Kerri-Ann Richard, October 7, 2015. Image credit: Biomass Magazine
When considering the conversion of their existing coal-fired combined heat and power (CHP) plant in 2011, Finnish utility Vaskiluodon Voima evaluated a number of technical options before settling on biomass gasification as the best alternative. Integrating a biomass gasification plant with the existing coal-fired boiler was a relatively inexpensive option with the advantages of short project duration and less plant downtime. In addition, because the biomass fuel is locally sourced, there was an immediate cost savings on the importing of coal as well as regional economic and employment benefits. On the other hand, there were some anticipated process and financial risks associated with the project, and those were evaluated after the installation.
Gasification of wood biomass products results in the formation of corrosive alkali chlorides, which cause a lot of harm to the superheater tubes. To mitigate the corrosion process, the wood biomass must be mixed with a fuel that has high sulphur content, such as coal or peat. Upon startup of the plant in 2013, the initial fuel mixture was kept below 50% wood biomass. In 2014, a 24-hour test using 100% biomass with coal ash as a protective layer on the superheater tubes was a success while the plant was operating at a low load. Additional testing is being done in 2015, with the goal of creating a fuel mixture that is 70% wood biomass and 30% peat without burning any coal.
Emission regulations and taxes on coal have put a lot of pressure on the utility to switch to biomass for the long-term viability of the plant. However, as it stands, coal is cheap compared to wood chips, and the profitability of running the biomass plant is low. The hopes are that biomass will became a more appealing option with support from the European Emissions Trading System (ETS) and a higher national feed-in tariff (FiT).
Biomass gasification rises to the challenge
September 17, 2015
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.
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 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 CO2emissions 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.