United States Air and Radiation
Environmental Protection (6202-J)
Agency
430-R-99-012
August 1999
Conceptual Design for a
Coal Mine Gob Well Flare
\ETHAN
OUTREACH
9 R O C R A M
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Gob Well Flaring: Design and Impact
Daniel J. Brunner, Resource Enterprises
Karl Schultz, U.S. Environmental Protection Agency
(Note: This paper initially was presented at the International Coalbed Methane Symposium, held
in Tuscaloosa, Alabama, May 3-7, 1999, and then at the Mine Ventilation Symposium at the
University of Missouri-Rolla, June 7-10, 1999. Subsequently, it was published in its current form
in the September 1999 issue of CBM Review (World Coal, Palladian Publications, Ltd, UK.)
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Gob Well Flaring: Design and Impact
Currently, over 30 U.S. coal mining operations employ a system of degasification to assist in reducing
the emission of methane into their mine ventilation systems. All of these mines use vertical gob wells.
This is an effective gob degasification technique for U.S. longwall coal mining operations, particularly
when prime movers apply suction to the wellheads (active gas extraction).
Under ideal conditions, operators collect gob gas (methane in air mixture) directly at the wellhead for
sale or on-site use. However, because of vertical gob well gas production characteristics (variable gas
quality and quantity), difficulties in coordinating commercial gas recovery with underground mine
degasification requirements, and because of the economics of commercializing gob gas, coal mine
operators commonly vent gas from gob wells to the atmosphere. This practice raises safety and global
environmental concerns, and wastes a potential resource.
This article presents a safe and controlled system of gob well flaring that would provide substantial
global environmental benefits. It presents a conceptual design of a gob well flare that incorporates safety
features and operating practices based on petroleum industry standards. It summarizes the safety
benefits, the global environmental benefits, and the potential financial benefits to mine operators of
application of this system in the U.S. In conclusion, it presents an actual application of gob well flaring at
a mining operation in Australia.
INTRODUCTION
In the U.S., and throughout the world, a growing number of companies are looking at low cost or
profitable means of lowering or offsetting their greenhouse gas emissions. Since the United States
signed and ratified the Earth Summit Treaty in Rio de Janeiro, Brazil, the Climate Change Action Plan
was developed to provide partnerships between industry and the government to identify and realize
economically viable measures to reduce greenhouse gas emissions. The U.S. Environmental Protection
Agency's (U.S. EPA) Coalbed Methane Outreach Program is one of these programs, and has focussed
its efforts on identifying and working with the coal industry to develop profitable projects to use coal mine
methane, a potent greenhouse gas, rather than venting it to the atmosphere and contributing to global
climate change. However, without external incentives, it is not always economic to employ all of the gas
conning from degasification systems, in particular the gob gas of compromised quality and significant flow
fluctuations. In these cases, companies interested in realizing significant reductions in greenhouse gas
emissions could benefit from a low capital expenditure technology to combust this methane.
The U.S. EPA commissioned a conceptual design of a single gob well flare to constructively engage
labor, industry and regulatory entities on the safety, technical, and cost aspects of constructing and
operating a flare at an active gob well. Additionally, the U.S. EPA has been corresponding with technical
experts in Australia regarding their experience with flaring, and the benefits that Australian coal mines
have realized through their recent activities. Now the U.S. EPA seeks to help develop, in partnership with
industry and labor, a demonstration facility in the U.S.
BENEFITS OF METHANE FLARING
Gas flaring is a standard safety practice in many industries. For example, methane and other
associated gasses are routinely flared during processing and production of oil and gas, and are
continuously flared from landfill collection systems. The petroleum industry flares for safety reasons
during system upsets when high concentrations and volumes are released in the vicinity of potential
sources of ignition. In the landfill industry, methane contributes to approximately 50 percent of the gas
recovered. Flaring is conducted to combust it and other associated toxins (hydrogen sulfide and non-
methane organic compounds) which are ground-level ozone build-up gases. Unlike landfills, coal mine
gob gas consists of a methane mixture in air and does not contain many toxins.
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Mine Benefits
Incorporating a controlled flaring system at gob wells would minimize the potential of an unconfined
deflagration occurring on surface at well discharge locations brought about by natural or man-made
sources. An unconfined deflagration, under appropriate conditions, may lead to a confined deflagration or
detonation which may propagate in processing pipeline, and potentially through gob wells to the mining
horizon. Incorporating a controlled flaring system would minimize this risk to the underground mine and
to the public.
Continuous monitoring provisions, necessary with a gob well flare, would provide uninterrupted
records of gob well performance. These would be invaluable in comparing gob well production with
underground conditions, investigation of mine incidents such as mine fan failures, changes to the
ventilation system, or accidents. Currently most active gob wellhead installations in the U.S. do not use
continuous monitoring equipment.
Controlled flaring would reduce background concentrations of methane and allow mine operators to
locate mine intakes closer to production areas.
Environmental Benefits
As the global warming potential of methane is approximately 21 times that of CO2 (over a 100-year
time frame) (IPCC, 1996), combusting the methane released from vertical gob wells with a controlled
flaring system would result in emission of a significantly less harmful gas. Assuming stochiometric
combustion of methane in air, combusting methane through flaring releases 7.5 times less greenhouse
gas than venting.
Methane also contributes to tropospheric ozone problems and harms vegetation at high
concentrations. Flaring coal mine methane may also alleviate local air quality problems.
PROPOSED GOB WELL FLARE
A controlled flare system is proposed with flare flash-back prevention features, a prime gas mover, an
elevated stack, a controlled pilot, and a continuous monitoring system. A concept design, suitable for
demonstration to a single, actively extracted gob well is presented. The concept design is also suitable,
with some modification, for connection to a multiple'gob well gathering system.
Flare Design Parameters
The gob well flare concept design was derived for typical gob well performance characteristics.
Methane Concentration. The flare was designed for combustion of methane concentrations (in air)
ranging from greater than 30 percent to 100 percent by volume.
Gas Flow Rate. The flare design was developed to accommodate a variable range of gas flows (methane
and air mixture). Standard gas flows ranging from 0.007 m3/s to 0.661 m3/s (14 to 1400 scfm (20 mscfd
to 2 mmscfd)) were specified.
Gas Heating Values. Flare performance was specified for gas heating values ranging from 11.17 MJ/m3
to 37.3 MJ/m3 (300 Btu/scf to 1000 Btu/scf).
Codes and Guidelines
Applicable codes and guidelines for utility, landfill, and flares used in the petrochemical industry were
incorporated in the gob well flare concept design.
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40 CFR 60.18 General Control Device Requirements. These are control requirements to achieve EPA
air emission standards and specify:
• no visible emissions (except for 5 minutes every 2 hours);
• flame presence at all times when emissions are vented;
• minimum gas quality (7.5 MJ/m3 (200 Btu/scf) - unassisted flare);
• maximum gas exit velocity as a function of flare type and gas quality (18.3 m/s (60 fps)
unassisted, variable quality);
• flares must be monitored for design conformance;
• the pilot flame must be continuously monitored.
Industry Handbooks
Applicable guidelines were obtained from flare gas systems handbooks.
Flare Height. The height of the flare is based on ground level limitations of thermal radiation intensity.
These are determined from maximum gas flows and heating values, including wind factors.
Recommended limiting radiation intensities are:
1.4 kW/m2 (440 Btu/hr-ft2) for unlimited time exposure by personnel;
9.5 kW/m2 (3000 Btu/hr-ft2) maximum at base of flare;
4.7 kW/m2 (1500 Btu/hr-ft2) minimum fenced boundary limit;
2.4 kW/m2 (750 Btu/hr-ft2) maximum at property lines
4.7 kW/m2 (1500 Btu/hr-ft2) at digital equipment and controls.
Noise. Noise emissions result from combustion of the turbulent gas stream. The emitted decibel level is
proportional to the second power of the quantity of the hydrocarbon burned. In populated areas, a closed
flare system may be necessary to reduce noise emissions.
Luminance. Sufficiently mixed air and fuel gases will burn with a blue non-luminous flame. If insufficient
mixing occurs, the flame will become luminous. Where luminance is a concern, an assisted, or closed
flare system is recommended.
