Enhancement of Biodiesel Oxidation Stability Using Additives

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Enhancement of biodiesel oxidation stability using additives obtained from sewage sludge fast pyrolysis liquids. Lucía Botella, Marina Sierra, Fernando Bimbela, Gloria Gea, Jose Luis Sanchez, and Alberto Gonzalo Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b01681 • Publication Date (Web): 03 Dec 2015 Downloaded from http://pubs.acs.org on December 6, 2015

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Enhancement of biodiesel oxidation stability using additives obtained from sewage sludge fast pyrolysis liquids. Lucía Botella1,2, Marina Sierra1, Fernando Bimbela1,2, Gloria Gea1,2, José Luis Sánchez1,2, Alberto Gonzalo1,2*. 1

: Chemical and Environmental Engineering Department, Universidad Zaragoza,

Campus Río Ebro, c/María de Luna 3, 50018-Zaragoza, Spain 2

: Thermochemical Processes Group (GPT), Aragon Institute for Engineering Research

(I3A), Universidad Zaragoza, Campus Río Ebro, c/Mariano Esquillor s/n, 50018Zaragoza, Spain

KEYWORDS: oxidation stability, antioxidant, FAME, blending

ABSTRACT In the present work, bio-oil derived from the catalytic pyrolysis of sewage sludge has been blended in small amounts with sunflower biodiesel with the aim of evaluating its potential as a novel, low cost and renewable biodiesel additive that could replace costly commercial biodiesel antioxidants normally used to date. The effect of blending small 1

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amounts of bio-oil with sunflower biodiesel on the biodiesel properties (oxidation stability, the cold flow properties, flash point and viscosity) has been analysed. Furthermore, apart from studying the effect of adding low bio-oil concentrations (0.1, 1.8 and 3.5 mass %), the effect of other operating conditions, specifically the temperature (278-333 K) and mixing time (5-60 min), during the bio-oil and biodiesel blend preparations have also been analysed. As regards the oxidation stability, blends prepared adding 3.5% mass fraction of bio-oil complied with the limits imposed by the ASTM D6751 and EN 14214 standards. Blending sewage sludge bio-oil and sunflower biodiesel did not result in an enhancement of the biodiesel cold flow properties in the studied range, while the flash point of these blends was lower than that of pure sunflower biodiesel. The viscosity was barely affected in all cases. The oxidation stability enhancement achieved by the addition of bio-oil obtained from sewage sludge catalytic pyrolysis was higher than the enhancement obtained with bio-oil from non catalytic pyrolysis.

1. INTRODUCTION Biodiesel production still presents some challenges and hurdles that need to be overcome if it is to become a cost-competitive replacement for conventional diesel. The main issues that have yet to be solved are related to the high production costs caused by the use of expensive raw materials, with up to 90% of the manufacturing costs of biodiesel being associated with purchasing the oily feedstock used as a source of fatty acids1. In June 2014, the European Union reached a political agreement on the draft directive on indirect land-use change (ILUC) amending the fuel quality (98/70/EC) and 2


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renewable energy (2009/28/EC) directives2, and by 2020 EU governments are expected to financially support only 2nd and 3rd generation biofuels3. In order to produce sustainable non-food competitive second-generation biodiesel, alternative energy crops and/or organic wastes and residues should be taken into consideration as potential sources for fatty acids intended for biodiesel production. These alternative raw materials may have the disadvantage of being more complicated to work with, while yielding in many cases biodiesel with worse values for some properties. Furthermore, in order to be commercialised, biodiesel must also comply with highly restrictive regulatory norms and standards, such as EN 14214 and ASTM 6751 imposed by the European Union and the USA, respectively, and extended to other countries. Thus, the established requirements regarding certain properties such as the oxidation stability or cold flow properties must be met. This poses a problem to producers, especially if low cost and low quality raw materials are used in the biodiesel manufacture. The oxidation stability is a property strongly dependent on the raw material composition. Biodiesel suffers from ageing upon storage because of its biodegradable nature, hence it loses quality with time. This problem becomes worse depending on the storage conditions, particularly in relation to its exposure to air and/or light, high temperatures and the presence of substances that could have a catalytic effect on the oxidizing process4–6. Thus, as a consequence of the oxidation process, the physical and chemical properties of biodiesel become altered. For example, an increase in its viscosity, density and polymeric content occurs, resulting in the production of gums and sediments7,8. 3


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The cold flow properties of biodiesel are related to its freezing point and the flowability of the fuel at low temperatures. These properties are highly dependent on the raw material selected for producing the biodiesel, and especially on the degree of unsaturation. Whilst saturated compounds are responsible for the poor cold flow properties of biodiesel, unsaturated esters are mainly responsible for its oxidation9. In order to overcome these problems, costly synthetic additives are added to biodiesel10– 16

