Performance and Simulations of Small-Scale Solid Waste Incinerators at Njokerio, Ng’ondu and Green Valley Areas in Njoro, Kenya

Abstract

Solid waste management is challenging and incineration technique is more preferred to other methods in reduction of mass and volume, removal of odour and energy recovery in both industrial and residential environments. The challenges facing residents at Njokerio, Ng’ondu and Green Valley areas included poorly designed open-wastes collection systems, exceeding incinerator loading rates, inappropriate operating temperature levels and inadequate design specifications. Objectives of this study were to characterize solid wastes, determine and assess factors influencing incineration performance and to simulate air flow patterns and velocity profiles for small-scale incinerators. Solid wastes collected from study areas were sun dried for three days, chopped into small pieces then separately packed into containers. Equipment used were eight small-scale incinerators, two muffle furnaces, flue gas analyser, electronic weighing balance, dryer, vibrator and chopping machines and air flow metre. Data collected was statistically analysed to determine trends, means, F-values and Least Significant Different at 5% confidence level. Characterized mean values for moisture content, volatile matter, ash content, fixed carbon were 41, 33, 15 and 11%, respectively while density had 257 kg/m3 and calorific values had 10 MJ/kg. Incinerating wastes at varying moisture contents (MC) from 15 to 75% produced mean emission values for carbon monoxide (CO), carbon dioxide (CO2), and hydrocarbon (HC) ranging between 5 and 11 ppm, 5 and 14%, and from 508 to 1168 ppm, respectively. Varying incinerator loading rates from 15 to 75 kg/h yielded means CO ranging between 5 and 12 ppm, CO2 from 5 to 14%, and HC between 252 and 1096 ppm. Moreover, increasing operating temperatures from 180 to 900oC contributed to mean values for CO, CO2 and HC emissions ranging from 14 to 5 ppm, 15 to 6% and 1253 to 316 ppm, respectively. Simulation of Egerton University dispensary incinerator had a maximum air flow velocity of 5.2 m/s resulting into the best incineration performance while Community Resource Centre had the lowest of 1.9 m/s. Air flow and velocity profiles simulations of circular base-shaped incinerator model, projected best performance yielding maximum velocity of 6.4 m/s, whereas triangular base-shaped had the lowest of 4.3 m/s. High moisture contents, overloaded incinerators and low operating temperatures contributed to high gases emissions, leading to dark and dense smoke which resulted into incomplete combustion implying poor incineration performance. Wastes incineration at low loading rates, low moisture content and high operating temperatures produced finest and grayish white bottom ash, low levels of carbon and complete combustion. The small-scale incinerators are not incorporate with air pollution control devices hence cannot fully meet the emissions standards, although can lower if operated effectively.