Waste Management xxx (2014) xxx–xxx

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Processing and properties of a solid energy fuel from municipal solid waste (MSW) and recycled plastics JeongIn Gug, David Cacciola, Margaret J. Sobkowicz ⇑ Department of Plastics Engineering, University of Massachusetts Lowell, One University Ave., Lowell, MA 01854, USA

a r t i c l e

i n f o

Article history: Received 11 April 2014 Accepted 30 September 2014 Available online xxxx Keywords: Briquette MSW Plastics recycling Refuse-derived fuel

a b s t r a c t Diversion of waste streams such as plastics, woods, papers and other solid trash from municipal landfills and extraction of useful materials from landfills is an area of increasing interest especially in densely populated areas. One promising technology for recycling municipal solid waste (MSW) is to burn the high-energy-content components in standard coal power plant. This research aims to reform wastes into briquettes that are compatible with typical coal combustion processes. In order to comply with the standards of coal-fired power plants, the feedstock must be mechanically robust, free of hazardous contaminants, and moisture resistant, while retaining high fuel value. This study aims to investigate the effects of processing conditions and added recyclable plastics on the properties of MSW solid fuels. A well-sorted waste stream high in paper and fiber content was combined with controlled levels of recyclable plastics PE, PP, PET and PS and formed into briquettes using a compression molding technique. The effect of added plastics and moisture content on binding attraction and energy efficiency were investigated. The stability of the briquettes to moisture exposure, the fuel composition by proximate analysis, briquette mechanical strength, and burning efficiency were evaluated. It was found that high processing temperature ensures better properties of the product addition of milled mixed plastic waste leads to better encapsulation as well as to greater calorific value. Also some moisture removal (but not complete) improves the compacting process and results in higher heating value. Analysis of the post-processing water uptake and compressive strength showed a correlation between density and stability to both mechanical stress and humid environment. Proximate analysis indicated heating values comparable to coal. The results showed that mechanical and moisture uptake stability were improved when the moisture and air contents were optimized. Moreover, the briquette sample composition was similar to biomass fuels but had significant advantages due to addition of waste plastics that have high energy content compared to other waste types. Addition of PP and HDPE presented better benefits than addition of PET due to lower softening temperature and lower oxygen content. It should be noted that while harmful emissions such as dioxins, furans and mercury can result from burning plastics, WTE facilities have been able to control these emissions to meet US EPA standards. This research provides a drop-in coal replacement that reduces demand on landfill space and replaces a significant fraction of fossil-derived fuel with a renewable alternative. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction The volume of municipal solid waste (MSW) generated from residential, commercial and institutional locations is increasing due to population growth and the ‘‘throw away’’ culture that persists throughout much of the world. The ability of landfills to handle our waste is limited due to the space required and resulting pollutions to soil, water, and air. According to a US EPA (Environmental Protection Agency) report in 2011, total MSW ⇑ Corresponding author. Tel.: +1 978 934 3433. E-mail addresses: [email protected] (J. Gug), david_cacciola@ student.uml.edu (D. Cacciola), [email protected] (M.J. Sobkowicz).

generation was 250 million tons in 2010 and only 34% of waste was recycled over the same period in the US. Although 12% of trash was converted through waste-to-energy (WTE) operations, around 54% MSW was still going to landfill without any treatment (US EPA, 2011a). According to another report from the Organization for Economic Co-operation and Development (OECD) fact book, the United States was ranked as the largest generator of MSW in the world in 2010 (OECD, 2013). Importantly, the volume of plastic wastes in total MSW has increased from 0.5% to 12.5% between 1960 and 2010. Less than 10% of that plastic is utilized, either through recycling, re-use, or energy recovery (Subramanian, 2000; US EPA, 2011a). This statistic represents a shortcoming of

