NO{sub x} formation and reduction during combustion of wet sewage sludge in the circulating fluidized bed -- Measurements and simulation
- Technical Univ. Hamburg-Harburg, Hamburg (Germany)
Mechanically dewatered municipal sewage sludge with a water content between 70 and 80 wt% was incinerated in a pilot-scale circulating fluidized bed (CFB). The combustion chamber had a total height of 15 m and a circular cross-section of 0.1 m diameter. In order to compensate for heat losses, the combustor was electrically heated from the outside. The combustion temperature of 850 C could thus be maintained along the whole length of the riser. Axial concentration profiles of the gaseous species O{sub 2}, CO{sub 2}, NO, N{sub 2}O, NH{sub 3} and organic carbon measured as CH{sub 4} were recorded for a variety of operating conditions. Since a major part of the investigation was devoted to the elucidation of the mechanisms of NO{sub x} formation and reduction in the case of combustion of wet sewage sludge, the combustor was also used for reaction kinetic studies whereby different gases, e.g., CO, NH{sub 3}, NO and N{sub 2}O were injected into the combustor which was during these experiments fluidized with nitrogen. The kinetic studies lead to a simplified kinetic scheme for NO{sub x} formation and reduction under conditions of sludge combustion. The combustor model divides the combustion chamber into four stages, i.e., a bottom zone, a splash-zone, an upper dilute zone and an exit zone. The bottom zone is modeled as a bubbling fluidized bed, whereas for the upper dilute zone a two-phase structure with an upflowing dilute suspension phase and a downflowing dense phase is assumed. This model is combined with a combustion model and the NO{sub x} formation and reduction model to form the CFB sludge combustor model. The calculations yield axial distributions of the different gaseous species which are shown to be fairly good agreement with the measurements. The model is able to explain the effects of different operating variables on the NO{sub x} emissions. IN particular, the model is able to explain why two-stage combustion, which in the case of coal combustion is considered to be a good means for keeping the NO{sub x} emissions low, is not favorable in the case of wet sewage sludge combustion. Furthermore, the model is able to explain the low efficiency of NH{sub 3} injection into the upper part of the combustion chamber for NO{sub x} reduction.
- Sponsoring Organization:
- Deutsche Forschungsgemeinschaft, Bonn (Germany); New Energy and Industrial Technology Development Organization, Tokyo (Japan)
- OSTI ID:
- 357877
- Report Number(s):
- CONF-9705116--; ISBN 0-7918-1557-9
- Country of Publication:
- United States
- Language:
- English
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