For synthesis reaction, a high degree of gas purity is needed, so the gas must
be cleaned of particulates and other contaminating gases. The raw syngas may
contain three principal types of impurity: (1) solid particulates (unconverted
char, ash); (2) inorganic impurities (halides, alkali, sulfur compounds, nitrogen);
and (3) organic impurities (tar, aromatics, carbon dioxide).
At high temperatures, the equilibrium shifts toward hydrogen-producing
hydrogen-rich gas. The ash in the biomass appears as slag. At low temperatures,
the ash remains in the product gas as dry ash. Cleaning has two aspects: removing
undesired impurities and conditioning the gas to get the right ratio of H2
and CO for the intended use. This use determines the level of cleaning and
conditioning. Table 9.1 presents examples of product-gas specifications for
different end uses.