ENERGY-SAVING PNEUMATIC CONVEYING PIPE SYSTEM By:

Mario Dikty, Product Line Manager Pneumatic Conveying, Peter Hilgraf, Senior Manager Technical Center, and

Ray Worthington, Proposal Manager-Claudius Peters Technologies GmbH

Abstract

The cement industry is increasingly expected not only to produce high quality products competitively, but to do so utilizing less energy and reducing dedusting requirements. This paper will present current experi-ences for an efficient pneumatic pipe conveying system that consumes less energy and requires smaller dedusting equipment at the conveying pipe discharge. This system combines the advantages of an air-activated gravity conveyor plus those of a pneumatic transport pipe. The material characteristics that can be conveyed by this system will be presented and discussed using the Geldart diagram. System design guidelines relating to applicable types of feeding devices, horizontal runs, inclines, bends and vertical lifts will also be presented. The paper will conclude with results from several of these systems being presented with the aid of actual measured operating results.

IntroductionThe bulk materials in cement plants can be transported by mechanical or pneumatic systems. Comparison of different methods from the two groups generally leads to the following all-embracing statements: The energy demand of pneumatic conveying processes is many times greater than that of mechanical proc-esses. The capital costs for mechanical systems are significantly higher than for the corresponding pneu-matic systems. When compared to mechanical conveying systems the routing of pneumatic systems can be adapted for more flexibly to suit existing factors. It is substantially easier to achieve safe transport of combustible or explosive bulk materials with pneumatic systems. Pneumatic systems offer the option of conveying using inert gases. The primary disadvantage of pneumatic conveying is its high energy de-mand. This is due to the nature of the system. This can only be reduced further if the conveying procedure used is optimally suited to the properties of and/or the particular class of bulk material solids to be trans-ported.

Figure 1: Energy comparisons for conveying 100 t/h cement over different conveying distances; reference basis: belt conveyor

P Pne

u

/ PB

elt

978-1-4244-2081-0/08/$25.00 2008 IEEE

Figure 1 shows cement transport over different distances using various pneumatic conveying systems. It shows that the all-embracing statementhigh energy consumption for pneumaticsmust be examined with discrimination [1]. In the diagram the power demand figures Ppneu of pneumatic pipe conveying systems with optimized energy demand (feeding devices are pressure vessel and screw feeder) versus those of air-activated gravity conveyor systems. The bulk material in an AAGC is transported in the form of a highly aerated fluidized bed. This compares with the respective drive power demand of a belt conveyor Pbelt. Fig-ure 1 shows that the energy demand of an air-activated gravity conveyor is roughly identical with that of a belt conveyor system. The pneumatic pipe transport requires the expenditure of more than 20 times the energy consumption of the corresponding belt conveyor. This relationship also applies to other bulk materi-als [1]. The air-activated gravity conveyor systems (Figure 2) energy advantage is offset by a disadvantage of needing to be sloped downwards in direction of material flow. This is required to achieve stable transport and emptying resulting in limitations on the routing, e.g. no vertical conveying.

Figure 2: Schematic layout of an air- activated gravity conveyor

This suggests that the advantages of air-activated gravity conveyors, in particular the low energy require-ment, should be combined with those of pneumatic pipe conveyance. In particular the flexibility available with pipe routing with its nearly unlimited flexibility. This approach is implemented rigorously in the energy saving pneumatic conveying pipe system with fluidized elements described below. A conveying pipe that can be fluidized completely in certain areas (air-activated gravity conveyor principle) is also traversed in the direction of flow by a stream of driving gas(pipe conveying principle). The pressure drop of the driving gas replaces the downwards slope of the air-activated gravity conveyor.

