Abstract
Photo-chemical smog and acid rain formed from many pollutants including NOx are serious problems that have attracted much attention due to their negative influences on the atmosphere, plants, animals and even building materials. Effective measures of controlling NOx emissions are necessary. In this study, a computational fluid dynamics (CFD) software, Ansys Fluent 14.5, has been applied to research the processes of South American coal (SAm) reburning and cornstalk reburning. The influences of reburn zone excess air coefficient, reburning fuel fraction (Rff) and the secondary air temperature on the furnace combustion and NOx reduction have been determined. Results show that, in the simulated data range, the lower reburn zone excess air coefficient, the greater the rate of denitration for both SAm reburning and cornstalk reburning. The highest rates of denitration for SAm reburning and cornstalk reburning were 56.15% and 66.89%, respectively, in comparison to the conventional combustion. The denitration rate increases with the increase of reburning fuel fraction. However, when the reburning fuel fraction increases beyond a certain level, fuels within the furnace will undergo incomplete combustion. Under the premise of the fuel burnout, a relatively good case occurs at the reburning fuel fraction of 20% for the two kinds of reburning, and the NOx removal rate is 60.57% for cornstalk reburning, which is 7.61% higher than that of the SAm reburning. Temperature also has certain influences on the denitration effect and it shows that, in the lower temperature range, the higher the temperature of the secondary air, the higher the denitration rate. However, as temperature reaches a certain value, the denitration effect is no longer enhanced but reduced. For the two kinds of reburning, the case with the secondary air at 500 K is a relatively good one, and the NOx removal rate reaches 66.36% for cornstalk reburning, while it is 55% for SAm reburning. Overall, cornstalk reburning provides a higher NOx reduction rate in comparison to SAm reburning.
Highlights
Fuel reburning is one of the most promising NOx control technologies [1]
South American coal (SAm) combustion without reburning, SAm combustion with SAm reburning, and SAm combustion with cornstalk reburning at different reburn zone excess air coefficients (α), 0.87, 0.9, 0.93 and 0.96, are discussed respectively
In SAm reburning and cornstalk reburning cases, temperatures with different reburn zone excess air coefficients decrease from the primary zone down to the burnout zone
Summary
Fuel reburning is one of the most promising NOx control technologies [1]. In the past few years, a lot of scholars have dedicated themselves to gas reburning and found that using gas as reburning fuel could decrease the NOx emissions to a certain extent [2,3,4,5,6], but the rise of gas prices inevitably restricts the utilization of gas reburning [7]. SAm is derived from the paper written by Singh and Nimmo [30], and cornstalk used in this paper is from Hebei province of China, with its proximate and ultimate analyses provided by the combustion research laboratory of Hebei University of Technology It shows that the calorific value (CV) of SAm is greater than that of cornstalk, which determines that a higher feed rate of the cornstalk is needed in order to generate the same heat input as the SAm. The proximate analysis reveals a significantly higher volatile content in the cornstalk than that in the SAm. The cornstalk with higher volatile content may be of benefit in the reburning process. The influences of reburn zone excess air coefficient (α), reburning fuel fraction (Rff), and the secondary air temperature on the denitration effect for both SAm and cornstalk reburning are conducted in this work.
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