Sorbent enhanced hydrogen production from steam gasification of coal integrated with CO2 capture

In this work, CO₂ capture and H₂ production during the steam gasification of coal integrated with CO₂ capture sorbent were investigated using a horizontal fixed bed reactor at atmospheric pressure. Four different temperatures (650, 675, 700, and 750 °C) and three sorbent-to-carbon ratios ([Ca]/[C] = 0, 1, 2) were studied. In the absence of sorbent, the maximum molar fraction of H₂ (64.6%) and conversion of coal (71.3%) were exhibited at the highest temperature (750 °C). The experimental results verified that the presence of sorbent in the steam gasification of coal enhanced the molar fraction of H₂ to more than 80%, with almost all CO₂ was fixed into the sorbent structure, and carbon monoxide (CO) was converted to H₂ and CO₂ through the water gas shift reaction. The steam gasification of coal integrated with CO₂ capture largely depended on the reaction temperature and exhibited optimal conditions at 675 °C. The maximum molar fraction of H₂ (81.7%) and minimum CO₂ concentration (almost 0%) were obtained at 675 °C and a sorbent-to-carbon ratio of 2.     

Calcium looping gasification for high-concentration hydrogen production with CO2 capture in a novel compact fluidized bed: Simulation and operation requirements

The coal/CaO/steam gasification system is one of the clean coal technologies being developed for hydrogen production with inherent carbon dioxide separation. A novel reactor configuration for the system is proposed in this paper. It consists of three major counterparts: a gasifier, a riser and a regenerator. A regenerable calcium-based sorbent CaO is used to remove carbon dioxide. In the gasifier, the coal-steam gasification reaction occurs with in situ carbon dioxide removal by carbonation reaction. The removal of carbon dioxide favors the gasification and water-shift reaction equilibrium and enables the production of a hydrogen-rich gas stream. CaO is regenerated in the regenerator by burning the unreacted char with oxygen, and a pure stream of carbon dioxide is separated after a cyclone. The regenerated CaO then flows into the riser above the gasifier, and removes the carbon dioxide in the outlet gases from the gasifier and drives the water-gas shift reaction forward, further improving the hydrogen purity. In this work, the feasibility and optimum process conditions of the proposed system were described. The hydrogen purity can reach 96 vol% at a steam flow 80 mol/s and CaO recycle rate 30 mol/s when the carbon conversion rate is 0.50. Increasing the steam flow and CaO recycle rate can enhance the hydrogen yield and purity. With the rise of operation pressure from 1 bar to 10 bar, the hydrogen yield and purity decrease and methane yield increases. High pressure leads to higher calcination temperature. At 10 bar, the temperature for CaCO₃ decomposition is approximately 1100 °C, at such temperature, the sorbent is easy to deactivate. The appropriate temperatures in the gasifier and the riser are 700 and 600 °C, respectively. An analysis of heat integration is conducted. The maximum carbon conversion rate is ∼0.65. A hydrogen production efficiency of 58.5% is obtained at a carbon conversion rate 0.50, steam flow 60 mol/s and CaO recycle rate 30 mol/s, with a hydrogen purity of 93.7 vol%.     

Co-production system of hydrogen and electricity based on coal partial gasification with CO2 capture

To solve the problems of high cost and low efficiency of conventional co-production system of hydrogen and electricity with low hydrogen-to-electricity ratio, a novel co-production system based on coal partial gasification with CO₂ capture is proposed and thermodynamically analyzed. The new system integrates the conceptions of cascade conversion of coal and cascade utilization of syngas to realize the system with high efficiency, low cost, environmental friendliness and flexible hydrogen-to-electricity ratio. The performance of the new system is evaluated by an Aspen Plus model and effects of the operating conditions are also studied. It is found that the system with capturing CO₂ of 59.7% and hydrogen-to-electricity ratio of 4.76 holds a high exergy efficiency of 54.3% when the carbon conversion ratio of the pressurized fluidized bed (PFB) gasifier is equal to 0.7. The carbon conversion ratio of the PFB gasifier is a dominant factor to decide the performance of system. In comparison with the series-type co-production system, the parallel-type co-production system and separate production system, the new system proposed in this study has exergy-saving efficiency of 17.7%, 15.1% and 8.9%, respectively.     