Recommended Petroleum Industry Flaring Practices
The petroleum industry provides guidelines for flare flash-back protection. Flare flash back protection
is achieved by either (1) ensuring a minimum purge gas flow at all times out the stack, or (2) incorporating
a passive protective system which mitigates air inflow into the top of the stack. These measures are
recommended in addition to a liquid seal which effectively arrests flame and detonation propagation
upstream of the flare stack.
Purge Gas Requirement. A purge gas flow prevents air from entering back down into the stack due to
wind or thermal effects and potentially creating an explosive mixture.
Gas Seals. Gas seals, commonly denoted as fluidic, or diode seals, are recommended to reduce the
purge gas volume flow requirements. These seals are typically comprised of stacked conical orifices
installed inside the flare stack below the burner tip which impede vortex back-flow generated by wind or
thermal effects.
Liquid Seal. With a liquid seal, the gas process stream is introduced via a header into a vessel typically
containing an ethylene glycol - water mixture and discharged through a submersed perforated diffuser.
With this system, the gas is released as a series of distinct bubbles with liquid intervals between them
which ensures mitigation of flame propagation through the seal. Standards recommend a minimum liquid
head of 0.15 m (0.5 ft) above the diffuser outlet. A maximum of .30 m (1 ft) is recommended as gas
pulsation occurs at higher liquid levels. The total volume of fluid in the vessel must also be equivalent to a
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minimum of 3.05 m (10 ft) of the gas inlet line. Should a detonation occur in the stack, the detonation
would displace a liquid volume into the inlet header, and provide a minimum ethylene glycol -water seal
of 3.05 m (10 ft) in the line separating the flare from the rest of the system.
PROPOSED GOB WELL FLARE DESIGN
Figure 1 illustrates the gob well flare concept design. The design incorporates the initial gas
processing equipment which is typically in place at an actively extracted gob well, a by-pass gas venting
system, the flare with flare flash-back prevention features, and the monitoring and control system. The
estimated capital cost of the proposed flare design is US $60,000.
Characteristics
Active Flare. The design incorporates a mechanical blower/exhauster, as is typically fitted to an equipped
gob wellhead assembly, to maintain a positive gas pressure through the flare system.
Open Flare. The design employs an open flare, where gas is burned at the tip of an elevated stack at
combustion efficiencies of 98 percent, rather than an enclosed ground-level flare. Enclosed ground-level
flares are applied at some landfills and burn low quality gas more efficiently and emit less NOX (suitable
for use in EPA designated "ozone non-attainment areas"), but have higher capital and operating
requirements.
Unassisted Flare. Because of the lower heat content methane and air mixture extracted from a typical
gob well, an unassisted flare with continuous burning pilot would readily combust the gob gas without
producing significant visible smoke.
Flare Tip Diameter. The concept design recommends a minimum flare tip diameter of 24 mm
(approximately 8 inches) based on the expected gas flow range and the requirements of 40 CFR 60.18.
Flare Height. Based on a 4.7 kW/m2 (1500 Btu/hr-ft2) criteria at the base of the stack, the concept design
specifies a 6.1 m (20 ft) overall stack height. The heat distribution profile at grade, based on worst case
wind conditions, will establish the equipment (and well-head) to flare spacing.
Pilot System. The design incorporates a continuously monitored and operating pilot.
Safety Features
Isolation of Potential Sources of Ignition. The blower/exhauster and the by-pass vent are two potential
sources of ignition within the flare system. As indicated on Figure 1, an in-line detonation arrester
isolates the blower/exhauster from the gob well. This arrester stops low speed confined deflagrations and
high speed and high pressure flame fronts (sonic detonation and overdriven detonations) travelling in
either direction. This unit is manufactured of spiral wound crimped metal which provides flame quenching
elements of appropriate lengths and materials to adequately absorb or dissipate heat and retard and
quench propagating flame. Anticipated pressure losses for a 0.254 m (10 inch) diameter unit are 2.4 kPa
(0.35 psi) for the largest flow specified for the flare system design.
An end-of line flame arrester is fitted on the vent by-pass discharge stack. This arrester incorporates
a crimped stainless steel foil element to prevent flash back from unconfined deflagrations.
The flame arresters and their arrangement are typical of gob well installations equipped with blower-
exhausting equipment.
Isolation of Potential Ignition from Flare. The proposed design mitigates the potential of flashback from
the flare by incorporating (1) an active positive pressure system, (2) an API recommended fluidic seal, (3)
an API recommended liquid seal, and (4) a monitoring and control system with valve and equipment
activation capability.
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The blower/exhauster is the prime mover of the gas through the flare system and maintains a positive
pressure to the liquid seal at the base of the flare. The liquid seal acts as a damper maintaining constant
back pressure on the system. A pressure sensor between the blower/exhauster and the liquid seal
continuously monitors for positive pressure to detect blower/exhauster operation.
The flare stack is equipped with a fluidic seal which prevents inflow of air into the stack with gas flows
as low as 4.0 x 10"4 m3/s (0.75 scfm or 1.08 mscfd), well below the minimum design flow of 0.007 m3/s (14
scfm or 20 mscfd). As indicated under Monitoring and Control, the system measures gas quality and flow
rate and activates an alarm should flows drop below 0.009 m3/s (20 scfm or 28.8 mscfd), and valves for
by-pass mode should gas flows reach 0.007 m3/s (14 scfm or 20 mscfd).
The flare stack incorporates an API recommended liquid seal at the stack base to stop a confined
deflagration and/or a detonation from propagating upstream. Gas is bubbled through a perforated
diffuser maintained at least 0.15 m (0.5 ft) below a water-ethylene glycol seal. A head tank provides a
positive pressure supply of the water-ethylene mixture for the liquid seal. The flare's control system
activates the inlet valve (V5) based on the indication of the water level sensor. A discharge valve is
provided for manual activation (V6) should visual inspection detect excessive liquid levels.
Isolation from Natural and Man-made Sources of Ignition. The design proposes that the flaring facility be
protected from vandalism and unauthorized entry with a 2.4 m (8 ft) high perimeter fence and
appropriately protected from lightning by elevated perimeter static wires.
Monitoring and Control System
The concept design incorporates a continuous monitoring system with active control capability.
Table 1 illustrates proposed sensor set points and system actions during normal flare operations.
Sensor
Gas Quality
Static Pressure
Gas Flow
Liquid Level in
Seal
Flame
lonization
Settings
@ 30% Methane in Air
@ 25% Methane in Air
Max @ 100% Methane in Air
Min @ 250 Pa (1.0 in. w.g.)
Normal >1500 Pa ( 6.3 in. w.g.)
Max @ 3250 Pa (13 in. w.g.)
Min @ .007 m3/s (14 scfm)
Normal > .009 m3/s (20 scfm)
Max@ .66 m3/s (1400 scfm)
Min @ .15 m (6 in.) >
Discharge
Normal .1 5-.2S m (6 - 9 in)
Max@ .305m (12 in) Above
Pilot Flame not Detected
Pilot Flame Detected
System Action
Actuate By-Pass Mode, Alarm
De-Energize Blower/Exhauster
None
Activate By-Pass Mode, Alarm
Alarm if Below
Activate By-Pass Mode, Alarm
Activate By-Pass Mode, Alarm
Alarm if Below
Activate By-Pass Mode, Alarm
Activate Supply Valve
None
Activate By-Pass Mode, Alarm
Ignite Pilot
None
Table 1. Set points and system actions during normal flaring operations.
Sensors. Transmitting sensors monitor gas quality, static pressure, temperature and flow rate of the
process stream, in addition to pilot operation. Analog output from the sensors are routed to a data logger
with programmable activation and data recording features.
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Control. Power is supplied to the blower/exhauster, ail solenoid valves, and the pilot ignition system. At
programmed sensor conditions, the data logger activates relays as appropriate. The design incorporates
a cellular modem which provides for retrieval of performance data from any computer site.
Fail Safe Valves. The system design incorporates three principal compressed air activated fail safety
valves (V2 through V4 as shown on Figure 1). Compressed air at 550 kPa (80 psi) is supplied by small
diameter lines connected to a storage tank and integrated compressor. Manual and data logger activated
solenoid valves are connected to the compressed air lines at the Valve Controls (Figure 1) to either bleed
or provide positive air pressure to the actuators.
