Tesi sul tema "Underground coal gasification"

Mathematical models were developed to understand cavity growth mechanisms, heat and mass transfer in combination with chemical reaction, and the factors which affect gas production from an underground coal gasifier. A model for coal gasification in a one-dimensional spatial domain was developed and validated through comparison with experimental measurements of the pyrolysis of large coal particles and cylindrical coal blocks. The effects of changes in operating conditions and coal properties on cavity growth were quantified. It was found that the operating conditions which have the greatest impact on cavity growth are: temperature, water influx, pressure and gas composition, while the coal properties which have the greatest impact are: the thermo-mechanical behaviour of the coal, the coal composition and the thickness of the ash layer. Comparison of the model results with estimates from field scale trials, indicate that the model predicts growth rates with magnitudes comparable to those observed. Model results with respect to the effect of ash content, water influx and pressure are in agreement with trends observed in field trials. A computational fluid dynamics model for simulating the combined transport phenomena and chemical reaction in an underground coal gasification cavity has been developed. Simulations of a two-dimensional axi-symmetric cavity partially filled with an inert ash bed have shown that when the oxidant is injected from the bottom of the cavity, the fluid flow in the void space is dominated by a single buoyancy force due to temperature gradients established by the combustion of volatiles produced from the gasification of carbon at the cavity walls. Simulations in which the oxidant was injected from the top of the cavity reveal a weak fluid circulation due to the absence of strong buoyancy forces, leading to poor gasification performance. A channel model of gas production from underground coal gasification was developed, which incorporates a zero-dimensional cavity growth model and mass transfer due to natural convection. A model sensitivity study is presented and model simulations elucidate the effects of operating conditions and coal properties on gas production.

Underground coal gasification (UCG) has the potential to access vast resources of stored fossil energy in a safe, clean and environmentally sound manner. Previous experiments have however led to concerns around surface subsidence, groundwater pollution and water table lowering. These issues can be prevented through the use of appropriate site selection and an understanding of the processes which cause these effects. Numerical simulations provide a cost effective means of predicting these issues without the need for costly and publically opposed field trials. This work uses a commercially available discrete element code to simulate the coupled thermal, hydraulic and mechanical phenomena which cause environmental damage. Surface subsidence is predicted through the displacements of fully deformable discrete elements separated by a network of fractures. The flow of groundwater through these fractures is simulated in order to predict the effects of water table lowering and the inflow of groundwater into the UCG cavity. Heat conduction from the cavity walls is simulated using an explicit finite difference algorithm which predicts both thermal expansion effects and the influence of temperature on rock material properties. Comparison of results with experimental observations in the literature show good agreement for subsidence and groundwater behaviour, while initial predictions for a range of designs show clear relationships between environmental effects and operating conditions. Additional work is suggested to incorporate groundwater contaminant transport effects, and it is envisioned that the overall model will provide a valuable screening tool for the selection of appropriate site designs for the future development of UCG as an economically viable and environmentally sound source of energy.

Most of the energy produced in the world comes from fossil fuels: coal, oil and gas. Amongst them, coal is the most abundant and widespread fossil fuel in the world. Underground Coal Gasi cation (UCG), an in situ method to extract the calori c value of the coal, has been known for a century but has had very limited implementation throughout the world, mainly due to the availability of cheap oil over that period. It is now gaining relevance in order to unlock vast resources of coal currently not exploitable by conventional mining. However, growing concern on increased levels of carbon dioxide concentration in the atmosphere is pointing out the necessity to reduce the use of fossil fuels. Since alternative sources of energy (e.g. nuclear and renewables) are not in a position to meet the constantly increasing demand in a short term, carbon capture and its geological sequestration (CCS) is considered the best remedial option. An environmental risk assessment framework has been developed for coupling UCG to CCS accounting for bene ts and cost from both global and local perspectives. A UCG site presents signi cant di erences from other typical CCS projected scenarios, most notably the injection of CO2 into a heavily fractured zone. A model which accounts for ow in fractures represented by dual-porosity ow (TOUGH2) is coupled to a geomechanical model (FLAC3D). The impact of this fractured zone in the CO2 injection pressure buildup and stress eld is evaluated. Furthermore the effect of stress-dependent fracture permeability is assessed with the hydro-mechanically i coupled compositional simulator GEM. Simulation results suggest that in such a scenario, CO2 injectivity and dissolution improve though con nement is compromised and commercial injection rates seem unattainable. The e ects of miscibility and relative permeability on pressure buildup implemented in semianalytical solutions are also evaluated. Albeit further research is required, a UCG operation may, therefore, not be able to accommodate the produced CO2 in the gasi ed cavity and its surroundings in a safe and economical fashion. Rigorous studies and management practices are needed to establish the requirements for secure long-term con nement of the carbon dioxide in such scenario.

