Self-Aspirating Radiant Tube Burner

Transcription

1 Self-Aspirating Radiant Tube Burner Chanon Chuenchit and Sumrerng Jugjai * Combustion and Engine Research Laboratory (CERL), Department of Mechanical Engineering, Faculty of Engineering, King Mongkut s University of Technology Thonburi, Bangkok, Thailand, * Corresponding Author. Tel: (662) , Fax: (662) , sumrueng.jug@kmutt.ac.th Abstract: Radiant tube burners (RTB) are widely used in heating process of industrial production. The important advantage of RTB is providing a more uniform temperature profile along the axial direction. Most of the RTB burners operate at high capacity that require a lot of fuel and combustion air to obtain high temperature (>1,2 C). Therefore, the air compressor is needed in order to supply the sufficient air for a complete combustion. The objective of this research is to present the concept of self-aspirating radiant tube burner (SRTB) for a small-scale food processing industries at lower capacity (<1 kw), in which the combustion air is naturally entrained through the mixing tube by the momentum of fuel jet. Thus, the air compressor is not required for this method. Moreover, the energy recirculation principle for energy saving is applied by employing a porous heat exchanger to the burner. The results show that flame could be stabilized inside the SRTB within the range of studied firing rate (3-8 kw). The maximum temperature inside SRTB is up to 1,481 C with heat recirculation, while NO x and CO emission are less than 18 and 24 ppm, respectively. Keywords: Radiant tube burner, Self-aspirating, Porous medium, Heat recirculation, Combustion 1. INTRODUCTION Radiant tube burners appear well suited for the industrial processes that require a high and homogeneous temperature distribution within the furnace. However, most of the used radiant burner as the heaters are powered by electricity, and thus, the radiation energy is quite expensive. However, if the radiation energy achieved from the combustion of fossil fuel, energy cost can be expected to decrease. Increasing of energy efficiency is the most important topics in order to reduce fuel consumption and pollutant emissions. Preheating of combustible mixture by recycled heat from exhaust gas has been considered as an effective method for fuel conservation. This type of combustion has been called excess enthalpies or super-adiabatic flame temperature combustion in which the reactants (or the combustion air alone) are preheated using heat borrowed from beyond the flame zone, without mixing the two streams [1]. The heat recirculation systems have been extensively applied for increasing the combustion air temperature. Moreover, it expands the combustion stability. The new type of glass melting regenerative furnace at Nippon Furnace Kogyo (NFK) makes it possible to preheat the combustion air, typically above 1 C, by recirculating the hot exhaust gas resulting in fuel saving up to 5% over cold air systems, as well as a significant decrease of greenhouse gas CO 2 and NO x emissions [2]. Fig. 1 Heat exchanger based on energy conversion between convection and thermal radiation by porous medium Heat-recirculating combustion based on the porous medium technology has now been considered as the emerging furnace design methodology for the next generation of high performance combustion systems. Base on the prominent feature of the porous medium in effectively converting energy between flowing gas enthalpy and thermal radiation, the development of a high performance heat exchanger using a porous medium was proposed by Echigo [3]. The basic concept consists of a pair of porous mediums separated by a solid wall as shown in Fig. 1. Enthalpy of the hot flowing gas (combustion products) is effectively converted to thermal radiation emitted by the high temperature (heating) side 524

