Alternative Solid Fuel Production from Paper Sludge Employing

Jan 27, 2014 - There is no corresponding record for this reference. 4. Lens , P. N.; Vochten , P. M.; Speleers , L.; Verstraete , W. H. Water Res. 199...
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Alternative Solid Fuel Production from Paper Sludge Employing Hydrothermal Treatment Chinnathan Areeprasert,*,†,‡,∥ Peitao Zhao,*,†,§,∥ Dachao Ma,† Yafei Shen,† and Kunio Yoshikawa† †

Department of Environmental Science and Technology, Tokyo Institute of Technology, G5-8, 4259 Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa 226-8502, Japan ‡ Department of Mechanical Engineering, Faculty of Engineering, Kasetsart University, 50 Ngam Wong Wan Road, Ladyaow Chatuchak, Bangkok 10900, Thailand § Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, No. 2 Sipailou, XuanWu District, Nanjing 210096, China ABSTRACT: This paper aims to investigate the alternative solid fuel production from paper sludge employing hydrothermal treatment (HTT) in a lab-scale facility for implementation of the pilot-scale plant. The paper sludge was subjected to the HTT under subcritical hydrothermal conditions. In the lab-scale experiment, the temperature conditions were 180 °C, 200 °C, 220 °C, and 240 °C, respectively, while it was 197 °C in the pilot plant as the optimum condition. The holding time was 30 min in both cases. The hydrothermally produced solid fuel was evaluated for the fuel property, the water removal performance, and the mass distribution. Furthermore, the energy balance of the process was studied. The higher heating value of the HT pretreated paper sludge was slightly improved. In addition, the produced solid fuel had comparable H/C and O/C atomic ratios with that of coal, indicating the presence of carbonization during the HTT process. Using the mechanical dewatering, only 4.1% of moisture in the raw paper sludge can be removed while the solid fuel production from paper sludge by HTT at 200 °C showed 19.5% moisture reduction. According to the energy balance of the pilot plant, the recovered energy was significantly higher than the energy input, showing the feasibility of employing the HTT to produce alternative solid fuel from paper sludge.

1. INTRODUCTION The pulp and paper industry generates a large amount of wastes.1,2 Briefly, the waste stream starts from wood preparation process, such as debarking and chipping processes.2−4 Then, woodchip is subjected to pulping process.5 After that, the final step is paper production and product conversion processes.6 The waste from wood preparation process is mainly wood residues, and they are normally utilized as biomass whereas the waste obtained from the pulping process is very problematic.2 Paper sludge is generated from two sources: main and secondary clarifiers.7 The main clarifier, where precipitated particles are collected, produces high fiber concentration sludge, which is called primary sludge. It consists of wood fibers: cellulose, hemicellulose, and lignin.8 Moreover, specific substances for paper making process, such as kaolin and calcium carbonate, were also found in the primary sludge.9 For the secondary sludge, it is produced from a secondary clarifier and a wastewater treatment facility and contains fewer amounts of solid particles. The characteristics of the paper sludge depend on raw materials, techniques employed, water treatment processes, and designed property of paper products in each manufacturer.2 Many approaches have been developed to manage the paper sludge. Nowadays, landfilling is one of the most common practices.7,10,11 However, it brings about lots of severe problems in current society, such as groundwater contamination caused by leachate.7 Thermo-chemical conversion is also adapted to deal with the paper sludge. The study of cofiring applications of the paper sludge with either a subbituminous coal12 or a semianthracite coal13 showed some benefits in view of energy © 2014 American Chemical Society

recovery and environmental friendliness. Nevertheless, the bad fuel characteristics of the paper sludge, such as high moisture content, could unfavorably affect an incineration process. Supercritical water gasification, which is commonly known as a utilization of water above 374 °C and 22 MPa as a medium, was reported that it was able to generate valuable gases, such as hydrogen and methane, from the pulp and paper sludge.14 It showed a satisfying energy recovery; however, due to its severe operational condition and corrosion problem, the supercritical water gasification might be impractical in the real industry especially for the developing countries.15−17 The comparatively lenient operating condition of the subcritical hydrothermal treatment (HTT) would be more practical, which has been investigated by several researchers in recent years.18−20 This innovative treatment process can convert waste to value-added resources such as coal-like solid fuel or organic fertilizer.19,21 Namioka et al.18 applied the HTT to produce solid fuel and studied the effect of HTT on sewage sludge dewaterability. When increasing the HTT temperature, more moisture could be reduced at the same dewatering condition. Sakaguchi et al.22 upgraded high moisture content brown coal by HTT and reported that fuel property of the treated brown coal was improved due to the increase in the calorific value during HTT. In a 3-m3 demonstration plant, the HTT has been proven to reduce organic chlorine and improve Received: December 2, 2013 Revised: January 23, 2014 Published: January 27, 2014 1198

