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Gasification of landfill leachate in supercritical water: Effects on hydrogen yield and tar formation
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e1 1
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Gasification of landfill leachate in supercritical water: Effects on hydrogen yield and tar formation Yanmeng Gong a,b,c, Jiaang Lu b, Weili Jiang b, Shuyang Liu b, Wei Wang b, Aimin Li a,* a
State Key Laboratory of Pollution Control and Resources Reuse, School of the Environment, Nanjing University, Nanjing, Jiangsu, 210023, PR China b Jiangsu Provincial Academy of Environmental Science, Jiangsu Province Key Laboratory of Environmental Engineering, Nanjing, Jiangsu, 210036, PR China c 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, PR China
article info
abstract
Article history:
Landfill leachate was gasified in supercritical water (SCW) in a batch reactor made of 316
Received 29 June 2018
SS. The effects of temperature, pressure, reaction time and oxidation coefficient (OC) on
Received in revised form
the pollutant removal efficiencies and gasification characteristics were investigated. To
27 August 2018
observe the formation of tar and char visually, a capillary quartz reactor was also used.
Accepted 4 September 2018
Results indicated that CO2, H2 and CH4 were the most abundant gaseous products. Tem-
Available online xxx
perature has an appreciable effect on the gasification process. Increasing temperature
Keywords:
influence of reaction time on the fractions of gaseous products was negligible at time above
Landfill leachate
300 s, the yields of H2, CH4, and CO2 increased with reaction time whereas the CO, C2H4 and
enhanced the H2 yield (GYH2) and TOC removal efficiency (TRE) significantly. Although the
Supercritical water
C2H6 yields decreased. Tar and char formation was evident on the interior surface of
Gasification
capillary quartz reactor. Adding a little oxidant could increase H2 and CH4 yields and
Partial oxidation
decrease tar and char formation. GYH2 reached up to the maximum of 231.3 mmol L1
Hydrogen
leachate at 500 C, 25 MPa, 600 s and 0.2 OC, which was 2.4 times of that without oxidant. © 2018 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.
Introduction The gradual depletion of fossil energy and the deterioration of the ecological environment have motivated the pursuit of renewable energy alternatives to fossil fuels. Converting biomass to fuel gases (e.g., H2 and CH4) is one such alternative pathway [1]. Hydrogen production by supercritical water gasification (SCWG) from biomass sources has recently received much attention, due to the unique physical
properties of water above its critical conditions (T > 374.15 C, P > 22.12 MPa), such as low viscosity, high diffusivity, variable water density, low dielectric constant and special solubility for organics and gases [2,3]. This technology is well suited for processing high-moisture biomass since no drying step is needed allowing to reach hydrogen-rich gas with short residence times [4,5]. Landfill leachate is generated during the process of stack and landfill of municipal solid wastes. Being a refractory wastewater, leachate is often a mixture of high-strength
* Corresponding author. E-mail address: [email protected] (A. Li). https://doi.org/10.1016/j.ijhydene.2018.09.020 0360-3199/© 2018 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC. Please cite this article in press as: Gong Y, et al., Gasification of landfill leachate in supercritical water: Effects on hydrogen yield and tar formation, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.09.020
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organic and inorganic contaminants. For instance, young landfill leachate, which often has an age less than 5 years, is characterized by high concentrations of biodegradable organic matter, usually with a COD value greater than 10,000 mg/L [6]. Moreover, carboxylic acids, considered as the intermediates of biomass in SCWG process [7,8], are often found in leachate [8,9], which suggest that leachate may be suitable for H2 production in supercritical water (SCW). Currently, landfill leachate has been proposed as a biomass source for H2 and CH4 production by SCWG. For instance, Ferreira-Pinto et al. [10] firstly studied the gasification of landfill leachate in SCW. They found that temperature was the most important factor on the degradation of leachate and H2 was the most abundant gaseous product under all the experimental conditions. In another study reported by Molino et al. [8], the H2 and CH4 compositions in gaseous product of landfill leachate SCWG varied in the range of 25%e47% and 11%e18%, respectively, and the gas yield decreased with the increasing leachate flow rate. Furthermore, several alkali catalysts were proved to promote the hydrogen production for landfill leachate SCWG, and the catalyst efficiency was in the following order: NaOH > KOH > Na2CO3 > K2CO3 [11,12], despite the fact that the alkaline catalyst may cause corrosion and plugging of reactor [13,14]. However, significant amounts of tar and char are produced during SCWG process of high concentration feedstock [1,15,16]. Once converted into char, the feedstock is hardly gasified, resulting in lower gas yield and gasification efficiency. Moreover, the SCWG reactors could be plugged by the accumulation of ash and char [15]. Calzavara et al. [17] even suggested that the formation of tar and char might be the most significant technological problem in supercritical water process for H2 production. To date, supercritical water partial oxidation (SCWPO) may be the alternative method to solve this problem by adding certain amount of oxidant less than the stoichiometric requirements for complete oxidation [18,19]. In a SCWPO process, pure oxygen or air is adopted as the typical oxidant. One key advantage of SCWPO is that the in-situ heat generated from the oxidation reaction can rapidly heat up gasification medium through the sensitive temperature range, leading to less char formation and higher H2 yield [20]. Another advantage of this process is the negligible emission of criteria pollutants, including particulates, NOx, SOx and so on. Furthermore, the aqueous environment in SCWPO with high pressure and high density is favorable for the reaction and gasification of organics. In short, SCWPO process involves endothermic gasification reaction and exothermic oxidation reaction, thereby accelerating refractory substances degradation and improving the gasification efficiency [21]. Our previous study found that the TOC removal efficiency could reach up to 94.56% under SCWPO condition. However, no carbon balance was reached throughout the process due to the formation of tar and char, and carbon balance increased with the increase of temperature and oxidation coefficient [20]. To date, the experiments about gasification and partial oxidation of landfill leachate in SCW are still insufficient. Therefore, aiming to contribute to the deficient knowledge regarding this area, gasification and partial oxidation of landfill leachate in SCW were carried out in a batch reactor in
the present study, respectively. The effects of temperature (T), pressure (P), reaction time (t) and oxidation coefficient (OC) on the contaminant removal and gasification characteristics of leachate were investigated. Furthermore, a capillary quartz reactor was used to observe the formation of tar and char visually.
Materials and methods Leachate characteristics Leachate from municipal solid urban waste was obtained from a landfill site in Nanjing, Jiangsu Province, China. The main characteristics of the leachate are listed in Table 1. As shown, leachate is a carbon-rich high concentration organic wastewater with large amounts of suspended solids. 30 wt % hydrogen peroxide (analytical reagent grade), usually used as a source of free radicals to help decompose the compounds which are difficult to be gasified, was chosen as the oxidant [18]. The chromatographically pure dichloromethane was used as the extraction solvent. Both hydrogen peroxide and dichloromethane were purchased from Sinopharm Chemical Reagent Co., Ltd.
Apparatus and procedure The experimental device used in this work is schematically represented in Fig. 1. A batch reactor (length ¼ 300 mm, I.D. ¼ 20 mm, O.D. ¼ 35 mm) made of 316-stainless steel was equipped in the experimental systems, with the designed temperature and pressure of 625 C and 30 MPa, respectively. After loading a certain amount of landfill leachate and H2O2 solution, the reactor was sealed carefully and connected to a vacuum pump to remove the residual air in the reactor. The decomposition of H2O2 under negative pressure condition can be neglected [22]. The quantities of leachate and H2O2 solution were calculated based on the defined reaction temperature, pressure and oxidation coefficient. Subsequently, the reactor was pressurized to 0.1 MPa by helium, which served as an internal standard for gas product analysis [23]. Considering fluidized sand has a faster heating rate and a better heat transfer mechanism, the sealed reactor was heated in an isothermal fluidized sand bath [24]. Once the temperature and pressure in the reactor reached the desired values, we started the timer. After the desired reaction time elapsed, the reactor was quickly removed from the sand bath and immersed into cool water to quench the reaction. Considering the solid
Table 1 e Raw Leachate composition. Leachate characteristics (mg$L1, except pH) COD NH3eN TN TOC TC pH SS
35,500 ± 1800 2500 ± 180 3100 ± 80 14,100 ± 340 15,500 ± 480 8.09 ± 0.04 5000 ± 170
Dry basis composition (wt. %) C H O N S Moisture content Ash content
63.14 1.14 13.44 6.27 0.78 5.81 9.42
Please cite this article in press as: Gong Y, et al., Gasification of landfill leachate in supercritical water: Effects on hydrogen yield and tar formation, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.09.020
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leachate and liquid products were measured by a TOC analyzer (EURO TECH, ET 1020A). Ammonia nitrogen (NH3eN) and total nitrogen (TN) were determined by an ultraviolet spectrophotometry. Detailed experimental analysis methods can be seen in the literature [23]. In this work, TOC removal efficiency (TRE) and oxidation coefficient (OC) are defined as follows [25]. TRE ¼
OC ¼
Fig. 1 e Schematic diagram of experimental apparatus (1) high-pressure gauge; (2) safety valve; (3)e(4) thermocouple; (5) temperature controller; (6) power; (7) air compressor; (8) sand bath; (9) reactor; (10)e(12) needle valves; (13) vacuum gauge; (14) vacuum pump; (15) pressure gauge; (16) helium tank.
products were scarce for quantitative analysis, only gaseous and liquid products were collected and measured. All experiments reported in this work were replicated with deviations lower than 10%. In order to observe the formation of tar and char visually, a 200 mm length of quartz tube reactor (I.D. ¼ 2 mm, O.D. ¼ 6 mm) was also employed in this work. After one end of the quartz tube reactor was sealed by oxy-acetylene flame, landfill leachate and H2O2 solution (if necessary) were injected into the reactor by a micro syringe. If the oxidation coefficient was specified as zero, only landfill leachate was injected into the reactor. In addition, the effect of residual air in the reactor on the gasification reaction could be neglected [24]. Subsequently, the other open end of quartz tube reactor was sealed by oxy-acetylene flame as well. The sealed quartz tube reactor was heated by a tube heating furnace (SHENGLI TEST, SL140020), whose temperature was measured by a K type thermocouple and maintained by a temperature controller. For each test, sealed reactor was put into the preheated furnace. In the end of the reaction, the reactor was taken out from the furnace after a specified time and cooled to room temperature by a fan.
