Article pubs.acs.org/EF
Study of Oxy-fuel Coal Combustion in a 0.1 MWth Circulating Fluidized Bed at High Oxygen Concentrations Wei Li,†,‡ Shiyuan Li,*,† Qiangqiang Ren,† Li Tan,†,‡ Haoyu Li,† Jingzhang Liu,† and Qinggang Lu† †
Institute of Engineering Thermophysics (IET), Chinese Academy of Sciences (CAS), Beijing 100190, People’s Republic of China University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China
‡
ABSTRACT: The combustion and emission characteristics under three kinds of firing modes [O2/N2, O2/CO2, and O2/ recycled flue gas (RFG)] at high oxygen concentrations were investigated in a 0.1 MWth circulating fluidized-bed combustion system. The 0.1 MWth circulating fluidized-bed combustion unit was built in 2010 and retrofitted to use RFG in 2012, which was specially designed for combustion at high oxygen concentrations (up to 55%). The experimental results showed that CO2 concentrations in the flue gas reached a maximum of 93% when the overall oxygen concentration was in the range of 46.4−54.6% during the combustion. Under the O2/RFG firing mode, the CO concentration dropped from 803.4 to below 100 mg/MJ with the increase of the concentration of oxygen in the flue gas from 1.1 to 7.5%. The fuel N to NO conversion was in the range of 3.5−4.6% under the O2/RFG firing mode, which was much less than that observed under the O2/N2 mode (8.1−8.4%) and O2/ CO2 mode (6.5−7.6%). The limestone utilization in the O2/RFG firing mode was found to be much lower than that for both O2/N2 and O2/CO2 modes. Sulfur capture efficiencies were 70.2 and 93.7% under O2/RFG and O2/N2 firing mode conditions, respectively, with a Ca/S molar ratio of 2.2. An increase in the Ca/S molar ratio to 3 resulted in sulfur capture efficiencies of 83.9% under the O2/RFG firing mode.
1. INTRODUCTION Oxy-fuel combustion is widely considered as one of the most promising technologies for the capturing of carbon dioxide (CO2) emissions from coal-fired power plants. Up to now, most of the research in this field has primarily been focused on pulverized coal (PC) combustion methods,1−3 which have effectively demonstrated that no technical barrier exists to prohibit the construction of oxy-fuel pulverized fuel combustion units. Oxy-fuel circulating fluidized-bed combustion (CFBC) has several advantages that often make it a better choice for CO2 capture; CFBCs have the benefit of fuel flexibility, inherently low NOx emissions, the potential to achieve SO2 emission reductions of 90% or more by in situ addition of limestone, etc. Because the circulating fluidized-bed (CFB) boiler does not need sophisticated burner designs and burner management, it is considerably easier to convert existing CFB boilers from air- to oxy-fired operation compared to the PC boiler. For the above reasons, CFB oxy-fuel combustion technology has recently attracted further attention from investigators. Researchers at Foster Wheeler and VTT reported the experimental results of oxy-fuel combustion and sulfur capture at oxygen concentrations in the range of 28.6−39.8% in a 100 kW CFBC unit.4,5 VTT also reported pilot-scale tests at a oxygen concentration of 45%.6 CanmetENERGY (Canada) has reported extensively on developments in oxy-fuel technology since 2006, starting with a 100 kW mini-CFBC unit;7 further, a 0.8 MW CFBC8,9 unit at CanmetENERGY has been retrofitted for oxy-fuel research, with tests carried out at oxygen concentrations of ca. 29%. The Southeast University of China reported tests conducted in a 50 kW CFBC unit with bituminous coal at oxygen concentrations of 21−25%.10 A report from the Czes̨ tochowa University of Technology © 2014 American Chemical Society
(Poland) has also described trials employing a 100 kW CFBC unit;11 however, a recycled flue gas (RFG) was instead simulated with mixtures of pure O2 and CO2. The construction of a 30 MW oxy-CFBC unit installation12 in Spain (CIUDEN) has been reported to include a CO2 processing unit among other associated equipment. One of the issues under investigation regarding CFB systems under an enriched oxygen supply stream is the formation of pollutants, such as SO2 and NOx. Many investigations on the desulfurization characteristics of limestone under the oxy-fuel combustion have been conducted, with the consensus that the atmospheric composition strongly affects the sulfur retention capabilities of limestone. Zhao et al.10 found higher values of sulfur capture efficiencies under the oxy-fuel combustion compared to air combustion in a 50 kW CFBC unit. Czakiert et al.13 also observed the same result under the O2/CO2 atmosphere. However, Jia et al.7 obtained just the opposite outcome in a 100 kW mini-CFBC unit at CanmetENERGY. Experimental observations conducted in CFBC units differ on these atmospheric effects. With regard to NOx emissions, lower N conversion is observed under oxy-fuel combustion conditions in comparison to conventional air combustion.9,10 In addition, NO emissions increase with the O2 concentration in flue gas.14 Therefore, the combustion atmosphere is a very important factor that affects emission characteristics and limestone utilization during oxy-fuel combustion. The 0.1 MWth CFB combustor at the Institute of Engineering Thermophysics, Chinese Academy of Sciences (IET/CAS) was built in 2010 and, subsequently, retrofitted for Received: October 14, 2013 Revised: December 17, 2013 Published: January 6, 2014 1249
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Figure 1. Oxy-fuel CFBC system (0.1 MWth) at IET/CAS.
