Chapter 3 Experimental Apparatus and Procedures
3.3 Waste Gas Analysis
measured data. When R2 equals to 1, it is called perfect fit, meaning that the regressive model does not exist the residuals.
3.3 Waste Gas Analysis
The contents of waste gases include O2, CO2, CO and NOx. The gas analyzer (IR-208) can measure the concentrations of waste gases.
However, the gas turbine needs most of the air to cool the hot gas to avoid damaging the turbine. Thus, the concentration of NOx is too low to measure by instrument.
The measured O2 data can be applied to estimate CO2, excess air ratio and mole number of waste gas composition. Because the cooling air is mixed with produced CO2, the measured concentration of CO2 is larger than actual one. Moreover, the quantity of air is much higher than CH4 in exhaust gases, hence, the term of methane does not appear in actual reaction formula. Eq. (3.1) is modified by the excess air ratio for obtaining the actual reaction formula. Eq. (3.17) can find the concentration of CO2 and excess air ratio by the O2 mole fraction.
36
The actual reaction formula is expressed as following:
2
The mole fraction of O2 is applied to deduce the excess air ratio, and then the excess air ratio is used to deduce theoretical mole fraction of instrument and the coefficient a will be found.
The percentages of CO2 in waste gases can be calculated by:
z
3.4 Theoretical Calculation of Performance for Micro-Gas Turbine
The theoretical calculation of performance is calculated by isentropic process and rated efficiency of component given from Table 3.1, such as compressor and turbine. The temperature and pressure points are marked in Fig. 3.11. In fact, the exit temperatures of components cannot be measured by instruments directly due to the restrictions of AIDC, hence, those above isentropic efficiencies are applied for estimating the actual temperatures of components.
c is the isentropic efficiency of the compressor, and
T is isentropic efficiency of the turbine. It is expressed as:C compressor and
W
T is realistic work output by turbine. Eqs. (3.20) and (3.21) are applied for calculating the actual outlet temperature of compressor and the actual inlet temperature of turbine, respectively.mech
38 where
mech is mechanical efficiency.The Wnet is the net output that work of turbine minus compressor. It is The hot gas mass flow rate that drives blades of turbine is expressed as:
biogas software and the biogas mass flow rate is obtained from:
C The specific heat capacity mixing air with biogas is expressed as:
biogas
In ideal gas reversible adiabatic process, the isentropic compressor outlet temperature and turbine inlet temperature can be expressed as following:
k k
k k
ratio. Due to the pressure drop, so
5 4
P
P is expressed as following:
combustor
The heat exchanger effectiveness HE is calculated by:
)
The efficiency of combustion is expressed as:
4 The calculated heat exchanger outlet temperature (gas side) is:
gas ideal heat input that is:
ideal
40 output of a device to the calculated heat input. Its formula is expressed as:
th cal
,cal gas pgas air pair biogas pbiogas biogas
th m C T m C T m C T
Q
(3.39) where Qth,cal is the calculated heat input, generator generator efficiency,
generator output of a device to the measured heat input, its formula is expressed as:
actual
3.5 The Effect of Varying Loads and Ambient Temperature The power generation of the gas turbine engine is affected by main two conditions, one is operating loads and the other is ambient temperature. Thus, the thermal efficiency and power generation will be investigated in this research. The designed range of rated power output of engine is 15kW to 30 kW, and the increment of power output is 1 kW in five minutes interval under the same environmental conditions for ensuring the system in steady state. The operating load will affect the performance of engine, such as power output. Finally, the all of measured data make average to obtain the more accurate values.
The ambient temperature is an important parameter for engine performance, so it is recorded in each load. Fig. 3.12 shows the average ambient temperature of swine farm in Taichung. The temperature range of swain farm is about 17oC to 30oC. According to the data of CR30 given by Aerospace Industrial Development Corporation (AIDC), the net power output and electrical efficiency are affected by ambient temperature seriously. Thus, the performance of MGT affected by ambient temperature is analyzed in this research.
The experimental procedure is as follows:
1. Record ambient temperature and measure the relative humidity, temperature and pressure of treated biogas.
2. Measure the treated biogas constitutes and concentrations of methane 3. Warm up the engine at least 10 minutes in 15kW so it would be
steady.
