Conversion of Coal in a Fluidized Bed Chemical Looping Combustion

Article pubs.acs.org/EF

Cite This: Energy Fuels XXXX, XXX, XXX−XXX

Conversion of Coal in a Fluidized Bed Chemical Looping Combustion Reactor with and without Oxygen Uncoupling Kirsten M. Merrett and Kevin J. Whitty*

Energy Fuels Downloaded from pubs.acs.org by UNIV OF WINNIPEG on 01/27/19. For personal use only.

Department of Chemical Engineering, The University of Utah, 50 S. Central Campus Dr. Room 3290, Salt Lake City, Utah 84112, United States ABSTRACT: Chemical looping combustion (CLC) is a carbon capture technology that involves circulating metal-based oxygen carrier particles between two reactors, where the metal is alternately oxidized by air and then reduced by fuel as the fuel combusts. In the variant of CLC known as chemical looping with oxygen uncoupling (CLOU), the oxygen carrier releases gaseous O2 in the fuel reactor, allowing for a rapid reaction with solid fuels. This study focuses on improving the understanding of mechanisms responsible for carbon conversion of coal and coal char in a bench-scale fluidized bed reactor using oxygen carriers with and without oxygen uncoupling properties. Effects of coal particle size and temperature on the rate of carbon conversion are evaluated in order to quantify performance and to aid in design and modeling of large-scale fully circulating chemical looping systems. The data confirms that conversion with a copper-based CLOU carrier is faster and results in lower CO and CH4 concentrations than the natural ore ilmenite, which does not have CLOU properties. However, with coal as fuel, the increase in the rate of char carbon conversion with CLOU is lower than that previously reported because of competition for oxygen by volatiles. Conversion is also faster with smaller particles but results in higher CO concentrations as small particles gasify and release volatiles more quickly.

1. INTRODUCTION Coal is the second largest fuel source in the world and accounts for approximately one-third of the electricity generated.1 It is also a major source of anthropogenic CO2 emissions. To continue to benefit from coal-fired energy, technologies for energy production with CO2 capture are needed. One promising technology is chemical looping combustion (CLC), which inherently isolates the combustion product CO2, reducing the cost for carbon capture from power plants. 1.1. CLC with Solid Fuels. Chemical looping involves circulating a metal oxide, identified as an oxygen carrier, between two reactors: a fuel reactor where the oxygen carrier is reduced by combusting the fuel and an air reactor where the oxygen carrier is regenerated by the reaction with oxygen from air. This process naturally separates oxygen and nitrogen, limiting nitrogen dilution in the fuel reactor’s exhaust stream, so that the main fuel reactor products are water vapor and CO2. Water vapor is easily condensed, leaving a concentrated stream of CO2 ready for further processing. The heat produced during the CLC process is used to generate steam, which is fed to a turbine for production of electric power. A diagram of the CLC process for solid fuels can be seen in Figure 1.

Achieving good conversion of carbon in solid fuel char creates a particular challenge because the oxygen required for combustion is bound to the oxygen carrier and solid−solid reactions are ineffective. There are two primary approaches for combusting solid fuel in a chemical looping system: in situ gasification CLC (iG-CLC) and CLC with oxygen uncoupling (CLOU).2 In iG-CLC, solid fuel is fed into the fuel reactor where it is gasified in situ by steam and/or CO2 to produce gaseous H2 and CO that react heterogeneously with the solid oxygen carrier. The efficiency of this pathway is limited by the gasification step, which is slow at fluidized bed chemical looping reactor temperatures. This makes achieving good carbon burnout challenging. The most well-studied oxygen carrier for iG-CLC is ilmenite, a naturally occurring, relatively low-cost iron−titanium ore. However, ilmenite has been found to have relatively low reactivity, resulting in poor fuel conversion.3 Copper-based oxygen carriers, whether operating under iGCLC or CLOU conditions, are more reactive and convert fuel more efficiently but are more costly to manufacture and have a greater tendency toward agglomeration.4−9 With CLOU, the thermodynamics of the oxygen-carrying metal are such that the reduced form is strongly favored in the low O2 fuel reactor environment, so the reverse of the metal oxidation reaction occurs and gaseous O2 is spontaneously generated. The released oxygen readily reacts with the solid char as well as combustible gases created during coal devolatilization in the same manner as in conventional fuel combustion. Combustion with oxygen is efficient and fast at chemical looping reactor temperatures, and CLOU oxygen carriers have been Received: October 15, 2018 Revised: January 7, 2019

Figure 1. Diagram of the chemical looping process. © XXXX American Chemical Society

A

DOI: 10.1021/acs.energyfuels.8b03581 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels found to convert solid fuel char up to 50 times the rate of iGCLC.10 The different pathways for solid fuel combustion explored in this paper can be seen in Figure 2.

complexity to the plant. Achieving high conversion in the fuel reactor itself would be preferred. CLOU promises to dramatically improve char conversion in the fuel reactor. Only a few pure metal oxides have the thermodynamic properties to function as CLOU oxygen carriers. The main CLOU oxygen carriers studied are either Cu-based,9,17,39 Mn-based,40,41 or a combination of the two.42 CLOU carriers typically combine these metals and an inert support material (e.g., ZrO2, SiO2, and MgAl2O4) to get the desired CLOU behavior on a strong porous particle with desirable physical characteristics. Copper, cycling between cuprous and cupric oxide as per reaction 1, is recognized as the most reactive CLOU metal and most research on CLOU has focused on copper-based carriers. Since being recognized as a CLOU material, ample research on development of copper-based CLOU oxygen carriers has been conducted.7,8,10,43−47 Fundamental studies have demonstrated the fast oxygen release capabilities of CuO at CLC temperatures.10,48,49 Efficient conversion of solid fuel in lab-scale fluidized beds have been demonstrated under well-controlled conditions.10,43,47 Investigations of copper-based CLOU carriers in continuously operating fluidized bed reactors have demonstrated solid fuel conversions up to 96%, and CO2 capture efficiencies have achieved 100% at temperatures above 935 °C with high volatile fuels.17,39,50−52 These studies do not specifically compare carbon conversion of volatiles and char from coal in iG-CLC and CLOU mode, and more importantly, the influence of significant variables such as fuel particle size was not systematically evaluated.

Figure 2. Different pathways for solid fuel combustion by CLC [adapted from Adánez et al.].2

There have been many CLC studies carried out with various oxygen carriers, ranging from lab to semi-industrial scale.10−17 Well-performing oxygen carriers are the key to the success of CLC technology, and much of the research to date has focused on the development of oxygen carriers.2,3,18 For investigations focusing on solid fuel conversion in iG-CLC units, oxygen carriers studied have included ilmenite,11,14,19−27 manganesebased ores,11,23,28,29 and iron-based ores.16,30−36 It has been determined that char gasification is the rate limiting step in the iG-CLC process.37 If char is not converted in the fuel reactor, it is carried with the flowing oxygen carrier particles to the air reactor where it combusts, but the CO2 product is not captured. A method to address this is to add a so-called carbon stripper reactor between the fuel and air reactors so that any unconverted char particles are further converted or separated and returned to the fuel reactor rather than being carried over to the air reactor. It has been shown that addition of a carbon stripper improves fuel conversion and increases CO2 capture efficiency.38 However, addition of a carbon stripper adds an extra level of

2Cu 2O + O2 ↔ 4CuO

(1)

The aims of the study reported here are to better understand coal conversion during CLC under industrially relevant conditions, to compare performance of different types of oxygen-carrier materials and conversion mechanisms, to evaluate the influence of temperature and fuel particle size, and to acquire data useful for reactor design, modeling and scaleup. To achieve these objectives, we evaluated carbon conversion of coal by iG-CLC and CLOU using a variety of oxygen carriers, including ilmenite and copper-based manufactured materials. The effects of coal particle size and temperature on the rate of carbon conversion were explored.

Figure 3. Diagram of the bench-scale fluidized bed CLC reactor. B


Article

Energy & Fuels

2. EXPERIMENTAL SECTION

Table 1. Oxygen Carrier Properties

2.1. Apparatus and Procedure. A bench-scale bubbling fluidized bed chemical looping reactor (Figure 3) was used in this study. The reactor is manufactured using 310 stainless steel and has an inner diameter of 10 cm, a bed height of approximately 23 cm, and a total height of 136 cm. It can be heated to 950 °C using gas preheaters and electric heaters on the reactor. The system includes a screw feeder that introduces coal into the bottom of the reactor to maximize fuel and volatile residence times. Thermocouples are placed at different heights in the reactor (1, 9, 18, and 30 cm) to monitor temperature distribution in the bed. Downstream of the reactor, a hot filter catches any elutriated particles before the hot gases enter a water-cooled condenser to remove steam. A sample slipstream travels through a sample conditioner to fully dry the gas before entering a ZRE NDIR/O2 multichannel gas analyzer from California Analytic Instruments that continuously measures CO2, CO, CH4, and O2 concentrations. To balance system pressures, the system outlet is equipped with an induced draft fan. Steam was chosen as a fluidizing agent for this set of experiments instead of CO2 because it introduces challenges with data analysis. Some tests were carried out in nitrogen in order to more clearly see gas concentration profiles, but the conversion analysis concerns steam experiments. The bed inventory was kept at a bed aspect ratio of 2.25 for all experiments and superficial velocities ranged between 0.08 and 0.10 m/s. The oxidizing environment of 21% O2 was created by adding pure oxygen to steam for 300 s to ensure complete oxidation of the oxygen carrier particles. To guarantee that the oxygen supplied to the fuel was solely from the oxygen carrier, 30 s was allowed to pass before approximately 3 g of carbon (either coal or coal char) was introduced into the reactor through the feeder, effectively operating the reactor as a batch system. 2.2. Oxygen Carrier Properties. Three oxygen carriers were investigated, two without oxygen uncoupling properties (non-CLOU) and one copper-based CLOU carrier. • Ilmenite: One non-CLOU carrier was the natural ore ilmenite, for which iron is the active oxygen-carrying metal. Ilmenite’s oxygen-carrying capacity (percent mass of oxidized carrier that is transferrable oxygen) is associated with the reduction of Fe2O3 to FeO53 in a chemical looping system. For the ilmenite used in these experiments, the oxygen-carrying capacity was 3.5%. • Copper-on-alumina: The other non-CLOU carrier was prepared by impregnating a highly porous γ-Al2O3 (Puralox NWa-155, Sasol Germany GmbH) support with an excess of a 0.45 M copper ammonia nitrate solution for 10 min before it was rinsed with deionized water. A vacuum pump was used to filter the excess solution for 30 min. The oxygen carrier particles were then heated in air at 300 °C for 2 h to decompose the copper nitrate to CuO. The copper-on-alumina particles were then loaded into the fluidized bed chemical looping reactor and heated to 950 °C cycling through reduction and oxidation states for three days, allowing CuO to interact with the Al2O3 support forming the CuAl2O4 spinel.5,6,8 Although the thermodynamics of CuAl2O4 suggests that it has CLOU properties, experiments have shown that the rate of oxygen release is so slow that it effectively does not function as a CLOU carrier when manufactured with the precursor γ-Al2O3.5,6,54 In-house thermogravimetric analysis (TGA) data also indicate the copper-on-alumina particles, which will be indicated by CuAl2O4, do not possess CLOU properties. • Copper-on-silica: A copper-based CLOU carrier was manufactured by impregnating a porous silica support (CARiACT-Q10C, Fuji Silysia Chemical LTD) with an excess of a 1.0 M copper nitrate solution. The excess solution was removed by filtration before the particles were dried, and the process was repeated once more. The copper-on-silica particles, indicated as CuO/SiO2, were heated in air at 300 °C for 4 h to decompose the copper nitrate to CuO. Properties of the different oxygen carrier particles are summarized in Table 1. The oxygen transport capacity of the oxygen carriers was determined by TGA. The gas environment for the CLOU carrier was

