Energy Fuels 2010, 24, 288–294 Published on Web 10/29/2009
: DOI:10.1021/ef9007888
Low-Emission Premixed Porous Inert Media (PIM) Burner System Fueled with Vegetable (Rapeseed) Oil Using a Flow Velocity Flame Stabilization Technique )
Ayman Bakry,† A. Al-Salaymeh,‡ Ala H. Al-Muhtaseb,*,§ A. Abu-Jrai,§ D. Trimis, and F. Durst^
)
† Mechanical Engineering Department, Faculty of Engineering and Technology, University of Tanta, Tanta, Egypt, ‡Mechanical Engineering Department, Faculty of Engineering and Technology, University of Jordan, Amman, Jordan, §Department of Chemical Engineering, Faculty of Engineering, Al-Hussein Bin Talal University, Ma’an, Jordan, Institute of Thermal Engineering, Technische Universit€ at Bergakademie Freiberg, D-09599 Freiberg, Germany, and ^Institute of Fluid Mechanics, Friedrich-Alexander-Universit€ at Erlangen-N€ urnberg, 91058 Erlangen, Germany
Received July 24, 2009. Revised Manuscript Received October 1, 2009
One of the main targets of all current combustion systems, in addition to high efficiency and robust performance, is the ability to meet current and future pollutant emission regulations. In the present work, the suitability of vegetable (rapeseed) oil as a renewable, biodegradable, and environmentally friendly source of energy to operate porous inert medium (PIM) atmospheric burners was examined. A prototype integrated vaporizer system with a PIM burner based on the flow velocity flame stabilization technique was built and experimentally investigated. The operation was based on using two vaporizers working in a mutual mode, so that one of the vaporizers supplies the burner with the oil vapor while the other is cleaned. The PIM burner was successfully tested at power values from 5 to 20 kW, which correspond to a power modulation range of 1:6. Using a special oxidation cleaning technique, a free-residual vaporizer has been obtained. Qualitatively, a very good homogeneous and stable combustion shape across the burner was obtained. During the whole experiment, CO emissions recorded a zero value continuously for the whole range of power and relative air ratio tested. Furthermore, no flashback condition or smoke or soot formation was noticed during operation. The results of NOx and CO emission levels prove the high degree of repeatability and reliability of combustion with this new combustion technique in PIM burners. These results prove an excellent emission performance with respect to environmental pollution legislation.
industrial and residential applications have been proposed, e.g., household water heating systems,4 radiant burners,5-8 and burning organic pollutants with extremely low heat content.9,10 However, the depletion of conventional petroleum resources as a major source of energy in combustion systems motivates the use of new alternative renewable, biodegradable, and environmentally friendly sources of energy to replace the conventional sources.11 Vegetable oils and their derivatives (especially methyl esters), commonly referred to as “biofuel”, are the most prominent candidates of renewable energy resources to replace petroleum fuels. They have advanced from being purely experimental fuels to the initial stages of commercialization. Besides being a renewable and domestic resource, combustion of biofuel has a neutral effect on the global CO2 cycle and has reduced emissions compared
1. Introduction Recent research and development in the field of combustion system design have been motivated by the current and projected pollutant regulations to preserve a clean environment. Therefore, innovative concepts of combustion techniques have been proposed to replace the conventional and classical ones. Porous inert media (PIM) combustion has become increasingly important, specifically, from the viewpoints of technical interest and practical applicability.1,2 PIM combustion has many superior advantages over the conventional combustion processes, such as low NOx and CO emissions, wide variable dynamic power range, high power density, and high combustion stability.3 A diverse number of outstanding *To whom correspondence should be addressed. Telephone: þ962-77791-2255. Fax: þ962-3-217-9050. E-mail:
[email protected]. (1) Kamal, M. M.; Mohamad, A. A. Combustion in porous media. Proc. Inst. Mech. Eng., Part A 2006, 220, 487–508. (2) Mohamad, A. A. Combustion in porous media: Fundamentals and applications. In Transport Phenomena in Porous Media III; Ingham, D. B., Pop, I., Eds.; Elsevier: Amsterdam, The Netherlands, 2005. (3) Abdul Mujeebu, M.; Abdullah, M. Z.; Abu Bakar, M. Z.; Mohamad, A. A.; Muhad, R. M. N.; Abdullah, M. K. Combustion in porous media and its applications;A comprehensive survey. J. Environ. Manage. 2009, 90, 2287–2312. (4) Trimis, D.; Durst, F. Combustion in a porous medium; Advances and applications. Combust. Sci. Technol. 1996, 121, 153–168. (5) Tong, T. W.; Sathe, S. B.; Peck, R. E. Improving the performance of porous radient burners through use of sub-micron size fibers. Int. J. Heat Mass Transfer 1990, 33, 1339–1346. (6) Mital, R.; Gore, J. P.; Viskanta, R. A study of the structure of submerged reaction zone in porous ceramic radient burners. Combust. Flame 1997, 111, 175–184. r 2009 American Chemical Society
