Controlling NOx Emission of Crude Rice Bran ... - Wiley Online Library

Clean – Soil, Air, Water 2011, 39 (6), 515–521 Subramani Saravanan1 Govindan Nagarajan2 Radhakrishnan Ramanujam3 Santhanam Sampath4 1

Department of Automobile Engineering, Sri Venkateswara College of Engineering, Sriperumbudur, Chennai, Tamilnadu, India 2 ICE Division, Department of Mechanical Engineering, College of Engineering, Anna University, Chennai, Tamilnadu, India 3 Department of Mechanical Engineering, Sri Venkateswara College of Engineering, Sriperumbudur, Chennai, Tamilnadu, India 4 Department of Automobile Engineering, Rajalakshmi Engineering College, Thandalam, Chennai, Tamilnadu, India

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Research Article Controlling NOx Emission of Crude Rice Bran Oil Blend for Sustainable Environment The main objective of this work is to investigate the factors influencing the NOx control of a stationary diesel engine fuelled with crude rice bran oil blend with lesser effect on smoke density and brake thermal efficiency (BTE). Fuel injection timing, percentage of EGR, and fuel injection pressure are chosen as the factors for the objective and NOx emission, smoke density, and BTE are considered as the response variables. To critically analyze the effects of the chosen factors on the objective three levels were chosen in each factor and the experiments were designed by following the design of experiments method. Taguchi’s L9 orthogonal array was used to conduct the tests with different combination of factor levels. Through analysis of variance (ANOVA) method, the most influencing factors and also the significance of each factor affecting each response variable were found out. Response graph was drawn for each response variable to determine the optimum combination of factor levels in achieving the objective and the obtained combination was confirmed experimentally. Keywords: Diesel engine; NOx; Smoke density; Taguchi’s orthogonal array Received: July 20, 2010; revised: August 26, 2010; accepted: August 29, 2010 DOI: 10.1002/clen.201000283

1 Introduction Biofuels were one of the promising renewable sources of energy which can be utilized to meet the growing demand of the petroleum products. Since the consumption of diesel is higher than gasoline, many biofuel research works were focused to find an alternate to diesel fuel. In the present scenario of biofuel research, attention was focused on non-edible vegetable oils cultivated from the land not suitable for agriculture to test their suitability as an alternative to diesel fuel [1]. However, NOx emission of vegetable oil and biodiesel blends (oxygenated fuels) was comparatively higher than that of diesel [2–9] which needs further investigation to reduce the same. NOx emission of an I.C.Engine can be controlled by retardation of fuel injection timing, exhaust gas recirculation (EGR), fuel additives, and water injection [10, 11]. In these methods, NOx formation in the engine cylinder was prevented through modification of combustion process. NOx emission can also be controlled by the treatment of exhaust gases [12] with the help of different catalysts [13–17] to remove it completely and this method can be considered when the emission standards cannot be met by the combustion process modification alone. When compared with exhaust gas treatment, incylinder control is the most economical method for NOx reduction [12] and the same is discussed in this paper.

Correspondence: Dr. S. Saravanan, Department of Automobile Engineering, Sri Venkateswara College of Engineering, Post Bag No.3.Pennalur, Sriperumbudur, Chennai, Tamilnadu, India E-mail: [email protected] Abbreviations: ANOVA, analysis of variance; BTE, brake thermal efficiency; CRBO, crude rice bran oil; EGR, exhaust gas recirculation; FFA, free fatty acid; OA, orthogonal array.

ß 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

NOx reduction of oxygenated fuels through retardation of fuel injection timing and EGR has been attempted by many researchers [18–24]. In their investigations they reported that reduction in NOx emission is accompanied by increase in smoke density and decrease in brake thermal efficiency (BTE). They have concluded that retarding the fuel injection timing by 38 crank angle (CA) and more and increasing the EGR by more than 15% will increase the smoke density with decrease in efficiency [11, 18–24]. Hence there is a need for optimization of fuel injection timing and percentage of EGR to reduce the NOx emission without increasing smoke emission and fuel consumption [10]. It was also inferred that among the fuel injection and EGR, the most influencing factor in the NOx control has to be studied. In the present work both the fuel injection timing and percentage EGR were varied to study the effect of these two methods in reducing the NOx emission of a stationary diesel engine fuelled with vegetable oil blend. Since considerable work was done with these two methods individually in controlling the NOx emission, the present work concentrates on the combined effect in controlling the NOx emission. Since fuel injection pressure also plays an important role in I.C.Engine combustion, it was also varied in combination with injection timing and percentage EGR. Vegetable oil used in this investigation is high free fatty acid (FFA) crude rice bran oil (CRBO) which is a non-edible vegetable oil derived from rice bran. CRBO with high FFA content is a non-edible vegetable oil which can be utilized in CI engine in blended form as an alternate to diesel fuel [25]. High FFA CRBO has comparable properties as that of diesel [26] and the properties of CRBO blend compared with diesel are given in Tab. 1. High FFA CRBO blends were tested successfully in a stationary diesel engine [27]. CRBO blend contains 20% of CRBO and 80% of No.2 petroleum diesel on volume basis. From the earlier research work on CRBO blend it was found that CRBO blends have www.clean-journal.com

