Estimations of Gasifier Wall Temperature and ... - ACS Publications

Subscriber access provided by Columbia University Libraries

Article

Estimations of Gasifier Wall Temperature and Extent of Slag Penetration Using a Refractory Brick with Embedded Sensors Qiao Huang, Prokash Paul, Debangsu Bhattacharyya, Rajalekshmi C Pillai, Katarzyna Sabolsky, and Edward M. Sabolsky Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02604 • Publication Date (Web): 11 Aug 2017 Downloaded from http://pubs.acs.org on August 13, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Industrial & Engineering Chemistry Research is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Industrial & Engineering Chemistry Research

Estimations of Gasifier Wall Temperature and Extent of Slag Penetration Using a Refractory Brick with Embedded Sensors

Qiao Huang1, Prokash Paul1 and Debangsu Bhattacharyya1*, Rajalekshmi C Pillai2, Katarzyna Sabolsky2, Edward M. Sabolsky2 1

Department of Chemical and Biomedical Engineering, West Virginia University, Morgantown, WV 26506, USA 2

Department of Mechanical & Aerospace Engineering, West Virginia University, Morgantown, WV 26506, USA

Abstract The short service life of refractory lining in slagging gasifier in integrated gasification combined cycle (IGCC) results in low availability and high operating cost. For longer life of the refractory lining, estimation of slag penetration length and monitoring of wall temperature are important. In this paper, we have investigated two types of embedded sensors in the refractory lining of gasifier, namely thermistor and interdigital capacitor (IDC) to estimate wall temperature profile and extent of slag penetration. Conventional correlation-based approaches are not satisfactory for estimating outputs of interest from the raw sensor data for these systems due to high temperature gradient along the sensor as well as temporal change in the refractory properties due to slag penetration. Therefore, thermal model of refractory brick, slag penetration model, and models of the embedded sensors are developed and used to estimate temperature and slag penetration profile by using linear and nonlinear estimators.

Keywords smart refractory, modeling, estimation, slag penetration, temperature, embedded sensor, gasifier

_____________________________________________________________________________ *

Corresponding author. Tel:1-304-293-9335, E-mail: [email protected]

1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 30

1. Introduction Integrated gasification combined cycle (IGCC) technology is a promising technology for producing electricity from fossil fuels and biomass with high efficiency. This technology offers superior environmental performance and high energy efficiency.1,2 The gasifier is the heart of IGCC plant.3-5 The operating temperature of a gasifier is one of the key variables.6,7 Lower temperature can lead to lower carbon conversion and increase the viscosity of slag eventually leading to disruption in the slag flow.8 Higher operating temperature can improve carbon conversion, but reduce the life of the refractory.9,10 The lifetime of the refractory lining of the gasifier is a major concern for both cost11 and availability. If the gasifier is operated at the optimum temperature, the lifetime of the high chromia refractory can be prolonged to almost 2 years.10,12 Therefore, it is desired that the operating temperature of a gasifier is strictly monitored and controlled. One important issue in the operation of the entrained-flow gasifier is penetration of molten slag into the refractory lining leading to chemical and physical degradation.9,13-15 Replacement of the liner costs significant money and time affecting the profitability of IGCC plants. Thus a measure of the extent of slag penetration can be very useful not only for process monitoring, but also for developing effective mitigation strategies to reduce extent of slag penetration. However due to the harsh environment inside gasifier,2,16 it is nearly impossible to measure any variable using current state-of-the-art measurement technology. If the traditional temperature measuring devices, such as thermocouples, are inserted into gasifier through open ports within the refractory, they survive for only couple of weeks.17 Several types of temperature sensors have been proposed for gasifier temperature measurement. Based on the flame emission spectroscopy (FES) technology, Parameswaran et al. proposed an optical temperature sensor for detecting the flame temperature of the pressurized entrained flow gasifier.7 Tunable diode laser (TDL) absorption sensor was used by Sun et al. to measure the gas temperature in slagging gasifier.18 Taking advantage of the temperature-dependent birefringence and thickness of the sapphire disk (part of the sensor), Huang et al. have developed optical sapphire temperature sensors.19 However, many of these novel sensors are yet to be tested under sustained, real-life conditions. In addition, the access ports needed for these sensors provide paths for slag penetration reducing the refractory life further. Furthermore, the light or the laser beam may be blocked by flowing slag. Finally, due to the high temperature and corrosive properties of slag, the sensor properties may degrade over time. One possibility to get around with the issue of defects due to access ports is to make the sensor integral to the refractory by co-firing them. The materials for embedded sensors are carefully selected by matching 2 ACS Paragon Plus Environment

