Sustainable Energy Conversion from Biomass Waste Combustion

Licentiate thesis, 2019

Various biomass-type conversions into sustainable heat and power, from dedicated crops to solid biomass waste via the combustion process, are at the center of scientific and industrial focus today. However, biomass combustion contribution to global particulate matter (PM) emission has become one of the biggest challenges that remain in the development of sustainable biomass combustion systems. Particulate matter emission is well-known to be a strongly threatening agent to human health, causing 3.7 million premature deaths in 2012 according to the World Health Organization. Based on this fact, developing knowledge on how to reduce PM emission in biomass combustion systems is the main concern of this thesis. This work includes the development of models and experimental methods on different scales.

Designing an advanced biomass combustion furnace is the first step to developing the knowledge on a large scale. The constructed furnace allows detailed observation of different combustion parameters and PM characteristics. The furnace is fueled with waste biomass and equipped with a combination of a PM sampling system and accurate, real time, and flexible measurements of temperature and exhaust gas composition.

In order to produce more correct predictions of PM precursors, the development of a computationally efficient and accurate thermally thick particle model for a combustion system is performed on a smaller scale. Partial differential equations (PDEs) for heat and mass balance are transformed into ordinary differential equations (ODEs) with the utilization of orthogonal collocation as the particle discretization method. The transformation allows the current model to be implemented into the computational fluid dynamics (CFD) environment using sub-grid-scale modelling.

A detailed study of the effect of Stefan flow on particle surface using CFD analysis allows improvement of the boundary layer heat and mass transfer in particle modelling. The effect of Stefan flow is found to be pronounced since it reduces the oxidative gas mass transfer rate to the particle surface, and at the same time, it has significant effects on the convective heat flux from or to the bulk gas. The efficiency of the current particle model is demonstrated through the low usage of computational power. The employed particle model is also proven to be accurate and stable through its high degree of agreement with simulation results for particle pyrolysis and combustion experiments using different particle moisture contents, shapes, and sizes.