Techniques for measurement of heat flux in furnace waterwalls of boilers and prediction of heat flux – A review

Accepted Manuscript Techniques for measurement of heat flux in furnace waterwalls of boilers and prediction of heat flux – A review G. Sankar, A. Chandrasekhara Rao, P.S. Seshadri, K.R. Balasubramanian PII: DOI: Reference:

S1359-4311(16)30304-0 http://dx.doi.org/10.1016/j.applthermaleng.2016.03.013 ATE 7880

To appear in:

Applied Thermal Engineering

Received Date: Accepted Date:

29 October 2015 4 March 2016

Please cite this article as: G. Sankar, A. Chandrasekhara Rao, P.S. Seshadri, K.R. Balasubramanian, Techniques for measurement of heat flux in furnace waterwalls of boilers and prediction of heat flux – A review, Applied Thermal Engineering (2016), doi: http://dx.doi.org/10.1016/j.applthermaleng.2016.03.013

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TECHNIQUES FOR MEASUREMENT OF HEAT FLUX IN FURNACE WATERWALLS OF BOILERS AND PREDICTION OF HEAT FLUX – A REVIEW Sankar G*, Chandrasekhara Rao A, Seshadri P S and K.R.Balasubramanian& Bharat Heavy Electricals Limited, Tiruchirappalli and &National Institute of Technology, Tiruchirappalli Authors Contact: [email protected] (Tel.: + 91 431-257 5116), [email protected], [email protected] and [email protected]

Abstract Computation of metal temperatures in a furnace waterwall of a boiler is necessary for the proper selection of tube material and thickness. An adequate knowledge of the heat flux distribution in the furnace walls is a pre-requisite for the computation of metal temperatures. Hence, the measurement of heat flux in a boiler waterwall is necessary to arrive at an optimum furnace design, especially for high ash Indian coal fired boilers. Also, a thoroughly validated furnace model will result in a considerable reduction of the quantum of experimentation to be carried out. In view of the above mentioned scenario, this paper reviews the research work carried out by various researchers by experimentation and numerical simulation in the below mentioned areas: i) furnace modeling and heat flux prediction, ii) heat flux measurement techniques and iii) applications of heat flux measurements. Keywords: Applications of heat flux measurement, Furnace modeling, Heat flux measurement techniques, Heat flux prediction, Indian-coal fired boilers. 1. Introduction A boiler furnace is an enclosure of tubes carrying water/steam. Tube failures are imminent when there is excessive heating due to reduction of fluid flow in the furnace wall or due to a sudden increase in incident heat flux. Judicious selection of waterwall tube thickness is required to avoid these failures. Moreover, the fluid pressure drop in the furnace wall is also affected by the thickness of the tube. Hence arriving at an optimum thickness is required to achieve a trade-off between pressure drop and metal temperatures. This requires a lucid understanding of heat flux profile in the furnace, especially for boilers firing high ash Indian coal. Heat flux and metal temperature are to be measured for a large permutation of operating variables like furnace size, operating load, water wall flow, type of coal fired and ash deposition for obtaining a complete Page 1 of 28

understanding of the heat flux absorption in the boiler. But a large scale experimentation requires meticulous planning and a sufficiently large number of measurements which will lead to the temporary shutdown of the powerplant. A furnace model capable of predicting heat flux based on the specified operating parameters will be able to mitigate this problem. A thoroughly validated model with a set of experimental results will be an effective tool in the determination of boiler furnace heat flux for a variety of operating variables. Hence, a comprehensive review of research work involving furnace modeling, heat flux prediction, heat flux measurement techniques and its applications is essential. The aim of this review is to identify research gaps in the above mentioned areas and suggest suitable methods for heat flux measurement and prediction for the boiler industry. Accordingly, the review has been divided into three sections: 

