共同利用型バイオガスプラントのエネルギー利用方法に関するライフサイクル的評価

• Material Type: 博士論文
• Title: 共同利用型バイオガスプラントのエネルギー利用方法に関するライフサイクル的評価
• Author/Editor: 中山, 博敬
• Publication Date: 2014-03-31
• Publication Date (W3CDTF): 2014-03-31
• Alternative Title: Evaluation of Energy Usage in Centralized Biogas Plant from Life Cycle Approach
• Degree grantor/type: 酪農学園大学
• Date Granted: 2014-03-31
• Date Granted (W3CDTF): 2014-03-31
• Dissertation Number: 甲第142号
• Degree Type: 博士（農学）
• Conferring No. (Dissertation): 甲第142号
• Text Language Code: jpn
• Note (General): I. Introduction As the number of dairy cattle per dairy farm increases in Hokkaido, an appropriate treatment of liquid feces and urine generated at cowsheds is essential. One of the facilities that treat such a liquid manure is a methane (anaerobic) fermentation-based biogas plant. It is the site where feces and urine is fermented under an anaerobic condition with a certain temperature. After fermentation, digested slurry left over is spread over fields as fertilizer. In the course of fermentation, organic matters are bio-decomposed into gas, which can be used as energy. There are many biogas plants operated in Europe. In those plants, the combined heat and power (CHP) system converts biogas into electricity to sell to power companies and households. Thermal energy is used under the local heating network. Biogas is also purified to use as fuel for natural gas vehicle. In Japan, Hokkaido leads the country's biogas plant operation. The form of biogas utilization is categorized into using heat converted by gas boiler, and using both heat and electricity generated by CHP system. In addition, a portable equipment has developed to purify, compress and replenish biogas, which has been tested to demonstrate that the purified gas can be used for natural gas vehicle and gas burners. Biogas can be treated by gas-boiler, CHP system and purification equipment. However, there are no previous reports that assess those three forms of biogas utilization by comparing their energy productive efficiency. In this paper, the authors have conducted a life cycle assessment of the productive and economic efficiency of biogas between the three forms of utilization (gas boiling, CHP system and purification), given that biogas is used in centralized biogas plants in cold regions. We particularly focused on the following objectives: (1) to establish a program in which biogas plant operation is simulated, calculate the energy balance of biogas under the cold climate of Hokkaido, and clarify the energy volume and energy productive efficiency of biogas utilized in each form; (2) to clarify the energy balance of biogas utilized in each form in consideration of the energy volume required for plant construction and maintenance, and conduct a life cycle assessment of each energy utilization system; (3) to conduct a life cycle assessment of biogas' economic efficiency given that the biogas energy replaces fossil energy, with consideration of costs for plant construction and maintenance; (4) to propose possible methods for utilizing biogas energy generated at centralized biogas plants in cold regions, toward future community development in Hokkaido. II. Form of Utilizing Biogas and Energy Balance Assessment A simulation model was created based on measurement data gathered from centralized biogas plants in operation. The effects of biogas utilization methods on energy balance during plant operation were compared quantitatively for cases in which a gas boiler, a CHP system or a purification system is used, to determine the most efficient biogas utilization method. The greenhouse gas emissions were also measured from the amounts of fossil energy input and methane in the off-gas discharged during refinement, to clarify the environmental burden. The results revealed that the energy production efficiency was the highest for biogas plants treating waste from 1,000 dairy cows using a CHP. The greenhouse gas emission was the smallest in the case using a CHP and the largest in the case using a purification system. The greenhouse gas emission per GJ of output energy in the case of using a purification system was more than ten times that of the case using a CHP. III. Evaluation of energy balance on biogas plant operation from Life Cycle Approach Energy is used in constructing and renovating a biogas plant and in transporting excreta, which is the raw material. However, using the digested slurry that is produced by such plants as fertilizer can reduce the usage of chemical fertilizers. Therefore, it is possible to regard the use of digested slurry for fertilizer as offsetting the energy required to manufacture chemical fertilizer. In light of the above, this section compares the amount of energy that is consumed when a plant is constructed and when excreta is transported to the amount of energy that is produced at a plant (covered in Section II). The aim is to clarify the life-cycle energy balance in consideration of the durable years the facility. It was revealed that the energy output exceeds the energy consumption from the sixth or seventh year of plant operation, regardless of the gas utilization method, i.e., those using a gas boiler, a combined heat and power (CHP) system and a purification system. IV. Form of Utilizing Biogas and Economic Efficiency Assessment Biogas-made energy is supplied from the plant in three forms: heat, electricity and purified gas. These forms of biogas energy can replace several types of fossil energy as alternative. The price and the amount of energy vary according to the fossil energy. In light of this, the economic value of the biogas energy supplied from the plant must be assessed by fossil energy type. Also, the economic efficiency in an energy-generating system needs to be assessed in consideration of expenses for constructing, maintaining and renewing biogas plants. In this section, we compared biogas energy and replaceable fossil energy, and then clarified the life cycle economy of biogas in consideration of the plant's durable years. When we conducted a life cycle assessment of the biogas plant in economic efficiency, its economic balance was found to be negative over 35 years