(7) Kesting, A.; Picken€acker, O.; Trimis, D.; Durst, F. Development of a radiation burner for methane and pure oxygen using the porous burner technology. Proceedings of the 5th International Conference on Technologies and Combustion for a Clean Environment, Lisbon, Portugal, 1999. (8) Kayal, T.; Chakravarty, M. Modeling of a conceptual self-sustained liquid fuel vaporization-combustion system with radiative output using inert porous media. Int. J. Heat Mass Transfer 2007, 50, 1715– 1722. (9) Hoffman, J. G.; Echigo, R.; Yoshida, H.; Tada, S. Experimental study of combustion in porous media with a reciprocating flow system. Combust. Flame 1997, 111, 32–46. (10) Wood, S.; Harris, A. T. Porous burners for lean-burn applications. Prog. Energy Combust. Sci. 2008, 34, 667–684. (11) Labeckas, G.; Slavinskas, S. Comparative performance of direct injection diesel engine operating on ethanol, petrol and rapeseed oil blends. Energy Convers. Manage. 2009, 50, 792–801.
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the heat energy added to complete the vaporization process.21 The crack reactions in the vegetable oils are taken place at temperatures lower than 300 C, where the oil starts to dissociate, leaving cracking products.22 The hard deposits adhere and then accumulate on the vaporizer internal surfaces and finally block it after some time, leaving it unusable. Another problem that has to be resolved before using vegetable oils in PIM burners is the flame stability. As discussed in a previous work,23 the quenching zone and flow velocity control are the only stability techniques that were successfully applied to PIM burners for power densities >650 kW/m2 under atmospheric operation. Hoping to prove the universal validity of the first approach, an attempt was carried out to burn rapeseed oil using the quenching zone approach to stabilize the flame. It was found that the combustion was characterized by dark smoke formation, high levels of interrupted noise in the upper part of the burner, large droplets falling from the downstream end of PIM, extremely complicated ignition, etc. This led to a rapid functional failure of this layer and, hence, a failure of this stabilization technique during the very early phase of operation. The flow velocity technique, introduced in a previous work,23 circumvents the above-mentioned difficulties by combusting the oil vapor directly in the PIM matrix. Hence, the condensation of the vapor is avoided because it is not obstructed by any intermediate cold object. Because of the advantages of porous medium burner technology and rapeseed oil, the combination leads to a system with outstanding environmental friendliness. Therefore, the following study was systematically carried out for a controllable evaporation of rapeseed oil to collect information on the crack reaction residual products that are accumulated on the vaporizer walls to examine its validity for making it usable for combustion within PIM with reasonable operating conditions and insignificant side effects.
to conventional fuels. Rapeseed oil methyl ester (RME) is considered to be one of the best alternatives among biofuels.11,13 It has been reported that RME is nontoxic and easily biodegradable in an aquatic environment.14 It was determined that, during a 21 day period, 98% of pure RME and only 60% of pure fossil diesel fuel were biologically decomposed, which means that RME fully meets the main requirements of international standards for biological degradation.15 Aiming toward gaining both the outstanding characteristics of premixing combustion in the PIM burners and the use of vegetable oils as alternative and renewable fuels, a research project has been conducted at the Institute of Fluid Mechanics of ErlangenN€ urnberg University. Its aim is the development of a state-ofthe-art PIM burner, which can be operated with neat vegetable oils. However, because of the completely different physical and chemical properties, especially the high boiling and ignition temperatures and the high viscosity, vegetable oils show totally different combustion characteristics to fossil fuels,16 so that complete evaporation or gasification without any residuals in premixed burners is a very challenging task. While