Combustion characteristics of Mui and Taru basin coal in a fluidized bed combustor

Abstract

Coal reserves at Mui and Taru in Kitui and Kilifi counties respectively are estimated to provide over 400 million tons of coal. Being new discoveries, their combustion characteristics and emission performance need to be investigated for grading and design of combustion equipment. Therefore, the main objective of this study was to investigate their combustion characteristics using a fluidized bed combustor. Specific objectives were to determine the physical and chemical properties, determine the single particle combustion characteristics (Temperature - weight) and to determine the emissions and temperature profiles. The properties were investigated using the American Standards for Testing Materials, while the combustion characteristics were studied in a fluidized bed combustor. Proximate analysis of the Mui well 1, Mui well 2 and Taru coal samples were: Moisture Content 3.75, 5.48 and 3.53 %, Volatile Matter 59.25, 58.05 and 55.10 %, Ash content 9.25, 11.48 and 24.63 % and Fixed Carbon 27.80, 25.00 and 16.75 % respectively. Ultimate analysis: Sulphur wt. % 1.94, 1.89 and 1.07, Carbon 65.68, 60.98 and 51.00 %, Hydrogen 5.97, 5.70 and 5.09 %, Nitrogen 0.92, 0.94 and 1.00 % and Oxygen 11.62, 12.33 and 11.13 % for Mui well 1, Mui well 2 and Taru coal samples respectively. Temperature - weight analysis showed that devolatilization starts at 200 °C and 250 °C and combustion was complete at 750 °C and 650 °C for Mui and Taru basin coal respectively. The maximum temperature obtained in the fluidized bed combustor was 855 °C at height of 700 mm from the base, just above the point of fuel feed, while the minimum was 440 °C at 2,230 mm height showing a greater percentage of coal volatiles and char combustion takes place at freeboard. Maximum pressure drop was 1.02 mbars at 150 mm from the base, while minimum was 0.67 mbars at 700 mm from the base the point which recorded maximum temperature; this showed that pressure was highest at point of feed and least at the point of combustion. Gross Calorific Values were for Mui well 1 coal 27,090 kJ/kg (Grade A), Mui well 2 coal 25,196 kJ/kg (Grade B), and the Taru coal 21,016 kJ/kg (Grade C). Flue gas analysis by digital gas analyzer showed the presence of H2S as 20 ppm and 6 ppm for Taru and Mui coal respectively; hence Taru coal utilization will have greater environmental impact compared to Mui coal therefore need for pre-treatment. CO, percentage rose to a maximum of 2,000 ppm at 600 °C suggesting incomplete combustion. The percentage level of O2 decreased as combustion progressed to a minimum of 15 % and then increased to 20.3 % suggesting depletion of coal. The experiments were done in batch form. Comparing these findings with coal from other regions, this coal meets the required Calorific Values and temperatures ranges for use as a solid fuel in commerce and industry. It’s therefore recommended that this Kenyan coal be considered for power generation.