environment impact evaluation of a new type continuous mixing

ENVIRONMENT IMPACT EVALUATION OF A NEW TYPE CONTINUOUSMIXING PLANT FOR DAM CONSTRUCTIONTakayuki OBARA, Xuehui AN, Feng JINTsinghua UniversityABSTRACT: On concrete dam construction work, it is necessary to manufacture a large amount of concreteand CSG (Cemented Sand and Gravel). The manufacture plant of concrete and CSG is operated in very longterm, therefore the effect of its environmental impact could be serious. In general, there are two types ofmanufacture plant on the process of manufacturing concrete or CSG. The one is batch type plant; its mixingprocess is not continuous and it is necessary to repeat the processes of measuring and mixing every batch.The other is continuous mixing plant; mixing process is completely continuous, at the same time, themeasurement and supplement of materials are also carried out continuously. Since continuous mixing plantcan manufacture a large amount of products at short time, continuous mixing plant is suitable formanufacture plant of concrete dam construction. But it is hard to control the quality of products, so batchtype plant used to be adopted as the mixer of the manufacture plant. In recent years, the new continuousmixing plant which can control the quality of products accurately has developed and been put to practical useon several projects of concrete and CSG dam construction. The best feature of this plant is its energy savingeffect. This plant is equipped with a static continuous mixer for mixing concrete or CSG. This mixer is set upin vertical direction, and then it is possible to mix materials with no electric power but only by gravity force.In this study, environmental impact due to manufacture concrete and CSG by conventional batch type plantand new continuous mixing plant are estimated and compared, and then the advantage of new continuesmixing plant for environment is discussed.KEYWORDS: environmental impact, energy saving, continuous mixing plant1. INTRODUCTIONIn present-day life, various kinds of industrialproducts are produced all over the world, and itcauses to consume a large amount of naturalresources and energy, and causes to dischargeexhaust gas and industrial waste. Environmentalassessment is very important in every field ofindustry, and many research and investigating workshave been carried out. Life cycle assessment is amethod of evaluating the environmental impactsystematically and quantitatively in the entire lifecycle of product and processes or activities. In thefield of construction, this method of life cycleassessment is also applied to a system ofenvironmental assessment, and is carried out actively.In project of dam construction, because damconstruction project is very big in scale, the projectof dam construction has great influences on waterquality and natural environment of neighboring area.On the concrete dam construction project, themanufacture plant of concrete and CSG is operatedin very long term, and its environmental impactcould be serious. From this point of view,environmental impacts due to operating manufactureplant of concrete and CSG on dam construction workis discussed in this research. And this research

mixing plants were hard to control the quality ofproducts, so batch type plant used to be adopted asthe manufacture plant for concrete dam construction.In recent years, the new continuous mixing plantwhich can control the quality of products accuratelyhas developed by Maeda et al. in Japan, and was putto practical use on several projects of concrete andCSG dam construction in Japan and China.Therefore our choice of manufacture plant becamewider. Fig.2 shows the outline of new continuousmixing plant, this new continuous mixing plant ismainly composed of material feeders, belt conveyers,continuous mortal mixer and continuous concretemixer. In this system, the supply condition of eachconstituents of concrete is shown Fig.2 and managedby real time monitoring system in real time whenmixing. And a continuous static mixer, witch iscalled “MY-mixer”, is adopted as concrete mixer.MY-mixer was developed by Maeda et al. Thestructure and mixing concept of MY-mixer is shownin Fig.3, it is composed of box shape units whichhave two vertically paralleled inlets and twohorizontally paralleled outlets, and the units areconnected in series. When materials get throughevery MY-mixer unit, materials are kneaded andlapped, as a result, two layers of materials at inletincrease by double of that to four layers (2 2 =4). If nnumber of MY-mixer units is connected, the numberof material layers after getting through MY-mixer isequal to 2 n , this principle is called 2 n mixing theoryof MY-mixer. In case of mixing concrete or CSG, theMY-mixer is set up in vertical direction as Fig.2, andthen it is possible to mix concrete without electricpower but only by gravity force. Tek Raj Gyawali etal. conducted comparisons of this new continuousmixing plant and ordinary batch mixing plant forproductivity and quality of concrete, and theyconcluded that this system has capacity of producinglarge quantity of concrete with its precise quality,and this system not only rationalizes the concreteproduction work, but also helps to protect theenvironmental condition by decreasing electricconsumption in large scale.3. ENVIRONMENTAL INPACT DUE TOMANU- FACTURING CONCRETE AND CSGIn this research, the environmental impacts dueto manufacture concrete and CSG of damconstruction in case of operating batch mixing plantMortar mixer and feederWater+Add.SementFine AggregateCourse AggregateSmaller SizeMedium SizeLager SizeCourse AggregatefeederContinuesMortar MixerMonitoring!Monitoring!BeltConveyerCourseAggregateMixed MortarBeltConveyerContinuously!(connecting (connecting each each devices) devices)Monitoring!Fig.2 Outline of new continuous mixing plant.Mixed Mortar +Course AggregateContinuesConcreteMixerMY-mixerTo Site!

