Discuss about the Estimation of Energy Content in Solid Municipal Waste.
The aim is to understand how the energy content in solid municipal waste can be estimated, in order to understand if these can be suitable as an alternate source of energy. The purpose of this study is to identify methods of estimation of energy, and the tools needed to do so, in order to develop a project to analyze the energy contained in solid municipal waste in the Melbourne municipality region. The report will discuss the current situation of energy crisis and how solid waste can be used to meet the increasing demand for energy, and also, how solid waste impacts our environment (thereby highlighting the dual advantage of reusing solid waste for energy), outline findings from existing literature on this topic and identify methods and tools for estimating energy in the solid municipal wastes, including organic and inorganic wastes.
Energy is an important factor which has enabled the growth of human civilization since ages. Energy is essentially needed to drive any process, and humanity has been dependant of abundant, convenient and efficient sources of energy to power the technological advancements through the ages (mitpress.mit.edu 2017). Our dependency on energy has only increased with time and development in technology and civilization. Energy is needed for generating electivity, driving vehicles, or powering the various machineries in industries (dep.pa.gov 2018). Most of this energy is comes from fossil fuels likeCoal, oil or natural gas, which are the products of a process spanning for millions of years that converted organic materials into fossil fuels. These resources are non-renewable (that is, once depleted, cannot be replenished), and our continued dependence on these fuels have resulted in these resources nearing their depletion (f.waseda.jp 2018; nationalgeographic.org 2018). Moreover, with the continuous development of technology, global economy, per capita income and an ever increasing global population, the energy consumption and energy demands has also increased significantly, which stretches the available sources of energy (Coyle and Simmons 2014). This highlights the necessity of using alternate sources of fuel to address the extra energy demand.
Municipal Solid Waste (MSW) which is also known as garbage is the a waste discarded by the public in a municipality region, and can comprise of different types such as: biodegradable waste (like organic waste), recyclable waste (like glass, paper, cardboard, jars, tin cans, fabrics, tires and batteries), inert waste (like dirt, debris, construction or demolition materials), composite waste (like clothing, packaging, toys), hazardous wastes (such as chemicals, toxic wastes, fertilizers, paints, electrical appliances, aerosols) and biomedical wastes (like medicines, biological samples, syringes and medical devices) (Cfpub.epa.gov 2018; Hossain et al. 2014)). Most of these wastes end up in landfills, and significantly affects the environment. The environmental impacts of Solid Municipal Waste includes emissions of greenhouse and hazardous gases like methane, toluene, methylene chloride from the landfills, increasing the risks of contamination of water and degradation of water quality due to leaching from the landfills, and increase in the consumption of energy to collect and transport the solid waste from the urban areas to the landfills, degradation of the habitat due to the landfill and even slowing down the degradation of organic waste in the landfills. Moreover, MSW also affects the economy by the devaluation of land near the landfills, due to the costs for disposal of wastes into the landfills, which requires many vehicles for collection and transportation as well as affecting the efficient usage of land for development (Cmap.illinois.gov 2013; Cremiato et al. 2018). This shows that an increase in MSW can have negative effects on the environment and also increase the use of landfills, which can further impact the economical growth of the region.
Studies have shown that MSW can be used as a source of energy, as different types of solid wastes can have different amount of energy stored in them. Ozbay and Durmusoglu (2013) showed that the net calorific value of solid waste bales for MSW ranged from 1 to 16.4 MJ per kilo (of dry weight). Studies by Scarlat et al. (2015) showed that MSW can be used as a locally available energy resource from the process of waste incineration and through landfill gas (LFG) and the authors pointed out that the total waste generated in Africa can provide 2199 PJ of energy by 2025, and using LFG an additional 363 PJ of energy can be produced by 2025. This can be used to produce up to 122.2 TWh of electricity, compared to an estimated consumption of 661.5 TWh of energy as of 2010.
Utilizing MSW therefore can prove us with an additional energy source, as well as reduce the amount of wast5e reaching the landfills, and thus reducing the impact of MSW on the environment as well as on the economy.
