Of the processes investigated, the circulating fluidized bed process was found to be the most economical. A1.3 Provisions in the European Communities Large Combustion Plant Ordinance (LCPO): emission targets for existing plants.
INTRODUCTION
OBJECTIVES
A review of current and flue gas technologies to reduce sulfur dioxide, nitrogen oxides and particulate matter from coal-fired power plants.
INTERNATIONAL GAS EMISSION LEGISLATION
Eskom power stations collectively burn about 500 million tonnes of coal on the eastern Transvaal Highveld and produce about 1 million tonnes of sulfur dioxide gas (Tyson et ai, 1988). The tall stacks fitted to Eskom power stations are very effective in limiting ground level concentrations of sulfur dioxide (S02) and nitrogen oxides (NOx).
EFFECTS ON HUMAN HEALTH
If all the sulfur dioxide produced by the power stations over a full year in this volume of. At this concentration, it would take a person living on the Eastern Transvaal Highveld 20 years to circulate just one liter of sulfur dioxide through their lungs.
EFFECTS ON PLANTS AND MATERIALS
Compared to the 16 OOOL of naturally occurring hydrogen, methane, nitrous oxide and carbon monoxide that would also be inhaled over the same period, the concentration of sulfur dioxide is so low and within the limits considered harmless. Sulfur dioxide can remain in the air for 2 to 4 days and can be transported up to 1000 km away.
Excessive exposure to sulfur dioxide accelerates corrosion rates for metals such as iron, zinc, copper, and nickel, especially at relative humidity above 70% (8allinger Energy Series, 1979). Germany has strict legislative requirements for a maximum of 400 mg/m3 of sulfur dioxide emitted into the atmosphere.
FLUE GAS DESULPHURISATION
- Magnesium oxides
Heat from the process evaporates the remaining moisture before the hydrated lime reacts with sulfur dioxide (SO2), sulfur trioxide (SO3) and hydrochloric acid (HCl). Using FGC, the efficiency of ESP is improved quite significantly (Bosch and Hansen, 1997).
NITROGEN OXIDES
EFFECTS ON HUMAN HEALTH
- Selective Catalytic Reduction
- Lurgi Circulating Fluidized Bed Reactor Process
The choice of NOx removal generally depends on the stringency of the legislation requirements. The purpose of the catalyst is to improve efficiency for NOx reduction. The location of the plant can cause problems in retrofit situations due to the limited space between the boiler and the ESP.
As the catalytic action deteriorates, additional layers are used until the full life of the catalyst is reached. The sulfur content of the coal has a direct and noticeable effect on the electrical resistance of the fly ash produced.
USES OF FLY ASH
Wilkelstein (1996) in a study found a strong positive correlation between levels of suspended particles and stomach and prostate cancer and respiratory diseases. He found a weaker association with cardiovascular disease and a strong suspicion of a link between pollution and cirrhosis. Coal dust is considered a hazard because it can cause black lung disease, medically known as Coal Workers Pneumoconiosis.
Some of the signs of black lung disease are difficulty breathing, focal exacerbation, progressive formation of scar tissue (fibrosis), heart failure and increased risk of rheumatoid arthritis with a form of black lung disease ie.
EFFECTS ON PLANTS AND ANIMALS
REMOVAL OF PARTICULATE (FLY ASH) EMISSIONS
- Cyclones and Multicyclones
- Electrostatic Precipitators (ESP)
- Flue Gas Conditioning by Sulphur Trioxide
- Process Description
- Advantages
Also, the presence of moisture in the ash causes blockage, thus reducing the efficiency of the device. Discharge of the collected dust cake can be achieved by changing the flow or applying. Flue gas conditioning (FGC) is used to improve electrostatic precipitator (ESP) performance by reducing fly ash resistance and increasing fly ash cohesiveness.
The efficiency of ESP collection depends on the chemical and physical properties of fly ash, sulfur trioxide produced from coal sulfur, excess air and flue gas temperature. One of the main factors affecting ESP performance is the electrical resistivity of the fly ash.
ELECTROMEMBRANE DESULPHURISATION
- Process Equations
On the anion exchange side of the bipolar membrane, hydroxyl ions and sodium bisulfate react to form sodium sulfite. The catalyst is used at the boundaries between the cation and anion exchange parts of the bipolar membrane, for the generation of hydrogen and hydroxyl ions. For current densities greater than 100 Alm2, the current yield of hydrogen and hydroxyl ions is constant and equal to 0.95:.
The voltage reduction on the MB-3 bipolar membrane can be described by the equation: eMS = surface conductivity of the bipolar membrane. The electromembrane method has great potential due to the three-layer bipolar membrane, which has a catalytic effect in the water dissociation reaction.
ISHIKAWAJIMA- HARIMA HEAVY INDUSTRIES IN-LINE TYPE FGD
This method is based on the use of standard absorption apparatus for removing sulfur dioxide from flue gases by means of a sodium bisulphite solution. The resulting sodium bisulfite is converted to sulfite and sulfur dioxide using standard electrodialysis installations. After desulfurization, the flue gas enters a U-shaped channel where the absorbent liquid is separated from the gas by centrifugal force.
Majuba is located in the southeastern highlands of Mpumalanga province at an altitude of 1,709 meters above sea level. Particulate removal from the boiler flue gas is achieved using a pulsed jet fabric filter (PJFF) which, when in full commercial operation, will be the largest of its kind in the world (Smith, 1998).
