This research aims to find a potential sustainability power plant that can be implemented in reality. The COOLCEP-C system proposed by Zhang et al. (2010) is zero-carbon emission and takes good advantage of LNG cold energy. Since the design of the COOLCEP-C system corresponds with the objectives of this research, some modifications and system optimizations are done based on the COOLCEP-C system.
After the modifications, there are four kinds of derivative systems proposed in this research, and they are MCOOLCEP-C system, RMCOOLCEP-C system, HIMCOOLCEP-S system and HIRMCOOLCEP-S system. Each system has its own features. First of all, the H2O separation section, CO2 capture section and LNG regasification cycle in the MCOOLCEP-C system are intensified based on the COOLCEP-C system, and the improvement makes the MCOOLCEP-C system more practical. Secondly, based on the MCOOLCEP-C system, the RMCOOLCEP-C system increases the net power output by incorporating reheat procedure. Thirdly, in the HIMCOOLCEP-S and the HIRMCOOLCEP-S, more modifications are done based on the COOLCEP-S system. Both of these two systems consumes less power because the CO2 compressor is eliminated.
In order to assess the feasibility of the systems, AspenPlus is used in this research to simulate the four systems with 100 kg/s recirculating CO2, and the simulation result can be discussed in two dimensions. Firstly, in the aspect of the engineering and environmental protection, there are four assessment items, and they are net power output, energy efficiency, CO recovery and the CO capture purity. More electricity can be
generated if there is higher net power output. Since the power output of NG turbine is a fixed at 3.52 MW in every system, the net power output discussed only focuses on CO2
turbine located in the power generation cycle. In other words, the values of the net power output shown in the result of this research do not include the power output of NG turbine, which is 3.52 MW. But since the COOLCEP-C system is without the NG turbine, the values of power output of NG turbine is not related to it. Besides, if the energy efficiency is high, this represents less energy loss when the system transforms chemical energy in LNG into electrical energy. Furthermore, CO2 is generated by the system and the CO2
recovery represents the percentage of CO2 that is captured. Secondly, in the economic aspect, the payback period is set as three years. And the higher the net profit, the more lucrative the system in the first three years. The two aspects mentioned above is used to evaluate the COOLCEP-C system, the result shows that the net power output in the COOLCEP-C system is 32.79 MW, the energy efficiency is 51.11%, the CO2 recovery is 100% and its net profit is 3.04 MUSD. And in the following paragraph, the result for the proposed systems in this research is illustrated in two aspects as well.
The result in the aspect of engineering and environmental protection shows that the optimized RMCOOLCEP-C system has the highest net power output, CO2 recovery and CO2 capture purity. For the optimized RMCOOLCEP-C system, the net power output is 134.61 MW, the CO2 recovery is 98.60%, and the CO2 capture purity is 99.94 mol%.
However, the highest energy efficiency is created by the HIMCOOLCEP-S system, and the value is 69.01%. After the elimination of the compressor in the HIMCOOLCEP-S system, its energy consumption decreases obviously, and this leads to the least energy loss. But since the compressor is eliminated, the volume expansion is limited, leading to the fact that the net power output decreases enormously. On the other hand, the result in
the economic aspect shows that the RMCOOLCEP-C system is the most profitable one.
The net profit of optimized RMCOOLCEP-C system is 52.58 MUSD. This proves that its profitability is directly proportional to the net power output. But the profitability is weakly related to the energy efficiency.
After summarizing all of the assessments mentioned above, the RMCOOLCEP-C system is suggested to be implemented in the future since it is environmental-friendly and profitable, making it the most suitable system for the development. However, both of the operating temperature and pressure are extremely high. Therefore, it is suggested that the conditions of operation should be carefully maintained in the safe range. On the other hand, the HIMCOOLCEP-S system, which has a considerable energy efficiency, is also a good choice to develop, and the barrier of low net power output can be easily resolved by increasing its recirculating flowrates of CO2.
