•Reaction kinetics of CO2 with PZ/AEEA blends were studied.•A ‘zwitterion bridge’ reaction pathway for the interaction in the blend was proposed.•Simulation through density functional theory revealed the mechanism of interaction.Amine blends have been extensively studied as advanced solvents modification for CO2 absorption. Due to the lack of knowledge on mechanism of amines interaction, reaction in blends was traditionally regarded as parallel reactions between CO2 and different amines. In this work, the kinetics of CO2 reaction with blends of two promising diamines, piperazine (PZ), N-(2-aminoethyl) ethanolamine (AEEA), were studied in a wetted wall column. The reaction rate constants of single amine were derived from kinetic data based on the zwitterion and termolecular mechanism for PZ and AEEA, respectively. It is interesting to find that the overall reaction rate of the blend is 12.0–28.1% lower than the calculated value through parallel reaction mechanism, which indicates the strong interaction between PZ and AEEA. A ‘zwitterion bridge’ reaction pathway was proposed as the microscopic mechanism of the interaction in the blend. Based on the kinetic models and experimental data, the interaction that PZ can promote the reaction in amines blend by transferring CO2 to AEEA through the PZ-CO2 intermediate was revealed. Besides, the interaction was proved to be a fast reaction with the reaction rate of same magnitude compared with the zwitterion hydrolysis reaction. The mechanisms of interaction between the two amines are further investigated through density functional theory (DFT). Results of simulation, in terms of activation energy and molecular geometry deformation, indicate that the PZ prefers a direct reaction with CO2 rather than the reaction with the AEEA-CO2 intermediate.

•Novel, direct non-aqueous gas stripping process is proposed.•Pentane, hexane, and cyclohexane are identified as good carrier gas candidates.•Stripping with pentane yields a 38% reduction in energy consumption.•Low temperature CO2 stripping (<90 °C) is realized.•CO2 stripping with organic gases helps break the gas-liquid interface.Using an inert, immiscible organic component as the purging gas is an innovative technology in the CO2 stripping process. It allows stripping at a lower temperature and overcomes the inherent problem of high energy consumption in steam-based CO2 stripping processes. This work focused on developing a novel, direct non-aqueous gas stripping process and exploiting its potential for energy saving through experiments. An elaborate pre-screening of carrier gases was conducted through hydrocarbon, alcohols, aldehydes, ketones, and their isomers, etc. Pentane, hexane, and cyclohexane were selected as the carrier candidates for their good immiscibility with water/amine, low vaporization energy, mild condensation conditions, and low toxicity. The effects of carrier gas category, carrier gas flow rate, and rich solvent feeding temperature on the stripping performance were investigated. Pentane was found to be the superior carrier gas due to its low heat of vaporization, high regeneration rate, and low energy requirement. A unique phenomenon of water evaporation from the solvent was discovered in the organic gas stripping process, providing a promising approach to break the gas-liquid interface for enhanced diffusion kinetics. The feasibility of low temperature CO2 stripping was analyzed by varying the rich solvent feeding temperature (70–100 °C). With pentane as the stripping gas, it was interesting to find an optimum rich-feed temperature lower than 90 °C, indicating great potential for a reduction in the overall energy requirement. Under the optimal conditions, the energy consumption during CO2 regeneration, using pentane as the carrier gas, was 2.38 GJ/t CO2, 38.8% lower than that of the conventional stripping process. Furthermore, parasitic issues, such as water loss and solvent loss, were discussed. An improved direct non-aqueous gas stripping process with two-stage compression was used to tackle the carrier gas recovery problem. The simulation results show that more than 98% of the carrier gas can be recovered through preliminary separation approaches.Download high-res image (127KB)Download full-size image

In this study, three different nanoparticles—SiO2, TiO2, and Al2O3—were employed to enhance the CO2 gas absorption by monoethanolamine (MEA) solvent. It is observed that, with increased solids loading, the total mass-transfer enhancement has a tendency to be dominated by the bubble breaking effect. It is concluded that nanoparticles result in an increased CO2 absorption rate of >10%. On the other hand, nanoparticles exhibit much more attractive impact on CO2 desorption. Under the same desorption extent, solvent with 0.1 wt % TiO2 nanoparticles saved 42% desorption time, compared to that without nanoparticles. Under higher heat flux density, more input heat would be supplied to the heat of desorption, rather than the heat of water evaporation, which is due to the enhancement of desorption rate by nanoparticles. The issue of particle aggregation was also investigated by analyzing the size distribution of nanoparticle clusters and zeta potential of MEA solvents.

