HomeRenewablesBiomassBiomass gasification rises to the challenge

Biomass gasification rises to the challenge

The 2013 conversion of a Finnish coal-fired power and district heating plant to biomass involved a number of technology and financial risks. Tildy Bayar visited the plant to find out what its owners have learned during the first two years of commercial operation.

In 2011, Finnish utility Vaskiluodon Voima undertook the conversion of its 230 MWe, 175 MWth Vaskiluoto coal-fired combined heat and power (CHP) plant to biomass – the largest such conversion project at the time. The chosen solution – integration of a large-scale biomass gasification installation with the existing coal-fired boiler – was completed in 2013.

COSPP visited the utility in the city of Vaasa and spoke with Mauri Blomberg, managing director, about the learnings and outcomes from the converted plant’s first two years of operation.

Project background

As well as power production of 0.9-1.7 TWh per year, the plant provides around 450 GWh per year in district heating, covering around 60% of the Vaasa region’s heating demand. Its coal-fired boiler dates from 1982, with a desulphurisation plant added in 1993 and a new turbine plant in 1998. The plant is now fit for use until 2030, the utility said.

The main drivers for the fuel conversion included high taxes on fossil fuels used for district heating, pressure from Finland’s government to decrease the use of coal in CHP plants, expected returns from the European Emissions Trading System (ETS) and a national feed-in tariff (FiT) of €15 ($17)/MWh for forest biomass. The hoped-for outcomes were extension of the existing plant’s lifetime, emissions reduction, cost-efficiency in power and heat production, and use of domestic resources. And, says Blomberg, without some kind of retrofit ‘we couldn’t run the plant for too long – we thought we might be obliged to close down in 2020’.

The Vaskiluoto plant Credit: Vaskiluodon Voima
The Vaskiluoto plant Credit: Vaskiluodon Voima

Several technical options were considered, Blomberg noted. A new circulating fluidised bed (CFB) boiler to replace the existing pulverised coal-fired boiler was a good option from a technical viewpoint, as it would enable the use of a wide range of fuels. However, it would also feature high investment costs and significant downtime. Adding biomass grinding equipment to the existing plant would be a cheaper solution, but it would require higher-cost dry biomass for grinding to a sufficiently fine grade (normal forest chips would not be feasible). And, with the use of local biomass, only a relatively small amount of coal could be replaced.

Integration of a biomass gasification plant with the existing coal-fired boiler would enable the use of a wide range of biomass and other fuels; would be a relatively inexpensive retrofit solution which would enable the use of the existing boiler; and would need a much shorter project time, with only a few weeks’ downtime. Negative aspects included the technical risks associated with a new technology and some process risk with the existing boiler. However, this solution was seen as the best option.

The overall project budget was just under €40 million, with a renewable energy and new technology grant of €10.8 million from Finland’s Ministry of Employment and the Economy. The main equipment supplier was sustainable technology and services firm Metso (now Valmet Technologies). Handover of the completed plant took place in January 2013.

The project in operation

The gasification plant has a fuel input of 140 MW and can replace between 25% and 40% of the coal used by the power plant. Gasification of wood, peat and field biomass results in product gas that is fired together with coal in the existing boiler. In addition, the gasification plant can be fired exclusively with forest biomass, and uses peat as a reserve fuel.

From the fuel receiving depot, the biomass passes through sampling, crushing and sieving, and is then stored in two silos before passing through a dryer to the gasifier. The dryer is 50 metres long and 10 metres wide. Inside it, warm air is sucked through a layer of wood chips on a conveyor belt, drying and reducing it to a size that can be burned in the boiler. Blomberg said the plant’s employees have nicknamed the dryer ‘the Titanic’ because of its outward resemblance to the ship.

In operation, he said the gasification plant has reduced the power plant’s coal use by 25% to 40%; has reduced its CO2 emissions by 230,000 tonnes per year; and has positively impacted the Vaasa region’s economy and employment. As the biomass fuel is produced locally, the regional economy has gained around €15 million per year – money that was previously spent on imported coal and CO2 emissions rights, Blomberg said. And the plant has resulted in a direct, permanent net employment effect of around 100 person-years.

The first two years

According to Blomberg, the gasification plant has fulfilled all expectations in its first two years of operation, with just two minor issues. First, a mechanical dimensioning issue with the biomass scraper chain conveyors required reinforcement of the original fixings between the chains and the scrapers, which were not strong enough. Next, there were minor slagging problems at the gas burners. Slagging is a slow process, he emphasised, with an easy solution: mechanical cleaning of the burners at two-month intervals. This process does not require any downtime, and the slag is easy to remove as it is porous and brittle.

To minimise the risk of corrosion at the boiler superheaters, the share of wood biomass is kept below 50% of the total fuel input. Corrosion risk is constantly monitored with online measurement tools.

In September 2014 a test was undertaken to see if the plant could run on 100% biomass, during which the coal feed to the boiler was stopped completely for 24 hours and only wood chip gas from the gasification plant was used, with the plant operating at low load.

