Full-scale Boiler Research Articles

Equilibrium model approach to predict local chemical changes in recovery boiler deposits

A step-by-step equilibrium model approach was developed to estimate and describe the enrichment of K and Cl within recovery boiler deposits. The model predicts free melt movement towards colder temperatures within deposits. Due to a temperature gradient, the melt amount and composition differ across the deposit. The melt movement affects the local composition, which leads to changes in the local phase composition. The model predicts how the deposit profile changes due to melt migration into pores within the deposits. Changes in the deposit composition profile also affect the melting behavior locally, resulting in lower local melting temperatures, which can be detrimental to heat exchanger materials. The modeling results were compared to earlier published laboratory and full-scale boiler measurements, and there is a good agreement between the results. The model predicts a local decrease in the first melting temperature of recovery boiler deposits by ∼30 °C. These findings closely align with experimental results, shedding light on the intricate mechanisms of melt percolation and intra-deposit aging processes. The proposed step-by-step model offers a means to achieve more accurate estimations of locally prevalent first melting temperatures in recovery boiler deposits.

Characteristics of fine particle formation during combustion of high-alkali coal in a new full-scale circulating fluidized bed boiler

High-alkali coal generated an increased number of fine particles during the combustion process. Understanding the transformative characteristics of particulate matter (PM) in a full-scale boiler that burns 100 % Zhundong (ZD) coal is key for controlling pollutant emissions. This paper investigated PM formation in a novel full-scale Circulating Fluidized Bed (CFB) boiler. PM samples were collected from various points along the steam-cooled flue at 50 %, 67 %, and 100 % boiler load. Utilizing Low-pressure impactors and scanning electron microscopy coupled with energy disperse spectroscopy (SEM-EDS) were applied to analyze the mass-based particle size distribution(MPSD), elemental compositions, and morphology of PM. The results show that sub-micron particles produced under low boiler load consist primarily of AAEMs sulfates, whereas under high load, they consist primarily of alkali metal chlorides. Super-micron particles are predominantly sulfates and silicates. The concentrations of Na, K, and Cl in PM0.4 decreased as the flue gas cooled down in the steam-flue of the CFB. At 100 % boiler load, Na, K, Cl and S account for more than 90 % of the total elements in the sub-micron particles. However, these four elements demonstrated a 25 % decrease at a boiler load of 67 % compared to 100 % load. Interestingly, the compositions of super-micron particles seemed to be minimally affected by the boiler load.

Synergistic reduction of SO2 emissions while co-firing biomass with coal in pilot-scale (1.5 MWth) and full-scale (471 MWe) combustors

The objective of this study was to understand the synergistic effect of sulfur dioxide (SO2) emission reduction during the combustion of biomass-coal blends in comparison to pure coal by recording the gaseous SO2 concentrations, analyzing the composition of combustion ash by energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD), and simulating and verifying the fate of sulfur by a thermodynamic equilibrium software (FactSage). In Set A experiments, five combinations of coal and steam-exploded biomass (100/0, 75/25, 50/50, 25/75, 0/100 in percentages by mass) were co-fired in a 1.5 MWth pilot-scale combustor. In Set B, pure coal (100/0) and an 85/15 percent mass blend of coal and torrefied biomass were co-fired in a 471 MWe utility boiler. Woody biomass contains lower amounts of sulfur compared to coal and the linear lowering of SO2 emissions as a result of co-firing is called the dilution effect. For Set A tests, FactSage predicted, and ash analysis confirmed that calcium was primarily responsible for the synergistic emission reduction (beyond dilution effect) in all the blends, except in pure biomass, where potassium was responsible. During pure coal combustion, 41.1% of the sulfur in the system was absorbed in the ash, compared to 97.0% during pure biomass testing. In Set B tests, the 85/15 coal-biomass blend, showed a 28.1% reduction in SO2 emissions along with a 22.1% reduction in the lime slurry utilization at flue gas desulfurization (FGD) towers compared to pure coal, which is equivalent to saving 7.4 L of lime slurry per MWe-hour. The results suggest that blending woody biomass with coal can help coal-fired utility boilers meet environmental emission standards and reduce desulfurization costs. These results are important since the current literature is deficient in research conducted at suspension-fired full-scale boilers to reduce SO2 emissions by co-firing coal and biomass blends.

