Hydrothermal Gasification of Biomass - Biomass Gasification, Pyrolysis and Torrefaction - page 341

hot spots in the tube. In horizontal tubes while bottom part experience the
sharp rise in heat transfer coefficient was the pseudo critical temperature the
upper part does not experience the change to that extent due to buoyancy
effect (Shang et al. 2008).

9.7.4.1 Heat Transfer in SCW
Table 9.5 illustrates the designed performance of a typical heat-recovery
exchanger for SCWG. The data are taken from a large operating near-SCWG
plant. The fluid-to-wall heat-transfer coefficient in clean SCW in the vertical
tube may be calculated by the correlation of Mokry et al. (2011):

0 : 564

p w
p b

061 Re 0 : 904
b

Pr 2 0 : 684
b

N u 5 0 : (9.14)

Based on experiments and reviews of 15 correlations Jager et al. (2011)
recommended the following correlations for horizontal tubes in SCW.

0 : 43

p w
p b

2 : 4

0069 Re 0 : 9
b

Pr 2 0 : 66
b

N u 5 0 : 1 (9.15)

1

x = d

where x is length and d is diameter of tube.
Heat transfer in SCWG may vary because of solids in the fluid. Thus,
applicability of these equations to SCWG is uncertain. Information on this
aspect of heat transfer is presently unavailable.

9.7.5 Carbon Combustion System
Because gasification and pyrolysis reactions are endothermic, heat from
some external source is required for operation of the reactor. In thermal gasi-
fication systems, the reaction temperature is very high (800 1000 C), so a

large amount of energy is required for production of fuel gases from biomass
or other feedstock. This heat is generally provided by allowing a part of the

TABLE 9.5 Sample Data from Product to Feed Heat Exchanger Pilot Plant

(VERENA)

Reactor
Temperature
( C)

Flow-Rate kg/h
(Methanol%)

Product
In ( C)

Product
Out ( C)

Feed In
( C)

Feed
Out ( C)

100 (10%) 561 168 26 405 582

90 (20%) a

524 155 22 388 537

a Heat exchanger surface area—1.1 m 2 , heat transfer coefficient—920 W/m 2 C.
Source: Boukis et al. (2005).