Page 157 - Entrophy Analysis in Thermal Engineering Systems

152 Entropy Analysis in Thermal Engineering Systems

i.e., Eq. (10.1). The energy and mass conservations are two fundamental
laws of nature whose validity has been supported by experiments. The Gibbs
criterion, however, rests heavily on theoretical arguments.
The predictability of the equilibrium-based models is examined by
reviewing the scholarly reports where the composition of the reaction prod-
ucts is calculated by minimization of Gibbs function. The survey has rev-
ealed significant discrepancies, both quantitatively and qualitatively,
between the equilibrium calculations and experimental measurements in
several studies. Table 10.1 provides a comparison between the equilibrium
model prediction and measured compositions of the products of biomass
gasification. These data are collected from Refs. [4–12].
An average discrepancy between the measured and predicted data of
Table 10.1 is calculated for the gaseous species and depicted in Fig. 10.2.
The traditional equilibrium-based estimated nitrogen, carbon dioxide,
and carbon monoxide yields are within 10% of the experimental values.
However, the hydrogen yield is 57% overpredicted, whereas that of meth-
ane is roughly 90% underpredicted. The failure of the equilibrium-based
models to correctly predict the compositions of biomass gasification can also
be found in Refs. [13–15].
The discrepancy between the experiments and predictions of the equi-
librium models is commonly reasoned to be due to the reaction products

Table 10.1 Producer gas compositions obtained from biomass gasification.
Species Experiment Model Notes Source
12.5 18.03 MC: 14% Jarungthammachote
H 2
1.2 0.11 Temperature: 1273K and Dutta [4]
CH 4
CO 18.9 18.51 Feedstock: Rubber wood
8.5 11.43 AFR: 2.29
CO 2
59.1 51.92 Gasifier: Downdraft
N 2
8.43 14.99 Temperature: 1148.7K Jarungthammachote
H 2
2.52 0 Feedstock: Coconut shell and Dutta [5]
CH 4
CO 11.61 20.68 ER: 0.35
14.95 10.42 Gasifier: Spout fluid bed
CO 2
61.55 53.9
N 2
14.1 21.0 Feedstock: Sawdust Ruggiero and Manfrida
H 2
3.5 1.3 Gasifier: Circulating [6]
CH 4
CO 18.7 13.2 fluidized bed
CO 2 14.7 15.8