COAL BOILER

Classification of coal quality are generally divided into two, namely the division in this scientifically based pembatubaraaan level, and the division based on the intended use.Based on sequence pembatubaraannya, coal is divided into a young coal (brown coal or lignite), sub-bituminous, bituminous, and anthracite. While based on the intended use, divided into coal steam coal (steam coal), coal coke (coking coal or metallurgical coal), and anthracite.Steam coal is coal which use the most extensive scale.

Based on the method, be using steam coal consist of direct utilization of coal that meets certain specifications used immediately after going through the process of crushing (crushing / milling) as the first coal power plant, then use the first process to facilitate handling (handling) such as CWM (Coal Water Slurry), COM (Coal Oil Mixture), and CCS (Coal Cartridge System), and then pemanfataan through a conversion process such as gasification and liquefaction of coalIn the coal power plant, the fuel used is coal steam which consists of sub-bituminous and bituminous class. Lignite also started to get a place as a fuel in power plant lately, along with the development of technology that can accommodate the generation of low-quality coal.

Figure 1. Electric generation scheme on coal power plant

(Source: The Coal Resource, 2004)

At the power plant, coal is burned in the boiler produces heat which is used to change the water in the pipe that is passed in the boiler into steam, which then is used to drive turbines and generators rotate. Electricity generation in power plant performance is largely determined by the thermal efficiency in coal combustion process, because in addition to an effect on the efficiency of generation, can also lower the cost of generation. Then in terms of environment, it is known that the amount of CO2 emissions per unit of calories from coal is the highest when compared with other fossil fuels, with a comparison to coal, oil, and gas is 5:4:3. So based on trials that get the results that the increase in thermal efficiency by 1% will be able to reduce CO2 emissions by 2.5%, then the thermal efficiency will be improved significantly reduce the environmental burden caused by burning coal.Therefore, it can be said that the combustion technology (combustion technology) is a major theme in the effort to increase coal utilization efficiency is directly at the same time anticipating the future environmental issues.Basically the method of burning the plant is divided into three, namely the burning of a layer of fixed (fixed bed combustion), the burning of coal powder (pulverized coal combustion / PCC), and the burning of a floating layer (fluidized bed combustion / FBC). Figure 3 below shows the type - the type of boiler used for each - each combustion method.Figure 2. A typical boiler by combustion method(Source: Idemitsu Kosan Co.., Ltd.)Combustion Layer FixedCoating method still uses stoker boiler for combustion processes. As the fuel is coal with ash content that is not too low and the maximum size of about 30mm. In addition, because of the limitation of coal grain size distribution is used, it is necessary to reduce the amount of fine coal that come mixed into coal. The reason does not use coal with ash content is too low is because the method of this combustion, coal is burned on top of a thick ash layer formed on the lattice of fire (fire traveling grate) in stoker boilers. If levels of very little ash, ash layer will not be formed on the lattice so that combustion will occur directly on the lattice, which can cause severe damage in that section. Therefore, the ash content of coal is preferred for this type of boiler is about 10-15%. The minimum thick layer of ash that is needed for combustion is 5cm.Figure 3. Stoker Boiler(Source: Idemitsu Kosan Co.., Ltd.)In this stoker combustion, ash from burning of small amounts of fly ash, only about 30% of the total. Then with an effort such as the burning of two levels of NOx, NOx levels can be lowered to about 250-300 ppm. Meanwhile, to reduce SOx, still needed additional facilities such as flue gas desulfurization equipment.Combustion of Coal Powder (Pulverized Coal Combustion / PCC)Today, most especially the large-capacity power plant is still using the PCC method on the combustion of fuel. This is because the PCC system is a proven technology and has a high level of reliability. Efforts to improve plant performance is mainly done by increasing the temperature and pressure of the steam produced during the combustion process.Development starts from the sub-critical steam, then super-critical steam, steam and ultra super critical (USC). As an example of USC power plant which uses technology is generating no. 1 and 2 belong to J-Power in Tachibana Bay, Japan, which boilernya respectively - each with a capacity of 1050 MW Babcock made by Hitachi. The resulting vapor pressure is 25 MPa (254.93 kgf/cm2) and the temperature reached 600 ℃ / 610 ℃ (1 stage reheat cycles). The development of steam conditions and graph generation efficiency improvement at PCC is shown in figure 4 in below.Figure 4. The development of steam power plant conditions(Source: Clean Coal Technologies in Japan, 2005)At PCC, crushed coal by using coal PULVERIZER used (coal mill) up to a 200 mesh (74μm diameter), and then together - the same with the combustion air is sprayed into the boiler to be burned. Combustion method is sensitive to the quality of coal being used, especially the nature ketergerusan (grindability), slagging properties, properties fauling, and water content (moisture content). Coal is preferred for PCC boilers that have properties ketergerusan with HGI (Hardgrove Grindability Index) above 40 and the water content of less than 30%, and the ratio of fuel (fuel ratio) is less than 2. Combustion with the PCC method will produce ash which consists itself of clinker ash as much as 15% and the rest of the fly ash.Figure 5. PCC Boiler(Source: Idemitsu Kosan Co.., Ltd.)...

