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Operation of a 50-kWth chemical looping combustion test facility under autothermal conditions
International Journal of Greenhouse Gas Control 87 (2019) 211–220
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International Journal of Greenhouse Gas Control journal homepage: www.elsevier.com/locate/ijggc
Operation of a 50-kWth chemical looping combustion test facility under autothermal conditions
T
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Samuel Bayham , Douglas Straub, Justin Weber National Energy Technology Laboratory, US Department of Energy, Morgantown, WV, 26507, USA
A R T I C LE I N FO
A B S T R A C T
Keyword: Chemical Looping Combustion
The United States Department of Energy is developing transformational fossil fuel combustion technologies that result in more efficient power cycles and/or substantial reductions in GHG emissions. Chemical Looping Combustion (CLC) has the potential to achieve a lower cost of electricity than a supercritical pulverized coal power plant with an amine absorption system. Although the concept and research on chemical looping has a long history, CLC has numerous technical issues that must be addressed prior to a commercial debut. This effort is directed toward the following major issues for CLC: (1) demonstrating continuous solids handling, operability, and reactor performance at autothermal conditions and (2) preliminary assessments of oxygen carrier attrition and relative material make-up costs. These are critical issues that should be addressed under realistic conditions prior to directing resources toward larger scale demonstrations. This paper will also describe NETL’s small 50kWth natural gas circulating CLC test facility located in Morgantown, WV. Despite its small size, this experimental test unit has operated at autothermal conditions for 11 h using a copper-iron-alumina oxygen carrier developed at NETL. This paper will describe the test facility and operating experiences using NETL’s “Gen 2.0” oxygen carrier material. Natural gas conversion to CO2 ranged from 70 to 90% over approximately 75 oxidationreduction cycles during the autothermal operating period. Estimates for the process specific attrition rate during autothermal operation are also presented. Finally, estimates for the oxygen carrier makeup cost, specific to the NETL process configuration, are several orders of magnitude greater than the NETL target. Future work should focus on reducing the oxygen carrier material cost and improving the attrition resistance of the oxygen carrier material under autothermal test conditions.
1. Introduction Electricity generation from fossil fuel power plants accounts for onethird of anthropogenic CO2 emissions in the United States (Agency, 2018). In addition, the electricity industry is changing as renewables and low-cost natural gas supplies become more prominent. Thus, future fossil energy power generation must consider alternatives that are a significant departure from conventional power plants. The United States Department of Energy’s National Energy Technology Laboratory (NETL) is studying novel concepts and power cycles to meet the needs for reliable and sustainable future electric power. Reducing greenhouse gas emissions is one objective for these future power plants, and alternative approaches to address energy penalties associated with postcombustion CO2 separation are being investigated. Chemical Looping Combustion is a promising technology for mitigating greenhouse gas emissions from fossil fuel applications. According to NETL studies, a circulating fluidized bed coal-CLC boiler/
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steam generator has the potential to achieve cost-of-electricity targets of $110/MW-h while simultaneously capturing CO2 at less than $40/ tonne (National Energy Technology Laboratory, 2014). In a similar study, Ekstrom et al. (Ekström et al., 2009) investigated several concepts to mitigate greenhouse gas emissions from natural gas and bituminous coal electric power generators, concluding that CLC concepts had the lowest cost-of-electricity and the lowest cost of CO2 avoidance. Others (Adanez et al., 2012; Tsupari et al., 2014; Zhu et al., 2018) have reported similar economic benefits, and these potential benefits have stimulated international interest in this transformational technology for combustion and other potential applications. This paper will focus on fossil-fuel combustion applications with an initial interest in benchscale evaluations of oxygen carrier materials under self-sustaining levels of solid oxygen carrier heat release. In a Chemical Looping Combustion (CLC) process, the fuel and air do not mix (see Fig. 1). Since the fuel and the oxidant do not mix, an oxygen carrier material is needed to transport oxygen from an air
Corresponding author. E-mail address: [email protected] (S. Bayham).
https://doi.org/10.1016/j.ijggc.2019.05.022 Received 14 February 2019; Received in revised form 13 May 2019; Accepted 17 May 2019 Available online 26 May 2019 1750-5836/ Published by Elsevier Ltd.
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There are numerous CLC test facilities at different scales, ranging from 300-Wth fuel input to 3-MWth (Adánez et al., 2018; Weber, 2019). Chalmers University of Technology has studied hundreds of oxygen carriers in their 300 Wth, 10 kWth, and 100 kWth units. Chalmers University has investigated various fuels such as natural gas, coal, petroleum coke, and wood char (Berguerand and Lyngfelt, 2008; Markström et al., 2013), and a number of low-cost “commodity” oxygen carriers including ilmenite, iron ore, and manganese ore (Linderholm et al., 2013; Schmitz et al., 2016). Instituto de Carboquímica of Zaragoza, Spain has also operated several small pilot units ranging from 0.5 to 50 kWth using lignite, bituminous, and anthracite coals (Cuadrat et al., 2011; Pérez-Vega et al., 2016). Southeast University of Nanjing, China has studied oxygen carriers at scales ranging from 1 to 50 kWth, and the 50 kWth unit has operated at pressures of 5 atm (Shen et al., 2009; Song et al., 2013). Other continuous CLC test facilities include Hamburg University (Thon et al., 2014), Technische Universitat Wien (Kolbitsch et al., 2009), and the Korean Institute of Energy Research (Baek et al., 2016). The fuel conversion and/or combustion efficiency has typically been reported in all previous work. However, the oxygen carrier losses, deactivation, and attrition rates are not always reported. Adanez and Abad (Adánez and Abad, 2019) have provided a thorough review of attrition rates that have been reported in the literature. The oxygen carrier attrition rates ranged from 0.0023 – 0.09%/h for some of the manufactured materials tested (Abad et al., 2006; Adanez et al., 2012), while ores tested range from 0.0625–0.67%/h (Abad et al., 2006). Thousands of hours of cumulative operation have been performed in these small integrated units. However, due to the high surface area to volume ratio for these small test unit, many of these units have been heated externally using electrical heaters. This external heating does not produce the same oxygen carrier environment that would be present in a larger scale demonstration unit. Several test facility have been constructed that are capable of CLC operation without adding heat directly to the solids path. The largest unit to date has been a 3-MWth unit operated by Alstom Power. This test unit consisted of two interconnected circulating fluidized bed reactors and a limestone-based oxygen carrier material. This 3-MWth unit successfully operated on Adaro and Pittsburgh #8 coals under autothermal conditions for approximately 40 continuous hours (Andrus, 2017). Other units include Darmstadt’s 1-MWth unit (Ströhle et al., 2015) that has reported oxygen carrier attrition rates of ilmenite and iron ore at 0.4–2.3 %/hr (Strohle et al., 2018). A 250-kWth unit at the University of Utah (Whitty and Lighty, 2018) has also been constructed to test chemical looping oxygen uncoupling (CLOU) carrier materials. This paper will describe a 50-kWth natural gas Chemical Looping Combustion rig that has operated under self-sustaining, or autothermal, CLC conditions with the intent of measuring the fuel conversion, heat release, and oxygen carrier losses as a function of time during autothermal operation. Although the gases flowing into this test unit can be preheated, no external heat can be added directly to the solid oxygen carrier flow path. Autothermal operation has been defined by the following conditions: 1) all inlet gas preheaters are de-energized, 2) the oxygen carrier reactions (fuel and air) are the only source of internal heat generation, 3) the oxygen carrier experiences natural heat losses during circulation, 4) the oxygen carrier experiences rapid heating when make-up material is added to maintain a constant carrier inventory. In the following sections, the experimental 50-kW test facility will be briefly described, with additional detail can be found in Bayham et al. (Bayham et al., 2017). Following a description of the test facility, the oxygen carrier material will be described, and additional details can be found in Siriwardane et al. (2018). These sections will be followed by a discussion of the data analysis approach and the results. The final section will describe estimated oxygen carrier losses that have been observed during autothermal test conditions in which used oxygen carrier material elutriated from the process were recovered, sieved to the original particle size distribution, and returned to the system to
Fig. 1. Diagram describing chemical looping combustion.
reactor to a fuel reactor. Common oxygen carriers include relatively common metals, such as iron, copper, manganese, nickel, and others. A CLC process can produce a nitrogen-free flue gas comprised of water vapor and CO2 similar to an oxy-combustion process. However, the CLC process does not require an air separation unit, and most CLC configurations can achieve significantly lower levels of excess oxygen in the flue gas. To capture the CO2 from a CLC process, the water vapor can be condensed to produce a high purity CO2 stream, and a portion of the flue gas can be recycled to fluidize the fuel reactor. Although CLC has been studied extensively since the 1990’s, significant technical challenges still exist (Gauthier et al., 2017). One of the first major challenges is the oxygen carrier material, specifically the cost, performance, stability, and durability. Low cost minerals such as hematite, ilmenite, and other “commodity” carrier materials have been investigated, but these materials generally lack the required reactivity to achieve complete fuel conversion, and the oxygen carrying capacity is generally lower than other alternatives. Others have focused on synthetic materials that can be manufactured using conventional catalyst manufacturing processes (i.e., spray drying, co-precipitation, etc.). According to Adanez and Abad (Adánez and Abad, 2019), more than 2,000 oxygen carrier materials have been studied, but only about 15 have been tested in continuous CLC environments. This paper will describe continuous CLC testing of an NETL developed bimetallic copper-iron oxygen carrier supported on alumina. This material has shown promising results under different testing techniques, such as TGA (Siriwardane et al., 2013), a two-inch diameter fluidized bed (Siriwardane et al., 2015), and an ASTM attrition method (Siriwardane et al., 2018). Two generations of this material with the same chemical formulation have been developed. The first generation (Gen 1.0) has been manufactured using a commercial spray-drying method, but improvements in the cost and durability were needed. The second generation (Gen 2.0) has been manufactured using a wetgranulation method that has been more cost effective and has produced larger particles (Siriwardane et al., 2018). Both of these materials have exhibited a high oxygen carrying capacity (> 10 wt%), and the heats of reaction in the fuel reactor have ranged from thermally neutral to slightly exothermic. Another major technical challenge for the CLC technology involves solids handling and control. Depending on the oxygen carrying capacity and the oxygen carrier solid conversion, the required carrier circulation rate can be orders of magnitude higher than the fuel mass flow. Solids circulation processes are inherently unsteady, and the CLC operating temperatures are in the 800–1000 °C range. Many of these issues have been addressed by commercial-scale circulating fluidized bed combustion processes, but some CLC configurations require the added complexity of controlling two interconnected carrier circulation loops (i.e., an air reactor loop and a fuel reactor loop). Some issues like capacity factors for commercial scale applications will need to be addressed by larger scale demonstrations, so the current research focus is directed toward evaluating whether existing oxygen carrier materials can achieve acceptable performance metrics to justify larger scale demonstrations. 212
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Fig. 2. Sketch of the chemical looping rig (refractory not shown).
