Thermal Characterization, Kinetic Analysis and Co-Combustion of Sewage Sludge Coupled with High Ash Ekibastuz Coal

: Efﬁcient utilization of natural resources and possible valorization of solid waste materials such as sewage sludge into secondary materials via thermal conversion and simultaneously recovering energy is vital for sustainable development. The continuous increase in metropolises leads to an enormous production of wet sewage sludge, which creates major environmental and technical issues. In this paper, the samples of sewage sludge from Astana’s waste water treatment plant are analyzed for their thermochemical properties, followed by thermogravimetric and kinetic analysis using the Flynn–Wall–Ozawa and Kissinger–Akahira–Sunose methods. Overall, the caloriﬁc value of sewage sludge sample was 18.87 MJ/kg and was comparable to that of the bituminous coal samples. The activation energy varied from 140 to 410 kJ/mol with changing conversion from 0.1 to 0.7. Further, mono-combustion and co-combustion experiments of the sewage sludge with high ash bituminous coal were conducted using the laboratory scale bubbling ﬂuidized bed rig, respectively. The difference in NO x emissions between mono-combustion of sewage sludge and co-combustion with coal were at around 150 ppm, while this value for SO 2 was similar in average, but ﬂuctuates between 150 and 350 ppm. Overall, the ﬁndings of this study will be useful in developing a co-combustion technology for a sustainable disposal of municipal sewage sludge.


Introduction
Renewable energy is currently undergoing active development through various approaches, including georesources' development [1], electrification of transport, advanced energy storage technologies [2], and the development of solar panels [3].Alternative source to recover energy is the municipal sewage sludge (SS), via various thermochemical methods [4].SS is the residual solid waste produced during the treatment of wastewater [4].SS typically consists of organic and inorganic compounds, along with toxic and hazardous substances [5].SS is derived from wastewater treatment plants (WWTP) and thus has a high moisture content, which leads to the accumulation of large volumes and subsequently high operational costs during treatment [6].It is reported that the handling and disposal of SS roughly takes 50% of the operational cost of secondary wastewater treatment plants in Europe [7].The growing population and the associated rate of urbanization have led to an increase in sewage sludge volumes.For example, considering local demography, the population of Kazakhstan is expected to reach around 23.5 million by 2050 [8], and thus high volumes of SS are produced.The high amount of SS can cause serious environmental method, the projected thermal decomposition may not align with the real decomposition in SS due to complexity of structure [29].Therefore, the disagreement can be adjusted by the modeling of the projected results at three different heating rates and with two isoconversional methods, KAS and FWO.
Furthermore, several experiments are available in the literature on the co-combustion of SS with different types of coals in various reactors as summarized in Table 1.The results of the literature review indicate that the experiments were performed in similar experimental conditions, and the temperature used in the experiment in this work is within the range presented in the literature.In addition, the literature studies demonstrate that the proportions of SS and coal in the feed affect the emissions, and this information was applied in the analysis of the experimental results.
Table 1.Literature review on co-combustion of SS samples with different coal samples.

Type of Reactor
Blend Solid Fuel Temperature Key Observations TGA [30] 10% SS/coal, 50% SS/coal 800 • C 50 wt% blends had two regions of reactivity.At T < 350 • C reactivity were similar to sludge, while for T > 350 • C had reactivity close to coal Bubbling fluidized bed [31] SS with High, medium, and low volatile bituminous coal 850 • C NO x increased during SS feeding, while only slight change was noted N 2 O emissions Fluidized bed combustor [25] 100% SS 720-930 • C NO x and SO 2 increased with an increase in bed temperature; N 2 O slightly reduced.
Fluidized bed reactor [32] 100% SS, 30% SS/62% coal/8% CaCO 3 800 • C Decline of SO 2 emissions during the co-combustion of SS and coal with CaCO 3 Fluidized bed reactor [33] 100% SS 700 To the best of our knowledge, there is a lack of data in the literature on SS and its co-combustion behavior with high ash bituminous coal similar to those produced in Kazakhstan.To this end, this work presents the mono and co-combustion of sewage sludge.In addition, thermogravimetric (TGA) analysis was used to investigate thermal degradation and the combustion behavior.The weight loss of material, kinetics, and combustion characteristics were precisely revealed via TGA.This analysis facilitated the comprehension of the synergistic effects of co-combustion of coal and of sewage sludge.Furthermore, flue gas measurements in terms of SO 2 , NO x and CO 2 from mono-combustion of SS and co-combustion of SS and blended with coal with proportion of 75%/25% in a laboratory scale bubbling fluidized bed (BFB) unit were presented.

