Chemical and structural characterization of char development during lignocellulosic biomass pyrolysis

The chemical and structural changes of three lignocellulosic biomass samples during pyrolysis were investigated using both conventional and advanced characterization techniques. The use of ATR-FTIR as a characterization tool is extended by the proposal of a method to determine aromaticity, the calculation of both CH2/CH3 ratio and the degree of aromatic ring condensation ((R/C)u). With increasing temperature, the H/C and O/C ratios, XA and CH2/CH3 ratio decreased, while (R/C)u and aromaticity increased. The micropore network developed with increasing temperature, until the coalescence of pores at 1100 °C, which can be linked to increasing carbon densification, extent of aromatization and/or graphitization of the biomass chars. WAXRD-CFA measurements indicated the gradual formation of nearly parallel basic structural units with increasing carbonization temperature. The char development can be considered to occur in two steps: elimination of aliphatic compounds at low temperatures, and hydrogen abstraction and aromatic ring condensation at high temperatures

Lumped chemical kinetic modelling of raw and torrefied biomass under pressurized pyrolysis

International audienceAccurate prediction of the yield and composition of pyrolysis products is an important requirement for the design and operation of pyrolysis reactors and gasifiers. In this paper, a new semi-global kinetic reaction scheme is proposed to predict the composition of pyrolytic volatiles (classified as main chemical family groups), non-condensable gases and char derived from both raw and torrefied biomass for a wide range of operating conditions. The model is based on an adjustable mechanistic reaction scheme, which includes a combination of three different sub-mechanisms for the primary pyrolysis of three reference biopolymers (cellulose, hemicelluloses and lignin) and the secondary pyrolysis of their respective intermediates. The extent of primary/secondary reactions is varied according to the main process features (linear heating rate, temperature, pyrolysis time, volatile residence time and pressure). The secondary reactions in the scheme involve liquid-phase reactions of high molecular weight intermediates (producing non-condensable gases, water vapour and secondary char) as well as homogeneous and heterogeneous gas-phase conversion of primary volatiles. The model predictions were validated using experimental data obtained from fast pyrolysis in different micropyrolyzers (at 500–600 °C and heating rates of 27 and 110 °C/s, respectively) and slow pyrolysis in a laboratory-scale high-pressure fixed bed reactor (at 400–600 °C, 7 °C/min and 1, 15 and 30 bar). In general, the comparison of model outputs and experimental data were satisfactory, and the model predicted accurate trends in product distribution for changes of the lignocellulosic composition (the pre-removal of hemicelluloses in the case of torrefied biomass), heating rate and pressure. The model correctly predicted a significant increase in char yield (14.6 wt%) when torrefied instead of raw biomass was pyrolyzed due to the significance of char-forming reactions during pyrolysis of torrefied biomass. Moreover, the model’s reliability was proven through its accurate prediction of various condensate groups in bio-oil produced in a micropyrolyzer (maximum deviation < 4 wt%). Corresponding to the experimental data, the model predictions showed that the effect of pressure was most significant in the range of 1–15 bar (bio-oil yield decreased by 5.4 wt%), whereas no significant effect in bio-oil yield was evident for a pressure increase in the range of 15–30 bar. The CO2 and CO yields were slightly under-predicted by around 3 wt%, attributed to the catalytic effects of inherent inorganics on secondary cracking reactions which were not considered in the reaction scheme. Future work should focus on the validation of the model at temperatures below 400 °C

Improved reactivity of large coal particles by K2CO3 addition during steam gasification

In this study, the excess solution impregnation method was used to impregnate large coal particles (5 and 10 mm) with K2CO3, and the effect of the additive on steam gasification reactivity was investigated. A washed bituminous, medium rank-C Highveld coal, with an ash content of 12.6 wt.% (air-dried basis), was used for experimentation. The excess solution method was used to impregnate coal particles with the selected additive, K2CO3, and results from XRF analysis indicated that the potassium loading increased from 0.05 wt.% (raw coal) up to 0.83 wt.% (impregnated coal), on a coal basis. The potassium-impregnated large coal particles were used for low temperature (800–875 °C) steam gasification experiments. Results obtained for the reactivity of the parent coal were compared to that of the impregnated coal, which indicated that the addition of K2CO3 increased the reaction rate of large coal particles by up to 40%. It was also found that the addition of K2CO3 decreased the activation energy, from 191 kJ/mol (raw coal) to 179 kJ/mol (impregnated coal)

Transformation of nitrogen functional forms and the accompanying chemical-structural properties emanating from pyrolysis of bituminous coals