Heat flux prediction and furnace modeling



Heat flux measurement techniques



Applications of heat flux measurement

2. Furnace modeling and heat flux prediction 2.1. Numerical simulation and experimental analysis Rajaram and Abraham [1] described an iterative procedure for the calculation of furnace heat flux from the measured metal temperatures for a boiler with tangent tube construction. The authors formulated finite-difference equations using the procedure given by Adams and Rogers [2] and solved the same using relaxation technique and obtained a relationship between outside heat flux and total temperature drop. Heat flux obtained using the iterative procedure was compared with heat flux arrived using enthalpy pick up method and was found to be in good agreement. Hwang and Howell [3] calibrated a furnace model based on PCGC-3 (Pulverized coal gasification/ combustion – 3 dimensions) code developed by Brigham Young University. The authors used measurements of local gas temperature distributions, local radiative and total wall heat flux distributions and stack NOX for model calibration. Measurements were done on the Unit-2 boiler of Fayette power project which was a Combustion Engineering designed cornerPage 2 of 28

fired pulverized-coal boiler of 606 MW capacity. The model was based on generalized threedimensional Cartesian co-ordinates with Reynolds or Favre averaging of Navier-Stokes equations for turbulent flow. Radiative heat transfer was modeled using discrete ordinates approach that accounted for variable properties along each ordinate. Wall emissivities of 0.38 and 0.6 were used for the furnace models. Radiant heat flux was measured using ellipsoidal radiometer and total heat flux (conduction + convection + radiation) was measured using total heat flux meter. The authors concluded that the higher emissivity model gives better agreement with the experimental results. Adam and Marchetti [4] developed a dynamic simulator to model large boilers with natural recirculation. Two non-linear models were used separately for the evaporation in the vertical waterwall tubes and phase separation in the drum. Many research papers [5-15] are available on the modeling of boiler furnace using computational fluid dynamics (CFD). However most of these researches are on the modeling of the gas side behavior as discussed henceforth. Yamomoto et al. [5] developed a computer program for the analysis of combustion, radiation heat transfer, changes in flow rate distribution and waterwall temperature in pulverized-coal fired boilers. Radiation heat transfer was modeled using discrete transfer method. Kouprianov [6] studied the temperature and heat flux patterns in the furnace of a 500 MW opposite-wall fired boiler firing high ash coal by considering the furnace zone-byzone using the advanced zonal computational method. Xu et al. [7, 8] developed a numerical model for predicting furnace performance parameters like temperature distribution, oxygen concentration and heat flux profile of a front wall fired utility boiler. The authors used k-ε eddy viscosity model for solving turbulent mixing in the furnace. The authors also compared the predicted incident heat flux with the actual measured values and both were in good agreement. Fan et al. [9] compared both the standard k-ε model and RNG k-ε model in the modeling of tangentially fired furnace and concluded that the latter leads to a reasonably accurate prediction of the aerodynamic field. Yin et al. [10] studied the gas flow patterns, temperature distribution and NOx distribution in a 609 MW tangentially fired utility boiler firing pulverized coal. The model was developed using FLUENT. Diez et al. [11] provided a review of online simulation techniques used in the modeling of pulverized coal boilers. Vuthaluru [12] studied the effects of operating parameters like gas temperature profile, radiation heat flux and residence time in a wall fired furnace using FLUENT. The non-premixed combustion model was used for reaction chemistry. Zone method of analysis was used by Bordbar and Hyppanen [13] for the prediction Page 3 of 28

