Coal-derived Syngas Research Articles

Shock-tube water time-histories and ignition delay time measurements for H2S near atmospheric pressure

Hydrogen sulfide combustion has been of interest in recent years due to its presence in coal-derived syngas and in sour gas. However, there are limited data available for the calibration of chemical kinetics models of high-temperature H2S oxidation. Ignition delay times and laser-absorption water time histories were therefore measured in two different shock tubes around atmospheric pressure over a range of temperatures from 1445 to 2210K for H2S-O2 mixtures diluted in 98% Ar. While modern H2S models from the literature predicted the ignition delay times with fair accuracy, the water profiles were initially very poorly predicted with the same kinetic mechanisms. A model analysis showed that the reaction R62, SH+HO2 ⇆ H2S+O2 (in reverse), was chiefly responsible for these poor predictions. It was possible to obtain better water predictions when the rate for R62 was divided by 10. A tentative model is proposed herein, based on this assumption for R62, and good predictions were obtained for both the ignition delay times and water profiles, as well as for former high-pressure shock-tube data with H2-H2S mixtures. The present study shows that more accurate rates for the reactions R62 (SH+HO2 ⇆ H2S+O2), R75 (SH+O2 ⇆ HSO+O), and R46 (SH+SH (+M) ⇆ HSSH (+M)) would be needed to better predict H2S combustion chemistry.

Comparison of the efficiency of carbon dioxide capture by sorption-enhanced water–gas shift and palladium-based membranes for power and hydrogen production

Pre-combustion capture of carbon dioxide requires the industrial separation of carbon dioxide from hydrogen-rich streams. The present study analyses the thermodynamic efficiency penalty of this separation step and the achievable carbon capture ratio, with particular focus on high-temperature separation technologies: sorption-enhanced water–gas shift (SEWGS) and palladium membranes. Twelve different cases have been simulated, starting from coal-derived syngas or from natural gas derived reformate, using carbon dioxide capture by conventional absorption, SEWGS, and palladium membranes, and producing hydrogen-rich fuel for power production or pure hydrogen. For the production of decarbonised fuel from coal syngas, SEWGS always yields the lowest efficiency penalty per unit of carbon dioxide captured. For the production of pure hydrogen from coal syngas, SEWGS has a significantly higher carbon capture ratio than the alternatives while palladium membranes yield the lowest efficiency penalty per unit of carbon dioxide captured. For the production of decarbonised fuel from natural gas reformate, SEWGS is the most efficient technology in terms of efficiency penalty. For the production of pure hydrogen from natural gas syngas, palladium membranes yield the lowest efficiency penalty.

Experimental and numerical studies of pressure effects on syngas combustor emissions

Pressure effects on NOx and CO emissions of a model syngas combustor were experimentally and numerically studied. The validated numerical method was employed to analyze pressure effects on the combustor emissions such as different mixing levels and extra reactant addition. The model combustor, fueled with 10MJ/Nm3 coal-derived syngas, was installed in a pressurized test-rig, and emissions were measured within 0.1–0.35MPa. Based on the flow, temperature and species fields calculated by the CFD method, a chemical reactor network (CRN) model was established. With a detailed chemical scheme, the emissions calculated by the CRN model agreed well with experimental results. The model was then employed to calculate emissions within pressure range from 0.1 to 2.0MPa, with the adiabatic flame temperature in the primary zone varied from 1477 to 2317K. The calculated NOx and CO emissions generally showed exponential relationship with the operating pressure, except that the NOx emission decreased at higher pressure when the primary adiabatic flame temperature was lower than 1800K. Through the analyses of the NOx formation pathways, it was found that the higher pressure would generally enhance the N2O pathway, which dominated NOx formation at low temperature, but NOx emission concentration through N2O pathway decreased with pressure above 1.0MPa at low temperature, which could explain the NOx emission behavior. The CRN modeling method was applied to analyze the effects of pressure on emission behaviors under the different mixing modes and with the extra reactant such as H2O and NH3 in fuel. Premixed and steam dilution burning modes showed better performance on reducing emissions at elevated pressures. NH3 could obviously increase the emission levels, especially at low pressure. Comprehensive understanding of the relationship between the emissions and the operating pressure in full pressure range was obtained, which could be valuable for the predicting and analyzing the syngas combustor pollutant emissions.