Table 1. Fuel Analyses for Datong Coal LHV (MJ kg−1)
ultimate analysis (wt %)
proximate analysis (wt %)
Qnet,ar
FCar
Mar
Aar
Var
Car
Har
Oar
Sar
Nar
22.61
44.38
2.2
26.05
27.37
58.08
3.73
8.58
0.32
1.04
Table 2. Composition of the Ash for Datong Coal composition
SiO2
Al2O3
Fe2O3
CaO
MgO
TiO2
SO3
P2O5
K2O
Na2O
content (wt %)
45.23
37.83
4.02
5.42
0.66
1.62
2.50
0.18
0.32
0.14
flue gas recycling in 2012. The facility is presently used to study combustion characteristics and emissions at high oxygen concentrations (up to 55%). The aim of the paper was to compare the combustion and emission characteristics under different firing modes at high oxygen concentrations in a 0.1 MWth CFBC system.
a fuel feeder and a sorbent feeder. The maximum coal feed rate is 54.5 kg/h. The RFG is extracted after passing through the bag filter, which then passes through the flue gas condenser, a heater, and a recirculation fan and finally enters the buffer tank. The RFG exits the buffer divided into primary and secondary fluidizing gases and passes into the mixers; oxygen, air, and RFG are then mixed at different ratios corresponding to the conditions examined. The flows, temperatures, and pressures of oxygen, air, and RFG are all measured and logged. The oxygen is supplied by an air separation unit (ASU) or a liquid oxygen tank (LOT). The capacity of oxygen generation for the ASU is 42.9 kg/h. The measurement system consists of thermocouples, pressure sensors, flow meters, and gas analyzers. The temperatures and pressures in the oxy-CFBC unit are measured at distances of 250, 800, 1520, 2500, 4000, and 5700 mm along the height of the combustor above the distributor and at the cyclone and the loop seal. The oxygen concentrations of the flue gas are measured by a zirconia oxygen analyzer. The concentrations of CO2, CO, SO2, and NOx in the flue gas are monitored online by a Fourier transform infrared (FTIR) analyzer (GASMET DX4000) before the bag filter. 2.2. Fuel and Bed Compositions. Datong coal was used in all tests. The fuel analyses are shown in Tables 1 and 2. The coal was
2. MATERIALS AND METHODS 2.1. Oxy-fuel CFBC System (0.1 MWth) at IET/CAS. The oxyfuel CFBC system is shown schematically in Figure 1, which consists of a refractory combustor equipped with water-cooling tubes and cyclone, U valve, flue gas cooler, bag filter, fuel and sorbent feed units, gas mix and supply units, flue gas recycling unit, and measurement and data acquisition systems. The combustor employed has a height of 6.0 m and an inside diameter of 100 mm in the lower portion, which is gradually expanded to 140 mm in the upper part. Four water tubes cool the refractory material for a total heat duty of 70 kW, with a water flow rate of 4 m3/ h. The CFBC is equipped with an oil burner for preheating. The hightemperature flue gas generated by the oil burner is used to heat the bed materials for fuel ignition and startup. The feeding unit consists of 1250
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Table 3. Limestone (CaCO3) Composition composition
CaO
SiO2
Al2O3
Fe2O3
K2O
TiO2
SrO
Cr2O3
content (wt %)
54.87
2.06
0.81
0.53
0.30
0.07
0.41
0.04
sieved to a diameter between 0.355 and 4 mm. Sand (5 kg) with particle diameters between 0.2 and 0.9 mm was used as the starting bed material. Additional fine sand was then added during the startup process to build up material circulation. Limestone (CaCO3) was fed continuously for sulfur removal performance studies under different combustion atmospheres. A limestone material having diameters between 0.1 and 1 mm was used; the composition is shown in Table 3. 2.3. Oxy-fuel Startup. The startup procedure differed from the usual oxy-fuel CFBC operations. Typically, the procedure for oxy-fuel CFBC startup uses air; once the combustor is in fully air-firing mode and solid circulation has been established, the switch from air- to oxyfuel-firing mode is made.8−10 In the present tests, the startup procedure used the enriched-air-firing mode (Figure 2). The high-
concentrations in the feed gas increased from 21 to 50 vol %, as temperature was increased to the test condition, as shown in Figure 2. When solid circulation was fully established, the switch from enriched air (O2/N2) to O2/RFG firing mode commenced; this was accomplished by gradually increasing flow rates of RFG and decreasing the air-flow rate to zero. In the test, this transition took less than 15 min, with virtually no temperature fluctuation observed within the combustor. No operational difficulties were encountered. The time for the total startup was nearly 7 h, which is attributed to heating of the refractory material and cyclone, as well as the presence of numerous fixed water-cooling tubes.