Control Solenoid Valves. Two additional solenoid valves are incorporated to activate the fuel gas supply
(V7) and maintain fluid level control in the liquid seal at the base of the flare (V5).
Manual Operation Provisions. The system employs manual over-ride provisions, including sight
monitoring gauges for pressure, gas flow and gas quality. Of particular concern during start-up of the
system is ensuring sufficient gas flow through the stack prior to ignition of the pilot. Although this can be
done automatically, the design recommends manual system re-activation when switching from by-pass to
flare, and when initiating from the shut-in position.
EVALUATION OF POTENTIAL ENVIRONMENTAL AND ECONOMIC BENEFITS OF FLARING
Methane is a "greenhouse gas," meaning that its presence in the atmosphere affects the earth's
temperature and climate system. Methane's chemically active properties have indirect impacts on global
warming as the gas enters into chemical reactions in the atmosphere that not only affect the period of
time methane stays in the atmosphere (i.e., its lifetime), but that also play a role in determining the
atmospheric concentrations of tropospheric ozone and stratospheric water vapor, both of which are also
greenhouse gases. These indirect and direct effects make methane a large contributor, second only to
carbon dioxide, to potential future warming of the earth. Over a 100-year period it is 21 times more
effective at trapping heat in the atmosphere than carbon dioxide. In 1996 178 Mt (196 mm tons) of carbon
equivalent emissions (using a 100 year global warming potential) came from anthropogenic methane
sources in the U.S., or over ten percent of total greenhouse gas emissions. Put in perspective, emissions
of carbon dioxide attributed to the entire U.S. industrial sector totaled 466 Mt (514 mm tons).
In the United States, methane emissions from coal mines are the fourth largest anthropogenic source,
after landfills, agricultural activities, and fugitive emissions from natural gas lines. EPA estimates that
emissions in 1996 equaled 18.9 Mt (21 mm tons). Because coal mine methane is a valuable energy
resource, its economic use should always be the first option to consider. However, in some instances this
gas is either of too low a quality, or is too far from a market to make this choice viable. In these
instances, flaring can be an attractive means of reducing greenhouse gas emissions at low cost. A single
gob well flare, as described in this paper, allows for additional flexibility in destroying the particular gas
sources that do not have a market. EPA estimates that of the 1.6 Gm (57 bcf) of methane drained from
U.S. mines in 1997, 1.2 Gm3 (42 bcf) was used, leaving 425 Mm3 (15 bcf) of vented gas (U.S. EPA,
1998). Assuming that 50 percent of this vented gas could not be viably employed, 121 M m3(7.5 bcf) of
methane, or nearly 1 Mt (1.1 mm tons) of carbon equivalent emissions could be handled with flares.
The Value of Flaring to Coal Mine Operators
Flaring is a very cost effective means of reducing methane emissions. The expected average cost to
reduce greenhouse gas emissions has become the subject of much debate. There is significant
disagreement between economists on this question, with average cost estimates ranging from dollars per
ton of carbon equivalent reductions to well over $180/t ($163/ton) (Yeller, 1998, and Mining Week, 1998).
Translated into methane volumes, $10/t ($9.07/ton) of carbon equivalent would equal $38.85 per 1000 m3
($1.10/mcf); $50/t ($45.36/ton) carbon would equal $194.23 per 1000 m3 ($5.50/mcf) in the potential
value of the emissions offsets. Brokers cite current trades of carbon emissions at around $5.50/t
($4.99/ton).
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Economic Analyses
Preliminary after-tax economic analyses, performed for a range of average methane flow rates from
a typical vertical gob well, determine the carbon equivalent break-even cost of flaring. These analyses
assume:
• A installed cost of U.S. $85,000 which includes project development, installation and permitting;
• An operating cost of U.S. $17,000 per year which includes monitoring, maintenance, and
relocation (assume once every two years);
• A twelve (12) year project life;
• Cost of capital of 15 percent;
• Inflation at 3 percent per year;
• Escalation in value of carbon of 6 percent per year (Natsource, 1999);
• An average annual tax liability of 40 percent.
Figure 2 presents the estimated discounted cost of gob well flaring on a value of carbon equivalent basis
for average gob well methane flows of between 2,830 and 16,990 m3 per day (100 and 600 mscfd)
(averaged over one year). The figure illustrates that discounted costs are well below $10/t ($9.07/ton) of
carbon equivalent for the average range of methane flows considered. The figure shows that flaring is
economic for current carbon values of $5/t ($4.54/ton) with average daily methane flows of greater than
5,663 m3 per day (200 mscfd). Estimated internal rates of return for a range of carbon values ($1.83/t to
$7.33/t) and range of average methane flow rates are presented on Figure 3. The figure demonstrates
that the return on investment may be significant for the values of carbon offsets discussed herein.
$1.00
$0.00
2000 4000 6000 8000 10000 12000 14000 16000 18000
Average Methane Flared (Cubic Meters per Day)
Figure 2: Discounted Break-even Cost of Flaring.
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140%
2000 4000 6000 8000 10000 12000 14000
Average Methane Flared (Cubic Meters per Day)
16000
18000
Figure 3: Internal Rate of Return for Flaring for a Range of Carbon Values.
GOB WELL FLARING EXPERIENCE TO DATE
Capricorn Coal Development Joint Venture (Capricorn) commissioned a gob well flare similar to the
design presented herein at the Central Colliery in Queensland, Australia. Capricorn submitted a detailed
design of the flare to Australian mine safety authorities for comments which were addressed by Capricorn
to the satisfaction of the authorities. Capricorn constructed the flare and it became operational in
December of 1998. The flare combusts methane from a number of vertical gob wells and is rated for
102,000 m3 per day (3.6 mmscfd) of gob gas. Presently it combusts an average 90 percent methane and
air mixture (by volume), and is reducing carbon emissions by more than 10,000 t (9,078 tons) per year
(Shell Coal, 1999). As shown on Figure 4, the flare is 20 meters tall (65.6 ft). It implements a flame
arrester below the flare tip for flare flash-back protection, as opposed to the liquid seal proposed in the
concept design. It is continuously monitored, and is equipped with fail-safe controls that by-pass the flare
during low gob gas flows, and alarm if high static pressures are monitored upstream of the flare. As of
the date of this article, the flare has successfully operated as designed.
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Figure 4: Gob Well Flare Commissioned by Capricorn at Central Colliery in the Bowen Basin.
CONCLUSION
While installation of the proposed flare design may bring significant economic, safety, and operational
benefits to a coal operator, to develop a flare in the United States, any system will require the approval of
the Mine Safety and Health Administration (MSHA). As such, the U.S. EPA has presented the gob well
concept design to MSHA who indicated that a pilot flaring project would need to be approved at the
district level upon request by a mine operator. The U.S. EPA is interested in partnering with mine safety
authorities, coal operators, and labor to ensure that all real and perceived safety concerns are addressed
in a demonstration project.
REFERENCES
Intergovernmental Panel on Climate Change (IPCC), 1996, Radiative Forcing of Climate Change, The
1996 Report of the Scientific Assessment Working Group of IPCC, Summary for Policymakers.
Mining Week, 1998, "GCC criticizes Administration analysis of climate treaty's impacts", August 10,
1998p.3.
Natsource, Personal Communication.
Shell Coal, 1999, "Capricorn Coal meets greenhouse gas challenge", "in site" publication by Shell Coal,
1999.
U.S. EPA, 1998, Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1996± Washington, DC
Office of Policy, Planning and Evaluation, U.S. Environmental Protection Agency.
Yeller, J., 1998, The Kyoto Protocol and the President's Policies to Address Climate Change:
Administration Economic Analysis
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E-TO-vtUHEAD EQUIPMENT SPACING - *3'
DETONATION AMESTER
SINO.E STAGE tCPADATCR
r TAB. CLOSED VALVE vz
Figure 1: Conceptual Design of the Proposed Gob Well Flare.
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Conceptual Design for a
Coal Mine Gob Well Flare
Coalbed Methane Outreach Program
Atmospheric Pollution Prevention Division
U.S. Environmental Protection Agency
August 1999"
*This document was originally prepared in July 1995; its first printing was in August 1999.
