Energy balance and techno-economic assessment of algal biofuel production systems

266 Pages·2013·4.78 MB·English

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UNIVERSITY OF SOUTHAMPTON

ENGINEERING & THE ENVIRONMENT

Water and Environmental Engineering

Energy Balance and Techno-economic Assessment of Algal Biofuel
Production Systems

by

John James Milledge BSc MPhil

Thesis for the degree of Doctor of Philosophy

August 2013

UNIVERSITY OF SOUTHAMPTON
Abstract
ENGINEERING & THE ENVIRONMENT
Water and Environmental Engineering
Thesis for the degree of Doctor of Philosophy
ENERGY BALANCE AND TECHNO-ECONOMIC ASSESSMENT OF ALGAL
BIOFUEL PRODUCTION SYSTEMS
John James Milledge BSc MPhil
.
There has been considerable discussion in recent years about the potential of
micro-algae for the production of sustainable and renewable biofuels.
Unfortunately the scientific studies are accompanied by a multitude of semi-
technical and commercial literature in which the claims made are difficult to
substantiate or validate on the basis of theoretical considerations.

To determine whether biofuel from micro-algae is a viable source of renewable
energy three questions must be answered:
a. How much energy can be produced by the micro-algae?
b. How much energy is used in the production of micro-algae?
c. Is more energy produced than used?

A simple approach has been developed that allows calculation of maximum
theoretical dry algal biomass and oil yields which can be used to counter some
of the extreme yield values suggested in the 'grey' literature.

No ready-made platform was found that was capable of producing an energy
balance model for micro-algal biofuel. A mechanistic energy balance model
was successfully developed for the production of biogas from the anaerobic
digestion of micro-algal biomass from raceways. Preliminary calculations had
suggested this was the most promising approach. The energy balance model
was used to consider the energetic viability of a number of production
scenarios, and to identify the most critical parameters affecting net energy
production. These were:
i
a. Favourable climatic conditions. The production of micro-algal biofuel in
UK would be energetically challenging at best.
b. Achievement of ‘reasonable yields’ equivalent to ~3 % photosynthetic
efficiency (25 g m-2 day-1)
c. Low or no cost and embodied energy sources of CO and nutrients from
2
flue gas and wastewater
d. Mesophilic rather than thermophilic digestion
e. Adequate conversion of the organic carbon to biogas (≥ 60 %)
f. A low dose and low embodied energy organic flocculant that is readily
digested, or micro-algal communities that settle readily
g. Additional concentration after flocculation or sedimentation
h. Exploitation of the heat produced from parasitic combustion of micro-
algal biogas in CHP units
i. Minimisation of pumping of dilute micro-algal suspension

It was concluded that the production of only biodiesel from micro-algae is not
economically or energetically viable using current commercial technology,
however, the production of micro-algal biogas is energetically viable, but is
dependent on the exploitation of the heat generated by the combustion of
biogas in combined heat and power units to show a positive balance.

Two novel concepts are briefly examined and proposed for further research:
a. The co-production of Dunaliella in open pan salt pans.
b. A 'Horizontal biorefinery' where micro-algae species and useful products
vary with salt concentration driven by solar evaporation.

