Optimisation of costs and
performance of a sludge destruction plant
M D Everest 1 MSc CEng MInstE. Meniscus
Systems Ltd. Tel: 01480 433714. Fax: 01480 433715.
E-mail: sales@meniscus.co.uk
S Harris 2 . Severn Trent Water Ltd. Tel: 01162 340340. Fax: 01162 671887.
E-mail: Sam.harris@severntrent.co.uk
First published in CIWEM 2001
1. Abstract
To help drive down operating costs at the Coleshill Sludge Destruction Plant in
Birmingham, Severn Trent Water Ltd use existing operational data to understand
process interactions within the plant and to obtain a "helicopter" view of site
performance.
A data management service is used that collects the routine operational data
and routine analysis undertaken by the operators. This data is used to
automatically derive cost and performance information for each separate process
unit. The system also calculates a heat and mass balance throughout the plant
to help measure and monitor key performance parameters. Using these techniques
it is possible, for example, to determine increased operating costs caused by
non availability of plant. Data mining techniques are used to identify "hidden"
relationships in the data and in conjunction with the operators, used to derive
a set of operating rules for the plant. These rules are input into an expert
system that automatically applies them to the incoming data on a day to day
basis.
2. Keywords
Sludge destruction, incineration, optimisation, key performance parameters,
data mining, expert system, data collection, best operating practice.
3. Introduction
The sludge destruction plant (SDP) operated by Severn Trent Water Ltd at
Coleshill in Birmingham, processes over 28,000 tonnes of digested sludge a year
from Minworth STW and the Black Country Trunk Sewer - having a combined
population equivalent for sludge treatment of 2.5 million.
The plant comprises 5 centrifuges producing thickened sludge with target dry
solids content of 20%. Thickened sludge passes through rotary driers to
increase the dry solids to a target of 30% before passing to two fluidised bed
incinerators, rated at 2.5 tonnes of dry solids per hour each. The incinerators
are supplemented with natural gas to maintain a temperature of 870 deg C in the
incinerator head space. An economiser provides hot water used in the driers and
an air pre-heater raises combustion air from ambient temperature to around 550
deg C. Comprehensive gas cleaning ensures that the flue gases are compliant
with the operating conditions as set out by the Environment Agency.
Ash and any thickened sludge not burnt in the incinerator are sent to landfill.
The overall plant, and in particular the sludge thickening centrifuges,
incinerator and flue gas handling systems are fully automated, monitored and
controlled with a sophisticated Yokagawa control system. However, in order to
continually drive down operating costs at the site, Severn Trent Water Ltd are
investigating how day to day changes in operating conditions affect the overall
cost and performance of the plant. The monitoring programme to evaluate these
opportunities is two thirds complete. This paper highlights some of the interim
findings of this work.
1 Meniscus Systems Ltd. Unit 3 Blotts Barn,
Brooks Road, Raunds Northants. NN9 6NS. http://www.meniscus.co.uk
2 Severn Trent Services Ltd. Senior Technician, Service Delivery East, Wanlip
STW, Leicester.
4. Macro vs micro view of data
The Yokagawa control system on the plant provides comprehensive alarm
management for the plant and undertakes monitoring and control of the
incinerator operating conditions on a real time basis.
This form of "micro" control of the plant is absolutely necessary for the
control of complicated process plant. However, for process monitoring and to
obtain overall information on costs and performance criteria a more "macro" or
"helicopter" view of the data is helpful. This presents data in a more concise
format and allows calculations that use data not easily collected by standard
monitoring instrumentation, e.g. dry solids readings. Presentation of data in
this way also facilitates the use of more complex data analysis techniques such
as data mining. Traditionally, process or plant managers have undertaken such
analysis using spreadsheets or simple databases. By adopting this "macro" view
of cost and performance, the plant operator has a much better understanding of
the individual costs of each process within the SDP and of the total overall
operating cost of the site. This in turn helps to justify modifications and
improvements to the plant so as to reduce operating costs.
