Iron-Based Coal Direct Chemical Looping Combustion Process: 200-h

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Iron-Based Coal Direct Chemical Looping Combustion Process: 200‑h Continuous Operation of a 25-kWth Subpilot Unit Samuel C. Bayham, Hyung R. Kim, Dawei Wang, Andrew Tong, Liang Zeng, Omar McGiveron, Mandar V. Kathe, Elena Chung, William Wang, Aining Wang, Ankita Majumder, and Liang-Shih Fan* William G. Lowrie Department of Chemical and Biomolecular Engineering, The Ohio State University, 140 West 19th Avenue, Columbus, Ohio 43210, United States ABSTRACT: The coal direct chemical looping (CDCL) combustion process using an iron-based oxygen carrier has been developed and demonstrated in a 25-kWth subpilot unit. The CDCL subpilot unit is the first chemical looping demonstration unit with a circulating moving bed for the solid fuel conversions. To date, the CDCL subpilot unit at OSU has been operated for more than 550 h. The feasibility of the subpilot unit with various types of solid fuels including sub-bituminous coal and lignite coal has been tested. This article discusses the operational experience of a successful 200-h integrated, continuous demonstration with sub-bituminous coal and lignite coal. Throughout the 200-h continuous operation, the CDCL subpilot unit showed steady behavior in terms of solid circulation, coal handling, and oxygen carrier reactivity and recyclability. Tests with both coals confirmed more than 90% coal conversion with 99.5 vol % purity of CO2 achieved in the reducer. The sound design of the reducer allowed for nearly full coal conversion with a high purity of CO2, eliminating the need for additional downstream fuel polishing and separation units. The combustor gas contained lean oxygen concentrations with minute amounts of carbonaceous gases (CO2, CO, and CH4) detected. The combustor gas analysis implied the proper regeneration of iron-based oxygen carriers, good gas sealing between the reducer and combustor, and no indication of unconverted carbon carry-over. Moreover, the fates of coal pollutants such as NOx and SOx that are commonly observed in the conventional coal combustion process were also investigated during the subpilot unit operation. The NOx analysis showed that the CDCL process is capable of significantly reducing NOx emissions by avoiding thermal NOx formation. The sulfur analysis indicated SO2 generation in both reducer and combustor, agreeing with the sulfur chemistry in the CDCL scheme. emissions from power plants.21 The addition of the ability to directly convert solid fuels expands the vast potential and fuel flexibility of the CLC process. One method to utilize solid fuel in the CLC process is “chemical looping oxygen uncoupling” (CLOU), which is a two-step oxidation mechanism for solid fuel conversion using metal oxides. The metal oxide oxygen carriers in the CLOU scheme release gaseous oxygen at high temperatures in a nonoxidizing atmosphere. This gaseous oxygen then is available for the solid fuel conversion in the same reactor. For the CLOU applications, researchers considered many candidates for oxygen carriers such as CuO, Mn2O3, and Co3O4. The concept of CLOU using CuO-based oxygen carriers has been extensively studied by researchers at Chalmers University,15 the University of Utah,19 and CSIC in Spain.20 While CLOU for solid fuel conversion is a promising alternative to traditional CLC by providing a decreased fuel residence time, the CLOU requires further development in the oxygen carrier development, such as preventing particle agglomeration and controlling the oxygen release. Another method of the CLC process for the solid fuel conversion is to use gasification enhancers such as steam and CO2. Using gasification enhancers, the coal direct chemical looping (CDCL) process developed at the Ohio State University (OSU) circulates iron-based oxygen carriers in a

1. INTRODUCTION With efforts to reduce greenhouse gas emission from the fossil fuel combustion process, the carbon capture and sequestration (CCS) concept has been proposed to mitigate the imminent effects of global warming.1 Among the carbon capture processes proposed to reduce the cost of capturing CO2, the chemical looping combustion (CLC) process has developed a promising economical advantage because of its potential for inherent carbon capture with minimal energy penalty.2−6 The CLC concept uses a metal−oxide-based (e.g., NiO, CuO, Fe2O3) or metal sulfate-based (CaSO4) oxygen carrier to oxidize carbonaceous fuels in the absence of atmospheric nitrogen, allowing for the production of nearly pure CO2 while reducing the metal oxide to a lower oxidation state. The reduced metal oxides then are regenerated with air in a second reactor as heat is released from this exothermic, regeneration reaction. The inherent CO2 separation capability enhances the commercial potential to the CLC concept by significantly reducing the energy and economic penalties associated with carbon capture technologies. The U.S. Department of Energy (U.S. DOE) identifies the CLC process as one of the most innovative carbon capture processes that can deliver significant cost reduction in the carbon capture technologies.7 Initially, extensive research has been made on the CLC process for gaseous fuels conversion, with synthesis gas and methane.8−13 Recent research interests have shifted to the direct conversion of solid fuels using the CLC concept14−20 because inexpensive coal is considered the main source of CO2 © 2013 American Chemical Society