, increasing production costs while demeaning the renewable nature of the biodiesel

since these additives are usually obtained from fossil-based feedstocks. Due to the high cost of commercial additives, it has become necessary to seek low cost alternative sources of antioxidant compounds. In this context, the liquid product obtained from the fast pyrolysis of sewage sludge from waste-water treatment plants (WWTP)17–19 could be an interesting source of biodiesel antioxidants. This liquid, , when obtained from a lignocellulosic biomass, contains a significant amount of phenolic compounds that have been demonstrated to have a high antioxidant power9,20– 22

. This could make its possible use as an additive interesting in order to enhance the

stability of biodiesel against oxidation. Pyrolysis liquid is a complex and heterogeneous mixture of organic compounds and water23–26. Liquid from sewage sludge has a strong, unpleasant and characteristic odour due to its content in acids and aldehydes, among other compounds19,23. It is not homogeneous, containing two or three phases depending on the operating conditions during the pyrolysis process, one aqueous phase and one or two organic phases. The organic phases are herein referred to as bio-oils. The main applications envisaged for this liquid are related to its potential use as a fuel and as a source for the production of 4


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chemicals27–29. In this context, the use of sewage sludge bio-oils, obtained via catalytic pyrolysis, as an additive source for transportation fuels is a possible alternative that must be evaluated, taking into account the significant amount of phenolic compounds present in this type of bio-oil30. The present work studies bio-oil derived from the catalytic pyrolysis of sewage sludge with the aim of evaluating its potential as a low cost and renewable biodiesel additive, and its effect on the oxidation stability of biodiesel. Other relevant biodiesel properties such as the cold filter plugging point, the flash point and viscosity have also been evaluated. Furthermore, apart from the effect of the bio-oil concentration, the effect of other operating conditions, specifically the temperature and mixing time, during the biooil and biodiesel blend preparations have also been analysed. Finally, for the purposes of comparison, the effect has been measured of the addition of small amounts of bio-oil derived from the non-catalytic pyrolysis of sewage sludge on biodiesel oxidation stability. 2. EXPERIMENTAL SECTION 2.1. BIODIESEL PRODUCTION The biodiesel used in this work was produced by transesterification in the Thermochemical Processes Group (GPT) laboratories. The experimental setup at lab scale is described elsewhere31. Refined sunflower oil with acidity below 0.5% was selected as raw material. Thus, it was not necessary to apply a preliminary esterification treatment. Other chemicals used in the biodiesel production process include methanol (99%), potassium hydroxide (98%), sulphuric acid (98%) and magnesium sulphate (anhydrous, 98 %), all of them supplied by PANREAC. 5


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The transesterification reaction took place under typical reaction conditions (1:6 oil-tomethanol ratio, 333 K reaction temperature, and 1% (m/m) potassium hydroxide with respect to the oil as catalyst). After the transesterification, the glycerol produced is removed, and the FAME is cleaned with acidulated water (using sulphuric acid). The product is finally dried in a rotary evaporator, and the last traces of water are removed using magnesium sulphate. After its production, the biodiesel was stored in a high density polyethylene container, during a maximum time period of 7 days, at 248 K to prevent it from being oxidised before adding bio-oils, as it has been observed in the preliminary study about the storage conditions, carried out in this work. 2.2. PRELIMINARY BIODIESEL AGEING STUDY In order to adequately evaluate the oxidation stability of the bio-oil/biodiesel blends, a preliminary ageing study of the non-blended biodiesel from refined sunflower oil was carried out. The storage temperature of biodiesel has proven to be a crucial parameter in biodiesel stability over time5. Hence, sunflower oil biodiesel samples were stored at three different temperatures (248, 279 and 298 K) during 8 days. Changes in the oxidation stability of the samples over time were recorded. The worst stability over time was found for the biodiesel that had been stored at 298 K. However, the sample stored at 248 K remained almost unaltered during storage (see Figure 1). Consequently, 248 K was selected as the biodiesel storage temperature in this work. It was established that no more than 7 days should elapse from the biodiesel production to the preparation of the biodiesel/bio-oil blends, and this was taken into account in the preparation of the different blends in this work. These results are in accordance with the observations 6


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described by Berrios et al. on the effect of temperature (288-318 K) on the storage stability of biodiesel/diesel blends5.

Figure 1. Change in biodiesel oxidation stability versus storage time.

