Structural collapses and inhomogeneous flow conditions in fixed-bed conbustion

Combustion in fixed beds, such as in grate furnaces, often suffers from problems with inhomogeneous flow conditions through the fuel bed. The effect is also referred to as channelling and it is believed to cause elevated emission levels and increased ware on grate and furnace material. The inhomogeneous flow is primarily caused by inhomogeneous fuel properties in the fuel bed, or as a result of the design of the air injection through the grate. In addition, the fuel properties change during the conversion processes. During the conversion of a batch of wood pellets, it has been observed that both a linear increase in bed porosity and sudden structural collapses occur. This dynamic behaviour is believed to play an important role for the formation of inhomogeneous bed properties. In this work, a computational fluid-dynamics model is used to evaluate the combustion of a batch of wood char. It takes into account inhomogeneous flow conditions, inhomogeneous fuel-bed properties and also the collapse processes that occur in fuel beds. The results show that the inhomogeneous flow conditions, in combination with bed collapse, causes a channel-like flow in the fuel bed. When disabling the collapse processes and letting the conversion result only in linear shrinkage at constant porosity, the channelling is dampened. It can, hence, be concluded that the collapse processes inside a fuel bed play have influence on the formation of channelling flow in fixed beds. Further investigations of the structural dynamics of fuel beds are needed to learn more about how different fuel properties affect the collapses in fuel beds and how it influences the combustion