instruction bed lab

8/2/2019 Instruction bed Lab

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Laboratory Notes

Heat transfer measurements in fluidized bed combustionreactor

(approx. 2-3 hours laboratory exercise)


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Introduction

Fluidised Bed Combustion has became a matured technology today. This has beenintroduced and developed in last 20 years. The main working principle is that the fuelis combusted in a turbulent bed of sand and ash which implies good heat transferand mixing. Since 1990 a coal fired Pressurised Bed (PFBC) is used as a base loadunit in Stockholm. Similar units are reported in operation in Spain, Germany, Japanand United States.

The main advantage with PFBC is the low combustion temperature which impliesvery low emissions of sulphur and NOx. PFBC units are compact in size. The PFBCunit, which is operating in Stockholm has been built as a part of combined cyclepower generation and it produces heat and power.

Disadvantages are the price and the complexity of the system. Mainly for thesereasons no more PFBCs were built in Stockholm.

In Sweden, there are a lot of Circulating Fluidized Beds (CFB) in operation usingbiomass as fuel. In CFB applications, the air entering from the bottom has a highervelocity and it makes whole bed blowing away from the furnace. At the top the bed,material is separated from the flue gases and it returns to the bottom.

One disadvantage using biomass could be high temperature corrosion on steam tubepackages.

Characteristics

A way of describing the process of fluidisation is to imagine a bed of solid particleskept in position on a plate inside a vertical tube. The plate is perforated so that aircan enter from the bottom through the plate and pass through the bed. Different air

flows give different properties to the bed.


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At very small flows the air will pass between the particles without disturbing theirposition. The bed is then called static bed and acts like a filter. By increasing the air

flow to a higher level, some particles in the bed will start to move depending on theuneven air distribution in the bed.

At higher air flows particles will be separated from each other and the bed volumeincreases. This is incipient fluidisation. A higher contact area between the air and theparticles will be the result, which is an advantage during combustion.

Even at higher air flows the particles will be mixed with each other and the bed ismore like a boiling fluid. Now we are talking about full fluidisation. This is the domainwhere most advantages with fluidisation can be found.

Continuous increase of the air flow means bigger size of the air bubbles and at theend these will occupy the whole width of the bed. The phenomenon is calledslugging and it is very common when using gas as a fluidisation medium.

When the flow is increased further more, the particles will be blown away togetherwith the air stream. This is called pneumatic transportation of particles. CFB boilerswork in this domain.

Combustion in a fluidised bed

During combustion in a fluidised bed the air is supplied through a distribution plateplaced at the bottom of the bed. The air fluidises the bed. The best way of using thebed is to use the right amount of air that makes the full fluidisation in the bed.

The bed itself consists of fuel, ash, sand and a sorbent (could be dolomite). Themass fraction of fuel is very small, only around 1 %. Sand is only used if there is notenough amount of ash or sorbent which could be the case if oil is used as a fuel. Thesorbent is used for capturing the sulphur through reactions in the bed.

There are 7 important parameters which determine the characteristics of the bed:

1. Bed Temperature. Normal temperatures used are around 750C-950Cdepending on the use and the fuel characteristics. To control the temperature

f th b d b d t b ti l l d Thi i d i l ti t


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4. Bed height. Common values are 1 metre 4 metre. (4 m in PFBC). Increasedfluidisation velocity gives higher bed level.

5. Tube location. I the case with immersed cooling tubes, the tube size and thetube placement strongly affect the effectiveness of the bed. Wrongdimensioning of tube placement will lead to pressure drops and worse heattransfer.

6. Heat Transfer Coefficient. Heat transfer from the bed to the cooling tubes isdue to convection and radiation. The radiation part is affected by thetemperature only. The convection part is affected by particle size, temperature,distance between tubes and the fluidisation velocity. Convective heat transferincreases when the tube temperature increases, smaller particles and higherdistance between the tubes. The fluidisation affects the convective heattransfer in such a way that the particles destroy the thin air layer surroundingthe tubes. Increased convective heat transfer will be the result.

7. Pressure drop. The pressure drop in a fluidised bed is dependent on the fluid-isation velocity at low air flows. The pressure drop increases with increasingvelocity up to incipient fluidisation. Increasing the velocity from this point doesnot affect the pressure drop anymore. The pressure drop remains constantuntil pneumatic transportation starts. In a power plant there are also pressuredrops through eventual dust separators and distribution plates. These dropsincrease continuously with increasing velocity.

Advantages with combustion in a fluidised bed

In the fluidised bed combustion the combustion temperature is kept low compared toconventional combustion. This gives several advantages. Since the temperature is

kept in the interval of 750C-950C (which is around 1000C-1600C for conventionalcombustion furnaces), the following advantages can be achieved.

Low NOx emissions. There are no thermal NOx at all (from air nitrogen). Thisproduction needs a temperature from 1500C, which means that the NOxproduced in a fluidised bed comes from fuel nitrogen only


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Good heat transfer due to particle convection. This means that the heattransfer surface can be constructed smaller for the same power. Even better if

the unit is pressurised.

Fuel flexibility

Disadvantages with fluidised bed combustion

Erosion problems on immersed tubes

Ash fusion because of wrong air distribution

Fuel feeding is complex (PFBC)

Ash removal (PFBC)

Production of N2O which is a very strong greenhouse gas.

Description of Measurement equipment

In the fluidised beds, which are used for steam production the bed is cooled by steam

tube packages. In this laboratory exercise, the operation of the bed is set at opposite.The bed is heated with electrical elements. The characteristics of the bed will anyhowbe the same.

