Scope of Present Study

This study carries out the 30kW-generator experiments on a small biogas plant in a swine farm to collect data to serve as a preliminary study for constructing a 300-KW power plant of a bigger scale biogas plant in the near future. The energy balance for overall biogas into the energy cycle was tested and calculated. The three-step piggery wastewater treatment system and H2S removal system need energy to operate, so the net energy output from biogas-powered generator should deduct the one consumed by the systems mentioned above. In this study, the first task is to find the optimum air-fuel ratio for the 30kW engine under the different methane (CH4) concentrations in biogas for the best efficiency. Secondly, build a waste heat recovery system to maintain the minimal temperature (should be greater than 15℃) in winter to maintain the generation rate of biogas as that in summer, and calculate the respective efficiency with and without waste heat recovery system, and make a comparison. The last one is to apply oxygen-enriched combustion for the engine with different oxygen (O₂) concentrations to justify whether the efficiency increase is comparable with the cost of increasing oxygen.

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Chapter 2

Biogas System in Swine Farm

2.1 Swine Manure Management

Swine production is very important in the agriculture of Taiwan. If the amount of wastewater produced by a pig is estimated as 20 liters per day, then the total wastewater produced by 6.6 million pigs (the total number of pigs in Taiwan) is 19 tons per day, making as the third most pollution source in Taiwan that is behind the sewage and industrial wastewater.

In 1987, the quality of waste water draining from livestock farms has been required to meet the governmental standards. Since then, a great variety of wastewater treatment methods have been developed and tried.

Among them, the three-step waste treatment system developed by the Taiwan Livestock Research Institute (TLRI), which includes solid-liquid separation, anaerobic treatment and aerobic treatment (activated sludge treatment system), is regarded and accepted as one of the best systems in Taiwan, and has been extended to hog farms since 1987. Via the three-step treatment, both the Biochemical Oxygen Demand (BOD) and Suspended Solids (SS) of treated water can be less than 100 mg per liter.

2.2 Three-step Piggery Wastewater Treatment (

)

The three-step piggery wastewater treatment system is based on a typical continuous plug-flow design, and the volume of raw wastewater remains constant over each 24-hour period. Under optimal operation conditions, wastewater thus flows into the system and is discharged continuously. Anaerobic treatment is conducted after solid/liquid

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separation, and occurs inside of anaerobic basins covered with “red-mud plastic cover” (1.2~1.8mm of thickness), made of a kind of PVC material, which is corrosion-resistant and gas-and-water impermeable. Anaerobic treatment is generally slower than forced aeration, but consumes less energy. The anaerobic treatment system of TPWT process can also salvage a part of chemical energy content of wastewater by generating methane, a useful fuel. The optimal hydraulic retention time is around 4–6 days, and BOD removal is expected to be more than 80%.

2.2.1 Solid-liquid Separation

Separation of the solid fraction from the wastewater is to reduce the content of solids for subsequent handling and treatment, and to recover the solids for using as fertilizer, etc. This physical process is accomplished by using various kinds of screens. The efficiency of this treatment is a 15-30% decrease in BOD and a 50% decrease in SS. The moisture content of the separated solids is 70-80%. An extruder is often added to reduce the water content of the solids to 70% or below so that the material is suitable for composting.

2.2.2 Anaerobic Treatment

Since hog wastes are biodegradable, biological treatment is generally an economical way of handling them. The horizontal tent-type anaerobic fermenter is a modification of the Red Mud Plastic (RMP) bag fermenter which was also developed by the Taiwan Livestock Research Institute.

Among its merits are the fact that it is easy to construct, has a low investment cost, is easy to maintain, and can be separated into several divisions as desired. These fermenters can be sealed from either inside or

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outside. The four sides of RMP sheet, which make up the top of fermenter, are in tubular shape, so that PVC pipes may be inserted inside them to give extra strength. The strengthened sheet is then fixed to the wall of the fermenter with hooks.

