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A comparative study of combustion performance and emission of biodiesel blends and diesel in an experimental boiler
Applied Energy 88 (2011) 4725–4732
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
A comparative study of combustion performance and emission of biodiesel blends and diesel in an experimental boiler Afshin Ghorbani a,⇑, Bahamin Bazooyar a, Ahmad Shariati a, Seyyed Mohammad Jokar b, Hadi Ajami a, Ali Naderi a a b
Ahvaz Faculty of Petroleum Engineering, Petroleum University of Technology, Ahvaz, Iran Chemical Engineering Department, School of Chemical and Petroleum Engineering, Shiraz University, Iran
a r t i c l e
i n f o
Article history: Received 31 January 2011 Received in revised form 28 May 2011 Accepted 8 June 2011 Available online 12 July 2011 Keywords: Biodiesel Combustion Boiler emission
a b s t r a c t Recently, biodiesel has become more attractive since it is made from renewable resources and also for the fact that the resources of fossil fuels are diminishing day by day. This study compares combustion of B5, B10, B20, B50, B80 and B100 with petroleum diesel over wide input air flows at two energy levels in an experimental boiler. The comparison is made in terms of combustion efficiency and flue gas emissions (CO, CO2, NOX, and SO2) and influence of air flow at two energy levels 219 kJ/h and 249 kJ/h is studied. The findings show that at higher level energy diesel efficiency was a little higher than that of biodiesel, but at lower level biodiesels are efficient than diesel. Except B10, Biodiesel and other blends emitted less pollutant CO, SO2 and CO2 than diesel. B10 emitted lower CO2 and NOX, but emitted higher SO2 than diesel. Despite studies reporting an increase in the NOX level resulting from burning of biodiesel over conventional petroleum diesel fuels in engines, our findings indicated at the second energy level a reduction in the NOX level in the flue gases resulting from burning of biodiesel. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Due to the reduction in fossil fuel resources and the increasing consumption of fuels, the use of alternative fuels instead of them seems to be necessary. In Iran, oil and gas are the major resources of energy, however, there are much potential for producing energy from renewable resources such as biomass. It is an established fact that if sufficient water is available, most of Iran’s land can be utilized to produce crops. Currently, about 12% of the total land area is utilized for the purpose [1]. About 17–20% of crops such as wheat, sugar, and rice produced in Iran are wasted as agricultural residues [2]. These residues can be used as a potential for production of biofuels. Biodiesel has recently become more attractive among biofuels because of some benefits such as reducing greenhouse gas emissions, safety of the supply, availability of resources, increasing rural development and biodegradability. Biodiesel, an alternative diesel fuel, is made from renewable biological resources such as vegetable oils and animal fats. Biodiesel is made through transesterification reaction of these resources with methanol or ethanol in the presence of a catalyst. Generally, methanol is preferred for transesterification because it is less expensive than ethanol. So, biodiesel is known as fatty acid methyl ester. Although
⇑ Corresponding author. Tel.: +98 917 106 8034; fax: +98 611 555 2255. E-mail address: [email protected] (A. Ghorbani). 0306-2619/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2011.06.016
there is much research on the use of biodiesel in diesel engines [3–21] research on the use of biodiesel blends in boilers is rare. Although the use of biodiesel in boilers showed a reduction in NOX emission [22–24] but others reported to the contrary [25– 27]. Tashtoush [22] did not use blends and Batey [23], Lee et al. [25] and Vanlaningham et al. [26] used only B20 blend of soybased biodiesel and diesel in their works. Although Krishna et al. [24] tested B10, B20, B30 and a blend of 50% biodiesel by volume with kerosene, studies on blends of biodiesel and diesel need to be carried out and the advantages of these fuels in boilers and furnaces are to be demonstrated. The objective of this study is to verify if the overall performance of biodiesels is comparable to that of diesel. Besides two pure biodiesels, soybean and sunflower biodiesel, we tested various blends of biodiesel and diesel such as B5, B10, B20, B50 and B80. Besides the performance, gas emissions and percent reduction of emissions relative to diesel are compared. 2. Materials and methods Several types of vegetable oils, with a variable composition in fatty acids, can be used for the preparation of biodiesel. Two common vegetable oils used in biodiesel production are soybean and sunflower oil, which were used in this work as well. Vegetable oils were transesterified to produce the methyl ester using a catalyst, potassium hydroxide. Methanol chromatographic grade (99:5%) and potassium hydroxide was purchased from Merck Company.