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Table of Contents
Page No.
1.0 INTRODUCTION 1
1.1 Benefits of Methane Flaring 1
1.1.1 Mine Benefits 2
1.1.2 Environmental Benefits 2
1.2 Mine Safety and Health Administration (MSHA) Response Request 2
2.0 PROPOSED FLARE DESIGN 3
2.1 Design Parameters 3
2.1.1 Methane Concentration 2
2.1.2 Gas Flow Rate 2
2.1.3 Gas Heating Values 3
2.1.4 Flare Location 3
2.2 Applicable Codes and Guidelines 3
2.2.1 40CFR60.18 General Control Device Requirements 4
2.2.2 Industry Handbooks 4
2.2.3 American Petroleum Institute (API) 931, Manual on Disposal
of Refinery Wastes Volume on Emissions, Chapter 15
Flares, and API Recommended Practices (RP) 521 4
2.3 Proposed Gob Well Flare Characteristics 5
2.3.1 Active Flare 5
2.3.2 Open Flare 5
2.3.3 Unassisted Flare 6
2.3.4 Flare Safety Features 6
2.3.5 Flare Tip Diameter 6
2.3.6 Flare Height 6
2.3.7 Pilot System 6
2.4 Proposed Gob Well Flare Design 6
2.4.1 Initial Gas Processing Equipment 9
2.4.2 By-Pass Venting System 9
2.4.3 Flare 9
2.4.4 Monitoring and Control 10
2.4.5 Flare System Operating Procedures 12
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Table of Contents (Continued)
Page No.
2.5 Safety Features 14
2.5.1 Isolation of Potential Sources of Ignition 14
2.5.2 Isolation of Potential Ignition from Flare 14
2.5.3 Isolation from Natural and Man-Made Sources of Ignition 15
3.0 SUMMARY 17
3.1 Proposed Gob Well Flare 17
3.1.1 Prime Mover 17
3.1.2 Passive Flare Safety System 17
3.1.3 Active Flare Safety System 18
3.2 Proposed Gob Well Flare Captial Costs 18
4.0 REFERENCES 19
List of Figures
Figure No. Page No.
1. General Layout of the Flare Facility 8
List of Tables
Table No. Page No.
1. Flare Components and Specifications 9
2. Monitoring and Control System Components and Normal Settings 11
3. Process Stream Valve Configuration for all Modes of Operation 11
4. Set Points for Sensing Equipment and Corresponding System Actions During
Normal Flare Operation 12
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1.0 INTRODUCTION
Currently, about 30 U.S. coal mining operations employ a system of degasification to assist in
reducing the emission of methane into their mine ventilation systems. All but one of these
mines utilize vertical gob wells as a form of degasification. This is an effective technique for
longwall coal operations, particularly if a vacuum is applied to the wellhead using a mechanical
prime mover such as a liquid seal extractor or blower/exhauster (an active as opposed to
passive system). In most cases, gas recovered from gob wells is discharged directly to the
atmosphere. Not only does this pose safety and environmental concerns, but it is a waste of a
potential resource.
In the U.S. there are no standards for equipping actively extracted or passive gob wellheads.
Some active installations are fitted with flame arresters, backflow check valves, and monitoring
and control systems, while others may feature only a short vent stack with minimal ancillary
equipment. Passive gob wells have, under certain situations, reversed flow and supplied intake
air to the gob. As coal mines extend farther under public lands and as residential areas
continue to spread out from urban zones toward coal mines, gob wellhead safety becomes
more of a concern, not only for the protection of the public, but for the mine as well.
From an energy perspective, the most viable solution would be to recover the gob gas at the
wellhead for utilization. However, because of technical and economic circumstances, this is not
always possible. In these cases, both safety and environmental objectives could be satisfied by
flaring the emitted gas. An active, controlled system of flaring, similar to that used in other
industries, is proposed.
This report presents a conceptual design for a flare system for a single actively extracted
longwall gob well. This concept is intended to be applicable to mines where for either technical
or economic reasons, it may not be possible to utilize the gas recovered. The objective of this
project is to develop a flare that would be suitable for igniting methane-air mixtures extracted
from coal mine gob wells, with particular concern to eliminating the possibility of a confined
deflagration and/or detonation through the flare system and/or gob well and mine.
1.1 Benefits of Methane Flaring
Gas flaring is a standard safety practice in many industries. For example, methane and other
associated gases are routinely flared during processing and production of oil and gas, and are
continuously flared from landfill collection systems. The petroleum industry flares for safety
reasons during system upsets when high concentrations and volumes are released in the
vicinity of potential sources of ignition. In the landfill industry, methane constitutes
approximately 50 percent of the gas recovered. Flaring is conducted to combust it and other
associated toxins (hydrogen sulfide and non-methane organic compounds), which are ground-
level ozone build-up gases. Unlike landfills, coal mine gob gas consists of a methane mixture in
air and does not contain many toxins.
Page 1
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1.1.1 Mine Benefits
Incorporating a controlled flare system would minimize the potential of an unconfmed
deflagration occurring on surface at the gob well discharge location, brought about by natural or
man-made sources. This would mitigate risk to the public as well as the underground mine.
Recommended design practices for gob wellheads, with options for incorporation of an active
flare system, would improve the current level of gob wellhead safety in the U.S. and minimize
the implementation of passive gob wells, which may be susceptible to air reversal.
In addition, the concern of introducing mine intake air of higher methane background
concentrations would be minimized. Localized air supply shafts, driven to connect main entries
more closely to longwall workings, may now be viable to mine operators with constrained mine
ventilation systems.
Continuous monitoring provisions, necessary with a gob well flare, would provide uninterrupted
records of gob well performance. These would be valuable in comparing gob well production
with underground conditions, and investigation of mine incidents such as mine fan failures,
changes to the ventilation system, or accidents. Currently most active gob wellhead
installations do not use continuous monitoring equipment.
1.1.2 Environmental Benefits
As the global warming potential of methane is approximately 24.5 times that of CO2 (over a
100-year time frame), combusting the methane released from coal mines using an active and
controlled flaring system, would result in emission of a significantly less harmful gas (IPCC,
1994). Flaring 35 percent of the methane emitted from just one of the gassiest coal mines in
the U.S. would result in an emission reduction, based on CO2 equivalent, of one million tons
annually. Flaring coal mine methane would then alleviate local air quality problems in many
cases. Additionally, methane contributes to tropospheric ozone problems.
A controlled flare system is proposed with redundant safety features, a prime gas mover, an
elevated stack, a controlled pilot, and a continuous monitoring system.
1.2 Mine Safety and Health Administration (MSHA) Response Request
The conceptual design for an active, controlled and monitored, single gob well flaring system
was developed for presentation to MSHA. The Coalbed Methane Outreach Program
anticipates that MSHA will view favorably a gob well flare design incorporating standard
practices and safety features recommended by the American Petroleum Institute.
Page 2
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2.0 PROPOSED FLARE DESIGN
This section presents a conceptual design of a flare system, suitable for application to a single,
actively extracted gob well. The concept design is also suitable, with some modification, for
connection to a multiple gob well gathering system. This application could be investigated
following field performance verification of a single gob well pilot.
2.1 Design Parameters
Gob well flare system design parameters were derived using typical gob well performance
characteristics.
2.1.1 Methane Concentration
The flare system was designed for combustion of methane concentrations (in air) ranging from
greater than 30 percent to 100 percent by volume. At a methane concentration of 30 percent
by volume the flare will be by-passed (see Monitoring and Control, Section 2.4.4).
2.1.2 Gas Flow Rate
The flare design was developed to accommodate a variable range of gas flows (methane and
air mixture) extracted from a typical gob well by a blower/exhauster. Gas flows ranging from 20
mscfd to 2 mmscfd (14 to 1400 scfm) were specified. At high gas flows, high methane
concentrations are expected, while lower methane concentrations are expected at lower gas
flows.
2.1.3 Gas Heating Values
Flare performance was specified for gas heating values ranging from 300 Btu/scf to 1000
Btu/scf based on pure methane concentrations in air.
