Final Burnout Research Articles

CFD study of charcoal combustion in a simulated ironmaking blast furnace

Biomass is a carbon-neutral solid fuel and has the potential to replace coal in pulverised coal injection (PCI) operation of ironmaking blast furnaces (BFs). In this study, a three-dimensional (3D) computational fluid dynamics (CFD) model is used to evaluate the charcoal combustion behaviours under a range of PCI conditions. The key PCI variables include blast temperature, O2 concentration in the blast, mean particle size of charcoal, and charcoal injection rate. In particular, the combustion behaviours are investigated and compared at two locations, namely, along the centreline and over the entire chamber. The simulation results indicate that the final burnout along the centreline is more sensitive to the changes of operational conditions compared to the burnout over the entire chamber under the BF conditions. Furthermore, the effects of key operational variables on the two burnout calculations are identified quantitatively. For example, the increased blast temperature or the reduced mean particle size can improve the two burnouts both, due to the extra energy provided and large surface area, respectively. However, the increased O2 concentration in the blast from 21% to 23% can improve the final burnout only from 64% to 72%, but the further increase of O2 concentration in the blast cannot lead to a further increase for both two burnouts. High blast temperature, high oxygen concentration and fine particle size will allow for a higher burnout. This study provides an effective method of understanding and optimising biomass injection in BF practice.

A sophisticated model to predict ash inhibition during combustion of pulverized char particles

Final burnout of char particles from practical fuels such as coal and biomass occurs in the presence of a large ash component. Also, newly utilized coal resources, such as those from India, often contain much larger ash fractions than have traditionally been utilized. In the past, the inhibitory influence of ash on pulverized coal particle combustion has been most frequently modeled using an ash film model, though such films are rarely found when examining partially combusted particles. Conversely, some measurements have suggested that mineral components exposed on the surface of burning pulverized coal (pc) particles may diffuse back into the char matrix, the effect of which can be modeled as an ash dilution effect. To explore the implications of these different ash inhibition models on the temporal evolution of char combustion during burnout, we have developed a new computational model that considers the possibility of an ash film effect, an ash dilution effect, or some arbitrary combination of the two effects acting in tandem, which is the most realistic scenario. This new model predicts that restricted diffusion through the ash film has a significant impact on the char burnout rate throughout its lifetime, whereas char dilution only inhibits combustion significantly when most of the char has been consumed and the combustion mode shifts from predominantly external diffusion control to mixed diffusion control, with sensitivity to both external and internal diffusion resistance. The comparison of the model predictions with experimental results also confirms the previously suggested need to include gasification reaction steps when modeling coal char combustion.

Effects of Stefan flow and CO oxidation on char particle combustion in O2/CO2 atmosphere

Stefan flow occurring at the surface of char and CO oxidation in the boundary layer are often neglected in the conventional pulverized-coal air combustion. In this work, effects of Stefan flow and CO oxidation in the oxy-fuel burning are investigated numerically and experimentally. A mathematical model is firstly developed for the combustion of a carbon particle. Comparisons with the experiment data exhibit better agreement for the present model than that neglecting the effects of Stefan flow and CO oxidation. Predictions also show that neglect of the Stefan flow and CO oxidation results in overestimating the particle temperature, shortening the burnout time, and especially, being unable to predict the particle extinguishment and the final burnout rate correctly. Notable effects of Stefan flow and CO oxidation on the burnout time generally appear under conditions of lower O2 concentration, lower reactivity, lower gas temperature and larger particle size where the combustion is prone to be controlled by reaction kinetics and gas diffusion simultaneously (Regime II). On the contrary, the effects of Stefan flow and CO oxidation are relatively insignificant on the burnout time in the diffusion-controlled combustion (Regime III). However, whether the Stefan flow and CO oxidation could be neglected in such cases still depends on the actual combustion conditions and simulation results.