parameter

non-CLOU carriers

oxygen-carrying capacity (g O/g)a targeted CuO content (wt %) bulk density (g/mL) BET surface area (m2/g) size (μm)

CLOU carrier

ilmenite

CuAl2O4

CuO/SiO2

0.035 N/A 2.5 9.3 106−250

0.004 10 1.04 17 180−425

0.010 20 0.75 217 300−425

a

Measured by cycling air with 50% CO2, 25% CO, and 5% CH4 in helium for non-CLOU carriers and air and N2 for the CLOU carrier. alternated between air and N2, and the non-CLOU carriers were cycled between air and a gas composed of 50% CO2, 25% CO, and 5% CH4 in helium at 900 °C. 2.3. Coal and Coal Char Sample Preparation. The fuel used in this study was a sub-bituminous powder River Basin coal from the Black Thunder mine. The coal was sieved into three size ranges, shown in Table 2.

Table 2. Fuel Particle Size Range approximate average fuel size (μm) fuel size range (μm)

50 25−75

400 295−500

1800 1180−2360

The coal was dried in an oven until the moisture content was less than 0.5 wt %. Coal char was made by heating crushed coal in a nitrogen-purged furnace at 900 °C for 1 h. The char was then sieved into the three size ranges specified in Table 2. The proximate and ultimate analyses of the fuel are specified in Table 3 (analysis carried out by Horizon Laboratories). 2.4. Data Evaluation. This study focuses on carbon conversion of coal and its associated char. The fate of fuel carbon during CLC was determined by tracking the composition of the product gas in the fuel reactor phase. There are two main pathways for carbon conversion explored in this paper. The first is the conversion of carbonaceous volatiles released from coal and the second is the conversion of the residual char. To determine the conversion of carbon in the fuel (either coal or coal char) into gas at time t, XC,fuel(t), the mass flowrate of carbon (ṁ C) was integrated from the time the fuel enters the reactor (t0) to time t, divided by the total mass of carbon converted during the entire fuel reactor phase, where tend,FR is the time elapsed when the fuel reactor phase was ended, according to eq 2. For tests in which full conversion was not achieved during the reducing period, it was necessary to base the total mass of carbon converted in the fuel reactor phase on the amount of carbon fed into the reactor. To compare conditions, carbon conversion at 90 s was considered because earlier analyses have determined that 90 s is a reasonable solid residence time for a fuel reactor in a chemical looping system.48,55,56 t

XC,fuel(t ) =

∫t ṁ C dt 0

t

∫t end,FR ṁ C dt 0

(2)

To determine the contribution of volatiles to carbon conversion at time t, XC,vol(t), carbon conversion of char at time t was subtracted from the carbon conversion of coal at time t (eq 3).

XC,vol(t ) = XC,coal(t ) − XC,char(t )

(3)

If incomplete combustion occurs, the carbonaceous gases exiting the fuel reactor can be a combination of CO2, CO, and CH4. To quantify the distribution of carbon among gaseous species generated, the mass fraction of carbon present as each gaseous species i (f C,i) was determined (eq 4) by integrating the mass flowrate of carbon in each species i (ṁ C,i) from the time the fuel enters the reactor to the end of the reducing period over the total mass of gaseous carbon that has exited the reactor during the reducing period. Mass flowrates were determined C


Article

Energy & Fuels Table 3. Fuel Properties fuel

proximate analysis (dry basis %)

coal coal char

ultimate analysis (dry basis %)

fixed carbon

volatile matter

ash

C

H

N

S

O

ash

46.25 80.48

47.26 6.78

6.49 12.74

65.58 83.02

4.28 0.67

0.94 0.89

0.36 0.43

22.35 2.25

6.49 12.74

represents the air reactor where oxygen is added to the fluidizing gas to make its incoming concentration 21% O2, oxidizing the carrier and combusting any unburnt fuel particles. Shortly, after time zero, when the batch of coal is injected into the bed, a large spike of CO2 is observed, indicating the conversion of coal by chemical looping. Small concentrations of CO and CH4 are also observed, which result from the release of volatiles with ineffective oxidation by the carrier resulting from poor gas− solid mixing or slow kinetics. On the right side of the curve, a small peak of CO2 results from combustion of residual coal char, and the O2 content increases to the inlet concentration of 21%. During this period, some of the input oxygen is consumed by the carrier as it reoxidizes, but the effect is too small to evaluate in this experiment. It should be noted that the analyzer signal drifts off the baseline for some gases, such as the CH4 curve seen in the bottom figure of Figure 4. The baseline was adjusted to zero by subtracting the offset by the raw data. In some cases, the carbon conversion showed greater than 100% as a result of the analyzer’s slow response time and the long tails. 3.1. Carbon Conversion with the Non-CLOU Carriers. Carbon conversion of the 400 μm fuel particles at 90 s at different reactor temperatures for ilmenite and CuAl2O4 particles is presented in Figure 5. Both non-CLOU carriers show a distinct increase in carbon conversion with temperature. This trend is directly related to char conversion because the nonCLOU carriers require that the char be gasified by steam to combustible H2 and CO, and the rate of gasification is strongly dependent on temperature. As seen in the figure, carbon conversion of char with CuAl2O4 is better than that of ilmenite at all temperatures. Copper-based CuAl2O4 is known to be more reactive than iron-based ilmenite. The higher char conversion is likely due to a combination of enhanced reaction between the solid char and oxygen carrier as well as more efficient consumption of gasification products H2 and CO by CuAl2O4. Hydrogen and CO are known inhibitors of steam gasification reactions, and it has been reported that reactive oxygen carriers will limit such inhibition, thus improving conversion by gasification.57 Carbon conversion of coal at lower temperatures with both non-CLOU carriers depended predominately on conversion of carbonaceous volatiles because char conversion was inefficient. For ilmenite, the conversion of carbonaceous