Sample Preparation
Samples of SS were collected from the WWTP located in Astana, the capital city of Kazakhstan.The samples were preheated in a muffle furnace (Carbolite, Neuhausen, Germany) at 105 • C for 24 h to remove the excess moisture and deactivate potential bacteria and microbes, as has been suggested in the literature [36,37].Despite pre-drying, which consequently resulted in a significant weight loss due to the moisture evaporation, the remaining SS fraction was unsuitable for the batch feeding.As seen in Figure 1A, it had a slurry-like consistency with non-uniform particle sizes.Therefore, a hand press tool was used to make uniform pellets.The final shape of the SS pellet is demonstrated in Figure 1B.The mass of the individual pellets varied in the range of 0.55 ± 0.05 g.The bituminous high ash coal from the North of Kazakhstan (Ekibastuz) was used as a supplementary fuel during co-combustion experiments and was pelletized along with SS.Details thermal and chemical properties of the Ekibastuz coal can be found elsewhere [38].
Samples of SS were collected from the WWTP located in Astana, the capital city of Kazakhstan.The samples were preheated in a muffle furnace (Carbolite, Neuhausen, Germany) at 105 °C for 24 h to remove the excess moisture and deactivate potential bacteria and microbes, as has been suggested in the literature [36,37].Despite pre-drying, which consequently resulted in a significant weight loss due to the moisture evaporation, the remaining SS fraction was unsuitable for the batch feeding.As seen in Figure 1A, it had a slurry-like consistency with non-uniform particle sizes.Therefore, a hand press tool was used to make uniform pellets.The final shape of the SS pellet is demonstrated in Figure 1B.The mass of the individual pellets varied in the range of 0.55 ± 0.05 g.The bituminous high ash coal from the North of Kazakhstan (Ekibastuz) was used as a supplementary fuel during co-combustion experiments and was pelletized along with SS.Details thermal and chemical properties of the Ekibastuz coal can be found elsewhere [38].

Thermochemical Properties of the SS and Coal
The thermal properties of SS, in general, are non-uniform [23,39] and, hence, characterization is essential for the local sludge.Standards including ISO 18134-3:2023, ISO 18122:2022, ISO 18123:2023were used to determine proximate analysis and were performed in the muffle furnace [40][41][42].Elemental analysis was performed using a UNICUBE ® (Paris, France) micro elemental analyzer.Calibrated bomb calorimeter (Etalon ® , Almaty, Kazakhstan) was used to determine the calorific values.These analyses were repeated three times and the average values were reported and the standard deviation was less than 1%.Table 2 shows the thermal properties of SS in comparison with the high ash bituminous coal used in the experiments [43].As seen the SS samples have low fixed carbon but ~three times higher volatile matter than the coal.In addition, the ash content of SS was around 32%. Ultimate analysis demonstrated that oxygen, sulfur, and nitrogen presence in SS, considering its organic origin, were also approximately three times higher than the bituminous coal, whereas the carbon content was similar.The HHV value for the SS samples was 18.87 MJ/kg and was comparable to coal samples.

