The co-annular laminar diffusion flame burner used in the present study is based on the design of Santoro et al. Figure 3.1 shows the design of the burner that generates this flame. Ethylene (C2H4) was chosen as the fuel for its high sooting tendency.
The coflow diffusion flame burner consists of an upright stainless tube (11.1 mm i.d.) which is used as a fuel passage, surrounded by an air-coflow annulus (i.d. 101.6mm). The fuel tube protrudes 4mm out of the coflow exit plane, so that the small jets generated by the perforated plate can relax to plug flow. The length of the fuel tube (12 inches in length) was sufficient to obtain the fully developed laminar pipe flow for lower fuel flow rates.
The velocity profile in the coflow is homogenized by a settling chamber 10 cm high filled with 3 mm glass beads, and by using a perforated plate at the exit plane to increase the pressure drop. The glass beads fit into the burner cup and are not visible from the face of the burner as they are under the ceramic honeycomb.
The flame is enclosed in a quartz chimney, with a diameter that matches the outer diameter of the burner to shield it from laboratory air currents. A Teflon cap is used to enclose the chimney from top.
The air and the fuel lines are heated so that the temperature of fuel and air entering the burner is 35°C and hence nullifying any effect of the change in room temperature and to help the dopants vaporize and to keep them from subsequently condensing. The same heating was used for all dopants to maintain a uniform initial temperature for the fuel mixture entering the flame. There is a provision for introducing additives in the fuel line after the mass flow controller through a port. The combustion products formed from a laminar diffusion flame are drawn through the system by maintaining a steady vacuum using a high flow high vacuum pump. The laminar diffusion flame allows combustion products to be generated with a high level of reproducibility.
Figure 3.1 Laminar Diffusion Flame Burner
3.1 Experimental set up:
The Experimental set up consists of two lines: Main or Primary line and By-pass or Secondary line. The Primary line consists of a dual chamber reactor assembly (Figure 3.2). The first chamber of this reactor assembly houses a laminar diffusion flame burner.
The products are drawn through a sampling probe placed at a height of 5 cm above the flame tip. The sampling probe is comprised of a sintered metal porous tube (1.25 inch. i.d.) housed in a secondary stainless steel tube. The secondary tube allows a positive flow of nitrogen gas (3 L/min) to be introduced through the porous tube and thereby minimizes wall deposition, serves as a carrier gas and helps control the temperature of products entering the post-combustion reactor. A uniform internal diameter is maintained from the sampling probe to the post-combustion zone reactor to minimize wall deposition and maintain a uniform flow distribution. Connected at the top of the sampling probe is a junction probe from which the secondary or the by-pass line branches out. The junction probe has two doors: a slant sliding door and a horizontal sliding door to control the flow of primary line and secondary line respectively.
Cooled
N
2
Fuel
Air
Air
Quartz Chimney
Teflon Cap
Condenser
Flow meter
Pressure Gauge
Control Valve
Vacuum Pump
Sampling Probe
Condenser
XAD Trap
Brass Mesh
Filter
Slider
BY
-
PASS LINE
PRIMARY LINE
Flow Separator
Figure 3.2. Schematic of the Experimental Set-up
Table 3.1. Gas Parameters
Ethylene
Air
Nitrogen
Head pressure(psi)
40
60
20
Flow
260 (mL/min)
45 (L/min)
3 (L/min)
The second chamber is operated at cool zone conditions (temperatures < 700°C, residence time 2.0 to 6.0 sec). An in-line filter assembly after the cool zone reactor, connected at the top of the junction probe, captures the particles formed from combustion before capturing the gaseous effluents on the Amberlite XAD-2 trap after passing through a condenser. The length of the XAD-2 was maintained at ~7cm length since it is found that a 5-7.5 cm deep XAD-2 bed was sufficient for the collection of three and four ring PAHs with high efficiency even at high flow rates [65]. Prior to its use the XAD-2 is extracted by Soxhlet extraction for 24 hours to remove any impurities and stored in an air tight container. A unique advantage of this reactor system is that the reactor in the second chamber can be a 3.5 cm I.D. fused-silica reactor used to investigate reactions in the gas-quench region, a fixed bed reactor used to investigate gas-surface reactions, or a quartz reactor with entrained particles (based on a design of the Gullett Group, EPA-RTP Lab; to study reactions of particles in flue gas. This flow reactor system provides highly-controlled thermal environments. After particle separation, the resulting effluents from the second chamber are trapped then extracted and analyzed using GC-MS. There is also a provision for direct in-line GC-MS analysis using on-column cryo trapping. This flow reactor system has been used to investigate the formation of several high molecular weight pollutants. This reactor system has heated transfer lines and has been designed to ensure quantitative transport of high molecular weight products. The Secondary or the By-pass line is similar to the primary line's second chamber and is used only to pre-heat the sampling probe to thermal equilibrium.