Ash deposition and fine particle formation when utilizing biomass and coal fly ash in lab-, pilot- and full-scale

Fine particles were sampled at the 800 MWth Avedøre Power Station Unit 2 (AVV2) as well as at the 900 MWth Studstrup Power Station Unit 3 (SSV3), both utilizing wood pellets together with coal fly ash (CFA), for alkali capturing. In parallel, fine particle formation was quantified in the BoCTeR pilot-scale pulverized-fuel biomass combustion test-rig, at the Technical University of Munich, using the same wood pellets and CFA-additive as was applied in full-scale, to compare the impact of the CFA-addition. A comparison between fine particle formation and concentration in the BoCTeR and in full-scale, revealed a higher quantity of submicron particles (PM1) at AVV2, compared to SSV3, i.e. differences between the two full-scale boilers, while the PM1-values at BoCTeR was found to be substantially higher than what was measured in full-scale, mainly due to differences in temperature–time history between these two combustor scales.Ash deposition during wood pellets firing with CFA-addition, was quantified by an advanced deposit probe at SSV3. The results showed that with a probe surface temperature of 550 °C, and 2 %wt or 3 %wt CFA-addition, no significant amount of deposit was formed. When increasing the probe surface temperature to 650 °C, at 3 %wt CFA-addition, deposit was formed on the upstream side of the probe. No Cl was detected in the deposit samples, indicating that Cl-related corrosion can be effectively suppressed by 2 %wt or 3 %wt coal fly ash addition.Finally, deposit formation when firing different pre-treated biomasses; K- and Cl-rich wheat straw, milled wood pellets, hydrothermally carbonized (HTC)-leaves and steam exploded (SE)-bark, with and without a K-capturing additives, was also investigated in the entrained flow reactor at Technical University of Denmark (DTU-EFR). In terms of deposit formation, the K- and Cl-rich wheat straw proved to be the most problematic fuel, while milled wood pellets, HTC-leaves and SE-bark, had significantly lower deposition propensities. When applying kaolin and two types of CFA (both rich in Al and Si), as K-capturing additive, the deposition propensity of wheat straw decreased significantly, when applying a molar ratio of fuel-K/additive-Al of 1.0.This paper was originally presented at the recent 28th International Conference on Impacts of Fuel Quality on Power Production, the, Aare, Sweden, September 19–23, 2022.

Numerical study on coal/ammonia co-firing in a 600 MW utility boiler

Co-firing NH3 in coal-fired power plants is an attractive method to accelerate the pace of global decarbonization. However, the contradiction between achieving elevated temperature within the furnace and maintaining low NOx emission constrains the utilization of NH3 as fuel. In this study, 3-dimensional numerical simulations on coal/NH3 co-firing cases were conducted in a full-scale boiler for the first time. The influences of NH3 blending ratio, O2 enrichment combustion and deep air-staging technology were investigated. The results show that the burnout properties of NH3 are excellent in co-firing boiler. Higher NH3 blending ratio leads to lower temperature in the furnace. Enriching O2 concentration to 30% in the secondary air can compensate the temperature decline caused by 50% NH3 co-firing, while it brings an undesired surge in NOx concentrations. The high temperature and strong reducing atmosphere (HT&SRA) could be created by combining the O2 enrichment and deep air-staging combustion. The NO emission drops by 49.6% due to HT&SRA. Then, high flue gas temperature and low NOx emission can be achieved simultaneously. HT&SRA improves the overall exergy efficiency for 50% NH3 co-firing case from 51.65% to 51.78%. The findings open up a promising strategy for utilizing NH3 as a stationary fuel.

