8. Combustion Characteristics

Mineral concentrations are reported to be low at the time of the early spring harvest: 0.09-0.34% N; 0.37-1.12% K; 0.03-0.21% Cl (Lewandowski and Kicherer 1997). Other reports suggest that miscanthus has low mineral and ash content compared with other lignocelluolosic species, and that the ratio of useful fertilizer nutrients to heavy metal contaminants in cyclone and grate ashes makes miscanthus, hay, and hemp ash more useful than wood ash (Hasler et al. 1998). The mineral content is low compared with wheat straw, and comparable with willow/poplar coppice. Like other biomass fuels, reactivity/ignition stability is high compared with coal. Overall, the CO₂ balance shows a 90% reduction in emissions compared with coal combustion (Lewandowski et al. 1995).

The composition of miscanthus ash includes approximately 30-40% SiO₂, 20-25% K₂O, 5% P₂O₅, 5% CaO, and 5% MgO -- a range of values is reported from different studies (e.g., Moilanen et al. 1996; Hallgren and Oskarsson 1998). Ash behavior (sintering) is no worse than many other biomass ashes, with potassium content a significant factor. Choice of thermal process may be more important (e.g., gasification with gas clean-up) (Moilanen et al. 1996). Sintering of ash under fluidized-bed gasification may cause agglomeration (or, at worst, alkali-induced defluidization). Miscanthus ash showed clear sintering tendencies at temperatures as low as 600 C, compared with reed canary grass and willow (the latter of which was inert up to 900 C). This may be due to the combination of relatively high silica content in miscanthus together with potassium and fluxing agents such as iron (Hallgren and Oskarsson 1998). However, sintering can be controlled by replacing common fluidized bed materials (silica sand) with calcium-based materials, such as dolomite.

Miscanthus has been successfully burned on a commercial scale in Denmark, using a 78-MW circulating fluidized bed combustor (50% co-firing with coal) and a 160-MW powdered fuel combustor (20% co-firing). The plants were already adapted for co-firing with straw: 17 t of miscanthus bales (Heston type, 450 kg, 12% moisture) were burned without major problems in the fluidized bed combustor, and 100 t in the powdered fuel combustor (Visser 1996).