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The results of the physical analysis of values of the two biodiesels and diesel are summarized in Table 1. The transesterification was carried out in the LR 2000 P Laboratory Reactor, with Eurostar Power Control-Visc P7 Overhead Stirrer, LR 2000.10 Anchor Stirrer, and a thermocouple probe. The mixture was agitated by using a stainless steel stirrer. Two liters of oil were added to the reactor. Then the temperature of the reactor was adjusted to 55 °C by Heating Circulator Bath. 14 g of KOH pellets were dissolved in 500 ml methanol before being poured into the reactor. The mixture was heated and mixed continuously for 45 min. The reaction conditions that choosing in this work were optimum conditions for production of biodiesel that researchers reported in their studies [28,29]. Then the mixture was poured into two separator funnels. Two layers began to form after some minutes with the darker glycerol layer sinking to the bottom. Glycerol was separated from the upper layer (unwashed biodiesel). The Upper layer should be washed because of some impurities such as glycerol, excessive methanol and soap. The ester layer was washed to ten times with hot distilled water. Combustion laboratory unit made by P.A. Hilton, England, which was used to compare the combustion efficiencies, is shown in Fig. 1. The combustion chamber is a steel-horizontal cylinder one meter long with a 45 cm inner diameter. A stainless steel water jacket with an outer diameter of 50 cm was used to cool the combustion chamber. The combustor is designed such that a range of liquid fuels can be tested and compared. The fuel is forced from a pump at high pressure out of a nozzle within a tubular air director. A rotational force was applied to the fuel within the nozzle causing it to break up into small droplets as it exits the nozzle to form a spray cone. The air fan of the boiler has damper numbers that each number refers to various ranges of air flow. Therefore, rate of input air to the combustion chamber was adjusted by fan damper setting. Increasing in the fan damper number led to further air flow to the boiler. The air fan and fuel pump are coaxial and driven from an electric motor. Electrodes cause ignition and are switched off after the flame begins self sustaining. The combustor’s exit is flanged to an exhaust section, which is a straight pipe of 160 mm diameter. This section, in turn, exhausts into a 6 m high chimney of 200 mm diameter. Feeding and metering of cooling water, liquid fuel, and combustion air were achieved by auxiliary systems attached to the unit. Rota meters, which are integral parts of the unit, were calibrated and used to measure flow rates of the fuel and the cooling water. The instrumentation attached to the water heater allowed direct read-out of inlet and outlet water temperatures, water mass flow rates, inlet air temperature, exhaust gas temperature, and fuel flow rate. Technical characteristics and instrumentation specifications of the boiler are listed in Table 2 and Table A1, respectively. All fuels were tested at two levels of energy input rate and different fan damper numbers that refer to various ranges of air to fuel ratio. In all runs, data
was collected under steady-state conditions. The fuels were compared at the same levels of the energy input. The pressure of fuel at lower and higher level was 120 (827) and 200 psi (1379 kPa), respectively. After using pure biodiesels and diesel in the boiler, blends of biodiesel and diesel were burned and compared with one other. Soybean biodiesel was used in these blends and blends were burned at a fuel pressure of 1034 kPa. The data measured during the tests included water coolant and exhaust temperatures, and exhaust emissions, including O2, CO, CO2, NOX, and SO2. Inlet and outlet temperatures of the cooling water were measured by thermocouples utilizing digital thermometers. Water and fuel flow rates and exhaust temperature were read directly from the control panel. Samples of the exhaust gas were drawn for emission analysis. A flue-gas analyzer made by Kane, model KM9106 measured the emitted gases, including O2, CO, CO2, NOX, and SO2. Gas conditioning module (KMDM220) is fitted in the front compartment of a standard Quintox carry case and comprise from a Peltier fan cooled chiller assembly, a peristaltic pump removes the condensate, the control electronics and a power supply module. The module is supported on an aluminum alloy chassis. The chiller is connected to a flue mounted electrically heated probe by a 3 m long heated line with automatic temperature control. Because the gas that is extracted from the flue is maintained at 120 °C, no condensation occurs in the probe and the hose and so no sample gas is lost in the condensate. The chiller flash cools the sample gas to below the ambient dew point and any water in the gas condenses immediately. Then the condensate is pumped away using a peristaltic pump. Because the gas has no chance to remain in contact with the condensate, volatile sample gas is not lost into the condensate. The chilled gas then naturally warms up as it passes through the sampling pump to the sensors. Therefore, its humidity reduces and there is no risk of future condensation. The analyzer technical characteristics are indicated in Table A2. 3. Results and discussion 3.1. The effect of different air flows on the exhaust gas temperature The trends of variation of the exhaust gas temperature, Texh, with the fan damper number, are shown in Figs. 2a and 2b. As it is expected, the exhaust temperature decreases as air flow increases. This observation may be explained by the fact that the entering excess air to the combustion chamber is cooler than the gases in the boiler. Despite this obvious trend in the exhaust temperature, Batey [23] reported no significant difference in the flue gas temperatures. It is obvious from Fig. 2b that at 200 psi (1379 kPa) diesel has higher exhaust temperature than biodiesel. But for 120 psi (827 kPa), it is different to 200 psi (1379 kPa) and biodiesels have higher exhaust temperature.
Table 1 Physical characteristics of diesel and biodiesel. Property
Diesel
Soybean
Sunflower
Standard
Flash point (°C) Kinematic viscosity @ 40 °C (mm2/s) Cetane Number Specific gravity @ 60 °C Cloud point (°C) Pour point (°C) Copper corrosion Carbon residues Water and sediment Sulfated ash Acid value Total glycerol Free glycerol Gross Caloric Value (kJ/kg)
52 3.5 45 0.8367 6 19 1 0.01 Trace 0.001 Trace Trace Trace 43,640
163 5.8 49 0.8821 2 2 3 0.038 0.002 0.008 0.34 0.01 0.120 41,118
175 6 47 0.8934 3 6 3 0.035 0.003 0.005 0.41 0.01 0.190 41,005
ASTM ASTM ASTM ASTM ASTM ASTM ASTM ASTM ASTM ASTM ASTM ASTM ASTM ASTM
D93 D445 D 613 D1298 D2500 D97 D130 D4530 D2709 D874 D664 D6584 D6584 D240
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1
16
15
2
14 13 3 12
4 8
5 6
11
7
10
9
1- Exhaust
2- Exhaust temperature
3- Water inlet temperature
4- Cooling water flow meter
5- Condensate drain
6- Cooling water
7- Drain
8- Water outlet temperature
9- Fuel tank
10- Fuel control
11- Oil filter
12- Pump pressure gauge
13- Air flow
14- Oil pump
15- Air temperature
16- Observation window
Fig. 1. Combustion laboratory unit made by P.A. Hilton, England.
