Experimental study on performance of flow & desulfurisation of a gas-liquid screen scrubber for wet

Journal of harbin Institute of Technology( New Series), Vol 14, No 5, 2007xperimental study on performance of flow desulfurisation ofa gas-liquid screen scrubber for wet flue gas desulfurizationFANG Li-juan HUI Shi-en2方立军,惠世恩(l. School of Energy and Power Engineering, North China Electric Power University, Baoding 071003, China;2. School of Energy and Power Engineering, Xi'an Jiaotong University, Xi'an 710049, China)Abstract In the paper, the gas-liquid two-phase flow performance and desulfurisation performance of the gasliquid screen scrubber were experimentally studied when limestone was used as absorbent. Experiments werecarried out at varying the Mue gas velocity and slurry Aux in concurrent and countercurrent tower respectivelyThe experimental results showed that the flow resistance of absorber increased rapidly with an increase of theflue gag velocity whether in concurrent or in countercurrent tower, and the up trend of the flow resistance in theountereurent tower was higher than those in the concurrent one. The influence of the lue gas velocity on theflow resistance of absorber was more than those of the slurry flux density. Whether in the concurrent tower or inthe countercurrent one, increasing the flue gas velocity or the slurry flux density would enhance the desulphurization efficiency. The influence of the slurry thuu density on the desulfurisation efficiency was greater than thoseof the flue gas velocityKey words: gaB-liquid screen scrubber; wet lue gas desulfurisation( WFGD); low resistance; desulphurizationCLC number: TQ701 3Article II:10059113(2007)05072705Combustion of sulphur-containing fossil fuel, such sion amount of China has kept a high level about 20as coal and oil, results in sulfur dioxide emissions. The million tons annually. Pollution control of coal combusSO, is known to have detrimental effects on human tion in China is a very urgent task. New low-NO com-health and the environment, and as a consequence, bustion and flue gas desulfurization FCD techniqmany countnes have impsuitable for China should be researched and developed.coal-fired power plants over the past two decades 13. These techniques should be comparably effective, butFlue gas desulfurisation( FGD)is useful to decrease have low investments, operating cost and water cethe amount of SO, emitted from fired-plants. a number sumption, so that they can be widely used in Chinof different types of wet scrubbers have been developeMany effective factors can affect the desulfurisationin the past 20 years. Date on worldwide applications efficiency, and a main factor is the interfacial area bereflect that wet FGD technologies have been used at tween gas and liquid. How to obtain a high interfacialmost of the installations, 522 out of 668, completed area between gas and liquid and a minimum ratio bethrough 1998 2. Common examples include spray tween liquid and gas(L/G)is the way of exploring norubbers, packed towers, jet bubbling reactors, and vel wet flue gas desulfurisation devices. Based on thedouble-loop towers. The most commonly used and best- liquid-column WFGD, gas-liquid screen scrubber,astudied wet scrubber is the countercurrent spray scrub- new wet FGD facility has been developed and testedber employing liquid distribution at different levels in with limestone as sorbent in the paper. In the tower ofthe absorber. Wet scrubbers can properly control the e- the gas-liquid screen scrubber, the number of the nozmission of SO, from coal-fired plants, but their high ales is more than those in the liquid-column tower. Socost has discouraged their installation on existing units. the liquid aux density of the tower section increases andCoal combustion is the greatest atmospheric pollutionthe turbulent intensity between liquid and gas becomessource in China. The consumed energy in China takes more intense. The flow type between liquid and gas beabout 8-9% in the world, but the SO2 emission takes comes more complexity, and it is different from the oth-about 15.1%. The total SO, emission amounts in Chi- er two-phase now, such as the bubble flow, slug flowna were 23. 