USING ENTRAINED FLOW PRE-DRYING TO IMPROVE THE THERMAL EFFICIENCY OF LIGNITE POWER STATIONS

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1 USING ENTRAINED FLOW PRE-DRYING TO IMPROVE THE THERMAL EFFICIENCY OF LIGNITE POWER STATIONS F.Nazra Hameed 1, Andrew F.A. Hoadley 1 ABSTRACT Department of Chemical Engineering Bldg 36, Monash University,Clayton,VIC Address: nazra.hameed@monash.edu, andrew.hoadley@monash.edu Brown Coal power stations have a low thermal efficiency due to the high moisture content of the coal. If Carbon capture and storage (CCS) is required, there will be a significant energy penalty, which further reduces the energy efficiency and therefore, increases the amount of CO 2 which must be captured per unit of electricity produced. Using flue gas to pre-dry the coal provides a way of removing waste heat without requiring as expensive heat transfer equipment as the rotary and fluidised bed dryer. In this study, a flue gas entrained flow dryer is modelled. The drying model considers different particle size distributions, flue gas velocities (which dictate the dryer cross sectional area) and a range of drying heights. It also analyses the variation in moisture content and particle temperature at regular intervals along the column. As the height of the dryer increases, the residence time increases, which provide an increase in the heat transfer area, which in turn increases the overall extent of drying. One objective is to reduce the moisture content of the coal exiting the dryer from 62wt% to 50wt% wet basis. To achieve this moisture reduction, flue gas at around 270 o C is cooled to 120 o C. The lower the flue gas exit temperature achieved, the higher the thermal efficiency of the boiler. One of the challenges of an entrained flow dryer is to ensure sufficient coal is entrained to achieve this. Finer coal size distributions ensure that a greater proportion of particles are entrained. INTRODUCTION Low Rank Coal (LRC) such as lignite account for a high proportion of the global coal reserves. The advantages of using LRC are low mining costs, high reactivity and high amount of volatiles with low pollution-forming impurities. However, their uses are restricted due to spontaneous combustion, low calorific value and high amounts of moisture. As a consequence of the moisture content, carbon dioxide emissions from brown coal fired plants are much higher than for comparable black coal plants. Carbon capture and storage (CCS) is an important method for reducing CO 2 emissions to the atmosphere. However, regardless of the exact CCS process employed, there is a significant energy penalty, which further reduces the energy efficiency and increases the amount of CO 2 which needs to be captured. Drying the coal before it enters the boiler is

2 therefore important, not only in improving the efficiency of the power generation, but also in reducing the energy penalty associated with CCS. Another feature of most CO 2 capture processes is that the flue gas must be cooled to ambient temperature in order to condense out any water and thus there is a significant amount of heat in this flue gas which must be removed. Using the flue gas to dry the coal makes use of the waste heat, without requiring expensive heat transfer equipment. In addition, flue gas drying can achieve higher plant efficiencies and reduce the risk of spontaneous combustion compared with some other drying techniques. LITERATURE REVIEW Pre-drying of coal is necessary to increase process efficiency prior to its combustion in the generation of electricity. There are various drying techniques that have been researched in relation to their impact on increasing the overall process efficiency. These methods range from evaporative drying methods to non-evaporative drying methods (Allardice et al., 2004, Bhattacharya and Tsutsumi, 2004). Evaporative drying methods include mill drying, fluidised bed drying, solar drying and hot gas or steam in entrained flow drying(mujumdar, 1987, Allardice et al., 2004). Non-evaporative drying methods such as thermal dewatering processes, mechanical thermal expression or solvent dewatering have also been researched as possible methods to dry coal. Superheated Steam Drying (SSD) technique involves the use of superheated steam in a convective dryer to supply heat and carry off the moisture (Kozanoglu et al., 2006). There are different types of dryers that have been previously tested using superheated steam such as fluidized bed dryers, sprayer dryers, impinging jets, agitated bed dryers, packed beds and vibrated fluid bed dryers with immersed heat exchangers (Mujumdar, 1995). One such SSD dryer is the Superheated Fluidized Bed Dryer (SFBD) is where the coal is dried using steam at atmospheric pressure or at elevated pressures. This technique offers an improved heat transfer between the phases, leading to short residence times and a compact design (Berghel et al., 2008). The Vibratory Fluid Bed Dryer technique was designed as it was suitable to combine coal transport with the drying process. Drying is achieved as the steam or flue gas passes through the vibrating coal. Coal that needs to be dried is introduced into a perforated trough which is inclined at an angle of and is vibrated using an electromagnetic oscillator. The velocity with which the coal is transported is dependent on the angle of inclination and the amplitude and frequency of the oscillator. An advantage of this dryer is the low heat requirement due to good use of the heating medium (Cheremisinoff, 2000). A direct rotary SSD uses Loy Yang lignite which is passed through a 13.2mm sieve. The average particle size is found to be 6mm. The technique involves a known feed rate of coal fed into the rotating drum via a hopper. The action of the rotating drum along with the flow of superheated steam results in the movement of the coal along the drum, exiting from the outlet of the drum. Coal fines are separated from the superheated steam using a cyclone(clayton et al., 2007). The energy consumption was relatively high, but this was 2

