Catalytic Materials for High-Temperature Combustion

Catalytic combustion, as an alternative to conventional thermal combustion, has received considerable attention during the past decade. Research efforts have been promoted by the need to meet governmental demands concerning pollution and the wish to use energy sources more efficiently. The two main advantages offered by catalytic combustors over flame combustors apply to these goals:

1. Catalytic combustion can be carried out over a wide range of fuel concentrations in air and at low temperatures.
2. These low temperatures result in attaining NO, emission levels substantially lower than possible with conventional combustors.

     

MINIMISATION OF NOx EMISSION BY FLAME TEMPERATURE CONTROL

Emission control technology has advanced to an extent whereby pollutant levels are now controlled by legislation. Techniques for controlling emissions vary and include improved fuel mixture preparation, multi stage combustion, exhaust gas recirculation, inhibitors such as ammonia and other techniques which reduce the intensity of combustion and hence flame temperature.

This paper examines ways in which ultra low NO_x, emission combustors can be produced by applying technology developed for burning poor quality fuels.     

催化助热燃烧—一项重要的燃烧新技术

催化助热燃烧利用催化剂点燃和保持气相反应,保证燃料完全燃烧,提高了热效率,稳定地调节火焰温度,减少NO_x排放量,对提高能源利用率和保护环境有好处。采用蜂窝状陶瓷负载贵金属,(复)氧化物活性组分,或以多种材质的活性独石,可满足许多燃烧场合的要求。今后在催化剂体系选取时,应把高温热稳定性放在首位。     

Modified combustion efficiency curve for high-velocity model combustors integrated with the inlet

The method of obtaining combustion efficiency curves for fuel mixtures proposed by Annushkin is modified on the basis of data of high-altitude tests of high-velocity combustors integrated with the inlet. The combustion efficiency curve of a hydrocarbon–air mixture obtained in this study can be applied for design of high-velocity combustors.     

MINIMISATION OF NOx EMISSION BY FLAME TEMPERATURE CONTROL

Emission control technology has advanced to an extent whereby pollutant levels are now controlled by legislation. Techniques for controlling emissions vary and include improved fuel mixture preparation, multi stage combustion, exhaust gas recirculation, inhibitors such as ammonia and other techniques which reduce the intensity of combustion and hence flame temperature.

This paper examines ways in which ultra low NO_x, emission combustors can be produced by applying technology developed for burning poor quality fuels.     

An experimental study on the flame stability of LFG and LFG-mixed fuels

Landfill gas (LFG), having sufficient chemical energy to sustain the operations of commercial combustors, has become a subject of special interest recently. In this study, the flame stabilities of LFG, LFG-mixed fuel I and II were investigated for their applications to domestic appliances using premixed combustion and swirl combustors using non-premixed combustion. LFG-mixed fuel I and II are made by mixing LFG with liquefied propane gas (LPG). LFG-mixed fuel I has higher-heating value (HHV) equivalent to that of liquefied natural gas (LNG), and LFG-mixed fuel II has Wobbe index (WI) equivalent to that of LNG. Thermodynamic calculations and experimental results on burning velocity and flame stability confirmed that LFG-mixed fuel I and II may be used as interchangeable gas of LNG without any modifications of domestic combustion appliance. For the non-premixed type combustor, the flame stability of LFG-mixed fuels is lower than that of LNG in the weak swirl condition. But in the strong swirl condition, the LFG-mixed fuels show similar stable flame zone to that of LNG and thus, can be used as an interchangeable gas of LNG. In addition, LFG itself can also be used as an alternative fuel of LNG, except that the turndown ratio goes down by half.     

Catalysis in Combustion

Catalysis and combustion have long been linked. In fact, the science of catalysis stems from Davy's discovery [1] that platinum wires could promote the flameless combustion of flammable fuel-air mixtures. Today, catalysis is a mainstay of our modern chemical industry. Oxidation catalysts are used not only for the complete oxidation of fuels to carbon dioxide and water, as in radiant catalytic tent heaters and fume abatement devices, but also for the selective partial oxidation of hydrocarbons or other “fuels” to produce basic chemicals such as ethylene oxide (from ethylene), terephthalic acid (from p-xylene), and nitric acid (from ammonia). However, despite the long-known capability of catalysts to oxidize hydrocarbons without significant production of carbon monoxide, soot, or thermal NO_x, there seemed little possibility that catalytic oxidation reactors could ever displace conventional flame combustors as primary fuel combustors. This is because the volumetric heat release rates of conventional catalytic oxidation reactors are far too low to be competitive with the flame combustor.     