Received: January 3, 2013 Revised: February 28, 2013 Published: March 2, 2013 1347

dx.doi.org/10.1021/ef400010s | Energy Fuels 2013, 27, 1347−1356

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uniquely designed counter-current moving bed to convert solid fuels to sequestration-ready CO2.17,22−27 The CDCL process consists of two reactors, that is, the reducer and the combustor reactors. Solid fuels are converted in the reducer by the ironbased oxygen carriers with the help of gasification enhancers, such as steam and CO2. The enhancing gas aids the gasification of solid fuels into CO and H2, which reduces the iron oxide oxygen carriers and forms a high purity stream of CO2 in the reducer gas outlet. The mechanism of gasification enhancer in the CDCL process was previously introduced in past work.23,24 The reduced oxygen carriers from the reducer are regenerated by air in the combustor to produce heat for potential electricity generation from a steam cycle. Reactions (a) and (b) represent the reactions in the reducer and combustor of CDCL process, respectively. Fe2O3 + coal → Fe/FeO + CO2 + H 2O

(a)

Fe/FeO + air → Fe2O3 + heat

(b)

In general, the challenge for solid fuel CLC processes lies in the development of the reactor design that produces adequate fuel conversion and high CO2 purities with a reasonable process complexity and economics. In this regard, the fluidized bed for solid fuel conversion has been extensively studied because of the ease with which solid fuels can be injected; however, limited work has been performed on systems using moving bed technology for solid fuels conversion. The focus of this article is on the OSU CDCL combustion process that uses the ironbased oxygen carriers to convert solid fuels in a countercurrent moving bed configuration with a gasification enhancer. This article discusses the demonstration of the 25-kWth CDCL subpilot unit operated in a 200-h continuous and integrated manner. Previous research discussed the design of 25-kWth subpilot unit in more detail.25,26 In this work, iron-based oxygen carriers with 1.5−5 mm particle diameter size and two types of solid fuels (sub-bituminous and lignite coals) were used for the 200-h demonstration. More details of the ironbased oxygen carriers were presented in previous works.8,9,25−30 On the basis of the evaluations of the in situ gas analysis, the carbon conversion in the solid fuels, the CO2 purity in the reducer outlet, the combustor performance, and the fates of NOx and sulfur in the CDCL process were investigated.

Figure 1. General layout of the 25 kWth coal direct chemical looping unit (A, reducer; B, combustor; C, riser; D, cyclone).

oxygen carrier, it is possible for ash to be separated in situ in the reducer reactor and entrained out through the reducer outlet. The reduced oxygen carrier particles exit the reducer through a nonmechanical L-valve and flow into the combustor reactor. The role of the nonmechanical L-valve is to regulate the oxygen carrier circulation rate while providing the gas sealing between the combustor and the reducer.31 In the fluidized combustor reactor, the reduced iron particles are regenerated with air at high temperatures. The oxygen carrier particles reaching above the freeboard region of the combustor reactor become entrained into the riser section and pass through a cyclone to replenish the oxygen carrier at the top of the reducer reactor. Two noteworthy feedstocks tested during the 200-h continuous operation of the 25-kWth subpilot unit were sub-bituminous coal and lignite coal, whose proximate and ultimate analyses are shown in Table 1. The source of the sub-bituminous coal and lignite coal was the Powder River Basin (PRB) of Wyoming and North Dakota, respectively. Before injection into the unit, the two raw coals were preprocessed by drying in air and then pulverizing in a ball mill to a nominal particle size of ∼100 μm. The carbon quantity on a mass basis with moisture was used to determine the carbon consumption during the runs. The carbon conversion of the solid fuel was calculated using the following equation:

2. EXPERIMENTAL SETUP The 25-kWth subpilot unit for the CDCL process has been constructed and extensively demonstrated at OSU with solid fuels such as metallurgical coke, lignite, and sub-bituminous coals with successful continuous performance results. To date, more than 550 h of operation experience has been achieved. Of the operational experience, 200 h was the longest integrated continuous operation with a voluntary shut-down. A general sketch of the 25-kWth CDCL subpilot unit constructed at OSU is shown in Figure 1. Kim et al. have described the unit design in detail in a previous work.25,26 A review of the system setup is described here. The subpilot system consists of two main reactors, a reducer (or fuel reactor) and a combustor (or air reactor) that are connected by a nonmechanical L-valve to form a circulating moving bed setup. The reducer in the system adapts a counter-current moving bed design in which the solid oxygen carriers move down by virtue of gravity in a dense phase, while gases, such as the fuel gasification enhancer (steam and/or CO2) as well as the gasification products, move upward, counter-current to the solids. The gas−solid counter-current moving bed provides many advantages over a fluidized bed such as a greater control of the fuel residence time and conversion as well as of the oxygen carrier conversions.8,9,22−24 Because of the coarse nature of the

