ENA Kraft, Enköping Sweden

Transcription

1 THE ANALYSIS REPORT OF PLANT NO. 6 Cofiring of biomass - evaluation of fuel procurement and handling in selected existing plants and exchange of information (COFIRING) - Part 2 ENA Kraft, Enköping Sweden Evaluated by Bengt Hillring, SLU

2 1. General information about the plant ENA-Kraft Ltd is a energy producer located in Enköping, Sweden. The plant is owned by the companies Enköping energy and Västerås Energy. Energy has been produced at the plant since The company owns a steam boiler at 75 MW supplied fuel and it supplies the district heating network with heat and produce electricity for the grid. The main CHP boiler was delivered in 1994 by Burmeister & Wain Aktiesellskap and was and commissioned in The boiler is a vibration grate boiler with the following data: - Boiler capacity 55 MW th electrical output 23 MW e (including flue gas cooler). - Steam data: 27 kg/s, 100 bar and 540 C. The number of employees at ENA Kraft AB/Enköpingsenergi AB are 35 persons. 2. Process description The main power plant is a 55 MW CHP boiler with a 23 MW e combined with a flue gas cooler under the company name ENA Kraft Värme. The main fuel is wood fuel. There are also a 22 MW wood powder boiler, a 50 MW oil or LPG fuelled boiler and a 36 MW electricity boiler. There is also a 50 kw biogas boiler from the municipal water cleaning plant and an accumulator for heat with a capacity of 25 MW and 325 MWh to level out the load. Back-up for this plant is two 2x6 MW wood fuelled boilers including flue gas cooler and a 10 MW oil fuelled boiler. Biofuel from the city dump supplying a 200 kw boiler. There are another back up with 2x23 MW oil fuelled boilers in a third location. Data for district heating 1997 Connected power is 140 MW Connected subscribers are of which 947 single family houses Delivered heat was MWh (754 TJ) and houses connected to the plant Total grid network is 72 km Total water volume in the network is m³ Total water volume in the accumulator is m³ The water has been heated in the boiler and is pumped around the network in a closed system to the subscribers in the town of Enköping. 2

3 The normal data for the CHP plant (ENA Kraft) is for the district heating water feed line is 85 C and for the return line is 45 C. The heat losses in the network is about 10 %. 3. Fuels The main boiler was originally designed for burning of wood-fuel. The operating strategy today is to burn only biofuels including bark, sawdust, residues from logging operations and Salix (short rotation coppice). In table 1 the supply of fuels for the years are shown. 3

4 Table 1. Supply of fuel , TJ Fuel Bark - * Sawdust - * Logging residues - * Salix - * Wood pellets - * Methane - * LPG - * Oil - * Electricity - * Total *Supply of different fuels are not available. The strategy at Enköping is to burn biofuels in the period October - April with a supply of Salix during the 4 mid-winter month (November - February), co-fired with other biofuels. The main fuel supplier is the regional based biofuel company Naturbränsle Ltd. and the average moisture content is 46%. The biofuel supplied is saw dust and bark. The contractor for the Salix chips is the specialised company Agrobränsle Ltd. The share of Salix in the fuelmix is between 15-60% of the fuel supplied. Pellets is used in the summer for heat water production in a rebuilt oil boiler. Typical fuel mixture is 50% wood chips, 15% sawdust, 15% Salix and 20% bark. The moisture content is between 40 60%. Peak consumption is 125 m 3 /h, up to 40 trucks are delivering fuel in the winter day. The overall biofuel consumption is 100 m³/h and they are produced in the neighbourhood of the city of Enköping. It is mainly the forest owners in a radius of 70 km supplying the plant with residues from normal logging operations such as branches and tops. Saw dust and bark from local saw mills is also in the mix together with Salix chips produced on farmland. During the winter season around 15% Salix chips is burned and that is a amount of Salix that is at an safe before problems with fouling and deposit when the ash melting point is decreased of the higher content of potassium and sodium. Tests are still carried out at the plant to find the optimum supply of Salix to the CHP boiler. 4

5 4. Fuel handling and conveyor system The biofuel is delivered in chipped form on lorries and the singled is weighed and analysed with respect to moisture content to measure the energy content in the load. The fuel is dumped in the fuel reception in the form of a ground pocket with a maximum delivery time of 8 days. After that the biofuel is transported to a covered fuel stock for intermediate storing with rubber conveyors and scrape transports. There is also a disc-riddle mounted before the storing. The maximum fuel transportation capacity to the boiler is 500 m 3 /hour. The heat load in the city of Enköping is then based on the amount of fuel transported from the intermediate store to the boiler. Deliveries by lorries is made every Monday to Friday in daytime due to problems with disturbance for the neighbours. The capacity for the lorries is about 100 m 3 of fuel each and during the burning season in wintertime that means 40 lorries per day. Lorries transporting green-chips from forest fuels is tipped backwards while conventional chip lorries with by-products from saw-mills is side tipped. The fuel supplying company, Naturbränsle Ltd. is responsible for storing outside the CHP-plant while the plant owner, ENA Kraft Ltd themselves is preparing the right blend for the boiler. Each lorry is moisture defined manually by the Swedish standard with drying in 105 C for 12 hours and the sample is weighed before and after the drying procedure. The outside storage is developed as a loaf and the fuel is pre-screened with a disc. The power company is measuring the steam (moister) content in the flue gases before the flue gas condenser. Thus they get an indirect control of the moister content in the fuel. There are possible dust problems for the personnel in the fuel handling and the company has control programs for this. There has been some initial starting-up problems for the fuel handling system but there has not been any unplanned stop for five years now. 4.1 Screening Screening is made directly on the conveyor with the disc-riddle with a capacity of 500 m 3 /hour. The screening construction is made of ordinary carbon steel. The power of the screening system is 200 kw. 5

