Waste Management xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Waste Management journal homepage: www.elsevier.com/locate/wasman
Partial oxidation of landfill leachate in supercritical water: Optimization by response surface methodology Yanmeng Gong, Shuzhong Wang ⇑, Haidong Xu, Yang Guo, Xingying Tang Key Laboratory of Thermo-Fluid Science and Engineering, Ministry of Education, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China
a r t i c l e
i n f o
Article history: Received 24 December 2014 Accepted 13 April 2015 Available online xxxx Keywords: Landfill leachate Partial oxidation Supercritical water Response surface methodology
a b s t r a c t To achieve the maximum H2 yield (GYH2), TOC removal rate (TRE) and carbon recovery rate (CR), response surface methodology was applied to optimize the process parameters for supercritical water partial oxidation (SWPO) of landfill leachate in a batch reactor. Quadratic polynomial models for GYH2, CR and TRE were established with Box–Behnken design. GYH2, CR and TRE reached up to 14.32 mmolgTOC1, 82.54% and 94.56% under optimum conditions, respectively. TRE was invariably above 91.87%. In contrast, TC removal rate (TR) only changed from 8.76% to 32.98%. Furthermore, carbonate and bicarbonate were the most abundant carbonaceous substances in product, whereas CO2 and H2 were the most abundant gaseous products. As a product of nitrogen-containing organics, NH3 has an important effect on gas composition. The carbon balance cannot be reached duo to the formation of tar and char. CR increased with the increase of temperature and oxidation coefficient. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Sanitary landfill is widely employed in the disposal of municipal solid waste throughout the world with landfill leachate formation being an inevitable consequence. In the past several decades, some conventional landfill leachate treatments have been developed. However, a combination of chemical, physical and biological steps or an advanced method is required to meet the strict quality standards for direct discharge of leachate into surface water. Supercritical water (SCW, T > 647.15 K, P > 22.12 MPa) presents complete miscibility with oxygen and organics, making SCW a very suitable medium for the oxidation of organics (Bermejo and Cocero, 2006). Supercritical water oxidation (SCWO) has been regarded as a promising technology to treat landfill leachate effectively in recent years (Gong and Duan, 2010; Wang et al., 2011; Williams and Onwudili, 2006). During SCWO process, organics can be degraded into CO2 and H2O finally in a few minutes or even several seconds via homogeneous oxidation reaction with oxidant in SCW. Gong and Duan (2010) reported that chemical oxygen demand (COD) and biochemical oxygen demand (BOD) removal efficiencies of landfill leachate could reach up to 99.23% and 98.06%, respectively, at 703 K and 30 MPa in a continuous transpiring-wall SCWO reactor. It has been verified by Williams and Onwudili (2006) that almost complete oxidation of the organic components of the leachate could be achieved under SCWO ⇑ Corresponding author.
conditions. Moreover, MnO2 was an effective catalyst in destruction of landfill leachate (Wang et al., 2011). To date, Aquarden Technologies in Denmark has developed an operational SCWO-based solution for on-site treatment of leachate and recognized that SCWO technology could be instrumental in resolving the landfill leachate treatment challenge (Aquarden website, 2015). Typically, in a SCWO system, landfill leachate is preheated above critical temperature of water before it reacts with oxidant in a subsequent reactor, as shown in Fig. 1. It has been confirmed that significant tar and char will be generated in the preheating process of high concentration feedstock (Antal et al., 2000; Guo et al., 2013; Matsumura et al., 2006b). Sricharoenchaikul (2009) observed that the portion of tar and char reached 57.6% at 773 K and 30 MPa in the supercritical water gasification (SCWG) of 10 wt.% black liquor, whereas the portion decreased to 24.6% at 923 K. Actually, landfill leachate is a high concentration organic wastewater, in which the COD concentration could reach up to 35,000–50,000 mg L1 (Renou et al., 2008). Thus, it can be predictable that the leachate preheater and SCWO reactor might be subject to plugging by tar and char in a long running system, because once tar and char form they are difficult to be converted (Chuntanapum and Matsumura, 2010). Some research suggests that the plugging problem caused by tar and char might be solved by increasing heating rates of feedstock (Matsumura et al., 2006a; Sınag˘ et al., 2004; Watanabe et al., 2005), adding catalyst (Matsumura et al., 2005; Xu and Lancaster, 2008) or adding a small amount of oxidant (Guo et al.,
http://dx.doi.org/10.1016/j.wasman.2015.04.013 0956-053X/Ó 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Gong, Y., et al. Partial oxidation of landfill leachate in supercritical water: Optimization by response surface methodology. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.04.013
2
Y. Gong et al. / Waste Management xxx (2015) xxx–xxx
Fig. 1. Reaction fields in SCWO process for landfill leachate.
