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Resource utilization of landfill leachate gasification in supercritical water
Chemical Engineering Journal 386 (2020) 124017
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Resource utilization of landfill leachate gasification in supercritical water ⁎
T
Yunan Chen , Youyou He, Hui Jin, Liejin Guo State Key Laboratory of Multiphase Flow in Power Engineering (SKLMF), Xi’an Jiaotong University, Xi’an 710049, Shaanxi, China
H I GH L IG H T S
landfill leachate was gasified in supercritical water with high heating rate batch reactor. • Original temperature and long residence time improved original landfill leachate gasification efficiency. • High catalytic activity decreased in the order: KOH > K CO > MnO > KMnO • The highest carbon gasification efficiency was 99.2% and hydrogen yield was 26 mol/kg at 650 °C with KOH. • The • The highest chemical oxygen demand removal rate was 94.56% at 650 °C with KOH. 2
3
2
4.
A R T I C LE I N FO
A B S T R A C T
Keywords: Landfill leachate Supercritical water Hydrogen production COD removal NH3-N removal
In this paper, original landfill leachate gasification in supercritical water was studied in high heating rate batch reactor. The experiments were carried out in the temperature range of 500–650 °C, residence time range of 0–30 min, and pressure range of 22.5–26.0 MPa. The effects of temperature, pressure, residence time, catalysts and oxidant on gasification, chemical oxygen demand (COD) and NH3-N removal rate were examined. As a result, high heating rate could promote landfill leachate gasification in supercritical water better. High temperature and long residence time could improve gasification efficiency. The highest carbon gasification efficiency (CE) reached 99.2% and hydrogen yield reached 26 mol/kg with KOH at 650 °C, 30 min. KOH could also improve COD and NH3-N conversion significantly. The highest COD and NH3-N removal rate was 94.6% and 51.0%, respectively. The addition of oxidant had the same effect on NH3-N elimination obtained with KOH. The results of experiments might predicate that the combination of catalytic and partial oxidation might be an effective method to remove NH3-N for landfill leachate gasification in supercritical water.
1. Introduction
Supercritical water gasification (SCWG) was an innovative thermochemical treatment method for organic waste and wastewater, which had drawn great attention of related researchers. The temperature and pressure of supercritical water exceeded the critical point (374.15 °C, 22.1 MPa). It was a unique nonpolar solvent, and it also had high diffusivity, low viscosity, excellent transfer properties and high organic substances solubility [12,13]. Supercritical water could rapidly and efficiently decompose organic substances which were due to the elimination of the potential interphase mass transfer limitations [14,15]. It had been proven to destroy a wide variety of organic wastes, such as sewage sludge, navy excess hazardous materials (EHM), colored dyes and semiconductor manufacture wastes and so on [16–20]. SCWG was a clean method with no nitrogen oxide or sulfur oxide emission [21]. Additionally, it did not need any costly pre-process for the treatment of landfill leachate. However, the employment of this method for the treatment of landfill leachate was investigated in only
Sanitary landfill was the main method currently used for municipal waste disposal because of its economic advantage. However, the landfill would generate large quantities of leachate which continues to be a pressing problem. Leachate was the waste water which was from waste and rainwater percolating through the landfill. It exhibited high concentration of COD, strong odor and dark caramel color. The presence of organic compounds, ammonia, inorganic salts and heavy metals made the leachate a potential threat to soil, ground water and surface water [1–3]. The present treatment of landfill leachate was a combination of physicochemical and biological methods, but these techniques were complicated and the universal method had not yet been established due to the discrepancy and variable composition of leachate [4–6]. Advanced treatments such as Fenton, wet air oxidation, supercritical oxidation and gasification had been developed in recent years [7–11].
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Corresponding author. E-mail address: [email protected] (Y. Chen).
https://doi.org/10.1016/j.cej.2020.124017 Received 23 October 2019; Received in revised form 26 December 2019; Accepted 2 January 2020 Available online 03 January 2020 1385-8947/ © 2020 Elsevier B.V. All rights reserved.
Chemical Engineering Journal 386 (2020) 124017
Y. Chen, et al.
five articles. Molino et al. [8] studied municipal waste leachate gasification in supercritical water, and they found that H2 and CH4 compositions in gaseous product varied in the range of 25.0–47.0% and 11.0–18.0% respectively. Ferreira-Pinto et al. [22] studied the landfill leachate gasification of lower COD in supercritical water. They found that temperature was the most important factor for the degradation of leachate and the production of hydrogen. Although the heating rate was fast and the COD removal rate reached 97.4%, the COD concentration of landfill leachate was lower than original landfill leachate. Gong et al. [23] found that the addition of oxidant could increase H2 and CH4 yields, and the temperature was range from 200 °C to 600 °C. The maximum H2 yield reached 231.3 mmol/L at 500 °C, 25 MPa, 600 s and 0.2 oxidation coefficient (OC). The maximum TOC removal efficiency was 85.5% at 600 °C. They also found that H2 yield, TOC removal rate and carbon recovery rate reached 14.32 mmol/gTOC, 82.5% and 94.6% respectively with lower COD of landfill leachate [24]. The catalytic gasification of landfill leachate in supercritical water was studied by Gong et al. [25], and the temperature was ranged from 380 °C to 500 °C. In catalytic gasification, four homogeneous alkali catalysts (NaOH, KOH, Na2CO3, K2CO3) which were often used in SCWG of coal, biomass and biomass wastes et al. [26–29] were also used in these experiments, and it was proved that alkali catalysts could promote the formation of hydrogen better. In the only studies, SCWG of original landfill leachate had been carried out in a low heating rate batch reactor, and the heating rate was in the range of 3 K/min to 20 K/min. Considering that tarry production was favored at lower temperatures and gasification at higher temperatures, so low heating rate was unfavorable for gasification [30,31]. Low heating rate could increase the residence time of landfill leachate in the zone of low temperature, and it could hinder the gasification and increase the formation of tar. Although the homogeneous alkaline catalysts were used in the experiments of original landfill leachate SCWG and the temperature was below 500 °C, the complete gasification of original landfill leachate did not achieve. The maximum CE of the original landfill leachate gasification in supercritical water was 85.5% at 500 °C [23], and the maximum COD and NH3-N removal rate were only 82.1% and 6.6% respectively. In order to promote original landfill leachate gasification and hydrogen production in supercritical water better, high heating rate batch reactor (70 K/min) was used in this work. The effects of high temperature (500–650 °C), pressure, reaction time, catalysts and OC on the contaminant removal and gasification characteristics of original landfill leachate were investigated.
China. The water used in the experimental runs was high-purity water which was obtained by treating de-ionized water with pure water instrument (PL5242-PALL). 2.2. Experimental apparatus and procedure The experiments were conducted in a high-throughput batch reactor system [32]. The system was composed of six channels and the parameters can be controlled individually. The batch reactor was made of Hastelloy C276 and the volume was 10 ml. The designed temperature and pressure were 750 °C and 30 MPa, respectively. The system pressure was detected with a pressure transducer and a thermocouple was employed to monitor the temperature. The original landfill leachate was used without diluting or drying. It was measured and then injected into the reactor with a syringe, and the catalyst or oxidant was also loaded into the reactor. Then, the reactor was swept by argon for 5 min to remove the air, and the purging process was repeated for 3 times to make sure that air was replaced by argon. Finally, the reactor was filled with argon and made an initial pressure. Then it was placed into the electric furnace. The average heating rate was about 70 °C/min. The reactor was held for a specified residence time when the desired temperature and pressure were achieved. After that, the reactor was taken out and cooled to ambient temperature rapidly by cooling water. The gaseous products were sampled and measured with a wet gas flowmeter. The reaction mixtures were washed with deionized water and separated into aqueous products and insoluble residues for further investigations. Two to three duplicate runs and the average values were calculated. The uncertainty of the experimental data was calculated and the results showed that the uncertainty of gas yield is less than 2.5%, and the uncertainty of pressure and temperature were less than 2.3% and 2.6%, respectively [33]. 2.3. Analytical methods An Agilent 7890A gas chromatograph equipped with thermal conductivity detector was used to analyze the composition of gaseous products. A PLOT C-2000 capillary column was used. The column temperature was held at 50 °C for 8 min and the thermal conductivity detector (TCD) temperature was held at 280 °C. Argon was employed as a carrier gas at a constant flow of 5 ml/min. A standard gas mixture with four kinds of composition was used to calibrate the TCD. The proximate analysis of landfill leachate was obtained from the laboratory in Coal Geological Bureau in Shanxi province. The ultimate analysis of andfill leachate were determined by a vario MACRO cube Elemental analyzer. The TOC contents of liquid were determined by Elemental High TOCII. The values of NH3-N and COD of liquid were measured by Lovibond MultiDirect multi-parameter photometer (Lovibond, ET99722).
