2. Experimental study
The wet leaves samples were cut directly from the banana tree
and the semi-dried samples were obtained from leaves that had
fallen from the banana tree and were already undergoing a natural
degradation process. The samples were grinded and sieved to produce
particles less than 2.5 mm in size, according ASTM E828-81.
To correlate the respective composition with the thermal
behavior of the biomass under characterization, chemical and
physical analysis were performed. The heating value of biomass
and the conditions for thermochemical conversion of biomass by
pyrolysis and combustion are highly correlated with the content
of ash, moisture, volatiles and lignocellulosic content including
the elemental composition of carbon, hydrogen, sulfur and oxygen.
The proximate analysis was performed on the biomass samples
for the determination of moisture, volatile matter, ash and fixed
carbon contents following the ASTM standardized procedures.
The moisture content was determined as the weight loss in a stove
at 103 C using the procedures given in ASTM E871-82. The volatile
matter was measured as weight loss after burning in muffle furnace
at 950 C following procedures given in ASTM E872-82. The
ash content was determined as the residue after burning to constant
weight in a muffle furnace at 575 C according to ASTM
E1755-01. This process allows removal of the volatiles and fixed
carbon. Fixed carbon was determinate using the data previously
obtained in the proximate analysis and according García et al.
[10] using the formula % FC = 100–(% Ash–% VM). All analyses were
performed in triplicate.
Ultimate analysis was performed in order to determine the basic
elemental composition of biomass. The carbon, nitrogen and
hydrogen contents of the samples were measured in the Perkin-Elmer
CHN 2400 elemental analyzer. The samples were burnt in a
pure oxygen atmosphere and the combustion gases are automatically
measured. The sulfur content was determined in the Spectro
Ciros CCD equipment (atomic emission spectrometry with inductively
coupled plasma, ICP-AES). The oxygen content was determinate
by subtracting the sum of the other element contents from
100%. The analyses were performed in duplicate.
The higher heating value (HHV) of the samples was determined
in a static and adiabatic bomb calorimeter, Parr 1241, according to
NBR 8633 and NBR 11956 based on the ASTM D2015 for determination
of the gross calorific value of coal and coke. The samples
were ground until obtaining a homogenous fine powder. The powders
were recombined and dried at 110 C and 110 mmHg for 6 h
and stored in a desiccator for 12 h. Finally, the powders were
pressed in the form of pellets 1 cm in diameter, with mass approximately
0.5 g. The combustion of samples was done in the presence
of O2 at a pressure of 20–30 atm. Benzoic acid was used as a standard
for determining of heat capacity of the calorimeter. The analyses
were performed in duplicate.
The lignocellulosic fractions (cellulose, hemicellulose and lignin)
of the samples were determined by procedures based on the
Van Soest and Wine method [21]. The hemicellulose content was
determined from the difference between acid detergent fiber and
neutral detergent fiber. The lignin was determined by mass difference
between the sample digested with detergent acid and the oxidation
carried out with the buffered solution of acetic acid and
potassium permanganate. The cellulose content was determined
by the difference in mass between the dry residue generated in
the lignin analysis and the same residue calcined in a muffle furnace
at 500 C for 2 h. The analyses were performed in triplicate.
The thermal behavior of the samples was evaluated through
thermogravimetric and derivative thermogravimetric analyses
(TG/DTG) and differential scanning calorimetry (DSC) in the simultaneous
analyzer STA449C NETZSCH. The analyses were conducted
in the temperature range from room temperature up to 900 C
with a heating rate of 10 C/min under oxidizing (synthetic air)
and inert (N2) atmosphere at a gas volumetric flow of 30 mL/min.
While the DSC characterized the physical and chemical processe