Potential Environmental Impacts of Vapours

Transcription

1 Potential Environmental Impacts of Vapours Learning Outcome When you complete this module you will be able to: Explain the impact of gases and vapours on the environment. Learning Objectives Here is what you will be able to do when you complete each objective: 1. List the common domestic, industrial, and naturally occurring gases and vapours that have environmental impact. 2. List the sources of the common gases and vapours. 3. Describe the current methods of gas and vapour conditions and disposal. 4. Describe alternative methods of reducing gas and vapour pollution. 1

2 GASEOUS POLLUTANTS The gases and vapours responsible for environmental pollution are numerous and the full impact of individual gases in combination with other gases and elements is not thoroughly understood. This module will emphasize gases that are responsible for acid rain and the so called greenhouse gases. Pollutant Sulphur oxides Nitrous oxides Carbon dioxide Carbon Monoxide Methane Chlorofluorocarbons Ozone Problem produced Responsible for about 70% of acid rain. Responsible for about 30% of acid rain, produce smog and trap heat that increase greenhouse effect The main heat trapping gas responsible for the greenhouse effect A very poisonous gas. A green house gas. Destroy ozone after rising to the stratosphere Hazardous to health and corrosive to many materials in the lower atmosphere Table 1 Common Gaseous Pollutants HOW POLLUTANT GASES ARE PRODUCED Sulphur Oxides Sulphur oxides [sulphur dioxide (SO 2 ) and sulphur trioxide (SO 3 )] are produced whenever a fuel containing sulphur is burned. Sulphur is contained in most coals and fuel oils used by industry. Low sulphur fuels are much more expensive than high sulphur fuels hence the use of high sulphur fuel has a great advantage from an economic standpoint. Some mining and smelting operations discharge great quantities of sulphur oxides to the atmosphere. Pulp mills are also a source of sulphur oxides. Sulphur oxides also enter the atmosphere from natural sources. Some hot springs emit sulphur oxides, and during a volcanic eruption millions of tons of sulphur oxides may be introduced into the atmosphere. 2

3 Nitrogen Oxides (NO x ) Oxygen and nitrogen can react to form several different compounds; nitric oxide (NO), nitrous oxide (N 2 O), nitrogen dioxide (NO 2 ), nitrogen trioxide (N 2 O 3 ), and nitrogen pentoxide (N 2 O 5 ) are some of the possibilities. These gases have varying degrees of stability and are referred to collectively as NO x. In many cases more than one of these gases is given off by a single source. Nitric oxide (NO) and nitrogen dioxide (NO 2 ) are the major NO x gases formed in the combustion process (e.g. in boilers and internal combustion engines. NO 2 formation is favoured at temperatures below 1000 K, while NO formation is favoured above that temperature. The total NO x formed during a combustion process increases with an increase in temperature and an increase in excess O 2 in the burning zone. Usually more than 90% of the NO x formed in the combustion zone is NO. Some of this NO then combines with the excess O 2 to form NO 2. This latter reaction occurs as the temperature of the exhaust gases drops below 1000 K. The rate of reaction slows until it ceases as temperatures approach ambient, and some of the NO will decompose back to O 2 and N 2 before being discharged from the exhaust system. Some decomposition of NO does occur slowly in the atmosphere. NO and NO 2 convert back and forth as components in the photochemical smog problem. When water is added, NO 2 is a contributor to the formation of nitric acid (HNO 3 ), which accounts for roughly 30% of the acidity in most acid rain. Carbon Dioxide (CO 2 ) Any time a fossil fuel is burned CO 2 is produced. It is one of the most important gases on our planet because all plant life depends on CO 2. All animal life produces CO 2 and forest fires are also large contributors of this gas. CO 2 cannot be eliminated from the combustion process as long as the fuel contains carbon. Carbon Monoxide (CO) Carbon monoxide, an extremely poisonous gas, is produced in any combustion process that does not have sufficient air. One of the greatest producers of CO is the internal combustion engine because there is insufficient time for complete combustion to take place. Overloaded boilers, forest fires, and burning refuse can also produce CO. Methane (CH 4 ) Methane is the main component of natural gas. It is a product of decomposed organic matter such as in swamps and sewage lagoons and is formed in the digestion processes of animals, especially ruminants. 3