2. Materials and methods 2.1. Materials The leachate used in the experiment was supplied from Jiangcungou refuse landfill located in Xi’an, Shaanxi, China. The main properties of leachate including proximate analysis, ultimate analysis, total organic carbon (TOC), COD and NH3-N value were listed in Table 1. K2CO3, KOH, MnO2, KMnO4 and 30.0 wt% H2O2 (analytical reagent grade) were purchased from Sinopharm Chemical Reagent Co., Ltd. in
2.4. Data interpretation In order to evaluate the results of landfill leachate gasification, the Gas yield, hydrogen yield potential (HYP), H2 selectivity [34], CE,
Table 1 Properties of leachate sample. Water content (wt.%)a
96.61 a b c
COD (mg/L)
36950
NH3-N (mg/L)
2480
TOC (mg/L)
9357
Proximate analysis (wt %)b
Ultimate analysis (wt %)b
Mt
Ash
VM
FC
C
H
Oc
N
S
2.12
32.37
50.16
15.35
27.30
4.08
31.15
2.31
0.67
Water content = 100% — dry matter content. On air dry basis. By difference (O% = 100% — Mt%—Ash% — C% — H% — N% — S%). 2
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removal rates of the COD concentration (XCOD), removal rates of the NH3-N concentration (XNH3-N) and the lower heating value (LHV) [35] were proposed for indicators. In the above indicators, HYP (mol/kg) was defined as the sum of measured hydrogen and hydrogen which could theoretically be formed by completely shifting carbon monoxide and completely reforming hydrocarbon species [36]. The Gas yield, LHV, GE, H2 selectivity, CE, XCOD and XNH3-N were calculated by the following equations:
CO + H2O ↔ CO2 + H2 (Water-gas shift) CO + 3H2 ↔ CH4 + H2O (Methane conversion)
= the mole number of certain gas product (1)
CE (%) = the total carbon in gaseous products/the total carbon in dry
matter in feedstock × 100%
(2)
H2 selectivity = moles of H2 /(2 × moles of CH 4)
(3)
X COD = (1 − COD in liquid product/COD in feedstock) × 100%
(4)
XNH3 - N = (1 − NH3 - N in liquid product/NH3 - N in feedstock) × 100% (5)
LHV(MJ/Nm3)
= 0.108 × VH2 + 0.126 × VCO + 0.358 × VCH4
(6)
where Vi is the percentage of the molar fraction of each gas component and i represents H2, CO, and CH4 respectively. In this work, OC are defined as follows [23]:
OC = MO2,add /MO2,need
(7)
3.2. Effect of pressure
where MO2,add is the mass of O2 generated from H2O2 loaded into the reactor and MO2,need is the mass of oxidant for complete oxidation by stoichiometry calculation.
Fig. 3 showed the effect of pressure on SCWG of landfill leachate at 650 °C and 20 min. The main components of gas products were H2, CO, CH4 and CO2. As the pressure increased from 22.5 MPa to 25.5 MPa, the molar fraction of H2 and CH4 showed a slight increase, but the change was not significant. The mole fraction of CO2 decreased with the increase of pressure. These results were similar to the variation trend obtained by Gong et al. [23] and it indicated that the change of pressure had no significant effect on the molar fraction of gas products. Although the change of pressure had little effect on the change of molar fraction of gas products, the yield of H2 and CH4 increased slightly, while the yield of CO2 decreased slightly. Lu et al. [41] found that the yield of H2 increased and the yield of CO2 decreased with the increase of pressure. It indicated that higher pressure promoted the water-gas conversion reaction better. With the increase of pressure, the CE showed a slight decrease and the HYP showed a slight increase. It also indicated that high pressure was not conducive to free radical reaction but could promote the water-gas conversion reaction better. The LHV increased slightly with the increase of pressure and reached 12.1 MJ/Nm3 when the pressure was 25.5 MPa. Although the H2 selectivity was gradually decreased with the increase of pressure, the value of H2 selectivity was 1.39 at 25.5 MPa which was still greater than 1.0. It indicated that high pressure could also promote the formation of hydrogen better. Fig. 4 showed the effect of pressure on the removal rates of COD and NH3-N concentration. As the pressure increased, the removal rates of COD and NH3-N concentration was almost no change at 650 °C and 20 min. The removal rates of COD reached about 80.0% and the removal rates of NH3-N reached about 19.0%. In general, the effect of pressure on SCWG of landfill leachate could be almost ignored when the pressure was in the range of 22.5–25.5 MPa.
3. Results and discussion 3.1. Effect of temperature The effects of temperature on SCWG of landfill leachate were shown in Fig. 1. It could be observed that main components of gaseous products were H2, CO, CH4 and CO2. The molar fraction of CO was the lowest and the molar fraction of CH4 almost maintained a constant 16.7%. The molar fraction of H2 kept increasing from 44.8% to 50.4% with the increase of temperature, and the maximum value was over 50.0%. In contrast, the molar fraction of CO2 decreased from 37.0% to 31.0% with the increase of temperature. As the temperature increased, hydrogen yield increased from 9.1 mol/kg at 500 °C to 15.4 mol/kg at 650 °C, and the rate of increment was nearly 70.0%. The yield of CH4 and CO2 increased slowly and the yield of CO was almost unchanged. The reason was that the shift of ionic degradation to free radical degradation for landfill leachate gasification in supercritical water with the increase of temperature. The free radical degradation promoted the reaction and produced gaseous products better [37], and it was dominated at lower pressures and/or higher temperatures in supercritical water [25]. Many studies had indicated that steam reforming reaction (8), water gas shift reaction (9) and methanation reaction (10) were main reaction in SCWG. The steam reforming was the major route of H2 production. The increase of temperature was favorable to the reforming reaction and enhanced the formation of H2 because the reforming reaction was an endothermic reaction. As the temperature increased, the water gas shift and methanation reaction were prevented. However, the yield of CH4 and CO2 increased which might indicate that more decomposition of carbohydrates in landfill leachate at higher temperature. This result was consistent with that obtained by Gong et al. [23]. C + H2O → CO + H2 (Steam reforming conversion)
(10)
As shown in Fig. 1, the values of LHV and H2 selectivity were almost unchanged with the increase of temperature, and the values of LHV reached about 11.5 MJ/Nm3. The H2 selectivity was greater than 1.0 which indicated that high temperatures are better for hydrogen production. The CE and HYP increased with the increase of temperature, and the maximum values of them were 66.9% and 36.6 mol/kg respectively. Steam reforming and water gas shift reaction were thought to be the main pathways to form gases in noncatalytic SCWG, especially for H2. The steam reforming reaction was the most important for CE in the gasification process and high temperature accelerated the steam reforming reaction. Bühler et al. [38] also proposed that free radical degradation dominated and led to higher CE and hydrogen yield at higher temperature. Fig. 2 showed the effect of temperature on the removal rates of COD and NH3-N concentration. As the temperature increased, the removal rates of COD and NH3-N concentration increased from 72.2%, 0.5% to 79.3%, 19.4% respectively. The NH3-N concentration was almost no change at 500 °C and it changed a little with the increase of temperature because supercritical water could donate proton for converting N in organics to NH3 [39], and NH3 was generally considered to be a product of nitrogen-containing compounds in SCWG [40]. In general, higher temperature was helpful to obtain higher carbon gasification efficiency and COD removal rate. However, the NH3-N could not be effectively removed by SCWG in high temperature.
Gas yield(mol/kg)
/the mass of dry matter in feedstock
(9)
3.3. Effect of residence time The effects of residence time on SCWG of landfill leachate were
(8) 3
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Fig. 1. Effect of temperature on landfill leachate gasification in supercritical water (feed amount: 1.66 g, residence time: 20 min, pressure: 23–26 MPa.)
reverse process of the water gas shift reaction and methanation reaction would be reinforcement with the increase of residence time [42]. The values of LHV were almost unchanged with the increase of residence time and reached about 12.0 MJ/Nm3. The H2 selectivity increased from 1.0 to 2.1 which indicated that longer residence time was better for hydrogen production. The results of COD and NH3-N removal rate were also shown in Fig. 6. The XCOD increased from 74.1% at 0 min to 81.9% at 30 min. It suggested that the increase of residence time could promote the degradation of landfill leachate and decrease COD concentration. As residence time increased from 0 to 10 min, the removal rate of NH3-N was slow. The removal rate of NH3-N increased significantly when residence time increased from 10 to 20 min, and the change of NH3-N removal rate increased slowly. In general, the increase of residence time would also favor the treatment of landfill leachate overall. Fig. 2. Effect of temperature on XCOD and XNH3-N (feed amount: 1.66 g, residence time: 20 min, pressure:23–26 MPa.)