When done burning, nitrogen compounds present in coal will oxidize to form the so-called fuel NOx NOx, whereas nitrogen in the combustion air will oxidize to form NOx too high temperature is called thermal NOx. In total NOx emissions in flue gas, fuel NOx content reaches 80-90%. To overcome this NOx, denitrasi action (de-NOx) in the boiler during the combustion process takes place, by utilizing the properties of NOx reduction in coal.Figure 6. Denitrasi process in PCC boilers(Source: Coal Science Handbook, 2005)In the combustion process, the speed of injection of coal powder and air mixture into the boiler is reduced so that the ignition and combustion of fuel also slows. It can lower the combustion temperature, which resulted in decreased levels of thermal NOx.In addition, as shown in Figure 6 above, the fuel is not all feed into the main combustion zone, but some included in the section on the upper main burner. NOx is produced from primary pembakara subsequently burned through 2 levels. In the reduction zone which is a first-degree arson or arson is also called reduction (reducing combustion), nitrogen content in the fuel is converted to N2. Next, do a second degree burning or combustion oxidation (oxidizing combustion), the form of complete combustion in the combustion zone. With this action, NOx in exhaust gas can be compressed up to 150-200 ppm. As for the desulfurization still requires additional equipment ie flue gas desulfurization equipment.Floating layer combustion (Fluidized Bed Combustion / FBC)In the combustion method FBC, coal crushed first by using the maximum-sized crusher to 25mm. Unlike combustion using coal stoker who put on the lattice heat during combustion or spray PCC method of coal and air mixture during combustion, coal grains kept in a floating position, by passing a certain wind speed from the bottom of the boiler. The balance between the upward push of the wind and gravity will keep the grains of coal remain in the floating position so as to form a layer of a fluid is always moving. This condition will cause the fuel combustion is more perfect because of the position of coal is always changing so that air circulation can be run properly and sufficient for the combustion process.Due to the nature of such combustion, the fuel specification requirements that will be used for the FBC is not as restrictive as in other combustion methods. In general, there are no special restrictions for levels of fly substances (volatile matter), the ratio of fuel (fuel ratio) and ash content. In fact all kinds, including low rank coal can be burned with either though using the method of this FBC. Only when the coal will be incorporated into the boiler, the water content attached to the surface (free moisture) are expected to not more than 4%. In addition to the above advantages, the value added of the FBC method is a tool used coal crusher is not too complicated, and the size of the boiler can be reduced and made compact.When the combustion temperature in the PCC is around 1400 - 1500 ℃, then the FBC, the combustion temperature range between 850-900 ℃ course so that the levels of thermal NOx that arise can be suppressed. In addition, the mechanism of combustion of 2 levels as in the PCC, the total NOx levels can be reduced again.Then, when the desulfurization equipment is still required for the handling of SOx in combustion method fixed and PCC, then at FBC, desulfurization can occur simultaneously with the combustion process in boilers. This is done by mixing limestone (lime stone, CaCO3) and the coal then simultaneously inserted into the boiler. SOx produced during the combustion process, will react with lime to form gypsum (calcium sulfate). In addition to the desulfurization process, limestone also serves as a medium for the fluidized bed due to its software so that the pipe heater (heat exchanger tubes) is installed in the boiler is not easy to wear.Figure 7. A typical FBC boiler(Source: Coal Science Handbook, 2005)Based on the working mechanism of combustion, the method is divided into two namely Bubbling FBC FBC and circulating FBC (CFBC), as shown in Figure 7 above. It could be argued that the Bubbling FBC FBC is a basic principle, while the CFBC is development.In CFBC, there is another tool installed on a boiler is a high temperature cyclone. Fluidized bed of media particles that have