Gasification Step Research Articles

Oxygen Release and Oxidation Rates of MgAl2O4-Supported CuO Oxygen Carrier for Chemical-Looping Combustion with Oxygen Uncoupling (CLOU)

The chemical-looping combustion (CLC) and chemical-looping with oxygen uncoupling (CLOU) processes are novel solutions for efficient combustion with inherent separation of carbon dioxide. These processes use a metal oxide as an oxygen carrier to transfer oxygen from an air reactor to a fuel reactor, where the fuel reacts with the solid oxygen carrier. When solid fuel is used in CLC, the char must be gasified by, e.g., steam to form H2 and CO, that can be subsequently oxidized to H2O and CO2 by the oxygen carrier. In the case of CLOU, the oxygen carrier releases oxygen gas in the fuel reactor. This enables a high rate of conversion of char from solid fuels, because it eliminates the need for the gasification step needed in normal CLC with solid fuels. In this work, the rate of oxygen release and oxidation of an oxygen carrier consisting of CuO supported by MgAl2O4 (40/60 wt %) for the CLOU process is investigated. The oxygen carrier was produced by freeze-granulation, calcined at 950 °C, and sieved to a size range of 125–180 μm. The reaction rates were obtained in a laboratory-scale fluidized-bed reactor in the temperature range of 850–900 °C, under alternating reducing and oxidizing conditions. The rate of oxygen release was obtained using devolatilized wood char as the fuel in N2 fluidization. Care was taken to obtain reliable measurements not affected by the availability of the fuel and temperature increase in the bed during combustion of the fuel with the released oxygen from the carrier. The Avrami–Erofeev mechanism was used to model the rates of oxygen release and the values of ko and Eapp were estimated to be 2.5 × 105 s–1 and 139.3 kJ mol–1, respectively. The rates of Cu2O oxidation were investigated in a flow of 5% O2 at the inlet of the reactor. However, it was observed that the oxidation rate is limited by the oxygen supply, indicating rapid conversion of the oxygen carrier. From the obtained reaction rates, the minimum total amount of the investigated oxygen carrier needed in the air and the fuel reactor is estimated to be between 69–139 kg MWth–1.

Kinetic study of polish hard coal and its char gasification using carbon dioxide

The kinetics of polish hard coal and its char gasification using carbon dioxide as gasifying agent was studied at atmospheric pressure between 293 and 1223K at different linear heating rates. Calculations using isoconversional method showed that coal gasification has two separate steps: pyrolysis in the range from 600 to 850K and gasification in the range from 1000 to 1273K. This proves the additivity of pyrolysis and gasification steps. The apparent activation energy calculated using the F1 model for coal char gasification is equal to 245kJ/mol, and logA is equal to 7.41/s, which is similar to the value obtained by the isoconversional method. Coal gasification reaction with carbon dioxide is faster and takes place in a lower temperature range than the gasification process of the char obtained in a separate process. The stabilized and chemically deactivated char is harder to gasify, and the same conversion is achieved at the higher temperature. The estimated shift may be as high as 150K.

Catalytic gasification of a Powder River Basin coal

Catalytic gasification of a Wyodak low-sulfur sub-bituminous coal from the Powder River Basin of Wyoming was investigated using an inexpensive catalyst, Na2CO3, widely available in Wyoming. Experiments were performed in an atmospheric pressure fixed-bed laboratory gasifier. The effects of different factors including feed gas composition, catalyst loading and reaction temperature on the associated coal pyrolysis and char gasification were evaluated. Na2CO3 was found to be active during both pyrolysis and gasification steps and it can considerably decrease the activation energy of gasification step. The optimal Na2CO3 addition level is approximately 3wt.%. Shrinking core model and random pore model can be used to fit the kinetic data obtained under both non-catalytic and catalytic conditions.

