Italy’s first biomass fired fluidised bed power plant

The original power plant in Ospitale di Cadore was a fuel oil fired boiler plant with a condensing turbine. It generated electricity for the furnaces of a neighbouring ferro-silicon plant, the whole operating in island mode.

After 10 years of working life the necessity to carry out technical improvements for environmental reasons rendered the operation of this power plant no longer possible, and so in 1979 it was shut down.

But in the light of the Italian government’s policy to support electricity from renewable energy sources, operation of the plant took on a new perspective. AE Energietechnik – centre of competence for biomass power plants within the Babcock Borsig Power group – was awarded the contract for retrofitting the power plant with a biomass fired fluidised bed steam generator, including fuel preparation and flue gas treatment system.

Overhaul

The existing condensing turbine with its 17.5 MW synchronous speed medium voltage generator was thoroughly overhauled and the maximum load increased to 19.4 MW to operate using the live steam from the fluidised bed steam generator. The turbine is equipped with a water cooled condenser. For re-cooling an open-cycle cooling tower is installed.

The power plant, which is shown in Figures 1 and 2 is now operated by the independent power producer SICET (Societá Italiana Centrali Elettro Termiche) and feeds its electrical output into the ENEL grid at Ospitale di Cadore in parallel mode.

Besides the capability to burn wood based biomass the plant is also designed for co-firing sludge and RDF in preparation for the period after the year 2008, when subsidised energy pricing ends. The addition of a dry flue gas cleaning plant and adequate feeding systems would be necessary.

Trial run

The plant consists of the following systems:

• Fuel preparation, storage and transportation

• Fluidised bed boiler plant including flue gas treatment (SNCR, dust filter)

• Turbine plant with generator and feedwater preheaters

• Cooling system

• Auxiliary systems

• Electric power distribution

• Process control system.

Commissioning started in April 1999 and the plant was handed over to the customer after the successful trial run in November 1999.

Since then the plant has been in operation for about 15 000 hours. It has attained all the design parameters in terms of capacity, efficiency and emissions. The acceptance test was successfully performed in May 2000. Working with Sicet, AE Energietechnik is now optimising the plant to allow for the combustion of fuels with a wider range of calorific values and to maximise reliability.

Fuel preparation, storage and transportation

The system is shown in Figure 3. Biomass fuel is delivered as wood chips within the necessary range of fragment size for the combustion process, as well as logs, and offcuts and trimmings (log heads and refills) from the sawmills. A sliding bar discharging system and a belt conveyor transport the fine material. The logs and other coarse material are transported by a chain feeder and a belt conveyor to the chipper. The capacity of the chipper is 55 tonne per hour. Then the whole fuel mass is passed over a disc classifier. The oversize part is fed back to the chipper and the fine material is stored in a loose pile. From there the wood is conveyed to the fuel silo in the boiler house by a reclaimer screw and a belt conveyor. The boiler silo is equipped with a sliding bar system that unloads the wood chips into two metering screws. Finally, the fuel is fed into the furnace via four air pressurised nozzles

Fluidised bed

To handle the wide heating value range of the fuel and to meet the severe emission limits the patented fast internally circulating fluidised bed combustion system (FICB) was chosen (Figure 4).

The fluidised bed is located in the lower part of the steam generator and is integrated into the evaporator water walls. The steam generator has a vertical three-pass design with natural circulation. The first pass consists of the FICB furnace and the post combustion chamber. The second pass contains the superheaters and an evaporator. The economiser and the air preheater are installed in the third pass.

Primary combustion air is fed by an open type air distributor into the fast fluidised section of the bed (“lift”) and moves the bed material up to the impact particle separator. This type of air distributor allows the spreading of coarse inert material over the entire cross-section of the bed during operation.

The secondary air is brought in above the slow fluidised bed; this is also where the separated bed material falls down and passes through a gap back into the lift. This secondary air is mixed in with the high turbulence flue gas flow entering the post-combustion chamber.

By means of two air pressurised fuel feeders the biomass is distributed over the slow bed and is mixed into the hot material separated from the fast fluidised bed. This fuel-bed material mixture passes into the fast zone, where the main combustion takes place during the upwards transport of material. The sand is separated from the flue gases at the outlet of the fast zone and falls onto the slow bed again. To achieve more flexibility with different fuel properties it is also possible to feed some biomass directly into the fast bed.

Post combustion

Complete burn-out of organic compounds takes place in the post-combustion chamber. For firing of biomass with a high content of nitrogen, urea can be injected into this zone, to reduce the NOx emissions below the prescribed limiting value.

Downstream of the second pass a special type of particle separator is installed. This separator collects entrained fine material at a temperature of 400°C and reduces the synthesis of dioxins in the following flue gas path. This collected finer fraction is recirculated into the fluidised bed to maintain the bed inventory. Finally the flue gas is cooled down to 160°C, purged of fly ash in a bag-house filter. and removed from the plant via an induced draft fan and the stack.