atw Vol.

atw Vol. 63 (2018) | Issue 4 ı April

evaporator are elevated are that the

efficiency of power plant is increased.

When the temperature of main evaporator

and reheating steam increases

by 10 °C, the efficiency increases

by 0.5 %; and pressure increases by

10 kg/cm 2 , the efficiency increases by

about 0.2 %. Domestically, in 1990’s,

500 MW-grade standard coal thermal

power plant was designed and built,

and its operation condition was pressure

246 kg/cm 2 and temperature

538 °C.

In the case of Dangjin Thermal

Power No. 9, No. 10 and Samcheok

Thermal Power No. 1, No .2 that have

been being built, the pressure of

250 kg/cm 2 , temperature of 600 °C

were accomplished [4].

SCO 2 cycle is the power generation

technology of the Gas Brayton Cycle

method where pressurized carbon

dioxide is heated by the pressure

greater than critical condition to high

temperature and turbine is driven.

Presently, CO 2 power generation cycle

can be applied to most heat sources

used, and also it can be used for large

power plant, small scale distribution

power supply, or power supply for

marine plant.

Super critical condition means the

conditions for temperature and pressure

greater than critical point in the

general material state where liquid-gas

phase change occurs, and the

temperature and pressure at the lower

pressure part is greater than 32 °C, 74

atm, and all parts of cycle are maintained

over critical condition. While

operation is carried out at high

pressure, volumetric flow decreases,

so the size of overall heat conversion

cycle can be decreased; accordingly,

construction period and production

unit price can be lowered to secure

high economic feasibility.

Besides, compared to water vapor,

the compatibility with existing material

is excellent, so it can be supplied

to turbine at the temperature higher

than evaporator cycle. From this, the

increase of additional power generation

efficiency can be possible [5].

2.3 Heat Conversion Model

Design

IHX loop of VHTR that is studied in

the present study is the system where

the high temperature heat generated

in the reactor by connecting hydrogen

generation equipment and power

generation equipment in series can be

supplied in the same manner.

IHX loop nuclear reactor shown in

Figure 1 provides 350 MWt heat output,

and the heat generated from

| | Fig. 1.

IHX Loop Modelling.

nuclear fission is supplied to helium

fluid. For heat transfer to produce

hydrogen, heat exchanger, steam generator

for the power generation via

USC cycle, and in the power generation

via SCO 2 , one heat exchanger is

provided. In order to utilize the result

of the study regarding the existing

VHTR, the major principle and

variable if heat conversion model

were set as follows. Temperature and

pressure at No. 1, 2, 3, 4, 10 were

presumed by reference literature [8].

Temperature and pressure of ultrasuper

critical cycle No. 5, 6 and SCO 2

cycle, No. 8 were assumed by using

reference literature [9]. The model to

be explained below was defined as

reference model, and then the present

authors will plan to develop a model

that considers a variety of heat

efficiency improvement method. In

the present study, in the concept

similar to general Rankine cycle’s

reheating cycle, bypass mode was

proposed.

To begin with, the reference model

is as follows. After 910 °C helium fluid

discharging from VHTR carries out

heat exchange with heat exchanger 1,

hydrogen is produced by receiving

heat from high temperature helium

fluid in the heat exchanger 1. 846 °C

helium fluid passing heat exchanger 1

enters into steam generator 2 and go

through heat exchange. The fluid of

this steam generator is ultra-super

critical state water, and produces

power. The temperature of helium

fluid that passes through steam

generator 2 is 614.8 °C, this helium

fluid enters into heat exchanger 3

where heat exchange is carried out.