of incident radiative heat fluxes on the furnace walls of a natural circulation boiler. Belosevic et al. [14, 15] studied the effects of the coal quality and grinding fineness on the operation characteristics like flue gas temperature and furnace wall radiation flux using k-ε gas turbulence model. The authors used six-flux method for radiation modeling. The predictions were compared with full scale measurements and were found to be in good agreement. Zhang et al. [16] conducted a systematic investigation on the heat flux distribution inside the furnace of a 300 MW CFB boiler. Temperature was measured at 177 locations on the rear and left walls at 5 elevations in a 300 MW CFB boiler. At each location three thermocouples were welded on the insulation side to measure the temperature distribution. Heat flux profile was calculated by means of finite element method. Chinsuwan et al. [17] developed a mathematical model for predicting the profile of heat flux on the crest of membrane wall along the height of circulating fluidized bed furnaces. In the above mentioned researches, there was little mention about the effect of ash deposition on the heat absorption. Ash deposition related parameters like deposition thickness, its growth and the physical state of deposition are vital parameters that affect the heat absorbed by the tube. Hence some light is thrown on literature available with respect to ash deposition modeling and the effect of ash deposition on heat transfer in the next section. 2.2. Models related to study of ash deposition Wang and Harb [18] developed an ash deposition model and integrated the same into the combustion code PCGC-3. The ash sub-model included the effects of operating conditions and also the chemistry of ash on the formation of deposits. Particle impaction modeling was based on statistical particle cloud model by Litchford and Jeng [19], [20] which was suitable for a heterogeneous turbulent flow field as in a utility boiler. Particle sticking and growth of deposition on the boiler walls was modeled using a modified version of deposition model developed by Richards et al. [21]. In this mode, the deposit growth was simulated using the incident heat flux, particle impaction rates and the size and composition of fly ash distribution. Lee and Lockwood [22] developed a predictive scheme based on CFD to study the slagging propensity of coal. The predicted heat flux to the slag panel was compared with actual measurements in a pilot scale test facility and the variation was within 8%. Simulation of ashPage 4 of 28

deposits in pulverized coal fired boiler was also attempted by Fan et al. [23], where the authors mentioned about the usage of k-ε and the RNG k-ε models for closure of turbulence equations. Sufficient research [24-27] has also been done on investigating the effect of ash deposits on heat transfer. Adams et al. [28] developed a CFD based model using GLACIER CFD code to study the properties like gas temperatures, CO and NOx concentrations, deposit thickness, sintering and surface temperature and furnace heat flux applicable to bio-mass firing generated ash deposits. Chernetskii et al. [29] developed a mathematical model for studying the slagging of furnace of a pulverized coal fired boiler. Fly ash formation, particle transport to surface, particle sticking, deslagging, deposition growth and properties were all included in the model. Mu et al. [30] carried out numerical simulations on characterizing the ash particle deposition on an industrial scale heat recovery steam generator (HRSG) using FLUENT. Field measurements from an industrial scale HRSG were used to validate the model. From the review of ash deposition modeling it is observed that most of the authors suggest using the k-ε model for closure of turbulence equations. Also, most of the authors used the Eulerian approach for continuum phase and Lagrangian approach for the coal particles. The coal particles were considered as various computational sections and characterized by a normal Gaussian probability density function (PDF) in space. The data available in the above literature will be helpful in modeling ash deposition on the waterwall tube or the measurement device. 2.3. Thermal-hydraulic modeling A few authors [31-33] have also attempted thermal-hydraulic modeling of boiler wherein the waterwall of the boiler is treated as a flow network containing series and parallel loops, pressure grids and connecting tubes and a mathematical model is developed based on mass, momentum and energy conservation of the above said components. These simultaneous non-linear equations are solved to obtain the distribution of mass flux, pressure drop, void fraction, metal temperature and other properties along the furnace wall. Tucakovic et al. [31] modeled a 350 MW lignite fired boiler with outlet steam parameters of 18 MPa and 540 °C with predominantly rifled tube waterwalls. Jie-Pan et al. [32] performed a thermal-hydraulic analysis of vertical water wall of the 1000 MW ultra-supercritical boiler at Yuhuan power plant. In another research, Jie-Pan et al.

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[33] analyzed the thermal-hydraulic performance of a low mass flux 600 MW CFB supercritical boiler. From the review of furnace modeling, it can be summarized that most of the researchers have focused on modeling the furnace side of the boiler even though a few researchers have concentrated on the thermal-hydraulic modeling of the waterwall. Nevertheless, very few research articles are available on the accurate prediction of heat flux for high ash coal fired boilers. Hence, a research involving the development of a model for the prediction of heat flux for high ash coal fired boilers and validation of the same will be of high importance in arriving at an optimum boiler design. 3. Heat flux measurement techniques It is essential to explore the various methods available for heat flux measurement and then weigh in on the merits and demerits to enable selection of an appropriate method for measurement. In the forthcoming section, heat flux measurement techniques available in literature from the late 1960s to present are discussed. 3.1. Widely used heat flux measurement principles and devices Extensive review on various heat flux measurement techniques applicable to the scientific community is available in [34-40].

