ThermogravimetryâMass Spectrometry Analysis of Nitrogen

Article pubs.acs.org/EF

Thermogravimetry−Mass Spectrometry Analysis of Nitrogen Transformation during Oxy-fuel Combustion of Coal and Biomass Mixtures Xin Wang,†,‡ Qiangqiang Ren,*,† Wei Li,†,‡ Shiyuan Li,† and Qinggang Lu† †

Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China

‡

ABSTRACT: The nitrogen transformation during the oxy-fuel combustion of coal and biomass mixtures was studied through thermogravimetry−mass spectrometry (TG−MS). Influence factors, such as atmosphere and biomass share, were selected to study the release of four nitrogenous gases HCN, HNCO, NH3, and NO. With the presence of ≥20% O2, the combustion of Datong coal and biomass mixtures releases less HCN, NH3, and NO and more HNCO in the O2/CO2 atmosphere compared to the O2/N2 atmosphere. HCN and HNCO compete mutually during the formation of nitrogenous gases. The mixtures of Datong coal and biomass release more nitrogenous gases than Datong coal alone in both O2/CO2 and O2/N2 atmospheres during the devolatilization. With the O2 concentration in the O2/CO2 atmosphere increasing from 20 to 100%, the relative yields of NH3, HCN, and HNCO produced from the mixtures decrease, while the NO yield increases. The HNCO yield changes greatly with the increase of the O2 concentration because of the oxidation by OH. Increasing the biomass share in the mixtures improves the relative yields of nitrogenous gases. The biomass share can greatly affect the HCN and HNCO yields, owing to the different forms of N-containing structures between coal and biomass.

1. INTRODUCTION The carbon capture and storage (CCS) technology can realize CO2 enrichment for industrial utilization, which is very favorable for reduction of greenhouse gas emissions and development of low carbon economy in China. Among various carbon capture technologies, the oxy-fuel combustion (O2/ CO2) technology shows great advantage and feasibility because of its low investment and cost. The oxy-fuel combustion technology is also suitable for the current industrial and technological levels and, thus, can be used to transform the existing combustion equipment.1 The utilization of biomass, one of renewable energy, is considered as zero CO2 emission. The biomass−CCS (BioCCS) technology, which combines the biomass utilization technology and the CCS technology, can capture and store CO2 formed during biomass utilization.2 This technology can largely reduce the amount of atmospheric CO2, which is characterized by negative CO2 emission and has become a new research focus in the field of low-carbon energy.2 Biomass fuels (e.g., agricultural straw) containing abundant alkali metal K are very likely to cause corrosion and agglomeration during the direct combustion in furnaces.3−5 Recent studies show that some elements in coals (e.g., Al and S) can form high-melting-point compounds with alkali metal in biomass fuels and, thus, can improve corrosion and agglomeration during the co-combustion with biomass fuels.6,7 Moreover, the cost of retrofitting an existing coalfired power plant to a co-combustion plant is considerably lower than building a new dedicated biomass-fired plant.8 The co-combustion of coal and biomass is a low-cost and low-risk method for CO2 reduction for fossil energy utilization systems and enables large-scale biomass utilization with high combustion efficiency and stability for renewable energy © 2015 American Chemical Society

utilization systems. Furthermore, co-combustion can be operated in a flexible mode, which minimizes the fluctuating supply of biomass and secures the power generation.8 At present, the research on the oxy-combustion of coal and biomass mixtures and the research on the conversion rules of combustion-produced nitrogen oxides are both at the preliminary phase. The combination of the combustion test and numerical simulation was adopted to study the effect of mixing with sawdust on NO generation under an O2/N2 or O2/ CO2 atmosphere.9−11 Although the addition of sawdust reduced the nitrogen content in the fuel, the conversion rate of NO increased during the combustion.9−11 The cocombustion of coal and biomass in a furnace tube reactor was also studied.12 The results show that, under the O2/CO2 atmosphere, as the mixing ratio of wheat straw and other biomass increased, NO release was reduced. With the same O2 concentration, NO release during the combustion of coal and biomass mixtures in the O2/CO2 atmosphere was less than that in the O2/N2 atmosphere. A study on NO formation during oxy-fuel combustion of coal and biomass chars13 shows that the influence of CO on NO is more significant at 850 °C than at 1050−1150 °C. Moreover, the co-combustion of coal and biomass in a circulating fluidized-bed (CFB) combustor was tested.14,15 The results show that the emissions of NOx and SO2 were reduced in the co-combustion condition, owing to the low fuel nitrogen content in the biomass. NOx emissions from a coal−bagasse blend burning in the O 2 /CO 2 atmosphere16 did not clearly reflect the nitrogen content, and Received: January 5, 2015 Revised: February 16, 2015 Published: March 16, 2015 2462

