Gas Turbine

Biogas can be converted to current via gas turbines of medium and large capacity (20 MW and more) at a maximum temperature 1200 °C.

The tendency is to go to even higher temperatures and pressures, whereby the electrical capacity and thus the efficiency can be increased. The main parts of a gas turbine are the compressor, combustion chamber, and turbine.

Ambient air is sucked and compressed in the compressor and transmitted to the combustion chamber, where biogas is introduced and burnt with the compressed air. The flue gas that is so formed is passed to a turbine, where it expands and transfers its energy to the turbine. The turbine propels the compressor on the one hand and the power generator on the other hand. The exhaust gas leaves the turbine at a temperature of

approximately 400~600 °C. The waste heat can be recovered by driving a steam turbine downstream for heating purposes or for preheating the air that is sucked into the combustor

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Chapter 3

Experimental Apparatus and Procedures

3.1 Experimental Layout

The Experiment layout and biogas pretreatment system are shown in Figure 3.1. The flow meter measuring the biogas flow rates is installed in front of inlet of combustor. The flow meter is automatically adjusted according to the change of engine load. The biogas and air flow rates are shown in flow meter and sucked into the combustion chamber when the turbine engine starts. In order to prevent turbine blade from heat damage, most of air will be used to cool the hot gas which is from outlet of combustion chamber. The desulfurized biogas is moved to biogas storage for this experiment.

First, the biogas will pass through the cyclone and filter for removing the liquid water and impurities which damage the engine. Then, the front compressor which treats biogas will increase the pressure and temperature of biogas by reducing its volume for corresponding pressure of combustor. The compressor outlet temperature is about 40 ^oC, and the pressure of biogas is 5 kg_f / cm². Secondly, the biogas will pass through Freeze dryer to remove water vapor for enhancing power output [3], the biogas temperature is reduced to 36 ^oC. Finally, the biogas will be stored in the biogas tank whose capacity is 800 liters for maintaining the pressure (5.6 kg_f/cm²), and then the biogas is mixed with air and ignited in the combustor. Besides, the compositions of waste gases are measured by gas analyzer (IR-208), which is set at the engine outlet, and the waste gas temperature is measured by K-type thermocouple.

The electricity produced by micro-gas turbine (MGT) will supply to biogas pretreatment system for reducing energy consumption (~ 0.7 kW) from other power sources. Those devices include freeze dryer and compressor. Finally, the electricity is recorded by the power meter and supplied to parallel electric grid.

3.1.1 Micro-Gas Turbine Engine (CR30)

Figure 3.2 shows the schematic procedure of micro-gas turbine engine. The main components include centrifugal compressor, radial turbine, annular combustor and annular recuperator. The compressor, turbine and generator are mounted on the same shaft which is supported by patented air bearings and can spin at up to 96,000 RPM. The turbine provides power to drive compressor and generator.

First, the air passes through the air filter to remove the impurities, and then absorbs the heat from cooling fin of generator to protect generator from heating damage. Afterward, the air will be accelerated and pressured by compressor for attaining the limitations of pressure in combustion chamber, and then the compressed air will pass through the recuperator to enhance its temperature for reducing the consumption of fuel and increasing thermal efficiency. The fluid of heat exchanger is exhaust gases which are exhausted from outlet of turbine engine. Next, the air will be mixed with treated biogas and sucked into combustor for igniting. Finally, the hot gases drive the blades of turbine to generate electricity.

CR30 is controlled by digital power controller (DPC), which mainly controls the fuel valve, engine speed and turbine outlet temperature. In order to control the net power output, the DPC commands the fuel valve

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(Woodward Valve) to achieve the rated power output by adjusting the engine speed. Moreover, the turbine outlet temperature is fixed at 594°C by turbine exhaust temperature sensor (TET). The limited temperature value is set by Capstone Turbine Corporation for protecting the turbine.

The biogas consists of CH4 and CO2 mainly that leads to a low heating value, so the biogas inlet velocity is higher than those of the natural gas and propane for obtaining the same input heat under the same workload. Thus, the fuel injector is designed as premix type. The single premix solenoid can control fuel flow and increase flame stability when medium or low BTU content fuels are used.

