Development of combustion tube experimental setup for underground coal gasification

Abstract

Traditional methods of energy production using coal are mainly based on obtaining coal by surface and underground mining. Later, the obtained coal is burned in thermal power plants and used to produce steam in steam boilers, but it is also used in homes for heating purposes. During surface and underground mining, miners work in very difficult conditions, risking their lives. Unfortunately, there have been many fatal accidents in coal mines in the past. In addition, not using enough filters in the chimneys of thermal power plants and houses, and insufficient control cause various environmental problems. The underground coal gasification process provides relatively cleaner and safer utilization of coal compared to traditional methods. The underground coal gasification process is mainly based on the production of synthesis gas (syngas) formed by gasification of the coal in situ. In the former Soviet Union, important studies were carried out, but these studies were interrupted by the cheaper production of natural gas. Significant field and laboratory work have subsequently been carried out in the United States, Europe, China, and Australia. The only operating plant on an industrial scale is the Angren underground coal gasification plant in Uzbekistan, a remnant of the former Soviet Union. After a suitable coal seam is determined for the underground coal gasification process, the production and injection well pairs are drilled into the determined coal seam. Then if it is not adequate, a connection is created between the wells to ensure gas flow between the wells. Air, oxygen-enriched air, or steam-oxygen mixture can be injected from the injection well to partially oxide coal. During the gasification of coal, several reactions take place. The drying of coal, pyrolysis and gasification processes are the main processes. First, the moisture inside the coal evaporates in the drying phase and is followed by pyrolysis. Second, with the increase in the temperature coal decomposition occurs which is also called pyrolysis. Char forms as a result of pyrolysis and char gasification take place as the final step. These processes occur simultaneously as the oxidant injection continues. As the coal gasifies, a cavity is formed in the gasification region and the cavity grows during gasification. As a result of the gasification of coal, syngas is released. Syngas mainly contains carbon dioxide, carbon monoxide, methane, and hydrogen. The released syngas is produced from the production well and transferred to the facility on the surface. Later, the produced syngas is used for various purposes as electricity generation, methanol, hydrogen, and synthetic fuel production. The underground coal gasification process depends on several different parameters, especially the coal type. Coal properties vary between coals of different ranks. Due to the complex structure of coal, it can exhibit a heterogeneous behavior even within the same seam. For these reasons, the efficiency of underground coal gasification may differ in different coal ranks and different coal fields of the same rank. The depth of the coal seam, the formation, and the geological structures around it also affect operational efficiency. In addition to the formation characteristics, the operation parameters also affect the efficiency of the underground coal gasification process. Producing a maximum amount of syngas with a high heating value by gasification of minimum coal can be described as process efficiency. Injected fluid, injection pressure, connection method between wells, and the method used for production are the operational variables that affect the efficiency of the process. Field and laboratory studies are carried out to improve the understanding of the physical and chemical properties of the underground coal gasification process and to make industrial-scale applications with the information obtained. Field studies initiated with the former Soviet Union later gained momentum in the United States. The effects of variables such as distance between injection and production wells, connection method between wells, different coal ranks, different operating pressures, and the method used for production on underground coal gasification are tried to be better understood with these field and laboratory studies. At the same time, the growth rate and geometry of the cavity that is to be formed during underground gasification are also examined in this field and laboratory studies. In this way, possible subsidences are tried to be prevented. Before initiating the pilot-scale experiments and industrial-scale operations, laboratory studies are required to determine the optimum operation parameters and coal properties. Thus; the underground coal gasification process can be accomplished in an efficient and secure way. In laboratory experiments for underground coal gasification, the coal sample can be taken as a block and block tests can be carried out. However, it is not always possible to obtain block samples. In the case of deeper seams, the samples are obtained through the well as the core. Gasification experiments can also be performed by packing the coal particles in a tube. Previous combustion tube experiments have some drawbacks in terms of their design. First, some of the previous combustion tube experiments for the UCG were executed by providing an adiabatic environment in the reactor. In these experiments, the system is designed such that heat losses are almost zero. The temperature outside the reactor tried to be equal to the temperature inside the reactor by heating the reactor from outside. While this experimental design provides fine temperature behavior throughout the experiment, it does not represent the underground conditions. There is a considerable amount of heat loss through the surrounding formations. Second, previous combustion tube experiments do not have any H2S filter. The experimental design of this study includes an H2S filter. Furthermore, heat losses were tried to minimize; however, the reactor used in this study is not adiabatic.In adiabatic operation of combustion tubes it is not clear if a self sustained combustion front is achieved since the reactor is heated externally. The provided heat could have an impact on the combustion front behavior. It is hard to differentiate the effect of external heating. In this study, a combustion tube experimental setup was developed for underground coal gasification experiments through the adaptation of combustion experiments conducted for in situ combustion of oil petroleum fıelds. Content of the syngas and the progression speed of the combustion front were followed. The amount of produced gas was not measured in this study. Dimensions of the combustion tube to be used for coal gasification experiments were determined by examining the synthesis gas content and the slopes of the temperature profiles in front of and behind the combustion zone. While determining these measurements, the situation where deviations in the composition of the syngas are small, and the slope of the temperature profile in front of and behind the combustion zone does not change was taken as a stable period. During the study, parameters such as injected fluid, coal rank, and operating pressure were also examined. One of the important factors affecting the UCG operation is the content of the injected fluid. In the literature and different field applications, gasification has been carried out using air, oxygen-enriched air, and a vapor-oxygen mixture. In a part of this study, the effect of the amount of oxygen in the injection fluid on the gasification of coal was investigated. According to the results of the experiment, it is observed that the CH4, H2, CO, and CO2 contents of the produced syngas increases with increasing oxygen content in the injected fluid. Hence, the heating value of the syngas increases with increasing oxygen content in the injected fluid. The syngas produced during the gasification of coal contains various corrosive gases, primarily H2S. The main source of released H2S is sulfur in coal. There are different techniques such as caustic scrubbing, metal oxides, and alkaline impregnated activated carbon to trap the released H2S. In this study, a caustic scrubbing column is used to mitigate H2S production. Before the syngas is released into the atmosphere, it enters this column and is purified as much as possible from the H2S. However, the H2S amount at the inlet and the outlet of the scrubber could not be measured since neither in the gas chromotagraph or the gas analyser H2S measurements are available. After the introduction of the scrubber, a substantial decrease in the rotten egg smell indicated qualitatively the decrease in the H2S concentration. In conclusion, a combustion tube experimental setup that was developed for the in-situ combustion of oil experiments was modified for the underground coal gasification experiments. The experimental setup is updated by adding an H2S scrubber and appropriate sand filters. Recommendations are provided for the common problems faced in the experiments such as clogging of the sand filter and condenser and occurrence of the second combustion front. Using a liquid trap with a longer length with a higher volume can aid the condensation of the fluid inside the gas. This can be a solution for the clogging of the sand filter and condenser since the liquid trap also acts as a condenser that keeps the excess amount of liquid. Furthermore, the effect of the O2 content of the injected fluid was also examined. It is experienced that an increase in the O2 content of the injected fluid enhances the methane, hydrogen, and carbon monoxide content of the syngas. The stable behavior can be observed for a 1 m length 7 cm diameter combustion tube with an injection rate of 6 lt/min for the lignite samples used.