Predicting mixed-gas adsorption equilibria on activated carbon for precombustion CO2 capture.

We present experimentally measured adsorption isotherms of CO2, H2, and N2 on a phenol-formaldehyde resin-based activated carbon, which had been previously synthesized for the separation of CO2 in a precombustion capture process. The single component adsorption isotherms were measured in a magnetic suspension balance at three different temperatures (298, 318, and 338 K) and over a large range of pressures (from 0 to 3000-4000 kPa). These values cover the temperature and pressure conditions likely to be found in a precombustion capture scenario, where CO2 needs to be separated from a CO2/H2/N2 gas stream at high pressure (~1000-1500 kPa) and with a high CO2 concentration (~20-40 vol %). Data on the pure component isotherms were correlated using the Langmuir, Sips, and dual-site Langmuir (DSL) models, i.e., a two-, three-, and four-parameter model, respectively. By using the pure component isotherm fitting parameters, adsorption equilibrium was then predicted for multicomponent gas mixtures by the extended models. The DSL model was formulated considering the energetic site-matching concept, recently addressed in the literature. Experimental gas-mixture adsorption equilibrium data were calculated from breakthrough experiments conducted in a lab-scale fixed-bed reactor and compared with the predictions from the models. Breakthrough experiments were carried out at a temperature of 318 K and five different pressures (300, 500, 1000, 1500, and 2000 kPa) where two different CO2/H2/N2 gas mixtures were used as the feed gas in the adsorption step. The DSL model was found to be the one that most accurately predicted the CO2 adsorption equilibrium in the multicomponent mixture. The results presented in this work highlight the importance of performing experimental measurements of mixture adsorption equilibria, as they are of utmost importance to discriminate between models and to correctly select the one that most closely reflects the actual process.

Hydrogen production from food wastes and gas post-treatment by CO2 adsorption.

The production of H(2) by biological means, although still far from being a commercially viable proposition, offers great promise for the future. Purification of the biogas obtained may lead to the production of highly concentrated H(2) streams appropriate for industrial application. This research work evaluates the dark fermentation of food wastes and assesses the possibility of adsorbing CO(2) from the gas stream by means of a low cost biomass-based adsorbent. The reactor used was a completely stirred tank reactor run at different hydraulic retention times (HRTs) while the concentration of solids of the feeding stream was kept constant. The results obtained demonstrate that the H(2) yields from the fermentation of food wastes were affected by modifications in the hydraulic retention time (HRT) due to incomplete hydrolysis. The decrease in the duration of fermentation had a negative effect on the conversion of the substrate into soluble products. This resulted in a lower amount of soluble substrate being available for metabolisation by H(2) producing microflora leading to a reduction in specific H(2) production. Adsorption of CO(2) from a gas stream generated from the dark fermentation process was successfully carried out. The data obtained demonstrate that the column filled with biomass-derived activated carbon resulted in a high degree of hydrogen purification. Co-adsorption of H(2)S onto the activated carbon also took place, there being no evidence of H(2)S present in the bio-H(2) exiting the column. Nevertheless, the concentration of H(2)S was very low, and this co-adsorption did not affect the CO(2) capture capacity of the activated carbon.

Mechanical durability and combustion characteristics of pellets from biomass blends.

Biofuel pellets were prepared from biomass (pine, chestnut and eucalyptus sawdust, cellulose residue, coffee husks and grape waste) and from blends of biomass with two coals (bituminous and semianthracite). Their mechanical properties and combustion behaviour were studied by means of an abrasion index and thermogravimetric analysis (TGA), respectively, in order to select the best raw materials available in the area of study for pellet production. Chestnut and pine sawdust pellets exhibited the highest durability, whereas grape waste and coffee husks pellets were the least durable. Blends of pine sawdust with 10-30% chestnut sawdust were the best for pellet production. Blends of cellulose residue and coals (<20%) with chestnut and pine sawdusts did not decrease pellet durability. The biomass/biomass blends presented combustion profiles similar to those of the individual raw materials. The addition of coal to the biomass in low amounts did not affect the thermal characteristics of the blends.

Thermal behaviour and kinetics of coal/biomass blends during co-combustion.

