Combustion Side Research Articles

Simulation of Dynamic Characteristics of Supercritical Boiler Based on Coupling Model of Combustion and Hydrodynamics

To accommodate the integration of renewable energy, coal-fired power plants must take on the task of peak regulation, making the low-load operation of boilers increasingly routine. Under low-load conditions, the phase transition point (PTP) of the working fluid fluctuates, leading to potential flow instability, which can compromise boiler safety. In this paper, a one-dimensional coupled dynamic model of the combustion and hydrodynamics of a supercritical boiler is developed on the Modelica/Dymola 2022 platform. The spatial distribution of key thermal parameters in the furnace and the PTP position in the water-cooled wall (WCW) are analyzed in a 660 MW supercritical boiler when parameters on the combustion side change under full-load and low-load conditions. The dynamic response characteristics of the temperature, mass flow rate, and the PTP position are investigated. The results show that the over-fire air (OFA) ratio significantly influences the flue gas temperature distribution. A lower OFA ratio increases the flue gas temperature in the burner zone but reduces it at the furnace exit. The lower OFA ratio leads to a higher fluid temperature and shortens the length of the evaporation section. The temperature difference in the WCW outlet fluid between the 20% and 60% OFA ratios is 11.7 °C under BMCR conditions and 7.4 °C under 50% THA conditions. Under the BMCR and 50% THA conditions, a 5% increase in the coal caloric value raises the flue gas outlet temperature by 32.7 °C and 35.4 °C and the fluid outlet temperature by 6.5 °C and 9.9 °C, respectively. An increase in the coal calorific value reduces the length of the evaporation section. The changes in the length of the evaporation section are −2.95 m, 2.95 m, −2.62 m, and 0.54 m when the coal feeding rate, feedwater flow rate, feedwater temperature, and air supply rate are increased by 5%, respectively.

Thermal deviation mechanisms for coupled heat transfer between the combustion side and the furnace tube side in the tubular heating furnace

Temperature deviation significantly affects the safe operation of tubular heating furnaces. This is attributed to the inevitable temperature difference between the flue gas and the working medium on the tube side. To date, little research has considered the dynamic heat transfer between the furnace body and the tubes in tube furnaces, and even less attention has been paid to the thermal deviation within tube furnaces. To optimize the operational parameters of the heating furnace, a comprehensive heat transfer model is established in this paper to numerically couple the heat transfer between the combustion side and the working medium on the tube side in tube furnaces. A hydrogenation feed heating furnace with a processing capacity of 600,000 tons per year in a chemical enterprise is selected as the subject of study. Using Fluent software, on the basis of coupled simulation, a non-uniformity coefficient of temperature distribution is introduced to characterize the thermal deviation within the furnace. The impact of the thermal load and hydrogen doping in the fuel gas on the thermal deviation within the furnace is analyzed. The numerical results demonstrate that with an increase in thermal load, the flame structure in the furnace gradually becomes concentrated, the average temperature of the flue gas increases, and the outlet temperature of the furnace chamber increases by 19%, with the thermal efficiency decreasing by 5.11%; at the same time, the overall non-uniformity coefficient of the temperature distribution in the furnace fluctuates irregularly, indicating a nonlinear relationship between the thermal load and the thermal deviation within the furnace. With an increase in the hydrogen content in the fuel gas, the flame structure in the furnace becomes more dispersed, the average temperature of the flue gas increases, but the combustion efficiency of the heating furnace exhibits irregular fluctuations; the thermal deviation within the furnace initially increases and then decreases, with a lower non-uniformity coefficient of temperature at 50% hydrogen content, which is 2.48% lower than at 40% hydrogen content. Under both conditions, the thermal deviation primarily leads to localized hot spots in the radiant furnace tubes and uneven temperature distribution in the convection section, with a minimal impact on the thermal efficiency of the heating furnace.

