Supercritical Water Gasification of Eucalyptus Wood Chips Using NiFe 2 O 4 as a Catalyst

. In this work, the supercritical water gasification of eucalyptus wood chips has been investigated in relation to reaction temperature and presence of catalyst. Experiments were performed in a batch reactor at 450 ºC and 500 ºC with two different feed concentrations. This work wanted to evaluate mainly the gases were formed during the reactions. The gas products were analysed by gas chromatography. According to the results, it was found that eucalyptus wood chips reacted to form mainly H 2 , CH 4 gases with little yield of CO 2 . Increasing the reaction temperature beyond 500 ºC led to the increasing production of both CH 4 (around 31.1 mol%) and H 2 (up to 38 mol%) gases and the liquid sample and solid residue have decreased. Generally, this work suggests that the SCWG has improved significantly the production of H 2 but more experiments still necessary to verify the effects of other experimental parameters and to characterise the liquid sample and solid residue.


Introduction
Searching for renewable energy sources to ensure economic development with social inclusion without environmental aggression is a challenge all over the world. An alternative is biomass, a primary and renewable energy source. Brazil has 5,673,783 hectares of eucalyptus planted, 70.74 million m³ of all this eucalyptus was consumed by the pulp and paper industry in Brazil in 2016 [1]. In 2016, Brazil generated 47.8 million tons of solid waste, with 14.1 million tons (29.5 %) from industrial activities (pulp and paper sector). In the paper industry, this waste is burned to generate energy, avoiding the use of fossil fuels but more waste is generated than can be used for the process energy requirements. Nevertheless, it is still necessary to discover new technologies to generate clean energy using this waste.
Hydrogen from waste biomass is considered to be a clean gaseous fuel and efficient for heat and power generation due to its high energy content [2]. A good choice to produce hydrogen-rich-gas is biomass gasification, involving a thermochemical conversion process of a solid into fuel gas, which maintains 70-80% of the original energetic fuel. The process occurs at high temperatures (700-1500 °C) and in the presence of a gasification agent (air or oxygen, steam, carbon dioxide). However, during the gasification process, some of the fuel is not converted into gas remaining as a condensate product (tar) [3]. Tar formation is one of the major problems to deal with during the biomass gasification process because it is a complex mixture of condensable hydrocarbons with low condensation temperatures [3][4][5]. A new and promising technology to ensure lower tar formation and higher hydrogen yield is the supercritical water gasification (SCWG). The main difference between SCWG and other gasification techniques is related to the gasification medium i.e., supercritical water compared to inert gas and/or steam. Supercritical water gasification is a process that can also convert wet biomass, which reduces pre-treatment costs. Further, the high solubility of organic compounds in supercritical water enhances their reactivity, which leads to an improvement of the SCWG performance. These advantages make SCWG an even more attractive technology, because one of the major disadvantages of the traditional gasification is the large production of tar. The SCWG also produces higher hydrogen content at high temperatures (> 700 °C) and higher methane content at lower temperatures (< 600 °C) [6,7]. Even though the SCWG increases the selectivity of hydrogen production, it requires high pressure and temperature for the reaction without catalysts. Catalytic supercritical water gasification (CSCWG) is a promising 1 solution to improve the efficiency of the gasification process, producing a mixture of gases and values-added liquids products depending on the process conditions. The choice of catalyst to gasify lignocellulosic biomass in supercritical water depends on its capability to break C-C bonds, promote water gas shift (WGS) reaction, accelerate the cleavage of the C-O bond, and decrease the activation energy in SCWG reaction [8]. Heterogeneous catalyst has higher selectivity and recyclability compared with the homogeneous catalyst. The transition metals are typical catalysts for CSCWG because they offer high selectivity, high catalytic activity, and high level of carbon conversion to gas at lower temperature [9].
The present work aimed to perform the supercritical water gasification process of eucalyptus wood chips in the presence of a novel NiFe 2 O 4 for increased hydrogen production using a batch reactor. The investigation involved study of the influences of biomass concentration in addition to reaction temperature on the gasification process.