Implementation of the ConceptFine-grained bulk materials, such as cement, raw meal, fly ash, kiln bypass dust and gypsum can be fluid-ized with low gas velocities. They then behave like fluids. If the fluidization/gas flow is maintained they can be easily transported on slightly inclined air-activated gravity conveyors under the influence of gravity [2, 3]. The air-activated gravity conveyor structure is shown schematically in Figure 2. This consists of a lower casing through which the fluidizing gas is supplied through a gas-permeable media. The upper casing is used to carry away the material and fluidizing gas. Depending on the particular bulk material the specific fluidizing gas flows WSq and the inclination angles WS are of the order of WSq (1.0 to 3.0) m3/(m2min),

WS (4 to 8). In this case the axial transport velocity is Sv 2.0 m/s. The aeration system used for the fluidizing conveying pipe system (FLC) process is shown schematically in Figure 3.

Figure 3: Schematic layout of a FLC pipe

Fluidization gas Bulk solid

Bulk solid Exhaust gas

Gas-permeable distributor

Figure 4: Schematic layout of a FLC plant

The structure of a FLC conveying plant is shown schematically in Figure 4. It is a positive pressure convey-ing system with bulk material supplied through different types of single feeders. Multi-point feeding is also possible and discussed further below. The total conveying gas flow FM coming from the pressure genera-tor is divided into a fluidizing gas flow fluidFM , running parallel to the transport section with take-offs to sup-ply the fluidizing elements and an axial driving gas flow driveFM , fed into the start of the conveying pipe. The fluidization transforms the bulk material into a fluid-like state with low internal friction. This raises the material from the bottom of the pipe up into the flow of the driving gas. These are optimum conveying con-ditions that can achieve the same transport velocities as with an air-activated gravity conveyor. Flow con-trollers set the fluidizing gas flow fluidFM , to a predetermined target value and/or limit it to a permissible maximum value. If there were no such control then an incipient blockage in the conveying pipe, i.e. a pres-sure rise at the start of the conveying line would occur. This would cause a diversion of the driving gas through the fluidizing gas line by-passing the position of the blockage. This would intensify the formation of the blockage and must therefore be prevented. The limitation/control of the gas flow in the fluidizing gas line is also an essential precondition for all the measures used to monitor the conveying process. Figure 5 shows an example of one possible circuit for avoiding blockages with a screw feeder as the feeding device.

Figure 5: Schematic layout of FLC circuit with screw feeder as the feeding device

FLC Pipe

The length of the individual, independently aerated, pipe sections/fluidization units along the FLC pipe is currently set as L 3.0 m. A suitable manual valve for regulating the local fluidizing gas flow and a non-return flap are located in each supply line.

The supply of fluidizing gas increases the conveying gas flow along the conveying route from driveFM , at

the start of the pipeline to +=xL

fluidFdriveFxF MdMM0

,,,

after a conveying distance Lx, i.e. the gas veloc-

ity grows in the direction of conveying due both to the gas expansion and to the increase in the gas mass flow. If the local gas velocity vF,x exceeds a critical value vF,crit the conveying gas flow xFM , at that point is capable of conveying the bulk material even without fluidization [4]. The supply of fluidizing gas is no longer necessary and the conveying line can be continued as a normal, non-aerated, pipe. However, it is often more advantageous to maintain the air-activated gravity conveyor character of the conveying system over the entire conveying section with its resulting low transport velocities. In this case the FLC pipe line has to be staggered, i.e. the pipe diameter will be enlarged section by section.

Conveying Air ConsumptionFLC plants that are in operation or are currently offered are designed with conveying gas velocities of vF, A (2 to 3) m/s at the start of the line. The specific fluidizing gas flows of WSq (0.5 to 1.0) m3/(m2min) de-pend on the particular bulk material. Compared to conventional pipe conveying one can find here the big advantage regarding the necessary total conveying air quantity. In conventional pipe conveying the mini-

