CMM flaring

Introduction

Flaring is a controlled combustion process in which the combustible material to be flared is piped to a remote, usually elevated location, and burned in an open flame in the air. The specially designed burner tip and supporting equipment as auxiliary fuel feed system and steam or air mixing device makes nearly complete (>98%) destruction efficiency possible. The usual methane concentration of the utilised gas is 20% and above. The combustion process in a flare is controlled through the flame temperature, combustion zone residence time, the mixing turbulence of the gas stream components to complete the oxidation reaction and available oxygen for free radical formation.

CMM flaring can be performed in either open or closed systems, and the technique is similar to that implemented in the oil and gas industries. This method of methane disposal is relatively cheap compared to the extra costs connected with power generation infrastructure or incorporating recovered methane into a region’s natural gas pipeline network. However, the big disadvantage of this process is the heat energy which can not be recovered.

Open / Closed systems

As mentioned above flares can be closed or open, whereas a closed (thermal oxidizer flare) flare offers more benefits compared to an open (utility) one. The closed flare consists of a vertical, refractory-lined combustion chamber that effectively eliminates any visible flame. As the flame is enclosed, there is no thermal radiation from the flare at ground level, ensuring safety of work in the surrounding. Moreover, the closed design reduces noise associated with the flare. The burner is located at ground level and is designed to ensure the biggest destruction efficiency under maximum burner turndown. The combustion air enters the combustion chamber from below the burner by automatically controlled louvers or dampers.

The advantages of a closed flare are

• the not visible flame,
• the large volume flows to be dealt with,
• a better burner/flame control and
• a burning of all flammable gas components.

Unfavourable are the relatively bad portability and higher overall costs.

Components of Flaring Systems

A flaring system generally consists of the following major components and subsystems

• Collection piping within a unit (including a mix of pressure reliefs and vents)
• Components of a flare to the site
• A liquid separator to keep away water drops from the gas stream

In some degassing systems the CMM contains water drops. These may be caused by the water ring pumps or by the condensation of moister in the coal mine gas. This water should be separated in order to avoid damages in the hot parts of the flare and special burner parts.

• A flare stack with flare tip

The flare stack for burning CMM is mostly a closed system to guarantee a very good destruction of the methane. The enclosure covers the whole flame and is big enough to allow a complete combustion of the CMM.

• A fuel gas system incl. pilot burners with ignition

There is priming needed for start of the combustion in CMM flares. It is mostly a starting ignition plug with power delivered by the switchgear. In most cases CMM is used as supporting burner fuel, so that a special compression is not needed. Especially for CMM with very low calorific value a special supporting fuel is needed. If

the location is not connected to natural gas supply, propane or butane (or acetylene) in compressed gas cylinder can be used to have an ignition flame to start the main

burners.

• Controls and instrumentation

The switchgear contains the whole measuring and control technology to guarantee a save burning process. Especially the control of the flame and the flame temperature is very important and has to be done continuously. In a case, where flame dies down, by too high or too low flame temperature the feeding of fuel (CMM) has to be stopped immediately. Discharge of CMM into a hot combustion chamber without ignition may lead to gas explosions. Too high temperature may cause damages of the materials in the combustion room. The burning temperature should be between 800 °C and 1200 °C, whereby the materials and the isolation should be stable for this temperature level.

• Compressor to feed the flare (option)

Compact stations can flare the CMM independently from a central degassing station. A compressor is included in the system to extract the gas from underground and feed it to a burning system in a flare. In this case the switchgear and the measuring and control technology has to deal with both: the gas extraction and the gas burning.

The flare components have supporting role and are necessary or optional for the operation.

Advantages of flare plants

The advantages of flare plants have to be weighted against the main disadvantage of impossibility to use the thermal energy of the combustion:

• Flares are a low cost technology with a short manufacture and installation lead time.
• Flares are simple, low cost and reliable in the operation. Gas flaring offers a very high potential for significant GHG reduction by transferring the methane in CO₂.
• Flares require only low maintenance.
• Flare units can be installed, operated and maintained at a significant cost reduction on conventional utilisation systems.
• Flaring is the ideal option where time constraints, low cash flows and low electricity price might be significant barriers for power production.
• Flares can automatically be controlled.
• Distance (i.e. internet) remote monitoring and control is possible.

Examples of use

• 11.08.2009 a JI-Project in an active mine shaft in a mine in Saarland in Germany was started up by Evonik New Energies. The expected activity level amounts to 39,200 t CO₂eq during the period from 2009-2012
• In October 2007 the first flare plant was started up in Molodogvardeysk in the Ukraine. The flare was delivered by A-TEC Anlagentechnik GmbH with the capacity for destruction of over 2800 t methane per year
• In the mine 22 Kommunarskaya a flare starts up by ECO-Alliance. The expected activity level amounts to 40.000 MWh/a. Start operation Jan. 2009
• Rivne Landfill, proposed by the Renewable Energy Agency. The ambition of the reduction amounts to 20.000 mtCO₂eq per year
• Russia: North- Danilovsk oil field, Khanty- Mansiysk Okrug, the ambition of the reduction amounts to 814.860 mtCO₂eq during 2008-2012

Further information

Jonathan Kelafant, “Global CMM/CBM Development: Examples of Successful Projects” India Coal Mine/Coalbed Methane (CMM/CBM), Clearinghouse Kick-Off Event, November 17, 2008
Neil Butler, “Coal Mine Methane Drainage and Utilisation”, Methane to Markets,Beijing, 30th October 2007
ATEC, “Technical equipment for coal mine gas suction and utilisation, Mobile gas compressor and flare system (KGUU)”
Hofstetter “HOFGAS – CFM4c, The Coal Mine Methane Flare”
“Benefits of an Enclosed Gob Well Flare Design for Underground Coal Mines” EPA 430-R-99-012; August 1999
Robert E. Schwartz and Dr. Shin G. Kang, “Flare System Design – What is important?“