Thermochemical Properties of the SS and Coal
The thermal properties of SS, in general, are non-uniform [23,39] and, hence, characterization is essential for the local sludge.Standards including ISO 18134-3:2023, ISO 18122:2022, ISO 18123:2023were used to determine proximate analysis and were performed in the muffle furnace [40][41][42].Elemental analysis was performed using a UNICUBE ® (Paris, France) micro elemental analyzer.Calibrated bomb calorimeter (Etalon ® , Almaty, Kazakhstan) was used to determine the calorific values.These analyses were repeated three times and the average values were reported and the standard deviation was less than 1%.Table 2 shows the thermal properties of SS in comparison with the high ash bituminous coal used in the experiments [43].As seen the SS samples have low fixed carbon but ~three times higher volatile matter than the coal.In addition, the ash content of SS was around 32%. Ultimate analysis demonstrated that oxygen, sulfur, and nitrogen presence in SS, considering its organic origin, were also approximately three times higher than the bituminous coal, whereas the carbon content was similar.The HHV value for the SS samples was 18.87 MJ/kg and was comparable to coal samples.Prior to the batch combustion of SS pellets in the bubbling fluidized bed rig, a series of thermogravimetric analyses were conducted to explore the weight loss behavior of the samples.Dry and powdered SS and its blends with coal particles were used in Simultaneous Thermal Analyzer (STA) 6000 between 30 and 900 • C, with a heating rate of 15 • C/min and constant air flow rate.Furthermore, the kinetic analysis of TGA data for SS samples were performed using the free-model iso-conversional methods based on the 10, 15, 20 • C/min heating rate in the nitrogen environment.The proximate and ultimate analyses performed demonstrate similar results of Kazakhstan's SS compared to those of China's SS (range of C = 30.9-40.5%;H = 5.06-5.8%;N = 5.36-6.84%;S = 0.61-1.28%;O = 12.3-19.7%)[44] and those of Thailand's SS (range of C = 9.0-31.1%;H = 2.0-4.6%;N = 1.5-4.3%;S = 0.7-1.8%; Energies 2023, 16, 6634 5 of 13 O = 15.7-27.5%)[45].The variations can be explained by heterogeneous compositions of sewage sludge samples, atmospheric conditions, and different moisture content in case of the proximate analysis.

Experimental Apparatus and Test Conditions for Combustion
A laboratory scale BFB rig was used for the combustion experiments.The schematic illustration of the rig was shown in Figure 2. The rig was heated electrically with a threezone heater (Nabertherm) to maintain the furnace temperature.The BFB rig consists of two vertically aligned stainless steel tubes with diameters of outer tube being 89 mm and inner tube being 48 mm.Other elements of the rig include a fuel feeding point at the top, pressure drop manometers, gas supply cylinders/flowmeters, filters to collect fly ash, and a flue gas analyzer.Inert sand was placed as the bed material, which was held by the twolayer stainless steel meshes.The gas supply system was split into primary and secondary streams, connected to the bottom of the rig and to the feeding point, respectively.The gas inlet was controlled by mass flow meters (Bronckhorst).Mechanical batch feeding was organized from the top of the rig and was supported by pneumatic transport, if necessary, based on the fuel density.Combustion products at the exit line of the furnace were filtered from fly ash particles and measured in real-time mode using the calibrated Gasmet Dx 4000 FTIR (Vantaa, Finland).

Experimental Apparatus and Test Conditions for Combustion
A laboratory scale BFB rig was used for the combustion experiments.The schematic illustration of the rig was shown in Figure 2. The rig was heated electrically with a threezone heater (Nabertherm) to maintain the furnace temperature.The BFB rig consists of two vertically aligned stainless steel tubes with diameters of outer tube being 89 mm and inner tube being 48 mm.Other elements of the rig include a fuel feeding point at the top, pressure drop manometers, gas supply cylinders/flowmeters, filters to collect fly ash, and a flue gas analyzer.Inert sand was placed as the bed material, which was held by the twolayer stainless steel meshes.The gas supply system was split into primary and secondary streams, connected to the bottom of the rig and to the feeding point, respectively.The gas inlet was controlled by mass flow meters (Bronckhorst).Mechanical batch feeding was organized from the top of the rig and was supported by pneumatic transport, if necessary, based on the fuel density.Combustion products at the exit line of the furnace were filtered from fly ash particles and measured in real-time mode using the calibrated Gasmet Dx 4000 FTIR (Vantaa, Finland).
Operating conditions of the lab-scale BFB rig were summarized in Table 3. Combustion tests were conducted at the atmospheric pressure and the bed temperature was kept at 850 °C for all tests.Primary air supply was kept constant at 3.0 L/m (at STP) to ensure the superficial gas velocity being above the minimum fluidization velocity.The pressure drop between the bottom and the top of the reactor was measured using a U-tube manometer and was around 2.05 kPa.Operating conditions of the lab-scale BFB rig were summarized in Table 3. Combustion tests were conducted at the atmospheric pressure and the bed temperature was kept at 850 • C for all tests.Primary air supply was kept constant at 3.0 L/m (at STP) to ensure the superficial gas velocity being above the minimum fluidization velocity.The pressure drop between the bottom and the top of the reactor was measured using a U-tube manometer and was around 2.05 kPa.
Experimental results on flue gas analysis presented in this work reported the maximum peak values among each batch for NO x , SO 2 (in ppm), CO 2 and O 2 (in vol%).The completion of the batch pellet combustion was monitored using stabilization of the measured gases and recovery of oxygen in the flue gas, which took approximately 5 min per batch.