mum start velocity vF,min follows the equation nR

mR

MatF pDKv =min, . A material factor KMat is multiplied by the

pipe diameter DR raised to the exponent m divided by the absolute pressure pR raised to the exponent n. Example: for cement the minimum start velocities at the same pressure are for a ND100(4) pipe: approx. 4m/s, for a ND200 (8) pipe: approx. 6m/s, for a ND450(18) pipe: approx.11m/s. In a FLC pipe the mini-mum start velocity vF,min is not a function of the pipe diameter and the pressure, the minimum start velocity vF,min = const., it is similar to air-activated gravity conveyors. That means that independent of the pipe di-ameter the start velocity for cement, raw meal, fly ash or gypsum is in the range of 2-3m/s. and specific fluidizing gas flows of WSq (0.5 to 1.0) m3/(m2min). This is much lower compared to the air-activated gravity conveyor. The result is lower conveying air quantity for the FLC pipe compared to conventional pipe. Detailed data are shown in the comparisons with conventional pipe conveying section later in this paper.

Bends and Vertical Pipe SectionBends are not aerated. It is important to use small r/d ratios (Bend Radius / Pipe diameter) to reduce the retention time of the material in the bend. For example, Cement has a de-aeration time of 20-50 sec/ 2kg. That means that fluidized cement needs about 20-50 sec to come from the aerated density to bulk density when the fluidization is switched off. In a ND200 (8) pipe with a r/d ratio of 3 and a transport velocity of 3.0m/s the retention time is < 1 sec. Compared with the 20-50 sec de-aeration time it is clear that fluidiza-tion of the bend is only necessary for materials with extreme low de-aeration times. Prior to the bend outlet a ramp is installed ahead of the following fluidized element. This is done to avoid wear on the edge of the fluidizing element and to prevent material build up. A horizontal bend DN150(6) is shown in Figure 6.

Figure 6: Examples of Horizontal and Vertical bends in a FLC pipeline

Vertical sections and changes in direction/bends in a FLC pipe conveying route are made as normal with non-aerated pipes. Depending on the position of a vertical section along the conveying distance, its pipe diameter DR,vert.,, is made correspondingly smaller than the diameter DR of the aerated horizontal conveying pipe in order to set up stable operating conditions. The resulting locally increased gas velocity prevents gas/solids segregation and the associated pressure pulsations. An example for a pipe system ND100(4) vertical to a ND150(6) horizontal is shown in Figure 6. Inclined ConveyanceThe FLC pipe is capable of conveying uphill at angles of up to R 30 (tested and in operation so far) above the horizontal. An inclined section can be located anywhere along the line except directly at the start of the conveying line out of the feeding device. The bulk material feed should always be made into a hori-zontal, or alternatively vertical, pipe. The transition to the inclined section can then be made after a suffi-ciently long horizontal acceleration section. Figure 7 shows a FLC installation for fly ash with a pipe inclina-tion upwards of 30.

Figure 7: 30 FLC pipe for fly ash

Feeding DevicesThe bulk material feeding devices that are suitable for a FLC plant and that have been tested are: pressure vessels, screw feeders, rotary-valve feeders and various flap type feed gates. Energy demands, capital costs, plant height and maintenance requirements decide the type of feeding device used for a given appli-cation. Types of bulk material feeders are shown in Figure 4.

Multi Feeding PointsThe requirement for pneumatic conveying systems to have multi-point feeds arises for example in the power generating industry. This is more or less simultaneous feed of bulk material into one transport line through several feeders in parallel. The fly ash collected in the several filter hoppers is fed into a common transport line that connects a larger number of these hoppers. Underneath the multiple feed points, the irregular batch wise and frequently simultaneous discharge of solids from several feeders is only mani-fested in short peaks of the conveying pressure. This is at the start of the line but dies away rapidly due to the fluid like behaviour of the bulk material in the FLC pipe. The accumulation of any bulk material disap-pears again almost immediately. With the large number of feed points under a power stations filter the aim is to use simple and inexpensive feeding devices like double flap valves, double rotary flap systems, or

FLC pipe

aeration pipe

wear-resistant rotary-valve feeders. Their use is supported, and in some cases only possible, due to the special FLC pipe characteristics (high loadings at low conveying pressures). For example one realized plant is equipped with 18 rotary feeders which feed a ND400 (16) FLC pipe with 57.5 t/h of fly ash over a 130 m conveying distance, including 47 m vertical height. The transport underneath filters is possible in the same way, in cement plants, to prevent the installation of large collecting screw conveyors.

