Investigations on PAHs in Malkara lignite and its ex-situ gasification char residues

Coal is the longest-lasting fossil fuel and has an indispensable place in energy production. Turkey has 16 billion tons of lignite reserves. However, since these reserves cannot be effectively processed and extracted, half of the electricity need of the country is supplied by the import of coal and natural gas. Here, the inability to effectively process and extract energy resources results in energy imports. However, due to their high ash and moisture content, the calorific value of the vast majority of Turkish lignite reserves are around 1000-2000 kcal / kg. Therefore, in some instances these resources are not economically feasible to be extracted and processed by conventional mining activities. It also causes environmental and air pollution due to its high sulfur content and much higher CO2 emissions than other sources. Clean coal technologies are important for minimizing external energy dependence. In this sense, these technologies allow for the economical feasibility of the processing of Turkish lignites and the minimization of environmental pollution. Pulverized coal combustion, circulating fluidized bed combustion and coal gasification technologies stand out in terms of efficient and clean energy obtainment. With the syngas produced by coal gasification technology, it is possible to produce electricity in Integrated Gasification Combined Cycle (IGCC) plants in the most efficient way, as well as to produce many chemicals such as methanol, ammonia, hydrogen and liquid fuels with Fischer-Tropsch method. Underground Coal Gasification (UCG) technology specific to the gasification technology is another clean coal technology that is a combination of coal mining, transport, processing and gasification processes. It provides in-situ assessment of reserves that are especially low rank and at marginal depths. Gasification agents are injected from the injection well which is opened into the coal seam, and the coal is ignited. The syngas (H2, CO) is collected from the production well which is connected to the injection well via the coal seam, and utilized for electricity and/or chemicals production. Through 3 chemical processes (drying, pyrolysis and combustion and gasification of carbonized char) it is intended to produce CO and H2 gases in oxygen deficient environment with a number of homogeneous and heterogeneous reactions. The prominent aspects of the technology are that it enables the coal to be utilized in an environmentally friendly manner; the great majority of the ash produced remains underground; it is advantageous in terms of health and safety since it is not underground human power; it eliminates the need for gasification reactors; low investment and operating costs; bypassing of operations such as coal mining, transportation, and storage; and applicability to reserves that are low rank and not feasible to utilize. However, even if it is defined as clean coal technology, the UCG process also needs to be considered in the environmental aspect. The most prominent environmental aspects of the process are carbon dioxide emissions, and the risk of contamination of groundwater due to byproducts such as ash, tar and char residues, and process water which is formed as a result of surface subsidence or the process and left underground. The risk of pollution of underground and surface waters is seen as the most important environmental risk. Many relevant studies have been done in the literature. The ash left in the cavity area, the process water, the tar and char residues are discussed as inorganic and organic contaminants. These byproducts become source of pollution by the fact that after the gasification process is concluded, the groundwater occupies the cavity region. In the works carried out, process water, ash and tar are discussed as both organic and inorganic contaminants. Almost all of the studies related to char residues are interested in the inorganic part, and there are minute amount of studies related to the organic part. In this study, the contents of Malkara lignite and its char residues in count of polycyclic aromatic hydrocarbons (PAH) resulting from simulations of ex-situ underground coal gasification of the lignite block of Malkara Pirinççesme region were investigated. In this respect, 16 PAH compounds, which are prioritized by EPA in the contents of chars and coal samples and the seven of which are carcinogenic to humans, have been qualitatively and quantitatively analyzed by GC/MS with in the scope of EPA 8270D method and literature research. Beyond the EPA 16 PAH compounds; within the scope of the DIN CEN/TS 15439 Technical Specification (Tar Protocol), PAH classes contained in char and coal samples were semi-quantitatively examined in terms of naphthalene and total C equivalents. Based on the EPA 3540C method and and literature research, sample preparation procedures were carried out using Soxhlet extraction experiments. The 8 hour extraction experiments were carried out using dichloromethane (DCM) and toluene/methanol (10:1, (v/v) (TM10:1) that EPA recommended, and toluene/methanol (1:6, v/v) (TM1:6) solvent systems suggested in the literature. Extracts obtained from Soxhlet extraction experiments were analyzed by GC/MS. In addition, functional groups of original samples, extracts and extraction residues were examined by FT-IR spectroscopy. Analysis by GC/MS in selective ion monitoring (SIM) mode revealed that the char residues contained 2-6 rings and a total of 2.758 mg/kg of 12 EPA PAH compounds. 