Chimney effect induced by smoldering fire in a U-shaped porous channel: A governing mechanism of the persistent underground coal fires
Graphical abstract
Introduction
Underground coal fires (UCF, see Fig. 1), referring to sites of slow combustion (smoldering) of subsurface coal, are an intractable hazard threatening the safety of coal mining industries and polluting the atmosphere and the lithosphere environments (Lu et al., 2018; Tang et al., 2019; Wang et al., 2018, 2016; Zhuo et al., 2019). UCF produce toxic products (CO (O’Keefe et al., 2010), tar (Engle et al., 2012), Polycyclic Aromatic Hydrocarbons (PAHs) (Liu et al., 2012), volatile organic compounds (VOCs) (Yan et al., 2015), Hg (Hower et al., 2009; O’Keefe et al., 2010), etc.), resulting in heavy air pollution and soil contamination. UCF release massive thermal energy, which is transferred to the ground surface (Wessling et al., 2008b) and results in severe vegetation degradation (Kuenzer et al., 2012, 2007). UCF can also lead to geo-hazards such as collapse, subsidence, and landslides (Ide et al., 2010; Jiang et al., 2011; Kuenzer and Stracher, 2012). Additionally, UCF are a significant source of ancient carbon release from coal seams to the atmosphere. Understanding the role of greenhouse gas emissions (GHGs) emitted from UCF in the global climate change is still an emerging research topic (Engle et al., 2011; O’Keefe et al., 2010; Song et al., 2020; van Dijk et al., 2011).
For the persistence of UCF, abundant fuel in a coal reservoir is a precondition; slow burning (propagation) rate of smoldering fire provides a beneficial condition (Rein, 2013). Once underground deposited coal is ignited, the persistence of UCF is determined by certain extinct conditions of smoldering fires, which are mainly associated with the heat and mass (air) transfer processes (Rein, 2013). Underground rocks/sediments perform fine thermal insulation conditions for UCF, but air in underground fractured or porous zones is very depleted. Thus, air supply is a crucial process governing the persistent UCF.
The natural driving forces of air supply to subsurface porous zones include wind, atmospheric pressure fluctuation (Song et al., 2015b), topographic effect (Song et al., 2014), and thermal buoyancy (Krevor et al., 2011; Li et al., 2018; Song et al., 2019). Recently, it has demonstrated that thermal buoyancy is a significant force for air supply to UCF (Krevor et al., 2011; Li et al., 2018; Song et al., 2019). For subsurface fires, the fire depth (H) is an important element determining the accessibility of air supply to UCF. Under different driving forces, the responses of the air flow to increasing the fire depth may behave differently. We acknowledge that under natural forces such as wind, topographic effect, and atmospheric pressure fluctuations, air supply to underground space should decay with increasing the depth. However, with thermal buoyancy generated by UCF itself (Krevor et al., 2011; Li et al., 2018; Song et al., 2019), responses of the air supply and UCF to the fire depth are still uncertain.
Thermal buoyancy driven natural convection is widely applied to improve air conditions in buildings (Lei et al., 2016) and to generate green energy, e.g., the solar chimney (Zamora and Kaiser, 2009; Zhou et al., 2009) and the channel-chimney system (Auletta et al., 2003; Nasri et al., 2015); on the other hand, it has destructive influences on, for instance, fire protection of buildings (Gao et al., 2017; Ji et al., 2017), the gas-cooled reactors (Chen et al., 2017), and underground coal reservoirs (Krevor et al., 2011; Li et al., 2018; Song et al., 2019). It has demonstrated that geometrical configurations (Auletta et al., 2003; Nasri et al., 2015; Zamora and Kaiser, 2009; Zhou et al., 2009) and free/porous (Li et al., 2018; Sharma et al., 2018; Song et al., 2019) media of the channel have crucial effects on the behaviors of the natural convection. For thermal buoyancy driven natural ventilation in a vertical free channel, the chimney effect (the stack effect) has been well recognized; gas flow induced by the chimney effect increases with increasing the height of the channel. Recently, we reported the characteristic behavior of thermal buoyancy driven air flow through thin porous media in a U-shaped channel (Song et al., 2019). The chimney effect of such U-shaped configuration with porous media widely occurs in the fields of underground thermal engineering (Liu et al., 2018) as well as in the nature like underground coal fires (UCF) (Krevor et al., 2011; Li et al., 2018; Song et al., 2019). But investigation on this topic has been paid rare attention (Sharma et al., 2018; Song et al., 2019) and our understanding on its thermo- and fluid-dynamics is still poor. In this paper, we aim to characterize the dependences of the gas flow and the smoldering fire in a U-shaped porous channel on the channel’s height, and then attempt to reveal the governing mechanism of persistent UCF.
