EFFECT OF MULTI-AIR STAGED ON NOx EMISSION FROM COAL
COMBUSTION IN CIRCULATING FLUIDIZED BED COMBUSTOR
Chanwit Kamonrat1,*, Prapan Kuchonthara1,2,#
1
Fuels Research Center, Department of Chemical Technology, Faculty of Science,
Chulalongkorn University, Phayathai Road, Pathumwan, Bangkok 10330, Thailand
2
Center of Excellence on Petrochemical and Materials Technology, Chulalongkorn
University, 254, Phayathai Road, Pathumwan, Bangkok 10330, Thailand
*e-mail: Chanwit.Ka@student.chula.ac.th, #e-mail: prapan.k@chula.ac.th
Abstract
Coal combustion in circulating fluidized bed combustor (CFBC) is an interesting
process for generating power. NOx emission is an important problem from coal combustion
that causing the air pollution. Thus, the reduction of NOx emission should be considered. In
this study, multi-staged air injection in coal combustion was carried out in the CFBC to
minimize the NOx emissions. The effects of primary air and secondary air ratios and the
position of secondary air injection on NOx emissions were investigated. It was found that
NOx emission was reduced when the ratio of primary air and secondary air was decreased. In
addition, the extension of distance between primary and secondary injection points can
reduce the NOx emission. It can be concluded that NOx emission was reduced by operation
with the multi-air staged combustion mode in the CFBC.
Keywords: multi-air staged, coal combustion, NOx emission, CFBC, NOx reduction
Introduction
Coal is the widely used solid fuels in combustion process due to the high reserves and
its high heating value. One of the problem from coal combustion is NOx emission that
causing the air pollution. NOx in the atmosphere could be dissolved in rain or mist, that
resulting in acid rain. Moreover, NOx is destroyed stratospheric ozone, which served
protection UV rays [1]. NOx formation can be divided into 3 types [2]. The first is “thermal
NOx” that forms by the oxidation of atmospheric nitrogen at high temperature as following
equations,
O + N2  NO + N
N + O2  NO + O
N + OH  NO + H
(1)
(2)
(3)
The second is “prompt NOx”, mainly as NO (nitric oxide). This NO is generated from
the reaction between atmospheric nitrogen and hydrocarbon radicals from the fuels at hightemperature and fuel-rich combustion zone. The last type is “fuel NOx”. It is produced from
the combustion of coal-bounded nitrogen that releasing the N-containing volatiles such as
NH3, HCN. These N-volatiles can be oxidized to form NO. The fuel NOx is generated more
readily than thermal NOx [3].
Recently, the studies of NOx elimination have been widely investigated. From the
previous studies [4, 5], air-staged combustion was applied for coal combustion to minimize
the emission of NOx. Fan et al. [6] reported that the expanding distance between burnout air
injection port and main-burner nozzle could reduce NOx emissions. Spliethoff et al. [7] also
stated that under air-deficient condition, the NOx emissions could minimize. In addition,
Qian et al. reported that NOx emissions would be increased with the decreasing of oxygen
ratio [8]. Hence, air-staging design in the coal combustion process was an important
consideration for determining the emission of NOx. Wang et al. [9] has been studied the
multi-staged combustion of coal. The coal combustion was carried out in a multi-path air
inlet one-dimensional furnace. They reported that the multi-air staged was more effective to
reduce NOx emission than that the single-air staged combustion. However, the multi-air
staged combustion has not been carried out in the circulating fluidized bed combustor
(CFBC) that is the effective combustor and can be operated at the lower temperature
comparing with the one-dimensional furnace.
Consequently, in this study, the multi-staged air injection was applied in coal
combustion to minimize NOx emission. The experiment was carried out in the CFBC. The
effects of primary and secondary air ratios and the position of secondary air injection on NOx
emissions were investigated.
Methodology
A. Coal preparation
Coal was supported by Banpu Public Company Limited. Coal was ground by an
electric grinding machine and sieved for the size less than 1.2 mm. Proximate and ultimate
analyses of coal sample are shown in Table 1.