XC = (NCO2,Red − NCO2,Enh + NCO,Red + NCH4,Red)/NC,in

(1)

where NC,in is the molar flow rate of carbon from the solid fuel, NCO2,Red, NCO,Red, and NCH4,Red are the molar flow rates of the carbonaceous gases CO2, CO, and CH4 from the reducer outlet, and NCO2,Enh is the flow rate of the CO2 enhancer gas. The molar flow rates of the carbon species were determined on the basis of the species concentrations. The average carbon conversion of solid fuel in the reducer was also determined using the average concentrations of O2 from the combustor gas outlet analysis as shown in equation 2.

XC = (NO2,Cons − (NCO2,Comb + NCO,Comb + NCH4,Comb))/NC,in (2) 1348


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Table 1. Ultimate and Proximate Analysis of the Coals Used in the Testing proximate (dry basis) ash volatile matter fixed carbon energy value energy valuea moisture a

sub-bituminous

lignite

5.61% 44.89% 49.5% 27.52 MJ/kg 29.16 MJ/kg 13.96%

7.14% 44.15% 48.71% 25.60 MJ/kg 27.57 MJ/kg 13.59%

ultimate (dry basis) sub-bituminous

lignite

69.08% 4.61% 0.96% 0.0013% 0.34% 19.4%

65.3% 4.48% 0.96% 0.0012% 0.55% 21.57%

carbon hydrogen nitrogen chlorine sulfur oxygen

Moisture and ash free. filter, chiller, and desiccant bed were installed to condition the gas sample prior to measurements. A list of the fuels, flow rates, and conditions for the 200-h operation is shown in Table 2. For the sub-bituminous coal, the first and second

where NO2,Cons is the molar oxygen consumption for regenerating the particle in the combustor, and NCO2,Comb, NCO,Comb, and NCH4,Comb are the molar flow rates of carbonaceous species measured in the combustor reactor, which are carried over to the combustor from incomplete fuel oxidation in the reducer reactor. The molar oxygen consumption was calculated from knowledge of the difference in oxygen concentration from atmospheric air as well as the inlet air flow rate taking into account dilution with nitrogen from the L-valve gas region. For determining the purity of the CO2 at the reducer gas outlet, the normalized gas concentrations were calculated from the following equation:

Yi = x i /(xCO2 + xCO + xCH4)

Table 2. Condition Testing Parameters for Sub-bituminous Coal and Lignite Coal

(3)

trial no.

solid fuel type

flow rate (g/min)

enhancing gas (CO2, LN/min)

TRed (°C)

TComb (°C)

1

subbituminous subbituminous subbituminous subbituminous subbituminous lignite lignite

23

5

970

890

23

3

970

890

32

5

960

890

46

5

960

890

56

5

970

890

22 46

5 5

940 940

860 880

2

where xi indicates the concentration of species i = CO, CO2, or CH4, measured by the analyzer. Minute species such as SO2 and NOx were not included in the calculation because it is assumed that these species would be removed by traditional pollutant mitigation methods, such as flue gas desulfurization for SO2 and selective catalytic reduction for NOx, as opposed to an oxygen polishing step required for CO and CH4, which is more cost-intensive. The carbon molar concentrations were determined in the downstream gas analysis system where slipstream samples were collected from the gas outlets of the reducer and combustor. Figure 2

