Leckner, Bo

Lyngfelt, Anders

Åmand, Lars-Erik

Chalmers Publication Library

Chalmers Publication Library
Obtaining Low N2O, NO, and SO2 Emissions from
Circulating Fluidized Bed Boilers by Reversing the Air Staging Conditions
This document has been downloaded from Chalmers Publication Library (CPL). It is the author´s
version of a work that was accepted for publication in:
Energy & Fuels
Citation for the published paper:
Lyngfelt, A. ; Åmand, L. ; Leckner, B. (1995) "Obtaining Low N2O, NO, and
SO2 Emissions from Circulating Fluidized Bed Boilers by Reversing the Air
Staging Conditions". Energy & Fuels, vol. 9(2), pp. 386-387.
http://dx.doi.org/10.1021/ef00050a027
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386 Energy & Fuels l9!35,9, 386-387 
Obtaining Low N20, NO, and SO2 Emissions from 
Circulating Fluidized Bed Boilers by Reversing the Air 
Staging Conditions 
Anders Lyngfelt,* Lars-Erik h a n d , and Bo Lecher 
Department of Energy Conversion, Chalmers University of Technology, 
412 96 ateborg, Sweden 
Received September 21, 1994 
A method for decreasing NzO without increasing the 
emission of the other pollutants NO, and SOz, reversed 
staging, was investigated in the 12 Mw circulating 
fluidized bed boiler at Chalmers University of Technol- 
ogy. It was possible to reduce the emission of NzO to 
one-fourth (25 ppm) and NO to half (about 40 ppm) 
compared to normal staging and normal temperature, 
without significantly affecting the sulfur capture ef- 
ficiency (being about 90%). 
Air staging, which is normally used in circulating 
fluidized bed boilers, means that only a part of the 
combustion air, primary air, is added to the bottom zone, 
resulting in a lower oxygen concentration in the bottom 
part, while the secondary air results in more oxidizing 
conditions in the upper part of the combustion chamber 
and the cyclone. The principal idea with reversed 
staging is to reverse the conditions in top and bottom, 
Le., to decrease the oxygen concentration in the upper 
part and to increase it in the bottom part. 
It is well-known that the emissions of NO,, SOz, and 
N20 can be significantly decreased or increased by 
changes in operational parameters like bed temperature 
and air supply. The problem is that, while a measure 
taken to decrease one of the emissions may prove 
successful, it has the opposite effect on one or two of 
the others.' 
The method is based on the difference in the effect of 
oxygen on the various emissions depending on the 
location in the combustor (bottom part vs upper part 
and cyclone) and a reversal of the conditions normally 
used was found to give a considerably lower emission 
of NO and Nz0 without increasing SOz. 
A circulating fluidized bed boiler is normally operated 
with a significant amount of secondary air, and the 
bottom part of the combustion chamber is, to a great 
extent, under reducing conditions, while oxidizing con- 
ditions in the upper part are necessary to obtain 
acceptable sulfur capture and combustion. In order to 
reverse the situation, i.e., to obtain more oxidizing 
conditions in the lower part and less oxidizing conditions 
in the upper part, the following strategy was used. 
The combustor air ratio, i.e., the air ratio in combus- 
tion chamber and cyclone, was kept close to unity. No 
secondary air was used and all air was added in the 
bottom zone, except for some air which was added for 
final combustion after the cyclone giving a total air ratio 
of 1.2. 
The increased air ratio of the bottom part makes this 
part more oxidizing compared to normal staging. The 
gradual consumption of oxygen with height decreases 
(1) Lyngfelt, A,; k k n e r , B. Fuel 1993, 72, 1553-1561. 
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Figure 1. Emissions of NzO, NO, and SO2 for normal air 
staging and reversed staging. 
the average oxygen concentration from the bottom and 
upwards, approaching very low oxygen concentrations 
in the top zone of the combustion chamber and the 
cyclone, since the combustor air ratio is kept at about 
unity. 
The 12 MW circulating FBB used for the experiments 
has the feature that air can be added in the outlet of 
the cyclone for final combustion. The coal was a 
bituminous coal with a sulfur content of 1.6% (dry, ash- 
free). 
The test cases were run a t constant load and the total 
air ratio was kept at 1.2. The bed temperature was 850 
"C and the limestone addition molar CdS ratio was 
about 2.5. 
About 60% primary air and 40% secondary air (2.2 
m above air distributor) was used in the reference case 
(normal staging). In the case of reversed staging there 
was no secondary air in the combustion chamber, but 
about 20% of the total air was added after the cyclone 
for final combustion. The combustor air ratio, before 
addition of final combustion air, was kept a t about unity. 
The emissions of Nz0, NO, and SOz in the two cases 
are shown in Figure 1. Additional runs of the reversed 
staging case under slightly varied conditions gave 
somewhat lower NzO emissions and somewhat higher 
NO emissions than in Figure 1. 
0887-0624/95/2509-0386$09.00/0 0 1995 American Chemical Society 
Communications 
0 
Energy 13% Fuels, Vol. 9, No. 2, 1995 387 
fected. The CO emission, however, increased from 
about 50 to 300 ppm. The CO emission was found to 
be strongly dependent on the combustor air ratio and 
could be halved just by increasing this with about 1% 
without the low N2O and NO emissions being affected. 
This is seen in Figure 2, where the effect of varied 
combustor air ratio on the emissions is shown. 
The results indicate that it is possible to separate the 
effects of reducing/oxidizing conditions on the emissions 
by producing these conditions selectively in the bottom 
and top parts of the combustor. Thus, a major decrease 
of the N2O and NO emissions was obtained without 
increasing the SO2 emission. 
A more detailed description of the test conditions and 
test results is given by Lyngfelt et aL2 
Acknowledgment. This work has received financial 
support from the Swedish National Board for Industrial 
and Technical Development (NUTEK). 
EF940179X 
(2) Lyngfelt, A.; h a n d , L.-E.; Lecher, B. Low N20, NO and SO2 
emissions from circulating fluidized bed boilers. To be presented at 
the 13th International Fluidized Bed Combustion Conference, Orlando, 
FL, May 7-10, 1995. 
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Obtaining Low N<sub>2</sub>O, NO, and SO<sub>2</sub> Emissions from Circulating Fluidized Bed Boilers by Reversing the Air Staging Conditions

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