Solid biomass oxy-fuel combustion with external recirculation was demonstrated in a 100-kW atmospheric, down-fired combustor operated close to stoichiometry. The external recirculation setup comprised a particulate filter, a condenser, a fan and O2 addition before the burner inlet. A theoretical description of the recirculation process is presented and validated. Two fuels, softwood (SW) and forest residues (FR), with similar residence times, were compared. Gaseous species, including potassium (K) compounds (atomic K, KOH, and KCl), and gas temperature were quantified in real-time by tunable diode laser absorption spectroscopy (TDLAS) and photofragmentation TDLAS at two locations in the reactor core. Major species (CO2, H2O, O2, and N2) were also measured at the exhaust. Flue gas particles collected with a low-pressure impactor at the exhaust were analyzed by X-ray powder diffraction and scanning electron microscopy. The average CO2 purity (dry) was 90 % for SW and 86 % for FR. The NO concentration was higher for FR due to the larger nitrogen content in the fuel. The gaseous K species concentrations were higher for FR than for SW (factor 2–3), but not as high as expected from the difference in fuel K content (factor 7), likely due to the high content of Si and Al in FR. Gas-phase K was significantly lower than predicted by thermodynamic equilibrium calculations (TEC), probably due to K adsorption by soot particles. The fine and coarse particle concentrations were significantly higher for FR than for SW due to the higher ash content of FR. The FR fine mode particles consisted mainly of K2SO4 and KCl, in good quantitative agreement with TEC of gas phase condensation. Apatite, Ca5(PO4)3OH, likely formed from vaporized Ca and P, was found in the fine mode in all recirculation cases

A mid-IR TDLAS sensor for NO measurements in high-temperature environments is presented. The diagnostic targets the strong NO transition at 1977.27 cm−1, which provides low spectral interference and weak temperature sensitivity, thus reducing the effect of temperature inhomogeneity along the optical path on the retrieved path-averaged NO concentration. The diagnostic was validated in plasma-heated gases with H2O vapor and in pilot-scale biomass/hydrogen combustion at temperatures up to 1450 K and H2O mole fractions up to 30%. NO concentration obtained by fitting HITEMP-based spectra agreed with FTIR measurements within 15%, with estimated uncertainties of 20%. The sensor achieved a detection limit of about 50 ppm·m. Neighboring H2O transitions were negligible at lab-scale but required in pilot-scale fitting

Methane pyrolysis (MP), where CH4 is thermally decomposed to H2 and solid carbon is a CO2 free H2 production technique. By performing non-catalytic MP experiments in the temperature range of 1100°C-1700 °C we showed that the produced carbon nanoparticles (CNP) have a similar elemental composition (C, H, O, and N) and turbostratic nanostructure compared to rubber grade carbon black (CB) if the reactor temperature is ∼1400 °C with 100% conversion of the CH4 to CNP, C2H2, and H2 with product yields of ∼0.53 g/gfuel, ∼0.23 g/gfuel, and ∼0.24 g/gfuel, respectively. By increasing the reactor temperature above 1400 °C, further decomposition of the C2H2 to CNP and H2 occurs, and the theoretical yield of 0.75 g/gfuel of CNP and 0.25 g/gfuel H2 could be reached. However, the turbostratic nanostructure of the CNP at higher temperatures was too graphitized compared to rubber grade CB and instead more like electro conductive CB

Quantitative laser-based diagnostics like Raman spectroscopy are essential for studying high-temperature processes, but their application in intensely luminous and transient environments such as plasma torches is severely limited by overwhelming background emission. This study focuses on the quantitative thermometry of a 7 kW atmospheric air plasma jet, an environment where such measurements are notoriously difficult. To enable these measurements, a Polarization Lock-In Filtering (PLF) Raman technique is used to suppress the intense and fluctuating plasma background. The method successfully yields high-quality N<inf>2</inf> ro-vibrational spectra along the jet's central axis. Model-based fitting of these spectra produces a detailed axial temperature profile, showing a decay from over 3700 K near the nozzle. Furthermore, the high signal quality enabled the detection of singly ionized nitrogen (N<inf>2</inf>+) in the plasma core, providing direct evidence of its ionized state. These results represent the first application of PLF for thermometry in a plasma torch and provide critical experimental data for validating magnetohydrodynamic simulations