Pandey et al. (2016) studied the use of MSW as a source of renewable energy. The authors argued that there is an ever increasing generation of solid waste, accelerated by the fast pace of modernization, population growth, and urbanization, as a result of which billions of tons of solid waste is prodiced every day across the globe. This solid waste is also causing contamination of the environment and the ecosystem (Rajkumar et al. 2018; Ali et al. 2014; Chadar and Chadar 2017). The authors point out that some of the developing countries in the world produces millions of tonnes of solid waste is produced every day (siteresources.worldbank.org 2018). Most of the municipal solid wastes produced consist of a large proportion of biomass wastes such as food, paper, clothes, wood, vegetable, rubber, plastic, and other discarded materials. These substances increases environmental hazards, as well as increase the need for landfills for disposal of the waste. The authors also point out reports from the World Bank which suggested that the solid waste generation will reach 2.5 billion tonnes per year at the end of 2025 (siteresources.worldbank.org 2018). In India, an average of 1, 20,000 tonnes of MSW is produced per day, per capita, 94% of which reaches the landfills, and the metropolitan cities contributes to 30,000 tonnes of waste every day (Pandey et al. 2016). The authors propose that energy can be produced from this waste by converting it into biogas, syngas or heat, and three types of methods can be utilised to make energy from waste, such as physical method, thermal method and biological method.
In the physical method, solid waste is mechanically processed to produce an energy source which is suitable, durable and handle able which can be then used as a form of fuel. This can include wood chips, wood briquettes and pellets. In thermal method, the waste is processed to generate heat and then used in various processes as a source of energy. This can be done by four techniques: a) direct combustion: here the solid wastes are first dried and then burnt to produce heat. The heat so produced can then be used to convert water to steam and run turbines (Hossain et al. 2014). b) Pyrolysis: here the solid wastes are heated to a high temperature in anaerobic condition (atmosphere edoid of oxygen) to make combustible gases, liquid or solid residue (Wang et al. 2017). Here the chemical composition and physical state of the waste is simultaneously changed in an irreversible reaction to produce substances like as hydrogen, methane, charcoal, hydrocarbons, vinyl chloride, coke and carbon monoxide, which can be also used for the creation of power. This process is also known as dry distillation, destructive distillation or cracking (Ates et al. 2013). c) Thermal gasification: in this process, organic wastes are converted into carbon monoxide, carbon dioxide and hydrogen at temperatures above 700 degrees Celsius in limited oxygen supply to prevent combustion. The resultant mix of gases is called syngas or synthesis gas which can be used as fuel (Panepinto et al. 2015). d) Plasma Gasification: here a plasma arc torch at temperatures of 2200 to 13900 degree Celsius is used to convert organic waste to syngas (Janajreh et al. 2013; Fabry et al. 2013; Woolcock and Brown 2013)). In biological method, microbes are used to make fuel from wastes. Here, the microorganisms processes the waste, and breaks the molecules down in the absence of oxygen. It can be done by two methods: Fermentation: which can be used in industrial as well as domestic purposes to manage MSW and make. It is the process in which yeast and bacteria acts on organic matter, in the absence of oxygen to produce alcohol which is an efficient fuel source (Qian et al. 2016). b) Biogas: which can be formed when microorganisms an aerobically digests organic wastes to produce gases such as methane (Pandey et al. 2016).
Zhou et al. (2014) studied the properties of MSW in context of its physical and chemical composition and their heating values. The authors used statistical indices like mean value, coefficient of variation, standard deviation, and t test to test for the analysis of the physical composition, proximate, ultimate analysis and the heating value of the solid wastes (Babatunde et al. 2013; Ciuta et al. 2015). The authors pointed out that thermal conversion of Municipal Solid Waste (MSW) is challenged by several factors such as: a) the composition of MSW is highly variable and complicated, which is affected by several factors, and is shown to vary across regions and times. b) There is also a variation in the content of moisture and ash in the MSW from referent regions. The amount of volatile matter present in the MSW affects how well it can be ignited by the process of pyrolysis or gasification. c) Owing to the complex nature and composition of the MSW, most studies focused on the thermo chemical aspects of different types of substances present in the MSW. For component such as plastic, which can be of different types like polyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC) can have different energy values (Rochman et al. 2013). d) The stable operation of the incinerator is greatly affected by the heating values of the MSW (Zhou et al. 2014) e) while calculating the proximate and ultimate heating values, the answers might involve terms such as accepted, dry air, dry or ash free. Their data are essentially transferrable however, might create problems while comparison of different data (Zhou et al. 2014). f) Variation in the composition and energy content in MSW across different cities might also be due to the usage of different techniques for estimation of the data (Zhou et al. 2014).
