Large-scale Laboratory Furnace Research Articles

Particle history from massively parallel large eddy simulations of pulverised coal combustion in a large-scale laboratory furnace

A study on the coal particle history during combustion in a large-scale furnace using large eddy simulation is presented. The massively parallel execution produces a high-resolution representation of the fluid mixing and particle dispersion throughout the whole computational domain. The coal combustion is modelled using well-established, cost-effective combustion models. A specific feature of the devolatilization model is the optimisation of the kinetic constants for the furnace operating condition, which were obtained through an iterative procedure between particle heating rates from full large eddy simulation runs and the advanced model Chemical Percolation Devolatilization. In a previous work, we showed that the classical coal combustion models, when used in a high-resolution massively parallel large eddy simulation, lead to satisfactory predictions of the in-flame gas properties, namely gas temperature and gas species concentrations. In this work, we went beyond the comparisons between gas phase measurements and predictions. Single particles were tracked over time and instantaneous ensambles were collected to obtain a better understanding of the conditions that coal particles are subjected to in the investigated test case. The particles trajectory, combustion history and instantaneous state distribution were analysed. The volatile flame features were related with the characteristic trajectory of different sized particles. The combustion history revealed that particles are subjected to large variations of heating rates, including very short sequential periods alternating between heating and cooling during the early stages of combustion, due to the high turbulence intensity in the near burner region. Finally, the state distribution of the ensamble provided a global picture of the instantaneous coal combustion process.

Large-eddy simulation of ash deposition in a large-scale laboratory furnace

A computational fluid dynamics model is presented that allows for the investigation of the ash deposition and provides an economical approach for studying design changes in new boilers and retrofit options for existing units. This study proposes a detailed description of ash deposition integrating three separate particle-sticking criteria: melt fraction, viscosity, and energy conservation upon collision. Also, a detailed model for predicting the thermal properties of existing deposit layers (thermal conductivity and emissivity) is implemented into a one-dimensional wall heat-transfer model. The coupled ash-deposition and wall heat-transfer model is implemented into a large-eddy simulation (LES) framework to predict the heat-flux profile, deposition rates, slagging and fouling for industrial boilers. The results of this approach are validated with experimental data from the University of Utah’s 100 kW down-fired, oxy-fuel combustion (OFC) furnace. Two OFC cases with different geometries are studied for their coal combustion and dynamic ash-deposit growth in this large-scale laboratory furnace. Comparisons of the deposition rates and gas temperature agree within 4.82% and 17.58%, respectively, of the measured data.

Fire resistance of high strength fiber reinforced concrete filled box columns

This paper presents an investigation on the fire resistance of high strength fiber reinforced concrete filled box columns (CFBCs) under combined temperature and loading. Two groups of full-size specimens were fabricated. The control group was a steel box filled with high-strength concrete (HSC), while the experimental group consisted of a steel box filled with high strength fiber concrete (HFC) and two steel boxes filled with fiber reinforced concrete. Prior to fire test, a constant compressive load (i.e., load level for fire design) was applied to the column specimens. Thermal load was then applied on the column specimens in form of ISO 834 standard fire curve in a large-scale laboratory furnace until the set experiment termination condition was reached. The test results show that filling fiber concrete can improve the fire resistance of CFBC. Moreover, the configuration of longitudinal reinforcements and transverse stirrups can significantly improve the fire resistance of CFBCs.

Impact of OFA on combustion and NOx emissions of a large-scale laboratory furnace fired by a heavy-oil swirl burner

Experiments were carried out on a horizontal test furnace to assess the overall air-staging combustion performance and NOx emissions of a large-scale laboratory furnace fired by a heavy-oil swirl burner. Impacts of over-fire air (OFA) parameters (including the OFA position and ratio) on NOx and CO emissions were experimentally and numerically evaluated in the test furnace. The experimental results indicated that both the OFA ratio and positioned location affected NOx emissions during the heavy fuel oil (HFO) combustion. For the OFA positioned location downstream from the burner nozzle outlet, it was found that a relatively larger distance between OFA and the burner outlet could generate lower NOx emissions and almost yield no increase in the burnout loss. A high-temperature region above 1773 K was obtained in a downstream reference section of the burner jet and introducing OFA shrank significantly the high-temperature region. Additionally, industrial-size measurements performed in an industrial furnace were used to confirm further OFA impacts. In the industrial furnace, a 20% OFA ratio application (the primary combustion equivalence ratio of 0.88) reduced NOx emissions from 540 to 420 mg/Nm3 at 3% O2; this NOx reduction proportion (i.e., 22.22%) was close to levels determined in lab-scaled experiments.

Experimental studies on heat transfer of oxy-coal combustion in a large-scale laboratory furnace

A number of issues arise in the transition from air-firing to oxy-firing. In situations where the furnace should be retrofitted for oxy-firing, a similar heat transfer performance is crucial for operation of power plants. A survey of recent literature turned up very little detailed research on the characterization of pulverized oxy-coal flames generated by staged feed-gas burners and the impacts of burner settings on heat transfer performance. A large number of studies applied either O2 fraction upstream of the burner or recycle ratio as main parameter to characterize oxy-fired conditions. For this reason, this study applies other characteristic parameters, as for example swirl number and feed gas flow ratio between the burner registers, rather than only O2 concentration for operation of the test facility. Experiments were conducted in a test facility with a rated capacity of 0.40 MWth fired by an industry-type burner. A theoretical study was also carried out to calculate combustion parameters, e.g., adiabatic flame temperature, in order to define burner settings and assess the experimental results. The burner was set to operate at three levels of secondary swirl number (1.15, 1.65, and 2.05), while part of the feed gas was divided between the secondary and tertiary registers using flow ratios between 0.40 and 2.00. For completeness, three levels of O2 fraction upstream of the burner (29, 31, and 33 vol% O2) were also tested under oxy-fired conditions. Measurements of peak flame temperature, absorbed heat flux at the water-cooled walls, and total radiative heat flux were performed and applied besides heat balance calculations to evaluate heat transfer performance. Experimental data indicated the feed gas distribution among the burner registers control the heat transfer in the furnace in parallel with the swirl strength of the secondary flow for both air-fired and oxy-fired conditions. Even though lower flame temperatures were obtained in oxy-firing, the results suggest the possibility of matching similar heat uptake in the furnace to that found under air-fired conditions. Experimental data also indicated the choice of an O2 fraction of 31 vol% upstream of the burner, which enables a very close adiabatic flame temperature as air-firing, is an appropriated parameter for maintaining similar heat transfer conditions in the furnace.

Combustion of biodiesel in a large-scale laboratory furnace

Combustion tests in a large-scale laboratory furnace were carried out to assess the feasibility of using biodiesel as a fuel in industrial furnaces. For comparison purposes, petroleum-based diesel was also used as a fuel. Initially, the performance of the commercial air-assisted atomizer used in the combustion tests was scrutinized under non-reacting conditions. Subsequently, flue gas data, including PM (particulate matter), were obtained for various flame conditions to quantify the effects of the atomization quality and excess air on combustion performance. The combustion data was complemented with in-flame temperature measurements for two representative furnace operating conditions. The results reveal that (i) CO emissions from biodiesel and diesel combustion are rather similar and not affected by the atomization quality; (ii) NOx emissions increase slightly as spray quality improves for both liquid fuels, but NOx emissions from biodiesel combustion are always lower than those from diesel combustion; (iii) CO emissions decrease rapidly for both liquid fuels as the excess air level increases up to an O2 concentration in the flue gas of 2%, beyond which they remain unchanged; (iv) NOx emissions increase with an increase in the excess air level for both liquid fuels; (v) the quality of the atomization has a significant impact on PM emissions, with the diesel combustion yielding significantly higher PM emissions than biodiesel combustion; and (vi) diesel combustion originates PM with elements such as Cr, Na, Ni and Pb, while biodiesel combustion produces PM with elements such as Ca, Mg and Fe.

Large Eddy Simulation of coal combustion in a large-scale laboratory furnace

A detailed Large Eddy Simulation (LES) of pulverised coal combustion in a large-scale laboratory furnace is presented. To achieve a detailed representation of the flow, mixing and particle dispersion, a massively parallel LES was performed. Different phenomenological network models were applied and compared to each other in order to obtain the most adequate devolatilization kinetic data for the LES. An iterative procedure allowed to optimise the devolatilization kinetic data for the studied coal and operating conditions. The particle combustion history is studied by analysing particle instantaneous properties giving a perspective on coal combustion that currently is not available by other means than LES. Predicted major species and temperature were compared with measurements and a good agreement was obtained. The finely resolved near burner region revealed that the flame is stabilised very close to the burner. Furthermore, two distinct zones of CO2 production were found – one in the internal recirculation zone (IRZ) due to gaseous combustion, and one downstream of the vortex breakdown, due to intense char combustion. It was found that particle properties are inhomogeneous within the IRZ, whereas in the external recirculation zone (ERZ) and downstream of the vortex breakdown they were found to be homogeneous.

