The demand for increased overall efficiency, improved fuel flexibility, and more stringent environmental legislations promotes the development of new fuel- and technology-related concepts for the bioenergy sector. Previous research has shown that careful consideration of the fuel ash composition and the adjustment of the same via various routes, i.e., fuel design, have the potential to alter the ash transformation reactions, leading to, e.g., a reduction of the formation of slag or entrained inorganic ash particles. The objective of the present work was, therefore, to demonstrate the use of fuel design as a primary measure to reduce the emission of PM1 during combustion of woody biomass in medium-scale grate-fired boilers while keeping the slag formation at a manageable level. This was achieved by designing fuel blends of woody biomass with carefully selected Scandinavian peats rich in Si, Ca, and S. The work includes results from three experimental campaigns, performed in three separate grate-fired boilers of different sizes, specifically 0.2 MWth, 2 MWth, and 4 MWth. In one of the campaigns, softwood-based stemwood pellets were copelletized with different additions of peat (5 and 15 wt %) before combustion. In the other campaigns, peat was added in a separate fuel feed to Salix chips (15 wt % peat) and softwood-based stemwood pellets (10 and 20 wt % peat). Particulate matter and bottom ashes were characterized by scanning electron microscopy-energy-dispersive X-ray spectroscopy for morphology and elemental composition as well as by powder X-ray diffraction for crystalline phase composition. The results show that the fuel design approach provided PM1 reduction for all fuel blends between 30 and 50%. The PM1 reduction could be achieved without causing operational problems due to slagging for any of the three commercial boilers used, although an expected increased slagging tendency was observed. Overall, this paper illustrates that fuel design can be implemented on an industrial scale by achieving the desired ash transformation reactions, in this case, leading to a reduction of fine particulate emissions by up to 50% without any operational disturbances due to slag formation on the grate.

Molten compositions in the CaO-K₂O-SiO₂ system relevant to woody biomass ash slags were simulated with molecular dynamics to extract structural characteristics. Multivariate analysis elucidated correlations of these structural characteristics with viscosity measurements. The simulations show SiO₄/silicate tetrahedral units (STUB) diffusing slowly and forming flexible networks via oxygen bridges. The degree of STU polymerization varies linearly with the (K₂O + CaO)/SiO₂ ratio. Ca depolymerises stronger than K, but K diffuses quicker. Depolymerization and diffusion cause network disruptions and agitations that promote collective atomic mobility of the system. This imposes structural characteristics in the slag that correlate with viscosity. The inter-STU Si-O-Si angle narrows with decreasing viscosity, while the Si-O bond length of these bridges increases. Attributes related to atomic mobility, such as the variations in the Si-O-Si angle and the distance of nearest Si-Si pairs, also correlate with viscosity. The discussion provides insight into the connection between structural characteristics and viscosity.

Biomass fuels with calcium and potassium as the main ash-forming elements are expected to form ash consisting mainly of carbonates and oxides. These carbonates are stable in a rather narrow temperature range, which in turn depends on the Ca/K ratio, as well as on the surrounding atmosphere. The objective of the present study was to perform a detailed characterization of ash formation and transformation at a single-pellet level during combustion of silicon-poor woody biomass fuel. Combustion tests were performed with poplar in a single-pellet isothermal thermogravimetric analyzer operated at different temperatures and atmospheres and quenched at different stages of fuel conversion. The char and residual ashes were characterized for morphology and chemical composition. The focus of the experimental work in this study was on the time (conversion) resolved ash formation and transformations at the late part of the char combustion phase. Thermodynamic equilibrium calculations were used both to design the experiments and to support the interpretation of experimental results. It was concluded that carbonates were, in general, stable at low temperatures (here, 600–800 °C), identified as CaCO₃, K₂Ca₂(CO₃)₃, and K₂Ca(CO₃)₂, and decomposed at higher temperatures. In addition, a combined carbonate and phosphate phase in the form of carbonate apatite, Ca_9.9(PO₄)₆(CO₃)_0.9, was also found, mainly at lower temperatures. However, for char/ash samples quenched before full conversion, CaCO₃ was still found at temperatures higher than expected, possibly explained by the stabilizing effect of locally higher CO₂ partial pressure within the burning fuel particles. Thus, the results of the present study provide new insights into conversion-based ash formation and transformation in a burning fuel particle with relevance for combustion of Si-poor woody biomass fuels.

