Appendix C Second Year EngD Dissertationepubs.surrey.ac.uk/809984/43/Mills - Appendix C.pdf · 2nd Year EngD Dissertation Nick Mills October 2012 University of Surrey 247 Thames Water

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Appendix C – Second Year EngD Dissertation

2nd Year EngD Dissertation Nick Mills October 2012

University of Surrey 241 Thames Water

UNIVERSITY OF SURREY AND THAMES WATER UTILITIES LIMITED

Second Year EngD Dissertation Optimising Sustainable Energy Production

within the Water Industry

Nick Mills

October 2012

Engineering Doctorate in Sustainability for Engineering and Energy Systems



Summary

Background

The UK water industry has huge, but as yet under-developed, potential to generate sustainable

energy in the form of biogas, heat, electricity and other fuels from by-products created in the

treatment of wastewater. Sewage sludge is the main energy rich by-product, a sustainable

biomass resource with a similar calorific value to woodchip.

Objectives

This project will research, analyse, design and implement methods of increasing the

sustainable energy production from wastewater. Specifically anaerobic digestion processes are

to be analysed and optimum configurations developed. Sustainable thermal sludge drying is to

also be trailed to demonstrate that a granular renewable solid fuel can be produced and used

beneficially within existing combustion plant. The project aims to challenge common

understanding and attitudes; barriers to change that exist within policy will also be explored.

Justification

Although currently delivering in the order of only 1.5% UK’s renewable electricity, the water

industry could be responsible for delivering over 7% today or 3.5% of the 2020 renewable

target; whilst making significant economic and environmental savings to UK water companies

such as Thames Water, its customers and ‘UK PLC’.

Outcomes

A genuine contribution to knowledge has been made through the modelling of existing

advanced anaerobic digestion processes this has resulted in a number of recommendations to

optimise configuration, which has justified the construction of a new pilot plant. The sludge

drying trial has proven that the technology is safe, efficient and sustainable and that the dried

product can be used successfully within a fluidised bed combustor. The early trials have shown

that combustion conditions and system efficiency of a wet sludge combustor can be improved

significantly.

Next Steps

Life cycle analysis of anaerobic digestion process configurations will be refined and a journal

paper will be published in early 2013. The advanced anaerobic digestion pilot plant will be

commissioned to prove and develop the process ready for full scale implementation.

Dried sludge fuel combustion trials are to be completed; data processed and analysed to

quantify the effect before reporting the results in an appropriate journal.

Through analysis and further trials, understand the benefit of separating sludge process

streams to optimise energy recovery through drying, disseminate the results and influence

strategy.

Research and engage with various industry and academic experts to understand policy barriers

which are preventing the UK water industry unlocking the renewable energy potential of

sewage sludge, publicise any conclusions.



Contents

1 Introduction ...................................................................................................................... 245

1.1 Aims & Objectives ..................................................................................................... 245

1.2 Overall Research Plan ............................................................................................... 245

2 Background ....................................................................................................................... 250

2.1 Wastewater treatment ............................................................................................. 250

2.2 Sludge ........................................................................................................................ 251

2.3 Existing Energy Recovery Technologies .................................................................... 252

2.3.1 Anaerobic Digestion .......................................................................................... 252

2.3.2 Advanced Anaerobic Digestion ......................................................................... 253

2.3.3 Incineration ....................................................................................................... 254

2.4 The Influence of UK Renewable Energy Policy.......................................................... 255

3 Analysis of Existing Processes ........................................................................................... 256

3.1 Advanced Anaerobic Digestion Analysis ................................................................... 256

3.2 Optimum THP Configurations ................................................................................... 257

3.3 Preliminary Life Cycle Analysis THP Configurations .................................................. 258

4 Processes under Development ......................................................................................... 262

4.1 Sustainable Thermal Drying for Fuel Production ...................................................... 262

4.1.1 Background & Justification ............................................................................... 262

4.1.2 Slough Dryer Process ........................................................................................ 264

4.1.3 Slough Dryer Performance ................................................................................ 266

4.1.4 Experience ......................................................................................................... 267

4.1.5 Site Integration.................................................................................................. 267

4.2 Alternative Fuel Trial ................................................................................................. 268

4.2.1 Initial results ...................................................................................................... 270

4.2.2 Quantification ................................................................................................... 271

4.3 Intermediate-Thermal Hydrolysis Process ................................................................ 273

4.3.1 Laboratory Trials ............................................................................................... 274

4.3.2 I-THP Process Modelling ................................................................................... 275

5 Conclusion ......................................................................................................................... 277

References ................................................................................................................................ 278

Appendix A ...................................................................................... Error! Bookmark not defined.



Appendix B ...................................................................................... Error! Bookmark not defined.

Appendix C ...................................................................................... Error! Bookmark not defined.

Appendix D ...................................................................................... Error! Bookmark not defined.

Appendix E ...................................................................................... Error! Bookmark not defined.



1 Introduction

1.1 Aims & Objectives

The UK water industry has huge, but as yet under-developed, potential to generate sustainable

energy in the form of biogas, heat, electricity and other fuels from by-products created in the

treatment of wastewater. Sewage sludge is the main energy rich by-product and has a similar

calorific value1 to woodchip and some forms of coal and is a sustainable biomass resource. It is

currently underutilised, only 10% is converted into useful energy typically electrical power.

Although currently delivering in the order of only 1.5% UK’s renewable electricity, the water

industry could be responsible for delivering over 7% today or 3.5% of the 2020 renewable

target2; whilst making significant economic and environmental savings to UK water companies

such as Thames Water and its customers.

This collaborative research and development project between the University of Surrey and

Thames Water is being untaken by a Doctoral Research Engineer. This project will research,

analyse, design and implement methods of increasing the sustainable energy production from

wastewater. There are four main outputs from the work which are explained in this document:

Modelling of the existing and optimised processes - including economic &

environmental life cycle analysis;

A full scale demonstration of sustainable thermal drying for the production of granular

renewable solid fuel;

Granular renewable solid fuel support dosing trial to replace natural gas within a full

scale fluidised bed incinerator;

Pilot development of an advanced anaerobic digestion process.

The project is continually challenging common understanding and attitudes in academia and

the industry as a whole. The aim is to change the accepted norm from treatment and disposal;

to a situation in which the optimum energy recovery is continually strived for. The research

engineer has been awarded an Industrial Fellowship from the Royal Commission for 1851; this

additional support will enable the project to explore barriers to policy change in the later

stages.

1.2 Overall Research Plan

A literature study has been undertaken which has explored and studied published material to

understand what has already been achieved. This has concentrated on the high level analysis

of Anaerobic Digestion (AD), incineration & drying processes. In particular the study has

focused on environmental and economic life cycle analysis but inevitably a better

understanding of policy drivers has been required. References to this study can be found

throughout this document and an extended literature review can be found in Appendix A.

1 Typically 20 MJ/kg dry 2 Based on current UK renewable electricity output of 64TWh and 2020 prediction of 91TWh (DECC 2011) and assuming current output of 0.8TWh from sewage sludge and 3.2TWh (40% efficient) in 2020.



Gaps in knowledge were identified before and at the start of the EngD which helped refine the

project scope and direction. In particular it identified a clear need to better understand the use

of energy, specifically heat, in advanced anaerobic digestion processes such as thermal

hydrolysis. This along with a PhD project has enabled a pilot plant to be designed to trial and

develop an optimum configuration for advanced anaerobic digestion. The work has also shown

that sludge incineration is very expensive process and the fluidised bed incineration within

TWUL is operating far from optimum. This justified the funding release from Thames Water to

build the sludge drying demonstration plant and the alternative fuel trial, so as to understand

how these processes could fit into and modify strategy for sludge and energy within Thames

Water.