Techno-economic investigation of a chemical looping combustion based power plant.

Among the well-known state-of-the-art technologies for CO2 capture, Chemical Looping Combustion (CLC) stands out for its potential to capture CO2 efficiently from a fuel power plant. CLC involves the combustion of carbonaceous fuel such as coal-derived syngas or natural gas via a redox chemical reaction with a solid oxygen carrier circulating between two fluidised beds. Avoided NOx emissions, high CO2 capture and thermal efficiency are the key concepts that make worth the investigation of this technology. One of the main issues about CLC might concern the impact of the solid metal oxides price and lifetime on the Levelised Cost Of the Electricity (LCOE). A natural gas fired power plant embedding a CLC unit is presented in this work. Detailed fluidised bed models are implemented in Aspen Plus software. Kinetics and hydrodynamics are taken into account to evaluate their effect on the total solid inventory required for full fuel conversion. The models are incorporated into a power plant and a detailed economic evaluation is undertaken by varying two relevant parameters: fuel price and lifetime of the solid particles. The effect of these parameters on the LCOE is investigated and a comparison between CLC and a post-combustion technology employing amines (e.g. monoethanolamine, MEA) is presented. It is shown that the CLC power plant under study leads to a lower LCOE compared to the current MEA post-combustion solution.

Design and Assessment of an IGCC Concept with CO2 Capture for the Co-Generation of Electricity and Substitute Natural Gas

The focus of this work is on the modeling and the thermodynamic evaluation of an integrated gasification combined cycle (IGCC) for the co-production of electricity and substitute natural gas (SNG). At first, an IGCC with CO2 capture for electricity generation is analyzed. Coal-derived syngas is conditioned in a water gas shift unit (WGS), and cleaned in an acid gas removal system including carbon capture. Eventually, the conditioned syngas is fed to a combined cycle. A second case refers to a complete conversion of syngas to SNG in an integrated commercial methanation unit (TREMP™ process, Haldor Topsøe, Kgs. Lyngby, Denmark). Due to the exothermic reaction, a gas recycling and intercooling stages are necessary to avoid catalyst damage. Based on a state-of-the-art IGCC plant, an optimal integration of the synthetic process considering off-design behavior was determined. The raw syngas production remains constant in both cases, while one shift reactor in combination with a bypass is used to provide an adequate H2/CO-ratio for the methanation unit. Electricity has to be purchased from the grid in order to cover the internal consumption when producing SNG. The resulting heat and power distributions of both cases are discussed.

Radiative heat release from premixed oxy-syngas and oxy-methane flames

The abundant supply of coal globally and its energy content per unit mass make the use of this fuel an attractive option for power generation. However, with increasingly strict environmental regulations imposed on greenhouse gas emission technologies the future use of coal in its current form is questionable. One solution to reduce pollutant emissions is oxy-combustion with coal-derived syngas, which can provide a method of using coal efficiently with carbon capture. This paper presents a study of the radiative heat release properties from oxy-syngas and oxy-methane flames. This is relevant since the radiative properties of prospective oxy-combustion systems are not fully known. In many oxy-fuel systems the flue gas stream is recycled and premixed with the fuel and oxidizer to reduce flame temperatures. It is estimated that the radiative heat transfer from these recirculated gases will significantly increase due to the increased emission of radiation from CO2 and H2O in the 1.2μm to 5μm wavelength range, which can impact future combustor design models. Motivated by this, this work aims to provide a study of the global and spectral radiation properties of oxy-syngas combustion and how relevant variables affect radiative emissions. Theses variables include the effect of CO2 acting as a diluent, percentage of H2 in the fuel, firing input, and the effects of equivalence ratio on the flame’s radiative heat release and spectral radiation of CO2 and H2O. The current study reveals that the radiative heat release factor of syngas flames decreases at higher firing inputs. It was also observed that the radiative heat factor decreased at high hydrogen concentrations. An increase in heat release factor is also measured at higher recirculation ratios of CO2.

