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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.

Fuel drying is an energetically and economically expensive pretreatment process, which may not be worth the investment in the case of small-scale generation plants. This paper presents an investigation on the air dryer feasibility to enhance the operation of biomass steam boiler. In the proposed approach, the external drying technology using preheated air and the biomass steam production system is modelled in terms of energy and an economical analysis. A focus is given to the system size influence on the dryer economic suitability: the smallest size of the biomass combustion system for which fuel drying is a suitable solution, from the economic point of view, is computed. In the computations, the heat used for drying is assumed to be part of the cost for operating the dryer and the thermal balance of the system is assumed to be previously verified. According to the model results, if the steam production plant operational time is above 8000 h/y, wood chips drying is feasible if the system size is larger than 1.78 tdaf/h of fuel processed.