since operation commenced, regardless of converted biogas forms and replaced fossil fuels. We assumed three possible methods for improving the economic balance: reducing expense with construction cost subsidy, increasing revenue by accepting auxiliary feedstock, and increasing unit costs of replaced fossil energy. We then re-assessed the economic balance under these conditions. In subsidizing half the plant construction cost, the economic balance was positive in all forms until when main facilities were renewed. With no subsidization for renewal, the balance subsequent to the facility renewal fell negative. In accepting auxiliary feedstock equivalent to 41.52 million yen based on previous studies, the long-term economic balance was positive in all forms. In defining the unit cost of fossil energy as 11.4 yen per MJ based on the feed-in tariff (FIT) system, the long-term economic balance was positive in the form of gas boiler, whereas it was negative in the co-generating system. V. Discussion 1. Proposal on Technical Improvement of Centralized Biogas Plants in Hokkaido Digestive slurry heated to 55 °C in a sterilization tank is transported to a storage tank outside, where the heat is lost into the air. If thermal energy of the digested slurry is used to heat feedstock fermented, this may help reduce the thermal heat supply from gas boiler in winter. It may also allow more amount of energy to be supplied out of the plant. To verify this, we calculated the energy balance of the three biogas forms, given that the heat lost when the sterilized digestive slurry is cooled down from 55℃ to45℃, 2,093 MJ/d, was used via a heat converter to heat the material slurry. We found from the calculation that the CHP system was the most efficient in energy production, showing that the electricity supplied out of the plant increased by 7%, and the heat by 89%. Thermal recycling from digested slurry can increase the amount of electricity and heat supplied outside of the plant. Thus, it is necessary to introduce the thermal recycling system. 2. Biogas Transportation to Facilities that Utilize Heat In Denmark, biogas generated is transported to district heating facilities and is used as fuel for the CHP system. In urban areas of Hokkaido, meanwhile, there are district heating facilities in some areas, remote from the places in which farm animal manure is collected. This present condition makes it hard to introduce the Denmark style of biogas transportation. Building district heating facilities may be an option in areas where the manure is collected. However, it is not practical in that most conventional houses use oil stoves instead of district heating system. In today's local infrastructure, one possible facilities where biogas heat can be used yearly is a heated swimming pool. We assessed the energy balance and economic efficiency of biogas when transporting biogas from the plant through the pipe line to the heated pool in the city centre. By assessing three forms of biogas utilization, we found that the CHP system is the most energy-efficient. However, the life cycle assessment of economy revealed that the sales of electricity in the FIT system to replace A heavy oil would never make the economic balance positive. Therefore, it is clear that the revenue increase by accepting auxiliary feedstock is necessary to achieve a long-term economic balance of the biogas plant. It is considered that if auxiliary feedstock is not accepted, a system similar to FIT should be built to promote the use of thermal energy from biogas. 3. Proposal of Utilizing Energy Generated from Centralized Biogas Plant in Hokkaido In converting biogas into thermal energy to heat water, gas boilers are simple-structured and easy to install and maintain. In light that thermal energy is generated more in summer than winter, operators should ensure that there are facilities that consume a sufficient amount of heat in summer so as not to have excess heat in that season. The operators also should ensure that alternative type of energies are available to supply heat in winter to compensate the shortage. CHP system of biogas can bring us with both electricity and heat. Electricity is easy to be transmitted via electric wires. The amount of heat generated is subject to season: it increases in summer and decreases in winter. Selecting facilities which use heat needs to be carefully made. Biogas purified is equivalent to natural gas, but it is not appropriate, in terms of energy efficiency, to transport the purified gas to remote areas and to use it by gas boilers or CHP system. Therefore, some systems are required so that consumers can receive purified gas on site, for example, consumers drive a natural gas vehicle to the site and refuel the purified gas. It must be noted, however, that purified gas generated in summer is twice that of winter. It means that the amount of feedstock manure and the resulting purified gas to fill the demand in summer cannot satisfy the winter demand, and then natural gas needs to be purchased from other areas to supplement the shortage. 4. Significance of the Economic Assessment of Biogas by Utilization Form and the Proposal of Utilization of Energy Generated In a biogas plant capable of managing cattle manure of 1,000 heads of cattle, the most energy-efficient method of utilizing biogas is using biogas by the CHP system. In terms of economic efficiency, biogas management never gains a positive figure by solely replacing conventional fossil energy by the energy from manure, assuming that the revenue is gained through manure management at the plant and economic value of digested slurry as fertilizer. For an economic benefit, the electricity generated needs to be sold under FIT, and the heat needs to be sold with 11.4 yen per MJ. These findings can be applicable when community associations and farming managers construct centralized biogas plants, as well as in designing the methods for promoting renewable biogas energy.; 開始ページ : 1; 終了ページ : 162
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• Date Accepted (W3CDTF): 2016-06-03T01:05:16+09:00
• Date Created (W3CDTF): 2017-09-19
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