Chapter 2 Biogas Generation System
2.1 Process of Biogas Production
Fig. 2.1 shows the process of biogas production. The manure of swine after collection goes to wastewater treatment. The first step is solid/liquid separation. Separation of the solid from the wastewater 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. Anaerobic treatment 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. The anaerobic treatment system can also salvage a part of chemical energy content of wastewater by producing methane.
Biogas from the anaerobic tank contains very high degree of hydrogen sulfide (H2S), which can corrupt the power generator, so the desulfurization process is needed in advance. 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 mitigated greatly by biological method. After desulfurization process, the biogas will store in a red plastic bag.
2.2 Utilization of Biogas
Biogas can be used either for the production of heat only or for the generation of electric power. Normally heat and power are produced in
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the same time for higher energy efficiency. Such power generators are called combined heat and power (CHP) generation plants, and it is normally used in a four-stroke or a Diesel engine. CHP generation is an efficient way for energy conversion of biogas at small- and large-scale plants of biogas production. A Stirling engine or gas turbine, a micro gas turbine, high- and low-temperature fuel cells, or a combination of a high-temperature fuel cell with a gas turbine are alternatives.
Biogas can also be used by burning it to produce steam, by which can drive an engine in the Organic Rankine Cycle (ORC) or the Cheng Cycle, the steam turbine, the steam piston engine, or the steam screw engine.
Fig. 2.2 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 current 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
Fig. 2.3 lists some engines that can be operated with biogas. These have been improved during the recent years by following the development works inspired by the worldwide boom in biogas plants.
The performance by some manufacturers even has already exceeded that
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of those given in this figures.
2.3.1 Four-stroke Gas Engine and Diesel Engine
Today’s four-stroke biogas engines were originally developed for natural gas and are therefore well adapted by the special features of biogas. Their electrical efficiencies normally do not exceed 34~ 40%, as the nitrogen oxide (NOx) output has to be kept below the prescribed values. The capacity of the engines ranges from 100 kW to 1 MW.
Four-stroke biogas engines often run in the lean-burn range (ignition
In small agricultural plants, ignition oil Diesel engines are frequently installed. These engines are more economical and have a higher efficiency than four-stroke engines in the lower capacity range. However, higher NOx emissions are produced by Diesel engine. Their lifetimes usually are given as 35,000h of operation.
In general, Diesel engines burning gas fuel can be operated by direct injection because pre-chamber engines develop hot places, resulting in uncontrolled spark failures with biogas. Owing to the internal formation of gas mixtures, Diesel engines can be faster controlled. The ignition oil Diesel engine is operated ideally at λ < 1.9. The efficiency is then up to
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15% better than that in a four-stroke engine.
2.3.2 Stirling Engine
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 in.
2.3.4 Micro Gas Turbine
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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. 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.
2.3.5 Fuel Cell
Comparing to combustion engines, the fuel cell converts the chemical energy of hydrogen and oxygen directly into current and heat. Water is formed as the reaction product.
In principle, a fuel cell works with a liquid or solid electrolyte held between two porous electrodes–anode and cathode. The electrolyte lets 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
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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.
Depending on the type of fuel cell, 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 Experimental Equipment Layout
The experimental equipment layout is shown in Fig. 3.1 When the engine is running, the air and the biogas are sucked into the engine automatically. The water vapor of biogas is removed by dehumidifier, marked by D, before biogas flows through the flow meter, F1. The flow meters, marked by F1 and F2, measure the air and the biogas flow rates, respectively, which are controlled by valves at the engine inlets. The crank angle degree can be recorded by rotary encoder, marked by R. The spark timing controller, marked by ST, can adjust spark timing according to the correspondent crank angle degree. The in-cylinder pressure can be obtained at each crank angle degree by the spark-plug pressure sensor, marked by P. The engine gets the power by combustion to drive the generator to produce the electricity. A waste gas analyzer places at the engine outlet to measure the compositions of waste gases, and the gas temperature is measured by a thermocouple.
