Scope of Present Study - 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

Chapter 1 Introduction

1.3 Scope of Present Study

The scope of this research is presented in Fig. 1.5. First, the treated biogas stored in the tank will be sucked into the combustion chamber when turbine engine is working. The components of biogas are measured by Gas Analyzer (IR-208) before entered into the engine, which is Capstone CR30 Micro Turbine engine, provided by the Aerospace Industrial Development Corporation (AIDC). The workloads vary from 15 to 30 kW and air flow and biogas supply rate change with the load accordingly and automatically. The biogas and air flow rates are recorded for analyzing performance of turbine engine. Since the influences of ambient temperature on the performance of gas turbine engine is very important, hence the performance of gas turbine engine are tested under different ambient temperatures (15~35^oC) and loads (15~30 kW). The components of exhaust gases are also recorded. The gas turbine performance is analyzed by using thermodynamic formula to investigate the energy balance and entrance temperatures, which cannot be measured directly. Then, a comprehensive comparison with the work of Vidal et al.

[19] is given to justify the effect of ambient temperature. Finally, the economic benefits are estimated by using C30 data, such as the electricity generation, equipment cost, maintenance cost et.al, to obtain the payback period and electricity cost per kWh and to estimate the potential application of biogas in Taiwan by using gas turbine engine.

Chapter 2 Biogas Generation System

2.1 Process of Biogas Production

The manure of swine is pretreated by using the three-step piggery wastewater treatment system to produce biogas, which has a hydrogen sulfide (H2S) will corrupt the engine. Fig. 2.1 and Fig. 2.2 show the process of biogas production including solid/liquid separation, anaerobic treatment and aerobic treatment (activated sludge treatment system). The manure of swine is collected and treated with wastewater treatment system. First, the separation of the solid from the waste water is to reduce the content of solids for subsequent handling and treatment, and to recovered solids can be used as fertilizer, etc. This physical process is accomplished by using various kinds of filters. Secondly, the anaerobic treatment which is conducted after solid/liquid separation, and occurs inside of anaerobic basins enclosed with “red-mud plastic (RMP) cover”

(1.2~1.8mm of thickness), made of a kind of PVC material, which is corrosion-resistant and gas-and-water impermeable. Besides, the anaerobic treatment system can salvage a part of chemical energy content of wastewater by producing methane.

The anaerobic tank contains very high content of hydrogen sulfide (H2S), which can corrupt the power generator, so the desulfurization system is needed to remove H₂S. The common method for reduction of hydrogen sulfide is biological desulfurization. In the process, the H2S is absorbed in water and then its content is reduced effectively by using

biological method. Finally, the biogas will be stored in a red plastic bag after the desulfurization process.

2.2 Utilization of Biogas

From 1990, the animal husbandry has been blooming in Taiwan, so the pollution of manure become more serious. In order to solve this problem, the three-step piggery waste water was built, which can produce biogas. There are some usages of biogas, ex: domestic fuel of gas stove, water heater. Besides, the application of power generator by using biogas had paid attention and expanded the scale gradually. The combined heat and power (CHP) generation plants is popular used in a four-stroke or a Diesel engine. The CHP generation can produce heat and power for higher energy efficiency simultaneously. It is a general way to transform energy of biogas at small or large-scale plants of biogas production.

Fig. 2.3 shows the range of capacities for the power generators, which are available on the market for the pilot-plant or industrial scale. The efficiency is defined as the ratio of the electrical power generated to the total energy content in the biogas. Efficiency figures are also provided by different manufacturers. Small-capacity engines generally can result in the lower efficiencies than that of high-capacity engines.

The generated electricity and heat can supply to the bioreactor itself, associated buildings, and neighboring industrial companies or houses. The power can be fed into the public electricity network, and the heat into the network for long-distance heat supply.

2.3 Engines

Table 2.1 presents some engines that can be operated with biogas.

These have been improved during the recent years by developing the works which are inspired by the worldwide boom in biogas plants. Some manufacturers have already had the engine performances better than presented the Table 2.1.

Table 2.1 Comparison of Different Power Generators

Feature Four-stroke

Liquid gas Any Natural gas Natural gas Natural gas, Fuel oil

2.3.1 Micro Gas Turbine

Micro gas turbines are small high-speed gas turbines with low combustion chamber pressures and temperature, which are designed to generate the electrical powers between 28kW to 200kW. They are operated on a Brayton cycle, consisting of a gas compressor, a combustion chamber and an expansion turbine. This study use the CR30, which is micro gas turbine. For normal operation, the compressor sucks

in the combustion air. The fuel is normally supplied to meet the combustion air in the combustion chamber. When biogas with a low calorific value is used, it must be adjusted to a flammable mixture of biogas and air before it is supplied into the combustion chamber.

The electrical efficiency of 15~25% for today’s micro gas turbines is still unsatisfactorily low. An attempt to increase the efficiency has been made by preheating the combustion air in heat exchange with the hot turbine exhaust gases. But great improvements are still necessary before micro gas turbines can be introduced into the market of industrial biogas plants. However, the coupling of a micro gas turbine with a micro steam turbine to form a micro gas-steam turbine seems already interesting and economical today because of its high electrical efficiency.

2.3.2 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

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

(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

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.

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

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

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:

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

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 32-0)