Dispersion Data

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Temperature	-4.30000000000001°C
Mixing Height	499.1 m
Wind Direction at 50 metres (Xstrata Tower)	241°
Wind Speed at 50 metres (Xstrata Tower)	21.0 km/h
SO2 Dispersion Conditions (Smelter)	NEUTRAL-1

Xstrata Nickel Sudbury Smelter SO2 emissions

Control Procedures

The operator is required to make decisions on the extent and timing of control action required to ensure that the GLC (ground level concentration) limit of 0.34 ppm is not exceeded on the monitoring network or within the population centres surrounding the Smelter for any one hour period. Under the new Control Order, there is also a requirement that fugitive emissions from the plant not cause an exceedence of 0.30 ppm for the half hour average. This additional requirement will affect only the Edison and Parkinson monitors.

Since the only practical control option is on the amount of converter blowing time within a given time period, the operator must decide how much impact the SO2 emissions are likely to have at the ground level and determine the operating schedule for converter blows accordingly. For the fugitive half hour limit, the main control available is to stop casting and any transfer operations which can lead to SO2 fugitive emissions.

As outlined in the Operations Manual, the operator has a number of sources of information to lead him/er to the decision, including:

- 5 minute average SO2 and wind data
- running hourly average data for SO2 (running half hour on Edison and Parkinson monitors)
- trends in GLC and wind data
- forecasts of meteorological conditions
- estimate of worst case concentrations (Look-up table)

In using this information, the operator must bear in mind that he/she has the ultimate responsibility to decide which of the indicators are likely to be most accurate in certain circumstances, and that he must in many instances make a judgement on the basis of past experience how the information is to be used.

For example, the worst case concentration estimate provided in the Look-up table can only serve to guide him/her in making the judgment but it cannot on its own provide the final decision on control of operations. This is due to the fact that the table values represent the worst case condition rather than the present actual one, as well as the fact that the models used to derive the table elements do not provide a highly accurate estimate for a given hour. For some of the same reasons, the operating limit advice on number of minutes of converting available, which is based on the trend predictors for GLC and wind, should be used with care as a guide to making the decisions but not as absolute criteria for taking action.

The CCR supervisor monitors the predictive model display data on an on-going basis in order to assess the level of operation which he should authorize for the converters. The decision is based on a prediction of the maximum concentration level that can be expected for the predicted meteorological situation, as well as the current monitor readings and projected trends of SO2 concentration and of wind speed.

It should be noted that because of the lowered GLC limit in the new Control Order, all of the conditions listed in the table can lead to an exceedence of the hourly average concentration if the emission condition were to persist for the entire hour. The fact that the converting periods are limited to only a part of the hour permits the operator to authorize converter operation as long as it does not persist throughout the hour.

Meteorological Conditions

Unstable

Unstable (convective) conditions will occur primarily during the Spring and Summer days when skies are mostly clear and winds are moderate to light. In order for convective conditions to develop, there must be sufficient heating of the ground to produce rapid warming of the air near the ground. Typically, the lower level temperature on the Xstrata Tower will reach a minimum in the morning before sunrise and will increase rapidly after sunrise to a maximum by early afternoon. This develops an unstable layer which grows in depth as the heating continues through the day. The critical period for the plume is around mid-to-late morning, depending on the time of year, when the mixing height grows to a level such that the plume is just trapped within the unstable layer. Following this period, as the convective regime is firmly established, the plume undergoes looping, continuing the high concentrations at ground level, generally within several kilometres of the stack.

During the convective dispersion regime it is found that the GLC's fluctuate rapidly due to wind direction variability as well as the intense level of turbulence. This makes it difficult to predict where the plume will be impacting or the level of the resulting concentrations, which can vary over the range from 0.0 ppm to 2.0 ppm for single converter operation.