In ideal state, the system of turbine engine is Brayton cycle, which has four steps. They are isentropic compression, isobaric heating, isentropic expansion and isobaric heat rejection. The theoretical thermal efficiency calculation is analyzed by Brayton cycle. Figure 3.3 shows the CR30 equipments and the following Table 3.1 shows the detailed data of engine.

Table 3.1 Engine Technical Data Capstone Turbine Engine

Engine model CR30

Electrical Power Output 30 kW

Voltage 400 to 480 VAC

Electrical Service 3-Phase, 4 Wire

Maximum Engine Speed 96000 rpm

Rated Efficiency

Maximum Output Current 46A, grid connect operation

Electrical Efficiency 26 %

Dry weight 405 kg

Power-to-weight (specific power)

0.074 kW/kg (0.045 hp/lb)

3.1.2 Biogas Flow Meter (TBT-FT004)

Fig 3.4 shows the mass flow transmitter, TBT-FT004, used for measuring the mass flow rate. The mass flow transmitter is used almost entirely for gas flow applications, such as compressive gas, mixed gas and unexplosive gas. The minimum length ahead the sensor along the pipe should be 10 times of pipe diameter and 5 times behind sensor for

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forming the fully developed flow. The principle of flow meter is thermal-mass flow, which measures fluid mass flow rate by means of the heat convected from a heated surface to the flowing fluid. It uses heat to measure flow, and then it introduces heat into the flow steam and measures how much heat dissipates using one or more temperature sensors, hence, the heated temperature sensor is controlled by power supply and the temperature difference between these two sensors have to keep constant under a fixed mass flow rate. The different mass flow rate will result in different temperature difference. Therefore, it can deduce the mass flow rate of fluid by the quantity of power supply to maintain the temperature difference between two sensors. The TBT-FT004 data are shown in Table 3.2.

Table 3.2 TBT-FT004 Flow Meter Data

Measuring object Gas (40 ^oC 5.6 kgf / cm²G) Measured unit

, min , min

, ³min

3 m l kg

m h

Power supply 12~30 VDC, 100 mA

Range ability 300 : 1

Accuracy 3%

Temperature Range -30 ^oC ~50 ^oC

Max. Pressure 1.6 MPa

Scale Range 0.2~90 m³/h

Material SUS304

3.1.3 Dehumidifier (RD-20A)

Figure 3.5 shows the dehumidifier, GTT RD-20A, 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 dehumidifier is R-134a. The detailed data of RD-20A are given in the Table 3.3.

Table 3.3 RD-20A Dehumidifier Data

Dehumidifier Model RD-20A

Inlet Temperature 80°C

Inlet Pressure 7 kg/cm²

Air Volume Rate 2.5 Nm³/Min

Refrigerant R-134a

Power 220V, 1Hz

Horsepower 1/2 HP

3.1.4 Air Compressor (H-50)

Figure 3.6 shows the compressor (H-50). It is used to compress the biogas for complying to the pressure of preheated air, which is compressed by inner compressor. If the biogas cannot attain the need of pressure level, the control system of CR30 will shut compressor down for protecting the machine. The detailed data of H-50 are shown in the Table 3.4.

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Table 3.4 H-50 Air Compressor Data

Air Compressor Model H-50

Air Volume 0.51 m³/Min

Rated discharge Pressure 7 kg/cm²

Driver 5HP, 220V, 60Hz

Net Weight 322 kg

3.1.5 Gas Analyzer (ECA450)

Figure 3.7 is the gas analyzer, BACHARACH ECA 450 that is used for measuring waste gas component data, which include the concentrations of oxygen, NOx and carbon dioxide. The measured and calculated data are shown in the following Tables 3.5 and 3.6

Table 3.5 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^oF (-20 to 1315^oC) Primary / Ambient Temp. -4 to 999^oF (-20 to 537^oC)

Pressure / Draft -27.7 to 27.7 inches of Water

Table 3.6 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

NOx (ref. to % O2) 0 to 17,000 ppm CO (ref. to % O₂) 0 to 99,9999 ppm

NO (ref. to % O2) 0 to 14,900 ppm

NO2 (ref. to % O2) 0 to 2,100 ppm SO2 (ref. to % O2) 0 to 17,000 ppm

3.1.6 Methane Concentration Analyzer (GuardCH4)

Figure 3.8 shows guardian plus infra-red gas monitor GuardCH4, which is used for measuring the methane concentration of the inlet treated biogas.