Farm-level reproduction number during an epidemic of infectious salmon anemia virus in southern Chile in 2007–2009

An epidemic of infectious salmon anemia virus (ISAV) has greatly impacted salmon production in Chile with devastating social and economic consequences. The epidemic is analyzed here and is likely the largest ISAV outbreak reported affecting one of the most productive regions for salmon farming activities in the world. After re-emerging in 2007, ISAV rapidly expanded the following two years, both in magnitude and geographic range, affecting about 65% and 50% of salmon farms located at the 10th and 11th regions of Chile, respectively. A useful metric for the control of infectious diseases that quantifies the progression of an epidemic is the reproduction number at the farm level ( R f), which describes the mean number of secondary cases generated by an infectious farm. The parameter in this study was estimated for individual farms ( R f i ), specific phases ( R tf), and for the entire epidemic ( R f) by using several analytical approaches based on the characterization of the epidemic curves for the two regions. For the initial spread and the epidemic growth phase, initial and intrinsic growth rates were used to estimate R tf. In addition, two approaches (epidemic final size and nearest neighbor analyses) were used to obtain an individual ( R f i ) and overall estimate of R f for the complete epidemic. In general, two distinct regional patterns of spread were identified. In the 10th region, after an explosive initial spread of ISAV in which R tf reached 12.0–16.9, a smaller epidemic growth of 1.6 ≤ R tf ≥ 2.5 and a final burnout with R tf < 1 were observed. For the 11th region, R tf only reached 2.4 during the initial spread phase, ranged from 1.6 ≤ R tf ≥ 4.4 during the epidemic growth phases and ended when R tf was <1.0. The epidemic was characterized by clustering of ISAV ‘superspreaders’ farms i.e., farms with statistically significantly ( P < 0.047) higher R f i values. Distances between pairs of infected farms were statistically significantly ( P = 0.003) shorter in the 10th compared to the 11th region. Overall, R f ranged from 1.6 to 2.5 and 1.3 to 1.7 in the 10th and 11th regions, respectively. Our findings suggest that control efforts were able to protect 38–60% and 23–41% of the farms in the 10th and 11th regions, respectively, and may have resulted in the epidemic not spreading further. In addition, control strategies in highly populated areas using a control zone of at least 10 km radius may be more effective than the 5 km zone recommended by the World Animal Health Organization.

Sorting efficiency and combustion properties of municipal solid waste during bio-drying

One aerobic and two combined bio-drying processes were set up to investigate the quantitative relationships of sorting efficiency and combustion properties with organics degradation and water removal during bio-drying. Results showed that the bio-drying could enhance the sorting efficiency of municipal solid waste (MSW) up to 71% from the initial of 34%. The sorting efficiency was correlated with water content negatively (correlation coefficient, r=-0.89) and organics degradation rate positively (r=0.92). The higher heating values (HHVs) were correlated with organics degradation negatively for FP (i.e. the sum of only food and paper) (r=-0.93) but positively for the mixing waste (MW) (r=0.90), whereas the lower heating values (LHVs) were negatively correlated with water content for both FP (r=-0.71) and MW (r=-0.96). Other combustion properties depended on organics degradation performance, except for ignition performance and combustion rate. The LHVs could be greatly enhanced by the combined process with insufficient aeration during the hydrolytic stage. Compared with FP, MW had higher LHVs and ratios of volatile matter to fixed carbon. Nevertheless, FP had higher final burnout values than MW.

Model Study of the Effects of Coal Properties and Blast Conditions on Pulverized Coal Combustion

High coal burnout within the raceway is important for the operation of a blast furnace. It is usually achieved by adjusting some operational parameters in practice. In this work, a three-dimensional model we developed recently is used to investigate the effects of some key operational parameters on coal burnout. The results confirm that notable improvements in final burnout can be achieved for coals with more fine particles and high volatile matter, and by higher oxygen enrichment. The use of high blast temperature can increase coal burnout, but the further increase in blast temperature over 1200°C has little effect on final burnout. The effects of these parameters on other combustion characteristics are also analysed, in terms of volatile content, temperature field and gas species distribution, aiming to understand the underlying mechanisms behind these improvements. It is demonstrated that local oxygen supply is very important for high burnout in addition to coal properties. In addition, it is necessary to consider the raceway region when investigating the effects of these variables on coal burnout. This study helps identify appropriate and cheaper coals and optimise operating conditions to maximize the benefits of pulverized coal injection.

