Environmental Engineering Reference
In-Depth Information
(Ni-based) steam reforming catalysts (Wang et al., 1999, 2000), and Fe-based
catalysts have all been shown to be able to largely convert NH
3
(Leppälahti and
Koljonen, 1995; Leppälahti et al., 1991; Stevens, 2001). Carbon deposition may be
a problem, although high steam contents can prevent this. A high H
2
concentration
limits the conversion. A novel development is the application of monoliths to remove
tar and NH
3
as applied by Corella et al. (2004a, 2005); these require less strict cleaning
of the gas from particles, though sticky ashes may cause deposition and related
activity decrease. Catalytic filtration using ceramic filters impregnated with a
Ni-based catalyst can also decrease tar and NH
3
concentrations simultaneously;
the configuration might also consist of an annular packed bed on the clean side of
the ceramic candles.
When chlorine is present in the raw syngas, this might result in the formation
of a condensable (fouling) species, NH
4
Cl, which solidifies below 250
280
C and
presents a fouling risk. Reaction of NH
3
with H
2
S can result in the formation of
ammonium (poly)sulfide, with a melting point below 150
C.
−
CHAPTER SUMMARY AND STUDY GUIDE
This chapter deals with gasification of biomass as a technology that is flexible
regarding the production of a spectrum of products, such as heat, power, chemi-
cals, and biofuels. Gasification is treated from different points of view: chemical
thermodynamics, reaction kinetics of heterogeneous char-oxidizer reactions, and
heat and mass transfer; reactor technologies are reviewed based on the different
applications and properties of biomass and the qualities of the produced gas.
A distinction is made in technologies for dry biomasses and wet biomass. In
the last case, hydrothermal gasification is a promising, relatively new conversion
technology making use of the water content in the fuel so that drying is not needed.
Finally, in view of requirements of emissions and demands imposed by down-
stream equipment, gas cleaning options and devices are addressed. Of particular
importance is the cleaning of biomass gasification product gas with respect to par-
ticles, tar, sulfur species, chlorine compounds, and alkali metal species, as well as
trace elements.
KEY CONCEPTS
Differences between combustion and (hydrothermal) gasification of biomass
Chemical equilibrium; trends with actual oxidizer amount compared to stoichiometric
combustion
Reaction kinetics; description of char conversion during gasification with H
2
O
and CO
2
Models for char surface development with conversion
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