Power Generation through Surface Coal Gasification

Transcription

1 Paper ID : Power Generation through Surface Coal Gasification Sri Tapas Maiti, Sri S. Mustafi IEOT, ONGC, MUMBAI, INDIA maiti.tapas@gmail.com Abstract Introduction India s oil reserve shall be exhausted by the year It is emphasized to use Syngas from coal as fuel for power generation as coal is the most abundant fossil fuel. Coal Gasification Technology Gasification is a process that converts coal to carbon monoxide and hydrogen which is called synthesis gas or syngas. Coal gasification processes available as on date can be classified into three major categories i.e. the Fixed/moving bed, the Fluidized bed and the Entrained bed systems. Moving bed gasifier is the best suited for Indian scenario. Moving-bed gasifiers are characterized by a bed in which the coal moves slowly downward and gasified by a counter-current blast to the coal. The hot synthesis gas comes out from the top of the gasifier and the impurities present in the syngas (e.g. Sox, NOx etc.) are removed downstream before using it to run the turbines. Integrated Gasification Combined Cycle (IGCC) Technology The syngas after purification is used to run the gas turbine to produce electricity. The exhaust gases exchange heat with boiler feed water in a heat recovery system generator(hrsg) to produce HP superheated steam which is fed to a steam turbine to produce additional power. This combination of Brayton cycle & Rankine cycle integrated with gasification of coal is called IGCC power generation. Overall efficiency and plant availability are much higher than that of normal coal based power plant. It is much more environment friendly. Conclusion Japan is engaged in developing advanced IGCC technology and achieved efficiency of 46%. The efficiency can be increased upto 57%in the process of development of new metallurgy for Gas turbines.the efficiency can be achieved upto 65% in the future on the development of newer methods using fuel cells, hybrid systems, advanced combustion systems etc. Main Paper: Fuel consumtion is considered as the measure of a country s growth and development.the consumtion of petroleum products in India is increasing very rapidly. India's oil imports is around 12,00,000 barrels/day (as on 2007). India s own oil production as on 2007 was 8,80,500 barrels/day against oil reserve of around 570, barrels which means that India s oil reserve shall be exhausted by the year Significant success to utilize other energy sources e.g. Nuclear, Solar, Wind etc. has not been achieved in India as indicated below: India is suffering from a severe uranium fuel shortage due to a lack of domestic uranium availability, which has held up its ambitious nuclear energy programme. Without uranium fuel, 1

2 its existing reactors are running at partial capacity producing less electricity and new plants have been delayed repeatedly. Despite an extraordinary investment of money and resources, nuclear energy is a small blip on India's energy horizon, providing barely 3 percent of the electricity produced in the country. Power generation through solar energy needs to have a bunch of solar collectors for a meager requirement. Solar energy may be very cheap but the solar collectors are relatively expensive and require regular upkeep in order to work properly and efficiently. Moreover it is not available continuously and hence hybrid system is required to cover up the deficiency. The main disadvantage regarding wind power is the unreliability factor. In many areas, the winds strength is too low to support a wind turbine or wind farm. In view of the above energy scenario it is emphasized to use the clean coal gasification technology to achieve the energy security of the country. World wide, coal is the most abundantly available fossil fuel accounting for 69% of world s fossil fuel reserve (against only 17% for oil). India is the third largest coal producing country with an estimated reserve of 245 billion tons which is likely to last for more than 200 years at current rate of consumption. Gasification became a commercial process in 1812 and was initially used to produce town gas for lighting and heating purposes. Since the early 1900s syngas has also been used as a chemical feedstock. The last years have seen the start of renaissance in gasification technology. There are several different reasons for this but first and foremost is the dramatic increase in energy cost. There are various range of products such as power, chemicals, fertilizers and transport fuels which could be obtained from gasification of coal. Electricity generation has emerged as a large new market since gasification is seen as a means of enhancing the environmental acceptability of coal, as well as of increasing the overall efficiency of the conversion of the chemical energy in the coal into electricity. One of the most attractive options to achieve extremely low environmental pollution is the Integrated Gasification Combined Cycle (IGCC). The IGCC concept opens the well proven combined cycle concept to so called dirty fuels such as coal, refinery residues etc. by adding gasification, air separation and gas cleaning processes to the upstream gas turbine combustor. Coal Gasification technology Gasification is basically a partial oxidation process that converts any carbonaceous feedstock such as coal, petroleum coke, heavy oil & oil tars, biomass etc. into a gaseous product called synthesis gas (syn-gas). Syn-gas mainly consists of carbon monoxide and hydrogen (with some CO 2, H 2 O and contaminants) with the composition depending on fuel and type of gasifier). In a gasification reactor, first drying of the feedstock particles takes place. As the temperature rises, devolatilization occurs and tars, oils, phenols and hydrocarbons are formed. With further increase in temperature these devolatilized substances further decomposed to lighter products like CO, H 2 and CH 4. The fixed carbon which remains after devolatilization is gasified via reactions with O 2, H 2 O and CO 2. Beyond hydrocarbons other elements like sulphur, nitrogen and ash are of major consideration because they also react and produce H 2 S, COS, HCN etc. have to be removed either out of gasifier or downstream in a gas cleaning process. There are three types of gasification technologies developed for IGCC application. The three processes are Fixed/Moving bed processes, Fluidized bed processes and Entrained flow processes. All types of gasifiers can be designed to operate with air or pure oxygen as gasification agent. Oxygen blown gasification is preferred against air as it gives higher cold gas efficiency and carbon conversion rate. It is also useful because it gives smaller plant component dimensions due to absence of nitrogen surplus. Gasifier technologies differ in temperature, carbon conversion rate, efficiency, feedstock size and hydrocarbon content in raw gas starting from fixed bed to entrained flow gasification. 2

