The influence of moisture and ash content of palm fibre and shell towards particulate emission in palm oil mills

Abstract

This study aims is to evaluate the influence of moisture and ash content of palm fibre and shell on particulate emission in palm oil mill boilers. The particulate mass emission concentration of size segregated particulate of PM2.5 (particulate with diameter less than 2.5µm) and PM10 (particulate with diameter less than 10µm) was estimated from the total particulate mass. The influence of the operational condition of the boiler on particulate emission was scrutinized based on fuel feeding rate, palm fibre and shell and air to fuel (A/F) ratio. The particulate morphology and the elemental composition of the fly ash were also investigated. A total of five biomass fired boilers found in palm oil mills, with steam capacity ranging from 17 to 35 ton/hr were selected in this study. Samples of palm fibre and shell, bottom ash, retained and stack fly ashes were collected from each of the boilers. The stack fly ash was sampled iso-kinetically following the USEPA Method 17, while the others were collected on the day of the sampling. The result showed that the proximate analysis of the palm fibre and shell had the highest moisture and ash content as compared to other biomass fuels. The study also showed that the average total particulate (TP) emission concentration emitted from the stacks was 2.2±0.9 g/Nm3, ranging from 0.4 to 3.8 g/Nm3 (corrected to 7% O2). A high ash content of the biomass fuel contributed to higher particulate emission and similarly, high A/F ratio also promoted high particulate emission. The particulate size analysis showed that the under size 50% cumulative particulate distribution of the stack and retained fly ash ranged from 1.3 to 38 and 60 to 350µm, respectively. Meanwhile, the particulate emission of PM2.5 and PM10 were estimated to range from 0.03 to 0.30 and 0.37 to 0.73 g/Nm3, respectively, which represented 0.8% to 71% and 13% to 95% of the total particulate emission concentration. The calculated Particulate Emission Factor (PEF) based on palm fibre and shell feeding rate for TP, PM10 and PM2.5 were found to be 12, 1.5 and 0.5 g/kg, respectively. A total of 13 elements were determined in the stack fly ash, with Zn was found to be extremely enriched. The maximum predicted ground level concentration (GLC) of PM10 and PM2.5 based on the air dispersion modelling was found in average of 25.9±8.7 and 12.5±11.8 µg/m3, respectively occurring between 500 and 600m distance away from the stack. The predicted GLCs for both particulate pollutants were significantly lower than the Malaysia Ambient Air Quality Standards based on 24-hour averaging time and were compared with the AERMOD modelling. The health risk assessment impact of PM2.5, PM10 and selected elemental components such As, Cr and Ni found in the stack fly ash indicated that the emission from the boiler was unlikely to pose any significant impact towards human population in the vicinity.