Heat-affected zone analysis of high ash coals during ex situ experimental simulation of underground coal gasification V. Prabu a , S. Jayanti b,⇑ a Department of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati 781039, India b Department of Chemical Engineering, Indian Institute of Technology Madras, Chennai 600036, India highlights  Laboratory scale ex situ simulation of underground coal gasification (UCG).  Identification and characterization of heat-affected zone during UCG.  Comprehensive sampling of remnant coal surrounding cavity.  Establishment of depth-wise profiles of volatile matter depletion.  Testing with coals, lignite and wood. article info Article history: Received 25 September 2013 Received in revised form 7 November 2013 Accepted 8 January 2014 Available online 28 January 2014 Keywords: Underground coal gasification (UCG) Cavity formation Heat affected zone Ash layer resistance Proximate analysis abstract Underground coal gasification (UCG) is a transient and evolving phenomenon in which many chemical and physical changes occur simultaneously and/or sequentially in various regions throughout the coal seam. Heat affected zones of dry and volatile depleted porous zones are formed up to a certain depth of the coal block leading to coal spalling. In the present work, we present a proximate analysis of the coal samples from various locations of the heat-affected zone (HAZ) during borehole combustion and gasifi- cation studies which simulate experimentally underground coal gasification. Results from HAZ studies of wood and four coals show that there is a considerable change in the volatiles to fixed carbon ratio in the depth direction and that the extent of variation depends on the coal as well as on the conditions prevail- ing during the experiment. These results have a potential bearing on the reactivity of the coal and the product gas composition. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Underground coal gasification (UCG) is an important technol- ogy for the exploitation of the large fraction of the coal reserves that are too deep underground to be mined economically and of- fers possibilities of coupling with technologies for clean coal utili- zation and carbon capture and sequestration [1–3]. Development of the UCG technology is essential for countries like China and In- dia which are large consumers of coal for power generation and which have large reserves of deep underground coal. For example, estimates reported by Ministry of Power in India in 2005 show that up to 13% of proven Indian coal reserves are at a depth of more than 600 m and a further 25% lie at a depth of between 300 and 600 m [4]. Indian coals are bituminous or sub-bituminous or lig- nite coals and typically contain a large amount of ash which cannot be easily washed. Large ash content in the coal leads to higher transportation cost of coal per MJ of energy, higher rate of erosion of power plant equipment as well as higher cost of fly ash disposal. Due to their high ash content, low calorific value and location at uneconomical depth, UCG is the most suitable technique to exploit these reserves. While UCG has been seen as a promising technol- ogy for a long time and has been used for commercial power pro- duction in the former USSR [5,6], there is limited experience at the level of large scale implementation. The cumulative amount of coal gasified in over forty years of operation in the USSR is about 15 million tones while in the US and Australia, it is only of the or- der of 50 and 35 kilo tonnes only [6]. This level of exploitation compares poorly with the current annual utilization of about 500 million tones of coal in India alone. Further, much of the field experience with UCG has been limited to rather shallow depths of 200 m or less. In order to develop UCG as a commercial technology making a significant contribution to the current energy needs, it is necessary to study a number of related issues such as the http://dx.doi.org/10.1016/j.fuel.2014.01.035 0016-2361/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +91 44 2257 4168; fax: +91 44 2257 4152. E-mail address: sjayanti@iitm.ac.in (S. Jayanti). Fuel 123 (2014) 167–174 Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel