Proceedings World Bioenergy 2010

IMPROVED FLEXIBILITY AND ECONOMY BY USING SMALL FLUIDISED BED BOILERS IN DISTRICT
HEATING
Alpo Sund 1 , Matti Lilja 2
1 Nurmijärven Sähkö Oy, Kauppanummentie 1, 01900 Nurmijärvi, Finland
2 Renewa Oy, Teknobulevardi 3, 01530 Vantaa, Finland
matti.lilja@renewa.fi
ABSTRACT: The Finnish heating company Nurmijärven Sähkö generates annually over 80 GWh heat with wood based
fuels. To ensure optimal flexibility and minimal emissions, company has chosen fluidised bed combustion for two of its
below 12 MW heating plants. These plants have shown a capability to operate at very low partial loads during low
demand seasons, thus reducing the need to use oil fired reserve boilers. Exploiting of lower cost fuels, like slash from
local forests, has been possible due to the flexible combustion process. Emissions have been successfully kept clearly
below set limits, also at partial load operation or when using variable quality of fuels.
Keywords: fluidised bed, heat generation, district heating, operating experience
1 INTRODUCTION
Nurmijärven Sähkö Oy is a municipality owned
energy service company in Southernmost Finland. It
provides 9500 inhabitants with 90 GWh heat
annually. About 95 % of the heat is generated with
wood based fuels in company’s three plants. This
equals to 120 000 cubic meters of wood.
To ensure economical heat for its customers,
Nurmijärven Sähkö needs heat generation with is
flexible to meet fluctuating heat demands, as well as
digest versatile fuel and take the advantage of lower
priced slash or waste wood batches from near-­‐by
sources.
In order to be able to maximise the operational
flexibility, the company has chosen bubbling fluidised
bed (BFB) combustion for two of its heating plant. The
8 MWth and 11 MWth units have been operated since
2002 and 2007. The units are normally unmanned
and controlled remotely.
2 COMBUSTION TECHNOLOGY
Heating plants in the localities Nurmijärvi and
Rajamäki have a capacity of 8 MW th and 11 MW th,
respectively. Their combustion system is based on
bubbling fluidised bed technology, where the fuel is
introduced in the furnace where a 4 ton sand bed and the
primary combustion air form a homogenous mixture with
about 800 o C of temperature.
The furnace and fluidized bed with the boiler form an
integrated structure which is completely enclosed in a
water cooled structure. There are refractories on the side
walls in order to achieve correct combustion temperature
and to prevent erosion. The boiler has a fully welded
hermetic structure of membrane walls. The furnace is in
the shape of rectangle. The structure is self-supporting
and propped up from below which allows thermal
expansion upwards.
Because only few kilos of fuels are in the furnace at
each moment, the gasification and combustion is fast.
The heat capacity of the sand is so big that any amount of
fuel humidity is easily evaporated and doesn’t harm the
combustion itself.
Fluidized bed temperature is controlled by
combustion air distribution to primary, secondary and
tertiary air. The well controlled furnace temperature is
important for minimising of the temperature-derived
NOx creation. With dry fuels, when the fluidized bed
temperature might otherwise become too high, circulating
gas is used in the fluidising air system.
Figure 1. Hot water boiler with a Renewa bubbling
fluidised bed
Fluidised bed combustion is better known for utility
world bioenergy 2010
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