Application of subcritical and supercritical fluids in coal extraction and analysis

Sammanfattning: Coal is used to generate electricity and produce high-value chemicals through coal gasification, liquefaction, and coke. During combustion sulfur is released, which leads to acid rain formation. Mineral matter present in coal is undesirable because it causes technological problems such as corrosion of equipment and reduction of overall rate of combustion of coal. Also, after combustion, the mineral matter which has been transformed to solid waste (ash) is disposed of in landfills. The reported pre-combustion extraction methods for desulfurization and demineralization are time consuming, require the use of large volumes of environmentally toxic reagents at high temperature and they are not selective.The main aim of this thesis was to systematically develop desulfurization and demineralization methods based on non- toxic solvents using pressurized liquid extraction (PLE) and supercritical fluid extraction (SFE). The main emphasis was placed on achieving high selectivity for total sulfur and mineral matter during solvent extraction. Therefore, optimization of extraction methods in terms of solvent composition, extraction temperature and extraction time using Box Benhken design was carried out. The aim was to maximize solubility of target analytes, to improve mass transfer properties of the extraction solvent as well as reducing extraction temperature and time.Water with ethanol as a co-solvent was used during PLE method development. The results showed that increasing ethanol content resulted in higher extracted amount of total sulfur at mild temperature in a short time, corresponding to decrease in viscosity which resulted in fast diffusion of solvent and fast mass transfer. At optimum conditions, total sulfur extraction efficiency of 79 wt.% from the raw coal was achieved. SFE utilized supercritical carbon dioxide (scCO2) with ethyl lactate (EL) as a co-solvent. Higher temperature and lower ethyl lactate content significantly increased extraction of total sulfur even though increasing temperature reduces the density of scCO2. Higher solubility of sulfur compounds may be due to their increased vapour pressure in addition to the more polar solvent. At optimum conditions, total sulfur extraction efficiency of 93 wt.% from the raw coal was achieved. Neither of the methods were efficient in extracting mineral matter from the raw coal in comparison to extraction with acids/bases such as nitric acid and sodium hydroxide.Overall, the two methods have similar effects on the raw coal. Additionally, the properties of the raw coal were slightly improved. In addition to the reduction of the sulfur content, the thermal decomposition and burnout temperatures were decreased to a more considerable extent compared to the combustion temperature and the temperature at which maximum conversion of coal occurs. The calorific value, fixed carbon and volatile content have also increased and these parameters are all important in the combustion efficiency of coal.The research presented here serves as a base for the scaling up of extraction methods that utilizes green and environmentally sustainable solvents for reduction of pollution from coal combustion.