recommended a new electrostatic filter unit to replace the old system and the addition of a flue gas condenser to remove the sulphur oxides,” said Aro. “To improve boiler availability and efficiency, we advised replacing the old rotary flue gas air preheater with our HTS unit. We also recommended replacing an old fin-tube hot economiser that was plugged most of the time and adding some new boiler surface.” The HTS that Aro refers to is the Andritz Heat Transfer System. This is a simple, closed-circuit water system for preheating combustion air indirectly using flue gases.

It is much easier to operate and maintain than the old rotary preheater and keeps the flue gas outlet temperature constant regardless of the load. The gas- to-water heat transfer in the HTS is four times more efficient than air-to-gas transfer. Timo Mäkinen, Plant Manager, who also served as Commissioning Manager for the

project, commented on the HTS technology. “This system is much simpler, steady-state and makes it easy to operate the boiler. From a maintenance perspective, it keeps the flue gas above corrosion temperatures regardless of the load. Plus, our old hot economisers were often plugged and that is not the case anymore. So, we now have much better efficiency and availability.” For emissions control, Andritz delivered an electrostatic precipitator (for dust) and a flue gas condenser for SO2 removal. Hot flue gas is delivered to the bottom of the condenser. Inside, a large-volume fine water mist is sprayed into the flue gas, which binds the sulphur and the rest

of the dust into the condensate. Sodium hydroxide is dosed with the water to keep a consistent pH in the 4.5-5.5 range. Condensate is treated and the hot water passes through a heat exchanger before being recirculated through the condenser. After the flue gas condenser, the clean and cold flue gases are led into the 130 metre high stack.

“This heat exchanger provides a big benefit to us,” Mäkinen said. “It is basically free heat that we use in our district heating network and also to warm combustion air before the HTS.”

Rönkkä agreed, pointing out that the flue gas condenser design was impressive – simple, but effective. “We love the

Autumn 2017 9 9

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