Corrosion properties of bio-oil and its emulsions with diesel

Transcription

1 Chinese Science Bulletin 2008 SCIENCE IN CHINA PRESS Springer Corrosion properties of bio-oil and its emulsions with diesel LU Qiang, ZHANG Jian & ZHU XiFeng Key Laboratory for Biomass Clean Energy of Anhui Province, University of Science and Technology of China, Hefei , China Bio-oil is a new liquid fuel but very acidic. In this study, bio-oil pyrolyzed from rice husk and two bio-oil/diesel emulsions with bio-oil concentrations of 10 wt% and 30 wt% were prepared. Tests were carried out to determine their corrosion properties to four metals of aluminum, brass, mild steel and stainless steel at different temperatures. Weight loss of the metals immersed in the oil samples was recorded. The chemical states of the elements on metal surface were analyzed by X-ray photoelectron spectroscopy (XPS). The results indicated that mild steel was the least resistant to corrosion, followed by aluminum, while brass exhibited slight weight loss. The weight loss rates would be greatly enhanced at elevated temperatures. Stainless steel was not affected under any conditions. After corrosion, increased organic deposits were formed on aluminum and brass, but not on stainless steel. Mild steel was covered with many loosely attached corrosion materials which were easy to be removed by washing and wiping. Significant metal loss was detected on surface of aluminum and mild steel. Zinc was etched away from brass surface, while metallic copper was oxidized to Cu 2 O. Increased Cr 2 O 3 and NiO were presented on surface of stainless steel to form a compact passive protection film. The two emulsions were less corrosive than the bio-oil. This was due to the protection effect of diesel. Diesel was the continuous phase in the emulsions and thus could limit the contact area between bio-oil and metals. bio-oil, emulsion, corrosion, XPS Liquid fuels produced by fast pyrolysis of biomass offers an alternative way to solve the liquid fuel shortage and severe environmental problems at present [1]. The liquid product, usually known as the bio-oil, is a complex mixture of compounds derived from depolymerization of cellulose, hemicellulose and lignin. Chemically, it comprises quite a lot of water, more or less solid particles and hundreds of organic compounds that belong to acids, alcohols, ketones, aldehydes, phenols, ethers, esters, sugars, furans, nitrogen compounds and multifunctional compounds. Bio-oil has been regarded as a promising candidate to replace petroleum fuels to be used in various thermal devices [2]. As a liquid fuel, one of the basic requirements is that it will not corrode metals and other materials during the storage and utilization processes. However, bio-oil usually contains about 7 wt% 12 wt% acids, and has a ph value of or an acid number of mgkoh/g. It has been reported that bio-oils are very corrosive to aluminum, mild steel and nickel-based materials [3]. Aubin et al. [4] found that the corrosion rate of bio-oil would be enhanced with the increase of water content in it. Fuleki [5] reported that brass and stainless steel would not be affected by bio-oil at any temperatures. Darmstadt et al. [6] revealed that copper could be slightly corroded by the bio-oil derived from vacuum pyrolysis of bark residues, while stainless steel was anticorrosive. Moreover, they studied the changes of metal surface chemistry with the purpose to Received September 29, 2007; accepted February 21, 2008 doi: /s Corresponding author ( xfzhu@ustc.edu.cn) Supported by National Natural Science Foundation of China (Grant No ), Knowledge Innovation Program of Chinese Academy of Science (Grant No. KGCX2-YW-306-4) and National Basic Research Program of China (Grant No. 2007CB210203) csb.scichina.com Chinese Science Bulletin December 2008 vol. 53 no

