Gero Di Piazza
Encouraged by legislation, producing power through waste burning is on the increase in Europe. But in spite of its ability to reduce landfilling, environmental groups remain sceptical towards waste to energy technology such as that used by Sydkraft in Sweden.
Iroducing power from household rubbish seems an ideal way of making good use of something that could cause harm. Not only does landfilled waste release gases into the atmosphere, but landfill space is in short supply in some parts of the world.
Waste-to-energy (WTE) technology has proven to be safe, environmentally friendly and economical. Burning rubbish effectively destroys waste stream bacteria, pathogens and other harmful elements. The WTE process also reduces waste volumes by about 90 per cent. The remaining ash is either used as a road bed material or can be landfilled. The US, for example, generates more than 225 million t of rubbish annually. About one-third of that is recycled or composted, leaving 150 million t to be landfilled or used in WTE plants. WTE turns rubbish into steam or electricity to power homes and industry through the process of combustion, just as coal, oil or natural gas is burned in boilers.
Side view of the Händelö/Norrköping P14 CFB boiler
Global engineering and construction firm, Kvaerner, has in October this year implemented its latest WTE combined fluidized bed (CFB) boiler system at Sweden’s Händelö power plant. The 75 MW plant, owned by Sweden’s Sydkraft, has undergone several fuel changes over the years since it was built in 1982. The plant originally comprised twin coal fired travelling-grate boilers producing 82 MW electricity and 180 MW heat.
The early 1990s saw Sweden legislate taxes on heat produced by fossil fuel. These taxes lead Sydkraft, in 1993, to operate a new biomass fired CFB boiler. The main fuel was forest residue. Three years later, one of the coal fired boilers was retrofitted with a vibrating grate and a spreader designed for combustion of demolition wood waste. The other grate boiler was fired with coal mixed with 25 per cent tyre derived fuel.
Today it is fitted with a boiler for combustion of various types of waste. The change was made due to increasing biomass prices, large amounts of waste on the market and Sweden’s dislike of landfilling. The boiler is designed for fuel flexibility, using a fuel mix of 30-50 per cent combined household waste, 50-70 per cent classified industrial waste and up to 20 per cent sewage sludge.
The P14 boiler
The new waste boiler is situated in Norrköping, a town located 250 km southwest of Stockholm with 100 000 habitants. Bengt-Ake Andersson, engineer at Kvaerner said: “The boiler is designed to burn 200 000 t per annum and that isthe quantity Sydkraft plans to use. The boiler, titled P14 (the 14th boiler at the facility), is the first at this plant that is designed and used for combustion of municipal solid waste (MSW).The plant producessteam for localindustry, heat for the district heating network in Norrköping and electricity to the national grid.”
The boiler was chosen based on its performance at La Coruna’s Sogama plant operated by Soc Galega do Medio Ambientehas in Spain. Additional plans for the P14 unit include the firing of sewage sludge. The sludge will be pumped by a hydraulic piston pump directly into theloop seal prior to the furnace. The sand circulating through the cyclone via the loopseal into the furnace willfacilitate the transport of the sludge into the furnace.This type of feeding sewage sludge was developed in Germany during the last decade. It is used on several CFB boilers and bubbling fluidized bed (BFB) boilers. “It will be the first plant in Sweden burning sewage sludge on a regular basis. The co-combustion of this sludge with the other waste streams may require some fine tuning,” says Andersson, adding: “The aim of the [Swedish] government has been to use the sludge on farmers’ fields but more and more of the farmers do not accept sludge since consumersdo not want to buy products fertilized by sludge.”
At Norrköping, the sludge is delivered to two feeding points by a conveyor and a pump system
In Norrköping, the sewage sludge is delivered to a 45 m3 receiving bin and transported immediately by a conveyer to a 200 m3 storage silo. From the storage silo, the sludge is pumped by one pump, with the capacity range 2-16 m3/h, to the two feeding points, one in each cyclone loop seal.
In order for the incinerator to work efficiently without producing unacceptable levels of air pollution, it must be a multiple chamber incinerator, and be sealed from the environment so that no gases can escape the chamber before combustion is completed. One of the rules religiously followed is that CO must not exceed 60 to 160 ppmv on new units and 50 to 250 ppmv for existing units.
When emissions of SO2, NOx and CO become too high the proper action must be taken to reduce them. When the SO2 or and NOx content is too high it implies that either the scrubber needs more lime, the combustion temperature is too high, or there is too much excess air in the incinerator. When the CO content becomes too high, it suggests that the combustion temperature is too low, or that there is not enough air in the incinerator.
The Sydkraft plant is equipped with a Kvaerner SNCR system using ammonia for NOx reduction. The main component in the external flue gas treatment system is an Alstom/NID system with a mixer, reactor and bag filter. Lime is mixed with water and introduced to the mixer along with fly ash from the boiler and more water. The moistened particles are then injected into hot flue gas in the reactor, in which activated carbon is also added. The reactor ensures an even distribution of particles in the gas flow. The flue gas then passes through a bag filter where the particles are removed.
The Sydkraft plant is designed for fuel flexibility, using a fuel mix of household, industrial waste and sewage sludge
The lime additive binds chlorine and sulphur, while the activated carbon is used for separation of dioxins and heavy metals. Some of the fly ash is deposited in a silo, but most of it is recirculated through the mixer and reactor to give the additives enough time to do their work.
Ashes to ashes
Bottom ash is the unburned and non-burnable portion of MSW. In a mass-fired facility, bottom ash can contain considerable amounts of metals and glass. A well-operating combustor should be able to achieve a 95-99 per cent ash burnout. Fly ash is the ash that is produced from the combustion and it is filtered in the bag house. The better the air pollution control of the plant the more fly ash is collected. Fly ash can be moistened and mixed with bottom ash prior to disposal.
Against this whole process is the Greenpeace organization. Mark Strutt, senior campaigner, said: “We are trying to phase it out [waste to energy incineration]. It is very polluting to the atmosphere, especially with the fly ash and bottom ash contaminating air residue. When you incinerate waste it doesn’t just disappear, certain toxic materials are left at the bottom such as batteries, lead, mercury, light bulbs etc. Items such as plastics produce chlorine, which leads to harmful dioxins that are cancerous to humans. The people that run these plants argue that these harmful poisons are reduced when they add carbon to it, which in turn eradicates a high percentage of the dioxins. But you are still left with the harmful ash content.” The ash used for landfilling, he argues, still contains vaporized harmful chemicals that can leach.
The future of WTE systems seems strong according to both enthusiasts and critics. Increasing the percentage of waste that is landfilled would certainly help to take the pressure off landfill sites. Andersson envisages a continuation of strong global growth: “I foresee a similar development in the rest of the world as we now experience in Europe, namely towards increased recovery of material and energy from waste.”
Strutt has confidence in EU legislation, to be implemented by 2004, which pledges to radically change waste disposal habits. Although a little optimistic, he wants to carry out his policy of ‘zero waste’. He said: “Recycling should be composted into four components – organic, dry material – glass etc., left overs from the above – such as used nappies, and household waste like paints, mobile phones electronics.
“One part of the EU directive which we welcome is that manufacturers of electrical equipment will be obliged to take back their equipment for them to recycle once consumers have no use for them.”