Chapter 3 Experimental Apparatus and Procedures
3.4 Oxygen-Enriched Combustion
Biogas contains 20~30% carbon dioxide, which can feed algae in advance in other subproject. After feeding the algae, there is a little pure oxygen left in the biogas so that it can make an oxygen-enriched burning in the combustion chamber.
The extra oxygen mixes with fuel and the mixture flows into the engine.
After the engine is operated steadily, collect the data include biogas flow rate, air flow rate, power output, waste gas temperature, carbon monoxide concentration, oxygen concentration and carbon dioxide concentration.
Find the new air-fuel ratio. Then, calculate the engine thermal efficiency.
The experimental parameters are the biogas flow rates at 220L/min, 240L/min and 260L/min, respectively, and each is with five different excess air ratios.
34
Chapter 4
Results and Discussion
The biogas was from the anaerobic tank made of a red plastic bag. The original biogas from the tank contains high concentration of H2S, around 3000ppm. Such high concentration H2S will corrode the engine, so a H2S removal system developed by Tseng and his colleagues [15] was installed such that it could effectively reduce H2S concentration from 3000ppm to 300ppm. After the reduction of H2S concentration, the biogas was measured by using Cosmos XP-3140 (the high concentration methane and the carbon dioxide analyzers), and found it containing 59%~62% of CH4 and 20%~30% of CO2. This work regarded the treated bigas after H2S removal as consisting of 60% of CH4 and 40% of CO2, by which all calculations were based on this base line. The engine operated with the biogas as fuel and generated electric power for farm using.
4.1 Effect of Excess Air Ratio (λ)
In this section, the effects of fuel supply rate together with corresponding different excess air ratio on power generation are studied.
The fuel (biogas) supply rates tested were 180, 200, 220, 240, and 260 liters per minute, respectively.Under each fixed biogas flow rate, it tests five different excess air ratios, ranged from 0.8 to 1.2. The excess air ratio (λ) was defined in Eq. (3.5) of section 3.2. Note that the mole of fuel counted in this study was the one of methane but not the one of biogas itself. The reason is that the CO2 existed in the biogas did not particate the combustion. Another important issue is that because the maximum allowable total volume flow rate (sum of biogas and air flow rates) into
35
the engine is about 1800 L/min, the maximum air supply rate is limited by the fixed biogas one that can be observed in the last columns of Table 4.1a~e. In other words, the experiment with the higher biogas flow rate carrys out a narrower range of air flow supply rate.
4.1.1 Power Generation
Figure 4.1 shows the power generation rates as a function of excess ratio ratio at different biogas supplies. The detailed experimental values are given in the third row in Table 4.1a~e.
It can be seen from this figure that under the constraints of the engine, the higher the fuel supply rate, the higher the power generation at a given excess air ratio. The maximum power output is 26.8 kW at biogas supply of 260 L/min with an excess air ratio of 1.01 (nearly stoichiometric), equivalent to 1500 L/min of air supply. For the biogas supply rates above 240 L/min, the power generation increases with the increase of excess air ratio, whereas for those of 200 and 220 L/min the power generation starts to drop after λ around 0.95 (approximately the stoichiometric ratio), which is in fuel-rich range (λ is the reciprocal of equivalent ratio). For the case of 180 L/min, it even decend from very beginning λ =0.78. From the power point of view, the three fuel supply rates, shuch as 180, 200 and 220 L/min, cannot produce the ecectricity more than 20kWe that may not be appropriate in real application.
According to these data, it might conclude that the optimumal biogas flow rate to the present engine is around 240 to 260 L/min because the corresponding thermal efficiency (shown in next figure) needs to be considered as well.
