Combustion is an exothermic reaction that releases the chemical energy (HHV) of a fuel trans­forming it into heat that is transferred to the surrounding environment (combustion chamber) and to the combustion products (flue gases and ashes) (Obernberger, 1998). Combustion of a solid fuel is actually carried out as the exothermic release of the chemical energy contained in two different fuels: a volatile fuel mixture of permanent gases (syngas) and vapors (tars) which burns rapidly in the gaseous phases (volatile matter) and a carbonaceous solid fuel (char) which burns slowly in the solid-gas interface. However these two different fuels must be extracted from the original solid fuel and this is accomplished in the preliminary phases (heating, drying and pyrolysis) providing thermal energy (heat) to the feedstock.

A combustor device for solid fuels must then be designed to guarantee an adequate heat exchange between the feedstock and the reactor in order to allow drying and extraction of volatile matter within the residence time. Since the two fuels obtained have different combustion behavior being respectively a gas (volatiles) and a solid (char) the oxidant (usually air) must be provided in different quantities and in different places inside the reactor to guarantee complete combustion.

Char glowing combustion is carried out in the boundary layer between the solid surface and the gaseous phase and therefore it requires an active surface available for oxygen to combine with carbon. The external layer of each particle is burnt leaving an ash deposit which partly shields the new active layer of carbon from further oxidation. Char combustion is then slowed by the incomplete availability of new active layers of carbon due to its solid geometry and ash deposits. Combustion air to oxidize char is called primary air or underfire air because it must be provided within the feedstock bed with an adequate velocity to optimize turbulence and mechanical stress on the particle for ashremoval. It will also be provided in high excess with respect to stoichiometric conditions given the disadvantaged mixing conditions between fuel and oxidant.

Considering the previous general mass composition of biomass and a generic content x [kgC/kgbio, db] of fixed carbon in the dry feedstock the following equation provides the primary air mass flow:

Rp x x x 32/12 x 100/23.3 [kg air/kg fixed carbon] (5.24)

where Rp [kgalr/kgalr st] is the ratio between the mass of primary combustion air provided and the theoretical stoichiometric primary combustion air.

Volatiles flaming combustion, on the other hand, takes place in the gaseous phase above the solid fuel bed and it is fairly more advantaged with respect to char combustion, given the high miscibility of fuel gases and gaseous oxidant. Combustion air to oxidize volatile products is called secondary air or overfire air because it must be provided above the solid fuel bed with an adequate turbulence to guarantee adequate mixing with the gaseous fuel.

As considered in section 5.2, given the generic biomass mass composition CpHqOr, the following quantity represents the stoichiometric amount of air needed to oxidize hydrogen:

(8q — r) x 100/23.3 [kg air/kg H] (5.25)

While the following quantity represents the stoichiometric amount of air needed to oxidize volatile carbon:

(p — x) x 32/12 x 100/23.3 [kg air/kg volatile carbon] (5.26)

Therefore the following equation provides the secondary air mass flow:

Rs x ((8q — r) x 100/23.3 + (p — x) x 32/12 x 100/23.3) [kg air/kg volatile matter]

(5.27)

where (kgair/kgairst) is the ratio between the mass of secondary combustion air provided and the theoretical stoichiometric secondary combustion air.

The sum of primary air and secondary air mass flows represents the total combustion air:

Rs x 100/23.3 x ((8q — r) x p x 32/12) + x x (Rp — Rs)

x 32/12 x 100/23.3 [kg air/kg biomass] (5.28)

which was already determined in 5.2 for stoichiometric conditions, and which can be easily obtained from the previous when considering Rp = Rs = 1.

Combustion performance depends strongly on the geometry of the reactor, on the air and fuel inlet and resulting turbulence inside the reactor and also on the size of the solid fuel given that the ratio between external surface of the particles, which determines the heat exchange rate and char oxidation rate and the particle volume increase with decreasing particle size. Adequate heat transfer to the solid fuel and char/volatiles mixing with air is also guaranteed by an adequate movement of the fuel inside the reactor, which must also provide a pathway for ash removal from the combustion chamber.

Biomass combustion may be used in a power cycle for CHP application or to provide process heat in a boiler or for heating and air conditioning for households or larger scale applications. Whatever the application the general process scheme would be the one in (Figure 5.11) where a generic feedstock is burnt, flue gases are cleaned of particulates in a dedicated device (e. g. cyclone) and flow into a heat exchanger providing heat to a working fluid which runs a power cycle for CHP or is conveyed directly to a thermal user. Given the relatively high biomass ash content and their possible low melting point, usually the heat exchanger is not installed directly inside the combustion chamber and it is not directly exposed to the flame to avoid high temperature corrosion and fouling of the tubes. Cooled gases are then conveyed to the emission abatement section, which can be as simple as a filter for particulate matter, and eventually reach the stack.

As described by various authors (Baukal, 2004), there are four mainly diffused typologies of biomass combustors: pile burners, grate burners, suspension burners and fluidized bed burners; depending on the application (industrial or household) the concept may be modified to fit the different size of the combustion chamber.

Pile and grate burners are often referred to as fixed bed combustors while suspension and fluidized bed burners are often referred to moving bed combustors. Their different concept and performance will be described in the following sections.

5.3.2 Fixed bed combustion

As mentioned above, fixed bed combustion is one of the most used technologies in biomass combustion thanks to the following advantages: it can fire a wide range of fuels (of varying moisture content, particle size and ash content) and requires less fuel preparation and handling. Fixed bed combustors usually consist of a two-stage combustion chamber with a separate furnace and boiler located above the secondary chamber where the oxidation of volatilized products is completed. They can be divided into pile burners and grate burners.