The Experimental and Simulations Effect of Air Swirler on Pollutants from Biodiesel Combustion

In the present study the effect of air swirl on the combustion characteristics and pollutant emission of biodiesel B5, B10 and gasoil combustion is studied. The experiments are carried out on an axisymmetric cylindrical combustion chamber. Numerical investigation is conducted using fluent computer code. The RNG, k-ɛ model is used for the modelling of the turbulence phenomena in the combustion chamber .The eddy dissipation model is used for the simulation of transport combustion. The experimental and numerical result show that the exhaust gas temperature, the levels of NOx and CO2 emission increase with increase of swirl number and then decrease. The CO emission declines with increase of swirl number. The numerical and experimental results are in good agreement.


INTRODUCTION
Industrial development and social growth and as a result steep rise for the demand of fossil fuels in one hand and on the other hand pollutant emission of petroleum-based fuels and its effects on environment has led to numerous investigation on alternative fuels which can be produced from local resources within the country.Biofuels are a broad range of fuels which are derived from biomass.The term covers solid biomass, liquid fuels and various biogases including charcoal, vegetable oil, bioethers, biodiesel, syngases, etc. Biodiesel refers to mono alkyl esters of long-chain fatty acids derived from the transesterification of vegetable oil or animal fat feedstock, for use in liquid burners and diesel engines as fuel (Panwar et al., 2010;Skoglund et al., 2010).A by-product of the transesterification process is the production of glycerol which can be used in pharmaceutical and personal care applications.Properties of biodiesel are similar to common diesel fuel.The main difference between biodiesel and diesel fuel is oxygen content, which is biodiesel contains 10-12 weight percentage oxygen which has improved its performance attributes such as increased cetane number and high fuel lubricating value.However, the calorific value of biodiesel is lower than regular diesel fuel.Biodiesel have negligible sulfur and ash content, so sulfur dioxide emission and toxic pollutants of biodiesel is less than diesel fuel.Many researches have been carried out on biodiesel and its characteristics.The behavior of biodiesel in internal combustion engines is well documented in the literatures (Senatore et al., 2000;Roska et al., 2005;Agarwal, 2007;Shi et al., 2005Shi et al., , 2006;;Sharp et al., 2000Sharp et al., , 2005;;Lapuerta et al., 2008;Monyem et al., 2001;Nabi et al., 2006;Laforgia and Ardito, 1995), but a few studies are conducted on the behavior of biodiesel in liquid burners and furnaces.Furthermore, the effects of different parameters such as air swirling flow on biodiesel combustion is not studied completely.Swirling flows are applied in a wide range of application both non-reacting and reacting system (Laforgia and Ardito, 1995).
In combustion systems, it is used in various systems such as industrial furnaces, utility burners, gas turbines, internal combustion engines and many other practical heating devices, in order to enhance mixing and improve combustion and its characteristics (Lilley, 1977;Syred and Beer, 1974).Swirling flows affect flame shape, flame size, stability and combustion intensity by formation of secondary recirculating flows (Chen and Driscoll, 1988).A study by Krishna (2003) examined the effect of replacing heating oil with biodiesel blends in residential heating equipment and commercial boilers.Here, CO emissions for blends of soy methyl ester with No. 2 fuel oil were found to be comparable to that of the fuel oil at fixed fuel pump pressure and at various flue gas oxygen levels.Most significantly, NOx levels reduced as the percentage of biodiesel in the fuel blends was raised.Hoon and Suyin (2010) evaluated levels of exhaust species from the  Their results indicated an improvement in combustion and the potential use of palm oil biodiesels in smallscale liquid fuel burners.Datta and Som (1999) investigated combustion and emission characteristics in a gas turbine combustor at different pressure and swirl conditions.They reported that an increase in swirl number reduces the NO x emission level at all combustor pressures.However, though at lower pressure and increase in swirl number decreases combustion efficiency, the trend is exactly the reverse at higher pressure.The effect of swirl on combustion dynamics in a lean-premixed swirl-stabilized combustor is studied by Huang and Yang (2009).They found out that a high swirl number tends to increase the turbulence intensity and the flame speed and consequently shorten the flame length.However, excessive swirl often causes the central recirculating flow to enter into the inlet annulus and leads to the occurrence of flame flashback.Bashirnezhad et al. (2007) studies the effect of fuel angles and air swirling flow on soot formation.They showed that the maximum temperature of flame has increased with increase of swirl number and remained constant with further increase of swirl number.The aim of this study is measuring regulated emissions such as CO, CO 2, NO x and flame temperature from boiler fueled with biodiesel and gasoil at different swirl number.