There are various methods of classifying heat flux

measurement techniques based on the available literature. One such classification is as follows: (i) gradient method and (ii) transient method. In the gradient method, temperature is measured at discrete locations in the material with known thermal conductivity and then heat flux is computed based on the Fourier’s law of heat conduction. This method is mostly applied to conditions where steady state prevails. In the transient method, the amount of thermal energy absorbed by the device is measured as a function of time. Rate of change of temperature of the sensing element is measured and then related to heat flux on applying heat balance. Heat flux measurement devices based on the gradient technique were developed by Gardon [41, 42], Hager [43] and Godefroy et al. [44] to measure the total heat flux. The principle used by Gardon [41, 42] has been used to developed heat flux meters for measuring the heat flux in boiler waterwalls as discussed in section 3.2.3. For applications involving unsteady state measurements Page 6 of 28

transient method is more suitable since it involves measurement of temperature as a function of time and research work focused on the same are discussed in brief henceforth. Diller et al. [35] developed a slug calorimeter which involved the measurement of temperature at a single location of a ‘slug’ which was thermally insulated from the surroundings. Heat flux was computed using lumped mass approximation method. Another version of slug calorimeter for high temperature environments was developed by Hubble [45]. But calorimeters are best suited for fire related research and using the same for extensive heat flux measurement in a boiler waterwall is expensive. Anson and Godridge [46] developed a heat flux sensor for measuring heat flux in an oil-fired boiler. This device was based on the layered gauge method of measuring heat flux where two separate heat flux gauges were used to measure the radiation and convection component of the heat flux. Water-cooled configuration was used to enable measurements in high temperature applications such as boiler. But usage of this device for a coal-fired boiler which has contamination in the form of ash remained questionable. This led to the development of transpiration radiometers [47-54] in which air was blown through a porous plug surface that was exposed to radiation. The measurement of the temperature difference between the air flow and the porous surface exposed to the radiation was proportional to the incident heat flux which has been explored by many researchers. More recently, a hybrid method for measuring heat flux by combining the gradient and transient method was developed by Hubble and Diller [55]. Also, Ploteau et al. [56, 57] developed thermoelectric flux meters for heat flux measurements in industrial infrared furnaces. 3.2. Heat flux measurement principles applicable to boiler waterwall Even though heat flux measurement principles could be classified as discussed above, a classification more related to heat flux measurement in boiler waterwall was provided by Marner and Henslee [34]. The authors have discussed in brief about the use of heat flux meters in measuring gas side fouling using two major principles viz. radial disk principle and guarded cylinder technique. The radial disk principle employs the principle of measuring temperatures at two locations on the disc at the center and the periphery as shown in the Fig. 1. The difference in temperature due to the thermal resistance of disc is a direct indication of the heat flux to the fouled water wall.

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Fig. 1. Schematic of radial disk principle In a guarded cylinder technique as shown in Fig. 2, the temperature gradient across a cylinder of known thermal conductivity is measured using a number of thermocouples and the heat flux is computed. A thermal guard is used to ensure that the heat flux within the cylinder is axial.

Fig. 2. Schematic of thermally guarded measuring cylinder principle Even though the radial disk principle is easy to use, the method does not always guarantee an accurate measurement of heat flux, since it is mounted on the surface of the tube. This is because, the measurement depends on the heat flux on the top and bottom surface of the device. Any change in the varied operating conditions mainly due to ash deposition will lead to a considerably erroneous indication of heat flux. The guarded cylinder technique does not have this limitation, but it is difficult to implement owing to requirement of high manufacturing accuracies. Page 8 of 28

