Abstract
Volumetric combustion has been developed to realize a high substitution ratio of biomass in co-firing boilers, which features an intensive flue gas internal recirculation inside furnace. However, the characteristics of NOx and SOx emissions in large-scale boilers with volumetric combustion were not fully clear. In this paper, an Aspen Plus model of volumetric combustion system was built up based on a co-firing boiler. In order to characterize the reductions of NOx and SOx, three biomass substitution ratios were involved, namely, 100% biomass, 45% biomass with 55% coal, and 100% coal. The effects of flue gas recirculation ratio, air preheating temperature, oxygen concentration, and fuel types on pollutants emission in the volumetric combustion system were investigated. According to the results, it was concluded the higher substitution ratio of biomass in a co-firing boiler, the lower emissions of NOx and SOx. Moreover, flue gas internal recirculation is an effective pathway for NOx reduction and an increased recirculation ratio resulted in a significant decreasing of NOx emission; however, the SOx increased slightly. The influences of air preheating temperature and O2 concentration on NOx emission were getting weak with increasing of recirculation ratio. When 10% or even higher of flue gas was recycled, it was observed that almost no NOx formed thermodynamically under all studied conditions. Finally, to reach a low emission level of NOx, less energy would be consumed during biomass combustion than coal combustion process for internal recirculation of flue gas.
Highlights
In 2007, EU set up ambitious targets of raising the share of renewable energy in the EU to 20% and reducing greenhouse gases by 20% in the year 2020 compared to 1990 [1]
To reach a low emission level of NOx, less energy would be consumed during biomass combustion than coal combustion process for internal recirculation of flue gas
According to the predicted results, flue gas internal recirculation is effective method for reduction of NOx, and NOx was rapidly decreased with increase in recirculation ratio, while unexpectedly, the SOx emission increased simultaneously
Summary
In 2007, EU set up ambitious targets of raising the share of renewable energy in the EU to 20% and reducing greenhouse gases by 20% in the year 2020 compared to 1990 [1]. Biomass combustion process becomes more complicated than that of fossil fuels due to its physical and chemical properties, such as high moisture content, low bulk density, low melting point of the ash, and high content of volatile matter. Those properties result in a serial of technological problems, such as feedstock problem, load or flame instability, slagging, corrosion phenomena, and so on [5]. All those potential problems significantly limit the application of biomass combustion in large scales. A key challenge for application of biomass based co-firing systems is to develop the efficient combustion technologies, to make sure biomass is able to replace fossil fuels in a large portion
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