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Oxy-fuel biomass combustion can facilitate carbon capture in heat and power plants and enable negative carbon dioxide (CO₂) emissions. We demonstrate oxy-fuel combustion (OFC) of softwood powder in a 100-kW atmospheric down-fired pulverized combustor run at a global oxidizer-fuel equivalence ratio of around 1.25. The simulated oxidizer was varied between oxygen (O₂)/CO₂ mixtures of 23/77, 30/70, 40/60 and 54/46, and artificial air. The concentrations of the main gaseous potassium (K) species: atomic K, potassium hydroxide (KOH) and potassium chloride (KCl), were measured at two positions in the reactor core using photofragmentation tunable diode laser absorption spectroscopy (PF-TDLAS). Major species were quantified by TDLAS in the reactor core and with Fourier transform infrared spectroscopy and mass spectrometry at the exhaust. Flue gas particles were collected at the exhaust employing a low-pressure impactor and analyzed by X-ray powder diffraction and scanning electron microscopy. The measured individual K species concentrations in the reactor core agreed with predictions by thermodynamic equilibrium calculations (TEC) within one order of magnitude and the sum of K in the gas phase agreed within a factor of three for all cases. Atomic K was underpredicted, while the dominating KOH and KCl were slightly overpredicted. The ratios of measured to predicted total K were similar in artificial air and OFC, but the distributions of the individual species differed at the upper reactor position. The gaseous K species and fine particle concentrations in the flue gas were directly proportional to the O₂ content in the oxidizer. The crystalline phase compositions of the coarse mode particles were rich in K- and calcium-containing species. The fine mode particles, which contained most of the K, consisted mainly of K₂SO₄ (94%) and K₃Na(SO₄)₂, which is in excellent agreement with TECs of gas phase condensation. As supported by the solid phase analysis, complete sulfation of K species was achieved for all studied cases. A CO₂ purity (dry) of up to 94% was achieved for OFC.

Potassium (K) plays a key role in biomass combustion as its presence in the gas phase may lead to slagging, agglomeration and corrosion. To facilitate feedstock flexibility, a detailed understanding of the K chemistry during solid fuel conversion is needed. Here, we present photofragmentation laser absorption imaging for simultaneous, quantitative wide-field imaging of gaseous atomic K, potassium hydroxide (KOH) and potassium chloride (KCl) using a high-speed camera. Concentration images of the K species, at levels down to below parts-per-million, were acquired in a physical domain of 3.8-by-16.4 mm with a spatial resolution of 0.13 mm and a temporal resolution of 0.5 s after averaging. The technique was applied for imaging of the K species release from solid coffee husk and wheat straw samples combusted in a laboratory flame. Conversion of KCl salt was investigated as a reference. Significant differences in K species concentration and spatial distribution were observed between the fuels and between the devolatilization and char conversion stages. We also demonstrate spatial mapping of the atomic K fragment recombination rates with 1.2 µs time resolution, which constitutes a step towards reaction kinetics imaging.

In this study, a Eulerian–Lagrangian model is used to study biomass gasification and release of potassium species in a 140 kW atmospheric entrained flow gasifier (EFG). Experimental measurements of water concentration and temperature inside the reactor, together with the gas composition at the gasifier outlet, are used to validate the model. For the first time, a detailed K-release model is used to predict the concentrations of gas-phase K species inside the gasifier, and the results are compared with experimental measurements from an optical port in the EFG. The prediction errors for atomic potassium (K), potassium chloride (KCl), potassium hydroxide (KOH), and total potassium are 1.4%, 9.8%, 5.5%, and 5.7%, respectively, which are within the uncertainty limits of the measurements. The numerical model is used to identify and study the main phenomena that occur in different zones of the gasifier. Five zones are identified in which drying, pyrolysis, combustion, recirculation, and gasification are active. The model was then used to study the transformation and release of different K species from biomass particles. It was found that, for the forest residue fuel that was used in the present study, the organic part of K is released at the shortest residence time, followed by the release of inorganic K at higher residence times. The release of inorganic salts starts by evaporation of KCl and continues by dissociation of K2CO3 and K2SO4, which forms gas-phase KOH. The major fraction of K is released around the combustion zone (around 0.7–1.3 m downstream of the inlet) due to the high H2O concentration and temperature. These conditions lead to rapid dissociation of K2CO3 and K2SO4, which increases the total K concentration from 336 to 510 ppm in the combustion zone. The dissociation of the inorganic salts and KOH formation continues in the gasification zone at a lower rate; hence, the total K concentration slowly increases from 510 ppm at 1.3 m to 561 ppm at the outlet.

