Int. J. Renewable Energy Technology, Vol. 2, No. 2, 2011
On the thermal oxidation stability of pyrolysis biomass oil Xianguo Hu*, Chuan Li, Yufu Xu and Qiongjie Wang School of Mechanical and Automotive Engineering, Hefei University of Technology, 193 Tunxi Road, Hefei, 230009, China Fax: +86(0)551 290 2956 E-mail:
[email protected] E-mail:
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[email protected] *Corresponding author
Xifeng Zhu Anhui Key Laboratory of Biomass Clean Energy, University of Science and Technology of China, Hefei 230026, China E-mail:
[email protected] Abstract: The rice husk-based biomass-oil is composed of complex organic compounds and water, and its oxidation stability has a direct influence on its physico-chemical property and application. The advanced oxidation experiments were carried out through the flowing of air and oxygen respectively. The influences of four factors, including gas flux, oxidation time, temperature and copper foil, on the oxidation process of rice husk-based biomass-oil, by measuring the pH value variation and the weight of deposit before and after oxidation were studied. The results showed that the pH value of biomass-oil decreased slowly with the increases of the gas flux and operation time. The presence of copper in the oil made the sediment weight increase after oxidation. Based on the GC-MS analyses of the compositions of biomass-oil before and after oxidation, it was found that the acetic acid was the main component in the oxidised biomass-oil, which was attributed to the pH value decrease of biomass-oil after oxidation. At the same time the content of phenolic compound was also increased compared with that before oxidation. It was also proposed a chain-reaction-based mechanism for the biomass oxidation. Keywords: biomass-oil; oxidation stability; GC-MS; chain reaction. Reference to this paper should be made as follows: Hu, X., Li, C., Xu, Y., Wang, Q., and Zhu, X. (2011) ‘On the thermal oxidation stability of pyrolysis biomass oil’, Int. J. Renewable Energy Technology, Vol. 2, No. 2, pp.155–168.
Copyright © 2011 Inderscience Enterprises Ltd.
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X. Hu et al. Biographical notes: Xianguo Hu is currently a Professor at the School of Mechanical and Automotive Engineering of Hefei University of Technology, China. He received his BSc and MSc from the Hefei University of Technology in 1985 and 1988, respectively. His PhD degree was awarded in Szent Istvan University, Hungary in 2002. As a visiting scientist he researched at Technical University of Budapest, Hungary and Technical University of Berlin, Germany from 1994 to 1997. His research area includes the preparation and characterisation of new fuel, lubricity of fuel, tribology of nanocomposites, etc. He is the author or co-author of more than 100 published technical papers. Chuan Li is a MSc student at the School of Chemical Engineering of Hefei University of Technology, China. His main focus is on the oxidation of biomass fuel. Yufu Xu is a Lecturer and PhD student at the School of Mechanical and Automotive Engineering of Hefei University of Technology, China. His main focus is on the upgrading biomass fuel. Qiongjie Wang was a MSc student at the School of Mechanical and Automotive Engineering of Hefei University of Technology, China. She is currently a PhD student in Nanjing University, China. Her main focus was on the oxidation and corrosion performance of biomass fuel. Xifeng Zhu is currently a Professor at the Anhui Key Laboratory of Biomass Clean Energy, University of Science and Technology of China, Hefei 230026, China. His research area includes preparation and application of new energy.