it is very simple to mix gaseous fuels with air in premixing combustion technology, the situation is more difficult in the case of liquid fuels (i.e., vegetable oils), which, however, need sophisticated technologies for the preparation of an ignitable homogeneous mixture from the fuel and air.17 The generation of this homogeneous mixture constitutes a key factor in terms of the quality of combustion. The idea is the transformation of the large liquid fuel continuous flow into very fine droplets with immense surface area and, thus, an increase of the vaporization and the mixing process with air and finally the combustion rate. Prevaporization and atomization processes are the two main employed methods that are provided for this purpose. The two systems have been applied in a wide range of applications the field of the burner.18-20 In the case of vegetable oils, the ignition temperatures are relatively higher than in the liquid fossil fuels. On one hand, their ignition temperatures are high to the degree where the conventional ignition systems are not enough to ensure sustaining the flame in an ignitable mixture. On the other hand, the high temperature of the ignitable preferable premixing mixtures of these fuels with air is subjected to the danger of either the spontaneous autoignition or the thermal flashback burning conditions. Furthermore, the vaporization of middle and heavy hydrocarbons, such as vegetable oils, undergoes crack reactions because of the thermal stresses resulting from
2. Materials and Methods 2.1. Rapeseed Oil Vaporization Experimental Setup. The rapeseed oil from the oil tank is pumped into helical stainlesssteel coils, with a coil diameter of 50 mm and an inner and outside tube diameter of 3 and 4 mm, respectively. The coil was manufactured from a 2 m long tube. This length proved sufficient for complete vaporization of an oil flow rate corresponding to 4.5 kW output burner power. This was fixed for all of the experiments in the current investigation. The oil mass flow rate was controlled by a mass flow controller (Liqui Flow, Bronkhorst High-Tech B.V., The Netherlands), which was calibrated for rapeseed oil in advance by measuring a collected quantity of oil versus time. The coil was kept in a uniform, controllable temperature environment by embedding it in a packed bed of 5 mm diameter Al2O3 beads. A stream of hot air, controlled by a 4.5 kW regulated electric air heater (Heissluft model 5000, Steinel, Germany) and mass flow controller, was
(12) Houghton, R. A. Global circulation of carbon. In Biomass Handbook; Kitani, O., Hall, C. W., Eds.; Gordon and Breach Science Publisher: New York, 1989; pp 56-61. (13) San Jose Alonso, J.; L opez Sastre, J. A.; Romero-Avila, C.; L opez Romero, E. J. Combustion of rapeseed oil and diesel oil mixtures for use in the production of heat energy. Fuel Process. Technol. 2006, 87, 97–102. (14) Korbitz, W. The biodiesel market today and its future potential. In Plant Oils as Fuels, Present State of Science and Future Development; Martini, N., Schell, J., Eds.; Springer-Verlag: Berlin, Germany, 1998. (15) Makareviciene, V.; Janulis, P. Environmental effect of rapeseed oil ethyl ester. Renewable Energy 2003, 28, 2395–2403. (16) Hazar, H.; Aydin, H. Performance and emission evaluation of a CI engine fueled with preheated raw rapeseed oil (RRO)-diesel blends. Appl. Energy 2009, in press. (17) Kamal, M. M.; Mohamad, A. A. Investigation of liquid fuel combustion in a cross-flow burner. Proc. Inst. Mech. Eng., Part A 2007, 221, 371–385. (18) Schramek, E. R. Paperback for Heating and Climate Technology; Oldenbourg Verlag: Berlin, Germany, 1999. (19) Rutsche, A. Development of a procedure of the vaporization and combustion of fuel oil in porous inert media. Diplomarbeit, FriedrichAlexander-Universit€ at Erlangen-N€ urnberg, Erlangen, Germany, 1997. (20) Combustion Engineering, 1st ed.; Borman, G. L., Ragland, K. W., Eds.; McGraw-Hill: New York, 1998.
(21) Lucka, K.; K€ ohne, H. Usage of cold flames for the evaporation of liquid fuels. Proceedings of the 5th Conference on Technologies and Combustion for a Clean Environment, Lisbon, Portugal, 1999; pp 207-213. (22) M€ uhlbauer, W.; Esper, A.; Stumpf, E.; Baumann, R. Plant oilbased cooking stove;A technology update. In Rural Energy, Equity and Employment: Role of Jatropha Curcas, the Rockefeller Foundation; Scientific and Industrial Research and Development Centre: Harare, Zimbabwe, 1998; pp 13-15. (23) Bakry, A.; Al-Salaymeh, A.; Abu-Jrai, A.; Al-Muhtaseb, A. H.; Durst, F. Adiabatic premixed combustion in a gaseous fuel porous inert media under high pressure and temperature: Novel flame stabilization technique. Combust. Flame 2009, doi: 10.1016/j.combustflame.2009.09.007.
