A Novel Boiler Design for High-Sodium Coal in Power Generationb-dig.iie.org.mx/BibDig2/P17-0164/data/pdfs/trk-10/POWER2015-49167.pdf1 A NOVEL BOILER DESIGN FOR HIGH. SODIUM COAL IN

A NOVEL BOILER DESIGN FOR HIGH-SODIUM COAL IN POWER GENERATION

Song Wu, Wengang Bai, Chunli Tang, Xiaowen Tan, Chang'an Wang, Defu Che* State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University

Xi’an, Shaanxi, 710049, P.R. China. *Tel: +86-029-82665185, Fax: +86-029-82668703

Email：[email protected]

ABSTRACT Considering the severe fouling of high temperature

convection pass in the boilers using high-sodium coals, a novel

boiler design with a furnace exit gas temperature (FEGT) below

800 °C was proposed. The design was evaluated in different

kinds of boilers with various capacities by examining thermal

system arrangement, heat transfer, ignition and combustion, and

steel consumption. The results indicate that, more radiation

heating surface should be used in the thermal system

arrangement of the novel boiler besides the volume-enlarged

furnace. A marked decrease in the converted coefficient of

radiation heat transfer is found in a volume-enlarged furnace

due to the reduction in the average temperature of the flame.

Moreover, the volume-enlarged furnace can adversely affect

ignition and combustion. The cyclone-fired boiler is considered

to be the most appropriate application for the novel design, for

its combustion and heat transfer in furnace are carried out

in divided chambers. A comparison of steel consumption

demonstrates the expense of the novel boiler is approximately

increased by 10% relative to the conventional one. In addition,

an improved application with flue gas recirculation is described

in detail, owing to its advantages of controlling FEGT and

maintaining the level of convection heat transfer capability of

the boiler.

Keyword: Fouling; High-sodium coal; Furnace exit gas

temperature; Thermal system arrangement; Cyclone furnace;

Flue gas recirculation

INTRODUCTION

There is a wide distribution of high-sodium coals in

America, Australia and China. For example, Zhundong

coalfield, found in Xinjiang of China several years ago,

contains 390 billion tons of coal resources approximately.

Moreover, the Zhundong coal also has the advantages of low

ash content, excellent performance of coal ignition and burnout,

and strong combustion stability. However, when used in the

fossil-fired power plant, the high-sodium coal can result in

severe fouling of high temperature convection pass in the boiler

[1-4]. The fouling of heating surfaces will threat both economy

and safety of the boiler, such as decreasing boiler output and

efficiency, corroding pressure parts, causing unit outages for

cleaning and repairs etc [5, 6]. As compared with the

conventional coals, the fouling caused by the high-sodium coals

is much more excessive and can’t be controlled with

sootblowers. As shown in Fig. 1, the high temperature reheater

in a 125 MW unit was fouled heavily after firing the Zhundong

coal. The bonded deposit covered almost all the tubes of the

bank in Fig. 1(b) and what’s worse, the flow passages of flue

gas have been partly blocked. Therefore, the fouling has been

the key problem that is restricting the efficient use of high-

sodium coals. The ultimate analysis and ash composition

analysis of the Zhundong coal are shown in Table 1 and Table 2,

respectively.

The forming of high temperature bonded deposit leads to

the severe fouling while the fuel with high alkali metal content

is burned. The majority of this deposit occurs in the high

temperature convection zone. It is formed with chemical

reactions and can’t be removed easily [7]. It’s generally

believed that the alkali metal in fuel plays a very important role

in this deposition process [8-12]. Volatile forms of the alkali

metal are vaporized in the furnace during combustion. Then the

vaporizations diffuse onto the tube walls and condense, making

significant contribution to the formation of the bonded deposit

on convection heating surface.

Proceedings of the ASME 2015 Power Conference POWER2015

June 28-July 2, 2015, San Diego, California

POWER2015-49167

1 Copyright © 2015 by ASME

(a) Before firing the Zhundong coal

(b) After firing the Zhundong coal

Fig. 1 Fouling of the high temperature reheater in a 125 MW unit

Table 1 Ultimate analysis of the Zhundong coal (wt %, as-received base)

Car Har Oar Nar Sar Aar Mt Qnet,ar /(MJ/kg)

54.99 2.32 9.16 0.41 0.55 5.27 27.30 19.13

Table 2 Ash composition analysis of the Zhundong coal

Ash Composition (wt %) Ash Melting Point /°C

SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O TiO2 SO3 DT ST HT FT

17.08 6.99 11.6 27.87 7.66 6.08 0.46 0.61 21.65 1320 1330 1340 1350

Much work has been devoted to the formation mechanisms

of high temperature bonded deposit related to the alkali metal.

