Measurements in laboratory units and large-scale plants
confirm that high NOx emissions can be generated during the
combustion of agricultural residues. NOx emissions in the
range of 300–800 mg/m3 have been reported from straw,
grass and miscanthus [53] whereas from pine wood, beech
wood and chipboard some 173, 231 and 921 mg/m3 NOx
were reported from a laboratory furnace [59]. Preto et al.
[34] measured some 200–369 mg/m3 of NOx during the
combustion of rice husks (0.27 wt% N) in a bubbling fluidized
bed. Lin et al. [39] also recorded around 400 mg/m3
NOx during the combustion of straw pellets (1.27 wt% N)
burnt in a fluidized bed at around 8308C and 6 vol% oxygen
concentration in the flue gas. The measurements by the
current authors during the combustion of coffee husks
(1.8 wt% N) in the bubbling fluidized bed gave around
400 mg/m3 NOx during single-stage combustion at 8508C
and 11 vol% oxygen concentration in the flue gas. From
these relatively high NOx emissions it may be concluded
that there is a need to apply emission reduction techniques.
NOx emission reduction techniques which have been found
useful during the combustion of wood or related biomass,
may be applied to the combustion of agricultural residues.
These techniques include staged combustion and injection
of NH3 or urea [59,62].