Experimental analysis The composition (H2, CH4, CO, CO2 and C2 gases) of gaseous product was analyzed by a gas chromatograph (Agilent 6890), equipped with a thermal conductivity detector (TCD), a TDX01 chromatographic column (3 m 3 mm) and a GDX-502 chromatographic column (3 m 3 mm). Argon was served as the carrier gas. The temperatures of sample injector, column and detector were set to be 50 C, 50 C and 100 C, respectively. In addition, the qualitative analysis of organic components in landfill leachate and liquid product were performed by a gas chromatograph-mass spectrometer (Agilent GC 6890-MS 5973) equipped with a HP-5MS capillary column (30 m 0.25 mm I.D., 0.25 mm film thickness) and using helium as the carrier gas. Total organic carbon (TOC) in landfill
mTOC;0 mTOC 100% mTOC;0
mO2; add mO2; need
(1)
(2)
where mTOC,0 (mg) is the initial mass of TOC in leachate, mTOC (mg) represents the mass of TOC in liquid products. Moreover, mO2,add (mg) is the mass of O2 generated from the decomposition of H2O2 loaded into the reactor and mO2,need (mg) is the mass of oxidant for complete oxidation by stoichiometry calculation, respectively. In order to quantify the performance of SCWG process in the case of leachate treatment, gas fraction (4i, %) and gas yield (GYi, mmol$L1 leachate) of gas i were assessed: 4i ¼
the molar number of gas i 100 the molar number of all gas products
GYi ¼
the molar number of gas i the volume of leachate
(3)
(4)
Carbon recovery rate (CR, %) is defined in Eq. (5), to evaluate the amount of carbon in gaseous and liquid products. CR ¼ wC;G þ wC;L
(5)
where wC,G (%) and wC,L (%) are the mass fractions of carbon in gaseous product and liquid product to total carbon in leachate, respectively.
Results and discussions Effect of temperature Fig. 2(a) illustrates the concentrations and removal efficiencies of TOC in liquid products, which were obtained from landfill leachate SCWG at 200e600 C and 25 MPa for 600 s when OC was equal to 0. The error bars in the figures represent the standard deviation obtained over three tests at each condition. As shown in Fig. 2(a), it is worth noting that, compared with TOC concentration in raw leachate (14,100 mg L1), TOC in liquid products increased rather than decreased under 450 C, leading to the negative values of TRE. For instance, maximum TOC in liquid product reached up to 16,500 mg L1 at 350 C; meanwhile, TRE was 16.9% correspondingly. This could be attributed to solid particles in raw landfill leachate. Before the measurement of TOC in landfill leachate and liquid products, the samples were filtered to protect the instrument from solid particles. However, the feedstocks were gasified without filtration in this work. In SCW, organics in suspended solids dissolved into liquid phase at lower temperature before gasification process, as shown in Eq. (6). When the temperature reached above 450 C, a
Please cite this article in press as: Gong Y, et al., Gasification of landfill leachate in supercritical water: Effects on hydrogen yield and tar formation, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.09.020
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Fig. 2 e Effect of temperature on gasification of landfill leachate at 600 s, 25 MPa and 0 OC: (a) TOC and TRE, (b) NH3eN and TN, (c) gas composition, (d) gas yield.
significant decrease of TOC was observed as temperature increased, indicating that organics in leachate were degraded remarkably at higher temperature. At 600 C, the concentration of TOC in liquid products dropped to 2000 mg L1 with 85.5% of TRE. SCW
Cm Hn Op Nq ðsÞ!Cm Hn Op Nq ðlÞ
(6)
Fig. 2(b) shows the effect of temperature on the concentrations of NH3eN and TN in liquid products. The concentrations of TN and NH3eN in liquid products were always higher than those in landfill leachate under all experimental conditions. The enhancement of TN in liquid products was another evidence of dissolution of solid organics in landfill leachate during SCWG process. Moreover, NH3eN concentration was close to that of TN consistently, indicating that TN in liquid products was almost entirely composed of NH3eN. This result is understandable because NH3 is generally the intermediate product of nitrogen-containing compounds in SCWG [1,26]. When temperature rose from 200 C to 450 C, both TN and NH3eN demonstrated a weak improvement. The maximum NH3eN and TN reached up to 4200 and 4600 mg L1 at 450 C, respectively. When temperature increased to 600 C, NH3eN and TN decreased to 3900 and 3600 mg L1, respectively, indicating that only a very small amount of NH3 was converted to N2. There was no obvious trend to reveal that NH3 was converted into N2 under our experimental conditions. In this work, gaseous product could not be detected until the temperature reached above 400 C. The temperature dependence of the gas composition and gasification yield (GY) from SCWG tests for 600 s and 25 MPa are shown in Fig. 2(c) and (d), respectively. As Fig. 2(c) shows, the gaseous products of landfill leachate in SCW were composed of CO2, H2, CH4, CO,
C2H4 and C2H6, in which CO2, H2 and CH4 were the major components. Generally, the most predominant reaction during SCWG is steam reforming reaction [27], which is expressed as Eq. (7). The steam reforming reaction of organics is endothermic reaction. Thus, an increase in temperature directly promotes the steam reforming reaction, and further facilitates the generation of H2. When temperature increased from 400 C to 600 C, H2 gas fraction (4H2) increased from 19.4% to 29.0%. Besides, CO2 was always the main gaseous product in our experimental conditions. Except for the generation from water gas shift reaction between CO and H2O (Eq. (8)), CO2 can also be converted from the carboxylic acids in landfill leachate via decarboxylation reaction in SCW [3,28]. Various carboxylic acids in landfill leachate can be identified from GC-MS analysis in Appendix A, such as butanoic acid, hexanoic acid and so on. However, 4CO decreased from 18.6% to 0.9% as temperature rose from 400 to 600 C. This can be explained by the water gas shift reaction between CO and H2O, in which CO is consumed and the hydrogen atom in H2O is converted into H2. Although this reaction is exothermic, it is thermodynamically beneficial when the temperature is less than 815 C [27]. On the other hand, the increase of temperature from 400 to 600 C shifts the equilibrium in methanation reactions (Eqs. (10) and (11)) to the left side, whereas 4CH4 increased from 11.3% to 41.8%. Although the mole fractions of C2H4 and C2H6 also increased from 0.37% and 0.74% to 1.05% and 3.82% when the temperature increased from 400 to 600 C, respectively, these two gases were relatively scarce in the gaseous product compared with other gases. Thus, the generation of C2 gases can be neglected in the SCWG process of landfill leachate. The trends of gas product distribution from SCWG of landfill leachate in this study are in agreement with those reported in previous studies [3,10]. As shown in Fig. 2(d), GYH2, GYCH4 and
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GYCO2 enhanced dramatically when the temperature increased from 400 C to 600 C, which indicates that high temperature promotes the steam reforming reaction in SCW medium. At 600 C, the yields of H2, CH4 and CO2 reached up to 245, 255 and 296 mmol L1 leachate, respectively. This result suggests that landfill leachate is a potential feedstock for hydrogen production by SCWG. Minor compounds of the gaseous products (such as C2 species) followed the same trend of GYH2. However, the yields of CO and C2 gases are negligible, compared with GYH2, GYCH4 and GYCO2. Cm Hn Op Nq þH2 O/CO þ H2 þ NH3 þ other compounds
(7)
CO þ H2 O/CO2 þ H2
(8)
RCOOH/RH þ CO2
(9)
CO þ 3H2 /CH4 ðor C2 H4 ; C2 H6 Þ þ H2 O
(10)
CO2 þ4H2 /CH4 ðor C2 H4 ; C2 H6 Þ þ 2H2 O
(11)
Carbon recovery (CR) was adopted to evaluate the carbon distribution in products, and the results were exhibited in Table 2. As shown in Table 2, wTOC decreased from 76.6% to 9.40% rapidly while wTIC, wCH4 and wCO2 increased steadily when temperature rose from 400 C to 600 C, indicating that more than 90% of organics in leachate had been degraded into inorganic carbon species and carbonaceous gases at 600 C. Furthermore, CO2 produced in the gasification process reacts with alkaline substances (such as NH3) in feedstock to generate carbonate and bicarbonate, leading to the increase of TIC in liquid products, as shown in Eq. (12). Meanwhile, CR changed from 82.0% to 54.4%. Similarly, Kıpc¸ak et al. [2] also found that the carbon balance only closed within 32% to 86% in hydrothermal gasification of olive mill wastewater in SCW. The missing carbon in the balance may be owing to the formation of tar and char via carbonization reaction [19,29]. Gong et al. [11] confirmed that solid phase products were mainly composed of travertine, ankerite and calcite, and tar and char were not detected in leachate SCWG. However, we observed solid particles on the filter after each run and sticky brown tar adhering on the walls of the sample tubes. Moreover, Xu et al. [19] proposed that the formation of C3 - C4 light gases was likely to be another reason for the missing carbon. However, C3 - C4 light gases products were not identified in our work. H2 O
H2 O
2 þ CO2 þ 2NH3 !2NHþ 4 þ CO3 % 2NH4 þ 2HCO3
(12)
Table 2 e Effect of temperature on CR and carbon distribution at 600 s, 25 MPa and 0 OC. T/ C
400 450 500 550 600
wC,L/%
wC,G/%
wTIC/%
wCH4/%
wCO2/%
76.6 70.6 45.3 25.3 9.40
2.16 1.68 2.54 11.2 12.1
0.31 0.88 3.92 10.6 14.0
2.12 5.82 7.79 12.0 16.2
Effect of reaction time Four reaction times (300, 600, 900 and 1200 s) were employed for evaluating the time dependency of SCWG reaction of landfill leachate. As shown in Fig. 3(a), a significant improvement of TRE was observed with increasing time when the reaction time was less than 900 s. Then, the enhancement on TRE slowed down gradually. When time increased from 300 s to 1200 s at 500 C, the concentration of TOC in liquid products decreased from 13,200 mg L1 to 7300 mg L1; meanwhile TRE increased from 6.4% to 48.2%. It indicates that increasing time favors the degradation of organics. However, increasing time had no significant effect on NH3eN concentrations, as shown in Fig. 3(b), which was probably because ammonia, known as a ‘‘refractory substance’’, was still stable in SCW environment [26]. The effect of time on variation trends of gas fraction as well as gas yield are depicted in Fig. 3(c) and (d) at 25 MPa and 500 C, respectively, when reaction time changed from 300 s to 1200 s. As shown in Fig. 3(c), the fractions of gaseous products have changed weakly in the experimental range. It indicated that reaction time did not significantly influence the product distribution. D'Jesu´s et al. [30] also found that the gas composition did not change with time when reaction time was longer than 1.5 min in SCWG of corn silage. Moreover, Kersten et al. [31] concluded that, when the reaction time was longer than 40 s, it did not influence gas composition significantly for glucose SCWG. Fig. 3(d) shows the effect of time on GY. It can be observed that as time increased from 300 s to 1200 s, GYH2, GYCH4 and GYCO2 increased from 120, 72 and 135 mmol L1 leachate to 163, 155 and 193 mmol L1 leachate, respectively. A possible explanation for GY in Fig. 3(d) is that the reverse process of methanation reaction and the water gas shift reaction will be reinforcement with increasing reaction time [19]. However, GY of CO, C2H4 and C2H6 declined gradually. It indicates that although increasing reaction time has little effect on molar fraction of gaseous products, it is helpful for the gas yield of leachate SCWG. Besides, Zhu et al. [32] found that GY of H2, CO, CH4 and CO2 increased as time increased from 10 s to 1800 s in the gasification of glucose. Table 3 summarizes CR and carbon distribution in liquid and gaseous products under different reaction times. As demonstrated, wCO2 and wCH4 increased from 7.46% and 4.00% to 10.6% and 8.57% as time increased from 300 s to 1200 s, respectively. Meanwhile, wTOC reduced from 60.6% to 33.5%, suggesting that most of the organic carbon had been degraded. Correspondingly, CR fell inside the range of 53.3e75.5%. In SCWG of papermaking black liquid, Sricharoenchaikul [33] observed that the yield of tar and carbon deposition reduced greatly and mass fraction of gas products increased as time increased.