4 Chlorofluorocarbons (CFCs) These are man-made products that are used as refrigerants, as a foaming agent for some plastics, as propellants in aerosol sprays, as cleaning agents and in Halon fire extinguishers. Ozone (O 3 ) Ozone is a form of oxygen which contains three oxygen atoms or O 3. In the upper atmosphere, O 3 is formed by the action of high intensity sunlight on O 2. Only a small percentage of the resulting ozone works down to the lower atmosphere and ground level. The upper level ozone is of critical importance as it shields the earth s surface from hazardous ultraviolet and other radiation. CFCs not only produce a greenhouse effect, but they also destroy this upper level ozone layer. Ozone at the lower levels is produced by electrical arcs such as from brushes in electric motors, arc welding, and lightning. The action of sunlight on a mixture of NO x and volatile organic compounds (frequently referred to as photochemical smog) also produces ozone in this zone. ACID RAIN Acid rain is formed when NO x and SO 2 in the atmosphere undergo complex and little understood changes in a medium of water vapour and sunlight. The resulting interaction produces sulphuric acid and nitric acid which precipitate with rain, snow, hail, sleet, fog, and in some cases deposit on the ground in a dry form. Since the acid can come down in either a wet or dry form, the condition is better referred to as acid deposition. The contaminants which form acid rain can be carried great distances from their source. The smogs of California deposit their pollution in the snows of Colorado. The snows melt and the acid runoff leaches hazardous minerals from the mountains. Some of the meltwater returns in the rivers as a water source for California. Effects of Acid Rain on Lakes and Rivers When the acid level increases in a lake, microscopic life dies and interference with the reproductive cycle of aquatic animals becomes evident. Metals leached from the soil, in higher concentrations than normal, restrict the ability of some animals to breathe properly, for example, fish gills become fouled and the fish suffocate. 4

5 The first melt of spring releases a surge of acid from the snow. In this toxic meltwater many eggs and hatchlings cannot survive. In lakes and streams, clams, snails and crayfish are first to die, then insects such as the mayfly, dragonfly, damsel fly and other larvae. Then by acid attack or starvation, the amphibians and fish die off. In Ontario, Quebec, and the Northeastern United States about 2500 lakes per year are dying. In other parts of the world acid rain is also a problem; for instance in Sweden acid rain is blamed for as many as dead lakes. Effects of Acid Rain on Plant Life Forests are being killed by acid rain. Some problems are directly created by acid deposition on the leaves, however the greater problem appears to be the effects of acid deposition on the soil. Nutrients normally dissolved and used by the tree roots have either been leached away and lost, or toxic quantities of other materials are taken into solution by the acidic groundwater. These materials interfere with the plants ability to acquire proper nourishment. The result is that plants are weakened and die. We must realize that these effects impact on all plant life, not just the forests. Effects of Acid Rain on People Acid rain causes both economic and health impacts. The effect on maple trees in Eastern Canada and the Northeastern United States is resulting in a diminishing maple sugar industry. Logging operations are being curtailed, and reforestation efforts are futile in heavily impacted areas; fruit trees produce substandard fruit, or die off; vegetables become discoloured and unhealthy, making them difficult to market; fish from certain waters may be banned for human consumption due to the impact of acid rain on the watercourse. The full extent of the economic impact of acid rain is difficult to assess, but the above examples provide some indicator of how widespread the impact may be. The effects of acid rain on human health are varied. Acid water can leach toxic heavy metals from the soil, as well as copper and lead from plumbing systems causing a toxic hazard to humans. Studies suggest that sulphur dioxide in the air causes bronchitis, emphysema, and a strain on the heart and circulatory system. Some recent reports suggest a possible relationship between acid haze and an increase in some types of cancer; probably due to the blocking of sunlight at the wavelength needed by humans to produce Vitamin D in the body. 5

6 THE GREENHOUSE EFFECT The earth has existed for a considerable period of time in an equilibrium state, where the amount of energy received from the sun is balanced with an equal amount of energy radiated from the earth back into space. Some high energy radiation incoming from the sun (such as X-rays and high energy ultraviolet light) encounters upper atmosphere ozone and is absorbed or reflected back into space. Slightly lower energy radiation (such as visible light and infrared radiation) passes through the atmosphere to ground level. At ground level, this radiation is largely absorbed, and eventually radiated back towards space at a lower energy level. Some gases known as the greenhouse gases will allow high energy radiation to pass through but restrict the transmission of the lower energy radiation that would normally move towards space. The relatively stable concentration of greenhouse gases accounts for the earth s overall average temperature being reasonably stable for many centuries. With an increase in greenhouse gases, more outgoing radiation is trapped by the atmosphere causing an increase in average temperatures. The increasing average temperatures can cause drought conditions in presently fertile growing regions; and polar ice masses may be reduced, causing rising ocean levels which would impact severely on man s activities. AI5_fig1.gif Figure 1 Greenhouse Effect 6