6 4.2 Conveyors The conveyors are made of steel cord of a special smooth type with an adjusting system with weights. The belt velocity is 3 m/s and the cleaning is made with scrapes. The diameter of the two screw conveyors is 1 meter with a length of 10 meter each. 4.3 Boiler feeding There are two boiler silos at the plant with an effective volume of 100 m 3 each. The distributor consists of a plate made of ordinary carbon steel and made of stainless steel in the screws. The discharge adjusting is on/off. Distributing and feeding pipes have a damper. The fire control is sprinkler and temperature guards. There could be splinters origin from the wood fuel passing the disc-riddle. 5. Combustion Combustion is made with wood-fuels during the whole combustion season. Continuous combustion with Salix is made during the period of November to March with a share between %. The temperature on grate is about C and the temperature of the boiler outflow is between C. Flue gas condensing is installed and the recipient for the condensation water is the municipal water cleaning plant. The CHP boiler has four fuel feeding points. There have been some clinker and fouling problems which are handled in projects with consultants. The combustion is stable and the ash feeding is continuous like the steam sweeping. The steam producing stability is good. There has been some fouling in the super-heater close to the fire-place because of the high temperature in that area. When the super-heater is covered the steam temperature was decreased. The conditions before the introduction of Salix combustion is unclear. Analysis was made of the cover in the super-heater and the following results is given by Hjalmarsson & Ingman (1998): 6

7 Table 2. Analyses of deposits in ENA-krafts boiler at Enköping (Hjalmarsson, A-K. & Ingman, R. from Salix combustion (in Swedish). VärmeForsk, 1998), % of dry matter Super-heater Fuel feeding area Boiler gable SiO Al 2 O CaO Fe 2 O K 2 O MgO MnO Na 2 O P 2 O TiO Cl S The problems with deposits in the super-heater has been solved with decreased amounts of Salix in the fuel mix and at the moment the practical maximum seems to be around 15 % of Salix in the fuel mix. 6. Ash handling Large particles of ash falls through the grate during the combustion while small particles remains in the air suspension are burned there and land in the fly ash. Of the ashes about 50 % will be fly ash and 50 % will be bottom ash. The fly ash contains nearly no unburned fractions (0 %) wile the bottom ashes contains about 5 % unburned fractions. The fly ash is scrapped dry from the electric filter and is removed by a pneumatic conveying system and is then transported to landfill. The bottom ash lands in a water container from two removal points, and is transported by a water cooled chain transporter from there to another container and is transported to landfill with special lorries. Heavy metals is vaporised in the combustion area and condensed at the fly ash particles, which gives a concentration in the fly ashes of heavy metals. The content of cadmium in fly ash is 40 times the content of the bottom ash. 7

8 Test have been carried out since May 1999 spreading a blend of bottom ashes and sewage sledge in Salix plantations. The ashes contain about 25 % lime and the bark burned in the boiler has different contents of trace materials. The origin of the fuel is of great significance for the metal content for the fly ash. 7. Control and cleaning Neither the company nor the CHP-plant is certified. The power control systems are computerised, cameras with monitors and fire detectors is used for fire safety control Electrical precipitator and the flue gas condenser are cleaning the flue gases. There are continuos measuring of the flue gas with respect to: NO, NO 2 (=NO x ), N 2 O, SO 2, HCl, NH 3 and the steam (moisture) content in the flue gas. The results are reported to the regional authorities (the National County Board) every quarter of the year and is summed up in a yearly report. The plant at Enköping have the following border values: NO x = 100 mg/mj fuel supplied CO = 90 mg/mj fuel supplied Dust = 40 mg/normal cubic meter at 6% O 2 NH 3 = 5 ppm N 2 O = 5mg/MJ fuel supplied Suspension = 10 mg/litre water Caesium 5000 becerel per kilogram fly ash The plant is clearly under the values given in the concessions given by the authorities. Sometimes the value for caesium is over the boarder values. 8

9 8. Investment and maintenance cost The fuel cost for the ENA Kraft company is 35 MSEK (4.1 MEUR) per year. Operational cost and maintenance cost is 4 MSEK (0.47 MEUR) each per year while the personnel cost is 7 MSEK (0.82 MEUR) per year. The flexible cost is with respect to the costs above about 130 SEK/MWh (15.5 EUR) energy output. Investment costs were 350 MSEK (41 MEUR). The total cost of 77 MSEK/year (9 MEUR) equals the cost of 280 SEK/MWh (32.9 EUR/MWh) per year which gives a fixed cost of 150 SEK/MWh (17.6 EUR/MWh) per year. Table 3. Operating time and availability, Hours/year Operating time - 5,600 5,600 5,600 5,600 Down time - Renovations, abt. - The accessibility during the winter season is %. 9