Introduction
The strong economic growth exhibited by industrialized countries in the past three decades has been accompanied by an increase in energy consumption. This has made the operation of Large Combustion Plants (LCP) to generate electrical energy and/or heat a cornerstone of our modern society.
The BAT Reference Document (BREF) (2006) declares that the economy of all industrialized countries is largely dependent on LCPs. However, these plants generate a significant amount of residues and waste and these emissions have an effect on the environment.
Gas Combustion installation use the raw material taken from earth and convert them into useful energy. However, the burning process results in a significant impact on the environment since the combustion process results in emissions to air, water and soil.
One of the large combustion installations commonly used is the Gas Combustion Plant. This paper will set out to describe the major pollutants emitted from Gas Combustion Plants which are oxides of nitrogen (NOx). Specifically, the paper will analyze the process through which this pollutant is produced and proceed to access the environmental impact that the pollutant has. The manner in which the pollutant is monitored will them be elaborated. The paper will conclude by describing methods used to control the emissions.
Sources of the Pollutant
The major pollutant(s) in any industry are described as those which produce the highest pollution index during the industrial process. Oxides of nitrogen (NOx) emissions are of primary concern in gas combustion plants. NOx is formed by a number of processes in Large Gas Combustion Plants.
The first process which produces a significant amount of NOx is when nitrogen and oxygen in the combustion air mix with one another at high temperatures in a flame (Sutton, et al. 2011). The high temperatures results in production of increased amounts of thermal NOx.
The formation of thermal NOx is dependent on the temperature of the combustion chamber. When the furnace temperatures are above 10000C, emissions of NOx are significantly high. Keeping the peak flame temperature below 10000C significantly lowers NOx emission. Thermal NOx is the dominant manner through which NOx is generated in Gas Combustion Plants.
The second process is when the nitrogen oxides in fuel react with oxygen in the combustion air. The formation of fuel NOx depends on the nitrogen content in the fuel as well as the level of oxygen in the reaction medium.
The quantity of fuel NOx produced is therefore greater in installations that have fuels with high nitrogen contents. The third albeit less significant mechanism is prompt NOx which is formed through the conversion of molecular nitrogen in the presence of hydrocarbon compounds at the flame front (BREF 2006).
Environmental Impact of NOx
While road transport and international shipping are the largest contributors to global NOx emissions, Large Combustion Plants also contribute a significant percentage of the global NOx emissions. These emissions have a number of significant negative effects on the environment (Sutton et al. 2011, p.19).
Nitrous Oxide contributes to the greenhouse effects which have been blamed for global warming (Steen 2001). While carbon dioxide is the primary greenhouse gas of concern, NOx also has some greenhouse effect. IEA (2001) denotes nitrous oxide (N2O) as the second important greenhouse gas emitted from LCPs with the first gas being Carbon dioxide (CO2).
Nitrogen oxides are toxic substances that contribute to the formation of acid rain as well as photochemical smog (Institute of Mechanical Engineers 1997). Park (1997, p.32) declares that the two basic ingredients of the acid rain are Sulphur dioxide and NOx.
These key components dissolve in rainwater and/or are oxidized to produce acid rain. Acid rain results in ecological changes in aquatic ecosystems with acidification. Lakes and rivers become acidified and this contributes to a damage of marine life. While some marine life can tolerate acidic water, the water is toxic to many aquatic animals such as crayfish and fish.
NOx emissions can aggravate some respiratory conditions. The United States Environmental Protection Agency (2011) documents that NOx exposure has been liked with airway inflammation in health individuals and aggravation of symptoms in people suffering from asthma.
NOx reacts with ammonia and moisture in the air to form particles which can get into the lungs of a person and lead to respiratory diseases in a healthy person or worsen the symptoms for a person who is suffering from a respiratory disease.
Monitoring of Pollutants
The desirable outcome is to reduce the emission of NOx to the lowest level possible. To realize such ambitions, monitoring which involves sampling and analysis has to take place. Monitoring also ensures that NOx emission reduction goals for individual plants are met.
Sutton et al (2011) notes that NOx emissions in Europe have reduced markedly over the last decade as a result of stringent emission controls applied to large combustion plants. Continuous emission monitoring systems are used to measure and record the emissions of NOx as well as volumetric flow and opacity (Lane 2003, p.17).
Continuous monitoring of NOx emissions is preferred due to the complex formation of NOx throughout the combustion process. However, continuous monitoring costs are high which make it feasible only for large combustion plants.
LCPs have to apply for an emission permit from the National Licensing Board for Environmental Protection and this permit gives the company the right to release a quantified amount of pollutants (Gale et al 2000).
Before the license can be issued, Initial equipment certification procedures is required for all LCPs and after that, periodic quality assurance and quality control procedures are undertaken to ensure that best practices are adhered to (OECD 2007).
If annual emissions by a particular plant exceed the number of allowances held, then the plant is made to pay a penalty depending on the excess ton of NOx emissions (United Nations Economic Commission for Europe 2007).