by a nitrogen balance, accounting for the variation in volumetric gas flowrates and all units are in mass percent. Multiple replicates were performed for most conditions. Error bars on graphs are based on standard error of these replicates. For conditions where only one test was performed, no error bars are shown. t

fC, i =

∫t end,FR ṁ C, i dt 0

t

∫t end,FR ṁ C dt 0

(4)

3. RESULTS AND DISCUSSION A gas evolution profile generated during a typical batch experiment (50 μm coal particles and CuAl2O4 carrier) is presented in Figure 4. The left-hand side simulates the fuel

Figure 4. Gas evolution curves for conversion of the small coal particles, fluidizing CuAl2O4 in N2 at 950 °C. The gas concentration scale is magnified in the lower figure.

reactor environment in which the reactor is fluidized with N2 (steam fluidization for data analysis) and the right-hand side

Figure 5. Carbon conversion at 90 s for the 400 μm fuel particles at varying temperatures for non-CLOU carriers. D


Article

Energy & Fuels

Figure 6. Influence of fuel particle size on carbon conversion at 90 s at 900 °C for the non-CLOU carriers.

Figure 7. Influence of temperature on the distribution of carbon in carbon-containing gases (mass basis) with 400 μm coal particles for non-CLOU carriers.

Figure 8. Influence of coal particle size on the distribution of carbon in carbon-containing gases at 900 °C for non-CLOU carriers.

volatiles was relatively constant at all temperatures, indicating that the increase in carbon conversion of the coal resulted from the increase in the gasification rate with temperature. CuAl2O4, on the other hand, showed reduced volatiles conversion at higher temperatures at 90 s. The apparent contribution of volatiles to carbon conversion at 950 °C was lower than the trend would suggest. However, this is due to the particularly high char conversion and the way the volatiles contribution is calculated in eq 3. Figure 6 shows the influence of fuel particle size on carbon conversion at 90 s at 900 °C for the non-CLOU carriers. Carbon conversion of char is higher for smaller particles as their surface area-to-volume ratio is greater than that for the large particles, resulting in more intimate contact between the steam and the solid char for both non-CLOU carriers. For ilmenite, carbon conversion of the 50 μm coal particles is less than the 400 μm

coal particles at 90 s (even though char conversion is slightly higher) because the smaller coal particles release volatiles at a faster rate than the 400 μm coal particles, and ilmenite (not being a very reactive oxygen carrier) cannot convert all of the volatiles that are being released from the 50 μm coal particles. The opposite trend is seen for the CuAl2O4 particles, which convert volatiles released from the 50 μm coal particles more efficiently because of the higher reactivity of the carrier. The influence of temperature on the distribution of carbon in carbon-containing gases for the 400 μm coal particles is presented in Figure 7. Carbon as CH4 did not change significantly with temperature for both non-CLOU carriers with roughly 5% of the carbon as CH4 with ilmenite and approximately 1.7% with CuAl2O4. Carbon as CO increases at elevated temperatures with both non-CLOU carriers. This is a consequence of both the higher volatile release and increase in E


Article

Energy & Fuels the gasification rate with temperature. It is interesting to note that the contribution of carbon as CO from volatiles decreases with temperature and the contribution of carbon as CO from gasification increases, suggesting the main cause for the increase of carbon in CO is the increase in the gasification rate of char and the inability of the non-CLOU carriers to convert the additional gasification products. Figure 8 shows the influence of fuel particle size on the distribution of carbon in carbon-containing gases at 900 °C. For CuAl2O4, carbon as CH4 does not vary significantly with the fuel particle size. For ilmenite, a slight increase in carbon as CH4 is seen with smaller coal particle sizes but is within 1% standard error. Carbon as CO decreases with increasing coal particle size because large particles gasify and devolatilize more slowly, allowing more opportunity for product gases to contact the carrier. Less CH4 and CO is seen with CuAl2O4 compared to ilmenite because of the higher reactivity of the carrier. 3.2. Carbon Conversion with a Cu-Based CLOU Carrier. Figure 9 shows the influence of temperature on carbon

Figure 10. Carbon conversion of different sized fuel particles at 900 °C using a CLOU carrier.

the CLOU carrier is a much more reactive carrier than the nonCLOU carriers as almost all of the carbon in the small coal particles is converted in 90 s. However, the distribution of how carbon is converted with the fuel particle size for the CLOU and non-CLOU carriers is similar, that is, more char is converted in the smaller fuel particles and larger coal particles rely mainly on the conversion of volatiles. The distribution of carbon in carbon-containing gases did not change significantly with temperature because nearly all the gas was in the form of CO2. Carbon as CH4 was less than 1% and carbon as CO was less than 2%. Fuel particle size had a slight influence on gas composition and only for the smallest fuel size because the rate of CO generation exceeded the rate of oxygen release, shown in Figure 11.