H2 enriched fuels from co-pyrolysis of crude glycerol with biomass

Pyrolysis of glycerol has been identified as a possible route for producing high added value fuels like renewable hydrogen (H2). Crude glycerol (CG) is the main byproduct of biodiesel industry and without purification it is a low added value material due to the presence of impurities. Co-pyrolysis of CG with biomass may improve the efficiency of the process and as a primary step of gasification give important information concerning the maximization of H2 concentration in the produced gas. Moreover, the thermochemical treatment of crude glycerol–biomass mixtures may offer several economic and environmental advantages in biodiesel industry and reduce the cost of biodiesel production. A mixture of CG with olive kernel (OK) was used as pyrolysis feed material. Pyrolysis of a 25wt% mixture of CG with OK at high temperature (T=720°C) seemed to promote steam reforming reactions leading to an increase of H2 concentration of 11.6 vv% in the pyrolysis gas in comparison to H2 in gas obtained by low temperature pyrolysis (T=520°C).

Thermodynamic Assessment of a Novel Concept for Integrated Gasification Chemical Looping Combustion of Solid Fuels

A novel integrated gasification chemical looping combustion (IGCLC) process is proposed here for combustion of solid fuels, particularly coal. The proposed process incorporates an ex situ step for gasification of the solid fuel, but unlike conventional ex situ methods, the gasification process is fully integrated with the combustion process. This is achieved using a three-step chemical loop for the production of hydrogen, combustion of gaseous fuels, and regeneration of metal oxides. A detailed thermodynamic chemical equilibrium assessment of the IGCLC process was carried out to evaluate its technical viability. The relevant analyses were performed using Aspen Plus process simulation software. The IGCLC process was found to be thermodynamically feasible. More specifically, it was uncovered that the gasification process can operate at an adequate temperature (above 1023 K) at thermoneutral conditions with high coal conversion (95%). It was also found that, to achieve the highest hydrogen production, the steam/hydrogen to carbon ratio (SHTCR) had to be set to 2, at which the gasification temperature was around 1070 K, coal conversion was 95%, gaseous fuel reactor (i.e., combustor) temperature was 1140 K, and H2 productivity was 0.85 mol/mol carbon. Mass and energy balance calculations were also performed. It demonstrated that the proposed IGCLC system can achieve an electricity efficiency of 49.5% at SHTCR ∼ 2 and feed temperature of 1100 K with some appropriate assumptions, which is 80% higher than a conventional coal-fired power station with carbon capture and storage (CCS) measures.

Sugarcane bagasse gasification: Global reaction mechanism of syngas evolution

Steam gasification of sugarcane bagasse has been investigated. A semi batch reactor with a fixed amount of sugarcane bagasse sample placed in steady flow of high temperature steam at atmospheric pressure has been used. The gasification of bagasse was examined at reactor and steam temperatures of 800, 900 and 1000 °C. The evolution of syngas flow rate and chemical composition has been monitored. The evolution of chemical composition and total flow rate of the syngas has been used to formulate a global reaction mechanism. The mechanism combines pyrolysis reaction mechanisms from the literature and steam gasification/reforming reactions. Steam gasification steps include steam–hydrocarbons reforming, char gasification and water gas shift reactions. Evidence of fragmentation, secondary ring opening reactions and tertiary reactions resulting in formation of gaseous hydrocarbons is supported by higher flow rates of syngas and hydrogen at high heating rates and high reactor temperatures. Increase in carbon monoxide flow rate at the expense of carbon dioxide flow rate with the increase in reactor temperature has been observed. This increase in the ratio of CO/CO 2 flow rate confirms the production of CO and CO 2 from the competing reaction routes. At 1000 °C gasification a total merging between the pyrolysis step and the char gasification step has been observed. This is attributed to acceleration of char gasification reactions and acceleration of steam–hydrocarbons reforming reactions. These hydrocarbons are the precursors to char formation by aromatization and repolymerization.

A review on devolatilization of coal in fluidized bed

Devolatilization is an important step in fluidized bed combustion and gasification of coal. ‘Devolatilization’ is a general term that signifies the removal of volatile matters from the coal matrix. It is an extremely important step because the combustion of volatile matter can account for 50% of the specific energy of fluidized bed combustion of a high-volatile coal. Significant insights into the complex physicochemical phenomena that occur during devolatilization have been obtained in the recent years. This review focuses on the devolatilization of coal in an inert gas, air, and oxygen-enriched air, with emphasis on the effects of the operating parameters (e.g. temperature, heating rate, pressure, and gas velocity) on the yield of volatile matter. Particle size, oxygen content of the fluidizing gas, volatile content of coal and specific heat are some of the other important parameters for the devolatilization of coal. This review also explains the development and application of structural and empirical models. The structural models (e.g. FG-DVC and CPD models) are fairly complex. However, they can accurately predict the yields of gas and tar. It is observed from the review of the literature that the mechanism of coal devolatilization needs further study. Although the shrinking-core model can describe the devolatilization in the beginning and toward the end of the process, major deviations are often observed. The economic studies reveal that the capital cost of fluidized bed combustion reduces upon doubling the capacity. Some problems associated with bubbling fluidized bed combustion (e.g. the increase in freeboard temperature) have been explained with the present knowledge of devolatilization. Copyright © 2011 John Wiley & Sons, Ltd.