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Figure 1. Rapeseed oil vaporization setup investigation.
used to heat the packed bed to a predefined temperature. Temperature uniformity along the coil was ensured through six K-type thermocouples (R€ ossel Messtechnik, Germany), which were welded to the coil at its beginning and outlet ends, and every two windings. A seventh thermocouple was inserted inside the packed bed for the purpose of determining its temperature. A schematic diagram of the experimental setup can be seen in Figure 1. The current series of investigations aimed to establish the appropriate operating conditions for the vaporization of rapeseed oil and to design a cheap, durable, and residualfree vaporizer. Because a residual-free vaporizer was deemed unreasonable, as will be seen later, a chemical approach was suggested in attempting to remove these residuals from the adherent vaporizer surfaces. A chemical reaction was initiated between the residuals and regulated flowing fluids introduced into the vaporizer from time to time. Therefore, a three-way valve was installed at the inlet to the coil to switch between oil and reacted fluid operation. After the backed bed had been preheated to the desired temperature value, the oil was pumped at the prescribed flow rate through the coil. A waste porous media burner was attached downstream of the vaporizer unit to eliminate the oil vapor by burning during the entire run time of each experiment. At the end of each experiment, the coil was dismounted and opened in many locations along it using a flat file. In this way, the residuals could be examined and appropriately described and classified. The experiments were carried out at evaporation temperatures ranging from 350 to 730 C (Table 1) and by varying the temperature, the cleaning fluid, and the operating time. Experiments with short operating times were conducted to determine whether residuals were formed or not. For such short-time experiments with low residual levels, additional experiments with longer operating times (between 18 and 26 h) were executed. During these experiments, the evaporation unit was cooled after 6-8 h and reheated the next day to continue the experiment. Oil inside the coil was washed out before starting and after finishing the experiment to find out the reasons for residual formation. Additionally, some experiments were carried out with oil, water, and air cleaning before cooling the test section to remove residuals. 2.2. Flow Velocity Flame Stabilization Experimental Setup. A prototype integrated rapeseed oil vaporizer with a PIM burner based on the flow velocity control technique was built and experimentally investigated. The experimental setup included the test apparatus facilities, control facilities, and measurement facilities. The test apparatus consisted of the PIM burner with the integrated vaporizer and other ancillary components. The
temperature (C)
length of the coil (m)
350 400 450 450 450 475 500 500 500 500 525 525 525 550 550 600 600 720 730
2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 1 2 2 2
cleaning fluid nitrogen nitrogen nitrogen nitrogen water nitrogen air air air air air rapeseed oil nitrogen nitrogen nitrogen
operating time (h) 6 6 6 6 24 6 18 18 6 6 8 8 20 24 6 26 6 18 6
Figure 2. Schematic diagram of the rapeseed oil PIM prototype burner.
control and measurement facilities used in this investigation were identical to that described in previous work23,24 and the previous experiment for the vaporization of rapeseed oil. Figure 2 provides a schematic diagram of the PIM burner with its different parts. The porous medium burner consists of conical and cylindrical parts with equal large pore size, so that the resulting Peclet number throughout the whole porous medium is greater than 65. The fuel/air mixture enters the conical part with a flow velocity, which is higher than the flame velocity inside the porous medium. Thus, combustion near the gas inlet inside the conical part of the porous material cannot take place. The burner is composed of two porous ceramic parts of the same size and material, made from Al2O3 corrugated lamellae (Walter E.C. Pritzkow Spezialkeramik, Germany) and similar to that used before in the gas burner. The first part was fabricated in the
(24) Bakry, A.; Genenger, B.; Schmidt, V.; Trimis, D. Low emission combustion of vegetable oils with the porous burner technology. In New and Renewable Technologies for Sustainable Developments; Afgan, N., Carvalho, M., Eds.; Kluwer Academic Publisher: Norwell, MA, 1998.
