Influence of Degree of Unsaturation on Combustion Efficiency and

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Influence of degree of unsaturation on combustion efficiency and flue gas emissions of burning five refined vegetable oils in an emulsion burner María Ascensión Sanz-Tejedor, Yolanda Arroyo, and Julio San-José Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01183 • Publication Date (Web): 05 Aug 2016 Downloaded from http://pubs.acs.org on August 6, 2016

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Influence of degree of unsaturation on combustion efficiency and flue gas emissions of burning five refined vegetable oils in an emulsion burner M. Ascensión Sanz-Tejedor*†, Yolanda Arroyo†, Julio San José*‡ †Department of Organic Chemistry, School of Industrial Engineering, University of Valladolid, Paseo del Cauce 59, 47011 Valladolid, Spain ‡Department of Energy Engineering and Fluid Mechanics, School of Industrial Engineering, University of Valladolid, Paseo del Cauce 59, 47011 Valladolid, Spain KEYWORDS: Refined vegetable oils, Emulsion burner, Combustion, Degree of unsaturation, Fatty acid composition, Nuclear Magnetic Resonance spectroscopy

ABSTRACT. This work presents experimental studies performed on a low-pressure auxiliary air fluid pulverization burner fueled with refined vegetable oils to research the impact of fatty acid profile on combustion and regulated emissions. The vegetable oils used were, coconut, palm, rapeseed, sunflower and soya. Firstly, the fatty acid profile and the degree of unsaturation of these vegetable oils were determined by high resolution Nuclear Magnetic Resonance spectroscopy. The physicochemical properties (density, kinematic viscosity, heating value and elemental analysis) were also determined and correlated with the degree of unsaturation. It was

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found that the higher heating value of vegetable oils increases as the degree of unsaturation also increases. In this experimental study, the influence of varying fuel flow rate at three input air flows on combustion efficiency and flue gas emissions was investigated. The nitric oxide and carbon oxide emissions obtained in all the tests performed are well below the permitted minimum levels. Combustion efficiencies equal to or above 80 % were achieved for soya, sunflower and rapeseed oils. A comparison between the degree of unsaturation of the vegetable oils and some combustion parameters is also established. In most of the experiments carried out, it was found that carbon oxide emissions decrease and combustion efficiency increases as the degree of unsaturation of vegetable oils increases.

1. INTRODUCTION Ever-increasing global energy consumption coupled with the dwindling stock of fossil oil reserves and the need to reduce greenhouse emissions has made the use of alternative and renewable energy sources a key necessity. In this context, the use of vegetable oils (VOs) is of particular relevance, above all in countries lacking oil and gas resources. Thus, the use of VOs as bioliquids for heating purposes not only helps to take advantage of agricultural surpluses but also to reduce polluting gases. Nevertheless, VO combustion is not without its problems due to the high viscosity that makes it difficult to achieve suitable atomization and complete combustion. For this reason, most research has focused on the combustion of methyl esters of long-chain fatty acids (FAMEs) derived from VOs (biodiesel) in diesel engines1-7 and commercial burners8-17 whereas the use of pure VOs for heating purposes has received far less attention. As an alternative, the combustion of VOs-diesel fuel blends has been carried out in order to decrease the oil’s viscosity and increase its volatility. Blends containing up to 40% of VOs allow the use


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of non-modified heating burners with good combustion efficiency and acceptable gas emission levels.18-20 What is interesting, however, is that the use of straight VOs in burners offers certain advantages compared to their use in engines. Thus, existing facilities can be switched from fossil fuels to VOs by simply varying the burner adjustments. Moreover, no transesterification reaction is required and significant improvements in the sustainability rates of crops and VO extracting factories can be achieved by using pure VOs. This is because no modifications need to be made to the oilseed crops or existing oil extracting industries, thereby enabling international community objectives linked to the use of biomass for energy purposes to be achieved without any investment. However, despite this interest, few works address the use of pure VOs in burners. First, Vaitilingom et al.21 carried out the combustion of rapeseed oil in an adapted commercial burner, achieving good combustion efficiency and low emission levels, after preheating the oil and raising the pressure in the spray nozzle. Later, the same group studied the combustion of cottonseed oil in a modified burner (type Riello 40N10). Optimal conditions require preheating the oil up to 125 ºC and using a fuel pressure of 28 bar.22 Use of preheated cottonseed oil in a multi-fueled burner has also been studied, although high emission levels were observed.23 We recently reported a preliminary study on the combustion of four vegetable oils, refined and crude oils, in a facility equipped with an emulsion pulverization burner and a combustion chamber which operates at constant pressure.24 We found that CO emissions and combustion efficiency depend on the burner’s operating conditions and to a certain extent on fatty acid (FA) composition. However, there seems to be no clear relationship between these parameters, probably because all the VOs studied displayed a similar degree of unsaturation (DU) and due to


