The process of underground coal gasification (UCG) is considered to be one of the forms of clean coal technologies for obtaining gaseous fuel and synthesis gas from hard coal (Burton at al. 2006; Bhutto at al. 2013; Cough 2009). In the situation of increasingly smaller coal resources available for economically justified mining and the unfavourable impact of coal combustion on the environment, with a simultaneous prospect of continuous growth of energy demand, it seems advisable to undertake research on this technology.
The idea of underground gasification of coal directly in the coal seam was developed at the beginning of the 20th century in England (Review of the feasibility 2004; Gregg and Edgar 1978) and in the former Soviet Union (Klimenko 2009; Kreinin at al. 1982), and these works were continued after World War II. Currently, after many years of research and trials on an industrial scale conducted around the world (mainly in the USA, China, Australia, South Africa, Russia, Canada and Poland), the technology of underground coal gasification is still not fully mature for its widespread use on an industrial scale (Greg 2018). The only active installation in which underground coal gasification is carried out on an industrial scale is located in Angren (Uzbekistan), located on the territory of the former Soviet Union (Olness 1982). This installation has been operating since 1961, supplying low-calorific gas to a nearby power plant. At present, this installation is operating at a low efficiency of approximately 20%, more as a demonstration facility for potential foreign investors. Therefore, further research is needed to make these technologies applicable on a mass scale.
The coal gasification process is a combination of several simultaneous reactions: pyrolysis, combustion and gasification with complex kinetics, which are difficult to define unequivocally (Cena and Thorness 1981). In this process, apart from gas, which is its main product, liquid products are also obtained in the form of tar with water and solids, which are the remainder of the process. The content of tar products in the obtained gas depends on the process conditions and ranges from 0.5 to 15.5 g/m3 (Pavlovich and Strakhov 2013; Chiranjeeva at al. 2015; Wiatowski at al. 2017). Condensing liquid products constitute an impurity of the produced gas and should be removed when using gas as a chemical raw material practically completely. The most important factors affecting the amount and composition of tars in the process gas are the temperature conditions in the reactor and in the pipeline transporting hot gases to the surface. The tar starts to evolve from coal at temperatures of 350–400 °C (Elliott 1981; Karabon 2002; Vreugdenhil and Zwart 2009) and ends at approximately 1000 °C. During this time, tar undergoes many processes (Barbour and Cummings 1986; Barbour at al. 1988), as a result of which its quantity and composition change.
With a short residence time and lower temperatures in the underground reactor (which depend mainly on the size of the gasified seam, intensity and flow conditions of the gasifying agent and its concentration), the tar will be less affected by secondary processes (Akbarzadeh and Chalaturnyk 2014), e.g. thermal cracking. As a result, such tar will contain heavier compounds with two or more ring structures and fewer BTEX compounds compared to the one that has been in the reactor for a longer time. After leaving the reactor, tar is transported in a process gas stream to the surface. Depending on the temperature in the outlet pipeline, tar may continue to undergo the process of secondary cracking. The higher the intensity, the higher the temperatures and the longer the residence tar in the pipeline. Moreover, due to the temperature difference between the beginning and end of the pipeline, the heavier components of tar are liquefied (fractional distillation), and only the lighter components reach the surface (Liu at al. 2020; Philips and Muela 1977). Condensation of tar in the pipeline is very unfavourable because the pipeline diameter decreases; in extreme cases, it may become clogged (Department of Energy 2009). On the other hand, a high content of tar products in the gas negatively affects the properties of the produced gas and makes it necessary to build an effective system of gas purification and tar separation. Additionally, high tar content causes problems with its management (it is a dangerous and troublesome waste), and there may also be a possibility of groundwater contamination in the case of migration of tar components into the rock mass (Liu at al. 2007).
All factors that occur in tar from its formation to the transport in the process gas to the surface are the cause of the variable yield of tar products in the gas and the fluctuation of their properties in a way that is difficult to predict. Since the main purpose of UCG is to obtain as much process gas as possible with a high calorific value, the gasification conditions must be optimized so that the amount of tar obtained is as low as possible.
Information on the quantity and properties of tars obtained in the UCG process is the basis for the decision to dispose of tar products separated from the process gas. One of the potential ways of using tars from the coal gasification process is to connect it to the stream of mass-produced coal tar produced in coking plants and processed by distillation. However, such management of coal gasification tars requires their appropriate adaptation to a quality that does not differ from the parameters of coal tar. This is necessary from the point of view of the quality requirements for the products of tar processing, especially coal tar pitch, to avoid deterioration of commercial products. This is mainly due to the excessive content of ash and inert parts, which are determined as substances insoluble in toluene and quinoline. As an alternative, other uses of tars with UCG can also be considered, including the most unfavourable option of disposal as hazardous waste. To test this possibility, it is necessary to know the properties of tars produced during the real process of underground coal gasification. There is little information in the available literature about this topic. Therefore, the aim of this study was to determine the range of variability of the basic properties of tars from the process of underground coal gasification, which will be the basis for indicating potential methods of their utilization. The obtained results were compared with the properties of typical coal tar, which made it possible to adopt a certain reference level for the tested tars from the UCG process.