Increasingly stringent limits for NOx and SO₂ emission are forcing the investigation of new reduction methods. This study was conducted to determine the combination of primary measures, i.e., flue gas recirculation (FGR) and excess air effects on sulfur, nitrogen and hydrocarbon emissions, in boiler flue gas. Experimental research was performed using an experimental, small-scale (20 kW) model of an industrial biofuel boiler. During combustion of sunflower seed hulls at different FGR ratios and incomplete combustion regimes, the composition of flue gas (NO, NO₂, N₂O, HCN, NH₃, SO_2, SO₃, H₂S, CO, and C_xH_y) was compared, allowing an explanation of the determinants of emission concentration changes to be provided. Increasing the flue gas recirculation ratio in the primary air had a positive effect on reducing NO_X and CO with certain organic compounds. However, an opposite effect on SO₂ was observed. NO_X and SO₂ concentrations were found to be approximately 500 mg/m³ and 150 mg/m³ under a regular combustion regime. When the FGR ratio of 50% NO_X concentration decreased by 110 mg/m³, the SO₂ concentration increased by 60 mg/m³. The incomplete combustion regime reduced NO_X concentration by 70 mg/m³, whereas SO₂ concentration increased by 100 mg/m³. The influence of primary measures presented an unclear relationship to hydrocarbon emissions, with concentrations not exceeding 18 mg/m³.

The transition from fossil fuels to renewable energy sources often requires shifting toward biomass fuels such as agriculture residues and waste, which tend to emit higher emission rates during combustion, and one of them is sulfur compounds. The main objective of this study is to clarify the regularities of the formation of sulfur compounds depending on the technological factors when burning sulfur-containing biomass. The experiments were conducted on two experimental stands—models of 20 kW and 25 kW capacities of industrial boilers equipped with reciprocating grates—by burning sunflower husk pellets and meat bone meal. The influence of incomplete combustion (indicator CO concentration), flue gas recirculation, and combined effects of both factors on concentrations of SO₂, SO₃, and H₂S were investigated during experiments. In addition, 20–90% of the sulfur in the fuel is converted to SO₂, contingent upon the combustion conditions. These findings have practical implications for the design and operation of biomass combustion systems. The highest SO₂ emissions were observed when primary air was mixed with flue gas recirculation and at the highest content of CO. The correlation of SO₂ and SO₃ and SO₂ and H₂S concentrations in flue gases of boilers was investigated. The conversion ratio of SO₂ to SO₃ was determined under different combustion modes and showed that this ratio can reach up to 5%. The sulfur content in ash deposits in different areas of the actual industrial boiler was analyzed. The highest percent of sulfur (S = 20%) in ash was found on the first boiler pass.

Due to the pollutant regulations of the Directive (EU) 2015/2193, the need to reduce the emissions of pollutant compounds such as NOx is of a great importance for industrial boilers. This study was performed to determine the effect of primary measures in reducing said pollutants by mixing primary or secondary air with FGR and supplying the mixture for combustion. The studies were conducted on low-scale industrial biomass grate-firing furnace and boiler stand of 25 kW. Sunflower husk pellets, shredded pine bark and wood pellets were combusted during these experiments and emissions of NOx in their combustion products were analyzed. The most significant reduction of emissions was achieved by mixing primary air and FGR and supplying it to the primary combustion zone – under the grate. The emissions of NOx were reduced by 29% and 15% for shredded pine bark and sunflower husk pellets respectively. The emissions of shredded pine bark were reduced enough to meet the standards of the EU Directive, unlike those of sunflower husk pellets. The possible ways to reach greater reduction of NOx by using primary measures are implementing staged combustion, supplying pure FGR and/or mixing agriculture waste with fuels containing less N in its composition.

Measurements of pollutant concentrations in the flue gases of a biofuel boiler show that pollutants concentrations fluctuate by 10-100% and more at short time intervals. Large fluctuations in concentrations hinder the effective application of pollution reduction measures. During the research, flue gas composition measurements were performed in 4 MW, 8 MW industrial biofuel boilers and 20 kW experimental model. The results of the research showed that the change of CO, NO, NO₂, SO₂, H₂S, O₂ and particulate matter concentrations is related to the periodicity of the boiler grate movement. During the movement of the grate, the surface of the fuel bed is mechanically disrupted. The different composition of the fuel layers is mixed and this affects the changes in the concentration of combustion products. The clarity of the concentration fluctuations depends on the emissions, the type of fuel and the combustion parameters.

Present EU regulations promote search for efficient means of reducing environmental pollution caused by fuel‐burning equipment. Primary means of NO_x reduction are of a limited efficiency. The aim of this study is investigation and development of Selective Non‐Catalytic Reduction (SNCR) for NO_x Control. This method can be applied in the existing boilers that have no external flue gas cleaning equipment, and that is the main advantage of the method. Experimental investigation of the influence of various reagent solutions on NO_x concentration was carried out in a stand simulating a boiler furnace. Reagents and their operation conditions were selected, desirable efficiency of SNCR method was determined. The experimental results indicated positive application perspectives of this method. Reduction of NO_x concentration by 40 % was reached. It was determined that improperly selected SNCR technology can even increase NO_x concentration. Katilų degimo produktuose susidarančių NO_x koncentracijų mažinimas, įpurškiant selektyviųjų reaktyvių Santrauka ES normos skatina ieškoti efektyvių priemonių atmosferos taršai iš kurą deginančių įrenginių sumažinti. Pirminės NO_x mažinimo priemonės yra riboto veiksmingumo. Darbas skirtas NO_x selektyviajam nekatalitiniam valymui (SNKV) tirti ir tobulinti. Metodo privalumas ‐ gali būti taikomas esamuose katiluose be išorinių valymo įrenginių. Stende, imituojančiame katilo kūryklą, atlikti įvairių įpurškiamų tirpalų įtakos NO_x koncentracijai eksperimentiniai tyrimai. Parinkti reaktyvai, jų naudojimo sąlygos ir nustatytas tikėtinasis SNKV metodo efektyvumas. Tyrimai parodė šio metodo taikymo perspektyvas, NO_x sumažėjo 40%. Nustatyta, kad netinkamai parinkus SNKV technologiją, NO_x išmetimai gali net padidėti. Reikšminiai žodžiai: azoto oksidai, selektyvusis nekatalitinis dūmų valymas, NO_x redukcija, degimas First Published Online: 14 Oct 2010 Keyword : nitrogen oxides, Selective Non‐Catalytic Reduction (SNCR), NOx reduction, combustion

A significant portion of energy losses in industrial systems arises from the inefficient use of high-temperature exhaust gases, emphasizing the need for enhanced heat recovery strategies. This study aims to improve energy efficiency by examining the effects of radiation-intensifying inserts on combined radiative and convective heat transfer in flue-gas heated channels. A systematic literature review revealed a research gap in understanding the interaction between these mechanisms in flue-gas heat exchangers. To address this, analytical calculations were conducted for two geometries: a radiation-intensifying plate between parallel plates and the same insert in a circular pipe. The analysis covered a range of gas-flue and wall temperatures (560–1460 K and 303–393 K, respectively), flow velocities, and spectral emissivity values. Key performance metrics included Reynolds and Nusselt numbers to assess flow resistance and heat transfer. Results indicated that flue-gas temperature has the most significant effect on total rate of heat transfer, and the insert significantly enhanced radiative heat transfer by over 60%, increasing flow resistance. A local Nusselt number minimum at a length-to-diameter ratio of approximately 26 suggested transitional flow behavior. These results provide valuable insights for the design of high-temperature heat exchangers, with future work planned to validate the findings experimentally.