1
Introduction
To reduce the demands for the limited fossil resource, more and more attention was paid to the alternative and regenerative energy materials. As one of the regenerative and clean energy materials, biomass-oil has many advantages such as zero emission of carbon dioxide, high energy density, easy storage and transportation. So far, most researches were focused on the preparation methods to increase the yield of biomass oil. Although biomass pyrolysis oils have potential to be used as a fuel oil substitute and combustion tests have shown that the oils burn efficiently in standard or slightly modified boilers and engines with rates similar to those for commercial fuels, the biomass oil contains complex chemical components with high oxygen (30–40%) and low pH value (between 2 and 3), including acid, alcohol, aldehyde, ketone, phenolic, water (15–30 wt%) etc. (Czernik and Bridgwater, 2004; Wang et al., 1997; Ozcimen and Karaosmanoglu, 2004; Wang et al., 1998). The complex compositions in the biomass oil make it unsteady, because different components are easy to reaction each other, and the reactions can be accelerated by inletting air due to the existence of oxygen, especially as stored with a long time at high temperature (Ba et al., 2004; Zhang et al., 2007). Over time, reactivity of some components in the oils leads to formation of larger molecules that result in high viscosity and in slower combustion (Oasmaa and Czernik, 1999). There are some reactions like the esterfication between hydroxyl and carboxyl groups, and etherification between hydroxyl and carbonyl groups during the storage and applications. At the same time, the polymerisation can be taken place because of unsaturated bonds which reactions will lead the increments of average molecular weight of biomass oil and rate of water content. It was reported that viscosity and average molecular weight will be increased
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lineally during storage and application. The bigger the molecular weight, the higher the viscosity of biomass oil (Oasmaa and Czernik, 1999; Czernik et al., 1994). The results showed the viscosity of biomass oil at 40°C could be reduced half and decreased the variation rate of viscosity of biomass oil with time. Methanol is the best additive. The viscosity of the biomass oil with 10 wt% methanol can meet the need of ASTM number four diesel fuel after exposure 96 hours at 90°C. Some solvents should be added to biomass oil in order to store longer with stable properties. Diebold and Czernik (1997) studied to add different additives such as 10 wt% acetic ether, 5 wt% methylisobutylketone and 5 wt% methonal, 10 wt% acetone, 5 wt% methanol and 5 wt% acetone, 10 wt% ethonal, in order to decrease the viscosity of biomass oil and enhance the stability of biomass oil. In contrast without methanol, the viscosity of the biomass oil can not meet the standard of ASTM number four diesel fuel. Another way to control the oxidation if biomass oil is to add the antioxidants, such as adding phenolic substances like hydroquinone (Diebold, 2000). Boucher et al. (2000) studied the ultimate objective of which was to provide background information on biomass pyrolysis oils (biomass oils) regarding their use as a gas turbine liquid fuel. The biomass oil was obtained by vacuum pyrolysis of softwood bark residues. The stability and ageing of the biomass oil and mixtures thereof were evaluated. The samples were stored at 40, 50 and 80°C for up to 168°C and at room temperature for up to one year, period after which the phase separation time, viscosity, solid and water content and average molecular weight were measured. The results indicated that the properties of the biomass oil were significantly altered when the biomass oil was heated at 80°C, but that the variations after heating at 40 and 50°C were not critical. It was found that the molecular weight increase after heating the biomass oil for one week at 80°C was equivalent to keeping the sample for one year at room temperature. The addition of aqueous phase to the biomass oil lowered its thermal stability significantly. A rapid phase separation occurred after heating at 80°C and, therefore, the total aqueous phase concentration in the biomass oil must be limited to 15%. Ageing of the raw biomass oil at room temperature resulted in a dramatic viscosity increase during the first 65 days, period after which a plateau was reached. The addition of methanol to the biomass oil was beneficial for the biomass oil properties as well as for the stability of the biomass oil and its mixtures. Methanol dissolved some structured components of the biomass oil and thus reduced the viscosity increase rate. Moreover, the addition of methanol to the biomass oil/pyrolytic aqueous phase mixtures delayed the phase separation process. In the present paper, we studied four factors affecting the oxidation process, including temperature, time, air and oxygen fluxes and placement of copper foil in the biomass oil. The mechanism of the oxidation of biomass oil was discussed based on analysing the variations of components of biomass oil before and after oxidation from GC-MS measurement.
2
Materials and methods
2.1 Chemicals All chemicals were analytical grade unless with special specification. Biomass-oil from rice husk was produced by fast pyrolysis process in the absence of air. The pyrolysis
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conditions were as follows: normal atmospheric pressure, 450–550°C with a high heating rate (103–104 K/s) for a short gasification time. The final product was cooled to liquid rapidly to decompose the biomass into short chain molecules (Zhang et al., 2007). The detailed preparation process was described in reference (Zhu et al., 2006). The pure copper (99.95%) foil was made from commercially pure copper mill products and selected to place inside the biomass oil.
