investigation on the effect of moisture content on conveying ......liang cai et al. investigation of...

9th International Freiberg Conference on IGCC & XtL Technology, 3-8 June 2018, Berlin

Investigation on the effect of moisture

content on conveying and resistance

characteristics of dense-phase pneumatic

conveying Zhen Liu

Department of Energy and Power Engineering

Tsinghua University

Beijing, China


Outline

Background

Experimental system

Results and discussion

Conclusion


Pulverized coal

Gas

Gas

Carrier gas

Oxygen

Stock bin

Lock hopper

Feeding hopper

Gasifier

Background

– Dense-phase pneumatic conveying is key technology to dry-fed entrained flow coal gasifier

Pneumatic

conveying

• Dense phase

• High pressure The original start-up curve of a dry-fed gasifier. Pulverized coal flow fluctuations often produce the so-called "positive feedback" effect, which can in consequence lead to unstable operation for the gasifier, and finally triggered the shutdown interlock in just 30 min.


Background

Influencing factors

Structure

• Top/down/side

discharge

• Pipeline

diameter

• Pipeline layout

and distance

Carrier

gas

• N2/CO2

Material

• Coal rank

• Particle size

• Moisture

content

Operating

parameter

•Pressure

•Total differential

pressure

•Gas velocity

• Solid/gas mass flow rate

• Solid concentration / Solid

loading ratio

• Particle velocity

• Pressure drop

Moisture and energy consumption

• Current dry-fed gasification process

requires the moisture content of

pulverized coal shall be lower than 2%;

• The milling & drying unit cost about 480

MJ to dry 1 ton coal from a moisture

content of 16% to 2%;

• If higher moisture content coal powders

could be transport, it would be a great

energy-saving for the dry-fed gasifiers

and improve the plant economics.


Background

Effect of moisture content

• Small amount: lubrication, friction force between particles decreases (Xie et al, 2007); enhances

electrical conductivity of particles and attenuates the effects of static electricity (Liang et al, 2012)

• Large amount: forming liquid bridge, agglomerability and cohesiveness of particles increases,

flowability decreases (Wang et al, 2008)

• Unstable conveying and blockage when moisture content exceeds 6% for bituminous coal and 9%

for lignite coal (Liang et al, 2013)

Xie Xiaoxu et al. Effect of moisture content on the flow characteristics of pulverized coal (in Chinese). Energy Research & Utilization. 2007, (02): 21～23

Liang Cai et al. Investigation of influence of coal properties on dense-phase pneumatic conveying at high pressure. Particuology, 2012,10(3):310-316.

Wang Chuanghong et al. Effects of particle size and moisture content of Shenfu soft coal powder on its fluidity parameters. Journal of East China University of Science and Technology

(Natural Science Edition) (in Chinese). 2008, 34(03): 377～382

Liang Cai et al. Flow characteristics and stability of dense-phase pneumatic conveying of pulverized coal under high pressure. Experimental Thermal and Fluid Science, 2012,41(10):149-

157.


Background

Experimental system


Conclusion


Experimental facility

Vessel volume,V 5 m3

Max. operating

pressure, P 6 MPa(G)

Operating

temperature, T Room temperature

Carrier gas Nitrogen

• nitrogen supply

• screw feeder

• coal delivery

• conveying and measurement

• coal receiving

• data acquisition and control

Storage

tank

Nitrogen tank

P

P

P

P dP

Pressure reducing valves

Pressurizing gas

Fluidizing gas

Supplementary gas

Conveying pipeline

Relieving gas

Pressure transducer

Differential pressure

transducer

Solid mass flowmeter

Gas flowmeterRegulating

valve

Load cell

Gas

Gas and particles

Delivery

tank

Receiving

tank

Dust

collector


Layout of conveying pipeline

Main parameters of measuring instruments

Instrument Measurement range Accuracy

Gas vortex flowmeter 0~20m3/h ±1%

Gas mass flowmeter 0~100Nm3/h ±0.5%

Pressure transducer 0~10MPa ±0.05%

Differential pressure

transducer 0~30kPa, 0~60kPa ±0.05%

Load cell 0~4t ±0.03%

Solid mass flowmeter Velocity: 0~15m/s,

concentration: 0~700kg/m3 ±2%

• Inner diameter of pipeline: 25mm, 15mm, 10mm

• Conveying distance: 197m, 108m

• Straight pipe: horizontal, vertical upward/downward

• Bends: various orientations

dPP

dP

P

dP

dP

P

dP P

dP P

dP

P dP

P

14m (H)

8m (H

)

10.3m

(VU)

2.3m (H

)

1.5m (H

)

2.3m (H

)

10m

(VU)

10m

(VD)

5m

(VD)

