Heat transfer characteristics of HiTAC heating furnace

Heat transfer characteristics of HiTAC heating furnaceusing regenerative burnersNabil Rafidi *,Wlodzimierz BlasiakRoyal Institute of Technology (KTH),Division of Energy and Furnace Technology,S-10044Stockholm,SwedenReceived 20June 2005;accepted 27December 2005Available online 21February 2006AbstractThe aim is to experimentally study the various modes of heat transfer and to investigate the effect of the HiTAC flame characteristics on the heat transfer intensity and uniformity inside a semi-industrial test furnace using various industrial regenerative burners and var-ious flame configurations namely;single-flame,twin-flame counter,twin-flame parallel and twin-flame stagger.Measurements of local instantaneous and average temperatures,heat fluxes and gas composition at several locations inside the furnace were carried out.It was observed that the HiTAC flame with highly reduced temperature fluctuations,turbulent intensity and combustion intensity have a larger reaction zone than a conventional flame.This large flame emits more thermal radiation in spite of its uniform and reduced tem-perature.Furthermore,the convective heat transfer was found to be uniform and as high as 30%of the total heat transfer to an object surface in the furnace.On the other hand,the very high reduction of NO x emission is a consequence of the low temperature and tem-perature fluctuation levels of the HiTAC flames.The above findings are valid to a similar extent in all burners and configurations but to less extent in the twin-flame counter configuration.Ó2006Elsevier Ltd.All rights reserved.Keywords:Heat transfer;Combustion;Furnace;Regenerative burner1.IntroductionHigh temperature air combustion (HiTAC)is character-ized by reactants of high-temperature and low-oxygen con-centration.Many researchers have recently realized significant energy savings,NO x emissions reduction and heat transfer uniformity in industrial heating furnaces resulting from such novel combustion [1].In practical application,cyclic regenerative burners operate on the principle of short-term heat storage using ceramic regener-ative heat exchangers are utilized in such technology.In fact,the dynamic operation of such burners makes simple measurements difficult to make.Therefore,most of the knowledge about HiTAC was generated either by using small laboratory scale furnaces in which a single jet flamewas studied [2]or by using semi-industrial furnace were the HiTAC process is simulated by using steady-state burn-ers [3,4].In all cases,the use of the dynamically operated regenerative burners was avoided and consequently mea-surements were made easier.Since a complete understand-ing of the heat transfer in industrial furnaces has always been required in the design and optimization of heating processes and to establish combustion and heat transfer conditions prevailing in an industrial furnace,our ambition in this work was to carry out measurements in a semi-industrial furnace equipped with commercial regenerative burner systems.Experimental evaluation of the effect of HiTAC flame characteristics on the heat transfer inside the furnace from different types of burners and at different regenerative burner configurations was carried out.Local probe measurements of heat fluxes,temperature,tempera-ture fluctuation and flue gas composition inside the furnace were carried out.To compensate for the dynamic operation of the burners,the raw data were compensated for the1359-4311/$-see front matter Ó2006Elsevier Ltd.All rights reserved.doi:10.1016/j.applthermaleng.2005.12.016*Corresponding author.Tel.:+4687908366;fax:+468207681.E-mail addresses:nabilr@mse.kth.se ,nabil.hanan@tele2.se (N.Rafi-di)./locate/apthermengApplied Thermal Engineering 26(2006)2027–2034inertia effect of the measuring equipments.Furthermore,in furnaces operating at high temperatures,convective heat transfer is insignificant and rarely investigated and mea-surement of convective heat transfer coefficient has always been challenging.This mode of heat transfer was investi-gated experimentally with the radiative one.2.Experimental apparatusThe experimental facility is a semi-industrial test furnace equipped with two different HiTAC regenerative burning systems both equipped with the‘‘honeycomb’’regenerator. The top view of the test stand(Fig.1)shows the locations of the burners and the observation and test openings.The outer dimensions of the furnace body are2.2·2.2·3.5m. The furnace body is insulated with a0.3m thick layer of ceramicfiber material.Heat is taken away from the furnace by four horizontal air-cooled tubes,heat sink,made of a special temperature-resistant kanthal alloy.Thefirst HiTAC burning system is a so-called single-flame regenerative burner,in which each unit composed of a single burner that comprises three pairs of regenera-tors.In this type,switching occurs between the regenera-tors while the fuel isflowing continuously.The switching time of the burner was constant and set to10s.In fact, the same burner was used for conventional cold-air-com-bustion.Thefiring capacity of this system was200kW. The other system is a twin-type regenerative