Nitrogen balances can vary a lot between different mills and recovery boilers.This presentation is based on nitrogen balance measurements for 17 boilers.They show that in modern new boilers the N conversion factor from black liquor N into nitrogen in NO_x is typically in the range of 20-30%.When the boiler design has been done specially for low-NO_x emissions,the range is 15-20%.The nitrogen split can be with low-NO_x design for instance 15% into NO_x,10% into smelt and 75% into N2 in flue gases.Low NO_x emission does not correlate with high concentration of N in out flowing smelt.On the other hand,N concentration in smelt can represent over 40% of the incoming N in black liquor.The emission figures correlate with excess air,temperatures,staging,and in some cases with carry-over.The balance figures of the presentation vary with the firing system,with the loadings,with liquor firing properties including dry solids,with the operation parameters etc.The missing data for reliable numerical modeling,without experience related parameters,are in pyrolysis and lower furnace oxidation processes,including deep temperature,concentrations and velocity gradients.The effect of weak and strong odorous gases into NO_x emission,depend on system design.With right design,the N input via NH3 in the odorous gases does not effect the NO_x emissions of recovery boilers.The important,practical issue is that NO_x emission and other emissions have cross effects,data for BAT has to be from the same recovery boiler in this respect.Lower NO_x emission may generate S02 emission,and the operation mode,which results into low NO_x emission may affect on the melting behavior of carry-over.Lower melting temperature can induce fouling and corrosion in the superheater;more sootblowing steam is needed and the generation capability of greenhouse gas free power is reduced.Naturally the corrosion issue can be compensated with more expensive materials.The other important practical issue is that generally boiler loading has effect on NO_x emission via temperatures.If the permit is for the today's firing capacity,the capacity increase will result into higher specific NO_x releases,and into higher emissions per ADt pulp.
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