2010; Jin et al., 2010; Xu et al., 2011). The heat resistance exists inevitably in the process of external heating method, showing that it is difficult to receive a rapid heating of landfill leachate (Jin et al., 2010). Although Na2CO3, K2CO3 and NaOH can be used to inhibit the formation of char and tar in SCWG of biomass, these alkaline catalysts may cause corrosion and plugging of reactor (Sınag˘ et al., 2004). Supercritical water partial oxidation (SWPO) involves carrying out oxidative reactions in SCW in the presence of sub-stoichiometric quantities of an oxidant. The key advantage of SWPO process is the use of partial oxidation in-situ to flash heat the feedstock through the sensitive temperature range, resulting in less char or tar formation and high hydrogen yield (Johanson et al., 2001). To date, little attention has been paid to partial oxidation of landfill leachate in SCW. In this work, landfill leachate SWPO was performed in a batch SCWO reactor investigating the effects of oxidation coefficient, reaction time, temperature and pressure on the process with response surface methodology (RSM). Another major goal was to obtain the optimum operating parameters to achieve the maximum hydrogen yield and TOC removal rate and minimum generation of tar and char. 2. Materials and methods 2.1. Leachate characteristics Landfill leachate was collected in November from a municipal landfill site on Xi’an Shaanxi Province, China, whose characteristics are listed in Table 1. The landfill is received only municipal solid waste since it was in operation in 1994 with the total volume of 49 million m3. Hydrogen peroxide solution with mass fraction of 30% served as the oxidant. 2.2. Apparatus and procedure Fig. 2 illustrates the experimental apparatus in this study. The batch reactor was made of Hastelloy-C276 alloy with a net capacity of 350 mL which was designed to a maximum temperature and pressure of 923 K and 32 MPa, respectively. The temperature was
Fig. 2. Schematic diagram of the batch reactor (1) gas bag; (2) low-pressure gauge; (3) high-pressure gauge; (4) safety valve; (5) thermocouple; (6) cooling water inlet; (7) cooling water outlet; (8) cooling coil; (9) kettle; (10) kettle cover; (11) electric heater; (12) temperature controller; (13) power source; (14) nitrogen cylinder; (15) needle valve; (16) check valve.
monitored by a thermocouple and adjusted by a temperature controller automatically. A high-pressure gauge was applied to determine the inner-pressure of reactor when the reactor was heating, and another low-pressure gauge was used to measure the pressure of products after the reactor was cooled to room temperature. The measuring ranges of these two pressure gages were 40 MPa and 1.6 MPa, with accuracy of ±0.64 MPa and ±25.6 kPa, respectively. A dip tube was extended to the bottom of reactor for purging the whole reactor by N2. The experiments were performed using the following procedure for safety and operation convenience. According to the thermodynamic property of water, the volumes of landfill leachate and H2O2 solution can be calculated for a given oxidation coefficient, temperature and pressure. At the beginning of each experiment, quantitative landfill leachate and H2O2 solution were charged into the reactor before the reactor was sealed. The whole system was purged by N2 for 10 min to sweep the air. Afterward, the switch of heating power was turned on and the temperature controller was adjusted to the selected values. As soon as the temperature was reached the setting value, the reactor was held for a specified time. When the desired reaction time has elapsed, the electric heater was stopped immediately and the reactor was cooled to room temperature rapidly by cooling water. Gaseous and liquid products were collected and analyzed at the end of each experiment. In this work, we repeated each experiment three times. 2.3. Experimental analysis The analysis of gaseous products occurred on a gas chromatograph (GC-112A, Shanghai) with a thermal conductivity detector (TCD) and a 3 m 3 mm TDX-01 packed column for the detection of H2, CO, CH4 and CO2. Helium served as the carrier gas, with the flow rate and pressure of 18.5 mLmin1 and 0.42 MPa, respectively. The total organic carbon (TOC) and total carbon (TC) of the liquid products were monitored by a TOC analyzer (ET1020A, EURO