3 due to the pilot scale design. The water from this technique is good quality which can be used in other processes and the coal in this technique retains most of the organic and inorganic components(clayton et al., 2007).The dryer is able to achieve a moisture reduction from 61-17% in lignite. In Indirect Heat Exchange Steam Tube Drying, raw coal of size 6mm is crushed and injected to the inside of the heating tubes of a rotating steam tube drum via a hopper. While the drum is rotating, the coal dries as it was heated by low pressure steam which condenses in the shell side of the tubes. The coal is then ejected onto a Redler conveyor followed by a cooling conveyor where it was cooled by air circulation. The exhaust gas which contains water vapour, fine coal and air is cleaned by an electrostatic precipitator and vented to the atmosphere (Potter, 1984). Screw Conveyor Dryer (SCD) is a another type of drying where the wet feed is moved through a series of stages which involve heating and drying with the help of a screw conveyor while hot fluid is circulated in a jacketed vessel. It can be used for indirect heating of coal and this technique provides high heat transfer area to volume ratio. Another new technology for the pre-combustion drying of high moisture fuel is the RWE developed superheated steam fluidized bed known as WTA (Wirbelschicht-Trocknung mit interner Abwärmenutzung). It consists of a fluidized bed drying with internal heat recovery to obtain 110tph of dried lignite from an input of 210tph of raw lignite. Its principle is based on a stationery fluidised bed and the energy needed for the drying process is supplied by heat via heat exchangers that are incorporated in the dryer and heated by low pressure steam recompressed from the steam turbine or by recompressed evaporated moisture. Coal particles of size 2mm are injected into the equipment and the coal moisture which is evaporated is compressed between 45-60psi to utilize the latent heat. A superheated environment is provided in this technology and a final moisture content of 12% is achieved at C, with a reduction in grain size to 1mm as the particles exit the dryer (Sarunac, 2010). A recently developed technology by the U.S. is the GRE (Great River Energy) Low Temperature Coal Dryer. It uses a moving fluidized bed dryer and that is fluidized by preheated air. The continuous system is capable of processing 100t/hr of raw lignite. The evaporated moisture is discharged through the dryer stack and the evaporation process is achieved by using waste heat (Sarunac, 2010). This technique helps to reduce a quarter of the moisture content and it also reduces NO x by 7.5%, CO 2 by 0.5%, SO2 by 1.9% and increases boiler efficiency by 0.5% per point. However, the coal used has much lower moisture than Low Volatile (LV) coals; therefore amount of moisture removal if applied to LV coals would be lesser. HRL Technology Ltd. designed an entrained flow process in which the coal is dried using hot gas before it is fed to the gasifier. The cooled gas with the evaporated moisture is directed to the gas turbine after separation from the coal in a cyclone (Bhattacharya and Tsutsumi, 2004). 3

4 In this study, an entrained flow dryer using flue gas with pneumatic conveying is investigated and thermal efficiency for different dryer configurations is estimated. With the exception of the GRE dryer, the other drying methods all consume energy, either as steam or electricity, or degrade the quality of steam and this offsets the benefits of drying the coal. However, when waste heat is used, the power generation efficiency may be increased. MODELLING A schematic of the entrained flow dryer is shown in Fig. 1. The coal particles were modelled as spherical particles with a uniform cross sectional area. The model assumes that the coal particles exchange heat between the flue gas and each particle, but there is no heat or momentum transfer between individual particles themselves. The flue gas flowrate is calculated based on a stoichiometric calculation of the mass flowrate of flue gas produced per mass of pre-dried coal assuming a 2%vol residual O 2 flue gas concentration. Tab 1. summarises the system of equations used for modelling. Eqn. 1-3 is used to determine which size fractions of the coal are conveyed and which fractions pass out as underflow from the bottom of the dryer. The dryer is modelled as a series of vertical sections with a section height of 0.1m. Since the density of the coal reduces as it dries, it is reasonable to assume that all of the coal which is pneumatically conveyed in the first section continues through all the subsequent sections of the dryer. Each drying section is modelled by an energy balance between the coal particles and the flue gas, described by Eqns. 5 and 6. However a heat transfer constitutive relationship given by Eqn. 7 is also required to solve this system of equations and this relationship relates the total heat transfer rate to a particle surface heat transfer coefficient (assumed constant), the particle surface area and the average temperature differences between particles and flue gas within this dryer section. 4

