Ranking of coal seams for underground coal gasification (UCG) in Mazino coal deposit, Tabas coal field, Iran

The underground coal gasification (UCG) has a potential for converting the world’s coal resources into energy, liquid fuels and chemicals. The UCG process involves the injection of steam and air or oxygen into an underground coal seam and igniting and burning of coal in-situ to produce the combustible gas. Previous studies showed that many criteria affect site selection of UCG. The criteria include coal seam properties, faulting, discontinuity, properties of hanging wall and footwall of coal seam and hydrogeological regime. In this paper, considering proper UCG site selection criteria, coal seam was ranked and selected for the UCG based on the controlled retraction injection point (CRIP) configuration in Mazino coal deposit. The result of this investigation showed that the M2 coal seam has great advantage (considering the seam thickness and reservoir) related to the other coal seams for the gasification by CRIP configuration.


INTRODUCTION
Coal is the largest fossil fuel resource in the world, with proven reserves that are adequate to meet the expected demand, without much increase in production costs (Couch, 2009).With the depletion in the oil and gas reserves, coal is expected to play an important role in the global energy sector in the near future (BP, 2010).Underground coal gasification (UCG) offers the potential for using the energy stored in coal in an economical and environmentally sensitive way, particularly from deposits that are not mineable by conventional methods (Couch, 2009).Therefore, UCG is a candidate process for converting the world's coal resources into energy, liquid fuels, and chemicals.If the UCG process is developed commercially, it would increase coal reserves by 60% (Sarraf, 2012).The process of UCG eliminates the costs of mining, lowers water consumption and transportation needs, and generates possible sites for CO 2 sequestration, and gasification installation, which are required for traditional surface gasification process (Gregg and Edgar, 1978;Burton et al., 2006).However UCG has some challenges such as process stability, aquifer contamination and ground subsidence.The procedure for in-situ gasification of coal is as follows (Figure 1 (i) Injection and production wells are drilled from the surface to the coal seam.(ii)Injection and production wells are linked together under ground.(iii) Air or oxygen is sent to the coal seam through the injection well.(iv) The coal is ignited in a controlled manner.(v) The gas products, such as H 2 , CO, CH 4 , and CO 2 , flow to the surface through the production well.The gas products are sent to the end users after cleaning.The gas products can be used for power generation or to synthesize chemicals, such as methanol, ammonia, and liquid fuels (Khadse et al., 2007;Daggupati et al., 2011).
The concept of UCG was first suggested by Sir William Siemens in 1868.At about the same time, in Russia, Dmitry Mendeleyev suggested the idea for drilling injection and production wells (Burton et al., 2006).Since the 1930s, more than 50 pilot UCG plants have been conducted worldwide.These developments have been concentrated in the former USSR, Europe, USA, South Africa, Australia and China.The economic attractiveness of a commercial venture depends on a number of factors including socioeconomic conditions and geologic and hydrologic characteristics (Oliver and Covell, 1989).Therefore, the most important factor in success of UCG operation is site selection based on geological and hydrogeological parameters.
Today, based on successes and failures of previous experiments and pilot studies, the site selection criteria are developed (Couch, 2009;Burton et al., 2006;Oliver and Covell, 1989;Ag Mohamed et al., 2012;Irwin et al., 2009;Shafirovich et al., 2008;Białecka, 2009).This criteria take into account the coal rank, coal seam thickness, seam depth, ash content, coal seam permeability, fault density, coal moisture, coal aquifer characteristic, dip and discontinuities.In this study, Mazino coal deposit was selected for UCG site selection in Tabas coalfield, Iran.The aim of the present paper is ranking of coal seams for controlled retraction injection point (CRIP) configuration by considering important criteria in UCG site selection.

CASE STUDY AND METHODOLOGY
Nowadays, it is expected that coal seam energy have a very important in view of government and investors in Iran because of the elimination of fuel subsidy.Tabas coal resources are estimated to be about 2 Gt, which are located in the Parvadeh and Mazino.Mazino is the largest thermal coal deposit in Tabas coalfield and its area is about 8800 km 2 .This deposit is located in 85 km west of Tabas, Yazd (Figure 2) (Yazdani et al., 2012).The Mazino formation consists of sandstone, shale, siltstone, and carbonate rocks.A general stratigraphic column at Mazino coal deposit is shown in Figure 3.The thickness of this formation is about 1200 m and gradually decreases in some strata and increases in the other towards the depths (that is, towards Coal-bearing strata in Mazino deposit is within the Middle Jurassic formations.These sediments have been developed across alluvial plain and coastal environment in Tabas coalfield (Yazdiand, 2012).The rank of the Mazinocoal deposit is semi-anthracite and all of the coal seams are formed within the complicated monoclines and synclinal folds dipping to east and have been cut by several faults (Anon, 2002).Several coal seams (75 coal seams) with different thickness are interbedded with these sediments.The coal seams thicknesses are varied from 0.5 to 6 m in different place (Anon, 2002).The overall dip angle of coal seams varies from 6 to 26°.The most important properties of the coal seams are shown in Table 1.It should be noted that total reserve of the coal seams was estimate by Russian geologists (Anon, 2002(Anon, , 2003)).The Mazino area is located far from the populated areas, national parks, wildlife habitat, agricultural fields,   Two steps are considered for ranking of coal seams in Mazino coal deposit.In the first one, suitable coal seams were selected only by considering UCG operation in traditional scale.In the second one, by considering final result of Step 1, coal seams ranking was performed for UCG based on CRIP configuration.

