Abstract
Co-combustion technologies of coal with biomass have been applied in several practical coal boilers in order to reduce CO2 emission, fuel cost and so forth. Furthermore, the biomass may be able to enhance the combustion performance and to control NOx emission, since the biomass contains high volatile matter and evolves NH3 as volatile-N species. Emission of NOx from coal combustion has recently become a subject of intense research, because of its increasing concentrations in the atmosphere and its known ability to deplete the ozone layer and also contribute to the Greenhouse effect. However, fundamental combustion and NOx emission characteristics for biomass and mixtures of coal with biomass have not been elucidated precisely yet. From these viewpoints, co-combustion technology is one of the candidates to reduce the NOx emission and enhance the effective utilization of both low-grade coals and biomasses. In this study, fundamentals of pyrolysis characteristics of several types of biomass were studied first, using a thermo-gravimetric analyzer. The combustion behaviour of biomass, coal and its mixture were tested, using electrically heated drop tube furnace. In the combustion test, the ignition behaviour, Combustion efficiency and NOx (NO and N2O) emission behaviour were focused, based on the results of gas compositions along the furnace axis. The main results obtained show that biomass can enhance the ignition characteristics of the low-volatile coal during co-combustion. NO and N2O formation during the co-combustion affects effect of coal combustion even if the input fuel nitrogen becomes half of that under coal combustion condition. At least NO behavior can be simulated by the homogeneous reaction scheme relating to NOx even at constant temperature. During biomass combustion, fine particulates less than 2 µm are produced as ash particles. Burning biomass with coal shifts the particle size distribution from fine particles to coarse particles.
