- Email: [email protected]
Integration of Time Domain Reflectometry in a smouldering reactor
Chemical Engineering Research and Design 1 3 9 ( 2 0 1 8 ) 34–38
Contents lists available at ScienceDirect
Chemical Engineering Research and Design journal homepage: www.elsevier.com/locate/cherd
Integration of Time Domain Reflectometry in a smouldering reactor Luis Yermán a,∗ , Thierry Bore a , Marcelo Llano Serna a , Sergio Zárate a , Tilman Bittner a , Mathieu Bajodek b , Alexander Scheuermann a a b
School of Civil Engineering, The University of Queensland, Brisbane, Australia Ecole Normale Superieure de Cachan. University Paris-Sacaly, Cachan, France
a r t i c l e
i n f o
a b s t r a c t
Article history:
Self-sustaining smouldering is a novel technology that has been efficiently applied for soil
Received 17 April 2018
remediation and the treatment of waste at very high moisture content. During smouldering,
Received in revised form 20 August
combustion of an organic phase occurs within an inert porous matrix. Simultaneously, pro-
2018
cesses such as evaporation, condensation and further migration of water take place. Due
Accepted 9 September 2018
to the dynamics of the smouldering, all these processes are interconnected. Water evap-
Available online 19 September 2018
oration and re-condensation is one of the main mechanisms responsible for smouldering quenching. The understanding of water migration within the porous medium is crucial for
Keywords:
the design and development of self-sustaining smouldering reaction systems. This paper
Smouldering combustion
introduces the first smouldering reactor performing Time Domain Reflectometry (TDR) mea-
Time Domain Reflectometry
surements during smouldering combustion experiments, aiming to measure water content
Waste treatment
distributions within the porous matrix. An existing smouldering reactor is transformed into
Integration
a coaxial transmission line designed for the propagation of electromagnetic signals. Wet artificial faeces mixed with sand were used for the tests, as its behaviour in smouldering was extensively studied. TDR signals and temperatures at different positions along the reactor were recorded simultaneously. This allows for the determination of the temporal changes of the water content distribution within the smouldering reactor. This development is the first step towards the determination of moisture content profiles during waste smouldering treatment applications. © 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1.
Introduction
Smouldering requires that a fuel is porous, as this promotes a high surface area for heat and mass transfer, insulates the reaction front
the surface of a condensed phase fuel. Smouldering occurs in porous
to reduce heat losses, and allows the flow of oxygen to the reaction zone (Pironi et al., 2009). By mixing the waste with an inert granular material, a porous matrix is created, with the necessary heat retention
media, like in a burning cigar (Rein, 2009). Smouldering combustion has been demonstrated as an effective and efficient method for the treatment of waste with high water content (Rashwan et al., 2016; Yermán
and air permeability properties for smouldering. Sand is commonly used because it has previously been identified as an effective agent for smouldering treatments (Pironi et al., 2009). Smouldering has also
et al., 2017a, 2016, 2015).
been extensively used for soil remediation (Scholes et al., 2015; Switzer et al., 2014, 2009), where the soil and contaminant, air, and groundwater
Smouldering combustion is a slow, low-temperature, flameless form of combustion sustained by the heat evolved when oxygen reacts with
High energy-efficiency is the key of this process, which allows treatment of waste with high water content in a self-sustaining manner. This means that after ignition, no energy is required and the combustion propagates through the material.
∗
coexist in the smouldering matrix. Research on smouldering combustion lacks diagnostic methods, and usually does not go beyond temperature profiles and chemical
Corresponding author. E-mail address: [email protected] (L. Yermán). https://doi.org/10.1016/j.cherd.2018.09.018 0263-8762/© 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Chemical Engineering Research and Design 1 3 9 ( 2 0 1 8 ) 34–38
35
Fig. 1 – Schematic representation of the smouldering reactor transformed into a coaxial cell. analyses. To the authors’ knowledge, the work of Tse et al. is the only example using high-level diagnostics for smouldering applications (Tse et al., 1999). However, this uses is a dry material and therefore there is no study of water migration. For waste applications, the downward migration of water due to increased hydrostatic pressure when water condenses, enables a critical condition that quenches the reaction (Yermán et al., 2015). The effect of moisture content on smouldering has never been systematically addressed, due to the lack of suitable observation methods. The advancement of innovative methods that can determine the temporal changes in moisture content distribution within a porous media, where concentrations and temperatures are changing, can shed light on the phenomena involved during smouldering. Time Domain Reflectometry (TDR) is an acknowledged method for determining the water content of porous media (Robinson et al., 2003). The TDR method can monitor changes in water content and density in porous media by taking advantage of the dipolar character of the water molecules, which results in a high permittivity compared to other phases. A TDR device measures the propagation velocity of a step pulse voltage along a transmission line: the velocity of this signal is a function of the effective permittivity of the material through which it travels. Conventional TDR technology is restricted to the point-wise determination of soil moisture, or mean water content, along with a TDR probe with a typical length of 10–30 cm. Recent developments have focused on Spatial Time Domain Reflectometry (STDR), which seeks the determination of water content profiles (Schlaeger, 2005). This approach requires inversion algorithms of the wave propagation along a transmission line due to an incident voltage step. The use of STDR for observing changes in water content during the smouldering process could optimise the application of this emerging technology, and even trigger new applications in the relevant industry. This paper introduces, for the first time, an innovative prototype of a smouldering process applying STDR method. A smouldering reactor is transformed into a coaxial line that allows the propagation of electromagnetic signals. A proof-of-concept to simultaneously measure temperatures and TDR signals is presented, essential for understanding water migration and smouldering quenching mechanisms.