Table 2 Technical characteristics of the boiler. Nozzle Power output up to Flow rate Acceptable fuel viscosity Fuel outlet pressure
1.35 US gal/h–60° H KH (Hollow) 150 kW 4–7 l/h 1.5–7.5@ 40 mm2/s 8–15 bar
3.2. Comparison of the performance of biodiesels and diesel in the boiler The efficiency of combustion, gc, is the ratio of the amount of heat transferred to the water in the jacket, Qw, to the amount of
heat input, Qin. It was mentioned that the fuels were burned at two levels of energies and at different air flows. For comparison of fuels, two levels of energy were selected that include wide ranges of air flows. Selection of these levels of input energies, 120 psi (827 kPa) and 200 psi (1379 kPa), was due to the limitations of the firing capacity of the fuels in the boiler. These levels of input energies were close to the minimum and maximum levels of the energy that could be generated with a nozzle containing a capacity of 1:35 gal/h of fuel. Qin for the first and second energy levels was about 219 kJ/h and 249 kJ/h, respectively. Variations in the combustion efficiency with the fan damper number for all fuels at two levels of input energies are shown in Figs. 3a and 3b. It can be seen in these figures that the combustion efficiency for all fuels
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Fig. 2a. Variation of exhaust gas temperature of diesel and biodiesels with fan damper number at 120 psi (827 kPa).
Fig. 3b. Variation of combustion efficiency of diesel and biodiesels with fan damper number at 200 psi (1379 kPa).
than diesel at lower energy. It may be related to higher exhaust temperature of biodiesel than diesel. At extreme cases, the differences were in the range of 1–15%. Among sunflower and soybean biodiesel, at higher energy, soybean biodiesel was efficient than sunflower biodiesel. Nevertheless, at lower energy, it seems there was not much difference between them at this boiler. These observations related to the trend of exhaust temperature of fuels.
3.3. Comparison of pollutant emissions of different biodiesels and diesel
Fig. 2b. Variation of exhaust gas temperature of diesel and biodiesels with fan damper number at 200 psi (1379 kPa).
decreases as air flow is increased. The reason might be related to the reduction of the exhaust temperature of the boiler with increasing of air flows as shown in Figs. 2a and 2b. At Fig. 3b, it was observed that diesel efficiency was a little higher than that of biodiesel. However, biodiesels were efficient
Fig. 3a. Variation of combustion efficiency of diesel and biodiesels with fan damper number at 120 psi (827 kPa).
3.3.1. CO and CO2 emissions Figs. 4 and 5 shows the measured emissions of carbon monoxide (CO) and carbon dioxide (CO2) in the exhaust gases for all fuels at the two fuel flow rates. The emission of CO is shown in Figs. 4a and 4b, which indicate that CO concentration of biodiesels are lower than diesel for two energy input levels. The CO emission has similar overall behavior for fuels. Increase input air results in uniform fuel distribution and poor spray characteristics. Consequently, CO emission intensifies as input air increases. The higher concentration of CO and CO2 for petroleum diesel is likely due to the more carbon content by weight of diesel relative to the biodiesel [30,31]. For the first level of the input energy, Figs. 5a shows that at fan damper 3, 11.7% reduction in CO2 are attainable if soybean biodiesel is burnt instead of petroleum diesel under the
Fig. 4a. Variation of CO concentrations of diesel and biodiesels with fan damper number at 120 psi (827 kPa).
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Fig. 4b. Variation of CO concentrations of diesel and biodiesels with fan damper number at 200 psi (1379 kPa).
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Fig. 6a. Variation of NOX concentrations of diesel and biodiesels with fan damper number at 120 psi (827 kPa).
conditions shown in the figures. For the second level of the input energy, Fig. 5b shows 7.3% reduction in CO2 for soybean biodiesel compare to diesel. Meanwhile, these figures also show that it is possible to reduce 12.6% and 6% in the CO2 contents of flue gases
Fig. 6b. Variation of NOX concentrations of diesel and biodiesels with fan damper number at 200 psi (1379 kPa).
Fig. 5a. Variation of CO2 concentrations of diesel and biodiesels with fan damper number at 120 psi (827 kPa).
Fig. 5b. Variation of CO2 concentrations of diesel and biodiesels with fan damper number at 200 psi (1379 kPa).
Fig. 7. Variation of SO2 concentrations of diesel and biodiesels with fan damper number at 120 psi (827 kPa) and 200 psi (1379 kPa).
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Fig. 8a. Variation of combustion efficiency with fan damper number.
Fig. 8d. Variation of CO2 with fan damper number.
Fig. 9a. Percent reduction of combustion efficiency by blends relative to diesel. Fig. 8b. Variation of NOX with fan damper number.
Fig. 9b. Percent reduction of NOX by blends relative to diesel. Fig. 8c. Variation of SO2 with fan damper number.
if sunflower biodiesel is burnt instead of petroleum diesel under the conditions shown in the figures. 3.3.2. NOX and SO2 emissions Figs. 6a and 6b show the measured emissions of nitrogen oxides, NOX, for all fuels at the two levels of input energies. The forma-
tion of NOX depends mainly on the availability of oxygen and the local combustion temperatures. The increase in temperature and oxygen causes more NOX to be produced. It was mentioned earlier that the exhaust and boiler temperatures decrease with increasing air flow. As a result, the decrease in temperature with air flow can compensate the effects of increased oxygen on NOX formation. The increase observed at the tails of the curves might be due to the effects of oxygen, which ultimately dominates the reduction effect of
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3.4. Comparison of the performance and pollutant emissions of different blends of biodiesels and diesel
Fig. 9c. Percent reduction of SO2 by blends relative to diesel.