Thermogravimetric Analysis of the SS and the Blends with Coal
Figure 3A,B shows the thermogravimetric analysis (TGA)/derivative gravimetry (DTG) curves of SS and blends with bituminous coal samples at different weight ratio: (a) pure SS, (b) 25%/75% (SS/coal wt%), (c) 50%/50% (SS/coal wt%) and (d) 75%/25% (SS/coal wt%).The final weights of the samples tested in the TGA varied between 30% and 34%, which was in good agreement with the ash content obtained during the proximate analysis.As seen from Figure 3A, no moisture release was seen until 100 • C for all samples in the thermograms, as the samples were pre-dried and stored in the air tight packs.A further increase in the temperature from 200 to 600 • C leads to a sharp weight loss curve for SS samples, followed by the blended ratio of 25 wt% SS, 50 wt% SS and 75 wt% SS.Chen et al. [46] has recently highlighted three main stages of SS combustion during TGA.The first stage of decomposition occurs in the temperature range of 100-250 • C that corresponds to the release of volatiles [46].In our experiments, the first peak of decomposition for SS was observed at 190 • C, while for blended tests, the peak shifted to higher temperature due to the presence of coal and its low volatile content.The peak was observed at a temperature of 220 • C and 230 • C for blend ratio 75 wt% and 50 wt% of SS, respectively.There was no sharp peak observed for the blend ratio of 25 wt% SS in which coal was predominant, because of the low volatile content of the coal.Experimental results on flue gas analysis presented in this work reported the maximum peak values among each batch for NOx, SO2 (in ppm), CO2 and O2 (in vol%).The completion of the batch pellet combustion was monitored using stabilization of the measured gases and recovery of oxygen in the flue gas, which took approximately 5 min per batch.

Thermogravimetric Analysis of the SS and the Blends with Coal
Figure 3A,B shows the thermogravimetric analysis (TGA)/derivative gravimetry (DTG) curves of SS and blends with bituminous coal samples at different weight ratio: (a) pure SS, (b) 25%/75% (SS/coal wt%), (c) 50%/50% (SS/coal wt%) and (d) 75%/25% (SS/coal wt%).The final weights of the samples tested in the TGA varied between 30% and 34%, which was in good agreement with the ash content obtained during the proximate analysis.As seen from Figure 3A, no moisture release was seen until 100 °C for all samples in the thermograms, as the samples were pre-dried and stored in the air tight packs.A further increase in the temperature from 200 to 600 °C leads to a sharp weight loss curve for SS samples, followed by the blended ratio of 25 wt% SS, 50 wt% SS and 75 wt% SS.Chen et al. [46] has recently highlighted three main stages of SS combustion during TGA.The first stage of decomposition occurs in the temperature range of 100-250 °C that corresponds to the release of volatiles [46].In our experiments, the first peak of decomposition for SS was observed at 190 °C, while for blended tests, the peak shifted to higher temperature due to the presence of coal and its low volatile content.The peak was observed at a temperature of 220 °C and 230 °C for blend ratio 75 wt% and 50 wt% of SS, respectively.There was no sharp peak observed for the blend ratio of 25 wt% SS in which coal was predominant, because of the low volatile content of the coal.The second stage corresponds to the decomposition of hemicellulose and cellulose, which contributes to a notable weight loss [47] as also seen in Figure 3B.In addition, the high rate of weight loss can be explained by the reaction of volatiles and other reactive structures in the presence of air [30].The second stage follows immediately after the main oxidation and has a small peak at ~420 • C.Moreover, due to the continuous weight loss, it can be noted that the occurrence of other reactions with more complex compounds.Some of these reactions can involve interactions with heavy metals, as most of the organic volatiles were decomposed during the main oxidation process.Overall, oxidation is the longest stage with more than 60% of weight loss between 200 and 600 • C. Figure 3A further showed a relatively small weight loss at temperatures higher than 800 • C. According to Chen et al. [48], this last stage corresponds to the burnout stage.Magdziarz and Wilk [49] suggested that the burnout happens due to the small part of the carbon left in the sample [49].However, Chen et al. [48] argued that the burnout stage corresponds to the decomposition of the remaining CaO, as reported to be 18.9 wt%, within the sludge as has been proposed by Folgueras et al. [30].Regression was performed for the data presented in Figure 3 and is available in the Supplementary Material (Figure S1, Equations (S1)-(S4)).

