Method for controlling the BTU content of a flare gas

A method for controlling the Btu content of a flare gas to combusted in a flare stack comprising a flare tip is provided. The method includes, introducing a first gas stream including nitrogen to be flared, the first gas stream having an initial Btu content, providing a supplemental fuel gas stream, and combining the first gas stream with the supplemental fuel gas stream, thereby obtaining a flare gas stream having a final Btu content measured at the flare tip.

BACKGROUND

Flares are regularly used to combust hydrocarbons and hazardous air pollutants in refineries, chemical plants and other oil and gas related operations. Certain of these facilities are required to meet minimum BTU Net Heating Values (NHVs) for gas streams fed to flares in order to assure combustion of hydrocarbons and hazardous air pollutants (HAPs). NHV requirements may apply to the gas in the feed line or at the flare tip depending on applicable rules for the specific facility type.

As an example, new rules applicable as part of the EPA's revised Refinery Sector Rule (RSR) eliminate previous compliance exemptions for process start up, shut down and malfunction (“SSM”) events which can all result in substantially increased gas flow to flares. Under the revised new rules, emergency flaring, flow from pressure relief device (PRD) venting, sulfur recovery plants catalytic cracking units and other processes must now meet new NHV requirements.

Most, but not all, PRD release and malfunction events will release hydrocarbon rich streams which are high in BTU NHV and inherently compliant with the rule. Conversely, nitrogen is regularly used to purge hydrocarbons and hazardous air pollutants from process equipment, feed lines, storage tanks and other areas in oil and gas facilities. These nitrogen purge streams can be very high flow and largely composed of Nitrogen which has a NHV of Zero (0) BTUs. Nitrogen dilution of gas streams will cause them to fall below regulatory NHVs and cause a facility to be out of compliance with regulatory standards.

Facilities will regularly need to feed higher BTU supplemental fuel gas to flare streams to increase the stream NHV in order to stay in compliance. Similar situations exist for chemical plants and other operations who must meet requirements outlined in other rules.

There are several source options for various supplemental fuel gases within plant operations including natural gas supplied by pipeline, process generated products such as hydrogen, propane, propylene or butane and light ends fuel gas produced from distillation and other processes. And, some facilities have already made needed connections to their flare streams to meet requirements for most or all cases.

Other facilities, though, are faced with technical and logistics challenges related to these requirements including the following:The expense of running connections within the facilities from fuel gas sources to flare feed lines. One example noted that the flares are on one corner of the site and the source gas is on the opposite corner with a resultant project cost that is too high to consider.Restricted and/or insufficient flow from natural gas utilities. Restricted flow could be a result of other critical users on a line that would be adversely affected by a plant pulling very high volumes of gasAn inadequate supply of surplus fuel gas from process sources may also prove to be a problem. For example a product that might otherwise be available is required to feed new or bottle-necked plant processes.Pipeline supplied natural gas exists and is possibly already connected, but the available volume is inadequate to meet highest flow, andInavailability of normal supplemental fuel gas supply due to partial or overall plant outage.

SUMMARY

A method for controlling the Btu content of a flare gas to combusted in a flare stack comprising a flare tip is provided. The method includes, introducing a first gas stream comprising nitrogen to be flared, the first gas stream comprising an initial Btu content, providing a supplemental fuel gas stream, and combining the first gas stream with the supplemental fuel gas stream, thereby obtaining a flare gas stream comprising a final Btu content measured at the flare tip.

The supplemental fuel gas flow is initiated when the initial Btu content falls below a predetermined minimum net heating value measured at the flare tip. The supplemental fuel gas flow is terminated when at least one of the following conditions has been met by the first gas stream: the hazardous air pollutants (HAP) level is less than 20 ppm, the pressure is below 5 psig, or the lower explosive limit (LEL) is less than 10%.

In a first operating mode the first gas stream has a first flow rate, wherein the first flow rate is measured; the supplemental fuel gas has a second flow rate, wherein the second flow rate is measured; and wherein the ratio of the second flow rate to the first flow rate is maintained at a predetermined value, thereby maintaining the final Btu content at or above a minimum net heating value measured at the flare tip.