Fig. 9d. Percent reduction of CO2 by blends relative to diesel.
the boiler temperature. At low air flow, the temperature in the boiler is too high and consequently more NOX was formed. The conflicting effects of temperature and oxygen on the NOX formation by increasing air flow cause the formation of NOX to be less at high air flow than low air flow. It seems that the effect of temperature is stronger in NOX formation than the oxygen rate. In these figures, it is also seen that the NOX contents of the flue gases from the burning of sunflower biodiesel in the boiler are less than those produced from the burning of petroleum diesel. This may be related to the lower temperature of the boiler that is observed during the burning of sunflower biodiesel. However, at the low energy level, diesel produces less NOX than soybean biodiesel and at the higher energy level, soybean biodiesel emits more. The trend of NOX emission maybe related to the trend of exhaust temperature. In lower energy level diesel has lower exhaust temperature than soybean biodiesels, therefore diesel emits lower NOX than soybean biodiesel. But in higher energy level due to higher exhaust temperature of diesel than soybean biodiesel, diesel emits higher NOX than soybean biodiesel. Fig. 7 compares the SO2 content of flue gases from the boiler when petroleum diesel and biodiesel are being burnt in the boiler. The figure shows that the SO2 contents of flue gases resulting from the burning of biodiesel are much less than those of petroleum diesel ones. Pinpointing the measured characteristics of biodiesels and petroleum diesel (Table 1) and the performance measurements of these fuels (Figs. 2–7), it is concluded that biodiesels can be a good substitute for petroleum diesel as burning fuels with their lower pollutant content properties.
Presently, the problems of higher cloud points and lower heating values and cost of biodiesels as compared with those of petroleum diesel have made them less attractive. However, blend of biodiesel and petroleum diesel could improve these problems. The amount of biodiesel in any fuel mixture is stated by a ‘‘B’’ factor. For example, a fuel mixture containing 20% biodiesel is labeled B20, while pure biodiesel is referred to as B100. To compare the combustion performances and the level of pollutant emissions, we made blends of biodiesel and diesel. The widespread use of soybeans in the USA for food products has led to the emergence of soybean biodiesel as the primary source for biodiesel in that country. Therefore, we prefer to use soybean biodiesel in these blends instead of sunflower biodiesel. All the blends were burnt under the same conditions. The combustion efficiencies of different blend were measured and plotted in Fig. 8a. The figure shows that the combustion efficiency drops with increasing the B factor. The pollutant contents of the flue gases from the boiler for each blend and the variations of their amounts with air flow are also plotted in Figs. 8b–8d. Figs. 9a–9d show the results of the experiments with different blend in the form of histograms to simplify the comparison of the performances of the blends used in this work. Research data for transportation diesel engines reported an increase in NOX emissions when biodiesel was used [31–37] but the results of researches on boiler are different. Lee et al. [25] used B20 blend in a residential-scale hot water boiler and reported similar or slightly higher NOX emissions with biodiesel. Vanlanigham et al. [26] also tested 20% degummed soybean oil and 80% petroleum fuel, to be comparable to petroleum fuel oil and detected that nitric oxide emissions at optimal furnace settings was same for fuel oil and B20. Furthermore, Macor and Paynaello [38] showed higher values of NOX for biodiesel than the domestic heating oil. On the contrary, Figs. 8b and 9b show that B20 in the boiler results in getting combustion efficiency 4.2% lower as compared with the pure petroleum diesel while it helped to reduce the pollutant contents of the flue gases. Batey [23] studied B20 blend and detected the same result but there are no research data on various combinations of biodiesel and petroleum blends for small residential and commercial combustion equipment in their work. Lee et al. [25] observed that combustion of B20 exhibited similar gaseous emissions to those of No. 2 fuel oil, with the exception that SO2 emitted by biodiesel are greater than those emitted by heating oil, due to the unexpected presence of sulfur in biodiesel. On the contrary, Figs. 8 and 9 shows that combustion of B20 helped to reduce the CO2 and SO2 emissions by 4.2% and 7%, respectively.
4. Conclusions This study revealed the potential of biodiesel and blends of biodiesel and diesel as a fuel for a boiler and a comparison is made by the combustion efficiency and flue gas emission at two energy levels for a wide range of air flow rate. In spite of small differences between combustion performances of biodiesel and diesel, biodiesel are efficient than diesel at the lower energy level. Furthermore, except NOX at the low energy level, biodiesel emitted less pollutant than diesel. Our findings show that except B10, other blends emitted less CO, SO2 and CO2. In spite of researcher findings on diesel fuels in engines, the results indicated a reduction in the NOX level at the second energy level in this boiler. Therefore, In addition to being available locally and renewable, biodiesel can make a good substitute for diesel fuel in those applications. As far as pollution
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potential is concerned, biodiesel makes a much better fuel than diesel over a wide range of air flows and energy inputs. Appendix A. Instruments’ specifications A.1. Specifications of the boiler instrumentation See Table A1.
Table A1 Specifications of the instrumentation for boiler performance measurement.
Fuel flow meter Water flow meter Temperature
Range
Accuracy
4–10 l/h 100–400 g/s 0–1100 °C
<1% 0.1% 0.3%
A.2. Gas analyzer specifications See Table A2.