Thermochemical energy conversion processes, such as combustion and gasification, are applied worldwide for generation of electricity, heat and synthesis of chemicals. Today, these processes are mostly run on non-renewable, fossil fuels and constitute a major source of carbon dioxide emissions. A promising renewable energy source with low net carbon dioxide emissions is biomass, in particular rest products from agriculture and forestry. However, biomass usually contains high amounts of volatile inorganic compounds, such as chlorine, potassium (K) and phosphorus (P), which lead to ash-related operational issues, including deposit build-up, slagging and corrosion. Therefore, efficient utilization of biomass requires knowledge of the chemistry and fate of the inorganic compounds during thermochemical conversion. Due to the reactive, high-temperature environments in those processes, gaseous compounds are preferably measured in situ using optical techniques.

This thesis mainly deals with the development of a laser-based technique for simultaneous in situ detection and quantification of the main gaseous K species in biomass combustion and gasification: atomic K, potassium hydroxide (KOH) and potassium chloride (KCl). The novel method combines photofragmentation (PF) with tunable diode laser absorption spectroscopy (TDLAS) and achieves sub-ppm detection limits for all three K species for a path length of 2 cm and a time resolution of 20 ms. Recording the ns-µs PF signal decay due to fragment recombination allows probing the K reaction kinetics. Together with TDLAS sensors for water, methane and gas temperature, the PF-TDLAS system was employed to characterize biomass reactors from laboratory- to pilot-scale. The results were compared to predictions by numerical models. In addition, PF-TDLAS was employed for quantitative wide-field imaging of K species in a laboratory flame during KCl salt and biomass conversion. Finally, in situ detection of phosphorus pentoxide (P₄O₁₀) with a time resolution of 140 ms was demonstrated using broadband infrared absorption spectroscopy. Absorption line strengths of P₄O₁₀ at temperatures relevant for combustion were determined for the first time. The techniques presented in this thesis can provide unique experimental data for validation and further development of numerical models and advance the understanding of K species chemistry during solid fuel conversion, which is needed to facilitate the utilization of biomass in the energy system.

Photofragmentation tunable diode laser absorption spectroscopy (PF-TDLAS) was used to simultaneously measure the concentrations of gas phase atomic potassium (K), potassium hydroxide (KOH) and potassium chloride (KCl) in the reactor core of a 140 kWth atmospheric entrained-flow gasifier (EFG). In two gasification experiments at air-to-fuel equivalence ratio of 0.5, the EFG was first run on forest residues (FR) and then on an 80/20 mixture of FR and wheat straw (FR/WS). Combustion at air-to-fuel equivalence ratio of 1.3 was investigated for comparison. A high K(g) absorbance was observed in gasification, requiring the photofragmentation signals from KOH(g) and KCl(g) to be recorded at a fixed detuning of 7.3 cm⁻¹ from the center of the K(g) absorption profile. In combustion, the fragments recombined instantly after the UV pulse within around 10 µs, whereas in gasification, the K(g) fragment concentration first increased further for 30 µs after the UV pulse, before slowly decaying for up to hundreds of µs. According to 0D reaction kinetics simulations, this could be explained by a difference in recombination kinetics, which is dominated by oxygen reactions in combustion and by hydrogen reactions in gasification. The K species concentrations in the EFG were stable on average, but periodic short-term variations due to fuel feeding were observed, as well as a gradual increase in KOH(g) over the day as the reactor approached global equilibrium. A comparison of the average K species concentrations towards the end of each experiment showed a higher total K in the gas phase for FR/WS, with higher K(g) and KCl(g), but lower KOH(g), compared to the FR fuel. The measured values were in reasonable agreement with predictions by thermodynamic equilibrium calculations.

Gaseous potassium (K) species play an important role in biomass combustion processes, and imaging techniques are powerful tools to investigate the related gas-phase chemistry. Here, laser absorption imaging of gaseous atomic K in flames is implemented using tunable diode laser absorption spectroscopy at 769.9 nm and a high-speed complementary metal oxide semiconductor (CMOS) camera recording at 30 kfps. Atomic K absorption spectra are acquired for each camera pixel in a field of view of 28 × 28 mm at a rate of 100 Hz. The technique is used to determine the spatial distribution of atomic K concentration during the conversion of potassium chloride (KCl) salt and wheat straw particles in a laminar premixed CH4/air flame with an image pixel resolution of up to 120 μm. Due to axisymmetry in setup geometry and, consequently, atomic K distributions, the radial atomic K concentration fields could be reconstructed by one-dimensional tomography. For the KCl sample, the K concentration field was in excellent agreement with previous point measurements. In the case of wheat straw, atomic K concentrations of around 3 ppm were observed in a cylindrical flame during devolatilization. In the char conversion phase, a spherical layer of atomic K, with concentrations reaching 25 ppm, was found within 5 mm of the particle surface, while the concentration rapidly decreased to sub-ppm levels along the vertical axis. In both cases, a thin (∼1 mm) layer without any atomic K was observed in close vicinity to the particle, suggesting that the potassium was initially not released in its atomic form.