-
[show abstract]
[hide abstract]
ABSTRACT: This paper describes the application of mathematical modelling, in particular computational fluid dynamics (CFD), to solve fluid flow, heat transfer, combustion and emissions problems for the cement industry. Whereas traditional physical modelling is limited to isothermal gas phase aerodynamics studies, CFD handles chemical reaction, convective and thermal radiation heat transfer, and the particle/gas flow interactions. In this paper, a new mineral interactive version (MI-CFD) is presented, in which mineral reactions are directly coupled to the flow, heat transfer and combustion processes. The procedure has been applied to over 100 plants worldwide. Example applications to cyclone, calciner and emissions- reduction problems are described herein. Very significant benefits to the cement industry have resulted in terms of fuel costs savings, reduced capital costs, increased production, the ability to comply with environmental legislation and the securing of environmental permits. MI-CFD has proven its value to both old and contemporary plants, including those under design/construction, where unconventional fuels or innovative pyro-processing arrangements have posed major engineering challenges. A specially-devised simpler mathematical model, enabling the effects of changing daily operational parameters to be forecast, is also described.
Cement Industry Technical Conference Record, 2008 IEEE; 06/2008
-
[show abstract]
[hide abstract]
ABSTRACT: Interest in the 'flameless oxidation' (FO) concept has escalated over the last decade because it has proved to be an effective method for reducing thermal NOx emissions from, and for improving combustion efficiency in, high temperature thermal processes. Thus far, the main areas of application have been in the steel, chemical and ceramic industries, where the technology has shown that NOx emissions can be lowered significantly while maintaining high efficiency. These advantages and other additional benefits are beginning to attract the attention of the gas turbine community in the power generation and aircraft industry sectors. In general, FO embodies partially premixed and diffusion reaction. Therefore, a validated methodology is needed, which can handle this complication in a manner enabling gas turbine combustor design calculations to be efficiently performed. The present paper concerns the validation of such a methodology and its application to a prototype gas turbine design. It follows a previous validation study of proposed FO modelling, covering a wide range of FO laboratory scale combustion facilities. Very useful agreement was demonstrated. In the first part of the present study, predictions of a recently constructed afterburner device, operated under gas turbine FO conditions, are performed to assess the validity of the model for gas turbine application. Thereafter, the method is applied to a new gas turbine combustor design. The conditions required to secure the FO regime therein are identified. Simulation of the new combustor shows encouraging results that demonstrate the potential for NOx reduction and improved pattern factors offered by FO combustion.
Journal- Energy Institute 05/2006; 79(2):75-83. · 0.59 Impact Factor
-
Combustion Science and Technology. 01/2000; 152:39-56.
-
01/2000;
-
Combustion Science and Technology 12/1996; 121(1-6):281-298. · 0.86 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The initial stages of the devolatilization of three United Kingdom pulverized coals have been studied in a coal-laden turbulent air jet emerging into the combustion gas environment produced by a methane-air flat flame burner. The temperature range provided by these flames was between 1670 and 1870 K and the corresponding particle heating rate was about 10(5) K/s. Data are reported for major gas species concentrations, particle weight losses, and nitrogen released, which quantify the effects of gas temperature, particle size, and coal type. Overall, the results show that the rate of devolatilization increases with temperature, significantly decreases with particle size and that the composition of the volatiles released is a strong function of the structure of the parent coal. The fuel nitrogen release is correlated to the total volatile yield. The formation of fuel-NO depends on the extent of the nitrogen release and the coal particle combustion mechanism. The turbulence intensity has an important effect on the extent of devolatilization; hence the total weight losses detected in the present experiment exceed those found under nonturbulent conditions.
