An investigation of the pollution risk of residues from a lab-scale underground coal gasification of Malkara-Pirinccesme lignite

Nowadays energy consumption is increasing so rapidly. Among all energy sources, coal is an abundant source in many parts of the world. With the depletion in oil reserves, coal can play a significant role in energy production. Clean coal technologies including gasification can contribute to a less polluted environment. Gas obtained from gasification is called Syngas or Synthetic gas. The clean product syngas resulting from the sulfur removal process has many potential uses. The syngas may be combusted in a gas turbine to produce electricity. In addition, carbon monoxide and hydrogen are basic chemical building blocks for production of many chemicals at chemical plants or refineries. Products that can be manufactured from syngas include: Hydrogen, methanol, synthetic natural gas (SNG) and fertilizers. In the case of un-worked, low rank and very deep underground coals for which mining may not be logical and economical, underground coal gasification (UCG) is a promising option for future use. As gasification proceeds, an underground cavity is formed and due to the pressure difference after the process is finished, water from the surrounding strata may enter the cavity. There are some strategies to a avoid unbalancing the quality of the underground water table like flushing the cavities with steam and/or water to remove pollutants from coal seams to prevent them from diffusing into surrounding water aquifers. Over time, the water table may return to a level close to that existing prior to the start of gasification. Even by considering such strategies there is a potential risk of water contamination during and after UCG which is a subject of investigation in terms of environmental concerns. The aim of this study is to examine the potential risk of inorganic arrays' transition such as heavy metals, sulfate, and ammonia from UCG solid residues (chars) and process water (liquid residues) to the groundwater and aquifers around the cavity which may decrease the underground water quality. In order to compare gasification with combustion in terms of water pollution, raw coals were converted to ashes through ASTM 3174-02 method. Solid samples (coals, ashes and chars) were digested with acids employing Microwave acid digestion (EPA 3052) and metal concentrations were determined by ICP-OES. Leaching tests including TCLP and EN 12457-2 were conducted on both UCG chars and ashes from burned coal and the leachability and mobility of the elements from ashes and chars to the water phase have been compared. The effect of leachant pH on the leachability have been observed using different extraction fluids with different pHs. The hazard level of UCG chars as waste has been determined through TCLP and EN 12457-2. Heavy metal, sulfate, and ammonia levels have been measured in the process water samples. The high concentration of toxic elements such as boron and antimony in the char eluates which exceeded standards suggest that toxic elements can leach from chars (solid residues of UCG) to the surrounding water aquifers after the process is ended. Chars from Underground Coal Gasification are considered to be non-toxic according to TCLP with leachants with the following pHs: 2.62 and 4.33. There are no landfill concerns about UCG solid residues (chars) as they can almost be categorized as inert waste according to EN 12457-2. Only in one case (Sb), they are considered to be non-hazardous waste. Considering process water samples, the metal concentrations are significantly higher than in the char leachates. Concentration of Cu in the process water highly exceeds the "Standards for Process Effluent Discharge to Municipal Sewage Systems". The ammonia amounts in liquid 1 and liquid 2 are extremely high (2400 and 1960 mg/l) which vastly exceeds the amount of 10 mg/l specified under "Non-Specific Source of Effluent Discharged to Environment" .The sulfate amount obtained from the liquid 1 is 4461 mg/l and from liquid 2 is 1322 mg/l which exceeds the limit (1500 mg/l) according to "Standards for Process Effluent Discharge to Municipal Sewage Systems". Gasification demonstrated a cleaner process to combustion in the terms of water contamination concerns (inorganic arrays) particularly in terms of boron contamination.