Production of Hydrogen and Nanocarbon by Catalytic Decomposition ...

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4th International Conference on Power and Energy Systems Engineering, CPESE 2017, 25-29 September 2017, Berlin, Germany The 15th International Symposium on District Heating and Cooling Production of Hydrogen and Nanocarbon by Catalytic Decomposition of Electrocracking Gasthe over Industrial Catalyst Assessing the feasibility of using heatandemand-outdoor under Integrated Reactor Conditions temperature function for a long-term district heat demand forecast a

a

b

c Ali A. Fayad I. Andrića,b,c *,S. A.Ismail Pinaa, *, P.Ahmed FerrãoaH. , J. Shukker Fournier,b.,Amina B. Lacarrière , O. Le Correc a

a Department of Chemistry, College of Education for Pure Sciences, University of Al Anbar, IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal P.O. Box 55, 55431 Al Ramadi, Iraq b Veolia College Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France Baghdad, Iraq b Department of Chemistry, of Education for Pure Sciences/Ibn Al Haitham, University of Baghdad, c Département Systèmes Énergétiques et Environnement - IMT Atlantique, 4 rue Alfred Kastler, 44300 Nantes, France

Abstract Abstract Hydrogen and nanocarbon were produced by the catalytic decomposition of electrocracking gas obtained by the pyrolysis of liquid organic waste via electric arc discharge. The GIAP-16 (NiO-Al2as O3one ) industrial catalyst was used to reduce maximum District heating networks are commonly addressed in the literature of the most effective solutions for the decreasing the decomposition temperature to 700 °C. In a fixed-bed reactor, under atmospheric pressure, reasonable amounts of high-purity greenhouse gas emissions from the building sector. These systems require high investments which are returned through the heat sales. Due to produced, the changed climate conditions building renovation policies, heat 2demand in the future could catalytic decrease, hydrogen were accompanied by depositsand of nanocarbon by-product. The NiO-Al O3 catalyst showed excellent prolonging thepowder investment return period. activity. X-ray diffraction analysis of the NiO-Al2O3 composite revealed the presence of cubic NiO and rhombohedral main scope of this paper is to assess the feasibility of using the heat demand – outdoor temperature function for heat demand AlThe 2O3, which were chemically stable. However, above 500 °C, NiAl 2O4 began to appear. The specific surface area of the catalyst forecast. The district of Alvalade, Lisbon with (Portugal), as a case study. The 4district is morphologies consisted of 665 was determined to be 65.45 m2/g, andlocated highly in dispersed, a pore was size used distribution centred around nm. The of buildings that vary in both construction period and typology. Three weather scenarios (low, medium, high) and three district GIAP-16 and the nanocarbon were investigated by scanning electron microscopy; the catalyst contained an agglomeration of renovation scenarios were developed (shallow, intermediate, deep). To estimate the error, obtained heat demand values were particles with thin well-formed filaments among much wider nanofibres and soot. compared with results from a dynamic heat demand model, previously developed and validated by the authors. © 2017 The Authors. Published by Elsevier Ltd. results showed that when only weatherLtd. change is considered, the margin of error could be acceptable for some applications ©The 2017 The Authors. Published by Elsevier Peer-review under responsibility of the scientific committee of the 4th International Conference on Power and Energy Systems (the error inunder annual demand was than 20% for all weather scenarios considered). However, afterand introducing Peer-review responsibility of lower the scientific committee of the 4th International Conference on Power Energy renovation Engineering. Systems Engineering. scenarios, the error value increased up to 59.5% (depending on the weather and renovation scenarios combination considered). The value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds to the Keywords: electric arc discharge; catalytic decomposition; electrocracking gas; GIAP-16 catalyst; hydrogen; nanocarbon deposit decrease in the number of heating hours of 22-139h during the heating season (depending on the combination of weather and renovation scenarios considered). On the other hand, function intercept increased for 7.8-12.7% per decade (depending on the coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and improve the accuracy of heat demand estimations. © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and * Corresponding author. Tel.: +964-770-994-4827. Cooling. E-mail address: [email protected]

Keywords: Heat demand; Forecast; Climate change 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 4th International Conference on Power and Energy Systems Engineering.

1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 4th International Conference on Power and Energy Systems Engineering. 10.1016/j.egypro.2017.11.112

Ali S. Ismail et al. / Energy Procedia 141 (2017) 315–331 Ali S. Ismail et al/ Energy Procedia 00 (2017) 000–000

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Nomenclature ATR CPO EDS FWHM GHSV NG POX SEM SR TCD TPR XRD 1.

autothermal reforming catalytic partial oxidation energy dispersive spectroscopy full width at half maximum gas hourly space velocity natural gas partial oxidation scanning electron microscopy steam reforming thermal conductivity detector temperature programmed reduction X-ray diffraction

