As mentioned before, the methane concentration of biogas can be affected by the organics concentration in wastewater. In the last year, Lin [3] tested different air-fuel ratio for 30kW generator with 60% methane concentration of biogas in a small swine farm in Miaoli. The results of Lin’s research is given in Table 4.4a~e. In this table, under each fixed biogas flow rate, the results include the power generation, deduced thermal efficiency and waste gas temperature. In this year, the experimental tests used the same engine/generator but with 73% methane concentration of biogas in Taichung.
Therefore, the comparison of using different methane concentrations is made in this section.
The power generation, deduced thermal efficiency and waste gas temperature for CH4 concentrations of 60% and 73% are presented in Figs.
4.8~10, separately, in which the solid lines indicates the present results, whereas the dash line indicates the Lin’s one [3]. Also remind that Lin’s work cannot apply the biogas flow rates of 140 and 160L/min because the CH4
concentration is too low to maintain the combustion in those biogas gas supply rates.
It is known by Eq. (3.5) of Section 3.2 when the methane concentration of biogas becomes higher under the same biogas supply rate, a higher air supply rate is needed in order to maintain a specific excess air ratio. In addition, the maximum allowable total volume flow rate (sum of biogas and air flow rates) into the engine is still retained that is about 1800L/min for the present engine.
These two reasons further narrow down the range of the maximum excess air ratio. Table 4.3 shows the maximum excess air ratio of different biogas supply
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rates with 60% and 73% CH4 of biogas. Take biogas supply rate of 260L/min for example, the maximun λ is 1.01 with 60% methane concentration of biogas, whereas it is 0.84 with 73% methane concentration of biogas. For 200 L/min and 180L/min, the maximum excess air ratios with 73% CH4 of biogas, 1.1 and 1.27, are greater than the ones, 1.09 and 1.13 with 60% CH4 of biogas. This is because an increase of methane concentraion in biogas can produce higher heat energy under the same biogas supply rate with enough air, and then the combustible range can be entended further. As a consequence, the lean misfire limit (maximum excess air ratio) is widen. This improvement can also be seen from Figs.4.8 and 4.9, which will be disscussed later.
Table 4.3 The Maximum Excess Air Ratio of Biogas Flow Rate with 60% and 73% CH4 of Biogas.
Biogas flow rate(L/min) λmax (CH4%)
180 200 220 240 260
λmax (60%) 1.13 1.09 1.11 1.13 1.01
λmax (73%) 1.27 1.1 0.99 0.89 0.84
Figures 4.8 and 4.9 show the respective power generation and thermal efficiency under 60% and 73% methane concentration of biogas.
The results show that when the methane concentration rises from 60% to 73%, the trend of power generation and thermal efficiency change. With 60%
CH4 of biogas, the power generation and efficiency start to decay after λ ≈ 0.95 for 200 and 220L/min. For 180 L/min, it even starts from the very beginning λ=0.78to to decrease. With 73% CH4 of biogas, as mentioned in section 4.1, for the biogas supply rates above 180L/min, the power generation
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and thermal efficiency increase with the increase of excess air ratio. As a result, for 180L/min (Fig. 4.8 and Fig. 4.9), it can be seen clearly that the lean misfire limit is widened from 1.13 to 1.27, when methane concentration of biogas increases from 60% to 73%.
In Figure 4.8, the power generation with 73% CH4 of biogas are higher than the ones with 60% CH4 of biogas, except the region around λ<0.85. However, in Figure 4.9, the thermal efficiency increases with the increasing methane concentration just when the excess air ratio is greater than 0.95 (near stoichiometric condition). In the region of λ>0.95, the thermal efficiencies with 73% CH4 are more than the ones with 60% CH4. The maximum difference can be achieved to about 11% at biogas supply rate of 180L/min.
This improvement enlarges the lean misfire limit, mentioned above. For the mixture on the relatively rich side (λ<0.95), there is no benefit. Moreover, in this region, the thermal efficiencies with 73% CH4 are much less than the ones with 60% CH4, and the maximum difference can be reached to about 7%. This is because incomplete combustion becomes serious in the fuel-rich region [11].
Besides, when the methane concentration increases, or when the excess air ratio decreases, where mixture becomes richer, the flame velocity is relatively faster comparing to lean mixture. It means that the spark timing should be delayed to avoid knocking, but the spark timing is fixed in this experiment. In order to get better performance, the ignition plug should ignite the mixture properly according to the position piston in the cylinder and the fuel/air ratio.
If the ignition time advances too much, leading to engine knocking. If the ignition time delays too much, the combustion pressure acting on the piston will decrease, leading to a loss of efficiency.
It might conclude that the incomplete combustion and improper ignition
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timing make the thermal efficiency not good as expected.
With 60% CH4 of biogas, the maximum power generation and thermal efficiency are 26.8 kW and 0.287 at the same biogas supply rate of 260L/min with λ=1.01. On the other hand, with present 73% CH4 of biogas, as mentioned before in Section 4.1, the maximum power generation and thermal efficiency occur at different biogas supply rates. The maximum power generation is 26.7 at biogas supply rate of 260L/min with λ=0.84, but the maximum thermal efficiency is 0.27 at biogas supply rate of 200L/min with λ=1.1. It indicates that different CH4 concentrations of biogases will change the combustion characteristics for the same engine.
The comparison of waste gas temperature is showed in Figure 4.10. The waste gas temperature with 73% CH4 of biogas is higher than the one with 60% CH4, because the higher heat energy are produced with higher CH4
concentration of biogas at the same biogas supply rate. So the waste gas temperature becomes higher. However, the increase of temperature with higher methane concentration does not mean the power generation and thermal efficiency increase, since the energy might be lost due to incomplete combustion and improper ignition timing.
To sum up, the rise of methane concentration can increase the power generation and thermal efficiency in the lean region. However, the the excess air ratio at higher biogas flow rate is restricted by allowable total volume of engine. Based on this reason, in order to get better engine performance, solving the problem of the limit excess air ratio for high biogas supply rate is needed.
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Table 4.4a Power Generation Rates as A Function of Excess Air Ratio at Biogas Volume Flow Rate 180L/min with 60% CH4 [3]
Biogas supply at 180L/min
Table 4.4b Power Generation Rates as A Function of Excess Air Ratio at Biogas Volume Flow Rate 200L/min with 60% CH4 [3]
Biogas supply at 200L/min
Table 4.4c Power Generation Rates as A Function of Excess Air Ratio at Biogas Volume Flow Rate 220L/min with 60% CH4 [3]
Biogas supply at 220L/min
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Table 4.4d Power Generation Rates as A Function of Excess Air Ratio at Biogas Volume Flow Rate 240L/min with 60% CH4 [3]
Biogas supply at 240L/min
Table 4.4e Power Generation Rates as A Function of Excess Air Ratio at Biogas Volume Flow Rate 260L/min with 60% CH4 [3]
Biogas supply at 260L/min
In this research, the waste heat is used to preheat the inlet gas (biogas and air), but not the water as done by Lin [3]. The results are shown in Figs.
4.11~16 and Table 4.5 a~c. The inlet gas are mixed in the heat exchanger and preheated to 80 and 120℃, while the inlet gas temperature without heating is around 40℃. The high pipe temperature of operating engine makes the inlet gas is heated to 40℃, even it does not pass through the heat exchanger. The volume of inlet gas expands with raised temperature, leading to a decrease in the aspiration ability of engine. At 120℃, the maximum allowable inlet gas volume starts to decrease from 1800L/min to about 1700L/min, leading to a