The thermal characteristics and kinetics of coal, biomass (pine sawdust) and their blends were evaluated under combustion conditions using a non-isothermal thermogravimetric method (TGA). Biomass was blended with coal in the range of 5-80 wt.% to evaluate their co-combustion behaviour. No significant interactions were detected between the coal and biomass, since no deviations from their expected behaviour were observed in these experiments. Biomass combustion takes place in two steps: between 200 and 360 degrees C the volatiles are released and burned, and at 360-490 degrees C char combustion takes place. In contrast, coal is characterized by only one combustion stage at 315-615 degrees C. The coal/biomass blends presented three combustion steps, corresponding to the sum of the biomass and coal individual stages. Several solid-state mechanisms were tested by the Coats-Redfern method in order to find out the mechanisms responsible for the oxidation of the samples. The kinetic parameters were determined assuming single separate reactions for each stage of thermal conversion. The combustion process of coal consists of one reaction, whereas, in the case of the biomass and coal/biomass blends, this process consists of two or three independent reactions, respectively. The results showed that the chemical first order reaction is the most effective mechanism for the first step of biomass oxidation and for coal combustion. However, diffusion mechanisms were found to be responsible for the second step of biomass combustion.

Co-gasification of different rank coals with biomass and petroleum coke in a high-pressure reactor for H(2)-rich gas production.

Four coals of different rank were gasified, using a steam/oxygen mixture as gasifying agent, at atmospheric and elevated pressure in a fixed bed reactor fitted with a solids feeding system in continuous mode. Independently of coal rank, an increase in gasification pressure led to a decrease in H(2) + CO production and carbon conversion. Gasification of the different rank coals revealed that the higher the carbon content and reactivity, the greater the hydrogen production. Co-gasification experiments of binary (coal-biomass) and ternary blends (coal-petcoke-biomass) were conducted at high pressure to study possible synergetic effects. Interactions between the blend components were found to modify the gas production. An improvement in hydrogen production and cold gas efficiency was achieved when the coal was gasified with biomass.

Kinetics of naphthalene adsorption on an activated carbon: comparison between aqueous and organic media.

The purpose of this work was to explore the kinetics of naphthalene adsorption on an activated carbon from aqueous and organic solutions. Kinetic curves were fitted to different theoretical models, and the results have been discussed in terms of the nature and properties of the solvents, the affinity of naphthalene to the solutions, and the accessibility to the porosity of the activated carbon. Data was fitted to the pseudo-second order kinetic model with good correlation coefficients for all the solution media. The faster adsorption rate was obtained for the most hydrophobic solvent (heptane). The overall adsorption rate of naphthalene seems to be controlled simultaneously by external (boundary layer) followed by intraparticle diffusion in the porosity of the activated carbon when water, ethanol and cyclohexane are used as solvents. In the case of heptane, only two stages were observed (pore diffusion and equilibrium) suggesting that the limiting stage is the intraparticle diffusion. The low value of the boundary thickness supports this observation.

Bio-syngas production with low concentrations of CO2 and CH4 from microwave-induced pyrolysis of wet and dried sewage sludge.

This paper assesses the feasibility of producing syngas from sewage sludge via two pyrolysis processes: microwave-induced pyrolysis (MWP) and conventional pyrolysis (CP). The changes in the composition of the produced gas as a function of the pyrolysis treatment and the initial moisture content of the sludge were evaluated. It was found that MWP produced a gas with a higher concentration of syngas than CP, reaching values of up to 94vol%. Moreover, this gas showed a CO2 and CH4 concentration around 50% and 70%, respectively, lower than that obtained in the gas from CP. With respect to the effect of moisture on gas composition, this was more pronounced in CP than in MWP. Thus, the presence of moisture increases the concentration of H2 and CO2 and decreases that of CO, especially when CP was used. In order to elucidate the behaviour of CO2 during the pyrolysis, the CO2 gasification kinetics of the char obtained from the pyrolysis were investigated. It was established that in microwave heating the gasification reaction is much more favoured than in conventional heating. Therefore, the low concentration of CO2 and the high concentration of CO in the microwave pyrolysis gas could be due to the self-gasification of the residue by the CO2 produced during the devolatilization of the sewage sludge in the pyrolysis process.

Microwave-assisted regeneration of activated carbons loaded with pharmaceuticals.