In a second operating mode the first gas stream has a first flow rate, wherein the first flow rate and the initial Btu content are measured; the supplemental fuel gas has a second flow rate, wherein the second flow rate is measured and the final Btu content is measured at the flare tip; and wherein the second flow rate is adjusted to maintain the final Btu content at or above a minimum net heating value at the flare tip.

DESCRIPTION OF PREFERRED EMBODIMENTS

ELEMENT NUMBERS

101=Flare Stack102=Flare Tip103=First Gas Stream104=Supplemental Fuel Gas Stream105=Flare Gas Stream106=Btu Content of First Gas Stream107=Btu Content at Flare Tip108=First Gas Stream Pressure109=First Gas Stream Flow Rate110=Supplemental Fuel Gas Stream Flow Rate111=Source of Supplemental Fuel Gas Stream112=Flare Assist Fluid113=Supplemental Fuel Gas Stream Valve114=HAP Measurement of First Gas Stream115=LEL Measurement of First Gas Stream

DEFINITIONS

As used herein, Lower Explosive Limit (LEL) is defined as the lowest concentration of a gas or a vapor in air that is capable of producing a flash of fire in the presence of an ignition source.

As used herein, Hazardous Air Pollutant (HAP) is defined as those pollutants that are known or suspected to cause adverse environmental effects. This definition includes those compounds or elements identified by the EPA.

As used herein, Flare Assist Fluid is defined as a smoke suppressing medium that is added to the flare gas stream in order to achieve a smokeless, or essentially smokeless, operation. Flare Assist Fluids may include, but are not limited to, steam or air.

As used herein, the term “combustion efficiency requirement” is defined as greater than 98% conversion of organic compounds to carbon dioxide.

As used herein, the minimum net heating value may be a particular value, values, or range of values specified by a relevant statute. The minimum heating value may be about 270 Btu/SCF measured at the flare tip. The minimum heating value may be at least about 270 Btu/SCF to about 350 Btu/SCF measured at the flare tip. The minimum heating value may be at least about 270 Btu/SCF to about 300 Btu/SCF measured at the flare tip. The minimum heating value may be less than about 500 Btu/SCF measured at the flare tip. The minimum heating value may be less than about 400 Btu/SCF measured at the flare tip.

The Federal regulations specify that the minimum exit velocity and or net heating value be determined at the flare tip, depending on the type of process and materials to be flared. With few exceptions, measurement and recording of these parameters requires either a Continuous Emissions Monitoring System (CEMS) or a Predictive Emissions Monitoring System (PEMS) is employed. A CEMS directly measures, records and reports on the flare gas stream in real time, as close to the flare tip as possible. A PEMS uses surrogate parameters, typically farther upstream, to calculate and estimate the required parameters and are used when flow rates and composition are generally well documented and predictable. The PEMS may be used as an alternative to a CEMS where approved by permitting authorities.

In one embodiment of the present invention, a temporary and/or portable based supplemental fuel gas supply is supplied for the duration of a project. This can vary from readily available tanker refill support for onsite stationary storage tanks to fully temporary systems for supply and delivery of supplemental fuel gas. Depending on the application, this supplemental fuel gas can be provided via portable gas vessels (Cylinders, Liquid Vessels, Tubes, Micro-Bulk or Bulk), skid-based systems or truck/trailer mounted supply. This basic form could also be a simple as an external source of gas with limited capacity used simply to “spike” NHV at the initiation of a flare flow event.

In another embodiment of the present invention data input may be input into control system operation. This could be as basic as initiating (for example) supplemental gas feed on receipt of a signal noting that a nitrogen heater has been activated indicating the start of a hot nitrogen sweep.

High level support would include automatically controlled feed of supplemental fuel gas based on use of direct compositional or calorimetric monitoring of the flare gas stream and/or use of nitrogen flow information to proportionally control the amount of supplemental fuel gas fed into a line or lines. On/off control could also be tied to LEL detection or other device providing information based control.