maintain a refractory heating rate of 50 °C/h. Air flows through both reactors during this initial preheating phase. Once the reactor temperatures exceed the autoignition temperature of natural gas, the second stage of operation is initiated where natural gas combustion is established in both reactors with air. Once the refractory reaches around 900 °C, solids are added via a lock hopper to the air or fuel reactor in small (nominally 2 kg) batches. Once the desired inventory is achieved, usually around 45 kg, circulation is initiated by adding aeration gas to the L-valve. After the circulation becomes steady, Lvalve cutoff tests are performed to determine the solids circulation rate (see Section 2.4). Typical carrier circulation rates for this test campaign are in the range of 160–300 kg/h. The final chemical looping combustion stage is initiated by 1) stopping the natural gas flow to the fuel reactor, 2) transitioning the air in the fuel reactor to nitrogen, and 3) injecting natural gas in the plenum of the fuel reactor. For the copperiron oxygen carrier used in this work, if the natural gas flow to the fuel reactor is sufficient enough, natural gas to the air reactor is gradually shut off along with the electric gas preheaters for each reactor. This initiates autothermal operation.
maintain constant inventory. These data can be used to estimate the oxygen carrier make-up cost specific to this process and form a basis for future comparisons and improvements.
2. Experimental setup 2.1. Rig design The 50-kWth natural gas chemical looping rig consists of refractorylined carbon steel components. A simplified schematic is shown in Fig. 2. The unit consist of a 20-cm diameter bubbling fluidized bed fuel reactor, a 15-cm diameter turbulent fluidized bed air reactor, a 5-cm diameter riser to convey solids to the cyclone, and a 20-cm diameter bubbling fluidized bed seal pot which prevents air from entering the fuel reactor and fuel from entering the spent air stream. A non-mechanical L-valve connects the solids outlet of the fuel reactor and the inlet of the air reactor. Although the solids circulation rate is affected by numerous factors, adjustments to the circulation rate were primarily controlled by two variables: the fuel reactor back pressure, and the aeration gas flow to the L-valve. The aeration gas added to the L-valve also serves the purpose of preventing gases from mixing between the air and fuel reactors. Solids in the air reactor are fluidized using primary air, and secondary air is injected above the bubble caps of the fluid bed to help entrain the solids into the riser. Global pressure in the unit is operated anywhere from 1.5 to 2.4 bar nominally. Reactions in the fuel reactor are diluted with nitrogen to help fluidize the oxygen carrier and prevent overreduction. In a commercial unit, the diluent nitrogen would be a mixture of recycled carbon dioxide and steam. For the autothermal tests reported in this paper, steam is not used due to potential issues with condensate when the preheaters are de-energized, and carbon dioxide is not used to prevent confounding effects with measuring fuel conversion to CO2. Natural gas can be introduced into one of three locations in the fuel reactor: 1) below the bubble caps, 2) directly into the bed, and 3) above the bed. When chemical looping is initiated, natural gas is injected below the bubble caps to provide some premixing with the fluidization nitrogen prior to entering the bed. Startup of the chemical looping rig involves three stages. The first stage involves heating the refractory by ramping electric preheaters to
2.2. Oxygen carrier characteristics The oxygen carrier used in this test consisted of a mixture of CuO and Fe2O3 supported on alumina. The material was manufactured at the catalyst manufacture Nexceris (Columbus, Ohio, USA) via a dry milling method, in a batch of 180 kg as described in (Siriwardane et al., 2018). A list of its properties such as mean particle size, density, and composition is presented in Table 1. The particle size distribution of the oxygen carrier tested during the campaign is shown in Fig. 3. The oxygen carrier tested in the campaign described in this work has been tested in various units, such as thermogravimetric analyzer and a small 5.08-cm fluidized bed with methane and coal (Siriwardane et al., 2015, 2013). The composition of the manufactured oxygen carrier was measured using X-ray Fluorescence (XRF) (Rigaku ZSX Primus II). As reported in previous works, the developed carrier composition consists of 30 wt% CuO, 30 wt% Fe2O3, and 40 wt% Al2O3 on a dry, premixed basis. However, as shown in Table 1, measurement of the delivered oxygen carrier from the catalyst manufacturer deviated from the premixed 213
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Table 1 Particle properties.
XCH4 =
Property
Value
Units
Particle density Sauter mean diameter D50 Sphericity Umf (at 298 K) Fe2O3 CuO Al2O3
2.9 343 397 0.91 14 31 37 32
g/cm3 μm μm – cm/s wt% wt% wt%
N˙ CH4,in − N˙ CH4,out N˙ CH4,in
This parameter determines the amount of methane that was oxidized by the oxygen carrier. The third performance parameter is an indication of the amount of fuel that is converted to CO2.
XCH4 → CO2 =
N˙ CO2,out N˙ CH4,in
Since the goal is to convert the methane fully to carbon dioxide, this expression is not affected by conversion of the gas to CO or carbon deposition. Finally, the carbon dioxide purity was calculated on the ratio of the molar flow rate of carbon dioxide and the total molar flow rate minus the molar flows of nitrogen and steam.
YCO2 =
N˙ CO2,out N˙ total,out − N˙ H2O,out − N˙ N2,out
Further detail on the performance parameters can be found in Bayham et al. (2017). 2.3.2. Solids analysis The solids circulation rate is measured in the test rig by correlating the pressure drop in the riser to the change in bed level during transient L-valve cutoff tests. An L-valve cutoff test involves circulating solids at temperature and shutting off the L-valve aeration gas to stop solids circulation. When circulation stops, the solids begin to accumulate in the fuel reactor while solids continue to elutriate from the air reactor. Example curves from the test campaign are shown in Fig. 4. The rate of solids accumulation and elutriation from the fuel and air reactors, respectively, can be correlated to the pressure drop (or solids inventory) measurement. The equation to measure the solids level in the air and fuel reactors is
Fig. 3. Particle size distribution of the Cu/Fe oxygen carrier used in this work.
composition. Despite this discrepancy, the authors elected to use the XRF composition in this work because it is the direct “as-received” measurement. 2.3. Data reduction 2.3.1. Gas analysis and gas parameters The species in the fuel reactor flue gas are measured using online continuous analyzers (non-disperse infrared) at a frequency of one sample per second and include CH4, CO2, CO, and O2 (paramagnetic). In addition, a gas chromatograph measures and records the same species in addition to nitrogen and hydrogen approximately every 90 s. The flue gas exiting the air reactor is measured for CO2 and O2, while the seal pot flue gas is measured for CO2 to monitor the amount of carbon slip. The sample gas from all three sampling points is filtered and chilled to 4 °C to condense any moisture in the line before reaching the analyzers. Further detail on gas analysis can be found in Bayham et al. (2017). The moles of each species exiting the fuel reactor are calculated assuming closed nitrogen and hydrogen mole balances. There are three parameters used to assess the performance of the material and the operation of the fuel reactor: the carbon balance, overall methane conversion and methane conversion to carbon dioxide. The carbon balance is the measure of the amount of carbon entering the fuel reactor in the form of methane and the amount exiting the fuel reactor in the flue gas.
Cbal =
dmbed, i A d ΔPi = i· dt g dt Where ΔPi is the bed pressure drop across reactor i (air or fuel reactor), Ai is the cross-sectional area of reactor i, and g is the acceleration of gravity. The rate of change at each period of time during the cutoff test was correlated linearly with the riser pressure drop.
m˙ OC = a⋅ΔPriser − b Where m˙ OC is the circulation rate and ΔPriser is the riser pressure drop. The units of a and b were correlated to be a = 897.4 and b = 91.19 if the circulation rate is in kg/hr and the pressure is in units of kPa. Results of the correlation are shown in Fig. 5, which was assumed to apply throughout the run since the values for the aeration gas to the Lvalve, the back pressure of the fuel reactor, and air flow rate through the riser did not change. The oxygen carrier conversion is estimated based on an oxygen balance of the gaseous species and the solids circulation rate.
XOC =
N˙ CH4,out + N˙ CO2,out + N˙ CO,out N˙ CH4,in
N˙ O,out N˙ O,total
The amount of atomic oxygen consumed by the fuel is
N˙ O,out = N˙ H2O,out + 2N˙ CO2,out + N˙ CO,out
where N˙ i,out is the moles of species i exiting the fuel reactor and N˙ CH4,in is the moles of methane entering the fuel reactor. Under ideal measurement and operating conditions, the value will be unity. In reality, the value deviates due to carbon deposition on the particle, error in measurement with the analyzers, and slight losses through the seal pot. The second performance parameter is the overall methane conversion.
The maximum amount of atomic oxygen to provide to the fuel is a function of the circulation rate and the composition of the oxygen carrier.