Effect of co-combustion on the burnout of lignite/biomass blends: A Turkish case study

Co-combustion of Turkish Elbistan lignite and woody shells of hazelnut was performed in a TGA up to 1173 K with a heating rate of 20 K/min. SEM images of each fuel revealed the differences in their physical appearances. Hazelnut shell was blended with lignite in the range of 2-20 wt% to observe the co-combustion properties. Maximum burning rates (Rmax), temperatures of the maximum burning rates (T(R-max)), and the final burnout values of the parent samples and the blends were compared. The results were interpreted considering lignite properties and the major biomass ingredients such as cellulosics, hemicellulosics, and lignin. Deviations between the theoretical and experimental burnout values were evaluated at various temperatures. Burnout characteristics of the blends up to 10 wt% were concluded to have a synergistic effect so the addition of hazelnut shell up to 8 wt% provided higher burnouts than the expected theoretical ones, whereas addition of as much as 10 wt% led to a decrease in the burnout. However, the additive effects were more favorable for the blend having a biomass content of 20 wt%. Apparent activation energy, Rmax, and T(R-max), were found to follow the additive behavior for the blend samples.

Devolatilisation behaviour of shredded tyre chips in combusting environment

Tyres shredded into chips represent a practicable alternative fuel for cement kilns. The devolatilisation and final burnout of tyre chips in a combusting environment in a refractory lined, uniform temperature, radiative furnace have been investigated. The weight losses of tyre chips suspended in the furnace for temperatures of 700, 800, 900 and 1000uC were monitored. As expected, the devolatilisation time is greatly influenced by the furnace temperature and by the thickness of the tyre chip. The tyre chips burn with a vigorous flame during devolatilisation. The results also showed that, although the chips fragment on the surface during devolatilisation the cord fibres of the tyre structure tend to maintain the integrity of the chip until devolatilisation is complete when the chip disintegrate resulting in a wide range of char particle sizes. Upon burnout of the volatiles, the residual char burns to extinction in a low intensity flame. A simple heat transfer controlled model to simulate the devolatilisation of tyre chips is proposed.

NUMERICAL SIMULATION ON PULVERIZED COAL COMBUSTION PROCESS IN A TANGENTIAL FIRING FURNACE

The computational fluid dynamics (CFD) code, PHOENICS 3.5, is employed to evaluate horizontal pulverized coal bias combustion process in a 200 MW tangential firing boiler. Multi-fluid two-phase model and coal comprehensive combustion model are integrated in the code. Gas temperature, particle distribution, gas species concentration and radiation heat flux at different sections of furnace are obtained from the numerical simulation at different secondary air (SA) vertical distribution. The calculation results show that a high particle concentration and high gas temperature zone exist, together with the separation of particle and gas downstream primary air (PA) nozzles, which ensure suitable ignition time, high temperature level of PA mixture and resistance to slag on the water-cooled walls. The calculated burnout of coal particles, O_2 concentration at furnace exit and gas averaged temperature in furnace are compared with in situ measurement data, resulting in good agreement. The simulation results of staged SA distribution case show that gas temperature of down part of furnace increase and the final burnout of char is decreased for the postponing mixing between fuel and oxygen. Simulation results are also compared with in situ measurement data, resulting in good agreement with each other. Thus the comprehensive coal combustion model has reasonably simulated PF combustion process in large boiler furnace, which is benefit for optimizing unit operation conditions and reducing pollutants emission.

A reduced NO x reaction model for pulverized coal combustion under fuel-rich conditions

A reduced NO x reaction model was developed for analysis of industrial pulverized coal firing boilers. The model was developed from experiments of laminar premixed combustion under a variety of stoichiometric ratios, burning temperatures, coal ranks (from sub-bituminous coal to anthracite) and particle diameters. Calculations agreed with experimental results for NO x and nitrogen species (NH 3 and HCN), if the model assumed that the hydrocarbon radicals were formed not only from pyrolysis of volatile matter, but also from char oxidation and gasification. The presence of hydrogen in char at the final burnout stage supported this assumption. NO x reduction by hydrocarbon radicals was the most important reaction in high temperature (>1500 K), fuel-rich, char combustion regions. NO x reduction from nitrogen species was sensitive to peak NO x concentration in volatile combustion regions, but NO x emission downstream had little influence from the peak NO x concentration. The heterogeneous reaction between char and NO x was important for fuel-lean or low-temperature conditions.