Exergetic performance assessment of gasification and pyrolysis processes of pre-treated wood board wastes

This paper investigates exergetic performance of gasification and pyrolysis processes and compares their exergy efficiencies, using the experimental data on a two-step process of thermo-chemical conversion of Wood Board (WB) wastes. The exergy values of biomass samples change from 17.49 MJ/kg to 24.57 MJ/kg, respectively. Hydrogen produced from pyrolysis step of gasification varies between 17 (%v/v) and 47 (%v/v) and ranges from 17.1 to 42.6 (%v/v) in pyrolysis. The exergy contents of hydrogen range from 2.20 kJ to 16.26 kJ with the highest efficiency of 93% in pyrolysis step of gasification of WB_1000 samples, and from 2.01 kJ to 10.43 kJ with the highest efficiency of 51% in pyrolysis of WB_250_1000 samples.

Experimental Results and Model for N‐Gas Compound Production in Pure Steam Gasification for Wastewater Sewage Sludge

AbstractSewage sludge produced in wastewater treatment plants represent a significant portion of waste resulting from human activity and it is important to offer new ways of treatment. Thermal gasification of sludge is a promising thermal treatment, particularly because it can lead to a recovery of energy and a drastic reduction in the initial waste volume. However, it is a subject that has rarely been treated in the literature for sewage sludge. In particular, the nitrogen compounds present in significant quantities in the sewage sludge and their reactions during a thermal gasification step, must necessarily be taken into account. In order to study nitrogen behavior, this paper presents experimental results that are included in a model to predict NH3 emission.

Surface tension measurements of coal ash slags under reducing conditions at elevated pressures

The aim of limiting the amount of CO 2 that is released together with other exhaust gases from power plants can be reached by technologies allowing for a systematic separation of this greenhouse gas. One such technology is the integrated gasification combined cycle power plant which makes use of a coal gasification step. For the gasification involving temperatures far higher than in typical pulverised combustion chambers, ash contained in the fuel is liquefied (slag) and must be removed from the cycle to guarantee safe operation of downstream equipment. To keep the efficiency of the power plant as high as possible, hot gas cleaning facilities are most desirable for this purpose. The design of these installations necessitates knowledge about thermophysical properties of coal ash slags, especially in reducing, pressurised atmospheres. In this work, the surface tension of 15 coal ash slags was measured in argon hydrogen gas of up to 10 bar absolute pressure according to the sessile drop method. Compared to experiments at 1 bar, surface tension values up to 42% lower were found on applying pressure. Additionally, shifts in the melting temperature interval of the ashes due to increased pressure were observed. The surface tension values obtained in pressurised atmospheres ranged from 270 to 490 mN/m with respect to temperature intervals where almost no data scattering occured.

