Sub-stoichiometric Combustion Research Articles

The Effect of Iron‐ and Manganese‐Based Oxygen Carriers as Bed Materials in Oxygen Carrier Aided Combustion

Combustion of organic materials in fluidized bed combustion is generally performed using an over‐stoichiometric air‐to‐fuel ratio. Despite excess air in the system, sub‐stoichiometric combustion regions are present in the fluidized bed because of nonperfect mixing of reactants in the system. These regions with oxygen‐deficient combustion contribute to increased levels of nonoxidized or partially oxidized carbon species. To enhance heat transfer and uniform heat distribution in the incineration chamber, an inert fluidized bed material such as silica sand is generally used. Substitution of silica sand in favor of an oxygen carrier could potentially be used to promote oxygen distribution in the incineration chamber. This is referred to as oxygen carrier‐aided combustion. In this work, three alternative bed materials, previously investigated in chemical looping combustion, are investigated and compared with silica sand in over‐ and sub‐stoichiometric combustion. The materials investigated as bed materials are a manganese ore, the mineral ilmenite, and a synthetic material mixture of Fe2O3 on a ZrO2 support. Results show that during combustion using sub‐stoichiometric air‐to‐fuel ratios, the amount of CO in the effluent gases can be reduced using an active bed material compared to inert silica sand.

Fired equipment safety in the oil & gas industry A review of changes in practices over the last 50 years

This paper reviews how the requirements of safety, availability, energy efficiency and environmental compliance have influenced the design and operation of fired equipment over the last 50 years. It presents the various norms and standards relevant to the classes of fired equipment used in the Oil & Gas industry and highlights the differences between prescriptive norms and performance based standards. The main hazards and common causes of accidents of process heaters, petrochemical furnaces and boilers are described. Finally, this paper reviews the evolution of the risk mitigations and design best practices over the last decades. It discusses in particular the particular challenges of improving the safety performance of existing equipment.Before the first oil crisis of 1973, the price of refinery fuels was very low and it was common practice to run heaters inefficiently with high excess air (e.g. 5 to 8 % O2 in the flue gas) and high draft to reduce the probability of sub-stoichiometric combustion and positive pressure in the combustion chamber. Since the safety margin was provided by operating with high excess air and high draft, control improvements were considered unnecessary. Fired equipment safety was essentially distributed between operator response to alarms (e.g. process upset conditions), instrumented protective functions programmed in the safety instrumented system and solutions such as explosion doors and snuffing steam to mitigate the consequence of explosions.In the last 25 years, the drive for safer operation with higher energy efficiency, lower NOx emissions and fewer nuisance trips has led operating companies to adapt their approach to fired equipment safety. The modern approach to fired equipment safety is to distribute the risk across independent protection layers. These safety barriers rely on a comprehensive control system with constraints and a safety instrumented system, but also on operational excellence with well-trained operators, good operating procedures and reliability-centered maintenance and risk-based inspection. As an important benefit, constraint controls with automated fuel cutbacks have proven effective at minimizing nuisance trips by keeping the heater within operational limits.

Gasification of pelletized renewable fuel for clean energy production

The main aim of the study was to develop and investigate a small-scale experimental gasification technique for the effective thermal decomposition of pelletized renewable fuels (wood sawdust, wheat straw). The technical solution of the biomass gasifier for gasification of renewable fuels presents a downdraft gasifier with controllable additional heat energy supply to the biomass using the radial propane flame injection into the bottom part of the biomass layer. From the kinetic study of the mass conversion rate of pelletized biomass and variations of the composition of produced gas it is concluded that the process of biomass gasification is strongly influenced by the amount of additional heat energy and air supply into the biomass. The results of experimental measurements of the composition of produced gas have shown that under the conditions of the sub-stoichiometric air supply into the layer of pelletized wood biomass (α<0.3) increasing additional heat energy supply in a range from 60kJ up to 130kJ leads to an enhanced mass loss of pelletized biomass and enhanced formation of volatiles (CO,H2) in the flaming pyrolysis zone. For the wood biomass the average content of CO in the products can be increased from 73g/m3 up to 97g/m3, while the average content of H2 increases from 4.7g/m3 up to 6.2g/m3. Similar variations of the composition of products are observed during the enhanced gasification of the wheat straw. At constant rate of additional heat energy supply and the sub-stoichiometric combustion conditions (α≈0.17−0.30), a faster thermal decomposition of the pelletized biomass and larger average amount of the produced volatiles (CO,H2) can be obtained by increasing the air supply rate from 0.27 to 0.43g/s, determining the variations of air-to-fuel ratio in a range from 1.3 up to 1.6.

