Prototyping the use of low calorific coal mine methane (CMM)

For utilization of coal mine methane (CMM) below 18% by volume methane concentration there are currently no economically viable technologies available. In order to avoid the emission of considerable amounts of greenhouse gas in the long term, the “LowCH4-MDT-Module“ project aims to develop, build and demonstrate a prototype of a modular plant with a lean gas burner, steam boiler and micro steam turbine. This prototype should enable a reliable utilization of CMM between five and 18 vol. % CH₄ and thus further avoid CH₄ emissions of today's approx. 270 million Nm³/a (corresponding to 3.6 million t CO₂ eq/a) for at least another decade.

For this purpose, the specification of a corresponding plant is derived by analyzing site data of CMM utilizations in the Ruhr Area. Based on this, the modular prototype will be designed, developed, planned, built and commissioned, optimized and put into trial operation at a CMM site of Stadtwerke Herne AG. Based on the technical and economic results, the potential of further use at CMM sites in the Ruhr Area will be evaluated.

Energy Recovery of CMM from abandoned coal mines using gas Otto engines today gives a significant contribution to the reduction of CH₄- or CO₂-eq emissions by avoiding methane emissions (GWP Global Warming Potential of factor 25 CO₂-eq for CH₄) and also significantly helps hazard control from diffuse surface CMM leaks.

This type of mine gas utilization boomed with including CMM as supported energy in Germany´s Renewable Energy Sources Act in 2000. Over the years, the CH₄ concentration in CMM in Ruhr Area decreased considerably from the original level of about 40 to 70 % CH₄ by volume. However, the usual utilization of CMM in gas engines requires minimum CH₄ concentration of approx. 18 vol. %, which is increasingly being undercut at CMM utilization sites. This project shall contribute to the development of a technical and economical alternative for a follow-up utilization at CMM concentrations below 18 vol. CH₄. Interested operators of CMM plants with deep decreased CH₄ concentration then can fall back on a new proven technology LowCH4-MDT module without much loss in time.

The role of micro steam turbines

Steam turbine plants have a lower power generation capacity compared to gas piston engines due to the lower efficiency of steam power plants compared to engines. However, the reduction in GHG emissions by further use of CMM at the majority of sites and the additional contribution to hazard prevention are retained for a longer period of time with this alternative technology approach. For economic reasons, the incentive to supply waste heat to local heat consumers near the CMM utilization from the steam power cycle at site increases.

If it will be possible to develop and establish an alternative technological energy recovery solution following the use in gas engines at those sites where the captured methane concentration falls below 18% by volume, the economic utilization of CMM can be prolonged in the longer term. In this way, the valuable contributions to avoiding anthropogenic methane emissions and averting the dangers of diffuse CMM leaks at the surface are preserved.

Development potential of a technologically redesigned mine gas utilization

In the project, the lean gas burners shall be designed in such a way that they can be operated with the widest possible range of low methane contents from five to 18% by volume. For CMM high and sometimes varying proportions of CO₂, N₂ and water vapor must also be taken into account.

Small steam boilers with low-gas burners in form of modular, displaceable units are required. The ability of CMM utilization modules to be moved to another site for adaptation to changing site parameters is of great economic importance in CMM utilization.

For this application, novel micro steam turbines from 150 to 300 kW_el are to be designed, constructed and tested.

For the future use of the new plant configuration at other sites, forecasting tools for future CMM composition and amount are being developed and an economic evaluation of the prototype plant operation is given.

• DMT GmbH & Co. KG (funded)
• Stadtwerke Herne AG (associated)