Waste to Renewable Energy Source: Achieving Sustainability in Waste Disposal
The development of human civilization and the rapid increase of the population in the last decade causing an enormous impact on the production of municipal solid waste (MSW). MSW is generally encompassed residential, industrial, commercial, municipal, and construction & demolition wastes. The population of cities and per capita income are decisive factors in the generation of solid waste.
The sources of waste are more than one and the garbage generated become more visible and complicated; as a result, the environment is becoming infected daily. It elevates the health risks due to air and water-borne pollution, especially in urban environments.
The global solid waste generation is more than 1.3 billion tons per year which are expected to increase to approximately 2.2 billion tons per year by 2025. The developed countries generate nearly 572 million tons of waste per year which is almost half of the world’s waste.
Unfortunately, middle-income countries dispose of their waste in open dumps while landfilling and thermal treatment are the most common methods for disposal of waste in high-income countries.
If we consider the Indian scenario of solid waste management which is a major concern, around 62 million tons of waste is being generated annually in India. Only 28% of collected waste is processed or treated from the total appx. 68% of collected waste and remaining waste goes for disposal through landfilling / open dumps.
As per some estimates, the projected waste generation in India shall be around 165 MMTPA by 2033. Indiscriminate disposal of waste at dump yards in an unhygienic manner is leading to health-related problems and environmental degradation.
The role of waste management comes into the light for effective implementation of waste disposal which appeals to sustainability in our environment. Waste management is the collection, transportation, recovery, recycling or disposal, and monitoring & analysis of waste, in an effort to reduce their effect on human health and the ecosystem.
It’s important to utilize the available waste-to-energy (WtE) technologies which are perhaps not fully applied in many parts of the globe. Waste management should take full advantage of the energy potential there in waste. The known recovery technologies from waste are thermal conversion (incineration and gasification) and bioconversion (landfill gas and anaerobic digestion).
The WtE systems reduce waste volume by 90% and the remaining residue can be safely disposed of in landfills. The waste is used as a renewable fuel to generate energy which helps in reducing reliance on electricity derived from coal-based thermal power plants. WtE systems collect and process the waste scientifically and convert waste into inert non-leachable ash while generating energy.
Basically, there are three major concepts of WtE namely Thermochemical, Biochemical, and Mechanical. The thermochemical concept involves Incineration, Conventional/Plasma gasification, and Pyrolysis while Biochemical encircles Biomethanation and Fermentation. The crushing, compressing, and pelletizing are the main processes of the Mechanical concept.
Heating Waste by Incineration Process
The main merit of Incineration is a waste reduction in volume and weight by about 90% and 75 respectively. Of course, it comes with challenges like the management of dioxins and furans formed in Incineration. The process involves the combustion of waste at very high temperatures in the presence of excess oxygen that produces ash, flue gas, and heat energy.
Source: M&C Tech
After the energy crisis in the 1970s, solid waste incineration techniques were quickly developed in countries like Japan, Denmark, Sweden, Switzerland, China, Spain, and Austria. The great majority of these WtE plants are based on moving grate combustion of as-received or post-recycled MSW and produce electricity and heat.
Gasification
It’s a thermos chemical conversion of a carbonaceous fraction of waste into syngas (CO, H2, CH4, and CO2) in an oxygen-deficient environment and at a high temperature (650-1600 deg C). Syngas can be used for various applications such as the generation of electricity, FT fuels, chemicals, and hydrogen.
Source: Biofuels Academy
Its initial cost is higher than the incineration process and requires skilled labor to operate. However, it is more efficient and environmentally friendly technology than incineration for converting waste to energy.
The main advantages of gasification are related to the fact that it is relatively faster than the other conventional processes. As a result, more waste can be treated in less time. This technology may prevent the formation of dioxins and reduce the emission of acid gases.
Pyrolysis
Pyrolysis of waste plastics is an upcoming technology for the conversion of plastics to either liquid fuels or chemicals. It’s a thermal decomposition of the organic fraction of waste in the absence of oxygen.
Source: Google
The conversion produces three components: Fuel gas, Fuel oil, and Char along with the inert materials in the waste feed.
Hydrolysis and Fermentation
The major challenges in hydrolysis and fermentation are the integration of hydrolysis and fermentation into a single step and availability of low-cost enzymes.
Source: greener-industry
There are two steps in this process, the first step in the conversion of cellulosic fractions of waste to ethanol is the hydrolysis of cellulose and hemicellulose into simple sugars using chemical/enzymes and another step is the fermentation of sugars into ethanol followed by distillation. The process produces Lignin which is a by-product.
Refuse Derived Fuel (RDF)
This process gives a high calorific value to the waste. RDF is easily storable, transportable, and more homogeneous fuel for either steam/electricity generation or as an alternative fuel in industrial furnaces/boilers and it is also used in cement rotary kilns as co-processing fuel and co-combustion in coal-fired power plants.
Source: wastebusters
RDF is produced by removing recyclables and noncombustible waste and producing a combustible material by shredding, compressing, and pelletization of remaining waste.
Biomethanation
This process is well established and matured technology, but suitable only for wet biodegradable wastes. It’s anaerobic digestion of biodegradable organic waste in an enclosed container under controlled conditions of temperature, moisture, pH, etc.
Source: slideshare
Decomposition takes place of organic fractions present in waste and it generates biogas comprising mainly of methane and carbon dioxide. The remaining material is rich in nutrients and can be used as fertilizer.
For gasification and anaerobic digestion to be successful, the MSW must be segregated in color-coded containers for each type of waste at the generation site. This step supports recycling efforts and it might reduce some MSW sorting costs.
From an economic point of view, MSW incineration and gasification present more advantages. Nevertheless, since waste management is often based on other aspects than purely economic criteria (i.e., social or environmental), anaerobic digestion should not be discarded as a future option to recover energy from MSW, although it would be necessary to reduce its investment costs.
Waste is a growing renewable source that can be used for the generation of energy. While selecting the WtE technology, one has to look at the scale of waste to be processed, existing emission norms, energy recovery, and economic factors.
Conversion of waste to energy does not only reduce the detrimental effects of land disposal but also avoids GHG emissions associated with fossil fuels combustion.
