CFD SIMULATION OF TEMPERATURE VARIATION IN CARBONIFEROUS ROCK STRATA DURING UCG

The numerical simulation was based on the computational fluid dynamics formalism in order to identify the change of temperature in rock strata during underground coal gasification (UCG). The calculations simulated the coal gasification process using oxygen and water vapour as a gasification agent in 120 hours. Based on the selected software (Ansys-Fluent) a model of underground coal gasification (UCG) process was developed. The flow of the gasification agent, the description of the turbulence model, the heat-exchange model and the method of simulation of chemical reactions of gasification are presented herein.


INTRODUCTION
Fuel acquisition, mainly underground fuels, has undeniably contributed to the development of civilisation in the modern world.Fossil fuels are currently the main source of energy, which were originally used as thermal energy, generally as a raw material for the generation of electricity.However, in the case of the exhaustion of natural resources of raw materials and the need to use zero-emission technology for their processing this has resulted in the most economically developed countries of the world being forced to search for new methods of energy production.This issue is very clearly seen in the countries of the European Union, and hence also in Poland.In addition, the situation is complicated by the prospect of future restrictions regarding the supply of crude oil and natural gas, which threatens the stability and energy security for many countries of the world (Białecka 2008;Borowiecki eds. et al. 2008).
In the case of a country that has huge amounts of deposits of hard or lignite coal, the development of clean coal technologies is invaluable.Poland is undoubtedly in the ranks of these states, as a country whose economy depends on energy manufactured from coal combustion.The forward-looking method, which opposes the mentioned threats, is the underground coal gasification process (Białecka 2008).According to the results of the model and experimental research (Białecka 2008;Kozaczka 1994;Stańczyk et al. 2012;Tomeczek 1991), the in situ coal gasification method could be an innovative method of chemical mining of out-of-balance coal deposits, unavailable within the usage of conventional extractive techniques.
The coal gasification process in in-situ conditions, poses a potential threat to the original state of the rock strata.The georeactor formed underground, in a coal seam, affects layers of rocks in the vicinity.One of the mechanisms of this interaction is the natural flow of heat from the gasification area (gasification channel) to the rock strata.The study of georeactor-rock strata interaction is important for the determination of the potential impact of gasification technologies by specifying the dynamism of changes of physical and mechanical properties of rocks constructing a rock mass.Such research can be carried out using computer simulation methods.The advantages of software for computational fluid dynamics (CFD) is the possibility of modelling the phenomena related to fluid flow (combustion, turbulence, multiphase flows, chemical reactions, heat conduction, radiation, etc.) (AN-SYS… 2009).

CHARACTERISTICS OF THE UNDERGROUND COAL GASIFICATION PROCESS
Under the conditions of the underground coal gasification process (UCG), along the gasification channel, three zones of gasification reaction were developed, namely (Białecka 2008): oxidation zone reduction zone pyrolysis zone The oxidation zone is determined by a number of exothermic chemical reactions between components of a gasification agent and a coal mass, accompanied by an energy effect in the form of heat.The products arising from the oxidation reactions constitute substrates of secondary reactions in the reduction zone.However, the pyrolysis zone includes thermal processes, as a result of which gas and fluid products are produced at the expense of heat collected from a stream of the flowing syngas (Białecka 2008;Borowiecki eds. et al. 2008;Kozaczka 1994;Tomeczek 1991).
The UCG process is determined thanks to a chemical interaction between the gasification agent (oxygen, a mixture of oxygen and water vapour, air or air enriched with oxygen) and coal, which accompanies large amounts of heat emitted to the rock strata.In the work of Białecka (2008), based on the results of experimental research on the gasification process with the participation of oxygen, the possibility of temperature along the gasification channel of 2270 K and higher, for a distance of 2-3 meters of its length were shown.Thus, high temperature disrupts the initial balance in the vicinity of the rock mass.The result of heat transfer into the rock strata is the change of the physical-chemical properties of rocks strata, which may result in their destruction.This creates a potential threat to mine crews and reduces the efficiency of the process.

MODEL OF COAL GASIFICATION PROCESS BASED ON THE APPLICATION OF CFD
The main assumption was to base the simulation on the reactive flow of fluid.Gas moving through the fire channel simulates the physical-chemical phenomena related to the transportation of mass and energy as well as the occurrence of chemical reactions.The description of chemical reactions is a kinetic description.In the framework of the prepared model, the interaction of gas from the gasification channel with the vicinity layer of the Carboniferous rocks has been taken into account.This description is related to the transportation of heat energy.The coal layers as well as the geological layers in the vicinity of a georeactor are treated as a homogeneous body.Due to the complexity of equations and the complexity of numerical methods used for solving them, it was decided to use the non-commercial CFD code.

Georeactor geometry
Studies on the underground gasification technology were shown that a useful form of syngas can be obtained even from coal with a thickness of at least 1 meter.Carrying out the gasification process at these types of coal seams enables eliminating the arising mining difficulties (Białecka 2008).
For the purpose of implementing a simulation of the underground coal gasification process, a georeactor model was developed and located in the coal seam of the thickness of 1 metre, the width of 2 metres and the length of 5 metres.The coal seam is available for the gasification process thanks to a gasification channel in the form of a cylinder with a diameter of 0.18 metres and a length of 5 metres, located in the symmetry plane of the georeactor.The gasification channel ensures the supply of the gasification mixture to the reaction area of the georeactor and ensures the reception of syngas.The effects of the adopted considerations led to the develop-ment of 5 geometrical models of the georeactor which were shown in Figure 1.

