Evaluation of Ash-Free Coal for Chemical Looping Combustion

• Author / Creator

  Shabani, Azar

• In this study, performance of ash-free coal (AFC) was evaluated for chemical looping combustion (CLC). Coal is the major source of power generation worldwide and release of ash forming minerals and CO2 emission are the major issues during its combustion. AFC is used in order to get rid of the problems related to release of ash forming minerals. CLC is considered as a promising technology with inherent CO2 capture due to its potential to reduce energy penalty and the cost associated with CO2 separation from combustion off-gas. Several oxygen carrier materials with suitable thermodynamic properties and high oxygen transfer capacity have been identified for the CLC and chemical looping with oxygen uncoupling (CLOU) processes. One of the most promising oxygen carrier materials is CuO/Cu2O. In this study, introduction of CLC and CLOU systems, status review of CLC of solid fuels, experimental results of CLC of AFC at different ratios, economic considerations and techno-economic evaluations followed by mass and energy balance of the system are presented. As background information, preparation process and properties of AFC were discussed. CLC combustion experiments of AFC were performed in a thermogravimetric analyzer (TGA) using CuO as an oxygen carrier to evaluate the CLC performance of CuO with AFC during reduction and oxidation processes and to explore the reaction mechanism of the CuO/AFC system. TGA experiments with a CuO/AFC mixture with different ratios (10:1 - 50:1) at various temperatures ranging from 450 to 1000 °C were performed and the results were analyzed in greater details for the close to stoichiometric ratio of CuO/AFC ratio of 30. Advanced analytical techniques such as XRD, SEM, EDX and ultimate analyses were employed to characterize the oxygen carrier and to understand the possible interaction of the oxygen carrier with volatile matter and char. Thermodynamic equilibrium calculations were performed for CuO/AFC system by FactSage. A combustion mechanism of AFC in CuO has been described in three stages as: Stage 1: most of the volatile matter was released from AFC at around 450 °C and combustion of these gases started at around 400 °C with CuO, which could be due to the induced gas-solid interactions. Stage 2: from 450 to 790 °C, the solid-solid interaction of CuO and AFC-char. Stage 3: auto-decomposition of CuO to Cu2O took place above 790 °C and oxygen was released, enhancing the solid-solid interaction between CuO and residual char. Performance of AFC in CLC during multicycle TGA experiments was also investigated. A ratio of CuO/AFC of 30 (close to stoichiometric ratio of 27.1) was selected for the consecutive reduction and re-oxidation cyclic experiments. The reactivity of the first cycle was slightly higher than the consecutive cycles due to the fresh CuO in the first cycle. The thermal behaviors, such as mass change in the consecutive cycles were almost similar and there was no residual ash deposition after each cycle. Furthermore, reduction and oxidation processes were performed with different isothermal times, after one hour the combustion was incomplete but longer combustion period (three hours) led almost the complete combustion. Fresh samples and solid residues were analyzed by several advanced analytical techniques including XRD, SEM and BET. XRD analysis of the residue of CuO/AFC showed the presence of CuO at the end of the cycles; CuO and SiO2 in residues from CuO/BL raw coal (parent coal of AFC). At higher temperatures (900 oC) increased sintering was observed after each cycle, as a result the surface area, total pore volume and average pore radius of the material decreased; however, the final masses of reduced and oxidized masses at 900 oC were constant for CuO/AFC ratio of 30 due to no ash content in AFC. Based on the data collected in TGA, application of CLC for power generation was assessed; mass and energy balance of the proposed system were also carried out. Overall, AFC as the solid fuel showed a promising oxidation/reduction performance and has great potential to be used in the CLC process.

• Subjects / Keywords

  • Ash-free coal
  • CO2 capture
  • Chemical looping with oxygen uncoupling
  • Chemical looping combustion

• Graduation date

  Spring 2016

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3R20S69X

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.