The study on alkali metal occurrence mode in coal [13-16]

indicated that the alkali metal in coal can be classified into

organic alkali metal and inorganic alkali metal. The organic

alkali metal exists in forms of carboxylates and nitrogen or

oxygen functional groups, while the inorganic alkali metal

exists in forms of chloride crystals, hydrated ions as well as

aluminosilicates etc. Especially, Weng et al. [17] conducted an

experimental investigation on alkali metal occurrence mode in

Zhundong coal by extraction method. Research results showed

that the sodium content in Zhundong coal is significantly higher

than other types of coals, but the potassium content is lower.

And the majority of the sodium in Zhundong coal exists in the

form of water soluble sodium. Some researchers [18-21]

reported that the soluble alkali metal in coal will generate

gaseous alkali metal compounds to be released in combustion or

gasification conditions, while the insoluble alkali metal that

exists in the form of aluminosilicates will be left in the ash.

Kosminski et al. [22] detected that the release of sodium is

nearly half of that of chlorine and inferred to be in the form of

sodium chloride vapor during gasification and pyrolysis of low-

rank coal. Tomeczek et al. [9] agreed that the chlorides are the

main existing form of the alkali metal in the high temperature

flue gases. As condensing on downstream heating surfaces or

gas-turbine vanes, they will react with SO3, O2 and water vapor

in the flue gases to produce alkali metal sulfates with low

melting points. Shimogori et al. [12] conducted a series of

slagging tests in a 1.5 MWth pilot plant and pointed out that the

predominant mechanism in the formation of the initial layer of

deposit differs for bituminous and sub-bituminous coals.

Additionally, many investigators [23-25] have explored the bed

agglomeration and defluidization during fluidized-bed

combustion and gasification of fuels containing high levels of

alkali metal. They agreed that the low melting eutectics formed

by alkali metal are responsible for these problems. These

research efforts are very useful to recognize how the high

temperature bonded deposit forms.

Currently, a number of measures have been proposed to

prevent excessive fouling of high temperature convection pass

in the boilers firing high-sodium coals. Water washing [26, 27]

is an effective pretreatment method to remove the alkali metal

in the fuel and much attention has been focused on it. Because

of high capital cost, high energy consumption as well as high

water resource consumption, water washing is not suitable to be

used as the preferred option in the coal-fired power plants.

Another method is to add the additives or adsorbents to the

furnace [28, 29]. However, the efficient additives and

adsorbents are still under research, and the huge consumptions

required for the power plants restrict the practical application of

this method deeply. The Babcock & Wilcox Company [30]

advises that the side spacings in the convection banks should be

widened in the design of the boilers using severe fouling coals.

Recently, the blending combustion has become popular in the

utilization of the Zhundong coal, but the blend ration of the

Zhundong coal is restrained and generally no more than 40% is

allowed [31]. Besides, some power plants are using the

Zhundong coal by reducing the load to a low level. Since these

measures can’t solve the fouling problem both effectively and

economically, it’s more desirable to find a better method for the

utilization of high-sodium coals.


The purpose of this study is to propose a novel boiler

design for high-sodium coal. By providing more radiation

heating surface in the furnace, this design can reduce the

furnace exit gas temperature (FEGT) below 800 °C to avoid the

high temperature bonded deposit. In order to explore the

feasibility of the design, different kinds of boilers with various

capacities are selected to discuss its thermal system

arrangement, heat transfer as well as ignition and combustion.

Also a comparison of steel consumption between a novel boiler

and a conventional one is conducted to consider the economy of

the design. In addition, for the convenience of adjusting and

controlling the FEGT, an improved application with flue gas

recirculation is described in detail.