Introduction

Fossil fuels will play a major role in hydrogen production in the near future, or will continue to be the world's main energy source. Over the past 50 years, global energy consumption has doubled every 10 years due to the ready availability of fossil fuels, their comparatively low cost, and the current infrastructure for delivery and distribution [1,2]. Hydrogen is highly desirable for the future. It is a major feedstock in many chemical and petrochemical industries and is positively perceived to be an environmentally clean source of energy, an efficient future transportation fuel, and for electricity generation. Hydrogen, with unique properties such as abundance, low mass density, high energy density, and non-polluting nature, has attracted significant attention from many researchers. For decades the major technologies for hydrogen production have been steam reforming (SR), partial oxidation reforming (POX), catalytic partial oxidation (CPO), and autothermal reforming (ATR), and include reforming (plasma and aqueous phase) and pyrolysis processes. In addition, electrolysis and other methods for generating hydrogen from water, hydrogen storage related approaches and hydrogen purification methods are also important [3]. There are many publications that report different methods for producing H2 and carbon from hydrocarbons. Hence, hydrogen and nanocarbon are two emerging research topics in the field of environmentally benign energy and material science, respectively [4-11]. Unfortunately, the main drawback of these decomposition methods is the energy loss associated with the capture of carbon. Thus, cracking may be the preferred option for natural gas and other hydrocarbon sources with high H2/C ratios [3]. The thermal decomposition of hydrocarbons can be represented by the simplified reaction: CnHm

nC(s) +

1 2

m H2

(ΔH is hydrocarbon dependent).

Other compounds are possibly formed, or oligomerisation occurs, due to hydrocarbon decay that depends on the reaction kinetics and on the presence of impurities in the raw materials. The above reaction yields a carbon-rich condensed phase and a H2-rich gas. For instance, the thermocatalytic decomposition of methane obeys the following reaction (that is controlled by strong C–H bonds): CH4

C(s) + 2H2

(ΔHo = + 75.6 kJ/mol).

Therefore, the chemical and petrochemical industries produce hydrogen on the large scale by reforming light hydrocarbons and natural gas. However, presently used reforming technologies face a variety of technical and scientific challenges that depend on the quality of raw materials. Impurity removal, cost, continuity of supply,

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conversion efficiencies and security require the integration of H2 production, purification and usage [12]. In this study, we harnessed the liquid organic waste from the chemical and petrochemical industries and, via electric arc discharge, produced electrocracking gas that was subsequently converted to reasonable amounts of hydrogen. 1.1 Processes for generating hydrogen from electrocracking gas 1.1.1

Decomposition of liquid hydrocarbons (liquid organic waste)

A summary of the literature highlighting the types of catalysts used, temperature, and carbon products from the decomposition of hydrocarbons is given below. Almost all of the papers reviewed address the decomposition of methane or natural gas into H2 and carbon black, while few papers discuss the pyrolysis of liquid hydrocarbons by electric arc discharge followed by the catalytic decomposition of the electrocracking gas produced in this manner, into H2 and carbon products for further applications. The electrocracking field effectively began when the Grand Prix was awarded to the Hungarian producers of the Erosimat spark machining apparatus at the 1958 World Exhibition in Brussels. The fundamental principles underlying the operation of a spark machining apparatus are provided by phenomena observed many years earlier [13] when it was noted that an electric arc, when produced between metal electrodes dipped below the surface of a dielectric liquid, produces minute metal particles that are detached from the electrodes. This process was already utilised at the beginning of the 20th century. About 20 years later, Kohlschütter developed this method further by applying a condenser in parallel with the electrode gap. Oscillating spark discharges ensued and the anode experienced significant weight loss compared to the cathode. Until then, spark machining investigations were carried out by electrical and mechanical engineers, as well as by physicists; only the co-workers of Lasarenko, namely Pechuro, Merkuryev, Grodzinsky, and Sokolova [13,14], report briefly on the decomposition of the dielectric medium, and on the products produced in that process. To the best of our knowledge, these are the only references from that time, or before, that discus electrocracking. The process of electrocracking, although similar to spark machining, is characterised by features that are quite different from those of the latter. It had been noted that the primary problem associated with the industrial application of spark machining is the filling of the gap between the electrodes with the dielectric liquid. In Hungary, kerosene is mainly used for this purpose. This is advantageous because the breakdown resistance of petroleum is almost as high as that of transformer oil, but its viscosity is considerably lower, therefore it can be applied more advantageously than transformer oil in fine machining, when the gap between the electrodes is comparatively small. During operation, metal particles enter the kerosene from the anode and the cathode; due to the decomposition of the kerosene, finely dispersed carbon, similar to carbon black, is also formed. In a spark machining apparatus, the dielectric liquid is circulated by a pump and the solid particles suspended in the liquid are removed continuously or intermittently. This removal can be carried out by filtration, separation, or more simply by settling [13]. The first reactor used for the decomposition of liquid hydrocarbons by electric arc discharge featured two graphite electrodes, and was constructed in the 20th century by Pechuro and Pesin, with the first patent for such a reactor being granted in the 1970s [15]. The use of this technique in Iraq by Al-Madfai and co-workers (Petroleum Research Centre) in 1985 then followed. These researchers reported briefly on the decomposition of light and heavy fuel oil by electrocracking, using a reactor with a sustained distance of 1.9–3.9 mm between the two electrodes. The gases produced were analysed using on-line gas chromatography, and the residual liquid by ultraviolet and infrared spectroscopic techniques. The evolved gases were mainly acetylene, lower olefins, paraffin, and hydrogen. The consumption of energy per acetylene was 28.5 kWh m-3 [16]. Most research on the decomposition of liquid organic substances in electrical discharges, for the purpose of acetylene manufacture, has been concerned mainly with the design of the plant, the compositions and yields of the products from various types of feedstock, and the effects of the electrical characteristics of the feedstock on yield and consumption of electricity; this method was found to be more environmentally friendly than the carbide method [17-20]. Pechuro and Pesin [14] reported results of experiments aimed at establishing how organic compounds, used as inter-electrode media, decompose when they are acted on by condensed discharges, paying particular attention to the


