PowerPoint Presentation

An alternative method for modelling flare emissions DMUG 201706 April 2017

Adam Clegg Associate Director, Environmental Assessment Technical Leader

3IntroductionComplexity of modelling flare emissionsReview of current methods for modelling flare emissionsAn alternative method for modelling flare emissionsEvaluation of current and alternative methods with monitoring data An alternative method for modelling flare emissions Presentation structure

Complexity of modelling flare emissions4

5The majority (not all) of flare systems fall in to two principal categories

Complexity of modelling flare emissionsType of flare

Enclosed ground flareGenerally used to dispose of low to medium volumes of sweet (low sulphur) gasesFlame enclosed within a shroud (low degree of radiative heat loss and more controlled/efficient combustion)Easy to model - treat as normal point source releaseRelease defined by physical stack height, discharge temperature and velocityOpen elevated flareGenerally used to dispose of sour/acid (high sulphur) gases or for emergency depressurisation (e.g. ESD). Also used for base loadFlame open to the environment (higher degree of radiative heat loss and less controlled/efficient combustion)More complex release to modelWhere do you take the point of emission to occur?What do you enter for a release height and diameter?How do you define the efflux characteristics?

6There are several factors which increase the complexity of modelling elevated flare emissions compared to a conventional point source release:Presence of the flame itself Models cannot be applied to the flaming region, only the buoyant region above it (ADMS User Guide). Dispersion assumed to begin at termination of the flame rather than flare tipRequires an estimate of the flame length in order to calculate an effective release heightFlame length and, hence, effective release height constantly changes due to factors such as wind speed, gas composition/volume flow, combustion efficiency etcFlame expansion affects the effective release diameter if we assume dispersion begins at the flame tipDischarge temperatureSignificant variation in temperature across a flame how do you define a typical release temperature?Not all of the heat released during combustion is in the form of sensible heat which produces plume rise. This needs to be taken in to account when defining a discharge temperatureDischarge velocityThe velocity at the flame termination point will not be the same as the initial jet velocity through the flare tip. How do you calculate the reduction?

Complexity of modelling flare emissionsSpecific issues modelling open flares

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Complexity of modelling flare emissionsDynamic nature of defining open flare release characteristicsCrosswinds cause a deflection of the flame from the vertical and can also affect combustion efficiency which, in turn, impacts plume buoyancyIncreasing the volume of gas flared or the calorific value of the gas increases the flame length and reduces the effects of crosswindsKey discharge parameters such as release height and location, diameter, buoyancy etc, will change for each hour of meteorological data, particularly if dispersion assumed to begin at the flame termination pointModels dont accept varying release heights or locations (why would they!?) in time-varying emission files. Create individual input files for each hour of data? Very time consuming

8There is (still) no formal guidance from UK regulators on how to model open flare emissions

Most will often refer back to approaches suggested by various federal and state regulatory agencies in the US and Canada

Numerous approaches which vary significantly in terms of complexity and level of assumptions made

All have their limitations

Creates uncertainty for the model user in selecting an appropriate approach to take

Complexity of modelling flare emissionsConsistency of approach

Current methods of modelling open flare emissions9

10Current methods of modelling open flare emissionsAlabama Department of Environmental Management (ADEM) method

11Current methods of modelling open flare emissionsOhio EPA/New Jersey DEP method

12Current methods of modelling open flare emissionsIowa DNR method/Trinity Consultants

13Current methods of modelling open flare emissionsSummary of current methodsParameterADEM MethodOhio EPA/New Jersey DEP MethodIowa DNR/Trinity MethodEffective release heightFlare stack heightStill air flame length added to flare stack heightStill air flame length added to flare stack heightEffective release diameterCalculated from heat releaseCalculated from heat releaseDerived by equating flare buoyancy flux to that of a conventional stack releaseDischarge temperature/buoyancyArbitrarily defined as 1,000C independent of process conditionsArbitrarily defined as 1,000C independent of process conditionsArbitrarily defined as 1,000C independent of process conditionsDischarge velocity/momentumArbitrarily defined as 20 ms-1 independent of process conditionsArbitrarily defined as 20 ms-1 independent of process conditionsCalculated from actual process conditionsCrosswinds consideredNoAssumes flame is bent over at 45 from vertical for all wind speedsNoModification to release locationNoNoNoRadiative heat lossesAssumed as 55%Assumed as 55%Defined by model user