The 4 year research project can be summarised in the flow chart shown in Figure 1.

Figure 1 - Research Project Summary - Flow Chart

The main body of the project has been broken down into 9 key milestones each with unique

deliverables; these are detailed below including completed milestones 1, 2 & 3. A Gantt chart

is shown in Figure 2 which should be read in conjunction with this section.

Milestone 1 – Knowledge Gathering (see section 2)

Literature search and review

Attend conferences and gather knowledge

Confirm project is focused and will have a ‘contribution to knowledge’

Completed deliverables (April 2011):

Refined project scope

6 month report

Conference feedback from 15th Bio-solids and Energy from Waste conference

Milestone 2 – Year 1 Academic Modules

Social Research

Environmental Science & Society

PRINCE 2 Project Management

Transitions to a Low Carbon Economy

EngD Conference

Completed deliverables (September 2011):

Successful completion of all modules (average mark - 66%)

Received excellent feedback from EngD conference presentation



Figure 2 - Full Programme for EngD

Milestone 3 - Process model phase-1 (see section 3.1 & 3.2)

Construction of a process model for existing AD processes

Construction of a financial model linked to process model

Benchmarking of model against operational sites

OpEx comparison of existing AD processes

Optimum configuration for energy extraction and lowest net OpEx



Completed deliverables (September 2011)

Conference Paper - presented at the 16th European Biosolids Conference

Paper presented at the EngD Conference, University of Surrey

Internal presentations & reports

Milestone 4 - Process model phase-2 (see section 3.3)

Gather full capital cost for existing AD processes and update models

Full NPV and LCA comparison of existing AD processes

Optimum configuration for energy extraction and lowest whole life impact

Recommend options for alternative process configurations optimised financially and

environmentally

Risk analysis of processes

Deliverables (Oct 2012 (now Jan 2012) – LCA is only partially complete and is unlikely to

finished until Dec 2012):

Functional Process and Economic Modelling Planning tool – complete

One or two conference papers - abstracts accepted for ‘Fourth International

Symposium on Energy from Biomass and Waste’ & ‘1st International IWA Conference

on Holistic Sludge Management’

A peer reviewed journal submission – not yet completed

Internal report and presentation to TW asset management & University of Surrey


Life Cycle Approaches

Environmental Auditing & Management Systems

Environmental Law

EngD Conference

Year 2 Dissertation and Viva

Deliverables (November 2012):

Pass all modules and maintain average mark – year 1+2 average mark 68% achieved

Received excellent feedback from EngD conference

Pass year 2 viva – on-going

Milestone 6 - Sustainable Thermal Drying Demonstration (see section 4.1 & 4.2)

Operate plant and monitor performance of unit and the overall impact within a

wastewater treatment site

Produce 200tonnes of solid bio-mass fuel and transport to Crossness incinerator,

including converting an incinerator feed system and conducting support fuel trials

Monitor impact on incinerator (product handling, combustion & emissions)

Assess data from trial and build a model to understand full scale implications (process

model phase-3)

Deliverables (April 2013):

An updated Functional Process and Economic Modelling Planning tool for integrated

sustainable thermal drying – preliminary assessment on-going



Quantification of benefits within fluidised bed combustor – simplified analysis has

begun

Conference paper – accepted for 17th European Biosolids Conference

A peer reviewed journal submission – journal identified


Environmental Economics

Integrated Assessment

EngD Conference

Deliverable (September 2013):

Pass all modules and maintain average mark

Receive excellent feedback from EngD conference

Milestone 8 - ‘End of Waste’ case for granular renewable solid fuel (see section 4.2)

Gather data from sustainable thermal drying trial

Conducted appropriate analysis potentially with academic resource support

Build case for ‘End of Waste’ application

Submit application to EA and maintain discussions


‘End of Waste’ status for a waste derived fuel process – initial feasibility complete

Peer reviewed journal submissions as main and co-authors

One or two conference papers

Internal report and presentation to multiple departments and the board

Discuss and obtain feedback with technical experts within industry, TW and academia

Milestone 9 - Sludge & Energy Innovation Centre (see section 4.3)

Project manage design, construction and operation of pilot facility

Overseeing commercial negotiations with third party with respect to IPR

Coordinate and Supervise EngD project to optimise and develop new process on the

pilot plant

Gather data and update the model to include the new process (process model ph-4)

Design reconfigurations for existing plants in Thames Water and justify investment


Designs for new and reconfigured full scale plants

One or more peer reviewed journal submissions as main and / or co-author

One or two conference papers

A detailed publishing plan describing the papers and the proposed journals can be seen in

Appendix E.

In addition to the milestones laid out above, the research engineer also intends to use the

support given by the Royal Commission to explore barriers to policy change. The 3 step action

plan seen at the bottom of Figure 2 will begin with the identification of key players, which will

involve a secondment to the Department of Energy & Climate Change (DECC). This



investigation period will be followed by a series of workshops and conference to lay out the

future of sustainable energy from sewage sludge in the UK and the policy interactions which

may need improvement to stimulate growth in the industry.

2 Background

2.1 Wastewater treatment

Wastewater treatment has been developed over the past 120 years employing a number of

different techniques (Cooper 2001). The activated sludge process was developed in the early

20th century; the first plant was commissioned in DavyHume, Manchester in 1914 (Coombs

1992). This process now treats 80% of wastewater in the Thames Water region and 60% across

the UK. This project will explore energy recovery options based on wastewater treatment plant

employing activated sludge, the configuration shown in Figure 3 and described below:

1. Preliminary treatment – grit and rag are removed from the wastewater (there are

potential opportunities for energy recovery at this stage, but is not within this scope);

2. Primary treatment – removal of settable solids from the waste water, typically this is

through settlement in a tank with low velocities to allow the solids to sink to the

bottom, this forms primary sludge that can be pumped, forming a separate sludge

stream. The wastewater flows over a weir at the top of the tank and on to Secondary

treatment. The settlement process typically removes around 60% of suspended solids

and concentrates this up to less than 1% of the total flow;

3. Secondary treatment – removal of biodegradable organic matter, suspended solids not

removed in primary treatment and soluble materials such as ammonia, phosphate and

soluble carbon compounds. In an activated sludge plant an aeration basin is used, in

which conditions are optimised for microorganisms which in turn:

a. Oxidise biodegradable constituents into acceptable end products;

b. Capture non settled solids and form a biological floc;

c. Transform or remove nutrients;

This stage of the process is mainly aerobic and requires the input of considerable

quantities of air, generally using large compressor/blowers piped through to diffuser

domes in the floor of the aeration basin.

4. After the biological treatment within the aeration basin, the wastewater enters the

final settlement tanks, allowing the biological sludge mass to sink forming a sludge

which can be pumped. More than 70% of this sludge is returned to the aeration basin

to ensure sufficient sludge retention time in the process to accommodate the slowest

growing micro-organisms. The surplus activated sludge (SAS) representing 50% of the

flow now forms a second separate sludge stream;

5. The effluent from these final settlement tanks is now generally allowed to flow to the

water course. On some sites a tertiary treatment stage follows secondary treatment

which is used to remove residual suspended solids (Metcalf_&_Eddy 2003) and/or to

achieve ever increasing standards required.



Figure 3 – Typical Configuration of an Activated Sludge Wastewater Treatment Process

To give an idea of scale of wastewater treatment, Figure 4 shows part of a STW in West

London that treats sewage for a population of 1.8 million people.

