Chapter 2 Biogas Generation System
2.3 Engines
2.3.5 Fuel Cell
It is worth noting that the fuel cell converts the chemical energy of hydrogen and oxygen directly into current and heat. Water is formed as the reaction product. Fuel cells may differ in a number of ways from batteries since they demand a steady source of fuel and oxygen to operate.
However, they can produce electricity continually as long as these inputs are kept supplying.
Simply stated, a fuel cell works with a liquid or solid electrolyte held between two porous electrodes–anode and cathode. The electrolyte lets
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ions pass only and allow no free electrons from the anode to the cathode side. The electrolyte is thus “electrically non-conductive.” It separates the reaction partners and thereby prevents direct chemical reaction. For some fuel cells, the electrolyte is also permeable to oxygen molecules. In this case the reaction occurs on the anode side. The electrodes are connected by an electrical wire.
Both reaction partners are continuously fed to the two electrodes, respectively. The molecules of the reactants are converted into ions by the catalytic effect of the electrodes. The ions pass through the electrolyte, while the electrons flow through the electric circuit from the anode to the cathode. Taking into account all losses, the voltage per single cell is 0.6 ~ 0.9 V. The desired voltage can be reached by arranging several single cells in series, a so-called stack. In a stack, the voltages of the single cells are added.
The data summarized indicate that the biogas has to be purified to remove CO and H2S especially before feeding into the fuel cell. Only a small number of fuel cell plants, mostly pilot plants, are in operation for the generation of electricity from biogas.
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Chapter 3
Experimental Apparatus and Procedures
3.1 Experiment layout
The Experiment layout is shown in Figure 3.1. When the turbine engine starts, the air and the biogas are sucked into the engine. The flow meters, marked by F1 and F2, measure the air and the biogas flow rates, which are automatically adjusted according to the change in engine speed. It is important that not all of the air for combustion, part of the total air is used for cooling the hot gas exhausted from combustor outlet, which prevent turbine blade from heat damage.
First of all, the compressor would increase the pressure and temperature of biogas by reducing its volume, the compressor outlet temperature is about 40°C, and the pressure of biogas is about 5.6kgf/ . In the second place, the water vapor of biogas is removed by Freeze dryer, and then the biogas will be stored in the biogas tank. Finally the fuel is mixed with air and ignited in the chamber. The waste gas diverging nozzle. For these processes, the temperature is increased. In an ideal system, this is an isentropic process. After that, gases go to a
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combustion chamber. Ideally, the combustion is isobaric process.
Furthermore, the combustion product gases are expanded and accelerated by nozzle guide vanes before the energy is extracted by a turbine. In an ideal system, these gases are expanded isentropically and leave the turbine at their original pressure, mentioned previously. This ideal power cycle is called Brayton cycle. Figure 3.3 shows the engine and its detailed data, which can be referred in the following table.
Table 3.1 Engine Technical Data
Maximum Output Current 46A, grid connect operation Digester/Landill Gas HHV 350 to 1,275 BTU/scf
Dry weight ~ 159 kg
Power-to-weight (specific power) 0.188 kW/kg(0.115 hp/lb)
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3.1.2 Air Flow Meter (VA-400)
The flow meter at air inlet is insertion CS flow sensor type VA-400 flow sensor, whose range varies with the installed pipe diameter. In order to maintain the accuracy stipulated in the data sheets, the sensor must be inserted in the center of a straight pipe section with an undisturbed flow progression. An undisturbed flow progression is achieved if the sections in front of the sensor and behind the sensor are sufficiently long, absolutely straight and without any obstructions such as edges, seams, curves etc. The minimum length ahead the sensor along the pipe should be 10 times of pipe diameter and 5 times behind sensor for the fully principle is to use the float, which moves up and down within the tapered pipe, to measure the amount of volume rate of fluid. As the flow rate increases, it will go up. On the contrary, the float will be lowered if the flow rate is decreased. The float can rotate that is why it is also called the rotameter.
3.1.4 Dehumidifier (RD20)
Figure 3.6 shows the dehumidifier, GTT RD20, used for removing the water vapor of biogas. The maximum inlet biogas flow rate is 44 L/sec. It is pre-cooled as biogas leaves from the evaporator. The coolant in the
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dehumidifier is R-134a.
3.1.5 Thermocouple
A thermocouple is a sensor for measuring temperature. It consists of two dissimilar metals joined together at one end, which can produce a small unique voltage at a given temperature. This voltage is measured and interpreted by a thermometer. Thermocouples are available in different combinations of metals or calibrations. The four most common calibrations are J, K, T and E. Each calibration has a different temperature range and environment.
Type K (Chromel–Alumel) is the most commonly used thermocouple with a sensitivity approximately 41 µV/°C. The voltage of Chromel is positive relative to the one of alumel. It is inexpensive and its temperature is wide, ranging from −200 °C to +1350 °C.
In this research, four K-type thermocouples is used for measuring waste gas temperature and inlet gas temperature. Figure 3.7 shows the picture of K-type thermocouple.
3.1.6Gas Analyzer (ECA450)
Figure 3.8 is the gas analyzer, BACHARACH ECA 450, used for measuring waste gas component data, which include the concentrations of oxygen, NOx and carbon dioxide. The measured data and calculated data are shown in the following table 3.2 and table 3.3.
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Table 3.2 The measured data of gas analyzer ECA450 Measured Data
Oxygen 0.1 to 20.9%
Carbon Monoxide (hydrogen compensated)
0 to 4,000 ppm
Carbon Monoxide High 4,001 to 80,000 ppm
Nitric Oxide 0 to 3,500 ppm
Nitrogen Dioxide 0 to 500 ppm
Sulfur Dioxide 0 to 4,000 ppm
Combustibles 0 to 5% (application dependent) Stack Temp. -4 to 2400°F (-20 to 1315°C) Primary/Ambient Temp. -4 to 999°F (-20 to 999°C)
Pressure/Draft -27.7 to 27.7 inches of Water Table 3.3 The calculated data of gas analyzer ECA450
Calculated Data
Combustion Efficiency 0.1 to 100.0%
Excess Air 1.0 to 250%
Carbon Dioxide(dry basis) 0 to fuel dependent maximum
NOx 0 to 4,000 ppm
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3.1.7 Methane Concentration Analyzer (GuardCH4)
Figure 3.9 is guardian plus infra-red gas monitor GuardCH4, which is used for measuring the methane concentration of the inlet biogas.
3.1.8 Temperature with Humidity Transmitter (JHTD3010-N)
Such transmitter is shown in Fig. 3.10, whose humidity accuracy covers the full range from 0 to 100% RH, allowing precise measurement of the humidity over the operating temperature from -40 to 80 °C. It is used for measuring the temperatures and humidities of biogas that with and without dehumidification.
3.1.9 Humidity Temperature Meter (Center 311)
The Center 311 humidity temperature meter is shown in Figure 3.11.
It is used to measure the humidity and temperatures of the environment and biogas.
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
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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 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)
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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:
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 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:
λ because in the engine experiments the fuel supply rate is fixed, whereas the air volume flow rate is changed. As a consequence, the excess air ratio is equal to ratio of AFact to AFstoich. Also remind that λ is reciprocal of equivalence ratio.
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3.2.2 Thermal Efficiency
The thermal efficiency is calculated for how much energy converting into electric power, its formulation is as following :
(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