Chen et al. [1] analyzed renewable energy situation in Taiwan, such as biomass energy, solar energy, wind power, geothermal energy and hydropower etc. They indicated that renewable energy has not yet fully developed in Taiwan because the fossil energy is cheaper than renewable one. However, the renewable energy will become more competitive in the energy market since Legislative Yuan passed ―Renewable Energy Development Bill‖ in June 2009.
Besides, the promotion of the renewable energy will offer positive economic benefits for the related industry.
Rasi et al. [2] researched biogas component and variation in landfill, sewage treatment plant sludge digester, and farm biogas plant to analyze its potential used as bio-energy. They found that the biogas compounds vary with different biogas plants: carbon dioxide ranges from 36% to 41%, methane from 48% to 65%, nitrogen from 1% to 17% and oxygen content is less than 1%. Sewage digester biogas contains highest methane content, landfill biogas contains lowest methane and highest nitrogen contents in winter. The total volatile organic compounds (TVOCs) range from 5 to 268 mg/m3, and the farm biogas plant has the lowest TOVCs. The sulphur compounds are found in all three places.
Lin [3] tested different air-fuel ratios for 30kW generator with 60% methane concentration of biogas in a small swine farm in Miaoli. The oxygen-enriched combustion and heat recovery were also applied to his research. The results showed that a higher power output and better thermal efficiency can be achieved by a greater conversion of CH4 in the combustion process. The engine performances are not improved much by 1% oxygen-enriched air, but
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with 3% oxygen-enriched air, the maximum power generation and thermal efficiency are increased, especially the engine now can be operated normally at a lower limiting fuel supply rate. The heat recovery system is used to heat water, leading to an improvement of overall efficiency.
Tsai and Lin [4] surveyed bio-energy from livestock manure management in Taiwan. With a practical basis of the total swine population from the farm scale of over 1000 heads, the analysis showed following benefits: emissions of methane reduce 21.5 Gg, total electricity is generated of 7.2 × 107 kW-h per year, equivalent to electricity charge saving of USD 7.2 × 106, and carbon dioxide mitigation is of 500 Gg per year.
Su et al. [5] established greenhouse gas production data from anaerobic livestock wastewater treatment processes in Taiwan, and made the difference between the livestock wastewater treatment system in Taiwan and that presented by the IPCC. The data revealed that anaerobic wastewater treatment systems of pig and dairy farms emit 0.768 and 4.898 kg CH4, 0.714 and 4.200 kg CO2, and 0.002 and 0.011 kg N2O per year per head during three temperature periods. Because animal manure is diluted before being treated with a solid/liquid separator and an anaerobic wastewater treatment system, the average emissions rates of CH4 in the selected pig and dairy farms are lower than the limits imposed by the IPCC.
Yang et al. [6] estimated of methane and nitrous oxide emissions from animal production sector in Taiwan during 1990~2000. Methane emission from enteric fermentation of livestock was 30.9 Gg in 1990, increased to 39.3 Gg in 1996, and decreased to 34.9 Gg in 2000. Methane emission from the waste management was 48.5 Gg in 1990, 60.7 Gg in 1996, and 43.3 Gg in 2000. In the case of poultry, methane emission from enteric fermentation and waste
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management were 30.6~44.1 ton and 8.7~13.2 Gg. Nitrous oxide emission from waste management of livestock was 0.78 ton in 1990, 0.86 ton in 1996, and 0.65 ton in 2000. Nitrous oxide emission from waste management of poultry was higher than that of livestock with 1.11 ton in 1990, 1.68 ton in 1999, and 1.65 ton in 2000.
Saiful Bari [7] used carbon dioxide and nature gas, which contained about 96% methane, to simulate the operation of diesel engine in dual-fuel mode with biogas containing various percentages of carbon dioxide. They found out when biogas contains more than 40% carbon dioxide, the engine runs harshly. The trend of bsfc (brake specific fuel consumption) curve decreases in 20 to 30 per cent carbon dioxide region, but raises beyond certain (20 to 30) per cent of carbon dioxide region. It is because the carbon dioxide can be dissociated into carbon monoxide and oxygen under high temperature, and the carbon monoxide is comparatively fast burning gas than other alternative fuels, in the meantime, the oxygen from the dissociation increases the concentration of oxygen in the gas air mixture, leading to reducing the ignition delay and enhancing the combustion of unburned carbon particles. But when the carbon dioxide content is beyond certain region, the carbon dioxide becomes higher for the superfluous carbon dioxide remains undissociated, making the curve of bsfc to rise with an increase carbon dioxide concentration.
Duc et al. [8] used a small IDI biogas premixed charge diesel dual fuelled CI engine to test Diesel fuel substitution, engine performance, energy consumption and long term operation. They obtained following results. First, although the diesel dual fuel (DDF) mode has lower energy conversion efficiency, it can be offset by the reduced fuel cost of biogas over diesel. At low and medium loads, the DDF engine produces higher UHC (unburnt hydrocarbon) and less soot,
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leading to a reduction in efficiency. When engine load increases to full load the conversion efficiency is good as that in diesel fuelling. Second, the engine operates in DDF mode has the lower exhaust gas, higher lube oil and higher cooling water temperatures than those temperatures in diesel mode. Last, as a gas mixer is installed, the temperatures of oil and cooling water increases and lube oil sucks to the cylinder, resulting in high lube oil consumption. So, the engine cannot withstand the higher thermal load under DDF mode at the engine speeds and loads proposed for diesel fuel.
