Inhibition of NO emission by adding antioxidant mixture in Jatropha biodiesel on the performance and emission characteristics of a C.I. engine

Abstract

In this paper, the effect of adding an antioxidant mixture in Jatropha biodiesel as fuel, in a single cylinder, direct injection compression ignition engine was experimentally investigated and the level of pollutants in the exhaust and performance characteristics of the engine were analyzed. Nine test fuels were prepared with three antioxidants, namely, Succinimide (C₄H₅NO₂), N,N-dimethyl-p-phenylenediamine-dihydrochloride (C₈H₁₄Cl₂N₂), and N-phenyl-p-phenylenediamine (C₆H₅NHC₆H₄NH₂) added to neat biodiesel at 500 parts per million (ppm), 1000 ppm and 2000 ppm and the observed experimental results were compared with those of neat biodiesel and neat diesel as base fuels. The comparison showed that NO emission was reduced drastically for the test fuels with the antioxidant addition of 2000 ppm. The maximum reduction of 10% of NO emission was observed for the antioxidant mixture in neat biodiesel, with a slight increase in unburned HC, CO and smoke opacity. In addition, the obtained experimental results reveal that the addition of two antioxidants as mixture in neat biodiesel caused improved NO emission reduction for all test fuels.

Keywords

NO emission / antioxidants / Succinimide / N,N-dimethyl-p-phenylenediamine-dihydrochloride / N-phenyl-p-phenylenediamine

Introduction

Biodiesel engines have the advantages of better fuel economy close to diesel and lower level of pollutants from the engine exhaust like hydrocarbons (HC) and carbon monoxide (CO) emission, when compared to diesel engines. The charming characteristics that biodiesel holds are lesser non-toxic emission [ 1], with a lower viscosity and a high vapor pressure [ 2]. However, the disadvantage of using biodiesel as fuel in compression ignition engines is the high level of oxides of nitrogen (NO_x) in the exhaust. Many studies have highlighted the increase in NO_x emission by using biodiesel as fuel in compression ignition diesel engines [ 3– 6]. To control such NO_x emission, many strategies have been proposed by the scientific community around the world who claim that the addition of antioxidants to biodiesel minimizes the NO_x emission and extends the storage stability of biodiesel [ 7, 8]. Antioxidants such as tert-butylhydroquinone (TBHQ), butylated hydroxytoluene (BHT), propyl gallate (PrG), butylated hydroxyanisole (BHA) and a-Tocoperol have been added separately to soybean biodiesel at 500 ppm (parts per million) and the oxidation stability efficiency has been found to be in the order of TBHQ>PrG>BHA>BHT>a-tocopherol [ 9]. An antioxidant added to biodiesel has shown an improvement in the oxidation stability of the biodiesel fuels such as Safflower [ 10], rapeseed [ 11], and jatropha biodiesel [ 12] which have been found to be stable for several months. Apart from longer storage stability of biodiesel, antioxidants have been used as additives in fuel to suppress the NO_x particularly for biodiesel fuels. The addition of antioxidants in biodiesel prevents the formation of a series of gas phase reactions and eliminates the formation of NO_x [ 13]. NO_x is formed by the reaction of nitrogen with hydrocarbon radicals [ 14] as shown in the equations below.

