Chapter 3 Experimental Apparatus and Procedures
3.1 Experimental Equipment Layout
3.1.7 Methane Concentration Analyzer (GuardCH4)
Figure 3.8 is guardian plus infra-red gas monitor GuardCH4, which is used for measuring the methane concentration of the inlet biogas.
3.1.8Data Acquisition
Data acquisition system can automatically collect signals from analog and digital measurement sources, such as sensors and devices, under tests.
It uses a combination of PC-based measurement hardware and software to provide a flexible and user-defined measurement system. Usually, the researcher must calibrate sensors and signals before a data acquisition device acquires them. The specifications of these modules of National Instruments are shown in Table 3.2.
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Table 3.2 Specifications of the Data Acquisition Modules Model Signal Type Channels
Max powerful and easy-to-use software that can extend to many languages and operating systems. NI CompactDAQ delivers fast and accurate measurements in a small, simple, and affordable system. A CompactDAQ Chassis shown in Fig. 3.9a, a product of NI, is adopted because of the following advantages: plug-and-play installation and configuration, AC power supply and USB cable connection, mounting kits available for panel, enclosure, DIN-rail and desktop development, A380 metal construction, more than 5 MS/s streaming analog input per chassis, and Hi-Speed USB-compliant connectivity to PC. Different types of signal process modules are chosen to complete the data acquisition system, including NI 9203 Analog Input Module, NI 9211 Thermocouple Differential Analog Input Module and NI 9401 TTL Input Module. All of these are shown in Fig. 3.9b, Fig. 3.9c and Fig. 3.9d.
3.1.9 Ignition System
Figure 3.10 shows the details of ignition system. When the spark plug starts to ignite, the ignition signal is recorded into NI recorder by the tachometer. The in-cylinder pressure is captured by the spark plug
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pressure sensor, and then the charge converter converts the charge signal to voltage signal by the supply of steady current of power unit. The rotary encoder is installed to record the crank angle of piston cylinder. The spark timing controller shows in Fig.3.11, which can be adjusted to change the spark timing by different supply rate of high voltage for spark plugs.
3.1.9.1 Tachometer (VC4000DAQ)
The VERICOM 4000DAQ tachometer is used for measuring the exact spark timing, shown in Fig.3.12. It is clamped onto the spark plug wire to capture the spark signal.
3.1.9.2 Spark Plug Pressure Sensor (BKR5E-11 and 112A05)
The spark plug pressure sensor is modified from NGK BKR5E-11 spark plug with PCB Piezotronics 112A05 pressure sensor, shown in Fig.
3.13.The pressure range is up to 350 bar and the operating temperature up to 240 to 310°C. It is used for measuring the in-cylinder pressure during the combustion process.
3.1.9.3 Charge Converter (PCB 422E05)
Such converter is shown in Fig. 3.14, which is designed to convert the high impedance of a charge mode piezoelectric transducer into a low-impedance voltage. The charge output of the transducers is scaled in term of pressure, mV/psi.
3.1.9.4 Rotary Encoder (HPN-6A)
The HONTKO HPN6A rotary encoder, shown in Fig. 3.15, is used to record the crank angle of piston cylinder during the cycle.
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3.2 The Theoretical Calculation
The following calculations include the excess air ratio, thermal efficiency, theoretical mole fraction of CO2 in waste gas, theoretical percentage of consumed CH4, the percentage of water vapor removed from biogas and combustion stability. These data will be used in the analyses of the following experiments.
3.2.1 Excess Air Ratio
The air-fuel ratio (AF) is defined as a ratio of the mole of air to the one of fuel in the combustion process. The composition of biogas in this study contains air, leaking from the atomosphere to the storage tank when the water line of anaerobic fermentation pool is too low. Hence, the stoichiometric reaction for combustion of biogas with standard air is given as:
(3.1)
where , and are the moles of CO2, air and water vapor in the biogas, respectively. Both and can be measured by instruments, and can be obtained from the absoulate humidity( ) of biogas. Since the water vapor is considered as an ideal gas, the percentage of vapor from biogas can be calculated as follows:
(3.2)
where , and stand for the percentages of CH4, CO2 in biogas and
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air in biogas, respectively. is the pressure of biogas and is the vapor pressure in biogas, which is obtained from:
(3.3)
where is the relative humidity, measured by instrument, and the saturation pressure of vapor at the same temperature.
