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Thermal Power Plant Part

1. Thermodynamic Basis

1.1 Essence and statements of second law of thermodynamics

Experience tells us that many natural processes are directional. All the spontaneous processes in

the natural world are directional.

Clausius statement (1850):

Heat can never pass from a colder to a warmer body without some other change, connected

therewith, occurring at the same time.

Kelvin statement (1851):

It is impossible, by means of inanimate material agency, to derive mechanical effect from any

portion of matter by cooling it below the temperature of the coldest of the surrounding objects.

("Clausius statement" is from the perspective of heat transfer, while "Kelvin statement" is from

the perspective of heat-work conversion.)

1.2 Cycle and its thermal efficiency

When a working medium starts from a certain state, and then passes a series of thermodynamic

processes, and finally returns to the initial state, we call the integration of these thermodynamic

processes experienced by the working medium as "thermodynamic cycle" or "cycle" for short.

Positive cycle: The effect of cycle can make the thermal energy continuously be converted into

mechanical energy under certain conditions. (clockwise direction)

1.3 Carnot cycle

Carnot cycle is an ideal reversible cycle of reversible heat engine with theoretical significance,

which was proposed by a French young engineer named Carnot in 1824. It consists of four reversible

processes, i.e. one reversible heat engine works in two heat source reservoirs with constant

temperature.

1

2

1

1q

q

q

wnett

2

d—a Reversible adiabatic compression

a—b Reversible isothermal heat absorption (Q1) at T1

b—c Reversible adiabatic expansion

c—d Reversible isothermal heat release (Q2) at T2

1.4 Composition of Rankine cycle

Thermodynamic process:

1→2 Adiabatic expansion in steam turbine Wt = h1-h2

2→3 Constant-pressure heat release in condenser ︱q2︱= h2-h3

3→4 Adiabatic compression of water in feedwater pump ︱Wp︱= h4-h3

4→5→6→1 Constant-pressure heat absorption of water in boiler q1 = h1-h4

1.5 Thermal efficiency of Rankine cycle

Rankine cycle

Work exerted by steam turbine

Waste work by water pump Wp= h4 -h3

Thermal efficiency of Rankine cycle

Neglected waste work by water pump

2. Principle Thermodynamic System of Thermal Power Plant

Regenerative cycle: Part of steam is extracted from certain intermediate stages of the steam

1

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turbine to heat the boiler feedwater to increase the temperature of feedwater. This process is called

"regenerative feedwater heating". The thermodynamic cycle of regenerative feedwater heating is

called "regenerative cycle". As the regenerative extraction steam exerts some work in the steam

turbine and all its heat release is absorbed by water, no cold source loss exists in this part of steam

extraction. In addition, since the condensing amount (Dc) is decreased and the cold source loss of the

whole unit can also be reduced, the thermal efficiency of cycle can be improved. This is the purpose

of the application of regenerative feedwater heating.

Intermediate reheating cycle: The steam in the high pressure cylinder of steam turbine, which

has exerted some work, is extracted to the reheater for temperature rise, and then returned to the low

pressure cylinder of the steam turbine for continuous work. This process is called "intermediate

steam reheating" and the cycle of the device is called "reheating cycle". Purpose of intermediate

steam reheating: The initial purpose of applying intermediate reheating is to reduce the final

humidity of expansion while the initial pressure of steam ( 0P) is increased, so that safe operation of

steam turbine can be guaranteed and the thermal efficiency can be improved.

Loss and make-up of steam and water: During normal operation of a power plant, loss of steam

and condensate is always found at the connections of the pipelines, accessories and equipment,

which affects the safe and economical operation of the power plant. The loss of steam and water also

causes noise, dirt and thermal pollutions in the main powerhouse, as well as the problem of the

make-up of working medium. For a high-parameter condensing power plant, all make-up water is led

into the condenser. Thus, the make-up water can realize vacuum deaeration in condenser to reduce

the corrosion effect of oxygen on the low pressure heater and its tubing. In addition, the multi-stage

heating of low pressure regenerative extraction steam of steam turbine can be carried out to achieve

high heat economy and small temperature difference between the make-up water and condensate

mixture. However, the disadvantage is that the regulation of the amount of make-up water is affected

by the water level in hot well and the water level in feedwater tank. Accordingly, the regulation is

rather complicated.