MSW can be made of a series of heterogeneous substances; the comical characteristics are related to their chemical properties. The waste can be organic or inorganic. The organic wastes can include food wastes or residues, textiles, paper, rubber and plastic inorganic wastes include glass, ash, and tiles and inter matters (Babatunde et al. 2013). The inorganic and inert substances do not have effect on the thermo chemical values of the waste. Zhou et al. (2014) suggested that changes in the composition of MSW can be seen with variance in climate, lifestyle of the people, economic status and region. The authors also proposed that the MSW from developed nations where, more paper, plastic and tecti8le can be found in the MSW, while MSW from coal based economies have more inorganic wastes. Also, the production of energy depends on the MSW makeup, especially upon the presence of plastics and paper. The study on thirty samples of MSW showed that they had an average moisture content of 48.12% (fluctuating between 25.95% and 61.74%), while the average ash content was found at 43.57% (fluctuating between 20.56% and 76.76%), the mean carbon content was 56.99%, the hydrogen content was 7.84%. IN average the every value of the MSW studied by Zhou et al (2014) was found to be at 5337 KJ/Kg, with the highest value at 9436 KJ/Kg. This study highlights the importance of taking into account the composition of the MSW, as affected by variables like region, climate, economic status, living conditions and time (Zhou et al. 2014).
Amin et al. (2012) examined the amount, composition and energy present in the solid waste in Iran. The authors used a time series model for predicting the amount of solid waste that will be generated in the future, using data on past waste changes to predict the future changes in the properties and makeup of the waste. The energy contained in the waste was measured by the common heating coefficient of each component of Doulonge’s Formula (oxfordreference.com 2018). The author’s supports that by converting the MSW into a source of energy can help to minimize the use of landfills by reducing the volume of waste as well as reduce dependency on conventional source of fuel such as fossil fuels (Amin et al. 2012). The authors collected data from previous years from the Organization for Waste Recycling and Conversion, on the general components of the waste. The study used standard methods for analyzing the physical properties of the waste, and the difference in the composition over the years, were compared from relevant resources to predict the change in the properties of the MSW. Calculation of the amount of waste generated was done as per the factory input waste weight, wastes of coarse and fine reject production line. 30% of the waste was found to be coarse and 7% of fine wastes. The waste was divided into three groups: a) Mixed waste which included all components of the waste b) combustible waste from the coarse rejects of the production line and c) non combustible waste or waste for the landfills. The common heating value was calculated by two processes, in the first process, the energy in each group was calculated by multiplying the dry weight of each of the group by its energy content and in the second process, the heating value was estimated the chemical composition of the waste using the Dulonge’s formula. The study showed that the physical composition of the waste comprised of organic waste (59.1%), plastics (24.4%), paper and cardboard (5.25%) and textile (4.56%). In thermal processing, waste is converted to solid, liquid or gas and release heat in the process. The authors suggest that this heating value can be measured to understand the energy content of the waste. Estimating the heating values of MSW from the dry weight, which can be expected in 2020, was found to be at 3230 MWh (for general or mixed waste), 1911 MWh (for coarse waste) and 370 MWh for fine reject per day from the samples studied by the authors. The estimation of heating values found using the Dulong’s method was found to be lower than from the first method, as 2656 MWh, 1160 MWh and 329 MWh per day, for the same categories of wastes mentioned above. The estimated heating value of MSW for 2020 was calculated using the Dulong’s formula was found as 14,500 kj/kg (for general waste), 14681 kj/kg (for coarse reject) and 15925 kj/kg (for fine reject). Estimations by other studies in the same region also showed approximate values of 87,000kj/kg to 11,883 kj/kg based on the dry weight of solid waste as of 2003. Based on the trend of change in the composition of solid wastes, the authors forecasted that there is a possibility of decrees in the growth of waste in the coming years through successful utilization of waste as source of energy. Also, the percentage of combustible materials in the waste is also likely to increase, while the percentage of the organic waste is likely to decrease (Amin et al. 2012). This is why it is important that effective method of management of the waste is necessary, and the process of using waste to generate energy should be further explored.
