4th Edition Unit A-5 Plant Operations & Environment PDF

Unit A-5 • Introduction to Plant Operations and the Environment Objective 3 Describe the common methods for monitoring and reducing gaseous pollutants. Pollution Monitoring Thousands of chemical compounds are produced as byproducts of industrial processes and released into the atmosphere. Although...

Unit A-5 • Introduction to Plant Operations and the Environment Objective 3 Describe the common methods for monitoring and reducing gaseous pollutants. Pollution Monitoring Thousands of chemical compounds are produced as byproducts of industrial processes and released into the atmosphere. Although many are also produced and released by ongoing natural processes, all emissions can be classified as contaminants or pollutants. Government regulations cover hundreds of emissions in various jurisdictions. However, only a few are so important and so widespread that they are regulated. The most significant Energy Plant emissions are part of a group called Criteria Area Contaminants (CAC). Environment Canada publishes an ongoing list of CACs on its website. Current CACs include: • Sulfur Oxides • Volatile Organic Compounds • Carbon Monoxide • Ground-level Ozone • Nitrogen Oxides • Particulate Matter • Ammonia Although many jurisdictions want to manage the output of CO2 from industrial sources and do require its measurement, as of 2017 it was not included on the CAC list. Carbon Dioxide monitoring is almost universally used as a measure to ensure complete combustion and therefore maximum energy transfer from the fuel to the process. Ambient air quality standards (at a specific distance from the source of the pollutant) are set and acceptable exposure limits are identified for each CAC. Exceeding the acceptable limit may cause excessive damage to the environment and increased health risks. Whether through general regulations or site-specific permitting, each energy plant has: a) Clearly defined limits for the maximum emissions (Source Emissions) that are allowed from its stacks for each of the CACs produced in its process. b) Requirements for the continuous monitoring of some of these emissions. c) Requirements for reporting on the results to a government agency. For emissions that are monitored, limits may vary between jurisdictions and from plant to plant. As the government and public demand stricter measures on pollution, the challenge of environmental control will continue to grow. Many pollutants cannot be totally eliminated; only their concentration can be controlled. To effectively fight environmental pollution, plant outputs are regulated and must be continually monitored to ensure compliance. This monitoring also serves as an indicator of how well the existing equipment is functioning. 2-16 4th Class Edition 3 • Part A Gas and Noise Emissions • Chapter 2 Gaseous Emission Monitoring For the energy plant, the boiler stack is usually the main source of atmospheric pollution. Ventilation and exhaust stacks also contribute. Whatever the case, a clear picture of what is being emitted from the stacks is of vital importance for effective pollution control. Routine emission monitoring usually involves both Source Emission Monitoring (at the facility) and Ambient Emission Monitoring (at a predetermined location away from the facility or a designated air monitoring location). Source emission monitoring often involves both in-stack emissions and fugitive emissions (leaks from within the facility). It is usually the responsibility of the Power Engineer to ensure all required emission monitoring is performed according to regulatory or site-specific requirements. Continuous Emission Monitoring Systems (CEMS) involve the installation of equipment to sample, analyze, and report data at predetermined times for a stack or duct. Regulatory agencies may require the inclusion of a CEMS to ensure compliance with emission standards. The following example is one which may be seen for a boiler stack. By using a selector switch (a three-way valve), the sampling train shown in Figure 5 may draw flue gas from the stack, or ambient air from a remote location. The pump operates continuously to fill the storing box with the gas being tested. The sample then moves through the sampling train. Through the manipulation of valves, flue gas is “pulled” from the stack and directed to various analyzers. Figure 5 – Automatic Sampling and Analyzing Train #1 #2 #3 Stack Sampling Exhaust Thermometer Filter A Selector Switch #4 #5 #6 C Storing Box B Pump D Air In Cooling Water Each analyzer contains a reagent or sensor designed to absorb or react with a distinct pollutant. This determines how much of a particular gas is present. Results from the analyzers are sent to recorders and indicators in the control room. Alarms may be installed and set to go off if a preset allowable limit of any gas is exceeded. Continuous monitoring of the plant emissions is vital for the following reasons: a) It ensures that emissions are kept at an acceptable level. b) The results can be used to see how well equipment is functioning. c) It provides records of plant emissions. Other methods of monitoring involve stations located around the facility that monitor secondary stacks and fugitive emissions. In some cases, stations are also installed outside the facility compound to measure ambient emissions. These can be portable stations where a plant technician performs site-specific monitoring. They could also be stations used by government officials to determine the accuracy of plant reporting, or