Research on the effects of the fly ash reburning on element migration and ash deposition characteristics of high-alkali coal in a full-scale slag-tapping boiler

Burning high-alkali coals in boilers can cause severe slagging problems and affect boiler safety, which restricts the utilization of high-alkali coals. Some studies have shown that high-alkali coals can be burned well in the laboratory or medium-scale slag-tapping boilers, but few studies have been conducted in a full-scale boiler. The effects of the fly ash reburning on element migration and ash deposition characteristics of high-alkali coal in a full-scale slag-tapping boiler were investigated in this work. The results showed that the primary components in different samples were SiO2, Al2O3, Fe2O3 and CaO. The liquid slag had high Si and Fe contents and low Na content, while the ash deposition showed an opposite trend. Moreover, element Cl was only found in the ash deposition. After the fly ash reburning system was put in, the element content changed greatly. The Si and Al contents increased, and the Fe and Ca contents decreased in the liquid slag and fly ash samples. At the same time, the Na content in #1 liquid slag (LS) and #2 LS increased by 10.6% and 8.86% and decreased by 14.76%, 8.05%, 2.27% and 21.49% at the fifth superheater, the secondary reheater, the primary reheater and the economizer, respectively. Fly ash reburning can enhance the absorption of alkali metals by liquid slag and reduce alkali metals in the flue gas. Microscopic morphology analysis showed that the ash particles had a smooth surface after fly ash reburning. NaCl acted as a binder between ash particles, causing particle aggregation in the ash deposition. Mineral composition analysis indicated that fly ash reburning could affect the mineral composition contents in the ash samples. The sodium in the flue gas mainly existed in the form of NaCl. This work can guide industrial boilers to burn high-alkali coal directly and is beneficial for improving boiler design and optimizing combustion operation.

Design of circulation system and downstream gas treatment of a full-scale boiler for Chemical-Looping Combustion

Design of circulation system and downstream gas treatment of a full-scale boiler for Chemical-Looping Combustion

Ash Aerosol and Deposit Formation from Combustion of Coal and Its Blend with Woody Biomass at Two Combustion Scales: Part 2─Tests on a 471 MWe Full-Scale Boiler

Ash Aerosol and Deposit Formation from Combustion of Coal and Its Blend with Woody Biomass at Two Combustion Scales: Part 2─Tests on a 471 MWe Full-Scale Boiler

Modelling of the pulverized coal plasma preparation for combustion

The main goal of this research is to show the action of pulverized coal in the plasma chamber throughcomputer simulation and numerical experiments carried out with the aid of developed well knownthermodynamic, kinetic and multi dimensional computational fluid dynamics based on mathematicalmodels. The data needed for the validation of the numerical procedure were obtained from a cylindricaldirect flow burner equipped with a plasmatron (plasma generator) with 100kW of electric power andmounted on a full-scale boiler (Gusinoozersk TPP, Eastern Siberia). The experiments were carried outusing ‘Tugnuisk’ bituminous coal. Two mathematical models were employed: the one (‘1D Plasma-Coal’) being one-dimensional, but with an emphasis on complex chemistry, the other (3D FAFNIR) beingfully three-dimensional with emphasis on the geometry and overall combustion processes. 1D Plasma-Coal numerical experiments gave the predicted temperatures and velocities of gas and solids along thechamber length; while the concentrations of the gas components (CO, CO2, H2, CH4, C6H6, N2, H2O)were reported for the chamber exit. The degree of coal gasification showed that 54% of coal carbon wasgasified within the plasma chamber. 3D numerical results for plasma jet spreading length were in goodagreement with the measured data, while the temperature profiles within the plasma chamber were overpredicted. The predictions of main species concentrations reveal that oxygen was completely consumedwith the exit product stream consisting of combustible gases, un-burnt volatiles and char particles.

