Preliminary Research on the Effects of Coal Devolatilization and Char

Dec 9, 2016 - †State Key Laboratory of Coal Combustion, School of Energy and Power Engineering and ‡Department of New Energy Science and Engineeri...
0 downloads 11 Views 3MB Size
Download PDF
Article pubs.acs.org/EF

Preliminary Research on the Effects of Coal Devolatilization and Char Combustion Processes on the Emission of Particulate Matter during Lignite Combustion under Air and Oxy-fuel Conditions Chang Wen,*,†,‡ Bin Fan,† Wenyu Wang,† Xianpeng Zeng,† Ge Yu,† Weizhi Lv,† and Minghou Xu*,† †

State Key Laboratory of Coal Combustion, School of Energy and Power Engineering and ‡Department of New Energy Science and Engineering, School of Energy and Power Engineering, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan, Hubei 430074, People’s Republic of China ABSTRACT: The pulverized coal combustion in both air and oxy-fuel conditions generally experiences successive coal devolatilization and char combustion processes. The effects of these two processes have important influence on the emission characteristics of particulate matter. With some different characteristics of lignite from the higher rank coal, including the high contents of organically bound cations and the apparent char−CO2 gasification reaction upon oxy-fuel combustion, the effects of both coal devolatilization and char combustion processes on the emission characteristics of particulate matter during oxy-fuel combustion of lignite would accordingly be distinctive. A typical Chinese lignite was devolatilized in a N2 or CO2 atmosphere to generate N2−char and CO2−char, respectively. Raw coal and the two chars were burned in a drop-tube furnace under both air and oxy-fuel conditions. Afterward, the yields and inorganic compositions of segregated PM10 were carefully analyzed. The results have shown that the devolatilization in the N2 condition promotes the emission of sub-micrometer particles (PM0.5) and coarse particles (PM1−10) but the devolatilization process in CO2 inhibits their emission. N2−char and CO2−char present entirely different combustion behavior and, thus, reflect the discrepant emission characteristics of PM10. The rough higher yields of PM0.5 and PM1−10 from N2−char combustion compared to raw coal combustion may be driven by the higher particle combustion rate and/or temperature of N2−char. Furthermore, the quite severe gasification reaction of CO2 with char during char preparation results in an inhibited combustion rate of CO2−char, which can explain the case in which much less PM0.5 and PM1−10 are generated from CO2−char combustion than those from raw coal and N2−char combustion. The apparent effects reveal that more detailed work can be continued to further explore this research. combustion. The research of Rathnam et al.17 found that the volatile yield measured in a DTF in a CO2 atmosphere was higher than that in N2. Devolatilization experiments in thermogravimetric analysis (TGA) revealed an additional mass loss in CO2 compared to that in N2, indicating an apparent char−CO2 gasification reaction at temperatures exceeding 1030 K. The similar char−CO2 reaction during the devolatilization of lignite in a CO2 atmosphere was also observed by Liu et al.18 The pulverized coal combustion in both air and oxy-fuel conditions generally experiences successive coal devolatilization and char combustion processes. These two processes have important influence on the emission characteristics of particulate matter, and the influence in air combustion has been considered by some earlier studies. Zhang et al. reported that the vaporized organically bound elements released during the pyrolysis process of bituminous coals and anthracite contributed to ultrafine particles (PM0.1) dominantly.19,20 The carbonaceous matrix in lignite is associated with many organically bound cations; therefore, the emission characteristics of particulate matter during the combustion of lignite should largely be affected by the devolatilization process. The effects of the char structure and char combustion behavior on

1. INTRODUCTION Currently, the emission of particulate matter is a serious environmental problem in China,1−6 where coal combustion contributes to approximately 20% of the total particulate matter with the aerodynamic diameter of less than 2.5 μm (PM2.5) emission.7,8 There is a huge reservation of lignite in the Inner Mongolia autonomous region of north China. It accounts for approximately 10% of Chinese coal reserves and is being burned in many thermal power plants in north China. Inorganic constituents in lignite generally include organically bound cations, dispersed fine particles, and discrete minerals.9,10 A portion of organically bound cations and fine particles are vaporized from coal particles to form ultrafine particles in the coal combustion process,11 while many discrete minerals are transferred to form coarse ash particles. Therefore, the apparent pollution of particulate matter must be caused by the combustion of Chinese lignite in a conventional air-fired boiler and retrofitted oxy-fuel boiler, which might be used for mitigating the emission of greenhouse gases in the near future.12,13 With the switch from air to oxy-fuel conditions, the influence of the atmosphere on char properties and combustion behavior has already been widely studied and some research has focused on lignite.14−18 Khatami et al.14 reported that, as the N2 was replaced with CO2, the combustion of lignite in a drop-tube furnace (DTF) was obviously less intense and increasing the oxygen content in a CO2 atmosphere increased the intensity of © XXXX American Chemical Society

Received: August 27, 2016 Revised: December 8, 2016 Published: December 9, 2016 A