DOI: 10.1021/acs.energyfuels.5b00019 Energy Fuels 2015, 29, 2462−2470

Article

Energy & Fuels Table 1. Proximate and Ultimate Analyses of Feedstock (wt %) Datong coal wheat straw cotton stalk

Car

Har

Oar

Nar

Sar

Mar

Aar

Vdaf

FCar

58.28 44.29 43.22

3.74 5.50 5.13

8.61 40.81 37.18

1.04 0.56 0.92

0.32 0.20 0.14

1.87 1.80 1.84

26.14 6.84 11.57

38.14 79.79 78.75

44.53 19.38 18.40

Table 2. Main Inorganic Elements in Original Feedstock (wt %) Datong coal wheat straw cotton stalk

Al

Ca

Fe

K

Mg

P

Si

Na

5.23 0.03 0.43

1.01 0.34 0.96

0.74 0.07 0.34

0.07 1.98 1.01

0.10 0.18 0.34

0.02 0.11 0.14

5.52 2.37 2.48

0.03 0.20 0.27

Figure 1. Release curves of HCN in O2/N2 and O2/CO2.

the conversion of the fuel nitrogen to NOx was 20−50% in all cases. To summarize, the conclusions on the release amount of nitrogen oxide during the oxy-combustion of coal and biomass mixtures are not consistent and the precursors of nitrogen oxide have been rarely studied. Nevertheless, research on oxycombustion of coal and biomass mixtures is of great practical significance. Thermogravimetry−mass spectrometry (TG−MS), which provides data of both TG and MS, can monitor gaseous emissions during a temperature-programmed process online. In this study, TG−MS was used to determine the nitrogen transformation during the co-combustion of coal and biomass mixtures. Different from the common way, we calculated the weight change of each batch by analyzing the signals of the nitrogenous products, although the quantitative analysis was modestly incomplete. With the TG−MS signals of nitrogenous

products, a semi-quantitative analysis was applied to illustrate the product concentration changes. We discussed the changes of nitrogenous products during the co-combustion of coal and biomass mixtures with different influence factors and aimed to summarize the rules of nitrogen transformation.

2. EXPERIMENTAL SECTION 2.1. Materials. Coal from Datong in Shanxi province (Datong coal) and two types of biomass fuels, including wheat straw and cotton stalk, from Hebei province were selected in this study. Before tests, all of the samples were dried in a muffle furnace at 105 °C for 2 h, then milled, and sieved to a powder of 150−200 μm. The results of proximate and ultimate analyses with the feedstock are listed in Table 1. The main inorganic elements in the feedstock are shown in Table 2. In comparison to the biomass, Datong coal contains much more ash and fixed carbon, less volatiles, and more N, S, and Al. 2.2. TG−MS Tests. TG−MS tests were performed on a simultaneous thermal analyzer (NETZSCH STA-449F3) coupled 2463


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Figure 2. Release curves of HNCO in O2/N2 and O2/CO2. with a quadrupole mass spectrometer (QMS403C Aeolos). The shielding gas was Ar at a constant flow rate of 20 mL/min. The carrier gas was O2/N2 (2:8, volume) or O2/CO2 (from 2:8 to 10:0, volume) at a constant flow rate of 100 mL/min. In each test, a sample (10 ± 0.1 mg) was put into an Al2O3 crucible and heated in the TG−MS apparatus from room temperature to about 1000 °C at a rate of 20 °C/min. Some typical nitrogen oxides and their precursors were selected in this study according to early reports.12,17,18 We focus on the mass spectra of atomic mass units 30 (NO+), 17 (NH3+), 27 (HCN+), and 43 (HNCO+), which represent gases NO, NH3, HCN, and HNCO, respectively. Before the TG−MS test, the MS signals were normalized using Ar. In formal tests, we assumed that the base signals of the MS meter did not vary during each heating process, because the running time was very short. Because each nitrogenous gas detected by the MS meter has unique response behavior, the signals of different gases even in the same sample cannot be compared.19 Nevertheless, the signals of the same gas could be compared among different samples, including the shapes, characteristic temperatures, and intensities of the peaks.19 Data and curves of ion current intensity− time/temperature were received from TG−MS after the tests. Relative yields of nitrogenous gases can be determined by integrating the normalized curve.