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the plastics recycling industry and an opportunity to find alternative reuse strategies. Various WTE technologies exist to convert the combustible MSW into useable heat and electricity; combustion, gasification and pyrolysis (Al-Salem et al., 2009; Arena, 2012; Bilitewski, 2007; Cheng et al., 2007; Cheng and Hu, 2010; Hernandez-Atonal et al., 2007; Hilber et al., 2007; Kakaras et al., 2005; Koukouzas et al., 2008; Marsh et al., 2007; Psomopoulos and Themelis, 2009; US EPA, 2011b). As of 2010, 86 WTE power plants were in operation in the US, incinerating more than 28 million tons of MSW per year (Michaels, 2010; Themelis, 2003; US EPA, 2011b; Williams and Helm, 2011). In the European Union (EU) waste recovery efforts include mechanical–biological treatment (MBT) plants for managing MSW as solid recovered fuel (SRF) (Psomopoulos, 2014; Rada and Andreottola, 2012; Rada and Ragazzi, 2014; Samolada and Zabaniotou, 2014). WTE power plants can be classified by three categories for the incineration of MSW: mass-burn, modular and refuse derived fuel (RDF) systems. In mass-burn facilities, the MSW is not sorted before it is fed to the furnace. In a modular system, unprocessed MSW is also used; however, the facility can be easily moved from one place to another because it is small and portable. In an RDF system, the MSW is shredded and some non-combustible materials are collected before the waste is burned in the combustion chamber (US EPA, 2011b). Thermoplastics contribute roughly 80% to the total plastic consumption in the US and they are generally used in packaging, construction, and consumer goods (Dewil et al., 2006; Subramanian, 2000; US EPA, 2011a). Over 30% of thermoplastic products are one-time-use; these plastics are excellent candidates for conversion to electrical power or local heating because of their higher heating value and easy handling (Al-Salem et al., 2009; Arena, 2012; Peter, 1992; Subramanian, 2000; Williams, 2005). Common plastic materials, such as PE, PP and PS, used in packaging have higher heat of combustion than commercial coal fuel and almost as much energy as fuel oil (Subramanian, 2000). WTE can be a viable option for some recycled plastics with lower environmental risk (Deriziotis, 2004; Psomopoulos et al., 2009; Themelis et al., 2002), (excluding PVC and other halogenated plastics that release dioxins and other toxic organics) because of the low intrinsic value of mixed and contaminated waste streams and the low cost of new raw material when compared with complex processing required to regenerate good properties in recycled flake (Hopewell et al., 2009). According to a recent EPA report, paper and plastic wastes represent the largest fraction of MSW generated, at around 42% (US EPA, 2011a), and it is clear that both are easily ignitable materials with high energy content compared with other waste categories. Mixing these two in appropriate ratios and forming compact shapes using briquette technology provides an optimized fuel that can be used as a coal substitute. This could have significant impact because in the US, coal is the primary resource for electricity generation, followed by natural gas. The US Energy Information Agency (EIA) has classified coal into four major ranks (in descending heating value order): anthracite, bituminous, sub-bituminous and lignite. Bituminous and sub-bituminous coal are the most commonly used, with 92% production among total coal categories (US EIA, 2013). Energy conversion from well-sorted MSW has many advantages. According to actual operating data collected by the US WTE industry, on the average, combusting one metric ton of MSW in a modern WTE power plant produces a net of 650 kW h of electricity, thus avoiding mining a quarter ton of high quality US coal or importing one barrel of oil (Psomopoulos et al., 2009). Furthermore fuels recycled from well-sorted MSW and plastics are concentrated in areas of high population, giving them transportation benefit