46 million tons in 1997, which was the first mass fleplace in the world. From 2000 to 2004, the so, emis- perfor中国煤化工 bydrokinetCNMHGReceived2005-07-12.Sponsored by the National Natural Seience Foundation of China( Grant No. $0476050)and the P. H D. Foundation o NCEPUJournal of Harbin Institute of Technology(Neu,Vo.14,Na.5,2007and the heat mass transfer in the tower has no repor- respectively. The measure points of the flue gas wereted. The objective of the paper is to find the perform. set at the inlet and outlet of the tower, and the compo-ance of the liquid-gas two-phase hydrokinetics and the nents of the flue gas were got from two gas analyzers redesulfurisation performance of the scrubber. The work spectively. The types of the two gas analyzer aremay give some help for the further research on the mass MRU95/3CD and KM9106transfer of the tower and is useful of the design and op-The experiments were performed using the follow-timum operation of this type of the absorbering procedure. Circulation slurry was bumped into theinner of the tower, and jetted vertical upwards through1 Experithein the bottom of the tower. The ligucolumn reached a certain height and dropped by theThe experimental setup is shown in Fig. 1. The gravity. During the course of the up and down,theexperimental system includes five parts, such as the sully broke into some large liquid masses and manytower, the flue gas simulation system, the flue gas small drops. The collision happened between the up-cooling system, slurry circulation system and testing fa- ward slurry and the down slurry, which can make thecility. The main components of the experimental setup large masses of liquid break into many small drops withare the absorber, the holding tank and the oil burner. different shapes. The small drops of liquid will flowThe height of the tower is about 8. 9 m, which section with the flue gas up and down. The large masses ofis square. The nozzles are set at the bottom of tower; slurry broke at the top of the liquid-column,andthe number of the nozzles is 8 in the concurrent flow dropped down at the gap of the liquid column. Thetower and 16 in the counter current flow tower as small drops will join up during the drop. This canshown in Fig. 2). The area of the square section of make a full contact between the slurry and gas.Theconcurrent tower is 0. 22 x0. 49 m, and the area of the slurry will drop into the tank and a circulation of thesquare section of the countercurrent tower is 0. 49slurry is completed. Produced by the oil bumer, the0. 49 m. Static pressures are typically measured along flue gas with a high temperature first goes to the fluethe inlet and outlet planes in the wet scrubber using gas cooler. The temperature of the flue gas is cooledwall taps equally spaced around the vessel. Average into the designed temperature about 80-90C.Theflue gas velocities at each plane can be established pure S02 was mixed to the flue gas and get the simulatefrom a measurement of the total mass flow through the flue gas with a certain $, concentrate that the experirubber by a micromanometer and knowledge of the ments needed. The simulate flue gas flowing into thecross-sectional area and the gas density. Slurry fux is tower contact fully with the slurry. By the physicsmeasured by the turbine flowmeter. The liquid flow di- chemical reaction, the slurry absorbs the so, in therection in the tower is defined the droplets fall to the flue gas. At the wet scrubber outlet, a demister is setbottom of the tower, and in concurrent tower flue gas and the small drops in the pure flue gas will be got ridhas the same flow direction with the slurry droplets flow of. At last the pure flue gas exhaust through stack bydirection, otherwise is in the counter current tower. In the draft fan. The gypsum produced in the bottom ofthe experiments, the flue gas flow through the water tank, which is drained by the slurry bump. The freshwith concurrent type firstly and then gas-liquid fleslurry was recruited into the tank to control the Ph valwith countercurrent type, and the two towers carry out ue of the tankthe concurrent and counter current tower experimentspointLimestoneabsorberf Bumer Slury Lovalve中国煤化工CNMHGOxidization fanFig. 