Showing papers by "Dongke Zhang published in 2021"

Propagation of glowing combustion front in a packed bed of activated carbon particles and the role of CO oxidation

Abstract: The propagation of a glowing combustion front in a packed bed of activated carbon (AC) particles was investigated with particular attention to the role of gas phase oxidation of carbon monoxide (CO). The AC particles were loosely packed in a cylindrical quartz column. A N2/O2 mixture of varying O2 concentration flowed through the bed from the bottom. Following ignition at the top of the bed, a glowing combustion front was formed and propagated downwards. Experiments were conducted at different oxygen mass fluxes with varying oxygen concentration and total flow rate. The bed mass loss rate, propagation velocity and temperature of the glowing front, were measured. A transient two-dimensional axisymmetric numerical model considering surface and gas phase reactions and transport properties were developed to describe the combustion phenomena. By comparing with the experiments, the validity of the proposed numerical model was confirmed. The results demonstrated that both the C O2 and C CO2 surface reactions contribute to carbon consumption and a constant mass loss rate is attained at a given oxygen flux. The gas phase oxidation of CO acted as a major heat source to sustain the propagation of combustion front through the bed. Suppressing CO oxidation would reduce the propagation velocity but may increase the combustion front temperature because of the prolonged residence time per unit of bed mass.

Experimental and kinetic modelling studies of flammability limits of partially dissociated NH3 and air mixtures

Abstract: This paper presents a set of experimental and kinetic modelling studies of the flammability limits of partially dissociated NH3 in air at 295 K and 1 atm. The experiments were carried out using a Hartmann bomb apparatus. The kinetic modelling was performed using Ansys Chemkin-Pro with opposed-flow premixed flame model employing three detailed reaction mechanisms, namely, the Mathieu and Petersen, Otomo et al., and Okafor et al. mechanisms. The degree of NH3 dissociation was varied from 0 to 25% (0 to 20%v/v H2 in the fuel mixture with a fixed H2/N2 ratio of 3). It was found that the lower (LFL) and upper (UFL) flammability limits of pure NH3 in air were 15.0%v/v and 30.0%v/v, respectively, consistent with the literature data. The flammability limits of the mixture widened significantly with increasing the degree of NH3 dissociation. At 25% NH3 dissociation, LFL decreased to 10.1%v/v and UFL increased to 36.6%v/v. All tested mechanisms were able to predict the extinction characteristics exhibited by the lean and rich mixtures of partially dissociated NH3 in air with non-unity Lewis numbers. While all three mechanisms predict well LFL, the Otomo et al. mechanism showed the best agreement with the experimental data of UFL. The rate of production of radicals, sensitivity, and reaction path analyses were performed to identify the key elementary reactions and radicals during combustion of partially dissociated NH3. The production of key radicals including OH, H, O, and NH2 was enhanced in the presence of H2 and thus the conversion of NH to NO and then NO to N2 near LFL and the conversion of NH2 and NO to N2 near UFL leading to wider flammability limits.

An experimental study of gasification kinetics during steam activation of a spent tyre pyrolysis char

Abstract: The pyrolysis char, produced in an industrial scale retort process, was subjected to steam activation under atmospheric pressure to assess carbon conversion as a factor of variations in temperature (750–1050 °C), concentration of activation agent (33.3–66.7 vol% steam/N2 mixture), and reaction time (0.5–4 h). The BET surface area was determined using N2 adsorption and compare against carbon conversion. The steam gasification reaction was shown to be kinetically controlled under the conditions studied. The behaviour of the solid-gas gasification reaction rate was modelled using non-catalytic heterogeneous models: Dutta model, random pore model, shrinking core model, and volumetric model. The experimental data were fit best by the volumetric model, indicating that carbon-steam gasification reactions throughout the particle were uniform and the gasification reaction was not influenced by the structure of the pores. The pre-exponential factor, activation energy and reaction order of the volumetric model were 5.42 × 10−3 s−1, 114 kJ mol−1 and 0.6, respectively, determined by non-linear least-squares fitting of the model to the experimental data.