Combustion and Flame 01/1994; 96(1-2):150-162. · 3.59 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: This paper reviews and analyses the major findings of research during the last 5 years in the Mechanical Engineering Department, Imperial College, on the chemistry of fuel-bound nitrogen in pulverized coal flames. The aim has been to lay the foundations for nitrogen-related laboratory research on p.f. flames of practical interest. The work has included both measurements and mathematical modelling. The experiments were conducted in both small-scale turbulent coal flames and large-scale laboratory flames (0.5 MW). The emphasis has been three-fold: (1) Study of the initial stages of the devolatilization, nitrogen release and subsequent nitric oxide formation from pulverized coal particles injected into a bench-top turbulent flat flame; (2) Identification of the combustion aerodynamic conditions in a large-scale pulverized-coal-fired laboratory furnace which favour reduced NO formation/emission, and determination of formation mechanisms of nitrogen oxides. To this end, measurements were made on two aerodynamically distinct industrial-type burners to assess the effect of furnace operating conditions and coal particle size distribution on NO formation and emission. Detailed in-flame measurements of nitrogen oxides, intermediate nitrogenous species, major gas species, gas temperature and particle composition were undertaken. (3) Development and validation of an NO post-processor coupled to a 2D mathematical model (FAFNIR) of pulverized coal combustion. The NO post-processor is based on available empirically based, simplified kinetic schemes. The modelling encompasses the separate contributions to fuel-NO formation of the volatiles and the char as well as NO reduction by the char. The most significant conclusions to date include the following: (1) A low-NO(x) burner has been developed, based on the knowledge gained on the effect of particle trajectories in the near-burner region on NO formation and reduction, which can successfully reduce NO emissions from 600 to 280 vpm without affecting flame stability and combustion efficiency; (2) N2O survives in the fuel-rich region where HCN, NH3, and NO are present. There is no evidence of a post-flame N2O formation 'window' between 877 and 1227-degrees-C as suggested in some earlier studies; (3) The NO predictions are generally in good agreement with a wide range of experimental data, except for the near-burner region of the low-NO(x) burner, where the model somewhat over-predicts the combustion reactions. This deficiency in the parent code, which appears to be shared by all other codes, is due to the poor predictions of the gas temperature and oxygen concentrations in the near-burner region.
Fuel 01/1994; 73(9):1423-1436. · 3.25 Impact Factor
-
Combustion Science and Technology 01/1993; 93(1):73-90. · 0.86 Impact Factor
-
Combustion and Flame 01/1991; 87(1):104-108. · 3.59 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: This paper describes the application of a mathematical model to predict the aerodynamics, combustion,and NO emission performances of a gas turbine combustor. The numerical basis of the model is a specially constructed code that uses a generalised nonorthogonal and boundary conforming coordinate system. The gas turbine is a component of the British Coal “Topping Cycle,” a coal-fired fluidised-bed-based combined-cycle technology. A distinctive feature of this study is therefore that the fuel for the gas turbine is produced from the partial gasification of coal. Since this fuel contains nitrogenous species derived from the fuel-bound nitrogen of the parent coal, there is concern that these species might give rise to high NO emissions from the combustor. In addition to the fuel NO, the prompt and thermal NO are modelled. The aerodynamic and combustion performances of the combustor, a Frame-9 unit, for this rather unusual fuel are found to be generally good, with no indication of flame stabilisation difficulties and only a relatively small temperature nonuniformity over the exit plane. This nonuniformity can be traced to products that manage to “skirt round” the dilution jet flows. The predicted level of NO emission corresponds remarkably well with that measured. Most importantly, a significant reduction of NO to N2 by NH3 occurs within the primary zone, with the result that the NO emission from the gas turbine component of the Topping Cycle is considerably diminished.
Symposium (International) on Combustion 25(1):251-259.
