Hydrothermal Gasification of Biomass - Biomass Gasification, Pyrolysis and Torrefaction - page 331

suggest that the conversion of carbon to gases in SCW declines rapidly when
the solid content in a liquid feed exceeds 50% (Prins et al., 2005), but exper-
imental results show this to occur for a much lower concentration.
Experimental data (Mettanant et al., 2009b; Schmieder et al., 2000) show
that gasification efficiency starts to decline when the solid concentration
exceeds a value as low as 2%.
Table 9.3 presents data (Mozaffarian et al., 2004) that show the effect of
solid content in feed. Although experimental conditions and feedstock vary,
we can broadly classify these results into groups of low, medium, and high
solid feedstock. For a lower feed concentration (

2%), carbon conversion
efficiency is in the range 100% to 92% and reduces to 60

,

90% for an inter-

mediate concentration (2 10% concentration.

An SCW gasifier, thus, needs a very low solid concentration in the feed for
high carbon conversion efficiency. This requires higher pumping costs and
liquid effluent disposal which may be a major impediment in commercializa-
tion of SCWG.
The reactor type also influences how solid concentration affects gasifica-
tion efficiency. For example, Kruse et al. (2003) noted that a stirred reactor
shows opposite results—that is, higher gasification efficiency at higher solid
content (1.8

10%) and to 68 80% for a

.

5.4%) in feed. This contrasts with data from Schmieder et al.
(2000) from tumbling and tubular reactors that indicate a decrease in gasifi-
cation efficiency with solid content (0.2

0.6 M). In stirred reactors, reactants
are very well mixed, resulting in a heating rate that is faster than that
achieved in other reactor types. This may be the explanation for the higher
gasification efficiency where there is a higher solid content. The exact reason
for this decrease is not clear and is a major issue in the development of com-
mercial SCW gasifiers. Catalysts, high gasification temperatures, and high
heating rates can avoid the drop in conversion of a high-solid-content feed-
stock (Lu et al., 2006).

9.4.5 Heating Rate
Limited data obtained by Sinag et al. (2004) suggest that at a higher heating
rate the yield of hydrogen, methane, and carbon dioxide increases while that
of carbon monoxide decreases. Matsumura et al. (2006) noted some improve-
ment on g carbon gasification efficiency.

9.4.6 Feed Particle Size
The effect of biomass particle size is not well researched. With limited data,
Lu et al. (2006) showed that smaller particles result in a slightly improved
hydrogen yield and higher gasification efficiency. However, Mettanant et al.
(2009b) did not observe any effect when they varied the size of rice husk
particles in the range of 1.25 to 0.5 mm. Even if

the size effect is