over fossil fuels that are distributed unevenly around the country. According to a US EIA report, the average total delivery cost for coal by rail, including extraction and processing was estimated at around $45/ton in 2009 (US EIA, 2013) and it is estimated that around $230M is spent per year to transport 14,000 tons of coal and generate 109 kW h of every year (Shapley, 2011). In addition, a landfill waste volume reduction of 90–99% could be achieved by burning waste (Al-Salem et al., 2009). Importantly, harmful emissions such as dioxins, furans and mercury. from WTE facilities have been reduced steadily in recent decades and now meet US EPA standards (DeAngelo, 2004; Deriziotis, 2004; Psomopoulos et al., 2009; Themelis, 2003; Tian et al., 2012). The briquetting technique involves several steps: shredding, heating, mixing and compressing with high hydraulic pressure into a compact shape. According to research on briquetting properties and quality from sawdust and biomass published in 2010 and 2011, several key elements affect briquette properties: flake size, moisture content, compacting pressure and process temperature (Kers et al., 2010b; Krizˇan, 2006; Krizˇan et al., 2010, 2011). These studies proved that smaller particle size (less than 6 mm), optimization of moisture content (around 2–3%) and higher compacting pressure (5–10 MPa press scale of the machine) are required to obtain high-density briquettes, and the strength of briquette samples was improved when the processing temperature was controlled between 120 °C and 160 °C. The focus of this study was to control the plastic additive composition, moisture content, compacting pressure and processing temperature, while keeping the particle size constant to optimize the briquetting process. 2. Materials and methods 2.1. Material preparation Starting materials consisted of a pre-sorted domestic solid waste (DSW) stream containing mainly paper, cardboard and fiber materials including clothing fabrics and wood ships/shreds (provided by our collaborator WERC-2) and recycled plastic material, PET, HDPE and PP (corresponding to recycling numbers 1, 2, and 5, respectively) and expanded PS (or Styrofoam) collected from typical home and office locations. To prepare for compounding, all of the collected plastic products were shredded into 8 mm (0.3 in.) flakes using a laboratory mill (Thomas WileyÒ Mill) with an 8 mm sieve. They were then mixed with the DSW flake using a high intensity blade mixer (Prodex Henschel Mixer) in the following ratio: 70% of DSW, 5% of PS and 25% of one commonly recycled thermoplastic (PET, PP or HDPE) as selected based on calculation of the energy efficiency of bituminous and lignite coal on a weight basis. Styrofoam was always added due to its excellent potential in the WTE application (and poor recyclability). In the final step of material preparation, a laboratory oven (Fisher Scientific Isotemp Vacuum Oven) was used to control the moisture content in materials in the following protocols because high density is a transportation benefit and low moisture content results in higher energy efficiency: (1) material was not dried (samples designated ‘‘un-dried’’), (2) material was vacuumed for 1 h without heating (vacuumed), (3) material was vacuum-heated for 1 h at 50 °C (S. dried), and (4) material was vacuum-heated for 4 h at 100 °C (L. dried). 2.2. Sample preparation A cylinder-shaped sample from the prepared material was produced using a hydraulic press (Dake) at two different temperatures: 125 °C and 150 °C chosen to be close to the melting point of recy-

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cled plastics used (with the exception of PET). A home-made cylinder mold with a borehole 3.81 cm (1.5 in.) in diameter, and upper and lower plungers for clamping around the sample was used to shape the briquettes. The mold and plungers were pre-heated for 20–30 min on the press platens, prepared materials of mass 7 g were packed into the borehole, and the top plunger was then inserted. The mold containing materials was held at press temperature for 5 min, followed by compression for 5 min between 1 and 10 MPa on the press scale (19,600–196,000 psi actual pressure on materials). The final briquette was obtained as shown in Fig. 1 (right) and many samples were produced for the property tests. Also other samples provided from our collaborator in Fig. 1 (left) were used to compare the properties of briquettes. These had same composition of DSW and plastics like our sample, but a different shape resulted using the industrial briquette equipment.

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heating value, and can affect physical stability of the briquette. The sample moisture was determined from the following equation:

W M ð%Þ ¼

Wi  Wd  100 Wi

ð2Þ

where WM is the percent moisture in analysis sample (%), Wi is the initial weight of sample used (g) and Wd is the weight of sample after drying at 105 °C for 1 h (g). Three briquettes of each condition were tested as-produced and the average value was used. 2.3.2.2. Ash test. The ash test was carried out to analyze residue remaining after burning the briquette according to ASTM D3174 (Ash in the Analysis Sample of Coal and Coke from Coal) (ASTM,

2.3. Test methods 2.3.1. Water uptake test The water uptake test was carried out to investigate the stability of the briquettes to moisture exposure. This test was performed in cylindrical plastic containers 53 mm in diameter and 67 mm in height filled with water at room temperature for a few hours. The dry briquettes were weighed using an electronic balance first and then placed into individual water baths. After pre-determined times and up to 82 h, the briquettes were picked out and dried using tissue paper before weighing. This test was repeated until the specimens had equilibrium water uptake content and the results were used to calculate the water content in the briquette.

W U ð%Þ ¼

Wt  Wi  100 Wi

ð1Þ

where Wu is the water content in the sample, Wt is the wet weight of sample at time t and Wi is the initial weight of sample before placing in water. Three briquettes of each condition were tested and the average value was obtained, with standard deviations representing the experimental error.

