Modeling a solar-heated anaerobic digester for the developing world using system dynamics

142 Pages·2016·4.8 MB·English

Rochester Institute of Technology
RIT Scholar Works
Theses Thesis/Dissertation Collections
7-1-2012
Modeling a solar-heated anaerobic digester for the
developing world using system dynamics
Johanna Bentley
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MODELING A SOLAR-HEATED ANAEROBIC DIGESTER
FOR THE DEVELOPING WORLD USING SYSTEM DYNAMICS

By
Johanna Lynn Bentley
A Thesis Submitted in Partial Fulfillment of the Requirements for the
Degree of Master of Science
in Sustainable Engineering

Supervised by

Professor Dr. Brian Thorn
Department of Industrial and Systems Engineering
Kate Gleason College of Engineering
Rochester Institute of Technology
Rochester, New York
July 2012

Approved by:

Dr. Brian Thorn, Associate Professor
Thesis Advisor, Department of Industrial and Systems Engineering

Dr. Andres Carrano, Associate Professor
Committee Member, Department of Industrial and Systems Engineering

Dr. James J. Winebrake, Professor and Dean of the College of Liberal Arts
Committee Member

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Thesis Release Permission Form

Rochester Institute of Technology
Kate Gleason College of Engineering

Title:

Modeling a Solar-Heated Anaerobic Digester for the Developing World Using System Dynamics

I, Johanna Lynn Bentley, hereby grant permission to the Wallace Memorial Library to reproduce
my thesis in whole or part.

Johanna Lynn Bentley

Date

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DEDICATION

Be the change you want to see in the world.
Mahatma Gandhi

To my loved ones, for your support and inspiration to do great things.

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DISCLAIMER

All modeling performed during analysis used Stella, a systems modeling program created by isee
systems. This software offers a practical way to dynamically visualize and communicate the various
aspects of the proposed design by showing interactions between variables with a combination of stocks,
flows, and converters. Please refer to the attached CD for all modeling trials.

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ABSTRACT

Modeling a Solar-Heated Anaerobic Digester for the Developing World Using System Dynamics
Johanna Lynn Bentley
Supervising Professor: Dr. Brian Thorn

Much of the developing world lacks access to a dependable source of energy. Agricultural societies such as
Mozambique and Papua New Guinea could sustain a reliable energy source through the microbacterial
decomposition of animal and crop waste. Anaerobic digestion produces methane, which can be used directly for
heating, cooking, and lighting. Adding a solar component to the digester provides a catalyst for bacteria activity,
accelerating digestion and increasing biogas production. Using methane decreases the amount of energy expended
by collecting and preparing firewood, eliminates hazardous health effects linked to inhalation of particles, and
provides energy close to where it is needed. The purpose of this work is two fold: initial efforts focus on the
development and validation of a computer-based system dynamics model that combines elements of the anaerobic
digestion process in order to predict methane output; second, the model is flexed to explore how the addition of a
solar component increases robustness of the design, examines predicted biogas generation as a function of varying
input conditions, and determines how best to configure such systems for use in varying developing world
environments. Therefore, the central components of the system: solar insolation, waste feedstock, bacteria
population and consumption rates, and biogas production are related both conceptually and mathematically through
a serious of equations, conversions, and a causal loop and feedback diagram. Given contextual constraints and initial
assumptions for both locations, it was determined that solar insolation and subsequent digester temperature control,
amount of waste, and extreme weather patterns had the most significant impact on the system as a whole. Model
behavior was both reproducible and comparable to that demonstrated in existing experimental systems. This tool can
thus be flexed to fit specific contexts within the developing world to improve the standard of living of many people,
without significantly altering everyday activities.