(1) {\mathrm{CH+N}}_2\leftrightarrow \mathrm{HCN+N}

(2) \mathrm{HCN+OH}\leftrightarrow {\mathrm{CN+H}}_2\mathrm{O}

(3) {\mathrm{CN+O}}_2\leftrightarrow \mathrm{NO+CO}

(4) {\mathrm{N+O}}_2\leftrightarrow \mathrm{NO+O}

Several studies have found that an antioxidant has the capability of reducing NO, which is discussed below. Studies on antioxidants [ 15] such as N,N-diphenyl-1,4-phenylenediamine (DPPD) and N-phenyl-1,4-phenylenediamine (NPPD) blended with soybean biodiesel separately at 1000 ppm and 2000 ppm (parts per million) have shown reduced NO_x emission with an increase in the smoke, CO and HC emission. The effect of antioxidants namely p-phenyle-nediamine, Ethylene-diamine, a-Tocopherol acetate, Butylated hydroxytoluene and L-ascorbic acid added separately at 0.005%, 0.015%, 0.025%, 0.035% and 0.050% volume in Jatropha biodiesel has been investigated [ 16] and NO reduction efficiency has been found in the order of p-phenylenediamine, ethylenediamine, a-tocopherol, butylated hydroxytoluene, ascorbic acid and neat biodiesel. The addition of antioxidants not only diminishes the NO_x during combustion, but also shows the effectual step-down of brake specific fuel consumption [ 9] with antioxidant additions such as TBHQ, BHA, BHT, PrG and the natural antioxidant tocopherol in soybean biodiesel. The addition of antioxidants namely, 1, 2, 3 tri-hydroxy benzene (Pyrogallol), 3, 4, 5-tri hydroxyl benzoic acid (Propyl Gallate) and 2-tert butyl-4-methoxy phenol in (Butylated Hydroxyanisole) croton megalocarpus biodiesel have resulted in a good reduction of brake specific fuel consumption in a four cylinder turbocharged direct injection (TDI) diesel engine and the antioxidant effectiveness has been found to be in the order of Pyrogallol, Propyl Gallate and Butylated Hydroxyanisole [ 12]. Recent experimental investigation in a four cylinder inline diesel engine by using the antioxidant, DPPD with Jatropha blends, has shown the maximum reduction of NO_x emission by 16.54%, along with increased brake specific fuel consumption, HC and CO emission [ 17].

Many researchers have reported that the single addition of antioxidant in biodiesel reduces the level of pollutants in the exhaust specifically NO (nitric oxide) emission, whereas the experimental investigation with the combined effect of two antioxidant addition in neat biodiesel is limited. To upgrade the significant reduction of NO emission, the addition of two antioxidants in biodiesel is assayed over the single antioxidant addition in biodiesel. The choice of antioxidant as additive in biodiesel are selected based on the criteria of unused and rarely antioxidants in biodiesel. So, in this paper the effects of adding two antioxidants to the neat biodiesel at different ratios that affect the engine performance and exhaust emissions are investigated and the observed results are compared with those of neat diesel and neat biodiesel as base fuel, under brake mean effective pressure for the performance and emission characteristics.

Materials and methods

Test fuel standardization

Jatropha biodiesel was prepared by a two step transesterification process, and its properties were tested by standard ASTM methods. Three different antioxidant additives, Succinimide (C₄H₅NO₂), N,N-dimethyl-p-phenylenediamine-dihydrochloride (C₈H₁₄Cl₂N₂), and N-phenyl-p-phenylenediamine (C₆H₅NHC₆H₄NH₂) were selected for this study, and were added equally at dosing ratios of 500 ppm, 1000 ppm and 2000 ppm to neat biodiesel. The specification of test antioxidants is listed in Table 1. The detailed proportions of test fuels and its properties are shown in Tables 2 and 3, respectively.

Engine test

The schematic layout of the experimental setup is demonstrated in Fig. 1. The experimental investigations were conducted in a single cylinder air cooled diesel engine test rig equipped with an electrical loading device/AC alternator (Applied in terms of the brake mean effective pressure ranging from 0 MPa to 0.53 MPa at a constant speed of 1500 r/min, injection timing of 26° before top dead center and injection pressure of 215 bar). The detailed technical specifications are listed in Table 4. The engine was allowed to run using diesel for 20 min for the start and shut down of the engine. Before proceeding with the experiments, the engine was run with test fuels for 30 min in order to stabilize the fuel and to consume the previous fuel used in the fuel injection system. All the tests were conducted three times to ensure the reliability and the uncertainties of calculated. The measured data are tabulated in Table 5. An exhaust gas analyzer, model AVL 444 Di-gas analyzer, was used for measuring the level of pollutants from the engine exhaust and a smoke meter, model AVL 437 smoke meter was used for measuring the smoke opacity level of the engine exhaust.