Process analysis of an oxygen lean oxy-fuel power plant with co-production of synthesis gas

This paper investigates new possibilities and synergy effects for an oxy-fuel fired polygeneration scheme (transportation fuel and electricity) with carbon capture and co-firing of biomass. The proposed process has the potential to make the oxy-fuel process more effective through a sub-stoichiometric combustion in-between normal combustion and gasification, which lowers the need for oxygen within the process. The sub-stoichiometric combustion yields production of synthesis gas, which is utilised in an integrated synthesis to dimethyl ether (DME). The process is kept CO 2 neutral through co-combustion of biomass in the process. The proposed scheme is simulated with a computer model with a previous study of an oxy-fuel power plant as a reference process. The degree of sub-stoichiometric combustion, or amount of synthesis gas produced, is optimised with respect to the overall efficiency. The maximal efficiency was found at a stoichiometric ratio just below 0.6 with the efficiency for the electricity producing oxy-fuel process of 0.35 and a DME process efficiency of 0.63. It can be concluded that the proposed oxygen lean combustion process constitutes a way to improve the oxy-fuel carbon capture processes with an efficient production of DME in a polygeneration process.

High-temperature reduction of nitrogen oxides in oxy-fuel combustion

The possibility of high-temperature reduction of nitrogen oxides (NO x ) in oxy-fuel combustion is investigated. A detailed gas-phase model identifies the effect of temperature, stoichiometric ratio, residence time and composition of the oxidizer on NO x reduction. It is concluded that an efficient high-temperature reduction of NO x is achieved with high-purity oxygen, negligible amount of air ingress, presence of a sub-stoichiometric combustion zone, and relatively long residence time. The temperature should be high but decreasing along with the NO x concentration throughout the reaction zone. The influence of these parameters is discussed, also with regard to combustion efficiency, corrosion and ash handling.

Three-way-catalyst induced benzene formation: A precursor study

The implementation of the three-way-catalyst technology was the major step for pollutant abatement of gasoline vehicles. There are, however, situations, where the engine has to be operated at sub-stoichiometric combustion. At such fuel-rich conditions, an intense formation of benzene was observed over a Pd/Rh-based three-way-catalyst (TWC), when operating the catalyst in a critical temperature window of 600–730°C. At least four different reaction pathways can lead to benzene formation on the catalyst, viz. (i) dealkylation of alkylbenzenes under steam reforming conditions, (ii) hydrodealkylation, (iii) aromatization of cyclohexanes, and (iv) cyclotrimerization of ethyne. Based on the engine-out exhaust composition only routes (i) and (ii) seem to be reasonable. The pre-catalyst application of 12 different alkylbenzenes indeed revealed that benzene formation is possible from all these precursors. At most up to 60% of the spiked precursors were converted to benzene. For meta- and para-substituted alkylbenzenes, a multi-step mechanism is proposed because partial dealkylation products such as toluene were formed as well. But a different, one-step mechanism is assumed for ortho-substituted alkylbenzenes, since no intermediates could be detected. No C–C-bond cleavage was observed within alkyl side chains. It is concluded that dealkylation reactions of alkylbenzenes are the major pathways leading to benzene formation in the TWC. Because fuel-rich combustion conditions have to be applied for the regeneration of deNOX traps or certain particulate traps as well, this chemistry might also be of relevance for these exhaust gas treatment systems.

Coal Gasification in a Transport Reactor

An advanced transport reactor gasifier was operated using three different coals: Illinois #6 bituminous, Wyodak Powder River Basin subbituminous, and a Sufco Utah bituminous coals. A steady-state model was developed to evaluate the relative contributions of coal combustion and gasification of coal/char in this high-throughput reactor. The tests employed in-bed calcium-based sorbent for sulfur capture. The model was based on elemental mass and energy balances and assumed instantaneous devolatilization and combustion and kinetically limited gasification reactions in a continuously stirred tank reactor. The experimental results on the reaction of these three coals were compared with model predictions. The simulated results compared favorably with the experimental results (e.g., gas composition, carbon conversion, and reactor temperature). The bituminous coal data were accurately simulated as substoichiometric combustion processes. The more reactive subbituminous coal exhibited significant gasification conve...