Table 1 Leachate characteristics. pH 8.04 ± 0.02
COD (mgL1)
NH4–N (mgL1)
TN (mgL1)
TC (mgL1)
TOC (mgL1)
6633 ± 330
393 ± 54
3800 ± 82
9650.3 ± 467.2
4079.0 ± 376.0
Please cite this article in press as: Gong, Y., et al. Partial oxidation of landfill leachate in supercritical water: Optimization by response surface methodology. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.04.013
3
Y. Gong et al. / Waste Management xxx (2015) xxx–xxx
TECH (Far East) Ltd., Shanghai). Chemical oxygen demand (COD), ammonia nitrogen (NH4–N) and total nitrogen (TN) were conducted by individual Merck cell test via the Spectroscopy NOVA 60 spectrophotometer. TOC removal rate (TRE, %), TC removal rate (TR, %), carbon recovery rate (CR, %) and oxidation coefficient (a) were defined as follows:
½TOC0 - ½TOC 100% ½TOC0 ½TC0 - ½TC TR ¼ 100% ½TC0 ½Cg þ½TC CR ¼ 100% ½TC0 ½O a ¼ 2 add ½O2 need TRE ¼
ð2Þ ð3Þ ð4Þ
2.4. Statistical analyses In this section, the effects of the independent variables on the responses and interactions between any two variables were investigated using a Box–Behnken design (BBD) coupled with an analysis following response surface methodology (RSM). BBD suggests an experimental number by Eq. (5). 2
N ¼ k þ 2k þ cp
Code
Real values of coded level 1
0
1
Temperature (T/K) Pressure (P/MPa) Oxidation coefficient (a) Reaction time (t/s)
A B C D
723 23 0.1 300
773 25 0.25 600
823 27 0.4 900
independent variables and levels are presented in Table 2. Table 3 shows the total number of 30 experiments as BBD method. Data in Table 3 show that the reaction time in this batch study is quite long compared with the continuous system in the literatures (Civan et al., 2015; Gong and Duan, 2010), so it might be considered that the results were in equilibrium condition. TRE in landfill leachate SWPO was invariably above 91.87%, with a maximum value of 96.34% at 823 K, 25 MPa, 600 s and a = 0.4. In contrary, TR only changed from 8.76% to 32.98% at the same time. These results suggest that adding small amounts of oxidant (a 6 0.4) could accelerate the degradation of organics, resulting in a liquid product rich in inorganic carbon, and carbonate and bicarbonate became the main carbon-containing material. Furthermore, NH4–N concentrations in liquid products changed from 2409.46 mgL1 to 3233.78 mgL1, which were far higher than that in landfill leachate (393 mgL1). NH3 is generally considered to be a product of nitrogen-containing compounds in SCWG (Guo et al., 2013; Lee and Park, 1996), because SCW can donate proton for converting N in organics to NH3 (Ogunsola, 2000). As shown in Table 1, TN in raw landfill leachate is 3800 mgL1, which is close to NH4–N concentrations in liquid products. Thus, it can be confirmed that NH3 is the main product of nitrogenous substance in landfill leachate SWPO. SWPO cannot effectively remove NH4– N in landfill leachate; on the contrary, it will lead to an increase in NH4–N concentration. NH4–N will be the primary reactant in a subsequent SCWO reactor.
ð5Þ
where N is the experimental number; k is the factor number and cp is the replicate number of the central point (Zhang et al., 2010). Each response can be described as a second-order polynomial model by using Design Expert software (version 8.0) in all experimental regions, and the quadratic equation for each response is shown in Eq. (6) (Liu et al., 2012) k k k X k X X X bi X i þ bii X 2i þ bij X i X j i¼1
Variables
ð1Þ
where [TOC]0 (mg) and [TC]0 (mg) represent the initial mass of TOC and TC in landfill leachate loaded into the reactor, while [TOC] (mg) and [TC] (mg) represent the mass of TOC and TC in liquid products, respectively. Furthermore, [C]g (mg) stands for the mass of carbon in gaseous products (mg). [O2]add (mg) represents the initial quality of O2 decomposed by H2O2 and [O2]need (mg) is the mass of oxidant for complete oxidation of landfill leachate by stoichiometry calculation, respectively. Gas yield (GY, mmolgTOC1) is defined as the mole of certain gas product divided by [TOC]0, such as hydrogen yield (GYH2).
Y ¼ b0 þ
Table 2 Independent variables and levels.
