Installation of a Biomass-fuelled Boiler at an Existing Coal-fired Power Plant

by P Kolbeck, CADDET Danish National Team

The new biomass boiler house.

Introduction

As part of the effort to reduce CO2 emissions, the Danish Parliament in 1993 directed Danish power stations to use 1.2 million tonnes of straw and 0.2 million tonnes of wood chips annually after the year 2000. The total amount of energy to be converted to biomass is 19.5 PJ/year, which corresponds to conversion of around 6% of the power stations' coal consumption. Consequently, ELSAM (the utility company for the Western part of Denmark) has initiated various pilot projects with biomass. One of these is at Sønderjyllands Højspændingsværk, a coal-fired power plant located in Aabenraa in the southern part of Jutland. The plant has been operating since January 1998.

A straw boiler equipped with a superheater fired by wood chips has been fitted parallel to an existing conventional coal-fired boiler. Both boilers provide steam to the same high-pressure steam turbine which generates electricity and supplies heat to a district heating system. The new biomass-fuelled boiler which contains two boilers, one for straw and one for wood chips (the superheater), will reduce the power station's coal consumption and CO2 emissions. The size of the new boiler has been based on an annual consumption of 120,000 tonnes of straw and 30,000 tonnes of wood chips.

The boiler (Benson type) is installed in an existing boiler-house where a conventional system has recently been taken out of service. Costs have been reduced by reusing the existing infrastructure such as electrostatic precipitators, chimney and service infrastructure (stairs, lighting etc).

Process diagram of the plant.

Fuel Mix

It is the intention to run the new boiler at full capacity when the coal boiler is operating between 40% and 100% of capacity. The new boiler can be started on its own and is able to supply district heating without having the coal-fuelled boiler running.

The installation of the biomass-fuelled boiler has not effected the existing coal-fuelled boiler's ability to operate alone, nor prompted any alteration in and around the high-pressure steam turbine.

The flexibility of the system gives the operator of the boilers the unique possibility of switching fuels and optimising the fuel feed line from both an economic and environmental point of view.

The System

Straw is a low-grade and non-homogeneous fuel characterised by being highly volatile with high-chlorine and high-alkaline content, thereby increasing the risk of corrosion and fouling of boiler systems. To avoid this, a number of operational and constructional measures have been taken. As shown above, the two boilers are only connected to each other via a common feeder water pipe and a common steam pipe connecting the boilers with the high-pressure steam turbine. This system facilitates the separation of the two boilers' fl ue gases and thereby eliminates the risk of corrosion of the coal-fuelled boiler.

Water from the feed pipe is heated in two stages in the biomass-fuelled boiler plant: first in the straw boiler to 470°C/215 bar and then in the wood chip superheater to 542°C/200 bar. This two-step heating procedure is maintained in order to reduce the risk of corrosion from straw combustion, which is more aggressive than wood chip combustion at high steam temperatures.

Steam from both the new boiler and the existing coal boiler is mixed before being led to the high-pressure steam turbine. Subsequently, the steam is returned to the coal boiler's reheater. Minor alterations in the coal boiler's heat flow and thermal absorption will occur. Calculations so far indicate that these do not impair the electrical efficiency of the coal boiler.

Straw bales are supplied by four parallel feeding units and are combusted on a grate firing system after being scarified. The wood chip superheater is also based on a grate firing system.

Energy Production

The electricity generated from straw and wood chips is 40 MWe (100% load). Electricity is led to the grid. Heat is supplied to the local district heating system.

Environment

The table below gives key figures for the new boiler at full load.

Parameters for the New Boiler at Full Load

Fired biomass (straw and wood chips)

Saved amount of coal (net calorific value = 24.7 MJ/kg)

Annual CO2 reduction (95g/MJcoal)

95.5 MJ/second

80,000 tonnes/year

190,000 tonnes CO2

Slag from each boiler is kept separate in order to enhance the recycling potential. Slag is sought after for use in agriculture.

Flue gas from the wood-chip-fired superheater is led into the straw-fired boiler's flue-gas duct where excess heat is used and the flue gases are mixed. Flue gas from the new boiler is purified by means of electrostatic precipitators before being emitted. Fly ash is transported by a pneumatic ash transportation system to a big-bag system. Ash from the electrostatic precipitators can be reused in industrial processes.

Economics

The entire project is calculated to cost DKK 400 million (1995 prices, where DKK is the Danish krone) including all expenditure on design and costs related to commissioning of the system. The system is financed by ELSAM.

Conclusion

The successful combination of a coal-fired boiler with a boiler fired by straw and wood chips at Sønderjyllands Højspændingsværk has proved that environmentally-sound technologies have a strong potential for being integrated into the infrastructure of an existing power plant without comprehensive alterations.

For more information contact the CADDET Danish National Team at Tølløse.

The CADDET Renewable Energy Newsletter is a quarterly magazine published by the CADDET Centre for Renewable Energy at ETSU, UK.

The articles published in the Newsletter reflect the opinions of the authors. They do not necessarily reflect the official view of CADDET.

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