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the impurities present in the crude VOs compared to the refined ones. Thus, fresh inquiry should be undertaken so as to achieve a better understanding of the influence of the FA composition of VOs on combustion results. Here, we present the results obtained in the combustion of five refined VOs with a FA profile covering a wide range of DU: coconut (CnO), palm (PlO), sunflower (SfO), rapeseed (RpO) and soya (SyO) oils. Refined VOs were selected to avoid the influence of minor components present in crude VOs such as free FAs, mono and diacylglycerides, phospholipids and water content.14 The oil’s FA content of these VOs was determined by proton Nuclear Magnetic Resonance spectroscopy (1H NMR) and its physicochemical properties were also evaluated. Combustion of the five VOs was carried out in a facility equipped with a mechanical oil atomizing system by air emulsion. The effect of varying the fuel flow rate and air flow on regulated gas emissions and combustion efficiency was studied in order to determine the optimum operating conditions for each VO. The influence of the degree of unsaturation on combustion performance was also analyzed.

2. MATERIALS AND METHODS 2.1. Materials All refined VOs used in this study are commercially available. VOs are triacylglycerides (TAGs) with different substitution patterns, lengths and saturation degrees of the chains as well as other minor components such as free FAs and lecithin. TAGs differ in the type of FAs bonded to the glyceride. The main FAs that occur most frequently in VOs are unsaturated oleic (O: C18:1), linoleic (L: 18:2) and linolenic (Ln: 18:3) together with saturated fatty acid (S), mainly


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palmitic (C16:0) and stearic (C18:0), although lauric acid (C12:0) and myristic acid (C14:0) are the main components in coconut oil (Figure 1). NMR spectroscopy (1H and 13C) is a robust, rapid and quantitative tool that has successfully been used in the identification and quantitative determination of the major FA components of VOs based on integrating individual proton (1H) or carbon (13C) signals.25-34 One significant advantage of NMR spectroscopy is that, unlike other analytical techniques, it does not usually require extraction, separation or chemical modification of the VOs to be analyzed, thus making the results obtained highly reproducible. Although gas chromatography (GC) is the most commonly used technique for the qualitative and quantitative determination of FA residues in VOs, accurate GC analysis is dependent on a large number of experimental variables that are more difficult to standardize, control and reproduce. Many reports evidence that NMR provides comparable results to GC whilst proving less time-consuming and offering more rapidly available results.35-37 Determining the oil’s FA content is based on the intensity of the resonance signal being directly proportional to the number of hydrogen atoms. This relation is clearly visible in Figure 1, which shows the integrated spectrum of a pure sample of trilinolein. The spectrum presents the signals due to the different hydrogens of the FA chains (A-G and J) and the hydrogens in the glycerol backbone (H and I). The signal at 4.29 ppm was calibrated to 2.00 (corresponding to 2 hydrogens). Consequently, signal A (methyl hydrogens) integrates by nine hydrogens, signal E (allylic hydrogens) by 12 hydrogens and signals F and G (bis-allylic hydrogens) by six hydrogens each. The glyceryl methine (signal I; one proton) overlaps with the olefinic hydrogens (signal J; 12 hydrogens) and both were integrated. The general 1H NMR chemical shift assignments to the different kinds of hydrogens of the FA chains are already well established in


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the literature.41-43 Several mathematical relationships have been reported to determine FA composition in VOs and there is general agreement in the quantification results obtained.28, 41-44 Experiments to determine FA profile were performed on a Varian (54 Premium Shielded) spectrometer (500 MHz) equipped with a cold probe, at 298 K. For sample preparation, VO was dissolved to a concentration of 20 mg in 600 µL of deuterated chloroform (CDCl3, with six individual samples of each VO being prepared). The following acquisition parameters were used for 1H NMR spectra: number of scans, eight, spectral width 3.9 kHz, a 90 degree pulse, and a 25 second relaxation delay. Spectra were analyzed using MestReNova 9.0 software. 2.2. Combustion equipment and procedure The experimental facility used to burn the VOs forms parts of the Industrial Heating and Cooling Laboratory in the School of Industrial Engineering at the University of Valladolid. Figure 2 shows a schematic diagram of the facility. It includes a burner, a combustion chamber and a device for analyzing combustion products. The burner used is a low-pressure auxiliary air fluid pulverization burner, manufactured by AR-CO model BR5.45 This burner is commercially available and suitable for achieving good combustion of high viscosity fuels at low pressure, as is the case of VOs (kinematic viscosity ranged from 50 to 118 mm2/s at 25 oC) added to which there is no need to preheat the liquid fuels. Its technical characteristics are indicated in table 1. Use of this burner has previously been justified by San José et al.46 The burner’s supply system comprises a network of pipes, valves and two different tanks (Figure 3). One of the tanks is full of diesel fuel (3: Figure 3) and is used to reach steady state conditions. The other contains the test samples and is equipped with an electrical resistance to liquefy VOs which are solid at room temperature (1 and 2: Figure 3). The fuel flow supplied to the burner can be adjusted to one of six different positions: C1 to C6, from