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Figure 1. Lab-scale experimental flowchart. experiment was performed three times to ensure the repeatability. Table 1 summarizes all the operating conditions in this work.

the natural drying performance of the Japanese municipal solid waste to produce safe and clean solid fuel.19,23 According to the previous successful studies and results, the HTT could be successfully implemented to the paper sludge. First, the fuel property of the hydrothermally produced solid fuel from paper sludge was investigated in the lab-scale experiment. Second, the present work studied the effect of the HTT on the water removal performance, including mechanical dewatering and thermal drying process. Additionally, the scanning electron microscopy (SEM) and Fourier transform infrared spectrometry (FTIR) were performed to characterize the produced solid fuel. Third, the mass balance, the energy consumption, and the energy recovery of the labscale process were evaluated as well. Then, the optimal condition from the lab-scale experiment was demonstrated in a 1-m3 pilot-scale plant. Similar to the lab-scale process, the effect of the HTT on the solid fuel property, the dewatering and drying performances were determined. Finally, the mass and energy balances of the alternative solid fuel production process were presented to evaluate the feasibility for the commercial plant.

Table 1. Summary of Experimental Conditions parameters

laboratory

Hydrothermal Treatment (HTT) sludge mass 60 ± 0.2 g pressure (MPa) 1.8−2.4 temp. (°C) 180, 200, 220, 240 holding period (min) 30 Dewatering Experiment dewatering method pressing dewatering pressure(MPa) 0.6 dewatering time (min) 15 Drying Experiment drying mechanism thermal air temp. (°C) 32 ± 0.4 air velocity (m/s) 1.2

pilot 351 ± 1 kg 1.9 ± 0.05 197 ± 3 30 centrifuging 0.5 5 natural 32 ± 3 N/A

The mechanical dewatering and thermal drying were integrated into the proposed alternative solid fuel production process. The treated paper sludge was tested on the dewatering performance after HTT. It was conducted by a mechanical pressing device (Figure 1b, details can be found in ref. 24), which is mainly constituted by a piston, a cylinder, and an orifice plate. The hydrothermally treated paper sludge was filled in the cylinder then it was pressurized at a pressure of 0.6 MPa for 15 min. The nitrogen gas was used as a pressure source. During the experiment, the seal and filter cloth were able to minimize errors. Subsequently, the dewatered solid was immediately taken into a convective force-drying process by an air blow dryer machine illustrated in Figure 1c for the drying test (details can be found in ref. 25). The machine generates airflow by a centrifugal air compressor assembled with an electric heater to increase the air temperature. The dewatered sludge was put on the ASP-4100 electronic balance (As One Corp., Japan) integrated with the data logger. Due to the sensitivity of the electronic balance, the drying apparatus was calibrated before testing to minimize errors. The temperature condition of the drying experiment was controlled at 32 °C with an average velocity of 1.2 m/s. After the sample weight was kept constant for an hour, the experiment was terminated. The dewatering and drying conditions are also shown in Table 1. 2.2.2. Pilot-Scale Experiment. For the pilot-scale HTT, the reactor is a 1-m3 cylindrical batch-type assembled with an automatic stirrer. Approximately 351 kg of the paper sludge were supplied to the reactor in each experiment. Similar to the lab-scale procedure, the sample was initially kept mixing by a stirrer with a rotational velocity of 20 rpm. Then, the treatment condition was achieved by injecting a saturated steam generated by a fire-tube boiler fueled by liquefied petroleum gas