Correlating NOx, smoke and wall temperature for a coal liquid and shale oil in a subscale combustion turbine burner

The testing of 18 alternative fuels was the basis for an extensive data bank of combustion parameters and fuel properties. This Paper represents an effort to correlate the three key dependent variables (NO_x, smoke, and wall temperature) to the process conditions and fuel properties. Correlation was facilitated by the use of a multiple linear regression computer program. Linear terms, second-order terms, and their interactions were investigated. Correlation development is discussed and the results presented. Determining the physical meaning of the correlations is attempted together with a discussion concerning extrapolation of the correlations over the range of current operational gas turbine combustors. The correlations can be described as excellent for NO_x and good-to-fair for smoke and wall temperature.     

Toxicologic and Physicochemical Characterization of High-Temperature Combustion Emissions

Particles and combustion gases produced by two different high-temperature combustors, which burned pulverized coal and a No. 2 fuel oil-fly ash slurry, respectively, at adiabatic flame temperatures greater than 2400 K, were characterized. Effluent samples were taken at locations along the product gas stream and within the stack. Measurements of the particle size distributions, number concentrations, and gas species concentrations were made. The toxicity and mutagenicity of the effluent particles were determined. A large number of submicrometer particles were found in both cases of high-temperature combustion. The product emissions differed significantly in their particle size distribution and final chemical composition from those of conventional combustion systems having lower combustion temperatures.     

Development of a catalytic combustor for a heavy-duty utility gas turbine

Catalytic combustion is an attractive technology for gas turbine applications where ultra-low emission levels are required. Recent tests of a catalytic reactor in a full scale combustor have demonstrated emissions of 3.3 ppm NO_x, 2.0 ppm CO, and 0.0 ppm UHC. The catalyst system is designed to only convert about half of the natural gas fuel within the catalyst itself, thus limiting the catalyst temperature to a level that is viable for long-term use. The remainder of the combustion occurs downstream from the catalyst to generate the required inlet temperature to the turbine.

Catalyst development is typically done using subscale prototypes in a reactor system designed to simulate the conditions of the full scale application. The validity of such an approach is best determined experimentally by comparing catalyst performance at the two size scales under equivalent reaction conditions. Such a comparison has recently been achieved for catalysts differing in volume by two orders of magnitude. The performance of the full scale catalyst was similar to that of the subscale unit in both emission levels and internal temperatures. This comparison lends credibility to the use of subscale reactors in developing catalytic combustors for gas turbines.     

Intensity and efficiency of spray fuel-fed well-mixed adiabatic combustors

Motivated by the insights it can provide, we revisit the classical problem of liquid fuel-fed idealized steady-flow combustors. New quadrature-based results are presented for the theoretical combustion intensity and corresponding efficiency for well-stirred adiabatic vessels fed with a prescribed polydispersed spray. Each droplet of the spray is assumed to evaporate according to a non-quasi-steady (non-QS) gas-phase energy/mass diffusion-controlled rate for the pseudo-single-component fuel. As a byproduct, we calculate the complete droplet size distribution (DSD) function exiting the chamber, of interest for the design of downstream components. We explicitly assume that the volumetric rate of chemical energy release in such “primary” combustion chambers is controlled by the liquid fuel physical vaporization process (with negligible lags due to propellant droplet heat-up or vapor-phase ignition). In this instructive asymptotic limit, two decisive non-dimensional parameters are shown to be: (1) a vaporization Damköhler number (defined by the ratio of the mean residence time of the chemically reacting vapor mixture in the combustion space, to the reference value of the vaporization lifetime of a droplet with the injector-Sauter-mean diameter, and (2) a single dimensionless non-QS parameter. Illustrative numerical results for a kerosene-like fuel burning in air at pressures up to 24 atm are displayed for the case of a log-normal feedstream DSD with a range of spreads. Our results reveal the existence of an optimum vaporization Damköhler number which maximizes the combustion intensity—with maximum intensities, occurring well before nearly complete fuel evaporation, being quite sensitive to the non-QS parameter at high pressures. These deliberately idealized mathematical model results, spanning more than a 1000 combinations of operational parameters, set instructive bounds to the achievable performance of “real” spray combustors. Even without tractable enhancements (see Section 5.2), this approach can be used to economically map the sensitivity of spray combustor performance to a large number of important design and control parameters.     

