Preparation of CaCO3 using mega-crystalline calcite in electrical furnace and batch type microwave kiln

Mega-crystalline calcite (m-CC) breaks apart easily during calcination, and cannot be easily converted to CaO due to its characteristic that requires massive heat consumption. To solve this problem, the calcination characteristics were compared using electrical furnace (EF) and batch type microwave kiln (BM). After hydrating the manufactured CaO, Ca(OH)₂ was produced, and through the carbonation process, CaCO₃ was synthesized.The results of the XRD pattern of CaO that was formed through calcinations indicated that decarbonation reaction occurred as 98.2 wt.% by EF for 240 min, and 97.8 wt.% by BM for 30 min at the same temperature of 950 °C. Hydration results revealed that CaO by EF was high-reactive whereas CaO by BM was medium-reactive. CaCO₃ was synthesized through the carbonation process. At 25 °C, in both cases, colloidal-shaped CaCO₃ was found, and the more spindle-shaped CaCO₃ by cubic-shaped self assembly was synthesized at higher temperatures. However, in case of EF, Ca(OH)₂ existed in products.     

Low temperature catalytic steam gasification of HyperCoal to produce H2 and synthesis gas

HyperCoal is an ultra clean coal with ash content <0.05 wt%. Catalytic steam gasification of HyperCoal was carried out with K₂CO₃ at 775–650 °C for production of H₂ rich gas and synthesis gas. The catalytic gasification of HyperCoal showed nearly four times higher gasification rate than raw coal. The major gases evolved were H₂: 63 vol%, CO: 6 vol% and CO₂: 30 vol%. Catalyst was recycled for four times without any significant rate loss. The partial pressure of steam was varied from 0.5 atm to 0.05 atm in order to investigate the effect of steam pressure on H₂/CO ratio. The H₂/CO ratio decreased from 9.5 at 0.5 atm to 1.9 at 0.05 atm. No significant decrease in gasification rate was observed due to change in partial pressure of steam. Gasification rate decreased with decreasing temperature and become very slow at 650 °C. The preliminary results showed that HyperCoal, an ash less coal, could be a potential hydrocarbon resource for H₂ and synthesis gas production at low temperature by catalytic steam gasification process.     

Characteristics of precipitated calcium carbonate by hydrothermal and carbonation processes with mega-crystalline calcite using rotary microwave kiln

In the conventional kiln, mega-crystalline calcite (m-CC) breaks apart easily during calcinations, and cannot be easily converted to CaO due to that it requiring a lot of heat. In this study, m-CC was calcined to CaO of around 1 mm using the rotary microwave kiln. Furthermore, CaCO₃ was produced by the carbonation process and hydrothermal process, and the form of CaCO₃ was characterized.Calcination of m-CC using the rotary microwave kiln resulted in CaO (97 wt%) of relatively fine size.CaCO₃ of colloidal-shaped and 6 μm in size could be prepared by applying the carbonation process to Ca(OH)₂ using a bubble reactor at 25 °C. As the carbonation temperature increased from 25 to 80 °C, the shape of prepared CaCO₃ changed from a colloidal-type to spindle-type of 1 μm due to self-assembly. Also, hexagonal-shaped aragonite could be prepared by the hydrothermal process with the supersaturated Ca(HCO₃)₂ solutions.     

Experimental investigation of role of steam in entrained flow coal gasification

Experimental investigations of the influence of excess oxygen coefficient, H₂O/coal mass ratio using high-temperature steam, mean mass diameter of pulverized coal and coal size fraction on basic characteristics of coal gasification were performed. Experiments were carried out on a laboratory scale (0.09 m i.d. × 1.5 m high) coal gasification apparatus with lignite type of coal. Influence of steam was realized through comparison of results obtained from experiments with (H₂O/coal = 0.287 kg kg⁻¹) and without steam addition (H₂O/coal = 0.024 kg kg⁻¹). High values of carbon conversion, obtained both for finely ground and for coarse pulverized coal points to the easiness of lignite gasification, i.e. to its high suitability for gasification.     

