4th Class Edition 3 Part A Gas & Noise Emissions PDF

Unit A-5 • Introduction to Plant Operations and the Environment Objective 1 Identify the sources and effects of common gases and vapours that have an adverse environmental impact. The Atmosphere The Earth’s atmosphere is divided into various layers. Each layer is comprised of various naturally occ...

Unit A-5 • Introduction to Plant Operations and the Environment Objective 1 Identify the sources and effects of common gases and vapours that have an adverse environmental impact. The Atmosphere The Earth’s atmosphere is divided into various layers. Each layer is comprised of various naturally occurring gases. Some gases, such as stratospheric ozone, protect the Earth. Others, such as CO2 and O2, directly support life. The following diagram shows some of the layers of the atmosphere. Though the layers are bounded by transition zones, each layer is recognizable by its own unique characteristics. The transition zones have characteristics of the layers on both sides of the transition. Figure 1 – Layers of the Atmosphere The outermost layer is called the exosphere. It extends from about 500 to 10 000 km from Earth. The next closer layer is the thermosphere. This is the location of the Northern Lights. This layer absorbs much of the solar X-ray and UV radiation that the Earth receives. Temperatures in the upper thermosphere can range from about 500 to 2000°C or higher. This layer extends from about 85 to 690 km from the Earth’s surface. The mesosphere is found between the thermosphere and the stratosphere. It is about 50 to 85 km from the Earth’s surface. Meteors and other objects travelling through the mesosphere heat up by interacting with the thickening atmosphere. Some material from meteors lingers in the mesosphere, causing this layer to have a relatively high concentration of iron and other metal atoms. Temperatures in the mesosphere drop with increasing altitude from around 0°C to about -100°C at its upper limit. 2-4 4th Class Edition 3 • Part A Gas and Noise Emissions • Chapter 2 The stratosphere is about 20 to 50 km above the surface of the Earth and is located between the troposphere and the mesosphere. The stratosphere is warmer at higher elevations than at lower elevations. Temperatures increase from about -60°C closer to the Earth to about 0°C at its outer region. This is due to absorption of ultraviolet (UV) light from the sun during the production of ozone. Ozone is created when ultraviolet rays react with oxygen molecules (O2) to create ozone (O3) and atomic oxygen (O). This process is called the Chapman cycle. Thus, harmful UV radiation is absorbed in the production of ozone. The layer of the stratosphere where ozone accumulates is called the ozone layer. Recently, scientists have reported a noticeable depletion of ozone levels. This has been attributed to the action of chlorofluorocarbon based refrigerants (CFCs). Scientists claim that this depletion reduces the ozone layer’s ability to protect the Earth from harmful UV radiation. Ozone in the stratosphere is beneficial to the Earth. However, ozone that occurs at the lowest layer, the troposphere can trigger a variety of health problems, particularly for children, the elderly, and people with lung diseases such as asthma. This ground level ozone is not emitted directly into the air. Rather, it is created by chemical reactions between nitrogen oxides (NOX) and volatile organic compounds (VOC) in the presence of sunlight. Industrial emissions, motor vehicle exhaust, gasoline vapors, and chemical solvents are some of the major sources of NOX and VOC. Low level ozone is also made by lightning and electrical equipment. The troposphere is found at the surface of the Earth. It reaches about 7 to 20 km in height. Its greatest thickness is near the equator and its least thickness is at the poles. Most atmospheric physical activity and almost all atmospheric water exists in the troposphere. Gaseous Pollutants Scientific research continues to raise awareness about the various gaseous pollutants that human activities emit, and concern about how these gases affect human health and the environment. Governments respond by enacting legislation and tightening emissions requirements. Researchers respond with technological advances in equipment and processes so that industry can comply with new regulations. Industry responds by using newer technology and processes to produce fewer or less harmful emissions. Energy plants must comply with government regulations. Therefore, they must limit the amount and duration of gaseous emissions. One of the Power Engineer’s most important responsibilities is to ensure compliance with regulatory requirements. The gases and vapours that are responsible for environmental pollution are numerous. Below are some of the most common gaseous pollutants addressed by jurisdictional regulation. • Carbon Monoxide • Carbon Dioxide • Sulfur Oxides • Nitrogen Oxides • Methane • Ozone • Chlorofluorocarbons • Fluorinated Gases Power Engineers must be aware of the effects of these pollutants, how to detect them and, most of all, how to control them. This objective will describe the pollutants mentioned above. Control measures will be discussed later in this chapter. 