by Anders Bjorklund, Rolf Gustavsson and Lennart Helmersson
Locally produced biomass has taken over from fuel oil as the main fuel used in the CHP district energy plant which serves the town of Eskilstuna, Sweden. The plant incorporates both circulating and bubbling fluidized bed technology to burn the fuel — as Anders Björklund, Rolf Gustavsson and Lennart Helmersson report.
Eskilstuna is a town of 93,000 inhabitants by Lake Mälaren, approximately 100 kilometres west of Stockholm in Sweden. The company Eskilstuna Energi & Miljö is owned by the municipality and builds infrastructure and supplies the utilities for living and working in Eskilstuna. Electricity, heating, cooling, water and sewage, broadband facilities, refuse handling and recycling services are all provided within a co-ordinated and financially efficient framework that puts the spotlight on environmental needs.
The district heating water network expansion was started in 1969, and today approximately 90% of properties are connected. A total of 750 GWh heat was produced during 2007, of which 55% was delivered to apartments, 10% to one-family houses and 35% to buildings, shops and industry.
Figure 1. Map of the city of Eskilstuna, showing the district heating pipe network
Until the summer of 1982, only fuel oil was used for the district heat production. The replacement of fuel oil was started with the installation of electric boilers and heat pumps. In 1986, a CFB (circulating fluidized bed) hot water boiler was built, which was complemented with a flue gas condensation unit. In 2000, the combined heat and power plant (CHP) started up, consisting of a BFB (bubbling fluidized bed) steam boiler, also with a flue gas condensation unit.
Today the main fuel is biomass, and in 2007 the consumption of biofuel was approximately one million m³, which corresponds to a heat input of 832 GWh. This amount consists of: 6% sawdust, 18% bark and sawmill residues, 74% forest residue (woodchips from branches and tops) and 2% energy forest (salix/willow). Biofuel prices are negotiated on a yearly basis — there are seasonal variations in biofuel humidity, and prices are based on the effective heat value per kWh.
Fuel oil, today only 5% of all fuel fired, is used mainly for peak production during a few cold winter days with very high heat consumption. A small part of the fuel oil is also used when the boilers are shut down and started up again for maintenance and service work.
All electricity production is based on biofuels. The production of heat from 1980 to 2007 used different types of fuel, as shown in Figure 2.
Eskilstuna Energi & Miljö, CHP and the accumulator
In 2007, Eskilstuna Energi & Miljö produced 185 GWh of electricity and 424 GWh of heat in the BFB boiler, and 121 GWh heat in the CFB boiler. Flue gas condensation provided an efficient way for increasing heat production at low costs. In total, 152 GWh was produced by flue gas condensing. For the BFB boiler, 136 GWh of heat was produced from the flue gas condensing scrubber, which is 32% of the heat produced in the turbine condenser. This was the best result ever in producing heat by flue gas condensing.
The overall plant emission values remain below the average in Sweden (see Table 1).
CHP AND OTHER PRODUCTION FACILITIES
The 50 MW hot water boiler combusts biofuel. This CFB boiler operates at 16 bar and 190°C. The flue gas passes through two cyclones, in which particles and bed sand are separated and circulated back to the furnace. Heat generated in the boiler is transferred via a large heat exchanger to the district heating water network.
Capacity was increased up to 57 MW, when the un-cooled cyclones were replaced by cooled ones. The boiler is equipped with flue gas condensation, which further increases the heating capacity by 8 MW.
Experience from biofuel combustion was good from an environmental, as well as economical, point of view. In 1998, half of the energy needed was generated in the CFB boiler, while the rest was produced by means of oil, electric boilers and heating pumps.
Figure 3. The BFB boiler at Eskilstuna Energi & Miljö
In 1998, a decision to build a biofuel-based combined heat and power plant (CHP) was made. It was continuing with the efforts of converting the fuel base from oil to biofuel. It also allowed for utilizing a heating plant for electricity production.
CHP with BFB steam boiler
The electric capacity of the CHP plant is 38 MW. The heating capacity is 72 MW, with an additional 25 MW gained through flue gas condensation. The turbine and generator were supplied by ABB Stal (now Siemens) Finspång, Sweden. The BFB steam boiler, supplied by Kvaerner (now Metso Power), Finland, has a capacity of 110 MW. The steam parameters of this boiler are 140 bar, 540°C, 41 kg/s. It features three-step superheating with two water injections and a water-cooled ‘Hydro Beam’ floor.
Heat production is increased by means of the flue gas condensation units. A flue gas condensing scrubber with a combustion air humidifier is connected to the BFB boiler at the CHP plant. The thermal power of the scrubber is 25 MW, while the district heating water return temperature is 54°C. Higher thermal power can be generated in case the district heating water return temperature is lower. With an inlet flue gas temperature of 139°C, and outlet gas temperature of 30°C after scrubbing, the heat recovery is 22.5%.
The plant is equipped with a hot water accumulator tank, which is connected to the district heating network. This 50 metre high atmospheric tank has a volume of 26,000 m³ for water, with a temperature range of 45°C to 100°C — 1200 MWh of heat can be stored in it. The pressure in the 50 metre high water column also guarantees good water pressure in the district heating network.
OBJECTIVES SET FOR THE BFB BOILER
The most important criterion for selecting a power boiler was high availability of production. Since the town of Eskilstuna is a client of the CHP plant and the majority of the inhabitants are end customers, the supply and distribution of district heating had to be secure.
The amounts of fuel defined in the yearly purchasing contracts must be received and fired at the CHP plant, due to a limited storage capacity. This underlines the importance of high availability. The operation had to be simple and reliable, especially for times when the boiler needed to be shut down.