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CONTENTS

DEDICATION..........................................................................................................................................................III
DISCLAIMER..........................................................................................................................................................IV
ABSTRACT................................................................................................................................................................V
CONTENTS..............................................................................................................................................................VI
1 INTRODUCTION....................................................................................................................................................1
2 BACKGROUND.......................................................................................................................................................3
2.1 ANAEROBIC CONDITIONS.....................................................................................................................................3
2.2 SUBSTRATE AND NUTRIENTS................................................................................................................................4
2.3 PH........................................................................................................................................................................5
2.4 TOXICS.................................................................................................................................................................5
2.5 TEMPERATURE.....................................................................................................................................................5
2.6 WATER.................................................................................................................................................................6
2.7 PARTICLE SURFACE AREA....................................................................................................................................6
2.8 LOADING RATE, RETENTION TIME, AND MIXING.................................................................................................6
2.9 DIGESTER SYSTEM DESIGN..................................................................................................................................8
2.9.1 Plug Flow.....................................................................................................................................................8
2.9.2 Complete Mix.............................................................................................................................................10
2.9.3 Covered Lagoon.........................................................................................................................................11
2.9.4 Fixed Film..................................................................................................................................................12
2.9.5 Two-Stage Digestion..................................................................................................................................12
3 LITERATURE REVIEW......................................................................................................................................14
4 FORMAL PROBLEM STATEMENTS AND METHODOLOGY....................................................................22
4.1 A SYSTEM DYNAMICS APPROACH......................................................................................................................22
4.2 UNDERLYING METHODOLOGY...........................................................................................................................25
4.3 IDENTIFYING THE IMPORTANT CONCEPTS..........................................................................................................25
4.4 CONSTRUCTING A SYSTEMS DIAGRAM...............................................................................................................25
4.5 DEFINING THE SYSTEM CONSTRUCTS.................................................................................................................26
4.6 VERIFYING THE MODEL.....................................................................................................................................27
4.6.1 Behavior Reproduction..............................................................................................................................28
4.7 VALIDATING THE MODEL...................................................................................................................................28
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4.7.1 Structure Assessment.................................................................................................................................29
4.7.2 Dimensional Consistency...........................................................................................................................29
4.7.3 Extreme Conditions....................................................................................................................................29
4.7.4 Sensitivity Analysis.....................................................................................................................................30
5 INITIAL CONDITIONS AND CONTEXTUAL COSTRAINTS......................................................................32
5.1 MOZAMBIQUE, AFRICA......................................................................................................................................35
5.1.1 Initial Conditions and Assumptions for Mozambique................................................................................37
5.2 PAPUA NEW GUINEA, OCEANIA.........................................................................................................................42
5.2.1 Initial Conditions and Assumptions for Papua New Guinea......................................................................44
6 DEVELOPING THE MODEL..............................................................................................................................48
6.1 WEEKLY TRIAL RUN RESULTS...........................................................................................................................53
6.2 ANNUAL TRIAL RUN RESULTS...........................................................................................................................56
6.2.1 Temperature Regulation............................................................................................................................57
6.2.2 Amount of Waste........................................................................................................................................57
6.2.3 Bacteria Population...................................................................................................................................58
6.2.4 Bacteria Growth and Death Trends...........................................................................................................59
6.2.5 Amount of Gas...........................................................................................................................................60
7 MODEL ANALYSIS..............................................................................................................................................62
7.1 MOZAMBIQUE ANALYSIS...................................................................................................................................62
7.1.1 Waste Input Ceases....................................................................................................................................62
7.1.2 Initial Waste Amount Doubles, Triples, in Same Time Interval.................................................................63
7.1.3 Severe Weather Patterns which Erratically Affect Solar Insolation..........................................................66
7.1.4 No Solar Component..................................................................................................................................69
7.1.5 No Overheat Control..................................................................................................................................71
7.1.6 Fertilizer Collection or Spent Slurry Removal Ceases..............................................................................74
7.1.7 Overpopulation of Bacteria in Digester.....................................................................................................76
7.1.8 Underpopulation of Bacteria in Digester..................................................................................................78
7.2 PAPUA NEW GUINEA ANALYSIS.........................................................................................................................80
7.2.1 Waste Input Ceases....................................................................................................................................80
7.2.2 Initial Waste Amount Doubles, Triples, in Same Time Interval.................................................................81
7.2.3 Severe Weather Patterns which Erratically Affect Solar Insolation..........................................................84
7.2.4 No Solar Component..................................................................................................................................87
7.2.5 No Overheat Control..................................................................................................................................87
7.2.6 Fertilizer Collection or Spent Slurry Removal Ceases..............................................................................91
7.2.7 Overpopulation of Bacteria in Digester.....................................................................................................92
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7.2.8 Underpopulation of Bacteria in Digester..................................................................................................94
7.3 SENSITIVITY ANALYSIS AND OVERALL CONCLUSIONS.......................................................................................96
7.3.1 Mozambique Summary...............................................................................................................................98
7.3.2 Papua New Guinea Summary..................................................................................................................100
8 DISCUSSION........................................................................................................................................................102
8.1 STUDY 1 - GREECE...........................................................................................................................................102
8.2 STUDY 2 - ITALY..............................................................................................................................................103
8.3 STUDY 3 - IRAN................................................................................................................................................105
8.4 STUDY 4 – UNITED STATES..............................................................................................................................107
8.5 STUDY 5 – UNITED STATES..............................................................................................................................110
8.6 STUDY 6 – EGYPT AND UNITED STATES...........................................................................................................112
9 CONCLUSIONS AND FUTURE WORK..........................................................................................................118
9.1 CONCLUSIONS..................................................................................................................................................118
9.2 FUTURE WORK.................................................................................................................................................120
9.2.1 Multiple Waste Streams...........................................................................................................................120
9.2.2 Bacteria....................................................................................................................................................121
9.2.3 Pre-treatment of Waste............................................................................................................................122
9.2.4 Internal Digester Factors.........................................................................................................................122
9.2.5 Filtering, Purification, and Storage of Biogas.........................................................................................123
9.2.6 Incorporating Economic Analysis............................................................................................................123
9.2.7 Environmental Factors............................................................................................................................124
REFERENCES........................................................................................................................................................125

