Coarse Char Research Articles

How the physio-chemical properties of char from the pyrolysis of Automotive Shredder Residue (ASR) influences its future uses

Abstract Pyrolysis of ASR is an emerging technology which may increase circularity in End-of-Life Vehicle (ELV) waste recycling. To scale up from the current pilot scale, physiochemical characterisation of the by-products is required to determine their utilisation. This paper explored how the physiochemical properties of char from different pyrolysis fractions influence secondary uses. ASR was pyrolysed in a pilot-scale unit capable of processing 100 kg h−1 at 900 °C, producing 480 kg of char from which samples were taken for analysis. Three types of pyrolysis char were identified: (i) coarse char (CC) (ii) medium char (MC) and (iii) fine char (FC). Physical (particle size, moisture content and surface analysis) and chemical (calorific value, organic and inorganic elemental analysis) analysis was conducted. Physical and chemical differences were determined between char fractions: CC had the highest particle size (800 µm; mean 353.5 µm) calorific value (14,544 kcal g−1) and metal concentration; the relationship was CC > MC > FC. Organic elemental analysis indicated %C was highest under FC (80.29 %) and %S was highest in CC (1.04 %). Findings from this experiment provided initial insight into the differences in properties of char fractions from ASR pyrolysis. Potential future uses in char commercial markets were evaluated, with upgrading recommendations provided.

Predicting the Conversion Efficiencies of Any Coal Type in CFBCs

This study uses simulations with detailed chemistry to characterize the conversion of the various fuel components—noncondensable gas mixtures, soot, char fines, and coarse char—and to identify the factors that determine unburned carbon emissions (a.k.a. LOI) from CFBCs. It covers virtually the entire operating domain for commercial CFBCs with 20 test cases in 11 full-scale CFBCs for a range of coal quality from brown coals through anthracite. Soot burnout was complete for all cases except one. LOI was entirely determined by incomplete burnout of coarse char particles, except that an anthracite produced fines that burned slower than coarse char. Simulations for a Chinese CFBC accurately depicted how hotter furnace temperatures and load variations affect LOI. The intrinsic char oxidation reactivity determines whether or not char fines and coarse char can effectively compete with noncondensable fuel mixtures for the available O2 at lower elevations, and thereby influence NO production. More reactive chars co...

Ignitability and combustibility of Yallourn pyrolysis char under simulated blast furnace conditions

In this paper we have examined the potential of Yallourn brown coal char (collected from an industry-scale pyrolyser) to be used as a pulverized coal injection (PCI) fuel, its size-dependent properties, ignitability and combustibility under the simulated conditions of the blowpipe-tuyere section in a blast furnace. The combustion of individual sizes for Yallourn char was tested in a lab-scale drop-tube furnace (DTF) using pre-heated hot gas with a temperature up to 1000°C, and a particle residence time as short as 0.6s. Computational fluid dynamics (CFD) modelling was further conducted to optimize the char combustion conditions via sensitivity analysis. Irrespective of the pyrolysis condition, Yallourn char is superior over bituminous coal for being used as a top grade PCI fuel, due to its higher calorific value (7500–8110kcal/kg), lower ash content (<10wt%), high ash melting temperatures (>1550°C), and abundance of iron (>40wt% in ash). The performance of Yallourn char is also superior over bituminous coal under the simulated blast furnace conditions, for a rapid ignition and burnout even for a coarse char size of 300μm under the stoichiometric O2/C molar ratio and using low blast temperatures of 800–1000°C. All these are beneficial for reducing the energy consumption related to particle pulverization and the amount of oxygen for the combustion. With regard to the Yallourn char ignition and combustion in the hot gas, a minimum 6wt% volatile content is essential for a stable and rapid ignition of the volatiles at a gas temperature of 1000°C or below, since homogeneous ignition is predominant at low temperatures. However, once the blast temperature rises to 1200°C, the dependence on volatile content turns insignificant due to the dominance of the heterogeneous ignition, high C-O2 reactivity for the solid char, as well as the minimized pore diffusion control due to the large porosity (52.0–63.1%) of the Yallourn char tested here.

