Environmental Engineering Reference
In-Depth Information
downstream equipment and catalysts. Tar formation in the gasifier can be minimized by increasing
the O
2
-to-fuel ratio, raising gasification temperature, or using low-cost dolomite (CaMg(CO
3
)
2
)
which acts as a base catalyst in steam reforming (C
n
H
m
+ H
2
O → nCO + ((m /n)/2 + 1) H
2
) of tars.
For example, Gil et al. (1999) showed that addition of dolomite to a biomass gasifier decreased tars
from approximately 20 to 2 g tar/Nm
3
, but was accompanied by higher production of dolomite
attrition dust. Higher gasifier temperatures have disadvantages that lead to decreased carbon yield
in syngas or with higher slag production (NSF 2008). Even with prevention steps in the gasifier,
aftertreatment is required to reduce tars to tolerable levels, as discussed in the next section. Other
minor products such as H
2
S and ammonia are derived from biomass-bound S and N and their
concentrations in gasifier effluent are feedstock dependent for the most part.
8.3.1.2 synthesis Gas clean-up from Biomass Gasifier
Tars are produced at levels between 10,000 and 20,000 mg/Nm
3
under typical biomass gasifier
operating conditions (Torres et al. 2007; NSF 2008). Tars are best removed using steam-reforming
(Ni catalyst) or cracking (dolomite, carbonate, or Ni catalysts) reactions, but cracking requires
higher temperatures (900°C) than normally achieved at gasifier exit (<800°C) and Ni catalysts can
be poisoned by low amounts of H
2
S. Tar concentrations as low as 2 mg/Nm
3
were observed over a
50-h trial when dolomite was used in the gasifier followed by two reactors containing Ni catalyst to
remove tar (Caballero et al. 2000). However, a recent review concludes that no tar removal process
has demonstrated long-term performance at a gasifier's normal exit temperature of less than 800°C
(NSF 2008).
Ammonia is produced at levels between 2000 and 4000 ppmv in a biomass gasifier (Torres et al.
2007) and is removed through decomposition to N
2
and H
2
using supported Ni, Ru, and Fe catalysts,
but carbides and nitrides of W, V, and Mo can also be used. Much attention has been given to NH
3
decomposition in synthesis gas in the presence and absence of H
2
S (Jothimurugesan and Gangwal
1998). NH3 can be effectively removed with commercial reforming catalysts at 650°C, but removal
of H
2
S below 10 ppmv is required.
Because sulfur content of biomass is relatively low compared with other solid gasification feedstocks,
the H
2
S (and COS-carbonyl sulfide) concentration is normally between 20 and 600 ppmv in biomass
gasifier outlets (Torres et al. 2007). But this H
2
S concentration is too high to avoid poisoning of Ni
catalysts for tar and ammonia removal, and although higher temperature operation can mitigate this S
poisoning effect in Ni catalysts, sintering/deactivation of catalyst and materials-of-construction issues
become important (NSF 2008). The most feasible option for sulfur removal is to use a high temperature
adsorbent/regeneration process using ZnO, but Zn ferrites and Zn titanates are also effective.
ZnO + H
2
S → ZnS + H
2
O (adsorption step)
(8.1)
ZnS + 3/2 O
2
→ ZnO + SO
2
(regeneration step)
(8.2)
Although gasification hot gas clean-up processes have been extensively studied in the past,
significant research and development is needed to overcome barriers to commercialization of
biomass gasification (Torres et al. 2007).
8.3.1.3 Water-Gas-shift reaction
The water-gas-shift (WGS) reaction is used to adjust the CO/H
2
ratio in synthesis gas by the
mechanism:
CO + H
2
O ⇆ CO
2
+ H
2
(8.3)
Commercial catalysts are available for WGS reactions in the temperature ranges of low (225-250°C,
Cu-Zn), medium (350-375°C, Co-Mo sulfide form), and high (450-475°C, Fe-Cr). Because the