Blowdown and utilization system: The purpose of this system is to keep the quality index of

drum boiler water in the allowable range, so as to further guarantee that the quality of steam

evaporated from the boiler can meet the requirement. According to the provisions of DL 5000-94,

the blowdown rate of drum boiler shall not be less than 0.3% of the boiler maximum continuous

rating and not more than the following values: 1% for a condensing power plant which uses

chemically demineralized water as make-up water; 2% for a thermal power plant which uses

chemically demineralized water or distilled water as make-up water; and 5% for a thermal power

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plant which uses chemically softened water as make-up water.

The blowdown water is released from a drum position where there is the maximum

concentration, depressurized by two in-series valves and throttling device (throttling orifice or

reducing valve) and then sent into the flash tank. Under the pressure of flash tank, part of water is

evaporated into steam, and then the steam is sent into a deaerator with relevant pressure to allow the

recycled working medium to utilize the steam heat. With large salt concentration and its temperature

more than 100℃, the unevaporated blowdown water in the flash tank is generally arranged to pass

through the blowdown water cooler to heat the make-up water before being discharged into the

underground drainage ditch.

3. Regenerative Extraction Steam System and Its Equipment

Regenerative feedwater heating is applied to all steam turbine units of a modern thermal plant.

Regenerative system is the core of thermodynamic system of the whole plant. The performance

optimization of regenerative extraction steam system plays a significant role in the improvement of

thermal cycle efficiency of the entire steam turbine units.

3.1 Overview of regenerative extraction steam system

Regenerative extraction steam system is a system in which the boiler feedwater (main

condensate) is heated by extraction steam of the steam turbine. In order to form a regenerative

extraction steam system with good performance, careful analysis and selection should be made on

the number of stages and parameters (temperature, pressure and flow) of steam extraction of the

regenerative extraction steam system, the form and performance of the heater (heat exchanger), the

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guiding of extraction steam condensate, as well as the performance of the tubes and valves within the

system.

3.2 Regenerative heater

It can be seen from the figure that the steam comes from the Stage 1, 2, 3, 4, 5, 6, 7 and 8 of

steam extraction, and they are led into the high pressure heaters (No. 1, No. 2 and No. 3), the

deaerator and the lower pressure heaters (No. 5, No. 6, No. 7 and No. 8) respectively. The main

condensate enters the deaerator after going through the low pressure heaters (No. 8, No. 7, No. 6 and

No. 5), then the feedwater (i.e. the main condensate) is transported to the high pressure heaters (No.

3, No. 2 and No. 1) by the feedwater pump, and finally sent into the boiler. A large bypass is

provided at (the water side of) the three high pressure heaters, another large bypass is provided at

(the water side of) the low pressure heaters (No. 8 and No. 7), and an independent bypass (for

shutdown) is provided for the low pressure heaters (No. 6 and No. 5).

The boiler feedwater temperature will rise along with the increase in the number of stages for

regenerative steam extraction. On one hand, this can save fuel and correspondingly reduce the

investment in fuel supply system, pulverizing system and flue gas and dust removal system, but it

will enlarge the flue gas loss of the boiler and increase the investment in tail heating surface of the

boiler. On the other hand, due to the increase of steam consumption by the steam turbine, the flow in

the high pressure cylinder will increase, too; and the reduction of exhaust steam in low pressure

cylinder will lead to the increase of investment in the boiler, steam turbine body, main steam system,

feedwater system and regenerative system and the decrease of investment in the condenser and

circulating cooling water systems. With comprehensive consideration, high boiler feedwater

temperature generally leads to the increase of total investment. Therefore, the regenerative feedwater

temperature should not be too high, and should generally be lower than the optimum feedwater

temperature in thermodynamics.

With the same number of steam extraction stages, steam extraction parameters show obvious

impact on the thermal cycle efficiency of the system. Arrangement of steam extraction parameters:

Less steam at high-grade (high enthalpy and low entropy) sections should be extracted. The steam at

low-grade (low enthalpy and high entropy) sections should meet the performance requirement of

heater in the regenerative extraction steam system as much as possible. This can be summarized as

that the difference between steam and feedwater (i.e. main condensate) should be diminished as far

as possible, which means the heat transfer temperature difference should be diminished as far as

possible. To get a good heat transfer effect, two methods are mainly used currently.