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yield and composition of the decomposition products, and to the proportion of the discharge energy expended on chemical reactions. The hydrocarbons chosen were representative of the main classes of organic compounds found in various petroleum products that are most commonly used as inter-electrode media. Table 1 lists the typical chemical composition of gases obtained by the decomposition of selected organic materials [14]; the character of the feedstock has little effect on gas yield and composition. This may be due to the fact that the cracked products consist mainly of individual paraffinic hydrocarbons and low-boiling petroleum products. Table 1. Chemical composition of gases obtained by the decomposition of selected organic petroleum products [14]. Initial feedstock Light

Gas composition, vol% Heavy

H2

CH4

C2H6

C2H4

C3H8

C3H6

C2 H2

C4H8

Petroleum product I (ibp 72 °C, fbp 127 °C) Petroleum product II (ibp 98 °C, fbp 139 °C) Petroleum ether

Diesel oil

50.5

6.9

1.0

10.7

0.3

3.3

26.4

0.9

Diesel oil

50.4

7.1

1.3

12.3

0.9

3.7

26.4

0.9

Transformer oil

52.1

9.3

1.0

6.9

-

3.8

26.2

0.7

n- Hexane

Vacuum-gas oil

52.5

6.2

0.5

12.9

0.2

1.1

25.8

0.8

n- Hexane

n- Undecane

53.0

6.2

0.8

14.5

0.3

1.3

23.4

0.5

n- Hexane

n- Decane

51.0

6.3

0.9

14.3

0.2

1.7

24.9

0.7

The no-load potential of the power supply: Unl ≈ 10 KV, N ≈ 200 W, Ip = 0.026 A. Inter-electrode gap, lig ~ 5 mm. Rate of low-boiling feedstock feeding ~ 1.0 L/h. The gases were analysed by gas adsorption chromatography (ibp = initial boiling point, fbp = final boiling point).

The increasing demand for unsaturated hydrocarbons as raw materials in the petrochemical industry has encouraged the development of new processes aimed at utilizing new raw materials, increasing yields, and minimizing energy requirements. A current topic of investigation is the cracking of high-molecular-weight hydrocarbons at high temperatures and over very short times using electrocracking techniques and energy beams [16,17,21]. The process of electrocracking is based on the excitation (by specific methods) produced by an electric arc discharge between electrodes placed inside a liquid feedstock. The plasma formed as a result of the electric discharge, with a temperature in the range of 5000 to 10000 K, interacts with the feedstock and induces its thermal decomposition, yielding gases composed of hydrogen, acetylene, methane, ethylene, as well as fine carbon black [18]. 1.1.2

Decomposition of electrocracking gas to hydrogen and nanocarbon

The reforming processes discussed use methane, or thermocatalytically decomposed (or pyrolysed) natural gas (NG), as the raw material, since they are the main sources of hydrocarbons available. However, in various reforming processes, electrocracked methane is used as the source since it contains more than 50 vol% hydrogen, but this depends on its composition. Thus, most research into methane reforming can also be adapted to the remaining hydrocarbons. H2 is preferred as the energy source in fuel cells, which have been developed for transportation, as well as for power generation in stationary or portable installations. Fuel cell systems have several applications such as large power stations, power distribution generators for buildings and homes, small portable power supplies for microelectronic devices, and auxiliary power units for vehicles, among others [19,14,22]. These cells transform the chemical energy of H2 into electrical energy, with up to 60% efficiency [14,19]. In addition, when H2 is used in vehicular fuel cells, it can provide two to three-fold improvements in efficiency than current devices used in internal combustion vehicles (20–30%). These qualities make H2 one of the major alternatives to the fossil fuels consumed worldwide [21]. Currently, H2 is widely used as a raw material in chemical industries, in hydrogenation processes, in the production of ammonia and methanol, in the Fischer-Tropsch synthesis, and in the pharmaceutical industry,


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among others [22]. Our previous work provided a method for the synthesis of carbon nanofibres through the decomposition of liquid organic waste from the chemical and petrochemical industries, using the Ni-Al2O3-catalysed decomposition of electrocracking gas at temperatures of 250–550 °C. The components of the produced gas were: H2 (64 ± 1.0 vol%), CH4 (6.4 ± 1.0 vol%), C2H6 (0.7 ± 0.1 vol%), C2H4 (6.0 ± 1.0 vol%), C3H6 (0.8 ± 0.1 vol%), and C2H2 (22.1 ± 1.0 vol%) [17,19,20]. In this case, due to the presence of hydrogen among the electrocracking gas components, the proportion of hydrogen gas released after formation of carbon nanofibres contains additional hydrogen produced through the breakage of carbon-hydrogen bonds. For this reason, it is of importance to develop an efficient catalyst in terms of high catalytic activity, stability, selectivity and lower activation energy requirements. This catalyst system and its optimum operating conditions are expected to contribute effectively towards the large-scale production of hydrogen through the catalytic cracking of electrocracking (hydrocarbon) gases as raw materials; the ideas presented here also take advantage of previous work. In this paper was selected GIAP-16 industrial catalyst because of its high activity for methane steam reforming [22]; this high catalytic activity and stability during reforming ensures that reductions in the high temperatures normally used will be accompanied by increased reaction rates, as well as less catalyst deactivation through poisoning (chemisorption of carbon deposits and/or other impurities), which are the main problems usually encountered [23-29]. Catalysts have been successfully used for the thermal decomposition of hydrocarbons. The most common catalysts are noble and transition metals such as Ni, Fe, Pd, Co, and Mo, among others, either unsupported, or supported on high-surface-area ceramic substrates such as Al2O3 and SiO2 [3]. Among them, Ni catalysts have gained significant attention because of advantages such as availability, low cost, good activity, and stability [30]. On the other hand, several publications have described Ni as having greater deactivation susceptibility through coke formation resulting from the high temperatures used. In the present study, we transformed liquid hydrocarbons (liquid organic waste) to electrocracking gas through pyrolysis by electric arc discharge, with subsequent decomposition of the electrocracking gas to H2 and carbon products over an industrial catalyst in a fixed-catalyst-bed reactor, at different temperatures (300– 700 °C). The catalytic performance of the industrial catalyst, GIAP-16 (which is usually used for methane steam reforming) for the decomposition of electrocracking gas to hydrogen and carbon over the catalyst layer, was investigated. 2.

