Evaluation of the Fuel Properties and Thermal Efficiency of Sub-Bituminous Coal-Biomass Blends

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Abstract

The improvement of the combustion properties of coal and biomass by blending and carbonization was investigated. Proximate and ultimate analysis of coal, sawdust, corn cob and their blends were carried out using ASTM methods. The proximate and ultimate analyses were repeated on the five blends after carbonization at 500oC for one hour. Ten mixtures of coal-sawdust and coal-corn cob blends were made into briquettes using starch binder. The calorific values of the samples were determined using a bomb calorimeter, while the thermal efficiency of the briquettes was obtained using the water boiling test analysis. Pollution potential of the fuel samples were derived using a hypothetical power plant simulation. The quantity of CO2, NO2 and SO2 that would be emitted per hour in a 20MW power plant were calculated. The result of the proximate analysis of the raw samples (coal, sawdust and corn cob) showed that coal had the highest fixed carbon (42.38%) and the lowest moisture content (4.28%). Sawdust had the lowest fixed carbon (12.35%) while corn cob had a fixed carbon content of 15.65%. The results obtained showed considerable correlation between the uncarbonized coal-sawdust and coal-corn cob blends. The carbonized blends of both the coal-sawdust and coal-corn cob blends showed an improved fixed carbon content and volatile matter, relative to the uncarbonized. The calorific values and other fuel properties were of similar trends. The results of the ultimate analysis for coal were 70.04% carbon, 5.32% hydrogen, 2.03% nitrogen and 1.02% sulphur. Corn cob had 48% carbon, 5.79% hydrogen and 0.89% nitrogen while sawdust had 48.78% carbon, 5.79% hydrogen and 0.89% nitrogen. Corn cob and sawdust had no sulphur content, while the uncarbonized coal-sawdust and coal-corn cob blends showed decreasing carbon content. This study revealed that the fuel properties of coal and biomass can be improved by blending and carbonization. The simulated power plant analysis of the fuel showed that blending of coal with biomass reduced the SO2 and NO2 emissions to an extent. For NO2, the value of coal-sawdust blends ranged from 82.8-190.8 kg/hr, reduced from 198 kg/hr in coal, while the SO2 content was reduced from 60.5 kg/hr in coal to 5.6 – 50.4 kg/hr in different blends. Therefore, blending of coal with either sawdust or corn cob should be encouraged in coal fired power plants to reduce environmental pollution.

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