Energy Recovery from Municipal Solid Waste by Thermal Treatment
Municipal Solid Waste (MSW) is one of the most pressing challenges to date with direct effects on both the environment and public health. The general principles of solid waste management include prevention of the generation of waste or reducing the consumption of non-essential products, reusing of items, recycling of the recyclable products and composting of organic waste, recovering energy like heat or fuel through the waste treatment process, and disposal of waste at landfills. It is important to first select recyclable materials from the waste and then recover energy from the residual waste.
Thermal decomposition leads to the formation of new molecules. This allows us to receive products with a different, often superior character than the original residue. Thanks to this feature, pyrolysis has become an increasingly important process for today's industry – as it allows us to bring far greater value to common materials and waste.
Pyrolysis is frequently associated with thermal treatment. But in contrast to combustion and gasification processes, which involve entire or partial oxidation of material, pyrolysis is based on heating in the absence of oxygen. This makes it mostly an endothermic process that ensures high energy content in the products received. Pyrolysis products always produce solid (charcoal, biochar), liquid and non-condensable gases (H2, CH4, CnHm, CO, CO2 and N). As the liquid phase is extracted from pyrolysis gas only during its cooling down, in some applications, these two streams can be used together when providing hot syngas directly to the burner or oxidation chamber,
During pyrolysis, a particle of material is heated up from the ambient to a defined temperature. The material remains inside the pyrolysis unit and is transported by screw conveyor at a defined speed, until the completion of the process. The chosen temperature of pyrolysis defines the composition and yields of products (pyrolysis oil, syngas and char).
Energy recovery from solid waste refers to the processing of non-recyclable waste materials into usable heat, electricity, or fuel through different technologies including anaerobic digestion, incineration, gasification and pyrolysis. Anaerobic digestion is a process through which microorganisms break down organic matter in the absence of oxygen. Incineration, gasification, and pyrolysis, on the other hand, involve the direct application of thermal energy in the conversion process of waste into solid, liquid, and gaseous products.
Incineration process is the combustion of waste under controlled conditions, which can typically reduce the volume and weight of waste by up to 90% and 70%, respectively. Generally, MSW combustion produces different residues, including fly ash, bottom ash, and air pollution control (APC) residues as by-products. Research results show that the incineration of per ton MSW produces 250–300 kg of bottom ash and 25–30 kg of APC residues and fly ash. The fly ash contains highly toxic heavy metals and polychlorinated dibenzo dioxins/furans, which are a type of hazardous waste. Therefore, various countries, such as the USA, France, Canada, Japan, Italy, Germany, Sweden, etc., have introduced policies for the proper control and management of MSW fly ash.
In contrast to incineration, pyrolysis is the thermal degradation of waste in the absence of oxygen. Pyrolysis has a lower process temperature (between 300 ◦C to 900 ◦C) and lower pollution emissions than incineration. The main products obtained from pyrolysis of MSW are a synthesis gas (syngas), a biofuel (bio-oil or pyrolysis oil) and a solid residue (char). The syngas has a high calorific value, so it can be used as a fuel to generate electricity or steam, or as a feedstock in the petrochemical and refining industries. Pyrolysis oil may also be used as liquid fuel in different applications for heat and power production, for example in burners, boilers, furnaces, diesel engines and gas turbines. The solid residue from pyrolysis, known as char, is a combination of carbon and non-combustible materials, which can be used as a filter medium for water treatment or as an agricultural soil amendment.
Gasification can be considered as a process between pyrolysis and incineration in that it involves the partial or incomplete oxidation of a substance at high temperature ranging between 500°C and 1800°C. The main product of gasification is a syngas, which contains hydrogen, carbon monoxide, carbon dioxide, and methane. In some cases, the plasma stage may follow on from a conventional gasification. Plasma arc applies a higher temperature in gasification, and occasionally to pyrolysis processes. Plasma technology is used in industries that require disposal of hazardous wastes at high temperatures.
Pyrolysis vs. Other Methods for Thermal Treatment of Municipal Solid Waste
The gasification and pyrolysis of MSW are more efficient processes than incineration. They may have some advantages, such as lower pollutant emissions, the production of more useful products, more flexible fuel source, and modularity. However, pyrolysis is more preferable than the gasification process in terms of environmental impacts and carbon footprint, and its potential for conversion of the plastic waste into useful char, gas, and liquid oil. The main challenge of gasification of plastic waste is the formation of tar, which causes the major operational challenges, thus reducing the gas yield and influencing the total process productivity. The pyrolysis process also has drawbacks, such as the need for pre-treatment of feedstock, the formation of coke during the MSW pyrolysis process, and the high-water content of liquid products from pyrolysis.
At present, the most commonly used technique for thermal treatment of MSW in Europe is incineration, whereas the least common one is pyrolysis. However, recent findings show that pyrolysis is an economical and environmentally friendly alternative to incineration for MSW disposal, which not only solves the problem of the excessive amount of landfilled waste, but also helps to recover valuable energy. One of the advantages of the process of pyrolysis is the possibility of thermal treatment of various types of waste, both industrial and household. Pyrolysis can be used for different types of municipal waste such as: plastics; rubber waste (e.g. tires); textiles; sewage sludge, biomass and waste of biomass nature; and cellulose waste (paper, cardboard).