Environmental performance and energy recovery potential of five processes for municipal solid waste treatment
Introduction
Treatment and processing of municipal solid waste (MSW) should target minimizing the volume of landfilled waste whilst recovering as much resources out of it as possible. MSW is actually a resource with huge potential in terms of material and energy recovery. Thus, waste-to-energy operations have the advantages of resource generation and the minimization of landfilled waste. MSW is a heterogeneous resource that is a bundle of different waste types. The portion of each waste stream within the total amount of MSW differs according to several factors (Arafat and Jijakli, 2013). Waste streams that are classified as organic can be combusted or composted, whereas, waste streams that are classified as inorganic cannot. Organic waste streams include paper, plastics, textiles, wood, food wastes, and yard wastes; while the inorganic waste streams include glass and metals.
To fully understand the condition of MSW and its potential in energy generation, proximate and ultimate analyses are usually undertaken. The ultimate analysis of different MSW streams is presented in Table 1, which was obtained from two studies (Niessen, 2010, Themelis et al., 2002). Results of the proximate analysis are presented in (Niessen, 2010). While the analysis results will not be exactly the same for different countries, since MSW is a heterogeneous resource, a review of published results at various localities showed only slight discrepancies (Niessen, 2010, Themelis et al., 2002). Relative amounts of the MSW constituents in the MSW, on the other hand, can vary significantly by locality.
Incineration is a direct combustion technology in which the feedstock is directly transformed into energy. Carbon dioxide and water vapor are the major compounds emitted through the incineration of MSW (Johnke, 2012). Additionally, the incombustible ash usually constitutes a concentrated inorganic waste that has to be disposed of properly.
Gasification is the process of converting organic compounds, under controlled oxygen flow, into a mixture of gaseous species that is dominated by carbon dioxide (CO2), carbon monoxide (CO), hydrogen (H2), and methane (CH4). A summary of the products of gasification is given in Table 2 (Higman and van der Burgt, 2008).
Anaerobic Digestion is used to treat organic waste with the ability to recover energy in the form of biogas (mainly methane) (Tchobanoglous et al. 2004). Residence times of anaerobic digestion reactors can be greater than 30 days (Tchobanoglous et al. 2004). However, an advantage of anaerobic digestion is that the process will produce less solid sludge than aerobic digestion (Henze et al. 2008).
In composting, organic waste is transformed aerobically into soil conditioners and water, with some emissions of NH3 and CO2 (Polprasert, 2007). In landfills, on the other hand, the organic fraction of MSW can decompose through an anaerobic digestion pathway, since the landfills are covered and void of large amounts of air, leading to biogas formation. Some landfills (usually termed bioreactor landfills) are designed and operated under conditions that will enhance biodegradation and biogas production (Davis and Cornwell, 2008).
LCA is a cradle-to-grave analysis of the environmental impacts associated with a product or system. It analyzes all the stages in the life of the product/system including raw material extraction, production, usage, and disposal, focusing on the environmental impact of those stages. LCA's are now standardized through the ISO14000 standards (ISO, 2006a, ISO, 2006b). Impact assessment methods congregate different scientific methods and models to calculate the environmental impact. An example is the International Panel on Climate Change impact model (Pachauri and Reisinger, 2007).
Several LCA studies on MSW treatment are found in literature. A comprehensive summary of other LCAs found in literature is provided in Table 3 and in (Cleary, 2009). A major finding in most studies listed in Table 3 is that the production of energy or replacement of virgin materials associated with waste to energy and recycling technologies has tremendous environmental benefits over landfilling. In all the LCA studies on MSW encountered in literature, the focus was on applying the LCA methodology to assess specific MSW treatment scenario for a particular locality and as practiced in a given city with existing facilities. Hence, these studies have classically been too site specific. Yet, the analysis of waste management technologies from a technology centered perspective that extends beyond conditional location specific analyses could elucidate the true performance of those technologies.
The objective of this research work is to evaluate and compare different MSW treatment methods with energy recovery potential, from an energy, CO2 footprint and environmental performance viewpoints. To generate the inventory for the LCA, energy generation from MSW was first modeled based on thermodynamic and process models. Next, this inventory was used for environmental impact assessment, within the framework of LCA. In addition to attempting to present a holistic comparison, the novelty of the work presented here is that the analysis is based on different waste treatment technologies as they apply technically to different waste streams, not to a fixed MSW mix at a specific location. In other words, the inventory for the LCA was created based on technical modeling and not on specific case accounting. This means that the energy model, life cycle and environmental impact assessments were generalized as to accommodate a generic MSW stream not exclusive to a given locality and accommodating as little conditional parameters as possible. This way, the results of this study, along with the assessment framework, can be applied to different cases in different places; thus, providing a valuable tool to decision makers and interested parties when evaluating their waste to energy and MSW management options. This approach takes advantage of the reported observation that the composition of individual MSW constituents, or streams, (but not the MSW as a mixture) does not change much by location (Niessen, 2010, Themelis et al., 2002). This validates the attempted reduction in specificity of the work and increases its future global and unconditional applicability. It is also worthy to reiterate here the value of the framework followed in this study; from a technical evaluation that relies on engineering principles, to a product value assessment (in this case in terms of energy), to an environmental impact assessment. When combined with an economical and financial evaluation of MSW conversion to energy (discussed in detail in the literature), this work is a good first attempt at providing a holistic decision enabling framework for waste to energy operations.
Section snippets
Process modeling
In this study, five technologies (incineration, gasification, anaerobic digestion, composting, and bio-landfilling) were considered, in addition to a sixth alternative; recycling. Appropriate models were established for evaluating each alternative. Those models, briefly described next, are based on chemistry/thermodynamics and engineering principles. The goal of the models is to quantify the major inventory components (e.g., material emissions and energy yields) associated with each of the
Energy and CO2 comparisons
The energy saved from recycling a product as compared to producing it from raw virgin materials is presented in Table 7 for the three recyclable MSW components; paper, plastic, and wood. Those figures were obtained from (Morris, 1996) and account for energy required to segregate the MSW into separate streams. The efficiency adjusted energy (electrical) output from the different technologies as they apply to different waste streams is shown in Fig. 1, along with energy savings from recycling.
Conclusions
In this work, we explored five possible options for treatment processes with energy recovery potential, namely; incineration, gasification, anaerobic digestion, bio-landfills, and composting. These processes were compared to recycling where applicable (plastic, paper, and wood wastes). For optimal energy recovery, the municipal waste was assumed to be segregated into six major waste streams; food, yard, plastic, paper, wood and textile waste. The waste treatment options were investigated as
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