Biomedical Engineering Reference
In-Depth Information
Chapter 9
Hydrothermal Gasification of
Biomass
9.1 INTRODUCTION
This chapter deals with conversion of biomass into liquid or gaseous pro-
ducts in hydrothermal medium. Here the hydrothermal medium is a water-
rich phase above about 200
C at sufficiently high pressures to keep the water
in either a liquid or supercritical state (Peterson et al., 2008).
In the mid-1970s, Sanjay Amin, a graduate student working at the
Massachusetts Institute of Technology (MIT), was studying the decomposi-
tion of organic compounds in hot water (steam reforming):
C
6
H
10
O
5
1
7H
2
O
-
6CO
2
1
12H
2
(9.1)
While conducting an experiment in subcritical water, he observed that in
addition to producing hydrogen and carbon dioxide, the reaction was produc-
ing much char and tars. Herguido et al. (1992) also made similar observa-
tions in the steam gasification of biomass at atmospheric pressure.
Sanjay interestingly noted that when he raised the water above its “criti-
cal state,” the tar that formed in the subcritical state disappeared entirely
(Amin et al., 1975). This important finding kick-started research and devel-
opment on supercritical water oxidation (SCWO), for disposal of organic
waste materials (Tester et al., 1993), which has now become a commercial
option for disposal of highly contaminated organic wastes (Shaw and
Dahmen, 2000).
Biomass in general contains substantially more moisture than do fossil
fuels like coal. Some aquatic species, such as water hyacinth, or waste pro-
ducts, such as raw sewage, can have water contents exceeding 90%. Thermal
gasification, where air, oxygen, or subcritical steam is the gasification
medium, is very effective for dry biomass, but it becomes very inefficient
for a high-moisture biomass because the moisture must be substantially
driven away before thermal gasification can begin; in addition, a large
amount of the extra energy (
B
2242 kJ/kg moisture) is consumed in its evap-
oration. For example, Yoshida et al. (2003) saw the efficiency of their ther-
mal gasification system reduce from 61% to 27%, while the water content of
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