3fFe2O3 f + CuO ⎞⎟ N˙ O,total = m˙ OC ⎜⎛ MW MW Fe O CuO ⎠ 2 3 ⎝ 214
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period of time under autothermal conditions with the copper-iron oxygen carrier to assess its reactivity and attrition rate. This would allow for a preliminary estimation of the oxygen carrier makeup cost. A secondary objective was to observe the effect of the amount of natural gas added to the fuel reactor has on global system variables such as the fuel and air reactor temperatures, gas concentrations, and the solids circulation rate. In the test plan the methane inlet concentration (and thermal input) was set as the independent variable while maintaining the total reactor inventory as close to 45 kg as possible. The fuel reactor inlet gas velocity was kept constant, namely 0.276 m/s. The goal was to increase the methane flowrate from approximately 30 to 60 kWth in approximately 10-kWth intervals. To ensure a roughly constant fuel reactor inlet gas velocity, the nitrogen flowrate was decreased to make up for the increased methane into the fuel reactor. Additionally, to allow for the oxygen carrier material to be exposed to reaction and system conditions for as long as possible, the makeup solids consisted of ejected solids sifted to a particle size of 150 μm and greater (+100 mesh). 3. Results 3.1. Operations profile The unit was operated from April 30th to May 4th, 2017 to generate data for this work. The temperature profile of the fuel reactor, air reactor, and seal pot is shown in Fig. 6, along with total estimated solids inventory and system pressures. The first three shaded areas indicate periods where L-valve cutoff tests were performed to correlate circulation rate (rate of change of bed inventory) to the riser pressure drop. The remaining shaded areas in the figure indicate when chemical looping was being performed during the operation (i.e., air is not flowing into the fuel reactor for natural gas combustion). The numbers indicate the trial periods described in Table 2. The periods were performed either as a ramp to autothermal or under autothermal conditions, whereby the heaters were turned off. The inlet heaters were shut off between 04:00 am and 05:00 am in Fig. 6. The pressure profile is steady throughout the autothermal trials, indicating the system circulated solids smoothly. The dips in temperature in the air reactor are the result of adding cold makeup solids. The system underwent an upset condition after Trial 5, losing more than half the inventory through the gas exhausts. This required the unit to be shut down for maintenance, as the exhaust lines needed to be cleared to safely operate the unit.
Fig. 4. (a) Solids inventory in the air reactor and (b) riser pressure drop during the third L-valve cutoff test.
3.2. Effects of the methane input on the fuel reactor Fig. 7 plots the reactor temperatures, gas concentrations, gas molar flowrates, gas and solid conversions, and inventories as a function of the natural gas input into the fuel reactor. The thermal fuel input ranged from 28 to 63 kWth based on higher heating value. According to the test plan, as the fuel flow was increased in the fuel reactor, the nitrogen flow required to sustain fluidization was decreased proportionally. Attempts were made to keep the total solids inventory around 45 kg throughout the autothermal period. As shown in Table 2, once the fuel flow into the unit increased above 30 kWth, the inlet gas preheaters were turned off to start autothermal operation. Once the thermal input design limit of the unit was attained, the natural gas was increased further to 62 kWth for a brief period, and was reduced to 55 kWth. Fig. 7(a) shows that increasing the natural gas feed to the fuel reactor causes the temperature to increase, up to the fuel input of 48 kWth. This is the result of the slightly exothermic nature of the reduction reactions with natural gas and the copper-iron material. When the fuel input was increased further, the air and fuel reactor temperatures drop slightly due to the decrease in the methane conversion to carbon dioxide. Fig. 7(b) clearly shows that natural gas conversion breakthrough occurs above 48 kWth, where the fuel reactor is unable to fully
Fig. 5. Measured riser pressure drop as a function of the solids circulation rate.
The weight fractions of iron oxide and copper oxide in the oxygen carrier are shown in Table 1 and it is assumed that the oxygen carrier is fully oxidized in the air reactor.
2.4. Operational plan The main objective of the operation was to run the rig for a long 215
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Fig. 6. Operations profile for the autothermal trials.
throughout the autothermal period, the transition between Trial 4 and Trial 5, where the fuel flow is decreased, shows that both methane conversions increases again, suggesting the effect is more due to methane throughput rather than the slight change in bed height or degradation of the carrier. Fig. 7(c), shows the relationships between the thermal input and the carbon balance, three conversion efficiencies and the resulting CO2 purity at the outlet of the fuel reactor. Average fuel reactor carbon balances where greater than 94%. Both the CH4 conversion and the CH4 conversion to CO2 increased with thermal input until 48 kWth, then decreases due to methane breakthrough of the bed. Similarly, the CO2
convert the products of methane decomposition or gasification. For fuel inputs of 48 kWth and less, almost no hydrogen or carbon monoxide are present, and a small amount of methane bypasses through the bed unreacted. The methane slip is likely due to the formation of large bubbles in the fluid bed that prevent full contact of the methane with the solids. The point of breakthrough occurs when the gas residence time (bed height) may not be sufficient to convert the hydrogen and carbon monoxide products due to the excess natural gas added to the feed. The total gaseous flow rate through the fuel reactor increases due to the reactions generating more moles of gas products, causing the gas residence time to drop. While the bed height decreases slightly Table 2 Operations profile.
Trial time FR Pressure FR Temp AR Temp Total Inventory FR Inventory Fuel Reactor Inventory over Fuel Input Solids Circ Rt FR Gas RTime FR Solid RTime Moles CH4 in FR Moles N2 in FR Moles CH4 in AR Moles Primary Air in AR Moles Secondary Air in AR Carbon Balance CH4 Conv. CH4 Conv. (CO2) OC Conv. CO2 Purity Total Moles Leaving Air Reactor N2 in Spent Air O2 in Spent Air CO2 in Spent Air
min kPa °C °C kg kg kg/MWth kg/h s min mol/h mol/h mol/h mol/h mol/h % % % % % mol/h % % %
1
std
2
std
3
std
4
std
5
std
60 165 807 876 45 26.9 925 163 2.06 10.8 118 378 6.4 632 1768 93.9 87.8 81.7 25.3 87.1 2131 86.4 12.3 0.7
0 1 18 27 1.3 0.9 – 49 0.08 3.2 0 0 8.1 0 0 5.6 2.4 4.1 7.5 2.3 59 2.3 1.5 0.7
62 165 823 928 45.9 27.8 713 251 1.99 6.8 158 340 0 632 1768 95.6 89 84.4 21 88.3 2031 90.5 9.3 0.2
0 1 11 18 1.4 0.9 – 44 0.06 0.9 0 0 0.4 0 0 1.1 0.7 1.6 2.7 0.8 15 0.6 0.6 0
418 165 848 965 43.7 25.9 529 288 1.72 5.5 198 300 0 632 1768 96.7 90.8 86.8 23.3 88.1 1958 93.9 5.9 0.2
0 1 4 19 2.1 1.4 – 39 0.09 0.7 0 0 0 0 0 3.3 0.9 3.3 2.8 1.9 12 0.5 0.5 0
30 165 837 959 40.9 23.8 386 290 1.55 5 249 249 0 632 1768 95.4 82.4 71 24.5 63.6 1945 94.5 5.4 0.1
0 1 3 22 1.9 0.9 – 32 0.08 0.5 0 0 0 0 0 16.1 2.4 15.1 3.6 5.1 10 0.4 0.4 0
170 169 841 962 40.4 23.3 421 309 1.57 4.6 224 274 0 632 1768 97.5 86 80.4 22.8 75.8 1933 95.1 4.8 0.1
0 3 5 23 2.8 2.5 – 44 0.17 0.8 0 0 0 0 0 11 3.2 12.3 3.8 8 15 0.7 0.7 0
“std” = Standard deviation. 216
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Fig. 7. Effect of natural gas input on different variables in the circulating system. (a) Average reactor temperature, (b) molar flow rates, (c) methane conversion, (d) solids inventory and circulation rate.
mi, outlet = ai,1 t 3 + ai,2 t 2 + ai,3 t + ai,4
purity follows the conversion curves. As shown in Fig. 7(d), the circulation rate also increased as more natural gas was added to the fuel reactor. The increased natural gas flow produced more gaseous products, resulting in a slightly greater pressure in the fuel reactor. This affects the solids circulation rate. As the solids residence time in the fuel reactor decreases, the gas conversion decreases while maintaining a constant oxygen carrier conversion as seen in Fig. 7(c). The error bars on the solids circulation rate are due to the chaotic nature of solids transport through the riser, and the slugs that form from the air reactor affect the pressure drop measurement in the riser. The oxygen carrier conversion stays roughly constant, although initially it is slightly higher due to the low solids circulation rate. While the methane conversion could be improved, the reactor design is more important when it comes to methane conversion than the reactivity of the oxygen carrier. For example, making the beds taller by increasing the gas residence time and adding baffles to improve the gassolid contact or staging fluidized bed reactors will improve the methane conversion. The design of the 50-kWth unit is not specific to an oxygen carrier and is not optimized for full fuel conversion.
Where i is either the air reactor, fuel reactor, or seal pot gas outlet, ai, j are polynomial constants for reactor i, and t is the time in seconds. The derivative of these polynomial curves is taken to estimate the rate of solids entrainment from the fluidized beds.
m˙ i, outlet = 3ai,1 t 2 + 2ai,2 t + ai,3 The cumulative weight of solids collected during the autothermal trials is plotted in Fig. 8(a), along with the fitted polynomial function. The fluidized beds are not designed ideally to retain all the solids and when solids are added using the lock hopper, the pulse of gas and solids disturbs the delicate pressure balance, causing solids to leave through the outlets. To take this into account, the fraction of mass flow of solids from each stream with a particle cut size less than 150 μm (-100 Tyler mesh) measured after the run is considered to be products of attrition. Thus, the attrition rate of solids was determined using the following formula. m m m m˙ attr = m˙ FR, outlet f FR + m˙ AR, outlet f AR + m˙ SP, outlet f SP , fines , fines , fines
Where fim , fines is the weight fraction of solids less than 150 μm for reactor outlet i. From the run presented here, the final weight fractions of fines for each reactor is 92.4 wt% from the fuel reactor, 11.8 wt% from the seal pot, and 7.1 wt% from the air reactor. The individual solids attrition rate curves and the total attrition rate is plotted in Fig. 8(b). The weight fraction of fines was highest from the fuel reactor because the disengagement height above the fluid bed is high, ensuring that the percentage of intact particle loss is low. As shown in Fig. 8(b), the solids attrition rate reaches around 0.45 kg/h.
