Chapter 2 Biogas System in Swine Farm
2.4 Engines
2.4.5 Fuel cell
Comparing to combustion engines, the fuel cell converts the chemical
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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 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 Experiment layout
The Experiment layout is shown in Figure 3.1a. When the 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 controlled by valves at the engine inlets. The engine gets the power by combustion to drive the generator to produce the electricity. The thermocouple at the engine outlet measures waste gas temperature, and followed by the gas analyzer to measure the concentration of CO in waste gas.
Figure 3.1b is the layout of waste heat recovery. The heat exchanger is installed following the exhaust pipe. The waste gases flow into the exchanger and transfer heat to water in a separated pipe. There are four thermocouples established at exchanger inlets and outlets. T1 and T2 measure the temperature of waste gases before and after the heat exchanger. T3 and T4 are for water temperature at inlet and outlet. A flow meter is also set up for measuring water flow rate.
Later, oxygen is added to the engine for oxygen-enriched combustion test, whose layout is shown in Fig. 3.1c. Note that oxygen is mixed with biogas before they are sent to engine. There exist a valve and a flow meter to control and measure oxygen flow rate.
3.1.1 Engine
The original four-stroke diesel engine was operated with diesel fuel,
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using compression to ignite the fuel. In order to use biogas gas as fuel, the spark ignition system was installed to the engine. In other words, The ignition way was changed into spark ignition instead of comprssion one.
Figure 3.2 shows the refurnished engine and its detailed data can be referred in the 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 x Stroke 104 x 115 mm
Compression ratio 17 : 1
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
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3.1.2 Air Flow meter (VA-400)
The flow meter at air inlet is insertion 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 developed turbulent flow profile, so the measured flow rate can be accurate enough. Figures 3.3a and 3.3b show the flow meter and its detailed data.
3.1.3 Biogas Flow Meter (TF-4000)
The flow meter at biogas inlet is TF-4000 thermal-mass flow meter.
The measuring object generally is a mixture of 60% of CH4 and 40% of CO2 for biogas produced in current swine farm. Figure 3.4a and Figure 3.4b show the flow meter and its technical data. Operation principle is as following: Two temperature sensors are put on along the flow path of gas.
One of them is heated by a controlled power supply, and the other 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.
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3.1.4 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.
Type J (iron–constantan; −40 ~ +750 °C) has a more restricted range than type K, but with a higher sensitivity about 55 µV/°C. The Curie point of the iron (770 °C) causes an abrupt change in the characteristic, which determines the upper temperature limit.
In this research, K-type thermocouple is used for measuring waste gas temperature and J-type thermocouple is used for water temperature in waste heat recovery. Figures 3.5a and 3.5b show the pictures of these two kinds of thermocouples.
3.1.5 Water Flow Meter (VF-2000)
The VF-2000 Flow Sensor is assembled with a few pieces of components. The sensor body and Shedder bar (vortex generator) are molded as one component. This design approach has reduced the cost as well as the size and weight of the flow meter. Sensor body is made of PPS
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(Polyphenylene sulfide) and is designed to eliminate deposits. The operating principle of VF-2000 uses a bluff body or shedder bar in the flow to generate a street of vortices downstream. The VF-2000 Flow Sensor measures the flow rate by counting the number of vortices with a piezoelectric sensor. In this research, VF-2000 is used for measuring water flow rate in waste heat recovery. Figure 3.6a and Figure 3.6b show the picture of VF-2000 and its detailed data.
3.1.6 Gas Analyzer (IMR 1400)
Figure 3.7 is IMR 1400, which is used for waste gas component data. It can measure the concentrations of oxygen and carbon monoxide, with which it can deduce the concentration value of carbon dioxide.
3.1.7 Water Pump
The water pump is used to pump cooling water through the heat exchanger. The water is heated by waste gas exhausted from engine.
Figure 3.8 shows the pump and its maximum water flow rate.
3.1.8 Heat Exchanger
The heat exchanger (Fig. 3.9a) is placed at waste gas outlet to heat the water. It has a casting surrounding four sets of copper pipes of water, which each outer wall is welded by many copper fins (see Fig 3.9b) to enhance the heat transfer between the hot waste gas and the cooling water. The void space between the casting and pipes is for the passage of waste gas.
3.1.9 Data Acquisition
Data acquisition system can automatically gather signals from analog
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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.
Table 3.2 Specifications of the data acquisition modules
Model Signal Type Channels
Max Sampling
Rate
Resolution Signal Input Ranges NI 9203 Current 8 500 k/s 16 bits ±20 mA NI 9211 Thermocouple 4 15 k/s 24 bits ±80 mV
National Instruments, a leader in PC-based data acquisition, offers a complete family of proven data acquisition hardware devices and the 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 Figure 3.10a, 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
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Differential Analog Input Module. Both are shown in Figure 3.10b and Figure 3.10c.