4. Record all of the measured data, such as above all and prepare all of
42
the instruments.
5. Adjust the power output at demanded quantity and record the net power output, biogas flow rate, air mass flow rate, and so on.
6. Repeat the procedure for different power output.
7. Repeat the above procedure at different ambient temperature.
Besides, if the ambient temperature is too low, the biogas supply will get some troubles. We check the condition of biogas before carrying out the experiment. There are two problems about biogas and they lead micro-gas turbine not to work. Firstly, the swine farm is not usually clean the swine house in the winter, otherwise, pigs may catch cold. Thus, the waste water, which flows into the anaerobic fermentation tank, is not enough to achieve the standard level of water. This situation causes the quantity of biogas to decrease. In addition, the level of water is too low (<
90 cm) that makes outside air to leak into anaerobic fermentation tank, so the concentration and quantity of biogas are affected by above reasons.
Secondly, the biogas is treated by biological desulphurization system, the low temperature will cause decreasing activity of desulfurization bacteria, thus, the concentration of H2S, which can corrode the turbine engine, increases up to 500 ppm.
3.6 Uncertainty Analysis
The accuracy of the measured data should be confirmed before the analyses of experimental results are carried out because the exactitude of the data may not be very good. Error analysis is a method applied to quantify validity and accuracy of measured data. The devices of experiment have deviation of measurement, which affects the accuracy of
measured data, and other errors are from the improper operation. There are three reasons to cause these errors including instrument error, method error and artificial error. The experimental errors can be defined as determination errors and indeterminate errors. The determination errors can be called systematic errors that have constant value caused by devices themselves, so the measured values have same tendency. Furthermore, the indeterminate errors can be said random errors, which must use statistical method to solve and the values irregular.
3.6.1 Uncertainty Analysis of Mass Flow Meter
In this study, the mass flow meter is thermal mass flow meter (TBT-FT004). The disturbed flow and inside component sensors will cause the measurement deviation, the measurement range of TBT-FT004 adopted in this study is 5~2830 L/min3%. The biogas flow rate is used to calculate the thermal efficiency, so the error will affect the result.
3.6.2 The Experimental Repeatability
To verify experimental accuracy, the measured data are recorded five times in each load. Then the standard deviation and coefficient of variation (CV) are applied to evaluate the accuracy of measured data. The standard deviation shows how much variation or dispersion from the average value. It is defined as:
average value of measured data.The CV is a dimensionless number that can be used to show the extent
44 system needs more commands to adjust the fuel valve for maintaining the net power output, consequently, the variations are high in lower load.
Table 3.8 Experimental Repeatability for Thermal Efficiencyat 31.4 oC
Net Power
Table 3.9 Error Analysis for Thermal Efficiency at 31.4 oC
Net Power Output
(kW) Thermal Efficiency (%) Standard Deviation (%) CV (%)
15.03 15.7 0.19 1.19
15.86 16 0.14 0.86
17.04 16.6 0.80 4.85
18.23 17.3 0.45 2.64
19.05 17.7 0.33 1.85
20.00 17.9 0.25 1.37
21.14 18.4 0.24 1.32
21.96 18.4 0.12 0.64
22.98 18.7 0.21 1.14
24.01 18.9 0.15 0.79
24.00 18.9 0.14 0.77
3.6.3 CR30 System Stability
Figures 3.16 and 3.17 show the system stability in 15 and 22 kW, respectively. The data points are obtained after users increase the rated power output for analyzing stability of engine and ensuring the timing to record the data. The net power output is obtained from power meter, whose current is recorded after it is consumed by compressor (H-50) and freeze dryer (RD-20A). Therefore, the recorded power outputs are lower than the real rated power output, generated by MGT. The MGT system approaches stable after it runs for two minutes, which offers the standard of time to consult for recording those data in this study.
