Contamination of groundwater due to underground coal gasification

Abstract

Underground coal gasification (UCG) generates potential groundwater pollution because it changes local hydrogeological parameters. Groundwater pollution is caused by diffusion and penetration of contaminants generated by UCG processes towards surrounding strata and possible leaching of residue by natural groundwater flow after gasification. A large number of hazardous water-borne contaminants were identified during different UCG operations conducted so far, and in some locations long-term groundwater contaminations were observed. Organic pollutants were detected after UCG process are phenols, benzene with its derivatives, polycyclic aromatic hydrocarbons (PAHs), heterocycles etc. and inorganic pollutants includes ammonia, mercury, zinc, sulphates, cyanides, heavy metals etc. Adsorption function of coal and surrounding strata makes a significant contribution to decrease the contaminants over time and distance from the burn cavity. Possible pollution control measures regarding UCG include identifying unsuitable zone, sitting a hydraulic barrier and pumping contaminated water out for surface disposal. The paper enumerates major pollutants identified in different UCG sites and its mitigation measures during gasification processes, and groundwater remediation after gasification.

Innovative thermodynamic underground coal gasification model for coupled synthesis gas quality and tar production analyses

“…Using the established commercial simulator Aspen Plus®, we introduce a new UCG synthesis gas quality related tar model, which considers main chemical in-situ sub-processes of drying, pyrolysis, reduction/partial oxidation, related mixing of gas fractions, water influx, optional gas losses and respectively a selected model tar set including typical single pollutants [1,3,8,10,[24][25]. The model is based on chemical and phase equilibrium balances under minimization of Gibbs energy referring to the Peng Robinson Equation of state [35,36].…”

“…With focus on the growing public debate on gas leakage affiliated with organic groundwater pollution risks as well as general tar plugging problems, a key future UCG by-product challenge thus lies in better tar production control and aligned organic pollutant minimization in UCG gases [20][21][22][23]. Principle UCG tar related organic pollutant groups, primarily consisting of BTEX, Phenols, PAHs and heterocyclic compounds have been well documented in diverse preceding publications [10,11,21,[24][25][26]. Besides environmental motives, the tar control topic may also be deeper analyzed from an economic valuable tar perspective, regarding tar yield and spectrum optimization for marketable products [27].…”

Approaches to the Gas Control in Ucg

“…During the gasification process, air or oxygen is injected with high pressure equal to or greater than the surroundings hydrostatic pressure. Some of the gas products are therefore lost to the surrounding permeable media and perhaps to overlying strata, as a result of cracks in the overburden [3]. Both air and oxygen gasification has been tried in the world.…”

“…The basis of the control algorithm is the controller equation (3), which calculates a new addition of the control variable in each step of the control algorithm. The control variable u is the percentage change in the position of the servo valve or exhaust fan motor power frequency.…”

“…In the first case, the control algorithm calculates in each step the control variable percentage to the number of pulses opening or closing the servo valve according to equation (3). The connection diagram of the feedback controller for the stabilization of O 2 concentration by air flow is shown in Figure 18.…”