Herein we develop a 1/20-scale laboratory experimental framework (see Fig. 2) to mimic buoyancy-driven UCF in the field (Song et al., 2019) and unravel the relationship between UCF and the fire depth. In Section 2, the experimental setup and procedures are illustrated; approaches to quantify the burning rate and the fire spread rate are also detailed. Section 3 presents the dependences of the air supply, the burning rate, the fire spread rate and the maximum burning temperature on the fire depth. A one-dimensional model correlating the air supply and the fire depth, uncertain analyses on permeability and spatial scale, as well as the governing mechanism of persistent UCF are discussed in Section 4.
Section snippets
Experimental setup
In-situ UCF (see Fig. 1) is covered by complex geological rocks and propagates along multiple directions (three dimensions), which results in poor repeatability and controllability for experimental research. This work focuses on the chimney effect induced by UCF, in which the vertical direction is the most important. For the sake of simplicity, one-dimensional laboratory experiment framework was established (Song et al., 2019).
A schematic diagram of experimental setup was shown in Fig. 2. The
Chimney effect
Dragged by the buoyant smoke in the exhaust pipe, fresh air flows downward to the coalbed (Krevor et al., 2011; Song et al., 2019). The evolution of air velocity (Va) with the smoke temperature (T7) and the fire depth (H = 1.6–4.6 m) is presented in Fig. 3. The air velocity increases with increasing either the smoke temperature or the fire depth. Such enhancement of the air supply results from the stronger buoyancy, that is, the chimney effect of UCF.
Air flow through a thermally decomposed thin
Comparisons of chimney effects induced by smoldering fire in a U-shaped channel and by flaming fire in a vertical channel
The above results indicate that an increase of the fire depth can unexpectedly facilitate UCF (see Fig. 3, Fig. 4). Such an unusual behavior is attributed to the chimney effect induced by smoldering fire in a U-shaped porous channel (see Fig. 3). The correlations of air velocity with the smoke temperature and the fire depth have been presented in the previous work (Song et al., 2019). For the conventional chimney effect of vertical free channel e.g., building fires, a strong dependence on the
Conclusions
The chimney effect of smoldering fire in a U-shaped porous channel, representing a simple scenario of UCF in the field, is examined in this paper. With increasing the fire depth, air flow dragged by buoyant smoke generated by UCF increases, and the burning rate, the fire spread rate, as well as the burning temperature of UCF are facilitated. Both air flow and burning status are associated with the permeability of porous media in the channel. At an extremely harsh condition like percolation
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by National Natural Science Foundation of China (No. 51804168 and No. 51876183), Natural Science Foundation of Jiangsu Province (No.: BK20171005 and No. 17KJB620003), the Project of Priority Academic Program Development of Jiangsu Higher Education Institutions, and the Fundamental Research Funds for the Central Universities under project 2017CXNL02. Zeyang Song acknowledges financial support from China Postdoctoral Science Foundation (No. 2018T110492 and No. 2017M620209
References (63)
- et al.
Thermal design of symmetrically and asymmetrically heated channel–chimney systems in natural convection
Appl. Therm. Eng.
(2003) - et al.
Air ingress analysis of chimney effect in the 200MWe pebble-bed modular high temperature gas-cooled reactor
Ann. Nucl. Energy
(2017) - et al.