Results and discussion

Effect of antioxidants on the performance characteristics

Figure 2 shows the effect of the test fuels on the brake specific fuel consumption (BSFC) and brake thermal efficiency (BTE) under brake mean effective pressure (BMEP). It is observed that the brake specific fuel consumption decreases with an increase in load for all the test fuels, whereas the brake thermal efficiency of the test fuels increases with an increase in load, due to lower calorific value results in more consumption of the fuel to make the engine run. The brake thermal efficiency and brake specific fuel consumption observed for neat diesel and neat biodiesel are 32.5% and 0.263 kg/kWh, and 28.6% and 0.318 kg/kWh, respectively, where a lower brake thermal efficiency and higher brake specific fuel consumption are observed for neat biodiesel than for neat diesel, causing a larger amount of fuel to be burned in the premixed combustion stage [ 18]. The rich oxygen molecule present in biodiesel helps in the easier combustion of the fuel, whereas for the test fuel due to the quenching character of the antioxidant, a lower brake thermal efficiency and higher brake specific fuel consumption are observed. Lower brake thermal efficiency and higher brake specific fuel consumption values are observed for all test fuels DN1, DN2, DN3, SD1, SD2, SD3, SN1, SN2 and SN3 as 27.6% and 0.412 kg/kWh, 27% and 0.428 kg/kWh, 26.12% and 0.432 kg/kWh, 27.5% and 0.413 kg/kWh, 26.8% and 0.421 kg/kWh, 26.12% and 0.432 kg/kWh, 27.2% and 0.421 kg/kWh, 26.7% and 0.432 kg/kWh and 25.2% and 0.441 kg/kWh, respectively. The reduction in brake thermal efficiency for the test fuels is mainly caused by the suppression of the oxygen molecule in biodiesel by the addition of antioxidants.

Effects of antioxidants on emission characteristics

Figure 3 shows the effect of test fuels on NO under BMEP. In general, NO emissions are formed at high combustion temperatures, in the rich local oxygen region inside the engine cylinder. The rich oxygen content of the fuel increases the oxidation reaction at higher combustion temperatures and results in a higher NO_x emission called thermal NO_x. It can be witnessed with NO emission values of neat diesel as 1320 ppm and neat biodiesel as 1390 ppm due to the increased temperature inside the combustion chamber [ 19]. On the other hand, the higher amount of oxygen present in neat biodiesel improves the combustion of the fuel, and results in lesser HC, CO and smoke emission. The higher viscosity and oxygen content of neat biodiesel increases the exhaust gas temperature (EGT) for neat biodiesel to 339°C than that of neat diesel of 325°C. Figure 4 depicts the effect of test fuels on EGT under BMEP. During the oxidation process, the peroxyl and hydrogen peroxide radicals that are present in biodiesel absorbs heat during combustion and gets converted into hydrocarbon radicals [ 20]. The nitrogen present in air combines with the hydrocarbon radicals [ 17, 21], and forms a series of gas phase reactions by forming NO emission [ 14]. But antioxidant has the ability of quenching the free radicals present in neat biodiesel, which are responsible for the formation of NO emission [ 15, 17]. From the results, it is observed that the NO emission decreases with an increase in the parts per million additions of the antioxidants for the test fuels and the maximum reductions of NO emission observed for the test fuels are in the order of SN3>SN2>SD3>DN3. At the rated load of the engine, the maximum NO reduction percentage is 10% less for SN3, 9.3% less for SN2, 8.9% less for SD3 and 7.9% less for DN3 compared to neat biodiesel of 1390 ppm. The observed NO values for the test fuels such as SN3, SN2, SD3 and DN3 are 1250 ppm, 1260 ppm, 1265 ppm and 1280 ppm respectively, and the corresponding values of exhaust gas temperature observed for SN3, SN2, SD3 and DN3 are 311°C, 313°C, 314°C and 317°C respectively, which registers the maximum reduction of exhaust gas temperatures for the test fuels. A good reduction of NO emission and the exhaust gas temperature are observed for the test fuels as 1285 ppm and 329°C for SN1, 1305 ppm and 322°C for DN2 and 1320 ppm and 325°C for SD2 test fuels respectively, which are less than the NO emission of neat diesel by 1320 ppm and 325°C. The SN3 test fuel shows the maximum reduction of NO emission among the test fuels, which is due to the suppression of the oxygen molecules, nitric oxides, superoxide ions and hydroxyl radicals, by the addition of antioxidants [ 15].

