Töreboda – home to glulam and VänerEnergi

Founded in 1989 and jointly owned by the municipalities of Mariestad and Töreboda, VänerEnergi AB is in many ways a typical municipal district heating company that seeks to adapt and evolve to provide local, low-cost and eco-friendly heat to its citizens. The company owns and operates biomass-fired district heating operations in the towns of Mariestad, Töreboda, and Lyrestad.

In Mariestad, the largest of the three towns (population c. 16 600), heat is produced by Katrinefors Kraftvärme AB (KKAB), a combined heat and power (CHP) plant developed in 2002 and jointly owned by Metsä Tissue AB that operates the Katrinefors tissue mill in Mariestad, and VänerEnergi.

Consisting of two biomass-fired co-generation units with flue gas condensing, KKAB supplies steam, district heat, and electricity using forestry residues, woodchips, and fibre sludge from the tissue mill as fuel. In addition, residual heat from the wastewater treatment plant (WWTP) is recovered for the heat network.

History in the making

In the town of Töreboda (population c. 4 600), VänerEnergi’s plant was originally built as a heat-only plant. That changed in mid-2018 when the company decided that it would invest around SEK 1.8 million (≈ EUR 176 000) to install a 50kW ORC turbine from Againity AB.

This is a good investment for the environment. One of the major advantages of being able to produce electricity while producing heat is that electricity is produced when we need it the most, that is, during the winter. Furthermore, both fuel efficiency and total plant efficiency are improved. So as far as we are concerned, there was really no reason not to make the investment, said Kjell-Åke Wallström, District Heating Manager for VänerEnergi.

The Töreboda heat plant is located on the outskirts of the town on a site owned by and adjacent to a wood processing plant operated by Moelven Töreboda AB. Part of the Norway-headed solid wood processing group Moelven Industrier ASA, Moelven Töreboda produces glue-laminated (glulam) timber structures for construction, as evidenced in the heating plant building itself.

As a “by the way”, the visible ceiling beams in Stockholm Central train station main hall also came from the Töreboda glulam plant. Spanning 24 metres, the arched glulam beams were made and installed from 1925 to 1928.

The glulam plant itself was established in 1918 and operational in 1919, which by some accounts, makes it the world’s oldest operational glulam facility.

Briquette producer

Commissioned in 2011, VänerEnergi’s Töreboda heat plant consists of two hot water boilers, a primary baseload 4 MWth boiler that operates year-round, and a secondary seasonal load 4 MWth boiler that comes online during the heating season. In addition, there is an 11 MWth oil-fired back-up boiler that uses bio-oil.

The entire heat plant, including ancillaries such as storage silos, hot water buffer tank, conveyors, ash removal, and flue gas filters were supplied by the Danish company Weiss ApS, now part of compatriot Linka Energy A/S.

The primary load 4 MW boiler uses dry wood shavings and sawdust from Moelven’s production process. This is supplied directly to a storage silo from where it is fed to the boiler.

Since the supply from Moelven exceeds the boiler demand, the excess volume of shavings and sawdust is briquetted and stored onsite by VänerEnergi. The briquettes are used as fuel in VänerEnergi’s smallest heat plant, the Lyrestad facility, with the odd spare tonne sold externally.

We’re marginal producers with around 1 200 to 1 600 tonnes of briquettes annually of which at least half we use ourselves in Lyrestad. The amount available to third parties any given year really depends on Moelven’s glulam production and how cold and long the winter heating season is, explained Kjell-Åke Wallström.

Flue gas condenser and bio-oil backup

The secondary 4 MWth boiler uses forest chips (chipped logging residues) a “wet” fuel with 20 to 40 percent moisture content (MC) compared to the dry (8 to 12 percent MC) wood shavings. Sourced from the region, these are delivered to the plant where they are stored in a covered fuel bunker.

This boiler, which comes into service when the heating demand increases for the winter season, is also equipped with a flue gas condenser to recover additional heat from the moist flue gases.

The backup oil boiler is prepped to use bio-oil and is designed to come into operation only if for some reason the load demand cannot be met from the solid fuel boilers.

– It is a clear, common-sense environmental choice to use bio-oil, produced locally from locally grown rapeseed instead of burning imported fossil heating oil. However, the current tax policy proposal will make it an expensive choice for all the wrong reasons, remarked Wallström, alluding to the Swedish government’s proposal to apply the same energy- and carbon tax levels on crop-based bio-oils as their fossil counterparts from 2021.

