Abstract
Burning of rice straw can emit considerable amounts of atmospheric pollutants. We evaluated the effect of rice straw moisture content (5%, 10% and 20%) on the emission of carbon dioxide (CO2) and on the organic and inorganic constituents of released particulate matter (PM): dioxins, heavy metals, and polycyclic aromatic hydrocarbons (PAHs). Four burning tests were conducted per moisture treatment using the open chamber method. Gaseous emission profiles of carbon monoxide (CO), sulfur dioxide (SO2), nitrogen monoxide (NO), and nitrogen dioxide (NO2) were also measured. Additionally, combustion characteristics, including burning stages, durations, combustion efficiency, temperature, and relative humidity, were recorded. Burning tests showed flaming and smoldering stages were significantly longer in 20% moisture treatment (P<0.05) compared with the rest. The amount of burned straw and ashes decreased with increasing straw moisture content (P<0.001). Carbon dioxide was the main product obtained during combustion with emission values ranging from 692 g CO2 kg dry straw-1 (10% moisture content) to 835 g CO2 kg dry straw-1 (20% moisture content). Emission factors for PM were the highest in 20% moisture treatment (P<0.005). Fine PM (PM2.5) accounted to more than 60% of total PM mass. Emission factors for dioxins increased with straw moisture content, being the highest in 20% moisture treatment, although showing a wide variability among burning tests (P>0.05). Emissions factors for heavy metals were low and similar among moisture treatments (P>0.05). Emission factors for individual PAHs were generally higher in 20% moisture treatment. Average SO2 and NO emissions decreased with moisture content. In contrast, CO emissions increased with moisture content. Overall, emission factors of atmospheric pollutants measured in our study were higher in the 20% moisture content. This difference could be attributed to the incomplete combustion at higher levels of rice straw moisture content. According to our results, rice straw burning should be done after straw drying and under minimal moisture conditions to lower pollutant emission levels.
