John F. Stubington

Release of volatiles from large coal particles in a hot fluidized bed

Abstract Coal particles with diameters of 3–11 mm were injected into a small, hot bed of sand fluidized by nitrogen. Volatiles evolution was followed by sampling the exit gas stream and subsequent analysis by gas chromatography. Three Australian coals covering a range of volatile matter were studied and the effects of coal particle size and bed temperature were determined. The yields of gaseous components, char and tar are explained by consideration of the competitive reactions for coal hydrogen and oxygen and secondary reactions of the volatile species within the coal particle. The pore structure developed during devolatilization has a significant effect on the extent of these secondary reactions. It is concluded that heat transfer is the main process controlling the volatilization time in fluidized bed combustors. The time required for heat transfer into the coal particle, determined by calculation and experiment, agrees with the measured volatilization time. Significant factors are external heat transfer to the surface of the particle, internal conduction through the coal substance and radiation through the pores, and the counterflow of volatiles out of the coal particle. For different coals, variations in the volatilization time appear to be caused by the development of different pore structures, which affect radiant heat transfer through the pores.

The effects of fragmentation on devolatilization of large coal particles

Spherical coal particles (5 to 12 mm diameter) from three Australian bituminous coals were devolatilized in an inert atmosphere of nitrogen at 850 °C, using a specially designed thermogravimetric rig based on an electrically heated stainless steel tubular reactor. Weight loss histories of the coal particles were recorded and the simultaneous concentrations of gases evolved were determined chromatographically. It was concluded that the particle size had no effect on the ultimate yields of gas, tar plus water and char. Tar was the major volatile component and evolved before the gaseous volatiles. Therefore, it is recommended that the devolatilization time should be measured as the time for 95% of the ultimate weight loss, rather than the time for 95% of the ultimate gaseous volatiles yield. Fragmentation of the coal particles, which varied with different coals depending on the coal plasticity, occurred early in the devolatilization process and increased the rate of devolatilization and decreased the devolatilization time. The increase in devolatilization time correlated with particle diameter by an empirical equation tv = Adpn for each coal. Comparison of the data with previous work using a fluidized bed reactor showed that the devolatilization time was shorter in the fluidized bed because of a greater degree of fragmentation.

Devolatilization times of mm-size coal particles

The devolatilization time of mm-size coal particles affects the in-bed combustion efficiency of volatiles in fluidized bed combustors and has been studied by numerous workers. Conflicting effects of the various parameters on devolatilization time have been reported, when different rigs, measurement methods and conditions were used. Therefore, this work aimed to study the effects of experimental parameters using the same rig, procedure and measurement method to eliminate these differences. Four coals, ranging from lignite to bituminous, from the Penn State coal sample bank were studied in a thermogravimetric analysis (TGA) apparatus. Higher heating rates, closer to those in a fluidized bed combustor, were obtained by inserting the coal particles directly into the already hot furnace, rather than using the relatively slow heating rate programmed into the apparatus. The devolatilization time was measured from the weight loss curve for individual coal particles in the 1–5 mm size range. The effect of particle size was correlated by equations of the form, tv = Adp1.5, consistent with a heat transfer-controlled regime. No effects of coal type or gas flow rate on the devolatilization time were found. The effects of atmosphere (inert or air) and devolatilization time definition (90 or 95% weight loss) were expressed as ratios of the values of the correlation parameter, A, and the effect of temperature was presented as an Arrhenius-type plot for A. The decreases in devolatilization time with increasing temperature or oxygen concentration again reflect the controlling nature of the heat transfer process. No fragmentation of the coal was observed in this work, but the effect of primary fragmentation was deduced by comparison with literature data. It is suggested that an effect of coal type on devolatilization time is only observed when different degrees of primary fragmentation occur for different coals, and this may explain the differing observations for coal type effect in the literature.

Significant factors affecting devolatilization of fragmenting, non-swelling coals in fluidized bed combustion

Abstract A theoretical study has been made to identify those variables having a statistically significant effect on the devolatilization of fragmenting coals. To achieve this objective, a simple mathematical model has been developed for prediction of the devolatilization time of fragmenting coals and the number of fragmentations during devolatilization in conditions relevant to fluidized bed combustion. It incorporates important phenomena occurring during coal devolatilization, namely heat transfer from the bed to the surface of the coal particle, heat transfer inside the devolatilizing coal particle, primary decomposition reactions, transport of released volatiles from the coal matrix to the surface of the coal particle, and devolatilization-induced fragmentation. The model has been used for a factorial evaluation to determine significant variables affecting the devolatilization of fragmenting coals. Eight factors and two responses were selected, the responses were devolatilization time and number of fragmentations. Results of the factorial evaluation show that for both responses, the devolatilization of 1–20 mm diameter coal particles at 1023–1223 K is predominantly governed by coal particle diameter. The devolatilization time is also significantly affected by coal thermal properties, and less significantly by bed temperature and reaction rate of devolatilization. In addition to coal particle diameter, the number of fragmentations is significantly affected by variables related to volatiles transport to the coal particle surface, i.e. the diameter of coal convective pores and the viscosity of volatiles.