Underground Coal Gasification (UCG), although not a new concept, is now attracting considerable global attention as a viable process to provide a â cleanâ and economic fuel from coal. Climate change legislation and the declining position of coal reserves (i.e., deeper and thinner seams) in many parts of the world are promoting and fueling the UCG renaissance. This research presents an analysis of operational parameters of UCG technology to determine their significance and to evaluate the effective range of values for proper control of the process. The study indicates that cavity pressures, gas and water flow rates, development of linkage between wells, and continuous monitoring are the most important operating parameters. A protocol for the selection of suitable sites for UCG projects is presented in this study. The site selection criteria are developed based on successes and failures of previous experiments and pilot studies. The criteria take into account the site characteristics, coal quality parameters, hydrology of the area, availability of infrastructure and regulatory and environmental restrictions on sites. These criteria highlight the merits and demerits of the selected parameters, their importance in site selection and their economic and environmental potentials. Based on the site selection criteria, a GIS model is developed to assist in selecting suitable sites for gasification in any given area of interest. This GIS model can be used as a decision support tool as well since it helps in establishing the tradeoff levels between factors, ranking and scaling of factors, and, most importantly, evaluating inherent risks associated with each decision set. The potential of UCG to conform to different frameworks defined to assess the capability and potential of any project that merits the label, â sustainable,â has been evaluated. It has been established that UCG can integrate economic activity with ecosystem integrity, respect for the rights of future generations to the use of resources and the attainment of sustainable and equitable social and economic benefits. The important aspects of UCG that need to be considered for its sustainable development are highlighted. In addition, the environmental benefits of UCG have been evaluated in terms of its potential for reduction in greenhouse gas (GHG) emissions. The findings indicate that UCG significantly reduces GHG emissions compared to other competitive coal exploiting technologies. A model to compute the life cycle greenhouse emissions of UCG has been developed, and it reveals that UCG has distinctive advantages in terms of GHG emissions over other technologies and competes favorably with the latest power generation technologies. In addition to GHG emissions, the environmental impacts of these technologies based on various impact assessment indicators are assessed to determine the position of UCG in the technology mix. It is clear from the analysis that UCG has prominent environmental advantages and has the potential to develop and utilize coal resources in an environmentally friendly and economically sound manner.
Ph. D.

The key controlling factor in the effective energy conversion of coal to combustible gases during the UCG process is the behaviour of the pyrolysed char in the reduction zone of the UCG cavity, which has not been published in available academic literature. This study investigates the impact of the operating parameters during the reduction zone of UCG using a bespoke high pressure high temperature rig which was developed as part of this research work. This rig, operating at temperatures of up to 900 oC and at pressures up to 5.0 MPa, simulates the UCG process including each UCG zone individually for a broad range of underground conditions to a depth of 500 m. Carbon dioxide and steam were used as the primary reductants with char derived from dry steam coal and anthracite sample. Carbon dioxide and steam were injected at a variety of pressures and temperatures, plus at a range of relative H2O/CO2 proportions. The composition of the resulting product gas of both coals was measured and subsequently used to calculate carbon conversion (X), carbon conversion of combustible gases ( ), cold gas efficiency (CGE) and low heating value (LHV) of the product gas. Optimal operating conditions were determined for the dry steam coal and anthracite that produced the best gas composition both at atmospheric and elevated pressure and are unique for each UCG system. A shrinking core model was employed to describe the behaviour of the pyrolised char to determine the activation energy and pre-exponential factor at atmospheric pressure for both coals. The evolution of the volatile matter of both coals and its contribution to the overall UCG performance was also determined. An optimum H2O/CO2 ratio was determined for both coals which enhanced the gasification rate of both coal chars up to the ratio of 2:1, above this ratio the effect saturated for both coals. It was shown that pressure increases the reduction-gasification process of the chars which suggests that there is an optimum operating pressure which produces a peak in carbon conversion, CGE and LHV for the product gas over the conditions tested that differs for each coal. Therefore UCG projects aiming at reaching higher pressures will not achieve an increase in the output, unless there are some new effects occurring above 4.0 MPa. Pressure enhances the gas solid reactions and almost doubles the max carbon conversion ( of combustible gases achieved at elevated pressure compared to that at atmospheric pressure. A shrinking core model was modified to take into account the effect of total pressure to the gasification rate of dry steam coal at 900 oC and pressures ranging from 0.7 to 1.65 MPa. Reaction constants for various pressures at 900 oC were determined for both coal chars. Analysis of data shown that typical UCG operations on low rank coals provides a combustible product gas that relies heavily on releasing the volatile matter from the coal and does not depend on the carbon conversion of char to gas which justifies the high CGE and LHV of the product gas found in the field trials. It was found that carbon conversion X is not significantly affected by the type of coal and that the carbon converted during UCG is between approximately 45% for high rank coals up to 55% for low rank coals. Experimental results were used to calculate the output, size and UCG model of a potential power plant which produced realistic solutions and proves that high rank coals can be suitable for UCG projects. Anthracite can produce almost the same amount of combustible gases as the dry steam coal operating under specific conditions but with a lower CGE and LHV which suggests that anthracite may be found to be more suitable for producing hydrocarbons with UCG than energy.