i¼1
ij
ð6Þ
j
In Eq. (6), Y represents response, Xi and Xj are the coded values of the independent variables, b0 is the constant coefficient, bi, bii and bij are the coefficients of linear, quadratic and interaction term, respectively. The results were completely analyzed by using the analysis of variance (ANOVA). 3. Results and discussion 3.1. Experimental results Temperature, pressure, oxidation coefficient and reaction time have significant effect on the partial oxidation of waste or model compounds in SCW (Guo et al., 2010; Youssef et al., 2010). Guo et al. (2010) has been confirmed that the maximum H2 mol fraction was obtained in municipal sludge SWPO when oxidation coefficient was 0.3. Hence, the experimental central point in BBD in this work was set to be 25 MPa, 600 s, 773.15 K and a = 0.25, because of operational security and maximum hydrogen yield. The four
3.2. Analysis of response surface for TRE Table 4 shows the ANOVA results for TRE. Generally, values of p-value less than 0.05 indicate that model terms are significant, whereas the values greater than 0.1 are usually considered as insignificant (Arslan-Alaton et al., 2009). Adequate precision (AP) is a measure of the range in predicted response relative to its associate error and its required value is 4 or more (Aghamohammadi et al., 2007). In the present study, the AP of 22.676 was obtained for TRE. The square of correlation coefficient for each response was computed as the determination coefficient (R2). A high R2 means that the calculated model agrees well with experimental data within the range of experiments (Bashir et al., 2010). According to Joglekar and May (1987), R2 should not be less than 0.8 for a reasonable model. Furthermore, the predicted determination coefficient (Pred R2) should be in a reasonable agreement with adjusted determination coefficient (Adj R2) (Lee et al., 2010). The coefficient of variation (CV), defined as the percent ratio between the standard deviation (SD) and the mean value is a measure of the reproducibility of the model and a model can be considered reasonably reproducible if its CV is not larger than 10% (Li et al., 2010). As Table 4 shows, the Model F-value of 32.93 and a low p-value ( F
Model A-T (K) B-P (MPa) C-a D-t (s) AB AC B2 C2 Residual Lack of fit Pure error
30.23 11.33 1.23 15.37 0.45 0.48 0.41 0.34 0.51 2.41 2.17 0.24
8 1 1 1 1 1 1 1 1 21 16 5
3.78 11.33 1.23 15.37 0.45 0.48 0.41 0.34 0.51 0.11 0.14 0.047
32.93 98.78 10.69 133.95 3.93 4.18 3.6 2.98 4.42
CO, even the CO yield can be neglected compared with other gases, but the amount order will change to CO2 > H2 > CH4 > CO only if the oxidation coefficient is large enough. As shown in Table 3, the variation ranges of GYH2, GYCH4, GYCO2 and GY were 2.37– 14.58, 0.64–3.36, 3.24–14.41 and 7.07–34.53 mmolgTOC1, respectively. The maximum GYH2 occurred at 823 K, 27 MPa and 600 s with a fixed oxidation coefficient of 0.25. A possible explanation for the absence of CO in gaseous products can be written as below: Steam reforming reaction
Cm Hn Op Nq þ H2 O ! CO þ H2 þ CO2 þ NH3 þ other hydrocarbon
ð8Þ
Water gas shift reaction
CO þ H2 O $ CO2 þ H2
ð9Þ
Methanation reaction
CO þ 3H2 $ CH4 þ H2 O
ð10Þ
Oxidation reaction Fig. 5. Perturbation plot of independent variables for TRE.
CO þ 1=2O2 ! CO2
ð11Þ
Please cite this article in press as: Gong, Y., et al. Partial oxidation of landfill leachate in supercritical water: Optimization by response surface methodology. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.04.013
6
Y. Gong et al. / Waste Management xxx (2015) xxx–xxx
Fig. 6. 3D and contour plots of TRE as a function of P and T (a = 0.25, t = 600 s).
Fig. 7. 3D and contour plots for TRE as a function of T and a (P = 25 MPa, t = 600 s).
H2 þ 1=2O2 ! H2 O
ð12Þ
Other reactions þH2 O
þH2 O
! CO2 þ2NH3 ! 2NHþ4 þ CO2 2NHþ4 þ 2HCO3 3
ð13Þ
Early work presented that biomass gasification in SCW mainly includes three reactions: steam reforming reaction, water gas shift reaction and methanation reaction (Kıpçak and Akgün, 2012). In the present study, we also assume that these three types of reactions are involved in partial oxidation of landfill leachate in SCW, as shown in Eqs. 8–10. High concentrations of NH4–N were detected in liquid products. Thus, it can be speculated that NH3 might be the product of steam reforming reaction of nitrogen-containing organics in leachate (Eq. (8)). The amount of CO will decrease as water gas shift reaction (Eq. (9)) and methanation reaction (Eq. (10)) carry out. Meanwhile, GYCH4 is significantly lower than GYH2 and GYCO2, suggesting that methanation reaction in Eq. (10) is weak. If a small amount of oxidant is added, CO and H2
will be converted to CO2 and H2O via oxidation reaction, respectively (Guo et al., 2010; Kıpçak and Akgün, 2012). Further, NH3 may have an important effect on the components of gaseous products for the process. As shown in Eq. (13), NH3 reacts with CO2 in water to form ammonium, carbonate and bicarbonate continuously. The decrease of CO2 in gas will reinforce the water gas shift reaction and oxidation reaction of CO, leading to the absence of CO ultimately. The generation of carbonate and bicarbonate may explain the reason why a low TR (8.76–32.98%) occurred when TRE was invariably above 91.87% in the experiments. The ANOVA results for GYH2, GYCH4, GYCO2 and GY were summarized in Table 5. F-values of 29.24, 10.64, 111.34 and 58.38 with a low p-value ( F