3.2 Excess Air Ratio (λ), Thermal Efficiency and Exergy
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 stoichiometric reaction for combustion of methane with standard air is given as:
CH 2 O 3.76N CO 2H O 7.52N (3.1)
The stoichiometric air-fuel ratio, AFstoich, is
AF . 9.52 (3.2)
On the other hand, AFact is the air-fuel ratio of the actual mole of air to mole of methane into the engine. Because the mole ratio is equal to the volume flow rate ratio, AFact can be also expressed as:
AF M A f f (3.3)
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). An example is given as follows:
If the air and biogas (with 60% of CH4) flow rate were measured as 1400 and 240 liters per minute, respectively, then, the actal air-fuel ratio is:
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AF L/ L/ % 9.72 (3.4)
The Excess Air Ratio (λ) is the ratio of the actual mole of air used to the stoichiometric mole of air, defined as:
λ
AFAF (3.5)
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. Also remind that λ is reciprocal of equivalence ratio.
From those definitions, the resultant Excess Air Ratio (λ) for the above example is
AF
AF .
. 1.02 (3.6)
The thermal efficiency is calculated for how much energy converting into electric power, its formulation is as following :
(3.7)
where Energy Input is calculated from the lower heating value (LHV) of
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methane, whose value is 50020kJ/kg, in the biogas. It is expressed as:
mCH LHV of CH (3.8)
where mCH is the methane mass flow rate in biogas, and it is calculated by:
mCH 60% ρCH (3.9)
where ρCH is the density of methane, which is 0.717 g/m3 at STP.
The second law efficiency can be briefly shown as follow:
(3.10)
In calculating the second law efficiency for heat exchanger, the Exergy supplied is from the different flow exergy (availability) between inlet and outlet of waste gas. It is calculated as follow:
Exergy from waste gas ∑ ∆φ (3.11)
where ∆φ is the flow exergy difference between the heat exchanger at inlet and outlet for four different major components in waste gas. The flow exergy is calculated as follow:
φ h h T s s (3.12)
where h , T and s are the enthalpy, temperature and entropy for the
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surrounding, and s is the entropy for the component. Its calculated as follow:
s s ° R ln (3.13)
where s ° is the absolute entropy, and y p is the partial pressure for the component. p is the pressure for the surrounding. R is the universal gas constant.
Testing and finding the optimum excess air ratio is important for gaining a higher thermal efficiency design. The collected data are expected to be used in the future design of pilot and real scale plants.
The experimental parameters include biogas flow rate and excess air ratio. The different biogas flow rates are 180 L/min, 200L/min, 220L/min, 240L/min and 260L/min. Under each fixed biogas flow rate, it tests five different excess air ratios, ranged from 0.8 to 1.2. The collected data include biogas flow rate, air flow rate, power generation, waste gas temperature, carbon monoxide concentration, oxygen concentration and carbon dioxide concentration. Before taking the measurement, the engine was operating continuously until all conditions were ensured to be steady.
Then, all measurements were tested twice and took an average. The experimental procedure is as follows:
1. Operate the engine at least 20 minutes so it would be steady.
2. Fix the biogas flow rate at demanded quantity.
3. Control the air flow rate for specific excess air ratio.
4. Collect the corresponding data, mentioned above.
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5. Repeat the procedure for different excess air ratio.
3.3 Waste Heat Recovery
The heat recovery is calculated by the following :
m C ∆T (3.14)
where m is water mass flow rate, C is calorie (specific heat of water), and ∆T is the water temperature difference between inlet and outlet. The heat exchanger effectiveness is to describe the percent of energy from a higher temperature flow is transferred to the lower temperature flow. It is defined as follow:
(3.15)
The energy transfer from gas is the energy lost from the gas between the inlet and outlet.
∑ ∆Q (3.16)
where ∆Q is the enthalpy difference between inlet and outlet for each major component in waste gas (H2O, CO2, O2, N2).
Q m h h (3.17)
where m is mole flow rate for the component, and h and h are the enthalpies at inlet and outlet in joule per mole. Sum up the energy transfer from four major gas and heat recovery, the effectiveness of the heat exchanger can be known.
Overall efficiency displays energy conversion rate for waste heat and power output. It can show how many energy utilization increasing with heat exchanger.
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(3.18)
Temperature affects biogas production quantity, therefore, to maintain the temperature in winter to retain the biogas production rate is crucial.
Recovering waste heat from engine for biogas production process or for swine farm usage can save more energy from power supply.
The air temperature at outlet of heat exchanger cannot be lower than 100℃ such that water vapor in the waste gas will not be condensed.
Liquid water with H2S will corrode the engine and heat exchanger.
