Air and oxy-fuel combustion kinetics of low rank lignites

The air and oxy-fuel combustion processes of two low-grade lignite coals were investigated by thermogravimetric analysis (TGA) method. Coals were provided from two different coal mines in the Aegean region of Turkey. Oxy-fuel combustion experiments were carried out with three different gas mixtures of 21% O₂–79% CO₂; 40% O₂–60% CO₂ and 50% O₂–50% CO₂ at 950 °C and heating rates of 10 °C/min, 20 °C/min and 40 °C/min. The kinetics of the oxy-fuel combustion of coals were studied by using four different methods namely, Coats-Redfern (model-fitting method), Friedman (FR), Flynn–Wall–Ozawa's (FWO) and Kissinger–Akahira–Sunose's (KAS) methods. The apparent activation energies of combustion process calculated by FWO method are slightly but systematically higher than that calculated by the KAS and FR methods for the oxy-fuel atmospheres. Combustion behavior of both coals in the oxy-fuel combustion environment could vary significantly, likely due to their characteristics such ash and volatile matter contents.     

Experimental investigation on the kinetics of biomass combustion in vertical tube reactor

The paper presents results of experimental investigation performed in order to examine kinetics of loose biomass combustion in vertical tube reactor. The investigation conducted included continuous measurement of the fuel mass loss rate, with two biomass combustion models (piston and batch model) proposed, each relying on appropriate theoretical postulates. Results obtained indicated that piston combustion model had shown better agreement between theoretical and experimental data and was therefore used to further analyse effects of excess-air on the combustion kinetics, as well as associated effects of flue gas recirculation. Recirculation of cold flue gases is used to lower peak temperature inside the furnace, as well as to reduce a zone where ash melting problems may potentially occur. During the investigation performed, effects of flue gas recirculation on the combustion process were simulated by simultaneously injecting nitrogen and air flows into the furnace. This was deemed appropriate to simulate real-life conditions prevailing in the furnace with gas recirculation. Experiments were conducted on specially designed and constructed apparatus that enabled kinetic parameters to be determined for the combustion of different types of biomass. Results obtained have indicated that quantity of air affects kinetics of biomass combustion and that increased recirculation leads to reduced biomass reaction rate. The same conclusion was reached based on the results of experiments conducted with two different types of agro-biomass, namely wheat straw and corn stalks, which are most commonly used for energy generation. Results achieved are deemed particularly important when it comes to design of new plants that utilize cigarette type combustion system, but also for development of numerical models used to simulate combustion of biomass bales, with special emphasis placed on the impact of recirculation gases on the combustion kinetics.     

Advances in catalytic hydrogen combustion research: Catalysts,mechanism, kinetics,and reactor designs

Compared with conventional hydrogen-air combustion, catalytic hydrogen combustion (CHC) exhibits higher safety and efficiency and ultra-low NOx emissions. Significant advances in CHC have been achieved in recent years through fundamental research. Therefore, the state-of-the-art CHC technology is comprehensively reviewed herein, including catalyst development, catalytic reactors, and the factors impacting the kinetics of various CHC systems. Furthermore, the progress made in CHC catalyst design over the years is examined, and detailed information regarding their synthesis, structure, and characteristics are presented. The comparison of several types of CHC reactors, including fixed-bed, monolithic, and microchannel reactors, is presented in terms of their operational features and performances. The effects of several operating parameters, including the reaction temperature, hydrogen-to-oxygen stoichiometric ratio, space velocity, residence time, and operating pressure, of the CHC processes are reviewed. The catalytic reaction pathways and the kinetics of the CHC processes are also summarized, advancing the understanding of the underlying chemistry on the catalyst surfaces. This review analyzes the various catalysts, reactor types, operating factors, and underlying mechanisms involved in CHC to better understand the mass transfer, heat transfer, and chemistry during this process. Finally, future research directions are proposed to explore the design of catalytic reactors for CHC.     

