Enhanced H2 Production from Municipal Solid Waste Gasification

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Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Enhanced H2 Production from Municipal Solid Waste Gasification Using Ni−CaO−TiO2 Bifunctional Catalyst Prepared by dc Arc Plasma Melting Muhammad Irfan,† Aimin Li,*,† Lei Zhang,† Muhammad Javid,‡ Mengya Wang,† and Shujauddin Khushk†

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†

School of Environmental Science & Technology, Dalian University of Technology, Key Laboratory of Industrial Ecology and Environmental Engineering (MOE), Dalian 116024, Liaoning, China ‡ School of Material Science & Engineering, Dalian University of Technology, Dalian 116024, Liaoning, China S Supporting Information *

ABSTRACT: The gasification of municipal solid waste (MSW) in the presence of a Ni−CaO catalyst is a promising approach for in situ CO2 capture along with enhanced H2 syngas production. However, the instability of the catalyst−sorbent matrix is a challenging obstacle for the gasification process. The direct current (dc) arc plasma melting method was employed to prepare a series of bifunctional catalysts (Ni−CaO) with varied Ni loadings, while TiO2 as polymorphic transition material was added to promote the stability of the catalyst. The catalyst samples were evaluated for cyclic CO2 capture and catalytic gasification of MSW using thermogravimetric analysis (TGA) and a fixed-bed reactor, respectively. The obtained results showed that the addition of TiO2 into Ni−CaO catalyst−sorbent led to improvements in the stability of catalyst for CO2 capture. The gasification performance of a bifunctional Ni−CaO−TiO2 catalyst was evaluated on the basis of Ni loading, the catalyst to MSW (C/M) ratio, temperature, and steam rate. The results indicated that, after the addition of a Ni−CaO−TiO2 catalyst (20% Ni loading), the dry gas yield (DGY) increased from 0.75 to 1.74 Nm3/kg and H2 concentration increased from 35.1 to 57.7%. Meanwhile, the tar content decreased from 9.38 to 2.55% with the addition of the catalyst. The significant influences of Ni loading, C/M ratio, gasification temperature, and steam on product composition and characteristics were also evaluated. Tar analysis indicated that steam and catalyst addition resulted in tar reduction, which led to a higher DGY. Moreover, an unconventional method (i.e., thermal plasma technique) was employed for the catalyst preparation, which demonstrated promising results for MSW gasification with an industrial application prospect.

1. INTRODUCTION The global energy demand has been increasing rapidly due to massive population growth and industrialization. To meet this rising energy demand, fossil fuels (oil, natural gas, and coal) are being consumed extensively causing depletion of these resources as well as severe environmental problems. Approximately 80% of the total global energy demand is being fulfilled by fossil fuels.1,2 Therefore, there is an increasing need to explore some sustainable alternatives to fulfill future energy demands. Hydrogen has received much attention as a renewable and sustainable substitute to fossil fuels. The pyrolysis and gasification of municipal solid waste (MSW) appear to be a promising approach for producing hydrogen to fulfill future demands.3,4 Gasification technology offers great flexibility in exploiting various kinds of feedstock materials including coal, biomass, and MSW. MSW as a feedstock can be converted into syngas (comprised of H2, CO, CO2, and CH4) by gasification; later this syngas can be used for energy, power, heat, biofuels, and chemical production. Gasification of MSW in steam atmos© XXXX American Chemical Society

phere leads to an enhanced syngas quality with higher dry gas yield (DGY) and H2 concentration. However, generally tar is produced more with steam than with air gasifying agent. Similarly, gasification of MSW feedstock also produces high tar content than coal or biomass gasification due to the presence of plastic waste in MSW.5 The use of syngas is entirely determined by its purity; for example, syngas utilization in the fuel cell or synthesis reactions (i.e., Fischer−Tropsch) demands high purity.6 Nevertheless, there are challenges in achieving high purity syngas and efficient thermochemical conversion, which includes poor tar cracking, CO2 production, unreformed volatile contents, and carbon deposition. The CO2 decreases the energy density of the syngas, consequently increasing the production cost. Furthermore, the tar content tends to block the pipeline and equipment, leading to an Received: Revised: Accepted: Published: A

April 12, 2019 June 29, 2019 June 29, 2019 June 30, 2019 DOI: 10.1021/acs.iecr.9b01999 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 1. Schematic diagram of the dc arc plasma reactor (left) and internal view of the arc plasma reactor chamber (right).