Seeling and Schneider (2012) proposed method for energy estimation for MSW, based on its physical makeup (heat of combustion) in the household solid wastes. The authors used an approach of using a predictive equation for the estimation of the energy content in MSW, analyzing the physical composition, and then multiplying the percent of each type of waste with its default heating values. The authors used values from studies by Komilis et al. (2012). The authors supported the view proposed by other studies that the energy of MSW is associated to the amount of plastic, paper, food, vegetation and textile contained in it. The authors also pointed out that the default heating values generally refer to the total heat value measured using bomb calorimeters. It is also important to note whether the values are expressed on the basis of dry weight or wet weight of the solid waste, where wet weight refers to the dry weight plus the moisture content in the solid waste. With the water content in the fuel, the energy content decreases, since a part of the heat is used up to evaporate the moisture contained the waste. The author’s identified the calculation to measure the HHV: Higher Heating Value, wet mass content (MJ/Kg) as:
HHV= (1-M)HHVd
Where M is the moisture content and HHVd is the Higher Heating Value. Dry mass content (MJ/Kg). Different methods are available for describing and predicting the energy content in MSW. The authors conducted search on the Elsevier’s Science Direct Database for models that correlate the physical composition of MSW to its energy content, and identified three models for their study. In the first model, the MSW was found to be composed on 16% plastic, 11% paper, 63% food and 60% moisture and the average gross heating (HHV) value was found to be 11.5 MJ/kg. I the second model, the MSW was found to be composed on 21% plastic, 11% paper, 52% food, 4% grass, 2% textile and 55% moisture, and the waste had an average gross heating value (HHV) of 17 MJ/kg. In the third model, the waste was composed of 19% plastic, 25% paper, 22% food, grass and wood, 7% textile and HHV of 18 MJ/kg. Authors points out that the equations predicts higher heating values (HHV) assuming that all the moisture in the waste is in liquid form, while LHV considers that water is not present in liquid form in the waste. More energy is liberated from HHV scenario, since condensation is an exothermic reaction, due to which its value can be higher than LHV. The authors therefore point out that LHV is a better measure of calculating the energy content than HHV. HHV contains the heat of condensation of water vapor formed in a combustion reaction. The authors propose that such a measurement is realistic for industrial combustion equipments as the water in the final product is in the form of vapor. LHV can be measured from HHV by subtracting the heat of vaporization of water in the waste as per the equation below. In the equation, the wet weight, heating values at constant pressure, wet basis moisture content and the latent heat of vaporization of water is considered. The difference between HHV and LHV is mainly due to the content of hydrogen, and biomass generated from feedstocks generally contains about 6% to 7% hydrogen.
LHV= HHV (1-M)-2.443M.
The calculations by Seeling and Schneider (2012) found the default gross heating value (in MJ/kg) for different types of components in the MSW as: organic material (4.65), paper waste (15.24), and plastics (44.04). Textiles (17.45), Glass (0.14), metals (0.70), and estimated contributions (MH/kg) as organic material (2.66), paper waste (1.77), and plastics (4.94). Textiles (0.59), Glass (0), metals (0.01) and an over value of 9.98 MJ/kg. The three models identified by the authors showed the following variations between the HHV and LHV as: model 1: HHV=10.64 MJ/kg, LHV=5.16 MJ/kg; model 2: HHV=16.86 MJ/kg and LHV=8.77 MJ/kg and model 3: HHV=17.01 MJ/kg and LHV=8.85 MJ/kg.
