Investigation of Enhanced Coal Bed Methane (ECBM) Processes

Gravimetric Adsorption Measurements under Realistic Conditions

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Abstract

Coal bed methane is a promising alternative to conventional gas. The ECBM technology combines improved coalbed methane recovery with underground storage of CO2. This process can be studied by gravimetric measurements of adsorption isotherms using the Rubotherm IsoSORP© systems with magnetic suspension balances. Accurate gas adsorption data are required for planning ECBM projects for enhanced coalbed methane recovery.

Index Terms - Natural Gas, ECBM, CO2, Coal Bed Methane, Gravimetric Measurement, Coealbed Methane Recovery

Introduction

As a result of increasing energy prices, the investigation of unconventional sources for oil and gas is of key economic importance. Large resources of natural gas are present as coal bed methane (CBM) in coal seams. Enhanced coal bed methane recovery/ coalbed methane extraction (ECBM) is a technique that has been developed to use the injection of carbon dioxide to improve methane recovery from coal seams [1].

Besides improved natural gas recovery, ECBM offers another advantage: CO2 from CCS (carbon capture and storage) processes is safely stored underground and is not emitted in the atmosphere [2].

However, the displacement of methane by CO2 is a very complex process: gases are not only adsorbed onto the coal surface but absorbed in the coal matrix as well, which causes the coal sample to swell. As a consequence, the development of the ECBM technology requires careful studies with individual coal samples under realistic conditions [3]. This application note explains the use of Rubotherm IsoSORP© systems to study the ECBM process by gravimetric measurements. [Measure CBM Recoverx/ Measure CBM Extraction]

Experimental

The Rubotherm IsoSORP© system uses the magnetic suspension balance (MSB) for the gravimetric determination of adsorption isotherms.tl_files/rubotherm/bilder/de-isosorp-series/Application Note 2/isoSORP MIX STATIC (SC-HP)_CD.JPG

A gas dosing system is used to supply pure or mixed gases at the required experimental conditions. Coal bed methane is present in coal seams at pressures between 30 and 300 bar and temperatures between 30 and 100°C.

The laboratory scale measurements have to cover these pressure and temperature ranges.  Creating a defined gas atmosphere with CO2 at elevated pressures is not a trivial task: CO2 needs to be compressed from cylinder pressure (60 bar) by means of a syringe pump [4] and the complete dosing system including all valves and tubings needs to be heated to avoid condensation.

Fig. 1 shows a schematic of the complete IsoSORP© system.

Results

Fig. 2 Absolute Adsorption of CH₄ and CO₂ on an Sardinian coal sample at 45°C and 60°CECBM studies were performed on coal samples from the Sulcis coal province in southern Sardinia. The Adsorption isotherms of pure CO2 at 45° and 60°C are shown in Fig. 2: Adsorption for CO2 is higher than for methane which is an important precondition for ECBM. [5]

The next step is to measure the adsorption of mixtures of CO2 and methane. In this case, the magnetic suspension balance is used to measure the total adsorption isotherm gravimetrically.

 

Based on these values, the adsorption data for the individual components can be gained by GC analysis of the remaining non-adsorbed mixture in the gas phase.

Samples for GC analysis can be taken after depressurization, the expanded the gas flow is directed through a 6-port gas sampling valve. Analysis by MS is another alternative.Fig. 3 Total and component-wise (selective) adsorption of CH4/ CO2 mixtures on Sadinian coal at 45°C

The data gained from these experiments (Fig. 3) show that more CO2 than methane is adsorbed in the coal even when CO2 is the minor component in the mixed gas phase [6]. This confirms that CH4 can be removed from coal seam deposits by CO2 injection.

tl_files/rubothFig. 4 IsoSORP SC Mix static system for high accuracy adsorption analysis of gas mixtureserm/bilder/de-isosorp-series/Application Note 2/isoSORP MIX STATIC (SC-HP)_CD.JPG

In order to generate gas mixtures with exactly defined composition, Rubotherm has developed MIX-modules as additional option: MIX instruments include storage tanks with calibrated volumes, a gas circulating pump and a gas sampling volume with sampling valve for analysis (Fig. 4) [7].

 

Conclusion

Coal bed methane (CBM) is a valuable future alternative to conventional natural gas. ECBM, Enhanced Coalbed Methane Recovery, the technology to use carbon dioxide injection for improved natural gas recovery, offers the additional advantage of long term CO2 storage.

It could be shown that Rubotherms IsoSORP© instruments can provide valuable data on gas storage capacity (absorption) and the dynamics of methane replacement by CO2 for the planning and design of ECBM projects.

Rubotherm Setup Reqired For This Measurement Of ECBM Recovery

IsoSORP© MSB system

High measuring load of up to 60 g

Fluid Density measurement

Pressure Range HP II up to 350 bar

Temperature Range ambient to 150°C

 

SC-HPII STATIC Dosing System

Heated to 100°C to avoid condensation

Teledyne ISCO Syringe Pump for CO2

Optional: MIX module

References

[1] R. Pini, D. Marx, L. Burlini, G. Storti, M. Mazzotti: Coal characterization for ECBM recovery: gas sorption under dry and humid conditions; Energy Procedia, Vol. 4 (2011) 2157-2161

[2] Ch. Garnier, G. Finqueneisel, T. Zimny, Z. Pokryszka, S. Lafortune, P.D.C. Défossez, E.C. Gaucher: Selection of Coals of different maturities for CO2 Storage by modelling of CO2 and CH4 adsorption isotherms; International Journal of Coal Geology, Vol. 87 (2011) 80-86

[3] J.S. Bae, S.K. Bhatia: High-Pressure Adsorption of Methane and Carbon Dioxide on Coal; Energy & Fuels, Vol. 20 (2006) 2599-2607

[4] Supercritical Fluid Applications in Manufacturing and Materials Production, Teledyne Isco, Syringe Pump Application Note AN1

[5] S. Ottiger, R. Pini, G. Storti, M. Mazzotti, R. Bencini, F. Quattrocchi, G. Sardu and G. Deriu: Adsorption of Pure Carbon Dioxide and Methane on Dry Coal from the Sulcis Coal Province (SW Sardinia, Italy); Environmental Progress, Vol. 25 (2006), 355-364

[6] S. Ottiger, R. Pini, G. Storti and M. Mazzotti: Competitive adsorption equilibria of CO2 and CH4 on a dry coal; Adsorption, Vol. 14 (2008)

[7] FlexiDOSE Series Gas & Vapor Dosing Systems, Rubotherm 2013

Authors

Frieder Dreisbach holds a doctoral degree in mechanical engineering thermodynamics and is managing director of Rubotherm GmbH, Bochum, Germany. Frieder.Dreisbach@rubotherm.de

Thomas Paschke holds a doctoral degree in analytical chemistry and is application specialist at Rubotherm GmbH, Bochum, Germany. Thomas.Paschke@rubotherm.de