increase in maintenance cost and overall process efficiency.5,7 Moreover, less tar content is required for downstream applications (i.e., internal combustion engines, gas turbines, fuel cells, and Fischer−Tropsch synthesis), which is difficult to achieve through thermal cracking.2 To overcome the CO2 production and tar formation during the gasification process, different catalysts and sorbents have been studied extensively by many researchers.8−10 Catalytic gasification can improve gas composition, eliminate/reduce tar content, and increase gasification efficiency.11 Ni-based catalysts have presented decent reactivity toward C− C and C−H bond cleavages; hence Ni-based catalysts are effective in tar cracking and reforming reactions.12,13 Moreover, Ni-based catalysts can reach high reforming activity at low cost. However, the main issue associated with monometallic catalysts is the deactivation and low stability caused by coke deposition and sintering at high temperature.14 To boost the catalyst activity and to suppress the carbon deposition, the addition of catalyst supports and/or additional metals (i.e., CaO or other active substances) has been highly acknowledged.11 The incorporation of the catalyst with sorbent into a bifunctional material has the potential to serve two functions, i.e., catalytic reforming and CO2 sorption. Furthermore, CaO can enhance the catalytic activity and the stability, and has been widely used in biomass, coal, and MSW gasification. It can also reform the tar and CO2 sorption in the gasification process.15,16 Additionally, CaO is not only cheap but also available in abundance. However, CaO shows a loss of CO2 sorption capacity after repeated use due to sintering caused by high temperature. Therefore, there is a strong desire to develop a bifunctional catalyst/sorbent that can maintain its performance over multiple cycles. The stability of catalyst/sorbent can be enhanced by the addition of the polymorphic transition particles (spacer) to inhibit densification of the catalyst/ sorbent matrix.16 Many studies have been carried out by different researchers to extend the stability of the CaO by adding spacer promoters. It has been acknowledged that the performance of CaO can be enhanced by the addition of different metal oxides (Al2O3, ZrO2, TiO2, CaZrO3, Na2ZrO3, etc.) to the sorbents.16−25 In this study, a Ni−CaO based bifunctional catalyst was prepared and its stability was improved by the addition of a TiO2 polymorphic transition spacer material in order to inhibit densification of the catalyst−sorbent matrix. Theoretically, TiO2 has the potential to inhibit densification by (i) acting as a physical spacer between CaO particles and (ii) also stresses

due to differential thermal expansion and sintering rates of CaO and TiO2.16 Wu et al.26 investigated the incorporation of TiO2 with nano-CaO via hydrolysis and indicated that TiO2 addition in CaO resulted in improved cyclic CO2 sorption durability. Besides, Yu et al.27,28 also demonstrated better CO2 capture stability at high temperature due to the TiO2 incorporation. Therefore, the bifunctional catalysts of Ni− CaO with varied Ni loadings (0, 10, 20, and 30 wt %) were prepared by the addition of TiO2 with a direct current (dc) arc plasma melting method. Compared with conventional techniques, thermal plasma techniques have several advantages for the catalyst preparation, including (a) less preparation time, (b) high catalyst activation, (c) less energy need, and (d) high distribution of active species.29 That is why application of plasma techniques for catalyst preparation has the potential to obtain highly active catalytic materials for industrial processes and particularly for the gasification process. The prepared catalyst/sorbent samples were evaluated for CO2 cyclic capture with multiple carbonation−calcination cycles using a thermogravimetric (TG) analyzer. Later, the performance of a Ni− CaO−TiO2 bifunctional catalyst in MSW gasification was evaluated in a fixed bed reactor. The influence of different parameters, i.e., Ni loading, catalyst to MSW ratio (C/M), gasification temperature, and steam rate, were considered to analyze the influence of the Ni−CaO−TiO2 bifunctional catalyst on the product gas composition, DGY, carbon conversion efficiency (CCE), and tar content. Also, the application of the Ni−CaO−TiO2 bifunctional catalyst has not been studied before in the thermochemical conversion process (i.e., MSW gasification).

2. MATERIALS AND METHODS 2.1. MSW Sample. In this study, different samples of MSW were collected from the residential areas surrounding the Dalian University of Technology, Dalian, China. Each component fraction of MSW was separated manually (i.e., kitchen/food, paper/cardboard, textile, plastic, and grass/ leaves/wood). However, components such as metal/cans, dust, and glass were excluded. Each component fraction was sun-dried, ground (particle size = 1−2 mm), and stored separately. The components of each fraction were mixed as per mass ratio (kitchen waste 48.5 wt %, paper 13.3 wt %, textile 9.9 wt %, grass/wood 7.2 wt %, and plastic 21.1 wt % air-dry basis) before the gasification experiments were carried out. The elemental analysis, proximate analysis, and calorific value B

DOI: 10.1021/acs.iecr.9b01999 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 2. Experimental setup.

of 350 °C and was generated by the steam generator. The steam rate was determined by supplying a defined quantity of water (1, 2, and 3 mL/min) by a peristaltic pump into the steam generator. The gasification experiments were performed at atmospheric pressure. Before gasification experiments were carried out, the reactor tube was heated to the required gasification temperature. The N2 gas at 40 mL/min and steam at the desired rate were supplied into the reactor. Once the gasification temperature was stabilized, the perforated stainless steel sample bucket (30 × 80 mm) containing a mixture of catalyst (as per C/M ratio) and MSW (2 g) was inserted into the middle of the reactor tube. The product gas flowed through the condenser, the tar/liquid trap flask, and the purifying/ drying tube, and was finally collected in a gas sampling bag. The condensed liquid/tar was collected in a 250 mL flask dipped in the container filled with ice−water, and later the tar/ liquid fraction was separated from aqueous solution and weighed. Each gasification experiment was carried out for 15 min (retention time). Later, the product gas samples were analyzed by gas chromatographic (GC) analysis. For all experiments, two repetitions with the same conditions were performed to confirm the repeatability. The calculation method for DGY, CCE, LHV, and char content is given in section S2 of the Supporting Information. 2.5. Analytical Methods. The proximate analysis, elemental analysis, and calorific value of MSW samples were carried out by an SDTGA-5000 proximate analyzer (Sande Co. Ltd., China), an Elementary Vario ELIII CHNS analyzer (Germany), and a bomb calorimeter, respectively. For the characterization, the prepared catalyst samples were analyzed with powder X-ray diffraction (XRD; model D/MAX-2400, Rigaku, Japan). Analysis of surface and texture properties was carried out with a Quantachrome Autosorb AS-1 (USA). The composition of product gas was analyzed using an Agilent 490 micro gas chromatograph (USA), whereas gas chromatography−mass spectroscopy (GC−MS) analyses of tar samples were carried out on an HP 6890 (GC) coupled to a MS detector (HP 5975 series, Agilent, USA) with an Agilent 7683 injector/autosampler. The tar compounds from GC−MS spectra were identified using the NIST mass spectra library. Moreover, the morphologies of prepared and used catalysts were determined with a scanning electron microscope (SEM; FEI Quanta 450, USA).

