Pilot-Scale Investigation of Heat Flux and Radiation from an Oxy-coal Flame Part 1: Development of Instrument Models

Conference paper

Pilot-Scale Investigation of Heat Flux and Radiation from an Oxy-coal Flame Part 1: Development of Instrument Models Andrew Fry, Jennifer Spinti, Oscar Diaz, Ignacio Preciado, Kaitlyn Scheib, Stan Harding, Eric Eddings, Philip J. Smith Department of Chemical Engineering and Institute for Clean and Secure Energy, University of Utah, 50 South Central Campus Drive, Salt Lake City, Utah 84112, United States Abstract The work described in this paper is part of the larger mission of the Carbon-Capture Multidisciplinary Simulation Center (CCMSC) (http://ccmsc.utah.edu) at the University of Utah. The purpose of this center is to demonstrate the use of exascale computing with verification and validation/uncertainty quantification (V&VUQ) as a means of accelerating deployment of advanced, low cost, low emission coal-fired power generation technologies. This paper focuses on measurement techniques and the supporting instrument model developments used to determine wall temperature, radiation and heat flux during oxy-combustion experiments in the 1.5 MWth L1500 pulverized coal furnace located at University of Utah's Industrial Combustion and Gasification Research Facility. Introduction Computational Fluid Dynamic (CFD) modeling of high temperature reacting flows has become a complicated fusion of advanced algorithms to capture turbulence, complex boundary conditions, detailed physics and chemistry, and supercomputers. Application of these types of tools for solving difficult problems first requires validation against quantified and well characterized data sets. To accomplish the mission of the CCMSC, a hierarchal validation approach is used to obtain simultaneous consistency among a set of selected experiments at different scales embodying the key physics components (large eddy simulations, multiphase flow, particle combustion and radiation) of a 500MWe oxy-fired boiler. This overall hierarchy is shown in Figure 1. A crucial component of this hierarchal validation approach consists of combustion testing and model validation/uncertainty quantification (VUQ) in the University of Utah's 1.5 MWth pulverized coal furnace. As part of the second experimental campaign of the program, detailed instrument models were developed to quantify potential errors associated with combustion operating conditions and measurements to be used in the VUQ analysis, also known as quantities of interest (QOIs). The QOIs for this test campaign were wall temperatures, cooling surface heat flux, and radiation. Therefore, prior to the testing period, multiple measurement techniques and their associated instrument models were developed and evaluated. The instrument model is used to calculate the desired output values from the raw measured data. This analysis also evaluated instrument uncertainties in an effort to reduce potential bias errors in the measurement. These data will then be used to calculate a heat balance on the furnace and to quantify uncertainty in the collected data. The data will then be used to perform a consistency analysis between model and experiment, through a formalized validation/uncertainty quantification approach (V&VUQ). #12;Figure 1. Carbon Capture Multidisciplinary Simulation Center (CCMSC) program hierarchy with the current effort's placement identified Materials and Methods (Experimental) L1500 Combustor The 1.5 MWth pilot-scale combustor (L1500) at the University of Utah is a pulverized coal-fired furnace that was designed to simulate combustion in low emission, pulverized coal-fired boilers. This unit has been used for many investigations of technologies for NOx and particulate control, including staging, reburning, SNCR and burner development. The reaction zone of this furnace has a one meter, square cross section and is approximately 14 meters in length. The length is divided into 10 sections, each with various sampling and injection ports. The furnace is refractory lined, with cooling panels in the first four sections to maintain realistic boiler temperature profiles. Multiple ports are located in each of the reactor sections, allowing for numerous configurations of sampling, reagent injection and overfire air. The pilot-scale combustor is represented in Figure 2 with some of its features and sample locations detailed. #12;Figure 2. Model representing University of Utah's 1.5 MWth pilot-scale pulverized coal furnace (L1500) As detailed in Figure 2, the L1500 has been retrofitted with a stainless steel pipe, fan and control system to allow flue gas recycle (FGR) back to the burner. These modifications, along with an O2 and CO2 supply and control system, make oxy-combustion experimentation possible. Combustor Modifications The first four sections of the L1500 have heat transfer surfaces on the inside wall next to the refractory. These devices remove a portion of the total heat input to the furnace in a similar ratio to what is removed in the radiant section of a coal-fired utility boiler. Historically these devices have been ½" stainless steel pipe bent into a coil shaped like an accordion. Cooling water flowed through these coils at a rate sufficient to avoid