Simulations of Multiphase Particle Deposition on a Gas Turbine Endwall With Impingement and Film Cooling

Replacing natural gas fuels with coal-derived syngas in industrial gas turbines can lead to molten particle deposition on the turbine components. The deposition of the particles, which originate from impurities in the syngas fuels, can increase surface roughness and obstruct film cooling holes. These deposition effects increase heat transfer to the components and degrade the performance of cooling mechanisms, which are critical for maintaining component life. The current experimental study dynamically simulated molten particle deposition on a conducting blade endwall with the injection of molten wax. The key nondimensional parameters for modeling of conjugate heat transfer and deposition were replicated in the experiment. The endwall was cooled with internal impingement jet cooling and film cooling. Increasing blowing ratio mitigated some deposition at the film cooling hole exits and in areas of coolest endwall temperatures. After deposition, the external surface temperatures and internal endwall temperatures were measured and found to be warmer than the endwall temperatures measured before deposition. Although the deposition helps insulate the endwall from the mainstream, the roughness effects of the deposition counteract the insulating effect by decreasing the benefit of film cooling and by increasing external heat transfer coefficients.

Development of composited rare-earth promoted cobalt-based Fischer–Tropsch synthesis catalysts with high activity and selectivity

The objective of this research is to find an optimal ratio of rare earth elements (RE), lanthanum (La) and cerium (Ce), as promoters of Co-based Fischer–Tropsch synthesis (FTS) catalysts to achieve maximal diesel yield. These synthesized rare-earth promoted catalysts were characterized with Brunauer–Emmett–Teller (BET), scanning electron microscopy (SEM), X-ray diffraction (XRD) and hydrogen temperature programmed reduction (H2-TPR). The effects of La/Ce molar ratio on the FTS performance of silica gel supported cobalt based catalysts were investigated in a fixed bed reactor at temperatures of 220 and 240°C. The evaluation tests show that the catalysts promoted by composited rare-earth (CRE) with appropriate molar ratio La to Co have better FTS performance than unpromoted and individual rare-earth (IRE) promoted catalysts. The results show that the CRE promoted FTS catalysts are promising. The CO conversion is increased significantly and the selectivity for methane and C2–C4 is decreased while the selectivity for long chain hydrocarbons is greatly increased (C5+ increased from 70.36 to 80.25% at 220°C and from 69.94 to 78.02% at 240°C, respectively). Among all CRE modified catalysts, based on the result of distribution of C12–C18 (weight% of hydrocarbons), Co-1La2Ce/S.G (n RE: n Co=0.1 and n La: n Ce=1:2) is proven as a highly efficient catalyst for production of diesel rich synthetic oil from coal-derived syngas.

Effect of air/fuel nozzle arrangement on the MILD combustion of syngas

The effect of air/fuel nozzle arrangement on MILD (Moderate or Intense Low-oxygen Dilution) combustion of coal-derived syngas under lean operating condition was evaluated in a parallel jet combustor. The results were presented on flow field using non-reactive numerical calculation and on OH* chemiluminescence images and exhaust emissions using experiments. The increase in equivalence ratio aids in the reduction of peak flame temperature and in the distribution of reaction volume. The critical equivalence ratios above which MILD combustion occurred were identified for four MILD configurations. The MILD configuration with both air and fuel injected from the opposite side of the combustor exit was observed to achieve MILD combustion at leaner condition than other three configurations. The MILD scheme was established for syngas fuel under highly lean condition with low NOx and CO emissions.

Modeling and simulation of H2S effect in high-temperature water–gas shift reaction using coal-derived syngas

In this study, a model including hydrogen sulfide (H2S) effect on high-temperature water–gas shift reaction (WGSR) performance was built using coal-derived syngas as feedstock. Based on this model, it was found that the H2S coverage rate increases with the decrease in reaction temperature, leading to more significant carbon monoxide (CO) conversion reduction at low reaction temperature. It was also found that the H2S effect can be ignored when the reaction temperature is in the range at which reverse WGSR occurs. In such a temperature range, CO conversion for both with and without the H2S effect approaches equilibrium result. Through parametric studies using this model, it was found that CO conversion depends significantly on the residence time which can be adjusted by varying the flow rate, steam to carbon molar ratio, inlet pressure of syngas and reactor size. For syngas with fixed CO/H2 molar ratio, CO conversion was found to decrease with the increase in carbon dioxide (CO2) content.

