NEW TECHNOLOGIES IN COAL-FIRED THERMAL POWER PLANTS FOR MORE EFFECTIVE WORK WITH LESS POLLUTION

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1 UDK : Dip.el.eng. Igor SEKOVSKI NEW TECHNOLOGIES IN COAL-FIRED THERMAL POWER PLANTS FOR MORE EFFECTIVE WORK WITH LESS POLLUTION Abstract Today people make a lot of analysis, of work of thermal power plants, to get more effective thermal power plants and to find means to make work of thermal power plants to be with less pollution. Thermal power plant are plants who turning primary energy into heat than heat into electricity. Thermal power plants can be : gas plants, fossil-fired plants and nuclear power plant. In this text I will analyze work of thermal power plant and tell how some technologies can be used to make power plants better. Coal-fired thermal power plant In thermal power plant most used fuel is coal. In Balcan countries most coal resources is lignite. Lignite is generally considered as low rank fuel. Lignite have low heating valve and high moisture and ash content. For that reason boiler will have poor combustion, and high maintenance. To have more better performance is used method to pulverize coal. Pulverized coal firing has become a dominant combustion technique owing to its great flexibility. Also, a wide range of coal types and furnaces may be used with this method. The great use of pulverized coal is in power production (in thermal power plants and industrial boilers for heat and power cogeneration. In many laboratory of thermal engineering and energy are carry out the examination of various coal and burner types, to get more better performance. A coal-fired power plant converts thermal energy into electric energy with the help of steam, for that reason they are called steam power plants. The combustion of coal produces hot flue gases which are used to generate steam. The steam flow through a turbine and passes on its energy to the turbine blades. This energy is converted into rotational energy. 1

2 The turbine shaft drives a generator which generates electricity. The spent steam is cooled in the condenser to create water, which is that pumped back into the steam generator. In figure1 we can see this process. figure1 1-desulphurisation, 2-dedusting, 3-dentrification,4-steam generator, 5-steam turbine, 6-induced draught fan, 7-burner, 8-coal mill, 9-feed water pump, 10- condeser, 11-generator, 12-transformator, 13-cooling water cleaning. We can see that coal is pulverized with coal mill and put into burner. Water is heated and steam is made. Steam go to steam turbine. Steam turbine is rotate and is connected with generator. Generator is producing el. energy. That el. energy is transform in el. energy with high level of voltage and that energy is distributed to people. A coal-fired power plant converts thermal energy into electric energy with help of steam which is why it is also called a stream power plant. Thermal process is known as Rankine cycle. 2

3 In next figure is diagram of temperature and pressure. If we analyze the diagram temperature-pressure, we can see that above critical point of water 374 O C and 22,06 M Pa there is no phase transition from water to steam. We need to improved that process and with that to improve efficiency of power plants. To improve that process and increase thermodynamic efficiency we make some improvement. Steam from boiler is heat with super heater than super heated steam go to turbine. After turbine steam is condensed with condenser and with pumps go to economizer. With economizer we heat water and that water enters into boiler, with that we increase efficiency of thermal power plant. 3

4 Now diagram will be : 4

5 We now can see that Rankine cycle is improved. To understand the process of making el. energy in thermal power plants I will analyze that process. Coal is burn in boiler and heat is created. To use all heat, we must use heat from boiler wall. That technology is called water wall. From tank (drum) with water with tubes water go down and is heated from boiler. Tubes are near the walls of boiler, thay are heated and then heated water is return back into drum and from heated water steam is created figure3. 5

6 figure 3 Since the efficiency of the plant is limited by ratio of absolute temperature of the steam at turbine input and output. Efficiency improvements require use of higher temperature and pressure steam. For that reason there are some improvements that are used to get more efficiency. Water from steam drum goes to the water wall heaters. From wall heaters the water rises through the water walls and is eventually turned in to steam, heat is generated by burners. As the water is turned into steam in the water walls, steam one again enters the steam drum. That steam pass through a series of steam and water separators and then dries inside the steam drum. The steam drum has internal devices that removes moisture from the wet steam entering the drum from the steam generating tubes, steam flows into the super heater coils. The steam is heated more from hot flue gases from boiler. Temperature is now super heated above the saturation temperature. The superheated steam is then piped through the main steam lines to valves before the high pressure turbine. Turbine can be with high pressure part, middle pressure part and low pressure part, but in some plant can be only with high and low pressure part. I will analyze turbine with three parts. To improve work of plant and turbine is used re-heater. Steam from high pressure turbine go to re-heaters tubes, is heated by hot flue gases, and with more energy steam now is used in turbine. From low pressure turbine steam is going to condenser. Hot water from condenser go to economizer. Condenser use cold water from river or cooling towers. Water is heated again in to economizer with flue gases. From economizer water go to steam drum. In some big thermal plants steam drum is not in use, they use only pipes and with that efficiency of power plant is increase. All these process can be seen in picture 4. All these is to increase efficiently of power plant. 6

7 Picture 4 1 -water wall, 2-boiler, 3-steam drum, 4-super heater, 5-economizer, 6- reheater, 7-high pressure turbine, 8-middle pressure turbine, 9-high pressure turbine, 10-generator, 11-condenser, 12-cooling towers, 13-hot water, 14-cold water. Additional increase of efficiency of thermal power plant is made when we use thermal power plant for district heat. In that type of power plant heat is extracted from the turbine in the form of hot steam. It is that formed via heat exchanges to a distribution system than supply the heat private households and industrial companies. With these effectively is improve the power plant. 7

