The excess air ratio (λ), a reprocipal of equivalence ratio (), was defined in Eq. (3.5) of Section 3.2. λ=1 represents the stoichiometric condition. When λ>1, it means the mixture is fuel-lean, and λ<1, the mixture is fuel-rich.
The biogas supply rates in this reserach were ranged from 140 to 260 liters per minute and the corresponding excess air ratios were from 0.8 to 1.6. Note
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that the maximum allowable total volume flow rate (sum of biogas and air flow rates) into the engine is about 1800 L/min, therefore, the maximum air supply rate is limited by the biogas supplied flow rate. In other words, the experiments with the higher biogas flow rate carried out with a narrower range of air flow supply rate. So it restricted the maximum excess air ratio for each biogas supply rate. Such limitations in experiments are summarized in the following Table.
Table 4.1 Limiting Value of Air Supply for Each Biogas Supply Rate Biogas Flow
The detailed resultant experimental data for each specified biogas flow rate are given in Table 4.2a~g. In these tables, the measured results include the power generation, waste gas temperature and waste gas species concentrations (O2, CO2 and NOx). In addition, the thermal efficiency, CH4 consumption and CO2 concentration deduced from measurements described in Sec. 3.2 are provided as well.
Figures 4.1 is the power generation as a function of excess ratio ratio at different biogas supply rates with 73% methane concentration. From the data of 140L/min of biogas flow rate, it seems provide a complete flammability
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atmospheric pressure and temperature, in a vertical glass tube, the flammability limits of pure methane are 0.5 and 1.68. Increasing pressure above atmospheric pressure usually widens the flammability range.
Most of the widening occurs at the fuel-rich side, the flammability limits on the fuel-lean side is not strongly pressure-dependent [27]. When the combustion occurs in engine, the pressure is higher than standard atmospheric pressure. In theory, the flammability range should be wider. However, in this study, it has a narrower flammability range, due to following reasons. First, 73% CH4 of biogas was used as fuel instead of pure methane. The CO2 in the biogas act as dilution would absorb part of heat energy, so the flammability range is narrower. Secondly, the flammability range is deduced by the operation range of engine, instead of real air-fuel limit of flame propagation.
The mechanical structure of engine is more complicated than glass tube, so the heat loss is higher than that of glass tube. This heat loss of engine makes flammability range become narrower. Due to above reasons, the present biogas with 73% of CH4 is as expected to have a narrower flammability range. As the biogas flow rate increases, the lower flammability limits in terms of are no longer achieved due to the restricted air supply rate, mentioned previously.
On the other hands, the upper one still can be maintained as a fixed vale of = 0.8 (or1.25).
From the case of 140L/min biogas flow rate, the optimal excess air ratio occurs at = 1.2 (or 0.83) with a maximumu power of 12.9kW. In the case of 160L/min, the optimum is = 1.29 (or 0.77) with a maximumu power of 16.4kW. The optimal no longer can be obtained as the biogas flow rate
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increases further for this engine. Therefore, the engine can be operated at the part of fuel-lean regime from 160~200 L/min of biogas supply rates. For the biogas supply rate greater than 200 L/min, it can only operate at fuel rich regime, where the generated power increases with an increase of excess air ratio.
It can be seen that at a given excess air ratio, the higher the biogas supply rate, the higher the power generation. The maximum power output that the present engine can achieve is 26.7 kW at biogas supply of 260 L/min with an excess air ratio of 0.84. However, its corresponding thermal efficiency is 0.235; see Fig.
4.2 (the deduced thermal efficiency verse ), which is not the highest one. It is because of the limited air supply, unable to lead to the optimal . The maximum thermal efficiency is 0.27 occurred at the biogas supply rate of 200 L/min with λ= 1.1.
In general, the trends for both Figs. 4.1 nd 4.2 are quite similar, since the thermal efficiency is directly proportional to power generation (see Eq. (3.7) in Section 3.2) for each fuel flow rate.
The biogas supply rate affects the optimal , behind which the performance starts to decline as discussed previously. When the biogas supply rates are above 180L/min, under each fixed biogas flow rate, the power generation and thermal efficiency increase with an increase of excess air ratio. When the biogas supply rates are below 180L/min, the lower biogas supply rate is given, the smaller optimal excess air ratio . This is because the total heat energy released from the combustion process is less at lower biogas supply rate. For the case of 160L/min, the power and thermal efficiency start to descend at λ
=1.29. For the case of 140L/min, they even starts to descend at λ =1.20.
