Chapter 3 Experimental Apparatus and Procedures
3.3 The Effect of varying loads
(3.7)
The actual power generation of this study is the net output of turbine generator.The energy input is calculated from the lower heating value (LHV) of methane, whose value is 50020kJ/kg, in the biogas. It is expressed as:
of CH4 (3.8)
Where is the methane mass flow rate in biogas, and it is calculated by:
(3.9)
where is the density of methane
3.3 The Effect of Varying loads
With the increase in operating load, engine speed is also rise. To study
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the operating load and the thermal efficiency of gas turbine engine, the design rated powers are set from 15 to 30kW, and the increment of power output is assigned as 1 kW for each test under the same environmental conditions. Variation of operating load will make impact on the generator performance. When the output power is adjusted to the 30kW, then the engine speed reaches 96,000 rpm. At a higher operating load, the heat energy is used fully, resulting in higher thermal efficiency [16]. As mentioned above, this study used the hydraulic free piston engine, which is different from the turbine engine.
One of the experimental parameters is methane concentration of biogas.
Before experiment, the methane concentration of biogas is measured. The collected data in experiment include compositions of biogas, engine speed, air flow rate, biogas flow rate, air-fuel ratio and actual power generation. The measurements start as the engine is operating continuously until all conditions are ensured to be steady. Then, all measurements are taken twice and make an average. The experimental procedure is as follows:
1. Measure the methane concentration of biogas.
2. Operate the engine at least 10 minutes so it would be steady.
3. Collect all of the measured data.
4. Adjust the output power at demanded quantity.
5. Repeat the procedure for different output power.
6. Repeat the above procedure for different methane concentration.
3.4 Uncertainty Analysis
The accuracy of the experiment data should be confirmed before
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the analyses of experimental results are carried out because the exactitude of the data may not be very good. Error analysis is a procedure used to quantify data validity and accuracy. Experimental measuring results are always in errors. Experimental errors can be classified into fixed error and random error respectively. Fixed error is the same for each reading and can be removed by proper calibration and correction. Random error is different for every reading and hence cannot be removed. The objective of uncertainty analysis is to estimate the probable random error in experimental results.
3.4.1 Uncertainty Analysis of Volume flow Meter
The apparatuses must be corrected by other standard instrument to make sure that they can normally operate and let the inaccuracy of the experimental results reduce to minimum. In this study, the major sensor in the experiment was the volume flow meter. The measurement range of P-050 Flow Meter adopted in this study was 45~450L/min±5%.
3.4.2 The Experimental Repeatability
To verify experimental accuracy, perform one test under the loads varying from 15 to 30 kW two times to ensure experimental repeatability.
The evaluation included three measurements for volume flow rate.
Standard deviation is defined as the absolute difference among the three volume flow rates. Table 3.4, 3.5 and Fig. 3.12, 3.13 show the coefficients of variation (CV) and experimental error bars. The CV is defined as the ratio of standard deviation S to mean , where S is derived by S=
The CV is a dimensionless number that can be used to specify the
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variation of data points in a data series around the mean. As the experiments were conducted outdoors, environmental conditions were difficult to control, for safety reason. As a consequence, the errors (＜4) in these experiments were expected to be higher than general experiment errors, but they should be acceptable.
Table 3.4 Experimental Repeatability for Thermal Efficiency for Piston Engine
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Table 3.5 Experimental Repeatability for Thermal Efficiency for Turbine Engine
Power generation(kW) Biogas Flow
Rate(L/min) CV(%)
14.83 184.9 3.41
15.89 189.7 3.22
16.94 193.2 3.42
18.05 200.1 3.82
18.97 207.0 3.61
19.92 213.9 2.63
20.89 220.8 3.01
21.98 227.7 2.22
22.91 234.6 2.13
24.04 241.5 1.81
24.84 248.4 2.42
25.17 248.4 2.14
25.23 251.8 2.10
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Chapter 4
Results and Discussion
The experimental study, a continuous effort of Wu [6], is carried out with a 30kW gas turbine, whose operation, as mentioned previously, is quite different from the 30kW piston engine used previously by Lin [4], Huang [5] and Wu [6]. Both engines operate in different way because of the inherent design pholosophies.
4.1 Power Generation by Turbine Engine
The biogas used in this research is supplied from the anaerobic tank made of red plastic bag. The original biogas from the tank contains high concentration of H2S, around 5000ppm. H2S would corrode the engine severely without proper treatment. Therefore, an H2S removal system is built up by using biological process, which is favorable to environment protection and cost friendly. The removal rate of screened microorganism could remove at most 99% of H2S from the biogas. In other words, the H2S concentration in the biogas is effectively reduced from 5000ppm to 50ppm.
