Technical Article - November 5, 2019
In the classification of urban waste production, Switzerland is among the worst countries in the world with more than 700kg of waste per capita produced per year (previous article : Swiss waste-management policy, a peek behind the curtains of one of the most efficient country in the world). Around 50% of this waste is recycled and the rest is incinerated, the energy produced is then recovered in the form of electricity and heat.
Common source of waste being incinerated comes from households, offices, small businesses, courtyards and gardens, public facilities and roads. What is the journey of this un-wanted material? To further explain I will analyse the situation in Ticino, a region located in southern Switzerland, with a particular focus on the waste-to-energy plant located in Giubiasco (more info here).
According to statistics, urban waste collected in Switzerland is composed as follows:
To further explain, "Biogenic waste" refers to organic waste for instance material of plant, animal or microbial origins. This term includes numerous types of waste from different sectors such as agriculture, agri-food industry, private consumption and energy production.
"Composite materials" are materials composed of at least two different materials that cannot be separated manually. An example is the Tetra Pack (the milk package) which consists of cardboard, plastic and aluminum. According to the Swiss average, the calorific value of the waste is around 3.5 MWh per ton.
The plant was designed for a thermal power of 67 MW, calculated to treat 140’000 tons of waste per year. It is aimed at recovering the energy contained in the waste through a cogeneration system (boiler, turbine, generator) that uses the steam produced by combustion and transforms it into electrical and thermal energy. A forced air flow is put into combustion to supply the necessary quantity of oxygen and maintain the constant temperature. No other additives are added to feed the fire.
The fumes generated by the combustion of waste (with temperatures over 1000 ° C) circulate from the furnace through the boiler. The heat exchangers inside the boiler recover the great heat contained in the fumes. Thanks to the heat exchangers, the thermal energy of the fumes is transferred to the inlet water of the boiler which thus vaporizes. The boiler can produce about 39 tons per hour of steam at 400 °C at a pressure of 40 bar. The steam generated in this way drives a turbine which, coupled with a generator, transforms thermal energy into electrical energy. The electricity produced is fed into the electricity grid (around 100’000’000 kWh per year) equal to the annual requirement of approximately 23’000 families (considering an average consumption of 4’500 kWh/year). A part is also used for the electrical needs of the plant itself.
District heating is an urban heating system consisting of a network of pipes, insulated and buried, for the distribution of heat, in the form of hot water, produced by a single central plant. This heat is distributed to residential, commercial, hospital, artisanal and industrial customers. The heat produced by the combustion of waste (residual heat) in the Waste-to-Energy plant furnaces is used to produce hot water at 105 °C.
As a heat transfer medium, the heated water is transported from the plant to the individual buildings by means of a pumping station and a network of insulated and underground closed-circuit steel pipes. The district heating network consists of two pipes side by side and thermally insulated, one of delivery and one of return.
Once the building is reached, the heat is transferred through a heat exchanger, which replaces the boiler, to the internal distribution plant. In the exchanger, the thermal energy is transferred between two fluids, making sure that the hot water reaches the domestic system for heating and sanitary use thanks to the delivery pipe, and that the cold water returns to the plant with the return piping, to be heated again. All without changing the distribution systems inside the building.
The plant can serve facilities located several kilometers away. The length of the couple of pipes of the analyzed plant, are about 20km long. The thermal energy recovered is enough to heat simultaneously up to 2'500 families.
The slag, that are the components that resist to combustion, are collected in an extractor downstream of the oven. Incineration reduces waste volume by 90% and weight by 75-80%, destroying pathogenic germs. The slag, before their final storage in landfills, is screened in order to extract, and therefore recycle, the metals contained, with significant environmental benefits. The recovered material (iron, aluminum and stainless steel) amounts to about 11% of the quantity of waste.
We are a society that produces more and more waste. Unfortunately, currently, not all wastes are recycled. As this example demonstrates, wastes that are not recycled can also be valorized and this, for example, through cogeneration. While ideally we should aim at preventing and reducing waste generation to generate minimal to zero output, it is not always achievable. Thus maximising waste use, improving our recycling strategies, as well as ensuring a safe and environmental friendly disposal are crucial implementation to our system.