2. EXPERIMENTAL SECTION 2.1. Raw Material. In this study, the paper sludge, which was provided by the Siam Kraft Industry Company Limited, Thailand, is the mixture of the primary and secondary sludge. The preliminary dewatered paper sludge was the raw material for both lab-scale and pilot-scale experiments. 2.2. Alternative Solid Fuel Production. 2.2.1. Lab-Scale Experiment. In the laboratory, the paper sludge was treated in a batch-type electric heater autoclave machine (MMJ-500, OM LabTech Co., Ltd., Japan), and its schematic is simply illustrated in Figure 1a. The experimental system consists of a reactor, an electric heater, and a condenser with a water-cooling bath. The size of the vertical reactor is 500 mL. At the beginning, pure water (Wako Pure Chemical Industries, Ltd. Japan) was mixed with the paper sludge as the mass ratio of 1:1 to simulate the HTT condition. The prepared sample was poured into the reactor assembled with a stirrer. It was kept rotating during the experiment to ensure the uniformity of the product. Before heating up, argon gas was introduced to create an oxygen free environment. The paper sludge was subjected to four temperatures variation, 180 °C, 200 °C, 220 °C, and 240 °C with a holding time of 30 min. After treatment, the pressure inside the reactor was released by discharging the steam through the condenser. Shortly after the pressure was reduced to the atmospheric condition, the product was thoroughly removed from the reactor as well as the condensate left in the condenser, and its piping for calculating the mass balance. Each 1199

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2.4. Process Analysis. To understand the process characteristics and ensure economic feasibility, the process analysis, which are mass balance, energy consumption, and energy recovery, was calculated. The mass balance was calculated by determining the mass input and output interactively between each process in both lab-scale and pilotscale experiments: (1) HTT, (2) mechanical dewatering, and (3) drying. The energy consumption and energy recovery were also determined to evaluate the sustainability of the process. The net energy consumption for treating the paper sludge was estimated based on both theoretical assumptions and the experimental data. The energy efficiency was calculated by using the ratio of energy output/ input determined by eq 126 and eq 2 shows the calculation of the energy input for the HTT in the laboratory.

(LPG). The operating condition was 1.9 MPa with an average temperature of 197 °C. When the temperature reached the desire state, the treatment process was instantly kept holding for 30 min same as the lab-scale condition. After the reaction was finished, the steam was flushed out to reduce the pressure inside the reactor. Finally, the products were extracted from the reactor directly by a drain valves after the pressure fell down to 0.5 MPa. In parallel to the lab-scale water removal process, the dewatering test of the pilot-scale experiment was performed by a 1.5-kW centrifugal decanter. After the hydrothermally treated paper sludge was taken out from the reactor, it was filled into a three-layer textile-made bag, which has 650-μm mesh in each layer, to prevent the loss of the sample. Before starting the dewatering test, two bags, which have the same amount of samples, were tested together across from each other in order to balance the centrifugal machine during the operation. The rotational speed was 960 rpm and the dewatering time was 5 min. It should be mentioned that the pressure exerting on the sample was calculated from the division of centrifugal force to the circular area of the sludge cake after centrifuging. After the dewatering test, the sludge cake was immediately taken out. To perform the natural drying test, the sludge cake was first manually crushed then it was filled in a 30 × 30 cm plate and placed indoors with an average temperature of 32 °C. The 24-h natural drying of both the raw material and treated paper sludge was done under the same conditions. All the operating conditions mentioned above are also summarized in Table 1. 2.3. Fuel Analysis. In the laboratory, the raw sludge and the product were dried at 105 °C in an electric oven and pulverized before analysis. The proximate and ultimate analysis were carried out as the dry basis with a Shimadzu 50 TGA/DTA analyzer and PerkinElmer 2400 Series II CHN organic elemental analyzer. The higher heating value (HHV) was determined by a Shimadzu CA-4PJ bomb calorimeter. In case of the pilot-scale raw material and product, the proximate analysis was conducted by the Leco’s TGA701 Thermogravimetric Analyzer. Leco’s TruSpec CHNS/O was used for the ultimate analysis and the HHV was measured by the Leco’s AC600 Bomb Calorimeter. The proximate analysis, ultimate analysis, and HHV of both raw and treated paper sludge are presented in Table 2.

energy output/input ratio =

energy input = [msludgec p(TT − Trm)] + [mmoisture(hg,T − hl,Trm)] (2) where msludge is the dried mass of the paper sludge; cp is the specific heat of the sludge; TT is the specific temperature condition for each experiment; Trm is the room temperature; hg,T is an enthalpy of the saturated vapor at the specific temperature; hl,Trm is the enthalpy of the saturated liquid at the room temperature, and mmoisture is the moisture content in the paper sludge. The calculation of the lab-scale experiment has been done under the following assumptions: (1) The water was heated from 25 °C to the saturated vapor water state in each specific temperature. (2) The energy consumption of the electrical stirrer and other electrical utilities were neglected. (3) The energy utilized in the dewatering experiment was negligible; however, the energy used for water evaporation was estimated by multiplying the amount of the moisture evaporated in the drying experiment with the latent heat for evaporation of the water (2,260 kJ/kg). (4) The specific heat of the sludge was assumed to be 1.7 kJ/(kg· K).27,28 For the pilot-scale experiment, the energy consumption, energy recovery, and energy balance of the entire solid fuel production process were determined based on the following assumptions: (1) The energy output was calculated from the energy recovery from the dried mass of the final product, which is the multiplication of the HHV and the dried mass of the product. (2) The energy input was the summation of the energy from the steam that used for achieving the HTT condition, the total electrical energy consumption, and the energy used for water evaporation.