The release curve of each nitrogen gas show two phases at 200−400 and 400−900 °C. More HCN, NH3, and NO and less HNCO were released in the O2/N2 atmosphere compared to the O2/CO2 atmosphere. Among these nitrogen gases, the release amounts of HCN and HNCO vary obviously when it changes from the O 2 /N 2 atmosphere to the O 2 /CO 2 atmosphere. One possible reason is that the reaction C + CO2 = 2CO happens more easily in the O2/CO2 atmosphere than in the O2/N2 atmosphere, leading to the increase of the CO content and the reduction of the char content. Thus, the low char content retrains the heterogeneous reduction HNCO + C = HCN + CO on the char surface.20 On the other hand, the formation of HCN mainly occurs through the thermal cracking of volatiles and is mainly affected by the concentration of the H radical and the interaction between the H radical and N-containing structures on the char surface, while the active N sites with the potential of HCN formation will be consumed rapidly by CO2.21 In addition, the reductions of NO/CO/ char,20,22,23 2C + 2NO = 2CO + N2 and CO + NO = CO2 + N2, also happen more easily in the O2/CO2 atmosphere, which leads to the reduction of NO. The release amount of nitrogenous gas during the first release phase from the mixture fuels is more obvious compared to coal. Nitrogen in biomass mainly comes from proteins, amino acids, and nitrogenous bases 24 and is mostly decomposed and released at a low temperature. As showed in Figure 4, the NO peak at the second phase in the O2/CO2 atmosphere shows a higher temperature than that of the second NO peak in that O2/N2 atmosphere, regardless of the coal or mixture fuel. This is because CO2 has a larger heat capacity than N2. CO2 absorbs more heat during the combustion, which leads

3. RESULTS AND DISCUSSION 3.1. Release Amounts of Nitrogenous Gases in Different Atmospheres. 3.1.1. Release Amounts of Nitrogenous Gases in O2/N2 and O2/CO2 Atmospheres. The release curves of the four nitrogenous gases in O2/N2 and O2/ CO2 during the heating process are presented in Figures 1−4. Datong coal alone was also tested for comparison. In each test with the mixture fuel, the mass share of biomass was 20% and the mixing ratios of O2/N2 and O2/CO2 were both 2:8. 2464


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Figure 3. Release curves of NH3 in O2/N2 and O2/CO2.

to the decrease of the surface temperature in the particles.25 In this way, NO release in the O2/CO2 atmosphere occurs later than in the O2/N2 atmosphere. 3.1.2. Release of Nitrogenous Gases in O2/CO2 with Different Mixing Ratios. The release curves of the four nitrogenous gases in O2/CO2 with different mixing ratios during the heating process are presented in Figures 5−8. The mass percent of biomass in the blends is 30%, which fits the general biomass share in the subsequent pilot CFB experiments. With the increase of the O2 ratio, the intensity of the first release peak of NO is improved, while that of HNCO is reduced. For HCN and NH3, however, their trends can only be described as complex patterns. On one hand, with the increase of the O2 ratio in the low-temperature region, a high O2 ratio will promote the decomposition of amino acids, proteins, pyrrole, and pyridine in the biomass to release HCN and NH3. On the other hand, the increase of the O2 ratio also promotes the oxidation of HCN and NH3 to form NO. As a result, for HCN and NH3, the intensities of the first release peaks are affected by the above two factors and show complex patterns.26 The temperatures of the first release peaks under different O2 ratios are almost the same for each nitrogenous gas. Except HNCO, the intensities of the second release peaks of all nitrogenous gases rise with the increase of the O2 ratio. As known, HNCO is an unstable compound, and with the presence of a high O2 concentration, it can react easily and quickly to form NO as follows: H2O + O = 2OH, HNCO + OH = NH2 + CO2, and NH2 + 3O = NO + 2OH.20,21,27 As shown in Figure 8, the second peak intensity of the coal/wheat straw mixture is enhanced with the increase of the O2 concentration, while that of the coal/cotton stalk mixture