CO2 membrane stripping is a novel method for CO2 desorption at low temperature (around 348K) for amine-based CO2 capture, which has the potential to reduce regeneration energy requirement. In this work, we investigated CO2 membrane stripping with two different commercial hollow fiber membranes, polypropylene (PP) and Polyvinylidene fluoride (PVDF). CO2 membrane stripping in PVDF membrane showed a faster CO2 desorption rate than in PP membrane, but PP membrane presented a better stability on long-term running. Energy consumption for CO2 membrane stripping with 20 wt% MEA solvent was evaluated. Compared with conventional thermal regeneration, 28% energy can be saved if regeneration pressure of CO2 membrane stripping operated at 20 kPa.

Promoted aqueous ammonia is a potential solvent for CO2 separation processes. In this work, we investigated the effect of temperature, sarcosinate concentration, and CO2 loading on the mass transfer coefficients of CO2 absorption in a sarcosinate-promoted aqueous ammonia solution on a wetted-wall column. We further investigated the kinetics of the reaction between CO2 and a blended NH3/sarcosinate absorbent using stopped-flow spectrophotometric techniques, following the pH changes via coupling to pH indicators. Our study revealed that the mass transfer coefficient for CO2 absorption in a 3 M ammonia + 1.5 M sarcosinate blended solution at 288 K is close to that in 5 M monoethanolamine absorbent at 313 K. We did not observe any synergistic or catalytic effects between NH3 and sarcosinate in the blended solution; the mechanism of the reaction of CO2 with the NH3/sarcosinate mixture is the simple combination of the individual reactions of NH3 and sarcosinate with CO2.

Trace carbon dioxide (CO2) removal from an enclosed space was studied experimentally and theoretically in a hollow-fiber membrane contactor using monoethanolamine (MEA) solution. Changes in trace CO2 removal performance with liquid flow rate, gas flow rate, absorption temperature, and CO2 loading were investigated individually in an apparatus under well-defined and controlled experimental conditions. We developed a two-dimensional (2-D) mathematical model to predict and further analyze the experimental results. The modeling results agreed well with the experimental work. Liquid flow rate positively influences trace CO2 removal, but is not recommended for operation at a very high level. Increasing the gas flow rate improves CO2 absorption flux at the cost of reducing CO2 removal efficiency; the optimal gas flow rate for the trade-off between CO2 removal efficiency and absorption flux is presented. The optimal absorption temperatures change with liquid CO2 loading for trace CO2 removal. CO2 removal efficiency is a decreasing function of CO2 loading; the recommended CO2 loading of MEA solution is below 0.35 mol CO2/mol MEA. To predict the influence of membrane wetting on trace CO2 removal, we also propose a model that incorporates membrane wetting. The minimum breakthrough pressure are 0.38 and 0.0581 MPa for fresh membrane and old membrane, respectively. Membrane wetting significantly deteriorates trace CO2 membrane absorption performance, with CO2 removal efficiency decreasing suddenly once the membrane is wetted.

To understand and optimize CO2 absorption in binary amine systems, we experimentally and theoretically investigated CO2 absorption using typical amines and blended amines in a polypropylene hollow-fiber membrane contactor. The amines studied were monoethanolamine (MEA), diethanolamine (DEA), and N-methyldiethanolamine (MDEA), and their aqueous blends of MEA/MDEA, DEA/2-amino-2-methyl-1-propanol (AMP), and MDEA/piperazine (PZ). The predicted results, including overall mass transfer coefficients and CO2 removal ratio, agreed very well with those determined experimentally. For single amines, the optimal concentration was around 30 wt % for MEA and 20 wt % for DEA. MDEA concentration had little effect on the overall mass transfer coefficient. We optimized the formulation of blended amines using theoretical analysis. The optimal compositions in MEA/MDEA, DEA/AMP, and MDEA/PZ systems were respectively 30 wt % MEA, with MDEA in proportions from 0.1 to 0.3; 15 wt % DEA, with AMP in proportions from 0.5 to 0.8; and 20 wt % MDEA, with PZ in a proportion of 0.3. To further understand the CO2 membrane absorption process, we also analyzed individual mass transfer resistances as a function of additive concentration in blended amines and the effects of liquid velocity on the overall mass transfer coefficient. This shows that CO2 absorption is controlled by the liquid side for DEA/AMP blends and by combined liquid–gas phases for MEA/MDEA blends. For MDEA/PZ blends, control of CO2 absorption is characterized by a gradual transition from liquid side controlled to liquid–gas combined controlled as the concentration of PZ increases.

Membrane stripping is a promising method for carbon dioxide (CO2) desorption with low energy demand. We developed a mathematical model to simulate the CO2 stripping from the CO2-rich monoethanolamine (MEA) solvent in a hollow fiber membrane contactor. The modeling results agreed well with literature experimental results. To improve the understanding and facilitate the optimization of the membrane based CO2 stripping process, we investigated the effects of different operating variables including liquid flow velocity, sweeping gas flow rate, regeneration pressure, and temperature on CO2 desorption performance. The effects of membrane’s length and diameter of membrane fiber on CO2 desorption were also studied. Results showed that increasing membrane’s length will improve CO2 stripping performance, but not infinitely, because of the thermodynamic limitation of absorbent, and shorter membrane module is preferred for thinner membrane. Membranes with a smaller diameter favor CO2 desorption. To predict the influence of membrane wetting on CO2 stripping performance, the change of CO2 lean loading with the wetting ratio was investigated. Membrane wetting significantly deteriorates CO2 membrane desorption performance, with CO2 lean loading increasing suddenly once the membrane is wetted.

