Article pubs.acs.org/EF
Low-Temperature Chemical Looping Combustion for Removing Unburnt Gaseous Components with a Cement-Supported CuO Oxygen Carrier Lei Xu, Zhenshan Li,* Hongming Sun, Jinhua Bao, and Ningsheng Cai Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Beijing Municipal Key Laboratory for CO2 Utilization and Reduction, Department of Thermal Engineering, Tsinghua University, Beijing 100084, People’s Republic of China ABSTRACT: Chemical looping combustion (CLC) is a novel CO2 capture technology with inherent separation of N2 with CO2 produced by fossil fuel combustion. Fe-based oxygen carrier is one of the most promising oxygen carriers for solid-fuel CLC with respect to its low cost. However, most Fe-based oxygen carriers have very low reactivity and would result in a large portion of unburnt gases at the outlet of the fuel reactor. The fuel combustion is less efficient with unburnt components in the product, and the CO2 concentration is not high enough for the subsequent capture and storage. To eliminate the unburnt gas and improve the CO2 purity, a two-stage CLC process is proposed by combining the normal CLC with the low-temperature CLC, and a cementsupported Cu-based oxygen carrier was developed for the low-temperature CLC procedure. Thermogravimetric analysis (TGA) results indicated that the Cu/cement oxygen carrier had relatively fast oxidation and reduction rates at low temperatures (∼300 °C). Experimental results obtained from the single fluidized bed showed that unburnt CO can be converted fully with this oxygen carrier at low temperatures with no agglomeration. The developed oxygen carrier was also tested for more than 18 h in a dual fluidized-bed reactor and demonstrated that CO can be converted fully in the fuel reactor, and the oxygen carrier particles did not show any tendency to form agglomeration.
1. INTRODUCTION Chemical looping combustion (CLC) is a combustion technology that enables the combustion of fossil fuels with inherent capture of CO21,2 and becomes a promising combustion technology for power generation and industrials. CLC is also a more efficient combustion technology with less exergy loss according to the principle of cascading utilization of chemical energy, proposed by Jin and Wang.3 The CLC process is generally put into implementation by two interconnected fluidized-bed reactors, the air reactor and the fuel reactor, with oxygen carriers circulating between them. In the fuel reactor, the oxygen carriers are reduced by fuels, as shown by reaction 1. The reduced particles are reoxidized in the air reactor, as shown by reaction 2. Overall, the oxygen from the air is transferred to the fuel reactor. In this way, theoretically, the outlet gases from the fuel reactor consist of mainly CO2 and steam. Steam can be easily removed by condensation, and CO2 is concentrated for sequestration. Ideally, a stream of 100% CO2 can be obtained by CLC for gas separation because the mixing of air and CO2 is avoided. fuel reactor
CLC. The conversion of solid fuels in the fuel reactor usually consists of three steps: pyrolysis, gasification, and combustion. When the coal is introduced into the fuel reactor, pyrolysis occurs immediately to produce the volatiles and char. Then, the char is gasified into syngas (mainly containing CO and H2) by steam and/or CO2. Afterward, gaseous products are burned to produce CO2 and H2O. If the reactivity of the oxygen carrier is not high enough, a certain amount of CO and H2 would leave the fuel reactor before it is burned. Increasing the inventory of oxygen carriers is a way to mitigate the loss of syngas. The lower the reactivity, the larger the inventory required. However, a large inventory causes a large pressure drop of the reactor, and more power would be consumed to drive the solid fluidization and circulation.4−6 Moreover, even if the inventory was enlarged, the segregation between solid fuel particles and oxygen carriers would also lead to a low conversion of syngas in the fuel reactor.7 The segregation cannot be avoided by enlarging the bed inventory for the reason that particle properties, such as density and size, also have effects on the extent of segregation besides operating parameters, such as gas velocity and solid circulation rate.8 The coarser and denser particles settle down at the bottom, while the finer and lighter particles move upward. As a result of segregation and low oxygen carrier reactivity, the low conversion of syngas has been observed in many CLC experiments.9−14 The unburnt gas concentrations at the outlet obtained by Cuadrat et al.12 were about 17 vol % syngas (CO and H2) for the 125−200 μm coal and 49 vol % syngas for the 74−125 μm coal. The
(2n + m)MexOy + CnH 2m → (2n + m)MexOy − 1 + mH 2O + nCO2
(1)
air reactor MexOy − 1 +
1 O2 → MexOy 2
(2)
Received: June 3, 2013 Revised: October 30, 2013 Published: October 31, 2013
Solid fuels, such as coal and biomass, and gaseous fuels, such as natural gas and syngas, are most commonly investigated for © 2013 American Chemical Society
6872
dx.doi.org/10.1021/ef401035b | Energy Fuels 2013, 27, 6872−6879
Energy & Fuels
Article