Investigation on ash deposit formation during the co-firing of coal with agricultural residues in a large-scale laboratory furnace

This article describes an experimental investigation on ash deposit formation during the co-firing of coal with agricultural residues (wheat straw, olive stones and peach stones), whose main objective was to relate the deposition rate, morphology and composition of the deposits with the type of biomass used in the co-combustion process. The study was conducted in a large-scale laboratory furnace for relatively high co-firing ratios (0.4, energy basis). For comparison purposes, tests have also been performed for pure coal firing and co-firing of coal with pine branches. During the tests, the deposits were collected with the aid of an air-cooled stainless steel deposition probe, whose surface temperature was maintained at around 500°C, and of an uncooled ceramic probe with a surface temperature of about 850°C. Both probes were placed in a region inside the furnace in which the mean gas temperature was about 1000°C, being the deposits collected after 2h of exposure. A number of deposit samples were subsequently analyzed on a scanning electron microscope equipped with an energy dispersive X-ray detector. The results reveal that, in contrast with the co-firing of coal with pine branches, co-firing of coal with wheat straw and olive stones increase the ash deposition rate, as compared to pure coal firing. In addition, deposits resulting from the use of wheat straw and olive stones as secondary fuels present higher degree of adherence to surfaces. This is due to the high concentration of alkali metals present in the deposits composition, leading to compounds of low melting points. In particular, these two biomass fuels originated deposits with relatively high levels of K and S, which facilitate their sintering degree. The use of peach stones in the co-firing process led to a deposition rate similar to that of pure coal firing. However, the deposits present a relatively higher degree of adherence to the surfaces, which is probably due to the presence of higher levels of Na in its composition.

Validation of an effervescent spray model with secondary atomization and its application to modeling of a large-scale furnace

The present work consists of a validation attempt of an effervescent spray model with secondary atomization. The objective is the simulation of a 1 MW industrial-type liquid fuel burner equipped with effervescent spray nozzle. The adopted approach is based on a double experimental validation. Firstly, the evolution of radial drop size distributions of an isothermal spray is investigated. Secondly, the spray model is tested in a swirling combustion simulation by means of measured wall heat flux profile along the flame. In the first part of the paper, both experiments are described along with the measuring techniques. Drop sizes and velocities measured using a Dantec phase/Doppler particle analyzer are analyzed in detail for six radial positions. Local heat fluxes are measured by a reliable technique along the furnace walls in a large-scale water-cooled laboratory furnace. In the second part Euler – Lagrange approach is applied for two-phase flow spray simulations. The adopted spray model is based on the latest industrially relevant (i.e. computationally manageable) primary and secondary breakup sub-models complemented with droplet collision model and a dynamic droplet drag model. Results show discrepancies in the prediction of radial evolution of Sauter mean diameter and exaggerated bimodality in drop size distributions. A partial qualitative agreement is found in radial evolution of drop size distributions. Difficulties in predicting the formation of small drops are highlighted. Comparison of the predicted wall heat fluxes and measured heat loads in swirling flame combustion simulation shows that the absence of the smallest droplets causes a significant elongation of the flame.

Detailed measurements in a laboratory furnace with reburning

This article presents a detailed experimental characterization of the reburning process in a large-scale laboratory furnace. Natural gas, pine sawdust and pulverized coal were used as reburn fuels. Initially, the study involved the collection of in-flame combustion data, without reburning, in order to define appropriate locations for the injection of the reburn fuels. Next, flue-gas data were obtained for a wide range of experimental conditions using the three reburn fuels and, subsequently, detailed measurements of local mean O2, CO, CO2, HC and NOx concentrations, and gas temperatures have been obtained in the reburn zone for three representative furnace operating conditions, one for each reburn fuel studied. The flue-gas data revealed that the sawdust reburning leads to NOx reductions comparable or even higher than those attained with natural gas reburning, while coal reburning yields much lower NOx reductions. The detailed data obtained in the reburn zone indicates that the reburning process remains active throughout all the reburn zone in the cases of natural gas and sawdust reburning, while in the case of coal reburning its relatively low volatile matter content is insufficient to establish an effective reburn zone. In the cases of the sawdust and coal reburning the burnout levels remain approximately constant, regardless of the NOx emissions reduction, with the sawdust reburning leading to higher particle burnout performance than the coal reburning.

