From pollutant to energy – methane as fuel

Materials World magazine
,
1 Jul 2008

Michael Forrest talks to Dr Cameron Davies, Chief Executive of Alkane Energy plc, Nottingham, UK, about the company’s work to recover methane.

Energy is key to economic development. Although nuclear energy accounts globally for 16% of electrical power, fossil fuels are the primary energy source, and are likely to remain so for the medium term. The result is increased output of carbon dioxide (CO2) and a consequent growth in the rate of global warming.

Other greenhouse gas (GHG) production is also rising in line with economic growth, and after CO2, the second most damaging, in terms of climate change, is methane (CH4). It has a global warming potential 23 times more effective (for a given weight, averaged over 100 years) than CO2 and now accounts for 16% of all GHG production. Of this, over half originates in the natural decomposition of organic material, including 30% from enteric fermentation. In the extractive industries, oil and gas account for 16% of global CH4 production, while coal mining accounts for six per cent.

Coal mine methane (CMM) results from the geological process that metamorphoses organic matter into coal. Along with other mine gases, they are collectively known as firedamp and their explosive nature has claimed many lives.

The largest CH4 emissions occur where there is the most underground coal mining. China takes the lead (34%), followed by the USA (13%), with Ukraine and Russia producing seven per cent each. In 2005, worldwide CMM emissions were estimated at nearly 400Mt of CO2 equivalent, or about 30 billion cubic meters (BCM). By 2020, the world’s coal mines are expected to produce annual emissions of 450Mt, or about 40BCM.

Fuelling change

Methane is also a valuable fuel, and compared with other hydrocarbons it produces less CO2 per unit of energy generated. Although it is not a particularly energy-rich hydrocarbon, at 891kJ/mol, its low mass compensates by producing more energy per equivalent mass. These factors combine to make capturing CH4 and using it as fuel a positive step in reducing its effect on climate change.

The UK Department for Environment, Food and Rural Affairs has commissioned research into current and future coal mine gas emissions until 2050. Its consultants concluded that the CH4 emitted each year in the UK from abandoned coal mines is equivalent to about 1.4Mt of CO2 and that emissions of this gas will continue at significant levels beyond 2050. These estimates are likely to be included in the revised Kyoto baseline, and will ultimately be traded within the Europe Union Emissions Trading Scheme.

One UK company that has exploited this is Alkane Energy plc, a Nottinghamshire-based company that was AIM-listed in 2000. The company has developed technology to recover CH4 from abandoned coal mines and has installed it on old mine sites as standalone, containerised (mounted in a standard 20ft or 40ft ISO shipping container) power-generation plants.

Methane recovery

Mine shafts and inclined drifts into closed/abandoned coal mines are sealed and, for safety reasons, fitted with vent pipes to prevent CH4 build-up underground and escape at locations remote from the mine site. Driveways and faces tend to collapse over time and gas pressure increases. With no ventilation, CH4 concentrations can reach 75%. The remainder of the gas mixture is made up of CO2 and nitrogen (N). Early pilot projects extracted CH4 from the shaft top vents but when a vacuum was applied using compressors, the CH4 percentage dropped as air was drawn in.

In order to avoid this problem and keep the energy content high, recent projects have avoided the shaft tops and drilled boreholes directly into the mine workings. These boreholes are lined with steel tubing to the top of the coal seam and with slotted liners for the final section, and are tested for CH4 flows using Alkane’s own design of transportable containerised gas extraction equipment. Estimates of the potential power generation capacity are made from the measured gas flow rates and CH4 concentrations.

Some mines are under positive pressure due to rising mine water or adjacent gassy sandstone reservoirs, but usually the gas must be extracted with compressors. Typical flow rates are up to 3,000m3/hour at CH4 concentrations up to 75%. The gas is pumped through a cyclone and a series of safety valves, including a slam shut valve with a failsafe closed position to clean the gas and isolate the mine in case all power fails.

Alkane’s mine gas production plants are containerised to allow them to be expanded or moved to a new site depending on the CH4 flow rate. The plants are fully automated and can be monitored and operated through the Internet from a 24-hour control centre at Markham, UK, or at any remote location by Alkane’s technicians via a laptop and a web link.

The company and a software partner have developed computerised controls that allow stop/start, remote maintenance operations and viewing of copies of the onsite plant control computer screens. These show gas pressures, engine and pumping systems’ temperatures, gas flow rates and concentration, and the pressure in the mine. Alarms are set for a range of safety and engine health parameters, and the system automatically alerts the duty technician to breakdowns or potential problems.

Typically CMM contains 60-75% CH4, the remainder being CO2 and nitrogen. This gas is not suitable for compression or direct injection into the UK natural gas grid as its calorific value is below pipeline specification. Its primary use, therefore, is as a fuel source for gas engines and to generate renewable electricity. Alkane’s sister company in Germany, Pro2 Anlagentechnik, containerises Deutz and MAN gas engines as transportable units, and Alkane installs these as small-scale power stations on the abandoned mine sites.

Firmly planted

Alkane has seven generating plants located in the East Midlands and South Yorkshire coalfields and one in Germany. The East Midlands coals are high-volatile bituminous and strongly gas-producing with an in situ gas content of between four and six cubic metres per tonne of coal. They were mainly worked as underground mines and following the retrenchment of the industry in the 1980s and 1990s now represent large strategic gas reserves located in several hundred abandoned mines.

The most recent plant, completed in May 2008, has an electrical generation capacity of 4.05MW, consisting of three 1.35MW Pro2 Anlagentechnik containerised generation systems supplying electricity to nearby businesses at 11kV via the local distribution grid. In addition, a 150m pipeline supplies one megawatt of CH4 to a local textile factory where it has replaced natural gas in a large Hamworthy boiler.

Also in 2008, the capacity of the Warsop, UK, plant was doubled with the installation of a second 1.35MW generator. Its electricity is contracted at £51/MWh until September 2009. This compares with an average price of £42/MWh achieved in 2007. In recent weeks the forward price of electricity has risen even further with medium term contracts now being signed at over £65/MWh.

Recent additions to the Alkane’s generating capacity will bring the company’s total energy capacity to 133mkWh, a significant increase over the 80mkWh produced in 2007. Alkane’s coal mine CH4 capture plants eliminate the equivalent of about 600,000t of CO2 per annum.

Although the use of coal mine CH4 as a fuel for energy generation produces CO2 and water via the exhausts, the process saves the equivalent of 18.5 times more CO2 than letting the CH4 escape to the atmosphere. Dr Cameron Davies, Chief Executive of Alkane, says ‘the conversion of methane from abandoned and operating coal mines to eco-friendly energy is at the forefront of technologies that are reducing greenhouse gas emissions and slowing the impact of climate change’.