EXPERIMENTAL AND MODELLING INVESTIGATION OF THE PILOT-SCALE OXY-FUEL COMBUSTION OF VICTORIAN BROWN COAL

摘要

Victorian brown coal is the single largest source contributing to the power generation in Victoria, Australia.Due to the presence of abundant moisture, i.e. up to 70 wt%, Victorian brown coal exhibits a greenhouse gasemission rate as high as ～1300 kg-CO_2/MWh-electricity sent out in the existing air-fired power plant,relative to ～900 kg-CO_2/MWh for the Australian black coal. Oxy-fuel combustion is a cost-effective optionto deliver a CO_2-rich gas stream that is ready for direct CO_2 sequestration, when compared with the postcombustioncapture, e.g. amine absorption for CO_2 recovery. As the youngest lignite in the world, Victorianbrown coal is supposed to possess a variety of distinct characteristics during the oxy-fuel combustion, whichare to varying extents linked with the abundant moisture within the coal matrix. On the one hand, theabundant moisture is believed to enhance coal volatile ignition delay, due to the required evaporation of it atthe initial heating step for coal particles. On the other hand, once being evaporated, the steam possesses alarger specific heat capacity than nitrogen and oxygen. Its presence in the furnace thus promotes theradiative heat transfer from coal flame to flue gas, and consequently to the water in the heat exchange tubes.The use of recirculated flue gas (RFG) in place of air could further complicate these two issues, consideringthat CO_2 also has a large specific heat capacity that is comparable to steam, whilst the adjustable oxygenconcentration in the furnace is in favor of shortening the ignition time of coal particle, either dry or wet. Inlight of this, it is of great interest for us to investigate the oxy-fuel combustion of Victorian brown coalparticularly on a pilot scale.Oxy-fuel combustion of Victorian brown coal is being carried out in a 3MW pilot-scale boiler to clarify itsdistinct combustion characteristics. The residual moisture in brown coal plays a critical role on the ignitiondelay and expansion of the flame length as well, which subsequently affects flame stability, heat-transfer infurnace and even the stability of the feeding of the pulverized coal. Through a preliminary test in the pilotscaleplant, a variety of essential data for the combustion of Victorian brown coal under the oxy-fuel modehave been obtained, which are useful for the future design and optimisation of an industrial-scale oxy-fuelsystem and burner for Victorian brown coal. Apart from experimental investigation, the computational fluiddynamic (CFD) modeling have also carried out to confirm the remarkable influence of moisture on thecombustion of this distinct low-rank coal in both air and oxy-fuel mode. As has been confirmed, 3MW pilotscaleoxy-fuel combustion of Victorian brown coal has been successfully, leading to the production of 80%CO_2 at the 5% air ingress in the system. The moisture in brown coal plays a crucial role on coal ignitiondelay and the flame stability. Upon the pilot-scale tests, the valuable data for the oxy-fuel plant and burnerdesign for Victorian brown coal are being gained, including coal oxidation rate, heat flux and distribution inthe furnace, pollutant emission, and its ash fouling and slagging propensities.