|Methane Emissions within Natural Gas Processing and Oil Refinery Units
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Typically natural gas processing plants treat the well head natural gas to remove water, CO2, sulfur and other
by products such as ethane, propane or other heavy hydrocarbons before transferring the gas to the pipeline.
Within the plant process the use of an amine stripper emits tail gas with residual hydrocarbons that will have to
be treated before emission discharge to comply with EPA regulations.
Solution That Is Cost Effective
Natural Gas and Refinery Plants have most often made use of Direct Fired Thermal Oxidizers with no heat
recovery systems. Direct Fired Thermal Oxidizers use considerable amounts of auxiliary fuel to maintain
temperatures at or above 1,500°F. Maintaining such high temperatures within the direct fired thermal oxidizer
ensures that all volatile organics are combusted. However, the added fuel costs are significant in the daily
Many Regenerative Thermal Oxidizer systems are now finding their way into the Natural Gas Processing and Oil
Refinery industries. Regenerative Thermal Oxidizers offer a far less expensive means to regulatory compliance
in these industries.
Regenerative Thermal Oxidizers offer efficiencies more applicable to these industries than other types of plant
emissions. These industry types provide a fairly consistent flow of tail gas and RTO systems can be easily
designed. Natural Gas and Oil Refinery processing units provide ample VOC’s in the form of off-gas to auto
sustain the regenerative equipment. These RTO systems operate without any additional fuel while limiting the
energy costs associated with obtaining high negative static duct pressures.
The structured ceramic media within the heat recovery bed can be reduced and generally provides between 80
to 85% thermal efficiency. Standard two chamber or (bed designs) are implemented with destruction efficiencies
of 98% to 99% meeting all the necessary permitting requirements.
Regenerative Thermal Oxidizers should be designed to prevent corrosion from any residual acid gases while at
the same time able to operate on high VOC loading with lower oxygen levels. Oxygen levels of 3% or greater are
preferred and should be monitored to minimize any CO formation. Caution should be given to provide a reliable
system that may contain some Hydrogen Sulfide (H2S) and larger amounts of CO2. Moisture (RH) from the
added ambient make up air may cause CO2 to form carbonic acid within the RTO system. Typical residue from
any carbonic acid (H2CO3) may form on the bottom of the media beds, interior duct and valves. Periodic
flushing or system burn outs should be employed to reduce any media fouling. To lessen any carbonic acid
formation a fresh air heater may be used to provide heated supplemental oxygen without condensing any
moisture into the process stream. This typically occurs in the cooler regions of the country.
The adoption of a hot air by-pass system should be utilized to prevent RTO high temperature shut downs during
high VOC fluctuations within the tail gas. Using the hot air by-pass system, when VOC spikes occur in the tail
gas, enables the RTO system to expel any excess heat while continuing the oxidation process and maintaining
compliance. The hot air by pass system assures continuous plant operation while not having to circumvent any
tail gas to a stand by flare system. Circumvention of the tail gas to the flare reduces the effective costs savings
while causing momentary upset conditions to occur.
Energy Saving Systems
Applying a power generation turbine system to low heat waste stream from the regenerative thermal oxidizer
offers 50kw of continuous power on a typical 10,000 SCFM system. This micro turbine system provides enough
power to completely self sustain the system. The system consists of a single small skid-mounted unit, containing
all the equipment required for the power skid to be operated (i.e. Heat exchanger's, piping, working fluid feed
pump, turbine, electric generator, control and switch-gear). All systems are easy to transport and install, and
they are easy to interface with the hot and cold sources on the RTO system. Systems are low in cost and
provide years of maintenance free operation. Many states are now offering tax incentives to apply power
generation systems within plant operations.
Expected Cost Saving
The typical plant savings are approximately $550,000 per year in natural gas cost when comparing a
regenerative thermal oxidizer to a direct-fired thermal oxidizer system. General paybacks are within one year and
a few equipment manufacturers’ offer energy performance programs designed to guarantee saving and
payouts. These programs accept the difference in the current direct fired operating costs as payment for the
equipment purchase. Generally these savings will pay for the equipment through saving in 12 to 18 months and
allows companies to comply without any capital investments.
PSD GHG Regulations
As stated by the United States Environmental Protection Agency, new major stationary sources of certain air
pollutants, defined as “Regulated NSR pollutants and major modifications to existing major sources are required
to, among other things; obtain a PSD permit prior to construction or major modification. Once major sources
become subject to PSD, these sources must, in order to obtain a PSD permit, meet the various PSD
requirements for new stationary emission guild lines.
PSD GHG Example
A proposed emissions unit emits five of the six GHG compounds in the following amounts:
50,000 TPY of CO2
60 TPY of methane
1 TPY of nitrous oxide
5 TPY of HFC-32 (a hydrofluorocarbon)
3 TPY of PFC-14 (a perfluorocarbon)
The GWP for each of the GHGs used in this example are:
Carbon Dioxide 1
Nitrous Oxide 310
* As of the date of this document (see 40 CFR Part 98, Subpart A, Table A-1)
The GHGs mass-based emissions of the unit are calculated as follows:
50,000 TPY + 60 TPY + 1 TPY + 5 TPY + 3TPY = 50,069 TPY of GHGs
The CO2e-based emissions of the unit are calculated as follows:
(50,000 TPY x 1) + (60 TPY x 21) + (1 TPY x 310) + (5 TPY x 650) + (3 TPY x 6,500)
= 50,000 + 1,260 + 310 + 3,250 + 19,500 = 74,320 TPY CO2e
Note: Short tons (2,000 lbs), not long or metric tons, are used in PSD applicability calculations.29
For Complete Document, Download PSD GHG Emission Guidlines
Air Pollution Control Equipment
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