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the lowest to the highest (1.5 to 5 kg/h). The fuel flow is mixed with the primary air in a rotary vane compressor. The primary air volume flow depends on inlet air temperature, the compressor capacity and the revolutions per minute of the engine. In our system, the inlet air temperature is the only parameter that can be adjusted in intervals of 5 ºC. Thus, the primary air volume flow remains nearly constant in all the experiments performed although it does not generally exceed 10% of the stoichiometric air required for combustion. Secondary air is supplied by a separate fan that spreads it concentrically on the spray nozzle with axial and radial speed components to achieve a perfect blend. Secondary air flow can be adjusted by a fan damper setting that controls the amount of air entering the combustion chamber. Combustion takes place in the air-cooled combustion chamber which is a stainless steel horizontal cylinder which has 32 cm inner diameter and one meter long. The temperature and pressure of the combustion chamber can be adjusted and controlled by changing the cooling air flow in the combustion chamber and the dumper aperture located in the chimney, respectively. The main regulated emissions (O2, CO2, CO, NOx, SOx and unburned hydrocarbons) as well as the fume temperature were measured using a calibrated TESTO model 350M/XL instrument. To analyze solid non-combusted material, the measuring equipment used was a TESTO model 207 opacity pump. Technical characteristics of the gas analyzer are shown in table 2. Experiments were performed over several days under similar weather conditions (the same temperature and relative humidity). Combustion of the five VOs studied was carried out under steady state conditions, for which the temperature and overpressure of the combustion chamber are required to reach 200 ºC and 240 Pa, respectively. Diesel fuel is generally used to start up the burner and the facility is run for about 30 minutes until steady-state is reached. After the warmup period, the diesel fuel tank is shut off and the VO is introduced by using a suitable valve


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system. For each run, the fuel flow and the secondary air ratio were adjusted. The three fuel flow rates evaluated for this study were 3.3 kg/h, 4.1 kg/h and 5 kg/h which correspond to C3, C4.5 and C6 positions for the burner. Excess air to the combustion chamber was adjusted by fan dumper setting to max, mid and min positions. For all tests, the fuel’s injection pressure in the emulsion remained constant at about 1.105 Pa. Assuming factors to be orthogonal, assays were performed on the five oils at three fuel flows, and each emulsion at three air flows, giving a total of 45 different assays. Before collecting the fume temperature and gas emissions, the burner is allowed to work for enough time (about five minutes) to ensure the system is stabilized.

3. RESULTS AND DISCUSSION 3.1 Fatty acids (FAs) profile and physicochemical characterization of the VOs used. The FA profile of the VOs was determined by proton 1H NMR. The 1H NMR spectra of the five VOs studied are reported in Figure 4. To determine the proportions of the different acyl groups of TAG molecules in VOs, we propose four equations that utilize the signal area H of the glycerol skeleton at 4.29 ppm, which corresponds to two hydrogens, together with integrals of the B, G and E signals. The integral of the H signal was calibrated at 200.00 (four significant digits) and used as a reference value to integrate the other signals for all the samples. The proportions of Ln, L, O and S were calculated by applying the following simple equations (1-4).

