A challenge to expand biogas production by psychrophilic digestion to colder regions lies in the fact that gas production decreases with temperature and becomes insignificant when temperature drops below 15℃. The thesis assess the feasibility of operating a biogas reactor in colder climates of Bhutan by simulating the thermal loss of modified reactor designs. The digestion temperature is set to 35℃, an optimum temperature using cow manure fed on daily basis. The analysis emphasesto the regions where the monthly average temperature drops below 12 ℃ .
The designed underground fixed dome digester of 5.5 m3 capacity is analyzed by applying the numerical thermal model. First the digester is insulated with 0.4 m thick rice straw which is cheap and locally available material. From the daily biogas yield of 1.44 m3 comprising 60% of methane, the energy content derived is 6.64 kWh/m. For the uninsulated digester, external heating elements would have to deliver 181.27 kWh/day to compensate the thermal losses. Increasing the insulation thickness to 1 m is insignificant. Secondly, the reactor is placed inside a greenhouse to increase ambient temperature by 10℃. The average heat loss obtained is 22.50 kWh/day. The calculation shows that even with the proposed improvements in reactor design, it is not energetically feasible to operate in the assessed colder region. The alternative method is to increases the hydraulic retention time but this increases the volume of the digester and cost which limits the affordability of the biogas plant by the rural people. Most reasonable method is to decrease the operating temperature between 18℃ and 28℃. Hence, to expand the dissemination of biogas plants in cold regions in Bhutan further improvements in the design and process efficiency and feasibility study of the heating system are necessary.
Table of Contents
Dedication
Acknowledgement
Abstract
List of Abbreviations
Lists of Table
Lists of Figure
Chapter 1: Introduction
1.1 History of Biogas
1.2 P roblem state ment
1.3 Aim and Objectives
1.4 Methodology
1.5 Li mitations
1.6 Thesis Outline
Chapter 2: Literature Review
2.1 Biological P rocess of Anaerobic Digestion
2.1 Factors Affecting Biogas Production
2.3.1 Temperature
2.3.2 Feedstock
2.3.3 Hydraulic Retention Time
2.3.4 Organic loading rate
2.3.5 pH
2.3.6 Carbon-Nitrogen Ratio
2.3.7 Total Solids and Volatile Solids
2.2 Types of Anaerobic Digestion sys tems
2.2.2 Batch Type System
2.2.3 Continuous Plant
2.4 Biogas P lant in Cold Regions and Methods to Overcome Low Temperature
2.5 Selection of Biogas Plant for Cold Regions of Bhutan
Chapter 3: Scaling of Digester Systems
3.1 Sizing of Reactor
3.2 Sizing of Gasholder
3.3 Construction Materials
3.4 Site Layout
3.4.1 Constructing Cylindrical Reactor
3.4.2 Constructing Gasholder
3.4.3 Constructing Compensation or Outlet Tank
3.4.4 Constructing Mixing Tank
Chapter 4: Thermal Analysis and Simulation Results
4.1 Formulation of the Digester
4.1.1 Thermal Models of each Digester Components
4.1.2 Soil Temperature
4.2 Results and Discussions
4.2.1 Total Heat Loss from the Digester
4.2.2 Alternatives Heating Methods
Chapter 5: Cost -Benefits Analysis
5.1 Cost Saving Analysis
5.1.1 Electricity Cost Equivalent
5.1.2 Liquefied Petroleum Gas (LPG) Cost equivalent
5.1.3 Cost Saving using Biogas over Fuelwood
5.2 Benefits of Biogas Plants
5.2.1 Social benefits
5.2.2 Environmental Benefits
Chapter 6: Conclusions and Recommendations
6.1 Conclusions
6.2 Recommendations
References
Appendix
Dedication
I dedicate my thesis to: My parents for sacrifices, prayers, support and advices.
My lovely wife Pelden Demo for encouragement and being an inspiration for me. My daughter Sonam Tshogyal for being my motivation.