2.
Materials and methods
2.1.
Materials
The material selected for these tests is a well-characterized (Yermán et al., 2016, 2015) artificial faeces recipe, which has been extensively studied in smouldering applications (Yermán et al., 2017b, 2017a, 2015). The electromagnetic properties, and a dielectric spectroscopy for smouldering applications, were studied in Bore et al. (2016). A fresh batch of surrogate faeces is prepared prior to each test and mixed with sand (0.6–1.2 mm, 1700 kg/m3 bulk density). A pre-existing upwards forward smouldering combustion column (Yermán et al., 2017b, 2017a, 2015) with an inside diameter of 16.0 cm is shown in Fig. 1. The design requires a transition section that enables the “injection” of an electromagnetic wave into the reactor. This transition has three different coaxial line sections. From top to bottom, the first section (1) is made from a commercial connector, and consists of a 50 conical transition filled with a low loss dielectric material (PTFE) providing the transition from the connector (N-type) to the dimension of the reactor. The second section (2) is a safety gap to protect the connector from potential high gas temperatures. Second and third sections are separated by a partition made of PVC. Section (3) is the exhaust system, where combustion gases leave the reactor through a set of holes. Section (4) is where the smouldering reaction and water migration take place. This section holds 10 horizontal thermocouples (TC) at different heights. A data logger (Agilent/HP 34980A) records the temperature from the TCs. The first TC (TC1) is located at 2 cm from the heater and is used to control the ignition of the medium (Yermán et al., 2015). It is worth noting that this system (i.e. the coaxial feeding line, transition, and reactor) represents a large one-port
36
Chemical Engineering Research and Design 1 3 9 ( 2 0 1 8 ) 34–38
Fig. 2 – Temperature histories from smouldering experiments (A). Relevant zones enlarged (B, C). coaxial cell, which is the best configuration for the propagation of an electromagnetic wave (Pozar, 2013). The reactor is placed over a stainless steel base that houses a heater and air diffuser. Compressed air flow is controlled by means of a mass flow controller. Waste is mixed with sand using a mechanical mixer and placed inside the reactor. Porosity is adjusted by controlling the mass-to-volume ratio. The reactor is connected via the coaxial feeding line with a TDR device (STDR-65 from Sequid).
2.2.
Methods
The coil heater at 500 W achieves initial heating of the bottom portion of material. Once the temperature at TC1 reaches 250 ◦ C (z = 2 cm) the heater is turned off and the airflow is initiated, allowing for self-sustaining smouldering to propagate upwards. The STDR-65 records the TDR trace in time-lapse
Table 1 – Smouldering experimental conditions. Parameter
Units
Value
Waste moisture Sand-to-dry waste ratio Airflow Waste pack height
%wt (wet basis) gsand /gwaste L/min cm
60 24 80 60
fashion, every 2 min. Temperatures are measured every 5 s. Experimental conditions (3 repetitions) are summarized in Table 1.
3.