Experimental setup:
The laboratory furnace which is used in this research includes a horizontal cylinder to the length of 170 cm and diameter of 50 cm.On the rims of furnace some orifices in different spaces from burner nozzle have been made for measuring temperature and sampling combustion gases.The liquid burner is a pressure jet oil burner with 400 KW maximum power rating.The fuel is supplied to the nozzle at pressure of 12 bars.The fuel and air flow rate are adjustable using the oil pump and the burner air valve, respectively.A variable swirl burner provides near-burner zone high mixing rates of air and fuel.Fuel is injected to the furnace through a 60° hollow-cone nozzle.A K Type thermocouple, which stands high temperatures, is used to provide temperature measurements within the furnace.The thermocouple is directly coupled with a voltmeter which shows the temperature in Celsius. Figure 1 shows the laboratory combustion chamber.Table 1 compares the properties of diesel fuel and biodiesel that is used as fuel.
In order to produce swirling flow, five air swirlers having different vane angles are applied.They are made from mild steel.The inner diameter of air swirlers is 20 mm and their outer diameter is 60mm.The vane angles of air swirlers are 0°, 30°, 45°, 60° and 75° that their swirl number are 0, 0.42, 0.72, 1.25 and 2.7 respectively.Figure 2 demonstrates the swirlers.All of the measurements are made after stabilization of furnace temperature.During the experiments, the air and input fuel temperatures have been controlled and maintained.The sampling device has been placed in the stack for analyzing exhaust gases, on the 160cm from furnace vent.The gas stream produced from combustion like CO, CO 2 and NO x is registered each 5 minutes using a Testo 350 XL for analyzing gas.Table 2 demonstrates a list of instruments and their specifications.

Numerical simulation:
The continuity, momentum and energy equations in cylindrical coordinates can be expressed in the form of general form of governing equation: where, φ is a dependent variable and can be mass, momentum, Turbulence kinetic energy and its dissipation rate and enthalpy.Г φ is the diffusion coefficient and S φ is the source term.
Swirl flow affects the combustion and the characteristics of flame by enhancing the mixing of fuel where, NO x Post-processing: In the present study, thermal-NO x and prompt NO x is considered which their modeling is carried out in the post-processing stage.A single transport equation for mean NO mass fraction with a source term is solved after obtaining converged solution for the flow and mixing field through the turbulent flow calculations: The kinetics of the thermal NOx formation rate is much slower than the main hydrocarbon oxidation rate and most of the thermal NO x is formed after completion of combustion.Therefore, the thermal NO formation process can often be decoupled from the main combustion reaction mechanism and the NO formation rate can be calculated by assuming equilibration or partial equilibration of the combustion reactions.
The thermal NO x is formed by the oxidation of atmospheric nitrogen at high temperatures and prompt NO x is formed by reactions of intermediate species at the flame front (Ilbas et al., 2005).The principal reactions governing the formation of thermal NOx from molecular nitrogen are given by the extended Zeldovich mechanism (Zeldovich, 1947): k1, k2 and k3 are the rate constants for the forward reactions 11-13 respectively and k-1, k-2 and k-3 are the corresponding reverse rates.The overall thermal NO formation rate can be calculated as: The concentration of O and OH is given by: The prompt NO x formation is significant in most hydrocarbon fuel combustion conditions and the prompt NO x route is generally accepted as: The prompt NO formation rate is calculated from the (De Soete, 1975) global model as:

Table 1 :
Properties of biodiesel and diesel fuel combustion of Palm Oil Methyl Ester (POME) and its blends with No. 2 diesel in a non-pressurized, watercooled combustion chamber.They explored the correlations between emission species and fuel pumping pressures over a range of equivalence ratios.

Table 2 :
List of measurement instruments and their specifications