Kinetic Analysis of SS Samples
Next, the kinetic analysis of the sewage sludge was carried out using three different heating rates and in nitrogen environment.The rate of solid phase reactions for heterogeneous sewage sludge samples can be expressed as follows: where dα/dt is the rate of conversion, k(t) is the rate of constant, f (α) depends on the reaction model, α is the fractional conversion, A is the pre-exponential Arrhenius factor (1/s), E is the apparent activation energy, R is the universal gas constant, T is the reaction temperature (K).Herein, the iso-conversional method with the FWO and KAS models was used to calculate the activation energy from experimental data on TGA analysis of sewage sludge at 10, 15, and 20 • C/min [50].The result for TGA in the nitrogen environment is presented in the Supplementary Material (Figure S1).The KAS model obeys the following expression: where β is the heating rate (K/min), A is the pre-exponential factor (1/min), α is the conversion factor, and g(α) is a decomposition mechanism function.The FWO model [51] employs a correlation between the sample heating rate, activation energy, and inverse temperature, which was first approximated using Doyle's method [52] for the temperature integral.The equation is as follows: Using Equations ( 2) and (3), the activation energy of pure SS at conversion factors from 0.1 to 0.7 was evaluated, as shown in Figure 4a (FWO) and Figure 4b (KAS).The average apparent activation energies determined via FWO and KAS are 236.9kJ/mol and 239.4 kJ/mol, respectively, with R 2 above 0.9.The deviation of 1.04% in mean activation energies showed a good agreement between the KAS and FWO methods.A relationship between the activation energy and a conversion factor was depicted in Figure 5.As observed from Figure 5, an activation energy shows an increase with a conversion factor, particularly at a conversion of 0.6~0.7.According to Liu et al. [26], the maximum activation energy was determined to be 184.14kJ/mol at the conversion value A relationship between the activation energy and a conversion factor was depicted in Figure 5.As observed from Figure 5, an activation energy shows an increase with a conversion factor, particularly at a conversion of 0.6~0.7.According to Liu et al. [26], the maximum activation energy was determined to be 184.14kJ/mol at the conversion value of 0.2, and the reason for this difference could be different environments: 30% O 2 with 70% CO 2 , and 100% N 2 in this study.The activation energy depends on several parameters including activated molecule concentration, diffusion limitation, and organic contaminants [53].Syed-Hassan et al. [54] reported that SS is a complex mixture of organic and inorganic compounds, as well as diverse live and dead microbes [54].Proteins, carbohydrates, and lipids are the significant components of sludge and correspond to 76-97% of the sludge's volatiles [55].Up to 60% of weight loss accounts for the volatile release and organic fraction combustion, while the sharp increase in activation energy from 0.6 to 0.7 corresponds to the degradation of organic compounds in sludge composition.The correlation coefficients were shown in Figure S3, accounting for almost the same R 2 values for both models.A relationship between the activation energy and a conversion factor was depicted in Figure 5.As observed from Figure 5, an activation energy shows an increase with a conversion factor, particularly at a conversion of 0.6~0.7.According to Liu et al. [26], the maximum activation energy was determined to be 184.14kJ/mol at the conversion value of 0.2, and the reason for this difference could be different environments: 30% O2 with 70% CO2, and 100% N2 in this study.The activation energy depends on several parameters including activated molecule concentration, diffusion limitation, and organic contaminants [53].Syed-Hassan et al. [54] reported that SS is a complex mixture of organic and inorganic compounds, as well as diverse live and dead microbes [54].Proteins, carbohydrates, and lipids are the significant components of sludge and correspond to 76-97% of the sludge's volatiles [55].Up to 60% of weight loss accounts for the volatile release and organic fraction combustion, while the sharp increase in activation energy from 0.6 to 0.7 corresponds to the degradation of organic compounds in sludge composition.The correlation coefficients were shown in Figure S3, accounting for almost the same R 2 values for both models.