3 Gasification Process Fixed-bed gasifiers are characterized by a bed in which the coal moves slowly downward under gravity as it is gasified by a blast that is generally, but not always, in a counter-current blast of steam & oxygen, to the coal. In such a counter-current arrangement, the hot synthesis gas from the gasification zone is used to preheat and pyrolyse the downward flowing coal. With this process the oxygen consumption is very low, but steam consumption is high and pyrolysis products are present in the product synthesis gas. The operating pressure is from bars and the outlet temperature of the synthesis gas is from deg. Celcius. Moving-bed processes operate on lump coal which sizes vary from 5-40 mm. An excessive amount of fines, particularly if the coal has strong caking properties, can block the passage of the up-flowing syngas. In this process carbon conversion time is relatively on higher side and it contains tar and phenols which are very difficult to handle. The moving bed processes are the oldest process. Right from sixties this was adopted by Lurgi at Sasol Fluidized-hed gasification is characterized by gasification temperature below the ash melting point and subsequently lower oxygen consumption. This process offers extremely good mixing between feed and oxidant, which promotes both heat and mass transfer. This ensures an even distribution of material in the bed, and hence a certain amount of only partially reacted fuel is inevitably removed with the ash. This places a limitation on the carbon conversion of fluid-bed processes. The operation of fluid-bed gasitiers is generally restricted to temperatures ( deg celcious) below the softening point of the ash, since ash slagging will disturb the fluidization of the bed. Sizing (1-5mm) of the particles in the feed is critical; material that is too fine will tend to become entrained in the syngas and leave the bed overhead. This is usually partially captured in a cyclone and returned to the bed. The lower temperature operation of fluid-bed processes means that they are more suited for gasifying reactive feed stocks, such as low-rank coals and biomass. The operating pressure is from bars. BHEL has developed a 6.2MW IGCC demonstration power plant based on coal gasification through 150 TPD capacity Fluidised bed indigenously at their trichy plant in the year

