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Cogeneration: Secure and Sustainable

Alf Thomas
Mott Macdonald Ltd., London, UK

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In January 2000, PSEG Global, through Polish subsidiary Elcho Sp. z.o.o., concluded a contract with a consortium of Foster Wheeler Energia oy of Finland and Foster Wheeler Energia Polska Sp. z.o.o. for the turnkey engineering, procurement and construction of a combined heat and power (CHP) plant in Poland.

Located adjacent to an existing plant in Chorzow near Katowice in Upper Silesia, the plant will enter commercial operation in 2003. It will eventually replace the existing plant, which is currently being operated by EC Chorzow and comprises two coal-fired steam boilers. The existing plant exports 140 MWth of thermal heat to the local district heating company.

Construction of the new plant began in late 2000. It will burn coal supplied from nearby mines and will satisfy environmental requirements through the use of circulating fluidized bed (CFB) boilers, which use limestone to control sulphur dioxide (SO2) emissions and relatively low combustion temperatures to control nitrogen oxide (NOx) emissions.

The new plant will involve construction of two new power blocks, each consisting of one CFB boiler and an extracting/condensing steam turbine. During the heating season when the power station will be supplying heat and electrical power, each power block will supply 180 MWth of heat to the district heating system and 65 MWe to the local grid. During the summer when the sets will be fully condensing, each power block will export 102 MWe to the grid.

Each power block will be able to operate independently from the other. In addition, each unit will have a peak capability of exporting 250 MWth of heat to the district heating system.

PSEG Global selected engineering consultants Mott MacDonald as owner’s engineer for the project and Foster Wheeler as the engineering, procurement and construction (EPC) contractor. As owner’s engineer, Mott MacDonald, together with its sub-consultants TechWise of Denmark, will carry out project management, design review, QA audits and works inspection, supervision of construction, commissioning and acceptance tests.

Before the EPC contractor was appointed, Mott MacDonald carried out the technical evaluation of tenders and assisted in the final contract negotiations. In addition, Mott MacDonald has provided technical assistance to Elcho during its negotiations with Polish companies for coal, limestone and raw water supplies, ash utilization, sewage disposal and transportation of coal, limestone and ash.

Coal handling

Coal will be delivered to the new plant by rail. The coal stockyard will have a storage capacity for 100 000 t of coal, equivalent to 30 days’ supply.

Each boiler will have two 100 per cent capacity coal crushers located between the coal stockyard and the coal day silos to deliver a coal product in the size range required. Electromagnetic separators will detect and remove metals, and two sets of screens will separate oversize material from the delivered coal to protect the crushers and bypass undersize coal around the crushers. The coal will then be transported by conveyors to the reversible tripper conveyors to distribute coal to the boiler bunkers.

Boilers

The two identical Foster Wheeler boilers will be of the natural circulation drum type and will, at the maximum continuous rating (MCR), produce 420 t/h of live steam flow at 538à‚°C and 135 bar with feedwater at a temperature of 233à‚°C.

The boilers will operate over a range to enable the turbine generator output to be varied from 30 per cent to 100 per cent of turbine maximum continuous rating (TMCR), when firing coal only, under automatic control. The boilers will be able to maintain the live steam temperature over the load range from 70 per cent to 100 per cent TMCR.

The ‘Compact’ design selected by Foster Wheeler is based on the boilers at the 70 MWe Jaworzno power station in Poland which burns coal similar to the coal that will be burned at Chorzow. The boilers have integral water-cooled cyclone separators, which will reduce the overall footprint of the boiler compared with the classical external cyclone designs. In addition, the boilers will not require expansion joints and will have thinner refractory than required by traditional designs. Both these features are expected to reduce maintenance and start-up times. The boilers will be top supported.

The superheaters will be located in the convective rear pass section of the boiler together with a non-steaming, bare tube economizer and a horizontal tubular type air heater. The design includes a superheater located in the solids return part of the compact separator. This design of superheater was chosen for this application due to the chlorine content in the coal and the corresponding potential for fouling and corrosion. Half-track type sootblowers will be located in the convective pass to keep the superheaters, economizers and air heaters free from ash fouling.

