Study on Calculation Method for Partition of Heat Transfer in an Ultra-supercritical Boiler

doi:10.4236/epe.2013.54B061

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Energy and Power Engineering, 2013, 5, 311-314

doi:10.4236/epe.2013.54B061 Published Online July 2013 (http://www.scirp.org/journal/epe)

Study on Calculation Method for P artition of Heat

Transfer in an Ultra-supercritical Boiler

Ye Teng1, Zhongxiao Zhang1,2, X u da n Liu2, Wei Liu2, Tuo Zhou1, Ming Zhu1

1School of Energy and Power Engineering, University of Shanghai for Science and Technology, Shanghai, China

2School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai, China

Email: [email protected]

Received March, 2013

ABSTRACT

Use a 1000MW ultra-supercritical tower boiler as the research object. On the basis of one dimensional model, simplify

the tube heat transfer model and the radiation heat transfer model; establish the two-dimensional area calculation model

with the regional method; summarize the heat load distribution of flue gas temperature and water wall surface; and

compare with the measured data. The error range of the result is acceptable on the project. The distribution of water

wall surface heat load along the furnace width and the area where heat transfer deterioration cause easily along the

furnace height direction are studied with the model and algorithm on different boiler load conditions. All these provide

the reference for the design and operation of the ultra supercritical boiler.

Keywords: Ultra-supercritical; Flue Gas Temperature; Heat Load; Heat Transfer Deterioration

1. Introduction

Ultra-supercritical unit with the features of large capacity,

high parameter and low energy consumption, has become

an important development direction in China [1]. Part of

heat, which is given off by burning pulverized coal, is

transferred to the water wall. However, the process is

quite complicated. Under the supercritical pressure, the

increase of surface heat flux on water wall will cause the

tube heat transfer deterioration [2], due to the large spe-

cific heat region [3]. In addition, uneven temperature

field in furnace, and flame center deviation, will lead to a

series of accidents such as high local temperature of the

tube surface, boiler slogging and so on.

In order to solve or improve the above problems, we

need to establish a mathematical model to study the heat

transfer process inside the furnace. Boiler thermody-

namic calculation standard of furnace heat transfer is

based on a variety of zero-dimension models, but it can

only provide a few parameter values, and this method

will generate large errors during the calculation of large

capacity boiler. We can study on heat transfer character-

istics along height, width, depth direction in furnace by

promoting the zero-dimension model to multi-dimension

model.

In this paper, we use zone method to establish th e two-

dimensional area calculation model on the basis of one-

dimensional partition model [4]. And we analyze the

distribution characteristic of the water wall temperature;

witch can provide a reference for the large capacity boi-

ler design and operation.

2. Two-dimensional Area Mathematical

Model

This paper studies a 2955 t/h ultra-supercritical boiler

(once-through boiler with spiral pipes by variable pres-

sure operation, single furnace tower layout, tangential

firing, controlling temperature by swing nozzle, balanced

ventilation, suspension structure of all steel). The char-

acteristics of design coal are shown in Table 1, and basic

design parameters of the boiler are shown in Table 2.

2.1. Simplifying Assumptions

On consideration of the extremely complex process in-

side furnace, we need to simplify the heat transfer proc-

ess in a reasonable manner to get the law of heat load on

water wall surface and water wall temperature [5, 6].

This paper made the following assumptions:

Table 1. The design data of coal.

Item Data

Car % 64.15

Har % 3.61

Oar % 0.78

Nar % 0.71

Sar % 0.43

Aar % 12

Mar % 14

Qar,net kJ·kg-1 23420

Y. TENG ET AL.

312

Table 2. Design parameters at BMCR.

Item Data

Superheated steam flow/t·h-1 2955

Superheater outlet steam pressure/MPa 27.9

Superheater outlet steam temperature/℃ 605

Reheat steam flow/t·h-1 2443

Reheater inlet steam pressure/MPa 6.2

Reheater outlet steam pressure/MPa 6.03

Reheater inlet steam temperature/℃ 367

Reheater outlet steam temperature/℃ 603

 Separate heat transfer and combustion process in-

side the furnace, and get flame temperature distribution

along the height direction by one-dimensional model [4,

7].

 View flame located in the center of furnace as a

blackbody, and cylindrical distribution in the main com-

bustion zone.

 Furnace flue gas and water wall are regarded as

gray body, Area method is used to study heat transfer in

the furnace.

 Water wall is diaphragm wall, and the unilateral

surface gets different flame radiation.

 The flux of working medium in tube is homogene-

ous, and the boiler operation belongs to the supercritical

area; the coefficient of heat transfer in the tube is se-

lected according to working state (pressure, flow, heat

load), the physical properties of working medium and

experience parameters.

2.2. Mathematical Model

In this paper, two-dimensional area simplified furnace

water wall heat transfer model is established through

regional method. Area division is shown as Figure 1.

Yellow circle area represents fireball, and white area

represents flue gas around the fireball. We will take one

small area among the divisions into consid eration an d the

radiation heat from this area is transferred to the sur-

rounding water surface.

Figure 1. Furance partition figure.

Division calculation area of the object of study:

 Each layer is divided into regions on the basis of the

one-dimensional model of 18 layer partition, that is,

fireball center and fireball outside.

 Each layer of the wall was divided into 40 areas;

each side wall has ten regions.

