Article pubs.acs.org/EF
Copper-Decorated Hematite as an Oxygen Carrier for in Situ Gasification Chemical Looping Combustion of Coal Weijing Yang, Haibo Zhao,* Jinchen Ma, Daofeng Mei, and Chuguang Zheng State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan, 430074 Hubei, People’s Republic of China ABSTRACT: Iron ore is a cheap and nontoxic oxygen carrier in chemical looping combustion (CLC) systems. However, iron ore exhibits low reactivity with solid fuels. In this work, hematite decorated with Cu via impregnation was used as an oxygen carrier. Experiments were conducted in a batch fluidized bed reactor using Shenhua (SH) bituminous coal and Gaoping (GP) anthracite as fuels. The effects of Cu loading in the hematite, reaction temperature, supply oxygen coefficient, and coal type on the CLC performance of coal were investigated. It was found that hematite loaded with 6% Cu (6CuHem) had better reactivity with the gasification products (CO and H2) of coal than hematite (Hem) alone. Furthermore, 6CuHem was found to accelerate the gasification rate of GP anthracite char. A reaction temperature higher than 900 °C was conducive to fast and complete conversion of carbon in coal. As the supply oxygen coefficient was increased, the rate of carbon conversion and the combustion efficiency of coal increased. Multiple redox cycles were performed to study the stability of these oxygen carriers. It was found that the reactivity of 6CuHem was mostly stable after three redox cycles. SEM-EDX analysis revealed a shell enriched with copper on the outside of fresh 6CuHem particles. After 29 cycles, the shell nearly disappeared, and the copper content on particle’s surface was decreased due to surface abrasion and migration of copper toward the internal part of the particle.
1. INTRODUCTION Since the Industrial Revolution, the amount of CO2 in the atmosphere has significantly increased. CO2 is the main greenhouse gas that causes increases in average global temperatures.1 Controlling CO2 production is arguably the most challenging environmental policy issue facing the world.2 Current CO2 capture technologies have the disadvantage of large energy inputs required to separate CO2 from flue gas. Chemical looping combustion (CLC) is an innovative combustion technology for heat and power production with inherent CO2 capture.3 The CLC process uses two reactors and circulating oxygen carriers (OCs), which are reduced in a fuel reactor (FR) and regenerated by oxidation with air in an air reactor (AR) to provide the oxygen required for fuel combustion. In the CLC process, the flue gas leaving the FR mainly consists of CO2 and H2O, allowing high-purity CO2 to be produced for storage or utilization after steam condensation4 CLC with gaseous fuels has been developed over the past few years. However, CO2 emissions mainly result from the combustion of solid fuels such as coal in large-scale power plants. The use of coal in CLC is an attractive strategy to restrict CO2 emissions from fossil fuel utilization.5 There are three ways to use coal in CLC:6 in the first way, coal is gasified in the gasifier, and the gasification gas is then introduced into the CLC system. Because additional equipment (the gasifier) and expensive pure oxygen are required, this way is not economical. The second way is to introduce coal directly into the FR, where the devolatilization and in situ gasification of coal as well as reduction reactions between these combustible gases and OCs occur simultaneously. This way is called in situ gasification chemical looping combustion (iG-CLC). The third way is chemical looping with oxygen uncoupling (CLOU), which utilizes a special OC that is able to release gas-phase © 2014 American Chemical Society
oxygen in the FR and regenerate in the AR. This paper focused on iG-CLC. A schematic of an iG-CLC configuration is shown in Figure 1. In the FR, a mixture of OC and coal is fluidized by
Figure 1. Chemical looping combustion using coal as fuel.
a gasification agent, which is usually CO2 or steam. According to reactions 1-3, initially coal quickly releases volatiles, then char is gasified by CO2 and/or steam at the high FR temperature. H2 and CO are the main gasification products generated in this process. The gasification products and volatiles are then oxidized by the OC. Reactions 4-6 show the OC reduction reaction and the main products of coal devolatilization and gasification. Subsequently, the OC is Received: May 23, 2013 Revised: May 7, 2014 Published: May 12, 2014 3970
dx.doi.org/10.1021/ef5001584 | Energy Fuels 2014, 28, 3970−3981
Energy & Fuels
Article
extensive CLC studies with Cu-decorated hematite and coal as the fuel have not been reported in the literature. This paper aimed to investigate the CLC of two typical coals using Cu-decorated hematite as the OC in a batch fluidized bed reactor. Cu-based OC has many advantages, including high reactivity with fuels20 and high oxygen transport capacity. Additionally, the reduction and oxidation reactions of CuO are both exothermic, and Cu-based materials are much cheaper than Ni-based materials. However, Cu-based OCs have rather low melting points (1083 °C for Cu, 1235 °C for Cu2O, and 1326 °C for CuO) and easily sinter and agglomerate at high temperatures. In this paper, Cu was impregnated into the hematite to improve the reactivity of hematite, and the resistance to sintering and agglomeration of these Cucontaining materials was investigated.