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Clean – Soil, Air, Water 2011, 39 (6), 515–521

Table 1. Properties of CRBO blend and diesel

Property 2

Kinematic viscosity at 408C (mm /s) Calorific value (kJ/kg) Specific gravity Flash point (8C) Aniline point 8C Distillation temperature T90 (8C) Calculated cetane index

Testing method

Diesel oil

CRBO blend

Redwood viscometer ASTM D 240-02 – ASTM D93 ASTM D611 ASTM D86 ASTM D 4737

3.63 43000 0.840 70 74 335 50.97

12.34 40136 0.874 85 61 379 45.61

Table 2. Factors influencing the objective with chosen levels

Factor no

1 2 3

Factors influencing the objective Injection timing Percentage of EGR Injection pressure

Level of factors 1

2

3

Standard timing 0 Normal pressure (200–210) bar

Advanced timing (by 2.58 CA) 10 (220–230) bar

Retarded timing (by 2.58 CA) 15 (240–250) bar

the potential to replace diesel oil and their NOx emission was higher than that of diesel [26, 27]. Hence a method to reduce NOx emission of CRBO blends has to be investigated. The main objectives of the present work are: (1) To study the combined effect of fuel injection timing, percentage EGR, and fuel injection pressure in reducing the NOx emission of diesel engine fuelled with CRBO blend. (2) To investigate the most influencing factor in reducing the NOx emission, smoke density, and BTE of CRBO blend. (3) To find an optimum combination of injection timing, percentage EGR, and fuel injection pressure in reducing the NOx emission with a lesser effect on smoke density and efficiency.

2 Materials and methods 2.1 Taguchi design Before starting the experimentation, the experiments were designed by employing design of experiments (DOE) method. For the present problem, fuel injection timing, percentage EGR (by volume), and fuel injection pressure were considered as the factors influencing the objective. Three levels were chosen in each factor to study the significant effects of these factors on the set objective. Retarded and advanced fuel injection timing was taken as 2.58 CA since increasing the angle further will increase the smoke density and NOx emission [19]. Hence for fuel injection timing, standard timing, retarded, and advanced timing of 2.58 CA were taken as the three levels of factor. The upper level for EGR was fixed as 15% and within that 0 and 10% have been chosen as the other two levels. For stationary diesel engine, it was suggested that, the fuel injection pressure has to be maintained within 250 bar for smooth operation of the engine [28]. Hence by fixing 250 bar as upper limit, two more levels were chosen within that including standard pressure. The three levels of the chosen factors are given in Tab. 2.


2.1.1 Taguchi orthogonal array (OA) In full factorial experiment for three factors with three levels, the number of experiments will be 33 ¼ 27. To reduce the number of experiments to be conducted, experiments were designed by using Taguchi OA technique. For more than two numbers of three level factors the recommended OA is L9 [29] which is given in Tab. 3. In Tab. 3 column 1 indicates the levels of factor 1 (fuel injection timing), column 2 the levels of factor 2 (percentage EGR), and column 3 the levels of factor 3 (fuel injection pressure).

2.2 Experimental setup The schematic diagram of the experimental set-up is shown in Fig. 1. The technical specifications of the engine used in this investigation are given in Tab. 4. A swinging field electrical dynamometer was used to apply the load on the engine. This electrical dynamometer consisted of a 5-kVA AC alternator (220 V, 1500 rpm) mounted on bearings and on a rigid frame for the swinging field type loading. The output power was obtained by accurately measuring the reaction torque by a strain gauge type load cell. A water rheostat with an adjustable depth of immersion electrode was provided to dissipate the power generated.