3. RESULTS AND DISCUSSION Table 4 presents the conditions for tests using the 0.1 MWth oxy-fuel CFBC under the different firing modes. The average temperature of the furnace was 850 °C in all tests. The velocities at the lower part were in the range of 3.85−4.47 m/s, and the overall oxygen concentrations were in the range of 46.4−54.6% [overall oxygen concentration = (the flow rate of oxygen in primary gas + the flow rate of oxygen in secondary gas)/(the flow rate of primary gas + the flow rate of secondary gas) × 100]. The recycle ratio was nearly 50%, which was lower compared to oxy-PC units. The oxygen was supplied by an ASU or a LOT during the different tests. The value of Ca/S in Table 4 is the molar ratio of the additional Ca and the total sulfur in the coal. All combustion test conditions resulted in smooth operation. The axial temperature profiles along the combustor in O2/RFG firing mode were stable and reasonably uniform. Stable particle fluidization is essential for the CFB combustion process; during the O2/RFG firing mode tests, overall oxygen concentrations were between 46.4 and 54.6% and the oxygen concentrations of the primary gas were between 46.6 and 56.7%. Good fluidization was achieved in the test. Furthermore, no sintering was observed during the test conditions. The purity of oxygen obtained from the ASU was between 83 and 90%; accordingly, the lower CO2 concentration measured for condition 1 (Table 4) was apparently caused by N2 within the system, which was generated by the ASU. Once the O2/ RFG firing mode combustion was established, the CO2
Figure 2. Oxygen concentration in the feed gas and coal feed rate during the startup stage. temperature flue gas generated by the oil burner was used to heat the bed materials. When the temperature of the lower part of the combustor warmed to 500 °C, intermittent coal feeding began. As the coal feed rate gradually ramped up, the oil feed rate was reduced until the solid fuel feed was completely established, at which point the oil supply was stopped. During this period, oxygen was added to ensure a temperature increase in the lower part because of the large number of fixed water-cooling tubes in this area. Therefore, the oxygen
Table 4. Experimental Conditions condition
1
2
3
4
combustion mode average combustor temperature (°C) O2 source O2 purity (%) primary O2 concentration (%) secondary O2 concentration (%) overall O2 concentration (%) Ca/S flue gas recycle ratio (%) flue gas O2 concentration (%) RFG O2 concentration (%) CO2 (vol %) fuel (kg/h) fluidization velocity at the upper part (m/s) fluidization velocity at the lower part (m/s)
850 ASU 79.1 49.5 45.3 48.0 0 37.5 1.1 2.1 67.1 17.8 3.5 4.5
O2/RFG 852 LOT 99.9 56.7 51.5 54.6 2.2 47.3 7.5 7.3 84.1 17.9 3.2 3.9
851 LOT 99.9 55.2 51.7 53.8 2.6 48.0 7.2 6.7 92.2 17.9 3.3 3.9
856 LOT 99.9 55.3 51.1 53.6 3 49.2 5.9 5.9 93.9 18.2 3.3 3.9
1251
5
6
7
8
O2/CO2 850 ASU 79.0 46.6 46.2 46.4 0
863 LOT 99.9 50.0 50.1 50.0 2.2
850 ASU 94.9 52.2 51.7 52.0 0
852 ASU 90.1 50.1 49.9 50.1 2.2
4.3
7.5
5.3
6.6
84.1 17.2 3.6 4.3
91.0 16.5 3.32 3.93
44.4 16.8 3.6 4.4
43.3 17.3 3.4 4.0
O2/N2
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concentration in the flue gas quickly rose to above 67.1%. Once the purity of oxygen introduced from the liquid oxygen tank reached 99%, a maximum CO2 concentration of greater than 90% was obtained; in contrast, the CO2 concentration in the O2/N2 firing mode was only 43.3%. Figure 3 depicts CO2 and
Figure 4. Emissions under the O2/RFG firing mode (conditions 2, 3, and 4).