-
[show abstract]
[hide abstract]
ABSTRACT: An assessment has been made of the technical feasibility of co-firing pulverized sewage sludge in a utility hoiler with regard to the change in envisaged combustion performance and the possible additional impact on the environment from toxic metals and NOx emissions. Data have been reported for three PF flames— bituminous coal, sewage sludge, and an equal blend, by mass, of the two fuels in a 0.5-mW down-fired furnace.The experimental results, supported by complementary predictions of the fuel particle trajectories, show that the flame stability performance of the blended fuel is slightly improved as compared with the singlefuel flames. The overal combustion characteristics of the sewage sludge flame were found to be similar to those previously observed for low-rank, coals (e.g., lignite). The nitric oxide emissions were higher for the sewage and the blended flame—an observation that is not solely reflected in the nitrogen content of the fuels but also in subtle changes in initial ignition behavior. A comparative analysis of the concentrations of selected metals (n, Cu, Cr, Mn, Ni, Cd, Pb) in various solid particle fractions collected near the furnace exit for the three flames shows that the metal enrichment on submicron ash particles was lowest for the sewage sludge flame and increased with the proportion of coal (fuel rank). The formation of a higher submicron fraction of ash particles from lignite coal reported in the literature, however, was not observed for the sewage sludge flame. These observations showed that the vapor pressure alone does not provide a sufficient explantation for metal partitioning in the presence of large quantities of minerals. Other processes such as fragmentation, mineral/metal interactions, and metal oxidation/chlorination become increasingly important.From an operational view-point, the co-firing of dried, pulverized, sewage sludge (DPSS) slightly enhanced flame performance, while the metal emission values in the flue gas, as well as the metal leachability values, remained lower than recommended EU legislative limits. The 25% increase in NOx observed could in part be ameliorated through available NOx reduction techniques.
Symposium (International) on Combustion.
-
[show abstract]
[hide abstract]
ABSTRACT: Biomass has always been used as a localized energy source. Its seasonal availability, low calorific value, and density make it less attractive as a fuel in a centralized power generation system mainly because of higher costs associated with its storage and transportation. Notwithstanding these constraints, biomass fuels may be used together with coal in existing combustion systems. The focus of this paper is therefore on transferring the experience that has been gained from pulverized coal combustion to that of the cocombustion of biologically derived solid fuels. The substitution of these green fuels has the potential to significantly ameliorate the environmental impact of coal utilization. This lecture comprises a few comments on equipment, and on fuel characterization, something on the relevant physical processes and experimentation, and some discussion of the mathematical modeling of the combustion of solid fuels.
Symposium (International) on Combustion.
-
[show abstract]
[hide abstract]
ABSTRACT: Coal is complex and heterogeneous, with extremely variable properties. As a result, it has proved very difficult to construct generalized physical descriptions of pulverized coal combustion for incorporation into reliable mathematical models suited to industrial applications. There are many processes to be simulated: pyrolysis, char kinetics, particle/turbulence interaction, etc. This paper is concerned with the early stages of pyrolysis, which significantly affect flame stability, NO formation, soot formation, and ultimately, char burn-out. In most of the existing predictive procedures for devolatilization, combustion and emissions are modeled by a single-step global chemical reaction, with the yield of volatile matter presumed to experience mixing-controlled combustion. Several more detailed multi-step coal devolatilization models have recently emerged, having a range of capabilities, e.g., predicting the thermal decomposition of a coal under practical conditions. A common shortcoming of these models is that they require a large set of input data, involving kinetic parameters, gas precursor compositions, and additional parameters describing the coal’s polymeric structure. The input data must be generated from an extensive series of experimental measurements for each coal of interest. Very significant computational expense and application restricted to coals, which have already been studied, are implied. All of these problems are exacerbated when coal blending or co-firing with renewable solid fuels, such as forest and agricultural waste, and sewage sludge, is considered.In this paper, a new approach based on neural networks is proposed; it is capable of handling a range of solid fuels. The model considers heating rate, fuel atomic ratios, and the temperature of the fuel particles to predict the volatiles released by the particles. The “learning” properties of the model implicitly facilitate all the physical conditions of devolatilization experiments, which were used during its training and validation phases. The neural-network model was implemented into an existing 3D CFD combustion code. The predictions for high- and low-NOx burners demonstrate improved prediction of in-flame data for reduced computational effort, one-fifth of that with the standard single-global-reaction devolatilization model. Its devolatilization predictions have also been compared with a detailed devolatilization model (FLASHCHAIN) and were found to be comparable.
Combustion and Flame.
