Proceedings World Bioenergy 2010

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oilers of 100 MWth or larger. The Renewa BFB technology, used in the Nurmijärven Sähkö’s plants, is based on same principle as any bubbling bed. However, since the first application in 1985, the design has been optimised for small and medium sized boilers and thus can be implemented cost-efficiently. There are more than 30 applications of steam and hot water BFB boilers in the capacity range of 3 – 30 MWth. 3 OPERATION EXPERIENCE The two Nurmijärven Sähkö’s boilers have been in commercial operation for 8 and 3 years. The utility operates also another biomass fired boiler with reciprocating grate so observations between different combustion technologies have been made. The most visible advantage of the BFB boilers is their fast response to load change needs. The output can be decreased or increased by 12 % units within 10 minutes. This is a clear advantage in district heating plants where the daily demand curve has major variations. One just has to secure evenly chipped fuel. Flexibility to utilise different fuels has practical benefits e.g. in cold winter days when high calorific fuels can be used to achieve outputs even above nominal point, and thus postpone the start of expensive peaking plants. The same benefit there is during the summer when very low output level can be achieved with high calorific biomass. This saves costs of operating fossil fuelled plants and thus buying emission credits can be avoided. Figure 2. Rajamäki heating plant with 11 MWth fluidised bed boiler Nurmijärven Sähkö has successfully used fuels like forest slash, fresh and dry, from final forest felling, nontrimmed and trimmed small forest slash, saw dust, bark, grain sorting residues and even oat kernels with 10 % mixture. The non-trimmed forest slash, including green particles with chlorophyll, has not caused any problems in occasional use, even at operation on 100 % load, if the humidity has been inside the guaranteed window (± 12 % range around nominal point). No findings of chlorine caused corrosion have been detected. Very long time operation on full load with only such fuel has not for the time being been performed, however. During typical operation period, the main advantage of BFB flexibility comes from optimising fuel economy. 18 world bioenergy <strong>2010</strong> Cheaper low quality fuel lots can be exploited which reflects also as a better bargaining power when buying fuel. Nurmijärven Sähkö’s policy is to use only renewable biomass based fuels, but many Finnish BFB operators actively optimise their fuels costs by allowing biomass and peat suppliers to offer their best prices. Fluidised beds, however, have more stringent requirement than grates on the impurities coming with the fuel. Therefore magnetic separators and disc screens are highly recommended to screen out coarse particles and metal pieces before they enter the furnace. If such material, however, get inside the combustion chamber, they can be removed through the bottom funnels which are normally used for bed sand replacement. The inclined shape of the furnace bottom helps the coarse particles to roll towards the hoppers. 4 FLUIDISED BED BOILER MAINTENANCE Concerning maintenance costs, a certain advantage has been recorded in a smaller need to replace components. Normally only bearings of motors and pumps need to be replaced, while fuidised bed boiler internals don’t have any moving mechanical components which could be subject to any wear and tear. Only the bed temperature sensors need regular replacement. Another advantage has been the possibility to use short shut-downs for maintenance works. This is possible because after shutting down there remains no fuel in the furnace and the structures can be cooled down in 3-4 hours. The service persons can then enter the boiler or make the convection section soot blowing during one operation shift. In this way the scheduled annual maintenance outages can be shortened. Typically annual service outages have taken some 120 hours. 5 OPERATION COSTS The main costs of operation, except the fuel, come from power autoconsumption, bed sand make-up and ash disposal. The recorded power demand of the two plants is 33 kW per produced MWth of heat. The figure includes also the energy consumed by the district heat pumps. The ash amount from the electrostatic precipitator is below 4,4 kg per MW th produced, representing thus below 1 % of the consumed fuel mass. Bottom ash volume is negligible in the BFB boilers. The small ash volumes have been most economical to bring to a landfill but the company is now actively looking for alternative uses for the ash. The nutrient contents of the ash might help to recycle it. Part of the bed material, which is normal equigranular construction quality sand, is replaced daily. The recorded sand consumption has been 3,8 kg/MWth (at the 11 MW boiler) and below 2 kg/MWth (at the 8 MW boiler). This means that the sand silo, capable to receive a full truckload, needs to be filled only a couple of times a year. Presently, the rejected sand is transported to a landfill. Nurmijärven Sähkö is looking for possibilities to sell this practically very clean sand to potential users or recycle it in the process by screening. The manpower costs are quite reasonable. Both plants have an advanced control system and they can be remotely controlled from each other’s control room. Therefore only one of them, normally the Nurmijärvi
plant, is permanently manned. Rajamäki plant is visited shortly once per day, due to requirements of legislation, but all operations are performed from the control centre. During the night time and weekend the operator can control the plants with a portable PC even from his home. 6 EMISSIONS OF FLUIDISED BED COMBUSTION Emissions of Rajamäki plant have been measured by an independent authority. Table 1 shows emissions on 3 MW partial load, i.e. below 30% of nominal capacity. Table I: Rajamäki heating plant emissions at 3 MW partial load Rajamäki Actual load: 3 MWth heating plant Performed by: Suomen Analyysipalvelut Oy Date: 14.3.2008 Parameter Value unit Flue gas temperature 51,8 o C CO2 in flue gas 12, 3 ±0,7 % O2 in flue gas 8, 3 ±0,4 % Dust content 20 ± 4 mg/m 3 n Dust emission 0,14 ± 0,05 kg/h Dust contents red. at 6% O2 23 ± 5 mg/m 3 n Specific emissions 9 ± 3 mg/MJ NOx 97 ppm NO2 contents red. at 6% O2 236 ± 18 mg/m 3 n SO2
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