2.4 Low Level Environmental Impacts

This section discusses low level (i.e. near the earth's surface) environmental impacts that are associated with thermal energy production. Such low level impacts include acid rain, particulate matter deposition and local air quality concerns such as urban smog.

Acid Rain is created through the process of acidification of natural precipitation by oxides of sulphur (SO,,) and nitrogen (NOJ, both Foes of fossil fuel combustion. SO_x is formed during combustion through the oxidation of sulphur and sulphur compounds present in the fuel. SO_x formation depends almost exclusively on the amount of sulphur in the fuel, as opposed to such factors as burner design or combustor capacity. Commonly used fuels, coal and oil, contain small but significant percentages of sulphur, while natural gas contains only trace levels. Historically, 90% or more of the sulphur present in these fuels has been released to the atmosphere during combustion. Thus, even though the weight percentage of sulphur in coal and oil fuels is relatively small (normally less than 2.5% and often below 0.5%), the total emissions are significant because of the significant quantities of coal and oil that are burned throughout the world to produce power and heat energy.

NO_x is formed during the combustion process through the oxidation of both atmospheric nitrogen (the combustion air is 80% nitrogen) and nitrogen contained in the fuel used. The rate of formation of NO_x during combustion depends on many factors including combustion chamber temperatures and oxygen levels, the degree of turbulence and/or the extent of stratification of the combustion air, fad and combustion products in the combustion chamber, and the combustion products cooling rate within, and downstream of, the combustion chamber. Each of these factors are affected by, or resulted from, specific system characteristics such as the type and size of combustor, the fuel being burned, and the actual operating conditions at the time. NO_x emissions are formed during combustion of coal, oil and natural gas.

Once released to the atmosphere, the SO_x and NO_x emissions chemically react with moisture in the air and can then return to earth as acidified precipitation . This precipitation may be in the form of rain, snow, fog or mist. The acidified precipitation is not restricted to the proximity of the emission sources. These emissions can be carried a considerable distance by prevailing winds before reacting with moisture laden air and before the precipitation event is experienced.

The impacts associated with acid rain are significant, affecting flora and fauna. Acid rain has resulted in major ecological damage to, and even the "death" (with the destruction of life form habitats) of, thousands of lakes around the world. Forests and crops have also been extensively damaged and their continued existence threatened by acid rain effects. In addition, acid rain causes damage to man-made materials, modern and historical buildings, monuments, etc. Evidence also indicates that acid forming air pollutants can contribute to respiratory problems in children and other susceptible groups.

Important steps have recently been initiated by many of the industrialized nations to minimize the emission of acid rain causing air pollutants. These initiatives take the form of commitments to reduce and/or avoid any further increase of SO_x and NO_x emissions considering both existing and new sources.

Although the depletion of ozone in the upper atmosphere has been linked to negative environmental impacts, as discussed in Section 2.3, elevated low level ozone levels (i.e. occurring normally in the lower atmosphere), have also been identified as a pollution problem. Ground level ozone is a major component of what is commonly referred to as smog. Smog describes conditions that negatively impact the local air quality and are of concern to public health in many densely populated areas. Ozone related smog is normally associated with the urban environment with the increased concentration of both mobile and fixed emission sources and is therefore often termed urban smog. However, since the pollutants necessary to the formation of ozone can be transported downwind to rural areas, rural smog Is also a concern, albeit In most parts of the world, to a lesser degree.

Low level ozone Is primarily formed by photochemlcal reactions with two pollutants; volatile organic compounds (VOCs) and oxides of nitrogen (NO_x).

VOCs are emitted from both human related and natural sources with the latter being the major contributor in rural and open spaces. The former predominates in the urban environment. Human related sources of VOCs include, among others, combustion of fossil fuels, although thermal energy production facilities are not significant contributors.

Elevated ozone levels, and hence smog levels, are known to have adverse effects on human health, vegetation and materials. Human health concerns centre around respiratory ailments such as coughing, decreased lung function and premature aging of the lungs. Impacts on vegetation usually occur as damage to foliage, resulting in problems with plant growth and productivity. Observed man-made material damage associated with low level ozone include hardening of rubber materials and bleaching of paints.

Control and reduction of low level ozone concentrations is being pursued by some industrialized nations, through the establishment of VOC and NO_x emission reduction targets.

| Basics of District Heating and Cooling |