Fig. 2. Compressive strength test by cleft failure.

2.3.2. Proximate analysis test The proximate analysis is an important characterization method to determine the grade and fuel quality of coal and biomass (Beamish, 1994; García et al., 2013). A deviation from the ASTM standard (ASTM, 2009) should be noted here for the result of proximate analysis: the burning tests in this study were performed in air without purge gas. There are some different test procedures for proximate and ultimate analysis of coal vs. biomass such as heating time in moisture test and set temperature in ash test; however, most steps are the same in both standards. 2.3.2.1. Moisture content test. This test was performed to investigate the moisture content in the briquette in accordance with ASTM D3173 (Moisture in the Analysis Sample of Coal and Coke) (ASTM, 2011a). Higher moisture content negatively affects fuel

Fig. 3. TGA results for starting material of DSW based on paper type.

Fig. 1. The sample produced by our collaborator (WERC-2) with industrial briquette equipment (left) and the briquette from paper type DSW and recycled plastics by compression molding in this research (right).

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2011b). The residual ash weight fraction was determined using the following equation:

W A ð%Þ ¼

Wb  100 Wi

ð3Þ

where WA is percent ash in analysis sample (%), Wb is the weight of ash after burning the sample under 720 °C for 3 h after heating gradually to 420 °C over 1 h (g), and Wi is the original weight of sample used (g). Only one briquette of each condition was tested due to time limitations (7 h for one sample). 2.3.2.3. Volatile matter test. The fraction of volatile matter was determined by calculating the loss in weight from burning the briquette in a starved oxygen environment by sealing with the cover to avoid any contact with oxygen under the atmosphere in accordance with ASTM D3175 (Volatile Matter in the Analysis Sample of Coal and Coke) (ASTM, 2011c). The weight loss percent was calculated as follows:

W L ð%Þ ¼

Wi  Wh  100 Wi

ð4Þ

2.3.4. Combustion test A mass loss cone calorimeter (MLCCal) (Fire Testing Technology) was used to investigate the heat release rate (HRR) and effective heat of combustion (EHC) of samples. The sample holder was a square box filled with fiberglass insulation with the sample (or the methane calibration flame) resting on top for consistent distance from the ignition source. The sample was placed on a microbalance and burned under a 25 kW/m2 heat flux that was previously calibrated with methane. The HRR and EHC were collected during combustion using the MLCCalc software program connected to MLCCal equipment. Four briquettes of each condition were tested and the average HC result was obtained. 2.3.5. TGA test The starting DSW material was tested with thermogravimetric analysis (TGA) equipment to investigate the composition of material as provided by our collaborator. Samples were heated from 30 °C up to 900 °C under a nitrogen atmosphere with the heating rate of 20 °C/min and the weight loss was recorded. Total four random samples between 4 and 7 mg were analyzed.

where WL is the weight loss percent in analysis sample (%), Wi is the initial weight of sample used (g), and Wh is the weight of sample after heating at 925 °C for 7 min (g). Three briquettes of each condition from the sample protocols prepared were tested and the average value was used. The volatile matter percent was obtained using both the weight loss percent and moisture percent as follows:

W V ð%Þ ¼ W L  W M

ð5Þ

where WV is the volatile matter percent in analysis samples (%). 2.3.2.4. Fixed carbon value. From the proximate analysis test results above, the fixed carbon value could be calculated according to ASTM D5142 (Proximate Analysis of the Analysis Sample of Coal and Coke by Instrumental Procedure) (ASTM, 2009) for proximate analysis of briquette using the following equation:

F C ð%Þ ¼ 100  ðW M þ W A þ W V Þ

ð6Þ

where FC is the fixed carbon percent in the analysis sample (%). 2.3.3. Mechanical test The compressive strength of the briquettes was determined using a cleft failure technique with an Instron machine (Model 6025 and 4444) in compression mode. For the compressive strength, the intact sample was placed between round plates exposing the circular cross section to the pressure as shown in Fig. 2 and compressive force was applied to the sample in the direction perpendicular to the press direction (Kakitis et al., 2011; Kers et al., 2010a,b; Krizˇan et al., 2011). The compression force was ramped up to increase the stress inside the specimen until briquette failure by cleft or splitting and the maximum force applied specified the sample compressive strength. The compressive strength of each specimen was calculated from the ratio between maximum applied force and the sample length with following equation:

rc ¼

Pm L

Fig. 4. The briquette density variation under different processing conditions.

