Composition Of Organic Carbon/Elemental Carbon, Water-soluble Ions And Organic Matter From Crop Straw Combustion

Emission from major agricultural residue was measured through laboratory simulations using a self-designed combustion system. Emission factors (EFs) of organic carbon (EFoc), element carbon (EFEC), eight ions (K+, NH4+, Na+, Mg2+, Ca2+, Cl-, NO3-,SO42-) and organic comprounds were measured using Thermal Optical Carbon Analyzer (Model 2001A), Ion Chromatograph (ISC 2000/ISC 3000) and Gas chromatography-Mass spectrometry (GC-MS) for burning wheat and rice straw in flaming and smoldering conditions. The results indicated that:The mean values of EFOC and EFEC were 9.24,2.24 g·kg-1 (wheat straw), and 6.37, 1.07g·kg-1(rice straw) in flaming condition, while 40.8,5.8g·kg-1 and 37.63,4.98g·kg-1 in smoldering condition. EFs of particulate matter (PM), OC, and EC in flaming smoke were evidently lower than those in smolder though varying trend of curves for EFPM, EFoc and EFEC under both flame and smolder were same. Corresponding average ratios of OC/EC for wheat and rice straw in flame were 4.04,5.96, and 7.07,7.80 in smolder, respectively. OC and EC contents in the PM mass were 43.1%,10.5% for wheat straw and 47.28%,7.94% for rice straw in flame, while 44.7%,6.4%; 46.79%,6.2% in smolder, respectively. The combustion status had no impact on the proportion of OC in PM, whereas, values of EC/PM varied significantly depending on combustion types, which were higher in flaming condition. Additionally, whether the measured data for two combustion type were plotted or together the correlation between OC and EC were significant (p<0.01, R2=0.9 for wheat straw; p<0.01, R2=0.95 for rice straw).Water-soluble ions emitted from crop straw burning were dominated by Cl- and K+, contributing 3.4%,2.4% in PM mass in flaming condition for rice straw and 2.2%,1% for wheat straw, respectively, much higher than the ratios measured under smoldering condition. The corresponding EFs of Cl- and K+ were 0.726,0.514 g·kg-1 for wheat straw and 0.246,0.118 g·kg-1 for rice straw in flame, while 0.418,0.120 g·kg-1 and 0.301,0.053 in smolder, respectively. Na+, Mg2+ and NH4+ were also vital in PM, and the total proportion in the PM mass ranged from 0.8% in smolder (wheat straw) to 3.14% in flame (rice straw). No matter which cultivar of crop straw it was, EFs of K+ in flaming smoke were always higher than that in smolder, while EFs of Na+ in flaming always lower than that in smolder. Strong correlations between the concentrations of Cl-with K+ and NH4+ were observed in rice straw in flame, and it appeared that cations mostly existed as chloride. Additionally, the ratios determined of OC/EC, K+/Na+, Cl-/Na+ of crop straw were much less than that measured from trees burning, and the OC/EC ratios were elevated in comparison to fossil fuels. The distribution might be useful in distinguishing this source from other biomass source and fossil fuels burning.High concentrations of carbohydrates in PM were detected in the smoke samples of crop straw burning, and levoglucosan was the most abundant anhydrosugar. The concentrations and EFs of carbohydrates in flaming smoke evidently lower than those in smolder, and the highest ratio of EFs under flame and smolder was 17 while the levoglucosan/OC ratio makes a great difference in various combustion sources. Sterols were present in all smoke samples, they were generally comprised of Campesterol, stigmasterol and β-sitosterol in PM of rice straw straw burning, and the β-sitosterol was the major compound among the sterol which detected in the smoke samples. The EFs of sterols in smolder higher than the values detected in flaming condition. EFs of Alkanes varied significantly depending on the cultivars of crop straw. The change of EFs in rice straw in two combustion condition was similar to Sterols and carbohydrates though the same varying tendency for EFs of Alkanes under both flame and smolder. Whereas, the values of Alkanes varied distinctly depending on cultivars of crop straw. The concentrations of 2,6-dimethoxy phenol and Acetosyringone were abundant in crop straw under flaming condition. The content of phenolic compounds in smodering smoke were evidently lower than those in flame, while the EFs phenolic compounds in smodering comdition were significantly higher than the value detected in flame.