2 porous medium designated as the emitter and directed to the solid wall. In thermal equilibrium, the reversed conversion from the incident thermal radiation emitted from the solid wall to the cool gas (or combustion air) enthalpy takes place in the low temperature (cooling) side porous medium designated as the absorber, leading to an efficient method in preheating the combustion air. Jugjai and Rungsimuntuchart [4] applied the heat recirculation principle to the self-aspirating burners by employing a porous heat exchanger in combination with the swirling central flow burner (SB). The preheating temperature of the mixture can be increased up to 3 C without flashback. The results showed that the thermal efficiency of the proposed burner is higher than that of a conventional radial flow burner (CB) by about 3 percentage points (5% relative). Scribano et al [5] have been introduced the heat recirculation system by using of exhaust gas recirculation (EGR) techniques in order to enhance the performance of the industrial radiant tube burner. It was shown that the NO x emissions at the exhaust significant decrease up to 5% with respect to the original burner without the EGR. However, because almost all the RTB burners operate at high capacity that use a lot of fuel and combustion air to receive high temperature (>1,2 C), therefore the air compressor is needed in order to supply the sufficient air for a completely combustion. The main objective of this research is to integrate a self-aspirating gas burner and radiant tube in one unit that is a new innovation for radiant tube burner. It is called the self-aspirating radiant tube burner (SRTB). A unique advantage of the self-aspirating burner, that the combustion air is naturally entrained through the mixing tube by the momentum of fuel jet, therefore the air compressor is no longer desired by this burner. Moreover to save energy in the world, the energy recirculation principle with porous medium technology is selected to use in this work, which is called the air preheating system (APS). It is developed for using in a small-scale drying process and heating process at lower firing rate (lower than 1 kw), which has never been studied before. 2. EXPERIMENTAL SET-UP Fig.2 shows the schematic diagram of the experimental set-up for the SRTB. Fig. 3 shows a photograph of the SRTB. The flow rates of LPG (1) (4% of propane and 6% of butane), low heating value of about 46 MJ/kg, are controlled by a pressure regulator (2). The flow meter (3) is installed downstream of a pressure regulator. The flow meter is used to measure the FR of the fuel gas. A mercury manometer (4) is used to measure the fuel pressure. Then the gas emerges from an injector orifice (5). On leaving the injector, the gas entrains the primary air by a momentum-sharing process between the emerging gas and ambient air, in case of without preheated air, and preheated air, in case of with APS. The gas/air mixture enters the mixing tube (6) which is constructed of cast iron and then flow into the radiant tube (7), which is constructed of stainless steel tube of.1 m in inside diameter, 1 mm in wall thickness, and 1.25 m in total axial length. In order to simplify this system is adiabatic condition to study the fraction of heat recirculation of APS, the radiant tube was covered by ceramic fiber insulator of 3 mm in thickness. The oxygen concentration in the mixture is measured by using the sampling mixture line (14), which is located at the burner head, and then carried to the portable emission analyzer, Messtechnik Eheinm model Visit1L (16). The measuring range is 1-1, ppm for CO and - 4, ppm for NO x with a measuring accuracy of about 5 ppm (from the measure value) and a resolution of 1 ppm for both CO and NO x All measurement of emissions in the experiment is those corrected to % excess oxygen and dry basis. All data are recorded by computer (17). The temperature T at different locations on the axial axis of the radiant tube as shown in Fig. 2, are also recorded. Eight locations of temperature measurement, T 1 - T 8 from the radiant tube inlet to the radiant tube exit, T 1 - T 3 are performed with B-type and T 4 - T 8 are performed with N-type thermocouples. End of the U-tube is connected to the heat recirculation section, in which the porous emitter (8), made from stainless steel in conical shape with 2 mm in hole diameter, extract a part of energy and then radiate to the separating wall (9) and porous absorber (1), respectively. The porous absorber was constructed from stainless steel wire mesh with 34 meshes per inch. It is cylindrical shape with 17.7 cm in diameter and 24.7 cm in length. The energy that absorbs at the porous absorber is transferred to the incoming air. T a,, T a,1 T a,2 and T exh are N-type thermocouple to measure the ambient temperature, porous emitter out let air temperature, the preheated air temperature, and the exhaust gas temperature, respectively. The upper cover (11) and the lower cover (12) are steel of 1 mm in wall thickness, which protect the heat loss from the heat recirculation section. The preheating box (13) was installing to control the flow direction of the ambient air from surrounding to the inlet of mixing tube (in case of APS). The exhaust gas composition is measured by using the sampling gas line (15). 525