Page 3 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


their thermal coefficient of expansion (COE) with the host brick thus resulting in a brick with embedded sensors, which is denoted as the ‘smart’ refractory brick here. The COE for high chromia refractory is about 8.154 × 10 /℃ in the temperature range of 21-1427 ℃, and COE for sensor material used in this

study ([75-25] vol% WSi2-Al2O3) is about 9.288 × 10 /℃ in the temperature range of 25-1000 ℃.

Therefore, it is expected that the longevity of the brick will not be affected much by the embedded sensors. Embedding sensors in the component of an equipment item is an active area of research for several years. Since the speed of sound in solids depends on its temperature, Jia et al. proposed an embedded ultrasound sensor to measure the temperature distribution across the refractory wall.20 Li et al.21 developed a passive resonant sensor which is embedded into a ceramic material by co-firing it. This sensor is used to detect the temperature in the harsh environment. For turbine blades, embedded fiber Bragg grating (FBG) sensors are being investigated.22,23 These FBG sensors can be embedded in the wind turbine rotor blades to monitor the strain and estimate the lifetime of the blade. 22 While ‘smart’ refractory brick is a promising concept, three issues need to be solved for their commercial application. First, spatial gradient in the temperature along the sensor length is typically neglected in most applications. However, due to the very high temperature gradient in the gasifier wall24, temperature gradient along the active sensor length cannot be neglected. Using typical correlation-based approaches, it is not possible to estimate the temperature distribution along the sensor length by using the single lumped measurement from a thermistor. A model-based approach can be useful to address this issue. Second, as mentioned earlier, the molten slag that penetrates into the refractory lining leads to changes in the refractory thermal and electrical properties. 9,13-15 Therefore, the response of the embedded sensors will also be affected by slag penetration. In order to address this issue, interdigital capacitor (IDC) sensors can be used to provide information about the slag penetration depth. Finally, model inaccuracies and noise in the measurement data are unavoidable. The Kalman filter (KF) can be very useful in estimating the wall temperature profile and slag penetration depth in presence of process and measurement noises.25 This paper is arranged as follows: Section 2 briefly describes the experimental procedure for manufacturing the ‘smart’ brick. Section 3 presents the thermal, slag, and sensor models. These models are used in the KF-based estimation. In Section 4, the proposed KF algorithm for the differential algebraic system is briefly described. Results are presented in Section 5. Finally, the conclusions are drawn in Section 6.

2. Experimental Setup The Al2O3 substrates were prepared by mixing coarse Al2O3 (A20 SG, SA: 1.2 m2/g, Alcoa, USA) and fine Al2O3 (A1000 SG, SA: 8.6 m2/g, Alcoa, USA) powders by a weight ratio of 80:20. Mixing of the 3 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 30