DOI: 10.1021/acs.energyfuels.6b02165 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels the ash formation were also studied. Kang et al.21 prepared cenospheric and non-cenospheric chars from bituminous coal and found that the air combustion of cenospheric char yielded fewer coarse particles and more sub-micrometer particles than the other. In comparison to the combustion of raw biomass, the air combustion of its corresponding biochar studied by Gao and Wu causes the reduction of the PM1 yield but the increase of PM1−10.22 With some different characteristics of lignite from the higher rank coal, including the high contents of organically bound cations and the above-mentioned apparent char−CO2 gasification reaction upon oxy-fuel combustion,18,19 the effects of both coal devolatilization and char combustion processes on the emission characteristics of particulate matter during oxy-fuel combustion of lignite would accordingly be distinctive. However, the work has not yet been considered by any research. The direct quantification of particulate matter emitted from the stage of volatile combustion is confronted with the practical difficulty of experimental design. Instead, in this paper, a typical lignite in Inner Mongolia (IM lignite) and its chars devolatilized in a N2 or CO2 atmosphere were combusted in both oxy-fuel and air conditions to collect and analysis the particulate matter. The comparison of results from the combustion of coal and chars was attempted to understand the effects of the coal devolatilization and char combustion processes on the emission characteristics of particulate matter. It is also a broad effort to match the scientific and industrial interest in the oxy-fuel combustion of Chinese lignite.

represent the intermediate state of coal combustion. The influence of the devolatilization process in the emission of particulate matter would be discussed via the comparison between the results of raw coal combustion and char combustion, considering the practical difficulty in collecting the inorganic particulate matter emitted from the combustion stage of volatile. Table 1 shows the properties of coal and chars. Note that a fraction of volatile matter (VM) still remains in the chars, possibly because it is difficult to be released from isolated pores in the short residence time of the DTF. Similar results were found in some previous studies.22,23 Raw coal, N2−char, and CO2−char were combusted in the same DTF at the same temperature of 1573 K. The gas ambience of combustion was 21 vol % O2/79 vol % N2 (AIR), 21 vol % O2/79 vol % CO2 (OXY21), and 29 vol % O2/71 vol % CO2 (OXY29), respectively. Note that the OXY29 condition was performed to match the adiabatic flame temperature with the AIR condition.24,25 The coal feeding rate was 0.3 g/min, and the feeding rates of N2−char and CO2−char were 0.18 and 0.07 g/min, respectively, to ensure equal ash input with raw coal. After the combustion of coal and chars, a watercooled isokinetic sampling probe with nitrogen dilution was used to collect the PM10, which was segregated into 13 size fractions by a lowpressure impactor (LPI, Dekati, Ltd., Finland) after removing coarse ash particles of >10 μm using a cyclone. The sectional area of the furnace is approximately 10 times that of the isokinetic sampling probe; therefore, the ash balance in the experiments is difficult to be calculated. The detailed methodology of char preparation and PM10 collection can be found in our earlier research.26−28 2.2. Methods of Sample Analysis. The Brunauer−Emmett− Teller (BET) surface area and total pore volume of chars were analyzed by a Micromeritics ASAP 2020, which tests pore sizes between 0.17 and 300 nm via the N2 isothermal adsorption− desorption (77 K) method. The PSDs of the studied coal and chars were measured by the Malvern 2000 particle size analyzer. The coal and chars were ashed in an Emitech K1050X plasma asher at 150 °C to generate low-temperature ashes (LTAs), and the inorganic elemental compositions of LTAs were measured by Eagle III X-ray fluorescence (XRF). The above tested data are shown in Table 1. A Sartorius microbalance (0.001 mg) was used to weigh the alumina foils placed in every stage of the LPI, and the weight difference of each stage before and after the experiment was calculated to receive the PSDs and yields of PM10. Another batch collected PM10 through polycarbonate foils to analyze the elemental compositions using XRF. computer-controlled scanning electron microscopy (CCSEM) was chosen to identify the occurrence mode of typical elements in inorganic minerals of the IM lignite, and the methodology of CCSEM analysis has been introduced in our previous research.29−31

2. EXPERIMENTAL SECTION 2.1. Conditions of Sample Preparation and Combustion. Pulverized and sieved to 45−100 μm, the IM lignite is named as “raw coal”. It experienced a devolatilization process in either a N2 or CO2 condition in a DTF to generate “N2−char” and “CO2−char”, respectively. The furnace temperature for char preparation was 1573 K, and the in-furnace residence time of char particles was less than 1.0 s. The char products were collected by the glass fiber filter placed at the bottom of the DTF. The particle size distributions (PSDs) of raw coal and two chars were plotted in Figure 1; it presents that the chars experience slightly swell rather than fragmentation. Artificial changes to the physical characteristics of char during the collection process were avoided as much as possible. Therefore, the slightly swelled chars can be considered as the intermediate product of raw coal combustion, and the particle size and carbon contents of chars can accordingly