ii
Contents
Abstract ........................................................................................................................................... i
Contents ....................................................................................................................................... iii
List of tables ............................................................................................................................. ix
List of figures ........................................................................................................................... xi
List of accompanying materials................................................................................ xiii
Declaration of authorship ............................................................................................... xv
Acknowledgements .......................................................................................................... xvii
Abbreviations and nomenclature .............................................................................xix
1. Introduction .................................................................................................................... 1
1.1 Fossil fuels and global warming ............................................................ 1
1.2 Renewable and bioenergy ...................................................................... 3
1.3 Algae and algal biofuels ......................................................................... 8
1.4 Aims and objectives .............................................................................13
1.5 Structure of thesis ................................................................................14
2. Algal biofuel process operations ...................................................................15
2.1 Growth systems ...................................................................................15
2.1.1 Open systems ................................................................................16
2.1.1.1 Algal wastewater stabilisation ponds ......................................17
2.1.1.2 Circular algal cultivation ponds ..............................................19
2.1.1.3 Raceway systems ....................................................................19
2.1.2 Closed systems – Photo-bioreactors (PBRs) ...................................20
2.1.3 Comparison of open and closed systems ......................................22
2.2 Engineering aspects of raceway systems .............................................24
2.2.1 Materials of construction ...............................................................24
2.2.2 Raceway channels ..........................................................................26
2.2.2.1 Mixing and fluid velocity in raceways .....................................26
2.2.2.2 Reynolds number and turbulence ...........................................28
2.2.2.2.1 Effect of Reynolds number and turbulence on algal
productivity ..........................................................................................29
iii
2.2.2.3 Head loss ................................................................................ 30
2.2.2.4 Depth of raceways .................................................................. 31
2.2.2.5 Width of raceways ................................................................... 32
2.2.2.6 Raceway bends ....................................................................... 34
2.2.3 Fluid propulsion in raceways ......................................................... 36
2.2.4 Light penetration and light climate in raceways ............................ 39
2.2.5 Gaseous transfer zones and sumps in raceways ........................... 41
2.2.6 Raceway size ................................................................................. 43
2.2.7 Requirements for micro-algal growth ............................................ 44
2.2.7.1 Nutrients and water ................................................................ 44
2.2.7.2 Carbon dioxide ....................................................................... 46
2.3 Algal harvesting ................................................................................... 48
2.3.1 Sedimentation ............................................................................... 50
2.3.2 Flocculation ................................................................................... 53
2.3.2.1 Non-organic flocculants .......................................................... 54
2.3.2.2 Organic flocculants ................................................................. 55
2.3.2.3 Flocculation by pH and environment modification ................. 57
2.3.2.4 Bio-flocculation ....................................................................... 58
2.3.2.5 Alternative methods of flocculation ........................................ 59
2.3.3 Flotation ........................................................................................ 60
2.3.4 Filtration ........................................................................................ 62
2.3.4.1 Micro-filtration ........................................................................ 62
2.3.4.2 Ultra-filtration ......................................................................... 62
2.3.4.3 Macro-filtration ....................................................................... 63
2.3.5 Centrifugation ............................................................................... 65
2.3.5.1 Disc-stack centrifuges............................................................. 65
2.3.5.1.1 Energy requirements for disc-stack centrifugation ............ 66
2.3.5.2 Disc-stack centrifugation for combined cell separation and
disruption …………………………………………………………………………….69
2.3.5.2.1 Micro-eddies and algal cell disruption ............................... 69
2.3.5.2.2 Cell disruption in disc-stack centrifuges ............................ 70
2.3.5.3 Solid bowl and other centrifuge types .................................... 72
iv
2.3.6 Materials handling .........................................................................74
2.3.7 Drying ............................................................................................75
2.4 Energy extraction from micro-algal biomass .......................................76
2.4.1 Direct combustion .........................................................................77
2.4.2 Pyrolysis ........................................................................................79
2.4.2.1 Methods of pyrolysis ...............................................................79
2.4.2.2 Pyrolysis of micro-algae ..........................................................79
2.4.2.3 Refining of pyrolysis bio-oil ....................................................80
2.4.3 Gasification....................................................................................81
2.4.3.1 Gasification of micro-algae .....................................................82
2.4.4 Liquefaction and hydro-thermal upgrading ...................................84
2.4.5 Bio-hydrogen .................................................................................88
2.4.6 Micro-algal fuel cells ......................................................................89
2.4.7 Bioethanol .....................................................................................90
2.4.7.1 Micro-algal bioethanol ............................................................92
2.4.8 Biodiesel and trans-esterification ..................................................94
2.4.9 Anaerobic digestion ......................................................................98
2.4.10 Co-production and biorefineries................................................. 101
2.5 Conclusions ...................................................................................... 103
