Isothermal combustion kinetics of synthetic refuse plastic fuel (RPF) blends by thermogravimetric analysis

Applied Thermal Engineering 104 (2016) 16–23

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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng

Research Paper

Isothermal combustion kinetics of synthetic refuse plastic fuel (RPF) blends by thermogravimetric analysis Yousuf Jamal a,b, Minho Kim a, Hung-suck Park a,⇑ a

Department of Civil and Environmental Engineering, University of Ulsan, Ulsan 680817, South Korea Institute of Environmental Sciences and Engineering, School of Civil and Environmental Engineering, National University of Sciences and Technology, H-12, Islamabad 44000, Pakistan b

h i g h l i g h t s
Combustion behavior of wastewater sludge, plastic and synthetic blends are investigated.
Experiments are conducted in a large-scale thermogravimetric analyzer.
Maximum weight loss percent, rate and time are reported.
Using Isothermal model kinetic parameters of co-combustion are determined.

a r t i c l e

i n f o

Article history: Received 24 September 2015 Revised 9 April 2016 Accepted 30 April 2016 Available online 30 April 2016 Keywords: Isothermal kinetics Wastewater sludge Plastic Co-combustion Thermogravimetric analysis Activation energy

a b s t r a c t Thermal behavior of wastewater sludge, plastic and synthetic blends during combustion process was studied to identify potential of refused derived fuel application. Experiments were conducted in a large-scale thermogravimetric analyzer at temperatures ranging from 400 °C to 700 °C in the presence of air flow at 8 L/min. Isothermal modeling was performed to find system kinetic parameters including combustion reaction rate constant (k), order of reaction (n), Arrhenius pre-exponential factor (A) and activation energy (E). Results revealed that with increase in temperature from 400 °C to 700 °C, maximum rate of combustion was observed to increase from 1.6%/min to 7.12%/min and 2.09%/min to 21.9%/min for wastewater sludge and plastics, respectively. The corresponding time for maximum weight loss rate was observed to decrease from 15 min to 3.1 min for wastewater sludge and from 47 min to 3.8 min for plastic. Thermogravimetric profile and activation energy indicated that refuse plastic fuel blends with higher percentage of plastic required higher activation energy than wastewater sludge from 12.43 kJ/mol to 58.60 kJ/mol. Experimental results show the potential to explain and predict combustion behavior and activation energy of combustion of wastewater sludge and plastic blends in practical RPF applications. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction Rapid industrialization and urbanization, in the last decade, have put tremendous pressure on the society to search for comprehendible renewable energy resources [1,2]. Countries around the world are now paying serious attention for their domestic energy security. At the same time, many countries have also made it obligatory to reduce the greenhouse gas (GHG) emissions [3]. In Korea among the wastes generated in industrial complexes, the percentage of sludge resulting from wastewater treatment is very high (42.1%) [4], most of which had been disposed off into ⇑ Corresponding author. E-mail address: [email protected] (H.-s. Park). http://dx.doi.org/10.1016/j.applthermaleng.2016.04.151 1359-4311/Ó 2016 Elsevier Ltd. All rights reserved.

the sea in many parts of the world. However, sea disposal of organic wastes including wastewater sludge is now banned [5,6], which warrants a new method of its disposal. On the other hand, a large amount of waste plastics, identified as hazardous wastes, are also generated daily. Reduction in disposal of plastics can be achieved by recovering the scrap plastic monomer and reusing it [7]. Reduction in volume of contaminated low quality scrap plastics can also be achieved by incarnation which also helps to recover plastic hidden energy. This also can reduce the problem of effective land use. The disposal of these wastes by blending with other hazardous wastes could cause severe pollution for the environment [8], and thus necessitate their safe disposal. Given the situation, combustion of wastewater sludge under appropriate control may be a

