Modelling of atmospheric dispersion of gas (SO2)

MODELLING OF ATMOSPHERIC DISPERSION OF GAS (SO2)

1.0 Background

This report is based on atmospheric dispersion modelling of SO2 gas, released into the atmosphere as a waste product of H2S. Due to the toxic nature of H2S, it is usually removed from crude before usage. This process of removal leads to the emission of sulphur dioxide (SO2) into the atmosphere. The released sulphur dioxide then contributes, amongst other sources an environmental problem. A dispersion model is therefore necessary in ascertaining the manner in which SO2 is transported downwind and dispersed to some levels of concentration. These different levels of concentration are then studied to know its effects on people and also its environmental impacts. Subsequently, some steps are taken to reduce the magnitude of its potential effects in real situations. (American Institute of Chemical Engineers, 2000).

This modelling is done using the Pasquill-Gifford steady state equation. Chin et al (2000) reported that the dispersion of sulphur dioxide gas in air has played an important role in the study of atmospheric chemistry, global climate change, and environmental health because it is the most predominant pollutant in the troposphere having severe consequences on anthropogenic component.

The report discusses the environmental consequences of air pollution by SO2 emissions, its impact on human health at various toxicity levels and how these effects can be mitigated.

1.1 Atmospheric dispersion modelling of SO2 gas

A model is a simplified picture of what happens in reality. It may not contain all the features that would naturally be obtained in a real system but it contains the specific features of interest the problem that needs to be solved. In science, models are widely used to make predictions and/or solve problems, and are most times used to determine the best solutions for the management of specific environmental related problems. Air dispersion modelling is said by the United States Environmental Protection Agency to be a primary regulatory tool for source impact prediction.

An atmospheric dispersion model is a mathematical simulation of the physics and chemistry governing the transport, dispersion and transformation of pollutants in the atmosphere. Aside being a means of estimating the downwind air pollution concentrations, it is also a means by which the air quality impact of a facility in a given area is assessed (National Institute of Water and Atmospheric research).

Atmospheric dispersion models can assume any form with the simplest being found in the form of graphs and tables but in recent times air dispersion models commonly take the form of computer programs with user friendly interfaces.

Air dispersion models calculate pollutant concentrations downwind of a source using information obtained from the contaminants emission rate, characteristics of emission source, local topography, ambient or background concentration of pollutant and meteorology of the area.

The process of atmospheric dispersion modelling contains four stages of:

  1. data input,
  2. dispersion calculations,
  3. deriving concentrations, and
  4. analysis).

As common with other model types, atmospheric dispersion models are not devoid of uncertainties. The accuracy and uncertainty of each stage must be known and evaluated to ensure a reliable assessment of the significance of any potential adverse effects which may arise due to inadequate study, review or inappropriate application. An important element for effective atmospheric dispersion modelling is choosing an appropriate modelling tool that matches the scale of impact and complexity of a particular emission.

Modelling of sulphur dioxide (SO2) dispersion can be carried out using either mathematical or analytical models. While analytical models deal with practical approaches to the physical effects or properties noticed or observed over time, mathematical models on the other hand, determine rates of emission using experimental simulations. Different models are required for different scenarios as there is no single model for all applications and the best model will be what is that which will give a solution when sets of data are computed and modelled.

1.2 Importance of atmospheric dispersion of SO2 gas

The development of a community and the protection of its ecosystem will require an effective and efficient air quality assessment. This has given rise to the need of studying and developing models that will be suitable for predicting atmospheric dispersion of sulphur dioxide (SO2) when the overall effect it has on the environment humans is considered.

Modelling is usually done to determine or estimate the downwind concentrations of pollutants over varying periods of time-either long term or short term. The development of any community including the protection of the ecosystems requires an effective and efficient air quality assessment.

The most common use of dispersion modelling is to assess the potential environmental and health effects of emissions to air from industrial or trade premises.

These models, according to the National Institute of Water and Atmospheric Research, are very valuable tools for:

  1. planning new facilities
  2. assessing compliance of emissions with air quality standards, guidelines and criteria
  3. identifying the main contributors to existing air pollution problems
  4. determining appropriate stack heights
  5. forecasting episodes of pollution
  6. designing ambient air monitoring networks
  7. evaluating policy and mitigation strategies
  8. estimating the influence of geophysical factors on dispersion (e.g. terrain elevation, presence of water bodies and land use)
  9. saving cost and time over monitoring as modelling costs are a fraction of monitoring costs and a simulation of annual or multi-year periods may only take a few weeks to assess
  10. assessing the risks of and planning for the management of rare events like accidental hazardous substance discharges
  11. running numerical laboratories for scientific research involving experiments that would otherwise be too costly in the real world (e.g. tracking accidental hazardous substance releases, including foot-and-mouth disease)