-
[show abstract]
[hide abstract]
ABSTRACT: Detailed measurements have been performed for two distinct pulverized-coal-fired burners in a large-scale laboratory furnace. Comparative in-flame data are archived and include gas temperature, O2, CO concentration, and an inventory of stable fuel nitrogen species and solids (HCN, NH3, N2O, NO, nitrogen release, mass flux, and particle burnout). A significant decrease in the NO concentration in the near burner region and a substantial decrease in the furnace exit values are observed when the “central tube” from a single annular orifice burner jet (normally the location of a gas or oil burner for light-up purposes) is replaced with a single central orifice burner jet of same cross-sectional area. The latter burner exhibits the delayed combustion phenomena normally associated with a tangentially fired system. The particle burnout remains unaffected due to the longer particles' residence time in the all-important oxygen lean internal recirculation zone. The difference in NO emissions is not reflected in the on-line N2O emission values that are found to be low (2–3 ppm) for both flames. Data recorded nearer to either burner configuration, however, reveal that the maximum N2O values for both burners are found in the flame region and are between 1%–2% of the corresponding maximum NO concentrations.
Combustion and Flame.
-
[show abstract]
[hide abstract]
ABSTRACT: A mathematical model for predicting the fate of toxic heavy metals during combustion is presented. The model accounts for the formation of new particles through nucleation and the growth of existing particles through the combined effects of condensation and coagulation. A key simplification enabling the development of a usable methodology is the subdivision of the particle size distribution into just two modes, one comprising fine particles and the other coarse particles. The toxic heavy metal calculations are embodied in a post-processor appended to a parent code for the prediction of the aerodynamics and combustion. Three flames are studied for a coal fuel, sewage sludge fuel and a blend of these two fuels. The partitioning and emissions of the semi-volatile metals, lead and cadmium, are predicted and compared with data collected in a large-scale laboratory combustor. The predicted metal enrichments on the small particles are remarkably well-predicted. However, it is concluded that heterogeneous condensation of the metal vapours alone is insufficient to explain fully the fate of metal vapours. Surface reaction and gas phase chemical reaction should also be considered, but the necessary chemical kinetic data are currently lacking. The results indicated that ash mineral matter, such as kaolinite, is an effective sorbent for lead, but not for cadmium. As expected, the coagulation of fine particles is influenced more by the particles number density than by combustion variables. It is also found that the scavenging of fine particles by coarse particles has little effect on the metal partitioning.
Fuel Processing Technology.
-
[show abstract]
[hide abstract]
ABSTRACT: The results of extensive experimental and predictive studies of nitric oxide (NO) and particulates (unburned coke) emissions from a large-scale laboratory furnace, fired by a heavy fuel oil (HFO) swirl burner with a rotary cup air blast atomizer are presented. A detailed in-flame data archive of gas temperature and O2, CO, CO2, and NO concentrations has been obtained for five flames for differing excess air levels (15% and 20%), swirl numbers (1.05 and 1.2), primary air-to-fuel ratios (2.5 and 3.0), and atomizer cup speeds (1.0×104 and 2.0×104 rpm). A wider range of operating parameters has been established to quantify their effects on NO and particulate concentrations at the exit of the furnace. In a parallel modeling study, a two-dimensional computational fluid dynamics code for the prediction of HFO spray combustion and NO and particulates emissions has been constructed. Validation of the code against the experimental data reveals reasonably good quality predictions in the near burner region. The code is capable of simulating the measured trends of flue-gas NO and particulates emissions with useful precision for a wide range of atomizer/burner operating conditions. A scrutiny of the in-flame and flue-gas data, with the aid of the predictions, has provided an enhanced understanding of combustion and combined NO/particulate emissions characteristics of the HFO flames generated by the rotary cup atomizer and establishes the foundation for future work in optimizing combustion and emissions performance.
Symposium (International) on Combustion.