Sadef et al. (2016) tried to analyse the waste to energy (WTE) and recycling value of municipal solid waste, which can help in the create an integrated waste management system. The authors pointed out that as of 2012, approximately 1.3 billion tons of solid waste was produced around the globe, and the amount is expected to get doubled by 2025. Similarly, the costs of waste management will also increase from 205.5 billion USD to 375 billion USD in 2025. It is also highlighted that the MSW are sources of biomass, recycled materials, energy and revenue if used and managed properly. The authors suggested methods for the conversion to waste to energy, such as pyrolysis, anaerobic digestion, and incineration and refuse derived fuel can help in the generation of energy in the form of electricity, fuel, heat, organic fertilizers and chemicals.
The study analyses researches from across the globe to identify different methods for the analysis of energy content in MSW and in the conversion of MSW to energy. The study will describe the process of estimating energy in solid wastes, and the tools, equipments and methods that can be used to generate energy from these waste products. This can be used to design waste to energy estimation and conversation project to be set up in Melbourne.
Described next is the proposed project to be set up in Melbourne, Australia for the estimation and transferring of Municipal Solid Waste to energy, based on the studies above to design a hypothetical model.
The process of estimation of energy content of MSW can be done in a multi step process. In step 1, the composition of the waste can be differentiated into its components based on their wet weight and percentage in MSW. In step 2, energy content can be measured (for example using the average heat values, or other values discussed later), which can be expressed on its dry weight. In step 3, the dry weight of the waste components can be calculated by adjusting the water content. In step 4: the total energy content can be estimated by multiplying the amount of solid waste with the average energy content of the material per unit weight. IN step 5, the specific energy content is calculated by dividing the total energy of the MSW sample by the total waste of the sample (Msw.cecs.ucf.edu 2018). The required tables and formula for calculation are given below:
Table 1: For waste composition in MSW
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Component |
Wet Weight |
Percentage |
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Food waste |
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Cardboard |
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Paper |
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Textile |
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Plastic |
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Rubber |
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Leather |
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Trimmings from gardens |
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Wood |
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Glass |
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Metal |
Table 2: Average Heat Values
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Components |
Heat Value/ dry weight |
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Food waste |
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Cardboard |
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Paper |
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Textile |
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Plastic |
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Rubber |
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Leather |
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Trimmings from gardens |
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Wood |
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Glass |
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Metal |
Table 3: Dry Weight of MSW:
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Component |
Wet Weight |
Dry weight (after adjusting average water content of each component from its wet weight) |
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Food waste |
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Cardboard |
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Paper |
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Textile |
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Plastic |
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Rubber |
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Leather |
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Trimmings from gardens |
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Wood |
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Glass |
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Metal |
Formula 1:
Total energy= weight of solid waste x energy content
Using this formula, the total energy content in a sample of MSW can be estimated by using the weight of different components of the MSW and the average energy content of each component. Calculating the total energy in each component in the waste can therefore help in the estimation of the total energy, by using the percentage of each component in the MSW, which can be identified through a physical analysis.
Table 4: Total energy content
|
Component |
Wet Weight |
Dry weight (after adjusting average water content of each component from its wet weight) |
Total Energy content |
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Food waste |
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Cardboard |
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Paper |
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Textile |
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Plastic |
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Rubber |
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Leather |
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Trimmings from gardens |
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Wood |
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Glass |
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Metal |
Formula 2:
Specific energy content= total energy content/ total waste of MSW expressed in unit of weight
This equation can help to analyze the specific energy content of each component, which can be used to understand the importance of each of these components in the generation of energy, and thus usability of the MSW sample as an alternative energy source.
Studies by different authors (such as Sadef et al.; Ozbay et al.; Pandey et al.; Zhou et al. and Amin et al.) have highlighted various methods of calculating the energy of MSW, discussed below is the strategies identified from these studies to develop a system of estimating the energy content of MSW:
The methods used by Sadef et al. (2016) utilized mathematical formulas for the energy estimations. The analysis of the MSW was done by measuring various factors such as the water content, dry matter (DM), volatile solids (VS), fixed carbon and ash contained in the waste. Elemental analyzer was used to measure the Carbon/Nitrogen ratio, and the content of carbon, nitrogen, hydrogen, sulfur and oxygen. Atomic Absorption Spectrometer was used for the estimation of heavy metals like copper, cadmium, lead and iron and bomb calorimeter was utilized to calculate the calorific values of the waste. The authors calculated the dry matter (DM) using the formula:
DM%= 100x WDM/Wsample
This equation shows that the dry weight percentage of MSW can be easily derived from the Dry weight of the sample and the total weight of the sample. Calculating the dry weight can allow the estimation of energy without the loss of energy due to the presence of water vapor. This method has been found to be useful according to many authors.