7 GREENHOUSE GASES Water Vapour The water vapour content of the air depends on its temperature. The warmer the air the more moisture it can hold and water vapour can hold a tremendous amount of heat. By itself it cannot produce a greenhouse effect but water vapour amplifies the effect of other gases. Carbon Dioxide Carbon dioxide is the principal green house gas, accounting for about 50% of the total problem. The amount of CO 2 in the atmosphere is estimated to be increasing by about 1/2% per year. That may not sound like much but it means an addition of 5 x 10 9 tons of carbon/year. About one-half of that carbon is used by plants or absorbed by the oceans, and the rest stays in the atmosphere. Methane Methane is believed to comprise about 20% of the greenhouse gas problem. The carbon in methane makes it a heat trapping gas like carbon dioxide, and in fact it has about thirty times the activity of CO 2. Although the greenhouse effect of methane is much higher than that of CO 2, methane has an estimated half-life of about 7-10 years compared to 500 years for CO 2. Some scientists are concerned that methane is responsible for preventing the atmosphere from ridding itself of CFCs. Chlorofluorocarbons CFCs contribute about 15% of the greenhouse gas problem. CFCs are very stable man-made compounds that help destroy ozone after they reach the stratosphere. Ozone shields the planet from harmful cosmic radiation that can cause cancer. In the lower atmosphere CFCs are able to absorb infrared rays about 10,000 times as effectively as carbon dioxide making them powerful greenhouse gases. Nitrous Oxide Nitrous oxide (N 2 O) (laughing gas) is very stable, lasting an estimated 150 years or longer in the atmosphere. This gas is one of the gases contributing to the green house effect, accounting for about 10% of the problem. When it rises to the stratosphere it is responsible for destroying ozone. Nitrous oxide levels in the atmosphere are estimated to be rising at a rate of 0.25% per year. 7

8 Ozone Ozone in the lower atmosphere is a pollutant that is responsible for about 5% of the greenhouse effect. This gas is an irritant to the eyes and respiratory system. In the stratosphere, ozone is very beneficial because it blocks out cancer causing ultra violet radiation. In the lower atmosphere it is a hazard in that it is dangerous to health and corrosive to many products including rubber. WHAT IS BEING DONE TO REDUCE HARMFUL GASES? Sulphur Dioxide Removal At this point the student may wish to refer to Learning Module ENVS 6011 ( ), Gaseous and Noise Pollutants, for additional information on the removal of sulphur dioxide from stack gases. Many plants have met environmental regulations by utilizing naturally occurring low sulphur, western source coal. Others have achieved the required emission levels by installing flue gas desulphurization (FGD) systems. The FGD systems are categorized as nonregenerable or regenerable. Nonregenerable systems are the best developed and most widely applied. Most of the processes involve wet scrubbing of the combustion flue gas in a gasliquid contractor using lime or limestone, alkaline fly ash with supplemental lime or limestone, sodium carbonate and dilute sulphuric acid as the scrubbing medium. A typical limestone system is shown in Fig. 2. Efficiency for these systems can be as high as 90 to 95% with combustion gases containing up to 5,000 ppm SO 2. A regenerable FGD system is shown in Fig. 3 its advantage being that the SO 2 recovered is converted into a marketable by-product such as sulphur, sulphuric acid, or liquid SO 2. These units have a higher initial cost, some of which may be recovered by the sale of the by-product. Some of these processes have been installed in systems over 100 MW capacity. Wet scrubbing units use a solution of magnesium oxide to remove the SO 2. 8

9 AI5_fig2.gif Figure 2 Limestone Wet Scrubbing System (Courtesy of Babcock & Wilcox) AI5_fig3.gif Figure 3 Regenerable Wet Scrubbing System (Courtesy of Babcock & Wilcox) 9

