喜歡就充值下載吧，，資源目錄下展示的全都有，，下載后全都有，dwg格式的為CAD圖紙，有疑問(wèn)咨詢(xún)QQ：414951605 或1304139763 Flue gas purification and heat recovery: A biomass fired boiler suppliedwith an open absorption systemLars Westerlunda, Roger Hermanssona, Jonathan FagerstrmbaDivision of Energy Engineering, Lulea University of Technology, S-971 87 Lulea, SwedenbEnergy Technology and Thermal Process Chemistry, Ume University, SE-901 87 Ume, Swedena r t i c l ei n f oArticle history:Received 30 August 2011Received in revised form 14 December 2011Accepted 29 February 2012Available online 27 March 2012Keywords:Open absorption systemParticle reductionHeat recoverya b s t r a c tA new technique for energy recovery combined with particle separation from flue gas has been tested inthis project. A conventional small boiler for biofuel produces besides heat also particles to the environ-ment through the flue gas. Decreasing the impact on the environment is desirable. Increased efficiencycan be obtained if the temperature and water content of the flue gas can be further reduced. Installingan open absorption system in the heat production unit fulfils both these demands. An experimental unithas been built and tested in the last 2 years. The results show a reduction of particles in the flue gas by3344% compared to the ordinary system. At the same time the heat production from the unit increasedby 40% when fired with wet biofuels.? 2012 Elsevier Ltd. All rights reserved.1. IntroductionSmall biofuel boilers (?100 kW) have a high efficiency todaybut still the amount of particles in the flue gas is too high 1,2.The purification methods available on the market are too expen-sive for this size of boiler 3. The open absorption system can bebeneficially used in such heat production units. At the same timeas the amount of particles in the flue gas is reduced the main partof the remaining heat in the flue gas after the convection part isrecovered. The energy supplied to the system is mainly heat thatis produced by the boiler, and only a small amount of electricityis needed for circulation pumps.Sorption technology has advantages compared to conventionalheat recovery systems since latent heat in the water vapor canbe utilized better 4. The dew point limitation, a major obstacleto ordinary heat recovery appliances, does not apply to sorptionsystems. Ref. 4 also concludes that sorption systems have bettereconomic and environmental benefits than condensing boiler sys-tem. To our knowledge the open absorption system has not earlierbeen used for heat recovery and particle reduction in flue gas frombiomass boilers. It is assumed that the technique used will give thesame particle removal efficiency as a wet scrubber but a more effi-cient heat recovery.The open absorption system consists mainly of three compo-nents: the absorber, the generator and the condenser (see Fig. 1).The working medium (water) is produced by an external system(primarily from biofuel). The flue gas is brought into contact withthe absorption solution in the absorber. Water vapor is absorbedby the solution and the flue gas is dried, cooled and scrubbed ofparticles. The diluted solution is pumped to the generator wherethe absorbed water is separated from the solution by primary heatsupply. The concentrated solution is transported back to the absor-ber in a closed loop. The water vapor is condensed in the condensergiving off primarily the latent heat. The condensed and chilledwater is separated from the system after the condenser.2. Test facilityAt Lule University of Technology comprehensive researchwork concerning the open absorption system has been performedfor a long time 57. Development plants for different applicationshave been constructed, mainly for drying purposes 8,9. In the ac-tual case the conditions for the absorption solution were more se-vere, since particles and chemical reactions could ruin the stabilityof the solution. The facility was connected to an existing boiler andwith limited space for the equipment. A long distance between theabsorber and generator caused large heat losses during theexperiments.2.1. System descriptionThe open absorption system was constructed with stainlesssteel and integrated with the boiler as shown in Fig. 2 where theabsorber and generator are clearly seen. The third main part, thecondenser, is the heat exchanger (HEX 4). The cross-currentabsorber consists of a channel, filled with packings, to create good0306-2619/$ - see front matter ? 