Modelling and experimental studies of a water–gas shift catalytic membrane reactor

A 2-dimensional, axis-symmetric CFD model of a tubular CMR has been developed using a commercial software package FLUENT for the purposes of guiding the design and operation of a HTWGS-CMR for the processing of coal-derived syngas. Development of the model has been approached in a stepwise manner through the successive incorporation of sub-models for the CMR processes. For each step, performance of model was checked and validated against measurements using a prototype CMR with same set-up applied in simulation.The optimum catalyst loading, which yield the maximum CO conversion within targeted operating reactor temperatures (350–450°C), was found to be 11.6kg/(COmol/s) for the inlet syngas temperature of 350°C with a reactor having a 1″ shell diameter. The CMR model was validated experimentally with a simulated coal-derived syngas (64.5% of CO, 33.0% of H2 and 2.5% of CO2 with a 3:1 steam to carbon (S:C) ratio) at a total dry gas flow of 4 LN/min and a feed pressure of 15barg. These tests were performed using a prototype reactor which incorporated with a tubular (0.1mm thick, 150cm2, 3/8″ OD) Pd/Ag23wt% membrane. The CMR model was simulated using a wider range of operating parameters (namely permeation rate, inlet temperature, catalyst loading, pressure at permeate side and S:C ratios) to examine its sensitivity to these variables. Outcomes of these parametric analyses have enhanced our understanding of CMR operation in order to optimise its performance.

A comprehensive performance assessment study of pilot-scale Pd and Pd/alloy membranes under extended coal-derived syngas atmosphere testing

The present study encompasses the results of a comprehensive performance assessment of different pilot-scale Pd/Alloy composite asymmetric membranes under long-term coal-derived syngas atmosphere testing. The membranes were developed and tested first at the CIMS-WPI laboratory under pure H2 and later at NCCC under a mixture of N2/H2 and H2 enriched syngas coming from a TRIG™ gasification system. Thirteen membranes were examined for 4275 h and classified in four types: Pd, Pd/Au, Pd/Pt and Pd/Au/Pt membranes. Their respective H2 permeance and purity profiles, permeance difference under pure hydrogen and syngas conditions as well as stability of operation were assessed. The membranes showed good stability in syngas although exhibiting an initial drop in permeance when compared to that under pure hydrogen. Pure Pd membranes displayed the highest permeability, but showed pinholes after being exposed to syngas. Pd/Pt membranes showed a reduced permeance drop under syngas conditions, indicating sulfur resistance properties. Furthermore, Pd/Au/Pt membranes, presented for the first time in the literature, displayed stable permeance and purity profile. The inhibition of the initial permeance drop and the long-term operation of these ternary membranes were demonstrated. Finally, Pd/Au membranes were shown to display the most stable permeance and purity characteristics. In this case, the initial permeance drop was significantly reduced, exhibiting the lowest decline amongst all cases considered. It is hypothesized that Au may act as a patch paste blocking defects on the Pd layer.

Experimental and numerical studies of pressure effects on syngas combustor liner temperature