High Wind Neutral Conditions

When the weather situation leads to significant cloudiness either during the day or at night, or where the winds are sufficiently strong to break down significant stratification, the dispersion is said to be neutral, by which it is meant that turbulence is at a moderate level leading to a cone shaped plume which is fairly steady in size and direction, generally producing sufficient dilution of the plume so as not to cause high ground level concentrations.

For the stack conditions, which provide little buoyancy to the plume, if the wind speed exceeds about 25 km/hr the plume will downwash on the stack, leading to GLC higher than the limit within a few km of the stack. Similarly, high winds which follow the passage of a cold front can lead to the same kind of down-washing and higher GLC levels. For most of the moderate wind cases there will be no problem in maintaining the limit concentration level for single converter operation if the converting is not continuous over the entire hour. The neutral condition is by far the predominant stability.

Stagnation/Light Wind Condition

When the weather system is such that very little wind is generated, the material released from the stack cannot be transported out of the region and the concentrations will build up over time. If this condition persists for several hours, then it is likely that the 0.34 ppm limit will be exceeded at some location.

This situation is particularly difficult to control since, to be effective, the shut down of converter operation must come before significant SO2 concentrations are seen on any of the monitors. If action is delayed to the point that the stagnation condition has already set in, then the material already released from the stack has no chance to move out of the area, and will cause an exceedence.

The stagnation condition can, under certain circumstances, last for the better part of the day; however, its development is well predicted by the forecast program. A new alarm has been set up to alert the CCR Supervisor that the wind speed is decreasing and likely to stay below the 4 km/h level, indicating a stagnation condition.

The primary control criterion in this situation is the trend on wind speed at the upper level of the tower. The wind direction in this situation is not a reliable measurement, and for really stagnant conditions the air will be moved from one direction to another over the period of minutes, so that the affected region cannot be predicted in this case. After a prolonged period of this sloshing around, the plume forms a mushroom around the stack.

When the wind speed trend indicates that within the next period of one hour the wind speed will fall below 4 km/hr then visual checks are done on the plume behaviour. If it is evident that the plume is not dispersing then all converter blows are disallowed until the trend indicates that the wind speed will exceed 7 km/hr within the next period of 60 minutes. After control has been initiated on the basis of the trend on wind speed, it is necessary to monitor the onset of the predicted stagnation condition.

If it is apparent that the condition does not materialize as predicted, the converter operation can be reinstated at the one converter level if the other indicators do not rule this out.

A second condition for control in stagnation situations occurs when the actual wind speed, as a rolling hourly average, is less than or equal to 4 km/hr. This again requires visual observation of the plume in order to determine if the plume is dispersing properly or whether a stagnation situation has arisen. If a stagnation period has set in then a shutdown is required.

Thereafter, if the actual wind speed, as a rolling hourly average, returns to 4 km/hr. or greater for three or more five minute periods, then resume operating.

Non-Critical Conditions

The non-critical conditions are those which do not lead to exceedences of 0.34 ppm for the hourly average when the converter is operated on a 40 minute basis for the hour. These include some of the neutral regimes as well as the stable conditions for single converter operation at remote monitors and other conditions when the plume is in unpopulated areas. If a non-critical regime is identified, then the converters can be operated without any constraints on the amount of blowing time within the hour.

Since the meteorological forecasts can be amended at any time when conditions warrant it, and since the observed meteorological data can indicate a change in the conditions on a 5 minute update basis, it should be kept in mind that the non-critical regime can be terminated as abruptly as it starts; accordingly, the CCR Supervisor must maintain a constant watch on all of the indicators, even during the non-critical regimes.

Visual Observations

Visual observations should be made on a regular basis when meteorological conditions are such that stack or converter aisle monitor emissions can impinge on the townsite. For example, for easterly winds and periods of stagnation extreme caution should be taken regarding all activity in the converter aisle. The aisle should be prepared to stop casting, skimming and/or blowing. If casting or skimming is stopped the converter openings should be under the off-gas hoods. Visual observations should be made from the townsite or a good look-out point. All decisions and data used must be logged including the date and time the observations were made.

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