THE ANALYSIS REPORT OF PLANT NO. 3 Cofiring of biomass - evaluation of fuel procurement and handling in selected existing plants and exchange of information (COFIRING) - Part 2 Stora Enso Fors Ltd Sweden Evaluated by Bengt Hillring, SLU
1. General information about the plant Stora Enso Fors Ltd is a card board mill located in Fors, 150 km northwest for Stockholm, Sweden. The plant is owned by the company Stora Enso Ltd, which is a Swedish/Finnish owned company, one of the largest in the pulp and paper industry. Paper products have been produced at the mill since 1950. The company owns an integrated combined heat and power plant (CHP) of the paper mill and supplies the mill with process steam and electricity. There are four different boilers within the mill: one circulating fluidised bed (CFB) boiler using biofuel and coal, two oil fired boilers at 12.5 MW and 65 MW and an electric boiler at 20 MW. The main CFB boiler, studied in this project, was delivered in 1985 by Ahlstrom (currently owned by Foster Wheeler) with the following data for combined heat and power production: - Boiler capacity 55 MW th electrical output 9.6 MW e (back-pressure steam turbine). - Steam data: 20 kg/s, 60 bar, 475 C. - Process steam taken off at both 12 bar and 4.5 bar. - Bed area: 275 m 2 The mix of fuels used in the mill, as a whole, is shown in figure 1 (pie chart). Each year, 6000-7000t (147 171.5 TJ) of coal are used. It has an average net heating value of 24.5 GJ/t. Around 150,000t/year (1230 TJ) of biofuel are used with an net energy value, LHV of 8.2 GJ/t. The biofuel is burned in various forms including wood chips, bark and 2
sawdust and rejects from the card board production. The average moisture content in the biomass is 46% and 10% in the coal. The production of CTMP pulp was in 1998 120 722 adt and of card-board 271 045 t. The number of employees at Stora Enso Fors AB is about 700. The power plant personnel is in total 16 persons operating the boilers in shift. Biofuels 1998 Chips 27% Dry chips 44% Bark purchased 6% Bark own 14% Sludge 2% Sawdust 7% Figure 1. Biofuel mix used in the paper mill. Figures of 1998-Proportions expressed on energy basi (Total solid biofuel use 1230 TJ). 2. Process description The CFB boiler produces steam for both the production of electricity and for the plant s process steam requirements. It is possible to burn a wide range of fuels in the boiler including coal, biofuel, oil and waste materials such as paper, paper board and plastics. The combustion temperature within the boiler is controlled to between 810 C and 930 C. When running at full load (55MW th ), 26t/h (6-8 kg/s) of biofuel is being fed into the furnace. Coal is currently used only as a support fuel when the biofuel heat value is too low to match energy demand. Normal temperature in the furnace is 850 C and in the top of the boiler before the cyclones about 900 C. The flue-gas temperature after the economiser and air preheater of the boiler is at 150 C giving a total efficiency of 89%. 3
Figure 2. The production process at Stora Enso Fors. 3. Fuels The CFB boiler was designed for burning of coal which was the only fuel burnt during the first two years. In 1987, the first steps were taken towards combustion of a mixture of coal and biofuel. The amount of biofuel has increased since then. The operating strategy today is to burn only biofuel with coal used as an support fuel. In table 1 the supply of fuels for the years 1994-1998 are shown. Table 1. Supply of fuel 1994-1998, TJ Fuel 1994 1995 1996 1997 1998 Wood 479 1 206 1 159 1 134 1 184 Coal 254 136 129 178 212 Oil 203 119 196 328 236 Total 936 1 460 1 485 1 640 1 633 Stora Enso Fors AB has established a joint venture with the Forest division of Stora Enso, which supply the wood-fuel and with Swecox Ltd, which supply coal. Bark from the mills debarking plant as well as bark from local sawmills are used. Reject from the paperboard production, paper and non-polluting plastics from the process are also burnt in the boiler. 4
The important factors for emissions and the operation of the boiler are the mixture of different fuels and the fuel is clean from ice and scrap which might cause accidental stops in the feeding system. 4. Fuel handling and conveyor system The biofuel and the coal have separate fuel feed systems. The coal, being screened to less than 15 mm, is fed from the silo to a rotary feeder and thereby into the furnace. The biofuel is delivered chipped or crushed, or chipped at site before entering a silo from which it is transferred by means of a screw feeder onto a conveyor. Finally, it is injected into the furnace using two rotary feeders. The size of the fuel particles is between 1 and 50 mm. The feeding of fuel is made by chain conveyors of chain type. The fuel is stored outside the plant and in silos placed at the plant. The loading system from the silo are feeding conveyors. The biofuel wear on the screw inside the silo and the screws transporting the fuel into the boiler. These parts are regularly maintained at every planned stop. 5. Combustion The steam boiler is of conventional design and the gases leave the heat exchanger at a temperature of 150 C. The CFB boiler was designed for burning coal which was the only fuel burnt during the first two years. In 1987, the first steps were taken towards co-firing with coal as the major fuel and biofuel as supply fuel. The amount of biofuel has increased since then and today the fuels are used in the opposite way; main fuel is biofuel and supply fuel is coal. The operational strategy today is to burn only biofuel with coal used as an occasional support fuel. The air feeding system to the furnace has been redesigned to cope with the increasing amount of biofuel. An extra upper level of secondary-air injectors has been installed high in the furnace in order to accelerate the final combustion of the particles and flammable gases. 5