Monoliths in catalytic oxidation

Catalytic combustion is useful to avoid emission of nitrogen oxides, to combust fuel gas of different calorific levels, and to combust low contents of badly smelling or hazardous gaseous compounds. After dealing with some characteristics of catalytic combustion it is argued that catalytic combustion to a final temperature lower than about 800°C calls for a rapid transport of thermal energy out of the reactor. A fluidized bed in which combustion has been successfully performed is dealt with as well as a reactor filled with metal bodies sintered to each other and to the wall of the reactor. To achieve a sufficiently high catalytically active surface area a thin layer of silicone rubber is applied to the surface of the metal bodies and subsequently pyrolyzed to a highly porous layer of silica. To raise the thermostability alumina can be added to the silica layer.

To establish a final temperature above 900°C the homogeneous gas-phase combustion can be ignited by a solid catalyst or the reaction can be performed completely catalytically. Since the combustion reaction proceeds very rapidly at elevated temperatures, a large gas flow can be utilized, which calls for a reactor exhibiting a low-pressure drop. For catalytic combustion monoliths and gauzes are appropriate. The chemical composition of ceramic and metallic monoliths is dealt with as well as the cell densities and wall thicknesses of commercial monoliths. The application of active components to the surface of the walls of monoliths is subsequently discussed. Since monoliths do not allow radial mixing, a homogeneous gas mixture has to be fed to the monolith to prevent very high temperature levels moving randomly over the channels of the monolith and deactivating the catalyst.

With monoliths in gas turbines often catalytic ignition is used. To limit the temperature a fraction of the fuel feed is injected into the homogeneous combustion chamber. A number of alternatives of transporting the fresh fuel to the homogeneous combustion zone is mentioned. The cause of the catalyst temperature being higher than that of the gas flow is dealt with as well as the low volatility at elevated temperatures required for the catalytic components. Selection of the catalytically active materials is discussed and the procedure to bring the gas flow at the light-off temperature of the catalyst.

Monolithic combustors used in radiant heaters display often an oscillatory behavior. After dealing with the cause of the oscillations, prevention by means of a flame arrestor is mentioned.     

Problems of Implementing Ramjet Operation

This paper discusses an approach to implementing the operation and designing the flow duct of a ramjet combustor. Methods of air compression, ignition, flame stabilization, and fuel combustion in the flow are considered for the purpose of implementing an effective process on a short length with moderate total pressure losses. Advantages of narrow-mode ramjet engines are noted. Comparative results of experiments on the burning of hydrocarbon fuels in the ramjet combustors are given.     

Catalysis in Combustion

Catalysis and combustion have long been linked. In fact, the science of catalysis stems from Davy's discovery [1] that platinum wires could promote the flameless combustion of flammable fuel-air mixtures. Today, catalysis is a mainstay of our modern chemical industry. Oxidation catalysts are used not only for the complete oxidation of fuels to carbon dioxide and water, as in radiant catalytic tent heaters and fume abatement devices, but also for the selective partial oxidation of hydrocarbons or other “fuels” to produce basic chemicals such as ethylene oxide (from ethylene), terephthalic acid (from p-xylene), and nitric acid (from ammonia). However, despite the long-known capability of catalysts to oxidize hydrocarbons without significant production of carbon monoxide, soot, or thermal NO_x, there seemed little possibility that catalytic oxidation reactors could ever displace conventional flame combustors as primary fuel combustors. This is because the volumetric heat release rates of conventional catalytic oxidation reactors are far too low to be competitive with the flame combustor.     

PVC AND INSTABILITY IN SWIRL COMBUSTORS

This paper describes the precessing vortex core (PVC) and other instabilities that are present in swirl generators/burners/combustors at high degrees of swirl to the flow and at high Reynolds number. PVC generates a characteristic sinusoidal signal of velocity and pressure and yields very high levels of regular oscillations of pressure, acoustic oscillations in addition to the high levels of turbulence. A mathematical model is described of the PVC and instability in swirl flows under cold flow conditions by treating the flow to be 2-Dimensional, incompressible, and at low Mach number. Relationships are derived for the various forces acting on the central core of the flow and also the conditions under which PVC is accelerated and hampered. Experimental data are obtained from a swirl combustor in which the degree of swirl and mode of gaseous fuel introduction could be changed. Measurements were made of the noise amplitude-frequency spectra and PVC frequency under cold and hot flow conditions and the radial distribution of temperatures. Comparative data are presented for the effect of combustion upon PVC. Good agreement is found between the calculated and experimentally measured acoustic frequency of the PVC.     