not reacted and unburned coal which flew with the flow of exhaust gas will be separated in the cyclone is then channeled back to the boiler. Through this circulation process, fluidized bed height can be maintained, denitrasi process may take more optimal, and higher combustion efficiency can be achieved. Therefore, in addition to low-quality coal, materials such as biomass, sludge, plastics, and scrap tires can also be used as fuel in the CFBC. The ash residue almost entirely of fly ash with the flue gas flow, and will be arrested first by using the Electric Precipitator before the flue gas exit to the chimney (stack).Figure 8. CFBC Boiler(Source: Idemitsu Kosan Co.., Ltd.)At FBC, when the pressure inside the boiler the same as the outside air pressure, called the Atmospheric FBC (AFBC), whereas when the pressure is higher than the outside air pressure, about 1 MPa, called the pressurized FBC (PFBC).Combustion air pressure factors influence the development of this FBC technology. To Bubbling FBC develops from PFBC to Advanced PFBC (A-PFBC), while for CFBC thereafter developed into the Internal CFBC (ICFBC) and then pressurized ICFBC (PICFBC).PFBCIn PFBC, in addition to the heat generated is used to heat water into steam to turn a steam turbine, combustion gas is also produced which has a high pressure gas turbine that can play, so that using a PFBC power plant generation has a better efficiency compared to AFBC due to a combination of mechanisms (combined cycle) is. Gross value generation efficiency (gross efficiency) can reach 43%.In accordance with the principles of combustion in FBC, SOx produced at PFBC can be suppressed by the mechanism of desulfurization along with combustion in the boiler, while the NOx can be suppressed by combustion at relatively low temperatures (about 860 ℃) and the burning of 2 levels. Because the gases of combustion are used again by running into the gas turbine, the combustion ash that come flowing out along with the gas needs to be removed first. Use CTF (Ceramic Tube Filter) can effectively capture these ashes.Pressurized condition that produces a better combustion will automatically reduce levels of CO2 emissions so as to reduce the environmental burden.Figure 9. Working principle of PFBC(Source: Coal Note, 2001)To further improve thermal efficiency, gasification unit partially (partial gasifier), which uses gasification technology floating layer (fluidized bed gasification) was then added to the PFBC unit. With the combination of gasification technology is the effort to increase the temperature of the gas at the entrance (inlet) gas turbine allows it to be done.In the process of partial gasification in the gasifier, the carbon conversion is achieved is about 85%. This value can be increased to 100% through a combination with the oxidizing agent (oxidizer). Further development of PFBC is called the Advanced PFBC (A-PFBC), the working principle is shown in Figure 10 below. Efficiency of net generation (net efficiency) which produced the A-PFBC is very high, can reach 46%.Figure 10. The working principle of A-PFBC(Source: Coal Science Handbook, 2005)ICFBCSectional boilers ICFBC shown in figure 11 below.Figure 11. Sectional boilers ICFBC(Source: Coal Note, 2001)As shown in the figure, the main combustion chamber (primary combustion chamber) and the decision space heat (heat recovery chamber) separated by a barrier wall mounted sideways. Then, because the pipe heater (heat exchange tube) is not attached directly to the main combustion chamber, then no worries about wear and tear of the pipe so that the silica sand is used instead of limestone for FBC media. Limestone is still being used as a reducing agent, SOx, only the numbers pressed in accordance with the purposes only.At the bottom of the main combustion chamber windbox attached to the wind flow to the boiler, where the small-volume air flows through the middle to create the layer moves (moving bed) is weak, and large-volume air flow through both sides of the windbox is to create a strong layer moves. Thus, in the middle of the main combustion chamber will form a layer moves down slowly, while on both sides of the room, the media will be lifted FBC strong upward toward the center of the main combustion chamber and then come down slowly - land, and then raised again by the large volume of the windbox wind. This process