Oxy-fuel combustion

vattenfallCarbon dioxide (CO2) and sulphur dioxide (SO2) emissions are a major concern in combustion processes using fossil fuels, as for example coal; the former is implicated in global climate change and the latter produces acid rain. CO2 is one of the major contributors to the build-up of greenhouse gases in the atmosphere. At the same time, fuels containing sulphur produce SO2 pollution during combustion. The capture of CO2, emitted in large quantities from power stations, is considered an option to be explored in the medium term for reducing CO2 levels released to the atmosphere. Oxy-fuel combustion is one of the possibilities under investigation within the different options for CO2 capture. This technology uses for combustion O2 mixed with recirculated flue gases, instead of air used in conventional combustion, to produce a flue gas stream with high concentration of CO2. Until now, most of research has been directed towards the development of oxy-fuel systems in pulverised fuel boilers. However, it is believed that a circulating fluidised bed combustor (CFBC) will be an important candidate for new coal fired power plants because solids recirculation can help to an effective control of the temperature. In-situ desulphurisation is another of the best known advantages of fluidised bed combustion. However, sulphur capture by calcium compounds is a process highly dependent on the temperature and CO2 concentration. In oxy-fuel combustion, COconcentration in the bed may be enriched between 40 and 90%. Under so high CO2 concentration, different from that in conventional coal combustion atmosphere (0-15% CO2), the calcination and sulphation behaviour of the sorbent must be defined previously to decide the optimum operating temperature in the combustor. For oxy-fuel operating conditions, with high CO2 concentration in the combustor, the SO2 retention could occur depending on the operating temperature by different processes: direct sulphation, simultaneous calcination/sulphation or sulphation of calcines. The objectives of this research line are to study these ways of sulphur capture and to optimize the operating temperature for sulphur retention in oxy-fuel operating conditions during fluidised bed coal combustion. In addition, because the characteristics of the calcium sorbent (as specific surface area, pore size, composition, etc.) have a very strong influence on the rate of sulphation, it will be analysed the behaviour of different limestones and dolomites for application in oxy-fuel coal combustion in fluidised beds. Test runs will be carried out in a continuous oxy-fuel fluidised bed combustor with different coals, sorbents and test conditions. Special attention will be paid to the SO2 retention and emission. Finally, a mathematical model of a circulating fluidised bed combustor will be developed to simulate and to optimise the sulphur retention in this type of combustors working in oxy-fuel conditions. The model will be a useful tool for the design and scale-up of this type of combustors. Specific objectives may be summarised as follows:
  • To model the calcination and sulphation processes happening in the calcium based sorbents (limestones and dolomites) and to determine the kinetic parameters of these processes during oxy-fuel fluidised bed combustion.
  • To find the optimum operating temperature of the combustor to maximise the sulphur retention during oxy-fuel fluidised bed combustion.
  • To analyse the effect of the main operating conditions of the oxy-fuel fluidised bed combustors (coal type, O2/CO2 ratio fed to the combustor, sorbent type, Ca/S molar ratio, O2 excess, etc.) on the behaviour of the calcium based sorbents and on the SO2 retention in this kind of reactors.

3 kWth Oxy-fuel Pilot plant for solid fuels