Characterisation of torrefied carbon For carbon dioxide capture and cofiring application

  • Odira Vanatius Atueyi

Student thesis: Doctoral Thesis

Abstract

Increased carbon dioxide (CO2) emissions across the globe, and the resulting atmospheric levels, have become the subject of many scientific studies in recent times. Managing and reducing CO2 emissions has remained a challenge for scientists and researchers in carbon capture science, despite technology advancements. Although recent technologies deployed suggest an improvement from the classical approaches, there is a need to explore other alternatives to optimise process performance and to reduce the cost of carbon capture and sequestration processes. In this study, torrefaction technology was employed to develop 'torrefied carbon' using renewable carbonaceous materials, such as Iroko (IR - hardwood) and Scottish Pine (SP - softwood), for CO2 capture from the combustion stacks of coal-powered plants. The study was divided into two parts: (a) developing the torrefied carbon using selected torrefaction conditions, at temperatures of (290 °C, 320 °C, 350 °C and 380 °C), a residence time of 60 min and heating rate (10 °C min-1), under CO2 atmosphere. The second is testing the torrefied carbons for CO2 adsorption potential and cofiring applications. The physicochemical characteristics of the torrefied carbons, such as hydrophobicity, calorific values and ultimate analysis, as well as the torrefaction performance indicators, such as energy gain, energy consumption, mass density and mass yield, amongst others, were assessed, allowing the fuel quality and potential use of the torrefied carbon once entirely spent for CO2 capture in same power plant to be evaluated. Given the results obtained, the torrefaction performance indicators showed there is energy gain for the selected torrefaction conditions. The highest energy gain values of 104 and 102 were found for the SP and IR, respectively, at the torrefaction condition of 320 °C, at a residence time of 60 min. The calorific values of the torrefied carbons developed at 320 °C and 350 °C, where - IR (26.49 MJ kg-1 and 26.75 MJ kg-1) and SP (26.13 MJ kg-1 and 29.12 MJ kg-1), respectively, which were higher than those of the low-ranked coal (23.20 MJ kg-1) investigated. For the adsorption studies, the torrefied carbons developed at 350 °C showed the highest CO2 adsorption capacity for both IR and SP carbons. The thermodynamic study of the CO2 adsorption using the Langmuir and isosteric heat of adsorption suggests the existence of heterogeneous surface sites on the torrefied carbon surfaces. The CO2 adsorption shows low heat of adsorption, given the values of the isosteric heat, for IR320 (-45 KJ mol-1), IR350 (-58 KJ mol-1), SP320 (-28 KJ mol-1) and SP350 (-41 KJ mol-1), an indication that the CO2 adsorption process is governed by physisorption. The kinetics of the CO2 adsorption of the torrefied carbons followed the Double Exponential Model, described by two distinct rate-determining steps. The rate of CO2 adsorption on the torrefied carbons appeared fast, given the equilibration time of an average of < 8 min for the IR and 11 min for the SP carbon, suggesting that the short time of equilibrium based on the Pressure Swing Adsorption process indicates a good potential from the materials on a kinetic basis. Within the study context, it was determined that the torrefied carbons could be employed for cofiring in coal-powered plants following a CO2 capture process. Although the structural features exhibited by the torrefied carbons were not fully explored in this work, due to the research limitations, the study opens up an opportunity into the potentials of torrefied carbon utilisation as a cost-intensive alternative in CCS applications.
Date of Award10 May 2021
Original languageEnglish
Awarding Institution
  • University Of Strathclyde
SupervisorAshleigh Fletcher (Supervisor) & Jun Li (Supervisor)

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