One of the world-leading countries for CHP, Finland is now about to open a facility fuelled with biogas produced at the site from municipal solid waste and then cleaned of impurities. John Pagni, in Helsinki, and Kitty So explain.

Graphic by the architects UKI of how Kylmäjärvi 1 (K1) and 2 (K2) will
look this winter, when the second plant is completed. From left to right:
K2, K1, CFB gasifier building with SRF conveyor connection extreme right.

When officially opened in May, Finland’s new 90 MW Kymijärvi 2 power station will break ground in its novel energy efficiency and environment technology: a waste-to-energy CHP production unit using clean biogas as its fuel. The launch is being touted by operator Lahti Energy as a technological showcase for Finland, using innovative technology to cool the extracted biogas to temperatures where impurities can more easily be removed than when the gas is hot.

Based in Lahti, a compact southern city with 102,000 inhabitants, the plant’s raw material is rubbish collected within a 200 km radius from households, industry and construction sites and then processed by waste management companies to meet detailed standards detailed by Lahti Energy, a municipality-owned utility. In Finnish terms, that radius has significant critical mass, given that it includes Helsinki, 100 km away, where 10% of Finland’s 5.2 million population live. Households are supposed to separate waste, which is collected and shredded by waste management companies. Those collecting industrial waste, such as Päijat-Häme Waste Disposal, use BMH Technology’s Tyrannosaurus SRF separator-shredder to do both jobs. Processed waste is delivered to the plant’s fuel depot.

FROM SOILD RECOVERED FUEL TO BIOGAS

This sorted waste or ‘solid recovered fuel’ (SRF) consists of plastic, wood and paper that cannot be recycled but is sufficiently energy-rich to be gasified and burnt. After separating the waste, suppliers to the utility are required to cut it into a size of about 6 cm and reduce any moisture content to below 20–30%. A sample from each truckload is tested by a laboratory at the entrance to the power plant’s weigh station. Kymijärvi 2 can receive two truckloads – or 360 m3 of waste – per hour for gasification, totalling 250,000 tonnes of waste annually that would otherwise go to landfill.

‘Household waste is the best quality and people want to separate it so it can be turned into energy,’ said Jaana Lehtovirta, Lahti Energy’s spokesperson: ‘Consumers are the best at this although our suppliers have invested a lot in equipment to remove metals and other material.’

The plant operator is confident of a steady supply as it will pay for the waste, although it would not specifiy how much. ‘The recycling philosophy advances with this new system as Lahti Energy now pays for the fuel and the waste collectors get money for collecting it. This motivates everyone when even rubbish has value,’ said Jukka Manskinen, Lahti Energy’s production director.

After approval and unloading, the solid fuel is fed into two 7500 m3 silos from where it is conveyed 240 metres to the gasifiers. And it is here where the innovative alchemy begins, with the solid fuel entering a circulating fluidized bed (CFB) reactor where it mixes with sand, limestone and air at a temperature of 900°C, under which conditions the solid fuel breaks down into gas. The hot biogas rises to the top of the gasifier and thence into a cooling system where its temperature falls to 400°C.

‘The gas does not burn in the reactor – as there is insufficient oxygen for combustion – while cooling is necessary to clean the gas of unwanted particles (metal compounds and alkalis) that re-solidify and fall to the bottom and are then removed,’ said Manskinen. ‘This is the novelty – gasification is an old technology [for instance developed by the Technical State Research Centre of Finland in the 1970s], but the cooling and cleaning components, supplied by Metso Corporation, are unique. They purify the gas so that emissions are practically zero except for carbon dioxide produced during gasification.’

The weigh station (left) with SRF reception building
(foreground right) with Kymijärvi power station in the
background

The gas impurities are removed inside 12 cooling chambers containing 300 ceramic candle filters, each collecting unwanted particles while allowing the biogas to pass through. A nitrogen pulse every minute ejects collected dust, which falls to the chamber floor for removal. Manskinen and Lehtovirta added that studies are underway to find uses for all the ash – for example in agriculture or re-burning.

ELECTRICITY AND DISTRICT HEATING

Combustion of the now pure biogas fuel proceeds as normal, with gas fed into the power station’s boiler to produce electricity and district heating hot water. This it does highly efficiently – Kymijärvi 2’s production capacity is 50 MW of power plus 90 MW of heat, operating at 540°C under 121 bars, achieving nearly 90% overall efficiency.

‘The temperature and pressure are high because the corrosive metals and alkalis have been extracted during cleaning. Accordingly, other waste burning units cannot work as such levels,’ said Manskinen. ‘So our energy efficiency is 30% when others, like hybrid fossil-SRF ones, are below 20%.’

The Metso boiler burns 22 m3 of biogas every second or 86,000 m3 per hour; 10 m3 of biogas is equivalent in energy to 1 m3 of natural gas. The supplier also claims efficiency in manpower. In the control room only six people (per eight-hour shift) can monitor the entire process and also manage the original Kymijärvi 1 hybrid fuel CHP station, which will still operate.

The set-up of Kymijärvi 1, which was also built by Metso, shows how it provided a precursor to Kymijärvi 2. Lahti Energy has been operating its Kymijärvi 1 unit since 1998, using coal and gasified wood solid waste in an 85:15 ratio. Although 15% may appear a small figure, it represents 1.2 million tonnes of wood, and a reduction of 700,000 tonnes in the need for imported coal. More importantly, the technology cuts both carbon dioxide and sulphur dioxide emissions by 10%, nitrogen oxides by 5% and released particulates by 40%. Heavy metal emissions were also slashed.