2.2 Advanced oxidation experiment Figure 1
Illustration diagram of oxidation apparatus
Notes: 1 – oxidation tubes; 2 – safe bottle; 3 – gas flowmeter; 4 – hot oil; 5 – gas pressure controller; 6 – sulphuric acid; 7 – sodium hydroxide; 8 – gas absorption bottle
According to Chinese Petroleum Standard SY2652, the intensified oxidation experiments were carried out by using the Oil Anti-oxidation Capability Testing Machine (Dalian Analysis Apparatus Co.), and the pH value of the tested oil was analysed by pH-metery (Shanghai Precision & Scientific Instrument Co.). The illustration diagram of experiment apparatus was shown in Figure 1. 30 g biomass oil was placed in the oxidation tube and the air or oxygen was purified by passing through sodium hydroxide and sulphuric acid bottles, the gas flux was controlled as flow velocity by flowmeter. Four typical oxidation conditions were selected as follows except for special descriptions in the text: 1
25°C, by air, 150ml/min, 4 h
2
55°C, by air, 150ml/min, 4 h
3
25°C, by O2, 150ml/min, 4 h
4
55°C, by O2, 150ml/min, 4 h.
2.3 Composition analysis The biomass oils before and after oxidation were analysed by gas chromatography-mass spectrometry (GC-MS; GC, HP5890; MSD, HP5972A) which was conducted to analyse the compositions of biomass-oil. The separation of components was realised on a column of DB-5MS, 30 m × 0.25 mm × 0.25 μm, and the oven temperature program was 40°C for 5 min at 6°C/min to 295°C for 10 min. The total ion current was automatically
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obtained by the computer software according to molecular mass from 10 to 1,000 and element type of each component.
3
Results and discussion
3.1 Role of gas A more important reaction that affects the storage of pyrolysis oils is that of forming hydroperoxides and alkylperoxides by autoxidation with air. Air oxidation to form more acids and reactive peroxides that catalyse the polymerisation of unsaturated compounds (Diebold, 2000). Figure 2 shows the pH value variations with air flux at different temperatures. It was found the pH value decreased with the air flux either at higher or lower temperature. The reduction of pH value should be attributed to the oxidation of biomass oil. Furthermore, the pH value reduction variation range of biomass oil at 55°C was bigger that that at 25°C. The pH value of biomass oil decreased with air flux after 100 ml/min of air flux at 25°C. The pH value of biomass oil decreased gradually after air flux 50 ml/min at 55°C and its variation rate became quickly at the flux of 250 ml/min. It shows that the effect of temperature on the oxidation of biomass oil is obvious, and the oxidation at high temperature is easier that at low temperature. Figure 2
Variation of pH value with air flux
Figure 3 shows the variation of pH value of biomass oil with the air transflux time. It was found that pH value of biomass oil decreased with air transflux time. In general, the rate and extent of pH value decrease of biomass oil at 55°C were higher those at 25°C. At 25°C the decrease of pH value became smoother after six hours. It was estimated that the oxidation was completed after 6 hours at 25°C. In contrast, the pH value variation became greater after six hours at 55°C; it showed that the more biomass oil was oxidised at higher temperature. It was also concluded that temperature is a very important factor for oxidation of biomass oil as well; it can accelerate the oxidation velocity and make the deeper oxidation.
