Maximal efficiency

Pekka Posti, the project manager responsible for the recovery boiler and the evaporation plant in Metsä Group’s bioproduct mill project, is enthusiastic about the qualities of the recovery boiler being built on the site.

Delivered by Valmet, the boiler will produce 363 kilos of steam per second. It operates at a pressure level of 100 bars, at a temperature of 515 °C,” Posti explains.

The boiler 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,” says Posti.

This new type of high-power boiler is an essential element of the bioproduct mill concept.

This new type of high-power boiler is an essential element of the bioproduct mill concept. All of the solutions employed in the recovery boiler aim to ensure maximal electricity production.

“We have invested a great deal in maximising the mill’s electricity production. It will produce 2.4 times as much energy as it needs.”

Massive structure

The recovery boiler has become something of a landmark. The Finnish company Ruukki is delivering its frame. At 82 metres tall, the frame is around the same height as Hotel Torni in Tampere.

The actual boiler is 60.5 metres high, with a surface area of 323 square metres at the bottom. Its thermal surface is 52,501 square metres.

At 82 metres tall, the frame of bioproduct mill´s recovery boiler is around the same height as Hotel Torni in Tampere.

Efficiency through many factors

Combined with innovative solutions, the high dry-matter content of the black liquor makes the recovery boiler extremely energy-efficient.

“High combustion-air temperature, optimised feed-water preheating and heat recovery from the vapour from the solvent tank significantly improve efficiency. In addition, the steam bled from the turbine is used for cleaning pipes,” Posti explains.

Heat exchangers are used to recover heat from flue gases.

“The flue gases are cooled from 214 °C to 130 °C, and the heat is recovered in the feed water for the boiler.”

The flue gases are recovered effectively through four 5-field electrical filters. The mild malodorous gases generated by the mill are burned in the recovery boiler.

“Overall, the emission levels of the boiler will be very low.”

In addition, an ash recrystallisation system delivered by Andritz will be built to remove chlorine and potassium from the ash. Sodium and sulphates will be recovered in the process.

“Known as ARC, the system will reduce chemical expenses and protect the superheaters in the recovery boiler,” says Posti.

Automatic air nozzles and retractable liquor sprayer stations also improve safety at work.

Smelt spout robot for increased user-friendliness

With regard to the recovery boiler, special attention has been paid to working conditions. Thanks to a smelt spout robot, a traditionally arduous and sweaty work phase will be handled by machines.

“Automatic air nozzles and retractable liquor sprayer stations also improve safety at work,” says Posti.

A water pressure test was run on the recovery boiler in March 2017, meaning that the pressure vessel has been completed.

Read more: The recovery boiler is the heart of the bioproduct mill

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