NOVEL BOILER DESIGN FOR HIGH-SODIUM COAL AND THERMAL PERFORMANCE CALCULATIONS Control principle of fouling

In the boilers firing high-sodium coals, the severe fouling

is defined as the formation of high temperature bonded deposit

on convection heating surfaces. As previously noted, it is the

volatile alkali metal that should be responsible for the formation

of high temperature bonded deposit. Thus, in order to control

the excessive fouling, it is necessary to focus on the

transformation of the volatile alkali metal in coal along the flue

gas flow. As illustrated in Fig. 2, volatile forms of the alkali

metal are vaporized in the hot flue gas when coals are burned in

the furnace. Later the vaporizations leave the furnace outlet and

enter the high temperature convection pass. With diffusing onto

the cool tube walls, they condense and produce a glue which

results in the formation and growth of the bonded deposit. Areas

where the high temperature bonded deposit occurs are marked

in grey. As the flue gas temperature is reduced, the

vaporizations that are not captured by the banks condense on

ash particles in flue gas and are solidified. Hence, the fouling of

low temperature convection pass is mainly in the form of loose

deposit which can be removed by sootblowers easily.

According to the above analysis, the key to successfully

prevent the formation of the high temperature bonded deposit is

to reduce the vaporizations content in the flue gas passing

through the high temperature convection banks. The phases of

the vaporizations are mainly determined by temperature, so it is

available to transform these vaporizations by flue gas

temperature control. A feasible and effective method is to

control the FEGT. If the FEGT is reduced to a desired level at

which the majority of the vaporizations are solidified, there will

be few vaporizations entering the high temperature convection

pass. As a result, the formation of the high temperature bonded

deposit can be avoided.

Fig. 2 Transformation of the volatile alkali metal in coal along the flue

gas flow

Determination of FEGT Generally, the determination of FEGT primarily depends

on deformation temperature of the ash in a conventional boiler

design. However, considering the control of high temperature

bonded deposit, a specific FEGT is required to solidify the

vaporizations of the alkali metal in the flue gas leaving the

furnace outlet. Melting points of common sodium compounds

released in the flue gas are shown in Table 3. Since the release

of sodium is mainly in the form of sodium chloride vapor, the

desired FEGT should be lower than the melting point of NaCl.

Accordingly, a novel boiler design for high-sodium coal is

proposed, providing an FEGT below 800 °C during normal

operation. Table 3 Melting points of common sodium compounds

Sodium Compound Melting Point /°C

Na2O 611

Na3Fe(SO4)3 623

Na3Al(SO4)3 646

NaCl 801

Na2CO3 851

Na2SO4 884

Na2SiO3 1088

Na4SiO4 1088

Boiler configuration for high-sodium coal The furnace of a large pulverized coal, oil or gas fired

boiler is both the place where combustion of fuel occurs and the

place where heat transfer occurs. Radiation basically controls

heat transfer to the furnace enclosure walls. An important

function of the furnace is to reduce the hot flue gas temperature

to a level acceptable to superheaters. For the purpose of firing

high-sodium coal, a decreased FEGT below 800 °C is required

in the novel boiler design. To meet this design requirement, the


novel boiler provides more radiation heating surface to cool the

combustion products in the furnace. As shown in Fig. 3, the

width, depth and height of the furnace are all increased in the

novel design, and the platen cooled by superheat steam in the

upper furnace is also enlarged. As a result, more heat of

combustion products can be removed in the furnace and the

desired FEGT can be achieved. In addition, with the total

convective heat transfer reduced, the novel boiler provides a

decreased scale of convection pass correspondingly, though the

configuration of a single bank may change a little.

Fig. 3 Furnace configuration comparison between a novel boiler and a

conventional one

Thermal performance calculations A well-designed and operated boiler results from an

accurate thermal performance calculation. The thermal

performance calculation of a boiler can be classified into design

calculation and verification calculation, depending upon

whether a new boiler is being designed or an existing piece of

equipment is being analyzed. Both design calculation and

verification calculation are based on the same heat transfer

principles. Besides, identical equations and diagrams are used.

Heat transfer of radiation heating surface in furnace is

expressed by: 4

radiation fur 0 flaQ A a T (1)

where A is the furnace enclosure wall area, ψ is defined as the

thermal effectiveness factor, afur is known as the furnace

emissivity which is a hypothetic emissivity corresponding to the

radiosity of the flame, σ0 is the Stefan-Boltzmann constant and

Tfla is the average temperature of the flame. Heat transfer of

convection heating surface can be given as:

convection cQ UA t (2)

where U is the heat transfer coefficient, Ac is the area of

convection heating surface and Δt is the temperature difference.