46
Chapter 4
Results and Discussion
The experimental study, a continuous effort of Ge [4], is carried out with 30kW micro-gas turbine (MGT) in a swine farm in Taichung. It is one of products of Capstone so no refit can be allowed. Note that the turbine outlet temperature is always fixed at 594 oC under any operation, assigned by Capstone. The effect of ambient temperature on engine performance is investigated with an aid of theoretical analyses. The biogas used in this research is supplied from the anaerobic tank made of red plastic bag. It is treated in advance with H2S removal system due to the high concentration of H2S (~5000 ppm) that will corrode the engine severely. By this process, the concentration of H2S in biogas is decreased to 50 ppm. Furthermore, the biogas constituent concentrations at engine inlet are measured by using gas analyzers (IR-208), which can measure the concentrations of methane, oxygen, carbon dioxide and NOx; see Section 3.1.8 for details. The contents of desulfurized biogas at each ambient temperature are shown in Table 4.1. In real situation, the biogas should not contain any O2 after anaerobic process, however, it shows a lot of air in biogas, indicating that there are leakages from atmosphere to storage tank and biological desulphurization process. Moreover, the concentrations of biogas are 64, 51.7, 60 and 47.8% at ambient temperature 21.8, 23.5, 29.5 and 31.4oC, respectively. It indicates that the content of CH4 in the treated biogas changes day by day because such gas is not produced by an industrial process. In addition, the water vapor in biogas cannot be not removed completely even it passes through the dryer.
However, its quantity is approximated by using Eq. (3.2) in Section 3.2.1.
Table 4.1 Compositions of Biogas at Inlet of Turbine Engine at different Ambient Temperature
Ambient
Temperature CH4 (%) CO2 (%) Air (%) H2O (%) Residues (%)
21.8 oC 64 19.3 10.42 1.3 4.98
23.5 oC 51.7 20.1 25.6 1.44 1.16
29.5 oC 60 12.73 18.37 1.44 2.36
31.4 oC 47.8 22.3 23.13 3.17 3.6
4.1 Theoretical Calculation of Performance for Micro-Gas Turbine Engine
Because many inlet and outlet temperatures and pressures in the MGT
components cannot be measured by instruments directly, therefore, a theoretical analysis is adopted to obtain these data by incorporating with the applicable measurements. Now, the processes of MGT are approximated by Brayton cycle together with the applications of the thermodynamic isentropic efficiency and the actual component efficiencies, provided by Ref. [18].
A case of rated power output of 25kW (corresponding a maximum engine rotational speed 96,000rpm) at ambient temperature 31.4 oC is given to demonstrate the theoretical analysis. Figure 4.1 shows the corresponding MGT cycle. The system is assumed as in steady state, and Cps’ do not change with temperature. Then, it follows air standard cycle, internally reversible one, and the fluid is ideal gas. The locations of
48
temperatures and mass flow rates are marked in Fig. 4.1, and the input data are shown in Table 4.2. The air, methane and biogas mass flow rates and compressor inlet and turbine outlet temperatures are measured by sensors. The other parameters are obtained from AIDC and reference [18].
It will be demonstrated next. The entire calculation procedure to determine the unknown data (not able to measure) is given in section 3.4.
As to the pressure ratio and isentropic efficiencies of compressor and turbine, they are found by using the compressor and turbine performance maps [18] under the specified corrected mass flow rate and engine speed ratio. The corrected mass flow rate is expressed as:
ambient
T ambient temperature, Tstandard temperature at standard condition,
dard
Pstan pressure at standard condition and Pambient ambient pressure.
The engine speed ratio is defined as
Nmax
N Npresent (4.2)
where Npresent is present engine speed and Nmax maximum engine speed of turbine engine in the experiment.
