Thumbs up for Sorø Bioenergi

Found in 1161 by Bishop Absalon, the town of Sorø is located in Sorø municipality (population c. 8 000) in Region Sjælland on the island of Zealand (Sjælland), about 80 km east of the capital city Copenhagen.

Incidentally, Copenhagen too was founded by Bishop Absalon as the then settlement lay midway between the Roman Catholic cathedrals built in Roskilde, and Lund, the latter across the strait (Öresund) in present-day Sweden.

As previously reported in Bioenergy International, Gert Jørgensen, Mayor of Sorø held a groundbreaking ceremony on June 3, 2020, and on October 14, 2021, Mayor Jørgensen was back again to “cut the branch” to mark the official inauguration of Sorø Fjernvarme’s CHP plant Sorø Bioenergi.

It is an architectural gem – a new bespoke landmark for the town of Sorø, thereby fulfilling one of several specific criteria set by the town council. More on that further on.

AffaldPlus- about exacting value

A novel project on many levels, another key criterion is that the plant had to be capable of utilizing locally sourced park- and garden waste as fuel. Hence the reason for starting the visit in the town of Næstved, about 40 km south of Sorø in neigbouring Næstved municipality.

It is here, at Næstved Biobrændsel, where the fuel is supplied from.

Næstved Biobrændsel is a centralized biomass fuel processing terminal owned and operated by AffaldPlus. AffaldPlus is a waste management company serving the municipalities of Faxe, Næstved, Ringsted, Slagelse, Sorø, and Vordingborg, and is jointly owned by the said municipalities.

The municipality-owned non-profit company receives processes and ensures the appropriate recycling or treatment of residential and commercial waste arisings from the municipalities.

At its disposal so to speak, AffaldPlus has six facilities for handling and processing collected waste from the six. municipalities. This includes two waste-to-energy (WtE) plants, located in Næstved and Slagelse respectively, in which non-recyclable solid waste is used to provide districting heat and electricity to each city respectively.

The 50 MWth and 13 MWe Næstved Affaldsenergi plant has three lines with a total capacity to treat around 130 000 tonnes per annum while Slagelse Affaldsenergi has a single line that can treat approximately 40 000 tonnes annually.

The latter is also the only remotely monitored WtE facility in Denmark – monitored from Næstved Affaldsenergi’s facility.

In addition, around 18 000 tonnes per annum of source-separated food waste is pre-processed into a pulp, screened to remove contaminants, and pumped to storage at Næstved Affaldsenergi. This pulp is used by third-party biogas plant operators as substrate.

AffaldPlus has 19 recycling sites distributed across the six municipalities where household and commercial recyclables are received and aggregated.

The Næstved Recycling Terminal receives sorts, and forwards recyclables from the recycling centres such as cardboard, paper, and plastics back into the market as secondary materials.

The company runs three secondhand shops (PlusButikkerne), as well as a DIY store for recycled construction materials store, PlusByg.

Hazardous consumer waste such as batteries, paints, etc, are collected, aggregated, and forwarded to third-party treatment specialists. AffaldPlus also operates two technical landfills, in Faxe and Forlev respectively, for non-recyclable and non-combustible residual materials, and from which landfill gas (LFG) for energy is extracted.

Finally, AffaldPlus owns and operates the new 12 MWth/1 MWe Sorø Bioenergi biomass-fired combined heat and power (CHP) in Sorø.

National resource strategy

As a regulatory background, it should be mentioned that the Danish government’s resource strategy stipulates that 25 percent of all garden and park waste should be converted into biomass fuels to replace fossil fuels.

This led AffaldPlus to look for alternatives to composting green waste several years ago. Recycling and recovery efforts at AffaldPlus thus also include green waste such as hedge- and roadside trimmings, grass cuttings, bush and tree “lop’n’top” and pruning from private gardens, public parks, and other urban green areas.

As mentioned, Næstved Biobrændsel is a centralized biomass fuel processing terminal that supplies the fuel to Sorø Bioenergi. The terminal is also one of thirteen park- and green waste-receiving depots located throughout the six municipalities.

As a citizen or business in one of our municipalities, you can able to use any of AffaldPlus’ facilities. Citizens are charged via the annual service fee, while companies a charged a fee per delivery, said Per Madsen, Operations Coordinator at AffaldPlus.

The material from the other twelve depots is aggregated and then trucked to Næstved Biobrændsel for further processing as it is a central location in relation to the other depots. Most of these garden waste depots are open 24/7.

Generally speaking, people respect the fact that it is just park- and garden waste such as hedge trimmings, grass, lop’n’top, stumps, etc, and not a dump site for general waste. The depots are remotely monitored, Per explained.