In this paper, the converted coefficient of radiation heat transfer

is defined as:

radiationrad

fla wf( )

Q

A T T

(3)

where Twf is the average temperature of the working fluid. αrad

reflects the intensity of radiation heat transfer in furnace, which

is related to many different factors, including flue gas physical

properties, temperature, furnace configuration, variation of slag

or ash buildup on heating surface, and so on. In the novel

design, since radiation in furnace takes more proportion, it is

more significant to increase αrad in range of reasonable

parameters.

RESULTS AND DISCUSSION Thermal system arrangement

Fig. 4 illustrates the effect of the FEGT on relative heat

absorption of furnace and convection pass when the Zhundong

coal is used. As the FEGT declines, the heat absorbed by

furnace increases. Conversely, the heat absorption of convection

pass decreases. A reduction of 100 °C in the FEGT

approximately leads to a 5% shift in relative heat absorption of

furnace or convection pass. Especially, when an FEGT of

800 °C is determined in the novel boiler design, heat absorption

of furnace is nearly twice more than that of convection pass.

Thus, more heating surface should be arranged in the furnace to

accomplish the heat absorption distribution of boiler system.

Fig. 4 Relative heat absorption of furnace and convection pass at

different FEGTs (Heat absorbed by air heater is included in the total

absorption)

With such a low FEGT, the thermal system arrangement of

a boiler also should be adjusted simultaneously. Taking a 220

t/h boiler system for instance, flue gas outlet temperatures of

heating surfaces along the flue gas flow are shown in Fig. 5. If

the FEGT is maintained at a level of 800 °C, the removal of

heat absorbed by secondary superheater and part of high

temperature superheater will be accomplished in the furnace.

That means the secondary superheater and part of the high


temperature superheater should be arranged in the furnace as

radiant superheaters, taking the form of widely spaced (700 mm

or larger side spacing) superheat platens or wall superheaters

which are similar to water-cooled wall in configuration. This

analysis is based on the assumption that the heat absorbed by

water-cooled wall remains unchanged. In practice, when the

furnace is enlarged, the water-cooled wall absorbs more heat.

Correspondingly, the heat which should be absorbed by the

economizers is decreased. Eventually, more heating absorption

and superheating absorption will be obtained in the furnace by

radiation heat transfer.

Fig. 5 Flue gas temperature distribution in a 220 t/h boiler system

The effect of boiler capacity on the relative heat absorption

of economizer, water-cooled wall, superheater, reheater and

novel furnace (FEGT = 800 °C, if applied) is shown in Fig. 6.

As the capacity increases, the amount of heat required to

generate saturated steam or reach the critical point declines.

However, the variation in the heat absorption distribution of

boiler system is not significant when the boiler capacity is more

than 1000 t/h. If the novel design is applied to the boilers, the

furnace will achieve about 80% total heat absorption, regardless

of boiler capacity. To meet the requirement, a considerable part

of heat absorbed by superheater, reheater and economizer

should be transferred in the furnace instead. Thus, more radiant

superheaters or reheaters and even radiant economizers are used

in the thermal system arrangement of the novel boiler besides

the volume-enlarged furnace.

Fig. 6 Relative heat absorption for selected boilers with various

capacities

Heat transfer Heat transfer in furnace is closely related to both the

effective thickness of radiant layer (s) and the radiant

attenuation factor (k). Fig. 7(a) illustrates the effect of furnace

volume and shape (H/d) on s, where H is the furnace height and

d is the averaged equivalent diameter of the furnace cross

section. As the furnace volume increases, s increases. The

reason is that the increase in volume is faster than that in

enclosure wall area when a furnace is enlarged. For a given

furnace volume, an increase in H/d leads to a fall in s, but the

variation is small. This means the shape of furnace has a very

weak effect on the furnace emissivity. In addition, if the platens

are arranged in a furnace, s will be decreased due to the

significant rise in furnace enclosure wall area. It can be seen

from Fig. 7(b) that as the FEGT is reduced, k is raised. When an

FEGT is given, a decrease in k is found with an increase in s.

therefore, k may not have a marked variation in a volume-

enlarged furnace, owing to both a fall in the FEGT and a rise in

s.


(a)

(b)

Fig. 7 Effect of furnace configuration and FEGT on radiation

parameters

Fig. 8 shows the radiation heat transfer characteristic in the

volume-enlarged furnace. These calculations are carried out in a

1000 t/h subcritical space-fired boiler and the variation of slag

or ash buildup on heating surface is elided. As d increases, both

the furnace volume and enclosure wall area (A) are raised.