does not change. For either coal/wheat straw or coal/cotton stalk, as the O2 ratio increases, the temperatures of second release peaks drop down and the peak width narrows. Increasing the O2 concentration can strengthen the combustion and reduce the heat capacity of O2/CO2. Thus, the high O2 concentration improves the release rate of nitrogenous gas and shortens the release time. With the same O2 ratio, the second release peaks for NH3, HCN, and HNCO almost show the same temperatures, which are significantly lower compared to that of NO. However, as the O2 ratio increases, the temperature of the second release peak of NO is closer to those of other nitrogenous gases. With the integral value of the curve in O2/CO2 = 2:8 as a reference, the other integral values are normalized. After that, the relative yields of nitrogenous gases in different O2/CO2 atmospheres are determined and shown in Figure 9. With the increase of the O2 concentration, the relative yields of NH3, HCN, and HNCO decrease and that of NO increases. As reported,28,29 below 650 °C, HCN and NO mostly come from the heterogeneous reactions of the feedstock rather than the secondary reaction during devolatilization or from the homogeneous oxidation. Moreover, most HNCO is released directly by the feedstock.30,31 Thus, the decrease of NH3, HCN, and HNCO concentrations and the increase of the NO concentration may be interpreted by three points: first, the increase of the O2 ratio leads to the heterogeneous reactions to form NO; second, the high O2 ratio promotes the oxidation of the NO precursors NH3, HCN, and HNCO; and third, the low CO2 ratio complicates the reaction CO2 + C = 2CO and, thus, inhibits the reductions of NO/CO/char.22,23 As a consequence, 2465


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Figure 4. Release curves of NO in O2/N2 and O2/CO2.

Figure 5. Release curves of HCN in O2/CO2 with different mixing ratios.

biomass in the same temperature region. However, the hightemperature region (400−800 °C) has two release phases, which represent biomass char N release and coal char N release, respectively. Moreover, when the biomass share reaches 30%, two obvious peaks occur in each of the two release phases for HCN, HNCO, and NH3. For NO, however, the two peaks occur at the biomass share of 50%. The above results indicate that, with the increase of the biomass share, the difference between the two release phases in the high-temperature region becomes more obvious. The phenomenon indicates that the synergistic effects of nitrogenous gas releases between coal and biomass are weakened gradually and their specific release characteristics become more evident. As showed in Figures 10−13, the addition of biomass increases the release amounts of nitrogenous gases in the low-

increasing the O2 ratio accelerates the conversion from NO precursors to NO. 3.2. Release Amounts of Nitrogenous Gases from Coal and Biomass with Different Mixing Ratios. The release curves of the four nitrogenous gases during the combustion of coal and biomass with different mixing ratios are presented in Figures 10−13. The mixing ratio of O2/CO2 is 5:5. With the increase of biomass share, the intensities of the first release peaks are improved, while their temperatures are almost all at about 300 °C for the four nitrogenous gases. The final temperatures of nitrogenous gas releases are nearly the same, regardless of biomass shares. As showed in Figures 10−13, all nitrogenous gases show only one release peak in the low-temperature region (200−400 °C), which indicates that volatile N is released from coal and 2466


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Figure 6. Release curves of HNCO in O2/CO2 with different mixing ratios.

Figure 7. Release curves of NH3 in O2/CO2 with different mixing ratios.

Figure 8. Release curves of NO in O2/CO2 with different mixing ratios.

pyridine N and the residual pyrrole N are decomposed to HCN and further oxidized to NO and nitrile N is converted into HNCO, while quaternary N is decomposed to NH3. N oxide may be converted to pyridine N to form HCN or converted to cyanuric acid [(HNCO)3] and further decomposed into HNCO.30,31,34 Nitrogenous gases can be oxidized into NO, which can be reduced to N2 or N2O. Therefore, the increase of the biomass share results in an improvement in the contents of amino N and protein N, and a reduction in the contents of pyrrole N and pyridine N. In the low-temperature region, the release amounts of nitrogenous gases are mainly controlled by amino N and protein N and can be significantly improved by the addition of biomass.35 In the high-temperature region, the release amounts of nitrogenous gases are controlled

temperature region but complex patterns appear in the hightemperature region. The N-containing forms in the biomass are mainly protein N and amino N, with a small amount of pyridine N and pyrrole N.24 The main N-containing forms in coal are pyrrole N, pyridine N, quaternary N, and N oxide.32 In the lowtemperature region, pyrrole N is mostly converted into heterocyclic N with the release of a small amount of HCN.26 Pyridine N almost does not decompose, despite the slight effect of O2.33 Amino N is condensed mutually into diketopiperazine (DKP) with the release of NH3. Protein N is converted into amino N first and then releases NH3 with the formation of heterocyclic compounds (e.g., DKP and nitrile N). In addition, DKP is decomposed into HCN or HNCO in the lowtemperature region.26,34 In the high-temperature region, 2467


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Figure 9. Relative yields of nitrogenous gases in O2/CO2 with different mixing ratios.