My brothers, Phub Dorji, Sonam Chheda, Karma Sherub and sister Kinley Wangmo for their livelihood support and protection.
In memory of My Grandmothers Zamin and Tomki and to my great grandparents.
Acknowledgement
Fore sincere gratitude to Dr. Martin Elborg for his continuous support during my master thesis, for his patience, enthusiasm and immense knowledge. His guidance help me to complete my thesis on time. I would also extend my profound gratitude to Dr. Tshewang Lhendup, the program coordinator, who is also my co-supervisor for making accessible from his eventful administrate exertion and believing in me. Dr. Nadim Reza Khandaker from Bangladesh for being Bioenergy Module tutor. Besides, I would also like to thank Manoj Sharma, Lecturer for his insightful comments and obliging questions.
My sincere thanks to the National Centre for Meteorology and Hydrology for providing the required data and Department of Renewable Energy. I would also like to thank Mr. Karchung, lecturer of Jigme Namgyal Engineering College for his vibrant response during my need.
I thank my classmates for stimulating discussions, for sleepless nights and for all the fun we had together in the last two years. Last but not the least, I am grateful to the College of Science and Technology (CST), the Royal University of Bhutan for giving me the opportunity to study in Renewable Energy Master’s course, of which I am proud to be graduating as first batch.
Abstract
A challenge to expand biogas production by psychrophilic digestion to colder regions lies in the fact that gas production decreases with temperature and becomes insignificant when temperature drops below 15°C. The thesis assess the feasibility of operating a biogas reactor in colder climates of Bhutan by simulating the thermal loss of modified reactor designs. The digestion temperature is set to 35°C, an optimum temperature using cow manure fed on daily basis. The analysis emphases to the regions where the monthly average temperature drops below 12 °C. The designed underground fixed dome digester of 5.5 m3 capacity is analyzed by applying the numerical thermal model. First the digester is insulated with 0.4 m thick rice straw which is cheap and locally available material. From the daily biogas yield of 1.44 m3 comprising 60% of methane, the energy content derived is 6.64 kWh/m3. For the uninsulated digester, external heating elements would have to deliver 181.27 kWh/day to compensate the thermal losses. Increasing the insulation thickness to 1 m is insignificant. Secondly, the reactor is placed inside a greenhouse to increase ambient temperature by 10°C. The average heat loss obtained is 22.50 kWh/day. The calculation shows that even with the proposed improvements in reactor design, it is not energetically feasible to operate in the assessed colder region. The alternative method is to increases the hydraulic retention time but this increases the volume of the digester and cost which limits the affordability of the biogas plant by the rural people. Most reasonable method is to decrease the operating temperature between 18°C and 28°C. Hence, to expand the dissemination of biogas plants in cold regions in Bhutan further improvements in the design and process efficiency and feasibility study of the heating system are necessary.