High quality syngas production from pressurized K2CO3 catalytic coal gasification with in-situ CO2 capture

The enhanced K-catalytic coal gasification by CO₂ sorption reaction (EKcSG) was proposed to produce syngas with high content of H₂ and CH₄ and perform in-situ CO₂ capture. CO₂ is reduced dramatically with the introduction of the CaO into the reactor under typical K-catalytic coal gasification condition (3.5 MPa, 700 °C). The carbonation reaction of CaO can promote the syngas production by improving the equilibrium of the water-gas shift reaction and supplying heat for coal gasification reaction. In the presence of the CaO sorbent (Ca/C = 0.5), the CO₂ concentration in the product gas decreased from 25.61% to 12.80% compared with that without CaO. Correspondingly, the total concentration of H₂ and CH₄ is improved from 65.61% to 82.99% and the carbon conversion reached above 95%. The effect of Ca/C ratio and reaction temperature was investigated during the EKcSG process. It is considered that Ca/C ratio of 0.5 is the best proportion in terms of carbon conversion and CO₂ absorption in our experimental conditions.     

High capacity potassium-promoted hydrotalcite for CO2 capture in H2 production

This paper presents an experimental study for a newly modified K₂CO₃-promoted hydrotalcite material as a novel high capacity sorbent for in-situ CO₂ capture. The sorbent is employed in the sorption enhanced steam reforming process for an efficient H₂ production at low temperature (400–500 ^°C). A new set of adsorption data is reported for CO₂ adsorption over K-hydrotalcite at 400 °C. The equilibrium sorption data obtained from a column apparatus can be adequately described by a Freundlich isotherm. The sorbent shows fast adsorption rates and attains a relatively high sorption capacity of 0.95 mol/kg on the fresh sorbent. CO₂ desorption experiments are conducted to examine the effect of humidity content in the gas purge and the regeneration time on CO₂ desorption rates. A large portion of CO₂ is easily recovered in the first few minutes of a desorption cycle due to a fast desorption step, which is associated with a physi/chemisorption step on the monolayer surface of the fresh sorbent. The complete recovery of CO₂ was then achieved in a slower desorption step associated with a reversible chemisorption in a multi-layer surface of the sorbent. The sorbent shows a loss of 8% of its fresh capacity due to an irreversible chemisorption, however, it preserves a stable working capacity of about 0.89 mol/kg, suggesting a reversible chemisorption process. The sorbent also presents a good cyclic thermal stability in the temperature range of 400–500 °C.     

Boiling coal in water: Hydrogen production and power generation system with zero net CO2 emission based on coal and supercritical water gasification

Coal is the single most important fuel for power generation today. Nowadays, most coal is consumed by means of “burning coal in air” and pollutants such as NO_x, SO_x, CO₂, PM2.5 etc. are inevitably formed and mixed with excessive amount of inner gases, so the pollutant emission reduction system is complicated and the cost is high. IGCC is promising because coal is gasified before utilization. However, the coal gasifier mostly operates in gas environments, so special equipments are needed for the purification of the raw gas and CO₂ emission reduction. Coal and supercritical water gasification process is another promising way to convert coal efficiently and cleanly to H₂ and pure CO₂. The gasification process is referred to as “boiling coal in water” and pollutants containing S and N deposit as solid residual and can be discharged from the gasifier. A novel thermodynamics cycle power generation system was proposed by us in State Key Laboratory of Multiphase Flow in Power Engineering (SKLMFPE) of Xi'an jiaotong University (XJTU), which is based on coal and supercritical water gasification and multi-staged steam turbine reheated by hydrogen combustion. It is characterized by its high coal-electricity efficiency, zero net CO₂ emission and no pollutants. A series of experimental devices from quartz tube system to a pilot scale have been established to realize the complete gasification of coal in SKLMFPE. It proved the prospects of coal and supercritical water gasification process and the novel thermodynamics cycle power generation system.     