Figure 9. Carbon conversion of the 400 μm fuel particles at 90 s during CLOU at different temperatures.

conversion with 400 μm fuel particles at 90 s using the CLOU carrier. At 850 °C, the rate of oxygen release is slow so that carbon conversion is primarily a result of conversion of volatiles and char gasification. Increasing temperatures to 900 °C increased overall carbon conversion and enhanced char conversion by approximately a factor of 12 because the rate of oxygen release from the CLOU carrier increases with temperature. Char and volatile conversion were similar at 900 and 950 °C. TGA analysis shows that CuO/SiO2 releases all of its oxygen in approximately 60 s at 950 °C. The fuel is introduced at 30 s, at which time there is 24% of the total available oxygen left on the carrier to be released at 950 °C and 55% at 900 °C. This is a shortcoming of introducing the fuel after 30 s for the CLOU experiments because the most reactive state of the CLOU carrier is not captured; however, this delay was necessary to ensure fuel conversion was solely from the carrier and not oxygen in the fluidizing gas. In a continuous circulating system, there would not be this discrepancy with the amount of oxygen available for conversion because the oxygen carrier entering the fuel reactor is fresh. The influence of fuel particle size on carbon conversion at 90 s, fluidizing in steam at 900 °C for the CLOU carrier is presented in Figure 10. Carbon conversion was highest with the small fuel particles because they have a greater surface area-to-volume ratio than the larger fuel particles. The large particles did not achieve full conversion because the oxygen carrier had exhausted all of its oxygen before the particles could be fully converted. It is clear

Figure 11. Influence of fuel particle size on the distribution of carbon in carbon-containing gases using a CLOU carrier at 900 °C.

3.3. CLOU Versus Non-CLOU Oxygen Carriers. Figure 12 compares carbon conversion of the 400 μm fuel particles at 900 °C for the three oxygen carriers used in this study. Carbon conversion of the volatiles and char was much more efficient with the CLOU carrier CuO/SiO2 than for the non-CLOU carrier’s ilmenite and CuAl2O4. For the two smallest fuel sizes tested, nearly all the carbon was completely converted at 90 s with the CLOU carrier at temperatures above 900 °C, and less than 6% of the carbon ended up in the form of CH4 and CO for all fuel sizes and temperatures (Figure 13). In a continuous system, this would result in a high CO2 purity as well as high CO2 capture efficiencies. The non-CLOU carriers were unable to achieve complete carbon conversion and 7−27% of the carbon-containing gases were in the form of CH4 and CO, depending on conditions. Large amounts of unconverted gases F


Article

Energy & Fuels

more reactive than ilmenite, resulting in higher carbon conversions at 90 s and lower carbon as CH4 and CO. Conversion by copper-based CLOU is far more efficient than with conventional non-CLOU carriers, particularly the commonly used ilmenite. The CLOU carrier resulted in higher carbon conversions as well as less unconverted carbon gases compared to the nonCLOU carriers as the mechanism by which carbon is converted in a CLOU system is more favorable. Chemical looping systems have many competing influences on the fate of carbon, including elutriation of solid fuel particles, gasification rate, temperature, and oxygen carrier reactivity. Future research will focus on distinguishing and quantifying the relative influences of various conversion mechanisms.

Figure 12. Carbon conversion of the 400 μm coal particles at 90 s at 900 °C for CLOU and non-CLOU oxygen carriers.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Kirsten M. Merrett: 0000-0003-1093-9786 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This material is based on the work supported by the United States Department of Energy under Award Number DEFE0025076. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. The authors are grateful for experimental assistance from Dana Overaker, Alex Godun, and Giovanna Roth.

Figure 13. Distribution of carbon in carbon-containing gases for the 400 μm coal particles at 900 °C for CLOU and non-CLOU carriers.

■

off the fuel reactor will increase the cost of operation as an oxygen polishing step would be necessary to completely convert these gases before further CO2 processing can take place. The inability of the non-CLOU carriers to fully convert char is unfavorable as unburnt char particles will be carried over to the air reactor, decreasing CO2 capture. These results agree with previous investigations that a CLOU oxygen carrier converts solid fuel more readily than non-CLOU carriers. It has been reported that char conversion rates can be up to 50 times faster with a CLOU carrier. However, these studies were carried out under ideal conditions using only char as the fuel. In the system studied here, where volatiles compete with char for oxygen, char conversion was 2−22 times greater conversion with the CLOU carrier versus the two non-CLOU carriers at 90 s. These results are promising for larger scale design and operation of CLOU units for solid fuels.

NOMENCLATURE

Abbreviation Name

CLC = chemical looping combustion CLOU = chemical looping with oxygen uncoupling iG-CLC = in situ gasification chemical looping combustion TGA = thermogravimetric analysis Symbol Name and Units

f C,i = fraction of carbon converted in species i compared to the total carbon converted (g/g) ṁ C = mass flowrate of carbon (g/s) ṁ C,i = mass flowrate of carbon in each species i (g/s) t = time at which carbon conversion is measured (s) t0 = time solid fuel is introduced into the reactor (s) tend,FR = time at the end of the reducing period (s) XC,fuel (t) = carbon conversion of the fuel (coal or coal char) at time t (g/g) XC,vol (t) = carbon conversion of volatiles at time t (g/g)

