CRANFIELD UNIVERSITY CAROLINE AMY HEPBURN Removal of Siloxanes from Biogas ...

162 Pages·2014·4.64 MB·English

CRANFIELD UNIVERSITY
CAROLINE AMY HEPBURN
Removal of Siloxanes from Biogas
SCHOOL OF ENERGY, ENVIRONMENT AND AGRIFOOD
Cranfield Water Science Institute
PhD
Academic Year: 2011 - 2014
Supervisors: Ewan McAdam and Nigel Simms
October 2014

CRANFIELD UNIVERSITY
SCHOOL OF ENERGY, ENVIRONMENT AND AGRIFOOD
Cranfield Water Science Institute
PhD
Academic Year: 2011 - 2014
CAROLINE AMY HEPBURN
Removal of Siloxanes from Biogas
Supervisors: Ewan McAdam and Nigel Simms
October 2014
This thesis is submitted in partial fulfilment of the requirements for
the degree of Doctor of Philosophy
© Cranfield University 2014. All rights reserved. No part of this
publication may be reproduced without the written permission of the
copyright owner.

ABSTRACT
Economic utilisation of biogas arising from sewage sludge is hampered by the
need to remove siloxanes, which damage gas engines upon combustion. This
thesis applies on-line Fourier transform infrared spectroscopy to measure
siloxanes in biogas upstream and downstream of the activated carbon vessels
designed to adsorb siloxanes. On-line analysis provides accurate
measurement of siloxane concentrations with a detection limit below the
siloxane limits set by engine manufacturers, high data intensity and timely
identification of breakthrough. Cost savings of up to £0.007 kWh-1 may be
realised compared to existing grab sampling. Using on-line analysis, the
performance of full-scale and bench-scale carbon vessels were measured.
Full-scale carbon contactors are typically operated at Reynold’s numbers close
to the boundary between the laminar and transitional regimes (Re = 40 - 55).
This thesis demonstrates, at full- and bench-scale, that increasing the Reynold’s
number to site the adsorption process in the transitional regime increases
media capacity, by 36% in dry gas and by 400% at 80% humidity. It is
postulated that the change in gas velocity profile which occurs as Reynold’s
number increases reduces the resistance to siloxane transport caused by gas
and water films around the carbon particles, and therefore increases the rate of
the overall adsorption process. In the laminar regime (Re = 31) increasing
humidity from zero to 80% led to the classical stepwise reduction in adsorption
capacity observed by other researchers, caused by the increasing thickness of
the water film, but in the transitional regime (Re = 73) increasing humidity had
no effect as no significant water film develops. It is therefore recommended that
siloxane adsorption vessels should be designed to operate at Reynold’s
numbers above 55. By choosing a high aspect ratio (tall and thin) both
Reynold’s number and contact time can be optimised.
Keywords: Activated carbon; Adsorption; Anaerobic digestion; FTIR
spectroscopy; Hydrodynamic regime
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ACKNOWLEDGEMENTS
First of all, a great big thank you to my supervisors, Ewan McAdam and Nigel
Simms, for keeping me on the straight and narrow when I was lost in realms of
inexplicable data and encouraging me when it seemed my test rig should be
consigned to the scrap heap.
Thanks also to the rest of my thesis committee, Pete Jarvis and Paul Morantz,
for pushing me to know my mechanisms and golden nuggets, and ensuring that
I kept to my Gantt chart (more or less).
Thanks to the EPSRC and Severn Trent Water for funding this project and
thanks to my contacts at STW, especially Pete Vale and John Griffiths, for
tolerating my requests for carbon samples, data and more. I’m keeping the
hard hat.
Many people have helped me in more informal roles – too many to name – but
thanks to all the CWSI technicians, especially Rukhsana, Nigel, Alan and Paul,
for suggesting better ways to do things when I was tapping a measuring
cylinder on the bench for hours and finding a lighter coolbox when I broke my
wrist and struggled to lift the big one. Thanks to everyone in the Pilot Hall and
the PhD office for assistance and companionship and helping me realise it
wasn’t just me… especial thanks goes to Rachel, Siobhán and Catherine – girly
coffees kept me sane!
Finally, thanks to my parents for supporting and encouraging me through a long
three years, and to Alex for feeding me up when I was down and dragging me
away to the hills by bike and on foot (but not electric tram).
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TABLE OF CONTENTS
ABSTRACT......................................................................................................... i
ACKNOWLEDGEMENTS...................................................................................iii
LIST OF FIGURES...........................................................................................viii
LIST OF TABLES..............................................................................................xii
LIST OF EQUATIONS......................................................................................xiv
LIST OF ABBREVIATIONS...............................................................................xv
1 Introduction......................................................................................................1
1.1 Background...............................................................................................1
1.2 Aim and objectives....................................................................................6
1.3 Thesis structure........................................................................................7
2 Adsorbent selection for environmental gas applications..................................9
Abstract...........................................................................................................9
2.1 Introduction.............................................................................................10
2.2 General process characterisation...........................................................11
2.3 Adsorbent characteristics........................................................................15
2.4 Adsorbate characteristics........................................................................17
2.5 Conclusions............................................................................................25
3 On-line siloxane detection for adsorption vessel characterisation using