Effect of CO2 on oxy-fuel combustion of coal-char particles in a fluidized bed: Modeling and comparison with the conventional mode of combustion

A char combustion model is developed to study the effect of CO2 on the combustion of coarse char particles under oxy-fuel conditions in a fluidized bed (FB). It is a transient one-dimensional model, taking into account the heat and mass transfer from the bed to the particle and the heterogeneous combustion and gasification of char. The model shows good ability to predict the char temperature history measured in our previous work for different combinations of O2/CO2 and O2/N2 with various coal types. Simulations are carried out to establish the role of CO2 in oxy-fuel conversion at different O2 levels, particle sizes, and bed temperatures. The model is used to analyze the relative contribution of carbon in the char consumed by CO2 (gasification) and O2 (combustion), as well as the differences of the peak temperatures and the burnout times in O2/CO2 and O2/N2 for char particles in a commercial FB combustor. The results indicate that the conversion of coarse (mm size) char particles in an oxy-FB is controlled by the diffusion of O2 both in the O2/CO2 and O2/N2 case. The burn-out time decreases with the bed temperature also in both cases. The lower O2 diffusion rate in CO2 compared to N2, is the main reason for the longer burnout time and lower peak temperature found using O2/CO2 at bed temperatures of 1073–1173K. In that temperature window, the contribution of the CO2-char gasification is limited, being notable only at high bed temperature in O2/CO2, e.g. 1223K. In such high temperature conditions (rarely expected to be found in commercial coal FBC) the predicted burnout time of a lignite char-particle becomes shorter in O2/CO2 than in O2/N2.

Studies of Biochars Generated from Pilot-Scale Downdraft Gasification

<abstract><title><italic>Abstract.</italic></title> Biochars were generated with a pilot-scale downdraft gasifier using wood chips as the raw material. Biochars were collected from the bottom of the gasifier (coarse char) and from the filter (fine char). They were characterized by scanning electron microscopy (SEM), elemental analysis, thermogravimetric analysis (TGA), and higher heating value (HHV) and compared with the raw material and commercial activated carbon. Coarse char showed a larger average particle size than fine char. Coarse char also had higher carbon content (47.2%) than fine char (32.4%). Both biochars had much lower carbon content but higher mineral content than commercial activated carbon (Filtersorb 200; 88.1%). Both coarse char and fine char retained considerable HHV (14.5 and 10.0 MJ kg^-1 respectively), but their HHVs were lower than that of wood chips (17.7 MJ kg^-1) and much lower than that of Filtersorb 200 (30.7 MJ kg^-1). Fine char showed higher thermal stability than coarse char. Both biochars were more thermally stable than wood chips but less stable than activated carbon.

Characterisation of residual carbon from entrained-bed coal water slurry gasifiers

Gasification slag is a by-product of the coal gasification process. To achieve “zero emissions” from coal gasification technology, the environmentally safe utilization of slag by-products from gasifiers must be addressed and developed. In this study, work has been carried out to characterize residual carbon in slag together the morphological study on vitreous material present in slag. The compositions of particles in coal, char and slag were determined by using SEM/EDX. An elemental analyzer (Flash EA 1112) was adopted to analyze CHNS on bulk coal, char and slag. The size distribution of carbon in slag was obtained by analyzing slag sample blocks using an image analysis program developed in this study. It was found that the coarse and fine slags both have a relatively high content of unburnt carbon which hinders their utilization as additives in cement and concrete. There was no evidence that coarse slag is the final destination for coarse char particles since large unburnt chars were found in both the coarse slag and the fine slag. The morphology of the residual carbon in the coarse slag is similar to that found in the fine slag, and is also comparable to partially combusted char produced in a drop tube furnace (DTF) operating at 1300 °C, 600 ms and 2% oxygen. It is also found that the vitreous particles in fine slag tends to be below 150 μm and the majority of residual carbon (75.6 wt%) in fine slag tends to be greater than 150 μm, which implies that, a simple screening operation might make the slag usable in concrete industries.