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Appendix
The cost per unit electricity produced
In order to figure out the unit cost of the electricity produced, the calculation results are shown in Table A-1. The E-cost is obtained after the price of the CO2 and cold energy is deducted from the total annual cost (TAC). Then, the unit cost is obtained by calculating the ratio of E-cost to the electricity produced. Since the calculation of the E-cost is based on TAC, which does not include any expenses such as the maintenance cost, labor cost, taxes and so on, the unit cost shown in the table is just a possible result between different systems discussed in this research. The results show that the optimized RMCOOCEP-C system can produce the electricity with lowest unit cost, which is 0.031 USD/kWh, and the unit cost of the proposed systems are lower than the average electricity cost of Taiwan Power Company in 2018, which is 0.072 USD/
kWh. Therefore, the proposed systems have superiority to be the low-cost power systems.
Table A-1 The electricity cost for COOLCEP-C and optimized proposed system.
System
Cold energy recovery
From the perspective of the LNG cold energy recovery in each system, the result is also obtained in Table A-2 by calculating the ratio of the recovered cold energy to the potential cold energy carried by the LNG. The recovered cold energy is the cold energy that is supplied to the H2O separator, CO2 capture section and the CO2 condenser and the energy that can be absorbed by the LNG when its condition is changed from -161.5 oC, 1 bar to 8 oC 70 bar. With the basis of 100 kg/s LNG, the cold energy recovery in MCOOLCEP-C and RMCOOLCEP-C are very high. However, it is relatively low in the HIMCOOLCEP-S and HIRMCOOLCEP-S. This result represents that the amount of the cold energy provided to the MCOOLCEP-C and RMCOOLCEP-C is moderate, but the excessive amount exists in HIMCOOLCEP-S and HIRMCOOLCEP-S. Hence, the flowrate of LNG in HIMCOOLCEP-S and HIRMCOOLCEP-S can be reduced and the cold energy recovery will be increased accordingly.
Table A-2 The cold energy recovery for COOLCEP-C and optimized proposed system.
System
HIRMCOOLCEP-S 61.72 85.50 72.19
Capital cost calculation CE = CB(Q
QB)MfMfPfT (A-1)
C1
C2= INDEX1
INDEX2 (A-2)
Where
C1 : equipment cost in year 1 (USD)
C2 : equipment cost in year 2 (USD)
CB : known base cost for equipment with capacity QB (USD)
CE : equipment cost with capacity Q (USD)
INDEX1 : cost index in year 1 (-)
INDEX2 : cost index in year 2 (-)
fM : correction factor for materials of construction (-)
fP : correction factor for design pressure (-)
fT : correction factor for design temperature (-)
M : constant depending on equipment type (-)
Table A-3 Typical equipmentcapacity delivered capital cost correlations.
Equipment Material Capacity QB CB (USD) Size range M
Column CS Mass (t) 8 6.56×104 8-300 0.89
Heat exchanger CS Heat Transfer
Area (m2) 80 3.28×104 80-4000 0.68
0.01 2.0
0.1 1.3
0.5 to 7 1.0
50 1.5
100 1.9
Table A-6 Correction factors fT.
Design temperature (oC) Correction factor, fT
0-100 1.0
300 1.6
500 2.1
Table A-7 Detailed numerical calculations for economic evaluation.
kUSD/y COOLCEP-C MCOOLCEP-C RMCOOLCEP-C HIMCOOLCEP-S HIRMCOOLCEP-S
Annualized
Note: The payback period is set at 3 years.
Standard Cubic Meter (SCM) A unit used in natural gas.
Vs = Va× Fp× Ft× (Fpv)2 (A-3)
Fp= Absolute Pressure
Standard Pressure (A-4)
Ft= Absolute Standard Temperature
Standard Line Temperature (A-5)
Where
Absolute Pressure : measured pressure (atm)
Absolute Standard Temperature : 298.15 (K)
Fp : pressure factor (atm/atm)
Fpv : super compressibility factor (=1) (-)
Ft : temperature factor (K/K)
Standard Line Temperature : measured temperature (K)
Standard Pressure : 1 (atm)
Va : actual volume (m3)
Vs : standard cubic volume (under 15oC and 1 atm) (m3)