3 4 5 6 7

trials began with a feed rate of approximately 23 g/min of fuel, and for each subsequent trial, the flow rate was increased stepwise, held at 32 and 46 g/min for 1 h each, until 56 g/min was reached. For the lignite coal, the first trial (trial 6) began at 23 g/min, and the feed rate was increased to 46 g/min for trial 7. The increase in the fuel flow rate was performed stepwise to carefully monitor the effects of fuel injection on the pressure balance to maintain full stability of the system. A volumetric screw feeder from Schenck Accurate was used to control the feeding rate of the coal. The hopper walls of the coal feeder were agitated, and a stem-less screw was used to avoid coal aggregate formation and maximize the coal flowability. The feed rate of coal was adjusted by varying the voltage to the screw feeder (0−90 VDC), and a flow rate calibration curve was developed for use in operation. The flow rate and the corresponding voltage were checked after every trial to verify that the proper flow rate was set. All of the tests in this campaign were performed with CO2 as the enhancing gas, which was supplied from standard gas cylinders rated at 99.9% purity. The flow rate of CO2 corresponded to approximately 8−12 mol % of carbon input from the coal. The concentration, pressure, and temperature measurements were performed at a frequency of one sample per second with a 60 s moving average, and the measurements were monitored continuously to observe the effects of fuel and various gas flow rates on the system. Furthermore, the recyclability of iron-based oxygen carriers from the 200-h operation was investigated. Particles were sampled from the bottom of the reducer after the experiment, and 70 mg was used for recyclability testing. A Setaram Setsys Thermogravimetric Analyzer (TGA) was used to determine the ability of the particle to maintain reduction with H2 and oxidation with air at 900 °C. A reducing gas and oxidizing gas that contained 16.6% H2 balanced with N2 and 7% O2 balanced with N2, respectively, was used for the recyclability with a total gas flow rate of 300 mLN/min. The sample weight change was

Figure 2. Down-stream gas cleanup and sampling system for gas analysis at 25-kWth subpilot unit. represents a sketch of the gas sampling system configuration. Two nondisperse infrared analyzers, model ZRE Infrared Analyzer and the Chemiluminescence Analyzer by California Analytical Instrument, were used for measuring the concentrations from the reducer and combustor. The infrared-based analyzer was used to measure CO2, CO, CH4, SO2, and O2, and the chemiluminescence-based analyzer was for the NOx measurement. Gas concentration data were continuously recorded via a data acquisition system at a rate of 1 sample per second. A diaphragm pump and needle valve were used to control the sample gas flow rate to the gas analyzer. A 2 μm sintered 1349


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recorded, and the iron oxide conversion (XOC) was calculated using the following equation:

XOC = (1 − (W0 − Wt )/(W0MOx,exp/MOx,total))

(4)

where W0 is the original sample weight, Wt is the sample weight at any given time t, MOx,exp is the expected oxygen weight loss with reduction, and MOx,tot is the total oxygen weight loss of the iron-based oxygen carrier.

3. RESULTS AND DISCUSSION This section highlights the key experimental results using subbituminous and lignite coals from the 200-h continuous operation. The circulation of the oxygen carrier in the system during the operation was shown to be smooth throughout the 200-h run. A representative snapshot of the pressure profile during the run is shown in Figure 3. The figure shows the

Figure 4. Temperature and pressure profiles of the reducer (a) and combustor (b) upon initiation of fuel injection into the system (trial 1).

pressure drop across the reducer fluctuates because of the slight variability of the coal feeding rate due to the small size of the coal feeder. The riser pressure drop, which is a function of the solids holdup or flux, shows a slight increase upon injection of the fuel into the reducer. The temperature of the reducer reactor gradually decreases over time with the endothermic reactions occurring in the reducer, the gasification reactions and the reduction of the oxygen carrier, showing a moderate temperature drop of 10 °C over a period of 20 min. Likewise, the combustor reactor shows a moderate increase in temperature as a result of the combustion reaction with the reduced oxygen carrier. 3.1. Reducer Performance. As was previously described, the first set of testing involved injection of sub-bituminous coal as the fuel feedstock. Calculation of the once-through fuelcarbon conversion using equation 1 indicates high conversion of the sub-bituminous coal to gaseous species in the moving bed reducer reactor, as plotted in Figure 5, which shows the first 40 min of the trial. The carbon conversion fluctuated over a range of values. However, after the first 30 min, the conversion steadied out to an average conversion of 96.9% with oscillating fluctuations. The high conversion of solid fuel is the result of the design of the moving bed reducer reactor. The effective gas−solid and solid−solid contacting patterns in the moving bed design allow for the complete conversion of the coal char to gaseous fuel products. The reducer reactor also showed excellent performance in terms of CO2 purity in the gas outlet. Figure 6 shows the normalized outlet gas concentrations from the reducer after the initiation of sub-bituminous coal injection. After one oxygen carrier circulation cycle (around 30 min), the gas concentrations in the reducer outlet showed to be steady with a high purity of CO2, averaging around 99.65 vol %, with insignificant unreacted reducing gases, such as CO and CH4, measured at an

Figure 3. Pressure profile of the CDCL system.

