India is looking towards a massive rise in the volume of waste from 60 million to 150 million tons by 2030. Unregulated garbage dumping in landfills without much scientific treatment has led to more problems. Surrounding areas of landfills are often heavily polluted (e.g. Ghazipur landfill, Delhi) as it is difficult to stop the hazardous chemicals from leaching into the surrounding land. In addition, these landfills could contribute to global warming by releasing methane (CH4), which is 20 times more dangerous as a greenhouse gas than CO2. Besides that, another widely used practice, incineration, has its issues like ash disposal post-treatment, environmental pollution and damage to public health.
What is Plasma ?
Not to be confused it with blood plasmas, plasma is frequently described as the fourth state of matter after solid, liquid and gas. In the plasma state, the matter is of charged particles with electrons, ions and neutrals. It is interesting to know the fact that more than 99% of the known universe is in the plasma state. A plasma state can be attained at very high-temperature ranges. A typical example is the sun, whose interior temperatures exceed 107K.
A plasma’s high energy content lends itself to several important applications like cutting, welding, spraying, metallurgy, and waste treatment.
Use of Plasma for Waste Treatment
Thermal plasmas (plasmas with the same electro and ion temperature) with their high energy content can vaporize anything. Thermal plasma torches, used for plasma generation, act as a plasma source in waste treatment. These torches generate plasma jets with a very high temperature (5000-14000K), enthalpy and a high energy density ranging from 106-107 J/m3.
Thermal plasma jets obtained via torches convert waste into syngas (synthesis gas) and dissociate other materials into constituent chemical elements that are then collected and vitrified to produce an inert glass-like slag retaining most of the heavy andalkali metals from the waste. The vitrified slag can be used as construction materials. Because of oxygen-starved conditions and high temperature in the reactor, this process does not emit NOx, SOx, and dioxins, respectively. Thus, high energy can be extracted from the syngas generated for plasma treatment/gasification compared to other gasification processes.
Advantages of using Plasma for Waste Treatment
When directed towards the waste, the high-temperature plasma jet can vaporize anything and destroy all sorts of chemical bonds. Such a temperature in plasmas can allow synthesizing or degrading chemical species in some conditions unreachable by conventional combustion/incineration. This characteristic of thermal plasma makes them more viable for treating highly toxic wastes such as air pollutant control (APC) residues, radioactive and medical wastes.
Since plasma torches are operated electrically, they provide the enthalpy control i.e. heat supplied to waste, by adjusting the electrical power. While combustion, on the other hand, because of its thermochemistry, does not allow precise control of the enthalpy injected into the waste.
Because of their high energy density and temperature, plasma offers rapid heating and can efficiently deal with materials with high melting temperatures. Importantly, for a given waste quantity, a plasma treatment unit requires a small size compared to other methods. This trait of this technology can potentially deal with the issue of landfills.
The incinerator residue i.e. ash contains the most hazardous materials and can be treated using plasma technology. Plasma treatment reduces ash to a vitrified slag which is not harmful and can be used as a graded material for the construction of roads.
Challenges in using the technology
The main issue faced by the technology is its economic viability. The use of electricity makes the cost of operations of one such unit higher than conventional options. For E.g. it requires 1.5 kWh/kg of waste processed for small plants (< 100 MT capacity) and ~ 1.2 kWh/kg of waste processed for plants with greater than 100 MT capacity.
Secondly, the electrodes in plasma torches erode with the operation and have a specific life. Thus needs to be replaced with the new ones after a few operation. This further leads to an increase in the cost of operation.
A good future
Locally, countries like Korea, Russia and Japan are running plasma plants ranging from 10TPD (tonnes per day) to 100TPD and working towards making the processes more economical. Another approach that many follow is combining incineration and plasma treatment. Thus, plasma only treats the fly ash coming out of the incinerator. So, that environment and cost can both be looked at simultaneously.
Also, in India, Maharashtra Enviro Power Limited is operating a plasma plant with a capacity of 72TP in Pune. A small-scale plant has also been set up at Jaffarabad Delhi with the help of IIT Delhi for medical waste. Research studies are ongoing on this unit to increase it’s efficiency. Researchers in IIT Delhi are analyzing plasma torches via fluid dynamics modelling to understand the flows and electrode interactions thoroughly. All this could aid in the design processes of the torches.