Introduction
Waste management refers to the process of collecting, transporting and disposing trash, sewage and other waste materials. There are different sources of waste including residential, industrial, commercial, institutional, construction and demolition, municipal, agriculture and manufacturing and all these waste products whether solid, gaseous, liquid or radioactive come under the realm of waste management. Due to the rapid growth and fast development in Australia, the country has produced waste on humongous amount. As per the data provided by Australian Bureau of Statistics, the growth of average annual rate of waste production in Australia is about 5.4% between 1996-97 and 2006-07. If Australians produced 1,200 kg of waste per person in between 1996-1997, it escalated to 2,100kg per person in between 2006-07. So far the most common way of solid waste disposal in Australia is landfill because of the availability of abundant lands and low population density in the country. Though thermal waste treatment which involves treating waste at high temperatures is not popular yet in Australia, due to various environmental hazards involved, but considering the huge quantities of waste production by Australians, time is near when thermal waste treatment would be the only solution left to the government for dealing with the problem of solid waste disposal. This paper would touch upon four kinds of thermal waste treatment technologies that are popularly used around the world – Incineration, Gasification, Plasma Technology and Pyrolysis.
Incineration
Incineration is a waste minimization technique which involves combustion of waste materials at high temperature. Incineration converts the waste materials into ashes, flue gas and heat. The heat produced during incineration at times is used for electric power generation. Currently, countries like Sweden, Denmark, USA, UK, and France use incineration for waste treatment process. Incineration is not a popular waste treatment technology in Australia and there are at present no large scale thermal treatment facilities available for the treatment of non-hazardous MSW (Australian Bureau of Statistics, 2006).
Incinerator is a furnace to burn wastes. There are many types of incinerators. Burn pile is the oldest technique for incineration where the waste is piled on the ground far away from urban and rural areas and then set on fire. Many municipalities across the world still use this technique for small wastes. Burn barrels are metal chambers inside which the combustion of the waste happens. Here the combustion is controlled and there is no chance of fire getting spread to the surrounding areas in case of strong winds and dry conditions around. This process is also for small quantity of waste disposal. Moving Grate incinerators are the most used incinerators and often is also called Municipal Solid Waste Incinerators (MSWIs). These are big combustion boilers where solid waste is burned under controlled conditions and also different techniques can be used to reduce flue gases and reduce the amount of organic toxins. Rotary Klin incinerators are used in big industrial plants and are more sophisticated than the moving grate machines (Moustakas & Loizidou).
Fig 1: A Schematic Diagram of Incineration Process (Moustakas & Loizidou)
Incineration has few advantages. It reduces the volume of waste materials by 90% and weight by 70% before they are disposed in landfill. Incineration also fully destroys and detoxifies hazardous and clinical wastes like pathologically contaminated waste, combustible carcinogens and toxic organic compounds which cannot be treated using conventional method of sewage treatment (Petts). Incineration also helps destroy the organic compounds of biodegradable waste which if landfilled directly produces LFG or landfill gas. Landfill gas accounts for larger amount of methane emissions into the atmosphere. The heat generated during incineration can be used for electric power generation. Denmark and Sweden have been using the energy produced by incineration for electricity for over a century.
Incineration has few disadvantages as well. Firstly it is a lot more expensive process than landfill as building of incineration plant and maintenance of air pollution control facilities require a huge sum of cost. Further, incineration generates three kinds of hazardous pollutants including heavy metals like manganese, nickel, chromium, arsenic, cadmium and lead, toxic gases such as carbon dioxide, carbon monoxide, acid gases, nitrogen oxide, sulfur oxide and fine particles. Lastly, incineration may be effective in reducing the volume of the waste materials, but it cannot totally eliminate the waste. Besides these, incineration process could also lead to other hazards as described in the table below (Petts):
Fig 2: Sources and Nature of potential environmental impacts from the operational phase of incineration (Petts)
Incineration can leave a hazardous impact on the environment. Incineration generates two kinds of ash - bottom ash and fly ash. Though fly ash quantities are quite lower compared to bottom ash generated by MSW incinerators, fly ash due to its more hazardous components cause more harm to human health. Especially dioxins and furans are considered to be very dangerous as they cause cancer, neurological damage, respiratory problems, thyroid problems and damage reproductive systems (Alternative Energy, 2008). People living nearby the incineration plant may get exposed to toxic compounds of incinerators by eating foods including vegetables, fish and wildlife contaminated by toxic emissions and breathing toxic air.
