(463e) Modelling Studies for Cavity Growth in Underground Coal Gasification

Coal is a sedimentary rock and it is primarily composed of carbon along with sulfur, oxygen, nitrogen and hydrogen. Coal is ranked on the basis of its composition, properties and level of organic metamorphism. Lignite has the lowest rank with 60 – 70 % carbon content on dry ash basis. Sub bituminous has higher rank than Lignite. It has 71 – 77% carbon on dry ash basis. Anthracite is the highest rank coal with 77 – 87% carbon on dry ash basis.

India is the third largest producer of coal and has 12.65% of coking coal and 87.35% of non coking coal of total coal deposits. India is rich in sub bituminous coal followed by bituminous and then lignite. The ash content of Indian coal is about 30 – 50%. A large portion of Indian coal is deep seated and has poor quality. Underground coal gasification (UCG) is an effective method to utilize such coal. ^[1]

In coal gasification process, coal reacts with carbon dioxide and steam to produce syngas (hydrogen, carbon monoxide and carbon dioxide). The gasification mainly consists of three stages. In the first stage pyrolysis occurs through de-volatilization of coal which gives char. This is followed by combustion is the second stage, and then steam or carbon dioxide gasification is the third stage. UCG is a method where the gasification takes place in-situ. Two vertical wells (or boreholes) are drilled, one for injection of gasifying agents while the other one is for recovering the product gas. These wells are connected by using method like directional drilling. The main motivation in the development of UCG is to eliminate mining.

The output of UCG is influenced by many factors like reaction kinetics, hydrodynamics of gases, heat and mass transfer between the gas phase and solid phases, coal characteristics and geo-mechanical behavior of coal seams. Being an underground process, it is affected by many factors. It also makes visualization of the process undergoing at the coal seam difficult. These complexities led to the necessity of a model which will predict the product gas composition, calorific value and temperature variation. Since cavity growth, product composition and heating value depend on many factors, it is difficult to solve all equations using one single tool or model. Thus it is better to divide the model into sub models, solve them separately and then integrate them. ^[2]

In our work, we are proposing a model capable of predicting composition of product gas and its calorific value while taking chemical kinetics and spalling in to consideration. In UCG, as gasification proceeds, the cavity forms and grows. This cavity is considered as gasifier where the reactions take place. The cavity growth takes place in two directions, one around the injection well and one towards the production well. ^[4] Our focus is mainly on the first case where the growth is around the injection well. To model the phenomenon, we have considered a one dimensional hemispherical model with an assumption that the coal gets consumed only along the radial direction. The cavity is assumed to be a packed bed reactor and gases to be in pseudo steady state. Pyrolysis, combustion, Boudouard reaction (carbon dioxide gasification), steam gasification, methanation and water gas shift reaction are considered. ^[3] The variation of product gas composition and density of coal and gas along the radius is obtained. Effects of thermo-mechanical spalling on the product gas composition are also examined.

References:

1. Khadse A, Qayyumi M, Mahajani SM, Aghalayam P. “Underground coal gasification: a new clean coal utilization technique for India.” Energy (2007); 32:2061–71.
2. J. J. Nitao, D. W. Camp, T. A. Buscheck, J. A. White, G. C. Burton, J. L. Wagoner, M. Chen. “Progress on a New Integrated 3-D UCG Simulator and its Initial Application.” International Pittsburgh Coal Conference, Pittsburgh, 2011
3. Thorsness, C.B, Greens, E.A. and Sherwood, A., “A One Dimensional Model for in situ Coal Gasification”, Lawrence Livermore National Laboratory Report, UCRL 52523, (1978).
4. Ganesh Samdani, Preeti Aghalayam, Anuradda Ganesh, R.K. Sapru, B.L. Lohar, Sanjay Mahajani. “A process model for underground coal gasification – Part-I: Cavity growth”. Fuel 181 (2016); 690-703.