Coal is one of the most important energy sources in the world for electricity and other applications. Coals are generally classified as high and low rank coals depending on their properties , especially heating value , moisture content , impurities etc. Above all , lignite or sub-bituminous coal is a type of low-rank coal (LRC) , and it has high moisture content (20% – 50%) , lower heating value (3 ,000 – 4 ,500kcal/kg on an as-received basis) , and high volatile content. These fuels hinder the operation of power plants because the high moisture content reduces the power plant's efficiency and increases flue gas emissions. Also , there are some disadvantages to using LRC such as costly transportation , potential safety hazards during transportation and storage , and operational problems. However , LRC is a competitive primary energy source for power generation. If these fuels are upgraded , those can be used to replace relatively expensive bituminous coals or be blended components with high-rank coal in existing boilers or in new designed boilers. Hence , an appropriate drying process is necessary to develop an energy-efficient power generation systems with lower environmental impacts. Among the other drying systems , the fluidized bed drying system has the advantage of temperature control due to uniformity of bed temperature and high drying rate since the fluidized bed drying involves high heat transfer and mass transfer rates through excellent mixing and gas-solid interactions. It offers a way of drying coal in more economical and environmentally acceptable means.
Recently , many power plants plan to build a CO2 capture and storage (CCS) process because regulations for greenhouse gas emissions have been reinforced. The CO2 concentration in earth atmosphere is increased by combusting fossil fuels to generate electricity. Capture of CO2 in flue-gas streams is essential for the control of CO2 emissions. Especially , one of the advanced concepts for capturing CO2 is an absorption process with dry re-generable sorbents. This CO2 capture process discharges CO2/steam mixture gas around 150oC of temperature after regeneration reactor. Therefore , we are to develop the fluidized bed dryer using the outlet gas out of the CCS process.
In this study , the purpose of our work is to integrate a CCS and a fluidized bed coal drying system. We produced the upgraded Indonesian coal which has a high moisture content of 35 wt.% using a batch type laboratory-scale fluidized bed dryer.
Figure 1. Schematic diagram of integrated CCS/coal-drying system.
Figure 1 shows the concept of integrated CCS/coal-drying system. As shown in the process flow diagram , the CO2 in the flue gas from fossil fuel-fired power plants is captured through CO2 capture process. Next , the CO2 and steam after a CCS process is used as heat sources for drying LRC. Furthermore , CO2 is used as fluidizing gas and steam is used for indirect coal drying through the heat exchanger. The experiment was performed using CO2 as a fluidization gas in a fluidized bed dryer (a 500 mm tall pipe with an inner diameter of 80 mm) at a gas velocity ranging from 4 – 6 Umf using a compressed carbon dioxide gas cylinder. The inlet gas temperature was varied from 100 to 200oC using the preheater. The drying rate of the coal was investigated in terms of the inlet gas temperature and the gas velocity in order to determine the optimum operating conditions. Changes in the moisture content of the coal , before and after the experiments , were characterized by a proximate analysis , an ultimate analysis , the higher heating value (HHV) , the lower heating value (LHV) , a particle size analysis , a thermo gravimetric analysis (TGA). In summary , we are to implement the integrated CCS/coal-drying system using CO2 and steam as heat sources for LRC drying.
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