The flow rates of all the gases were measured using calibrated mass flow meters and monitored on a display module. Both the lines i.e. primary and by-pass lines are controlled by separate flow regulating valves and are connected to a main flow regulating junction valve. After the junction valve the line passed through a condenser to cool the out flowing gases into the flow meter. The line from the flow meter is then connected to a pressure gauge and finally connected to the pump through a main flow regulating valve.
All the ice baths are filled regularly with water and crushed ice with salt added on the top surface of the crushed ice to maintain the cold temperature for longer periods.
Air
Ethylene
Additives
Diffusion
Flame
Post-combustion
Reactor
Particle Separator
XAD
Trap
Pump
GC-MS
Figure 3.3. Overall Schematic of experimental layout
3.2 Baseline Experiment: (No additives)
The baseline experiments are carried on with ethylene fuel to analyze the amount of soot and the PAHs produced. It is carried at thermal equilibrium of the system. To bring the system into thermal equilibrium sampling is done on the secondary or by-pass line by closing the slant sliding door and opening the horizontal sliding door. The individual flow regulating valves of primary and the secondary lines are closed and opened respectively.
For the diffusion flame, Air flow is maintained at 45 L/min (with a head pressure of 60 psi) and the ethylene flow at 260 ml/min (with a head pressure of 40 psi) while the Nitrogen flow through the sampling probe is maintained at 3 L/min (with a head pressure of 20 psi).
The products of combustion are drawn by the pump at a rate of 50 L/min by adjusting the valve in the by-pass line. The junction valve is always fully opened. Due to the accumulation of the soot on the by-pass filter the flow decreases, so the flow is monitored continuously and adjusted back to 50 L/min. Sampling is done on the by-pass line till the system reaches thermal equilibrium. As soon as the system reaches thermal equilibrium, the flow is directed to the primary line by opening the slant sliding door and the primary line flow regulating valve while shutting off the horizontal sliding door and the by-pass line flow regulating valve.
Sampling is continued on the primary line for one hour. Throughout the sampling, the flow is adjusted to 50 L/min as the pressure and the flow drops due to the accumulation of the soot on the filter of the in-line filter assembly. After one hour of sampling on the primary line, the flow is switched back to the by-pass line and shutting of the primary line's slant sliding door and valve and opening the secondary line's horizontal sliding door and valve and sampled until thermal equilibrium is attained. Meanwhile, the filter of the primary line is replaced with another baked filter.
When the by-pass line reached thermal equilibrium again, the flow is directed towards the primary line as explained before. In this way sampling on the primary line is done for four hours using four different baked filters and the same XAD trap. The XAD is extracted using soxhlet extraction to analyze for PAHs. The collected filters were then stored in aluminum pans and closed to prevent any atmospheric deposition or condensation. The filters were pre-cleaned by baking them at 4200C to remove any organics from the surface of the filters that could contribute to a bias reading from oxidation of surface contaminants during carbon analysis. The mass of the collected particle samples was determined using a Leco Model RC 412 Multiphase Carbon Determinator (MCD) that thermally desorbs the samples from ambient temperature to 10000C.
3.3 Additive addition setup
Additives are added to the fuel stream to evaluate their effects on soot formation as well as the PAHs. Candidate aromatic and aliphatic additives include Benzene, Bromo-Benzene, Chloro-Benzene, Bromo-Butane, Chloro-Butane, mixture of Bromo-Benzene and Chloro-Benzene (in the ratio 1:2.2) and mixture of Bromo-Butane and Chloro-Benzene (in the ratio 1:2.2). The additives are injected from a syringe pump directly into the fuel stream after the fuel passes through the mass flow controller. The fuel line is heated to 350C by heating tapes throughout its length from the cylinder to the inlet of the burner to nullify the effect of the changes in the room temperature.
Mass Flow Controller for Fuel
Syringe Pump
Additive
To Burner From Cylinder
Fig 3.4 Additive Additional set-up: not to scale