Table A2 Technical characteristics of the Quintox KM9106 gas analyzer. Parameter
Measuring range
Precision
Resolution
Temperature of smokes Oxygen (O2) Carbon oxide (CO)
0–1100 ° C
1.0 °C ± 0.3% of reading
0.1 °C
0–25% 0– 10,000 ppm
0.1% + 2% ±20 ppm < 400 ppm
0.1% 1 ppm
Nitric oxide (NO)
0–5000 ppm
Sulfur dioxide (SO2)
0–5000 ppm
5% of reading < 2000 ppm ±10% of reading > 2000 ppm ±5% of reading > 100 ppm ±5 ppm < 100 ppm ±5% of reading > 100 ppm ±5 ppm < 100 ppm
1 ppm
1 ppm
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
A comparative study of combustion performance and emission of biodiesel blends and diesel in an experimental boiler Afshin Ghorbani a,⇑, Bahamin Bazooyar a, Ahmad Shariati a, Seyyed Mohammad Jokar b, Hadi Ajami a, Ali Naderi a a b
Ahvaz Faculty of Petroleum Engineering, Petroleum University of Technology, Ahvaz, Iran Chemical Engineering Department, School of Chemical and Petroleum Engineering, Shiraz University, Iran
a r t i c l e
i n f o
Article history: Received 31 January 2011 Received in revised form 28 May 2011 Accepted 8 June 2011 Available online 12 July 2011 Keywords: Biodiesel Combustion Boiler emission
a b s t r a c t Recently, biodiesel has become more attractive since it is made from renewable resources and also for the fact that the resources of fossil fuels are diminishing day by day. This study compares combustion of B5, B10, B20, B50, B80 and B100 with petroleum diesel over wide input air flows at two energy levels in an experimental boiler. The comparison is made in terms of combustion efficiency and flue gas emissions (CO, CO2, NOX, and SO2) and influence of air flow at two energy levels 219 kJ/h and 249 kJ/h is studied. The findings show that at higher level energy diesel efficiency was a little higher than that of biodiesel, but at lower level biodiesels are efficient than diesel. Except B10, Biodiesel and other blends emitted less pollutant CO, SO2 and CO2 than diesel. B10 emitted lower CO2 and NOX, but emitted higher SO2 than diesel. Despite studies reporting an increase in the NOX level resulting from burning of biodiesel over conventional petroleum diesel fuels in engines, our findings indicated at the second energy level a reduction in the NOX level in the flue gases resulting from burning of biodiesel. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Due to the reduction in fossil fuel resources and the increasing consumption of fuels, the use of alternative fuels instead of them seems to be necessary. In Iran, oil and gas are the major resources of energy, however, there are much potential for producing energy from renewable resources such as biomass. It is an established fact that if sufficient water is available, most of Iran’s land can be utilized to produce crops. Currently, about 12% of the total land area is utilized for the purpose [1]. About 17–20% of crops such as wheat, sugar, and rice produced in Iran are wasted as agricultural residues [2]. These residues can be used as a potential for production of biofuels. Biodiesel has recently become more attractive among biofuels because of some benefits such as reducing greenhouse gas emissions, safety of the supply, availability of resources, increasing rural development and biodegradability. Biodiesel, an alternative diesel fuel, is made from renewable biological resources such as vegetable oils and animal fats. Biodiesel is made through transesterification reaction of these resources with methanol or ethanol in the presence of a catalyst. Generally, methanol is preferred for transesterification because it is less expensive than ethanol. So, biodiesel is known as fatty acid methyl ester. Although
⇑ Corresponding author. Tel.: +98 917 106 8034; fax: +98 611 555 2255. E-mail address: [email protected] (A. Ghorbani). 0306-2619/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2011.06.016
there is much research on the use of biodiesel in diesel engines [3–21] research on the use of biodiesel blends in boilers is rare. Although the use of biodiesel in boilers showed a reduction in NOX emission [22–24] but others reported to the contrary [25– 27]. Tashtoush [22] did not use blends and Batey [23], Lee et al. [25] and Vanlaningham et al. [26] used only B20 blend of soybased biodiesel and diesel in their works. Although Krishna et al. [24] tested B10, B20, B30 and a blend of 50% biodiesel by volume with kerosene, studies on blends of biodiesel and diesel need to be carried out and the advantages of these fuels in boilers and furnaces are to be demonstrated. The objective of this study is to verify if the overall performance of biodiesels is comparable to that of diesel. Besides two pure biodiesels, soybean and sunflower biodiesel, we tested various blends of biodiesel and diesel such as B5, B10, B20, B50 and B80. Besides the performance, gas emissions and percent reduction of emissions relative to diesel are compared. 2. Materials and methods Several types of vegetable oils, with a variable composition in fatty acids, can be used for the preparation of biodiesel. Two common vegetable oils used in biodiesel production are soybean and sunflower oil, which were used in this work as well. Vegetable oils were transesterified to produce the methyl ester using a catalyst, potassium hydroxide. Methanol chromatographic grade (99:5%) and potassium hydroxide was purchased from Merck Company.