Y. Jamal et al. / Applied Thermal Engineering 104 (2016) 16–23

17

Nomenclature °C E J k kg kJ K max

centigrade degree activation energy (kJ/mol) joule combustion reaction rate constant (1/min) kilogram kilojoule kelvin temperature (K = 273 + °C) maximum

secure method of disposal, especially when it is co-utilized with waste plastics in existing facilities that are already equipped with appropriate devices for emission control. In this way, use of biomass and other locally generated plastic wastes can also be promoted as fuel [9]. The refuse derived fuel (RDF) technology of wastes is the process that consider blending of combustible wastes, (such as waste plastic, waste papers, waste wood, and wastewater sludge after moisture removal), and manufacture a RDF solid block by going through several processes such as pulverization, fragmentation, sorting and dehydration [10]. Manufactured solid fuel, RDF, with a heating value greater than 6000 kcal/kg is known as refuse plastic fuel (RPF) [11,12]. Through waste-to-RPF conversion process, the wastes are utilized as a source of energy [13]. At the same time, several other problems are also addressed up, such as lack of landfill space to manage huge amount of waste and transportation of wastes for their disposal. Consequently, it contributes to the reduction of GHG emissions, which is important for the ‘low-carbon green growth’ vision that targets co-benefit of economic and environmental effects. Therefore, higher energy efficiency can be expected by producing RDF/RPF and by utilizing them as fuel appropriately. Combustion of wastes in oxygen-enriched environment has been proven to be a technique that provides both operative and environmental benefits [14,15]. Previous works have shown the co-combustion of wastewater sludge with coal is viable from the energy, economic and environmental point of view, especially when it is carried out in existing infrastructures [16–19]. Furthermore, it has been reported that up to 50% of dried sludge can be handled together with the base fuels without any technical problem from the view-point of gaseous emissions [20–22]. Thermogravimetric Analyzer (TGA) has been widely employed to characterize the thermal degradation processes of coal and/or other biomass fuels [23–25], and more recently it has been used to describe the degradation profiles of sludge’s [26–28]. This information is of great importance to enhance the knowledge of RDF/ RPF blending and to estimate the activation energy required for combustion, which can subsequently be used to establish the optimum combustion operational conditions. Due to the high water content and low calorific value of wastewater sludge [29], it is necessary to improve the calorific value of wastewater sludge by dewatering and mixing with waste plastic, to sustain the combustion and emission stability. Liu et al. [30] has shown high oxygen concentration in TGA gas increases decomposition of municipal solid waste and reduces its activation energy requirement. Recently Zhang et al. [31] has done a comparison on isothermal and non-isothermal kinetics of municipal sewage sludge and concluded that less activation energy is required in isothermal arrangement. A four step incineration (drying, pyrolysis, gasification, and combustion) of sludge plastic blends in a two chamber inclinator have been reported earlier but without determining activation energy for the blends [32].

mol M n R RDF RPF

mole blending ratio of plastic in sludge by weight order of reaction universal gas constant (8.3145 J/mol K) refuse derived fuel refuse plastic fuel

Due to the increased use of co-combustion of waste in coal power plant, a great deal of attention has been given to the kinetic analysis of co-combustion of biomass or wastewater sludge in conjunction with coal [33,34]. However, co-combustion of wastewater sludge with plastic waste has not been studied in detail due to lack of application cases. However, to utilize the renewable energy of dried sewage sludge to meet the renewable portfolio standard (RPS) economically, low calorific energy of dried sludge fuel (2300–3200 kcal/kg) is experimentally tried to improve in this study by mixing it with waste plastics. Therefore the purpose of this work is to investigate overall isothermal combustion kinetics of wastewater sludge, plastic and synthetic blends to find the potential of producing RFF with blending waste sludge and plastics. This experimental work report the system kinetic parameters including combustion reaction rate constant (k), order of reaction (n), Arrhenius pre-exponential factor (A) and the activation energy (E). Also highlight how the addition of plastic in wastewater sludge affects the overall combustion characteristics. 2. Materials and methods 2.1. Preparation of blend samples In this study, wastewater sludge was collected from Ulsan wastewater treatment plant, South Korea. To make a synthetic blend of wastewater sludge with plastic, commercially available polystyrene plastic pallet was used. Dehydrated cake sample of waste sludge (dried at 100 ± 5 °C) with 10% moisture content and plastic were first reduced in size and then gently mixed in a beaker with a mixing ratio, M = [weight of plastic/weight of (plastic + sludge)], of M = 0, 0.3, 0.6, and 1.0 by weight of plastic and sludge respectively. Homogenized blended samples of 7 ⁄ 2 (diameter ⁄ height, cm) size were then prepared by using a mounting press (MP145, Dong Yang Corp., Seoul, Korea) (Fig. 1). The conditions maintained in the mounting press to prepare the samples were: temperature: 80 °C, pressure: 150 kgf/ cm2, press time: 10 min. 2.2. Experimental set-up Large scale thermogravimetric analyzer (LS-TGA) having simulation functions of incineration of industrial wastes was used. Weight of non-homogeneous waste material sample used in LSTGA was 10 gram (g). A pictorial view of this analyzer and the detailed specification of the LS-TGA are given in (Fig. 2, Table 1). The analyzer was composed of a high temperature electric furnace with silicon carbide heating element, real-time weight measurement balance, exhaust gas collection unit, and the temperature monitoring apparatus. The measurement on weight loss rate for each blend of sample was based on the applied heating temperature.

