Biomass combustion emissions can be divided into two main groups: particulate matter (PM) and gaseous emissions. Primary PM particles consist of a complex mixture of soot (elemental carbon, EC), organic material (OM) or condensable organic compounds (COCs) and inorganic material in the forms of salts and metal oxides. In addition, incomplete wood combustion generates substantial amounts of volatile organic compounds (VOCs). VOCs are precursors for secondary organic aerosol (SOA) (Heringa et al., 2011; Wu et al., 2017).
In complete combustion conditions, the particles are formed mainly from salts. Neverthe-less, during the start–up phase and improper operation, the amounts of COCs and soot can be elevated substantially in automatic wood combustion plants. Typically, incomplete com-bustion conditions occur in manually operated wood appliances, which result in the for-mation of COCs and/or soot, VOCs and CO (Nussbaumer et al., 2003).
Continuously fired appliances have lower CO and CxHy emissions than conventional wood stoves (Lamberg et al., 2011; Ozgen et al., 2014) due to the fully controlled combustion process. The PM emissions and the particle size distributions from small-scale combustion appliances strongly depend on the combustion conditions. The optimized and automated (continuously controlled) wood fuel combustion process induces a decrease in the total sus-pended particle (TSP) emissions and changes the PM size distribution (Kubica et al., 2004).
Based on studies of Boman et al. (2004), Hays et al. (2003) and Ehrlich et al. (2007), modern and optimized small-scale boilers emit mostly submicron particles (< 1 m), and the mass concentration of particles that are larger than 10 m is typically < 10% for small combustion appliances. Particulate emissions in pellet boilers are very low and mainly consist of inor-ganic matter, in contrast to wood stoves, for which the particle emissions are dominated by soot and organics (Van Loo & Koppejan, 2008).
3.5.1 Particle formation during biomass combustion
Aerosols are defined as two-phase systems that consist of particles and the gases in which they are suspended (Hinds, 1999). Aerosols ranges in particle size from 0.001 to over 100 m. There are many types of aerosols with varied chemical and physical properties and formation principles. Combustion aerosols can be classified according to PM size into
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coarse particles (> 1 m), fine particles (< 1 m) and ultrafine particles (< 0.1 m), and according to origin, formation principle and chemical composition.
Figure 4: Images of coarse fly ash (left) and aerosol (right) particles from wood combustion in a large furnace (Van Loo & Koppejan, 2008).
3.5.1.1 Coarse fly ash particle formation
The major types of ashes that are formed in biomass combustion are bottom and fly ashes.
The fly ash fraction includes a fine mode and a coarse mode. The coarse mode constitutes a small portion of the ashes that are formed during the particle entrainment from the fuel bed;
thus, it might depend strongly on the primary air flow in a grate combustion system (Pagels et al., 2003). The coarse fly ashes vary in size from a few micrometres to 250 m (Obern-berger & Thek, 2010). For various boiler types and combustion conditions, one portion of the coarse ashes precipitate in plant sections, whereas the other portion forms the coarse fly-ash emissions. Typically, these particles are formed from low-volatility metals, such as Ca, Si, and P, along with smaller amounts of K, Na and Mn, in the form of oxides, sulphates or phosphates (Kelz et al., 2010). In small-scale combustion, these particles represent only 10 wt% of total particulate emissions (Kelz et al., 2010). In addition, the coarse particle mode strongly depends on the fuel type or fuel load (Wierzbicka et al., 2005). According to Jo-hansson et al. (2003) and Carlsson (2008), the coarse mode is absent from the flue gas or is present in very small amounts. Coarse PM with an aerodynamic diameter in the range of 2,5-10 m can be deposited mainly in the upper respiratory system (Cormier et al., 2006).
3.5.1.2 Fly ash particle formation
In comparison to coarse fly ash particle formation, the mechanism of fly ash particle for-mation is much more complex. Under suitable combustion conditions, inorganic elements such as K, Na, P, S, Cl, Zn and Pb, are released into the gas phase, where they may form small fly ash particles (< 1 m). Sulphates, chlorides, carbonates and oxides form in the gas phase. Once one of these compounds becomes supersaturated due to flue gas cooling or immoderate formation of the corresponding compound, gas-to-particle conversion via
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nucleation and condensation begins. The growth of aerosol particles predisposes the system to vapour condensation, coagulation, agglomeration, surface reactions and adsorption (Sip-pula, 2010). Furthermore, the growth of particles via condensation depends on the chemical composition and initial diameter of the particles, along with the saturation ratio. Aerosol particle growth via coagulation and agglomeration occurs due to collisions of aerosol parti-cles with each other. Collisions occur due to Brownian motion, turbulence or external forces (Hinds, 1999). All these aerosol particle growth mechanisms might occur simultaneously.