3.3. Devices used for heat flux measurement in boiler waterwall From the literature available, devices used for heat flux measurement in boiler water-walls can be classified as: (i) radiometers (ii) portable heat flux meters inserted in inspection ports (iii) Gardon type heat flux meters welded to the section of boiler tubes (iv) tubular type instruments placed between two adjacent boiler tubes. 3.2.1. Radiometers Martins et al. [50] proposed a hemispherical radiation heat flux meter to measure the difference between hemispherical radiation heat flux and convective heat flux. The device was based on the principle of determination of temperature difference of the gas flowing through the porous filament exposed to the respective heat flux to be measured. A numerical analysis of the instrument was also carried out with the help of two mathematical models: i) integral energy and mass balance ii) numerical solution of the continuity, momentum and energy equations. The authors suggested to use highly porous element with large diameter, small thickness and low heat transfer efficiency for measuring radiation heat transfer and lowly porous element with small diameter, large thickness and high heat transfer efficiency for measuring convective heat transfer. Martins et al. [51] experimentally verified and calibrated a blow-off heat flux sensor for measuring total-hemispherical radiation with the heat flux range of 40-150 kW/m2. However, the hemispherical radiation heat flux meter cannot be used for coal-fired boilers, since the heat flux involved is considerably higher of the order of 350 kW/m2. Martins et al. [52], [53] also developed a transient transpiration radiometer for measuring separately the convective and radiative components of heat flux in the range of 20-700 kW/m2. Calisto [40] developed a transpiration radiometer for the measurement of convective and radiative components of the heat flux separately. The meter was capable of measuring transient response as well with minimal response time on account of the lower overall thermal inertia. The author formulated an analytical model and solved the same concurrently using Green’s function method and finite difference method. Numerical simulations were also carried out using computational fluid dynamics (CFD). The results were experimentally validated by measuring the heat flux in a blackbody tubular furnace open to atmosphere.

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Even though these kind of sensors can be used as a part of online boiler and furnace diagnostic systems, the usage of these for the measurement of heat flux in a boiler waterwall for the purpose of understanding the heat flux behavior is not recommended. This is because large number of sensors are required for this purpose and using a sophisticated sensor such as a radiometer will be expensive. Moreover a measurement device like radiometer which is mounted on the surface of the boiler tube or inserted through the peep holes in the boiler is bound to have some inaccuracies when there is excessive ash deposition as discussed in section 4. This problem will be aggravated when using these devices in high-ash Indian coal fired boilers. 3.2.2. Portable heat flux meters Portable heat flux meters [58, 59] which are free from variable attenuation effects of ash deposition can be used for measuring incident radiation heat flux in a boiler furnace. A device named fluxprobe [59] was designed for measuring values of incident radiation flux primarily in membrane walls with an accuracy of +5%. Even though heat flux probes provide the direct means of monitoring local slag deposits in a boiler, they have the following demerits which leads to a conclusion that they are not ideal for an application involving heat flux measurement in large scale: 

Multiple probes are needed to measure the heat flux accurately and also to monitor the critical locations within a boiler.



The heat absorbed by the probe vary based on plane of measurement, orientation of flux probe and ash deposition effects.



Non-uniform furnace cleanliness during the measurement period leads to measurement of heat flux not corresponding to a single cleanliness state of the boiler



Limitations regarding to accessibility of various areas of the furnace.

3.2.3. Gardon type heat flux meters Gardon type heat flux meters have been the subject of research of many researchers [60-67]. These are heat flux measuring devices welded to the surface of the boiler tube. They predominantly work on the radial disc principle. Semenovker et al. [61] developed a heat flux meter to measure the heat flux in waterwall tubes of supercritical boiler. The device comprised of machined cylinder of thickness around 10mm with two bores for inserting two chromel-alumel Page 10 of 28

thermocouples insulated by quartz. 40 such meters developed by Semenovker et al. [61] were used by Nosov et al. [65] in their investigation of heat flux in waterwall of once through boiler. TEC (Technology for Energy Corporation) developed a thermal response probe and installed the probe at the TVA Bull Run Steam Plant and TVA Kingston Unit 5 [38]. The probe was designed to sense instantaneous heat flux and skin temperature of tube to provide data about slag build up in the boiler. CEGB (Central Electricity Generation Board) in partnership with Land Combustion Corporation developed a Fluxdome [38] as shown in Fig. 3. This device was used for absorbed heat flux measurement in the 1970s. Heat flux was determined from the temperature difference across the pair of thermocouples whose leads were protected from furnace atmosphere by a shield welded to the waterwall tubes. The dome shaped profile was used since it was most likely to collect a representative thickness of ash.