The diameter of the fireball D can be calculated by the

following formal:





0.56 0.87 0.7

0.25

1.45 dl

Ddmd Dhbsb



We calculate the radiation characteristic parameters of

flame and flue gas through the real furnace operating

parameters, and radiation projection received by water

wall can be calculated by the following formal:

iijgiijsi

GGST SST









Radiation heat transfer area can be expressed as:

/cos 32

cos cos

cos coscos cos

Ks KB

iij i

ijij ij

KeV A2

GS sB

AA bh

SS ss







 













3. Water Wall Tube Heat Transfer

Calculation Model

The heat transfer calculation of membrane type water

wall is on the basis of calculating heating load and me-

dium enthalpy value in each section by energy equation.

Then metal wall temperature and fouling wall tempera-

ture can be calculated:



()

iiii

gzgz gzgz

js gzi

hw gzi

qdl

hfpthh m

tt q



 













 







As the physical properties of working medium change

dramatically under ultra-supercritical pressure condition,

the selection of heat transfer coefficient can be fitted by

reference [8]. According to the measured date [8], the

calculation condition in this paper is given in Table 3.

Table 3. Calculation condition.

ConditionBoiler load/MWSteam pressure/MPa Steam flow/t·h-1

1 970 27.85 2869

2 662 19.64 1980

3 507 14.82 1477

Y. TENG ET AL. 313

4. Results and Discussions

The distributions of calculated value and measured data

on three conditions are shown as Figure 2. It did not

consider the influence of flame migration on the heat

load on water wall in calculation, so there is a certain

error compared to the measured values. However, it still

can show the same change tendency, and calculated val-

ues and measured values have better alignment. Two-

dimensional area model can reflect the basic change rule

of one-dimensional model, and it can give better features

of flue gas temperature, as well as heat load on water

wall after adjusting some parameters. However, more

factors will produce more errors which have more im-

pacts on the calculation results.

It can be concluded that the average heat load and

temperature on water wall surface along the furnace

height by one-dimension calculation model. But for real

boiler, the heat load distribution on water wall is not

uniform along the furnace width direction. The distribu-

tion of tangential firing boiler is generally high in the

middle and low on both sides.

Two representative cross-sections were selected in order

to compare the calculated value and measured data. The

two cross-sections are 34 m elevation (main burner cen-

ter section) and 54 m elevation (SOFA burner center sec-

tion). The fo rmer is located in burning area, and the latter

is located in burnout area. The distribution of heat load

along the width direction in 34 m elevation is shown as

Figure 3. The distribution of heat load along the width

direction in 54 m elevation is shown as Figure 4.

As shown in Figure 3 and Figure 4, along the water

wall width direction, both calculated values and meas-

ured data on back wall show the same distribution fea-

tures. The heat load on water wall surface is high in the

middle and low on both sides on these three conditions in

Figure 2. Heat load on water w al l on back wall .

Figure 3. Distribution of heat load along width direction in

34 m elevation.

Figure 4. Distribution of heat load along width direction in

54 m elevation.

34 m elevation, so it is on left wall. By contrast, the arc

in 34 m elevation is more apparent, but in 54 m elevation,

such law is not obvious, especially on the low load con-

dition 3. This means that the heat load on water wall sur-

face in 34 m elevation is more significantly than that in

54 m elevation along the furnace width direction. The

highest heat load on water wall on condition 1 is 501

kW/m2; 356 kW/m2 on condition 2 and 332 kW/m2 on

condition 3 in 34 m elevation. It agrees with the meas-

ured data. In 54 m elevation, the highest heat load on

water wall surface decreases obviously which shows the

unsatisfied agreement between the calculated values and

measured data.

Y. TENG ET AL.

314

Figure 5. Curve for heat load on water wall.

The expected highest position of heat load is generally

in the same height in the middle of each side wall in de-

sign, and the heat load of the four corners is low. Fig-

ure 3 reflects this rule, but Figure 4 does not. The main

reason is that 54 m elevation is at the SOFA wind vents

and the air temperature is only 334℃ in SOFA burner,

but its air volume accounts for 23% of the total air vol-

ume. Thus it will produce a certain cooling to the flue

gas of high temperature in furnace. As a result, it reduces

the heat load in this area.

Figure 5 shows the heat load distribution of water wall

surface on condition 1. The heat load distribution of the

four walls is the same when the flame center does not

deflect. The highest head load appears in the center of

each side wall which agrees with the design intention.

5. Conclusions

The complicated calculation model can reflect the calcu-

lation results of simple model. This strategy is closer to

the actual situation in terms of predicting larger variation

range and furnace area. However, there are some aspects

that can influence the precise calculation. The value and

distribution are reasonable by calculating the

two-dimensional area model of 1000 MW USC tower

boiler.

In the same height, the maximum value of heat load on

water wall surface appears in the central position which

shows radial distribution. Of all the calculation condi-

tions, the most dramatic change of heat load appears in

34 m elevation, and it gives a range from 237 kW/m2 to

501 kW/m2 which is 264 kW/m2 in difference.

USC boilers need to avoid heat transfer deterioration

in large specific heat region. The calculation results of

flue gas temperature show that the highest flue gas tem-

perature and heat load occur in the combustion area. The

phase transition point of working medium should be con-

trolled to be away from these areas under the supercriti-

cal pressure.

Calculation results show that the highest heat load on

water wall surface occurs in 34 m elevation. The maxi-

mum value of heat load on water wall surface is 501

kW/m2 on 970 MW condition. The maximum value of

heat load decreases with the decrease of boiler load. The

value becomes 332 kW/m2 on 507 MW condition.

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