The following section provides a brief discussion of the main issues regarding the different combustion technologies that have been described previously and an insight into main design procedures for combustion chambers and some operational criticalities in terms of corrosion, slagging and fouling.

5.3.4.1 Design principles

In grate furnaces primary air passes through a biomass bed in which drying, devolatilization and char combustion take place, while secondary air to oxidize volatiles is added in a combustion zone over the bed. In the fluidized bed biomass is burned in a suspension of gas and solid bed material in which combustion air enters from below (Basu, 2006; Van Loo and Kopperjan, 2002). A comparison between grate furnaces and fluidized beds is seen in Figure 5.22.

Mass flows of air and flue gases are determined by combustion calculations. Boiler efficiency (which includes combustion efficiency) and excess air are the parameters chosen by the designer. For a biomass pile burner an efficiency ranging from 65 to 75% may be expected, while a stoker

Figure 5.21. Schematic of a Circulating Fluidized Bed (CFB).

• fuel

• bed material

fixed bed combustion bubbling fluidised circulating fluidised

(grate furnace) bed combustion bed combustion

Fixed bed Bubbling bed Circulating bed

Figure 5.22. Comparison between grate furnaces and fluidized beds.

boiler an efficiency of 70 to 80% can be considered while for a biomass Fluidized Bed based boiler an efficiency ranging from 75 to 85% can be considered (US EPA, 2007). Dealing with excess air, an excess air coefficient of 40% can be considered for a bubbling fluidized bed (Basu, 2006).

Once the required power of the boiler is fixed, the amount of fuel can be determined knowing its Lower Heating Value LHV, hence assuming that the water vapors produced exit the furnace above their condensation temperature (Oka and Anthony, 2004).

Q0

n0LHV

where:
m f	— fuel mass flow [kg/s]
Qo	— heat power output [kW]
По	— boiler efficiency [-]
LHV	— lower heating value [kJ/kg]

The furnace and combustion system have to be designed to optimize the following aspects: stable ignition, complete burnout; prevention of slagging and corrosion inside the furnace; prevention of fouling and corrosion on the convective heating surfaces.

The depth and breadth of the furnace must adapt to the flame form in a way that it can expand as freely as possible, also ensuring that the walls will not be touched. Beside this the design ofthe furnace must take into account the generation of heat (heat release) and the absorption of heat by the walls ensuring that the designed amount of fuel can be burnt in the given furnace volume, that is the fuel burnout must be completed (Baher, 1985). Besides the cross-section and height of the furnace the performance will depend on the fuel type and on the geometry and location of the heating surfaces. This means that the ash melting temperature of the fuels defines the necessary furnace outlet temperature at the furnace end before the convective heating surfaces in order to avoid sticky deposits on them (Khan et al., 2009).

Dealing with heat release, each type of fuel has its specific heat release rate (Prabir et al., 2000), which can be expressed in three different bases: furnace volume, furnace cross section area and water wall area in the burner region. The volumetric heat release rate is the amount of heat generated by the combustion of fuel in a unit effective volume of the furnace. It is given by:

where:

qv — volumetric heat release rate [kW/m3]

B — designed fuel consumption rate [kg/s]

V — furnace volume [m3]

LHV — lower heating value [kJ/kg].

A proper choice of volumetric heat release rate will ensure that fuel particles are burnt substan­tially and the flue gas is cooled to the required safe temperature before leaving the furnace. This temperature is known as furnace exit gas temperature (FEGT) and it is critical for safe operation of downstream heat exchanger surfaces.

The heat release rate per unit cross-sectional area is also indicated as grate heat release rate or grate thermal load and is given by:

where:

qF — thermal load [kW/m3]

F — cross sectional area of the furnace grate [m2].

Typical thermal loads for coal fired furnaces are given in Table 5.8.

The burner zone heat release is based on water wall area in the burner region and it is defined as:

B ■ LHV

2(a + b)Hb

where:

qb = borner zone heat release [kW/m3]

a and b = width and depth of the furnace [m]

Hb = distance between top edge of the uppermost burner and lower edge of the lowest

burner [m].

The heat absorbed by the surrounding walls can be defined by the corrected Stefan-Boltzmann equation and thus the following equation can be written:

20.53 * Fr * BBSA

Hout = Heat absorbed by furnace wall + Heat carried by the gas leaving the furnace

5.3.4.2 Deposit and slagging problems

Deposition (i. e. slagging and fouling) and corrosion problems are major issues in the design and operation of a combustion system. In solid fuels combustion the particulate and vapors formed during combustion can deposit on heat exchanger tubes or furnaces walls exposed to radiant heat. If the deposit is formed by molten ashes in the boiler section that is directly exposed to the flame (fire-side) the phenomenon is referred as slagging; if the deposit is formed by vapors that form a sticky layer in the convective part of the boiler the phenomenon is named fouling (Tortosa-Masia et al., 2005).

Table 5.9. Ash chemistry deposition indexes (Yin et al., 2008; Juniper, 1996).