will create a spiral flow (spiral flow) that occurs continuously in the main combustion chamber. The mechanism of spiral flow of media FBC can keep floating layer so that a uniform temperature. In addition, because the flow is moving at a very dynamic, the disposal of unburnt material is also easier.Then, when the media is a powerful FBC raised up at the top of the barrier wall, some will be turned toward the heat collection chamber. Because the space is also taking a hot air flow from the bottom, then the space will be formed layers move down slowly as well. As a result, the media FBC will flow from the main combustion chamber leading to the capture chamber heat and then back again into the main combustion chamber, forming a circulation flow (circulating flow) between the two spaces. Using a heating pipe installed in the room taking the heat, the heat from the primary combustion chamber flows through the mechanism of circulation taken earlier.In general, changes in the volume of air supplied to the heat collection chamber is directly proportional to the coefficient of thermal conductivity as a whole. Thus it is only by setting the volume of the wind, heat and temperature levels keterambilan on floating layer can be well controlled, so that the load settings can be done easily as well.To further improve the performance of the generation, the process on ICFBC then pressurized by entering the unit ICFBC into pressurized container (pressurized vessel), hereinafter referred to as pressurized ICFBC (PICFBC). With this mechanism in addition to water vapor, will be produced also a high-pressure combustion gases that can be used to rotate so that the generation of gas turbines in combination (combined cycle) can be realized.Generation Coal Gasification Combined WithIncreasing the efficiency of generation with a combination of mechanisms through the use of synthetic gas gasification process results as in A-PFBC, the next generation of technology lead to further intensify the use of coal gasification technology into the generation system. This effort eventually resulted in the generation system called the Integrated Coal Gasification Combined Cycle (IGCC).Since this paper only discusses the development of power generation technology, then an explanation of how the coal gasification process takes place will not be described here.IGCCAn outline flow chart IGCC power generation system is shown in figure 12Figure 12. Typical IGCC(Source: Clean Coal Technologies in Japan, 2005) below.As shown in the figure, there are tools on the gasification system (gasifier) ​​used to produce gas, generally entrained flow type. Available on the market today for those types such as Chevron Texaco (now owned by GE Energy's license), E-Gas (formerly owned by Dow's license, then Destec, and last Conoco Phillips), and Shell. The working principle is the same all three devices, namely coal and high levels of oxygen incorporated into it and then performed the reaction of partial oxidation (partial oxidation) to produce synthetic gas (syngas), which is composed of over 85% of H2 and CO. Because the reaction takes place at high temperatures, ash in coal will melt and form a slag in a molten state (glassy slag). The heat generated by the gasification process can be used to generate high pressure steam, which then flowed into the steam turbine.Oxygen is used for the gasification process generated from the facility Air Separation Unit (ASU). This unit serves to separate the oxygen from the air through cryogenic separation mechanism, producing a yield of about 95% oxygen. In addition to oxygen, the ASU also produced nitrogen used as inert media for feeding coal into the gasifier, but can also be used to lower the temperature of the combustor so that NOx emissions can be controlled.In the synthesis gas, in addition to H2 and CO is also produced other elements that are not environmentally friendly such as HCN, H2S, NH3, COS, mercury vapor, and char.Therefore, the gas must be processed first to remove the part before it is sent to the gas turbine. Flue gas from the gas turbine and then flows to the Heat Recovery Steam Generator (HRSG) which serves to change the heat of the gas into water vapor, which then flowed into the steam turbine. With this mechanism, the efficiency of the resulting net generation is also far exceeds the generation of the