1.() = 9 · (% );

Solving for Ln:

% =

2.() = 12 · (% ) + 6 · (% );

Solving for L:

% =

! "

− 2 · (% )

3.$() = 12 · [(% + % + % &';

Solving for O:

% & =

( )*

− (% + % )


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4.+ = 100 − [(% + % + % &';

Eq. 1 directly gives the percentage of Ln in the sample by measuring the area of signal B (AB), which is exclusive from methyl hydrogens of the Ln. Signal B is only present in the RpO and SyO spectra. To determine the L content, eq. 2 was considered. Thus, the area of signal G (AG) refers to bis-allylic hydrogens of Ln (12 protons) and L (6 protons) acyl chains present in the same glycerol moiety. The O content was calculated from eq. 3, and takes into account the area of the E signal (AE) which refers to the 12 possible allylic hydrogens of all unsaturated FAs (O, L and Ln). Finally, the amount of S will be 100 minus the percentage of unsaturated acids (eq. 4). The proportions calculated of each FA in the VOs studied are shown in table 3. Taking into account the percentage of each FA, it is possible to estimate the degree of unsaturation (DU) which measures the average of carbon-carbon double bonds per triglyceride molecule (table 3). SyO displays the highest DU, followed by sunflower and rapeseed oils. DUs for PlO and CnO are significantly lower. A further parameter to be considered when analyzing combustion results is the saturated to unsaturated FA ratio (S/U). As expected, CnO displays the highest S/U given its high content in saturated FAs whereas RpO exhibits the lowest. The remaining oils also give very low values (from 0.8 to 0.1). The final row shows the percentage of polyunsaturated FAs (the sum of L and Ln). The elemental composition and physicochemical properties of the VOs studied were determined in the Castilla and León Regional Laboratory, which is an accredited laboratory for fuel analysis. The results of this analysis and the standards applied to determine each property are shown in table 4. Density, kinematic viscosity, heating value (HV) and elemental analysis are important fuel properties since they determine the applications that can be allocated and the most suitable


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technology for each bioliquid. As can be seen in table 4, the elemental analysis and physicochemical properties of the VOs studied differ slightly from one another due to their different FA profile. Several groups have researched correlations between physicochemical properties and chemical composition by studying pure FAs and their esters.2,47,48, and have found that chain length and the average of the number of C-C double bonds of the FAME profiles have an impact on density, viscosity and heating value. Density increases with decreasing chain length and increasing number of C-C double bonds, whereas viscosity increases with chain length and decreases as the number of pi-bonds increases. This correlation between physical properties and chain length and number of C-C double bonds is not easy to establish for VOs, not only due to their different FA profile but also because of FA positional distribution in the glycerol skeleton, which exerts a major influence on inter and intramolecular interactions. Inspection of data in table 4 reveals that density increases slightly in the order PlO < RpO < SfO < [CnO] < SyO, coinciding with the increase in DU, with the sole exception of coconut oil, which has the lowest DU. Contrastingly, viscosity increases in the order [CnO] < SyO < SfO < RpO < PlO, in the reverse order to DU, except for CnO. The high density and low kinematic viscosity of CnO can be explained taking into account that the main FA esterified to the glycerol unit is lauric acid (C12:0), the FA with the shortest chain length. This evidences that the length of the FA chain as well as the degree of unsaturation has a marked influence on density and viscosity in line with the previous results reported for neat FAs and esters. The higher heating value (HHV) is an important property defining energy content and therefore the efficiency of VOs as bioliquids. It has been established that the HHV for FAME increases as the chain length increases, and decreases with an increased number of C-C double bonds and that there is a direct correlation between HHV and viscosity.49,50 As shown in table 4,


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the HHVs of the five VOs studied increase in the order CnO < PlO < SfO < RpO < SyO, which proves that the HHV of vegetable oils increases as the DU increases, unlike what has been observed for FAME. CnO presents the lowest HHV value, in line with its high content in lauric acid and the lower number of carbon atoms in the molecule (73.5 %); while SyO has the highest HHV as well as the highest DU. Figure 5 shows the relationship between HHV, in kJ/kg, and the degree of unsaturation. 3.2. Analysis of the combustion processes: experimental data Nine different combustion experiments for each VO were carried out. The effect of varying the input air and fuel flows on regulated gas emissions (CO2, CO, NOx, SO2, CxHy), carbon soot particles and combustion efficiency has been studied. The results are shown in table 5 and Figures 6 to 10. In this work ASHRAE expressions have been used.51 All calculations have been expressed per unit of mass and we only determined combustion efficiency, consequently it is not necessary to introduce the same thermal power to conduct a comparative analysis. SO2 emissions depend on the sulfur content of the fuel, the amount of sulfur in VOs being negligible (see table 5). Consequently, SO2 was not detected in any combustion process experiments performed on VOs. As regards the concentration in carbon dioxide (CO2) in the combustion fumes, we found that concentrations ranged from 3.5 to 7.6 % for all the assays carried out over the five refined VOs. In addition, the CO2 emissions from combustion of renewable resources contribute little to global warming since these emissions come from biomass. 3.2.1. CO Emissions In order to find the best conditions and to study the possible influence of oil’s FA content on CO concentration in the combustion fumes, a study was first made of the evolution of CO