Figure 3. CO2 and O2 concentrations during the O2/RFG firing mode (conditions 2, 3, and 4).
O2 concentrations in the flue gas during the O2/RFG firing mode, which includes three test conditions (conditions 2, 3, and 4). Each test condition lasts about 1 h. Because of plenty of fly ash under high oxygen concentration combustion, the sampling filters need to be changed frequently during the test, which lead to the intermittent fashion of the CO2 concentration curve in Figure 3. In addition, during the switch between test conditions, it can be shown that there is a peak of the O2 concentration in flue gas because of coal feeding and operational parameter changing. Table 5 shows the pollutant emissions under the different firing modes. All data for gas pollutant emissions are reported as time-averaged values on a dry basis. Figure 4 shows emissions under the O2/RFG firing mode. As evidenced by the data presented in Table 5, CO concentrations were markedly affected by excess oxygen; during the O2/RFG firing mode, as the concentration of oxygen in the flue gas increased from 1.1 to 7.5%, the CO concentration dropped from 803.4 to below 100 mg/MJ. Similar results were also obtained in the O2/CO2 firing mode. Moreover, the effect of excess oxygen on fuel N to NO conversion during the O2/RFG firing mode was apparent, as shown in Figure 5. NO emissions decreased gradually, followed
Figure 5. Fuel N to NO conversion for the different firing modes.
by a noticeable increase with higher concentrations of excess oxygen (>6%) in our tests, which are similar to the results presented by Lupianez et al.15 In addition, the higher the excess oxygen, the higher the NOx emissions under the O2/CO2 and O2/N2 firing modes. The calculation of fuel N to NO conversion in Figure 5 is based on the following formula:
Table 5. Pollutant Emissions under Different Test Conditions condition combustion mode CO (ppm) CO (mg/MJ) NO (ppm) NO (mg/MJ) fuel N to NO conversion (%) SO2 (ppm) SO2 (mg/MJ) sulfur capture efficiency (%)
1
2
5957.2 803.4 137.2 19.0 4.4 640.5 197.0 70.7
O2/RFG 616.8 92.2 142.6 21.3 3.5 666.6 227.6 70.2
3 472.9 71.2 128.6 19.4 4.1 461.9 159.0 79.7 1252
4
5
699.5 99.6 111.9 15.9 4.6 365.0 118.8 83.9
O2/CO2 1009.5 166.6 213.2 36.0 6.5 473.4 87.0 80.1
6
7
8 O2/N2
292.0 43.7 253.7 38.0 7.6 152.6 52.2 92.6
709.7 104.7 270.3 42.0 8.1 689.5 231.0 71.6
1103.3 164.5 278.2 41.5 8.4 149.8 51.1 93.7
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Article |NO|fg
fuel N to NO conversion = 1 −
that in O2/N2 and O2/CO2 modes. The effect of the CO2 partial pressures on the sulfur capture of the limestone cannot be seen clearly in these tests. However, it could be deduced that limestone utilization efficiency is greatly affected by the RFG, which has a high sulfur oxide content. It is worthy to note that Jia et al.7 observed similar results in comparison to the air-fired operation mode, whereas opposite results were observed by Zhao et al.10 A possible reason for this phenomenon in the present study is that all of the tests were conducted with furnace temperatures around 850 °C, which is not the optimum temperature for sulfur capture. A report by de Diego et al.17 describing tests conducted using a small-scale bubbling fluidized-bed (BFB) rig has shown that, under oxy-fuel combustion conditions, the optimum temperature to achieve the highest sulfur retention was 900−925 °C, whereas operations using enriched air required optimum combustion temperatures of 850−870 °C. The IET/CAS facility continues to conduct research in the field of high-concentration oxy-fuel CFB technology. Currently, we have finished the design of the 1.0 MWth pilot-scale oxy-fuel CFB combustor, which can be operated at total oxygen concentrations in the range of 21−50%. The commission of this pilot-scale facility is set to commence in August 2014. At the present time, bench-scale work focuses mainly on the sulfation behavior of limestone under oxy-fuel CFB conditions, especially at high oxygen concentrations. Additionally, oxy-fuel combustion characteristics of blends of biomass and coal are also currently under investigation; further reports on these studies are forthcoming.