3.4. Role between temperature and residence time
shown in Fig. 5. It showed that molar fraction of H2 substantially increased and reached 61.8% at 650 °C and 30 min. The molar fraction of CH4 and CO2 had decreased with different degrees when residence time increased from 0 to 30 min, and they decreased from 36.0%, 20.9% to 20.9%, 14.6% respectively. The molar fraction of CO was below 3.0% in these experiments. Although the maximum residence time was 30 min, the molar fraction of gas products was not stable. This was perhaps that four gas products could mutually convert via water gas shift and methanation reaction, which were reversible reactions and needed a longer residence time to reach equilibrium. From this figure, it also showed that hydrogen yield increased notably with the increase of residence time at 650 °C, and it reached 26.0 mol/kg at 30 min which was nearly three times larger than 8.9 mol/kg at 0 min. The CE, HYP were 56.1%, 27.5 mol/kg at 0 min, and they reached 70.7%, 51.7 mol/kg at 30 min respectively. The above results indicated that longer residence time made more feedstock react and complete gasification in supercritical water. A possible explanation for the phenomenon was that the
Although temperature and residence time could both promote landfill leachate gasification in supercritical water, their role on SCWG of landfill leachate was still unclear and further studies were needed. The effect of residence time on variation trends of H2 yield, HYP and CE at specified temperature were shown in Fig. 7. As can be seen from Fig. 7 H2 yield increased with the increase of temperature and residence time at almost all experimental conditions except at 0 min. The yield of hydrogen reached 5.3 mol/kg at 500 °C and 0 min, while the yield of hydrogen reached 26 mol/kg at 650 °C and 30 min. The yield of hydrogen had an increase of nearly 4 times at 650 °C and 30 min compared that at 500 °C and 0 min. It indicated that higher temperature and longer residence time could enhance the formation of hydrogen better. The CE and HYP also increased with the increase of temperature and residence time at all experimental conditions, and the maximum CE, HYP reached 70.7%, 51.7 mol/kg respectively at 650 °C and 30 min. At the same residence time, the hydrogen yield, HYP and CE had maximum increases of 2.3, 2.1 and 1.3 times at 650 °C and 30 min 4
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Fig. 3. Effect of pressure on landfill leachate gasification in supercritical water (feed amount: 1.66 g, temperature: 650 °C, residence time: 20 min.)
it was necessary to add catalysts for the reaction. The homogeneous catalysts were widely used in related experiments of SCWG. The alkali catalysts including K2CO3, KOH, Na2CO3 and NaOH were investigated and found that they were favorable for gasification in supercritical water, especially K2CO3 and KOH. The oxidation catalyst (KMnO4) had also been used effectively in the experiments of supercritical water partial oxidation gasification [48]. The heterogeneous catalysts were also used in related experiments of SCWG. In order to produce hydrogen and degrade pollutants better, the high-efficiency homogeneous alkali catalysts (K2CO3 and KOH), partial oxidation catalyst (KMnO4) and heterogeneous catalysts (MnO2) were used. These catalysts were widely used in SCWG [49] and supercritical water oxidation (SCWO) [50].
Fig. 4. Effect of pressure on XCOD and XNH3-N (feed amount: 1.66 g, temperature: 650 °C, residence time:20 min.)
3.5.1. Effect of catalyst types The experiments with various catalysts were carried out at 650 °C and 20 min, and the pressure range was from 23 to 26 MPa. The results were illustrated in Fig. 8 and it showed that molar fraction of gas production had a little impact when K2CO3 was used. The molar fractions of H2 and CH4 increased and the molar fraction of CO2 decreased slightly when KOH was used. Although K2CO3 and KOH could promote the water-gas shift reaction better [26], KOH had a better function than K2CO3 on landfill leachate gasification in supercritical water. The molar fraction of CO2 increased and molar fraction of H2 decreased in the presence of MnO2, which indicated that MnO2 was not only a catalyst but also had an oxidation [51]. KMnO4 could increase the molar fraction of CH4 and reduce the molar fraction of H2, which was probably due to its catalysis for methanation reaction. On the whole, the molar fractions of H2, CO2 and CH4 were in the ranges from 46.0% to 51.5%, 29.0% to 35.5%, 16.7% to 21.4% respectively. The molar fraction of CO remained nearly constant whenever the catalysts were used or not. As shown in Fig. 8, it could be observed that the yields of H2, CH4 and CO2 increased with catalysts, and the yield of CO was almost unchanged. The yield of H2 increased significantly to 20.5 mol/kg with K2CO3 and 22.3 mol/kg with KOH. The K2CO3 could increase the yield
compared that at 500 °C and 30 min, respectively. At the same temperature, the hydrogen yield, HYP and CE had maximum increases of 2.9, 1.9 and 1.2 times at 650 °C and 30 min compared that obtained at 650 °C and 0 min, respectively. It indicated temperature was better for gasification than residence time, and residence time was better for hydrogen production. The above results were consistent with other works which were done by some researchers [23,43–47]. They indicated that high temperature enhanced the steam reforming and water gas shift reaction, and long residence times speed up the reaction process.
3.5. Effect of catalyst addition From the above analysis, it could be seen that temperature and residence time could effectively promote gasification of landfill leachate in supercritical water. But simply increasing the temperature and residence time did not achieve full gasification and pollutant removal, so 5
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Fig. 5. Effect of residence time on landfill leachate gasification in supercritical water (feed amount: 1.66 g, temperature: 650 °C, pressure: 23–26 MPa.)
had an oxidation. The CE and HYP had a perceptible increase with catalysts, and it had a great improvement. Above all, the CE exceed 90.0% with K2CO3 and KOH, comparing that without catalyst which was 66.9% and 54.8% respectively. The catalytic activity of CE decreased in the order: KOH > K2CO3 > MnO2 > KMnO4. Fig. 9 showed the effect of various catalysts on the removal rates of COD and NH3-N concentration. It showed that the removal rates of COD increase significantly when KOH and KMnO4 were used and the removal rates of COD reached 92.8% and 91.0% respectively. The maximum removal rate of NH3-N concentration reached 48.4% with KOH. However, the removal rates of NH3-N concentration decreased when other catalysts were used compared to that obtained without catalyst. It indicated that K2CO3, MnO2 and KMnO4 could also convert N in organics to NH3, and NH3 was the main product of nitrogenous substance in landfill leachate at the condition of these experiments. From the above results, it found that KOH was the optimum catalyst of these four catalysts. It could not only have the best catalysis for gasification but also have the best ability to remove pollutants.
Fig. 6. Effect of residence time on XCOD and XNH3-N (feed amount: 1.66 g, temperature: 650 °C, pressure:23–26 MPa.)
of H2 by the water gas shift reaction and produce the intermediate potassium formate (HCOOK). The HCOOK reacted with water to produce H2 and potassium bicarbonate (KHCO3). The thermal decomposition of KHCO3 releases CO2 and K2CO3 [26,52]. The KOH might react with CO2 and form carbonates. It enhanced the water gas shift reaction and steam reforming reaction [26,52]. It also showed that the addition of KOH had better selectivity for H2 in Fig. 8. These results were consistent with the findings of Sinag et al. [53] who discovered that alkali salts could catalyze the water gas shift reaction and thereby increased the H2 yield. The MnO2 could promote the CO2 production and KMnO4 enhanced the formation of CH4. The yield of CH4 increased from 5.2 mol/kg without catalyst to 8.1 mol/kg with KMnO4. The total gas yield had greatly increased with catalyst addition, especially for KOH, and the total gas yield increased from 30.6 mol/kg to 43.2 mol/ kg. The value of LHV increased when KOH and KMnO4 were used, and the maximum value of LHV reached 13.0 MJ/Nm3 with KMnO4. The H2 selectivity decreased when MnO2 and KMnO4 were used because they
3.5.2. Effect of catalyst loading The effects of KOH loading on landfill leachate gasification in supercritical water were investigated and the results were shown in Fig. 10. As the catalyst loading increasing from 0.4 to 1.3, the molar fraction of H2 decreased and CO2 increased slowly, and the molar fraction of CH4 and CO had a little change. The molar fraction of CH4 remained at about 17.0% and the molar fraction of CO was below 1.0% with the increase of catalyst loading. The molar fraction of gas products were nearly the same when the addition proportion of catalyst increased from 1.3 to 1.6. The yields of H2, CH4 and CO2 increased with the increase of catalyst loading. The maximum yield of H2 and CH4 reached 24.3 mol/kg and 8.3 mol/kg respectively when the addition proportion of catalyst was 1.6. This change trend was consistent with the results obtained by Kruse et al. [54]. She reported that the yield of H2 and CO2 increased with the increase amount of KOH. It clearly indicated that WGS reaction was improved by the addition of KOH. The CE and HYP increased significantly with the increase of catalyst 6
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Fig. 7. Effect of residence time on landfill leachate gasification in supercritical water at specified temperature (feed amount: 1.66 g, temperature: 500–650 °C, pressure: 23–26 MPa.)
Fig. 8. Effect of catalyst types on landfill leachate gasification in supercritical water (feed amount: 1.66 g, catalyst loading (g catalyst/g dry matter): 1:1, residence time: 20 min, pressure: 23–26 MPa.) 7
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Fig. 9. Effect of catalyst types on XCOD and XNH3-N (feed amount: 1.66 g, catalyst loading (g catalyst/g dry matter): 1:1, residence time: 20 min, pressure: 23–26 MPa.)