Easy installation

Operational since mid-2019, the ORC turbine was installed quite literally on top of the primary load boiler.

Supplied by Againity ready mounted on a compact skid measuring 3.5 m in length, 1.6 m in width, and 2.1 m in height, the installation boils down to connecting two pipes from the boiler circuit, and two pipes from the district heating system, and a power cable.

– For us, it made sense to place the ORC unit on the same level as the operations room, which is one floor up, and avoid using ground floor space. Keeping free space around the boilers makes for easier maintenance access, for example, when replacing grate bars. Besides, there was plenty of space between the top of the boiler and the ceiling, and it was quite easy to slide the ORC skid into position through a hole in the wall. The installation itself was simple, there are only four pipes and a cable to connect to the ORC, said Kjell-Åke Wallström.

High outgoing temperature

Againity’s ORC turbines can utilize heat from hot water or steam as long as it exceeds 90°C, in this case, 105 °C hot water, is used.

– Our outgoing water temperature to the network is 80 – 90 °C depending on the ambient temperature, which is high. This is because we have a food industry client on the network that uses the heat for pasteurization purposes, explained Kjell-Åke Wallström.

The ORC has accumulated over 12 months of operational data supplying 210 MWh of electricity which begs the obvious questions on performance and availability.

We’re delighted with the unit. It is robust and works as it should and there is no real operational difference to running the plant as a CHP compared to before as a heat-only plant, remarked Harri Magnusson, Production Manager at VänerEnergi.

The electricity generated is used for self-consumption and covers about 40 percent of the power needed to operate the entire facility including the briquetting plant.

– From the boiler perspective, we could have opted for a slightly larger ORC unit, perhaps 65-70 kW to maximize the power potential, but the administrative regulations on self-generation at the time of the investment decision in 2018, made it economically unfeasible for us to go above the 49.9 kW threshold. Should this change, then I’m quite confident that we can come to some sort of upgrading arrangement with Againity, said Kjell-Åke Wallström.

Resolving dry fuel challenges

Using wood shavings and sawdust as fuel is not without its own challenges as Kjell-Åke Wallström reveals.

The small particle size in combination with low moisture content makes for a readily ignitable fuel with high mobility.

Therefore, the fuel is metered into the boiler with the assistance of a fan to avoid the risk of back fire but maintaining residence time within the furnace to ensure complete combustion is tricky.

– We were having problems with a relatively high amount of unburnt particles in the flue gases and ash. These hot particles got caught up in the textile filters in the flue gas cleaning systems causing fire incidents in the filter. This we solved by installing a cyclone before the filter to catch these particles and recirculate them to the boiler. The result has been quite significant, with improved boiler efficiency, less ash, and no filter fires since, explained Kjell-Åke Wallström.

There is though one challenge remaining, closing the ash loop given that the solid fuels used at Töreboda are clean biomass fuels.

The ash is not used in agriculture or forestry. From the woodchip boiler the ash is used as landfill cover material, said Kjell-Åke Wallström.

The ash from the shavings and sawdust boiler however cannot be used at all as it is classed as a hazardous residue.

– It came as a surprise to us too, that the ash contained high levels of chromium which can leach if untreated. Apart from going against the whole circular economy principle, it means extra costs. We test our fuel and know they are clean, so the theory now is that the boiler grate bars, which are wear parts, are the source. The investigation is ongoing to resolve this final challenge, said Kjell-Åke Wallström.

Address untapped local baseload power potential

For Againity, the Töreboda installation represents the company’s eighth ORC delivery to a Swedish district heat plant and part of its overall mission to “power-up” Swedish heat plants.

Recently, the order for unit number 20 was received.

– We are of course delighted that VänerEnergi too had chosen to upgrade its heating plant to a CHP plant. We have an increasingly urgent need for weather-independent, dispatchable electricity generation in our energy system, yet turbines are missing in more than 400 Swedish district heating plants. Many of these are smaller plants located in rural towns across the country. They could help alleviate local power constraints by installing self-generation capacity like VänerEnergi and others have done, remarked David Frykerås, CEO of Againity referring to the current electricity installed capacity and generation discussions in Sweden.