2.3. SEWAGE SLUDGE BIO-OIL PRODUCTION Two different sewage sludge bio-oils, non-catalytic (NCSS) and catalytic (CSS), were produced as described in previous works29,32. Both liquids were produced from the pyrolysis of thermally dried anaerobically digested sewage sludge. The characterisation of the sewage sludge is described in detail in previous works17–19,29. Pyrolysis was performed at 723 K in a lab scale fluidised bed reactor ( 0.05.

For comparison purposes, several blends of non-catalytic bio-oil (NCSS bio-oil) and biodiesel were also prepared. Both bio-oils show significant differences in composition, which is expected to result in different effects on the fuel properties of the biodiesel-rich phase. Three different blends were prepared by adding to biodiesel the same amounts of NCSS bio-oil as in the case of CSS bio-oil (0.1, 1.8, and 3.5% (m/m)) and mixing during 32.5 min at 305.5 K. The oxidation stability obtained by adding bio-oils to the biodiesel were also compared with the oxidation stability reached by adding 4-allyl-2,6dimethoxy-phenol. The additions of this compound were done under the same operating conditions as used for the NCSS bio-oil. 3. RESULTS AND DISCUSSION 3.1. CHARACTERISATION OF BIO-OIL 13


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Catalytic bio-oil has much lower viscosity and water content, and higher energy content (expressed as lower heating value, LHV) than non-catalytic bio-oil, as can be observed in Table 1. Furthermore, the lower water content of the catalytic bio-oil could enhance the miscibility of the bio-oil in biodiesel. Ultimate analyses for both bio-oils are displayed in Table 1. Both bio-oils have a relatively high nitrogen and sulphur content which is a drawback for fuel applications, since this may lead to NOx and SOx generation34 during the use of the biodiesel rich phase as a biofuel. Thus, the use of this material would require a previous step to remove these heteroatoms or an extraction to separate phenolic compounds35, As mentioned above, the results of GC-MS/FID analyses of NCSS and CSS bio-oil samples were used for determining the chemical composition of bio-oils by means of semiquantitative analysis. Figure 2 shows the compounds detected and identified in both chromatograms: a) catalytic bio-oil and b) non-catalytic bio-oil.

Figure 2. GC/MS chromatogram of a) catalytic bio-oil and b) non-catalytic bio-oil.

As shown in Figure 2, a total of 25 compounds were identified in the case of NCSS biooil, whereas up to 29 compounds were elucidated in the case of CSS bio-oil. For NCSS bio-oil, around 60% of the total sum of peak areas corresponds to peak areas of identified compounds, whilst the percentage of peak areas corresponding to identified compounds increases to 64% in the case of CSS bio-oil.

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Particular attention has been paid to those compounds that could present antioxidant properties, namely, amines and phenolic compounds. The presence of amines in both bio-oils could not be assured, according to the peak identification conducted in the GCMS qualitative analyses in which no peaks could be attributed to amines. On the other hand, the phenolic compounds identified by GC/MS for both bio-oils are shown in Table 4.

Table 4. Phenolic compounds identified in the bio-oil samples. The number in the parentheses is the identification number used in Figure 2.

Up to five different phenolic compounds could be identified in both bio-oils, though, according to the analysis of the peak areas obtained, they are present in different proportions. Phenol, 3-methylphenol, 4-methylphenol, 2,5dimethylphenol and 4ethylphenol were identified in both types of bio-oil. According to the results, the amount of phenolic compounds in CSS bio-oil is around 60% greater than in NCSS biooil, revealing the beneficial effect of subjecting the pyrolysis vapours to a catalytic posttreatment. 3.2. CHARACTERISATION OF BIODIESEL Some important fuel properties of the biodiesel used have been measured, specifically the oxidation stability (1010 ± 20 s), cold filter plugging point (269 ±1 K), flash point (448 ±1 K) and kinematic viscosity (4.27x10-6±10-8 m2s-1). The values are expressed as average ± standard deviation (6 replicates). 15


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The quantitative composition of the biodiesel prepared from refined sunflower oil is presented elsewhere33. The most abundant compounds are the linoleic (C18:2) and the oleic (C18:1) acid methyl esters, with 62 and 28% (m/m) respectively, followed by the palmitic (C16:0, 7%) and stearic acid methyl esters (C18:0, 4%). Other fatty acid methyl esters were also identified, though in very small amounts. 3.3 EFFECT OF THE BIO-OIL/BIODIESEL RATIO IN THE CONCENTRATION OF BIO-OIL DISSOLVED IN BIODIESEL The bio-oil concentration (% mass fraction) in the biodiesel-rich phase can be estimated according to equation [1], accepting the hypothesis that the mass of biodiesel dissolved in bio-oil is negligible, as other authors have confirmed when blending biodiesel with bio-oil from lignocellulosic biomass pyrolysis36:

=

− × 100 [1]