Tippayawong et al. [9] used biogas and diesel mixed fuel to feed a small diesel engine and examined its endurance over 2000 hours. The results showed that engine has 7% of higher power output and higher efficiency compared to that in normal diesel operation.
Alasfour [10] used 30% iso-butanol-gasoline blend and preheated the inlet air to investigate the NOx emission in a spark ignition engine. The results showed that the maximum level of NOx emission is reduced by 9% in 30%
iso-butanol-gasoline blend comparing to gasoline. When the inlet air temperature increases from 40 to 60°C, the NOx emission will increase 10% at a fuel/air equivalence ratio of 0.9.
Porpatham et al. [11] tested the effect of CO2 concentration in biogas on the performance of constant speed spark ignition (SI) engine. A lime water scrubber was used to absorb carbon dioxide (CO2) in biogas. They found when carbon dioxide (CO2) in biogas is reduced from 41% to 30%, then 20% of engine performance is improved, unburned hydrocarbons (HC) is reduced and lean limit of combustion is extended. However, such improvement occurs just in the lean-fuel region. Increasing methane concentration plays a significant role in lean-fuel region because the flame velocities are low in such region. There is
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no benefit for power and efficiency on the rich-fuel side due to incomplete combustion of engine.
Borjesson and Berglund [12] analyzed the emissions of nitrogen oxides (NOx), carbon dioxide (CO2), carbon oxide (CO), sulphur dioxide (SO2), hydrocarbons (HC), methane (CH4), and particles from a life-cycle for different biogas systems on six different raw materials. They identified that the biogas systems emit lots of gases mentioned above, and the emissions are affected by the properties of the raw material digested, the energy efficiency of the biogas production, and the end-use technology. Between two biogas systems that provide an equivalent energy service, fuel-cycle emission may vary by a factor of 3–4, and for certain gases the factor by up to 11. There are extensively significant source of emissions, for example, waste-products or ley cropping.
Abd-Alla et al. [13] operated a high speed indirect injection dual fuel engine, using methane and sometimes propane as main fuel and Diesel fuel as pilot fuel.
The effects of exhaust gas recirculation (EGR), diluents admission (N2 and CO2), and intake air temperature on combustion and emissions were concerned. The results showed that the admission of diluents reduce the NOx emissions, the higher intake temperature increases the NOx emissions but reduces the unburned hydrocarbon emission.
Tsagarakis [14] analyzed optimal number of energy generators for biogas utilization in wastewater treatment facility. The data of this analysis was based on the first generator for energy production from biogas, and it had been operated for 5.5 years. If one generator is used, the cost per kWh produced is 0.0876 €/kWh covering 15.9% of the facility’s needs. If two generators are used, the average cost for energy production is 0.0881€/kWh covering 32.6% of the facility’s needs. The estimations of six generators have been made. The
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economic analysis is calculated by total annual economic cost, which is the sum of the annuitized construction cost and the annual operation and maintenance costs. The results showed that the cost of each kWh produced may increases when the number of generators is increased, the cost decreases when the lifetime of generator increases.
Semin et al. [15] compared the cylinder pressure and maximum pressure of the compressed natural gas engine with original diesel engine. The result showed that the transform of diesel engine into compressed natural gas engine decreases the cylinder pressure.
Torregrosa et al. [16] tested the effect of coolant and inlet charge temperature on the emission reduction and performance of DI Diesel engines. The coolant temperature ranged from 65 to 97℃. The intake charge temperature ranged from 44 to 68 ℃. The results show that the coolant temperature has low effects on engine operation. There is no effect at medium or higher load. The NOx
emissions decrease and HC emissions increase for lower wall temperature at low loads. Intake air temperature influences NOx formation for all the loads, as the temperature increases the NOx emissions increases. But its influence on HC emissions is restricted to low loads, as the temperature increases the HC emissions decrease. For engine, HC emissions are the result of incomplete combustion, and the higher HC emissions imply the lower fuel conversion efficiency.
Zarante et al. [17] operated four-cylinder, flexible fuel engine, using gasoline and nature gas as fuels, to evaluate the exhaust emissions of carbon monoxide (CO) and carbon dioxide (CO2). Due to the low carbon-hydrogen ratio of nature gas with regard to gasoline, the CO2 emission of nature gas is less than that of gasoline. Also the CO emission of nature gas is less than that of gasoline,
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because the engine can operate with leaner mixtures when natural gas is used instead of gasoline.
Ga et al. [18] used biogas-gasoline hybrid engine to test the conversion efficiency of the biogas. The results showed that 1 m3 of biogas can produce 1kW-h electricity and reduce 1kg CO2 emission in the atmosphere.
