Utilization of Sewage Sludge Using Multiple Thermal Conversion Processes

Received Sep 09, 2021 Revised Nov 11, 2021 Accepted Dec 27, 2021 Sewage sludge is a usual waste from urban areas that can be utilized in many renewable energy sources. In this study, we examine the sewage sludge utilization using pyrolysis process to produce pyrolytic oil using Taguchi methods, combustion characteristic of sludge pyrolytic oil (SPO) blend with heavy fuel oil (HFO), and cogasification of sewage sludge with CO2/steam as the gasification medium using Taguchi methods. The best-operating conditions for the pyrolysis of sewage sludge are a heating rate of 10C/min, temperature of 450C, the residence time of 60 min, and N2 flow rate of 700 mL/min. Under these conditions, the obtained pyrolytic oil yield is very close to the result from the Taguchi method calculation. In the combustion characteristic of sludge pyrolytic oil (SPO) blend with heavy fuel oil (HFO), a higher SPO in the fuel blend enhances the occurrence of micro-explosion and reduces the size of the residual. Higher SPO content in the fuel blend increases the combustion rate and increases the ignition delay due to moisture evaporation. In the co-gasification of sewage sludge and palm kernel shell optimization using Taguchi method, the best operational condition for maximum H2/ CO syngas ratio reaches at the gasification temperature of 900 C, a blending ratio of 30%, a CO2/(CO2+H2O) ratio of 70%, and a catalyst addition of 20% bed material mass. The best operating condition for maximum concentration of H2 reach with gasification temperature of 800 C, a blending ratio of 40%, a CO2/(CO2+H2O) ratio of 70%, and a 15% catalyst addition of bed material mass. The CO2/(CO2+H2O) ratio is the most important parameter among both case.

Sewage sludge is a usual waste from urban areas that can be utilized in many renewable energy sources. In this study, we examine the sewage sludge utilization using pyrolysis process to produce pyrolytic oil using Taguchi methods, combustion characteristic of sludge pyrolytic oil (SPO) blend with heavy fuel oil (HFO), and cogasification of sewage sludge with CO2/steam as the gasification medium using Taguchi methods. The best-operating conditions for the pyrolysis of sewage sludge are a heating rate of 10 o C/min, temperature of 450 o C, the residence time of 60 min, and N2 flow rate of 700 mL/min. Under these conditions, the obtained pyrolytic oil yield is very close to the result from the Taguchi method calculation. In the combustion characteristic of sludge pyrolytic oil (SPO) blend with heavy fuel oil (HFO), a higher SPO in the fuel blend enhances the occurrence of micro-explosion and reduces the size of the residual. Higher SPO content in the fuel blend increases the combustion rate and increases the ignition delay due to moisture evaporation. In the co-gasification of sewage sludge and palm kernel shell optimization using Taguchi method, the best operational condition for maximum H2/ CO syngas ratio reaches at the gasification temperature of 900 C, a blending ratio of 30%, a CO2/(CO2+H2O) ratio of 70%, and a catalyst addition of 20% bed material mass. The best operating condition for maximum concentration of H2 reach with gasification temperature of 800 C, a blending ratio of 40%, a CO2/(CO2+H2O) ratio of 70%, and a 15% catalyst addition of bed material mass. The CO2/(CO2+H2O) ratio is the most important parameter among both case.

INTRODUCTION
Nowadays, the main energy resources still have a huge dependency from the fossil fuels. This dependency may cause the significant consumption of fossil fuel, also it has a huge effect in the production of green house effect which leads to the climate change in the future. Therefore, it is necessary to improve the renewable energy utilization efficiency to make renewable energy become sustainable to replace the fossil fuel dependency [1]- [4]. Biomass is one of promising fuel to replace the fossil fuel due some advantages such as huge abundance, environmentally friendly, and low cost. The biomass mainly produced from forestry residual, biological waste, agricultural waste, municipal waste, and many other wastes. Among those example of waste, municipal waste become a promising source of biomass due to its availability. Sewage sludge is a major municipal waste that has a complex element such as organic materials, inorganic materials, and moisture. The usual treatment of sewage sludge utilization is use in agricultural, incineration, and landfill gas production. thermal pyrolysis apparatus; combustion characteristics of the SPO, HFO and its blending; and co-gasification of sewage sludge and PKS using different medium.

