Mills, N.1, Perrault, A.1, Pearce, P.1, Thorpe, R.2, 1Thames Water Innovation, 2University of Surrey(free)
Thames Water owns and operates two sludge incinerators, Crossness and Beckton which have respective throughputs of 90 and 190TDS/day. The units employ a total of five fluidised bed combustors with steam recovery and electrical power generation (50GWh pa). Since construction these units have required the use of supplementary support fuel (natural gas) to maintain combustion. This is mainly due to the poor performance of sludge/fuel preparation systems, compromising throughput, efficiency and revenue generation.
A highly efficient thermal sludge drier was constructed on a satellite site which used waste energy to dry digested sewage sludge to 95%DS and produce a granular renewable solid fuel (GRSF). The GRSF was used at the Crossness site and mixed with indigenous sludge cake (26-32% DS). The mixed sludge feed was then combusted in the normal method. Initial trials have indicated that a small feed (<10%) of GRSF improves the combustion characteristics. During the trials natural gas consumption reduced, bed and freeboard temperatures improved, combustion air preheating could be reduced and cake feed increased.
The trial has proven the value that sludge has as a solid fuel and that small changes in the fuel preparation can make large differences to the efficiency of a fluidised bed incinerator. The system model, including the off-site drying, has proven to be economic and sustainable.
Keywords: Fluidised Bed Incineration, Sludge Drying, Combustion Optimisation
This paper presents some preliminary results from a four year collaborative research project between Thames Water and the Centre for Environmental Strategy at the University of Surrey; employing an engineering doctorate student, the primary author of this paper.
Introduction – Project 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.
This Engineering Doctorate project will research, analyse, design and implement methods of increasing the sustainable energy production from wastewater. Specifically anaerobic digestion processes are being analysed and optimum configurations developed. Sustainable thermal sludge drying, the subject of this paper, is also being trailed to demonstrate that a granular renewable solid fuel can be produced and used beneficially within existing/future combustion plant. The project aims to challenge common understanding and attitudes; and explore barriers to change that exist within policy.
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 target1 ; whilst making significant economic and environmental savings to UK water companies such as Thames Water, its customers and ‘UK PLC’.