Simulation grid
Figure 2 shows the results of the numerical grid.It was decided to generate unstructured grids in the form of tetrahedron in a pyramid shape with the base of a triangle, namely: an unstructured grid of a solid model of the georeactor heat system (system I -Fig.2a where: where: equation of the conservation of the chemical reaction: where: In addition, the description of the gas flow in the reaction area, which is composed of the gasification channel of the given geometry and changes in the fluid properties resulting from the turbulent nature of the flowing gas were taken into account.Thus, the most widely used CFD turbulence model k-ε was included in the calculations.The phenomenon related to the exchange of heat between the flowing fluid (syngas) and the rock strata, is solved on the basis of the following expressions (9) (ANSYS… 2009): ) ( ) ( T k h t (5) where: The left side of the equation ( 5) interprets the phenomena related to the convectional heat transport in the syngas flow, while the right side of the equation describes the transport of heat energy.
The amount of heat conducted to the rock mass determines the thermal conductivity coefficient λ.The value of this parameter for the ground of the Upper Silesian Coal Basin was examined (Pawiński, Roszkowski, Strzemiński 1995), and the results are presented in table 1. Basic gasification process reactions are expressed in the form of three equations of primary reactions between coal and the gasification agent, as well as three equations of secondary reactions between products of primary reactions.Gasification reactions do not occur individually and their effect, expressed by a specific volume of product gas components, involves the resultant changes to the course in terms of preferences for certain chemical reactions and the creation of specific chemical compounds, the decisive role of such parameters as temperature, pressure or mass fraction of gasification agent compounds.The chemical composition of coal is listed in table 2. The following chemical reactions of the gasification process were adopted for the calculations: C+0.5O The following parameters of the kinetic model, expressed in the form of the dependence (12), were adopted (ANSYS… 2009):

NUMERICAL SOLUTIONS OF THE UCG PROCESS
The following local conditions of the numerical solution, separately for each of the discretization areas, were included, namely: a) for the gasification channel (fluid element): -

STUDY OF MODEL TEST RESULTS
Below, the temperature field change results were presented, which is the solution of the numerical model of the UCG process.The results of tests showed the possibility to reach the temperature of 1000°C after 120 hours, in a distance of 0.5 metres above the gasification channel.At the height of 1.5 metres the temperature was about 375°C (Fig. 3), for the claystone layer 475°C (Fig. 5), while for the claystone and sandstone layer about 325°C was observed (Fig. 7-9).At the height of 2.5 metres above the gasification channel, in all analysed cases, the same values of temperature of about 75°C were observed.The obtained model results are similar to the results of experimental research presented in work (Stańczyk et al. 2012).According to authors (Stańczyk et al. 2012), in ex-situ conditions of coal gasification processes, with the participation of oxygen and water vapour as the gasification agent, the temperature of about 1200°C at the height of 0.4 metres above the gasification channel was observed, after of 110 hours.

SUMMARY AND CONCLUSIONS
The capabilities of Ansys-Design Modeller were used to develop the model of georeactor and rock strata.The next step was to import the geometrical model to the Ansys-Fluent.Based on the geometrical model the discretization area was prepared, which was used to develop a numerical model of UCG.
The research results presented in this work leads to the following conclusions: 1.The applied software turned out to be a tool enabling the creation of models of coal gasification processes occurring in different conditions and, in particular, of the process occurring beneath the ground surface.2. Due to difficulties arising from the choice of the proper way to implement and carry out coal gasification processes related to the appropriate location in the vicinity of the rock mass, it is justified to predict each time the influence of the adopted solution for the model thermal layout of a georeactor by applying the numerical methods of CFD.

Fig. 1 .
Fig. 1.The geometric model of the georeactor developed for the simulation of the UCG process The geometry used in the calculations is based on the following information (Fig. 3): solid model of the coal layer -1 × 2 × 5 m solid model of the gasification channel -ø0.18 × 5 m solid model of claystone, mudstone and sandstone layer.

Fig. 3 .
Fig. 3. Change of average temperature in the sandstone layer under the conditions of the UCG process in time interval of 120 hours

Fig. 4 .Fig. 5 .Fig. 6 .
Fig. 4. The shape of the cavity (red area) and the changes of temperature in a layer of sandstone during UCG after 120 hours

Fig. 7 .
Fig. 7. Change of average temperature in the sandstone and claystone layer under the conditions of the UCG process in time interval of 120 hours

Fig. 8 .Fig. 9 .Fig. 10 .
Fig. 8.The shape of the cavity (red area) and the changes of temperature in a layer of sandstone and claystone during UCG after 120 hours

Fig. 11 .Fig. 12 .
Fig. 11.Change of average temperature in the sandstone, claystone and mudstone layer under the conditions of the UCG process in time interval of 120 hours