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Y. Jamal et al. / Applied Thermal Engineering 104 (2016) 16–23

in LS-TGA as shown in Fig. 2 and was automatically adjusted during operations. 2.4. Kinetic modeling Considering the complexity of the combustion process with the non-homogeneous mixture at different mixing ratios and temperatures, an isothermal combustion was considered, rate of combustion reaction was described as shown in Eq. (1) [18].

dX ¼ kðTÞf ðXÞ dt

ð1Þ

In Eq. (1) k(T) is temperature dependent combustion reaction rate constant and f(X) = (1  X)n is the reaction model. n is the order of reaction and X is the conversion ratio of the sample on combustion Eq. (1) assumes that combustion reaction occurring in the isothermal state is affected by the mass and heat transfer is dominated by the materials burning. Thus conversion rate is proportional to mass, and rate constant k(T) is established from the equation. Where conversion ratio (X) in the process is defined as

X¼

Wi  Wt Wi  Wf

ð2Þ

Wi is the initial, Wt is at any time t and Wf is the final or equilibrium sample weight.

dX ¼ kð1  XÞn dt

ð3Þ

For Isothermal kinetic modeling two cases were considered 2.4.1. Case 1: Assuming n is fixed (n = 1) Integrating Eq. (3) from X to Xo and time t to to



ln

 1  Xo ¼ kðt  t o Þ 1X

ð4Þ

where Xo is the conversion at time t = 0 and X is the conversion at time t = t. Plotting a graph between the values of left side of Eq. (4) versus time (t  t0) gives a straight line with slope value equal to k. 2.4.2. Case 2: Assuming n is not fixed (n – 1) Similarly integrating Eq. (3) from X to Xo and time t to to, when n – 1, gives Eq. (5) as following

ð1  XÞ1n  ð1  X o Þ1n ¼ kðn  1Þðt  t o Þ

ð5Þ

Plotting left side of Eq. (5) versus time (t  t0) gives a straight line with slope value equal to k(n  1). Where k is the combustion reaction rate constant and n is the order of reaction. Fig. 1. View of prepared RPF samples (a) M = 0, (b) M = 0.3, (c) M = 0.6, and (d) M = 1.0.

2.3. Experimental method Thermogravimetric analyzer was heated at a heating rate of 10 °C/min up to the temperatures 400 °C, 500 °C, 600 °C and 700 °C. The targeted temperatures in the analyzer were then maintained for a specific time range. Experiments were carried out under these isothermal conditions in the presence of air flow rate of 8 L/min, and at the known temperature. Weight loss from each sample was then noted at the selected temperature over the time range. This time range was kept constant for all the samples. Combustion temperature was controlled by thermostat operation

2.4.3. Activation energy determination Arrhenius temperature dependence on combustion reaction rate constant was used to determine activation energy (E) required for the combustion of wastewater sludge, plastic and their blends as shown in Eq. (6).

  E k ¼ A  exp  RT

ð6Þ

where, in Eq. (6), k is the rate constant (1/min), A is the Arrhenius pre-exponential factor; E is the activation energy (J/mol) and R is the universal gas constant (J/mol K) Taking logarithmic response of Eq. (6) gives



Ln ðkÞ ¼ Ln ðAÞ 

E RT



ð7Þ

Y. Jamal et al. / Applied Thermal Engineering 104 (2016) 16–23

19

(a)

(b) Fig. 2. Experimental set-up (a) Large scale thermogravimetric analyzer (LS-TGA) with gas analyzer. (b) Details of LS-TGA.