Exit Gas Analysis
Lastly, to examine the combustion characteristics of the SS blend fuel, the combustion of SS-bituminous briquettes was performed.A total of 40 batch combustion tests carried out done with pellets of pure SS and 20 with a blend ratio of SS/coal 75%/25% in the BFB. Figure 6a shows the concentrations (maximum concentration for CO2 and minimum for O2 during the batch run) of O2 and CO2 in the exit gas line during the series of batch feeding.As can be observed, the lowest concentrations of O2 were observed between 1.6 and

Exit Gas Analysis
Lastly, to examine the combustion characteristics of the SS blend fuel, the combustion of SS-bituminous briquettes was performed.A total of 40 batch combustion tests carried out done with pellets of pure SS and 20 with a blend ratio of SS/coal 75%/25% in the BFB. Figure 6a shows the concentrations (maximum concentration for CO 2 and minimum for O 2 during the batch run) of O 2 and CO 2 in the exit gas line during the series of batch feeding.As can be observed, the lowest concentrations of O 2 were observed between 1.6 and 4.0 vol%, which ensured a complete combustion of the dropped pellets in the BFB rig.It can be seen from Figure 6a that the general trend of O 2 peaks demonstrated a slight fluctuation with an average concentration being around 2.5%.These fluctuations in the consumption of O 2 peaks can be explained by the heterogeneous nature of sewage sludge.
Meanwhile, the CO 2 concentration was obtained within the range of 5 to 8%, with an average value of 6.5 vol.% between the 40 batch experiments that were carried out.Figure 6b further illustrates the highest detected concentrations of SO 2 and NO x during the mono-combustion of SS samples.Peak value for SO 2 concentration was determined to be 347 ppm, while for NO x , the value was 442 ppm.Among different batches, the trend showed a slight increase for NO x , while SO 2 emissions did not demonstrate any significant deviations.The average values and standard deviations were found for the data presented in Figure 6 and are available in the Supplementary Material (Figures S4 and S5).
4.0 vol%, which ensured a complete combustion of the dropped pellets in the BFB rig.It can be seen from Figure 6a that the general trend of O2 peaks demonstrated a slight fluctuation with an average concentration being around 2.5%.These fluctuations in the consumption of O2 peaks can be explained by the heterogeneous nature of sewage sludge.Meanwhile, the CO2 concentration was obtained within the range of 5 to 8%, with an average value of 6.5 vol.% between the 40 batch experiments that were carried out.Figure 6b further illustrates the highest detected concentrations of SO2 and NOx during the monocombustion of SS samples.Peak value for SO2 concentration was determined to be 347 ppm, while for NOx, the value was 442 ppm.Among different batches, the trend showed a slight increase for NOx, while SO2 emissions did not demonstrate any significant deviations.The average values and standard deviations were found for the data presented in Figure 6 and are available in the Supplementary Material (Figures S4 and S5).
The combustion of blends with 75 wt% SS was further carried out for another 20 batches after the mono-combustion of SS.The results for emissions in terms of NOx and SO2 gases are presented in Figure 7a,b.As can be seen in Figure 7a, NOx emissions highlighted a sharp decrease in concentrations under co-combustion mode.The trend was in line with the amount of nitrogen available in coal as compared to the pure SS.Likewise, Figure 7b shows the SO2 concentrations in the blends, which fluctuated between 150 and 350 ppm, with an average value remaining similar to those in the case of mono-combustion of SS.A possible explanation for the fluctuation could be the interconversion of SO2 to H2SO3 in the presence of coal, within the stipulated time of batch feeding; however, further analysis is required to explore that fluctuation.Average values and standard deviations were found for the data presented in Figure 7 and is available in the Supplementary Material (Figures S6 and S7).Table 4 summarizes the results based on mean and standard deviation values based on 60 experimental data points for each gas mentioned above.The combustion of blends with 75 wt% SS was further carried out for another 20 batches after the mono-combustion of SS.The results for emissions in terms of NO x and SO 2 gases are presented in Figure 7a,b.As can be seen in Figure 7a, NO x emissions highlighted a sharp decrease in concentrations under co-combustion mode.The trend was in line with the amount of nitrogen available in coal as compared to the pure SS.Likewise, Figure 7b shows the SO 2 concentrations in the blends, which fluctuated between 150 and 350 ppm, with an average value remaining similar to those in the case of mono-combustion of SS.A possible explanation for the fluctuation could be the interconversion of SO 2 to H 2 SO 3 in the presence of coal, within the stipulated time of batch feeding; however, further analysis is required to explore that fluctuation.Average values and standard deviations were found for the data presented in Figure 7 and is available in the Supplementary Material (Figures S6 and S7).Table 4 summarizes the results based on mean and standard deviation values based on 60 experimental data points for each gas mentioned above.