-
[show abstract]
[hide abstract]
ABSTRACT: During coal combustion various toxic compounds are generated from its sulphur content. Their environmental impacts are considered to be very important. While there are various conventional preparation methods to remove the sulphur in the fuel, recent work reveals that newly-isolated micro-organisms, naturally present in coal, have the ability to reduce its sulphur content. The removal of sulphur using biological leaching involving acidophilic iron oxidising bacteria like Acidithiobacillus ferrooxidans and Leptospirillum ferrooxidans are examined and a computational technique based on computational fluid dynamics is developed to model the biological leaching of sulphur from coal.The model was validated against a pack-column experiment carried out for iron separation during 60 days. The mathematical model predicted iron separation over time is similar to experimental measurements, with an average difference of 5.5%. According to the experimental results, there was an overall reduction of 33% of pyrite, whereas the model prediction was 32%. The model results shows overall good agreement with pack-column experimental data.
Minerals Engineering.
-
[show abstract]
[hide abstract]
ABSTRACT: A new dual fuel burner designed for the co-firing of waste-derived solid fuels (e.g., biomass, refused-derived fuel, sewage sludge) with pulverized coal in practical combustors was evaluated through trials undertaken in a 0.5 MW down-fired furnace. A new mathematical procedure was also constructed that accounts for multimode combustion of these fuels. It includes the influence of the heating and devolatilization rates of each fuel on the effective stoichiometry of the volatiles in the combustion domain depending on their respective particle trajectories. Results included for sawdust-coal flames, show the sognificant effect of co-firing ratio and fuel injection mode on flame ignition, combustion aerodynamics, and nitric oxide emissions. Predicted indices of the coal devolatilization rate along the particle trajectories emphasize the influence of the faster devolatilization and ignition of the sawdust on coal combustion in the near burner region. When the sawdust particles are injected through the center of the burner, surrounded by an annular coal jet, they immediately ignite thereby enhancing the combustion intensity of the coal within the internal recirculation zone. This injection mode leads to a subsequent reduction in the nitric oxide formation along with a higher combustion efficiency as compared with a flame where the sawdust and coal injection positions are reversed. An optimum co-firing ratio in which the sawdust provided 30% of the total heat input was found to exhibit the maximum particle burnout and minimum nitric oxide emissions. Co-firing results obtained for a lower reactivity and higher nitrogen content fuel (pulverized sewage sludge) as compared with sawdust, show that the fuel injection mode had a marginal effect on burnout and NO emissions. The sawdust and sewage sludge co-firing results emphasize the need to consider both the reactivity and nitrogen content of the fuel prior to selecting an injection mode.
Combustion and Flame.
-
[show abstract]
[hide abstract]
ABSTRACT: During coal combustion various toxic compounds are generated from its sulphur content. Their environmental impacts are considered to be very important. While there are various conventional preparation methods to remove the sulphur in the fuel, recent work reveals that newly-isolated micro-organisms, naturally present in coal, have the ability to reduce its sulphur content. The removal of sulphur using biological leaching involving acidophilic iron oxidising bacteria like Acidithiobacillus ferrooxidans and Leptospirillum ferrooxidans are examined and a computational technique based on computational fluid dynamics is developed to model the biological leaching of sulphur from coal. The model was validated against a pack-column experiment carried out for iron separation during 60 days. The mathematical model predicted iron separation over time is similar to experimental measurements, with an average difference of 5.5%. According to the experimental results, there was an overall reduction of 33% of pyrite, whereas the model prediction was 32%. The model results shows overall good agreement with pack-column experimental data.
-
[show abstract]
[hide abstract]
ABSTRACT: The temperature and flow rate are estimated of chimney gases emitted from the accidental fire in the Pile 1 nuclear reactor at the Windscale Works in October 1957. These are used in the study of the atmospheric dispersion of the chimney plume and of the radionuclides in it. The accident was one of great physical complexity, compounded by the fact that the number of installed sensors was, by modern standards, rather limited. Sophisticated computer modelling (CFD) is now available. However, the scope for validation in a one-off accident is limited. The mathematical simulation of the accident presented here is based on physically acceptable assumptions, chosen to represent a realistic worst case. These give a chimney gas temperature of 577 °C and a corresponding flow rate of 416 kg s-1. The uncertainties in these quantities are assessed in a sensitivity study.