Effects of CaCO3 content on the densification of aluminum nitride

The effect of adding up to 13.4 wt.% CaCO₃ on the densification behavior of aluminium nitride (AlN) was investigated during pressureless sintering between 1100 and 2000 °C. The presence of second-phases, weight losses, Ca contents, and microstructures of sintered samples were correlated with the densification curves. Two microstructural aspects determined the densification of aluminum nitride with CaCO₃: second-phase evolution path and formation of large pores. Additions of small amounts of CaCO₃ caused the formation of higher melting point calcium aluminates (mainly CA₂) that increased the temperature at which liquid-phase sintering process started, but once activated rapid densification was observed. For larger CaCO₃ amounts, liquid-phase started to form at lower temperature, but the initial densification was slow, diminishing the advantage of lower C₁₂A₇ related eutectic temperature. Irrespective of the initial CaCO₃ content, all second-phase evolution paths converged to CA phase above 1600 °C, suggesting that during sintering of AlN with CaO at high temperatures, a liquid phase with composition of CA phase is more stable than others compositions. The effect of this composition changing on densification is discussed. Large pores were formed in the sites originally occupied by large particles of CaCO₃ and retarded the bulk densification in samples with high additive contents.     

Mineralogical and engineering characteristics of dry flue gas desulfurization products

Fifty-nine coal combustion products were collected from coal-fired power plants using various dry flue gas desulfurization (FGD) processes to remove SO₂. X-ray diffraction analyses revealed duct injection and spray dryer processes created products that primarily contained Ca(OH)₂ (portlandite) and CaSO₃·0.5H₂O (hannebachite). Most samples from the lime injection multistage burners process contained significant amounts of CaO (lime), CaSO₄ (anhydrite), and CaCO₃ (calcite). Bed ashes from the fluidized bed process were often dominated by CaSO₄ but also contained CaCO₃, CaO (lime), and MgO (periclase). Cyclone ashes were similar in composition to the bed ashes but contained more unspent sorbent and CaSO₄ and less MgO. Fly ash in all samples ranged from 10 to 79 wt%. Samples usually exhibited two distinct swelling episodes. One occurred immediately after water was applied due to hydration reactions, especially the conversion of CaO to Ca(OH)₂ and CaSO₄ to CaSO₄·2H₂O (gypsum). The second began between 10 and 50 d later and involved formation of the mineral ettringite (Ca₆[Al (OH)₆](SO₄)₃·26H₂O). The final pH after 112 d ranged from 10.0 to 12.1. If samples are incubated under ‘closed’ (i.e. incomplete recarbonation with atmospheric CO₂) and alkaline weathering conditions, gypsum and portlandite are initially formed followed by the conversion of the gypsum to ettringite. Closed, alkaline conditions typically can occur when FGD products are placed in confined settings such as a road embankment or buried as a discrete layer as occurs in some surface mine reclamation projects.     

Effect of steam and sodium hydroxide for the production of hydrogen on gasification of dehydrochlorinated poly(vinyl chloride)

Dehydrochlorinated poly(vinyl chloride) (PVC) and activated carbon were pyrolyzed with sodium hydroxide in a flow of steam and nitrogen at 3.0 MPa and 560–660 °C. In both cases, hydrogen and sodium carbonate were the main products, and methane, ethane, and carbon dioxide were minor products. The gasification rate increased with partial steam pressure, and the reaction order with respect to steam partial pressure was 0.69. For both dehydrochlorinated PVC and activated carbon, the gasification rate increased with the NaOH/C molar ratio. However, the rate became saturated at NaOH/C ratios higher than 2.0. The activation energy of gasification of dehydrochlorinated PVC or activated carbon was 178 kJ/mol, assuming first-order reaction rate. These experimental results indicate that hydrogen was produced from the reaction: C+2NaOH+H₂O→Na₂CO₃+2H₂.     

Hydrogen production from coal by separating carbon dioxide during gasification

Hydrogen generation during the reaction of a coal/CaO mixture with high pressure steam was investigated using a flow-type reactor. Coal, CaO and CO reactions with steam, and CO₂ absorption by Ca(OH)₂ or CaO occurred simultaneously in the experiment. It was found that H₂ was the primary resultant gas, comprising about 85% of the reaction products. CO₂ was fixed into CaCO₃ and CO was completely converted to H₂. Pyrolysis of the coal/CaO mixture carried out in N₂ was also examined. The pyrolysis gases were compared with gases produced by general coal pyrolysis. While general coal pyrolysis produced about 14.7% H₂, 50.5% CH₄, 12.0% CO and 12.0% CO₂, the gases produced from coal/CaO mixture pyrolysis were 84.8% H₂, 9.6% CH₄, 1.6% CO₂ and 1.1% CO.     