5 Tab 1: Equations used for modelling the system EQUATION EQUATION NO. F w =m i.g 1 F d =0.5.C d.a i.v rel 2 V rel =V g -Vi 3 q icoal = m icoal.(h iout - H iin ) 4 Q coal = q icoal 5 Q fg = m fg. Cp fg. T diff 6 qi = U.Ai. T diff 7 [ T diff= ½.((Tg in -Tg out )+(Tc out- Tc in )] Notation: i- Mass of coal for each size F w - Weight Force m i - Mass of coal in each size fraction F d - Drag Force Ai- Interfacial area of coal for each size fraction V rel - Relative velocity between the particles and gas V g - Velocity of the gas V i - Velocity of the solids in a given size fraction U- Heat Transfer Coefficient H- Enthalpy of coal Q fg Energy in the flue gas m fg - Mass of flue gas Cp fg Heat capacity of the flue gas Tc Temperature of coal Tg Temperature of flue gas An iterative solution technique is required for each dryer section due to the non-linearity associated with coal particle enthalpy. 5

6 RESULTS AND DISCUSSION Fig 1: Schematic Diagram of Coal Dryer The drying trends with three different approximately normal size distributions are analysed. The size distributions 1,2 and 3 had D 50 of 2.5, 0.68 and 0.34 mm respectively.fig. 2 and 3 illustrate the variation in the flue gas and coal particle temperatures of size distribution 1 and 3 for each section of the dryer. The lighter particles are entrained and dried more quickly compared to the larger particles. The particles that are completely dried are those that are heated above 100 o C. The temperatures of these completely dried coal particles can potentially heat to the same temperature as the flue gas. As the coal passes up the dryer the flue gas temperature decreases and as this occurs the heat transfer driving force with the coal particles also decreases. Comparison of Drying and Entrainment for Different Size Distributions As a function of dryer height, Fig. 2 and 3 show that larger particles dry more slowly. There are two factors affecting this. From Eqn. 3, it can be seen that larger particles have lower velocities and therefore pass more slowly along the dryer. However, they also have a 6

7 lower heat transfer area per unit mass of water to be evaporated and it is this lower heat transfer area that restricts their rate of drying. The effect of particle size is clearly observed by comparing Fig. 2 and 3. Size distribution 3 having a D 50 of 0.34 had a greater portion of coal particles entrained. The flue gas temperature for Distribution 3 is reduced from C to C, whilst it is only reduced to 220 o C for Distribution 1. Comparison of Drying for Different Gas Velocities The modelling analysed two different gas velocities as shown in Fig. 2 and 4. Flue gas velocities of 5 and 10m/s were chosen to observe the behaviour of coal and the variation in coal and flue gas temperatures for different sections of the dryer. The greatest temperature change for flue gas occurred at the lowest gas velocity of 5m/s. Also the temperature change was largest towards the end of the dryer which signifies that the length of the drying section plays a significant role in determining the extent of drying. Lower gas velocities increased the residence time of the coal particles, thus allowing for more heat transfer between the flue gas and coal Temperature(degC) coalsize=0.151mm coalsize=0.1855mm coalsize=0.2mm coalsize=0.22mm coalsize=0.35mm coalsize=0.45mm coalsize=0.53mm coalsize=0.6mm FlueGas Height of Dryer (m) Fig 2: Variation in flue gas and coal temperatures for entrained particles of size 1 distribution with gas velocity of 5m/s for different sections of the dryer 7

8 Temperature(degC) coalsize=0.0755mm coalsize=0.1mm Fluegas Height of Dryer(m) Fig 3: Variation in flue gas and coal temperatures for entrained particles of size 3 distribution with gas velocity of 10m/s for different sections of the dryer Temperature(degC) coalsize=0.151mm coalsize=0.1855mm FlueGas Height of Dryer (m) Fig 4: Variation in flue gas and coal temperatures for entrained particles of size 1 distribution with gas velocity of 10m/s for different sections of the dryer 8