4 Entrained-flow gasifiers operate with feed and blast in co-current flow. The residence time in these processes is short (a few seconds). The feed is ground to a size of 100 µm or less to promote mass transfer and allow transport in the gas. Given the short residence time, high temperatures are required to ensure a good conversion (>98%), and therefore all entrained-flow gasifiers operate in the slagging range. The high-temperature ( deg. Celcious) operation creates high oxygen demand for this type of process. Entrained-flow gasifiers do not have any specific technical limitations on the type of coal used, although coals with a high moisture or ash content will drive the oxygen consumption to levels where alternative processes may have an economic advantage. Shell advocates this technology and is presently commissioning an Entrained Bed Gasifier in China with coal having 25% ash content. One important point to note throughout all the above is the significance of the slagging behavior of the ash. At temperatures above the ash-softening point, the ash becomes sticky and will agglomerate, causing blockage of beds or fouling of heat exchange equipment. Once above the slagging temperature, at which point the ash has a fully liquid behavior with low viscosity, it is possible again to remove it from the system reliably. Removal Of Impurities Removal of the impurities takes place immediately at the outlet of the reactor by means of a quench cooler in which most of the high-boiling hydrocarbons and dust carried over from the reactor are condensed and/or washed out with gas liquor from the downstream condensation Stage. 4

5 COAL FEED EXPANSION GAS LP STEAM BFW ASH ASH GAS BFW GAS WATER LIQUOR LIQUOR The gas liquor from the quenched cooler contains suspended matter, sulphur, chloride, NH 3 and NH 4 ions, phenols etc. The gas liquor is further processed for separation of tar and phenol. Then sour gas and ammonia are selectively & separately stripped from the liquor. The sour gas free of ammonia is further processed for sulphur recovery. Ammonia and Sulphur can be used for production of fertilizer and Sulphuric acid etc. The CO 2 can also be removed selectively before combustion and captured carbon dioxide gas can be put to good use, even on a commercial basis, for enhanced oil recovery. This is well demonstrated in West Texas, and today over 5800 km of pipelines connect oilfields to a number of carbon dioxide sources in the USA. Overall in USA, 32 million tonnes of CO 2 is used annually for enhanced oil recovery. The world's first industrial-scale CO 2 storage was at Norway's Sleipner gas field in the North Sea, where about one million tonnes per year of compressed liquid CO 2 separated from methane is injected into a deep reservoir (saline aquifer) about a kilometre below the sea bed and remains safely in place. The US$ 80 million incremental cost of the sequestration project was paid back in 18 months on the basis of carbon tax savings at $50/tonne. Power Generation The syngas, after purification, is burnt in the combustion chamber of the gas turbine with the air from the air compressor. The hot flue gases are expanded in the gas turbine to produce electricity. The exhaust gases still at a very high temperature exchange heat with boiler feed water in a heat recovery system generator(hrsg) to produce HP superheated steam which is fed to a steam turbine to produce additional power. This combination integrated with gasification of coal is called IGCC power generation. 5

6 Advantage Of Igcc Against Conventional Power Plant Conventional thermal power plants have the disadvantage of causing pollution of the environment at alarming levels. Emissions include oxides of sulphurs, nitrogen, carbon, dust and solid wastes apart from waste heat etc. In case of coal gasification these are eliminated down stream of gasification process and before combustion in GT. The overall efficiency in case of IGCC is more than 40% whereas it is around 33% in case of conventional thermal power plants The Plant Availability in case of IGCC is 85% against 50% in case of conventional thermal power plants because of frequent failure of boiler tubes. The IGCC plant can be commissioned in short duration (about 3 years) due to its modular configuration and capacity addition, if necessary at a later stage, is easy. High ash coal, which are abundantly available in India, are no problem for IGCC whereas they cause considerable problem in conventional thermal power plants. Water requirement is much less for IGCC plant in comparison to conventional thermal power plants Combination of GT & ST the combined cycle is the most efficient & environment friendly. So IGCC method has an edge over the conventional power generation. 6

7 Conclusion Petrotech-2010 Coal in conventional power plant releases about 75% more carbon dioxide per unit energy produced as compared to the other fossil fuels during the combustion process due to its higher carbon to hydrogen ratio. It is so far regarded as a low quality fuel. However,IGCC method eliminates most of the disadvantages of conventional power plant. It is to mention that Japan is engaged in developing advanced IGCC technology for enhancement of overall efficiency of the system. They have achieved efficiency of 46% and in the process of development of new metallurgy for Gas turbines (upto 1700 deg. Celcius) the overall efficiency could be achieved upto 57%. Finally R&D efforts on the development of newer methods that are more efficient and cleaner such as fuel cells, hybrid systems, advanced combustion systems etc. the overall efficiency can be achieved upto 65% in the near future. 7