One method is to use a mixing heater: inside the heater, directly mix the steam extracted from

the steam turbine and the feedwater (i.e. main condensate) coming into the heater. The steam

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condenses into water and its latent heat of vaporization is released into the feedwater to make them

become a unity. As both the pressure and the temperature are the same, the heat transfer efficiency is

very high. However, every heater using this method must be equipped with a water pump to regulate

the feedwater pressure to make it consistent with the steam extraction pressure of corresponding

section. In consideration of the high failure rate and standby requirement of the water pump, the

system will be quite complex. Therefore, few power plants use mixing heaters nowadays. At present,

this method can be applied to the deaerator which is used as the first-stage heater. Some

manufacturers (e.g. manufacturers in Russia) adopt mixing heaters as the last two low pressure

heaters, which is a useful attempt.

Another method also uses surface heater, however, in consideration of the features of steam and

water, necessary measures should be taken from the structural perspective to improve the heating

effect of the heater as far as possible. Generally speaking, certain degree of superheat can always be

found in the steam extracted from the high and intermediate pressure cylinders of the steam turbine.

A superheated steam cooling section ("superheat section" for short) can be provided at the steam

inlet of the heater. Since the temperature of condensate (drain water), which has finished heat

transfer through the heater, is higher than the temperature of main condensate entering the heater, a

drain cooling section may be provided. In this way, it is able to reduce the terminal (temperature)

difference between the inlet and outlet of the heater to the greatest extent by fully utilizing the steam

extraction energy, which is helpful to improve the efficiency of the whole regenerative system. See

the figure below.

In the superheated steam cooling section, the superheated steam is cooled and its heat is

absorbed by the main condensate whose temperature rises. The temperature of superheated steam

drops down, close to or equal to its saturation temperature under relevant pressure. It is important to

note that the application of superheat section is conditional. Relevant conditions are: in case of full

load operation of the unit, the degree of superheat of steam should be ≥ 83°C, the steam extraction

Main condensate

at outlet

Superheated steam

cooling section

Superheated steam

Body part (condensing

section) Drain cooling section

Main condensate

at inlet

Saturated

steam

Saturated drain

water

Fig. 7-5 Schematic Diagram of Working

Process of Regenerative Heater

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pressure ≥ 1.034 MPa, flow resistance ≤ 0.034 MPa, terminal temperature difference of heater in the

range of 0 ~ -1.7 °C, and residual degree of superheat of steam at outlet of superheat section ≥

30 °C.

In the drain cooling section, as the drain water temperature is higher than the inlet water

temperature, the drain water temperature drops and the temperature of main condensate rises due to

heat absorption during the heat transfer process. The drop of drain water temperature leads to the

increase of steam extraction capacity of adjacent low pressure heater, while the rise of inlet water

temperature leads to the reduction of steam extraction capacity of the current stage. As a result, less

extraction of high-grade steam and more extraction of low-grade steam should be arranged, because

the efficiency of regenerative system can be improved in this way.

For the provision of a drain cooling section, there are no restricted conditions like those for the

superheated steam cooling section. For this reason, most heaters of 600 MW units are provided with

drain cooling sections at present.

The provision of drain cooling section for a heater can not only improve the economical

efficiency, but also be good for safe operation of the system. This is because that as a kind of

saturated water, the drain water must pass through the throttling device for pressure reduction when

it flows to the next stage of heater with lower pressure, and that once the saturated water passes

through the throttling device for pressure reduction, it will generate steam and form two-phase flow,

which will have impact, vibration and other negative effects on the pipeline and the next stage of

heater. Since the cooled drain water is unsaturated water, the possibility of two-phase flow during the

throttling process can be greatly decreased.

Furthermore, for a high pressure heater, its drain water always flows to the deaerator by gravity

in the end. The heat carried by uncooled drain water will dramatically reduce the steam extraction

capacity of the deaerator, even cause spontaneous boiling of the deaerator. With the provision of a

drain cooling section, the drain water temperature can be reduced, which is helpful to guarantee the

steam extraction capacity of the deaerator and able to rule out the possibility of its spontaneous

boiling.

Other necessary conditions to guarantee the efficiency of regenerative system include provision

of sufficient heat exchange area and selection of materials with good heat-conducting property for a

heater. Because these conditions can make the terminal temperature difference of the heater be as

less as possible, so that the efficiency of the system could be higher.