Pressure effects on the liner temperature of a syngas model combustor are studied by experimental and numerical methods, with an attempt to analyze the influence mechanism. The model combustor, fueled with 10 MJ/Nm3 coal-derived syngas, is installed in a pressurized test-rig. Several diffusion flames with thermal powers up to 180 kw at pressures within 0.1–0.35 MPa are studied using a variety of measurement techniques. The liner temperature varies monotonically along the flow direction, and the maximum temperature appears around the dilution holes. Moreover, the liner temperature increases in the dilution zone but decreases in the primary zone with the increase of operating pressure, when the combustor exit temperature is maintained at 1073 K. Then, the combustion behaviors of the model combustor within pressure range of 0.1–2.0 MPa are simulated using CFD method, and the computed liner temperatures of testing cases agree well with the experimental data. The liner temperature varies approximately from −20 K to 150 K depending on locations when the pressure rises from 0.1 MPa to 2.0 MPa. It shows notable variation with operating pressure under low pressure conditions. However, as the operating pressure continues to increase, its impact on the liner temperature and the maximum temperature turns out to be very small. In order to explain the influence mechanism of operating pressure on the linear temperature, its effects on the flame structure, the liner heat flux, the convection and radiation are analyzed under different pressures. This study could be valuable for predicting and analyzing temperature distribution of the liner of gas turbine combustor.

Three-dimensional numerical modeling on high pressure membrane reactors for high temperature water-gas shift reaction

This study presents a three-dimensional numerical model that simulates the H2 production from coal-derived syngas via a water-gas shift reaction in membrane reactors. The reactor was operated at a temperature of 900 °C, the typical syngas temperature at gasifier exit. The effects of membrane permeance, syngas composition, reactant residence time, sweep gas flow rate and steam-to-carbon (S/C) ratio on reactor performance were examined. Using CO conversion and H2 recovery to characterize the reactor performance, it was found that the reactor performance can be enhanced using higher sweep gas flow rate, membrane permeance and S/C ratio. However, CO conversion and H2 recovery limiting values were found when these parameters were further increased. The numerical results also indicated that the reactor performance degraded with increasing CO2 content in the syngas composition.

Ignition delay time measurements behind reflected shock-waves for a representative coal-derived syngas with and without NH3 and H2S impurities

The composition of a representative coal-derived syngas was determined by averaging 40 practical coal syngas compositions from the literature and corresponds to a departure from many recent studies which only focus on syngas blends containing just CO and H2. Ignition delay times have been measured behind reflected shock waves for this averaged mixture with an equivalence ratio of 0.5 (0.4554% CO/0.3297% H2/0.1032% CO2/0.0172% CH4/0.2407% H2O/0.8538% O2 in 98% Ar (mol.%)) at around 1.7, 13, and 32atm. The same mixture was also investigated with impurities (200ppm of NH3 and 50ppm of H2S). Care was taken when working with the blends containing H2O and NH3 to avoid errors in the shock-tube composition; direct measurement of the water vapor mole fractions were performed using a tunable diode laser absorption diagnostic near 1.38μm. The effect of the various constituents on the ignition delay time was also investigated by comparing to results from a baseline mixture (H2/CO/O2/Ar) and results with this baseline mixture with only one of the other constituents of the syngas (i.e., CO2, CH4, H2S). Experimental data were compared with recent detailed kinetics mechanisms from the literature. Results showed that, under the conditions of this study, extending the mixture composition to include realistic concentrations of species beyond just the CO and H2 does not have a very large effect on the ignition delay time for a coal-derived syngas. However, a comparison of this coal-derived syngas with a syngas derived from biomass, tested in an earlier study by the authors, exhibited large differences due to the larger CH4 concentration in the bio-derived syngas. Two chemical kinetic models from the literature were found suitable to reproduce these data over most of the range of mixtures, temperatures, and pressures investigated, namely the mechanisms associated with Galway and with Princeton.

Coal-derived syngas MILD combustion in parallel jet forward flow combustor

The effect of air-fuel momentum flux ratio on MILD (Moderate or Intense Low-oxygen Dilution) combustion of coal-derived syngas was examined in parallel jet forward flow combustor. The results were presented on flow field using non-reactive numerical simulations and on OH∗ radicals distribution and exhaust emissions using experiments. The predicted gas recirculation ratios in the combustor are high enough to establish the reaction condition of MILD scheme. Lower air-fuel momentum flux ratio associated with higher heat load benefits the drop of peak flame temperature and the increase of reaction zone volume. The critical air-fuel momentum flux ratios below which MILD combustion occurred were identified for three MILD configurations. The MILD configuration equipped with larger air nozzles and smaller fuel nozzles was observed to achieve MILD combustion at leaner condition. The MILD regime was established for syngas fuel with lean operational limit and ultra-low NOx and CO emissions.

