Options for re-burning bottom ASH at "Lakvijaya" coal power station

Abstract

This thesis discusses about the options for re-burning of bottom ash which is currently discarded as a waste from Lakvijaya power station that has got the largest pulverized coal fired boilers in Sri Lanka. This study has been conducted to evaluate the contents of the bottom ash and the possibilities of re-burning the bottom ash to recover the energy of the bottom ash before sending to the dumping yard. Thereafter the advantages in terms of economy are evaluated. The results illustrate that re-burning of the bottom ash will recover the loss of chemical energy due to the unburned carbon content of the bottom ash, loss of ash enthalpy due to the sensible heat of the bottom ash and loss of radiation through boiler throat in spite of the opening area of the boiler throat to the submerged scrapper conveyer, recovering of these energy sectors will increase the boiler efficiency while other operational costs such as unscheduled outage of the boiler, ash disposal costs, chemical costs, maintenance costs including spare parts are also discussed. To achieve the above process of re-burning the bottom ash will require some process modification to the existing system which is also proposed. Finally the investigation is extended to evaluate and implementing of such a project to find out the project IRR. The results shows that the potential recoverable energy of the bottom ash is expected to be 1,369kW which will lead to 0.18% increase of efficiency in the boiler. This will result in 1,699 MT of annual total coal saving which has a monetary value of 29.6 million rupees. The introduction of this re-burning process will make an effect on the fly ash output which has a remarkable monetary value due to the demand of cement and roofing manufacturers in Sri Lanka. The increase of the fly ash production will generate further 2.22 million rupees per year. Anyway this revenue is expected to be enhanced after the introduction of new price strategies for the bidders by competitive basis. The introduction of new process has some challenges to overcome specially the incombustible contents in the bottom ash specially the SiO2. The percentage increase of the SiO2 in the fuel is discussed and this will result in decrease of boiler efficiency in the degree of 0.01% which is negligible according to the Harbin boiler manufacturer. Finally a supplier was found who manufactures such systems which have a compatibility with vast range of boiler manufacturers’ including Harbin and feasibility study has been done accordingly which gives a result of 20.6% IRR for 240.1 million rupees of investment during a 30 year period. Key words: Submerged scrapper conveyer, Recovery, Boiler efficiency, Bottom ash, Fly ash, Coal, Re-burning