Raw Materials
The raw material used was Eucalyptus grandis chips with approximately 30 % moisture content and size of 710 μm obtained from Federal University of Bahia. The biomass was characterised before use. All analysis was performed according to the Standard Test Method (ASTM). The characteristics of the wood chips were determined by physic-chemical analyses as ultimate analysis; determination of immediate analysis and Higher Heating Value (HHV). The methodology of these analyses has been discussed in an earlier paper [10]. The results of these analyses are summarized in Table 1. The catalyst used, NiFe 2 O 4 , was synthesized by combustion reaction method using ferric nitrate Fe(NO 3 ) 3 .9H 2 O, nickel nitrate Ni(NO 3 ) 2 .6H 2 O and urea CO(NH 2 ) 2 as fuel. The methodology of that synthesis and characteristics of the catalyst has been discussed in an earlier paper [11].

Reactor system and experimental procedure
The supercritical water gasification experiments were performed in a non-stirred Hastelloy-C reactor obtained from Parr Instruments Co., Inc. USA. The reactor has 75 ml of volume capacity and the maximum operating conditions of temperature and pressure are 600 ºC and 45 MPa, respectively.
In each experiment, first the feed material was loaded into the reactor containing deionized water, then the catalyst was loaded. The amount of water loading was 20 mL at 450 °C and 15 mL at 500 °C, giving corresponding operating pressures of 33 MPa and 28 MPa, respectively. Experiments were performed using two different amounts of biomass, 2.22 g (10 wt %) and 5.0 g (20 wt %) and amounts of catalyst (1.0 g and 2.0 g). These were designed to evaluate some of the main parameters of the process (temperature, catalyst, feed concentration).
Once the reactor was loaded, it was sealed. It is placed into an electrical furnace fitted with temperature controller. The reactor was heated fast at a rate of 20 °C min −1 up to 450 and 500 °C, such that the reaction temperature was reached after just 15 minutes. The reaction time was mostly 30 min. At the end of each reaction, the reactor was rapidly withdrawn from the heater and quickly cooled to ambient temperature with compressed air. After cooling, the gas outlet valve was opened to collect the gas samples. After that, the reactor was opened to collect the liquid sample and solid residues.
The results have been obtained are only preliminary. It is necessary to carry out experiments in duplicates/triplicates to guarantee a good reproducibility in terms of product balance and physical experiments parameters. In this work only, the gases formed during the reactions were analyzed properly.

Gas products analysis
The gases were collected from the reactor using a gas sampling bag then 1 μL were sampled into a gas tight syringe and analysed using gas chromatograph (Shimadzu GC-2014 gas chromatograph) fitted with two columns and detectors were used. Usually, two samples were taken per experiment and the average reported. The hydrocarbon gases (methane, ethene, ethane, propene, propane, butane and butene) were separated on a 2 m long by 2 mm diameter and packed with 80 -100 mesh Hayesep and analysed using a flame ionization detector (FID). The permanent gases (hydrogen, oxygen, nitrogen, and carbon monoxide) were separated on a 2 m packed molecular sieve (60 -80 mesh) column and analyzed using a thermal conductivity detector (TCD) while a packed Hayesep column of similar dimensions was used for analysing carbon dioxide. The GC was first calibrated with standard gas mixtures (1 % H 2 , 1 % CO, 1 % CH 4 , 1 % CO 2 , 1 % ethene, 1 % ethane, 1 % propane, 1 % propene, 1 % butane and 91 % N 2 as the swing gas) at levels similar to the composition of the produced gases. Argon was used as the carrier gas. The methods of analysis applied are in agreement with previous work [12]. The results from the GC were obtained in volume percent and were converted to moles of each gas using the general gas equation.

Results and discussion
The parameters investigated included the influence of feed concentration, feed catalyst and the temperature on the supercritical water gasification of eucalyptus wood chips. Mostly of the experiments were performed at 450 ºC and 30 min of residence time, only the test 10 was carried out for 60 min. Only three experiments were performed at 500 ºC. The experiments performed, and the gas analysis results obtained from the experiments can be observed in Table 2 and Table 3, correspondingly.