AvailabilityWear is the primary factor for the reduction in availability of pneumatic conveying systems. In the case of pneumatic lean-phase conveying, a very substantial wear rate can be caused by the systems inherent high velocity of the bulk material passing along inside the wall of the pipe. The use of dense phase conveying generally causes far less wear [5]. Wear loss, WCP, in pneumatic conveying piping increases with an expo-nent k of 23 of the conveying velocity vF (depending on the combination of bulk material and pipe material) according to kFCP vW .

Start Up with Full Conveying LineRestarting after the conveying process has been interrupted by, for example, a power failure, i.e. starting up with a full line, is absolutely no problem with the FLC pipe. The conveying gas is fed to the conveying system at different times. After the fluidizing gas has been applied, the driving gas flow is switched on after a time delay. This transports the deposited bulk material that has been transformed to the fluidized state and takes it away evenly and without significant pressure fluctuations. The procedure has proved success-ful with all the bulk materials investigated so far and is implemented as a standard procedure in all FLC plants.

Suitable Bulk MaterialsThe bulk materials that are particularly suitable for FLC pipe are all those that can be fluidized with low gas velocities and that expand substantially homogeneously. High gas retention is also an advantage. Bulk materials with appropriate properties are to be found in the entire hatched area of the Geldart diagram (Figure 8). The difference (particle density S - fluid density F) is plotted on the ordinate against the aver-age particle diameter dS,50 on the abscissa, measured as the sieve residue R = 50 %. The relationship (S - F) S (fluid = gas) applies in this case [6,7]. The various bulk materials plotted in Figure 8 and 9 have already been transported successfully with the FLC. They cover the entire recommended application range.

Figure 8: Tested bulk solids for FLC pipe shown on the Geldart diagram

No. Bulk Material Blaine [cm/g]

Average Particle-dS,50 [m]

Bulk Density SS [kg/m3]

Solids DensityS [kg/m3]

1 Cement 3938 13 1100 3100 2 Cement 2588 29 1260 3110 3 Raw meal 4188 30 925 2800 4 Cement kiln dust 9292

Figure 11: Transparent glass pipes in the FLC plant in the test centre

Comparison of FLC with Conventional Pipe Pneumatic ConveyanceExamples of results from operating FLC pipe conveying systems are discussed below. These are shown for two typical plants.

Plant WietersdorfCase one is a FLC plant for transporting raw meal in Austria. The focal point of the plant modernization at Wietersdorf in 2005 was to complete and start up a new preheater system. The selected solution for feed-ing of the preheater was a FLC system in combination with a bucket elevator.

Figure 12: Route of the raw meal from the raw meal blending silo to the preheater feed bin

Horizontal trans-parent glass pipe

Vertical transparent glass pipe

FLC Pipe

The performed system extensions led to a significant increase in energy efficiency while simultaneously raising the production capacity [9]. Figure 12 shows the route of the raw meal from the raw meal blending silo to the preheater feed bin. The 125 t/h raw meal is transported continuously over a conveying distance of 194 m(636 ft.) to the bottom of the preheater and from there goes up 95 m(311 ft.) vertically via bucket elevator to the feed bin of the preheater. In case of failure of the bucket elevator the FLC plant is designed to be able to transport 90 t/h raw meal up to the feed bin of the preheater via a vertical pneumatic convey-ing pipe connected to the FLC pipe.