pressure drops across the moving bed reducer, the fluidized bed combustor, and the L-valve, the locations of which are illustrated in Figure 1. Four coal injection trials are shown in the profile, where the reducer pressure increases and plateaus. As shown, the pressure profiles during these times are shown to be steady, with minor variations a result of the screw feeder delivering the coal. The pressure across the L-valve decreases upon injection of fuel because of the increase in the pressure of the reducer, which increases the pressure on the negative pressure side of the L-valve transducer. The unsteady condition during the time frame between 3 am and 6 am on 9/15/12 is a result of the testing of the pressure at the reducer gas outlet line outside of the reactor. The gas outlet line pressure was dynamically varied during the test leading to a variation of the reactor pressure and to an unsteady operating condition. The test indicates the resilience of the system for which it could readily return to its steady operating condition after the testing. A representative profile of the reducer and combustor temperature and pressure upon injection of a sub-bituminous Powder River Basin coal is shown in Figure 4a and b. The injection rate of sub-bituminous coal was 23 g/min with a CO2 enhancing gas flow rate of 5 LN/min (trial 1 in Table 2). The plots show how fuel injection affects the reducer and combustor reactor temperature profiles and the reducer and riser pressure drop. Immediately upon fuel injection, the pressure drop across the moving packed bed reducer increased by 5 in. of water column, an indication of coal devolatilization and in situ gasification of the coal char with the CO2 enhancer gas. The 1350


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Table 3. Summary of Testing with Sub-bituminous Powder River Basin Coal trial no.

flow rate (g/min)


TRed (°C)

TComb (°C)

CO2 purity in reducer

carbon conversion

1 2 3 4 5

23 23 32 46 56

5 3 5 5 5

970 970 960 960 970

890 890 890 890 890

99.65% 99.63% 99.73% 99.72% 99.45%

96.9% 96.5% 99.2% 96.9% 97.7%

time to steady carbon conversion is shown in Figure 7. In theory, the effect of the enhancer gas is to increase the kinetics Figure 5. Carbon conversion profile for Powder River Basin coal (trial 1).

Figure 6. Reducer outlet concentration (nitrogen free basis) profile for injection of Powder River Basin coal (trial 1).

Figure 7. Effect of different CO2 enhancer gas flow rate on the singlepass carbon conversion profile (trial 1, 5 LN/min CO2,; trial 2, 3 LN/ min CO2).

average concentration of 0.09 and 0.22 vol %, respectively. The CO2 to reducing gas ratio is shown to be very high, indicating near-complete oxidation of the reducing gases in the moving bed reducer. Sufficient hold-up of oxygen carrier in the moving bed allows for the complete oxidation of coal volatiles and reducing gases to produce CO2 and H2O. The operation of the moving bed reducer of the CDCL subpilot unit was demonstrated to be more effective than the operation of fluidized bed reactor configurations in terms of generating high purity of CO2 from the reducer. From the literature, 10−15 vol % of the gases from the gas outlet of a fluidized bed reducer are reducing gases such as CO and CH4, which is significantly higher than the reducing gas concentration found in the moving bed system.15,20 Overall testing conditions with sub-bituminous coal, CO2 purity, and single-pass carbon conversion measured using two different calculation methods (equations 1 and 2) are summarized in Table 3. As described earlier, the CO2 purity for all of the tests is shown to be very high, with the lowest CO2 purity of 99.45 vol % for trial 5. The single-pass carbon conversion was also determined to be high for all trials showing 96.5% or higher conversion by reducer and combustor measurements. During the run with sub-bituminous coal, the effect of enhancing gas flow rate on the carbon conversion was also tested. Coal injection was stopped for two circulation cycles, and the flow rate of the CO2 enhancer gas was decreased from 5 to 3 LN/min before the next testing condition (trial 2 in Table 3). A plot of the effect of enhancing gas flow rate on the

of the in situ gasification of the coal char product in the reducer reactor. Thus, increasing the flow rate of the enhancer gas should improve the overall char conversion, but only up to a point where the entire quantity of coal char is gasified within the residence time of the reducer reactor. The enhancer gas flow rate after this point should have little effect on the overall steady-state conversion of the fuel char. As can be seen in Figure 7, the carbon conversion increased dramatically within the first ∼5 min of fuel injection; this jump accounts for the conversion of the volatile products of the sub-bituminous coal. After that, however, the slow step of in situ gasification took over, requiring a reasonable amount of time to reach a steady state. Eventually, however, the slow in situ gasification dominates, requiring a longer time period to reach a steady state. Trial 2 with 3 LN/min of CO2 enhancer gas shows that the presteady-state carbon conversion is lower than trial 1 with 5 LN/min of enhancer gas, which is the result of the lower rate of in situ gasification of the coal because of the lower enhancer gas flow rate. After 37 min, both curves merge and produce one high steady-state conversion, indicating the steady-state production of gasification products with the influx of devolatilized fuel char. The second set of testing involved the injection of the North Dakota Lignite coal. The operational results with lignite coal in terms of CO2 purity in the reducer gas outlet and coal conversion are similar in magnitude to the operational results with sub-bituminous coal. Figure 8 shows a typical normalized gas concentration profile for lignite coal injection for trial 7, 1351


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Figure 10. Combustor outlet concentration profile for Powder River Basin coal injection (trial 1).