Effect of ammonia addition on combustion characteristics of hydrogen/air using passive turbulent jet ignition

Hydrogen (H2) and ammonia (NH3) are ideal carbon-free fuels, and the application of NH3/H2 in internal combustion engines offers the potential to reduce carbon emissions. And turbulent jet ignition (TJI) is an advanced ignition strategy that can effectively enhance the combustion. This work aims to experimentally investigate the effect of NH3 addition on the combustion characteristics of H2/air mixtures under passive TJI conditions. Combustion images and instantaneous pressure were collected for processing and analysis. The effect of NH3 substitution and equivalence ratio were investigated. The results indicate that the peak combustion pressure decreases with the addition of NH3, accompanied by longer combustion duration and ignition delay, as well as lower jet velocity and flame area growth rate. The hot jet will be quenched by the strong heat transfer, causing the change in the ignition mechanism as the NH3 substitution ratio increases. In addition, compared to lean conditions, the combustion appears to be less sensitive to the equivalence ratio on the rich side and always shows high jet velocity and flame propagation speed. However, a relatively small amount of NH3 addition changes the ignition mechanism on the rich combustion side compared with lean conditions, resulting in a longer ignition delay.

Thermal matching characteristics in an autothermal methanol reforming microchannel reactor for hydrogen production

To ensure efficient operation of an onboard hydrogen production reactor, it is essential to synchronize heat generation and consumption in the channels. A computational fluid dynamics study is conducted to simulate a base case of the microchannel reactor that couples hydrogen catalytic combustion with methanol steam reforming in response to this problem. Our results show poor thermal matching of the reactor under the base case, with hydrogen combustion concentrated in the first half of the channel, creating a hot spot (796.09 K) and incomplete catalyst utilization (89% and 60% for reforming and combustion, respectively). Compared to the base case, when reducing the combustion side’s pre-exponential factors to 1.5×106, the local hot spots are eliminated, and the catalysts in the reforming and combustion channels are fully utilized. Furthermore, the maximum temperature of the reactor decreases to 691 K, producing more hydrogen while less hazardous carbon monoxide.

Mathematical procedure for predicting tube metal temperature in the radiant superheaters of a tangentially and front fired utility boilers

The superheaters can be indicated as one of the most critical components of the utility boilers experiencing highest temperatures. The local overheating of the heat exchangers material is a significant operational issue. The current and future market conditions implicate new unfavourable regimes of boiler operating conditions bringing the need for prediction capability of tube wall temperature in either design or retrofit process. There is a need to obtain a realistic metal temperature distribution based on uneven heat flux absorbed and complex steam flow. The current paper demonstrates a computational methodology of predicting tube metal temperature of radiant superheaters platens. It couples a full scale 3-D boiler simulations based on CFD technique on the combustion side with an in-house 1-D heated pipe flow model of the steam side. The methodology was applied to two different boilers with different firing strategy (front fired and tangentially fired) to demonstrate the model flexibility. The model is capable of identifying the exact location of specific platen in which the metal maximum temperature occurs. A validation study was conducted by utilising the available measurements. The computations are in qualitative and quantitative agreement with measurement. The modelling approach can be easily extended to boilers utilising all types of fuels as well as heat recovery steam generators.

Research on combustion and emission characteristics of circulating fluidized bed during load changes

The conventional design of circulating fluidized bed (CFB) focuses more on high combustion efficiency and low pollutant emission. However, as the proportion of new energy generation in China continues to increase, the power grid is placing higher demands on the operational flexibility of thermal power units. The CFB has the advantage of a wide range of load regulations, and it is an inevitable choice to participate in the deep and flexible peak regulation of the power grid. In order to conduct research on the features of CFB during load changes, this work used 0.1 MWth CFB as the research object and examined its combustion and emission characteristics at different loads, as well as the change in temperature and pollutant emissions during load changes. The results showed that the temperature distribution along the height direction of the furnace was not uniform at lower load, while the inhomogeneity of the temperature distribution decreased, and both the temperature and pressure of the furnace increased as the load increased. Moreover, NOx emission and combustion efficiency both rose with the load. When the load increased from 30% to 100%, the NOx emission increased from 228 mg/m3 to 324 mg/m3, the CO emission decreased from 1422 mg/m3 to 104 mg/m3, and the carbon content of the fly ash decreased from 39.2% to 27.6%. If the temperature stability was used as the reference standard for load stability, the load-change rates on the combustion side of the experiment platform from 50% load to 75% load and from 75% load to 100% load under conventional combustion were 0.69%/min and 0.81%/min, respectively. In general, the load variation range of the CFB was wide, but the load-change rate on the combustion side and the combustion efficiency at low loads need to be further improved.

