Proceedings World Bioenergy 2010

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 2010
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