Table 3. L9 orthogonal array (OA) [29]

Trial no 1 2 3 4 5 6 7 8 9

Column 1

Column 2

Column 3

1 1 1 2 2 2 3 3 3

1 2 3 1 2 3 1 2 3

1 2 3 2 3 1 3 1 2

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2.3 Testing procedure Tests were conducted on the engine, with the selected factors at different levels to determine the effect of the factors on the objective. The engine was operated nine times with the combination of different levels of influencing factors as given in Tab. 2. Two replicates were conducted for each trial and the order of the trial was selected randomly. In each trial the engine was tested at different loads and at each load the responses (NOx emission in ppm, smoke density in mg/m3, and time taken for fuel consumption [s]) were measured. NOx emission was measured with MRU 1600 exhaust gas analyzer and the smoke density was measured with AVL smoke meter.

2.4 Error analysis The errors associated with various measurements and in calculations of performance parameters were computed. The maximum possible errors in BTE were estimated by using the method proposed by Moffat [30]. Errors were estimated from the minimum values of output and the accuracy of the instrument. This method is based on careful specification of the uncertainties in various experimental measurements. If an estimated quantity, S depends on independent variables like (x1, x2, x3. . .xn) then the error in the value of ‘‘S’’ is given by

Figure 1. Experimental set up.

Table 4. Specifications of engine

Make

Kirloskar

Model Type Bore Stroke (mm) Compression ratio Cubic capacity (L) Rated power (kW) Rated speed (rpm) Start of injection (bTDC) Injector operating pressure (bar)

TAF 1 Direct injection and air cooled 87.5 110 17.5:1 0.661 4.4 1500 23.48 200–205

@S ¼ S

( 2 2 )12 @x1 2 @x2 @xn þ þ þ x1 x2 xn

where @xx11 , @xx22 , etc. are the errors in the independent variables, x1 the accuracy of the measuring instrument, and x1 is the minimum value of the output measured.

2.4.1 Errors in brake thermal efficiency and exhaust gas emissions

2.2.1 Exhaust gas recirculation (EGR) system A piping arrangement of length 8 m was provided to tap the exhaust gases from the exhaust pipe and to connect it into the inlet airflow passage. This arrangement is shown schematically in Fig. 1. The exhaust gases were tapped from the exhaust pipe which is 10 m away from the engine. While entering into the inlet airflow passage the temperature of the exhaust gases was approximately equal to that of the ambient air which eliminates any additional cooling requirements. A control valve was provided in the pipeline to control the flow rate of exhaust gases and the mixture of exhaust gases and fresh air were admitted into the inlet manifold. Percentage EGR was calculated by using the expression: % EGR ¼

mass of air without EGRmass of air with EGR 100 mass of air without EGR

2.2.2 Injection timing and injection pressure Injection timing was changed by changing the thickness of advance shim. The spring tension of the injector needle with setting screw was varied to get the different fuel injection pressure.


Brake specific fuel consumption was calculated from the fuel consumption and BTE. The maximum possible error in the calculation of BTE is: !12 @Torque 2 @rpm 2 @time 2 þ þ Torque rpm time 2 2 2 !12 0:021 0:15 0:01163 þ þ ¼ 0:0033 ¼ 0:33% 7:0 1500 9:77

@BTE ¼ BTE ¼

As per the specifications of the analyzer the maximum possible error in the measurement of smoke density and NOx emission is 5%.

3 Analysis of data Obtained responses in each trial were analyzed through analysis of variance (ANOVA) and the results were tabulated to determine the significance and contribution of the selected factors in achieving the objective. Response graph was drawn for each response variable to determine the combination of factors in achieving the objective and based on the response graph and ANOVA table the optimum level of combination was arrived. www.clean-journal.com

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3.1 Analysis of variance ANOVA is a statistical method used to interpret experimental data and make necessary decisions. The total variability of the NOx emission, smoke density, and BTE is measured by the sum of squares of these values by using the formula given below [29]:

SST ¼

" N X

# y2i

i¼1

T2 N

In the above formula N is the total number of experiments, T the sum of all experiments response variable, and yi is the ith response variable. The total sum of squares includes the sum of squares due to each factor (SSf) and the sum of squares of errors (SSe). The ratio of SSf to SST is the percentage contribution (P) by the factor. Ftest has to be performed to identify the significant effect of each factor on the response variables. F-test is the ratio of mean squared factor (MSF) to the mean squared error (SSem), where MSF is equal to the SSf divided by the number of degree of freedom (DF) associated with the factors. The larger the F-value, the greater the effect on the response variable due to the change in factor.

