes the following items per line:

Two spray absorbers, in which the flue gas and suspension are in cocurrent flow. The injection of lime slurry and cooling water is achieved via nozzles that are carefully designed in order to be able to handle the two-phase flows. These injection nozzles are mounted on lances. The pollutants HCl, HF and SO2 are absorbed by the injected lime slurry.

Adsorbent. A fine-grained additive mixture containing blast furnace coke is injected into the flue gas flow either between the spray absorber and the inlet of the fabric filter, or directly into the spray absorber. This promotes the adsorbtion of the organic compounds and heavy metals. The adsorbent is then removed in the downstream fabric filter together with the mixed salts.

Fabric filter. The mixed salts that are contained within the flue gas are removed downstream of the spray absorber by the fabric filter. The raw gas is then distributed to the individual chambers of the baghouse. A vertically-suspended cylindrical fabric filter is connected to each chamber, and the raw flue gas flows from the outside in. Various salts collect on the outside of the fabric filters, while the cleaned flue gas passes inwards. The cleaned flue gas then exits the baghouse via a header duct.

Induced-draft fan.

Lime silo shared between the two lines. This silo is designed to be able to supply both lines, containing adsorbent for at least 240 hours of full-load operation. The absorbent (CaO) is quenched with water in the slaking tank, and then fed to the lime slurry tank. Water is added for further dilution in the lime slurry tank to achieve the concentration required by the process. Lime slurry is atomised in the spray absorber.

SNCR process with NH3 water system

To meet NOx emissions requirements, a deNOx system was implemented which employs selective non-catalytic reduction (SNCR). Nitrogen oxides are reduced by injecting ammonia water into the combustion zone. A pump forwards this reagent via a pipe to a stationary storage tank farm. The capacity of these tanks is sufficient to operate both combustion lines under normal load conditions for 14 days. An ammonia water pump station with reciprocating pumps is near the storage tanks, and forwards ammonia water via a distribution station near the injection points.

Each combustion line is equipped with a dosing unit at the inlet of the distribution station. A control valve is used to automatically regulate the flow of ammonia water to the boiler outlet.

SNCR was chosen because it offers high operational reliability and availability.

Electrical systems

The separate components of the electrical equipment perform the following tasks:

Interconnection of the plant with the 22 kV grid.

Distribution of electrical power for unit auxiliary supply.

Generation of electrical energy.

Monitoring of components of unit auxiliary supply system.

The unit auxiliary supply system is interconnected with the 22 kV grid via two transmission lines. The terminal points of supply in each case are the terminals at the transfer point on the old OLO auxiliaries.

The waste incineration plant is supplied with electrical power from the 22 kV grid during startup and shutdown, and when the turbine generator is at a standstill.

The turbine generator feeds the electricity to a set of 6 kV auxiliary supply switchgear. The 22 kV switchgear is interconnected with the 6 kV switchgear via a 7 MVA oil-filled transformer.

A two-train system is provided for unit auxiliary supply. The 400 V main distribution board consists of two feeder breakers and one tie breaker, with appropriate interlocks.

Main components

22 kV switchgear

The 22 kV switchgear interconnects the waste incineration plant with the power grid. The switchgear is made up of the following cubicles: two cable feeder cubicles; one tie cubicle with instrumentation; three transformer bays with fused interrupter; one transformer bay with power breaker; and one cable feeder cubicle as reserve cubicle.

6 kV switchgear

The 6 kV switchgear distributes the energy and consists of the following cubicles: one generator feeder cubicle with power breaker; one transformer outgoing feeder cubicle with power breaker; one metering panel with outgoing cable to ground fault reactor; two motor circuit cubicles with power breaker; and one motor circuit cubicle with power breaker as reserve cubicle.

400 V switchgear.

The LV switchgear is implemented as metal-enclosed, isolated-phase indoor single-bus switchgear.

The switchgear for outgoing feeders is mounted in withdrawable units in order to give a modular arrangement. All the power and control leads are fitted with connectors to permit the replacement of the withdrawable units without needing to de-energise the busbar. When equipment is in the test position, the power contacts are isolated from the circuit to allow the switchgear to be tested safely.

The protective devices comprise: thermal trips, fuses, motor protection switches or power breakers. Faults on the busbar and the outgoing feeders result in group alarms in the control room. Resolution of the group alarm into individual alarms can be performed locally at the switchgear.

Combustion air system

The combustion air system essentially comprises of the primary and secondary air systems. Due to their optimised interaction, combustion is highly uniform and produces only low levels of pollutants.

Primary air intake is via the waste pit, preventing the annoyance of fugitive odours in the plant environs.

After preheating, air is fed through the grate which distributes it as required by the combustion process.

Slag removal system

The slag tap is used for the removal of combustion residues. A water bath quenches the slag, and in the process, prevents the ingress of leakage air into the combustion chamber. Slag removal has low makeup water requirements and it involves neither drip and dust losses nor unpleasant release of heat or odours.

Slag treatment

Slag which is ejected from the slag tap slides onto a conveyor belt which transports it to the existing slag pit. A hopper, to be fed by the existing slag crane, will be installed adjacent to the slag pit. A transverse conveyor belt will be provided downstream.

Two ferrous metal separators ensure that these materials are retrieved from the slag. Separated ferrous metals are conveyed to an outside storage area, and the remainder of the slag is conveyed inside the existing structure by another conveyor belt for loading onto dump trucks.

Less residues, lower emissions

Relative to the old plant, the amount of solid combustion residues will be reduced by a factor of 50 per cent. The new flue gas cleaning system is designed so that not only existing but also future, more stringent, EU emissions standards will be fulfilled on plant start-up, which is scheduled for May 2002.
Tables

Flue gas cleaning design data for the Bratislava plant