4. CONCLUSIONS Carbon conversion and product gas compositions were evaluated versus temperature and fuel particle size in a fluidized bed chemical looping reactor using non-CLOU and CLOU oxygen carriers. Higher temperatures and smaller fuel particle sizes resulted in better conversion under most conditions. For very fine fuel particles, the fast rate of volatiles released can overwhelm the ability of the oxygen carrier to convert the gases, particularly CO and CH4. For the non-CLOU ilmenite and copper aluminate oxygen carriers, conversion depends almost entirely on the rate of gasification, which is sensitive to temperature and the fuel particle size where smaller particles that provide a lot of surface area for heterogeneous reactions are more efficient. Of the non-CLOU carriers, CuAl2O4 is much

■

REFERENCES

(1) International Energy Agency. Market Report Series Oil 2017, 2017; pp 1−12. (2) Adánez, J.; Abad, A.; Mendiara, T.; Gayán, P.; de Diego, L. F.; García-Labiano, F. Chemical Looping Combustion of Solid Fuels. Prog. Energy Combust. Sci. 2018, 65, 6−66. (3) Adánez, J.; Abad, A.; Garcı ́a-Labiano, F.; Gayán, P.; de Diego, L. F. Progress in Chemical-Looping Combustion and Reforming Technologies. Prog. Energy Combust. Sci. 2012, 38, 215−282. (4) Mendiara, T.; Adánez-Rubio, I.; Gayán, P.; Abad, A.; de Diego, L. F.; García-Labiano, F.; Adánez, J. Process Comparison for Biomass Combustion: In Situ Gasification-Chemical Looping Combustion (IGG


Article

Energy & Fuels CLC) versus Chemical Looping with Oxygen Uncoupling (CLOU). Energy Technol. 2016, 4, 1130−1136. (5) Arjmand, M.; Azad, A.-M.; Leion, H.; Lyngfelt, A.; Mattisson, T. Prospects of Al2O3 and MgAl2O4-Supported CuO Oxygen Carriers in Chemical-Looping Combustion (CLC) and Chemical-Looping with Oxygen Uncoupling (CLOU). Energy Fuels 2011, 25, 5493−5502. (6) Arjmand, M.; Azad, A.-M.; Leion, H.; Mattisson, T.; Lyngfelt, A. Evaluation of CuAl2O4 as an Oxygen Carrier in Chemical-Looping Combustion. Ind. Eng. Chem. Res. 2012, 51, 13924−13934. (7) Adánez-Rubio, I.; Gayán, P.; Abad, A.; de Diego, L. F.; GarcíaLabiano, F.; Adánez, J. Evaluation of a Spray-Dried CuO/MgAl2O4 Oxygen Carrier for the Chemical Looping with Oxygen Uncoupling Process. Energy Fuels 2012, 26, 3069−3081. (8) Adánez-Rubio, I.; Gayán, P.; García-Labiano, F.; de Diego, L. F.; Adánez, J.; Abad, A. Development of CuO-Based Oxygen-Carrier Materials Suitable for Chemical-Looping with Oxygen Uncoupling (CLOU) Process. Energy Procedia 2011, 4, 417−424. (9) de Diego, L. F.; Garcı ́a-Labiano, F.; Adánez, J.; Gayán, P.; Abad, A.; Corbella, B. M.; Palacios, J. M. Development of Cu-Based Oxygen Carriers for Chemical-Looping Combustion. Fuel 2004, 83, 1749− 1757. (10) Mattisson, T.; Lyngfelt, A.; Leion, H. Chemical-Looping with Oxygen Uncoupling for Combustion of Solid Fuels. Int. J. Greenhouse Gas Control 2009, 3, 11−19. (11) Berguerand, N.; Lyngfelt, A. Design and Operation of a 10 KWth Chemical-Looping Combustor for Solid Fuels - Testing with South African Coal. Fuel 2008, 87, 2713−2726. (12) Ströhle, J.; Orth, M.; Epple, B. Design and Operation of a 1 MWth Chemical Looping Plant. Appl. Energy 2014, 113, 1490−1495. (13) Ströhle, J.; Orth, M.; Epple, B. Chemical Looping Combustion of Hard Coal in a 1 MWth Pilot Plant Using Ilmenite as Oxygen Carrier. Appl. Energy 2014, 157, 1−11. (14) Pérez-Vega, R.; Abad, A.; García-Labiano, F.; Gayán, P.; de Diego, L. F.; Adánez, J. Coal Combustion in a 50 KWth Chemical Looping Combustion Unit: Seeking Operating Conditions to Maximize CO2 Capture and Combustion Efficiency. Int. J. Greenhouse Gas Control 2016, 50, 80−92. (15) Linderholm, C.; Schmitz, M.; Knutsson, P.; Lyngfelt, A. Chemical-Looping Combustion in a 100-KW Unit Using a Mixture of Ilmenite and Manganese Ore as Oxygen Carrier. Fuel 2016, 166, 533−542. (16) Bayham, S. C.; Kim, H. R.; Wang, D.; Tong, A.; Zeng, L.; McGiveron, O.; Kathe, M. V.; Chung, E.; Wang, W.; Wang, A.; et al. Iron-Based Coal Direct Chemical Looping Combustion Process: 200-h Continuous Operation of a 25-KWth Subpilot Unit. Energy Fuels 2013, 27, 1347. (17) Abad, A.; Adánez-Rubio, I.; Gayán, P.; García-Labiano, F.; de Diego, L. F.; Adánez, J. Demonstration of Chemical-Looping with Oxygen Uncoupling (CLOU) Process in a 1.5kW Th Continuously Operating Unit Using a Cu-Based Oxygen-Carrier. Int. J. Greenhouse Gas Control 2012, 6, 189−200. (18) Lyngfelt, A. Oxygen Carriers for Chemical Looping Combustion - 4000 h of Operational Experience. Oil Gas Sci. Technol. 2011, 66, 161−172. (19) Berguerand, N.; Lyngfelt, A. The use of petroleum coke as fuel in a 10kWth chemical-looping combustor. Int. J. Greenhouse Gas Control 2008, 2, 169−179. (20) Berguerand, N.; Lyngfelt, A. Chemical-Looping Combustion of Petroleum Coke Using Ilmenite in a 10 Kwth Unit-High-Temperature Operation. Energy Fuels 2009, 23, 5257−5268. (21) Berguerand, N.; Lyngfelt, A. Operation in a 10 KWth ChemicalLooping Combustor for Solid Fuel-Testing with a Mexican Petroleum Coke. Energy Procedia 2009, 1, 407−414. (22) Berguerand, N.; Lyngfelt, A.; Mattisson, T.; Markström, P. Chemical Looping Combustion of Solid Fuels in a 10 KWth Unit. Oil Gas Sci. Technol. 2011, 66, 181−191. (23) Linderholm, C.; Lyngfelt, A.; Cuadrat, A.; Jerndal, E. ChemicalLooping Combustion of Solid Fuels - Operation in a 10 KW Unit with