(HHV) of MSW samples used in experiments are presented in Table S1 (Supporting Information). 2.2. Catalyst Preparation. The dc arc plasma melting method with argon atmosphere was employed to prepare the Ni−CaO−TiO2 bifunctional catalyst with varied Ni loadings (i.e., 0−30%). Initially, the Ni, CaO, and TiO2 were mixed (Table S2) using a mortar and pestle for 20 min. Then, the pellet was made with a mixture of Ni, CaO, and TiO2 by a hydraulic press under the pressure of 30 tons. The pellet was placed on the water-cooled Cu platform, which served as an anode in the dc arc plasma reactor chamber, whereas a tungsten rod served as the cathode (see Figure 1). Subsequently, the reactor chamber was sealed and evacuated to a vacuum of 5.0 × 10−3 Pa, and Ar was introduced to reach 0.04 MPa in the chamber. To ignite the arc, the graphite piece was placed beside the pellet. The plasma arc current was set at 120 A, while the voltage was controlled at ∼25 V. The targeted pellet was melted by arc plasma, and a homogeneous bulk ingot was achieved. The approximate temperature inside the chamber reached up to 104 K, while inside the arc zone the electron temperature reached up to 11 × 103−23 × 103 K.30,31 Later, the ingot was ground with ball milling. For comparison, Ni−CaO was also prepared under the same conditions as mentioned above. The prepared catalyst samples were calcined in air at 700 °C for 1 h. 2.3. Cyclic Carbonation−Calcination Test. To study the CO2 cyclic capture ability, the prepared catalyst samples were evaluated in a thermogravimetric analyzer (Exstar, SII, TG/ DTA 6300). Approximately 20 mg of a prepared catalyst sample was placed in an alumina pan and initially heated to 700 °C at a rate of 50 °C/min under 40 mL/min N2 flow. When the carbonation temperature (700 °C) was stabilized, a CO2 flow was added at 20 mL/min to carbonate the sample for 12 min. Subsequently, the CO2 flow was stopped and calcination was carried out at 850 °C for 10 min (N2 = 40 mL/min). The carbonation and calcination holding temperature and time were determined by blank tests as discussed in section S1 of the Supporting Information. The cycles of carbonation and calcination were repeated 10 times. 2.4. Gasification Experiments. The scheme of the experimental setup consisting of a fixed bed reactor (batch type) is shown in Figure 2. The reactor tube was made of stainless steel (500 mm length and 48 mm i.d.), and an electric furnace was used to heat the reactor tube to the desired temperature. Steam was supplied with a constant temperature C


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3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization. XRD analysis of the prepared catalyst samples was carried out to identify the compounds and the phases in catalyst samples (Figure 3). The