Effect of pressure CR/%
wTOC/%
5
82.0 80.1 61.1 62.2 54.4
Fig. 4 shows the effect of pressure on gasification of landfill leachate in SCW at 500 C, 600 s and 0 OC. As shown in Fig. 4(a) and (b), pressure has negligible influence on the concentration of TOC and NH3eN in liquid products. However, Kıpc¸ak [2] found that the concentration of TOC in liquid products augmented from 874.3 mg L1 to 1270.0 mg L1 as pressure rose from 100 bar to 300 bar at 550 C, whereas the degradation of
Please cite this article in press as: Gong Y, et al., Gasification of landfill leachate in supercritical water: Effects on hydrogen yield and tar formation, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.09.020
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Fig. 3 e Effect of reaction time on gasification of landfill leachate at 500 C, 25 MPa and 0 OC: (a) TOC and TRE, (b) NH3eN, (c) gas composition, (d) gas yield.
TOC gradually decreased as pressure increased. This discrepancy might be explained by the differences of the investigated pressure interval. In our study, the tested pressure changed from 23 to 29 MPa, and the narrow pressure interval had negligible influence on the degradation of organics. Fig. 4(c) and (d) show the effect of pressure on molar fraction of gas and GY of landfill leachate, respectively. As shown in Fig. 4(c), 4CH4 increased while 4CO2 decreased with increasing pressure. This result agrees well with the finding of Kıpc¸ak et al. [2], where an increase of CO2 content as well as a decrease of CH4 content in the gas effluent was observed with deceasing pressure. However, no obvious variation of 4H2 was observed in the range of 23 MPae29 MPa, indicating that the effect of pressure on gasification products is complicated [34]. Furthermore, 4CO, 4C2H4 and 4C2H6 were consistently lower than 5% under experimental pressure. As Fig. 4(d) shows, GYCO2 decreased with system pressure. GYCO2 was 200 mmol L1 leachate at 23 MPa, and reduced to 122 mmol L1 leachate at 29 MPa. Although the yields of H2 and CH4 were weakly affected by pressure, the yield of all gaseous products decreased from 433 to 328 mmol L1 leachate, implying that high pressure appears to depress the gasification reaction of landfill leachate. Lu et al. [34] found GYH2 increased with
Table 3 e Effect of reaction time on CR and carbon distribution at 500 C, 25 MPa and 0 OC. t/s
300 600 900 1200
wC,L/%
wC,G/%
CR/%
wTOC/%
wTIC/%
wCH4/%
wCO2/%
60.6 45.3 34.3 33.5
3.48 2.54 5.12 12.1
4.00 3.96 4.56 8.57
7.46 7.87 9.31 10.6
75.5 59.7 53.3 64.8
increasing pressure, but GYCH4 and GYCO had a tendency to decrease with pressure, which suggests that higher pressure favors the water gas shift reaction.
Effect of oxidation coefficient In this section, we investigated the effect of oxidation coefficient (OC) on the gasification of landfill leachate in SCW. As shown in Fig. 5(a), when OC increased from 0 to 1.0, TOC in liquid products decreased from 9900 mg L1 to 5753 mg L1 at 500 C, 25 MPa and 600 s. Correspondingly, TRE increased from 30.0% to 49.0%, indicating that the increase of OC could improve the removal efficiency of organics remarkably. Fig. 5(b) shows effect of OC on the concentration of NH3eN. Although the concentration of NH3eN in liquid products decreased significantly with the increase of OC, it was still higher than that in leachate. As OC increased from 0 to 1.0, the concentration of NH3eN reduced from 4105 mg L1 to 2649 mg L1. The result implied that NH3eN was likely to convert into N2 after adding a small amount of oxidant, as shown in Eq. (13). However, NH3eN was still the stable pollutant in SCWPO of landfill leachate. 4NH3 þ3O2 /2N2 þ 6H2 O
(13)
As shown in Fig. 5(c), the main products of SCWPO of landfill leachate were CO2, H2 and CH4. Meanwhile, trace amount of CO, C2H4 and C2H6 were detected with sum of mole fraction of these three gases consistently lower than 5%. This result agrees with that of the previous study. Jin et al. [18] found that in the partial oxidative gasification of biomass and its model compounds in SCW, the amount order of produced gases was H2>CO2>CH4>CO with small amount of oxidant, while the amount order of produced gas was
Please cite this article in press as: Gong Y, et al., Gasification of landfill leachate in supercritical water: Effects on hydrogen yield and tar formation, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.09.020
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Fig. 4 e Effect of pressure on gasification of landfill leachate at 500 C, 600 s and 0 OC: (a) TOC and TRE, (b) NH3eN, (c) gas composition, (d) gas yield.
Fig. 5 e Effect of OC on gasification of landfill leachate at 500 C, 600 s and 0 OC: (a) TOC and TRE, (b) NH3eN, (c) gas composition, (d) gas yield. CO2>H2>CH4>CO only OC was comparatively large. In this work, as OC increased from 0 to 1.0, 4CO2 rose gradually from 41.9% to 81.2%; meanwhile, 4CH4 reduced from 21.0% to 4.2%. This can be explained by the fact that the higher amount of oxidant may promote the oxidation of CO and CH4 into CO2. Furthermore, 4H2 increased as OC increased when OC is lower
than 0.2; on the contrary, it decreased as the increase of OC once the OC was greater than 0.2. Excessive oxidant in gasification system may convert CO into CO2 to inhibit water gas shift reaction, or even react with H2, which will lead to low GYH2 [19]. When OC equaled 0.2, 4H2 reached its maximum value of 32.6% at the investigated experimental conditions.
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The stoichiometric complete oxidation of landfill leachate was achieved at OC of 1.0; however, 12.1% H2 was still detected in gaseous products. This is mainly because lots of organics in suspended solids in landfill leachate were not considered when we measured the COD value of raw landfill leachate. In conclusion, in addition to steam reforming reaction, other reactions, including water gas shifting reaction, methanation reaction and oxidation reactions, also existed among organics, CH4, CO, H2 and oxidant in SCWPO of landfill leachate [18,21,35], as shown in Eqs (14)e(17). Cm Hn Op þ ðm þ n pÞ=2O2 /mCO þ nH2 O
(14)
CH4 þ 1=2O2 /CO þ 2H2
(15)
CO þ 1=2O2 /CO2
(16)
H2 þ 1=2O2 /H2 O
(17)
Fig. 5(d) shows the effect of OC on GY. As OC increased from 0 to 1.0, GYCO2 increased rapidly from 143.0 to 850.8 mmol L1 leachate. However, GYH2 reached the maximum of 231.3 mmol L1 leachate at 0.2 OC, which was 2.4 times the GYH2 in the absence of oxidant. When OC exceeded 0.2, GYH2 started to decline and reduced to 95.8 mmol L1 leachate when OC equaled 1.0. Furthermore, similar to GYH2, GYCH4 reached to the maximum of 147.6 mmol L1 leachate at 0.2 OC. These results suggest the optimal OC in SCWPO of landfill leachate should exist, in which the largest GYH2 can be available. In this work, the optimal OC was 0.2. Adding a small amount of O2 could notably contribute to steam reforming reaction of organics, thereby improving GYH2, GYCH4 and GYCO2. However, oxidation reaction will gradually dominate in the presence of much oxidant, and H2 and CH4 will be oxidized into H2O and CO2, respectively [36]. The data in Table 4 can be used to estimate the effect of OC on carbon distribution and CR in products. wCO2 increased with increasing OC, but the wTOC showed the opposite trend. These trends for wCO2 and wTOC are fully consistent with the increase of TRE and GYCO2 in Fig. 5(a) and (d), respectively. The CR increased from 59.7% to 88.7% when OC increased from 0 to 1.0, indicating that the formation of tar and char was inhibited in the presence of oxidant.