Ethylene Electrooxidation into Ethylene Oxide on Nanostructured Ag/GDC Electrocatalysts

Ethylene oxide (EO) is nowadays a chemical intermediate of paramount importance. As one of the highest volume petrochemicals, it serves as a precursor for the further production of added value molecules, including plastics, polyester and ethylene glycol. EO is industrially produced by the partial oxidation (epoxidation) of ethylene with air (or oxygen) on Ag/α-Al2O3 catalysts at low reaction temperatures (around 220 oC) and high pressures (10-30 bar). Silver is the only one known active heterogeneous catalyst for the epoxidation of ethylene. The epoxidation is in competition with the combustion side reactions of both ethylene and EO, leading to the production of CO2 and H2O. The mechanism of the epoxidation and, in particular the nature of the involved oxygen species are under debate.We have designed by reactive magnetron sputtering a solid oxide electrochemical cell based on nanostructured Ag/Gadolinia-Doped Ceria (a well-known O2- conductor) catalyst. We found an unprecedent linear increase of the selectivity towards EO (Fig. 1) and of the ethylene conversion. These results demonstrate that EO can be electrocatalytically produced in a solid oxide electrochemical cell between 220 °C and 300 °C on a nanostructured Ag/GDC electrocatalyst prepared by reactive magnetron sputtering. Despite low values of Faradaic efficiencies, the EO selectivity and the ethylene conversion can be increased at 220 °C with the application of small positive currents (< 1 mA) that correspond to an energy consumption of around ~ 1 mW/cm². This proof-of-concept opens the way for a new environmentally-friendly route (without utilization of chlorinated hydrocarbons) for the epoxidation of ethylene in a solid oxide cell via direct electrooxidation of ethylene. These results also underline the key role of O2- species in the mechanism of ethylene epoxidation on Ag. Figure 1

Numerical Simulation of the Effect of Combustion Process on Flue Gas Thermal Deviation in 660MW Boiler

In order to analyze the effect of the air volume on the combustion side and the non-uniformity of pulverized coal on the temperature distribution of the boiler outlet flue gas, and then analyze the thermal deviation characteristics of the steam heating surface, the following five different working conditions are calculated and analyzed in a 660MW boiler. Results show that the deviation of pulverized coal content of each burner and the deviation of primary air volume are the important factors that cause the temperature deviation of flue gas outlet in the furnace. Therefore, it is an important way to improve the temperature distribution and reduce the thermal deviation of flue gas outlet by improving the uniform combustion and air distribution of the burner in the furnace.

Effective Implementation of low thermal conductivity material Yttrium Stabilized Zirconium Coating on a Diesel Engine Components Fuelled with neat Waste Cooking Oil-An Assessment Study

In this study, an attempt was made on the consumption of neat waste cooking oil (WCO) in a thermal barrier coated (TBC) diesel engine. During the first stage of the study, the standard engine fuelled with diesel and WCO with different power outputs (25%, 50%, 75% & 100%) to measure the performance, combustion and emission parameters. In the second stage of the study, the engine components coated with 8% yttrium stabilized zirconium (8%YSZ) ceramic coat and NiCrAl bond coat using plasma spray coating. In the third stage of the study, the base engine convert to TBC mode and fuelled with diesel and neat waste cooking oil and reading were compared. From the experimental study, it came to know that, the brake thermal efficiency of uncoated diesel and WCO, 31.2%, and 25.7% were as for TBC diesel and WCO it was improved to 32.8 % and 27%. In the combustion side, the cylinder peak pressure for uncoated diesel and WCO 62 bar and 55.8 bar were as for TBC diesel and WCO improved to 63 bar and 57 bar. Further on the emission side, the CO, HC, and smoke were decreased in the TBC mode along with the penalty of increase in the NOx for both diesel and WCO. From the experimental study, the TBC mode reduces the heat flux from the combustion chamber and traps the heat inside the cylinder as an effect it reduces the ignition delay and helps to utilize highly viscous fuel like WCO in a diesel engine.

