India generates about 62 million tonnes of municipal solid waste annually. Of the 43 million tonnes collected, only 11.9 million is treated while 31 million is dumped in landfill sites.
What is Plasma Arc Gasification?
Plasma arc gasification (PAG) is a waste-treatment technology that uses a combination of electricity and high temperatures to turn municipal waste (garbage or trash) into usable by-products without combusting (burning). Although the technology is sometimes confused with incinerating or burning trash, plasma gasification does not combust the waste as incinerators do. Instead, it converts the organic waste into a gas that still contains all its chemical and heat energy and converts the inorganic waste into an inert vitrified glass called slag. The process can reduce the volume of waste sent to landfills and generate electricity.
In the PAG process an electrical arc gasifier passes a very high voltage electrical current through two electrodes, creating an arc between them. Inert gas, which is under high pressure, then passes through the electrical arc into a sealed container (called a plasma converter) of waste materials. Temperatures in the arc column can reach more than 14,000 °C (25,000 °F), which is hotter than the surface of the Sun. Exposed to such temperatures, most waste is transformed into gas consisting of basic elements, while complex molecules are torn apart into individual atoms.
The by-products of plasma arc gasification consist of the following:
- Syngas – It is a mixture of hydrogen and carbon monoxide. Waste materials, including plastics, contain high amounts of hydrogen and carbon monoxide, and the conversion rate of those materials into syngas can exceed 99 percent. Before the syngas can be used for power, it must be cleansed of harmful materials such as hydrogen chloride. Once cleaned, the syngas can be burned like natural gas, with a portion going to power the plasma arc gasification plant and the remainder being sold to utility companies, which also use it primarily for producing electricity.
- Slag – It is a solid residue resembling obsidian, can be cleaned of contaminants, including heavy metals such as mercury and cadmium, and processed into bricks and synthetic gravel.
- Residual heat – It emanates from the process and can be used to produce steam for electrical generation.
- Environmental Concerns – Some scientists have argued that syngas produced via this method, if burned for energy without proper treatment, could emit toxic acids, dioxin, and other pollutants, and the slag could retain high levels of mercury and other hazardous materials that can create challenges for solid-waste disposal.
- Cost concerns – A small plant employing this technology will cost around 60 lakh and can go up to Rs. 30 crore. These costs are too high for a country like India.
- No waste segregation – Lack of waste segregation is a huge challenge for Indian cities. The composition of the waste stream does affect the effectiveness of the gasification procedure. Garbage that is high in inorganic materials, such as metals and construction waste, will yield less syngas, which is the most-valuable by-product, and more slag.