Ecological Aspects in Control of Small-scale Biomass Fired Boilers

Control of combustion in small-scale boilers has been standing outside interest for a long time. More attention has been paid to design of the boilers than to economic and ecological aspects of their operation. To the question under which operating conditions of the boiler a controller carries out its activities has been paid a limited attention. The quick by developing use of biomass fired boilers, local shortage of quality biomass, legal restriction on emissions and available cheaper instrumentation have caused a turn in replacing older simple control solutions by more sophisticated technology. The final goal is evident: to achieve for small-scale boilers to be able to operate automatically fulfilling ecological limits comparable with those usual in fullscale boilers, even when firing biomass of lower quality and no skilled service can be expected. Some experience obtained from experiments aimed at this goal and improvements carried out on pilot boilers are reported in this paper.

Performance and Flexibility Improvements of Staged Pressurized Oxy-Combustion

Staged Pressurized Oxy-Combustion (SPOC) is a low-carbon coal combustion power technology being developed by Washington University in St. Louis (WUSTL). Oxy-combustion plants enable straightforward capture of carbon dioxide (CO2) by removing most of the nitrogen in the combustion air prior to use, thereby burning fuel in near-pure oxygen instead of air, producing a flue gas containing primarily CO2 and water. CO2 capture at amounts > 90% is possible, often using cryogenic air separation. Oxy-combustion typically relies on flue gas recycle (FGR) to reduce the peak temperature and radiation that would otherwise occur in a fuel/oxygen only flame. SPOC reduces the peak temperatures of combustion by utilizing two or more pressurized boiler modules connected in series to produce fuel staging; hence, only a portion of the fuel is combusted in any given furnace module. This means that the thermal energy released at each stage can be captured and removed from the gases prior to subsequent stages, when more fuel is introduced. This allows the SPOC process to operate with minimal FGR, avoiding the associated efficiency losses and additional costs. Also, the process operates at an elevated gas-side pressure, reducing boiler size, enhancing heat transfer to achieve a compact boiler configuration as compared to an atmospheric-pressure boiler design, and allowing for recovery of the latent heat of the water from the flue gas at a temperature useful to the steam cycle. The resultant net efficiency of the system is over 3 percentage points greater than traditional atmospheric-pressure oxy-combustion, and 7 percentage points greater than the post combustion variant, representing a step-change improvement over first-generation capture technologies. To further develop the concept, WUSTL and the Electric Power Research Institute, Inc., organized a project with American Air Liquide, Inc., Doosan Babcock Limited, and the U.S. Department of Energy to investigate a practicable and workable boiler design. The team has identified the potential for enhanced process flexibility for controlling power generation over a wider load range than is normally available to conventional coal-fired power plants due to the staged nature of the heat release. With increasing intermittent renewable generator contribution, on-demand generators need to be highly flexible to participate in the future energy market, requiring extensive operation at reduced load. Conventional coal-fired steam generators typically face challenges in maintaining temperature control of the reheat steam and main steam at reduced loads. This results in inefficient operation, both in terms of the boiler efficiency and steam turbine heat rate. The results of this project show the SPOC process is capable of exceptional turndown, both on a stage basis and with the ability to bypass entire stages. Oxygen-supply flexibility was also investigated, as this is also a key consideration for the overall flexibility of the SPOC process given the operating constraints of conventional air separation units. A boiler design concept assessment was conducted and was focused on delivering compact and constructible design. The assessment checked appropriate tube operating metal temperatures at full load and at lower operating loads, balanced against the needs of efficient coal combustion, and the resultant slagging and ash environments. Combustion testing in the 100-kWth pressurized combustion test rig at WUSTL was carried out to validate the combustion, heat flux profiles and burnout at multiple loads. Combustion parameters investigated were flame stability, fuel burnout, ash composition, radiative heat flux, and temperature profiles. The results of these tests formed the basis of a full-scale boiler design that will encompass improvements in both efficiency and flexibility over conventional oxy-combustion processes. The air separation unit flexibility was investigated, and associated cost implications were addressed. Detailed economic assessment results for a 550 MWe net power block are also provided, allowing for a comparison against the baseline NETL oxy-combustion and post combustion capture cases.

