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
Chapter 1: Introduction
1.1 History of Biogas
1.2 Problem statement
1.3 Aim and Objectives
1.4 Methodology
1.5 Limitations
1.6 Thesis Outline
Chapter 2: Literature Review
2.1 Biological Process 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 systems
2.2.2 Batch Type System
2.2.3 Continuous Plant
2.4 Biogas Plant 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
Research Goal and Topics
The primary goal of this thesis is to evaluate the energetic and economic feasibility of operating an anaerobic biogas digester in the cold regions of Bhutan, specifically Bumthang, by analyzing thermal performance and potential heating solutions to maintain an optimum digestion temperature of 35°C.
- Thermal performance simulation of underground biogas digesters.
- Evaluation of insulation materials and greenhouse integration for thermal retention.
- Assessment of external heating systems (solar PV and thermal collectors).
- Economic and cost-benefit analysis of biogas plants compared to conventional fuels like fuelwood and LPG.
- Development of design and sizing guidelines for cold-climate biogas systems.
Excerpt from the Book
4.1 Formulation of the Digester
The digestion temperature (T) inside the reactor is maintained constant to operate the systems throughout the year by the external heating system. To minimize the heat loss and the load to the external system, digester is insulated with rice straw which is cheap and available locally. As the digester is directly in contact with the soil, the steady state one-dimensional heat conduction is evaluated by using Fourier law of Conduction (Cengel & Boles, 2016).
The mathematical equations for the heat loss are being evaluated base on the following general assumptions;
1. Factors affecting the biogas production are maintained in dynamic equilibrium.
2. The temperature of the slurry inside the digester is maintained constant and uniform at 35°C for the optimum gas production (Singh, Jain, & Sing, 2017)
3. The heat dissipation to the surrounding (soil) consists of heat loss from the gasholder, the cylindrical reactor, and the digester floor.
4. The internal heat generation from microbes reaction is negligible (Hashimoto, Chen, & Hruska, 1979; Prasad & Sathyanarayan, 1979; Sobel & Muck, 1983)
5. Physical properties of the digester and insulation materials are uniform.
6. The temperature of input slurry is equal to ambient temperature (Perrigault, Weatherford, & Marti, 2012; Kishore, 1989).
7. The flow rate of the slurry doesn’t influence heat loss from the digester (Gebremedhin, Wu, Gooch, & Wright, 2013)
8. The wall thickness of the digester is uniform throughout.
9. The physical properties of the soil are uniform. But the temperature differs with the depth of the soil.
Summary of Chapters
Chapter 1: Introduction: This chapter provides an overview of the history of biogas and the current state of its dissemination in Bhutan, while defining the problem statement and research objectives.
Chapter 2: Literature Review: The chapter explores the biological stages of anaerobic digestion, factors affecting gas production in cold climates, and evaluates various digester types and optimization techniques.
Chapter 3: Scaling of Digester Systems: This section details the mathematical procedures for sizing the reactor, gasholder, and other components, along with selection criteria for construction materials.
Chapter 4: Thermal Analysis and Simulation Results: This chapter presents the mathematical thermal models used to evaluate heat loss under various conditions and discusses the potential of greenhouse and solar heating integration.
Chapter 5: Cost -Benefits Analysis: The author compares the costs of biogas systems against the use of conventional fuels like electricity, LPG, and fuelwood, highlighting social and environmental benefits.
Chapter 6: Conclusions and Recommendations: This chapter summarizes the research findings on the energetic feasibility of heating digesters and provides recommendations for future R&D and implementation strategies in Bhutan.
Keywords
Anaerobic Digestion, Biogas, Cold Climate, Bumthang, Thermal Analysis, Heat Loss, Rice Straw Insulation, Solar Greenhouse, Renewable Energy, Cost-Benefit Analysis, Hydraulic Retention Time, Methanogenesis, Bhutan, Energy Economics, Sustainable Development
Frequently Asked Questions
What is the core focus of this research?
The research focuses on the technical and economic feasibility of operating household-sized anaerobic biogas digesters in cold-climate regions of Bhutan, specifically addressing the challenge of maintaining optimal digestion temperatures.
What are the primary themes discussed in the thesis?
The thesis covers anaerobic digestion biological processes, thermal modeling for underground reactors, heat loss mitigation techniques, and cost-benefit analysis against traditional fuel sources.
What is the main research objective?
The objective is to design and analyze the thermal performance of a biogas system in Bumthang and determine if integrating external heating systems to reach a digestion temperature of 35°C is energetically and economically feasible.
Which scientific methodology is applied?
The study utilizes numerical thermal modeling, TRNSYS 17 for soil temperature simulation, MATLAB for heat loss calculations, and an Energy Return on Energy Invested (EROEI) analysis.
What topics are covered in the main section of the paper?
The main section details the scaling of digester components, thermal formulations for gasholders and reactors, and an analysis of how insulation and greenhouse integration influence heat loss.
What are the characterizing keywords for this work?
Key terms include Anaerobic Digestion, Thermal Analysis, Cold Climate, Renewable Energy, Bhutan, and Biogas Plant Feasibility.
Why is biogas performance currently low in northern Bhutan?
The low ambient temperatures in regions like Bumthang significantly reduce the biological activity of methanogenic bacteria, leading to insufficient gas production during winter months.
Is heating the digester to 35°C economically viable?
No, the study concludes that using external heating systems to maintain 35°C is not energetically or economically feasible for rural households, as the energy invested in heating exceeds the energy output generated by the plant.
What are the social and environmental benefits of biogas plants in Bhutan?
Benefits include reduced indoor air pollution, improved health outcomes by replacing toxic fuelwood smoke, and the production of high-quality fertilizer that improves soil fertility for farmers.
- 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