Study of kinetics and thermal decomposition of ammonia borane in presence of silicon nanoparticles

Ammonia borane (AB, NH₃BH₃) is a promising material by virtue of its high gravimetric hydrogen storage capacity of 19.6 wt%. Hydrogen release from AB initiates at around 100 °C and as such is compatible to meet the present-day requirements of a PEM fuel cell. The thermal decomposition of AB is a complex process involving several reactions. Major issues include poor reaction kinetics, leading to delayed commencement of hydrogen generation i.e. long induction period, and the small amount of hydrogen released at optimal temperature. In the current paper the thermal decomposition of AB is studied at different temperatures. Further the effect of Si nanoparticles on the induction period and kinetics as well as the gas release reaction is studied in detail using different characterization techniques. It was found that the induction period reduced and the amount of gas released increased as a result of Si nanoparticle addition. This was facilitated by a reduction in the activation energy of decomposition and improved kinetics with the addition of silicon nanoparticles.     

Artificial intelligence for the modeling and control of combustion processes: a review

Artificial intelligence (AI) systems are widely accepted as a technology offering an alternative way to tackle complex and ill-defined problems. They can learn from examples, are fault tolerant in the sense that they are able to handle noisy and incomplete data, are able to deal with non-linear problems, and once trained can perform prediction and generalization at high speed. They have been used in diverse applications in control, robotics, pattern recognition, forecasting, medicine, power systems, manufacturing, optimization, signal processing, and social/psychological sciences. They are particularly useful in system modeling such as in implementing complex mappings and system identification. AI systems comprise areas like, expert systems, artificial neural networks, genetic algorithms, fuzzy logic and various hybrid systems, which combine two or more techniques. The major objective of this paper is to illustrate how AI techniques might play an important role in modeling and prediction of the performance and control of combustion process. The paper outlines an understanding of how AI systems operate by way of presenting a number of problems in the different disciplines of combustion engineering. The various applications of AI are presented in a thematic rather than a chronological or any other order. Problems presented include two main areas: combustion systems and internal combustion (IC) engines. Combustion systems include boilers, furnaces and incinerators modeling and emissions prediction, whereas, IC engines include diesel and spark ignition engines and gas engines modeling and control. Results presented in this paper, are testimony to the potential of AI as a design tool in many areas of combustion engineering.     

A study on municipal solid waste (MSW) combustion in N2/O2 and CO2/O2 atmosphere from the perspective of TGA

In this paper, the combustion behavior of municipal solid waste (MSW) is carried out in a thermogravimetric analyzer under different N₂/O₂ and CO₂/O₂ atmospheres with temperature ranging from 100 °C to 1000 °C. TG (thermogravimetric) and DTG (derivative thermogravimetric) curves are analyzed. The nth order reaction fitting model is used to yield the activation energy of reduction process according to the degree of conversion. The results indicate that all samples lose most their weight between 200 °C and 540 °C. As the oxygen concentration increased, conversion rate curves and DTG curves shift to lower temperature without significant change in its shape. At the same oxygen concentration, the peak values in CO₂/O₂ atmosphere are smaller than those in N₂/O₂ atmosphere, indicating that CO₂ has a higher inhibitory effect than N₂ on MSW combustion. After 600 °C, the weight loss peak appears much later in CO₂/O₂ atmosphere than it does in N₂/O₂ atmosphere. With the increase of heating rate, the maximum weight loss rates of samples increase obviously. The three-step reaction of nth order reaction model fits the weight loss very well.     

Dissolution kinetics of carbon in aluminum droplet combustion: Implications for aluminized solid propellants

An analytical model describing the kinetics of carbon dissolution in burning aluminum droplets has been developed in order to simulate its effects under solid rocket motor conditions. A carbon dissolution rate (k) was introduced in different droplet regression laws and depending on the heterogeneous kinetics between the Al surface and the surrounding gases. The model was validated using previous experiments performed by the authors on millimeter-sized Al droplets burning in several CO₂-containing atmospheres at atmospheric pressure (P=1 atm). It has been shown that the carbon dissolution is affected by the presence of hydrogen due to competition between CO and H₂ chemisorption. The model was then applied to aluminized propellants (AP/HTPB) at high pressures (P=60 atm) and high temperatures (T=3000 and 3500 K), as well as at various burning rates and adsorption conditions. Though the accuracy of the extrapolation results needs further improvement, it has been shown that the carbon dissolution process should not be neglected in order to achieve global understanding of the combustion of Al particles, particularly agglomerates.     