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Figure 3
Variation of pH value with air flowing time
Figure 4
Effects of oxygen flux and temperature on pH value of biomass oil (4 h)
In order to study the oxidation of biomass oil because of transfluxing different gas, it was analysed the role of oxygen transflowed into biomass oil. The pH values of the biomass oils changed from 2.97 to 2.83 and 2.67 after oxygen-oxidation at 25°C and 55°C for 4 h respectively, as shown in Figure 4. The pH value decreased gradually with increasing gas flux and temperature. At the same time, it was found that the dropping range of pH value at higher temperature was larger. It was suggested that the oxidation process was accelerated by the increasing temperature and gas flux. The biomass oil might be in an unstable state as storage in the summer days. It was concluded that higher oxygen flux and longer transflux time can accelerate the oxidation of biomass oil compared with air’s role according to Figures 2, 3 and 4. Figure 5 shows that pH value of biomass oil decreased with the air flowing time either at 25 or 55°C. At flowing time two hours the difference of pH value variation of biomass oil is smaller at 25°C and 55°C. It identified the oxidation of biomass oil is lighter at the beginning of two hours. However, the decrease rate of pH value of biomass oil became quicker after two hours at 55°C. It shows the oxidation role of oxygen is
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obvious at higher temperature. It is similar with the effect of air in the role of temperature. The effects of oxidation time on the pH value of biomass oil by through air or oxygen were shown in Figure 6. It was found that the pH values of biomass oil decreased from 2.97 to 2.68 and 2.58 after operated at 55°C for 8 h respectively. It also shows the oxygen, comparing to the air, can enhance the oxidation of biomass oil. Figure 5
Relation of pH value and oxygen flowing time
Figure 6 Effect of oxidation time on pH value of biomass oil under air and oxygen (55°C, 150ml/min)
3.2 Role of antioxidant Hydrocarbon oxidation reactions are central to numerous processes that convert bulk chemicals into useful and higher-value products. Zhang et al. (2008) investigated an efficient metal-free catalytic system for aerobic oxidation of aromatic hydrocarbons was successfully established by synthesising a series of aryl-tetrahalogenated N-hydroxyphthalimides and applying these compounds with 1,4-diamino-2, 3-dichloroanthraquinone (DADCAQ). A highly efficient metal-free catalytic system consisting of DADCAQ was developed for the oxidation of aromatic hydrocarbons with
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molecular oxygen. It was known that deeper oxidation might lead lower pH value compared with that before oxidation. If an antioxidant can resist the reduction of pH value of biomass oil, it means the antioxidant can play a role of antioxidation in biomass oil. One of commercial antioxidant in lubricating oil in China T501, 2,6-Di-tert-butyl 4-methylphenol, was selected to evaluate the antioxidation in biomass oil in the present study. It was found that addition of T501 can resist the tendency of pH value decrease of biomass oil, as shown in Table 1. From Figure 7, we can find that after addition of antioxidation additive T501 used widely as lubricant’s anti-oxidation agent, the pH value of biomass-oil after oxidation was increased after oxidation, while the increase extent is limited. These phenomena may be caused from by 2,6-Di-tert-butyl-4-methylphenol’s limited resolvability and dispersion ability in the biomass oil at 25°C. Therefore, it could not be oxidised firstly to protect the biomass-oil. Table 1
Variations of pH value of biomass oil with or without antioxidation additive T501 at 25°C pH value of biomass oil with T501
State
Test 1
Test 2
Average
pH value of biomass oil without T501 Test 1
Test 2
Average
Before oxidation
3.52
3.29
3.405