Effect of feed concentration and catalyst concentration
The effect of feed concentration on gasification of eucalyptus wood chips, in supercritical water gasification was carried out in the presence of NiFe 2 O 4 at 450 ºC for 30 min reaction time. The wood sample concentrations used were 10 wt% and 20 wt%. The concentration of eucalyptus wood chips and the presence of catalyst especially have a significant effect on the gasification process. It can be observed that increasing feed concentration the CH 4 (33 %) gas product yield rises significantly (Table 3). In general, the concentration of the other gaseous products (CO, CO 2 , and hydrocarbons) usually increase at higher concentrations of feed samples, as some researchers have reported [13,14,15]. But it was not noticed during the experiments. It could be explained by the presence of the catalyst, which probably helped to keep the predominance of water gas shift and methanation reactions instead of the alternative reaction pathways during the reaction at higher biomass. Nickel catalyst is a good option not only because of the cost-effective but is also known promote water-gas shift reaction and to largely suppress tar formation by cracking it. Many researchers have been introducing it into a supercritical water gasification reaction to understand better the activity and stability of its. Most searches found that using nickel catalyst the conversion of biomass can be accelerated. Researchers [8,16,17] have done experiments to evaluate the hydrothermal gasification performance using Ni as a catalyst. According to these authors, chemical and physical structure of Ni catalyst importantly changes at hydrothermal conditions, these changes can improve the gasification reactions of biomass under supercritical even subcritical water conditions. According to the " Fig. 1 (A)" the presence of catalyst during the experiments at 450 ºC increased potentially the H2 and CH4 gas product yields by 49.3 % and 10.2 %, respectively. The results were consistent with earlier literature reports [8,12,16,17].
About the liquid obtained from each experiment, at lower feed concentration the liquid samples were clear. As the feed concentration was increased, a dark brownish oil layer was present in the liquid sample. This indicated that the range of reaction temperature and reaction time were not enough to complete degraded the biomass used during the experiments.

Effect of temperature
" Fig. 1 (B)" illustrates the effect of reaction temperature increasing from 450 to 500 ºC on the composition of gas products from hydrothermal gasification of eucalyptus wood in the presence of NiFe 2 O 4 catalyst. The feed of biomass was kept at 2.2 g (10 wt %). According to the "Fig 1 (B)", the temperature has a significant effect on the gas yields, the higher temperature favour gaseous production, especially H 2 and CH 4 yields due to the promotion of free-radical reactions which stimulate gas formation. The highest hydrogen yield (around 38 %) was obtained by conversion of eucalyptus wood chips at 500 ºC. even during the experiments carried out without catalyst.

Possible uses of gas products
The gasification process led to the production of hydrogen and methane fuel gases, which have various existing applications. First, the gas product can be fed into a gas engine for electricity production and combusted directly for clean energy in the form of heat for the cellulose industry. Second, the gas products are obtained at high pressure, which makes separation using e.g. pressure-swing absorption or membranes, highly applicable. Hydrogen may be obtained pure and used in hydrogen fuel cells, while methane can be used in gas engines to produce electricity. Pure methane can also be fed into domestic gas networks for homes or converted to syngas for further processing. All these added value activities from the SCWG process can potentially increase the economic and environmental viability of the cellulose and other biomass process industries

Conclusions
According to the preliminary results, it was observed that the supercritical water gasification is a viable process for convert eucalyptus biomass in H 2 and CH 4 . It was found that eucalyptus wood chips decompose to H 2 , CH 4 gases with little yield of CO 2 . Increasing the reaction temperature beyond 500 ºC led to the increasing production of both CH 4 (around 31.1 %) and H 2 (around 38 %) gases and the liquid sample and solid residue have decreased. It was also observed the influence of the catalyst. The NiF 2 O 4 increased the C-C bonds decomposition of eucalyptus. Using the nickel ferrite led to the increasing production of H 2 (49 %). This work proposes that the supercritical water gasification has improved expressively the production of H 2 but more experiments still necessary to verify the effects of other parameters and to characterise the liquid sample and solid residue.
Detailed feasibility studies will be carried in collaboration with the cellulose industry in Brazil, to evaluate the potential yields of these gases and associated costs of the process in a typical cellulose plant in consideration for technology take-up.