Conclusions
This work presents a systematic study on the evaluation of the thermochemical properties of local sewage sludge and investigates the mono and co-combustion of SS and with high ash Ekibastuz coal in the TGA and on a laboratory-scale BFB rig.The thermo-chem-

Figure 2 .
Figure 2. The bubbling fluidized bed apparatus with supplementary equipment.Figure 2. The bubbling fluidized bed apparatus with supplementary equipment.

Figure 2 .
Figure 2. The bubbling fluidized bed apparatus with supplementary equipment.Figure 2. The bubbling fluidized bed apparatus with supplementary equipment.

Figure 3 .
Figure 3. (A) TGA, (B) DTG curves for SS and bituminous coal at different blend ratios at heating of 15 °C/min in air.

Figure 3 .
Figure 3. (A) TGA, (B) DTG curves for SS and bituminous coal at different blend ratios at heating of 15 • C/min in air.

Figure 4 .
Figure 4. Kinetic plots for pure sewage sludge in the N2 environment based on heating rate of 10, 15, and 20 °C/min for (a) FWO and (b) KAS.

Figure 4 .
Figure 4. Kinetic plots for pure sewage sludge in the N 2 environment based on heating rate of 10, 15, and 20 • C/min for (a) FWO and (b) KAS.

Figure 4 .
Figure 4. Kinetic plots for pure sewage sludge in the N2 environment based on heating rate of 10, 15, and 20 °C/min for (a) FWO and (b) KAS.

Figure 5 .
Figure 5.The relationship between activation energy vs. conversion for both FWO and KAS models.

Figure 5 .
Figure 5.The relationship between activation energy vs. conversion for both FWO and KAS models.

Figure 6 .
Figure 6.(a) O2 and CO2 concentrations (b) SO2 and NOx emissions during the mono-combustion of SS in BFB rig.

Figure 6 .
Figure 6.(a) O 2 and CO 2 concentrations (b) SO 2 and NO x emissions during the mono-combustion of SS in BFB rig.

Figure 7 .
Figure 7. Effect of blending with coal on (a) NOx emissions (b) SO2 emissions.

Figure 7 .
Figure 7. Effect of blending with coal on (a) NO x emissions (b) SO 2 emissions.

Table 2 .
Proximate and ultimate analysis of Kazakhstan's SS and Ekibastuz bituminous coal.

Table 3 .
Operating conditions of the laboratory-scale BFB rig during the SS pellets combustion.Can be operated up to 950 • C; ** Used only for pneumatic transport. *

Table 4 .
Mean and standard deviation values for flue gas compositions during batch combustion of SS and blend.

Table 4 .
Mean and standard deviation values for flue gas compositions during batch combustion of SS and blend.