20.13 nm, and the catalyst with 30% Ni loading had the highest value (i.e., 20.13 nm). The morphology of the prepared catalyst samples and after the carbonation−calcination cyclic test was explored by SEM (Figure 4). The SEM images of prepared catalyst samples showed that the morphology did not change prominently after 10 cycles of severe cyclic carbonation−calcination testing. This showed that the thermal stability of the prepared catalyst samples was improved due to the presence of TiO2 in the catalyst−sorbent matrix, which inhibited densification of the catalyst−sorbent matrix and effectively reduced the sintering. In contrast, the morphology of the fresh and cycled CaO showed substantial change and the cycled CaO exhibited clear signs of sintering, which resulted in poor cyclic stability for CO2 cyclic capture (refer to section 3.2). 3.2. Cyclic CO2 Capture Performance. The net CO2 capture performance of the catalyst/sorbent matrix, expressed as grams of CO2 per gram of sorbent (CaO) is shown in Figure 5. In this study, the prepared catalyst/sorbent matrix with varied Ni loadings (i.e., 0, 10, 20, and 30%) was evaluated for CO2 uptake performance up to 10 cycles of carbonation− calcination under a severe condition (i.e., 100% CO2 for carbonation and 100% N2 for calcination). In addition, for the purpose of comparison, the prepared catalyst/sorbent matrix was compared with CaO. Each sample of the catalyst/sorbent matrix blended with TiO2 showed an increase in the CO2 uptake capacity up to the second cycle and then decreased slightly. However, after the fifth cycle, each catalyst/sorbent sample was relatively stable. An improved cyclic CO2 capture performance of the TiO2-blended catalyst/sorbent matrix was achieved due to inhibited densification caused by (i) physical separation of CaO by TiO2 particles and (ii) stresses attributed to differential sintering and thermal expansion rates of CaO and TiO2.16 With an increase in Ni content in the catalyst/ sorbent matrix, the net CO2 uptake was slightly declined. These results suggested that the high Ni loading could increase the diffusion barrier and hinder the contact between CaO and CO2.18 On the other hand, the CO2 uptake for CaO was increased initially and then decreased rapidly after each cycle. This can be explained by two reasons: (1) The sintering of CaO due to high temperature produces a dense CaCO3 film, which prevented further CO2 absorption into the CaO interior. (2) The porous structure of CaO was destroyed leading to the reduction of pore volume and surface area.32 Besides, an initial increase in CO2 uptake was observed for CaO as well as for the prepared catalyst/sorbent matrix and it might be due to selfactivation.16 3.3. MSW Gasification Performance. 3.3.1. Influence of Catalyst. In this study, the prepared catalyst (Ni−CaO−TiO2) with varied Ni loadings was used to evaluate the catalytic effect of Ni loading on the syngas composition and overall MSW gasification performance. The effect of varied Ni loadings in the catalyst on syngas composition and DGY is shown in Figure 6a. In the figure, it is clear that the H2 and CO concentrations increase from 46.7 to 58.4% and from 10.8 to 16.1%, respectively, with the increase in Ni loading from 0 to 30%. The CO2, CH4, and CnHm show a decreasing trend (from 26.3 to 20.1%, from 9.6 to 4.1%, and from 6.6 to 1.3%, respectively. The increase in H2 and CO concentrations can be explained by reactions 1−3. NiO was reduced to Ni(0) (1). Later, the reduced Ni(0) reacted with tar and hydrocarbons to form the carbide (2) and finally produced gaseous products (H2 and CO) in the presence of steam (3). The increase in

Figure 3. XRD patterns of prepared catalyst samples.

results of the XRD patterns indicated that the catalyst samples prepared with the dc plasma arc melting method were composed of elemental Ni, NiO, CaO, and TiO2. The diffraction peaks at 37.2, 43.3, 44.5, 51.8, 76.3, and 79.6 represented NiO and elemental Ni, while the diffraction peaks at 32.2, 37.3, 53.8, 64.1, and 67.4 showed the presence of CaO. The NiO and elemental Ni diffraction peaks gradually increased with the increasing Ni-loading value or concentration in the bifunctional catalyst. Among all NiO and elemental Ni diffraction peaks, the peak at 44.5 showed the highest intensity, suggesting that elemental Ni was available in large quantity compared to NiO. Moreover, according to the previous literature, elemental Ni showed higher decomposition ability toward tar and higher activity toward the steam reforming and water gas shift reactions compared to NiO.32 The remaining diffraction peaks (i.e., 17.9, 25.9, 33.1, 47.5, 59.2, 69.7, and 78.6) were associated with TiO2. The textural characteristics of prepared catalyst samples and CaO are shown in Table 1. The BET surface area of CaO and Table 1. Textural Characteristics of Prepared Catalyst Samples catalyst sample

BET surf. area (m2/g)

total pore volume (cm3/g)

average pore size (nm)

CaO CaO−TiO2 10Ni−CaO−TiO2 20Ni−CaO−TiO2 30Ni−CaO−TiO2

11.36 8.10 8.94 9.58 7.81

0.11 0.06 0.08 0.09 0.05

15.91 14.73 13.37 15.65 20.13

that of CaO−TiO2 were observed as 11.36 and 8.10 m2/g, respectively. However, the surface area of the catalyst samples with varied Ni loadings showed dramatic behavior. Initially, it increased up to 9.58 m2/g at 20% Ni loading and then significantly decreased to 7.81 m2/g at 30% Ni loading. Meanwhile, the total pore volume of the catalyst samples also exhibited a pattern similar to that of the BET surface area. The results of average pore size indicated that the prepared catalyst had mesoporous characteristics, with a range from 13.37 to D


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Figure 4. SEM images of prepared catalysts and CaO before and after CO2 capture test.

reactions were fortified due to the presences of Ni content in the catalyst as explained in reactions 1−3. Wang et al.33 indicated that high Ni content could reduce the catalyst performance. Similarly, in this study, the concentration of H2 was not significantly increased above 20% Ni loading (Figure 6a); therefore, 20% Ni loading could be considered as an optimum value for the catalyst. Thus, Ni loading of 20% was used in the experiments to study the influence of gasification temperature and steam rate. To study the influence of C/M ratio on MSW gasification, the experiments were carried out without the catalyst and with varied C/M ratio of Ni−CaO−TiO2 (20% Ni loading), while keeping other conditions constant (Table 2). The effect of the C/M ratio on the product distribution, DGY, tar yield, CCE, and LHV is shown in Table 2. The syngas percentage increased after using the catalyst from 70.79 to 102.16% and further increased up to 116.37% at a C/M ratio of 1.0. Consequently, the DGY and CCE increased from 0.75 to 1.74 Nm3/kg and from 58.01 to 82.90%, respectively, with the increasing C/M ratio. This increase of DGY and CCE can be explained as the tar and char gasification reactions were fortified and led to reduction in tar and char content with the catalyst application (Table 2). Figure 6b shows the effect of different C/M ratios on product gas composition. The H2 content was significantly increased from 35.1 to 49.9% after using the catalyst and further increased up to 57.7% with the increasing C/M ratio. However, CO decreased from 16.9 to 13.5% after catalyst application (C/M = 0.5), while it slightly increased up to 15.4% with the increasing C/M ratio. The CO2, CH4, and CnHm contents showed an overall decreasing trend with increasing C/M ratio. The concentrations of CO2, CH4, and CnHm were decreased from 30.4 to 26.9%, from 11.5 to 6.3%, and from 6.1 to 3.4% after using the catalyst (C/M = 0.5), respectively, and these concentrations further decreased with the increasing C/M ratio. These results indicated that the RWGS, steam methane reforming, hydrocarbon reforming, and carbonation reactions were enhanced when the catalyst was added and further boosted with an increase in the C/M ratio. Likewise, the catalytic activity for the tar cracking and hydrocarbon cracking in vapor phase was greater when the bifunctional catalyst (Ni−CaO−TiO2) was applied. This could be explained as the bifunctional catalyst enhanced the chances of contacting solid particles with gas and extended gas