2 understand the oxidation mechanism of metals by bio-oil. Bio-oil is known as a low-grade liquid fuel. Besides the strong acidity, it is also characterized by a low heating value, high oxygen content, chemical and thermal instability, as well as non-miscibility with petroleum fuels [7]. These poor fuel properties make bio-oil only to be directly used in furnaces and kilns. Whereas many problems will be encountered when bio-oil is employed alone in diesel engines and gas turbines, such as ignition difficulty and formation of char deposits on engine walls [8]. One of the methods to solve this problem is to mix bio-oil with diesel with the help of surfactants to make homogeneous bio-oil/diesel emulsions. The emulsions have much better fuel properties than crude bio-oil [9,10]. Successful tests have been reported on employing emulsions of different bio-oil concentrations in internal-combustion systems [11]. Because of these facts, it is very necessary to study the corrosion properties of bio-oil and its emulsions with diesel and find out corrosion resisting materials. 1 Experimental 1.1 Bio-oil preparation The bio-oil for experimental tests was produced from rice husk by the autothermal fast pyrolysis pilot set with the capacity of 120 kg/h in our laboratory. The highest bio-oil yield of about 50 wt% was obtained at the pyrolysis temperature of 475. The crude bio-oil was filtered by centrifugation to get rid of large solid particles, and then analyzed for its elemental and chemical composition as well as basic fuel properties. 1.2 Bio-oil/diesel emulsion preparation Commercial 0# diesel was purchased. Two bio-oil/diesel emulsions were prepared by dispersing 10 wt% and 30 wt% of bio-oil into 88.5 wt% and 68.5 wt% of 0# diesel oils with an ultrasonic emulsifier. The contents of surfactant were 1.5 wt% in the two emulsions. Diesel made up the larger portion and formed the continuous phase in the two emulsions. Bio-oil droplets were dispersed into the diesel matrix. Stability analysis revealed that they could keep stable for over 10 days at room temperature. In this paper, the two emulsions are referred to as emulsion A (10 wt% of bio-oil) and emulsion B (30 wt% of bio-oil). 1.3 Corrosion test The corrosion tests were performed by immersion of four different metals into the bio-oil and emulsion samples. The metals used were aluminum, mild steel (Q235A), brass (H62) and austenite stainless steel (SS321, 1Cr18Ni9Ti). For each metal, three metal strips were prepared for the three oil samples. All the metal strips were machined into 2 cm in length and 1 cm in width, 1.5 mm in thickness for stainless steel strips and 2 mm in thickness for the other metal strips. The metal strips were cleaned and polished by silicon carbide paper and weighed, then immersed in 50 ml glass bottles containing 30 ml oil samples. After that, the bottles were sealed and placed at room temperature (~25 ), 50 and 70 respectively. At specific intervals (6, 12, 24, 48, 72 and 120 h), the strips were taken out of the bottles, and washed in ethanol for two minutes. After wiped with tissue paper, the strips were weighed and then taken back to the bottles until the next weight measurement time. 1.4 Corroded surface analysis The metal strips corroded at 70 for 120 h in the bio-oil and emulsion B, as well as the un-corroded metal strips were chosen for surface analysis. Before the analysis, all the metal strips were washed by the methanol/dichloromethane mixture for 30 min, and then gently wiped with tissue paper so as to remove any loosely attached materials. The chemical states of the elements on metal surface were determined by XPS (Thermo ESCALAB 250). The XPS analysis was performed with a monochromatized Al KΑα source ( ev). The pass energy was 20 ev. Internal calibration was referenced to the C-(C, H) components of the C 1s spectra at the binding energy (BE) of ev. After subtraction of a nonlinear background, the spectra were deconvoluted by the peak fitting software. 2 Results and Discussion 2.1 Properties of bio-oil and emulsions The rice husk bio-oil is a homogeneous liquid. Its elemental composition is presented in Table 1. The main organic acids detected by gas chromatography/mass spectrometry (GC/MS) are listed in Table 2. The most abundant acid in the rice husk bio-oil is acetic acid. The total amount of acids detected accounts for 6.50 wt% of the whole bio-oil. ARTICLES ENERGY TECHNOLOGY LU Qiang et al. Chinese Science Bulletin December 2008 vol. 53 no

