Waste recovery is key to sustainability in modern industrialized society
In 2010, mean municipal solid waste production in European countries was around 502 kg per person according to figures provided by Eurostat. Waste management is undoubtedly one of the main challenges faced by more advanced societies given the steady increase in its production and environmental, economic and social impact.
Despite the fact that this is the environmentally least sustainable option, most of this waste continues to be disposed of in landfill sites. However, the current trend is to reduce this practice in favor of environmentally and economically more interesting options. The Waste Framework Directive of 2008 introduced a waste management hierarchy in which the indicated options from higher to lower priority are prevention, reuse, recycling, material and energy recovery and, finally, disposal of the waste. As would be expected, the first option is based on reducing waste generation either by discouraging the sale of disposable articles, limiting the use of plastics, encouraging the return of glass packaging, etc.
The second best option is reuse, which can be undertaken depending on the specific product concerned (packaging, toner cartridges, shopping bags, clothing, etc.). Although the product itself may occasionally not be able to be reused, it can nevertheless be recycled for another use, such as the case of paper and glass. If none of these alternatives are feasible, and rather than simply depositing waste in a landfill site, the only sustainable means of gaining some economic benefit from it is to recover the valuable products from it. Recovery can be either material or energy-based. Material recovery involves using the waste as a raw material in another process. This is the case of slag from blast furnaces and the rubble produced during building demolitions etc., which can be used in cement production as they contain the same minerals present in the traditional raw materials. Energy recovery is another means of extracting some benefit from waste by using it to obtain renewable energy while solving an environmental problem.
The various energy recovery technologies available can be classified as either biological or thermal processes. The former can be applied when the waste contains a significant biodegradable fraction, whereas the latter will be viable when the calorific value of the waste, which is measured by way of the lower calorific value (LCV), is medium or high.
Most widely used energy recovery processes are:
- Disposal and exploitation of landfill gas
- Combustion with excess oxygen (incineration)
Disposal and exploitation of landfill gas
Given current legislation, it is not advisable to consider this alternative as viable as the amount of biodegradable waste deposited in landfill sites continues to decrease. However, the energy contained in landfill gas should be taken advantage of despite the technical drawbacks (variable calorific value, presence of numerous contaminants in the gas, aggressive conditions for cogeneration motors or microturbines, etc.).
The biodegradable fraction of waste is transformed into biogas and digested sludge by way of an anaerobic digestion process. Biogas is a mixture of carbon dioxide, methane and other minority gases (H2S, etc.) which, after a scrubbing process, can be used to produce electricity by way of a cogeneration process. The residual calorific energy of the process can be recovered and, in part, used to concentrate the wastewater generated by way of a vacuum evaporation and/or concentration process. This results in high-quality water and a highly concentrated waste.
This thermal process involves the transformation of organic matter into other compounds that are easier to treat and is performed at high temperature (between 500 and 900 ºC) in the absence of air. A gas with a high LCV (a mixture of hydrogen, carbon monoxide, methane, ethane, ethylene, etc.) is obtained, although some of the energy obtained from the gas must be used in the pyrolysis process itself, which is endothermic. In addition to the gas, a solid carbon known as coke, which is eliminated by way of an incineration process secondary to the main pyrolysis process, is also formed.
This is a thermal process in which partial combustion of the waste is performed under oxygen-deficient conditions. The process produces syngas (a mixture of hydrogen, carbon monoxide, water and light hydrocarbons), the composition of which varies depending on the waste and operating conditions. This syngas must also be scrubbed prior to subsequent use. Solid waste, mainly tar and ash, which must be incinerated, is also generated. Syngas can be used to produce electricity in combustion engines or microturbines, can be transformed into a liquid fuel which can be used as a diesel substitute, can be injected into the natural gas grid if CO2 and traces of oxygen are removed beforehand, and the hydrogen it contains can also be used in a fuel cell to generate electricity. This is therefore a very interesting and efficient option that is currently being studied further.
Combustion with excess oxygen (incineration)
This is a fast thermal process in which complete combustion takes place and the waste is oxidized to carbon dioxide and water. In order for the waste to react with oxygen to produce energy it must contain carbon, hydrogen and sulfur. This is the most widely used energy recovery technology.
In summary, waste-based energy recovery systems are a sustainable option for waste management that allow energy saving and reduce greenhouse gas emissions.
Moreover, the ever increasing number of technologies available means that a wide variety of waste of all types can be used in energy recovery processes.