Experimental verification of the impact of the air staging on the NOx production and on the temperature profile in a BFB

Abstract

The results of an experimental research on air staging in a bubbling fluidized bed (BFB) combustor are presented within this paper. Air staging is known as an effective primary measure to reduce NOX formation. However, in the case of a number of industrial BFB units, it does not have to be sufficient to meet the emission standards. Then selective non-catalytic reduction (SNCR) can be a cost-effective option for further reduction of the already formed NOX. The required temperature range at the place of the reducing agent injection for an effective application of the SNCR without excessive ammonia slip is above the temperatures normally attained in BFBs. The aim of this paper is to evaluate the impact of staged air injection on the formation of NOX in BFB combustors and to examine the possibility of increasing the freeboard temperature. Several experiments with various secondary/primary air ratios were performed with a constant oxygen concentration in the flue gas. The experiments were carried out using wooden biomass and lignite as fuel in a 30 kWth laboratory scale BFB combustor. Furthermore, the results were verified using a 500 kWth pilot scale BFB unit. The results confirmed that the air staging can effectively move the dominant combustion zone from the dense bed to the freeboard section, and thus the temperatures for an effective application of the SNCR can be obtained.

Experimental verification of the efficiency of selective non-catalytic reduction in a bubbling fluidized bed combustor

“…As mentioned above, the temperatures in the dense bed of BFB are usually in the range 800 − 900 °C, and therefore it is evident that it is difficult to reach sufficiently high temperatures in the freeboard zone for efficient SNCR application. However, our previous work [14] that has been done in a laboratory-scale BFB combustor confirmed that such temperatures can be reached in the freeboard section of a BFB combustor through an intensively staged supply of combustion air. Such an intensive air staging, which means that understoichiometric conditions are established in the primary combustion zone in the dense bed, is not typically applied in the industrial practice.…”