Table 4.2 The Input Data in 25kW at 31.4oC
Notation Denotation Values
c
Compressor isentropic efficiency 0.767 [18]
T Turbine isentropic efficiency 0.83 [18]HE Heat exchanger effectiveness 0.786 [24]
generator
Generator efficiency 0.96 [18]
C mech,
Compressor mechanical efficiency 0.97 [18]
T mech,
Turbine mechanical efficiency 0.97 [18]
m air Air mass flow rate 16.02 kg/min (measured)
CH4
m
Methane mass flow rate 0.1519 kg/min (measured)biogas
m
Biogas mass flow rate 0.3478 kg/min (measured)m
gas Hot gas mass flow rate 16.368 kg/min (measured)air
C
p, Air specific heat capacity at constantpressure 1.005 kJ/kg [29]
biogas
C
p, Biogas specific heat capacity at constantpressure 2.2 kJ/kg [29]
,CH4
C
p Methane specific heat capacity atconstant pressure 1.45 kJ/kg [29]
gas
C
p, Hot gas specific heat capacity atconstant pressure 1.0145 kJ/kg [29]
rp Pressure ratio 3.85 [18]
T1 Compressor inlet temperature 311.7 K (measured)
T5 Turbine outlet temperature 866 K (measured)
LHVCH4 Lower heating value of methane 50020 kJ/kg [29]
50
Figure 4.1 shows the calculated data based on above input data (Table 4.2). The values in green are measured temperature, the ones in red are isentropic temperatures and blue ones are actual temperatures calculated by isentropic processes and rated efficiency [18], respectively. The calculated values are summarized in Table 4.3 and the works done by turbine and required by compressor, generator power output, theoretical total input heat and thermal efficiency are presented in Table 4.4.
Table 4.3 The Calculated Temperature at 31.4 oC
Notation Denotation Value
T2 Compressor outlet temperature 503 K
T2s Isentropic compressor outlet temperature 458 K T3 Heat exchanger outlet temperature (air) 788 K T3s Ideal heat exchanger outlet temperature (air) 866 K
T4 Combustor outlet temperature 1185 K
T4s Ideal combustor outlet temperature 1250 K
T5 Turbine outlet temperature 866 K
T5s Isentropic turbine outlet temperature 866 K T6 Heat exchanger outlet temperature (gas) 589.2 K T6s Isentropic heat exchanger outlet temperature (gas) 458 K
Table 4.4 The Calculated Data at 31.4 oC
Notation Denotation Value
WT Turbine output work 85.66 kW
WC Compressor input work 52.82 kW
Wgenerator Generator power output 31.54 kW
Wconsumption MGT internal consumption 2.35 kW
Qideal Ideal heat input 111.1 kW
Qth,cal Calculated heat input 113.9 kW
Qth,actual Actual heat input 126.7 kW
isen Isentropic thermal efficiency 60.4 %
cal
th, Calculated thermal efficiency 25.62 %
actual
th, Actual thermal efficiency 23.04 %
Table 4.5 shows the comparison of the calculated temperatures with Capstone data at full power (96000 rpm). It indicates that the exit temperatures for each component in this research are very close to the Capstone data except the combustor outlet temperature. It is because that the adiabatic assumption is applied in combustor performance in this study that it leads to the present combustor outlet temperature is greater than the one given by Capstone.
Table 4.5 Comparison of the Calculated Temperature with Capstone data
This Study Capstone
Turbine Outlet Temperature
T5 (K) 865.9 866
52 different workload. The input data are shown in Table 4.6, and the results for each workload by using the data of Table 4.6 are shown in Table 4.7.
Table 4.7 Results of the Calculation in 15~30 kW at 31.4 oC
Figure 4.2 shows the actual and theoretical generator power outputs, they are almost parallel and the difference is around 5 kW. The discrepancy is attributed to that the combustor is assumed as adiabatic in calculation and the pressure drop, occurred in air passage (piping loss), does not consider as well. As expected, the theoretical powers are higher than experimental ones.
The theoretical performances of gas turbine for other ambient temperature are also calculated in order to understand its effect on power output and the related reason. The input data in Table 4.8 except the measured compressor inlet temperature (T1) are obtained from the
54
theoretical calculation mentioned above. The rated power outputs shown in this are selected from the maximum engine speed, 96000 rpm, at each ambient temperature.