Characterisation of simultaneous changes in nitrogen functionalities and condensed aromatic crystallites during pyrolysis of bituminous coals was conducted. X-ray photoelectron spectroscopy (XPS) was utilised to determine nitrogen functional forms in three South African bituminous coals and the subsequent transformation in respective chars. Corresponding structural properties of coal and char were deduced through X-ray diffraction (XRD) analysis. Carbon structural properties in parent coals were also determined by solid state 13C nuclear magnetic resonance (ss NMR). The chars were prepared by pyrolysis at 740–980 °C in a bench-scale fluidised-bed (FB) and at 1000–1400 °C in a drop-tube furnace (DTF). The changes in XPS N 1s spectra of the coals through the respective chars were used to determine the nitrogen functionality transformations. Deconvolution of the XPS N 1s spectra revealed that pyrrolic nitrogen decreased with increasing pyrolysis temperature while quaternary nitrogen increased appreciably. Simultaneously, information deduced from XRD spectra showed that aromaticity () and average crystallite diameter () increased with severity of pyrolysis temperature in all the chars, while the fraction of amorphous carbon () and degree of disorder index (DOI) decreased significantly. Chars derived from the vitrinite-rich (also high in total reactive macerals) coal were more susceptible to thermal treatment with regard to nitrogen functional forms and other carbon crystallite transformations; high temperature chars only contained pyridinic and quaternary nitrogen, and exhibited a significant increase in crystallite height () and the average number of aromatic carbons (). Aromaticity of coals determined from ss 13C NMR and XRD corresponded. Comparison of structural changes brought by pyrolysis, as measured by XPS and XRD, showed that a good correlation existed between increasing quaternary nitrogen and . In a marked contrast, the diminishing of pyrrolic nitrogen displayed a good efficacy with DOI and . The reported concomitant transformations of nitrogen functional forms with char morphological changes are considered precursors to nitrogen release. This will inform future detailed studies on the conversion of coal nitrogen in solid fuel fired systems, such as in the applications of low-NOx burner technologies towards the release and reduction of nitrogen oxides in pulverised coal combustio

Hydrogen production from ammonia decomposition over a commercial Ru/Al2O3 catalyst in a microchannel reactor: experimental validation and CFD simulation

In this work, an integrated experimental and CFD modelling technique was used to evaluate a microchannel reactor producing hydrogen from ammonia decomposition using a commercial Ru/Al2O3 catalyst. The microchannel reactor performance was first assessed in a series of experiments varying the reaction temperature (723–873 K) and ammonia flow rates (100–500 Nml min−1) at atmospheric pressure. A global rate expression based on Temkin-Pyzhev kinetics that accurately predicts the entire experimental operating space was established using a model-based technique with parameter refinement and estimation. The kinetic model provided the reaction source term for subsequent CFD simulations aiming to obtain a more fundamental understanding of the reaction-coupled transport phenomena within the microchannel reactor. The transport processes and reactor performance were discussed in detail using velocity, temperature, and species concentration profiles. Finally, the influence of mass transport limitations within the various regions of the microchannel reactor was evaluated and discussed by means of dimensionless numbers vis-à-vis Damköhler and Fourier numbers. Overall, results presented in this paper provide valuable data for the efficient design of ammonia-fuelled microchannel reactors for hydrogen generation aimed at portable and distributed fuel cell application

Performance evaluation of a high-throughput microchannel reactor for ammonia decomposition over a commercial Ru-based catalyst

In this work, the prospect of producing hydrogen (H2) via ammonia (NH3) decomposition was evaluated in an experimental stand-alone microchannel reactor wash-coated with a commercial Ruthenium-based catalyst. The reactor performance was investigated under atmospheric pressure as a function of reaction temperature (723–873 K) and gas-hourly-space-velocity (65.2–326.1 Nl gcat−1 h−1). Ammonia conversion of 99.8% was demonstrated at 326.1 Nl gcat−1 h−1 and 873 K. The H2 produced at this operating condition was sufficient to yield an estimated fuel cell power output of 60 We and power density of 164 kWe L−1. Overall, the microchannel reactor considered here outperformed the Ni-based microstructured system used in our previous workDST Hydrogen Infrastructure Centre of Competence, and the North-West University (under the following Grant numbers: KP5-I05-Chemical Hydrogen Production Technologies; KP4-Hydrogen Fuelling Options; NRF grant 85309

Reactivity study of fine discard coal agglomerates

In this laboratory study thermo-gravimetric analyses were conducted to determine the influence of the addition of an alkaline metal catalyst (K2CO3 incorporated into a fine discard coal agglomerate mixture by physical mixing) on the CO2 gasification char reactivity of 10 mm coal pellets subjected at high temperatures (900–1000 °C). It was found that catalyst addition significantly increased the reaction rate of the carbon conversion. The reaction rate was doubled for the 5% catalyst addition runs compared to the base case (0% catalyst at 900 °C), 120% faster at 950 °C and, 93% faster at 1000 °C. The temperature influenced the reaction rate, thereby decreasing the reaction time from 6.4 h at 900 °C to 3.4 h at 950 °C and, 1.9 h at 1000 °C for the 5% catalyst addition. The fastest time needed for conversion was with the use of 5% catalyst addition at 1000 °C, which required only 13% of the total time needed in comparison with the base case (14 h). The conversion plots were further analyzed, and it was concluded that the lines followed the homogeneous model (REF) up till a conversion of 50% for the pelletized particles and 20% for the raw coal.The observed activation energy was also found to be lower in the coal pellet catalyzed system when compared to raw run of mine (R.O.M.) coal of the same dimensions as for the coal agglomerates (199 kJ/mol) and was calculated to be 195 kJ/mol, 184 kJ/mol, and 156 kJ/mol for the 1%, 3% and 5% catalyst addition runs, respectively. These results show that there is an opportunity to possibly improve on the fluidised-bed/large particle gasifier throughput by speeding up the time needed for the rate limiting CO2 gasification reaction in a catalysed system comprising of fine discard coal agglomerates containing an alkaline additiveSouth African Research Chairs Initiative (SARChI) of the Department of Science and Technology and National Research Foundation of South Africa (Coal Research Chair Grant No. 86880