The stoichiometric air-fuel ratio, AFstoich, is:
(3.4)
On the other hand, AFact is the air-fuel ratio of the actual mole of the air to the summation of moles of the methane, CO2 and air in biogas into the engine. Because the mole ratio is equal to the volume flow rate ratio, and the summation of the methane, CO2, air and water vapor in biogas flow rate is equal to the biogas flow rate. AFact can be also expressed as:
(3.5)
The air flow rate can be measured by air flow meter directly, whereas the methane flow rate is obtained by the measured biogas flow rate
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multiplied by the mole fraction of methane (both flow meters were demonstrated in sections 3.1.2 and 3.1.3).
The Excess Air Ratio (λ) is the ratio of the actual mole of air used to the stoichiometric mole of air, defined as:
CO2 in waste gas in the combustion process are calculated as follows:
The balanced reaction is:
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percent of O2 in waste gas as follow:
(3.8)
where is the total moles in waste gas, is obtained from the atom balance as:
(3.9)
The theoretical percent of CO2 in waste gas can be calculated by:
(3.10) The theoretical percent of used CH4 is defined as:
(3.11)
3.2.3 Thermal Efficiency
The thermal efficiency is defined as the ratio of the fuel conversion efficiency to the combustion efficiency, and its formulation is as following :
(3.12)
Fuel Conversion Efficiency is defined as:
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(3.13)
where W is the electric power generated and LHV the lower heating value.
Combustion Efficiency is expressed as the ratio of the enthalpy difference between the the products and reactants to the LHV of biogas:
(3.14)
where the numerator stands for the real heat release rate between inlet and outlet of the biogas, and the denominator represents the ideal heat release rate. Now,
4[ +( ) ] 4+ 2[ +( ) ] 2+ 2 [ +( ) ] 2 + 2[ +( ) ] 2} (3.15)
where the unit of enthalpy is kJ/kmole, and is the mole flow rate of biogas, calculated by:
(3.16)
in which and refer to the density and the mole of biogas, respectively.
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Eventually, the thermal efficiency can be obtained by dividing the fuel conversion efficiency (Eq. 3.13) by combustion efficiency (Eq. 3.14); that is
(3.17)
3.2.4 Combustion Stability
The process of spark-ignition engines includes suction, compression, expansion and exhaust strokes. The combustion stability, represented by knock, can be detected by many ways in which three of them are introduced as follows. The ionization current measurement circuit is installed with spark plug electrodes to obtain current intensity. The high frequencies contain to the current signal due to variation of pressure when combustion stability becomes bad. Hence, the combustion stability can be analyzed through current intensity. Second one is the engine vibration method. By the way of an accelerometer fixed on the top-surface of the engine cylinder. The last one is the in-cylinder pressure method. The in-cylinder pressure is measured by pressure sensor. It is much more reliable than other two methods because the fact that in-cylinder pressure method directly measure the pressure of in-cylinder.
The indicated mean effect pressure (IMEP) is calculated by integrating pressure with respect to response volume during the combustion process, and Vd is the effective working volume. It is expressed as:
(3.18)
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The combustion stability, coefficient of variation in indicated mean effective pressure ( ), is defined as:
(3.19)
where is the average of indicated mean effect pressure and the standard deviation of IMEP. Their formulations are as following:
(3.20)
(3.21) where n is the number of combustion cycle.
3.3 Dehumidifying Water Vapor of Intake Fuel
The compositions of biogas have more water vapors when it ferments under higher temperature. The water vapor will block the pipe by fueling biogas, making impact on the generator performance.