to investigate public concerns. Fixed monitoring sites may be specified in the jurisdictional environmental legislation. These sites would be chosen specific to the location of the facility, the geographic landscape, and the 4th Class Edition 3 • Part A 2-17 Unit A-5 • Introduction to Plant Operations and the Environment location of other emission emitters in the same area. 2-18 4th Class Edition 3 • Part A Gas and Noise Emissions • Chapter 2 Measuring Pollutants There are different methods of testing stack emissions, both manually and using CEMS. Some methods are better suited to certain emissions. Here is a brief description of three techniques used to detect four major pollutants: NOX, CO2, CO, and SO2. Chemiluminescence Chemiluminescence refers to the emission of light as a result of a chemical reaction. For example, when NO oxidizes in the presence of ozone, a certain amount of light is produced in direct proportion to the amount of NO that is oxidized. This light can be measured and a reading produced. This is the predominant technique used for NOx measurement. Non-Dispersive Infrared Detection (NDIR) Non-Dispersive Infrared Detection (NDIR) is used to detect both carbon dioxide and carbon monoxide in the emissions stream, using different wavelengths. The basis of this technique is the absorption of IR light by CO2 and CO. In this type of detector, infrared light, at different wavelengths, is passed through two tubes. One tube (the reference tube) contains a non-IR absorbing gas such as nitrogen. The other tube contains the sample gas. CO2 or CO in the sample absorbs IR radiation. The energy that passes through the reference tube is compared to the energy that passes through the sample tube by a detector that measures the difference in IR absorption. The results of the measurement indicate the CO2 or CO concentration. Pulsed Fluorescence Testing When sulfur dioxide is exposed to ultraviolet light it becomes “excited” at one wavelength. It will then decay to a different (lower) energy state. In doing so, it will emit a UV light at a different wavelength. This light is then converted into an electrical signal, which is representative of the level of SO2. There are standards to be met when testing emissions for official purposes. The Environmental Protection Agency of the United States as well as the American Society for Testing and Materials have developed standards that lay out precisely how these tests are to be carried out. Both standards may be referred to in regulations. Regulations also mandate: a) Minimum CEMS availability requirements. b) How long CEMS data must be retained. c) Annual evaluations to ensure the quality of procedures. Reducing Levels Of Pollutants Before researchers understood the long term damaging effects of low levels of pollutants, they believed “The solution to pollution is dilution”. To reduce pollution in the immediate area, much higher “super” stacks were built. As a result, the local pollution was reduced because emissions were discharged higher into the atmosphere. This “solution” only spread the pollution over a greater area; it did not reduce the negative environmental impacts. Today, the emphasis is on reducing or eliminating emissions in a variety of ways. 4th Class Edition 3 • Part A 2-19 Unit A-5 • Introduction to Plant Operations and the Environment Current Methods of Pollution Removal or Reduction Carbon Monoxide Carbon monoxide is produced when a fuel is burned with insufficient oxygen. In a furnace, different conditions can create a shortage of oxygen. The most obvious is that there is not enough air being supplied to the furnace. Another reason is insufficient turbulence. The air and the fuel do not mix well enough for the carbon from the fuel to react completely with the oxygen in the air. Localized shortages of oxygen result in the production of carbon monoxide. Also, overloading of the boiler can cause quick combustion. This deprives the fuel of the time needed to be completely burned in the hot enclosure of the furnace. Power Engineers can control all three causes of incomplete combustion mentioned above. Most plants are equipped with automatic flue gas analyzers, which continuously monitor the composition of the flue gas leaving the furnace. Even where there are no flue gas analyzers, a Power Engineer should know by experience: a) How to determine proper combustion from the appearance of the flame. b) The temperature of the gas at various points in its path. c) The appearance of the stack emissions. Carbon Dioxide (CO2) Carbon dioxide presents a problem for Power Engineers. On one hand, when a fossil fuel is burned, CO2 must be released as part of the basic combustion process. On the other hand, it is gas with a serious downside – its global warming effect. As long as hydrocarbon based fuels are used, carbon dioxide will be released. It is necessary to figure out how to reduce the impact that facilities producing carbon dioxide have on the environment. Maximizing efficiency is one way Power Engineers can minimize CO2 production. This ensures that only the required amount of fuel is burned. In turn, no more CO2 than is absolutely necessary is released. There are several different ways to improve efficiency. Ensure that: a) Burners are in good condition (i.e. make sure that burner nozzles are kept clean and burners are tuned). b) Controls are operating correctly. c) Stack emission monitoring systems are maintained. Methods of removing carbon dioxide from flue gas have been developed, but are not currently in widespread