3 T a, T a,1 T 5 T 7 T 8 T 6 15 T exh 13 4 T 4 T 3 T 1 T T a, x Fuel (LPG) 7. Radiant Tube 13. Preheating Box 2. Pressure regulator 8. Porous Emitter 14. Sampling mixture line 3. Flow meter 9. Separating wall 15. Sampling gas line 4. Mercury manometer 1. Porous Absorber 16. Gas Analyzer 5. Injector orifice 11. Upper Cover 17. Computer 6. Mixing tube 12. Lower Cover Fig. 2 Schematic diagram of self aspirating radiant tube burner Fig. 3 Photograph of the SRTB with APS 3. EXPERIMENTAL RESULTS AND DISCUSSION 3.1 Temperature distribution Fig. 4(a) shows typical temperature profiles along the axial axis of the radiant tube in case of SRTB at varying firing rate FR from 3.66 to 7.52 kw. The porous absorber, the porous emitter, and preheating box were removed to neglect the air preheating system. The combustion air at the ambient temperature was entrained into the mixing tube by the momentum jet of LPG. Then combustion occurred inside the radiant tube, temperature distributions inside radiant tube were measured. Then the exhaust gas flow out at the exit. At a relative low FR = 3.66 kw the maximum temperature 526

4 located at T 2 of about 1,344 C. The flame is still stabilized at this location accompanied by increasing the FR up to 7.52 kw, the maximum temperature of about 1447 C. The temperature distributions also increase with increasing the FR. Fig. 4(b) shows typical temperature profiles along the axial axis of the radiant tube in case of SRTB with APS to preheat the combustion air. In this case, the porous absorber, porous emitter, and preheating box were installed to produce the energy recirculation from exhaust gas to the combustion air. The combustion air at T a, was induced by the momentum jet of LPG from surrounding air, then flow through the porous absorber. Therefore, the combustion air was preheated and flow into the preheating box at T a,1 and then flow to the mixing tube inlet at T a,2. At the radiant tube exit, the porous emitter was installed for the exhaust gas flow pass. A part of energy of exhaust gas was absorbed and radiated to the separating wall and the porous absorber for use to preheat combustion air respectively. The firing rates are the same in case of SRTB. The result shows that temperature distributions in case of SRTB with APS are higher than in case of SRTB. Because of the combustion with preheated air introduce to higher flame temperature. That means the energy saving occurred. 16 T, o C T, o C T a,2 T 1 T 2 T kw 4.69 kw 6.13 kw 7.52 kw T 4 Radiant tube T a, Absorbing Porous Medium (a) (b) T a,1 Ta,2 T 1 T 2 X, m 3.66 kw 4.69 kw 6.13 kw 7.52 kw T5 SRTB T 6 T 7 T 8 SRTB With APS Radiant tube X, m Fig. 4 Temperature distribution in radiant tube (a) SRTB and (b) SRTB with APS T3 T exh T 4 T 5 T Emission Fig. 5 shows the effect of FR on CO and NO x emissions. The measured value of CO and NO x do not reflex actual combustion situation within the radiant tube because the sample probe is located at the exit of the radiant tube. In case of SRTB, almost complete combustion with relatively low CO emission of less than 19 ppm can be achieve because T 7 T8 Emitting Porous Medium T exh 527

5 sufficient combustion air and high temperature to complete combustion. NO x is increase with FR but this is a relatively small in absolute value [4-6]. However, the NO x average maximum value does not exceed 186 ppm within the relative wide experiment range of FR. In case of SRTB with APS, CO and NO x emission are higher than that of the SRTB. Most of the CO and NO x emissions are below 24 ppm and 18 ppm, respectively, which are relatively low values According to Sullivan and Kendall [7], it was considered that the thermal NO x is sensitive to the firing rate and the prompt NO x is much more sensitive to the theoretical air percent. Thus, at low FR, the prompt NO x level in this experiment is expected to be a major contribution of the measured value because of relatively low PA. The increasing of NO x at high FR might be caused by thermal NO x. CO, ppm, %O CO SRTB with APS CO SRTB NO x SRTB with APS NO x SRTB NO x, ppm, %O FR, kw Fig. 5 Variation of CO and NO x emission with FR 3.3 Primary Aeration The primary aeration, PA, is used to indicate quantitatively whether a fuel-oxidizer mixture is rich, lean, or stoichiometric. It is defined as the ratio of the oxidizer-to-fuel ratio to the stoichiometric oxidizer-to-fuel ratio, which could be expressed in the form PA A/ F 21 % O stoi % O where %O 2 is the oxygen concentration of mixture in the mixing tube, (A/F) stoi is the air-fuel ratio of LPG at stoichiometry. Fig. 6 shows variation of PA of SRTB in case of with and without APS. Within the range of FR studied, PA in all cases of SRTB is almost linearly decreased with increasing FR, because of an increasing in air viscosity. Therefore, the primary air is difficult to entrain into the mixing tube. In case of SRTB with APS, all PA values are lower than that of the SRTB, because the preheating effect causes expansion of the mixture and an increase in its viscosity [8]. 1 (1) 8 SRTB SRTB with APS PA, % FR, kw Fig. 6 Variation of PA with FR 528