The desulfurized biogas passes a methane concentration analyzer and gas analyzer, which can measure the concentrations of oxygen, carbon dioxide and NOx. The resultant measurements indicates that the biogas comprises 67% of CH4, 9.2% of CO2 and 5.6% of O2. It is unlikely that the biogas contains O2 after the anaerobic process, indicating an existense of air leakage from atmosphere to storage tank. According to the concentration of O2 in the biogas, the corresponding concentration of Air is deduced as 19.38%. Besides, the content of water vapor in biogas is
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1.6% at 30°C derived from Eq. (3.15) in Section 3.2.4, while the relative humidity of biogas is considered to be 100%. These data are summarized in Table 4.1.
Table 4.1 Compositions of Biogas flown into Turbine Engine
CH4 CO2 Air H2O Residues
67% 9.6% 19.38% 1.61% 2.41%
Table 4.2 shows the measured and derived data as a function of specific power demand of CR30 gas turbine. The rated power output, actual power generation and engie rotational speed are directly read from the engine itself. Biogas flow rate is obtained from measurement and both CH4 consmption rate and thermal efficiency are derived from the reading data. It can be seen that the the rated power output is greater than actual power generation because part of the turbine work generated is needed to drive the compressor. The corresponding data are illustrated in Fig. 1.
The maximum power output is only 25.23kW under the rated power output of 30kW, since the rotational speeds behind 25kW can no longer increase propotionally but approach a limit value, 96000rpm approximately, as shown in Fig. 4.2. Therefore, it may conclude that the upper operation limit for this type engine is 25kW. On the other hand, the lower limit is 15kW, below which the engine cannot be drived.
As expected, the required biogas gas flow rate increases with an increase of power generation. As shown in Fig. 4.3, the their relatioship appears as a nearly postive linearity as the power output less than 25kW.
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The CH4 consumption rate is calculated directly by multiplying the biogass flow rate with 67% given in Table 4.1 under an assumption of fuel-lean combustion. There are two main reasons for turbine engine not able to calculate the consumption ratio of CH4 as that for piston engine used by Lin [4], Huang [5] and Wu [6]. First, the air flow rates, measured by the flow meter at the engine inlet, are listed in Table 4.2. It is obvious that the amount of inlet air of turbine engine is much much larger than the one for piston engine given in the Table 4.4. It is because that most of inlet air is used to cool the turnine liner and downstream combustion product gas to avoid the heat damage to the turbine blades, and only small portion, whose quantity cannot be justified, is used for combustion.
Therefore, it is impossible to evaluate Figure 4.4 shows a schematic diagram of a turbine combustion chamber with its main components, burning and cooling zones. Furthermore, the concentration of CH4 in flue gas is diluted so greatly by the secondary cooling air, illustrated in Fig.
4.3, that it is not expected to be measurable exactly. In the meantime, the percentage of O2 consumed in combustion cannot be measured either, because the inlet is not known in advance. As consequence, the real consumption ratio of CH4 cannot be calculated for turbine engine.
The calculation of thermal efficiency in Table 4.2 is obtained by Eqs.
3.7, 3.8 and 3.9 of Sec. 3.2.2. As mentioned previously, the input energy is assumed to be complete combustion of CH4 in the biogas. It is found that the thermal efficiency increases with an increase of power generation.
Figure 4.3 shows that the relationship is almost linear between 17 and 25kW of power generation.
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Table 4.2 The measured and derived data as a function of specific rated power of CR30 gas turbine at 30°C
The rated from the work of Wu [6]. Table 4.3 shows the details of compositions of biogas which is used by Wu [6]. With 69% CH4 of biogas, the maximum power output for piston engine is 26.5kW and the corresponding biogas supply is 225 L/min. The minimum power output is 5.4kW at biogas supply of 181 L/min. Apparently, the power generation range is wider for the 30kW piston engine, which can be seen in Fig. 4.5 as well (The effective range is from 15 to 25kW for turbine engine).
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Table 4.3 Compositions of Biogas flown into Piston Engine; Wu [6]
CH4 CO2 Air H2O Residues
69% 13.3% 12.38% 1.99% 3.33%
The corresponding experimental results are given in next table.
Table 4.4 The experimental data of Piston Engine under various output power with 69% CH4; Wu [6]
Figure 4.3 shows the biogas volume flow rate under different output powers for these two engines. Remind that for piston engine, the obtained power generation is resulted from a specific biogas flow rate, whereas for piston engine, the biogas flow rate is obtained under a specific rated
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power. It can be seen that the power generation increases as the biogas flow rate increases for piston engine.
The detailed performance comparions for both engines are summarized in Table 5, which is illustrated graphically in Fig. 5.