Fig. 11. Effect of KOH loading on XCOD and XNH3-N (feed amount: 1.66 g, temperature: 650 °C, residence time: 20 min, pressure: 23–26 MPa.)
the addition of excessive oxidant could effectively remove NH3-N, but not conducive to hydrogen production [51,55]. The addition of excess oxygen would cause serious corrosion to the reactor. In order to avoid corrosion, produce hydrogen, and effectively remove NH3-N, partial oxidation gasification was a desirable method. The effects of oxidant on SCWG of landfill leachate were shown in Fig. 12. It showed that molar fraction of H2 increased firstly and then decreased with the increase of OC. It reached the maximum value of 59.6% when the OC was 0.4. The molar fraction of CH4 and CO2 also decreased firstly and then increased with the increase of OC, showing a change trend opposite to that of H2. The molar fraction of CO did not change with the increase of OC. The yield of hydrogen first increased and then decreased with the increase of OC, and reached the maximum value of 21.9 mol/kg when the OC was 0.6. The change of OC had little effect on the production of other gases. These results suggested that the addition of small amount O2 could notably enhance the steam
loading. When the catalyst loading was 1.0, the CE reached 92.1%. The CE reached 99.2% when the catalyst loading was 1.6, and it was better than other studies. The LHV and H2 selectivity had a little effect with the increase of catalyst loading. The effect of catalyst loading on the removal rates of COD and NH3N concentration were showed in Fig. 11. The COD removal rate was not affected much by the catalyst loading, and the maximum COD removal rate reached 95.0% when the addition of KOH was 1.6. The NH3-N removal rate was also not affected much by the catalyst loading, and the maximum NH3-N removal rate reached 51.0%. 3.6. Effect of oxidant addition Although the addition of catalyst could effectively promote the gasification of landfill leachate in supercritical water, NH3-N could not be completely removed. In the process of supercritical water oxidation,
Fig. 10. Effect of KOH loading on landfill leachate gasification in supercritical water (feed amount: 1.66 g, temperature: 650 °C, residence time: 20 min, pressure: 23–26 MPa.) 8
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Fig. 12. Effect of oxidation on landfill leachate gasification in supercritical water (feed amount: 1.66 g, temperature: 650 °C, residence time: 20 min, pressure: 23–26 MPa.)
Table 3 Comparison of XCOD and XNH3-N with catalyst and oxidant.
KOH H2O2
T (°C)
Time (min)
Catalyst amount(g/g)
Oxidation coefficient
XCOD (%)
XNH3-N (%)
650 650
20 20
1.6 0
0 0.6
94.6 82.2
51 50.4
that COD had little difference compared with that without adding oxidant and it indicated that partial oxidation had no effect on COD removal and remained at about 79.0%. The removal rate of NH3-N concentration was significantly higher than that of no oxidant. The removal rate of NH3-N concentration was only 20.0% with no oxidant, but up to 50.0% after adding oxidant. This indicated that partial oxidation had a positive effect on ammonia nitrogen removal. In general, the addition of oxidant in partial oxidation experiments had some effects on gasification and the removal rate of NH3-N concentration. The removal rate of COD did not change much with the increase of OC.
Fig. 13. Effect of oxidation on XCOD and XNH3-N (feed amount: 1.66 g, temperature: 650 °C, residence time: 20 min, pressure: 23–26 MPa.)
reforming reaction of organics and improve the yield of H2, CH4 and CO2. However, H2 and CH4 would be oxidized into H2O and CO2 respectively in the presence of more oxidants [56]. Except for the point where the OC was 0.6, the CE was both lower than that when no oxidant was added. The HYP increased with the increase of OC in the range of 0–0.6. It reached the maximum of 43.9 mol/kg when OC was 0.6, and then decreased when OC further increased. The effects of oxidant on the removal rates of COD and NH3-N concentration were shown in Fig. 13. It could be seen from the figure
3.7. Comparison with catalyst and oxidant From the above analysis results, it could be seen that catalyst and oxidant play a good role in gasification and treatment of landfill leachate in supercritical water. However, which method was more suitable for the gasification and treatment of landfill leachate in supercritical
Table 2 Comparison of gasification with catalyst and oxidant.
KOH H2O2
T (°C)
Time (min)
Catalyst amount(g/g)
Oxidation coefficient
CE (%)
H2 (mol/kg)
HYP (mol/kg)
650 650
20 20
1.6 0
0 0.6
99.2 68.2
24.3 21.9
57.6 43.9
9
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Table 4 Comparison of landfill leachate gasification between this work and others. Feed
Reactor
Heating rate (K/min)
COD (mg/l)
T (°C)
P (MPa)
Time/flowrate
catalyst
OC
CE (%)
XCOD (%)
XNH3-N (%)
Refs.
Landfill leachate
Continuous Batch Batch Batch Batch
3 20 20 70
1580 42536 35500 6633 36950
650 500 600 550 650 650 600 500
25 29 25 25 25.9 25.3 25.2. 23.6
6 g/min 25 min 10 min 10 min 25 min 25 min 25 min 25 min
– – – – KOH
– – – 0.25 – 0.6
– 44.8 55 85 99.2 68.2 65.0 49.5
97.4 76.8 – – 94.6 82.2 76.4 72.2
– 6.5 7.6 – 51.0 50.0 12.3 0.5
Ferreira et al. [22] Gong et al. [25] Gong et al. [24] Gong et al. [23] This work
44.8% which was lower than the result obtained at the heating rate of 70 K/min. The NH3-N removal rate obtained at the low heating rate was higher than that obtained at the high heating rate. It may be due to the low heating rate (3 K/min) which resulted in the increased reaction time of landfill leachate gasification in supercritical water, and it was beneficial to the removal of COD and NH3-N, but not beneficial to gasification at the same conditions. Although the CE and COD removal rate with the addition of oxidant were lower than the results obtained with the addition of KOH at the high heating rate (70 K/min) and 650 °C, the NH3-N removal rate was consistent with the results obtained by the addition of KOH. It also indicated that the addition of oxidant could accelerate the removal of NH3-N.
water had not been compared. Therefore, the catalytic gasification and partial oxidation gasification of landfill leachate were compared. Table 2 showed the comparison of gasification with catalyst and oxidant. At the same condition, the maximum CE with the addition of oxidant reached 68.2% and it was far lower than the values of 99.2% when KOH was added. The maximum yield of hydrogen by adding oxidant was 21.9 mol/kg which was lower than that obtained by adding KOH. The maximum HYP was 43.9 mol/kg and 57.6 mol/kg with the addition of oxidant and catalyst, respectively. It could be seen that the role of gasification improvement by adding oxidant in the range of the experiments was far less than that by adding KOH catalyst. Table 3 showed the comparison of the removal rates of COD and NH3-N with catalyst and oxidant. The removal rate of COD was greatly improved by adding KOH, but it remained unchanged with the addition of oxidant, which showed that the removal effect of COD was obviously better with KOH than that with oxidant. For NH3-N removal, KOH and oxidant were equally effective. In general, KOH was more effective in removing COD and NH3-N from landfill leachate. According to the above analysis, the gasification and decontamination of landfill leachate with KOH in supercritical water were better than that of partial oxidation gasification. Although KOH could promote gasification and COD degradation of landfill leachate well, only about 51.0% of NH3-N could be eliminated. Although partial oxidation could not enhance gasification and COD degradation well like the role of KOH, its effect on NH3-N elimination was similar to that obtained by using KOH. The mass of KOH addition was higher than the mass of oxidizer addition. It indicated that the less addition of oxidant could play a good role in NH3-N elimination, but had no obvious effect on gasification. The experiments of SCWO of landfill leachate showed that the excessive oxidant can completely eliminate COD and NH3-N. But the excessive oxidant would cause serious corrosion to the reactor and no hydrogen production. In order to avoid the problem caused by excessive oxidant and produce hydrogen better, combined with the above experimental results, the mixed catalytic (namely the combination of catalytic gasification and partial oxidation) was very desirable. Followup work was being carried out, and relevant reports would be carried out later.
4. Conclusions In this work, landfill leachate gasification in supercritical water with high heating rate batch reactor was proved to be feasible. These results showed that high heating rate could promote landfill leachate gasification in supercritical water better. High temperature and long residence time could improve gasification efficiency. The catalytic activity decreased in the order: KOH > K2CO3 > MnO2 > KMnO4. The addition of KOH could increase CE and H2 yield greatly, and the highest CE was 99.2% and hydrogen yield was 26 mol/kg. At the same time, KOH could also improve the COD and NH3-N conversion significantly, and promote the degradation of organic compounds in landfill leachate. The highest COD removal rate was 94.6% and NH3-N removal rate was 51.0%. The addition of oxidant had the same effect on NH3-N elimination which was similar to that obtained by using KOH. The results of experiments demonstrated that landfill leachate gasification in supercritical water could not only degrade the organic compounds but also generate hydrogen efficiently. The combination of catalytic gasification and partial oxidation might be an effective method to remove NH3-N. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
3.8. Comparison with literature
Acknowledgement
Table 4 showed a comparison of present work with published literatures on landfill leachate gasification in supercritical water. In this work, original landfill leachate was gasified at 650 °C by using KOH and the best CE (99.2%) was reached. The COD and NH3-N removal rate reached 94.6% and 51.0% respectively. These results were much higher than those obtained by other studies. Although the COD removal rate of landfill leachate in continuous reactor reached 97.4% at 650 °C, the COD concentration of landfill leachate was only 1580.0 mg/l which was lower than that of original landfill leachate. At 600 °C, the CE obtained at the heating rate of 70 K/min was higher than that obtained at the heating rate of 20 K/min. Although the COD removal rate reached 76.8% at the heating rate of 3.0 K/min at 500 °C which was higher than the result obtained at the heating rate of 70 K/min, the CE was only
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Resource utilization of landfill leachate gasification in supercritical water ⁎
T
Yunan Chen , Youyou He, Hui Jin, Liejin Guo State Key Laboratory of Multiphase Flow in Power Engineering (SKLMF), Xi’an Jiaotong University, Xi’an 710049, Shaanxi, China
H I GH L IG H T S
landfill leachate was gasified in supercritical water with high heating rate batch reactor. • Original temperature and long residence time improved original landfill leachate gasification efficiency. • High catalytic activity decreased in the order: KOH > K CO > MnO > KMnO • The highest carbon gasification efficiency was 99.2% and hydrogen yield was 26 mol/kg at 650 °C with KOH. • The • The highest chemical oxygen demand removal rate was 94.56% at 650 °C with KOH. 2
3
2
4.