FTIR spectroscopy...........................................................................................27
Abstract.........................................................................................................27
3.1 Introduction.............................................................................................28
3.2 Materials and Methods............................................................................31
3.2.1 Instrument and set up.......................................................................31
3.2.2 Calibration and verification...............................................................32
3.2.3 Data analysis....................................................................................34
3.3 Results....................................................................................................34
3.3.1 FTIR calibration using reference gas and real biogas......................34
3.3.2 Volatile organic carbons in biogas samples.....................................40
3.3.3 Finalised calibration using partial least squares analysis and
verification using external reference gas...................................................40
3.3.4 Application to full scale contactors...................................................42
3.4 Discussion ..............................................................................................42
3.5 Conclusions............................................................................................49
3.6 Acknowledgements.................................................................................49
4 On-line management of siloxane breakthrough in carbon contactors using
changepoint detection......................................................................................51
Abstract.........................................................................................................51
4.1 Introduction.............................................................................................52
4.2 Materials and Methods............................................................................54
4.2.1 The FTIR spectrometer and case study site.....................................54
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4.2.2 The changepoint detection algorithm and calculations.....................54
4.3 Results and discussion...........................................................................56
4.3.1 Changepoint detection for broad-fronted breakthrough curves........56
4.3.2 Changepoint detection for sharp-fronted breakthrough curves........59
4.3.3 Effectiveness of changepoint detection algorithm............................61
4.4 Implementing changepoint detection for broad-fronted breakthrough
profiles..........................................................................................................63
4.5 Conclusions............................................................................................65
5 Characterisation of Full Scale Carbon Contactors for Siloxane Removal
from Biogas using On-line Fourier Transform Infrared Spectroscopy...............67
Abstract.........................................................................................................67
5.1 Introduction.............................................................................................68
5.2 Materials and Methods............................................................................69
5.2.1 Site description.................................................................................69
5.2.2 On-line Fourier transform infrared (FTIR) analysis...........................70
5.2.3 Data analysis....................................................................................72
5.3 Results....................................................................................................72
5.3.1 Evaluation of siloxane concentration in the treated biogas...............72
5.3.2 Analysis of siloxane uptake in the GAC contactors..........................75
5.3.3 Analysis of GAC contactor operational conditions............................77
5.4 Discussion ..............................................................................................77
5.5 Conclusions............................................................................................84
6 Siloxane adsorption from biogas in the transitional regime...........................85
Abstract............................................................................................................85
6.1 Introduction.............................................................................................86
6.2 Materials and Methods............................................................................88
6.2.1 Test rig.............................................................................................88
6.2.2 FTIR.................................................................................................91
6.3 Results....................................................................................................91
6.3.1 Effect of EBCT and Reynold’s number on uptake in dry gas............91
6.3.2 Influence of humidity........................................................................95
6.3.3 Influence of competition between siloxane species..........................97
6.4 Discussion ..............................................................................................99
6.5 Supporting information..........................................................................103
6.5.1 Materials and methods...................................................................103
6.5.2 Capillary condensation...................................................................104
6.5.3 Use of the modified Wheeler equation to calculate rate constants.107
7 Practical implementation.............................................................................109
7.1 Is on-line FTIR assessment an effective tool?......................................109
7.2 How do we best enhance media capacity in practice?..........................114
7.3 What is the optimum carbon vessel design for siloxane removal?........117
7.3.1 For new carbon contactors.............................................................117
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