Combustion Performance Evaluation of Air Staging of Palm Oil Blends

Article pubs.acs.org/est

Combustion Performance Evaluation of Air Staging of Palm Oil Blends Mohammad Nazri Mohd Jaafar,*,† Yehia A. Eldrainy,† Muhammad Faiser Mat Ali,† W. Z. Wan Omar,† and Mohd Faizi Arif Mohd Hizam† †

Department of Aeronautical Engineering, Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, 81310 UTM, Johor, Malaysia ABSTRACT: The problems of global warming and the unstable price of petroleum oils have led to a race to develop environmentally friendly biofuels, such as palm oil or ethanol derived from corn and sugar cane. Biofuels are a potential replacement for fossil fuel, since they are renewable and environmentally friendly. This paper evaluates the combustion performance and emission characteristics of Refined, Bleached, and Deodorized Palm Oil (RBDPO)/diesel blends B5, B10, B15, B20, and B25 by volume, using an industrial oil burner with and without secondary air. Wall temperature profiles along the combustion chamber axis were measured using a series of thermocouples fitted axially on the combustion chamber wall, and emissions released were measured using a gas analyzer. The results show that RBDPO blend B25 produced the maximum emission reduction of 56.9% of CO, 74.7% of NOx, 68.5% of SO2, and 77.5% of UHC compared to petroleum diesel, while air staging (secondary air) in most cases reduces the emissions further. However, increasing concentrations of RBDPO in the blends also reduced the energy released from the combustion. The maximum wall temperature reduction was 62.7% for B25 at the exit of the combustion chamber. Murthy5 conducted a short-term diesel engine performance and emission tests at varying loads of diesel and soap-nut oil (10%, 20%, 30%, and 40%) fuel blends. They reported that among the blends, S10 showed a better performance with respect to thermal efficiency and fuel consumption. All blends have shown higher HC emissions above three-quarter load. S10 and S20 showed lower CO emissions at full load. NOx emission for all blends was lower, and S40 blend achieved a 35% reduction in NOx emission. In addition, they concluded S10 has an overall better performance with regards to both engine performance and emission characteristics. Staging means that a part of the fuel or oxidizer or both are added downstream of the main combustion zone. In a fuel staging combustor, the fuel is divided to two or more streams and directed into different locations in the combustion chamber in order to burn the fuel in the primary zone under a lean or a rich condition, which is less conducive to NOx formation compared to a stoichiometric condition.6 NOx reduction attainable with air or fuel staging usually ranges from 30 to 40% than that resulting from conventional combustors because the most important factor affecting NOx formation is the flame temperature.7−10 In addition, staging decreases the emissions of CO and unburned hydrocarbon due to the longer residence time and the burning of CO and hydrocarbon in the final lean stage.11

1. INTRODUCTION Biofuel refers to liquid fuel derived from vegetable oils or animal fats. Palm oil is one of the highest yielding crops, and Malaysia being the biggest producer of palm oil has a genuine and rare opportunity to explore and exploit the biofuel market. Biofuels would be the potential replacement of fossil fuel due to the rising levels of greenhouse-gas CO2, global warming, and decreasing supplies of petroleum.1 In particular, biodiesel usage is a must in many European Union (EU) countries, and their application has already increased in several states in the US and Canada. Rapeseed oil, sunflower, soybean, and other oils which are suitable for biofuels are abundant in European Union countries.2 Recently, some developing countries had introduced energy policies, which include the use of the locally available biofuel. Field trials using Malaysian Palm Oil Board (MPOB)’s in-house vehicles are being conducted to evaluate blends of RBDPO/petroleum diesel (up to 10% of the former) as a diesel substitute. No technical problems have been reported so far.3 Diesel engine tests were conducted by Shehata and Razek4 for sunflower oil (S100) and 20% jojoba oil/80% diesel fuel (B20) to evaluate their emission and performance compared to commercial diesel fuel. The results indicated that S100 and B20 gives lower brake thermal efficiency, brake power, and higher brake specific fuel consumption due to lower heating values compared to diesel. However, S100 and B20 emitted lower NOx concentrations due to their lower flame temperatures. These showed that biofuel blends could be a promising alternative fuel for diesel engines. However, biofuels have disadvantages of high viscosity, drying with time, thickening in cold conditions, and bad atomization characteristics. Misra and © 2012 American Chemical Society

Received: Revised: Accepted: Published: 2445

July 27, 2011 January 16, 2012 January 26, 2012 January 26, 2012 dx.doi.org/10.1021/es2025005 | Environ. Sci. Technol. 2012, 46, 2445−2450