Effect of air preheat temperature on the MILD combustion of syngas

The effect of air preheat temperature on MILD (Moderate or Intense Low-oxygen Dilution) combustion of coal-derived syngas was examined in parallel jet forward flow combustor. The results were presented on flow field using numerical simulations and on global flame signatures, OH∗ radicals distribution and exhaust emissions using experiments. The discrete and high speed air/fuel injections into the combustor is necessary for the establishment of MILD conditions, because they cause strong gas recirculation and form large mixing region between the air and fuel jets. The critical equivalence ratio above which MILD combustion occurred was identified. The MILD regime was established for syngas fuel under air preheating conditions with lean operational limit and suppressed NOx and CO emissions. In the MILD combustion regime, the air preheating resulted in higher NOx but lower CO emissions, while the increase of equivalence ratio led to the increase of NOx and the decrease of CO emissions.

Multiwalled carbon nanotube mixed matrix membranes containing amines for high pressure CO2/H2 separation

CO2-selective mixed matrix membranes capable of facilitated transport were synthesized for CO2/H2 separation at high pressures of at least1.52 MPa and high temperatures above373.15 K. A significant improvement in membrane stability was achieved by thoroughly dispersing multiwalled carbon nanotubes (MWNTs) as mechanical reinforcing fillers in a polyvinylalcohol matrix containing amines. With 2wt% MWNTs incorporated, the membrane performance, including a CO2/H2 selectivity of 43 and a CO2 permeability of 836Barrers (1 Barrer = 7.5×10−11 cm3(STP) cm/(cm2 s kPa)), showed no change for 444h (18.5 days) when operated at1.52 MPa and380.15 K. At the same conditions, the membrane containing 4wt% acid-treated MWNTs also showed good stability. The membranes synthesized in this work are the first of this kind, displaying exceptional CO2/H2 separation performance and high tolerance to feed gas at high pressures and high temperatures. Potentially, they could be used in a stand-alone membrane unit for energy-efficient precombustion carbon capture from coal-derived syngas or in conjunction with water-gas-shift reaction for CO clean-up to produce high-purity H2 for fuel cells and to simultaneously capture CO2.

Effect of fuel injection velocity on MILD combustion of syngas in axially-staged combustor

The role of fuel injection velocity on MILD (Moderate or Intense Low-oxygen Dilution) combustion of coal-derived syngas was examined in an axially staged combustor, where the secondary air was mixed with the flue gases from the gas generation zone to produce hot and diluted oxidant prior to its mixing with the secondary fuel. The global flame signatures, OH∗ radicals distribution, and exhaust emissions were obtained through experimental measurements, while the mixing behavior between the secondary fuel and oxidant was numerically studied. Higher secondary fuel injection velocity within 199–299 m/s facilitated the earlier entrainment of oxidizer into the secondary fuel and increased the flame lift-off height, resulting in a lower flame temperature, a more distributed reaction zone and reduced NOx emissions, but higher pressure loss and CO formation. The MILD regime yields lower NOx emissions compared to the traditional diffusion combustion mode, and the N2O-intermediate mechanism dominates the NO production in the syngas MILD flame with adiabatic flame temperature lower than 1565 K according to the prediction of the chemical reactor network model.

Gasoline from Coal via DME with Electricity Co-production and CO2 Capture

Abstract Synthetic gasoline may be produced from coal-derived synthesis gas via the methanol-to-gasoline (MTG) process of ExxonMobil. The combination of commercial syngas-to-methanol technology with the MTG process thus provides a ready synthetic route for liquid hydrocarbon fuels. An alternative process has been proposed for gasoline from coal that would involve producing dimethyl ether (DME) directly from coal-derived syngas in a liquid-phase synthesis reactor and then converting the DME to gasoline. This paper presents energy and carbon balances and cost estimates based on detailed Aspen Plus process simulations for five plant designs to co-produce gasoline and electricity from coal via a DME-to-gasoline (DTG) process.