Step 1: Suitable coal seam based on UCG in traditional scale
In the first step, by considering criteria for UCG site selection, suitable coal seams were selected.The results of Step 1 are shown in Table 2.It should be noted that in the step one each stage performed sequence.This means, when a coal seam is unsuitable in each stage, it removed and not considered in other stages.According to Table 2, the most important points areas follow: (i) Based on coal seam thickness criteria for UCG site selection (Couch, 2009;Burton et al., 2006), in stage 2, the M2, M23-1, M23, M22-1, M17, M14-1, M10-1, M10-2, M10, M9 and M8 coal seams are unsuitable for UCG because their thickness is less than 1 m.(ii) The M26 coal seam depth is lower than 100, therefore considering the gas leakage to the surface andsubsidence problem, the M26 coal seam is unsuitable for UCG in stage 4.

Step 2: Coal seam ranking based on CRIP configuration
There are several configuration including vertical well, εUCG, long-tunnel large-section two-stage, steeply dipping and CRIP for UCG method (Couch, 2009;Burton et al., 2006).Nowadays, in UCG methods, the CRIP configuration is considered and has remarkable advantage compared with the others.In CRIP configuration, the UCG panel has a large dimension and intends to the huge cost drilling.Moreover, two in-seam holes are drilled in parallel in the lower part of the seam, and then turned near the end to meet at a point, maybe as much as 500 to 700 m from where they have started.
Where they intercept, a vertical ignition well is drilled.Then the seam is ignited at the base of the vertical well and the air flow established from the injection well to the vertical well, drawing the fire to the end of the liner (Couch, 2009).
The CRIP configuration was used at Rocky Mountain 1, in the USA, and is developed in larger scale trial at Bloodwood Creek in Australia by CSIRO and Carbon Energy Pty Ltd (CEPL) from 2008 to 2010 (Figure 4)   Depending on the amount of electricity and the life cycle of the power plant, varying amounts of coal are needed to meet the energy requirement.
In UK 5Mt of coal in resource to provide 20 years of operation and in USA is 3.5 Mt for 15 years (Irwin et al., 2009) All suitable seams in the step 1 have a suitable resource.

Coal seam thickness
The thickness of coal is an important factor in CRIP configuration.Considering the economic issue, it is better that the thickness of coal seam should be more than 2m.
All suitable seam thickness in the stage 1is more than 1m.

M6 andM25
M5, M4,M2-1, M2, M1 (Couch, 2009).As clearly seen in Figure 4, the UCG by the parallel CRIP is similar to the longwall mining method, while with more of the coal removed, and the ash staying behind underground.According to the aforementioned text, at the second step, by considering final result of Step 1, coal seams ranking was performed for UCG based on CRIP configuration.The results of Step 2 are shown in Table 3.According to Table 3, the most important points are as follow: (i) The reserve of coal seam is an important factor in the CRIP configuration method.Shafirovich and Varma (2009) concluded for 20 years continuous operation of a 300 MW UCG-based combined-cycle power plant (efficiency, 50%), it is necessary to produce 75.6 × 10 9 Nm 3 of syngas with a heating value of 5 MJ/m 3 .It means 33 million metric tons coal needs to be gasified for this purpose.We can conclude that a 200 MW UCG-based combined-cycle power plantrequire to 1.2 million tons of coal per year.Therefore all coal seams in stage 1 are suitable for gasification.The thickness of coal seam is an important factor in CRIP configuration.Considering the economic issue, it is better that the thickness of coal seam should be more than 2 m (Couch, 2009).Forasmuch as the thickness of coal seam has important role in the CRIP configuration method, Therefore, the M6 and M25 seams with thickness less than 2 m, are unsuitable in the CRIP method.Because the M2 seam has suitable thickness and reserve, it has great advantage related to the other seams for the gasification by CRIP method.

Conclusion
Underground coal gasification has the potential to harness energy from coal seams in an economic, environmental and sustainable manner.This technology can be applied to abandoned coal mines and deposit considered uneconomic or technically difficult for conventional mining methods.The selection of suitable coal seams for UCG project is the most important step and has a significant effect on the successful of whole process.This procedure involves taking into account geological and environmental parameters.The present work gives rise to be ranked coal seams in Mazino coal deposit.The most important results as are follow: (i) The M2 coal seam has a highest ranking for UCG by CRIP configuration in Mazino coal deposit and M1, M5, M4 and M2-1 coal seams are in the other ranking, respectively.(ii) The M2 coal seam reserve is 139Mt.Thus considering 1.2 million tons of coal per year for a 200 MW UCGbased combined-cycle power, it can be predicted that this seams supply the coal for power plant more than 100 years.
water wells, underground aquifers and oil and gas fields.Based upon the factual data, it seems that Mazion deposit is a candidate for UCG method.

Table 2 .
Coal seam qualification for UCG in step 1.

Table 3 .
Coal seams ranking for CRIP configuration in Step 2.