On the heating rate and volatile yield for coal particles injected into fluidised bed combustors

Abstract A numerical model for devolatilisation of a single coal particle was used to estimate the heating rates of coal particle sizes fed to atmospheric-pressure industrial fluidised bed combustors. This model incorporates an appropriate level of detail for each of the processes occurring during the devolatilisation: external heat transfer, internal heat transfer, devolatilisation, and transfer of volatiles. The volatiles flux out of the coal particle decreases the heating rates significantly for the fluidised bed combustion of coal particle sizes of medium- to high-volatile coals. The model was shown to predict accurately both the detailed centre temperature-time profile and the overall devolatilisation times as a function of particle size. The heating rates for coal particles fed to industrial fluidised bed combustors, calculated from this model, are of the order of 2–150 K s−1 for 2 to 20 mm diameter particles, and should not be confused with the much higher heating rates of pulverised fuel size particles. Since the heating rates are similar, it is not surprising that the volatile yields in a fluidised bed combustor are approximately the same as (or marginally greater than) the Proximate Volatile Matter determination. Further research is necessary to measure these volatile yields more accurately in a fluidised bed environment.

Comparison of experimental methods for determining coal particle devolatilization times under fluidized bed combustor conditions

Numerous studies have measured the devolatilization times of coal particles ‘under fluidized bed conditions’, either in a hot fluidized bed or by holding the particle(s) stationary in a hot reactor. Here, two fluidized bed methods and one stationary particle method were compared, using the same three coal samples. Devolatilization times and fragmentation were studied. A new devolatilization endpoint definition from CO2 evolution profiles gave good agreement with flame extinction time measurements, so either of these methods may be used in fluidized bed experiments to measure devolatilization time. The different environment around the coal particle in thermogravimetric experiments gave substantially longer devolatilization times, so the results from stationary particle experiments are not relevant to fluidized bed combustion. Coal particle fragmentation behaviour differed for the three coals studied, but no effect of coal properties on the devolatilization time in fluidized beds was observed, and an earlier correlation fitted these results well.

The effects of design factors on emissions from natural gas cooktop burners

The effects of the following burner design factors on the emissions of NO2, NOX, CO, and hydrocarbon were investigated: cap material, cap size, port shape, port size, port spacing, central secondary aeration, and flame insert. The approach used in this study was designed to overcome the possibility of ambiguous or contradictory conclusions encountered in earlier studies. The factorial experimental design method was used to arrange the experiments, and the results were statistically analysed using the analysis of variance. Flame stability was found to be crucial; a slight instability promoted a large increase in emissions of CO, hydrocarbon, and NO2. Port shape was found to be the most significant burner design factor affecting the emissions, and hence it is a potential factor to be studied further for lower emissions.

Unburnt carbon elutriation from pressurised fluidised bed combustion of Australian black coals

Abstract A batch-fed bench-scale pressurised fluidised bed combustion (PFBC) facility was built to research the in-bed processes generating fine char particles during PFBC of Australian black coals. Elutriation of such particles is responsible for the combustion inefficiency and their combustion in the filter cake on high-temperature ceramic filters may contribute to the formation of ‘sticky ash’ and consequent cleaning difficulties. Measured carbon elutriation varied from coal to coal and the results agreed qualitatively with large-scale PFBC performance, allowing definition of satisfactory and unsatisfactory performance criteria. Carbon elutriation in PFBC was correlated tentatively with crucible swelling number although the range of values of the crucible swelling number needs to be extended. Carbon elutriation in PFBC did not correlate with volatile matter for the five Australian coals investigated in contrast with previous PFBC pilot plant studies on four Northern Hemisphere coals. The higher swelling coal produced a significantly larger average macro-pore diameter after devolatilisation in PFBC, which caused greater attrition of fine char from the particle surface during the char burnout stage.

Premixed Methane-Air Flame Spectra Measurements Using UV Raman Scattering

Abstract The underfed combustion of dried bagasse in the forms of loose fibre, dense fibre, small pellets and large pellets has been investigated. The large pellets ignited rapidly on the surface and each pellet burnt slowly as an individual fuel lump, so that a bed depth greater than ∼700 mm would be necessary to reduce primary air to the stoichiometric rate. The other three forms exhibited the characteristic ignition and burn-out stages at low air rates. Their maximum combustion rates were ignition limited and decreased as the bulk density increased. Since the corresponding primary air flow rates were less than or equal to stoichiometric, their combustion at the maximum rate would be practicable. The ash from the small pellets fused into clinker, so they should only be fired, in existing bagasse boilers, as a supplement to wet bagasse at temperatures below the ash fusion temperature.

Gas flow regimes in fluidized-bed combustors

The effects of pressure and temperature on the gas flow regimes in the dense beds of fluidized-bed combustors were reviewed and investigated. Diagrams were developed to identify the gas flow regimes in both bubble and particulate phases, since these significantly affect the combustion processes occurring within the bed. In commercial pressurized fluidized-bed combustion, bubbles were effectively cloudless and no gas backmixing or slugging occurred: so the gas flow in these beds could be modeled by assuming two phases (bubble and particulate) with plug flow through each phase. For industrial-scale atmospheric fluidized-bed combustion (AFBC), different gas flow regimes occurred in the bubbling bed and circulating bed technologies. The bubbles in bubbling AFBC were cloudless with no gas backmixing or slugging, so the gas flow in these beds could also be modeled by assuming two phases (bubble and particulate) with plug flow through each phase. However, the dense bed formed at the base of the riser in industrial-scale circulating fluidized-bed combustion operating at atmospheric pressure contained clouded bubbles and experienced significant gas backmixing, so the gas flow should be modeled by assuming three phases (bubble, cloud, and emulsion) with perfect mixing in each phase, providing the primary air fluidizing velocity is below the exploding bubble regime.