3.1.7 Humidity Temperature Meter (Center 311)

The Center311 humidity temperature meter is shown in Fig. 3.9. It is used to measure the humidity and temperature of the environment and biogas.

3.1.8 Gas Analyzer (IR-208)

Figure 3.10 shows the IR-208 Gas Analyzer. It integrates two different types of gas measurement into one instrument. A multiple channel infrared detector array utilizing a single beam infrared optical

30

system detects target gases using specially designed narrow band-pass optical filters. Comparing the infrared absorption of the reactive detectors to the nonreactive detector in the array provides the comparative for measuring the gas concentration in the sample stream. With a choice of more than 270 gases, up to 3 gases can be measured under infrared and up to 3 additional gases can be measured utilizing electrochemical cell, paramagnetic, or other sensors. The specification data of gas analyzer (IR-208) are shown in the following Table 3.7.

Table 3.7 The Specification Data of Gas Analyzer IR-208

Specification Value

Measuring method NDIR single beam

Response time 2 seconds

Pressure 5 Psig

Maximum load impedance 4-20mA isolated output 500 ohms

Power source 120/240 VAC, 50/60 Hz

Sample flow Standard: 0.2 to 2.0 L/Min Sample temperature 32° to 150°F (0° to 70°C)

Weight 16 lbs. (7.3kg)

Resolution 0.1 ppm

Repeatability + or – 0.25% of full scale

3.2 The Theoretical Calculation

The following calculations include the excess air ratio, thermal efficiency, theoretical fraction of mole of CO2 in waste gases, the percentage of water vapor removed from biogas. These data will be used in the analysis of the following experiments.

3.2.1 Excess Air Ratio

The air-fuel ratio (AF) is defined as a ratio of mole of air to the one of fuel in the combustion process. The treated biogas contains air, which leaks from atmosphere to the storage tank when the pipe of anaerobic fermentation pool is too low. Hence, the stoichiometric reaction for combustion of biogas with standard air is given as:

𝐶𝐻₄+ 𝑥𝐶𝑂₂+ 𝑦(𝑂₂+ 3.76𝑁₂) + 𝑧𝐻₂𝑂 + (2 − 𝑦)(𝑂₂+ 3.76𝑁₂) → (1 + 𝑥)𝐶𝑂₂+ (𝑧 + 2)𝐻₂𝑂 + 7.52𝑁₂ (3.1)

where x, y and z are the moles of CO2, air and water vapor in the biogas, respectively. Both x and y can be measured by instruments, and then z can be obtained from the absolute humidity () of biogas. Since the water vapor is considered as an ideal gas, the percentage of vapor from biogas can be calculated as follows:

𝑀𝑜𝑙𝑒 𝐹𝑟𝑎𝑐𝑡𝑖𝑜𝑛 𝑜𝑓 𝐻₂𝑂 𝑖𝑛 𝐵𝑖𝑜𝑔𝑎𝑠(%) = ¹⁸

16𝛼+44𝛽+28.8𝛾 𝑃_𝑣

𝑃_{𝑏𝑖𝑜𝑔𝑎𝑠}−𝑃_𝑣 (3.2) where , and  are the percentages of CH4, CO2 in biogas and air in biogas, respectively. P_biogas is the pressure of biogas and P_v is the vapor pressure in biogas, which is obtained from:

𝑃_𝑣 = 𝛷𝑃_𝑔 (3.3)

where 𝛷 is the relative humidity , measured by instrument, and 𝑃_𝑔 the

32

saturation pressure of vapor at the same temperature. The stoichiometric air-fuel ratio, AFstoich is :

