Carbon dioxide can be removed from syngas by chemical and physical absorption with a washing liquid or by adsorption with solid absorption. The choice for chemical or physical absorption (or a combination of both) depends on thepartial pressure in the gas. For chemical absorption in commercial processes substituted amines are used, while solvents, like methanol, polyethylene glycol, and dimethyl ether, are used for physical absorption. The CO2 concentration can be removed to approximately 0.1 vol% by these processes. When the syngas contains significant concentrations of other gases besides H2 and CO2, adsorption on solid materials, such as silica gel, active carbon, zeolites, and molecular sieves, is preferred.

As discussed earlier, the produced syngas often contains high amounts of prob­lematic impurities, such as sulfur, chlorine, and alkalis. The gasification process has several cleaning units and its total efficiency depends on the heat management of the various steps. One of the most important challenges for an efficient gas cleaning process is developing a hot gas cleaning technology, which works at or close to the gasifier temperature. These techniques include the development of novel particu­late removal techniques, an improved catalyst for tar cracking to produce a tar — and particulate-free product gas and a higher degree of process integration. For example,

one possible way to increase the efficiency of hot gas cleaning and also reduce the cost is to decrease the number of gas cleaning stages by combining different physical and chemical processes in the same equipment, such as catalytic tar cracking in a particular barrier filter. This was first proposed for combustion applications [62], but further applied to the gasification process by many research groups, such as the deposition of the Ni/MgO catalyst onto the pore walls in a-alumina in a candle [63] and a catalytically active fixed bed in a cylindrical catalytic filter element [64]. Still, there is continuous research and development being done to improve particulate fil­tration, various sorbents and associated equipment to achieve a high efficiency of gas cleaning, especially at high temperatures.