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emissions as this is a limiting environmental factor. Table 5 shows the measured emissions of CO in ppm, in the flue gases for all VOs studied at three fuel flows and at three input air flows. From these data, it can be observed that in all tests performed the CO concentrations in fumes are well below 300 ppm, the maximum acceptable limit for a combustion process. The lowest CO emissions were obtained for SyO, except when combustion was carried out with a fuel flow of 5 kg/h under min air conditions. By contrast, CnO showed the highest CO emissions in seven of the nine assays performed on it. We found that in all the experiments conducted at fuel flows of 4.1 and 5 kg/h under max and mid air conditions, CO emissions decrease as the degree of VO unsaturation increases. This trend has also been observed in combustions carried out at a fuel flow of 3.3 kg/h under min air conditions. Figure 6 depicts CO emission trends in relation to the degree of unsaturation. These results can be explained bearing in mind that the presence of C-C double bonds, which are oxidized more easily than single C-C bonds, in the chains of unsaturated FAs favors combustion processes. Oxidation is even more favorable for the conjugated double bonds of the L and Ln. The saturated FAs present in CnO are more difficult to oxidize due to the greater energy of a sigma carbon-carbon bond compared to another pi-bond. It is important to note that the relatively low levels of CO observed reveal the potential of refined VOs as a fuel for an emulsion burner. 3.2.2. NO emissions Figure 7 shows the variations of NO emissions, in ppm, when adjusting the opening of the air valve and the fuel flow for the five VOs studied. The combustion fume analyzer distinguishes NO2 and NO emissions, although only NO values are presented in this work, since the combustion of all the VOs studied produced zero NO2 emission. This could be explained considering that the low fuel injection pressure used to form the emulsion precludes dissociation


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of the nitrogen molecule and the subsequent chemical reaction with oxygen from air to give NO2. From these data, it can be observed that in most of the experiments performed, NO emission displays a similar overall behavior for all VOs. NO emissions remained almost constant under max and mid air conditions at the three fuel flow rates studied. The lowest NO emissions were achieved under min air conditions. CnO is the only vegetable oil which diverges from this behavior in one of the nine experiments. CnO emitted higher amounts of (NO) as the fuel flow increased, proving that these conditions are less favorable vis-à-vis achieving good combustion performance. It is important to emphasize that the NO emissions obtained were extremely low, ranging between 14 and 46 ppm (± 5) in all the assays carried out. These values are well below permissible limits, evidencing that low pressure emulsion burners contribute efficiently to reducing nitrogen oxide emissions. 3.2.3. CxHy and soot emissions The presence of unburned hydrocarbons in fumes is because larger size drops fail to evaporate before crossing the burner flame front and end up as unburned material. Figure 8 shows unburned hydrocarbon emissions, in ppm, when adjusting the opening of the air valve and the fuel flow for the five VOs studied. These emissions are extremely high and, as expected, increase as the amount of fuel used rises and air is reduced. In most of the experiments, SfO and RpO emitted higher amounts of CxHy than CnO and PlO. This is partly due to the higher carbon/hydrogen ratio for the first two relative to CnO and PlO. Once again, as occurred with CO emissions, SyO performs better in the burner with much lower unburned hydrocarbon emissions than the other VOs assayed. While it is true that SyO shows a high C/H ratio, it seems that its lower viscosity and high degree of unsaturation compared to sunflower and rapeseed oils improves the atomization characteristics of the spray and leads to a more complete and cleaner


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combustion which could justify this behavior. One possible solution for decreasing unburned hydrocarbons would be to increase the temperature of the combustion chamber walls. Further research is needed to solve this problem. Concentrations of soot particles were measured applying the ASTM D 2156-94 standard method. We found that soot emissions were negligible for most of the tests performed on five refined VOs. Only for combustions carried out at a fuel flow of 5 kg/h under min input did air soot emissions exceed 10 mg/m3. PlO displayed the highest concentration in line with its higher viscosity relative to the other VOs studied. 3.2.3. Combustion Efficiency Calculations of the quantity of air required for combustion and the quantity of flue gas products generated during combustion are frequently needed for sizing system components and as input to efficiency calculations. Combustion calculation can often be simplified by using molecular mass as the basis for calculations. Table 6 shows the theoretical mass of dry flue, mgo, required for combustion with excess air per kilogram of fuel. The combustion efficiency of the combustion equipment is defined as: -./01234. 566474578, % = 100

:; − +52403

Influence of Degree of Unsaturation on Combustion Efficiency and

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