There are a number of ways in which efficiency can be measured and BREF (2006) documents that there are a number of national guidelines such as [51, DIN, 1996] which describe acceptable tests and measures for efficiencies in LCPs.
Thermal efficiency is calculated as the ratio of useful mechanical output to the heat flow transferred to the cycle process media. Monitoring of combustion efficiency is important since poor combustion translates to lower economic viability as well as increased environmental impacts by the plant (OECD 2011).
Operation practices at a LCP are carried out to ensure that they adhere to best practice and that the equipment used is well tuned. Monitoring of operational practices is effective since these practices affect the levels of NOx. Optimization of fuel and air ratios in the furnace influence NOx emissions.
The National Research Council (2006) states that a well-tuned furnace results in substantially lower NOx emissions than one that is not well tuned. LCPs are issued with calibrating procedures as well as maintenance procedures which are supposed to be followed at all times.
Measurements are made by an authorized laboratory and the examination and declaration of equipment is made if necessary (Gale et al. 2000). Since the fuel quality has a direct impact on the amount of NOx emitted, monitoring is done of the fuel used. Fuel quality standards necessitate the use of fuels that comply with certain standards (OECD 2003).
Control Methods and how they Work
Pollution control is an expensive affair and as of 1997, NOx controls were costing the US up to 90 million per year (McCormick, 1997). Even so, the negative impacts of NOx make it necessary for control measures to be taken up so as to safeguard public health as well as protect the environment. There are a number of control methods that are currently applied in Gas Combustion Plants to mitigate NOx emission.
NOx content can be reduced during the combustion process since NOx is formed by the effect of combustion of nitrogen in the air and in fuel. This can be accomplished by use of low NOx burners (LNB) which modify the means of introducing air and fuel to delay the mixing and reduce oxygen levels.
By reducing the amount of time that air stays in the combustion chamber can effectively reduce NOx formation (Krishnan 2002). LNBs reduce the temperature of the flame and retard the conversion of nitrogen to NOx as well as the formation of thermal NOx.
All this is done while maintaining high combustion efficiency. McCormick (1997) reveals that LNBs ensure that the fuel and air mix more slowly therefore reducing the temperature of the combustion. Reduction of furnace temperature leads to lower NOx formation.
Another easy to implement technique for reducing NOx emissions in lowering excess air. Reducing the amount of oxygen available in the combustion chamber to the minimum levels needed to complete the combustion process ensures that fuel bound nitrogen conversion is reduced.
A considerable emission reduction is therefore achieved by ensuring low excess air. While this technique does not cause any reduction in power availability, it may result in incomplete combustion and hence increase the amount of unburned carbon in the ash.
As has been noted, the presence of oxygen in the furnace contributes to the formation of NOx. Off-stoichiometric combustion is a process which exploits this fact to reduce NOx formation (McCormick 1997). This process limits the amount of oxygen in the furnace in a number of ways.
One variation of implementing the off-stoichiometric combustion process is by recirculating some of the flue gas back to the furnace. Recirculation of flue-gas reduces the available oxygen in the combustion zone by cooling the flame thus preventing formation of thermal NOx. This technique is said to cut NOx formation by up to 75%.
Another method is air staging in which case air supply to the furnace is limited and more air is added outside the furnace to complete combustion. Air staging, involves the creation of two divided combustion chambers with one chamber lacking oxygen while the other has an excess of oxygen.
The secondary combustion chamber which has excess oxygen ensures complete burnout. Air staging has the merit of not increasing the energy consumption of the LCP and there is no adverse effect on the plants infrastructure.
Since the formation of NOx is inevitable, the final levels emitted can be reduced by reburning where some of the NOx formed by the initial combustion are reduced back to nitrogen by injecting fuel such as oil or gas into a secondary combustion zone.
This method is based on the creation of different zones in the furnace by staged injection of fuel and air with the aim being to reduce back to nitrogen the nitrogen oxides formed so far. Natural gas is the preferred reburning fuel of choice. This is because it does not lead to the formation of NOx in the burnout zone as coal and oil do.
Post-combustion approaches can also involve the injecting of reactants such as ammonia into the flue gas. This ammonia reacts with NOx and a catalyst can be used to increase the effectiveness (National Research Council, 2006).
This process is known as selective catalytic reduction (SCR) and it is widely used in many LCPs in Europe and the US to reduce NOx in exhaust gases.
Combustion modification aimed at reducing NOx emissions may have an adverse impact on boiler operation and even result in the formation of other pollutants. Care should therefore be taken to ensure that other pollutants such as particulate organic matter are not formed.
Conclusion
This paper has described the major pollutant in Gas Combustion Plants, NOx and articulated the impacts this pollutant has on the environment. It has been noted that the negative impacts can only be alleviate by curbing the release of NOx in the environment.
With this understanding, a review of the various ways in which monitoring can be carried out has been made. The paper has then engaged in a detailed discussion at the control methods currently applied to reduce NOx emissions has been made. Reducing NOx emissions results in positive environmental outcomes which benefit the entire society.
References
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