8 We must know that fossil fuel combustion is one of the reason for the greenhouse effect. In the burning process, during transformations of fossil fuels into electrical power are generate CO, CO2, NOx, SOx and solid gas substances dust and they are emitted in atmosphere. Thermal power plant also emitted in atmosphere waste heat and make big pollution. For that reason there are some methods and technology to reduce pollution. In some clean coal technologies purify the coal before it burns. One type of coal preparation is coal washing. With that from coal are remove unwanted minerals by mixing crushed coal with a liquid and allowing the impurities to separate and settle. Other systems control the coal burn to minimize emissions of sulfur dioxide, nitrogen oxides and particulates. With wet scrubbers is remove sulfur dioxide. Sulfur dioxide is major cause of acid rain. Wet scrubbers work by spraying the gas with limestone and water. the mixture reacts with sulfur dioxide to form synthetic gypsum, a component of drywall. In the wet scrubbing process the capture rate for sulfur dioxide is in excess of 90 %. That process we can see on next figure. 8

9 Low NOx (nitrogen oxide) burners reduce the creation of nitrogen oxide. NOx is bad for ozone. These burners are restricting oxygen from NOx. Electrostatic precipitators removes particulates that aggravates asthma and cause respiratory ailments by charging particles with electrical field and then capturing them on collection plates. The de-nitrification systems uses the so called selective catalytic reduction method : ammonia is sprayed into the hot flue gas and the mixture is than routed via catalysts in which a chemical reaction takes places. In these place process over 70 % of nitrogen oxides are converted in to harmless nitrogen ( N 2 ) and water (H 2 O). During the de-dusting process, the fly ash is removed from flue gas from boiler. These process takes place in large electrostatic precipitators, were discharge electrodes create a strong electric field. The ash particles electrodes create a strong electric field. The ash particles are negatively charged and settle on the positively charged separators plates before being removed by beaters mechanisms. This process removes more than 99,9% of the fly ash from the flue gas. 9

10 Carbon capture and storage is perhaps the most promising clean coal technology. With that we catch carbon dioxide (C 02 ) emissions from thermal power plant. Since C 02 contributes to global warming, reducing its release into the atmosphere has become major concern. First technology is with flue gas separation of the C 02 with steam, and condenses the steam into a concentrated stream. Another process is oxy-fuel combustion. Oxy-fuel combustion burns the fuel in pure or enriched oxygen to create a flue gas composed primarily of C 02 and water. Third most used technology is precombustion capture. It removes O 2 before it s burned as part of gasification process. To increase boiler efficiency and reduce pollution there are two new technologies : pressurized fluidized bed combustion and integrated gasification combined cycle. Pressurized fluidized bed combustion is fully contained technology for processing solid fuels where fuel is suspended in a mixture of superheated air and sand, collectively called the,, fluid bed. In that mixture reagents like limestone are added and temperatures are controlled to directly capture the sulfur and reduce formation of nitrogen oxide with these technology can burn very dirty coal and we can remove 90 % or more of sulfur and nitrogen pollutants while the coal is burning. With integrated gasification combined cycle (IGCC) systems, stream and hot pressurized air or oxygen combine with coal are reaction that forces carbon molecular apart. That is resulting syngas. Syngas is mixture of carbon monoxide and hydrogen, is than cleaned and burned in gas turbine to make electricity. The heat energy from the gas turbine also powers a stream turbine. IGCC power plants create two form of energy and they have potential to reach a fuel efficiency of 50 %. 10

11 To reduce waste heat from thermal power plant we use cooling towers or river. Cooling tower can reduce waste heat with two methods dry or wet. In modern power plants to increase efficiency of cooling tower is use combination of these two methods. they are called hybrid cooling towers. By combination wet and dry cooling methods, hybrid technology allows low structural heights and ensures greatly reduced plume formation. First, the cooling water gives off part its waste heat to air via finned tubes. It than falls down into the cooling towers basin, were it cools further as a result of evaporative cooling, while giant fans press the cooling air through the wet and dry sections. Moist and dry air mix in the third of the cooling towers and these prevents nearly all plume formation. During the day the cooling tower is always operated in hybrid mode in other to minimize plume formation. At night the fan of dry section are switched off reducing self-consumption of power plant and therefore increasing efficiency levels. The amount of cooling required by any steam-cycle power plant is determined by its thermal efficiency. It has essentially nothing to do with whether it is fuelled by coal, gas or uranium. A nuclear or coal plant running at 33 % thermal efficiency will need to dump 14 % more heat than one at 36 % efficiency. That is one of many reasons to have power plant with more efficiency. If coal power plant is near to big river, lake or sea cooling can be achieved by simply running water trough the power plant and discharging it at a slightly higher temperature, but we must look what is critical temperature of river, lake or sea and keep temperature to below critical temperature. Conclusion When thermal power plant coal fired work it make pollution. In this paper I give short review and description of work of thermal power plant, how pollution can be low and how to have improved power technologies. Literature - Babcock & Wilcox Co. (2005) Steam: It s Generation and use 11

12 - PFBC clean-coal technology. A new generation of combinedcycle plants to met the growing world need for clean and cost effective power 1995,ABB Carbon AB - Поповиќ М : Топлинске турбине, Научна књига, Београд 1960 г. - British electricity International (1991), Modern Station Practice: Incorporating modern power system practice (3 rd edition) - Арсен Арсенов,Производство на електрична енергија,1996 ( Скопје) - Thermie , Technical annex 4.1 Solid fuels, European commission, Edition