Experimental Investigation of LPG/H2/Air Premixed Flame Stability Zone

Flame stability, environmental changes and fossil fuel shortage represent major challenges to any successful combustion device utilization. In this study, the stability zone of laminar premixed ILPG/ -air flames was investigated experimentally. Non-swirling burners with different diameters (10, 12.5, and 17 mm) were employed to characterize the flashback and blow-off limits. Different hydrogen blends (0%-50%) at equivalence ratios (ER) (0.6-1.4) were used. The results show that maximum flashback limits occurred at ER slightly richer than stoichiometric, with the mixture flow rate at a flashback of (3.75, 7.25 and 14) LPM for the 50% hydrogen blending ratio and a burner diameter of (10, 12.5 and 17 mm), respectively. When hydrogen blending was 50% at stoichiometric condition, the critical velocity gradient at flashback increased from (469.9-650.8 1/s) with 10 mm diameter, and the critical velocity gradient at blow-off increased from (1538-2936 1/s). It was observed that the flashback limits decreased with increasing burner diameter. Its limit increased with increasing hydrogen addition to the ILPG. The blow-off limit increased with increasing fuel concentration. This paper further presents the stability zone for ILPG/air combustion for a non-swirling burner with a 10-mm diameter and different hydrogen blends. It was found that the stability zone was narrow on the lean combustion side and enhanced with increasing diameter and hydrogen addition.

Emissions of dioxins and furans during steam gasification of Automotive Shredder residue; experiences from the Chalmers 2–4-MW indirect gasifier

Emissions of dioxins and furans during the gasification of Automotive Shredder Residue (ASR) were investigated. The experimental work was carried out in a Dual Fluidized Bed (DFB) system, which consists of a 2–4-MWth gasifier that is fluidized with steam, and an interconnected fluidized bed combustor that is fluidized with air. Two different ASR fractions with higher and lower contents of plastic were tested. Measurements were carried out in the flue gas stream exiting the combustion side of the DFB, as well as in the raw gas stream exiting the gasifier side. A calcium (lime) coat was applied to the flue gas filter to ensure compliance with the emissions regulations regarding the retention of HCl and dioxins. The results showed lower emissions of polychlorinated dibenzo-p-dioxins/dibenzofurans (PCDD/PCDF) in the flue gas when the raw gas derived from the ASR gasification was combusted, as compared to the direct combustion of ASR. The level of polychlorinated compounds in the flue gas before the lime-coated filter was 0.11 ng/m3N dry gas (I-TEQ) when gasification was used as a pre-step, as compared to 0.27 ng/m3N dry gas (I-TEQ) when the ASR was directly combusted. The raw gas produced by gasification contained very low levels of PCDD/PCDF, whereby the toxicity per kg of ASR was 0.17 ng/kgASR, as compared to 3.44 ng/kgASR after passage through the combustion and cooling sections and 0.34 ng/kgASR at the outlet after the lime-coated filter. A higher content of plastic in the ASR led to an increase in the levels of dioxins and furans in the raw gas, with the highest yield seen for highly chlorinated compounds, while higher temperature in the gasifier is shown to be beneficial in reducing dioxin formation.

Nitridation of grate in a biomass‐fired boiler

AbstractAfter a straw‐fired power plant in Denmark had been in operation for 16 years with 93,815 hr operation, there was a need for overlay welding repair of the grate due to a decrease in wall thickness from erosion/corrosion. The grate consisted of 15Mo3 (Werkstoff nr. 1.5415) tubes where water/steam flows through, which were welded together with fins, and where one side faced the combustion zone coming into contact with the fuel (the side that required weld overlay). The water/steam temperature was 320°C and the surface metal temperature was estimated to be 350°C. However, there were difficulties when trying to overlay weld the grate as the weld pool was unstable. The microhardness measurements on cross‐sections of the tubes revealed a higher hardness of the tube facing the combustion zone compared to opposite the combustion zone. In addition, there was higher hardness adjacent to the combustion side than the steam/water side. Analysis of etched cross‐sections with light optical and scanning electron microscopy revealed the presence of acicular structures at the convex surface. Further analysis has been conducted with X‐ray diffraction and a LECO nitrogen analyser indicating the presence of iron nitrides. Reasons for their evolution and implications for the repair welding are discussed.

