Incinerator

(Residual / Municipal Waste processing plant Spittelau, Austria)

Flashback: As controverse as it may sound, but incineration can be viewed under the aspect of " energy on loan"; i.e. the reversal of the manufacturing process by giving off the potential energy content that is trapped: within. From a thermodynamic point of view, any incineration is the least feasible option, as the energy released is set free as heat and if not further used wasted for ever!
Incineration of represents just one of several approaches to deal with the residual waste issue. Even though the combustion of garbage cannot be considered an environmental sound way, in certain cases it represents a sustainable remedy if landfill space is scarce and fossil fuel bills are breaking off huge chunks of a city's budget. The city of Vienna (AUT) operates two municipal waste incineration plants and employ "state of the art" filter technology. In order to deal with the waste of about 300 thousand households, the plants operate 24h a day, and almost 365 days a year. Their total capacity is designed to handle around 450 000 t/year. One of the incinerators - the "Spittelau" plant - is located in the heart of the city. Combustion of chlorinated waste matter (e.g. PVC, salt, etc.) generates volatile halogen species that readily react with hydrocarbons to form highly toxic poly-chlorinated dibenzo-dioxins (PCDDs) and poly-chlorinated dibenzo-furans (PCDFs) - generally known as dioxins. Therefore, the filter ash from exhaust air purifiers are highly contaminated with these mutagenic chemicals. The filter ash of the Spittelau plant has a dioxin concentration of about 2.160mg TEQ/kg on average, which means a yearly output of more than 10g TEQ (toxic equivalent of 2,3,7,8,tetrachlor dibenzo-dioxin = TCDD). To reduce dioxin leaching into the environment, the filter ash is mixed into cement. The hardened blocks enable legal deposition along with household waste at Viennese landfills. Although glassification of filter ash would be a lot safer, it is currently not executed, as it is more expensive than the concrete alternative. Although the concrete blocks enriched with dioxin levels that high should be considered as hazardous waste and therefore not discharged in a common household landfill site, it is obviously common legal practice.
The plants location within the city boundaries is ideal from a logistical point of view; it cuts short the garbage truck collection and delivery radius, contributing to less greenhouse gas emission, as well as lowering fuel costs. As mentioned before, incineration of waste is not a genius approach but it considerably lowers the all over volumetric amount of the combusted material.

Though, the plant gains further recognition as the thermal energy output as a result of the combustive reaction feeds its process heat directly into the district-heating network of the city of Vienna, heating hospitals, homes, offices, etc. At current rates, the entire heating capacity of 60MW is able to heat about 15000 homes, each with an average floor space of about 80m2. This significantly reduces the total greenhouse gas output of the entire community and is also well in phase with the limits proposed by the Kyoto protocol - which aims to freeze global greenhouse gas production at levels of the early 1990's.
Even though recycling is practiced throughout the city of Vienna, the waste delivered still contains a high enough combustible fractions in form of paper, plastic, and compound products to maintain a self-combusting reaction. The furnace itself is kept at a minimal combustion temperature of 800C; and fed with a flowrate of about 18 tons of waste per hour. In very rare cases, when the minimal caloric value of the garbage (8.6MJ/kg) is not reached, additional natural gas is used to overcome the activation energy required starting the reaction.

RWI at Spittelau - Vienna

The solid fraction resulting from the incineration process includes ashes, scrap metals, and other incombustible materials. In general, some 280kg of ashcake are obtained when 1000kg of waste are burnt; i.e. the equivalent of 28%. It is filtered, and screened for any metallic compounds, before being partly used as an additive in the cement industry (used only in special industrial applications - like the inner/outer sealing strips of landfills, etc.). The scrap metal fraction is forwarded to the smelter industry, while the remaining mineralised fraction is discharged in landfills.
Þ For final deposition, refer to the landfill-section.

Filter cake

The hot combustion gases at the site of generation - peak at 850C. Being too hot to be used safely, they are cooled down by heating up a substantial mass of water. This pool of water in turns drives a steam turbine, which provides enough electricity to cover the energetic requirements of the entire plant. To further increase the plant's efficiency, the remaining heat contained in the condensed water column is then fed into the district-heating network of the city of Vienna.

In a final step, the cooled combustion gases are further processed by passing them through several stages of electro-mechanical filter stages, sprinklers, and neutralization chambers to eliminate and hinder any re-formation of toxic gasses (mostly dioxins, furans, HCl, SO2, etc.). The currently used filtering technique yields cleansing results that are far below current legal limits.
The water required for the cleansing of the combustion gasses are purified within the plant itself and discharged into the nearby Danube river.
Þ For waste water treatment, see also the sewage-section.

Public exhaust gas monitoring display station

References: Fernwärme Wien (1998); Spittelauer Lände; A-1090 Wien - AT
25 Jahre Spittelau (1996); Fernwärme Wien GmbH; Spittelauer Lände; A-1090 Wien - AT
For additional information visit the RWI web-site or any of the other sites listed below:
http://www.kunsthauswien.com
http:// www.kunsthauswien.com/english/joingame.htm
http://www.fernwaerme.co.at
http://www.ubavie.gv.at
http:// www.greenpeace.org/~toxics/html/content/aust2info.html
http://www.epa.gov/ttn/atw
http:// www.metsoautomation.com
http://www.rvf.se/ - solidly fixing dioxin.pdf

Intro / Paper / Glass / Plastic / Metal / Compost / Toxic / Residual / Sewage / Landfill