3.2 Verification Confirmation experiment was conducted to verify the optimum combination obtained through ANOVA and response graph. Response variable of the confirmation experiment was verified by comparing it with the variables of normal operating conditions.

4 Results and discussion 4.1 ANOVA table Table 5 shows the ANOVA table for NOx emission for the tested OA. It can be seen that the percentage contribution of injection timing (53.5%) is higher than that of EGR (36.9%). Hence, when compared with EGR, injection timing is the most influencing factor in controlling the NOx emission of CRBO blend. It can also be seen that the calculated F-value of all the factors are higher than the tabulated Fvalue which shows the significance of all the three factors in con-

trolling the NOx emission of CRBO blend. It is well known that modification in the fuel injection timing and recycling a portion of exhaust gases will alter the maximum gas temperature attained in the cylinder. Since the rate of NOx formation is a function of combustion temperature, fuel injection timing, and percentage EGR plays a major role in NOx control as obtained through testing and ANOVA. It is also evident from Tab. 5 that fuel injection pressure has significance on the NOx emission of CRBO blend and comparatively it has less effect since its percentage contribution is much lower when compared with the other two. Results of ANOVA for smoke density are given in Tab. 6. It can be seen that fuel injection pressure is the most influencing factor in controlling the smoke density of the engine fuelled with CRBO blend. It has more significance on the smoke density since its Fcalculated (12.80) is higher than Ftable (2.98) and its percentage contribution (36.0%) is higher than the other two factors. I.C.Engine combustion process is affected by fuel atomization and vaporization which has an effect on the smoke density. Since fuel atomization and vaporization are influenced by fuel injection pressure, it is the most influencing factor in controlling the smoke density. It can also be seen that all the three factors have significance in controlling the smoke density of CRBO blend since their Fcalculated is higher than Ftable. As discussed in the NOx emission, change in fuel injection timing and EGR alters the maximum gas temperature attained and also causes an effect on the smoke emission. From the table it can be observed that injection timing has least significance on smoke emission when compared with EGR and injection pressure since its percentage contribution is lesser than that of the other two. Table 7 shows the effect of the selected factors on efficiency. From the table it is observed that fuel injection pressure (29.1%) is more influential than the other two factors, i.e., injection timing and injection pressure. It is also observed that all the three factors have no effect on the efficiency since their Fcalculted value is less than that of Ftable.

4.2 Response graph Figures 2 and 3 show the response graphs for NOx emission and smoke density, respectively, for the chosen factors at various levels. It

Table 5. ANOVA Table for NOx emission

SS

DF

MSF

Fcal

Ftab

P-value

61 424.22 42 315.74 5399.636 5639.369 114 779

2 2 2 11 17

30 712.11 21 157.87 2699.818 512.6699 6751.704

59.9062109 41.2699695 5.26619133

2.98 2.98 2.98

0.535152 0.368672 0.047044 0.049132

Factor Injection timing (SSf) Percentage EGR (SSf) Injection pressure (SSf) Error (SSe) Total (SST)

Table 6. ANOVA table for smoke density


SS

DF

MSF

Fcal

Ftab

P-value

1496.116 2114.302 2678.996 1150.676 7440.089

2 2 2 11 17

748.0578 1057.151 1339.498 104.6069

7.15113441 10.1059436 12.8050652

2.98 2.98 2.98

0.201088 0.284177 0.360076 0.154659





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Table 7. ANOVA table for BTE

SS

DF

MSF

Fcal

Ftab

P-value

0.025051 0.144554 0.612113 1.832674 2.614392

2 2 2 11 17

0.012526 0.072277 0.306056 0.166607 0.153788

0.07518075 0.43381775 1.83699927

2.98 2.98 2.98

0.009582 0.055292 0.234132 0.700994


Figure 4. Response graph for BTE. Figure 2. Response graph for NOx emission.