Figure 5. Cyclic CO2 capture performance.

CO along with CO2 decrease might be attributed to the reverse water gas shift reaction (RWGS) (4). Moreover, CO2 content was also reduced in the presence of CaO due to carbonation reaction (5). NiO + H 2 → Ni(0) + H 2O

(1)

Ni(0) + Cx Hy(tar/hydrocarbons) → NiCx + Hy

(2)

steam

NiCx + Hy ⎯⎯⎯⎯→ Ni + CO + H 2

(3)

RWGS CO2 + H 2 → CO + H 2O

(4)

carbonation reaction CaO + CO2 → CaCO3

(5)

Ni loading had a significant effect on product distribution (Table 2). The weight percent (equivalent to MSW feed) of syngas increased rapidly with the increase in Ni loading, while tar and char content reduced considerably. Consequently, DGY and CCE increased from 1.15 to 1.79 Nm3/kg and from 74.65 to 83.39%, respectively, with an increase in Ni content in the catalyst. These results indicated that the steam reforming E


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Figure 6. Influence of Ni loading (a) and C/M (b) on gasification performance.

Table 2. Influence of Ni Loading and C/M on Gasification Performance Ni loading C/M temp steam rate product distrib syngas tar char DGY tar yield CCE LHV

% °C mL/min wt % equivalent to MSW feed

Nm3/kg g/Nm3 % MJ/Nm3

0 1 850 2.0

10 1 850 2.0

20 1 850 2.0

30 1 850 2.0

20 0 850 2.0

20 0.5 850 2.0

20 0.75 850 2.0

20 1 850 2.0

91.49 5.82 11.95 1.15 50.61 74.65 9.85

106.94 3.74 11.64 1.48 25.27 82.76 9.61

116.37 2.55 10.15 1.74 14.66 82.90 9.70

116.93 2.37 9.61 1.79 13.24 83.39 9.81

70.79 9.38 13.55 0.75 125.07 58.01 10.03

102.16 4.95 11.24 1.31 37.79 75.99 9.37

111.01 3.85 10.79 1.49 25.84 80.45 9.52

116.37 2.55 10.15 1.74 14.66 82.90 9.70

3). The effect of gasification temperature on product gas characterization and distribution could be attributed to three main reasons: (1) rapid initial pyrolysis at a higher temperature; (2) the char gasification (endothermic reactions) favored at the higher temperature; (3) the steam reforming/ cracking of the tars amplified by the higher temperature. The tar and char content significantly decreased due to the elevation of gasification temperature (Table 3). The tar yield rapidly decreased from 15.13 to 1.47 g/Nm3 when the gasification temperature increased from 700 to 850 °C. As tar cracking and reforming reactions are endothermic reactions, therefore, these reactions were fortified with the rising

residence time in the reactor, which promoted hydrocarbon cracking and tar cracking.34 3.3.2. Influence of Gasification Temperature. For gasification, the temperature is an important variable because the gasification reactions are generally endothermic. Figure 7a shows the effect of gasification temperature (700−850 °C) on syngas gas composition, whereas Table 3 represents the product distribution, DGY, and carbon conversion efficiency (CCE) for MSW steam gasification (steam rate = 2.0 mL/min) with Ni−CaO−TiO2 catalyst (C/M = 1). Increase in the gasification temperature (700−850 °C) resulted in increased DGY from 0.79 to 1.74 Nm3/kg (Table F


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Figure 7. Influence of gasification temperature (a) and steam rate (b) on gasification performance.

