Chemical-looping combustion of Victorian brown coal
thesisposted on 08.02.2017, 01:16 authored by Saha, Chiranjib
Victoria has over 500 years of brown coal resources at present consumption rate. Current utilization of brown coal through conventional pulverized coal-fired power generation results in large CO2 emission. Capture of CO2 at a lower energy and cost penalty is important especially when the consumption of this huge resource is considered necessary for economic development of the state and country. Currently three major approaches (oxy-fuel, pre and post-combustion capture) are under investigation for easier CO2 separation and capture. However, all these technologies are cost and energy intensive. Chemical looping combustion (CLC) is considered to be an emerging alternative technology to facilitate easier capture of CO2 at lower energy and cost penalty. In this process, a metal oxide is used to transfer oxygen from the combustion air to the coal. Therefore, the direct contact between air and coal is avoided preventing CO2 from being mixed with combustion gases, thus concentrating the CO2 in the flue gas and resulting in its easier separation. The spent metal oxide, after re-oxidation in fresh air, is reused in the next cycle. A majority of the international research efforts have been focused on CLC of natural gas and to an increasing extent on high-rank coals relevant to their regions. The highly reactive, high volatile matter and oxygen content, low ash Victorian brown coals in general are considered to be suitable for CLC. However, there is no technical data and scientific understanding of such process using Victorian brown coals. This first-ever study on CLC using Victorian brown coals attempts to fill this gap and includes the following investigations: · Selection of metal oxide for Victorian brown coal CLC · Optimization of CLC operating conditions using best performed oxygen carrier · Effect of coal volatile matter and ash on performance of oxygen carrier · Performance of oxygen carrier at high pressure during combustion · CLC process simulation for poly-generation concept · Determination of redox kinetics during CLC Identifying the best oxygen carrier in terms of oxygen transfer ability and its longer-term performance during combustion is the key to a successful CLC process. Through modeling and experiments at atmospheric and high pressure, complemented with analytical techniques, this study aims to assess the performance of three different oxygen carriers (NiO, CuO and Fe2O3) of varying particle sizes and identify the practical issues of brown coal CLC over multiple continuous cycles. Also used in the experiments is a mineral of iron oxide. CLC of different Victorian brown coals and international low-rank coals of varying ash content are investigated in this work using a Thermogravimetric analyzer and a purpose built bench scale experimental setup. The experiments are performed in both batch and continuous mode up to 950°C. Online monitoring of temperature and gas composition has been used to examine carbon conversion, reactivity of metal oxides, CO2 production and its purity. The solid residues of combustion are characterized using scanning electron microscopy, energy dispersive X-rays, X-ray diffraction, Mossbäuer spectroscopy and Xray photo electron spectroscopy to identify the structural, morphological, compositional and surface oxygen level changes in the metal oxides. Thermodynamic, process and mathematical modeling of the CLC system was carried out with the experimental data. The performance of NiO, in terms of extent of coal combustion and reactivity, degraded in progressive cycles due to increase in weight loss. This weight loss was observed due to interaction with ash during combustion. However, the loss in weight of CuO and Fe2O3 was negligible in every cycle. More than 1000 hours of continuous operation is estimated based on the weight loss values per cycle of CuO and Fe2O3 at current experimental conditions. Agglomeration was observed between CuO and coal minerals when operating at 950°C, but not in case of Fe2O3. However, at low operating temperature of 800°C high coal combustion and good reactivity was observed for CuO. At 950°C high (~99%) carbon conversion and high reactivity of Fe2O3 were achieved towards concentrating CO2. Also highest reactivity of Fe2O3 during combustion was achieved at high conversion levels implying less solid inventory requirement in a practical CLC system compared to CuO. Fe2O3 particles of 100-150μm size resulted in best performance towards oxygen transfer and carbon conversion. The chemical and the structural integrity of the used particles were same as the fresh particles as observed by solid characterization analysis. The effect of the coal volatile matter on the conversion of Fe2O3 was identified. The CO/CO2 ratio throughout the reduction period was very close to zero indicating that high purity CO2 could be achieved with high carbon conversion. It was also identified that the low ash content of Victorian brown coals are advantageous for a CLC process. The Fe2O3 particles interacted less with ash in Victorian brown coal compared to high ash coals tested in this study. These particles maintained high reactivity and their separation from ash particles are expected to be easier in a practical system. Iron oxide based low cost minerals showed good performance at high pressure meaning potential for application in a combined cycle power plant. The process modeling of CLC showed the possibility of heat-power generation along with hydrogen production utilizing Victorian brown coals. Also this thesis makes a preliminary attempt to model oxygen transfer during a CLC process for prediction of oxygen carrier conversion in such a process. This mathematical model showed that the reduction mechanism of metal oxide is controlled by chemical reaction to certain conditions and the oxidation is controlled by product layer diffusion mechanism. The predicted results are validated experimentally to less than 1% error. This study has identified CLC as a prospective process for cleaner utilization of Victorian brown coals. It has identified iron oxide as the most prospective oxygen carrier along with its preferred particle size and temperature. This study sets the basis for further focused investigations using iron oxide and its minerals. Further modeling and experimental work should include tests in a larger reactor for scaling up this process to combust wider variety of Victorian brown coals.