Materials and methods

2.1 Materials and generation of electrocracking gas The organic liquid waste used in this investigation was obtained from a local refinery, and its physicochemical characteristics are listed in Table 2. Table 2. Physicochemical characteristics of the liquid organic waste used in this study. Specification

Value

Specific gravity at 15.5 °C

0.917

APIa gravity

21.410 2 -1

Viscosity at 40 °C (mm s )

4.539

Distillate at 300 °C (vol%)

66.400

Flash point (°C)

113.000

Salt content (mg/L) H2S content (ppm) Total sulfur content (wt%) Water content (vol%)

23.300 11.410 2.890 0.160

The apparatus used in our experiments is similar to that used in previous studies, as depicted in the schematic

320


diagram in Fig. 1 [20]. The electrocracking gases were obtained through the decomposition of the organic liquid wastes in an electric-arc laboratory reactor designed for the pyrolysis of organic liquids in low-voltage electrical discharges, as shown in Fig. 1.

Figure 1. Schematic diagram of the laboratory setup for gas electrocracking.

The reactor is a stainless-steel vessel composed of a vertical cylinder with a water jacket designed for the loading of 700 mL of organic liquid waste. This container is fitted with fixed stationary graphite electrodes in the form of graphite rods arranged in parallel, with a spacing of 1 mm. There are mobile intermediate contacts on the electrodes, and a graphite ball of diameter ~6.5 mm is positioned between the rods. Fig. 2 presents a clearer schematic diagram of the stainless-steel vessel. When voltage from the power supply is applied to the stationary electrodes and a ballarc discharge occurs, the organic liquid waste decomposes to produce gas and soot. The gas was collected to determine the composition of the decomposed electrocracked liquid organic waste fractions; the gas composition is listed in Table 3.

Figure 2. Schematic diagram of the stainless-steel vessel used in the decomposition of organic liquid waste.


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The reactor is filled with a specific amount of the raw materials, after which the installation is sealed. Voltage is then applied and the resulting gaseous decomposition products that accumulated over the reaction period were analysed by gas chromatography every 20 min. The compositions of the inlet and outlet hydrocarbons gas mixtures were analysed on-line by a LHM-8 gas chromatograph (Meta-chrom Co., Russia), fitted with a thermal conductivity detector (TCD) and a 90 mA current bridge detector. Al2O3 promoted with a 5% solution of NaOH was used as the stationary phase, with a carrier gas (nitrogen) flow rate of 20 mL/min. The column was a 7 m thermostat column that was operated at 80 °С. Table 3. Composition of the electrocracking gas from the organic liquid waste fraction. Component

vol%

H2 CH4 C2H6 C2H4 C3H8 C3H6 C2H2 C4H10

55.4 ± 1.0 10.3 ± 1.0 1.6 ± 0.1 11.8 ± 1.0 0.2 ± 0.1 0.8 ± 0.1 17.5 ± 1.0 2.4 ± 0.1

The industrial GIAP-16 catalyst used in this work (State Institute for Nitrogen Industry (GIAP), Russia) is composed of NiO/Al2O3. The catalyst, which was obtained in the form of cylindrical extrudates ~5 mm in diameter and 6–7 mm in length, were crushed and sieved. The catalyst particle size used in the present study ranged 0.5–1.0 mm. Before the experiment, the GIAP-16 catalyst was activated by roasting in a furnace at 600 °C for 3 h, to remove coke and other impurities. 2.2 Production of hydrogen and nanocarbon As shown in Fig. 3, a laboratory flow setup with an integrated reactor was used for the production of hydrogen and nanocarbon. The electrocracking gas was displaced from storage by water through a valve, regulated by a control valve, and operated at atmospheric pressure, as determined by a water U-tube manometer. The flow of gas to the reactor was precisely regulated by calculating the gas hourly space velocity (GHSV), which was determined from the data obtained from a rheometer, to ensure that each process was subjected to the selected GHSV.

Figure 3. Schematic diagram of the laboratory setup for the production of hydrogen and nanocarbon.