The results show that the engine can produce higher power and greater
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thermal efficiency with higher biogas supply rates with larger excess air ratios, but maximum inlet gas flow rate of engine limits these trends to increase.
Figure 4.3 shows the waste gas (or flue) temperatures for different biogas supply rates as a function of excess air ratio. Similar to the power generation and thermal efficiency, the exhaust temperature is higher as the biogas supply rate is higher at a specified excess air ratio. It is because more heat can be released in combustion when biogas supply rate increases, leading to the higher waste gas temperature. Also the maximum waste gas temperatures occurs at λ=0.9~1.1 (near stoichiometric condition) except the ones of 240 and 260L/min biogas supply rates, which cannot reach the stoichiometric condition.
Figures 4.4, 4.5 and 4.6 are the O2, CO2 and NOx concentrations in the waste gas at different biogas supply rates as a function of excessive air ratio. These data are presented in the row 6, 7 and 8 in Table 4.1 a~g as well. In Fig. 4.4, approximately 27% of CO2 in the biogas gas supplied. The extra CO2 is from combustion. When the combustion is more completed, such around λ=0.9, the generated CO2 can outweigh the dilution effect by other combustion product gases, leading to a peak appearance near stoichiometric condition. In Fig. 4.6, the NOx concentration reaches to peak value in the range around λ=0.9~1.1(near stoichiometric condition) that is coincident with the waste gas temperature of Fig. 4.3, indicating that the main source of NOx is from thermal NOx.
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This study applies the measured O2 data in the waste gas to estimate the corresponding CO2 concentration and the consumed percentage of CH4. They are derived by using Eqs. (3.13) and (3.14). The consumption percentage of CH4 in combustion and the corresponding estimated CO2 concentration are presented in the tenth and eleventh rows of Table 4.1 a~g, respectively. The maximum discrepancy of CO2 concentration between the estimations and the ones measured by the gas analyzer are around 3%, showing that the agreements are quite well.
Figure 4.7 shows the deduced CH4 consumption ratio as a function of excess air ratios. It can be seen that the maximum consumption ratio occurs in the neighborhood of stoichiometric ratio (0.9 <λ< 1.1), then it descends toward the upper and lower flammability limits. The highest CH4 consumption ratio is 96.03% at biogas supply rate of 200L/min with λ=1.1, consistent with highest thermal efficiency (see Fig.4.2).
To sum up, the optimum biogas supply rate to the present engine with 73%
CH4 of biogas seems to be 200 L/min because it has the highest thermal efficiency and the highest CH4 consumption ratio. However, it cannot provide the maximal power output (26.7kW), which occurs at biogas supply rate of 260L/min withλ= 0.84.
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Table 4.2a Power Generation Rates as A Function of Excess Air Ratio at Biogas Volume Flow Rate 140L/min
Table 4.2b Power Generation Rates as A Function of Excess Air Ratio at Biogas Volume Flow Rate 160L/min
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Table 4.2c Power Generation Rates as A Function of Excess Air Ratio at Biogas Volume Flow Rate 180L/min Table 4.2d Power Generation Rates as A Function of Excess Air Ratio at
Biogas Volume Flow Rate 200L/min
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Table 4.2e Power Generation Rates as A Function of Excess Air Ratio at Biogas Volume Flow Rate 220L/min
Biogas supply at 220L/min
Air flow (L/min) 1220 1370 1520 Excess air ratio 0.79 0.89 0.99
Power (kWe) 17.5 21.5 24
Thermal efficiency 0.182 0.223 0.25 Waste gas
Table 4.2f Power Generation Rates as A Function of Excess Air Ratio at Biogas Volume Flow Rate 240L/min
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Table 4.2g Power Generation Rates as A Function of Excess Air Ratio at Biogas Volume Flow Rate 260L/min
Biogas supply at 260L/min
Air flow (L/min) 1440 1520 Excess air ratio 0.79 0.84
Power (kWe) 23.6 26.7
Thermal efficiency 0.208 0.235 Waste gas
temperature (˚C) 533.3 537.5
O2(%) 1.05 0.91
NOx(ppm) 554 907
CO2(%) 14.74 14.65
Estimation values
Used CH4 (%) 74.99 79.85
CO2 (%) 12.5 12.45
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