CV
R2
Adj R2
Pred R2
AP
GYH2 GYCH4 GYCO2 GY
29.24 10.64 111.34 66.03
Contents lists available at ScienceDirect
Waste Management journal homepage: www.elsevier.com/locate/wasman
Partial oxidation of landfill leachate in supercritical water: Optimization by response surface methodology Yanmeng Gong, Shuzhong Wang ⇑, Haidong Xu, Yang Guo, Xingying Tang Key Laboratory of Thermo-Fluid Science and Engineering, Ministry of Education, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China
a r t i c l e
i n f o
Article history: Received 24 December 2014 Accepted 13 April 2015 Available online xxxx Keywords: Landfill leachate Partial oxidation Supercritical water Response surface methodology
a b s t r a c t To achieve the maximum H2 yield (GYH2), TOC removal rate (TRE) and carbon recovery rate (CR), response surface methodology was applied to optimize the process parameters for supercritical water partial oxidation (SWPO) of landfill leachate in a batch reactor. Quadratic polynomial models for GYH2, CR and TRE were established with Box–Behnken design. GYH2, CR and TRE reached up to 14.32 mmolgTOC1, 82.54% and 94.56% under optimum conditions, respectively. TRE was invariably above 91.87%. In contrast, TC removal rate (TR) only changed from 8.76% to 32.98%. Furthermore, carbonate and bicarbonate were the most abundant carbonaceous substances in product, whereas CO2 and H2 were the most abundant gaseous products. As a product of nitrogen-containing organics, NH3 has an important effect on gas composition. The carbon balance cannot be reached duo to the formation of tar and char. CR increased with the increase of temperature and oxidation coefficient. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Sanitary landfill is widely employed in the disposal of municipal solid waste throughout the world with landfill leachate formation being an inevitable consequence. In the past several decades, some conventional landfill leachate treatments have been developed. However, a combination of chemical, physical and biological steps or an advanced method is required to meet the strict quality standards for direct discharge of leachate into surface water. Supercritical water (SCW, T > 647.15 K, P > 22.12 MPa) presents complete miscibility with oxygen and organics, making SCW a very suitable medium for the oxidation of organics (Bermejo and Cocero, 2006). Supercritical water oxidation (SCWO) has been regarded as a promising technology to treat landfill leachate effectively in recent years (Gong and Duan, 2010; Wang et al., 2011; Williams and Onwudili, 2006). During SCWO process, organics can be degraded into CO2 and H2O finally in a few minutes or even several seconds via homogeneous oxidation reaction with oxidant in SCW. Gong and Duan (2010) reported that chemical oxygen demand (COD) and biochemical oxygen demand (BOD) removal efficiencies of landfill leachate could reach up to 99.23% and 98.06%, respectively, at 703 K and 30 MPa in a continuous transpiring-wall SCWO reactor. It has been verified by Williams and Onwudili (2006) that almost complete oxidation of the organic components of the leachate could be achieved under SCWO ⇑ Corresponding author.
conditions. Moreover, MnO2 was an effective catalyst in destruction of landfill leachate (Wang et al., 2011). To date, Aquarden Technologies in Denmark has developed an operational SCWO-based solution for on-site treatment of leachate and recognized that SCWO technology could be instrumental in resolving the landfill leachate treatment challenge (Aquarden website, 2015). Typically, in a SCWO system, landfill leachate is preheated above critical temperature of water before it reacts with oxidant in a subsequent reactor, as shown in Fig. 1. It has been confirmed that significant tar and char will be generated in the preheating process of high concentration feedstock (Antal et al., 2000; Guo et al., 2013; Matsumura et al., 2006b). Sricharoenchaikul (2009) observed that the portion of tar and char reached 57.6% at 773 K and 30 MPa in the supercritical water gasification (SCWG) of 10 wt.% black liquor, whereas the portion decreased to 24.6% at 923 K. Actually, landfill leachate is a high concentration organic wastewater, in which the COD concentration could reach up to 35,000–50,000 mg L1 (Renou et al., 2008). Thus, it can be predictable that the leachate preheater and SCWO reactor might be subject to plugging by tar and char in a long running system, because once tar and char form they are difficult to be converted (Chuntanapum and Matsumura, 2010). Some research suggests that the plugging problem caused by tar and char might be solved by increasing heating rates of feedstock (Matsumura et al., 2006a; Sınag˘ et al., 2004; Watanabe et al., 2005), adding catalyst (Matsumura et al., 2005; Xu and Lancaster, 2008) or adding a small amount of oxidant (Guo et al.,
http://dx.doi.org/10.1016/j.wasman.2015.04.013 0956-053X/Ó 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Gong, Y., et al. Partial oxidation of landfill leachate in supercritical water: Optimization by response surface methodology. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.04.013
2
Y. Gong et al. / Waste Management xxx (2015) xxx–xxx
Fig. 1. Reaction fields in SCWO process for landfill leachate.
