Applications of Accidental Release Modeling for Environmental Risk

Assessment and Emergency Response in China

Hui Guo, PhD

Chengzhi Wu, Weiping Dai, Lixian Dong

May 12, 2010Xi’an, China

trinityconsultants.com

A&WMA International Specialty Conference

Background

� 14,742 environmental pollution

accidents occurred from 1998 to

2006 in China; air pollution

accidents accounted for 34.4%.

� Emergency response plan and

environmental risk assessment are

now required in China.

� Both regulatory and technical

guidance on the subject is not as

comprehensive as needed.

� The role of air dispersion modeling

for accident release.

Objective of this Study

� Compare Chinese technical guidance of Risk

Management Plan (RMP) with that in the U.S.

� Discuss the applicability of four refined models to

Chinese RMP guidance.

� Introduce different levels of accidental release

models from relatively easy-to-use model to

sophisticated models.

Comparison of Regulatory and Technical Guidance in China and in the U.S.

� Accidental Release Prevention Program, also referred

to as the Risk Management Program (RMP) rule

released by U.S. EPA

� Hazard assessment

� Prevention program

� Emergency response

� Technical Guidelines for Environmental Risk

Assessment on Projects

� Current version– release in 2004

� New proposed version – proposed in 2009

Table 1. Detailed Comparison between Chinese and U.S. RMP Guidance (1)

Parameter

U.S. EPA RMP GuidanceNewly Proposed Chinese RMP

Guidance

Worst Case Scenario Alternative ScenarioMaximum Credible Accident

Scenario

Meteorological

Condition

Mete data source

The past three years’ mete data

at the site or from the nearby

mete station

The past three years’ mete data at the

site or from the nearby mete station

One year’s mete data in the past

three years at the site or from

station

Wind Speed/Stability 1.5 m/s and F stability

Most frequent stability class; the

most frequent non-calm wind speed

in that stability classConduct air dispersion modeling

by using whole year mete data to

get the worst hourly

meteorological condition.Ambient Temperature

/Humidity

Highest daily maximum

temperature and average

humidity

Average temperature and humidity

Wind Direction All directions (circle) All directions (circle) Wind direction at that hour

Topography Surface roughness

For obstructed terrain, use

urban – 1meter; For flat terrain,

use rural – 0.3 meter

For obstructed terrain, use urban –

1meter; For flat terrain, use rural –

0.3 meter

Not mentioned

Source

Release Quantity Largest quantity Reasonable quantity Reasonable quantity

Release Duration

10 minutes for gas release;

instantaneous spill for toxic

liquids

Be estimated based on the length of

time it would take to stop the release

5-30 minutes for release

15-30 min for pool evaporation

Height of release Ground level release Any appropriate release height Any appropriate release height

Temperature of released

substance

Highest daily temperature or

process temperature, whichever

is higher; For gases liquefied by

refrigeration, use chemical’s

boiling point.

Process or ambient temperature that

is appropriate for the scenario

Process or ambient temperature

that is appropriate for the scenario

Mitigation Mitigation system Passive mitigation system Passive and active mitigation system Not mentioned

Table 1. Detailed Comparison between Chinese and U.S. RMP Guidance (2)

Parameter

U.S. EPA RMP GuidanceNewly Proposed Chinese RMP

Guidance

Worst Case Scenario Alternative ScenarioMaximum Credible Accident

Scenario

Endpoints

Endpoints for toxic substances Be specified in the Regulation Be specified in the Regulation LC50 and IDLH

Endpoints for flammable

substances

Overpressure of 1 psi for vapor

cloud explosions

� Overpressure of 1 psi for vapor

cloud explosions;

� Radiant heat of 5kW/m2 for 40

seconds for fireballs or pool fires

� Lower flammability limit (LFL) for

vapor cloud fires.

Not required

Models

Dispersion model

Any appropriate model, including

plume models, puff models and

dense gas dispersion model

Any appropriate model, including plume

models, puff models and dense gas

dispersion model

Gaussian puff model, adjusted

puff model for dense gas

modeling

Fire model N/AVapor cloud fire, pool fire, jet fire and

BLEVENot required

Explosion model

Vapor cloud explosion (TNT-

equivalent model or other models

taking into confinement of the

vapor cloud)

Vapor cloud explosion (TNT-equivalent

model or other models taking into

confinement of the vapor cloud)

Not required

Modeling

Results

Maximum distances to the

endpoints

The geographical areas (the

circles) that could be affected

according to the endpoints

The geographical areas (the circles) that

could be affected according to the

endpoints

The geographical areas (the

circles) that could be affected

according to the endpoints

Maximum concentrations at

receptorsNot required Not required

Maximum concentration at each

receptor, time, and concentration

contour

Centerline concentrationConcentration distributions on

centerlineConcentration distributions on centerline

Maximum concentration on

centerline and the corresponding

location

Others

Incompletely combusted toxics Not required Not requiredIncomplete combustion fraction

based on empirical methods

Toxics generated from

combustionNot required Not required

SO2, CO and other toxic

substances from polymer

combustion

Application of Appropriate Models for Different Accidental Release Scenarios

Possible Release Scenarios

� One phase or two phases

� Choked or unchoked

� Cloud buoyancy

Source Term Analysis� Source Term Analysis:

� Basic equations in the technical guidelines

� Commercial software

� Accidental release of toxic aqueous solutions– heat and

mass transfer mechanisms need to be considered when

calculating emission rates

� Specific Source Term Model: SOURE5 model for

LNG Modeling

� In the U.S., computer simulations instead of the basic

equations is required to model LNG accidental releases.