Thermal infrared imagery of the Burning Mountain coal fire
Remote Sens. Environ.
(1974) - et al.
Quantifying greenhouse gas emissions from coal fires using airborne and ground-based methods
Int. J. Coal Geol.
(2011) - et al.
Gas emissions, minerals, and tars associated with three coal fires, Powder River Basin, USA
Sci. Total Environ.
(2012) - et al.
Impact of thermal convection on CO 2 flux across the earth–atmosphere boundary in high-permeability soils
Agric. For. Meteorol.
(2014) - et al.
Influence of stack effect on flame shapes of gas burner fires
Appl. Therm. Eng.
(2017) - et al.
Paralava and clinker products of coal combustion, Yellow River, Shanxi Province, China
Lithos
(2009) - et al.
The Tiptop coal-mine fire, Kentucky: preliminary investigation of the measurement of mercury and other hazardous gases from coal-fire gas vents
Int. J. Coal Geol.
(2009) - et al.
Modeling of gas flow and temperature fields in underground coal fires
Fire Safety J.
(2001)
Comparison of methods to estimate the rate of CO2 emissions and coal consumption from a coal fire near Durango
CO. Int. J. Coal Geol.
Fissure formation and subsurface subsidence in a coalbed fire
Int. J. Rock Mech. Min. Sci.
Deflection characteristic of flame with the airflow induced by stack effect
Int. J. Therm. Sci.
Potential of small-baseline SAR interferometry for monitoring land subsidence related to underground coal fires: Wuda (Northern China) case study
Remote Sens. Environ.
Geomorphology of coal seam fires
Geomorphology
Uncontrolled coal fires and their enviornmental impacts: investigating two arid mining regions in north-central China
Appl. Geogr.
Coal fires revisited: the Wuda coal field in the aftermath of extensive coal fire research and accelerating extinguishing activities
Int. J. Coal Geol.
Enhancement of natural ventilation of a novel roof solar chimney with perforated absorber plate for building energy conservation
Appl. Therm. Eng.
A lab-scale experiment on low-temperature coal oxidation in context of underground coal fires
Appl. Therm. Eng.
Formation and distribution of polycyclic aromatic hydrocarbons (PAHs) derived from coal seam combustion: a case study of the Ulanqab lignite from Inner Mongolia, northern China
Int. J. Coal Geol.
The formation of multi-steady-states of buoyancy ventilation in underground building
Tunn. Undergr. Sp. Tech.
Dynamic evolution of spontaneous combustion of coal in longwall gobs during mining-stopped period
Process Saf. Environ. Prot.
Flow characteristic investigation of inhibition foam used for fire extinguishment in the underground goaf
Process Saf. Environ. Prot.
Natural convection enhancement in an asymmetrically heated channel-chimney system
Int. J. Therm. Sci.
CO2, CO, and Hg emissions from the Truman Shepherd and Ruth Mullins coal fires, eastern Kentucky, USA
Sci. Total Environ.
Small-scale forward smouldering experiments for remediation of coal tar in inert media
Proc. Combust. Inst.
A remote sensing and GIS based investigation of a Boreal Forest Coal Fire
Int. J. Coal Geol.
Influence of staircase ventilation state on the airflow and heat transfer of the heated room on the middle floor of high rise building
Appl. Energy
Mine fire gas indices and their application to Indian underground coal mine fires
Int. J. Coal Geol.
Coal fires in China over the last decade: a comprehensive review
Int. J. Coal Geol.
Numerical study on effects of air leakages from abandoned galleries on hill-side coal fires
Fire Safety J.
Cited by (51)
Experimental study of coal spontaneous combustion high-temperature region spreading characteristics
2023, Process Safety and Environmental ProtectionOxidation and exothermic properties of long flame coal spontaneous combustion under solid-liquid-gas coexistence and its microscopic mechanism analysis
2023, Science of the Total EnvironmentNumerical study on thermal-hydro coupling characteristics of underground coal combustion under heterogeneous void fraction
2023, Case Studies in Thermal Engineering