Results and discussion
Fig. 2 shows the temperature-time profiles in the smouldering reactor. The preheating period lasted 20 min. When the airflow is initiated, the portion of mixture closest to the heater (2 cm)
Chemical Engineering Research and Design 1 3 9 ( 2 0 1 8 ) 34–38
37
Fig. 3 – TDR signals during smouldering experiments: reflection coefficient as a function of time (A). Conventional TDR waveform, reflection coefficient as a function of travel time (B). Enlargement of B (C). experiences a sharp increase in temperature up to ∼550 ◦ C as rapid exothermic oxidation occurs. The adjacent thermocouples experience a temperature increase due to the convective heat transfer from the reaction zone to the virgin material ahead. The sand-waste mixture is thus pre-dried ahead of the smouldering front’s arrival. As the fuel is consumed and the reaction at that location stops, the temperature falls as it is cooled by incoming air. The successive peaks (in the absence of external energy being applied) indicate the advancement of the smouldering front in a self-sustaining manner. A more detailed description of the temperature profiles for similar experiments can be found elsewhere (Yermán et al., 2017b, 2015). The temperature of the exhaust gases is measured by means of TC10, which is located outside the sand-waste bed (100 cm). Only when the smouldering front reaches 57 cm does the temperature of the gases rise above 100 ◦ C, which highlights the efficiency of the treatment. Results from the TDR measurements are presented in Fig. 3. The TDR device delivered classic TDR signal: the reflection coefficient ‘’ as a function of time (generally in nanoseconds, called TDR time). It should be noted that the time represents the distance along the transmission line. To focus on the evolution of signals during smouldering experiments, the curves were stacked and represented in a 3D graph: the X-axis represents the experiment time in minutes, the Y-axis represents the TDR time in nanoseconds and the Z-axis represents the value of the reflection coefficient time. Based on the temperature histories and TDR analysis, five stages can be identified:
Stage 1 (0–60 min): Heating up, no chemical reaction. No changes are recorded in TDR signals, temperatures are almost constant (room temperature). Stage 2 (60–80 min): A rise in temperatures can be observed for the whole reactor from room temperature to a plateau around 70–80 ◦ C. This effect was already seen and explained in Yermán et al. (2015). TDR signals are slightly impacted by this increase. Stage 3 (80–140 min): The smouldering front is about to reach the TDR zone. Nevertheless, significant changes in the TDR signals are observed. As reactor temperature is almost constant during this stage, the change of TDR signal is mostly due to the change of moisture content profile induced by water migration. The shape of TDR signals is changing slightly, which can be related to a heterogeneous electric permittivity induced by water migration. Stage 4 (140–340 min): The smouldering front enters the TDR zone. A drastic change in the TDR signals is observed. The analysis of these signals is complex. In addition to temperature change and water migration, the transformation of the material into ash affects the TDR signals. A two-layered medium is identified, separated by the smouldering front: atop is a layer composed of almost dry material, whereas the bottom is composed of dry ashes and sand. The shape of the TDR, showing some ‘steps’ (between 215 and 318 min) is typical for layered media. This added complexity emphasises the urgent need of a quantitative analysis based on a combined model. Stage 5 (340 min-onwards): The smouldering reaction has finished. All the material has been transformed into ash, and cools down. The TDR signals do not show changes. The electrical permittivity of dry ashes embedded in sand is only slightly
38
Chemical Engineering Research and Design 1 3 9 ( 2 0 1 8 ) 34–38
dependent on temperature. Previous studies have shown a variation of permittivity of dry ash between 1.5–2, with temperature varying from 20–400 ◦ C (Baum and Ghorbani, 2016). These results can be utilized to compute the water content profiles based on measured TDR and temperature signals. An STDR algorithm is necessary to compute the apparent permittivity profile from TDR signals. Comprehensive details of the inversion procedure can be found in Schlaeger (2005). The effect of water content and temperature on apparent permittivity will be separated using suitable mixing equations (Robinson et al., 2003). The material can be considered as a three-phase mixture (gas, solid and water). For each phase, the dielectric permittivity can be computed according to theoretical formulas or empirical equation (Bore et al., 2018, 2017). An example of application of such an algorithm can be found in Scheuermann (2012). The development of this technology can enhance opportunities beyond monitoring water content profiles in complex porous media.
4.
Conclusion
This paper reports the design of a new prototype of smouldering reactor aiming to measure online dynamic water content profiles in porous media for smouldering combustion processes. An existing reactor was transformed into a large coaxial cell in order to propagate electromagnetic signals. In this context, the reactor itself acts as a TDR sensor. This is the first time TDR measurements have been performed at such high temperatures. The advancement of these online systems can shed light on the development of integrated sensors for smouldering applications. The inversion of the TDR signal is the next step of the proposed research, and is currently under investigation.
Acknowledgements Authors thank Raul Govind-Vanmali and Kieran Jackson for their assistance with the experiments, Prof. Jose Torero for his constructive advice, and Ian Pope for his proofreading.