1 Schematic illustration of the wet FGD pilot plant based on the Gas-liquid screen728Journal of Harbin Institute of Technology( New Series),Vol. 14, No 5, 2007droplets, Ceth the twoperiment data show flow resistance produced by thedisperse phase of the liquid mass and droplets is muchmore than those produced by the tower wall. So the lat应府ter one can be neglected. In the paper, the friction be-tween the disperse phase of liquid mass and dropletsand the flue gas mainly brings the flow resistance of thetower. Fig. 3 shows the influence of the flue gas velocities on the flow resistance coefficient of the tower. The(a)The position of noles (b)The position o nozzles ininfluence of L/G ratio on the flow resistance coefficientof the concurrent and counter current tower are preseFig 2 Distribution of the nozzles in the bottom of the towin Fig. 4(a)and (b), respectively. It can be seenFrom the present experiments, the simulant fluethat whether in the concurrent tower or in the countergas flux ranged from 1500 to 3000N. m'/h and the current tower the flue gas velocity has a prominent imcirculatory slurry flux ranged from 5 to 30 m/h. In the pact on the flow resistance of the tower. The low re-concurrent tower the circulatory shurry flux is from 5 tostance coefficient decreased as the lue gas velocity30 m/h and in the counter current tower the flux is increase, however, the flow resistance of the absorberfrom 20 to 55 m/h. The circulating slurry tank Ph is increased with an increase of the flue gas velocity rebetween 5. 4 and 5.9, and SO2 concentration of fluemarkably. The slurry flux density can also get the samegas at the wet scrubber inlet is between 2000 and 3000effects on the flow resistance of the absorber like thmin. All the experimental conditions are shown in flow resistance coefficient decreases as the liquid fluxincreases. At a high flue gas velocity the decreasingTab. 1 The experimental conditionsdegree of flow resistance coefficient become smallFlue gas flum/(N·m3·h)Comparing the effects on the flow resistance betweenslurry fluxthe flue gas velocity and the slurry flux density, the/(m3·h-1)1∞o20002500SGK16former is larger than the latter. So a suitable flue gasvelocity and a high slurry flux density may get an opti-SGK2 SGK7 SGK12 SGK17 SGK22mum work condition in low flow resistance. In the con-current towerSGK3 SGK8 SGK13 SGK18 SGK233. 87 m/s to 9. 02 m/s, the maximun pressure differSGK9 SGK14 SGK19 SGK24ence at a fixed slurry flux is 183.8 Pa. While in theSCK10 SCK15 SGK20 SGK25 counter current tower, as the flue gas velocity is from1. 74 m/s to 4. 05 ms, the maximum pressure differNGK6 NGK11 NGK16 NCK21ence at a fixed slurry flux is 172. 5 Pa. So the effectsNGK7 NGK12 NGKI7 NGK22oned by the fluNGK8 NGK13 NGK18 NGK2the counter curent tower are greater than the ocity irent orNGK4 NGK9 NGK14 NGKI9 NGK24Fig. 4 illustrates whether in the concurrent towerNGK5 NGK10 NGKI5 NGK20 NGK25or in the counter current one the flow resistance coefficient increases slowly with the increase of L/G ratio. At2 Results and Discussionlow flue gas velocities, the linearity relationship between flue gas velocity and flow resistance is agreed2.1 Flow Resistance of the Wet Scrubber and An. well, but with the flue gas velocity increase the in-of the flow resistance is greater thanThe now resistance of the tower is one of the main thoseG ratio. In the concurrent tower. theparameters about the flow performances of absorber. In flowincreases smoothly with the flue gas vepractice, LG ratio is usually as a key designing param- locity in the counter current tower, the flow resistanceeter. In the paper, effects of flue gas velocity and L/G increases greatly with an increase of flue gas velocityratio on the flow resistance of the concurrent and count- when the experimental conditions in low L/G ratios ander current tower are studied.high 1-In the tower, the flow resistance is mainly pro- the hi中国煤化工 1duced by two parts friction. One is the friction of flue L/GCNMHGAue gas velocitiesgas with tower wall and the other one is the friction of theice ot WG ratios on the tlow resistance is noAue gas with disperse phase of the liquid mass and obvious. In the same flue gas velocity, the main influ-Journal of Harbin Institute of Technology( New Series), Vol 14, No 5, 2007ence factors on the flow resistance are the slurry fluxdensity and the velocity of jetted slurry at the mouth ofnozzles. The slurry flux density or the velocity of jetted=1.74mcrease of L/G, which may increase the flow resistance.ship between flue gas velocity and flow resistance is notagreed well, the increase degrees of the flow resistanceis greater than those of L/G ratio.L=30(b)The counter current towerFig 4 Infuence of L/G ratio on the flow resistance of the2.2 SO, Absorption from Flue Gas and AnalysesThe effects of flue gas velocity, slurry flux densitywere studied. Fig. 5 showed the influence of the fluevelocities on the desulfurisation efficiency, andVelocity of flue gas flow/(m-g)ffects of LG ratio on the desulfurisation efficiency(a) The concurrent towerwere showed in Fig. 6.