OpenFOAM을 활용한 촤 고유 반응 모델 구현 및 영향 평가

Three intrinsic char reaction models of unreacted shrinking core, random pore and char burnout kinetics (CBK/E) were implemented in the open source code OpenFOAM. Evaluation of the models was performed for a 2.1 ㎿e laboratory-scale furnace and a 500 ㎿e tangential-fired full-scale boiler. For the laboratoryscale furnace there was minor difference among the char reaction models except for the random pore model. For the industrial boiler the influence of the char reaction models was found insignificant in the overall temperature distribution. On the other hand results showed that the CBK/E model predicts the most accurate char burnout rate in the industrial boiler.

Modelling VOCs Emissions in Coal Fired Power Stations using Perfectly Stirred Reactor Approach

The main objective of this study is to evaluate the emissions of volatile organic compounds (VOCs) from coal-fired power stations. A quantitative understanding of the chemistry controlling the formation and destruction of these intermediate species is a prerequisite for the realistic modelling of pollutant formation in flames. Therefore, well investigated skeletal reaction mechanism has been built and introduced into perfectly stirred reactor model in order to accomplish prediction of some of these hazardous important intermediate species. This can be great help in considering lack of experimental VOCs data from fossil fuel fired power stations. Predicted results have been validated against ICSTM Furnace data where possible. The performance of the model against the large laboratory scale experimental data has resulted in considerable confidence in the use of this model for a full-scale boiler configuration. But, more confidence will be forthcoming from an increase in the amount of validation data, which is unluckily lacking at the moment. Furthermore, the model can be used in order to provide deeper insight into the formation processes of VOC species emitted from coal-fired power stations. However, this can be accomplished far better by including more elementary reactions into the skeletal reaction mechanism.

Understanding the effects of oxyfuel combustion and furnace scale on biomass ash deposition

Recycled wood oxyfuel combustion is attractive for the advantages of reusing the waste bioenergy and reducing the carbon emissions. However, the changes in the fuel properties and combustion conditions can lead to uncertainties in the ash deposition. In addition, the understanding of the differences in the ash deposition between the pilot-scale and full-scale furnaces is very limited. We have performed ash deposition experiments on a 250 kW pilot-scale furnace for recycled wood air and oxyfuel combustion along with the EI Cerrejon coal combustion as a reference. A CFD-based ash deposition model, which uses the excess energy based particle sticking model, has been developed and the predictions are in qualitative agreement with the measurement data. The results suggest that, besides furnace temperature, the aerodynamics and ash physicochemical properties dictate the ash deposition. The recycled wood has a much higher deposition rate than the coal in the pilot-scale furnace; however, the biomass can numerically have a lower deposition rate under high velocities close to the full-scale boilers. This is mainly due to the biomass having a much lower sticking efficiency since it has high calcium and silicon concentrations and low potassium concentration. Although the effect of oxyfuel combustion is small and within the experimental uncertainties, it is found that oxyfuel combustion can affect the particle impaction and sticking behaviours depending on the fly ash properties and these effects occur in different ways in the pilot-scale and full-scale conditions. Great care should be taken to perform the transfer of the deposition observations from the pilot scale to the full scale and this is because the furnace scale has an effect on the selective deposition behaviour. In this paper a relationship between the fly ash properties (ash composition, size, etc.) and ash deposition for the woody biomass has been proposed. Additionally, the uncertainty analysis of the CFD modelling is undertaken, which indicates that the fly ash size distribution and the heterogeneity are responsible for the major source of errors along with the experimental uncertainties.