2.1.4 Flare Location
For this design, the flare was assumed to be located in areas that are not designated by the
EPA as "ozone non-attainment areas", or where noise or luminance ordinances are imposed.
2.2 Applicable Codes and Guidelines
Applicable codes and guidelines for utility, landfill, and flares used in the petrochemical industry
were incorporated in the gob well flare design.
PageS
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2.2.1 40 CFR 60.18 General Control Device Requirements
These are control requirements to achieve EPA air emission standards and specify the
following:
• No visible emissions (except for 5 minutes every 2 hours)
• Flame presence at all times when emissions are vented
• Minimum gas quality (200 Btu/scf - unassisted flare)
• Maximum gas exit velocity as a function of flare type and gas quality (60 fps unassisted,
variable quality)
• Flares must be monitored for design conformance
• Pilot flame must be continuously monitored
2.2.2 Industry Handbooks
The following applicable guidelines were provided from flare gas systems handbooks.
Flare Height: The height of the flare is based on ground level limitations on thermal radiation
intensity as determined from maximum gas flows and heating values, including wind factors for
a 20 mph speed. Limiting radiation intensities are:
• 440 Btu/hr-ft2 unlimited time exposure by personnel
• 3000 Btu/hr-ft2 maximum at base of flare
• 1500 Btu/hr-ft2 minimum fenced boundary limit
• 750 Btu/hr-ft2 maximum at property lines
• 1500 Btu/hr-ft2 digital equipment and controls
Noise: Noise emissions result from combustion of the turbulent gas stream. The emitted
decibel level is proportional to the second power of the quantity of the hydrocarbon burned. In
populated areas, a closed flare system may be necessary to reduce noise emissions.
Luminance: Sufficiently mixed air and fuel gases will burn with a blue non-luminous flame. If
insufficient mixing occurs, the flame will become luminous. Where luminance is a concern, an
assisted or closed flare system is recommended (Section 2.3).
2.2.3 American Petroleum Institute (API) 931. Manual on Disposal of Refinery Wastes Volume
on Atmospheric Emissions, Chapter 15 - Flares, and API Recommended Practices
(RP) 521
Guidelines for flare flash-back protection design are provided by API. Flare flash-back
protection is achieved by either (1) ensuring a minimum purge gas flow at all times out the
stack, or (2) incorporating a passive protective system that mitigates air inflow into the top of
the stack, in addition to (3) incorporating a liquid seal, which effectively arrests flame and
detonation propagation upstream of the flare stack.
Page 4
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Purge Gas Requirement: A purge gas flow prevents air from entering back down into the stack
due to wind or thermal effects (caused by ambient air and gas density differences during low
stack flows) and potentially creating an explosive mixture.
Gas Seals: Gas seals, commonly denoted as fluidic, or diode seals, are recommended to
reduce the purge gas volume flow requirements. These seals are typically comprised of
stacked conical orifices installed inside the flare stack below the burner tip. The gas seals
successfully impede vortex back-flow generated by wind or thermal effects.
Liquid Seal: API recommends the use of a liquid seal at the base of the flare stack to prevent
flame and detonation propagation upstream. The gas process stream is introduced via a
header into a vessel typically containing an ethylene glycol - water mixture and is discharged
through a submersed perforated diffuser. With this system, the gas is released as a series of
distinct bubbles with liquid intervals between them, which ensures mitigation of flame
propagation through the seal. Standards require a minimum liquid head of 6 inches above the
diffuser outlet. A maximum of 12 inches is recommended as gas pulsation occurs at higher
liquid levels. API RP 521 also recommends that the gas inlet header height above the liquid
level be at least 1.5 times the diameter of the header. This is required in order that a seal be
maintained should a vacuum form inside the header as a result of sudden gas cooling during
discharge. The total volume of fluid in the vessel must also be equivalent to a minimum of 10
feet of the gas inlet line. Should a detonation occur in the stack, the liquid volume is displaced
into the inlet header, providing a minimum 10-foot water seal in the line separating the flare
from the rest of the system.
In addition, the height of the vapor space above the liquid line should be a minimum of twice the
diameter of the vessel in order to allow for disengagement of entrained liquid before gas entry
into the stack.
2.3 Proposed Gob Well Flare Characteristics
The following characteristics were stipulated for the gob well flare system. They incorporate the
design parameters and Petroleum and Landfill Industry guidelines and regulatory criteria.
2.3.1 Active Flare
Only an active gob well flare system should be employed. A mechanical blower/exhauster, as
is typically fitted to an equipped gob wellhead assembly, will maintain a positive gas pressure
and serve as the prime gas mover.
2.3.2 Open Flare
An open flare, where gas is burned at the tip of an elevated stack at combustion efficiencies of
98 percent, is more suitable for a gob well application than an enclosed ground-level flare.
Enclosed ground-level flares are used typically at landfills and burn low quality gas more
efficiently and emit less Nox (suitable for use in EPA-designated "ozone non-attainment
areas"), but have higher capital and operating requirements.
Page 5
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2.3.3 Unassisted Flare
Because of the readily combustible and lower heat content methane-and-air mixture extracted
from a typical gob well, an assisted flare system, where steam or air is injected at the burner tip
to promote mixing and therefore enhance combustion, is not required. Instead, an unassisted
flare with continuous burning pilot would readily combust the gob gas without producing
significant visible smoke (cooled carbon particles).
2.3.4 Flare Safety Features
A flare design that incorporates all of the API 152 flare protection alternatives is recommended.
Operability will be ensured with a continuous monitoring and control system with the capability
of activating a system of fail-safe valves.
2.3.5 Flare Tip Diameter
A minimum flare tip diameter of 8 inches is recommended based on the expected gas flow
range and the requirements of 40 CFR 60.18.
2.3.6 Flare Height
Based on a 1500 Btu/hr-ft2 criterion at the base of the stack, a 20-foot overall stack height is
specified. The heat distribution profile at grade, based on worst case wind conditions, will be
used to establish the equipment (and wellhead) to flare spacing.
2.3.7 Pilot System
A continuously monitored and operating pilot with a separate pilot gas fuel source is
recommended.
2.4 Proposed Gob Well Flare Design
A general layout drawing illustrating the proposed gob well flare facility is shown inFigure 1.
The facility is comprised of (1) the initial gas processing equipment that is typically in place at
an actively extracted gob well, (2) a by-pass gas venting system, (3) the flare, and (4) the
monitoring and control system. The monitoring and control system will be capable of activating
fail-safe valves and equipment shut-off features. Table 1 provides a detailed list of all
components and specifications, including estimated costs.
Page6
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I
ruwE-TQ-vame*fl CQUIPMENT SPACING - 43-
Figure 1: General Layout of the Flare Facility
Page 7
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Table 1 : Flare Components and Specifications
COMPONENT
Processing Equipment
vvellneao HssemDiy
bhut-in vatve v i
Air Activaiea Fan uose vaive V2
Water Separator/Fines Filtration
Detonation Arrester
Blower/Exhauster
Air Activated Fail Open Valve V3
Air Activated Fail Close Valve V4
End-of-Line Flame Arrester
Piping
Flanged Fittings
Head Tank
Solenoid Contol Valves V5 and V7
Manual Valve V6
Flare
Liquid Seal
Flare Stack
Pilot and Burner Tip
Pilot Ignition and Cable
Monitoring and Control
Compressor
Valve Controls
Monitoring and Control System
Relay Panel
AC-DC Power Transtormer
Liquid Level Sensor
Velocity Sensor
Static Pressure Sensor
Temperature sensor
Gas Quality Meter
Sight Gauges
Air Lines and Conduit
Facility
AC Power to Site
Site Preparation
Perimeter Fencing
Lightning protection
' existing equipment tor active got) well
Component Specification and Costs for Gob Well Flare System
MAKE/TYPE
iio. uasing
Manual ouuerriy
Aciuaiefl Butterfly
Anderson Separators
Protecto Seal No. 2500
Dry Extractor
Actuated Butterfly
Actuated Butterfly
Protecto Seal 860
HOPE
HDPfc
150 Gal
0.25 in. two-position
0.25 in
KALDAIR A-516-70
KALDAIR P-8-E
KALDAIR KEP
KEP-100 w/Nema 4X
1 hp Electric
Solenoid Valves
Data Logger
2 x AC and 4 DC
With Battery BacKup
Exp-Proof 2 Wire Trans
Orifice DP
Static Tap to Manometer
Thermocouple w/Trans.
Infrared Gas Monitor
Magnehelic
220 V AC
Pad for Flare
Cyclone 8 ft.
Rod/Perim.
SPECIFICATION
B men i-iange
Meaium pressure
Mefliurfl Pressure
2 mmsctd max
10 in. w.g. max @ 2 mmsctd
0.43 psi min. Pd @ 20 mscfd
Low Pressure
Low Pressure
8 in. Dia. Flange
8 in. ID
8 in. ID
Sight Glass - Press. Relief
0.25 ID
2 ft ID, 6 ft fit.
20 ft TOH, 8 inch ID
Single with Flame lonization
220 V AC
Integrated with 30 gal Tank
12 V DC Activated - 2 Position
8 Channel with DC Activation
5 VDC or Manual Activation
220 V AC - 12/24 V DC
6 in. Range, 4-20 mA Output
2, 4, 6 In. ID, 0-1.5 In. w.g. DP
0-25 in. w.g., DC in, 4-20 mA
0-125 F, DC in, 4-20 mA
0-100% CH4, DC in, 4-20 mA
0-30 in. w.g. ,0-2 in. w.g.
Weatherproof
8 ft Diameter 6 in. Poured
120 ft
QUANTITY
1
1
1
1
2
1
1
1
1
60ft
10
1
2
1
1
1
1
1
1
2
1
6
1
1 Ea.
COST PER UNIT
$ 1,bUU.UU
$ 7,400.00
$ 1,600.00
$ 1,600.00
$ 3,200.00
$6.00 per foot
$ 30.00
$ 400.00
$ 35.00
$ 15.00
Subtotal
Subtotal
$ 800.00
$ 98.00
$ 3,075.00
5 600.00
$ 325.00
$ 850.00
5 724.00
$ 304.00
$ 150.00
$ 3,818.00
$ 125.00
$ 300.00
Subtotal
S 1,000.00
subtotal
Total
TOTAL COST
$ l.bUU.UU
$ 7,400.00
$ 1,600.00
$ 1,600.00
$ 3,200.00
$ 360.00
$ 300.00
$ 400.00
$ 70.00
$ 15.00
$ 16,545.00
$22,725
$ 800.00
$ 98.00
$ 3,075.00
$ 600.00
$ 325.00
$ 850.00
$ 724.00
$ 304.00
$ 150.00
$ 3,818.00
$ 125.00
$ 300.00
$ 10.869.00
$ 1,000.00
$ 1,000.00
$ 51,139.00
BASIS
Existing
Existing
Existing "
1 is Existing "
Existing "
S&J Sales
Estimated
Skinner
Estimated
KALDAIR
Sepor
Skinner
CampDell Scientific
Campbell Scientific
Campbell Scientific
Davis Instruments
DavisS Davis
Cole-Parmer
Cole-Parmer
MSA Instruments
Davis Instruments
Estimate
Existing *
Estimated
Existing *
Existing *
-------
2.4.1 Initial Gas Processing Equipment
For a typical extracted gob well installation, a single-stage water separator with fine particulates
filtration capability is installed prior to inlet to the blower/exhauster. The separator is equipped
with a water discharge tap fitted at the base to release accumulated water. The
blower/exhauster, as indicated on Figure 1, typically provides a vacuum of up to - 5 in. Hg at the
wellhead and discharges at positive pressures slightly above atmospheric. For the flare system
design indicated on Figure 1 and maximum liquid levels in the seal drum, a minimum positive
pressure of 0.85 psi is required at the maximum design flow of 1400 scfm, and a minimum
positive pressure of 0.45 psi is required at the minimum design flow of 14 scfm.
2.4.2 By-Pass Venting System
As indicated in Figure 1, a by-pass venting system is incorporated in the flare design. Activated
valves (see Section 2.4.4) enable the flare to be by-passed and the gas to be vented through
an end-of-line flame arrester. For this design, 8-inch nominal internal diameter pipeline is used
downstream from the blower/exhauster.
2.4.3 Flare
During normal operations the gas is routed to the base of the flare.
Liquid Seal: The gas is discharged into the liquid seal drum, shown at the base of the flare,
through a perforated diffuser, and it is allowed to percolate to the surface of the liquid. The
specified liquid is an ethylene glycol - water mixture (50/50), which is maintained at the required
level (minimum 6 inches, maximum 12 inches above the diffuser outlet) by a float level control
system. The level control system operates an inlet valve through the control panel, as shown
on Figure 1. A 150-gallon head tank is provided for a positive pressure supply of water-
ethylene glycol which will need to be checked on a routine basis. A manual valve is used for
purging or for over-fill conditions as indicated.
Flare Stack: Gas bubbles emanating from the liquid seal surface move to the top of the seal
drum and ascend into the flare stack. As shown on Figure 1, the overall stack height was sized
at 20 feet with a projected maximum radiation intensity of 1500 Btu/hr-ft2 at the base of the
stack. This is within the tolerance for digital equipment and controls, although for the proposed
design, all controls will be situated at a distance of at least 45 feet from the base of the flare,
where radiation intensities of below the 440 Btu/hr-ft2 level are projected. This is used as the
minimum flare-to-wellhead equipment distance shown on Figure land is based on projected
radiation profiles assuming a maximum wind speed of 20 mph. The stack shown on the figure
is self-supporting for wind velocities up to 70 mph.
Fluidic Seal: As shown on Figure 1, the stack is equipped with a fluidic seal below the burner
tip. For this design, multiple conically shaped orifices are used. With this seal, purge gas flows
as low as 0.75 scfm (45 scfh) can be accommodated without air reversal. This is well below the
14 scfm minimum gas flow specified for the design.
Page 9
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Pilot Assembly: The pilot will be supplied by a feed gas source such as a fuel gas storage tank,
as shown on Figure 1. For continuous operation, the pilot will require 2.4 mscfd of fuel gas
(propane or methane). Air for combustion is introduced into the pilot gas stream through an
inspirator located near the flare tip. Pilot operation will be detected using a flame ionization
sensor system. This is an industry-accepted detection system and is preferable due to high
reliability and low maintenance and operating requirements. Pilot ignition is achieved through
an electric ignition cord leading from the control panel and mounted alongside of the stack.
Control systems maintain pilot operation at all times except as indicated in Section 2.4.4,
Monitoring and Control.
2.4.4. Monitoring and Control
The design incorporates a continuous monitoring system with active control capability.
Sensors: Transmitting sensors will monitor gas quality, static pressure, temperature and flow
rate of the process stream, in addition to pilot operation. Analog output from the sensors will be
routed to an 8-channel data logger with programmable activation and data recording features.
Control: Power will be supplied to the blower/exhauster, all solenoid valves, and the pilot
ignition system, through relays with manual and data logger activation capability. At
programmed sensor conditions, the data logger will activate relays as appropriate. The data
logger incorporates a cellular modem that enables retrieval of performance data from any
computer site.
Fail-Safe Valves: The system design incorporates three principal compressed-air-activated fail-
safe valves (V2 through V4 as shown on Figure 1). Compressed air at 80 psia is supplied by
small diameter lines connected to a storage tank with integrated compressor. Manual and data-
logger-activated solenoid valves are connected to the compressed air lines at the Valve
Controls (Figure 1) to either bleed or provide positive air pressure to the actuators.
Control Solenoid Valves: Two additional solenoid valves are incorporated to activate the fuel
gas supply (V7) and maintain fluid level control in the liquid seal at the base of the flare (V5).
Manual Operation Provisions: The system will be equipped with manual over-ride provisions
and sight gauges for pressure, gas flow and gas quality.
The purpose and normal operating condition (flaring) of all monitoring and control components
are itemized in Table 2. The configuration of the fail-safe valves for all modes of operation is
presented in Table 3. The process required to achieve each of the operating modes indicated
in this table are described in Section 2.4.5 (Flare System Operating Procedures).
Table 4 presents the set points for the proposed sensing equipment and the operation modes
activated by the control system should these settings be attained during normal flaring
operations.