burner,in which each unit composed of a pair of burners and each burner has a regenerative heat exchanger.The burners work alternately,when one is in combustion mode the other is in regeneration mode to recover energy from the exhaust gases.Thus,as seen in Fig.1,the test furnace is equipped with two units with totalfiring capacity of 200kW.The switching time of the two-burner system was set to30s.Depending on the selection of the individ-ual burners for each pair,there can be three configuration of operation,namely counter,parallel and,stagger.In counter configuration(Fig.2a),the pair of burners A and B comprises one unit and burner C and D comprises another.The parallel and stagger configurations are self explained when looking at Fig.2b and c,respectively. Moreover,the fuel used was LPG(>98%n-propane).2.1.Measurement equipmentsSeveral meters and equipments were used in this study;(1)suction pyrometer to measure the gas temperature,(2) thin-wire thermocouple(80l m in diameter)with a short response time(time constant=25ms)to measurefluctua-tion temperature,(3)ellipsoidal radiometer,first proposed by Nils-Erik Gunners in1967[5],to measure the irradia-tion,i.e.,radiation heatflux incident per unit area falling on a surface,(d)the plug type total heatflux meter to mea-sure the rate of heatflow per unit area on the meter surface, andfinally(e)gas sampling probe to quench the sample down to140°C in a very short time.The samples from this probe were then analysed using a micro gas chromatograph to measure the molar fraction of H2,O2,N2,CH4,CO, CO2and hydrocarbons up to C8.NO x was measured using a chemi-luminescent analyser.The above meters were held inside water-cooled probes. The probes were mounted at the top of the furnace and moved by a computer-controlled traversing system.They Fig.1.Top view of semi-industrial test furnace,openings for probe measurements.2028N.Rafidi,W.Blasiak/Applied Thermal Engineering26(2006)2027–2034were inserted at various points inside the furnace through 13openings at the furnace roof,Fig.1.In each opening,the measurements were performed at every 100mm from the ceiling down to the burner centerline.At locations close to the reaction zones,distances between measuring points were even less (down to 25mm)especially when measuring the flue gas composition.In single-flame system,as an example,more than 930gas samples were analysed using a micro gas chromatograph.However,only 23samples were rejected because the ratios of maximum acceptable deviation to standard deviation were above 1.65according to Chauvenet’s criterion [6].The origin point (0,0,0)was located at the single-flame burner.Moreover,temperatures and flow rates of all streams entering and leaving the furnace were measured to perform the furnace energy balance.2.2.Experimental conditionsThe measurements were performed with one conven-tional cold-air-combustion and four different regenerative HiTAC configuration and conditions.The four HiTAC cases were single-flame,twin-flame counter,twin-flame parallel and twin-flame stagger.Table 1lists the operating conditions of each configuration.The furnace temperature is an average wall temperature measured using six thermo-couples,three along one side and three along the ceiling of the furnace.In all cases,the firing rate,the furnace temper-ature,and cooling-tubes surface area were kept constant.The furnace temperature is controlled by varying the cool-ing air flow rate.3.Results and analysisSimultaneously to probe measurements,energy balance was measured for the test furnace for the cases of single-flame,conventional and HiTAC,and twin-flame configura-tions.Energy output distributions are shown in Fig.3.The percentage of useful energy,i.e.,energy taken by the heat sink (cooling air pipes),resulted by HiTAC configuration was higher than that resulted by conventional flame by 35%and 44%.The relatively high surface heat losses are because the test furnace contains numerous openings for observations and measurements.3.1.TemperatureFig.4shows the measured temperature profiles of single flame combustion using the pyrometer.The half plane to the left is for HiTAC flame and the other half is for conven-tional cold-air combustion.The measured locations are shown with points.In spite of the dynamic operation of switching devises and the regenerators,time variation of the measured temperatures were not observed in all points and the profiles shown are the time average temperature profiles.However,this situation was different in the cases of twin-flame configurations.Therefore,thin-wire thermo-couple was used to measure the instantaneous temperature at various locations.Fig.5shows the temperature profiles of the HiTAC twin-flame system for two configurations;counter and parallel.The profiles are shown only for one unit (burners pair B and D)and the dimensions are relative to burner B.The profile to the left of the burner is for the average temperature during combustion period of burner B while the one to the right is for the averagetemperatureFig.2.Scheme showing the three different configuration of the twin-regenerative burning system.