(60d) Ash Partitioning and Deposition Mechanisms in Rice Husk Combustion

Utilization of biomass when combined with carbon capture and sequestration (CCS) is one feasible solution for ‘Bio-energy with carbon capture and storage’ (BECCS), which can reduce the CO₂ emissions in the atmosphere. Compared to other biomasses, rice husk is very unique because its ash content is typically higher than 15% and more than 88% of these mineral matters are composed of silica. This work mainly focus on the investigation of ash partitioning and deposition mechanisms during rice husk combustion.

Two different rice husk resources were used in this work, one from China and another one from United States, which are abbreviated as RH_CN and RH_US separately. The experimental work was performed in a 100 kW down-fired oxy-fuel combustor (OFC) and both fuels were co-fired with natural gas in order to achieve combustion conditions that are comparable with coal combustion. Two conditions were tested: 1) air combustion; 2) oxy-combustion with 70% O₂ and 30% CO₂ in oxidant gas (denoted ash OXY70). For the ash aerosols, the particle size distribution and size segregated composition were investigated though multiple techniques (e.g. SMPS, APS and BLPI). For the ash deposits, the formation rates were studied though a temperature-controlled deposition probe. It was found similar submicron accumulation modes and supermicron fragmentation modes appeared for both rice husks. Although the composition of supermicron particles were similar and enriched in silica for both rice husks, the submicron particles from RH_CN are mainly composed of volatile elements (K, Na, Cl and P), while these from RH_US are mainly composed of Si. This composition difference in submicron aerosols subsequently affects the ash deposit formation, for example, the tightly bound inside deposits from RH_CN have much more volatile elements than that from RH_US. More importantly, apparent ash shedding on deposits occurred after 0.5 hour for RH_CN, but this cannot be observed in RH_US until the maximum sampling time (2 hours). The reason causing the difference in ash partitioning and deposition between these two rice husks will be discussed in this paper.