322

The gas flow was directed toward the quartz reactor. Catalyst samples (0.2 g) were placed in a fixed bed in the centre of the reactor tube (inner diameter, 10 mm) inside the split furnace. The temperature in the reaction zone was measured by a thermocouple and recorded. The reaction was followed over time by sampling both the input and output gas streams at regular intervals followed by analysis using the gas chromatography equipment described above. This chromatograph was specially configured (columns and microvalve switching) for the analysis of light gases that include alkanes (C1–C5), alkenes (C2H4, C3H6), and acetylene (C2H2). These experiments had previously been performed using quartz as a non-catalyst reference material [19], under analogous conditions and with the same weights used in the catalyst experiments. These previous results were used to evaluate reaction selectivity toward the production of hydrogen and nanocarbon. The total amounts of carbon deposited as a result of exposure to the stream was determined by material balance using the data in this work and those obtained from the quartz experiments [19], and were calculated according to the procedure described in Appendix A. 2.3 Characterization Powder X-ray diffraction (XRD) (Model Bruker-AXS D8 Advance/Ultima IV, Germany) experiments were conducted with an accelerating voltage and an applied current of 40 kV and 40 mA, respectively, using Cu Kα radiation (λ = 1.5406 Å) in the range (10–90°). Scanning electron microscopy (SEM, S-4800, Hitachi, Japan), along with energy dispersive spectroscopy (EDS), at an accelerating voltage of 15 kV, was used to study the surface morphologies of the catalyst and nanocarbon, in addition to elemental analysis. The specific surface area of the catalyst was determined using a surface area and porosity analyser (Micrometrics, Model ASAP 2460, USA), with nitrogen (N2) adsorption–desorption at –196 ºC and the Brunauer-Emmett-Teller (BET) equation. An adsorption model was included in the instrument software. 3.

Results and discussion

3.1 Flow reactor studies In an initial series of experiments, an electrocracking gas mixtures was passed over the GIAP-16 catalyst, in a fixed-bed reactor at atmospheric pressure, with the hydrogen produced accompanied with nanocarbon deposit as byproduct. The electrocracking gas was composed of H2 (55.4 ± 1.0%), C2H2 (17.5 ± 1.0%), CH4 (10.3 ± 1.0%), C2H4 (11.8 ± 1.0%), C4H10 (2.4 ± 0.1%), C2H6 1.6 ± 0.1%, C3H6 (0.8 ± 0.1%), and C3H8 (0.2 ± 0.1%), and was passed over 200 mg of GIAP-16 at total flow rates of 6.976 or 13.330 cm3/min, corresponding to GHSVs of ~1744 or 3333 h-1, respectively. In general, all cracking gases formed hydrocarbons and were always rich in hydrogen. Another predominant product was acetylene followed by methane and ethylene, with negligible concentrations of ethane, propane, propylene and butane. Literature reports demonstrate that the catalytic decomposition of acetylene is the major route for the growth of carbon nanofibres and carbon nanotubes [14]. We saw the formation of nanocarbon at low temperatures (up to ~400 °C), presumably because of the decomposition of acetylene. At ~400 °C, increases in the concentrations of methane and ethane in the gas mixture suggested that acetylene is being hydrogenated and decomposed. As the temperature was further increased, other cracking-gas hydrocarbons become involved in carbon forming reactions; only H2 was present in the effluent gases at 500–700 °C. The corresponding gas-phase product distributions are presented in Table 4. It can be seen that optimum conditions for the formation of hydrogen gas are realised at approximately 500–700 °C. As the temperature is shifted either above or below this level, specifically 500–700 °C with GHSVs of 1744 and 3333 h-1, the output gas is only composed of hydrogen, with a drop in the yield of solid carbon observed. The data presented in Table 4 show that the optimum yields of solid carbon occurs at 300 °C with a GHSV of 1744 h-1. As the hydrogen content in the gas mixtures increases to 90%, and even up to 100%, the amount of carbon nanofibre generated reached minimum values. Increases in the percentage of hydrogen appear to significantly decrease the amounts of carbon or the stability of the deposited carbon at high temperatures in the gas stream. This explains why the majority of carbon deposition occurs in the upper part of the tube, near the feed end, specifically on the walls of the glass tube of the catalytic decomposition chamber, instead of the catalyst itself, although the catalyst layer was placed in the centre of the reactor furnace, since this region is at a relatively low temperature. It is


323

desirable that the catalyst in this region is selective and sufficiently resistant to undesirable side reactions. Table 4. Gas-phase product distributions as functions of synthesis conditions. All reactions were carried out for 40 min over GIAP-16. Temperature, °С

GHSV, h-1 1744 3333 1744 3333 1744 3333 1744 3333 1744 3333

300 400 500 600 700

Н2 69.22 76.88 77.84 84.46 100.00 100.00 100.00 100.00 100.00 100.00

СН4 12.20 11.53 13.67 13.49 0.00 0.00 0.00 0.00 0.00 0.00

Component of gas, %vol. С2H6 C2H4 C3H8 C3H6 0.56 7.22 0.08 1.12 0.89 7.88 0.00 2.52 3.14 5.23 0.00 0.00 1.22 0.83 0.0 0.0 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

C2H2 9.26 0.30 0.12 0.00 0.00 0.00 0.00 0.00 0.00 0.00

C4H10 0.34 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Yield of C, g/Lgas 0.3007 0.2917 0.2892 0.2776 0.2792 0.2760 0.2717 0.2774 0.2776 0.2779

Hydrogen Yield (%)

100 90 80 1744 (h-1)

70 60

3333 (h-1) 200

300

400 500 Temperature (oC)

600

700

Figure 4. Hydrogen yields from the catalytic decomposition of electrocracking gas at gas hourly space velocities (GHSVs) of 1744 and 3333 h-1, as functions of reaction temperature.