(a)Counter configuration,(b)parallel configuration and (c)stagger configuration.Table 1Operational conditions in the five studied cases Configuration Fuel flow rate (N m 3/h)Fuel type Combustion air flow rate (N m 3/h)Furnacetemperature (°C)Combustion air temperature (°C)Conventional7.7LPG 200110025Single-flame HiTAC 7.7LPG 2101100940Twin-flame HiTAC7.7LPG21011001040N.Rafidi,W.Blasiak /Applied Thermal Engineering 26(2006)2027–20342029during regenerative period of burner B.The temperature profiles of a stagger configuration,not shown,were very similar to the parallel configuration probably because the flame in both configurations has straight axis.It is evidence from Figs.4and 5that the spatial temperature distribu-tions of all HiTAC configurations are more uniform that that of a conventional cold-air combustion.HiTAC flames have significant lower peak temperatures and the time vari-ations of these profiles are also uniform in all the cases except for the twin-flame counter configuration.The spatial temperature uniformity ratio,R T ,was calcu-lated according to Eq.(1)and found to be 1.1,0.33and 0.23for the cases conventional single-flame HiTAC sin-gle-flame,and HiTAC twin-flame counter,respectively.R T ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiX ðT i ÀT ÞT2s ð1Þwhere T i is the measured time average temperature at a cer-tain location or point and T is the time average temperature of all measured points in the plane for a complete cycle.The total error of the measurement was found to be 27°C.3.2.Fluctuation temperature mapsIn order to calculate flame fluctuation temperature,the raw temperature data were compensated for the thermal inertia effects of the thin wire thermocouple using the time constant value obtained from the manufacturer (25ms).Fig.6presents these fluctuations for the twin-flame config-urations and shows small fluctuations in the measured points about one order of magnitude compared to that of other conventional-air flames of [7].Moreover,these fluc-tuations are smaller when the flame axis is straight,i.e.,parallel and stagger configurations.The fluctuation profiles are relatively even and highest fluctuation is in the region closest to the burner exit (opening 6).The low values of fluctuations can be a result of non-intense presence of unburned fuel/air mixture at any measured location (low combustion intensity).The combustion products are first entrained to the combustion air and fuel flow jets and co-located with the unburned gases [3].Thermodynamically,the temperature increase due to combustion is mild when the air and fuel jets are diluted first.Gupta et al.[7]indi-cated that when the fluctuation is small the integral and micro thermal time scales are high and therefore the turbu-lence intensity in the flame is low.3.3.Thermal radiationFig.7shows the measured thermal radiation incidence on the ceiling of the test furnace for the five configurations using the ellipsoidal radiometer.These are the time average values with 8kW/m 2uncertainty.In the twin-flame config-uration,the time variation of the radiative heat flux due to switching was between 5%and 10%varying from point to point.The highest and most uniform radiation was in the case of HiTAC single flame.The uniformity ratios were found to be 0.28,0.18,0.26,0.25and 0.18for the conven-tional,single HiTAC,counter,parallel and stagger config-uration,respectively.Although all test were performed under the same firing capacity and furnace wall tempera-ture,the heat carried out by the air-cooling tubes,the heatFig. 4.Temperature profile of a single flame.(a)HiTAC and (b)conventional-air combustion.2030N.Rafidi,W.Blasiak /Applied Thermal Engineering 26(2006)2027–2034sink,were two to two and a half times higher in HiTAC cases.Consequently,the tubes’outer surface temperature were lower and hence,affecting and reducing the radiation incident on the meter.Therefore,the overall impression about the heat transfer to a heat sink is much higher in the case of HiTAC cases and cannot be judged by the sole reading of this meter.3.4.Thermal convectionMeasurements of the total heat flux (q T ),i.e.,the rate of heat flow per unit area on the surface located in the fur-nace,using the plug-type heat flux meter were performed at different locations.In fact,this total heat flux meters serve to complement the data obtained above by the radi-ative heat flux meter,the irradiance,G r ,and gas tempera-ture measurements,T g ,in order to obtain the convective heat transfer coefficient,h c ,in the interface between the meter surface and its environment.This is done by making an energy balance on the meter plug surface:q T ¼q r þq c ¼a s G r Àe s r T 4s ÀÁþh c ðT g ÀT s Þð2Þwhere q c is the net convective heat flux received by the sur-face (kW/m 2),q r is the net radiative heat flux betweentheFig.5.Temperature profile of a twin-flame in a plane at burner center.(a)Parallel and (b)counter.surface and the hot environment.a s is the absorptivity of the plug surface,e s is the emissivity of the plug surface,r is the Stefan–Boltzmann constant and T s is the temperature of the plug frontal surface.The absorptivity a s in Eq.(2)is nearly always equal to the emissivity e s (gray surface)then we can write Eq.