36
Table 4.1a Power Generation Rates as A Function of Excess Air Ratio at Biogas Volume Flow rate 180L/min
Biogas supply at 180L/min
Air flow (L/min) 800 910 960 1030 1160
Excess air ratio 0.78 0.89 0.93 1 1.13
Power (kWe) 13.6 13.5 13.1 11.9 8.5
Thermal Efficiency 0.211 0.209 0.203 0.184 0.132
Waste GasTemperature (°C) 459 460 465 466 450
Table 4.1b Power Generation Rates as A Function of Excess Air Ratio at Biogas Volume Flow rate 200L/min
Biogas supply at 200L/min
Air flow (L/min) 910 1000 1080 1150 1250
Excess air ratio 0.8 0.875 0.943 1.01 1.09
Power (kWe) 16.2 16.6 16.5 15.3 13.8
Thermal Efficiency 0.226 0.231 0.230 0.213 0.192
Waste gas temperatre (°C) 474 473 474 476 472
O2(%) 0.7 2.1 2.6 3.6 4.1
37
Table 4.1c 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) 1000 1100 1185 1250 1400
Excess air ratio 0.8 0.88 0.94 0.99 1.11
Power (kWe) 18.9 19.2 19.1 18.5 15.5
Thermal Efficiency 0.240 0.243 0.242 0.234 0.196
Waste GasTemperature (°C) 491 481 480 482 480
O2(%) 0.4 1.9 2.7 3.3 5.5
Table 4.1d Power Generation Rates as A Function of Excess Air Ratio at Biogas Volume Flow rate 240L/min
Biogas supply at 240L/min
Air flow (L/min) 1050 1100 1250 1400 1550
Excess air ratio 0.77 0.8 0.91 1.02 1.13
Power (kWe) 22.5 22.8 23.5 24.0 24.4
Thermal Efficiency 0.261 0.265 0.273 0.279 0.283
Waste GasTemperature (°C) 517 513 506 506 506
O2(%) 0.4 0.6 2.1 2.5 3.1
38
Table 4.1e 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) 1200 1250 1300 1400 1500
Excess air ratio 0.81 0.84 0.88 0.94 1.01
Power (kWe) 25.4 25.6 26.1 26.5 26.8
Thermal Efficiency 0.272 0.275 0.280 0.284 0.287
Waste GasTemperature (°C) 533 533 531 528 522
O2(%) 0 0.2 0.5 0.65 1
The thermal efficiency is calculated by Eq. (3.7) given in section 3.2.
It is deduced from the related measurements, such as power generation rate and methane volume flow rate. Knowing the thermal efficiency, it can estimate the power generation from the biogas quantity, which can also be estimated from swine farm population. The economic benefits for the swine farm after generator installed, such as elecrictity generation and CO2 reduction, can also be deduced.
Figure 4.2 shows the thermal efficiency at different biogas supplies as a function of λ. The values can be seen in the fourth row of Table 4.1a~e.
In general, the trends for both Figs. 4.1 and 4.2 are quite similar because for each fuel flow rate the thermal efficiency is directly proportional to power generation (see Eq. (3.7) in section 3.2.). Note that the thermal
39
efficiency curve of 240 L/min is very close to that of 260 L/min. However, the operation range (0.77 < λ < 1.13) of 240 L/min is wider comparing to the one of 260 L/min (0.81 < λ < 1.01).
The maximum thermal efficiency now can be reached by this engine is 28.7% at the maximum fuel supply rate of 260 L/min. It is lower than the ones (30~40%) for commerical biogas engines [18]. Apparently, the present engine needs more adjustments or improvements to increase its thermal efficiency. Therefore, an improvement is carried out in a later section.
Figure 4.3 shows the waste gas temperautres for 5 different fuel supply rates after the engine exaust as a function of excess air ratio. Similar to the power generation and thermal efficiency, the exhaust temeprature is higher as the biogas supply rate is higher at a specified excess air ratio.
For 240 and 260 L/min, the waste gas temperature decreases with an increase of λ that is unexpected intitutely. However, the maximum temperature differences are only 11˚C between λ= 0.77 and = 1.13 for 240 L/min and λ= 0.81 and = 1.01 for 260 L/min, respectively. If considering the total enthalpy of the gas flow (~m• CPΔT), then the one with larger λ is greater. Apparently, it is resulted from the dilution effect by the larger amout of gas due to higher air supply rate. For the cases of 180 and 200 L/min, their trends agree with that the maximum temperature occurs around stoichiometric point and it is decendent toward the fuel-rich (λ < 1) and fuel-lean (λ > 1) regions. The behavior of 220 L/min is somewhat in between of 240 and 200 L/min.
