Effects of biomass briquetting and carbonization on PM2.5 emission from residential burning in Guanzhong Plain, China

In this study, maize straw, wheat straw, and wood branches and their processed products (i.e., briquettes and charcoals) were used to investigate the emission factors (EFs) of PM2.5 components and the influence of form changing. The EFs of organic carbon (OC) was 241 +/- 39.7 to 3920 +/- 792 mg kg(-1); OC was the largest contributor in PM2.5. The EFs of PM2.5, OC and elemental carbon (EC) demonstrated that charcoal was the most efficient in reducing PM2.5 and carbonaceous fraction emissions, followed by briquettes compared with raw fuels in residential burning use. Among cations, K+ had the highest EF, whereas Cl- has the highest EF among anions; the presence of both K+ and Cl- was attributable to high abundance in biomass. Cl, K, S, and Na (in descending order) contributed more than 95% of total elements with high EFs; the other elements, particularly heavy metals, had very low EFs. The EFs of total polycyclic aromatic hydrocarbons (PAHs) and saccharides were in the following order of magnitude: raw fuels > briquettes > charcoals. PAHs with four rings (i.e., pyrenes) dominated the total PAHs in PM2.5-, whereas among saccharides, levoglucosan had the largest EFs (0.03-110.41 mg kg(-1)). Briquettes, with simple process and reasonable cost, demonstrated a good ability in decreasing PM2.5 emission with potential of 53% (765,000-427,000 t year(-1)), and their emission reduction potential were much higher for straws than for branches. Charcoals demonstrated considerable emission reduction potential for straw (93.7% averagely for PM2.5 and compositions) and comparable for wood branches (80% averagely) while its complex process and high cost should be considered. More than 600,000 t of PM2.5 and thousands of tons of PAHs and saccharides are estimated to be eliminated annually if all raw biomass fuels are processed through carbonization. Considering the efficiency-cost ratios, biomass briquetting may aid clean heating in rural areas, in addition to the "natural gas and electricity replacement strategy," in China and other countries facing similar problems