Pretreatment of flue gas prior to CO2 capture via amine scrubbing is important for maintaining CO2 capture efficiency and avoiding formation of toxic byproducts due to the presence of NOX. One flue gas cleaning method for that has increasingly gained attention is NOX scrubbing, in which NO is pre-oxidized to NO2 which can be absorbed using aqueous solutions containing suitable additives. While commonly used sulfur-based additives achieve high absorption efficiencies, they suffer from high consumption rates due to oxidative consumption. A recent study on phenolic additives demonstrated that e.g. 4-methylaminophenol could be used as efficient additive which has much lower consumption rates. This compound however is limited for industrial application due to environmental concerns. The present study aims to demonstrate the usage of bio-based phenolic polymers for NO2 absorption which should be more suitable for industrial usage on the example of lignin sulfonate. Additionally, vanillin was studied as a model compound to better understand NO2 absorption chemistry with such additives. Tests were conducted using lignin sulfonate at different pH values and additive concentrations with 50 ppm inlet NO2 at a total gas flow rate of 2.3 NL/min. Lignin sulfonate at 500 mg/100 mL and pH 9 maintained high NO2 absorption efficiency of approximately 90 %. Vanillin at 5 mM showed efficiencies of 91 % and 94 % at 50 and 100 ppm inlet NO2, respectively, at the same gas flow rate. With vanillin, absorption efficiency remained at a high level even after more than 1 mol NO2 per mole additive was absorbed, while sulfur-based additives showed a much lower ratio than 1 when fully consumed. NO2 absorption by the sulfur-based additives was successfully modeled, highlighting the lower chemical consumption of vanillin. Overall, these results demonstrate the high potential of lignin-based additives for NO2 scrubbing.

Additive consumption during NOx removal via NO2 absorption into aqueous solutions remains a critical challenge for sustainable flue gas cleaning and minimizing process waste. While sulfite additives are widely used to enhance NO2 absorption rates, their effectiveness is severely compromised under aerobic conditions due to rapid oxidation of sulfite to sulfate via a NO2-induced radical chain mechanism. This not only reduces the process efficiency but also leads to excessive additive consumption and waste formation. Thiosulfate, acting as a radical scavenger, offers strategies to mitigate these undesired side reactions by interrupting the radical chain mechanism. In the present study, sulfite and thiosulfate consumption were systematically investigated under aerobic and anaerobic conditions in a laboratory-scale NO2 scrubber for NO2 concentrations ranging from 50 to 200 ppm and additive concentrations between 0 and 10 mM sulfite and 0-3 mM thiosulfate. The novelty of this work lies in the development of a comprehensive kinetic model that predicts absorption efficiency as well as additive consumption across varying operating conditions. Unlike previous empirical approaches, this model provides mechanistic insights into the interplay between absorption reactions and side reactions, enabling good estimations of additive requirements and their effective contribution to the NO2 removal process. The strong agreement between modeled and experimental data highlights its robustness and applicability. This modeling framework represents a step toward optimizing additive usage in flue gas cleaning, reducing chemical waste, and improving the sustainability of industrial flue gas cleaning processes.

Thermal conversion of agricultural biomass residues poses a great opportunity to valorize waste materials by recovering energy and valuable elements such as phosphorus. Utilizing biomass residues in thermal conversion is, on the other hand, often coupled with operational challenges due to particle emissions, deposit formation, corrosion and slagging caused by the ash-forming elements in the biomass. A detailed understanding of the ash chemistry is required when utilizing those fuels to reduce these operational problems and recover valuable elements from the ash. However, predictions for ash transformation are often always reliable when using existing thermodynamic data and ash transformation mechanisms. The present work investigated the release of phosphorus and potassium during gasification of two seed-originated agricultural biomass residues, wheat bran and sunflower seed shells, at 900-1100 degrees C in 3 % O2 or 10 % CO2 (rest N2). The residues were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and inductively coupled plasma optical emission spectroscopy (ICP-OES). During the gasification of wheat bran, phosphorus and potassium were partly released to the gas phase, while only potassium was released to the gas phase during the gasification of sunflower seed shells. The residues from the gasification of wheat bran contained mainly K-Mg-phosphates, while phosphorus was identified as hydroxyapatite in the sunflower seed shell residues. The experimental observations for wheat bran are in contradiction with predictions from thermodynamic equilibrium calculations, which suggest that all phosphorus remains in the residues. The discrepancy between the experimental and calculated results may be due to carbothermic reduction of phosphates, i.e. reactions between phosphates and carbon. As the occurrence of carbothermic reduction reactions is connected to the kinetics of the carbon consumption, it is suggested that thermodynamic data alone is not sufficient to correctly predict the ash chemistry in thermal conversion processes of phosphorus rich biomass fuels.