Figures 4.4 and 4.5 are the O2 and CO concentrations in the waste gas
40
at different fuel supply rates, and Fig. 4.6 is the CO2 one, deduced directly from the previous two measuements inside the analyzer. These data are also presented in the row 6 to row 8 in Table 4.1a~e. Because they are expressed by in percentage values but not absolute ones, this study applies the measured O2 data in the waste gas by neglecting CO concentration (in ppm) to estimate the consumed percentage of CH4 in the combustion process. The procedure is given as follows:
The balanced reaction is:
CH 0.67CO 2λ O 3.76N O CH 2 2 H O
1.67 CO 7.52λN (4.1) where a is the moles of O2 and b is the moles of CH4 in waste gas. a can be calculated from the percent of O2 in waste gas as follow:
mole fraction % of O . AF λ (4.2)
where 1.67+AF λ is the total moles in waste gas. b is obtained from the atom balance as:
1 λ (4.3)
The percent of CO2 in waste gas can also be calculated by:
mole fraction % of CO . .AF λ (4.4)
The percent of used CH4 is defined as:
41
Percentage of consumed CH % CH CH CH (4.5)
The estimated vaules of consumption percentage of CH4 in combustion reaction are presented in the tenth row, and followed by the corresponding estimated CO2 mole fractions (11th row), served as comparison with the ones calculated from analyzer, in the same table. It can be seen that the differences of CO2 mole fraction between the estimations and the ones calaulated from the analyzer are around 5%, which should be acceptable for experimental works.
Figure 4.7 shows the estimated CH4 consumption ratio as a function excess air ratios. It can be seen that the consumption ratio increases with an increase of fuel supply rate, which is consistent with the trends of power generation and thermal efficiency in Figs. 4.1 and 4.2, except for the last points for 180 and 220L/min. It indicates that a higher power generation and better thermal efficiency need to be accomplished by a greater conversion of CH4 in the biogas. As to the deviation for the two points, it is because that for normal operation, the lower limit of biogas supply rate into the engine is around 220~240L/min, whereas the engine is about to shut down at excess air ratio over 1.1 when biogas supply is below 220L/min, resuting in the abnormal behaviors.
To sum up the results of this section, for biogas supply rates of 240 and 260L/min, the power generated increases with the excess air ratio (λ) until the limiting ones at 1.13 for 240L/min and 1.01 for 260L/min are reached. Because of the limit of engine volume, it is impossible to know the critical point that the performance starts to drop. For biogas
42
supply rates below 220L/min, there is not enough biogas supply for engine to operate. It is a dilemma that decease the biogas supply rate for the higher limit of excess air ratio, but the amount of biogas is not enough for burning, or incease the biogas supply rate for engine normal operation, but the excess air ratio is limited. To slove this problem, this study proposes two suggestions. First, redesign the engine to increase the volume limit of gas into the engine. Because the oringinal engine is designed for deisel as fuel, the capacity is not suitable enough for lower low-heating-value fuel, such as biogas. Second, use the biogas with higher concentration of methane. With higher concentration methane in biogas, the lower fuel supply rate may also fit the engine needs, so the limit of the air flow rate can be increased. Of course, the higher excess air ratio by adding pure oxygen can also be reached by this way.
4.2 Oxygen-Enriched Combustion
As the biogas passing through the algae, which the system was developed by Lin and his colleagues [16], it could obtain 1 ~ 3% of O2
due to photosynthesis reaction. Using such oxygen mixed with biogas for the engine could have a better thermal efficiency. Therefore, the effect of oxygen-enriched combustion for engine was tested that the parameters were 1% and 3% oxygen-enriched airs.
The stoichiometric air-fuel ratio is different from normal one. For example, with 3% of O2 addition, the oxidizer (air) contains 24% O2 and 76% N2, so the methane chemical balance equation changes into
CH 2 O 3.17N CO 2H O 6.34N (4.6)
43
The stoichiometric air-fuel ratio becomes:
AF 8.34 (4.7)
It is less than the original value, 9.52, obtained from Eq. (3.2), meaning that the combustion needs less air because of the increase of O2
concentration. Knowing the stoichiometric air-fuel ratio, the used percentage of CH4 can be calculated by Eq. (4.2) and (4.3)
The experimental results of 1% oxygen-enriched air are shown in Figs.