Thesis (Ph. D.)--University of Missouri-Columbia, 1998.
Trademark symbol follows Chemchar in title. Typescript. Vita. Includes bibliographical references (leaf 130). Also available on the Internet.

Com o aumento crescente das restrições ambientais acompanhado do aumento crescente da demanda energética e matéria-prima pela população que cresce em proporções assustadores com poucos indícios de sua descida fizeram com que buscassem alternativas com viabilidade econômica e reduzisse os impactos ambientais. Para o carvão mineral, a alternativa encontrada é a Gaseificação do Carvão em Subsolo. Das vantagens encontradas com o processo, as mais interessantes são: a segurança operacional e pouca infraestrutura necessária, competitividade no preço do produto gerado (gás sintético) e pouco gerenciamento do rejeito produzido já que as cinzas são deixadas nas cavidades em subsolo. Uma das dificuldades encontradas é mostrar a mudança do comportamento mecânico e acústicos das rochas e maciço rochoso quando submetido a alta temperatura ou pós-operacional com o resfriamento das cavidades geradas durante o processo. O maciço rochoso, o sistema de fraturas e as suas propriedades mecânicas (resistência à compressão e resistência à tração) e as propriedades física (permeabilidade e anisotropia) influênciam o design operacional do processo. Com os resultados obtidos foi possível uma interdependência linear entre as velocidades das ondas P e S, essa mesma interrelação foram observadas antes e depois do ciclo de aquecimento e resfriamento com coeficiente de determinação (R²) de 0,9177 e 0,9472, respectivamente. As velocidades das ondas P e S são reduzidas com a temperatura. A redução é mais evidente na onda P com redução máxima de 39% do valor inicial. A velocidade da onda S é reduzida continuamente a partir dos 800°C, passando de 7 % para 3% da velocidade inicial. A regressão feita com a resistência à compressão dos ensaios triaxiais diverge dos resultados obtidos nos ensaios uniaxiais. Os resultados da resistência à tração e os de resistência à compressão apresentaram aumento e redução da resistência em diferentes temperaturas. A resistência à compressão não apresentou qualquer regressão com as velocidades ultrassônicas, enquanto que o módulo de Elasticidade estático apresentou uma regressão linear crescente com a velocidade da onda P com coeficiente de determinação (R²) de 0,7922.
With the increasing increase of environmental restrictions, accompanied by an increasing increase in energy and raw material demand by the population that grows to frightening proportions with little evidence of their descent, they have sought to find alternatives with economic viability and reduce environmental impacts. For coal, the alternative found is Coal Gasification in Subsoil. Of the advantages found in the process, the most interesting are: operational safety and little infrastructure required, competitiveness in the price of the product generated (synthetic gas) and little management of the waste produced since the ashes are left in the underground cavities. One of the difficulties is to show the change in the mechanical and acoustic behavior of rocks and rock mass when submitted to high temperature or postoperational with the cooling of the cavities generated during the process. The rock mass, the fracture system and its mechanical properties (compressive strength and tensile strength) and physical properties (permeability and anisotropy) influence the operational design of the process. With the results obtained, a linear interdependence between the P and S velocities was possible. This same interaction was observed before and after the heating and cooling cycle with coefficient of determination (R²) of 0,9177 and 0,9472, respectively. P and S wave velocities are reduced with temperature. The reduction is more evident in the P wave with a maximum reduction of 39% of the initial value. The S wave velocity is continuously reduced from 800 ° C, from 7% to 3% of the initial velocity. The compressive strength with the triaxial tests differs from the results obtained in the uniaxial tests. The results of the tensile strength and the compressive strength showed increase and reduction of the resistance with different temperatures. The compressive strength did not show any regression with the ultrasonic velocities, while the static elasticity modulus presented an increasing linear regression with the P-wave velocity with determination coefficient (R²) of 0,7922.