The present invention may provide a temporary and/or portable connected source of supplemental fuel gas to elevate a flare stream BTU NHV to a level which meets plant and/or regulatory compliance requirements for combustion. A system as noted above with auto-actuation of flow based on some external device or controller. Either system discussed above with automatically controlled feed of supplemental fuel gas based on control data including, but not limited to, flow, pressure, direct read of flare line or flare tip NHV, LEL data or some other source of process and/or measurement information.

Turning now to the sole FIGURE, a method for controlling the Btu content of a flare gas is provided. In a first operating scenario, the initial Btu content106of the first gas stream103has a value greater than a predetermined minimum net heating value. In such an operating condition, no supplemental fuel104will be needed, and the first gas stream103may be sent to the flare101as it is.

In a second operating scenario one of the following three conditions has been met by the first gas stream. Either the hazardous air pollutants (HAP) level114is less than 20 ppm The pressure108is below 5 psig Btu/SCF. Or the lower explosive limit (LEL)115is less than 10%. In such an operating condition, no supplemental fuel104will be needed, and the first gas stream103may be sent to flare101as it is.

However, in a third operating scenario, the initial Btu content106falls below a threshold minimum net heating value107measured at the flare tip102. In such as scenario the following method is initiated. The threshold minimum net heating value may be 300 Btu/SCF. The threshold minimum net heating value may be 270 Btu/SCF.

A first gas stream103to be flared, comprising nitrogen is provided. The first gas stream103has an initial Btu content106. A supplemental fuel gas stream104is provided and combined with the first gas at a combination point111, thereby obtaining a flare gas stream105. The flare101may be equipped with an assist fluid112. The flare gas stream105has a final Btu content107measured at the flare tip102, after the addition of any assist fluid112. The first gas stream103initial Btu content may be measured upstream of the combination point111.

In one embodiment of the present invention, the predetermined minimum net heating value may be the minimum heating value required to achieve combustion efficiency requirements.

In one embodiment of the present invention, a feed-forward type operating mode is utilized. In this mode the first gas stream103has a first flow rate109, wherein the first flow rate109is measured. The first flow rate109may be measured by any means know to the art. The supplemental fuel gas stream104has a second flow rate110, wherein the second flow rate110is measured. The first flow rate109and the second flow rate110may be measured by any means know to the art. The ratio of the second flow rate110to the first flow rate109is maintained at, or above, a predetermined value. Since the first flow rate109is typically outside of the control of this system, it is the second flow rate110that is modulated to maintain the flow rate ratio.

This flow rate ratio may be calculated based on the measured first gas stream flow rate109, and the known heating value of that particular stream. This flow rate ratio may be calculated based on the measured first gas stream flow rate109, and the measured heating value, the initial Btu content, of the first gas stream106. The flow rate of the supplemental fuel gas stream104is adjusted by control valve113. This flow rate ratio allows the final Btu content107to be maintained at or above a minimum net heating value measured at the flare tip102. The minimum net heating value may be 270 Btu/SCF measured at the flare tip102.

In another embodiment, a feed-back type operating mode is utilized. In this mode the first gas stream103has a first flow rate109, wherein the first flow rate109and the initial Btu content106are measured. The supplemental fuel gas104has a second flow rate110, wherein the second flow rate110is measured and the final Btu content107is measured at the flare tip102. The first flow rate109and the second flow rate110may be measured by any means know to the art. The flow rate of the supplemental fuel gas stream104is adjusted by control valve113. The second flow rate110is adjusted to maintain the final Btu content107at or above a minimum net heating value at the flare tip102.

The frequency with which flow rates and the Btu contents may be measured and reported may be dependent upon the measurement type that is chosen, or the overall requirements of the processing system. For example, if a gas chromatograph or a calorimeter is used, it is possible that only one Btu measurement may be possible within the target 15 minute time period. If, however, a mass spectrometer is employed, the Btu content may be measured and reported many times in the target 15 minute period. The flow rates and the Btu contents may be measured and reported at least every 15 minutes.

The supplemental fuel gas stream may be provided by a local, external bulk source111. The bulk source111may be portable and temporary. The supplemental fuel104may be hydrogen, ethane, propane, propylene, butane and/or any suitable hydrocarbon with a sufficient heating value.