The volatile solids were calculated using the equation:
VS%= (100 (WDM -Wash)/WDM)
Calculating the volatile solids can help to understand the content of volatile substances in MSW. The volatile substances increase the energy content of the MSW, and hence it is vital to analyze its content.
For food waste, the energy content was calculated with the equation (Dulong Formula):
Btu/lb= 145C +601 (H2 – 1/8O2)+ 40S +10N
Considering that MSW contains a significant amount of food waste, it is also important to analyze the energy contained in this waste, to understand the energy efficiency of the MSW sample.
The RDF value of the MSW was calculated using the equation:
E (MJ/kg)= 0.051 (F) + 3.6 (CP) + 0.352 (PLR)
This equation gives the estimation of the energy content in MJ/kg of the MSW, Where E is the energy content, F is the percent of weight of the food waste, CP is the percentage by weight of paper, and PLR is the percentage by weight of rubber and plastic.
The crude fiber in MSW was estimated using the formula:
BF= 0.83-0.028 LC (where LC is the percent by weight of crude fiber)
This helps to analyze the energy content of the crude fibers, which is also found in significant amounts in MSW. These equations can be used in the estimation of energy of various components of MSW.
Studies by Seelig et al. (2012) also provide additional mechanisms for estimation such as: Higher heating Value (HHV), LLV (Lower heating Value) for estimating the energy content in MSW. The HHV of wet mass content of the MSW is calculated by multiplying HHVd (which is the HHV of the dry mass content) with the difference in value of the moisture content from 1.
HHV= (1-M)HHVd
While, LHV is calculated by deducting the value of 2.442 times the moisture content, from the product of HHV and the difference between M and 1.
LHV= HHV (1-M)-2.443M.
Both these values can be used to find different approximation of the energy content of MSW (based on the presence or absence of moisture). The authors suggested that the LHV shows a lower value, since it does not calculate the latent heat of vaporization of water, and hence is a better estimation for industrial purpose of estimation of solid waste energy content.
Sadef et al. (2016) discussed the different technologies for the transfer of waste to energy (WTE). They have outlined the need of equipments required for incineration, production of refuse derived fuel and for anaerobic digestion of the MSW components. For Incineration, is used in several countries as a part of waste management system. The process can have an overall energy efficiency of 25%-30% and can reduce the mass of the solid waste by 70% and volume by 80%. However, the process also creates air pollutants. To produce Refuse Derived fuels (RDF), which is a combustible or high calorific fraction derived from MSW can be an alternative to conventional fuel (Sarc et al. 2013; Taylor et al. 2013). In this process, the waste is processed by reducing the volume (compacting), separation, crushing and then drying. After drying, the RDF is then mixed with binders like calcium hydroxide in order to increase the calorific value of the RDF. The final product is in the form of chalk like pellets (Sadef et al. 2016). Anaerobic digestion can be used to prepare biogas and organic fertilizers, and can be used especially for organic waste like food waste (Ariunbaatar et al. 2014). The process can provide an energy efficiency of 25%.
Pandey et al. (2016) outlined three methods for the conversion of waste to energy, namely Physical Method, Thermal Method and Biological Method. Each of these methods requires different tools, equipment and strategies for extracting the energy from MSW.
In this method, basic mechanical processes and equipments that can be used to process the MSW to convert it into a form that are usable as fuel, such as making wood pellets or woodchips from wasted wood. This process requires mechanically or manually sorting the waste to identify components which has high energy content, such as wood, paper or plastic, and then mechanically convert them into a form that is easy to be stored and used as fuel. Therefore, no specialized tools or equipments are needed for this mechanism (Pandey et al. 2016).
