Supervisor at Vattenfall: Jinying Yan.
In this work the recycled wood chips are presented as a biomass fuel for combined heat and power (CHP) production with focus on the major emissions that come from their combustion. To investigate the emission problems that arise, the recycled wood chips, being a contaminated biomass fuel, are compared with the common wood fuels. The results illustrate that, due to their contaminated nature, they contain more acidic components than the common ones; therefore retention processes in the furnace and the flue gas must be considered. These acidic components in the fuels, especially fuel-S, fuel-Cl and fuel-N, produce major pollutants during combustion.
In order to prevent the problems caused by the acidic components, several processes have been used such as primary and secondary (SNCR) de-NOx, chemical dosing for protection from high temperature corrosion and alkaline dosing to neutralize the main acidic gases (e.g. SOx and HCl) during flue gas cleaning. It is important to understand the conversion of the acidic components to different chemical forms during combustion and how they interact with the other fuel components (e.g. ashes) and chemicals used.
In this study, the impacts of the wood fuel contamination on the emissions are evaluated for combustion that takes place in a typical bubbling fluidized bed (BFB) boiler. Furthermore, a model is developed, with the use of Microsoft Excel, to simulate the biomass combustion, including the conversion and retention of the major acidic components that happen in the boiler and during the flue gas cleaning. The model is calibrated by using boiler operation data for key retention processes and is validated by the monitoring data (i.e. flue gas compositions and main emissions) obtained from the boiler operation. Moreover, several cases are defined so as to simulate the changes in the fuel compositions (fuel-S, fuel-Cl and fuel-N) and chemical dosing (mainly hydrated lime dosing) for the flue gas cleaning. Additionally, impact analysis is performed by the simulations of the defined cases. The results show that:
- Fuel quality, especially fuel-Cl, plays an important role in the concentration of the acidic components in the flue gas.
- The different variations of the fuel define the demands of chemical dosing.
- The ash that is produced can result partially in the retention of the acidic components.
- Efficient primary de-NOx process should be implemented for the fuels with high fuel-N content.