transferred to the AR, where it is reoxidized (see R7). The released heat of chemical looping combustion in the AR and FR is the same as for normal combustion. coal → volatile + char
(R1)
C + CO2 → 2CO
(R2)
C + H 2O → H 2 + CO
(R3)
MexOy + CO → MexOy − 1 + CO2
(R4)
MexOy + H 2 → MexOy − 1 + H 2O
(R5)
4MexOy + CH4 → 4MexOy − 1 + CO2 + 2H 2O
(R6)
2MexOy − 1 + O2 → 2MexOy
(R7)
2. EXPERIMENTAL SECTION
The oxygen carrier significantly affects CLC performance. A suitable OC should at a minimum possess the following characteristics: high reactivity toward the fuel, good stability, high resistance to agglomeration, attrition and fragmentation, low cost, and environmental friendliness. At present, metal oxides (Fe-, Mn-, Cu- and Ni-based OCs) with different support materials have been widely investigated.6−10 In many publications, the OC was manufactured from pure chemicals. Although synthetic materials are still interesting for CLC (because synthetic materials could serve as reference points for the selection or modification of natural materials for OC), the cost of these synthetic materials is too high if used in large-scale CLC plants with solid fuels. Moreover, the lifetime of the OC is reduced in CLC applications with solid fuels due to deactivation by ash, or loss of materials during the separation of ash and OC or mechanical attrition between OCs and/or the reactor wall. Usually, the reactivity of gaseous fuels is better than that of solid fuels. For coal, the gasification of coal char is the rate-limiting process in iG-CLC. Therefore, accelerating the gasification of coal char is important for fast and complete conversion of solid fuels in the FR. A number of publications have attempted to improve the gasification rate of coal char by adding catalysts to the OC. These catalysts are usually alkali metals or alkaline earth metals.11−13 However, the loss of catalysts during the repeated redox process and the sintering/ agglomeration of OC particles due to the low melting points of alkali components remain problems to be solved. Iron ore is inexpensive and environmentally friendly but exhibits low reactivity when used with solid fuels. Iron ore has emerged as a very attractive OC candidate for the CLC of solid fuel. The use of iron ore as the OC in CLC has been investigated by many researchers.5,14−17 It was found that the reactivity of iron ore should be sufficient to be used in the CLC system. However, the redox rate of Fe2O3, the main component of iron ore, is slower than that of CuO or NiO.18 OCs with high reactivity can enhance the gasification rate and combustion efficiency of coal.5 Thus, it may be possible to improve the reactivity of iron ore via doping with highly reactive metal oxides. Hematite decorated with NiO was investigated by Chen et al.19 It was found that NiO-decorated hematite produced by mechanical mixing possessed better reactivity than the original hematite. Recently, the synergetic effect of mixed copper−iron oxide OCs for CLC was investigated using a thermogravimetric analyzer (TGA) and a bench-scale flow reactor.18 It was found that the agglomeration resistance of the CuO species and the reactivity of the Fe2O3 species were both improved by the synergetic effect of the bimetallic CuO−Fe2O3. However,
2.1. Preparation and Characterization of OC. Hematite is a common mineral in the iron-smelting industry and is mainly composed of Fe2O3. In this work, hematite particles subjected to thermal treatment were used as the original hematite: fresh hematite was calcined at 500 °C in air for 2 h and at 1000 °C in air for 10 h, producing calcined hematite. This calcination was performed to improve the mechanical strength and initial reaction rates of natural minerals as OCs.21 Additionally, some publications have shown that the preoxidation of iron ore helps prevent defluidization problems.22 The components of hematite were measured by X-ray fluorescence (XRF) and the results are shown in Table 1.
Table 1. Chemical Composition Analysis of the Calcined Hematite components
Fe2O3
SiO2
Al2O3
others
content (wt %)
81.89
8.42
8.37
1.32
Cu-decorated hematite was prepared by wet impregnation. In this work, the mass fractions of Cu were set to 3%, 6%, 10%; mass fractions were calculated as Cu/(hematite + Cu). The equivalent mass fractions of CuO in the impregnated materials were 3.7%, 7.4%, and 12.2% in 3%, 6%, and 10% Cu−hematite, respectively. Copper nitrates were dissolved in water, and the solutions were separately added to fresh hematite. The mixture was allowed to evaporate naturally at room temperature for 24 h without stirring, then stirred in a water bath at 80 °C until a dried paste was formed. The dried paste was further dried in air at 105 °C to remove residual water. Finally, the dried material was calcined in air at 500 °C for 2 h and at 1000 °C for 10 h. The calcined material was sieved through stainless steel screens to yield particles in the range of 180−280 μm. The compositions of all OCs reported in this work are weight percentages. OCs were labeled based on their composition as follows: the calcined hematite was labeled as Hem, 3% Cu−hematite was labeled as 3CuHem, 6% Cu−hematite as 6CuHem, and 10% Cu−hematite as 10CuHem. To facilitate the manufacturing process, only one impregnation was performed for each sample. Hematite and Cu-decorated hematite particles were physically and chemically characterized with several techniques. The crushing strength of the particles was measured as the strength needed to fracture a single particle sized between 180 and 280 μm. Twenty different measurements were performed on a Shimpo FGJ-5 crushing strength apparatus and averaged to yield the final measurement. The attrition index of the calcined particles was measured with an abrasion tester. Approximately 34 g of OC particles was placed into a stainless cylinder (length 14.5 cm, diameter 12.0 cm) with a 1.5 cm baffle and rotated on a ball-mill roller for 50 min at 10 rpm. After attrited OC particles were sieved and weighed, the attrition index was calculated as δ = (m1 − m2)/m1 × 100%, where m1 is the mass of the OC before the test and m2 is the mass of the OC after the test. The Brunauer− Emmett−Teller (BET) surface area of the particles was evaluated by 3971
dx.doi.org/10.1021/ef5001584 | Energy Fuels 2014, 28, 3970−3981
Energy & Fuels
Article
Table 2. Characterization of Hematite and Cu-Decorated Hematite before Reactions Hem crushing strength (N) attrition index (%) BET surface area (m2/g) true density (kg/m3) particle diameter (mm) oxygen transport capacity (%) crystalline phases
3CuHem
2.4 ± 0.6 14.0 2.6 4730 0.18−0.28 2.72 Fe2O3, Al2O3, silicon−iron oxide, MnO2
6CuHem
2.8 ± 0.5 5.7