Table 1. Proximate and ultimate analyses of coal sample
Proximate analysis
Fixed carbon
Volatile matter
Moisture
Ash
Ultimate analysis
C
H
N
Oa
Sb
a
b
wt% (as received)
43.93
41.16
12.08
2.83
wt% (dry ash free)
71.75
7.08
1.27
19.66
0.24
by difference
bomb washing method
B. Coal combustion in CFBC
The coal combustion was carried out in a circulating fluidized bed combustor
(CFBC). The combustor consists of a riser column, a cyclone and a downcomer.
Configuration of the CFBC is illustrated in Fig. 1. The height of riser was 300 cm and the
inner diameter was 10 cm. Secondary air was fed into the riser at three difference position; at
the height of 100 cm, 200 cm and 240 cm, respectively by using the additional pipes
mounting at the riser. Temperature in riser was measured by K-type thermocouple. LPG was
used as the starter fuel for the riser by heating at 600 oC. Primary air was injected by a blower
and coal was fed into the riser by screw feeder. After temperature in the riser reached at 800
o
C, sand beds were fed into the riser then waiting until the temperature in CFBC became
steady. Flue gas during the process was collected and subsequently subjected to a flue gas
analyzer (Madur model GA-40T plus). Ratios of primary air and secondary air were varied as
80:20, 70:30 and 60:40 by volume. In addition, the secondary air was injected into the riser at
the different position at the height of 100 cm, 200 cm and 240 cm, respectively.
Figure 1. Schematic diagram of circulating fluidized bed combustion.
Results
A. Single-air staged coal combustion: Effect of secondary air position on NOx emissions.
The secondary air was supplied into the combustor as the ratio of primary air and
secondary air of 70: 30. The positions of secondary air injection were varied from as 1 m, 2 m
and 2.4 m, respectively.
Figure 2. Effect of position of secondary air injection on NOx and CO emissions.
From the experiments found that the increasing distance between primary air and
secondary air caused NOx emissions decreased. At the positions 1 m, 2 m and 2.4 m of the
secondary air injection, NOx emissions released 284 ppm, 213 ppm and 178 ppm,
respectively. In contrast, when the position of secondary air feeding increased that result in
CO emission increased which the concentrations of CO emission are 0.07%, 0.14% and
0.33%, respectively.
B. Single-air staged coal combustion: Effect of primary air and secondary air ratio on NOx
emissions
The primary air and secondary air ratio was adjusted while the total air was kept
constant and the position of secondary air injection was fixed at a height of 2.4 m.
80:20
70:30
60:40
Figure 3. Effect of primary air and secondary air ratio on NOx and CO emission.
The proportion between primary air and secondary air were varied as 80:20, 70:30
and 60:40. The result demonstrated that NOx emissions reduced with enhancing the
proportion of secondary air. NOx emissions were emitted during combustion as 213 ppm,
178 ppm and 119 ppm by following the proportion 80:20, 70:30 and 60:40, respectively.
Trend of the CO emission concentrations was different from NOx emissions. CO emission
increased with increasing of the proportion of secondary air which CO emission are released
0.11%, 0.33% and 0.69%, respectively.
C. Multi-air staged coal combustion
The operating conditions used in this part are shown in Table 2. Primary to secondary
air ratio was kept 80:20 for all experiment in this part. Secondary air was fed through three
injection ports. Air-staging ratio is proportion of secondary air feed into the reactor at
position of 1m, 2m, and 2.4m, respectively.
Table 2. Operating conditions for multi-staged combustion
Condition
M0b
M1
M2
M3
a
Ratio of secondary air
SA1 : SA2 : SA2.4a
0 : 0 : 1
0.25 : 0.25 : 0.5
0.25 : 0.5 : 0.25
0.5 : 0.25 : 0.25
SA1, SA2 and SA2.4 are the position of secondary air injection at 1 m, 2 m and 2.4 m, respectively.