Thus, more heat of combustion products is removed in the

furnace and the FEGT is reduced, as shown in Fig. 8(a).

Further, Fig. 8(b) indicates the furnace emissivity (afur)

increases with d. As previously discussed, the variation in k is

small, so the rise in afur is mainly caused by the increase in s.

Additionally, it is found that the converted coefficient of

radiation heat transfer (αrad) falls as d increases. Notably, when

d is raised, though the increase in afur is beneficial for enhancing

radiation heat transfer in furnace, αrad still has a significant

decrease. This results from the reduction in the average

temperature of the flame that affects the intensity of radiation

heat transfer strongly.

(a)

(b)

Fig. 8 Radiation heat transfer characteristic in the volume-enlarged

furnace (H/d = 3)

Heat transfer principle in convection pass is unchanged

between a novel boiler and a conventional one. However, such a

low FEGT in the novel boiler can lead to a remarkable decrease

in the temperature difference of convection pass, which

increases the steel consumption and makes it difficult to

maintain the rated steam temperature. Thus, an optimized

thermal system is required in the novel boiler, considering the

heat transfer of both furnace and convection pass.

Ignition and combustion Ignition and combustion are very important factors in the

boiler furnace design. A volume-enlarged furnace is used to

reduce the FEGT in the novel design, which can adversely

affect ignition and combustion. Considering a space-fired

boiler, as the FEGT is reduced, both the average temperature of

the flame (Tfla) and the heat release rate per unit furnace cross-

sectional area (qF) decline (see Fig. 9). The lower qF and Tfla

are, the smaller the intensity of mix and combustion will be.

Eventually, the boiler will not be able to maintain the normal

operation if the FEGT is reduced to such a low level. For a

grate-fired boiler, the volume-enlarged furnace has a small

effect on the ignition and combustion due to the arrangement of

furnace arches, but the low boiler efficiency immensely limits

its utilization in power generation. Similarly, when the novel


design is applied in a circulation fluidized bed boiler, the

difficulty in ignition and combustion still exists. Moreover, the

bed agglomeration and defluidization are also exacerbated

during the combustion of high-sodium coals. In a cyclone-fired

boiler, combustion and heat transfer in furnace are carried out

in divided chambers. Combustion mostly occurs inside the

cyclone barrel while radiation heat transfer mostly occurs inside

the main furnace. When the main furnace is enlarged, ignition

and combustion within the cyclone barrel are almost unaffected.

Thus, the cyclone-fired boiler is considered to be the most

appropriate application for the novel design. Because of high

combustion temperatures, cyclone furnaces have high NOx

emissions. Considering an increasing public demand for

reducing pollution, the cyclone units became less popular.

However, with the development of air staging technology,

considerable reductions in NOx levels have been demonstrated

in some cyclone boilers using staged combustion.

Fig. 9 Effect of FEGT on ignition and combustion in a space-fired

boiler

Steel consumption A comparison of steel consumption between a novel boiler

and a conventional one is demonstrated in Fig. 10. This

evaluation is conducted in a 220 t/h boiler, using the Zhundong

coal as designed coal. Physical arrangement parameters of the

boiler components are listed in Table 4. In the novel boiler, the

secondary superheater is arranged in the furnace as a platen

radiant superheater and the FEGT is maintained at a level of

800 °C. The steel consumption is measured by the heating

surface area. For the welded membrane water-cooled wall used

here, the area is calculated in terms of the single-side exposed

surface size of the enclosed furnace volume. For the sake of

comparison, the double area of water-cooled wall is counted

when the whole boiler is considered. It can be seen that the steel

consumption of water-cooled wall and secondary superheater is

raised in the novel boiler. Conversely, other main components

of the novel boiler cost less. According to the results of the

whole boiler, the expense of the novel boiler is approximately

increased by 10% relative to the conventional one.