3. RESULTS AND DISCUSSION 3.1. PSDs of PM10. The PSDs of PM10 generated from three combustion conditions are given in Figure 2. They present some similar characteristics. The peak size of ultrafine mode particles is ∼0.136 μm, and the trough is 0.353−0.535 μm; meanwhile, the peak size of coarse mode particles is ∼8.8 μm. The ultrafine mode particles are mainly generated by the vaporization, nucleation, and subsequent condensation and coagulation of inorganic constituents1,32,33 and are generally less than ∼0.5 μm (PM0.5) according to the observation of PSDs. A clear tendency of the concentration with the sequence of N2−char > raw coal > CO2−char is presented in the range of particle sizes between 1 and 10 μm, for at least the AIR and OXY29 conditions. This implies that those particles (PM1−10) possess similar formation mechanisms, which were generally reported to be the char fragmentation and mineral coalescence.1,34−36 Owing to the low concentration (Figure 2) and relatively complicated formation mechanisms, this paper pays no attention to the emission characteristics of PM0.5−1.

Figure 1. PSDs of raw coal and its prepared N2−char and CO2−char. B

DOI: 10.1021/acs.energyfuels.6b02165 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels Table 1. Properties of IM Lignite and Its Prepared N2−Char and CO2−Char proximate analysis (wt %, db) raw coal N2−char CO2−char

raw coal N2−char CO2−char a

Aa

VMb

FCc

13.60 25.58 66.05

33.80 15.22 13.66

52.60 59.20 20.29

ultimate analysis (wt %, db) C

Od

H

N

S

BET (m2/g)

pore volume (cm3/g)

63.53 4.57 15.65 0.84 1.81 64.49 0.64 6.48 0.66 2.16 265.8 26.71 0.33 4.55 0.03 2.33 124.2 chemical composition analysis of low-temperature ashes (wt %)

0.111 0.133

Na2O

K2O

SO3

P2O5

CaO

Fe2O3

MgO

Al2O3

SiO2

TiO2

others

0.86 2.70 1.92

0.62 0.72 1.02

25.43 13.26 7.19

0.48 0.49 0.53

6.50 5.88 6.64

11.99 8.26 13.34

2.26 3.11 2.96

12.47 13.23 15.47

38.49 49.87 47.63

0.38 0.37 0.46

0.52 2.13 2.83

Ash. bVolatile matter. cFixed carbon. dCounted by difference.

Figure 2. PSDs of PM10 generated from the combustion of the IM lignite and chars: (a) AIR, (b) OXY21, and (c) OXY29.

Figure 4. Yields of (a) PM0.5 and (b) PM1−10 generated from the combustion of the IM lignite and chars.

Figure 3. Elemental compositions of (a) PM0.5 and (b) PM1−10 generated from the combustion of the IM lignite and chars.

formation of PM0.5 from raw coal combustion appears to be markedly promoted in the OXY29 condition compared to the AIR and OXY21 conditions. This phenomenon can be explained from the following perspectives. The reaction of CO2 gasification with char can be enhanced in the high CO2 concentration and high combustion temperature,37 especially for the lignite. In contrast to the AIR and OXY21 conditions, the gasification reaction in the OXY29 condition of raw coal can promote a stronger local reducing environment and then facilitate the vaporization of char-bound elements.38,39 Consequently, the release of alkali metals and the reaction with SO2 would be promoted to form the sulfates,40 which causes the high mass of Na and S in PM0.5, shown in Figure 5. In addition, Figure 5a shows that the vaporization of Ca, Mg, Si, and Al are also promoted by the stronger local reducing environment in the OXY29 condition of raw coal. In contrast, with the comparable adiabatic flame temperature between the OXY29 and AIR conditions, a similar degree of char fragmentation and mineral coalescence causes relatively similar yields of PM1−10 generated from the combustion of raw coal in the OXY29 and

3.2. Compositions of Inorganic Elements in PM0.5 and PM1−10. Figure 3 reveals that Na and S are the predominant elements in PM0.5, regardless of the combustion condition. The mass of Na and S accounts for >50% in PM0.5, of which the other 30% is Fe, Mg, and Si. The PM1−10 is mainly composed of Ca, Fe, Si, and Al; these elements account for >80% in all combustion conditions. It appears that Fe−Ca aluminosilicate is the predominant composition in PM1−10 from the IM lignite combustion. Furthermore, the mass fractions of Ca and Fe in PM1−10 from N2−char combustion are higher than those from raw coal combustion, whereas the mass fraction of Si is apparently lower (Figure 3b). These observations may be related to the effect of the coal devolatilization process on the formation of PM10 and will be discussed below. 3.3. Yields of PM0.5 and PM1−10. The yields of PM0.5 and PM1−10 are calculated and shown in Figure 4. The effects of the combustion condition on the emission of PM0.5 and PM1−10 are first observed. For raw coal, both the yields of PM0.5 and PM1−10 present the tendency of OXY29 > AIR > OXY21. The C

DOI: 10.1021/acs.energyfuels.6b02165 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