M0 is refer to single-staged combustion.
b
In this part, the multi-staged combustion of coal was studied to compare with singlestaged combustion. From the experiments reflected that the multi-staged combustion released
NOx emissions less than single-staged combustion. Multi-staged combustion discharged NOx
emissions in the range 95-140 ppm while single-staged combustion released NOx emissions
was 213 ppm. The lowest NOx emissions achieved in condition M1 (95 ppm) and increased
when the proportion of secondary air at lower position increased. Moreover, the CO emission
found that single-staged combustion obtained CO emission higher than multi-staged
combustion. CO concentration was 0.11% in single-staged combustion and CO
concentrations were in range 0.01-0.07 % for multi-staged combustion.
Figure 4. Effect of multi-air staging in coal combustion on NOx emissions and CO emissions.
Figure 5. Temperature profile in CFBC with various operating conditions.
The temperature profiles show that the temperature of single-staged combustion was
higher than multi-staged combustion. The highest temperature of single-staged combustion
was 971 oC while the multi-staged combustion was in range 850-880 oC.
Discussion and Conclusion
A. Single-air staged coal combustion: Effect of secondary air position on NOx emissions.
Effect of position of secondary air injection on NOx and CO emissions in the outlet
stream was showed in Fig. 2. The result indicated that when extending the distance of
secondary air injection, the emission of NOx was decreased, whilst, the CO emission was
increased. At the secondary air injection point of 2.4 m, the NOx emission achieved to the
lowest value about 180 ppm. It can be explained that, at higher secondary air injection point,
the residence time of the flue gas in primary combustion zone was increased then the Ncontained gases were more decomposed [9]. Moreover, the expansion of primary combustion
area might enhance the Boudouard reaction (Eq. 4) which was the reaction between the
generated CO2 during combustion and carbon of char [10].
CO2 + C  2CO
(4)
B. Single-air staged coal combustion: Effect of primary air and secondary air ratio on NOx
emissions
Fig. 3 shows effect of primary air and secondary air ratio on NOx and CO emissions.
It was found that NOx emissions decreased with decreasing the ratio of primary air and
secondary air from 80:20 to 60:40. The lowest NOx emission was obtained at the ratio of
primary air and secondary air ratio of 60 : 40 with the NOx concentration about 130 ppm.
This can be explain that limited amount of oxygen in the primary combustion area might
reduce the oxidation of nitrogen resulting in reducing the NOx emission. However, airdeficient condition in the primary zone induced the higher CO emission due to the incomplete
combustion. In addition, the higher concentration of CO can be promoted the reaction
between NO and CO to generate CO2 and N2 as shown in Eq. (5) [11]. Then the
concentration of NO was reduced.
CO + NO  CO2 + (1/2)N2
(5)
C. Multi-air staged coal combustion
Fig. 4 shows NOx and CO emissions from the multi-air staging combustion in CFBC.
The result showed that the multi-air staging (M1 – M3) gave the lower NOx concentration
than that the single-air staging combustion (M0), especially secondary air feeding mode M1.
It also stated that, for multi-air staging combustion, the high portion of secondary air and at
high injection position would be obtained the lowest NOx emission. Consider the temperature
profile in the riser, as shown in Fig. 5, it can be observed that the multi secondary air
injection mode M1 gave the lowest temperature profile compared with the other modes. The
drop of temperature in the riser could not be promoted the formation of NOx. In addition, in
the multi-air staging combustion, amount of oxygen in each combustion zone would be
limited. Conversely, for multi-air staging combustion, the concentration of CO was increased.
This was probably due to the promotion of the reaction between NO and carbon of char (Eq.
6) when adding the secondary air [12].
C + NO  CO + (1/2)N2
(6)
Multi-air staging coal combustion is applicable in the CFBC. This technique is an
effective operation to control the emission of NOx. The longer distance between primary air
and secondary air injection could reduce the formation of NOx. Moreover, the multi-air
staging combustion provided an amelioration of NOx reduction as well as combustion
efficiency, restraining the CO problem.
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Acknowledgements: This research was supported by Fuels Research Center, Department of
Chemical Technology and Center of Excellence on Petrochemical and Materials Technology,
Chulalongkorn University.