Coal gasification with in situ CO2 capture by the synthetic CaO sorbent in a 1 kWth dual fluidised-bed reactor

Coal gasification with in situ CO₂ capture is believed to be able to produce highly concentrated H₂ with little or no CO₂ compared with the conventional process. This has been demonstrated by other researchers working on a single fluidised bed by continuously feeding the CaO sorbent. This work presents the results of coal gasification with in situ CO₂ capture by a synthetic CaO sorbent in a 1 kW_th dual fluidised-bed reactor at atmospheric pressure, which has not been reported in the literature. The synthetic CaO sorbent is cyclically used by going through multiple carbonation/calcination cycles during coal gasification.     

Enhanced hydrogen-rich gas production from steam gasification of coal in a pressurized fluidized bed with CaO as a CO2 sorbent

Steam gasification of a typical Chinese bituminous coal for hydrogen production in a lab-scale pressurized bubbling fluidized bed with CaO as CO₂ sorbent was performed over a pressure range of ambient pressure to 4 bar. The compositions of the product gases were analyzed and correlated to the gasification operating variables that affecting H₂ production, such as pressure (P), mole ratio of steam to carbon ([H₂O]/[C]), mole ratio of CaO to carbon ([CaO]/[C]) and temperature (T). The experimental results indicated that the H₂ concentration was enhanced by raising the temperature, pressure and [H₂O]/[C] under the circumstances we observed. With the presence of CaO sorbent, CO₂ in the production gas was absorbed and converted to solid CaCO₃, thus shifting the steam reforming of hydrocarbons and water gas shift reaction beyond the equilibrium restrictions and enhancing the H₂ concentration. H₂ concentration was up to 78 vol% (dry basis) under a condition of 750 °C, 4 bar, [Ca]/[C] = 1 and [H₂O]/[C] = 2, while CO₂ (2.7 vol%) was almost in-situ captured by the CaO sorbent. This study demonstrated that CaO could be used as a substantially excellent CO₂ sorbent for the pressurized steam gasification of bituminous coal. For the gasification process with the presence of CaO, H₂-rich syngas was yielded at far lower temperatures and pressures in comparison to the commercialized coal gasification technologies. SEM/EDX and gas sorption analyses of solid residues sampled after the gasification showed that the pore structure of the sorbent was recovered after the steam gasification process, which was attributed to the formation of Ca(OH)₂. Additionally, a coal-CaO–H₂O system was simulated with using Aspen Plus software. Calculation results showed that higher temperatures and pressures favor the H₂ production within a certain range.     

Characteristics of cardboard and paper gasification with CO2

Evolutionary behavior of syngas chemical composition and yield have been examined for paper and cardboard at three different temperatures of 800, 900 and 1000 °C using CO₂ as the gasifying agent at constant flow rate. Specifically the evolution of syngas chemical composition with time has been investigated. Pyrolysis of the sample was dominant at the beginning of the gasification process as observed from the high initial devolatilization of the sample followed by char gasification of material to form syngas for a long period of time. Results provided the role of gasification temperature on kinetics of the CO₂ gasification process. Increase in gasification temperature provided increased conversion of the sample material to syngas. Thus the sample conversion to syngas was low at the low temperature of 800 °C while at elevated temperatures of 900 and 1000 °C substantial enhancement of the kinetics process occurred. The evolution of extensive reaction rate of carbon-monoxide was calculated. Results show that increase in temperature increased the extensive reaction rate of carbon-monoxide. The global behavior of syngas chemical composition examined at three different temperatures revealed a peak in concentration of H₂ to exhibit after few minutes into the gasification that changed with gasification temperature. At 800 °C gasification temperature peak in H₂ was displayed at 3 min into gasification while it decreased to only 2 min, approximately, at gasification temperatures of 900 and 1000 °C. The effect of reactor temperature on CO mole fraction has also been examined. Increase in the gasification temperature enhances the mole fraction of CO yields. This is attributed to the increase in forward reaction rate of the Boudouard reaction (C+CO₂2CO). The results show important role of CO₂ gas for the gasification of wastes and low grade fuels to clean syngas.     