Production of pyrolytic oil from sewage sludge
This experiment aimed to investigates the influence of different important parameters on the pyrolysis process of sewage sludge for producing sludge pyrolytic oil (SPO) using the Taguchi method.

Experimental setup
In the experiment, 90 g of dried sewage sludge was packed in a quartz tube cylindrical holder and put into the furnace, as shown in Fig.2a. Initially the tube vacuumed for pyrolysis, then the tube fill with N2 gas to the ambient pressure. Then the furnace heat the sample to the specific temperature and maintained for a selected residence time. The heated carrier gas flow into the holder and interact with the sample. The generated volatile gas was collected and condense at 25 •C to produce a liquid oil. Figures 2b show pyrolysis operating process. It must undergo two steps: heated up from room to the target temperature at a specific heating rate and held at the targeted temperature for a designated residence time. The production of pyrolytic oil occurs in a certain condition and is influenced by the combination of four operational parameters: heating rate, target pyrolytic temperature, residence time, and nitrogen flow rate. The Taguchi method will analyze the influence of this parameter combination on the pyrolytic oil production from sewage sludge using a fixed-bed tubular furnace.

Taguchi method
The Taguchi method was used to find the maximum yield of SPO from the pyrolysis experiments. The feature of the Taguchi method uses an orthogonal array experimental design with a simple analysis of variance. In general, the Taguchi method is not aimed at finding the optimal conditions as a full factorial method, but it can analyze the best trend with fewer experimental data and thus is commonly applied to optimize industrial processes.
Taguchi method was previously applied to analyze caster meal pyrolysis by Chen et al. and this group. The Taguchi method uses the S/N ratio, the signal to noise ratio, to measure the quality 100 characteristics deviating from the desired value. Different strategic categories of nominal-the best (NTB), smaller the better (STB), and larger the better (L.B.)-are used to describe the S/N ratio characteristics. For example, in this study, the "larger the better" characteristic must be adopted to obtain the maximum yield of pyrolytic oil. The expression of the S/N ratio is shown as follows: Where N is the test number, yi is the value of the pyrolytic oil yield in the ith test. The experiment is conducted three times and take its average results.

Combustion characteristics of sewage sludge pyrolysis oil, heavy fuel oil, and their blends
The SPO is taken from the previous experiment and blended with HFO with different blending ratios. The fuel blends were mixing SPO and HFO with the pure HFO, 20% SPO/80% HFO, 40% SPO/60% HFO 60% SPO/40% HFO, 80% SPO/20% HFO and pure SPO. We used Tween 80 and Span 80 surfactants to mix the SPO and HFO, and the fuel blends were also prepared using a magnet and a magnetic stirrer. Fig. 3 shows the suspended droplet experiment setup used in this study. The component of this setup is a motorized stage linear guideway, two heating plates, a moving thermocouple attached in the linear guideway and a fixed thermocouple attached into the heating plate, an LED backlight, a DC power supply, a temperature controller, an electrical switch, a data collection card, a high-speed camera, and a computer. The temperature data acquired from the thermocouple and temperature controller were measured and recorded using LabVIEW software. The droplet in the linear guideway at the thermocouple was captured by the high-speed camera using a Phantom Miro C110 camera and a SIGMA MACRO 105 mm F2.8 EX DG OS HSM lens. The two heating plates had 70 mm width and 45 mm height separated by around 6.5 mm. The two plates were allowed to heat to 500 o C,550 o C, and 600 o C as the required experimental temperature. Once the required experimental temperature was reached, the LED backlight and high-speed camera were automatically turned on, and a linear guideway was moved into the gap between the heating plates to adjust the focus of the high-speed camera. The fuel droplet has a diameter of around 1000 mm at the junction of the moving thermocouples. The droplet's twodimensional area was calculated by selecting a suitable edge detection function to discover the droplet's edge accurately. Then, the diameter of the spherical droplet could be estimated to obtain the D 2 value during the droplet heating process.