The experimental parameters include biogas flow rate and excess air ratio. Before experiment, the intake biogas constitutes and their concentrations are measured. The biogas flow rates are set as 200, 220 and 240 L/min, respectively. Under each fixed biogas flow rate, it tests different excess air ratios, ranged from 0.8 to 1.2. The collected data include biogas flow rate, air flow rate and power generation. The measurement starts when the engine is operating continuously until all conditions are ensured to be steady. Then, all measurements are tested twice and take an average. The experimental procedure is as follows:
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1. Dehumidify water vapor in biogas.
2. Measure the relative humidity, temperature and pressure of biogas.
3. Measure the intake biogas constitutes and their concentrations.
4. Operate the engine at least 20 minutes to warm up.
5. Fix the biogas flow rate at demanded quantity.
6. Control the air flow rate for specific excess air ratio.
7. Collect the corresponding data, mentioned above.
8. Repeat the procedure for different excess air ratio.
3.4 The Effect of Spark Timing
The spark timing adjustment is an important parameter for engine performance. The optimum spark timing gives a maximum brake-torque, and leads to the maximum power output. In this study, the maximum power output of spark timing can be found. The advance or delay from the optimum spark timing lead to improper performance of engine.
The experimental parameters are spark-timing, biogas flow rate and excess air ratio. The biogas flow rates are 220, 240 and 260 L/min, and excess air ratios are ranged from 0.8 to 1.2. The optimum spark timing is adjusted in this study. Besides, the advance and delay of the optimum spark timing are investigated as well. At each specific spark timing, it tests different biogas flow rates and each flow rate is accompanied with different excess air ratios. The collected data include biogas flow rate, air flow rate, resultant power generation, pressure of in-cylinder, and concentrations of methane, oxygen, carbon dioxide and NOx. The experimental procedure is as follows:
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1. Dehumidify water vapor of intake biogas.
2. Measure the relative humidity, temperature and pressure of biogas.
3. Measure the intake biogas constitutes and their concentrations.
4. Operate the engine at least 20 minutes for warm up.
5. Control the spark timing at a fixed degree.
6. Fix the biogas flow rate at demanded quantity.
7. Control the air flow rate at specific excess air ratio.
8. Collect the corresponding data, mentioned above.
9. Repeat the procedure for different excess air ratio.
10. Repeat the above procedure for different spark timing.
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Chapter 4
Results and Discussion
This study is a continuous effort of Lin’s[4] and Huang’s[5] works. It carries out with three major modifications, which are the measurements of the detailed intake biogas constitutes and their concentrations, dehumidification of the water vapor in intake biogas and installation of the complete ignition system, consisting spark-plug pressure sensor, and rotary encoder to record the crank angle of piston cylinder.
The biogas used in this research was supplied from the anaerobic tank made of red plastic bag. The original biogas from the tank contains high concentration of H2S, around 4000ppm. It would corrode the engine severely if without proper treatment. Therefore, an H2S removal system, was built up by using biological process, which is environment and cost friendly. The removal rate of screened micro-organism could remove H2S of biogas up to 99%. In other words, the H2S concentration in the biogas was effectively reduced from 4000ppm to 50ppm.
4.1 Effect of Water Vapor of Intake Fuel
The desulfurized biogas passed a methane concentration analyzer, temperature with humidity transmitter and gas analyzer that the concentrations of CH4, O2, and CO2, temperature and relative humidity of biogas can be measured. From the measurements, it found that the biogas comprised O2. It is impossible for the biogas containing O2 after the anaerobic process, so the existence of O2 must be from the air, leaking from atmosphere to the storage tank. According to the concentration of O2, the corresponding N2 concentration can be deduced. In addition, the water
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vapor of biogas with and without dehumidification can be derived by using Eq. (3.2) in Sec. 3.2.1. Since the temperature of intake biogas was 30°C, therefore, the relative humidities of intake biogases without and with dehumidification were 85.2% and 52.7%, respectively. Table 4.1 shows the details of biogas compositions.