Collect inlet and outlet temperature data of water flow and waste gas from heat exchanger so the total energy recovered by heat exchanger can be calculated. With the fuel consumption rate, the overall efficiency by waste heat recovery now can be calculated. The experimental procedures as following:
1. Set up the heat exchanger at waste gas outlet.
2. Start the water pump to let water flow through heat exchanger.
3. Start the engine at least 20 minute.
4. Collect water flow rate, water inlet and outlet temperatures and waste gas inlet and outlet temperatures.
Note that the pump must be started before the engine. Because the heat exchanger without flowing water inside, the fin and pipe will be over heated and could melt or be damaged as they directly contacted the hot waste gas.
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3.4 Oxygen-Enriched Combustion
Biogas contains 20~30% carbon dioxide, which can feed algae in advance in other subproject. After feeding the algae, there is a little pure oxygen left in the biogas so that it can make an oxygen-enriched burning in the combustion chamber.
The extra oxygen mixes with fuel and the mixture flows into the engine.
After the engine is operated steadily, collect the data include biogas flow rate, air flow rate, power output, waste gas temperature, carbon monoxide concentration, oxygen concentration and carbon dioxide concentration.
Find the new air-fuel ratio. Then, calculate the engine thermal efficiency.
The experimental parameters are the biogas flow rates at 220L/min, 240L/min and 260L/min, respectively, and each is with five different excess air ratios.
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Chapter 4
Results and Discussion
The biogas was from the anaerobic tank made of a red plastic bag. The original biogas from the tank contains high concentration of H2S, around 3000ppm. Such high concentration H2S will corrode the engine, so a H2S removal system developed by Tseng and his colleagues [15] was installed such that it could effectively reduce H2S concentration from 3000ppm to 300ppm. After the reduction of H2S concentration, the biogas was measured by using Cosmos XP-3140 (the high concentration methane and the carbon dioxide analyzers), and found it containing 59%~62% of CH4 and 20%~30% of CO2. This work regarded the treated bigas after H2S removal as consisting of 60% of CH4 and 40% of CO2, by which all calculations were based on this base line. The engine operated with the biogas as fuel and generated electric power for farm using.
4.1 Effect of Excess Air Ratio (λ)
In this section, the effects of fuel supply rate together with corresponding different excess air ratio on power generation are studied.
The fuel (biogas) supply rates tested were 180, 200, 220, 240, and 260 liters per minute, respectively.Under each fixed biogas flow rate, it tests five different excess air ratios, ranged from 0.8 to 1.2. The excess air ratio (λ) was defined in Eq. (3.5) of section 3.2. Note that the mole of fuel counted in this study was the one of methane but not the one of biogas itself. The reason is that the CO2 existed in the biogas did not particate the combustion. Another important issue is that because the maximum allowable total volume flow rate (sum of biogas and air flow rates) into
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the engine is about 1800 L/min, the maximum air supply rate is limited by the fixed biogas one that can be observed in the last columns of Table 4.1a~e. In other words, the experiment with the higher biogas flow rate carrys out a narrower range of air flow supply rate.
4.1.1 Power Generation
Figure 4.1 shows the power generation rates as a function of excess ratio ratio at different biogas supplies. The detailed experimental values are given in the third row in Table 4.1a~e.
It can be seen from this figure that under the constraints of the engine, the higher the fuel supply rate, the higher the power generation at a given excess air ratio. The maximum power output is 26.8 kW at biogas supply of 260 L/min with an excess air ratio of 1.01 (nearly stoichiometric), equivalent to 1500 L/min of air supply. For the biogas supply rates above 240 L/min, the power generation increases with the increase of excess air ratio, whereas for those of 200 and 220 L/min the power generation starts to drop after λ around 0.95 (approximately the stoichiometric ratio), which is in fuel-rich range (λ is the reciprocal of equivalent ratio). For the case of 180 L/min, it even decend from very beginning λ =0.78. From the power point of view, the three fuel supply rates, shuch as 180, 200 and 220 L/min, cannot produce the ecectricity more than 20kWe that may not be appropriate in real application.
It can be seen from this figure that under the constraints of the engine, the higher the fuel supply rate, the higher the power generation at a given excess air ratio. The maximum power output is 26.8 kW at biogas supply of 260 L/min with an excess air ratio of 1.01 (nearly stoichiometric), equivalent to 1500 L/min of air supply. For the biogas supply rates above 240 L/min, the power generation increases with the increase of excess air ratio, whereas for those of 200 and 220 L/min the power generation starts to drop after λ around 0.95 (approximately the stoichiometric ratio), which is in fuel-rich range (λ is the reciprocal of equivalent ratio). For the case of 180 L/min, it even decend from very beginning λ =0.78. From the power point of view, the three fuel supply rates, shuch as 180, 200 and 220 L/min, cannot produce the ecectricity more than 20kWe that may not be appropriate in real application.