The aim of this study is to investigate how pre-treatment of herbaceous straw biomass for ash control affects the release of nitrogen species during combustion and gasification. To comprehend the formation of NO and its precursors, NH3 and HCN, the release of these species was investigated and compared under both combustion and gasification-like conditions at 950 degrees C. The effects of various upgrading methods, such as torrefaction, water -leaching, a combination of leaching and torrefaction, and CaCO3 addition, were studied. The assessment of ni-trogen release was divided into two consecutive conversion steps - devolatilization/pyrolysis and ash/char re-actions. The release of nitrogen is highly dependent on the reaction conditions. For instance, the emissions of NO from the combustion conditions (3 vol% O2) for all fuel samples were, on average, six times higher than under gasification conditions (14.5 vol% H2O and 5 vol% CO2). The emissions of NO from the combustion and gasi-fication of torrefied biomass were, on average, 20 % higher than those from raw biomass. Water-leaching had a suppressing effect on NO formation during char conversion. Approximately 62 % of the char-N formed NO for raw and torrefied material, whereas only 26 %-35 % was formed for pre-or postwashed samples. The effect of the applied pre-treatment approaches on the release of nitrogen was particularly significant during char con-version. Increasing calcium and decreasing potassium content had catalytic effects, mainly on the conversion of volatile-N to NH3. The Ca-doped biomass feedstock showed approximate 10 % increase in volatile-N to NH3 conversion compared to the source material.

CO2 capture experiments were carried out in a small-scale bubble column reactor using alkaline solutions of sodium hydroxide. The results of the experiments show that a higher NaOH content in the liquid phase and elevated CO2 concentrations in the gas mixture led to a higher CO2 removal efficiency. A mathematical model was developed to describe the CO2 absorption as a function of CO2 gas inlet and OH- concentrations at low CO2 and OH- concentrations. The CO2 uptake at high OH- concentrations (high pH) was accurately predicted by the model, whereas the CO2 uptake at lower OH- concentrations could not be predicted with the same accuracy.

NO2 absorption has recently gained attention as a promising alternative to conventional NOx reduction methods. One key challenge in NO2 absorption is that high amounts of additives are required to maintain satisfactory absorption efficiency. The present study investigates the absorption of NO2 from flue gases in aqueous solutions with low concentrations of phenolic additives. Absorption tests with hydroquinone and 4-methylaminophenol sulfate were performed in a laboratory-scale absorption system with test gases containing 0 or 5% O2 (N2 as a balance gas) and 50, 100, or 150 ppm of NO2, representing typical flue gas conditions in a range of industrial processes. The results show that NO2 was absorbed efficiently with small concentrations of 4-methylaminophenol sulfate, both with 0 or 5% O2 in the test gas. Hydroquinone also enhanced NO2 absorption but to a lesser degree than 4-methylaminophenol sulfate. Interestingly, the presence of O2 did not influence the consumption of the phenolic additives, in contrast to commonly used additives such as sulfite that require oxidation inhibitors such as thiosulfate. With a 2 mM solution of 4-methylaminophenol sulfate, more than 90% of the incoming NO2 was absorbed. Thus, NO2 absorption with 4-methylaminopheno sulfate appears to be able to remove NOx emissions from flue gases with high efficiency.

Post-flame sulfation of gaseous sodium hydroxide (NaOH) and sodium chloride (NaCl) was investigated with optical in situ measurements at 850 to 1475 degrees C. A multi-jet burner was used to generate well-controlled com-bustion environments. The multi-jet burner also enabled the separate feeding of the sodium species and SO2 to the combustion environment where the sulfation reactions occurred. Concentrations of NaOH(g) and NaCl(g) were measured in the product gas using broadband UV absorption spectroscopy to follow the degree of sulfation. At 1475 and 1275 degrees C almost no sulfation occurred with an initial NaOH(g) concentration of 20 ppm and SO2 concentrations between 0 and 150 ppm. At 985 degrees C, the NaOH(g) concentration decreased to less than 5 ppm with SO2 concentrations above 50 ppm and at 850 degrees C almost all NaOH(g) was sulfated under these conditions. The experimental results for the gas-phase sulfation of NaOH were compared to previous results for the sulfation of KOH under the same conditions and the results were shown to be similar for NaOH and KOH under these conditions. Sulfation of NaOH(g) generally occurred to a more significant extent than the sulfation of NaCl(g). At 1115 to 1475 degrees C, no sulfation of NaCl(g) was observed. At the lowest investigated temperature, 850 degrees C, the NaCl (g) concentration decreased from 20 ppm to 12 ppm after the addition of 150 ppm SO2. Chemical equilibrium calculations and kinetic modeling using an updated kinetic model for the detailed Na-Cl-S chemistry were compared to the experimental results. Above 1100 degrees C, the system can be described by chemical equilibrium, implying that equilibrium is reached in less than 100 ms. At temperatures below 1100 degrees C, the measured con-centration indicated kinetic control. Under these conditions, the kinetic model was in good agreement with the experimental results for NaOH(g) but over-predicted the sulfation of NaCl(g). The combined experimental data, chemical equilibrium calculations and kinetic modeling of the present study support that sulfation of alkali species can occur in the gas phase through homogeneous reactions.