� Source5 model – developed by Gas Technology Institute

(GTI) to predict the vaporization and spreading rate of

instantaneous and continuous LNG spills over land or

water.

Accidental Release Dispersion Models

Table 2. General and Specific Characteristics of Four Refined Accidental Release Dispersion Models in the U.S. (1)

INPUFF AFTOX SLAB DEGDIS

Sponsor U.S. EPA

U.S. Air Force,

supported by U.S.

EPA

U.S. Department of Energy

(DOE), supported by U.S.

EPA

U.S. Coast Guard,

U.S.EPA and Gas

Research Institute (GRI)

Algorithm Gaussian puff Gaussian puff

Conservation equations of

mass, momentum, energy

and species. The

conservation equation are

spatially averaged to treat

the cloud as a steady state

plume, a transient puff or a

combination of the two

depends on the release

duration

Jet plume model (Ooms

model) first, then dense

gas dispersion model is

used to model the

subsequent dispersion

when jet plume reaches

ground level

BuoyancyNeutrally/positively

buoyant gas

Neutrally/positively

buoyant gasDense gas Dense gas (or aerosol)

Source TypeSingle or multiple

point source

Evaporating pool,

or point source

Evaporating pool, elevated

vertical jet release, elevated

horizontal jet release and

instantaneous volume source

Ground-level area

source dense gas (or

aerosol) clouds, vertical

jet release plume

Release type

Finite duration, or

continuous plume

from a stack

Instantaneous, finite

duration and

continuous release

Instantaneous, finite duration

and continuous release

Transient (not for jet

release), finite duration

and continuous release

Table 2. General and Specific Characteristics of Four Refined Accidental Release Dispersion Models in the U.S. (2)

INPUFF AFTOX SLAB DEGDIS

Emission RateNot calculate, support

variable emission rates

Calculate the

evaporation rate of

liquid pool internally

Not calculate Not calculate

Mete Data

Constant mete condition

or variable mete

conditions for each mete

period with the same

length

Constant mete

condition for one run

Constant mete condition

for one run

Constant mete condition

for one run

Deposition/Decay Yes No No No

Output

Concentration vs. time

for a given receptor; and

puff trajectories after

each simulation period

Maximum distance to

specified endpoints at

each time step;

Concentration at

specified location and

time;

Concentration at specific

location at each time step;

time averaged maximum

concentration at different

downwind distance

Concentration field at

specified times;

concentration time

histories at specified

positions.

Other Strengths

Handle time-dependent

release rates, spatially

and temporally variable

wind field and moving

point source

Calculate 90%

confidence interval

(CI) for toxic corridor;

Also can simulate

neutrally buoyant release

including lofting of the

cloud if it becomes

lighter-than-air

Specified by U.S.GTI to

conduct LNG accidental

modeling combined with

SOURCE5 model;

account for heat and

water transfer

Limitations No consideration of chemical reaction, no complex terrain and no building downwash effects

Accidental Release Dispersion Models

� INPUFF & AFTOX – Gaussian Puff Model,

Neutrally/Positively Buoyant Gas

� Most applicable to Chinese newly proposed RMP

guidance.

� INPUFF – more suitable for modeling toxic gas or soot

generated from fire or explosion

� AFTOX – more suitable for modeling liquid evaporation

pool

� DEGADIS & SLAB – Dense Gas Model

� DEGADIS – vertical jet release

� SLAB – horizontal & vertical jet release

� Important – Concentration Averaging Time

INPUFF

AFTOX

SLAB

DEGDIS

Application of Accidental Release Models

� Easy-to-use models

� Aloha

� RMP*Comp

With a chemical database, simple source term

calculation module and simple dispersion, fire and

explosion models

� Refined models

� INPUFF, AFTOX, SLAB, DEGADIS etc.

� Sophisticated models

� CHARM (Complex Hazardous Air Release Model)

CHARM� A sophisticated 3D Eulerian grid model developed by

Mark W.Eltgroth.

� CHARM can treat: complex terrain (e.g. valleys);

complex meteorological condition (e.g., mountain-valley

flows, sea breezes); aerodynamic effects of nearby

buildings (building downwash) and chemical reactions.

� CHARM can calculate dispersion of dense gas, neutrally

buoyant and positively buoyant gas from all kinds of

release, and also predicts thermal radiation and

overpressures caused by fire and explosion.

� Grid wind field with multiple met sites, Convective heat

transfer, gas deposition and liquid flow over terrain.

CHARM

Summary

� When dispersion modeling analyses are

conducted for RMP, accurate source parameters

and appropriate models are important.

� This study provides discussions on consideration

and selection of appropriate models for accidental

release scenarios.

� Regulatory RMP requirements in China and the

U.S. are compared and discussed.

Contact Information

� Ms. Hui Guo, Trinity Consultants, China

86-0571-28828368

hguo@trinityconsultants.com

� www.trinityconsultants.com