Experimental studies of ash transformation processes in combustion of phosphorus-rich biomass fuels

Abstract: The present thesis is a summary of eight papers dealing with experimental studies on bed agglomeration, slag formation, formation of deposits and fine particulate matter during combustion of phosphorus-rich biomass fuels. The experimental procedures were performed in a bubbling fluidized bed (5 kW), two fixed bed appliances (20 and 15 kW) and in a powder burner (150 kW). The phosphorus-rich fuels studied (separately and in mixtures with typical woody or straw biomass fuels) included; rapeseed meal, rapeseed cake, wheat distillers dried grain with solubles (DDGS) and, oat grains. Phosphoric acid (H3PO4 (PA)), kaolin (Al2Si2O5(OH)4) and calcite (CaCO3) were used as fuel additives. The bed materials used in fluidized bed experiments included, quartz (SiO2) and, olivine ((Mg, Fe)2SiO4). During fluidized bed combustion of the phosphorus-rich fuels; i.e. DDGS, rapeseed meal and, rapeseed cake, ash particles rich in K-Ca/Mg-phosphates were formed during combustion, leading to the formation of non-continuous bed particle layers and subsequently bed agglomerates. For woody fuels; i.e. logging residues, willow and, bark, K-compounds in gas/liquid phase reacted with the quartz bed material, and formed an inner bed particle layer rich in K-silicates. The melting behaviour of this layer was found responsible for the initiation of the bed agglomeration. The addition of a high enough amount of phosphorus to the woody fuels (by co-firing with a P-rich fuel or adding PA additive), to convert the available fuel ash basic oxides into phosphates, reduced the amount of K available for the reaction with the quartz bed material particles, thus preventing the formation of an inner reaction bed particle layer. The phosphate-rich ash particles formed during combustion adhered and reacted with the bed material forming non-continuous coating layers, and subsequently agglomerates. During combustion of straw fuels (rich in Si and K), partially molten K-silicates formed non-continuous bed particle layers and subsequently bed agglomerates. Adding phosphorus to the last fuel, changed the composition of the bed ash from being dominated by low melting temperature K-silicates, to a system dominated by crystalline K-Ca-phosphates. The phosphate-silicate ash particles formed during the combustion were found responsible for the initiation of the bed agglomeration process. No significant difference in the bed agglomeration tendency/characteristics was found between olivine and quartz bed materials when combusting the phosphorus-rich DDGS fuel. The bed agglomeration mechanism for this fuel in quartz bed therefore seems to be directly applicable in olivine beds, and can be described as direct adhesion of bed particles by partially molten K-Mg-phosphates in both bed materials. In fixed bed combustion of phosphorus-rich fuels, it was found that the relation between alkali and alkaline earth metals in the fuel ash has a key role in the slag formation. DDGS (rich in S, K, P and Mg), formed high amounts of molten material. Fuels with higher Ca content, i.e. rapeseed meal, showed low slagging tendency. The effect is attributed to the formation of low melting temperature K-Mg-phosphates, or more stable K-Ca/Mg-phosphates, respectively. The slag formed during combustion of woody and straw fuels, consisted mainly of K-rich silicates. The addition of phosphorus (by co-firing with a P-rich fuel or adding PA additive), promoted the formation of K-Ca/Mg-phosphates, thereby reducing the amount of K-rich-silicates formed during combustion. The formation and composition of deposits and fine particulate matter during combustion of phosphorus-rich fuels were also studied. In general, during fluidized bed and to a minor extent in fixed bed combustion, a reduction of fine particulate matter containing KCl as the main component was achieved by increasing the phosphorus content in woody or straw fuels. As a consequence, an increased amount of potassium was found in the coarse ash particle fractions principally as KMgPO4, CaK2P2O7, CaKPO4, and KPO3, while the levels of HCl and SO2 in the flue gases increased. It was found that the relationship between alkali and alkaline-earth metals (i.e., (K + Na)/(Ca + Mg)) in the overall fuel ash composition must be considered, since both Ca and Mg are needed for the formation of refractory ternary phosphate phases containing K. The addition of excessive amounts of phosphorus to P-poor fuels with high (K + Na)/(Ca + Mg) molar ratio resulted in the formation of low melting temperature alkali-rich phosphates, which increased the bed agglomeration tendency and release of alkali and phosphorus from the bed. Powder combustion of the DDGS-fuel resulted in the formation of high amounts of fine particulate matter and deposits rich in KPO3. During fixed bed combustion of oat grains, slag rich in K-silicates and fine particulate matter rich in K-phosphates and KCl was formed. The result of using kaolin additive was that no slag was formed, and the effect on the formation of fine particulate matter was an increased content of condensed K-phosphates at the expense of K2SO4 and KCl. Consequently, higher levels of HCl and SO2 in the flue gases were obtained. The addition of calcite increased the amount of slag formed. Phosphorus was captured to a higher degree in the slag and bottom ash, compared to the combustion of pure oat. The molten phase formed during combustion consisted of both phosphates and silicates and probably had a low melting temperature. The effect of the calcite additive on the fine particle emissions was that the content of KPO3 decreased considerably, while the content of K2SO4 and KCl increased. Consequently, the levels of HCl and SO2 in the flue-gas decreased. A general observation was that phosphorus is the controlling element in ash transformation reactions during biomass combustion because of the high stability of ternary phosphate compounds.