Investigation of novel mixed metal ferrites for pure H2 and CO2 production using chemical looping

The mixed metal oxides NiFe₂O₄ and CoFe₂O₄ are candidate materials for the Chemical Looping Hydrogen (CLH) process, which produces pure and separate streams of H₂ and CO₂ without the use of complicated and expensive separations equipment. In the CLH process, syngas reduces a metal oxide, oxidizing the H₂ and CO in the syngas to H₂O and CO₂, and “stores” the chemical energy of the syngas in the reduced metal oxide. The reduced metal oxide is then oxidized in steam to regenerate the original metal oxide and produce H₂. In this study, we report thermodynamic modeling and experimental results regarding the syngas reduction and H₂O oxidation of NiFe₂O₄ and CoFe₂O₄ to determine the feasibility of their use in the CLH process. Modeling predicts the oxidation of nearly all the CO and H₂ in syngas to H₂O and CO₂ during the reduction step for both materials, and regeneration of the mixed metal spinel phase during oxidation with excess H₂O. Laboratory tests in a packed bed reactor confirmed over 99% conversion of H₂ and CO to H₂O and CO₂ during reduction of NiFe₂O₄ and CoFe₂O₄. Powder XRD analysis of the reduced materials showed, in accordance with thermodynamic predictions, the presence of a spinel phase and a metallic phase. High reactivity of the reduced NiFe₂O₄ and CoFe₂O₄ with H₂O was observed, and XRD analysis confirmed re-oxidation to NiFe₂O₄ and CoFe₂O₄ under the conditions tested. When compared with a conventional Fe-based CLH material, the mixed metal spinels showed a higher extent of reduction under the same conditions, and produced four times the H₂ per mass of active material than the Fe-based material. Analysis of the H₂ and CO consumed in the reduction and the H₂ produced during the oxidation showed over 90% conversion of the H₂ and CO in syngas back to H₂ during oxidation.     

Integrated gasification combined cycle and carbon capture: A risky option to mitigate CO2 emissions of coal-fired power plants

The power sectors of many big economies still rely on coal-fired plants and emit huge amounts of carbon dioxide. Emerging countries like Brazil, China and South Africa plan to expand the use of coal-fired thermal plants in the next decade. Integrated gasification combined cycle (IGCC) is an innovative technology that facilitates the implementation of carbon capture (CC). The present work analyzes the maturity and costs of the IGCC technology, with and without CC, and assesses the effect of the technology risk on its economic viability. Findings show that the inclusion of the risk in the economic analysis of IGCC plants raises the cost of CO₂ avoided from 36 US$/t_CO2 to 106 US$/t_CO2 in the case of Shell Gasifiers and from 39 US$/t_CO2 to 112 US$/t_CO2 in the case of GE Gasifiers. Thus, the introduction of IGCC with CC on a wider scale faces huge uncertainties. The feasibility of these plants will rely heavily on the overcoming of the technology risk. Besides, its implementation in the short run will depend on government incentives to bear with the additional cost incurred in the first-generation plants.     

Thermodynamic analysis of a membrane-assisted chemical looping reforming reactor concept for combined H2 production and CO2 capture

There is great consensus that hydrogen will become an important energy carrier in the future. Currently, hydrogen is mainly produced by steam reforming of natural gas/methane on large industrial scale or by electrolysis of water when high-purity hydrogen is needed for small-scale hydrogen plants. Although the conventional steam reforming process is currently the most economical process for hydrogen production, the global energy and carbon efficiency of this process is still relatively low and an improvement of the process is key for further implementation of hydrogen as a fuel source. Different approaches for more efficient hydrogen production with integrated CO₂ capture have been discussed in literature: Chemical Looping Combustion (CLC) or Chemical Looping Reforming (CLR) and membrane reactors have been proposed as more efficient alternative reactor concepts relative to the conventional steam reforming process. However, these systems still present some drawbacks. In the present work a novel hybrid reactor concept that combines the CLR technology with a membrane reactor system is presented, discussed and compared with several other novel technologies. Thermodynamic studies for the new reactor concept, referred to as Membrane-Assisted Chemical Looping Reforming (MA-CLR), have been carried out to determine the hydrogen recovery, methane conversion as well as global efficiency under different operating conditions, which is shown to compare quite favorably to other novel technologies for H₂ production with CO₂ capture.     