Preparation and characterization of CaO nanoparticles from Ca(OH)2 by direct thermal decomposition method

CaO is an important material because of its application as catalyst and effective chemisorbents for toxic gases. In this research CaO nanoparticles were prepared via direct thermal decomposition method using Ca(OH)₂ as a wet chemically synthesized precursor. Nanocrystalline particles of Ca(OH)₂ have been obtained by adding 1 and 2 M NaOH aqueous solutions to 0.5 M CaCl₂·2H₂O aqueous solutions at 80 °C. The precursor [Ca(OH)₂] was calcined in N₂ atmosphere at 650 °C for 1 h. Samples were characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA), infrared spectrum (IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Brunaure–Emett–Teller (BET). SEM images showed that CaO nano-particles were nearly spherical in morphology. TEM images illustrated that produced CaO nano-particles had the mean particle size of 91 and 94 nm for 1 and 2 M NaOH concentration, respectively. As a result, this method could be used for production of CaO nano-particles on large-scale as a cheap and convenient way, without using any surfactant, organic medium or complicated equipment.     

Effects of components on the rate of heat liberation of the hydration in the system of glass/gypsum/lime

Slag-simulated glasses in the system of CaO–Al₂O₃–SiO₂ were prepared. Their rates of heat liberation were measured in the system with gypsum and lime, especially in the excess gypsum system. They have a period with constant rate of heat liberation. These rates depend on the components of the glass. The effects of the components on the rate of heat liberation were discussed.The rates of constant heat liberation depend on the composition of glass especially the contents of CaO and Al₂O₃. The period of this process was controlled by the amount of lime addition. During this stage, the solid Ca(OH)₂ was completely consumed. In this sulfate-excess system, the reaction equation, during Ca(OH)₂ consumption, should be written as; glass + gypsum + lime ⇒ C–S–H with a C/S ratio of 1.42 and Ettringite.     

The effects of raw materials particle size and salt type on formation of nano-CaZrO3 from molten salts

Nano-CaZrO₃ was successfully synthesized at 800 °C using the molten-salt method, and the effects of salt type and raw materials particle size on the formation of CaZrO₃ were investigated. Na₂CO₃, CaCl₂, nano-ZrO₂ and micro-ZrO₂ were used as starting materials. On heating, Na₂CO₃ reacted with CaCl₂ to form NaCl and in situ CaCO₃. Na₂CO₃–NaCl molten eutectic salt provided a liquid medium for reaction of CaCO₃ and ZrO₂ to form CaZrO₃. The results demonstrated that in both nano- and micro-ZrO₂ inclusive samples, CaZrO₃ started to form at about 700 °C and that, after the temperature was increased to 1000 °C, the amounts of CaZrO₃ in the resultant powders increased with a concomitant decrease in CaCO₃ and ZrO₂ contents. After washing with hot-distilled water, the samples containing nano- and micro-ZrO₂ heated for 3 h at 800 °C and 1000 °C, were single-phase CaZrO₃ with 70–90 nm and 400–450 nm particle size, respectively. Also, the synthesis process was completed in lower temperatures using eutectic salts. Furthermore, the synthesized CaZrO₃ particles retained the size and morphology of the ZrO₂ powders, which indicated that a template formation mechanism dominated the formation of CaZrO₃ by molten-salt synthesis.     

Lime enhanced gasification of solid fuels: Examination of a process for simultaneous hydrogen production and CO2 capture

The lime enhanced gasification (LEGS) process uses CaO as a CO₂ carrier and consists of two coupled reactors: a gasifier in which CO₂ absorption by CaO produces a hydrogen-rich product gas, and a regenerator in which the sorbent is calcined producing a high purity CO₂ gas stream suitable for storage. The LEGS process operates at a pressure of 2.0 MPa and temperatures less than 800 °C and therefore requires a reactive fuel such as brown coal. The brown coal ash and sulfur are purged from the regenerator together with CaO which is replaced by fresh limestone in order to maintain a steady-state CaO carbonation activity (a_ave). Equilibrium calculations show the influence of process conditions and coal sulfur content on the gasifier carbon capture (>95% is possible). Material balance calculations of the core process show that the required solid purge of the sorbent cycle is mainly attributed to the necessary removal of ash and CaSO₄ if the solid purge is used as a pre-calcined feedstock for cement production. The decay in the CaO capture capacity over many calcination–carbonation cycles demands a high sorbent circulation ratio but does not dictate the purge fraction. A thermodynamic analysis of a LEGS-based combined power and cement production process, where the LEGS purge is directly used in the cement industry, results in an electric efficiency of 42% using a state of the art combined cycle.     

