From trash to gas: Waste gasification trumps incineration in Finland
An ultramodern facility in the city of Lahti maximises the energy recovered from municipal solid waste.
A render of the KYVO2 plant currently under construction. (Image: UKI Arkkitehdit)
Landfills are the final destination for much of the world’s solid waste. With growing stress on the world’s finite resources, it has become clearer that the practice is a waste of space, materials and energy. Recycling helps to mitigate these issues, but as our energy demand rises, energy recovery from waste has become increasingly popular. Incineration is the most common method of energy recovery, but burning waste comes with its own price. It is not particularly efficient, and the diversity of the fuel can result in harmful air pollution. Gasification is a more complex method of energy recovery offering improvements in both efficiency and air emissions.
The city of Lahti, Finland, has a long tradition of sorting waste. Their goal is that waste management and energy production should be mutually supportive, and they provide residents with a separate container for energy-rich waste. This has made it possible for Lahti Energy, an electricity and district heating company, to mix in a fraction of solid waste in their coal gasification plant. The waste has replaced nearly 700 000 tonnes of coal since they started in 1998.
Based on their experience from mixing waste with coal, Lahti Energy decided to build a new gasification plant running entirely on solid waste. The plant is currently under construction alongside the original plant, to make use of the existing waste transportation infrastructure. When ready in 2012, it will use 250 000 tons of solid waste to produce 300 GWh of electricity and 600 GWh of heat annually, enough to cover the energy needs of the 100 000 citizens of Lahti for most of the year.
The technology that makes it possible
With gasification, solid fuel goes through a controlled heating that transforms it into a mix of combustible gases. This allows impurities to be filtered out of the gas before combustion, facilitating efficient burning and avoiding harmful emissions.
At Lahti Energy’s new KYVO2 plant, the filtered gases will be mixed with crushed limestone while suspended in an upward-blowing jet of air. This creates a chemical reaction between the fuel and the limestone under combustion, removing sulphate from the gas and preventing it from reacting with oxygen to form sulphur oxide emissions. By tumbling and mixing the burning fuel in a jet of air, a better heat transfer is achieved than in conventional boilers. This allows the plant to burn at lower temperatures, reducing the reaction between nitrogen and oxygen that creates nitrogen oxide emissions. After transferring its heat to the boilers, the flue gas goes through a final filtration before being discharged.
The heat produces steam for electricity generation. With a combined cycle turbine, some of the excess heat can be reused to generate even more electricity. According to Lahti Energy, this makes it possible to get 40% more electricity per ton of fuel compared to state-of-the-art incinerators. The remaining heat is used to heat water for a district heating system. The overall efficiency of heat and electricity production combined could potentially be as high as 89 %, making it by far the most efficient way to recover energy from waste.
"Since we are an electricity producing company, we want to produce as much electricity as possible. With gasification we are able to raise the amount of electricity produced to much higher than traditional incineration plants." - Jaana Lehtovirta, Head of Communications, Lahti Energy.
With the KYVO2 project, Lahti Energy stands out as an international forerunner on waste gasification. Based on commercially available technologies, this Finnish concept could easily be used in other places around the world. With innovative solutions like this, the City of Lahti is now competing to become the Nordic Energy Municipality of 2011.
