The design of a combustion system is important for achieving optimum efficiency from the process. During the combustion process, slagging and fouling of the furnace and the boiler occurs. This is more serious when biomass contains a high proportion of alkali metals. The alkalis volatilize during combustion and condense as alkali metal salts on the relatively cool furnace walls. These elements react with other compounds to form a sticky lining on the furnace and boiler wall surface. Regular cleaning of these deposits is required which usually involves process shut­down, reducing the efficiency of the process. The design of the combustion equipment should be such that a minimum of fouling takes place. A number of different designs of combustion systems have evolved in an attempt to get max­imum combustion efficiency with minimum fouling. These are summarized along with the salient features of each design in Table 1.3.

Fixed-bed combustion

In this type of combustion system, the biomass is fed in the form of a bed on grates at the bottom of a furnace. The grates may be either inclined or horizontal. Air is passed through the grate (on which the fuel is present) at a restricted rate such that the fuel is not stirred and there is no relative movement of the fuel solids. The stokers used for feeding the fuel may be either overfeed stokers or spreader stokers.

The overfeed stokers were originally designed for firing coal. These feed the fuel by gravity onto the moving grate at one end. The grate travels slowly across the furnace, carrying the fuel along, as combustion takes place. The residual ash and slag are continuously discharged at the opposite end.

Table 1.3 Designs of combustion systems [7, 8] Combustion method Salient features Fixed bed Florizontal grate: Overfeed Grate is level and moving in different manners. Biomass is fed by gravity onto the moving combustion — Forward moving grate — Reverse moving grate stokers grate at one end. It ignites and burns as surface combustion. Residual ash and slag is continuously discharged at the opposite end. — Reciprocating grate Spreader Grate is level and moving in different manners. Stokers distribute the comminuted — Step grate — Louvre grate Inclined grate stokers biomass onto the furnace above an ignited fuel bed on an air cooled travelling grate. Suspension firing occurs partially. Fine particles tend to bum in suspension while larger particles fall onto the travelling grate where they are burnt. Most common design selected for biomass combustion systems. Biomass is fed at the upper part of the grate. Pre-drying of fuel occurs at the upper part of the furnace after which it slowly tumbles down under gravity onto a reciprocating grate lower in the furnace where combustion takes place. The grate is either water cooled or air cooled. Suitable for biomass fuels with lower ash contents. Fluidized Bubbling fluidized bed combustion Circulation fluidized bed combustion Finely comminuted biomass particles fed onto a bed of sand at the bottom of the furnace and subjected to an evenly upward flow of air which fluidizes the biomass. Initial drying followed by ignition takes place. Rotary hearth furnace combustion Kiln furnace Suitable for combustion of high moisture fuel such as liquid organic sludge and food residue. Burner combustion Burner Used for burning wood powder and fine powder such as bagasse and pith.

Spreader stokers distribute the comminuted and dried biomass fuel over an ignited fuel bed on an air cooled traveling grate. These stokers can be made responsive to heat load changes by automatic adjustment of grate travel speed, fuel feed rate, and air intake. A major disadvantage with this type of a system is that an ash layer needs to be maintained on the grate in order to protect it from thermal degradation. Biomass ash may have a high silica content which may cause a greater abrasion of the grate, resulting in a higher maintenance cost of the grate. Another disadvantage with this type of a combustion design is that there can be a significant amount of fly ash and unburned carbon in the flue gas, resulting in lower combustion and boiler efficiencies and higher costs of emission controls.