The simplest technology for biomass combustion is the pile burner which is the engineered version of the primitive bone fire solution. Biomass is piled inside a refractory chamber through a screw conveyor, in an underfeed system, or dropped on top of the pile in overfeed systems (Fig. 5.12) and it is ignited manually or with an oil or gas start-up burner. Primary air is blown directly inside the bed through holes in the refractory lining while secondary air is provided above the bed through chute openings. Combustion is mainly surface driven, therefore adequate radiation from the combustion chamber walls is necessary and vaulted ceilings are usually utilized. The conical pile settles according to the friction angle of the material and in underfeed burners new fuel pushes inside the pile causing a bottom-up vertical gradient of the mass loss and the ashes on top to fall at the sides of the pile where the ash pits are located. In overfeed burners no forced movement of the pile is present and a gravity driven top-bottom gradient of mass loss is present causing the ashes to fall into the ash pit which is positioned below the bed.

Ash is usually removed from the ash pit by manually extracting the container as a drawer, or else by cleaning the grate directly when the ash is cooled. In either case the cyclic operation of the burner results in high maintenance requirements which contribute to the low efficiencies (<70%) and difficulties in controlling the process. Automated ash extraction through augers may be present in bigger scale applications to increase availability. Pile burners are not suitable for load following operation while they are simple, inexpensive and suitable to burn wet and dirty fuels.

A typical overfeed pile burner for small to medium scale application is the so called Dutch oven which is the oldest technology still in use, especially in the forestry products and sugar industry.

With reference to Figure 5.13 the Dutch-oven technology is a two-chamber furnace made of refractory walls with holes (tubes) for combustion air inlet and fuel is dropped on a grate from a

hole on the ceiling. Drying and gasification takes place in the first chamber where radiation from the walls and ceiling is essential to speed up the process. The construction characteristics vary considerably according to the different biomass size and humidity, although the Dutch-oven can handle easily fuel with up to 50% humidity thanks to the heat reservoir of the pile which however does not allow abrupt load changes.

When a single chamber pile combustor is considered, the underfeed solution in a cylindrical combustion chamber is the so-called cyclonic furnace (Fig. 5.14) which is still used in northern Europe. Combustion gases are extracted from the top of the brick lined furnace and are conveyed to the heat exchanger while biomass is pushed by an auger through a hole at the center of a circular grate on top of which the pile is formed. For this reason the cyclonic furnace may also be considered as a grate combustor. During combustion the solid fuels proceeds radially from the center to the periphery where ashes are discarded. Primary air is fed preheated through the grate while secondary air is provided tangentially on the top of the furnace as to provide a cyclonic vortex that optimizes the combustion of volatiles. The cyclonic furnace shows a relatively high efficiency, also on wet fuels, and low dust emission while its disadvantages are mainly related to its high maintenance costs and poor automation possibilities.