N2−char and raw coal for all three combustion conditions. It seems that the devolatilization process in a N2 and CO2 atmosphere presents a converse effect on the char combustion and the formation of particulate matter. 3.4. Possible Mechanisms To Affect the Emission of PM10 Generated from the Combustion of IM Lignite and Chars. The effects of coal devolatilization and char combustion processes on ash formation are quite complicated. First, during coal devolatilization, the organically bound elements may be released to cause ash loss of chars, which is shown in Table 2. Second, the particle combustion temperature of coal and char may be affected by different causes, such as the combustion condition of volatile and char. Finally, the gasification reactions of CO2 with char during high-temperature combustion of lignite in the oxy-fuel condition may influence the transformation of inorganic elements and the formation of PM10. As presented in Table 2, the calculated ash loss of N2−char during coal devolatilization is 7.44 g of ash/kg of coal, whereas CO2−char has a high ash loss of 25.11 g of ash/kg of coal, which reaches up to ∼19% of the ash content of the IM lignite. Apparently, the ash loss of vaporized elements fails to explain the higher yield of PM0.5 from N2−char combustion than raw coal combustion; therefore, other factors would affect the formation of PM0.5. The results of several studies indicate that the major reason to promote the formation of ultrafine and coarse particles at higher furnace temperatures19,44 and higher oxygen mole fractions42,44,45 is the increase of the char particle combustion rate and/or temperature. Similarly, in comparison to the oxy-fuel combustion, the greater formation of particulate matter in O2/N2 combustion with the same oxygen mole fraction is also attributed to the increased char combustion temperature.41,42,46 In a nutshell, the combustion rate and/or temperature of char particles may be the predominant factor affecting the formation of particulate matter for the same coal. The relatively higher yields of PM0.5 and PM1−10 generated from N2−char combustion (Figure 4) imply that the combustion of N2−char may be promoted in comparison to the raw coal combustion. This result seems to be contradictory to our earlier result from the oxy-fuel combustion of bituminous coal, which reported a lower PM10 yield from N2−char combustion than raw coal combustion.27 An important work from Khatami et al.14 studied the overall temperature histories of single coal particles by three-color optical pyrometry. Their results including two lignites and one bituminous coal are repeated in Figure 6. Figure 6c shows that the particle temperature during the devolatilization stage of bituminous coal was clearly higher than that during the char combustion stage, which was also observed by Timonthy et al.47 using two-color pyrometry to test Illinois no. 6 bituminous coal. This is consistent with our earlier discussion that the combustion temperature of bituminous coal was higher than that of its corresponding char.27 The traces of particles of two lignites did not reveal a clear difference between the volatile and char combustion stages (Figure 6). Timonthy et al.47 also found a similar temperature history in Montana lignite. The volatile

Figure 5. Elemental mass of PM0.5 generated from the combustion of (a) raw coal and (b) N2−char.

AIR conditions (Figure 4b). Recent studies41−43 have claimed that O2/CO2 combustion at the same oxygen concentration decreased the yields of the sub-micrometer and coarse particles compared to air combustion because it resulted in a lower particle combustion temperature, which would slow the vaporization of volatile species and the degree of char fragmentation. Our results of raw coal in the OXY21 and AIR conditions comply well with the previous finding. The effects of the combustion atmosphere on the PM10 emission from the combustion of chars were also observed. Table 2 shows the ash loss of chars caused by the release of the organically bound elements. It is worthwhile to notice that approximately 5.7% of inorganic elements is already vaporized and released during the preparation of N2−char (Table 2); therefore, the difference in the release of vaporized elements of N2−char in the three combustion conditions is unobvious. Accordingly, similar PM0.5 yields in the three conditions are found (Figure 4a). The emission of CO2−char combustion presents the trend of AIR ≈ OXY29 > OXY21 for both the PM0.5 and PM1−10. It is noteworthy that the devolatilization process in CO2 causes quite a severe gasification reaction between CO2 and char, which gives rise to extremely high carbon consumption of CO2−char (Table 1). As a result, the reaction of CO2 and carbon in char is largely weakened during the combustion of CO2−char; thus, the local reducing atmosphere cannot be provided effectively, and accordingly, the vaporization of inorganic matter and the formation of PM0.5 from the OXY29 condition are similar as those from the AIR condition. The yields of PM0.5 and PM1−10 from the CO2−char combustion in the OXY21 condition are lowest in three combustion conditions, with the similar reasons mentioned in the last paragraph. In comparison to the combustion of raw coal and chars, Figure 4a presents that the PM0.5 yield from N2−char combustion is obviously higher than that from raw coal combustion in the conditions of AIR and OXY21 but these two have similar yields in the OXY29 condition. For PM1−10, the yields of N2−char and raw coal combustion are relatively similar in the condition of OXY21, whereas N2−char combustion produces more PM1−10 than raw coal combustion in AIR and OXY29 conditions (Figure 4b). The combustion of CO2−char produces much less PM0.5 and PM1−10 than that of

Table 2. Ash Loss of CO2−Char and N2−Char during the Coal Devolatilization Process assumed initial coal mass (g) raw coal N2−char CO2−char

