| Power generation by co-firing biomass and coal is an efficient way of reducing CO2 emission; this technology can be widely utilized for its low risks and higher efficiency. The main technical obstacle for the generalizing is the higher risks of ash deposition and slagging caused by co-firing of biomass. Variable factors including biomass species, blending ratio of biomass, combustion temperature, excess air rate, particle size of fly ash, velocity of gas flow, the gas flow and temperature distribution in boiler can affect the ash characteristics and the ash deposition. This article is attempting to explain the correlation between these multiple factors and how they work cooperatively on the ash character. This article also provides some suggestions on ash deposition and slagging performance of boiler with a CFD numerical simulation.Firstly, an experiment is conducted on the drop tube furnace platform to analyze the effect of biomass species?blend ratio?combustion temperature and excess air rate on the biomass and coal co-combustion ash characteristics. It is found that the alkali metal potassium is playing a leading role in the ash fusion characteristics. The volatile potassium reacts with the Si and Al from coal ash to form a low melting point material KAlSi2O6 which is supposed to be the key substance for ash fusion and aggregation. With the same blend ratio of biomass cotton stalk is capable of causing the most obvious ash fusion while sawdust is much more inconspicuous and rice husk could rise the co-combustion ash fusion temperature. The suggested blend ratio for co-combustion of cotton stalk?saw dust are below 20% and below 30%. Rice husk can be blended on a high ratio during co-firing without causing severe ash fusion and deposition. It is also found that, an incomplete mix of biomass and coal fuel may lead to the imperfect reaction between potassium(g) and ash component during which the volatile potassium may reach the heat exchange surface and cause an even worse ash deposition and corrosion. KAlSi3O8 is decomposed to KAlSi2O6 with the combustion temperature of 1220?. A lower combustion temperature not only reduces the formation of low melting point material and also contributes to the reduction of melt phase for its lower temperature. Keeping a lower combustion temperature is of great significance to the reduction of ash deposition and slagging. When the excess air rate is raised, sulfur content is becoming more active and form CaSO4 with calcium in the ash, potassium however cannot be fixed in K2SO4 by sulfur(g) on its initial reacting stage. Therefore, a higher excess air rate cannot help reduce the ash deposition and slagging risk.Secondly, the Principal component factor analysis is adopted to make a weight analysis of different factors working on the ash fusion characteristics. It is demonstrated that the content of KAlSi2O6 in ash and the content of potassium in biomass are the most effective factors. The content of SiO2 and CaAl2Si2O6 are less effective factors. The linear analysis of B/A slagging index and tow principle factors shows that, the slagging index is effective on estimating the slagging situation of co-firing ash with the most alkali metal fixed in ash and form low melting point material. The boundedness of coal slagging index for estimating the co-firing ash is that the index is incapable of reflecting the effect of combustion temperature on ash characteristics.Thirdly, the numerical simulation proves that, a bigger particle size means a stronger inertia and a bigger gas flow velocity means a stronger rigidity, both stabilizing the gas flow and the particle movement. A more stable flow of gas and fly ash particle reduce the affection of eddy current, making fewer fly ash particles to reach the back flow side of heat exchange face pipes. The burner zone in boiler furnace has a much higher risk of slagging in comparison with no burner zone. The back rows of pipes are facing a higher risk of ash deposition than the front rows pipes. |
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