Table 3. Influence of Gasification Temperature on Gasification Performance gasification temperature (°C) 700 steam rate C/M Ni loading prod distrib syngas tar char DGY tar yield CCE LHV

mL/min % wt % equivalent to MSW feed

Nm3/kg g/Nm3 % MJ/Nm3

750

800

850

850

850

850

850

2 1 20

2 1 20

2 1 20

2 1 20

0 1 20

1.0 1 20

2.0 1 20

3.0 1 20

74.36 11.95 16.32 0.79 151.27 58.19 9.11

91.58 7.36 14.06 1.05 70.10 70.11 9.12

105.11 4.32 11.95 1.36 31.76 78.77 9.40

116.37 2.55 10.15 1.74 14.66 82.9 9.70

68.29 17.50 11.74 0.71 246.48 61.96 10.59

99.44 3.92 10.82 1.34 29.25 74.45 9.68

116.37 2.55 10.15 1.74 14.66 82.9 9.70

118.91 2.41 10.04 1.83 13.17 83.06 9.54

gasification temperature and resulted in less tar yield at the higher temperature ranges. Similarly, the char content also experienced a decreasing trend with the rising gasification temperature due to the endothermic nature of the Boudouard and char gasification reactions (6)−(8). Moreover, increasing gasification temperature can improve mass and heat transfer rates of catalytic reactions, which will enhance the catalytic activity of the bifunctional catalyst Ni−CaO−TiO2 by reducing the activation energies for the tar and hydrocarbon steam reforming reactions (11) and (12).

Boudouard reaction C + CO2 F 2CO

(6)

char gasification C + H 2O → CO + H 2

(7)

C + 2H 2O → CO2 + 2H 2

(8)

water gas shift CO + H 2O F CO2 + H 2 G

(9) DOI: 10.1021/acs.iecr.9b01999 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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steam, more tar participated in steam gasification, which resulted in higher DGY and CCE. With regard to the syngas composition (see Figure 7b), the H2 concentration was recorded as 30.1% without steam addition; however, steam introduction resulted in a rapid increase of H2 concentration (52.1%) and kept increasing with the rising steam rate. This can be explained by the char gasification ((7), (8)), water gas shift (9), steam methane reforming (10), and tar/hydrocarbon reforming ((11), (12)) reactions. Meanwhile, the CO content decreased (from 17.7 to 13.9%) with the introduction of steam and later gradually increased with the steam rate from 1.0 to 3.0 mL/min. Furthermore, the concentrations of CO2, CH4, and CnHm showed a decreasing trend, and suggested that the RWGS (4), steam methane reforming (10), tar reforming (11), and hydrocarbon reforming (12) reactions were fortified due to the presence of steam. Therefore, lower CO2, CH4, and CnHm concentrations were achieved. In this study, the steam rate of 2 mL/min was the optimum value. Therefore, the steam rate of 2 mL/min was used in this work to study the influence of catalyst and gasification temperature on the product gas composition and characterization. 3.4. Tar Analysis. Tar is a complex mixture of aliphatic, heterocyclic, and aromatic hydrocarbons produced during the thermochemical conversion of MSW, biomass, etc., which can be classified as aliphatic compounds (ALP), heterocyclic compounds (HCC), light aromatic hydrocarbons (LAH), light polyaromatic hydrocarbons (LPAH), and heavy polyaromatic hydrocarbons (HPAH), along with oxygen-containing compounds (OCC). The tar produced during MSW gasification was analyzed through GC−MS to study the effects of steam and catalyst. The corresponding results of relative tar content (ALP, OCC, HCC, LAH, LPAH, and HPAH) and main tar compounds detected during GC−MS analysis are presented in Figure 8 and Tables S3−S5, respectively.

steam methane reforming CH4 + H 2O F CO + 3H 2

(10)

tar reforming tar + H 2O → H 2 + CO + CO2 + CnH m

(11)

hydrocarbon reforming CnHm + H 2O → H 2 + CO + CO2

(12)

The results for gas composition at different gasification temperatures are presented in Figure 7a. The main gasification reactions, i.e., Boudouard reaction (6), char gasification reactions (7) and (8), and steam methane reforming reaction (10) are endothermic; hence these reactions are fortified with the rising temperature. The H2 and CO concentration were significantly increased from 38.7 to 57.7% and from 10.0 to 15.4% with the rising gasification temperature, respectively. This can be explained as the Boudouard (6) and char gasification (7) reactions are endothermic in nature and are strengthened by increasing the gasification temperature. On the other hand, with increasing the gasification temperature, the concentrations of CO2, CH4, and CnHm hydrocarbons decreased from 34.9 to 21.2%, from 10.2 to 4.3%, and from 6.2 to 1.4%, respectively. Since endothermic reactions are favored at the higher temperatures, therefore, a higher temperature produces more H2 and CO contents and lower CO2, CH4, and CnHm contents. The CO2 is mainly produced by water gas shift (9) and oxidation reactions that only take place at a lower temperature, while CO2 is consumed through the Boudouard reaction (6) that occurs at a higher temperature. Additionally, the CaO in catalyst also lowers the CO2 content by carbonation reaction (5). Therefore, the content of CO increased, while the CO2 content showed a decreasing trend with the rising gasification temperature. A higher gasification temperature favored the steam methane reforming reaction (eq 10), enhancing the methane decomposition; therefore, the CH4 concentration accounted for a sharp decrease. 3.3.3. Influence of Steam. To study the influence of steam on the catalytic gasification of MSW in the presence of Ni− CaO−TiO2 catalyst (20% Ni loading), the experiments were carried out without and with varied steam rates (1.0−3.0 mL/ min). Other conditions were kept constant (see Table 3). Table 3 shows the effect of steam rate on product distribution, DGY, CCE, and LHV, while the syngas composition is presented in Figure 7b. During the experiments, the flow of steam was controlled by the peristaltic pump, which fed the water into the steam generator at a required quantity. It is evident from Table 3 that, after the steam addition, the DGY significantly increased from 0.71 to 1.34 Nm3/kg and further improved with increasing steam rate. It was attributed to the introduction of steam and greatly enhanced the steam reforming reactions (10)−(12). Therefore, the tar and char contents decreased from 17.50 (246.8 g/Nm3) to 2.41% (13.17 g/Nm3) and from 11.74 to 10.82%, respectively, with the introduction of steam and further decreased with increasing steam rate. The decrease in the tar yield suggested that the tar content was greatly reduced in steam environment, which favored the tar reforming and cracking reactions (11). Correspondingly, the mass balance of product distribution (syngas + tar + char) presented in Table 3 exceeded 100% after the introduction of steam, suggesting that steam was incorporated into the product gas by means of steam reforming reactions (7)−(12). Furthermore, due to the presence of