Fig. 3. Schematic of CERL / Land Fluxdome SAIC (Scientific Applications International Corp.) developed an instrument named Boiler Thermowell [38] in the early 1990s. The device as shown in Fig. 4 consisted of four thermocouples located in a housing with an open space to permit heat transfer from outer thermocouples which are surface response thermocouples to inner thermocouples which are gradient thermocouples. 18 SAIC probes were installed in the combustion zone at TVA-Bullrun powerplant in 1992 to supplement the previous study done with TECs thermal response probe. Page 11 of 28

The study also employed usage of chordal thermocouples for evaluating soot blower effectiveness.

Fig.4. Schematic of SAIC Boiler thermowell Since the fluxdome, thermowell and the heat flux meters are mounted to the surface of the waterwall tube, the effect of non-uniform ash deposition on the measuring device and the tube leads to inaccuracies in the computed values of heat flux. Heat flux instrument used for measuring absorbed heat flux should represent the boiler tube as closely as possible in terms of both radiant heat transfer and surrounding atmosphere like ash deposition. This requirement is satisfied by tubular type instruments. 3.2.4. Tubular type instruments Tubular type instruments were explored by [39], [68-78]. Taler [68] compared two methods of determining heat flux from the measurement of temperature. The first method as shown in Fig. 5 was based on measurement of tube temperature at two locations viz. crown and fin tip and fluid temperature at the rear of the tube. The second method was based on measurements made at multiple locations. Taler concluded that both methods can be used to measure accurately the heat flux. However, the first method is relatively simple, since installation of thermocouples is simpler and computation time required is less.

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Fig. 5. Detail of thermocouple installation Flux tube as shown in Fig. 6 constructed by Taler et al. [72] consisted of a short length of eccentric tube containing four thermocouples on the furnace side below the inner and outer surface and a fifth thermocouple at the rear of the tube.

Fig. 6. Schematic representation of flux tube Taler et al. [76] developed three different type of flux tubes to identify steady-state boundary conditions in water walls of steam boilers. The first one was constructed from a short length of eccentric smooth tube containing three or five thermocouples and was used to measure the heat flux to water walls made from bare tubes. The second one had two longitudinal fins which were welded to the eccentric smooth tube but were not welded to the adjacent water wall. The third Page 13 of 28

flux tube had the fins attached to the waterwall tubes instead of being welded to the flux tubes. The second and third variant were used to measure the heat flux to membrane walls. In both the variants flux tube was not attached to the adjacent waterwall tubes, thereby making the temperature distribution in the flux tube independent of temperature fluctuations in the water wall tubes. Since significant difference in water temperature can occur in neighboring tubes in a once-through supercritical boiler with vertical wall configuration, flux tubes can be specifically used in those cases to identify steady state boundary conditions. In all the three variants, the thermocouple placed on the rear of the tube was used to measure the bulk fluid temperature. This is because the heat flux on the insulated rear side of the wall is zero in membrane walls which leads to a very little difference in temperature between the wall and fluid. Sobota and Taler [77] proposed a simplified method of determining the absorbed heat flux and heat transfer coefficient on the inner surface of the waterwall tubes. As the fluid temperature, the tube wall temperature on the rear side was assumed, measured temperature was approximated using the appropriate functions and the least squares method. The functions describing the temperature measured within the insert at four locations are functions of the heat flux and heat transfer coefficient on the internal surface of the insert. Method of determining absorbed heat flux and heat transfer coefficient was very fast, as there was no need for the determination of the temperature field in the measuring insert at every iteration step. Two of the currently manufactured sensors include the Applied Synergistics heat flux sensor and Chordal thermocouples [38]. Applied Synergistics sensor was based on the previously used CERL/Land Fluxtube. It was offered as a part of smart soot blowing system to measure heat flux in the furnace area using probes and also measure gas temperatures in the convection pass. The device was designed to use as a permanently installed device in the furnace waterwall. Heat flux measurement is done using thermally guarded cylinder technique. Chordal thermocouples (Fig. 7) can be used in heat flux evaluation by measuring tube wall temperatures. Holes are drilled along the chords of a tube wall and the thermocouple wire is installed through these holes. These sensors are generally fabricated in the shop because of the necessity to maintain a level of accuracy.

