This article discusses the thermal recycling of plastic solid waste (PSW) as an environmentally sustainable method for the disposal of PSW, including the advantages and disadvantages that it presents.
Due to the increase in global population, the consumer demand for plastic products has steadily increased over the last forty years due to their versatility and relatively low cost (Panda, 2010). This consumer demand inevitably caused increased production, consumption and waste generation rates of PSW over the last few decades (Al-Salem, 2009). The various sources of PSW include domestic items such as food containers, packaging foam, disposable cups, plates, cutlery, compact disc boxes, electronic equipment cases, drainage pipe, carbonated drinks bottles, plumbing pipes and guttering, flooring and cushioning foams (Panda, 2010). Such plastic items now form a significant component in a diversity of waste streams (Keane, 2007).
The adverse environmental impacts of PSW, including production energy costs, stricter regulations, limited lifespan, increasing landfill content and inability to biodegrade, present an important challenge in the pursuit of sustainable consumption as a precursor to achieving sustainable development (Ritch, 2009). In fact, governments, such as the one in Kenya, have collaborated very closely with the United Nations Environment Programme in respect of the serious environmental problem that PSW presents (Njeru & Njeru, 2006) that is clearly not addressed by recourse to landfilling (Keane, 2007). Indeed, the suitable treatment of PSW is one of the key questions of the sustainable waste management (Panda, 2010).
There are three main alternatives to PSW disposal in addition to landfilling:
1. Thermal Recycling, which is the direct incineration of plastic wastes for energy recovery;
2. Mechanical Recycling, which is the method by which plastic wastes are recycled into new resources without affecting the basic structure of the material; and
3. Chemical Recycling, which is the method that breaks down polymers into other chemicals (Dodbiba & Dogbiba, 2008).
Thermal recycling with heat recovery involves complete or partial oxidation of the plastic material, producing heat, power or gaseous fuels, oils and chars besides by-products that must be disposed of, such as ash (Al-Salem, 2009). Also referred to as waste-to-energy (WTE), it involves the recovery of the inherent energy value of PSW waste through incineration and processed fuel applications (Panda, 2010).
Life cycle assessment (LCA) is a tool for measuring the environmental sustainability and environmental performance-improvement opportunities of various products and processes. It includes looking into various end-of-life options including thermal recycling. (Khoo, H., 2010).
In conducting an LCA on thermal recycling, many advantages have emerged that recognize thermal recycling as one of the preferred methods of PSW disposal (Stehlik, & StehlÌk, 2009).
First, thermal recycling of PSW often has a low initial burden. Only some collection efforts and transportation resources are required because PSW is available fuel close to the end user (Eriksson, 2009). Due to urbanisation people tend to live in cities or greater residential areas. This means high waste generation per kilometre hich in turn makes the cost for waste collection less expensive than in sparsely populated areas (Eriksson, 2009).
Second, incineration plants are robust with respect to waste quality. The plants are designed to be able to treat bulky heterogeneous waste (Eriksson, 2009). In many cases, PSW are mixed with varying qualities or are contaminated, in which case, the plastic should be used for energy utilization (Astrup, 2009)
Third, in respect of fuel economy for the owner of an incinerator, it becomes very cost effective (Eriksson, 2009). Even if some costs are higher due to expensive cleaning equipment, revenue from sold energy is supported by a reception fee for the waste fuel. This makes PW very competitive as base load in district heating systems (Eriksson, 2009).
Fourth, thermal recycling is considered one method of waste management that has the benefit of energy recovery. PSW has been considered a good candidate for feedstock for energy production due its high heating values (Singhabhandhu, 2010).
Fifth, thermal recycling produces valuable petrochemicals as feedstock, it also produces energy in the form of heat and steam (Al-Salem, 2009). Energy generation by incineration of plastics waste is in principle a viable use for recovered waste polymers since hydrocarbon polymers replace fossil fuels and thus reduce the CO2 burden on the environment. The calorific value of polyethylene is similar to that of fuel oil and the thermal energy produced by incineration of polyethylene is of the same order as that used in its manufacture. Incineration is the preferred energy recovery option of local authorities because there is financial gain by selling waste plastics as fuel (Panda, A K., 2010). Production of liquid fuel would be a better alternative as the calorific value of the plastics is comparable to that of fuels, around 40 MJ/kg (Panda, A K., 2010).