3.3. Attrition rate during autothermal period Because of the combined effect of mechanical- and chemical-induced attrition of the oxygen carrier, the goal was to measure the attrition rate of the oxygen carrier during the autothermal trials. The attrition rate was estimated based on the solids collected from the secondary cyclones in each of the gas outlets (fuel reactor, air reactor, seal pot). The solids were lock-hoppered out using hand valves occasionally by an operator into bins that are on scales that continuously measure the weight. Because of the step-nature of the weight measurements, the noise of the weight signal when solids are dropped into the drum, and the need to take a smooth derivative of the signal to attain an attrition rate, the solid weight curves were fit to a third-degree polynomial function:
4. Discussion NETL has performed technoeconomic studies for chemical looping technologies, and these studies suggest that the capital costs for equipment like the air and fuel reactors have a much smaller impact on the overall cost of electricity than the operating cost associated with replacing oxygen carrier (National Energy Technology Laboratory, 217
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Fig. 8. Attrition rate plot during the autothermal trials. (a) Polynomial fit of the raw solids data collected from the fuel reactor, air reactor, and seal pot flue gas outlets. (b) Rate of solids collected from each flue gas outlet. Shaded area is the region where chemical looping is occurring.
converted by the oxygen carrier. One of the big uncertainties in this performance metric is the cost of the oxygen carrier material. Additional work is currently underway to refine the oxygen carrier makeup cost, but the results in this paper will assume a fairly large range for this parameter. Fig. 9 shows a plot of the cost of oxygen carrier makeup based on the autothermal test campaign as a function of the unit cost of the oxygen carrier. The horizontal black dashed line is the makeup cost goal of $5/MWth-hr, and the sloped lines are the makeup costs based on the minimum and maximum attrition rates, respectively, measured during the autothermal test. There are two unitcost points plotted in the figure to show cost extremes. The high unit cost point is the actual cost of the oxygen carrier as manufactured by the catalyst vendor using a wet granulation method (Siriwardane et al., 2015). The low-cost value is the cost of the oxygen carrier if the catalyst production process could be scaled to a process comparable to a taconite production process (Heller and Yang, 2001). As can be seen from
2014). In other words, the cost for oxygen carrier makeup can significantly alter the feasibility and competitiveness of CLC technology. NETL’s internal research group has set a goal to achieve an oxygen carrier makeup rate that is less than US$5 per megawatt thermal hour (National Energy Technology Laboratory, 2014). If this goal is achieved for a coal-fired chemical looping combustion system, it has the potential to exceed the US Department of Energy’s goal of reducing the cost of electricity for a carbon-capture plant to below 35% the increase in cost of electricity over a supercritical pulverized coal power plant without CO2 capture (NETL, 2010). The attrition results presented in the previous section are used to establish a baseline oxygen carrier makeup cost for NETL’s Gen 2.0 material in this specific test unit. Although these attrition rates were measured under autothermal conditions, there is no basis for extrapolating these rates to a different test unit, particularly a larger scale process configuration. The intent is to use this data as a basis to make relative comparisons between NETL’s Gen 2.0 oxygen carrier and more advanced materials that will be tested in the future under similar conditions. For this specific test unit, the following performance metric is used to establish our current state-of-the-art baseline for NETL’s Gen 2.0 oxygen carrier material. The normalized oxygen carrier makeup cost in US dollars per megawatt hour thermal is estimated using the following formula.
Makeup Cost =
WOC m˙ attr N˙ CH4,in XCH4 → CO2 HHVCH4
Where WOC is the unit cost of the oxygen carrier ($/kg), m˙ attr is the oxygen carrier loss rate from the system, N˙ CH4,in is the molar flowrate of methane into the fuel reactor, and HHVCH4 is the higher heating value of methane (889 kJ/mol). The methane conversion to CO2 is used for derating the rig performance since the unreacted methane is not
Fig. 9. Estimated cost of makeup oxygen carrier based on experimental results. 218
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developed this oxygen carrier material. The authors would also like to acknowledge the hard work and dedication of the operational staff of Dave Reese, Jeff Riley, Mark Tucker, and Richard Eddy. And finally, the Activity Manager, Steve Carpenter, and the Senior Research Fellow, Geo Richards, have provided unwavering support through the highs and lows, and their leadership is greatly appreciated.
Fig. 8, the goal is not attained unless the unit cost of makeup carrier is below US$0.20/kg. There are several approaches to attain the goal. One is to improve the process side by reducing the mechanical attrition, such as by lowering the circulation rate, reducing the number of sharp bends in pneumatic conveying, using a disengagement section to separate gas from solid instead of a cyclone, etc. A second approach would be to improve the inherent attrition resistance of the oxygen carrier. Another approach may include reducing oxygen carrier manufacturing costs in combination with the previously approaches. In summary, all options will be considered in future work to reduce the makeup cost of oxygen carrier. It should also be noted that time can play a role in the attrition rate. If the unit were run longer in autothermal chemical looping mode, there is the possibility that the material attrition rate will increase. However, the baseline number will not significantly change unless the material attrition rate changes drastically, since this work is looking at the economics on a log-log scale.
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Techno-economic evaluations and benchmarking of pre-combustion CO2 capture and oxy-fuel processes developed in the european ENCAP project. Energy Procedia 1, 4233–4240. Gauthier, T., Yazdanpanah, M., Forret, A., Amblard, B., Lambert, A., Bertholin, S., 2017. CLC, a promising concept with challenging development issues. Powder Technol. 316, 3–17. Heller, K.B., Yang, J.-C., 2001. Taconite Mining and Processing Industry Profile.Taconite Mining and Processing Industry Profile. Kolbitsch, P., Bolhar-Nordenkampf, J., Proll, T., Hofbauer, H., 2009. Comparison of two Ni-based oxygen carriers for chemical looping combustion of natural gas in 140 kW continuous looping operation. Ind. Eng. Chem. Res. 48, 5542–5547. Linderholm, C., Lyngfelt, A., Dueso, C., 2013. Chemical-looping combustion of solid fuels in a 10kW reactor system using natural minerals as oxygen carrier. Energy Procedia 37, 598–607. Markström, P., Linderholm, C., Lyngfelt, A., 2013. Chemical-looping combustion of solid fuels – design and operation of a 100kW unit with bituminous coal. Int. J. Greenh. Gas Control. 15, 150–162. National Energy Technology Laboratory, 2014. Guidance for NETL’s Oxy-combustion R& D Program: Chemical Looping Combustion Reference Plant Designs and Sensitivity Studies. https://www.netl.doe.gov/energy-analysis/details?id=758. NETL, 2010. Cost and performance baseline for fossil energy plants. Bituminous Coal Nat Gas Electr. 1. https://doi.org/10.2172/1480987. Pérez-Vega, R., Abad, A., García-Labiano, F., Gayán, P., de Diego, L.F., Adánez, J., 2016. Coal combustion in a 50kWth chemical looping combustion unit: seeking operating conditions to maximize CO2 capture and combustion efficiency. Int. J. Greenh. Gas Control. 50, 80–92. Schmitz, M., Linderholm, C., Hallberg, P., Sundqvist, S., Lyngfelt, A., 2016. Chemicallooping combustion of solid fuels using manganese ores as oxygen carriers. Energy Fuels. Shen, L.H., Wu, J.H., Gao, Z.P., Xiao, J., 2009. Reactivity deterioration of NiO/Al2O3 oxygen carrier for chemical looping combustion of coal in a 10 kW(th) reactor. Combust. Flame 156, 1377–1385. Siriwardane, R., Riley, J., Bayham, S., Straub, D., Tian, H., Weber, J., Richards, G., 2018. 50-kWth methane/air chemical looping combustion tests with commercially prepared CuO-Fe 2 O 3 -alumina oxygen carrier with two different techniques. Appl. Energy 213, 92–99. Siriwardane, R., Tian, H., Miller, D., Richards, G., 2015. Fluidized bed testing of commercially prepared MgO-promoted hematite and CuO–Fe2O3 mixed metal oxide oxygen carriers for methane and coal chemical looping combustion. Appl. Energy 157, 348–357. Siriwardane, R., Tian, H., Simonyi, T., Poston, J., 2013. Synergetic effects of mixed copper–iron oxides oxygen carriers in chemical looping combustion. Fuel 108, 319–333. Song, T., Shen, T., Shen, L., Xiao, J., Gu, H., Zhang, S., 2013. Evaluation of hematite oxygen carrier in chemical-looping combustion of coal. Fuel 104, 244–252. Strohle, J., Ohlemuller, P., Epple, B., 2018. Chemical looping pilot operation in 1 MWth scale – lessons learnt and the way forward. 5th International Conference on Chemical
5. Conclusions A bimetallic carrier containing copper and iron oxides developed at US DOE/NETL (Gen 2.0) has been tested under autothermal conditions in a 50-kWth natural-gas fired CLC unit. More than 11 h of continuous operation were completed, and several key performance parameters were measured. Carbon balances were greater than 90% for all conditions. Gas conversion results ranged approximately from 70 to 90% during autothermal trials (40–60 kWth). Oxygen carrier circulation rates varied from 160 to 300 kg/h. The attrition rate of the material estimated during the long period of operation reached around 0.45 kg/ h and over 75 oxidation-reduction cycles. An oxygen carrier makeup cost performance metric has been presented that includes several other key characteristics of an oxygen carrier material. Using NETL’s Gen 2.0 oxygen carrier material as the current state-of-the-art, a baseline performance has been described. The baseline oxygen carrier makeup cost (see Fig. 9) indicates that significant progress is required to meet the internal NETL research goal of US$5 per thermal megawatt-hour. This baseline performance will be used to make relative comparisons to more advanced oxygen carrier materials that will be developed and tested in the future. The baseline oxygen carrier makeup cost indicates that significant progress is required to meet the internal NETL research goal, and more advanced oxygen carriers will be compared to this baseline in the future. 6. Disclaimer This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. Acknowledgements This work was funded by the U.S. Department of Energy’s Advanced Combustion Program. The support of Rich Dennis, John Rockey, Steve Markovich, and Briggs White are greatly appreciated. Ranjani Siriwardane, Jarrett Riley, William Benincosa, and Hanjing Tian have also played a critical role in this effort as the research team that 219
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U.S. Environmental Protection Agency, 2018. Sources of Greenhouse Gas Emissions. https://www3.epa.gov/climatechange/ghgemissions/sources/electricity.html. Weber, J., 2019. Single and double loop reacting systems. In: Breault, R.W. (Ed.), Handbook of Chemical Looping Technology. John Wiley and Sons. Whitty, K., Lighty, J., 2018. Integrated oxygen production and CO2 separation through chemical looping combustion with oxygen uncoupling. NETL CO2 Capture Tehcnology Project Review Meeting. Zhu, L., He, Y., Li, L., Wu, P., 2018. Tech-economic assessment of second-generation CCS: chemical looping combustion. Energy 144, 915–927.
