REPUTACIÓN DE MARCA

The use of electricity has been an essential part of the South Africa economy for the last four decades. Coal power, an established electricity source that provides vast quantities of inexpensive, reliable power has become more important as supplies of oil and natural gas diminish. ESKOM, South Africa power utility company has a nominal capacity about 44, 193 MW. It generates approximately 95 % of electricity used in South Africa and almost 45 % of electricity used in Africa continent (ESKOM Abridge Report, 2009). The most economical method available is the use of abundant supplies of low-quality coal in Mpumalanga and the Northern Province; most of their power stations were sited next to the coal deposits. Coal power is a rather simple process. In most coal fired power plants, chunks of coal are crushed into fine powder and are fed into a combustion unit where it is burned. Heat from the burning coal is used to generate steam that is used to spin one or more turbines to generate electricity. ESKOM used conventional or pulverized coal fired power generation technology. Cyclone furnaces is another coal fired power generation technology available for use in electricity generation stations.

2.2.1 Conventional coal fired power station

In the 1920s, the pulverized coal firing was developed. This process brought advantages that included a higher combustion temperature, improved thermal efficiency and a lower requirement for excess air for combustion. The idea of burning coal that has been crushed into a fine powder stems from the conviction that if the coal is made fine as much as necessary, it will burn roughly as easily and efficiently as a gas. The feeding rate of coal according to the boiler demand and the amount of air available for drying and transporting the pulverized coal fuel is controlled by

computers. Pieces of coal are crushed between balls or cylindrical rollers that move between two tracks or "races." The raw coal is then fed into the pulveriser along with air heated to about 650 º F from the boiler. As the coal gets crushed by the rolling action, the hot air dries it and blows the usable fine coal powder out to be used as fuel. The crushed coal from the pulverizer is delivered to the boiler, usually by a conveyor belts. Air is blown into the boiler to burn the coal, creating heat energy.

The burning of coal heats water in the boiler to produce high pressure steam (transferring the heat energy from combustion into the steam). The combustion of coal in the boiler creates hot gases (including CO2) and ash. The ash is disposed of and the hot gases are released into the

atmosphere. The steam drives turbines that generate the electricity while the ‘smoke’ from the boiler is carefully filtered to remove as much of the unwanted emissions as possible. The steam passes through the blades of a turbine, causing the blades to rotate (like a fan). In the process, the steam’s heat energy is converted to kinetic (movement) energy. The spinning turbine blades are attached to a shaft (axle) that also spins a generator shaft. The low-grade coal produces a large amount of ash, which is returned to the ground and isolated from the environment in long-term storage.

2.2.2 Cyclone furnaces

In the 1940s, the cyclone furnace was developed. This new technology allowed the combustion of poorer grade of coal with less ash production and greater overall efficiency. Cyclone furnaces were developed after pulverized coal systems and require less processing of the coal fuel. They can burn poorer grade coals with higher moisture contents and ash contents to 25 %. The crushed coal feed is either stored temporarily in bins or transported directly to the cyclone furnace. The furnace is basically a large cylinder jacketed with water pipes that absorb the some of the heat to make steam and protect the burner itself from melting down. A high powered fan blows the heated air and chunks of coal into one end of the cylinder. At the same time additional heated combustion air is injected along the curved surface of the cylinder causing the coal and air mixture to swirl in a centrifugal "cyclone" motion. The whirling of the air and coal enhances the burning properties producing high heat densities (about 4700 to 8300 kW/m2) and high combustion temperatures.

The hot combustion gases leave the other end of the cylinder and enter the boiler to heat the water filled pipes and produce steam. Like in the pulverized coal burning process, all the fuel that enters the cyclone burns when injected once the furnace is at its operating temperature. Some slag remains on the walls insulating the burner and directing the heat into the boiler while the rest drains through a trench in the bottom to a collection tank where it is solidified and disposed of. This ability to collect ash is the biggest advantage of the cyclone furnace burning process. Only 40 % of the ash leaves with the exhaust gases compared with 80 % for pulverized coal burning. Cyclone furnaces are not without disadvantages. The coal used must have relatively low sulfur content in order for most of the ash to melt for collection. In addition, high power fans are required to move the larger coal pieces and air forcefully through the furnace and more nitrogen oxide pollutants are produced compared with pulverized coal combustion. Finally, the actual burner requires yearly replacement of its liners due to the erosion caused by the high velocity of the coal.

2.2.3 Environmental impacts of coal fired electrical generation

Many of the commonly held concepts about the environmental impacts of coal fired electrical generation are listed below. However, we believe that the benefits far outweigh any potential negative impacts.

1. Coal mining causes severe erosion, resulting in the leaching of toxic chemicals into nearby streams and aquifers, and destroys habitants.

2. About two-thirds of sulfur dioxide, one-third of carbon dioxide emissions and one quarter of the nitrogen oxides emissions in the U.S. are produced by coal burning.

3. Coal burning also results in the emission of fine particles matter into the atmosphere. Nitrogen oxide and fine airborne particles exacerbate asthma, reduce lung function and cause respiratory diseases and premature death for many thousands of Americans.

4. Smog formed by nitrogen oxide and reactive organic gases causes crop, forest and property damage. Sulfur dioxide and nitrogen oxides both combine with water in the atmosphere to create acid rain. Acid rain acidifies the soils and water killing off plants, fish, and the animals that depend on them.

Combustion generated pollutants, such as oxides of nitrogen (NOx), of sulfur (SOx), and

particulates, if uncontrolled and emitted into the atmosphere represent environmental and health hazards, such as acid rain. Environmental regulations supported by intensive research and developments have reduced pollutant emissions significantly. Improvements in efficiency and emissions come by increasing steam pressure and temperature in the steam cycle, and by increased turbine inlet temperature in the gas turbine cycle (Beer, 2010). Coal gasification produces a fuel gas that is capable of being used in the gas turbine. By integrating coal gasification with gas turbine and steam cycles, advantage can be taken of high efficiency and low pollutant emission while using coal, an inexpensive, secure and indigenous fuel in many countries throughout the world. A potential supplementary advantage of the Integrated Gasification Combined Cycle (IGCC) is the capability of capturing carbon dioxide (CO2) from

the fuel gas and making it ready for high-pressure pipeline transportation to a carbon sequestration site (Beer, 2010). This will be key to the commercial and clean co-production of electricity and hydrogen from coal.