Bench-scale production of liquid fuel from woody biomass via gasification

The bench-scale production of hydrocarbon liquid fuel was achieved from woody biomass via gasification. The daily production capacity of the biomass-to-liquid (BTL) plant used in this study was 7.8 L of hydrocarbon liquid from 48 kg of woody biomass (on a dry basis), corresponding to 0.05 barrels. The BTL process involved the following steps: oxygen-enriched air gasification of the woody biomass, wet and dry gas cleaning, gas compression, carbon dioxide removal, and the Fischer–Tropsch (FT) synthesis reaction. In the gasification step, oxygen-enriched air gasification was carried out using a downdraft fixed-bed gasifier. The content of oxygen, which acts as the gasifying agent, was increased from 21.0 to 56.7 vol%; maximum values of the conversion to gas on a carbon basis and cold gas efficiency–approximately 96 C-mol% and 87.8%, respectively–were obtained at an oxygen content of around 30 vol%. With the increased oxygen content, the concentrations of CO, H 2, and CO 2 increased from 22.8 to 36.5 vol%, from 16.8 to 28.1 vol%, and from 9.8 to 14.8 vol%, respectively, while that of N 2 decreased from 48.8 to 16.0 vol%. The feed gas for the FT synthesis reaction was obtained by passing the product gas from the gasification step through a scrubber, carbon dioxide removal tower, and desulfurization tower; its composition was 30.8 vol% CO, 25.2 vol% H 2, 0.9 vol% CO 2, 2.5 vol% CH 4, 40.6 vol% N 2, < 5 ppb H 2S, and < 5 ppb COS. The hydrocarbon fuel was synthesized in a slurry bed reactor using hexadecane as the solvent and a Co/SiO 2 catalyst. For hydrocarbons with carbon chain lengths of more than 5 carbon atoms (collectively referred to as C 5+) in the liquid fuel, a selectivity of 87.5% was obtained along with a chain growth probability of 0.84 under the following conditions: 4 MPa, 280 to 340 °C, and a ratio of catalyst weight to feed gas rate (W/F) of 9.3 g·h/mol.

Development of an Entrained Flow Gasifier Model for Process Optimization Study

Coal gasification is a versatile process to convert a solid fuel in syngas, which can be further converted and separated in hydrogen, which is a valuable and environmentally acceptable energy carrier. Different technologies (fixed beds, fluidized beds, entrained flow reactors) are used, operating under different conditions of temperature, pressure, and residence time. Process studies should be performed for defining the best plant configurations and operating conditions. Although “gasification models” can be found in the literature simulating equilibrium reactors, a more detailed approach is required for process analysis and optimization procedures. In this work, a gasifier model is developed by using AspenPlus as a tool to be implemented in a comprehensive process model for the production of hydrogen via coal gasification. It is developed as a multizonal model by interconnecting each step of gasification (preheating, devolatilization, combustion, gasification, quench) according to the reactor configuration, that is an entrained flow reactor. The model removes the hypothesis of equilibrium by introducing the kinetics of all steps and solves the heat balance by relating the gasification temperature to the operating conditions. The model allows to predict the syngas composition as well as quantify the heat recovery (for calculating the plant efficiency), “byproducts”, and residual char. Finally, in view of future works, the development of a “gasifier model” instead of a “gasification model” will allow different reactor configurations to be compared.

A predictive multi-step kinetic model of coal devolatilization

Devolatilization is the first step in coal combustion and gasification, thus an accurate kinetic modeling is relevant for the optimal design of these processes. In this work a relatively simple but flexible kinetic model is used to predict the thermal degradation of different coals in a wide range of operating conditions. The main feature of the model lies in its predictive capability: the elemental composition of the starting coal and the operating conditions are the only information required. Three reference coals are used to characterize the devolatilization process. The pyrolysis of each reference coal is described with a multi-step kinetic mechanism effective both at high and low heating rates. The devolatilization of the actual coal is simply obtained as a linear combination of the thermal degradation of the reference coals. The complete kinetic model refers to ∼30 reactions and lumped species, which makes this scheme suitable for being adopted in fluidynamic computations. A wide collection of comparisons between model prediction and experimental data validates this model both in terms of residual char and in terms of detailed gas and tar composition. The importance of secondary gas-phase reactions, mainly at high pressure, is also discussed and verified on the basis of an existing detailed kinetic scheme of pyrolysis and oxidation of hydrocarbon fuels.

Thermodynamic analysis of gasification-driven direct carbon fuel cells

The gasification-driven direct carbon fuel cell (GD-DCFC) system is compared with systems using separate gasification steps prior to work extraction, under autothermal or indirect constraints. Using simple system exergy analysis, the maximum work output of the indirect gasification scheme is 4–7% lower than the unconstrained direct approach, while the work output of the autothermal gasification approach is 12–13% lower than the unconstrained case. A more detailed calculation for the DCFC and indirect gasification plants, using common solid fuel compositions, gives conversion efficiencies in the range of 51–58% at an operating voltage of 0.7 V selected for both systems in this study. In contrast, the conversion efficiency of the autothermal gasification approach is estimated to be 33–35% at 0.7 V. DCFC efficiencies can be increased to over 60% by an increase in operating voltage and/or inclusion of a bottoming cycle. The thermodynamic model also indicates that steam gasification yields similar work output and thermal efficiency as for CO2 gasification. Open circuit potential measurements agree with equilibrium calculations both for the C–O and C–H–O gasification systems, confirming the governing mechanism and feasibility of the GD-DCFC. Current–voltage measurements on an un-optimized system demonstrate power densities of 220 mW cm−2 at 0.68 V during operation at 1178 K.