A dissertation submitted to the Faculty of Engineering and the Built Environment, University of Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science in Engineering Johannesburg, 29 August 2016
This study discusses underground coal gasification (UCG) and the analysis thereof. Two main methods were used. The first is the Bond Equivalent Diagram, which gives an ideal of where operations should take place in relation to their coal and product gas compositions. This method was used to analyze several real life sites for their idealized and actual operations. The second consisted of a comparative exergy simulation study. This was done for an air-blown UCG plant with a downstream Fischer-Tropsch reactor and an oxygen-blown UCG plant with upstream air separation. The plants were analyzed by their overall exergy efficiency as well as their exergy outputs with respect to coal inputs (fuel). It was discovered that the air-blown simulation with downstream Fischer-Tropsch was the better choice from an exergy point of view due to it having higher efficiencies (1.5 for overall, 1.38 for fuel) as opposed to the oxygen-blown simulation (0.77 overall, 0.8 for fuel). This coupled with other design and safety factors led to the conclusion that the air-blown simulation was better.
MT2017

This paper examines an overview of the economic and environmental aspects of Underground Coal Gasification (UCG) as a viable option to the above ground Surface Coal Gasification (SCG). In addition, some highlights, hurdles and opportunities from early investment to successful commercial application of some worldwide UCG projects will be discussed. Global energy demands have prompted continual crude oil consumption at an astronomical pace. As such, the most advanced economies are looking for local and bountiful resources to challenge crude oil's dependence for which coal provides the best alternative so far. In the U.S, the Department of Energy (DOE), the National Energy Transportation Laboratory (NETL) along with the Lawrence Livermore National Laboratory (LLNL) continue to support pilot programs that develop improved methods for clean coal technologies to produce coal derived fuels competitive with crude oil fuels at about $30 per barrel. Lignite, the softest of the four types of coal, is the best candidate for underground coal gasification due to its abundance, high volatility and water to carbon content in its rock formation. The biggest challenge of modern humans is to find a balance of energy consumption, availability of resources, production costs and environmental conservation. Additionally, UCG has environmental benefits that include mitigating CO₂ emissions through Carbon Capture and Storage (CCS) and reduced overall surface pollutants, making it the preferred choice over SCG.
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In a global climate where sustainable development is being prioritized for the benefit of current and future generations, it is necessary to make informed decisions about the type of technologies being deployed. Because coal plays such a significant role in the generation of electricity, understanding the impact it has on the environment is an important part of understanding energy and environmental issues in general. To this end, a Life Cycle Assessment (LCA) was performed on two methods of electricity generation which employ coal as a primary source of energy. This assessment focused on the impacts from the power plants such as the emissions, resource consumption, and energy use of all processes required for the power plant to operate, including any necessary waste disposal and material recycling. Two technologies were selected. A PCC plant which represents the average emissions and efficiencies of currently operating coal-fired power plants in the world, and an IGCC plant which uses combustible gas derived from an UCG plant, hence a UCG-IGCC plant. The results of the LCA suggest that UCG-IGCC technology is more sustainable than the conventional PCC technology. The results yielded significant reductions in environmental stressors related to air, water, resource consumption and waste used in the impact assessment. Ultimately, LCA can be seen to be an ideal integrated environmental management tool to facilitate decision making on competing technologies to be deployed by assessing them throughout their entire life cycles.

Underground coal gasification (UCG) is believed to be one of the cleaner coal exploiting technologies for energy generation. In this study UCG is assessed as a technology for unlocking coal seams that are too deeply buried underground for extraction and those which are enclosed by complex geological settings making it impossible to extract using conventional mining methods. The assessment of the UCG technique was based on a desktop study from previous UCG trials globally. The objective of this study was mainly focused on considering factors which could be useful in the selection of potential UCG sites in South Africa. It was noted that the coal geology, coal properties, geological and geotechnical condition are crucial parameters to consider when selecting a UCG site. Three boreholes from the Highveld coalfield were used for the subsurface evaluation by means of geophysical wireline logging. Five coal samples from these borehole cores were studied using different characterisation techniques to understand the nature of coal and determine the coal properties suitable for UCG. The information acquired by wireline logging gave an insight into the geological and geotechnical condition of the area, and the properties of coal determined show some degree of suitability for UCG process in terms of their physical and chemical composition.