Hydrogen production from coal gasification for effective downstream CO2 capture

The coal gasification process is used in commercial production of synthetic gas as a means toward clean use of coal. The conversion of solid coal into a gaseous phase creates opportunities to produce more energy forms than electricity (which is the case in coal combustion systems) and to separate CO₂ in an effective manner for sequestration. The current work compares the energy and exergy efficiencies of an integrated coal-gasification combined-cycle power generation system with that of coal gasification-based hydrogen production system which uses water-gas shift and membrane reactors. Results suggest that the syngas-to-hydrogen (H₂) system offers 35% higher energy and 17% higher exergy efficiencies than the syngas-to-electricity (IGCC) system. The specific CO₂ emission from the hydrogen system was 5% lower than IGCC system. The Brayton cycle in the IGCC system draws much nitrogen after combustion along with CO₂. Thus CO₂ capture and compression become difficult due to the large volume of gases involved, unlike the hydrogen system which has 80% less nitrogen in its exhaust stream. The extra electrical power consumption for compressing the exhaust gases to store CO₂ is above 70% for the IGCC system but is only 4.5% for the H₂ system. Overall the syngas-to-hydrogen system appears advantageous to the IGCC system based on the current analysis.     

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.     

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.     

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.     

Optimization of H2 production with CO2 capture by steam reforming of methane integrated with a chemical-looping combustion system

Methane steam reforming (SR) integrated with a chemical-looping combustion (CLC) system is a new process for producing hydrogen from natural gas, allowing carbon dioxide capture with a low energy penalty. In this study, mass and enthalpy balances of an SR-CLC system were carried out to determine the autothermal operating conditions for optimal H₂ production. The evaluation was conducted using iron-based oxygen carriers. Two configurations were analysed, firstly with the reformer tubes inside the fuel reactor and, secondly, with the reformer tubes inside the air reactor. This paper analyses the effect of two parameters affecting the SR process, namely the conversion of methane in the reformer (XCH₄$X_{{\text{CH}}_4}$) and the efficiency of the hydrogen separation of a pressure swing adsorption (PSA) unit (η_PSA), as well as two parameters affecting the CLC system, namely the Fe₂O₃ content in the oxygen carrier and its conversion variation (ΔX_OC), on the H₂ yields. Moreover, it also analyses the reduction of Fe₂O₃ to Fe₃O₄ or to FeAl₂O₄. The results shown that a H₂ yield value of 2.45 mol H₂ per mol of CH₄ can be obtained with the reformer tubes located inside the air reactor and with Fe₂O₃ being reduced to Fe₃O₄. This corresponds to a CH₄ to H₂ conversion of 74.2%, which is similar to state-of-the-art H₂ production technologies, but with inherent CO₂ capture in the SR-CLC process.     

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.     

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.     

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.     

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.     

Integrated catalytic adsorption (ICA) steam gasification system for enhanced hydrogen production using palm kernel shell

This paper investigates the integrated catalytic adsorption (ICA) steam gasification of palm kernel shell for hydrogen rich gas production using pilot scale fluidized bed gasifier under atmospheric condition. The effect of temperature (600–750 °C) and steam to biomass ratio (1.5–2.5 wt/wt) on hydrogen (H₂) yield, product gas composition, gas yield, char yield, gasification and carbon conversion efficiency, and lower heating values are studied. The results show that H₂ hydrogen composition of 82.11 vol% is achieved at temperature of 675 °C, and negligible carbon dioxide (CO₂) composition is observed at 600 °C and 675 °C at a constant steam to biomass ratio of 2.0 wt/wt. In addition, maximum H₂ yield of 150 g/kg biomass is observed at 750 °C and at steam to biomass ratio of 2.0 wt/wt. A good heating value of product gas which is 14.37 MJ/Nm³ is obtained at 600 °C and steam to biomass ratio of 2.0 wt/wt. Temperature and steam to biomass ratio both enhanced H₂ yield but temperature is the most influential factor. Utilization of adsorbent and catalyst produced higher H₂ composition, yield and gas heating values as demonstrated by biomass catalytic steam gasification and steam gasification with in situ CO₂ adsorbent.     

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.     

Enhanced ethanol steam reforming by CO2 absorption using CaO,CaO*MgO or Na2ZrO3

This paper presents results of thermodynamic analysis and experimental evaluation of hydrogen production by steam reforming of ethanol (SRE) combined with CO₂ absorption using a mixture of a solid absorbent (CaO, CaO*MgO and Na₂ZrO₃) and a Ni/Al₂O₃ catalyst. Thermodynamic analysis results indicate that a maximum of 69.5% H₂ (dry basis) is feasible at 1 atm, H₂O/C₂H₅OH = 6 (molar ratio) and T = 600 °C. whereas, the addition of a CO₂ absorbent at 1 atm, T = 600 °C and H₂O/C₂H₅OH/Absorbent = 6:1:2.5, produced a H₂ concentration of 96.6, 94.1, and 92.2% using CaO, CaO*MgO, and Na₂ZrO₃, respectively. SRE experimental evaluation achieved a maximum of 60% H₂. While combining SRE and a CO₂ absorbent exhibited a concentration of 96, 94, and 90% employing CaO, CaO*MgO, and Na₂ZrO₃, respectively at 1 atm, T = 600 °C, SV = 414 h⁻¹ and H₂O/C₂H₅OH/absorbent = 6:1:2.5 (molar ratio).     

