Energy has been universally recognized as one of the most important inputs for economic growth and human development. In terms of energy, coal is a major energy source which provides about 40% of the total electricity generation of the world. Additionally, coal reserves significantly exceed oil and gas. Thus the estimates of total world coal resources (including non-mineable coal) are usually stated in trillions of tons and recent estimates of the total remaining coal resources in the world quote a figure of 18 trillion tons. However, the extraction of coal and production of electricity in coal-fired power stations significantly pollutes the environment and is detrimental to the global climate. Therefore, a lot of effort has been made in recent decades to promote the use of renewable sources in the energy market. Moreover, taking into consideration that the use of renewable resources in the energy market also has negative impact on environment and is more expensive in comparison with conventional power stations, it appears appropriate to develop clean coal technologies.
Although the concept of Underground Coal Gasification (UCG) is not new, as it dates back more than 100 years, renewed interest in UCG technology has recently occurred in most coal-producing regions of the world. This is a technology permanently developed in a world where coal is burnt in a controlled manner and gasified under in situ conditions to produce syngas which can be converted into various types of fuel, such as gas for electricity generation or even oil. Development of UCG continued for 78 years in the Soviet Union and included successful commercial production at numerous sites. During this time, the formerly Soviet Union conducted roughly 200 field tests and several commercial projects producing over 15 million tons of coal. Much of this was at the electric power plant in Angren in Uzbekistan that is still in operation after 53 years. The World Energy Council estimates that UCG could enable the economic recovery of 600 billion tons of coal reserves, allowing companies to access coal that has previously been considered ‘non-minable’. The UCG process could be more economical than conventional surface coal gasification because no mining, processing and transportation of coal are involved. Further, no ash/slag removal or disposal is required. The environmental impact of UCG is relatively low compared to conventional mining methods, as major disturbances in landscape and surface disposal of ash are not required. The main motivation for moving toward UCG as the future coal utilizing technique is the environmental and other advantages over the conventional mining process. UCG is particularly advantageous for deep coal deposits and steeply dipping coal seams since under these conditions there are less gas leakages to the surroundings, the high pressures favor methane formation and the subsidence at the surface may be minimal. Of course, UCG involves some environmental impacts such as land subsidence, ground water reserve pollution, air pollution and climate change exacerbation which represent disadvantages1.
UCG has been demonstrated on almost all types of coal, although coal with lower ash content is preferable. Compared to surface gasification, UCG requires much smaller gas clean-up equipment, because both the tar and ash content of UCG-based syngas is substantially lower than that obtained from a surface gasifier. Because the processing of the coal is kept underground, surface and air emissions of sulphur, nitrous oxides, and mercury are dramatically reduced.
The UCG implies a controlled, quiet and incomplete onsite combustion, without any personnel involved, and equipment placed underground. Also, there is no need for investments in expensive process equipment, as well as conveyer or tube transportation systems for slag and ash to landfills. Considering these benefits indicates how significant economic effects of the UCG application are. In parallel, environmental benefits are also exceptional. Analysing UCG process as a way of obtaining a suitable and useful energy source, some of the advantages are: – the absence of heavy mining in underground, as well as in surface mining conditions, – minimum costs of transporting gas to consumers, – significantly lower cost of the final product (normalized to 1 MJ of energy), – there is no significant degradation of the terrain, since all the non-combustible substances (slag or ash) remain in the deposits beneath the surface of the earth, – there is no substantial pollution of the environment and the working environment, as there are no solid residues in the combustion of gas from UCG process in the boiler furnace, – there is no need for storing about 50% of the total mass is waste material (slag and ash remain underground), – there is no need to secure significant areas necessary for the collocation and disposal of slag and ash, there is no combustible dust that additionally pollute the atmosphere by the products of self-ignition on the landfills, and – there is no need for recultivation of the terrain, since after the completion of the gasification, the terrain again resumes the same purposes from before gasification, and considerable areas are used during the UCG in continuity with agro-cultural activities1.
The commercial sustainability of the UCG process primarily depends on geological constraints. In addition, the competitiveness of these products is changing with the cost of technology, or the level of development of this technology.