Localized fire in a gallery: Model development and validation

A numerical simulation of a wood fire in a natural gallery is compared with experimental data. Temperatures, velocities, gases and particles concentrations as well as soot deposits are the measured quantities of the experiment. We expose in this article the comparison with a simulation performed by the open-source solver FireFOAM. In order to improve the accuracy of the modeling, the initial numerical tool has been modified. A boundary condition for temperature based on energy conservation has been added to better estimate thermal losses at the walls. A hypothesis regarding vertical velocity in a confined geometry containing a localized fire is also made. By assuming that the velocity is exclusively horizontal, the error between simulation and experiment is reduced. With the aim of comparing the temperature measurements with the simulation accurately, the thermocouple model commonly used in FireFOAM has been changed. The limitations of the previous models are also discussed. Then, two combustion side issues are presented. First, the model of Beresnev-Chernyak computes soot deposit on walls. Second, the fractional effective doses (FED) for toxicity, radiation and heat hazards have been implemented to evaluate the safety of the environment during the fire.

FAILURE MODE AND EFFECT ANALYSIS (FMEA) AS TREATMENT OF PREDICTIVE PREVENTION AND LEAKAGE OF BOILER TYPE BALANCE DRAF FAN

Risk analysis aims to prevent the incompatibility of a system used in the testing process including the Boiler system. At the Labuan 2 Steam Power Plant (PLTU) 2, the Boiler component was declared very often to have a pipe leak. Failure Mode And Effect Analysis (FMEA) is a method used to analyze the risk of failure. This method is represented by calculating the value of RPN (Risk Priority Number) obtained from the results of the multiplication of S (severty), O (occurrence), and D (detection). The highest analysis results are shown at 480 (in the steam drum component in the form of abnormal steam and water level failure). The cause is human error. Next three levels below are (1) Economizer component, which is 360 causes of combustion side ash erosion, material lifetime, and economizer tube fibration; (2) super heater tube and re-heater tube components which are 320 due to Corrosion, Overheat Flyash erosion, Sub blower erosion, Welding error; and (3) Walltube which is 320 causes of overheating and sub blower erosion. The recommendation for the analysis of FMEA testing to predict mantenance is done by applying remaining life assessment after 8,000 hours / operational.

Performance analysis of a membrane‐based reformer‐combustor reactor for hydrogen generation

Hydrogen is mostly produced in conventional steam methane reforming plants. In this work, we proposed a membrane-based reformer-combustor reactor (MRCR) for hydrogen generation in order to improve heat recovery and overall thermal efficiency. The proposed configuration will also reduce the complexity in existing steam methane reforming (SMR) plants. The proposed MRCR comprises combustion zone, hydrogen permeate zone, and SMR zone. A computational fluid dynamics model was developed using ANSYS-Fluent software to simulate and analyze the performance of the proposed MRCR. Results show that high hydrogen yields were observed at high reformer pressures (RPs) and low gas hourly space velocities (GHSVs). Furthermore, by increasing the steam to methane ratio and addition of excess air in the combustion side, the hydrogen yield from the MRCR decreases. This is attributed to the reduction in the effective temperature of the hydrogen membrane. High RP, low GHSV, and low steam to methane ratio that increased the hydrogen yield also decreased carbon monoxide (CO) emissions. For an increased RP from 1 to 10 bar, the CO emission decreased by about 99%. The reduction in CO emission at high RP would be attributed to the effect of water gas shift reaction in the MRCR. Results of the extensive parametric study presented in this work can be used to determine the operating conditions based on tradeoffs between hydrogen yield (mole H2/mole CH4), hydrogen production rate (kg of H2/h), allowable CO emissions, and exhaust gas temperature for other applications such as gas turbine.

Productivity versus product quality: Exploring the limits of autothermal microchannel reactors in methane steam reforming

Energy-intensive endothermal gas phase reactions such as methane steam reforming constitute an essential class of chemical processes that provide feedstocks for the chemical industry. Autothermal reactor designs that couple endothermal reforming with exothermal combustion reactions are an effective means of process intensification towards a more sustainable production. Besides a proof of these reactor concepts, however, there is a need for optimization of the reactor design and operating parameters. Using a high-fidelity first-principles reactor model that includes the solution of the radiative transfer equation the optimal design of an autothermal channel reactor is identified emphasizing the heat transfer inside the reforming compartment. A two-step approach is proposed starting with the design of the reforming side followed by the design of the combustion side with two-compartment model. Highest product purity in combination with minimum heat transfer barriers dominated by conduction is achieved with a microchannel reactor with widths up to few millimeters. An increase in catalyst productivity, however, is only achieved with a reactor width of 8 mm and more where radiative heat transfer becomes significant. A technological optimal design is thus subject to the plant and economical constraints at a specific site and therefore balancing of purity, energy efficiency and productivity is required in any given scenario.