regular system (PCC) that currently dominate. In addition to the generation efficiency, another advantage IGCC is very low emission levels of pollutants generated, fuel flexibility that can be used, water usage is 30-40% lower than conventional power plant (PCC), a significant level of CO2 capture, slag can be utilized to construction materials, and others - others.An example is the Nuon IGCC located in Buggenum, the Netherlands, with a capacity of 250mW. The plant produces a net efficiency of 43% (Low Heating Value), with the performance of environmental quality standards are very good. NOx emissions are produced very low at less than 10 ppm, then the sulfur removal efficiency above 99%, the level of flyash emissions, chloride compounds and volatile heavy metals that can be practically zero, and the waste water can be recirculated back so that no waste water disposal into the environment.In addition to these advantages, there are also weaknesses in the IGCC system developed at this time, for example, the amount of generation capacity is determined based on the number of units and gas turbine model to be used. Examples for GE Frame 7FA gas turbines with a capacity of 275MW. If IGCC will be operated with a generating capacity of 275MW, is quite a unit that is installed. When the second unit to be used, means the generation capacity to 550MW, and if 3 units it will be 825MW. Then when the desired generation capacity is under 200MW, then the model used is no longer the GE Frame 7FA, but GE 7FA with a capacity of 197MW. Similarly, if the generation capacity requires a smaller, then the GE 6FA a capacity of 85MW can be used.With the combination of model and number of gas turbine units to be used this, but will limit the generation capacity in the IGCC, is actually also will narrow the operating range. For example when going to lower the load at peak operation, it should be done by reducing the load on the gas turbine. Decrease the burden of this gas turbine will automatically lower the efficiency of generation and the consequences are less well on emissions of pollutants generated. Another weakness that need to be observed from the IGCC system today is the generation cost per kW and operation & maintenance (O & M) are more expensive, as well as the availability factor (AF) is lower than the PCC.IGCC history began in 1970 when the company STEAG of West Germany the expandable capacity of 170MW IGCC. Much later, demonstration project called Cool Water IGCC plant was launched in the U.S. in 1984, which operates a 120MW IGCC capacity until 1989. As of this writing, there is actually not a purely commercial IGCC units. The main cause is a large construction investments, as well as IGCC technology that has not been proven. IGCC technology here means the circuit of the entire building process (building blocks) that form the IGCC system intact. This needs to be emphasized because of their technology - for example, each unit in IGCC gasifier, HRSG, gas turbines, steam turbines, and the other is a proven technology. During the development of which lasted about 20 years since the Cool Water project, IGCC units are in commercial operation today both in the U.S. and in Europe in the first demonstration plant status. Examples of some of the IGCC plant is1. Tampa Electric Polk Power Station IGCC 250mW, located in Florida, USA. IGCC is operating since September 1996 under the Tampa project, using a gasifier of Chevron Texaco (now GE Energy). The fuel used is coal and petroleum coke (petcoke). The problem faced is more low carbon conversion rate compared with the planned value.Fauling had also occurred in the gas cooler.2. 260MW IGCC Wabash River Power Station, located in Indiana, USA. Operation since September 1995 under the Wabash River project, this plant uses gasification technology from Global Energy (now part of Conoco Phillips). Since the end of the project from the U.S. Department of Energy (DOE) in 2001, the fuel used is 100% petcoke.3. 250mW Nuon IGCC Power Station, located in Buggenum, Netherlands. This stems from IGCC Demkolec project that began in January 1994. The technology used is from Shell, the fuel is coal mixed with biomass (sludge and waste wood) to further reduce CO2 emissions. The problem that ever happened was a gas leak, the onset of cooler and cooler fauling on gas when mixed sludge of about 4-5%....