Two Fuels, above-Bed and in-Bed Fuel Feed and Two Oxygen Carriers, Manganese Ore and Ilmenite. Fuel 2012, 102, 808−822. (24) Cuadrat, A.; Abad, A.; García-Labiano, F.; Gayán, P.; de Diego, L. F.; Adánez, J. Ilmenite as Oxygen Carrier in a Chemical Looping Combustion System with Coal. Energy Procedia 2011, 4, 362−369. (25) Cuadrat, A.; Abad, A.; García-Labiano, F.; Gayán, P.; de Diego, L. F.; Adánez, J. Relevance of the Coal Rank on the Performance of the in Situ Gasification Chemical-Looping Combustion. Chem. Eng. J. 2012, 195-196, 91−102. (26) Cuadrat, A.; Abad, A.; García-Labiano, F.; Gayán, P.; de Diego, L. F.; Adánez, J. The Use of Ilmenite as Oxygen-Carrier in a 500Wth Chemical-Looping Coal Combustion Unit. Int. J. Greenhouse Gas Control 2011, 5, 1630−1642. (27) Cuadrat, A.; Abad, A.; Adánez, J.; de Diego, L. F.; GarcíaLabiano, F.; Gayán, P. Behavior of Ilmenite as Oxygen Carrier in Chemical-Looping Combustion. Fuel Process. Technol. 2012, 94, 101− 112. (28) Linderholm, C.; Lyngfelt, A.; Dueso, C. Chemical-Looping Combustion of Solid Fuels in a 10kW Reactor System Using Natural Minerals as Oxygen Carrier. Energy Procedia 2013, 37, 598−607. (29) Schmitz, M.; Linderholm, C.; Hallberg, P.; Sundqvist, S.; Lyngfelt, A. Chemical-Looping Combustion of Solid Fuels Using Manganese Ores as Oxygen Carriers. Energy Fuels 2016, 30, 1204− 1216. (30) Bayham, S.; McGiveron, O.; Tong, A.; Chung, E.; Kathe, M.; Wang, D.; Zeng, L.; Fan, L.-S. Parametric and Dynamic Studies of an Iron-Based 25-KWth Coal Direct Chemical Looping Unit Using SubBituminous Coal. Appl. Energy 2015, 145, 354. (31) Gu, H.; Shen, L.; Xiao, J.; Zhang, S.; Song, T. Chemical Looping Combustion of Biomass/Coal with Natural Iron Ore as Oxygen Carrier in a Continuous Reactor. Energy Fuels 2011, 25, 446−455. (32) Gu, H.; Shen, L.; Zhong, Z.; Niu, X.; Ge, H.; Zhou, Y.; Xiao, S. Potassium-Modified Iron Ore as Oxygen Carrier for Coal Chemical Looping Combustion: Continuous Test in 1 KW Reactor. Ind. Eng. Chem. Res. 2014, 53, 13006. (33) Jiang, S.; Shen, L.; Niu, X.; Ge, H.; Gu, H. Chemical Looping CoCombustion of Sewage Sludge and Zhundong Coal with Natural Hematite as the Oxygen Carrier. Energy Fuels 2016, 30, 1720−1729. (34) Mendiara, T.; de Diego, L. F.; García-Labiano, F.; Gayán, P.; Abad, A.; Adánez, J. On the Use of a Highly Reactive Iron Ore in Chemical Looping Combustion of Different Coals. Fuel 2014, 126, 239−249. (35) Song, T.; Shen, T.; Shen, L.; Xiao, J.; Gu, H.; Zhang, S. Evaluation of Hematite Oxygen Carrier in Chemical-Looping Combustion of Coal. Fuel 2013, 104, 244−252. (36) Tong, A.; Sridhar, D.; Sun, Z.; Kim, H. R.; Zeng, L.; Wang, F.; Wang, D.; Kathe, M. V.; Luo, S.; Sun, Y.; et al. Continuous High Purity Hydrogen Generation from a Syngas Chemical Looping 25 KWth SubPilot Unit with 100% Carbon Capture. Fuel 2013, 103, 495−505. (37) Dennis, J. S.; Scott, S. A. In Situ Gasification of a Lignite Coal and CO2 Separation Using Chemical Looping with a Cu-Based Oxygen Carrier. Fuel 2010, 89, 1623−1640. (38) Markström, P.; Linderholm, C.; Lyngfelt, A. Chemical-Looping Combustion of Solid Fuels - Design and Operation of a 100kW Unit with Bituminous Coal. Int. J. Greenhouse Gas Control 2013, 15, 150− 162. (39) Adánez-Rubio, I.; Abad, A.; Gayán, P.; de Diego, L. F.; GarcíaLabiano, F.; Adánez, J. Biomass Combustion with CO2 Capture by Chemical Looping with Oxygen Uncoupling (CLOU). Fuel Process. Technol. 2014, 124, 104−114. (40) Schmitz, M.; Linderholm, C.; Lyngfelt, A. Chemical Looping Combustion of Sulphurous Solid Fuels Using Spray-Dried Calcium Manganate Particles as Oxygen Carrier. Energy Procedia 2014, 63, 140− 152. (41) Schmitz, M.; Linderholm, C. J. Performance of Calcium Manganate as Oxygen Carrier in Chemical Looping Combustion of Biochar in a 10 KW Pilot. Appl. Energy 2016, 169, 729−737. (42) Adánez-Rubio, I.; Izquierdo, M. T.; Abad, A.; Gayán, P.; de Diego, L. F.; Adánez, J. Spray Granulated Cu-Mn Oxygen Carrier for H