Table 1 Specification of large scale thermogravimetric analyzer. Item

Condition

Max. power Acc. voltage Temperature Weighing scale Sample weight

5 kW 220 V 1500 °C (max.) 0.1 g (on-line measurement system) 100 gram (max.)

Value of combustion reaction rate (k), for all order of reaction (n) at mixing ratios (M), was selected based on the highest value of R2 (linear regression) from the above two cases. Plot of Ln (k) versus inverse temperature (1/T) from on Eq. (7) was used to calculate the activation energy (E) from the slope value

(E/R) and Arrhenius pre-exponential factor (A) from Ln (A) for the combustion behavior.

3. Results and discussion 3.1. Combustion behavior Fig. 3a–d illustrates the time series TGA profiles of sample weight loss (%) for the temperatures studied from 400 °C to 700 °C at different plastic mixing ratios, M = 0, 0.3, 0.6, and 1.0 respectively. It can be seen that higher percentage of plastic in RPF increases the weight loss in TGA studies at all temperatures. Thus more reduction in waste volume is possible by mixing higher content of plastics in the RPF blend. Real RPF blend are very

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Y. Jamal et al. / Applied Thermal Engineering 104 (2016) 16–23

T: 400 °C

T: 500 °C

1.2

T: 600 °C

M=0 M = 0.3 M = 0.6 M = 1.0

Sample weight (%)

1.0

0.8

0.6

0.4

0.2

0.0 0

10

20

30

40

50

60

Time (min) 1.2

O‹P T: 700 °C

M=0 M = 0.3 M = 0.6 M = 1.0

1.0

Sample weight (%)

complicated and the combustion reaction of individual plastic constituent in the blend are susceptible to follow radical chain reactions. So, analysis of all such type of chain reactions are difficult to understand and follow, to explain the combustion reaction. As in this study, we use commercial polystyrene to simplify thermal decomposition of RPF blends with sludge, thermal decomposition of the RPF blend is interpreted as the overall reaction. Fig. 4 illustrates the trend of maximum weight loss rate (%/minute) and time (minutes) corresponding to maximum loss rate at 500 °C analysis. In detail data obtained for all mixing ratios, M = 0, 0.3, 0.6, and 1.0 respectively at all temperatures 400–700 °C along with total sample weight loss (%) at the corresponding time in different blend samples are presented in Table 2. It is clear from the table that with increase in temperature from 400 °C to 700 °C, maximum weight loss rate (%/min) increased in all the samples under same mixing ratios. Similarly an increase in maximum weight loss rate was also observed at increasing mixing ratios combusted at the same temperature. Moreover, for sample blend (M = 1.0), only plastic; a maximum of 100% weight loss was noted with decrease in time (minutes) corresponding to an increase in maximum weight loss rate at all the temperatures studied 400–700 °C. A maximum weight loss of 89% was noted in the plastic blend sample of M = 0.6 at 500 °C. This proves that co-combustion of wastewater sludge with plastic can remarkably reduce the mass of the wastewater sludge which showed only 44% weight loss for sample M = 0 at the same temperature. This may be because more energy is required for degrading the mix blend of wastewater sludge and plastic than wastewater sludge (M = 0) only. However, this trend of overall high weight loss from the sample was not noticeable at higher temperatures of 600 °C and 700 °C. Xieo et al. [35] has reported similar TGA findings while studying co-combustion of sewage sludge with straw and coal blends emphasizing a decrease in the total weight loss of sample blend at higher temperatures due to the formation of ash near the wastewater sludge. This ash formation at higher temperatures hinders the further contact of oxygen with the sludge waste blend [36]. This explains the phenomena of less combustion and reduction in the total mass loss of the wastewater sludge and plastic blend at very high temperatures. Menad et al. [37] has also reported the maximum TGA degradation of polystyrene plastics around 500 °C which proves the finding of this work. Fig. 3, also gives an indication (graphs are with similar shape) that the order of sample combustion reactions of different mixing ratios at different temperatures would not differ much. Considering the minimal effect in weight loss at 700 °C in comparison to 600 °C (Table 2) the values of rate constant, k, were not calculated at 700 °C for activation energy (E) and Arrhenius pre-exponential factor (A) calculations.