the formation of steam. During coal-fired combustion, the surface of these coils accumulates mineral matter deposits (ash) which over time degrade the effectiveness of removing heat and do not allow the reactor to come to a thermal steady state. The shape and orientation of the surfaces made it difficult to remove the deposits by "sootblowing" because of their location and orientation. In addition, the complex geometry of the coils made them difficult to model and to interpret the total heat removal through instrument models. To circumvent all of these difficulties, the coils in the first two sections of the furnace were removed. Holes were cut through the shell and refractory of the first two sections on each of the side walls. These 6" tall by 36" wide openings allowed access for the insertion of flat plate heat exchangers. These devices are much simpler in geometry, which allows for ease of deposit removal through sootblowing and for more straight forward and robust interpretation of the data through instrument models. The cooling coils were left installed in sections 3 and 4 of the furnace. Figure 3 contains a model cross section of the first four sections of the L1500 detailing the heat removal devices and the sootblowing hardware. The figure and also includes a photograph of a flat plate heat exchanger and coil while the furnace is operating. The flat plate heat exchanger is a device that is heavily leveraged to determine the flame heat release profile and to quantify the total heat removal and local heat flux in the furnace. #12;Figure 3. Model detailing a cross section of the first four sections of the L1500 (above) with cooling panels (sections 1 and 2) and cooling coils (sections 3 and 4) identified and pictures of the cooling panels and cooling coils during operation (below) Measurement Modifications In past experimentation, the temperature of the inside walls of the furnace have been determined using shielded type-B thermocouples. These devices were installed into the furnace through 1" holes in the shell and refractory with the tip of the ceramic shield in line with the inside refractory surface. A model and picture depicting these devices in the furnace is included in Figure 4. Figure 4. Model and picture representing old devices for measuring wall temperature in the L1500 The measurements made by the type-B thermocouples depicted in Figure 4 Error! Reference source not found.were difficult to interpret and not representative of a true wall temperature due to uncertainties associated with the installation of the device, the 1" hole through the refractory, and the two cavities of gas between the actual refractory and the thermocouple bead due to the ceramic shield. For the purpose of better quantifying the inside refractory surface temperature, the shielded type-B thermocouples were replaced with multi-depth type-B thermocouples installed in the refractory. The detailed geometry of these devices is included in Figure 5. #12;Figure 5. Model depicting the new wall temperature measurement device with geometry detailed which influences heat transfer and temperature profiles The remaining QOI for L1500 experimentation is the radiative intensity profile of the flame. This can be measured using narrow angle radiometers installed in the first three sections of the furnace. The radiometers employed by the University of Utah include a probe with a cylindrical cavity, a focusing lens, an irradiated and a non-irradiated thermistor and electronics to determine the difference in resistance between the two thermocouples as shown in Figure 6. Use of the radiometers requires calibration by a radiation source (blackbody radiator) that relates a surface of known temperature and emissivity to an output voltage signal. Figure 6. Model depicting narrow angle radiometer with devices and geometry influencing the measurement of incident radiation Instrument Models Heat Removal by Cooling Coils and Cooling Panels The total heat removal from the furnace using both the flat-plate heat exchangers and the coils is determined by measuring the instantaneous mass flow of cooling water and the temperature of the water as it enters and as it exits the device. The heat flux is then computed from Equation 1 where 𝑚𝑤 ̇ is the measured mass flow rate of water, 𝑐𝑝 is the heat capacity of water, and 𝑇𝑖 and 𝑇𝑜 are the measured inlet and outlet water temperatures respectively. #12;𝑇 𝑄 = 𝑚𝑤 ̇ ∫𝑇 𝑜 𝑐𝑝 𝑑𝑇 𝑖 Eq. 1 Further work is needed to develop an instrument model that analyzes uncertainties associated with 𝑚̇ and with the measurement of inlet and outlet temperatures. Multi-depth Wall Thermocouples - Steel Plates The flat-plate heat exchanger was constructed using a hot-side steel plate with a ½" thickness. This geometry allows the insertion of thermocouples at multiple depths into the hot-side plate, which can be used to quantify the local heat flux and to extrapolate to a hot steel surface temperature. A model depicting all of the relevant geometry of the multi-depth thermocouples installed in the flat plate heat exchangers is included as Figure 7. Figure 7. Model depicting the multi-depth thermocouples installed in the hot-side stainless steel plate of the cooling panels with geometry features influencing heat