Gasification
Gasification refers to the process which without combustion changes organic substances or fossil based high carbon materials like rice hulls, chicken litter, crop stalks, wood shavings, tree bark and saw dust into hydrogen, carbon monoxide and carbon dioxide by making the materials go through reaction at high temperature with a restricted amount of oxygen. The end results of gasification are ash, solids, liquids and syngas. Syngas is a renewable source of energy if its gasified compounds derive from biomass. Gasification involves several processes. First dehydration takes out the water content from the waste materials, then pyrolysis done at 200-300o C produces char which then undergoes combustion process to produce carbon dioxide. If the combustion happens in the presence of less oxygen, it produces carbon monoxide. To reduce the content of carbon monoxide, the gas is mixed with heated steam to produce carbon dioxide.
Fig 3: Diagram of the Waste gasification Process (The Blue Ridge, 2009).
There are many types of gasifiers available in the market like fluidized bed, counter-current fixed bed, co-current fixed bed, plasma and entrained flow. Counter current fixed bed gasifiers are the ones in which the gasifier agent flows in the counter direction, achieving higher temperatures. At higher temperatures even complex hydrocarbons break down to form fine char which then are broken down into carbon monoxide and hydrogen. In co-current fixed bed gasifiers the fuel and the gas flow in the same direction. In this technique the tar output is less and energy efficiency is more than counter current fixed bed. In fluidized bed reactors the fuel and air both are fluidized into the gasification chamber. The temperate of the reaction is much lower than other types and generally low grade coals are used. Entrained flow gasifiers use pulverized solid, fuel slurry and oxygen (Worley & Yale, 2012). This type of gasifier operates at very high temperatures and so the output does not contain any methane and tar.
Gasification too like incineration contributes to toxic emission of acid gases including hydrochloric acid, nitrogen oxide and heavy metals like cadmium and mercury. During the cooling process of gasification, dioxins and furans are created and these toxins when released into the environment cause air pollution, water pollution, food contamination and different health hazards. Gasification may reduce the volume of solid waste by 85%-90% but the residual ash which comprises 8-15% of the original waste is dumped in landfill and this highly acidic and toxic residual ash easily percolates into the ground through precipitation and contaminates the ground water. Gasification also impairs composting and recycling programs because the organic solid waste like paper, wood, food scraps which are used mainly in gasification are valuable for composting and recycling. Further gasification contributes to greenhouse gases in the atmosphere. The gasification of petroleum based plastic waste materials adds greenhouse gases into the atmosphere in a similar manner as combustion of fossil fuel like coal, natural gas and oil does (The Blue Ridge, 2009).
Fig 4: Air Pollutants from Starved-air Combustion/Gasification (The Blue Ridge, 2009).
Gasification has few benefits nevertheless. The wide-ranging heat values of gasification enables the production of syngas not only through coal, but through low-carbon feedstock like municipal wastes, high-sulfur fuel oil, biomass etc. This flexibility of gasification gives these resources good economic value and lowers costs. The syngas generated by gasification can be transformed into an array of valuable products including electricity, steams, liquid fuels, hydrogen and basic chemicals which are used for various industrial applications. Gasification combined with other technologies for advanced power generation can give up to 60% efficiency to power plants compared to 40% efficiency of conventional power plants (NETL).
Plasma Technology
Plasma technology which was developed and used mainly for the production of high grade steel is currently being extensively used for municipal waste treatment. Currently the development of this technology is underway in Italy, Spain and many European countries. Japan and USA have already started using this technology by installing plasma converters. Australia and Canada are yet to take initiatives for developing this technology.
Plasma technology involves creating a thermal plasma field by directing an electrical current of 2-20 megawatts through a low pressure gas stream. Such plasma has very high temperature of 5000 to 15000 o C. This intense temperature is useful for melting all the waste components including toxic substances, metals and silicon (Kowalski & Kopinski).
Fig 5: Plasma Utilization of Toxic Municipal Waste and Specification and Usefulness of Output products (Kowalski & Kopinski).
Plasma technology is useful for treating all kinds of toxic, hazardous and lethal waste because of its use of high temperature to dissociate molecular bonds. Municipal waste treatments through this method do not generate harmful toxic ash or gaseous substances as is common with gasification and incineration. Metals regained from the process of plasma technology are used in metallurgic industry for creating slag which is useful as preservative for road construction materials. The created non-toxic gases are sealed in gas cylinders to be used as fuel and energy producer. Plasma technology is useful for treating 10 to 500 tons of municipal waste in a day. If the cost of waste treatment through incineration is $100/ton, plasma technology does the same at $40/ton and brings the costs down to zero by creating green byproducts.