An oxy-fuel mass-recirculating process for H2 production with CO2 capture by autothermal catalytic oxyforming of methane

A new oxy-fuel H₂ generation process with CO₂ avoidance is provided. The process utilizes mass recirculation of CO and H₂O to the oxyforming reactor. A comparison between non-recirculating and mass-recirculating oxyforming reactor operation is given. Main benefits of mass recirculation are emphasized. The oxyforming reactor is integrated with the H₂ and CO₂ separators, fuel cell and O₂ generator. In the process C/O is equal to 0.5 while C/H determines the temperature level in the reactor. The reaction system includes combustion, steam reforming and water–gas shift reactions. The oxyforming process is found to be mass transport controlled with O₂ as the limiting reactant. It is emphasized that under MR conditions the decomposition of H₂/CO₂ by water–gas shift reaction is suppressed by means of CO/H₂O-enrichment and hence MR conditions allow for higher temperatures beneficial to endothermic steam reforming reaction. Under MR conditions the thermodynamic equilibrium limits are overcome and all reactions are forced to proceed to the completion which enables 100% selectivities to H₂ and CO₂. The effects of operation parameters such as temperature, flow rate, pressure and composition are examined. The derived S-terms enable for the concise interpretation of the effect of pressure on the concentration gradients transverse to the flow. The consistent control algorithm of the oxyforming reactor is provided.     

Low temperature sugar cane bagasse pyrolysis for the production of high purity hydrogen through steam reforming and CO2 capture

Biomass pyrolysis offers a fast route to produce elevated yields towards highly valued liquid products. This research aims the determination of optimal experimental conditions for a slow and low temperature pyrolysis to produce the highest yield towards condensable (CVM) and non-condensable (NCVM) volatile matter from Mexican cane bagasse and to quantify and characterize the compounds that constitute CVM and NCVM obtained. Results indicate that yield towards volatiles is strongly dependent on temperature. The highest yield was achieved at temperatures greater than 500 °C at a heating rate of 10 °C/min, residence time of 60 min and a particle size between of 420 and 840 μm. Product quantification under isothermal conditions determined that at 550 °C the NCVM, CVM and solid residue was of 26, 57 and 16%, respectively. Preliminary thermodynamic analysis of steam reforming and CO₂ absorption reactions using one of the main CVM products resulted in a potential high hydrogen production yield.     

Recent developments in membrane-based technologies for CO2 capture

Developing new methods and technologies that compete with conventional industrial processes for CO₂ capture and recovery is a hot topic in the current research. Conventional processes do not fit with the current approach of process intensification but take advantage due to their maturity and large-scale implementation. Acting in a precombusion scenario or post-combustion scenario involves the separation of CO₂/H₂ or CO₂/N₂, respectively.     

Biomass-fired cogeneration systems with CO2 capture and storage

In this study, we estimate and analyze the CO₂ mitigation costs of large-scale biomass-fired cogeneration technologies with CO₂ capture and storage. The CO₂ mitigation cost indicates the minimum economic incentive required (e.g. in the form of a carbon tax) to make the cost of a less carbon intensive system equal to the cost of a reference system. If carbon (as CO₂) is captured from biomass-fired energy systems, the systems could in principle be negative CO₂ emitting energy systems. CO₂ capture and storage from energy systems however, leads to reduced energy efficiency, higher investment costs, and increased costs of end products compared with energy systems in which CO₂ is vented. Here, we have analyzed biomass-fired cogeneration plants based on steam turbine technology (CHP-BST) and integrated gasification combined cycle technology (CHP-BIGCC). Three different scales were considered to analyze the scale effects. Logging residues was assumed as biomass feedstock. Two methods were used to estimate and compare the CO₂ mitigation cost. In the first method, the cogenerated power was credited based on avoided power production in stand-alone plants and in the second method the same reference output was produced from all systems. Biomass-fired CHP-BIGCC with CO₂ capture and storage was found very energy and emission efficient and cost competitive compared with other conversion systems.     

