Kinetics of heterogeneous NO and N{sub 2}O reduction at FBC conditions

Conference · Thu Jul 01 04:00:00 EDT 1999

OSTI ID:20006770

Johnsson, J E; Jensen, A; Nielsen, J S

A new method for reduction of N{sub 2}O at the same time as the emissions of NO and SO{sub 2} are kept low has been proposed by Chalmers University of Technology. The method involves a reversing of the conventional air staging strategy, i.e., running at almost stoichiometric conditions in the bottom of the combustion chamber and adding secondary air in the top or in the cyclone. The change in reaction pathways for N{sub 2}O and NO formation and reduction is very complex, and the catalytic activity of the solid material in the boiler may vary with the air staging. Representative samples of solids taken from large scale tests in a 12 MW CFB boiler under different operating conditions (no, severe and reversed air staging) have been tested in small scale laboratory fixed bed reactors. The activity of char and bed material (a mixture of sand, ash and partly sulfated limestone) for decomposition of N{sub 2}O and simultaneous catalytic reduction of N{sub 2}O and NO was measured. The char was found to be very active compared to bed material under inert conditions. There was no influence of operating conditions on the activity of the char. The reduction of N{sub 2}O and NO at 1,076 K was not influenced by pore diffusion for char particle sizes below about 0.1 mm and 0.2 mm, respectively. For particles above 1 to 2 mm the reduction of NO and N{sub 2}O were in the strong pore diffusion regime. The presence of CO had only minor effect and the increase in reduction rate leveled off at concentrations above about 2 vol%. Bed material is an active catalyst for N{sub 2}O decomposition, but the activity is lower than for char when compared on a mass basis. Rate equations for the decomposition of N{sub 2}O over bed material and reduction of N{sub 2}O and NO over char were estimated. It was not possible to determine rate constants for the reduction of N{sub 2}O and NO over bed material was very fast. The final conclusion was that char and bed material will both play a role for the reduction of N{sub 2}O and NO in a CFB boiler and bed material will be the most important solid material under reducing conditions when reductive decomposition of sulfated limestone takes place.

• Research Organization: Technical Univ. of Denmark, Lyngby (DK)
• OSTI ID: 20006770
• Report Number(s): CONF-990534--
• Country of Publication: United States
• Language: English

Related Subjects

01 COAL, LIGNITE, AND PEAT
AIR POLLUTION ABATEMENT
COMBINED SOXNOX PROCESSES
DECOMPOSITION
FLUIDIZED-BED COMBUSTION
LIMESTONE
NITRIC OXIDE
NITROUS OXIDE
REDUCTION
STAGED COMBUSTION