𝐴𝐹_{𝑠𝑡𝑜𝑖𝑐ℎ} = 𝑚𝑜𝑙𝑒 𝑜𝑓 𝑎𝑖𝑟

𝑚𝑜𝑙𝑒 𝑜𝑓 𝐶𝐻₄+𝑚𝑜𝑙𝑒 𝑜𝑓 𝐶𝑂₂+𝑚𝑜𝑙𝑒 𝑜𝑓 𝑎𝑖𝑟 𝑖𝑛 𝑏𝑖𝑜𝑔𝑎𝑠+𝑚𝑜𝑙𝑒 𝑜𝑓 𝐻₂𝑂 = (2−𝑦)×4.76𝑚𝑜𝑙𝑒

(1+𝑥+𝑦×4.76+𝑧)𝑚𝑜𝑙𝑒 (3.4)

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:

𝐴𝐹_𝑎𝑐𝑡 = (𝑚𝑜𝑙𝑒 𝑜𝑓 𝑎𝑖𝑟 )_𝑎𝑐𝑡 rate multiplied by the mole fraction of methane.

The Excess Air Ratio () is the ratio of the actual mole of air used to the stoichiometric mole of air, defined as:

𝜆 = (𝑚𝑜𝑙𝑒 𝑜𝑓 𝑎𝑖𝑟)_𝑎𝑐𝑡

Note that the actual mole of fuel is equal to stoichiometric mole of fuel 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. The  is reciprocal of equivalence ratio. In this study, the most of air is used to cool the hot gas for protecting the blades of turbine.

3.2.2 Thermal Efficiency

The thermal efficiency is defined as the ratio of the actual power generation to the energy input, and its formula is as following:

𝑇ℎ𝑒𝑟𝑚𝑎𝑙 𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 = 𝐴𝑐𝑡𝑢𝑎𝑙 𝑃𝑜𝑤𝑒𝑟 𝐺𝑒𝑛𝑒𝑟𝑎𝑡𝑖𝑜𝑛

𝐸𝑛𝑒𝑟𝑔𝑦 𝐼𝑛𝑝𝑢𝑡 (3.7)

The actual power generation of this study is the net power output of turbine generator. The energy input is calculated from the lower heating value (LHV) of methane, whose value is 50020 kJ / kg. It is expressed as following:

34

where ^V_biogas is measured biogas volumetric flow rate, biogas@1atm,25C _is

density of biogas in normal condition. mfCH _@5atm

4 is mass fraction of CH4 in biogas at five atmospheric pressure.

3.2.3 Least Square Method

The least square method is applied to find the curve which represents the relationship between the measured data, and the curve has minimum value that the sum of the square of the distance which is all the data points to the curve. This study uses first-order linear curve to do the least square method for finding the representative curve. The equations are expressed as following:

In order to ensure whether the curve can represent the measured data, the goodness of fit (R²) is a good indicator for examining the linear regression. The goodness of fit is given as following:

yy



 measured data. When R² equals to 1, it is called perfect fit, meaning that the regressive model does not exist the residuals.

3.3 Waste Gas Analysis

The contents of waste gases include O2, CO2, CO and NOx. The gas analyzer (IR-208) can measure the concentrations of waste gases.

However, the gas turbine needs most of the air to cool the hot gas to avoid damaging the turbine. Thus, the concentration of NOx is too low to measure by instrument.

The measured O2 data can be applied to estimate CO2, excess air ratio and mole number of waste gas composition. Because the cooling air is mixed with produced CO2, the measured concentration of CO2 is larger than actual one. Moreover, the quantity of air is much higher than CH₄ in exhaust gases, hence, the term of methane does not appear in actual reaction formula. Eq. (3.1) is modified by the excess air ratio for obtaining the actual reaction formula. Eq. (3.17) can find the concentration of CO₂ and excess air ratio by the O₂ mole fraction.

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The actual reaction formula is expressed as following:

2

The mole fraction of O2 is applied to deduce the excess air ratio, and then the excess air ratio is used to deduce theoretical mole fraction of instrument and the coefficient a will be found.

