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Rebuild of backup heat plant offers biomass fuel flexibility

Since 2013, the Danish district heating supplier Ølgod Fjernvarmeselskab Amba (Ølgod DH) has been using wood pellets as fuel for its 2 MW peak load and backup hot water plant. Soon they will be able to combine pellets with woodchips thanks to its Linka boiler. By investing in a woodchip receiving, storage, and feeding equipment, the company is ensured future fuel flexibility at the facility benefiting both the company and their 1 600 customers in the town of Ølgod.

Expected to be completed by the end of 2018, work on the project to retrofit Ølgod DH’s 2 MW wood pellet-fired hot water boiler plant in Ølgod, Denmark to use woodchips as the main fuel is being carried out by Linka Energy that built the original plant in 2013. Originally designed as a peak load and backup plant, increased heat demand will see the facility begin operating more as a baseload plant (photo courtesy Linka Energy).

Founded in 1952, Ølgod DH currently supplies heat to over 1 600 customers, equivalent to more than 98 percent of the households and companies in Ølgod. Although heavy fuel oil (HFO) and subsequently natural gas were the original fuels used, the heat generation today is almost entirely, 99 percent, derived from carbon dioxide (CO₂) neutral sources such as biomass and industrial waste heat.

In 2013, Denmark-headed biomass boiler technology provider Linka Energy A/S delivered a 2 MW pellet based hot water boiler plant to be used as peak load and backup for Ølgod DH’s main supply. Replacing an oil-fired unit, the order included the boiler house and a separate building for ash storage.

The plant consists of a 2 MW Linka H boiler, which is designed to achieve full combustion and efficient utilisation of the biomass fuel. A cylindrical, highly efficient channel flue hot water boiler, it has a moveable, water- and air cooled step grate installed in the front of the boiler and features a smooth flame channel and water-cooled rotation chambers.

According to Linka, it is dimensioned to achieve full combustion and efficient utilisation of the radiated heat in the flame channel, whilst convection heat is utilised to the maximum in the subsequent flue gas sections. Through a proprietary combustion system, secondary air is added to the process in a downstream principal, to create the correct amount of turbulence in the combustion area.

The boiler is equipped with automatic pneumatic soot removal so manual cleaning is reduced to a few times per year. An ash extraction system, mounted inside the boiler, transports the ash from the boiler to an 8 m3 ash container located in a separate building.

The 2 MW Linka H biomass boiler is designed to operate on biomass fuels such as woodchips and pellets and can be easily switched from one fuel to run on the other. The boiler is equipped with an automatic pneumatic soot removal and internal ash extraction. The bag filter removes dust from the flue gases (photo courtesy Linka Energy).

The pellets are augered to the boiler from an 85 m3 storage silo. As the heating plant is located inside the town of Ølgod, close to residential areas, the requirements on emissions and noise are particularly stringent. To accommodate these requirements, a bag filter adjacent the boiler removes dust from the flue gases and a silencer is mounted on the 25-metre high flue stack.

From pellet peak load to woodchip baseload

The pellet-fired hot water boiler plant replaced an older oil boiler unit that was used for peak demand during especially cold spells and as a backup to the main woodchip-fired baseload heating plant located outside of the town.

By switching from oil to wood pellets Ølgod DH could significantly reduce heating costs and its fossil carbon footprint. Since 2013, wood pellets have accounted for around 3 percent of Ølgod DH’s annual heat production, and the pellet boiler has had an annual operational runtime average of 300 hours.

Earlier this year, a nearby factory opted to remove their old boiler and connect to theØlgod district heating network instead. This will increase the baseload heating demand on the heating network putting the backup pellet boiler plant into full use. However, to reduce future fuel costs – pellets are more expensive than woodchips – Ølgod DH chose to retrofit the boiler plant to use woodchips as the main fuel instead.

Linka Energy was tasked with effectuating the fuel switch by adjusting the boiler parameters accordingly and building the frontend woodchip receiving, storage and boiler feed. A sunken woodchip receiving and storage bunker is being built next to the existing boiler house. The pellet silos will be kept, thus allowing for combined fuel combustion, with both woodchips and pellets.

As the existing boiler was already prepared for additional fuel types, an only minor rebuild is required. The control system is adjusted for combined fuel combustion and this means that the boiler can alternate between woodchips and pellets, with an easy switch on the control panel. The entire retrofit project is expected to be completed by the end of 2018.

Concrete formwork of the new sunken woodchip bunker. The top of the bunker will be equipped with a hydraulic roof enabling easy access and refill as delivery trucks can unload directly into the bunker. Scraper arms will move the fuel forward to a sunken auger, from where it will be transported to the boiler via inclined augers. Photocells will keep the sunken auger filled at all times (photo courtesy Linka Energy).

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