1000 1000 1000

char yield (%)

ash content (wt %)

theoretical ash mass (g of ash/kg of coal)

ash loss (g of ash/kg of coal)

49.398 16.063

12.947 24.704 64.969

129.47 122.03 104.36

7.44 25.11

D

DOI: 10.1021/acs.energyfuels.6b02165 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

Figure 6. Whole temperature histories of a single particle combusted in the air condition, with the temperature deduced from three-color pyrometry by Khatami et al.:14 (a) Beulah lignite, (b) Wilcox lignite, and (c) Pittsburgh no. 8 bituminous coal.

and Ca in PM1−10 are expected to originate from the inorganic minerals in lignite rather than the organically bound cations and ultrafine particles, which tend to form sub-micrometer particles. Accordingly, the occurrence mode of Fe and Ca in inorganic minerals of IM lignite is identified by CCSEM and shown in Figure 7. This figure indicates that the element Fe exists in the form of siderite (∼60%) and pyrite (∼24%), whereas Ca is associated with siderite (∼28%), calcite (∼8%), pyrite (∼5%), Fe−Ca aluminosilicate (∼11%), and complex unclassified species (∼11%). Much higher fractions of Fe and Ca in PM1−10 from N2−char combustion should be attributed to the decomposition and fragmentation of the above-mentioned Feand Ca-rich minerals, i.e., siderite, pyrite and calcite, which were reported to facilitate the formation of coarse particles apparently via fragmentation.48,49 To some extent, the decomposition and fragmentation of those minerals are promoted by the higher particle combustion rate and/or temperature of N2−char in AIR and OXY29 conditions, resulting in higher yields of PM1−10 than that from raw coal combustion. In contrast, the fragmentation of those minerals is likely to be inhibited by the lower combustion temperature of the OXY21 condition, which possibly gives rise to the relatively similar yields of PM1−10 from the raw coal and N2−char combustion. Figure 3b also reveals that the mass fraction of Si in PM1−10 from N2−char combustion is much lower than that from raw coal and CO2−char combustion, which also implies a higher combustion rate and/or temperature of N2−char particles. Si exists as quartz (∼38%), Si-rich (∼8%), illite (∼11%), and kaolinite (∼9%) in the IM lignite (Figure 7c). The high combustion rate and/or temperature of N2−char particles is beneficial to the melting, coalescence, and interaction between the Si-bearing minerals (mainly quartz and Si-rich) and the Fe/ Ca-rich fine fragmented particles; consequently, plenty of Si is expected to be transferred to PM10+, owing to the obvious coalescence. In contrast, the thermally stable quartz and Si-rich are relatively difficult to melt, owing to the moderate combustion temperature of raw coal and CO 2 −char; accordingly, much Si remains in PM1−10 without obvious coalescence, causing a higher Si content than that in PM1−10 from N2−char combustion.

combustion stage of lignites seems to provide a similar temperature to the char combustion stage (Figure 6). The improvement of N2−char combustion should be caused by other reasons. It is noteworthy that the preparation of N2−char in a DTF should experience a longer residence time than the in situ char of raw coal; the pore structure and BET surface area of N2−char can accordingly be promoted to lead to a higher combustion rate and/or temperature compared to the in-situ char of raw lignite possibly. A similar phenomenon was also found by Gao et al.,22 who burnt the biochars and their raw biomass and found a higher yield of PM1−10 from the combustion of biochars. The devolatilization process in CO2 causes quite a severe gasification reaction between CO2 and char. This effect of the gasification reaction gives rise to extremely high carbon consumption and, thus, leaves quite a high ash content in CO2−char (Table 1). As a result, the combustion of CO2−char is expected to be difficult and the formation of ultrafine and coarse particles is largely inhibited in comparison to raw coal and N2−char. Figure 4a also shows that the PM0.5 yield from N2−char combustion is obviously higher than that from raw coal in the conditions of AIR and OXY21 but these two have similar yields in the OXY29 condition. The discussion in section 3.3 has indicated that it is the formation of PM0.5 from raw coal combustion that appears to be markedly promoted in the OXY29 condition compared to the AIR and OXY21 conditions, owing to the gasification reaction in the OXY29 condition of raw coal promoting a stronger local reducing environment and facilitating the vaporization of char-bound elements.38,39 In contrast, approximately 5.7% of inorganic elements are already vaporized and released during the preparation of N2−char (Table 2); therefore, the effect of CO2 on the release of vaporized elements of N2−char in the OXY29 condition is unobvious (Figure 5b). As a result, the PM0.5 yields from N2− char combustion are similar under the three combustion conditions (Figure 4a). Figure 4b reveals that the yields of PM1−10 from the combustion of N2−char and raw coal are relatively similar in the condition of OXY21, whereas the N2−char combustion produces more PM1−10 than raw coal combustion in the AIR and OXY29 conditions. Figure 3b suggests that the major ashforming elements of PM1−10 from AIR and OXY29 combustion of N2−char are Ca, Fe, Si, and Al and the mass fractions of Ca and Fe are much higher than that from raw coal combustion. Fe

4. CONCLUSION The air and oxy-fuel combustion of a Chinese lignite and its chars devolatilized in the N2 or CO2 condition was performed, E

DOI: 10.1021/acs.energyfuels.6b02165 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

heavy carbon consumption and a fairly high ash content of CO2−char. The inhibited combustion rate of CO2−char can be deemed as the possible explanation for the much lower yields of PM0.5 and PM1−10 from CO2−char combustion than those from raw coal and N2−char combustion.