Two-stage process

We’ve had these depots operational for quite a number of years. Now with Sorø Bioenergi operational, we can extract a substantial biomass fuel fraction from the material to heat homes and businesses in Sorø, replacing natural gas that was previously used, said Per Madsen.

On an annual basis, around 53 000 tonnes of garden waste is aggregated. Of this, approximately 11 000 tonnes become biomass fuel, 32 500 tonnes end up as raw compost while the balance evaporates.

The raw compost is sold as a soil conditioner for agriculture only as it is not heat treated making it unsuitable as garden compost.

The biomass fuel is extracted in a two-stage process. The incoming material is put through an Eggersmann Recycling Teuton ZS 55 stationary shredder and screened by an Ecostar HEACT 7000 3F stationary screener into three fractions – fines, fuel, and oversized.

The fines fraction comprises soil, leaves, needles, moss, etc, and is put into raw compost. The oversized fraction is put back into the incoming material cycle for re-shredding while the fuel fraction is put into intermediary storage.

Following a period of several weeks, the fuel fraction is put through a Peterson Pacific Terra Select T6 trommel screen to remove remaining sand, soil, and leaves or needles.

The challenge is finding the right balance. On the one hand, extracting as much fuel as possible while keeping processing and handling costs to a minimum. On the other hand, keeping the fuel to within specification, said Per.

Although regulated by a supply contract, the latter has proved to be something of a moving target, defined by experience over the last year.

Typically, biomass plants have quite detailed fuel specifications, and there are quality standards, for example, for various grades of woodchips to base supply contracts on. In the case of Sorø Bioenergi, it is a bit of a paradox since the business case and combustion technology are based on using a very heterogenous fuel, park- and garden waste, for which there are really no quality standards. Our furnace can accept a wide range of biomass in terms of particle size and moisture content. Therefore, it really is a joint effort to establish what the optimal fuel specification for both parties, is, said Peter Olsen, Senior Project Manager at Dall Energy.

This is compounded by the fact that green waste arisings vary considerably in material and volume throughout the year – lots of grass, moss, and hedge trimmings in the spring and summer to stumps, roots, lop’n’top, and Christmas trees in the autumn and winter months.

Although both Næstved Biobrændsel, and Sorø Fjernvarme, the owner and operator of the Sorø Bioenergi plant, are in turn owned by AffaldPlus, fuel supply is on a market basis.

This is an important benefit-sharing aspect. Citizens from all six municipalities are supplying the fuel while only those connected to Sorø Fjernvarme’s heat network benefit from its use. By paying a market-based price for the fuel to AffaldPlus, all citizens benefit, said Per Madsen.

By the same token, Sorø Fjernvarme is an independent, non-profit company charged with the task of supplying district heat to Sorø on behalf of Sorø municipality.

Thus, any eventual financial ups and downs do not benefit or burden the other AffaldPlus owner-municipalities.

Sustainable construction

From Næstved Biobrændsel, the fuel is trucked the 40 or so kilometres to Sorø Bioenergi. Situated on a greenfield site earmarked for commercial development, the bespoke building is fully visible at quite a distance from the E20 motorway that runs past it.

As such there were a number of planning permit specifications regarding the design of the plant to make it an architectural feature, a positive landmark addition for Sorø.

For instance, the building itself is a “G” or “U” enclosed courtyard shape in keeping with traditional farmsteads found in the region instead of a more conventional “Thor’s Hammer” T-shape.

Instead of ‘cassettes’ in steel and aluminum, wood has been used. The eye-catching wooden slat façade is a nod to the use of sustainable woody biomass and the slats are made of heat-treated wood instead of chemically impregnated wood to prevent decay.

The roof supports a green sedum top to manage rainfall while strengthening biodiversity.

In the construction of Sorø Bioenergi, we have done everything we could within our budgetary constraints to make the construction as sustainable as possible. The building is designed with sustainable material choices. The amount of concrete is, for example, reduced by at least 25 percent compared to traditional construction. Concrete is a carbon-intensive construction material, and therefore we used alternative materials where possible such as the façade, explained Tommy Fer, Project Manager for Sorø Fjernvarme.

Innovative technology

The fully enclosed receiving bunker where incoming trucks self-unload is one example, it is made of steel instead of concrete. A fully automatic overhead Danish Crane Building (DCB) clam grab loads the partitioned infeed hopper.

Here you can see an amendment, the hopper is now inclined. We found that, unlike woodchips, which flow freely onto the infeed pushers on account of their uniform particle size, the irregular and varied particle size of green waste tended to bridge, explained Peter Olsen.

The heart of the plant is a 12 MWth Dall Energy biomass gasification furnace and a hot thermal oil boiler that powers a 1 MWe GMK high-temperature Organic Rankine Cycle (ORC) turbine.