|CO2 |fg − |CO2 |in + |CO|fg
Nfuel /Cfuel
(1)
where |NO|fg and |CO|fg are the NO and CO concentrations in the flue gas, respectively, |CO2|fg and |CO2|in are the CO2 concentration in the flue gas and in the inlet gas, respectively, and Nfuel/Cfuel is the molar ratio of N and C in the fuel. The fuel N to NO conversions were determined to be in the range of 3.5−4.6% under the O2/RFG firing mode, which was much lower than that observed under the O2/N2 mode (8.1−8.4%); similarly, Jia et al.9 also observed low N conversion under the O2/RFG firing mode. In addition, the N conversion under the O2/RFG firing mode was also lower than that of the O2/CO2 mode. From these data, it can be deduced that the chemical reaction between fuel N and recycled NOx and the reduction of recycled NOx from NOx to N2 contribute to the decrease of NO emissions under O2/RFG firing mode. In the volatilization stage of coal volatile matter, the intermediate product NH reacts with recycled NO to produce N2, instead of reacting with O and OH to produce NO, which is the dominant effect for the decrease of NO emissions in coal oxy-fuel combustion with RFG.16 The calculation of sulfur capture efficiency is based on the following formula: |SO2 |fg
sulfur capture efficiency = 1 −
|CO2 |fg − |CO2 |in + |CO|fg
Sfuel /Cfuel
(2)
where |SO2|fg is the SO2 concentration in the flue gas and Sfuel/ Cfuel is the molar ratio of S and C in the fuel. Comparisons of sulfur capture efficiencies for different firing modes for different Ca/S molar ratios are shown in Figure 6. Sulfur capture
4. CONCLUSION Oxy-fuel combustion experiments employing high oxygen concentrations under various gaseous atmospheres were successfully carried out at the IET/CAS facility using the 0.1 MWth oxy-fuel CFBC unit. The experimental overall oxygen concentration used was in the range of 46.4−54.6%, while the oxygen concentration of the primary gas was in the range of 46.6−56.7%. It has been shown that this facility operates reliably in a high-concentration oxy-fuel mode and is able to produce flue gas highly concentrated in CO2 (93.9%). A startup operation in enriched air (O2/N2) firing mode was required because of the large number of cooling tubes in the furnace area; however, the transition from O2/N2 to O2/RFG firing mode was easily completed in a very short time (about 15 min). The sulfur capture efficiencies were found to increase with an increase of the Ca/S molar ratio. At a Ca/S molar ratio of 2.2, the desulfurization efficiencies were determined to be 70.2 and 93.7% in O2/RFG and O2/N2 operations, respectively, indicative of the greatly reduced limestone utilization in the O2/RFG mode in comparison to O2/N2 and O2/CO2 modes. The sulfur capture efficiencies were about the same for both O2/CO2 and O2/N2 firing modes. NO emissions were measured to be in the range of 15.9−21.5 mg/MJ in the O2/ RFG firing mode, apparently lower than that for O2/N2 and O2/CO2 modes. During the O2/RFG firing mode, as the concentration of oxygen in the flue gas increased from 1.1 to 7.5%, the CO concentration dropped from 803.4 to below 100 mg/MJ. The N conversion were determined to be in the range of 3.5−4.6% in the O2/RFG firing mode, which was much less than that observed in the O2/N2 mode (8.1−8.4%).
Figure 6. Comparison of sulfur capture efficiencies for the different firing modes.
efficiencies were observed to increase with an increase in Ca/S molar ratios, as expected. At a Ca/S molar ratio of 2.2, the sulfur capture efficiencies were 70.2 and 93.7%, in the O2/RFG and O2/N2 firing modes, respectively; however, with an increase of Ca/S to 3, the sulfur capture efficiencies only reached a maximum of 83.9% under the O2/RFG firing mode. The sulfur capture efficiencies were about the same for both O2/CO2 and O2/N2 firing modes, indicating that limestone utilization in the O2/RFG mode was significantly lower than 1253
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AUTHOR INFORMATION
Corresponding Author
*Telephone: +86-10-82543055. Fax: +86-10-82543119. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge the support by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant XDA07030200) and the External Cooperation Program of the Bureau of International Cooperation (BIC), Chinese Academy of Sciences (Grant GJHZ201301).
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REFERENCES
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