References Baum, T.C., Ghorbani, K., 2016. Measurements on the effects of moisture on the complex permittivity of high temperature ash. IEEE Trans. Microw. Theory Tech. 64, 607–615, http://dx.doi.org/10.1109/TMTT.2015.2512595. Bore, T., Schwing, M., Serna, M.L., Speer, J., Scheuermann, A., Wagner, N., 2018. A new broadband dielectric model for simultaneous determination of water saturation and porosity. IEEE Trans. Geosci. Remote Sens., 1–12, http://dx.doi.org/10.1109/TGRS.2018.2835447. Bore, T., Wagner, N., Cai, C., Scheuermann, A., 2017. Broadband electromagnetic analysis of compacted kaolin. Meas. Sci. Technol. 28, 14016, http://dx.doi.org/10.1088/1361-6501/28/1/014016. Bore, T., Yerman, L., Serna, M.L., Wall, H., Torero, J.L., Scheuerman, A., 2016. Dielectric spectroscopy of artificial
faeces for smouldering applications. SAS 2016 — Sensors Applications Symposium Proceedings, http://dx.doi.org/10.1109/SAS.2016.7479902. Pironi, P., Switzer, C., Rein, G., Fuentes, A., Gerhard, J.I., Torero, J.L., 2009. Small-scale forward smouldering experiments for remediation of coal tar in inert media. Proc. Combust. Inst. 32 (II), 1957–1964, http://dx.doi.org/10.1016/j.proci.2008.06.184. Pozar, D.M., 2013. Microwave Engineering. John Wiley & Sons., http://dx.doi.org/10.1017/CBO9781107415324.004. Rashwan, T.L., Gerhard, J.I., Grant, G.P., 2016. Application of self-sustaining smouldering combustion for the destruction of wastewater biosolids. Waste Manag. 50, 201–212, http://dx.doi.org/10.1016/j.wasman.2016.01.037. Rein, G., 2009. Smouldering combustion phenomena in science and technology. Int. Rev. Chem. Eng. 1, 3–18. Robinson, D.A., Jones, S.B., Wraith, J.M., Or, D., Friedman, S.P., 2003. A review of advances in dielectric and electrical conductivity measurement in soils. Vadose Zone J., 444–475. Scheuermann, A., 2012. Determination of porosity distributions of water saturated granular media using spatial time domain reflectometry (spatial TDR). Geotech. Test. J. 35, 441–450, http://dx.doi.org/10.1520/GTJ103056. Schlaeger, S., 2005. A fast TDR-inversion technique for the reconstruction of spatial soil moisture content. Hydrol. Earth Syst. Sci. 9, 481–492, http://dx.doi.org/10.5194/hess-9-481-2005. Scholes, G.C., Gerhard, J.I., Grant, G.P., Major, D.W., Vidumsky, J.E., Switzer, C., Torero, J.L., 2015. Smoldering remediation of coal-tar-contaminated soil: pilot field tests of STAR. Environ. Sci. Technol. 49 (24), 14334–14342, http://dx.doi.org/10.1021/acs.est.5b03177. Switzer, C., Gerhard, J.I., Pironi, P., Rein, G., Torero, J.L., 2009. Self-sustaining smouldering combustion: a novel remediation process for non-aqueous phase liquids in porous media. Environ. Sci. Technol. 43, 5871–5877, http://dx.doi.org/10.1021/es803483s. Switzer, C., Pironi, P., Gerhard, J.I., Rein, G., Torero, J.L., 2014. Volumetric scale-up of smouldering remediation of contaminated materials. J. Hazard. Mater. 268, 51–60, http://dx.doi.org/10.1016/j.jhazmat.2013.11.053. Tse, S.D., Anthenien, R.A., Fernandez-Pello, A.C., Miyasaka, K., 1999. An application of ultrasonic tomographic imaging to study smoldering combustion. Combust. Flame 116, 120–135, http://dx.doi.org/10.1016/S0010-2180(98)00053-4. Yermán, L., Cormier, D., Fabris, I., Carrascal, J., Torero, J.L., Gerhard, J.I., Cheng, Y.-L., 2017a. Potential bio-oil production from smouldering combustion of faeces. Waste Biomass Valor. 8, http://dx.doi.org/10.1007/s12649-016-9586-1. Yermán, L., Hadden, R.M., Carrascal, J., Fabris, I., Cormier, D., Torero, J.L., Gerhard, J.I., Krajcovic, M., Pironi, P., Cheng, Y.-L., 2015. Smouldering combustion as a treatment technology for faeces: exploring the parameter space. Fuel 147, http://dx.doi.org/10.1016/j.fuel.2015.01.055. Yermán, L., Wall, H., Torero, J., Gerhard, J.I., Cheng, Y.L., 2016. Smoldering combustion as a treatment technology for feces: sensitivity to key parameters. Combust. Sci. Technol. 188, 968–981, http://dx.doi.org/10.1080/00102202.2015.1136299. Yermán, L., Wall, H., Torero, J.L., 2017b. Experimental investigation on the destruction rates of organic waste with high moisture content by means of self-sustained smoldering combustion. Proc. Combust. Inst. 36, http://dx.doi.org/10.1016/j.proci.2016.07.052.
