Thermal power plants (waste to energy) represent an alternative for the waste disposal through a combustion process at high temperatures.
In modern systems, such as those designed by SMEA Engeneering, it uses the heat generated by the combustion of materials to recover energy, which can be in form of electricity or heat (district heating), all this taking advantage of the expansive power of steam.
The WTE plant, dispose various types of waste: they can be solid waste (MSW) understood only in their dry component, doctors and hospital waste, sewage sludge or waste generated by the chemical industry.
It’s highly recommendable that the material at the input, before being introduced in the incinerator, is carefully treated to ensure they are removed all those materials which have a low or zero calorific value, maybe because they are non-combustible (metals, glass, etc.) or because they are wet. The material ready to take part in this process is called “refuse derived fuel” or “fuel from waste”.
The most used thermal power plants in Italy and Europe are “grid” type (hereinafter we will also examine othes kinds). The main operational steps of a wte power plant of this type, summarized in simple and schematic manner, are the following:
- Placing of the incoming the waste into a aspirated and ventilated place to prevent the spread of odors. After that the material is transported into the furnace by means of a spider or a hopper. As already anticipated, the better is the waste pre-treatment (dehydration and separation from inert), the higher is the calorific value;
- The waste reach then into the oven ready for combustion, and move on the grids in movement mixing the contents. From the bottom the air is pushed to allow the best process, maximizing the yield by pushing the temperature up to 1000 degrees centigrade and beyond, if necessary using the methane, if the fuel is not sufficient. After the first combustion chamber, there is another one, (defined post-combustion chamber), that has the aim to further perfect the result, reducing pollution to a minimum;
- The great heat generated by the process described above, leads to evaporation of the water contained in the boiler by developing the high-pressure steam. The gas is used in a turbine to generate electricity, with the aid of an alternator and a geared motor;
- Once the waste has completed their combustion process, the ashes are discharged into a container of water at the bottom of the grids, so that they are cooled to be then transported in special dumps. The water instead must undergo a purification process before being returned to the external environment;
- 5. The smoke generated by the disposal process, must be carefully purified before being released into the air, and this happen through numerous types of filters.
WASTE POWER PLANTS TYPES
Depending on the different type of combustion chamber, the WTE plants can be divided into several categories:
- Grids: here the waste is positioned above the grids that can be fixed or moved (this to favor a progressive combustion of the material). The air can be driven into the chamber from different directions, but in any case a part always from the bottom to facilitate the process and to cool the metal components;
- fluid bed. At first, a strong jet of air is push from the bottom upwards through a deposit of sand, and place it in a turbulent suspension; at this point the fuel from waste is introduced and begins to float along with the sand and burning, giving the impression this is actually a fluid. This method is used to treat waste from the petrochemical industry: the advantage is that it requires little maintenance, thanks to the absence of machinery in motion inside the furnace;
- rotary kiln. This type of system is particularly applicable in the treatment of hazardous waste, such as industrial and hospital one; in the first combustion chamber is applied a cylinder partially inclined that rolling deposit outside the ashes or the not flammable material, the gas instead complete its process in the second chamber before being thoroughly purified (you can also enter an afterburner to involve also the finest particles), sorted and be then emitted into the atmosphere;
- multiple steps hearth. The incoming fuel from waste pass through different burners, by actuating mechanical toothed arms installed at a certain height from the flame. With this type you can also treat sludge.
In the modern plants, such as those designed by SMEA Engineering, a good part of the produced heat is recovered and this allows to use steam to generate electric energy to resell or to produce hot water to be internally used or for offices and homes. Just about the last point is worth noting that while if you retrieve only electricity energy efficiency is 19-27%, it rises to 58% if it’s possible to captures even a part of the energy from heating (CHP).
The Italian example: the incentives recognized by the Italian republic are of two types:
- Green certificates: These are certificates issued to those, producing renewable energy, are able to emit less CO2 than what a fossil fueled central would (such as coal and oil). These are then sold to those who are obliged to produce a share of renewable energy and cannot.
- CIP 6, (recently banned for plants that dispose of the inorganic waste component): consists of an inflated revenue from the sale of electricity for 8 years
WTE POWER PLANTS’ ADVANTAGES
The reasons distinguishing WTE power plants are the following:
- First of all, the process described above greatly reduces the material to be deposited in landfills, we are talking about a reduction of at least 70% of the weight and 90% of the volume, not to mention that a good part of the waste must be separated again before entering the combustion, to be recycled and reused. You do not burn material that can be recovered;
- The VLE (very low emission) power plant can recover energy and therefore money, which can go in favor of the citizens as a tax on waste reduction;
- The non-recyclable waste are waste that if not used in energy plants should still dumped in landfill feels far more polluting. Subjecting them to combustion instead, you have the ability to filter an enormous amount of pollutants, and avoids burning oil: think that on average one ton of waste replaces two barrels of crude oil. Thanks to SMEA Engineering design solutions, an heavy reduction of the release of pollutantshas been reached, remaining well below the limits set by law (see table below)