Pore Development During CO2 and Steam Activation of a Spent Tyre Pyrolysis Char

Abstract: The activation of a spent tyre pyrolysis char using CO2 and steam was experimentally investigated, focusing on the pore development of the char during activation. The pyrolysis char, produced in an industrial scale retort process, was ground and sieved to a particle size fraction < 150 μm, and activated in a fixed bed reactor under CO2 and steam, respectively. The effect of temperature (750 to 1050 °C), reaction time (0.5 to 4 h for steam activation, 1 to 6 h for CO2 activation) and activation agent concentration (33.3 to 66.7 vol% of CO2 or steam in N2) on the carbon conversion and reaction rate was measured. The activated chars were characterised for the BET surface area, pore volume and average pore size of the activated chars using N2 adsorption and morphology using SEM. Higher temperature and activation agent concentration, and longer reaction time led to higher carbon conversion. As the carbon conversion increased, the BET surface area initially increased linearly and then decreased, reaching a maximum surface area of 666.6 m2 g−1 (0.60 conversion) for steam and 434.5 m2 g−1 (0.52 conversion) for CO2. Micropores were created in the early stage of activation, increasing first until carbon conversion reaching ca. 0.30. Steam-activated chars showed higher BET surface areas than CO2-activated chars at the same carbon conversion. Steam was found to generate both greater microporosity and mesoporosity than CO2.

The transformation and fate of sulphur during CO2 gasification of a spent tyre pyrolysis char

Abstract: The transformation and fate of sulphur (S) in a spent tyre pyrolysis char during CO2 gasification were studied by following the S species and contents using X-ray photoelectron spectroscopy (XPS). The spent tyre pyrolysis char (particle size fraction ≤150 µm), without and with 1 M HCl acid washing to remove inorganic S, were gasified in a fixed bed reactor. The effect of temperature (850, 950, 1050 °C), reaction time (1, 2, 3, 6 h) and CO2 concentration (33.3, 50.0, 66.7 vol% in N2) on the S species in the char samples were investigated. The main S species in the spent tyre pyrolysis char were ZnS and aliphatic sulphide. After CO2 gasification, aliphatic sulphide, thiophene, sulphoxide and sulphone became the dominant organic S while ZnS and CaSO4 were the main inorganic S. The percentage of total S increased with increasing gasification temperature, time and CO2 concentration. The content of organic S increased with increasing gasification temperature and time, while, the content of inorganic S decreased. Increasing CO2 concentration had negligible effect on the content of organic S but led to significant reduction in the content of inorganic S since ZnS reacted with CO2 to produce ZnO and SO2. Aliphatic sulphide, sulphoxide and sulphone were shown to have transformed to more stable thiophene. ZnS decomposed to release SX at > 900 °C while CaSO4 reacted with CO and carbon to produce COS. Both SX and COS reacted with the organic matrix in the char to form sulphoxide and sulphone.

Performance of activated carbons prepared from spent tyres in the adsorption of rhodamine B in aqueous solutions

Abstract: Activated carbons were produced from spent tyre pyrolysis char by steam or CO2 activation and evaluated for their performance in rhodamine B (RhB) adsorption in aqueous solutions. The effect of RhB starting concentration (80-150 mg L-1), contact time (0-80 min), temperature (298-318 K) and initial pH on the adsorption process was examined. Pseudo-first-order and pseudo-second-order models were carried out to fit the experimental data to derive RhB adsorption kinetics. Langmuir, Freundlich and Temkin isotherm models were applied to depict RhB adsorption behaviour of the prepared activated carbons. Gibbs free energy (ΔG), enthalpy (ΔH) and entropy (ΔS) were calculated. It has been found that the activated carbons can effectively adsorb RhB due to high mesoporosity and RhB equilibrium adsorption capacity (qe) increased almost linearly with increasing total mesopore volumes, regardless of the activation agents. When BET surface areas are similar, CO2-activated carbon obtained higher qe than steam due to higher mesoporosity of CO2-activated carbon. The results show that pseudo-second-order well fitted the experimental data. RhB starting concentration increased from 80 to 150 mg L-1 causing qe increased from 158 to 251 mg g-1 but RhB removal decreased from 99.7 to 84.5%. The RhB adsorption process follows the Langmuir model and thermodynamic calculation, indicating RhB adsorption is an endothermic, spontaneous process, dominated by both chemisorption and physisorption.

Preparation and characterization of carbon black (CB) using heavy residue fraction of spent tyre pyrolysis oil

Abstract: In the interest of utilisation of industrial waste products, this paper investigated the preparation and characterization of carbon black (CB) made via partial oxidation of a heavy residue fraction (HRF) of spent tyre pyrolysis oil in a drop tube furnace. The effect of process temperature, residence time (Rtime), and O2 concentration on the yield and quality (elemental composition, functional groups, morphology, thermal behavior, and surface area) of HRF-CB were determined using elemental analyzer (EA), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), thermogravimetric analyzer (TGA), and Brunauer-Emmett-Teller (BET) nitrogen adsorption/desorption technique. Higher temperatures led to higher yields (41–60%) and better quality of the resultant CB. Above 1150 °C, the CB samples were comparable with commercial CB products, with low ash (

Effect of oil shale semi-coke on deposit mineralogy and morphology in the flue path of a CFB burning Zhundong lignite

