The recovery boiler is the heart of the bioproduct mill

One of the most visible structures at Metsä Group’s bioproduct mill in Äänekoski is the 82-metre-high recovery boiler. Delivered by Valmet, the boiler is also a vital part of the bioproduct mill’s operations.

“The recovery boiler has two important functions: it is used to convert the chemicals used in the pulp production into a reusable form, and it generates heat, which is used to produce electricity,” says Pekka Posti, the project manager responsible for the recovery boiler and the evaporation plant.

The boiler combines all of the solutions required for high electricity production in a new way.

The solutions employed in the recovery boiler aim to ensure maximal electricity production.

“The boiler combines all of the solutions required for high electricity production in a new way.”

In terms of structure, the recovery boiler differs considerably from boilers intended for burning oil or wood, for example.

“It’s a chemical boiler, meaning that the materials requirements are strict. Special attention must be paid to quality in its construction and installation,” says Posti.

According to Pekka Posti, the project manager responsible for the recovery boiler and the evaporation plant, the boiler uniquely combines all of the qualities required for high electricity production. Photos: Metsä Group

Chemicals recovered

In the pulp production process, wood chips are cooked to create pulp. White liquor is used as the cooking chemical. White liquor consists of sodium hydroxide and sodium sulphide.

“The pulp is washed after it has been cooked. The remaining solution, black liquor, is burned in the recovery boiler,” Posti explains.

Before the black liquor is burned, the excess water is evaporated, which increases the dry-matter content of the black liquor to 83 per cent.

“The organic components of the wood in the black liquor are burned in the boiler. In the process, the cooking chemical in the black liquor melts at the bottom of the boiler.”

The molten cooking chemical flows away from the boiler along smelt spouts. Cooking chemical contains sodium sulphide, sodium carbonate and unreacted sodium sulphate. As its name suggests, the recovery boiler is used to recover chemicals.

“The smelt is dissolved in water, and weak white liquor is added to create green liquor. In the causticizing plant, the green liquor is turned into white liquor, which is returned to the pulp production process,” says Posti.

The steel frame of the bioproduct mill’s recovery boiler is 82 metres high. The boiler itself is more than 60 metres tall. The boiler will be suspended inside the frame, as the heat makes it expand by dozens of centimetres.

Energy efficiency taken to a new level

The combustion process in the recovery boiler generates a great deal of heat, which is recovered effectively.

“The heat is transferred to water, and the steam that is generated is superheated and fed into a turbine that produces electricity,” Posti explains.

Thanks to the efficient recovery boiler, the bioproduct mill will produce 2.4 times as much energy as it needs.

The boiler generates 363 kilos of steam per second. It operates at a pressure level of 100 bars, at a temperature of 515 °C. It burns up to 7,200 dry-matter tonnes of black liquor per day.

“The boiler generates 4.35 kilos of steam per kilo of dry matter. This is an excellent result, even in global comparison.”

Thanks to the efficient recovery boiler, the bioproduct mill will produce 2.4 times as much energy as it needs.

“In addition, the process has a very low environmental impact.”

Casting was carried out in the turbine building in June 2016. The heaviest parts of the turbine weigh 270 tonnes, which calls for extremely strong foundations.

Read more: Maximal efficiency

Return to frontpage