Figure 4 – Picture of Mogden STW showing the final Settlement Tanks in the fore ground with Aeration Basins

visible in the background (TWUL-i 2011)

2.2 Sludge The sludge or waste bio-solids from the wastewater treatment process (described above)

require further treatment before it can be safely returned to the environment. Historically it

has been treated to reduce pathogens and disposed of to agriculture which is encouraged by

the EU sewage sludge directive 86/278/EEC; and most sludge treatment processes have been

designed to meet the Sludge Use in Agriculture Regulations 1989. As a result process streams

are not suitable for optimum renewable energy extraction, but instead are designed for least

cost sludge disposal. Traditionally, only the simplest attempts have been made to recover

energy such as biogas from digestion. Barber observes that “currently, the Water Industry

generates the majority of this Biogas [renewable energy] using infrastructure which was not

designed for either, energy generation or carbon footprint reductions” (Barber 2010). This is

evident from Figure 5 which shows the potential energy available from the wastewater

organics.



Figure 5 - Potential Energy from a typical (150,000 Population Equivalent - PE) Wastewater Plant with AD -

adapted from (Pearce 2009)

2.3 Existing Energy Recovery Technologies Despite conflicting drivers several techniques have been deployed by the UK water industry

that allows energy recovery from sludge, currently producing around 0.77 TWh pa of electricity

from 1.6 M Tonne Dry Solids (TDS) pa of sewage sludge (Andrews 2008; DECC-ii 2011). The

energy content of the sludge is approximately 4.7MWh/Tonne Dry Solids (TDS) (Lee 2010)

assuming this value is true for all 1.6 MTDS pa, the UK as a whole produces sludge with a gross

energy content of 7.52 TWh pa. Only 0.77 TWh pa is converted to electrical energy resulting in

a UK wide annual conversion efficiency of just 10%.

Thames Water currently produces 391,311 TDS pa of sewage sludge (June rerturn to OfWat)

and generates 166GWh pa (TWUL-ii 2011) of renewable electricty, which means TW has only a

9% conversion efficiency, generally the processes are more than 10% efficient but overall

efficiency is reduced because a proportion of sludge is not processed through a recovery

opertaion, generally on smaller remote sites.

The conversion, across the UK, is currently achieved using a combination of anaerobic

digestion, advanced AD and incineration with energy recovery, these are briefly described

below. A more detailed description of these processes and other aspects of the industry can be

found in Appendix A.

2.3.1 Anaerobic Digestion

Anaerobic Digestion (AD) can achieve the required pathogen kill to allow the sludge to be

disposed of to land, under the current UK regulations over 60% is recycled to agriculture

(Kelessidis and Stasinakis 2012). AD has the added benefit of reducing the volume of sludge

and producing a methane rich biogas which can be used as fuel. The most common variant is

mesophilic anaerobic digestion (MAD); it is a complex biological process involving a diverse

bacterial consortium (Appels, Baeyens et al. 2008) shown in Fugure 5.



Figure 6 - Stages of Mesophilic Anaerobic Digestion (Tchobanoglous 1993)

In a typical process both sludge streams are thickened and combined before being heated to

approximately 37degC inside a mixed digester tank with retention times of 12 to 20 days. The

volatile solids destruction is approximately 50% which yields 350m3/TDS of biogas and

translates to 40% mass reduction (Appels, Baeyens et al. 2008). The final digestate is then

dewatered to a cake of around 20% Dry Solids (DS) and transported off site for agricultural

land use (Suh and Rousseaux 2002). To gain a better understanding of AD and the parameters

that influence the process see Appendix A.

2.3.2 Advanced Anaerobic Digestion

Although AD is widespread and effective sludge treatment technique for the water industry, it

has limitations. For this reason there are a number of process variations which have been

under development and have begun to be applied during the last 10 years, these all pre-treat

the sludge aiming to improve the digestibility, the benefits of advanced AD (Pickworth 2006;

McNamara, Wilson et al. 2012) can be summarised as:

Increased Biogas yields;

Increased volatile solids destruction;

Reduction in mass;

Process allows increased organic loadings in existing assets reducing capital costs;

Enhanced dewatering characteristics reducing transport costs and increasing the quality of

product for farmers.

The most developed and applied advanced AD techniques are thermal and biological

hydrolysis, as hydrolysis is the typically the rate limiting step of AD these variants attempt to

reduce this bottleneck. The Thermal Hydrolysis Process (THP) is the most widespread and the

technology of choice Thames Water to achieve short term generation and carbon mitigation

targets (TWUL 2009).

THP involves using high temperature (165degC) and pressure (7Barg) to disrupt and solubilise

sludge before a conventional digester. The process homogenises the sludge so that it is more

digestible resulting in increased methane production and a smaller volume of digestate (Kepp

2000). Across the world there are 23 full scale Thermal Hydrolysis sites either in operation or



construction that will process 445,000 Tonnes Dry Solids (TDS) pa (Cambi 2010). However, the

increase in biogas does not necessarily result in an overall net increase in energy yield; as the

process demands an input of high grade heat and additional electrical energy, when compared

with conventional AD. The high grade heat demand typically outweighs the heat available from

Combined Heat & Power (CHP) and all of the UK THP installations currently require a support

fuel (typically natural gas) to maintain the process (Mills-ii 2011). Due to the way in which the

industry is financially regulated one of the major drivers for THP is that it maximises

throughput through existing assets and this maybe at the detriment to energy recovery.

Research is limited in this area, presenting an opportunity for the project to make a

contribution to knowledge by optimising the application of THP.

2.3.3 Incineration

Incineration involves the complete conversion of sewage sludge to oxidised end products such

as carbon dioxide and other gases, water and ash. There are clear advantages to complete

conversion which are high volume reduction, disinfection and the recovery of heat to produce

steam which can drive turbines to produce electricity. However, high costs and adverse

environmental effects limit the application of this process to large works with limited disposal

options where the economics are more favourable (Metcalf_&_Eddy 2003). Thames Water

own and operate two sludge fluidised bed incinerators in East London and around the UK

there are six similar plants that remain in operation as shown in Table 1 that process about

10% of the total sludge produced across the UK.

Operator Name Installation

Name

Permit

Capacity

(TDS pa)

Through-

put

2006

Through-

put

2007

Through-

put

2008

Through-

put

2009

United Utilities Widnes 100,000 24,654 26,006 1,737 7,1773

Thames Beckton 90,500 59,441 59,291 67,342 71,540

Thames Crossness 53,500 31,035 31,186 30,191 31,186

Severn Trent Coleshill 40,000 17,574 15,550 04 0

Severn Trent Roundhill 15,000 6,737 888 0 0

Yorkshire Knostrop 28,500 22,290 24,040 25,064 22,514

Yorkshire Esholt 25,500 17,256 14,500 17,842 17,253

Yorkshire Blackburn 18,000 10,052 11,913 13,160 12,936

Yorkshire Calder

Valley

16,500 7,395 10,451 12,377 12,505

Totals 387,500 196,434 190,825 167,713 175,111

Table 1 - Sewage Sludge Incinerators in England & Wales (Hand-Smith 1999; Abbott 2004; Smith 2008; EA 2009)

It is unlikely that many new sludge incinerators would be built due to the public perception

and subsequent planning restrictions. These existing UK facilities were typically built to replace

sludge dumping at sea which was banned in the UK and the EU in 1998 (EC 1998). The reaction

to the changes enabled the high capital and operational costs to be justified (Werther and

Ogada 1999). Some believe that much of the UK’s sewage sludge incineration will be replaced

3 Throughput low due to plant being upgraded 4 Plant mothballed



with the more economic AD process by 2030 (AEA 2010) and the trend in Table 1 indicates this

has begun, Yorkshire Water has announced plans to replace incineration assets with advanced

AD processes it is estimated this will save £120/TDS in operational costs (Bigot 2011).