Table 2. Fuel Properties pilot-scale (°C)

lab-scale (°C) items/conditions

raw

initial moisture ash volatile matter fixed carbon

76.0 34.1 62.1 3.9

carbon hydrogen nitrogen sulfur oxygen

31.3 4.6 2.2

H/C O/C HHV (MJ/kg)

1.76 0.67 12.7

180

200

220

240

raw

197

35.8 56.5 7.7

76.4 27.0 62.1 10.9

30.5 59.4 10.1 35.2 4.2 3.1 0.55 27.0 1.43 0.58 14.7

Proximate Analysis (%)

27.8

34.3 62.0 5.3 Ultimate 31.7 4.2 2.0

37.2 57.8 5.0 Analysis 30.8 3.9 1.7

37.0 56.5 6.5 (%) 32.4 3.9 1.4

35.6 4.2 2.0

27.8 26.4 Atomic Ratio 1.59 1.52 0.66 0.64 13.4 13.4

25.3

22.4

34.8 4.3 4.0 0.58 29.9

1.44 0.59 13.4

1.42 0.47 13.6

1.48 0.64 14.1

(HHV of product) × (dried mass) (energy input) (1)

3. RESULTS AND DISCUSSION 3.1. Product Analysis. 3.1.1. Appearance. The first row in Figure 2 shows the appearance of the HT pretreated paper

Scanning electron microscopy (SEM) was performed by JSM6610LA scanning electron microscope (JEOL Co., Ltd., Japan) to study surface morphology of the raw and treated paper sludge. Furthermore, Fourier transform infrared spectrometry (FTIR) was done by JIR-SPX200 FT-IR spectrometer (JEOL Co., Ltd., Japan). The fine particle samples were mixed with KBr and pelletized. Then, it was scanned from 400 to 4000 cm−1 with the resolution of 4 cm−1.

Figure 2. Appearance of raw and treated paper sludge from lab-scale experiment. 1200

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sludge from different treatment temperatures. It appears that the paper sludge became slurry after subjected to the HTT and the color became much darker. These trends are more and more noticeable when the treatment temperature increased. Moreover, it was observed that the paper sludge was liquefied at 240 °C since higher amount of liquid was obtained comparing with other lower temperature conditions. It could be ascribed to that the fiber structure of sludge, mainly in the forms of cellulose and hemicellulose,29 was broken and decomposed due to heat and pressure during the HTT process. Furthermore, the dried products from the different treatment temperatures are comparatively presented at the lower part of Figure 2 for a comparison. The nonuniformity of the raw paper sludge was found and the shrinkage was clearly observed after drying. The shrinkage was occurred because of the moisture evaporation. Comparing with the treated paper sludge, the shrinkage of the raw paper sludge after drying was identical, implying that the former had lower moisture content and was uniform. Figure 3 presents a comparison of the raw material, pilotscale hydrothermally treated paper sludge, and naturally dried

Figure 4. SEM images: (a) Raw; (b) 180 °C; (c) 200 °C; (d) 220 °C; (e) 240 °C; (f) 197 °C.