Combustion and Attrition of Biomass Chars in a Fluidized Bed

The fluidized bed combustion of char from three different biomass fuels, pine seed shells, olive husk, and wood chips, was investigated in a bench scale combustor. A combination of experimental techniques was used to characterize the relevance of attrition phenomena during the combustion of the chars and their impact on the fuel particle size distribution and overall carbon conversion. Results showed that, depending upon the biomass, extensive primary and secondary fragmentation could be experienced by the char particles, significantly influencing the particle size distribution of the fuel in the bed. This is the result of the mechanical properties of the raw fuel particles and the large porosity of the char after devolatilization. Char conversion closely followed the shrinking-particle constant-density model and occurred to a large extent via the generation of carbon fines by percolative fragmentation followed by postcombustion during their residence time in the bed. Approximately 25−45% of the initial fixed carbon followed this pathway, with the remainder being directly burnt as coarse char. This resulted in an enhancement of the combustion rate of the char particles. The significance of the particle shape was also highlighted in the evaluation of the particle-exposed surface. Finally, indications were given on how to account for attrition phenomena in a simplified population balance on biomass char.

The influence of fine char particles burnout on bed agglomeration during the fluidized bed combustion of a biomass fuel

The combustion of biomass char in a bubbling fluidized bed is hereby addressed, with specific reference to the influence that the combustion of fine char particles may exert on ash deposition and bed agglomeration phenomena. Experiments of steady fluidized bed combustion (FBC) of powdered biomass were carried out with the aim of mimicking the postcombustion of attrited char fines generated in the fluidized bed combustion of coarse char. Experimental results showed that the char elutriation rate is much smaller than expected on the basis of the average size of the biomass powder and of the carbon loading in the combustor. Samples of bed material collected after prolonged operation of the combustor were characterized by scanning electron microscopy (SEM)–EDX analysis and revealed the formation of relatively coarse sand–ash–carbon aggregates. The phenomenology is consistent with the establishment of a char phase attached to the bed material as a consequence of adhesion of char fines onto the sand particles. Combustion under sound-assisted fluidization conditions was also tested. As expected, enhancement of fines adhesion on bed material and further reduction of the elutriation rate were observed. Experimental results are interpreted in the light of a simple model which accounts for elutriation of free fines, adhesion of free fines onto bed material and detachment of attached fines by attrition of char–sand aggregates. Combustion of both free and attached char fines is considered. The parameters of the model are assessed on the basis of the measured carbon loadings and elutriation rates. Model computations are directed to estimate the effective size and the peak temperature of char–sand aggregates. The theoretical estimates of the effective aggregate size match fairly well those observed in the experiments.

Fluidized Bed Combustion of a Biomass Char (Robinia pseudoacacia)

Fluidized bed combustion of char from a biomass, Robinia pseudoacacia, was investigated in a bench scale combustor. Different experimental techniques have been adopted to characterize the combined role of combustion and comminution phenomena (primary, secondary, and percolative fragmentations, attrition by abrasion) in determining fixed carbon conversion and the rate of carbon elutriation. Comparison of experimental results obtained under steadily oxidizing conditions and under alternating oxidizing/inert conditions suggested mechanistic aspects of the fluidized bed combustion of biomass char. Fixed carbon combustion was almost always complete. Conversion occurred to a large extent via the generation of carbon fines followed by postcombustion during their residence time in the bed. Approximately half of the initial fixed carbon followed this pathway, the remainder being directly burnt as coarse char. The prevailing mechanism of carbon fines generation in the bed was percolative fragmentation rather than attrition by abrasion. In spite of the extensive generation of elutriable carbon fines, the combined effect of high fuel reactivity and of relatively long fines residence times in the reactor determined the large combustion efficiency. It is inferred from experimental results that char fines adhesion onto bed solids might be relevant to the observed phenomenology.

