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Alkali species have been under intensive research in thermal conversion applications due to their abundance especially in biomass fuels. Alkali metals, sodium (Na) and potassium (K), are known to cause severe operational problems in combustion units, such as slagging, fouling, and corrosion. In this work, we present a monitoring method to follow alkali behavior from vapor to particles in a pilot-scale reactor. In our approach we combine Tunable Diode Laser Atomic Spectroscopy (TDLAS) for atomic potassium monitoring, Collinear Photofragmentation and Atomic Absorption Spectroscopy (CPFAAS) for KCl and KOH detection, and Surface Ionization Detection (SID) for monitoring of total flue gas and aerosol alkali content. Experiments were carried out in the Chalmers 100 kW oxy-fuel combustion unit that, during these experiments, used propane as fuel. Alkali species were injected as a water solution directly to the flame. In addition, SO2 was used to alter the conditions for alkali species formation injecting it directly to the combustion feed gas. Due to the alkali monitoring system described, we were able to monitor the alkali behavior during nucleation and sulfation processes. The conditions for dimer formation and heterogeneous nucleation were observed when the temperature conditions were changed by lowering the thermal input to the unit. publishedVersion

Biomass and waste derived fuels contain large amounts of sodium, potassium, and chlorine that form NaCl and KCl, that is, compounds that cause operational problems, such as slagging, fouling, and high-temperature corrosion. Therefore, alkali chlorides are the main reasons that explain why steam parameters are less advanced and efficient in biomass and waste-based power generation when compared to coal. These problems can be mitigated by introducing sulphur into the system to form alkali sulphates that are not as problematic on steel surfaces as alkali chlorides. However, the alkali sulphation process in realistic combustion environments needs further exploration. Thus, new diagnostic methods for in-situ monitoring of alkali sulphation kinetics in combustion systems are required. In this work, the simultaneous monitoring of KCl and KOH concentrations in a pilot-scale combustor using Collinear Photofragmentation and Atomic Absorption Spectroscopy (CPFAAS) during stationary and transient operation of the combustor, is introduced. The CPFAAS information is complemented by monitoring SO2 and HCl concentrations using Fourier-transform infrared spectroscopy (FTIR). The temporal performance of the system is demonstrated by measuring the temporal combustor response curves for KCl sulphation for different Cl/K ratios during rapid changes in gaseous SO2 concentrations. The temporal concentration curves obtained imply that the Cl/K ratio has a significant impact on the temporal alkali sulphation behaviour. The measurement system described enables further exploration of K–Cl–S chemistry in realistic large-scale power plant environments.

The focus of this paper is on effects of chlorine and sulfur on coal ash deposition rates, under practically relevant but systematically controlled combustion conditions. This problem is important, not so much for coal, but to understand and predict deposition rates for biomass combustion where chlorine contents can be high. To this end, ash deposition rates on a controlled temperature surface were measured for controlled amounts of chlorine and sulfur added to a pulverized coal, doped with potassium and burned in a 100 kW rated combustion rig. Previous work with 35 tests on 11 coal, biomass and petroleum coke fuels burned under a range of operating conditions had strongly suggested that the deposition rate of the tightly bound inside deposits was independent of the ash aerosol composition, and depended only on PM1 in the flue gas. The loosely bound outside deposition rate was dependent primarily on the total alkali content in the flue gas. The new results using chlorine added to the fuel (in the form of ammonium chloride) required these previous conclusions to be drastically revised. They showed that chlorine, not alkali alone, had large effects on the deposition rate of the inside deposits, which now were orders of magnitude higher than without chlorine addition, and did not fit previous (multi-fuel) correlations with PM1. Sulfur addition, together with chlorine, did not affect deposition rates much, although it did lower the chlorine content of the deposit. These results are interpreted in terms of the ash aerosol size segregated composition, which was also measured, and potential sulfation reactions within the deposit.

The grate-kiln process is employed for sintering and oxidation of iron-ore pellets. In this process, a fuel (typically coal) is combusted with a large amount of excess air in a rotary kiln, and the high air-to-fuel ratio leads to significant NOx formation. The current Article is an assessment of NOx reduction measures that have been tested in pilot-scale and in full-scale by the Swedish iron-ore company Luossavaara-Kiirunavaara Aktiebolag (LKAB). The results show that the scaling between the full-scale kiln and the pilot-scale kiln is crucial, and several primary measures that reduce NOx significantly in pilot-scale achieve negligible reduction in full-scale. In the investigated full-scale kiln, thermal NOx formation is efficiently suppressed and low compared with the NO formation from the fuel-bound nitrogen (especially char-bound nitrogen). Suppressing the NO formation from the char-bound nitrogen is difficult due to the high amounts of excess air, and all measures tested to alter mixing patterns have shown limited effect. Switching to a fuel with a lower nitrogen content is efficient and probably necessary to achieve low NOx emissions without secondary measures. Simulations show that replacing the reference coal with a biomass that contains 0.1% nitrogen can reduce NOx emissions by 90%.