Figure 1. Schematic of the two-stage CLC process. cement is high alumina CA70 for its super capability of hightemperature resistance. The main components in the cement are Al2O3 (72.84 wt %) and CaO (24.61 wt %). First, powders (less than 10 μm) of copper oxide (25 wt %) and cement (75 wt %) were uniformly mixed. Then, water (water cement mass ratio of 0.5) was added to the mixed powders, and then the obtained slurries were dried in air at room temperature for 15 days, during which the cement-hardening process was undergone. Finally, the hardened samples were crushed into particles and sieved to the size range 125−300 μm. Then, some of the particles were calcined at 1150 °C for 3 h. The phase composition was analyzed by X-ray diffraction (XRD). The main phases are copper oxide and calcium aluminate, as shown in the previous paper.16 The bulk density of the uncalcined oxygen carrier is 1050 kg/m3. 2.2. Reduction and Oxidation on a Cement-Supported CuO Oxygen Carrier with TGA. The reduction and oxidation kinetics of the oxygen carrier were examined in a thermogravimetric analyser (TA SDT Q600). The reaction temperature and sample weight were continuously recorded by a data acquisition system connected to the thermogravimetric analyser. A small amount of each sample (about 7 mg) was placed in a quartz sample holder in each test. The reduction agent was CO (10, 5, 3, and 1 vol %) mixed with N2 with a total flow rate of 100 mL/min [standard temperature and pressure (STP)]. The oxidation agent was air with a flow rate of 100 mL/min (STP). Both the reduction and oxidation were tested at different temperatures. A total of 20 cycles were run in the thermogravimetric analyser to examine the stability of the oxygen carrier. 2.3. Experiments in the Fluidized-Bed Reactor. The cycling reduction and oxidation tests were also carried out in both a batchscale single-fluidized bed and a dual-fluidized bed, as shown in Figure 2. The flow rates of introduced gases were controlled by mass flow controllers. In the batch-scale single-fluidized bed, the reduction test was conducted with CO (10 vol %) mixed with N2 (90 vol %) as the fluidizing gas and the oxidation test was conducted with air as fluidizing gas. A total of 100 redox cycles were performed at 400 °C isothermally, and a total of 20 redox cycles were performed with reduction at 300 °C and oxidation at 500 °C. The minimum fluidization velocity of the oxygen carrier is 0.055 m/s obtained from test in the batch-scale fluidized bed. Both the reduction and oxidation were conducted at a flow rate of 2.5 L/min (standard state), corresponding to around 0.145 m/s at 400 °C. For the 100 cycles test, the reduction period is around 160 s and the oxidation is around 300 s. For the 20 cycles test, the reduction period is around 320 s and the
concentration of unconverted gases (CO and CH4) could reach 21 vol % from the test on the 10 kWth CLC combustor at Chalmers University of Technology.15 The gas conversion was even lower in the test by Chalmers on a 100 kW unit with solid fuels; the concentration of unburnt gases (H2, CO, and CH4) in the flue gas was up to 32 vol %.9 To fully convert the unburnt gases, a two-stage CLC process by combining the normal CLC with the low-temperature CLC is proposed in this paper, as shown in Figure 1. The outlet gas from the normal CLC system is introduced into a secondary CLC system operating at lower temperatures. For solid fuel CLC, the outlet gas from the fuel reactor always contains fine particles, such as ash, and should go through the dust-removing equipment, which usually works at around 150 °C. If the outlet gases from the dust-removing equipment go to the lowtemperature CLC system, the high-temperature increase can be avoided. In the low-temperature CLC, the gas volume is small because of the lower temperature and, thus, can minimize the reactor size. Moreover, the dust-free gases do not contaminate the oxygen carriers. To implement the low-temperature CLC, the oxygen carriers should have good reactivity at low temperatures. Cement was chosen as a binding material or support material for oxygen carriers for its low cost.16−18 In this study, the cement-supported copper-based oxygen carrier was used to fully convert the unburnt gas components. The fuel reactor operated at as low as 300 °C, and the air reactor operated at as low as 400 °C. The feasibility of the copperbased oxygen carrier was examined in a thermogravimetric analyser, a batch-scale single-fluidized bed, and a dual-fluidized bed.
2. EXPERIMENTAL SECTION 2.1. Preparation of the Cement-Supported CuO Oxygen Carrier Particles. Two oxygen carriers were used for the thermogravimetric analysis (TGA) test and the batch-scale fluidized test. One oxygen carrier is calcined at 1150 °C, and the other oxygen carrier skipped the calcination procedure during preparation. Both of the oxygen carriers were prepared by mechanical mixing. The type of 6873
dx.doi.org/10.1021/ef401035b | Energy Fuels 2013, 27, 6872−6879
Energy & Fuels
Article
into the gas analyzer. The process of the reduction and oxidation steps was followed by monitoring the CO, CO2, and O2 contents in the product gases. Two types of tests were conducted in the singlefluidized bed. One was conducted at 400 °C isothermally for 100 cycles, and the other was conducted with reduction at 300 °C and oxidation at 500 °C for 20 cycles. The sketch of the tri-fluidized bed is shown in Figure 2. In this study, only the left riser (RL) and the middle reactor were used. The primary air entered the system through the inlet RL1 with a flow rate of 4 L/min, and the secondary air entered through the inlet RL2 with a flow rate of 18.3 L/min. The fuel (5 vol % CO and 95 vol % N2) was introduced to the middle reactor with 6.5 L/min. N2 was introduced into the loop seals with a flow rate of 3.4 L/min for the upper part and 3.5 L/min for the lower part. Oxygen carriers (2 kg) were added to the system and tested continuously for about 18 h. The fuel and air reactor were heated by an electric oven. Other parts, including the riser, cyclone, and loop seals, are heated by electric wires with insulation. The pressure can be measured at points named I1, I2, O1−O8, G1, G3, and G5. The solid circulation rate GS was measured on the basis of the pressure signals of G1, G3, and G5. During a stable operation, the gas flow rate of the upper loop seal (
