Co-combustion of miscanthus and calcium rich brown macroalgae
Conference contribution, 2016
The high ash content and varying ash composition from aquatic biomass is often mentioned as problematic if used for thermal energy conversion. This paper suggests a fuel design approach where detailed information on ash composition is the starting point for mixing and using fuels considered to be difficult. The procedure is demonstrated on brown macroalgae grown for biorefinery purposes in sea water. The fuel fingerprint (concentrations of the main ash forming elements) showed an interesting profile with very high Ca content together with significant amounts of Mg, K, Na, Cl, S, and also some minor contributions from Si and P. After careful considerations, it was concluded that this specific alga would be suitable for co-combustion with a silicone rich biofuel that would typically require some additive to avoid ash melting. One such fuel is Miscanthus.
The aim of this study was to evaluate and compare algae as a renewable source of Ca with mineral CaCO3 to reduce the risk of alkali silicate melt formation in combustion of the energy crop Miscanthus. The Miscanthus was co-pelletized with algal biomass and CaCO3, both at Ca/(K+Na) molar ratios of 1.5 and 3.0, and combusted in a bubbling fluidized bed. in. The ash reactions were assessed by analyzing samples from bed, deposit probe, cyclone, and particulate matter with SEM-EDS and P-XRD.
The results showed that Ca from the algae reacted with the Miscanthus ash, forming less problematic silicate ash fractions. At the low combustion temperatures used (< 720°C) stable CaSO4 was formed, capturing some of the S that would otherwise have been available for alkali sulfation. Comparing the Ca rich algae with adding pure CaCO3 to the Miscanthus pellets indicated that the Ca in the algae ash was more prone to react with the K-silicate, and thereby more efficiently preventing ash melting.