NO Reduction Scale of Three Typical Chinese Coals during Oxy-Coal Combustion

Oxy-coal combustion has become one of the most promising technologies for its ability to capture carbon dioxide and produce low pollutant emissions. In particular, the NO emission from oxy-coal combustion is found to be much lower than that in air combustion, but the NO reduction degree varies. In order to ascertain a rough NO reduction potential for Chinese coals, three types of coals (lignite, bituminous coal and anthracite) were combusted in a drop-tube furnace (DTF) in air and oxy-coal atmosphere during this study. The results showed that oxy-coal combustion could largely reduce NO emission down to 50% for the three coals. The NO reducing potential of oxy-coal technology for the lignite was higher than that for the bituminous coal and the anthracite. High content of volatiles in the lignite was considered as the major reason. NO removal was suppressed in the bituminous coal combustion due to the impacts of CaO and Fe2O3.


INTRODUCTION
Severe environmental issues such as green-house effect are mainly caused by the pollutant emission from coal combustion (Chiu et al., 2012;Yu et al., 2012).However, coal-fired power plants that account for more than 70% of the national electricity generation cannot be replaced for several decades in China (International Energy Agency, 2012).Therefore, it is imperative and eco-sustainable to control coal combustion emissions such as CO 2 , particulate matter (PM) and NO x efficiently.
Oxy-coal combustion technology uses recycled flue gas (RFG) instead of N 2 together with pure O 2 as the atmosphere, which could generate high concentration of CO 2 for compression afterwards (Toftegaard et al., 2012).Compared to the traditional air combustion combined with the ammonia adsorption CO 2 recovery method, oxy-coal combustion is more economical (Chen et al., 2012).Oxy-coal combustion has become one of the most applicable technologies in future for its ability to control the pollutants cheaply and effectively (Chan et al., 2012;Pan et al., 2012).
Previous researches explored nitrogen chemistry due to the potential to reduce NO in oxy-coal combustion but the decrements vary.When comparing the NO reduction capacity of oxy-coal combustion for different coals, the ratio of NO emission in oxy-coal to that in air combustion is introduced rather than the NO emission differences between oxy-coal and air combustion in order to avoid the influence of the nitrogen content in coal.Fig. 1 shows the ratio of NO exhausts from oxy-coal to that from air combustion ranging from 1.25% to 81.8% (Andersson et al., 2008;Shaddix and Molina, 2011;Stadler et al., 2011;Zhang et al., 2011;Liu et al., 2012).The great difference indicates that the coal properties and experimental conditions may largely influence NO reducing potentials.Lignite coal combustion in oxycoal atmosphere only produced 20% of that in air from the experiment of Andersson et al. (2008).Shaddix and Molina (2011) put forward that 20% NO was removed in oxy-coal combustion.And while Zhang et al. (2011) pointed that the NO x emission amount was 40% lower in oxy-coal than in air combustion, Stadler et al. (2011) revealed NO emission from oxy-coal combustion shares two-thirds proportion of that from air when German lignite was combusted in a vertical pilotscale furnace.Still Liu et al. (2012) stated that NO emission from oxy-coal combustion with partial CO 2 removal from the RFG was decreased to 1.25% of that in air combustion.
The NO decrement in oxy-coal combustion is mainly caused by the reduction of recycled NO.And it is further determined by the properties of the coal used.However, the relationship between the NO reduction capacity and the coal properties is still controversial.The coal rank or nitrogen content was considered as the important factor influencing the NO reduction from different researches.It is difficult to speculate on the reduction scales of different types of coal, although they may play an instructive and guiding role in NO x control in oxy-coal combustion.It is essential to have a rough idea of the NO decrement in Chinese coals before the utilization of oxy-coal combustion in China.

T o k y o I n s t . T e c h . S o u t h e a s t U n iv . R W T H C A N M E T I H I A N L
C h a lm e s U n iv .T e c h .Fig. 1.The ratios of NO exhausted in oxy-coal to that in air combustion from the literatures (Andersson et al., 2008;Shaddix et al., 2011;Stadler et al., 2011;Zhang et al., 2011;Liu et al., 2012).
In this work, three different types of Chinese coals were investigated in a drop-tube furnace (DTF) in air and in oxycoal conditions separately.The NO concentration in the flue gas was measured to understand the ratios of NO exhausts in oxy-coal to that in air combustion.The relationship between the oxy-coal NO reduction capacity and coal properties was also discussed.