3.51
3.29
3.40
After oxidation
3.48
3.27
3.375
3.42
3.23
3.325
pH difference
0.04
0.02
0.03
0.09
0.06
0.075
Figure 7
Effect of anti-oxidation additive T501 on the pH value of biomass oil (25°C, 4 h)
3.3 Role of metal In general metal can play a role of catalyst in lubricating oil. It is not avoid contacting metal for biomass during its storage, transport and applications. It is worth studying the effect of metal on the stability of biomass oil. The effect of the presence of metal copper on the oxidation of the biomass oil was shown in Figure 8. It was found that the pH value
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of the biomass oil with copper was lower than that without copper. This showed that copper, probably as an oxidation catalyst, could accelerate the oxidation of biomass oil, especially between 100 and 200°C. In order to know the more details of the effect of copper on the oxidation of the biomass-oil, the weights of depositions in the period of oxidation under different conditions were obtained, as shown in Figure 9. When the oxygen flow velocity exceeded to 100ml/min, the copper could enhance the weight of deposition, that is, the existence of copper enhanced the oxidation of biomass oil. Figure 8
Effect of copper on the pH value of biomass oil (55°C, 4 h)
Figure 9
Effect of copper on the deposition of biomass oil (55°C, 4 h)
3.4 Oxidation mechanism These peroxides because of the exposure of biomass oil to air, hydroperoxides and alkylperoxides by autoxidation with air, are not very stable and spontaneously decompose to form free-radicals. The importance of forming free-radicals from peroxides is that they can catalyse the polymerisation of olefins. Thus, exposure to air would be expected to
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increase formation of polyolefins during biomass oil storage (Diebold, 2000; Wang et al., 2006). The total ion currents of the original biomass oil and the biomass oil under four kinds of oxidation conditions were shown in Figure 10. It was seen that the ion peak of biomass oil changed significantly after oxidation. Some peaks disappeared, some peaks arisen and others intensity changed, and most peaks arisen in five minutes. On the other hand, the biomass oil under four kinds of oxidation conditions showed the same peak position but with different intensities after oxidation. The data of total ion current peak and experiments conditions were dealt with by the computer software database including plenty of standard chemical information, according to molecular mass from 10 to 1,000 and element type of every component. The detailed results were listed in Tables 2 and 3. It was found that the acetic acid content in biomass oil exceeded 25% either before or after oxidation, which was the main reason for lowering pH value of the biomass oil; additionally, the content of the multi-hydroxy levoglucosan dropped after oxidation, and p-methoxyphenol appeared in the oxidation process of biomass oil. Figure 10 Total ion currents of biomass oils under different oxidation conditions
Table 2
Components of crude biomass oil Crude biomass oil
Main components
RT/min
Area w/%
Formaldehyde
1.42
3.14
Aldehyde
1.51
6.52
Hydroxyacetaldehyde
1.61
3.14
Hydroxypropanone
1.72
2.70
Butyric acid
1.82
0.96
Acetic acid
2.07
29.76
glyceraldehyde
2.6
3.54
3,4-dihydroxy-dihydro-furan-2-one
2.77
3.27
2,2-dimethoxy-ethanol
2.86
6.83
Furfural
3.13
6.56
2,5-dimethoxy-tetrahydro-furan
3.5
3.47
On the thermal oxidation stability of pyrolysis biomass oil Table 2
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Components of crude biomass oil (continued) Crude biomass oil
Main components
RT/min
Area w/%
4-hydroxy-butyric acid
4.27
0.43
5H-furan-2-one
4.51
0.74
2,3-dimethyl-cyclohexanol
4.76
1.31
3-methyl-5H-furan-2-one
5.19
0.38
Corylon
6.15
1.18
Phenol
6.59
1.57
o-cresol
6.8
1.12
m-cresol
7
1.46
2-methoxy-6-methyl-phenol
7.79
1.78
3,4-dimethyl-phenol
8.99
1.14
4-ethyl-phenol
9.7
1.31
