ADORA models the atmospheric emissions and dispersion resulting from three types of release scenarios:

 

1. Open Burn Open Detonation (OBOD)

OBOD is a common means of disposing of conventional weapons and wastes containing energetic materials (propellants, explosives, and petrochemicals). ADORA's OBOD module can be used to predict the emissions from OBOD operations, their dispersion in the atmosphere and deposition on ground. The OBOD modules of ADORA can also be used to model other release scenarios such as conflagrations involving chemical warehouses, rail cars, tanks, pools, and petroleum products.

Other codes neglect after-burn reactions in the cloud, and either assume thermo-chemical equilibrium or complete combustion at time zero. These techniques provide reasonable results at high temperatures. However, as the cloud cools down due to heat losses and ambient air entrainment, the thermo-chemical equilibrium assumption breaks down. To overcome this problem, we developed a unique technique called constrained equilibrium that is based on a kinetic approximation to the conventional thermo-chemical equilibrium approach. ADORA uses this technique to represent the chemical reactions in a more realistic way and obtain accurate emission data. This technique has been validated against available emission factor data from literature.

Moreover, other models assume that a fixed amount of air is entrained into the cloud at zero time. In reality, air entrainment takes place through out the evolution of the cloud (Figure 1). The presence of excess air at initial times (as assumed by other models) changes the reaction path of the cloud constituents and affects the temperature-time history of the cloud. This results in erroneous emission predictions and the cloud size and trajectory. ADORA on the other hand, calculates the entrainment rate as the cloud evolves, thus resulting in more accurate cloud trajectory.

ADORA uses the after-burn reactions and air entrainment calculations mentioned above to calculate emission factors (EF). For a specific release of interest and meteorological conditions, if EF are known the use of those values is recommended. However, emission factors are available only for a very small number of source compositions. Therefore, when EF data are not available they need to be calculated by using accurate physics based models. Once emission factors are calculated passive dispersion can be used to calculate atmospheric dispersion. ADORA has a standard passive dispersion routine integrated with the emission factor calculation scheme to provide the user with a seamless calculation method.

 

2. Moisture Reactive Chemicals

ADORA can model both continuous and instantaneous releases of chemicals that may react with atmospheric moisture. Some chemicals that are initially dense may become positively buoyant due to the exothermic reactions with ambient moisture and lift off the ground. Examples of chemicals that fit into this category of release are BCl3, UF6, COCl2, F2, TiCl4, PCl3, SF4, SO3/H2SO4, SO2Cl2, HF, ClF3, SiBr3H, CH3COCl, SbCl5, and SiCl4

 

3. Non-Reactive Chemicals (Buoyant & Dense Gases)

Neglected by other codes and EPA's Off-site Consequence Analysis (OCA) tables, ADORA accounts for the thermodynamic transformations (such as phase changes and polymerization) associated with the release of non-reactive buoyant or dense gases. Examples of chemicals that are considered in this type of release are refrigerated NH3, HF, CH4, H2, hot process gases, fire gases and heavier-than-air gases.

Depending on the chemical reactions, and thermodynamic transformations that occur in the cloud, the cloud may become positively buoyant and lift off the ground, reducing the hazard distance of toxic materials on the ground. In some special situations where the reaction products are more toxic than the reactants, neglecting the chemical reactions may result in the under prediction of hazard distances.

 

ADORA runs on IBM PC with Windows/NT operating systems. Combined with powerful plotting capabilities and a Graphical User Interface (GUI) for ease of use, ADORA has all the necessary requirements to be an effective environmental tool.

 

Some of ADORA's key features are the following:

I. Various modes of heat transfer between the cloud and the ambient surroundings such as convection, conduction and radiation are included in the ADORA model. In addition, the cooling of the cloud due to the entrainment of ambient air is also accounted. From experience, we found that the cloud temperature is the most crucial factor that determines the accuracy of model predictions. Accordingly, significant effort has been made to develop accurate heat transfer and cloud chemistry algorithms.

II. For spills of moisture reactive chemicals, ADORA describes the chemical reaction using a global one-step reaction scheme. The reaction kinetics can be specified by the user, if available in literature. Otherwise, ADORA provides two options (entrainment limited or mixing limited) for the calculation of reaction rate. For OB/OD releases, ADORA provides three methods for describing chemical reaction, all coupled with cloud dynamics: simple thermochemical equilibrium, constrained equilibrium (an effective kinetic approximation), and direct specification of a reaction.

III. For OBOD operations, ADORA can model the fireball resulting from a high temperature reaction as either a spherical puff based on an integrated puff formulation or as a spherical puff with a tail (elongated puff). The latter is a more realistic representation of the cloud shape and it allows for a much more accurate determination of the near-field dispersion close to the release point. This is very desirable especially when the fence line is close to the source.

IV. The temperature-time history is modeled accurately in ADORA. The resulting buoyancy and momentum imparted to the cloud are accounted in the plume rise algorithms.

V. Different dispersion parameters are used in ADORA for continuous and instantaneous releases. Most other models use continuous release dispersion parameters even for instantaneous releases.

VI. ADORA models the formation of particulate matter (PM) during the cloud cool down from OBOD operations. For larger condensates it calculates the deposition contours.

VII. ADORA currently assumes dispersion on a flat surface. However, if required a complex terrain model can be integrated into ADORA as a post processor.

VIII. ADORA has been developed as a generic model that is species independent. New reactants that are not currently covered in ADORA can be modeled just by adding their properties to the ADORA database.

IX. ADORA can also calculate emissions of detonations that occur underground.

In addition to the above features, ADORA has thermo-chemical, physical, and toxicity property databases for over 2100 species and phases. Users have the options to add, edit, and remove this information. ADORA can perform automatic parametric studies to identify the worst-case scenarios.

 

More details on the contract can be found at the following web site: http://www.blazetech.com/ADORA/GENERAL_SERVICES_ADMINISTRATION.doc