5.2.1 Introduction

Combustion can be defined as “an exothermic oxidation process occurring at a relatively high temperature” (Basu, 2001). A simplified stoichiometry of the reaction for a biomass of generic composition is the following (Tillman, 1991):

CpHqOr + (p + q/4 — r/2)O2 ^ p CO2 + 1 /2q H2O + heat (5.11)

O2 used as an oxidant is usually provided through combustion air therefore assuming a standard volume composition of air (79% N2 and 21% O2) so also (3.76 x n) N2 has to be considered, n being the number of moles of oxygen required to complete the combustion of the fuel.

Stoichiometric or ideal combustion for a biomass (with the following composition: p carbon mass fraction; q hydrogen mass fraction, r oxygen mass fraction) can therefore be simplified as:

1 kgCpH? Or + 1/0.233(8q — r + 8/3p)kgairst

^ 11/3pkgCO2 + 9 x qkgH2O + 0.767/0.233(8q — r + 8/3p)kgN2st (5.12)

The combustion of biomass can be described as the steps followed by the biofuel to undergo a complete oxidation. Four steps can be identified (Browne, 1958): drying and heating, solid particle pyrolysis, char oxidation and volatile oxidation (Fig. 5.3) (van Loo and Kopperjan, 2002; He et al., 2006).

A biomass particle that enters a hot combustion chamber is rapidly heated from the outside to its internal core. Heat is transferred from the furnace to the particle outer layer through radiation- convection from flame and flue gases and conduction from the hot biomass bed while conductive heat transfer brings heat inside the particle. The temperature increases abruptly in the outer layer but slowly towards the core of the particle therefore humidity evaporation begins in the external layer and proceeds towards the inside with an evaporation front which is considered to happen conventionally when the layer reaches 105°C. Water evaporates and its expansion cracks the particles producing micro and meso-pores through which steam is ejected. The dried layers increase further their temperature, but cannot burn because oxygen does not reach the inner layers, eventually hemicellulose first and cellulose-lignin after start to decompose thermally. Long polymeric chains are cracked into smaller ones which vaporize or become permanent gases leaving the particles through the same paths followed by steam. This mixture of permanent gases

(syngas) and vapors (tars) constitute the volatile content of the biomass and when ejected into the combustion chamber reacts with oxygen producing a flaming combustion.

Volatile extraction and combustion continues while the pyrolysis front moves towards the inner core of the particles, the same as the evaporation front did previously, leaving a charred layer on the outside which burns when in contact with oxygen. Char combustion does not produce a flame (glowing combustion) and it is particularly slow since oxidation happens only in the solid-gas boundary layer leaving a layer of insulating ashes which will eventually be removed by mechanical actions of the flue gases combined with gravity to allow oxygen to attack a new fresh layer of char.

This series of events which happen continuously during the combustion of a solid fuel, such as biomass, depends on the temperature reached by a certain area of the particle and on the exposure to oxygen, and are illustrated in Figure 5.4.

According to the temperature gradient the combustion of a solid fuel may then be divided into four steps that occur at different temperatures (Williams etal., 2012):

STEP 1: Below 200°C (heating and drying) biomass absorbs heat in the heating and drying process. The sample loses weight steadily, but it does not ignite.

STEP 2: From 200°C to 280°C (torrefaction) the sample continues to increase its tempera­ture while releasing preliminary volatiles deriving from low temperature decomposition, mainly hemicellulose; gases evolved are still not fully ignitable, however some exothermic reactions happen. The temperature at which the reaction of pyrolysis and oxidation become exothermic can be considered as the definition of the ignition point of wood. There are several studies examining the ignition point (Janssens, 1991; Li and Drysdale, 1992; Masank, 1993; Fangrat et al., 1997; Babrauskas, 2001) that could be considered to happen at a temperature of around 250°C.

STEP 3: From 280°C to 500°C (pyrolysis and volatile combustion): pyrolysis is the ther­mal degradation of a solid in the absence of oxygen; the global pyrolysis combustion model is represented in Figure 5.5. Pyrolysis is a process that is mainly endothermic and happens in two phases:

• the primary reactions are endothermic reactions that transform biomass in GAS (syngas),

CHAR (fixed carbon + ashes) and TAR (condensable gases);

• the secondary reactions are exothermic reactions (cracking) that break tar in syngas and char.

During this phase pyrolysis gases copiously evolve from the particle and when they meet oxygen they burn with a flaming combustion in the gas phase, provided that the mixing with air happens within the lower and upper limits of flammability (LaGrega et al., 1994). Self-sustaining diffusion flames from biomass can burn at 1100°C and more; one-half to two thirds of the heat of combustion is due to flaming combustion, the rest to glowing combustion of char. If pyrolysis

I

GAS ——— 1—— ►—- FLAMING COMBUSTION

♦ I

I

TAR

CHAR ——— і—— >— GLOWING COMBUSTION

I

Figure 5.5. Simplified pyrolysis and combustion process (Tillman, 1991).

gases are liberated rapidly they consume oxygen around the particle surface therefore there is no oxygen left for char combustion which then accumulates.

Since char has only one-third to one-half the conductivity of wood (Browne, 1958) the layer of char decreases the progress of the pyrolysis front towards the inside of the particle (Fig. 5.5) and a temperature decreasing trend is observed passing from the surface of the particle to the center. This turns into a diversified timing of combustion within the particle which may be still expelling water from the inside core while the mid core is pyrolyzing and the outer layer is already charred. For this reason usually a strong initial flaming is followed by a decrease until sufficient heat has reached a deeper portion of wood to activate pyrolysis reaction.

STEP 4: above 500°C: Glowing combustion begins and it occurs with and without flame. When the surface temperature has reached 1000° C the char at the surface reacts as fast as the pyrolysis layer moves to the center of the particle (Martin, 1956). The luminous diffusion flames due to primary pyrolysis gases and tars are substituted by non-luminous diffusion flames due to the combustion of carbon and hydrogen. When even the production of those gases is ended the remaining char glows almost without flame.

The four steps of biomass combustion will be described with more detail in the following sections.