Co-gasification of sewage sludge and palm kernel shell
In this experiment, the co-gasification of sewage sludge and palm kernel shell is investigated using CO2/steam as the gasification medium. A laboratory-scale bubbling fluidized bed gasifier with continuous feed is designed to study the co-gasification characteristics. Experiments following the Taguchi method are carried out to find the optimal operation parameters using the fluidized bed gasifier with maximum H2/CO ratio or maximum concentration of H2 in the syngas.   Fig. 4 shows a schematic of the lab-scale bubbling fluidized bed gasifier (~1 kWth). It comprised several components, including a fluidized bed reactor, a gas preheater, a steam generator, a continuous feeder, an electric heater, a filtration system, a Venturi tube, a syngas analyzer (MRU VARIOluxx), and a jet burner. The fluidized bed reactor was made of a stainless (310s) cylindrical tube with a diameter of 50 mm and a height of 800 mm. The gas distributor plate was 1 mm thick and perforated with 460 holes. Fuel particle size was about 1 mm. Approximate 150 g of quartz sand (with particle sizes ranging between 250 and 300 mm) was used as the bed material in the reactor. The minimum fluidization velocity was calculated from empirical formulas. The olivine (ranging in size between 250 and 300 mm) was also added as a catalyst to promote tar reduction. The amount of catalyst is between 0% -25% bed material mass. In this study, the operating parameters and conditions were as follows: The flow velocity was fixed at 0.225 m/s; the bed temperature ranged between 750 and 900 o C, and the gasifying agents were steam, CO2, or their blends.

Gasification using fluidized bed gasifier
Before the gasification experiment, the carrier gas was preheated to 500 C using a gas preheater and then passed through the gas distributor to enter the gasifier. The bed material was heated and maintained at the target temperature by the electric heater. The continuous feeder was composed of a fuel tank, a motor, a screw, stainless steel pipes, and a water jacket. The motor controlled the rotation speed of the screw, and the fuel feeding rate was fixed at 0.18 kg/h. In order to avoid heat transfer from the reactor to the fuel feeder, a water jacket was used to maintain the fuel feeder below 40 o C. Also, a nitrogen stream was used to deliver the fuel and prevent the product gas's back-flow from the gasifier. When the gasifier temperature reached the desired value with the help of the electric heater, the hot steam produced from the steam generator was fed into the gasifier. Once the fuel had dropped into the dense phase zone in the gasifier, it was converted to syngas through the gasification process. After that, the syngas passed through the filter system and Venturi tube for gas cleanup and flowrate measurement, respectively. The venturi tube was used to estimate the syngas flow rate by a height difference of a U-shape tube. Finally, the syngas composition was detected with a gas analyzer and then was consumed using a jet burner, which has an LPG/Air flat flame for syngas burnout.

Taguchi method
The Taguchi method was used to find the optimal co-gasification conditions for sewage sludge and PKS. The Taguchi method in this experiment is similar to the pyrolytic oil production experiment. However, in this experiment, the Taguchi method was used to determine the maximum H2/CO ratio and maximum H2 concentration from the gasification; the larger better (LTB) characteristics in the Taguchi method were used. The S/N ratio for LTB is defined in Equation (1).

RESULTS AND ANALYSIS
This section explains the research results and, at the same time, gives a comprehensive discussion. Results can be presented in figures, graphs, tables, and others that easily understand the reader. The discussion can be made in several sub-chapters. Table 1 shows the approximate analysis of the sewage sludge used in this study, and it is composed of 6.94 wt % moisture, 45.45 wt % volatiles, 9.94 wt % fixed carbon, and 37.67 wt % ash.   Figure 3 shows the TGA thermograph of sewage sludge. The heating rate is 10 o C/min, and the nitrogen flow rate is 50 mL/min. The purple curve shows thermogravimetry (T.G.), the blue curve shows differential thermogravimetry (DTG), the red curve shows differential scanning calorimetry (DSC), while the green curve represents T.G. in the air atmosphere. The mass loss of sewage sludge increases along with the temperature, and the major reactions occur between 120 o C and 500 o C There are three stages of weight loss for the purple T.G. curve in Figure 5. The first stage has a weight loss of 9.33% in the temperature range of 30-200 • C due to removing the water and organics with low boiling points. In the second stage, there are two distinct peaks at 273 •C and 323 •C in the DTG curve. About 33.57% of the original sewage sludge is lost, and this is mainly from the decomposition of aliphatic compound protein and carbohydrates contained in the sewage sludge. In the third stage, the weight loss tends to be mild when the temperature exceeds 500 •C, which might be from the decomposition of residual organic matter and inorganic materials, such as calcium carbonate. Finally, the residue is about 46.79% of the original weight of the sewage sludge when the temperature reaches 1000 •C