2010; Jin et al., 2010; Xu et al., 2011). The heat resistance exists inevitably in the process of external heating method, showing that it is difficult to receive a rapid heating of landfill leachate (Jin et al., 2010). Although Na2CO3, K2CO3 and NaOH can be used to inhibit the formation of char and tar in SCWG of biomass, these alkaline catalysts may cause corrosion and plugging of reactor (Sınag˘ et al., 2004). Supercritical water partial oxidation (SWPO) involves carrying out oxidative reactions in SCW in the presence of sub-stoichiometric quantities of an oxidant. The key advantage of SWPO process is the use of partial oxidation in-situ to flash heat the feedstock through the sensitive temperature range, resulting in less char or tar formation and high hydrogen yield (Johanson et al., 2001). To date, little attention has been paid to partial oxidation of landfill leachate in SCW. In this work, landfill leachate SWPO was performed in a batch SCWO reactor investigating the effects of oxidation coefficient, reaction time, temperature and pressure on the process with response surface methodology (RSM). Another major goal was to obtain the optimum operating parameters to achieve the maximum hydrogen yield and TOC removal rate and minimum generation of tar and char. 2. Materials and methods 2.1. Leachate characteristics Landfill leachate was collected in November from a municipal landfill site on Xi’an Shaanxi Province, China, whose characteristics are listed in Table 1. The landfill is received only municipal solid waste since it was in operation in 1994 with the total volume of 49 million m3. Hydrogen peroxide solution with mass fraction of 30% served as the oxidant. 2.2. Apparatus and procedure Fig. 2 illustrates the experimental apparatus in this study. The batch reactor was made of Hastelloy-C276 alloy with a net capacity of 350 mL which was designed to a maximum temperature and pressure of 923 K and 32 MPa, respectively. The temperature was
Fig. 2. Schematic diagram of the batch reactor (1) gas bag; (2) low-pressure gauge; (3) high-pressure gauge; (4) safety valve; (5) thermocouple; (6) cooling water inlet; (7) cooling water outlet; (8) cooling coil; (9) kettle; (10) kettle cover; (11) electric heater; (12) temperature controller; (13) power source; (14) nitrogen cylinder; (15) needle valve; (16) check valve.
monitored by a thermocouple and adjusted by a temperature controller automatically. A high-pressure gauge was applied to determine the inner-pressure of reactor when the reactor was heating, and another low-pressure gauge was used to measure the pressure of products after the reactor was cooled to room temperature. The measuring ranges of these two pressure gages were 40 MPa and 1.6 MPa, with accuracy of ±0.64 MPa and ±25.6 kPa, respectively. A dip tube was extended to the bottom of reactor for purging the whole reactor by N2. The experiments were performed using the following procedure for safety and operation convenience. According to the thermodynamic property of water, the volumes of landfill leachate and H2O2 solution can be calculated for a given oxidation coefficient, temperature and pressure. At the beginning of each experiment, quantitative landfill leachate and H2O2 solution were charged into the reactor before the reactor was sealed. The whole system was purged by N2 for 10 min to sweep the air. Afterward, the switch of heating power was turned on and the temperature controller was adjusted to the selected values. As soon as the temperature was reached the setting value, the reactor was held for a specified time. When the desired reaction time has elapsed, the electric heater was stopped immediately and the reactor was cooled to room temperature rapidly by cooling water. Gaseous and liquid products were collected and analyzed at the end of each experiment. In this work, we repeated each experiment three times. 2.3. Experimental analysis The analysis of gaseous products occurred on a gas chromatograph (GC-112A, Shanghai) with a thermal conductivity detector (TCD) and a 3 m 3 mm TDX-01 packed column for the detection of H2, CO, CH4 and CO2. Helium served as the carrier gas, with the flow rate and pressure of 18.5 mLmin1 and 0.42 MPa, respectively. The total organic carbon (TOC) and total carbon (TC) of the liquid products were monitored by a TOC analyzer (ET1020A, EURO