3. Software evaluation .............................................................................................. 105
3.1.1 FP7 All-Gas project ..................................................................... 105
3.1.2 Life Cycle Assessment software ................................................. 106
3.1.3 SuperPro software ...................................................................... 107
3.1.4 UNISIM software ......................................................................... 108
3.1.5 Aspen Plus software ................................................................... 110
3.1.6 Pinch analysis ............................................................................. 113
3.1.7 Conclusions ................................................................................ 114
4. Initial investigations ............................................................................................ 117
4.1 Micro-algal yield ................................................................................ 117
4.1.1 Photosynthesis and photosynthetic efficiency ............................ 117
4.1.2 Solar insolation ........................................................................... 119
4.1.3 Micro-algal calorific value and calculated biomass yield ............ 120
4.1.4 Results and discussion ............................................................... 120
4.1.4.1 Calorific value of micro-algae .............................................. 120
4.1.4.2 Maximum theoretical micro-algal biomass yield.................. 121
v
4.1.4.3 Maximum theoretical micro-algal oil yield ............................ 123
4.1.4.4 Potential achievable micro-algal biomass and oil yield ......... 123
4.1.5 Conclusions ................................................................................. 125
4.2 Operational energy returns on paddlewheel mixing .......................... 126
4.2.1 Energy return on operational energy investment EROOI ............. 126
4.2.2 Basis of calculation ...................................................................... 127
4.2.3 Results and discussion ................................................................ 127
4.2.4 Conclusions ................................................................................. 130
5. Model construction ............................................................................................... 131
5.1 Model structure and assumptions ..................................................... 131
5.1.1 Micro-algal biomass and biogas yields ........................................ 131
5.1.2 Growth system ............................................................................ 132
5.1.2.1 Paddlewheel ‘mixing’ energy ................................................ 133
5.1.3 Gaseous transfer energy ............................................................. 133
5.1.4 Outgassing of CO ....................................................................... 134
2
5.1.5 Accumulation of O ..................................................................... 136
2
5.1.6 Pumping ...................................................................................... 136
5.1.7 Harvesting ................................................................................... 138
5.1.8 Anaerobic digestion .................................................................... 139
5.1.8.1 Digester volume and dimensions ......................................... 139
5.1.8.2 Heating ................................................................................. 139
5.1.8.3 Mixing ................................................................................... 140
5.1.9 Spreadsheet ................................................................................. 141
5.2 Validation and calibration .................................................................. 142
5.2.1 Comparison with an existing AD model ...................................... 142
5.2.1.1 Method and conditions of comparison ................................. 142
5.2.1.2 Results and discussion ......................................................... 142
5.2.2 Loading rate ................................................................................ 144
5.2.3 Mass balance ............................................................................... 144
5.3 Associated studies during model development ................................. 144
5.3.1 Estimation of relative outgassing of oxygen in a raceway .......... 144
5.3.1.1 Method of estimation ........................................................... 145
5.3.1.2 Results and discussion ......................................................... 145
5.3.2 Night-time de-oxygenation by micro-algal respiration ................ 146
vi
5.3.2.1 Method of calculation .......................................................... 146
5.3.2.2 Results and discussion ........................................................ 147
5.3.3 Bacterial CO production and nitrogen utilisation ...................... 149
2
5.3.3.1 Method of estimation ........................................................... 149
5.3.3.2 Results and discussion ........................................................ 149
6. Modelling of scenarios ....................................................................................... 151
6.1 Head loss in raceways ....................................................................... 151
6.2 Energy input relative to biomass energy production and outgassing in
a raceway .................................................................................................... 152
6.2.1 Effect of depth and velocity on energy ratio............................... 152
6.2.2 Effect of depth and velocity on outgassing of CO in raceways .. 154
2
6.2.3 Effect of width and length on outgassing and energy ratio ....... 154
6.2.4 Effect of micro-algal photosynthetic efficiency on outgassing and
energy ratio in raceways .......................................................................... 156
6.3 Effect of selected process parameters on the concentration factor
required to achieve an EROOI of 1 .............................................................. 157
6.3.1 Effect of raceway dimensions ..................................................... 157
6.3.2 Concentration factor to achieve an EROORI equal to 1 for various
process options ........................................................................................ 158
6.3.2.1 ‘Benchmark’ case ................................................................. 160
6.3.2.2 CO Concentration ............................................................... 162
2
6.3.2.3 Effect of factors increasing micro-algal concentration......... 162
6.3.2.4 Effect of paddlewheel operation factors .............................. 163
6.3.2.5 Anaerobic digestion factors ................................................. 164
6.3.2.6 Climate conditions ............................................................... 165
6.3.2.7 Harvesting and equipment efficiencies ................................ 165
6.3.2.7.1 Harvesting efficiency ....................................................... 166
6.3.2.7.2 Other equipment efficiencies .......................................... 166
6.3.2.8 Summary of scenario results................................................ 167
6.4 Pragmatic case .................................................................................. 167
6.4.1 Heat Recovery ............................................................................. 168
6.4.2 Evaporation ................................................................................ 168
6.5 Dissolved oxygen .............................................................................. 169
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