6 3.4 Recirculation Fraction The recirculation fraction is a parameter use for indicate the preheating performance of the burner, which could be defined as the ratio of energy for air preheating to heat of combustion, as expressed by Tpre Ta, r (2) Tmax Ta, where T pre is the preheated air temperature, T max is the maximum temperature in radiant tube and T a,o is the ambient temperature. The calculated of r from Eq. (2) is shown in table 1. Table 1 Calculation of recirculation fraction FR, kw T pre, o C T max, o C r , , , , , , , , Fig. 7 shows an effect of FR on r for SRTB with APS. The calculation shows that r increase with FR because the increase of preheated air temperature more increases than an increasing in the maximum temperature in radiant tube, as shown in Fig. 4. A high r results in a higher energy saving [1]. It means that a fuel usage in the SRTB with APS is less than the SRTB at the same T max r FR, kw Fig. 7 Effect of FR on r for SRTB with APS Preliminary experimental results of SRTB with and without APS are depicted in this research. It shows that the selfaspirating burner technique can be integrated with the radiant tube burner and provides a uniform temperature distribution in radiant tube and low emissions. So the SRTB is possible to apply in the small and medium scale enterprises (SMEs) in Thailand, especially in drying process. 4. CONCLUSION A study and preliminary test of the self-aspirating radiant tube burner both with and without APS has been carried out. Burner performance and combustion characteristics of SRTB were clarified through measured thermal structure in term of temperature distribution along the axial axis. The following conclusions can be drawn from the experimental results; 1. The SRTB can be operated with high turndown ratio, i.e. FR in range of 3 to 8 kw and without flame flashback into the mixing tube. 2. Temperature along the axial locations in case of SRTB with APS is higher than those of without APS for all FR. 529

7 3. The emission of CO and NO x were found to be low about of less than 24 ppm and 18 ppm, respectively. 4. The recirculation fraction increase as FR increase. Increasing in the primary aeration in order to obtain a more complete combustion and find optimum dimension of the SRTB and in case of with load are the topic of study in near future. 5. REFERENCES [1] Weinberg, F.J. (1971) Combustion temperatures: the future, Nature, 233, pp [2] Katsuki, M. and Hasegawa, T. (1998) The science and technology of combustion in highly preheated air, in: 27th Symposium (International) on Combustion/ The Combustion Institute, pp [3] Echigo, R. (1982) Effective energy conversion method between enthalpy and thermal radiation and application to industrial furnace, Proceedings of the 7th Heat Transfer Conference, Munich, 6, pp [4] Jugjai, S. and Rungsimuntuchart, N. (22) High efficiency heat-recirculating domestic gas burners, Experimental Thermal and Fluid Science, 26(5), pp [5] Scribano, G., Solero, G., and Coghe, A. (26) Pollutant emissions reduction and performance optimization of an industrial radiant tube burner, Experimental Thermal and Fluid Science, 3, pp [6] Yoksenakul, W. and Jugjai, S. (211) Design and development of a SPMB (self-aspirating, porous medium burner) with a submerged flame, Energy, 36(5), pp [7] Sullivan, J.D. and Kendall, R.M. (1992) Thermal performance and NO x emissions from porous surface radiant burners, International Gas Research conference, pp [8] Namkhat, A. and Jugjai, S. (21) Primary air entrainment characteristics for a self-aspirating burner: model and experiments, Energy, 35(4), pp