Meteorite ablation products and their contribution to the atmospheres of terrestrial planets: An experimental study using pyrolysis-FTIR

We have used a recently developed quantitative pyrolysis-Fourier transform infrared spectroscopy method to measure the production of water and carbon dioxide during 250 °C desorption and 1000 °C gasification steps for a range of carbonaceous chondrites. Greater yields of water and carbon dioxide during gasification are associated with meteorites believed to have experienced more aqueous alteration on their asteroid parent body (i.e. gas yields for petrographic type 1 > type 2 > type 3). Volatile yields most likely reflect quantities of hydrated mineral phases and partially oxidised organic matter. Methane was not detected in the gasification products of the meteorites, allowing an upper limit on its production of around 100 ppm to be calculated based on the sensitivity of the pyrolysis-Fourier transform infrared spectroscopy technique employed. When considered alongside rates of infall of cosmic dust throughout Earth history, the data can be used to evaluate the production of volatiles during the thermal ablation of dust upon atmospheric entry, and to estimate their contribution to a terrestrial planet’s atmosphere and hydrosphere. Over the long term, it appears that contributions of this nature to the Earth’s volatile inventory are small, although production rates are calculated to have been substantially higher before and during the Late Heavy Bombardment of 3.8–4.0 Ga. Moreover, ablation of carbonaceous chondritic material does not appear to be a plausible source of the atmospheric methane budget of Mars.

Hydrothermal Gasification of Waste Biomass: Process Design and Life Cycle Asessment

A process evaluation methodology is presented that incorporates flowsheet mass and energy balance modeling, heat and power integration, and life cycle assessment Environmental impacts are determined by characterizing and weighting (using CO2 equivalents, Eco-indicator 99, and Eco-scarcity) the flowsheet and inventory modeling results. The methodology is applied to a waste biomass to synthetic natural gas (SNG) conversion process involving a catalytic hydrothermal gasification step. Several scenarios are constructed for different Swiss biomass feedstocks and different scales depending on logistical choices: large-scale (155 MW(SNG)) and small-scale (5.2 MW(SNG)) scenarios for a manure feedstock and one scenario (35.6 MW(SNG))for a wood feedstock. Process modeling shows that 62% of the manure's lower heating value (LHV) is converted to SNG and 71% of wood's LHV is converted to SNG. Life cycle modeling shows that, for all processes, about 10% of fossil energy use is imbedded in the produced renewable SNG. Converting manure and replacing it, as a fertilizer, with the process mineral byproduct leads to reduced N20 emissions and an improved environmental performance such as global warming potential: -0.6 kg(CO2eq)/MJ(SNG) vs. -0.02 kg(CO2eq)/MJ(SNG) for wood scenarios.

Synthesis of Liquefied Bio-Dimethyl Ether (DME) from Woody Biomass via Gasification

For the synthesis of bio-dimethyl ether (DME) from woody biomass via gasification, we have developed and promoted the biomass-to-liquid (BTL) process. This process consisted of oxygen-enrichment gasification of woody biomass by a bench-scale fixed-bed gasifier, wet gas cleaning, gas compression, desulfurization, removal of CO2, DME synthesis reaction by a fixed-bed reactor. The composition obtained at gasification step was as follows: 25.6 vol% of H2, 31.2 vol% of CO, 2.7 vol% of CH4, 0.4 vol% of CO2, 40.1 vol% of N2, 2S, and 2O3 catalyst for dehydration. It was found that the Cu-Zn and γ-Al2O3 catalysts exhibited higher activity when used as mixed powder form than used as separated pellet form. DME concentration 5.2 vol% in the product gas was obtained under the following conditions: 0.98 MPa, 230 °C, W/F=37.9 cat.-kg h/m3. It was possible to recover the almost all DME in the gas phase by using the activated carbon trap with dry ice. Finally, we succeeded in the continuous synthesis of DME over 200 h and recovery of approximately 120 g of liquefied bio-DME.