UCG Serbian perspective
The energy crisis, which the world is currently facing, has caused a series of disruptions in the electricity, oil and gas markets and requires a more efficient operational approach in response to it, as well as responses to similar events in the future. As to Serbia, the impacts of climate change on the global economy and society have already lead to a number of new negative consequences for society and its development resulting in the need to adapt to changing climatic conditions and reduce greenhouse gases. Primary task of the energy system of the Republic of Serbia is to provide safe and continuous access to energy resources at sustainable prices, respecting all the principles of a healthy environment. So far, in addition to importing, Serbia has mostly relied on and used its own limited fossil resources. Approximately 75% of energy in the Republic of Serbia comes from fossil fuels. However, the combustion of fossil fuels releases carbon dioxide, which is the main cause of the greenhouse effect. The use of fossil fuels, as well as their extraction from the ground, contribute to air pollution and can cause serious damage to our health and the environment. Serbia is determined to minimize or completely eliminate these effects in the possible dynamics.
In the structure of primary energy production in Serbia, coal participates with 68.5%. The Republic of Serbia has reserves of low-calorie lignite coal, with low thermal power and high percentage of ash and moisture, which is ranked as the worst type of coal, primarily in the Kolubara and Kostolac basins, which requires larger amounts of other energy-generating products during its combustion process. Coal production exceeding 30 million tonnes during the year due to reduced coal quality and increased specific consumption of coal in thermal power plants is not enough to balance the planned electricity production due to reduced thermal capacity of coal. Moreover high specific consumption of coal affects the increase of carbon dioxide emissions into the atmosphere, which can be a particularly aggravating fact in the coming period1.
It is the fact that already in 2021 the Republic of Serbia defined the investments in the energy and mining sectors as one of the pillars of energy development and used all its resources in defining the basis and conditions for energy transition and establishing a new green growth model. However, increasing energy costs and energy demand have renewed global interest in clean coal technologies. The question of energy resources has become more urgent this year as global energy prices have soared. Global demand for gas has grown sharply since the Russian-Ukrainian war. Underground Coal Gasification (UCG) as a promising technology and industrial process that converts coal into product gas, with a lot of health, safety and environmental advantages over conventional mining techniques, due to the prevailing global appears as the promising solution.
According to the prominent Serbian expert in the field of UCG method implementation, in the name of a safer energy future of Serbia, it is necessary to save on imported liquid and gas fossil fuels, as well as coal on which energy security of Serbia is based. Thus, the rationalization in the use of available energy resources certainly could positively affects economy and the environment of the country1.
While the world is looking at alternative energy resources to meet many of the imposed challenges, fossil fuels (primarily natural gas and gases related to it) will likely remain an important part of global energy consumption for a long time. Regarding climate change and other environmental concerns, significant investments in clean coal technologies are evident in the world, with the aim of making rational use of this significant and relatively inexpensive energy source, while at the same time reducing its negative impact on the environment. Therefore, the use of these resources with a properly defined technology has a central place in the overall feasibility of UCG technology. Moreover, there are numerous applications of the gas generated from the UCG process. It could be utilized as a fuel in thermal power plants, as well as district heating plants, in chemical industry, in systems for the hydrogen generation, and also for obtaining petroleum products as fuels. Today there are more than 40 countries which is dealing with UCG. Some of the countries are only involved from the perspective of research and applied investigations, while some are developing semi-industrial systems, and others are already at the stage of the full industrially proven manufacturing set-up.
In Serbia, in order to be able to additionally strengthen the environmental aspect of UCG application, various additional renewable energy resources have been considered. The key one are solar photovoltaic systems that are proposed to be utilized to provide necessary onsite clean energy generation option. Stand-alone or grid-connected photovoltaic systems could be installed to deliver onsite power for running dedicated pumps, and other UCG process equipment creating complete UCG energy generation furthermore environmentally friendly. Preliminary studies are showing that the planned photovoltaic system can completely power all the required onsite UCG loads. Needless to mention, solar photovoltaic systems are completely environmentally friendly, generating power cleanly onsite and without any noise or any other negative effects on the environment. Due to all this, after the termination of the UCG development in a certain location, the land is returned to its original purpose without any need for the special field recultivation.
Petrović, D. T., et al.: Valorization of Non-Balanced Coal Reserves in Serbia for ... Thermal science: Year 2019, Vol. 23, No. 6B, pp. 4067-4081