List of Abbreviations
Abbildung in dieser Leseprobe nicht enthalten
Lists of Table
Table 2.1: Three Different Operating Temperature of Biogas Plant
Table 2.2: Comparisons of Commonly Used Digester Types
Table 3.1: Design Parameter of Gasholder
Table 3.2: Summary of the Plant's Dimensions
Table 4.1: Digester Dimensions and Parameters with their Respective Values
Table 4.2: Daily Total Heat Energy Loss from the Digester
Table 4.3: Comparison of Insulated and Uninsulated Digester
Table 5.1: Energy Consumption by Electric Cooking Equipment
Table 5.2: Daily and Monthly Energy Consumption by Electric Cooking Equipment
Lists of Figure
Figure 1.2: Bhutan Map with the Location of Bumthang Highlighted
Figure 1.3: Thesis Methodology
Figure 2.1: Biological Stages in Anaerobic Digestion
Figure 2.2: Potential Feed Sources for Biogas Plants
Figure 2.3: General Overview of Control Household-Size Anaerobic Digestion
Figure 2.4: Commonly Built Digester Types
Figure 2.5: General Orientation and Shapes of Greenhouse
Figure 2.6: Hot Water Pot Being Heated on a Bukhari Wood Stove
Figure 3.1: General Components of Digester System
Figure 3.2: Layout of Digester Design
Figure 3.3: Layout and Dimensions of Proposed Biogas Digester
Figure 4.1: Layout of the Digester Showing Heat Loss to the Surrounding
Figure 4.2: Heat Conduction through the Double Layer Gasholder
Figure 4.3: Heat Conduction through Composite Cylindrical Reactor
Figure 4.4: Heat Conduction through Reactor Floor
Figure 4.5: Daily Average Temperature of Bumthang with Fitted Sinewave Function
Figure 4.6: Soil Temperature at Different Depth against Ambient Temperature
Figure 4.7: Daily Average Energy Loss from Digester to the Ground
Figure 4.8: Heat Loss due to Input and Removal of Manure
Figure 4.9: Ambient Temperature and the Temperature Inside Greenhouse
Figure 4. 10: Energy Loss in Underground Digester Built Inside Greenhouse
Figure 4.11: Greenhouse Integrated with Underground Biogas Plant
Figure 4.12: Daily Average Solar Insolation for Bumthang
Figure 4.13: Biogas Plant Integrated with Solar Flat Plate Collector
Figure 4.14: Lowering the Digestion Temperature
Figure 5.1: Energy Trade Balance...60 Figure 5.2: LPG import and Growth Trend
Chapter 1: Introduction
Biogas has been used since 1980 in Bhutan but due to lack of technical support and unavailable materials for maintenance, it is not popular among the rural areas. In 2011, the Department of Renewable Energy (DRE) reintroduced the biogas program to promote renewable energy. Today, about 4,173 biogas plants are operating benefiting about 15,400 farmers within the country (Palden, 2017). There are particularly few nationals studies and reports papers on biogas production since biogas technology is not as widespread as in other developing countries such as India, China, and Nepal, hence, less research on biogas has been conducted.
History of Biogas In 17th century, Jan Baptita van Helmont from Netherlands first noticed the flammable gas present in decaying organic matter. In 1776, Italian physicist, Alessandro Volta succeeded in correlating the amount of organic matter decayed and the quantity of gas produced. He became also know for discovering methane gas. But the construction of biogas plant was not popular because of cheap fossil fuel until in 1973 when oil crisis occurred. The developed countries such as France, British and Germany initiated research and development to build different types of biogas plants to counteract the fuel crisis. Thereafter, the use of biogas plant spread to other developing countries. Today, millions of household-sized biogas plants are operating in India, China, Bangladesh, Nepal and other developing countries.
The household-sized bioreactor was first started in Bhutan in the early 1980s. Initially, the biogas technologies failed due to poor technical designs, lack of spare parts, and poor maintenance (Palden, 2017). In 2011, Bhutan Biogas Project (BBP) under DRE with financial support from Asian Development Bank (ADB) and technical support from SNV Netherlands Development Organization has reintroduced the digester in eight districts, Chhukha, Samtse, Sarpang, Tsirang, Mongar, Samdrup Jongkhar, Pemagatshel, and Trashigang. The project was initially started to promote and market digesters to farmers living in areas with not too cold climate and favorable terrain. The highest attitude where biogas is constructed is at Gasa at the elevation of 2,560 masl (MoAF, 2017). On a personal site visit to two biogas reactor in Damji Gewog, it was found that the gas production is very low in winter (P. Lham, personal communication, June 20, 2019).
