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of tar formation; (2) promote several other chemical reactions to change the production rate, composition, and heating value of the gas; (3) promote char gasification;
(4) prevent active agglomeration of the feedstock, char, and tar that can lead to reactor
choking; and (5) remove carbon dioxide through the active adsorption process. The
steam reforming catalysts also reform tar and produce gas of high quality.
4.2.3.1 dolomite, Olivine, and alkali metal-Based Catalysts
These are generally cheap and disposable catalysts. Dolomite is a magnesium ore
with the general formula MgCO 3 . CaCO 3 is considered to be a good catalyst for biomass gasification. Dolomite is also a good adsorbent for carbon dioxide and capable
of removing tar very efficiently. It is, however, a very fragile substance and may
quickly attrite in highly turbulent conditions within a fluidized bed. CaO additive
was studied by Dalai et al. [41], who showed that the use of this additive reduced the
gasification temperature to about 150°C to get the same level of gas production. Both
carbon conversion and hydrogen production increased with impregnation of CaO in
cellulose, cedar, and aspen. The production rates of gas and hydrogen also depended
on the nature of feedstock; cedar and aspen performed better than cellulose.
Hu et al. [42] tested calcined olivine and dolomite in a fixed-bed reactor and found
higher activities of calcined catalysts compared to those of natural catalysts. Other
literature also showed that in the presence of olivine, tar conversion increased with
an increase in temperature from 800°C to 900°C, and at 900°C and higher, all watersoluble heterocyclic compounds get converted [21] (Barrio et al., 2012, pers. comm.).
With 17 wt% olivine in the sand at 900°C, the conversion of heavy polyaromatics
increased from 48% to 71%. Calcined dolomite, however, increased the conversion
up to 90%. Aznar et al. [44–46] showed that dolomite was very effective in removing tar coming from a blend of plastic waste with pinewood sawdust and coal in
the temperature range of 750°C–880°C. xu et al. [47] demonstrated that at 700°C,
hydrogen concentration in the product increased by the use of CaO as an adsorption
agent for carbon dioxide.
Monovalent alkali metals such as Li, Na, K, Rb, Cs, and Fr were also found to be
catalytically active in steam gasification. Both K and Na are a part of biomass and
accumulate in the ash, which in turn can act as a catalyst. This solves the problem
of ash handling and the ash reduces the tar content in the gas phase. The ash catalytic activity, however, can be lost due to particle agglomeration. Sutton et al. [43]
pointed out that direct addition of alkali metals can require (1) expensive recovery
of catalyst, (2) increased char content after gasification, and (3) ash disposal problems. Lee [48] and Lee et al. [49] found that the addition of Na 2 CO 3 enhances the
catalytic gasification of rice straw over a nickel catalyst and the additive increases
the gas formation. They also found that the gas production rate is affected by the
nature of the additive and follows the order: Na > K > Cs > Li. The use of activated
alumina as a secondary catalyst was found to be effective by Simell et al. [50–52];
however, this catalyst deactivated faster due to coking compared to dolomite. Sami
et al. [29] showed that both zirconia and alumina promoted toluene and ammonia
conversions at lower temperatures, indicating enhanced oxidation activity of zirconia with alumina. Furthermore, H 2 S had a little effect on the activity of aluminadoped zirconia.