0.8

0.6

0.4

0.2

0.0 0

10

20

30

40

50

60

Time (min)

Fig. 3. Large scale TGA profiles of samples weight loss at various mixing ratios and at temperatures (a) T: 400 °C, (b) T: 500 °C, (c) T: 600 °C, and (d) T: 700 °C.

Fig. 4. Temporal RPF weight loss rate (%/minute) at 500 °C.

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Y. Jamal et al. / Applied Thermal Engineering 104 (2016) 16–23 Table 2 Maximum weight loss rate, corresponding time and total weight loss at different temperatures. Mixing ratio (M)

400 °C

500 °C

600 °C

700 °C

WMr

tp

WT

WMr

tp

WT

WMr

tp

WT

WMr

tp

WT

0 0.3 0.6 1.0

1.6 1.67 2.18 2.09

15 26 34 47

39 59 86 100

3.17 3.82 5.07 5.87

6 14 15 17

44 68 89 100

4.67 7.4 8.82 11.62

6 5 6 8

44 58 78 100

7.12 13.02 14.63 21.9

3.1 3.3 3.9 3.8

47 59 79 100

Note: WMr: Maximum weight loss rate (%/min); tp: time corresponding to maximum weight loss rate (min); and WT: total weight loss (%).

3.2. Kinetic parameters Based on experimental results the combustion reaction rate constants were investigated considering both the order of reaction (n), equal and not equal to one. Fig. 5 shows the trend plot for calculating k value when the order of reaction is n = 1 and n – 1. Table 3 shows in detail values of rate constant, k, calculated for plastic blending ratios M = 0, 0.3, 0.6, and 1.0 at temperature range 400–600 °C. Variation in reaction order (n) was noted for the isothermal combustion temperatures. It is worth mentioning that only for wastewater sludge order of reaction (n) was near to one whereas for pure plastic order of reaction was near to zero for the case where reaction order was considered not equal to one. For plastics order of reaction near zero means that rate is only dependent on temperature and not on concentration. R2 values above 0.90 are fair in each case. It reflects how fair data fit into regression line. Similar trends of variation in order of reaction have been reported earlier while studying combustion of different type of biowaste sludge’s, but at different combustion temperatures of 300–800 K [28]. It is also to report that combustion reaction rate constant, k, increases with temperature for all the sample mixing ratios. Table 4 shows the maximum values of R2, between the rate constant (k) and temperatures (1/T, K1) for plastic blend ratios (M).

Table 3 Estimated values of rate constants at different reaction order for wastewater sludge, plastic and for their blends co-combustion. n=1

k

R2

n–1

k

R2

400 500 600

1

0.078 0.086 0.120

0.984 0.987 0.982

1.07 1.07 1.05

0.089 0.099 0.132

0.976 0.985 0.984

400 500 600

1

0.103 0.115 0.159

0.917 0.985 0.981

0.54 1.06 1.08

0.046 0.132 0.190

0.986 0.987 0.984

400 500 600

1

0.089 0.098 0.114

0.854 0.952 0.944

0.30 1.25 1.30

0.033 0.183 0.255

0.968 0.977 0.978

400 500 600

1

0.052 0.095 0.399

0.709 0.657 0.893

0 0.05 0.70

0.021 0.054 0.240

0.964 0.849 0.899

Mixing ratio (M)

Temp. (°C)

0

0.3

0.6

1.0

Note: n: order of reaction; k: reaction rate constant; and R2: co-relation co-efficient.

This lead to the investigation of activation energy (E) and Arrhenius pre-exponential factor (A) as explained in the methods section. Fig. 6 shows the method for determining activation energy (E) and Arrhenius pre-exponential factor (A) by plotting Ln k values against temperature (1/T, K1). Arrhenius activation energy for combustion was found lower for dehydrated wastewater sludge cake than for plastic combustion (Table 4). High R2 values show the linearity of data fit. Findings are in accordance with Cepeliogullar et al. [38] who have earlier worked on kinetics of various biomass and plastic wastes. The blend with higher percentage of plastic similarly was noted to require higher activation energy than wastewater sludge from 12.43 to 58.60 kJ/mol in this study. Liu et al. [39] has reported a similar increasing trend in activation energy with increase of waste plastic ratio in the co-combustion blend of waste plastic and anthracite coal. However contrary results have been reported earlier for co-combustion of waste plastic and sludge [40] having high 30% moisture content in the waste water sludge sample. Park et al. [41] working on refuse plastic fuel under isothermal conditions has reported an activation energy of 39.44 kcal/mol (165 kJ/mol) for plastic decomposition. Higher activation energy values for plastic ranging between 100 and 225 kJ/mol are also reported earlier [42], showing more activation energy requirement for thermal decomposition of plastics that mainly helps in the volume reduction of blended wastewater sludge samples.