Problem statement
The dissemination of biogas plant is not so popular in Bhutan due to cheap electricity and abundant fuelwood available. Farmers with low income cannot afford to build biogas plant even though the government provides two-third of total cost. Few decades ago in early 1980s, the promotion of biogas plant failed due technical designs, lack of spare parts and poor maintenance. In 2011, BBP reintroduced building biogas plants limited to higher ambient temperature in southern parts of the country. Production of biogas is directly related to the ambient temperature. The places where the ambient temperature falls below 15°C suffers from low gas production. The low temperature reduces the growth rate of the bacteria responsible for the anaerobic digestion. In Northern parts of the country, most regions have an average ambient temperature below 15°C. Bumthang is one of the coldest district (Dzongkhag) in Bhutan which lies above 2,600 masl (Figure 1.2). The annual average temperature falls below 12°C. MoAF recorded the region as one with the highest fuelwood consumption used for cooking and heating. People don’t have enough access to Liquefied Petroleum Gas (LPG) due to scattered settlement. To promote biogas plant in such region, digestion temperature need to be increased significantly above ambient temperature in order to produce sufficient biogas. Therefore, researchers have suggested many methods to increase digestion temperature in the literature such as digging compost pit around the plant, using hot water with substrate, greenhouse canopy, solar Photovoltaic, and solar thermal. The thesis will evaluate the feasibility for a biogas reactor in Bumthang. Compared to an uninsulated underground reactor, thermal insulation of the reactor, additional greenhouse construction, and external heating system are evaluated. With the goal to keep the reactor temperature at 35°C, the reactor designs will be carried out.
Abbildung in dieser Leseprobe nicht enthalten
Figure 1.1: Bhutan Map with the Location of Bumthang Highlighted
Aim and Objectives
The aim of the thesis is to design and analyze the thermal performance of anaerobic digestion for the regions where average ambient temperature falls below 15°C and to find out the economic feasibility of integrating external heating system to increase the digestion temperature up to 35 °C. The objective of this study is to;
- Identify the limitation of the anaerobic digestion and its implementation in cold climate regions.
- Compare the different design of anaerobic digestion, investigate the applicability of anaerobic digester in cold climate.
- Review the literature in order to find a way to implement anaerobic digestion in efficient and improved way with low investment.
- Provide a guideline with valuable information regarding the anaerobic digestion design process, sizing, control parameters inside the digester for further optimization of biogas production, and most importantly for the process control.
- To apply a mathematical model that predicts heat loss from the designed anaerobic digester to operate at optimum temperature and study the economic feasibility of integrating external heating system such as solar photovoltaic (PV) and solar thermal collectors.
- Study the general benefits of the digester and biogas cost equivalents to other common fuels used for cooking in rural areas of Bhutan.
Methodology
In this thesis, the site location and problem is identified base on the comprehensive literature survey. The digestion temperature is set to 35°C which is proved to be the optimum temperature by many researchers and scientists. The objective is to evaluate different methods of lowering heat demand of biogas reactor and observe the economic feasibility of the external heating systems on a designed digester in cold places based on energy return on energy invested (EROEI). Insulation can help to decrease the thermal loss from the reactor chamber. The following chart explained the brief methodology of the thesis.
Abbildung in dieser Leseprobe nicht enthalten
Figure 1.2: Thesis Methodology
Limitations A practical construction and analysis of such experimental plant was impossible due to,
1. The time constraint of the thesis that did not allow for practical implementation although it was anticipated at the outset.
2. Research and development of biogas in the country is very limited.
3. The dissemination of biogas in Bhutan is not well understood by the users particularly by rural people, therefore, lack of monitoring the operating parameters.
The organic feed use for the biogas plant is limited to cow manure because it is easily available and commonly use feed compare to other types of feedstock. The biological process of digestion in the cow stomach already consists of methanogens.
The simulation results of the soil temperature simplified to smooth sine function, however this is sufficient to know the temperature variation of the ground and it is appropraite to understand and analyze the simulation for the underground digester.
The ambient temperature increase by constructing greenhouse over a reactor is not practically observed at the selected place rather it is assumed an average temperature increase of 10°C from literature insignificant of solar insolation and construction materials used. For this evaluation, the designed digester should be semi-buried.
It was obvious to increase the digestion temperature higher than ambient temperature. Therefore, with the aim to reach the digestion temperature at 35°C, the cost feasibility of an external heating system particularly solar PV and solar thermal is discussed, but the detailed evaluation has to be part of future studies.