Corrosion in Recycled Wood Combustion—Reasons, Consequences, and Solutions

Combustion of recycled wood leads to increased corrosion problems of furnace walls. One of the causes is known to be the elevated concentrations of heavy metals and chlorine and rather low levels of sulfur in the fuel. A detailed understanding of the boiler environment, deposit formation, and corrosion mechanisms are essential factors in finding the most cost-effective solutions for reducing corrosion problems for full-scale boilers. The presence and behavior of lead in bubbling fluidized bed boilers firing recycled wood were studied using a fine particle measurement technique and short-term deposit probe measurements. The main compounds present in the fine particles were determined with the use of ion chromatography and inductively coupled plasma mass spectrometry. Deposit samples were analyzed with scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy and an X-ray diffractometer. The results were compared to the similar deposit samples taken from a circulating fluidized bed boiler. The analyses revealed that the major part of lead was combined with chlorine and, in most cases, also with potassium. Two compounds were identified (KPb2Cl5 and K2PbCl4) in the boiler environment as well as in laboratory-scale corrosion tests. Thermodynamic calculations also showed the formation of these compounds. The corrosivity and the corrosion mechanism of potassium lead chlorides were further studied with the laboratory-scale measurements. The results showed that iron, potassium, lead, and chlorine will form sintered particles together. This supports the theory that corrosion observed below the first melting temperature of the deposit could be driven by the melt formation between the deposit and the corrosion product.

Numerical and experimental study on biased tube temperature problem in tangential firing boiler

In this study, a numerical and experimental investigation on the flow field in a 500-MW unit of a tangentially fired boiler has been performed to understand the effect of residual swirl on the temperature distribution and inconsistency of flow velocity in the crossover pass. The k–kL–ω turbulence model was used in this study to analyse the possible formation of transition flow in the tangential firing boiler. Furthermore, 1D and 3D hybrid modelling were used for the simulation of the convective heat transfer in the boiler tube assembly. The commercially available code ANSYS Fluent and COMSOL were used for the studies, respectively. The numerical results for the velocity profile and residual swirl in the boiler were validated using the experimental results from the cold air velocity test, in which the flow velocity was directly measured in the full-scale boiler. The computational results agreed well with the experimental findings. Furthermore, from a comparison of the simulation and experiment, it was found that the biased gas flow induces non-uniform tube temperatures at the crossover pass area of the tangential firing boiler. These results aid in the detailed study of the residual swirl effects on the temperature or flue gas at various locations of the boiler. This study indicates that a non-uniform tube temperature at the crossover pass is an inherent problem resulting from residual swirl in the tangential firing boiler. Therefore, the pulverised coal burner should be fine-tuned to avoid tube rupture due to local overheating.

Deposit Shedding in Biomass-Fired Boilers: Shear Adhesion Strength Measurements

Ash deposition on boiler surfaces is a major problem encountered in biomass combustion. Timely removal of ash deposits is essential for optimal boiler operation. In order to improve the understanding of deposit shedding in boilers, this study investigates the adhesion strength of biomass ash from full-scale boilers, as well as model fly ash deposits containing KCl, K2SO4, CaO, CaSO4, SiO2, K2CO3, Fe2O3, K2Si4O9, and KOH. Artificial biomass ash deposits were prepared on superheater tubes and sintered in an oven with temperatures ranging from 500 to 1000 °C. Subsequently, the deposits were sheared off by an electrically controlled arm, and the corresponding adhesion strength was measured. The effect of sintering temperature, sintering time, deposit composition, thermal shocks on the deposit, and steel type was investigated. The results reveal that the adhesion strength of ash deposits is dependent on two factors: ash melt fraction, and corrosion occurring at the deposit–tube interface. Adhesion strength inc...

