Characterization of PM2.5 and gaseous emissions during combustion of ultra-clean biomass via dual-stage treatment

Biomass combustion technology is widely considered as an attractive option to remove carbon dioxide emissions and thus could become a viable option for next generation co-combustion power plants with coal. Among biomasses, the faster growing herbaceous biomass (Miscanthus) is relatively inexpensive as a solid fuel but has higher mineral contents (especially high levels of alkali metals) than that of a lignocellulosic biomass (Pitch pine) which leads to ultrafine particle emissions during combustion. This study proposes a dual stage treatment method to extract the inherent minerals in a herbaceous biomass prior to the combustion test. The method consists of an initial treatment with a basic (NaOH) solution and a sequential treatment with an acidic (CH3COOH) solution. To verify the proposed method, we investigated the variations in both the mineral contents and the PM, NOx and SO2 emissions between raw- and treated samples. The experimental results show that sodium, potassium and chloride in the treated herbaceous sample, which are the major minerals that generate ultrafine particles, were removed up to 96.3, 99.4 and 98.3%, respectively. The proposed method is more effective for herbaceous biomasses due to its higher Brunauer–Emmett–Teller (BET) value. Furthermore, the treated herbaceous biomass is even environmentally superior to raw lignocellulosic biomasses currently used in co-combustion power plants with coal in terms of the PM2.5, NOx and SO2 emissions, for which their rejection was measured as 60, 25 and 46%, respectively. These results will help to reduce the energy and costs for De-NOx (selective catalytic reduction, SCR system) and De-SO2 (scrubber) units.