This environmental performance encouraged Lahti Energy to look at building a biogas-only plant, especially as a biogas-only system would yield 40% more electricity per fuel tonne of solid waste than hybrid firing. In addition, using solid waste would replace the nominal import of 170,000 tonnes of imported Siberian coal, which would need to be trucked from Loviisa harbour.

After its launch, Kymijärvi 2 will become the plant’s main power supplier – operating all year round – with Kymijärvi 1 used for backup in peak demand periods such as winter, when the mercury can drop to -30°C. ‘We foresee that it may be operating for five-to-seven months depending on the weather and market situation,’ said Manskinen.

‘Our district heating customers are mainly apartment blocks and industry, where the majority of our 7600 customers are, as almost every building here is connected,’ said Lehtovirta.

‘The power goes into the grid and is sold on the national power exchange there at the going rate, but we have 87,000 customers nationwide, though mainly in the Lahti area. This was an ideal solution for Lahti, but it may not be for foreign cities. We have been building our district heating network here since the 1960s, which many do not have nor need and CHP produces a lot of heat – 600 GWh a year versus 300 GW of power. Therefore they must consider how to use that heat.’

The financial side looks solid too. Kymijärvi 2’s total cost was €160 million (US$209 million), of which the European Union granted €7 million and Finland’s Ministry of Employment and the Economy donated €15 million as a ‘new technology’ subsidy. Loans from the European Investment Bank (€70 million) and the Nordic Investment Bank (€50 million) made up the bulk of the funding. ‘These institutions would not have been involved if they were not convinced of its environmental and economic benefits,’ said Lehtovirta.

Not only is burning the most efficient way of utilizing this waste, the system also offers investment costs that are lower and better than mixed-waste and even biomass-burning boilers, as these lack a cooling-cleaning system. But it is the environmental benefits that could be the most eye-catching: annual carbon dioxide emissions from Kymijärvi 1 and 2 operating together will total 230,000 tonnes, down from 410,000 tonnes with just Kymijärvi 1.

Lahti Energy’s Kymijärvi CHP plants last September,
showing the old K1 unit (centre) surrounded by
construction work for the new biogas K2 unit (left)
and CFB gasifier building (right) Source: Lahti Energy

Recognition of the project’s importance has led to presentations abroad. The project also won Finland’s Ilmastoteko 2011 prize for its contribution to countering climate change. The plant will also help the country meet the EU’s 2020 green energy targets of cutting greenhouse gases by 20%, improving energy efficiency by 20%, and raising renewable energy consumption to 38%.

ADVANTAGES OF BIOMASS GASIFICATION

In 2016, the EU’s landfill directive stipulates that only treated waste can go to landfill to prevent waste generation, encourage recycling and the incineration of fibre for energy. Currently, Lahti recycles 94% of all waste and recovers valuable metals in the process.

Cogeneration specialists agree that processing biomass with CFB gasifiers offers several production and environmental advantages over direct combustion. They also welcome the innovation involved in cleaning waste gas.

In many coal-fired power stations, the feedstock must be pulverized into a powder and squirted into the boiler: ‘you could do the same with biomass but coal crushes much more easily than biomass,’ said Dr Geraint Evans, head of Biofuels and Bioenergy at the UK’s National Centre for Biorenewable Energy (NNFCC). This Finnish gasifier bypasses this challenge because it can use wood as chips.

Gasification is much more flexible in terms of the final product, said Richard A. Olliver, executive director of the US-based Gasification Technologies Council. ‘You can convert that syngas to other valuable products, one of which is feeding it to a gas turbine to make power,’ he said. Other options include: converting syngas into steam; producing chemicals such as hydrogen, methanol and ammonia; and creating synthetic natural gas or transportation hydrocarbon liquids.

Gasifying biomass is also more efficient and cheaper than solid combustion because of challenges in burning material such as ash, which melts and can cause corrosion. Dr Evans said burning biomass converts 25% of its energy into power. Gasification can increase this figure to as much as 40%.

As a result, several power operators have been developing waste-only power plants. Olliver points to a few smaller plants already using waste-fuelled gasifiers. For instance, Ansaldo Aerimpianti constructed a gasification plant in Grève-in-Chianti, Italy, using a TPS Termiska Processer gasification boiler. US-based Outotec Energy Products has provided a gasifier for a waste-fuelled plant in England.

Many other plants implementing gasifier boilers use biomass to supplement fossil fuel-based feedstock such as pulverized coal. Metso also plans to supply Vaskiluodon Voima, in Vaasa, Finland, with a 140 MW biogasification plant using forestry waste plus coal for commissioning in December. Its size makes it the ‘crown jewels’ of biomass power projects worldwide, said Salman Zafar, manager of the renewable energy advisory group BioEnergy Consult. He describes it as one of the biggest biomass power plants to be built in the world. Its most striking feature is the exceptionally large scale of its feedstock drying process for preparing wet biomass for gasification. ‘This would facilitate the use of low-cost moist materials like wood chips and green trash,’ said Zafar.

Meanwhile, Biossence, owned mostly by Switzerland-based Network Economy AG, is working to accommodate Metso’s technology in a planned sustainable energy facility in east London, UK. It is planning two more facilities in the UK that will use gasification processes from Canada’s Enerkem to turn municipal waste into syngas.

But transporting solid biomass fuel to gasification plants can also present challenges. Biomass’s low-energy density can make transporting it expensive. For example, producing around 200,000 tonnes per year of diesel from biomass would require 1.2 million tonnes of wood, said Dr Evans. ‘So, how much biomass we can sustainably get and how to move that biomass are key issues.’ These issues are leading to emerging, enabling technology such as ‘torrefaction’, which involves turning wood or similar biomass into bio-coal to facilitate its movement.

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