ð7Þ

where rc is the compressive strength (N/mm) in cleft failure, Pm is the maximal applied compressive load at yield (N) and L is the sample thickness (mm). Five briquettes of each condition were tested and the average value was obtained, with standard deviations representing the experimental error.

Fig. 5. The briquette density variation by changing forming pressure.

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3. Results and discussions

3.2. Density variation

3.1. TGA test

High material density leads to a greater ratio of energy efficiency to volume (Kers et al., 2010b). Incineration of waste materials for power generation does not require a high density fuel, but denser product has some advantages such as reduced transportation and storage costs, more consistent burning, and greater mechanical stability of the briquettes (Krizˇan et al., 2011). From a comparison of the data shown in Fig. 4, it is clear that the briquette density increases with higher compression temperature, and after removal of air bubbles in the styrene foam using vacuum. Interestingly, however, a drying procedure prior to compression consistently resulted in lower density. This may be due to insufficient moisture content to bind particles together, analogous to

TGA results present the purity of composition in starting materials of DSW provided by our collaborator in Fig. 3. The huge weight loss occurred between 300 and 400 °C, and it indicates the decomposition of cellulose (280–400 °C) and lignin (320– 450 °C) (Strezov et al., 2004). The last mass loss corresponds to release of chemically bonded CO2 and chemically formed water (450–600 °C) (García et al., 2013). According to the TGA results, we concluded that the starting materials of DSW from our collaborator were fairly homogeneous and consisted primarily of cellulose and lignin.

Fig. 6. Weight change of the briquettes over time in water bath.

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because the mold began to deform under high pressure conditions (10 MPa on machine pressure scale, or 196,000 psi actual sample pressure); the result is shown in Fig. 5. The two different pressure scales shown indicate machine scale and actual pressure on the sample. The briquette density increased from 1 MPa (19,600 psi of actual pressure on sample) to 2.5 MPa (49,000 psi), but after 2.5 MPa only slight improvement was gained up to 10 MPa (196,000 psi). There was no significant difference in properties after 5 MPa (98,000 psi). 3.3. Water uptake test

Fig. 7. Height change of the briquettes over time in water bath.

sand or dirt that does not form a cohesive mass unless water is present. To make a denser product, higher compacting pressure, some residual moisture and removal of air in added Styrofoam are required. Fig. 4 shows that heated drying of the starting material decreased the density significantly especially when PET or HDPE was added, and that vacuum did not significantly change the density when PP was added. The briquettes mixed with HDPE flakes likely presented the highest improvement in density variation because HDPE has low melting temperature (around 130 °C) closer to process conditions. The melting allowed the added plastic to flow and particles to settle and compact. Density variation data for all briquettes is shown next to the graph statistically. The density was improved 7–10% by increasing the compressing temperature from 125 °C to 150 °C. The vacuum pre-treatment to remove air trapped inside styrene foam also caused an increase of 8% and 11% for PET and HDPE, respectively, whereas PP had no big improvement in density. It should be noted, however, that density values were not statistically significantly different for the briquettes prepared with different thermoplastics within a given pre-processing condition. On the other hand, the heated drying step prior to compression showed a 2–6% decrease in density after short drying time at low temperature, and further decrease of 6–15% was observed in longer drying at high temperature. Briquette density was also improved with higher compacting pressure as shown in Fig. 5. Only one set of briquettes (70% MSW + 5% PS + 25% PP, un-dried) was investigated for this test