Figure 10. Release curves of HCN from different mixing ratios of coal and biomass.

Figure 11. Release curves of HNCO from different mixing ratios of coal and biomass.

is almost twice as high as that in wheat straw, which is opposite the test results. A possible explanation is that the contents of fixed carbon and char nitrogen in Datong coal are larger compared to wheat straw and cotton stalk, and thus, reductions of NO at high temperature are quite important: 2C + 2NO = 2CO + N2 and CO + NO = CO2 + N2.20,22,23 Because more N2 is involved in the reaction, the contents of other nitrogenous gases decrease. As a result, with the increase of the biomass share, the contents of fixed carbon and char nitrogen in the mixtures decrease and two reactions mentioned above are complicated; therefore, the yields of nitrogenous gases instead of N2 increase. Moreover, previous studies show that inorganic elements Ca and Fe could promote the generation of nitrogenous gases,

by pyridine N, pyrrole N, protein N, and amino N together; therefore, complex patterns appear with the addition of biomass. With the integral value on the curve of Datong coal as a reference, other integral values are normalized. After that, the relative yields of nitrogenous gases from different mixing ratios of coal and biomass are determined and shown in Figure 14. With the increase of the biomass ratio, the relative yields of nitrogenous gases increase, owing to volatile nitrogen in the biomass. Among the nitrogenous gases, the relative yields of HCN and HNCO change greatly, which indicates that the generation of HCN and HNCO is a delicate balancing act along with the biomass ratio. As showed in Table 1, the nitrogen content in Datong coal is higher than in biomass and 2468


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Figure 12. Release curves of NH3 from different mixing ratios of coal and biomass.

Figure 13. Release curves of NO from different mixing ratios of coal and biomass.

Figure 14. Relative yields of nitrogenous gases from different mixing ratios of coal and biomass.

especially NO and HCN.36,37 Fe and Ca can catalyze both the oxidation of fuel N and the reduction of NO on the char surface.36,38 Increasing the biomass share leads to the decrease of Ca and Fe contents in the mixtures. With the increase of the biomass share, the second peaks of nitrogenous gases (mainly the release of char N) are gradually weakened, especially for HCN, NH3, and NO. As a result, the catalytic oxidation of fuel N by Ca and Fe is stronger than the catalytic reduction of NO in the high-temperature region (400−800 °C).

about the nitrogen transformation during the oxy-fuel combustion of coal and biomass mixtures. The coal−biomass mixtures release more nitrogenous gases during the devolatilization compared to coal alone. Among the nitrogenous gases, HCN and HNCO compete mutually. The main precursors of NOx are NH3 and HCN in the O2/N2 atmosphere and NH3 and HNCO in the O2/CO2 atmosphere. The high O2 concentration accelerates the release of nitrogenous gases and shortens the release time. In comparison to HCN and NH3, the O2 concentration has a great effect on HNCO because of the oxidation by OH. Increasing the O2 ratio in the O2/CO2 atmosphere accelerates the conversion from nitrogen oxide precursors to NO.

4. CONCLUSION The release of nitrogenous gases from combustion of coal− biomass mixtures under the O2 /CO 2 atmosphere was characterized. We provide some preliminary conclusions 2469


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Raising the biomass ratio in mixtures improves the relative yields of nitrogenous gases. With the increase of the biomass share, the contents of fixed carbon and char nitrogen in the mixtures decrease and the reductions of NO/CO/char are hindered; therefore, the yields of nitrogenous gases are improved.

■

AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-010-82543055. E-mail: renqiangqiang@iet. cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is funded by the National Natural Science Foundation of China (51476169) and the External Cooperation Program of the Bureau of International Cooperation (BIC), Chinese Academy of Sciences (Grant GJHZ201301).

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ThermogravimetryâMass Spectrometry Analysis of Nitrogen

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