Looping. Ströhle, J., Orth, M., Epple, B., 2015. Chemical looping combustion of hard coal in a 1MWth pilot plant using ilmenite as oxygen carrier. Appl. Energy 157, 288–294. Thon, A., Kramp, M., Hartge, E.-U., Heinrich, S., Werther, J., 2014. Operational experience with a system of coupled fluidized beds for chemical looping combustion of solid fuels using ilmenite as oxygen carrier. Appl. Energy 118, 309–317. Tsupari, E., Tähtinen, M., Kärki, J., 2014. Feasibility of solid fuel CLC plant investment in the future energy systems including high share of solar and wind. Energy Procedia 63, 7508–7516.
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International Journal of Greenhouse Gas Control journal homepage: www.elsevier.com/locate/ijggc
Operation of a 50-kWth chemical looping combustion test facility under autothermal conditions
T
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Samuel Bayham , Douglas Straub, Justin Weber National Energy Technology Laboratory, US Department of Energy, Morgantown, WV, 26507, USA
A R T I C LE I N FO
A B S T R A C T
Keyword: Chemical Looping Combustion
The United States Department of Energy is developing transformational fossil fuel combustion technologies that result in more efficient power cycles and/or substantial reductions in GHG emissions. Chemical Looping Combustion (CLC) has the potential to achieve a lower cost of electricity than a supercritical pulverized coal power plant with an amine absorption system. Although the concept and research on chemical looping has a long history, CLC has numerous technical issues that must be addressed prior to a commercial debut. This effort is directed toward the following major issues for CLC: (1) demonstrating continuous solids handling, operability, and reactor performance at autothermal conditions and (2) preliminary assessments of oxygen carrier attrition and relative material make-up costs. These are critical issues that should be addressed under realistic conditions prior to directing resources toward larger scale demonstrations. This paper will also describe NETL’s small 50kWth natural gas circulating CLC test facility located in Morgantown, WV. Despite its small size, this experimental test unit has operated at autothermal conditions for 11 h using a copper-iron-alumina oxygen carrier developed at NETL. This paper will describe the test facility and operating experiences using NETL’s “Gen 2.0” oxygen carrier material. Natural gas conversion to CO2 ranged from 70 to 90% over approximately 75 oxidationreduction cycles during the autothermal operating period. Estimates for the process specific attrition rate during autothermal operation are also presented. Finally, estimates for the oxygen carrier makeup cost, specific to the NETL process configuration, are several orders of magnitude greater than the NETL target. Future work should focus on reducing the oxygen carrier material cost and improving the attrition resistance of the oxygen carrier material under autothermal test conditions.
1. Introduction Electricity generation from fossil fuel power plants accounts for onethird of anthropogenic CO2 emissions in the United States (Agency, 2018). In addition, the electricity industry is changing as renewables and low-cost natural gas supplies become more prominent. Thus, future fossil energy power generation must consider alternatives that are a significant departure from conventional power plants. The United States Department of Energy’s National Energy Technology Laboratory (NETL) is studying novel concepts and power cycles to meet the needs for reliable and sustainable future electric power. Reducing greenhouse gas emissions is one objective for these future power plants, and alternative approaches to address energy penalties associated with postcombustion CO2 separation are being investigated. Chemical Looping Combustion is a promising technology for mitigating greenhouse gas emissions from fossil fuel applications. According to NETL studies, a circulating fluidized bed coal-CLC boiler/
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steam generator has the potential to achieve cost-of-electricity targets of $110/MW-h while simultaneously capturing CO2 at less than $40/ tonne (National Energy Technology Laboratory, 2014). In a similar study, Ekstrom et al. (Ekström et al., 2009) investigated several concepts to mitigate greenhouse gas emissions from natural gas and bituminous coal electric power generators, concluding that CLC concepts had the lowest cost-of-electricity and the lowest cost of CO2 avoidance. Others (Adanez et al., 2012; Tsupari et al., 2014; Zhu et al., 2018) have reported similar economic benefits, and these potential benefits have stimulated international interest in this transformational technology for combustion and other potential applications. This paper will focus on fossil-fuel combustion applications with an initial interest in benchscale evaluations of oxygen carrier materials under self-sustaining levels of solid oxygen carrier heat release. In a Chemical Looping Combustion (CLC) process, the fuel and air do not mix (see Fig. 1). Since the fuel and the oxidant do not mix, an oxygen carrier material is needed to transport oxygen from an air
Corresponding author. E-mail address: [email protected] (S. Bayham).
https://doi.org/10.1016/j.ijggc.2019.05.022 Received 14 February 2019; Received in revised form 13 May 2019; Accepted 17 May 2019 Available online 26 May 2019 1750-5836/ Published by Elsevier Ltd.
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There are numerous CLC test facilities at different scales, ranging from 300-Wth fuel input to 3-MWth (Adánez et al., 2018; Weber, 2019). Chalmers University of Technology has studied hundreds of oxygen carriers in their 300 Wth, 10 kWth, and 100 kWth units. Chalmers University has investigated various fuels such as natural gas, coal, petroleum coke, and wood char (Berguerand and Lyngfelt, 2008; Markström et al., 2013), and a number of low-cost “commodity” oxygen carriers including ilmenite, iron ore, and manganese ore (Linderholm et al., 2013; Schmitz et al., 2016). Instituto de Carboquímica of Zaragoza, Spain has also operated several small pilot units ranging from 0.5 to 50 kWth using lignite, bituminous, and anthracite coals (Cuadrat et al., 2011; Pérez-Vega et al., 2016). Southeast University of Nanjing, China has studied oxygen carriers at scales ranging from 1 to 50 kWth, and the 50 kWth unit has operated at pressures of 5 atm (Shen et al., 2009; Song et al., 2013). Other continuous CLC test facilities include Hamburg University (Thon et al., 2014), Technische Universitat Wien (Kolbitsch et al., 2009), and the Korean Institute of Energy Research (Baek et al., 2016). The fuel conversion and/or combustion efficiency has typically been reported in all previous work. However, the oxygen carrier losses, deactivation, and attrition rates are not always reported. Adanez and Abad (Adánez and Abad, 2019) have provided a thorough review of attrition rates that have been reported in the literature. The oxygen carrier attrition rates ranged from 0.0023 – 0.09%/h for some of the manufactured materials tested (Abad et al., 2006; Adanez et al., 2012), while ores tested range from 0.0625–0.67%/h (Abad et al., 2006). Thousands of hours of cumulative operation have been performed in these small integrated units. However, due to the high surface area to volume ratio for these small test unit, many of these units have been heated externally using electrical heaters. This external heating does not produce the same oxygen carrier environment that would be present in a larger scale demonstration unit. Several test facility have been constructed that are capable of CLC operation without adding heat directly to the solids path. The largest unit to date has been a 3-MWth unit operated by Alstom Power. This test unit consisted of two interconnected circulating fluidized bed reactors and a limestone-based oxygen carrier material. This 3-MWth unit successfully operated on Adaro and Pittsburgh #8 coals under autothermal conditions for approximately 40 continuous hours (Andrus, 2017). Other units include Darmstadt’s 1-MWth unit (Ströhle et al., 2015) that has reported oxygen carrier attrition rates of ilmenite and iron ore at 0.4–2.3 %/hr (Strohle et al., 2018). A 250-kWth unit at the University of Utah (Whitty and Lighty, 2018) has also been constructed to test chemical looping oxygen uncoupling (CLOU) carrier materials. This paper will describe a 50-kWth natural gas Chemical Looping Combustion rig that has operated under self-sustaining, or autothermal, CLC conditions with the intent of measuring the fuel conversion, heat release, and oxygen carrier losses as a function of time during autothermal operation. Although the gases flowing into this test unit can be preheated, no external heat can be added directly to the solid oxygen carrier flow path. Autothermal operation has been defined by the following conditions: 1) all inlet gas preheaters are de-energized, 2) the oxygen carrier reactions (fuel and air) are the only source of internal heat generation, 3) the oxygen carrier experiences natural heat losses during circulation, 4) the oxygen carrier experiences rapid heating when make-up material is added to maintain a constant carrier inventory. In the following sections, the experimental 50-kW test facility will be briefly described, with additional detail can be found in Bayham et al. (Bayham et al., 2017). Following a description of the test facility, the oxygen carrier material will be described, and additional details can be found in Siriwardane et al. (2018). These sections will be followed by a discussion of the data analysis approach and the results. The final section will describe estimated oxygen carrier losses that have been observed during autothermal test conditions in which used oxygen carrier material elutriated from the process were recovered, sieved to the original particle size distribution, and returned to the system to
Fig. 1. Diagram describing chemical looping combustion.
