THERMOCHEMICAL CONVERSION: (CO)GASIFICATION AND HYDROTHERMAL GASIFICATION - Biomass as a Sustainable Energy Source for the Future - page 347

10.5.6 Alkali and Trace Metal Cleaning

Many biomass types among the agricultural residues show high contents of alkali
salts, in particular potassium based (straw, perennial grasses). When the temperature
exceeds approximately 700 C, these (eutectic) salts are evaporated into the gas phase
(Stevens, 2001). This gives rise to problems of deposition on downstream surfaces
that have lower temperatures (below

650 C). Herewith, aerosols with fine particle

sizes (<5 m) are formed, or the species condense directly on any surface, such as

other particulate matter or the walls of process equipment. During biomass gasifica-
tion, alkali vapors can be removed by cooling the hot producer gas to below 600 C
to allow for condensation of the material into solid particulates (Stevens, 2001).
These solids are subsequently removed using various dry or wet particle removal
systems. In the design of these particle removal systems, not only the chemical
behavior of the condensed alkali salt has to be taken into account but also the
effect of tar condensation (Zwart, 2009). Ash stickiness, often resulting from
lower-melting alkali species, is also impacting dust filtration as sticky ash is difficult
to remove.
The capture of alkali species upstream of the gas cleaning section, and thus within
the gasifier, is preferred. This also retards or prevents agglomeration in case of flui-
dized bed gasification and is possible using

μ

“ alkali getters, ” which are clay minerals,

e.g., kaolinite (Bartels, 2008).

10.5.7 Nitrogen Compounds

The main nitrogen compounds in gases produced by biomass gasification are
ammonia (NH 3 ) and to a lesser extent hydrogen cyanide (HCN) and other species like
aromatic nitrogen compounds (e.g., pyridine) and HNCO. These nitrogen compounds
originate from fuel-bound nitrogen present in the feedstock (see, for instance,
De Jong, 2005; Leppälahti and Koljonen, 1995). 0.2 wt% of nitrogen in sawdust
led to concentrations of 300 400 ppm NH 3 in the fuel gas of a pressurized

fluidized bed gasifier with fuel-bound nitrogen to NH 3 conversion values in the range
of

-

70% at a CC of >90% (Wang et al., 1999).
In IGCC concepts for (combined) power and heat production, a substantial part
of the nitrogen species (NH 3 , HCN, etc.) is converted into NO x in the gas turbine
combustors. NO x is difficult

50 -

to remove and highly undesirable as atmospheric

pollutant.
Given the experience obtained in coal to FT diesel production (Sasol company),
HCN is indicated to be less tolerable than NH 3 ; HCN exposure leads to the deactiva-
tion of the FT catalyst (Leibold et al., 2008; Olofsson et al., 2005). Thus, conversion or
removal of NH 3 and HCN prior to exposure of the catalyst to these species is required.
Regarding removal of NH 3 , wet scrubbing can be applied, but this creates a liquid
waste stream. An alternative is dry, hot gas cleaning. Catalysts used for tar conversion
also show activity toward ammonia conversion in which harmless N 2 is formed.
At temperatures approaching those of fluidized bed gasifiers ( 900 C), dolomite,