transfer detailed Figure 8 is a simplified schematic of the heat transfer pathway through the flat-plate heat exchanger (stainless steel plate) to the thermocouple bead. Figure 8. Schematic of the multi-depth thermocouple instrument model for the flat plate heat exchangers #12;The purpose of this model is to compute the heat flux at the hot wall and to estimate the temperature at the metal plate location (Ts) for the thermocouple configuration detailed in Figure 7. The model assumes unidimensional heat flux through each layer and the following layers and temperatures are considered: • • • • Layer 1: Stainless steel plate with thickness X1 and hot-side temperature Ts and cold-side temperature Tc2 Layer 2: Thermal paste with thickness X2 and hot and cold-side temps Tc2 and Tc3, respectively Layer 3: Inconel sheath with thickness X3 and hot and cold-side temps Tc3 and Tc4, respectively Layer 4: MgO layer with thickness X4 and hot and cold-side temps Tc4 and Tc5, respectively The dot represents the thermocouple bead, which is the actual location where the temperature (Tc5) is measured. The heat flux q and temperatures Ts (hot surface), Tc2, Tc3, Tc4 and Tc5 can be calculated using equations 2 through 6. q Ts  Tc 2  * K ss X1 Tc 2  Ts  q X1 K ss Eq. 2 Eq. 3  X 1   X 2     Tc3  Ts  q   K K 2   ss    Eq. 4  X 1   X 2   X 3     Tc4  Ts  q    K K 2 K 3      ss  Eq. 5  X 1   X 2   X 3   X 4     Tc5  Ts  q     K K 2 K 3 K 4       ss    Eq. 6 Here, Kss, K2, K3 and K4 are the thermal conductivities of the stainless steel plate, the thermal paste, the Inconel sheath and the MgO, respectively. Multi-depth Wall Thermocouples - Refractory Wall Figure 9 is a simplified schematic representing the region through the top wall of the furnace where the multi-depth wall thermocouple instrument model will be applied. The detailed physical description of this device was included as Figure 5. #12;Figure 9. Three layer system through top furnace wall In this model the heat flux and the hot refractory surface temperature can be determined using Equations 7 and 8. The variables in these equations are detailed in Figure 5. q  k ref T1  T2  X1  X 2   X Ts  T1  q 1 K  ref     Eq. 7 Eq. 8 Radiometers The instrument model for the radiometer is somewhat more complicated and represents the physical configuration described in Figure 10. Figure 10. Model depicting narrow angle radiometer with devices and geometry influencing the measurement of incident radiation The amount of energy hitting the thermistor, ri is found using the lens optics equations (9 and 10). In the equations shown, do is the distance of the object from the lens and ro is the object radius. The variable f is the lens refractive index. The image distance (di) is the distance from the lens to the irradiated thermistor. #12;𝑑𝑖 = 𝑟𝑖 = 1 Eq. 9 1 1 + 𝑑𝑜 𝑓 𝑑𝑖 𝑟𝑜 Eq. 10 𝑑𝑜 The heat flux arriving at the thermistor can be calculated using Equations 11 and 12 where:  is the reflectivity of the lens, rlens is the refractive index of the lens and Io is the heat flux from the black body radiator. 𝑟𝑙𝑒𝑛𝑠 ) (1 𝑟𝑖 − 𝜌) Eq. 11 𝑞𝑟𝑎𝑑 = 𝜋𝑟𝑖2 𝐼𝑖 Eq. 12 𝐼𝑖 = 𝐼𝑜 ( An energy balance can be calculated for the thermistor according to Equation 13 and Figure 11. Figure 11. Schematic for the energy balance on the irradiated thermistor qrad + qrad3 + qrad4 = qcond + qconv + qrad2 Eq. 13 The resistance of the irradiated thermistor can be determined using Equation 14, which is dependent on the thermistor temperature and the multiple coefficients supplied by the thermistor manufacturer. 𝐵 𝐶 𝐷 𝑡 𝑡 𝑡 𝑅𝑡 = 𝑅𝑟𝑒𝑓 𝑒𝑥𝑝 (𝐴 + 𝑇 + 𝑇 2 + 𝑇 3 ) Eq. 14 The output voltage from the Wheatstone bridge, which is the electronic component that amplifies the difference in resistance between the irradiated and non-irradiated thermistors, can be calculated from Equation 15 with the values of each of the variables provided in Figure 10. 𝑅𝑛𝑜𝑛 𝑛𝑜𝑛 +𝑅1 𝑉𝑚𝑒𝑎𝑠 = 𝑉𝑎𝑝𝑝 (𝑅 −𝑅 𝑅𝑖𝑟𝑟 ) 𝑖𝑟𝑟 +𝑅2 Eq. 15 Summary and Conclusions The mission of CCMSC at the University of Utah is to demonstrate the use of exascale computing with verification, validation, and uncertainty quantification as a means of accelerating deployment of advanced, low cost, low emission coal-fired power generation technologies. A task within the CCMSC hierarchy exists where pilot-scale oxy-combustion experimental data and their uncertainties are #12;generated. These information will be used to validate state-of-the-art computational fluid dynamic (CFD) models which will in turn be used for advanced generation system design. Of particular interest to this program are temperatures and heat flux profile in the near flame region of University of Utah's 1.5 MWth pulverized coal combustor. Devices have been developed on the L1500 that will allow measurement of the QOIs with the least impact and also ease of interpretation. These devices include: heat transfer surfaces with measurement of total heat removal and localized heat flux, multi-depth thermocouples in the refractory walls, and narrow angle radiometers. Instrument models were developed that are used to relate the measured values to the QOIs through rigorous physical relationships and mathematics. These Instrument models will be used to facilitate the comparison of collected data and CFD model predictions.