Article

Energy & Fuels Chemical Looping with Oxygen Uncoupling (CLOU) Process. Int. J. Greenhouse Gas Control 2017, 65, 76−85. (43) Leion, H.; Mattisson, T.; Lyngfelt, A. Using Chemical-Looping with Oxygen Uncoupling (CLOU) for Combustion of Six Different Solid Fuels. Energy Procedia 2009, 1, 447−453. (44) Tian, X.; Zhao, H.; Wang, K.; Ma, J.; Zheng, C. Performance of Cement Decorated Copper Ore as Oxygen Carrier in ChemicalLooping with Oxygen Uncoupling. Int. J. Greenhouse Gas Control 2015, 41, 210. (45) de Diego, L. F.; Gayán, P.; García-Labiano, F.; Celaya, J.; Abad, A.; Adánez, J. Impregnated CuO/Al2O3 Oxygen Carriers for Chemical-Looping Combustion: Avoiding Fluidized Bed Agglomeration. Energy Fuels 2005, 19, 1850−1856. (46) Xu, L.; Wang, J.; Li, Z.; Cai, N. Experimental Study of CementSupported CuO Oxygen Carriers in Chemical Looping with Oxygen Uncoupling (CLOU). Energy Fuels 2013, 27, 1522−1530. (47) Mattisson, T.; Leion, H.; Lyngfelt, A. Chemical-Looping with Oxygen Uncoupling Using CuO/ZrO2 with Petroleum Coke. Fuel 2009, 88, 683. (48) Clayton, C. K.; Whitty, K. J. Measurement and Modeling of Decomposition Kinetics for Copper Oxide-Based Chemical Looping with Oxygen Uncoupling. Appl. Energy 2014, 116, 416−423. (49) Arjmand, M.; Keller, M.; Leion, H.; Mattisson, T.; Lyngfelt, A. Oxygen Release and Oxidation Rates of MgAl2O4- Supported CuO Oxygen Carrier for Chemical-Looping Combustion with Oxygen Uncoupling (CLOU). Energy Fuels 2012, 26, 6528. (50) Adánez-Rubio, I.; Abad, A.; Gayán, P.; de Diego, L. F.; GarcíaLabiano, F.; Adánez, J. Performance of CLOU Process in the Combustion of Different Types of Coal with CO2 Capture. Int. J. Greenhouse Gas Control 2013, 12, 430−440. (51) Pérez-Vega, R.; Adánez-Rubio, I.; Gayán, P.; Izquierdo, M. T.; Abad, A.; García-Labiano, F.; de Diego, L. F.; Adánez, J. Sulphur, Nitrogen and Mercury Emissions from Coal Combustion with CO2 Capture in Chemical Looping with Oxygen Uncoupling (CLOU). Int. J. Greenhouse Gas Control 2016, 46, 28. (52) Adánez-rubio, I.; Abad, A.; Gayán, P.; García-labiano, F.; Diego, L. F. De; Adánez, J. Coal Combustion with a Spray Granulated Cu-Mn Mixed Oxide for the Chemical Looping with Oxygen Uncoupling ( CLOU ) Process. Appl. Energy 2017, 208, 561. (53) Leion, H.; Lyngfelt, A.; Johansson, M.; Jerndal, E.; Mattisson, T. The Use of Ilmenite as an Oxygen Carrier in Chemical-Looping Combustion. Chem. Eng. Res. Des. 2008, 86, 1017−1026. (54) Hu, W.; Donat, F.; Scott, S. A.; Dennis, J. S. The Interaction between CuO and Al2O3 and the Reactivity of Copper Aluminates below 1000 °C and Their Implication on the Use of the Cu-Al-O System for Oxygen Storage and Production. RSC Adv. 2016, 6, 113016. (55) Sahir, A. H.; Sohn, H. Y.; Leion, H.; Lighty, J. S. Rate Analysis of Chemical-Looping with Oxygen Uncoupling (CLOU) for Solid Fuels. Energy Fuels 2012, 26, 4395. (56) Eyring, E. M.; Konya, G.; Lighty, J. S.; Sahir, A. H.; Sarofim, A. F.; Whitty, K. Chemical Looping with Copper Oxide as Carrier and Coal as Fuel. Oil Gas Sci. Technol. 2011, 66, 209−221. (57) Keller, M.; Leion, H.; Mattisson, T.; Lyngfelt, A. Gasification Inhibition in Chemical-Looping Combustion with Solid Fuels. Combust. Flame 2011, 158, 393.

I


Conversion of Coal in a Fluidized Bed Chemical Looping Combustion

Recommend Documents