Investigation of characteristics and formation mechanisms of deposits on different positions in full-scale boiler burning high alkali coal

Zhundong coalfield are huge in China; however, high alkali contents induce severe slagging and fouling problems in boiler. To obtain the characteristics and formation of deposits in boiler burning high alkali coal, the deposits at different positions were sampled in a 350MW boiler burning Zhundong coal, and analyzed by X-ray Fluorescence. The results indicated that the slagging was deteriorating from bottom to top at water wall. The slag with obvious layer structure only presented at platen superheater. The deposits in horizontal flue were mainly formed from the solidified particles attachment. To further understand the formation mechanism of layer structure in slag at platen superheater, the slag was separated according to the morphology and each layer was analyzed by X-ray Diffraction. The results suggested that through the condensed CaSO4 was the main compounds for the strong thermophoresis in layer 1, the little fused particles with high sodium content played the key bonding role. For layer 2, the fused particles contain high sodium content adhered to deposit surface and formed melt surface, which capture the other particles. The Fe species captured by melt surface lower the melting temperature of the formed deposit, and then aggravated the deposit fusibility in layer 3.

Coal-nitrogen release and NOx evolution in the oxidant-staged combustion of coal

Air-staged combustion is one of the most sophisticated and effective technologies for reducing NOx emissions. To better understand NOx formation and destruction in air-staged combustion, measurements of the major gas species and NOx profiles along the axial distance of a furnace were obtained in a 20 kW down-fired pilot-scale combustion facility. The furnace of this facility has a representative configuration of a full-scale boiler with a staged combustion system in terms of residence times and temperature characteristics. The combustion air is divided into primary air, secondary air, and burnout air, which can be heated to 80 °C, 200 °C, and 200 °C, respectively by electric heaters before entering the furnace. Under the staged case, a usual NOx trend was obtained by the measurements: coal nitrogen can be rapidly converted to NOx and then reaches a peak concentration in the primary combustion zone, followed by a significant reduction in NOx in the reduction zone. The greater the staged level is, the earlier the reduction stage starts. For the deepest air-staged case with ratio of burnout air to total air of 0.42, the NOx profile curve does not exhibit the features of the NOx peak and a pronounced reduction, as the NOx values are always at very low levels. There exists an NO reducing saturation phenomenon in an overlong reduction zone. For providing data associated with the oxygen-enriched combustion technology that will be available in the near future, low-level oxygen-enriched combustions of coal (21–30%O2) have been performed to assess their NOx emission characteristics compared to the air-staged configuration. There is no essential difference in the mechanism of NO formation and destruction between the staged combustion with oxygen enrichment and the air-staged combustion. With the increase of the staged degree, the impact of O2 concentration promotion on the rise of NOx emission gradually decreases. The NOx rises again at the point where burnout oxidizer addition is caused by the oxidation of some unknown nitrogen-containing intermediates prevailing in the oxygen-poor reduction zone.

The ash deposition mechanism in boilers burning Zhundong coal with high contents of sodium and calcium: A study from ash evaporating to condensing

The high contents of sodium and calcium in Zhundong coal induce severe slagging and ash deposition in boilers. In this study, the ash deposition mechanism was investigated based on the results obtained from a full-scale boiler (350 MW) burning Zhundong coal, and a fixed bed reactor used for ash evaporating-condensing. In the full-scale boiler, the condensing and depositing of sodium and calcium sulfates play an important role on ash depositing on convection heating surfaces. Sulfates start to significantly condense and deposit at the flue gas temperature of about 850 °C on the medium and high temperature reheater surfaces. Ash evaporating tests proved that, with the increasing in temperature from 400 °C to 1200 °C, the ash evaporating process is divided into three stages: 1) 400–800 °C, 80% of sodium, and 100% of chlorine are released; 2) 800–1000 °C, all the left sodium evaporates and sulfur starts to be released with the formation of partial aluminosilicates; 3) 1000–1200 °C, all the left sulfur is released through the decomposition of calcium sulfates and then calcium starts to evaporate, while silicon oxides disappear due to the formation of new complex silicates. Ash condensing tests further proved that, the sodium in Zhundong coal was released mainly in the forms of atom, oxide, and chloride, in which sodium chloride account for about 50%. When the evaporating temperature increased higher than 1000 °C, partial alkali and alkaline earth metals were released as gaseous sulfates, and afterward condense and deposit on the heating surfaces. At last, a temperature-dependent ash deposition mechanism in Zhundong coal combustion was proposed.