The pellet-fired steam plant in the city of Harjavalta is the crown jewel of STEP

Well built, smoothly functioning and tidy. This is how Kimmo Sandberg, the Operational Manager of Suomen Teollisuuden Energiapalvelut – STEP, describes the pellet-fired steam plant – the crown jewel of STEP ­– completed in 2016 at Harjavalta Industrial Park.

“This is a plant we’re happy to introduce to others,” Sandberg praises.

A new steam generating plant was needed when Nornickel Harjavalta Ltd. expanded its operations and increased the energy demand of the Industrial Park. The plant investment was made by STEP and the plant was delivered by KPA Unicon Oy. STEP supplies steam to Nornickel under a long-term partnership agreement.

The plant was supplied on a turnkey basis, including all process equipment, building, commissioning and training of operating personal.

A significant act for the climate

The new steam boiler plant, with a capacity of 30 megawatts, will produce approximately 160–180,000 MWh of steam for Nornickel´s production per year. The plant uses wood pellets as fuel. The new steam generating plant will reduce the carbon dioxide emissions of the energy production in the Industrial Park by approximately 70,000 tonnes per year.

“The new plant has increased the security of steam supply and replaced the use of heavy fuel oil in the Industrial Park. Old heavy fuel oil plants now serve as back-up plants and are converted to use liquefied natural gas (LNG) and light fuel oil as fuel,” Sandberg explains.

A pellet-fired plant was chosen for Harjavalta because of its low investment costs, rapid reactivity and efficient use of space.

“The pellet-fired plant responds very quickly to load changes, making it an excellent solution for both basic and peak load use. The plant’s adjustability is significantly better than, for example, a plant that uses wood chips as fuel, and storing pellets in vertical silos is much more efficient than storing wood chips,” says Timo Huusko, Development Director from KPA Unicon.

“In addition, the Harjavalta plant was built relatively quickly and is easy to operate: it is connected to the remote control room of STEP, and the plant does not require constant manning,” Sandberg adds.

Only the best pellets for burning

The plant in Harjavalta uses the best quality white pellets that are sourced from Finnish suppliers. The plant has two 500-cubic-meter silos. According to the design, when the plant operates at full capacity, the two full silos can fuel the plant for about four days.

Before combustion, the pellet is ground to dust with separate hammer mills, after which the excess air is filtered off from the dust. Finally, the wood powder is burned in a boiler with a dust burner.

The boiler is a water-tube boiler in which the combustion takes place in a furnace. There is a burner in the roof of the furnace which sustains the flame in the combustion chamber. From the furnace, the flue gases are transferred to the convection section, where the economizers and evaporators are located.

In this convection section, the heat from the flue gases is transferred from the pipes to the water, and when the water is heated enough, it evaporates. The steam is delivered to the end user along the pipes, and the flue gases are passed through an electrostatic precipitator to the chimney and further into the atmosphere.

“By means of the electrostatic precipitator, the plant complies with the ever-stricter emission limits, so no other flue gas treatment is needed,” states Huusko.

The boiler is equipped with a dual-fuel burner, so if necessary, the same boiler can also be used to burn the back-up fuels: LNG or light fuel oil.

“We don’t have our own LNG station – we buy the fuel from Nornickel when needed. That way, we don’t need to manage the boil-off gases,” says Sandberg.

Simple boiler, less maintenance

The Harjavalta plant is subject to one scheduled annual maintenance stoppage, usually during the summer break of the Industrial Park when the steam demand is low. Annual maintenance is carried out by STEP.

Maintenance of a pellet-fired boiler is less expensive than, for example, a boiler that uses wood chips: because of their simple structure, the pellet-fired boilers are cheap to maintain and install.

“The design of the boiler does not include moving parts, so in that sense it´s maintenance-free. Normal auxiliaries do instead require maintenance, such as bearing maintenance and filter replacement,” says Huusko.

The quality of the pellet has an impact on the operation and maintenance of the plant. According to Sandberg, the Harjavalta plant has avoided problems thanks to well-managed logistics.

“The pellets come directly from our fuel suppliers’ warehouses to our plant and are blown directly from the trucks into our silos. Humidity, for instance, has not caused any problems for us.”

“If the pellet gets wet, the milling result changes, and the powder becomes sticky. This causes problems in the process: the more water comes in, the more water vapor arises, and the combustion result changes. In turn, the ash content, which depends on the type of wood the pellet is made of, affects the fouling of the boiler. When a boiler gets dirty, its efficiency deteriorates,” Huusko says, and continues: “The pellets are strictly standardized, so there will be no problems when the pellet meets the standards.”