Numerical Investigation of Oxy-Mild Combustion of Pulverized Coal in a Pilot Furnace

摘要

The Conventional coal-fired plants are large contributors to air pollution and greenhouse gas. The combustion generates pollutants such as oxides of sulphur, nitrogen, and carbon as well as fine organic and inorganic particulates. The new technologies able to reduce drastically the pollutant emissions and facilitate to use of coal in an environmentally more friendly way, are commonly known as clean coal technologies (CCT). In this context the CCS technologies play an important role to reduce the CO2 emissions. The only form with truly zero CO2 emissions in existence today is pre-combustion gas separation, namely, the combustion of fuel using oxygen instead of air. It is well known that burning pulverized coal in pure oxygen increases the flame temperatures, thus also increases NO_x emissions. Therefore, to moderate the flame temperature and reduce NO_x the oxygen is mixed with recycled flue gas (RFG). This approach to reduce CO2 emissions is often called oxy-firing or oxy-fuel combustion. The purified CO2 stream is then compressed and condensed to produce a manageable effluent of liquid CO2, which can be sequestered for storage (CCS) or for use in subsequent processes (CCR). MILD (Moderate or Intensive Low Dilution) or HiTAC (High Temperature Air Combustion) is an innovative combustion technology and probably the most important achievement of the combustion technology in recent years. In MILD combustion the reactions take place in almost the whole volume of the combustion chamber. This leads to temperature and species concentration fields uniform in the chamber. The fuel is oxidized in an environment that contains a substantial amount of inert gases (N2, CO2, H2O) and low oxygen concentrations. This is caused by an internal recirculation of combustion products generated by injecting preheated air jets into the combustion chamber with very high momentum, bringing the temperatures close to the combustion products temperature, reducing the NO_x emissions. Because both technologies allow reductions of pollutant emissions, the aim of this work is to demonstrate the advantages of a combination of these two combustion technologies in order to analyze the temperature and specie concentrations field, the CO2 and NO_x emissions by means of CFD. The goal is understand if it is possible to combine the MILD combustion and OXY one in order to reduce the NO_x emissions, and capture the CO2.