99/03846 Dry bottom ash removal. Ash cooling vs. boiler efficiency effects: Carrea, E. et al. FACT (Am. Soc. Mech. Eng.), 1998, 22, (1), 501–505

This paper describes a combined experimental and theoretical study which was undertaken to quantify the impact of fuel and process parameters on reburning effectiveness and provide the scaling information required for commerical application of reburning under highly varied industrial conditions. Initially parametric screening studies were conducted in a 25 KW refractory-lined tunnel furnace. These studies were supported by large scale testing in a 3.0 MW pilot scale facility. The work at both scales focused on the importance and the fate of the reactive nitrogen species within the reburning zone.The results of this study confirm the potential of the reburning process for significant NOx reductions, but they also demonstrate that a constant reburning effectiveness cannot be assumed under all conditions. The NOx reduction possible through reburning depends primarily on the NO concentration at the end of the primary zone; the stoichiometry, temperature, and residence time in the fuel-rich reburning zone; the mixing and stoichiometry of the reburning fuel jet; and the temperature in the final burnout zone. At the optimum reburning stoichiometry (SR2=0.9) the exhaust emissions correlate linearly with the sum of the primary NOx and the equivalent reburning fuel nitrogen. Optimum effectiveness requires adequate primary zone residence time to insure complete combustion of the primary fuel. Reburning zone residence times of at least 400 ms are desirable and high temperatures favor molecular nitrogen formation. Rapid mixing of the reburning fuel enhances the effectiveness of the NOx destruction process but it can potentially detract from the overall process efficiency with coal reburning due to increased conversion of reburning fuel nitrogen unless the fuel is transported with an essentially inert gas stream. Extremely low exhaust emission levels can be achieved with coal reburning if the final burnout zone can be operated at a temperature low enough to promote in-situ thermal de-NOx.

Near-extinction and final burnout in coal combustion

The late stages of char combustion have a special technological significance, as carbon conversionsof 99% or greater are typically required for the economic operation of pulverized coal-fired boilers. In the present article, two independent optical techniques are used to investigate near-extinction and final burnout phenomena for Illinois #6 and Pittsburgh #8 bituminous coals. Captive-particle image sequences, combined with in situ optical measurements on entrained particles, provide dramatic illustration of the asymptotic nature of the char burnout process. Single-particle combustion to complete burnout is seen to comprise two distinct stages: (1) a rapid high-temperature combustion stage, consuming about 70% of the char carbon and ending with near-extinction of the heterogeneous reactions due to a loss of global particle reactivity, and (2) a final burnout stage occurring slowly and at lower temperatures. For particles containing mineral matter, the second stage can be further subdivided into (2a) late char combustion, which begins after the near-extinction event and converts carbon-rich particles to mixed particle types at a lower temperature and a slower rate, and (2b) decarburization of ash—the removal of residual carbon inclusions from inorganic (ash) frameworks in the very late stages of combustion. This latter process can be extremely slow, requiring over an order of magnitude more time than the primary rapid combustion stage. For particles with very little ash, the loss of global reactivity leading to early near-extinction is believed to be related to changes in the carbonaceous char matrix, which evolves over the course of combustion as a result of simultaneous oxidation and heat treatment. More realistic models are needed to predict the asymptotic nature of char combustion and to make accurate predictions in the range of industrial interest.