2012 Elsevier Ltd. All rights reserved.http:/dx.doi.org/10.1016/j.apenergy.2012.02.085Corresponding author. Tel.: +46 920 491000; fax: +46 920 491047.E-mail address: Lars.Westerlundltu.se (L. Westerlund).Applied Energy 96 (2012) 444450Contents lists available at SciVerse ScienceDirectApplied Energyjournal homepage: between the absorption solution and the flue gas. The flowdirection for the solution versus the gas gives the absorber itsname; the solution flows vertically while the gas flows horizon-tally. The generator consists of a number of vertical tubes wherethe hot flue gas flows inside. The tubes are surrounded by absorp-tion solution. Heat transferred from the flue gas is used for evapo-ration of absorbed water.A part of the hot gas from the boiler is used as heat input for thegenerator and the total gas flow passes the convection part of theboiler before entering the absorber. The by-pass over the absorberis closed during a normal run. The number of particles in the fluegas is reduced during contact with the absorption solution in theabsorber. At the same time the flue gas is cooled and dried. Afterthe absorber the flue gas flows to the chimney.The particles collected by the solution are separated by a smallcontinuous flow through the filter. The heat from the flue gas in-creases the temperature of the solution when it flows verticallythrough the absorber. In HEX 1 and 2 this heat is transferred tothe district heating system. The steam produced in the generatoris condensed and chilled in HEX 4 giving off heat to the districtheating system and the condensate is finally drained from the sys-tem. The district heating system is lastly heated through the con-vection zone in the boiler.Diluted absorption solution from the absorber to the generatoris preheated in HEX 3. Concentrated solution to the absorber iscooled when giving off the heat in this heat exchanger.2.2. InstrumentationFor an overall view of the system conditions, the temperatures(TTs) and liquid flows (FMs) were measured at the positions shownin Fig. 3. Psychrometer (dry and wet bulb temperature) was used toestablish the flue gas conditions after the absorber. Measurementswere taken at 10 s intervals.To control the system, the temperature in the generator shouldbe kept at a constant value. This is done by controlling the flow ofsolution to the generator and the amount of hot gas to the appara-tus. If the temperature in the generator is above the set point value,the diluted solution flow to the generator increases by increasingthe opening of the control valve (CV1). If it is fully open and thetemperature is still too high, the damper (CV2) decreases the flowof hot gas to the generator. The liquid level in the generator shouldbe constant and is controlled with the level transmitter (LT1) andcontrol valve (CV5).3. Method3.1. Heat and mass balancesA heat balance of the absorber consists of heat given off by theflue gas _Qabsorber 1 and heat taken up by the district heating systemand solution flows _Qabsorber 2 according to Eq. (1). The mass flow,temperature and heat capacity of the absorption solution changeand have to be included in a total balance._Qabsorber 1_mdryfluegas? hafter absorber? hbeforeabsorber?_Qabsorber 2qwater?_Vwater? Cp;water? TT3 ? TT1_msolutiontoabs? Cp;solutiontoabs? TT12 ? Tref?_msolutionfromabs? Cp;solutionfromabs? TT8 ? Tref?1The flue gas enthalpy (h) was calculated with normal psychro-metric correlations. The heat capacity value for the dry flue gaswas determined by adding each spices specific heat capacity valuemultiplied by their mass fraction. To estimate the conditions of theflue gas before the absorber the psychrometer could not be usedbecause of temperatures above 100 ?C. The water content wasdetermined through software Fluegas knowing the water contentin the fuel and the content of oxygen in the flue gas. The total massflow of dry flue gas from the boiler, was resolved with Fluegas andthrough manual measurements of the total volume of gas flow. Thecondensate volume flow was measured with flow meter (FM 2 inNomenclatureRoman letters_Qheat transfer rate (kW)_Vvolume flow (m3/s)Ttemperature (?C)TTtemperature transmitter and also temperature value(?C)_mmass flow (kg/s)Cpspecific heat capacity (J/kg K)hflue gas enthalpy (kJ/kg dry mass flow flue gas)qdensity (kg/m3)Subscriptsabsorber 1 energy from the flue gasabsorber 2 energy to liquid flowsrefreference value (0 ?C)convheat transfer rate from conventional boiler (kW)boilerheat transfer rate from boiler (kW)generator heat transfer rate to generator (kW)extraheat transfer rate according to increased area (kW)concconcentration of absorption solution (mass fraction)frac abspart of total flue gas flow which flows through absorber(mass fraction)AbbreviationsHEXheat exchangerTTtemperature transmitter and also temperature value(?C)LTlevel transmitterFMflow meterCUcontrol unitCVcontrol valvePpumpFig. 1. The open absorption system.L. Westerlund et al./Applied Energy 96 (2012) 444450445Fig. 2). The volume flow of concentrated absorption solution to theabsorber was determined by registration of the opening time forvalve CV5 in Fig. 3. During one opening period the volume of1.55 l flows to the absorber. The mass flow of diluted solution fromthe absorber could then be calculated by adding the mass flow ofcondensate to the concentrated solution mass flow.Heat balances for HEX 3 and 4 were also performed. On the pri-mary side of HEX 3 the latent heat for water vapor was included inthe balance.The total heat transfer rate was calculated with the mass flow inthe district heating system and the temperature difference be-tween TT1 and TT5.Amassbalanceforabsorbedwaterfromthefluegas_mwater absorbed and condensate _mcondensate was performed accordingto the following equation._mwater absorbed_mdryfluegas? xbeforeabsorber? xafter absorber?