Embed Size (px)
Citation preview
Proceedings of International Symposium on EcoTopia Science 2007, ISETS07 (2007)
System Evaluation for Power Plants Using Hydrothermal Oxidation
Kazuma Hirosaka1, Akira Ishikawa2, Tatsuya Hasegawa3
1. Dept. of Aerospace Engineering, Graduate School of Engineering, Nagoya University, Nagoya, Japan 2. CHUBU Electric Power Co., Inc., Nagoya, Japan
3. EcoTopia Science Institute, Nagoya University, Nagoya, Japan
Abstract: It is difficult to utilize wet biomass as a fuel for power plants because it contains high moisture and low calories. Exothermic reaction by hydrothermal oxidation of wet biomass could be used as a heat source. In this study, utilization of this heat source for power plants is considered and an indirect type power plant is investigated, where reactant is oxidized in a reactor and the reaction heat is conveyed to the main water, which is flowed into a turbine. The pressure of the reactor is varied between 5 and 25 MPa. One steam turbine with the maximum turbine inlet temperature of 650 oC and the minimum outlet steam quality of 0.9 is employed to generate electricity. The amount of electric power and the energy conversion efficiency are calculated by using ethanol as reactants. At a fixed reactor pressure, these values become higher as the ethanol concentration increases. When the ethanol concentration is around 15 wt%, higher electric power and efficiency are obtained at lower reactor pressure due to larger energy is consumed for the compression of oxygen. When the ethanol concentration is between 16 and 19 wt%, turbine outputs dominate oxygen compression work and higher electric power and efficiency are obtained at higher reactor pressure. The maximum efficiency is about 7.0 % at the concentration of 19.2 wt% at the reactor pressure of 25 MPa. Keywords: Hydrothermal Oxidation, Power Plant, Efficiency, Ethanol
1. INTRODUCTION Energy recovery from waste materials with high
moisture is a challenging issue. Burning them directly and utilizing the produced heat energy for any purposes such as generating electricity is not so efficient because a lot of heat energy is demanded for drying process and operation without additional fuel is impossible. Some other technologies such as gasification of high moisture waste materials in a fluidized bed or methanation of them to obtain methane gas for gas engine have been studied by many researchers [1,2].
Hydrothermal process is also one of the promising technologies for treatments of high moisture waste materials. The advantages of hydrothermal process are:
(1) No need for drying process for waste materials. (2) High reactivity near below and above critical point
of water (374.3 oC, 22.1 MPa) (3) Reaction media is water and low burden for
environment. (4) Carbon dioxide, nitrogen oxide and sulfur oxide,
which might be produced by hydrothermal oxidation, are contained in exhausted water and can be collected easily.
Supercritical water oxidation, which is one type of hydrothermal oxidation, is commercially utilized for treatment of hazardous waste material, such as PCBs by making use of its high reactivity [3]. Hydrolysis process in subcritical water has been studied as a preparation method before methanation of waste materials in order to fasten the methanation rate [4].
In our previous work [5], we have focused on a hydrothermal oxidation process as a waste material treatment and estimate the energy conversion efficiency. In that study, a power plant, which consists of one steam
turbine whose adiabatic efficiency is 90 % is considered. This value 90 % is decided by referring the value of the large size steam turbine commercially used in a fossil fuel-burning power plant Usually waste materials are preferred to be treated onsite because conveying them to one place for integrated treatment cost a lot for transportation, especially when those waste materials are just high moisture waste materials such as food leftover and animal manure and not hazardous materials like PCBs. Thus estimation of energy conversion efficiency by small steam turbine (often described as micro steam turbine) also needs to be conducted as a more practical estimation. An adiabatic efficiency of a micro steam turbine is not so high as 90 % and in this study we calculate the amount of electric power and the energy conversion efficiency when a micro steam turbine is used for power generation. Reactant and oxidizer are ethanol and oxygen, respectively, which are same as ref. [5] in order to make a comparison easier. 2. CONFIGURARION OF POWER PLANT
In ref. [5], direct type and indirect type power plant are considered. In this study, only indirect type power plat is considered since only indirect type is practical when real waste materials, which include many kinds of impure substances such as Na+ and K+, are used as reactants. Fig.1 illustrates the system configuration of the indirect type power plant. The heat produced by hydrothermal oxidation in the reactor is conveyed to the main water by a heat exchanger and the produced steam is flowed into a turbine. Each parameters concerning about steam turbine, transmission part and generator are tabulated in Table 1 and compared with those values used in ref. [5] for convenience.
