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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.

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. / text

Underground Coal Gasification (UCG), although not a new concept, is now attracting considerable global attention as a viable process to provide a "clean" 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, "sustainable," 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.

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.