For incineration of solid waste, in specialized furnaces, which allows heating the waste to incinerate it. Solid Waste Incinerators such as Thermal Oxidizer can be used in this process. This process allows both continuous and batch wise processing of the waste based on the volume of MSW to be processed. This system can allow both thermal combustion and pyrolysis. The waste can be heated using direct or indirect fired. The heating depends on the residual oxygen limits of the process (Massaro et al. 2014; Chalabi et al. 2018). System efficiency can be improved through recuperative or regenerative systems. Im the recuperative system, pre heated air or fume is used. The furnace can be set up in both horizontal and vertical configuration for processing batches with roller hearth, rotary drum and rotary hearth transport system. The system can help in the reduction of volume of the waste, recovery of metals, conversion of biomass to energy and even decontamination process. The types of materials that can be processed in the thermal oxidizer includes solid waste, tires, munitions, biomass, hospital waste, sludge’s and electrical wastes (.surfacecombustion.com 2018)
Different equipments are available for pyrolysis of solid wastes. BioGreen equipments have designed a pyrolysis equipment that can allow the conversion of different types of bulk materials like biomass, biosolids and waste into products of high energy content, such as syngas, biochar, oil compounds and solid fuels. The BioGreen system for pyrolysis consists of equipments like a Spirajoule (which helps in the regulation of the temperature up to a range of 800 degrees calcium) and helps to maintain the conditions of processing the waste and thus ensure a uniform conversion of the materials, cooling system (that uses chilled water) to ensure safe and efficient evacuation of products generated in the process of pyrolysis, kenki drier and belt dryer (to reduce the moisture content of there MSW) (biogreen-energy.com 2018).
For the Gasification of the solid wastes, different types of equipments are available, such as: Modular Biomass Gasification of Alternative Energy Solutions International Inc (AESI) which provides cost effective heating systems or Nexterra’s Gasification Technology (nexterra.ca 2018). The Nexterra system provides energy output in the range of 2 to 40 MWh (or 8 to 140 MMBtu/hr) as well as 2 to 15 MW of electricity. The setup consists of a metering bin for storage of fuel to supply the gasifier. In the gasifier, the fuel flows through different stages in the process of drying, pyrolysis and gasification and the reduction of waste to ash (Wang et al. 2017). The inner and outer cones at the base of the gasifier are used to introduce steam and oxygen. Pyrolysis and Gasification can occur at 815 to 980 degree Celsius to convert the substance to syngas. The system also as an automatic ash removal system using ash hopper, thus ensure prolonged use of the equipment (nexterra.ca 2018).
Recor utilizes Plasma Arc Flow Technology, where the liquid waste is passed through submerged eclectic arc between two electrodes. The arc breaks the liquid molecules to their atomic components and forms a plasma at around 5500 degrees Celsius (Fabry et al. 2013). The arc moves the plasma from the electrodes, and regulates the formation of syngas which is then collected. The syngas is a hydrogen based fuel (recor.co.za 2018; Nielsen et al. 2015).
Fermentation of organic waste JFE BIGDAN Type Biogas system can be used to produce methane. The equipment allows fermentation of slurry, livestock excrement, food waste, sewage sludge and other municipal organic wastes at a temperature around 37 degrees Celsius. The equipment can be used to recover electric energy as well as liquid fertilizers. The process consists of the first sterilization process by heating the mass up to 70 degrees Celsius for an hour and then heat is recovered from the slurry (jfe-eng.co.jp 2018; Qian et al. 2016).
For calorimetric estimation, Bomb Calorimeter can be used. This is a specific type of fixed volume calorimeter that can be used to calculate the heat of combustion in a given reaction. These calorimeters can withstand high pressures while measuring the reactions. In this process, electricity is used to ignite the fuel, which in turn superheats the surrounding air. The air then expands and flows out of a tube. The escaping air passes through a copper tube, which is used to heat water outside the tube, and the calorific content of the fuel is estimated by measuring the change in the temperature of water outside the copper tube. The apparatus essentially consists of a small cup for the sample, a bomb, an oxygen source, a stirrer, thermometer and an insulated container (learner.org 2018).