Effect of admixing different carbon structural variants on the decomposition and hydrogen sorption kinetics of magnesium hydride

The effect of admixing catalysts comprised of carbon nanostructures, specifically planar, helical and twisted carbon nanofibers, spherical carbon particles and multi-walled carbon nanotubes, on the hydrogen storage properties of magnesium hydride has been investigated. Optimum results were achieved with the mixture containing twisted carbon nanofibers (TCNF) synthesized by Ni catalyst derived by oxidative dissociation of catalyst precursor LaNi₅. The desorption temperature of 2 wt.% TCNF admixed MgH₂ is ∼65 K lower than that of pristine MgH₂ milled for the same duration. The enhancement in hydrogen absorption capacity of MgH₂ admixed with 2 wt.% TCNF has been found to be two-fold in the first 10 minutes at 573 K and under a hydrogen pressure of 2 MPa, i.e. 4.8wt% as compared to 2.5 wt% for MgH₂ alone. The increase in capacity by a factor of about two within the first 10 minutes as a result of the catalytic activity of TCNF is one of the exciting results obtained for hydrogen absorption in catalyzed MgH₂.     

Characteristics of non-premixed oxygen-enhanced combustion: II. Flame structure effects on soot precursor kinetics resulting in soot-free flames

A detailed computational study was performed to understand the effects of the flame structure on the formation and destruction of soot precursors during ethylene combustion. Using the USC Mech Version II mechanism the contributions of different pathways to the formation of benzene and phenyl were determined in a wide domain of Z_st values via a reverse-pathway analysis. It was shown that for conventional ethylene-air flames two sequential reversible reactions play primary roles in the propargyl (C₃H₃) chemistry, namely

(1)     

An investigation on the thermodynamics and kinetics of magnesium hydride decomposition based on isotope effects

This work investigates the thermodynamics and kinetics of magnesium hydride decomposition by analyzing isotope effects in hydride and deuteride samples. Complete pressure composition desorption isotherm measurements of MgD₂ are reported for the first time. Deuterium desorption enthalpy and entropy obtained from the van’t Hoff plot of the middle plateau fugacities are 73.8 ± 0.4 kJ/mol and 135.5 ± 0.6 J/mol K, respectively, which are in good accordance with the values obtained more than fifty years ago from plateau pressure measurements. This result reveals that the enthalpy of desorption of MgD₂ is slightly lower than that of MgH₂, whereas the entropy change is higher for the deuteride than for the hydride. Although the differences in the enthalpy and entropy of both isotopes are weak, the synergy of both effects is capable of explaining the higher equilibrium pressures for the deuteride than for the hydride.On the other hand, kinetics of magnesium hydride decomposition has been investigated by simultaneous H and D desorption experiments from mixed hydride-deuteride samples. The obtained results reveal that that decomposition is controlled by the nucleation and growth of the Mg phase. Because this reaction step is not affected by the isotopic replacement of H for D no isotope effect is observed in the kinetics of magnesium hydride decomposition. On the contrary, a marked isotope effect is observed in the kinetics of H₂(D₂) absorption by magnesium. In this case, the lighter isotope shows faster kinetics than the heavier one, what has been related to the fact that absorption is rate limited by H(D) diffusion through the hydride(deuteride) phase.     

Modelling the hydrodynamics and kinetics of methane decomposition in catalytic liquid metal bubble reactors for hydrogen production

Bubble reactors using molten metal alloys (e.g, nickel-bismuth and copper-bismuth) with strong catalytic activity for methane decomposition are an emerging technology to lower the carbon intensity of hydrogen production. Methane decomposition occurs non-catalytically inside the bubbles and catalytically at the gas-liquid interface. The reactor performance is therefore affected by the hydrodynamics of bubble flow in molten metal, which determines the evolution of the bubble size distribution and of the gas holdup along the reactor height. A reactor model is first developed to rigorously account for the coupling of hydrodynamics with catalytic and non-catalytic reaction kinetics. The model is then validated with previously reported experimental data on methane decomposition at several temperatures in bubble columns containing a molten nickel-bismuth alloy. Next, the model is applied to optimize the design of multitubular catalytic bubble reactors at industrial scales. This involves minimizing the total liquid metal volume for various tube diameters, melt temperatures, and percent methane conversions at a specified hydrogen production rate. For example, an optimized reactor consisting of 891 tubes, each measuring 0.10 m in diameter and 2.11 m in height, filled with molten Ni_0·27Bi_0.73 at 1050 °C and fed with pure methane at 17.8 bar, may produce 10,000 Nm³.h^?1 of hydrogen with a methane conversion of 80% and a pressure drop of 1.6 bar. The tubes could be heated in a fired heater by burning either a fraction of the produced hydrogen, which would prevent CO₂ generation, or other less expensive fuels.     