2.4 The Influence of UK Renewable Energy Policy The Electricity Act was implemented in 1989 to privatise the industry, within this legislation

the Non Fossil Fuel Obligation (NFFO), originally set up to support nuclear power, was

instigated. The auction based system opened the market to renewable generators, the early

system had many short falls and targets where never met, but this early legislation built a

small but successful renewables industry, mostly based around landfill gas and biomass

generation (Simmonds 2002). In 2002 the industry was reformed again and the Renewables

Obligation (RO) replaced the NFFO, the targets where more aspirational and the incentives

more encouraging, generators receive about £45/MWh. Figure 4 shows how biomass

generation has grown since 1990, the growth in landfill gas is quite incredible, and the

response to co-firing in 2002 is credit to the RO scheme that generally has received criticism

for not delivering (Gross 2010; Woodman and Mitchell 2011). The success in these other

industries has not been replicated in the water industry and sewage sludge generation is

dwarfed in comparison; 105% growth compared with 1696% seen in landfill gas (DECC-ii 2011).

Figure 7 - Renewable Electricity from Biomass in the UK, source data from (DECC-ii 2011)

The lack of growth from sewage sludge generation could be blamed, at least in part, on the

complexity of the regulation in the water industry and independent analysis for DECC claims

constraints on cash flow result in low payback on investment mainly due to financial

restrictions from regulation (AEA 2010), although there are signs that change is coming to

resolve some of these constraints (OFWAT 2011). This work by AEA in part has justified the

reduction in support for sewage sludge generation which now receives half the RO subsidy it

received in 2010. The basic analysis for DECC has no appreciation for the current and potential

advances in technology that would increase the invest-ability of future schemes but instead

the assumption has been that the industry renewable energy output is optimum. As the

analysis in section 2.3 shows only 10% conversion efficiency if it were feasible to achieve 40%



then the industry would generate about 3,200 GWh pa which would represent 7% today and

3.5% of the UK 2020 renewable electricity target5; mitigating climate change by reducing the

UKs greenhouse gas emissions (DECC-iii 2011) . One of the aims of the research is to change

attitudes across the industry including DECC and OFWAT to allow the potential to be

unleashed.

3 Analysis of Existing Processes Advanced AD technologies discussed previously enhance the performance by pre-

processing and implementing the rate limiting hydrolysis step upfront of AD.

Processes have been compared economically and environmentally due to gaps in

published material. This work will form a base line for future process improvements

being developed as part of the project. The conclusions from these studies have

informed the industry and academia on optimum configuration for advanced

anaerobic digestion and opened the debate around discrepancies in UK Energy

policy.

3.1 Advanced Anaerobic Digestion Analysis Thermal and biological hydrolysis are the two most commonly deployed advanced AD

processes. A model was created which compared conventional MAD, Acid Phase Digestion

(APD) a variant of biological hydrolysis and THP (Mills-i 2011) (see Appendix B for full paper). It

was found that THP AD produces more bio-gas than APD and MAD as seen in table 1. THP also

produces the smallest volume of sludge cake reducing the significant transport costs

associated with all AD processes.

MAD APD THP

Biogas Yield (m3/TDS) 300 353 450

Additional Energy Input

(MWh/TDS) - electrical

0.14 0.19 0.29

Additional Energy Input

(MW/TDS) - thermal

0.0 0.0 0.30

Efficiency (%) 12% 13% 11% (16%6)

OpEx (£/TDS) 34 12 17

Table 2 - Relative Performance of Advanced AD

However, when the operating expenditure (OpEx) is compared the THP does not have the least

cost operation and in fact the APD process is optimum. This can be explained by analysing the

energy input for each process, THP has a huge parasitic energy requirement which negates the

additional revenue and disposal savings bought about by the process.

THP forms a key part of Thames Water’s 25 year sludge strategy over MAD and APD. However

as the model reveals, the increase in biogas does not necessarily result in an overall net

increase in energy yield; as the process demands an input of high grade heat and additional

electrical energy, when compared with MAD and APD. The high grade heat demand typically

5 5 Based on current UK renewable electricity output of 64TWh and 2020 prediction of 91TWh (DECC-iii 2011) and assuming current output of 0.8TWh from sewage sludge and 3.2TWh (40% efficient) in 2020. 6 Ignoring support fuel requirements



outweighs the heat available from CHP and many of THP plants studied in the UK require a

support fuel to maintain the process (Bowen 2010).

According to the model APD does offer a solution which is more economical to operate on a

typical wastewater treatment site. However, operating experience at full scale has meant that

APD is not the preferred option for Thames Water. This is due to the long retention times and

inherent instability of APD when compared with THP, which means that a shut down on an

APD plant is likely to be measured in weeks instead of hours for a THP plant.

3.2 Optimum THP Configurations

The previous model was developed further to understand heat use within THP in more detail

(see Appendix C). In particular the amount of process steam that can be supplied by the CHP

and the additional quantity that is required from another source. The last study showed that

THP uses additional electrical power over conventional MAD which is due to THP being a more

complicated process with increased sludge handling i.e. more pumps and additional

dewatering. The additional thermal energy required is at least 0.3 MWh/TDS; this is provided

from a support fuel, natural gas in this case, representing 40% of the process steam energy.

Wilson observes a similar support fuel requirement for the Cardiff THP plant, which was

designed for 0.33MWh/TDS or 46% but the operational performance is closer 0.51-

0.53MWh/TDS (Wilson 2011). A Sankey diagram of the 84TDS/day designed process can be

seen in Figure 5 and shows the 28MWh of support fuel required to maintain the process.

Figure 8 - Sankey Diagram from Cardiff THP Site (Wilson 2011)

There are two options currently being used as a support fuel on operational THP plants across

the UK, these are natural gas and biogas diversion, shown as Option A and B respectively in

Figure 6.



Figure 9 - THP with Support Fuel Options

It was found that Option A was most economic when considering OpEx this is because of the

subsidised revenue is maximised by using all of the bio-gas in the CHP. Based on this it is

recommended that natural gas is used as a support fuel instead of bypassing the more

valuable bio-gas away from the CHP.

One of the benefits of this process study within this research project was the identification of

potential anomalies and these were investigated further. In particular analysis was conducted

to understand the best gas engine type. It was found that an engine with a high electrical

efficiency was optimum despite this having a lower high grade heat rejection and requiring

more support fuel. The difference in OpEx between the two engines modelled was >20%

(Mills-ii 2011).

3.3 Preliminary Life Cycle Analysis THP Configurations A literature search had found that previous studies had not explored advances in technology

but instead focused on traditional disposal routes for sewage sludge (Dalemo, Sonesson et al.

1997; Sonesson, Dalemo et al. 1997; Suh and Rousseaux 2002; Lundin, Olofsson et al. 2004).

These typically include land fill, compost, incineration and land application after conventional

AD during a period (1995-1999) when the EU was changing regulations (Houillon and Jolliet

2005), more recent studies used laboratory data to compare full scale processes, the reliability

of this approach is debatable (Carballa, Duran et al. 2011). In all papers reviewed there is

limited detail on plant configuration which has a large impact on energy balance, operational

economics and subsequently the environmental impact (Mills-ii 2011) further justifying the

need for LCA study and a contribution to knowledge potential.