Figure 8. Reducer outlet concentration profile for lignite coal injection (trial 7).

which has a fuel flow rate set at 46 g/min. The profile of normalized gas concentrations in the reducer outlet shows high purity CO2 with minimal reducing gases. The average steadystate concentrations are 99.7 vol % CO2, 0.19 vol % CO, and 0.12 vol % CH4. Moreover, the single-pass carbon conversion of the lignite coal is shown in Figure 9. The carbon conversion calculated for each of these trials is summarized in Table 4. The time-averaged carbon conversion was 97.9% for trial 6 and 96.3% for trial 7.

Figure 11. Combustor outlet concentration profile for lignite coal injection (trial 7).

average carbon in coal conversion in the reducer was calculated using equation 2. This calculation was able to corroborate the solid fuel conversion measured from the average reducer molar flow rate within an error of 0.1%. Figure 11 shows the combustor gas profile with the lignite coal injection for trial 7 (46 g/min of coal). Similar to the subbituminous coal experiment, Figure 11 also shows that a lean O2 concentration was observed as the reduced oxygen carriers were regenerated with air in the combustor. Furthermore, the combustor gas profile in Figure 11 was compared to the combustor gas profile for trial 6, with a lower flow rate of lignite coal (22 g/min), which is shown in Figure 12. As expected, the curve for the higher fuel flow rate shows a lower concentration of O2 due to a greater conversion of the oxygen carrier particle at higher fuel loading (i.e., 16.5 vol % O2 for 22 g/min fuel as compared to 12 vol % O2 for 46 g/min fuel). The residual carbon-based gases such as CO2, CO, and CH4 are expected in the combustor gas outlet if the unconverted carbon is transported with the oxygen carriers from the reducer, or if the nonmechanical L-valve fails to provide the good gas sealing between the combustor and reducer while transferring the oxygen carriers. Oxidation of unconverted coal with air in the combustor and CO2 enhancer gas leakage from the reducer

Figure 9. Carbon conversion profile for lignite coal injection (trial 7).

3.2. Combustor Performance. The gas outlet of the combustor was analyzed to observe the possible carry-over of unconverted coal into the combustor from the reducer and to monitor the regeneration of the oxygen carriers. A plot of the combustor outlet gases at the same conditions as measured for the reducer in Figures 6 and 7 (sub-bituminous coal at 23 g/ min injection) is shown in Figure 10. The CO2 concentration is below the error limit for the analyzer, so the CO2 may very well be near zero percent. The concentration profile of O2 in Figure 11 is shown to be approximately 16.5 vol %, which is less than atmospheric, indicating consumption of O2 by the reduced oxygen carrier. Accounting for the measured loss in oxygen based on the concentration measured in the combustor, the Table 4. Summary of Testing with Lignite Coal trial no.

fuel feed rate (g/min)


TRed (°C)

TCmbstr (°C)

CO2 purity in reducer

carbon conversion reducer measurement

6 7

22 46

5 5

940 940

860 880

99.53% 99.64%

97.9% 96.3%

1352


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Figure 12. Combustor outlet concentration profile for lignite coal injection (trial 6). Figure 13. Graph of the 150 h oxygen carrier particle’s oxygen carrying capacity over 10 cycles of reduction (30 min under H2) and oxidation (30 min under air) in a TGA.

