absorption of h 2 s over co 2 the caustic liquor sulfide rich white liquor stream 19 upon leaving the scrubber 236 is suitable for use as pulping liquor without any further treatment a typical fuel gas stream 4 composition entering the h 2 s removal system 205 is depicted in table 1 the ratio of h 2 s to co 2 in this example is 0 0866 recall from above that the h 2 s to co 2 ratio needs to be about 2 0 or higher before contacting the caustic liquor stream 17 the absorption stripping operation therefore has two distinct functions the first is to reduce the h 2 s concentration of the fuel gas 110 sufficiently so that when combusted in the gas turbine 110 the so 2 concentrations in the turbine exhaust will be environmentally acceptable the second function is to produce an acid gas stream with an h 2 s to co 2 ratio of at least 2 0 this means that the h 2 s selectivity over co 2 must be very high selectivity in this context is defined as the ratio of mass transfer coefficients e g k g a h 2 s k g a co 2 using u03c3 to represent this selectivity it can be shown that the selectivity can be expressed in terms of transfer units ntu where ntu can be.