As described above the biogas produced in AD has traditionally been utilised in a spark ignition

gas engine or dual fuel engine and converts 30 - 40% of the energy into renewable electricity.

A proportion of the waste heat from the exhaust gas and the water jacket is recovered for

utilisation by the process forming CHP (Hawkes 2011). In the UK this form of generation is

incentivised under the RO scheme which rewards generators with additional financial revenue.



Figure 10 – Biogas Upgrading with Pressurized Water Scrubbing (Dirkse 2010)

A new technology, Gas to Grid (GtG), cleans up and injects all of the bio-gas produced in AD

into the gas network and is financially supported under the Renewable Heat Incentive (RHI)

(DECC-i 2011). A number of technologies are available, to remove the carbon dioxide and

hydrogen sulphide but water absorption has been deployed in UK to date, the resulting gas has

a methane content of >99%. Once upgraded the bio-gas requires the addition of propane and

odourizer to be compliant with gas quality standards before final compression into the gas

network (Greer 2010; Starr, Gabarrell et al. 2012). A disadvantage of this process is that heat is

no longer supplied from a waste source and has to be supplied by either bypassing some of the

biogas or purchasing supplementary natural gas. The UKs first plant was commissioned at

Didcot STW on 3rd Oct 2010 with a capability of 100m3 of biogas per hour employing a water

scrubbing system (Shah 2010).

A Life Cycle Analysis (LCA) study was conducted that compared the two options (CHP & GtG)

for conventional MAD and THP. Figure 8 shows the system boundaries used within this study,

the full paper can be found in Appendix D.

Figure 11 - System Boundaries

Table 1 shows the main output parameters which affect the LCA model.



It can be observed that THP generates more energy than the MAD options, but requires

additional electricity and natural gas. This is detrimental in the CHP configuration as the net

energy output is less than all of the alternatives. The best configuration from a pure energy

point of view is the THP GtG followed by the traditional MAD CHP scenario.

Inventory Item Units MAD CHP MAD GtG THP CHP THP GtG

Electricity consumption kWh/TDS 144 274 288 489

Electricity generation kWh/TDS 660 n/a 920 n/a

Bio-methane kWh/TDS n/a 1,661 n/a 2,564

Natural gas kWh/TDS 0 600 520 910

Propane kWh/TDS n/a 350 n/a 530

Diesel kWh/TDS 2.4 2.4 1.3 1.3

Net energy7 kWh/TDS 516 437 112 635

Polymer kg/TDS 6.8 6.8 12.9 12.9

Sludge disposal volume m3/TDS 3.5 3.5 1.9 1.9 Table 3 - LCA Inventory

From the system inventory (Table 3) Global Warming Potential (GWP), measured in CO2eq

emissions was calculated, Figure 8 shows the results which have been divided and displayed as

6 discrete areas to aid analysis and focus any follow up work.

The lowest global warming impact can be seen from the MAD CHP configuration. Despite THP

having almost 50% more energy production the benefit of increased displacement is masked

by the increased energy requirements (electricity and heat) of the process.

The process with the highest global warming impact is the THP with GtG configuration and is

also due to the plants high demand for electricity and heat.

Figure 12 - Global Warming Potential

7 Site only, ignores transport and consumables



The whole life cost was calculated, a technique used to inform investment decisions, Figure 13

shows that THP GtG, the least sustainable option, has the best return on investment due to the

incentive scheme.

Figure 13 - NPV over time (years) with full incentives

Figure 14 - NPV over time (years) with no incentives

However, Figure 14 shows that with subsidy (RHI & RO) removed both GtG options are no

longer economically viable and the CHP configurations clearly offer the least risk option.

This analysis should open up opportunities for debate on how the economics and incentives

should be considered in relation to the outcomes they are aiming to achieve. In the short term

THP with CHP appears to be reasonable compromise. This study has clearly demonstrated the

need for further detailed investigation, to enable greater understanding of these relationships,

dependencies and levels of risk (Mills 2012). This is important because an asset owner, like

Thames Water, must understand the risk because these assets are very expensive and involve

very long payback periods. If the subsidy is grossly mismatched with reality then government

departments are likely to change the policy in the future resulting in expensive assets which

are uneconomical to operate. Therefore this research presents an opportunity to further

contribute to knowledge for the benefit of the UK’s renewable energy generation target and

policy and to help inform asset owners of potential risks to their future strategies and

investments.



4 Processes under Development From the previous section it is obvious that existing processes have limitations and are not

optimised for energy recovery, net OpEx or environmental impact. Therefore, the project is

developing several processes through pilot or demonstration trials that aim to change the

situation. AD has the advantage of being a very simple process which produces bio-gas and

digested sludge in which a lot of calorific value remains. It is therefore logical to assume that

extracting a ‘second bite‘ of energy from the digested sludge would be a good development.

AD has the advantage of being able to recover energy from sludge with the water present, but

it will never be able to recover all energy from sludge. So turning sludge into fuel is likely to be

more lucrative as AD will never be able to recover 100% of the energy.

4.1 Sustainable Thermal Drying for Fuel Production To access the remaining energy after AD the sludge needs to be dried to produce a solid fuel

product (Flaga 2005). However, sludge drying in the UK has had a troubled past with several

dust explosions and fires (HSE 2011). These issues are mainly associated with direct drying

equipment particularly the hot air drum dryer type which creates a lot of dust within a rotating

drum at over 400˚C. It was therefore important to demonstrate a different technology could

be operated safely and efficiently. Wessex Water has recently built a new dryer installation

that will produce a product that “…will be disposed of as a fuel to a third party.” (Jones 2008).

This project is different because it is not treating the dried sludge as a waste. Instead the

process creates a granular renewable solid fuel (GRSF) product which has an inherent value as

a fuel. The long term aspiration is to separate the sludge streams and dry SAS only which has

knock on benefits to downstream processes and produces a better fuel product.

The project has seven specific goals, these are:

1. Prove that viable solid fuel can be manufactured from sewage sludge;

2. Prove that viable solid fuel can be used within existing combustion plant for support;

3. Prove that the dryer plant is a practicable solution;

4. Understand the energy balance and operational economics in STW context;

5. Attempt above using Surplus Activated Sludge (SAS) only with a novel dewatering step;

6. Apply for ‘end of waste’ status for fuel;

7. Market product as a fuel.

4.1.1 Background & Justification

The main driver which allowed for the short term business justification for the project is the

desire to improve the operation of the two Sludge Power Generators (SPGs) in East London

which Thames Water own and operate. Beckton and Crossness SPGs process 200TDS/day and

110TDS/day respectively, both consume large quantities of natural gas to maintain combustion

inside 5 fluidised bed incinerators (2 at Crossness and 3 at Beckton). Crossness consumed 1.1M



m3 of natural gas in 2011, which represented 8-12% of the thermal input. Burning natural gas

has several impacts:

1. The SPGs are thermally limited burning natural gas reduces the volume of sludge cake

that can be burnt reducing throughput and forcing sludge to go elsewhere;

2. Natural gas also reduces the oxygen content of the combustion air, further reducing

the sludge throughput;

3. Natural gas is costly (£319k in 2011) and natural gas is not getting cheaper;

4. RO incentive is reduced proportionally to the natural gas consumed and over 10% by

energy the incentive is significantly reduced, this explained below.

