고압압송식 분체 수송장치에서 미분탄의 수송특성

김장규, 이성수, 이정수*, 김종진*

포항산업과학연구원 환경에너지연구부문,

한국전력공사 기술연구소 전력연구실*

Pulverized Coal Conveying Characteristics in a

Pressurized Pneumatic Coveying System

J.G.Kim, S.S.Lee, J.S.Lee*, J.J.Kim*

Environment & Energy Research Division, RIST,

Electric Power Research Dept., Research Center, KEPCO*

Introduction

Pneumatic conveying is defined as the transportation of dry powdered and granular solid particles in a gas stream. Advantages of pneumatic conveying are dust free transportation, flexibility in routing, distribution to many different areas and pick-up from several areas, low maintenance and low manpower costs, multiple use with one line, security, and ease of automation and control.

Total pressure loss in pipelines for pneumatic conveying is sum of pressure losses due to gas and solid. In this section pressure loss due to gas is mentioned. Pressure loss in pipes due to gas flow can be expressed as the following Darcy equation;

[eqn: DELTA P ~=~ 2` f_g` L over D rho` v^2 ~=~ lambda_g` L over D {rho` v^2} over 2]                         (1)

where, friction factor is functions of Reynolds number and relative roughness of pipes. In the above equation, □g = 4fg. There are various empirical equations for friction factor for fluid and in this paper correlation suggested by Haaland is used.1)

When solid is conveyed by a gas stream, the solid can be considered as a fluid dispersed among the gas stream and the pressure drop contributed by the solid stream can be expressed in the same form as that used for gas stream. The total pressure loss is expressed simply as the summation of both pressure drops contributed by gas stream and solid stream;

[eqn: DELTAP~=~ lambda_g L over D {rho_g `V_f^2} over 2 ~+~ lambda_s L over D {rho_s0 (1-epsilon) `V_s^2} over 2]               (2)

where, [eqn: lambda_s``] is friction factor for solid.

When solid particles are uniformly distributed in gas for dilute phase conveying, the difference in velocity of gas and solid particles is negligible. By applying the relation [eqn: m = {rho_s0 (1-epsilon)} over rho_g], the pressure drop becomes;

[eqn: DELTAP~=~ (` lambda_g `+` m`` lambda_s ) L over D {rho_g `V_f^2} over 2]                             (3)

When particle to gas mass ratio is large that there is serious difference between particle and gas velocities, above equation can not be used and particle velocity should be estimated. One of the equations calculating particle velocity can be derived by considering drag force to be the source of the pressure drop caused by particle2) and this can be written as follows when the Stokes' law is applied:

[eqn: {V_s / V_f} over {1-V_s /V_f}`=`{g G_s L} over {AV_t (DeltaP-DeltaP_g )}]                       (4)

Above equation takes into account amount of particle transported and pressure drop along with terminal velocity. When we don't know both particle velocity and the protion of the total pressure drop from particle contribution we can first calculate particle portion of the pressure drop from equation (2) by assuming uniform velocity in the pipe and then by using this pressure drop particle velocity can be calculated from equation (4) and this particle velocity is used to correct the particle portion of the pressure drop. This iteration process is continued until both the paticle portion of the pressure drop and the particle velocity are not changing within the error range.

Experiment

Bays Water(BS) coal and Wit Bank(WB) coal is used for the experiments. Pulverized coals prepared in POSCO Pohang Works for Injection to Blast Furnace are used in the present without further treatment. From the particle size distribution analysis of pulverized coal used in the present experiments it is found that average particle size of BS is 23[eqn: rmmum] and average particle size of WB is 29[eqn: rmmum].

As to the flowability and floodability properties of the coal powder, these properties are estimated by the Hosokawa integrated estimatioin instrument. In case of Wit Bank, the flowability is in not-good region except when it is without 200mesh under portion, and in case of Bays Water, the flowability is in not-good region even without 200mesh under portion.

The coal feeding system consists of storage hopper, lock hopper and feed hopper. Pulverized coal stored in storage hopper is fed by gravity to lock hopper which is at atmospheric pressure. After lock hopper is pressurized to the pressure of the feed hopper, pulverized coals stored in lock hopper are feed by gravity to feed hopper which is kept at operation pressure for feeding. Pulverized coals stored in feed hopper are fluidized by supplying air through fluidizing plate to prevent bridge and agglomeration and fluidized particles are discharged through pipelines connected to the bottom of the feed hopper. Additional air is supplied to the pipelines connected to the bottom of the feed hopper and solid gas ratio is regulated by this additional air.

Coal receiving system consists of two receiving hoppers and pulverized coals supplied from feed hopper are stored in the first receiving hopper and are returned to the storage hopper at the top of the coal feeding system via the second receiving hopper which takes the role of a buffer tank. Load cells are installed at feed hopper and receiving hopper and coal conveying rate can be calculated by weighing the feed hopper and receiving hopper in a specified time interval. Pressure gages and flowmeters are installed to measure feed hopper pressure, pressure at the specified location of the pipe and air flow rate to feed hopper and conveying pipe. Fluidizing air is supplied to the feed hopper and additional air is supplied to the exit of the feed hopper, that is, to the inlet of the pipeline. Additional air is not for pickup but for dilution of the solid gas mixture discharge from the feed hopper to control the phase of the conveying as well as for preventing from blockage in pipeline. Flowmeters and pressure guage are installed to measure the flow rate and pressure of fluidizing air and additional air.

Continuous coal feeding experiment can be carried out in this coal transport system. The pressure of the receiving hopper can be regulated so that the receiving hopper can simulate the gasifier.

Results and Discussions

Series of experiments are performed with lock hopper type pulverized coal feeding system. Experimental correlations on solid friction factor for dilute phase conveying suggested by Konno and Saito3), Reddy and Pei4), and Yang5) are employed to compare with present experiment. The correlation by Konno and Saito shows best agreement with the present experiment. In dense phase transport, applying Konno and Saito could not well predict the experimental result but the pressure drop calculation model for dense phase suggested by Welschof showed better agreement. Both results are shown in Fig. 1 and 2. And application of the particle velocity calculation equation (4) improved the accuracy of the pressure drop calculation employing Konno and Saito model.

Conclusion

1) Konno and Saito model for solid friction factor is in best agreement with experimental data, and this model was used throughout all calculations.

2) In dense phase conveying the pressure drop calculation model suggested by Welschof showed better agreement.

3) Particle velocity calculating equation derived from Welschofs suggestion improved the pressure drop prediction accuracy.

References

1) Haaland, S.E., Trans. ASME, JFE 105, 89 (1983)

2) Welschof, G., VDI-Forsch.-Heft 492 (1962)

3) Konno, H., and S. Saito, J. Chem. Eng. Japan 2, 211 (1969)

4) Reddy, K.V.S., and D.C.T. Pei, I.E.C. Fund 8, 490 (1969)

5) Yang, W.C., International Powder and Bulk Solids Handling and

Processing Conference Exposition, Chicago, May 11-14, 1976.