The criteria pollutants (NOx, SOx, CO, and particles) that are most affected by biomass are NOx and SOx. In pulverized coal systems, these are dominated by NO and SO2. SOx emissions generally decrease essentially in proportion to the amount of biomass being fired, since most biomass fuels contain a trivial amount of sulfur compared to coal. Additionally, the inorganic components of many biomass fuels react with sulfur to form sulfates that are collected in the ash, with potassium and calcium being the most important examples. This further reduces the SOx emissions, but generally by a small amount.
NOx emissions are more difficult to generalize than are SOx emissions. Most of the NOx formed during combustion of both biomass and coal derives from fuel nitrogen. Many of the best and most abundant biomass fuels are wood residues with low fuel nitrogen contents compared with coal. These fuels produce relatively little NOx and generally lead to reductions in NOx when cofired. Some biomass fuels, in particular those that are rapidly grown or cultivated, have high fuel nitrogen contents. These can produce relatively high NOx emissions. Furthermore, the forms of nitrogen in biomass differ from those in coal, with biomass fuel nitrogen to NOx conversion efficiencies being higher than those for coal under many circumstances. Therefore, some biomass fuels can produce more NOx than coals, even when the biomass fuels have lower fuel nitrogen contents on either a mass or energy basis.