The optimal condition for sludge pyrolysis
The optimal pyrolysis conditions for the maximum yield SPO are investigated using the Taguchi method in the fixed bed reactor. The table gives the parameters and their levels. Four pyrolysis parameters are the heating rate, pyrolysis temperature, residence time, and nitrogen flow rate. These parameters are assumed to be independent, and each parameter has four levels. Table 2 shows the experimental layout and the mass balance of all experimental conditions, including aqueous, oil, and solid yields from the calculation. Each experiment was performed three times and took the average to assure the reliability of the results. The optimal operation conditions are determined based on the experimental results and the calculated S/N ratio, as shown in Fig. 6. In Fig. 6, these four pyrolysis parameters are labelled as "A" for heating rate, "B" for pyrolytic temperature, "C" for residence time, and "D" for nitrogen flow rate. It can be deduced from Figure  6 that the optimal pyrolysis conditions are A1, B1, C2, and D3, which represent a heating rate of 10 C/min, the pyrolytic temperature of 450 •C, the residence time of 60 min, and nitrogen flow rate of 700 mL/min. The results show that oil yield was affected by the combination of four operational parameters. The nitrogen flow rate has the most obvious effect among these parameters, followed by the pyrolytic temperature, heating rate, and residence time. In the Taguchi method, the confirmation experiment is the last step, and the aim is to validate the yield of SPO. After obtaining the optimum conditions and predicting the response under these conditions, a new experiment was performed with the optimum levels of these pyrolysis parameters. The theoretical optimal S/N ratio can be estimated as: From the experimental results, the theoretical optimization S/N value is -19.63, and it can be deduced that the theoretical maximum pyrolytic oil yield is 0.1044 g/g-sewage sludge (18.85% on a dry ash-free (daf) basis). The real yield of the confirmation experiment is 0.1019 g/g-sewage sludge (18.4% on a dry ash-free (daf) basis). The difference is only 0.0025 g/g sewage sludge, and the thus real experimental result is very close to the theoretical value. In addition, under the optimal conditions, the solid product is about 54.83%, the aqueous product is about 15%, and the gas product is 19.98% by mass balance. Fig. 7a shows the TGA results for the SPO fuel. The weight loss curve contains three important stages. Stage 1 is the moisture evaporation stage ranging from 30 o C to 134 o C. The DTG curve has peaks at 105 o C and 115 o C, and an endothermic reaction is indicated by the DSC curve. Stage 2 is the volatile decomposition and burning stage, with temperatures ranging from 134 o C to 485 o C. The released components include lightweight components (LWC) and middleweight components (MWC). The DSC curve shows a slight exothermic reaction between 300 o C and 450 o C, with a peak at a temperature of 395 o C. It is speculated that the combustion heat released during this period results from volatile burning. In addition, the total weight loss was approximately 77.19 wt%. Stage 3 is the primary combustion stage at temperatures higher than 485 C. In this stage, the DTG curve had a significant peak at 624 o C due to further decomposition of the HWC, which subsequently reacted with the oxygen and exhibited an intense release of combustion heat. Finally, the total weight loss in this stage was approximately 11.90 wt%, with almost no remaining residue. Fig. 7b shows the TGA results for the HFO. The weight loss curve comprises only two main regions in the HFO. The moisture evaporation stage (stage 1) of the HFO is vague due to the very low moisture content in the HFO. Stage 2 is the volatile decomposition and burning stage, with temperatures ranging from 30 o C to 582 o C. Initially, the DSC curve remains approximately constant until 287 o C. A noticeable exothermic peak occurs at 359 o C, indicating a volatile burning heat release. The DTG curve has a peak located at 354 o C. At higher temperatures, the DTG curve shows another peak. The second peaks at 396 o C indicate the primary 104 endothermic reaction, and the last two peaks are located at 467 o C and 485 C, respectively. Stage 3 is the combustion stage, with temperatures higher than 525 o C. The DTG curve shows a significant peak at 671 o C and shows an intense combustion heat release for the DSC curve. The exothermic reactions result from the further decomposition and burning of the high weight component (HWC). The total weight loss is approximately 10.21 wt%. Fig. 7c and 7d show the TGA results for the different SPO/ HFO fuel blends. As shown, the weight loss in the moisture evaporation stage increases with increases in the SPO content due to high moisture content in the HFO. In the early volatile decomposition and burning stage, the fuel blend with a higher SPO content shows a more significant weight loss rate than the higher HFO content. However, the weight loss increases significantly in the higher HFO content in the high-temperature region. The higher LWC content mainly causes this result in the SPO compared with HFO, and these LWC components vaporize easier in the lower temperatures. Since the HFO contain more MWC, some MWC is released near the end of the volatile decomposition and burning stage, which cause more intense weight loss than is the case for SPO.