Visual observations We further conducted the gasification process of landfill leachate with quartz tube reactor to observe the generation of
tar and char virtually. The quartz reactors were shown in Fig. 6, after 600 s SCWG reaction at 400 C and 600 C in the absence of oxidant. At 400 C, it can be observed clearly that suspended carbon deposition and sticky tar adhered to the inner wall of the quartz tube. This result demonstrates that the polymerization and carbonization reactions might take place among the complex organics in landfill leachate during the SCWG process, leading to the generation of tar and char. Although tar and char are insoluble in water and difficult to be degraded, they can be greatly reduced in SCW at high temperature. For instance, Sricharoenchaikul [33] found that a raise in reaction temperature led to a decrease in fractions of char and tar at black liquor gasification in SCW. The reduction of char and tar at higher temperature could be visually confirmed. However, the formation of tar in leachate SCWG was still obvious at 600 C in our study. Generally, oxidant is introduced into the SCWG system to inhabit the formation of char or tar and promote gasification yield [18,19]. As can be seen from Fig. 7, compared with the quartz tube reactor image at 500 C, 600 s and 0 OC, less char and tar were observed on the reactor interior surface when OC was equal to 0.5. Virtually, no tar or char products were identified at OC of 1.0. Unfortunately, further analysis of these solid and liquid products observed in this study was not possible due to their extremely small quantities.
Identifying liquid phase products In order to further study the transformation of organics during the gasification process, the organic compounds extracted from leachate and liquid product were listed in Appendix A and B, respectively, which were determined by GC-MS from comparison with standard MS library (NIST 98). In this work, 18 organic components were detected from leachate, including fatty acids, aromatic acids, phenols and other N-containing and/or C-containing compounds, indicating that the organic contaminates in landfill leachate were very complicated. Composition analysis of leachate revealed that leachate was a C-rich biomass with a relevant composition of carboxylic acids. This result suggests that leachate is suitable for energy recovery. Considering acids are the intermediates of any biomass gasification, a significant bio-syngas yield can be expected [8]. After gasified at 500, 25 MPa and 600s in the absence of oxidant, 22 products were identified, as shown in Appendix B. Compared with raw leachate, the organic in liquid product was mainly composed of ketones, phenols, pyridines, anilines and indoles, which was consistent with the study by Gong et al. in the treatment of landfill leachate using SCWG [11]. In
Table 4 e Effect of OC on CR and carbon distribution at 500 C, 600 s and 25 MPa. OC
0 0.2 0.4 0.6 0.8 1.0
wC,L/%
wC,G/%
CR/%
wTOC/%
wTIC/%
wCH4/%
wCO2/%
45.3 42.1 42.1 36.9 40.1 33.0
2.54 4.77 5.44 8.54 5.54 8.47
3.96 8.13 4.79 2.94 1.07 0.44
7.87 18.0 23.7 34.3 39.8 46.8
59.7 73.0 76.0 82.7 86.5 88.7
Fig. 6 e Images of landfill leachate SCWG in quartz tube reactors after 600 s at 400 C and 600 C in the absence of oxidant.
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Fig. 7 e Images of landfill leachate SCWG in quartz tube reactors after 500 C and 600 s when OC was equal to 0, 0.5 and 1.0, respectively.
their study, gasification liquid phase products of leachate were mainly composed of cyclopentanone, 2-octanone, phenol, p-cresol and nitrogenous compounds [11]. In our work, 4 types and 12 kinds of N-containing compounds were supposed to be the products of gasification of landfill leachate. It is not surprising that N-containing compounds are identified among the products from gasification of biomass. For instance, Guan et al. [37] reported that indoles were the major components of the liquid phase products from the gasification of Nannochloropsis sp. Pentanone and hexone rather than organic acids were detected in the liquid products, implying that the ketones may be the products of leachate SCWG. Generally, the ketones are considered to be the intermediate compounds in the reaction of biomass to gases [38,39]. On the other hand, ketones can be converted from organic acids in leachate by bimolecular decarboxylation [28]. For example, under the attack of H free radical, CeO bond in a molecular acetic acid is broken, resulting in the formation of equimolar amounts of CH3CO and H2O. Subsequently, CH3CO can react with CH3 to form acetone [28]. The proposed mechanism is shown in Eq. (18): þH$
þCH3 $
CH3 COOH ! CH3 CO$!CH3 COCH3 H2 O
and CH4 were the most abundant gaseous products under the investigated reaction conditions. Increasing temperature enhanced the H2 production and TOC removal efficiency greatly. However, pressure and reaction time had little influence on organics removal, tar and char generation. (2) Adding a small amount of oxidant favored the formation of H2 and CH4. GYH2 was up to the maximum of 231.3 mmol L1 leachate at 500 C, 25 MPa, 600 s and 0.2 OC, which was 2.4 times the GYH2 in the absence of oxidant. TRE and CR increased as OC increased. TRE and CR were 49.0% and 88.7%, respectively, when OC equaled 1.0. Adding a small amount of oxidant could reduce the formation of tar and char. (3) Suspended carbon deposition and sticky tar could be observed visually on the inner wall of the quartz tube reactor. NH3 was very stable, and NH3eN were hardly converted into N2 under our experimental conditions. Numerous compounds were detected in the liquidphase products after gasification, such as NH3, pyridines, anilines, phenols and indoles.
Acknowledgment The work was supported by National Natural Science Foundation of China (51708262), Jiangsu Province Natural Science Foundation of China (BK20151040), Post-Doctoral Fund of Jiangsu Province (1501124B) and Open Fund of Jiangsu Province Key Laboratory of Environmental Engineering (ZX2017002).
Appendix A. Organic compounds in leachate.
(18)
Besides, strong responses of phenols (33.655% peak area at 7.166 min, 12.024% peak area at 9.431 min and 5.365% peak area at 11.717 min, respectively) were observed in the GC-MS spectrum. The result implies that phenols are stable and hard to be degraded in SCW, and phenols are the main intermediate compounds during gasification process of leachate. Moreover, it is possible to form tar and char in the presence of quinoline, indole, naphthol and other polycyclic aromatic hydrocarbons. Guo et al. [1] presented that polycyclic aromatic hydrocarbons, such as fluorene and phenanthrene, were probably char precursors.
No.
Time/min
Area/%
Name
1 2 3 4 5 6 7 8 9 10 11 12 13
3.397 3.531 5.065 7.153 7.333 8.678 9.413 9.602 10.569 11.759 13.972 15.656 17.710
2.132 1.152 6.686 17.72 12.127 2.573 21.065 12.159 1.498 6.888 1.751 1.305 1.697
14
17.929
0.78
15
18.011
3.056
16 17
19.463 19.601
0.757 0.872
18
19.773
1.757
butanoic acid 2-butanamine pentanoic acid phenol hexanic acid 1-methyl-2-pyrrolidinone 4-methyl-phenol heptanoic acid cyclohexanecarboxylic acid octanoic acid nonanoic acid benzenepropanoic acid 1,3,3-trimethy-l-2-((E)-3methylbuta -1,3-dienyl) cyclohexanol methyl 2-(1,4, 4-trimethylcyclohex-2-enyl) acetate 2-isocyanato-1,3-dimethyl benzene ethyl citrate 2-(4-methylcyclohexyl) prop-2-en-1-ol N,N-dimethyltetradecanamine
Conclusions According to the experimental investigations presented in this paper, the following conclusions can be drawn: (1) Temperature has a crucial effect on the gasification process. Dissolution of solid organics in leachate resulted in the increase of TOC in liquid products below 450 C. Gas products were detected above 400 C. CO2, H2
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Appendix B. Organic compounds in liquid effluent when operated at 500, 25 MPa, 600 s and in the absence of oxidant.
[2]
[3]
NO. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
Time/min
Area/%
Name
2.586 3.600 4.051 4.803 5.164 6.127 7.166 7.304 7.794 8.253 8.928 9.336 9.431 9.487 9.607 10.153 11.717 13.655 14.888 16.731 16.791 18.441
1.045 1.065 2.767 5.480 1.731 2.389 33.655 1.586 2.297 1.553 1.783 2.158 12.024 6.623 1.643 1.358 5.365 2.830 6.642 1.296 0.396 1.476
2-pentanone 2-hexanone 2-methyl-pyridine 3-methyl-pyridine 2,6-dimethylpyridine 3,5-dimethylpyridine phenol 2,3-dimethylpyridine 3,5-dimethylpyridine 2-ethyl-5-methyl-pyridine 2-methyl-phenol acetophenone 3-methyl-phenol 2-methyl-benzenamine 3-methyl-benzenamine 3-pyridinamine 2-ethyl-phenol quinoline indole 2-methyl-indole 5-methyl-indole 2-naphthalenol
[4]
[5]
[6]
[7]
[8]
[9]
[10]
List of symbols [11]
SCW SCWG SCWPO COD TN TC TOC TIC SS NH3eN TRE OC 4i GYi CR mTOC,0 mTOC mO2,add mO2,need wC,G wC,L
Supercritical water Supercritical water gasification Supercritical water partial oxidation Chemical oxidation demand (mg$L1) Total nitrogen (mg$L1) Total carbon (mg$L1) Total organic carbon (mg$L1) Total inorganic carbon (mg$L1) Suspended solids (mg$L1) Ammonia nitrogen (mg$L1) TOC removal efficiency (%) oxidation coefficient gas fraction of gas i (%) gas yield of gas i (mmol$L1 leachate) Carbon recovery rate (%) initial mass of TOC in leachate (mg) the mass of TOC in liquid products (mg) O2 generated from the decomposition of H2O2 (mg) the mass of oxidant for complete oxidation (mg) the mass fraction of carbon in gaseous products (CH4, CO and CO2) to total carbon in leachate (%) the mass fraction of carbon in liquid product (TOC and TIC) to total carbon in leachate (%)
[12]
[13]
[14]
[15]
[16]
[17]
[18]
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Gasification of landfill leachate in supercritical water: Effects on hydrogen yield and tar formation Yanmeng Gong a,b,c, Jiaang Lu b, Weili Jiang b, Shuyang Liu b, Wei Wang b, Aimin Li a,* a
State Key Laboratory of Pollution Control and Resources Reuse, School of the Environment, Nanjing University, Nanjing, Jiangsu, 210023, PR China b Jiangsu Provincial Academy of Environmental Science, Jiangsu Province Key Laboratory of Environmental Engineering, Nanjing, Jiangsu, 210036, PR China c 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, PR China
article info
abstract
Article history:
Landfill leachate was gasified in supercritical water (SCW) in a batch reactor made of 316
Received 29 June 2018
SS. The effects of temperature, pressure, reaction time and oxidation coefficient (OC) on
Received in revised form
the pollutant removal efficiencies and gasification characteristics were investigated. To
27 August 2018
observe the formation of tar and char visually, a capillary quartz reactor was also used.
