ANALYSIS AND COMPARISON OF THE FUEL PROPERTIES OF BIO-OILS PRODUCED BY CATALYTIC FAST PYROLYSIS OF Tectona

: This study analyzed the fuel properties of bio-oils produced by catalytic fast pyrolysis of Tectona grandis in a fixed bed reactor at different temperatures (400 – 600 o C) and biomass to catalyst (b/c) weight ratios (90/10 – 60/40). Magnesium oxide (MgO) was used as catalyst. The product yields were determined. Bio-oils were characterized with their elemental composition and their Higher Heating Values (HHVs) as well as their basic fuel properties at maximum bio-oil yields conditions, including viscosity, flash point, moisture content, pH value and Conradson Carbon Residue (CCR), were determined and compared with those of non-catalytic pyrolysis bio-oils. The maximum yields of bio-oil at 400, 500 and 600 o C were 31.53, 40.87 and 29.30 wt.%, respectively, obtained at b/c ratios of 70/30, 80/20 and 70/30. Catalytic pyrolysis bio-oils possessed higher carbon and hydrogen but lower oxygen and sulphur contents than non-catalytic pyrolysis bio-oils. The HHVs of catalytic pyrolysis bio-oils (40.31 – 42.08 MJ/kg) were higher than those of non-catalytic bio-oils (36.47 – 36.76 MJ/kg). Catalyst reduced the viscosity (at 400 and 500 o C), moisture content and CCR (at 400 and 600 o C), and increased the pH value of bio-oils (at 400 and 600 o C). Catalytic pyrolysis deoxygenates and enhances the fuel properties of bio-oils.


INTRODUCTION
Advances in global economy and sustainable development have necessitated the need to make energy readily available in the most environmentally friendly way. Energy generation from fossil fuels is no longer business as usual as humans are now well aware of its various consequences on the environment (Thangalazhy-Gopakumar et al., 2010), humans and aquatic lives (Latake et al., 2015). These include acid rain, depletion of the ozone layer, global warming (Zabeti et al., 2012) and destruction of microalgae, to mention a few. Sequel to these, regulations on carbon emissions into the atmosphere have become more stringent than ever. Besides, the depletion of fossil fuel resources suggests that fossils will not be able to meet the ever increasing energy demand in the future (Kraiem et al., 2014).
Renewable energy resources are now being considered as substitutes for fossils. Amongst the promising renewable resources, biomass is seen as a viable option. Although there are thermochemical and biochemical technologies for biomass conversion to energy fuels, thermochemical processes are often preferred. Pyrolysis, one of the thermochemical processes, has been shown to be a viable means for production of solid, liquid and gaseous biofuels, with the liquid fuel accounting for about 70% of the energy content of the feedstock (Bridgwater and Peacocke, 2000).
However, the liquid fuels (bio-oils) from pyrolysis have high moisture, oxygenated compounds, acidity and viscosity. Besides, they are unstable during storage and are characterized with low H/C ratios (Pütün, 2010). These shortcomings have brought about the use of catalysts in upgrading bio-oils from biomass pyrolysis (Samolada, 2000). Zhou et al. (2013) studied the effect of zinc oxide on the characteristics bio-oil derived from pyrolysis of rice husk in a fixed bed reactor. Pütün et al. (2008) investigated the effect of alumina on the product yields from pyrolysis of Euphorbia rigida biomass sample in inert and steam atmospheres and analysed the compositions of the bio-oil yields. French and Czernik (2010) evaluated the performance of commercial and laboratory-synthesized catalysts on hydrocarbon production during pyrolysis of aspen wood, Avicel PH-105 cellulose and straw lignin. Pütün (2010) studied the effect of magnesium oxide on pyrolysis of cotton seed and compared the yields of asphaltenes, n-pentane solubles, aliphatic sub-fraction, aromatic sub-fraction and polar sub-fraction from non-catalytic and MgO catalyst-assisted pyrolysis. Güllü (2003) studied the effect of sodium and pottasium carbonate on the yield of liquid products from pyrolysis of hazelnut, tea factory waste, tobacco stalk and yellow pine wood. Onay (2014) studied the effect of Ni-Mo/ − alumina catalyst on pyrolysis of Laurel (Laurus nobilis L.) seed in a fixed bed tubular reactor. Samolada et al. (2000) presented an evaluation of some catalysts commonly used for catalytic pyrolysis of biomass. In many of the studies discussed above, the basic fuel properties of the derived bio-oil were not determined. Qiang et al. (2008) analysed the chemical and physical properties of bio-oil from fast pyrolysis of rice husk and determined the basic fuel properties of the bio-oil. However, the effect of catalyst on these properties was not considered. Mythili et al. (2017) studied the effect of pyrolysis temperature on physicochemical properties of Prosopis juliflora bio-oil but did not also consider the effect of catalyst on these properties. . Therefore, in this study, the effect of magnesium oxide catalyst on the physicochemical properties of bio-oil from Tectona grandis pyrolysis was studied. The basic fuel properties, including density, viscosity, pH value, flash point, pour point and Conradson carbon residue of the bio-oil were also determined and compared with those of the bio-oil from non-catalytic experiments.