Figure 13. Nuon IGCC, Buggenum(Source: Thomas Chhoa, Shell Gas & Power, 2005)4. Elcogas 300MW IGCC Power Station, located in Puertollano, Spain. IGCC plant is in operation since June 1996 under the project Puertollano, using gasification technology from Prenflow (currently part of Shell). Fuel is a mixture of petcoke and coal ash 40% yield with a ratio of 50:50. Under the program of the European Union, this plant is planned as a place for the project taking CO2 (CO2 recovery) and H2 production.Taking into account various factors, including the generation of high efficiency, environmentally friendly factor, and a proven gasification technology, an effort to further reduce the weaknesses of IGCC have been started.Apart from cost, effort has also been conducted to further improve the efficiency of generation, namely by adding a fuel cell (fuel cell) into the IGCC system. Thus, there will be three types of combinations of generation in this new system of gas turbines, steam turbines, and fuel cell. Generation method is called with Integrated Coal Gasification Fuel Cell Combined Cycle (IGFC), the diagram shown in figure 16 alirnya below.Figure 14. Typical IGFC(Source: Clean Coal Technologies in Japan, 2005)In fuel cells, electricity generation is done directly through an electrochemical reaction between hydrogen and oxygen so that the energy loss rate and efficiency pembangkitannya little high. Hydrogen can be derived from natural gas, bio gas, or gases of coal gasification. Based on the material used for the electrolyte, the fuel cell is divided into 4-Phosphoric Acid Fuel Cell (PAFC), Molten Carbonate Fuel Cell (MCFC), Solid-Oxide Fuel Cell (SOFC) and Proton-Exchange Membrane Fuel Cell (PEFC). Below is shown the characteristics of the four types of fuel cells.Table 1. Characteristics of Fuel Cells(Source: Clean Coal Technologies in Japan, 2005)From the table above shows that fuel cells are suitable for combination with the generation of gas turbines is the SOFC, because the reaction produces very high temperatures.Compared with the PCC, the generation with IGFC method is theoretically capable of reducing CO2 emissions by 30%. Another plus is the high efficiency of the generation that can be achieved is at least 55%. Besides these advantages, there are several things to consider before IGFC really - really can be applied commercially. The first is the urgency of IGCC technology maturation, because IGFC basically is the development of IGCC. Then, the need for fuel cell development but low-cost high-efficiency, to support the generation cost competitive in the future.CoverDevelopments in power plant coal combustion technology has been presented above. In general it can be said that a growing technology does not depart from the principal so-called 3E, namely Engineering (technical side), Economy (the economy), and the Environment (the environment). In the early stages, Economy factor may be the primary consideration for the construction of generation facilities, followed by Engineering, and the last Environment. But along with efforts to reduce pollution or environmental contamination that caused the tightness of environmental quality standards, it appears that the order of 3E is starting to change. Environment factors are slowly ranks first in the consideration of technology development, and engineering, and last precisely Economy.Taking the example of IGCC, it is natural that the early stage of development would require a large fee. But along with the strengthening of environmental issues and the technology matures, it will decrease the cost and at a certain time would be competitive against existing technologies. Instead, the existing generation technologies, for example, which currently dominates the PCC, gradually will be more expensive to accommodate the environmental quality standards are increasingly stringent, and in the end it actually would cost in terms of economics. Showing below the generation cost comparison between IGCC and PCC in the U.S. over the last 20 years, and predictions in the future.Figure 15. Generation Cost Comparison IGCC and PCC per kW in the U.S.(Source: JCOAL Journal, vol.3, January 2006)From the chart above shows that over the last 20 years, the cost of generation for the PCC increased by about 50%. This increase is caused by the addition of equipment to reduce the environmental burden, such as facilities desulfurization (FGD). In contrast, the generation cost per kW at IGCC actually decline, and expected in 2010, its value will be equal to the PCC, which is about $ 1200.Reference1. Amick, Phil, Flexibility Coal Gasification for Fuels & Products, ConocoPhillips, 20052. Baardson, John A., Coal to liquids: Shell Coal Gasification with Fischer-Tropsch Synthesis, Baardson Energy LLC, 2003.3. Chhoa, Thomas, Shell Gasification Business in Action, Shell Gas & Power, 2005.4. JCOAL, Coal Science Handbook, the Japan Coal Energy Center, 2005.5. JCOAL, JCOAL Journal Vol. 2, nov. 2005, the Japan Coal Energy Center, 2005.6. JCOAL, JCOAL Journal Vol. 3, January 2006, the Japan Coal Energy Center, 2006.7. JCOAL, JCOAL Journal Vol. 4, mar. 2006, the Japan Coal Energy Center, 2006.8. Presentation Materials, Idemitsu Kosan Co.., Ltd., 2003.9. Sekitan no Kiso Chishiki, Sekitan Shigen Kaihatsu Kabushiki Kaisha.10. Shigen Enerugi Shigen Nenryou Bu-Chou, Ko-to-ru No. 2001 Nen Ban, Shigen Sangyou Shinbunsha, 2001.11. Sema, Tohru, Karyoku Hatsuden Souron, Denki Gakkai, 2002.12. WCI, The Coal Resource, World Coal Institute, 2004.

see also previous article "Chemical recovery boiler"