Page 233 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

210 Biomass Gasification, Pyrolysis and Torrefaction

Heat required for endothermic reaction per mol of C 5 131 kJ/mol
Heat required for drying 5 0.5 3 131 5 65.5 kJ
Total heat required 5 131 1 65.5 5 196.5 kJ.
If p moles of carbon participate in the exothermic reaction, R4:
pC 1 0:5pO 2 -pCO 2 111p (ii)
Then, we have 111p 5 196.5 or p 5 1.77.
Adding reactions (i) and (ii), we get the net reaction:
2:77C 1 H 2 O 1 0:88O 2 -1:77CO 1 H 2
Thus, for (2.77 3 12) kg of carbon, we need (2 1 16) kg of steam and
(0.88 3 32) kg of oxygen. If we add more oxygen, the combustion reaction, R5,
may take place and the temperature of the combustion zone may rise further.

7.3.5 Catalytic Gasification
Use of catalysts in the thermochemical conversion of biomass may not be
essential, but it can help under certain circumstances. Two main motivations
for catalyst’s use are as follows:
1. Removal of tar from the product gas, especially if the downstream appli-
cation or the installed equipment cannot tolerate it (see Chapter 6 for
more details).
2. Reduction in methane content of the product gas, particularly when it is
to be used as syngas (CO, H 2 mixture).
The development of catalytic gasification is driven by the need for tar
reforming. When the product gas passes over the catalyst particles, the tar or
condensable hydrocarbon can be reformed on the catalyst surface with either
steam or carbon dioxide, thus producing additional hydrogen and carbon
monoxide. The reactions may be written in simple form as:

Steam reforming reaction:
catalyst
C n H m 1 nH 2 O !ðn 1 m=2ÞH 2 1 nCO (7.20)
Carbon dioxide (or dry) reforming reaction:
catalyst
C n H m 1 nCO 2 ! 2nCO 1 ðm=2ÞH 2 (7.21)

As we can see, instead of undesirable tar or soot, we get additional fuel
gases through the catalytic tar-reforming reactions (Eq. (7.20)). Both gas
yield and the heating value of the product gas improve.
The other option for tar removal is thermal cracking, but it requires high
(.1100 C) temperature and produces soot; thus, it cannot harness the lost

energy in tar hydrocarbon.