A R T I C LE I N FO
A B S T R A C T
Keywords: Landfill leachate Supercritical water Hydrogen production COD removal NH3-N removal
In this paper, original landfill leachate gasification in supercritical water was studied in high heating rate batch reactor. The experiments were carried out in the temperature range of 500–650 °C, residence time range of 0–30 min, and pressure range of 22.5–26.0 MPa. The effects of temperature, pressure, residence time, catalysts and oxidant on gasification, chemical oxygen demand (COD) and NH3-N removal rate were examined. As a result, high heating rate could promote landfill leachate gasification in supercritical water better. High temperature and long residence time could improve gasification efficiency. The highest carbon gasification efficiency (CE) reached 99.2% and hydrogen yield reached 26 mol/kg with KOH at 650 °C, 30 min. KOH could also improve COD and NH3-N conversion significantly. The highest COD and NH3-N removal rate was 94.6% and 51.0%, respectively. The addition of oxidant had the same effect on NH3-N elimination obtained with KOH. The results of experiments might predicate that the combination of catalytic and partial oxidation might be an effective method to remove NH3-N for landfill leachate gasification in supercritical water.
1. Introduction
Supercritical water gasification (SCWG) was an innovative thermochemical treatment method for organic waste and wastewater, which had drawn great attention of related researchers. The temperature and pressure of supercritical water exceeded the critical point (374.15 °C, 22.1 MPa). It was a unique nonpolar solvent, and it also had high diffusivity, low viscosity, excellent transfer properties and high organic substances solubility [12,13]. Supercritical water could rapidly and efficiently decompose organic substances which were due to the elimination of the potential interphase mass transfer limitations [14,15]. It had been proven to destroy a wide variety of organic wastes, such as sewage sludge, navy excess hazardous materials (EHM), colored dyes and semiconductor manufacture wastes and so on [16–20]. SCWG was a clean method with no nitrogen oxide or sulfur oxide emission [21]. Additionally, it did not need any costly pre-process for the treatment of landfill leachate. However, the employment of this method for the treatment of landfill leachate was investigated in only
Sanitary landfill was the main method currently used for municipal waste disposal because of its economic advantage. However, the landfill would generate large quantities of leachate which continues to be a pressing problem. Leachate was the waste water which was from waste and rainwater percolating through the landfill. It exhibited high concentration of COD, strong odor and dark caramel color. The presence of organic compounds, ammonia, inorganic salts and heavy metals made the leachate a potential threat to soil, ground water and surface water [1–3]. The present treatment of landfill leachate was a combination of physicochemical and biological methods, but these techniques were complicated and the universal method had not yet been established due to the discrepancy and variable composition of leachate [4–6]. Advanced treatments such as Fenton, wet air oxidation, supercritical oxidation and gasification had been developed in recent years [7–11].
⁎
Corresponding author. E-mail address: [email protected] (Y. Chen).
https://doi.org/10.1016/j.cej.2020.124017 Received 23 October 2019; Received in revised form 26 December 2019; Accepted 2 January 2020 Available online 03 January 2020 1385-8947/ © 2020 Elsevier B.V. All rights reserved.
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five articles. Molino et al. [8] studied municipal waste leachate gasification in supercritical water, and they found that H2 and CH4 compositions in gaseous product varied in the range of 25.0–47.0% and 11.0–18.0% respectively. Ferreira-Pinto et al. [22] studied the landfill leachate gasification of lower COD in supercritical water. They found that temperature was the most important factor for the degradation of leachate and the production of hydrogen. Although the heating rate was fast and the COD removal rate reached 97.4%, the COD concentration of landfill leachate was lower than original landfill leachate. Gong et al. [23] found that the addition of oxidant could increase H2 and CH4 yields, and the temperature was range from 200 °C to 600 °C. The maximum H2 yield reached 231.3 mmol/L at 500 °C, 25 MPa, 600 s and 0.2 oxidation coefficient (OC). The maximum TOC removal efficiency was 85.5% at 600 °C. They also found that H2 yield, TOC removal rate and carbon recovery rate reached 14.32 mmol/gTOC, 82.5% and 94.6% respectively with lower COD of landfill leachate [24]. The catalytic gasification of landfill leachate in supercritical water was studied by Gong et al. [25], and the temperature was ranged from 380 °C to 500 °C. In catalytic gasification, four homogeneous alkali catalysts (NaOH, KOH, Na2CO3, K2CO3) which were often used in SCWG of coal, biomass and biomass wastes et al. [26–29] were also used in these experiments, and it was proved that alkali catalysts could promote the formation of hydrogen better. In the only studies, SCWG of original landfill leachate had been carried out in a low heating rate batch reactor, and the heating rate was in the range of 3 K/min to 20 K/min. Considering that tarry production was favored at lower temperatures and gasification at higher temperatures, so low heating rate was unfavorable for gasification [30,31]. Low heating rate could increase the residence time of landfill leachate in the zone of low temperature, and it could hinder the gasification and increase the formation of tar. Although the homogeneous alkaline catalysts were used in the experiments of original landfill leachate SCWG and the temperature was below 500 °C, the complete gasification of original landfill leachate did not achieve. The maximum CE of the original landfill leachate gasification in supercritical water was 85.5% at 500 °C [23], and the maximum COD and NH3-N removal rate were only 82.1% and 6.6% respectively. In order to promote original landfill leachate gasification and hydrogen production in supercritical water better, high heating rate batch reactor (70 K/min) was used in this work. The effects of high temperature (500–650 °C), pressure, reaction time, catalysts and OC on the contaminant removal and gasification characteristics of original landfill leachate were investigated.
China. The water used in the experimental runs was high-purity water which was obtained by treating de-ionized water with pure water instrument (PL5242-PALL). 2.2. Experimental apparatus and procedure The experiments were conducted in a high-throughput batch reactor system [32]. The system was composed of six channels and the parameters can be controlled individually. The batch reactor was made of Hastelloy C276 and the volume was 10 ml. The designed temperature and pressure were 750 °C and 30 MPa, respectively. The system pressure was detected with a pressure transducer and a thermocouple was employed to monitor the temperature. The original landfill leachate was used without diluting or drying. It was measured and then injected into the reactor with a syringe, and the catalyst or oxidant was also loaded into the reactor. Then, the reactor was swept by argon for 5 min to remove the air, and the purging process was repeated for 3 times to make sure that air was replaced by argon. Finally, the reactor was filled with argon and made an initial pressure. Then it was placed into the electric furnace. The average heating rate was about 70 °C/min. The reactor was held for a specified residence time when the desired temperature and pressure were achieved. After that, the reactor was taken out and cooled to ambient temperature rapidly by cooling water. The gaseous products were sampled and measured with a wet gas flowmeter. The reaction mixtures were washed with deionized water and separated into aqueous products and insoluble residues for further investigations. Two to three duplicate runs and the average values were calculated. The uncertainty of the experimental data was calculated and the results showed that the uncertainty of gas yield is less than 2.5%, and the uncertainty of pressure and temperature were less than 2.3% and 2.6%, respectively [33]. 2.3. Analytical methods An Agilent 7890A gas chromatograph equipped with thermal conductivity detector was used to analyze the composition of gaseous products. A PLOT C-2000 capillary column was used. The column temperature was held at 50 °C for 8 min and the thermal conductivity detector (TCD) temperature was held at 280 °C. Argon was employed as a carrier gas at a constant flow of 5 ml/min. A standard gas mixture with four kinds of composition was used to calibrate the TCD. The proximate analysis of landfill leachate was obtained from the laboratory in Coal Geological Bureau in Shanxi province. The ultimate analysis of andfill leachate were determined by a vario MACRO cube Elemental analyzer. The TOC contents of liquid were determined by Elemental High TOCII. The values of NH3-N and COD of liquid were measured by Lovibond MultiDirect multi-parameter photometer (Lovibond, ET99722).