Thesis Outline
Chapter 1 briefly discusses the discovery of biogas and its usefulness in the world and in Bhutan. The objectives, problem statement, and methodology is framed in this chapter.
Chapter 2 discusses the environmental conditions for the production of biogas. Four important stages of anaerobic digestion are explained. For the maximum production of biogas, the controlled parameters are mentioned with their functions in the anaerobic digestion process. The chapter also discusses the difficulty in a cold climate for producing biogas. The external methods for optimizing the biogas systems are mentioned briefly.
Chapter 3 discusses the general factors affecting biogas production. Sizing and designing procedures of each components of the digester system are mathematically explained. The standards construction materials are selected for the efficient operation of biogas. Layout drawing is provided with its dimensions and detailed constructions process of each biogas components is stated.
Chapter 4 studies thermodynamics and a basic derivations of a mathematical thermal model for underground digester are developed. With average assumptions of the parameters from the literature review the heat dissipation through conduction to the environment (soil) is calculated using TRNSYS 17 and MATLAB software for a year. TRNSYS 17 is used for the deriving soil temperature at different depths and MATLAB to simulate the values for heat losses from the insulated and non-insulated digester. The energy conversion for the solar flat plate collector from the hourly solar radiation is briefly explained for the user understanding of system and component design.
Chapter 5 discusses the cost-benefit analysis. The used of conventional fuels in Bumthang such as fuelwoodd and Liquefied Petroleum Gas is compared with the biogas and cost saving per household is calculated. The amount of carbon dioxide and methane emitted to the atmosphere is also calculated. The social and environmental benefits of using biogas are briefly described.
Chapter 6 summarize the thesis and gives recommendations for future studies.
Chapter 2: Literature Review
The process of producing biogas from the microbial breakdown of organic compounds in a controlled environment in the absence of oxygen is called anaerobic digestion. It is one of the primogenial microbiological process exploited by manhood to treat and sanitize sewage waste from waste water and animal manure (Yu, Wensel, Ma, & Chen, 2013) and at the same time addressing mankind’s energy need. Recently, it is increasingly being used as an alternative renewable energy in developing countries.
2.1 Biological Process of Anaerobic Digestion
The quality of the products formed by anaerobic digestion process depend on the balance of the four stages, viz. hydrolysis, acidogenesis, acetogenesis and methanogenesis (Yadvika, Santosh, Sreekrishnan, Kohli, & Rana, 2004). Each step is governed by different groups of microbial community. The products and co-products formed at each stage have an effect on subsequent stages. Consequently, maintaining the balance in these four microbial community is important. Hence, the types of feedstock and its composition should not impact the progression of these four stages.
Abbildung in dieser Leseprobe nicht enthalten
Figure 2.1: Biological Stages in Anaerobic Digestion [McCarty, 1964]
1. Hydrolysis This process is the first stage in anaerobic digestion which the microbes break down the complex organic compounds (carbohydrates, proteins, and fats) into monomers such as monosaccharaides, amino acids, and fatty acids. The rate of this process depends on parameters such as type of organic feed, size of particles, pH, and production of enzymes, diffusion, and adsorption of enzymes on the particles of wastes subjected to the digestion process (Conrab, 1999).
2. Acidogenesis During this second stage, the remaining organic components including the hydrolysis products are further broken down by acidogenic bacteria creating acetic acids and volatile fatty acids along with hydrogen, carbon dioxide, ammonia, hydrogen sulfide, and other byproducts.
3. Acetogenesis The third stage of anaerobic digestion is called acetogenesis. The simple molecules created by acidogenic bacteria are digested by acetogenic bacteria converting them into larger acetic acids, carbon dioxide, and hydrogen. Hydrogen released during acetogenesis helps the acetogens to break down the remaining compounds in this stage. The autotrophic methane bacteria use this hydrogen for a symbiosis.