The purpose of this work was to explore the application of microwaves for the regeneration of activated carbons spent with salicylic acid, a metabolite of a common analgesic frequently found in wastewater from the pharmaceutical industry. The exhausted carbon was treated in a quartz reactor by microwave irradiation at 2450 MHz at different temperatures and atmospheres, the regeneration efficiency being highly dependent on the operating conditions. Quantitative desorption of the pollutant was achieved at high temperature and oxidizing atmosphere, with regeneration efficiencies as high as 99% after six cycles. The stripping efficiency was superior to 95% at high temperatures and decreased at 450 degrees C. The incomplete desorption of the adsorbate at low temperature was further confirmed by the changes in the porosity observed by N2 and CO2 adsorption isotherms. Hence, micropores remain blocked which results in a reduction in loading capacities in successive cycles.

Removal of naphthalene from aqueous solution on chemically modified activated carbons.

The aim of this work was to correlate the textural and chemical features of carbonaceous adsorbents with the adsorption capacity of naphthalene from aqueous phase, at the concentration in which this compound is usually found in wastewater from coke ovens. The study reveals that the adsorption capacity in different carbon materials depends not only on the textural characteristics of the material but also on the functionalities of the activated carbons. The micropores of the adsorbents, particularly those of narrower diameter, were found to be active sites for the retention of naphthalene. In contrast, the modification of the surface chemistry of the carbon materials led to a decrease in the adsorption capacities. Dispersive forces play an important role, and adsorbents with a higher non-polar character have proven to be more efficient for the naphthalene adsorption. This behaviour has been linked to the presence of specific interactions between the basal planes and the polyaromatic structure of the naphthalene molecule.

Production of bio-fuels by high temperature pyrolysis of sewage sludge using conventional and microwave heating.

The pyrolysis of sewage sludge was investigated using microwave and electrical ovens as the sources of heat, and graphite and char as microwave absorbers. The main objective of this work was to maximize the gas yield and to assess its quality as a fuel and as a source of hydrogen or syngas (H2 + CO). Both gases were produced in a higher proportion by microwave pyrolysis than by conventional pyrolysis, with a maximum value of 38% for H2 and 66% for H2 + CO. The oils obtained were also characterized using FTIR and GC-MS. The use of conventional electrical heating in the pyrolysis of sewage sludge produced an oil that could have a significant environmental and toxicological impact. Conversely, microwave pyrolysis still preserved some of the functional groups of the initial sludge such as aliphatic and oxygenated compounds, whereas no heavy PACs were detected.

Gas chromatographic-mass spectrometric study of the oil fractions produced by microwave-assisted pyrolysis of different sewage sludges.

The pyrolysis of sewage sludge was studied in a microwave oven using graphite as microwave absorber. The pyrolysis temperature ranged from 800 to 1000 degrees C depending on the type of sewage sludge. A conventional electrical furnace was also employed in order to compare the results obtained with both methods. The pyrolysis oils were trapped in a series of condensers and their characteristics such as elemental analysis and calorific value were determined and compared with those of the initial sludge. The oil composition was analyzed by GC-MS. The oils from the microwave oven had n-alkanes and 1-alkenes, aromatic compounds, ranging from benzene derivatives to polycyclic aromatic hydrocarbons (PAHs), nitrogenated compounds, long chain aliphatic carboxylic acids, ketones and esters and also monoterpenes and steroids. The oil from the electric oven was composed basically of PAHs such as naphthalene, acenapthylene, phenanthrene, fluoranthene, benzo[a]anthracene, benzofluoranthenes, benzopyrenes, indenepyrene, benzo[ghi]perylene, and anthanthrene. In contrast, these compounds were not produced in the case of microwave-assisted pyrolysis.

Microwave-induced pyrolysis of sewage sludge.

This paper describes a new method for pyrolyzing sewage sludge using a microwave furnace. It was found that if just the raw wet sludge is treated in the microwave, only drying of the sample takes place. However, if the sludge is mixed with a small amount of a suitable microwave absorber (such as the char produced in the pyrolysis itself) temperatures of up to 900 degrees C can be achieved, so that pyrolysis takes place rather than drying. Microwave treatments were also compared with those carried out in a conventional electric furnace, as well as the characteristics of their respective carbonaceous solid residues.