Figure 8. Relative tar content.

The relative tar contents (after thermochemical conversion of MSW without steam and catalyst) of HPAH, LPAH, and LAH were recorded as 19.13, 52.93, and 8.96%, respectively (Figure 8). However, after the addition of steam into the process, the HPAH content decreased to 11.10%, while the relative tar contents of LPAH, LAH, and HCC increased to 68.77, 10.04, and 6.0%, respectively. The LPAH compounds H


a

FXB-2S FXB FXB

TG, 800; TC, N/A; S/B, 0.95; ER, N/A

wood waste PE citrus peel residues

32−58 28−39 54−65 37−43 50.4 51 49.2 53.5 55.4 40.6 53.9 17−37 58.8

TG, TG, TG, TG, TG,

FXB-2S FLB FXB-2S FLB FXB-2S FLB

TG, 600; TC, 800; S/B, N/A; ER, N/A TG, 700−900; TC, N/A; S/B, 1.33; ER, N/A TG, 800; TC, N/A; S/B, 1.5; ER, N/A

30−15 43−49 20−21 22−23 23.3 28 17.6 16.2 7.1 15.2 28.1 20−27 25.3

25−28

10−15.4

CO

19−22 24−12 19−13 26−18 25.2 22 22.1 26.9 29.1 26 15.2 35−21 15.4

33−26

34.9−21.2

CO2

product gas composition (%)

16−3 5−0.5 6−0.3 10−11 0.75 0.79 11.1 3.4 8.4 18.2 2.1 21−10 0.5

7−4

10.2−4.3

CH4

1.03−1.85 1.2−2.3 − 1.51−1.66 − − − − − − − 1.2−2 −

0.87−1.23

0.79−1.74

gas yield (Nm3/kg)

41 42 43

40

4 36 37 38 39

15.34−2.19 1.85−0.18 g/Nm3 28−3.5 g/Nm3 − 5.91−2.94 0.55 g/Nm3 0.17 g/Nm3 11.20 11.10 12.7 16.9 − 12.9−2.62 −

ref this study 35

tar content (%) 11.95−2.55

FXB, fixed bed reactor; FXB-2S, two-stage fixed bed reactor; FLB, fluidized bed reactor. bTG, gasifier temperature; TC, catalytic reactor temperature; S/B, steam ratio; ER, equivalent ratio.

Ni−dolomite Ni−dolomite Ni−dolomite Ni−CeO2−Al2O3 NiO−SBA15−CaO NiO−SBA15−CaO Ni−dolomite Ni−Pt−dolomite Ni−Fe−dolomite Ni−CO−dolomite Ni−Mg−Al−CaO NiO−γ-Al2O3 Ni−CaO−Al2O3

700−850; TC, 850; S/B, 1.23; ER, N/A 700−900; TC, N/A; S/B, 1.0; ER, 0.25 950; TC, 850−1050; S/B, 1.37; ER, N/A 823; TC, N/A; S/B, 0.71; ER, N/A 750; TC, 750; S/B, 1.5; ER, 0.2

35−46

TG, 800; TC, 600−900; S/B, N/A; ER, N/A

FXB-2S

sewage sludge−pine sawdust MSW MSW rice husk wood residues wood RPF−wood coconut shell

NiO− dolomite

38.7−57.7

TG, 700−850; TC, N/A; S/B, 2 mL/min; ER, N/A

FXB

H2

conditionsb (°C, °C, −, −)

reactor typea

MSW

feedstock

Ni−CaO−TiO2

catalyst

Table 4. Comparison of Previous Ni-Based Catalyst Studies with This Study

Industrial & Engineering Chemistry Research Article

I


Article


scale microreactor, which does not address the technical issues (i.e., coke deposition, sintering and interaction with ashes, etc.) associated with the industrial scale application of the bifunctional catalyst Ni−CaO−TiO2. Therefore, it demands a large pilot scale study to address the technical issues associated with its industrial application.