The water uptake test was performed to investigate the briquette stability under severe weather conditions by simulation with total water immersion. Both weight increase and height increase were tracked as water entered the briquette as shown in Figs. 6 and 7. A more gradual change in slope was observed in weight compared to height, likely due to the water entry on the sides of the cylindrical samples. In general, the higher compression temperature reduced total water uptake. As observed in the density measurements, the rate of water uptake was further reduced when material was pre-treated using vacuum without heat, while the water absorption significantly increased again by drying for short and long time. On the other hand the production line samples from WERC-2, which were prepared using industrial briquetting equipment (Fig. 1, left), were very stable without expansion and weight change of sample during the same test condition. The ‘‘pillow’’ shaped briquette geometry appeared to provide maximum sealing on the sides of the sample. This sample shape prevents absorption of water, improving stability under extended moisture exposure. Water absorption could be observed as expansion of the briquettes when they were placed in the water bath, because the domestic solid waste that makes up 70% of the briquette contains a large amount of paper and cardboard. The slope of the graph increased rapidly in all cases, except when material was pre-treated using vacuum without heat or without drying steps at 150 °C processing condition. Plots with the steepest slopes indicate that the samples exhibit poor stability and disintegrate immediately as soon as the water is absorbed. Briquettes that were vacuumed only, on the other hand, exhibited more stable behavior with less swelling. This corresponds to a more gradual increase in height. Therefore, it is essential to remove air bubbles inside the materials to maintain sample stability under diverse weather conditions. In contrast, the briquettes that were dried for 4 h with heating had larger weight gain due to water absorption with ineffective compression sealing of low moisture content. This result indicates that the extremely dry fibrous material in the fuel becomes hygroscopic. The best result combined the advantages of air removal (or densification) and optimizing moisture content: a vacuum pre-treatment without heating, followed by molding at 150 °C. Concerning the addition of recycled plastics, the best result in the vacuum pre-treatment scenario was from the addition of PP, compared to PET and HDPE. Other pre-treatments showed mixed results for the plastics addition. 3.4. Proximate analysis test Burning tests were carried out to investigate the moisture content, ash residue, volatile portion and fixed carbon in the briquettes. The results of proximate analysis for all briquettes are compared with literature data of coals and biomass materials in Table 1. In general, the briquettes containing HDPE flakes presented slightly higher moisture content compared to those with PET and PP when there was no extra drying step; It was surmised that HDPE materials flowed more easily due to low melting

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temperature encapsulating the other components and holding moisture in. The briquettes containing PET flakes showed higher moisture content compared to those with PP and HDPE when materials were dried with vacuum and heating for long time. It would be expected that PET would contain more residual moisture because it is more hydrophilic. The water affinity of the different added plastics is reflected in their different water contact angles (HDPE = 92.4° > PP = 90.0° > PET = 79.2°) (Fávaro et al., 2007). In summary, briquettes with PET appeared higher in moisture content even after drying due to trapped water, while HDPE moisture content decreased gradually from drying materials prior to compression molding. The briquettes containing PP did not change significantly.

After burning for ash quantification, a similar weight reduction was observed for all briquettes, around 93–96 weight%, and volume decreased significantly as well, producing a white colored ash. This low ash residue results in an advantage for transportation and landfilling after incineration. The proximate analysis results of tested briquettes and reference data for biomass and coal are presented in Fig. 8 as stacked columns. The DSW briquettes that were produced using only DSW particles without addition of plastics pellets have a significantly different partition between volatile matter and fixed carbon content as compared to coal. The investigated briquette composition was closer to biomass, with high volatile matter and lower fixed carbon content. Biomass resources also have much higher

Table 1 Proximate analysis result of investigated briquettes and reference data. Samples 125 °C (un-dried)

150 °C (un-dried)

150 °C (vacuumed)

150 °C (S. dried)

150 °C (L. dried)

DSW (no Plastics) Biomass

Coal

PP PET HDPE PP PET HDPE PP PET HDPE PP PET HDPE PP PET HDPE Pistachio Shell Wheat Grain Wood Pellets Anthracite Bituminous Lignite

M (%)

VM (%)

Ash (%)

FC (%)

1.73 2.13 2.46 1.36 1.70 2.23 1.40 2.35 2.42 1.41 1.95 2.01 1.03 1.86 1.63 5.6 8.75 10.3 7.96 0.4 1.4 7.6

87.02 82.50 82.29 84.94 83.16 82.91 83.66 81.31 82.00 83.49 82.72 84.29 84.46 82.47 84.39 75.0 82.5 80.0 82.0 6.2 38.5 36.8

5.40 4.99 5.80 5.00 5.09 5.69 6.19 5.69 5.99 6.28 5.89 5.58 4.28 5.39 5.38 9.5 1.3 2.8 1.3 6.1 7.7 7.9

5.85 10.38 9.45 8.71 10.05 9.17 8.75 10.65 9.59 8.82 9.44 8.12 10.23 10.28 8.60 9.9 7.45 6.9 8.74 87.4 52.4 47.7

References –







– – García et al. (2013)

Donahue and Rais (2009)

Note: DSW = domestic solid waste (not including plastics), M = moisture content, VM = volatile matter, Ash = ash residue, FC = fixed carbon

Fig. 8. Proximate analysis comparison of literature results for biomass (García et al., 2013), coal (Donahue and Rais, 2009) and briquettes from this study.

