Technical, Microbial, and Economic Study on Thermophilic Solid-state Anaerobic Digestion of ...

231 Pages·2017·2.77 MB·English

by  Long Lin

Technical, Microbial, and Economic Study on Thermophilic Solid-state Anaerobic
Digestion of Lignocellulosic Biomass

DISSERTATION

Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy
in the Graduate School of The Ohio State University

By
Long Lin, M.S.
Graduate Program in Environmental Science

The Ohio State University
2017

Dissertation Committee:
Dr. Zhongtang Yu, Advisor
Dr. Harold M. Keener, Co-advisor
Dr. Frederick C. Michel Jr.
Dr. Ajay Shah
Copyrighted by
Long Lin
2017
Abstract

Yard trimmings (leaves, grass, wood chips, etc.) are abundant biomass wastes,
which need to be treated and/or utilized properly. Composting is a conventional approach
to divert yard trimmings from the landfill. Alternatively, solid-state anaerobic digestion
(SS-AD) can convert yard trimmings to biogas (renewable fuel) and digestate.
Thermophilic SS-AD can be more favorable for degradation of lignocellulosic biomass,
such as yard trimmings, than mesophilic digestion due to a higher hydrolysis of the
recalcitrant lignocellulose biomass. It is promising to develop thermophilic SS-AD that
can both produce energy and reduce waste to treat yard trimmings. However, several
technical challenges need to be addressed prior to scale up of this technology. One of the
challenges is the requirement of nitrogen-rich amendment and effective inoculum.
Another challenge is the regional and seasonal variability in the composition of yard
trimmings, which can affect the thermophilic SS-AD performance. Moreover, digestate
of SS-AD needs to be properly handled or effectively utilized. Optimization of the
process is needed to make it both technically and economically feasible. Therefore,
interests are raised from technical, microbial and economic aspects to examine the
process.
This research included five inter-related projects: 1) examination of performance
of thermophilic SS-AD and composting of yard trimmings with liquid anaerobic effluent
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as inoculum; 2) evaluation of the effect of yard trimmings on thermophilic SS-AD and
prediction of methane yield based on feedstock component ratios; 3) development of a
sequential batch thermophilic SS-AD with recirculated digestate as inoculum; 4)
investigation of microbial community dynamics in the sequential batch thermophilic SS-
AD; and 5) techno-economic comparison of thermophilic SS-AD and composting of yard
trimmings.
The first project compared the performance of thermophilic SS-AD and
composting of yard trimmings with effluent from liquid anaerobic digestion at different
total solids (TS) contents and feedstock-to-effluent (F/E) ratios. High total solids content
negatively affected both performances. The greatest total carbon loss was observed at
35% TS in composting, which was about 50% higher than that in SS-AD; while, using
SS-AD, more than half of the degraded carbon was converted to methane with the highest
methane yield of 121 L/kg VS . The preferred F/E ratio for SS-AD was 4–6.
feedstock
Methane production from SS-AD was low at F/E ratio of 2 and 3, likely due to inhibitory
effect of high concentrations of ammonia nitrogen (up to 5.6 g/kg N). Nutrient-rich (N, P,
K) end products with non-detectable coliform were generated from both SS-AD and
composting.
The second project evaluated the effect of three components in yard trimmings,
i.e. wood chips, lawn grass, and maple leaves, on thermophilic SS-AD performance.
Digestion of mixed feedstocks resulted in sooner and more methane production than
digestion of single components. The favorable peaking time (14 days) and methane yield
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(143 L/kg VS) were achieved with equal fractions of the three components, increasing the
methane yield by 80–200% compared to digestion of single components. Concentrations
of volatile fatty acids and ammonia varied with component ratios and correlated with
system performance. While organic components in mixtures were degraded differently,
they were mostly degraded within 24 days and agreed with methane yield. A mixture
design model was established to predict methane yield and results showed that all
interactions were synergistic, with the ternary interaction having the strongest effect. The
model was verified using experimental data, which showed good agreement.
A sequential batch thermophilic SS-AD of yard trimmings was developed with
recirculated digestate as the inoculum in the third project. The SS-AD consisted of 4
consecutive runs (30 days/run), with digestate from the previous run being used as the
inoculum of the subsequent run. The substrate-to-inoculum (S/I) ratio of 1 (TS basis) was
favored over 2 and 3 due to significantly higher methane yield and volumetric
productivity. At an S/I ratio of 1, sequential batch SS-AD gradually reached steady state
by 3 runs with increases in both methane yields (up to 11.5%) and cellulose degradation
(up to 55%), indicating that recirculated digestate could be a feasible inoculum to
establish long term stable SS-AD of lignocellulosic biomass. The initial sharp increases
of volatile fatty acids during runs 2–4 indicated faster hydrolysis of organic matter than
during run 1, suggesting that microbes were probably more acclimated due to digestate
recirculation. At steady state, 51% (w/w) of the digestate was recirculated as the
inoculum.
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In the fourth project, the bacterial and archaeal communities (day 0, 4, 8, 12, 20,
and 30) in the sequential batch SS-AD were examined using Illumina sequencing of 16S
rRNA genes to assess the effect of recirculated digestate as inoculum on the microbial
community dynamics. Microbes shifted substantially toward a stable state with increased
diversity in the first 3 runs, which was consistent with the findings from the third project.
The proportions of Firmicutes that contained cellulolytic bacteria increased from 40% to
80% from run 1 to run 3, which might explain the concomitantly increased cellulose
degradation and volatile fatty acids (VFAs). The VFA accumulation likely induced
dynamic shifts of methanogens. Proportions of archaea rose from 1% to 5% from run 1 to
run 4 when methane peaked. Particularly, hydrogenotrophic Methanothermobacter was
enriched at volatile fatty acid (VFA) levels of 6–14 g/kg, implying that non-acetoclastic
oxidative pathway dominated during the steady-state thermophilic SS-AD. Results
suggested that recirculating SS-AD digestate might be an effective way for inoculation.
Finally, a techno-economic analysis was conducted to compare the economic
feasibility of commercial-scale SS-AD and composting for a 20,000 MT/year capacity of
yard trimmings and L-AD effluent. Composting (NPV $2 million) was shown to be more
economically competitive than SS-AD (NPV $0.2 million) without financial incentives.
SS-AD had higher capital cost but lower labor associated operational cost than
composting. The addition of digestate drying improved the economics of SS-AD, but it
was also the most energy intensive step relying on heat recovery to reduce cost
($18.8/MT). Operating costs were estimated be to $44/MT capacity for SS-AD and
$31/MT for composting. The revenues were comparable at $48/MT for both systems.
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With financial incentives, SS-AD was shown to be slightly more profitable than
composting. However, RINs and RECs had minor effects on SS-AD economics
compared to investment tax credits and grants. Both systems were most sensitives to
plant size and tipping fees.
The results of this research indicate that thermophilic SS-AD of yard trimmings is
technically feasible and its economics could be comparable with composting with some
financial incentives. The knowledge obtained from these studies could be used to assist in
the optimization and scale-up of SS-AD and enhance the renewable energy from
lignocellulosic biomass.