4. Methanogenesis
This is the terminal stage of anaerobic digestion where the intermediate products from preceding stages are converted into methane, carbon dioxide, water and traces of other gases by methanogenic bacteria. The hydrogen is also used in this process which creates favorable conditions for the acidification microbes to produce short-chain organic acids, consequently producing low hydrogen in acetogenic phase. The remaining indigestible materials and dead microbes remain as digestate which is rich in minerals favorable for agriculture use. As a results of this stage of digestion biogas is produced. Biogas consists of 55-65% methane, 35-45% carbon dioxide, 0-3% nitrogen, 0-1% hydrogen and hydrogen sulfide (Balat & Balat, 2009).
2.1 Factors Affecting Biogas Production
The rate of biogas yield and efficiency of the anaerobic digestion is influenced by some critical parameters discussed in (Ebunilo, Aliu, & Orhorhoro, 2015) and are listed in the following sections.
2.3.1 Temperature
The rate of biogas production is proportional to digestion temperature. It is one of the most important factors affecting the activity of methane-forming bacteria. Scientific studies by many researchers show that anaerobic digestion can occur at temperatures between 0 to 97°C in nature and 0 to 57°C in control volume. Anaerobic digestion can be classified according to the temperature it is operated and therefore which bacterial are dominant:
i) Psychrophilic
ii) Mesophilic
iii) Thermophilic Refer directly to the table: as given in Table 2.11. In control digester it is critical to maintaining a constant temperature at the optimum range. Operating the biogas plant in the mesophilic range is more robust because the bacteria are less sensitive to the change in operating routine. A decrease in temperature results in less enzymatic activity and reduced biogas production which can be offset to some extent by increasing the hydraulic retention time (HRT) to allow more contact time between substrate and bacteria, but requires a slower feed-rate or larger digester volume.
Table 2.1: Three Different Operating Temperature of Biogas Plant Digester type Operating temperature [ °C ] Source
Abbildung in dieser Leseprobe nicht enthalten
2.3.2 Feedstock
A wide range of biomass can be used as feedstock for anaerobic digestion which consists of organic compounds to provide sufficient nutrients for the anaerobic digestion (AD) bacteria. However, for economic and technical reasons, some materials are more favored as inputs than others. All the existing digesters in Bhutan use cow manure. Similar suiattion is the case in India, Nepal and Cambodia (Daxiong, Shuhua, Baofen, & Gehua, 1990; GTZ, 1999). Feed such as leachate from landfills which is a serious issues in Bhutan can also serve as a viable input feedstock. However, great care has to be taken of present poisonous substances.
Abbildung in dieser Leseprobe nicht enthalten
Figure 2.2: Potential Feed Sources for Biogas Plants
2.3.3 Hydraulic Retention Time
The hydraulic retention time (HRT) is an average period (in days) over which the feedstock is kept inside the digester. It is also an important parameter for sizing the digester volume. HRT depends on the digestion temperature. A HRT shorter than its optimum range will remove the active microbes at a higher rate than the microbes reproduced. Longer HRT will result in building a large volume of the digester which increases the capital costs. HRT will differ with climatic conditions of the place and inversely proportional with the inside digester temperature.
2.3.4 Organic loading rate
It is defined as the number of volatile solids fed per unit volume of digester per day. The gas production from the digester highly depends on the organic loading rate. For the different volume of the plant, there is an optimum loading rate. For the minimum loading, the methane production will increase but the lower loading rate will decrease the bacteria metabolic rate resulting in lower gas production. Similarly, the increasing of the loading rate from its optimum will reduce gas production.
2.3.5 pH
Unless the external factors are involved, the pH in the digester remains undisturbed. This is because the process involved in AD naturally buffered the slurry maintaining the pH. The optimum pH value inside the digester is in the range between 6.8 and 7.2 (Budiyona, Widiasa, & Sunarso, 2010). Any deviation from the optimum pH can kill the microorganisms resulting in low gas production.