2.0 DATA SET

The modelling uses Pasquill-Gifford formula for steady state dispersion given below. Emission of SO2 was considered from ground level conditions to varied distances in the x, y, z co-ordinates.

where:

  • C(x,y,z) is the concentration of the emission (in micrograms per cubic meter) at any point x meters downwind of the source, y meters laterally from the centreline of the plume, and z meters above ground level.
  • Q is the quantity or mass of the emission (in grams) per unit of time (seconds)
  • u is the wind speed (in meters per second)
  • H is the height of the source above ground level (in meters)
  • sy andszare the standard deviations of a statistically normal plume in the lateral and vertical dimensions, respectively.

The input are data used for this modelling are:

wind velocity, u = 8ms-1;

rate of emission of SO2,Qp = 0.06KgS-1;

effective stack height, H = 30m

stability condition of day, medium sunlight which corresponds to D because wind velocity exceeded 6m/s.

Data sets used in this report consist of values of sy and sZ obtained from predicted values of X (distance from source) which ranged from 200-10000m. Also predicted to depict various scenarios were values of y and z which are distances in the lateral direction.

In all, four different scenarios were taken under consideration to determine SO2 concentration in air. While scenario 1 consists of values generated at varied distances of x(m), and y(m) with z at ground level, scenarios 2, 3 and 4 consists still of varied distances of x(m) and y(m) but with z being increased to 50m, 100m and 150m respectively. It should be noted that though lateral distance in the z- direction was increased for these scenarios it was kept constant at various distances in the y- direction.

3.0 CONCENTRATION AND CONTOUR PLOTS

Atmospheric dispersion modelling of a gas is used to predict ground level pollutant concentrations in an area around the source(s). The results which can be used to identify areas of unacceptable impact are usually presented as concentration and contour plots. Below are plots obtained from the above sets of data.

4.0 AIR POLLUTION

Air pollutionis the introduction ofchemicals, particulate matter or biological materialsthat can adversely affect, harm or cause any form of discomfort to humans or other living organisms, or damages thenatural environment, into theatmosphere. It modifies the natural characteristics of the atmosphere.

The atmosphere is a complex, dynamic natural gaseous system that is essential to support life on planetearth. Air pollution has long been recognized as a threat to human health as well as to the Earth'secosystems.

Air Pollution can be defined as the presence in the outdoor atmosphere of one or more contaminants in such quantities and of such duration as may be injurious to human, plant, or animal life, or to materials, or which may unreasonably interfere with the comfortable enjoyment of life or property, or the conduct of business. (Harrop,2002)

4.0.1 AIR POLLUTANTS

Many substances present in the air can in one way or the other have adverse effects on humans, other living organisms and the natural environment. Air pollutants are pervasive, and are responsible for a wide range of adverse health and environmental effects putting the lives of humans and other living organism at constant risk when these emissions exceed set air quality standards.

Substances that are naturally not found in the air or present in amounts that exceed the normal are often referred to as pollutants. Pollutants can exist in the form of liquids, solids, particulate matter or gases. Typical pollutants include oxides of nitrogen (NOx), volatile organic compounds (VOCs), hydrocarbons (HC), hydrogen sulfide (H2S), carbon monoxide (CO), particulate matter (PM), sulphur dioxide (SO2), toxic air contaminants such as lead (Pb),nitrous oxide (NO) as well as greenhouse gases such as carbon dioxide (CO2), and methane (CH4).

Air pollutants can be classified based either on the state of matter or by their origin. Under the state of matter based classification, the gaseous and particulate sub-classes are recognised while the primary and secondary sub-classes of pollutants are known to exist under classification by origin. While primary pollutants are those substances that are emitted to the atmosphere directly from a process, such as nitrogen oxide emitted from high temperature combustions, sulphur dioxide released from factories, carbon monoxide gas from a motor vehicle exhaust or ash from a volcanic eruption, secondary pollutants on the other hand are indirectly emitted (forms in the air when primary pollutants react or interact) into the atmosphere by chemical reactions.

4.0.2 SOURCES OF AIR POLLUTION

This refers to those locations, factors or activities responsible for the emission of pollutants into the air. The sources are grouped into two broad categories which are:

Natural sources- production of particulate ash, chlorine and sulphur volcanic activities; emission of methane by the digestion of food by animals; dust from natural sources like lands with little or no vegetation lacking; carbon monoxide and smoke from wildfires; and radioactivity of radon with the earth crust.