Cbio-oil is the estimated bio-oil concentration in the biodiesel-rich phase, mbio-oil is the mass of bio-oil added, mnon-miscible is the mass of the bio-oil-rich phase and mbiodieselrichphase is the mass of the biodiesel-rich phase. Figure 3 displays the estimated bio-oil concentration in the biodiesel-rich phase versus bio-oil added for the blends prepared. The results have not been analysed by ANOVA because the bio-oil concentration could not be properly determined. However, the results represented in Figure 3 seem to 16


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indicate an increase in the concentration of bio-oil in the biodiesel-rich phase by augmenting the bio-oil added for both types of bio-oil analysed. Nonetheless, catalytic bio-oil is more miscible than non-catalytic bio-oil in biodiesel, as can be observed in Figure 3. The lower water content and the higher proportion of non-polar compounds in the catalytic bio-oil32 could explain its higher miscibility in biodiesel. Bio-oil solubility in biodiesel has not been determined in this study. Bio-oil is a complex mixture of different organic compounds having different solubility in biodiesel. For certain bio-oil/biodiesel ratios used in the preparation of the blends, some organic compounds can reach their equilibrium solubility in biodiesel but others have not reached saturation yet. This fact explains that the concentration of bio-oil dissolved in biodiesel increases with the bio-oil/biodiesel mass ratio used in the preparation of the blends in the range of study. Similar results have been obtained by other authors37.

Figure 3. Bio-oil concentration in biodiesel-rich phase versus percentage of bio-oil added to pure biodiesel. The type of bio-oil used and the operating conditions during the preparation of blends are indicated in the legend.

3.4. BIODIESEL-RICH PHASE COMPOSITION Figure 4 shows the identification of phenols by GC/MS of the biodiesel-rich phase obtained with 3.5% of catalytic bio-oil added at 333 K during 5 min. Other compounds identified correspond to FAME and are not shown in the figure.

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Figure 4. Identification of phenols in GC/MS chromatogram of the biodiesel-rich phase (operating conditions during the blend preparation: 3.5 % of catalytic bio-oil added at 333 K during 5 min).

The GC/MS analyses of the biodiesel-rich phases of the different blends revealed that all phenolic compounds identified in the raw bio-oil (see Table 4) appear in the chromatograms. The GC/MS analysis performed on biodiesel-rich phases with different bio-oil percentages added showed that the greater the amount of bio-oil added to the biodiesel, the greater the amount of phenols that were dissolved in the biodiesel-rich phase. 3.5. FUEL PROPERTIES OF THE BIODIESEL-RICH PHASES PREPARED WITH CATALYTIC BIO-OIL 3.5.1. OXIDATION STABILITY The oxidation stability of biodiesel-rich phases measured using the PetroOXY method is plotted in Figure 5 together with the PetroOXY oxidation stability of the pure biodiesel. These results indicate that the bio-oil addition produces an increase in the biodiesel oxidation stability, except for blends with 0.1 % of bio-oil added at the extremes of the temperature interval studied (278 and 333 K). This fact could be due to the biodiesel aeration during mixing, which favours oxidation8. The aeration may be important if the bio-oil amount added is too small in order to notice its antioxidant effect.

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In a previous work33 it was established with a 95% confidence level that, in order to fulfil the minimum oxidation stability limits according to the ASTM D6751 (3 hours) and EN14214:2013 V2+A1 (8 hours) standards, biodiesel samples containing additive should have PetroOXY oxidation stabilities over 1206 and 2796 s, respectively. From the results in Figure 5, it can be concluded that with the highest amount of CSS bio-oil added, 3.5 %, all the biodiesel samples blended with CSS bio-oil comply with the limits imposed by both American and European standards. The improvement in oxidation stability caused by adding bio-oil can be explained by the dissolution of some bio-oil compounds in biodiesel, which can act as antioxidants, as for example the phenolic compounds observed by GC/MS in the biodiesel-rich phase (Figure 4).

Figure 5. Oxidation stability of biodiesel-rich phase for different percentages of catalytic bio-oil added. The plot legend indicates the operating conditions during the preparation of blends. Error bars indicate standard deviation. In order to gain further knowledge about the antioxidant effect of phenolic compounds present in the bio-oil, chemical composition of the biodiesel-rich phase obtained with 3.5% of catalytic bio-oil added at 333 K during 5 min mixing time was analysed by GC/MS after conducting the oxidation stability test. All phenolic compounds identified in the raw bio-oil (Table 4) appear in the chromatograms of the biodiesel-rich phase before and after the oxidation stability test. When comparing the peak areas of these phenolic compounds in the analyses conducted in the same conditions in both oxidation stability tests, all of them showed diminished peak areas in the spent samples: by 2 % in 19


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the case of phenol, 3 % for 3-methylphenol, 13 % for 4-methylphenol, 4 % for 2,5dimethylphenol, and 10 % for 4-ethylphenol, thus indicating that these compounds are consumed during the oxidation stability test. This decrease could explain the antioxidant activity of phenolic compounds on the stability of biodiesel. The enhancement of the oxidation stability (∆OxSt) when bio-oil is added to biodiesel is expressed as follows (equation [2]): ∆ =

!" !" !"