2. Materials and methods 2.1. Materials The leachate used in the experiment was supplied from Jiangcungou refuse landfill located in Xi’an, Shaanxi, China. The main properties of leachate including proximate analysis, ultimate analysis, total organic carbon (TOC), COD and NH3-N value were listed in Table 1. K2CO3, KOH, MnO2, KMnO4 and 30.0 wt% H2O2 (analytical reagent grade) were purchased from Sinopharm Chemical Reagent Co., Ltd. in
2.4. Data interpretation In order to evaluate the results of landfill leachate gasification, the Gas yield, hydrogen yield potential (HYP), H2 selectivity [34], CE,
Table 1 Properties of leachate sample. Water content (wt.%)a
96.61 a b c
COD (mg/L)
36950
NH3-N (mg/L)
2480
TOC (mg/L)
9357
Proximate analysis (wt %)b
Ultimate analysis (wt %)b
Mt
Ash
VM
FC
C
H
Oc
N
S
2.12
32.37
50.16
15.35
27.30
4.08
31.15
2.31
0.67
Water content = 100% — dry matter content. On air dry basis. By difference (O% = 100% — Mt%—Ash% — C% — H% — N% — S%). 2
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removal rates of the COD concentration (XCOD), removal rates of the NH3-N concentration (XNH3-N) and the lower heating value (LHV) [35] were proposed for indicators. In the above indicators, HYP (mol/kg) was defined as the sum of measured hydrogen and hydrogen which could theoretically be formed by completely shifting carbon monoxide and completely reforming hydrocarbon species [36]. The Gas yield, LHV, GE, H2 selectivity, CE, XCOD and XNH3-N were calculated by the following equations:
CO + H2O ↔ CO2 + H2 (Water-gas shift) CO + 3H2 ↔ CH4 + H2O (Methane conversion)
= the mole number of certain gas product (1)
CE (%) = the total carbon in gaseous products/the total carbon in dry
matter in feedstock × 100%
(2)
H2 selectivity = moles of H2 /(2 × moles of CH 4)
(3)
X COD = (1 − COD in liquid product/COD in feedstock) × 100%
(4)
XNH3 - N = (1 − NH3 - N in liquid product/NH3 - N in feedstock) × 100% (5)
LHV(MJ/Nm3)
= 0.108 × VH2 + 0.126 × VCO + 0.358 × VCH4
(6)
where Vi is the percentage of the molar fraction of each gas component and i represents H2, CO, and CH4 respectively. In this work, OC are defined as follows [23]:
OC = MO2,add /MO2,need
(7)
3.2. Effect of pressure
where MO2,add is the mass of O2 generated from H2O2 loaded into the reactor and MO2,need is the mass of oxidant for complete oxidation by stoichiometry calculation.
Fig. 3 showed the effect of pressure on SCWG of landfill leachate at 650 °C and 20 min. The main components of gas products were H2, CO, CH4 and CO2. As the pressure increased from 22.5 MPa to 25.5 MPa, the molar fraction of H2 and CH4 showed a slight increase, but the change was not significant. The mole fraction of CO2 decreased with the increase of pressure. These results were similar to the variation trend obtained by Gong et al. [23] and it indicated that the change of pressure had no significant effect on the molar fraction of gas products. Although the change of pressure had little effect on the change of molar fraction of gas products, the yield of H2 and CH4 increased slightly, while the yield of CO2 decreased slightly. Lu et al. [41] found that the yield of H2 increased and the yield of CO2 decreased with the increase of pressure. It indicated that higher pressure promoted the water-gas conversion reaction better. With the increase of pressure, the CE showed a slight decrease and the HYP showed a slight increase. It also indicated that high pressure was not conducive to free radical reaction but could promote the water-gas conversion reaction better. The LHV increased slightly with the increase of pressure and reached 12.1 MJ/Nm3 when the pressure was 25.5 MPa. Although the H2 selectivity was gradually decreased with the increase of pressure, the value of H2 selectivity was 1.39 at 25.5 MPa which was still greater than 1.0. It indicated that high pressure could also promote the formation of hydrogen better. Fig. 4 showed the effect of pressure on the removal rates of COD and NH3-N concentration. As the pressure increased, the removal rates of COD and NH3-N concentration was almost no change at 650 °C and 20 min. The removal rates of COD reached about 80.0% and the removal rates of NH3-N reached about 19.0%. In general, the effect of pressure on SCWG of landfill leachate could be almost ignored when the pressure was in the range of 22.5–25.5 MPa.
3. Results and discussion 3.1. Effect of temperature The effects of temperature on SCWG of landfill leachate were shown in Fig. 1. It could be observed that main components of gaseous products were H2, CO, CH4 and CO2. The molar fraction of CO was the lowest and the molar fraction of CH4 almost maintained a constant 16.7%. The molar fraction of H2 kept increasing from 44.8% to 50.4% with the increase of temperature, and the maximum value was over 50.0%. In contrast, the molar fraction of CO2 decreased from 37.0% to 31.0% with the increase of temperature. As the temperature increased, hydrogen yield increased from 9.1 mol/kg at 500 °C to 15.4 mol/kg at 650 °C, and the rate of increment was nearly 70.0%. The yield of CH4 and CO2 increased slowly and the yield of CO was almost unchanged. The reason was that the shift of ionic degradation to free radical degradation for landfill leachate gasification in supercritical water with the increase of temperature. The free radical degradation promoted the reaction and produced gaseous products better [37], and it was dominated at lower pressures and/or higher temperatures in supercritical water [25]. Many studies had indicated that steam reforming reaction (8), water gas shift reaction (9) and methanation reaction (10) were main reaction in SCWG. The steam reforming was the major route of H2 production. The increase of temperature was favorable to the reforming reaction and enhanced the formation of H2 because the reforming reaction was an endothermic reaction. As the temperature increased, the water gas shift and methanation reaction were prevented. However, the yield of CH4 and CO2 increased which might indicate that more decomposition of carbohydrates in landfill leachate at higher temperature. This result was consistent with that obtained by Gong et al. [23]. C + H2O → CO + H2 (Steam reforming conversion)
(10)
As shown in Fig. 1, the values of LHV and H2 selectivity were almost unchanged with the increase of temperature, and the values of LHV reached about 11.5 MJ/Nm3. The H2 selectivity was greater than 1.0 which indicated that high temperatures are better for hydrogen production. The CE and HYP increased with the increase of temperature, and the maximum values of them were 66.9% and 36.6 mol/kg respectively. Steam reforming and water gas shift reaction were thought to be the main pathways to form gases in noncatalytic SCWG, especially for H2. The steam reforming reaction was the most important for CE in the gasification process and high temperature accelerated the steam reforming reaction. Bühler et al. [38] also proposed that free radical degradation dominated and led to higher CE and hydrogen yield at higher temperature. Fig. 2 showed the effect of temperature on the removal rates of COD and NH3-N concentration. As the temperature increased, the removal rates of COD and NH3-N concentration increased from 72.2%, 0.5% to 79.3%, 19.4% respectively. The NH3-N concentration was almost no change at 500 °C and it changed a little with the increase of temperature because supercritical water could donate proton for converting N in organics to NH3 [39], and NH3 was generally considered to be a product of nitrogen-containing compounds in SCWG [40]. In general, higher temperature was helpful to obtain higher carbon gasification efficiency and COD removal rate. However, the NH3-N could not be effectively removed by SCWG in high temperature.
Gas yield(mol/kg)
/the mass of dry matter in feedstock
(9)
3.3. Effect of residence time The effects of residence time on SCWG of landfill leachate were
(8) 3
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Fig. 1. Effect of temperature on landfill leachate gasification in supercritical water (feed amount: 1.66 g, residence time: 20 min, pressure: 23–26 MPa.)
reverse process of the water gas shift reaction and methanation reaction would be reinforcement with the increase of residence time [42]. The values of LHV were almost unchanged with the increase of residence time and reached about 12.0 MJ/Nm3. The H2 selectivity increased from 1.0 to 2.1 which indicated that longer residence time was better for hydrogen production. The results of COD and NH3-N removal rate were also shown in Fig. 6. The XCOD increased from 74.1% at 0 min to 81.9% at 30 min. It suggested that the increase of residence time could promote the degradation of landfill leachate and decrease COD concentration. As residence time increased from 0 to 10 min, the removal rate of NH3-N was slow. The removal rate of NH3-N increased significantly when residence time increased from 10 to 20 min, and the change of NH3-N removal rate increased slowly. In general, the increase of residence time would also favor the treatment of landfill leachate overall. Fig. 2. Effect of temperature on XCOD and XNH3-N (feed amount: 1.66 g, residence time: 20 min, pressure:23–26 MPa.)
