This paper reports an experimental investigation of the influence of operating parameters on the formation and reduction of NOx and N20 from coal combustion, ...
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PII: S0016-2361(96)00045-2
Fuel Vol. 75 No. 8, pp. 971-978, 1996 Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0016-2361/96 $15.00+0.00
Influence of operating parameters on NOx and N20 axial profiles in a circulating fluidized bed combustor Luis F. de Diego*, Carlos A. Londonot, Xiao S. Wang and Bernard M. Gibbs Department of Fuel and Energy, University of Leeds. Leeds LS2 9JT, UK (Received 23 October 1995; revised 19 February 1996) This paper reports an experimental investigation of the influence of operating parameters on the formation and reduction of NOx and N20 from coal combustion, carried out in a circulating fluidized bed combustor 6.2 m high and 0.161 m i.d. The main operating parameters studied were temperature, excess air factor, secondary/total air ratio, limestone addition and coal particle size. It was found that the NOx emission increased whereas the N20 emission decreased when the temperature increased, but the effect of the temperature depended on the excess air factor. Both the N O x and the N20 emissions increased with increasing excess air factor and decreased with increasing secondary/total air ratio. Moreover, the NOx emission increased and the N20 emission decreased with limestone addition and with increasing coal particle size. Unlike previous studies in which only the exhaust gases were the focus, here an attempt was made to analyse the NOx and N20 axial profiles to improve understanding of the formation and destruction of these pollutants. Analyses of the NO~ and N20 concentration profiles along the riser height suggested that the NO~ formed in the bottom was gradually reduced along the height of the combustion chamber and that the N20 concentration increased from the bottom to the top of the combustion chamber for all the operating conditions. Copyright © 1996 Elsevier Science Ltd. (Keywords: nitrogen oxides; circulating fluidized bed combustors; axial profiles)
The combustion of fossil fuels in fluidized bed combustors (FBCs) has seen rapid development during the past decade. This technology coupled with sorbent addition allows coals of different rank, even with high sulfur and ash contents, to burn in a clean way. Coal combustion in circulating fluidized bed combustors (CFBCs) is a relatively new technology for a great variety of fuels and installation scales. This technology has some advantages over bubbling fluidized beds, such as higher combustion efficiencies and more effective control of SO2 and NOx emissions. However, these advantages have been offset by higher emissions of N2 O1-4, which has been recognized as contributing to the stratospheric ozone depletion and the greenhouse effect. The emissions of NOx and N 2 0 from FBCs depend on many operating parameters 1'3'4 and fuel types 3'5'6, and many homogeneous and heterogeneous reactions are responsible for the formation and destruction of these pollutants 7. Although recent experimental and modelling studies have clarified some of the important issues concerning the formation and destruction of NOx and N 2 0 in the combustion processes, the exact mechanism of coal-nitrogen conversion is still unknown. It is now generally agreed that a higher temperature * Current address: Instituto de Carboqimica (CSIC), P.O. Box 589, 50080 Zaragoza, Spain t Current address: Universidad Nacional de Colombia, Apdo a6reo 1027, Medellin, Colombia
leads to a lower emission of N 2 0 and a higher emission of NOx 1'3'5'6'8-10, and both the NOx and N 2 0 emissions increase with increasing excess air factor 1.3. 6. .8 9 . Air staging is a techni.que used successfully to reduce the 9 II 13 NOx emission ' - , but its effect on the N 2 0 emission is unclear, Lu e t al. 13 found in a pressurized FBC that the N 2 0 emission increased with increasing secondary/total air ratio. Some researchers 3'6 found no significant change in the N 2 0 emission, whilst others'9 H 1' 4 reported a decrease in the N 2 0 emission together with the NOx reduction. Lyngfelt and Leckner 11 found that the effect of air staging depended on the bed temperature. At a higher temperature, the air staging effect on the NOx emissions was found to be more pronounced, whereas its effect on the N 2 0 was less significant. Moreover, some workersl2'15 found that the effect of air staging depended on staging position. Limestone addition to an FBC system has been shown to reduce the N 2 0 emission and to increase the NOx emission 6'9'11'16-18. However, Brown and Muzio 19 and Cabrita e t al. 2° found that limestone addition did not affect the N 2 0 emission, whilst Mj6rnell e t al. 21 reported that the change in the N 2 0 emission on limestone addition depended on the type and size of the limestone, and Zhao e t al. 12 observed that the NOx emission for a petroleum coke decreased on limestone addition. This investigation was intended to examine the effects of major operating parameters, which are sometimes confusing and misleading in the literature, on the NOx
Fuel 1996 Volume 75 Number 8
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Influence of operating parameters on NOx and N20: L. F. de Diego et al. and N20 emissions from a CFB combustor. Unlike other studies in which the exhaust gases were the focus, detailed analyses of the axial profiles of NOx and N20 in the riser of a CFBC under different operating conditions are reported here. The objective of this study was to improve understanding of the formation and destruction of these pollutants under fluidized bed conditions. EXPERIMENTAL Figure 1 shows a schematic diagram of the CFB combustor used in this investigation. The system comprised a riser 0.161 m i.d. and 6.2 m high, two highefficiency cyclones in series, an external heat exchanger (EHE) and an L-valve. The whole system was made of stainless steel and was insulated with ceramic fibre on the outside surface. Primary air was supplied at the riser bottom through an air distribution plate, while secondary air was injected through opposing injectors located 1.83 m above the primary air distribution plate. Solids entrained by the combustion gases from the riser were collected by the primary cyclone and fed to the EHE via a dip leg. After exchanging heat with water flowing
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explosion relief device
secondary cyclone - primary cyclone
ash collector
- EHE exhaust - water tube coils E