× 100

[2]

where OxSt bioedieselrich is the oxidation stability of the biodiesel-rich phase and OxSt biodiesel is the oxidation stability of the biodiesel without bio-oil. The experimental results obtained for each blend are shown in Table 5.

Table 5. Experimental results for the properties of the biodiesel-rich phases.

The ∆OxSt versus the amount of CSS bio-oil added is presented in Figure 6. The enhancement of the oxidation stability rises by increasing the amount of added bio-oil. The bio-oil added is the most influential factor, as can be observed in Table 3. According to Figure 3, the amount of bio-oil dissolved in biodiesel seems to increase by augmenting the amount of the bio-oil added. Therefore, the concentration of the compounds from bio-oil with antioxidant properties also increases with the amount of bio-oil added.

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Although, according to the ANOVA analysis, the blending temperature and mixing time do not significantly affect the oxidation stability enhancement, the curvature appears as a significant term in Table 3, which means that the variation of the enhancement of the oxidation stability with at least one of the factors analysed is not linear. The temperature and the mixing time could favour the oxidation reactions but could also favour the miscibility of the antioxidant compounds. Therefore the enhancement of the oxidation stability could show an optimal value with one or both parameters. This fact could explain the slight curvature observed.

Figure 6. Enhancement of the oxidation stability by CSS bio-oil addition (mixing time 32.5 min). The results are represented as the model value ± 0.5·Fisher LSD interval, taking into account all the runs. Centre points are experimental data.

The effect of the bio-oil addition on the biodiesel storage stability has also been analysed. Some of the biodiesel samples with additive blends were stored in a polyethylene container at ambient temperature during 45 days. The containers were exposed to daylight. Figure 7 shows the variation of the oxidation stability for three biodiesel-rich phases, prepared with different amounts of added bio-oil, after a storage time of 45 days. The variation suffered over the same period of time by pure biodiesel is also presented. The results reveal that the addition of 0.1 % of bio-oil is not sufficient to control the oxidation process. However, additions of 1.8 % and 3.5 % of bio-oil allow the oxidation stability of the biodiesel to remain essentially unaltered during 45 days.

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Figure 7. Oxidation stability of the biodiesel-rich phase over time (45 days) for different percentages of bio-oil added. Blending operation conditions are indicated over the bars. Error bars indicate standard deviation.

3.5.2. COLD FLOW PROPERTIES The cold filter plugging point (CFPP) can be used as an indicator of the operability of the biodiesel at low temperatures. For example, EN 14214 fixes a range of maximum values from 253 K to 278 K depending on the geographic area. As can be seen in Table 5, the CFPP values obtained for the different blends of biodiesel with additives analysed in this study remain basically unaltered, ranging between 271 K and 269 K. Therefore, the CFPP of all the samples in this study fall within the range indicated in the EN 14214 standard for this property. However, the statistical analysis indicates a slight enhancement of this property by adding higher percentages of bio-oil to biodiesel. The biodiesel-rich phases obtained by adding the highest amount of bio-oil show the lowest CFPP values (Figure 8), which are similar to the CFPP obtained for the pure biodiesel (269 ± 1 K). The addition of higher amounts of bio-oil could dilute the concentration of methyl esters of saturated fatty acids, which are responsible for the crystallisation, causing a decrease in the CFPP. Other authors9 have discussed this dilution effect on the CFPP obtained in blends of biodiesel and bio-oil from pine pyrolysis, observing a slight improvement of the cold flow properties of biodiesel by adding between 5 % and 34 % 22


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mass fraction of bio-oil. Regarding the effect of the other factors analysed in this work on the CFPP, the results reveal that the blending temperature and the mixing time used during the blend preparations do not significantly affect the cold flow properties of the biodiesel-rich phases (see Table 3).

Figure 8. Cold filter plugging point of the biodiesel rich phases (mixing time 32.5 min). The results are represented as the model value ± 0.5·Fisher LSD interval, taking into account all the runs. Centre points are experimental data.

