Development of Plasma Mediated Waste Pyrolysis Technology
An international scandal involving a vast intra-European traffic in medical waste originating from France and culminating in eventual redistribution in other European markets in the late 1980s led to the resignation of the then French minister of health. As a result, the world was shocked into recognising the magnitude of the medical waste disposal problem.
India is no stranger to such horror stories. Recycled syringes and quilts packed with used surgical cotton have a thriving market. In 1991, Pioneer reported a well-knit racket operating from the backyards of the All India Institute of Medical Sciences in Delhi transporting syringes to Meerut. Pathogens of deadly diseases like hepatitis B find a ready and fertile breeding ground in the piles of undisposed medical waste.
Public concern over disposal and treatment of medical waste has resulted in increased regulations and court actions on a global scale. The fundamental reason is the phenomenal growth in the quantity of medical waste generated in the hospitals, attributed to the growing use of disposable as precautions against exposure to infectious diseases such as AIDS and the growth of medical and public health facilities. The generators include hospitals, clinics, and medical research facilities. A rule of thumb for medical waste production in affluent countries seems to be 1 kg per bed per 8-hour shift.
Historically, landfilling was the most preferred means of disposal of medical waste. However, public opposition and positive correlation with groundwater contamination have resulted in this option steadily going out of favour. Burning the waste material in the open air can never be complete, with small quantities of many organic and chlorinated organic compounds and pathogens surviving and leading to dispersal of dangerous diseases that can spread through the air.
Incineration is the most popular method of medical waste disposal. About 85% of medical waste are incinerated, while only 6–15% is waste that requires special handling and disposal. The fundamental problem of incineration is that the chemistry of combustion determines the generation of heat. Efficient combustion demands airflow far above the stoichiometric requirement. The very high flow rate generates airborne pollutants and limits the attainable temperature. The effectiveness of incineration, measured in terms of the destruction and removal efficiency, is low. In addition, the performance of the emissions control equipment to meet the stringent requirement of safe disposal is poor.
PVC (Polyvinyl Chloride) plastic, which contains chlorine, constitutes many disposable products used in health care. When PVC products burn, they serve as a primary source of chlorine for dioxin formation. “Dioxin” refers to a family of compounds that can form when chlorine combines with organic material in a reactive environment. Dioxins and related chlorinated organic compounds are potentially toxic substances that produce a remarkable variety of adverse effects in humans and animals at low doses. These compounds are persistent in the environment and accumulate in magnified concentrations as they move up the food chain, concentrating on breast milk. Dioxin is carcinogenic, interacting directly with DNA through a receptor-based mechanism.
In the late 1990s, the Technology Information Forecasting and Assessment Council (TIFAC) approached IPR to develop a Plasma Mediated process for medical waste destruction. With dramatic developments in high-temperature plasma sources, we can apply plasma heat to highly toxic waste, and the final products can be harmless gases. The large flux of ultraviolet radiation in thermal plasma can dehydrogenate organic chlorine. The reactors can process gaseous, liquid, and solid materials.
Plasma-based medical waste treatment is highly complex since it must contend with extreme temperatures and a corrosion-prone environment. The process depends on complex pyrochemistry resulting in toxic and dangerous products. Furthermore, it deals with high volume, low packing density waste with a nonstandard composition consisting of various plastics, organic materials, and liquids. As a result, compliance with environmental emission standards is complex. In addition, there are capital and operating cost constraints imposed by inferior competitor technologies.
Being an internationally competitive technology with very high commercial stakes, critical information on many crucial aspects was not readily available. No peer group with expertise in this field existed in India for problem-solving consultations. Under a development programme with intense time pressure, many problems will have to be solved concurrently with development.
The workhorse of plasma-based waste destruction technology is the plasma torch. Plasma torches are electrical discharge plasma sources with the plasma extracted as a jet through an opening in the electrode and out of the confines of the cathode-anode space. The arc column’s inherent thermal and electromagnetic instabilities are stabilised by forced gas flow along the current path. Interaction with a guiding wall or external magnetic fields also stabilises the plasma. DC arc, RF and microwave plasma sources can be converted into plasma torches. Plasma temperatures can easily reach tens of thousands of degrees, and high enthalpy gas flows get generated in large volumes.
The driver for developing plasma torches was the space race in the 1960s. Missiles re-entering the atmosphere create shock-ionised air plasma. Laboratory simulation of these conditions was necessary to create materials capable of withstanding the searing heat of re-entry. Arc systems can generate re-entry conditions using clean, high enthalpy gases at high stagnation pressures. Many present-day plasma torches are derivatives of the plasma jet sources built for this application and now meet the need for intense heat sources for waste treatment.
Pyrolysis is the thermal disintegration of carbonaceous material into fragments of compounds in an oxygen-starved environment. The presence of charged and excited species renders the plasma environment highly reactive, catalysing homogeneous and heterogeneous chemical reactions. The most likely compounds to form are methane, carbon monoxide, hydrogen, carbon dioxide, and water when the process is optimised. The high temperature and high enthalpy inhibit the formation of hydrocarbons.
The product gas is high in hydrogen and carbon monoxide, with traces of methane, acetylene, and ethylene; therefore, it can be combusted very efficiently, resulting in carbon dioxide, nitrogen and water vapour being the only gaseous exhaust to the atmosphere. The slag is a homogeneous, silico-metallic monolith with leachate toxicity levels orders of magnitude lower than current landfill regulations. Emission and leachate results demonstrate convincingly that plasma gasification is a far more environmentally friendly method of disposing of waste than any competing technology. Plasma gasification provides more than a 95% volume reduction ratio of slag to input material. Other technologies offer an 80% reduction typically.
The prototype plant was built by Ganesh Prasad and Sudhir Nema using a conventional plasma torch and was installed at the Gujarat Cancer Research Hospital for field trials. For ruggedness and energy efficiency, we decided to use an in-house developed graphite electrode plasma torch. A commercial version of a Plasma Reactor came after a series of prototypes built to improve the system’s reliability. We installed this in the Goa Medical College in 2000. Manohar Parrikar, the then Chief Minister of Goa came to see the facility and complimented our team on the excellent engineering of the plant.
Conventional plasma torches require large gas throughput to stabilise the arc, resulting in product gas dilution and reduction in energy efficiency. We have Innovated the graphite torch exploiting the gas generation in the Pyrolysis of organic matter. An inline suction pump sucks the product gas and sends it through a filter to remove soot particles. It is then fed to the plasma torch. We had a 35 % gain in energy efficiency with this torch. It also increases electrode life by reducing the electrode erosion rate. We received a Patent for the Endogenous Gas Feed concept in 2007.
After the successful trial of the system in the Goa Medical College, the Department of Science and Technology sponsored a programme of demonstration of this technology by installing several plasma systems all over the country, including one at the Shri Chitra Institute in Trivandrum. A large number of systems enabled extensive field trials of this technology. The technology was transferred to several manufacturers.
The final battle that Pyrolysis Technology had to fight was with the Regulatory Bureaucracy. The powerful incinerator lobby had put in many obstacles in the path of CPCB declaring Pyrolysis as an approved technology for medical waste destruction. However, we finally won the battle with the issue of a Gazette notification in 2016 endorsing Plasma Pyrolysis for medical waste destruction.
Plasma pyrolysis technology is one of the many societally beneficial applications developed by FCIPT. One important lesson that we learnt was that while it was relatively easy to master technology development, it was complicated to fight the battle with entrenched forces that resisted the introduction of advanced technologies, citing various reasons. Even government departments sometimes become tools of these retrograde forces.