Biomass can be used to generate heat, power, or biofuels in thermal conversion processes such as combustion, gasification and pyrolysis. However, some types of biomass contain high levels of phosphorus, which can be released to the gas phase and cause operational or environmental problems. The mechanism(s) responsible for phosphorus release has not been convincingly established. Understanding the high-temperature phosphorus chemistry is also important in order to enable efficient recovery of phosphorus in residues from thermal con-version of biomass. In this work, the release of phosphorus from wheat bran char and sunflower seed char in different gas environments (100 % N2, 1-20 % O2, and 10 % CO2) and temperatures (900-1100 degrees C) was studied. The chars were converted in a horizontal tube reactor and characterized using ICP-OES, XRD, SEM-EDS, and 31P NMR. The release of ash-forming elements was determined using ICP-OES analysis of the char and sample res-idues, whereas the release of carbon was determined using CO and CO2 gas analysis. In both chars, phosphorus was present primarily together with potassium and magnesium, mainly as pyrophosphates in the wheat bran char, and largely as orthophosphates in the sunflower seed char. For wheat bran char, the release of phosphorus increased from 27 % at 900 degrees C to 71 % at 1100 degrees C in N2, whereas the release was at least 20 % lower in the oxidizing atmospheres (1-20 % O2, or 10 % CO2). The sunflower seed char reached a maximum release of 55 % at 1100 degrees C in N2. For wheat bran char, the molar ratio of released carbon/phosphorus was close to 2.5, which fits well with the theoretical value for carbothermic reduction of phosphates (P2O5(s, l) + 5C(s) -> P2(g) + 5CO(g)). At 1100 degrees C, in N2, the release of phosphorus, potassium and sodium occurred mainly during the first 10 min. It was shown that KMgPO4, used as a model compound, could be reduced by carbon starting from 950 degrees C, but that some of the phosphorus remained in the condensed phase. The work provides a better understanding of phos-phorus release and presents evidence showing that carbothermic reduction reactions can be an important phosphorus release mechanism for seed-and grain-based biomass char.

Nitrogen dioxide (NO2) absorption in aqueous sulfite solutions has recently gained more attention to reduce NOx emissions. The NO2 removal efficiency is strongly influenced by the competing reactions of NO2 absorption and sulfite oxidation. The present study investigates the role of sulfite oxidation in NO2 absorption at neutral pH. The effects of sulfite concentration, thiosulfate, pH, NO2 inlet concentration, oxygen concentration on NO2 absorption efficiency, and sulfite oxidation rates were investigated under well-controlled conditions at 25 degrees C in a laboratory-scale wet scrubbing system. NO2 absorption rates showed a strong dependence on the sulfite concentration and increased linearly with increasing NO2 inlet concentrations between 25 and 150 ppm. At the highest investigated sulfite concentration, 20 mM, the NO2 removal efficiency was around 80%. Oxidation rates of sulfite during NO2 absorption in the presence of 5 vol % O-2 increased with increasing sulfite concentrations in the scrubbing solution and ranged from 20 mg.L-1.min(-1) with 1 mM sulfite to 200 mg.L-1.min(-1) with 20 mM sulfite. The oxidation rate decreased significantly when thiosulfate was added, while the oxidation rates were independent of the oxygen concentration between 2-10 vol % O-2. The NO2 absorption rate decreased with decreasing pH between pH 6 and 8 proportionally to the sulfite concentration as predicted by the sulfite-bisulfite equilibrium. The implication from these results is that significantly less sulfite feeding is needed in industrial NO2 absorption if the pH can be maintained at neutral pH, instead of lower pH such as in simultaneous SO2 and NO2 scrubbing.