In the absence of a breakthrough technology it is likely that H2 will continue to be produced from fossil fuels for some time. The aim of this study was to explore whether or not industrial catalysts that have been used for decades for reforming methane and natural gas, can also be used to produce hydrogen from surplus organic liquid waste produced by oil refinery sites. We chose to use the GIAP-16 industrial catalyst for the catalytic production of hydrogen, through the decomposition of hydrocarbons that have already been obtained through an electrocracking process, despite the lack of literature that discusses the nature of the GIAP-16 industrial catalyst. It is known that the initial rate of hydrocarbon decomposition depends on the nature of the catalyst and its support. The industrial, GIAP16 catalyst is often used without any pre-treatment, due its activity. Fig. 4 shows that GIAP-16 exhibits effective catalytic activity and stability, with the production of high percentages of hydrogen over the 300–700 °C temperature range, which remained almost unchanged over the entire duration of the experiment (180 min). In addition, no phase transformation was observed, except for the formation of carbon nanofibre, which may be the main reason for the significant catalytic activity and stability of the GIAP-16 catalyst; this is discussed in further detail below. The catalyst components are NiO and Al2O3; the latter is considered to be the support. Some previous studies have reported that different types of nickel oxides exist in the


324

as-prepared catalyst, which was demonstrated by the temperature programmed reduction (TPR) [31], at high temperature, of NiO to Nio. Therefore, the high activity exhibited is due to the presence Nio, and can be ascribed to the formation of NiO.Al2O3 or NiAlO4 as new oxides that are well dispersed and have long conversion lifetimes, and exhibit high resistance to sintering and coking [32]; inspection of the XRD data (below) reveal the presence of peaks corresponding to NiAlO4.

Solid Carbon Yields , g/Lgas

0.32 0.30 0.28 0.26 1744 (h-1)

0.24

3333 (h-1) 0.22

200

300

400 500 Temperature (oC)

600

700

Fig. 5. Solid carbon yields from the catalytic decomposition of electrocracking gas at gas hourly space velocities (GHSVs) of 1744 and 3333 h-1, as functions of reaction temperature.

Fig. 5 depicts the relative amounts of solid carbon from the catalytic decomposition of the electrocracking gas mixture. Subsequent analysis showed that the catalytic decomposition of acetylene was the major route for the growth of carbon nanofibres. At low temperatures (300 °C), carbon was formed because of the decomposition of acetylene. Inspection of the data presented in Table 4 show that above ~300 °C increases in the concentrations of methane and ethane were observed, suggesting that acetylene hydrogenation had occurred along with decomposition. As the temperature was further increased, other electrocracking-gas hydrocarbons became involved in the carbon forming reactions, and only H2 was present in the effluent gas at 500–700 °C. In addition, in this set of experiments we found that the yield of carbon nanofibres at high temperatures mainly consisted of amorphous carbon especially at 500–700 °C. It is also evident that catalytic activity remains constant at temperatures up to 700 °C. When the reaction was performed above 700 °C, a sudden drop in activity was observed approximately 40 min into the reaction. According to a mechanism proposed in the literature [33], the nature of the carbon-containing gas, as well as catalyst composition, plays an important role in the formation of the microstructure and morphology of carbon nanofibres. 3.2 Textural, structural and morphological properties The GIAP-16 catalyst and carbon nanofibre samples were analysed by XRD. Fig. 6 shows the typical powder XRD pattern of GIAP-16, before use, which is structurally composed of several different metastable states, and exhibits three prominent peaks at 2θ angles of 37.4°, 45.7°, and 68.3°, which are well matched to the (d104), (d113), and (d300) spaces of the Al2O3 phase (JCPDS card file no. 04-0875), respectively. In addition another phase is observed that corresponds to NiO particles, with peaks at 2θ angles of 37.4°, 43.5°, and 62.92°, belonging to the (d111), (d200), and (d220) spaces of the NiO phase (JCPDS card file no. 04-0835), respectively. Trace amounts of impurities were also detected in the GIAP-16 catalyst, included SiO2, with a prominent peak at a 2θ angle of 26.0° assigned to the (d111) space of SiO2 (JCPDS card file no. 05-0490). The XRD (2θ) data show that Al2O3 has a


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structurally complex oxide metastable phase, α-Al2O3. The peaks corresponding to Al2O3 and NiO gradually broadened over time, and their intensities decreased; the peaks are mainly due to smaller average crystalline sizes and lattice strain. The peaks from rhombohedral α-Al2O3 and cubic NiO overlap appreciably at 37.4° and form an intermediate single phase, NiAl2O4 nickel aluminate spinel, suggesting that the composite is formed in the catalyst during its high temperature preparation. Moreover, a detectable solid-state reaction occurred between NiO and Al2O3 in the industrial catalyst, due to increasing temperature. The d104 spacing of the GIAP-16 catalyst was 0.2397 nm, and the crystallite size (Lc) was estimated by analysing the catalyst peak using the Scherrer equation, Lc = kλ/(β cosθ), where k is a constant (the shape factor, approximately 0.9), λ is the X-ray wavelength (0.15418 nm), β is the full width at half maximum (FWHM) of the diffraction line, and θ is the diffraction angle. The catalyst crystal size was determined to be 25.36 nm.

Fig. 6. Representative X-ray powder diffraction pattern for the GIAP-16 industrial catalyst.