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LIST OF FIGURES

FIGURE 2.1. DIGESTION PHASES FOR STANDARD-RATE DIGESTER.................................................................................7
FIGURE 2.2. PLUG FLOW DIGESTER DIAGRAM...............................................................................................................9
FIGURE 2.3. COMPLETE MIX DIGESTER DIAGRAM.......................................................................................................10
FIGURE 2.4. COVERED LAGOON DIGESTER DIAGRAM..................................................................................................11
FIGURE 2.5. FIXED FILM DIGESTER DIAGRAM..............................................................................................................12
FIGURE 2.6. TWO-STAGE DIGESTER DIAGRAM..............................................................................................................13

FIGURE 3.1. PLUG FLOW DESIGN AS DESCRIBED BY DALE IN U.S. PAT. NO. 4,274,838................................................14
FIGURE 3.2. NON-MIXED VERTICAL TOWER DIGESTER AS DESCRIBED BY CHYNOWETH IN U.S. PAT. NO. 4,735,724...15
FIGURE 3.3. PLUG FLOW DIGESTER WITH RAMP AS DESCRIBED BY VARANI IN U.S. PAT. NO. 4,208,279......................16
FIGURE 3.4. RHOADES DESIGN AS SEEN IN U.S. PAT. NO. 4,221,571............................................................................17
FIGURE 3.5. SCHEMATIC DIAGRAM OF EXPERIMENTAL GREENHOUSE SETUP AS DESCRIBED BY THOMAS.....................18
FIGURE 3.6. SCHEMATIC DIAGRAM OF THE BIOGAS PRODUCTION SYSTEM AS DESCRIBED BY AXAOPOULOS................19
FIGURE 3.7. PROPOSED SYSTEM WITH VARIABLE COMPONENTS...................................................................................20

FIGURE 4.1. SYSTEM DYNAMICS FRAMEWORK AS DESCRIBED BY FORRESTER..............................................................24

FIGURE 6.1. SYSTEM DYNAMICS MODEL OF ANAEROBIC DIGESTION SYSTEM AS GENERATED BY STELLA……………..49
FIGURE 6.2. WEEKLY TRENDS OF FOUR SUBSYSTEMS OF ANAEROBIC DIGESTER SYSTEM.............................................53
FIGURE 6.3. WEEKLY TRENDS OF SOLAR COMPONENT AND AD TEMPERATURE............................................................54
FIGURE 6.4. WEEKLY INTERACTION BETWEEN GAS AND BACTERIA POPULATION.........................................................55
FIGURE 6.5. WEEKLY INTERACTION BETWEEN WASTE, BACTERIA, CONSUMPTION RATE, AND TEMPERATURE..............56
FIGURE 6.6. HISTOGRAM OF TEMPERATURE REGULATION INSIDE DIGESTER FOR TRIAL RUN........................................57
FIGURE 6.7. AMOUNT OF WASTE INSIDE DIGESTER FOR TRIAL RUN..............................................................................58
FIGURE 6.8. BACTERIA POPULATION INSIDE DIGESTER FOR TRIAL RUN........................................................................59
FIGURE 6.9. BACTERIA GROWTH AND DEATH TRENDS INSIDE DIGESTER FOR TRIAL RUN..............................................60
FIGURE 6.10. AMOUNT OF GAS INSIDE DIGESTER FOR TRIAL RUN.................................................................................61

FIGURE 7.1. IMPACT OF WASTE INPUT CESSATION ON DIGESTER...................................................................................63
FIGURE 7.2. ERROR MESSAGE GENERATED BY STELLA UPON WASTE INPUT CESSATION...............................................63
FIGURE 7.3. EFFECT OF DOUBLING, TRIPLING WASTE INSIDE DIGESTER ON BACTERIA POPULATION.............................64
FIGURE 7.4. EFFECT OF DOUBLING, TRIPLING WASTE INSIDE DIGESTER WITHIN FIRST 24 HOURS ON BACTERIA
POPULATION........................................................................................................................................................64
FIGURE 7.5. EFFECT OF DOUBLING, TRIPLING WASTE INSIDE DIGESTER ON GAS PRODUCED..........................................65
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