9 The dimensions of the dryer play a vital role in the extent of drying. However, the issue of a large cross sectional area will be of great concern if a lower gas velocity is used. The dryer could be arranged as a series of dryers to provide for a longer residence time during the initial stages of drying, where most of the moisture removal for finer coal particles take place. Comparison of Moisture Contents and Entrainment for Different Size Distributions Fig. 5 and 6 illustrate how the moisture content of the particles changes with increasing flue gas velocity at the exit of a 40m high dryer. At lower gas velocities, less than 50% of the particles are entrained, but the low gas velocity provided sufficient residence time for drying and as a result the average moisture content decreases. However at higher gas velocities, as more particles are entrained and the residence time reduces, there is not adequate time for heat transfer between the coal and flue gas and the moisture content increases and a large portion of the particles leave the system un-dried or partially dried. At 20m/s the average moisture content was approximately 1.4(db) for size distribution 3 which indicated the majority of the particles are entrained, but only the finest particles are dried. Tab. 2 gives the percentage of particles entrained with two different flue gas velocities. The finest particles had a greater portion of coal entrained in comparison to the coarser particles. A maximum of 80% entrainment was achieved with size distribution 3. Finer coal particles reduce the need for beater mills which dry and pulverize the coal. Also, this helps to save on energy and economic costs involved in running the mills Average Moisture Content (db) All particles Entrained Particles Flue Gas Velocity (m/s) Fig 5: Average Moisture content (db) of coal particles of size distribution 1 with flue gas of different velocities for a 40m drying column. 9

10 2 1.8 Average Moisture Content (db) Size distribution 1 Size distribution 2 Size distribution Flue Gas Velocity (m/s) Fig 6: Comparison of Average Moisture content (db) of all the coal particles of three different size distributions with flue gas of different velocities for a 40m drying column. Tab 2: Percentage of Entrained Particles for three different size distributions for Flue gas velocity of 5 and 10m/s D 50 (mm) Percentage of Particles Entrained by Mass Flue Gas Velocity (m/s) 5 10 Distribution Distribution Distribution CONCLUSION Drying coal involves a reduction in flue gas exit temperatures facilitating the integration of a carbon capture process. The modelling of three different coal size distributions in an entrained flow dryer showed a reduction in moisture content of brown coal from 62% to 50% as well as a reduction in flue gas exit temperature from C to C, which reduces the temperature of the flue gas which would need to be reduced in any case in a carbon capture process. In addition, the modelling investigated the coal temperatures at 10

11 regular intervals and the lighter particles were entrained while the heavier particles required further milling to ensure entrainment. Although the modelling involves brown coal having 62% moisture (wb), the drying model could be used to assess other low rank coals with different properties. ACKNOWLEDGEMENTS The authors would like to acknowledge the financial support of HRL Ltd. and in particular the involvement of and comments from Terry Johnson and Tony Campisi. REFERENCES ALLARDICE, D. J., CHAFFEE, A. L., JACKSON, W. R. & MARSHALL, M Water in Brown Coal and Its Removal. In: CHUN-ZHU, L. (ed.) Advances in the Science of Victorian Brown Coal. Amsterdam: Elsevier Science. BERGHEL, J., NILSSON, L. & RENSTRÖM, R Particle mixing and residence time when drying sawdust in a continuous spouted bed. Chemical Engineering and Processing: Process Intensification, 47, BHATTACHARYA, S. & TSUTSUMI, A An Overview of Advanced Power generation Technologies Using Brown Coal. In: CHUN-ZHU, L. (ed.) Advances in the Science of Victorian Brown Coal. Amsterdam: Elsevier Science. CHEREMISINOFF, N. P Evaporating and Drying Equipment. Handbook of Chemical Processing Equipment. Woburn: Butterworth-Heinemann. CLAYTON, S., DESAI, D. K. & HOADLEY, A. F. A Evaluation of Pilot Scale Drying of Lignite in a Keith Rotary Superheated Steam Dryer. Chemeca Sofitel Melbourne, Victoria. KOZANOGLU, B., VAZQUEZ, A. C., CHANES, J. W. & PATIÑO, J. L Drying of seeds in a superheated steam vacuum fluidized bed. Journal of Food Engineering, 75, MUJUMDAR, A. S Handbook of Industrial drying, Marcel Dekker. MUJUMDAR, A. S Handbook of Industrial Drying, Marcel Dekker. POTTER, O. E Comparison of Technologies For Brown Coal Drying, Davy Mckee. SARUNAC, N Power 101: Flue Gas Heat Recovery in Power Plants, Part II [Online]. Available: Flue-Gas-Heat-Recovery-in-Power-Plants-Part-II_268.html [Accessed ]. BIOGRAPHY Ms.Fathima Nazra Hameed is a First Class Honours graduate from Monash University. Her project focuses on coal drying. She loves food and travel and has lived in a few countries before coming to Australia. Dr.Andrew Hoadley is a senior lecturer at Monash University. His prime areas of interest are Wastewater cleanup, dewatering processes and biofuels. 11