operating condition. In other words, when the HTT temperature was increased, more damages were occurred to the structure of the paper sludge leading to the decomposition. 3.1.2. Fuel Characteristic. Fuel properties of both raw and treated paper sludge given in Table 2 contain the proximate analysis, ultimate analysis, and HHV. The raw paper sludge has very high amount of ash and volatile matter while fixed carbon was considerably low resulting in poor fuel property in view of the energy density. However, after the HTT at 240 °C, the volatile matter was significantly decreased from 62.1% to 56.5% and the fixed carbon was approximately doubled from 3.9% to 7.7%, indicating the predictable higher energy density. The loss of the volatile matter or the combustible part could be attributed to the hydrolysis of the polysaccharide into glucose.33,34 Then, in an oxygen free environment, the polysaccharide can be decomposed into CO2 and water at the temperature of 180−250 °C.33 Additionally, the enhancement of the polymerization reaction due to the high temperature condition (240 °C) leads to the increase of fixed carbon. In the pilot-scale experiment, the fixed carbon was slightly decreased from 10.9% to 10.1% after subjected to the HTT. Besides, the volatile matter was also decreased from 62.1% to 59.4%. Table 2 also presents the ultimate analysis result. After the lab-scale HTT, the carbon content was slightly increased to from 31.3% to 35.6% after treated at 240 °C. The hydrogen and nitrogen content were reduced after the HTT. Nitrogen was originated from proteins contained in the secondary sludge (sewage sludge) as a composition of paper sludge. It was reported that proteins were decomposed above 150 °C; as a result, an organic nitrogen in protein was degraded to ammonium-N presence in the liquid part.35 Therefore, the reduction of nitrogen content in paper sludge after the HTT resulted from the decomposition of the organic nitrogen. The lower amount of nitrogen content after the HTT would benefit on the reduction of NOx emission during the combustion.36 In case of the pilot-scale experiment, the paper sludge after the

Figure 3. Appearance of raw and treated paper sludge from pilot-scale experiment.

product from the pilot-scale experiment. After the HTT, the product became slurry and more uniform similar to the labscale experiment. Furthermore, the particle size seemed to be smaller and stuck together like mud and the odor was much less, which was attributed to the suppression of the production of the smelly gases production (e.g., hydrogen sulfide) resulting from the sterilization of microbes in the raw paper sludge.30 Additionally, SEM micrographs were used to study the fibrous morphology of the paper sludge as shown in Figure 4. A large fibrous material can be observed in the raw paper sludge illustrated in Figure 4a. For the paper sludge treated at the temperature of 180 °C (Figure 4b) and 200 °C (Figure 4c), the big fibrous particle was vanished while the long chain fibrous material was still observable. Parts d and e of Figure 4 illustrate that most of the fibrous material was degraded when it was treated at higher treatment temperature especially at 240 °C. The SEM image of the pilot-scale product presented in Figure 4f also shows the same characteristic with the lab-scale product. The degradation of the paper sludge by HTT was devoted to its composition. Since the main components of the paper sludge is woody material, cellulose, hemicellulose, and lignin, it was reported that those materials were decomposed at temperature around 160−200 °C for hemicellulose and 240−350 °C for cellulose.31 Moreover, the essential decomposition at a temperature below 200 °C was attributed to the depolymerization of cellulose.32 Hence, the appearance and the morphology of the paper sludge after the HTT clearly indicated the effect of HTT on the decomposition depends on the severity of the 1201

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HTT showed the same characteristic with the lab-scale test as carbon content was increased from 34.6% to 35.0% while nitrogen content was decreased from 4.6% to 3.1%. Additionally, Table 2 shows the decrease of H/C and O/C atomic ratios, which can be visualized by Van Krevenlen diagram in Figure 5. The carbonization process of the paper sludge after

Figure 6. FTIR spectra of raw and treated paper sludge from lab-scale experiment.

(2) The broad band between 3600 and 3200 cm−1 was attributed to the characteristic band of cellulose that was the composition in the paper sludge. Particularly, the rounded tip band appeared in this region was a vibration of hydroxyl functional group, −OH stretching. Moreover, the HTT seemed to have no effect on the −OH band in cellulose since the reduction of peak intensity was not observed. However, compared to the cellulose,10,40 it was clearly shown that the peak of the paper sludge became less intense. The former might be ascribed to the source of paper sludge, which is derived from woody material that had already subjected to the pretreatment process by either thermo-mechanical or thermo-chemical processes, commonly known as pulping process.41,42 (3) The band around 2800 to 3000 cm−1 was observed, and it was assigned to a vibration of aliphatic −CH x stretching. There were two peaks at 2922 and 2852 cm−1. They could be attributed to a vibration of asymmetric C−H stretching in cellulose.43 (4) In this region, there were two identical peaks at 1645 and 1430 cm−1 and one tiny band at 1540 cm−1. The peak at 1645 cm−1 was originated by CN stretching vibration of amides.43 The absorption peak of the paper sludge after the HTT became weak as it was rounded especially when treated with the high temperature. Thus, it could be suggested that amides were hydrolyzed through hydrolysis reaction during the HTT. The band around 1430 cm−1 were attributed to −CHx aliphatic compound such as −CH2 and −CH3.10 For the treated paper sludge, the intensified of the band around 1430 cm−1 implied that the aliphatic compounds or nonaromatic compounds were obtained after the treatment. Finally, the tiny band around 1540 cm−1 was assigned to CO asymmetric stretching in carboxylic group.44,45 It was eliminated in case of the hydrothermally treated paper sludge indicating an occurrence of decarboxylation reaction. (5) The peaks 1160, 1112, and 1030 cm−1 were appeared in the paper sludge, and they became more intense in case of the HTT paper sludge. It could be attributed to C− O−C asymmetric stretching in aliphatic ether or C−O− C stretching in ether because of dehydration reaction of

