Dispersed Carbon in Basalts of the Altered Oceanic Crust: Isotope Composition and Mechanisms of Formation

Carbon contents and isotopic compositions were compared in the basalt groundmass of the oceanic crust of different age in the zone of the East Pacific Rise. In samples the basalt groundmass of the ancient oceanic crust (~270 Ma, ODP Site 801C) in which a carbonate phase was found, the isotopic composition of the oxidized carbon (δ13C = ±1.5‰) indicates that this carbon was formed by the precipitation of seawater dissolved inorganic carbon (DIC). In the samples in which no carbonate phase was identified, the low concentration (<0.1 wt % CO2) of oxidized dispersed carbon and its isotopic composition (δ13C < –7‰) are in the range of values typical of carbon dissolved in basalt glasses without crystallinity. This makes it possible to relate the oxidized dispersed carbon to residual carbon dissolved in the magmatic melt after CO2 degassing. The precipitation of DIC results in a positive correlation between the concentration of total carbon and its δ13C values, along with the formation of a carbonate phase. The application of this criterion to basalt groundmass samples of the young crust (~15 Ma, ODP Site 1256D) showed that oxidized dispersed carbon in the young oceanic crust groundmass was not formed by the precipitation of DIC, contradicting the generally accepted paradigm. Constant concentration and δ13C values of the reduced dispersed carbon in the basalt groundmass of the young and ancient oceanic crusts, including lithological zones where microbial activity has not been recorded, indicate that the most probable model is high-temperature abiogenic generation of reduced dispersed carbon near the ridge axis. The Fischer–Tropsch synthesis and/or Bell–Boudouard reaction provide a possible basis for the abiogenic model. The Bell–Boudouard reaction 2CO = C + CO2 leads to the formation of an adsorbed layer of elemental carbon on the fresh surfaces of minerals during background alteration of the oceanic basalt crust. The CO2–CO gas-phase equilibrium maintains the necessary depletion of the newly formed elemental carbon in the 13C isotope to δ13C < –20‰. Abiogenic models for the origin of the isotopically light reduced dispersed carbon in the basalt groundmass do not assume the presence of carbon depleted in the heavy 13C isotope in the magmatic melt.

Gasification of Coal by CO2: The Impact of the Heat Transfer Limitation on the Progress, Reaction Rate and Kinetics of the Process

This paper presents the impact of thermal lag on the progress of different coal types’ gasification by CO2. The analysis was performed using thermogravimetry and numerical modeling. Experiments were carried out at a heating rate of 1–50 Kmin-1 and a temperature ranging from 383 to 1173 K. The developed numerical model enabled the determination of a true sample temperature considering the gasification process to consist of two single-step consecutive reactions. Analysis revealed that the average thermal lag in CO2 is about 11% greater than that in N2, which is related to the properties of CO2 itself and the occurrence of the char–CO2 reaction. The onset temperature of the reverse Boudouard reaction depends on the type of fuel; however, no simple relationship with the coal rank was found. Thermal lag has an impact on the kinetic parameter Aα0.5 describing devolatilization, up to 19.8%, while in the case of the char–CO2 reaction, this influence is expected to be even greater. The performed analysis proved that disregarding thermal lag may significantly hinder the interpretation of the analyzed processes; thus, TG experiments should be carried out with a low heating rate, or at the post-processing stage, a thermal lag model needs to be employed.

Acid and Alkali Pre-treatment Effects on CO2 Assisted Gasification of Pine Wood

Biomass gasification in CO2 is a promising thermochemical pathway to help assist with growing issues of CO2 in the environment. However, high reaction temperature requirement and low reaction rate is limiting its development. To resolve these issues, the effect of acid and alkali pre-treatment on the pyrolysis and CO2-gasification of pine wood was examined using a semi-batch reactor. The temporal behavior of syngas components, energy, and their yield, and energy efficiency was quantified. Results showed that while acid pretreatment of biomass with lower alkali and alkaline earth metals (AAEMs) content was beneficial for the CO and syngas yield, the alkali pretreatment with higher AAEM content showed the opposite trend. In contrast, the CO2 assisted gasification of alkali pretreated biomass improved the CO and syngas yield due to catalytic influence of AAEM on the Boudouard reaction, while the acid washed biomass yielded the lowest syngas yield. During gasification, the syngas yield, energy yield and overall energy efficiency were enhanced by 83.44 %, 44.64 % and 44.58 %, respectively using alkali treatment. The results revealed that alkali pre-treatment is an effective catalytic incorporation pathway to improve the syngas, energy output, and reactivity to CO2 gasification.