Suspended droplet experiments
In general, various stages can be observed during the suspended droplet experiments, and these stages can be described as follows: (i) The surface micro-explosion stage: The LWC near the surface is vaporized and nucleated. The LWC induces a continuous micro explosion with a low intensity that distorts the droplet shape. (ii) The interior micro-explosion stage: The LWC in the droplet's interior becomes superheated. When the superheating limit is exceeded, homogeneous nucleation and vaporization occur, resulting in an accumulation of pressure within the droplet. This pressure causes swelling or expansion of the droplet, leading to the droplet being pulled into smaller droplets, broken into pieces, or released as fuel oil vapour into the ambient environment.
(ii) The vaporization stage: The droplet boiling point is governed mainly by the MWC. However, if the ambient temperature is not excessively high, the MWC near the droplet surface may be vaporized with the LWC in the droplet's interior. The pressure accumulated within the droplet is lower than in the interior micro- 105 explosion stage. Consequently, the surface micro-explosion and interior micro explosion's intensity is reduced, and droplet shrinkage occurs. (iv) The expansion micro-explosion stage: If the ambient temperature is very high, the MWC near the droplet surface and the LWC in the droplet's interior are heated rapidly and vaporized. The resulting pressure accumulation within the droplet is like that in the interior micro-explosion stage. However, the magnitude of the accumulated pressure is greater. Consequently, high-intensity expansion and an explosion occur, leading to a dramatic distortion in the droplet shape. It should be noted that the expansion micro explosion stage always precedes the combustion stage.
(v)The combustion stage: The released fuel vapor mixes with the oxygen in the environment and is ignited if the ambient temperature is sufficiently high. The resulting flame propagates from the surroundings to the main droplet and forms a non-premixed flame that wraps around the main droplet. This combustion event increases the heating rate of the droplet and the intensity of the micro-explosion. As a result, a dramatic distortion in the droplet shape occurs, which results in the release of fuel oil vapor and therefore enhances the existing flame. The combustion process proceeds continuously until all the volatile components in the droplet are consumed.
(vi) The smolder stage: Following the combustion stage, the volatile components in the droplet are completely burned. However, the combustible solid particles continue to burn. While no obvious flame exists, the solid residue is red or orange, gradually extinguishing as the solid particles are burned. The ambient temperature is not high enough to vaporize the droplet, ignite the fuel vapor surrounding the droplet, or burn out the remaining combustible solid particles. The residual product is typically a liquid phase, solid phase, or sludge-like phase. Fig. 8 show the results for the suspended droplet experiment for the SPO/HFO fuel blends with ambient temperature of 550 o C at different blending ratios, where (a) the change in the droplet size during the heating or combustion period, and (b) shows the image sequence of the droplet at ambient temperatures of 550 C. The other stages are also marked on the figures to indicate the various heating behaviors. For the fuel blends with 60% SPO, this stage occurred in period A-B-C-D-E (0 -1.335 s); the 80% SPO fuel blend and pure SPO occurred in period A-B-C-D (0 -1.193 s and 0 -1.45 s), respectively. Overall, the results showed that the intensity of the micro-explosion event increased with an increase in the SPO content.
The fuel blends with a higher SPO ratio (i.e., 60 -80% SPO and pure SPO) exhibited a combustion stage in period E-F-G (1.329 -1.705 s) for the 60% SPO fuel blends and in period D-E-F (1.114 -1.59 s and 1.45 -1.75 s) for the 80% SPO fuel blend and pure SPO fuel, respectively. Finally, all the fuels exhibited a residual stage. The residual product of the fuel blends with a higher SPO content (i.e., 60 -80% SPO and pure SPO) ranged in size between 0.18 -0.43 times that of the original diameter Table 3: Heating performance of the different SPO/HFO fuel blends in the different ambient temperature