Energy Consumption

Bulk material Raw mealConveying gas AirType of solid feeding device Screw FeederType of conveying system FLC Pipe Conventional PipeSolids mass flow [t/h] 125Total conveying distance [m] 194Including: total height [m] 7Pipe diameter [mm] 273.0 Total gas volume flow [m3/h at 20C, 1bar] 2130 3920Dedusting air, percentage [%] 54 100Gas velocity at pipe inlet [m/s] 2.5 11.6 Gas velocity at pipe outlet [m/s] 12.2 21.7 Solid/air ratio at pipe inlet [kgS/kgF] 134.5 27.9 Solid/air ratio at pipe outlet [kgS/kgF] 54 27.9 Pipe pressure difference [bar] 1.15 1.25 Total pressure difference [bar] 1.45 1.55 Power consumption of compressor [kW] 68 134Power consumption of screw feeder [kW] 50 58Total power consumption [kW] 118 192Total specific power consumption [kWh/(t100m)] 0.49 0.79 Total specific power consumption [%] 62 100

Figure 13: Summary of the raw meal FLC pipe data compared with conventional pipe

Figure 13 shows the summary for the raw meal FLC pipe conveying data compared with a conventional pipe conveying. It can be seen that the FLC pipe has a total energy consumption of 118kW compared to 192kW for the conventional pipe. In Austria 1 kWh costs 0.18 $. The plant is in operation for a minimum of 8000h per year. Wietersdorfer & Peggauer Zementwerke GmbH save 8000h 0.18 $/kWh (192kW-118kW) = 106560 $ per year due to the FLC equipped system.

Conveying / Dedusting Air ConsumptionThe highest energy saving is due to the low air quantity required, as already explained above. The FLC pipe equipped system saves 74 kW in energy consumption while at the same time also reducing the de-dusting air quantity. Instead of 3920 m/h (at 20C, 1bar) only 2130 m/h (at 20C, 1bar) have to be de-dusted. The FLC pipe system thus only needs 54 % of the conveying air / dedusting air compared to the conventional pipe conveying system. As a result a smaller pressure generator as well as a smaller dedust-ing device is required.

Wear reductionThe wear ratio between the FLC pipe and the conventional pipe conveyance system for an average wear exponent of 2.5 is:

5.2

,

,

==

systemFLCtheofvelocityconveyingaverageconveyancealconventiontheofvelocityconveyingaverage

WW

ratioWearsystemFLCCP

conveyancealconventionCP

For the case of plant Wietersdorf the wear factor is:

7.102.76.18 5.2

=

=ratioWaer

The wear ratio shows that a conventional pipe conveying system would have approximately 11 times more wear than the FLC pipe system. This means that the lifetimes of the wear parts of the FLC pipe system are approx. 11 times longer. For this plant, after more than 2 years of operation, no equipment (no FLC pipe, no bend) had to be exchanged.

Plant Hannibal (under construction)The screw feeder will receive cement from the packhouse silos and will convey it to the two existing river silos for barge loading at a conveying rate of 281 t/h (Figure 14). The conveying distance is 282 m(925 ft.) [including 43.5 m(141 ft.) vertical lift] with three changes in elevation, ten (10) pipe bends and two (2) two-way diverter valves.

Figure 14: Schematic Plant Hannibal FLC pipe

Energy consumptionFigure 15 shows the summary of the FLC pipe cement conveying system data compared with a conven-tional pipe conveying system. It can be seen that the FLC pipe system has a total energy consumption of 427 kW compared to 796 kW for the conventional pipe conveyance. With energy costs of 0.1 $ / kWh and 8000h operation time per year the expected savings are 8000h 0.10 $/kWh (796 kW-427 kW) = 295200 $ per year.

River Silos

Screw Feeder

Energy consumption

Bulk material CementConveying gas AirType of solid feeding device Screw Feeder Type of conveying system FLC Pipe Conventional PipeSolids mass flow [t/h] 281Total conveying distance [m] 265Including: total height [m] 66Pipe diameter [mm] 457.2 Total gas volume flow [m/h at 20C, 1bar] 5670 14640Dedusting air quantity [m/h at 100C, 1bar] 7220 18640Dedusting air, percentage [%] 39 100Gas velocity at pipe inlet [m/s] 3.0 11.0 Gas velocity at pipe outlet [m/s] 11.3 26.2 Solid/air ratio at pipe inlet [kgS/kgF] 68.8 16.2 Solid/air ratio at pipe outlet [kgS/kgF] 43.0 16.2 Pipe pressure difference [bar] 1.37 1.43 Total pressure difference [bar] 1.67 1.73 Power consumption of compressor [kW] 250 610Power consumption of screw feeder [kW] 177 186Total power consumption [kW] 427 796Total specific power consumption [kWh/(t100m)] 0.57 1.07 Total specific power consumption [%] 53.2 100

