Oxy-fuel combustion

Carbon 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, CO₂ concentration 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