Wirbelschichtverbrennungsanlagen in Österreich und ihre Rauchgasreinigungsanlagen

Within the scope of the multi‐lateral technology initiative Fluidized Bed Conversion of the International Energy Agency a status report on fluidized bed technology in Austria, with the main focus on combustion, is under preparation. The investigated fluidized bed combustors are situated in Austria and have one Megawatt thermal capacity at minimum. With the exception of some demonstration plants the investigated objects can be categorized by industrial aspects. With knowledge of their fuel consumptions, date of activation and commercial purpose general statements about the flue gas cleaning system within their category can be made.     

Modeling of fuel mixing in fluidized bed combustors

This paper presents a three-dimensional model for fuel mixing in fluidized bed combustors. The model accounts for mixing patterns which were experimentally shown to govern mixing in risers with geometry and operational conditions representative for furnaces in fluidized bed combustors. The mixing process is modeled for three different solid phases in the furnace and the model, which includes the return leg, can be applied both under bubbling and circulating regimes. The semi-empirical basis of the model was previously validated in different large-scale fluidized bed combustors and is combined with a model for fuel particle conversion to obtain the fuel concentration field. Model results are compared with experimental data from the Chalmers 12 MW_th CFB combustor, yielding a reasonable agreement.     

Numerical and experimental study of oscillatory processes in small-size combustion heaters of air

Results of a comprehensive numerical and experimental study of oscillatory processes in combustors of small-size combustion heaters of air are reported. Methods for prediction and experimental determination of regular features of changes in spectral characteristics of pressure oscillations in the combustor in the nominal operation mode are presented. The data obtained in the study can be used for the development of various combustion heaters, including those for testing combustors of air-breathing engines for advanced flying vehicles.     

Partitioning of trace elements during the combustion of coal and biomass in a suspension-firing reactor

A novel, quartz ‘suspension-firing’ reactor is described for monitoring trace element release during solid fuel combustion under conditions relevant to fluidised bed combustors. The new design allows the examination of fuel particle combustion in the absence of bed solids. Experiments have been conducted using two coals, a sample of wood bark and one of straw. Ash from the reactor walls and base have been analysed separately from ash collected on a sintered disc in the path of exit gas. Trace element concentrations in these samples were analysed by Inductively Coupled Plasma (ICP)-mass spectrometry and ICP-atomic emission spectrometry (AES). The fractions of original trace elements retained by the ash have been reported; relative enrichment in the ‘sinter-ash’ was calculated by comparing with ‘bottom ash’. Mercury was almost completely volatilised from all fuels, as was selenium for all except wood-bark. Chromium, manganese and thallium were partially volatilised and nickel mostly retained in all samples. The behaviour of beryllium, lead, molybdenum, vanadium and zinc varied, depending on the fuel sample. Beryllium was released to a greater extent from coal/straw than the other fuels. Vanadium was partially volatilised from wood-bark and coal/straw, while the largest proportion of the zinc released was from the wood-bark. Lead and molybdenum were retained to a greater extent by ‘Colombian coal’ and wood-bark, respectively. Evidence of the enrichment of certain trace elements on the finer ‘sinter-ash’ particles has also been observed, e.g. for As, Cd, Pb and Tl during the combustion of the ‘Colombian-coal’.     

Recent advances in high temperature catalytic combustion

Catalytic combustion of methane has been investigated for the application to gas turbines. As the combustion is operated at high temperatures and high space velocity, heterogeneous reaction and surface-initiated gas phase reaction proceed concurrently. Thermal resistance to maintain large surface area is, therefore, requested to attain high combustion efficiency above 1000°C. Hexaaluminate compounds were effective in maintaining large surface area. On the other hand, palladium catalysts were generally employed for the combustion of methane below 1000°C. The prototype catalyst combustors were successfully tested with their high combustion efficiency and low NO_x emission by using Pd based- and/or hexaaluminate catalysts.