powders was achieved by ball milling the powders in isopropanol for 24 hour using Al2O3 milling medium in polyethylene jars. The resultant slurry was dried in air and subsequently screened using a sieve of 150 µm into a fine bimodal Al2O3 powder. The slurry required for tape casting was prepared by ballmilled bimodal Al2O3 powder, with benzyl butyl phthalate (TCW, Tape casting warehouse, NJ) as dispersant, polyalkylene glycol and polyvinyl butyral (TCW, Tape casting warehouse, NJ) as binder, in an ethanol/xylene solvent with a 1: 1 weight ratio for 5 h using Al2O3 milling medium in polyethylene jars.27,28 The resultant slurry was then de-aired by rolling for 24 hour. Tape casting was done on a TCW (TTC-1200) tape caster with a gap of 0.8 mm height and 12 cm width. The speed of the carrier film was 28 cm/min. The tape was casted on silicone-coated mylar substrates by using a single doctor blade. Drying was conducted at 25 °C in air with a relative humidity of 65%. These thin alumina composite tapes were then cut and laminated at 93 ºC under pressure to fabricate ~600 µm thick laminates. Finally, the as-prepared alumina laminates were laser cut into two pieces of 8 ¼” x 1 ½” substrates. WSi2-Al2O3 is used to manufacture the embedded sensor. WSi2-Al2O3 powder was prepared by the solid state route. WSi2 (Alfa Aesar, USA) and Al2O3 powders (80:20 coarse vs fine) were mixed in 75:25 vol% and then ball-milled in isopropanol. The well mixed slurry was dried and sieved with a sieve of 150 µm. The resulting WSi2-Al2O3 powders were casted into ~ 80 µm thin tapes by tape casting technique and subsequently laminated into 800 µm thick laminates. The long temperature sensors (thermistors) tape were laser cut into 8 ¾” x 2” x 2mm by a press laminator. Figure 1(a) shows the photographs of WSi2Al2O3 thermistor tape and alumina substrates. The laser cut long thermistor tapes were later embedded in between two alumina substrates and pre-sintered at 1600 °C in Argon. This embedded WSi2-Al2O3 thermistor shown in Figure 1(b) was embedded into a high-chromia refractory brick to form a smart brick.


Page 5 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a)

(b)

Figure 1. Photographs of (a) [75-25] vol% WSi2-Al2O3 thermistor tape, alumina substrates and (b) embedded thermistor manufactured by embedding the thermistor into the alumina substrates as shown in (a)

3. Model description 3.1. Thermal model. It is anticipated that the ‘smart’ brick will be placed in the high chromia refractory layer near the hot face of an entrained-flow gasifier as shown in Figure 2. Therefore, a thermal model of the high chromia layer has been developed by considering heat transfer with the bulk of the gasifier and with other bricks on the wall. In addition, thermal models of other layers on the wall in the direction from hot face to cold face, namely the alumina castable layer, the silica fire brick layer, and steel shell layer have also been developed.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 30

Figure 2. Schematics of the refractory wall with smart refractory brick The dominant heat transfer mechanism through the gasifier wall is by conduction. Therefore, the same governing equation shown in eq 1 is used for all the layers. The boundary condition on the hot face shown in eq 2 is based off our earlier work which includes wall-gas, wall-solid, and wall-wall heat transfer.29 Effects of the radiative heat transfer, convective heat transfer, and heat loss from the outer shell are all considered in achieving this hot surface temperature profile. 29 Temperature and flux continuities are assumed at all interface boundaries. The governing equation is shown in eq 1:

!

(1)

where T is temperature, K is thermal conductivity, " is density, #$ is specific heat. The boundary condition (BC) for the hot surface is given by: %&' %()*+,+- ./

(2) ℃

where %()*+,+- is a constant temperature, . is the slope, which is set to be -28.6 0. The BCs on all interfaces are given by: 1

2

13

4

(3)


Page 7 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


% %3

(4)

The BC on the cold surface is given by:

ℎ)+ %* 1 %)+ 16*

(5)

where ℎ)+ denotes the convective heat transfer coefficient. The thermal model has been developed by considering properties of the pristine refractory brick and slagpenetrated refractory brick. Temperature-dependent thermal properties for all the layers are provided in Table S1 and S2 (Supporting Information), where temperature T is in ℃. Table S3 under Supporting Information provides densities of all layers. Thicknesses of the layers in the gasifier wall are given in Table S4 under Supporting Information. Besides the temperature disturbance introduced on hot face, slag penetration will also change the temperature profile of a gasifier wall by affecting the heat capacity, density, and thermal conductivity of the refractory brick. Mixing rules are used in this work to evaluate effective properties of the slaginfiltrated refractory brick based on its composition. More details about the mixing rules can be found in Table S6 under Supporting Information. 3.2 Slag penetration model. The main driving force to be considered in the process of bulk slag intrusion is the capillary pressure.30 In the gasifier application, the pore system of the refractory lining is oriented in horizontal direction. The velocity of capillary flow through the horizontal pores can be calculated by the Washburn equation.31 7

89: 4 ;

Estimations of Gasifier Wall Temperature and ... - ACS Publications

Recommend Documents