言80velocity of flue gas flow/(mg">(a) The concurrent towerFig 3 Influence of the flue gas velocities on the flow resist.ance of the tower0p=3.87m/a246810121416182022中国煤化工(mCNMH Gerties on the desulfurrisation efticiencyJournal of Harbin Institute of Technology( New series), vol. 14, No 5, 2007From Fig. 6, whether in the concurrent flow toweror in the counter current tower the desulfurisation efficiency increased quickly with an increase of L/G ratioat the same flue gas velocity, and in the counter cur-rent tower, the up trend of the desulfurisation efficien-cy is quicker than those in the concurrent one. Comparing Fig. 4 with Fig. 6, we found that L/G ratio hasmore influence on the SO, absorption efficiency than itdid on the flow resistance of the absorber. IncreasingLG ratio obtained high desulfurisation efficiency andhe flow resistance did not increase more which w1012141618suitable to the experimental conditions at the high flueL/G(a)The concurrent towergas velocities.Conclusions1)Whether in the concurrent tower or in thecounter current tower of the gas- liquid screen scrubberthe flue gas velocity has prominent influence on theflow resistance of the tower. The flow resistance of theabsorber increased with an increase of the fluee gas vlocity remarkably. The slurry flux density has the sameeffects on the flow resistance of the absorber like theflue gas velocity. The lue gas velocity has more influ-ence on the flow resistance than the slurry flux density510152025303540did2)The desulfurisation efficiency of the absorber(b) The counter cument towerFig 6 Influence of LG ratio on the desulfurisation emincreased smoothly with an increase of the flue gas ve-locity. Whether in the concurrent tower or in thecounter current tower, the desulfurisation efficiency in-Whether in the concurrent flow tower or in the creased with an increase of flue gas velocity or slurrycounter currentthe desulfurisation efficiency in-flux density. L/G ratio has more influence on the SOcreases with an increase of slurry flux density at the absorption efficiency than it did on the flow resistancesame flue gas velocity. The slurry flux density influ- Increasing DG ratio obtained high desulfurisation effiences the interfacial area and the mass transfer coeffi ciency and the flow resistance did not increase morecient. So the desulfurisation efficiency increased. Ashich was suitable to the experimentalconditions at thethe flue gas velocity increased, the up trend of the des- high flue gas velocitiesulfurisation efficiency smoothly. The velocity of flue Referencesgas can influence both mass transfer and the turbulenceof the gas and liquid flow. On the one hand, the in[1] Soud H N. Developments in FGD. CCC/29. IEA Coal Recreased flue gas velocity can enhance the reaction ofgas-liquid mass transfer. On the other hand, the in-[2]Search, 2000creased Que gas velocity can enhance the turbulence oftechnologies: a review. Environment Progress, 2001,20(4):219-227the gas and liquid flow, the collision between the gas [3] Xu Xuchang, Chen Changhe, Qi Haiyin, et al. Develop-and the flue gas may be fiercer than ever and bringment of coal combustion pollution control for S0, and NO. inmore small diameter drops which can enlarge the interChina. Fuel Processing Technology, 2000, 62: 153-160facial area between gas and liquid. So we can draw the[4] Lin Zonghu. The Gas-liquid Two-phase Flow and Bubblingsaton efliciencyHeat Transfer. X’an:xi’ an Jiaotong University Pressby increasing the flue gas velocity is similar to the one1987.45-48the slurry flux. Th[5] Kong Hua, Gao Xiang, Liu Tongbo, et al. Experimentalconsistent with those reported earlier in impinge stream中国煤化工m,201,CNMHG