The fluid of this heat exchanger is

super critical-state carbon dioxide,

and it produces power by the heat

supplied. The temperature of helium

fluid coming out of heat exchanger 3

is 450 °C. The heat output that is produced

in heat exchanger 1 producing

hydrogen is 37.37 MWt. The mass flow

of helium from IHX is m 1 , and the

mass flow of water flowing in heat

exchanger 1 is m 2 , the mass flow of

water flowing in steam generator 2 is

m 3 , and the mass flow of CO 2 flowing

in heat exchanger 3 is m 4 . In this

study, the temperatures and pressures

from No.1 to No.10 in Figure 1 were

assumed, and m 1 and m 2 were calculated

by using the Equation (1), and

m 3 and m 4 were calculated by using

the Equation (2). Besides, considering

the characteristics of general longitudinal

temperature difference of heat

exchanger, the temperature at No. 6

and No. 9 was assumed to decrease by

10 °C compared to the temperature at

No. 4 and No. 7 of the steam generator

inlet.

Major equation or relationship for

heat equilibrium analysis is as follows:

• Equation used for calculating m 1

and m 2

: W = m∆h = m(h out – h in )... (1)

Here,W : Thermal power (MWt)

m : Mass flow (kg/hr)

h : Enthalpy (kJ/kg)

in : Entrance of the equipment

out : Outlet of equipment

• Equation used for calculating m 5

and m 8

∑m in h in = ∑m out h out ... (2)

In the case of hydrogen production, it

was assumed that all heat was

converted to work required, and in

OPERATION AND NEW BUILD 231

Operation and New Build

Heat Balance Analysis for Energy Conversion Systems of VHTR ı SangIL Lee, YeonJae Yoo, Deok Hoon Kye, Gyunyoung Heo, Eojin Jeon and Soyoung Park