■

AUTHOR INFORMATION

Corresponding Authors

*Telephone: +86-27-87544779. Fax: +86-27-87545526. E-mail: [email protected]. *Telephone: +86-27-87546631. Fax: +86-27-87545526. E-mail: [email protected]. ORCID

Chang Wen: 0000-0002-8539-8630 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors acknowledge the National Natural Science Foundation of China (Grants 51506067 and 5166112501) and the National Key Research and Development Plan of China (Grant 2016YFB0600601). The authors also thank the Analytical and Testing Center at Huazhong University of Science and Technology for the XRF and CCSEM tests.

■

REFERENCES

(1) Xu, M.; Yu, D.; Yao, H.; Liu, X.; Qiao, Y. Coal combustiongenerated aerosols: Formation and properties. Proc. Combust. Inst. 2011, 33 (1), 1681−1697. (2) Yao, Q.; Li, S.-Q.; Xu, H.-W.; Zhuo, J.-K.; Song, Q. Studies on formation and control of combustion particulate matter in China: A review. Energy 2009, 34 (9), 1296−1309. (3) Pui, D. Y.; Chen, S.-C.; Zuo, Z. PM2.5 in China: Measurements, sources, visibility and health effects, and mitigation. Particuology 2014, 13, 1−26. (4) Zhou, K.; Xu, M.; Yu, D.; Liu, X.; Wen, C.; Zhan, Z.; Yao, H. Formation and control of fine potassium-enriched particulates during coal combustion. Energy Fuels 2010, 24 (12), 6266−6274. (5) Wen, C.; Gao, X.; Yu, Y.; Wu, J.; Xu, M.; Wu, H. Emission of inorganic PM10 from included minerals during the combustion of pulverized coals of various ranks. Fuel 2015, 140, 526−530. (6) Wen, C.; Zhang, P.; Yu, D.; Gao, X.; Xu, M. Loading identical contents of sodium and quartz into different ash-removed coals to elaborately investigate the real effects of coal particle combustion on the emission behavior of PM 10 . Proc. Combust. Inst. 2016, DOI: 10.1016/j.proci.2016.08.052. (7) Zhang, R.; Jing, J.; Tao, J.; Hsu, S.-C.; Wang, G.; Cao, J.; Lee, C.; Zhu, L.; Chen, Z.; Zhao, Y.; Shen, Z. Chemical characterization and source apportionment of PM2.5 in Beijing: Seasonal perspective. Atmos. Chem. Phys. 2013, 13 (14), 7053−7074. (8) Song, Y.; Zhang, Y.; Xie, S.; Zeng, L.; Zheng, M.; Salmon, L. G.; Shao, M.; Slanina, S. Source apportionment of PM2.5 in Beijing by positive matrix factorization. Atmos. Environ. 2006, 40 (8), 1526−1537. (9) Matsuoka, K.; Suzuki, Y.; Eylands, K. E.; Benson, S. A.; Tomita, A. CCSEM study of ash forming reactions during lignite gasification. Fuel 2006, 85 (17), 2371−2376. (10) Zygarlicke, C.; Steadman, E.; Benson, S. Studies of transformations of inorganic constituents in a Texas lignite during combustion. Prog. Energy Combust. Sci. 1990, 16 (4), 195−204. (11) Gao, X.; Rahim, M. U.; Chen, X.; Wu, H. Significant contribution of organically-bound Mg, Ca, and Fe to inorganic PM10 emission during the combustion of pulverized Victorian brown coal. Fuel 2014, 117 (PartA), 825−832. (12) Toftegaard, M. B.; Brix, J.; Jensen, P. A.; Glarborg, P.; Jensen, A. D. Oxy-fuel combustion of solid fuels. Prog. Energy Combust. Sci. 2010, 36 (5), 581−625.

