1. Introduction
Greenhouse gas (GHG) emissions from fossil fuel combustion for power generation represent a major contribution to climate change. For this very reason, a switch from conventional to renewable power resources, i.e., solar, wind, hydroelectric energy and biomass is necessary [
1].
Biomass can consistently provide energy and fuels and has an advantage over other renewable energies sources as it is more homogeneously distributed over the earth and is an abundant resource [
2]. International Energy Outlook 2017 reported that biomass could provide over 14% of the world’s primary energy consumption, which is the highest among renewable energy resource, and it will contribute a quarter or third of the global primary energy supply by 2050 [
3].
For all the above and as a consequence of unstable oil prices and the alarming climate change, biomass gasification has increasingly received interest [
4]. Indeed, this is a versatile and interesting way to re-use different wastes (e.g., agricultural and urban wastes, energy crops, food and industrial processing residues) to produce bio-syngas, which can be used for electrical power generation (fuel cells, gas turbine or engine), or as feedstock for the synthesis of liquid fuels and chemicals such as methanol [
5]. Furthermore, the necessary technology for this process can be adapted from old coal gasification units [
6]. However, one of the most critical technical challenges in biomass gasification is the formation of tars. Tar condensation can cause serious risks to downstream equipment. Therefore, tars should be removed from the effluent stream of biomass gasification [
7].
Existing techniques for tar removal after a gasifier include separation either by physical (mechanical) methods, using ceramic candle filters or wet scrubbers, or thermochemical conversion methods using high temperature thermal or catalytic cracking to convert tar into syngas [
8]. Physical separation methods would cause secondary pollution since they only remove tar from gas products, resulting in a waste stream that needs treatment. Conversely, thermal cracking has received increasing attention because tar can be converted into useful gas products and increases the overall efficiency of the gasification process [
9]. Thermal cracking without catalysts operates at high temperature (>1000 °C) to decompose the tars in smaller non-condensable molecules. The high energy consumption makes this process less interesting. By contrast, catalytic cracking of tars can be carried out at lower temperatures converting tars into useful gases in a more efficient manner and is being widely studied as a principal method for tar removal [
10]. As a steam reforming reaction, the proposed reaction could remove tar by a catalytic process and produce fuel H
2 and CO at relatively low temperatures. At the same time, tar steam reforming poses some challenges that must be addressed related to the reaction conditions. Reforming catalysts can lose activity over time due to carbon deposition and sintering over the active phases [
5]. These problems can be minimised with optimal operating conditions in the presence of the right catalyst. Real tar composition is highly complex and most studies use model tar compounds such as toluene, benzene or naphthalene to ascertain the catalytic mechanism [
11,
12,
13]. However, previous studies have shown that these compounds represent a worst-case scenario in the tendency of the system to form carbon deposits [
14,
15].
Noble metals and Ni are most widely active phases used in reforming catalysts. Noble metal catalysts including Pt, Rh and Ru are known for their exceptionally good activity and stability in tar steam reforming. However, these catalysts have had limited use due to their high costs [
16].
Nowadays, the aim is to develop an economically viable material, ideally not containing noble metals, which produces the same high levels of conversion and reaction performance as the noble ones. Ni is an attractive choice as steam reforming catalytic metal thanks to its good performance in the conversion of different types of hydrocarbons [
16], being, for instance, the most popular active phase in methane steam reforming [
17,
18]. In particular, Ni/Al
2O
3 catalysts are considered as the state-of-the-art materials for steam reforming processes. Furthermore, the high surface area of alumina and its mechanical properties result in an excellent choice as support for nickel nanoparticles [
19].
Under these premises, this work showcases the application of a Ni/Al
2O
3 catalyst in the steam reforming of toluene (C
7H
8) as a tar model compound in a fixed bed reactor. Until now, the catalytic performance in the steam reforming of toluene has been mainly evaluated as a function of catalyst design variables, such as the nature of the support [
20,
21,
22] and metal [
23,
24], but little attention has been paid to the reaction conditions, especially for this benchmark catalytic formulation [
25]. Identification and optimisation of the reaction parameters in the presence of a commercial-like catalyst (Ni/Al
2O
3) are vital to achieving the best catalytic performance. However, few studies involving parameter screening have been carried out [
8].
Herein we analyse the influence of reforming temperature, steam to carbon molar ratio (S/C) and gas hourly space velocity (GHSV) on the toluene reforming performance. These are considered the key parameters to fine-tune the reaction and maximise the overall performance.
Parallel reactions in this system can be numerous. The main reactions that can occur during toluene steam reforming are represented as follows:
Steam reforming of toluene is irreversible and Reactions (1) and (2) are dependent on the S/C ratio used. CH
4 is produced from hydroalkylation (Reaction (4)) and pyrolysis of toluene. Methane reforming followed by a water–gas shift reaction converts the produced CO with steam to H
2 and CO
2. A Boudouard reaction is an exothermic reaction that produces C from CO. All of these reactions are heavily conditioned by the temperature, space velocity and reactants ratio [
26,
27,
28]. Hence, a careful assessment of the impact of these parameters will allow us to identify the optimum conditions towards the generation of added value products.
For all the above, this work fills an essential gap in tar reforming literature via the systematic study of key reaction parameters using a state-of-the-art catalyst to reveal the optimum process conditions.