Contents lists available at ScienceDirect
Chemical Engineering Research and Design journal homepage: www.elsevier.com/locate/cherd
Integration of Time Domain Reflectometry in a smouldering reactor Luis Yermán a,∗ , Thierry Bore a , Marcelo Llano Serna a , Sergio Zárate a , Tilman Bittner a , Mathieu Bajodek b , Alexander Scheuermann a a b
School of Civil Engineering, The University of Queensland, Brisbane, Australia Ecole Normale Superieure de Cachan. University Paris-Sacaly, Cachan, France
a r t i c l e
i n f o
a b s t r a c t
Article history:
Self-sustaining smouldering is a novel technology that has been efficiently applied for soil
Received 17 April 2018
remediation and the treatment of waste at very high moisture content. During smouldering,
Received in revised form 20 August
combustion of an organic phase occurs within an inert porous matrix. Simultaneously, pro-
2018
cesses such as evaporation, condensation and further migration of water take place. Due
Accepted 9 September 2018
to the dynamics of the smouldering, all these processes are interconnected. Water evap-
Available online 19 September 2018
oration and re-condensation is one of the main mechanisms responsible for smouldering quenching. The understanding of water migration within the porous medium is crucial for
Keywords:
the design and development of self-sustaining smouldering reaction systems. This paper
Smouldering combustion
introduces the first smouldering reactor performing Time Domain Reflectometry (TDR) mea-
Time Domain Reflectometry
surements during smouldering combustion experiments, aiming to measure water content
Waste treatment
distributions within the porous matrix. An existing smouldering reactor is transformed into
Integration
a coaxial transmission line designed for the propagation of electromagnetic signals. Wet artificial faeces mixed with sand were used for the tests, as its behaviour in smouldering was extensively studied. TDR signals and temperatures at different positions along the reactor were recorded simultaneously. This allows for the determination of the temporal changes of the water content distribution within the smouldering reactor. This development is the first step towards the determination of moisture content profiles during waste smouldering treatment applications. © 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1.
Introduction
Smouldering requires that a fuel is porous, as this promotes a high surface area for heat and mass transfer, insulates the reaction front
the surface of a condensed phase fuel. Smouldering occurs in porous
to reduce heat losses, and allows the flow of oxygen to the reaction zone (Pironi et al., 2009). By mixing the waste with an inert granular material, a porous matrix is created, with the necessary heat retention
media, like in a burning cigar (Rein, 2009). Smouldering combustion has been demonstrated as an effective and efficient method for the treatment of waste with high water content (Rashwan et al., 2016; Yermán
and air permeability properties for smouldering. Sand is commonly used because it has previously been identified as an effective agent for smouldering treatments (Pironi et al., 2009). Smouldering has also
et al., 2017a, 2016, 2015).
been extensively used for soil remediation (Scholes et al., 2015; Switzer et al., 2014, 2009), where the soil and contaminant, air, and groundwater
Smouldering combustion is a slow, low-temperature, flameless form of combustion sustained by the heat evolved when oxygen reacts with
High energy-efficiency is the key of this process, which allows treatment of waste with high water content in a self-sustaining manner. This means that after ignition, no energy is required and the combustion propagates through the material.
∗
coexist in the smouldering matrix. Research on smouldering combustion lacks diagnostic methods, and usually does not go beyond temperature profiles and chemical
Corresponding author. E-mail address: [email protected] (L. Yermán). https://doi.org/10.1016/j.cherd.2018.09.018 0263-8762/© 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Chemical Engineering Research and Design 1 3 9 ( 2 0 1 8 ) 34–38
35
Fig. 1 – Schematic representation of the smouldering reactor transformed into a coaxial cell. analyses. To the authors’ knowledge, the work of Tse et al. is the only example using high-level diagnostics for smouldering applications (Tse et al., 1999). However, this uses is a dry material and therefore there is no study of water migration. For waste applications, the downward migration of water due to increased hydrostatic pressure when water condenses, enables a critical condition that quenches the reaction (Yermán et al., 2015). The effect of moisture content on smouldering has never been systematically addressed, due to the lack of suitable observation methods. The advancement of innovative methods that can determine the temporal changes in moisture content distribution within a porous media, where concentrations and temperatures are changing, can shed light on the phenomena involved during smouldering. Time Domain Reflectometry (TDR) is an acknowledged method for determining the water content of porous media (Robinson et al., 2003). The TDR method can monitor changes in water content and density in porous media by taking advantage of the dipolar character of the water molecules, which results in a high permittivity compared to other phases. A TDR device measures the propagation velocity of a step pulse voltage along a transmission line: the velocity of this signal is a function of the effective permittivity of the material through which it travels. Conventional TDR technology is restricted to the point-wise determination of soil moisture, or mean water content, along with a TDR probe with a typical length of 10–30 cm. Recent developments have focused on Spatial Time Domain Reflectometry (STDR), which seeks the determination of water content profiles (Schlaeger, 2005). This approach requires inversion algorithms of the wave propagation along a transmission line due to an incident voltage step. The use of STDR for observing changes in water content during the smouldering process could optimise the application of this emerging technology, and even trigger new applications in the relevant industry. This paper introduces, for the first time, an innovative prototype of a smouldering process applying STDR method. A smouldering reactor is transformed into a coaxial line that allows the propagation of electromagnetic signals. A proof-of-concept to simultaneously measure temperatures and TDR signals is presented, essential for understanding water migration and smouldering quenching mechanisms.