Table 4.5 Thermal Efficiencies and CH4 Consumption Rates of Turbine and Piston Engines under Various Output Power
Piston Engine Turbine Engine for both turbine and piston engines as a function of load. It indicates that for turbine engine, the thermal efficiency increases with increasing power generation and reaches its maximum value around 23.39%. For the piston
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engine [6], the maximum thermal efficiency can be reached is 26.37% at the CH4 consumption rate of 155.2 L/min. The maximum efficency of turbine engine is 2.97% lower. For gas turbine, most of the air is used for cooling, the hot gas exhausted from combustor outlet is cooled down by jet flow discharged from dilution holes, which prevent turbine blade from heat damage [21]. Because of lots of the air is used for cooling the exhaust from the combustor outlet, the partial energy is absorbed by air flow during cooling process. The result leads to lower thermal efficiency for turbine comparing to that for the piston engine under the higher load operating range (21 to 30kW). However, it can be seen from this figure that under the low load operating condition, the turbine engine can provide higher performance with lower efficiency variation compared to those of piston engine. It is because that under the lower loading, the fuel supply rate becomes less that leads to a lower volumetric efficiency with fixed cylinder volume. Furthermore, the lower volumetric efficiency will result in an decrease of in-cylinder pressure and make the mixture to become not richer. Therefore, the combustion efficiency is lower than in the high load operating condition. To sum up, the results show that the operation of turbine engine is more stable than that of piston one. Besides, the lower load limit is 15kW for turbine engine, whereas piston engine still can be operated as low as 5.4kW. Table 4.2 also reveals that the thermal efficiency of turbine engine increases negligibly when the power demand changes from 25 to 30kW, revealing a consistent trend with the engine speed. Therefore, the halt to the thermal efficiency indicates that the engine has reached its threshold speed.
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4.2.1 Waste Gas Analysis
The waste gas concentrations, including O2, NOx and CO2, the data are presented in Table 4.6.
Table 4.6 The Measurements of the Waste Gas Constitutes and their Concentrations for Turbine in the waste gas than the others. Apparently, under the higher loading that the combustion becomes more completed, so the exhaust gas will contain lower levels of O2.
For gas turbine, most of the air is used for cooling (Approximately 80 percent) , mention above, It is because that the waste gas contains large amounts of air, the O2 concentration in waste gas is too large.
BACHARACH ECA 450, used for measuring waste gas component data, which include the concentrations of oxygen, NOx and carbon dioxide, in order To calculate NOx, it referenced to a user defined Oxygen level of between 0 and 15%, however, we measure the concentrations of oxygen in the waste gas exceeds 15% that the analyzer can not calculate the concentrations of NOx.
This study applies the measured O2 data in the waste gas to estimate the
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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. (4.5). The percentages of CO2 in waste gas in the combustion process are calculated as follows:
The balanced reaction is: regarded as complete combustion that b equals to zero.
From the collected data (Table 4.1), the reaction for combustion of
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(4.4)
The percent of CO2 in waste gas can be calculated by:
(4.5)
To take two examples from the Table 4.6, the reaction becomes:
16.2% O2 in waste gas (25.23kW)
(4.6)
The percent of CO2 in waste gas is 2.4%
16.9% O2 in waste gas (14.83kW)
(4.7)
The percent of CO2 in waste gas is 2.1%
Apparently, the higher load operating provides higher CO2
concentration than the lower ones. The reason is the same as before that the combustion becomes more completed, therefore, more CO2 is generated.
For pistone engine, the waste gas concentrations, including O2, NOx
and CO2, are shown in Table 4.7.
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Table 4.7 The Measurements of the Waste Gas Constitutes and their Concentrations for Piston Engine; Wu [6]
Actual Power generation
(kW) 14.97 18.12 20.90 23.94 25.75
Excess air ratio 0.944 0.940 1.089 1.197 1.190 Waste gas temperature (°C) 498.3 502.4 504 506 506.5
O2 (%) 2.3 2.1 3.7 5.6 5.2
NOx (ppm) 586 860 1045 1479 1945
Estimation values
CO2 (%) 13.5 13.6 11.2 9.2 9.4
It can be found that the concentrations of oxygen and carbon as a function of excess air ratio. CO2 and O2 concentration decreases with excess air ratio increases. It also can be seen that lower concentration of carbon in waste gas for turbine comparing to that for the piston engine, it is because that the exhaust gas contain large amount of air for turbine, therefore, the ratio of carbon dioxide emissions is relatively lower in comparison to piston engine.
