Towards Miscanthus combustion quality improvement: the role of flowering and senescence

In commercially grown Miscanthus x giganteus, despite imposing a yield penalty, post-winter harvests improve quality criteria for thermal conversion and crop sustainability through remobilisation of nutrients to the underground rhizome. We examined 16 Miscanthus genotypes with different flowering and senescence times for variation in N, P, K, moisture, ash, Cl, and Si contents, hypothesising that early flowering and senescence could result in improved biomass quality and/or enable an earlier harvest of biomass, i.e. in autumn at peak yield. Ideal crop characteristics at harvest are low N and P to reduce future fertiliser inputs, low K and Cl to reduce corrosion in boilers, low moisture to reduce spoilage and transportation costs, and low Si and ash to reduce slagging and consequent operational downtime. Stems and leaves were harvested during: summer, autumn, and the following spring after overwinter ripening. In spring, stem contents of N were 30 to 60 mg kg-1, P were 203-1132 mg kg-1, K were 290-4098 mg kg-1, Cl were 10 to 23 mg kg-1, and moisture were 12-38%. Notably, late senescence resulted in increased N, P, K, Cl, moisture and ash contents, and should therefore be avoided for thermochemical conversion. Flowering and senescence led to overall improved combustion quality, where flowered genotypes tended towards lower P, K, Cl, and moisture contents; marginally less, or similar, N, Si and ash contents; and a similar HHV, compared to those that had not flowered. Such genotypes could potentially be harvested in the autumn. However, one genotype that did not flower in our trial exhibited sufficiently low N and K content in autumn to meet the ENplus wood pellet standards for those traits, and some of the lowest P, moisture and ash contents in our trial and is thus a target for future research and breeding.