There has been a problem in getting the exact amount of biofuel into each rotary feeder. The conveyer transporting the biofuel from the screw feeder to the rotary feeders was shortened and angled up in the front. It is feeding the biofuel from a bark silo to a storage of 4m3 placed between the silo and the boiler. From the storage the fuel is transported by two #varvtalsstyrda# screws, one for each #barkslussmatare#. 6. Ash handling The ash in the flue gases is removed by an electrostatic precipitator with two chambers; this captures 99% of the fly ash. There are two removal points of bottom ashes feed by a pneumatic conveying system. At the moment the bottom ashes are pelletised in different testing blends to be recycled to the forest land. Sludge is combusted in the boiler and there are some problems with the emptying the fly ash due to formation of cement when water is added. In the sludge 6
there are fractions of coating from the cardboard production containing chalk and clay. When the ash is put out by water the burned limestone will harden under production of heat. 7. Control and cleaning The NO x is removed using the selective non-catalytic reduction (SNCR) technique installed in 1994. The system uses an 25% ammonia (NH 3 ) solution to neutralise the acid gases at a flow rate of 150 litres/hour. Approximately 70% of the NO x reacts with the ammonia and the products are N 2 and water. Constant monitoring of the following products is carried out after the gas cleaning equipment: NO x, NH 3, SO 2, CO 2, CO, H 2 O, and fly ash. The mill is certificated by ISO 9000, ISO 14001 and EMAS so also the energy production There are power control systems with cameras and monitors for fire safety control. 8. Investment and maintenance cost The CFB boiler was built in 1984 and commissioned in May 1985. The investment cost was 6.47 MEUR, of which 2.12 MEUR was funded by the Swedish government. The investment appears to be low mainly due to the fact that the equipment needed to operate the plant, was already in place. The project to install the CFB boiler had a calculated pay back time of three years. The actual pay back time for the CFB boiler was 2 years. The air supply system was redesigned in 1996 to accommodate higher proportions of biomass at a cost of approximately 0.15 MEUR. Erosion of the heat transfer surfaces does occur and various measures are needed in order to overcome it s effects. Plasma coating, with a hard surface, has been applied to various parts of the boiler. Economiser No.1 was coated in 1992 and also in 1996 and economiser No.2 in 1995. Half of the ceiling in the furnace was plasma coated in 1993. In 1998 the economiser and the ceiling was replaced as part of the routine maintenance programme. In 1996, some tubes in the furnace, the rear wall of the cyclones and some convection parts were replaced as a part of the routine maintenance. The biofuel feeding system was redesigned 1998/99 because there were some problems with getting the exact amount of biofuel into each rotary feeder. 7
Total cost for the project was 0.24 MEUR. The SNCR NO x treatment, installed in 1994, was approximately 0.35 MEUR. The fluegas cleaning system costs 0.14 MEUR/year, while 0.12 MEUR/year is the cost for the NH 3 and 23,500 EUR is the maintenance cost. The fixed costs (excluding Recovery island) for the power plant is 3.1 MEUR and the capital cost at an interest rate of 13% is 1.5 MEUR. Table 2. Operating time and availability, 1994-1998 Hours/year 1994 1995 1996 1997 1998 Operating time 7697 7808 7362 7002 7593 Down time 482 594 576 475 273 Renovations, abt. 581 358 822 1283 894 9. Remarks and conclusions The CFB boiler at Stora Enso Fors AB was originally designed for coal combustion but due to changes in the national energy policy and environmental certification today s main fuel is biofuel. Co-firing takes place between different types of biofuels (wood chips, bark, paper etc.) as well as together with coal. No major redesign work has been needed to accommodate biofuel combustion except for the addition of the high-level air injection. There has not been any increased problem with corrosion. The benefits of co-firing with biofuel as major fuel and coal as a supply fuel gives lower CO 2 emissions. The investment in an air feeding system in 1996 gives lower NO x and CO emissions and the new installation of the biofuel feeding equipment will give future lower NO x an CO emissions. In the future, the aim is to continue to use biofuel and to further reduce the use of coal. 1 EUR = 8.5 SEK 8