reactor to a fuel reactor. Common oxygen carriers include relatively common metals, such as iron, copper, manganese, nickel, and others. A CLC process can produce a nitrogen-free flue gas comprised of water vapor and CO2 similar to an oxy-combustion process. However, the CLC process does not require an air separation unit, and most CLC configurations can achieve significantly lower levels of excess oxygen in the flue gas. To capture the CO2 from a CLC process, the water vapor can be condensed to produce a high purity CO2 stream, and a portion of the flue gas can be recycled to fluidize the fuel reactor. Although CLC has been studied extensively since the 1990’s, significant technical challenges still exist (Gauthier et al., 2017). One of the first major challenges is the oxygen carrier material, specifically the cost, performance, stability, and durability. Low cost minerals such as hematite, ilmenite, and other “commodity” carrier materials have been investigated, but these materials generally lack the required reactivity to achieve complete fuel conversion, and the oxygen carrying capacity is generally lower than other alternatives. Others have focused on synthetic materials that can be manufactured using conventional catalyst manufacturing processes (i.e., spray drying, co-precipitation, etc.). According to Adanez and Abad (Adánez and Abad, 2019), more than 2,000 oxygen carrier materials have been studied, but only about 15 have been tested in continuous CLC environments. This paper will describe continuous CLC testing of an NETL developed bimetallic copper-iron oxygen carrier supported on alumina. This material has shown promising results under different testing techniques, such as TGA (Siriwardane et al., 2013), a two-inch diameter fluidized bed (Siriwardane et al., 2015), and an ASTM attrition method (Siriwardane et al., 2018). Two generations of this material with the same chemical formulation have been developed. The first generation (Gen 1.0) has been manufactured using a commercial spray-drying method, but improvements in the cost and durability were needed. The second generation (Gen 2.0) has been manufactured using a wetgranulation method that has been more cost effective and has produced larger particles (Siriwardane et al., 2018). Both of these materials have exhibited a high oxygen carrying capacity (> 10 wt%), and the heats of reaction in the fuel reactor have ranged from thermally neutral to slightly exothermic. Another major technical challenge for the CLC technology involves solids handling and control. Depending on the oxygen carrying capacity and the oxygen carrier solid conversion, the required carrier circulation rate can be orders of magnitude higher than the fuel mass flow. Solids circulation processes are inherently unsteady, and the CLC operating temperatures are in the 800–1000 °C range. Many of these issues have been addressed by commercial-scale circulating fluidized bed combustion processes, but some CLC configurations require the added complexity of controlling two interconnected carrier circulation loops (i.e., an air reactor loop and a fuel reactor loop). Some issues like capacity factors for commercial scale applications will need to be addressed by larger scale demonstrations, so the current research focus is directed toward evaluating whether existing oxygen carrier materials can achieve acceptable performance metrics to justify larger scale demonstrations. 212
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Fig. 2. Sketch of the chemical looping rig (refractory not shown).
maintain a refractory heating rate of 50 °C/h. Air flows through both reactors during this initial preheating phase. Once the reactor temperatures exceed the autoignition temperature of natural gas, the second stage of operation is initiated where natural gas combustion is established in both reactors with air. Once the refractory reaches around 900 °C, solids are added via a lock hopper to the air or fuel reactor in small (nominally 2 kg) batches. Once the desired inventory is achieved, usually around 45 kg, circulation is initiated by adding aeration gas to the L-valve. After the circulation becomes steady, Lvalve cutoff tests are performed to determine the solids circulation rate (see Section 2.4). Typical carrier circulation rates for this test campaign are in the range of 160–300 kg/h. The final chemical looping combustion stage is initiated by 1) stopping the natural gas flow to the fuel reactor, 2) transitioning the air in the fuel reactor to nitrogen, and 3) injecting natural gas in the plenum of the fuel reactor. For the copperiron oxygen carrier used in this work, if the natural gas flow to the fuel reactor is sufficient enough, natural gas to the air reactor is gradually shut off along with the electric gas preheaters for each reactor. This initiates autothermal operation.
maintain constant inventory. These data can be used to estimate the oxygen carrier make-up cost specific to this process and form a basis for future comparisons and improvements.
2. Experimental setup 2.1. Rig design The 50-kWth natural gas chemical looping rig consists of refractorylined carbon steel components. A simplified schematic is shown in Fig. 2. The unit consist of a 20-cm diameter bubbling fluidized bed fuel reactor, a 15-cm diameter turbulent fluidized bed air reactor, a 5-cm diameter riser to convey solids to the cyclone, and a 20-cm diameter bubbling fluidized bed seal pot which prevents air from entering the fuel reactor and fuel from entering the spent air stream. A non-mechanical L-valve connects the solids outlet of the fuel reactor and the inlet of the air reactor. Although the solids circulation rate is affected by numerous factors, adjustments to the circulation rate were primarily controlled by two variables: the fuel reactor back pressure, and the aeration gas flow to the L-valve. The aeration gas added to the L-valve also serves the purpose of preventing gases from mixing between the air and fuel reactors. Solids in the air reactor are fluidized using primary air, and secondary air is injected above the bubble caps of the fluid bed to help entrain the solids into the riser. Global pressure in the unit is operated anywhere from 1.5 to 2.4 bar nominally. Reactions in the fuel reactor are diluted with nitrogen to help fluidize the oxygen carrier and prevent overreduction. In a commercial unit, the diluent nitrogen would be a mixture of recycled carbon dioxide and steam. For the autothermal tests reported in this paper, steam is not used due to potential issues with condensate when the preheaters are de-energized, and carbon dioxide is not used to prevent confounding effects with measuring fuel conversion to CO2. Natural gas can be introduced into one of three locations in the fuel reactor: 1) below the bubble caps, 2) directly into the bed, and 3) above the bed. When chemical looping is initiated, natural gas is injected below the bubble caps to provide some premixing with the fluidization nitrogen prior to entering the bed. Startup of the chemical looping rig involves three stages. The first stage involves heating the refractory by ramping electric preheaters to
2.2. Oxygen carrier characteristics The oxygen carrier used in this test consisted of a mixture of CuO and Fe2O3 supported on alumina. The material was manufactured at the catalyst manufacture Nexceris (Columbus, Ohio, USA) via a dry milling method, in a batch of 180 kg as described in (Siriwardane et al., 2018). A list of its properties such as mean particle size, density, and composition is presented in Table 1. The particle size distribution of the oxygen carrier tested during the campaign is shown in Fig. 3. The oxygen carrier tested in the campaign described in this work has been tested in various units, such as thermogravimetric analyzer and a small 5.08-cm fluidized bed with methane and coal (Siriwardane et al., 2015, 2013). The composition of the manufactured oxygen carrier was measured using X-ray Fluorescence (XRF) (Rigaku ZSX Primus II). As reported in previous works, the developed carrier composition consists of 30 wt% CuO, 30 wt% Fe2O3, and 40 wt% Al2O3 on a dry, premixed basis. However, as shown in Table 1, measurement of the delivered oxygen carrier from the catalyst manufacturer deviated from the premixed 213
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Table 1 Particle properties.
XCH4 =
Property
Value
Units
Particle density Sauter mean diameter D50 Sphericity Umf (at 298 K) Fe2O3 CuO Al2O3
2.9 343 397 0.91 14 31 37 32
g/cm3 μm μm – cm/s wt% wt% wt%
N˙ CH4,in − N˙ CH4,out N˙ CH4,in
This parameter determines the amount of methane that was oxidized by the oxygen carrier. The third performance parameter is an indication of the amount of fuel that is converted to CO2.
XCH4 → CO2 =
N˙ CO2,out N˙ CH4,in
Since the goal is to convert the methane fully to carbon dioxide, this expression is not affected by conversion of the gas to CO or carbon deposition. Finally, the carbon dioxide purity was calculated on the ratio of the molar flow rate of carbon dioxide and the total molar flow rate minus the molar flows of nitrogen and steam.
YCO2 =
N˙ CO2,out N˙ total,out − N˙ H2O,out − N˙ N2,out
Further detail on the performance parameters can be found in Bayham et al. (2017). 2.3.2. Solids analysis The solids circulation rate is measured in the test rig by correlating the pressure drop in the riser to the change in bed level during transient L-valve cutoff tests. An L-valve cutoff test involves circulating solids at temperature and shutting off the L-valve aeration gas to stop solids circulation. When circulation stops, the solids begin to accumulate in the fuel reactor while solids continue to elutriate from the air reactor. Example curves from the test campaign are shown in Fig. 4. The rate of solids accumulation and elutriation from the fuel and air reactors, respectively, can be correlated to the pressure drop (or solids inventory) measurement. The equation to measure the solids level in the air and fuel reactors is
Fig. 3. Particle size distribution of the Cu/Fe oxygen carrier used in this work.
composition. Despite this discrepancy, the authors elected to use the XRF composition in this work because it is the direct “as-received” measurement. 2.3. Data reduction 2.3.1. Gas analysis and gas parameters The species in the fuel reactor flue gas are measured using online continuous analyzers (non-disperse infrared) at a frequency of one sample per second and include CH4, CO2, CO, and O2 (paramagnetic). In addition, a gas chromatograph measures and records the same species in addition to nitrogen and hydrogen approximately every 90 s. The flue gas exiting the air reactor is measured for CO2 and O2, while the seal pot flue gas is measured for CO2 to monitor the amount of carbon slip. The sample gas from all three sampling points is filtered and chilled to 4 °C to condense any moisture in the line before reaching the analyzers. Further detail on gas analysis can be found in Bayham et al. (2017). The moles of each species exiting the fuel reactor are calculated assuming closed nitrogen and hydrogen mole balances. There are three parameters used to assess the performance of the material and the operation of the fuel reactor: the carbon balance, overall methane conversion and methane conversion to carbon dioxide. The carbon balance is the measure of the amount of carbon entering the fuel reactor in the form of methane and the amount exiting the fuel reactor in the flue gas.