5. Data collection and monitoring
A total of 19 parameters are collected from site on a daily basis with a
further 23 on a weekly basis. Daily readings comprise routine operational data
including sludge, polymer, water and centrate flow readings as well as inlet
and outlet dry solids readings for the centrifuge and the dry solids of sludge
leaving the driers.
Weekly readings comprise key hours run data and consumption figures for gas,
electricity, water and caustic.
Data is entered into a Psion Workabout handheld computer by the night shift
operations team and the data is downloaded to a central computer during the
night. The central computer derives a broad range of process and cost based
information and uploads a new data file and changes to the reading forms on the
Psion automatically.
Key parameters are assigned high/low limits which generate daily exception
reports for the operators. These can be uploaded to the Psion if required.
All the data sent to the Psion and all the derived parameters can be viewed
over the internet and customised reports summarise key performance information
and the daily operating costs for the plant.
The monitoring regime entailed no additional monitoring or data collection for
the operations team since all the data was collected prior to this monitoring
exercise and recorded on traditional paper log sheets.
6. Heat and mass balance
A mass balance derived around the whole of the plant, enables the calculation
of key performance information, such as mass of dry solids burnt in the
incinerator. Such information is not available under the standard monitoring
systems since it would require measurement of sludge flows from the centrifuge
and driers at high dry solids conditions. Such flow measurement is difficult to
achieve in a reliable and consistent way.
The mass balance uses inlet sludge flow and dry solids readings to derive the
mass of water and solids processed at each process in the plant. Readings
derived in this way are accurate to an estimated +/- 15%. The accuracy of the
mass balance is dependent upon the accuracy of the centrate flow meter (only
one such meter) and on the amount of thickened sludge "stock" held on site
ready for disposal at the landfill. Monthly results are summarised in a one
page report.
The heat balance is based around the incinerator and uses solids and water
loading data derived from the mass balance as inputs to calculate heat losses
in the flue gases and in the associated water vapour. These heat losses are
then compared to the heat inputs into the incinerator in the way of natural gas
(gross calorific value used), sludge solids load and pre-heated inlet air. The
difference in the heat balance between heat into and out of the incinerator is
5%.
For a sludge with a calorific value (gross) of 16,050 kJ/kg, the incinerators
require 4.7MJ of energy to evaporate each kg of water. 63% of the energy input
into the incinerator is required to evaporate the water with the balance
comprising heat losses in flue gases 35% and 1% each for heat losses in ash and
for general heat losses (estimated) around the plant.
Stoichiometric air requirements for the sludge (analysis per 100kg of dry
solids: C 34%, H 6.6%, O 13.8%, N 3.8%, S 1.3% and Cl 0.3%) are 5.7 kg of air
per kg of dry solids burnt. For the combined gas and sludge figures the
stoichiometric air requirements are 6.3 kg of air per kg of dry solids burnt.
7. Energy losses due to high excess air
values
Control of the combustion air required in the incinerator is managed by the
central Yokagawa control system on the plant. Control is based upon a key
assumption that dry solids into the centrifuge is 31% at which point the sludge
is assumed to be autothermic - this is confirmed with the heat balance. In
practice, the solids content of the sludge into the incinerators is always less
than this due to operational problems with the rotary driers. The plant
presently generates sludge with a dry solids content of 22-26% against a target
of 30%.
As a consequence, the control system continually overstates the combustion air
requirement. Based on a carbon balance which assumes that no residual carbon is
left in the ash, the total mass of air used is 12.7 kg per kg dry solids burnt
which equates to an excess air figure in the incinerator of over 100%,
considerably more than is required for optimal performance of the incinerator.
This high excess air figure is confirmed by the high oxygen readings in the
flue gases of between 12 and 15%.
Too much excess air wastes energy in two ways.