Inverse influence of initial diameter on droplet burning rate in cold and hot ambiences: a thermal action of flame in balance with heat loss

Isolated droplet burning were conducted in microgravity ambiences of different temperatures to test the initial diameter influence on droplet burning rate that shows a flame scale effect and represents an overall thermal action of flame in balance with heat loss. The coldest ambience examined was room air, which utilized a heater wire to ignite the droplet. All other ambiences hotter than 633 K were acquired through an electrically heated air chamber in a stainless steel can. An inverse influence of initial droplet diameter on burning rate was demonstrated for the cold and hot ambiences. That is, the burning rate respectively decreased and increased in the former and latter cases with raising the initial droplet diameter. The reversion between the two influences appeared gradual. In the hot ambiences the burning rate increase with increasing the initial droplet diameter was larger at higher temperatures. A “net heat” of flame that denotes the difference between “heat gain” by the droplet and “heat loss” to the flame surrounding was suggested responsible for the results. In low-temperature ambiences there is a negative net heat, and it turns gradually positive as the ambience temperature gets higher and the heat loss becomes less. Relating to luminous flame sizes and soot generation of differently sized droplets clarified that the flame radiation, both non-luminous and luminous, is determinative to the net heat in microgravity conditions. In addition, the work identified two peak values of soot generation during burning, which appeared respectively at the room temperature and at about 1000 K. The increase in ambience temperature made also bigger soot shells. The heat contribution of flame by both radiation and conduction was demonstrated hardly over 40% in the total heat required for droplet vaporization during burning in a hot ambience of 773 K.     

Characterization and kinetics of reduction of CaSO4 with carbon monoxide for chemical-looping combustion

Chemical-looping combustion (CLC) of fuels via the cyclic reduction and oxidation of an oxygen carrier is a novel process for CO₂ capture. CaSO₄ has emerged as an alternative material with much lower cost and higher oxygen storage capacity compared to metal oxide based oxygen carrier. In principle, CaSO₄ is reduced by CO and H₂ (coal gasification products) generating CaS, CO₂ and H₂O, and then the solid product CaS is regenerated back to CaSO₄ in air. Research on the reactions of CaSO₄ and CaS is not only important for understanding the reaction mechanism and parameters for the new CLC technology but also of high interest in understanding the sulfur chemistry in fluidized bed combustion and gasification. This paper focuses on the reduction reaction of CaSO₄ with CO to CaS and CO₂ (CaSO₄ + 4CO → CaS + 4CO₂) by thermodynamics, characterization and kinetics analysis. Phase diagram of CaSO₄–CaO–CaS indicates the operation regime of reduction–oxidation cycle by delicate control of temperature and partial pressure of reaction gases. The kinetics of the reduction reaction of a low cost natural anhydrite as oxygen carrier was investigated in an isothermal differential bed reactor where a thin layer of CaSO₄ particles were exposed to a plug flow of CO balanced by N₂. Prior to the kinetics modeling, extensive physical and chemical characterization analyses, such as X-ray diffraction (XRD), scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM–EDX), and N₂ adsorption–desorption were carried out to understand the reaction mechanism. External mass transfer and heat effects were minimized and the operation regime for intrinsic kinetics was determined. The experimental data was described with a gas–solid shrinking unreacted core model (SCM) with both chemical reaction control and product layer diffusion resistance considered.     

An experimental and modelling study of dual fuel aqueous ammonia and diesel combustion in a single cylinder compression ignition engine

The ability of ammonia to act as a hydrogen carrier, without the drawbacks of hydrogen gas-storage costs and low stability-renders it a potential solution to the decarbonisation of transport. This study combines both modelling and experimental techniques to determine the effect of varying the degree of aspiration of ammonium hydroxide (NH₄OH) solution, at different engine loads, in the combustion of a compression ignition engine. Ignition delay was extended as ammonia injection increased, causing an increase in peak in-cylinder temperature, but generally lower combustion quality-increasing incomplete combustion products, while decreasing particle size. The higher peak in-cylinder temperatures generally correlated with higher nitrous oxide (NO_x) emissions in the exhaust, though a fuel-bound nitrogen effect was apparent. Chemical kinetic modelling at equivalent conditions found increasing levels of unburnt ammonia with greater aspiration. Moreover, the ignitability of NH₄OH was found to improve in simulations substituting diesel with hydrogen peroxide direct injection.     

