Advanced modeling of nitrogen oxide emissions in circulating fluidized bed combustors: Parametric study of coal combustion and nitrogen compound chemistries
This paper describes work-in-progress aimed at developing an emission model for circulating fluidized bed combustors using detailed homogeneous and heterogeneous chemical kinetics. The main emphasis is on nitrogen oxides (NO{sub x}, N{sub 2}O) but also unburned gases (CO, C{sub x}H{sub y}) and sulfur dioxide (SO{sub 2}) will be investigated in the long run. The hydrodynamics is described by a 1.5-dimensional model where the riser is divided into three regions: a dense bubbling bed at the bottom, a vigorously mixed splash zone, and a transport zone. The two latter zones are horizontally split into a core region and an annular region. The solids circulation rate is calculated from the known solids inventory and the pressure and mass balances over the entire circulation loop. The solids are divided into classes according to size and type or particle. The model assumes instantaneous fuel devolatilization at the bottom and an even distribution of volatiles in the suspension phase of the dense bed. For addition of secondary air, a complete penetration and an instantaneous mixing with the combustor gases in the core region is assumed. The temperature distribution is assumed to be known, and no energy balance is solved. A comprehensive kinetic scheme of about 300 elementary gas-phase reactions is used to describe the homogeneous oxidation of the volatiles including both hydrocarbon and volatile-nitrogen components (NH{sub 3}, HCN). Heterogeneous char combustion to CO and CO{sub 2}, and char-nitrogen conversion to NO, N{sub 2}O, and N{sub 2} are described by a single particle model that includes 15 reaction steps given in the form of 6 net reaction paths. In the paper, the model is briefly described. A special emphasis is put on the evaluation of chemistry submodels. Modeling results on nitrogen oxides' formation are compared with measured concentration profiles in a 12 MW CFBC riser from literature. The importance of accurate chemistry description on predictions is illustrated by comparing modeling results using detailed kinetics to those obtained when hydrocarbon and volatile-nitrogen oxidation are described with empirical, global kinetic rate expressions from literature. Submodels that need further improvements are discussed.
- Research Organization:
- Aabo Akademi Univ., Turku (FI)
- OSTI ID:
- 20006810
- Report Number(s):
- CONF-990534-; TRN: IM200008%%417
- Resource Relation:
- Conference: 15th International Conference on Fluidized Bed Combustion, Savannah, GA (US), 05/16/1999--05/19/1999; Other Information: 1 CD-ROM. Operating system required: Windows 3.x; Windows 95/98/NT; Macintosh, Power Macintosh; UNIX. All systems need 2X CD-ROM drive.; PBD: 1999; Related Information: In: Proceedings of the 15th national conference on fluidized bed combustion, by Reuther, R.B. [ed.], [1800] pages.
- Country of Publication:
- United States
- Language:
- English
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