Fouling propensity of high-phosphorus solid fuels: Predictive criteria and ash deposits characterisation of sunflower hulls with P/Ca-additives in a drop tube furnace

• We propose a characterisation tool to describe the fouling of high-P solid fuels.
• The influence of H3PO4 and CaCO3 additives is studied in a drop tube furnace.
• Visual and SEM–EDX analyses are applied to the deposits.
• A proper addition of Ca-, P-based additives can reduce the sunflower hulls fouling.
• The experimental results confirm the added value of the characterisation approach.

Fouling from the processing of residual biomass fuels in combustion applications is a major concern. This paper discusses the fouling behaviour of sunflower hulls with a high phosphorus (P) content by means of a broad fuel characterisation strategy including advanced predictive indices, the fuel selective leaching, multiple deposition tests in a Drop Tube Furnace (DTF) and deposits analysis with scanning electron microscopy–energy dispersive X-rays spectroscopy (SEM–EDS). First, we summarise the P-role in the ash chemistry, with a focus on the fouling mechanisms. Second, a characterisation strategy of the ash, based on three indices, including some details from the fuel selective leaching, is proposed to describe the P-rich fuels propensity to foul. The developed approach could be used as a complement to chemical equilibrium models. Thirdly, the characterisation strategy is applied to sunflower hulls. Deposition tests in an industrial scale DTF are performed for the raw fuel, and for the fuel with phosphoric acid (H3PO4H3PO4) water solution and calcium carbonate (CaCO3CaCO3) as additives, to obtain different P/K and P/Ca ratios in the fuel composition. The results show that increasing the fuel P-content allows to capture the alkali metals in alkali–alkaline earths–phosphates and alkali–phosphates phases, reducing the occurrence of deposits of S- and Cl-compounds. Low melting temperature phases can be reduced enhancing the formation of coarser, high melting temperatures ash particles formed by K/Na–Ca/Mg–phosphates, by means of an optimised addition of phosphorus- and active calcium-based additives. The experimental results confirmed the added value of the high-P fuels predictive characterisation strategy.