The use of dolomite as a catalyst in biomass gasification has attracted much attention because it is a low cost material that significantly can reduce the tar content of the product gas from a gasifier. With suitable ratios of biomass to oxidant agent, almost 100% elimination of tars can be achieved. Dolomite is a magnesium-calcium ore with the general formula MgCO3 • CaCO3 that contain additional minerals at trace levels, such as SiO2, Fe2O3 and Al2O3. The chemical composition of dolomite, as well as many of its characteristics, such as surface area and pore diameter, varies from source to source. Dolomites are the most active with the calcination reaction (MgCO3 • CaCO3 = MgO • CaO + 2CO2). The activity can be directly related to the surface area, pore size and pore distribution. A higher activity has also been observed when iron oxide is present.

Dolomites are generally used in a secondary bed downstream from the gasifier, but they can be used as a primary catalyst, dry-mixed with the biomass. Claimed dolomites can be placed in the gasifier with good result, but they tend to be brittle and thus erode easily.

Dolomite decomposes in principle all tar compounds but naphthalene, thus naphthalene has been identified as the most abundant condensable compound after reforming of tars over dolomite at 800-900°C. This point to a limitation of the dolomites as catalysts if the overall aim is the total elimination of all tars from the product gas. The naphthalene conversion varies with the steam partial pressure, reaching a maximum for a steam concentration of 5-15% and overall naphthalene conversions have been reported to be 80-95% with dolomite (Sutton, 2001).

An additional drawback with dolomite is its eventual use at pressurized conditions. This because dolomite must be calcined for performing acceptably and calcined dolomites can be re-carbonated depending on the temperature and partial pressure of the carbon dioxide. For example, at 900°C, CaO will be carbonated to CaCO3 if the partial pressure of carbon dioxide exceeds 100 kPa. Thus, the use of dolomites as catalysts are, in principle, restricted to biomass gasification applications at atmospheric pressure.

Deactivation due to carbon deposition has been also reported. Nevertheless, the relatively high amounts of steam used in gasification can be effective in maintaining the activity of the dolomite catalysts. The catalyst is also sensitive towards chlorine in the gas phase since it easily forms CaCl2 at the actual temperature present in the gasifier or the tar cracking reactor (Nemanova,

2011) . CaCl2 has a relatively low melting point at 782°C, which is lower than the temperature normally used in the gasifier or a cracking reactor, > 800°C. The CaCl2 will form a soft outer layer at the bed particles. This layer will be more easily abraded and will also cause a blocking of the pore structure, i. e. the material will be completely non-active in terms of catalytic activity.