Ring deposits are common problems in rotary kiln operations. The ring is constantly subjected to thermal and mechanical wear counteracting the growth of the ring. If the ring hardens or if the growth of the ring is too rapid the kiln needs to be shut down and the ring removed, reducing the operational time and profitability of the process. In the present study, ring deposits from a limestone fed long rotary kiln producing quicklime was sampled and characterized in detail by SEM-EDS, dynamic rate TG and XRD. This work identifies three hardening mechanisms active in the kiln, an increased densification of the ring deposits near the refractory surface, the formation of calcite and spurrite through carbonation of the ring deposits, and the intrusion of molten fuel ash and product into the refractory, resulting in a strong attachment of the deposit to the refractory surface. The work also concludes that a significant part of the ring deposit has its origin in the fuel ash, contributing to deposit mass and increasing ring growth rate.

The use of small- and medium-scale combustion of biomass for energy utilization is expected to grow in the coming decades. To meet standards and legislation regarding particle emissions and to reduce corrosion and deposit formation, it is crucial to reduce the release of alkali species from the fuel. This can be achieved by capturing the volatile alkali in the residual bottom ash as more thermally stable compounds. In this work, we investigate the combination of primary measures, i.e., process parameters and fuel additives, for reduction of the release of K and Na from the fuel bed during fixed bed combustion. In addition, the influence of these combined measures on fine particle emissions was explored. The results showed a clear influence of the process parameters, herein bed temperature, and that a significant reduction of the alkali release and PM1 emissions can be achieved by correct settings. Furthermore, the application of additives (kaolin and diammonium sulfate) reduced both K and Na release even further. The observed effects on the release behavior was mainly explained by the formation of KAlSiO4 and K2SO4 during addition of kaolin and diammonium sulfate, respectively. This work therefore emphasizes the importance of good control over the fuel bed conditions, especially temperature, when these additives are applied. To reduce the potential deactivation (for kaolinite) and melting (for K2SO4), the control of bed temperature is vital. Thus, it was concluded that the release of volatile alkali species and related fine particle emissions in small- and medium-scale biomass heat and power plants using wood fuels could be significantly reduced by a correct combination of controlling the combustion parameters and the use of fuel additives.

Zirconia is known for its high strength but lacking translucency. Recently, a new type of high translucent zirconia, 5 mol% yttria partially stabilized zirconia (5Y-PSZ), with a larger fraction of cubic zirconia phase has become commercially available. However, the resistance to aging of these commercially available zirconia materials is not yet fully established.

Purpose: The aim of the present study was to analyze the effects of artificial aging on surface roughness, transparency, phase transformation and biaxial flexural strength of two 5Y-PSZ products, DD cubeX2 and Prettau Anterior.

Materials and methods: The artificial aging was performed in an autoclave under 2 bars of pressure at 134 °C for 10 hours, which is estimated to correspond to 30–40 years in vivo. Artificial aging for 10 hours had no significant effect on surface roughness, transparency, or phase transformation for either of the tested materials.

Results: DD cubeX2 had higher mean flexural strength than Prettau Anterior both before and after artificial aging for 10 hours (p < .05). DD cubeX2 showed, however, a significant reduction in flexural strength after artificial aging (p < .05), whereas Prettau Anterior showed a slight increase in flexural strength after artificial aging but not at a significant level.

Conclusion: Within the limitation of the present study, both DD cubeX2 and Prettau Anterior seems to be relatively resistant to aging. However, a wider range of measured flexural strength indicated that Prettau Anterior probably is a less stable material than DD cubeX2, which also means that the flexural strength of DD cubeX2 could be more predictable.