Solvent selection is the potential breakthrough for the chemical absorption method to capture CO2 from coal-fired flue gas. A total of 16 kinds of selected monoethanolamine (MEA)-based solvents were assessed by absorption at 40 °C up to 12 kPa CO2 partial pressure and desorption at 100 °C down to 1.0 kPa CO2 partial pressure. Results showed that the 20% MEA + 10% diethylenetriamine (DETA) system maintains the highest average absorption rates and reaches the highest loading of 0.556 kg of CO2/kg of solute during all of the tested systems. The absorption capacity of the 20% MEA + 10% DETA system is increased to 53%, and the corresponding average removal efficiency is enhanced more than 31% compared to 30% MEA solution. Furthermore, there is only a slight difference between the regeneration performance of the 20% MEA + 10% DETA system and that of the 30% MEA solution.

This paper presents the experimental results of CaO sorption enhanced anaerobic gasification of biomass in a self-design bubbling fluidized bed reactor, aiming to investigate the influences of operation variables such as CaO to carbon mole ratio (CaO/C), H2O to carbon mole ratio (H2O/C) and reaction temperature (T) on hydrogen (H2) production. Results showed that, over the ranges examined in this study (CaO/C: 0–2; H2O/C: 1.2–2.18, T: 489–740 °C), the increase of CaO/C, H2O/C and T were all favorable for promoting the H2 production. The investigated operation variables presented different influences on the H2 production under fluidized bed conditions from those obtained in thermodynamic equilibrium analysis or fixed bed experiments. The comparison with previous studies on fluidized bed biomass gasification reveals that this method has the advantage of being capable to produce a syngas with high H2 concentration and low CO2 concentration.

Absorption characteristics of CO2 into aqueous ammonia are the key factors to achieve simultaneous removal of pollutants from coal-fired flue gas by ammonia scrubbing. An experimental study with asemi-continuous flow reactor was conducted to investigate the CO2 absorption characteristic of aqueous ammonia solutions. Pure CO2 was chosen in order to avoid the effect by the other gas components in this paper. The effect on solution CO2 loading and CO2 absorption capacity, which is caused by ammonia concentration, CO2 inlet velocity and temperature, has been given. Furthermore, the effect on absorption characteristic and slippery characteristics of ammonia caused by additives is also covered. The experiment results show that, with increase of ammonia concentration, the solution CO2 loading increases while the CO2 absorption capacity decreases. Both the above two parameters decrease with the increase of CO2 inlet velocity, but the total reaction rate acts reversely. When the temperature reaches 15 °C or 35 °C, the absorption characteristic can reach the extreme points. Adding AMP into ammonia can enhance the absorption of CO2, and control the slippery of ammonia in the absorption process. What’ s more, 3% is a better choice for addition concentration.

Experiments on CO2 removal from flue gas using polypropylene (PP) hollow fiber membrane contactors were conducted in this study. Absorbents including aqueous potassium glycinate (PG) solution, aqueous solutions of monoethanolamine (MEA) and methyldiethanolamine (MDEA) were used to absorb CO2 in the experiments. Based on the wetting experimental results, aqueous PG solution can offer a higher surface tension than water, aqueous MEA and MDEA solutions. Aqueous PG solution has a lower potential of membrane wetting after a continuously steady operation for 40 h to maintain CO2 removal efficiency of about 90%. Under moderate operating conditions, effects of the temperature, flow rate, and concentration of absorbents, and the flow rate of flue gas as well as the volumetric concentration of carbon dioxide in the flue gas on the mass transfer rate of CO2 were studied on a pilot-scale test facility. Unlike conventional absorbents, the mass transfer decreases with an increasing liquid temperature when using aqueous PG solution. Results show that CO2 removal efficiency was above 90% and the mass transfer rate was above 2.0 mol/(m2 h) using the PG aqueous solution. It indicates that the hollow fiber membrane contactor has a great potential in the area of CO2 separation from flue gas when absorbent's concentration and liquid–gas pressure difference are designed elaborately.

Experiments on the thermal decomposition of wet wood in air were carried out in this work. The samples (typically 100×100 mm exposed surface, 15 mm thick) of several species with moisture content from 5% to 30% were subjected to a uniform heat flux 20–70 kW m−2. A one-dimensional pyrolysis model is proposed to examine the influence of heat flux, species and moisture content on the process of thermal decomposition of wet wood. Temperature profiles at different points and solid conversion are calculated and compared with experimental data. There is good agreement between the experimental and calculated results.