The hydraulic retention time (HRT) is calculated according to the amount of water used to wash the pig houses, as the following the formula:

HRT =

A 1:3 ratio of manure to washing water is suggested, which can easily be achieved with a flushing tank system. According to the work by Hong (1985), the daily excreta of a 100-kg pig is around 5 liters, so the total wastewater from one pig may be estimated as 20 liters. A HRT of 12-15 days is common for hog wastewater treatment.

A tent-type fermenter should consist of no fewer than two digesters.

The volume of the first digester is usually 1.2 times of the daily wastewater. Both acidogenesis and sedimentation take place in this first digester. Most of the methanogensis reaction occurs in the rear digester(s).

Biogas may be collected for the use as fuel. The excreta of each pig can generate 0.1-0.3 m³ of biogas per day. Biogas may be used in cooking stoves, water heaters, water pumps, electric power generators, gas lamps, warming piglets, vehicles, mowers, and incinerators for animal bodies, etc.

2.2.3 Aerobic Treatment

There are many kinds of aerobic treatments that may be utilized for livestock wastewater. Considering the environmental conditions of

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Taiwan in subtropical climate, activated sludge processing and oxidation ditches are recommended. In aerobic treatment, organic matter is decomposed solely through aerobic oxidation.

Activated sludge processes are versatile and flexible. Effluent of any desired quality can be produced by varying the processing parameters.

These processes require less land but more skilled management than simpler processes, such as oxidation ditches. Activated sludge is a complex biological mass, resulting from when organic wastes are aerobically treated. The sludge will contain a variety of heterotrophic microorganisms, including bacteria, protozoa, and higher forms of life.

The relative abundance of any particular microbial species will depend on the type of waste that is being treated, and the way in which the process is operated. For optimum treatment, raw waste must be balanced nutritionally. In three-step treatment, most of the easily biodegradable matter has already been decomposed in the anaerobic digester, therefore, operating an activated sludge treatment requires intensive care for good performance. It is best to control the BOD of anaerobic effluent at around 1000 mg/L. The growth conditions for microorganisms in activated sludge tanks are usually measured according to the mixed liquor suspended solids (MLSS) and sludge volume index (SVI). The HRT for an aerobic tank is normally 1.0-1.5 days.

While activated sludge tanks have a water depth of 2-5 m, this should not exceed 1.5 m in oxidation ditches. Oxidation ditches, therefore, require a larger land area, but have the advantages of being easy to operate and of generating less sludge.

A final clarifier to settle the activated sludge before the discharge of

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treated water is required in aerobic treatment. The settled sludge may be removed by mechanical methods for return to the aerobic unit, or be dehydrated for disposal. Usually the HRT in the clarifier should not exceed 6 hours.

2.3 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 the same time for higher efficiency. Such power generators are called combined heat and power (CHP) generation plants, and normally use a four-stroke or a Diesel engine. 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.

Figure 2.1 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. Vehicles can sometimes be

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driven by the power or the heat.

2.4 Engines

Figure 2.2 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 of those given in this figures.

2.4.1 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 (NO_x) 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 window 1.3 < λ < 1.6, λ = air-fuel ratio/stoichiometric air-fuel ratio), where the efficiency is expected to drop. The efficiency of lean-burn engines with turbocharger is 33~ 39%. The NO_x emissions can be reduced, however, by a factor of 4 in comparison to ignition (by compression) oil Diesel engines, and the limiting values can be met without further measures. Since the engines tend to knock with varying gas qualities, a methane content of at least 45% in biogas should be ensured.

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,

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their NO_x emissions are higher. Their lifetimes usually are given as 35,000h of operation.

In general, gas Diesel engines work 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 15% better than that in a four-stroke engine.

2.4.2 Stirling engine

An alternative to the commonly used four-stroke and the Diesel engines is the Stirling engine. The efficiency of the Stirling process is closest to that of the ideal cycle. The Stirling engine has been recommended for power generation for many years, but is seldom realized on an industrial scale because of technical problems in details.

Power generated from biogas in Stirling engines is not known yet in industrial scale installations.

2.4.3 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

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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.

2.4.4 Micro gas turbine

Micro gas turbines are small high-speed gas turbines with low combustion chamber pressures and temperatures. They are designed to deliver up to 200 kW electrical powers. 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.

2.4.5 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

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