Accepted 4 September 2018
Results indicated that CO2, H2 and CH4 were the most abundant gaseous products. Tem-
Available online xxx
perature has an appreciable effect on the gasification process. Increasing temperature
Keywords:
influence of reaction time on the fractions of gaseous products was negligible at time above
Landfill leachate
300 s, the yields of H2, CH4, and CO2 increased with reaction time whereas the CO, C2H4 and
enhanced the H2 yield (GYH2) and TOC removal efficiency (TRE) significantly. Although the
Supercritical water
C2H6 yields decreased. Tar and char formation was evident on the interior surface of
Gasification
capillary quartz reactor. Adding a little oxidant could increase H2 and CH4 yields and
Partial oxidation
decrease tar and char formation. GYH2 reached up to the maximum of 231.3 mmol L1
Hydrogen
leachate at 500 C, 25 MPa, 600 s and 0.2 OC, which was 2.4 times of that without oxidant. © 2018 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.
Introduction The gradual depletion of fossil energy and the deterioration of the ecological environment have motivated the pursuit of renewable energy alternatives to fossil fuels. Converting biomass to fuel gases (e.g., H2 and CH4) is one such alternative pathway [1]. Hydrogen production by supercritical water gasification (SCWG) from biomass sources has recently received much attention, due to the unique physical
properties of water above its critical conditions (T > 374.15 C, P > 22.12 MPa), such as low viscosity, high diffusivity, variable water density, low dielectric constant and special solubility for organics and gases [2,3]. This technology is well suited for processing high-moisture biomass since no drying step is needed allowing to reach hydrogen-rich gas with short residence times [4,5]. Landfill leachate is generated during the process of stack and landfill of municipal solid wastes. Being a refractory wastewater, leachate is often a mixture of high-strength
* Corresponding author. E-mail address: [email protected] (A. Li). https://doi.org/10.1016/j.ijhydene.2018.09.020 0360-3199/© 2018 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC. Please cite this article in press as: Gong Y, et al., Gasification of landfill leachate in supercritical water: Effects on hydrogen yield and tar formation, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.09.020
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organic and inorganic contaminants. For instance, young landfill leachate, which often has an age less than 5 years, is characterized by high concentrations of biodegradable organic matter, usually with a COD value greater than 10,000 mg/L [6]. Moreover, carboxylic acids, considered as the intermediates of biomass in SCWG process [7,8], are often found in leachate [8,9], which suggest that leachate may be suitable for H2 production in supercritical water (SCW). Currently, landfill leachate has been proposed as a biomass source for H2 and CH4 production by SCWG. For instance, Ferreira-Pinto et al. [10] firstly studied the gasification of landfill leachate in SCW. They found that temperature was the most important factor on the degradation of leachate and H2 was the most abundant gaseous product under all the experimental conditions. In another study reported by Molino et al. [8], the H2 and CH4 compositions in gaseous product of landfill leachate SCWG varied in the range of 25%e47% and 11%e18%, respectively, and the gas yield decreased with the increasing leachate flow rate. Furthermore, several alkali catalysts were proved to promote the hydrogen production for landfill leachate SCWG, and the catalyst efficiency was in the following order: NaOH > KOH > Na2CO3 > K2CO3 [11,12], despite the fact that the alkaline catalyst may cause corrosion and plugging of reactor [13,14]. However, significant amounts of tar and char are produced during SCWG process of high concentration feedstock [1,15,16]. Once converted into char, the feedstock is hardly gasified, resulting in lower gas yield and gasification efficiency. Moreover, the SCWG reactors could be plugged by the accumulation of ash and char [15]. Calzavara et al. [17] even suggested that the formation of tar and char might be the most significant technological problem in supercritical water process for H2 production. To date, supercritical water partial oxidation (SCWPO) may be the alternative method to solve this problem by adding certain amount of oxidant less than the stoichiometric requirements for complete oxidation [18,19]. In a SCWPO process, pure oxygen or air is adopted as the typical oxidant. One key advantage of SCWPO is that the in-situ heat generated from the oxidation reaction can rapidly heat up gasification medium through the sensitive temperature range, leading to less char formation and higher H2 yield [20]. Another advantage of this process is the negligible emission of criteria pollutants, including particulates, NOx, SOx and so on. Furthermore, the aqueous environment in SCWPO with high pressure and high density is favorable for the reaction and gasification of organics. In short, SCWPO process involves endothermic gasification reaction and exothermic oxidation reaction, thereby accelerating refractory substances degradation and improving the gasification efficiency [21]. Our previous study found that the TOC removal efficiency could reach up to 94.56% under SCWPO condition. However, no carbon balance was reached throughout the process due to the formation of tar and char, and carbon balance increased with the increase of temperature and oxidation coefficient [20]. To date, the experiments about gasification and partial oxidation of landfill leachate in SCW are still insufficient. Therefore, aiming to contribute to the deficient knowledge regarding this area, gasification and partial oxidation of landfill leachate in SCW were carried out in a batch reactor in
the present study, respectively. The effects of temperature (T), pressure (P), reaction time (t) and oxidation coefficient (OC) on the contaminant removal and gasification characteristics of leachate were investigated. Furthermore, a capillary quartz reactor was used to observe the formation of tar and char visually.
Materials and methods Leachate characteristics Leachate from municipal solid urban waste was obtained from a landfill site in Nanjing, Jiangsu Province, China. The main characteristics of the leachate are listed in Table 1. As shown, leachate is a carbon-rich high concentration organic wastewater with large amounts of suspended solids. 30 wt % hydrogen peroxide (analytical reagent grade), usually used as a source of free radicals to help decompose the compounds which are difficult to be gasified, was chosen as the oxidant [18]. The chromatographically pure dichloromethane was used as the extraction solvent. Both hydrogen peroxide and dichloromethane were purchased from Sinopharm Chemical Reagent Co., Ltd.
Apparatus and procedure The experimental device used in this work is schematically represented in Fig. 1. A batch reactor (length ¼ 300 mm, I.D. ¼ 20 mm, O.D. ¼ 35 mm) made of 316-stainless steel was equipped in the experimental systems, with the designed temperature and pressure of 625 C and 30 MPa, respectively. After loading a certain amount of landfill leachate and H2O2 solution, the reactor was sealed carefully and connected to a vacuum pump to remove the residual air in the reactor. The decomposition of H2O2 under negative pressure condition can be neglected [22]. The quantities of leachate and H2O2 solution were calculated based on the defined reaction temperature, pressure and oxidation coefficient. Subsequently, the reactor was pressurized to 0.1 MPa by helium, which served as an internal standard for gas product analysis [23]. Considering fluidized sand has a faster heating rate and a better heat transfer mechanism, the sealed reactor was heated in an isothermal fluidized sand bath [24]. Once the temperature and pressure in the reactor reached the desired values, we started the timer. After the desired reaction time elapsed, the reactor was quickly removed from the sand bath and immersed into cool water to quench the reaction. Considering the solid
Table 1 e Raw Leachate composition. Leachate characteristics (mg$L1, except pH) COD NH3eN TN TOC TC pH SS
35,500 ± 1800 2500 ± 180 3100 ± 80 14,100 ± 340 15,500 ± 480 8.09 ± 0.04 5000 ± 170
Dry basis composition (wt. %) C H O N S Moisture content Ash content
63.14 1.14 13.44 6.27 0.78 5.81 9.42
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leachate and liquid products were measured by a TOC analyzer (EURO TECH, ET 1020A). Ammonia nitrogen (NH3eN) and total nitrogen (TN) were determined by an ultraviolet spectrophotometry. Detailed experimental analysis methods can be seen in the literature [23]. In this work, TOC removal efficiency (TRE) and oxidation coefficient (OC) are defined as follows [25]. TRE ¼
OC ¼
Fig. 1 e Schematic diagram of experimental apparatus (1) high-pressure gauge; (2) safety valve; (3)e(4) thermocouple; (5) temperature controller; (6) power; (7) air compressor; (8) sand bath; (9) reactor; (10)e(12) needle valves; (13) vacuum gauge; (14) vacuum pump; (15) pressure gauge; (16) helium tank.
products were scarce for quantitative analysis, only gaseous and liquid products were collected and measured. All experiments reported in this work were replicated with deviations lower than 10%. In order to observe the formation of tar and char visually, a 200 mm length of quartz tube reactor (I.D. ¼ 2 mm, O.D. ¼ 6 mm) was also employed in this work. After one end of the quartz tube reactor was sealed by oxy-acetylene flame, landfill leachate and H2O2 solution (if necessary) were injected into the reactor by a micro syringe. If the oxidation coefficient was specified as zero, only landfill leachate was injected into the reactor. In addition, the effect of residual air in the reactor on the gasification reaction could be neglected [24]. Subsequently, the other open end of quartz tube reactor was sealed by oxy-acetylene flame as well. The sealed quartz tube reactor was heated by a tube heating furnace (SHENGLI TEST, SL140020), whose temperature was measured by a K type thermocouple and maintained by a temperature controller. For each test, sealed reactor was put into the preheated furnace. In the end of the reaction, the reactor was taken out from the furnace after a specified time and cooled to room temperature by a fan.