atw Vol. 63 (2018) | Issue 4 ı April OPERATION AND NEW BUILD 230 Heat Balance Analysis for Energy Conversion Systems of VHTR SangIL Lee, YeonJae Yoo, Deok Hoon Kye, Gyunyoung Heo, Eojin Jeon and Soyoung Park VHTR (Very High Temperature gas Reactor) which helium is used as a coolant can easily produce heat required in high-temperature thermochemical process, and because of low heat output density, the possibility of core melting is low. Helium has the advantage of safety, and the coolant can become super high temperature, thereby power production as well as hydrogen production application is possible. In this study, provided that VHTR is located in the primary system, the heat conversion system will be discussed in which hydrogen production and power supply are possible. In order to control the ratio between power and hydrogen production, the helium flowing through nuclear reactor is made to pass through heat exchanger for hydrogen production and steam generator or heat exchanger. Power production was made to be composed of ultra-super critical steam cycle (USC) and supercritical CO 2 (SCO 2 ) cycle so that efficient operation condition can be selected. This study proposed the whole heat conversion system model, and carried out thermodynamic feasibility calculation according to major design variable at each point and sensitivity analysis for efficiency optimization. 1 Introduction Recently, an interest on hydrogen as a clean energy source and a fossil fuel substitute has been increasing. From the viewpoint that hydrogen utilizes the energy system which uses the existing fossil fuel without the emission of environmental pollution material, contrary to fossil fuels, hydrogen is emerging as a promising future clean energy. Among hydrogen production methods, high-temperature pyrolysis hydrogen production method using heat chemical process is considered as a proper method for mass hydrogen production. Heat is required much for high-temperature heat chemical process, and lightwater reactor that uses water as coolant does not produce heat required for high-temperature heat chemical process. VHTR (Very High Temperature gas Reactor) which uses helium as coolant can easily produce heat required for high-temperature thermochemical process, so recently the study of the use of high temperature gas for hydrogen production has been the research trend [1, 2]. VHTR has no possibility of core melting due to low heat output density, and it does not use water, so there is no risk of explosion danger due to hydrogen generation in the case of coolant loss accident. Besides, it has the advantage that high-temperature coolant can be made compared to water-cooled reactor, so it has the advantage of power production and process heat supply [3]. Nuclear reactor is in charge of heat supply, and this can be converted variously to be used as the production of hydrogen or power. In this study, by borrowing general name in the atomic power field, VHTR is called as a primary system, the part which hydrogen production and power supply are possible through heat conversion, is defined as the secondary system. Helium flowing in nuclear reactor delivers the heat of the primary system to the secondary system through HX (Heat Exchanger). Helium flowing through the secondary system passes first through heat exchanger where hydrogen production occurs, and secondly and thirdly passes through steam generator and heat exchanger composed of ultrasuper critical cycle (Ultra- supercritical steam cycle: USC) and super critical carbon dioxide (Supercritical CO 2 : SCO 2 ) cycle, respectively, producing process heat and power. In this study, the authors proposed the overall heat conversion system model, and performed the thermodynamic feasibility calculation in accordance with major design variable at each point and sensitivity analysis for efficiency optimization. 2 Research methodology 2.1 Concept and methodology of hydrogen production equipment As a method of hydrogen production which uses water as a raw material by using 900 °C heat, high temperature electrolysis using heat energy simultaneously and the mixed method of using thermochemistry process method and electrolytic method. Recently, research has been focused on Sulfur- Iodine thermochemical cycle where iodide and sulfuric acid were used to break down water. This is because the required equipment can be scaled up and process handling material is only composed of gas and liquid so that continuous operation is possible. Besides, it is advantageous to use nuclear reactor where the safety of load change is demanded as heat source [2]. In the hydrogen production equipment where high temperature heat is used, according to the Reaction 1 below, sulfuric acid (H 2 SO 4 ) can be broken down into water vapor (H 2 O(g)), oxygen (O 2 (g)), and sulfur dioxide (SO 2 (g)). Reaction 1: 2H 2 SO 4 + Heat 2H 2 O + 2SO 2 + O 2 After decomposition, oxygen(O 2 (g)) is removed, and water vapor(H 2 O(g)) and sulfur dioxide (SO 2 (g)) are cooled down, reacting with iodide (I). According to Reaction 2 below, sulfuric acid (H 2 SO 4 ) and hydrogen iodide (HI) are formed. Reaction 2: 4H 2 O + 2SO 2 + 2I 2 2H 2 SO 4 + 4HI + Heat Finally, by using high temperature heat, hydrogen Iodide (HI) can be separated into hydrogen (H 2 ) and iodide (I) according to the reaction 3 below. Reaction 3: 4HI + heat 2I 2 + 2H 2 2.2 The concept and status of USC and S-CO 2 cycle USC power plant means the power plant where vapor pressure is 254 kg/ cm 2 or higher, and main vapor’s or reheated vapor’s temperature is 593 °C or higher. The reasons why pressure and temperature of the Operation and New Build Heat Balance Analysis for Energy Conversion Systems of VHTR ı SangIL Lee, YeonJae Yoo, Deok Hoon Kye, Gyunyoung Heo, Eojin Jeon and Soyoung Park

