Development of a mathematical model and experimental work and comments on the results for the underground coal gasification process

Underground Coal Gasification (UCG) is a method, which aims to produce flammable gas (syngas) from low quality or economically non-feasible coal seams if they were to be mined using conventional methods. This is achieved by drilling two wells into the coal seam and then these wells are linked to each other using various techniques. One of the wells is named the injection well, while the other one is named the production well. For the process to start, a combustion is initiated in-situ to increase the temperature of the coal seam first. After the temperature levels are elevated to optimal gasification conditions, injection of gasification agents starts. These agents are generally air, pure oxygen and for hydrogen oriented gasification, steam. Resulting syngas from the gasification mainly consists of CO2, CO, H2 and CH4. If air is used as the gasification agent, a high amount of N2 is inevitably found in the composition of the syngas, thus it yields a syngas with much lower heating values opposed to the gasification using pure oxygen. As the gasification progresses, a cavity forms. The shape and the expansion of this cavity is an important aspect of the UCG process. It growth should be able to be controlled and guessed beforehand. Due to the high temperatures and forming of the cavity, roof of the coal seam may collapse and cave-ins might occur. UCG processes can work at high pressure levels because of the hydrostatic pressure in the coal seam. The pressure in the cavity should be controlled. In the likely event of the cavity pressure being higher than the hydrostatic pressure around the seam, the production gasses would leak from the coal seam and would cause pollution such as underground water pollution. All things considered, it is obvious that the UCG process is hard to predict and control. Also as the pilot scale and on-site experiments are expensive, it becomes important to conduct laboratory scale ex-situ gasification experiments, which simulate the underground conditions to find more about the UCG process and to find optimal working conditions. It is also important to mathematically model the UCG process so that the effects of changing different parameters can be easily observed and assessed. This work's aim is to setup a laboratory scale UCG experimental setup, to conduct gasification experiments and to develop a mathematical model to numerically simulate the UCG process. For this purpose, an experimental setup was built in the Combustion Laboratories of the Istanbul Technical University Mechanical Engineering Faculty. The experimental apparatus consists of a reactor in which the gasification will occur, a cyclone to remove solid particles such as ash from the syngas, a heat exchanger to cool down the syngas and condense the water and tar in the syngas, a scrubber in order to remove the H2S from the syngas, a vacuum pump to overcome pressure losses in the experimental setup and a flare to burn the resulting syngas in order to safely remove the product. Reactor is designed to allow different well configurations, contains twenty temperature measurement points and is highly insulated in order to correctly simulate the underground conditions. Pressure measurements are taken along the experimental setup using transducers to keep track of the pressure levels and the pressure losses. Temperature measurements inside the reactor are taken using several specially coated thermocouple which are able the stand the elevated temperatures which are achieved during the process. Additionally, much like the pressure, temperature along the experimental setup. There is also one thermocouple located at the flare to check the pilot flame. Four orifices are used to measure the flow rate of air, oxygen, steam and syngas. Gas samples are gathered after the syngas passed from the scrubber to get its composition determined. Composition of the syngas is measured using a Gas Chromatogram. Three successful experiments are conducted using the lignite coal gathered from the Pirinççeşme, Malkara. For the main gasification agent, pure oxygen is used for all three experiments. First experiment last for 14 hours and in average produced syngas had 30% CO, 25% H2 and 3% CH4 in its composition. The second experiment consists of three phases. Gasification agent used in the first phase is pure oxygen. For the second phase, oxygen enriched air is used for gasification and for the last phase air feed is stopped and steam is fed into the reactor together with oxygen. For most part of the phase one, a syngas composition with 40% CO, 20% H2 and 3% of CH4 is achieved. These values started to drop during the last hours of the phase one. During the second phase composition of the syngas was devout of any burnable component. After the steam injection started with the third phase, the CO composition slowly increased to 30% while H2 composition increased to 25%. This regime, however, could not be sustained and the experiment terminated. The third and final experiment last for 14 hours. For the gasification agent pure oxygen is fed with a rate of 1.5 m3/h. This experiment consists of two phases in which for the second phase, an additional steam feed is used. For the first half of the experiment, CO levels was around 30% average while the average hydrogen level was about 15%. After the steam phase begun, the CO concentration increased to 35% and the H2 concentration increased to 20%. It shows that steam injection increases the overall quality of the syngas. Mathematical model developed in this work threats the coal seam as a cylinder and assumes the gasification channel is on the axis of this cylinder. Using this view, it is possible to model the UCG process in 2D axisymmetric coordinates. Another assumption is made for the gasification channel to be treated one-dimensional. Continuity, conservation of mass for chemical species and conservation of energy equations are written for this system. The reactions are taken into account using Arrhenius type chemical kinetics. The resulting non-linear equation system is discretized using Fine Volume Approach. Resulting system of algebraic equation systems are solved iteratively using appropriate numerical methods. Using the mathematical model developed in this work, it is possible to change the working parameters and discuss about their impact on the UCG process. Some of these parameters include the geometry of the coal block, flow rate and composition of the gasification agents, initial radius of the gasification channel and initial temperature of the coal block. Additionally, there are some model specific parameters such as heat transfer factor and the gas penetration depth. Although these parameters are not directly linked any physical property, one may loosely relate the heat transfer factor to the effect of turbulence on the heat transfer, and the gas penetration depth to the porosity of the coal. In this work, effects of these parameters are also discussed briefly. Lastly, experimental work, modelling results and the work done in literature is compared with each other. Comparing the experimental results of other works to the experimental results of this work, some variations are spotted relating to the syngas composition. These variations are addressed by making note of the different coal properties used in the experiments. Comparing the mathematical model results with the experiments, it is found that the model is able to predict some tendencies of the temperature field. Comparing the syngas compositions, it can be said that the model poorly predicted the experimental data. These inconsistencies are explained by the lack of drying and pyrolysis mechanism from the mathematical model. On the other hand, mathematical model was able to predict the cavity shape.