Corresponding author: T. Hasegawa, [email protected]
680
Proceedings of International Symposium on EcoTopia Science 2007, ISETS07 (2007)
3. RESULTS AND DISCUSSION The pressure inside the reactor is varied from 5 MPa to 25 MPa at 5 MPa intervals. The turbine inlet temperature (denoted as TIT) needs to be below 650 oC because the steam over 650 oC would erode the turbine blade. The steam passing through the turbine is just exhausted to the atmosphere, 0.1013 MPa. The work needed for separation of oxygen from air (Wsep) is 24.2 kJ/mol, which is the same value in ref. [5]. The works for high-pressure pump and feed pump could be ignored because they are much less than those of compressor and oxygen separation. The calculation methods for compressor works (Wcom) and turbine outputs (Etb) are same as the ones in ref. [5]. The heat loss from the reactor and that from the heat exchanger are estimated to be 10 %. Initial temperatures of reactant and that of water are 25 oC. Oxygen temperature before compression is also 25 oC and the initial pressure is 0.1013 MPa. The drain temperature of heat exchanger assumed to be 35 oC in order to maintain the temperature difference of 10 oC between high and low temperature sources, which is the condition of the heat exchanger. Energy conversion efficiencies of the power plant is calculated using the following equation.
3.1. Effect of the inlet steam pressure Figs.2 and 3 show the electric power and the conversion efficiency, respectively, at the ethanol concentration between 15 wt% and 21 wt% at different pressures of the inlet steam. The reactor pressure is 5 MPa. The concentration of the ethanol solution should be over 15 wt% to obtain the net profit of electric power since some amount of electric power is consumed for Wsep and Wcom. From these figures, the electric power and the efficiency increase at higher concentration. When the ethanol concentration is 15 wt%, there is a peak around the inlet steam presser of 3.6 MPa. The reason of this is illustrated in Fig.4. The enthalpy of the inlet steam is around 2936 kJ/kg when 15 wt% ethanol solution is oxidized. Turbine output is maximized when the pressure of the inlet steam is just fit into the line B-B’. Otherwise the steams, which have higher enthalpies are exhausted from the turbine outlet as indicated by line A-A’ (inlet steam pressure is 4.2 MPa) and C-C’ (inlet steam pressure is 2 MPa) so that the quality and the pressure of the exhausted steams can be over 0.9 and 0.1013 MPa (atmospheric pressure), respectively.
rehE
=η When the reactor pressure is 5 MPa, the upper limit of the inlet steam pressure is 4.25 MPa, which is the saturated pressure of the water at 253.94 oC since the temperature difference in a heat exchanger needs to be more than 10 oC and the saturated temperature of water at 5 MPa is 263.94 oC. Thus the maximums of the electric power and the conversion efficiency are obtained when the inlet steam pressure is 4.25 MPa at the ethanol concentration between 16 and 21 wt%.
)( sepcomtb WWEE +−=
Here, hre is the reaction enthalpy of reactant, E is the electric power, Etb is the turbine output, and Wsep and Wcom are the work for oxygen separation and compression, respectively.
In the next subsection, calculation results of the electric power and the conversion efficiency at each ethanol concentration at each reactor pressure, 5-25 MPa are indicated. These results are obtained at the optimal condition of the inlet steam pressure.
Reactant
Reactor
Compressor
High pressurepump
Feed pump
WaterHeat exchanger
Turbine Generator
Reactant
Reactor
Compressor
High pressurepump
Feed pump
WaterHeat exchanger
Turbine Generator
Figure 1 System configuration Table 1 Parameters for turbine, transmission part and generator This study
(micro turbine) In ref. [5]
Operational flow rate 2 ton/hour 300 ton/dayMax TIT 650 oC 650 oC Adiabatic efficiency 77 % 90 % Machine efficiency 89 % 100 % Generator efficiency 93 % 100 %
0
50
100
150
200
250
300
350
400
0 1 2 3 4Inlet steam pressure [MPa]
Ele
ctric
pow
er [k
W/k
g]
21wt%20wt%19wt%18wt%17wt%16wt%15wt%
Figure 2 Electric power at different inlet steam
pressures
681
Proceedings of International Symposium on EcoTopia Science 2007, ISETS07 (2007)
0
1
2
3
4
5
6
0 1 2 3 4Inlet steam pressure [MPa]
Effi
cien
cy [%
]21wt%20wt%19wt%18wt%17wt%16wt%15wt%
Figure 3 Efficiency at different inlet steam pressures
2000 3000
0.1
1
10
AB
C
300℃
200 ℃
T=100 ℃
400 500 ℃
7.5
[kJ/kg/K]s=5 6 7
8.5
9.5
9
8
Pre
ssur
e [M
Pa]
Enthalpy [kJ/kg]
2936 kJ/kg
Quality = 0.92000 3000
0.1
1
10
AB
C
300℃
200 ℃
T=100 ℃
400 500 ℃
7.5
[kJ/kg/K]s=5 6 7
8.5
9.5
9
8
Pre
ssur
e [M
Pa]
Enthalpy [kJ/kg]
2936 kJ/kg
Quality = 0.9
Figure 4 p-h diagram of expansion process
3.2. Effect of the reactor pressure
Figs.5 and 6 show the electric power and the conversion efficiency, respectively, at each ethanol concentration at different reactor pressures. When the ethanol concentration is below 14 wt%, the enthalpy of the inlet steam is not high enough to gain the net profit of the electric powers. If the condition of the quality at the turbine outlet (i.e. the quality should be over 0.9) is eased to some extent, e.g. 0.85, net profits would be obtained at even lower ethanol concentrations. When the ethanol concentration is around 15 wt%, the higher electric power and efficiency are obtained at lower reactor pressure due to lower compression work. On the other hand, the electric power and the efficiency increase at higher reactor pressure when the ethanol concentrations are
between 16 and 19 wt%. This is because the inlet steam pressure can be set to higher values when the reactor pressure becomes higher according to the condition of the heat exchanger, which leads to obtaining larger electric power. The maximum efficiency is about 7.0 % at 19.2 wt% at the reactor pressure of 25 MPa. When the ethanol concentration becomes higher and the enthalpy of the inlet steam increases, the inlet steam pressure should be lower as shown in Fig.7, where the inlet steam pressure is moving from A through B to C as the enthalpy increases along with the 650 oC line. Otherwise TIT would surpass the upper limit of the TIT condition, 650 oC. Consequently, the electric power is decreasing at higher concentration. This sharp drop is avoided by increasing the flow rate of the feed pump. In this study, this flow rate is assumed to be constant and fixed to the total flow rate of reactant and oxygen.