Experimental analysis The composition (H2, CH4, CO, CO2 and C2 gases) of gaseous product was analyzed by a gas chromatograph (Agilent 6890), equipped with a thermal conductivity detector (TCD), a TDX01 chromatographic column (3 m 3 mm) and a GDX-502 chromatographic column (3 m 3 mm). Argon was served as the carrier gas. The temperatures of sample injector, column and detector were set to be 50 C, 50 C and 100 C, respectively. In addition, the qualitative analysis of organic components in landfill leachate and liquid product were performed by a gas chromatograph-mass spectrometer (Agilent GC 6890-MS 5973) equipped with a HP-5MS capillary column (30 m 0.25 mm I.D., 0.25 mm film thickness) and using helium as the carrier gas. Total organic carbon (TOC) in landfill
mTOC;0 mTOC 100% mTOC;0
mO2; add mO2; need
(1)
(2)
where mTOC,0 (mg) is the initial mass of TOC in leachate, mTOC (mg) represents the mass of TOC in liquid products. Moreover, mO2,add (mg) is the mass of O2 generated from the decomposition of H2O2 loaded into the reactor and mO2,need (mg) is the mass of oxidant for complete oxidation by stoichiometry calculation, respectively. In order to quantify the performance of SCWG process in the case of leachate treatment, gas fraction (4i, %) and gas yield (GYi, mmol$L1 leachate) of gas i were assessed: 4i ¼
the molar number of gas i 100 the molar number of all gas products
GYi ¼
the molar number of gas i the volume of leachate
(3)
(4)
Carbon recovery rate (CR, %) is defined in Eq. (5), to evaluate the amount of carbon in gaseous and liquid products. CR ¼ wC;G þ wC;L
(5)
where wC,G (%) and wC,L (%) are the mass fractions of carbon in gaseous product and liquid product to total carbon in leachate, respectively.
Results and discussions Effect of temperature Fig. 2(a) illustrates the concentrations and removal efficiencies of TOC in liquid products, which were obtained from landfill leachate SCWG at 200e600 C and 25 MPa for 600 s when OC was equal to 0. The error bars in the figures represent the standard deviation obtained over three tests at each condition. As shown in Fig. 2(a), it is worth noting that, compared with TOC concentration in raw leachate (14,100 mg L1), TOC in liquid products increased rather than decreased under 450 C, leading to the negative values of TRE. For instance, maximum TOC in liquid product reached up to 16,500 mg L1 at 350 C; meanwhile, TRE was 16.9% correspondingly. This could be attributed to solid particles in raw landfill leachate. Before the measurement of TOC in landfill leachate and liquid products, the samples were filtered to protect the instrument from solid particles. However, the feedstocks were gasified without filtration in this work. In SCW, organics in suspended solids dissolved into liquid phase at lower temperature before gasification process, as shown in Eq. (6). When the temperature reached above 450 C, a
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Fig. 2 e Effect of temperature on gasification of landfill leachate at 600 s, 25 MPa and 0 OC: (a) TOC and TRE, (b) NH3eN and TN, (c) gas composition, (d) gas yield.
significant decrease of TOC was observed as temperature increased, indicating that organics in leachate were degraded remarkably at higher temperature. At 600 C, the concentration of TOC in liquid products dropped to 2000 mg L1 with 85.5% of TRE. SCW
Cm Hn Op Nq ðsÞ!Cm Hn Op Nq ðlÞ
(6)
Fig. 2(b) shows the effect of temperature on the concentrations of NH3eN and TN in liquid products. The concentrations of TN and NH3eN in liquid products were always higher than those in landfill leachate under all experimental conditions. The enhancement of TN in liquid products was another evidence of dissolution of solid organics in landfill leachate during SCWG process. Moreover, NH3eN concentration was close to that of TN consistently, indicating that TN in liquid products was almost entirely composed of NH3eN. This result is understandable because NH3 is generally the intermediate product of nitrogen-containing compounds in SCWG [1,26]. When temperature rose from 200 C to 450 C, both TN and NH3eN demonstrated a weak improvement. The maximum NH3eN and TN reached up to 4200 and 4600 mg L1 at 450 C, respectively. When temperature increased to 600 C, NH3eN and TN decreased to 3900 and 3600 mg L1, respectively, indicating that only a very small amount of NH3 was converted to N2. There was no obvious trend to reveal that NH3 was converted into N2 under our experimental conditions. In this work, gaseous product could not be detected until the temperature reached above 400 C. The temperature dependence of the gas composition and gasification yield (GY) from SCWG tests for 600 s and 25 MPa are shown in Fig. 2(c) and (d), respectively. As Fig. 2(c) shows, the gaseous products of landfill leachate in SCW were composed of CO2, H2, CH4, CO,
C2H4 and C2H6, in which CO2, H2 and CH4 were the major components. Generally, the most predominant reaction during SCWG is steam reforming reaction [27], which is expressed as Eq. (7). The steam reforming reaction of organics is endothermic reaction. Thus, an increase in temperature directly promotes the steam reforming reaction, and further facilitates the generation of H2. When temperature increased from 400 C to 600 C, H2 gas fraction (4H2) increased from 19.4% to 29.0%. Besides, CO2 was always the main gaseous product in our experimental conditions. Except for the generation from water gas shift reaction between CO and H2O (Eq. (8)), CO2 can also be converted from the carboxylic acids in landfill leachate via decarboxylation reaction in SCW [3,28]. Various carboxylic acids in landfill leachate can be identified from GC-MS analysis in Appendix A, such as butanoic acid, hexanoic acid and so on. However, 4CO decreased from 18.6% to 0.9% as temperature rose from 400 to 600 C. This can be explained by the water gas shift reaction between CO and H2O, in which CO is consumed and the hydrogen atom in H2O is converted into H2. Although this reaction is exothermic, it is thermodynamically beneficial when the temperature is less than 815 C [27]. On the other hand, the increase of temperature from 400 to 600 C shifts the equilibrium in methanation reactions (Eqs. (10) and (11)) to the left side, whereas 4CH4 increased from 11.3% to 41.8%. Although the mole fractions of C2H4 and C2H6 also increased from 0.37% and 0.74% to 1.05% and 3.82% when the temperature increased from 400 to 600 C, respectively, these two gases were relatively scarce in the gaseous product compared with other gases. Thus, the generation of C2 gases can be neglected in the SCWG process of landfill leachate. The trends of gas product distribution from SCWG of landfill leachate in this study are in agreement with those reported in previous studies [3,10]. As shown in Fig. 2(d), GYH2, GYCH4 and
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GYCO2 enhanced dramatically when the temperature increased from 400 C to 600 C, which indicates that high temperature promotes the steam reforming reaction in SCW medium. At 600 C, the yields of H2, CH4 and CO2 reached up to 245, 255 and 296 mmol L1 leachate, respectively. This result suggests that landfill leachate is a potential feedstock for hydrogen production by SCWG. Minor compounds of the gaseous products (such as C2 species) followed the same trend of GYH2. However, the yields of CO and C2 gases are negligible, compared with GYH2, GYCH4 and GYCO2. Cm Hn Op Nq þH2 O/CO þ H2 þ NH3 þ other compounds
(7)
CO þ H2 O/CO2 þ H2
(8)
RCOOH/RH þ CO2
(9)
CO þ 3H2 /CH4 ðor C2 H4 ; C2 H6 Þ þ H2 O
(10)
CO2 þ4H2 /CH4 ðor C2 H4 ; C2 H6 Þ þ 2H2 O
(11)
Carbon recovery (CR) was adopted to evaluate the carbon distribution in products, and the results were exhibited in Table 2. As shown in Table 2, wTOC decreased from 76.6% to 9.40% rapidly while wTIC, wCH4 and wCO2 increased steadily when temperature rose from 400 C to 600 C, indicating that more than 90% of organics in leachate had been degraded into inorganic carbon species and carbonaceous gases at 600 C. Furthermore, CO2 produced in the gasification process reacts with alkaline substances (such as NH3) in feedstock to generate carbonate and bicarbonate, leading to the increase of TIC in liquid products, as shown in Eq. (12). Meanwhile, CR changed from 82.0% to 54.4%. Similarly, Kıpc¸ak et al. [2] also found that the carbon balance only closed within 32% to 86% in hydrothermal gasification of olive mill wastewater in SCW. The missing carbon in the balance may be owing to the formation of tar and char via carbonization reaction [19,29]. Gong et al. [11] confirmed that solid phase products were mainly composed of travertine, ankerite and calcite, and tar and char were not detected in leachate SCWG. However, we observed solid particles on the filter after each run and sticky brown tar adhering on the walls of the sample tubes. Moreover, Xu et al. [19] proposed that the formation of C3 - C4 light gases was likely to be another reason for the missing carbon. However, C3 - C4 light gases products were not identified in our work. H2 O
H2 O
2 þ CO2 þ 2NH3 !2NHþ 4 þ CO3 % 2NH4 þ 2HCO3
(12)
Table 2 e Effect of temperature on CR and carbon distribution at 600 s, 25 MPa and 0 OC. T/ C
400 450 500 550 600
wC,L/%
wC,G/%
wTIC/%
wCH4/%
wCO2/%
76.6 70.6 45.3 25.3 9.40
2.16 1.68 2.54 11.2 12.1
0.31 0.88 3.92 10.6 14.0
2.12 5.82 7.79 12.0 16.2
Effect of reaction time Four reaction times (300, 600, 900 and 1200 s) were employed for evaluating the time dependency of SCWG reaction of landfill leachate. As shown in Fig. 3(a), a significant improvement of TRE was observed with increasing time when the reaction time was less than 900 s. Then, the enhancement on TRE slowed down gradually. When time increased from 300 s to 1200 s at 500 C, the concentration of TOC in liquid products decreased from 13,200 mg L1 to 7300 mg L1; meanwhile TRE increased from 6.4% to 48.2%. It indicates that increasing time favors the degradation of organics. However, increasing time had no significant effect on NH3eN concentrations, as shown in Fig. 3(b), which was probably because ammonia, known as a ‘‘refractory substance’’, was still stable in SCW environment [26]. The effect of time on variation trends of gas fraction as well as gas yield are depicted in Fig. 3(c) and (d) at 25 MPa and 500 C, respectively, when reaction time changed from 300 s to 1200 s. As shown in Fig. 3(c), the fractions of gaseous products have changed weakly in the experimental range. It indicated that reaction time did not significantly influence the product distribution. D'Jesu´s et al. [30] also found that the gas composition did not change with time when reaction time was longer than 1.5 min in SCWG of corn silage. Moreover, Kersten et al. [31] concluded that, when the reaction time was longer than 40 s, it did not influence gas composition significantly for glucose SCWG. Fig. 3(d) shows the effect of time on GY. It can be observed that as time increased from 300 s to 1200 s, GYH2, GYCH4 and GYCO2 increased from 120, 72 and 135 mmol L1 leachate to 163, 155 and 193 mmol L1 leachate, respectively. A possible explanation for GY in Fig. 3(d) is that the reverse process of methanation reaction and the water gas shift reaction will be reinforcement with increasing reaction time [19]. However, GY of CO, C2H4 and C2H6 declined gradually. It indicates that although increasing reaction time has little effect on molar fraction of gaseous products, it is helpful for the gas yield of leachate SCWG. Besides, Zhu et al. [32] found that GY of H2, CO, CH4 and CO2 increased as time increased from 10 s to 1800 s in the gasification of glucose. Table 3 summarizes CR and carbon distribution in liquid and gaseous products under different reaction times. As demonstrated, wCO2 and wCH4 increased from 7.46% and 4.00% to 10.6% and 8.57% as time increased from 300 s to 1200 s, respectively. Meanwhile, wTOC reduced from 60.6% to 33.5%, suggesting that most of the organic carbon had been degraded. Correspondingly, CR fell inside the range of 53.3e75.5%. In SCWG of papermaking black liquid, Sricharoenchaikul [33] observed that the yield of tar and carbon deposition reduced greatly and mass fraction of gas products increased as time increased.