At present, surface regenerative heaters are applied in most regenerative systems of steam

turbines. Structurally, surface regenerative heaters can be divided into two types, i.e. header –

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coil-tube type and tube-in-sheet – U-shaped tube (or straight tube) type. The most-frequently used

type is tube-in-sheet – U-shaped tube regenerative heater. This type is featured by compact structure,

material-saving property, small flow resistance and high heat exchange efficiency. In terms of

arrangement, regenerative heaters can be divided into vertical and horizontal types. For a vertical

heater, it covers a small area and it is easy to carry out maintenance work. However, its heat transfer

effect is poor. During the design of roof truss height for the steam turbine house, consideration

should be given to lifting-out of tube bundle and crossing of operating unit if necessary. In a

horizontal heater, due to the thin condensate film formed on the surface of heat exchange tube during

the flow of steam and liquid, the heat transfer effect is good, which is convenient to arrange a drain

cooling section, but its installation and maintenance is not as convenient as that of a vertical heater

and it covers a larger area too. At present, horizontal regenerative heaters are applied in most

regenerative systems of steam turbine units in China.

3.2.1. Structure and performance of high pressure heater

Fig. 7-16 is the outline of a high pressure heater and Fig. 7-17 is the structure diagram of the

same.

1 – relief port of water chamber; 2 – feedwater outlet; 3 – chemical cleaning connection; 4 –

manhole; 5 – feedwater inlet; 6 – water-side relief port (water-side safety valve seat at the other side);

7 – water chamber seat frame; 8 – water chamber outlet; 9 – pup joint drain port; 10 – operation

steam exhaust port; 11 – water gauge connection; 12 – high-low water level alarm connection; 13 –

regulated drain valve signal port; 14 – seat frame; 15 – temperature connection; 16 – steam-side

steam exhaust port; 17 – rolling support; 18 – emergency drain valve; 19 – drain inlet of high

pressure heater of previous stage; 20 – start-up steam exhaust port and chemical cleaning connection;

21 steam-side safety valve seat; 22 – pressure gauge connection; 23 – site cutting centerline; 24 –

heated steam inlet; 25 – start-up steam exhaust port

Fig. 7-16 Outline of High Pressure Heater

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1 – U-shaped tube; 2 – tie rod and spacer; 3 – end plate of drain cooling section; 4 – inlet of drain

cooling section; 5 – diaphragm of drain cooling section; 6 – feedwater inlet; 7 – manhole seal plate; 8

– independent split diaphragm; 9 – feedwater outlet; 10 – tube plate; 11 – insulation board of steam

cooling section; 12 –steam inlet; 13 – impingement baffle; 14 – tube bundle protection ring; 15 –

diaphragm of steam cooling section; 16 – diaphragm; 17 – drain water inlet; 18 – impingement baffle;

19 – drain water outlet

Fig. 7-17 Structure Diagram of High Pressure Heater

It can be seen from Fig. 7-17 that the regenerative heater consists of the shell, the tube plate, the

tube bundle, the diaphragm and other main components.

The heater shell is made of rolled steel plate and in all-welded structure. For easy pulling out of

the shell during its internal inspection, on-site cutting line is indicated on the shell. A stainless steel

protection ring is lined beneath the cutting line to prevent any damage to tube bundle during cutting.

A rolling support is provided in the middle of the shell for pulling out the shell at the time of

maintenance. The supporting point of the heater is at the shell position corresponding to the tube

plate and the rolling support is close to the shell tail. When the shell expands with heat, the heater

shell can roll freely in axial direction.

The water chamber of the heater is at right side of the shell (as shown in Fig. 7-17). It is in

hemispheric structure with small openings. A split diaphragm is provided to separate the incoming

and outgoing water. The split diaphragm is welded onto the tube plate and the end of split diaphragm

close to the outlet side is welded to the inner sleeve of feedwater tube, so that the sharp stress at the

connection between tube and shell can be avoided. Above the water chamber, there are also exhaust

adapter, safety valve seat and chemical cleaning connection.

For a high pressure heater, its tube bundle has a small wall thickness, but its tube plate is very

thick. The connection method of welding and explosive expansion is used to reliably connect the two

and guarantee no leakage under high temperature, high pressure and changes of operating conditions.