Effect of porosity on carbonation and hydration resistance of CaO materials

CaO pellets with different porosity were carbonated at 700 °C in CO₂ atmosphere. The carbonation rate was controlled by the diffusion of CO₂, regardless of the difference in porosities. For the low-porosity pellet, carbonation reaction only occurred on the surface, with a dense CaCO₃ film thus formed, which combined well with the substrate material; while for the pellet of high-porosity, the carbonation reaction occurred simultaneously both on surface and inside pores, and each CaO grain was surrounded by CaCO₃ film that contained microfissures. Hydration test results showed that carbonation treatment could effectively improve the hydration resistance of CaO materials regardless of porosity, but the carbonated high-porosity pellet was prone to breakage due to poor combination between the carbonated CaO grains. Therefore, for the purpose to improve the hydration resistance by carbonation treatment, it is recommended that the CaO materials should be either with less appreciable apparent porosity or with a limited carbonation ratio for the high-porosity CaO material.     

Gasification reactivity of char from dried sewage sludge in a fluidized bed

The gasification reactivity of char from dried sewage sludge (DSS) applicable to fluidized bed gasification (FBG) was determined. The char was generated by devolatilizing the DSS with nitrogen at the selected bed temperature and was subsequently gasified by switching the fluidization agent to mixtures of CO₂ and N₂ (CO₂ reactivity tests) and steam and N₂ (H₂O reactivity tests)._. The tests were conducted in the temperature range of 800–900 °C at atmospheric pressure, using partial pressure of the main reactant in the mixture (CO₂ or H₂O) in the range of 0.10–0.30 bar. Expressions for the intrinsic reactivity (free of diffusion effects) as a function of temperature, partial pressure of gas reactant (CO₂ or H₂O) and degree of conversion were obtained for each reaction. For the whole range of conversion it was found that the char reactivity in an H₂O–N₂ mixture was roughly three times higher than that in a mixture with the corresponding partial pressure of CO₂. The reactivity was only influenced by particle size greater than 1.2 mm in the tests with steam at 900 °C. It was demonstrated that the method of char preparation greatly influences the reactivity, highlighting the importance of generating the char in conditions similar to that in FBG.     

The effects of temperature,holding time and salt amount on formation of nano CaZrO3 via molten salt method

Nano-particles of CaZrO₃ were successfully synthesized at 800 °C using the molten-salt method, and the effects of processing parameters, such as temperature, holding time and amount of salt on the crystallization of CaZrO₃ were investigated. Na₂CO₃, CaCl₂ and nano-ZrO₂ were used as starting materials. On heating, Na₂CO₃ reacted with CaCl₂ to form NaCl and in situ CaCO₃. Na₂CO₃–NaCl molten eutectic salt provided a liquid medium for reaction of CaCO₃ and ZrO₂ to form CaZrO₃. The results demonstrated that CaZrO₃ started to form at about 700 °C and that, after the temperature was increased to 1000 °C, the amounts of CaZrO₃ in the resultant powders increased with a concomitant decrease in CaCO₃ and ZrO₂ contents. After washing with hot-distilled water, the samples heated for 3 h at 800 °C were single-phase CaZrO₃ with 70–90 nm particle size. Furthermore, the synthesized CaZrO₃ particles retained the size and morphology of the ZrO₂ powders, which indicated that a template formation mechanism dominated the formation of CaZrO₃ by molten-salt synthesis.     

The influence of different CaO source in the production of anorthite ceramics

The effects of starting raw materials and firing conditions on anorthite (CaAl₂Si₂O₈) phase formation are investigated by differential thermal analysis (DTA)–thermogravimetry (TG) and X-ray powder diffraction (XRD). Four different sources of CaO were used for anorthite production such as Ca(OH)₂, CaCO₃, marble powder and gypsum mould waste. The mixture of raw materials was prepared in stoichiometric ratio of anorthite. Sintering of samples was carried out at various temperatures (1000–1300 °C). In all samples before the formation of anorthite phase, formation of layered alimunosilicate phase (LAS) and of gehlenite phase were observed at low temperatures (<1200 °C). All the samples showed similar crystallization behaviour at 1200 °C. The densification characteristic and the flexural strength of samples were affected by the nature of starting raw materials. The maximum density (～80%) was reached in sample ACH which was prepared from Ca(OH)₂.     