Lastly, recovered plastic waste can be used for energy utilization. Energy utilization may involve the use of the plastic waste as fuel in industrial processes or the production of solid fuels for energy production facilities such as power plants. If plastic waste is used for energy utilization the substituted fuels could be fossil fuels such as coal, oil, natural gas, etc. depending on the industrial process and the energy technology in which the plastic is used as fuel. The use of plastic waste as fuels (energy utilization) is by far one of the most important options available for plastic waste in Europe (Astrup, 2009)
In conducting an LCA on thermal recycling, many disadvantages have likewise emerged.
First, PSW is a more troublesome fuel compared to other fuels due to its heterogeneity. The variable fuel quality gives rise to problems during the combustion process which requires an advanced and expensive air pollution control and management of residues (Eriksson, 2009).
Even if the overall degree of efficiency can be high due to flue gas condensation, the electricity-to-heat ratio is in general lower than for e.g. wood combustion, which in turn in general is lower than for oil and natural gas combustion (Eriksson, 2009).
Second, in most developed countries, public distrust of incineration limits the potential of waste-to-energy technologies as it produce greenhouse gases and some highly toxic pollutants. It has been suggested that the chlorine content in PVC and other plastics is related to the formation of dioxins and furans (Panda, 2010).
Lastly, heavily contaminated plastics waste collected from domestic waste stream can be utilized by energy recovery from waste incineration plants. Cost of this system of recovery is considered highest among all the alternatives. Again, incinerator design and operation depends upon the type of waste to be incinerated and another important factor to carry out this process is to minimize the harmful emissions. (Panda, A K., 2010).
Incineration of plastics has net emissions of greenhouse gases. These emissions are also in general higher for incineration than for landfill disposal. However in situations where plastics are incinerated with high efficiency and high electricity to heat ratios, and the heat and the electricity from incineration of plastics are replacing heat and electricity in non-combined heat and power plants based on fossil fuels, incineration of plastics can give a net negative contribution of greenhouse gases. The results suggest that efforts should be made to increase recycling of plastics, direct incineration of plastics in places where it can be combusted with high efficiency and high electricity-to-heat ratios where it is replacing fossil fuels, and reconsider the present policies of avoiding landfill disposal of plastics. (Eriksson, O., 2009). Incineration is to be preferred given a high efficiency, a high electricity-to-heat ratio and when energy from incineration replaces fossil fuels, at least for district heating. (Eriksson, O., 2009).
The United States is one jurisdiction that has taken advantage of thermal recycling of PSW in respect of the production of clean electricity. It has the highest consumption of plastics among other countries which is equal to 27.3 metric tons against the global consumption of 170 metric tons. The US consumption is expected to reach to 39 metric tons by 2010 (Panda, 2010). PSW in the US amounts to about twenty percent of the volume and eight percent of the weight of all municipal solid waste in USA during 2000 which increased to eleven percent by 2006 (Panda, 2010). It is important to note that the use of municipal solid waste (MSW) to generate electricity waste-to-energy (WTE) projects represents roughly 14%of U.S. non hydro renewable electricity generation (Kaplan, Pogze, 2009) in which there is direct combustion of MSW, where the resultant steam is used to run a turbine and electric generator. (Kaplan, P O z g e., 2009).
There are about WTE plants operating in 25 states in the US. Policy-makers appear hesitant to support new WTE through new incentives and regulation. Of the 30 states that have state-wide renewable portfolio standards, all include landfill gas as an eligible resource, but only 19 include waste-to-energy. There is a legitimate concern about the renewability of waste-to-energy because a significant fraction of the energy derived from WTE results from combusting fossil-fuel-derived materials, such as plastics. (Kaplan, P O z g e., 2009).
Interestingly, the US military established a system called "Tactical Garbage to Energy Refinery" or TGER. It powered a standard 60-kilowatt generator by turning the solid trash, made of plastics, papers and food garbage, into fuel pellets into an ethanol which creates usable energy that is a significant alternative clean fuel source for the military (Wingfield, 2009).
In conclusion, plastics are projected to play a strategic role in the global portfolio of clean energy solutions in this carbon-constrained world. The continued development of recycling and recovery technologies, investment in infrastructure, the establishment of viable markets and participation by industry, government and consumers are all considered priorities of the highest order (Al-Salem, 2009). Discarded MSW is a viable energy source for electricity generation in a carbon constrained world. WTE is capable of producing an order of magnitude more electricity from the same mass of waste. If the goal is greenhouse gas reduction, then WTE should be considered as an option under U.S. renewable energy policies. (Kaplan, P O z g e., 2009).
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