Table4.1 Compositions of Biogas with and without Dehumidification without Dehumidification with Dehumidification
CH4 72% 72.2%
CO2 18.6% 17.8%
O2 1.09% 1.39%
N2 4.1% 5.23%
H2O 3.14% 1.9%
Residues 1.07% 1.48%
From above table, there are two kinds of biogases due to an addition of dehumidifier (Sec. 3.1.4). Therefore, the respective stoichiometric air-fuel ratios based on the measured or deduced data are 5.57 (without dehumidification) and 5.31 (with dehumidification). Note that the maximum allowable total volume flow rate (sum of biogas and air flow rates) into the engine is about 2000L/min, therefore, the maximum air supply rate is limited by the biogas one. In other words, the experiments with the higher biogas flow rates carried out with a narrower range of air flow supply rates. So it restricted the maximum excess air ratio for each biogas supply rate.
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Figure 4.1 is the power generation rates as a function of excess air ratio with and without dehumidification. The detailed experimental values are given in the Table 4.2 a~f. In these tables, the measured results include the power generation, waste gas temperature and waste gas species concentrations (O2, CO2 and NOx). Besides, the thermal efficiency deduced from measurements, described in Sec.3.2.3, is provided as well.
It can be seen from Fig. 4.1 that the power outputs of dehumidified biogas are higher than those without dehumidification. Apparently, the power generations by dry intake biogas are better than those by untreated one. In general, the maximal power outputs occur at the excess air ratio approximately equal to 1.0 (stoichiometric condition). The maximum power outputs of biogas supply at 200, 220 and 240L/min after temperature. It is because more heat can be released during combustion as the biogas supply rate increases. The exhaust temperatures of dry biogas are higher than those of wet biogas at a specified excess air ratio. Also the maximum waste gas temperatures for the different biogas flow rates occur at λ~1.00.
There are two main reasons that power output is increased after dehumidification. First, from the composition of biogas (see Table 4.1.),
36 concentration and the avertion of energy absorption by water vapor, then the biogas flow enthapy increasing rate can be defined as:
(4.1) where , and can be measured and they refer to biogas flow rate, exhaust gas temperature and intake biogas temperature respectively.
Thus, the biogas flow enthapy increase rates of biogas supply at 200, 220 and 240L/min after dehumidification with an excess air ratio of 1.00 are 0.79%, 1.17% and 1.27% respectively. Obviously, the higher biogas supply flow provides more enthapy after the dehumidification.
Figures 4.3a~c show the thermal, fuel conversion and combustion efficiencies as a function of excess air of 240, 220 and 200L/min biogas supply rates with and without dehumidification. These are deduced by Eqs. (3.12), (3.13) and (3.14) given in section 3.2.3. It can be seen from these three figures that the thermal efficiency is higher than fuel conversion efficiency. This is because not all the fuel energy supplied to the engine is released by the combustion process since the combustion usually is incomplete. Therefore, it is necessary to cosider the combustion efficiency for evaluating the thermal efficiency. The average of combustion efficiency of the engine is about 0.85. In other words, about
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15% of energy is lost by the form of heat during combustion process.
Besides, it is obvious that the dry intake biogas has a better fuel conversion efficiency and thermal efficiency than the wet one at each biogas flow rate. However, the maximum power output corresponding to thermal efficiency for the biogas supply rate at 200 L/min and 220 L/min do not locate at λ=1.0. This is because the maximal fuel conversion efficiencies, displayed in Fig 4.3b and 4.3c, for both biogas supply rates are higher at λ=1.2 rather than at λ=1.0.
According to these data, it might conclude that the the engine can produce greater power and higher thermal efficiency by removing the water vapor in the intake biogas.
Figures 4.4, 4.5 and 4.6 are the O2 , NOx and CO2 concentrations in the exhaust gas at different biogas supply rates as a function of excess air ratio with and without dehumidification. These data are also listed in the row 8, 9 and 10 in Table 4.1 a~f as well. Figure 4.4 shows that O2
concentration in waste gas increases with increasing excess air ratio, because more O2 is left during combustion as the air is over supplied. In Fig. 4.5, NOx concentration reaches to a peak value in the range around λ=0.9~1.1 (near stoichiometric condition), coincident with the maximum waste gas temperature in Fig. 4.2. It indicates that the main source of NOx
is formed through high temperature oxidation of N2 in the air during combustion. Generally speaking, CO2 concentration in Fig. 4.6 decreases with excess air ratio when λ>0.9. All of them have a peak values at λ=0.9, except for the case 240L/min with dehumidification. Note that the dry and wet biogases have already contained about 18% of CO2, the extra CO2 is from the combustion. When the combustion is more completed,
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the generated CO2 can outweigh the dilution effect by other combustion product gases, leading to a peak appearance near the stoichiometric condition.