4th Class Edition 3 • Part A 2-5 Unit A-5 • Introduction to Plant Operations and the Environment Carbon Monoxide (CO) Carbon monoxide is an extremely poisonous gas. It can be produced by any of the following: a) When combustion occurs with insufficient air b) Insufficient mixing of fuel with air c) Insufficient time for combustion to be complete One of the greatest producers of CO is the internal combustion engine, because it does not provide enough time for complete combustion in the engine cylinder. Forest fires, burning refuse, and poorly tuned boilers can also produce CO. When fuel is burned, carbon reacts with oxygen. If there is enough oxygen, turbulence, and time for combustion, the carbon and the oxygen form carbon dioxide (CO2). If these elements are insufficient, carbon combines with the oxygen to form carbon monoxide (CO). Carbon monoxide is a serious pollutant because it is a deadly toxic gas. Its presence in stack emissions also represents an operating loss. The chemical energy in the fuel is not entirely converted to heat when the carbon element of fuel burns to carbon monoxide. This is because the oxidation process that releases heat is not completed. In high concentrations, carbon monoxide is very explosive. If it accumulates in boiler fireside locations, where flue gas circulation is less vigorous, then a source of ignition will cause a furnace explosion. Boiler-produced carbon monoxide is addressed by ensuring that: 1. There is adequate combustion air. 2. There is adequate turbulence. 3. The boiler fuel train is in a proper state of tune. Burners and their control systems (fuel control valves, air registers, dampers, and oxygen trim systems) require regular “set-up” to ensure peak combustion efficiency throughout the entire boiler firing range. To reduce the production of carbon monoxide, Power Engineers must be able to: a) Operate burners cleanly and safely, with the combustion controls set to “manual.” In this way, boiler emissions are minimized even during warm up, cutting-in, and cutting-out procedures. b) Recognize when burners have insufficient oxygen (are burning “rich”), without relying on instruments. c) Troubleshoot and take appropriate steps when burners are operating rich (such as reducing fuel flow, calibrating the O2 analyzer, cleaning burner nozzles, etc.). d) Recognize when a boiler is overloaded. When overloaded, there is insufficient time for combustion to occur in the furnace, and it is possible for fuel and air ratio to become rich. Carbon Dioxide (CO2) Any time a hydrocarbon fuel is burned, CO2 is produced. It is one of the most important gases on the planet because all plant life depends on it. Animal and plant life produce CO2 naturally through respiration and decomposition in the presence of oxygen. Forest fires are also large natural contributors. CO2 cannot be eliminated from the combustion process as long as the fuel contains carbon. Carbon dioxide, unlike carbon monoxide, has not historically been classified as a gaseous pollutant. However, growing concern over the effects of increased levels of atmospheric CO2 has prompted new interest in removing the gas from the smokestacks of large-scale sources. Jurisdictions across North America have been active in legislating limits on the production of CO2 from industrial sources. CO2 is now a commonly measured component of flue gases. 2-6 4th Class Edition 3 • Part A Gas and Noise Emissions • Chapter 2 CO2 is considered a greenhouse gas (GHG). The carbon dioxide released into the atmosphere by human activity has been identified as contributing to climate change. Countries around the world who are major CO2 emitters are recognizing more and more that regulatory steps must be taken to reduce and even eliminate CO2 emissions. Coal-burning central heating plants, industrial plants such as steel mills and electrical generating stations are large contributors of CO2 emissions. Plants that burn fossil fuels other than coal also release CO2, but at lower levels. The amount of CO2 produced when a fuel is burned is a function of the carbon content of the fuel. The heat content, or the amount of energy produced when a fuel is burned, is mainly determined by the carbon and hydrogen content of the fuel. Heat is produced when carbon and hydrogen combine with oxygen during combustion to produce CO2 and H2O. Natural gas is primarily methane (CH4), which has a higher energy content relative to other fuels, and thus, it has a relatively lower CO2-to-energy content. The relative amounts of CO2 emitted per unit of energy released is shown in Table 1. Note that the basis of comparison is the amount of CO2 produced by the combustion of natural gas. Table 1 – Relative CO2 Emissions of Various Hydrocarbon Fuels Type of Fuel Relative CO2 Emission Natural Gas 1.00 Anthracite Coal 1.95 Sub-Bituminous Coal 1.83 Diesel Fuel 1.38 Gasoline 1.35 Propane 1.19 The table data shows that for a given heat energy release, anthracite coal produces 1.95 times the amount of CO2 as natural gas and 1.07 times the CO2 as diesel fuel. Sulfur Oxides (SOX) During the combustion of a fuel that contains sulfur (such as coal or fuel oil), the sulfur in the fuel is oxidized into sulfur dioxide (SO2) and sulfur trioxide (SO3) gases. Because sulfur can burn to either dioxide or trioxide, these compounds are collectively called SOX, where the “X” can be a “2” or a “3.” Pulp mills, mining, and smelting operations can discharge great quantities of SOX. Sulfur oxides also enter the atmosphere from natural sources, such as hot springs and volcanic eruption. While not as toxic as carbon monoxide, these sulfur oxides contribute to acid rain, which is hazardous to all forms of life and corrosive to building structures. In the presence of moisture, they form a weak sulfurous or sulfuric acid which irritates skin, corrodes most metals, and disfigures the exterior appearance of most painted surfaces. Acid rain is responsible for the acidification of lakes and destruction of forests. As part of industrial smog, acids cause itchy skin, watering eyes, coughing and fatigue. Although the main concern for SOX in the air is the development of acid rain, it has also been identified as a greenhouse gas. Allowable emission levels for SOX vary from jurisdiction to jurisdiction. 4th Class Edition 3 • Part A 2-7 Unit A-5 • Introduction to Plant Operations and the Environment Nitrogen Oxides (NOX) Fossil fuel combustion also produces nitrogen oxides. NOX may be any one of, or a combination of, the following compounds: • Nitrogen monoxide (NO) • Nitrogen dioxide (NO2) • Nitrous oxide (N2O) In many cases, more than one of these gases is given off by a single source. Nitrogen monoxide and nitrogen dioxide are the major NOX gases formed during the combustion of fuels in boilers and internal combustion engines. The total NOX formed during combustion increases with temperature and with excess O2. Usually, more than 90% of the NOX formed in the combustion zone is NO. Besides the combustion process, nitrous oxide (N2O) also comes from the agricultural use of synthetic fertilizers. NOX compounds have many serious environmental effects. NO and NO2 have been shown to react with sunlight in a complicated fashion to form a photochemical smog. In the presence of volatile organic compounds, they may form lethal, toxic cyanides. When NO2 combines with water in the atmosphere, nitric acid (HNO3) is formed, which accounts for roughly 30% of acid rain. NO is a powerful greenhouse gas. The presence of NOX compounds in industrial emissions is now closely regulated. Methane (CH4) Methane (CH4) is emitted by natural sources such as wetlands, as well as human activities such as leakage from natural gas and petroleum processing systems and the raising of livestock. Natural processes in the soil and chemical reactions in the atmosphere help remove CH4 from the atmosphere. CH4 has 25 times the ability of CO2 at trapping radiation. Therefore, CH4 is a more significant greenhouse gas than CO2, despite the fact that it has a shorter atmospheric lifetime. Ozone (O3) Ozone (O3) is a compound that contains three oxygen atoms. Ozone is formed in the stratosphere, in a region called the ozone layer, by the action of high intensity ultraviolet radiation on O2. In this part of the atmosphere, ozone is produced by the catalytic decomposition of O2 with shortwave UV light. Once produced, ozone can absorb longwave UV light and revert back to O2. Thus, O2 and O3 maintain an equilibrium in the stratosphere. Stratospheric ozone is critically important since it shields the Earth’s surface from hazardous ultraviolet radiation. Only a small percentage of the resulting ozone works down to the lower atmosphere and ground level. Ozone is also produced in the lower atmosphere by electrical arcs such as brushes in electric motors, arc welding, and lightning. The action of sunlight on a mixture of NOX and volatile organic compounds (photochemical smog) also produces ozone in this zone. Chlorofluorocarbons (CFCs) Compounds which are composed of chlorine, fluorine, and carbon are called chlorofluorocarbons. They are man-made substances, commonly used as refrigerants, aerosol propellants, and solvents. When released into the atmosphere, these gases diffuse to the stratosphere where they are broken down by strong ultraviolet light. This releases chlorine which reacts with and destroys the ozone by converting it back to O2. The decomposition of the O3 through the reaction with chlorine prevents the absorption of longwave UV light, which can therefore reach the Earth’s surface. CFCs are therefore identified as ozone depleting substances (ODS). They are also greenhouse gases. 2-8 4th Class Edition 3 • Part A Gas and Noise Emissions • Chapter 2 Sources of CFCs in the atmosphere are: a) Mechanical refrigerant leaks b) Refrigeration system servicing c) Disposal of equipment that contains refrigerant gas Fluorinated Gases Fluorinated gases are entirely man-made; there are no natural sources of these gases. They were developed to replace chlorofluorocarbon and hydrochlorofluorocarbon (HCFCs) refrigerants. They include hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and others. Of the greenhouse gases released by human activity, fluorinated gases are the longest lasting and most powerful. Sources of fluorinated gases in the atmosphere are the same as those for CFC gases. Other Commonly Monitored Pollutants While the above pollutants are the most common, several other gaseous pollutants are also commonly monitored such as arsenic, chlorine, and mercury. Monitoring of these pollutants may be due to either regulatory or plant efficiency requirements. 4th Class Edition 3 • Part A 2-9