Effect of pressure CR/%
wTOC/%
5
82.0 80.1 61.1 62.2 54.4
Fig. 4 shows the effect of pressure on gasification of landfill leachate in SCW at 500 C, 600 s and 0 OC. As shown in Fig. 4(a) and (b), pressure has negligible influence on the concentration of TOC and NH3eN in liquid products. However, Kıpc¸ak [2] found that the concentration of TOC in liquid products augmented from 874.3 mg L1 to 1270.0 mg L1 as pressure rose from 100 bar to 300 bar at 550 C, whereas the degradation of
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Fig. 3 e Effect of reaction time on gasification of landfill leachate at 500 C, 25 MPa and 0 OC: (a) TOC and TRE, (b) NH3eN, (c) gas composition, (d) gas yield.
TOC gradually decreased as pressure increased. This discrepancy might be explained by the differences of the investigated pressure interval. In our study, the tested pressure changed from 23 to 29 MPa, and the narrow pressure interval had negligible influence on the degradation of organics. Fig. 4(c) and (d) show the effect of pressure on molar fraction of gas and GY of landfill leachate, respectively. As shown in Fig. 4(c), 4CH4 increased while 4CO2 decreased with increasing pressure. This result agrees well with the finding of Kıpc¸ak et al. [2], where an increase of CO2 content as well as a decrease of CH4 content in the gas effluent was observed with deceasing pressure. However, no obvious variation of 4H2 was observed in the range of 23 MPae29 MPa, indicating that the effect of pressure on gasification products is complicated [34]. Furthermore, 4CO, 4C2H4 and 4C2H6 were consistently lower than 5% under experimental pressure. As Fig. 4(d) shows, GYCO2 decreased with system pressure. GYCO2 was 200 mmol L1 leachate at 23 MPa, and reduced to 122 mmol L1 leachate at 29 MPa. Although the yields of H2 and CH4 were weakly affected by pressure, the yield of all gaseous products decreased from 433 to 328 mmol L1 leachate, implying that high pressure appears to depress the gasification reaction of landfill leachate. Lu et al. [34] found GYH2 increased with
Table 3 e Effect of reaction time on CR and carbon distribution at 500 C, 25 MPa and 0 OC. t/s
300 600 900 1200
wC,L/%
wC,G/%
CR/%
wTOC/%
wTIC/%
wCH4/%
wCO2/%
60.6 45.3 34.3 33.5
3.48 2.54 5.12 12.1
4.00 3.96 4.56 8.57
7.46 7.87 9.31 10.6
75.5 59.7 53.3 64.8
increasing pressure, but GYCH4 and GYCO had a tendency to decrease with pressure, which suggests that higher pressure favors the water gas shift reaction.
Effect of oxidation coefficient In this section, we investigated the effect of oxidation coefficient (OC) on the gasification of landfill leachate in SCW. As shown in Fig. 5(a), when OC increased from 0 to 1.0, TOC in liquid products decreased from 9900 mg L1 to 5753 mg L1 at 500 C, 25 MPa and 600 s. Correspondingly, TRE increased from 30.0% to 49.0%, indicating that the increase of OC could improve the removal efficiency of organics remarkably. Fig. 5(b) shows effect of OC on the concentration of NH3eN. Although the concentration of NH3eN in liquid products decreased significantly with the increase of OC, it was still higher than that in leachate. As OC increased from 0 to 1.0, the concentration of NH3eN reduced from 4105 mg L1 to 2649 mg L1. The result implied that NH3eN was likely to convert into N2 after adding a small amount of oxidant, as shown in Eq. (13). However, NH3eN was still the stable pollutant in SCWPO of landfill leachate. 4NH3 þ3O2 /2N2 þ 6H2 O
(13)
As shown in Fig. 5(c), the main products of SCWPO of landfill leachate were CO2, H2 and CH4. Meanwhile, trace amount of CO, C2H4 and C2H6 were detected with sum of mole fraction of these three gases consistently lower than 5%. This result agrees with that of the previous study. Jin et al. [18] found that in the partial oxidative gasification of biomass and its model compounds in SCW, the amount order of produced gases was H2>CO2>CH4>CO with small amount of oxidant, while the amount order of produced gas was
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Fig. 4 e Effect of pressure on gasification of landfill leachate at 500 C, 600 s and 0 OC: (a) TOC and TRE, (b) NH3eN, (c) gas composition, (d) gas yield.
Fig. 5 e Effect of OC on gasification of landfill leachate at 500 C, 600 s and 0 OC: (a) TOC and TRE, (b) NH3eN, (c) gas composition, (d) gas yield. CO2>H2>CH4>CO only OC was comparatively large. In this work, as OC increased from 0 to 1.0, 4CO2 rose gradually from 41.9% to 81.2%; meanwhile, 4CH4 reduced from 21.0% to 4.2%. This can be explained by the fact that the higher amount of oxidant may promote the oxidation of CO and CH4 into CO2. Furthermore, 4H2 increased as OC increased when OC is lower
than 0.2; on the contrary, it decreased as the increase of OC once the OC was greater than 0.2. Excessive oxidant in gasification system may convert CO into CO2 to inhibit water gas shift reaction, or even react with H2, which will lead to low GYH2 [19]. When OC equaled 0.2, 4H2 reached its maximum value of 32.6% at the investigated experimental conditions.
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The stoichiometric complete oxidation of landfill leachate was achieved at OC of 1.0; however, 12.1% H2 was still detected in gaseous products. This is mainly because lots of organics in suspended solids in landfill leachate were not considered when we measured the COD value of raw landfill leachate. In conclusion, in addition to steam reforming reaction, other reactions, including water gas shifting reaction, methanation reaction and oxidation reactions, also existed among organics, CH4, CO, H2 and oxidant in SCWPO of landfill leachate [18,21,35], as shown in Eqs (14)e(17). Cm Hn Op þ ðm þ n pÞ=2O2 /mCO þ nH2 O
(14)
CH4 þ 1=2O2 /CO þ 2H2
(15)
CO þ 1=2O2 /CO2
(16)
H2 þ 1=2O2 /H2 O
(17)
Fig. 5(d) shows the effect of OC on GY. As OC increased from 0 to 1.0, GYCO2 increased rapidly from 143.0 to 850.8 mmol L1 leachate. However, GYH2 reached the maximum of 231.3 mmol L1 leachate at 0.2 OC, which was 2.4 times the GYH2 in the absence of oxidant. When OC exceeded 0.2, GYH2 started to decline and reduced to 95.8 mmol L1 leachate when OC equaled 1.0. Furthermore, similar to GYH2, GYCH4 reached to the maximum of 147.6 mmol L1 leachate at 0.2 OC. These results suggest the optimal OC in SCWPO of landfill leachate should exist, in which the largest GYH2 can be available. In this work, the optimal OC was 0.2. Adding a small amount of O2 could notably contribute to steam reforming reaction of organics, thereby improving GYH2, GYCH4 and GYCO2. However, oxidation reaction will gradually dominate in the presence of much oxidant, and H2 and CH4 will be oxidized into H2O and CO2, respectively [36]. The data in Table 4 can be used to estimate the effect of OC on carbon distribution and CR in products. wCO2 increased with increasing OC, but the wTOC showed the opposite trend. These trends for wCO2 and wTOC are fully consistent with the increase of TRE and GYCO2 in Fig. 5(a) and (d), respectively. The CR increased from 59.7% to 88.7% when OC increased from 0 to 1.0, indicating that the formation of tar and char was inhibited in the presence of oxidant.