Page 10
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Table 2: Monitoring and Control System Components and Normal Operating Settings
Monitoring and Control
Component
Manual Valve - V1
Compressed Air Activated Fail
Close Valve - V2
Compressed Air Activated Fail
Open Valve - V3
Compressed Air Activated Fail
Close Valve - V4
Solenoid Activated Control
Valve - V5
Manual Control Valve - V6
Solenoid Activated Control
Valve - V7
30 Gallon Compressed Air Tank
Integrated Compressor
Compressed Air Activated Valve
Controls (Solenoid Valves)
Methane Quality Sensor
Gas Flow Sensor
Static Pressure Monitor
Temperature Sensor
Liquid Level Sensor
Flame lonization Detector
Pilot Ignition System
Monitoring and Control System
(Data Logger with Recording
Capability)
Purpose
Well Flow Control and Final Wellhead Shut-In
Facilitates Activation/Shut-in of Well
Flare By-Pass Relief Valve
Flare By-Pass Relief Valve Actuated with V3
Control Liquid Level in Seal Drum
Manual Discharge of Excess Liquid or Purging
Seal Drum
To Activate Fuel Gas Supply Manual
Maintains Actuation Pressure on all
Compressed Air Control Valves
Maintains Air Tank at 80 psig
Deactivates Compressed Air to Specific Fail
Safe Valves Throughout System
Monitors Methane Gas Concentration in
Process Stream
Monitors Actual Process Stream Flow Rate
Monitors for Positive Static Pressure
(Blower/Exhauster Operation) and
downstream restrictions (liquid seal fluid level)
Monitors Temperature of Process Stream to
Determine Gas Flows at Standard Conditions
Monitors Liquid Level in Seal Drum
Monitors Presence of Pilot
Ignites Pilot if Off
Programmed to Send Signals to Valve Control
System and Pilot Ignition Pending Sensor
Information
Normal Setting
Open
Pressurized - Open
Pressurized - Closed
Pressurized - Open
De-Energized - Closed
Closed
Energized - Open
80 psig
Off
Energized
Continuously Active
Continuously Active
Continuously Active
Continuously Active
Continuously Active
Continuously Active
Energized
Continuously Active
Table 3: Process Stream Valve Configuration for all Modes of Operation
Mode
Normal
By-pass
Shut-in
V1 Valve
Open
Open
Closed
V2 Valve
Open
Open
Closed
V3 Valve
Closed
Open
Open
V4 Valve
Open
Close
Closed
Page 11
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Table 4: Set Points for Sensing Equipment and Corresponding
System Actions During Normal Flare Operation
Sensor
Gas Quality
Static Pressure
Gas Flow
Liquid Level in Seal
Flame lonization
Settings
@ 30% Methane in Air
@ 25% Methane in Air
Max @ 1 00% Methane in Air
Min @ 1.0 in. w.g.
Normal > 6.3 in. w.g.
Max @ 13 in. w.g.
Min @ 14 scfm
Normal > 20 scfm
Max@ 1400 scfm
Min @ 6 in. Above Discharge
Normal Range 6 in. to 9 in. Above Discharge
Maximum @ 12 in. Above Discharge
Pilot Flame not Detected
Pilot Flame Detected
Siystem Action
Actuate By-Pass Mode, Alarm
De-Energize Blower/Exhauster
None
Activate By-Pass Mode, Alarm
Alarm if Below
Activate By-Pass Mode, Alarm
Activate By-Pass Mode, Alarm
Alarm if Below
Activate By-Pass Mode, Alarm
Activate Supply Valve
None
Activate By-Pass Mode, Alarm
Ignite Pilot
None
2.4.5 Flare System Operating Procedures
Programmed and manual system operation and control logic for flare start-up and shut-down
are presented. Of particular concern during start-up of the system is ensuring sufficient gas
flow through the stack prior to ignition of the pilot. Although this could be done automatically,
manual system reactivation is recommended when switching from by-pass to flare, and when
initiating from the shut-in position. A manual procedure is also recommended for switching the
system from normal operations or by-pass mode to shut-in.
Flare Start-Up From Shut-In: The following manual start-up procedure is recommended,
initiating with the monitoring and control system set to manual override (refer to Figure 1 and
Section 2.4.4, Monitoring and Control):
1. Engage by-pass mode (V4 closed, V3 open)
2. Open V1 and V2
3. Monitor gas quality
3.1 - If gas quality is greater than 30% methane proceed to 4
3.2 - If gas quality is less than 30% methane maintain by-pass mode and assess
continued operations
4. Activate blower/exhauster
5. Monitor gas quality again
5.1 - If gas quality is greater than 30% methane proceed to 6
5.2 - If gas quality is less than 25% methane deactivate the blower/exhauster
Page 12
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6. Deactivate by-pass mode (V4 open, V3 closed)
7. Verify flow, static pressure and level indicator
7.1 If within tolerances proceed to 8
7.2 If outside tolerances reactivate by-pass mode and check system
8. Activate fuel gas supply valve (V7)
9. Activate pilot ignition system
10. Set monitoring system to automatic mode
Flare Start-Up From By-Pass Mode: The following manual start-up procedure is recommended
initiating with the monitoring and control system set at manual override (refer to Figure 1 and
Section 2.4.4, Monitoring and Control):
1. Verify gas quality
1.1 - If gas quality is greater than 30% methane proceed to 2
1.2- If gas quality is less than 30% methane maintain by-pass mode and assess
continued operations
2. Deactivate by-pass mode (V4 open, V3 closed)
3. Verify flow, pressure and level indicator
3.1 - If within tolerances proceed to 4
3.2 - If outside tolerances reactivate by-pass mode
4. Activate fuel gas supply valve (V7)
5. Activate pilot ignition system
6. Set monitoring system to automatic mode
Normal to By-Pass Mode: The following remote procedure is recommended (refer to Figure 1
and Section 2.4.4, Monitoring and Control):
1. Deactivate pilot ignition system
2. De-energize and close fuel gas supply valve (V7)
3. Activate by-pass mode (V3 open, V4 closed)
4. Signal alarm
Shut-In: The following manual procedure is recommended for shutting-in the gob well from
normal flare operating conditions (refer to Figure 1 and Section 2.4.4, Monitoring and Control):
Page 13
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1. Deactivate pilot system
2. De-energize and close fuel gas supply valve (V7)
3. Activate by-pass mode (V3 open, V4 closed)
4. Deactivate blower/exhauster
5. Deactivate (close) shut-in valve (V2)
2.5 Safety Features
Throughout the gas process stream, protection is provided from all potential sources of ignition
and from flashback or detonation occurring in the flare stack, via (1) an integrated passive
safety system, and (2) an active monitoring and control system.
2.5.1 Isolation of Potential Sources of Ignition
The blower/exhauster and the by-pass vent are two potential sources of ignition within the flare
system. As indicated on Figure 1, the blower/exhauster is isolated on either side by in-line
detonation arresters. These arresters are designed to stop low speed confined deflagrations
and high speed and high pressure flame fronts (sonic detonation and overdriven detonations) in
either direction. The design incorporates redundancy as a liquid seal in addition to a detonation
arrester is incorporated between the blower/exhauster and the flare. The arresters specified for
this design are tested according to API 2000, Underwriters Laboratories 525, and Factory
Mutual Research Approval's FM Class No. 6061 standards. The specified in-line units
incorporate spiral wound crimped metal (of appropriate lengths and materials) to provide flame
quenching. Anticipated pressure losses using a 10-inch diameter unit are 0.35 psi for the
largest flow specified for the flare system design.
An end-of-line flame arrester is fitted on the vent by-pass discharge stack to protect the flare
system from a flame entering into the system should the by-pass gas be ignited. The arrester
specified for this design incorporates a crimped stainless steel foil element and is designed to
prevent flash back from unconfined deflagrations.
The flame arresters and their arrangement are typical of gob well installations equipped with
blower-exhausting equipment.
2.5.2 Isolation of Potential Ignition from Flare
The proposed design mitigates the potential of flash-back from the flare by incorporating (1) an
active positive pressure system, (2) an API-recommended fluidic seal, (3) an API-
recommended liquid seal, and (4) a monitoring and control system with valve and equipment
activation capability.