A Multi Fuelled Cyclone Combustor

The paper outlines a series of tests carried out on a prototype 1.5 MW vertical cyclone combustor with integral ash removal which removes in excess of 80% of the ash. For high calorific value fuels such as coal the system is run fuel rich to avoid slagging. The low calorific value exhaust gases are passed via a transfer duct into an inverted swirl burner/furnace arrangement where final burnout of the gasified products occur. The sytem, designed to utilise a wide range of solid fuels was evaluated for a range of biomass materials and coal. The coal work investigated the effects of crushed (dm ≃ 250 μm) and pulverised (dm ≃ 70–80 μm) bituminous coals on system performance whilst biomass trials investigated the effects of chopped straw, chicken litter, shredded paper and refuse derived fuels. The fuel and air were premixed and fired tangentially in all cases into the primary combustor. The combustor was operated over a range of mixture ratios (φ) from fuel rich (φ = 0.5) to fuel lean (φ = 2.0) with temperatures in the order of 1250°C, hence operated in a non-slagging mode. The whole system was operated with a minimum of secondary air, required only to burn the gasified products from the primary chamber. The trials included monitoring of exhaust gases for a range of emissions. In addition, isokinetic sampling of the exhaust gases was carried out to determine particulate emission levels. Results show that best fuel burnout is achieved with biomass material levels better than 99% being achieved. Satisfactory performance was achieved with coal, (ash retention emissions) with fuel burnout in the order of 80%. Ash retention values for the biomass materials was in excess of 80% up to 98%. Coal ash retention levels were lower when analysed on a mass balance basis but of the same order when considering particulate emissions.

Coal burnout in the IFRF No. 1 furnace

A coal combustion model is used to match burnout measurements for four coals of different rank in a one-dimensional furnace and is then combined with a flow and heat transfer model to predict burnout in the IFRF No. 1 furnace. The coals show a continuous decrease in high temperature volatility and char reactivity with rank. Predicted and measured flame temperatures are shown to depend on the high temperature volatility and the char reactivity, varying by 300°C for the coals. For the residence time available in the flame, a proportion of fine coal is burned; the low reactivity of the high rank coals is partially compensated by a grinding characteristic giving more fine coals. Final burnout levels are shown to depend principally on the char reactivity as well as the furnace cooling, as this determines furnace temperatures. The sensitivity to volatility, grind, the flow model, and uncertainties in the combustion model are quantified. It is shown that the furnace cooling must be reduced to obtain acceptable levels of burnout for the two coals of lowest volatility.

Turbulent flame propagation and combustion in spark ignition engines

Pressure measurements synchronized with high-speed motion picture records of flame propagation have been made in a transparent piston engine. The data show that the initial expansion speed of the flame front is close to that of a laminar flame. As the flame expands, its speed rapidly accelerates to a quasi-steady value comparable with that of the turbulent velocity fluctuations in the unburned gas. During the quasi-steady propagation phase, a significant fraction of the gas behind the visible front is unburned. Final burnout of the charge may be approximated by an exponential decay in time. The data have been analyzed in a model independent way to obtain a set of empirical equations for calculating mass burning rates in spark ignition engines. The burning equations contain three parameters: the laminar burning speed s l , a characteristic speed u T, and a characteristic length l T. The laminar burning speed is known from laboratory measurements. Tentative correlations relating u T and l T to engine geometry and operating variables have been derived from the engine data.

Burnout impulse may be thrust misalignment in disguise.

Torque-free precession of spin stabilized spacecraft, showing thrust misalignment as cause for final burnout impulse

Characteristic Control Requirements for Nuclear Rocket Engines and Vehicles

Because efficiert staging of rocket vehicles, in general, requires a rapid thrust buildup, a rapid startup will be essential in nuclear rocket engines. However, the startup and staging methods selected must not compromise system reliability or safety. The minimum startup time depends for the most part on the maximum allowable fuel element tempetature rise rates. In addition, the relatively long interstage section and the thrust buildup characteristics presert unique, if not complex, problems for the analysis of the staging sequence. The two thrust vector cortrol systems, although reflecting some degradation in over- all performance, do not exhibit enough advantage of one over the other to select the optimum system for the early phase of a nuclear vehicle program. Further evaluation, for proper selection, is highly dependent on the engine development approach and study of the engine environments and operational requirements. The instantaneous and sharp cutoff of thrust, experienced in the chemical engine, is not feasible in the nuclear rocket engine since the fission product heat must be removed to prevent material temperatures from subsequertly reaching unacceptable levels if engine integrity is to be mairtained with all componerts in operating condition. The tail-off performance produced by after-coo.,ng must be factored intomore » the determination of the final burnout velocity, but it must certainly be predictable and repeatable, in both magnitude and time, in order to establish acceptable vehicle performance, stability, and control. (auth)« less