Figure 7 shows a typical XRPD pattern of the carbon nanofibres. The intense peak at observed at 26.552° is attributed to the (002) diffraction plane of carbon (JCPDS card files, no. 41-1487). The diffraction peak of the carbon nanofibre was clearly visible, regardless of the catalyst or carbon-containing gas used for the synthesis, indicating that the peak corresponds to the basal plane of crystalline graphite that has crystallised into the hexagonal structure with lattice parameters: a = b = 2.47 Å and c = 6.72 Å, close to the ideal and highly ordered structure of graphite [34]. The d002 spacing of the carbon nanofibre was 0.336 nm, and the crystallite size (Lc) was estimated by analysing the carbon nanofibre peak using the Scherrer equation, as described above; the carbon nanofibre crystal size was determined to be 8.68 nm. The XRPD analysis of the powder also revealed peaks for Ni, suggesting that the Ni that overlaps with the deposited graphite forms a detectable NiC phase. In addition, the Ni peaks are much sharper because large Ni particles are easily dispersed in the fibres to give distinct nickel peaks at 2θ = 44.27°, 51.54°, and 76.51°, indexed to the Ni(111), Ni(200), and Ni(220) diffraction peaks of the cubic phase of nickel (JCPDS card files, no. 01-1258). Rodriguez reported that there is a subtle relationship between the degree of ordering in the deposited carbon nanofibres and the ability of the metal catalyst particle to participate in strong interactions with graphite [35]. He also found that the metal catalyst, which has good wetting characteristics with respect to graphite, forms highly graphitic carbon nanostructures.

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Fig. 7. Representative X-ray powder diffraction pattern for the carbon nanofibres produced. The pattern corresponds to nanofibres grown at 300 °C at a total electrocracking gas flow rate of 6.97 cm3/min.

The X-ray energy dispersive spectrum of the NiO-Al2O3 industrial catalyst is shown in Fig. 8. The atomic (at%) and weight (wt%) percentages refer to the main components of the catalyst, which are almost identical to bimetallic catalysts that exhibit similar activity, except that our catalyst has somewhat slightly anti-coking performance compared to other catalysts. In addition, its activity is enhanced due to reduced particle sizes of the active metallic species and the formation of stable filamentous carbon, even in small quantities. Despite this, the catalyst remains active for the production of hydrogen in reasonable volumes at high temperatures. The elemental analysis data of the industrial catalyst shows no impurities. Typical N2 adsorption-desorption isotherms for the GIAP-16 catalyst are shown in Fig. 9a. Analysis of these isotherms leads to the conclusion that this material has a type IV isotherm profile, according to the IUPAC classification system, consistent with mesoporous materials with pore diameters between 2.0 and 50.0 nm. The nitrogen isotherms exhibit very steep desorption curves at ~ 0.42 < P/Po < 0.50 nm, which reveal that a large proportion of adsorbents are in pores with sizes that fall in the narrow range of pore radii (rp) (~1.7 < rp < 2 nm) [36]. In fact, the observed sudden cut-off (located at around P/Pº ≈ 0.43), corresponds to the lower closure point of a large number of nitrogen hysteresis loops, especially if the pores are slit shaped, leading to the lower limit of stability for the particular capillary condensate at a given temperature [37-39]. The expression ‘micropore’ is reserved for pores no wider than ∼2 nm [36], while wider pores (2–50 nm), which are of specific significance in the context of capillary condensation, are known as ‘mesopores’. Here we suggest that the catalyst is mainly mesoporous and contains a small number of micropores, as the characteristic feature of the type IV isotherm is its hysteresis loop, which is associated with capillary condensation that takes place in mesopores that limits uptake to a range of high P/P° [36]. The hysteresis loop observed in Fig. 9A lies between H1 and H2 types, according to the IUPAC classification system, and is based on the shape of the boundary of the hysteresis loop, which gives morphology information. It is known that hysteresis loops of type H1 are characterised by a narrow pore-size distribution with non-connected cylindrical pore textures, while type H2 correspond to a large pore-size distribution that are disordered and highly connected [40]. In our study, the sudden cut-off at around P/Pº ≈ 0.43 (for nitrogen at 77 K), and assuming a sharp pore-filling or emptying step, then this catalyst is considered to be located at the lower limit of capillary condensation hysteresis. Generally the behaviour of the catalyst in this study is consistent with the SEM images (Fig. 10) that show conglomerates (or agglomerates) and adsorbents containing slit-shaped mesopores crossed by cylindrical pores, consistent with monolayer-multilayer adsorption in accordance with the available data


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that include a BET specific surface area of 65.45 m2 g-1, and a pore volume of 0.144 cm3 g-1.

Fig. 8. Scanning electron microscopy image and EDS spectrum of the NiO-Al2O3 industrial catalyst.

Fig. 9B details the textural properties of the industrial NiO-Al2O3 catalyst (GIAP-16) that are obtained from the N2-adsorption/desorption isotherms. Pore-size distributions were obtained by the Barrett–Joyner–Halenda (BJH) method and presume capillary condensation phenomena. Two pores-size distributions for the NiO-Al2O3 catalyst are observed; one monomodal centred at around 4 nm, and the other bimodal with a broad distribution extending from 6 to about 12 nm. The morphology and elemental distribution of the NiO-Al2O3 (GIAP-16) catalyst were determined with the foreknowledge of the specific surface area, pore volume, and pore diameter results, obtained using BET and EDS. It should be emphasised that a catalyst of natural morphology was used in this study; it was industrially prepared, rather than synthesised in the laboratory, and is intended to be used for syngas production by steam reforming, or as a layered catalyst for use in fixed-bed reactors to prevent coke formation and improve syngas yield, usually in oil refining sites [22]. Consequently, since the catalyst is used in this work for the production of H2 and solid nanocarbon, rather than the production of syngas, it is important that the catalyst is robust and does not deactivate rapidly.