2-ring naphthalene constitutes half of EPA PAHs in char, by the 1.186 mg/kg concentration value. By evaluating the extraction performance of the solvent systems, the best result in char samples was obtained with TM10:1, and it is concluded that DCM should not be preferred for char residues. It has been found that while the solvent system to be selected for the char residues should be in larger fractions with a similar structure to PAHs such as toluene and with a strong displacement capacity, the solvent should be in smaller fractions, giving the system a penetration ability such as methanol. Furthermore, for the char residues, it has been concluded that the choice of less polar solvent systems would be more appropriate. It is found that the total amount of EPA PAH (7.159 mg/kg) in the coal is quite higher than the char and contains all the EPA PAH compounds. The 4-ring pyrene compound constituted 84% of the total by 6.043 mg/kg. The most efficient result for coal is also obtained with the TM10:1 system, which gives the best result in char residues. It was seen that the TM1:6 system, with a small amount of toluene, gave the worst result with 40% recovery rate. It has been concluded that for PAH extraction from coal, aromatic structured toluen should constitute a larger fraction of the toluene / methanol binary systems and that a small amount of polar methanol fraction, which is capable of achieving effective swelling in the coal, is sufficient. Dichloromethane, which is another solvent recommended by EPA, has the extraction performance with 6.299 mg/kg concentration value and captures effective swelling in coal structure with its polarity. With similar results in the literature, this result shows that dichloromethane can be an alternative solvent for Malkara lignite. From the FT-IR results of the original samples and extraction residues, no interpretive information has been acquired in terms of solvent performance evaluation, except for some sharpening of the peaks by changing the solvent polarity in char and coal. The spectra of TM10: 1A and TM10: 1B extracts showed that aliphatic, aromatic and ar-aliphatic structures were diffused to the solvents. Coal samples were found to be composed of 92.1% of HMW EPA PAHs, whereas char residues were found to be composed of 81% of LMW EPA PAHs, and it is concluded that as a result of the UCG process, the EPA PAH content of coal decreased by 2/3. This is thought to be originates from the fact that during the stages of UCG process such as drying, pyrolysis and carbonized char gasification, LMW and HMW PAH's that are volatilized from coal generate tar and process water; and/or thermal cracking of HMW PAH's lead to the formation of LMW PAH compounds. This situation is also supported by the low fixed carbon and high volatile component content of the Malkara lignite. Furthermore, in the FT-IR spectrum, the attenuation of the band in the char with respect to the coal at 1615 cm-1, confirms the decrease of PAH content of coal due to the UCG process. It is thought that the difference in the OH strecthing bands of the FT-IR spectra of the original coal and char samples may be due to moisture contained in the coal; therefore, this indicates that char may have hydrophobic structure. In the spectra of char and coal extracts; presence of aliphatic C-H strecthing bands, aromatic C = C strecthing bands, out of plane hydrogens of substituent aromatics; especially, aromatic C = C bending bands and aromatic out-of-plane C-H bending bands of the char are indicative of aliphatic, aromatic and ar-aliphatic structures in the coal and char samples. The bands at 1000-1260 cm-1 in the coal showed the presence of heteroatom (N)-containing ring structures in the coal. In the calculations made according to the tar protocol; naphthalene and total C equivalent PAH and PAH derivative distributions were found to be close to each other. LMW PAHs and its derivatives were found to constitute 77% of the char, with 56% being 3 rings. In addition, it was found that 35% of the content of the coal was composed of 6 rings and the coal contained HMW PAHs and its derivatives with 76%. It has also concluded that coal may contain PAHs and its derivatives with 7 and larger than 7 rings, called gravimetric tar. Quantitative comparison of the contents revealed that char contains PAH compounds and its derivaitves up to 3% as the naphtahlene equivalent and up to 2% as the total C equivalent of the coal. This finding is confirmed by the difference in peak intensities of the FT-IR spectra of solvent extracts. This dramatic decline in PAH quantities confirms that gasification reactions take place at temperatures in excess of 1000 °C for 15 hours as well as drying and pyrolysis phases in the UCG simulation and that the LMW and HMW PAHs are separated from the coal and / or broken down into LMW PAHs and its derivatives to form process water and tar.