results in CO2 formation. In the combustor gas analyses in Figures 10, 11, and 12, these carbon-based gases in the combustor are shown to be minimal. For both operational cases with sub-bituminous coal and lignite coal, the concentrations of these carbon-based gases are less than 0.1 vol %. The insignificant carbon-based concentration in the combustor is an indication of minimal unconverted coal carry-over and gas leakage into the combustor from the reducer. The sound design of the moving bed reducer that accounts the hydrodynamics of oxygen carriers and pulverized coal prevented any unconverted coal and ash carryover into the combustor. The reducer reactor was carefully controlled by maintaining the gas velocities in the moving bed above the minimum fluidization velocity of the coal and ash so that the particles remain in the reducer rather than allowing the coal and ash particles to move into the combustor along with the oxygen carriers. The CDCL process is designed to elutriate the unconverted coal and/or ash through the reducer gas outlet with a CO2 stream. As the unconverted coal and coal ash removal is simultaneously achieved in the reducer operation, the CDCL process eliminates the use of a separate operating unit such as a carbon stripper for the oxygen carrier− solid fuel separation. The elimination of the carbon stripper unit operation simplifies the CDCL process scheme and provides an economic advantage. Additionally, the standpipe design of the nonmechanical L-valve prevented the leakage of reducing gases to the combustor while successfully transporting the oxygen carriers. 3.3. Post-Run Oxygen Carriers Analysis. The successful performance of the reducer and combustor demonstrated the ability of the CDCL reactor to achieve high fuel conversion and maintain continuous recyclability of the oxygen carrier particles. To further investigate the reactivity of the oxygen carrier, samples of the solid particles were analyzed prior, during, and after the 200-h operation. Oxygen carrier particles were sampled from the bottom of the reducer during the run after approximately 150 h of subpilot operation. These particles were tested in the TGA for their ability to reduce under H2 and oxidize under air. The 150 h-reacted oxygen carrier particles were able to regenerate continuously for a further 10 cycles as illustrated in Figure 13. Thus, the reacted oxygen carrier demonstrated its ability to retain reactivity. 3.4. Fate of Pollutants. The fates of pollutants from the coal combustion process such as fuel nitrogen and sulfur were also experimentally investigated during the subpilot scale CDCL process demonstration. These pollutants are two of

the most concerning pollutants in any coal combustion process. The fates of these pollutants were studied in trial 1 with the sub-bituminous Powder River Basin coal. Table 5 shows the concentrations of the SO2 and NOx pollutants measured from the outlets of reducer and combustor on a dry, nitrogen free basis. Table 5. Concentration of Pollutants in the Reducer and Combustor Reactors with Sub-bituminous Powder River Basin Coal Injection (Trial 1) reducer combustor

SO2 (ppm)

NOx (ppm)

600−1170 0−70

1148−1669 0

3.4.1. Fate of NOx. There are two mechanisms of NOx formation in the conventional coal combustion process, that is, thermal NOx and fuel NOx. Thermal NOx is generated by nitrogen oxidation at excessive temperatures. In a traditional coal combustion process, the thermal NOx formation is expected near the flame where the temperature is over 2000 °C.32 To reduce the thermal NOx formation from the coal combustion, the flame temperature needs to be controlled to avoid hot spots in the flame. The CDCL process is a flameless combustion process occurring within a manageable range of temperatures. Therefore, the CDCL process is capable of minimizing thermal NOx formation. Embedded nitrogen in coal is the main source of fuel NOx,33 and the major source of fuel NOx is its volatile release from the coal. The fuel bounded nitrogen is converted to HCN with hydrocarbons in the coal. Next, HCN is converted to NH or NH2, which then reacts with O2 to produce NO. When NH or NH2 reacts with NO, N2 is formed.34,35 For the reducer in the CDCL process, fuel NOx formations are expected to be minimal because no gaseous oxygen is available in the CDCL reducer and the fuel bounded nitrogen tends to pyrolyze to form N2.34,35 The fuel NOx is not expected in the combustor because there is no nitrogen bounded in the iron particles. Figure 14 summarizes the fate of NOx in the CDCL process. For the NOx analysis in the 25-kWth subpilot unit, the reducer produced a range of 1148−1669 ppm on a dry, nitrogen free basis. Taking into account the volumetric flow 1353


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Figure 15. Fate of sulfur in the CDCL process.

Figure 14. Fate of NOx in the CDCL process.

summarizes the path of sulfur in the CDCL process. For the sulfur analysis in the 25-kWth subpilot unit, it was observed that the SO2 concentration was 190−1170 ppm and 0−70 ppm in the reducer and combustor, respectively. The sulfur analysis in the subpilot unit demonstration confirms the chemistry of sulfur in the CDCL scheme.

rate of the reducer outlet gas, this corresponds to 0.043−0.063 kg/GJ of fuel (0.10−0.15 lb/MMBTU). This is lower than conventional pulverized coal combustion emission processes using low NOx burners, which produce NOx in a typical range of 0.086−0.22 kg/GJ of fuel (0.2−0.5 lb/MMBTU).36 Only fuel NOx formation is expected in the reducer from the embedded nitrogen in coal, because the operation temperature of the reducer was not sufficient enough to form significant thermal NOx. It was calculated from Table 5 that approximately 10−15% fuel bounded nitrogen was converted to NOx in the reducer. Additionally, the combustor gas analysis in the 25-kWth subpilot unit indicated no NOx formation. Because of the regeneration of oxygen carriers at fairly lower temperatures and no source of fuel nitrogen in the combustor, thermal and fuel NOx were not observed in the combustor, respectively. Experimental studies suggest that the CDCL process is effective in NOx control, and many studies have also reported the effectiveness of chemical looping process in NOx reduction.34,35 3.4.2. Fate of Sulfur. In a conventional coal combustion process, most of the sulfur in coal is oxidized to SO2 (reaction c), and further oxidation results in SO3 (reaction d). S (in fuel) + O2 → SO2