-
The combustion air blowers operate for longer than required
-
The energy required to heat the excess air to the operating temperature of the
head space is wasted. Note that the economiser and the air pre-heater
downstream of the incinerator will extract heat from the flue gases and so
recover a proportion of this heat. However, if the demands of these two units
are met with a reduced excess air requirement then any surplus excess air can
be considered as wasted.
For every 10% increase in the amount of
excess air used the annual savings on blower power are £7,200. If the target
excess air requirement in the incinerator is 75% (8 to 10%) of oxygen in the
flue gases) then the additional blower power costs are around £18,750 per
annum.
The additional energy required to heat all the excess combustion air from 550
to 870 deg C is 8% of the total energy input. Based on the target excess air
requirement of 75% then the wasted energy represents 21% of the natural gas
burnt in the incinerator or around £25,000 per annum.
8. Cost of non operation of the sludge
driers
As a consequence of operational problems associated with the sludge conveyors
to the driers, the reliability of the driers is poorer than required, resulting
in sludge with too low a dry solids content being fed to the incinerator.
By increasing the dry solids of sludge to the incinerator, the site reduces the
volume of sludge sent to landfill. With the use of the mass balance it is
calculated that a 1% increase in dry solids from the drier reduces the sludge
disposal to landfill by 63 tonnes per month.
In addition, by increasing the dry solids content of the sludge the process
moves towards an autothermic condition, whereby the sludge will burn without
the need for supplemental fuel, the gas use per kg of dry solids required to
maintain the head space temperature will reduce. At present it is difficult to
quantify this saving. There is a relationship between gas use per kg of dry
solids and the dry solids content of the sludge to the driers but its impact is
masked by a more dominant relationship between the gas use and the blower
hours.
It is feasible that cost savings identified by use of the heat and mass
balances may be sufficient to justify additional capital expenditure to improve
the reliability of the driers.
9. Polymer make up water
Polymer make up water contributes 11% of the total volume of sludge processed
by the centrifuges. The annual additional power cost to centrifuge this volume
of water increases the centrifuge power by some £10,000 per annum.
Whilst this volume is accepted as normal by experts in the industry, it
effectively means that a sludge with a 3% dry solids content will be reduced
down to 2.7% prior to thickening in the centrifuges. If additional polymer is
used to thicken digested sludge to 3% prior to processing then this represents
an additional cost
10. Future work
Future work involves incorporating operating knowledge gained from analysis of
the data and from the experience of the local operators and managers into a
site specific expert system. Such a system has two objectives
-
To apply the operating rules to the incoming data on a day by day basis so as to help identify potential operating conditions on a proactive basis. In this way the system can help establish and maintain best operating practice for the plant.
-
To develop a cost model for the entire plant that takes into consideration the
impact that one process has on the subsequent process rather than viewing each
process as an individual stage.
11. Conclusion
The monitoring programme has demonstrated that daily operational data,
collected as part of routine operational duties, can provide a valuable insight
into the overall cost and performance of a plant.
With minimal costs of acquisition of the data, this approach maximises the use
of existing data and helps to capture operating knowledge about the process.
The ability to derive operating information about individual processes
facilitates inter plant cost and performance benchmarking, helps to establish
and maintain best operating practice and provides managers and operators with
readily accessible cost and performance information.
12. References
Wastewater Engineering 3ed. Metcalf & Eddy
Kemps Engineers Yearbook. Edited by J P Quayle
Heat Transfer from Flames. W A Gray, I K Kilham and R Muller
Sewage Sludge II. Conditioning, Dewatering and Thermal Drying. Institute of
Water Pollution Control
Process Plant Design. J R Backhurst, J H Harker
Thermodynamic and Transport Properties of Fluids. G F C Rogers, Y R Mayhew
13. Acknowledgements
The authors would like to thank the operators and management at Severn Trent
Water, in particular those at the Coleshill Sludge Destruction Plant, for their
assistance and support throughout this project
Views presented in this paper are those of
the authors and do not necessarily represent views help by either Meniscus
Systems Ltd or Severn Trent Water Ltd.
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