Performance Analysis of a Mui Basin Coal Gasifier Based on Feed Oxygen, Water Concentration and Heat Regulation

Abstract

Kenya depends mainly on electricity from hydroelectric power, which is unreliable due to climatic change, and thermal generators whose power is expensive due to high diesel prices. Innovative ways of providing affordable and reliable energy services to Kenyan citizens is thus necessary, one of which is use of coal for energy generation. Gasification of coal from Mui Basin, Kitui County, can be one of the mitigating ways of providing clean, affordable and reliable energy to Kenyans. Traditionally coal power plants included grated coal firing plants where coal was crushed into gravel-like form and used in the boiler. Later, pulverized (powdered) coal power plants like Sub-critical, and Super-critical coal plants, were used. These plants, however, had low efficiencies (37%-45%) and generated undesirable emissions like SOx, NOx, CO2 and particulates. Currently, Integrated coal Gasification Combined Cycle (IGCC), is being used with oxidizing agents like steam, oxygen, and air to convert coal into clean syngas for power generation, giving better plant efficiencies. Integrated coal Gasification Combined Cycle includes tapping the unwanted emissions and using them in relevant industries, thus protecting the environment. The existing Mui Basin coal gasifier was developed based on coal gasification using air as the oxidizing agent. The syngas generated had high nitrogen content that led to low cold gas efficiency as well as the lower heating values. This study entailed performance improvement of a bench-scale fixed-bed gasifier using Mui Basin coal, based on regulation of reactor heat and the amounts of oxygen and steam inputs. Initially oxygen/air ratio into the gasifier was varied and an optimum ratio determined. Later the steam/oxygen ratio was varied and optimum ratio obtained. A Multi-gas Analyzer TY-6331P was used to analyze the syngas composition, while type-K thermocouples were used to monitor reactor temperatures. From the study, it was noted that, for air gasification, the optimum equivalence ratio was 0.30, with a mean reactor temperature of 748.8ºC and gasification efficiency was 43.1%. For gasification with oxygen-enriched air with no heat regulation, the cold gas efficiency was 73.1%, carbon conversion efficiency was 90.1%, mean temperature was 849.9ºC and syngas lower heating value was 8.58 MJ/Nm3. However, when reactor heat was regulated, the optimum oxygen/air ratio rose from 0.61 to 0.71, gasification efficiency rose to 78.7% and lower heating value rose by 8.7%. Inclusion of steam improved the syngas lower heating value to 9.33 MJ/Nm3, gasification efficiency to 87.0% and syngas yield rose by 14.53%. The optimum steam/oxygen ratio was 0.40. The data from this study is intended to play key role in the design of energy systems that utilize syngas for power generation help in developing small-scale coal gasifiers for domestic applications like small electric generators and small stationary engines. The gasifier can also be scaled up to industrial level for power generation.