The boilers will use limestone that will be fed into the combustion chambers to control SOx emissions and the relatively low combustion temperatures will limit NOx emissions. In addition, one electrostatic precipitator will be provided for each boiler to ensure particulate emissions are within environmental limits.

Steam turbine

The steam turbines are designed specifically for extracting/condensing service in the various operating modes to support district heating operation. The steam cycle will incorporate six stages of feed water heating (including deaeration) to optimize efficiency. Steam not extracted from the turbines will be condensed in surface condensers which are, in turn, cooled by water from two forced draft cooling towers.


Figure 1. The new plant will consist of two new power blocks, each consisting of one CFB boiler and an extracting/condensing steam turbine
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The steam turbine will be supplied by Siemens, with the design based on their established industrial modular range of turbines using reaction type blades.

The complex single turbine casing comprises a front high pressure/intermediate pressure (HP/IP) section of horizontally split casing type, bolted to a low pressure (LP) section of steel construction.

Within the main outer casing, the HP section, including inlet steam control stage, will be mounted within a reverse flow inner casing. This arrangement is used so that the outer casing can be designed for HP section exhaust conditions, rather than HP inlet steam conditions.

The turbine will have a control stage towards the LP end of the split casing and will be a rotary ring type with hydraulic actuation. The extraction steam connection for the district heating will be upstream of the control stage, with the control system permitting extraction pressures ranging from 1.02 bar to 3.45 bar depending on required level of district heating.

The single flow LP section of the turbine will be mounted within the fabricated section of the casing, with exhaust steam flowing downwards to a conventional under slung condenser. The last stage turbine blades will be 700 mm in length.

Electrical system

The grid system to which the new plant will be connected is considered reliable. However, the plant will be designed to shut down in a safe and controlled manner in the event of a grid black out, without undue distress to the boilers and turbines.


Figure 2. Foster Wheeler Energia Oy and Foster Wheeler Energia Polska will construct the plant in Chorzow, near Katowice
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The output from each 10.5 kV generator will be connected by an isolated phase busduct, generator circuit breaker and 142 MVA transformer to a 110 kV line. This line will be partly overhead and partly buried and will connect to an existing 110 kV substation approximately 800 m from the new plant. The generators will have a nominal voltage output range of +/-5 per cent from 10.5 kV, and the step-up transformers will include on-load tap-changers to accommodate for larger changes to the 110 kV system requirements.

The provision of generator circuit breakers will allow power to be imported for commissioning and plant start-up from the grid via the step-up transformer and unit transformers. When the generators are available to export power they will be synchronized to the grid by closing the generator circuit breakers. Thereafter increasing turbine-generator output will reduce the imported power from the grid supply and result in the export of power to the grid.

The unit transformers will be directly connected to the 10.5 kV system of the generator circuit breakers and will have a nominal ratio 10.5/6 kV with off-circuit tap-changers and ONAF cooling. Each will be rated at 31.5 MVA to supply the total load of the common services and the auxiliary load of both units. This will provide redundancy in case of failure of a unit transformer.

The unit transformers will be connected to a 6 kV board that can be coupled by a bus-section switch. This 6 kV system will provide supplies to motors above 200 kW and the auxiliary transformer supplies to the 400 V system.

Control and instrumentation

Supervisory control, protection and monitoring of the new plant will be implemented using the latest generation distributed control system (DCS). Normal operation of the plant will be carried out from a central control room (CCR) and will be highly automated so that staffing levels in the CCR and on the plant during normal operation will be optimized. The DCS will feature a diverse hierarchy in which the CCR operators will be able to initiate plant operations from high level automated sequences down to manual intervention with field devices.

Protection of the boilers, turbine generators and other systems will be implemented either by means of stand alone systems or the DCS will coordinate unit protection. Stress monitoring of the boilers and turbines and vibration monitoring of major drives will be implemented to ensure that thermal stresses are acceptable and that plant life monitoring and predictive maintenance can be carried out.

