10 points for best answer, thanks :)
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4 months ago
Stop complaining, about it not being a %26#039;serious question%26#039; why would it be on a exam paper, if you dont wanna answer it dont!
Best Answer
As the world begins to wake up to the dangers and the damage caused by pollution, the pressure to become more environmentally responsible increases for those of us that use the combustion process to produce our energy. The problems caused by pollutants such as nitrogen and sulphur oxides are now worldwide issues. Poor air quality, serious smog conditions in some urban areas, the formation of acid rain, and the subsequent damaged it causes to water supplies %26amp; timber are at the forefront of our environmental concerns. Add the perplexing theories regarding Co2 emissions and the %26quot;Greenhouse effect%26quot;. Politicians of all stripes rant %26amp; rave and create laws that effect our businesses. We really do need to have a basic understanding of the facts regarding air quality and what we are doing that has an effect on this aspect of the environment we live in now and in the future. The challenge for industry is to minimize these problems and create a safer future through a cleaner environment.POLLUTANTS - the abc’s
Before identifying methods to reduce and control various types of pollution, it is important to recognize exactly what these pollutants are and how they are formed. Understanding how pollution is created is a first step in developing techniques to combat it.
CO2 = CARBON DIOXIDE
As the best known of the so called %26quot;greenhouse%26quot; gases, carbon dioxide is under constant scientific scrutiny and is the focus of social and political concern. Produced by the process of combustion, particularly the burning of fossil fuels such as coal, oil and natural gas, CO2 is emitted in gaseous form in the flue gases originating from any combustion device.
C + O2 = CO2 (Burning of carbon based fuels)
CH4 + 2O2 = CO2 + 2H2O (Burning of gas)
Carbon dioxide molecules, unlike other components of pure air have the property of absorbing the infra-red (heat) radiation of the sun. Therefore, the more CO2 in the atmosphere, the more heat the atmosphere can absorb which could contribute to the melting of the polar ice caps and the expansion of the oceans into low lying areas. The objective of an efficient burner is to achieve combustion conditions as near to the Stoichiometric parameter of the fuel as is safely possible. The term stoichiometric literally means %26quot;measurement of elements%26quot; and is used to denote a condition where the equivalent weights of substances in a chemical reaction have been precisely determined. In layman’s terms, %26quot;Perfect Combustion%26quot;. At near stoichiometric level, the oxygen content of the flue gas is at the lowest percentage and the CO2 is at the highest. However, the more efficient the burner, the lower the amount of excess air used in the combustion process resulting in the least volumetric emission of CO2 in the flue gases. Perfect combustion involves using exact weights of air and fuel, although in practice this is not possible. Even the best burners available will use a certain amount of excess air. The idea is to use combustion equipment that is able to operate at the lowest levels of excess air.
SO2 = SULFUR DIOXIDE
Closely associated with major air pollution disasters, sulfur oxides have long been responsible for considerable environmental damage. However, these pollutants are generally the product of the fuel bound sulfur content and even sophisticated combustion techniques have not had any effect on its formation. Selection of low sulfur content fuel seems to be the answer to this problem. As probably the most significant single air pollutant, SO2 is produced when sulfur or fuels containing sulfur are burned:
S+O2 = SO2
SO3 is created in the atmosphere by the oxidation of SO2 under the influence of sunlight.
2 SO2 + O2 = 2 SO3
Additionally, some SO3 is introduced directly from combustion processes along with SO2.
The moisture in the air reacts rapidly with SO3 to form a sulfuric acid: ACID RAIN
SO3 + H2O = H2SO4.
When such conversions occur the material originally introduced to the atmosphere is called a primary air pollutant. The new materials produced by chemical reaction in the air are called secondary air pollutants.
CO = CARBON MONOXIDE
Usually formed when there is a lack of Oxygen in the combustion process or when a flame is ‘chilled’. This gas can prove fatal if inhaled but is really a greater danger as a combustible gas which is responsible for many of the accidental explosions that occur around combustion equipment.