Inlet 2(2 1 )layersOutlet Inlet 4(2 2 )layersOutlet Inlet 8(2 3 )Outlet layersnn unitsMY-mixer2 nlayersVerticalInletHorizontaloutletFlowof MaterialsKneadingProcessLappingProcessFig.3 Continuous concrete mixer “MY-mixer” and its mixing theory.and continuous mixing plant are estimated anddiscussed. In these estimations, electricityconsumption and CO 2 discharge are calculated asenvironmental impacts. First the total electric powerof each equipment, namely necessary power per hour(kW) for operating the plant, is calculated. And thencalculated power divided by production capacity ofthe plant is power consumption per unit volume ofproduct (kWh/m 3 ), and the power consumptionmultiplied by environmental impact rate of CO 2(kg/kWh) is CO 2 discharge per unit volume ofproduct (kg/m 3 ). In these calculations, the requiredpower for each equipment is referred to reference 6.And environmental impact rate of CO 2 is defined as0.371kg/kWh referred to the investigating result byconcrete committee of Japan society of civilengineers (Reference 1).4. ESTIMATION RESULTS4.1 Concrete plantFig.4 shows the outline of batch mixing plant.This plant equips 2m 3 pug mill type forced mixer.Since it is necessary to mix dam concrete composedof large size particle of aggregate, the cycle time ofmixing is set to 90 minutes, and thus its mixingcapacity is 80m 3 per hour. On its manufactureprocesses, first the materials are transported to themain tower of plant by each feeder and beltconveyers, and then measuring of materials andmixing of concrete are conducted repeatedly. Fig.6shows the outline of continues mixing plant. Thisplant equips a continuous mortar mixer and acontinuous concrete mixer “MY-mixer”. Thecapacity of continues mortar mixer is 70m 3 /h and thecapacity of MY-mixer is 150m 3 /h. In its manufactureprocesses, first the course aggregate are fed to thebelt conveyer 1 by each feeder continuously. At thesame time, powder materials, fine aggregate andwater are fed into continuous mortar mixer andmixed continuously, and then the mixed mortal is fedon the course aggregate on belt conveyer 1. And theall of materials on belt conveyer 1 are transported tothe top of the MY-mixer and dropped into the insideof MY-mixer, and then concrete mixing is conductedcontinuously at the inside of MY-mixer only bygravity force. The details of structure andequipments of their two plants are given in table 1.

Mixing plant(Pug mill mixer)CementFryashSand and gravelG1 G2 G3 SBelt conveyer 2Belt conveyer 1WaterFig.4 Outline of batch mixing plant for concrete.CementGravelG2G1FryashG3SMortar mixerGravelWaterMortarBelt conveyer 2Belt conveyer 1Mortar and GravelMY-mixerFig.5 Outline of continuous mixing plant for concrete.Table 1 Capacity and equipments of plant.Batch mixing plant Continues mixing plantMaximum production capacity 80 m 3 /h 150 m 3 /hEquipment for material suppliment belt conveyer-1 350t/h belt conveyer-1 350t/hbelt conveyer-2 180t/h belt conveyer-2 180t/hcement feeder 12t/h belt feeder 120t/h 4water pomp 0.5m 3 /mindust extractor 20m 3 /mincement feeder 12t/hwater pomp 0.5m 3 /mindust extractor 20m 3 /minMixermixer 2.0m 3 Continues mortar mixer 70m 3 /hMY-static mixer 650*650 L=6.5mNotesmixing cycle time 90s

