practice6

Firewood is the composition of hydrocarbons. Different species of trees has different compositions of carbon, hydrogen, oxygen, nitrogen, sulfur and ash.	Fuel	Ultimate analysis, %
	Fuel	Carbon	Hydrogen	Oxygen	Nitrogen	Sulfur	Ash
	Oak, Dry	50.2	6.0	43.3	0.1	……….	0.4
	Pine, Dry	52.6	7.0	40.1	………	……….	0.3

(Heat of combustion or Calorific value or heat of reaction). Heating value is the energy released per unit quantity of fuel when the combustible is burned and the products of combustion are cooled back to the initial temperature of combustible material. Heating value of the solid fuel depends upon the moisture content. Air dried firewood generally consists of 10 – 20 % of moisture.
	Fig1: Heating value of pinewood as a function of moisture content
	Heating values are recognized by higher heating value and lower heating value. The higher heating value is obtained when all the water formed by combustion is a liquid; the lower heating value is obtained when all the water formed by combustion is a vapor. The higher heating value exceeds the lower heating value by the energy that would required to vaporize the liquid formed.
Calorific values are also defined as Gross Calorific Value and Net Calorific value.	The Gross Calorific Value (GCV, MJ/kg, d.b.) of biomass fuels usually varies between 18 and 21 MJ/kg (d.b.), and can be calculated using following empirical formula
*GCV = 0.3491XC + 1.1783XH + 0.1005XS - 0.0151XN - 0.1034XO - 0.0211Xash [MJ/kg d.b.]* Where, xi is the content of carbon (C ), hydrogen (H), sulfur (S), Nitrogen (N), oxygen (O) and ash in wt% (d.b.).
The Net Calorific Value (NCV, MJ/kg, w.b.) can be calculated from GCV taking into account the moisture and hydrogen content of the fuel using following empirical formula
*NCV = GCV(1-w/100)-2.447w/100-2.447(h/100)9.01(1-w/100) [MJ/kg w.b.]*
	Where, w moisture content of the fuel in wt% (w.b.) h concentration of hydrogen Guiding values for woody biomass fuels: 6.0 wt % (d.b.) for herbaceous biomass fuels: 5.5 wt% (d.b.)