Figure 7. Occurrence mode of (a) Fe, (b) Ca, and (c) Si in inorganic minerals of the IM lignite.

which is an attempt to explore the effects of the coal devolatilization and char combustion processes on the emission of particulate matter. The conclusions are summarized as follows: The yields of sub-micrometer (PM0.5) and coarse (PM1−10) particles generated from N2−char combustion are generally higher than that from raw coal combustion, showing that the devolatilization in the N2 condition may promote the formation of particulate matter. In contrast, the PM10 yield from CO2− char combustion is much lower than that from raw coal combustion, indicating that the devolatilization process in CO2 inhibits their emission. N2−char and CO2−char generated from lignite present entirely different combustion behaviors and, thus, reflect the discrepant emission characteristics of PM10. The rough higher yields of PM0.5 and PM1−10 from N2−char combustion than raw coal combustion may be driven by the promoted combustion rate and/or temperature of N2−char particles. The effect is confirmed by the promoted decomposition and fragmentation of Fe- and Ca-rich minerals. Furthermore, owing to the obvious coalescence, the higher mass fraction of Si transferred into PM10+ also verifies this effect. Quite a severe gasification reaction of CO2 and char during char preparation in the CO2 condition results in a very F

DOI: 10.1021/acs.energyfuels.6b02165 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels (13) Scheffknecht, G.; Al-Makhadmeh, L.; Schnell, U.; Maier, J. Oxyfuel coal combustionA review of the current state-of-the-art. Int. J. Greenhouse Gas Control 2011, 5 (Supplement 1), S16−S35. (14) Khatami, R.; Stivers, C.; Joshi, K.; Levendis, Y. A.; Sarofim, A. F. Combustion behavior of single particles from three different coal ranks and from sugar cane bagasse in O2/N2 and O2/CO2 atmospheres. Combust. Flame 2012, 159 (3), 1253−1271. (15) Levendis, Y. A.; Joshi, K.; Khatami, R.; Sarofim, A. F. Combustion behavior in air of single particles from three different coal ranks and from sugarcane bagasse. Combust. Flame 2011, 158 (3), 452−465. (16) Bejarano, P. A.; Levendis, Y. A. Single-coal-particle combustion in O2/N2 and O2/CO2 environments. Combust. Flame 2008, 153 (1), 270−287. (17) Rathnam, R. K.; Elliott, L. K.; Wall, T. F.; Liu, Y.; Moghtaderi, B. Differences in reactivity of pulverised coal in air (O2/N2) and oxy-fuel (O2/CO2) conditions. Fuel Process. Technol. 2009, 90 (6), 797−802. (18) Liu, X.; Xu, M.; Si, J.; Gu, Y.; Xiong, C.; Yao, H. Effect of sodium on the structure and reactivity of the chars formed under N2 and CO2 atmospheres. Energy Fuels 2012, 26 (1), 185−192. (19) Zhang, L.; Ninomiya, Y.; Yamashita, T. Formation of submicron particulate matter (PM1) during coal combustion and influence of reaction temperature. Fuel 2006, 85 (10), 1446−1457. (20) Zhang, L.; Ninomiya, Y.; Yamashita, T. Occurrence of inorganic elements in condensed volatile matter emitted from coal pyrolysis and their contributions to the formation of ultrafine particulates during coal combustion. Energy Fuels 2006, 20 (4), 1482−1489. (21) Kang, S. G.; Sarofim, A. F.; Beér, J. M. Effect of char structure on residual ash formation during pulverized coal combustion. Symp. (Int.) Combust., [Proc.] 1992, 24, 1153−1159. (22) Gao, X.; Wu, H. Biochar as a fuel: 4. Emission behavior and characteristics of PM1 and PM10 from the combustion of pulverized biochar in a drop-tube furnace. Energy Fuels 2011, 25 (6), 2702−2710. (23) Russell, N.; Beeley, T.; Man, C.-K.; Gibbins, J.; Williamson, J. Development of TG measurements of intrinsic char combustion reactivity for industrial and research purposes. Fuel Process. Technol. 1998, 57 (2), 113−130. (24) Wall, T.; Liu, Y.; Spero, C.; Elliott, L.; Khare, S.; Rathnam, R.; Zeenathal, F.; Moghtaderi, B.; Buhre, B.; Sheng, C.; Gupta, R.; Yamada, T.; Makino, K.; Yu, J. An overview on oxyfuel coal combustionState of the art research and technology development. Chem. Eng. Res. Des. 2009, 87 (8), 1003−1016. (25) Buhre, B.; Elliott, L.; Sheng, C.; Gupta, R.; Wall, T. Oxy-fuel combustion technology for coal-fired power generation. Prog. Energy Combust. Sci. 2005, 31 (4), 283−307. (26) Wen, C.; Xu, M.; Yu, D.; Sheng, C.; Wu, H.; Zhang, P.; Qiao, Y.; Yao, H. PM10 formation during the combustion of N2−char and CO2− char of Chinese coals. Proc. Combust. Inst. 2013, 34 (2), 2383−2392. (27) Wen, C.; Yu, D.; Wang, J.; Wu, J.; Yao, H.; Xu, M. Effect of the devolatilization process on PM10 formation during oxy-fuel combustion of a typical bituminous coal. Energy Fuels 2014, 28 (9), 5682− 5689. (28) Yu, Y.; Xu, M.; Yao, H.; Yu, D.; Qiao, Y.; Sui, J.; Liu, X.; Cao, Q. Char characteristics and particulate matter formation during Chinese bituminous coal combustion. Proc. Combust. Inst. 2007, 31 (2), 1947− 1954. (29) Wen, C.; Xu, M.; Zhou, K.; Yu, D.; Zhan, Z.; Mo, X. The melting potential of various ash components generated from coal combustion: Indicated by the circularity of individual particles using CCSEM technology. Fuel Process. Technol. 2015, 133, 128−136. (30) Wen, C.; Gao, X.; Xu, M. A CCSEM study on the transformation of included and excluded minerals during coal devolatilization and char combustion. Fuel 2016, 172, 96−104. (31) Yu, D.; Xu, M.; Zhang, L.; Yao, H.; Wang, Q.; Ninomiya, Y. Computer-controlled scanning electron microscopy (CCSEM) investigation on the heterogeneous nature of mineral matter in six typical Chinese coals. Energy Fuels 2007, 21 (2), 468−476. (32) Buhre, B.; Hinkley, J.; Gupta, R.; Wall, T.; Nelson, P. Submicron ash formation from coal combustion. Fuel 2005, 84 (10), 1206−1214.