Combustion rate
All the single droplet heating behaviors for the different SPO/HFO blends are summarized in table 3. We can see that in there is no combustion happen in the ambient temperature of 500 o C. Some blending indicates that the changes in the droplet size can be approximated by a straight line using a linear regression analysis technique with the least square error in the ambient temperature of 550 o C and 600 o C. The slope of this straight line represents the vaporization or combustion rate.  Fig. 9 shows a suspended droplet experiment results for the fuel blend with different SPO/HFO ratios at an ambient temperature of 550 C. The figure also shows that the linear regression line indicates the combustion rate. The 60% SPO fuel blend had a combustion rate approximately of 2.226 mm 2 /s over the range of t/D0 2 = 0.961 -1.243. The 80% SPO fuel blend had a combustion rate approximately of 2.556 mm 2 /s over the range of t/D0 2 = 1.004 -1.433, while the pure SPO fuel had a combustion rate approximately of 3.047 mm 2 /s over the range of t/D0 2 = 1.05 -1.263. Higher SPO content increased the combustion rate of the fuel blends. Since sludge pyrolysis oil contains higher moisture, the water evaporates at the beginning of the combustion process leading to a longer ignition delay. The ignition delay time increased with the increment of SPO.  Fig. 10a shows T.G. and DTG curves of pure PKSs in N2 and CO2 environments, where four stages of weight loss occur. The first stage ended at 169 o C due to the weight loss from some remaining moisture content in the PKS. The second and third stages, which ended at 321 o C and about 400 o C were denoted as the pyrolysis of hemicellulose and cellulose, respectively. The final stage may have been the pyrolysis of lignin. It is worth noting that when PKS is in a CO2 environment, the fourth-stage thermal decomposition occurs earlier and has a higher weight loss rate. Fig. 10b shows the TG and DTG curves of pure sewage sludge in N2 and CO2 environments. Weight loss can be divided into three stages. The first stage ended near 181 C, and all the moisture had evaporated. The second stage ended at about 589 o C. It was speculated that the thermal decomposition of lipids, cellulose, and protein occurred in this temperature range. The last stage was the thermal decomposition of heavyweight components or inorganic matter. In a CO2 environment, the overall weight loss rate is enhanced, and the reaction is also completed earlier.

Thermogravimetric analysis
(a) (b) Figure 10: The T.G. and DTG results for PKS (a) and sewage sludge (b) under N2 and CO2 environments,