In a non-transferred plasma torch, the working gas becomes ionized and forms plasma as it interacts with the electric arc at the cathode tip. However, in certain cathode shapes, particularly flat ones, and under specific conditions, the gas flow can separate at the cathode tip, forming a vortex region. While this flow separation is influenced by geometric factors, it occurs in the critical zone where plasma is generated. Understanding the causes of this separation is essential, as it may significantly impact torch performance. If the separation proves detrimental, it is important to identify ways to mitigate it. This paper presents a computational analysis of a non-transferred plasma torch to investigate the physics behind flow separation. The results highlight the location and causes of the separation, as well as its potential advantages and disadvantages. Finally, the paper explores theoretical approaches to address flow separation in plasma torches, offering practical insights for enhancing their design and efficiency. 

Coal replacement with hydrogen is a strategy for reducing carbon emissions from high-temperature industrial processes. Hydrogen lancing is a direct way for introducing hydrogen to existing coal-fired kilns. This work investigates the effects of hydrogen lancing on nitrogen oxides (NOx) emissions and ignition behaviour in a pilot-scale furnace that employs a 30 % coal replacement with hydrogen lancing. The investigation encompasses the impacts of lancing distance, angling, and velocity. Advanced measurement techniques, including spectrometry and monochromatic digital cameras, characterise the flame and assess emissions. The results indicate that the 30 % coal replacement by hydrogen lancing enhances combustion and reduces the emissions of carbon monoxides (CO). The flame characteristics vary with the location of the hydrogen injection, generally becoming more-intense than during coal combustion. NOx emissions during lancing are similar or up to double the emissions observed for pure coal combustion, depending on the lancing configuration. Increasing the distance between the hydrogen lance and coal burner increases NOx emissions. 

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. 

This study explores how different electrode shapes affect plasma flow in a non-transferred plasma torch. Various cathode geometries—including conical, tapered, flat, and cylindrical—were examined alongside stepped anode designs. A 2D axisymmetric computational model was employed to assess the impact of these shapes on plasma behavior. The results reveal that different cathode designs require varying current levels to maintain a consistent power output. This paper presents the changes in electric conductivity and electric potential for different input currents across the arc formation path (from the cathode tip to the anode beginning) and relating to Ohm’s law. Significant variations in plasma jet velocity and temperature were observed, especially near the cathode tip. The study concludes by evaluating thermal efficiency across geometry configurations. Flat cathodes demonstrated the highest efficiency, while the anode shape had minimal impact.

The grate-kiln process for iron-ore pellet induration utilizes pulverized coal fired burners. In a developed infrastructure for H2, it might be desirable to heat the existing rotary kilns with renewably produced H2. Technical challenges of H2 heating of grate-kilns include high emissions of NOX and maintaining sufficient heat transfer to the pellet bed. This article examined cofiring (70% coal/30% H2) in 130 kW experiments using two different integrated burner concepts. Compared to pure coal combustion, cofiring creates a more intense, smaller flame with earlier ignition and less fluctuations. The process temperature and heat transfer are enhanced in the beginning of the kiln. The co-fired flames emit 32% and 78% less NOX emissions compared to pure coal and H2 combustion, respectively. We can affect the combustion behavior and NOX emissions by the burner design. H2/coal cofiring using integrated burners is probably an attractive solution for emission minimization in rotary kilns.

A machine vision driven sensor for estimating the instantaneous feeding rate of pelletized fuels was developed and tested experimentally in combustion and gasification processes. The feeding rate was determined from images of the pellets sliding on a transfer chute into the reactor. From the images the apparent area and velocity of the pellets were extracted. Area was determined by a segmentation model created using a machine learning framework and velocities by image registration of two subsequent images. The measured weight of the pelletized fuel passed through the feeding system was in good agreement with the weight estimated by the sensor. The observed variations in the fuel feeding correlated with the variations in the gaseous species concentrations measured in the reactor core and in the exhaust. Since the developed sensor measures the ingoing fuel feeding rate prior to the reactor, its signal could therefore help improve process control.