Anthropogenic sources- includes human activities such as burning different kinds of fuel; burning wood, stoves, fireplaces, incinerators and furnaces; smoke stacks of power plants, municipal waste incineration and manufacturing facilities; and oil refining and general industrial activities.

4.0.3 POLLUTION BY SO2.

Pollution of the atmosphere by sulphur dioxide has been ongoing for most part of the earths history. However, concern heightened over SO2 as a modern pollutant due to its increased concentration which is in turn due to increased industrialization in most developed parts of the world(Folinsbee,1992).

Sulphur dioxide is a colourless, toxic, dense, non-flammable gas with a characteristic suffocating and pungent odour that penetrates and irritates the eye (Institute of Medidicine,2007). It is a common combustion pollutant mostly given off by coal burning power plants. This is because some coal is contaminated with sulphur which when burnt is released into the air. It is heavier than air, readily soluble in water and belongs to family of gases called sulphur oxides (SOx). In the presence of moisture in the air, sulphur dioxide dissolves into the moisture giving rise to acid rain.

Though releases from biological decays, volcanoes, oceans and forest fires are natural means by which SO2 is released into the atmosphere, human activities such as combustion of fossil fuel, conversion of wood pulp to paper, smelting, production of elemental sulphur, incineration of waste (which is the case under investigation here) and coal burning has long been recognised as the major pollutants of air with coal burning being identified as the single largest anthropogenic source of SO2 that accounts for about 50% of annual global emissions and burning of oil accounting for a further 25-30%.

Electric Power Research Institute reports that in the United States, fossil fuel-fired power plants contribute 66% of all anthropogenic sources. Other significant emissions are from the industry and transportation sectors with recorded 29% and 5% emissions respectively.

The actual composition of emissions being released into the atmosphere vary seasonally depending on the location and on which waste that is being incinerated at any given time. In Phoenix for instance, SO2 peaks are usually recorded during the summer whereas in Philadelphia peaks are recorded during winter season (Electrical Research Power Institute report on health effects of sulphur dioxide).

The case reviewed here is the release of SO2 into the atmosphere by the process of H2S incineration. Incinerators, often sited in or near urban areas, are known to release SO2 amongst other substances like dioxins and furan. Having in most cases served its purpose, it still has led to increase public concern because of the great deal of harm it constitutes to the environment and atmosphere inhabited by humans along with other living things. Even with the use of modern incinerators that are equipped with rigorous pollution control technology that reduces the emission of hazardous substances, total elimination of pollutants into our surroundings cannot be completely ruled out.

4.1 ENVIRONMENTAL CONSEQUENCES

To evaluate the magnitude of health and environmental impact of SO2 emissions from incinerators, dispersion modelling using the Pasquil-Gifford method which takes into consideration the height of the stack (incinerators height), distance from source and the velocity of the wind that transport the plume was adopted here.

Results obtained from this have shown varying levels of concentration of the pollutant in the atmosphere. The concentration levels as it affects human beings and the environment deduced from the figures of concentration plots above are discussed below.

Concentration levels and its effects vary depending on the stack height, distance from source and the nature of material being incinerated. Areas near the source of emission are usually known to be most affected and as such considered hazardous to life support and the environment. Such areas are usually unsafe for habitation by human beings as constant inhalation of the SO2 gas leads to diverse health related problems. Areas with low concentrations will in the long run due to accumulation over a long period of time have its characteristic health effects as well.

The effect of exposure and inhalation of sulphur dioxide can be summarised below.

4.1.1 ACUTE TOXICITY TO HUMANS

The health of countless people is ruined or put in danger every year by SO2 pollution. Air pollution is responsible for major health effects. The lungs are particularly most susceptible to the acute and chronic effects of poor air quality due to pollution by SO2. Studies have estimated a record annual death of over 50,000 people in the US alone ( U.S Environmental Protection Agencys).The degree to which how people get affected depends on the concentration and length of time of inhalation of the pollutant. SO2 affects the health of humans when it is inhaled. Its effects are immediately felt with most people being reported to feel the worst symptoms some 10 15 minutes after inhalation. In cases where people have been exposed to very high concentrations, hypoxemia (a condition of insufficient blood oxygenation), pulmonary edema (life threatening accumulation of fluid in the lungs) and deaths in minutes has been caused. Other acute effects to high level concentrations include chest tightness, breathing problems, narrowing of lung air passages (bronchoconstriction) with a resultant wheezing effect, skin damage and permanent eye damage. Symptoms increase as sulphur dioxide level increases and/or breathing rate increases.