Table 1 Leachate characteristics. pH 8.04 ± 0.02
COD (mgL1)
NH4–N (mgL1)
TN (mgL1)
TC (mgL1)
TOC (mgL1)
6633 ± 330
393 ± 54
3800 ± 82
9650.3 ± 467.2
4079.0 ± 376.0
Please cite this article in press as: Gong, Y., et al. Partial oxidation of landfill leachate in supercritical water: Optimization by response surface methodology. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.04.013
3
Y. Gong et al. / Waste Management xxx (2015) xxx–xxx
TECH (Far East) Ltd., Shanghai). Chemical oxygen demand (COD), ammonia nitrogen (NH4–N) and total nitrogen (TN) were conducted by individual Merck cell test via the Spectroscopy NOVA 60 spectrophotometer. TOC removal rate (TRE, %), TC removal rate (TR, %), carbon recovery rate (CR, %) and oxidation coefficient (a) were defined as follows:
½TOC0 - ½TOC 100% ½TOC0 ½TC0 - ½TC TR ¼ 100% ½TC0 ½Cg þ½TC CR ¼ 100% ½TC0 ½O a ¼ 2 add ½O2 need TRE ¼
ð2Þ ð3Þ ð4Þ
2.4. Statistical analyses In this section, the effects of the independent variables on the responses and interactions between any two variables were investigated using a Box–Behnken design (BBD) coupled with an analysis following response surface methodology (RSM). BBD suggests an experimental number by Eq. (5). 2
N ¼ k þ 2k þ cp
Code
Real values of coded level 1
0
1
Temperature (T/K) Pressure (P/MPa) Oxidation coefficient (a) Reaction time (t/s)
A B C D
723 23 0.1 300
773 25 0.25 600
823 27 0.4 900
independent variables and levels are presented in Table 2. Table 3 shows the total number of 30 experiments as BBD method. Data in Table 3 show that the reaction time in this batch study is quite long compared with the continuous system in the literatures (Civan et al., 2015; Gong and Duan, 2010), so it might be considered that the results were in equilibrium condition. TRE in landfill leachate SWPO was invariably above 91.87%, with a maximum value of 96.34% at 823 K, 25 MPa, 600 s and a = 0.4. In contrary, TR only changed from 8.76% to 32.98% at the same time. These results suggest that adding small amounts of oxidant (a 6 0.4) could accelerate the degradation of organics, resulting in a liquid product rich in inorganic carbon, and carbonate and bicarbonate became the main carbon-containing material. Furthermore, NH4–N concentrations in liquid products changed from 2409.46 mgL1 to 3233.78 mgL1, which were far higher than that in landfill leachate (393 mgL1). NH3 is generally considered to be a product of nitrogen-containing compounds in SCWG (Guo et al., 2013; Lee and Park, 1996), because SCW can donate proton for converting N in organics to NH3 (Ogunsola, 2000). As shown in Table 1, TN in raw landfill leachate is 3800 mgL1, which is close to NH4–N concentrations in liquid products. Thus, it can be confirmed that NH3 is the main product of nitrogenous substance in landfill leachate SWPO. SWPO cannot effectively remove NH4– N in landfill leachate; on the contrary, it will lead to an increase in NH4–N concentration. NH4–N will be the primary reactant in a subsequent SCWO reactor.
ð5Þ
where N is the experimental number; k is the factor number and cp is the replicate number of the central point (Zhang et al., 2010). Each response can be described as a second-order polynomial model by using Design Expert software (version 8.0) in all experimental regions, and the quadratic equation for each response is shown in Eq. (6) (Liu et al., 2012) k k k X k X X X bi X i þ bii X 2i þ bij X i X j i¼1
Variables
ð1Þ
where [TOC]0 (mg) and [TC]0 (mg) represent the initial mass of TOC and TC in landfill leachate loaded into the reactor, while [TOC] (mg) and [TC] (mg) represent the mass of TOC and TC in liquid products, respectively. Furthermore, [C]g (mg) stands for the mass of carbon in gaseous products (mg). [O2]add (mg) represents the initial quality of O2 decomposed by H2O2 and [O2]need (mg) is the mass of oxidant for complete oxidation of landfill leachate by stoichiometry calculation, respectively. Gas yield (GY, mmolgTOC1) is defined as the mole of certain gas product divided by [TOC]0, such as hydrogen yield (GYH2).
Y ¼ b0 þ
Table 2 Independent variables and levels.