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Fig. 7. Typical chordal thermocouple installation It can be summarized that tubular type instruments are more accurate than Gardon type heat flux meters (surface mounted devices) when it comes to heat flux measurement in a boiler waterwall, since they represent the waterwall tube closely in terms of both heat transfer and ash deposition. Moreover, it is clear that using chordal thermocouple for measurement of heat flux is appropriate for large scale experimentation since it is less expensive. However, this area of heat flux measurement needs further exploration. 4. Applications of heat flux measurement in boiler waterwall Measurement of heat flux has been widely used in the boiler industry for monitoring of ash deposition on water wall tubes and the development of smart soot blowing system. Valero and Cortes [79] discussed about ash-fouling in coal-fired utility boilers and the ways to monitor ash fouling and online cleaning of ash-build up. Chambers et al. [62-64, 66] worked extensively on the development of a heat flux meter to monitor ash build-up in boiler furnaces. Neal and Northover [58] studied the effects of ash deposits on the measurement of radiant flux in large boiler furnaces using simple finite element computer models using heat conduction computer program FLHE. The effects of ash deposits on surface mounted device similar to a Gardon type heat flux meter and a tube wall inserted device were studied and tabulated as in Table 1. The authors concluded that, when ash deposits tend to occur, the heat absorbed reduces more in the surface mounted device than in the thickened tube device. This was aggravated when Page 15 of 28

the device operated with a higher surface temperature than the adjacent boiler surface. The authors also suggested the use of guarded cylinder technique, since it has an advantage that calibration is unaffected by modest amounts of surface corrosion. Table 1. Errors in boiler tube heat flux measuring device as a result of ash deposition [58]

Sl. No.

Boundary Conditions

Actual*

Surface mounted device

Tube wall inserted device

Indicated*

Indicated*

Clean surfaces (No ash) 1 120 120 Identical surface temperature on the measuring device and the boiler tube 1 mm ash deposit on device 2 1 mm ash deposit on tubes 100 100 Identical surface temperature on the measuring device and the boiler tube 2 mm ash deposit on device 1 mm ash deposit on tubes 3 100 72 Identical surface temperature on the measuring device and the boiler tube 2 mm ash deposit on device 4 1 mm ash deposit on tubes 100 70 Surface Temperature elevation 50 K 2 mm ash on device 5 1 mm ash on tubes 100 61 Surface Temperature elevation 180 K * - Normalised values (expressed as percentages) of heat flux into the boiler tube

120

100

100

96

72

In another research [59], authors used both flux tube and dometer for measuring heat flux absorbed locally by boiler tube under prevailing ash conditions and a flux probe to measure local values of incident radiation. Accuracy of the flux tube was around +5% and accuracy of dometer was around +10%. On the other hand, the flux probe operated at an accuracy of +5% while measuring incident heat flux.

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Bueters et al. [80] developed a soot blower system capable of operating selectively individual soot blowers on an independent basis in response to the build-up of ash deposition on the furnace wall in the vicinity of soot blower system. It was facilitated by the use of three or four heat transfer rate sensors for each soot blower as shown in Fig. 8. The device was mounted to the crown of the water wall tubes and also on the webs on the furnace side between adjacent waterwall tubes. The sensor consisted of two thermocouples spaced apart from each other. The hot thermocouple lead sensed one temperature and the cold temperature lead sensed a temperature less than the one sensed by hot thermocouple because of the presence of insulating material in between. This difference in temperature was a direct indication of local heat transfer rate passing through the ash deposit into the furnace wall.