Inlet

Outlet

Solid mass

flowmeter

Coal distributor

8m (H

)

Differential

pressure

transducer

Pressure

transducer

Valves for changing

conveying distance

H: horizontal

VU: vertical upward

VD: vertical downward


Material

• Chinese high-volatile bituminous coal

• HMC: coal with high moisture content

• LMC: coal with low moisture content

• Geldart A type particles, Pan PC1 type

particles, suitable for pneumatic

conveying

Coal property

Parameter HMC LMC Parameter HMC LMC

Mar/% 6.44 3.02 Cdaf/% 82.46 84.22

Vdaf/% 39.48 36.90 Hdaf/% 4.78 5.24

FCd/% 55.66 55.14 Odaf/% 11.49 9.00

Ad/% 8.03 12.62 Ndaf/% 0.96 1.10

Qnet, ar/(MJ·kg-1) 25.96 26.95 Sdaf/% 0.30 0.44

Property and flowability of pulverized coal

Parameter HMC LMC Parameter HMC LMC

External moisture content/% 4.84 1.63 Median particle diameter/μm 35.3 30.6

Internal moisture content/% 1.60 1.39 Compressibility/% 39.2 39.8

Particle density/(kg·m-3) 1449 1427 Angle of repose/° 48 41

Tap density/(kg·m-3) 852 758 Angle of wall friction/° 27 26

Loosely packed density/(kg·m-3) 518 456 Flow function 3.1 3.1


Data processing and experimental range

– The slopes of weight loss curves and

weight gain curves of the vessels were

calculated as average solid flow rate, and

were used to calibrate the instantaneous

mass flow rate Ms and concentration C

which were measured by the solid mass

flowmeter.

– Solid mass flux 𝐺𝑠 =𝑀𝑠

𝐴=

𝑀𝑠

0.25𝜋𝐷2

– The volume flow rate of carrier gas

𝑄𝑔 = 𝑄𝑝 + 𝑄𝑓 + 𝑄𝑠 −𝑀𝑠 𝜌𝑠

– Superficial gas velocity 𝑈𝑔 =𝑄𝑔

𝐴=

𝑄𝑔

0.25𝜋𝐷2

– Solid loading ratio 𝜇 =𝑀𝑠

𝜌𝑔𝑄𝑔

Experimental range

Pipe diameter (mm) 25 15 10

No. of experiments 226 183 63

Back pressure (MPag) 1~4 1~4 1~4

Solid mass flow rate(kg/h) 330~5327 171~2123 96~586

Solid mass flux (kg/(m2·s)) 187~3014 268~3337 338~2072

Solid concentration (kg/m3) 24~641 20~437 18~180

Solid loading ratio (kg/kg) 1~25 1~17 1~7

Superficial gas velocity (m/s) 1.5~14.4 4.3~20.5 8.8~22.6


Background

Experimental system


Conclusion


The effect of moisture content on conveying characteristics

D=25mm, L=197m

• In the region of high gas velocity (dilute phase), no effects of moisture, because particles are in

suspension state and particle-particle interactions are weak.

• In the region of low gas velocity (dense phase), solid mass flow rate and solid concentration is larger for

coal with lower moisture content, because resistance and cohesiveness increases with increasing

moisture content.

Ug vs C

D=25mm, L=197m

Ug vs Gs


The effect of moisture content on conveying characteristics

D=25mm, L=197m

Ug vs Ug –us, ∆P=0.25MPa

• At 1.0MPag, the slip velocity is larger for coal with low moisture content

• At 4.0MPa, the slip velocity is nearly the same for the two coals

D=25mm, L=197m

Ug vs Ug –us, ∆P=0.50MPa


Pipeline resistance – Straight pipes (calculation of 𝜆𝑧)

Solid loading ratio, μ

Density ratio, ρs/ρg

Reynolds number, Re

Froude number, Fr

Diameter ratio, dp/D

• Solid concentration

• Inelastic collision of

particle/particle and

particle/wall

• The conveying

ability of carrier

gas

• Re=ρgUgD/μg

• Ratio between inertia

force to viscous force

• Turbulent intensity

of gas phase

• Fr=Ug/(gD)0.5

• Ratio between

inertia force to

gravity

• The effect of

gravity on two-

phase flow

• Particle distribution

and wall effects

• Different friction

factor for different

pipe diameter

𝜆𝑧

𝜆𝑧 = 𝑥0𝜇𝑥1 (𝜌𝑠 𝜌𝑔 )𝑥2𝑅𝑒𝑥3𝐹𝑟𝑥4 (𝑑𝑝 𝐷 )𝑥5

𝜆𝑧 = 𝑥0𝐹𝑟𝑥4 (𝑑𝑝 𝐷 )𝑥5

Eliminate the dimensionless number

of small exponent

∆𝑃𝑇 ∆𝐿 = ∆𝑃𝑔 ∆𝐿 + ∆𝑃𝑠 ∆𝐿 = (𝜆𝑔+𝜇𝜆𝑧)𝜌𝑔𝑈𝑔

2

2𝐷


Pipeline resistance – Straight pipes

Horizontal pipe:

