3.2. Blank Test with raw coalBaseli

3.2. Blank Test with raw coal
Baseline tests were conducted with pure coal by heating it up to 900 C in atmosphere, and the weight loss and released heat from the combustion of coal with air in the TG-DTA are shown in Fig. 5. A broad peak from 250 to 700 C with a maximum at 700 C was observed, which indicated that the water in the coal begin to evolve at 150 C, meanwhile, the coal volatilization initiated around 433 C and the combustion reaction proceeds up to 800 C in air. This indicates that the coal volatilization reaction initiated at 260 C with the maximum rate at 500 C. The weight loss is about 40% because of coal volatilization in nitrogen.
3.3. TG experiments of Fe2O3/Al2O3 OC
Chemical reactivity is a crucial indicator of oxygen carrier in chemical looping combustion, which includes the conversion and oxidation reduction reaction rate, and only oxygen carrier with good reactive is available for chemical looping combustion. The higher conversion presents the better reactivity, which shows the oxygen carrier fully react with fuels in chemical looping combustion process. In order to study the conversions of Fe2O3/Al2O3 OC with coal after mixing, based on the literature
[37], the conversion rate (a) is defined as:
a ¼ W 0 À W
ðf =ð1 þ f ÞÞDW OC þ ð1=ð1 þ f ÞÞDWcoal ð3Þ

where a is conversion rate of the mixture (coal and Fe2O3/Al2O3 OC), W0 is the initial mass of the sample, W is the mass in reaction process, f is the quality ratio of the oxygen carrier and coal, DWOC is a mass loss of Fe2O3 into Fe3O4, DWcoal is the biggest weight loss of the coal samples completely reaction calculated by the ash.
Fig. 6 shows TG analysis of Fe2O3/Al2O3 OC with coal for one, five and ten cycles. The weight loss of the sample is more obvious after fist cycle, and the weight loss appear at 150 C shows structural water began to escape. The reaction is more significant from 400 to 700 C reaching peak at 650 C and ended at 850 C. DTA curve (Fig. 7) can also indicated the exothermic reaction ended at 850 C. After 5 cycles, the rate of weight loss was significantly reduced, significant weight loss and the mixed sample combustion heat become obvious at 600 C, which reached the maximum at 800 C and ended at 900 C. Nevertheless, weight loss and heat release become less significant after ten cycles, and there is a less significant exothermic peak only at 700–800 C. After 900 C, the reaction was essentially completed. Thus, along with the number of cycles increasing, the temperature of exothermic peak increased, weight loss and heat release obviously decreased, and the combustion rate decreases.
The circulating combustion conversion rate of the sample for one, five and ten cycles calculated by Eq. (3) are 44.56%, 24.37% and 11.39% respectively, which indicates that the reactivity of Fe2O3/ Al2O3 significantly reduced after many cycles. Another reason is that the coal used as solid fuel for CLC brings some ashes after combustion, which can induce more and more ashes by several