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The results of the physical analysis of values of the two biodiesels and diesel are summarized in Table 1. The transesterification was carried out in the LR 2000 P Laboratory Reactor, with Eurostar Power Control-Visc P7 Overhead Stirrer, LR 2000.10 Anchor Stirrer, and a thermocouple probe. The mixture was agitated by using a stainless steel stirrer. Two liters of oil were added to the reactor. Then the temperature of the reactor was adjusted to 55 °C by Heating Circulator Bath. 14 g of KOH pellets were dissolved in 500 ml methanol before being poured into the reactor. The mixture was heated and mixed continuously for 45 min. The reaction conditions that choosing in this work were optimum conditions for production of biodiesel that researchers reported in their studies [28,29]. Then the mixture was poured into two separator funnels. Two layers began to form after some minutes with the darker glycerol layer sinking to the bottom. Glycerol was separated from the upper layer (unwashed biodiesel). The Upper layer should be washed because of some impurities such as glycerol, excessive methanol and soap. The ester layer was washed to ten times with hot distilled water. Combustion laboratory unit made by P.A. Hilton, England, which was used to compare the combustion efficiencies, is shown in Fig. 1. The combustion chamber is a steel-horizontal cylinder one meter long with a 45 cm inner diameter. A stainless steel water jacket with an outer diameter of 50 cm was used to cool the combustion chamber. The combustor is designed such that a range of liquid fuels can be tested and compared. The fuel is forced from a pump at high pressure out of a nozzle within a tubular air director. A rotational force was applied to the fuel within the nozzle causing it to break up into small droplets as it exits the nozzle to form a spray cone. The air fan of the boiler has damper numbers that each number refers to various ranges of air flow. Therefore, rate of input air to the combustion chamber was adjusted by fan damper setting. Increasing in the fan damper number led to further air flow to the boiler. The air fan and fuel pump are coaxial and driven from an electric motor. Electrodes cause ignition and are switched off after the flame begins self sustaining. The combustor’s exit is flanged to an exhaust section, which is a straight pipe of 160 mm diameter. This section, in turn, exhausts into a 6 m high chimney of 200 mm diameter. Feeding and metering of cooling water, liquid fuel, and combustion air were achieved by auxiliary systems attached to the unit. Rota meters, which are integral parts of the unit, were calibrated and used to measure flow rates of the fuel and the cooling water. The instrumentation attached to the water heater allowed direct read-out of inlet and outlet water temperatures, water mass flow rates, inlet air temperature, exhaust gas temperature, and fuel flow rate. Technical characteristics and instrumentation specifications of the boiler are listed in Table 2 and Table A1, respectively. All fuels were tested at two levels of energy input rate and different fan damper numbers that refer to various ranges of air to fuel ratio. In all runs, data
was collected under steady-state conditions. The fuels were compared at the same levels of the energy input. The pressure of fuel at lower and higher level was 120 (827) and 200 psi (1379 kPa), respectively. After using pure biodiesels and diesel in the boiler, blends of biodiesel and diesel were burned and compared with one other. Soybean biodiesel was used in these blends and blends were burned at a fuel pressure of 1034 kPa. The data measured during the tests included water coolant and exhaust temperatures, and exhaust emissions, including O2, CO, CO2, NOX, and SO2. Inlet and outlet temperatures of the cooling water were measured by thermocouples utilizing digital thermometers. Water and fuel flow rates and exhaust temperature were read directly from the control panel. Samples of the exhaust gas were drawn for emission analysis. A flue-gas analyzer made by Kane, model KM9106 measured the emitted gases, including O2, CO, CO2, NOX, and SO2. Gas conditioning module (KMDM220) is fitted in the front compartment of a standard Quintox carry case and comprise from a Peltier fan cooled chiller assembly, a peristaltic pump removes the condensate, the control electronics and a power supply module. The module is supported on an aluminum alloy chassis. The chiller is connected to a flue mounted electrically heated probe by a 3 m long heated line with automatic temperature control. Because the gas that is extracted from the flue is maintained at 120 °C, no condensation occurs in the probe and the hose and so no sample gas is lost in the condensate. The chiller flash cools the sample gas to below the ambient dew point and any water in the gas condenses immediately. Then the condensate is pumped away using a peristaltic pump. Because the gas has no chance to remain in contact with the condensate, volatile sample gas is not lost into the condensate. The chilled gas then naturally warms up as it passes through the sampling pump to the sensors. Therefore, its humidity reduces and there is no risk of future condensation. The analyzer technical characteristics are indicated in Table A2. 3. Results and discussion 3.1. The effect of different air flows on the exhaust gas temperature The trends of variation of the exhaust gas temperature, Texh, with the fan damper number, are shown in Figs. 2a and 2b. As it is expected, the exhaust temperature decreases as air flow increases. This observation may be explained by the fact that the entering excess air to the combustion chamber is cooler than the gases in the boiler. Despite this obvious trend in the exhaust temperature, Batey [23] reported no significant difference in the flue gas temperatures. It is obvious from Fig. 2b that at 200 psi (1379 kPa) diesel has higher exhaust temperature than biodiesel. But for 120 psi (827 kPa), it is different to 200 psi (1379 kPa) and biodiesels have higher exhaust temperature.
Table 1 Physical characteristics of diesel and biodiesel. Property
Diesel
Soybean
Sunflower
Standard
Flash point (°C) Kinematic viscosity @ 40 °C (mm2/s) Cetane Number Specific gravity @ 60 °C Cloud point (°C) Pour point (°C) Copper corrosion Carbon residues Water and sediment Sulfated ash Acid value Total glycerol Free glycerol Gross Caloric Value (kJ/kg)
52 3.5 45 0.8367 6 19 1 0.01 Trace 0.001 Trace Trace Trace 43,640
163 5.8 49 0.8821 2 2 3 0.038 0.002 0.008 0.34 0.01 0.120 41,118
175 6 47 0.8934 3 6 3 0.035 0.003 0.005 0.41 0.01 0.190 41,005
ASTM ASTM ASTM ASTM ASTM ASTM ASTM ASTM ASTM ASTM ASTM ASTM ASTM ASTM
D93 D445 D 613 D1298 D2500 D97 D130 D4530 D2709 D874 D664 D6584 D6584 D240
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1
16
15
2
14 13 3 12
4 8
5 6
11
7
10
9
1- Exhaust
2- Exhaust temperature
3- Water inlet temperature
4- Cooling water flow meter
5- Condensate drain
6- Cooling water
7- Drain
8- Water outlet temperature
9- Fuel tank
10- Fuel control
11- Oil filter
12- Pump pressure gauge
13- Air flow
14- Oil pump
15- Air temperature
16- Observation window
Fig. 1. Combustion laboratory unit made by P.A. Hilton, England.