2. Experimental
2.1. Catalyst Preparation
The nickel-based catalyst was prepared following a wet impregnation method. The necessary amount of Ni(NO
3)
2·6H
2O (≥97.0%, Sigma-Aldrich) to obtain 20 wt.% NiO was dissolved in an excess of acetone (≥99.8%, Sigma Aldrich). Then, the support γ- Al
2O
3 (≥98.0% purity, Sasol) was added into the solution and, after stirring for 2 h, the solvent was removed under vacuum at 60 °C by using a rotating evaporator. The remaining mixture was dried overnight at 110 °C. Finally, the solid was calcined at 600 °C with a ramping rate of 2 °C·min
−1 for 4 hours. It has been reported that Ni/Al
2O
3 catalysts are stable under reaction conditions despite the calcination temperature being lower than those of the experiments [
29,
30]. Lower calcination temperatures have been shown to lead to better catalytic performance in steam reforming [
31]. The obtained sample was labelled Ni/Al
2O
3. The Ni content assuming complete reduction from NiO to Ni is 16.4 wt.%.
2.2. Characterisation
Thermogravimetric analysis (TGA) was carried out to investigate the coke deposition on the catalyst in a Pyris 1 thermogravimetric analyser from PerkinElmer (Waltham, MA, USA). The samples were ramped from room temperature to 900 °C at a rate of 10 °C·min−1 in air.
N2-adsorption-desorption analysis was conducted in a TriStar 3000 V6.07 A analyser from Micromeritics (Norcross, GA, USA). Before the analysis, the catalyst was degassed at 150 °C for 4 h in a vacuum. The Brunauer–Emmett–Teller (BET) method was used to calculate the surface area of the catalyst.
2.3. Catalytic Toluene Steam Reforming Tests
Toluene steam reforming was carried out in a fixed bed reactor used in previous bio-oil reforming work [
24]. Before the reaction, the reactor was purged with N
2 to remove air. The catalyst was reduced under 50 mL·min
−1 of H
2 up to 700 °C for 1 hour before each test.
Figure 1 shows the schematic diagram of the experimental reaction set-up, and the reaction zone is shown in
Figure 2. The reactor was heated up by two copper electrodes; toluene and steam were injected by two syringe pumps from the top of the reactor and preheated at 200 °C to the vapour phase in a preheating chamber. Toluene was carried by N
2 with a fixed concentration of 100 g Nm
−3. Then the reactant stream entered an incoloy alloy 625 tube (12 mm i.d., 2 mm thick, 253 mm long), equipped with an inner quartz tube (9 mm i.d., 1 mm thick, 300 mm long) to prevent any contact between the reactant gas stream and the incoloy internal surface. 500 mg of Ni/Al
2O
3 catalyst with a particle size in the range of 250–500 μm was placed right in the middle of the quartz tube. A K-type thermocouple was used to determine the catalytic bed temperature.
The product gases after reaction pass through two condensers in series to collect any liquid product as well as unreacted toluene and water. Ice and dry ice were used as coolant in the two condensers, respectively. The products identified in the gas phase were H2, CH4, CO2 and CO. Two on-line gas analysers were used to determine the product gas compositions: an MGA3000 Multi-Gas infrared analyser (ADC Gas Analysis, Herts, UK) for CO2, CH4 and CO, followed by a K1550 thermal conductivity H2 analyser (Hitech Instruments, Luton, UK).
The performance of catalysts was evaluated by the conversion into gaseous products (based on a carbon balance between the inlet and the outlet stream of the reactor), selectivity to main products (where ‘‘i” is CO
2, CO and CH
4 in moles) and hydrogen yield, which were defined as follows:
The experimental error in toluene conversion, gas selectivity and gas yield is ±2%. Toluene conversion and H2 production could be influenced by experiment conditions and parameters. Reforming temperature, S/C ratio and residence time are reported to be the key factors that would affect the total conversion and H2 yield. In this paper S/C ratios of 1, 2 and 3; temperatures of 700, 800 and 900 °C, and GHSV of 30,600, 61,200, 91,800 and 122,400 h−1 were investigated.
2.4. Thermodynamic Simulation
The ASPEN software package (AspenTech, Bedford, MA, USA) was used to determine the thermodynamic equilibrium of the toluene reforming reactions over the different reaction conditions. An ideal property method, an RGIBBS reactor (based on Gibbs free energy minimisation) was selected to investigate the thermodynamic equilibrium. Material flows into the reactor are identical to those from the corresponding experiment. The influence and effects of experimental parameters, including reforming the temperature and S/C ratio on the toluene conversion, the yield of main light gases and the carbon deposition was investigated.
4. Conclusions
To remove tar produced from biomass gasification, catalytic steam reforming was conducted for toluene as a model tar compound. Simulations of a thermodynamic equilibrium based on Gibbs free energy minimisation and experiments in a fixed bed reactor using a Ni/Al2O3 catalyst were carried out. The effect of reforming temperature, S/C ratio and GHSV on toluene conversion and product distribution was studied.
Increasing the temperature from 700 to 900 °C increased total conversion, with a potential risk of higher coke deposition. A temperature of 800 °C observed the highest H2 production, high toluene conversion (>94%) and relatively lower coke deposition.
The Ni/Al2O3 catalyst only requires a very short residence time (GHSV < 91,800 h−1) for toluene reforming with this catalyst and effectively removes low density toluene in a mixed gas stream. H2 yield and toluene conversion increased slightly and approached simulated thermodynamic equilibrium results when GHSV decreased. Coke deposition increased at a lower rate as GHSV increased.
A high S/C ratio would greatly increase total conversion, hydrogen production and reduce the coke formation on the catalyst. The presence of excess steam could shift the equilibrium of the water–gas shift reaction to produce more H2.
A temperature of 800 °C, GHSV of 61,200 h−1 and S/C ratio of three provided the most suitable reaction conditions for toluene conversion and H2 production in steam reforming of toluene, obtained a steady state of a toluene to gas conversion over 94%, a H2 production of 141.6 mol/mol toluene in a five-hour test, with no obvious deactivation observed in five hours. Based on these results, this condition would be suitable for tar model compound removal.