use. The most widely tested process, Carbon Dioxide Capture and Storage (CCS), is a method used to remove, transport, and store CO2 from large stationary emitters such as power plants and natural gas processing plants. CCS has worked on a small scale, and is now being implemented on a larger scale. The process involves a number of steps, which include: a) Removing CO2 from the flue gas by contacting it with a solution of amines. b) Separating the CO2 from the amine solution by heating. c) Compressing the separated CO2 into a liquid. d) Using the liquid CO2 to enhance other industrial processes. Reforestation ties up carbon in the form of plant material as the trees grow thus removing CO2 from the atmosphere. It is estimated that 10 000 000 acres of new forest would use up all the CO2 that would be emitted by power plants in the next 10 years. Other ideas are being researched, such as putting urea into the ocean so more CO2 will be absorbed into the water. However, altering the ocean’s chemistry might not be a viable solution, especially if it has a negative effect on the ecosystem. 2-20 4th Class Edition 3 • Part A Gas and Noise Emissions • Chapter 2 Until such processes are proved to be viable, Power Engineers can do their part by being mindful and efficient equipment operators. Sulfur Dioxide (SO2) Many facilities have met environmental regulations by using naturally occurring low sulfur coal. Others have installed Flue Gas Desulfurization (FGD) systems. The FGD systems are categorized as: • Non-Regenerable, where the medium used to absorb the SO2 is disposed of as waste. • Regenerable, where the medium is recycled. The advantage of regenerable systems is that the sulfur is recovered and can be sold. However, these systems are more expensive than non-regenerable systems (which are currently more widely available). Most of the processes involve wet scrubbing of the combustion flue gas using lime or limestone, alkaline fly ash or sodium carbonate and dilute sulfuric acid. A typical limestone system is shown in Figure 6. Efficiency for these systems can be as high as 90 to 95% with combustion gases containing up to 5000 ppm SO2. Figure 6 – FGD System using Wet Scrubbing of Flue Gases with Limestone Slurry SO2 generated in furnace Boiler Stack Precipitator Induced Draft Fan Absorber Quencher Limestone Feed SO2 out Flue Gas Path Slurry Feed Tank Recirculation Tank 4th Class Edition 3 • Part A 2-21 Gas and Noise Emissions • Chapter 2 Nitrogen Oxide (NOX) Nitrogen oxides (NOX) are formed from both fuel-bound nitrogen (fuel NOX) and the nitrogen contained in the combustion air introduced into the furnace (thermal NOX). Fuel NOX is dependent on the: a) Percent of nitrogen in the fuel b) Reactivity of nitrogen compounds contained in the fuel c) Oxygen availability in the combustion zone The use of fuel that contains less (or zero) nitrogen is the most effective means of reducing fuel NOX. It is difficult to control thermal NOX production in power plants. This is because favourable conditions for NOX production are created by the combustion practices developed to increase power plant operating efficiency and control other air pollutants. Boilers are designed to have high furnace temperatures and use excess air to ensure complete combustion. This design controls the amount of the undesirable by-products such as smoke and carbon monoxide. Unfortunately, the high temperatures and increased oxygen encourage the formation of high levels of NOX. There are two practical means of controlling emissions of nitrogen oxides from power plants. Control may be accomplished either by minimizing their formation in the first place, which is mostly dependent on high temperature and the availability of oxygen; or removing them from the flue gas after they have been produced but before they enter the atmosphere. Removing NOX from Flue Gas NOX can be removed from the flue gas stream by using technologies such as Selective Catalytic Reduction or Selective Non-Catalytic Reduction. These methods remove NOX by causing a reaction between it and ammonia, which produces nitrogen and water. Depending on the method used, a catalyst may or may not be involved in the reaction. NOX reductions of up to 80% or more are possible. Controlling NOX Before it is Produced The conversion of nitrogen in the air to NOX is highly dependent on temperature. The formation of NOX proceeds rapidly at combustion zone temperatures in excess of 1650°C. By maintaining the combustion temperature below 1650°C, the NOX emission can be reduced. Besides using low nitrogen fuels, the following are methods that can be used to reduce the formation of NOX: a) Two stage combustion b) Low excess air operation c) Gas recirculation d) Specially designed Low NOX Burners These methods either restrict the conditions under which the nitrogen and oxygen react to reduce available oxygen, or reduce peak flame temperatures to reduce the NOX produced. This may be done by one of the following. a) Introducing fuel rich atmospheres b) Fuel lean atmospheres c) Longer combustion times d) Dilution of combustion air with exhaust gases Depending on the method used, NOX can be reduced by 30 – 80%. 