4.1.2 LONG-TERM EFFECTS ON HUMAN HEALTH

Repeated long-term exposure to lower levels can be hazardous to the health. Changes in lungs function can result in workers at incineration plant who are exposed even to levels of sulphur dioxide over a long period of time. Long-term exposure can also result in respiratory disorders, decrease pulmonary function, alteration of the lungs mechanism of defense and aggravation of existing cardiovascular diseases. Other effects include reduced fertility, phlegm and temporary loss of smell. People suffering from cardiovascular diseases like the asthmatics, alongside older people and children are vulnerable to to air pollution induced diseases.

4.1.3 LONG-TERM EFFECT ON THE ENVIRONMENT.

Long-term release of SO2 has created perturbation in the ecosystem. These pollutants can cause acid rain when it reacts with moisture to form acid rain that adversely affects the ecosystems.

On a local environmental scale, sulphur dioxide is very toxic to a wide variety of plant life producing visible signs of injury and/or reduced yields in certain crops. Ironically, beneficial effects are sometimes seen in some plants where SO2 has reduced the incidences of fungal diseases- the incidence black spot on roses is low in areas with high level of SO2. Lichens are known to be good bio-indicators of pollution as they do not like to grow in areas polluted by sulphur dioxide. If released in appreciable amount at ground or low levels, this pollutant can combine with moisture to cause gradual damage to historic monuments, buildings, various fibres, paper, electrical components, and corrosion of metals.

SO2 affects the global environment by forming acid rain. Acid rain affects the natural balance of rivers(freshwater acidification), degradation of soils, wildlife and vegetation damage.(Delmelle et al,2002).

4.2 PROCESS IMPROVEMENT

Air pollution due to SO2 emissions has many disastrous effects that need curbing. To achieve this, environmentalists as well as scientists and the government are using or testing a variety of methods targeted at reducing pollution.

Air pollution can be curbed in two ways- input control and output control. Input control prevents a problem before it occurs or atleast limit the effect that will be produced by the process. Key input control methods that exist include restricting population growth, improved energy efficiency, moving to non-polluting renewable forms of energy production, using less energy, and reducing waste production by re-using and recycling materials.

The opposite of this, which is the output control method, seeks to fix problems resulting from air pollution. This often means cleaning an area that has already been damaged by air pollution.

Input controls are often more effective than the output controls and less expensive making them less desirable especially to polluting industries.

The following ways should be adopted as means of reducing the amount of emissions from incinerators into the air.

  • residue from sulphur refinery can be used in steam plants instead of being discharged indiscriminately into the atmosphere.
  • waste hydrogen sulphide gas from the refinery and elemental sulphur from other refinery sour gas can be used to produce sulphuric acid.
  • plans for incineration installations must include increased stack height so that the effect of the resulting emissions will not be felt immediately.

5.0 CRITICAL REVIEW

The aim of any modelling exercise is often if not all the time targeted at determining the significance of the effects of pollutants released from a given source. The results according to the National Institute of Water and Atmospheric Research in its report on the good practice for atmospheric dispersion modelling, must be reported effectively and concisely in a manner suitable for the purpose for which they were produced. This implies that the results must be communicated in such a manner that can be understood by other people who may not be experienced in interpreting model output. Two elements to this exist: firstly, to report the modelling results in an easy-to-understand manner; and secondly, to evaluate the implications of the results in terms of the potential effects of the predicted ground-level concentrations on peoples health and the environment also in a manner that would be easily understood.

Based on this, the following can be said about the various scenarios considered here.

SCENARIO 1

Graphical results obtained here shows that the concentration is highest 400m away from the source of the SO2 discharge and only reduced slightly as the downwind travelled in the x- direction. The effect of this high concentration experienced even up to a 5000m from the source will be highly critical. With concentration values reaching 896g/m3, the area will completely be unable to sustain humans other living organisms and the ecosystem. Even at 10000m away from the source where concentration level is 28 g/m3, humans will be risk due to long term exposure to this level of concentration.

However there will be no SO2 pollution effect on people at a farther distance from 200m because at such distance the SO2 concentration was at zero level.

SCENARIO 2

In like manner to scenario 1, concentration values obtained at various distances away from the source are still very much on the high. The effect will still be very dangerous even at 10000m away from the source area.

SCENARIO 3

At distances between 1500m and 4000m, the concentration of SO2 emission exceeds recommended safety level expected to support human life. This implies that human being and the ecosystem at these distances from the source will immediately experience the effect of this level of concentration even at short term.