Coke removal from deactivated Co–Ni steam reforming catalyst using different gasifying agents: An analysis of the gas–solid reaction kinetics

The reactivity of various gases, namely; O₂, air, CO₂, H₂ and N₂, with carbon deposited on alumina-supported Co–Ni catalyst during propane reforming in a fluidized bed reactor at 773–973 K using relatively low feed steam:carbon ratio (0.8–1.5) has been investigated in a thermogravimetric analysis unit. Analysis of the transient solid weight loss revealed that carbon removal mechanism is dependent on the type of gasifying agent. Carbon gasification kinetics using O₂ and air followed the Avrami-Erofeev (A2) model while data for both CO₂ and H₂ were captured by the geometrical (contracting area, R2) model. However, carbon gasification with inert N₂ proceeded at much slower rate (about 10 times lower than air) and was adequately fitted by the one-dimensional diffusion (D1) model. Specific reaction rates from these phenomenological models were also linearly correlated with the catalyst carbon content with reactivity coefficient of the gasifying agent decreasing in the order, O₂ > air > CO₂ > H₂ > N₂. In order to minimize energy consumption during catalyst regeneration, reduce greenhouse gas emissions and reduce catalyst sintering, it would be desirable to employ a mixture of air and CO₂ as the carbon gasifying agent to take advantage of the coupled exothermic (air oxidation) and endothermic (reverse Boudouard reaction involving CO₂ and carbon) nature taking place during the carbon removal operation.     

Controllable preparation of nano-CaCO3 in a microporous tube-in-tube microchannel reactor

In this work, nano-CaCO₃ particles with tunable size have been synthesized via CO₂/Ca(OH)₂ precipitation reaction in a microporous tube-in-tube microchannel reactor (MTMCR) with a throughput capacity up to 400 L/h for CO₂ and 76.14 L/h for liquid. The overall volumetric mass-transfer coefficient (K_La) of CO₂ absorption into Ca(OH)₂ slurry in the MTMCR has been deduced and analyzed. To control the particle size, the effect of operating conditions including initial Ca(OH)₂ content, gas volumetric flow rate, liquid volumetric flow rate, micropore size, and annular channel width was investigated. The results indicated that the mass transfer in the MTMCR can be greatly enhanced in contrast with a stirred tank reactor, and the particle size can be well controlled by tuning the operating parameters. The nano-CaCO₃ particles with an average size of 28 nm and a calcite crystal structure were synthesized, indicating that this process is promising for mass production of nanoparticles.     

Development of cobalt catalysts for the steam reforming of naphthalene as a model compound of tar derived from biomass gasification

The catalytic performances of Co/MgO catalysts for the steam reforming of naphthalene were investigated. The results of characterizations (TPR, XRD, CO adsorption, and CO-TPD) showed that large-sized Co metal particles were formed over the catalysts pre-calcined at 873 K with high Co loading via reduction of Co₃O₄ and MgCo₂O₄ phases. A few Co metal particles were obtained over the catalysts pre-calcined at 1173 K with all Co loading values after reduction.The catalytic performances data showed that 12 wt.% Co/MgO catalyst pre-calcined at 873 K exhibited the best catalytic performance (conv., 23%, 3 h) for the steam reforming of naphthalene among the catalysts tested in this study, due to the existence of Co metal and the low amounts of coke deposition. On the other hand, the data also revealed that the reaction of steam reforming of naphthalene proceeds over all Co-loaded catalyst pre-calcined at 1173 K initially; however, the deposition of the polymer of C_nH_m radicals and the oxidation of catalysts by H₂O led to the decrease of activity.It should be noted that 12 wt.% Co/MgO catalyst pre-calcined at 873 K showed high and stable activity under the low steam/carbon mole ratio (0.6), with H₂ and CO₂ as main products. These two excellent advantages serve to increase the overall biomass gasification system energy efficiency and allow using the product gas for fuel cell system. Thus, Co catalyst is a promising system for the steam reforming of naphthalene derived from biomass gasification as a second fixed catalytic bed.     

Experimental simulation of hard coal underground gasification for hydrogen production

The main objective of this study was the assessment of the feasibility of applying the underground hard coal gasification in the production of a hydrogen-rich gas. In the course of the experiment the so-called two-stage gasification process in which oxygen and steam were supplied to the reaction zone separately in alternate stages was investigated. For this purpose a special surface reactor has been constructed, in which the underground conditions were to be imitated both in respect to the coal seam and the surrounding rock layers. The surface simulation of the underground gasification was carried out on extra large coal samples, which allowed the recreation of near real underground conditions. In the reactor an experiment lasting almost 7 days has been performed, with the average hourly gas yields of 7.8 m³/h and 9.2 m³/h for the oxygen and the steam gasification stages, respectively. The average hydrogen concentration during the oxygen stage (heating up) was 15.28% with the maximum of 54.4%. The average hydrogen concentration during the steam stage was 53.77% with the maximum of 62.90%. Thus the feasibility of hydrogen-rich gas production in the process of underground gasification of hard coal has been demonstrated. During the course of the surface experiment an original and unprecedented database of temperature profiles has been acquired which now constitutes an invaluable source of thermodynamic information for the prospective underground gasification activities.