چكيده به لاتين
The present study seeks to investigate the effect of alkaline earth metals on the catalytic stability and the coke formation of nickel-based catalyst in the direct reduction process of iron. One of the challenges of this process is the carbon deposition due to Boudouard reaction that during this reaction, CO is decomposed and the carbon formed from this reaction by blocking the active sites of the catalyst, reduces the catalyst performance. Since CO is acidic, it is possible to increased CO2 adsorption on the catalyst surface by using alkaline promoters and increasing the catalyst's basicity and as a result, it prevented the progress of the CO decomposition reaction. For this purpose, in the first stage of this research, 10%Ni-Al2O3 catalyst without promoter and also with 5% by weight of each of the alkaline earth metals MgO, CaO, BaO, SrO and with a fixed amount of NiO loading (10%), was synthesized. According to the results of TPO analysis and catalytic performance test, the sample with MgO promoter had the best performance with 61.19% methane conversion at 700°C and was selected as a suitable sample. Then, in the next step, the amount of different loadings of MgO (25, 45, 65 and 85%) with a fixed amount of loading of NiO (10%) was investigated. The results of the reactor test showed that increasing the loading rate up to 25% was almost ineffective and increasing the loading rate up to 45 and 65%, increased the methane conversion, but increasing it up to 85% has the opposite result and has reduced methane conversion. According to the diagrams, the 10%NiO-45%MgO-Al2O3 catalyst had the highest catalytic conversion. Also the stability test of this catalyst was checked and the results showed that all the samples except the 10%NiO-85%MgO-Al2O3 catalyst had good stability. The results of TPO and SEM also showed that with increasing the amount of MgO loading, Carbon deposition is accompanied by a decrease. The results of the reactor test also showed that the 10%NiO-45%MgO-Al2O3 catalyst has the highest catalytic conversion with 66.57% of methane conversion at 700°C, and as a result, it was selected as the most suitable sample at this stage. Then in the next step, the amount of different loadings of NiO (2.5, 5, 10 and 15%) and considering the ratio of one to one for MgO and Al2O3 was investigated. The catalytic activity of the samples showed that the catalyst with 2.5% NiO loading had the lowest catalytic activity and the samples with 10 and 15% NiO loading had the highest methane conversion. The results of TPO and SEM analysis showed that with increasing the amount of NiO, due to the increase in the crystal size of nickel and the decrease in its dispersion, the amount of carbon formation has increased. According to the mentioned results, 10%NiO-45%MgO-45%Al2O3 catalyst was selected as the most suitable sample of this research. Finally, the catalytic activity of this sample was also investigated in the condition that CO2 and H2O are present in the reaction at the same time. And the results showed that in this situation, Carbon dioxide in the range of 500-600°C has no catalytic conversion due to the water-gas shift reaction (in this temperature range, carbon dioxide is produced due to the progress of the water-gas shift reaction in the forward direction).also due to the reaction of methane with water vapor and considering that more hydrogen than carbon monoxide is produced in this reaction; As a result, the H2/CO ratio is greater than one.