The final particle size varies in the range of 50 nm-300 nm (Pagels et al., 2003). Moreover, in incomplete combustion, carbonaceous aerosol formation occurs. Carbonaceous aerosol particles are divided into soot particles (elemental carbon, EC) and particles that are com-posed of organic matter (OM/COCs). The OM fraction of particles is formed via the con-densation of organic vapours. Soot formation depends on the flame type, temperature, air-to-fuel ratio and characteristics of the fuel. According to Flagan et al. (1988), the soot par-ticles are agglomerates of small spherical parpar-ticles. Moreover, the soot and inorganic parti-cles have a structural similarity, which is due to the common origin of these partiparti-cles. How-ever, in contrast to inorganic ash formation, the mechanism of soot particle formation is not yet well understood, in which the hydrocarbon chemistry in the flame is highly complex.
The challenges also include the ability of soot to burn if exposed to oxygen at high temper-atures. When the combustion flue gas cools, the soot absorbs highly toxic compounds, such as polycyclic aromatic hydrocarbons (PAHs). PAHs are mutagenic and carcinogenic to hu-mans (Rengarajan et al., 2015). In summary, the inorganic aerosol is strongly influenced by the fuel type, while the carbonaceous aerosol can be reduced by optimizing the combustion process.
Figure 5: Ash formation mechanism in biomass combustion (Obernberger et al., 2010).
22 3.5.2 Gaseous emissions
The gaseous emissions that are generated by the combustion process are CO, CO2, hydro-carbons (HC), oxides of nitrogen (NOx) and sulphur oxides (SOx). In addition, water vapour is formed due to the fuel drying phase or hydrogen oxidation. CO2 is regarded as a major product of complete combustion. In addition, the carbon dioxide from wood combustion is not classified as a greenhouse gas. Hence, the biomass combustion process is regarded as CO2-neutral if a sustainable utilization is assumed (Obernberger et al., 2006). However, bi-omass combustion is not carbon-neutral since direct and indirect emissions of greenhouse gases (GHGs) other than CO2and indirect CO2can influence the carbon balance (Meyer et al., 2012). Incomplete combustion may lead to carbon monoxide emissions, and it is re-garded as a benchmark for the combustion efficiency. CO reduction can be realized with high combustion temperatures (> 850 °C), stoichiometric (theoretical) or excedentary total
, satisfactory mixing and sufficient retention time (> 1.5 s).
NOx emissions include NO, NO2 and N2O. The amount of N2O in modern fuel appliances is very low (Van Loo & Koppejan, 2008). NOx emissions play an essential role in atmos-pheric reactions and contribute to ground-level ozone (smog), SOA and acid rains (Camre-don et al., 2007). There are three types of NOx: fuel-derived NOx, thermal NOxand prompt NOx. For the formation of thermal NOx and prompt NOx in wood combustion, high temper-atures of >1300 °C are required. Therefore, the amounts of nitrogen oxides that form in modern appliances are minor due to the lack of high temperatures and nitrogen concentra-tions in the fuel. However, NOx might increase with increasing fuel N content (Ramirez-Diaz et al., 2014).
SOx contributes substantially to the acidification of forests. In biomass combustion, fuel-derived sulphur forms SO2 (along with SO3) and alkali and earth alkali sulphates. Conse-quently, sulphur (>75%) is released into the vapour phase, and due to rapid flue gas cooling in the boiler, sulphates condense on the fly ash particles and/or on the boiler walls. The efficiency of S fixation in the ash depends on the amounts of alkali (K and Na) and earth alkali metals (especially Ca) in the fuel and on the combustion technology and flue gas cleaning techniques. In addition, in power plants, where fuels with high S content are uti-lized, lime (CaCO3) and quicklime (CaO) are spread over the burning fuel to minimize the SOx emissions. In the presence of high SO2 emissions, the Cl is released due to sulphation of alkali and earth alkali metals. During the biomass combustion the alkali metals as K (and Na) are released into the gas phase as KOH and/or KCl. If potassium hydroxide is present in the flue gas, it may react further to KCl or/and K2SO4 with the presence of HCl and SO2
in the flue gas. As a result, the formation of KCl may strongly accelerate the corrosion, while K2SO4 has a little influence on the corrosion rate. Therefore, the additional sulphur is sometimes added to combustion process in order to force formation of the less corrosive K2SO4compound (Antunes et al., .2013).
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3.6 Flue gas cleaning devices in small-scale wood-fired appliances