Table 2 Technical characteristics of the boiler. Nozzle Power output up to Flow rate Acceptable fuel viscosity Fuel outlet pressure
1.35 US gal/h–60° H KH (Hollow) 150 kW 4–7 l/h 1.5–7.5@ 40 mm2/s 8–15 bar
3.2. Comparison of the performance of biodiesels and diesel in the boiler The efficiency of combustion, gc, is the ratio of the amount of heat transferred to the water in the jacket, Qw, to the amount of
heat input, Qin. It was mentioned that the fuels were burned at two levels of energies and at different air flows. For comparison of fuels, two levels of energy were selected that include wide ranges of air flows. Selection of these levels of input energies, 120 psi (827 kPa) and 200 psi (1379 kPa), was due to the limitations of the firing capacity of the fuels in the boiler. These levels of input energies were close to the minimum and maximum levels of the energy that could be generated with a nozzle containing a capacity of 1:35 gal/h of fuel. Qin for the first and second energy levels was about 219 kJ/h and 249 kJ/h, respectively. Variations in the combustion efficiency with the fan damper number for all fuels at two levels of input energies are shown in Figs. 3a and 3b. It can be seen in these figures that the combustion efficiency for all fuels
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Fig. 2a. Variation of exhaust gas temperature of diesel and biodiesels with fan damper number at 120 psi (827 kPa).
Fig. 3b. Variation of combustion efficiency of diesel and biodiesels with fan damper number at 200 psi (1379 kPa).
than diesel at lower energy. It may be related to higher exhaust temperature of biodiesel than diesel. At extreme cases, the differences were in the range of 1–15%. Among sunflower and soybean biodiesel, at higher energy, soybean biodiesel was efficient than sunflower biodiesel. Nevertheless, at lower energy, it seems there was not much difference between them at this boiler. These observations related to the trend of exhaust temperature of fuels.
3.3. Comparison of pollutant emissions of different biodiesels and diesel
Fig. 2b. Variation of exhaust gas temperature of diesel and biodiesels with fan damper number at 200 psi (1379 kPa).
decreases as air flow is increased. The reason might be related to the reduction of the exhaust temperature of the boiler with increasing of air flows as shown in Figs. 2a and 2b. At Fig. 3b, it was observed that diesel efficiency was a little higher than that of biodiesel. However, biodiesels were efficient
Fig. 3a. Variation of combustion efficiency of diesel and biodiesels with fan damper number at 120 psi (827 kPa).
3.3.1. CO and CO2 emissions Figs. 4 and 5 shows the measured emissions of carbon monoxide (CO) and carbon dioxide (CO2) in the exhaust gases for all fuels at the two fuel flow rates. The emission of CO is shown in Figs. 4a and 4b, which indicate that CO concentration of biodiesels are lower than diesel for two energy input levels. The CO emission has similar overall behavior for fuels. Increase input air results in uniform fuel distribution and poor spray characteristics. Consequently, CO emission intensifies as input air increases. The higher concentration of CO and CO2 for petroleum diesel is likely due to the more carbon content by weight of diesel relative to the biodiesel [30,31]. For the first level of the input energy, Figs. 5a shows that at fan damper 3, 11.7% reduction in CO2 are attainable if soybean biodiesel is burnt instead of petroleum diesel under the
Fig. 4a. Variation of CO concentrations of diesel and biodiesels with fan damper number at 120 psi (827 kPa).
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Fig. 4b. Variation of CO concentrations of diesel and biodiesels with fan damper number at 200 psi (1379 kPa).
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Fig. 6a. Variation of NOX concentrations of diesel and biodiesels with fan damper number at 120 psi (827 kPa).
conditions shown in the figures. For the second level of the input energy, Fig. 5b shows 7.3% reduction in CO2 for soybean biodiesel compare to diesel. Meanwhile, these figures also show that it is possible to reduce 12.6% and 6% in the CO2 contents of flue gases
Fig. 6b. Variation of NOX concentrations of diesel and biodiesels with fan damper number at 200 psi (1379 kPa).
Fig. 5a. Variation of CO2 concentrations of diesel and biodiesels with fan damper number at 120 psi (827 kPa).
Fig. 5b. Variation of CO2 concentrations of diesel and biodiesels with fan damper number at 200 psi (1379 kPa).
Fig. 7. Variation of SO2 concentrations of diesel and biodiesels with fan damper number at 120 psi (827 kPa) and 200 psi (1379 kPa).
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Fig. 8a. Variation of combustion efficiency with fan damper number.
Fig. 8d. Variation of CO2 with fan damper number.
Fig. 9a. Percent reduction of combustion efficiency by blends relative to diesel. Fig. 8b. Variation of NOX with fan damper number.
Fig. 9b. Percent reduction of NOX by blends relative to diesel. Fig. 8c. Variation of SO2 with fan damper number.
if sunflower biodiesel is burnt instead of petroleum diesel under the conditions shown in the figures. 3.3.2. NOX and SO2 emissions Figs. 6a and 6b show the measured emissions of nitrogen oxides, NOX, for all fuels at the two levels of input energies. The forma-
tion of NOX depends mainly on the availability of oxygen and the local combustion temperatures. The increase in temperature and oxygen causes more NOX to be produced. It was mentioned earlier that the exhaust and boiler temperatures decrease with increasing air flow. As a result, the decrease in temperature with air flow can compensate the effects of increased oxygen on NOX formation. The increase observed at the tails of the curves might be due to the effects of oxygen, which ultimately dominates the reduction effect of
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3.4. Comparison of the performance and pollutant emissions of different blends of biodiesels and diesel
Fig. 9c. Percent reduction of SO2 by blends relative to diesel.