3 Table 1 Elemental composition of rice husk bio-oil (wt%) C H N O S Si Na K Ca Mg Fe Table 2 Organic acids in rice husk bio-oil and their contents Acid Content (wt%) Acid Content (wt%) Acetic acid Valeric acid Formic acid Hydroxy-tridecanoic acid ,3-Dihydroxy-propionic acid oxo-octanoic acid Hydroxy-dodecanoic acid Hydroxyl-acetic acid Non-2-enoic acid Hydroxy-2-methyl-propionic acid ,4-Dimethyl-hexanedioic acid Methoxy-2-naphthoic acid Hydroxy-butyric acid Table 3 Physicochemical properties of bio-oil, 0# diesel and their emulsions HHV (MJ/kg) Water (wt%) Density (g/ml) Kinematic viscosity (cst, 40 ) PH ( ) Bio-oil # diesel Emulsion A Emulsion B The basic fuel properties of the bio-oil, 0# diesel and the two emulsions are shown in Table 3. It is clear that the emulsions possess better fuel properties than the bio-oil, but their ph values are not changed correspondingly. 2.2 Weight loss of the metal strips Bio-oil is known to be thermally unstable. It is reported that storage of bio-oil at temperatures higher than 80 would totally alter its properties and cause phase separation [12]. Because of this, the highest storage temperature was chosen as 70 in this study. No phase separation was observed during the storage period. The corrosion tests revealed that mild steel, aluminum and brass could be corroded in the bio-oil and the two emulsions. Weight losses of the metal strips with storage time are given in Figures 1 3. The most significant weight loss was observed for mild steel, followed by aluminum, while brass exhibited only slight weight loss. At elevated temperatures, the weight loss rates would be greatly enhanced. Stainless steel was not affected under any corrosion conditions. These results agree well with previous studies except brass which was found to be resistant to corrosion of bio-oil by Fuleki [5]. In the next part, we will discuss why brass can be corroded by bio-oil. It can be seen from the results that the rate of weight loss for aluminum is relatively constant in Figures 1 and 2, but decreases with storage time in Figure 3. Mild steel only exhibits the constant weight loss rate when corroded by bio-oil at room temperature. In all the other corrosion conditions, the rate decreases with storage time. During the corrosion tests, when taking out the metal strips at specific intervals, all the mild steel strips were observed to be covered with many viscous corrosion materials. These corrosion materials were loosely attached on the surface and easy to be removed by washing and wiping. The quantities of the corrosion materials increased with storage temperatures. In addition, the strips corroded by emulsions accumulated more corrosion materials than the strips corroded by bio-oil at Figure 1 Weight loss of metals corroded by bio-oil LU Qiang et al. Chinese Science Bulletin December 2008 vol. 53 no

4 ARTICLES Figure 2 Weight loss of metals corroded by emulsion A. Figure 3 Weight loss of metals corroded by emulsion B. the same storage temperature. The aluminum strips corroded by emulsions were also observed to be covered with some corrosion materials, while the other two metals were much cleaner. It has been pointed out by Fuleki [5] that intensified accumulation of the corrosion materials may decrease the interaction between the metal and bio-oil, and thus reduce the corrosion rate. The results indicate that the two emulsions are less corrosive than the bio-oil. This is obvious since diesel is the continuous phase in the emulsions, and the contact area between the metal surface and bio-oil is limited. Therefore, the corrosion rates are reduced. In comparison of the two emulsions, the bio-oil concentration of emulsion B is three times higher than that of emulsion A, but the corrosion rates of emulsion B to the three metals are less than two times of those of emulsion A. Stainless steel is the only corrosion resisting metal of the four metals tested. Due to the corrosion properties, material modifications are still required for existing diesel engines and gas turbines when employing emulsions to replace petroleum fuels [13]. Finally, it is important to note that the corrosion tests were just carried out under static conditions over a short duration of only 120 h. Thus, the results can only be used for preliminarily screening of appropriate materials for bio-oil applications. 2.3 Corroded surface analysis (1) Elemental composition of metal surface before and ENERGY TECHNOLOGY after corrosion. Different color changes were observed for the metal strips after corrosion. All the aluminum strips became grey. The mild steel strips shaded from grey into dark grey. The color changes of the brass strips were interesting. The strip corroded by bio-oil at 70 became deep brown, while those under other conditions only darkened a little. The color changes of stainless steel were not distinct. The elemental composition of the metal surface before and after corrosion from XPS analysis is shown in Table 4. Carbon, oxygen and nitrogen were detected on the un-corroded metal surface. This is because