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moisture content; while the briquettes tested here have moisture content closer to coal. This factor of low moisture content is an advantage, combined with the better mechanical stability and heating value due to the high heat capacity of water that significantly depresses combustion energy values. According to the co-combustion research on lignite and biomass, the ignition temperature of a resource is lower with

increasing volatile matter content, and more char formation takes place with increased fixed carbon content (Varol et al., 2010). Hence, although the combustion process is complex and additional factors should be considered, it can be expected that the solidshaped fuels from paper-based solid wastes and plastics have the advantages of easy ignition and less char forming with some CO2 reduction benefit (Varol et al., 2010) compared with commercial coals. 3.5. Mechanical test

Fig. 9. Compressive strength values for briquettes produced at different process conditions.

The mechanical properties of the briquettes were investigated using a compressive strength test. Theoretically, denser products will lead to harder samples with good mechanical properties for transportation, manipulation and storage of product (Kers et al., 2010b). In this test, the compressive strength of briquettes was improved dramatically with increased press temperature (150 °C vs. 125 °C), and removing air bubbles in the styrene foam without heating, while strength decreased with heated drying pre-treatment step prior to compression as shown in Fig. 9. This result is consistent with the theory that density dictates compressive strength. The mechanical property improvement is likely due to the plastic melting and flowing at the higher press temperature, which allows the material to act as a binder with other components of the DSW. Plastics with lower melting temperature (HDPE Tm = 130 °C) exhibited greater improvement of compressive strength than the others that have higher melting temperature (PP Tm = 130–170 °C and PET Tm = 250–260 °C). Fig. 9 presents the comparison of mechanical properties for all the briquettes. In general, the compressive strength results show similar behavior to the density variation except in a few cases. As

Fig. 10. Combustion test results with HRR by burning time for three different type briquettes (PP, PET and HDPE) at same processing condition of 150 °C press temperature with vacuum treatment.

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in the density variation, the briquettes had higher compressive strength when materials were prepared with moderate moisture and removal of air bubbles at high processing temperature (150 °C), while a heated drying step was detrimental to the strength. Plastic type also influenced the final strength differently. Without heated drying (vacuum treatment only), PP and HDPE are good candidates for improving strength and stability, while with heated drying pre-treatment the addition of HDPE is negative due to dramatically decreased product strength. Therefore, it is clear that moisture content has to be controlled carefully to maintain optimized moisture level for high strength when amount of HDPE flakes are added solid fuels (reported between 10 N/mm and 45 N/mm for municipal waste briquettes (Krizˇan et al., 2011), and between 80 N/mm and 115 N/mm for biomass briquettes (Kakitis et al., 2011)).

Table 4 Specific energy of materials and fuel briquettes. Material

MJ/kg

References

PE PP PET PS DSW (based on paper) DSW + PS + PP

46.3 46.2 25.6 41.4 18.6 (0.7  18.6) + (0.05  41.4) + (0.25  46.2) = 26.6 (0.7  18.6) + (0.05  41.4) + (0.25  25.6) = 21.5

Subramanian Subramanian Subramanian Subramanian Subramanian

DSW + PS + PET

(2000) (2000) (2000) (2000) (2000)

calculated using reference energy values for the pure components, as shown in Tables 3 and 4. The overall trend agrees with calculated values and the heterogeneous DSW provided likely does not have the same value as our assumed ideal biomass material.