Namely, 5 mm of surfacing welding should be carried out at the position where the tube extends the

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tube plate before the application of fillet welding by means of all-directional automatic argon arc

welding. Tube expanding is carried out with fully explosive expansion method to eliminate the gap

between the tube and tube plate. In this way, it is able to prevent leakage and avoid intensifying of

internal corrosion of the gap, as well as reduce vibration during operation. In addition, the heat

conductivity between the tube and tube plate can also be improved and the temperature of the tube

and tube plate can be well-distributed more rapidly. As the heater tube of the unit is made of carbon

steel, prior to explosive expansion, the tube end at the water inlet side should wear a stainless steel

sleeve, which can expand tightly to the inner wall of the tube during explosive tube expanding.

The superheated steam cooling section is located at the downstream feedwater outlet end. It is

composed of all tube sections with given length at feedwater outlet end which is enclosed by

cladding. Superheated steam enters this section via the sleeve. The purpose of the sleeve is to

separate high-temperature steam from the root of inlet stub tube, the shell and the tube plate (to avoid

generating excessive thermal stress). The cladding of superheated section can expand freely all

around with the sleeve as its center. Diaphragms with appropriate form are provided in this section to

make the steam uniformly pass the tube with a given flow rate to get good heat transfer effect. A

stainless steel impingement baffle is provided below the steam inlet stub tube to prevent direct

impact on the tube bundle by steam. The degree of superheat should be designed as 30°C for the

superheated steam leaving this section.

Next, the steam from superheated section goes into the condensing section. In the condensing

section, feedwater is heated mainly by the latent heat of vaporization during steam condensing. The

provision of a group of diaphragms can make the steam uniformly distributed in length direction of

the heater. These diaphragms reserve a certain steam passage at the upper part of the heater to allow

the steam flow uniformly from top to bottom and then condense gradually, so that the steam can

change from vapor phase to liquid phase (convective heat transfer with phase change). At this

moment, this group of diaphragms mainly shows its supporting and anti-vibration functions. (For

design of the heater, a vibration analysis should be carried out for the whole tube system to prevent

the occurrence of vibration under all load conditions.)

3.2.2. Structure and performance of low pressure heater

Fig. 7-18 is the structure diagram of a low pressure heater.

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1 – U-shaped tube; 2 – tie rod and spacer; 3 – steam inlet; 4 – Y-impingement baffle; 5 – protection

panel; 6 – feedwater outlet; 7 – feedwater inlet; 8 – drain water outlet; 9 – diaphragm of drain

cooling section; 10 – sealing element of drain cooling section; 11 – optional bypass of drain cooling

section; 12 – tube support plate; 13 – heater support; 14 – water level

Fig. 7-18 Structure Diagram of Low Pressure Heater

Compared with that of a high pressure heater, the structure of a low pressure heater is largely

identical but with minor differences. Since no superheated steam cooling section is provided for low

pressure heater, each low pressure heater only consists of the condensing section and the drain

cooling section. Due to lower pressure, the structure of a lower pressure heater is simpler than that of

a high pressure heater, and the thicknesses of its shell and tube plate are smaller too. The water

chamber of low pressure heater can be either hemispherical or cylindrical. The tubes of low heater

pressure are made of stainless steel. Because the air content (mainly referring to the oxygen content)

is high in the main condensate before the deaerator and air may further come in the vacuum part of

the equipment and tube, corrosion-resistant material is required. Since corrosion-resistant stainless

steel is applied to the tube bundle, no other air exhausting device is provided for the heater and only

steam exhaust port is provided on the cylinder.

Moreover, without a superheated steam cooling section, the steam inlet is arranged in the

middle part of the heater.

3.3 Drain system

In terms of the guiding of heater drain water (condensate), most regenerative systems of 600

MW steam turbine applies the straight-flow cascade drainage method. The purpose of this method is

to simplify the system and mainly reduce the influence on heat economy through the drain cooling

section.

The gravity-flow cascade drainage method of heater means that under normal operation

conditions, the heater drain water is led from the heater with higher pressure into the next stage of

heater with lower pressure and finally discharged into the deaerator (high pressure heater) or the

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condenser (low pressure heater) to heat feedwater or condensate, so as to improve the thermal

efficiency of the unit. In case of water level high-high of heater, the heater drain water is directly

guided into the condenser to prevent water from entering the steam turbine. The purpose of blow-off

system is to discharge the non-condensed gas released during the cooling process of extracted steam

and drain water into the deaerator (high pressure heater), the condenser (low pressure heater) or the

atmosphere (deaerator).