Fig. 9d. Percent reduction of CO2 by blends relative to diesel.
the boiler temperature. At low air flow, the temperature in the boiler is too high and consequently more NOX was formed. The conflicting effects of temperature and oxygen on the NOX formation by increasing air flow cause the formation of NOX to be less at high air flow than low air flow. It seems that the effect of temperature is stronger in NOX formation than the oxygen rate. In these figures, it is also seen that the NOX contents of the flue gases from the burning of sunflower biodiesel in the boiler are less than those produced from the burning of petroleum diesel. This may be related to the lower temperature of the boiler that is observed during the burning of sunflower biodiesel. However, at the low energy level, diesel produces less NOX than soybean biodiesel and at the higher energy level, soybean biodiesel emits more. The trend of NOX emission maybe related to the trend of exhaust temperature. In lower energy level diesel has lower exhaust temperature than soybean biodiesels, therefore diesel emits lower NOX than soybean biodiesel. But in higher energy level due to higher exhaust temperature of diesel than soybean biodiesel, diesel emits higher NOX than soybean biodiesel. Fig. 7 compares the SO2 content of flue gases from the boiler when petroleum diesel and biodiesel are being burnt in the boiler. The figure shows that the SO2 contents of flue gases resulting from the burning of biodiesel are much less than those of petroleum diesel ones. Pinpointing the measured characteristics of biodiesels and petroleum diesel (Table 1) and the performance measurements of these fuels (Figs. 2–7), it is concluded that biodiesels can be a good substitute for petroleum diesel as burning fuels with their lower pollutant content properties.
Presently, the problems of higher cloud points and lower heating values and cost of biodiesels as compared with those of petroleum diesel have made them less attractive. However, blend of biodiesel and petroleum diesel could improve these problems. The amount of biodiesel in any fuel mixture is stated by a ‘‘B’’ factor. For example, a fuel mixture containing 20% biodiesel is labeled B20, while pure biodiesel is referred to as B100. To compare the combustion performances and the level of pollutant emissions, we made blends of biodiesel and diesel. The widespread use of soybeans in the USA for food products has led to the emergence of soybean biodiesel as the primary source for biodiesel in that country. Therefore, we prefer to use soybean biodiesel in these blends instead of sunflower biodiesel. All the blends were burnt under the same conditions. The combustion efficiencies of different blend were measured and plotted in Fig. 8a. The figure shows that the combustion efficiency drops with increasing the B factor. The pollutant contents of the flue gases from the boiler for each blend and the variations of their amounts with air flow are also plotted in Figs. 8b–8d. Figs. 9a–9d show the results of the experiments with different blend in the form of histograms to simplify the comparison of the performances of the blends used in this work. Research data for transportation diesel engines reported an increase in NOX emissions when biodiesel was used [31–37] but the results of researches on boiler are different. Lee et al. [25] used B20 blend in a residential-scale hot water boiler and reported similar or slightly higher NOX emissions with biodiesel. Vanlanigham et al. [26] also tested 20% degummed soybean oil and 80% petroleum fuel, to be comparable to petroleum fuel oil and detected that nitric oxide emissions at optimal furnace settings was same for fuel oil and B20. Furthermore, Macor and Paynaello [38] showed higher values of NOX for biodiesel than the domestic heating oil. On the contrary, Figs. 8b and 9b show that B20 in the boiler results in getting combustion efficiency 4.2% lower as compared with the pure petroleum diesel while it helped to reduce the pollutant contents of the flue gases. Batey [23] studied B20 blend and detected the same result but there are no research data on various combinations of biodiesel and petroleum blends for small residential and commercial combustion equipment in their work. Lee et al. [25] observed that combustion of B20 exhibited similar gaseous emissions to those of No. 2 fuel oil, with the exception that SO2 emitted by biodiesel are greater than those emitted by heating oil, due to the unexpected presence of sulfur in biodiesel. On the contrary, Figs. 8 and 9 shows that combustion of B20 helped to reduce the CO2 and SO2 emissions by 4.2% and 7%, respectively.
4. Conclusions This study revealed the potential of biodiesel and blends of biodiesel and diesel as a fuel for a boiler and a comparison is made by the combustion efficiency and flue gas emission at two energy levels for a wide range of air flow rate. In spite of small differences between combustion performances of biodiesel and diesel, biodiesel are efficient than diesel at the lower energy level. Furthermore, except NOX at the low energy level, biodiesel emitted less pollutant than diesel. Our findings show that except B10, other blends emitted less CO, SO2 and CO2. In spite of researcher findings on diesel fuels in engines, the results indicated a reduction in the NOX level at the second energy level in this boiler. Therefore, In addition to being available locally and renewable, biodiesel can make a good substitute for diesel fuel in those applications. As far as pollution
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potential is concerned, biodiesel makes a much better fuel than diesel over a wide range of air flows and energy inputs. Appendix A. Instruments’ specifications A.1. Specifications of the boiler instrumentation See Table A1.
Table A1 Specifications of the instrumentation for boiler performance measurement.
Fuel flow meter Water flow meter Temperature
Range
Accuracy
4–10 l/h 100–400 g/s 0–1100 °C
<1% 0.1% 0.3%
A.2. Gas analyzer specifications See Table A2.
Table A2 Technical characteristics of the Quintox KM9106 gas analyzer. Parameter
Measuring range
Precision
Resolution
Temperature of smokes Oxygen (O2) Carbon oxide (CO)
0–1100 ° C
1.0 °C ± 0.3% of reading
0.1 °C
0–25% 0– 10,000 ppm
0.1% + 2% ±20 ppm < 400 ppm
0.1% 1 ppm
Nitric oxide (NO)
0–5000 ppm
Sulfur dioxide (SO2)
0–5000 ppm
5% of reading < 2000 ppm ±10% of reading > 2000 ppm ±5% of reading > 100 ppm ±5 ppm < 100 ppm ±5% of reading > 100 ppm ±5 ppm < 100 ppm
1 ppm
1 ppm
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