(2)asq T ¼e s G r Àr T 4s ÀÁþh c ðT g ÀT s Þð3ÞThe temperature of the plug frontal surface,T s ,can becalculated by knowing the thermal resistance of the plug and the temperature gradient thereof or by calibration curves.Thus,h c is the only left unknown in Eq.(3).How-ever,since this method for determining h c involves several meters,it is not only costly and time consuming but also lacks the accuracy (25%uncertainty).Therefore,these investigations are only qualitative and were performed only on the single-flame HiTAC.Fig.8shows the measurement results of q T and the cal-culated h c in the right half and the left half of the plane,respectively.It is obvious that values of h c at distances 400mm away from the burner axis and further is uniform and in the range of 70W/m 2K.This corresponds to 60kW/m 2net convective heat flux to the meter surface compared to 135kW net radiative heat flux between the meter plug surface and the hot environment.Thus,the con-vective heat transfer accounts for 30%of the total heat transfer.However,the measurement of convective heat transfer is just qualitative and flow conditions around the meter surface does not prevail a real object or slab to be heated.3.5.Local flue gas compositionFig.9presents the molar fractions of some measured species in the single-flame configurations.In the HiTAC cases,higher concentrations of hydrocarbons and in broader regions in the furnace were detected.For example,the CO molar fraction in HiTAC (graph a)has a peak of 2.45%at 0.6m from the burner.While in the conventional flame (graph b),the peak is 0.6%and at shorter distance from the burner.Almost similar profiles of H2were also detected from the measurements.The propane was not detected at all when operating at conventional-air case while it was detectable downstream to 1.2m from the bur-ner (graph c)in the HiTAC flame.The gradual disappear-ance of the fuel and the appearance of the intermediate species hydrocarbons in relatively high concentrations and at broader regions inside the furnace is evidence of a larger reaction zone (or flame)and of high concentration of radicals that are responsible of the final burn out of these species.In our flame volume investigation [8],the flame volume of an HiTAC flame was about 40times larger than that of a conventional-air combustion flame.More-over,the small gradients of oxygen molar fractions in the furnace,graph d of Fig.9,shows how oxidation is pro-gressing to a large distance downstream the burner.This large flame volume has significant effect on the heat transfer inside the furnace,it compensate for the reduction of radiative heat flux due to the mild temperature increase of the combustion and reduced flame peak temperature (Fig.4).In some cases,it even intensifies the overall radia-tive heat transfer (compare a and b in Fig.7).3.6.NO x emissionIt was observed from the local NO measurements that nitric oxide is rapidly generated to about 40ppm at 350mm downstream from the fuel gas jet (opening 6).Then the NO molar fraction was levelled offdownstream the burner until it exit the furnace at 55ppm in the cases of parallel and stagger configuration and at 75ppm in the case of counter configuration.The results may indicate that prompt mechanism is the dominant one since the rapid generation is in the rich fuel region in which combustionisFig.7.Radiative heat flux profile at burner center levels (a)conventional cold-air,(b)single-flame,(c)twin-flame counter,(d)twin-flame parallel,and (e)twin-flame stagger.2032N.Rafidi,W.Blasiak /Applied Thermal Engineering 26(2006)2027–2034happening between the fuel and the excess oxygen that is entrained with the combustion products into the fuel jet [3].Moreover,in the counter configuration,the more intense combustion and consequently the higher the fluctu-ating temperature may be the reason of a higher thermal NO xgeneration.Fig.8.Calculated convective heat transfer coefficient,h c ,and measured total heat flux,q T.Fig.9.Molar fraction distribution of (a)CO-HiTAC,(b)CO-conv.,(c)C 3H 8-HiTAC,and (d)O 2-HiTAC.N.Rafidi,W.Blasiak /Applied Thermal Engineering 26(2006)2027–203420334.ConclusionMeasurements reveal that high temperature air combus-tion has the following benefits compared to conventional-air combustions:•Enhancement of temperature and heatflux uniformity.•Temperaturefluctuations were far below that of a con-ventional combustion.•The HiTACflame characteristics;low combustion intensity,larger volume,and higher concentration of radicals and intermediate compounds intensifies thermal radiation from theflame,in spite of the mild tempera-ture increase in HiTACflames.•The reduced level of NO is due to the uniform temper-ature profiles,the mild temperature increase due to com-bustion of diluted air and fuel,and the low temperature fluctuations.In fact,about80%of the total nitric oxide was generated at a short distance downstream from the fuel jet.This proposes that NO is mainly generated by the prompt mechanism in HiTAC process.Andfinally,these burners supply combustion air at high temperature and low oxygen concentration making the HiTAC combustion phenomena 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