Interchangeability of various agricultural residues in small-scale boilers based on their inorganic composition: combustion similarities, environmental impact and ash properties

Agricultural processing leaves behind many residues that could be further utilized, for example for heating. Among currently unused crop residues are those of Melilotus albus , Medicago sativa , Medicago lupulina , Sinapis alba , Hordeum vulgare and Trifolium incarnatum . Various samples of these plants were pelletized for the purposes of this study. All pellets were subsequently thoroughly analyzed and then combusted in a 25 kW boiler. Several similarities in their combustion behavior were observed: no slagging, high PM emissions (due to high K and Cl content), high NO x emissions and the production of soft, powdery ash rich in nutrients. The ash elemental composition of the raw fuels plotted in an SCK ternary diagram indicated such behavior was likely to occur. This method could thus be utilized by boiler operators when looking for possible fuel replacement or supplementation. A widely used biofuel of similar inorganic composition, in this case sunflower husks, was used to successfully confirm this hypothesis. The use of fertilizers on fodder crops increases the environmental impact of their subsequent combustion, however, the high retention rates of essential plant nutrients in the produced ash make it a valuable commodity in circular economy. • Fodder crop residues can be used as supplementary feedstock to similar biofuels. • The SCK ternary diagram serves as an accurate indicator of biofuel behavior. • CK and K-group fuels produce quality soft and powdery ash rich in nutrients. • CK and K-group fuels are prone to high PM emissions. • Bottom-feed retort burners are recommended for firing CK/K-group fuels.