3.6. Combustion test The heat release rate (HRR) of a fuel is one of the most important characteristics to understand the combustion process, fire behavior and fire propagation rates (Schemel et al., 2008). Fig. 10 presents the combustion test results with HRR vs. time for the burning samples. The HRR increased and total combustion time decreased for the higher processing temperature in all briquettes. As expected, samples which contain HDPE or PP showed better combustion behavior with higher HRR value and longer burning time than those containing PET. Another figure of merit, effective heat of combustion (EHC), which represents the total heat released throughout burning of the briquettes divided by their mass, yields comparison of the specific energy for a variety of fuel resources including the briquettes tested here (Tables 2 and 3). The briquettes mixed with PP and HDPE present similar energy value to sub-bituminous coal resource (20.5–27.2 MJ/kg), while the other type of briquette containing PET flakes had lower energy value close to that of lignite coal (11.6–17.4 MJ/kg). Experimental specific energy results for all briquette types were lower than expected by 2.3–4.7 MJ/kg compared with values

Table 2 Mean effective heat of combustion. Mean effective heat of combustion (MJ/kg for burning)

PP PET HDPE

125 °C (undried)

150 °C (undried)

150 °C (vacuumed)

150 °C (S. dried)

150 °C (L. dried)

23 ± 0.6 14 ± 1.2 21 ± 1.5

22 ± 1.9 13 ± 0.5 21 ± 2.1

22 ± 1.3 14 ± 0.7 23 ± 1.5

24 ± 0.8 15 ± 0.9 22 ± 1.0

21 ± 2.3 14 ± 0.9 19 ± 2.8

Table 3 Specific energy of common materials and fuel resources. Material

MJ/kg

References

Polyethylene (PE) Polypropylene (PP) Polystyrene (PS) Polyethylene terephthalate (PET) Newspaper Wood Yard wastes Food wastes Anthracite coal Bituminous coal Sub-bituminous coal Lignite coal Fuel oil Natural gas

46.3 46.2 41.4 25.6 18.6 14.0 7.0 6.0 29.1–30.2 27.4 20.5–27.2 11.6–17.4 48.6 46.5–51.2

Subramanian (2000) Subramanian (2000) Subramanian (2000) Subramanian (2000) Subramanian (2000) Subramanian (2000) Subramanian (2000) Subramanian (2000) Milici et al. (2013) Milici et al. (2013) Milici et al. (2013) (US EPA, 1995) Subramanian (2000) Boundy et al. (2011)

4. Conclusions In this study, solid-shaped fuels from well sorted DSW and plastics were produced using shred-and-press technology as a recycling method for energy conversion applications and their properties were investigated. To characterize the briquettes, density variation, water uptake, proximate analysis, mechanical testing and mass loss calorimetry were carried out according to ASTM procedures and the results were compared with reference data of commercial coals. The following best practices can be obtained from the study:  Higher compacting temperature, vacuum removal of air in added Styrofoam and optimized moisture content resulted in highest briquette density and compressive strength.  Higher compacting pressure resulted in denser and harder products up to a point (around 49,000 psi on sample), beyond which improvement was negligible.  Added HDPE improved the density and hardness of the briquettes due to its lower melting temperature.  Moisture uptake was minimized when PP was added and for encapsulated, ‘‘pillow-shaped’’ briquettes.  The DSW fuels had similar composition to biomass with a higher volatile matter fraction and low fixed carbon and moisture contents compared to coals.  Briquettes with added PP and HDPE exhibited the highest specific energy and burning time in the combustion testing. This is an added benefit since PET recycle streams have high value in other applications, whereas PP and HDPE do not.  The mean EHC values for briquettes mixed with PP and HDPE were close to that of sub-bituminous coal, while briquettes with added PET had EHC similar to lignite coal. Higher briquette density has the advantages of as a greater ratio of energy efficiency to volume as well as decreased transportation and storage costs. The briquettes have the advantages of easy ignition at lower temperature and fewer residues after burning as well as reduced CO2 emission compared with commercial coals. PP and HDPE are considered as good candidates for addition to MSWbased fuels where high-energy value is required. Type and fraction of added plastic can be customized based on source pricing and available alternative usage. Acknowledgments This research was funded in part by a collaborative project with WERC-2 Inc. We thank Bill Rhatigan and Jerry O’Brien for their support and helpful discussions.

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Please cite this article in press as: Gug, J., et al. Processing and properties of a solid energy fuel from municipal solid waste (MSW) and recycled plastics. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.09.031

Processing and properties of a solid energy fuel from municipal solid waste (MSW) and recycled plastics.

Diversion of waste streams such as plastics, woods, papers and other solid trash from municipal landfills and extraction of useful materials from land...
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