Figure 5. Van Krevelen diagram of raw and treated paper sludge compared with coals.

HTT was presented along with the atomic ratio of coal: anthracite, bituminous, subbituminous, and lignite. The atomic ratio moving from upper right to lower left illustrates the advancement of the carbonization process. It was clearly observed that the dehydration and decarboxylation reactions played important roles in the carbonization process of paper sludge. When increasing the HTT temperature, the degree of carbonization was intensified. The paper sludge treated at 240 °C was nearly approached the region of lignite which was considered as a low rank coal. Although the atomic ratios of the pilot-scale samples were different, the carbonization process had the same characteristic with the lab-scale result. The higher heating value (HHV) of the products are also provided in Table 2, from the lab-scale experiment, the HHV got improved by 5.2%, 5.5%, 5.7%, and 6.8%, respectively, via increasing the treatment temperature from 180 to 240 °C, respectively. The highest HHV can achieve about 13.6 MJ/kg using the highest HTT temperature of 240 °C. It is similar to the results from the pilot-scale experiment. The HHV of the pilot-scale product was increased from 14.1 to 14.7 MJ/kg (4.5%). The amount of fixed carbon, which has much higher energy content than volatile matter, was increased resulting in the improvement of energy density. Therefore, the fuel property of paper sludge after HTT was comparable or even better than that of the raw paper sludge. 3.1.3. FTIR Analysis. Figure 6 shows FTIR spectra of raw and treated paper sludge as the variation of the HTT temperature. It should be noted that the peak around 2300 to 2400 cm−1 was attributed to uncontrollable CO2 in the measurement environment. The characteristic of the peak could be explained as follows: (1) The two weak bands at 3697 and 3620 cm−1 implied the kaolinite were common additional substance for improving paper quality.37−39 With the increasing of HTT temperature, those two peaks had no remarkable difference. 1202

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Figure 7. Moisture content during the water removal process: (a) lab-scale; (b) pilot-scale.

the other hand, the paper sludge without the HTT still has a moisture content more than 25%. For the pilot-scale experiment, both the raw and hydrothermally treated paper sludge were subjected to the dewatering test by the centrifugal decanter, which generates the pressure approximately 0.5 MPa to the samples. Figure 7b presents the graphical result of the dewatering and drying performances. The moisture content of the raw paper sludge was initially 76.4%, and it was decreased to 72.3% after dewatering showing 5.4% moisture reduction rate. After the HTT, the moisture content was increased from 76.4% to 83.6% because of the steam injected to form the HTT condition. Then, it was reduced to 61.8% by the centrifugal dewatering presenting a reduction of 26.0%. The dewatering performance was significantly improved since the HTT paper sludge showed approximately five times higher than that of the raw paper sludge. After the 10-h natural drying, the moisture content of the raw material, which has 72.3% moisture left after dewatering, was decreased to 69.6%, reducing 3.7%. In case of the HTT paper sludge, the moisture content was reduced from 61.8% to 57.0%, performing 7.8% reduction rate. The drying performance of the treated paper sludge was doubled indicating that the HTT has a positive benefit to the drying process. The results from both lab-scale and pilot-scale experiments show that the paper sludge has better dewatering and drying performances after subjected to the HTT. It could be explained that cell structures of sludge have been crushed and bound water was released.19 Generally, the moisture in the sludge contains free water, surface water, interstitial water, and bound water. At ∼105 °C, the free and surface water can be evaporated whereas the interstitial and bound water need the higher temperature up to 400 °C to evaporate.48 Thus, the raw paper sludge has the difficulties in the water removal process due to the interstitial and bound water was still contained in the cell boundary. By employing the HTT, the interstitial and bound water could be dewatered and evaporated easily. The remanent moisture in the dewatered product was about 61.0% for both lab-scale (200 °C) and pilot-scale (197 °C) experiments. However, the results of the drying performance were quite different. From the pilot-scale result, the moisture left in the 24-h naturally dried product was approximately 52% while the lab-scale dried products contained about 15.0% moisture content within 10 h. The drying performance of the