Figure 15 - Natural Gas influence on ROC income, left – biomass ROC, right – co-fired ROC

If the natural gas consumption exceeds 10% of the thermal energy input to the SPG (monthly

average), the renewable incentive (ROCs) drop from 1.5ROC/MWh (£66/MWh) to

0.5ROC/MWh (£22/MWh). Factoring in sale of electricity the revenue generated using 100%

sludge (i.e. no natural gas) is £6,348/day (£190k/month). If >10% natural gas was consumed

the revenue was just £3,587/day (£107k/month) a difference of £83k/month or 44%.

1 TDS of dried sludge from Slough has been estimated to displace 250m3 of natural gas, based

upon this the savings are calculated as follows:

Figure 16 - Benefit from Dried Sludge

It is assumed that the SPG is operating with biomass ROCs as co-fired status is not guaranteed

and could not be claimed as part of the business justification. The trial relies on the supply of

free heat at Slough STW where spare biogas produced in AD has historically been flared.



Activity OpEx £k pa

Slough Operation (Labour) 0.8 FTE 27 cost

Slough Operation (Electricity) 60kW @ £65/MWh 35 cost

Slough Operation (Maintenance) 5% of capital 8 cost (n/a for initial

24month defect period)

Slough Cake Land Disposal Savings 21wT/day @ £18/wT 139 saving

Transport of dried sludge to Crossness SPG

£250/trip @ 1/week 13 cost

Crossness Nat Gas Savings 383,250m3 @ 29p/m3

111 saving

Increase in ROC revenue 1,789MWh @ 1.5ROC/MWh

118 revenue

Total 285 saving Table 4 - Economic Business Justification

Simplistically the net savings, shown in Table 3, of £285k equates to a payback period of about

5 years, a more complicated whole life cost model is used within Thames Water which is not

shown here but was used to justify the construction of the demonstration dryer plant at

Slough STW.

4.1.2 Slough Dryer Process

The process consists of five areas:

1. Sludge cake storage and feed system

2. Sludge Dryer

3. Product cooling and handling

4. Off Gas system

5. Thermal oil heating system

A 25m3 sludge cake hopper, with an agitator, provides 24 hours storage, at full throughput, of

sludge cake at 20%DS; the sludge is pumped with a progressive cavity pump to the dryer, the

pump is controlled through a variable speed drive so that the flow rate to the dryer can be

controlled. The dryer consists of two counter rotating heated paddles in a trough; the paddles

agitate the sludge whilst driving off the water. The sludge travels along the dryer by

displacement as water is removed and becomes progressively drier before exiting the trough

via a weir at the far end of the machine. The dried sludge or product is then cooled in a screw

conveyor with a water jacket to below 45degC. Finally the cooled product enters a drag

conveyor which lifts the product and drops it into 1m3 fabric bags, these bags are then ready

for transportation to the Crossness SPG. The plant can produce approx. 4.5TDS/day at full

throughput, the process flow diagram and a picture of the pant can be seen in Figures 17 & 18.



Figure 17 - Process Flow Diagram - Slough Dryer

Figure 18 - Slough Dryer under Construction

The production of odours is a significant problem for all methods of sludge treatment. One of

the main criteria for selection of this particular dryer is that it is totally enclosed. The ‘off-

gases’ that are evaporated from the sludge are minimised, captured and treated.

The off gas from the dryer is drawn by an exhaust fan; the off gas consists of a small amount of

solids, water vapour and leakage air. The gas first enters a condensing spray tower which cools

the gases and condenses the water vapour which is then drained away; this is followed by a

venturi separator which is designed to remove any remaining particles, these are washed to a

drain. Down steam of the exhaust fan is a carbon filter designed to remove odour before the

air is spent to the local atmosphere.



All of the heat required for drying will be sourced from a thermal oil heater that will consume

spare biogas currently being flared on site. The thermal oil will supplied to the dryer at around

190degC at full throughput.

4.1.3 Slough Dryer Performance

The installation of the dryer was completed in September 2011 and once commissioning was

complete, an acceptance test was conducted the results of which are shown in Table 5.

Actual value

Pass criteria

Comments

Throughput (TDS/day)

2.8 ≥2.6

Power consumption (kW)

48.2 (at ~57% feed rate)

50 Based on limited increase in power consumption at 75% throughput: ~50 kW, i.e. just 2 kW more than at 57%

Product Dry Solids (%)

97.6 >90

Dryer efficiency (tons of H2O/MWh of biogas input)

1.17 ≥1.10 Based on 63% methane in biogas

Shutdowns (#) 0 <3

Gas consumption (Nm3/h)

62.6 ≤69.8 Estimation of reference value based on design biogas consumption scaled down to 57% throughput

Dryer efficiency (MJ/kg H2O removed)

3.1 N/A

Table 5 - Results against the scale down acceptance test criteria (based on 57% throughput over two days)

The paddle dryer passed the acceptance criteria producing dried sludge product of 97%DS. The

dryer efficiency was 3.1 MJ/kg H2O removed this is among the best values found in the

literature for other type of dryers used for industrial applications (Error! Reference source not

found.6), which has confirms and supports the reasoning for selecting this particular type of

dryer.

Dryer Type Typical Heat loss sources

Typical Specific

Energy Consumption (MJ/kg of H2O evaporated)

Indirect Rotary Surface 3.0 to 8.0

Cascading Rotary Exhausts, leaks 3.5 to 12.0

Cross circulated tray / oven / band Exhaust, surface 8.0 to 16.0

Cross circulated shelf / tunnel Exhaust, surface 6.0 to 16.0

Through circulated tray / band Exhaust 5.0 to 12.0

Vacuum tray / band / plate Surface 3.5 to 8.0

Drum Surface 3.0 to 12.0

Fluidised / Sprouted bed Exhaust 3.5 to 8.5

Pneumatic conveying / Spray Exhaust 3.5 to 8.0

Two stage Exhaust, surface 3.3 to 6.0

Cylinder Surface 3.5 to 10.0

Stenter Exhaust 5.0 to 12.0 Table 6: Efficiencies of other type of dryer for industrial applications (Devki 2006)



4.1.4 Experience

The dryer has proven to be very reliable, robust and effective, although a few teething

problems have had to be overcome.

Odour is a problem that characterises all drying technologies. The off-gas from full-scale units

located on STWs is generally treated by diffusing it in the aeration lanes, which act as biological

scrubbers. This solution was not implemented at Slough STW because it was not considered

cost-effective for a pilot plant. Unfortunately the Granular Activated Carbon (GAC) utilised for

odour control has not performed. However, providing the condenser spray tower was

maintained the odour reduction from absorption is desirable.

The automated control uses a Proportional-Integral-Derivative (PID) control loop to

maintaining temperatures throughout the dryer, which has proven to be unstable. Figure 199

highlights the diverging trends in the sludge temperatures along the dryer. This form of

instability generally leads the dryer into a controlled shutdown and due to the long duration (9

hours) of these cycles it has proven to be difficult to tune the PID control. For the short term

the loop has been switched off and manual control initiated.

Figure 19: Example of diverging trends in the temperatures along the dryer and in the temperature of the thermal

oil supply (i.e. the control input)

4.1.5 Site Integration

A key point to this drying project being different to others is that it is truly sustainable. So an

assessment of the entire site and resulting mass and energy balance will be assessed

throughout the trial period. A model has been produced to assess the integration of the dryer

on site. This soon identified a potential pinch point on site, relating to the existing operational

plant. The primary sludge thickening is not performing and as a result is affecting the biogas

available to the sludge dryer. The model quantified the effect and showed that refurbishment

of the thickeners is necessary. Figure 20 shows the effect that improving the performance and

0

20

40

60

80

100

120

140

160

180

200

07/12/2011

13:12

07/12/2011

15:36

07/12/2011

18:00

07/12/2011

20:24

07/12/2011

22:48

08/12/2011

01:12

08/12/2011

03:36

08/12/2011

06:00

08/12/2011

08:24

08/12/2011

10:48

Date (dd/mm/yyyy hh:mm)

Te

mp

era

ture

s (

oC

)

Sludge at the dryer's inlet Sludge in the intermediate zone

Sludge at the discharge end Thermal oil supply



therefore the primary dry solids from 2% to 6%DS has on AD hydraulic retention time (HRT),

volatile solids destruction (VSD), spare biogas and the resulting drier throughput.