Table 4.8 Input Data at different Ambient Temperature
Ambient Temperature (oC) 21.8 23.5 29.5 31.4
Rated Power Output 27 28 25 24
Engine Speed 96044 96000 96004 96262
Pressure Ratio 3.73 3.73 3.8 3.85
Compressor Efficiency 0.77 0.77 0.767 0.767
Turbine Efficiency 0.831 0.831 0.83 0.83
The calculation results are given in Table 4.9. It shows that the combustor outlet temperature is increased with an increase of the ambient temperature. The reason is that the pressure ratio at higher ambient temperature has greater value than that at lower ambient temperature; see Table 4.8. The higher pressure ratio leads to a higher inlet temperature of combustor, causing a higher outlet temperature. Of course, the higher compressor ratio needs more input work. It indicates that decrease of power output between ambient temperatures 21.8 and 31.4 oC is around 1.48 kW due to the increased required power by compressor.
Table 4.9 Results of the Calculation at different Ambient Temperature
Compressor Outlet Temperature
T2 (oC) (calculated) 208.56 212.94 221.3 230
Figures 4.3 and 4.4 show the T-S and P-V diagrams for the ideal and actual cycles of the gas turbine engine at 31.4 oC. Those temperatures are obtained from Table 4.3. There are two useful equations developed by Gibbs equations [29] for computing the entropy change of an ideal gas.
vdP
56 where C is average specific heat at constant pressure. p
Eq. (4.5) can be applied to irreversible process because the properties of a substance depend only on the state. Thus, if it has an irreversible where subscript 1 and 2 represent initial and final states, respectively.
It can be seen the maximum entropy change occurs in recuperator due to the heat gain. Besides, the pressure drop of recuperator and combustor are considered in actual situation, so we can find the pressure difference in Fig. 4.4.
4.2 Power Generation by Gas Turbine Engine
The power generation produced by turbine engine is called net power output in this study. Tables 4.10 a~d show the measured and derived data as a function of specific power output of CR30 gas turbine under four different ambient temperatures. The rated power output, net power output, air flow rate and engine speed are obtained directly from Capstone remote monitoring software provided by AIDC. Biogas flow rate is measured by thermal mass flow meter, and the thermal efficiency is derived from experimental data by using Eq. (3.7). CH4 consumption rate is calculated directly by multiplying the biogas flow rate with concentration of biogas under the assumption of complete combustion.
Figure 4.5 shows the power consumption of the digital power controller (DPC), including pre-charge board, inverter and generator inductor, DPC power board, DPC heat sink fans and so on. It can be seen that the generator power output is higher than the net power output because part of the generator power output is consumed by control system (DPC), which needs around 2.3 kW for operation.
58
Table 4.10a The Measured and Derived Data as a Function of Specific Rated Power Output of CR30 gas turbine at 21.8 oC
Biogas Constituents CH4: 64 %, CO2: 19.3%, Air: 10.42 %, H2O: 1.3 %, Residues: 4.98 %
Table 4.10b The Measured and Derived Data as Function of Specific Rated Power Output of CR30 gas turbine at 23.5 oC
Biogas Constituents CH4: 51.7 %, CO2: 20.1 %, Air: 25.6 %, H2O: 1.44 %, Residues: 1.16 %
60
Table 4.10c The Measured and Derived Data as Function of Specific Rated Power Output of CR30 gas turbine at 29.5 oC
Biogas Constituents CH4: 60 %, CO2: 12.73 %, Air: 18.37 %, H2O: 1.44 %, Residues: 2.36 %
Table 4.10d The Measured and Derived Data as Function of Specific Rated Power Output of CR30 gas turbine at 31.4 oC
Biogas Constituents CH4: 47.8 %, CO2: 22.3 %, Air: 23.13 %, H2O: 3.17 %, Residues: 3.6 %
62
Figure 4.6 shows the net power output v.s. rated power output from 15~30kW under four different ambient temperatures. It shows that both are almost coincident until the engine speed reaches 96000 rpm in 27, 28, 25, 24 kW at 21.8, 23.5, 29.5 and 31.4 oC, respectively. After that, the maximum net power output apparently is influenced by the ambient temperature. The discrepancy between the rated and net power output
Figure 4.6 shows the net power output v.s. rated power output from 15~30kW under four different ambient temperatures. It shows that both are almost coincident until the engine speed reaches 96000 rpm in 27, 28, 25, 24 kW at 21.8, 23.5, 29.5 and 31.4 oC, respectively. After that, the maximum net power output apparently is influenced by the ambient temperature. The discrepancy between the rated and net power output