The main disadvantage for Plasma technology, however, is the large investment it requires to set up a plasma unit for waste management. Also plasma flame when used over time reduces the sampler orifice. This necessitates regular maintenance of the sampler orifice and time to time shut down of the plasma unit. The plasma unit chamber walls are its weak point in the design. However, it recent technological advancements new plasma chambers are using metal walls which are costly but have a far superior lifespan.
Plasma technology is a very new technology still in a developing stage. Therefore, the full impact of this technology on the environment is yet unknown but it has been seen that plasma units can be operated in a manner that will lead to fewer impacts on the environment compared to conventional thermal waste treatment technologies.
Pyrolysis
Pyrolysis is the process of thermochemical decomposition. Unlike the other methods where the process of decomposition or combustion happens with the help of oxygen, Pyrolysis happens in the absence of oxygen. Simply put, in pyrolysis the solid waste at high temperatures decomposes into char and tar (hydrogen and tar). This process is not reversible. Pyrolysis is a very old process and is frequently used in charring woods. Pyrolysis like gasification mainly depends on carbon-based waste like paper, petroleum-based waste like plastics and organic materials like food scraps (WSU, 2002). Pyrolysis too like gasification produces syngas consisted of 85% hydrogen and carbon monoxide. Most pyrolysis processes involve four stages. Firstly, the waste is pre-treated through sterilization or segregation from the recyclables. Then the remaining waste materials are heated to produce gas, char and tar. The gas is cleaned of particulates, hydrocarbons and other soluble. Cleaned gas is then used to produce electricity and sometimes heat.
Fig 6: Process Schematic for a Circulating Fluidized Bed Pyrolysis Design (Ringer, Putsche & Scahill, 2006)
Since pyrolysis is a process which like other high temperature processes such as incineration and gasification doesn’t involve oxygen or reaction with water and other agents, very fewer air emissions are produced in this process. Though in practice, it is impossible to keep a completely oxygen free atmosphere and therefore due to small amount of oxidation, the fewer emissions that are produced are controllable because the emissions are cleaned to remove pollutants. Pyrolysis plants are made of small units which can be adjusted according to changes in volume of waste streams. Hence these units offer more flexibility than incinerators. Further, pyrolysis is a way of producing fuel from the solid waste. Especially the bio-waste and bio-mass through pyrolysis process produce bio-fuel which can be used for various needs. Flash pyrolysis is a more modern process in which pyrolysis happens at higher temperature than normal pyrolysis and the biomass is heated only for a very small time. Anhydrous pyrolysis is used to produce similar type of liquid fuel like diesel fuel from plastic solid wastes.
Pyrolysis has some disadvantages like gasification and incineration. Pyrolysis undermines the process of recycling and composting as many of the waste products used in this process are valuable for recycling and composting. The amount of fuel produced by this process is not sufficient for the energy required to manufacture new products. If the recyclable waste used in pyrolysis could be recycled or reused then it would have been a lot better. Further, the air and gases emitted during this process include toxic acid gases, sulphur dioxide, nitrogen oxides, and heavy metals like cadmium, mercury and lead. Also this process leaves solid waste residues like char, ash and other unreformable carbon. Just like gasification when these solid wastes are dumped on landfill, precipitation carries the toxic and acidic waste materials into underground, contaminating the underground water. The wildlife and foods contaminated by these toxic wastes could lead to cancer and other detrimental health hazards for human.
Conclusion
The waste production in Australia is increasing every year by leaps and bounds. Thermal treatment technology which involves treating waste at high temperature could be a solution to the problem as thermal technologies can reduce the volume of waste to a large degree. The four types of thermal methods described in this paper include incineration, gasification, plasma technology and pyrolysis. Incineration is a waste minimization technique which involves combustion of waste materials at high temperature. Gasification without combustion changes organic and fossil based high carbon waste materials into hydrogen, carbon monoxide and carbon dioxide at high temperature with a restricted amount of oxygen. Plasma technology melting all the waste components including toxic substances, metals and silicon at high temperature of 5000 to 15000 o C. Pyrolysis is the process of thermochemical decomposition at high temperature without oxygen. All these technologies are useful for treating waste but all of them have some or the other impact on the environment. But taking into account that even landfill, the most common strategy for waste disposal in Australia, too has a lot of adverse effect on the environment; these thermal technologies could be a better substitute for landfill especially if these technologies can be used with caution.
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