Measure Efficiency: Calculated performance

Can I measure flare efficiency? > Measure Efficiency: Calculated performance

Summary

Measured flare data from existing equipment is used to calculate flare combustion zone net heating value (NHVCZ) or combustion zone net heating value dilution parameter (NHVdil). With this, the flare combustion/destruction efficiency (CE/DE) can then be inferred indirectly using equations derived from previous empirical studies. A high flare CE/DE is required to ensure sufficient destruction of VOCs sent to the flare.

Follow the link to the right of this page to access an online calculation tool

How it Works

  • Vent Gas’ NHV is calculated from its components’ NHV values or alternatively, obtained from a calorimeter.
  • A higher NHV contribution/adjustment is applied for hydrogen (~1212 BTU/SCF @ 60F 1 atm) as H2 was found to promote better destruction in the flare
  • The NHVCZ or NHVdil is then determined depending on the type and size of flare – volumetric average of the flare constituent’s NHV (air, steam & vent gas). Equations are listed below
  • NHVCZ is most commonly used for elevated flares with the exception of air-assisted flares and small steam-assisted flares (< 9”) which uses NHVdil
  • From past empirical studies, the higher the NHVCZ or NHVdil, the higher the flare CE/DE and vice versa

Equations:

Advantages

  • • Able to use existing equipment

  • • Quick to implement once required data is collected

  • • Suitable to be applied for elevated flares of variable size and designs

  • • Can be done remotely from outside process boundary as only involves workbook calculations

  • • Method can be integrated directly to distributed control system

Limitations

  • • Cannot be applied if required input data for calculations is unavailable

  • • Calculation method depends significantly on input data quality, ie: flow meter ranges and measurement errors.

Case study

A study was done to identify the impacts of steam/air assist as well as turn-downed vent gas flow rates on an elevated flare’s CE/DE. The study involved varying the steam/air and vent gas flow rates while directly measuring the VOC emissions from the flare stack to calculate CE/DE. This calculated CE/DE values were then validated by comparing against separate results obtained from the following installed remote sensing technologies:

  1. Infrared Hyper-Spectral Imaging
  2. Passive and Active Fourier Transform Infrared Spectroscopy

The NHVCZ or NHVdil of the flare is able to be calculated for each test cycle using the vent gas composition as well as the steam/air flow rate to assess the relationship between NHVCZ/NHVdil and the flare’s CE/DE.

Example Data – Steam Assisted Flare > 9’’ Diameter

# Vent Gas (lb/hr) Steam (lb/hr) Vent Gas Composition Calculated NHVCZ (BTU/SCF) Measured CE (%) Measured DE (%)
1 920 0 1:4 Natural Gas to Propylene Vol Ratio Diluted with N2 355.8 >99 >99
2 2342 1000 1:4 Natural Gas to Propylene Vol Ratio Diluted with N2 208.9 90 93
3 2342 2000 1:4 Natural Gas to Propylene Vol Ratio Diluted with N2 147.9 68 71
4 920 1000 1:4 Natural Gas to Propylene Vol Ratio Diluted with N2 127.6 35 39

*Dilution with N2 to target vent gas NHV of 356 BTU/SCF. Conditions for volume is at 60°F & 1 atm
*From TCEQ test series S3 & S4

 

Example Data – Air Assisted Flare

# Vent Gas (lb/hr) Air (lb/hr) Vent Gas Composition Calculated NHVdil (BTU/ft2) Measured CE (%) Measured DE (%)
1 900 20,000 1:4 Natural Gas to Propylene Vol Ratio Diluted with N2 30.0 >99 >99
2 900 90,000 1:4 Natural Gas to Propylene Vol Ratio Diluted with N2 6.9 78 72

*Dilution with N2 to target vent gas NHV of 356 BTU/SCF. Conditions for volume is at 60°F & 1 atm
*From TCEQ test series A3, 24’’ diameter of flare tip

 

Observations

  • As steam/air flow is increased or vent gas flow is decreased, the flare’s NHVCZ & NHVdil will decrease
  • The higher the flare’s NHVCZ or NHVdil, the higher the measured CE and DE

Predictive systems and calculations

The efficiency of combustion can be calcualted based upon the flare gas composition, volumes and data on the flare design from which exit velocities and mixing can be estiamted. These calcualtions can either be done by the operator directly or obtained from pre-built simualtion software. Recent advances incomputing have supported development of predictive systems providing real-time feedback on calculated values. In all cases, the quality of calculations is dictated by the quality of data that is available on flare volumes, compsition and flare design.

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