Optimum secondary combustion chamber through advanced CFD modelling

Waste-to-Energy (WtE) is a contributor to renewable energy production all over Western Europe with about 400 plants. In Norway, WtE stands for almost half of the district heating produced, with 2.3 TWh/year (2013). The Norwegian company Energos AS has developed a unique WtE technology, and the company is a successful spin-off founded after tight cooperation with the R&D sector. Energos' WtE technology can simultaneously achieve low NOx and CO emission levels without secondary gas cleaning measures. This is achieved through a two-stage combustion process, with gasification of the waste in the first stage, producing syngas, followed by combustion of the syngas in a separate secondary combustion chamber. Energos is a leading supplier of small-to-medium scale (15 - 50 MWth) WtE plants and technology, and has delivered several plants across Europe. The OptiCom project is a collaboration between Energos AS and the Norwegian research institute SINTEF Energi AS. The ambition of the project has been to reduce development and production costs of the WtE technology, and improve the cost-efficiency and environmental profile of the WtE plants using this technology. The focus of this project has been the aforementioned secondary combustion chamber, which is crucial to Energos' technology. The objective of this project has been to develop an improved secondary combustion chamber design for an Energos WtE plant, based on numerical simulations, including Computational Fluid Dynamics (CFD) simulations and ideal reactor simulations. With the aim to improve the combustion performance and gain better utilization of the chamber volume, the work has focused on analysis of the mixing and flow patterns in the secondary combustion chamber. The motivation for the project has been equipment size reduction (with potential reductions in capital costs - CAPEX) and improved WtE value chain sustainability. Improvements leading to reduction in NOx emissions have also been emphasized. The simulation results have revealed potential for improvement. Through this project, it has been shown that minor modifications to the process can improve the secondary combustion chamber with respect to utilization of the combustion chamber volume, and consequently cost-efficiency of the process. The proposed changes that were studied comprises: combination of air and flue gas registries, changing location of injection points, reduction of nozzle size, changing volume flow rate of secondary air, and combinations of these. Recommendations for future design are given. The modified combustion chamber will enable Energos AS to commercialize an improved process with better performance and improved competitiveness.

In OptiCom, the innovation will be realized by combining the long-term industrial experience and value creation gained by Energos during the last 17 years with the mathematical modeling and experimental expertise at SINTEF ER. In OptiCom, generic gasification and combustion knowledge will be coupled with advanced CFD modelling to achieve an improved secondary combustion chamber design aiming at improving the combustion performance and the plant efficiency, reducing harmful emissions, reducing the size of the secondary chamber and hence the construction cost, leading to increased sustainability of the WtE value chain. The underlying idea is, through advanced CFD modelling, to develop an optimized secondary combustion chamber for staged waste combustion, in order to achieve: 1)Improved secondary combustion chamber operation and control 2)Flexibility towards syngas heating value, temperature and composition at the secondary combustion chamber inlet 3)Improved mixing of fuel gas and secondary air through optimized secondary air and recirculated flue gas (RFG) injection (size and number of nozzles, residence time between injection points) 4)More stable plant operation, with reduced operational costs and lower emissions 5)Increased process understanding through modelling 6)Increased value chain sustainability OptiCom is highly relevant to the Norwegian bioenergy 2020 targets, EnergiX research priorities and EU strategies by contributing to (1) the doubling of bioenergy use in Norway by 2020 through the biogenic fraction of the waste; (2) energy efficiency improvement in industry processes; (3) improved environmental performance and (4) improved competence within the industry. Energos plans to implement design modifications in accordance with the simulation results. This new secondary chamber will be applied as an integrated part of a new technical/financial bid package to Energos future costumers.