This paper presents experimental and modeling work on the interaction between the K, Cl, and S components, and these chemical interactions are studied in both air-fuel and oxy-fuel atmospheres. Detailed kinetic modeling is conducted to examine the potassium chloride sulfation and its interaction with CO oxidation in both nitrogen- and carbon-dioxide-based atmospheres. The oxidation of CO enhances the kinetics of alkali sulfation for typical post-flame conditions, below 1000 °C, in both atmospheres. For higher temperatures, sulfation kinetics are promoted even further in CO2-rich atmospheres. Oxy-fuel atmospheres, i.e., CO2-rich atmospheres, also promote increased levels of CO in technical-scale flames. Therefore, in practical systems, enhanced sulfation kinetics will automatically be promoted by flue gas recirculation. Also, the availability of sulfur, in the form of an increased SO2 concentration, often enables complete sulfation of alkali in oxy-fuel atmospheres as a result of the flue gas recirculation. The availability of SO3 may increase in oxy-fuel compared to air-fuel atmospheres as a result of either elevated SO2 levels or different sulfation reaction patterns, as discussed in the modeling of this work. However, SO3 has no significant impact on the overall sulfation rates in oxy- compared to air-fired systems.

The use of low-quality fuels in power generation plants is typically motivated by the potential for reducing fuel costs or CO2 emissions, the latter in the case of a fuel based on biomass. These features make low-quality fuels attractive, although their use for power generation is usually problematic due to their composition. One of the main issues is high-temperature corrosion (HTC), which is caused by alkali-containing chlorides. The alkali chlorides, which are formed from alkali metals and chlorine released from the fuel during the combustion process, are a particular problem. HTC is often related to the combustion of fuels with a low sulfur-to-potassium ratio, such as biomass, and it has a significant effect on the thermal efficiency and/or the maintenance cost of the power plant. Sulfuric and alkali species not only influence the formation of highly corrosive salts but also affect other aspects of combustion chemistry. While the present work relates to HTC chemistry, it focuses on how potassium chlor...

Soot is an important component for heat transfer in combustion processes. However, it is also a harmful pollutant for humans, and strict emissions legislation motivates research on how to control soot formation and release. The formation of soot is known to be triggered by high temperature and high pressure during combustion, and it is also strongly influenced by the local stoichiometry. The current study investigates how the formation of soot is affected by increasing the oxygen concentration in the oxidizer, since this affects both the temperature profile and partial pressures of reactants. The oxygen-to-fuel ratio is kept constant, i.e., the total flow rate of the oxidizer decreases with increasing oxygen concentration. Propane is combusted (80 kWth) while applying oxygen-enriched air, and in-flame measurements of the temperature and gas concentrations are performed and combined with available soot measurements. The results show that increasing the oxygen concentration in the oxidizer from 21% to 27% slightly increases soot formation, due to higher temperatures or the lower momentum of the oxidizer. At 30% oxygen, however, soot formation increases by orders of magnitude. Detailed reaction modeling is performed and the increase in soot formation is captured by the model. Both the soot inception rates and surface growth rates are significantly increased by the changes in combustion conditions, with the increase in soot inception being the most important. Under atmospheric conditions, there is a distinct threshold for soot formation at around 1200 °C for equivalence ratios >3. The increase in temperature, and the slower mixing that results from the lower momentum of the oxidizer, have the potential to push the combustion conditions over this threshold when the oxygen concentration is increased.

A new method for real-time measurements of potassium and sodium containing aerosol particles is described and verified. The method is based on surface ionization technique and may be used to explore the alkali chemistry related to high temperature chemistry processes. The measurement device is a further development of the simple and cost-effective surface ionization detector previously used for online alkali measurements in combustion and gasification research. The discrimination between sodium and potassium is possible due to differences in their surface desorption kinetics and facilitated by rapidly reversing the field potential between the ion source and the nearby collector. The instrument is evaluated in a series of laboratory experiments using size-selected alkali salt particles containing KCl, NaCl, K2SO4, Na2SO4, KNO3 and NaNO3. The filament temperature was found to be a key influencing factor in order to optimize the strength and Na–K deviation of the observed ion current. The ability to simultaneously report absolute concentrations of Na and K makes the instrument attractive for solid fuel conversion of alkali-rich fuels such as low-grade biomass and to explore behavior deviations of Na and K in high temperature processes.