The percentages of CO₂ in waste gases can be calculated by:

z

3.4 Theoretical Calculation of Performance for Micro-Gas Turbine

The theoretical calculation of performance is calculated by isentropic process and rated efficiency of component given from Table 3.1, such as compressor and turbine. The temperature and pressure points are marked in Fig. 3.11. In fact, the exit temperatures of components cannot be measured by instruments directly due to the restrictions of AIDC, hence, those above isentropic efficiencies are applied for estimating the actual temperatures of components.



c is the isentropic efficiency of the compressor, and



_T is isentropic efficiency of the turbine. It is expressed as:

C compressor and

W

_T is realistic work output by turbine. Eqs. (3.20) and (3.21) are applied for calculating the actual outlet temperature of compressor and the actual inlet temperature of turbine, respectively.

mech

38 where



_mech is mechanical efficiency.

The Wnet is the net output that work of turbine minus compressor. It is The hot gas mass flow rate that drives blades of turbine is expressed as:

biogas software and the biogas mass flow rate is obtained from:

C The specific heat capacity mixing air with biogas is expressed as:

biogas

In ideal gas reversible adiabatic process, the isentropic compressor outlet temperature and turbine inlet temperature can be expressed as following:

k k

k k

ratio. Due to the pressure drop, so

5 4

P

P is expressed as following:

combustor

The heat exchanger effectiveness _HE is calculated by:

)

The efficiency of combustion is expressed as:

4 The calculated heat exchanger outlet temperature (gas side) is:

gas ideal heat input that is:

ideal

40 output of a device to the calculated heat input. Its formula is expressed as:

th cal

,cal gas pgas air pair biogas pbiogas biogas

th m C T m C T m C T

Q            

(3.39) where Qth_,cal is the calculated heat input, _generator generator efficiency,

generator output of a device to the measured heat input, its formula is expressed as:

actual

3.5 The Effect of Varying Loads and Ambient Temperature The power generation of the gas turbine engine is affected by main two conditions, one is operating loads and the other is ambient temperature. Thus, the thermal efficiency and power generation will be investigated in this research. The designed range of rated power output of engine is 15kW to 30 kW, and the increment of power output is 1 kW in five minutes interval under the same environmental conditions for ensuring the system in steady state. The operating load will affect the performance of engine, such as power output. Finally, the all of measured data make average to obtain the more accurate values.

The ambient temperature is an important parameter for engine performance, so it is recorded in each load. Fig. 3.12 shows the average ambient temperature of swine farm in Taichung. The temperature range of swain farm is about 17^oC to 30^oC. According to the data of CR30 given by Aerospace Industrial Development Corporation (AIDC), the net power output and electrical efficiency are affected by ambient temperature seriously. Thus, the performance of MGT affected by ambient temperature is analyzed in this research.

The experimental procedure is as follows:

1. Record ambient temperature and measure the relative humidity, temperature and pressure of treated biogas.

2. Measure the treated biogas constitutes and concentrations of methane 3. Warm up the engine at least 10 minutes in 15kW so it would be

steady.

4. Record all of the measured data, such as above all and prepare all of

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the instruments.

5. Adjust the power output at demanded quantity and record the net power output, biogas flow rate, air mass flow rate, and so on.

6. Repeat the procedure for different power output.

7. Repeat the above procedure at different ambient temperature.

Besides, if the ambient temperature is too low, the biogas supply will get some troubles. We check the condition of biogas before carrying out the experiment. There are two problems about biogas and they lead micro-gas turbine not to work. Firstly, the swine farm is not usually clean the swine house in the winter, otherwise, pigs may catch cold. Thus, the waste water, which flows into the anaerobic fermentation tank, is not

Besides, if the ambient temperature is too low, the biogas supply will get some troubles. We check the condition of biogas before carrying out the experiment. There are two problems about biogas and they lead micro-gas turbine not to work. Firstly, the swine farm is not usually clean the swine house in the winter, otherwise, pigs may catch cold. Thus, the waste water, which flows into the anaerobic fermentation tank, is not

在文檔中 養豬場環境溫度對30kW沼氣渦輪發電機發電影響之實驗研究 (頁 36-0)