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form of a cone with a divergence angle of 30, which is less than its counterpart used in the gas burner by 10. A numerical study to simulate the flow in PIM with a divergence angle using the Lattice-Boltzmann technique was carried out by ref 23, who confirmed the same results again. The inlet and outlet diameters of the conical part are 40 and 150 mm in diameter, respectively, and its height is 205 mm. The other part was cylindrical-like in shape, with a diameter of 150 mm and height of 150 mm. It was used to complete the combustion of the oil, reducing the emissions of CO and unburned hydrocarbons (UHCs) by increasing the residence time of the combustion products within PIM. The two parts were stacked together firmly with the same fiber orientation. They were also insulated by two separate parts of different insulating materials. The conical insulation part is made of Pyrostop 140 coating material (Didier-Werke AG, Germany). The cylindrical insulation is fabricated by Walter E.C. Pritzkow Spezialkeramik, Germany. The upper end of the cylindrical insulation part was formed to support the vaporizers embedded within it. Two stainless-steel vaporizers, each in the form of a tube, were used to vaporize the rapeseed oil in either a mutual or series mode of operation. The tube, which is 18 mm in diameter and 1 mm in thickness, is filled with stainless-steel wire windings (to augment the heat transfer by increasing the heated surface area/volume ratio) of 0.125 mm wire diameter and weighed 12 g each. Then, the tubes were pressed and turned to half ring, to fix their location on the insulation part. Both ends of the vaporizers were sealed by welding to prevent any oil leakage. The vaporizers were mounted in position, such that good contact with the insulation on their inner side is ensured. Three short tubes were welded at the ends of each vaporizer. Two are mounted at one end, where one of the tubes serves as the oil supply tube, while the other is used for cleaning with pressurized air. The third tube mounted on the other end is intended for discharging the oil vapor into the premixing tube. Proper cleaning proved to be a key element for proper operation of this kind of burner. When air is pumped into the burner during cleaning, the accumulated oil traces are swept by the stream and undergo a very fast and incomplete vaporization. A black viscous substance is then formed by agglomeration, eventually leading to a complete blockage of the vaporizer. Then, the airflow is stopped, resulting in a pressure rise to the back pressure of the mass flow controller (8 bar). At the stagnant interface between the oil and air, the combustion takes place, where the temperature field is higher than the ignition point of the oil. Thus, the resulting very high temperature accompanying by large thermal expansion at high pressure leads to explosion of the vaporizer at that position. Therefore, a pressure gauge was inserted in the oil line to detect the condition of vaporizer blockage if it happened. To start the burner, the vaporizers were wound on the outer surface, with a 2 m electrical band heater (Winkler Co., Germany), which is enough to generate 0.5 kW and can raise the temperature to 900 C. For ignition, a ceramic heater plug is used at the inlet section of PIM ceramic. Unfortunately, the oil vapor condensed on the surface of the igniter ceramic, which led to its destruction. An alternative method for starting the burner was used for fast operation and warming up the burner. The combustion air supplied via the mass flow controller (Bronkhorst High-Tech B.V., Netherlands) and the oil vapor from the vaporizer are mixed in a 40 mm diameter premixing tube. High-quality mixing is obtained by a ring of the vortices created by a sudden contraction directly upstream of the vapor inlet. The strong recirculation mixes the oil vapor with the air. The mixture flows through 30 mm short tube (to prevent condensation on its wall) to the ceramic, where it burns. The 30 mm diameter was calculated to prevent flashback formation at a minimum power of 3 kW and relative air ratio of 1.3 in a manner similar to that carried out in the gas burner. Four thermocouples were used to measure the temperature. One S type is located at the inlet section of the PIM ceramic to monitor the temperature at the interface between the free space in the
premixing tube and the PIM inlet section. The other three (K type) were installed as follows: one inside the 30 mm premixing tube to detect flashback formation and the other two maintained in contact with the outer surface of each vaporizer to monitor the vaporization temperature. The vaporizers, the connecting tubes to the premixing tube, and the entire outer surface of the burner were insulated using a 30 mm thick fiber mat (Didier-Werke AG, Germany), which has a coefficient of thermal conductivity of 0.15 W m-1 K-1. This is to maintain adiabatic combustion and keep the vaporizers at a high enough temperature to ensure complete oil vaporization (>500 C). As stated before, the exhaust gas analyzer equipment is turned on and allowed to warm for at least 2 h. The equipment is purged using zero gas for 20 min followed by individual calibration for all four channels by using bottled calibration gases. While all of the valves of oil and cleaning air are close, the combustion air is started and adjusted to a power value of 5 kW. Then, the methane is added and regulated to 5 kW output power. The methane and air pass through a premixing chamber and are fed to the burner through a 10 m long flexible rubber tube. After the mixture is ignited, the burner is left to warm for 15 min. The methane supply is then cut while turning on the oil pump. The oil mass flow controller is adjusted to the desired output power using only one vaporizer by opening only the oil valve before it. The combustion air is simultaneously adjusted to the desired relative air ratio. The flame is considered unstable if it propagates outside the burner either up- or downstream. Upstream propagation (flashback) is detected by the K-thermocouple inside the premixing tube. The downstream propagation (blow out condition) can be visually identified and is characterized by short blue flames (instead of the uniform orange flame) and excessive smoke. Once the flame is stabilized at a particular location, the flow rates of air and oil and the emissions are recorded. The experiments have been carried out by maintaining the oil flow rate constant (i.e., constant power) and varying the airflow rate to change the relative air ratio in the range between the flashback and blow out limits. The operation continues with one vaporizer for approximately 24 h. Then, the oil valve before the second vaporizer is turned on, and the first one is turned off. During the operation with the second one, the first one is allowed at least 15 min to evaporate the remaining quantity of oil inside it into the burner. Then, this vaporizer undergoes the cleaning process procedure. The cleaning air valve is opened, and the flow rate is adjusted (using the mass flow controller, with a maximum flow rate of 20 L/min) to ∼2.0 L/min for 10 min to oxidize the residuals inside the vaporizer. The flow rate is raised to the maximum flow rate for an additional 10 min to reject combusted substances outside the vaporizer into the burner, where they appear at the exit of the burner in the form of flying red particulates. At this step, the cleaning procedure for the first vaporizer is complete and becomes ready for use again. The same procedure is repeated for the second vaporizer. Hence, this cleaning procedure enables continuous operation and avoids sequential shutdown and startup.