2.
Materials and methods
2.1.
Materials
The material selected for these tests is a well-characterized (Yermán et al., 2016, 2015) artificial faeces recipe, which has been extensively studied in smouldering applications (Yermán et al., 2017b, 2017a, 2015). The electromagnetic properties, and a dielectric spectroscopy for smouldering applications, were studied in Bore et al. (2016). A fresh batch of surrogate faeces is prepared prior to each test and mixed with sand (0.6–1.2 mm, 1700 kg/m3 bulk density). A pre-existing upwards forward smouldering combustion column (Yermán et al., 2017b, 2017a, 2015) with an inside diameter of 16.0 cm is shown in Fig. 1. The design requires a transition section that enables the “injection” of an electromagnetic wave into the reactor. This transition has three different coaxial line sections. From top to bottom, the first section (1) is made from a commercial connector, and consists of a 50 conical transition filled with a low loss dielectric material (PTFE) providing the transition from the connector (N-type) to the dimension of the reactor. The second section (2) is a safety gap to protect the connector from potential high gas temperatures. Second and third sections are separated by a partition made of PVC. Section (3) is the exhaust system, where combustion gases leave the reactor through a set of holes. Section (4) is where the smouldering reaction and water migration take place. This section holds 10 horizontal thermocouples (TC) at different heights. A data logger (Agilent/HP 34980A) records the temperature from the TCs. The first TC (TC1) is located at 2 cm from the heater and is used to control the ignition of the medium (Yermán et al., 2015). It is worth noting that this system (i.e. the coaxial feeding line, transition, and reactor) represents a large one-port
36
Chemical Engineering Research and Design 1 3 9 ( 2 0 1 8 ) 34–38
Fig. 2 – Temperature histories from smouldering experiments (A). Relevant zones enlarged (B, C). coaxial cell, which is the best configuration for the propagation of an electromagnetic wave (Pozar, 2013). The reactor is placed over a stainless steel base that houses a heater and air diffuser. Compressed air flow is controlled by means of a mass flow controller. Waste is mixed with sand using a mechanical mixer and placed inside the reactor. Porosity is adjusted by controlling the mass-to-volume ratio. The reactor is connected via the coaxial feeding line with a TDR device (STDR-65 from Sequid).
2.2.
Methods
The coil heater at 500 W achieves initial heating of the bottom portion of material. Once the temperature at TC1 reaches 250 ◦ C (z = 2 cm) the heater is turned off and the airflow is initiated, allowing for self-sustaining smouldering to propagate upwards. The STDR-65 records the TDR trace in time-lapse
Table 1 – Smouldering experimental conditions. Parameter
Units
Value
Waste moisture Sand-to-dry waste ratio Airflow Waste pack height
%wt (wet basis) gsand /gwaste L/min cm
60 24 80 60
fashion, every 2 min. Temperatures are measured every 5 s. Experimental conditions (3 repetitions) are summarized in Table 1.
3.