Environmental Science & Technology

Article

Table 1. Fuel Characteristics of RBDPO, Petroleum Diesel, and Blends of RBDPO/Diesel on Volume Basis blends of RBDPO/diesel test conducted

petroleum diesel

10:90

30:70

50:50

70:30

90:10

RBDPO

density @ 15 °C (kg/L) ASTM D12989 sulfur content (wt %) IP 242 viscosity @ 40 °C (cSt) ASTM D445 pour point (°C) ASTM D97 gross heat of combustion (kJ/kg) ASTM D240 flash point (°C) ASTM D93 ASTM D92

0.8190 0.100 3.7 15 45,000 89

0.8275 0.090 3.8 15 43,100 90

0.8435 0.080 7.0 12 42,275 93

0.8600 0.060 8.6 12 41,450 99

0.8770 0.055 14.8 12 40,625 110

0.8940 0.035 29.5 9 39,800 142

0.9150 0.035 39.2 9 38,975 326

Figure 1. Calibration curve of specific gravity for various RBDPO blends.

Zhou et al.12 studied an air-staged and low NOx emission combustion technology with a SG-420/13.7-W756 boiler. They concluded that with the adjustment of the air proportion among different injectors, the nitrogen oxide emission of the boiler is reduced to 379 mg/m3 @ 6% O2 achieving a removal rate of 37.8%. Webster and Schmitz13 showed that thermal NOx is dependent on oxygen concentration inside the combustion chamber. They concluded that with a proper staging, an initial fuel-rich zone in the combustion zone can be created, and hence oxygen concentration decreases, which results in lower NOx emission. Jaafar et al.14 investigated the effect of the air staging (secondary air) in an industrial oil burner, and it showed NOx and CO emissions reduction of 30% and 16.7%, respectively, compared to the nonair staging tests. RBDPO/petroleum diesel blends at various volumetric ratios showed certain advantages as fuels compared to the individual RBDPO or petroleum diesel used solely as shown in Table 1.15 This paper investigates the effects of RBDPO/petroleum diesel blending ratio by volume and air staging (secondary air) on temperature distribution and emissions in an industrial oil burner.

2.1. Blending of RBDPO. The RBDPO was blended with the petroleum diesel at different volume ratios. A double impeller blender was used for dynamic blending purpose. The blending time was two hours to ensure homogeneous mixture. The specific gravity of the RBDPO blends was measured and calibrated by comparing it to the calculated value. A calibration graph of specific gravity of the RBDPO blends is shown in Figure 1. 2.2. Experimental Setup. A cylindrical mild steel combustion chamber with internal dimensions of 0.3 m diameter, 1.0 m length, and 2 mm wall thickness was used for this study as shown in Figure 2. The combustion chamber was insulated using 80 mm thick hy-cast cement. Secondary air was introduced at 400 mm downstream of the primary air entrance through 10 holes equally spaced circumferentially. The secondary air flow rate was set at 10% of the total air flow rate. The air and the fuel were admitted to the combustion chamber through an industrial oil burner. The air was controlled manually using a butterfly valve and measured using a turbine flow meter, while the fuel was controlled using a needle valve and was measured using a graduated cylinder. Nine thermocouples located on the combustion chamber wall and separated axially by 100 mm were used to measure the axial wall temperature profile. A Baltur BT14GW 140 kW was used to measure exhaust gas emissions. Carbon monoxide (CO), nitrogen oxides (NOx), sulfur dioxide (SO2), and unburned hydrocarbons (UHC) were measured during the experiment. The emissions of these gases were measured at equivalence ratios of 0.72, 0.84, 1.00, 1.12, and 1.34.

2. EXPERIMENTAL METHODOLOGY The experiment was conducted using RBDPO blends B5, B10, B15, B20 and B25 derived from RBDPO and petroleum diesel with different equivalence ratios by admitting secondary air to the combustion chamber. The combustion performance was obtained by measurements of combustion chamber wall temperature and of emission species of the exhaust gas using a gas analyzer. 2446

dx.doi.org/10.1021/es2025005 | Environ. Sci. Technol. 2012, 46, 2445−2450


Article

Figure 2. Schematics diagram of the combustion rig.

Figure 3. The wall temperature profile for different RBDPO/diesel blends at stoichiometric condition without air staging (solid lines) and with air staging (dashed lines).