CementMixing plant(Dram type mixer)Sand and gravelCement feederBelt feeder 1Belt conveyerWaterFig.6 Outline of batch mixing plant for CSG.Sand and gravelCementCement feederMY-mixerBelt feeder 1Belt conveyer 1WaterBeltfeeder 2Belt conveyer 2Fig.7 Outline of continuous mixing plant for CSG.Table 3 Capacity and equipments of plant.Batch mixing plant Continues mixing plantMaximum production capacity 60 m 3 /h 150 m 3 /hEquipment for material suppliment belt conveyer 350t/h belt conveyer-1 350t/hcement feeder 12t/h belt conveyer-2 350t/hMixerNotesbelt feeder 350t/hwater pomp 0.5m 3 /mindust extractor 20m 3 /mindram type mixer 3.0m 3mixing cycle time 180sbelt feeder-1 350t/hbelt feeder-2 350t/hcement feeder 12t/hwater pomp 0.5m 3 /mindust extractor 20m 3 /minMY-static mixer 650*650 L=5.2mTable 4 shows estimated results of powerconsumption and CO 2 emission of each plant forCSG. At the estimation of the electric consumptionof batch mixing plant, because its materialsupplement is not continuous, the duration ofoperating equipments for material supplement areconsidered as 24 minutes per hour (i.e. 0.4 times ofall duration). For example, total primal electricconsumption of belt conveyer is 30kW, and then itmultiplied by 0.4, thus it is equal to 12kW. As shownin table 4, the total powers of two types of plants arealmost same. However their maximum productioncapacity of continuous mixing plant is higher thanthat of batch mixing plant. Therefore the powerconsumption and CO 2 emission per unit volume ofproduction concrete of continues mixing plant is less

than that of batch mixing plant (i.e. batch mixingplant 1.05kWh/m 3 , 0.388kg/m 3 and continuousmixing plant 0.43kWh/m 3 , 0.158kg/m 3 ). The ratio ofthese values of continuous mixing plant to those ofbatch mixing plant is 54.9%. This may be concludedthat continuous mixing plant is more environmentfriendlythan the conventional batch mixing plant.5. ConclusionIn this research, the environmental impact due tooperating mixing plant of concrete and CSG in damconstruction project was estimated for conventionalbatch mixing plant and new continuous mixing plant.In both situation of manufacturing concrete and CSG,the estimated power consumption and CO 2 emissionof continuous mixing plant are less than that of batchmixing plant. As a result, continuous mixing plant ismore environment-friendly than the conventionalbatch mixing plant.Table 4 Estimated results of power consumption and CO 2 emission.Batch mixer plantContinues mixer plantMaximum production capacity60 m 3 /h 150 m 3 /hElectiric consumption Equipment Power Equipment PowerMaterial supplement belt conveyer 12.0 kW belt conveyer 44.0 kWbelt feeder 2.2 kW belt feeder 11.0 kWcement feeder 0.9 kW cement feeder 2.2 kWwater pomp 1.8 kW water pomp 4.5 kWdust extractor 0.9 kW dust extractor 2.2 kWMixer drum type mixer 45.0 kW MY-mixer 0.0 kWTotal power 62.8 kW 63.9 kWPower consumption per m 31.05 kWh/m 3 0.43 kWh/m 3CO 2 emission per m 30.388 kg/m 3 0.158 kg/m 3REFERENCESSubcommittee on Assessment for EnvironmentalImpact of Concrete, Concrete Committee of JSCE,Assessment for Environmental Impact of Concrete,Concrete Engineering Series 44, 2002, in Japanese.Maeda, M., Yamada, K., Uchida, A., Development ofContinues Mixer Based on New Kneading Theoryand the Experimental Study for its Efficiency, J.Struct. Constr. Eng., AIJ, No.505, pp.23-30, 1998, inJapanese.Gyawali, T.G., Yadama, K. and Maeda, M., Highproductivity continuous concrete mixing system,Proceedings of 17 th International Symposium onAutomation and Robotics in Construction, TaiwanChina, pp.218-223, 2000.An, Xuehui., Jin, Feng. and He, Shiqin., A NonstopSystem of Concrete Manufacturing & Conveying,Water Power Vol.32, No.3, pp.35-38, 2006, inChinese.Tian, Qi, 2005. Concrete plant and Asphalt concreteplant, China architecture and material press, China,in Chinese.