metals exposed in air would react with oxygen to form metal oxides and also absorb many organic contaminants. After corrosion, the carbon contents increased significantly on the aluminum, brass and mild steel strips. While the contents of metal elements decreased greatly except the copper content on the brass surface. Since aluminum and brass do not contain carbon, the significant increase of carbon content on the corroded surface indicates the formation of strongly attached organic deposits. These organic deposits were too adhesive to be removed by washing and wiping. In addition, more organic deposits were formed on the strips corroded by bio-oil than those corroded by emulsion B. However, it is unable to draw the same conclusion for mild steel, because mild steel consists of carbon. The chemical nature of the carbon on the corroded mild steel surface will be discussed below. LU Qiang et al. Chinese Science Bulletin December 2008 vol. 53 no

5 In the case of stainless steel, iron decreased in content, while chromium and nickel increased. The contents of carbon and oxygen did not show great changes, indicating that the stainless steel was not covered with significant organic deposits after corrosion. (2) Organic deposits on metal surface before and after corrosion. The chemical nature of the organic deposits on metal surface can be revealed by the detailed C 1s XPS spectra, which are shown in Figure 4. The spectra showed peaks profile indicative of different chemical forms of carbon present on the surface: (i) carbon bonded to carbon or hydrogen, C C or C H, at around the BE of ev; (ii) carbon singly bonded to oxygen, C OH or C O C, at around the BE of ev; (iii) carbon doubly bonded with one oxygen, C== O, or singly bonded to two oxygen, O C O, at around the BE of ev. The corrosion organic deposits might be formed as a result of various polymerization and condensation reactions in bio-oil [11]. The atomic contents of carbon existed in different chemical forms before and after corrosion are given in Table 4 Elemental composition of metal surface before and after corrosion Aluminum (at%) Element Al Fe Cu C O N Un-corroded Corroded by emulsion B Corroded by bio-oil Brass (at%) Element Cu Zn C O N Un-corroded Corroded by emulsion B Corroded by bio-oil Mild steel (at%) Element Fe C O N Un-corroded Corroded by emulsion B Corroded by bio-oil Stainless steel (at%) Element Fe Cr Ni Ti C O N Un-corroded Corroded by emulsion B Corroded by bio-oil Figure 4 C 1s XPS spectra of metal strips before and after corrosion LU Qiang et al. Chinese Science Bulletin December 2008 vol. 53 no

6 Table 5 Carbon contents on metal surface before and after corrosion Metal strips Atomic content (at%) C (C C) C (C O) C (C== O) Un-corroded Aluminum Corroded by emulsion B Corroded by bio-oil Un-corroded Brass Corroded by emulsion B Corroded by bio-oil Un-corroded Mild steel Corroded by emulsion B Corroded by bio-oil ud a) ud a) ud a) Un-corroded Stainless steel Corroded by emulsion B Corroded by bio-oil a) ud denotes that it was unable to determine the content. ARTICLES Table 5. Changes can be identified. The C 1s spectrum of the mild steel strip corroded by bio-oil is special. It can not be deconvoluted by using peak fitting software. The spectrum suggests that the presence of carbon is complex, including organic deposits and amorphous carbon. Moreover, the surface of this strip was observed to be very rough. According to these results, we might infer that during the corrosion process, iron was significantly etched away from the surface leaving amorphous carbon outside. Therefore, the carbon content detected is very high (Table 4). As mentioned above, the corroded mild steel surface was covered with loosely attached materials. Hence, even organic deposits were formed during storage, they would be completely removed by washing and wiping. However, the mild steel strip corroded by emulsion B differed considerably from the strip corroded by bio-oil. Its corroded surface was not so rough, and the C 1s spectrum indicates the carbon exists all in organic forms. These results confirm the fact that metals in emulsions can be partly protected by the continuous phase of diesel. (3) Chemical state of metal elements on metal surface before and after corrosion. The Al 2p spectra of the aluminum strips before and after corrosion are shown in Figure 5. For the un-corroded aluminum strip, the spectrum shows two peaks at around and 74.6 ev. The