3. RESULTS AND DISCUSSION

the blend due to the lower calorific value of RBDPO compared to the petroleum diesel. Generally, the air staging and the increase of the RBDPO to the diesel ratio lowered the bulk temperature inside the combustion chamber. 3.2. Gas Emission. Figure 4 shows the emissions of CO against the fuel equivalence ratio for B5, B10, B15, B20, and B25 with and without air staging. The figure shows the CO emission of blends is lower than that for the petroleum diesel. The maximum reduction of CO emission of RBDPO blends

3.1. Temperature Profile. Figure 3 shows the temperature profile of the combustion of RBDPO blends and the petroleum diesel at the stoichiometric conditions with and without secondary air. It can be seen from Figure 3 that the temperature increases due to combustion in the primary zone followed by a temperature reduction due to cooling effect of the secondary air (air staging). In addition, Figure 3 also shows that the temperature decreased with the increase of RBDPO content in 2447



Article

Figure 4. CO emission from the combustions of petroleum diesel and RBDPO blends without air staging (continuous lines) and with air staging (dashed lines) at different equivalence ratios.

Figure 5. NOx emission from the combustions of petroleum diesel and RBDPO blends without air staging (continuous lines) and with air staging (dashed lines) at different equivalence ratios.

compared to diesel was 56.9% for blend B25 at stoichiometric condition where a sufficient oxygen and highest flame temperature are available for complete reaction.16 The application of air staging decreased the level of CO emission for all blends. In addition, as the equivalence ratio increased the CO emission decreased owing to the increase in flame temperature, which increased the reactivity. The CO emission attained its lowest value at stoichiometric condition, and then it increased again due to the insufficient air for the complete combustion. At lower lean conditions, the CO levels are high because of the slow rate of oxidation.

Figure 5 shows the variations of emissions of NOx against the fuel equivalence ratio of blends with and without air staging. The emission of NOx decreased when the percentage of RBDPO in the blend increased owing to the lower heating value of RBDPO compared to the diesel fuel, which in turn led to lower temperatures within the combustion chamber and hence lower NOx. The maximum reduction of NOx emission was 74.7% for blend B25 at an equivalence ratio of 0.72. Applying secondary air lowered the combustion temperature further and, therefore, lowering the NOx emission. However, at too lean conditions, the NOx emission increased with air staging for B20 and B25. 2448



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Figure 6. SO2 emission from the combustions of petroleum diesel and RBDPO blends without air staging (continuous lines) and with air staging (dashed lines) at different equivalence ratios.

Figure 7. UHC emission from the combustions of petroleum diesel and RBDPO blends without air staging (continuous lines) and with air staging (dashed lines) at different equivalence ratios.

where there is sufficient air for complete burning of the fuel. However, at lean and rich conditions, UHC emission increased as observed in Figure 7. The secondary air reduced the level of UHC for each RBDPO blends due to availability of sufficient air required for complete combustion. On the other hand, the too lean conditions could lead to slow down of the combustion, and hence the UHC could increase as seen for diesel fuel. The maximum reduction in UHC emission due to blending was 77.5% for B25 at stoichiometric condition. These results showed that the increase in RBDPO concentration in the fuel blends decreased the bulk combustion

Figure 6 shows the SO2 emission of RBDPO blends compared to the petroleum diesel with and without secondary air at different equivalence ratios. RBDPO blends produced lower emission of SO2 compared to the petroleum diesel. This is because vegetable oils have a smaller amount of sulfur compared to diesel.17 The maximum reduction of SO2 emission was 68.5% for blend B25 at an equivalence ratio of 0.72. In addition, the emission level of SO2 was reduced further with the application of secondary air. Figure 7 shows the variation of UHC emission for the RBDPO blends under different fuel equivalence ratios. It was found that the minimum UHC emission occurs at the stoichiometric condition 2449



Article

(17) de Almeida, S. C. A.; Belchior, C. R.; Nascimento, M. V. G.; Vieira, L. d. S. R.; Fleury, G. Performance of a diesel generator fuelled with palm oil. Fuel 2002, 81, 2097−102.

temperature which means decreased combustion energy. However, it has a positive effect on the CO, NOx, SO2, and UHC emissions. In addition, the injection of secondary air further reduced the combustion energy as well as emissions. Therefore, it can also be concluded that low concentrations of RBDPO such as B5 and B10 could be potential replacement of diesel fuel.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Ministry of Science, Technology and Innovation (MOSTI) for funding this project (project number: 03-01-06-KHAS01).

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REFERENCES

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Combustion Performance Evaluation of Air Staging of Palm Oil Blends

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