3.3. Practical implications

Fig. 5. Method for determining k values when the order of reaction is (a) n = 1 (b) n – 1.

Valuable energy resources can be recovered from wastewater sludges and plastics. Petroleum based plastics are difficult to disintegrate in the environment. Moreover, to avoid sea disposal and land fill dumping of wastewater sludges, a conversion into a high quality solid refused plastic fuel (RPF) is advisable. To find the practical implication, wastewater sludges from nine different

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Y. Jamal et al. / Applied Thermal Engineering 104 (2016) 16–23

Table 4 Estimated kinetic parameters (E, A) for wastewater sludge, plastic and for their blends co-combustion from Isothermal modeling. E (kJ/mol)

R2

12.43

0.832

26.58

35.17

0.961

0.033 0.183 0.255

359.35

51.11

0.914

0.021 0.054 0.240

654.98

58.60

0.959

Mixing ratio (M)

Temp. (°C)

Temp. (°K)

(1/°K)

Reaction rate (k)

0

400 500 600

673 773 873

0.00149 0.00129 0.00115

0.078 0.086 0.132

400 500 600

673 773 873

0.00149 0.00129 0.00115

0.046 0.132 0.190

400 500 600

673 773 873

0.00149 0.00129 0.00115

400 500 600

673 773 873

0.00149 0.00129 0.00115

0.3

0.6

1.0

A 0.679

Note: A: Frequency factor; E: activated energy; and R2: co-relation co-efficient.

ment was determined for degradation of dehydrated wastewater sludge cake than plastic only during combustion ranging from 12.43 kJ/mole to 58.60 kJ/mole respectively. Producing RPF blend have the potential to optimize valuable energy resources in wastewater sludge’s and waste plastics with co-benefits in practical application. Acknowledgements This research was supported by a Grant (code 08 RTI B-03) from Regional Technology Innovation Program funded by Ministry of Land Transport & Maritime Affairs of Korean Government. Fig. 6. Arrhenius plot for activation energy of RPF with different plastic ratios.

locations of Ulsan metropolitan city, Korea, were collected with an average gross heating value of 14,900 kJ/kg and non-recyclable plastic was blended into wastewater sludges in ratios of 0.3 and 0.6 respectively. Plastics were collected from Ulsan municipal solid wastes with gross heating value of 32,042 kJ/kg. Making RPF fuel blend thus resulted in gross heating value of 20,043 and 23,397 kJ/kg, respectively. Energy of 23,397 kJ/kg was found higher than the hidden energy value, 22,700 kJ/kg, of coal on wet basis. Thus by creating a RPF, not only low quality resources can be upgraded for effective energy utilization but it also reduces the volume of municipal sludge’s and solid waste plastics for land fill sites. Though this research was done using commercial polystyrene plastic, further practical study and application to waste plastic can help to save coal reserves for better industrial uses and lead to low carbon footprint for future green societies. Though energy required for drying sludge is not considered in this work for making RPF, the drying could be integrated in waste heat utilization that further optimize heating value of RPF using energy network in ecoindustrial park [43,44].

4. Conclusion For isothermal combustion at different temperatures from 400 °C to 700 °C, maximum weight loss rate increased in all the samples of wastewater sludge, plastic and their blends under same mixing ratios. Similarly an increase in maximum weight loss rate was also observed at increasing mixing ratios combusted at the same temperature. A maximum weight loss of 89% was noted at 500 °C for the blend sample (M = 0.6) which proves that cocombustion of wastewater sludge and plastic can remarkably reduce the total mass of the waste. Less activation energy require-

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