Results and discussion
Fig. 2 shows the temperature-time profiles in the smouldering reactor. The preheating period lasted 20 min. When the airflow is initiated, the portion of mixture closest to the heater (2 cm)
Chemical Engineering Research and Design 1 3 9 ( 2 0 1 8 ) 34–38
37
Fig. 3 – TDR signals during smouldering experiments: reflection coefficient as a function of time (A). Conventional TDR waveform, reflection coefficient as a function of travel time (B). Enlargement of B (C). experiences a sharp increase in temperature up to ∼550 ◦ C as rapid exothermic oxidation occurs. The adjacent thermocouples experience a temperature increase due to the convective heat transfer from the reaction zone to the virgin material ahead. The sand-waste mixture is thus pre-dried ahead of the smouldering front’s arrival. As the fuel is consumed and the reaction at that location stops, the temperature falls as it is cooled by incoming air. The successive peaks (in the absence of external energy being applied) indicate the advancement of the smouldering front in a self-sustaining manner. A more detailed description of the temperature profiles for similar experiments can be found elsewhere (Yermán et al., 2017b, 2015). The temperature of the exhaust gases is measured by means of TC10, which is located outside the sand-waste bed (100 cm). Only when the smouldering front reaches 57 cm does the temperature of the gases rise above 100 ◦ C, which highlights the efficiency of the treatment. Results from the TDR measurements are presented in Fig. 3. The TDR device delivered classic TDR signal: the reflection coefficient ‘’ as a function of time (generally in nanoseconds, called TDR time). It should be noted that the time represents the distance along the transmission line. To focus on the evolution of signals during smouldering experiments, the curves were stacked and represented in a 3D graph: the X-axis represents the experiment time in minutes, the Y-axis represents the TDR time in nanoseconds and the Z-axis represents the value of the reflection coefficient time. Based on the temperature histories and TDR analysis, five stages can be identified:
Stage 1 (0–60 min): Heating up, no chemical reaction. No changes are recorded in TDR signals, temperatures are almost constant (room temperature). Stage 2 (60–80 min): A rise in temperatures can be observed for the whole reactor from room temperature to a plateau around 70–80 ◦ C. This effect was already seen and explained in Yermán et al. (2015). TDR signals are slightly impacted by this increase. Stage 3 (80–140 min): The smouldering front is about to reach the TDR zone. Nevertheless, significant changes in the TDR signals are observed. As reactor temperature is almost constant during this stage, the change of TDR signal is mostly due to the change of moisture content profile induced by water migration. The shape of TDR signals is changing slightly, which can be related to a heterogeneous electric permittivity induced by water migration. Stage 4 (140–340 min): The smouldering front enters the TDR zone. A drastic change in the TDR signals is observed. The analysis of these signals is complex. In addition to temperature change and water migration, the transformation of the material into ash affects the TDR signals. A two-layered medium is identified, separated by the smouldering front: atop is a layer composed of almost dry material, whereas the bottom is composed of dry ashes and sand. The shape of the TDR, showing some ‘steps’ (between 215 and 318 min) is typical for layered media. This added complexity emphasises the urgent need of a quantitative analysis based on a combined model. Stage 5 (340 min-onwards): The smouldering reaction has finished. All the material has been transformed into ash, and cools down. The TDR signals do not show changes. The electrical permittivity of dry ashes embedded in sand is only slightly
38
Chemical Engineering Research and Design 1 3 9 ( 2 0 1 8 ) 34–38
dependent on temperature. Previous studies have shown a variation of permittivity of dry ash between 1.5–2, with temperature varying from 20–400 ◦ C (Baum and Ghorbani, 2016). These results can be utilized to compute the water content profiles based on measured TDR and temperature signals. An STDR algorithm is necessary to compute the apparent permittivity profile from TDR signals. Comprehensive details of the inversion procedure can be found in Schlaeger (2005). The effect of water content and temperature on apparent permittivity will be separated using suitable mixing equations (Robinson et al., 2003). The material can be considered as a three-phase mixture (gas, solid and water). For each phase, the dielectric permittivity can be computed according to theoretical formulas or empirical equation (Bore et al., 2018, 2017). An example of application of such an algorithm can be found in Scheuermann (2012). The development of this technology can enhance opportunities beyond monitoring water content profiles in complex porous media.
4.
Conclusion
This paper reports the design of a new prototype of smouldering reactor aiming to measure online dynamic water content profiles in porous media for smouldering combustion processes. An existing reactor was transformed into a large coaxial cell in order to propagate electromagnetic signals. In this context, the reactor itself acts as a TDR sensor. This is the first time TDR measurements have been performed at such high temperatures. The advancement of these online systems can shed light on the development of integrated sensors for smouldering applications. The inversion of the TDR signal is the next step of the proposed research, and is currently under investigation.
Acknowledgements Authors thank Raul Govind-Vanmali and Kieran Jackson for their assistance with the experiments, Prof. Jose Torero for his constructive advice, and Ian Pope for his proofreading.
