8. Characteristics of Compacted Biomass [2].

Fuel	Moisture content %	Higher heating value kJ/kg (dry basis)	lower Heating Value kJ/kg (dry basis)	Bulk density kg/m3 (wet basis)	Energy content kJ/m3
Wood pellets	10	19 800	16 560	600	9 921 600
Hardwood chips (pre-dried)	30	19 800	12 240	320	3 938 400
Hardwood chips	50	19 800	7 920	450	3 632 400
Softwood chips (pre-dried)	30	19 800	12 240	250	3 078 000
Softwood chips	50	19 800	7 920	350	2 826 000
Bales of grass	18	18 360	13 680	200	2 700 000
Barks	50	20 160	8280	320	2 617 200
Cereal straw bales	15	18 720	14 400	175	2 530 800
Sawdust	50	19 800	7 920	240	1 936 800
Wheat straw bales	15	18 720	14 400	120	1 735 200

ash content or inorganic substances depend on the species of plant and soil contamination for plant growth, ranging from 0.4-0.5% (mass percentage on dry base) for wood to 22.3% for rice husks. The composition of ash depends on the plant species, growing conditions, the fraction of ash and combustion plant. Irrespective of the biomass resource, the fly ash has, in general, higher concentrations of heavy metals than bottom ash. Wood ash has a higher content of As, Cd, Pb and Hg than that of agricultural residues. Bottom ash contains 10-20 times more Pb and Cd than fly ash. Before recycling the ash collected from biomass boilers by using it in agriculture the bottom ash should be mixed with fly ash.

The moisture content ranges from one tree to another. It is lower in tree strain and higher in roots and crown. When the biomass is dried, the humidity reaches the equilibrium with the air relative humidity. The moisture content reaches usually about 20% when biomass is dried in air. Higher moisture content decreases the calorific value and the adiabatic combustion temperature, which increases the combustion time. High moisture content means higher amount of exhaust gases and thus combustion chamber with increased size. Humidity affects behavior biomass during pyrolysis, affects the physical properties and quality of the pyrolysis liquid. Also higher moisture content reduces the efficiency of biomass combustion process. To mitigate this effect an auxiliary heat exchanger to recover condensation heat of flue gases can be installed on the flue gas path.

Content of structural constituents (cellulose, hemi-cellulose and lignin) influences calorific value. It increases with the increase of lignin content. Cellulose and hemi-cellulose have a higher heating value of 18600 kJ/kg and lignin of (23260-26580) kJ / kg.

Higher heating value (HHV) or gross calorific value of solid biofuels can be determined by using oxygen bomb calorimeter, or can be estimated using data provided by proximate analysis, elemental analysis and chemical composition.

Calorific value of the biomass is closely related to lignin content. Thus, the higher heating value for an ash-free dry sample can be calculated using the following equation [4]:

, kJ/kg (1)

where (LC) is lignin content of ash free dry sample, %.

higher heating value of solid biofuels can be calculated as a function of fix carbon, C_f (%) content with equation [4]:

, kJ/kg (2)

Equations for estimating the calorific value of fuels from ligno-cellulosic materials and for vegetable oils based on their chemical analysis can be found in literature. For solid biofuels the modified equation of Dulong may be used as a function of carbon, hydrogen, oxygen and nitrogen content [4]:

, kJ/kg (dry basis) (3)

where C, H, O and N are carbon, hydrogen, oxygen and nitrogen content of dry sample, %.

Also, for a dry solid biofuel sample the following equation can be used [2]:

, kJ/kg (dry basis) (4)

where C, H, S, N, O and A are carbon, hydrogen, sulphur, nitrogen, oxygen and ash content of dry sample, %.

Lower heating value (LHV) or net calorific value of wet sample can be calculated by applying the equation:

, kJ/kg (wet basis) (5)

where HHV_db is higher heating value of dry sample, W is moisture content of wet sample (%) and H_db is hydrogen content of dry sample (%)

The higher heating values of some solid biofuels are given in Table IX.