first peak is attributable to metallic aluminum. The second one is assigned to Al 2 O 3. After corrosion in emulsion B, the two peaks can also be identified. However, after corrosion in bio-oil, only the peak corresponding to Al 2 O 3 appears on the spectrum. It is well known that aluminum exposed in air would react with oxygen to form Al 2 O 3 and also absorb organic compounds. Thus, metallic aluminum will be covered with a thin Al 2 O 3 layer, which will be covered with an organic deposits layer. It has been pointed out by Darmstadt et al. [6] that metallic aluminum could be detected when the combined thickness of the organic deposits and Al 2 O 3 layers was smaller than the XPS analysis depth (5 6 nm). Since only Al 2 O 3 is detected on the strip corroded by bio-oil, it confirms that more organic deposits were formed on aluminum surface after corrosion. Brass is mainly composed of copper and zinc. The Cu 2p 3/2 spectra of the brass strips before and after corrosion show the same single peak at around ev, indicating the presence of metallic copper or Cu 2 O. While the Zn 2p 3/2 spectra show the same single peak at around ev, indicating the presence of metallic zinc or ZnO. Auger Cu LMM and Zn LMM spectra allow differentiation of the present form of copper and zinc. The spectra are shown in Figures 6 and 7. In regard to the copper on the un-corroded brass strip, the spectrum consists of a main peak at around ev and a satellite peak. This is characteristic to metallic copper. After corrosion in emulsion B, the spectrum is composed of two main peaks. The first one at around ev is attributable to Cu 2 O. The second one of lower intensity is attributable to metallic copper. After corrosion in bio-oil, the spectrum only exhibits the peak corresponding to Cu 2 O. These results illustrate that metallic copper could be oxidized to Cu 2 O by bio-oil. In the case of the zinc on the brass strips, the spectra of un-corroded brass strip and the strip corroded by emulsion B show two main peaks at around and ev. The two peaks can be ascribed to metallic zinc and ZnO respectively. However, only ZnO is presented on the strip corroded by bio-oil. ENERGY TECHNOLOGY LU Qiang et al. Chinese Science Bulletin December 2008 vol. 53 no

7 Figure 5 Al 2p XPS spectra of aluminum strips before and after corrosion. Figure 6 Cu LMM Auger spectra of brass strips before and after corrosion. Figure 7 Zn LMM Auger spectra of brass strips before and after corrosion. The Fe 2p 3/2 spectra of the mild steel strips before and after corrosion are shown in Figure 8. The spectrum of the un-corroded mild steel strip consists of two peaks at around and ev. The first peak is attributable to metallic iron. Whereas the BE value of the second peak is between the BE values of Fe 3 O 4 and Fe 2 O 3, indicating the presence of Fe 3 O 4 and Fe 2 O 3. The intensity of the metallic iron peak decreases significantly after corrosion in emulsion B. Whereas only ferric oxides are detected after corrosion in bio-oil. Stainless steel is mainly composed of iron, chromium, nickel and titanium. The Fe 2p 3/2 spectra of the stainless steel strips before and after corrosion are shown in Figure 9. Both of metallic iron and ferric oxides are detected before corrosion. The intensity of the metallic iron peak increases after corrosion in emulsion B, and only metallic iron can be detected after corrosion in bio-oil. This change tendency of the present form of iron is just opposite to that of iron on the mild steel surface. The reasons for the phenomenon might be as follows. During the corrosion process of stainless steel, Fe 3 O 4 and Fe 2 O 3 on surface were quickly etched away. A compact passive film (Cr 2 O 3 ) was then covered on surface. Therefore, the metallic iron under this protection film was not influenced. However, there was no protection layer formed on the mild steel surface. Thus, iron of mild steel was easy to be oxidized. The Cr 2p 3/2 and Ni 2p 3/2 spectra are shown in Figures 10 and 11. The peaks located at around 577 ev on the Cr 2p spectra and 853 ev on the Ni 2p spectra are assigned to Cr 2 O 3 and NiO respectively. The spectra are almost identical before and after corrosion. It is known that metallic aluminum, zinc and iron, as well as their oxides are reactive with organic acids. Take acetic acid for example, the following reactions may take place during the corrosion process. Fe+2CH COOH=Fe CH COO +H Fe O +6CH COOH=2Fe CH COO +3H O Al+6CH COOH=2Al CH COO +3H Al O +6CH COOH=2Al CH COO +3H O Zn+2CH COOH=Zn CH COO +H ZnO+2CH COOH=Zn CH COO +H O Therefore, both of aluminum and mild steel will be seriously corroded by bio-oil and its emulsions. Moreover, dezincification corrosion will happen to brass, which seems to be the reason for the slight weight loss of brass. Metallic copper has a high electrode potential and is not reactive with the acids in bio-oil, but it is oxidized to Cu 2 O by bio-oil. It is not known at present 3732 LU Qiang et al. Chinese Science Bulletin December 2008 vol. 53 no