Influence of furnace combustion conditions on NOx emission from lignite flames

Coal combustion in boiler furnaces is followed by emission of pollutants, i.a. NOx compounds. The appropriate activities have been conducted lately for lowering this emission and satisfying the ecology requirements. Therefore, the knowledge of NOx formation mechanism in furnace conditions is of great importance. In the paper presented are the results of an application of the developed model on the computation of the chemithermodynamic equilibrium of a system formed by compounds and elements contained in the Kolubara basin lignite and the combustion air, at temperatures from 800 to 1700 K, pressure 1 bar, for two lignite compositions at excess air α = 1,25. The briefly outlined developed mathematical model used for shaping the computation algorithm takes into account only the thermal oxides formation mechanism which is for Kolubara lignites, having low nitrogen content, the dominant NOx formation mechanism even at lower combustion temperatures. The results obtained are of use, as thermodynamic indicators, for lowering NOx emission by adequately organizing and conducting the combustion process, i.e. by preventing formation of temperature zones with NOx emission above the allowed values. A sample computation of the equilibrium composition of the system has been performed for the stoichiometric combustion case, thus illustrating the possibility of NOx emission lowering by applying the so-called sub-stoichiometric combustion.

Prediction of Flue Gas Composition of an Incinerator Based on a Nonequilibrium-Reaction Approach

A nonequilibrium-reaction model is developed to predict the flue gas composition of an incinerator for solid waste combustion and gasification. The model may also be effectively utilized to predict the composition of exit gas from the primary chamber of an incinerator where invariably substoichiometric combustion conditions exist. The model assumes that the drying and pyrolysis of waste occurs in sequence followed by combustion of devolatilized gas and residue separately. The model is applied to the combustion of pine wood, and the flue gas compositions are computed as a function of operating parameters such as temperature (1,100 to 1,500 K), initial moisture content (0 to 40 percent), and percentage excess air (up to 100 percent). The model is extended to include substoichiometric combustion (to simulate the gasification process without steam injection) to compare its predictions with the available experimental data. The general agreement of theory and experiment is good. 8 refs., 5 figs., 4 tabs.

Hydrogen sulfide corrosion in low-nox combustion systems

Under lowNOx substoichiometric combustion conditions, sulfur in coal is released mostly as H2S gas, which will sulfidize boiler tubes at furnacewall temperatures. The corrosion rates of various alloys and coatings in simulated substoichiometric combustion gas were studied. Carbon steel and Type 304 stainless steel have reasonably good corrosion resistance up to 700 °F (371 °C) and 900 °F (482 °C), respectively. Type 309 and 310 stainless steels, INCONEL* 671, and an FeCrAl alloy have much better corrosion resistance. Diffusion and plasma coatings containing substantial amounts of chromium and/or aluminum suffer very little general corrosion. However, these coatings often have cracks and pores that lead to localized penetration. Effects of ash deposit, alloy sensitization, oxygen, thermal cycling, and alternate oxidizing and reducing conditions on the corrosion rates were also investigated.

4366668 substoichiometric combustion of low heating value gases : Ajay M. Madgavkar, Roger F Vogel, Harold Swift, Irvine, CA, United States assigned to Gulf Research & Development Company

4366668 substoichiometric combustion of low heating value gases : Ajay M. Madgavkar, Roger F Vogel, Harold Swift, Irvine, CA, United States assigned to Gulf Research & Development Company

Catalytic combustion of low heat value gas. 2. Reduction of carbon monoxide produced under substoichiometric combustion

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Corrosion Behavior of Metallic Materials in the Reducing Environments Characteristic of a Coal-Fired MHD Generator

Abstract Metallic materials selected for the balance of plant construction of components of an open cycle coal fired magnetohydrodynamic (MHD) generator must withstand the corrosive erosive conditions encountered in these systems. The corrosion behavior of carbon steel, chromium-molybdenum ferritic steels, austenitic stainless steels, and nickel base alloys, has been evaluated after exposure to complex gas mixtures that simulate a coal fired MHD environment with substoichiometric combustion. The results show that sulfidation was the predominant mode of interaction for the alloys in this environment. In general, carbon steel and Cr-Mo steels suffered significant sulfidation attack, while the austenitic stainless steels exhibited superior resistance to corrosion. It is concluded that low cost ferritic steels with a stainless steel cladding or a ramming refractory coating can be effectively used for protection of the radiant boiler components of a typical coal fired MHD generator system. The experimental obs...