District heating metering and dispatch will be integrated with the DCS to enable the plant to meet the demands of the district heating system. The DCS will also handle thermal load shedding and steam load levelling functions. Approved heat meters will be provided for custody transfer metering.

District heating

District heating water will be heated in the turbine heat exchanger by feeding extraction steam into the shell of the heat exchanger. The district heating water will circulate through the tubes of the heat exchanger.

A peak heat exchanger will be used to produce district heating during peak load situations (i.e. when the total district heating load is greater than 180 MWth per unit), in the event of a fault with the steam turbine and during the start-up of the system. During peak load operation the net thermal output of the unit will be 250 MWth.

In the peak heat exchanger district heating water will be heated by live steam via the pressure reduction valve. The pressure on the steam side of the peak heat exchanger will be used as the set point for the pressure reduction valve, which will in turn be determined by the required supply temperature of the district heating water.

District heating water will be circulated in the Chorzow network by five identical pumps connected in parallel that will be common to both units. During peak output four of the pumps will be in service with one in reserve. The reserve pump will be able to replace any of the operating pumps.

The volume changes of the district heating water in the network will be accommodated by water stored in the expansion tank, which will be common for both units. In order to avoid oxygenation of district heating water, the expansion tank will be kept at a slight over-pressure by a steam dome in the expansion space.

The pressurizing system maintains the required static pressure in the network. The static pressure in all parts of the network will be higher than the evaporation pressure of circulating district heating water plus a margin of about 10 m of water head.

The static pressure in the network will be maintained by running one pressure maintaining pump all the time and controlling pressure with feed and release control valves. One pump will be capable of pumping the required amount of make-up water during normal operation. The set point of static mean pressure is computed based on the requirements of the district heating network.


Coal and the environment: a balancing act

Poland has an expanding economy and is in the process of restructuring and reforming its energy industry. Its abundant reserves of coal provide a secure source of energy and foreign exchange, but heavy reliance on coal is also a major source of environmental pollution.

In April 1997, the Polish government passed a new Energy Act, which spells out long term energy forecasts and action plans for the Polish government. Key objectives include: increased security of energy supplies and diversification of sources; increased competitiveness for Polish energy sources in domestic and international markets; environmental protection; improving energy efficiency; and reducing energy-related emissions. Coal is Poland’s most important domestic energy source, and while its production is declining, it will remain a key energy source.

Coal is the dominant fuel in Poland’s economy, accounting for 94 per cent of primary energy production and over 65 per cent of total consumption in 1998. Together, hard coal (mostly bituminous) and brown coal (lignite) provide nearly all of the fuel consumed in Poland’s power plants. Coal exports, which go primarily to customers in Europe and the former Soviet Union, historically have been a major source of foreign exchange.

In May 1998, Poland announced a comprehensive restructuring programme for the coal industry aimed at maximizing efficiency and paying off some of the industry’s $4.5 billion debt. In March 1999, intentions were announced to shut down 15 hard coal mines over three years. In addition, nine mines are to be partially closed by 2002. The changes brought about by the coal restructuring programme have positive economic and environmental implications, which are important for Poland’s accession to the European Union (EU).

With installed electric capacity of 30 GW and 1998 electric generation of 135 000 GWh, the Polish power generation sector is the largest in Central and Eastern Europe. The Polish government expects electricity demand to grow by over 50 per cent by 2020. Poland’s electricity generation facilities are highly polluting and operate with outdated technology.

Poland’s status as an ascending EU member makes it more important that efficiency and environmental goals are met. In November 1998, Poland ambitiously committed to adapting its electricity market regulations to EU standards within four years. Renovation of the sector is expected to cost $15 billion by 2010. Multilateral lending institutions, most notably the World Bank and the European Bank for Reconstruction and Development, are involved heavily in financing and participating in projects ranging from building new, non-coal facilities to providing cleaner technologies for existing coal-fired plants.

Environmental issues have become increasingly important in Poland. During the 1980s, Poland was one of the most polluted countries in Europe, and while democratic reforms have brought about reductions in the level of air pollution, there remains much room for improvement. The EU has spotlighted Poland’s environmental record, making the country’s accession contingent on improvements in Poland’s environmental record.

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