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.

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.

Black liquor gasification is dependent on minimizing heat loss to the surroundings and thus needs to be well insulated. In combination with the high temperature and basic black liquor, a very corrosive environment is created on the hot face of such a reactor. Therefore the wall system is required to be chemically and thermally stable at the same time as it has insulating properties. These cannot easily be combined in the same material and therefore layers with different properties can be used in combination. Penetration of species through the lining can lead to further reactions with other construction materials, less suited for chemical resistance, corrosion of the pressure shell is an example with catastrophic consequences. This paper investigates two castable and one fused cast spinel (MgAl₂O₄) refractory after about 1 600 hours, and one fused cast material used for 15 000 hours of operation in a 3 MW_th black liquor gasifier. Infiltration of Na, followed by destruction of microstructure, and extensive formation of NaAlO₂ was observed throughout the whole castable materials, while it was mainly restricted to the hot face of the fused cast materials. Formation of NaAlO₂ leads to a volumetric expansion which eventually lead to an increased pressure on the steel shell. In addition, the expansion of the bricks can cause stress and by that spallation and material loss.

To gain more knowledge about possibly destructive effects of ash-forming elements in woody biomass on refractory materials in entrained flow gasification, an exposure study was performed on two high alumina refractories. The materials, a pre-fired castable consisting of about 63 weight-% Al₂O₃, and a phosphate bonded brick with 83 weight-% Al₂O₃ was exposed to synthetic ash mixtures at 1050°C and 1 atm CO₂ for 7 days. This paper presents distribution of ash-forming elements and morphology of the samples microstructure, while identification and distribution of crystalline compounds is presented in a separate paper. In the samples, potassium (K) had infiltrated the materials and reacted with different components, while calcium (Ca) did not seem to have any direct effect during these conditions. The matrix of the castable absorbed much K, became clogged and produced a distinct border between reacted and unaffected matrix. The coarser matrix of the phosphate bonded brick retained much of its porosity and had ash transported further into the material without a clear distinction between reacted and unaffected matrix. Grains with >30 atomic-% Si, formed a layer enriched in K, with a thickness up to 40 µm and cracks propagating through it. Grains mainly consisting of Al₂O₃ seemed unaffected by the exposure. When the ash was rich in SiO₂, a melt was produced that restricted the attack on the refractories to the surface and coarser pores.

Two high alumina refractories, one brick and one pre fired castable was exposed to pure K₂CO₃, K₂CO₃ + CaCO₃, and K₂CO₃ + CaCO₃ + SiO₂ at 1050°C and a CO₂ atmosphere. A stratified investigation of crystalline phases was made with polycrystalline x-ray diffraction, and thermodynamic equilibrium calculations were performed to explore possible formation paths. A monoclinic polymorph of KAlSiO₄ was formed to a large extent in both materials exposed to pure K₂CO₃. Throughout the affected part of the castable and a small layer close to the surface of the brick, a solid solution between KAlO₂ and KAlSiO₄ formed, K_1-xAl_1-xSi_xO₂, x = 0.19. The affected area of the castable had 30-50 %_wt new phases and made a sharp transition to unaffected material. The concentration of new phases in the brick was decreasing at an even rate from about 40 to 15%_wt throughout the whole material thickness of 14 mm. Exposure to K₂CO₃ and CaCO₃ showed the same phases and behavior, but no Ca-bearing phases could be detected. The mixture containing K₂CO₃, CaCO₃ and SiO₂ did not penetrate far into the material but formed the same phases in the affected areas. Wollastonite (CaSiO₃) formed in the slag on top of these materials. The major mechanism for formation of new phases is suggested to be the formation of an initial melt composed of K₂O and SiO₂. This liquid is then dissolving refractory components and forms a liquid in equilibrium with KAlSiO₄ and K_1-xAl_1-xSi_xO₂.

Managing slag properties is of utmost importance for successful operation of entrained flow gasifiers. The present study details some aspects of slag formed from a softwood bark fuel, and especially the situation with only small amounts of mineral contaminants, meaning composition is shifted from Si- towards P-dominated ash. Wood bark with and without KHCO₃ additive was gasified between 850 °C and 1300 °C at O2 stoichiometric ratio (λ) 0.6 to study the resulting ash properties and the influence of the additive. The ash particles collided with a flat impact probe inside the hot reactor, with particle impact angles varied between 90° to 30°. The reactor and probe were constructed to allow for long-distance microscope data collection close to the surface of the probe. In situ PIV and SEM-EDS of deposit samples from lab scale entrained flow gasification experiments were used for evaluation, while XRD was used to characterize carbonates. High potassium release was found but numerous spherical ash particles indicated lower ash melting temperatures than expected from the bulk ash composition. These new findings propose a mechanism for melt formations involving carbonates rich in potassium and phosphorous, followed by K-release and calcination leading to solidification.