_mcondensateqwater?_Vwater2Fig. 2. The open absorption system integrated with the boiler.Fig. 3. Control equipment and measurement points in the test facility.446L. Westerlund et al./Applied Energy 96 (2012) 444450The absorbed water from the flue gas and condensate flow outof the system should be equal when running the plant in constantconditions.The humidity ratio and other variables were determinedaccording to earlier descriptions.3.2. Heat recoveryTo compare the technique with a conventional boiler systemand a water scrubber the following concepts and calculations wereperformed.Conventional heat transfer rate: Heat transfer rate from the boi-ler without the open absorption system was calculated accordingto Eq. (3). The large area in the generator caused a decreased fluegas temperature which is not the case using only the convectionpart in the boiler. It had therefore to be withdrawn to get the totalheat transfer rate._Qconv_Qboiler_Qgenerator?_Qextra3_Qboilerqwater?_Vwater? Cp;waterTT5 ? TT4_Qgenerator_mconc solution? Cp;conc solutionTT11 ? TT10_mcondensaterwater Cp;steamTT13 ? TT11 Cp;waterTT11 ? TT10_Qextra_mdryfluegas? hordinary? hwithopenabssystem?4In Eq. (4) the generator heat power includes heating of the absorp-tion solution, heating of water (liquid) and superheating of steam.In_Qextrausing enthalpy values include the energy content in themoisture.Moisture content of the fuel: Water content in the fuel (masspart).The district heating system temperature to heating unit: Measuredbefore HEX 1 i.e., TT1.Theoretical possible improvement: Calculated as the energy pos-sible to retrieve from the flue gas if the energy content after theboiler is decreased to the level in the flue gas leaving the absorber.This value is divided by the conventional heat transfer rate._Qth:improv_mdryfluegashafter theboiler? hafter theabsorber?=_Qconv5Measured improvement: Total measured heat transfer rate in the dis-trict heating system, divided by the conventional heat transfer rate._Qmeasimprov_QHEX1_QHEX2_mfrac abs_QHEX4_Qboiler !,_Qconv6Improvement rectified by heat losses: Total measured heat transferrate increased by theoretically calculated heat losses, divided bythe conventional heat transfer rate.Improvement with a water scrubber: A theoretical calculation ofan ideal scrubber without heat losses. The apparatus works with atemperature equal to the district heating system temperature tothe heating unit (TT1) and a flue gas temperature after the scrub-ber 5 ?C above this temperature. Reheating of the flue gas afterthe scrubber is not included in the calculations._Qwater scrubber_mdryfluegashafter theboiler? hafter thescrubber?=_Qconv73.3. Gas flow through the by-passA leakage is normal for a damper (CV3) in a flue gas system. Todetermine the magnitude of this leakage measurements were per-formed with closed damper. The air temperature was 20 ?C duringthe measurements and several different volume flows were inves-tigated. The total gas flow from the boiler and after the absorberwas studied using a micro manometer and pitot tube in differentpoints on each cross-sectional area. The lengths of straight linesbefore and after each cross-sectional area were fulfilled.The results showed that only 64% of the total volume flowpassed through the absorber depending on a high pressure dropover the absorber. The flow resistance is proportional to the squareof the volume flow in each circuit (absorber/damper-CV3), and thequota for these flow resistances could then be determined. Even ifthe temperature changes, this quota is almost constant as long asthe flue gas can be treated as an ideal gas. The conditions for theflue gas before and after the absorber are known. With the idealgas law it is possible to calculate the specific volumes before/afterthe absorber. The part of mass flow of dry flue gas passing throughthe absorber can then be determined in each experiment.3.4. Particle sampling and analysisParticle collection was performed with a 13-stages low pressureimpactor from Dekati (DLPI). The particles were separated accord-ing to different aerodynamic diameters in the total range 0.0310lm. Two impactors were used at the same time, before andafter the absorber. The gas flow to the impactors was taken isoki-netically. By comparing these results the purification of the flue gascould be established.Analysis of the particles chemical composition was performedwith a scanning electron microscope with an attached energy dis-persive X-ray detector (SEM-EDS).4. ResultsAnalysis of the experimental values was essentially based onheat and mass balances. Heat losses from the total system to theroom were calculated at 11.4 kW. This value was used in the eval-uation of all experiments. An