Fig. 8. Heat transfer rate sensor- Bueters et al. [80] Afgan et al. [48, 49 and 81] worked on an air stream cooled ‘clean’ heat flux meter to monitor ash deposits in boiler. The flux meter was based on the disc type receiving element concept. The meter was protected from ash deposition by the air stream. Paist et al. [82] discussed about probes for measuring both incident and absorbed radiant fluxes in a boiler for investigation of fouling intensity. A separate hemispherical probe was developed for measuring incident heat flux. For measuring absorbed heat flux, various probes like sampling probe cooled by compressed air, calorimetric probe with four different detector surface temperatures and probes with potassium cooled heat pipe were suggested. All these probes were capable of being inserted into the peep holes of the furnace.

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Melvin and Scott [83] developed a heat flux meter as depicted in Fig. 9 to measure the heat flux absorbed by the furnace wall for the purpose of detection of deposition inside waterwall tubes. The meter attached to the membrane wall used two thermocouples inserted to different depths to sense the temperature difference between the two measuring points and thereby arrive at the heat flux absorbed at the membrane. A correction was applied to obtain heat flux absorbed at the crown. This meter was different from a chordal thermocouple or a fluxdome in a way that, drilling of holes on tube was avoided.

Fig. 9. Schematic of Heat flux meter- Melvin and Scott [83] Breen et al. [84] suggested a method of controlling the corrosion of furnace water wall tubes using a pair of thermocouples attached to the backside of the furnace wall. One thermocouple of each pair was attached to the tube and a second thermocouple was attached to the fin connected to that tube and temperature differential between two was measured at specific time intervals. A decrease in temperature difference indicated that ash has accumulated whereas an increase in temperature difference indicated that the formed slag has melted leading to further corrosion. Teruel et al. [85] developed a systematic approach to predict ash deposits in coal-fired boilers by means of neural networks. For the development of model, ash fouling was monitored in a 350 MW boiler with the help of 30 flux meters of flux tube design developed by Neal et al. [59]. Calisto et al. [86] developed a diagnostic system for boilers and furnaces by integrating CFD and artificial neural networks. Incident radiation heat flux on the boiler surface was used as an input for simulating the fouling phenomenon. Page 18 of 28

Numerous research papers and patents are available on the application of heat flux measurement in monitoring of ash fouling in waterwalls of boiler. Optimization of soot-blowing is the end requirement in many of the above discussed research work. The measurement techniques discussed are also based on deriving heat flux from basic conduction principle by measuring temperature at two different points. Various authors have used different locations for measurement. Since only an indication of ash fouling is required, very few of the above techniques explained above will result in an accurate measurement of heat flux. Hence, there is a need to develop a measurement technique concerning the application of understanding the heat flux variation over the furnace waterwall along the height and also laterally across the width of the furnace. This requires an accurate measurement of heat flux either directly or using temperature measurement using chordal thermocouple. However, the latter area need to be explored further. 5. Conclusion A comprehensive review of research work involving furnace modeling and heat flux prediction, heat flux measurement techniques and application of heat flux measurements is presented. From the review it is evident that, most of the heat flux measurement applications involve optimization of soot-blowing by monitoring the extent of ash fouling. But there is a need to attempt heat flux measurement to understand the heat flux distribution in the furnace both along the height of the furnace and laterally along the width of the furnace to ensure optimum furnace design. From the measurement techniques available in literature, it is clear that the usage of chordal thermocouple technique is more viable for obtaining large scale measurements. A valid computational procedure to estimate the heat flux from the measured temperatures also needs to be developed. The structural integrity of the drilled tube and the effect of ash deposition on the measurement also needs to be investigated. A research focusing on the above mentioned areas will benefit the boiler industry in achieving an appropriate understanding of the heat flux distribution in the furnace of the boiler and thereby result in an optimum design of boiler furnace. 6. Acknowledgement The authors wish to thank the management of Bharat Heavy Electricals Limited for having granted permission to publish this paper. Page 19 of 28

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REVIEW HIGHLIGHTS    

Heat flux measurement techniques applicable to boiler waterwall are elaborated. Applications involving heat flux measurement in boiler waterwall are discussed. Appropriate technique for usage in high ash Indian coal fired boilers is required. Usage of chordal thermocouple is suggested for large scale heat flux measurements.

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