For No. 1, No. 2 and No. 3 high pressure heaters, the normal drain water is led from the drain

cooling section of relevant high pressure heater into the next stage of high pressure heater (deaerator)

through a pneumatic normal water level control valve, and the emergency drain water is led from the

normal drain water pipe (upstream of the pneumatic water level control valve) to the condenser

through a pneumatic emergency drain control valve. The emergency drain pipe is provided to

prevent water from entering the steam turbine. In case of high water level of the high pressure heater,

the emergency drain control valve is forced to open. Both the normal drain control valve and

emergency drain control valve are provided with front and back drain valves for easy maintenance

and repair.

The low pressure heater also applies the gravity-flow cascade drainage method (i.e. drainage in

the sequence of No. 5, No. 6, No. 7 and No. 8 low pressure heaters), and the drain water finally flows

into the condenser. Each heater is provided with emergency drain ports to directly guide the drain

water into the condenser under emergency conditions.

4. Feedwater Deoxidation System and Its Equipment

During the operation of a thermal power plant, the dissolved oxygen in feedwater comes from

the air carried by make-up water or the air leaking into the system through the untight positions of

the vacuum equipment and pipe accessories. Air dissolving in water brings about some adverse

effects. On the one hand, the oxygen in air may have oxidation corrosion effect on the equipment and

dissolved oxygen in water may easily cause corrosion perforation which results in leakage and

bursting of pipeline. On the other hand, due to small heat conductivity coefficient of air, dissolved

oxygen in water may led to heat transfer deterioration which affects heat economy. For these reasons,

air in feedwater must be removed.

4.1 Working principle of deaerator

Deoxidation can be achieved by chemical deoxidation and physical (thermal) deoxidation

methods. Chemical deoxidation can remove oxygen thoroughly, but it can remove only one kind of

gas, requires high dosing cost and generates some salts, so that thermal power plants are less prone to

use this method. By the meanings of heating, thermal deoxidation can remove most kinds of gases in

water.

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Principle of thermal deoxidation: The dissolvability of a gas in water is in direct proportion to

the pressure component of the gas on water surface and the proportion factor is related to

temperature. The higher the temperature is, the smaller the proportion factor will be. The total

pressure of gas is the sum of all pressure components. When the water surface is full of steam, the

pressure component of steam on water surface is close to the total pressure of the gas mixture in

steam and the pressure component of other gases is close to zero, so that most of other gases

dissolved in water can be removed from water surface. Along with the heating of feedwater under

constant pressure in the deaerator, the water evaporation process keeps intensified and the pressure

component of steam on water surface gradually increases, but the pressure component of other gases

dissolved in water decreases. When the feedwater is heated to the saturation temperature under the

pressure of deaerator, the water will start to boil and the pressure component of steam will be close

to the total pressure on water surface and while the pressure component of other gases almost

approach to zero. Accordingly, the gases dissolved in water can escape from water. A deaerator can

not only remove oxygen, but also remove other gases.

4.2 Structure and System of Deaerator

Deaerator can be divided into vacuum, atmospheric and high pressure types based on its

working pressure, divided into spray-tray and spray-packing types based on its internal structure, and

divided into vertical and horizontal types based on the layout of deoxidation part.

The structure design of a deaerator must meet the following requirements:

(1) The feedwater should be heated to saturation temperature under the working pressure of

deaerator;

(2) The contact surface between water and steam should be large enough;

(3) The gases escape from water should be blown off rapidly;

(4) Between the heated steam and the water to be deoxidized, there should be a counterflow

path with enough length, i.e. a sufficiently large heat transfer area and a sufficiently long time for

heat and mass transfer should be provided.

High pressure deaerators are often used in the units with high parameters. A high pressure

deaerator can be sued as a stage of mixing heater and its heat transfer effect is very good. In addition,

when the high pressure heater is operated through a bypass, it is able to make boiler feedwater have a

relatively high temperature and easily prevent spontaneous boiling of the deaerator. Increasing

pressure means to increase the saturation temperature of water to reduce the dissolvability of gases in

water, which is more helpful to improve the deoxidation effect.