HMC 𝜆𝑧 = 2.990 × 10−3𝐹𝑟−0.946 (𝑑𝑠 𝐷 )−0.637

LMC 𝜆𝑧 = 0.06594𝐹𝑟−0.832 (𝑑𝑠 𝐷 )−0.103

Vertical upward pipe:

HMC 𝜆𝑧 = 2.047 × 10−4𝐹𝑟−1.095 (𝑑𝑠 𝐷 )−1.233

LMC 𝜆𝑧 = 0.03642𝐹𝑟−0.430 (𝑑𝑠 𝐷 )−0.003

Vertical downward pipe:

HMC 𝜆𝑧 = 1.018 × 10−13𝐹𝑟1.206 (𝑑𝑠 𝐷 )−3.519

LMC 𝜆𝑧 = 8.220 × 10−9𝐹𝑟0.129 (𝑑𝑠 𝐷 )−2.042

∆𝑃𝑇 ∆𝐿 = ∆𝑃𝑔 ∆𝐿 + ∆𝑃𝑠 ∆𝐿 = (𝜆𝑔+𝜇𝜆𝑧)𝜌𝑔𝑈𝑔

2

2𝐷

• The absolute value of exponents of the Froude number is

smaller for LMC, showing smaller effect of gas velocity on

solid friction factor for coal with lower moisture content

Range: pipe diameter 10~25mm, particle diameter 35μm,

carrier gas is N2, back pressure is 1~4MPag

Measurement(kPa/m)

Ca

lcu

lation

(kP

a/m

)

Horizontal

Vertical upward

HMC

LMC

Measurement(kPa/m)

Ca

lcu

lation

(kP

a/m

)

Vertical downward

HMC

LMC


Pressure drop due to gas phase:

∆𝑃𝑔 = 𝜉𝜌𝑔𝑈𝑔

2

2=𝜉

2∙𝜌𝑔𝑈𝑔𝐴

𝐴∙ 𝑈𝑔 =

𝜉

2∙ 𝐺𝑔𝑈𝑔 = ng ∙ 𝐺𝑔𝑈𝑔

Pressure drop due to the presence of solid particles:

∆𝑷𝒔 = 𝒏𝒔 ∙ 𝑮𝒔𝑼𝒈

Pipeline resistance – Bends

∆𝑃𝑇 = ∆𝑃𝑔 + ∆𝑃𝑠

1m

1m

R

b

Rb/D=15.2 (D=25mm)

Rb/D=21.3 (D=15mm)

Rb/D=26.7 (D=10mm)

Pressure measuring point

Pressure measuring point

– Additional pressure drop model

Measurement(kPa/m)

Calc

ula

tion

(kP

a/m

)

HMC

LMC 𝒏𝒔

Pipe diameter 25mm 15mm

Coal HMC LMC HMC LMC

Horizontal to horizontal 1.03 0.95 0.79 0.72

Horizontal to vertical upward 1.69 1.40 1.32 1.09

Vertical upward to horizontal 1.26 0.69 0.85 0.78

Horizontal to vertical downward 0.94 0.92 0.77 0.80

• LMC coal has smaller ns, due to smaller cohesiveness and

resistance


Background

Experimental system


Conclusion


Conclusion

– Pneumatic conveying of two kinds of high-volatile bituminous coal powder with different

moisture content was carried out on an experimental facility using 25mm, 15mm, and

10mm pipes , and at back pressures of 1.0~4.0MPag

– The characteristics of solid mass flow rate, solid concentration and particle velocity were

obtained. Resistance and cohesiveness increases with increasing moisture content, so

solid mass flow rate and solid concentration is larger for coal with low moisture content in

the dense-phase region. While in the dilute-phase region, moisture content has minor

effect on conveying characteristics.

– The pressure drop of the two-phase flow is modeled by additional pressure drop models

for straight pipes and bends. For coal with lower moisture content, the effect of gas velocity

on solid friction factor is smaller and ns is smaller, due to its smaller cohesiveness and

resistance


Thanks for your attention !

Zhen Liu

Department of Energy and Power Engineering

Tsinghua University

Beijing, China

(Tel.) +86-10-57339813

(Mob.) +86-13520030865

(E-mail) [email protected]

investigation on the effect of moisture content on conveying ......liang cai et al. investigation of...

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