Effect of Excess Iron on Oxidative Dehydrogenation of 1-Butene over a Series of Zinc Ferrite Catalysts

The influence of excess Fe3+ in ZnFe2O4 for the catalytic oxidative dehydrogenation of 1-butene to 1, 3-butadiene was investigated to try to clarify inconsistencies in the existing literature. A series of nanoscale zinc ferrite powders were produced with increasing Fe: Zn ratios. The materials were characterized by a range of techniques, which showed the presence of α-Fe2O3 as a distinct phase with an increasing excess of Fe3+ and SEM highlighted the increased presence of surface structures on the ferrites at higher Fe: Zn ratios. Reaction testing showed α-Fe2O3to be virtually inactive for the oxidative dehydrogenation of 1-butene. Results for the ferrite catalysts showed a significant decrease in both conversion and yield with an increasing excess of Fe3+. Therefore an excess of Fe3+ has a negative effect on catalytic activity and selectivity of zinc ferrite for the oxidative dehydrogenation of 1-butene, but acts as a promoter for competing hydrogenation and combustion side reactions.

Thermal Effect on the Automobile Piston: A Review

Background/Objectives: Mechanical and thermal properties of aluminum-based piston alloys are mainly depend on the heat treatment. Conventional heat treatment techniques are also not resulted to that level where it can be used for piston of downsized engine. Methods/Statistical Analysis: Around 30 papers were reviewed to understand the thermal effects and consequences on the Al Si piston alloy. Findings: From the literature it can be understood that two-step solution treatment of the Al Si alloy resulted the good mechanical properties then the single step solution treatment. Heat treatment at 540°C for 8 h and aging at 190°C for 8 h is correctly choice to achieve optimum mechanical properties by heat treatment. To calculate the temperature effect and heat transfer to the engine piston crown, the spatial and time averaged combustion side boundary condition is a most favorable and suitable treatment method within engineering approximations. For temperature and thermal stress distributions for a coated surface piston crown, the coating surface temperature increased with coating thickness by decreasing rate and the best results are at 1 mm coating. Application/Improvements: The coated piston has low thermal conductivity then the uncoated piston.

Characteristics of olivine as a bed material in an indirect biomass gasifier

The use of untreated olivine as the bed material in a biomass gasifier is investigated in this work, in which activation of the material is the main focus. The experiments were carried out in the Chalmers 2–4-MWth indirect biomass gasification unit and comprised analyses of the gas composition and bed material, as well as changes in tar yield. Starting from the raw material, the first signs of activation, in the form of a reduction in the tar yield, were observed already during the second day of the operation. The tar yield continued to decrease with time, and by the fourth day it was reduced by 30%, as compared to the yield on the first day of the operation. Analysis of the bed samples showed accumulation of inorganics within the bed material, with a share of potassium being present in leachable form. Thermodynamic calculations support the indications from the experiment that potassium can be released under gasification conditions and may play an important role in the activation of olivine. To examine the impacts of S and silica on the activity of olivine, two experiments were conducted. The addition of S to the combustion side gave a positive effect in terms of the tar levels in the raw gasification gas. The addition of silica sand revealed, as expected given the affinity of potassium for silicone, negative influences on the tar yield and gas composition that could not be attributed to mere dilution, as compared with the gas produced during operation with pure olivine.

Exergy analysis of coal fired tea drying furnace

Thermodynamic analysis of coal fired furnace attached to a vibro fluid bed dryer (VFBD) for hot air generation in the drying stage of tea processing installed in the Rosekandy tea estate factory, Cachar, Assam, India is studied with the application of the principles of energy and exergy analysis. The study explores that the process of obtaining hot air used in tea drying process is highly irreversible as envisaged by 82.41% of exergy destruction and most of the irreversibilities are located in the combustion side of the furnace. The exergetic efficiency of the furnace was found to be only 6.69% indicating that there is a great scope for improvement of the exergy performance of the drying furnace by taking appropriate measures. About 44.94% of the energy input is lost to the surroundings with heat transfer as revealed by energy analysis, which has got a lower thermodynamic value as explicit from exergy analysis.