Waste to Energy systems (WTE)

What to do with all the waste when none of the materials can be recovered? According to the hierarchic waste pyramid, the preferable option are the Waste to Energy systems, in other words incineration with energy and/or heat recovery, before disposal in the dump (where neither energy nor material can be recovered). In the Waste to Energy system, or in the Incinerator, waste is burnt, and the heat producing content of the waste is exploited (remember, plastic is produced from petroleum and therefore has a high heat producing power), heat is generated, water is heated to produce steam in order to obtain electric energy. This energy can be used to produce heat, to produce electricity or for the combined production of heat and electricity (cogeneration). Furthermore, with the Waste to Energy systems it is possible to decrease the mass of waste by 80-65%, and the waste volume by 96%. Up to about 20 years ago, waste was burnt only to decrease its volume and to make it inert without any energy recovery. However today the situation has changed and engineers, researchers and technicians study how to improve these plants from a technological point of view, making them increasingly safe and efficient. In many countries Waste to Energy systems are already a consolidated alternative (e.g. Japan, Sweden, Denmark), while in Italy, only 19% of the waste is incinerated. Which part of USW is burnt? The “combustible” fraction consists mainly of paper, plastic, organic waste (grass, wood, food leftovers) and from an energy point of view waste can, in some ways, be compared to fossil fuels, as these are organic materials which contain elements that can be oxidized (carbon and hydrogen). The Waste to Energy system is complex, and involves a number of chemical reactions, whose results depend on the operative conditions that are utilized and the technologies and processes that have been developed specifically for MSW, with the following possible operative solutions:

  • direct combustion, where waste is burnt and the thermal energy of the heat is transferred to a thermal vector (steam);
  • conversion into an intermediate liquid or gas fuel, by means of pyrolysis or gasification.

Combustion takes place in special furnaces, in 4 different stages: heating and drying, pyrolysis, combustion and/or partial oxidation, combustion and/or gasification of the carbon material. Besides the heat generated by the combustion, also ashes and gas emissions are produced; both these require special treatments to reduce their polluting load, so that they can be released in the environment without any risks for our health. The heat developed by combustion of waste is recovered and used to produce steam. In turn the steam that is generated activates a turbine that is coupled with an alternator and a gearmotor, and converts thermal energy into electric energy; alternatively the steam is used as a thermal vector.
How much energy do we obtain by burning waste? The yields of the Waste to Energy systems are however lower than the traditional electricity power plants, due to the low heating power of the waste: the efficiency is therefore variable and ranges from 17% to 25% (30% may also be reached in the more forced cycles), and increases to over 50% in case of heat recovery, producing indicatively 0.67 MWh electricity and 2 MWh heat for district heating systems per ton of treated waste. This has not prevented some cities from using this solution to optimize their energy demand and for their waste disposal, as for example in cities like Oslo, Paris and Vienna.

Sources (in italian):
Arena U., Leone U., Mastellone M. L. (a cura di), 2007. Recupero di energia e materia da rifiuti solidi: i processi, le tecnologie, le esperienze, le norme. AMRA s.c. ISBN 978-88-89972-07-6

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