alcohol. When the treatment temperature was higher than 200 °C, the peaks had no obvious difference. Additionally, the peak at 1030 cm−1 might be attributed to Si−O stretching vibration indicating the presence of SiO2 in the sludge.45,46 (6) In the last region, the small peak at 875 cm−1 might be devoted to CaCO3 as another additional substance for paper production process.10 3.2. Dewatering and Drying Performances. In this study, the water removal process contains dewatering and drying. As the moisture content in solid fuel greatly affects the combustion performance such as fuel consumption and combustion efficiency. The availability of moisture could benefit for the combustion process; however, when higher than 30%, it significantly decreased the burning rate and ignition front propagation.47 Thus, the effect of the HTT on the dewatering and drying performances was studied. Initially, the moisture content of the raw paper sludge was 76.0% (lab) and 76.4% (pilot), which was tremendously difficult to be dewatered. Figure 7a shows the graphical dewatering performance of the paper sludge before and after HTT. The moisture content in the raw paper sludge was reduced from 76.0% to 72.9% after the dewatering process; while it was decreased to 65.3% for paper sludge after treated at 180 °C. When the treatment temperature increased to 200 and 220 °C, the moisture content was reduced to 61.2% and 60.4% after the mechanical dewatering process. Moreover, it can be as low as 54.5% when it was treated with the 240 °C HTT temperature. After the mechanical dewatering, the moisture reduction rate of the raw paper sludge was 4.1% whereas the hydrothermally treated paper sludge showed, at least, 23%. It is five times higher than that of the raw paper sludge. Therefore, the dewatering performance was significantly improved after the HTT. Figure 7a also illustrates the forced convective drying results. The weight loss of the sample attributed to the evaporation of the moisture inside the sample. With the HTT, the moisture reduction rate was higher as can be observed by comparing the slope of the curve. The drying performance was slightly improved, since after 10-h drying test, the moisture content of the treated paper sludge was approximately reduced to 15%, which is low enough for utilization in a commercial boiler. On 1203

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temperature and pressure were used. Moreover, more steam was discharged before removing the product. Comparing the treated product with the dewatered product, a significant reduction was observed since majority of the product was the water, which was squeezed out. As a result, a large amount of dewatered liquid was obtained, and its amount was higher when treated at the higher HTT temperature. Finally, the recovered solid fuel after the drying test was reduced, and the evaporated liquid was decreased when increasing treatment temperature since the moisture was already removed during the mechanical dewatering process. The mass balance of the pilot-scale experiment was also presented in Figure 9. The raw paper sludge supplied to the HTT reactor was approximately 351 kg, and it was taken as the datum (100%). After the HTT, the total amounts of product from the reactor was about 361 kg (102.9%), which contained 256.3 kg (73.0%) of HT treated product and 104.9 kg (29.9%) of wastewater. After mechanical dewatering, the dewatered product was 109.8 kg (31.3%) while the dewatered liquid was 146.5 kg (41.7%). After natural drying, about 42 kg (12%) of the dried solid fuel was recovered while 67.9 (19.3%) of liquid was evaporated. The fuel recovery ratio was about 50.8% in the pilot-scale HTT process. When comparing the paper sludge treated at 200 °C (lab) and 197 °C (pilot), the fuel recovery was 94.6% and 50.8%. Even though the product extracting process during the pilot-scale experiment was carefully done, the mass loss seemed to be inevitable leading to the significant difference in the fuel recovery. 3.3.2. Energy Efficiency in Lab-Scale Experiment. Table 3 shows the estimated energy consumption and recovery result.

lab-scale product was much better because the drying condition were different. The drying condition in the laboratory was the convective force drying while it was the natural drying in the pilot-scale experiment. Obviously, the utilization of heated air with a well-circulated atmosphere can provide a better drying condition. 3.3. Process Analysis. 3.3.1. Mass Balance. Taking the raw paper sludge input as datum (100%), the mass distribution of the lab-scale experiments are shown in Figure 8. It should be

Figure 8. Mass distribution of lab-scale experiments as 100% raw paper sludge input.