Cbal =
dmbed, i A d ΔPi = i· dt g dt Where ΔPi is the bed pressure drop across reactor i (air or fuel reactor), Ai is the cross-sectional area of reactor i, and g is the acceleration of gravity. The rate of change at each period of time during the cutoff test was correlated linearly with the riser pressure drop.
m˙ OC = a⋅ΔPriser − b Where m˙ OC is the circulation rate and ΔPriser is the riser pressure drop. The units of a and b were correlated to be a = 897.4 and b = 91.19 if the circulation rate is in kg/hr and the pressure is in units of kPa. Results of the correlation are shown in Fig. 5, which was assumed to apply throughout the run since the values for the aeration gas to the Lvalve, the back pressure of the fuel reactor, and air flow rate through the riser did not change. The oxygen carrier conversion is estimated based on an oxygen balance of the gaseous species and the solids circulation rate.
XOC =
N˙ CH4,out + N˙ CO2,out + N˙ CO,out N˙ CH4,in
N˙ O,out N˙ O,total
The amount of atomic oxygen consumed by the fuel is
N˙ O,out = N˙ H2O,out + 2N˙ CO2,out + N˙ CO,out
where N˙ i,out is the moles of species i exiting the fuel reactor and N˙ CH4,in is the moles of methane entering the fuel reactor. Under ideal measurement and operating conditions, the value will be unity. In reality, the value deviates due to carbon deposition on the particle, error in measurement with the analyzers, and slight losses through the seal pot. The second performance parameter is the overall methane conversion.
The maximum amount of atomic oxygen to provide to the fuel is a function of the circulation rate and the composition of the oxygen carrier.
3fFe2O3 f + CuO ⎞⎟ N˙ O,total = m˙ OC ⎜⎛ MW MW Fe O CuO ⎠ 2 3 ⎝ 214
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period of time under autothermal conditions with the copper-iron oxygen carrier to assess its reactivity and attrition rate. This would allow for a preliminary estimation of the oxygen carrier makeup cost. A secondary objective was to observe the effect of the amount of natural gas added to the fuel reactor has on global system variables such as the fuel and air reactor temperatures, gas concentrations, and the solids circulation rate. In the test plan the methane inlet concentration (and thermal input) was set as the independent variable while maintaining the total reactor inventory as close to 45 kg as possible. The fuel reactor inlet gas velocity was kept constant, namely 0.276 m/s. The goal was to increase the methane flowrate from approximately 30 to 60 kWth in approximately 10-kWth intervals. To ensure a roughly constant fuel reactor inlet gas velocity, the nitrogen flowrate was decreased to make up for the increased methane into the fuel reactor. Additionally, to allow for the oxygen carrier material to be exposed to reaction and system conditions for as long as possible, the makeup solids consisted of ejected solids sifted to a particle size of 150 μm and greater (+100 mesh). 3. Results 3.1. Operations profile The unit was operated from April 30th to May 4th, 2017 to generate data for this work. The temperature profile of the fuel reactor, air reactor, and seal pot is shown in Fig. 6, along with total estimated solids inventory and system pressures. The first three shaded areas indicate periods where L-valve cutoff tests were performed to correlate circulation rate (rate of change of bed inventory) to the riser pressure drop. The remaining shaded areas in the figure indicate when chemical looping was being performed during the operation (i.e., air is not flowing into the fuel reactor for natural gas combustion). The numbers indicate the trial periods described in Table 2. The periods were performed either as a ramp to autothermal or under autothermal conditions, whereby the heaters were turned off. The inlet heaters were shut off between 04:00 am and 05:00 am in Fig. 6. The pressure profile is steady throughout the autothermal trials, indicating the system circulated solids smoothly. The dips in temperature in the air reactor are the result of adding cold makeup solids. The system underwent an upset condition after Trial 5, losing more than half the inventory through the gas exhausts. This required the unit to be shut down for maintenance, as the exhaust lines needed to be cleared to safely operate the unit.
Fig. 4. (a) Solids inventory in the air reactor and (b) riser pressure drop during the third L-valve cutoff test.
3.2. Effects of the methane input on the fuel reactor Fig. 7 plots the reactor temperatures, gas concentrations, gas molar flowrates, gas and solid conversions, and inventories as a function of the natural gas input into the fuel reactor. The thermal fuel input ranged from 28 to 63 kWth based on higher heating value. According to the test plan, as the fuel flow was increased in the fuel reactor, the nitrogen flow required to sustain fluidization was decreased proportionally. Attempts were made to keep the total solids inventory around 45 kg throughout the autothermal period. As shown in Table 2, once the fuel flow into the unit increased above 30 kWth, the inlet gas preheaters were turned off to start autothermal operation. Once the thermal input design limit of the unit was attained, the natural gas was increased further to 62 kWth for a brief period, and was reduced to 55 kWth. Fig. 7(a) shows that increasing the natural gas feed to the fuel reactor causes the temperature to increase, up to the fuel input of 48 kWth. This is the result of the slightly exothermic nature of the reduction reactions with natural gas and the copper-iron material. When the fuel input was increased further, the air and fuel reactor temperatures drop slightly due to the decrease in the methane conversion to carbon dioxide. Fig. 7(b) clearly shows that natural gas conversion breakthrough occurs above 48 kWth, where the fuel reactor is unable to fully
Fig. 5. Measured riser pressure drop as a function of the solids circulation rate.
The weight fractions of iron oxide and copper oxide in the oxygen carrier are shown in Table 1 and it is assumed that the oxygen carrier is fully oxidized in the air reactor.
2.4. Operational plan The main objective of the operation was to run the rig for a long 215
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Fig. 6. Operations profile for the autothermal trials.
throughout the autothermal period, the transition between Trial 4 and Trial 5, where the fuel flow is decreased, shows that both methane conversions increases again, suggesting the effect is more due to methane throughput rather than the slight change in bed height or degradation of the carrier. Fig. 7(c), shows the relationships between the thermal input and the carbon balance, three conversion efficiencies and the resulting CO2 purity at the outlet of the fuel reactor. Average fuel reactor carbon balances where greater than 94%. Both the CH4 conversion and the CH4 conversion to CO2 increased with thermal input until 48 kWth, then decreases due to methane breakthrough of the bed. Similarly, the CO2
convert the products of methane decomposition or gasification. For fuel inputs of 48 kWth and less, almost no hydrogen or carbon monoxide are present, and a small amount of methane bypasses through the bed unreacted. The methane slip is likely due to the formation of large bubbles in the fluid bed that prevent full contact of the methane with the solids. The point of breakthrough occurs when the gas residence time (bed height) may not be sufficient to convert the hydrogen and carbon monoxide products due to the excess natural gas added to the feed. The total gaseous flow rate through the fuel reactor increases due to the reactions generating more moles of gas products, causing the gas residence time to drop. While the bed height decreases slightly Table 2 Operations profile.
Trial time FR Pressure FR Temp AR Temp Total Inventory FR Inventory Fuel Reactor Inventory over Fuel Input Solids Circ Rt FR Gas RTime FR Solid RTime Moles CH4 in FR Moles N2 in FR Moles CH4 in AR Moles Primary Air in AR Moles Secondary Air in AR Carbon Balance CH4 Conv. CH4 Conv. (CO2) OC Conv. CO2 Purity Total Moles Leaving Air Reactor N2 in Spent Air O2 in Spent Air CO2 in Spent Air
min kPa °C °C kg kg kg/MWth kg/h s min mol/h mol/h mol/h mol/h mol/h % % % % % mol/h % % %
1
std
2
std
3
std
4
std
5
std
60 165 807 876 45 26.9 925 163 2.06 10.8 118 378 6.4 632 1768 93.9 87.8 81.7 25.3 87.1 2131 86.4 12.3 0.7
0 1 18 27 1.3 0.9 – 49 0.08 3.2 0 0 8.1 0 0 5.6 2.4 4.1 7.5 2.3 59 2.3 1.5 0.7
62 165 823 928 45.9 27.8 713 251 1.99 6.8 158 340 0 632 1768 95.6 89 84.4 21 88.3 2031 90.5 9.3 0.2
0 1 11 18 1.4 0.9 – 44 0.06 0.9 0 0 0.4 0 0 1.1 0.7 1.6 2.7 0.8 15 0.6 0.6 0
418 165 848 965 43.7 25.9 529 288 1.72 5.5 198 300 0 632 1768 96.7 90.8 86.8 23.3 88.1 1958 93.9 5.9 0.2
0 1 4 19 2.1 1.4 – 39 0.09 0.7 0 0 0 0 0 3.3 0.9 3.3 2.8 1.9 12 0.5 0.5 0
30 165 837 959 40.9 23.8 386 290 1.55 5 249 249 0 632 1768 95.4 82.4 71 24.5 63.6 1945 94.5 5.4 0.1
0 1 3 22 1.9 0.9 – 32 0.08 0.5 0 0 0 0 0 16.1 2.4 15.1 3.6 5.1 10 0.4 0.4 0
170 169 841 962 40.4 23.3 421 309 1.57 4.6 224 274 0 632 1768 97.5 86 80.4 22.8 75.8 1933 95.1 4.8 0.1
0 3 5 23 2.8 2.5 – 44 0.17 0.8 0 0 0 0 0 11 3.2 12.3 3.8 8 15 0.7 0.7 0
“std” = Standard deviation. 216
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Fig. 7. Effect of natural gas input on different variables in the circulating system. (a) Average reactor temperature, (b) molar flow rates, (c) methane conversion, (d) solids inventory and circulation rate.