If the thickeners do not function properly, the sludge feed to the digesters contains too much

water and not enough biomass, which affects the process in two ways. The additional water

results in consumption of additional biogas for heating, while the reduction in biomass results

in a shorter retention time resulting in a lower destruction of organics and therefore a reduced

biogas production.

Figure 20 – The Modelled Impact of Primary Sludge Thickener Performance

The model has shown that biogas for the dryer is not available until a DS of 3% or more is

produced in the thickeners. At 5%dS the dryer will be able to function at 80% of the designed

throughput. Improvements on site are underway to achieve consistent dry solids feed to the

digesters.

The drying of digested sludge is not considered to be the optimum configuration. SAS is very

problematic for AD as it does not digest or dewater readily when compared with primary

sludge. It requires more than 4 times the digester volume per unit of mass and there is also

evidence to suggest that SAS limits performance of primary sludge in AD (Winter 2010).

Therefore, if SAS or a proportion was separated pre AD and dried this would benefit the AD

process by improving HRT, VSD and freeing digester throughput capacity for increased imports

from satellite sites. Dried SAS would have a higher CV 18MJ/kg as instead of 14MJ/kg for

digested sludge. The modelling of this configuration will be explored and trialled if financial

support can be secured. It is important for this demonstration trial to prove the viability of

being able to consistently produce a fuel (GRSF) and that the dryer is reliable and effective.

Future work will include the investigation of other types of dryer, which can operate at lower

temperatures and utilise more abundant sources of waste heat.

4.2 Alternative Fuel Trial

The 5 fluidised bed combustors at Beckton and Crossness (Figure 21 - left) are of the same

design only differing in throughput. The Crossness process consists of the following steps:



1. Dewatering: plate presses take undigested sludge from 3%DS to 30%DS;

2. Fuel conveyors: several sets of drag and screw conveyors transfer the sludge cake to a

storage silo and then on to a single feed point on the combustors;

3. Combustors (Figure 21 – right): a 4.4m diameter bubbling fluidised bed with an air

flow of about 20,000 m3/hr, combust approximately 2 TDS/hr, natural gas support fuel

is used to maintain the bed temperature between 750-850˚C;

4. Heat Recovery: the exhaust gases, at about 900˚C, then pass through a waste heat

recovery boiler to raise steam (40Barg) which is used to pre-heat combustion air to

maintain bed temperatures, excess steam is used to drive a (5MWe) steam turbine;

5. 1st stage Gas Cleaning: the fly ash and mercury is removed in a cyclone with activated

lignite coke dosing;

6. 2nd stage Gas Cleaning: bag filters remove the final residue followed by a NaOH

scrubber to remove SO2.

Figure 21 - left - Crossness SPG, right - 1 of 2 Fluidised Bed Incinerators

Goal 2 of the project aims to prove that the dried sludge fuel (GRSF) can be used within SPG’s

for support. This is very important as it is one of the main business justifications for the

demonstration project. Due to the planning constraints on the Crossness SPG only three short

trial windows were allowed.

Phase-1 of the trial involved manually feeding dried product into the wet sludge cake at

Crossness pre combustion. Initially low feed rates were applied 1.1% by dry mass (0.3% by

volume, 0.8% by energy content) slowly building up to 8.7% by dry mass (2.6% by volume,

6.6% by energy content). This manual trial determined that the feed rate envelope for the

automatic feed equipment which was needed for phase-2 of the trial.

The GRSF was fed into a modified screw conveyor which normally transports indigenous sludge

cake from a silo and drops it on to a drag conveyor. The drag conveyor feeds a paddle feeder

which throws the cake into the combustion chamber and onto the fluidised bed, Figure 22 is a

representation of the arrangement.



Figure 22 – left – Auto Feed Equipment, right - Simplified PFD showing dried sludge feeding point

The screw conveyor was selected as it would mix the dried sludge granules with the wet cake

and minimise any issues arising from dust cloud formation.

Before and during the trial key parameters within the SPG were being logged at a high

frequency. These included combustion temperatures such as bed, wind box, pre-heat, and

freeboard, steam production, gas consumption, emissions, feed rates and turbine output.

4.2.1 Initial results

During phase-1 of the gas was consumption reduced in stream 2 (the trial stream) whilst in

comparison stream 1 did not change and required larger volumes of gas to maintain

combustion temperatures by pre-heating the combustion air with a gas burner.

The GRSF clearly had an impact on the combustion temperatures. Figure 23 shows data

collected from day 2 of phase-1, during the 150kgDS/hr & 200kgDS/hr feed the average bed

temperature increased by 30degC at the same time the freeboard temperature rose by

40degC. The wind box temperature was able to be dropped from 300degC to 230degC during

the feeding, resulting in significant benefits to the process.

Figure 23 - Combustion Temperatures



This result is most significant as it was not predicted the dried sludge would have quite this

effect on combustion. The dried sludge looks to be improving the combustion within the bed

and as a result it appears that more heat is being released into the bed. During the trials

operators can reduce the combustion air pre-heat in response to the higher bed temperatures.

This allows more steam to be fed to the steam turbine for electricity generation as less is being

used for combustion air heating. It has also allowed more sludge cake to be fed into the bed as

thermal capacity has been increased; Figure 24 aims to demonstrate this theory.

Figure 24 - Conceptual Thermal Energy Balance of Crossness, left - without GRSF, right - with GRSF

4.2.2 Quantification

The two SPGs have historically not performed as designed; unfortunately it has been difficult

to maintain optimum conditions within the combustors. The difference between bed

temperature and freeboard temperature is often large (>200˚C) this requires excessive support

fuel, reduction in sludge feed and water injection into the freeboard to prevent damage to

equipment from excessive temperatures. This is a common observation in sewage sludge

combustion and can be contributed to incomplete combustion of larger particles in the bed

which are combusting in the freeboard (Anthony 1995). Werther and Ogada go on to claim this

is an advantage of Fluidised Bed Combustion (FBC) technology referring to the freeboard as a

“post-combustion chamber” (Werther and Ogada 1999). Mathematical modelling of sewage

sludge, a complex fuel, also confirms that “released volatile matter burns in the freeboard in a

flameless fashion” (Urciuolo, Solimene et al. 2012). However the reality of operating a FBC in

this manner restricts the throughput and affects the efficiency of the plant.

