Due to concerns for climate change and future supply of phosphorus fertilizer within agriculture, there has been an increased interest in the combustion of phosphorus containing waste residues and opportunity biomass fuels. Previous research has shown that during combustion, phosphorus has large impact on ash transformation reactions and may decrease or increase ash-related problems such as slag formation and bed agglomeration. This is a serious concern if new types of biomass are to be added for heat and power production. Additionally, plant studies and leaching tests of P-rich biomass ash indicate that the plant availability of phosphorus varies greatly with its association in the ash. As such, the ash transformation behavior of phosphorus is of great importance for the success of such ventures. While several studies have been made on the behavior of phosphorus during combustion, no comprehensive study has been made evaluating the effect of fuel composition and combustion conditions. In this work, the behavior of phosphorus was determined for a wide range of fuels and combustion conditions. More specifically, the objective was to determine (i) the effect of fuel ash composition and combustion technologies on the fate of phosphorus during combustion, (ii) investigate potential difference in the behavior of phosphorus during combustion of sewage sludge and plant based biomassand (iii)the effect of phosphorus on slag formation and bed agglomeration for the co-combustion of a wide range of plant based biomasses. The investigation was carried out by comparing experimental data gathered from the combustion of 26different biomass fuelsor fuel blends in a bench scale bubbling fluidized bed (5 kW, 18 experiments), an underfed pellet burner (20kW, 10 experiments) and a swirling powder burner (150 kW, 7 experiments). This included chemical characterization of bed ash, bottom ash and fly ash fractions by X-ray diffraction (XRD), scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS) in addition to qualitative measures of slagging-and bed agglomeration tendencies. It was found that phosphorus, irrespective of combustion technology and fuel composition, was mainly found in bed-, and bottom ash fractions and/or coarse fly ash fractions (>1μm). Based on the crystalline phase composition of the phosphates found in bed-, bottom-and coarse fly ash samples, phosphate speciation was correlated to the molar ratio between P, Ca and Mg for all three combustion technologies. Based on these results, it would be possible to control the behavior of phosphorus during combustion and the plant availability of phosphates in biomassash by designing fuel blends based on their fuel ash composition. In fluidized bed combustion, it was found that for similar combustion conditions and fuel ash compositions (with respect to K, Ca and P), the speciation of phosphorus in coarse ash fractions was significantly different from experiments with plant based biomass compared to sewage sludge. Unlike ash from plant based biomass, the crystalline phase composition of ash from sewage sludge did not change with the relative concentration of K, Ca andP in the fuel. The results suggest that the reaction pathway of phosphorus during combustion of sewage sludge is different to plant based biomass due to difference in the association of phosphorus in the fuel. The effect of phosphorus on slag formation and bed agglomeration in biomass combustion was mainly related to the relative fuel ash concentration of K, Ca, Mg, Si and P. In fluidized bed combustion, P contributes to the formation of agglomerates through the melt induced mechanism, through complex interaction with K, Ca, Mg and Si. Similarly, in fixed bed combustion the composition of slag indicatedthatslag formation involves the formation of P and Si rich ash melt with a varying content of K, Ca and Mg. In both cases, the severity of problems was related to the melting behavior of the (CaO,MgO)-K2O-(SiO2,P2O5) multicomponent system.
| Identifer | oai:union.ndltd.org:UPSALLA1/oai:DiVA.org:ltu-71240 |
| Date | January 2018 |
| Creators | Falk, Joel |
| Publisher | Luleå tekniska universitet, Energivetenskap, Luleå |
| Source Sets | DiVA Archive at Upsalla University |
| Language | English |
| Detected Language | English |
| Type | Licentiate thesis, comprehensive summary, info:eu-repo/semantics/masterThesis, text |
| Format | application/pdf |
| Rights | info:eu-repo/semantics/openAccess |
| Relation | Licentiate thesis / Luleå University of Technology, 1402-1757 |
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