energy valorization of agro

444 Pages·2015·7.21 MB·English

ENERGY VALORIZATION OF AGRO-
INDUSTRIAL WASTES AND SWEET
SORGHUM FOR THE PRODUCTION OF
GASEOUS BIOFUELS THROUGH
ANAEROBIC DIGESTION

Dissertation
Submitted to
Department of Chemical Engineering
University of Patras
by

Margarita A. Dareioti

In Partial Fulfillment of the Requirements
for the Degree of

Doctor of Philosophy
University of Patras

Patras, 2015

Dedicated to my parents, Andreas & Anastasia
my brother, Tasos
& my Nikos!

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Acknowledgments

First, I would like to express the special appreciation to my advisor, Michael
Kornaros, for his excellent guidance, caring, patience and providing me with an
excellent atmosphere for doing research. I would like to thank him for encouraging my
research, for his brilliant comments and suggestions and for allowing me to grow as a
research scientist. I will be always thankful for everything he has offered me and very
fortunate having such a wonderful person as an advisor. I would also like to thank my
committee members, Professor G. Lyberatos, Professor S. Pavlou, Professor
D. Mantzavinos, Associate Professor I. Kookos, Associate Professor C. Paraskeva, and
Assistant Professor A. Stamatelatou.
I would also like to thank all the members of Kornaros group, and the undergraduate
students with whom I have cooperated during their diploma thesis. Especially, Thodoris
Vgenis, Dimitris Poupakis and my special friend Kostas Stavropoulos for scientific and
intimate support with whom I have enjoyed working together and sharing great
moments in the same laboratory. I would also like to acknowledge Dr. Aikaterini
Vavouraki for the creative collaboration and our scientific discussions and also
Dr. Spyros Dokianakis who shared with me his knowledge and give me invaluable help
during my early stages of my Ph.D research.
I dedicate this thesis to my loving family; my parents Andreas, Anastasia and my
brother Tasos as they were always there for me, giving me unconditional love and
supporting me throughout my life. I would also like to thank all of my friends, who
supported me in writing and incented me to strive towards my goal. Last but not the
least important, I would also like to dedicate this thesis to a special person, my fiance
Nikos Vrachatis, who was always standing by me during both the good times and those
when things haven’t gone so well. I give my deepest expression of love and
appreciation for the encouragement that he gave me all these years.

This research has been co-financed by the European Union (European Social Fund –
ESF) and Greek national funds through the Operational Program "Education and
Lifelong Learning" of the National Strategic Reference Framework (NSRF) - Research
Funding Program: Heracleitus II. Investing in knowledge society through the European
Social Fund.