8 ARTICLES Figure 8 Fe 2p XPS spectra of mild steel strips before and after corrosion. Figure 9 Fe 2p XPS spectra of stainless steel strips before and after corrosion. ENERGY TECHNOLOGY Figure 10 Cr 2p XPS spectra of stainless steel strips before and after corrosion. which compounds are responsible for this. Stainless steel is the only corrosion resisting metal of the four metals tested. The fundamental reason could be ascribed to the Figure 11 Ni 2p XPS spectra of stainless steel strips before and after corrosion. compact Cr 2 O 3 passive film formed on the metal surface. This integrated passive film could keep the underlying iron from contacting bio-oil. LU Qiang et al. Chinese Science Bulletin December 2008 vol. 53 no

9 3 Conclusion The corrosion tests show that aluminum and mild steel are totally non-resistant to corrosion of bio-oil and its emulsions with diesel. Brass will also be affected. Zinc on brass surface will be corroded, and metallic copper, which is hard to be oxidized in dry air, can be oxidized to Cu 2 O. After corrosion, many organic deposits will be formed on the corroded surface of aluminum and brass. Whereas, mild steel will be covered with many loosely attached corrosion materials which are easy to be completely removed by washing and wiping. Stainless steel seems to be the corrosion resisting metal. After corrosion, increased Cr 2 O 3 and NiO are presented on the metal surface. The compact Cr 2 O 3 passive film is able to prevent the underlying iron from being corroded. Emulsification of bio-oil with diesel seems to be a method to reduce the corrosion rate of bio-oil. The two emulsions are much less corrosive than the bio-oil. This may be due to the fact that the continuous phase of diesel in the emulsions could limit the contact area between metals and bio-oil, and thus protect the metals. Further studies will be carried out to investigate how the properties of the emulsions affect their corrosion properties. 1 Bridgwater A V, Peacocke G V C. Fast pyrolysis processes for biomass. Renew Sust Energ Rev, 2000, 4: Czernik S, Bridgwater A V. Overview of applications of biomass fast pyrolysis oil. Energ Fuel, 2004, 18: Oasmaa A, Leppamaki E, Koponene P, et al. Physical characterization of biomass-based pyrolysis liquid: application of standard fuel oil analyses. VTT Energy Publication 306, Aubin H, Roy C. Study on the corrosiveness of wood pyrolysis oils. Fuel Sci Technol Int, 1990, 8: Fuleki D. Bio-fuel system materials testing. PyNe Newsletter 7, Darmstadt H, Perez M G, Adnot A, et al. Corrosion of metals by bio-oil obtained by vacuum pyrolysis of softwood bark residue. An X-ray Photoelectron Spectroscopy and Auger Electron Spectroscopy study. Energ Fuel, 2004, 18: Oasmaa A, Czernik S. Fuel oil quality of biomass pyrolysis oils State of the art for the end users. Energ Fuel, 1999, 13: Chiaramonti D, Osamma A, Solantausta Y. Power generation using fast pyrolysis liquids from biomass. Renew Sust Energ Rev, 2007, 11: Chiaramonti D, Bonini M, Fratini E, et al. Developments of emulsions from biomass pyrolysis liquid and diesel and their use in engines. Part 1: Emulsion production. Biomass Bioenerg, 2003, 25: Ikura M, Stanciulescu M, Hogan E. Emulsification of pyrolysis derived bio-oil in diesel fuel. Biomass Bioenerg, 2003, 24: Baglioni P. Bio-emulsion development of a bio-crude-oil/diesel emulsion. PyNe Newsletter 10, Boucher ME, Chaala A, Pakdel H, et al. Bio-oils obtained by vacuum pyrolysis of softwood bark as a liquid fuel for gas turbines. Part II: Stability and ageing of bio-oil and its blends with methanol and a pyrolytic aqueous phase. Biomass Bioenerg, 2000, 19: Chiaramonti D, Bonini M, Fratini E, et al. Development of emulsions from biomass pyrolysis liquid and diesel and their use in engines. Part 2: Tests in diesel engines. Biomass Bioenerg, 2003, 25: LU Qiang et al. Chinese Science Bulletin December 2008 vol. 53 no