Oxy-fuel fluidized bed combustion of Victorian brown coal

Abstract

Coal contributes to almost forty percent of global power generation. As conventional coal-fired power generation technologies result in large CO2 emission, the pursuit for new technologies focuses on either reducing CO2 emission or that allows easier capture of the emitted CO2 from coal-fired power plants. Oxy-fuel fluidized bed (Oxy-FB) combustion is one such technology due to its ability to produce concentrated CO2 stream in the flue gas. This concentrated CO2 allows easier capture for subsequent transportation and storage. Other important benefits of this technology are the potential for using any type of fuel, and the ability to control SO2 and NOX emissions. Despite its perceived advantages over conventional technologies, very little is known about the applicability of Oxy-FB for brown coal. Brown coal accounts for 91% of Victoria’s current electricity needs. Since Victoria has an estimated reserve of over 500 years of brown coal at the current consumption rate, successful application of Oxy-FB can potentially result in environment friendly power generation in Victoria. This first-ever study investigates the Oxy-FB combustion using Victorian brown coal in a combined experimental and modelling approach. The research involves designing and commissioning of a 10 kWth fluidized bed rig, carrying out experiments in laboratory scale and bench scale equipment, and performing thermodynamic and process modelling. Laboratory scale experiments using single char particle were conducted to investigate the combustion characteristics of individual and large char particle under Oxy-FB conditions. Particle temperature was observed to be higher compared to bed temperature. Up to 48°C difference was noticed between the char particle temperature and the bed temperature using 15% (v/v) steam in oxy-fuel combustion atmosphere. The temperature of the char particle during Oxy-FB combustion has practical implication for agglomeration. The bench scale experiments were carried out to evaluate combustion efficiency, agglomeration characteristics, sulphation characteristics, carbonation characteristics, NOX (NO, NO2 and N2O) emission, SOX (SO2 and SO3) emission, and trace elements (Hg, Se, As and Cr) emissions during Oxy-FB combustion of Victorian brown coal. A high level of CO2 concentration (90-94% in dry flue gas), over 99% combustion efficiency and no bed agglomeration under oxy-fuel combustion conditions including those with the addition of steam at temperatures between 800°C and 900°C. Moreover, the measured NOX and SOX concentration levels in the flue gas are within the permissible limits for coal-fired power plants in Victoria. This implies that additional NOX and SOX removal systems may not be required with Oxy-FB combustion of Victorian brown coal. The gaseous mercury concentrations, however, are considerably higher under oxy-fuel combustion compared to air combustion suggesting that mercury removal system may be required to avoid corrosion in the CO2 separation units if CO2 capture and transportation is intended. These conventional pollutants and trace elements emission characteristics are of great importance for the design of the gas cleaning systems for CO2 capture and storage (CCS) purposes. Furthermore, these results also provide information for selecting the optimum operating condition. Thermodynamic equilibrium modelling was carried out to predict the compounds formed during the combustion of Victorian brown coal under different Oxy-FB combustion conditions. It was predicted that the amount of toxic gaseous Cr6+ species was greater for oxy-fuel combustion than for air combustion. The distribution of toxic Se4+ species, however, remained almost the same in both combustion conditions within the typical temperature range for Oxy-FB combustion (800 - 950°C). A process model on Oxy-FB combustion using Aspen Plus was also developed to predict combustion performance of any coal during Oxy-FB. It was observed that the concentrations of CO and SO2 were higher in the lower dense region of the bed. These levels, however, dropped significantly with the introduction of secondary oxygen. The simulation results were consistent with the experimental data. Overall, this thesis has identified several important issues, for the first time, on Oxy-FB combustion using brown coal. The information generated is useful for academics, industry and policy makers. Future research on Oxy-FB combustion can use the findings of this study while developing Oxy-FB combustion for brown coals.