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Dedication
This document is dedicated to my family.

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Acknowledgments

I want to address my utmost gratitude to my original advisor, Dr. Yebo Li, for his
professional guidance and endless encouragement throughout my Ph.D. study. This
dissertation would not have been possible without him. I really appreciated him offering
me this opportunity four years ago to study and do research in the Bioproducts and
Bioenergy Research Laboratory. This opportunity introduced me to the professional
training that will benefit me throughout the years. The time working with Dr. Li will
continue being one of the most valuable experiences in my life.
My sincere thanks go to my dissertation committee members including Dr.
Zhongtang Yu, Dr. Harold M. Keener, Dr. Frederick C. Michel Jr., and Dr. Ajay Shah for
their advice and collaboration on the work of my dissertation. I especially thank Dr.
Zhangtang Yu and Dr. Keener for their mentorship during the last year of my Ph.D.
work. I also thank Dr. Lingying Zhao for her service in my candidacy exam committee.
Other members of the Department of Food, Agricultural, and Biological Engineering and
Environmental Science Graduate Program are important, including Mrs. Mary Wicks,
Ms. Candy McBride, Mrs. Peggy Chrisman, Mr. Michael Klingman, Mr. Scott Wolfe,
Dr. Gil Bohrer, Ms. Sarah Straley, and Ms. Maura Eze for their academic, administrative,
and engineering support. I especially thank Mrs. Mary Wicks for reviewing my
publications as well as offering continuous encouragement throughout the study.
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Many thanks go to the talented members of the Bioproducts and Bioenergy
Research Laboratory that I have worked with, including Dr. Liangcheng Yang, Dr.
Xumeng Ge, Dr. Fuqing Xu, Dr. Stephen Park, Ms. Zhe Liu, Mr. John Sheets, Mr. Adam
Khalaf, Mr. Lu Zhang, Ms. Kathryn Lawson, Ms Danping Jiang, Ms. Juliana Vasco, Mr.
Josh Borgemenke, Ms. Lo Niee Liew, and several others. I especially want to thank Dr.
Liangcheng Yang and Dr. Xumeng Ge for their mentorship and generous help.
I am thankful for the funding supports to this research, including the United States
Department of Agriculture (USDA) National Institute of Food and Agriculture (NIFA)
Biomass Research and Development Initiative Program, the USDA NIFA Hatch
Program, the Ohio Agricultural Research and Development Center (OARDC)
Environmental Fellowship Program, the OARDC SEEDS program, and The Ohio State
University Environmental Science Graduate Teaching Assistantship program. Thanks to
the industry collaborators including quasar energy group and KB Bioenergy Inc. for
willingness to provide materials for this research.
I am extremely grateful to my parents, parents-in-law, siblings/siblings-in-law,
and extended family for their endless support. Most of all, I want to express my deepest
thanks to my loving husband Fenglong You and our precious daughter Anna Mu-Chen
You. You provide me major motivation throughout this journey. I especially thank my
mother Mrs. Xuetan Lan for taking care of Anna while I was busy with my study. Thank
you for always being there to support me.

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