4.3 Theoretical Thermal Efficiency for CR30 Micro Turbine
A picture of the experimental measurements of the temperature data is depicted in Fig. 4.6. η is the isentropic efficiency of the compressor, and η . It is expressed as:
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η Theoretical thermal efficiency Estimation values η c Isentropic efficiency of the
T1 Compressor inlet temperature 303 K
P1 Compressor inlet pressure 14.7 psi
P2 Compressor outlet pressure 70 psi
T3 Heat exchanger outlet temperature 711 K
T4 Turbine inlet temperature 1182 K
T5 Turbine outlet temperature 828 K
T6 Exhaust gas temperature 576 K
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Theoretical thermal efficiency is the ratio between the useful output of a device and the input, its formulation is as following :
(4.11) As shown in Figure 4.6:
= ＋ (4.12) 0.1743 kg/
Air 16.125 kg/
= × (4.13)
Cpair =1.011 kJ/ kg
= 2.304 kJ/ kg
＋
＋
= T1× =472k η c
=511.6k h4 h3= (T4 T3)
WC ×Cpair WT ×
η
0.306
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4.4 Energy Analysis for
CR30 Micro TurbineThe engine’s power output comes from the chemical energy released by combustion, but not all the resultant energy is used for turbine output.
A partial energy loss is due to waste heat and friction. The mechanical work generated by the turbine blades has four different paths. The first one is the power output that is also the actual output of the engine. The second path is the loss of heat transfer, and the third path is a friction loss, including nozzle loss and vane loss. Finally, the fourth path is for the attachments horsepower, including a compressor and dryer. 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 0.85~0.90. In other words, about 10~15% of energy is lost by the form of heat during combustion process.It shall comcentrate on the remaining energy. Figure 4.7 indicates that about 60% of energy is lost in the form of heat, 15% of
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energy is lost by the friction, and only about 25% of energy is is usedful.
4.5 Economic Benefits
In this section, the cost analysis is carried out for the piston and turbine engines. To evaluate the cost effectiveness of these two engines in order to know whose economic benefit is higher. In this part, the additional revenues are calculated as a result of additional electricity production in kWh. We can have a vision to know whether it is worth to replace piston engine by turbine engine in a swine farm.
The Benefit due to additional electricity generation in term of NT$
can be calculated as follows:
Benefit = Δ W(kWh)×2.7NT$ per kWh
From the study of Lin [4], the average biogas produced is around 0.078 per head pig per day, and the resultant erergy by using piston engine is 1.7 kWh per biogas. For turbine engine, the resultant energy is 1.55kWh per biogas according to the data in Table 4.1.
Based on the current test data, the economy benefits in a scale of 3,000, 5,000 and 10,000-head swine farm can be estimated and summarized in the following table.
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Table 4.9 Economy Benefits for 3,000 Scale Swine Farm per year Piston Engine Turbine Engine
Biogas Production 85,410 m3 / year
Electricity Generation 145,000kWh / year 132,000 kWh / year Electricity Charge saving 392,000 NT$ / year 356,000 NT$ / year
CO2 Emission Reduction 3,000 tons
Table 4.10 Captial Costs for 3,000 Scale Swine Farm per year Piston Engine Turbine Engine
Cost of electricity per kilowatt 4.23 NT$ 4.92 NT$
Table 410 shows equipment costs for the turbine engine and piston engine in a scale of 3000-head swine farm. The equipment cost of piston engine is 18% lower than that of turbine engine.
Table 4.9 shows the electric energy in electricity production for the turbine and piston engines per year. For the piston engine, the electric energy can reach about 145,000kWh per year, and it can save 392,000NT$ of electricity charge per year, providing that the present
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electricity purchase charge is 2.7NT$ per kWh. On the other hand, the electricity generated by turbine engine can reach about 132,000kWh per year, and it can save 356,000NT$ of electricity charge per year. It estimated that a swine farm with a scale of 3,000 heads in Taiwan can decrease 3,000 tons of CO2 per year.
The payback ratio may be calculated as follows:
maximum value of thermal efficiency is lower, leading to higher cost of electricity per kilowatt for turbine engine. So based on a cost benefit analysis, using the piston engine to generate electricity in a scale of 3000 swine farm has an advantage over the turbine engine.
If it increases from 3,000-head pigs to 5,000-head pigs in a swine farm, then it has different result and CO2 reduction is 5,000 tons. Table 4.11 indicates that the net electricity production for piston engine is around 182,000 kWh/ year, and for turbine one is around 219,000 kWh/
If it increases from 3,000-head pigs to 5,000-head pigs in a swine farm, then it has different result and CO2 reduction is 5,000 tons. Table 4.11 indicates that the net electricity production for piston engine is around 182,000 kWh/ year, and for turbine one is around 219,000 kWh/