Figure 3. Response graph for smoke density.

can be seen that 3-3-1 combination (third level in the injection timing, third level in the percentage EGR, and first level in the injection pressure) gives lesser NOx emission. It can also be seen that 2-1-2 combination shows a lesser smoke density. Optimum combination of factors for lower NOx emission, with less increase in smoke emission has to be chosen from these two combinations. It was observed from the Tab. 5 that injection timing is the most influencing factor for NOx emission when compared with EGR

and injection pressure. It was also observed from Tab. 6 that the injection timing is less significant in controlling the smoke density when compared with EGR and injection pressure. From Tabs. 5 and 6 it was inferred that the optimum level of injection timing has to be chosen based on lesser NOx emission only and the levels of EGR and injection pressure have to be chosen by taking into consideration the smoke density also. Hence the third level of injection timing can be taken as the optimum level for NOx emission. Tab. 6 shows that injection pressure is the most influencing factor for smoke density next to EGR and their levels have to be chosen based on the lesser smoke density. From the response graph it can be seen that first level of EGR and second level of injection pressure shows lower smoke density. From the results of ANOVA and response graphs, the combination 3-1-2 is chosen as the optimum one to reduce the NOx with minimum increase in smoke density. Response graph for efficiency at various levels of the chosen factors is shown in Fig. 4. It can be seen that the combination 3-2-2 gives higher efficiency when compared with the other levels of factors. It can also be seen from Tab. 7 that fuel injection pressure is the most influencing factor for efficiency when compared with injection timing and EGR and the percentage contribution of injection timing and EGR are much lower than the injection pressure. Hence the optimum level of injection pressure has to be chosen by taking into consideration the efficiency. From Fig. 4 it can be observed that second level of injection pressure gives higher

Table 8. Effect of optimization on response variables

Fuel

RB blend Diesel

Normal operating condition

Optimized condition

% Change

NOx (ppm)

Smoke (mg/m3)

BTE (%)

Combination

NOx (ppm)

Smoke (mg/m3)

BTE (%)

NOx (decrease)

Smoke (increase)

BTE (decrease)

689.8 671.2

53.2 71.8

17.49 16.77

3-1-2

550

60

17.1

20.27

12.78

2.24



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efficiency when compared with other two levels and that level is chosen as the optimum level for fuel injection pressure. Since the injection timing and EGR shows least significance on efficiency, their effect can be eliminated. It was also observed from Fig. 3 that the second level of injection pressure shows less smoke density and chosen as optimum level for lower NOx emission with minimum increase in smoke density. Hence the combination 3-1-2 can be considered as the optimum one for lower NOx and smoke emission with maximum efficiency for CRBO blend.


[2]

[3] [4]

[5]

4.3 Confirmation experiment Table 8 shows the comparison of response variables at predicted optimum combination with those of the normal operating condition. It can be seen that the combination 3-1-2 shows a reduction in NOx emission with minimum increase and decrease in smoke density and efficiency, respectively. As the injection timing is retarded, the start of combustion and hence the combustion process also retards which retards the occurrence of peak pressure. This decreases the peak combustion temperature attained in the cylinder and hence the NOx formation. Higher smoke density obtained as a result of retarded fuel injection was minimized by increasing the fuel injection pressure. Since the exhaust gases are not recycled it will not have any effect on smoke density. Hence, retarded injection timing, without EGR with 220–230 bar injection pressure is the optimum combination for lower NOx emission with lower smoke density and higher efficiency for CRBO blend.

[6]

[7]

[8] [9] [10] [11]

[12]

5 Conclusion In the present work ANOVA was employed to obtain the most influencing factor in controlling the NOx emission, smoke density, and efficiency of CRBO blend as a stationary CI engine fuel. Optimum combination of injection timing, percentage EGR and fuel injection pressure in reducing the NOx emission with lesser effect on smoke density and efficiency was arrived through ANOVA and response graph. From the experimental results and ANOVA, the following conclusions are drawn: (i) Combination of injection timing, EGR, and injection pressure can reduce the NOx emission with a trade-off on smoke and efficiency. (ii) Reduction in NOx emission is independent of a single factor and also depends upon the levels of the other factors. (iii) Fuel injection timing is the most influencing factor in reducing the NOx emission of CRBO blend. (iv) Fuel injection pressure is the most influencing factor in controlling the smoke density of the engine fuelled with CRBO blend. (v) Retarded injection timing without EGR with 220–230 bar injection pressure is the optimum combination for controlling the NOx emission with lesser effect on smoke density and efficiency.

[13]

[14] [15]

[16]

[17]

[18]

[19]

[20]

[21]

[22]

The authors have declared no conflict of interest. [23]

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