Hydrogen production by thermal partial oxidation of ethanol: Thermodynamics and kinetics study

In this study thermodynamics and kinetics analysis of the thermal partial oxidation (TPOX) of ethanol for producing hydrogen is performed. Equilibrium and kinetics calculations are performed in order to find the limiting parameters for the thermal partial oxidation. The effects of air ratio λ (the ratio of the oxidizer -to- fuel ratio to the stoichiometric oxidizer -to- fuel ratio) and mixture inlet temperatures (T_mix-in) on the reforming efficiency, the H₂ mole number, the reaction progress, the equilibrium time and the ignition delay time are investigated. Furthermore, the analysis is performed using different kinetics schemes and the results are compared. The optimum practical operating conditions of the partial oxidation process of ethanol are identified. In this way, the results of this work can be useful as a guideline in experimental work.     

Plant microbial fuel cell applied in wetlands: Spatial,temporal and potential electricity generation of Spartina anglica salt marshes and Phragmites australis peat soils

The plant microbial fuel cell (PMFC) has to be applied in wetlands to be able to generate electricity on a large scale. The objective of this PMFC application research is to clarify the differences in electricity generation between a Spartina anglica salt marsh and Phragmites australis peat soil based on experimental data and theoretical calculated potential. PMFC in salt marsh generated more than 10 times more power than the same PMFC in peat soil (18 vs 1.3 mW m⁻² plant growth area). The salt marsh reached a record power output for PMFC technology per cubic meter anode: 2.9 W m⁻³. Most power is generated in the top layer of the salt marsh due to the presence of the plants and the tidal advection. The potential current generation for the salt marsh is 0.21–0.48 A m⁻² and for peat soil 0.15–0.86 A m⁻². PMFC technology is potentially able to generate a power density up to 0.52 W m⁻², which is more than what is generated for biomass combustion or anaerobic digestion using the same plant growth area.     

Solution combustion synthesis of CoSx,MoSx, CoSx-MoSx and their catalytic activity in H2 production from H2S decomposition

Production of hydrogen from H₂S is desirable and can meet partly the ever-increasing demand on cheaper and greener hydrogen as a cleaner energy resource since it makes better use of H₂S than the various Claus processes. Medium temperature effective catalysts for H₂S decomposition would help developing a sulfur looping process to overcome the thermodynamic limitation for H₂S decomposition. However, developing such medium temperature catalyst for direct H₂S decomposition is still a great challenge. This work adapts solution combustion synthesis (SCS) method to synthesize oxide and sulfide of molybdenum and cobalt with fine crystalline and their activities in H₂S decomposition were investigated. The in-situ sulfurization with thiourea in SCS process leads to higher surface area and higher activity of the resultant catalysts. The presence of CoS_x enchances the activity of MoS_x in H₂S decomposition. The H₂S decomposition activity of the CoS_x-MoS_x composite is higher than that of each single component one. The H₂S decomposition kinetics analysis shows that an activation energy of 43.3 kJ/mol was achieved on 20%CoS_x-MoS_x which is much lower than that reported in literatures.     

Effects of hydrogen addition to methane on the thermal and ignition delay characteristics of fuel-air,oxygen-enriched and oxy-fuel MILD combustion

The present study investigated the effect of adding hydrogen to methane on the thermal characteristics and ignition delay in methane-air, oxygen-enriched and oxy-fuel MILD combustion. For this purpose, numerical simulation of MILD furnace is performed by k-ε turbulence, modified EDC combustion, and DO radiation models. Additionally, a well stirred reactor (WSR) analysis alongside with CFD simulations is used for getting the better insight of combustion process and numerical results. The results show that H₂ addition to CH₄ provides a more uniform temperature field with higher peak and average temperatures under a similar oxidizer atmosphere. Also, more ignition delay time (IDT) obtained by the replacement of CO₂ with N₂, can be compensated by consideration of H₂ in the fuel composition. This study implies that the use of H₂ as an additive to methane is an effective strategy for conversion of methane-air to oxy-fuel combustion system with almost identical thermal and ignition characteristics.