Calcined dolomite is the most popular and most investigated material as a tar crack­ing catalyst. Dolomite is a calcium magnesium ore with a general chemical formula CaMg(CO3)2. This catalyst is relatively inexpensive, abundant, and disposable. Az — nar, Corella and their research groups, in several publications, have investigated the effect of the in-bed use of dolomite, which can decrease the tar level from 6.5 %wt to

1.3 %wt [50]. The addition of 3-10 % calcined dolomite to biomass feed decreases the tar level by 40 % and improves gas quality significantly [51, 52]. Although dolomite has proved to be effective in terms of tar reduction, it has some critical limitations. Dolomite in its naturally occurring form is not very active in tar crack­ing, and it needs to be calcined. Calcination of dolomite involves its decomposition and the elimination of CO2 to form the MgO-CaO complex. Calcination reduces the surface area of the dolomite catalyst, and makes it more friable resulting in severe catalyst attrition and fine-particle production. While using dolomite in a fluidized bed, dust entrainment due to the eroding of soft dolomite particles necessitates the continuous feeding of dolomite into the reactor by mixing it with biomass fuel [51]. This requires a major gas-cleaning operation.

Another popular catalyst in this group is olivine, which could be an alternative for dolomite. Olivine is a mineral, which contains magnesium, iron oxide, and silica. In terms of attrition, olivine has certain advantages over dolomite. However, Corella et al reported that dolomite was 1.4 times more active than olivine in biomass gasification with air, but dolomite generated ~ 4-6 times more particulates in the gasification gas than olivine [53]. Nickel has also been used with an olivine support, which led to improved catalytic activity compared to unsupported olivine catalysts [54, 55].