NO x control through reburning using biomass in a laboratory furnace: Effect of particle size

This article describes an experimental study to evaluate the effect of the particle size distribution of the reburn fuel on the effectiveness of the reburning process to reduce NO x emissions in a large-scale laboratory furnace. Rice husk, with three distinct particle size distributions, was used as reburn fuel. Initially, flue-gas data (gaseous pollutants and particle burnout) was obtained for a wide range of experimental conditions with the “three” rice husks. Subsequently, in order to improve the understanding of the relationship between reburning effectiveness and reburn fuel particle size, detailed measurements of local mean O 2, CO, CO 2, HC and NO x concentrations, and gas temperatures have been gathered from the reburn zone for three furnace operating conditions that used the “three” rice husks. The results suggest that, for the present combustor and operating conditions, there is an optimum particle size range for enhanced NO x reduction through reburning. Below or above the optimum particle size range of the rice husk the reburn mechanism is less effective probably because the biomass devolatilization process either finishes before or extends well beyond the end of the reburn zone, respectively, impeding the exposure of NO to CH i and HCCO and facilitating the CH i and HCCO interaction with oxygen. In addition, the results show that the particle burnout decreases slightly as the NO x reduction increases, regardless of the rice husk particle size range. Given that the present combustor has only the roof section and the upper four segments lined with a layer of refractory, it can be anticipated that reburning using rice husk as reburn fuel in full-scale equipment would have even a lower impact in particle burnout than that observed in this study.

Impact of the air staging on the performance of a pulverized coal fired furnace

The aim of this work is to evaluate the impact of the air staging on the overall performance of a large-scale laboratory furnace fired by an industry-type pulverized coal swirl burner. Data have been obtained for pollutant emissions and particle burnout for a wide range of the furnace operating conditions. The influence of the axial position of the staged air injector, of the primary zone stoichiometric ratio ( λ pz), of the coal type, and of the configuration of the staged air injector on pollutant emissions and particle burnout are quantified on the basis of these results. The main conclusions are as follows: (i) NO x emissions decrease as the distance between the staged air injector and the burner exit increases; however, if the distance exceeds a given value, CO emissions increase significantly; (ii) in general, a reduction in λ pz causes a decrease in both NO x emissions and particle burnout and an increase in CO emissions. Moreover, this results suggest that the benefits in terms of reduction of NO x emissions obtained with values of λ pz < 0.9 can be considerably attenuated by its deleterious effect on the overall combustion efficiency; (iii) the type of coal can affect significantly the effectiveness of this NO x control method; it is not possible, however, to establish definitive correlations between the reduction in NO x emissions and the main coal properties; and (iv) the configuration of the staged air injector has little impact on NO x emissions, because the air staging effectiveness is a strong function of λ pz, but it is a key feature in establishing the overall combustion efficiency for λ pz < 1.

Detailed measurements in a pulverized-coal-fired large-scale laboratory furnace with air staging

The aim of the present work was to evaluate the performance of a pulverized-coal-fired large-scale laboratory furnace with air staging. New data are reported for gas phase species concentration, temperature and particle burnout for two primary zone stoichiometric ratios, 1.15 (unstaged flame) and 0.95 (staged flame), other operating conditions being fixed. The results revealed that the reduction in primary zone stoichiometric ratio caused a decrease in NO x emissions from 421 to 180 mg/N m 3@6%O 2, an increase in CO emissions from 51 to 168 mg/N m 3@6%O 2 and a reduction in particle burnout from 81.8% to 76.5%. It was concluded that the reduction of the O 2 availability in the primary zone inhibits the NO formation, mainly via the fuel mechanism, but it affects negatively both the CO and the char oxidation processes because, under staging conditions, both processes tend to occur in the vicinity of the over fire air injection region, where the temperatures are relatively low.