atw Vol. 63 (2018) | Issue 4 ı April evaporator are elevated are that the efficiency of power plant is increased. When the temperature of main evaporator and reheating steam increases by 10 °C, the efficiency increases by 0.5 %; and pressure increases by 10 kg/cm 2 , the efficiency increases by about 0.2 %. Domestically, in 1990’s, 500 MW-grade standard coal thermal power plant was designed and built, and its operation condition was pressure 246 kg/cm 2 and temperature 538 °C. In the case of Dangjin Thermal Power No. 9, No. 10 and Samcheok Thermal Power No. 1, No .2 that have been being built, the pressure of 250 kg/cm 2 , temperature of 600 °C were accomplished [4]. SCO 2 cycle is the power generation technology of the Gas Brayton Cycle method where pressurized carbon dioxide is heated by the pressure greater than critical condition to high temperature and turbine is driven. Presently, CO 2 power generation cycle can be applied to most heat sources used, and also it can be used for large power plant, small scale distribution power supply, or power supply for marine plant. Super critical condition means the conditions for temperature and pressure greater than critical point in the general material state where liquid-gas phase change occurs, and the temperature and pressure at the lower pressure part is greater than 32 °C, 74 atm, and all parts of cycle are maintained over critical condition. While operation is carried out at high pressure, volumetric flow decreases, so the size of overall heat conversion cycle can be decreased; accordingly, construction period and production unit price can be lowered to secure high economic feasibility. Besides, compared to water vapor, the compatibility with existing material is excellent, so it can be supplied to turbine at the temperature higher than evaporator cycle. From this, the increase of additional power generation efficiency can be possible [5]. 2.3 Heat Conversion Model Design IHX loop of VHTR that is studied in the present study is the system where the high temperature heat generated in the reactor by connecting hydrogen generation equipment and power generation equipment in series can be supplied in the same manner. IHX loop nuclear reactor shown in Figure 1 provides 350 MWt heat output, and the heat generated from | | Fig. 1. IHX Loop Modelling. nuclear fission is supplied to helium fluid. For heat transfer to produce hydrogen, heat exchanger, steam generator for the power generation via USC cycle, and in the power generation via SCO 2 , one heat exchanger is provided. In order to utilize the result of the study regarding the existing VHTR, the major principle and variable if heat conversion model were set as follows. Temperature and pressure at No. 1, 2, 3, 4, 10 were presumed by reference literature [8]. Temperature and pressure of ultrasuper critical cycle No. 5, 6 and SCO 2 cycle, No. 8 were assumed by using reference literature [9]. The model to be explained below was defined as reference model, and then the present authors will plan to develop a model that considers a variety of heat efficiency improvement method. In the present study, in the concept similar to general Rankine cycle’s reheating cycle, bypass mode was proposed. To begin with, the reference model is as follows. After 910 °C helium fluid discharging from VHTR carries out heat exchange with heat exchanger 1, hydrogen is produced by receiving heat from high temperature helium fluid in the heat exchanger 1. 846 °C helium fluid passing heat exchanger 1 enters into steam generator 2 and go through heat exchange. The fluid of this steam generator is ultra-super critical state water, and produces power. The temperature of helium fluid that passes through steam generator 2 is 614.8 °C, this helium fluid enters into heat exchanger 3 where heat exchange is carried out. The fluid of this heat exchanger is super critical-state carbon dioxide, and it produces power by the heat supplied. The temperature of helium fluid coming out of heat exchanger 3 is 450 °C. The heat output that is produced in heat exchanger 1 producing hydrogen is 37.37 MWt. The mass flow of helium from IHX is m 1 , and the mass flow of water flowing in heat exchanger 1 is m 2 , the mass flow of water flowing in steam generator 2 is m 3 , and the mass flow of CO 2 flowing in heat exchanger 3 is m 4 . In this study, the temperatures and pressures from No.1 to No.10 in Figure 1 were assumed, and m 1 and m 2 were calculated by using the Equation (1), and m 3 and m 4 were calculated by using the Equation (2). Besides, considering the characteristics of general longitudinal temperature difference of heat exchanger, the temperature at No. 6 and No. 9 was assumed to decrease by 10 °C compared to the temperature at No. 4 and No. 7 of the steam generator inlet. Major equation or relationship for heat equilibrium analysis is as follows: • Equation used for calculating m 1 and m 2 : W = m∆h = m(h out – h in )... (1) Here,W : Thermal power (MWt) m : Mass flow (kg/hr) h : Enthalpy (kJ/kg) in : Entrance of the equipment out : Outlet of equipment • Equation used for calculating m 5 and m 8 ∑m in h in = ∑m out h out ... (2) In the case of hydrogen production, it was assumed that all heat was converted to work required, and in OPERATION AND NEW BUILD 231 Operation and New Build Heat Balance Analysis for Energy Conversion Systems of VHTR ı SangIL Lee, YeonJae Yoo, Deok Hoon Kye, Gyunyoung Heo, Eojin Jeon and Soyoung Park