H2S will form sulfur dioxide (SO2) and sulfuric acid (H2SO4) during combustion which results in a very aggressive corrosion. The corrosion will literally reduce the lifetime of the downstream equipment by years! This is why gas engine manufacturers require that H2S in the clean gas must not exceed 100-250 ppm.Otherwise, the operating costs for change of engine oil, spark plugs and other maintenance will increase significantly.
Furthermore, there will be high costs for repairs and income lost during overhauls and break downs. Air quality standards is another driver as combustion of un-cleaned biogas will result in acid rain from emissions of sulfur dioxide (SO2). Also health and safety standards may require H2S removal as H2S is toxic even in small concentrations.