Temporal temperature measurement on burning biomass pellets using phosphor thermometry and two-line atomic fluorescence

We report accurate in-situ optical measurements of surface temperature, volatile gas temperature, and polycyclic aromatic hydrocarbon (PAH) emission over the whole burning history of individual biomass pellets in various combustion atmospheres. Two biomass fuels, wood and straw, were prepared in cylindrical pellets of ~300 mg. The pellets were burned in a well-controlled combustion atmosphere provided by a laminar flame burner with temperature ranging from 1390 K to 1840 K, and oxygen concentration from zero to 4.5%. The surface temperature of burning biomass pellets was accurately measured, for the first time, using phosphor thermometry, and the volatile gas temperature was measured using two-line atomic fluorescence thermometry. PAH emission was monitored using two-dimensional laser-induced fluorescence. During the devolatilization stage, a relatively low surface temperature, ~700 K, was observed on the burning pellets. The volatile gas temperature was ~1100 K and ~1500 K 5 mm above the top of the pellets in a gas environment of ~1800 K with 0.5% and 4.5% oxygen, respectively. PAH mainly released when the temperature of the pellet exceeded ~600 K with the highest concentration close to the surface and being consumed downstream. The weight of the released PAH molecules shifted towards lighter with a reduction of gas environment temperature. The wood and straw pellets had almost the same surface and volatile gas temperature but different compositions in the released volatile gases. The temperature information provided in the present work aids in revealing the reactions in the burning biomass fuels regarding species release, such as various hydrocarbons, nitrogen compounds, and potassium species, and is valuable for further development of biomass thermal conversion models.