4.8 to 4.10. The solid lines are the data with oxygen-enriched air, whereas the dotted line is the original one. From these figures, they clearly demonstrates that the resultant performances, such as power generation, thermal efficiency, exhaust temperature and percentage of CH4 used, by 1% oxygen-enriched air are not improved much, sometimes even becomes a litter worse that might be due to the experimental error.
However, the performances by 3% oxygen-enriched air show the quite different ones. Figures 4.11 to 4.13 illustrate the comparisons, and Table 4.2 summarizes the detailed tested data by using 3% oxygen-enriched air.
44
Table 4.2 Power Generation Rates as A Function of Excess Air Ratio with 3% Oxygen Addition at Biogas Volume Flow rate 220~260L/min
Biogas supply at 220L/min
Excess air ratio 0.9 1.01 1.03 1.09 1.17
Power generation(kW) 22.6 23.2 23.3 23.6 23.9 Thermal efficiency 0.286 0.294 0.295 0.299 0.302
Biogas supply at 240L/min
Excess air ratio 0.80 0.91 1.04 1.10 1.15
Power generation(kW) 24.7 24.9 25.5 25.5 25.8 Thermal efficiency 0.287 0.289 0.296 0.296 0.300
Biogas supply at 260L/min
Excess air ratio 0.84 0.90 1.00 1.06 1.10
Power generation(kW) 26.6 27.5 28 28.1 28.2 Thermal efficiency 0.285 0.295 0.300 0.301 0.302
The power generation and thermal efficiency for 240 to 260 L/min biogas supplies in Figs. 4.11 and 4.12 are slightly increased. The maximum power generation increase 1.3kW and 1.4kW for 240 and 260 L/min biogas supplies, respectively. The maximum thermal efficiency and the percentage of consumed CH4 also increased up to 30.2% and 100% approximately, respectively at λ = 1.1 for 260 L/min of fuel supply rate. Apparently, the increase of performance is insignificant for these two fuel supply rates.
On the other hands, the performance curves, such as power generation, thermal efficiency, and percentage of CH4 used, with 3%
oxygen-enriched air for 220 L/min of biogas supply are complete different from the original ones. As shown in Figs. 4.11 to 4.13, these performance curves increase with an increase of excess air ratio from 0.9
45
to 1.17, rather than drop from the very beginning point (0.8) without oxygen addition. In other words, the characteristiocs of the original lower fuel supply rate, such as 220 L/min, change dramatically by 3%
oxygen-enriched air. Apparently, with such extra pure oxygen addition from algae photosynthesis process into biogas, the engine can operate at a lower limiting fuel supply rate, such as 220 L/min, what was not allowed previously (see Figs. 4.1, 4.2 and 4.7).
4.3 Waste Heat Recovery
In this section, heat exchanger was installed to recover waste heat from the engine exhaust gas to increase the use of the energy. The detailed information for experimental layout, procedure and derivations of heat exchanger effectiveness and overall efficiency are given in section 3.3.
However, only one experiment was carried out to make an illustration.
The corresponding data are given as follows (Table 4.3).
Table 4.3 Waste Heat Recovery Data Waste Heat Recovery Temperature Data
Temperature at inlet (℃) Temperature at outlet (℃)
Water 22 45 Gas 502 120
Waste Heat Recovery Flow Rate Data
Water Flow Rate 9.6L/min
Biogas Flow Rate 240L/min
Air Flow Rate 1550L/min
Waste Heat Recovery Data
Heat Recovery 923kJ/min
Heat Exchanger
Effectiveness 94%
Overall Efficiency 47.3%
46
It is found experimentally that the designed heat exchanger can recover 923kJ/min of heat with a 94% of heat exchanger effectiveness, leading to an overall efficiency of 47.3%. The utilization of energy is obviously increased by 18.6% with an installation of waste heat recovery system.
The exergy (availability) from waste gas is 470.6kJ calculated by Eq.
(3.11) and the gained exergy from water is 34.6kJ that the second law efficiency of the heat exchanger is 7.4%.
With the waste heat recovery system installed for reuse of waste heat, the energy utilization can be increased. The energy drawn from waste gas, around 500°C, now can be reused to produce hot water rather than just emitted into surroundings.