3-(2-hydroxy-phenyl)-acrylic acid
10.1
1.53
Catechol
10.81
3.53
3-methyl-catechol
11.9
1.36
Vanillin
12.7
0.24
4-ethyl-catechol
12.86
0.71
Levoglucosan
14.73
9.95
2,3,4-trimethoxy-benzaldehyde
15.5
0.20
3-(4-hydroxy-2-methoxy-phenyl)propenal
15.8
0.15
Table 3
Components of biomass oil after oxidation test Biomass oil after oxidation conditions I to IV
Main components
RT/min
Area w/% I
II
II
IV
1,3-butanediol
1.70
9.02
4.69
3.79
3.18
Dihydroxy-ethylal
1.9
20.40
-
-
0.92
3-monohydric-ethacetic acid
2.03
3.25
1.73
1.40
0.83
Oxyacetone
2.32
16.60
9.03
7.01
-
Acetic acid
2.39
27.76
25.34
27.57
34.19
Furfural
4.16
2.90
2.01
2.15
2.63
2-methyl-5-ethyl-1,4-dioxane
4.71
3.57
-
-
-
3-butylene-1-alcohol
5.59
-
-
-
0.49
2-monohydric-3-methyl2-Cyclopentenone
7.61
-
-
1.33
1.20
5,6-dihydroxy-1,3-Cyclo hexadiene
7.63
1.50
1.50
-
-
Phenyl acetate
7.94
-
1.54
1.33
1.43
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Table 3
Components of biomass oil after oxidation test (continued) Biomass oil after oxidation conditions I to IV Area w/%
Main components
RT/min
I
II
II
IV
p-methoxyphenol
8.34
6.44
6.11
4.81
4.46
2-methoxyl-4-methylphenol
9.87
1.98
2.60
2.49
2.23
2-methoxyl-4-ethylphenol
11.02
-
1.77
1.88
1.66
2-monohydric-cinnamic acid
11.53
1.47
1.85
1.64
1.72
p-isobutyl phenol
11.71
-
1.77
2.66
2.32
2-methoxyl-5-propenylphenic acid
12.19
-
4.41
4.98
4.38
3-monohydric-4-methoxylbenzaldehyde
13.35
-
-
0.72
-
3-monohydric-5-methoxylphemethylol
12.24
3.86
-
-
-
2-methoxyl-4-propenylphenic acid
12.86
0.92
1.02
-
-
2-methoxyl-3-propenylphenic acid
13.47
2.65
2.56
1.98
1.72
3-methoxyl-4,5-xylenol
14.26
-
1.18
0.51
1.75
3-methoxyl-4-monohydricphenylallyl alcohol
14.62
-
0.71
0.65
0.49
Levoglucosan
15.75
3.19
4.65
8.63
9.27
2-methoxyl-4-monohydric– phenylacrolein
17.34
-
1.58
2.15
1.97
The oxidation mechanism of the biomass oil could be expressed as follow: Initiation: RH → R·+ H R·+ O2 → ROO·or RO Extension: ROO·+ RH → ROOH + R R·+ O2 →ROO ROOH → RO·+ OH 2ROOH → RO·+ ROO·+ H2O OH·+ RH → R·+ H2O
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Termination: 2R·→ R – R ROO·+ R·→ ROOR 2ROO·→ ROOR + O2 (where R means carbon chains) It could be deduced that the oxidation process was a chain reaction by free radicals, and the extension and termination of the chains were realised by free radicals. This suggested that the key method of the anti-oxidation of biomass oil was to remove the free radicals to terminate the chain reactions. Although the copper existence in the biomass oil can reduce the free radicals of hydrogen, the peroxide increased, accelerating the oxidation. The possible reactions were probably shown as follow: Cu2 + + H·→ Cu+ + H+ RO2+ Cu+ → RO2 + Cu2+ +
+
2+
Cu + H /Cu RO 2 ⎯⎯⎯⎯⎯⎯ → ROOH
4
Conclusions
1
The oxidation temperature, time, gas flux and type, and metal copper have significant effects on the oxidation process of the biomass oil. The oxidation was accelerated by increasing temperature, operation time and gas flux, changing inletting gas from air to oxygen and placing the copper foil inside biomass oil.
2
The pH value of biomass-oil changed from 2.97 to 2.68 and 2.58 after oxidised by through air and oxygen at 55°C for 8 h respectively. The presence of copper in the oil made the sediment weight increase after oxidation.
3
The content of acetic acid increased in the biomass-oil after oxidation, and the acetic acid was the main component in the oxidised biomass-oil.
4
The mechanism of the oxidation of biomass oil was a free radical reaction, which led to the decrease of pH value with the occurrences of more acid and phenolic compounds.
Acknowledgements The authors wish to express their thanks to Mr. W.J. Deng, Mr. M. Chen and Mr. M. Tian for their assistance in the present work. The financial support from the National Key Technology R&D Program (Grant No. 2007BAD34B02) and National Natural Science Foundation of China (Grant No. 50875071) are gratefully acknowledged.
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