9. Higher Heating Value of Solid Biofuels [8, 9, 10].

Biofuel	Higher heating value (dry basis), kJ/kg
Alfalfa (clover) stem	18 050-18850
Almond shell	20000-18220
Cotton stalk	15 830-19200
peanut shell	15 700-21 600
Olive pomace	21 400
Sunflower seed husk	16 120-19980
Sunflower strain	19230-21 800
Hazelnut shell	18300-20490
Wheat straw	16100-20750
Rice straw	14700-15950
Rye straw	16180-18990
Rape straw	17700-19330
Rice seed husk	15 500-19 800
Wood	15 500
Corn stalk	15 700-18 500
Corn cob	17000-17 400
Sorghum stalk	15400-17860
Sugarcane bagasse	17300-19400
Switchgrass	18000-19100
Bamboo	19000-19800
Tobacco stalk	16 400 (7% humidity)
Grapevine branches	16 500 (7% humidity)
Branches of apple	15 200 (7% humidity)
Miscanthus	18100-19600
Black locust	19500-19900
Eucalyptus	19000-19600
Poplar	1900-19700
Willow	18600-19700
Cattle manure (fresh)	11 300-17360
Pig manure (fresh)	13790-17890
Chicken manure (fresh)	12840-17140
Municipal solid waste	13100-19900
Refused derived fuel	15500-19900

Problems of biomass burning in boilers

Biomass combustion technologies present several problems [11]. The most important problems are related to fouling and corrosion of boiler heat exchanger. slagging and fouling reduce the heat transfer and cause corrosion. Corrosion and erosion lead to shortened equipment life. Deposits on heat exchangers surfaces are caused by inorganic matter present in burning biomass. Sodium, Na, and potassium, K lower the ash melting point, and therefore is enhanced ash deposition on boiler tubes. Calcium, Ca and magnesium, Mg increases the ash melting temperature. Silica, Si may combine with potassium, K producing silicates with low melting temperature in volatile particles. This process is important, on one hand to prevent ash sintering / melting and agglomeration on the grate of the fluidized bed combustion plant and, on the other hand to prevent ash slagging on the surface of the heat exchanger. In work Cereal straw and grass have a high content of K, Cl and sulphates and low content of Ca. Almond shells combustion is accompanied by a strong fouling and corrosion process of heat exchanger surfaces. They have a high content of alkali metals and reduced content of chlorine and sulphur compared to other fuels. Potassium and sodium combined with chlorine and sulphur have an important role in the corrosion mechanism. These elements evaporate during combustion forming chlorine that condenses on the heat exchanger tubes.

For boiler operators it is important to know the ash softening temperature, (deformation temperature), the temperature at which the first signs of rounding, due to melting, of the ash sample tip or edges occur. This temperature gives an indication of the melting behaviour of biomass ash. Work [12] gives an equation determining the use of the K-nearest neighbours (KNN) ordinary least squares (OLS) and partial least squares (PLS) approaches that predict the softening temperature (SOT) of biomass ash based ash composition:

[°C]

where CaO, Al₂O₃, K₂O and P₂O₅ are mass concentrations in biomass ash of calcium oxide, aluminium oxide, potassium oxide and phosphorus pentoxide, respectively (Table VIII).

Chlorine has a catalytic action on oxidation reaction of heat exchanger surface, particularly at low temperature (100-150C). Fuels with a molar ratio S:Cl less than 2 cause corrosion because alkali metal chlorides are formed. Metal volatilization followed by condensation leads to the formation of fly ash smaller than 1μm (aerosol), which is difficult to retain it in filtration plant. Ash deposition on heat exchange surfaces in biomass combustion boiler can take place in a more or less extent than in coal combustion boiler. At biomass co-combustion with coal the ash deposition takes place in a lesser degree than individual combustion. Deposits formed at biomass combustion have higher adhesion and hardness than those from coal combustion.