(33) Senior, C.; Flagan, R. Ash vaporization and condensation during combustion of a suspended coal particle. Aerosol Sci. Technol. 1982, 1 (4), 371−383. (34) Helble, J.; Sarofim, A. Influence of char fragmentation on ash particle size distributions. Combust. Flame 1989, 76 (2), 183−196. (35) Yan, L.; Gupta, R.; Wall, T. The implication of mineral coalescence behaviour on ash formation and ash deposition during pulverised coal combustion. Fuel 2001, 80 (9), 1333−1340. (36) Ninomiya, Y.; Zhang, L.; Sato, A.; Dong, Z. Influence of coal particle size on particulate matter emission and its chemical species produced during coal combustion. Fuel Process. Technol. 2004, 85 (8), 1065−1088. (37) Kim, D.; Choi, S.; Shaddix, C. R.; Geier, M. Effect of CO2 gasification reaction on char particle combustion in oxy-fuel conditions. Fuel 2014, 120, 130−140. (38) Zhang, L.; Binner, E.; Qiao, Y.; Li, C.-Z. In situ diagnostics of Victorian brown coal combustion in O2/N2 and O2/CO2 mixtures in drop-tube furnace. Fuel 2010, 89 (10), 2703−2712. (39) Jiao, F.; Chen, J.; Zhang, L.; Wei, Y.; Ninomiya, Y.; Bhattacharya, S.; Yao, H. Ash partitioning during the oxy−fuel combustion of lignite and its dependence on the recirculation of flue gas impurities (H2O, HCl and SO2). Fuel 2011, 90 (6), 2207−2216. (40) Li, W.; Wang, L.; Qiao, Y.; Lin, J.-Y.; Wang, M.; Chang, L. Effect of atmosphere on the release behavior of alkali and alkaline earth metals during coal oxy-fuel combustion. Fuel 2015, 139, 164−170. (41) Sheng, C.; Li, Y.; Liu, X.; Yao, H.; Xu, M. Ash particle formation during O2/CO2 combustion of pulverized coals. Fuel Process. Technol. 2007, 88 (11), 1021−1028. (42) Kazanc, F.; Levendis, Y. A. Physical properties of particulate matter emitted from combustion of coals of various ranks in O2/N2 and O2/CO2 environments. Energy Fuels 2012, 26 (12), 7127−7139. (43) Chen, L.; Yong, S. Z.; Ghoniem, A. F. Oxy-fuel combustion of pulverized coal: Characterization, fundamentals, stabilization and CFD modeling. Prog. Energy Combust. Sci. 2012, 38 (2), 156−214. (44) Liu, X.; Xu, M.; Yao, H.; Yu, D.; Gao, X.; Cao, Q.; Cai, Y. Effect of combustion parameters on the emission and chemical composition of particulate matter during coal combustion. Energy Fuels 2007, 21 (1), 157−162. (45) Buhre, B.; Hinkley, J.; Gupta, R.; Nelson, P.; Wall, T. Fine ash formation during combustion of pulverised coal−coal property impacts. Fuel 2006, 85 (2), 185−193. (46) Suriyawong, A.; Gamble, M.; Lee, M.-H.; Axelbaum, R.; Biswas, P. Submicrometer particle formation and mercury speciation under O2-CO2 coal combustion. Energy Fuels 2006, 20 (6), 2357−2363. (47) Timothy, L.; Sarofim, A. F.; Beér, J. M. Characteristics of single particle coal combustion. Symp. (Int.) Combust., [Proc.] 1982, 19, 1123−1130. (48) Yan, L.; Gupta, R.; Wall, T. Fragmentation behavior of pyrite and calcite during high-temperature processing and mathematical simulation. Energy Fuels 2001, 15 (2), 389−394. (49) McLennan, A.; Bryant, G.; Bailey, C.; Stanmore, B.; Wall, T. An experimental comparison of the ash formed from coals containing pyrite and siderite mineral in oxidizing and reducing conditions. Energy Fuels 2000, 14 (2), 308−315.

G

DOI: 10.1021/acs.energyfuels.6b02165 Energy Fuels XXXX, XXX, XXX−XXX