NOx = NITROGEN OXIDES
NOX is the collective term for nitrogen oxide gases. Thermal NOx is the most prevalent form of NOx pollutant and as the name implies it is formed at those areas of highest temperature during the combustion process. Chemically NOx is expressed as NO (Nitric Oxide) and/or NO2 (Nitrogen Dioxide). NO is formed when Nitrogen oxidizes in an oxygen rare atmosphere, but it is very unstable and will combine further with Oxygen whenever it becomes available to make NO2. Very little NO gets much further than the end of the exhaust stack. Oxides of nitrogen in the atmosphere, when exposed to sunlight, react chemically with other gaseous compounds in the atmosphere such as %26quot;volatile organic compounds%26quot; (VOCs) through photosynthesis to form a brownish yellow cloud of gases commonly referred to as smog. Add rain and the smog clears, but the chemical compounds then washed out by the H2O can form various harmful acids that over time do great damage.
The formation of NOx is a complex process which takes place in the pre-combustion, combustion and post-flame regions. It involves the reactive combination of nitrogen found within the combustion air and natural or organically bound nitrogen within the fuel itself. Attempts have been made to identify different NOx formation mechanisms such as thermal NOx, prompt NOx and fuel NOx, yet inconclusive finding have left NOx the subject of extensive research and analysis. It is worth noting that NOx can be produced in isolated sections of the flame and it is not unusual for over 80% of the combined NOx to be produced in only 10% of the flame volume. NOx is a thermally produced gas and therefore its reduction is largely dependent on the control of flame temperature. Distribution of the heat release areas of the flame that prevent concentrated heat release and high temperature points within the flame envelope is the key to lowering thermal NOx formation. Refractory surfaces reflect and concentrate heat and is therefore counter productive in lowering thermal NOx formation.
Minimizing NOx levels involves considering a number of factors including:
Furnace capacity (combustion area)
Temperature of furnace wall – Again refractory is counter productive
Furnace dimension ratios, the optimum length to diameter ratio being approximately 3.5 : 1
Pre-heated air
Humidity
Excess air also plays a major part in the creation of NOx. Greater excess air represents greater access to oxygen not taken up in burning the fuel, and it is therefore available to combine with the 80% nitrogen that is available in fresh combustion air. It has been found that NOx levels peak when excess air is at 40% and reduce rapidly as this excess air decreases. Indeed, as the combustion process of gas requires approximately 11 parts air to 1 part natural gas, it is clear that the real key to low NOx lies in the actual design of combustion equipment. A burner design with poor air/fuel distribution only serves to enhance the conditions necessary for NOx formation.
SMOKE
Excess fuel, incomplete combustion and flame impingement will result in the formation of smoke which discharges particles of carbon into the atmosphere and also causes fouling of the boiler surfaces which will impair efficiency. Smoke is almost always evidence of CO formation, as well as unburned carbon particulate matter.
SOLUTIONS
Reducing air pollution emissions depends on a number of factors – the fuel used, the local conditions, the design of the combustion chamber, heat release, and the design of the burner. (how well does it complement the design of the boiler). Weighing up the benefits and drawbacks of each available technique is therefore a necessary exercise in assessing suitable solutions for emission problems.
Although some methods claim to greatly reduce emissions, they may in fact cause a reduction in efficiency and can result in reduced heat outputs. Unrealistic emission regulations can actually increase pollutant emissions as it may require more fuel to produce the same amount of energy for the process at hand. Lowering air pollutant emissions can increase the cost of a process in terms of more expensive equipment to achieve compliant emission levels, and increased fuel cost due to some lowering of fuel efficiency. The trick is to balance the various factors and gain the most efficient combustion possible while complying with state %26amp; federal air quality codes.
NOx REDUCTION TECHNIQUES
External flue gas re-circulation