Differential Evolution Approach for Obtaining Kinetic Parameters in Nonisothermal Pyrolysis of Biomass

Pyrolysis, a first step in the biomass gasification, is the thermal decomposition of organic matter under inert atmospheric conditions, leading to the release of volatiles and formation of char. As pyrolysis is a kinetically controlled reaction, kinetic parameter estimation is very important in the design of pyrolysis reactors. In the proposed kinetic model of this study, the kinetic scheme of biomass decomposition by two competing reactions giving gaseous volatiles and solid charcoal is used. Four different models are proposed based on different possible relation of activity of biomass with normalized conversion. The corresponding kinetic parameters of the above models are estimated by minimizing the square of the error between the reported nonisothermal experimental data of thermogravimetry of hazelnut shell and simulated model predicted values of residual weight fraction using differential evolution (DE), a population-based search algorithm. Among the four different models proposed in this study, the model in which rate of change of activity of biomass with normalized conversion proposed as a function of activity itself gave the best agreement with the experimental data.

Chemical-looping with oxygen uncoupling for combustion of solid fuels

Chemical-looping with oxygen uncoupling (CLOU) is a novel method to burn solid fuels in gas-phase oxygen without the need for an energy intensive air separation unit. The carbon dioxide from the combustion is inherently separated from the rest of the flue gases. CLOU is based on chemical-looping combustion (CLC) and involves three steps in two reactors, one air reactor where a metal oxide captures oxygen from the combustion air (step 1), and a fuel reactor where the metal oxide releases oxygen in the gas-phase (step 2) and where this gas-phase oxygen reacts with a fuel (step 3). In other proposed schemes for using chemical-looping combustion of solid fuels there is a need for an intermediate gasification step of the char with steam or carbon dioxide to form reactive gaseous compounds which then react with the oxygen carrier particles. The gasification of char with H 2O and CO 2 is inherently slow, resulting in slow overall rates of reaction. This slow gasification is avoided in the proposed process, since there is no intermediate gasification step needed and the char reacts directly with gas-phase oxygen. The process demands an oxygen carrier which has the ability to react with the oxygen in the combustion air in the air reactor but which decomposes to a reduced metal oxide and gas-phase oxygen in the fuel reactor. Three metal oxide systems with suitable thermodynamic properties have been identified, and a thermal analysis has shown that Mn 2O 3/Mn 3O 4 and CuO/Cu 2O have suitable thermodynamic properties, although Co 3O 4/CoO may also be a possibility. However, the latter system has the disadvantage of an overall endothermic reaction in the fuel reactor. Results from batch laboratory fluidized bed tests with CuO and a gaseous and solid fuel are presented. The reaction rate of petroleum coke is approximately a factor 50 higher using CLOU in comparison to the reaction rate of the same fuel with an iron-based oxygen carrier in normal CLC.

Catalytic effects of potassium on lignin steam gasification with γ-Al2O3 as a bed material

The effects of potassium on the reactivity of biomass-char steam gasification with the presence of a porous material were investigated by using a thermogravimetric reactor with high-heating rates. Lignin was employed as a char-rich biomass model compound. The potassium carbonate (K2CO3) was added to lignin and a mixture of lignin and γ-Al2O3 porous particles by means of aqueous impregnation. The effects of K2CO3 and γ-Al2O3 addition on pyrol- ysis of lignin and steam gasification of lignin-derived char were evaluated in terms of lignin conversion and the gaseous products. Results showed that K2CO3 slightly increased the steam gasification rate of lignin-derived char, but it did not influence the conversion in both the pyrolysis and steam gasification steps. In addition, tar was reduced by adding K2CO3 because of the increment of carbon conversion to gas product. The presence of γ-Al2O3 was found to induce the lower reactivity of resulting char after pyrolysis, reducing the gasification rate and conversion. A significant im- provement in gasification conversion was observed with the presence of both K2CO3 and γ-Al2O3. Especially, almost complete gasification was achieved at a reaction temperature of 1,073 K.