12 14 16 18 20 220
100
200
300
400
Ele
ctric
pow
er [k
W/k
g]
Ethanol concentration [wt%]
5 MPa10 MPa15 MPa20 MPa25 MPa
12 14 16 18 20 220
100
200
300
400
Ele
ctric
pow
er [k
W/k
g]
Ethanol concentration [wt%]
5 MPa10 MPa15 MPa20 MPa25 MPa
Figure 5 Electric power at different reactor pressures
10 12 14 16 18 20 220
1
2
3
4
5
6
7
8
Effi
cien
cy [%
]
Ethanol concentration [wt%]
5 MPa10 MPa15 MPa20 MPa25 MPa
10 12 14 16 18 20 220
1
2
3
4
5
6
7
8
Effi
cien
cy [%
]
Ethanol concentration [wt%]
5 MPa10 MPa15 MPa20 MPa25 MPa
Figure 6 Efficiency at different reactor pressures
682
Proceedings of International Symposium on EcoTopia Science 2007, ISETS07 (2007)
2000 3000 4000
0.1
1
10
7.5
[kJ/kg/K]s=5 6 7
8.5
9.5
9
8
10
A
B
C
300℃
200 ℃
T=100 ℃
Pre
ssur
e [M
Pa]
Enthalpy [kJ/kg]
A’ B’ C’
500 ℃400 650 ℃
2000 3000 4000
0.1
1
10
7.5
[kJ/kg/K]s=5 6 7
8.5
9.5
9
8
10
A
B
C
300℃
200 ℃
T=100 ℃
Pre
ssur
e [M
Pa]
Enthalpy [kJ/kg]
A’ B’ C’
500 ℃400 650 ℃
Figure 7 p-h diagram of expansion process
4. CONCLUSIONS
Electric power and energy conversion efficiency of hydrothermal oxidation power plants using ethanol is investigated. Micro steam turbine is used for generating electricity. Findings from this study are listed as follows: (1) The electric power and the efficiency increase when
the inlet steam pressure increases up to the point, where the quality of the outlet steam equals to 0.9.
(2) Around the ethanol concentration of 15 wt% the electric power and the efficiency become higher as the reactor pressure decreases because the energy
needed for oxygen compression becomes larger as the reactor pressure increase. On the other hand, these values become higher as the reactor pressure increases at the ethanol concentration between 16 and 19 wt% because larger amount of turbine outputs obtained at higher pressure compensates the lager compression work.
(3) At the fixed reactor pressure, the electric power and the efficiency increase as the ethanol concentration increases as long as TIT does not surpass the upper limit of the condition of TIT, 650 oC.
(4) The maximum energy conversion efficiency of this power plant is about 7.0 % at the reactor pressure of 25 MPa and the ethanol concentration of 19.2 wt%.
ACKNOWLEDGEMENT This work is partially supported by the program “Asia Science and Technology Cooperation Promotion Strategy” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT) and by the Chubu Electric Power Co. in 2006. We also thank to the Central Research Institute of Electric Power Industry (CRIEPI) for allowing us to use the EgWin program. REFERENCES 1. K. Yoshikawa, H. Moritsuka, Advanced Technologies for
Biomass Power Generation, CMC Publishing Co.,Ltd, Tokyo (2007) (in Japanese).
2. Y. Yoneyama, K. Takeno et al., J. Japan. Soc. Waste Manage. Experts, 15-3 (2004), pp. 155-164 (in Japanese).
3. C. N. Stazszak, K. C. Malinowski, W. R. Killilea, Environmental Progress, 6-1 (1987), pp. 39-43.
4. M. Arakane, T. Imai et al., Environ. Eng. Res., 42 (2005), pp. 415-422 (in Japanese).
5. K. Hirosaka, K. Yuvamitra, A. Ishikawa, T. Hasegawa, Thermal Science and Engineering (submitting).
683