mi, outlet = ai,1 t 3 + ai,2 t 2 + ai,3 t + ai,4
purity follows the conversion curves. As shown in Fig. 7(d), the circulation rate also increased as more natural gas was added to the fuel reactor. The increased natural gas flow produced more gaseous products, resulting in a slightly greater pressure in the fuel reactor. This affects the solids circulation rate. As the solids residence time in the fuel reactor decreases, the gas conversion decreases while maintaining a constant oxygen carrier conversion as seen in Fig. 7(c). The error bars on the solids circulation rate are due to the chaotic nature of solids transport through the riser, and the slugs that form from the air reactor affect the pressure drop measurement in the riser. The oxygen carrier conversion stays roughly constant, although initially it is slightly higher due to the low solids circulation rate. While the methane conversion could be improved, the reactor design is more important when it comes to methane conversion than the reactivity of the oxygen carrier. For example, making the beds taller by increasing the gas residence time and adding baffles to improve the gassolid contact or staging fluidized bed reactors will improve the methane conversion. The design of the 50-kWth unit is not specific to an oxygen carrier and is not optimized for full fuel conversion.
Where i is either the air reactor, fuel reactor, or seal pot gas outlet, ai, j are polynomial constants for reactor i, and t is the time in seconds. The derivative of these polynomial curves is taken to estimate the rate of solids entrainment from the fluidized beds.
m˙ i, outlet = 3ai,1 t 2 + 2ai,2 t + ai,3 The cumulative weight of solids collected during the autothermal trials is plotted in Fig. 8(a), along with the fitted polynomial function. The fluidized beds are not designed ideally to retain all the solids and when solids are added using the lock hopper, the pulse of gas and solids disturbs the delicate pressure balance, causing solids to leave through the outlets. To take this into account, the fraction of mass flow of solids from each stream with a particle cut size less than 150 μm (-100 Tyler mesh) measured after the run is considered to be products of attrition. Thus, the attrition rate of solids was determined using the following formula. m m m m˙ attr = m˙ FR, outlet f FR + m˙ AR, outlet f AR + m˙ SP, outlet f SP , fines , fines , fines
Where fim , fines is the weight fraction of solids less than 150 μm for reactor outlet i. From the run presented here, the final weight fractions of fines for each reactor is 92.4 wt% from the fuel reactor, 11.8 wt% from the seal pot, and 7.1 wt% from the air reactor. The individual solids attrition rate curves and the total attrition rate is plotted in Fig. 8(b). The weight fraction of fines was highest from the fuel reactor because the disengagement height above the fluid bed is high, ensuring that the percentage of intact particle loss is low. As shown in Fig. 8(b), the solids attrition rate reaches around 0.45 kg/h.
3.3. Attrition rate during autothermal period Because of the combined effect of mechanical- and chemical-induced attrition of the oxygen carrier, the goal was to measure the attrition rate of the oxygen carrier during the autothermal trials. The attrition rate was estimated based on the solids collected from the secondary cyclones in each of the gas outlets (fuel reactor, air reactor, seal pot). The solids were lock-hoppered out using hand valves occasionally by an operator into bins that are on scales that continuously measure the weight. Because of the step-nature of the weight measurements, the noise of the weight signal when solids are dropped into the drum, and the need to take a smooth derivative of the signal to attain an attrition rate, the solid weight curves were fit to a third-degree polynomial function:
4. Discussion NETL has performed technoeconomic studies for chemical looping technologies, and these studies suggest that the capital costs for equipment like the air and fuel reactors have a much smaller impact on the overall cost of electricity than the operating cost associated with replacing oxygen carrier (National Energy Technology Laboratory, 217
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Fig. 8. Attrition rate plot during the autothermal trials. (a) Polynomial fit of the raw solids data collected from the fuel reactor, air reactor, and seal pot flue gas outlets. (b) Rate of solids collected from each flue gas outlet. Shaded area is the region where chemical looping is occurring.
converted by the oxygen carrier. One of the big uncertainties in this performance metric is the cost of the oxygen carrier material. Additional work is currently underway to refine the oxygen carrier makeup cost, but the results in this paper will assume a fairly large range for this parameter. Fig. 9 shows a plot of the cost of oxygen carrier makeup based on the autothermal test campaign as a function of the unit cost of the oxygen carrier. The horizontal black dashed line is the makeup cost goal of $5/MWth-hr, and the sloped lines are the makeup costs based on the minimum and maximum attrition rates, respectively, measured during the autothermal test. There are two unitcost points plotted in the figure to show cost extremes. The high unit cost point is the actual cost of the oxygen carrier as manufactured by the catalyst vendor using a wet granulation method (Siriwardane et al., 2015). The low-cost value is the cost of the oxygen carrier if the catalyst production process could be scaled to a process comparable to a taconite production process (Heller and Yang, 2001). As can be seen from
2014). In other words, the cost for oxygen carrier makeup can significantly alter the feasibility and competitiveness of CLC technology. NETL’s internal research group has set a goal to achieve an oxygen carrier makeup rate that is less than US$5 per megawatt thermal hour (National Energy Technology Laboratory, 2014). If this goal is achieved for a coal-fired chemical looping combustion system, it has the potential to exceed the US Department of Energy’s goal of reducing the cost of electricity for a carbon-capture plant to below 35% the increase in cost of electricity over a supercritical pulverized coal power plant without CO2 capture (NETL, 2010). The attrition results presented in the previous section are used to establish a baseline oxygen carrier makeup cost for NETL’s Gen 2.0 material in this specific test unit. Although these attrition rates were measured under autothermal conditions, there is no basis for extrapolating these rates to a different test unit, particularly a larger scale process configuration. The intent is to use this data as a basis to make relative comparisons between NETL’s Gen 2.0 oxygen carrier and more advanced materials that will be tested in the future under similar conditions. For this specific test unit, the following performance metric is used to establish our current state-of-the-art baseline for NETL’s Gen 2.0 oxygen carrier material. The normalized oxygen carrier makeup cost in US dollars per megawatt hour thermal is estimated using the following formula.
Makeup Cost =
WOC m˙ attr N˙ CH4,in XCH4 → CO2 HHVCH4
Where WOC is the unit cost of the oxygen carrier ($/kg), m˙ attr is the oxygen carrier loss rate from the system, N˙ CH4,in is the molar flowrate of methane into the fuel reactor, and HHVCH4 is the higher heating value of methane (889 kJ/mol). The methane conversion to CO2 is used for derating the rig performance since the unreacted methane is not
Fig. 9. Estimated cost of makeup oxygen carrier based on experimental results. 218
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developed this oxygen carrier material. The authors would also like to acknowledge the hard work and dedication of the operational staff of Dave Reese, Jeff Riley, Mark Tucker, and Richard Eddy. And finally, the Activity Manager, Steve Carpenter, and the Senior Research Fellow, Geo Richards, have provided unwavering support through the highs and lows, and their leadership is greatly appreciated.
Fig. 8, the goal is not attained unless the unit cost of makeup carrier is below US$0.20/kg. There are several approaches to attain the goal. One is to improve the process side by reducing the mechanical attrition, such as by lowering the circulation rate, reducing the number of sharp bends in pneumatic conveying, using a disengagement section to separate gas from solid instead of a cyclone, etc. A second approach would be to improve the inherent attrition resistance of the oxygen carrier. Another approach may include reducing oxygen carrier manufacturing costs in combination with the previously approaches. In summary, all options will be considered in future work to reduce the makeup cost of oxygen carrier. It should also be noted that time can play a role in the attrition rate. If the unit were run longer in autothermal chemical looping mode, there is the possibility that the material attrition rate will increase. However, the baseline number will not significantly change unless the material attrition rate changes drastically, since this work is looking at the economics on a log-log scale.
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5. Conclusions A bimetallic carrier containing copper and iron oxides developed at US DOE/NETL (Gen 2.0) has been tested under autothermal conditions in a 50-kWth natural-gas fired CLC unit. More than 11 h of continuous operation were completed, and several key performance parameters were measured. Carbon balances were greater than 90% for all conditions. Gas conversion results ranged approximately from 70 to 90% during autothermal trials (40–60 kWth). Oxygen carrier circulation rates varied from 160 to 300 kg/h. The attrition rate of the material estimated during the long period of operation reached around 0.45 kg/ h and over 75 oxidation-reduction cycles. An oxygen carrier makeup cost performance metric has been presented that includes several other key characteristics of an oxygen carrier material. Using NETL’s Gen 2.0 oxygen carrier material as the current state-of-the-art, a baseline performance has been described. The baseline oxygen carrier makeup cost (see Fig. 9) indicates that significant progress is required to meet the internal NETL research goal of US$5 per thermal megawatt-hour. This baseline performance will be used to make relative comparisons to more advanced oxygen carrier materials that will be developed and tested in the future. The baseline oxygen carrier makeup cost indicates that significant progress is required to meet the internal NETL research goal, and more advanced oxygen carriers will be compared to this baseline in the future. 6. Disclaimer This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. Acknowledgements This work was funded by the U.S. Department of Energy’s Advanced Combustion Program. The support of Rich Dennis, John Rockey, Steve Markovich, and Briggs White are greatly appreciated. Ranjani Siriwardane, Jarrett Riley, William Benincosa, and Hanjing Tian have also played a critical role in this effort as the research team that 219
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