3.1.1 Engine
The spark ignition system, adopted by Lin [4] and Huang [5], was installed to the four-stroke diesel engine. In other words, The ignition way was changed into spark ignition instead of comprssion one. For the original engine, the ideal power cycle is Diesel cycle. When the compression ignition system is converted to spark ignition system, the ideal power cycle of present engine becomes Otto cycle. Figure 3.2 shows the modified engine and its detailed data can be referred in the
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following table.
Table 3.1 Engine Technical Data Engine Technical Data
Engine model 8031i06
Diesel 4 stroke - Injection type direct
N° of cylinders 3 in line
Total displacement 2.9 L
Bore × Stroke 104 × 115 mm
Compression ratio 10 : 1
Engine speed 1800rpm
Aspiration natural
Cooling system liquid (water + 50% Paraflu11) Lube oil specifications ACEA E2-96 MIL-L-2104E
Lube oil consumption ~ 0.3% of fuel consumption
Fuel specifications EN 590
Speed governor mechanical (G2 class)
Engine rotating mass moment of inertia 0.942 kg m2 Dry weight ( standard configuration) ~ 370 kg
This study still uses the engine but with an important modification, which will be described in Section 3.1.9., to enhance its combustion stability.
3.1.2 Air Flow Meter (VA-400)
The flow meter at air inlet is insertion CS flow sensor type VA-400
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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, temperature sensors are put on along the flow path of gas. One of them is heated by a controlled power supply, and another one is not heated. The temperature difference between these two sensors should be always kept 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 flow by the quantity of power supply to maintain the temperature difference between these two sensors.
3.1.4 Dehumidifier (RD15)
Figure 3.5 shows the dehumidifier, GTT RD15, used for removing the water vapor of biogas. The maximum inlet biogas flow rate is 30 L/sec. It is pre-cooled as biogas leaves from the evaporator. The coolant in the
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dehumidifier is R22.
3.1.5 Temperature with Humidity Transmitter (JHTD3010-N)
Such transmitter is shown in Fig. 3.6, 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 humidity of biogas that with and without dehumidification and also measuring the temperatures and humidity of intake air.
3.1.6Gas Analyzer (ECA450)
Figure 3.7 is the gas analyzer, BACHARACH ECA 450, used for measuring waste gas component data, which include the concentrations of oxygen, NOx and carbon dioxide.
3.1.7 Methane Concentration Analyzer (GuardCH4)
Figure 3.8 is guardian plus infra-red gas monitor GuardCH4, which is used for measuring the methane concentration of the inlet biogas.
3.1.8Data Acquisition
Data acquisition system can automatically collect signals from analog and digital measurement sources, such as sensors and devices, under tests.
It uses a combination of PC-based measurement hardware and software to provide a flexible and user-defined measurement system. Usually, the researcher must calibrate sensors and signals before a data acquisition device acquires them. The specifications of these modules of National Instruments are shown in Table 3.2.
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Table 3.2 Specifications of the Data Acquisition Modules Model Signal Type Channels
Max powerful and easy-to-use software that can extend to many languages and operating systems. NI CompactDAQ delivers fast and accurate measurements in a small, simple, and affordable system. A CompactDAQ Chassis shown in Fig. 3.9a, a product of NI, is adopted because of the following advantages: plug-and-play installation and configuration, AC power supply and USB cable connection, mounting kits available for panel, enclosure, DIN-rail and desktop development, A380 metal construction, more than 5 MS/s streaming analog input per chassis, and Hi-Speed USB-compliant connectivity to PC. Different types of signal process modules are chosen to complete the data acquisition system, including NI 9203 Analog Input Module, NI 9211 Thermocouple Differential Analog Input Module and NI 9401 TTL Input Module. All of these are shown in Fig. 3.9b, Fig. 3.9c and Fig. 3.9d.
