Coal Fired Power Plants Engineering Essay

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In this chapter is going to be presented the function and some other aspects of a coal-fired power plant. First of all as coal-fired power plant it can be defined that plant which uses coal as fuel so as to produce electricity. Coal is a fossil fuel which is created through the compression of peat as it is buried under the earth. There are two general types of coal, the black coal and the brown coal. The typical mass of a black coal consists of [1]:

88% carbon

5% hydrogen

5% oxygen

1% nitrogen

1% sulphur

In this chapter it will be analyzed the thermodynamic principle on which the operation of a power plant is based and some other auxiliary functions which are significant for the proper operation. Furthermore it is presented the emissions of a coal-fired power plant and some efficient ways so as to be constraint.

3.1 Historical evolution of coal-fired power plants

The ever increasing demand for energy made it compelling the deployment of a technology which would have the ability to generate electricity in an effective and affordable way. On that basis the development of coal-fired power plants blocks started during 1950's when the first plants had a capacity of 60 MW and nowadays the capacity has raised up to 1010 MW in Europe and 1300 MW in the USA [2]. According to IEA [3] in year 2010 the total installed capacity of coal-fired power plants was more than 1600 GW and it is expected to be installed more 1000 GW until 2035. In Fig.1 it is presented the total capacity of coal-fired power plants installed through the years from 1920 up to 2004 worldwide an more specifically in countries such as the USA, China, Germany etc. where power demand is in very high levels. From the graph in Fig.1 it is obvious that the total capacity of the coal-fired power stations follows an upward trend. This enormous growth in coal-fired power plants can be explained on the grounds that coal is a very cheap fuel and in abundance in many places around the world as many studies have shown [4-7].

Fig. 1: Cumulative pulverized-coal plant installation between 1920 and 2004. Source: [8]

3.2 Clausius-Rankine Cycle

In this section it is presented the basic principle on which it is based the operation of a coal fired power plant. This principle is known from thermodynamics as the Clausius-Rankine cycle or steam cycle. In Fig.2 it is shown the four steps that conclude the steam cycle and the basic devices which are necessary so as to be implemented. More specific the working media is water and steam and in the first step (1-2) the pump increases water's pressure and therefore it is consumed work by the pump. Afterwards in the next step (2-3) input heat Qin from the combustion of pulverised coal is transferred to water which is evaporated and converted into steam, and steam is heated further. In the step (3-4) the steam is expanded from a high pressure turbine to a low pressure one and in this way mechanical work is generated in the shaft of turbines. Ultimately in the final step (4-1) the output heat is released and the steam is condensed into water again. Thus the work of the turbine gained is given by (1).

WT = Qin - Qout - WP

(1)

Fig.2: Steam cycle. Source: [2].

3.3 Operation of coal-fired power plants

In Section 3.2 it was presented the theory which applies in the function of a coal-fired power station. In this section it is described in more detail all stages of a coal-fired power plant and the way in which the basic principle is implemented in practice. In Fig.3 it is shown a schematic of a typical coal fired power plant and all devices that make it up.

The first step of the function of a coal-fired power plant is the supply of coal. This procedure is made through a conveyor belt which transfers coal to the coal hopper. After that coal is pulverized so as to become fine powder. In pulverized fuel boilers coal is pulverized into very small particles about 100 microns and this type of boilers is the most common [1]. The next step is coal to be burnt. Thus a preheated air stream drive the pulverized coal to the burners of the boiler, where fuel is burnt in short time and in this way it is produced a flue gas. This flue gas contains the chemical energy of the fuel (i.e. the coal) which has been converted into thermal energy. A portion of this thermal energy is transferred through radiation and convection into the water which circulates in a network of pipes inside the boiler and therefore the water is evaporated and converted into steam. This steam has very high temperature and pressure at this stage of the procedure (about 25 MPa and 500-600 oC [1]) and it is expanded from the high pressure turbine to the low pressure one. More specifically first the high pressure steam drives the high pressure turbine and the exhaust steam returns back to the furnace where it is reheated and drives the intermediate and low pressure turbines. This set of turbines rotates a shaft which is connected with a generator and in this way it is produced electricity. The exhaust steam which released by the low pressure turbine is cooled in the condenser and becomes water again. This water is pumped back to the network of pipes insight the boiler and thus the same procedure is iterated.

In the condenser cold water is circulated into tubes, which usually comes from a river or sea. Thus the heat of the exhaust steam is exchanged with this cooling water, which temperature is raised after that and respectively the steam is liquefied and becomes water again. If the plant is near the sea or river, then the cooling water flows back in the sea or river with a higher temperature which usually is 10-20 oC up [1]. Otherwise the warm cooling water should be processed through a cooling tower in order to be cooled. The cooling tower is a system, where the warm cooling water is driven in a higher altitude in the top of the tower and then it flows down, being exposed to an upward stream of air and in this way it is cooled.

As far as the flue gases are concerned, they are passed through different cleaning stages before discharged into the atmosphere through the stack. In more specific, the first step is to pass them through a device where the biggest amount of the dust particles is collected. This device is called precipitator. There are three kinds of precipitators which are bag filters, cyclone filters and electrostatic filters [1]. Next they pass into the desulphurisation unit so as the sulphur dioxide (SO2) to be removed.

C:\Users\George\Desktop\MSc SES\Carbon capture & transport\Assignment 1st\steam-power-plant.png

Fig.3: Schematic of a coal-fired power plant. Source: [9].

3.4 Efficiency of coal-fired power plants

The efficiency of power plant is a very significant factor, on the grounds that by improving it is needed less fuel to be consumed and CO2 emissions can be constrained. Of course it is not possible for every plant to have the same efficiency and there are many factors which can influence it [10]. In Fig.4 is presented in a flow chart which indicates the transformation of energy in one form to another, the losses in each stage and the total efficiency of a typical coal-fired power station. It can be inferred that the majority of losses occur during the conversion of thermal energy into mechanical in the turbines, where a big amount of thermal energy, i.e. heat is rejected through the condenser into the atmosphere. These losses are approximately 45% of the input energy and this fact is reasonable enough as it is explained by the second law of thermodynamics, which says that all heat engines have to reject some heat. Other significant losses occur in the boiler where about 6% of the input energy is lost in flue gas and in auxiliary procedures, such as the pumps where the losses are roughly 9%. Therefore a typical coal-fired power plant has about 30% to 40% percentage of efficiency [1, 2].

Fig.4: Conversion energy stages, losses and total efficiency of coal-fired power plants. Source: [2]

3.5 Emissions of coal-fired power plants

The typical emissions of plant which does not have any cleaning stages are [1]:

Carbon Dioxide (CO2): 700 tonnes/hour

Oxides of Nitrogen (NOX): 1t tonne/hour

Sulphur Dioxide (SO2): 1-20 tonnes/hour

Nitrogen (N): 2500 tonnes/hour

Steam: 150 tonnes/hour

Fly ash: 10-20 tonnes/hour

It is noticeable that about 2500 tonnes/hour of Nitrogen are released, nevertheless nitrogen is the major component of the air we breathe and therefore it is deemed harmless. Moreover about 700 tonnes/hour of Carbon dioxide are discharged during the combustion process and on world bases whole coal-fired power plants are responsible for 21% of global carbon dioxide emissions [10]. Despite the fact that CO2 might be harmless in small concentrations as it is a component of air mix, in bigger amounts it poses serious threats for the environment and contributes to the climate change as several studies have shown [11, 12]. Therefore it is compelling to reduce the emissions of carbon dioxide and for this reason it has been developed several techniques of capturing and storage carbon [2, 13]. Nitrogen oxides contribute to acid rain and harm people's health. They are discharged in bigger amount when the temperature of the boiler is higher [1]. Sulphur dioxide contributes also to the acid rain and therefore flue gases pass through the desulphurisation unit so as SO2 to be removed. Another very harmful emission of coal-fired power plants is the fly ash, which are known as particulates pollutes the environment in great extent and can also be responsible for respiratory problems in terms of people's health. However most plants are equipped with precipitators so as to remove this dangerous fly ash as it is referred in Section 3.3.

3.6 Advantages-disadvantages of coal-fired power plants

One major advantage of using coal for generating electricity is the reliability that offers. The coal-fired power plants can supply power to the grid with great reliability so as blackouts to be avoided during peak electrical loads. Except for that coal is very cheap fuel compared with other fuels and that fact makes this technology affordable enough and there is in abundance.

On the other hand the disadvantages of coal-fired power plants are that they release greenhouse gasses into the atmosphere

X. References

[1] BOYLE, G., EVERETT, B. and RAMAGE, J.: 'Energy systems and sustainability',(Oxford university press 2003).

[2] SPLIETHOFF, H.: 'Power generation from solid fuels', (Springer-Verlag Berlin Heidelberg 2010).

[3] FINKENRATH, M.,SMITH J. and VOLK D.: 'CCS retrofit. Analysis of the globally installed coal fired power plant fleet', (International Energy Agency 2012), p 17.

[4] ANDRULEIT, H., BABIES H.G., MEBNER, J., REHDER, S., SCHAUER, M. and SCHMIDT, S.: 'Reserves, resources and availability of energy resources 2011', (German Mineral Resources Agency, Hannover 2011).

[5] WORLD ENERGY COUNCIL: '2010 Survey of energy resources'. Available on: http://www.worldenergy.org/documents/ser_2010_report_1.pdf. Accessed in October 2012.

[6] BP: 'Statistical review of world energy June 2012'. Available on: http://www.bp.com. Accessed in October 2012.

[7] THIELEMANN, T., SCHMIDT, S. and GERLING J.P.: 'Lignite and hard coal: Energy suppliers for world need until the year 2100 - An outlook', International journal of coal geology, 2007, 72, pp. 1-14.

[8] YEH, S. and EDWARD, S.R.: 'A centurial history of technological change and learning curves for pulverized coal-fired utility boilers', Energy, 2007, 32, pp. 1996-2005.

[9] Image. Available on: http://electricalandelectronics.org/wp-content/uploads/2008/09/steam-power-plant.png.

[10] IEA: 'Power generation from coal: Measuring and reporting efficiency performance and CO2 emissions'. Available on: http://www.iea.org/ciab/papers/power_generation_from_coal.pdf. Accessed in October 2012.

[11] NORBY, R.J. and LUO, Y.: 'Evaluating ecosystem responses to rising atmospheric CO2 and global warming in a multi-factor world', New phytologist, 2004, 162, pp. 281-293.

[12] DELWORTH, T.L., MAHLMAN, J.D. and KNUTSON, T.R.: 'Changes in heat index associated with CO2-induced global warming', Climatic change, 1999, 43, pp. 369-386.

[13] GIBBINS, J. and CHALMERS, H.: ' Carbon capture and storage', Energy policy, 2008, 36, pp. 4317-4322.

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