3.4. Role between temperature and residence time
shown in Fig. 5. It showed that molar fraction of H2 substantially increased and reached 61.8% at 650 °C and 30 min. The molar fraction of CH4 and CO2 had decreased with different degrees when residence time increased from 0 to 30 min, and they decreased from 36.0%, 20.9% to 20.9%, 14.6% respectively. The molar fraction of CO was below 3.0% in these experiments. Although the maximum residence time was 30 min, the molar fraction of gas products was not stable. This was perhaps that four gas products could mutually convert via water gas shift and methanation reaction, which were reversible reactions and needed a longer residence time to reach equilibrium. From this figure, it also showed that hydrogen yield increased notably with the increase of residence time at 650 °C, and it reached 26.0 mol/kg at 30 min which was nearly three times larger than 8.9 mol/kg at 0 min. The CE, HYP were 56.1%, 27.5 mol/kg at 0 min, and they reached 70.7%, 51.7 mol/kg at 30 min respectively. The above results indicated that longer residence time made more feedstock react and complete gasification in supercritical water. A possible explanation for the phenomenon was that the
Although temperature and residence time could both promote landfill leachate gasification in supercritical water, their role on SCWG of landfill leachate was still unclear and further studies were needed. The effect of residence time on variation trends of H2 yield, HYP and CE at specified temperature were shown in Fig. 7. As can be seen from Fig. 7 H2 yield increased with the increase of temperature and residence time at almost all experimental conditions except at 0 min. The yield of hydrogen reached 5.3 mol/kg at 500 °C and 0 min, while the yield of hydrogen reached 26 mol/kg at 650 °C and 30 min. The yield of hydrogen had an increase of nearly 4 times at 650 °C and 30 min compared that at 500 °C and 0 min. It indicated that higher temperature and longer residence time could enhance the formation of hydrogen better. The CE and HYP also increased with the increase of temperature and residence time at all experimental conditions, and the maximum CE, HYP reached 70.7%, 51.7 mol/kg respectively at 650 °C and 30 min. At the same residence time, the hydrogen yield, HYP and CE had maximum increases of 2.3, 2.1 and 1.3 times at 650 °C and 30 min 4
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Fig. 3. Effect of pressure on landfill leachate gasification in supercritical water (feed amount: 1.66 g, temperature: 650 °C, residence time: 20 min.)
it was necessary to add catalysts for the reaction. The homogeneous catalysts were widely used in related experiments of SCWG. The alkali catalysts including K2CO3, KOH, Na2CO3 and NaOH were investigated and found that they were favorable for gasification in supercritical water, especially K2CO3 and KOH. The oxidation catalyst (KMnO4) had also been used effectively in the experiments of supercritical water partial oxidation gasification [48]. The heterogeneous catalysts were also used in related experiments of SCWG. In order to produce hydrogen and degrade pollutants better, the high-efficiency homogeneous alkali catalysts (K2CO3 and KOH), partial oxidation catalyst (KMnO4) and heterogeneous catalysts (MnO2) were used. These catalysts were widely used in SCWG [49] and supercritical water oxidation (SCWO) [50].
Fig. 4. Effect of pressure on XCOD and XNH3-N (feed amount: 1.66 g, temperature: 650 °C, residence time:20 min.)
3.5.1. Effect of catalyst types The experiments with various catalysts were carried out at 650 °C and 20 min, and the pressure range was from 23 to 26 MPa. The results were illustrated in Fig. 8 and it showed that molar fraction of gas production had a little impact when K2CO3 was used. The molar fractions of H2 and CH4 increased and the molar fraction of CO2 decreased slightly when KOH was used. Although K2CO3 and KOH could promote the water-gas shift reaction better [26], KOH had a better function than K2CO3 on landfill leachate gasification in supercritical water. The molar fraction of CO2 increased and molar fraction of H2 decreased in the presence of MnO2, which indicated that MnO2 was not only a catalyst but also had an oxidation [51]. KMnO4 could increase the molar fraction of CH4 and reduce the molar fraction of H2, which was probably due to its catalysis for methanation reaction. On the whole, the molar fractions of H2, CO2 and CH4 were in the ranges from 46.0% to 51.5%, 29.0% to 35.5%, 16.7% to 21.4% respectively. The molar fraction of CO remained nearly constant whenever the catalysts were used or not. As shown in Fig. 8, it could be observed that the yields of H2, CH4 and CO2 increased with catalysts, and the yield of CO was almost unchanged. The yield of H2 increased significantly to 20.5 mol/kg with K2CO3 and 22.3 mol/kg with KOH. The K2CO3 could increase the yield
compared that at 500 °C and 30 min, respectively. At the same temperature, the hydrogen yield, HYP and CE had maximum increases of 2.9, 1.9 and 1.2 times at 650 °C and 30 min compared that obtained at 650 °C and 0 min, respectively. It indicated temperature was better for gasification than residence time, and residence time was better for hydrogen production. The above results were consistent with other works which were done by some researchers [23,43–47]. They indicated that high temperature enhanced the steam reforming and water gas shift reaction, and long residence times speed up the reaction process.
3.5. Effect of catalyst addition From the above analysis, it could be seen that temperature and residence time could effectively promote gasification of landfill leachate in supercritical water. But simply increasing the temperature and residence time did not achieve full gasification and pollutant removal, so 5
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Fig. 5. Effect of residence time on landfill leachate gasification in supercritical water (feed amount: 1.66 g, temperature: 650 °C, pressure: 23–26 MPa.)
had an oxidation. The CE and HYP had a perceptible increase with catalysts, and it had a great improvement. Above all, the CE exceed 90.0% with K2CO3 and KOH, comparing that without catalyst which was 66.9% and 54.8% respectively. The catalytic activity of CE decreased in the order: KOH > K2CO3 > MnO2 > KMnO4. Fig. 9 showed the effect of various catalysts on the removal rates of COD and NH3-N concentration. It showed that the removal rates of COD increase significantly when KOH and KMnO4 were used and the removal rates of COD reached 92.8% and 91.0% respectively. The maximum removal rate of NH3-N concentration reached 48.4% with KOH. However, the removal rates of NH3-N concentration decreased when other catalysts were used compared to that obtained without catalyst. It indicated that K2CO3, MnO2 and KMnO4 could also convert N in organics to NH3, and NH3 was the main product of nitrogenous substance in landfill leachate at the condition of these experiments. From the above results, it found that KOH was the optimum catalyst of these four catalysts. It could not only have the best catalysis for gasification but also have the best ability to remove pollutants.
Fig. 6. Effect of residence time on XCOD and XNH3-N (feed amount: 1.66 g, temperature: 650 °C, pressure:23–26 MPa.)
of H2 by the water gas shift reaction and produce the intermediate potassium formate (HCOOK). The HCOOK reacted with water to produce H2 and potassium bicarbonate (KHCO3). The thermal decomposition of KHCO3 releases CO2 and K2CO3 [26,52]. The KOH might react with CO2 and form carbonates. It enhanced the water gas shift reaction and steam reforming reaction [26,52]. It also showed that the addition of KOH had better selectivity for H2 in Fig. 8. These results were consistent with the findings of Sinag et al. [53] who discovered that alkali salts could catalyze the water gas shift reaction and thereby increased the H2 yield. The MnO2 could promote the CO2 production and KMnO4 enhanced the formation of CH4. The yield of CH4 increased from 5.2 mol/kg without catalyst to 8.1 mol/kg with KMnO4. The total gas yield had greatly increased with catalyst addition, especially for KOH, and the total gas yield increased from 30.6 mol/kg to 43.2 mol/ kg. The value of LHV increased when KOH and KMnO4 were used, and the maximum value of LHV reached 13.0 MJ/Nm3 with KMnO4. The H2 selectivity decreased when MnO2 and KMnO4 were used because they
3.5.2. Effect of catalyst loading The effects of KOH loading on landfill leachate gasification in supercritical water were investigated and the results were shown in Fig. 10. As the catalyst loading increasing from 0.4 to 1.3, the molar fraction of H2 decreased and CO2 increased slowly, and the molar fraction of CH4 and CO had a little change. The molar fraction of CH4 remained at about 17.0% and the molar fraction of CO was below 1.0% with the increase of catalyst loading. The molar fraction of gas products were nearly the same when the addition proportion of catalyst increased from 1.3 to 1.6. The yields of H2, CH4 and CO2 increased with the increase of catalyst loading. The maximum yield of H2 and CH4 reached 24.3 mol/kg and 8.3 mol/kg respectively when the addition proportion of catalyst was 1.6. This change trend was consistent with the results obtained by Kruse et al. [54]. She reported that the yield of H2 and CO2 increased with the increase amount of KOH. It clearly indicated that WGS reaction was improved by the addition of KOH. The CE and HYP increased significantly with the increase of catalyst 6
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Fig. 7. Effect of residence time on landfill leachate gasification in supercritical water at specified temperature (feed amount: 1.66 g, temperature: 500–650 °C, pressure: 23–26 MPa.)
Fig. 8. Effect of catalyst types on landfill leachate gasification in supercritical water (feed amount: 1.66 g, catalyst loading (g catalyst/g dry matter): 1:1, residence time: 20 min, pressure: 23–26 MPa.) 7
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Fig. 9. Effect of catalyst types on XCOD and XNH3-N (feed amount: 1.66 g, catalyst loading (g catalyst/g dry matter): 1:1, residence time: 20 min, pressure: 23–26 MPa.)
