NO formation tendency characterization for biomass fuels

When a solid fuel, such as coal, biomass or a mixture of these fuels, enters a hot fluidized bed, the volatile carbon and nitrogen compounds are released, while some nitrogen and carbon remains in the solid char. Volatile nitrogen can form reactive species such as NH3, HCN and tar-nitrogen. These can react in the presence of oxygen to NO (and some N2O). Some part of volatile nitrogen is always reduced to N-2. During combustion of the char residue, some part of the char-nitrogen forms NO (or N2O) and the rest is converted to N-2.

Usually the standard fuel analysis is not enough to allow for accurate NO emission predictions in large scale fluidized bed combustion. This paper presents NO formation tendency characterization results from novel laboratory measurements in a small-scale fluidised bed combustor. The laboratory results of this paper give a good insight into the distribution of fuel nitrogen between reactive and non-reactive (N2) volatile components and char-nitrogen. A NO formation tendency database is formed based on the results, including data on biomass-, waste-, peat- and coal-type fuels. The combustion test results show that the cumulative conversion of fuel nitrogen to NO under lean, nonstaged fluidized bed combustion is 20 - 50 % (850 degrees C with O-2 in excess). For biomass and peat, nearly all reactive nitrogen (forming NO) is released from the fuel during pyrolysis. NO formation during char combustion is significant with coal.

With the help of the database, a reasonable estimate of the maximum non-staged NO emission in fluidized bed combustion can be obtained. Normally, air staging is utilized to reduce NO emissions. Effects of air-staging can be studied by means of modelling. In case of a BFBC boiler, the data can be used as input in the design or modelling of air staging for freeboard. For CFBC, the data can be used as input in the NO prediction where the once formed NO is further reduced by char carbon.