The central part of the equipment is the fluidised bed, which consists of a glass tubewith metal plates at the top and at the bottom. Air enters the bed through the lowerplate and the distribution filter. The purpose of the distribution plate is to distribute theair for bed operation and also to make sure that the bed material will not fall down

when the unit is switched off. To prevent the bed material to blow away at high airflows, there is a filter at the top plate.

The air from the pressurized system enters the unit through a reducing valve, fromwhich it is possible to regulate the air flow. Afterwards the air flows through tworotameters connected in series The meter has the measurement range from 0 4 to 4


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There are four thermocouples in the fluidised bed but there is only one temperaturedisplay panel available. By using the selection keys, it is possible to choose the

corresponding thermocouple to display the required temperature.

The keys corresponding to display temperature at different thermocouples:

Button 1 Thermocouple under the filterButton 2 Thermocouple in the bedButton 3 Thermocouple at the top of the heating elementButton 4 Thermocouple on the side of the heating element

When the buttons 1-3 are switched on, thermocouple 4 is connected to a temperaturesafety device which cuts off the power when the temperature reaches 200C. Whenbutton 4 is switched on, thermocouple 3 is connected to the safety device.

The pressure drop through the bed can be achieved with a movable probe, whichmeasures the static pressure. The pressure can be read on a U-pipe which contains

water.


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Voltimeter

TemperatureGuard

PowerRegulator

TempDisplay

Amperimeter

1 2 3 4

2

3

4

1

Rotameters

Fluidized

Bed

Up

ipe

Switch box

Fuse

Presurised AirValve

Figure 3: Layout of the equipment

V: Voltage indicatorA: AmmeterT: Temperature security deviceC: Temperature indicatorR: Power regulation

Button Display shows Security connected to1 Thermocouple 1 Thermocouple 4

2 Thermocouple 2 Thermocouple 4

3 Thermocouple 3 Thermocouple 44 Thermocouple 4 Thermocouple 3


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PERFORMANCE

1.0 Measurement of pressure drop and bed height

Adjust the flow according to the values given in table 2 and perform following at eachsetting:

- Measure the pressure drop through the bed. Please note that two values are

necessary because of the drop through the top filter- Measure the bed height and note special observations

Air flow Pressure drop mm vp

Rota meter

Meter 1 Meter 2

m3/h Sand bed+top filter

Top

filter

sandbed

Bedheight(mm)

Specialobservations

12.5 0.5

25 1.0

37.5 1.5

50 2.0

75 3.0

100 4.0

31 5.0

44 7.056 9.0

63 10.0

Table 2: Test conditions and data table for pressure drop and bed heightmeasurement

When the measurements are completed, mark those points on diagram 1.


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2.0 Determining the heat transfer coefficient

One of the main advantages of fluidised bed technology is the high heat transfercoefficient between the bed and the immersed tubes.

The students are supposed to calculate (1) the heat transfer coefficient between theheating element and the bed and (2) the heat transfer coefficient between theelement and the free air stream above the bed.

For heat transfer is given:

P = h *A * t

where

P = power [W]h = heat transfer coefficient [W/m2,K]

t = temperature difference during heat transfer [K]A = heating element area = 0.00165 m2

2.1 The element in the bed, constant surface temperature

Because of the long heating time for the sand at low air flow, it is advantageous to

start with the highest air flow. Before starting the measurements, the element issubmerged 4 cm into the bed. It is easier if the bed is fluidised a little bit.Perform the following measurements for the air flows due to table 3:

- Adjust the air flow

- Turn the power and try to find an equilibrium state when the surface

temperature (thermocouple 3) shows 190C. At the first measurement pointyou have to wait around 10 minutes to reach stability because of the heating ofthe sand. At a flow rate of around 6 m3/h the stable sand temperature wouldbe around 55C.


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When all the values are calculated, plot the heat transfer coefficient as a function ofthe flow in diagram 1.

Air flow

Rotameter

1st 2nd m3/h

CurrentA

VoltageV

BedtempC

Elementtemp

CtK

Heattransfer

coefficientW/m

2K

38 6.0

75 3.050 2.0

37.7 1.5

25 1.0

12.5 0.5

Table 3: Test conditions and data table for measurement part 2

2.2 The element in the air, constant surface temperature

Perform same measurements as in 2.1, but keep the element above the bed. Youonly need two measurements (two points), one in higher flow and other is smallerflow. Calculate and plot the heat transfer coefficient in diagram 1. Draw a straight linebetween the points.

Air flow

Rotameter

1st m3/h

CurrentA

VoltageV

BedtempC

Elementtemp

CtK

Heattransfer

coefficientW/m2K

25 1.0

100 4.0


2 3 Th l t i th b d i bl f t t


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Air flowRotameter

1st m3/h

ElementtempC

BedtempC

tK

CurrentA

VoltageV

Heattransfer

coefficientW/m

2K

75 3.0 190

75 3.0 150

75 3.0 100


REFERENCES

Fransson, T. H. et al.; 2002.

CompEduHPT: Computerized Educational Program in Heat and Power Technology

Division of Heat and Power Technology, KTH, SE-100 44 Stockholm, Sweden.

Kunii, D., Levenspiel, O.; 1991.

Fulidization Engineering

2nd

Edition, Butterworth-Heinemann Cooperation


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Diagram 1

0

200

400

600

800

1000

1200

1400

1600

0 1 2 3 4 5 6 7 8 9 10

Air flow (m3/h)

Heattransfe

rcoefficient(W/m2K)

0

2

4

6

8

10

12

14

16

Pressur

edrop(mmH2O)

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