3. Results and Discussion 3.1. Vaporization Analyses. 3.1.1. Vaporization without Cleaning. Dependent upon the vaporization temperature and operating time, residuals with different surfaces and mass are formed inside the coil. Operating the vaporizer at a temperature of 350 C for 6 h leads to black residuals with a shiny surface. The mass of the residuals is low. The residual layer looks like the surface resulting from a phosphorus treatment of steel. This type of residual layer resulted from the phosphorus compounds of the rapeseed oil. Operation of the evaporator at 600 C for 26 h leads to residuals with a black rough surface, which probably results from cocking 291
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Table 2. Residuals of Rapeseed Oil Vaporization without Cleaning of the Vaporizer
Table 4. Vaporization of Rapeseed Oil with Cleaning by Water, Rapeseed Oil, and Air
temperature (C)
length of the coil (m)
temperature (C)
length of the coil (m)
cleaning fluid
operating time (h)
350 450 500 600
2 2 2 1
500 550 500 525 525 525 550
2 2 2 2 2 2 2
water rapeseed oil air air air air air
18 6 6 8 8 20 24
a
cleaning fluid
operating time (h) 6 6 18 26
residualsa P P P, C C
P, shiny surface; C, formed by cooking.
residuals C P none none none none none
Table 3. Residuals of Rapeseed Oil Vaporization with Nitrogen Cleaning temperature (C)
length of the coil (m)
operating time (h)
400 450 450 475 500 600 720 730
2 2 2 2 2 2 2 2
6 6 24 6 6 6 18 6
a
residualsa none low P P P P C C C
P, shiny surface; C, formed by cooking.
during oil vaporization. The results of rapeseed oil vaporization without vaporizer cleaning are summarized in Table 2. The residuals are characterized by the letters P for a shiny surface and C for formed by cocking. An increasing temperature and increasing operating time lead to a greater mass of residuals formed by cocking. For the vaporization temperature of 600 C, no residuals of type P occur. Even at temperatures less than the temperature necessary for complete vaporization, oil residuals are formed. Therefore, a vaporizer without cleaning is not suitable for vegetable oil combustion in a porous medium burner, because the residuals probably will block the evaporator during the long time operation in practice. 3.1.2. Vaporization with Vaporizer Cleaning by Nitrogen. Some of the residuals formed in the vaporizer may be formed during starting and stopping of the vaporization test apparatus. During start and stop, without a cleaning procedure, oil remains inside the coil. Thus, experiments with nitrogen cleaning before starting and stopping for several vaporization temperatures and operating times were carried out. At 400 C, the rapeseed oil cannot be vaporized completely. There are no residuals after 6 h of operation at this temperature. The vaporizer material slightly changed in color because of the operation under elevated temperature. It can be concluded by comparing this result to results from experiments without cleaning that the residuals are at least partially formed during the cooling of the vaporizer. A part of the residuals grow during the experiments without cleaning because of oil, which remains inside the coil under elevated temperature with a nearly zero flow rate. The results of the complete set of experiments with cleaning by nitrogen are summarized in Table 3. Residuals are formed similar to the experiments without cleaning, depending upon the operating temperature and time. In the temperature range between 400 and 500 C with increasing temperature, more and more type P residuals with a shiny black surface are formed. In the temperature range between 600 and 730 C, the mass of type C residuals from cocking increases. Residuals are formed during stationary operation too. There is no vaporization operating window suitable for practical long time operation of a rapeseed oil porous medium burner with nitrogen cleaning.