3.5.3. FLASH POINT The flash point for the biodiesel-rich phases varies from 372 K to 402 K in the range of this study (Table 5), being inferior to the flash point of pure biodiesel (448 K). The addition of bio-oil to biodiesel reduces the flash point, which is not positively safe in terms of biodiesel safety during storage and transport. All the samples comply with the minimum flash point established by the ASTM D6751, 366 K, and only two of them have a value lower, by 2 K, than the minimum set by the EN 14214: 2013 V2+A1, 374 K. The high sensitivity of the flash point with respect to residual amounts of compounds with a lower flash point, as for example some alcohols, has previously been demonstrated38. Therefore, residual amounts of compounds from bio-oil could cause the decrease in the flash point.

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The amount of bio-oil added and the blending temperature do not significantly affect the flash point of the biodiesel–rich phases. However, a reduction in the flash point is observed when the mixing time increases during the preparation of blends (Figure 9). Therefore, the mixing time should be limited to reduce this inconvenience. The two samples with a flash point lower than the value established by the ASTMD6751 have been mixed during 60 min. Since other fuel properties analysed in this study are hardly affected by the mixing time, 5 min of mixing is suitable for the blends preparation.

Figure 9. Flash point of the biodiesel rich phases (blending temperature = 305.5 K). The results are represented as the model value ± 0.5·Fisher LSD interval, taking into account all the runs. Centre points are experimental data.

3.5.4. VISCOSITY Bio-oil has higher viscosity than biodiesel. However, the addition of low amounts of bio-oil hardly increases biodiesel viscosity. The viscosity of the biodiesel-rich phases obtained in this work varies from 4.24x10-6 m2·s-1 to 4.34x10-6 m2·s-1, similar to the biodiesel viscosity value (4.27x10-6 ± 10-8 m2·s-1). The experimental results for all the blends are shown in Table 5. All the viscosity values of the blends fall within the limits imposed by the standards, with values lower than 5 x10-6 m2·s-1. However, the statistical analysis shows, in the range of study, a slight increase in viscosity by raising the bio-oil concentration and this increase is slightly higher at the highest temperature, as can be observed in Figure 10. Other authors37 have observed small increases in biodiesel 24


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viscosity by the addition of bio-oil from lignocellulosic biomass due to the presence of heavy oligomers in its composition.

Figure 10. Viscosity of the biodiesel-rich phases (mixing time 32.5 min). The results are represented as the model value ± 0.5·Fisher LSD interval, taking into account all the runs. Centre points are experimental data.

3.6. CATALYTIC AND NON-CATALYTIC BIO-OIL COMPARISON The oxidation stability of the biodiesel-rich phase prepared from the addition of low amounts of catalytic and non-catalytic bio-oil is compared in Figure 11. CSS bio-oil enhances biodiesel oxidation stability to a larger extent than NCSS bio-oil for the same amount of bio-oil added. CSS bio-oil is more miscible with biodiesel than NCSS, as can be observed in Figure 3. In Figure 11, the oxidation stability of the biodiesel-rich phase by the addition of bio-oil is also compared to the oxidation stability obtained by adding 4-Allyl-2,6-dimethoxyphenol, a phenolic compound that has shown antioxidant properties in previous studies33. The addition of the CSS bio-oil does not improve the oxidation stability at the same level reached by the addition of the phenolic compounds, but these compounds are completely miscible with biodiesel.

Figure 11. Enhancement of the oxidation stability versus concentration of bio-oil added in biodiesel for both types of bio-oil, catalytic and non-catalytic. Comparison with the

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addition of 4-allyl-2,6-dimethoxyphenol. Blending temperature = 305.5 K and mixing time = 32.5 min.

4. CONCLUSIONS The addition of low concentrations of bio-oil obtained from sewage sludge pyrolysis enhances the oxidation stability of biodiesel, probably due to the presence of phenolic compounds in the bio-oil that can act as antioxidants. Catalytic bio-oil produces a higher enhancement of oxidation stability than non-catalytic bio-oil. The lower water content and the higher proportion of non-polar compounds in the catalytic bio-oil could explain its higher miscibility in biodiesel and therefore its better performance as an antioxidant. A higher percentage of added bio-oil results in a higher enhancement of the oxidation stability. Other fuel properties of the biodiesel-rich phases, such as CFFP, flash point and viscosity, are hardly affected by the addition of low sewage sludge biooil concentrations. The temperature and mixing time used during the preparation of blends affects the fuel properties of the biodiesel-rich phases to a lesser extent than the concentration of the bio-oil added in the range of this study. However, the flash point decreases when the mixing time increases during the preparation of blends. Therefore, the mixing time should be limited to reduce this drawback. In the range of this study, the best fuel properties are obtained for the blends prepared with the highest percentage of added bio-oil using the lowest temperature and mixing time. 26