Ali S. Ismail et al/ Energy Procedia 00 (2017) 000–000 Ali S. Ismail et al. / Energy Procedia 141 (2017) 315–331

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Fig. 9. (a) N2-desorption/desorption isotherms of the industrial GIAP-16 catalyst. (b) BJH pore-size distribution for the NiO-Al2O3 catalyst from adsorption-desorption isotherms.

The SEM micrographs of the catalyst are depicted in Figs. 10a and b and show that the catalyst contains large amounts of agglomerated particles. A layer of alumina particles can be seen to cover the surface of the nickel oxide, like sponge blocks in the form of peels. The BET surface area (65.4599 m2/g), pore volume (0.144346 cm3/g), adsorption-average pore diameter (4V/A, 8.82043 nm) also indicate the presence of exposed nickel layers between alumina blocks; these layers can significantly increase specific surface areas, as seen for nanostructured nickel. The surface area and active centres on the catalyst are associated with these layers. Meanwhile, the agglomerated catalyst exhibits remarkable catalytic activity in comparison to isolated Ni particles, and is highly effective in capturing carbon atoms from the electrocracking gas, and releasing hydrogen at high temperatures. Fig. 10c and d are typical SEM images of the solid product formed by reacting the electrocracking gas mixture over the NiO-Al2O3 catalyst, and show thin, well-formed filaments among much wider nanofibres and soot. While the specimen does not contain free soot, the non-uniformity of the amorphous carbon coating on some fibres has resulted in the formation of soot-like nanoballs along the fibres, which is caused by metal particles that are seeded over preformed nanofilaments. Large differences in the fibre diameters are generally observed, and despite the formation of these balls, the fibres are quite straight, with diameters ranging from 21.3 nm to 233.0 nm, with lengths that exceed several micrometres. Closer inspection reveals that most of the carbon nanofibres with smooth, round surfaces are bent, while a few are curly. Neither soot nor irregularities are observed along the length of the nanofibres, and the fibres appear quite uniform in size. 4.

Conclusions

In conclusion, we have demonstrated that electrocracking followed by thermal decomposition over GIAP-16 is a useful method for transforming liquid organic wastes, which are basically fossil fuel waste products, into hydrogen and nanocarbon. The electrocracking gas, composed mainly of hydrocarbons and a high proportion of hydrogen, is efficiently converted into hydrogen by the catalytic decomposition of hydrocarbons using the GIAP-16 industrial catalyst, at 500–700 °C, in a fixed-bed reactor at atmospheric pressure. The catalytic decomposition of hydrocarbons is drawing attention, not only for the production of hydrogen, but also for its ability to provide high-value carbon nanofibres. In our system carbon appears as solid deposits at 300 °C through the emergence of a NiC phase that promotes nucleation and growth of the carbon nanofibre on the surface of the catalyst pores, through a carbide mechanism. Subsequent analysis reveals that the catalytic decomposition of acetylene was the major route for the growth of carbon nanofibres.


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Figure 10. Scanning electron microscopy (SEM) images of (a, b) the NiO-Al2O3 industrial catalyst, and (c, d) carbon nanofibres synthesised using the NiO-Al2O3 industrial catalyst at 300 °C. The insets show the formation of soot-like nanoballs along the fibres associated with ‘fishbone’ structures.

Energy dispersive spectroscopic analyses (EDS) of these catalyst particles show the presence of aluminium, nickel and oxygen in sufficient quantities to enable the identification of NiAl2O4 resulting from the solid-state reaction between Al2O4 and NiO, with increasing temperature. X-ray powder diffraction (XRPD) and scanning electron microscopy (SEM) depth profiles analyses reveal that the peaks corresponding to Al2O3 and NiO are gradually broadened and their intensities decreased. These peaks are mainly due to average smaller crystalline sizes and lattice strain, with agglomerated catalyst particles and high average surface area (65.4599 m2/g). In addition the Ni peak was observed to overlap with the graphite deposit to form a detectable NiC phases. Acknowledgements The authors greatly appreciate the valuable help of Mr Yan Chaoyi (Latech Labs, Singapore) with the preparation of XRPD, SEM, EDS data, and BET data. Appendix A The relative amounts of solid carbon generated from the decomposition of electrocracking gas (EG) as a function of the reactant gas composition were calculated according to the following procedure. Consider the material balance


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in the deposition reactions: [Н2]imгi + [СН4 ]imгi + [С2Н6]imгi + [С2Н4]imгi + [С3Н6]imгi + [С2Н2]imгi → [Н2]omгo + [СН4]omгo + [С2Н6]omгo + [С2Н4]omгo + [С3Н6]omгo + [С2Н2]omгo + mC where

[M]i = concentration of component M in the input gas, [M]o = concentration of component M in the output gas, mri, mro = mass of gas (= M*n) for the input and output gases, respectively.

The yield of solid carbon from electrocracking gas decomposition can be written as: ∆C = mass of carbon (gm) from EGinput − mass of carbon (gm) from EGoutput

Yield of nanocarbon =

which is:

∆C (gm/L) Volume of gasinput

Total amount of nanocarbon = yield of C presence of the catalyst − yield of C presence of quartz. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]

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