Majed A. Toqan

Prediction of fly ash size and chemical composition distributions: The random coalescence model

In the course of combustion of pulverized coal, the coal minerals partially coalesce to form fly ash upon the complete burnout of the combustible matter. The probability of a fly ash particle impacting an obstacle in the flow e.g. heat exchange surface, depends largely on its size, and its retention in a deposit upon the chemical composition. The Random Coalescence Model (RCM) is an essential part of an overall model that follows the coal mineral matter transformation to deposit formation. Computer Controlled Scanning Electron Microscopy (CCSEM) of the size and chemical composition distributions of the mineral inclusions in the coal and analysis of the ion-exchangeable minerals form the inputs to the RCM calculations. A coal char combustion model provides limits for the coalescence of mineral inclusions as the char surface recedes during combustion. The calculation proceeds through steps determining the most probable values for the coal mineral size and chemical composition and the mean values and variants of the resultant fly ash size and chemical composition distributions. Experiments carried out with a Wyoming lignite in the pilot scale MIT Combustion Research Facility gave good agreement with results of the RCM calculations predicting the fly ash size and chemical composition distributions when CCSEM data of the coal minerals were used as input to the model.

The formation and destruction of aromatic compounds in a turbulent flame

Systematic, detailed studies of the effects of temperature and fuel type on polycyclic aromatic compound (PAC) emissions from turbulent flames were carried out in a well-controlled, pilot-scale turbulent flame. Benzene and toluene were injected as a secondary fuel into the hot combustion products of a fuel-lean natural gas air flame at 1350 and 1550 K. The experimental results show a strong combined influence of fuel type and temperatureon the PAC emissions. At 1350 K, the PAC yield was approximately 10 times higher than that of soot, while at 1550 K, the soot yield was 10 times higher than the PAC yield, indicating a much faster formation and conversion to soot of PACs. At the higher temperature, the product distribution of PACs shifted toward species containing cyclopenta-fused rings. At 1550 K, the fuel type had virtually no impact on either the total yield of PACs and soot or the product distribution, while at the lower temperature, a large difference in yield and product distribution were found between the experimental fuels. At 1350 K, toluene injection produced up to 25 times more PACs and 30 times more soot than the injection of benzene. Also, the peak concentrations of soot and PACs in the benzene flame occurred later than in the toluene flame. A numerical model based on a segregated flow analysis was used to quantitatively analyze the experimental data. The treatment of the experimental flames by the model showed that the PAC emissions from the flames were affected by both chemistry and mixing. The model correctly predicted the trends of formation, and destruction of most compounds included in the model, and the absolute concentrations of individual species vary from within a factor of 5 (for major species) to within two orders of magnitude (for trace pollutants).