(c)

SO2 + O2 → SO3

(d)

4. CONCLUDING REMARKS The operation of the 25-kWth CDCL subpilot designed and constructed at OSU has shown successful performance for conversion of two different coals (sub-bituminous and lignite) for 200 h of smooth and continuous operational experience with a voluntary shutdown. During this testing, the operation demonstrated high conversion of the solid fuels at full design capacity with high CO2 purity, which is attributed to the strategic design of the moving bed reducer, the combustor, solids circulating system, and the development of the oxygen carriers. The reducer gas analysis exhibited a CO2 purity of over 99 vol % with minimal reducing gases (CO and CH4), and the single-pass carbon conversions in the reducer were all greater than 90% for all cases. Gas analysis from the combustor reactor during fuel injection showed particle regeneration, and the measured consumption of O2 from the combustor was helpful for verifying the solid fuel conversion in the reducer. Insignificant amounts of carbon-based gases (CO2, CO, and CH4) were observed in the combustor, indicating no carbon carry-over from the reducer and good gas sealing between the reducer and the combustor. Furthermore, the system was able to demonstrate successful ash removal because of the size difference between the coarse oxygen carrier particles (1.5−5 mm in diameter) and the finer ash. The success of the subpilot unit can be attributed to the novel facets of its simple yet innovative design, allowing for a near-complete single-pass conversion of solid carbonaceous fuels to a sequestration-ready CO2 product with minimal carbon carry-over into the combustor reactor. The advancement of the CDCL process to the 25-kWth subpilot scale will further develop the CLC technology for potential commercialization.

The formation of SOx in the CDCL process follows a mechanism different from the conventional coal combustion process. The sulfur is introduced to the reducer with coal. In the reducer, the sulfur undergoes two different paths. First, a portion of sulfur in the coal reacts with Fe2O3 to form FeS and Fe3O4 solids that go to the combustor along with other reduced oxygen carrier particles as shown in reactions e and f. Fe2O3 + S (fuel) → FeS + SO2

(e)

Fe2O3 + S (fuel) → Fe3O4 + SO2

(f)

The rest of the sulfur becomes gaseous SO2, which is exhausted with CO2 and H2O from the reducer. In the combustor, the FeS particles from the reducer are reoxidized with air back to Fe2O3 as shown in reaction g. FeS + air → SO2 + SO3 + Fe2O3

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

(g)

As a result of the sulfur oxidation, the sulfur is mainly released as SO2 and a small amount of SO3. Figure 15

Notes

The authors declare no competing financial interest. 1354


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ACKNOWLEDGMENTS We would like to acknowledge the financial assistance provided by the United States Department of Energy (project no. DENT0005289) and Ohio Department of Development (project no. CDO/D-08-02). We would also like to acknowledge CONSOL Energy, Babcock and Wilcox Co., and ClearSkies for their partnerships, as well as Alan Wang and Zhenchao Sun for their assistance in the operation.

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NOMENCLATURE MOx,exp = expected oxygen weight loss with reduction of oxygen carrier (g) MOx,total = total oxygen weight loss for oxygen carrier (g) Nc,in = carbon molar input flow rate (mol/min) Nc,out = carbon output flow rate (mol/min) NCO2,Red = molar flow rate of CO2 in reducer gas outlet (mol/min) NCO2,Enh = molar flow rate of enhancer gas inlet (mol/min) NCO,Red = molar flow rate of CO in reducer gas outlet (mol/ min) NCH4,Red = molar flow rate of CH4 in reducer gas outlet (mol/min) NO2,Cons = molar rate of oxygen consumption in combustor (mol/min) NCO2,Comb = molar flow rate of CO2 exiting combustor (mol/ min) NCO,Comb = molar flow rate of CO exiting combustor Xc = single-pass carbon conversion (−) xi = gas fraction of species i (=CO2, CO, CH4) (−) Yi = normalized concentration of gas species i in reducer (−) W0 = initial oxygen carrier weight (g) Wt = weight of oxygen carrier at time t (g)

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