References Baum, T.C., Ghorbani, K., 2016. Measurements on the effects of moisture on the complex permittivity of high temperature ash. IEEE Trans. Microw. Theory Tech. 64, 607–615, http://dx.doi.org/10.1109/TMTT.2015.2512595. Bore, T., Schwing, M., Serna, M.L., Speer, J., Scheuermann, A., Wagner, N., 2018. A new broadband dielectric model for simultaneous determination of water saturation and porosity. IEEE Trans. Geosci. Remote Sens., 1–12, http://dx.doi.org/10.1109/TGRS.2018.2835447. Bore, T., Wagner, N., Cai, C., Scheuermann, A., 2017. Broadband electromagnetic analysis of compacted kaolin. Meas. Sci. Technol. 28, 14016, http://dx.doi.org/10.1088/1361-6501/28/1/014016. Bore, T., Yerman, L., Serna, M.L., Wall, H., Torero, J.L., Scheuerman, A., 2016. Dielectric spectroscopy of artificial
faeces for smouldering applications. SAS 2016 — Sensors Applications Symposium Proceedings, http://dx.doi.org/10.1109/SAS.2016.7479902. Pironi, P., Switzer, C., Rein, G., Fuentes, A., Gerhard, J.I., Torero, J.L., 2009. Small-scale forward smouldering experiments for remediation of coal tar in inert media. Proc. Combust. Inst. 32 (II), 1957–1964, http://dx.doi.org/10.1016/j.proci.2008.06.184. Pozar, D.M., 2013. Microwave Engineering. John Wiley & Sons., http://dx.doi.org/10.1017/CBO9781107415324.004. Rashwan, T.L., Gerhard, J.I., Grant, G.P., 2016. Application of self-sustaining smouldering combustion for the destruction of wastewater biosolids. Waste Manag. 50, 201–212, http://dx.doi.org/10.1016/j.wasman.2016.01.037. Rein, G., 2009. Smouldering combustion phenomena in science and technology. Int. Rev. Chem. Eng. 1, 3–18. Robinson, D.A., Jones, S.B., Wraith, J.M., Or, D., Friedman, S.P., 2003. A review of advances in dielectric and electrical conductivity measurement in soils. Vadose Zone J., 444–475. Scheuermann, A., 2012. Determination of porosity distributions of water saturated granular media using spatial time domain reflectometry (spatial TDR). Geotech. Test. J. 35, 441–450, http://dx.doi.org/10.1520/GTJ103056. Schlaeger, S., 2005. A fast TDR-inversion technique for the reconstruction of spatial soil moisture content. Hydrol. Earth Syst. Sci. 9, 481–492, http://dx.doi.org/10.5194/hess-9-481-2005. Scholes, G.C., Gerhard, J.I., Grant, G.P., Major, D.W., Vidumsky, J.E., Switzer, C., Torero, J.L., 2015. Smoldering remediation of coal-tar-contaminated soil: pilot field tests of STAR. Environ. Sci. Technol. 49 (24), 14334–14342, http://dx.doi.org/10.1021/acs.est.5b03177. Switzer, C., Gerhard, J.I., Pironi, P., Rein, G., Torero, J.L., 2009. Self-sustaining smouldering combustion: a novel remediation process for non-aqueous phase liquids in porous media. Environ. Sci. Technol. 43, 5871–5877, http://dx.doi.org/10.1021/es803483s. Switzer, C., Pironi, P., Gerhard, J.I., Rein, G., Torero, J.L., 2014. Volumetric scale-up of smouldering remediation of contaminated materials. J. Hazard. Mater. 268, 51–60, http://dx.doi.org/10.1016/j.jhazmat.2013.11.053. Tse, S.D., Anthenien, R.A., Fernandez-Pello, A.C., Miyasaka, K., 1999. An application of ultrasonic tomographic imaging to study smoldering combustion. Combust. Flame 116, 120–135, http://dx.doi.org/10.1016/S0010-2180(98)00053-4. Yermán, L., Cormier, D., Fabris, I., Carrascal, J., Torero, J.L., Gerhard, J.I., Cheng, Y.-L., 2017a. Potential bio-oil production from smouldering combustion of faeces. Waste Biomass Valor. 8, http://dx.doi.org/10.1007/s12649-016-9586-1. Yermán, L., Hadden, R.M., Carrascal, J., Fabris, I., Cormier, D., Torero, J.L., Gerhard, J.I., Krajcovic, M., Pironi, P., Cheng, Y.-L., 2015. Smouldering combustion as a treatment technology for faeces: exploring the parameter space. Fuel 147, http://dx.doi.org/10.1016/j.fuel.2015.01.055. Yermán, L., Wall, H., Torero, J., Gerhard, J.I., Cheng, Y.L., 2016. Smoldering combustion as a treatment technology for feces: sensitivity to key parameters. Combust. Sci. Technol. 188, 968–981, http://dx.doi.org/10.1080/00102202.2015.1136299. Yermán, L., Wall, H., Torero, J.L., 2017b. Experimental investigation on the destruction rates of organic waste with high moisture content by means of self-sustained smoldering combustion. Proc. Combust. Inst. 36, http://dx.doi.org/10.1016/j.proci.2016.07.052.