The optimal conditions for co-gasification of SS and PKS in the fluidized bed gasifier
In general, the temperature is one of the most crucial parameters in gasification, and it is usually located between 750 and 900 o C. It also dominates the char reaction rate, gasifier efficiency, secondary cracking of tar, and pollutant formation during gasification. In addition, the gasifying medium, such as Air, CO2, and H2O, can alter the H2/CO ratio of syngas, where, especially, CO2 and H2O can react with char and tar and generate more combustible gas. The effects of gasification temperature, the fuel blending ratio, CO2/(CO2+H2O) ratio, and catalyst addition on the H2/CO ratio and H2 concentration were investigated with the Taguchi method in a fluidized bed gasifier. These are important parameters in the practical operation of the gasifier. Moreover, some parameters were fixed for all cases, such as the fluidization velocity of 0.225 m/s (5 L/min), the fuel feeding rate of 0.18 kg/h, and the bed material (150 g). In addition, olivine was used as an additive catalyst with different weights relative to the bed material.
The gas compositions obtained in the 16 experiments are shown in Table 4. The experimental results are used to calculate the corresponding S/N ratios using Equation (1) to obtain the maximum H 2 /CO ratio and maximum H2 concentration. The optimum theoretical value could be predicted by substituting the S/N ratio of the parameters at optimal levels into Equation (2)  Table 5 shows the optimal operation conditions were obtained using the analytical results for the mean S/N ratio. From the table, we can see that the maximum H2/ CO ratio reach with the gasification temperature of 900 C, a blending ratio of 30%, a CO2/(CO2+H2O) ratio of 70%, and a catalyst addition of 20% bed material mass. The maximum concentration of H2 was 32.2% with gasification temperature of 800 C, a blending ratio of 40%, a CO2/(CO2+H2O) ratio of 70%, and a 15% catalyst addition of bed material mass. In the H2/CO ratio, the CO2/(CO2+H2O) ratio is the most significant variable, followed by catalyst addition, blending ratio, and finally, gasification temperature. The S/N ratio at the optimal level (S/Nopt) for H2/CO ratio was 4.81, and the 109 obtained maximum H2/CO ratio was 0.574. The confirmation experiment was then performed, and the result revealed that the maximum H2/CO ratio was 0.563, which was only a 0.011 difference from the theoretical value. In the case of maximum H2 concentration, CO2/ (CO2+H2O) ratio was the most significant parameter, followed by catalyst addition, gasification temperature, and finally, the blending ratio. The calculated S/Nopt ratio was 9.13, and the maximum concentration of H2 was 34.9%. The experimental confirmation results showed a value of 32.2% for the normalized H2 concentration.

CONCLUSION
The optimal production of pyrolytic oil, combustion characteristics of the blending between sludge pyrolysis oil (SPO) and heavy fuel oil (HFO) with different blending ratios and co-gasification of sewage sludge (SS) and palm kernel shell (PKS) using different gasification mediums was investigated. The findings of this study are summarized as follow: (1) The effective sequence of pyrolytic parameters for the yield of sludge pyrolytic oil is nitrogen flow rate, pyrolytic temperature, heating rate, and residence time. The best operating conditions for sewage sludge pyrolysis are a heating rate of 10 •C/min, pyrolysis temperature of 450 •C, residence time of 60 min, and nitrogen flow rate of 700 mL/min. Under these conditions, the obtained pyrolytic oil yield is 10.19% (18.4% on dry ash free (daf) basis), which is very close to 10.44% (18.85% on dry ash free (daf) basis.), the ideal value from the Taguchi method. (2) Various stages can be observed in the suspended droplet experiments for SPO/HFO blends, including (i) a surface micro explosion stage, (ii) an interior micro-explosion stage, (iii) a vaporization stage, (iv) an expansion micro-explosion stage, (v) a combustion stage, (vi) a smolder stage, and (vii) a residual stage. SPO contains a higher amount of water and LWC than HFO. Therefore, a higher SPO content in the fuel blend increases the frequency of micro-explosion and reduces the size of the residual product. An ambient temperature of 550 C with more than 60% SPO shows almost constant combustion rates. Higher SPO content in the fuel blend increases the combustion rate and the ignition delay time due to the moisture evaporation process. (3) Using the Taguchi method, different operating parameters, such as gasification temperature, the fuel blending ratio, the CO2/(CO2+H2O) ratio, and catalyst addition were investigated. The optimal operating conditions necessary to obtain the maximum H2/ CO ratio and the maximum H2 concentration in the bubbling fluidized bed gasifier were obtained. The maximum H2/ CO ratio reach with the gasification temperature of 900 C, a blending ratio of 30%, a CO2/(CO2+H2O) ratio of 70%, and a catalyst addition of 20% bed material mass. The maximum concentration of H2 reach with gasification temperature of 800 C, a blending ratio of 40%, a CO2/(CO2+H2O) ratio of 70%, and a 15% catalyst addition of bed material mass. Hydrogen is a promising energy carrier with large energy density and the hydrogen-rich syngas could help minimize the CO2 equivalent emission. The CO2/(CO2+H2O) ratio was always the dominant parameter in the selected conditions in the study. In addition, catalyst addition was the second dominant effect on the maximum H2/CO ratio and maximum H2 concentration. The amount of catalyst with 15 -20% bed material mass had the significant effect in this study. These experimental results of the 1 kWh continuous type fluidized-bed gasifier could be used as a reference for the practical operation of the largescale gasifier.