NOx emissions continues to be a major challenge in order to reduce the environmental impact of thermal conversion of non-recyclable waste and fuels with high nitrogen contents. Oxidation of NO to NO2 followed by absorption of NO2 in aqueous solutions with sulfur compounds as additives has recently gained more attention in order to reduce NOx emissions from flue gases. One major challenge in NO2 absorption is the high consumption of sulfite, requiring sulfite oxidation inhibitors such as thiosulfate. The present study clarifies the chemistry and consumption of thiosulfate in wet scrubbing of NO2 with buffered solutions under well-controlled conditions at neutral pH at 25 degrees C. Absorption rates for the reaction of thiosulfate and NO2 in the scrubber were determined, and the ability of thiosulfate to inhibit the sulfite oxidation was investigated and quantified. With 1 mM thiosulfate (no sulfite) in the scrubber solution, the NO2 absorption rate increased by 30% as compared to pure water and by 160% with 100 mM thiosulfate. The absorption rate increased with time in the presence of oxygen. With 1 mM sulfite (no thiosulfate), the initial NO2 absorption rate increased by 200% as compared to pure water. However, the absorption rate decreases significantly after short periods of time due to the high consumption of sulfite through undesired oxidation by O2. In tests with sulfite and thiosulfate as an oxidation inhibitor, high removal efficiencies could be maintained over extended periods of time. With a 10 mM sulfite solution, 1 mM thiosulfate decreased the sulfite oxidation rate from 130 to 31 mg center dot L-1 center dot min-1, and 2 mM thiosulfate decreased the rate to 16 mg center dot L-1 center dot min-1. For the investigated conditions, the consumption of thiosulfate did not change significantly when higher sulfite concentrations were used. The results of the study demonstrate high NO2 absorption rates for low concentrations of sulfite and thiosulfate at neutral pH.

We have investigated how CO2 and H2O influence NO release rates during char oxidation of biomass. In addition, the role of catalytic elements in these processes has been studied. NO release rates were determined from single-particle experiments of softwood and straw at 900 degrees C, in CO2/O-2 (34/3%) and H2O/O-2 (17/3%), with N-2 as the balance gas. The NO release profiles differed in CO2/O-2 and H2O/O-2. In CO2/O-2, the NO release increased as the char conversion proceeded. On the other hand, in H2O/O-2, the NO release decreased as the char conversion proceeded. In neither of these cases, the conversion rates of char-N to NO were proportional to the conversion rates of char-C. To investigate how the ash-forming matter influenced the NO release rates, the biomasses were demineralized and the experiments were repeated. For the chars of the demineralized biomasses, the NO release profiles were almost identical in CO2/O-2 and H2O/O-2. In addition, for the demineralized chars, the conversion rates of char-N to NO were proportional to the conversion rates of char-C. The conversions of char-N to NO were significantly higher for the demineralized chars. For softwood, the total conversion of char-N to NO was as high as 85%. These results show that (i) CO2 and H2O influence the formation of char-N to NO in different ways and (ii) the ash-forming matter contributes to the differences in the NO release rates.

The influence of two different biomass pre-treatment methods (torrefaction and combined treatment of washing and torrefaction) on the formation of NO and its precursors, NH3 and HCN, during combustion has been studied. Experiments were conducted in an electrically heated single particle reactor under well-controlled conditions in N-2 and O-2/N-2 at 850 degrees C. NO concentrations of the product gases were measured during devolatilization and char oxidation, and the total amounts of released NH3 and HCN were determined during devolatilization. The results show that the conversion of vol-N and char-N to NO changed due to the pre-treatments, influencing the NO emission factor (released NO/energy content) of the fuel. For torrefied straw, the NO emission factor was significantly higher than for raw straw. The change in NO formation can partly be explained by how the nitrogen contents change due to the pre-treatments: both for bark and straw, around 20% of the initial fuel-N was removed during the torrefaction; for straw, 50% of the initial fuel-N was removed during the combined torrefaction and washing. In addition, the changes in the NO formation can be explained by the release of NH3 and HCN during devolatilization. The HCN/NH3 ratio increased with increasing N content and decreasing H/N ratio due to the pre-treatments. The conversion of vol-N to NH3 increased with increasing Ca content and decreasing K content. In general, the devolatilization of the pre-treated biomass samples resulted in a higher conversion of vol-N to NH3 and HCN as compared to the raw biomasses.

In this thesis, the influence of pre-treatments on biomass combustion has been investigated. The main object was to clarify how those pre-treatments affect NO emissions and the formation of NO-precursors. The pre-treatments which have been investigated are steam explosion, torrefaction and a combination of washing and torrefaction. The investigated biomasses were spruce bark and wheat straw. Single particles of the investigated biomasses were combusted in a quartz glass single particle reactor. The total NO emissions were calculated based on measured NO concentrations as a function of time and flow of the combustion air. An additional set-up has been applied to the single particle reactor to investigate the formation of NH3 during pyrolysis of the biomasses. Wet chemistry methods have been used to quantify NH3.