This study applies the measured O2 data in the waste gas to estimate the corresponding CO2 concentration and to calculate the mole number of waste gas compositions during calculation of combustion efficiency. The estimated CO2 is derived by using Eq. (3.10) given in section 3.2.2. The corresponding estimated CO2 concentrations are presented in the last row of Table4.2a~f. The maximum discrepancy of CO2 concentration between the estimations and the ones measured by the gas analyzer is within 5%, showing that both agree quite well.
To sum up, it can conclude that the dehumidified biogas provides up to 1.17% extra enthalpy and enhances the power output up to 5.9% with respective to the humid biogas at biogas supply rate of 220L/min at λ=1.0.
Besides, the fuel conversion efficiency and thermal efficiency of dehumidified biogas are higher than the ones without dehumidifying.
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Table 4.2a Power Generation Rates as A Function of Excess Air Ratio without Dehumidification at Biogas Volume Flow Rate
200L/min
Biogas supply at 200L/min (without dehumidification)
Air flow rate (L/min) 1359 1236 1128 1024 957 Excess air ratio 1.21 1.10 1.01 0.91 0.86 Power generation (kW) 19.53 20.30 20.59 17.59 14.11
Thermal efficiency 0.285 0.274 0.267 0.226 0.212 Combustion efficiency 0.825 0.870 0.848 0.903 0.771 Fuel conversion efficiency 0.235 0.239 0.226 0.204 0.163 Waste gas temperature (°C) 482 484.6 505.7 491.4 477.4
Table 4.2b Power Generation Rates as A Function of Excess Air Ratio with Dehumidification at Biogas Volume Flow Rate 200L/min Biogas supply at 200L/min (with dehumidification)
Air flow rate (L/min) 1306 1250 1076 938 839 Excess air ratio 1.22 1.17 1.01 0.88 0.79 Power generation (kW) 20.51 21.24 21.55 18.33 13.15
Thermal efficiency 0.274 0.275 0.276 0.251 0.223 Combustion efficiency 0.897 0.905 0.860 0.843 0.681 Fuel conversion efficiency 0.246 0.249 0.237 0.212 0.152 Waste gas temperature (°C) 484 498 509.5 497.4 487
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Table 4.2c Power Generation Rates as A Function of Excess Air Ratio without Dehumidification at Biogas Volume Flow Rate
220L/min
Biogas supply at 220L/min (without dehumidification)
Air flow rate (L/min) 1540 1335 1233 1096 986 Excess air ratio 1.25 1.08 1.00 0.89 0.80 Power generation (kW) 22.10 22.96 23.41 19.71 15.25
Thermal efficiency 0.286 0.298 0.295 0.244 0.241 Combustion efficiency 0.847 0.827 0.821 0.849 0.847 Fuel conversion efficiency 0.243 0.247 0.242 0.207 0.161 Waste gas temperature (°C) 493.5 498.2 515.8 505.4 500.3
Table 4.2d Power Generation Rates as A Function of Excess Air Ratio with Dehumidification at Biogas Volume Flow Rate 220L/min Biogas supply at 220L/min (with dehumidification)
Air flow rate (L/min) 1360 1263 1178 1007 942 Excess air ratio 1.16 1.08 1.00 0.86 0.80 Power generation (kW) 23.74 24.33 24.78 19.90 16.96
Thermal efficiency 0.313 0.312 0.302 0.256 0.252 Combustion efficiency 0.832 0.820 0.825 0.817 0.707 Fuel conversion efficiency 0.261 0.256 0.250 0.209 0.178 Waste gas temperature (°C) 501 506.4 521.5 506 504
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Table 4.2e Power Generation Rates as A Function of Excess Air Ratio
Table 4.2e Power Generation Rates as A Function of Excess Air Ratio