Fuel from the infeed hopper is pushed onto the top of the circa two-metre grate-less inclined bed. The incoming biomass is dried and pyrolyzed on the top of the fuel layer and going down the layer the biomass is converted into a combustible gas, and becomes fine ash by the time it reaches the bottom of the bed layer.

The ash is removed by the characteristical screws that run the length of the hearth bed once the ash temperature reaches around 150 ^oC.

At full heat load we use about 4 tonnes of fuel per hour or 96 tonnes per day, which is about five or six truck movements depending on the fuel density, said Tommy Fer.

The heat for the drying and pyrolysis process is a combination of convective heat from the rising gasification gases below and the radiative heat from the gas combustion part above.

The gasification gas from the bottom part of the furnace is combusted in the top section. The gas combustion in terms of flow, temperatures, and emissions is very stable and is controlled via flue gas recirculation.

This control philosophy allows operation with lower excess air ratios than what is known from traditional incineration grate furnaces, and this is partly the reason why a higher overall efficiency, 110 percent (Lower Calorific Value), is possible. The gasification furnace also enables great flexibility on load, from 100 percent right down to 10 percent without compromising on emissions such as dust, NOx, and CO, said Peter Olsen.

In short, the Dall Energy furnace combines updraft gasification with gas combustion in the same chamber, which results in full performance and environmental benefits without the historical problems related to external combustion.

Primary air injection is about half of what is used in traditional grate incineration, and, with a large furnace bottom area, the gas velocity in the bottom part is very low which means particles remain here and dust emissions from the furnace are as low as 30 to 60 mg per Nm³ at the furnace outlet. Most of these particles are subsequently removed by a quench prior to the flue gas condenser bringing the levels down to 10 to 20 mg per Nm³ after flue gas condensing, explained Peter Olsen.

The quenching water, which contains the particles, is reused for ash moisturizing or temperature control of the furnace.

In doing so, the particles are removed from a high-concentration source preventing particle reinjection into the flue gas condenser.

The combination of low particle emissions from the furnace and the use of the quench eliminates the need for other dust-removal technologies such as electrostatic precipitators (ESPs) or textile filters to meet the EU Medium Combustion Plant (MCP) Directive’s statutory requirements of max 30 mg per Nm³ at 273.15 K, 101.3 kPa and 6 percent O₂ dry basis.

Do it all over again

With the blessing of Sorø town council, AffaldPlus acquired Sorø Fjernvarme from SEAS-NVE (now Andel Energi A/S, an energy distribution company), on January 1, 2018, and on January 1, 2020, it posted a tender for a new greenfield biomass-fired CHP plant.

The new plant would replace an existing fossil gas heat plant, located elsewhere in the town, as the baseload heat plant.

With the help of compatriot energy- and infrastructure engineering consultants Cowi, the tender was divided into nine lots. Having previously worked at Næstved Affaldsenergi, Tommy Fer has been involved in the project from the get-go.

While more complicated for us to manage nine lots compared to having an EPCM partner, it meant we could stagger the project timewise, and also get tenders from companies with relevant technologies but smaller balance sheets while reducing risk. Apart from thermal capabilities, the tender for the combustion island was specific in that the fuel to be used would primarily be green park- and garden waste along with woodchips or forest chips. The other specific was the overall height restriction imposed on the project, a maximum of 15 metres building height, recounted Tommy Fer.

The latter ultimately became a deciding factor. The furnace and boiler island design, procurement, installation, and commissioning, including all associated auxiliary equipment such as fans, piping, a fuel feeding system, an ash handling system, and electrical and control systems were the scope of supply for Dall Energy.

Dall Energy was one of several applicants for the combustion lot though at face value not the cheapest. However, the lot was so formed that a monetary value or discount was given to the applicant for every metre under the 15-metre threshold the housed boiler island needed, without digging a hole, remarked Tommy.

He added that there were no real concerns regarding the technology itself.

By operating waste-to-energy plants, we are pretty familiar with the pros and cons of grate-fired technologies and difficult fuels. At the time, Dall Energy had three of its biomass gasification furnaces operating in installations here in Denmark albeit smaller in size and using woodchips, so there was third-party experience available to consult with, Tommy Fer said.

In operation now for just over a year, has the project turned out as originally envisioned?

It has to a very large degree. It is a fantastic new landmark for Sorø, as we had hoped it would be. We are inundated with inquiries to visit the plant from all kinds of people, professions, and from many different countries, which we try to accommodate. Of course, we have had some minor teething issues along the way, for example, the infeed hopper, a few inadequate insulation details, and a leaky furnace seal but nothing that has impaired overall operations or performance of the plant, said a notable proud Tommy Fer, adding that family permitting, he would do it all over again.