Alternative method of modelling flare emissions14

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Alternative method of modelling flare emissionsOverview

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Alternative method of modelling flare emissionsHeat release

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Alternative method of modelling flare emissionsCombustion efficiencyCombustion efficiency influenced by several variables but key factors include:Heating value of components in the combustion zone, particularly the concentration of inerts (e.g. from N2, CO2 etc in the gas feed or from steam or air injection)Wind speed

Previous studies by the US EPA in the 1980s suggested appropriately designed and operated industrial flares would generally achieve a minimum combustion efficiency of 98%

More recent research by the US EPA in 2012 suggested combustion efficiency can be significantly reduced in high cross winds (> 22mph) or where there is a high concentration of inerts in the combustion zoneFewer studies looked at the effects of the highest crosswindsHowever, data availability on heating value of components in the combustion zone and resultant combustion efficiency is much improved, allowing a relationship to be determined

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Alternative method of modelling flare emissionsCombustion efficiency

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Alternative method of modelling flare emissionsFraction of heat radiated

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Alternative method of modelling flare emissionsFlame length and expanded diameter

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Alternative method of modelling flare emissionsEffective release height

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Alternative method of modelling flare emissionsEffective release locationIn a similar manner as the release height, API 521 and Cook et al (1990) can be used to calculate the shift in downwind position of the flame, with the incremental shift in downwind position of each flame segment defined by:

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Alternative method of modelling flare emissionsBuoyancy and momentum flux

Evaluation with monitoring data24

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Evaluation with monitoring dataMonitoring stationMonitoring data collected from a continuous AQMS near to a UK refinery provided an opportunity to evaluate the relative performance of each methodMonitoring station commissioned in response to complaints from local residentLocated near to complainant locationNot ideal from a model evaluation perspective (single station, predominately upwind/crosswind) and potential interference from complainant (see right!)Nonetheless, provides an opportunity to evaluate the relative performance of the various methodsMonitoring station recorded continuous measurements of SO2, NOX, H2S, THC, wind speed and directionMaintained to AURN standards15 month monitoring survey

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Evaluation with monitoring dataEvaluation methodologyCombustion, process and flare stack parameters varied for each hour of meteorological data using process and emissions data provided by the refinery

Evaluation undertaken using hourly SO2 only less influenced by background emission sources compared to some of the other pollutants monitored and easier to determine flare emission rates from S mass balance than pollutants such as NOx

Data filtered so that only those data points included when the monitoring station was downwind of the refinery. Defined as those wind directions which produced a modelled contribution of more than 0.1 g m-3 using R91 A-G stability class met file

Paired data points were also excluded from the evaluation study where either the model does not return any output; for example, resulting from incomplete meteorological data, or where monitored concentration data is not available from the AQMS for that particular period of time, such as in the event of equipment servicing or loss of power/malfunction

Evaluation completed using a combination of methods included in the Model Validation Kit (Olesen and Chang, 2010) and openair software (Carslaw and Ropkins, 2012)

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Evaluation with monitoring dataEvaluation results

nMeanrFA2MGEFBObservations1,23112.71100New method1,23116.20.4230.32912.7-0.242ADEM1,23116.60.4180.32213.0-0.266Ohio EPA1,23116.60.4180.32213.0-0.266Iowa DNR1,23116.50.4190.32413.0-0.260

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Evaluation with monitoring dataSummaryVery little difference between each method with all within the general range observed in other ADMS validation studies of conventional point source releasesSource apportionment indicated the combustion and process stacks contributed ~93% of the modelled mean concentration. This is a fixed contribution for each methodFurther interrogation of the data points reveal flare emission parameters were generally at base-load values. Base-load flaring generally not a significant contributor to off-site concentrationsSome evidence of improvement in performance in the new method There were several periods of abnormal operation flaring during the monitoring survey unfortunately the monitoring station was upwind of the refinery in each instance!Therefore, the performance of each method during abnormal operation flaring is more uncertain. This is where greater divergence in the performance of the methods would be expected e.g. Boger et al (2013) conclude that assigning arbitrary values for discharge temperature, velocity and fraction of heat radiated might be acceptable for base-load flaring but unlikely to be the case for scenarios with higher feed ratesRecommendation to evaluate the methods where flare emissions are the primary contributor to monitored levels

amecfw.com

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