Influence of degree of unsaturation on combustion efficiency and flue ...

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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,† and 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 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 the fatty acid profile on combustion and regulated emissions. The vegetable oils used were coconut, palm, rapeseed, sunflower, and soya. First, 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 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 helps not only 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 burners,8−17 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 of nonmodified 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 © 2016 American Chemical Society

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 °C and using a fuel pressure of 28 bar.22 Use of preheated cottonseed oil in a multifueled 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 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 Received: May 17, 2016 Revised: July 5, 2016 Published: August 5, 2016 7357

DOI: 10.1021/acs.energyfuels.6b01183 Energy Fuels 2016, 30, 7357−7366

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Energy & Fuels

Figure 1. Drawings showing the structure of a triacylglyceride and acyl chains of laurin, palmitin, stearin, olein, linolein, and linolenin, with the hydrogens detected for each signal by 1H NMR spectroscopy marked on the molecule (top). 1H NMR spectrum corresponding to a pure sample of trilinolein. The values of the integrals are indicated below each signal (bottom). 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 while 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 wellestablished in the literature.38−43 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

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 an 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 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 7358


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Energy & Fuels

Figure 2. Schematic diagram of the facility. scans, eight; spectral width 3.9 kHz; a 90° pulse; and a 25 s 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 BR 5).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 °C) 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

Table 1. Technical Characteristics of the Burner power rating flow rate acceptable fuel viscosity motor revolutions compressor heater oil pipe

Figure 3. Schematic diagram of the burner. 1: Tank for VOs; 2: Heating; 3: Tank for fuel; 4: Filter; 5: Pump; 6: Pressure regulator; 7: Fan; 8: Rotatory vane compressor; 9: Spray nozzle.

17−58 kW 1.5−5 kg/h 50−118 mm2/s at 25 °C 2800 rpm 120 W 1/2″A−1″R

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 noncombusted 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 °C and 240 Pa, respectively. Diesel fuel is generally used to start up the burner, and the facility is run for about 30 min until steady state is reached. After the warm-up period, the diesel fuel tank is shut off and the VO is introduced by using a suitable valve 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, 4.1, 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

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−C6, from the lowest to the highest (1.5−5 kg/h). The fuel flow is mixed with the primary air in a rotary vane compressor. The primary air volume flow depends on the 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 °C. 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 a 32 cm inner diameter and is 1 m long. The temperature and pressure of the 7359


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Energy & Fuels Table 2. Technical Characteristics of the Testo 350M/XL Gas Analyzer parameter

measuring range

accuracy

resolution

temperature of smokes oxygen (O2) carbon oxide (CO) (H2 compensated) nitric oxide (NO) sulfur dioxide (SO2) total hydrocarbons (CxHy)

−40 to 1200 °C 0−25% 0−500 ppm 0−3000 ppm 0−1450 ppm 0−6000 ppm

±0.5 °C (