Switching from coal to biomass challenges the operational stability of pulverised fuel burners. Differences in fuel properties affect ignition and losses (Loss On Ignition). A combustion model for pulverised biomass has been developed, providing accurate predictive capabilities of burner operation.
With increasing focus on fossil fuel impacts on our climate, supplanting coal by biomass in the production of power and heat is one of many pathways pursued by DONG Energy to reduce CO2 emissions. The most cost-effective way to implement the shift from coal to biomass is by utilizing existing power plant installations. Due to differences in fuel properties, these installations need to be adapted the new fuel to maintain a safe and economic performance.
One of many differences between coal and biomass is the comminution propensity of the fuels. Where coal is relatively easily grinded into a fine powder, the inherent fibrous structure of biomass makes this process more difficult and energy intensive. In practice, resulting biomass particles are much larger when injected through burners into the large boilers, and this poses a challenge – both in terms of ensuring a stable and safe operation of the boiler and ensuring that fuel losses are acceptable.
DONG Energy is currently adapting several power plants from coal to biomass combustion. Without the reassurance of extensive industrial experience, safety issues as well as economic risks are large. In the constant pursuit of improved decision grounds, DONG Energy has assigned Rambøll with the task of providing improved predictive tools for analysing burner operation and fuel combustion for converted coal burners. The objective is to assess the impact of planned changes on burner and boiler performance, and to provide guidelines for fuel quality specifications to be used when biomass pellets are procured.
Rambøll has developed an improved combustion model for pulverised fuel combustion prediction, which not only takes into account the larger size of biomass particles, but also the different morphology as this has been found to play a dominant role in the thermal degradation process. The model developed is adapted for use in the commercial CFD tool Fluent ® supplied by ANSYS. The principal model enhancements compared with existing models includes geometric resolution of individual fuel particles and improved kinetics for high heating-rate pyrolysis.
The project was awarded to Rambøll due to the extensive CFD and combustion experience of the Rambøll CFD group. The project group was composed of several partners, comprising DONG Energy, Vattenfall, Burmeister & Wain Energy and DTU Chemical Engineering (Technical University of Denmark).