재연소 과정을 적용한 연소로에서 공기 다단 연소기의 NOx발생 및 열전달에 대한 효과

An experimental study has been conducted to investigate the effects of a Multi air-staged burner on NOx formation and heat transfer in a 15kW large-scale laboratory furnace adopted the reburning process. The reburn fuel as well as burnout air was injected from each nozzle attached at the wall of the cylindrical furnace. Fuel in both main burner and reburn nozzle was LPG (Liquefied Petroleum Gas). The paper reports the influences on NOx reduction of reburn fuel fraction in reburning zine. Temparature distribution inside the overall region as well as total heat flux at the wall of the furnace has been measured to examine the heat transfer characteristics due to the reburning process. For comparison, the reburning effects were examined for a combustor with two types of burner; a regular single staged burner and a multi-air staged burner. A gas analysis was also performed to evaluate an appropriate condition for NOx emission in a promary zone for the excess air ratio of 1.1. As a result, combustion efficiency expected to become more efficient due to the reduction of heat loss in burnout zone decrease when multi air-staged burner in furnace adopted reburning technology was used.

THE EFFECTIVENESS OF REBURNING USING RICE HUSK AS SECONDARY FUEL FOR NO X REDUCTION IN A FURNACE

ABSTRACT The aim of the present work was to evaluate the effectiveness of the reburning process using biomass (rice husk) as reburn fuel in a large-scale laboratory furnace. For comparison purposes, tests were also conducted using natural gas and ethylene as reburn fuels. The paper reports new data for flue-gas emissions for a wide range of experimental conditions which quantify the effects of the reburn fuel fraction (energy basis), residence time in the reburning zone, and initial NO x concentration for the three secondary fuels on NO x reduction from the present combustor. The results show that at reburn zone residence times of about 0.7 s the reburning performance of the rice husk (1) is comparable to that of the natural gas reburning at high reburn fuel fractions, with almost 60% NO x reduction achievable at reburn fuel fractions of 25 and 30%, and (2) approaches those of the natural gas and ethylene at high initial NO x concentrations, with nearly 60% NO x reduction attainable at initial NO x concentrations between around 500 and 970 ppm. The results also reveal that there is a correlation between the extent of NO x reduction and particle burnout: the higher the reduction, the lower the burnout. However, the present results combined with previous data from the same furnace using coal as reburn fuel suggest that the use of rice husk as reburn fuel in full-scale equipment would have a lower impact in the carbon burnout as compared with coal.

CO-COMBUSTION OF PULVERIZED COAL, PINE SHELLS, AND TEXTILE WASTES IN A PROPANE-FIRED FURNACE: MEASUREMENTS AND PREDICTIONS

This paper describes an experimental and numerical investigation on the co-combustion of propane with pulverized coal, pine shells, and textile wastes. Measurements have been performed in a large-scale laboratory furnace fired by an industrial-type swirl burner. Data are reported for in-flame major gas-phase species concentration, including NOx, in-flame gas temperature, and overall char burnout for three flames: a propane/coal flame, a propane/pine shells flame, and a propane/textile wastes flame. For comparison purposes, data are also reported for a pure propane flame. The experimental results show that CO and unburned hydrocarbon emissions from propane cofiring with pine shells and textile wastes can be important due to the relatively large sizes of the solid particles and that NOx emissions data reproduce the impact on them of the fuel nitrogen content via fuel-NO formation for the three solid fuels studied. Also, the cofiring of propane with pine shells and textile wastes yields particle burnout values much higher than that of the propane/coal flame despite the similarities of the three flames revealed by the in-flame data. This is because of the higher volatile matter content of the pine shells and textile wastes, in spite of their much larger particle sizes, compared with coal. The experimental conditions were then simulated numerically. For the numerical predictions, gas-phase calculations were based on the Eulerian approach whereas the particulate phase predictions were based on a stochastic Lagrangian approach. The turbulence model used herein was the standard two-equation eddy viscosity model. The gaseous fuel and volatiles oxidation was assumed to be controlled by both turbulent mixing and chemical kinetics. Radiation was accounted for. The biomass/waste devolatilization was simulated by a single-reaction, first-order decomposition model. Thermal-NO, prompt-NO, and fuel-NO mechanisms were all considered as potential contributors to the NO formation. From the comparisons of the predictions with the experimental data, a good agreement was generally found, except in the near-burner region close to the furnace symmetry axis. The discrepancies are due to the limitations of the k–ϵ turbulence model to simulate high swirling flows. Moreover, the simplified fuel-NO model used herein requires further improvements to yield more accurate results.