Materials
Tectona grandis sawdust was procured from a sawmill in Ogbomoso, South-Western Nigeria. Magnesium oxide (MgO) catalyst (having 5% loss on ignition at 1000 o C) was procured from a chemical vendor in Ogbomoso, Nigeria.

Feedstock Processing
The procured sawdust sample was sundried for three days in order to reduce its moisture content. After sun drying, it was weighed and bagged in a cellophane bag for it to maintain its moisture content and it was kept at room temperature till when it was used for the experiments.

Feedstock Characterization
The biomass sample was characterized with its fixed carbon, volatile mater, moisture and ash by proximate analysis using a thermogravimetric analyzer ( Figure 1 shows the exploded view of the pyrolysis unit used for the experiments. The detailed description of this setup has been given elsewhere (Okekunle et al., 2016). Samples with different biomass/catalyst weight ratio (b/c ratio) of 90/10, 80/20, 70/30 and 60/40, thoroughly mixed together, were fed into the crucible, one at a time, and the crucible was covered and fastened with bolts and nuts in preparation for a run. The furnace was then plugged to the mains, pre-set and heated with the aid of an electric heating element to a temperature 50 o C higher than the desired temperature for pyrolysis in order to compensate for the heat loss during the insertion of the crucible. When the furnace attained the pre-set temperature, it was opened and the crucible was inserted into it. The furnace was covered again and reset to the actual pyrolysis temperature.

Experimental Setup and Procedure
This procedure was followed for the pyrolysis temperatures of 400, 500 and 600 o C and a residence time of 15 min. After each run, the liquid and char yields were weighed and expressed in percentages of the weight of the initial sample while gas yield was obtained by mass balance.

Ultimate Analysis
The elemental analysis of the bio-oils was done with the same equipment used for the feedstock. The sulphur content of the bio-oils was also determined by a Sulphur analyzer (TSHR TS 6000 Total Sulphur Analyzer). The Higher Heating Values (HHVs) of the bio-oils were determined using modified Dulong's formula according to Theegala  (2)

Fuel Characterization of Bio-oils
The fuel properties of the bio-oils were determined in accordance with standard methods.

Physicochemical Properties of Tectona grandis
The physicochemical properties of the feedstock are presented in Table 1. Proximate analysis showed a high percentage of volatile matter (78.42%), low moisture content (9.00%) and ash (0.58). Elemental analysis revealed the percentage of carbon (50.82%), hydrogen (5.88%) and oxygen (42.80%). The detailed analysis of the physicochemical properties of the feedstock has been given elsewhere (Okekunle et al., 2021). Table 2 shows the yields of char, bio-oil and gas from the catalytic pyrolysis at different b/c ratios and temperatures. From the table, the highest yields of char (52.45%), bio-oil (40.87%) and gas (46.95%) were obtained at 400 o C (60/40 b/c ratio), 500 o C (80/20 b/c ratio) and 400 o C (80/20 b/c ratio), respectively, while the lowest yields of char (25.65%), bio-oil (10.27%) and gas (21.83%) were respectively obtained at 600 o C (90/10 b/c ratio), 500 o C (60/40 b/c ratio) and 400 o C (60/40 b/c ratio).   Table 3 shows the elemental composition of the bio-oils from catalytic pyrolysis at different temperatures and b/c ratios compared with those of the bio-oils from non-catalytic experiments. As shown in the table, the bio-oils derived from catalytic pyrolysis are richer in carbon and hydrogen but leaner in oxygen than those obtained from non-catalytic (NC) runs at the same temperatures.These results are in agreement with the findings of Pütün (2010), who reported higher percentages of carbon (77.62 wt.%) and hydrogen (12.15 wt.%) but a lower oxygen percentage (4.9 wt.%) for catalytic pyrolysis bio-oil compared to the values obtained for non-catalytic pyrolysis bio-oil (C -74.24 wt.%, H -11.39 wt.% and O -9.56 wt.%). Zhou et al. (2013) also reported higher percentages of carbon (49.73 wt.%) and hydrogen (12.57 wt.%) with a lower oxygen percentage (35.62 wt.%) for bio-oil from catalytic pyrolysis than the values obtained for non-catalytic pyrolysis bio-oil (C -49.20 wt.%, H -11.65 wt.%, O -36.78 wt.%). Carbon and hydrogen enrichment with reduction in oxygen in the bio-oils from catalytic pyrolysis suggests they have better calorific values than those from non-catalytic experiments. This fact is substantiated by the HHVs presented in Table 3. These results have also shown that catalysts help in deoxygenating bio-oils (Pütün, 2010). Oxygen reduction in catalytic pyrolysis bio-oils will improve their stability and enhance their upgrading to hydrocarbons (Shah et al., 2012). This is because oxygen in bio-oil has been identified as a source of bio-oil instability (Bardalai and Mahanta, 2015) and also, bio-oil upgrading to hydrocarbons will require oxygen removal (Shah et al., 2012).