3.1.9 Ignition System
Figure 3.10 shows the details of ignition system. When the spark plug starts to ignite, the ignition signal is recorded into NI recorder by the tachometer. The in-cylinder pressure is captured by the spark plug
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pressure sensor, and then the charge converter converts the charge signal to voltage signal by the supply of steady current of power unit. The rotary encoder is installed to record the crank angle of piston cylinder. The spark timing controller shows in Fig.3.11, which can be adjusted to change the spark timing by different supply rate of high voltage for spark plugs.
3.1.9.1 Tachometer (VC4000DAQ)
The VERICOM 4000DAQ tachometer is used for measuring the exact spark timing, shown in Fig.3.12. It is clamped onto the spark plug wire to capture the spark signal.
3.1.9.2 Spark Plug Pressure Sensor (BKR5E-11 and 112A05)
The spark plug pressure sensor is modified from NGK BKR5E-11 spark plug with PCB Piezotronics 112A05 pressure sensor, shown in Fig.
3.13.The pressure range is up to 350 bar and the operating temperature up to 240 to 310°C. It is used for measuring the in-cylinder pressure during the combustion process.
3.1.9.3 Charge Converter (PCB 422E05)
Such converter is shown in Fig. 3.14, which is designed to convert the high impedance of a charge mode piezoelectric transducer into a low-impedance voltage. The charge output of the transducers is scaled in term of pressure, mV/psi.
3.1.9.4 Rotary Encoder (HPN-6A)
The HONTKO HPN6A rotary encoder, shown in Fig. 3.15, is used to record the crank angle of piston cylinder during the cycle.
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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 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
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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)
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:
(3.5)
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
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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:
CO2 in waste gas in the combustion process are calculated as follows:
The balanced reaction is:
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percent of O2 in waste gas as follow:
(3.8)
where is the total moles in waste gas, is obtained from the atom balance as:
(3.9)
The theoretical percent of CO2 in waste gas can be calculated by:
(3.10) The theoretical percent of used CH4 is defined as:
(3.11)
3.2.3 Thermal Efficiency
The thermal efficiency is defined as the ratio of the fuel conversion efficiency to the combustion efficiency, and its formulation is as following :
(3.12)
Fuel Conversion Efficiency is defined as:
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(3.13)
where W is the electric power generated and LHV the lower heating value.
Combustion Efficiency is expressed as the ratio of the enthalpy difference between the the products and reactants to the LHV of biogas:
(3.14)
where the numerator stands for the real heat release rate between inlet and outlet of the biogas, and the denominator represents the ideal heat release rate. Now,
4[ +( ) ] 4+ 2[ +( ) ] 2+ 2 [ +( ) ] 2 + 2[ +( ) ] 2} (3.15)
where the unit of enthalpy is kJ/kmole, and is the mole flow rate of biogas, calculated by:
(3.16)
in which and refer to the density and the mole of biogas, respectively.
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Eventually, the thermal efficiency can be obtained by dividing the fuel conversion efficiency (Eq. 3.13) by combustion efficiency (Eq. 3.14); that is
(3.17)
3.2.4 Combustion Stability
The process of spark-ignition engines includes suction, compression, expansion and exhaust strokes. The combustion stability, represented by knock, can be detected by many ways in which three of them are introduced as follows. The ionization current measurement circuit is installed with spark plug electrodes to obtain current intensity. The high frequencies contain to the current signal due to variation of pressure when combustion stability becomes bad. Hence, the combustion stability can be analyzed through current intensity. Second one is the engine vibration method. By the way of an accelerometer fixed on the top-surface of the engine cylinder. The last one is the in-cylinder pressure method. The in-cylinder pressure is measured by pressure sensor. It is much more reliable than other two methods because the fact that in-cylinder pressure method directly measure the pressure of in-cylinder.
The indicated mean effect pressure (IMEP) is calculated by integrating pressure with respect to response volume during the combustion process, and Vd is the effective working volume. It is expressed as:
The indicated mean effect pressure (IMEP) is calculated by integrating pressure with respect to response volume during the combustion process, and Vd is the effective working volume. It is expressed as: