NO Emissions from Anthracite Oxy-Firing in a Fluidized-Bed Combustor

Article pubs.acs.org/EF

NO Emissions from Anthracite Oxy-Firing in a Fluidized-Bed Combustor: Effect of the Temperature, Limestone, and O2 Carlos Lupiáñez, Luis I. Díez,* and Luis M. Romeo Centre of Research of Energy Resources and Consumptions, University of Zaragoza, Mariano Esquillor Gómez 15, 50018 Zaragoza, Spain ABSTRACT: Along with SO2, NOx emissions are considered the main pollutants from solid-fired combustion systems. In fluidized-bed boilers, injection of limestone pursues SO2 retention, achieving capture efficiencies over 90%. Nevertheless, CaO formed from limestone calcination has been identified as a catalyst of N-compound reactions, increasing NO emissions. This paper investigates the effect of limestone addition on NO emissions under oxy-fired conditions, as well as the influences of bed temperature, O2 concentration in the fluidizing gas, and excess oxygen. To this purpose, a set of experiments were conducted in a 90 kWth bubbling fluidized-bed reactor, testing two different limestones for a variety of operating conditions and using anthracite as the fuel. The limestone with the lower SO2 capture capacity has shown the higher impact on NO emissions for all of the O2/ CO2 atmospheres tested. It has also been observed that the higher the bed temperature and O2 excess, the higher the NO emissions. If compared to conventional air combustion, oxy-fired tests result in lower values of fuel−N conversion ratios.

1. INTRODUCTION Oxy-fuel combustion is one of the most promising technologies aimed at developing carbon capture in power plants.1−5 A lot of effort and investigation is being devoted to design new reactors and heat exchangers, improve process efficiencies, and reduce consumptions linked to oxygen production.6−10 Fluidized-bed combustion is a quite flexible technology because wide ranges of solid fuels and residues can be burnt with low emission levels. SO2 can be controlled by means of the addition of a sorbent, generally limestone or dolomite. As for NOx emissions, the usual bed temperatures (800−900 °C) bring along lower records in comparison to pulverized fuel combustion. The combination of both technologies, oxy-fuel and fluidized bed, can significantly contribute to the deployment of power plants without CO2 emission.11−13 Regarding NO emissions, several operating factors affect the oxidation of NO precursors in fluidized-bed combustion,14−17 namely, thermal load, type of limestone, Ca:S ratio, bed temperature and pressure, excess oxygen, and air staging. Miccio et al.15 reported that the main influences are due to the bed temperature, limestone addition, and excess oxygen, whose effects are surveyed in our paper. In order to check whether the influence of limestone is relevant in NO emissions in oxy-firing fluidized-bed combustion, a set of experiments have been designed and executed for a variety of operating conditions. Furthermore, temperature and O 2 (concentration in fluidizing gas and excess oxygen fed) influences are also investigated.

and N2 is only present because of uncontrolled air in leakages, the contribution is, in fact, negligible. Prompt NOx, as well as thermal NOx, requires temperatures over 1300 K, higher than those found in a fluidized-bed combustor. In addition, this sort of NOx formation mechanism is related to hydrocarbon-enriched atmospheres and then can also be considered negligible in solid-fired fluidized-bed applications. Nitrogen chemistry is a complex mechanism in which formation and depletion reactions competitively take place, being highly influenced by combustion conditions and the composition of the radical pool. The main reactions reported in fluidized-bed literature are summarized in Table 1, where homogeneous and heterogeneous reactions contribute to form and deplete NO and N2O. Moreover, some of these reactions can be catalyzed by some compounds present in the bed. Fuel N is released throughout devolatilization and char combustion. During devolatilization, HCN and tar N have been identified as the main products,21−23 but in the case of fluidizedbed combustion, other gases come in relevant concentrations, such as NH3 and CNO.24 The nitrogen bound in the resulting char mostly becomes N2, HCN, and NO. HCN is considered the main source of N2O, together with the reaction between char N and NO, through the mechanism given by reactions (R1)−(R3).14,23 N2O is stable below 800 °C, but over this temperature, it can be thermally decomposed or can react with other substances being reduced to N2 [reactions (R4)−(R7)].15,21,25,26 Thermal decomposition [reaction (R4)] is catalyzed by the presence of solid particles like CaO, char, and bed matter. HCN also contributes to the formation of NO, by either the mechanism given by reactions (R1), (R8), and (R9) or the mechanism given by reactions (R1) and (R10).

2. NOX CHEMISTRY Among the three mechanisms of NO formation, the main contribution in a fluidized bed operated under air-fired conditions is due to the conversion of fuel N to NO.18−20 Thermal NOx is not significant because the temperature levels in the reactor are not enough to dissociate N2 of the air. Under oxyfired conditions, where the fluidizing gas is a mixture of O2/CO2 © 2013 American Chemical Society

Received: March 27, 2013 Revised: November 6, 2013 Published: November 7, 2013 7619

dx.doi.org/10.1021/ef4019228 | Energy Fuels 2013, 27, 7619−7627

Energy & Fuels

Article

which has an important role in the NO depletion reactions, and produces conversion of HCN to NH3, increasing the probability of NO formation.34 Under air-fired conditions, free lime is usually found because of calcination of the limestone supplied for SO2 retention:

Table 1. Main Reactions Involved in NOx Chemistry for a Fluidized-Bed Reactor reaction

catalyst

1 N2O → N2 + O2 2

CaCO3 ⇆ CaO + CO2 CaO + SO2 + char, CaO, and 15, 21, bed material 25, 26

(R4)

N2O + char C → N2 + CO

(R6)

1 N2O + O2 → N2 + O2 2

(R7)

CNO + H → NH + CO

(R8)

NH + OH → NO + H 2

(R9)

⎛ −20474 ⎞ ⎟ Peq = 4.137 × 107 exp⎜ ⎝ T ⎠

1 N2 + CO2 2 1 NO + H 2 → N2 + H 2O 2 1 NO + NH3 + O2 → N2 + 2 1 NO + char C → N2 + CO 2 NO + CO →

(R12)

char, CaO, and 21, 29, bed material 30

CaCO3 + SO2 +

char, CaO, and bed material char and CaO

(R13)

(1)

1 O2 ⇆CaSO4 +CO2 2

(R19)

Direct sulfation is a slower process than indirect sulfation, and lower desulfurization efficiencies are usually found in the open literature.35,36 An option to improve the desulfurization ratios is an increase of the Ca:S molar ratio. Varonen et al.37 reported a decrease of NO emission when they increased the Ca:S ratio from 2.2 to 6 to reduce the high SO2 concentration obtained under noncalcining conditions. Notwithstanding this, calcination conditions can also be reached under oxy-firing if the bed temperature is increased. From the point of view of oxy-desulfurization, de Diego and coworkers38 found the optimum temperature at 925 °C, which falls within calcining conditions. This opens the possibility of operating oxy-fired fluidized beds under conditions in which the presence of CaO is again expected. Therefore, the actual effects of the desulfurization process and the operating conditions on NOx emissions are still to be well-established under O2/CO2 atmospheres.

25, 30, 31

CaO

(R14) 3 H 2O 2

(R18)

However, in oxy-fired conditions, the concentrations of CO2 significantly rise and calcination is inhibited for the usual range of bed temperatures, with a direct sulfation process then taking place:

21

1 CNO + O2 → NO + CO (R10) 2 3 1 3 NH3 + O2 → N2 + H 2O (R11) 4 2 2 5 3 O2 → NO + H 2O 4 2

1 O2 ⇆CaSO4 2

(R17)

The process depicted by reactions (R17) and (R18), the socalled indirect sulfation, takes place when the CO2 partial pressure is below the equilibrium pressure given by eq 1.

(R5)

N2O + CO → N2 + CO2

NH3 +

ref 14, 23

1 HCN + O2 → CNO + H (R1) 2 1 char N + O2 → CNO (R2) 2 CNO + NO → N2O + CO2 (R3)

(R15)

(R16)

Under oxy-fired conditions, Lasek et al.27 found a decreasing tendency of HCN to form when the O2 concentration in the fluidizing gas was increased, whereas Giménez-López et al.28 observed a decrease in the oxidation of HCN due to the reduction of free radicals −O induced by CO2-enriched atmospheres. Then, a decrease of NO and N2O formation from HCN is expected when the CO2 concentration in fluidizing gas is increased. The other main precursor released from the fuel, NH3, can be directly oxidized to form NO [reactions (R11) and (R12)].21,29,30 Both reactions are catalyzed by the presence of bed constituents like char, CaO, and bed material. At the same time, NO can be reduced to N2 by homogeneous reactions in the gas-phase reactions (R13)−(R15) or by heterogeneous reactions with carbon in char [reaction (R16)]. Reaction (R13) is of particular interest because high concentrations of CO can be found in the dense zone of a fluidized-bed combustor, being after oxidized along the reactor. Furthermore, the reaction is catalyzed by char and CaO, mainly found in the dense zone. In the case of oxy-fuel applications, the high CO2 concentrations favor char gasification reactions, so it is expected that the role of reaction (R13) is additionally enhanced.32,33 The presence of CaO can affect the formation of NOx emissions, namely, CaO catalyzes the N2O reduction, varies the HCN selectivity, enhancing the NO formation, increases free-radical concentrations (−O, −H, and −OH), which favors N2O depletion and NO formation, enhances CO oxidation,

3. EXPERIMENTAL FACILITY AND TESTS The research is based in experimentation carried out in a 90 kWth bubbling, oxy-fired fluidized-bed reactor (Figure 1). The rig is 2.5 m high with 0.2 m i.d., with the capability of operating under air-firing (AF) mode, oxy-firing (OF) mode, and oxy-firing with recycled flue gases (OFR) mode. Fluidizing gas is introduced through a distributor plate, and the reactor is instrumented to obtain online measurements of inlet gas flow rates, temperatures, and pressures. During the OF mode, fluidizing gas is supplied by a set of O2 and CO2 commercial bottles. A detailed description of the facility can be found elsewhere.11,39−41 A Spanish coal was selected to conduct the tests, and two highcalcium limestones were added as sorbents. Table 2 shows the composition of the fuel and limestones selected. The bed was initially formed by silica sand ranging within 175−1000 μm, with a mean size of 550 μm. Coal was sieved to 700−1200 μm and limestone to 100−1000 μm with a mean size of 500 μm. Gas compositions are online measured by a gas analyzer, determining the CO2, CO, SO2, and NO concentrations on a dry basis by nondispersive IR absorption (Siemens ULTRAMAT 6) and the O2 concentration with a paramagnetic sensor (Siemens OXYMAT 6). Two sampling points are available, one located at the reactor inlet to obtain the composition of the fluidizing gas and the other at the reactor outlet to measure the composition of flue gases. All measurements are taken on a dry basis, and the normalization given by eq 2 is adopted42 in order to 7620


Energy & Fuels

Article

Figure 1. CIRCE oxy-fuel fluidized-bed facility.11 by varying the air flow rate during the air-fired tests and by varying either the fluidizing gas flow rate or the fuel flow rate during the oxy-fired tests, always warranting a proper fluidization velocity (1.2−1.5 m/s). Details about the condition ranges during the tests can be seen in Table 3.

Table 2. Coal and Limestone Composition (wt %) coal proximate analysis (%) moisture ash volatile matter fixed carbon

1.00 31.55 7.55 59.90

carbon hydrogen nitrogen sulfur oxygen CaCO3 MgCO3 Other CaCO3 MgCO3 other

61.27 2.07 0.88 1.34 2.57 99.00 0.30 0.70 97.60 0.60 1.80

Table 3. Operating Conditions during the Tests test no.

ultimate analysis (%, d.b.)

limestone #1

limestone #2

1 2 3 4, 5 6 7−9 10, 11 12 13

compare the results in the following section of the paper, where the O2 concentration is expressed on a percent molar basis and fg stands for the flue gases and “inlet” for the feeding gas supplied to the reactor. [gas]6% = [gas]fg

[O2 ]inlet − 6 [O2 ]inlet − [O2 ]fg

fluidizing gas air

40:60 O2/ CO2 55:45 O2/ CO2 25:75 O2/ CO2

55:45 O2/ CO2

limestone

Ca:S ratio

bed temperature, Tbed (°C)

oxygen ratio, λ

none #1 #2 none

0 4 4 0

800−875 850 850 850, 875

1.6−1.7 1.6 1.6 1.6

#2 #2

2.5 2.5

850 850, 900, 950

1.6 1.6

#1 (calcined) #2 (calcined) #1 (calcined)

4

850, 900

1.1−1.7

4

850

1.7

4

900

1.3−1.7

All of the tests are started under air-fired conditions, and once the temperature and emissions are stable, the switch to oxy-firing is carried out by supplying O2/CO2 from the bottles (no flue gas recycling was used in this campaign). The duration of oxy-fired periods is between 1.5 and 2 h. A set of data are selected after pressures, temperatures, and flow rates have remained stable for at least 10−15 min. Combustion efficiencies during anthracite oxy-firing campaigns in the case-study facility are over 94.5%, as reported in detail by Guedea et al.43 Most of the oxy-firing experiments took place under calcining conditions; see Figure 2 (points to the right of the continuous line, see

(2)

The tests were executed under air-fired and oxy-fired conditions, the latter for three different O2 concentrations: 25%, 40%, and 55%. The temperature ranged within 800−950 °C, and then the influence of the noncalcining or calcining conditions could also be evaluated. Stoichiometric oxygen ratio was in the range 1.1−1.7; it was controlled 7621


Energy & Fuels

Article

as well as the concentration of free radicals (−O and −OH) promoting the oxidation of NO precursors.21 Nonetheless, and under specific combustion conditions, the effect of increasing temperature can be the opposite, reducing the total NO emitted.49 A comparison between the air-firing and oxy-firing numbers is not adequate in terms of concentration units because of the lower flue gas flow rates occurring during oxygen combustion. In order to avoid this distortion, a conversion ratio of fuel N to NO is commonly used in the literature.50,51 We have calculated this conversion ratio by closing the mass balance of the nitrogen, by using coal analysis and flow rate, and by flue gas composition and flow rate. The fuel flow rate is obtained from the calibrated discharge of the feeding screw (speed vs mass flow rate), and the flue gas flow rate is given by Pitot flowmeters. Figure 3b depicts the conversion ratios for the same tests as those displayed in Figure 3a. It is worth mentioning that the conversion ratios during oxy-fired tests are lower than those calculated during air-fired tests. While the former remained between 7.8 and 8.0%, the maximum conversion ratio during air firing surpassed 14.5%. This result is in good agreement with previous experiences published by Jia et al.36 and Czakiert et al.,52 and it is explained by the effect of CO2 in the atmosphere. Char gasification reactions are enhanced and the CO concentration in the dense zone increases, leading to NO reduction.53−57 Analysis of the influence of the temperature should be focused not only on the bed temperature but also on the free-board temperature profile.58 This is due to the fact that the kinetics of NO formation/depletion can still be of relevance for a temperature over 600 °C.29,59−62 In the case of the present work, this effect is neglected for a combination of reasons: first, the fuel is of very low volatile content, and most of the conversion happens in the dense zone;63 second, elutriation rates in the casestudy bubbling tests are very low, and heterogeneous reactions are not relevant beyond the splash region; finally, a sharp temperature decrease is observed in the free board because of a refractory-lined enclosure, inhibiting homogeneous conversion of nitrogen compounds. Values of this free-board temperature during the tests are shown in the forthcoming sections. 4.2. Limestone Addition and O2 Concentration. NO emissions obtained during AF experiments, without and with the two selected limestones, are represented in Figure 4. The effect of introducing the limestone is clearly observed: while limestone #1 (L1) hardly increases the emission about 40 mg/Nm3, the use of

Figure 2. Calcining or noncalcining conditions during oxy-fired tests, according to CO2 partial pressure and temperature. eq 1). Nevertheless, some of the tests are located in a region where calcination is a much slower process than sulfation, and then the availability of free CaO is quite low. This region falls between the continuous and dashed lines in Figure 2, according to the work by de Diego et al.44

4. RESULTS AND DISCUSSION 4.1. Temperature. First, the effect of the temperature is represented in Figure 3, where the results from air-fired and oxyfired tests (40:60 O2/CO2) without limestone addition are displayed. Under air firing, an increasing tendency is observed in a wide range of temperatures (800−875 °C); see Figure 3a. The NO concentration in flue gases rises from 265 to 306 mg/Nm3. The tendency is the same for oxy-firing, despite the narrower temperature range that is available. These results are in agreement with the data collected by Glarborg,25 who reported that the higher the bed temperature, the higher the NO emissions. Coal devolatilization is strongly influenced by the temperature, while volatile N/char N also depends on the temperature and fuel rank.25,45−47 In the case of high-ranking coals, like anthracite, the effect of the temperature is, nevertheless, attenuated in comparison to lignite because of the lower amount of volatiles.48 The improvement of char combustion due to higher temperatures increases char-N release,

Figure 3. Effect of the temperature on NO emissions and conversion factor. AF and OF tests without limestone addition: (a) concentration in flue gases; (b) conversion ratio of fuel N to NO. 7622


Energy & Fuels

Article

Figure 4. Effect of the limestone addition on the NO emissions and conversion factor during air-fired tests.

Figure 5. Effect of limestone addition on the NO emission and conversion factor during oxy-fired tests: 40:60 and 55:45 O2/CO2.

The observed increase of the NO levels due to limestone addition is coherent with previous works reported by different researchers. de Diego et al.14 found that the addition of limestone to reduce SO2 in a fluidized bed doubled the NO emission, whereas N2O diminished. A similar result was obtained by Hayhurst and Lawrence,64 who detected an important increase of NO emissions when limestone was added, with N2O almost unaltered. Zijlma and co-workers29,60,65 analyzed the influence of CaO on NH3 oxidation to NO, obtaining an important increase of the NO emitted. They also reported a sharp decrease of NH3 oxidation once CaO was sulfated. Differences among the tests rely on the catalyzing effect of free CaO existing in the bed. Limestone #1 possessed much more sulfur retention capacity than limestone #2, as expected because of previous experiences.42,44,66 SO2 emissions during L1 and L2 tests respectively were 427 and 1302 mg/Nm3, which supposed desulfurization efficiencies of 86% and 56%. Because the indirect

Figure 6. Effect of the temperature on the NO emissions during 55:45 oxy-fired tests, with limestone #2 addition (Ca:S = 2.5; λ = 1.6).

limestone #2 (L2) brings along a significant increase, more than doubling the NO emissions. 7623


Energy & Fuels

Article

Figure 7. Effect of calcined limestone addition on the NO emission and conversion factor during oxy-fired tests: 25:75 O2/CO2.

Figure 8. Effect of oxygen excess on the NO emission and conversion factor during oxy-fired tests: 25:75 and 55:45 O2/CO2 (Ca:S = 4; Tb = 900 °C).

fresh limestone does not show relevant variation of NO emission and fuel-N conversion, which yields a low effect of CaCO3 on the NO mechanism, in contrast that of CaO. This result is in good agreement with the findings reported by Johnsson.18 The enrichment in oxygen produces a significant increase of NO emissions, but this result is being distorted by the lower amount of flue gases. The comparison has to be better done in terms of the NO/fuel N conversion factor, and then the increase is not as high. This increase is explained by the higher O2 concentration rather than the CaO contribution. Even though the 55:45 O2/CO2 test is located to the right of the equilibrium line (see Figure 2), it is quite close and within the region where sulfation is much quicker than calcination. As for the effect of the CO concentration and free-board temperature, the same comments already given for Figure 4 can now be made. To seek the influence of the sulfation mechanism, additional 55:45 O2/CO2 tests were done with increasing bed temperature (Tb = 850, 900, and 950 °C): the higher the temperature, the higher the expected CaO available due to calcination kinetics. The results are shown in Figure 6: the increasing trend of the NO

sulfation mechanism is occurring, the availability of CaO is responsible for such NO augmentation. These results allow thinking on a quicker development of a CaSO4 on limestone #1 surface, which hides the remaining CaO in the sorbent particle. In this way, the catalyst effect of CaO is hidden. Hansen et al.67 also found that the more active the limestone (to capture SO2), the lower the NO induced by the presence of CaO. As already cited in section 2, the role of CO can also be of relevance. In the tests, quite different CO levels arose, as represented in Figure 4a. The observed trend is again coherent because the higher the CO, the lower the NO emissions, pointing out the reducing effect of CO. Free-board temperatures remained at low values, between 350 and 450 °C, so its influence on NO conversion can be assumed to be negligible. Figure 5 shows the results obtained under oxy-fired conditions. In this case, a 2-fold effect can be observed: the influence of adding L2 (Ca:S = 2.5) for a 40:60 O2/CO2 atmosphere and the effect of increasing the O2 concentration in the fluidizing gas from 40% to 55%. The experiments carried out for a 40:60 O2/ CO2 atmosphere fall in the noncalcining region. The addition of 7624


Energy & Fuels

Article

(i) The higher the bed temperature, the higher the NO emissions and nitrogen conversion ratios. In the case of oxy-fired tests, control of the bed temperature is outstanding because of its influence on the sulfation mechanism of the limestone. (ii) Operation under calcining conditions during oxy-fuel FB combustion can be a target concerning optimization of the desulfurization efficiency, but the increase of free CaO enhances NO emissions. The catalyzing effect of CaCO3 observed under noncalcining conditions is not as relevant compared to CaO. (iii) The limestone capability to retain SO2 significantly influences NO emissions. Under the same operating conditions, limestone #2 has always provided higher SO2 and NO emission levels. (iv) The higher the excess oxygen, the higher the NO emissions. Nevertheless, a larger influence is related to the percent of O2 in the fluidizing gas, prevailing over excess oxygen.

emission and conversion factor is coherent, although quite limited. Anyway, it is not possible to ascertain the isolated effect of CaO because the increase of the temperature also contributes to enhancement of the kinetics of other oxidation mechanisms. This points out the conclusion for the temperature trade-off: increasing the bed temperature can be a target from the point of view of sulfur retention because of the promotion of indirect capture, but it could eventually be a drawback because of enhancement of NO production. Two additional tests were executed, under a 25:75 O2/CO2 atmosphere, to further check the influence of the kind of limestone. Now, both limestones were calcined during a previous air-fired operating period, in order to increase the free CaO in the bed. The ted temperature, Ca:S ratio, and excess oxygen were kept the same. The results are shown in Figure 7. Despite the fact that the NO emissions are very similar, around 600 mg/Nm3, the conversion factor is higher for the L2 test, resulting in a value about 8% over the L1 test. This is explained by the different thermal inputs during the tests, 42 kW (L1) vs 32 kW (L2). In order to prevent load distortion, the normalized NO emissions per load unit were also computed: 157.8 mg/MJ (L1) vs 225.4 mg/MJ (L2). These values confirm the effect already observed for the AF tests, i.e., the larger influence of limestone #2 in promoting NO emissions. Again, the different capacities to capture SO2 of the limestones is confirmed by the SO2 emissions, 636 mg/Nm3 (L1) vs 1200 mg/Nm3 (L2), supposing a desulfurization efficiency of 86% and 63%, respectively. 4.3. Excess Oxygen. The effect of oxygen excess increasing NO emissions is well documented in the case of conventional combustion.14,68−70 For oxy-fuel fluidized-bed combustion, the analysis should also consider the composition of the fluidizing gas. For this purpose, tests under 25:75 and 55:45 O2/CO2 atmospheres have been executed for a range of oxygen excesses with the rest of parameters kept constant (calcined limestone #1, Ca:S ratio, and bed temperature). Figure 8 summarizes the results obtained. Regarding the results from the experiments with 25:75 O2/ CO2, the effect is quite moderate within the lowest range λ = 1.1−1.3 but much more relevant for λ = 1.5−1.7. Here emissions increase from 590 to 685 mg/Nm3 (Figure 8a) and conversion ratios from 18% to 19.2% (Figure 8b). A similar trend is observed when the O2 concentration in the fluidizing gas is increased to 55%. In this case, nitrogen conversion ratios are lower because of the higher records observed for the CO concentration; while CO remained within 200−250 mg/Nm3 during the 25:75 tests, it surpassed 500 mg/Nm3 for the 55:45 tests. The results provided by other researchers are in the same line (see Czakiert et al.,50 Duan et al.,71 and Hosoda and Hirama53) but report results for not as high O2 concentrations in the firing atmosphere. Because oxygen excess has to be maintained as low as possible to avoid unnecessary extra costs and dilution of leaving CO2, the eventual influence of this parameter in a full-scale facility should not be as crucial as the percent of O2 in the fluidizing gas.

■

AUTHOR INFORMATION

Corresponding Author

*Phone: +34 976 762 564. Fax: +34 976 732 078. E-mail: luisig@ unizar.es. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The work described in this paper was partially funded by the R +D Spanish National Program from the Spanish Ministry of Science and Innovation under Project ENE-2009-08246.

■

NOMENCLATURE AF air firing Ca:S calcium (in limestone) to sulfur (in coal) ratio OF oxy-firing Tb bed temperature (°C) Tfb free-board temperature (°C) λ oxygen ratio

■

REFERENCES

(1) Buhre, B. J. P.; Elliott, L. K.; Sheng, C. D.; Gupta, R. P.; Wall, T. F. Oxy-fuel combustion technology for coal-fired power generation. Prog. Energy Combust. Sci. 2005, 31 (4), 283−307. (2) 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. (3) 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. (4) Technology RoadmapCarbon capture and storage; International Energy Agency: Paris, Oct 2010. (5) Anthony, E. J. Oxyfuel CFBC: Status and anticipated development. Greenhouse Gases: Sci. Technol. 2013, 3 (2), 116−123. (6) Bolea, I.; Romeo, L. M.; Pallarés, D. The role of external heat exchangers in oxy-fuel circulating fluidized bed. Appl. Energy 2012, 94, 215−223. (7) Andersson, K.; Johnsson, F. Flame and radiation characteristics of gas-fired O2/CO2 combustion. Fuel 2007, 86 (5−6), 656−668. (8) Edge, P.; Gharebaghi, M.; Irons, R.; Porter, R.; Porter, R. T. J.; Pourkashanian, M.; Smith, D.; Stephenson, P.; Williams, A. Combustion modelling opportunities and challenges for oxy-coal carbon capture technology. Chem. Eng. Res. Des. 2011, 89 (9), 1470−1493. (9) Rezvani, S.; Huang, Y.; McIlveen-Wright, D.; Hewitt, N.; Wang, Y. Comparative assessment of sub-critical versus advanced super-critical

5. CONCLUSIONS The effect of temperature, limestone and excess oxygen on NO emissions under oxy-firing of coal in a fluidized bed reactor has been determined. For this purpose, an experimental campaign has been conducted in a 90 kWth nominal load reactor, for a combination of oxidation atmospheres −including air−, temperatures, type of limestone, Ca:S ratios and oxygen supplied. Conclusions obtained from the tests can be summarized in the following points: 7625


Energy & Fuels

Article

oxyfuel fired PF boilers with CO2 sequestration facilities. Fuel 2007, 86 (14), 2134−2143. (10) Simonsson, N.; Anheden, M.; Dubettier, R.; Joly, L.; Lockwood, F.; Tranier, J. P., Development Opportunities for Future Large Scale Lignite Oxyfuel Power Plants. Oxyfuel Combustion Conference 2, Queensland, Australia, 2011. (11) Romeo, L. M.; Díez, L. I.; Guedea, I.; Bolea, I.; Lupiáñez, C.; González, A.; Pallarés, J.; Teruel, E. Design and operation assessment of an oxyfuel fluidized bed combustor. Exp. Therm. Fluid Sci. 2011, 35 (3), 477−484. (12) 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. (13) Lupion, M.; Diego, R.; Loubeau, L.; Navarrete, B. CIUDEN CCS Project: Status of the CO2 capture technology development plant in power generation. Energy Proc. 2011, 4, 5639−5646. (14) de Diego, L. F.; Londono, C. A.; Wang, X. S.; Gibbs, B. M. Influence of operating parameters on NOx and N2O axial profiles in a circulating fluidized bed combustor. Fuel 1996, 75 (8), 971−978. (15) Miccio, F.; Löffler, G.; Wargadalam, V. J.; Winter, F. The influence of SO2 level and operating conditions on NOx and N2O emissions during fluidised bed combustion of coals. Fuel 2001, 80 (11), 1555− 1566. (16) Chen, Z.; Lin, M.; Ignowski, J.; Kelly, B.; Linjewile, T.; Agarwal, P. Mathematical modeling of fludized bed combustion. 4: N2O and NOx emissions from the combustion of char. Fuel 2001, 80, 1259−1272. (17) Lázaro, M. J.; Ibarra, J.; Moliner, R.; González de Andrés, A.; Thomas, K. M. The release of nitrogen during the combustion of coal chars: the role of volatile matter and surface area. Fuel 1996, 75 (8), 1014−1024. (18) Johnsson, J. E. Formation and reduction of nitrogen oxides in fluidized-bed combustion. Fuel 1994, 73 (9), 1398−1415. (19) Li, Y.; Lu, G.; Rudolph, V. The kinetics of NO and N2O reduction over coal chars in fluidised-bed combustion. Chem. Eng. Sci. 1998, 53 (1), 1−26. (20) Mahmoudi, S.; Baeyens, J.; Seville, J. P. K. NOx formation and selective non-catalytic reduction (SNCR) in a fluidized bed combustor of biomass. Biomass Bioenergy 2010, 34 (9), 1393−1409. (21) Hayhurst, A.; Lawrence, D. The Amounts of NOx and N2O Formed in a Fluidized Bed Combustor during the Burning of Coal Volatiles and Also of Char. Combust. Flame 1996, 105, 341−357. (22) Kazanc, F.; Khatami, R.; Manoel Crnkovic, P.; Levendis, Y. A. Emissions of NOx and SO2 from Coals of Various Ranks, Bagasse, and Coal−Bagasse Blends Burning in O2/N2 and O2/CO2 Environments. Energy Fuels 2011, 25 (7), 2850−2861. (23) Winter, F.; Wartha, C.; Löffler, G.; Hofbauer, H. The NO and N2O formation mechanism during devolatilization and char combustion under fluidized-bed conditions. Symp. (Int.) Combust. 1996, 26 (2), 3325−3334. (24) Konttinen, J.; Kallio, S.; Hupa, M.; Winter, F. NO formation tendency characterization for solid fuels in fluidized beds. Fuel 2013, 108, 238−246. (25) Glarborg, P. Fuel nitrogen conversion in solid fuel fired systems. Prog. Energy Combust. Sci. 2003, 29 (2), 89−113. (26) Wartha, C.; Winter, F.; Hofbauer, H. The Trade-Off between N2, NO and N2O under Fluidized Bed Combustor Conditions. J. Energy Resour. Technol. 2000, 122, 94−100. (27) Lasek, J. A.; Janusz, M.; Zuwała, J.; Głód, K.; Iluk, A. Oxy-fuel combustion of selected solid fuels under atmospheric and elevated pressures. Energy 2013, http://dx.doi.org/10.1016/j.energy.2013.04. 079. (28) Giménez-López, J.; Millera, A.; Bilbao, R.; Alzueta, M. U. HCN oxidation in an O2/CO2 atmosphere: An experimental and kinetic modeling study. Combust. Flame 2010, 157 (2), 267−276. (29) Zijlma, G. J.; Jensen, A.; Johnsson, J. E.; van den Bleek, C. M. The influence of H2O and CO2 on the reactivity of limestone for the oxidation of NH3. Fuel 2000, 79 (12), 1449−1454.

(30) Kiil, S.; Bhatia, S. K.; Dam-Johansen, K. Modelling of catalytic oxidation of NH3 and reduction of NO on limestone during sulphur capture. Chem. Eng. Sci. 1996, 51 (4), 587−601. (31) Winter, F.; Wartha, C.; Löffler, G.; Hofbauer, H. The NO and N2O formation mechanism during devolatilization and char combustion under fluidized-bed conditions. 26th Symp. (Int.) Combust. 1996, 3325− 3334. (32) Glarborg, P.; Bentzen, L. L. B. Chemical Effects of a High CO2 Concentration in Oxy-Fuel Combustion of Methane. Energy Fuels 2007, 22 (1), 291−296. (33) Mendiara, T.; Glarborg, P. Ammonia chemistry in oxy-fuel combustion of methane. Combust. Flame 2009, 156 (10), 1937−1949. (34) Liu, H.; Gibbs, B. M. The influence of calcined limestone on NOx and N2O emissions from char combustion in fluidized bed combustors. Fuel 2001, 80 (9), 1211−1215. (35) Liu, H.; Katagiri, S.; Kaneko, U.; Okazaki, K. Sulfation behavior of limestone under high CO2 concentration in O2/CO2 coal combustion. Fuel 2000, 79, 945−953. (36) Jia, L.; Tan, Y.; Anthony, E. J. Emissions of SO2 and NOx during Oxy-Fuel CFB Combustion Tests in a Mini-Circulating Fluidized Bed Combustion Reactor. Energy Fuels 2010, 24 (2), 910−915. (37) Varonen, M.; Hyytiäinen, I.; Palonen, M.; Ylä-Outinen, V. Results of oxyfuel combustion tests in 4 MWth CFB pilot boiler. 21st International Conference on Fluidized Bed Combustion, Naples, Italy, 2012. (38) de Diego, L. F.; Rufas, A.; García-Labiano, F.; de las ObrasLoscertales, M.; Abad, A.; Gayán, P.; Adánez, J. Optimum temperature for sulphur retention in fluidised beds working under oxy-fuel combustion conditions. Fuel 2012, http://dx.doi.org/10.1016/j.fuel. 2012.02.064. (39) Guedea, I.; Bolea, I.; Lupiáñez, C.; Cortés, N.; Teruel, E.; Pallarés, J.; Díez, L. I.; Romeo, L. M. Control system for an oxy-fuel combustion fluidized bed with flue gas recirculation. Energy Proc. 2011, 4, 972−979. (40) Guedea, I.; Díez, L. I.; Pallarés, J.; Romeo, L. M. Influence of O2/ CO2 mixtures on the fluid-dynamics of an oxy-fired fluidized bed reactor. Chem. Eng. J. 2011, 178, 129−137. (41) Font, O.; Córdoba, P.; Leiva, C.; Romeo, L. M.; Bolea, I.; Guedea, I.; Moreno, N.; Querol, X.; Fernandez, C.; Díez, L. I. Fate and abatement of mercury and other trace elements in a coal fluidised bed oxy combustion pilot plant. Fuel 2012, 95, 272−281. (42) Lupiáñez, C.; Guedea, I.; Bolea, I.; Díez, L. I.; Romeo, L. M. Experimental study of SO2 and NOx emissions in fluidized bed oxy-fuel combustion. Fuel Process. Technol. 2013, 106, 587−594. (43) Guedea, I.; Díez, L. I.; Pallarés, J.; Romeo, L. M. On the modeling of oxy-coal combustion in a fluidized bed. Chem. Eng. J. 2013, 228, 179− 191. (44) de Diego, L. F.; de las Obras-Loscertales, M.; García-Labiano, F.; Rufas, A.; Abad, A.; Gayán, P.; Adánez, J. Characterization of a limestone in a batch fluidized bed reactor for sulfur retention under oxy-fuel operating conditions. Int. J. Greenhouse Gas Control 2011, 5 (5), 1190− 1198. (45) Kambara, S.; Takarada, T.; Yamamoto, Y.; Kato, K. Relation between functional forms of coal nitrogen and formation of nitrogen oxide (NOx) precursors during rapid pyrolysis. Energy Fuels 1993, 7 (6), 1013−1020. (46) Baxter, L. L.; Mitchell, R. E.; Fletcher, T. H.; Hurt, R. H. Nitrogen Release during Coal Combustion. Energy Fuels 1996, 10 (1), 188−196. (47) Thomas, K. M. The release of nitrogen oxides during char combustion. Fuel 1997, 76 (6), 457−473. (48) Valentim, B.; Lemos de Sousa, M. J.; Abelha, P.; Boavida, D.; Gulyurtlu, I. Combustion studies in a fluidised bedThe link between temperature, NOx and N2O formation, char morphology and coal type. Int. J. Coal Geol. 2006, 67 (3), 191−201. (49) Tarelho, L. A. C.; Matos, M. A. A.; Pereira, F. J. M. A. Axial Concentration Profiles and NO Flue Gas in a Pilot-Scale Bubbling Fluidized Bed Coal Combustor. Energy Fuels 2004, 18 (6), 1615−1624. (50) Czakiert, T.; Sztekler, K.; Karski, S.; Markiewicz, D.; Nowak, W. Oxy-fuel circulating fluidized bed combustion in a small pilot-scale test rig. Fuel Process. Technol. 2010, 91 (11), 1617−1623. 7626