Visual observations We further conducted the gasification process of landfill leachate with quartz tube reactor to observe the generation of
tar and char virtually. The quartz reactors were shown in Fig. 6, after 600 s SCWG reaction at 400 C and 600 C in the absence of oxidant. At 400 C, it can be observed clearly that suspended carbon deposition and sticky tar adhered to the inner wall of the quartz tube. This result demonstrates that the polymerization and carbonization reactions might take place among the complex organics in landfill leachate during the SCWG process, leading to the generation of tar and char. Although tar and char are insoluble in water and difficult to be degraded, they can be greatly reduced in SCW at high temperature. For instance, Sricharoenchaikul [33] found that a raise in reaction temperature led to a decrease in fractions of char and tar at black liquor gasification in SCW. The reduction of char and tar at higher temperature could be visually confirmed. However, the formation of tar in leachate SCWG was still obvious at 600 C in our study. Generally, oxidant is introduced into the SCWG system to inhabit the formation of char or tar and promote gasification yield [18,19]. As can be seen from Fig. 7, compared with the quartz tube reactor image at 500 C, 600 s and 0 OC, less char and tar were observed on the reactor interior surface when OC was equal to 0.5. Virtually, no tar or char products were identified at OC of 1.0. Unfortunately, further analysis of these solid and liquid products observed in this study was not possible due to their extremely small quantities.
Identifying liquid phase products In order to further study the transformation of organics during the gasification process, the organic compounds extracted from leachate and liquid product were listed in Appendix A and B, respectively, which were determined by GC-MS from comparison with standard MS library (NIST 98). In this work, 18 organic components were detected from leachate, including fatty acids, aromatic acids, phenols and other N-containing and/or C-containing compounds, indicating that the organic contaminates in landfill leachate were very complicated. Composition analysis of leachate revealed that leachate was a C-rich biomass with a relevant composition of carboxylic acids. This result suggests that leachate is suitable for energy recovery. Considering acids are the intermediates of any biomass gasification, a significant bio-syngas yield can be expected [8]. After gasified at 500, 25 MPa and 600s in the absence of oxidant, 22 products were identified, as shown in Appendix B. Compared with raw leachate, the organic in liquid product was mainly composed of ketones, phenols, pyridines, anilines and indoles, which was consistent with the study by Gong et al. in the treatment of landfill leachate using SCWG [11]. In
Table 4 e Effect of OC on CR and carbon distribution at 500 C, 600 s and 25 MPa. OC
0 0.2 0.4 0.6 0.8 1.0
wC,L/%
wC,G/%
CR/%
wTOC/%
wTIC/%
wCH4/%
wCO2/%
45.3 42.1 42.1 36.9 40.1 33.0
2.54 4.77 5.44 8.54 5.54 8.47
3.96 8.13 4.79 2.94 1.07 0.44
7.87 18.0 23.7 34.3 39.8 46.8
59.7 73.0 76.0 82.7 86.5 88.7
Fig. 6 e Images of landfill leachate SCWG in quartz tube reactors after 600 s at 400 C and 600 C in the absence of oxidant.
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9
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Fig. 7 e Images of landfill leachate SCWG in quartz tube reactors after 500 C and 600 s when OC was equal to 0, 0.5 and 1.0, respectively.
their study, gasification liquid phase products of leachate were mainly composed of cyclopentanone, 2-octanone, phenol, p-cresol and nitrogenous compounds [11]. In our work, 4 types and 12 kinds of N-containing compounds were supposed to be the products of gasification of landfill leachate. It is not surprising that N-containing compounds are identified among the products from gasification of biomass. For instance, Guan et al. [37] reported that indoles were the major components of the liquid phase products from the gasification of Nannochloropsis sp. Pentanone and hexone rather than organic acids were detected in the liquid products, implying that the ketones may be the products of leachate SCWG. Generally, the ketones are considered to be the intermediate compounds in the reaction of biomass to gases [38,39]. On the other hand, ketones can be converted from organic acids in leachate by bimolecular decarboxylation [28]. For example, under the attack of H free radical, CeO bond in a molecular acetic acid is broken, resulting in the formation of equimolar amounts of CH3CO and H2O. Subsequently, CH3CO can react with CH3 to form acetone [28]. The proposed mechanism is shown in Eq. (18): þH$
þCH3 $
CH3 COOH ! CH3 CO$!CH3 COCH3 H2 O
and CH4 were the most abundant gaseous products under the investigated reaction conditions. Increasing temperature enhanced the H2 production and TOC removal efficiency greatly. However, pressure and reaction time had little influence on organics removal, tar and char generation. (2) Adding a small amount of oxidant favored the formation of H2 and CH4. GYH2 was up to the maximum of 231.3 mmol L1 leachate at 500 C, 25 MPa, 600 s and 0.2 OC, which was 2.4 times the GYH2 in the absence of oxidant. TRE and CR increased as OC increased. TRE and CR were 49.0% and 88.7%, respectively, when OC equaled 1.0. Adding a small amount of oxidant could reduce the formation of tar and char. (3) Suspended carbon deposition and sticky tar could be observed visually on the inner wall of the quartz tube reactor. NH3 was very stable, and NH3eN were hardly converted into N2 under our experimental conditions. Numerous compounds were detected in the liquidphase products after gasification, such as NH3, pyridines, anilines, phenols and indoles.
Acknowledgment The work was supported by National Natural Science Foundation of China (51708262), Jiangsu Province Natural Science Foundation of China (BK20151040), Post-Doctoral Fund of Jiangsu Province (1501124B) and Open Fund of Jiangsu Province Key Laboratory of Environmental Engineering (ZX2017002).
Appendix A. Organic compounds in leachate.
(18)
Besides, strong responses of phenols (33.655% peak area at 7.166 min, 12.024% peak area at 9.431 min and 5.365% peak area at 11.717 min, respectively) were observed in the GC-MS spectrum. The result implies that phenols are stable and hard to be degraded in SCW, and phenols are the main intermediate compounds during gasification process of leachate. Moreover, it is possible to form tar and char in the presence of quinoline, indole, naphthol and other polycyclic aromatic hydrocarbons. Guo et al. [1] presented that polycyclic aromatic hydrocarbons, such as fluorene and phenanthrene, were probably char precursors.
No.
Time/min
Area/%
Name
1 2 3 4 5 6 7 8 9 10 11 12 13
3.397 3.531 5.065 7.153 7.333 8.678 9.413 9.602 10.569 11.759 13.972 15.656 17.710
2.132 1.152 6.686 17.72 12.127 2.573 21.065 12.159 1.498 6.888 1.751 1.305 1.697
14
17.929
0.78
15
18.011
3.056
16 17
19.463 19.601
0.757 0.872
18
19.773
1.757
butanoic acid 2-butanamine pentanoic acid phenol hexanic acid 1-methyl-2-pyrrolidinone 4-methyl-phenol heptanoic acid cyclohexanecarboxylic acid octanoic acid nonanoic acid benzenepropanoic acid 1,3,3-trimethy-l-2-((E)-3methylbuta -1,3-dienyl) cyclohexanol methyl 2-(1,4, 4-trimethylcyclohex-2-enyl) acetate 2-isocyanato-1,3-dimethyl benzene ethyl citrate 2-(4-methylcyclohexyl) prop-2-en-1-ol N,N-dimethyltetradecanamine
Conclusions According to the experimental investigations presented in this paper, the following conclusions can be drawn: (1) Temperature has a crucial effect on the gasification process. Dissolution of solid organics in leachate resulted in the increase of TOC in liquid products below 450 C. Gas products were detected above 400 C. CO2, H2
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Appendix B. Organic compounds in liquid effluent when operated at 500, 25 MPa, 600 s and in the absence of oxidant.
[2]
[3]
NO. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
Time/min
Area/%
Name
2.586 3.600 4.051 4.803 5.164 6.127 7.166 7.304 7.794 8.253 8.928 9.336 9.431 9.487 9.607 10.153 11.717 13.655 14.888 16.731 16.791 18.441
1.045 1.065 2.767 5.480 1.731 2.389 33.655 1.586 2.297 1.553 1.783 2.158 12.024 6.623 1.643 1.358 5.365 2.830 6.642 1.296 0.396 1.476
2-pentanone 2-hexanone 2-methyl-pyridine 3-methyl-pyridine 2,6-dimethylpyridine 3,5-dimethylpyridine phenol 2,3-dimethylpyridine 3,5-dimethylpyridine 2-ethyl-5-methyl-pyridine 2-methyl-phenol acetophenone 3-methyl-phenol 2-methyl-benzenamine 3-methyl-benzenamine 3-pyridinamine 2-ethyl-phenol quinoline indole 2-methyl-indole 5-methyl-indole 2-naphthalenol
[4]
[5]
[6]
[7]
[8]
[9]
[10]
List of symbols [11]
SCW SCWG SCWPO COD TN TC TOC TIC SS NH3eN TRE OC 4i GYi CR mTOC,0 mTOC mO2,add mO2,need wC,G wC,L
Supercritical water Supercritical water gasification Supercritical water partial oxidation Chemical oxidation demand (mg$L1) Total nitrogen (mg$L1) Total carbon (mg$L1) Total organic carbon (mg$L1) Total inorganic carbon (mg$L1) Suspended solids (mg$L1) Ammonia nitrogen (mg$L1) TOC removal efficiency (%) oxidation coefficient gas fraction of gas i (%) gas yield of gas i (mmol$L1 leachate) Carbon recovery rate (%) initial mass of TOC in leachate (mg) the mass of TOC in liquid products (mg) O2 generated from the decomposition of H2O2 (mg) the mass of oxidant for complete oxidation (mg) the mass fraction of carbon in gaseous products (CH4, CO and CO2) to total carbon in leachate (%) the mass fraction of carbon in liquid product (TOC and TIC) to total carbon in leachate (%)
[12]
[13]
[14]
[15]
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Please cite this article in press as: Gong Y, et al., Gasification of landfill leachate in supercritical water: Effects on hydrogen yield and tar formation, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.09.020