Coal gasification using chemical looping with varied metal oxides as oxygen carriers
Graphical abstract
Introduction
Coal gasification using chemical looping (CGCL) has been suggested as an energy efficient method for producing high quality synthesis gas with high heat value and low tar content. CGCL is similar to traditional coal gasification technology in that the desired product is synthesis gas. However, for conventional coal gasification processes, a great amount of high temperature steam or oxygen-enriched air is needed as gasifying agent, which results in high cost and complicated process for pure oxygen production. In contrast, CGCL shares the same basic principles with chemical looping combustion (CLC), where coal is gasified into synthesis gas by lattice oxygen from OCs instead of molecular oxygen from air [1]. Therefore, compared with traditional coal gasification technologies, CGCL has a significant advantage for avoiding the cost and energy penalty of pure oxygen production as the CGCL involves the use of OCs [[2], [3], [4], [5]]. More importantly, CGCL can produce a high quality syngas production when the insufficient lattice oxygen supplied by OCs partially oxidizes coal to syngas rather than CO2 and H2O [6]. Mainly, the CGCL system comprises two interconnected reactors: an air reactor and a fuel reactor. As illustrated in Fig. 1, coal is converted into synthesis gas with the aid of oxidized OC particles in the fuel reactor. Then the reduced OC particles are transported into air reactor to be oxidized by air to achieve the regeneration and the regenerated OC particles subsequently start a second cycle in the fuel reactor. Hence, coal gasification process can be carried out through achieving a circulation loop of OC particles between the two interconnected reactors. Meanwhile, a concentrated synthesis gas stream without N2 dilution can also be produced in the dual reactor system as the indirect contact of coal with air. Therefore, unlike the syngas produced from traditional coal gasification using gaseous O2 in air, the concentrated synthesis gas stream, without N2, with catalysts can be used for producing useful chemicals including plastics, adhesives and fuels [7]. It's remarkable that the circulating OC particles act as not only solid heat carriers, which provide the required energy for gasification reactions of coal, but also catalyst in tar reduction during the gasification process [8]. Therefore, an ideal OC for the CGCL should possess the following properties: high reactivity, high resistance to attrition and stability under repeated reduction/oxidation cycles at high temperatures [9]. Additionally, the price of OC was an important feature which can be decisive for industrial application at large scale. Environmental and health aspects may also affect the choice. Consequently, the final determination as to the suitability of a material as an OC for the CGCL process was a combination of several relevant factors. Considering the importance of these desirable properties for CGCL, there is an increased interest in the study of OCs development since large-scale application of CGCL depends upon the availability of a suitable OC. Research into this field has been prolific, with many oxides of transition metals (e.g., Ni, Fe, Cu, Co, Mn) tested [10]. The reactivity of OCs depends on several aspects of the materials used, including the reducibility to a lower oxidation state, the oxidation potential and synthetic methods of such materials. Although the oxides of single metal and composite metal have already been studied in chemical looping processes as OCs [11], the impact of different oxides of metals on its performance has not been compared for the CGCL process. Consequently, this paper aims to perform a systematic study to screen the optimal oxide of transition metal used in the CGCL process.
CGCL is essentially a four-step process in which (a) coal pyrolysis and (b) coal gasification take place firstly in the presence of steam to produce gas productions according to reaction R1-R4.
(a) Coal pyrolysis:
(b) Coal steam gasification:
Simultaneously, (c) combustion reactions of coal-derived gas with OCs proceeds with the aid of reactions R5 and R6. The syngas derived from coal pyrolysis and steam gasification is partially oxidized by the OC particles to form CO2 and water (H2O). Finally, the reduced OCs particles are oxidized by gaseous O2 in air to achieve regeneration and then start a new cycle.
(c) Combustion reactions:
(d) Regeneration reactions:
Several reaction mechanisms have been proposed for the coal gasification with metal oxides and steam. It is usually assumed [12,13] that all the reactions proceed via indirect reduction by means of gaseous species. For CGCL, it has been reported that volatile release from coal may initiate reaction with OCs [12]. Solid-solid reactions between coal and OC particles have also been observed when there is fully contact between them [14]. However, there are only limited reports [[14], [15], [16]] in the literatures related to such information on the solid-solid reactions of coal with OC particles. Moreover, solid-solid reaction mechanisms between coal and the OC particles during CGCL process have rarely been discussed even though they are highly relevant for the final carbon conversion and syngas selectivity. Consequently, the emphasis of this work is not only on screening the optimal oxide of transition metals but also on the importance of the solid-solid reactions of coal with OC particles. Moreover, there is a lack of proper OCs with unique properties that would selectively produce synthesis gas since synthesis gas derived from coal pyrolysis and steam gasification can be further oxidized by the OCs to form CO2 and H2O. Therefore, we have devised a modified Stöber method for the preparation of core-shell structured OC particles, composed by four oxides of transition metals respectively [17]. These materials are unique relative to the reactivity with synthesis gas and with coal directly via solid-solid reactions. The influences of metal oxide species on the OC particles selectivity and activity are also investigated since OCs synthesized by different metals oxides exhibit different characteristics. Furthermore, the present study reports reaction mechanism studies on CGCL using thermo-gravimetric analysis (TGA) apparatus and fixed bed reactor.
Section snippets
Preparation of the oxygen carrier particles
The core-shell structured OCs samples were prepared by a modified Stöber method. Take Ni@meso-SiO2 for example, Ni(NO3)2·6H2O was dissolved in deionized water, forming 1 M of Ni(NO3)2 solution. Then, the solution was added to mixed solution of NaOH and polyethylene glycol (PEG) (average MW = 20,000) and the resulting solution was dried, and calcined at 600 °C in air for 2 h to obtain NiO nanoparticles. Afterward, NH3·H2O (25 wt%) was added into ethanol solution that contained the NiO
Thermogravimetric analysis (TGA)
To investigate the reaction process of the prepared mixtures, TG analyses were performed using N2 at the temperature range from 25 °C to 900 °C. TG analyse of raw coal was also conducted. The reaction curves are shown in Fig. 3. Considering the interactions between coal and OCs, the TG and DTG curves might be the superposition of several curves that correspond to coal and OCs. It is worth noting that the samples exhibited three weight loss peaks at around 100–300 °C, 300–650 °C and 650–900 °C,
Conclusions
The advantage of CGCL technology was to avoid the mixing of N2 from the air with synthesis gas, this mixing being the main source of difficulty, cost and energy penalty in any later separation process. A key issue for the large-scale development of this technology was the selection of OCs materials with suitable properties such as high reactivity with coal and high selectivity for synthesis gas. Therefore, the characteristics of OCs related to reactivity with coal and selectivity for synthesis
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