The relevance of attrition to the fate of ashes during fluidized-bed combustion of a biomass

The fate of ashes during the fluidized-bed combustion of a biomass fuel (Robinia pseudoacacia) has been investigated both experimentally and theoretically. Fluidized-bed combustion experiments with Robinia have been carried out with a bed of pure quartz at temperatures ranging from 700°C to 850°C at different oxygen concentrations in order to investigate the tendency of the biomass ashes to deposit on the bed particles and to give rise to bed agglomeration. SEM/EDX analysis of the silica bed particles after the tests was carried out to determine the formation of alkali-rich layers on inert bed particles and possibly of silicate melts. Results indicated that large quantities of biomass ashes are retained on the bed particles under all operating conditions, but only at the higher temperatures could molten surface layers and agglomerated bed particles be noticed. Experimental results have been interpreted on the basis of a single-particle combustion model applied to both fine and coarse char particles' burnout in a fluidized bed. Calculations show that extremely high temperatures are rapidly reached by fine particles even at very low oxygen concentrations. These temperatures are well beyond typical potassium silicate melting temperatures. On the other hand, coarse particles burn at temperatures only slightly above bed temperature. Experimental and theoretical results indicate that the following mechanism is relevant to the fate of ashes during fluidized-bed combustion of biomass: ash is mostly detached from the coarse char as attrited fines whose temperature is raised significantly by carbon afterburning. Fines can further adhere onto inert bed particles, with formation of alkali-rich surface layers. If the bed temperature is higher than the alkalisilicate eutectic, a melt forms that enhances bed particle stickiness and may ultimately lead to bed agglomeration.

Fluidized-bed combustion of a biomass char: The influence of carbon attrition and fines postocombustion on fixed carbon conversion

The fluidized-bed combustion of char from a biomass fuel (Robinia Pseudoacacia) has been studied with a focus on the fate of fixed carbon under the combined effects of coarse char combustion and attrition, of attrited carbon fines postcombustion and elutriation. Extensive postcombustion under oxidizing conditions prevents the measurement of the rates of carbon fines generation by attrition from collection of elutriated carbon. A novel technique is hereby presented to overcome this problem: the rates of coarse char attrition are evaluated from the comparison of the apparent carbon conversion rate of the actual test fuel , with that of a reference fuel , selected among those which have a negligible propensity to attrition. Results from the application of the proposed technique indicate that extensive fines generation by attrition, followed by their almost complete postcombustion, occurs during fluidized-bed combustion of Robinia char. About half of the fixed carbon burns along this pathway, the remainder being directly burnt as coarse char carbon. The occurrence of extensive attrition-postcombustion leads to enhancement of the coarse char particles' apparent Sherwood numbers by a factor of about two. Analysis of the dependence of the ratio between the fines generation rate and the parallel coarse char combustion rate on operating variables of the reactor is directed to shed light on the prevailing mechanism of carbon fines generation by attrition. It appears that, differently from low-volatile solid fuels, the percolative fragmentation mechanism plays a leading role on carbon attrition.

The fate of fixed carbon during the fluidized-bed combustion of a coal and two waste-derived fuels

The fate of fixed carbon during fluidized-bed combustion of a bituminous coal and two alternative fuels. a refuse-derived fuel (RDF) and a tyre-derived fuel (TDF), was investigated. A simple model was developed based on the assumption that fixed carbon present in the bed could be lumped into a coarse particles phase and a fine carbon phase. The model is based on a network of paths representing the generation of coarse and fine char particles from the parent fuel by primary fragmentation, the fine particle production by comminution of coarse char, the combustion of the coarse and of the fine char particles, as well as the elutriation of fines. Results of computations of carbon conversion were in good agreement with those measured in batchwise experiments with each of the three fuels. Conversion of the coal takes place mainly via fuel devolatilization to coarse char which further reacts to gaseous products. Conversion of TDF occurs via the generation, upon devolatilization, of amultitude of fines which eventually undergo combustion and elutriation. The phenomenology associated with RDF fluidized-bed combustion is intermediate between those of coal and TDF. The resulting framework for the evaluation of fixed carbon balance helps in identifying key factors in the conversion of such widely different fuels.

Carbon content of fly ash and size distribution of unburnt char particles in fly ash

The type and size of unburnt char particles in 13 fly ash samples were determined microscopically. The samples were collected from a power station fired from an inertinite-rich coal seam. The carbon content of the ash samples ranged from 2.3 to 25.3 wt%. When the carbon content in a fly ash was high, the proportion of coarse char particles, in particular > 10 6 μm 3 particles, was also high. These results suggest that the size distribution of the feedstock coal has a major effect on the carbon content of the fly ash.