EXPERIMENTAL
A Chinese lignite, a bituminous coal and an anthracite, with grain diameter ranges from 45 to 100 μm, were used as the raw samples.The proximate and ultimate analysis of the coals is presented in Table 1.The volatile contents in both lignite and bituminous coal are over 30% in this study.The alkali and alkali earth metal (AAEM) compositions from X Ray Fluorescence Spectrum Analysis (XRF) data of the three coals are also presented in Table 2. Na and K contents in the three coals are nearly the same.The contents of Ca and Fe in bituminous coal are relatively high and even higher than those in the lignite.All three coals were combusted in a DTF in both air (O 2 and N 2 , O 2 /N 2 ) and oxy-coal (O 2 with recycled flue gas, O 2 /RFG) atmosphere at 1373 K to avoid the impact of thermal NO.A brief schematic of the DTF is shown in Fig. 2. The height of the reactor tube was 210 cm, and the inner diameter was 6.2 cm.The feeding rate of each coal was 0.3 g/min.The total inlet gas flow rate was 5000 mL/min.The O 2 fraction was 21%, 27% and 32% (Vol/Vol) in each combustion atmosphere.The recycled ratio in O 2 /RFG was 0.8.
The flue gas was passed through a filter and a drying bottle.Then it was monitored by two gas analyzers.CO, CO 2 and O 2 were detected by a custom-made Madur gas analyzer with a detection range of 20%, 100% and 50%.CO and CO 2 sensors use non-disperse infrared (NDIR) method while O 2 sensor uses the electrochemical method for test.NO was measured by a gas analyzer using electrochemical sensors in the range of 0-5000 10 -6 Vol/Vol.The CO concentration (not shown here) revealed that the combustions of coal and char were complete in all combustion processes.

RESULTS AND DISCUSSION
Fig. 3 shows the ratios of NO exhausts from oxy-coal to that from air combustion for the lignite, bituminous coal and anthracite at O 2 fraction of 21%, 27% and 32%.It can also be seen that in O 2 /RFG atmosphere, lignite combustion emits 30%-40% NO of that in O 2 /N 2 .This proportion in bituminous coal and anthracite combustion is 36%-48% and 38%-42%, respectively.It indicates more than half of the NO is reduced in oxy-coal combustion.
Fig. 3 also shows the relationship of the ratios that NO released from oxy-coal to air combustion for the three coals.The ratio of NO emission between oxy-coal and air combustion for the lignite is the lowest under all conditions, which is corresponding to the conclusion that the higher volatile coal may lead to more NO reduction (Álvarez et al., 2012).However, their theory cannot apply to the result of the bituminous coal in the present study.For example, the ratio of NO exhausts from oxy-coal to that from air combustion for the bituminous coal is 48%, which is higher than that of anthracite (40%) at O 2 fraction of 32%.
In order to clarify the effects of these coal properties on NO reduction, the details of the reduction reaction are discussed.From the overall reducing reaction of NO (Eq.( 1)), the reducing substance could reduce NO into N 2 in the combustion process.The more reducing matter and longer reaction time, the more NO would be removed.The recycled NO may be reduced by both homogeneous and heterogeneous reactions.NO would be reduced by CH i (hydrocarbon radicals), H 2 and HCN, which are formed during the devolatilization process through homogeneous reactions, as shown in Eq. ( 2) and Eq. ( 3).In the present experiment, the homogeneous reaction time for the three  The lignite has the highest volatile content of the three coals, which may cause more NO to be reduced into N 2 through homogeneous reactions during its combustion than that in the bituminous coal and anthracite combustion.In addition, the reaction time of NO heterogeneous removal for the lignite char is longer than for other chars because the higher-volatile-content coal char is spongier (Gao et al., 2010).Both of these result in a large amount of NO removal by the lignite coal.The NO reduction potential of the anthracite is lower than that of the lignite because of the combined impacts of its lower volatile content and denser char structure.
CaO + coal-N → CaC x N y + CO (5) In the bituminous coal combustion the nitrogen chemistry could be affected by the high contents of CaO and Fe 2 O 3 .For example, the components of the Ca-N intermediates can be formed by the reaction of nitrogen in coal (coal-N) with CaO firstly (Eq.( 5)).Then the oxidation of these intermediates may lead to a direct formation of NO through Eq. ( 6) (Liu et al., 2014).Moreover, carbon as one of the reducing substances is also consumed in the reaction, which weakens NO heterogeneous removal afterwards.The oxidation of HCN (Ren et al., 2011) or NH 3 (Yang et al., 2010) may be accelerated by the catalytic effect of CaO.Their opportunities to reduce NO are decreased and consequently NO homogeneous reduction is suppressed.Fe element catalyzes the oxidation rate of nitrogen compounds (Niu et al., 2011).Ca and Fe could also enhance conversion of coal-N to NO (Gu, 2013).During the bituminous coal combustion, NO reduction amount may be decreased due to the changes caused by high proportions of CaO and Fe 2 O 3 .Therefore the ratio of NO emission between oxy-coal and air combustion for the bituminous coal is higher than that for the anthracite.

CONCLUSIONS
Three Chinese coals were combusted in a DTF under both air and oxy-coal atmosphere.Less than 50% NO was exhausted from oxy-coal combustion comparing with that in air combustion.The NO capacity of the lignite was higher than that of the bituminous coal and anthracite in the present study, which was mainly caused by the highest volatile content.High contents of CaO and Fe 2 O 3 suppressed NO reduction during the bituminous coal combustion, which resulted in a lower NO capacity of the bituminous coal than that of the anthracite.

Fig. 3 .
Fig.3.The ratios of NO exhausted in oxy-coal to that in air combustion for the three coals.

Table 1 .
Proximate and ultimate analysis of coals.