increased electrical input using theopen absorption system consists of four liquid pumps and an in-creased pressure rise for the flue gas fan. In the experimental unitthe increased electrical input was estimated at 2.2 kW. The dis-tance between different parts of the system and small volumeflows makes instantaneous comparison of measured values diffi-cult. The part of mass flow of dry flue gas that passed throughthe absorber was 6566% of the total mass flow of dry flue gas fromthe boiler for all experiments. The flue gas temperature after theboiler was for all experiments in the range of 150180 ?C.4.1. Heat transfer ratesHeat transfer rates to the district heating system are presentedin Fig. 4. Each heat exchanger (HEX 1, 2 and 4) and boiler is shown,Fig. 4. Heat transfer rates to the district heating system.L. Westerlund et al./Applied Energy 96 (2012) 444450447and the total heat transfer rate (total) and heat transfer rate fromthe absorber (abs) are included. The heat transfer rate from the ab-sorber includes HEX 1 and 2.4.2. The absorption solutionThe relative humidity of the flue gas leaving the absorber is 3845% RH. This value is somewhat higher than the design data due tothe high absorption rate and absorption of CO2in the solution. Theconcentration of CO2is stabilized at a low level and separated fromthe absorption solution during heat supply in the generator. Afterthe experiments the capability of the absorption solution fromthe generator was compared with new solution that had neverbeen in contact with flue gas. The results showed only small differ-ences in absorption capability.4.3. Heat transfer between flue gas and district heating systemFig. 5 shows the temperature of the district heating system(TT1) before heat exchanger (HEX 1) and the dry temperature ofthe flue gas leaving the absorber (TT25). The flue gas temperatureis only 12 ?C higher than water temperature even though thereare two drops in temperature, flue gas to absorption solution andabsorption solution to water in the district heating system. Theheat exchanger (HEX 1) has a large heat transfer area. Not onlythe temperature but also the relative humidity of the flue gas leav-ing the absorber is of interest for the heat transfer rate. If the rela-tive humidity of the gas leaving the absorber increases, the heattransfer rate will decrease. The appearance in Fig. 5 around 16 hwas caused by external interference.4.4. Mass balanceComparison of calculated absorbed amount of water from theflue gas in the absorber with measured condensate mass flow afterHEX 4 is illustrated in Fig. 6. Average values for these variables cor-respond well. The time difference is large between absorbed waterand condensate from the generator, since large volumes of absorp-tion solution are used in the absorber and generator and only asmall amount of water is absorbed. Instantaneous values cantherefore not be used.4.5. Heat recovery improvementResults from six different experiments are shown in Tables 1and 2. Used concepts in the table are explained in Section 3.2.The increased electrical input is not included in the comparison.Tables 1 and 2 show that increased moisture content of the fuelresults in improved heat recovery. A lower temperature in the dis-trict heating system has the same influence. The heat losses in theexperimental set-up were large. The last column in Table 1 consti-tutes the most accurate comparison for the open absorption sys-tem with a conventional boiler. These values are only for an idealunit, since some heat losses will always arise. With decreasingwater content in the fuel the improvement decreases of course.The open absorption system is superior to a water scrubbersince the flue gas is dried using the absorption system. The energyin the moisture is taken care of which increases the improvement.Including reheating of the flue gas after the scrubber increases thedifference between the systems. Increased return temperature ofthe district heating system gives a reduction of the heat recoverythat is less significant for the open absorption system comparedto the water scrubber.Fig. 5. Temperature levels for district heating system and flue gas leaving thesystem.Fig. 6. Absorbed water from flue gas and condensate mass flow.Table 1Increase of the heat transfer rate from the system during different moisture contents in the fuel and different temperatures in the district heating system to the heating unit.Conventional heattransfer rate (kW)Moisture content ofthe fuel (%)The district heating systemtemperature to heating unit (?C)Theoretically possibleimprovement (%)Measuredimprovement (%)Improvement rectified byheat losses (%)6550.946.45023447051.741.04629466152.144.44525457052.052.34124414248.035.15519478028.038.3261025Table 2Comparison of heat improvement between installation of an open absorption systemand installation of a water scrubber.Moisture content ofthe fuel (%)Improvement rectified byheat losses (%)Improvement with awater scrubber (%)50.9442351.7462752