10 Oxides of Nitrogen Unlike sulphur oxides, which are formed only from the sulphur contained in the fuel, nitrogen oxides (NO x ) are formed from both fuel-bound nitrogen and nitrogen contained in the combustion air introduced into the furnace. The NO x produced from nitrogen in the fuel is dependent on the: (i) Percent of nitrogen in the fuel. (ii) Oxygen availability in the combustion zone. (iii) Reactivity of nitrogen compounds contained in the fuel. The conversion of nitrogen in the air to NO x is highly dependent on temperature. The formation of NO x proceeds rapidly at combustion zone temperatures in excess of 1650 C. By maintaining the combustion temperature below 1650 C, the NO x emission can be reduced. There are several methods of reducing the formation of NO x as discussed in the following sections. 1. Two Stage Combustion This is the most effective method of reducing the formation of NO x. Initial combustion takes place in a fuel rich environment. The remaining air for combustion, and excess air, is introduced in an area away from the initial combustion zone to complete the combustion process. By reducing the availability of oxygen in the combustion zone the fuel-bound nitrogen is less likely to be converted to NO x. This process also prolongs combustion thus reducing the flame temperature which reduces the conversion of atmospheric nitrogen to NO x. 2. Low Excess Air Operation By firing with the least amount of excess air as possible, while still maintaining carbon losses to a minimum, the O 2 available is reduced. This means that the amount of fuel-bound nitrogen converted to NO x is reduced. The amount of NO x formed due to nitrogen in the air is also reduced due to the lower availability of O 2 and reduced quantities of nitrogen entering the combustion zone. 3. Gas Recirculation On gas- and oil-fired units, gas recirculation into the combustion zone provides dilution of the combustion air thus prolonging combustion and reducing the flame temperature. Since all the NO x from gas-firing and 50% of the NO x from oilfiring are produced by thermally converted nitrogen, gas recirculation to the burners appears to be effective on overall NO x reduction. With coal-firing the effect of this method of NO x reduction is minimal. 10

11 4. Dual Register Burner Fig. 4 shows a dual register burner developed by Babcock and Wilcox. This burner limits turbulence thus reducing the peak flame temperature at the burner. The reduced turbulence also delays combustion producing a slower burning flame in the combustion zone. While maintaining stable efficient operation the burner has demonstrated a 50% reduction in NO x over previous high turbulence burner designs. Utilizing this type of burner in conjunction with low excess air operation has produced an acceptable reduction of NO x. Figure 4 Dual Register Burner (Courtesy of Babcock & Wilcox) Diluting Pollution To reduce pollution in the immediate area of stack emissions, super stacks have been built. The local pollution has been reduced because the emissions are discharged higher into the atmosphere. This solution of the problem has only spread the problem over a greater area and has not reduced the amount of gases that have adverse environmental impacts. 11

12 ALTERNATIVES Solar Energy Solar energy is a nonpolluting source of energy. The energy is free but harnessing it is expensive. It was reported in 1989 in the New York Times that the conversion of solar energy to electrical power could become comparable in efficiency to conventional power generation. Tremendous gains in the development of solar cells has taken place in the last few years and if fossil fuel prices dictate, more research will take place. It is possible to reduce SO 2 and NO x emissions to an acceptable level, if laws are made strict enough, but as long as we burn fossil fuels the amount of CO 2 in the atmosphere will continue to increase. Hydrogen Fuel Using hydrogen as a fuel is not a new idea, but the methods of production, storage, and handling must be refined before it receives wide acceptance. Hydrogen is an ideal fuel in that the only product of combustion is water vapour. A kilogram of hydrogen delivers about 4.25 times more energy than a kilogram of carbon when burned completely. Hydrogen is often stored in cryogenic containers to keep the storage pressures from being unreasonably high. Even a small hydrogen leak poses an explosion hazard due to the wide range in the explosive limits. Hydrogen rich fuels such as methane and propane contribute less CO 2 pollution to the air than regular gasoline or diesel fuels. A greater usage of these fuels would reduce equipment costs, but the higher demand would likely result in a price increase. Carbon Dioxide Removal From The Atmosphere Sometimes we cannot see the forest for the trees. Many scientists are urging mass reforestation of the earth. Trees require CO 2 and it is estimated that 10,000,000 acres of new forest would use up all the CO 2 that would be emitted by power plants in the next 10 years. There are solvents that can remove CO 2 from power plant emissions but the process is cost prohibitive at the present. The CO 2 removed using this method could be injected into oil fields to maintain formation pressure. CFCs It appears that these man made substances will be phased out and alternate refrigerants will be used. Possibly refrigeration systems like the ammonia and lithium bromide absorption systems will be given more research and development. These systems produce no pollution and can be energy savers by making use of low level waste heat that is otherwise dumped into the atmosphere. 12