Technology II - physical trapping: Sorptive storage of CO2 on residual coal and coal dust in abandoned mines

Objectives

Due to their high gas sorption capacity, coal and dispersed organic matter are considered as promising targets for subsurface storage of CO2. The planned studies will explore whether a combination of this storage process with the production of coal bed methane or coal mine methane can be expected to provide synergetic effects. Further, and in contrast to earlier studies, focus lies on the injection of flue gas instead of pure CO2 in order to reduce costs associated with separating CO2 from flue gas. Additionally, joint deposition will be investigated of waste coal dust or sludge and CO2 in abandoned mines adsorbed to these substances. This procedure may provide an innovative option for both the deposition of waste coal dust or sludge and a productive future use of abandoned mines.

Scientific and technical goals

Utilization of gob zones of abandoned coal mines for CO2 storage

The use of gob areas and formation damage zones in and around abandoned coal mines as an alternative to storing CO2 in coal seams will be studied. These damage zones offer various advantages: (i) they have high permeability; (ii) they provide access to large volumes of residual coal, dispersed sedimentary organic matter and mineral surfaces with potentially high CO2 sorption capacities; (iii) the geological situation is usually well known; (iv) parts of the existing mining infrastructure (pumps, pipelines, ventilation shafts) may be used for gas injection; (v) long-term gas (methane) monitoring and water management plans are operational in these areas for public safety reasons (little additional investment required for CO2 monitoring). The work will involve initial laboratory sorption experiments on coals, dispersed organic matter and bedrocks to assess fundamental data on CO2 storage capacities and kinetics. The main challenge will then consist in the integration of physico-chemical data with engineering and mining information to arrive at a reliable feasibility assessment.

Flue gas injection

As a specific feature and timely topic, this project will consider the injection of flue gas rather than pure CO2. This may have several advantages: (i) it avoids costs for separating CO2; (ii) it reduces safety risks due to relatively low CO2 contents and low injection pressures. The concept involves using large volumes of residual organic matter in subsurface mining dam-age zones as - geologic filters - to remove CO2 from flue gas. Research will focus on the com-position and sorption properties of flue gas, such as: preferential sorption of individual flue gas components; rates (kinetics) of sorption and desorption on coals, dispersed organic mat-ter and mineral components under different temperature and pressure conditions. Further, the thermodynamic properties (equations of state) and the aqueous solubility of flue gas will be studied in detail.

Combination of coal dust or sludge and CO2 deposition

The third innovative aspect to be studied in this project is the combination of the underground deposition of waste coal dust or sludge with CO2 disposal. Large amounts of coal sludge are being produced every year as a waste product during the processing of mined hard coal and lignite (brown coal). This material contains significant amounts of organic carbon (>60 % of dry mass), and therefore can be expected to have a high sorption capacity for CO2. Its small particle sizes correspond to higher sorption rates compared to that in compact coal seams. On the one hand, deposition of this coal dust or sludge in abandoned coal mines appears as an attractive way to dispose this waste material while on the other to create additional gas sorption capacity in the underground mines. A combination of these two aspects - disposal of waste coal dust or sludge and underground deposition of CO2 - generates positive economic and ecologic synergy. It is proposed to use coal dust or sludge to extract CO2 on site by adsorption from flue gas. This process might require a high partial pressure to avoid de-sorption during deposition. However, an appropriate method should be developed to increase the economical and ecological benefits of the process. In case of success, this new and low-cost CO2 extraction method could replace existing traditional separation techniques which are uneconomical and deteriorate power production efficiency by about 15 %.

Numerical simulations

One of our major goals is to be able to predict the processes of gas transport and sorption/desorption in the workings (shafts and passages), gob areas, and damage zones of abandoned coal mines. Numerical modelling of gas transport processes in underground mines will be used as an integrative tool to: (a) assess the usefulness of conventional mine ventilation software for modelling flue gas injection; (b) to simulate flue gas injection using the MUFTE_UG simulation software (Helmig 1997, Helmig et al. 1998).

The CO2TRAP project was successfully completed in May 2008 and initiated these follow-up projects:

CO2 Seals

Integrity of sealing rock formations for CO2 storage
http://www.co2seals.de

CO2SINUS

CO2 Storage in in situ Converted Coal Seams
http://www.co2sinus.org

ALCATRAP

Optimisation of CO2 binding by reaction with alkaline residual materials through the ALCATRAP process
http://www.hydro.uni-bayreuth.de/pros/detail.php?lang=de&id=32