Repowering Elektrownia Szczecin for the next century of service
For almost a century the Elektrownia Szczecin plant has supplied energy to citizens of this Polish harbour city. At the end of December 2011 it commissioned the largest biomass-fired combined heat and power (CHP) plant in the country. With Finnish state-of-the-art biomass combustion and fuel handling technology it has replaced coal and is set to radically reduce its environmental impact while continuing to service its clients.
Situated on the mouth of the River Oder on the Baltic Sea coast, Szczecin is Poland’s second largest city in terms of land area. Located on the left bank in the harbour district of the city, the Elektrownia Szczecin CHP plant is part of the PGE Zespół Elektrowni Dolna Odra S.A (PGE ZEDO) group. PGE ZEDO is in turn owned by PGE Górnictwo i Energetyka Konwencjonalna S.A (PGE GiEK), itself a wholly owned subsidiary of Polska Grupa Energetyczna SA (PGE), a state-owned company. With its own hard coal and lignite resources, PGE is the largest vertically integrated power utility in Poland in terms of energy production and installed capacity. PGE ZEDO owns and operates three large combined heat and power (CHP) plants in the region with a total combined capacity of 586 MWth and 1.5 GWe. Two of these plants, Elektrownia Pomorzany and Elektrownia Szczecin are located in Szczecin. The latter, with its 183 MW thermal capacity is the smallest but the most modern of the three.
Fuel handling challenge
To say Elektrownia Szczecin is small is slightly misleading. The PLN 430 million (≈ EUR 100 million) investment into the 162 MWth and 68 MWe capacity biomass CHP is currently the largest of its kind in Poland. It has a design capacity to use up to 800 000 tonnes of biomass fuels consisting primarily (80 percent) of forest residue chips with the remainder made up of agro-fuels in the form of short rotation coppice (SRC) chips and straw pellets. There is no on-site facility for roundwood handling, storage or chipping.
The contract for the fuel receiving and handling facility went to Finnish company BMH Technology. The plant has both railway siding and port handling facilities on site. However these are not used since under current Polish regulations the fuels need to be sourced from within a 50-100 km radius, imports are not permitted. Instead the entire volume is trucked to the plant placing additional demands on one of the largest solid biomass receiving and handling facilities in Poland. Furthermore the system had to have two dedicated receiving stations and storage areas, a 30 000 m3 open area for forest chips, one 6000 m3 silo for SRC chips and one 2 000 m3 silo for straw pellets. The storage system is designed to keep the plant running for five to six days without any new supplies of fuel.
Unloading is to below-ground receiving pockets at a specified receiving station on site, depending on the biomass type. Pocket discharge is carried out by chain reclaimer. Each supplied batch is sampled according to the rules set out by the power plant. Unloaded material is screened in order to separate metals and oversize fraction out. Accepted material is directed to one of the silos or to open air storage. All three storage units have automatic screw reclaimers and from storage, material is transferred via a series of belt conveyors to the 160 m fuel infeed conveyor.
Fuel flexibility with BFB
The heart of the plant is the bubbling fluidised bed (BFB) boiler supplied by Valmet (then Metso), which also delivered the plant automation system. Existing flue-gas treatment, including an electrostatic precipitator along with steam turbine, were used. The BFB combustion technology is designed to use wet low-grade fuels such as peat, pulp and paper mill sludge, and agricultural and forest residues yet maintain high boiler efficiencies. The combustion principle is based on the fluidisation of a sand bed in the lower part of the furnace. Here the fuel and hot bubbling sand are mixed together. In contrast to a circulating fluidised bed (CFB) the sand remains as a one-metre deep bubbling layer on the bottom of the furnace.
The fuel is metered from four infeeds, two on either side above the bubbling bed. The high heat capacity of the sand bed enables rapid fuel ignition while managing fluctuations in fuel quality. The hot bed dries and volatilises the fuel. The volatile gases and fine fuel particles are combusted above the bed where secondary air is introduced, whereas larger particles and the residual char are combusted within the sand bed. According to Valmet its BFB technology enables a combustion efficiency of over 99 percent and boiler efficiencies typically in the 90 percent region depending on the fuel used.
A century of service
The first power plant in the city was opened in 1889 and an electric tram public transport system was established 1897. Construction of the original Elektrownia Szczecin plant began in 1911 and it was ready in 1916. During the 1930s it had a thermal capacity of around 85 MW. After WWII the plant was acquired by the Polish authorities. During the 1950s it underwent its first major modernisation and upgrade. This included installing its first grate-fired boiler and first pulverised coal (PC) boiler. In 1966 a decision was made to convert the power station to a combined heat and power (CHP) plant and a decade later Elektrownia Szczecin had an installed capacity of 320 MWth and 47 MWe. In 2000 the turbine was upgraded to increase the electrical output giving an installed capacity 162 MW th and 68 MWe. In 2007 plans were announced for a new dedicated biomass CHP unit to replace the coal boilers. At the end of 2011, after almost a century of using coal the new 183 MWth biomass CHP was commissioned.
The city currently has two district heat networks, a 240 km network on the left-bank to which both the PGE-owned Elektrownia Pomorzany and Elektrownia Szczecin are connected, and a smaller 46 km network on the right-bank of the river. Though the two networks are not connected work is underway to join them with a new 150 000 tonne-per-annum waste-to-energy plant due to be commissioned later this year.