The Combustion Process - Essay Example

The Combustion Process The combustion process, whether used for electri generation or in engines for transport, produce air-borne emissions of environmental concern. Overview of the solutions for reduction of emissions from both, power generation and transport, to comply with: Regulated pollutant levels and The UK commitments to the Kyoto Protocol The combustion process Emission control technologies 1. CO2 capture and sequestration 2. SO2 scrubbers 2.1. Wet scrubbing 2.2. Dry scrubbing 2.3. Seawater scrubbing 3. NOx 3.1. Primary sources 3.2. Selective catalytic reduction (SCR) 3.3. Selective non-catalytic reduction (SNCR) 4. Particulate matter 4.1. Electrostatic precipitators (ESP) 4.2. Fabric filters 4.3. Bag filters Vehicle emissions 5. Three-way catalysts Regulation Financial incentives and tax system Emissions trading References The combustion process. A simple equation that describes the combustion of a hydrocarbon (such as petrol) in air is: Fuel + Air Heat + H2O + CO2 + NOx Particulate matter (PM), or soot, is also emitted during combustion and when fossil fuels are burned, SO2 is obtained in significant amounts. NOx is the generic term used to refer to a mixture of mainly NO and NO2. CO2, SO2 and NOx are chemicals known to contribute to air pollution and global warming. Acid rain is due to the presence in the atmosphere of SO2 and NOx which, after a series of reactions, are deposited as nitric and sulphuric acid in the form of rain. Particulate matter is known to cause serious respiratory diseases. Emission control technologies. To avoid or minimize the emission of the aforementioned chemical species into the environment, the following methods have been designed for the clean-up of exhaust gases: 1.CO2 capture and sequestration Carbon sequestration is a method that allows the reduction of CO2 by capturing it at its source (e.g. a power plant) and storing it in non-atmospheric reservoirs such as geologic or oceanic reservoirs (Herzog). The Sleipner project is a working example of carbon sequestration technology. Off the coast of Norway in the North Sea, it is in fact the world's first commercial CO2 capture and storage project collecting about one million metric tons of CO2 each year since it was opened in 1996 (Sleipner CO2 project). 2. SO2 scrubbers The term "scrubber" is used to refer to a group of air pollution control devices that aid the removal of fine particles and/or gases (especially acid gases) from industrial exhaust streams. Scrubbers work by either physically removing substances from the exhaust gas stream or by chemically neutralising them so that they cannot do any harm once released into the environment. 2.1. Wet scrubbing Wet scrubbing is used to clean gases from pollutants and particulates by putting in contact the exhaust gas stream with the scrubbing solution. The scrubbing solution can simply be water for the removal of dust or a chemical solution which contains reagents that can specifically target certain compounds. Removal efficiency of pollutants is improved by increasing residence time in the scrubber or by the increase of surface area of the scrubber solution. 2.2. Dry scrubbing Dry scrubbing is generally used to remove acid gases, such as SO2, primarily from combustion sources. All dry scrubbing systems consist of two main devices: 1) a device to introduce a solid sorbent material into the gas stream; and 2) a particulate control device to remove reaction products, excess sorbent material, any particulate matter already in the flue gas. 2.3. Seawater scrubbing The natural buffering capacity of seawater can be taken advantage of by using it as a sorbent in a scrubbing system. When SO2 comes into contact with seawater a reaction takes place between the SO2 and the CaCO3 present in the seawater, producing CaSO4 and CO2. This reaction is complete in a very short time, so the equipment required for seawater scrubbing can be compact. 3. NOx 3.1. Primary measures Primary measures can be applied pre-combustion. In Bowin low NOx technology, air and fuel are pre-mixed at a ratio greater than is required for the combustion reaction. Water Injection technology is also available, where H2O is introduced into the combustion chamber, helping NOx reduction via achieving increased efficiency in the overall combustion process. 3.2. Selective catalytic reduction (SCR) SCR is a process where a gaseous or liquid reducing agent (normally NH3 or urea) is added to the exhaust gas stream and is absorbed onto a catalyst. The reducing agent then reacts with NOx in the exhaust gas to form H2O and N2. SCR catalysts function well only within a relatively narrow temperature window and process control systems must be programmed to keep the exhaust gas temperature within that range. For example, zeolites show the ability to withstand extended operation above 650 C and brief exposure to temperatures of 750 to 850 C (Lambert). However, vanadium catalysts are quickly deactivated at temperatures above 600 C. (SAE International). 3.3. Selective non-catalytic reduction (SNCR) SNCR involves injecting either NH3 or urea into a specific location of the combustion chamber to react with the nitrogen oxides formed in the combustion process. The resulting product of the chemical reaction is N2, CO2 and H2O. In conclusion, SNCR is operated in an identical manner to SCR with the difference that catalysts are not used in this case. 4. Particulate matter PM consists of particles of many different materials, such as unburned fuel or incombustible substances present in the fuel. PM can also contain secondary particulates formed from the reaction of SO2 or NOx with other material in the atmosphere. 