Page 14
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Active System: The blower/exhauster is utilized as the prime mover of the gas through the flare
system. A positive pressure is maintained between the discharge of the exhauster to the liquid
seal. The liquid seal acts as a damper maintaining constant back pressure on the system. A
pressure sensor between the blower/exhauster and the liquid seal continuously monitors for
positive pressure to detect blower/exhauster operation.
Fluidic Seal: The flare stack incorporates a fluidic seal that prevents inflow of air into the stack
with gas flows as low as 0.75 scfm. This is well below the design minimum flow of 14 scfm. As
indicated in Sectin 2.4.4, Monitoring and Control, gas quality and flow rate are measured at the
well head and the control system will activate an alarm should flows drop below 20 scfm, and
will properly activate valves for by-pass mode should gas flows reach 14 scfm.
Liquid Seal: The flare stack incorporates an API-recommended liquid seal at the stack base,
which will stop a confined deflagration and/or a detonation from propagating upstream of the
stack. Gas is bubbled through a perforated diffuser maintained at least six inches below a
liquid seal. The liquid is comprised of water-ethylene glycol mixture and the level is
continuously monitored. A 150-gallon head tank will provide a positive pressure supply of the
water-ethylene glycol mixture for the liquid seal. The control system will activate the inlet valve
(V5) based on the indication of the water level sensor. A discharge valve is provided for
manual activation (V6) should excess liquid levels be detected by visual inspection.
Monitoring and Control System: A continuously operating monitoring and control system,
programmed to activate fail-safe compressed air-actuated valves, is incorporated as an active
safety measure. Sensors monitor gas quality, blower/exhauster operation, gas flow, and liquid
seal level on a continuous basis. Set points for each sensor are programmed into the controller
as well as the appropriate valve activation logic. The control system will be able to remotely
switch the system from normal operation to by-pass mode. In the case where the system mode
of operation is changed, appropriate alarms will identify the tripped sensor. For this design,
reactivating the system from by-pass to normal, from shut-in to normal, and from normal to
shut-in can only be accomplished by manual operation. This is to ensure proper operating
conditions, as well as sufficient gas purge rates prior to flare re-ignition, and to provide the
operator the flexibility to determine shut-in at his discretion.
2.5.3 Isolation from Natural and Man-made Sources of Ignition
The proposed facility will be protected from vandalism and unauthorized entry by an 8-foot high
perimeter fence. It will be equipped with properly grounded lightning protection comprised of
elevated perimeter static wires. Grounding connections will be made to enhance potential
equalization to prevent arcing.
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3.0 SUMMARY
The conceptual design for a gob well flare conforms to all applicable codes and regulations and
was developed incorporating operational and safety guidelines from the American Petroleum
Institute (API). System features, including safety, and estimated capital costs are summarized
in this section.
3.1 Proposed Gob Well Flare
An open, unassisted flare system, comparable to that used in the petroleum and landfill
industries, was selected as the basis for the gob well flare. The design is for single gob well
applications, but could be modified for a multiple well system. The flare design incorporates an
active mechanical prime mover (a blower/exhauster), and both passive and active safety
systems with remote monitoring and control capability.
3.1.1 Prime Mover
A blower/exhauster, as that typically fitted to an actively extracted gob well, ensures that a
positive pressure process stream is maintained to the flare during operation. A relay controls
power to the prime mover. The relay can be either manually activated (On/Off), or remotely
exited (Off only) by the process control system. The process control system monitors for
positive pressure downstream of the blower/exhauster, gas flow rate, and gas quality to assess
blower/exhauster operation. System start-up, including activation of the blower/exhauster is a
manual process for the proposed design. As the prime mover is a potential source of ignition,
detonation arresters isolate it from the gob well and the downstream equipment.
3.1.2 Passive Flare Safety System
The gob well flare design incorporates all of the API-recommended safety features. The
primary feature designed to mitigated flash-back upstream from the flare is the liquid seal
located at the base of the flare stack. Gas from the process stream is discharged through a
perforated diffuser into the liquid. The perforated diffuser releases the gas as distinct bubbles
with liquid intervals between them. The design incorporates a liquid level sensor monitored by
the control system with supply valve activation capability. Recommended liquid minimum and
maximum levels will be programmed into the control system. The liquid, which serves to
mitigate flame propagation, or to seal the upstream equipment from the flare, is a water-
ethylene glycol mixture. The use of liquid seals is standard practice in the petroleum industry
where large volumes of gases are flared in a controlled manner during refining system upsets.
Of prime concern during flare operation is the potential for air to flow down the stack during low
gas emissions due to wind or thermal effects. This is usually solved by ensuring a minimum
gas purge velocity up through the stack at all times. To reduce the purge gas flow
requirements, the API recommends the use of fluidic seals below the burner tip. These seals
serve to mitigate vortex back-flow down the stack. The gob well flare design incorporates a
fluidic seal that reduces the purge flow requirement to 0.75 scfm, which is significantly below
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the minimum operating design limit of 14 scfm (20 mscfd). The control system will be
programmed to activate the by-pass mode and signal an alarm if process stream flows fall
below 14 scfm as measured by an orifice plate velocity sensor.
3.1.3 Active Flare Safety System
The gob well flare design incorporates a continuous monitoring system with active control
capability. The design's control system may remotely by-pass the flare and de-energize the
blower/exhauster. System start-up and shut-in is a manual process; for this the design includes
manual override provisions and sight gauges.
The control system is designed to monitor gas quality, static pressure downstream of the prime
mover, process stream temperature and velocity (flow), and the level of the liquid in the seal
drum at the base of the flare. Data transmitted from these sensors is input into a data logger
programmed for activation based on sensor information. If the sensors detect any of the
following (1) excessive back-pressure downstream of the blower/exhauster, (2) that gas flows
exceed the design limit of 1400 scfm (2 mmscfd), (3) that the gas quality is below 30 percent
methane by volume, or (4) the gas flows fall below the design minimum of 14 scfm (20 mscfd),
the control system activates the by-pass mode. If the gas quality drops below 25 percent by
volume with the system in by-pass mode, the controls de-energize the blower/exhauster. The
control system activates the by-pass mode by excitation of relays connected to solenoid valves
that maintain compressed air to the fail-safe system process control valves.
The monitoring and control system design provides visual system alarms and records process
stream data continuously during operation. Stored data and system status can be acquired
remotely from any modem-equipped terminal.
3.2 Proposed Gob Well Flare Capital Costs
This report includes the capital costs for the added infrastructure required to equip a single
actively exhausted gob well with a flare system. Future designs may consider connections to
multiple gob wells, with slight cost increases for additional gas collection and monitoring and
control equipment.
Manufacturers provided the capital costs for the additional processing equipment, the flare, the
monitoring and control system, and the additional facility requirements. These costs are
itemized in Table 1, and amount to a total of approximately $51,200 per system, not including
installation.
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4.0 REFERENCES
American Petroleum Institute Refining Department, Manual on Disposal of Refinery Wastes
Volume on Atmospheric Emissions, Publication 931, Chapter 15 - Flares, June 1977.
American Petroleum Institute and Refining Department, Recommended Practices, Publication
521,1969.
Banerjee, K., N.P. Cheremisinoff, P.N. Cheremisinoff, Flare Gas Systems Pocket Handbook,
Gulf Publishing, 1985.
Code of Federal Regulations, Protection of Environment 40, Part 60, Section 18, "General
Control Device Requirements", July 1994.
Intergovernmental Panel on Climate Change (IPCC), 1994, Radiative Forcing of Climate
Change, The 1994 Report of the Scientific Assessment Working Group of IPCC, Summary for
Policymakers.
KALDAIR, Inc, "Pipeflare Specifications: P-8-E", May 1995.
Nardelli, R., "The Wide World of Landfill Gas Flares", LFG Specialties, Inc., Cleveland, Ohio.
Stone, D. K., Lynch, S.K., Pandullo, R.F., "Flares. Part I: Flaring Technologies for Controlling
VOC-Containing Waste Streams", Air and Waste Management Association, Volume 42, No. 3,
March 1992.
•f
Straitz, J.F., "Flare Technology Safety", Chemical Engineering Progress, July 1987.
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