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Abstract

It is clear that renewable resources have received great interest from the international
community during the last decades and play a crucial role in the current CO -mitigation
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policy. In this regard, energy from biomass and waste is seen as one of the most
dominant future renewable energy sources, especially since that a continuous power
generation from these sources can be guaranteed, unlike other types such as solar energy
and wind energy. Thus, organic waste i.e. animal wastes, wastewaters, energy crops,
agricultural and agro-industrial residues are of specific importance since these sources
do not compete with food crops in agricultural land usage. The various technologies that
are available for power generation from biomass and waste can be subdivided into
thermochemical, biochemical and physicochemical conversion processes. Anaerobic
digestion (AD), classified within the biochemical conversion processes, is a robust
process and is widely applied. Various types of biomass and waste, can be anaerobically
co-digested to generate a homogeneous mixture increasing both process and equipment
performance. This technology is an attractive option to improve the yields of the
anaerobic digestion of substrates due to the positive synergisms established in the
digestion medium; a fact that increases the economic viability of the biogas plants.
This study focused on the valorization of agro-industrial wastes (such as olive mill
wastewater (OMW), cheese whey (CW) and liquid cow manure (LCM)) and sweet
sorghum stalks. Olive mills, cheese factories and cow farms are agro-industries that
represent a considerable share of the worldwide economy with particular interest
focused in the Mediterranean region. These industries generate millions of tons of
wastewaters and large amounts of by-products, which are in many cases totally
unexploited and thus dangerous for the environment. On the other hand, sweet sorghum
as a lignocellulosic material represents an interesting substrate for biofuels production
due to its structure and composition.
Anaerobic co-digestion experiments using different substrates were performed in a
two-stage system consisting of two continuously stirred tank reactors (CSTRs) under
mesophilic conditions (37°C) with a hydraulic retention time (HRT) of 19 days (3 d of
acidogenesis and 16 d of methanogenesis). The maximum methane production rate
(1.35 L CH /L ·d) was obtained using the mixture of 55% OMW, 40% CW and
4 R
5% LCM with methane yield of 467.53 mL CH /g VS added, whereas equally high
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methane production rate of 1.33 L CH /L ·d was obtained using the mixture of
4 R
90% CW and 10% LCM with 79% removal of total COD. Although the two-stage
anaerobic treatment process has several advantages over the conventional single-stage
process, experiments were conducted using CW and LCM as mono-substrates in order
to investigate the role of each step in their treatment. In particular, negligible difference
between single and two-stage process was observed treating the LCM, whereas using
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the CW, an easily-degradable substrate, the two-stage process was considered as a better
treatment system than single one.
Taking into account the aforementioned results, the mixture 55% OMW, 40% CW
and 5% LCM was selected for further study and optimization. Subsequently, two more
mixtures were studied, where sweet sorghum was added, in order to simulate the
operation of a centralized AD plant fed with regional agro-wastes which lacks OMW
or/and CW due to seasonal unavailability. The sorghum used in this dissertation was
either fresh or ensiled. The ensiled sorghum (ES) was firstly pretreated using an alkaline
hydrolysis. The aim of this study was to hydrolyze the ES targeting to carbohydrates’
solubilization and removal of lignin that hinders the access of enzymes to cellulose, thus
facilitating its subsequent biochemical conversion to fermentable sugars.
For the optimization of these mixtures, two operational parameters were examined
including pH and HRT. Batch experiments were performed in order to investigate the
impact of controlled pH on the production of bio-hydrogen and volatile fatty acids,
whereas continuous experiments (CSTRs) were conducted for the evaluation of HRT
effect on hydrogen and methane production. Firstly, using the mixture of 55% OMW,
40% CW and 5% LCM, the highest hydrogen production yield was observed at pH 6.0
(0.642 mol H /mol equiv. glucose consumed), whereas lactic acid was identified as a
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major metabolite which presented an intense accumulation before its further
bioconversion to butyric acid and hydrogen. Moreover, in continuous mode, the
maximum hydrogen production rate of 1.72 L/L ·d was achieved at HRT 0.75 d,
R
whereas at HRT 25 d the methanogenic reactor showed good stability with methane
production 0.33 L CH /L ·d. Secondly, using the mixture of 55% sorghum, 40% CW
4 R
and 5% LCM, the optimum pH value of 5.5 was obtained (0.52 mol H /mol equiv.
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glucose), whereas in continuous mode the maximum hydrogen and methane
productivities (2.14 L/L ·d and 0.90 L/L ·d, respectively) were observed at HRTs 0.5 d
R R
and 16 d, respectively. Finally, using higher percentage of sorghum in the mixture
(95% sorghum and 5% LCM), a lower optimum pH was obtained equal to 5.0 (0.92 mol
H /mol equiv. glucose). Moreover, higher HRT (5 d) than previous experiments gave
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maximum hydrogen yield may be due to lignocellulosic material, whereas the highest
CH production rate of 0.44 L/L ·d was achieved at the HRT of 25 d.
4 R
Moreover, further exploitation of digestate from an anaerobic methanogenic reactor
was studied using a combined ultrafiltration/nanofiltration system and further COD
reduction was obtained. On the other hand, vermicomposting was conducted in order to
evaluate the sludge transformation to compost and as a result, good results in terms of
increased N-P-K concentration values were obtained. Furthermore, simulation of
mesophilic anaerobic (co)-digestion of different substrates was applied, using the
ADM1 modified model, where the results indicated that the modified ADM1 was able
to predict reasonably well the steady-state experimental data. Finally, technical and
economic evaluation was performed for an integrated biogas plant (1.0 MW) in terms of
the amount of investment required, the operating costs, the income from electricity
production and the sale of the produced compost.
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