Figure 5 displays the effect of test fuels on CO under BMEP. CO emissions are formed due to the formation of improper fuel rich regions, resulting in incomplete combustion [ 22]. It is observed that the CO emission for neat biodiesel is lower than that of neat diesel, because the rich oxygen content in biodiesel results in a complete combustion with reduced CO emissions. At lower engine loads, reduced CO emission values are observed for the test fuels because the easier mixing of the air fuel mixture eliminates the formation of rich fuel regions which results in better combustion. But for the test fuels running at rated load, the availability of the oxygen molecule will be insufficient for complete combustion. Moreover, the antioxidant addition to the neat biodiesel bypasses the formations of the radicals and reduces the degree of formation of CO emission. Among the test fuels, 2000 ppm addition of antioxidants to the fuel increases the CO emission by 0.07% vol for SN3, 0.07% vol for DN3 and 0.06% vol for SD3, whereas for neat biodiesel and neat diesel it is 0.05% and 0.09% vol respectively. The higher viscosity of the test fuels results in poor atomization, leading to the formation of local rich mixtures inside the engine cylinder during combustion (i.e poor mixing of a homogeneous mixture), resulting in increased CO emission [ 23].

Unburned hydrocarbon emissions (HC) are formed by the incomplete combustion of the fuel, occurring mostly at the crevices of the engine cylinder. They are also formed by the partial oxidation of the air fuel mixture during combustion [ 24]. Figure 6 demonstrates the effect of test fuels on unburned HC under BMEP. It is observed that the test fuels emit higher unburned hydrocarbon emissions as compared to neat biodiesel at rated loads, whereas at low and medium engine loads, the unburned HC emissions for the test fuels are slightly lower than that of neat biodiesel. The test fuels that shows higher unburned HC emission at rated load are 26 ppm for both DN3 and SN3, 25 ppm for SN2 and 24 ppm for DN2, besides a marginal increase in unburned HC emissions for the test fuels SD2 as 19 ppm, SN2 as 20 ppm and SD1 as 21 ppm, when compared with the unburned HC emission for neat diesel and neat biodiesel by 25 ppm and 18 ppm, respectively. The additions of antioxidants in neat biodiesel increase the unburned hydrocarbon emission by the reduction of oxidative OH radical formation in the test fuels.

Figure 7 exhibits the effect of test fuels on smoke opacity under BMEP. It is observed that the smoke opacity for the test fuels increases with an increase in the loads, due to the consumption of more fuel in the diffusion phase. Under rated load conditions, the smoke opacity for neat biodiesel (37.6%) is lower than that of neat diesel (43.5%), because the plenteous oxygen molecules available for the improved combustion lower the smoke emission [ 25]. But for the test fuels, increased smoke opacity is observed for DN3, SD3, SD2, SN3, SN2 and DN2 as 48.8%, 50.5%, 43.5%, 50.2%, 44.6% and 45.5% respectively, due to the delay in the rate of oxidation of the free radicals with the hydroxyl group [26].

Conclusions

The following conclusions are drawn based on the experimental results:

Drastic reduction of NO emission is observed for SN (Succinimide and N,N-dimethyl-p-phenylenediamine-dihydrochloride) test fuels compared with the other test fuels. The maximum NO emission reduction percentage of 10%, 9.3%, 8.9% and 7.9% is observed for SN3, SN2 SD3 and DN3 test fuels, besides good reduction of NO emission for the test fuels, SN1, DN2 and SD2 of 7.5%, 6.1% and 5% respectively. Due to the lower calorific value and higher viscosity of the test fuels, the brake thermal efficiency of the fuel decreases with increased fuel consumption and the exhaust gas emissions such as smoke opacity, CO and unburned HC increase for the test fuels, with antioxidant addition greater than 1000 ppm. The addition of antioxidants in neat biodiesel holds well for the reduction of NO emission, whereas smoke opacity, CO and unburned HC increases on antioxidant addition at 2000 ppm.

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