Prof D Chandrasekharam

Coal, formed from organic matter, is a reservoir for significant quantities of  gases.  Methane is an important gas since it supports economic development of a country next to oil and natural gas. In fact, methane is considered as a hazard by the coal miners since it catches fire in the mines causing damage to life and property, disturbs the mining activity and reduces the economy of the mining operation.  Methane that is freely ejected in the coal mines, during and after the mining activity, is called coal mine methane (CMM). Underground coal fires that are common around the world is caused by this gas. Methane as such is a GHG that is harmful to the atmosphere and is 21 times more potent than CO₂! If this same methane is captured and stored, then it becomes  a tremendous energy source. This has been realized over the last few decades and technology is available now to extract methane from underground coal formations/beds. With declining oil resources, countries are frantically looking for other sources of energy to substitute oil. Methane is also generated from animal dung. The gas generated from the later escapes to the atmosphere while the methane trapped in the coal can be extracted and used for generating electric power with out polluting the atmosphere. What it means to a common man is, where ever there is coal there will be CBM. It is true!.  Major coal reserves occur inUSA,Australia,Indonesia,Russia,China andIndia.

 Until 2006 Chinawas using 1.4 billion cubic meters of CBM ( about 3 % of natural gas consumption) and is targeting an output of 10 billion cubic metric meters by the year 2015! CBM is one of the major thrust area projects in China’s current five year plan.USA has already extracting CBM and the maximum up tapped resources lie inAlaska(30 trillion cubic meters ??). Australiais second in the CBM production list fooled byChinaandIndia.Russiahas the maximum CBM resource, estimated to be of the order of 80 trillion cubic meters!

 It is not possible to extract to entire methane available in the coal formations. This very similar to oil exploitation. Complete extraction means, a few more households can be supported and a few more industries can be supported! Off late scientists and technologists are trying to take duel advantage in extracting CBM through pumping CO₂ into the coal beds. This way large amount of methane can be extracted from coal beds and at the same time, CO₂ that is harmful to the environment, can be stored in these coal beds. The CO₂ pumped into the coal beds will flush the methane by squeezing the pores filled with methane.  As said above, the advantage of this method is while extracting methane, CO₂ can be sequestered  in to the coal beds. This is one way of protecting the atmosphere and control global warming.

Methane commonly occurs in coal as a) adsorbed gas in the micropores, b) adsorbed into the molecular structure of coal, c) as free gas in the voids, cleats and fractures and d) also as dissolved gas in groundwater occurring in the coal beds.  CO₂ and coal have strong affinity! CO₂ can enter the finest pores in coal and get firmly adsorbed. When adsorbed, it releases methane from the coal beds. Coal can adsorb more CO₂ than methane and CO₂ is preferentially adsorbed onto the coal structure than methane, generally in the ratio of 2:1. But this ratio varies depending on the rank of the coal i.e. on the organic matter and mineral content, i.e. vitrinite, of coal. Published results on the maceral-CO₂ adsorption content show that telecollinite, a type of maceral has a suitable structure to adsorb maximum amount of CO₂. Pressure also plays an important role in CO₂ adsorption in coal.  Experimental results reported in the literature on coal-methane-CO₂ adsorption properties show that the CO₂/methane ratio is more at 20 bars than at 27 bars.  Adsorption is more under supercritical condition ( about 100 bars and 30 °C).

 While adsorption is the main mechanism of  methane retention by coal,  both adsorption and absorption are the main mechanism of CO₂ retention by coal.  Perhaps absorption of  CO₂ into the organic structure makes the coal to swell like phlogopite mica thereby changing the structural properties of coal. CO₂ ( 3.87Ã…) has a smaller molecular diameter than methane (4.09Ã…). This mechanism squeezes the methane out of coal. Many adsorption models involving adsorption of methane and CO₂ on coal do not consider this aspect.

 Methane is the cause for underground coal fires. The coal seams keep burning for years and controlling such fires is still a technological challenge. But in one way such fires are blessing in disguise as they provide a continuous heat source that can be utilised beneficially.

 For example, several coal seams in Raniganj, Jharkhand and Biharare burning underground. Coal mine fires are due to primary combustion (of methane) when oxygen and water are introduced through cracks and unsealed shafts. These coal fires continue through several years.  Most underground coal fires exhibit smouldering combustion and may only involve relatively small amounts of coal capable of burning in the presence of small amount (2%) of oxygen.  To give an example of the magnitude of this hazard, in USAthere are nearly 600 coal mine fires burning over a period of 80 years. Other under ground coal mines that are burning is located in Russiaand in several east European countries.  These fires are located at shallow depth and the depth in many cased do not go beyond 400-500 m.  Till now this heat energy available is not put to use. Heat exchanger technology commonly used in geothermal power generation can easily be adopted in regions where under ground coal mine fire is common and perennial.  Continuous heat source from burning coal seams underground will provide continuous electric supply. This method controls underground coal fires, controls CO₂ emission and generate electric power to million rural homes. We have the know how and need will power to implement it!! 

Extracting CBM by CO₂ injection into coal formations has several advantages and has the best solution to mitigate global warming!!

While pumping CO₂ in to coal formations needs technological breakthrough, capturing CO₂ is also a  “technology intense” process. Thus Carbon capture and storage (CCS) has attracted world wide attention during the last few decades as a major option to reduce carbon dioxide emissions.  Research in this field is being carried out by several countries and the technology is at various levels of development and in some they are in demonstration phase.  During the 2008Hokkaido summit, the G8 countries endorsed the recommendations by the International Energy Agency (IEA) that large scale demonstration projects in widely varied industrial sector settings need to be carried out by 2010. The aim of this exercise is to understand the uncertainties related cost, reliability and technologies related CCS.  

 It takes millions of years for oil, gas, coal and other natural resources to accumulate/form. It is a natural geological process that no one can duplicate.  One may be able to do it at a laboratory scale. For example, we may be able to produce oil, gas or coal in a laboratory over a few tens of year. But is it possible to produce billion barrels of oils by such laboratory process and supply the world.  This is one of the items in a wish list of humans. The present situation demands that large volumes of CO₂ need to be pumped in to geological formations within a short period of time so that the Earth’s atmosphere can be saved and several species on earth can be protected.  One has to wait and observe how far this can be achieved by the current and future technologies!