Elemental Analysis of Bio-oils
Moreover, Table 3 also shows that the presence of MgO catalyst reduced the percentage of sulphur in catalytic pyrolysis bio-oils (0.05 -0.07%) compared to the values for bio-oils from non-catalytic experiments (0.11 -0.21%). This implies that Sox emissions from the combustion of catalytic pyrolysis bio-oils will be much lower than those from non-catalytic pyrolysis bio-oils. Therefore, bio-oils from catalytic pyrolysis could be more environmentally friendly. H/C and O/C ratios for bio-oils from catalytic pyrolysis are respectively higher and lower than those for noncatalytic pyrolysis bio-oils. These results are in agreement with the findings of Pütün (2010) and Zhou et al. (2013), who reported higher H/C and lower O/C ratios for bio-oils from catalytic pyrolysis compared to the values obtained for bio-oils derived from non-catalytic pyrolysis.

Basic Fuel Properties of Bio-oil
The basic fuel properties of the bio-oils from catalytic pyrolysis at the b/c ratios where maximum yields were obtained for all the considered temperatures are compared with those of the bio-oils from non-catalytic experiments at the same temperatures. The maximum bio-oil yields from catalytic pyrolysis at 400 (31.53 wt.%), 500 (40.87 wt.%) and 600 o C (29.35 wt.%) were obtained at b/c ratios of 70/30, 80/20, and 70/30, respectively.   Figure 3 shows the viscosities of the bio-oils from catalytic pyrolysis at optimum b/c ratio for bio-oil yield at 400, 500 and 600 o C compared with those of the bio-oils from non-catalytic experiments at the same temperatures. As shown in the figure, the presence of catalyst reduced the viscosity of bio-oil at 400 and 500 o C. Shadangi and Mohanty (2014 a&b) have also reported reduction in viscosity of bio-oils obtained from catalytic pyrolysis compared to the values obtained for bio-oils from non-catalytic pyrolysis of the same feedstock. One of the main concerns in the use of bio-oil as fuel in diesel engines is high viscosity, which poses a challenge during atomization (Shadangi, 2014a). Catalytic pyrolysis, with the right process conditions and b/c ratio, can help address this concern. At 600 o C, however, the viscosity of catalytic pyrolysis bio-oil was higher than that of non-catalytic by 1 mPa.s. This may be attributed to repolymerization of lighter organics at this temperature. The viscosities of the catalytic pyrolysis bio-oils obtained in this  Figure 4 shows the comparison of the flash points of the bio-oils obtained from catalytic pyrolysis with those of the bio-oils from non-catalytic experiments. As shown in the figure, catalytic pyrolysis bio-oils have lower flash points than those from non-catalytic experiments, and are also lower than that of petroleum diesel (58.5 o C) (Khan et al., 2016). This implies that the catalytic pyrolysis bio-oils obtained in this study are more flammable than petroleum diesel. Therefore, for safety in handling and transportation, increasing the flash points of the bio-oils by removing lighter organic compounds may be necessary.   Figure 6 shows the comparison of the moisture content of the bio-oils produced by catalytic and non-catalytic pyrolysis. As shown in the figure, the moisture content in catalytic pyrolysis bio-oils was higher at 500 o C but lower at 400 and 600 o C than in non-catalytic pyrolysis bio-oils. Shadangi and Mohanty (2014 a & b) reported a higher moisture content in catalytic pyrolysis biooils while Zhou et al. (2013) reported a lower value than in non-catalytic pyrolysis bio-oils. Many factors can be responsible for these different observations, including the type of feedstock, the catalyst used, the temperature of pyrolysis as well as the feed to catalyst ratio. In this study, the moisture content of the bio-oils from catalytic and non-catalytic experiments were not significantly different at 400 and 500 o C. At 600 o C, however, there was a reduction from 15.8% to 13% with the use of MgO catalyst (b/c ratio of 70/30).   Figure 7 shows the comparison of the CCR of the bio-oils from catalytic and non-catalytic pyrolysis. Carbon residue is a measure of the coke-forming tendencies of fuel oils. From Figure  7, it is interesting to note that the CCR is the same for all the catalytic pyrolysis bio-oils at all temperatures. This may be due to the fact that the data presented are for the b/c ratios yielding maximum bio-oil at all the temperatures considered. Beisdes, the figure also shows that the CCR for the catalytic pyrolysis bio-oils is lower than the values obtained for non-catalytic pyrolysis bio-oils at 400 and 600 o C. This implies that at these temperatures and b/c ratios for maximum yield of bio-oil, the formation of heavy structures that are difficult to evaporate was reduced. The values of CCR for both catalytic and non-catalytic bio-oils are lower than those reported for other bio-oils (