noted that the additional water used to mock up HTT was also equal to the raw paper sludge input (one to one mass ratio). At 200 °C treatment condition, the product after the HTT was increased from 60.1 (100%) to 96.5 g (151.4%). After dewatering, the liquid part was 55.8 g (92.9%) while the solid product was approximately 35.1 g (58.5%). Then, in the drying test, 21.5 g (35.8%) of moisture was evaporated; the dried mass of the product was 13.7 g (22.7%). From the lab-scale solid fuel production process, the dried solid input was 24% (moisture content 76%), and 22.7% dried solid fuel was recovered accounting 94.6% of fuel recovery. When increasing the treatment temperature, the treated product by HTT was decreased. However, the condensate and blow-off gas were increased because the comparatively higher

Table 3. Energy Consumption and Recovery of the LabScale and Pilot-Scale Alternative Solid Fuel Production Processes energy input (%) condition (°C)

energy output (%)

hydrothermal

dewatering

drying

energy output/input ratio

180 200 220 240 197

102.0 104.0 103.0 101.0 141.0

67.3 72.3 74.6 79.6 63.9

N/A N/A N/A N/A 1.1

32.7 27.7 25.4 20.4 34.9

102.0 104.0 103.0 101.0 141.0

Figure 9. Mass balance of the pilot-scale experiment. 1204

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Figure 10. Energy balance of the pilot-scale alternative solid fuel production process.

4. CONCLUSION The results from the lab-scale and the pilot-scale experiments show that the HTT enhanced the fuel property of the paper sludge. The effects of the HTT on the paper sludge were reported as follows: (1) the produced solid fuel has slightly higher heating value; (2) the water removal performance, especially the mechanical dewatering performance, was significantly improved by the HTT; (3) the HTT consumed most of the energy in the alternative solid fuel production process and the high energy output/input ratio of the pilot plant implied the sustainability of the pilot-scale HTT process. Therefore, the HTT was not only able to produce the alternative solid fuel from paper sludge but also feasible for commercialization.

Both the lab-scale and pilot-scale solid fuel production processes show that the main energy consumption process was the HTT. It accounted for two-thirds and two-fifths of the total energy input for the lab-scale and pilot-scale experiments, respectively. When increasing the treatment temperature, the energy consumption of the HTT was higher whereas the energy required for the thermal drying was reduced gradually. This is because of the improvement in the mechanical dewatering performance, which dramatically reduced the moisture content of the treated paper sludge; therefore, the energy needed for evaporating the residual moisture was reduced. The electrical energy used for the centrifugal decanter was significantly small whereas the energy for drying was comparatively high in both experiments. The calculation of the ratio of energy output per input showed that the lab-scale result was feasible in theory; however, at high temperature as 240 °C, the energy consumption for HTT process seemed to be greatly increased leading to the impracticality since the recovery of the solid fuel would be reduced. The pilot-scale process had an encouraging high energy output/input ratio and indicated that it had potential to produce the alternative solid fuel from the paper sludge. 3.3.3. Energy Balance of the Pilot-Scale Plant. Figure 10 illustrates energy balance of the alternative solid fuel production process. The energy in the raw paper sludge as dry basis was utilized as the datum (100%). The main energy of the HTT process was for the boiler fuel, which accounted for 24.1% while the electrical energy for boiler utilities, motor of the reactor’s stirrer, and motor of the centrifugal decanter were significantly small. Its accumulation was lower than 1.0%. The recovered energy from the produced solid fuel was 53.1%. Half of the energy stored in the product could be comparatively utilized as the boiler fuel indicating the self-sustainability of the HTT pilot plant. Furthermore, the 200 °C (lab) and 197 °C (pilot) HTT conditions were compared in view of energy consumption since the small difference of the treatment temperature could be negligible. When scaling up, the required energy from the HTT was reduced by 15.4% contributing to the improvement of the energy output per input ratio.

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AUTHOR INFORMATION

Corresponding Authors

*Tel: +81-45-924-5507. Fax: +81-45-924-5518. E-mail: [email protected]. *E-mail: [email protected]. Author Contributions ∥

C.A. and P.Z. contributed equally. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is financially supported by the Siam Cement Public Company Limited (SCG). Mr. Areeprasert was financially supported by the Faculty of Engineering, Kasetsart University, Thailand. Dr. Zhao was also financially supported by the State Scholarship Fund of China under Grant No. 2011609050. The authors gratefully acknowledge Dr. Kriengkrai Suksankraisorn (SCG), Mr. Korakoch Phetdee (SCG) for the substantial support, and Dr. Masaru Tada from Center for Advanced Materials Analysis, Tokyo Institute of Technology, for FTIR analysis.

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