SCENARIO 4

Unlike the first 3 scenarios, emissions of SO2 from the incinerator will have no effect on people and the ecosystem even upto 10000m away. However, the effect that constant exposure and inhalation of this pollutant will have on inhabitants of this area in the long term should not be undermined build up of gas over say a period of ten to twenty years might not be very pleasing.

A critical evaluation of the general trend obtained from the considered scenarios shows that the area has got very high concentration level even at 10km away. This is due to the prevailing conditions of high wind velocity which transport the gas further in the x-direction and the low stack height used to release the gas into the atmosphere.

5.1 ACCURACY OF RESULTS

One of the common criticisms of dispersion modelling is, Its not at all accurate - its only a model. Three main sources of errors in atmospheric dispersion modelling are identified by the NIWAR (National Institute of Water and Atmospheric Research, Aurora). These are:

  • inaccurate input data
  • inappropriate use of the model (or expecting too much from it)
  • poor performance of the model itself.

The sum of uncertainty contained in a model result is the cumulative effect of these sources.

The presented in this report result is likely to contain one if not all of the above listed sources of errors as it did not follow key guidelines proposed by NIWAR for designing atmospheric dispersion modelling. These guidelines include:

  • showing that the model inputs are as correct as possible
  • knowing and stating the model performance limits
  • showing that the modelling has been carried out appropriately, and
  • including any validating information that might be available.

The Pasquil-Gifford formula used here is an effective tool for atmospheric dispersion modelling because it is a probabilistic model that assumes the Gaussian steady state plume model giving details on emissions (Qp, H), weather conditions( u, sy, sz) and predicts concentration levels in all values of x, y, and z.

Gaussian-plume models are easy to apply, well understood, widely used, and until more recently have received international approval (NIWAR). This notwithstanding, they may not always be the best models to use as they have some recorded restrictions associated which include:

  • at low wind speeds or conditions of calm, Gaussian models breakdown as a result of inverse wind speed dependence nature of steady state plume equations.
  • In modern terrains, Gaussian models will overestimate impingement effects at stable conditions as they do not account for the rising or turning wind caused by the terrain.
  • This model has no memory of calculating the hourly ground level concentration released in the previous hour(s). this particular limitation is very important in the proper simulation of morning inversion break-up, fumigation and diurnal recycling of pollutants over cities.

In order to ensure reliable data production, the use of other less restrictive models likes the advanced models is encouraged. The use of such advanced models like CALPUFF and TAPM (The Air Pollution Model) avoids most of the limitations listed above although their demands on resources (human, computational and data) usually far outweigh those of Gaussian-plume models.

6.0 SUMMARY/CONCLUSION

Air dispersion modelling is aimed at predicting the manner of pollutant dispersion in the atmosphere from different sources taking into consideration the height of the emission source , the degree of turbulence, the rate of emission, meteorology, downwind distances, terrain amongst other factors. Dispersion models are an important planning tool because of their ability to evaluate a variety of options for managing air quality.

Constant tolerance of practices that allow the discharge of pollutants from both hazardous and perceived harmless waste into the air, soil and water will only worsen the problem of pollution which will in turn take its toll on the health of human beings, the environment and its suitability for existence.

It will be applaudable if scientists, environmentalist and the government develop ways by which this menace of air pollution and environmental degradation can be curbed.

High sulphur refinery residue can be used in steam plants. The waste hydrogen sulphide gas from the refinery and elemental sulphur from other re finery sour gas can be used to produce sulphuric acid.

REFERENCES

Chin et al (2000), Atmospheric sulphur cycle simulated in the global model GOCART: in journal of geophysical research. Vol. 05, No.D20, pp 24,9671-24,687

Delmelle, P. et al (2002), Atmospheric dispersion, environmental effects and potential health hazard associated with the low-altitude gas plume of Masaya volcano,Nicaragua: in bull volcanol. Vol. 64, pp. 423-434.

Follinsbee, L. J (1992), Human health effects of air pollution: in journal of environmental health perspective, vol.100, pp 45-56.

Harrop, D. O (2002), Air quality assessment: a practical guide

IOM (Institute of medicine) (2007), Long term health effects of participation project in Shipboard hazard and defense. Published by the National Academy press, Washington D.C

National Institute of Water and Atmospheric Research: good practice guide to atmospheric dispersion modelling.

Think request (no date). The Environment, a global challenge. Available from http://library.thinkquest.org/26026/index.php3 (Accessed on 28 February,2010)

U.S. Environmental Protection Agency (1986), Guideline on air quality models (Revised), document, EPA 450/2-78-027R.

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