Energy & Fuels

Article

(51) Jia, L.; Tan, Y.; Wang, C.; Anthony, E. J. Experimental Study of Oxy-Fuel Combustion and Sulfur Capture in a Mini-CFBC. Energy Fuels 2007, 21, 3160−3164. (52) Czakiert, T.; Bis, Z.; Muskala, W.; Nowak, W. Fuel conversion from oxy-fuel combustion in a circulating fluidized bed. Fuel Process. Technol. 2006, 87 (6), 531−538. (53) Hosoda, H.; Hirama, T. NOx and N2O Emission in Bubbling Fluidized-Bed Coal Combustion with Oxygen and Recycled Flue Gas: Macroscopic Characteristics of Their Formation and Reduction. Energy Fuels 1998, 12, 102−108. (54) Niu, S. L.; Han, K. H.; Lu, C. M. Characteristic of coal combustion in oxygen/carbon dioxide atmosphere and nitric oxide release during this process. Energy Convers. Manage. 2011, 52 (1), 532−537. (55) Scala, F.; Chirone, R. Fluidized bed combustion of single coal char particles at high CO2 concentration. Chem. Eng. J. 2010, 165 (3), 902− 906. (56) Duan, L.; Zhao, C.; Ren, Q.; Wu, Z.; Chen, X. NOx precursors evolution during coal heating process in CO2 atmosphere. Fuel 2011, 90 (4), 1668−1673. (57) Abián, M.; Giménez-López, J.; Bilbao, R.; Alzueta, M. U. Effect of different concentration levels of CO2 and H2O on the oxidation of CO: Experiments and modeling. Proc. Combust. Inst. 2011, 33 (1), 317−323. (58) Tarelho, L. A. C.; Matos, M. A. A.; Pereira, F. J. M. A. Axial concentration profiles and N2O flue gas in a pilot scale bubbling fluidised bed coal combustor. Fuel Process. Technol. 2005, 86 (8), 925− 940. (59) Zevenhoven, R.; Hupa, M. The reactivity of chars from coal, peat and wood towards NO, with and without CO. Fuel 1998, 77 (11), 1169−1176. (60) Zijlma, G. J.; Jensen, A. D.; Johnsson, J. E.; van den Bleek, C. M. NH3 oxidation catalysed by calcined limestonea kinetic study. Fuel 2002, 81 (14), 1871−1881. (61) Desroches-Ducarne, E.; Dolignier, J. C.; Marty, E.; Martin, G.; Delfosse, L. Modelling of gaseous pollutants emissions in circulating fluidized bed combustion of municipal refuse. Fuel 1998, 77 (13), 1399−1410. (62) Jensen, A.; Johnsson, J. E.; Andries, J.; Laughlin, K.; Read, G.; Mayer, M.; Baumann, H.; Bonn, B. Formation and reduction of NOx in pressurized fluidized bed combustion of coal. Fuel 1995, 74 (11), 1555− 1569. (63) Tarelho, L. A. C.; Matos, M. A. A.; Pereira, F. J. M. A. Influence of limestone addition on the behaviour of NO and N2O during fluidised bed coal combustion. Fuel 2006, 85 (7−8), 967−977. (64) Hayhurst, A.; Lawrence, D. The Effect of Solid CaO on the Production of NOx and N2O in Fluidized Bed Combustors: Studies using Pyridine as a Prototypical Nitrogenous Fuel. Combust. Flame 1996, 105, 511−527. (65) Zijlma, G. J.; Jensen, A. D.; Johnsson, J. E.; van den Bleek, C. M. NH3 oxidation catalyzed by partially sulphated limestonemodelling and experimental work. Fuel 2004, 83 (2), 237−251. (66) García-Labiano, F.; Rufas, A.; de Diego, L. F.; de las ObrasLoscertales, M.; Gayán, P.; Abad, A.; Adánez, J. Calcium-based sorbents behaviour during sulphation at oxy-fuel fluidised bed combustion conditions. Fuel 2011, 90 (10), 3100−3108. (67) Hansen, P. F. B.; Dam-Johansen, K.; Johnsson, J. E.; Hulgaard, T. Catalytic Reduction on NO and N2O on Limestone during Sulfur Capture under Fluidized Bed Combustion Conditions. Chem. Eng. Sci. 1992, 47 (9−11), 2419−2424. (68) Svoboda, K.; Pohořelý, M. Influence of operating conditions and coal properties on NOx and N2O emissions in pressurized fluidized bed combustion of subbituminous coals. Fuel 2004, 83 (7−8), 1095−1103. (69) Kuprianov, V. I.; Permchart, W.; Janvijitsakul, K. Fluidized bed combustion of pre-dried Thai bagasse. Fuel Process. Technol. 2005, 86 (8), 849−860. (70) Varol, M.; Atimtay, A. T. Combustion of olive cake and coal in a bubbling fluidized bed with secondary air injection. Fuel 2007, 86 (10− 11), 1430−1438.

(71) Duan, L.; Zhao, C.; Zhou, W.; Qu, C.; Chen, X. Effects of operation parameters on NO emission in an oxy-fired CFB combustor. Fuel Process. Technol. 2011, 92 (3), 379−384.

7627


NO Emissions from Anthracite Oxy-Firing in a Fluidized-Bed Combustor

Recommend Documents