Different aspects need to be considered while thinking about utilizing MSW as energy source. Sadef et al. (2016) discussed the disadvantages of different waste to energy conversion processes such as, which highlights the negative aspects of each method. For composting, the disadvantages includes: the requirement for space, loss of nutrient, need for equipments and manpower and a significant problem with the odor of the compost. For anaerobic digestion or fermentation, the disadvantages include the presence of impurities in the product, and lack of feasibility of large scale process, the probability of overloads or shocks and the requirement of space. For refuse derived fuels, the disadvantages are the chance of air pollution while using refuse derived fuel, formation of ash, the high costs of production and need for land. For incineration the disadvantages are the creation of both airborne and waterborne pollutants, formation of carcinogenic chemicals, requires significant investments, can have adverse reaction from the local communities, and the production of solid wastes. For Pyrolysis, the disadvantages can include the formation of liquid products with low energy yield, formation of pyrolysis water from organic substances, production of coke, additionally, the cleaning process of the byproducts is difficult, and the process can also cause corrosion of the metal tubes. The operation and maintenance of these equipments are also cost intensive. These challenges show the different constrains in the process of conversion of solid wastes to energy.
Other constraints also include variance in the composition of the MSW with changes in the regions, time, culture and economic status of each place from where the MSW originates. This makes the process of estimation of energy complicated, as MSW from different places might have different ratios of its components. The content of water is another factor affecting the total energy content of the waste, as the presence of water results in the usage of more energy for its vaporization (Ozbay et al. 2013). The content of moisture in the MSW can also vary, as well as the content of ash, which also affects the efficiency of different MSW samples (Scarlat et al. 2015). Most studies that analyzed the energy content of plastics in MSW mostly categorized it in a single group, however, plastic can be of many different types, like polyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), each of which can have different energy contents, and hence should be analyzed separately (Zhou et al. 2014). While operating the incinerators, the stable operation of the incinerator is greatly affected by the heating values of the solid wastes, thus MSW producing lesser heat is not very applicable for incineration. Different studies of the MSW energy estimations from the wastes from different cities have used different strategies of estimation of the energy content, which have attributed to differences in the final energy content of the MSW, which might result in confusion while comparing the results (Zhou et al. 2014; Ozbay et al. 2013; Pandey et al. 2016; Amin et al. 2012).
Conclusion:
Energy is one of the most important requirements our developing civilization, which allows the equipments, vehicles, machinery and the different public infrastructures running. Energy to power these are generally in the form of fossil fuels or electricity. With further development in our civilization and rise in the global population, the need for energy is ever increasing, which is pushing the renewable resources like fossil fuels towards depletion (non renewable resources are near depletion). Moreover, usage of these fuels has also resulted in a significant emissions of greenhouse gases and a consequent impact on the environment (using fossil fuels causes’ global environmental impact). Also, with the growth of our civilization, and population, the amount of waste generated, especially in the form of Municipal Solid Waste (MSW) is also increasing rapidly. These waste can be composed of various types of organic and inorganic matter, and can cause contamination of the environment (land and water) and use a significant amount of land due to landfills (solid waste has environmental impact). Studies have shown that that the MSW contains significant amount of energy, which can be used to generate heat, and thus be used as a source of energy. Utilizing MSW as source of energy have two advantages, firstly it allows reducing dependency on non renewable source of energy such as fossil fuels, providing a renewable energy source and secondly, it helps to mitigate the environmental impact of the landfills, by reducing the amount of waste that ends up there.
Different studies have shown that MSW from various places can have various compositions, and thus the overall energy content also can vary depending on that. Different process are available to estimate the energy of MSW, however, values like Higher Heating Value, Lower Heating Value can provide dependable methods of analysis. Also, it is important to understand the difference between the dry weight and wet weight of the MSW in the estimation of the energy si9nce the presence of water can significantly affect the energy efficiency of the system. Different methods can be used for the conversion of waste to energy, such as physical process, thermal process and biological process, each can be done using different equipments, and can result in the formation of fuel in solid, liquid or gaseous state.
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