Ultrasonic spray pyrolysis synthesis of Ag/TiO2 nanocomposite photocatalysts for simultaneous H2 production and CO2 reduction

Simultaneous photocatalytic hydrogen production and CO₂ reduction (to form CO and CH₄) from water using methanol as a hole scavenger were investigated using silver-modified TiO₂ (Ag/TiO₂) nanocomposite catalysts. A simple ultrasonic spray pyrolysis (SP) method was used to prepare mesoporous Ag/TiO₂ composite particles using TiO₂ (P25) and AgNO₃ as the precursors. The material properties and photocatalytic activities were compared with those prepared by a conventional wet-impregnation (WI) method. It was found that the samples prepared by the SP method had a larger specific surface area and a better dispersion of Ag nanoparticles on TiO₂ than those prepared by the WI method, and as a result, the SP samples showed much higher photocatalytic activities toward H₂ production and CO₂ reduction. The optimal Ag concentration on TiO₂ was found to be 2 wt%. The H₂ production rate of the 2% Ag/TiO₂–SP sample exhibited a six-fold enhancement compared with the 2% Ag/TiO₂–WI sample and a sixty-fold enhancement compared with bare TiO₂. The molar ratio of H₂ and CO in the final products can be tuned in the range from 2 to 10 by varying the reaction gas composition, suggesting a viable way of producing syngas (a mixture of H₂ and CO) from CO₂ and water using the prepared Ag/TiO₂ catalysts with energy input from the sun.     

Prospect options of CO2 capture technology suitable for China

Reviewing the progress of CO₂ capture and storage (CCS) technology, the main obstacles and the potentials of greenhouse gas control in China are identified. An important point can be drawn is that the innovative energy systems, besides simple implementation of existing technology, are needed for CO₂ control in China. On the basis of integration principle of energy utilization and CO₂ separation, several innovative energy systems, including chemical-looping combustion with CO₂ capture, a partial gasification with O₂/CO₂ cycle, and a polygeneration system with CO₂ capture, are introduced. With synergetic integrating CO₂ into chemical energy conversion and utilization processes, these systems may make breakthrough in CO₂ capture with less or even zero energy penalty. Finally, according to the specific issue of China, a new scenario of Energy Network, which composed of energy source, transportation chain, and terminal user, is recommended for sustainable development in China.     

An experimental investigation into the CO2 gasification of deactivated activated-carbon catalyst used for methane decomposition to produce hydrogen

A series of experiments was conducted to study the CO₂ gasification of a deactivated palm-shell-based activated-carbon (ACPS) catalyst used for the thermocatalytic decomposition of methane to produce hydrogen. This catalyst becomes deactivated due to the accumulation of carbon deposits during the methane-decomposition process. The CO₂ gasification was carried out at 850, 900, 950 or 1000 °C to study the deactivated ACPS, which was used at methane-decomposition temperatures of 850 or 950 °C. A series of six methane-decomposition cycles at 950 °C alternating with five gasification cycles using CO₂ at 900, 950 and 1000 °C was also carried out to evaluate the stability of the catalyst. The experiments were conducted using a thermobalance by monitoring the change in mass of the catalyst with time, i.e., the mass gain during methane decomposition or the mass loss during CO₂ gasification. Gasification of the virgin and deactivated ACPS showed strong temperature dependence, with the half and complete gasification times having an exponential dependence on temperature. The gasification reactivity at different conversions was higher for the virgin ACPS and increased with increases in the decomposition temperatures used for deactivation of the ACPS. The activation energies of virgin ACPS and ACPS deactivated at a decomposition temperature of 850 °C decreased with an increase in conversion, while they increased for the ACPS deactivated at a decomposition temperature of 950 °C; the activation energies varied between 81 and 163 kJ/mol. The gasification reactivity changed with methane conversion, showing maximum values for both the virgin and deactivated ACPS at a decomposition temperature of 950 °C. The initial gasification reactivity of the catalyst decreased after three gasification cycles at 1000 °C, while no significant change was observed with gasification cycles at 950 or 900 °C.     

A novel approach for treatment of CO2 from fossil fired power plants. Part B: The energy suitability of integrated tri-reforming power plants (ITRPPs) for methanol production

In Part A of this two-paper work, a novel approach for treatment of CO₂ from fossil fired power plants was studied. This approach consists of flue gases utilization as co-reactants in a catalytic process, the tri-reforming process, to generate a synthesis gas suitable in chemical industries for production of chemicals (methanol, DME, ammonia and urea, etc.). In particular, the further conversion of syngas to a transportation fuel, such as methanol, is an attractive solution to introduce near zero-emission technologies (i.e. fuel cells) in vehicular applications. In fact, the methanol can be used in DMFC (Direct Methanol Fuel Cell) or as fuel for on-board reforming to produce hydrogen for PEMFC (Proton Exchange Membrane Fuel Cell).