Energy from waste
Refuse-derived Fuel
In the framework of the waste-to-energy strategy we find the so-called refuse-derived fuel (RDF), obtained from non-hazardous waste and used for energy recovery in incineration plants (also called waste-to-energy plants). There is a vast range of waste used and includes that which is excluded from the recycling processes, waste from industry and distribution, sludge from water purification, hazardous industrial waste, biomass waste, etc. Such waste must be treated adequately to be able to meet the criteria, regulations and industrial specifications aimed at achieving a calorific value suitable for use as RDF. One of the least expensive and most established methods of producing RDF is mechanical biological treatment (MBT).
In an MBT plant, metals (which are recycled) and inert materials (for example glass) and organic fractions (which are sent to composting plants, with or without an anaerobic digestion phase) are separated from MSW, choosing the fractions with a higher calorific value for the production of RDF. In addition to MBT, other solutions include biostabilization and bio-drying of the material, with metals and inert materials removed, where the organic fraction is stabilized and loses part of its moisture content, obtaining a final fraction with a higher calorific value, suitable for combustion and composed of paper and cardboard, wood, plastic and textiles that can be burnt directly. The product obtained from the treatments can be used as RDF only if it has certain characteristics and in particular a calorific value at least lower than 15 MJ and a moisture content of 25%. What are the current uses of RDF? There are a great number of uses: supplying waste-to-energy plants, cement factories, district heating plants, iron and steel plants, coal-fired power stations, etc. and, depending on the system, it is used both as a single fuel and as an auxiliary fuel. RDF can also be obtained by sterilizing sanitary waste with risk of infection as long as it is done under certain conditions and particular technologies are used.
Renewable energy
Biogas
Apple peel, fish bone, leftover pasta and a handful of corn waste. No, it is not some strange secret recipe! These are just some of the elements needed for the production of a rather specific type of fuel, biogas. Biogas is a gas, but, unlike methane extracted from the subsoil, it is produced by the decomposition of organic matter (the food part of our waste), municipal and livestock wastewater, agricultural biomass, etc. under anaerobic conditions, or in the absence of molecular oxygen (O2) or linked to other elements (for example, as in the case of nitrate NO3–). The concept is similar to that of compost production, given that it is just a case of decomposing organic matter, but the products and the ways in which this is done are different.
There are a host of benefits:
- biogas is a renewable energy source produced from waste, so it is a potential solution both from an energy and environmental point of view;
- there is no methane production and release into the atmosphere;
- the biogas production cycle is deemed carbon neutral, because the carbon dioxide contained in it is the same carbon dioxide previously fixed by plants, and it is not made from scratch as happens through the combustion of oil or coal.
Recover Energy
Waste-to-Energy
What is done with all the waste for which material recovery is not possible? According to the waste hierarchy pyramid, the preferable option is waste-to-energy, i.e. a process of thermal destruction, with recovery of energy and/or heat and with final residual production of ash that is then disposed of into a landfill to correctly close the cycle. In a waste-to-energy plant, or incinerator, the waste is burnt to exploit its calorific content (remember, for example, that plastic is produced from oil and therefore has a high calorific value), to generate heat, and to heat water to produce steam in order to obtain electricity. This energy can therefore be used to produce heat, to produce electricity or for the combined production of heat and electricity (cogeneration).
· Direct combustion, in which the waste is burnt, and the thermal energy of the heat is transferred to a heat carrier (water steam);
· Conversion into an intermediate liquid or gaseous fuel, by pyrolysis or gasification. Combustion takes place in special furnaces and is divided into 4 phases: heating and drying, pyrolysis, combustion and/or partial oxidation, combustion and/or gasification of the carbon material. In addition to the heat released by combustion, ash and gaseous emissions are produced; both require appropriate treatments to reduce their pollutant load and to be released into the environment without any health risk. The heat produced by the combustion of waste is recovered and used to produce steam. In turn, the steam generated drives a turbine which, coupled with an alternator and a gearmotor, converts thermal energy into electrical energy; alternatively, the steam will be used as a heat carrier. How much energy do we get from burning waste? The output of a waste-to-energy plant is in any case lower than that of a traditional power plant, given the low calorific value of the waste: efficiency therefore varies and ranges between 17% and 25% (it can even reach around 30% in more powerful cycles), but increases with the recovery of heat to over 50%, producing on average 0.67 MWh of electricity and 2 MWh of heat for district heating per ton of waste treated. This has not stopped a number of cities from using this kind of plant to optimize their energy demand and the disposal of their waste, take, for example, Oslo, Paris, Vienna and Copenhagen.