Figure 15: Summary of the cement conveying system data compared with a conventional conveying

Conveying / dedusting air consumptionThe biggest energy saving is due to the low air quantity required, as already explained previously above. The FLC pipe reduces the dedusting air quantity. Instead of 18640 m/h (at 100C, 1bar) only 7220 m/h (at 100C, 1bar) have to be dedusted. The FLC pipe needs only 39 % conveying air / dedusting air compared to the conventional pipe conveying. As a result a smaller pressure generator as well as a smaller dedusting device is required.

Wear reductionThe actual wear ratio factor is 10.9. This indicates that the conventional pipe conveying system would have approx. 11 times more pipe wear than the FLC pipe system. This also means that the lifetimes of the wear parts of the FLC pipe are approx. 11 times longer.

ConclusionFigure 16 shows a reference list for FLC plants for the cement industry. The total energy savings over all these 30 plants are about 4020 kW compared to a conventional pipe pneumatic conveyance. The reference list shows that the cement industry and other types of industry with increasing number of plants coming on line. Up to end of 2007 more then 45 FLC plants have been sold. Not only in the cement industry, but also for example in the power generating industry and in the alumina industry the installation of FLC plants in-creases more and more. The feeding devices installed in these plants are primary continuous feeding de-vices, like rotary valves or screw feeders.

The FLC pipe systems when compared to conventional pneumatic pipe conveying systems have the main advantages in energy savings, reduced dedusting quantities and lower wear behaviour. The two plants discussed in this paper, where the energy savings are in the range of 38 47 %. The dedusting air quantity was reduced up to 61 %. Additionally the wear behaviour is at 1:11 compared to a conventional pipe con-veying. The good results achievable with the FLC pipe system are shown in all installed plants. This makes the FLC pipe another available innovation, to the cement industry, for optimizing its pneumatic pipe convey-ing systems in regards to effectively lowering the electrical energy and dedusting air quantity requirements.

Figure 16: Reference list of FLC pipe plants in the cement industry

REFERENCES

[1] Hilgraf, P.: Energy consumption for pneumatic conveying compared with mechanical conveying. ZKG INTERNATIONAL 51 (1998) No. 12, pp. 660-673.

[2] Keuneke, K.: Fluidisierung und Fliebettfrderung von Schttgtern kleiner Teilchengre. VDI-Forschungsheft 31 (1965) No. 509.

[3] Muschelknautz, E.: Die Berechnung der pneumatischen Fliefrderung. transmatic 76, Teil II: Pneumatische und hydraulische Frderung, Krauskopf-Verlag, Mainz, 1976, C1, pp. 29-43.

[4] Hilgraf, P.: Minimum conveying gas velocities in the pneumatic transport of solids. ZKG Interna-tional 40 (1987) No. 12, pp. 610-616.

[5] Hilgraf, P.: Wear in pneumatic conveying systems Part 1: basic factors affecting wear, Part 2: wear measurement and prediction. Cement International No. 4, pp.5663 (2005), No. 6, pp. 5465 (2005).

[6] Geldart, D.: Types of gas fluidization. Powder Technol. 7 (1973), pp. 285-292. [7] Hilgraf, P.: Assessing the storage, transport and metering characteristics of bulk materials. ZKG

International 53 (2000) No. 1, pp. 28-43. [8] Hilgraf, P.: Review of pneumatic dense phase conveying, part 1 and 2. ZKG INTERNATIONAL 53

(2000) No. 12, pp. 657-662 and 54 (2001) No. 2, pp. 94-105. [9] Dikty, M.: Decision matrix for bulk solids transport. ZKG INTERNATIONAL 60 (2007) No. 7, pp. 56-

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