Fig. 11. Effect of KOH loading on XCOD and XNH3-N (feed amount: 1.66 g, temperature: 650 °C, residence time: 20 min, pressure: 23–26 MPa.)
the addition of excessive oxidant could effectively remove NH3-N, but not conducive to hydrogen production [51,55]. The addition of excess oxygen would cause serious corrosion to the reactor. In order to avoid corrosion, produce hydrogen, and effectively remove NH3-N, partial oxidation gasification was a desirable method. The effects of oxidant on SCWG of landfill leachate were shown in Fig. 12. It showed that molar fraction of H2 increased firstly and then decreased with the increase of OC. It reached the maximum value of 59.6% when the OC was 0.4. The molar fraction of CH4 and CO2 also decreased firstly and then increased with the increase of OC, showing a change trend opposite to that of H2. The molar fraction of CO did not change with the increase of OC. The yield of hydrogen first increased and then decreased with the increase of OC, and reached the maximum value of 21.9 mol/kg when the OC was 0.6. The change of OC had little effect on the production of other gases. These results suggested that the addition of small amount O2 could notably enhance the steam
loading. When the catalyst loading was 1.0, the CE reached 92.1%. The CE reached 99.2% when the catalyst loading was 1.6, and it was better than other studies. The LHV and H2 selectivity had a little effect with the increase of catalyst loading. The effect of catalyst loading on the removal rates of COD and NH3N concentration were showed in Fig. 11. The COD removal rate was not affected much by the catalyst loading, and the maximum COD removal rate reached 95.0% when the addition of KOH was 1.6. The NH3-N removal rate was also not affected much by the catalyst loading, and the maximum NH3-N removal rate reached 51.0%. 3.6. Effect of oxidant addition Although the addition of catalyst could effectively promote the gasification of landfill leachate in supercritical water, NH3-N could not be completely removed. In the process of supercritical water oxidation,
Fig. 10. Effect of KOH loading on landfill leachate gasification in supercritical water (feed amount: 1.66 g, temperature: 650 °C, residence time: 20 min, pressure: 23–26 MPa.) 8
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Fig. 12. Effect of oxidation on landfill leachate gasification in supercritical water (feed amount: 1.66 g, temperature: 650 °C, residence time: 20 min, pressure: 23–26 MPa.)
Table 3 Comparison of XCOD and XNH3-N with catalyst and oxidant.
KOH H2O2
T (°C)
Time (min)
Catalyst amount(g/g)
Oxidation coefficient
XCOD (%)
XNH3-N (%)
650 650
20 20
1.6 0
0 0.6
94.6 82.2
51 50.4
that COD had little difference compared with that without adding oxidant and it indicated that partial oxidation had no effect on COD removal and remained at about 79.0%. The removal rate of NH3-N concentration was significantly higher than that of no oxidant. The removal rate of NH3-N concentration was only 20.0% with no oxidant, but up to 50.0% after adding oxidant. This indicated that partial oxidation had a positive effect on ammonia nitrogen removal. In general, the addition of oxidant in partial oxidation experiments had some effects on gasification and the removal rate of NH3-N concentration. The removal rate of COD did not change much with the increase of OC.
Fig. 13. Effect of oxidation on XCOD and XNH3-N (feed amount: 1.66 g, temperature: 650 °C, residence time: 20 min, pressure: 23–26 MPa.)
reforming reaction of organics and improve the yield of H2, CH4 and CO2. However, H2 and CH4 would be oxidized into H2O and CO2 respectively in the presence of more oxidants [56]. Except for the point where the OC was 0.6, the CE was both lower than that when no oxidant was added. The HYP increased with the increase of OC in the range of 0–0.6. It reached the maximum of 43.9 mol/kg when OC was 0.6, and then decreased when OC further increased. The effects of oxidant on the removal rates of COD and NH3-N concentration were shown in Fig. 13. It could be seen from the figure
3.7. Comparison with catalyst and oxidant From the above analysis results, it could be seen that catalyst and oxidant play a good role in gasification and treatment of landfill leachate in supercritical water. However, which method was more suitable for the gasification and treatment of landfill leachate in supercritical
Table 2 Comparison of gasification with catalyst and oxidant.
KOH H2O2
T (°C)
Time (min)
Catalyst amount(g/g)
Oxidation coefficient
CE (%)
H2 (mol/kg)
HYP (mol/kg)
650 650
20 20
1.6 0
0 0.6
99.2 68.2
24.3 21.9
57.6 43.9
9
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Table 4 Comparison of landfill leachate gasification between this work and others. Feed
Reactor
Heating rate (K/min)
COD (mg/l)
T (°C)
P (MPa)
Time/flowrate
catalyst
OC
CE (%)
XCOD (%)
XNH3-N (%)
Refs.
Landfill leachate
Continuous Batch Batch Batch Batch
3 20 20 70
1580 42536 35500 6633 36950
650 500 600 550 650 650 600 500
25 29 25 25 25.9 25.3 25.2. 23.6
6 g/min 25 min 10 min 10 min 25 min 25 min 25 min 25 min
– – – – KOH
– – – 0.25 – 0.6
– 44.8 55 85 99.2 68.2 65.0 49.5
97.4 76.8 – – 94.6 82.2 76.4 72.2
– 6.5 7.6 – 51.0 50.0 12.3 0.5
Ferreira et al. [22] Gong et al. [25] Gong et al. [24] Gong et al. [23] This work
44.8% which was lower than the result obtained at the heating rate of 70 K/min. The NH3-N removal rate obtained at the low heating rate was higher than that obtained at the high heating rate. It may be due to the low heating rate (3 K/min) which resulted in the increased reaction time of landfill leachate gasification in supercritical water, and it was beneficial to the removal of COD and NH3-N, but not beneficial to gasification at the same conditions. Although the CE and COD removal rate with the addition of oxidant were lower than the results obtained with the addition of KOH at the high heating rate (70 K/min) and 650 °C, the NH3-N removal rate was consistent with the results obtained by the addition of KOH. It also indicated that the addition of oxidant could accelerate the removal of NH3-N.
water had not been compared. Therefore, the catalytic gasification and partial oxidation gasification of landfill leachate were compared. Table 2 showed the comparison of gasification with catalyst and oxidant. At the same condition, the maximum CE with the addition of oxidant reached 68.2% and it was far lower than the values of 99.2% when KOH was added. The maximum yield of hydrogen by adding oxidant was 21.9 mol/kg which was lower than that obtained by adding KOH. The maximum HYP was 43.9 mol/kg and 57.6 mol/kg with the addition of oxidant and catalyst, respectively. It could be seen that the role of gasification improvement by adding oxidant in the range of the experiments was far less than that by adding KOH catalyst. Table 3 showed the comparison of the removal rates of COD and NH3-N with catalyst and oxidant. The removal rate of COD was greatly improved by adding KOH, but it remained unchanged with the addition of oxidant, which showed that the removal effect of COD was obviously better with KOH than that with oxidant. For NH3-N removal, KOH and oxidant were equally effective. In general, KOH was more effective in removing COD and NH3-N from landfill leachate. According to the above analysis, the gasification and decontamination of landfill leachate with KOH in supercritical water were better than that of partial oxidation gasification. Although KOH could promote gasification and COD degradation of landfill leachate well, only about 51.0% of NH3-N could be eliminated. Although partial oxidation could not enhance gasification and COD degradation well like the role of KOH, its effect on NH3-N elimination was similar to that obtained by using KOH. The mass of KOH addition was higher than the mass of oxidizer addition. It indicated that the less addition of oxidant could play a good role in NH3-N elimination, but had no obvious effect on gasification. The experiments of SCWO of landfill leachate showed that the excessive oxidant can completely eliminate COD and NH3-N. But the excessive oxidant would cause serious corrosion to the reactor and no hydrogen production. In order to avoid the problem caused by excessive oxidant and produce hydrogen better, combined with the above experimental results, the mixed catalytic (namely the combination of catalytic gasification and partial oxidation) was very desirable. Followup work was being carried out, and relevant reports would be carried out later.
4. Conclusions In this work, landfill leachate gasification in supercritical water with high heating rate batch reactor was proved to be feasible. These results showed that high heating rate could promote landfill leachate gasification in supercritical water better. High temperature and long residence time could improve gasification efficiency. The catalytic activity decreased in the order: KOH > K2CO3 > MnO2 > KMnO4. The addition of KOH could increase CE and H2 yield greatly, and the highest CE was 99.2% and hydrogen yield was 26 mol/kg. At the same time, KOH could also improve the COD and NH3-N conversion significantly, and promote the degradation of organic compounds in landfill leachate. The highest COD removal rate was 94.6% and NH3-N removal rate was 51.0%. The addition of oxidant had the same effect on NH3-N elimination which was similar to that obtained by using KOH. The results of experiments demonstrated that landfill leachate gasification in supercritical water could not only degrade the organic compounds but also generate hydrogen efficiently. The combination of catalytic gasification and partial oxidation might be an effective method to remove NH3-N. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
3.8. Comparison with literature
Acknowledgement
Table 4 showed a comparison of present work with published literatures on landfill leachate gasification in supercritical water. In this work, original landfill leachate was gasified at 650 °C by using KOH and the best CE (99.2%) was reached. The COD and NH3-N removal rate reached 94.6% and 51.0% respectively. These results were much higher than those obtained by other studies. Although the COD removal rate of landfill leachate in continuous reactor reached 97.4% at 650 °C, the COD concentration of landfill leachate was only 1580.0 mg/l which was lower than that of original landfill leachate. At 600 °C, the CE obtained at the heating rate of 70 K/min was higher than that obtained at the heating rate of 20 K/min. Although the COD removal rate reached 76.8% at the heating rate of 3.0 K/min at 500 °C which was higher than the result obtained at the heating rate of 70 K/min, the CE was only
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