The Lahti waste gasification plant in Finland produces 50 MW of electricity and 90 MW of district heat from sorted industrial waste.
Non-recyclable waste can be a valuable fuel for energy production. For example, a city can use Valmet’s*) solutions to maximize its revenues from recycling materials and also from generating electric power, heating, and cooling.
“Valmet’s technology offers high fuel flexibility and an electrical efficiency that can be as high as in biomass-fired power plants. This gives the plant operator more options when choosing fuel and also generates more electricity. Both of these benefits greatly improve plant profitability,” says Pertti Petänen, Director, Products and Technology, Valmet. “Our technology gives very efficient combustion, which results in extremely good environmental performance.”
Valmet has long experience in combustion and gasification based on fluidized bed technology. Since the late 1970s, Valmet’s boilers and gasifiers have been used to combust various types of solid fuels, such as fossil fuels, biomass, and recovered fuels. Whether combusting wet biomass and other low-heating-value fuels or with high heating value fuels, the performance is excellent. In Valmet’s Circulating Fluid Bed waste-to-energy boilers, the high-temperature heat surfaces are protected from the corrosive substances in the flue gases. This gives high steam data and increased production of electricity.
At the E.ON Händelöverket plant in Sweden, good results have been achieved. In 2002, Valmet built a circulating fluidized bed boiler that produces electrical power, process steam, and hot water for district heating.
Valmet builds fluidized bed boilers with a fuel input of up to 1,000 MW, and the biggest waste-fired plant combusts over 400,000 tonnes of waste per year. Valmet also delivers smaller plants. “For plants with a fuel consumption of some 100,000 tonnes per year, we have a modularized waste-to-energy solution that makes project development relatively easy. The plant produces approximately 9 MW of electricity,” Petänen points out.
Valmet has also pioneered waste gasification. One of the biggest benefits of this technology is that electricity efficiency can be even higher than in traditional combustion.
If a fuel is not completely combusted, the risk of emissions of hazardous compounds can be high. “In our fluidized bed boilers, the fuel is exposed to a high temperature and mixed very well with oxygen. Combustion efficiency is excellent, with very low levels of unburned fuel in the ash and a low oxygen content in the flue gas,” Petänen explains. “And the hot flue gas has a long retention time at high temperatures, which keeps emissions levels very low.”
“Our fluidized bed boilers run well also when fuel quality varies, and this helps to keep the reliability high,” he continues. “Our boilers can also be run with varying loads, which, in some circumstances, can be very useful.”
Combustion in fluidized bed boilers enables a high-efficiency recovery process that fulfills today’s waste hierarchy targets.
Shredding and removing metals from waste are both essential parts of the recycling and recovering processes. In fact, this has created a new market for fuel preparation. Tighter quality control of the Solid Recovered Fuel (SRF) benefits other users, such as metal recycling companies, as well. Fuel suppliers will be able to invest in new machinery and create new jobs in the supply chain.
Stora Enso's Langerbrugge boiler in Gent, Belgium was designed for 125 MWth and the fuel is Refuse Derived Fuel (RDF), untreated and treated wood, and coal with a heating value range of 10–26 MJ/kg.
Solid Recovered Fuel (SRF) is one of the most challenging fuels because it typically contains a lot of chlorine, heavy metals, and incombustible debris. SRF often causes significant fouling, corrosion, and combined corrosion-erosion in the boiler.
Valmet has developed solutions that have proven to be suitable for demanding fuels, and good results have been achieved; for example, at the E.ON Händelöverket plant in Sweden. In 2002, Valmet built a circulating fluidized bed boiler that produces electrical power, process steam, and hot water for district heating. Up to 200,000 tonnes of waste per year is combusted and the fuel flexibility is high. The fuel mix is 30–50% combined household waste, 50–70% classified industrial waste. The boiler is designed for 65 bar (g) and 470°C with a capacity of 75 MWth, and a fuel input of about 83 MW.
Another example is the boiler at the Stora Enso Langerbrugge paper mill in Gent, Belgium. This is designed for 125 MWth (~137 MWfuel), 45 kg/s, 60 bar (g) and 475°C. The fuel is Refuse Derived Fuel (RDF), untreated and treated wood, and coal with a heating value range of 10–26 MJ/kg. Fuel flexibility is high here as well, and all fuels can be burned separately with a 100% energy share. The overall boiler availability and reliability have been very high; the latter is about 99%.
Currently, Valmet is delivering the world’s largest recovered fuel-fired boiler to Mälarenergi in Sweden. The boiler will be commissioned in the spring of 2014 and is designed to burn up to 70% household waste and up to 100% industrial waste. In addition, sludge, recovered wood, peat, and biomass are part of the design fuel palette. The boiler is designed for 155 MWth, 58 kg/s, 73 bar (g), and 470°C.
The main benefit of waste gasification, compared to other waste-to-energy processes, is the higher electrical efficiency that gasification offers.
In Valmet’s waste gasifier, waste-derived fuels are gasified into combustible product gas, which is cooled, filtered, and then combusted. The filtration removes the corrosive components from the gas so it can be combusted in a boiler with high steam parameters. This enables the higher production of electricity compared to traditional combustion.
In the spring of 2012, a 160-MW power plant based on Valmet’s gasification technology was started up in Lahti, Finland. The Lahti gasification plant is the largest and most efficient waste gasification power plant in the world. It processes 250,000 tonnes of SRF per year and produces 50 MW of electricity and 90 MW of district heat. In addition to SRF, the plant has been operating successfully with logging residue (up to 45% water content) and plastic-rich waste (18% water content). The experiences from the first year of operation have proven both the technology and cost structure to be viable for large-scale power generation.
“The key results in Lahti include high electrical efficiency achieved from waste-to-energy as well as a major reduction of fossil fuels and CO2 emissions,” Petänen comments.
Today, Valmet is a forerunner in the development of fluidized bed technology. Valmet’s main focus over the last 15 years has been on the utilization of renewable fuels and opportunity fuels. Valmet’s Research Center in Tampere, Finland, provides excellent possibilities to test new fuels. The center has three test reactors for fuel testing.
Since 1979, Valmet has supplied more than 300 boilers featuring fluidized bed technology. During the past ten years alone, the company has delivered more than 13 GWth of boiler capacity that utilizes renewable fuels. “This is a significant contribution to lower CO2 emissions. Our solutions really help our customers make a difference,” Pertti Petänen concludes.
*) Metso until Dec 31, 2013