Page 306 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

282 Biomass Gasification, Pyrolysis and Torrefaction

The O 2 supplied is:
Moles of O 2 3 32 5 12:5 3 32 5 400 kg of O 2 =min
From this, we can calculate:
ER 5 400=1848:75 5 0:22
ER 5 280=1848:75 5 0:15

The syngas constituents in the total product gas are CO (15.2%) and H 2
3
(42.3%). So, to produce 1000 N m /min of syngas, the amount of product gas,
Q pr , is:
3
Q pr 5 1000=ð0:152 1 0:423Þ 5 1739 N m =min
The cross-sectional area of the gasifier reactor, A, is:
2
A 5 π 4 =4 5 12:56 m 2
Assuming the operating temperature to be 1000 C and the pressure to be

25 bar, the volumetric flow-rate of product gas is:
1 1273

0 3
Q 5 Q pt 25 273 5 324 m =min
pt
0
The space velocity of the gas flow V g is Q pr /A 5 324/(12.56 3 60) 5 0.43 m/s.
3
The energy produced per N m of product gas is found by multiplying the vol-
ume fraction by the heating value of each constituent, which is taken from
Table C.2 in Appendix C. Adding together the contribution of all product gas con-
stituents, gives the total heating value, HHV, as:
HHV50:004325:110:15210:1523ð282:99=22:4Þ10:423
3ð285:84=22:4Þ10:0863ð890:36=22:4Þ10:008363:4511:33MJ=Nm 3
Thus, the total energy produced, E total, is Q pr 3 HHV 5 1739 3 11.33/60 5
328.3 MW th .
The hearth load is:
E total =A 5 328:3=12:56 5 26:14 MW=m 2

8.6.3 Energy Balance
Unlike combustion reactions, most gasification reactions are endothermic.
Thus, heat must be supplied to the gasifier for these reactions to take place
at the designed temperature. In laboratory units, this is not an issue because
the heat is generally supplied externally. In commercial units, it is a major
issue, and it must be calculated and provided for. The amount of external