WASTE-TO-ENERGY TECHNOLOGY

What do you think of your waste? Most people believe that their waste have no value at all. However our waste is a source of materials and energy, which remains unexploited when disposed to Landfills.

In order to explain the technologies available on waste management we should first set the required background. Waste produced by citizens is called Municipal Solid Waste (MSW). According to Eurostat, households in the EU-27 generated an average of 444 kg of waste per inhabitant in 2008. The quantity of household waste generated ranged between 300 kg and 500 kg per inhabitant in most of the EU Member States in 2008. Other waste streams that can be co-treated with MSW are Commercial non-Hazardous Waste and in some cases sludge coming from Waste Water Treatment Plants (WWTP), which should also be non-hazardous. The capacity of MSW produced and their composition varies in relation to the rate of growth of the society. Developed countries produce more waste, which contain more packaging material (plastics, paper etc). For instance in Greece the production of MSW was reduced due to the recession. Typical ranges of MSW composition are given below:

  • 28 – 32% Organics (food residuals, yard trimmings, etc)
  • 22 – 27% Paper & cartons
  • 15 – 18% Plastics
  • 8 – 10% Rubber, leather, textiles, wood
  • 3 – 5% Metals
  • 3 – 5% Glass
  • 3 – 6% Inerts
  • 4 – 6% Other

The biodegradable fraction of MSW (organics and partly paper and wood) which counts for around 50% of it, is considered as biomass which is a Renewable Energy Source (RES), according to the 2009/28 EU directive.

The European Union through its 2008/98 EU Directive proposed the following hierarchy on MSW management:

  1. Reduce
  2. Reuse
  3. Recycle
  4. Energy Recovery
  5. Landfill

In a few words, we should reduce the capacity of the waste produced, reuse what needs no further process in order to be able to return in production (ie. beer bottles), recycle packaging material, recover energy content in high efficiency power plants and landfill the residuals. According to this hierarchy, recycling comes first, followed by Energy from Waste technologies (EfW). Any waste management plant receives revenues for every ton of waste delivered in the tipping hall, called Gate Fee (€/ton). Focusing on EfW methods, there are two core technologies.

The first one is called Mechanical Biological Treatment (MBT) and the second Waste-to-Energy (WTE). MBT plants, among other products which are not subject of this article, produce secondary fuels called RDF (Refuse Derived Fuel) or SRF (Solid Recovered Fuel) depending on the technology implemented. The Lower Heating Value (LHV) of RDF/SRF produced by MBT plants varies between 15-20 MJ/kg. They can be utilized in energy intensive industries such as cement industries or coal power plants, but also in dedicated RDF/SRF incineration plants. However there are not many dedicated incineration plants for RDF/SRF, because they require the pretreatment of waste in MBT plants, resulting thus in higher treatment costs and gate fees.

On the other hand Waste-to-Energy plants receive waste, after recycling, without other intervenient plants. The LHV of MSW ranges from 7-10 MJ/kg with a typical value of 9 MJ/kg, resulting to an electrical energy production of 600-900 kWh per ton of MSW. WTE technology is the most proven and dominant technology in Municipal Solid Waste treatment with a history of more than 100 years and a reference of about 800 operating units worldwide, 435 of which are located in Europe.

The most dominant Waste-to-Energy technology is incineration. Other technologies are gasification and pyrolysis but with little references, most of which at a pilot scale. Independently the fuel treated (MSW or RDF/SRF) WTE plants utilize the Rankine thermodynamic cycle to produce electricity. The thermodynamic characteristics of the superheated steam vary from 400 – 480 °C and 40 – 125 bar. Higher temperatures of superheated steam, which would lead to higher energy production, are not selected due to corrosion problems on the water tubes of the boiler.

The products of WTE plants are energy, mainly electrical and in some cases thermal for teleheating or industrial uses. Thermal energy production most commonly appears in northern countries, where winter lasts for more than 6-8 months. Teleheating provides hot water to the local city for the central heating of buildings; hence diesel or natural gas usage for central heating purposes is avoided. In southern countries, the majority of WTE plants produce electrical energy only. However, there are a few cases, where apart from teleheating, telecooling is also utilized during summer, so that it may be installed in southern countries. Telecooling uses hot steam produced by the WTE plants to air-condition apartments, offices or hotels. Cold air production from hot steam seems to be paradox, but it happens by utilizing absorption cooling technology, like the gas refrigerators sold for households which do not consume electricity. Another product is metals recovered from ash, which can be recycled.

Ash is the only residual coming from WTE plants, the volume of which is 90% reduced compared to the volume of the incoming waste. As life of landfills is calculated according to the volume of waste disposed, the 90% rate of reduction is so important that the landfill lifetime is decupled. WTE ashes are divided in two main streams, the bottom- and the fly ash. Bottom ash, in most cases, is an inert non-hazardous by-product, the mass of which is 22-28% of the incoming mass. Bottom ash can be utilized as daily coverage material in landfills or in construction works (road works, substitute in cement etc), resulting to a further reduction of the residuals mass sent to landfills. On the other hand fly ash, which represents 3-5% of the incoming mass, is a hazardous material which needs further treatment in order to be disposed safely in exclusive landfill cells.

The main concern regarding WTE plants is the level of the flue gases emissions. The European Union has set limits for each pollutant produced by WTE plants through its 2000/76 EU directive. However modern WTE plants can accomplish emission rates far below the regulated limits by implementing state-of-the-art flue gas cleaning systems. For instance, dioxins produced by WTE plants is less than 0.07% of the total dioxin production in Europe (source: Waste Management World).

Waste-to-Energy is proven and environmental friendly technology, giving a final solution to waste management. According to the Confederation of European Waste-to-Energy Plants (CEWEP), in 2009, 70 million of MSW were treated in WTE plants across Europe, generating 28 billion kWh of electricity and 70 billion kWh of heat, which resulted to a substitution of 7-38 million tones of fossil fuels (gas, oil, hard coal & lignite). Replacing these fossil fuels, WTE Plants can supply annually about 13 million inhabitants with electricity and 12 million inhabitants with heat.

By Manolis Klados
iMBA, PT14

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