Electricity production had to be increased, with profit. It had to be optimized so the boiler could be operated as evenly as possible, without any sudden changes, plus the boiler had to comply with the emission limits set by authorities in Sweden and abroad. Safe operation without risk of human error was another criterion in order to secure a continuous supply of district heat.
In order to handle fluctuations in the district heat demand due to changes in the weather conditions, the CHP and the district heating network system required an accumulator tank; this enables electricity and heat production to be kept at a constant level. The occasional peak and decline in the district heat distribution network is handled by un-loading/loading hot (100°C) water from/into the accumulator.
EXPERIENCES AND RESULTS
Planning of operation and maintenance
The availability of the BFB boiler has been good from a time and efficiency point of view. To achieve this, both personnel and technology had to work together successfully.
The personnel at Eskilstuna have an extensive experience of biofuels, mainly wood based fuels — a spin-off from the forestry industry. The fuel includes a mix of sawdust and wood chips, the size of which ranges from a few millimetres up to 50 mm. These different fractions of different sizes and moisture content behave in a different way in the conveyor system of the plant. By mixing different fractions the fuel becomes more homogeneous. This is better for the boiler operation, since achieving optimal combustion and low emissions does not necessitate boiler adjustments.
In order to maintain an even boiler operation without sudden changes and variations, a number of different measures are used in order to ensure optimized energy production. Weather and air temperature prognosis is used as a basis for the production planning. Adjustments are made by means of the CFB boiler and the hot water accumulator, in which heat totalling around 1200 MWh can be stored and utilized when needed.
Figure 4. Flue gas condensing scrubber in the CHP plant
The accumulator enables personnel to plan the power plant operation in such a way that increased heating needed during cold winter days can be obtained from the accumulator without starting up a new boiler. In the autumn and spring, the accumulator adapts to the normal variations in heat demand between night and day without need for power boiler adjustments. Since there is a dependence on the heating plant for electricity production, the possibility to operate the boiler at low power in the spring, summer and autumn is important.
Unfortunately, the fuel also contains coarse material, such as sand and gravel, which wears the conveyors. It is important to take preventive measures to eliminate a need to stop the plant during high season. The coarse material finds its way to the boiler, and is fed out as bottom ash during the operation.
The BFB boiler is equipped with a patented Hydro Beam furnace floor, with water-cooled air beams and fluidizing air nozzles. With the water-cooled air beams it is possible to operate the boiler load down to around 20% boiler loads. The Hydro Beam floor has over 30% free removal space between the air beams. This means that heavier bed and coarse bed ash particles are easily extracted from the fluidized bed in order to prevent agglomeration and sintering problems. Operating with a wide loading range and reliability of the fluidized bed operation has provided a high availability and flexibility for the CHP plant in Eskilstuna.
In biofuel firing with steam temperatures above 500°C, there is a risk of high temperature corrosion of the low-alloyed steel materials of the final super-heaters. For this reason, austenitic steel (AC66) was selected as the material for the BFB boiler’s final super heaters. This material has proven to withstand any corrosion.
There is less refractory lined surface in the BFB boiler than in the CFB boiler. Due to this, there is less need for maintenance work. The low amount of refractory also makes the BFB boiler less sensitive to changes in temperature when the boiler is shut down and started up, so, if necessary, the boiler can be shut down for minor maintenance work and easily started up again without large disturbances in the production.
There is a scheduled yearly production stop of five weeks in the summer: The boilers are stopped for maintenance and service work for four weeks, and the shutdown and start-up process takes one week. There is also another scheduled two-day stop during winter.
In June 2007, in addition to the scheduled production stops, there was also a six-day stop due to hot weather, during which heat production was not needed. The 43 days of scheduled shutdown time leaves a remaining 322 days (7728 hours) available for production. Of these hours there were 48 hours of unscheduled stops due to different types of faults, resulting to an availability of 99%.
The boiler is designed for a fuel humidity range of 35% to 60%. The actual fuel humidity varies between 30% and 55%, and the guaranteed optimal point is at 48%. For the most part of the year the humidity varies from 48% to 52%.
The BFB boiler is equipped with two fuel conveyors in order to secure continuous fuel feed at all times. This minimizes a risk of a shutdown, should there be a disruption in one of the conveyors. The humidity of different fuels may vary a lot, but the quality of the fed fuel is evened out through mixing the different fuel loads that arrive in the plant. Good quality fuels contribute to good boiler efficiency.
Boiler efficiencies with biofuels are generally high (around 90%), and flue gas condensation plant efficiencies around 107% can be achieved. The amount of residue in the form of ashes is relatively low, plus they contain low amounts of unburned carbon, and are non-toxic and relatively inexpensive to store.
Biofuel is a green and renewable fuel. Unlike fossil fuels it does not cause carbon dioxide accumulation in the atmosphere. CHP plants in the EU, using fossil fuels like oil and coal, are forced into the EU’s system for trade with emission permits. With the latest EU and international agreements regarding cutting down the carbon dioxide emissions and the Electricity Certificate, it will be more attractive to invest in CHP plants based on biofuel firing.
There are technologies under development today for carbon capture and storage, in order to create carbon dioxide neutral emissions from fossil fuels, mainly coal. Such technologies may be realized within ten years from now, but these technologies will most probably suit very large CHP plants, since large investments are expected and plant efficiencies are likely to drop by approximately 10%. For smaller CHP plants like the one in Eskilstuna, biofuel firing could still be attractive, even though biofuel prices will go up, as well as the price for electricity and heat.
Anders Björklund and Rolf Gustavsson are with Eskilstuna Energi & Miljö, Eskilstuna, Sweden. Lennart Helmersson is with Metso Power, Sweden.