The test was a success, Blomberg said, although ‘we took a small risk’ of corrosion, which is usually mitigated by the sulphur in the coal, he explained. However, ‘we knew that the corrosion would not start very quickly’.

‘In our test we had protective old coal ash layers on top of the superheater tubes, which prohibited formation of corrosive alkali chlorides layers on the tubes,’ he said. ‘Another mitigating factor was that during the test we used good-quality wood chips which had low chloride content. Both these matters lowered the corrosion risks.’

Once the corrosion process starts, he added, the only way to stop it is to stop the boiler and wash the alkali metal layers off the superheaters with high-pressure water washing.

The plant produced 40 MWe and 50 MW of district heat during the test, Blomberg noted. ‘This year we intend to test the fuel mix where we have 30% peat and 70% wood chips; we intend to operate for a few weeks with this fuel mix without using coal,’ he said. ‘If we use just biomass there is corrosion risk – you need to have a fuel that emits sulphur to protect the boiler from corrosion.’

He explained: ‘Corrosion mechanisms related to biomass combustion are highly complicated chemical processes. In pure biomass combustion, alkali chlorides tend to form highly corrosive layers on superheaters. Luckily this can be hindered in a simple way. Co-firing fuel that contains sulphur has the benefit of converting corrosive low melting temperature alkali chlorides (for example KCl) into much less corrosive and higher melting temperature alkali sulphates (in this example into K2SO4).’

Peat, the plant’s reserve fuel, has some sulphur content, he noted. In subsequent tests, the utility plants to discover whether 30% peat is sufficient to protect the boiler from corrosion. ‘Peat is an inexpensive fuel,’ Blomberg says. ‘Most likely 30% of peat is enough. If somewhat more is needed, peat still remains a viable option.’

Sourcing the fuel

Running the plant on biomass became economically feasible ‘when we acquired the biomass gasification plant,’ Blomberg said. The coal used is imported from Russia and is purchased ‘at the world market price, so we are not exactly dependent on Russia,’ he added. How much coal is imported varies from 250,000 to 500,000 tonnes. ‘If the electricity market price is very low, we will use only a little amount of coal,’ he said.

The fuel required for the gasification plant, which produces 900 GWh per year, is procured within 100 km of the plant. It consists of 90% wood chips and 10% peat, delivered by 30 trailer-trucks per day. (‘We don’t use straw because its chloride and alkaline metal content is very high, and also there is no FiT for it,’ Blomberg says.) The wood chips are composed of forest residue chips, stem wood chips, and stump wood chips (mainly from spruce trees). By-products of the wood processing industry, such as sawdust and bark, are also used, as are recycled construction wood chips.

The peat used comes from local marshlands. The coastal area of the Vaasa region is ‘not a peat area as the peat layers are too thin here,’ Blomberg explains. To source the peat needed, ‘we go inland. We use peat,’ he continues, ‘because when you feed wood chips into a gasifier you have to be sure you get a 24/7 flow of wood chips, so peat is a reserve fuel for the gasification plant.’

However, he says, ‘the fuel competitiveness of wood chips is not very good at the moment, while coal is very cheap. But we don’t need to buy emission allowances for the wood fuel, and don’t need to pay any tax for that share of fuel.’ Vaskiluodon Voima has ‘chosen to make some losses with this biomass,’ he says, as ‘we don’t want to let down the people who work for us at this forest site in producing these wood chips and transporting them to us.’ But, ‘at the moment, profitability is not good compared to coal’.

‘If the competitiveness of biomass remains as it is now,’ he adds, ‘we must, at one point, reconsider the situation and maybe totally stop using wood biomass. At the moment we are positive and hope that this will not happen. We still have some belief that the ETS and/or a higher national FiT will improve the position of biomass in relation to fossil fuels.’

For the present, he says, ‘We can reduce the amount of wood chips or try to get them at a lower price.’

However, depending on various factors including developments in the power market, the plant could still be set to close in 2020, he said. When asked if this is likely, he refused to speculate, saying: ‘Looking at the market price of electricity in the Nord Pool area, it appears that coal condensing power is not profitable in the foreseeable future.

‘With coal-fired combined heat and power plants, the outlook is more complicated,’ he continued. ‘In general, it also affects if your power plant already meets the emissions levels of the IED, or if you need to make investments to fulfil those requirements.’

But, he noted, the plant’s continued viability is ‘not about emissions regulations from the EU. Politicians [in Finland] are putting a lot of pressure on the use of coal, and the tax for coal to be used in district heating production is very high’.

One cheaper fuel option could be energy from waste; however, Blomberg said that ‘for us there is a waste incinerator plant in Vaasa, so there is no waste left over – it collects all waste for a 100-metre radius from here’.

‘I hope emission trading would start to work to support us, and also that the FiT would be higher in Finland,’ he concluded, noting that he was hopeful that such a result could materialise at some point in the future depending on changes in government.

It is clear that the technology outcomes from the plant retrofit have been positive. However, the plant’s situation with regard to profitability, as it is dependent on larger factors, is less clear even in a country with abundant biomass resources. For biomass to be viable in the coming decades, Blomberg says, ‘we would quickly need some new decisions on the national and EU level’.