ARTICLE pubs.acs.org/EF
Effects of Hydrothermally Pretreated Sewage Sludge on the Stability and Dispersibilty of Slurry Fuel Using Pulverized Coal Se-Joon Park, Jong-Soo Bae, Dong-Wook Lee, Ho Won Ra, Jai-Chang Hong, and Young-Chan Choi* Clean Fossil Energy Research Center, Climate Change Technology Research Division, Korea Institute of Energy Research, 71-2 Jang-dong, Yuseong-gu, Daejeon, 305-343, Republic of Korea ABSTRACT: Some research has been carried out on the disposal of sewage sludge by gasification technology. Most of it was based on separate grinding and drying processes for pretreatment, since raw-sewage sludge contains considerable amounts of water and has extremely broad particle size distribution. In the present study, sewage sludge was hydrothermally pretreated—rather than being energy-intensively dried and ground—for CSWS (coalsludgewater slurry) preparation, which was assessed for suitability as a fluid fuel. Its apparent viscosity and solid concentration were compared with those of CWS (coalwater slurry). Sewage sludge enhanced coal slurry rheological properties, such as stability and dispersibility. The viscosity and solid content of CSWS remained constant in CSWS samples after 72 h stability testing, whereas those of CWS respectively decreased by ca. 1000 cP and 3.3%. Furthermore, there were no differences in particle size distribution and FT-IR spectra of the CSWSs between its top and bottom layers after 72 h.
1. INTRODUCTION Sewage sludge from wastewater is a potential energy resource, despite its high levels of pollutants, pathogenic bacteria, and odor. Combustion,1 carbonization,2 fertilization,3 anaerobic digestion4 and other techniques have been proposed to recycle sewage sludge, but these methods have disadvantages, including secondary pollution from heavy metals, polychlorinated biphenyls, and dioxin.5 Gasification technology involves the total conversion of its organic matter to syngas comprising H2, CO, CO2, and small quantities of CH4.6,7 Combustible gases, with calorific values between 2.55 and 3.2 MJ Nm3, can be obtained from sewage sludge gasification.8 Paterson et al.9 reported that sewage sludge increased both the calorific value of the syngas and carbon conversion during coal gasification. It has been reported that sewage sludge reduces ash fusion temperatures during cogasification with coal (Folgueras et al.10 and Li et al.11). Li et al.7 reported that sewage sludge formed a stable medium and prevented large coal particles from settling in CSWS (coalsludgewater slurry), allowing high solid content slurry to be economical pumped to entrainedflow gasifiers. Higher gasification efficiency is expected from the increased maximum solid concentration, since the thixotropy of sewage sludge can reduce the viscosity of CSWS.12 Gasification is a feasible sustainable energy conversion technology for the disposal of sewage sludge. It has been developed for industrial application in systems such as integrated gasification combined-cycle (IGCC) and coal-to-liquids (CTL) plants. This work reports the preparation and fluid fuel characteristics of CSWS to aid the development of sewage sludge use. Untreated sewage sludge contains ca. 80% moisture and has a broad PSD (particle size distribution) that affects fuel stability characteristics, such as viscosity, dispersion (or solid concentration), and atomization. It also contains pathogenic bacteria and odorous substances, which any thermochemical processing would be required to eliminate or reduce. Therefore, it requires pretreatment before use; hydrothermally pretreated sewage sludge was used in CSWS preparation, and the effects of the treated sewage r 2011 American Chemical Society
sludge on the stability and dispersibility of coal slurry were investigated.
2. EXPERIMENTAL SECTION 2.1. Materials. Shenhua coal was from Korea Southern Power Co., Ltd., and sewage sludge was from Daejeon Metropolitan City Facilities Management Corp. Proximate and ultimate analyses of the samples are listed in Table 1. The moisture content of the untreated sewage sludge was 81.71%. Ultimate analysis showed the carbon and heating values of sewage sludge to be less than half of those of coal, while its oxygen value was much higher. According to Jean-Henry,6 sewage sludge for some gasifiers, such as fixed or fluidized beds, requires a moisture content of less than 20%, otherwise energy consumption is too high. Moreover, entrained-flow gasifier fed with liquid-phase fuel positively requires pretreatment to reduce moisture content and cogasification with coal for energy-efficient gasification. 2.2. Methods. 2.2.1. Hydrothermal Pretreatment (HTP) Apparatus. The hydrothermal apparatus is outlined in Figure 1. It comprised a 1 L batch type reactor, a magnetic drive, an impeller, a pressure gauge, thermocouples on the top and bottom of the reactor, display and control panels, and valves for steam injection and ventilation. The reactor was designed to withstand up to 80 MPa and 300 °C. An external boiler injected saturated steam into the reactor, and a heating band was wired around the reactor to reduce water condensation upon direct contact of reactant with steam. Thermal treatment of activated and primary sewage sludge was first applied to reduce its water content.13 The water content of treated sludge can be reduced to less than 30% by mechanical methods such as filter pressing or centrifugal separation. The dewaterability of sludge is improved above 150180 °C;14 therefore, pretreatment was conducted at 200 °C for a 20 min holding period to avoid this controversy in the experiment. Received: March 28, 2011 Revised: August 8, 2011 Published: August 15, 2011 3934
dx.doi.org/10.1021/ef200893p | Energy Fuels 2011, 25, 3934–3939
Energy & Fuels
ARTICLE
Table 1. Proximate and Ultimate Analyses of Materials proximate analysisa (wt %)
a
ultimate analysisb (wt %) FC
C
H
N
O
S
HHVb (MJ/kg)
AFT (°C)c
7.26
0.72
28.94
5.01
3.97
59.63
1.73
12.17
1190
6.80
52.51
65.63
4.52
1.03
14.83
0.55
26.40
1150
sample
MC
VM
ash
sewage sludge
81.71
10.31
Shenhua coal
7.77
32.92
As received basis. b Dry basis. c Ash fusion temperature.
Figure 2. Hydrothermally treated sewage sludge.
Figure 1. The hydrothermal batch reactor.
Table 2. Particle Size Distribution of Coal and Sludge Sample volume under (%) particle size (μm)
Shenhua coal
sewage sludge after HTP