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Sewage sludge bio-oil could be a suitable low cost additive to enhance the oxidation stability of biodiesel. However, apart from the antioxidant compounds, the bio-oil contains other compounds that could reduce the flash point and increase the viscosity and nitrogen and sulphur content. Therefore, the bio-oil concentration added should be limited in order to reduce undesirable outcomes. 5. ACKNOWLEDGMENTS The authors thank the Gobierno de Aragón (Reference T-36 Grupo de Procesos Termoquímicos (GPT)), the European Social Fund, the former Spanish Ministry of Science and Innovation (MICINN), the Spanish Ministry of Economy and Competitiveness (MINECO) and FEDER (Project References CTQ2010-20137, CTQ2013-47260-R and ENE2013-41523-R) for providing financial support. The MINECO is also acknowledged for the FPI grant awarded to Ms. Lucía Botella (Ref. BES-2011-051093). 6. REFERENCES. (1) Ryu, B.-G.; Kim, J.; Kim, K.; Choi, Y.-E.; Han, J.-I.; Yang, J.-W. Bioresour. Technol. 2013, 135, 357–364. (2) European Commission. Land use change http://ec.europa.eu/energy/en/topics/renewable-energy/biofuels/land-use-change (accessed Jul 23, 2015). (3) Lane, J. biofuelsdigest. com/bdigest/2014/06/13/proposal-on-indirect-land-usechange-eu-council-reaches-agreement. “Proposal on indirect land-use change: EU Council reaches agreement” @ BiofuelsDigest: http://www.biofuelsdigest.com/bdigest/2014/06/13/proposal-on-indirect-landuse-change-eu-council-reaches-agreement/ (accessed Jul 23, 2015). (4) Knothe, G. Fuel Process. Technol. 2007, 88, 669–677. (5) Berrios, M.; Martín, M. A.; Chica, A. F.; Martín, A. Fuel 2012, 91, 119–125. (6) Karavalakis, G.; Stournas, S.; Karonis, D. Fuel 2010, 89, 2483–2489. (7) Pullen, J.; Saeed, K. Renew. Sustain. Energy Rev. 2012, 16, 5924–5950. (8) McCormick, R. L.; Westbrook, S. R. Energy & Fuels 2010, 24, 690–698. 27


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Garcia-Perez, M.; Adams, T. T.; Goodrum, J. W.; Das, K. C.; Geller, D. P. Bioresour. Technol. 2010, 101, 6219–6224. Almeida, E. S.; Portela, F. M.; Sousa, R. M. F.; Daniel, D.; Terrones, M. G. H.; Richter, E. M.; Muñoz, R. A. A. Fuel 2011, 90, 3480–3484. Dunn, R. O. Fuel Process. Technol. 2005, 86, 1071–1085. Karavalakis, G.; Hilari, D.; Givalou, L. Energy 2011, 36, 369–374. Chiu, C.-W.; Schumacher, L. G.; Suppes, G. J. Biomass and Bioenergy 2004, 27, 485–491. Kivevele, T. T.; Mbarawa, M. M.; Bereczky, A.; Laza, T.; Madarasz, J. Fuel Process. Technol. 2011, 92, 1244–1248. Lapuerta, M.; Rodríguez-Fernández, J.; Ramos, Á.; Álvarez, B. Fuel 2012, 93, 391–396. Moser, B. R. Renew. Energy 2012, 40, 65–70. Fonts, I.; Juan, A.; Gea, G.; Murillo, M. B.; Sánchez, J. L. Ind. Eng. Chem. Res. 2008, 47, 5376–5385. Gil-Lalaguna, N.; Fonts, I.; Gea, G.; Murillo, M. B.; Lázaro, L. Energy & Fuels 2010, 24, 6555–6564. Fonts, I.; Azuara, M.; Gea, G.; Murillo, M. B. J. Anal. Appl. Pyrolysis 2009, 85, 184–191. Santos, N. A.; Damasceno, S. S.; De Araújo, P. H. M.; Marques, V. C.; Rosenhaim, R.; Fernandes, V. J.; Queiroz, N.; Santos, I. M. G.; Maia, A. S.; Souza, A. G. Energy and Fuels 2011, 25, 4190–4194. Kang, S.; Li, X.; Li, B.; Fan, J.; Chang, J. Energy and Fuels 2011, 25, 2746– 2748. Medeiros, M. L.; Cordeiro, A. M. M. T.; Queiroz, N.; Soledade, L. E. B.; Souza, A. L.; Souza, A. G. Energy and Fuels 2014, 28, 1074–1080. García-Pérez, M.; Chaala, A.; Pakdel, H.; Kretschmer, D.; Roy, C. Biomass and Bioenergy 2007, 31, 222–242. Bimbela, F.; Oliva, M.; Ruiz, J.; García, L.; Arauzo, J. J. Anal. Appl. Pyrolysis 2007, 79, 112–120. Oasmaa, A.; Meier, D. J. Anal. Appl. Pyrolysis 2005, 73, 323–334. Piskorz, J.; Scott, D.; Radlein, D.; Scott, D. S. ACS Symp. Ser. 1988, 376, 167– 178. Samanya, J.; Hornung, A.; Apfelbacher, A.; Vale, P. J. Anal. Appl. Pyrolysis 2012, 94, 120–125. Cao, J.-P.; Zhao, X.-Y.; Morishita, K.; Wei, X.-Y.; Takarada, T. Bioresour. Technol. 2010, 101, 7648–7652. Azuara, M.; Fonts, I.; Barcelona, P.; Murillo, M. B.; Gea, G. Fuel 2013, 107, 113–121. Fonts, I.; Gea, G.; Azuara, M.; Ábrego, J.; Arauzo, J. Renew. Sustain. Energy Rev. 2012, 16, 2781–2805. 28