4.1. Electrostatic precipitators (ESP) An ESP is a particulate collection device that removes particles from a flowing gas using an induced electrostatic charge. These are highly efficient filtration devices. A basic electrostatic precipitator would include a stack of metal plates (ca. 1 cm apart) followed by a row of thin metal wires. A negative voltage of several thousand volts is applied between the wires and the plates. An electric discharge ionises the air around the electrodes and generates a flow of negative ions to the plates. This charges the particles present in the exhaust gas stream passing through the spaces between the wires and plates. The ionised particles, following the electric field, move to the grounded plates, where they can be collected (ASU Electrical Engineering). 4.2. Fabric filters A fabric filter is a device generally made from porous fabric through which exhaust gases are sent for filtration of particulate matter (Portland Cement Association). Bag filters may be several meters long and the choice of material will depend on the operating temperatures of the process where they are used (Intota). 4.3. Bag filters A bag filter is a fabric bag through which a gas stream is passed for the removal of particulate matter. Varieties of this device can also be used to filter particulate matter from liquid waste. The characteristics of the bag (e.g. porosity or material) will vary depending on the process. Vehicle emissions In countries like the USA, the pollution caused by cars, trucks and other vehicles accounts for almost a third of the total air pollution (US Environmental Protection Agency). 5. Three-way catalytic converters Catalytic converters are used in the exhausts of vehicle engines in order to reduce the amount of toxic gases released to the environment. A three-way catalytic converter has three simultaneous tasks: reduction of NOx and oxidation of CO and unburnt hydrocarbons. These three reactions occur most efficiently when the catalytic converter receives exhaust from an engine running at the point where the required stoichiometric amounts are present (100% conversion efficiency). When the equilibrium is upset, the conversion efficiency decreases. Regulation The Kyoto Protocol is a 'cap and trade' system that imposes limits on the emissions of a number of countries worldwide. On average, it is expected that the countries that take part in the Protocol will reduce their emissions 5.2% below their 1990 baseline over the 2008-2012 period (Kyoto Protocol) Financial incentives and tax system In terms of transport, financial incentives or tax release should be introduced in order to encourage car owners to travel using public transport and to purchase low fuel consumption vehicles. Tax reductions should also exist for cars that use alternative fuels, such as biodiesel. In terms of energy production and consumption, financial incentives in the form of tax reductions and grants should be made available to both small and big companies that want to adopt cleaner approaches (e.g. alternative energies) for their processes. Besides, research should be performed in an attempt to improve the energy efficiency of common combustion processes and the use of non-carbon energy sources should be promoted and encouraged financially. Emissions trading According to the Kyoto Protocol, individual countries or companies can purchase Carbon Credits if they expect that their emissions will exceed the allotted quota. These Credits are likely to be obtained from another country or company with excess allowances. The average market value of the Carbon Credits transacted in 2004 was 245 m. References -ASU Electrical Engineering. Electrical Engineering for Pollution Control. Date last updated: 15/01/03. Date last accessed: 10/03/07. http://www.eas.asu.edu/holbert/wise/electrostaticprecip.html -Herzog H., Golomb D., MIT. Carbon Capture and Storage from Fossil Fuel Use. Date last accessed: 12/03/07. http://sequestration.mit.edu/pdf/enclyclopedia_of_energy_article.pdf -Intota, Experts in "bag filter". Date last accessed: 11/03/07. http://www.intota.com/multisearch.aspstrSearchType=all&strQuery=bag+filter -Lambert, C. Urea SCR and DPF system for Tier-2 Diesel Light-Duty Tracks. August 24, 2006. http://www1.eere.energy.gov/vehiclesandfuels/pdfs/deer_2006/session5/2006_deer_lambert.pdf -MIT. Carbon Capture and Sequestration Technologies. Date last accessed: 10/03/07. http://sequestration.mit.edu/technology_overview/index.html -Portland Cement Association, Cement Manufacturing. http://www.cement.org/manufacture/man_dust.asp -SAE International. SAE Technical Papers. Date last accessed: 12/03/07. http://www.sae.org/technical/papers/2005-01-1860 -Sleipner CO2 project. CO2 storage. Date last accessed: 10/03/07. http://www.statoil.com/STATOILCOM/SVG00990.NSF/web/sleipnerenopendocument -Kyoto Protocol. Date last accessed: 12/03/07. http://unfccc.int/kyoto_protocol/items/2830.php -US Environmental Protection Agency, Green Vehicle Guide. Date last accessed: 12/03/07. http://www.epa.gov/autoemissions/faq.htm#Important Read More