Figure 3. Photograph of the exit section of the burner at 5 kW.
3.1.3. Vaporization with Water, Rapeseed Oil, and Air Cleaning. On the basis of the results described in subsections 3.1.1 and 3.1.2, it must be concluded that long time operation of a rapeseed oil vaporizer without periodical effective cleaning is impossible. The vaporizer would block in long time operation because of the residuals growing during operation at temperatures higher than 450 C. Therefore, it was investigated whether the residuals can be removed using fluids typically available in a household heating system based on a rapeseed oil porous medium burner. Water, rapeseed oil, and air were used to investigate residual removing before cooling the evaporation test section. Water and rapeseed oil did not show any cleaning of the vaporizer. Both experiments show no residuals after operation with air cleaning before cooling. The color of the coil material has changed because of the operation with high temperature. The residuals can be removed by cleaning with air. The residuals are oxidized at least partially by the oxygen of the air under elevated temperature, and the oxidation products are removed from the vaporizer by the airflow. The results of rapeseed oil vaporization with water, oil, and air cleaning are summarized in Table 4. Water and oil were not suitable for cleaning the evaporator coil. Cleaning with air under elevated temperature leads to clean vaporizer surfaces throughout the entire investigated temperature range. With air, the vaporizer can be cleaned periodically in long time porous medium burner operation. Caution, however, should be used when applying the air in the cleaning process inside the coil. Two possible problems may arise during the addition of cleaning air. First, a detonation wave may occur and propagate through the two-phase mixture of liquid fuel and gaseous oxidizer inside the coil. The fuel can be in the form of a nonvolatile liquid film on the walls of the tube, and ignition can take place because of the surrounding high-temperature field. This type of detonation wave is characterized by very high transient pressures, which are very destructive.17 Second, a sudden tube blockage may result from the fast pushing of the remaining oil quantity in the common tube between 292
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Figure 4. Photographs of the exit section of the burner at 20 kW.
the vaporizer and the three-way valve because of the high velocity of the air in comparison to oil, which is inversely proportional to the density ratio of both. Hence, a massive quantity of oil, depending upon the length of the common tube, enters the vaporizer and undergoes a very fast and incomplete vaporization process that leads to a partial polymerization reaction of the oil with air. This will cause the oil to agglomerate, producing gum-like material that leads to blocking the coil, leaving it useless. Special precautions should be carefully followed during the cleaning procedure. One has to make sure that the common connection tube is short and empty of oil by sucking the residual oil through an oil vent, or if possible, the oil and the air lines must be separate. Also, one has to wait enough time before applying the cleaning air inside the coil to ensure that the oil is completely vaporized. 3.2. Flame Stability. Figures 3 and 4 show two photographs of the exit cross-section of the rapeseed oil PIM burner at 5 and 20 kW output power, respectively. The regular and uniform flame distribution on the cross-section area indicates high-quality combustion without noticing black smoke or soot. The burner was successfully tested at power values from 5 to 30 kW, which correspond to a power modulation range of 1:6. The orange color ring around the sidewall of the conical part indicates the position of the flame stabilization. This ring was aligned horizontally throughout the entire investigation, indicating uniform and axisymmetric heat release across the burner. The measurement of emissions distribution and relative air ratio across the burner showed a wide variation with spatial and temporal fluctuations at the exit plane. These spatial and temporal discrepancies may be attributed to one or both of the following reasons: (1) Inadequate premixing of air and oil. This situation results in a non-uniform mixture distribution over the cross-section of the burner. This is characterized by the local dependency upon the relative air ratio, resulting in a wide spectrum of emission levels. The influence of mixture nonhomogeneity on NOx formation has been examined by several researchers. Lyons25 showed that spatial non-uniformity in the relative air ratio increases NOx emissions. The same has been found by Flanagan et al.,26 where they reported that a 5-fold increase in NOx emissions was recorded when going from well-mixed to incompletely mixed