CO2 capture performance of calcium-based sorbent doped with manganese salts during calcium looping cycle

The effects of manganese salts including Mn(NO₃)₂ and MnCO₃ on CO₂ capture performance of calcium-based sorbent during cyclic calcination/carbonation reactions were investigated. Mn(NO₃)₂ and MnCO₃ were added by wet impregnation method. The cyclic CO₂ capture capacities of Mn(NO₃)₂-doped CaCO₃, MnCO₃-doped CaCO₃ and original CaCO₃ were studied in a twin fixed-bed reactor and a thermo-gravimetric analyzer (TGA), respectively. The results show that the addition of manganese salts improves the cyclic carbonation conversions of CaCO₃ except the previous cycles. When the Mn/Ca molar ratios are 1/100 for Mn(NO₃)₂-doped CaCO₃ and 1.5/100 for MnCO₃-doped CaCO₃, the highest carbonation conversions are achieved respectively. The carbonation temperature of 700-720 °C is beneficial to CO₂ capture of Mn-doped CaCO₃. The residual carbonation conversions of Mn(NO₃)₂-doped and MnCO₃-doped CaCO₃ are 0.27 and 0.24 respectively after 100 cycles, compared with the conversion of 0.16 for original one after the same number of cycles. Compared with calcined original CaCO₃, better pore structure is kept for calcined Mn-doped CaCO₃ during calcium looping cycle. The pore volume of calcined MnCO₃-doped CaCO₃ is 2.4 times as high as that of calcined original CaCO₃ after 20 cycles. The pores of calcined MnCO₃-doped CaCO₃ in the pore size range of 27-142 nm are more abundant relative to clacined original one. That is why modification by manganese salts can improve cyclic CO₂ capture capacity of CaCO₃.     

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.     

Two novel oxy-fuel power cycles integrated with natural gas reforming and CO2 capture

Two novel system configurations were proposed for oxy-fuel natural gas turbine systems with integrated steam reforming and CO₂ capture and separation. The steam reforming heat is obtained from the available turbine exhaust heat, and the produced syngas is used as fuel with oxygen as the oxidizer. Internal combustion is used, which allows a very high heat input temperature. Moreover, the turbine working fluid can expand down to a vacuum, producing an overall high-pressure ratio. Particular attention was focused on the integration of the turbine exhaust heat recovery with both reforming and steam generation processes, in ways that reduce the heat transfer-related exergy destruction. The systems were thermodynamically simulated, predicting a net energy efficiency of 50–52% (with consideration of the energy needed for oxygen separation), which is higher than the Graz cycle energy efficiency by more than 2 percentage points. The improvement is attributed primarily to a decrease of the exergy change in the combustion and steam generation processes that these novel systems offer. The systems can attain a nearly 100% CO₂ capture.     

On the off-design of a natural gas-fired combined cycle with CO2 capture

During the last 15 years cycles with CO₂ capture have been in focus, due to the growing concern over our climate. Often, a natural gas fired combined cycle with a chemical absorption plant for CO₂ capture from the flue gases have been used as a reference in comparisons between cycles. Neither the integration of the steam production for regeneration of amines in the combined cycle nor the off-design behaviour of such a plant has been extensively studied before.     

Technoeconomic assessment of China’s indirect coal liquefaction projects with different CO2 capture alternatives

ICL (Indirect coal liquefaction), an alternative fuel-supplying technology, has drawn much attention and caused considerable debate in China’s energy sector. The hurdles to its development include the high risk of investment into large-scale installations, the high CO₂ emissions and water resource consumption. A comprehensive assessment of ICL is urgently needed. This study provides an economic assessment and a technical analysis based on process simulations. To address the future challenge of curbing CO₂ emissions, three absorption methods are compared for capturing the CO₂ released from the ICL process: DMC (a novel absorbent), MEA and Rectisol. The comparative results suggest that physical absorbents, represented by Rectisol and DMC, have a remarkable advantage over chemical absorption processes, represented by MEA. The Rectisol process costs the least, while the DMC process is close to the same level. As a novel absorbent, DMC has the potential to be widely used in the future. The economic analysis of ICL predicted a high capital cost of over 35 billion yuan and an overall product cost of approximately 3800 yuan/ton for the baseline. In addition, via a sensitivity analysis, coal price, electricity price and capacity factor were identified as the three most influential factors affecting the overall product cost.     

CO2 capture using carbide slag modified by propionic acid in calcium looping process for hydrogen production

Lime enhanced gasification (LEGS) process based on calcium looping in which CaO is employed as CO₂ sorbent is an emerging technology for hydrogen production and CO₂ capture. In this work, carbide slag which was an industrial solid waste was utilized as CO₂ sorbent in hydrogen production process. Modification of carbide slag by propionic acid was proposed to improve its reactivity. The CO₂ capture behavior of raw and modified carbide slags was investigated in a dual fixed-bed reactor (DFR) and a thermo-gravimetric analyzer (TGA). The results show that modification of carbide slag by propionic acid enhances its CO₂ capture capacity in the multiple calcination/carbonation cycles. The favorable carbonation temperature and calcination temperature for modified carbide slag are 680–700 °C and 850–950 °C, respectively. Prolonged carbonation treatment is beneficial to CO₂ capture of raw and modified carbide slags. The prolonged carbonation for 9 h in the 21st cycle increases the conversions of raw and modified carbide slags in this cycle. And then the carbonation conversions of the two sorbents were also improved in the subsequent cycles. Calcined modified carbide slag shows more porous microstructure compared with calcined raw one for the same number of cycles. Modification of carbide slag by propionic acid increases the surface area, pore volume and pore area. In addition, the volume and area of the pores in 20–100 nm in diameter were improved, which had been proved to be more effective to capture CO₂. The microstructure of calcined modified carbide slag favors its higher CO₂ capture capacity in the multiple calcination/carbonation cycles.     

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.