Co-combustion of biomass in a natural gas-fired furnace

The co-combustion of pulverized-biomass in a natural gas-fired large-scale laboratory furnace is investigated experimentally. The biomass fuels used included pine sawdust, pine shells, pine branches, and olive stones. For comparison purposes, the co-combustion tests have also included the study of natural gas/pulverized-coal flames. For each solid-fuel, flue-gas data were obtained for pollutant emissions and particle burnout as a function of the solid-fuel cofiring ratio. Ratios up to 20% (energy basis) were used in the experiments. Subsequently, in order to improve the understanding of the processes that occur during the co-combustion of natural gas and solid fuels, the study focused on detailed in-flame measurements of major gas species, including NO x , and gas temperatures for three representative furnaceoperating conditions. The results show that (1) cofiring of biomass fuels, compared with pure natural gas firing, has no significant effect on NO x emissions provided that the fuel-bound nitrogen is low because of the positive impact of the reduced in-flame temperatures of the cofiring in the thermal NO formation that serves to attenuate, to some extent, the fuel NO formation associated with biomass fuels; (2) cofiring of biomass fuels, compared with pure natural gas firing, may have an important impact on CO and hydrocarbon emissions if particle sizes are too large because this extends the volatilization process downstream into regions with relatively low temperatures; and (3) cofiring of biomass fuels, compared with cofiring of coal, allows for much larger particle sizes without a deleterious effect on particle burnout performance due to its higher content of volatile matter.

Characterisation of coal blends for pulverised fuel combustion

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.

00/02300 Coal transporters gain ground

This article describes an experimental study to evaluate the effect of the particle size distribution of the reburn fuel on the effectiveness of the reburning process to reduce NOx emissions in a large-scale laboratory furnace. Rice husk, with three distinct particle size distributions, was used as reburn fuel. Initially, flue-gas data (gaseous pollutants and particle burnout) was obtained for a wide range of experimental conditions with the “three” rice husks. Subsequently, in order to improve the understanding of the relationship between reburning effectiveness and reburn fuel particle size, detailed measurements of local mean O2, CO, CO2, HC and NOx concentrations, and gas temperatures have been gathered from the reburn zone for three furnace operating conditions that used the “three” rice husks. The results suggest that, for the present combustor and operating conditions, there is an optimum particle size range for enhanced NOx reduction through reburning. Below or above the optimum particle size range of the rice husk the reburn mechanism is less effective probably because the biomass devolatilization process either finishes before or extends well beyond the end of the reburn zone, respectively, impeding the exposure of NO to CHi and HCCO and facilitating the CHi and HCCO interaction with oxygen. In addition, the results show that the particle burnout decreases slightly as the NOx reduction increases, regardless of the rice husk particle size range. Given that the present combustor has only the roof section and the upper four segments lined with a layer of refractory, it can be anticipated that reburning using rice husk as reburn fuel in full-scale equipment would have even a lower impact in particle burnout than that observed in this study.

98/03199 Mathematical modelling of circulating fluidized-bed combustion plants of industrial scale

This article presents a detailed experimental characterization of the reburning process in a large-scale laboratory furnace. Natural gas, pine sawdust and pulverized coal were used as reburn fuels. Initially, the study involved the collection of in-flame combustion data, without reburning, in order to define appropriate locations for the injection of the reburn fuels. Next, flue-gas data were obtained for a wide range of experimental conditions using the three reburn fuels and, subsequently, detailed measurements of local mean O2, CO, CO2, HC and NOx concentrations, and gas temperatures have been obtained in the reburn zone for three representative furnace operating conditions, one for each reburn fuel studied. The flue-gas data revealed that the sawdust reburning leads to NOx reductions comparable or even higher than those attained with natural gas reburning, while coal reburning yields much lower NOx reductions. The detailed data obtained in the reburn zone indicates that the reburning process remains active throughout all the reburn zone in the cases of natural gas and sawdust reburning, while in the case of coal reburning its relatively low volatile matter content is insufficient to establish an effective reburn zone. In the cases of the sawdust and coal reburning the burnout levels remain approximately constant, regardless of the NOx emissions reduction, with the sawdust reburning leading to higher particle burnout performance than the coal reburning.