PULSE JET FABRIC FILTER
- Future enhancements
The PJFF is designed to limit boiler particulate emissions to less than 50 mg/Nm3 when burning coal at a boiler load between 50 and 100% Maximum Continuous Rating (MCR) with 7/8 of the PJFF in operation (Smith, 1998). They are fitted with pneumatically operated multi-valve dampers allowing half of the PJFF to be isolated for maintenance. The PJFF is designed to operate automatically under supervision via touch-sensitive displays from the Unit Control Room (UCR).
It ensures that the flue gas entering the PJFF remains below the maximum design level and adjusts the bag purge frequency to maintain the preselected pressure drop across the PJFF. Potential ash spills from PJFF hopper level detection piping and dust treatment plant riser hopper vent valve due to pressure build-up.
ESKOM'S ENHANCEMENT PROGRAM
ECONOMIC ANAL VSIS
COST COMPONENTS .1 CAPITAL COSTS
- FIXED OPERATING COSTS
- VARIABLE OPERATING COSTS
The maintenance function is both preventive and corrective to keep equipment failures to a minimum. The variable operating costs depend on the price of electricity, steam and water and to a large extent on the price of waste disposal. Include costs for water for slurry, cooling and cleaning, electricity for pumps, fans, valves, light controls, conveyors and mixers.
COST CALCULATIONS
ECONOMIC EVALUATION FOR A WET SCRUBBING AND SPRAY DRY SCRUBBING PROCESS
- OPERATING COSTS
The distance of transport and the price of fly ash are very important for the profitability of the process. The wet scrubbing process is the most commonly used desulfurization process to remove sulfur dioxide. The advantage of the wet scrubbing process is that it produces a high quality by-product that can be sold to offset some of the operating costs.
Comparing the results of Table 7.5 with Table 7.4, the spray dry washing process is a more competitive process than the wet washing process. This is due to the fact that both capital and operating costs are lower, making the spray dry washing process the more economical option to consider.
SELECTIVE CATALYTIC REDUCTION
Capital costs for CFB are lower than for a wet FGD plant (gypsum process). For a wet FGD process, the operational costs associated with CFB would be higher because the stoichiometric SOx absorption factor is higher for CFB than for wet FGD. Based on a typical flue gas volume of 532 Sm3/s for a 600 MWe Eskom boiler, Lurgi considers the CFB plant to be less economical than wet scrubbing (Tilly, 1996).
Capital costs: 75% of equivalent wet FGD Operating costs: 130% of equivalent wet FGD The above are estimated costs.
ELECTROMEMBRANE DESULPHURISATION
Membrane technology appears to be most promising in that it has low capital and operating costs making this a cost-effective option to consider. A review of the various desulfurization processes shows that the most commonly used technique is the wet scrubbing process. The operating costs of this process are high, yet it produces salable high-quality by-products and is characterized by high S02 removal efficiency.
Although capital costs are lower than the wet washing process, operating costs are still higher. The circulating fluidized bed process has lower capital and operating costs than the wet washing, spray dry washing process or the selective catalytic reduction process.
RECOMMENDATIONS
Khan, S R, Desai, M Sand Gavin, A F, "NOx Control Technologies: An Overview of the Effectiveness for Stationery Combustion Sources", Proceedings from the American Power Conference, Pennsylvania, 1990. Lowe, P A og Perlsweig, M, "Recent Experience" for SCR Systems at Coal-Fired Utility Boilers", Proceedings of the American Power Conference, 1990. Mackenzie, G A, "The Newsletter of the UNEP Collaborating Center on Energy and the Environment", Riso National Laboratory, Danmark, april 1996.
Presidents Council, "Report of the Three Committee's of the President's Council on a National Environmental Management System", Government Printers, Južna Afrika, 1991. Takeshita, M, "Air Pollution Control Costs for Coal-Fired Power Stations", Center for Applied Energy Raziskava, januar 1995.
CURRENT LEGISLATION LIMITS WORLD WIDE
COMPARISON BETWEEN SOUTH AFRICAN AND WORLD ENERGY COUNCIL MEMBER SOxAND NOx EMISSIONS
Beryllium Short-term Long-term Industrial: poison at high synthetic combustion of concentrations, poison at low coal, rocket especially toxic concentrations, fuels. Sulfur increased increased sulfur is coal combustion dioxide respiratory organs contained at the moment. impairment, diseases and fossil fuels, represent morbidity and decline in smelters and between 60% mortality in respiratory volcanoes. combination with function with Uranus, fixed. combination with particles and moisture, the concentrations exceed the concentrations set by the EPA. Carbon behavior Increased risk Incomplete Evidence is not monoxide changes, of coronary combustion of decisive;. nausea, heart disease- fossil fuels, effects short - drowsiness, arterial coal, oil, gas, term, reversible;.
Additionally, graphs were drawn to show how various parameters varied on a daily basis and comparisons were made between emissions from the power plant and the limits set by the Chief Air Pollution Control Officer (CAPCO). The table below shows the relevant guidance values for sulfur dioxide set by CAPCO with the Department of Environmental Affairs and Tourism (DEAT).
COST ANALYSIS
OPERATING COSTS Maintenance costs