conditions. Using a very similar experimental setup to that employed by Flanagan et al.26 to burn natural gas, Fric27 found that not only the spatial nonhomogeneity but also the temporal fluctuations in the relative air ratio could raise NOx emissions (10% temporal fluctuations produced a 200% increase in NOx). (2) Formation of large oil droplets by partial condensation of oil vapor on the cold ceramic in the region upstream of the flame. These droplets are burned in a diffusion flame mode producing very high NOx emissions. Even in lean flames, Rink and Lefebvre28 showed that an increase in droplet size means that a larger proportion of the total number of fuel drops in the fuel vapor are capable of supporting “envelope” flames. These “envelope” flames, which surround the large drops, combust in a diffusion mode at near stoichiometric air/fuel ratios. This gives rise to many local regions of high temperature, in which NO is formed at a high rate. Reduction in mean droplet size impedes the formation of envelope flames, so that a larger proportion of the total combustion process occurs in what is essentially a premixed mode, thereby generating less NOx. Thus, a mixed mode of combustion using premixing and diffusion flames is constituted. This leads to a discrepancy in the measured data to the degree that makes interpretation of the data awkward. This point needs further investigation to improve the efficiency of premixing. To validate the above discussion, the emission performance of this burner was examined with a well-premixed conventional gaseous fuel to isolate the premixing and the droplet size effects. To this end, a well-premixed methane fuel was employed instead of the rapeseed oil. This has been performed by extending the starting phase, in which the methane was used, to carry out a systematic investigation by measuring the emissions from the burner at different positions at its exit. The estimated uncertainty in the measurement of NOx was calculated on the basis of three replicates and found to be less than (2% from the measured value. The accuracy with which NOx can be measured is specified by the manufacturer of the exhaust gas analyzer equipment. The raw calibration data were curve-fitted using the computer software developed at LSTM-Erlangen. First of all, the measurements showed that the relative air ratio or the emission level results have no spatial or temporal dependency. Figure 5 depicts the NOx emission values versus the relative air ratio for different powers (5, 10, and 20 kW). The very low NOx emission level can be clearly seen. Also, the independence of the power value reflects again the balance
(25) Lyons, V. J. Fuel-air non-uniformity effects on nitric oxide emissions. AIAA paper 81-0327, 1981. (26) Flanagan, P.; Gretsingir, K.; Abbasi, H. A.; Cygan, D. Factors influencing low emissions combustion. ASME PD-39, fossil fuel combustion, 1992. (27) Fric, T. F. Effects of Fuel-air unmixedness on NOx emissions. AIAA paper 92-3345, 1992.
(28) Rink, K. K.; Lefebvre, A. H. The influence of fuel composition and spray characteristics on nitric oxide formation. Combust. Sci. Technol. 1989, 68, 1–14.
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: DOI:10.1021/ef9007888
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Figure 5. NOx emissions versus relative air ratio for operating methane gaseous fuel in the rapeseed oil burner at ambient temperature and atmospheric pressure.
between the effects of increasing the peak temperature, increasing the power, and decreasing the residence time, with an increase in the power on the NOx emission formation. However, Figure 5 shows that the blow out limit is remarkably extended (≈2.2, 2.1, and 1.9 at each power value, respectively) in comparison to the previous results. This may be due to the additional cylindrical PIM part. Furthermore, no flashback condition is noticed during operation, especially at the lower operating power of 5 kW and the relative air ratio of 1.3, as indicated from the temperature measurement and the NOx emission level shown in Figure 5. During the whole experiment, CO emissions recorded a zero value continuously for the whole range of power and relative air ratio tested. These results of NOx and CO emission levels prove the high degree of repeatability and reliability of combustion with this new combustion technique in PIM. Also, these results confirm the hypothesis of temporal and spatial dependency of the measured relative air ratio and emission levels when operating with rapeseed oil. Thus, to obtain good emissions using oil, enhancement of the premixing efficiency is essential. This can be achieved using a two-phase atomization system preceded by an integrated vaporizer to vaporize the oil. With this system, controllable fine droplets can be obtained, avoiding
the formation of envelope flames. It may also be beneficial to perform a prior estimation of the characteristics and suitability of the oil for combustion. Conclusions The high performance of combustion quality within the PIM burner was identified by burning vegetable (rapeseed) oil. Because of the high viscosity of rapeseed oil, a prevaporization technique was essential to obtain oil vapor suitable for the combustion process. Using a special oxidation cleaning technique, a free-residual vaporizer has been obtained. A remarkably stable combustion without any smoke or soot formation was obtained. During the whole experiment, CO emissions recorded a zero value continuously for the whole range of power and relative air ratio tested. The results of NOx and CO emission levels prove the high degree of repeatability and reliability of combustion with this new combustion technique in PIM. The concept of evaporation and periodical cleaning of the vaporizer is not limited to rapeseed oil. This concept can be used for several liquid renewable and fossil fuels to overcome the difficulties in liquid fuel evaporation for premixed combustion.
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