were decomposed into LPAH, LAH, and HCC compounds due to steam tar reforming reactions (11) after the introduction of steam into the process. Furthermore, after the application of the catalyst (Ni−CaO−TiO2) into the steam gasification of MSW, the relative tar contents of HPAH and LPAH decreased from 11.1 to 3.07% and from 68.77 to 55.83%, respectively, while LAH (from 10.04 to 28.63%) and HCC (from 6.00 to 9.51%) increased significantly. This indicates the catalytic reforming of tar content. The conversion of relatively large molecular weight PAHs compounds (HPAH and LPAH) into the light hydrocarbons (LAH) and heterocyclic compounds (HCC) suggested that the metal oxide (NiO) in the catalyst−sorbent matrix (Ni−CaO−TiO2) was reduced to Ni0 (metallic status) by reducing the gases (i.e., H2 and CO), which enhanced the catalytic performance of the catalyst for tar cracking and reforming (reactions 1−3). GC− MS analysis of the tar samples indicated that the large molecular weight PAHs compounds (i.e., HPAH and LPAH) were the amplest compounds in the tar (Table S3−S5). The HPAH compounds (pyrene, benz[a]anthracene, triphenylene, etc.) were significantly reduced and converted to LPAH (naphthalene, biphenyl, acenaphthylene, fluorene, anthracene, phenanthrene, etc.), LAH (benzonitrile, phenols, acetophenone, styrene, indene, etc.), and HCC (pyridine, quinoline, isoquinoline, indole, dibenzofuran, etc.) compounds after the introduction of steam into the process (Tables S3−S5). As the catalyst was applied to steam MSW gasification, the HPAH and LPAH greatly reduced to LAH and HCC compounds, which is evident from Figure 8 and Tables S4 and S5. Additionally, the total peak area corresponding to the thermochemical conversion of MSW without steam and catalyst was found to be 3.96 × 108, while it reduced to 3.29 × 108 after the introduction of steam into the process and further decreased after catalyst application (2.26 × 108), which indicated the overall reduction of tar content (Tables 2 and 3). Thus, the application of the Ni−CaO−TiO2 catalyst not only quantitatively reduced the tar content but also altered its chemical composition from larger PAHs to light PAH compounds. 3.5. Comparison with Previous Literature. To evaluate and analyze the performance of the bifunctional Ni−CaO− TiO2 catalyst prepared by the dc arc plasma melting method, the results (product gas composition, gas yield, and tar content) are compared with those for conventionally prepared Ni-based catalyst for steam gasification by different researchers. The comparison with previous studies is presented in Table 4. Different researchers have used different feedstocks and gasification systems; however, they all have studied catalytic gasification performances of Ni-based catalysts. Catalytic gasification with Ni-based catalysts under a steam environment resulted in hydrogen-enriched syngas (above 50%) along with a higher gas yield and lower tar contents. The minor differences are caused by several factors including catalyst chemical composition, reactor type, feedstock properties, and gasification conditions. From this comparison, we can assume that the bifunctional catalyst (Ni−CaO−TiO2) prepared by the dc arc plasma technique exhibits good performance and can be used as an alternative technique for the development of catalysts for gasification process. The present study focused on MSW gasification with a bifunctional catalyst (Ni−CaO− TiO2) prepared by a dc arc plasma technique, yet it offers the same potential for the catalytic gasification of other feedstocks (i.e., biomass and coal). This study was carried out on a lab

4. CONCLUSIONS In this study, a bifunctional Ni−CaO−TiO2 catalyst/sorbent was prepared by a dc arc plasma melting method with varied Ni loading. The prepared catalyst samples were characterized by XRD, BET, and SEM analysis. Cyclic CO2 capture in severe conditions was explored with a TG analyzer, and the results were compared with those for CaO. The stability of CaO for CO2 adsorption decreased with each cycle of carbonation− calcination, whereas the Ni−CaO−TiO2 catalyst exhibited relatively stable CO2 capture after the fifth cycle. Hence, the stability of CaO was improved due to TiO2 addition into the Ni−CaO matrix. However, with the increase of Ni content, the net CO2 uptake slightly declined. In situ catalytic gasification of MSW with steam and Ni−CaO−TiO2 catalyst was executed in a fixed bed reactor. The Ni loading, C/M ratio, gasification temperature, and steam rate of 20%, 1, 850 °C, and 2 mL/min were found to be optimum conditions in this study, respectively. The H2 concentration, CCE, and DGY significantly increased after the application of the bifunctional Ni− CaO−TiO2 catalyst. The addition of the Ni−CaO−TiO2 catalyst not only enhanced the syngas composition, CCE, and DGY but also reduced the tar amount and altered its composition. The Ni content in the catalyst played a vital role in improving the composition and characteristics of product gas while eliminating most of the tar content. Therefore, the Ni−CaO−TiO2 catalyst prepared by the dc arc plasma melting technique is a promising approach with a potential industrial application prospect for MSW gasification.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.9b01999. Temperature selection for carbonation−calcination test; methods of data processing; proximate and elemental analysis of MSW sample; recipe for series of bifunctional catalysts; main tar compounds identified with and without steam and catalyst (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 411 8470 7448. Fax: +86 411 8470 6679. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was financially supported by Liaoning Natural Science Foundation (201602182) and Fundamental Research Funds for the Central Universities (DUT16LAB04).

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ABBREVIATIONS BET = Brunauer−Emmett−Teller CCE = carbon conversion efficiency

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dc = direct current DGY = dry gas yield GC = gas chromatography GC−MS = gas chromatography−mass spectroscopy HHV = higher heating value LHV = lower heating value MSW = municipal solid waste SEM = scanning electron microscope TGA = thermogravimetric analysis XRD = X-ray powder diffraction

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Enhanced H2 Production from Municipal Solid Waste Gasification

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