4.4 Economic Benefits
In this section, the economic benefits are estimated by the data obtained by this research. With this estimation, we can have a vision to know whether it is worth to build a biogas power generation plant for a swine farm, how much clean energy it can produce, and how much economic benefits it can bring.
From the study of Tseng and his colleagues [15], the average biogas produced is around 0.1 m3 per head pig per day. The resultant erergy is 1.7 kWeh per m3 biogas according to the obtained data of section 4.1.
Based on the current tested data , the economy benefits in a scale of 1000 swine farm can be estimated and summarized in the following table.
Table 4.4 Economy Benefits for 1000 Scale Swine Farm per year
47
Biogas Production 36500 m3
Electricity Generated 62050 kWh
Electricity Charge Saved 186150 NT$
CO2 Emission Reduction 39.5 ton
The electric energy can reach about 62000kWh per year, and it can save 186000NT$ of electricity charge per year, providing that the present electricity purchase charge is 3NT$ per kWeh).
The carbon dioxide emission coefficient, from Taiwan Power Company, shows how many kilogram carbon dioxide produced per kWh electricity produced from power plant. According to the data of 2008, the carbon dioxide emission coefficient is 0.636 kg per kWhe. Bioengery is a kind of green energies because its carbon source is from the carbon dioxide in the air by photosynthesis, so the net carbon dioxide emission is zero in a cycle. The utilization of bioenergy from biogas can reduce the power generation from fossil fuel and also reduce the carbon dioxide emssion.
As a consequence, each kWhe elecricity generated by biogas can reduce 0.636 kg carbon dioxide emission in Taiwan. Therefore, the carbon dioxide reduction is around 40 tons per year for a 1000 scale swine farm.
If the heated water from waste heat recovery system is properly utilized, such as heating the anaerobic digester, it can save the heating energy from elecricity or nature gas.
The heating value from heat recovery can reduce the amount of nature gas used with same heating value. According to the data from CPC, the price of nature gas has been increased 86.3% within decade, from 8.05 NT$/m3 at 2001 to 15 NT$/m3 at 2010. With this trend, the waste heat recovery is expected to become more and more important in the near future. For the 1000-swine farm, the estimated energy recovery is around
48
1.5 108 kJ per year from Table 3.3. Converting the heating value into nature gas charge, it is around 60,000 NT$/ year.
Table 3.5 shows the statistics on swine farms in Taiwan from Council of Agriculture. In this table, the swine number in the farm scale over 1000 heads, which have economic potential for installing biogas electric generators, is around 4.3 million heads, which is about 66% of total swine population in Taiwan. Based on the data in this study, the overall economic benefits in Taiwan from biogas for the swine farms over 1000 pigs can be estimated as following:
z Electricity generation: 2.67 108 kWeh per year z Electricity charge saved: 800 million NT$ per year z Nature gas charge saved: 260 million NT$ per year z Carbon dioxide reduction: 170 thousand ton per year
Table 4.5 Statistics on Swine Farms in Taiwan
Swine farm scale (head) Numbers of swine farm Head on farm
1 – 19 2580 (22.89%) 18,563 (0.28%)
Total 11,271 (100.00%) 6,515,792 (100.00%)
49
Comparing the above estimated data with those of Tsai [1], the amount of electricity generated in this study is 3.7 times greater. This discrepancy mainly comes from two reasons. First, the amount of biogas product is different. Tsai [1] used the IPCC recommended coefficient for methane generation, which is 5kg head-1 year-1. In this study, the amount of biogas product was measured as 0.1m3 head-1 day-1, equivalent to about the methane generation of 15.33kg head-1 year-1. Second, Tsai [1] assumed the thermal efficiency of engine was 25%, but it was measured as 28.7%
in this study.
50
Chapter 5
Conclusions and Recommendations
5.1 Conclusions
This study carries out the 30kW-generator experiments on a small biogas plant in a swine farm to collect data to serve as a preliminary study for constructing a 300-KW power plant of a bigger scale biogas plant in the near future. It is divided into three parts. Firstly, the effects of fuel
This study carries out the 30kW-generator experiments on a small biogas plant in a swine farm to collect data to serve as a preliminary study for constructing a 300-KW power plant of a bigger scale biogas plant in the near future. It is divided into three parts. Firstly, the effects of fuel