From the data captured in phase-1 it was suggested/theorised that the addition of the dried

sludge is altering the combustion characteristics within the FBC; increasing the heat released in

the bed and reducing freeboard combustion, this in turn has resulted in the reductions in

natural gas support fuel used and air pre-heat required. An in-depth model of FBC combustion

concluded that “a range of 60/40 to 90/10 [bed/freeboard] depending on the fuel particle

distribution” can be expected (Yang, Sharifi et al. 2008). It was therefore logical to attempt to

prove/quantify whether the dried sludge had shifted the bed/freeboard combustion ratio. It is

however, much more difficult to do this with an operational asset due to the number of

unknowns. So a simplified heat balance was conducted for the incinerator bed and freeboard

the two equations are shown below. The following assumptions have been made as advised by

Professor Rex Thorpe (University of Surrey):

1. The system is in steady state;



2. The fluidised bed is well mixed;

3. There is enough oxygen;

4. Cp is roughly independent of combustion, temperature and phase

5. A fraction of the heat from sludge cake, Øc, is released within the bed

6. A fraction of the heat from gas, Øg, is released within the bed

Figure 25 – Heat Balance Equations for Fluidised Bed and Freeboard

A heat balance was performed using the two conditions seen in Figure 19, no GRSF feed @

08:17:28 and 200kg/hr of GRSF feed @ 11:59:14. Unfortunately missing data from the day

made it quite difficult to accurately conclude the heat balance and draw any conclusions to

whether the product has reduced the freeboard combustion. As a result the data collection for

the subsequent trials has been changed and this analysis will be repeated with a larger more

complete dataset.

However, the resultant energy inputs calculated during the energy balance, for the two

conditions in Figure 23, indicate that the GRSF has had a beneficial impact expected and

conceptualised earlier in the report. Showing a clear reduction in natural gas consumption,

increased sludge feed and reduced pre-heat needed for the combustion air, Figure 26 shows

the energy inputs to the system.



Figure 26 - Energy Inputs into the Incinerator during the Trial

4.3 Intermediate-Thermal Hydrolysis Process Thermal Hydrolysis is a well proven technology and its application is increasing across the

World and within Thames Water. However, it does have drawbacks in particular as described

above the support fuel that is required to sustain the process is significant.. Despite this the

thermal hydrolysis is very effective at improving AD and it is for this reason work is underway

on optimising the use of this process.

The Intermediate Thermal Hydrolysis Process (I-THP) involves a two stage AD process with THP

between two phases of digestion. The first stage of the process is similar to conventional MAD

with thickening followed by AD. After this first AD stage the digested sludge is dewatered to

15% DS or greater and fed into the THP process. Post THP the hydrolysed sludge is cooled and

diluted before a second AD stage which is a high rate digester. Digested sludge is dewatered

conventionally and spread on agricultural land.

Figure 27 - Intermediate Thermal Hydrolysis Process Flow Diagram

The main advantages of this process over conventional THP is that throughput through the

THP plant is reduced, due to the first AD stage which will reduce the mass and therefore

volume to be hydrolysed. This will reduce the energy requirements of the process



substantially. The process also achieves a higher VSD than conventional THP and subsequent

gas yield, which means that the revenue from the process is increased.

4.3.1 Laboratory Trials

The process has been trialled and proven at laboratory scale by a scientist working towards a

PhD in collaboration with Thames water and the University of Surrey. The trial has involved

using 10ltr digester vessels and a pilot scale THP rig. The I-THP process has been replicated and

repeated at a practical scale with a number of controls for comparison. Four processes were

trialled:

1. Conventional THP (THP) - Blue

2. Intermediate THP (I-THP) - Green

3. Double MAD – Identical to I-THP but without the heat, the sludge is dewatered and

water added to simulate the steam dilution affect - Black

4. Conventional MAD - Red

Figure 28 shows the VSD (referred to as Volatile Matter/Solids Reduction in the Figure) of the 4

processes over a 5 month period. It clearly shows that the I-THP process is achieving the

largest VSD. As expected the THP process performance is better than the conventional MAD.

However, what is unexpected is that the double MAD, the I-THP control, performs better than

the THP without a hydrolysis step. However, when examining the gas yield (Figure 29) the

double MAD does not appear to be converting the VS into gas and is therefore not a viable

solution.

Figure 28 - Volatile Solids Destruction across for the 4 Processes (Shana 2011)



Figure 29 - Specific Gas Production across the 4 processes (Shana 2011)

The gas yield shown in Figure 29 confirms that the VSD seen in the I-THP process is being

converted into Bio-gas. The gas yield for conventional MAD and THP are typical and confirm

that the I-THP process is a step change in a performance and is of a significant importance.

4.3.2 I-THP Process Modelling

I-THP clearly has huge potential, but this potential must be quantified before the process can

be applied at full scale and influence strategy. A process model was constructed that simulated

the process seen in Figure 27. The results show that energy balance is much improved for the

I-THP compared with THP, this can be seen in Figure 30. The consumption of natural gas as

support fuel is eliminated with this process. However, the electrical consumption has

increased due to the additional thickening, mixing and pumping required; both processes

require more electrical power when compared with MAD.

Figure 30 - Energy Input to the Process

The reduction in support fuel to the process is because the throughput through the THP has

reduced by 37% and the steam requirement is proportional to the throughput. There are also

additional benefits from increased gas production and subsequent electrical generation. In

addition the final cake volumes are reduced because the VSD is high; the dewaterability



performance is assumed to be equal to THP, but it is highly likely to improve. The new OpEx is

therefore greatly improved over conventional THP this can be seen Figure 31.

Figure 31 - Net OpEx

The net difference between THP and I-THP is a saving of £16.8/TDS, these savings when

extrapolated across the Thames Water future THP portfolio are significant. A business case

was formulated with the data above to justify the construction of a pilot plant to prove the I-

THP process at a realistic scale. The author has project managed the pilot plant project which

shall be in operation by December 2012, an additional Engineering Doctorate project has been

initiated that will focus on the development of the I-THP. It is the intention of the author to

conduct high level analysis including LCA of the I-THP process, whilst supervising the other

project.

Figure 32 - P&ID of the I-THP Pilot Plant Process



5 Conclusion The project is at the half-way point and from the research to date it can be concluded that the

industry is not set up to extract energy from sludge in an efficient way. The deployment of

advanced AD processes has not necessarily improved the situation and the research project

has identified gaps in knowledge which triggered investigative modelling. The initial modelling

has made a number of recommendations to optimise the configuration of THP; in summary

Gas to Grid technology is not as sustainable as CHP but the CHP needs to be selected carefully

to reduce high grade heat demand.

The intermediate THP process which has been proven in the laboratory is a very promising

alternative to the conventional THP and looks to be able to double the energy recovery. The

pilot plant will be critical in developing this technology further to enable it to be implemented

at full scale.

An optimised Advanced AD process will never be able to achieve full energy recovery at best it

may achieve 20%. To achieve higher conversion rates it is necessary to remove the water to

produce a granular renewable solid fuel which can be converted through combustion. The

demonstration trial has proven that drying technology can be safe, efficient and sustainable.

Drying of undigested SAS has a number of advantages and is to be explored during the second

half of the project.

The dried product produced has successfully been used within the Crossness Fluidised Bed

Combustor and has shown that combustion conditions and system efficiency can be improved

significantly. The data collection for later stages of the trial has been improved so that the

beneficial effect can be quantified and inform future investment decisions.

To date it can be claimed that genuine contribution to knowledge has already been made in

two areas:

1. THP process configuration analysis (including LCA)

2. Fluidised Bed Combustion - alternative fuel trial

The work in these two areas needs some further analysis and refinement before journal papers

are submitted in 2013. The project has a number of milestones remaining as described in this

report which will make a further contribution to knowledge specifically in the following areas:

1. SAS only drying configuration trial and analysis

2. I-THP process development

3. Policy barriers to unlocking sewage sludge renewable energy



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Documents

Appendix C Second Year EngD Dissertationepubs.surrey.ac.uk/809984/43/Mills - Appendix C.pdf · 2nd Year EngD Dissertation Nick Mills October 2012 University of Surrey 247 Thames Water