4th Class Edition 3 • Part A 2-21 Unit A-5 • Introduction to Plant Operations and the Environment Chlorofluorocarbons (CFCs) and Fluorinated Gases Currently, there are no proven methods for removing CFCs or fluorinated gases from the environment. The use of CFCs was discontinued by the Group of Seven (G7) countries after signing the Montreal Protocol in 1987. As a result, the release of CFCs has decreased. However, it has been estimated that it will take up to 1500 years for the compounds that are already in the atmosphere to decompose. There is a movement to regulate the use of fluorinated gases, as well. Proper maintenance, repair, and storage of machinery which use CFC and fluorinated gases will reduce the amount released to the atmosphere. Possible replacements for these gases include hydrofluoro-olefins (HFO) and hydrocarbon (HC) refrigerants. These gases have a lower impact on the environment than the refrigerants they would replace. Natural refrigerants, such as R-744 (CO2) and R-717 (ammonia) are replacing CFCs and fluorinated gases. CO2 is sourced from the atmosphere and is not ozone depleting. If accidentally released, it merely returns to its source. When contained in a refrigeration system, it does not contribute to global warming. In this regard, CO2 is considered a “green” refrigerant, with neutral environmental effect. Ammonia, though toxic and explosive, is a highly energy efficient refrigerant. If leaked to the environment, it is rapidly absorbed by water and enters the soil, where it provides essential nitrogen for plant growth. Reducing Pollution - Changing The Energy Source An important solution to reducing pollutants is to select energy sources that do not pollute. Ideally, these alternate sources are renewable and do not produce other economic or environmental problems. Some solutions offer energy generation that does not release compounds into the environment. Other solutions address natural sources of harmful emissions, and harness these natural sources for power production. An example is to use biomass as a fuel source (explained further below). There are six commonly used renewable or non-polluting energy sources: 1. Biomass 2. Hydro-Electric 3. Geothermal 4. Wind 5. Solar 6. Hydrogen Of the above, biomass and hydropower are traditional energy sources. A short overview (with the exception of hydro-electric power) follows. Biomass Biomass is a variety of plant-based organic material or municipal waste material. It has no commercial purpose other than to be burned as fuel. Much biomass is sourced from: • Forest residue • Wood chips and bark • Municipal solid waste 2-22 4th Class Edition 3 • Part A Gas and Noise Emissions • Chapter 2 • Bagasse 4th Class Edition 3 • Part A 2-23 Unit A-5 • Introduction to Plant Operations and the Environment Biomass is either burned directly as a fuel, or it can be converted into biofuel, such as biodiesel and bioethanol. Sugar cane grows rapidly. Therefore, when bagasse (waste sugar cane product) is used for power generation, the CO2 emissions can be quickly recaptured by planting more sugar cane. This reduces the overall environmental impact. Forests, on the other hand, grow slowly. Therefore, new forest growth is not as effective for rapid CO2 remediation. Geothermal Energy The technologies used to produce energy from geothermal sources are determined by the temperature of the liquid in the underground source rock. The most common geothermal power plants use reservoirs of water temperatures greater than 182°C. In this type of plant, hot water flows up through wells in the ground, under its own pressure. As it flows upward, the pressure decreases and some of the hot water boils into steam. The steam is then separated from the water and used to power a turbo-generator. Any leftover water and condensed steam are injected back into the reservoir, which makes this a renewable resource. Wind Energy Wind energy is generated when turbines, exposed to an appropriate speed and duration of wind, turn in the moving air and power electric generators. Generation of power from wind is increasing in popularity. Wind power has the advantage of being renewable but has the disadvantage of being intermittent. Because of this, wind generators rely on energy storage systems so that power is available even when there is insufficient wind. Solar Energy Solar energy is a nonpolluting source of energy. The energy is free, but harnessing it is expensive. In 1989, the New York Times reported 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 have taken place in the last few years and, if fossil fuel prices dictate, more research will take place. Solar energy sources generally utilize one of two types of solar technology: a) Solar Photovoltaic (PV) is the direct conversion of solar energy into electricity. b) Solar Thermal processes involve mirrors that focus and concentrate solar energy on a target point which is connected to a heat transfer medium, normally a liquid salt due to the temperatures achieved. The high temperature liquid salt is then circulated through a heat exchanger to produce high pressure steam to drive a turbo generator. Hydrogen as a Fuel Using hydrogen as a fuel is not a new idea. However, the methods used to produce, storage, and handle it must be refined before it receives wide acceptance. Hydrogen is an ideal fuel since 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. Fuel cells use hydrogen to produce electricity to drive cars and buses, and for portable power systems. But, fuel cell use is not yet widespread. Hydrogen is often stored in cryogenic containers to keep the storage pressures from being excessively high. Even a small hydrogen leak poses an explosion hazard due to the wide range in its explosive limits. Hydrogen rich fuels, such as methane and propane, contribute less CO2 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 price increases. 2-24 4th Class Edition 3 • Part A