Fig. 10 Comparison of steel consumption between a novel boiler and a

conventional one [1- Water-cooled wall, 2- Secondary superheater, 3-

High temperature superheater, 4- Low temperature superheater, 5-

Turning cavity, 6- Economizers, 7- Whole boiler (not including air

heaters)]

Table 4 Physical arrangement of the components in a 220 t/h novel (conventional) boiler

Furnace Other Components

Parameter Units Water-cooled

wall

Parameter Units Secondary

superheater

High

temperature

superheater

Low

temperature

superheater

Economizers

Width m 11 (7.6) Tube OD mm 42 (42) 42 (42) 38 (38) 32 (32)

Depth m 11 (7.6) Sidespacing mm 1000 (582) 100 (100) 100 (100) 60 (60)

Height m 31 (21.3) Backspacing mm 59 (59) 89 (89) 80 (80) 88 (88)

Volume m3 3518 (1116) Heating surface m2 790 (613) 920 (1060) 780 (815) 1680 (2038)

Surface m2 1154 (691) Free flow area m2 - (57) 35 (26) 30 (21) 27 (19)

Improved application with flue gas recirculation An alternate method to control FEGT is gas tempering by

flue gas recirculation. The cool recirculated flue gas is directly

mixed with hot flue gas near the furnace exit in this method.

The FEGT can be reduced with less furnace surface. However,

to achieve an FEGT below 800 °C, a recirculation ratio of 50%

is approximately required when the recirculated flue gas is from

boiler exhaust. As a result, the capital cost of boiler rises due to

a marked reduction in the average thermal head for heat transfer

of the whole boiler, and the expense on fan maintenance and


power requirement also reaches a high level. If the flue gas

recirculation is applied in the novel design, an improved

application is developed. The improved application provides a

two-stage control of FEGT. The first stage reduces the FEGT to

a relative lower level (such as 900 °C) by arranging more

radiation heating surface. Correspondingly, the second stage

reduces the FEGT to the desired level by flue gas recirculation.

Thus, the furnace surface or recirculation ratio can be

decreased, as compared with the cases which only adopt the

first stage or the second stage, respectively. In addition,

performances of the boiler will be improved significantly. The

FEGT can be maintained at a stable level by adjusting the

amount of recirculated flue gas when the operating conditions

change. On the other hand, the increased flue gas weight can

improve the level of convection heat transfer capability of the

boiler. Further, when the improved application is used in

practice, the optimization of thermal system arrangement and

operating parameters is of great significance for cost reduction.

CONCLUSION In this study, a novel boiler design was proposed to

prevent the severe fouling of high temperature convection pass

in the boilers firing high-sodium coals. The design was explored

and evaluated in different kinds of boilers with various

capacities by considering thermal system arrangement, heat

transfer, ignition and combustion, and steel consumption. The

following conclusions can be drawn:

(1) As the FEGT declines, the heat absorption of furnace

increases while the convection pass decreases. To achieve an

FEGT below 800 °C, more radiation heating surface should be

used in the thermal system arrangement of the novel boiler

besides the volume-enlarged furnace.

(2) When the furnace volume is enlarged, the effective

thickness of radiant layer increases while the radiant attenuation

factor varies slightly, which resulting in the increase in the

furnace emissivity. Moreover, the converted coefficient of

radiation heat transfer reduces significantly due to the reduction

in the average temperature of the flame.

(3) In a space-fired boiler, the volume-enlarged furnace

can adversely affect ignition and combustion. The cyclone-fired

boiler is considered to be the most appropriate application for

the novel design, for its combustion and heat transfer in furnace

are carried out in divided chambers.

(4) The steel consumption of the novel boiler is

approximately increased by 10% relative to the conventional

one.

(5) An improved application with flue gas recirculation is

probed owing to its advantages of controlling FEGT and

maintaining the level of convection heat transfer capability of

the boiler.

NOMENCLATURE afur furnace emissivity [-]

A furnace enclosure wall area [m2]

Ac area of convection heating surface [m2]

d averaged equivalent diameter of the furnace cross

section [m]

H furnace height [m]

k radiant attenuation factor [1/(MPa·m)]

qF heat release rate per unit furnace cross-sectional area

[MW/m2]

s effective thickness of radiant layer [m]

Tfla average temperature of the flame [K]

Twf average temperature of the working fluid [K]

U heat transfer coefficient [W/(m2·K)]

Greek symbols

αrad converted coefficient of radiation heat transfer

[W/(m2·K)]

Δt temperature difference [°C]

σ0 Stefan-Boltzmann constant [W/(m2·K4)]

ψ thermal effectiveness factor [-]

Subscripts

0 constant

c convection

F face

fla flame

fur furnace

rad radiation

wf working fluid

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A Novel Boiler Design for High-Sodium Coal in Power Generationb-dig.iie.org.mx/BibDig2/P17-0164/data/pdfs/trk-10/POWER2015-49167.pdf1 A NOVEL BOILER DESIGN FOR HIGH. SODIUM COAL IN

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