Index Formula

ase/acid ratio

Iron index Slagging factor Silica ratio

Critical viscosity at 1426°C Estimated ash temperature corresponding to and ash viscosity of 250 poise

Alkali index (the ratio of the total amount of sodium and potassium expressed as their corresponding oxides to the heating value of the fuel in the unit of kg/GJ Chloride-sulfate molar ratio

Multi-fuel fouling index Ash melting points

Also vapors and fine particles can cause deterioration of intrinsic properties of the materials in the combustion chamber (corrosion) (Khan et al., 2009) as it happens for example in straw — fired boilers. Based on fuel properties and in particular ash chemistry.

Several experimental tests have been performed to analyze the mechanism of fly ash and deposit formation and corrosion in grate-fired boilers (Zhou et al., 2007; Zbogar et al., 2006; Jensen et al., 1997; 2004; Michelsen et al., 1998; Montgomery and Karlsson, 1999; Nielsen et al., 1999; Hansen etal., 2000; Montgomery etal., 2002).

It has been observed that chlorine increases the volatility/mobility of alkali metals facilitating the formation of alkali silicates and alkali chlorides or hydroxides in the gas phase. When the flue gases are cooled passing the super-heater the alkali compounds condense on the heat exchanger tubes (Fig. 5.23). The initial deposit is formed by vapors and fine particles that arrive at the tube surface where alkali silicates melt at 700°C and thus provide a sticky condition on the entire surface of the tube. This sticky layer enhances further deposition of big fly ashes; this phenomenon affects only the upstream side of the tube.

The deposits consist mainly of fly ash particles and large amounts of condensable salts, forming a matrix that glues the fly ash particles together. In the deposit the presence of KCl and K2SO4 with a structure of iron oxide (FexOy) has been found. A suggested corrosion mechanism for chlorine corrosion is based on gaseous Cl attack, where Fe and Cl in the metal react with gaseous Cl, forming volatile metal chlorides. This mechanism can explain the shift in corrosion behavior observed experimentally (i. e. selective corrosion is negligible at 450°C while is significant if temperature is higher than 520°C) (Yin etal., 2008).

Corrosion mechanisms can be classified into three classes: corrosion associated with gas species; solid-phase corrosion and molten phase corrosion.

The same problems encountered in grate combustion can be found in fluidized bed boilers. Besides in fluidized bed reactors problems of bed agglomeration can be encountered. Agglomer­ation happens when a small part of the ashes remains in the bed, then ash sintering can happen through three main mechanisms: the presence of a liquid phase; solid-state sintering; and chemical reactions.

In biomass fired FBC partial melting is considered the main mechanism leading to bed agglom­eration. The first mechanism is also identified as partial melting and can be further classified as highly viscous melt and low viscous melt. Highly viscous melt happens in the presence of silica this kind of melt is the most problematic because a glassy phase is produced which does not crystallize upon cooling (Llorente etal., 2003; Skrifvars etal., 1998). In the case of low viscous melts alkaline compounds can melt and act like a bonding agent.

Possible remedies to fouling, slagging and corrosion problems are the use of additives or co-firing with coal, peat and sludge. High temperature corrosion can be mitigated also reducing the surface temperature of superheaters. The use of additives aims to raise the melting temperatures of ashes, prevent the release of gaseous KCl and/or react with KCl to form less corrosive compounds. The materials that have been found to raise the melting temperatures of ashes include Al2O3, CaO, MgO, CaCO3, MgCO3, and kaolin (Llorente etal., 2006; Van denBroek, 2000). For example the addition of 3% in weight of kaolin to chopped oats straw can raise the deformation of ashes from 700 to 1200-1280°C (Jenkins, 1994). ChlorOut (ammonium sulfate) a concept developed and patented by Vattenfall AB (Olanders and Steenari, 1995) is a very promising method. An aqueous solution of ammonium sulfate (NH4)2SO4 is sprayed in the combustion zone at a temperature of 800-900°C upstream of the super heaters. It effective converts alkali chlorides into alkali sulfates, that are much less corrosive. The spraying of ammonium sulfate can also reduce NOX formation. The main reactions involved are:

(NH4)2SO4 (aq) ^ 2NH3 (g) + SO3 (g) + H2O (g)

2KCl (g) + SO3 (g) + H2O (g) ^ K2SO4 (s) + 2HCl (g)

4NH3 (g) + 4NO (g) + O2 (g) ^ 4N2 (g) + 6H2O (g)

Possible remedies against bed agglomeration for FBC systems can be: co-combustion, pre­processing of the risky fuel, use of additives and use of alternative bed materials to increase the melting point of sintering compounds. As far as additives are concerned, kaolin, dolomite, lime­stone, lime, alumina have been tested even if they have not been so effective. The use of alternative bed material is the most promising solution; dolomite, magnesite, ferric oxide, alumina, feldspar and aluminum rich materials have been tested (Laitinen et al., 2000; Daavitsainen et al., 2001; Silvennoinen, 2003).