Abstract: The effect of oil shale semi-coke (SC) on the mineralogy and morphology of the ash deposited on probes situated in the flue path of a circulating fluidized bed (CFB) which burns Zhundong lignite (ZD) was investigated. 10 wt% or 20 wt% SC was added to ZD, which were then combusted in the CFB furnace at 950°C. Two probes with vertical and horizontal orientations were installed in the flue duct to simulate ash deposition. Both windward and leeward ash deposits on probes (P1W, P1L, P2W and P2L) were analyzed by using a scanning electron microscopy with energy dispersive X-ray (SEM-EDX), X-ray diffraction (XRD), an inductively coupled plasma optical emission spectrometry ICP-OES, and a particle size analyzer. When ZD was burned alone, the P1W deposit was comprised of agglomerates (< 30 µm) enriched in CaSO4 and Na2SiO3, incurring significant sintering. The P1L and P2W deposits, however, were of both discrete and agglomerated particles in similar mineral phases but with coarser sizes. The P2L deposit was mainly fine ash particles where Na2SiO3 and Na2SO4 were absent. As SC was added, the agglomerates in both P1W and P1L decreased. Moreover, SiO2 and Ca/Na aluminosilicates dominated the mineral phases whereas Na2SiO3 and Na2SO4 disappeared, showing a decrease in deposit stickiness. Likewise, the P2W deposit was found less spread on the probe, decreasing its deposition propensity. Na-bearing minerals turned into (Na, K)(Si3Al)O8 and (Ca, Na)(Si, Al)4O8 in the P2W deposit. Moreover, Na in the deposits decreased from 32 mg/g to less than 15 mg/g as SC presented. The addition of SC would therefore help alleviate the propensity of ash deposition in the flue path in the CFB combustion of ZD.

Counter-current flame propagation in a cylindrical bed of activated carbon saturated with artificial volatile matter in an upward flow of O2/N2 mixture

Abstract: A counter-current flame propagation process, or traverse combustion, in a cylindrical bed packed with activated carbon (AC) saturated with artificial volatiles was studied experimentally to appreciate the effect of homogenous volatile oxidation in the combustion process of packed solid. The experiments were conducted with an upward flow of a 50/50 v/v O2/N2 mixture. Flame propagation velocity and total carbon consumption during traverse combustion period, from ignition to the time when the flame propagated to the bed bottom, were determined. Ethanol, isooctane and benzene, were used respectively as the artificial volatile. Results show that compared to the case of AC alone, the flame propagation velocity was reduced while the combustion duration extended in the presence of volatile. The visible gas phase flame due to homogeneous volatile combustion traversed in the opposite direction of the oxidizing gas stream, competing with heterogeneous carbon oxidation for available oxygen. The proportion of oxygen consumed by homogeneous oxidation was the highest for ethanol, up to 87–94%, while isooctane and benzene accounted for 30–65% and 27–63%, respectively. Despite the competing homogeneous volatile combustion, the total amount of carbon consumed in the heterogeneous carbon oxidation was even increased in the cases of isooctane and benzene due to the extended combustion durations and relatively higher proportions of O2 consumed by heterogeneous carbon oxidation than in the ethanol case.

An experimental investigation into bed particle agglomeration and ash deposition during circulating fluidized bed gasification of Zhundong lignite

Abstract: Bed particle agglomeration and ash deposition in a 0.2 t/d circulating fluidized bed (CFB) gasifying high sodium Zhundong lignite (ZD) at different air equivalence ratios (ERs) were experimentally investigated. With quartz as bed material in the CFB at 900 °C, ZD within 3 mm in size was fed into the furnace and gasified at ERs 0.73–0.33. Two air-cooled probes were inserted in the furnace chamber (P1) and placed at the cyclone outlet (P2) respectively to simulate ash deposition. The bottom ash (BA), fly ash and ash deposited on probes were collected and analysed by using XRD, SEM-EDS, and an ash fusion analyser. At ER0.73 during gasification, agglomerates of bed particles were observed, since low melting-points sodium silicates sticking the bed particles together. Meanwhile, in the presence of NaFeSi2O6 and sodium silicate, char particles were found to have been coated by quartz and ash particles. Independent temperature-programmed ESEM imaging analysis showed these minerals became molten even at 800 °C, confirming their low melting-points. Agglomeration in BA became even severe as ER decreased, suggesting an aggravated agglomeration tendency. The fly ash mainly composed of calcium sulphate and Na-bearing minerals, whose morphological and mineralogical features were independent of ERs. In addition, ash deposited on P2 under all gasification conditions was found to be rich in Na2SO4. However, the contents of NaCl and unburned carbon increased significantly as ERs decreased. This decreased the sintering tendency of deposited ash but promoted corrosion of the P2 probe tip as observed.