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García, M.; Gonzalo, A.; Sánchez, L.; Arauzo, J.; Simoes, C. Chem. Ind. Chem. Eng. Q. 2011, 17, 91–97. Azuara, M.; Fonts, I.; Bimbela, F.; Murillo, M.; Gea, G. Fuel Process. Technol. 2015, 130, 252–262. Botella, L.; Bimbela, F.; Martín, L.; Arauzo, J.; Sánchez, J. L. Front. Chem. 2014, 2, 1–9. Fernando, S.; Hall, C.; Jha, S. Energy & Fuels 2006, 20, 376–382. Izhar, S.; Uehara, S.; Yoshida, N.; Yamamoto, Y.; Morioka, T.; Nagai, M. Fuel Process. Technol. 2012, 101, 10–15. Garcia-Perez, M.; Shen, J.; Wang, X. S.; Li, C.-Z. Fuel Process. Technol. 2010, 91, 296–305. Garcia-Perez, M.; Adams, T. T.; Goodrum, J. W.; Geller, D. P.; Das, K. C. Energy & Fuels 2007, 21, 2363–2372. Boog, J. H. F.; Silveira, E. L. C.; de Caland, L. B.; Tubino, M. Fuel 2011, 90, 905–907.

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TABLES. Table 1. Bio-oil properties and ultimate analysis32. The results are expressed as average ± standard deviation (3 replicates)

Catalytic

Viscosity x 106 (m2∙s-1) 35±1

Water Content (% m/m) 1.8±0.1

Higher Heating Carbon Value (MJ∙kg-1) (% m/m) 39.2±0.5 73.4

Hydrogena (% m/m) 8.6

Nitrogen (% m/m) 8.8

Sulphur (% m/m) 0.8

Non-catalytic

183±13

19.1±0.9

28.7±0.1

9.6

7.7

1.2

Bio-oil

a

64.0

The % of hydrogen includes hydrogen from the moisture.

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Energy & Fuels

Table 2. List of blends of biodiesel with the bio-oil obtained after catalytic posttreatment with γ-Al2O3. Sample

Percentage of bio-oil

Temperature (K) Time

added (mass %)

(min)

M1C01_T5_t60

0.1

278

60

M2C01_T5_t5

0.1

278

5

M3C01_T60_t60

0.1

333

60

M4C01_T60_t5

0.1

333

5

M5C18_T32_t32

1.8

305.5

32.5

M6C18_T32_t32

1.8

305.5

32.5

M7C18_T32_t32

1.8

305.5

32.5

M8C18_T32_t32

1.8

305.5

32.5

M9C18_T32_t32

1.8

305.5

32.5

M10C18_T32_t32

1.8

305.5

32.5

M11C35_T5_t5

3.5

278

5

M12C35_T5_t60

3.5

278

60

M13C35_T60_t5

3.5

333

5

M14C35_T60_t60

3.5

333

60 31


Energy & Fuels

Table 3. Relative influence of the factors on enhancement of oxidation stability, cold filter plugging point, flash point and viscosity. p-value (p) for each parameter and interaction is also indicated in parentheses. The effect is not considered significant for p-value > 0.05

Enhancement of oxidation stability (%)

Cold Filter Plugging Point (K)

Flash Point (K)

Viscosity 106 (m2·s-1)

89.2

270

383

4.29

Temperature

*(p = 0.8)

*(p = 0.3)

*(p = 0.8)

*(p = 0.5)

Bio-oil added

104.2 (p

Enhancement of Biodiesel Oxidation Stability Using Additives

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