Methane Emissions And Natural Gas Processors, Oil Refinery Units

More regulations, under PSD and Title V guidelines limit greenhouse gas emissions. 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 extensive amounts of auxiliary fuel to maintain temperatures at or above 1,500°F. Maintaining such high sustained temperatures within the direct fired thermal oxidizer ensures that all volatile organics are combusted. However, the added fuel costs are significant in the daily operating costs.

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 pertinent to these industries than other types of plant emissions. These industry types provide a fairly consistent flow of tail gas and systems can be easily designed. Oil Refinery and Natural Gas 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 needed permitting requirements~Standard two chamber or (bed designs) are employed with destruction efficiencies of 98% to 99% meeting all the necessary permitting requirements~Standard two chamber or (bed designs) are applied with destruction efficiencies of 98% to 99% meeting all the necessary permitting requirements~Standard two chamber or (bed designs) are implemented with destruction efficiencies of 98% to 99% meeting all the essential permitting requirements~Standard two chamber or (bed designs) are implemented with destruction efficiencies of 98% to 99% meeting all the compulsory permitting requirements~Standard two chamber or (bed designs) are implemented with destruction efficiencies of 98% to 99% meeting all the needed 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 ideal 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 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 and 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 any necessary supplemental oxygen without condensing any moisture into the process stream. This in general 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 fluctuations within the tail gas flow. Using the hot air by-pass system, when VOC spikes occur in the tail gas, permits 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 evades any tail gas to a stand by flare system. Circumvention of the tail gas to the flare lowers 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, including 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 site. Systems are low in cost and provide years of maintenance free operation. Many states are at present 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 calculated to guarantee saving and payouts. These types of 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.

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.

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:

GHG GWP*: Carbon Dioxide 1, Nitrous Oxide 310, Methane 21, HFC-32 650, PFC-14 6,500

* 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 the Complete PSD GHG Document and more information about obtaining a regenerative thermal oxidizer, please visit: American Environmental Fabrication & Supply, LLC.

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