2.3.6 Carbon-Nitrogen Ratio
For cow manure the C:N ratio, ranging from 20 to 30, is considered to be optimum for anaerobic digestion (Yadvika, Santosh, Sreekrishnan, Kohli, & Rana, 2004). Excessive C:N ratio results in rapid consumption of nitrogen by methanogens in order to fulfill their protein requirements, and decomposition of the left over carbon content results in a low gas production. In contrast to this, a low C:N ratio increases the pH due to an increase in concentration of ammonium inhibiting gas production.
2.3.7 Total Solids and Volatile Solids
The total solids (TS) means the concentration of organic and nonorganic solids in the solution. The optimum TS of 7% to 9% is best suited (Yadvika, Santosh, Sreekrishnan, Kohli, & Rana, 2004). If the TS value is not as required, it leads to separation of the slurry, the heavy particles will settle down on the digester floor to form the sludge layer and the lighter one floating on top forming scum which prevents the gas release and may block the gas pipes. If the TS concentration is too high, it will provide lodgings for the acetic acids, inhabiting the fermentation process. Volatile solids represents the number of organic solids in solution. The greater the concentration of VS, the more will be the gas production.
2.2 Types of Anaerobic Digestion systems
In developing countries there are several digesters in operation based on the mixing of feed in the reactor (Stalin, 2007). Figure 2.3 shows an overview of household-sized anaerobic digesters.
Abbildung in dieser Leseprobe nicht enthalten
Figure 2.3: General Overview of Control Household-Size Anaerobic Digestion
2.2.2 Batch Type System
Batch types biogas pant are built where there is difficulty in obtaining daily feedstock. The feed is filled and sealed to digester capacity and given sufficient retention time for digestion due to which it requires a large volume of the digester. The digestion process is slow with an unsteady gas production due to growing consumption of volatile solids. To overcome this problem, two or more digester can be aligned in sequence for more reliable biogas production. This type of digester is not very typical for household-size digester, but rather built in large volume and practiced on industrial scale.
2.2.3 Continuous Plant
A continuous system is fed and emptied on a daily basis or with a certain duration (GTZ, 1999) maintaining the time constant. The digester consists of an outflow route which empties automatically whenever fresh feedstock is fed. This type of digester is suitable for rural households as the necessary work fits well into the daily routine and reactor size is much smaller. Nearly all operating digesters are of continuous type. Some of the common continuous designs are described in Table 2.2 and depicted in Figure 2.4.
Table 2.2: Comparisons of Commonly Used Digester Types
Abbildung in dieser Leseprobe nicht enthalten
Figure 2.4: Commonly Built Digester Types. a) Chinese Fixed Dome Digester [Eze et al., 2012],
Abbildung in dieser Leseprobe nicht enthalten
2.4 Biogas Plant in Cold Regions and Methods to Overcome Low Temperature
Different studies give a lower bound of average ambient temperature for which biogas production is feasible: according to Gounot (1986), the regions where the average ambient temperature falls below 10°C, the production of biogas is insufficient because low temperature decreases or stops the growth of bacteria and Sodha, Ram, Bansal, and Bansal (1987) and GTZ (1999) determined the regions, where the digester temperature falls below 15°C, the gas production reduces drastically consequently limiting the use of an anaerobic digester to warmer regions. Similar cases have been observed in India, China and Nepal (Daxiong, Shuhua, Baofen, & Gehua, 1990; Yadvika, Santosh, Sreekrishnan, Hohli, & Rana, 2004; Gautam, Baral, & Heart, 2009).