Thermodynamics simulation performance of rice husk combustion with a realistic decomposition approach on the devolatilization stage

Rice husk has a great potential in its calorific value and silica content in ash which makes its valorization through combustion becomes important and interesting. This study presents the thermodynamics simulation performance of rice husk combustion using a realistic decomposition approach. A non-ideal gas approach and fugacity coefficient were also considered in the calculation. From this study, rice husk is devolatilized to form gases (63.37%), tar (8.69%), char (27.94%), and all of these are then oxidized to form flue gas. The realistic decomposition approach calculated that about 2.6 MJ/kg of specific combustion energy is produced, the maximum combustion temperature is up to 1457oC for perfect insulation condition, and up to 1400oC if there is a heat loss. It is found that combustion equipped with larger excess air could quench the heat produced and reduce the combustion efficiency but could maintain the temperature at 700oC. Furthermore, the thermodynamics simulation expressed that NO emission amount from rice husk combustion is negligible and there is still a probability for CO and H2 to be produced at above 500oC due to Boudouard reaction and homogeneous water gas shift reaction (WGSR). The study showed that a realistic decomposition approach could predict the rice husk combustion performance with a reasonable and logical result. Supplying excess air of about 180–200% is advantageous to keep the combustion temperature at 700oC in order to prevent silica crystalline formation which harms human health, as well as suppressing NO emission and reducing CO emission from the simultaneous Boudouard and WGSR.

A Microwave-Assisted Boudouard Reaction: A Highly Effective Reduction of the Greenhouse Gas CO2 to Useful CO Feedstock with Semi-Coke

The conversion of CO2 into more synthetically flexible CO is an effective and potential method for CO2 remediation, utilization and carbon emission reduction. In this paper, the reaction of carbon-carbon dioxide (the Boudouard reaction) was performed in a microwave fixed bed reactor using semi-coke (SC) as both the microwave absorber and reactant and was systematically compared with that heated in a conventional thermal field. The effects of the heating source, SC particle size, CO2 flow rate and additives on CO2 conversion and CO output were investigated. By microwave heating (MWH), CO2 conversion reached more than 99% while by conventional heating (CH), the maximum conversion of CO2 was approximately 29% at 900 °C. Meanwhile, for the reaction with 5 wt% barium carbonate added as a promoter, the reaction temperature was significantly reduced to 750 °C with an almost quantitative conversion of CO2. Further kinetic calculations showed that the apparent activation energy of the reaction under microwave heating was 46.3 kJ/mol, which was only one-third of that observed under conventional heating. The microwave-assisted Boudouard reaction with catalytic barium carbonate is a promising method for carbon dioxide utilization.

CO and CO2 Anode Gas Concentration at Lower Current Densities in Cryolite Melt

This work aims to study the CO-CO2 gas composition at low potentials and low current densities in cryolite melt with relatively low alumina content (≤2 wt%). There is a scarcity of data in the literature regarding the low current density region and also for bath low in alumina. The experimental setup was constructed to minimize the back reaction as well as the Boudouard reaction. For potentials up to 1.55 V and corresponding current densities up to 0.07 A cm−2, it was found that CO is the dominant product. Between 1.55 and 1.65 V (corresponding current density region 0.07 to 0.2 A cm−2), CO2 becomes the dominant gas product. These potential values are probably slightly large due to suspected Boudouard reaction between CO2 and carbon particles in the melt formed by disintegration of the graphite anode. The results are discussed in relation to the literature data and thermodynamic calculations.