Finding resilient refractory materials for slagging gasification systems have the potential to reduce costs and improve the overall plant availability by extending the service life. In this study, different refractory materials were evaluated under slagging gasification conditions. Refractory probes were continuously exposed for up to 27 h in an atmospheric, oxygen blown, entrained flow gasifier fired with a mixture of bark and peat powder. Slag infiltration depth and microstructure were studied using SEM EDS. Crystalline phases were identified with powder XRD. Increased levels of Al, originating from refractory materials, were seen in all slags. The fused cast materials were least affected, even though dissolution and slag penetration could still be observed. Thermodynamic equilibrium calculations were done for mixtures of refractory and slag, from which phase assemblages were predicted and viscosities for the liquid parts were estimated.

To stop the net emission of CO₂ to the atmosphere, we need to reduce our dependency of fossil fuels. Although a switch to a bio-based feedstock hardly can replace the total amount of fossils used today, utilization of biomass does still have a role in a future in combination with other techniques. Valuable chemicals today derived from fossils can also be produced from biomass with similar or new technology. One such technique is the entrained flow gasification where biomass is converted into synthesis gas. This gas can then be used as a building stone to produce a wide range of chemicals.

Slagging and corrosion problems are challenges presented by the ash forming elements in biomass during thermochemical energy conversion. The high temperature in the entrained flow process together with ash forming elements is creating a harsh environment for construction materials in the reactor. Severe corrosion and high wear rates of the lining material is a hurdle that has to be overcome to make the process more efficient.

The objective of this work is to investigate the nature of the destructive interaction between ash forming elements and refractory materials to provide new knowledge necessary for optimal refractory choice in entrained flow gasification of woody biomass. This has been done by studying materials exposed to slags in both controlled laboratory environments and pilot scale trials. Morphology, elemental composition and distribution of refractories and slag were investigated with scanning electron microscopy and energy dispersive X-ray spectroscopy. Crystalline phases were investigated with X-ray diffraction, and thermodynamic equilibrium calculations were done in efforts to explain and make predictions of the interaction between slag and refractory.

Observations of slag infiltration and formation of new phases in porous materials indicate severe deterioration. The presence of Si in the materials is limiting intrusion by increasing the viscosity of infiltrated slag. This is however only a temporary delay of severe wear considering the large amount of slag that is expected to pass the refractory surface. Zircon (or zirconium) (element or mineral?) based material show promising properties when modeled with thermodynamic equilibrium, but disassembling of sintered material and dissociation of individual grains was seen after exposure to a Si- and Ca-rich slag. Fused cast materials have a minimal slag contact where the only interaction is on the immediate hot face. Dissolution was however observed when exposed to a silicate-based slag, as was the formation of NaAlO₂ after contact with black liquor.

Pressurized entrained-flow gasification (PEFG) of bark and a bark/peat mixture (BPM) was carried out in a pilot-scale reactor (600 kW_th, 7 bar(a)) with the objective of studying ash transformations and behaviors. The bark fuel produced a sintered but nonflowing reactor slag, while the BPM fuel produced a flowing reactor slag. Si was enriched within these slags compared to their original fuel ash compositions, especially in the bark campaign, which indicated extensive ash matter fractionation. Thermodynamically, the Si contents largely accounted for the differences in the predicted solidus/liquidus temperatures and melt formations of the reactor slags. Suspension flow viscosity estimations were in qualitative agreement with observations and highlighted potential difficulties in controlling slag flow. Quench solids from the bark campaign were mainly composed of heterogeneous particles resembling reactor fly ash particles, while those from the BPM campaign were flowing slags with likely chemical interactions with the wall refractory. Quench effluents and raw syngas particles were dominated by elevated levels of K that, along with other chemical aspects, indicated KOH(g) and/or K(g) were likely formed during PEFG. Overall, the results provide information toward development of woody biomass PEFG and indicate that detailed understanding of the ash matter fractionation behavior is essential.