To tackle the problem of low production of biogas in a cold climate, several options have been proposed. The economical and affordable ways to heat the digester for the low-income rural people requires to look for a cheap and effective solution. Anand and Singh (1993) observed that coating the digester with charcoal increase the digestion temperature by 3°C. However, this has to be recoated every one and half month. Few digester are built to burn some biogas to maintain the temperature in the digester (Jayashankar, Kishor, Sawhney, & Sodha, 1989). Another method is to increase the hydraulic retention time (Safley Jr & Westerman, 1992) but this increase both the volume of the digester and the costs. The heat from external sources must be supplied to digesters to operate at optimum temperature or at a temperature above 20 °C (Anand & Singh, 1993). Approaches of insulating the digester, locating the digester inside a greenhouse, and heating methods are discussed in detail in the following sections.
2.4.1 Insulation of Digester
Insulating the biogas plant differ with its design. Insulation materials should possess good quality, be cheap and be readily available locally. The most abundantly available insulation in Bhutan will be rice straw, saw dust, pine leaves, and clay. There are many types of insulation in the market such as glass wool, minerals wool, polyurethane, polystyrene, polyisocyanurate, etc. The thickness of the insulating material layer around the reactor has to be sized appropriately. A country like Kyrgyzstan has built a biogas plant without the provision of insulation which allows the plant to operate only during summer time (Fluid, 2019).
2.4.2 Greenhouse
Building the biogas digester inside a greenhouse is a simple method of achieving temperature gain. The regions where the solar irradiation varies from 250 to 600 W/m2 (Kandasamy & Bai, 2008) reported that in the control ambient temperature of 17°C, the solar digester temperature increase with the average of 29.1 °C. Bumthang has an annual average temperature and solar irradiation of 12.1°C and 173.5 W/m2 respectively. Locating a greenhouse should be done by having large and exposed surface towards the south without any shading from buildings and trees especially for the lower winter sun.
Abbildung in dieser Leseprobe nicht enthalten
Figure 2.5: General Orientation and Shapes of Greenhouse [Based on Cakir & Sahin, 2015]
2.4.3 Heating Method
Circulating hot water inside the digester with the help of a heat exchanger will increase the temperature inside the digester. Stainless steel or PVC are recommended for the heat exchanger but corrosion issues should be considered properly due to the corrosive environment inside the digester. Installer can choose the heat exchanger depending on the layout of the pipes, shape of the digester, the type of substrate used, and the nature of the operating mode. The supplied heat can come from divers sources such as solar heating, electric heating, or wood burning to heat water. The solar PV and solar thermal system will favor heating of household digester in the place where the sunshine hours is long for the small scale plant.
Hot water from the vessel on a Bukhari, as depicted in Figure 2.6 can be used to treat the cold manure and fed to the digester (Monga & Lakhanpal, 1988). Almost 97% rural total population in Bumthang have this heating stove. Apart from bathing and washing schedule, we can use hot water once in a day with the feed. There is numerous biogas plant in China using cattle manure operating at low temperature using hot water (Monga & Lakhanpal, 1988).
Abbildung in dieser Leseprobe nicht enthalten
Figure 2.6: Hot Water Pot Being Heated on a Bukhari Wood Stove [DRE (2018)]
2.5 Selection of Biogas Plant for Cold Regions of Bhutan
Fixed dome digester are performing better in the cold regions because most of them are built underground where they are less exposed to the low ambient temperature. Floating drums are less resistant to low temperature because its gasholder is made of steel which is more heat conductive than the underground concrete fixed dome. Due to this reason India subsidized the building of fixed dome digester in cold regions in the last few decades (Kanwar, Gupta, Guleri, & Singh, 1994).
[...]
- Quote paper
- Ugyen Wangchuk (Author), 2019, Assessement of an Anaerobic Digester in Cold Region of Bhutan, Munich, GRIN Verlag, https://www.grin.com/document/900542
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Upload your own papers! Earn money and win an iPhone X. -
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Upload your own papers! Earn money and win an iPhone X. -
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Upload your own papers! Earn money and win an iPhone X. -
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Upload your own papers! Earn money and win an iPhone X. -
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Upload your own papers! Earn money and win an iPhone X. -
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Upload your own papers! Earn money and win an iPhone X. -
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Upload your own papers! Earn money and win an iPhone X.