Cho et al. [19] made a review for spark ignition of natural gas engines. In order to meet the emission standards and consider the stable combustion of engine, several methods can be used. First, lean burn is an effective way to reduce NOx emissions, but for recovering power output losses, turbocharging technology should be considered. Second, stoichiometric natural gas engine can equip with three-way catalyst to convert CO, HC and NOx, however, air-fuel ratio controller is needed. Third, EGR can improve knocking situation by reducing combustion temperature.
Huang and Crookes [20] diluted natural gas by using CO2 to simulate biogas as fuel in single-cylinder spark-ignition engine. The fraction of CO2 in simulated biogas was ranged from 0 to about 40%. They tested the effects of CO2 fraction, relative air-fuel ratio, engine speed and compression ratio on the engine performance and exhaust gas emissions. The measured results included power, thermal efficiency, exhaust temperature and mole fractions of the emissions CO, NOx and unburnt hydro-carbon in exhaust gases. The following are conclusions obtained from experimental results: First, increasing the fraction of CO2 in biogas can lower the NOx emission and enable the compression ratio to be increased. However, it would reduce cylinder pressure that results in the reductions of power and thermal efficiency, and the increases of unburnt hydrocarbon emissions at the same time. Second, the CO emissions are low and change little when running with fuel-lean mixture. As running
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with fuel-rich mixture, the CO emissionsincrease sharply when CO2 fraction is above 30% due to incomplete combustion. The CO emissions are almost unaffected by compression ratio and engine speed. Third, when the compression ratio becomes higher, the brake mean effective pressure, brake efficiency and emissions of NOx and unburnt hydrocarbon become higher. But when the compression ratio is above 13:1, the power and thermal efficiency increase slightly and the emissions of CO increase. As the compression ratio is further above 15:1, the detonations occur. Fourth, the highest power and thermal efficiency occur with compression ratio between 13:1 and 15:1 and the relative air-fuel ratio between 1.05 and 0.95. In this range, the emissions of CO and unburnt hydro-carbon are low but the NOx emissions are relatively high.
Nathan et al. [21] converted a single-cylinder, diesel engine to operate in homogeneous charge compression ignition (HCCI) mode with acetylene as fuel. They tested the effects of intake air temperature and exhaust gas recirculation (EGR) on the engine performance and exhaust gas emissions.
The intake air was heated by an electric heater in the range of 40~110℃ from no load to brake mean effective pressure(BMEP) of 4 bar. The results showed that the intake air temperature and amount of EGR have to be controlled according to engine output. At high engine output, engine is very sensitive to the intake air temperature and EGR. In order to get greater brake thermal efficiency, precise control is required. It is observed that the best charge temperature is reduced as BMEP increases, because the elevation of BMEP will lead to an increase of engine temperature and make the mixture become richer. When the mixture is rich, the self-ignition temperature reduces and the combustion rate increases. Besides, at high BMEPs, using EGR will lead to knock.
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Badr et al. [22] carried out a parametric study on the lean misfiring and knocking limits of gas-fueled spark ignition engine. They tested Ricardo E6 engine, using propane and liquefied petroleum gas (LPG) as fuels. The parameters included engine speed, compression ratio, spark timing, intake temperature, intake pressure, and relative humidity of intake air. The following are experimental results: Advancing the spark timing leads to the reduction of lean misfire and knocking limit. For low engine speeds, when the intake temperature increases, the lean misfire limit decreases. For high speeds, when the intake air temperature is up to 70℃ the lean misfire limit increases, beyond 70℃ the lean misfire decreases. As the relative humidity of the intake air increases, the lean misfire limit increases because the water vapors as a diluents will damp down the reaction rates during compression and combustion processes.
Sridhar et al. [23] reveled the misunderstanding when gas as a piston engine fuel. They converted multi-cylinder diesel engine into spark ignition engine, and the compression ratio ranged from 11.5:1 to 17:1. The results showed that the engine can run smoothly without auto-ignition tendency at high compression ratio (17:1). Besides, the engine can get higher efficiency and more brake power when working at higher compression ratio. However, comparing with diesel mode, the maximum de-rating in power is 16% and the overall efficiency drops down by almost 32.5% in gas mode. The analysis revealed that excess energy loses to coolant due to combustion chamber design.
In order to improve the efficiency, the combustion chamber design for gas fuel is needed in the future.
Tricase and Lombardi [24] evaluated the development of biogas in Europe and Italy. The amount of biogas increases gradually, and the usage of biogas
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depends on biogas quality. The percentage of biogas for generating electricity is 2/3 of total amount, and for generating heat is 1/3 nowadays. While Great Britain and Germany are the main producing countries so far, France has the highest production potential in the future. Most of Italian biogas is from landfills, and the biogas from animal waste is only small percentage of total.
Chung et al. [25] tested the chemical absorption and a biological oxidation process to remove high H2S concentrations. The results suggested that the liquid flow rate in the biological oxidation reactor was controlled at 3 mL/min, the volume ratio of biological reactor to chemical reactor was 13.5:1 when 150 g-S/m3/h of inlet H2S loading was introduced to the system.