i¼1
ij
ð6Þ
j
In Eq. (6), Y represents response, Xi and Xj are the coded values of the independent variables, b0 is the constant coefficient, bi, bii and bij are the coefficients of linear, quadratic and interaction term, respectively. The results were completely analyzed by using the analysis of variance (ANOVA). 3. Results and discussion 3.1. Experimental results Temperature, pressure, oxidation coefficient and reaction time have significant effect on the partial oxidation of waste or model compounds in SCW (Guo et al., 2010; Youssef et al., 2010). Guo et al. (2010) has been confirmed that the maximum H2 mol fraction was obtained in municipal sludge SWPO when oxidation coefficient was 0.3. Hence, the experimental central point in BBD in this work was set to be 25 MPa, 600 s, 773.15 K and a = 0.25, because of operational security and maximum hydrogen yield. The four
3.2. Analysis of response surface for TRE Table 4 shows the ANOVA results for TRE. Generally, values of p-value less than 0.05 indicate that model terms are significant, whereas the values greater than 0.1 are usually considered as insignificant (Arslan-Alaton et al., 2009). Adequate precision (AP) is a measure of the range in predicted response relative to its associate error and its required value is 4 or more (Aghamohammadi et al., 2007). In the present study, the AP of 22.676 was obtained for TRE. The square of correlation coefficient for each response was computed as the determination coefficient (R2). A high R2 means that the calculated model agrees well with experimental data within the range of experiments (Bashir et al., 2010). According to Joglekar and May (1987), R2 should not be less than 0.8 for a reasonable model. Furthermore, the predicted determination coefficient (Pred R2) should be in a reasonable agreement with adjusted determination coefficient (Adj R2) (Lee et al., 2010). The coefficient of variation (CV), defined as the percent ratio between the standard deviation (SD) and the mean value is a measure of the reproducibility of the model and a model can be considered reasonably reproducible if its CV is not larger than 10% (Li et al., 2010). As Table 4 shows, the Model F-value of 32.93 and a low p-value ( F
Model A-T (K) B-P (MPa) C-a D-t (s) AB AC B2 C2 Residual Lack of fit Pure error
30.23 11.33 1.23 15.37 0.45 0.48 0.41 0.34 0.51 2.41 2.17 0.24
8 1 1 1 1 1 1 1 1 21 16 5
3.78 11.33 1.23 15.37 0.45 0.48 0.41 0.34 0.51 0.11 0.14 0.047
32.93 98.78 10.69 133.95 3.93 4.18 3.6 2.98 4.42
CO, even the CO yield can be neglected compared with other gases, but the amount order will change to CO2 > H2 > CH4 > CO only if the oxidation coefficient is large enough. As shown in Table 3, the variation ranges of GYH2, GYCH4, GYCO2 and GY were 2.37– 14.58, 0.64–3.36, 3.24–14.41 and 7.07–34.53 mmolgTOC1, respectively. The maximum GYH2 occurred at 823 K, 27 MPa and 600 s with a fixed oxidation coefficient of 0.25. A possible explanation for the absence of CO in gaseous products can be written as below: Steam reforming reaction
Cm Hn Op Nq þ H2 O ! CO þ H2 þ CO2 þ NH3 þ other hydrocarbon
ð8Þ
Water gas shift reaction
CO þ H2 O $ CO2 þ H2
ð9Þ
Methanation reaction
CO þ 3H2 $ CH4 þ H2 O
ð10Þ
Oxidation reaction Fig. 5. Perturbation plot of independent variables for TRE.
CO þ 1=2O2 ! CO2
ð11Þ
Please cite this article in press as: Gong, Y., et al. Partial oxidation of landfill leachate in supercritical water: Optimization by response surface methodology. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.04.013
6
Y. Gong et al. / Waste Management xxx (2015) xxx–xxx
Fig. 6. 3D and contour plots of TRE as a function of P and T (a = 0.25, t = 600 s).
Fig. 7. 3D and contour plots for TRE as a function of T and a (P = 25 MPa, t = 600 s).
H2 þ 1=2O2 ! H2 O
ð12Þ
Other reactions þH2 O
þH2 O
! CO2 þ2NH3 ! 2NHþ4 þ CO2 2NHþ4 þ 2HCO3 3
ð13Þ
Early work presented that biomass gasification in SCW mainly includes three reactions: steam reforming reaction, water gas shift reaction and methanation reaction (Kıpçak and Akgün, 2012). In the present study, we also assume that these three types of reactions are involved in partial oxidation of landfill leachate in SCW, as shown in Eqs. 8–10. High concentrations of NH4–N were detected in liquid products. Thus, it can be speculated that NH3 might be the product of steam reforming reaction of nitrogen-containing organics in leachate (Eq. (8)). The amount of CO will decrease as water gas shift reaction (Eq. (9)) and methanation reaction (Eq. (10)) carry out. Meanwhile, GYCH4 is significantly lower than GYH2 and GYCO2, suggesting that methanation reaction in Eq. (10) is weak. If a small amount of oxidant is added, CO and H2
will be converted to CO2 and H2O via oxidation reaction, respectively (Guo et al., 2010; Kıpçak and Akgün, 2012). Further, NH3 may have an important effect on the components of gaseous products for the process. As shown in Eq. (13), NH3 reacts with CO2 in water to form ammonium, carbonate and bicarbonate continuously. The decrease of CO2 in gas will reinforce the water gas shift reaction and oxidation reaction of CO, leading to the absence of CO ultimately. The generation of carbonate and bicarbonate may explain the reason why a low TR (8.76–32.98%) occurred when TRE was invariably above 91.87% in the experiments. The ANOVA results for GYH2, GYCH4, GYCO2 and GY were summarized in Table 5. F-values of 29.24, 10.64, 111.34 and 58.38 with a low p-value ( F
CV
R2
Adj R2
Pred R2
AP
GYH2 GYCH4 GYCO2 GY
29.24 10.64 111.34 66.03
