Page 100 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

78 Biomass Gasification, Pyrolysis and Torrefaction

its yield. This conversion reaction, being the slowest, is used to determine
the size of the gasifier.

Char
Char, though a carbon residue of pyrolysis or devolatilization, is not a pure
carbon; it is not the fixed carbon of the biomass either. Known as pyrolytic
char, it contains some volatiles and ash in addition to fixed carbon. Biomass
char is very reactive. It is highly porous and does not cake.

3.6.3 Thermogravimetric Analysis
Because of the time and expense involved in proximate analysis by ASTM
D-3172, Klass (1998) proposed an alternative method using thermogravime-
try (TG) or differential thermogravimetry. It is discussed in Chapter 13.

3.6.4 Bases of Expressing Biomass Composition

The composition of a fuel is often expressed on different bases depending on
the situation. The following four bases of analysis are commonly used:
1. As-received
2. Air-dry
3. Total dry
4. Dry and ash-free.
A comparison of these bases is shown in Figure 3.15.

As-received basis
Air-dry basis
Dry basis
Dry and ash-free basis
A C H O N S M i M s

A FC VM M
Coke Volatile
A – Ash H – Hydrogen C – Carbon
O – Oxygen N – Nitrogen S – Sulfur
M – Inherent moisture Ms – Surface Moisture
i
FIGURE 3.15 Basis of expressing fuel composition.